High performance films: review of new materials and trends.
The meaning of the term "High Performance Polymer Materials" has changed during the last several years as a result of significant progress in polymer chemistry and the rapidly growing variety of polymer materials applications. The "classical" definition of high performance polymers and films as High Temperature Materials only has become too "narrow," and the researchers, producers, and users of the materials do not follow it anymore. The main reason is in a large number of new applications requiring high clarity, high or low heat conductivity, special surface properties, high purity, and other special performance properties not necessarily limited to the high temperature requirement. Moreover, in some new applications, properties other than temperature stability may become much more important for providing high performance.
The other "classical" approach in qualifying a polymer as a high performance material was based on business practicality, such as the price of the resin. According to this "logic," all polymers that are more expensive than a certain dollar amount, should, in reality, be considered high performance materials. Some experts even suggested using the price threshold of $10/lb to qualify a material as high performance. Without a doubt, this criterion is no longer realistic and practically has been abandoned. As an example, polyvinylbutyral (PVB) film is relatively inexpensive (much lower than $10/lb for the film; the resin is lower than $5/lb) and does not possess high temperature resistance capability. However, its bondability to glass and extremely high optical quality, i.e. super low haze and high light (luminous) transmittance, have made this thermoplastic material into one of the most successful polymers ever. PVB has dominated the field of high performance applications, such as interlayers for car windshields and architectural safety glazing, for nearly 70 years. In this case, the definition of high performance is based not on thermal but on optical and mechanical properties, along with low cost, enabling this polymer to be used in mass production of safety glass.
The modern industry demands advanced materials and products (including films and tapes) engineered to have special properties that guarantee performance in certain applications. There is a demand for advanced engineering polymers and plastics with various structures and properties tailored not only to meet high performance criteria, but also to "delight" customers by providing new and unexpected advantages.
An increased demand for flexible packaging and flexible printed circuit boards is driving significant growth in the high performance film markets (1). The unique properties and superior performance of polyesters, nylon, polycarbonates, fluoropolymers, liquid crystal polymers (LCP), and high temperature materials such as polyimides (Kapton by DuPont and Meldin by Saint-Gobain Performance Plastics) and other films will sustain demand from automotive, communications, and packaging industries (2). The world high performance film (HPF) market was estimated to be $7.3 billion in 1996. The revenues are projected to grow to $12.2 billion in 2003, with an average growth rate of 7%-8% during the forecast period (3). However, the higher cost and mature end-user markets for flexible materials could present challenges for vendors over the future period, and requires a creative approach in development of multilayer film structures combining different high-cost and low-cost polymers.
Nevertheless, practically all experts agree with the statement made by B. Mar in the June 2000 issue of Paper, Film, and Foil Converter, "It is definitely a bull market for high-performance films" (4).
In this paper, we review some of the modern trends in the development, processing, and applications of high performance thermoplastic materials based on various application requirements demanding high temperature resistance and other performance criteria.
Many polymer materials have abbreviations that are commonly used in the industry. These abbreviations, along with their chemical names, are listed in Appendix A.
ADVANCED HIGH PERFORMANCE FLUOROPOLYMER FILMS
New Modified Extrudable Fluoropolymers
The term "extrudable" or "melt-processable" fluoropolymers (FPs) is used to describe fluoropolymers. which can be processed as thermoplastic polymer melts because of relatively low viscosity, enabling the use of production units and lines based on screw extruders and injection molding machines. in contrast, "non-extrudable" (sometimes called "non-meltable") fluoropolymers are processed using the technology that was originally created for shaping metal and ceramic powders. These fluoropolymers (similar to ultrahigh molecular weight polyethylene, UHMWPE) can be molten; however, owing to the extremely high molecular weight (MW), the viscosity of their melts is so high that the use of conventional extrusion or injection molding techniques is impossible. These materials, in the form of fine powder, are precompressed, sintered at high temperatures (gelation point) into billets, and then skived into film and tapes or machined into parts according to the shape required by designers and application engineers.
New fluoropolymers with improved processability and performance, which were developed in the laboratories in late 1980s and early 1990s, are now making a real impact in the industry. These fluoropolymers are easier to process into film and other extruded items; they have better flexibility, surface smoothness, clarity, heat resistance, and purity essential for applications in the semiconductor industry. But scientists and engineers face two major problems when extruding fluoropolymers:
* Extremely high chemical aggressiveness, the property forcing processors to use very expensive equipment made of special steel alloys (Monel, Hastelloy, etc.).
* A very narrow processing "window," i.e., a small margin between the melting and degradation temperatures, which can be merely 10[degrees]F-20[degrees]F. Very precise temperature control and very skillful operators are mandatory.
In addition, two other difficulties have arisen lately, owing to the intention to increase productivity and to the eagerness to serve fast-growing and profitable high-technology markets, especially the semiconductor industry:
* A very low critical shear rate (typically from 1 [s.sup.-1] to 25 s'), at which melt fracture starts to cause surface distortion and damages the film and the quality of the other parts. The processors are unable to utilize the possible equipment capacity because of the limited speed at which they can process fluoropolymers without fracturing the surface of the extrudate.
* Constantly increasing purity requirements for the polymer film and other products (tubing and injection molding parts) in terms of metal ion content released by transported liquids used in semiconductor fabrication ("fabs").
Attempts to offer solutions (or at least improvements) to the above problems, along with the intention to minimize "classical" fluoropolymer deficiencies, such as limited bondability to other materials and a high creep (deformation under load, or "cold-flow"), are driving the chemistry development by the resin suppliers.
Typical melt-degradation temperature fields are shown in Fig. 1 for most important fluoropolymers. The processing window for PCTFE and ECTFE is very narrow, while a wider window between melting and degradation temperatures exists for THV-200 and THV-500 (typical fluoro-tercopolymers produced by Dyneon) than for other melt-processable fluoropolymers (5).
Improvements of fluoropolymers for extrudable high performance film are aimed at the reduction of thermal stress cracking, higher transparency, reduced creep and permeability, improved weldability, higher resistance to certain chemicals, and improved processability.
In terms of processability, the main problems besides the above-mentioned high melting and relatively low degradation temperatures and very low critical shear stress and shear rate (typically one or several [s.sup.-1]) are several problems typical for extrudable fluoropolymer in terms of general rheological phenomena, requiring a lot of experience and engineering skills for proper handling. A very long Newtonian region on the flow curves of fluoropolymers of different nature, in a wide range of MW values (or melt flow rates), leads to a relatively high viscosity of the melt, and shear thinning occurs at high shear stress only when the melt fracture is already heavily pronounced, forcing the slow-down of the extrusion process. Figure 2 shows the flow curves for FEP of different grades and different MW at the same temperature of 320[degrees]C. The prolonged Newtonian regions are exhibited for all samples. The extrusion of film is performed at shear rates corresponding to the end of the Newtonian region or in the very beginning of the area of the viscosity anomaly. Attempts to extrude fluoropolymers at shear stress higher than the melt fracture region (in "Region IV" on flow curves, i.e., beyond the melt fracture shear rate) have had limited success, mainly for tubing. It is Interesting that at high shear rates, the melt viscosities of FEP grades of different MW become very close (practically coinciding), so at the real speed of industrial film extrusion processes, different grades of FEP can be processed at the same (or very close) temperatures In the metering (pumping) section of a single-screw extruder. However, the temperature adjustment should be made for FEP of a very high MW (for grades with melt flow rate (MER) of 2 g/10 min and lower), a higher temperature is necessary to avoid a significant increase in the extruder's backpressure.
Another bothersome rheological characteristic of typical extrudable fluoropolymer is a very pronounced "stick-slip" phenomenon typical for fluoropolymer melts in general. It leads to the occurrence of another limit to the output and productivity: The increase In the rotation speed of the screw does not translate into the extruder output and can negatively affect quality of the film. Owing to an increase of the residence time for some melt portions, the polymer may be overheated, causing degradation and occurrence of "black specks" of carbonized material in the film.
The modern extrudable film grades of fluoropolymers are produced using different fluoromonomers, as described in many publications (see, for example. (6-8)). They are also summarized in Table 1. The chemical structures of the most common comonomers, perfluoropropylvinylether (PPVE), perfluorobutylethylene (PFBE), and perfluorohexylethylene (PFHE), used in quantities less than 5 mol% to modify polymers of tetrafluoroethylene (ETFE), are shown in Table 1, along with other alternating ethylene copolymers, such as HFIB, PPVE, and FMB introduced in FEP, PFA, ETFE, ECTFE copolymers and in a relatively new MFA terpolymer. The effect of comonomers on reducing the melting point ([T.sub.m]) of polymers with an increase of comonomer concentration is shown in Fig. 3 for three different comonomers. Primary effects of comonomers on the primary monomer is given in Table 2 for different modifiers. For example, the terpolymer MFA has properties between the "classical" FEP and PFA polymers; it is somewhat closer to the tougher PEA, as one can conclude from the comparative data presented In Table 3.
One of the advantages of MFA film and tubing is a much better surface smoothness in comparison to FEP and PEA. This was directly measured in a series of experiments conducted by Saint-Gobain Performance Plastics using a laser profilometer.
New, chemically modified ETFE grades have been introduced during the past several years in an attempt to improve its processability, mixability (compatibilization) with other polymers, and adhesion to different substrates.
New Generation of "Nonmeltable" Fluoropolymers (PTFE)
Major producers of non-melt-extrudable ("non-meltfabricable") polytetrafluoroethylene (PTFE) are moving forward using the same "chemical" approach. Several versions of modified PTFE-based resins are currently commercially available. All producers claim success in the recent development of a new generation of PTFE, including modified Teflon NXT by DuPont (USA), Hostaflon TFM by Hoechst (Germany), and Polyflon TFE by Daikin (Japan). For example, modem grades of TFM resin are based on PTFE modified with a small amount (approximately 0.1%) of perfluoropropylvinylether (PPVE) during polymerization. This comonomer (as well as hexafluoropropylene, HFP) can increase the amorphous phase without reduction of the MW. Such modification may also be tailored to lead to a significant reduction of the MW (up to 80%) and to branching of the molecules, which, in turn, reduces crystallinity.
New PTFE resins provide wider opportunities for designing and engineering high performance products that deliver new capabilities while retaining the exceptional chemical and thermal resistance, antistick (release) properties, low friction, good wear and abrasion resistance, negligible moisture absorption, weather resistance, toughness, and flexibility of conventional PTFE resins. The main improvements, that the new, advanced resins provide are as follows:
* Improved weldability due to lower melt viscosity
* A much higher creep resistance (lower deformation under load) and higher stiffness
* Reduced chemical permeability
* Better surface smoothness
* Higher clarity of thin film
* Better high voltage insulation (higher dielectric breakdown voltage).
All the improvements are attributed to lower viscosity (however, not low enough to enable direct extrusion) and a higher density ("denser polymer structure").
Film and tapes made from the modified PTFE grades by molding, sintering, and skiving billets have, in comparison to products from regular PTFE, better particle "adhesion" while sintered. Films and tapes of exceptional quality can be made from TFM and other modified PTFE resins filled with various fillers, especially with 25% carbon black.
It is important to emphasize that modified PTFE resins, for example DuPont's Teflon NXT, allow forming of very large billets with virtually no voids or distortion. This helps increase the yield, i.e., to slave the billets efficiently with little scrap loss. In addition, slaved film and sheets with a low microvoid content decrease transmigration of aggressive liquids and gases, which translates into reduced chemical permeability. A smooth surface for molded parts and much longer flexlife make these polymers valuable for many different applications. Some typical rapidly growing applications of film, sheets, and molded parts are: piping system components (diaphragms, bellows, gaskets, seals), containers, vessel and pipe linings for pure liquids, chemical bags, laminates for printed circuits, etc. Glass fabric backing can be securely embedded In the film and sheets to help attach linings to various substrates made of metal, concrete, and other construction materials.
Data on physical and mechanical properties presented in Table 4, creep resistance at various temperatures (Fig. 4), and results of flex fatigue evaluation using MIT's standard test (Fig. 5) illustrate some of the important properties and superiority of modified PTFE compared to conventional PTFE. The data show that new, advanced PTFE-type polymers are exhibiting advantageous mechanical properties, especially at elevated temperatures, and, in many cases, come very close to a tougher PFA.
One very important advantage of the film and molded parts made of the new generation of FTFE resins is high purity. It means not only minimized contamination content, but extraction of a lower amount of metal ions and other impurities to the levels decisive for applications in the semiconductor industry, as shown in Fig 6.
Modification of PTFE for creep reduction can also be achieved by its blending with other, mainly extrudable ("melt-fabricable") fluoropolymers. Mixtures of PTFE with up to 20 wt% of PFA have much less sagging in the sintering cycle and lower cold flow in the final film product.
Surface Modification of Fluoropolymer Films for Improved Bondability
Chemical inertness, surface smoothness, resistance to oxidation, all the distinctive advantages of fluoropolymers. become at the same time a real "nightmare" in applications requiring bonding and combinations of fluoropolymers with substrates of a different nature. Most fluoropolymers in the molten state are perfectly adherable to many polymers, metals, and other materials, but do not adhere to substrates of any nature in the solid state, even at elevated temperatures. Strong and economically justified adhesives, which would be applicable and reliable at low and room temperatures, have not yet been identified and introduced on a commercial basis. Modern science and engineering have turned their attention to research and design approaches, which may enable bypassing the direct adhesion problem. The main two developed routes are: a) modification of fluoropolymer surfaces to make them adherable to certain substrates, and b) incorporation of a transition layer (tie layer) bondable at the same time to both fluorop olymer and substrate surfaces.
Both basic technologies have been successfully implemented in the industry and are of great interest to scientists and application engineers. These technologies are based on surface treatment for adhesion improvement by changing the surface topography, increasing the surface energy, removal of boundary layers, creating electrical charges and chemically active sites. The modern treatments are helping to realize one or more of the following adhesion mechanisms, respectively: mechanical interlocking, interdiffusion, adsorption, electrostatic attraction, and chemical bonding.
Corona discharge and flame treatment processes as methods of surface modification are currently the dominant techniques used for treating the surfaces of most common thermoplastic films and other substrates, such as foils, paper, etc. This treatment increases the surface energy of substrates, which, in turn, improves wettability, printability, and adhesion (bondability) of the surfaces.
Fluoropolymer surfaces are very inert, and it is quite challenging to treat fluoropolymer films to enhance their surface energy using a dry process. It is recognized in scientific and patent literature that modification of fluoropolymer surfaces for improved bondability can be achieved by chemical treatment (etching and/or special grafting reactions), and physical treatment based on the influence of high energy phenomena. such as electrical-electronic (E-beam) and plasma effects. Plasma treatment of polymer materials is well described in books and reviews (9-12). An interesting low-pressure (atmospheric) plasma treatment technology has been recently developed and successfully tested for OPP, PE, PET, and PTFE (13). However, we would like to emphasize that it is difficult (if possible at all) to differentiate chemical and physical treatments due to chemical reactions accompanying any treatment considered to be strictly physical. In fact, only the chemical changes in the material surface are causing changes in reactivity, surface tension, improvement in wettability and adhesion ("cementability"). and capability of bonding to substrates. A number of experimental studies (14-16) have shown that, in fact, the only difference that can be used to identify the fluoropolymer treatment as chemical or physical is the depth of the treatment effectiveness: chemical treatment (for example, chemical etching) provides changes in the surface layer, several micrometers in depth, while known physical treatments such as "C"-treatment and plasma treatment can change the materials in a much more shallow depth measured in nanometers. One of the most effective and popular techniques in industry is the use of thermally activated bonding film to join fluoropolymers and metals and other various substrates to create desired sandwich-structures (14). However, one has to keep in mind that the tie-layer technology has its limitalions. Bonding films for fluropolymers (for example, PTFE) can be thermoplastics with convenient melting points, such as FEP with [T.sub.m] 280[degrees]C or commercially available chloro-fluoro-copolymer with [T.sub.m] 200[degrees]C. etc. (13-15). The reliability of the bond formed with adhesive film dissimilar to the given fluoropolymer (especially PTFE) is subject to many variables. In all cases, there are two main concerns regarding direct bonding of fluoropolymers with tie layers and substrates. The first is to exclude air during the bond cycle to prevent oxygen degradation of fluoropolymers at exposed edges or oxidation of metal substrates. The second is to handle the differences in thermal expansion of the components (layers), especially in the Z-direction, which is working against the bond and promotes delaminating of the "sandwich."
One of the most successful technologies was developed and widely patented many years ago (17-21). This is based on corona discharge treatment of the film surface. Corona treatment is known as a very efficient method of achieving bondability and improving surface receptivity to various materials for many polymers, and it is widely used for polyolefins and other thermoplastics. For fluoropolymers, the corona treatment of "high enough" energy leads to an increase of wettability and "cementability" as well, but the treatment effect disappears fairly quickly. A film made of a fluoropolymer after corona treatment loses the bonding characteristics in a period from minutes to hours, even if the film has been kept for the entire time in a black bag protected from UV-light, humidity, and elevated temperatures. It was discovered that corona treatment process in an atmosphere of different organic gases provides a much more durable treatment phenomenon: the wettability and cementability of fluoropolymer film treated at hi gh voltage in acetone, methanol, and many other gases last for many months and even years ("C"- treatment and "C"-film, where "C" stands for Cementable). The main patents related to corona treatment ("C"-treatment) of various fluoropolymers, suggested organic gases, and applications of treated films are briefly reviewed in Table 5. Figure 7 summarizes some typical data illustrating the efficiency and longevity of special corona treatment of FEP. Experiments confirm that the peel force (i.e. cementability measured according to ASTM's standard procedure for FEP "C"-film treated by corona in two different gases at optimal concentration and discharges) is very stable for many months (data has been presented for a two-year shelf-life period), and, in all cases, the cementability parameters are much higher than those required by industry standards.
The nature of the surface modification, which takes place during chemical etching by a solution of sodium and naphthalene (Na/Naphth) in tetrahydrofuran (THF), discovered in the l950s, as well as with other alkali metal mixtures in THF or liquid ammonia, has been under investigation for many years. It is recognized that the surface energy increases after sodium etching based on a similar mechanism for all fluoropolymers, due to defluorination, the main product being unsaturated hydrocarbon and oxygenated species.
To our knowledge, corona discharges of fluoropolymers in an organic gas atmosphere. which eventually leads to a dramatic improvement of surface wettability and bondability, have not been investigated in detail and are much less described for fluoropolymers in scientific literature. However, it is worth mentioning that the surface of a cementable film ("C"-film) is definitely enriched with oxygen and defluorinated to a substantial degree as is typical for chemically etched material. This conclusion can be undoubtedly made from the data in Table 6. High energy treatment in the atmosphere of inorganic gases, such as silane Si[H.sub.4] introduced by Air Liquide S.A. and Softal Electronic GmbH as the Aldyne" process (22), showed effectiveness for PP and other polyolefins; however, there is no data on the efficiency of this technology for fluoropolymers.
Chemical methods of modification and surface etching of various fluoropolymers were discussed in a large number of publications. A comprehensive review and investigation into this problem for PTFE, FEP, PEA, and PCTFE, including chemistry, surface topography, depth of etch, and depth profile for functional groups, can be found in many publications from the University of Connecticut and others (23-25). The etching using wet-chemical treatment with solvents and strong acids or bases, or the sodium/liquid ammonia for fluoropolymers is always carried out under pressure and eventually may become increasingly unacceptable because of environmental and safety considerations.
Chemical modification of fluoropolymer film surfaces is becoming more sophisticated because of grafting technology using various monomers and comonomers. The technology mostly used is based on radiation-induced grafting. For example, successfully developed radiation-induced grafting of styrene onto PTFE and properties of the grafted films have been described recently by Nasef and co-authors (26). Films grafted using gamma-radiation from a cobalt source at room temperature are used for ion-exchange membranes for various applications. Radiation-grafted acrylic acid onto PFA copolymer films has been also studied (27). Evaluation of rhodium led to a suggestion to use the graft copolymer in the field of rare metal waste treatment. Tough-to-bond polymers like fluoropolymers and UHMW PE are offered with primers. As an example, PTFE and other fluoropolymers, and UHMW PE films treated to be bondable to elastomers have been introduced (27). The films can be supplied with primers that further bond for various septa and pharmaceutical applications. However, these primers are considered to be trade secrets and are not described in the technical literature.
Fluoropolymer Film Surface Modification for Other Effects
There are many fluoropolymer film applications requiring special properties besides improved wettability and bondability. An important example is surface topography of film. In many applications the gloss of the film has to be reduced and a matte finish or geometrical patterns have to be introduced. Many techniques have been developed to achieve a certain surface structure. The "classical" method of reducing gloss is based on applying (pressing) to the solidifying film web a matte finished roll installed as one of the rolls of the roll-stack of the film take-off unit. The gloss of the final film in this case depends on roughness of the roll finish, temperature of the embossed film surface, speed of the film web, and pressure applied to the roll. Gloss reduction can be achieved by introducing special additives and mineral fillers. A combination of chemicals and physical treatment can be very efficient for melt-extruded fluoropolymer film; one example would be film made of PVDF and its copolymers and blends. In the case of solvent-cast film technology filler(s) with submicronized particle size are introduced into the solvent, and the achievement of a very uniform matted surface does not cause serious difficulties. Additives and fillers can provide a very smooth and satin-finished surface of PVF and other fluoropolymer films. It is important to take into consideration that any additives and fillers should be chosen and incorporated into the polymer very carefully because of their ability to influence mechanical, electrical, and other properties of film, and also because they can initiate degradation of the matrix resin. Gloss-reduction additives, which are very well known for low-temperature thermoplastic films and parts made of materials such as PMMA and various acrylic copolymers, have not yet been identified for fluoropolymers. It is very hard to believe that effective chemicals for gloss reduction will be available for fluoropolymers in the near future, owing to very high processing temperatures and chemical agg ressiveness of fluoropolymers, which cause decomposition of such additives.
A very interesting technology for forming fluoropolymer films with controlled gloss has been disclosed in a patent (28). A controlled-gloss fluoropolymer film is fabricated by casting a fluoropolymer dispersion onto a substrate, which has been coated with a solution containing a substantially inert particulate material, following by curing, drying, and stripping a fluoropolymer film that has a controlled-gloss or matte finish from the support substrate. The gloss of the matte fluoropolymer film is dependent upon and controlled by the amount of particulate material in the coating solution.
Water-soluble or water-dispersible crosslinkable polymeric surfactants having fluorocarbon and ionic components have been described (29). Such surfactants can be cured into tough, solvent-resistant coatings with low critical surface tension that are useful for protective finishes on fluoropolymer films.
A special and fast-growing area of surface modification is anti-glare (AG) film for liquid crystal displays (LCDs). Usually, AG films are multilayer film structures affixed to the LCDs to reduce the glare from external light. They are produced by laminating a transparent base material film and a hard transparent AG resin layer with an uneven surface (30). Production of fluorine-based AG films is rapidly growing. These films contain a fluorine-based film that acts as an anti-reflection layer. In comparison to glass, which reflects ~10% of external light, the AG film-coated glass reflects less than 1% of incidental light. Applications of AG films include wall-mounted TV sets, digital TV sets utilizing plasma-address liquid crystal (PALC) displays, and others.
Some other special applications of fluoropolymer film with a textured surface, such as adhesive thin film applique for replacement of paint on aircraft, have also been described (31).
Metallization of film (deposition of very thin metal layers on the surface) is usually used to improve barrier properties (lower gas and water vapor permeability), reduce water absorption, and improve optical properties such as light reflection along with aesthetics. Metallization of advanced film based on fluoropolymers is a difficult process because of the surface inertness of most of them, especially PTFE. Many studies on metallization of PTFE have been carried out (32-35); however, owing to the surface inertness, adhesion between PTFE and various metals deposited on the polymer surface generally fails to reach the level required by industrial applications. As in the case of fluoropolymer bondability, the surface treatment and modification, e.g. activation of the fluoropolymer film surfaces prior to metallization, have been shown to enhance metal adhesion. The most widely used method of activating the PTFE surface is a treatment with a reducing etchant, such as sodium naphthalenide and potassium sulfoxide (35). Apart from the wet chemical treatment, ion bombardment (e-beam treatment), X-ray irradiation, and cold plasma treatment have also been applied to the surface of PTFE for activation and improvement of meallization quality (review (35-36)). Shi and colleagues reported (37) improvement in adhesion between evaporated CU and ETFE modified with [N.sub.2], [O.sub.2], H2, and mixed-gas plasmas. Effectiveness of graft copolymerization with specific functional monomers for metallization improvement has been studied by a group of researchers (38, 39). Good adhesion of fluoropolymers to copper has been achieved by silanization of fluoropolymer film surfaces. The details of the chemistry and technology have been described in several publications (40-42). Studies have been carried out for various fluoropolymers (including PTFE) by silanization of their surfaces through chemisorbtion of some of the aminopropyl-trimethoxysilanes in a plasma environment. A comprehensive comparative analysis of different methods of pre-a ctivation of the PTFE surface for electroless deposition of copper, including plasma treatment, graft copolymerization, and silanization has been presented in recent publications (35-41.).
In concluding this section., it is worth mentioning that surface fluorination of the performance film surface by cold plasma treatment was developed and successfully implemented in the industry quite a while ago. The main direction was the creation of inex-pensive film with enhanced barrier properties, along with various fluorinated barrier coatings. Results of the investigation of barrier properties of fluorinated polyolefins, as well as fluoropolymers such as PVF and PVDF, have been discussed in detail in a number of articles and reviews. For example, the article by Corbin, Cohen, and Baddour (42) discusses elemental and plasma-enhanced reactions, conditions affecting surface fluorination, efficiency of solvent resistance, and solvent permeation in plasma-fluorinated polymers. An efficient modification of fluoropolymer films can be achieved with various coatings. One interesting development in this area has been disclosed in a patent (43). Fluoropolymer films, which combine the attributes of outdoor durabil ity, chemical resistance, and thermoformability with paint-like aesthetics, have a surface coated with a primer of amine-functional polymer and an overcoat of a thermoplastic adhesive of acid-modified polyolefin.
Orientation of Fluoropolymer Films
Orientation (stretching of the polymer in the viscoelastic state below its melting temperature) is a recognized method of improvement of mechanical, dielectric, optical, and permeability properties of many thermoplastic films; it was investigated and described in a large number of articles, books, and patents. There is no debate regarding the property improvements and usefulness of orientation for thermoplastic film such as PP, OPP, PET, PS, PE, their blends and alloys in terms of improvement of film clarity, dielectric, barrier, and other properties. These films are widely used, mainly in electrical and electronic industries and packaging. Expansion of fluoropolymer tubing is also recognized as a very efficient method for making shrinkable articles; some examples are tubes and sleeves. Examples include designed housing for roller blinds (44-47), roller covers for electro-photographic copying machines (toner image-transferring devices), and ink-jet printers (48, 49). For instance, expanded FEP tubular films a re used as a high-temperature, wear-resistant release roller protector, which is capable of preventing the sticking of ink and other substances in ink-jet printers and other units using solid or foamed rollers. Such fuser back-up rollers can be covered with a multilayer coextruded fluoropolymer film/tube combining an outer layer with high wear resistance, smoothness and anti-static properties, and an interlayer used to provide a bond with a metal shaft and as a "shape holder" during fabrication of the tubular intermediate product.
The situation with f tat fluoropolymer films is more complicated. There have been many arguments for pro- and contra- orientation of fluoropolymer films. Supporters for stretching of fluoropolymer films see additional advantages of fluoropolymer orientation in further improvement of properties essential for particular applications. However, design and construction of tenter-frame machines for fluoropolymer orientation needs heavy capital investment for the following reasons. First of all, toughness of fluoropolymers below melting/crystallization temperature requires a very high force for stretching and heavy-duty equipment. Second, the very high orientation temperature and chemical aggressiveness of fluoropolymers at elevated temperatures dictate the use of very expensive steel alloys, sophisticated design, and assembly technology. For example, the very high operation temperature of the gripping units ("clips") of the tenter-frames and complicated design of the mechanism outside the oven of the transverse-ori entation machine require unique engineering and high investment. That is why, as of today, the number of industrial scale lines producing oriented fluoropolymer films is very limited. One of the leaders in the field of manufacturing sequentially operating film orientation lines capable of stretching extrudable fluoropolymers such as FEP and ETFE is Burckner Maschlnenbau (Siegsdorf, Germany). One of the most interesting developments in the technology of oriented film is simultaneous stretching with the use of linear motors or USIM technology. It was developed by Bruckner in 1994-1998 and is described in many presentations (50-54). This technology was also reviewed by Breil (55). The LISIM technology has been scaled up for the production of BOPP and BOPET for up to 350 meters per minute (approximately 1150 feet per minute). However, the application of this technology for fluoropolymer film has not been described in literature.
The stretchability of fluoropolymers is limited; the commercialized units are capable of extending fluoropolymer films in the range from 30% to 250% only (56). However, there is always a valid question: "Is it worth it?" Are the advantages of oriented fluoropolymers so significant in comparison to the standard extruded fluoropolymer film that the expenses are justified? Answers to these questions are periodically revised, and, in some new applications, orientation of fluoropolymers is considered, nowadays, to have a high potential.
Research and development in orientation of fluoropolymers is moving in two main directions (for example, patents by Levy (57, 58)).
* Equipment: perfection of the existing longitudinal stretching roller units and transverse stretching tenter-frames, suggestion of novel designs of gripping mechanisms in an attempt to make all equipment less costly, more reliable, and requiring lower maintenance expenses.
* Development of special technologies capable of improving the process of stretching of fluoropolymers, providing a higher orientation ratio, and optimizing the ratio of stretching and shrinkage of produced film in longitudinal (machine) and perpendicular (transverse) directions.
Tsai and co-authors have disclosed a method of producing biaxially oriented multilayer fluoropolymer films with high orientation ratio (59, 60). It was proposed to produce films by coextruding or colaminating multilayer films having at least one layer of a fluoropolymer, one layer of a thermoplastic homopolymer or copolymer, and an intermediate adhesive layer. With this structure, the thermoplastic layer allows the fluoropolymer layer to be biaxially stretched many times its original length (from 1.5 up to 10 times). A high orientation ratio for the fluoropolymer film increases the mechanical strength, toughness, and water vapor barrier properties of the film while using a thinner-gauge fluoropolymer film. In addition, coextrusion can be done at higher temperatures in comparison to single-layer film processing (i.e., in the range from 280[degrees]C to about 40000), allowing film to be produced in absence of polymer degradation and melt fracture.
Advanced High-Purity Fluoropolymer Film
The "classical" meaning of and requirements for resin and film purity have changed dramatically because of the rapid increase of usage of polymer materials in the semiconductor industry. "Purity" is no longer only the absence (or at least minimization) of a number of mechanical (mineral) particulates and other contamination of a nonpolyrneric nature, which the application can tolerate. The traditional purity requirements are also becoming tougher every year, along with demand for gel-free film, and elimination of enclosures of carbon ("black specs") and brown spots indicating the possible degradation of the polymer material, which may occur during its processing into film or tape and reduce its performance.
The modern film industry serving semiconductor manufacturing is concerned primarily with purity in terms of metal ion content (MIC) and the possible "leakage" of metal ions from film, tubing, pumps, valves, and other parts used in the fabrication of semiconductors ("fabs"). This relatively new "MIC-purity" requirement has triggered serious scientific and engineering activities in various directions: changes in polymerization technology including modified catalytic systems, reactors redesign and reengineering, investigation into types of metal ions being released from different polymer materials when they are contacted by water and a number of other chemically active liquids, development of standard procedures of ion content and leakage measurements, etc. The importance of the "advanced polymer purity" is growing consistently, along with the reduction of the acceptable threshold of metal ion content in polymers for semiconductor applications.
Fluoropolymers are considered to be the most pure polymer materials from the point of view of MIC and unwanted chemical products generated during the degradation/destruction of the polymer while exposed to the aggressive chemical liquids used in the industry. That is why the investigation into purity and its improvements were mainly directed towards fluoropolymers. Practically all major producers of fluoropolymers have developed and launched production of special, high-purity grades of FEP, PFA, MFA. and other fluoropolymers, extruded film, sheets, and other extruded and molded products. Complete fluid-handling systems, including manifold distribution units and pumps, have been developed using fluoropolymers. All parts of these special plastic units are made exclusively of fluoropolymers and assembled in "clean rooms," It is critical for the semiconductor industry to avoid chemical contamination of the highly pure fluids, which are handled by fluoropolymer components.
The fluoropolymer purity is being measured as the quantity in PPM of certain metal ions being released by the material or film specimen during long-term extraction by submerging the specimen in HC1 or hot water, or its rinsing with clean and deionized water. The procedures are described in the industry standards, for example Semi Draft Doc. 3038 entitled "Method for Determining Trace Metals in Polymer Materials" (8/6/99). According to this document "... determining the metallic contamination concentration in bulk polymer materials used in either distribution systems for processing fluids or products in direct contact with the wafer is important criterion for deciding the suitability of a material." Data from different tests can be compared to determine the inorganic content in different materials and in the materials produced by different manufacturers. As a rule, the ion chromatography is applied after the specimen has been submerged in hot deionized water at temperature of 80[degrees]C for 24 hours or in HC 1.
In Table 7, results are presented of our "MIC-purity" measurements conducted for FEP and PFA, the most popular fluoropolymers in semiconductor applications. The statistics for nineteen metal ions have been measured for polymer samples from different major vendors. As one can see, the MIC varies for different metal ions in a relatively wide range (keep in mind that some metals are more harmful than others for the semiconductor liquid systems). Another conclusion is high-purity grades of the same fluoropolymer from different vendors may have a significant difference in certain MIC, and the application engineers may have to choose the resin based on the lowest content of the most harmful metal ions for their particular semiconductor fabrication ("fab"). Unfortunately, data described in many technical data sheets and brochures by the vendors are contradictory in terms of qualified comparison of different fluoropolymers. The superiority of MFA and special grades of PFA over other fluoropolymers is recognized; howe ver, the "battle" between these two polymers for the championship in metal ion purity is still ongoing.
HIGH PERFORMANCE (ADVANCED ENGINEERING) FILMS FOR ELECTRICAL AND ELECTRONIC APPLICATIONS
Properties and Applications
This group of films can be characterized in general as film made of polymer materials with high melting temperatures, high toughness, and superb dielectric properties. It is anticipated that these materials are filling the "use temperature space" between polyimides (Kapton-type film) having the highest thermal capabilities and the medium temperature engineering films made of such polymers as polyamides (nylons), polycarbonate, ABS. polypropylene oxide, polymethyl pentene (PMP), and polyesters (PET, PBT, PEN), which are not a subject of this review. This group comprises a large number of aromatic polycondensation, linear, semicrystalline thermoplastic polymers such as polyketones, including polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyaryleneether (PAE) polyaryletherketone (PAEK), polyamidimid (PAI), polyalcanimid (PACI), polyetherimide (PEI), polyethersulfide (PES or PESd), polyphenylene sulfide (PPSd), polysulfone (PPSf), polyethersulfone (PESF or PESf, polyphenylene sulfone (PPS), etc. The rmosetting polyphenyleneether is distinguished by a glass transition temperature ([T.sub.g]) as high as that of polyimides. a low dielectric constant close to that of fluorine resins, small water absorption, and good adhesion to metals (for example, to copper foil). It appears to be a promising material for printed circuit boards for multichip modules, which are the new generation high-density wiring boards. Probably one of the most known representatives of this group of high performance products is film made of poly (aryl-ether-ether-ketone), more commonly known in the plastics industry as polyetheretherketone (abbreviated as PEEK). This tough, thermally stable aromatic polymer with very attractive properties as an advanced high quality engineering thermoplastic is produced under the trade name "Victrex." PEEK is used in a wide variety of applications (61) demanding high dielectric strength (wire and cable coating and wrapping), high wear and abrasion resistance (motor vehicle bearings), high mechanical stre ngth and elasticity (tennis racquet strings), transportation of hot and aggressive liquids (special tubing and catheters). etc. When reinforced with up to 68% continuous carbon fibers, it is known as APC 2 (Aromatic Polymer Composite 2), a leading thermoplastic composite for aerospace applications.
According to some experts, liquid crystal polymers (LCP) and blends based on them should also be included in the group of advanced high performance materials (62-64).
The most industrially important high temperature engineering thermoplastics and their chemical structures are shown in Table 8.
All of the above high-temperature polymer materials are produced as neat, filled, reinforced, and modified versions and formulations known in the industry under their trade names. Here are some examples: GE (USA) is producing PEI under the trade name "Ultem"; Ticona (Germany) is offering PPS as "Fortron," and a family of oxygen-modified (oxidized) versions of PPS(d) and PPS(f) under the trade name "Ceramer" and "Ceramer Plus"; high-temperature aromatic polyester based LCPs are known under trade names "Ekonol" from Saint-Gobain Performance Plastics (USA) and "SumicaSuper" from Sumitomo Corporation (Japan); Oxford Performance Materials, Inc. (USA) is producing PEKK under the trade name OXPEKK.
A description of properties for all commercialized polymers can be found in technical data sheets and product literature from major vendors of advanced thermoplastics (62-65). It is worth emphasizing that practically only the neat resin grades of high performance materials are used for fabricating films and tapes, and the reinforced formulations with glass, carbon, and other fibers are mainly applicable in the production of thick sheets, injection and compression molded parts.
The above materials have many characteristics and applications in common. They are providing increased use temperature and toughness in comparison to the general engineering plastics, better rigidity, very high chemical resistance (including at high temperatures), and dielectric strength. Typically, the advanced engineering films for this group of high temperature and high dielectric strength are slit into tapes, which are applied in stretched and/or unstretched versions to wires and cables as a film wrap-jacketing to provide reliable performance of wires/cables at high temperatures. Such cable products are used in aircraft and space carriers. A number of other applications have been developed in an attempt to utilize the advantages of high performance polyketones. For example, PEEK and PAEK films are recommended (66) as an insulation material for two new products in the electrical sector: a metal core circuit and a copper clad laminate for control equipment. These materials were chosen for their chemical res istance, flame and radiation resistance, low smoke emission, and acceptable processability. A wide variety of applications for PEEK film are being described in an increased number of publications, patents, and technical literature. Films that are now finding applications in micro-motor parts for computers, laser printers and other office automation equipment, for optical discs and wear-resistant rings for washers are described in many journals (65-67). A very promising and efficient area is the development and fabrication of micro- and ultrafiltration membranes. A review of PEEK membrane fabrication, their properties and evaluation can be found in several reviews (68). According to one of the modem technologies, high performance asymmetric membranes are obtained by extruding blends of PEEK, polysulfone (PSF), and a small molecule solvent mixture, and then removing the PSF and solvent in a subsequent extraction step. The process for making ultrafiltration membranes differs from that for microfiltration membran es only in the blend component ratios and temperature of the film pick-up rolls.
Properties and applications of some new films and sheets made of PEI and its blends have been described (69. 70). An interesting application is the use of high-temperature, dimensionally stable plastic materials such as PEEK and PEI as substrates for the deposition of amorphous silicon thin-film solar cells (71).
Mechanical, thermal, and dielectric properties of advanced engineering polymers are widely published by producers and researchers. In Table 9, we compare different properties for various materials of the discussed group of high performance polymers measured mainly using film specimens. It is important to mention that properties for the majority of the high performance film group depend On the sub-molecular structure and morphology due to substantial crystallization capabilities, if the cooling of the molten film web is slow enough. For example, investigation into properties of PEEK films showing different thermal histories allows for the determination of the main microstructural features governing these properties. Data for tensile mechanical properties can be found (72) over a wide strain rate range, along with a number of references on this particular topic.
In addition to studies of general mechanical and dielectrical properties, researchers are now paying much more attention to such properties of advanced engineering materials as fatigue, aging, wear and tribological properties, structural changes under electrical charges. surface modifications, etc., requiring much more sophisticated devices and procedures. This trend is mainly triggered by the interest in high performance materials not only for above-mentioned and commonly identified applications, market segments, and niches, but by strong efforts that are being made in an attempt to provide, for these polymers, new opportunities to penetrate the automotive, medical, and consumer products industries. In these segments, food compliance, aging, friction, and structural (bulk and surface) changes may be of great importance. For example, PPS is a]ready offered and successfully used for a wide range of end-uses, including electronic connectors and such automotive parts as torque converters and servo components (65 --73). That is why a number of publications have appeared recently on topics like the influence of time and temperature on deformation of PEEK film (74), tribological properties of film during sliding wear of PEEK and its composites (75), effect of physical aging on the properties of film of amorphous PEEK (76), charge dynamics and morphology of PEI films (Ultem 1000 and 5000 grades) (77), electrical breakdown behavior of PEI films (78), surface modification of PAEK film by remote oxygen plasma (79), and effect of physical aging on properties of film (77).
In all cases, the properties of these materials depend on the following major factors: crystallinity, degree of degradation, presence of fillers, and reinforcement. Depending on the processing conditions, the measured crystallinity may be anywhere from 0% to 40% and the crystallization behavior is analogous to that of PET as an example of another rapidly crystallizing thermoplastic film material (64). It has been revealed that the maximum rate of crystallization for PEEK is about 230[degrees]C, and the spherulitic growth follows the classical cases known for other semicrystallinic thermoplastic materials. For example, the size of the spherulites increases with crystallization temperature, and the general spherulitic structure is similar to the more widely studied aliphatic crystallizable polymers, such as polyethylene.
Interesting conclusions are made by studying the crystallization process in the presence of fillers (carbon black, graphite) and especially fibers (typically glass and carbon fibers are used). In reinforced compositions, the nucleation of crystals may have several forms, including nucleation within the matrix itself, nucleation from contact points between fibers (or regions where fibers are almost in contact), and nucleation at a free fiber-polymer interface. This makes the cystallization process more complicated than for polymers without reinforcing fibers.
In most applications requiring optimum modulus and solvent resistance, the polymers are being processed in order to develop high crystallinity. These films are usually opaque and matte finished. However, quenching of the molten film web on the casting drum enables the manufacture of very clear film. It is possible to quench samples (practically very thick film and/or sheets up to 40 mil, or 1 mm) such that they are completely amorphous (63). This is realized on a commercial scale in production of amorphous PEEK suitable for thermoforming at a relatively low temperature of about 170[degrees]C compared to 350[degrees]C required for the thermoforming of crystalline PEEK film.
In conclusion, the key properties of film made from advanced engineering polymers may be summarized as follows:
* Thermal: continuous working temperature not lower than 250[degrees]C (> 450[degrees]F) with ability to retain mechanical properties at over 350[degrees]C (> 660[degrees]F).
* Mechanical: excellent flexural, tensile, impact, and fatigue properties (especially for oriented film).
* Dielectrical properties: high dielectric strength remains stable in a wide range of temperatures and frequencies.
* Chemical resistance: insoluble in all common solvents, extremely high resistance to water and steam at high temperature and pressure, crystallinity imparts excellent resistance to a wide variety of liquids.
* Radiation resistance: exceptionally high for plastics.
* Flame resistance: V-O flammability rating (organic additives may have a negative influence, while mineral fillers may enhance flame retardancy). Smoke and toxic gas emission during combustion is extremely low for a thermoplastic material.
* Wear and abrasion resistance: very high, excellent tribological properties (low coefficient of friction, high "lubricity") especially for formulations containing graphite and other special fillers and reinforcements.
The majority of vendors of high performance thermoplastics, such as PEEK, claim that the processing of these polymers is easy (at least "relatively easy") and does not require special equipment or equipment modifications. However, in reality, processing of PEEK, PES, PSE, and other high performance thermoplastics is complicated and requires deep knowledge of chemistry and rheology of these materials, high skills, and experience of processing engineers and extrusion operators. The main difficulties occurring as a result of specific properties of these polymers are as follows:
* Relatively high moisture absorption dictates the need for a deep drying of the resin before extrusion or injection molding to a certain level of residual moisture depending on the nature of the polymer and technical requirements to the film, tape, or sheet.
* Processing conditions have to be developed (optimized) and precisely maintained to avoid any degradation and/or crosslinking of the polymer, especially in a high-speed process.
* Very efficient filtration system is required to produce a gel-free film.
* Almost all polymers of this group have a limited processing time: A film with a smooth surface, free of "die lines," can be produced during a limited continuous manufacturing period in the range from several hours to several days, and eventually the extrusion line has to be stopped for complete cleaning; the time limitation is caused mainly by the residue of substances and matter ("buildup") on the lips of the extrusion die.
* Very careful monitored cooling of the film web has to be implemented in order to fabricate film with required crystallinity and with desired optical and mechanical properties: Fast quenching produces an almost completely amorphous clear film; on the contrary, slow cooling allows the crystallization of the polymer and produces a semi-opaque or a completely opaque crystalline film; the degree of the required quenching depends on the resin type and grade (MW), thickness of the film, and speed of the extrusion process.
* In many cases, PEEK, PES, PEI, and other film and tapes are being matte finished at least from one side for different reasons such as better winding ("non-blocking"), improved surface for adhesive coating, tighter winding-wrapping around a wire or cable, and improved handleability. However, in many cases the matte finish also helps hide the disadvantages of film such as "die lines," visible gels, and contamination.
* Cleaning of the equipment is complicated and labor intensive; purging with medium performance materials immediately after extrusion of high performance thermoplastics is usually used to simplify the cleaning procedure to some extent.
It is very important to study the effects of processing on high performance polymers since they are processed at very high temperatures, which in combination with kneading and shearing may strongly influence the structure and properties of the material. Some unexpected phenomena accompanying processing have been revealed for several advanced engineering polymers. For example, a careful study of changes in PES appearance, melt viscosity, MW, and mechanical properties after processing (80) showed that, similar to other advanced polymers, the PES can be greatly influenced by thermal-shear processing cycles, the use of regrind should be strictly controlled, and has been recommended that the number of cycles should be limited to four for injection molded parts and much less for film fabrication. The selection and control of the processing conditions is therefore essential in optimizing the structure and properties of film and other finished parts. Typically, the molecular weight (MW) and melt viscosity of thermopl astics decreases (the melt flow rate increases) after processing, which is usually attributed to thermal, thermal-oxidative, or mechanochemical degradation of the resin. A number of specific chemical reactions may also occur. However, the opposite phenomenon occurs during processing of some advanced thermoplastics. In fact, the thermal stability of the most high-temperature engineering polymers "holds up" well even at high shear deformation. For PES, the degradation temperature is over 500[degrees]C (80). It has been shown that the melt viscosity and MW of PES increases during kneading and extrusion, which became known as the phenomenon of "melt thickening." An increase in MW and melt viscosity of PES is not caused by degradation and crosslinking, since measurements have no distinct proven changes in glass transition point: [T.sub.g] practically remains constant, irrespective of processing conditions. Results (80) indicate that the reactions, probably grafting reactions, that take place as a consequence of pr olonged shear, heating, and thermo-oxidation during processing do not have any notable effect on the structure of PES, but may be responsible for melt thickening.
The problems indicated above may become more complicated when these high-temperature materials are coextruded with other thermoplastic polymers. Nevertheless, these advanced polymers can be successfully coextruded without the use of adhesive or tie layers with nylon-6, PVDF, or phenoxy resins (80). However, it is particularly interesting and important to note that ethylene/methyl acrylate (EMA) copolymers, which are commonly used as tie layers to join two dissimilar polymers, show no adhesion with polyketone polymers.
Blends, Alloys, and Modifications
Processing difficulties are one of the reasons for efforts to develop blends and alloys. Research in the field of blends (alloys) of PEI, PPS, PEEK, and other materials, as well as identification and application of processing additives, is aimed not only toward achieving property improvements, but in many cases, toward their influence on melt rheology and processability. However, the number of efficient melt processing aids for this group of advanced polymer materials is very limited because of very high processing temperatures and tough requirements on the additive's "survival" under the severe combination of high temperature and intensive shear deformation in modem high-speed film-extrusion equipment. That is why most success stories are related to the use of additives (processing aids) based on fluoropolymers and specific (unique) mineral fillers/modifiers. These additives are recommended in technical literature under various trade names. For example, incorporation of Dynamar fluoropolymer additives is des cribed (81) in order to overcome problems with "die lines" (or "weld lines") on the film surface. It is also reported that the free-flowing fluoropolymer additive Dynamar FX 5911X helps shift the start of melt fracture to higher shear rates and to reduce extruder torque. Some other fluoroelastomer processing aids are reviewed (82), but their applications are much more reliable for polymers requiring much lower processing temperatures. Vydax Fluorotelomers are offered (83) as a family of CFC-free dry film lubricants under the trade name Krytox DF and Krytox 2000. This aid, in a form of white waxy fluorotelomer particles, can be added to any formulation as a dry material or in a form of a dispersion either in a solvent (such as isopropyl alcohol) or water. The use of dry-mixed in additive is preferable, owing to elimination of the technolegical stage of preparation of the dispersion and difficulties in getting rid of the solvent or water base of such concentrates.
The effectiveness of some fine mineral additives as modifiers acting, to some extent, as internal lubricants has been described in recent publications. Among them, the addition of a relatively small quantity of boron nitride seems to be very useful, not only for fighting "melt fracture" (eliminating "sharkskin" on the film surface), but also for "unloading" extruders, as well by reducing the apparent viscosity, backpressure, and torque required for high-speed processing (84).
The most investigated and developed in the industry are binary blends of PEEK and polyethylene naphthalate (PEN), PEEK and polyetherimide (PEI), PAEK and PET, polysulfone (PSF) and polycarbonate (PC), and PEI and PC, Investigations into their morphology and properties have been described in a number of publications and patents. Kohlman and Petrie have described the mixability and mechanical properties of PPSf-PC and PEI-PC, including low-speed and high-speed impact resistance versus the content of components (85). The influence of blend composition on the deformation behavior of the cast amorphous PEN/PEI blends was investigated (86) above their respective glass transition temperatures ([T.sub.g]). PEN inherently shows a sharp necking phenomenon when it is subjected to stretching at temperatures as high as 20[degrees]C above its [T.sub.g]. This is attributed to highly localized rapid alignment of naphthalene planes parallel to the surface of the films. The addition of PEI is observed (87) to reduce this necki ng behavior, and the necking completely disappears when the content of PEI in the blends exceeds 10%. The increase of the PET hinders the rapid alignment of naphihalene planes, increases the friction between polymer chains in the blend, and prevents the formation of localized necks.
The deformation behavior of amorphous thin films of PEEK/PEI blends was investigated over a wide temperature range by use of optical and TEM microscopes (88). It was shown that in pure PEI and in blends with up to 60 wt% PEEK content, a transition from shear deformation to disentanglement crazing occurred as the temperature was raised. This transition has not been revealed for PEEK, which deformed by shear over the whole temperature range. Similar behavior was found for PEI/PEEK with 80 wt% of PEEK. The thin film deformation was found to be consistent with their bulk deformation. A high-temperature ductile-brittle transition was observed in tensile tests at low PEEK content.
The thermal, mechanical and optical properties of stretched cast film samples made from blends of PEI/PET as a function of composition and draw ratio were investigated (70, 71). Very interesting ternary blends of PEEK/PET/PEN were investigated (88, 89) in order to eliminate the neck formation in the rubbery state, while maintaining the strain-induced crystallizability, and also to increase the [T.sub.g]. The increase of PEI concentration elevated the [T.sub.g] along with the reduction of the ability to crystallize. It was revealed that to compensate for the latter, the concentration of PEEK should be increased to recover the strain hardening and to boost the [T.sub.g] of the final films. PEN crystallization was hindered in blends with 40% PEI. Although PEEK can be crystallized at concentrations as low as 10%, PEN remains amorphous. The annealing of film leads to an interesting phenomenon: Both PEEK and PEN separate from PEI and crystallize into their own phases (domains).
A large number of high performance blends and polymer modifications, especially with different fillers, have been developed recently for special applications as thermal interface management" materials (TIM). This direction in high performance materials, films, and tapes development is related to heat removal from microprocessors. Heat sink for semiconductor chips and other heat-removal applications are among the last areas where thermoplastics-inherent thermal insulators have yet to replace metals and solve problems of heat buildup in electronics, appliances, lighting, automotive, and industrial products. It is anticipated that bondable, flexible and lightweight films/tapes made of thermoplastic materials filled and modified by ceramic grains, high conductive fillers such as metal powder, carbon black, graphite, nanoparticulates, carbon fibers, nanofibers, nanotubes, "Buckytubes," and whiskers have an excellent chance to create interface products capable of beating the previous generation of TIM based on grea se and other "less user-friendly" materials.
Thermally conductive compounds are usually created based on crystalline advanced engineering polymer resins because of their high heat resistance, high thermal stability, and very low melt viscosity, which does not limit the filler load and does not create processability problems. It should be pointed out that films and tapes made of filled high performance resins have higher stiffness and tensile strength, but lower elongation at break and impact resistance than unfilled or reinforced compositions.
The most thermally conductive additives are specialty graphite fibers (conductivity values up of 500--1000 W/mK) in comparison to structural grade carbon fibers based on PAN having values less than 10 W/mK and electrically insulative ceramic fillers with 60--80 W/mK for boron nitride and 300 W/mK for aluminum nitride powders. High fiber loading (up to 70 wt%) can provide high-quality TIM materials based on LCP and PPS because of their low viscosity and excellent interface compatibility with graphite fibers.
Theoretical investigation and modeling of heat transfusivity of polymers and composites with different types of fillers and modifiers have been conducted and described many times in academic and technical literature devoted to "thermal packing" and "thermal management" analysis, modeling, and applications (89--96). Some interesting history and trends have been described in recently published reviews (93, 94). Thermal conductivity of heterophase polymer compositions has been carefully analyzed and reviewed in a cycle of original publications and reviews by Wong and Bollampally (97), and Bigg (98) for filled formulations, and by Godovski (99) and Kassiba and co-authors (100) for nanocomposites. Several comprehensive technical reviews of TIM principles, materials, and test methods have been recently published as well (100, 101). According to some experts (99). "... this ability to control heat build-up, yet also provide lightweight, flexible, and low-cost applications will make these plastics one of the most imp ortant technological developments for decades to come..."
Polymer range for TIM materials expands consistently, and in addition to LCP, PPSd, PEEK PPSf, and PEI, high performance films and tapes for TIM applications are being created using medium-temperature resins such as ABS, PBT, PA, PC, as well as lower-temperature commodity thermoplastics like PP, PS, and PMP. Even thermoplastic elastomers are getting thermal-conductivity treatment (102), offering various grades with conductivity in the wide range from 18 W/mK to 60 W/mK.
The following three general classes of fillers can be used to increase the thermal conductivity of thermoplastic films and tapes (95, 99--102):
* Carbon Fillers: carbon black powder, graphite powder, carbon fibers, graphite fibers, carbon and graphite nanofibers, nanotubes and nanowhiskers.
* Metallic Fillers: copper powder, steel and aluminum powders, and aluminum flake.
* Ceramic Fillers: boron nitride, aluminum nitride, and aluminum oxide.
Each filler type has advantages and disadvantages; for example, the carbon and metallic fillers are electrically conductive, as well as thermally conductive. The ceramic fillers will provide materials that are electrical insulators. The fiber or flake fillers have generally more efficient ingredients, in terms of loading needed to achieve conductivity; however, because of their high aspect ratio, film extruded at high speed (high shear rates) may have orientation dependence. The metallic fillers have high specific gravity, which leads to a weight disadvantage in some applications. Powdered fillers may require higher loading, but will not show an influence of particulate orientation.
With regard to the thermoplastic polymer matrixes, TIM are usually formulated based on crystalline and amorphous engineering resins because of their high heat resistance, lower melt viscosity, and high bond-ability to metal interfaces.
One of the most interesting developments would be a thermally conductive film placed between the coils of a radiant floor-heating system. Such a film could allow water to be run at lower temperatures.
The heat-transfer requirements of ever-smaller and "power-hungry" electronics have opened the door for this new generation of cooling materials. Unfilled thermoplastics have thermal conductivity around 0.2 W/mK, and most thermally conductive compounds have 10-50 times higher conductivity (in the range from 1 W/mK to 20 W/mK) (97, 100). Polymer-based products with 10-100 W/mK conductivity have been offered recently, i.e. 100-500 times higher than that for neat (basic) polymers. For comparison, some diecast metal alloys (magnesium or aluminum) are in the 50-100 W/mK range. It is emphasized (97, 102) that the metal's high thermal conductivity cannot be effectively utilized if it conducts heat faster than the airflow convection can remove heat from the surface. There are cases when the application (design) is convection-limited and (not conduction-limited) thermally conductive plastics can provide heat transfer with efficiency equivalent to those of aluminum and copper designs. In addition, the lower coefficient of thermal expansion (CTE) of plastics can be a plus capable of reducing the stresses occurring as a result of differential expansion, since the plastics more closely match the CTE of silicon or ceramics that they contact (99, 102). Specially formulated grease and silicone rubber has dominated the previous generation of TIM materials. The most advanced grease materials have been disclosed by Shin-Etsu Chemical Company in a series of patents published during 1999-2001. Their formulations have been based on silicone grease comprising high conductive fillers, preferably aluminum nitride (AIN) because of its higher conductivity than other fillers such as ZnO, BN, and SiC (103-108). A liquid silicone is being used to provide a better wetting effect on the substrates and increase the contact conductivity. In addition to creating a "moisture barrier" around the AIN to preserve the conductivity, the coating may also help keep the particulates dispersed in the liquid silicone through a kind of associative linkage mech anism. This high dispersion would help increase conductivity and its consistency over the entire article, forming a film media, rather than being concentrated in one spot.
In spite of successful implementation of silicone grease and rubber for forming TIM films, most experts now agree (102, 109) that various new TIM materials including highly filled advanced polymer tapes and phase change materials (cooling the microprocessor's units by absorbing heat during their melting) will soon take over for thermal grease type materials. Scientists and engineers have to solve several difficult technical problems to provide successful implementation of the next generation of TIM. Besides increased thermal conductivity, rheological aspects of processing of highly filled materials into film and tapes, and adhesion of these interface materials to metal parts of chips and sinks, a serious concern may be the thermal-expansion behavior of thermoplastic
composites (101, 102). This problem has been examined for a variety of thermoplastic composites based on PEEK and other materials by Barnes (110). It was shown that when each of the composites is raised above the [T.sub.g] of the polymer matrix, c onsiderable permanent distortion occurs, though the effect is not evident below [T.sub.g]. This was attributed to relaxation of process-induced residual stresses. A number of models are used to calculate the thermal-expansion behavior of the fiber transverse to their long axis. Unfortunately, in many cases, the results are inconsistent and in poor agreement with directly measured properties, and further perfection of the models is required.
LCP for High Performance Films
Liquid crystal polymers (LCP) are distinguished by high heat resistance, good mechanical properties, and moldability. Hoechst Celanese ("Vectra"), Amoco Polymers ("Xydar"), and DuPont ("Zenite") produce various liquid crystal polymers (LCPs) in increasing quantities. It is important to emphasize that a range of special grades of LCPs have been developed recently for use in mono- and multi-layer high performance film structures, as well as for blending with other thermoplastic polymers. These films were meant, at the beginning, for advanced barrier packaging applications. LCPs are often co-processed with conventional resins such as polyolefins, polyesters, and polyamides in multilayer films (111). Some experts believe that co-extrusion equipment capable of fabrication of multilayer film comprising polyvinylalcohol (PVOH) barrier grades can be suitable to switch to LCPs without any additional major outlay.
The main problem in processing of neat LCPs into film, limiting their consumption, has always been the strong orientation of the polymer in the longitudinal (extrusion or "machine") direction and the resulting anisotropy of properties in "machine" and transverse directions. The extremely rigid polymer molecules remain aligned in the melt and in the solid state. The orientation of LCPs in the melt "freezes" into the solid state because of the very long relaxation times. The strong orientation of LCPs does not create too many difficulties in injection molding processes, including injection molding and modern coinjection molding of filled and reinforced compositions. The amount of fillers and fibers may be quite high because of the very low viscosity of the neat LCP matrixes and a very sharp decline of the apparent viscosity with increase of temperature. These materials include grades that can be processed at temperatures as low as 220 [degrees]C and as high as 400[degree]C (111). Applications and properties of LCPs are reviewed in Table 10.
A number of interesting blends comprising LCPs have been deveLCPed and implemented in the last few years. Morphology of blends with LCPs influences the properties, along with additives and compatibilizers. On the other hand, the rheology of such blends, specifically the viscosity ratio of components, has a strong effect on morphology (112). Blends of LCPs with polyolefins, polyesters, polycarbonate, and other thermoplastics are used in increasing quantities. It is worth mentioning the following effects of LCPs on blend properties and their processing:
* At low levels, LCPs can be used as processing aids and reduce the viscosity of the main polymer "partner"; small quantities of LCPs may also improve the surface of parts, sheets, and film.
* LCPs can increase the modulus and stiffness of the matrix component and practically provide reinforcement for thin wall parts, sheets, and film; blends in this case may be considered "in-situ" composites; a good example is long duration improvement of weather balloons made of PE achieved by NASA together with Penn State University by incorporating 10% of LCP fibers into PE film (113, 114).
* Blends with LCPs as matrixes exhibit excellent barrier and enhanced thermal and physical properties versus non-LCP blends.
* LCPs and blends based on them display improved adhesion in various laminates.
* The interfacial tension between LCP and the thermoplastic component in the blend can be substantial; for example, it was shown by Kenig (115) that the tension on the interface LCP/PC is between 4 and 6 dyne/cm. This may negatively influence the compatibility and adhesion between components.
Various methods of reducing the undesirable flow-induced orientation and anisotropy of mechanical properties have been suggested for many applications. We cannot discuss them all in detail, because many companies are considering this information to be their "know-how." The most published and widely advertised method is the use of an extrusion die with a rotating mandrel or with counter-rotating die matrix and mandrel. For example, Superex Polymer, Inc., suggested a counter-rotating die for blow molding of multilayer tubing, beer bottles, and blown extruded film made of a combination of LCP with PET (116). R. Lusignea and his colleagues have described Superex technology in many technical publications and reviews for regular LCP films (117) and oriented films and structures (118). In spite of the fact that the theory and technology of extrusion through dies with rotating mandrels were developed quite a while ago (119-121) and the influence of helicoidal flow on the reorientation of the melt was proven theoretic ally and experimentally, the use of dies with high-speed rotating parts for LCP is still limited. A process for producing thin LCP films with improved tear resistance, balance of physical properties, and wrinkle-free surface has been disclosed (122). The use of an annular coextrusion die was suggested for lining the LCP with one or more surface layers of a nonadherent polymer before the LCP becomes shear-oriented. The multilayer tube (sleeve) formed by the coextrusion is sealed, stretched, and cooled by passing it over a sizing mandrel. The tube is slit open into a flat configured film.
A lot of attention has been paid to the development of isotropic LCP films for various applications, first of all for printed circuit boards. Potential applications targeted for the new films are copper-clad laminates, both single and double-sided (123). Thermal expansion equivalent to silicon, low dielectric constant, and extremely high hygroscopic capabilities make these films particularly suitable for packaging of integrated circuits. Surface topography and chemistry may be important in the above applications, and some inventions have disclosed processes for roughening LCP film surface (124) and for improving its bondability by modifying the polymer (125). The advanced blown and cast films containing blends of LCPs and PO and reinforcing fibers have been successfully fabricated for various applications requiring toughness and modulus enhancement of approximately 400% over the neat PO matrix (126).
ADVANCED LOW-TEMPERATURE POLYMER FILMS
As stated in the Introduction to this review, it is our belief that low-temperature thermoplastic materials are being increasingly considered as a valuable source for fabricating high performance products, and first of all film for advanced applications required by modern technologies. This approach has been strengthened during the last few years by the following very efficient "breakthrough" technologies:
* New polymerization chemistry and technology, first of all metallocene catalyzed polymerization of polyethylene (m-PE), polypropylene (m-PP), polystyrene (m-PS), and cyclic polyolefins like ethylene norbornene copolymers (m-ENC).
* Various catalytic modifications of low-temperature thermoplastics.
* Further progress in development of blends and alloys.
* A widespread increase in. efficiency of coextrusion of various thermoplastics including the design of multi-manifold coextrusion dies enabling the combination of polymers with significant differences in apparent melt viscosity and other rheological properties.
* Development of polyamides with higher melting and use temperatures or "high-temperature nylons" (HTN); the demand for HTN and semi-aromatic and aromatic family polyamides (aramid) has been growing recently in automotive and electrical/electronic applications because of their high flame resistance, arc and tracking resistance, and formability into thin-wall parts.
* Development and implementation of nanocomposites based on various thermoplastics such as PA (nylons), PP, PBT, PE. and progress in melt compounding of nanomaterials.
The main directions in development and application areas of low-temperature advanced film may be summarized as follows:
* High performance specialty mono and multilayer packaging (LLDPE, PA, PMP, PP with superb impact resistance, clarity, barrier, and heat-seal properties).
* Release film for low and medium temperature applications (high temperature PA. PMP, m-PE, etc.).
* Film with enhanced optical properties for solar panels, safety glazing, and special optical effects such as light-emitting materials.
For example, metallocene catalyzed low and super-low density PE recently enabled the introduction of various families of "enhanced" resins for blown film with very high impact resistance, outstanding hot-tack and heat-seal properties (127, 128), a line of thin (0.5-2.0 mil) high performance polyolefinic shrink films (129), and a novel "self-contracting," high elasticity seal and wrapping film (130).
The new chemistry and achievements in "in-line" quality control (131) have led to a strong improvement in optical properties of various polymers and opened the door for the creation of families of film and sheets with superb optical properties including films with extremely low haze (<0.4% at thickness of 15 mil and higher), which was achievable in the past by using PVB only. These mono and multilayer films based on ionomers (132), ethylenemethacrylates (133), PU, PC (134), and m-PO (135-137) may successfully compete in the future with PVB as a lower cost, plasticizer-free interlayer for optical laminates, protective, and safety glazing products. Multilayer sandwiches with EVA as "skin" layers coextruded to the core of the main m-PO resin (136) have successfully solved the adhesion problem in terms of lamination of m-PO to glass.
According to some experts (138), new advanced films, for example made of PMP, may successfully compete with high-end release liners of fluoropolymers (FEP and ETFE) in applications in which FP liners are "over-engineered" or as an alternative film when the users are facing FEP shortage. It was proven that one of the disadvantages of PMP film, its low elongation at break and the release properties of PMP. can be significantly improved by blending PMP grades having a low content of atactic fraction (< 2%) and a high MW (200,000-700,000), with PP of medium MW in the range from 150,000 to 350,000 (139). Elongation at break of such blends increases to 200%-350%, and for some optimized formulations up to 500%. in comparison to 20%-100% typically observed for neat PMP. At the same time, the release parameter (force required to peel the film specimen of a standard acrylic-based pressure sensitive adhesive coated substrate) decreases to the load less than 0.6 lb/inch in comparison to 0.6 lb/inch typical for a neat PMP film.
Unexpected capabilities of another polyolefin, namely metallocene catalyzed LLDPE (m-LLDPE), to perform as a release film (liner) was discovered and studied (140). It is known that m-PE is tacky because of ultra-low density. Processing of m-PO causes problems with winding in rolls owing to "self-blocking" of film layers. It is anticipated that m-PE may hardly be considered a good choice of material for application as a release liner. However, surprisingly, m-LLDPE with certain density, MW, and MWD shows pretty good release properties when applied to acrylic-based pressure-sensitive adhesives. A "window" in densities from about 0.865 g/ccm to 0.900 g/ccm has been determined for m-LLDPE with a narrow MWD (with index of polydispersity, PI less than 5.0), which produces a film providing release force values in an adhesive peel test lower than 39 g/cm (see Fig. 8). The film shows optimal release properties when the resin density is lower than 0.88 g/ccm and the PI is close to or less than 2.0. The film is useful i n manufacturing rolls and sheets of PSA tape especially when a silicone-free release film is desired.
There is no doubt that creation of new materials with improved properties and lower costs will continue, and new low-temperature advanced film products will be offered to various end-users. For example, it has recently been reported in the U.S. and the U.K. (141) that the newly developed family of iron-based catalysts is able to rival the effectiveness of both conventional organometallic and metallocene catalysts in the polymerization of olefins. The new catalyst family shares many of the advantages of metallocene catalysts in terms of activity and control of polymer molecular structure and properties. In addition, it offers the potential for producing a much broader range of polymeric materials at low cost.
Another new trend in advanced film and sheet development is the growing use of lightweight nanocomposite materials with just 2%-8% loading of fillers. For example, the organo-clay hybrid incorporated into nylon through a compounding process improves the oxygen transmission rate in film by four times compared to unfilled nylon 6 (142). The material also has properties equivalent to usual mineral-filled compounds with a much higher loading (20%-30%). The orientation of the organo-clay platelets prevents degradation of viscosity and allows lower viscosity at high shear rates. The stifness of the compound at 5% weight loading is comparable to a conventional 20% mineral-filled nylon with minimum rise in specffic gravity.
Advanced conductive and super-paramagnetic films may be possible to produce using a process that embeds magnemite nanoclusters into polyaniline and other electrical conductive polymers (143). These films are expected to have applications in a range of technologies including interference shielding and optical systems.
It is anticipated that nanofillers will help in developing films and coatings with "super-barrier" capabilities, improved flammability, rigidity, and toughness. Nanocomposite films are being developed as mono-layer items with various mineral nanofillers based on thermoplastic resins (144-147) and polyimides (148), as well as components in multilayer coatings and structures (149). It is worth mentioning that the significant progress in design and fabrication of multimanifold dies enables the coextrusion of polymers of a different nature with dissimilarity in apparent viscosity of two or more orders of magnitude. The triple-manifold extrusion system may also contribute to a high-quality appearance of film made of filled and unfilled components (150). However, scientists and engineers should keep in mind that, for some combinations of thermoplastic materials, best results could be achieved with coextrusion feedblocks rather than multi-manifold dies. The reason is related to the fact that different melt layers in a multi-manifold die may not be in contact long enough, just in the die land area to achieve a reliable mutual diffusion before being cooled to the solid state.
In most cases, the trend is to combine the two modem technologies: blending and coextrusion, i.e., to utilize the best combination of thermoplastic materials within each layer, and to utilize the best possible combination of different layers in the "sandwiched" film. For example, in addition to blends based on PMP (139), further development is moving in the direction of creating multilayer structures based on PMP and other thermoplastics in an attempt to make them more competitive with fluoropolymers in release film applications. One interesting recent invention (151) provides a multilayered polymethylpentene-containing release film for various applications with improved release properties when exposed to high temperatures in processes using epoxy resins. These films are non-oriented and comprise a polyamide layer, a PMP layer, and an adhesive layer between. It has been suggested to extrude the polyamide layer with a ternary blend of nylon 6, nylon 6,6 copolymer, and nylon 6,12 copolymer, and to combine this layer with a film made of a blend of PMP and PMP-based copolymer. It seems that combinations of sophisticated blending and coextrusion techniques are able to produce film variations practically without limits in developing advanced film structures.
Combinations of alloys and multilayered structures of engineering plastics are being thoroughly developed utilizing precise chemical structure and morphology control based on advanced compatibilizers and reactive processing technologies (152). Experts anticipate that these trends in developing engineering plastics and polyolefins, alloys, and blends coextruded in various combinations will be the focus for efforts of researchers and engineers for a long period of time. An important example below shows the summarized information collected by Okada (153) on polymer alloys and blends produced or being actively developed (Table 11), along with notable new compatibilizers, polymer blends, and alloy products (Table 12) recently commercialized in Japan.
BIOMIMETIC STRUCTURES-POTENTIAL FOR CREATION OF ADVANCED FILM
Biomimetic is the abstraction of a good design from nature and shows a very high potential for the creation of new advanced composite materials and film with intriguing properties. Biological materials are virtually all nanocomposites and have hierarchical structures, organized interactively at the molecular, nanoscopic, microscopic, and macroscopic levels (153). This integrated approach seems to be the key strategy by which nature balances strength and stiffness without sacrificing toughness of the materials. According to many experts, the mimicking of natural nanocomposite structures may open the way to the creation of materials for the future, including not only bones and shell-type materials, but film structures as well. Experiments are at an early stage, but already there are indications helpful to the design of synthetic nanoscale hybrid materials. There are some general "ideas of nature" that from our point of view may be very useful in the development of advanced film structures.
Experimental investigation by reflection spectroscopy, elipsometry, and transmission electron microscopy of optical properties of colored wing scales of butterflies and moths produced very interesting results (154, 155); the outer surfaces of these scales incorporate a regular layered structure of alternating dense and less-dense material, which reflects light by the "classical" interference process. In other words, without doubt, the coloration of the surface of biological materials is due to the structural architecture and is in no way related to pigmentation. Nature has found a way of fabricating sophisticated photonic bandgap structures that are more advanced than those fabricated in leading laboratories. Maybe the key to the creation of new film and coatings with intriguing optical properties such as brilliant colors, strong UV-reflection capabilities, and metallic appearance is in the microscopic geometry of surfaces as is typically observed in thin-film biological reflectors. This could enable fabricat ion of advanced optical film at "room temperature without nasty solvents" (156).
Investigation of mechanical properties and fracture mechanisms of shell, nacre, and other biomaterials in correlation to structural hierarchy showed the significant advantages of organo-mineral cross-lamellar microstructures in flexural strength and preventing crack propagation (157, 158). Nanocomposite assemblies achieved in efficient processing of layered organic-inorganic composites should be considered as a very difficult, but very desirable, way to fabricate films and coatings in the future. One proposed technology to mimic nature includes (159) organic polymerization (induced by light or heat), combined with continued inorganic polymerization, lock-in the nanocomposite architecture and covalently bonds the organic-inorganic interface. Another lesson from biology, which also proves the importance of hierarchical structures and highly specific interactions between the levels, has been derived from the investigation of tendon, intestine, vertebral disc, and other collagen composite systems (160). Functiona l macromolecular assemblies are considered to be important for the creation of multilayer and oriented composite systems (161), film and fibers (162).
It is understood that it would be very difficult (if possible at all) to cover in one paper, even one with many pages, all advanced high performance films and innovations influencing their development and applications. Any review is a "snapshot" of a "moving target," in this case on a changing picture of a very dynamic industry characterized by high-speed growth in terms of volume, as well as scientific and technical sophistication. Nevertheless, our opinion is that some trends in science and industry of high performance films will be retained and even strengthened as high-priority research and development directions. The most important trends, to our understanding, are described below:
* There is no doubt that further progress and development of high-tech applications in the electrical, electronic, and semiconductor industries, as well as in aircraft and space engineering. will fuel continuous efforts in the creation of high and ultra-high thermal resistant polymer films in the U.S., Europe, Japan, and other countries around the world. For example, a group of melt-processable rigid-rod polymers with polyphenylene backbone and relatively simple side groups, developed quite a while ago (163), may be implemented on an industrial scale in the next few years for various applications. In addition to aromatic high temperature materials. fluoropolymer films will continue to be in growing demand. This trend, which started in the early to mid- 1990s (164, 165), is still true today especially for the new generation of fluoropolymers. One interesting direction is further progress in tailoring the crystallinity of fluoropolymers. Materials like "amorphous fluoropolymers" (such as the Teflon AF family by DuPont) will be used for various coatings, providing a combination of excellent weatherability and easier processability in comparison to regular fluoropolymers. Amorphous fluoropolymers have shown limited solubility (3%--15%) in selected perfluorinated solvents, allowing solution-cast ultrathin coatings in the sub-micron thickness range. At the same time, these fluoropolymers are capable of retaining chemical resistance to all other solvents and process chemicals, along with outstanding optical clarity and transmission. Another example may be related to new developments of fluoroalkyl surfactant polymers providing clear, durable crosslinked film with an adhesive on one side and antistick properties on the other side (1.66).
* Another strong trend is based on further development and diversification of combinations of ultra-high temperature polymers and fluoropolymers. Multilayer structures made of polyimide (Kapton) and Teflon PTFE were presented early in the 1970s (167). This material was developed as a high-end insulation system with particular emphasis on the reduction of weight and space, and was spurred by the demanding requirements of space exploration efforts. More recently, this class of materials is being used in increasing quantities in military and commercial aircraft industries. A thin PI-FP film (tape) can be wrapped around a conductor and heat-sealed. The two leading technological trends are development and implementation of various combinations of thin PI overcoated with dispersions of mixtures of PTFE with FEP, PVF and other TFE copolymers (168, 169), or overcoated using separated dispersion layers of PTFE and FEP and other fluoropolymers (170-172). The addition of FEP promotes the bonding of PTFE to PI during the sintering of the multilayer tapes at high temperatures (around 700[degrees]F) when the FEP is being molten and adheres to PI from one side and PTFE from the other one. There is no doubt that these high performance films will gain many other applications.
* Future films include ultra-thin, highly oriented, and porous film (173) made of modified and new fluoropolymers, copolymers, blends and alloys, and chemically combined fluoro and alkane polymers (perfluoroalkanes). l3esides release film applications, researchers are focusing a lot of attention on the development of porous film and microporous electrolyte membranes for effective fuel cells (174).
* New high and ultra-high purity materials and film for semiconductor applications will constantly lower the level of acceptable metal ion linkage. New materials, their purification in polymerization and compounding, modified and perfected purity testing methods will be developed, along with "metal ion-clean" film-processing equipment.
* Films made of a group of mineral-organic materials comprising polymer-ceramic combinations, polymer nanocomposites, and "mineral polymers" (for example, polysilazenes) (175, 176) will be developed for more diversified applications, and will help solve many technical and medical problems.
These directions in advanced material development were firmly supported by the experts during the First World Congress "Nanocomposites--2001." which took place in Chicago (177).
* More advanced low-temperature films for a wide variety of applications with specific requirements will keep emerging. There is a very high probability that the demand for film made of novel resins produced using metallocene catalysts will grow not for packaging only, but also for applications requiring high optical clarity, extremely high elasticity, special surface properties, performance capabilities at extremely low temperatures. etc. Among them, super-low density PE. syndiotactic polystyrene (s-PS), cyclic m-ENBC, and various multilayer film structures. However, metal catalysts are "fighting back," and iron-based catalysts will soon be able to provide film materials, which may excite the industry.
* A large number of applications will need thermoplastic film filled with nanoparticulates, nanofibers, nanotubes, "Buckytubes," nanowhiskers, and combinations of particulated fillers and reinforcements, allowing the creation of materials with "many faces" typical for different composites. It is anticipated that nanocomposites will allow the development of films with improved properties, as well as new properties because of the fine particle disperse effect along with the interfacial phase increase phenomenon (178).
* Film with special optical, light, laser, and solar properties are being developed on an accelerated path. Ten years ago, no one imagined that polymers would be used with a light-emitting layer in light-emitting diodes. Now they form the basis of a new display technology that may develop to rival LOP (71, 179). Plastics are definitely expanding their opportunities in solar energy production, and this trend will grow thanks to low specific gravity, high clarity, dimensional stability, and dielectric properties of the modem polymers. especially metallocene PE and cyclic PO (180).
* Multilayer film structures will dominate many applications requiring combinations of materials with various high performance parameters; another trend would be the use of coextrusion and "co-lamination" for cost reduction of expensive high performance resin grades.
* Super-thin film (6-20 mcm and lower) made from polyimides, polyketones, polyesters, and other advanced engineering materials are increasingly attracting attention as cold and heat resistant coatings for various applications.
* Film made of mineral-organic hybrids with hierarchical structures mimicking "Mother Nature" will be developed, providing superb mechanical and optical properties along with high flex endurance and high resistance to fatigue for many applications in various industries.
APPENDIX A ABS Acrylonitro butyldienestyrene terpolymer BOPET Biaxially oriented polyethylene terephthalate BOPP Biaxially oriented polypropylene ECTFE Polyethylene chlorotrifluoroethylene copolymer ETFE Polyethylene tetrafluoroethylene copolymer EMA Ethylene methylacrylate copolymer EVA Ethylene vinylacetate copolymer EPDM Ethylene propylene diene monomer FEP Fluorinated ethylene/propylene copolymer FP Fluoropolymer HFP Hexafluoropropylene HIPS High impact polystyrene LCP Liquid Crystal Polymer LDPE Low density polyethylene LLDPE Linear low density polyethylene m-LLDPE Metallocene catalyzed linear low density polyethylene MFA Polytetrafluoroethylene-Polymethylvinylether OPP Oriented polypropylene PA Polyamide PAE Polyaryleneether PAEK Polyaryletherketone PACI Polyalcanimid PAI Polyamidimid PAN Polyacrylonitrile PBT Poly (butyl) terephthalate PC Polycarbonate PCTFE Polychlorotrifluoroethylene (mono chloro) PE Polyethylene PEEK Polyetheretherketone PEI Polyetherimide (Ultem) PEKK Polyetherketoneketone PES (PESd) Polyethersulfide PESF (PEStf) Polyethersulfone PFA Perfluoronated alkoxy resin PFBE Perfluorobutyl ethylene PFHE Perfluorohexyl ethylene PET Polyethylene terephthalate PEN Polyethylene naphthalate PI Polyimide PMMA Polymethyl methacrylate PMP Polymethyl pentene PO Polyolefin PP Polypropylene PPE Polyphenylene ether PPO Polypropylene oxide PPSf Polyphenylene sulfone PPSd Polyphenylene suffide PPS Polysulfone PPVE Perfluoropropyl vinyl ether PS Polystyrene PTFE Polytetrafluoroethylene PVB Polyvinyl butyral PVC Polyvinyl chloride PVDC Polyvinylidene chloride PVDF Polyvinylidene fluoride PVF Polyvinyl fluoride PVOH Polyvinyl alcohol THF Tetrahydrofuran TUP Thermoplastic urethane UHMWPE Ultrahigh molecular weight polyethylene
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Table 1 TFE & ETFE Based Copolymers and Effects of Copolymerization on PTFE Properties Basic Monomer Comonomer TFE HFP (Tetrafluoro-ethylene) (Hexafluoro-propylene) 5-15 [CF.sub.2] = [CF.sub.2] mol. % [FORMULA NOT REPRODUCIBLE IN ASCII] TFE PPVE (Tetrafluoro-ethylene) (Perfluoropropyl-vinylether) [CF.sub.2] = [CF.sub.2] [greater than or equal to] 1 mol. % [FORMULA NOT REPRODUCIBLE IN ASCII] TFE PMVE (Tetrafluoro-ethylene) (Perfluoromethyl-vinylether) [CF.sub.2] = [CF.sub.2] [FORMULA NOT REPRODUCIBLE IN ASCII] PPVE or PFMD (Perfluoromethoxydioxide) [FORMULA NOT REPRODUCIBLE IN ASCII] TFE PDD (Tetrafluoro-ethylene) (Perfluoro-2,2-dimethyl-1,3- [CF.sub.2] = [CF.sub.2] dioxole) >10 mol% TFE PPVE or PFMD (Tetrafluoro-ethylene) [less than or equal to] 1 mol% [CF.sub.2] = [CF.sub.2] ETFE PPVE, PFBE (Ethylene-tetrafluoro-ethylene) (Perfluorobutylether [CF.sub.3]-[([CF.sub.2]). sub.3] - CF = [CH.sub.2] PFHE [([CH.sub.2][CH.sub.2]).sub.n] (Perfluorohexylether) [([CF.sub.2][CF.sub.2]).sub.m] < 5 mol% [CF.sub.3]-[([CF.sub.2]). sub.5] - CF = [CH.sub.2] ECTFE PPVE (Ethylene-chlorotrifluoroethylene) PMD HFIB (Hexafluroisolbutylene) < 5 mol% [FORMULA NOT REPRODUCIBLE IN ASCII] Basic Resulting Monomer Copolymer TFE FEP (Tetrafluoro-ethylene) [CF.sub.2] = [CF.sub.2] TFE PFA (Tetrafluoro-ethylene) [CF.sub.2] = [CF.sub.2] TFE MFA (Tetrafluoro-ethylene) [CF.sub.2] = [CF.sub.2] TFE AF (Tetrafluoro-ethylene) (Amorphous fluoropolymer) [CF.sub.2] = [CF.sub.2] TFE (Tetrafluoro-ethylene) [CF.sub.2] = [CF.sub.2] ETFE Modified ETFE (Ethylene-tetrafluoro-ethylene) [([CH.sub.2][CH.sub.2]).sub.n] [([CF.sub.2][CF.sub.2]).sub.m] ECTFE Modifid ETFE (Ethylene-chlorotrifluoroethylene) Basic Crystalline Monomer Modification TFE Intra modecular conformation & (Tetrafluoro-ethylene) packing altered (helix inversion, [CF.sub.2] = [CF.sub.2] planar zigzag sequences. [CF.sub.3] group enters & distorts crystal lattice. Crystalline melting point ([T.sub.m]) reduced. Melt processable. TFE Helix inversion & Planar zigzag (Tetrafluoro-ethylene) sequences. Perfluoroalkoxy group [CF.sub.2] = [CF.sub.2] location; excluded from crystal lattice; enters chain folds or amorphous region. Crystalline melting point ([T.sub.m]) reduced; more effective than HFP in FEP. Melt processable. TFE Similar to FEP & PFA. (Tetrafluoro-ethylene) (see 1 & 2) Extrudable. [CF.sub.2] = [CF.sub.2] TFE Completely amorphous. (Tetrafluoro-ethylene) Melt processable. [CF.sub.2] = [CF.sub.2] TFE Compared to suspension PTFE: (Tetrafluoro-ethylene) 1. Not melt processable [CF.sub.2] = [CF.sub.2] 2. Reduced creep 3. Reduced permeability 4. Higher elastic modulus 5. Improved weldability 6. Higher light transparency ETFE Less organized crystalline (Ethylene-tetrafluoro-ethylene) domains. Reduced density of amorphous regions. Thinner lamellae. Enhanced mobility of crystallites. [([CH.sub.2][CH.sub.2]).sub.n] [([CF.sub.2][CF.sub.2]).sub.m] ECTFE Similar to ETFE modification. (Ethylene-chlorotrifluoroethylene) Basic Copolymer Monomer Properties TFE [T.sub.m] reduced from 327[degrees] (Tetrafluoro-ethylene) C to 260[degrees]C. MW & melt [CF.sub.2] = [CF.sub.2] viscosity reduced. Temperature rating reduced from 260[degrees]C to 204[degrees]C. Most physical properties retained. Poor creep vs. other melt processable FP. TFE [T.sub.m] reduced from 327[degrees] (Tetrafluoro-ethylene) C to 305[degrees]C. MW & melt [CF.sub.2] = [CF.sub.2] viscosity reduced. Temperature rating practically equal to PTFE (260[degrees]C). Most physical properties similar to PTFE. TFE [T.sub.m] reduced from 327[degrees] (Tetrafluoro-ethylene) C to 285[degrees]C (between FEP & [CF.sub.2] = [CF.sub.2] PFA). MW & melt viscosity reduced. Temperature rating reduced from 260[degrees]C to 235[degrees]C (between FEP & PFA). Most Physical properties between FEP & PFA and similar to PTFE. Higher light transmittance (twice more UV-light transparent vs. PFA). High surface smoothness. TFE High light transparency. (Tetrafluoro-ethylene) Soluble in fluorocarbons for [CF.sub.2] = [CF.sub.2] various coatings. TFE Creep reduced from > 10% to 6-6.2%. (Tetrafluoro-ethylene) Elastic moduls increased from [CF.sub.2] = [CF.sub.2] 80-90 GPSI to 100-110 GPSI. Dielectric strength increased from 85 kV/mm to 95 - 100 kV/mm. ETFE Improved thermal stress cracking (Ethylene-tetrafluoro-ethylene) properties. [([CH.sub.2][CH.sub.2]).sub.n] [([CF.sub.2][CF.sub.2]).sub.m] ECTFE Improved thermal stress cracking (Ethylene-chlorotrifluoroethylene) in the range from 135[degrees]C to 175[degrees]C. Table 2 Other Modified Polymers Primary Monomer Modifier Primary Effect(s) Vinylidene Fluoride HFP Flexibility, Impact Strength (VDF) CTFE Flexibility, Impact Strength TrFE Piezoelectric, Ferroelectric TFE Solubility, Flexibility HFA Optical Clarity Ethylene Base Resistance Chlorotrifluoroethylene VDF Solubility, Processibility (CTFE) Table 3 Physical Property Data for MFA 620 Film. Property Test Method MFA 620 Tensile Strength at Break * ASTM D-882 MD 5500 psi TD 4900 psi Elongation at Break * ASTM D-882 MD 500% TD 500% Specific Gravity ASTM D-792 2.12-2.17 Melting Point ASTM D-4591 536-554[degrees]F Thermal Exp. Coefficiency ASTM D-696 6.6-11.1 X [10.sup.-5] 73 - 300[degrees]F in/in/F Thermal Conductivity Specific Heat Oxygen Index ASTM D-2863 > 95% Flammability UL94 V-O Haze * ASTM D-1003 0.83% Water Absorption ASTM D-570 < 0.03% Volume Resistivity ASTM D-257 > 1 X [10.sup.17] [OMEGA].cm Surface Resistivity ASTM D-257 > 1 X [10.sup.17] [OMEGA]/sq Arc Resistance ASTM D-495 210 sec Dielectric Strength * ASTM D-149 3600 V/mil Dielectric Constant 23C ASTM D-150 at 50 Hz 2.0 at 100 kHz 1.95 Dissipation Factor ASTM D-150 at 50 Hz 0.0002 at 100 kHz 0.0002 Property PFA FEP Tensile Strength at Break * MD 5500 psi 3600 psi TD 5400 psi 3400 psi Elongation at Break * MD 450% 400% TD 480% 400% Specific Gravity 2.12-2.17 2.13-2.17 Melting Point 575-590[degrees]F 500-536[degrees]F Thermal Exp. Coefficiency 4.2-5.4 X [10.sup.-5] 5.2 X [10.sup.-5] 73 - 300[degrees]F in/in/F in/in/F Thermal Conductivity 0.109 0.109 Specific Heat BTU/(hr ft F) BTU/(hr ft F) 0.2799 0.2799 BTU/lb F BTU/lb F Oxygen Index > 95% > 95% Flammability V-O V-O Haze * 4.54% 0.89% Water Absorption < 0.03% < 0.01% Volume Resistivity > 1 X [10.sup.18] > 1 X [10.sup.18] [OMEGA].cm [OMEGA].cm Surface Resistivity > 1 X [10.sup.18] > 1 X [10.sup.18] [OMEGA]/sq [OMEGA]/sq Arc Resistance > 300 sec Dielectric Strength * 5000 V/mil 4500 V/mil Dielectric Constant 23C at 50 Hz 2.05 2.052 at 100 kHz 2.05 2.052 Dissipation Factor at 50 Hz 0.0002 0.0002 at 100 kHz 0.0002 0.0002 Property ECTFE Tensile Strength at Break * (1 mil film) MD 10,000 psi TD 8000 psi Elongation at Break * (1 mil film) MD 190% TD 300% Specific Gravity 1.68 Melting Point 464[degrees]F Thermal Exp. Coefficiency 4.4-7.5 X [10.sup.-5] 73 - 300[degrees]F in/in/F Thermal Conductivity 0.092 Specific Heat BTU/(hr ft F) Oxygen Index 60% Flammability V-O Haze * 4.18% Water Absorption < 0.1% Volume Resistivity > 1 X [10.sup.15] [OMEGA].cm Surface Resistivity Arc Resistance 130 sec Dielectric Strength * 6200 V/mil (1 mil film) Dielectric Constant 23C at 50 Hz 2.6 at 100 kHz 2.5 Dissipation Factor at 50 Hz < 0.0009 at 100 kHz 0.003 * Results obtained on 2-mil-thick samples. Table 4 Tensile Properties Versus Temperature for Different Grades of PTFE Test Temperature TFM Traditional PTFE Degrees Celsius N/[mm.sup.2] (%) N/[mm.sup.2] (%) -30 54 330 51 140 0 38 460 43 220 23 32 650 30 520 100 20 600 16 570 150 15 500 12 530 200 12 500 9 510 230 9 605 0 N/A ASTM 1708 die cut from 0.2 mm skived film Table 5 Main Patents on Corona Treatment of Fluoropolymers Patent # Gases & Chemicals 3,030,290 Hydrogen or anhydrous ammonia at abs. pressure 0.2 mm Hg 3,255,099 Inorganic agent having affinity for electrons (not for FEP) 3,274,089 [N.sub.2] + glycidyl methacrylate (0.5% vol.); or [N.sub.2] +; toluene-2, 4-diisocyanaer; Vinyl Acetate; 2-vinyl pyrrolidone; acrylonitrile; p-chlorostyrene; hexane; xylene: carbon tetrachioride; styrane sulfonic acid; tetra isopropyl titanate 3,274,089 For PTFE film: [N.sub.2] + dimethyl formamide, triethyl phosphite, vinyl ethyl ether, acetone, N-aminopropylmorpholine, acrylic acid 3,274,091 For PTFE, PVC, PE, PP, & FEP films: [N.sub.2] + acrylic acid or diethyl maleate 3,275,540 [N.sub.2] + glycidyl methacrylate and/or (diethylene triamine or ammonia or ethylene diamine) or various combinations of Organic/ amine; Methylmethacrylate/ Triethylene tetramine; Carbon tetrachloide.triethylamine; Methylvinylketone/ Tetraethylenepentamine; Tetrahydrofuran/Tertiary butyl amine; Vinyl acetate/Decyl amine; Vinyl acetate/Decyl amine; Zylene/ Methylisopropylamine; Vinyl butyl ether/Propylene diamine; Acrylonitrile/Ethylene diamine 3,291,712 [N.sub.2] + polyamine: diethylene triamine or etylene diamine or triethylene tetramine (0.7% vol.). Also: Cycloheryl amine; Tetraethylene pentamine; Decylamine; Tetriary butyl amine; Methylisopropyl amine; Propylene diamine; Triethylamine 3,296,011 [N.sub.2] + (0.5% glyc.methacr. Or acrylonitrile or p-Chlorostyrene or Toluene-2, 4-diisocyanate or Vinyl acetate or Xylene or Hexane or Carbon tetrachloride or Tetraisopropyltitanate 3,397,132 [N.sub.2] + (glycidyl methacrylate Or acrylonitrile or [CCI.sub.3]F3%): for metals and metallized films. 3,485,734 Air (for polyimide containing fluoropolymer) 3,507,763 [N.sub.2] + glycidyl methacrylate 3,676,181 [N.sub.2] + acetone (20% vol.); Acrylonitrile, hexane, methyl alcohol, methyl methacrylate, xylene 4,549,921 [N.sub.2] + acetone for lamination using adhesive Patent # Application 3,030,290 3,255,099 3,274,089 Liners for pipes & vessels for corrosive chemicals 3,274,089 Non-fogging films 3,274,091 Non-fogging films 3,275,540 Liners for pipes & vessels for corrosive chemicals 3,291,712 Liners for pipes & vessels for corrosive chemicals 3,296,011 Linears for pipes & vessels for corrosive chemicals 3,397,132 Condensers 3,485,734 Bearings 3,507,763 3,676,181 Printed electrical circuits 4,549,921 Printed circuits, flat electrical cable, corrosion protection, anti- stick surfacing Table 6 Surface Atomic Concentration for FEP Film Samples. (Data Obtained by X-ay Photoelectron Spectroscopy) Chemical Elements, % Sample C F O DuPont "C"-film 32 68 0.35 Non-cementable side Saint-Gobain Performance Plastics 30 70 -- Untreated side DuPont "C"-film 62 18 15 Cementable side Saint-Gobain Performance Plastics 59 21 12 Treated side Chemical Elements, % Sample N CI Si DuPont "C"-film -- -- -- Non-cementable side Saint-Gobain Performance Plastics -- -- -- Untreated side DuPont "C"-film 3 2 -- Cementable side Saing-Gobain Performance Plastics 3.4 -- 5 Treated side Table 7a FEP Resin Analysis--Extractable Metal Ions Average Bulk Elements Grade 1 Grade 2 Grade 3 ppm Al 0.0258 0.07 0.086 0.061 Ba 0 0 0 0.000 Ca 0.021 0.018 0.042 0.027 Co 0.0012 0.009 0.012 0.007 Cr 0.0012 0.0012 0.0012 0.001 Cu 0.0036 0.0036 0.0036 0.004 Fe 0.0024 0.0024 0.0024 0.002 K 0.0849 0.0849 0.0849 0.085 Mg 0.001 0.001 0.008 0.003 Mn 0.0006 0 0.001 0.001 Mo 0.0126 0.038 0.035 0.029 Na 0.0099 0.002 0.0099 0.007 Ni 0.0042 0.0042 0.0042 0.004 Sr 0 0.0001 0.001 0.000 Ti 0.0021 0.0021 0.0021 0.002 Zn 0.0003 0.0003 0.0003 0.000 Zr 0.0003 0.0003 0.0003 0.000 Pb 0.0345 0.0345 0.0345 0.035 Sn 0.0057 0.0057 0.0057 0.006 * All numbers are average ppm values measured for pellets ("bulk"). Table 7b PFA resin Analysis--Extractable Metal Ions Elements Grade 1 Grade 2 Grade 3 Grade 4 Grade 5 Grade 6 Grade 7 Al 0.017 0.0258 0.023 0.0258 0.355 0.12 0.474 Ba 0.0003 0.01 0.001 0.004 0 0.001 0.005 Ca 0.131 0.009 0.107 0.086 0.148 0.24 0.252 Co 0.0012 0.041 0.0012 0.112 0.0012 0.123 0.027 Cr 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 0.0012 Cu 0.0036 0.0036 0.0036 0.0036 0.0036 0.0036 0.059 Fe 0.0024 0.0024 0.0024 0.053 0.0024 0.21 0.0024 K 0.0849 0.0849 0.0849 0.0849 0.0849 0.0849 0.0849 Mg 0.003 0.002 0.004 0.008 0.005 0.026 0.007 Mn 0.001 0.009 0.0006 0.0006 0.0006 0.0006 0.001 Mo 0.0126 0.0126 0.063 0.0126 0.0126 0.0126 0.097 Na 0.0099 0.026 0.0099 0.0099 0.0099 1.292 0.0099 Ni 0.0042 0.0042 0.0042 0.0042 0.0042 3.45 0.0042 Sr 0.002 0.0001 0.001 0.004 0.002 0.01 0.006 Ti 0.0021 0.0021 0.0021 0.0021 0.0021 0.0021 0.0021 Zn 0.0003 0.0003 0.0003 0.004 0.0003 0.018 0.24 Zr 0.0003 0.035 0.0003 0.0003 0.0003 0.0003 0.025 Pb 0.0345 0.0345 0.0345 0.0345 0.0345 0.0345 0.0345 Sn 0.0057 0.0057 0.0057 0.0057 0.0057 0.0057 0.0057 Elements Grade 8 Grade 9 Grade 10 Grade 11 Grade 12 ppm Al 0.432 0.368 2.430 0.570 0.700 1.847 Ba 0.003 0.001 0.001 0.001 0.001 0.009 Ca 0.065 0.044 0.030 0.250 0.450 0.604 Co 0.03 0.0012 0.001 0.007 0.007 0.118 Cr 0.0012 0.0012 0.002 0.002 0.002 0.006 Cu 0.0036 0.0036 0.030 0.010 0.010 0.046 Fe 0.0024 0.0024 0.005 0.001 0.001 0.096 K 0.0849 0.0849 0.070 0.180 0.170 0.395 Mg 0.001 0.002 0.020 0.001 0.090 0.056 Mn 0.015 0.004 0.020 0.001 0.001 0.018 Mo 0.0126 0.0126 0.008 0.430 0.570 0.419 Na 0.106 0.0099 3.180 0.640 0.610 1.971 Ni 0.079 0.0042 0.160 0.004 0.004 1.242 Sr 0.0001 0.001 0.001 0.001 0.001 0.010 Ti 0.0021 0.0021 0.001 0.009 0.009 0.013 Zn 0.0003 0.0003 0.002 0.004 0.004 0.091 Zr 0.0003 0.0003 0.005 0.005 0.005 0.026 Pb 0.0345 0.0345 0.010 0.040 0.010 0.124 Sn 0.0057 0.0057 0.002 0.008 0.008 0.023 * All numbers are average ppm values measured for pellets ("bulk"). Table 8 High-temperature Polymer Materials Chemical Name Chemical Structure Polyethersulfone (PES) [FORMULA NOT REPRODUCIBLE IN ASCII] Polyetheretherketone (PEEK) [FORMULA NOT REPRODUCIBLE IN ASCII] Polyaryletherketone (PAEK) [FORMULA NOT REPRODUCIBLE IN ASCII] Polysulfone (PSF) [FORMULA NOT REPRODUCIBLE IN ASCII] Polyetherimide (PEI) [FORMULA NOT REPRODUCIBLE IN ASCII] Polyetherketoneketone (PEKK) [FORMULA NOT REPRODUCIBLE IN ASCII] Polyphenylenesulfide (PPSd) [FORMULA NOT REPRODUCIBLE IN ASCII] Oxidized Polyarylenes/Polyarylene [FORMULA NOT REPRODUCIBLE IN ASCII] Sulfide/Polyarylene Sulfone (PAS, Ceramer, PPSO) Table 9 Properties of High-Temperature Engineering Thermoplastics * Polymers Properties Units General Properties Specific Gravity g/[cm/sup.3] Water Absorption 24 hour immersion % Taber Abrasion (1kg load mg/1000 CS17 wheel) revalations Mechanical Properties Tensile Strength MPa at 20[degrees]C/68[degrees]F Elongation at Break % Flexural Modulus GPa at 20[degrees]C/68[degrees]F Rockwell Hardness (M scale) -- Izod Impact Strength kj/[m.sup.2] (6.4 mm specimen) Notched Un-notched Flexural Strength MPa Melting [degrees]C Temperature ([degrees]F) Glass Transition [degrees]C Temperature ([degrees]F) Thermal Properties Heat Distortion Temperature [degrees]C at 1.82 MPa ([degrees]F) Vicat Softening Point at Load 1 Kg [degrees]C([degrees]F) 5 Kg [degrees]C([degrees]F) Coefficient of Linear Thermal 1/[degrees]C Expansion (per [degrees]C) Mold Shrinkage % Electrical Properties Dielectric Strength kV/mm Volume Resistivity (dry) Ohm'cm Tracking Resistance (CTI) Sec High Voltage Arc Track Rate in/min Permitivity Relative -- (Dielectric Constant) -- at 60 Hz at [10.sub.6] Hz at [10.sub.4] Hz at [10.sub.3] Hz Loss Tangent at 60 Hz at [10.sub.4]Hz Continuous Service [degrees]C ([degrees]F) Temperature Properties Test Method/Standard General Properties Specific Gravity ASTM D792 (ISO 1183) Water Absorption 24 hour immersion ASTM D570 Taber Abrasion (1kg load ASTM D1044 CS17 wheel) Mechanical Properties Tensile Strength ASTM D638 at 20[degrees]C/68[degrees]F (DIN 53455-ISO 527) Elongation at Break ASTM D638 (DIN 53455-ISO 527) Flexural Modulus ASTM D790 at 20[degrees]C/68[degrees]F (ISO 178) Rockwell Hardness (M scale) ASTM D785 Izod Impact Strength ASTM D256 (6.4 mm specimen) ISO 170/1A Notched Un-notched Flexural Strength ASTM D790 Melting ISO 3146 Temperature Glass Transition Temperature Thermal Properties Heat Distortion Temperature ASTM D648 at 1.82 MPa (ISO R75) Vicat Softening Point at Load ASTM D1525 1 Kg (ISO R306) 5 Kg Coefficient of Linear Thermal ASTM D696 Expansion (per [degrees]C) Mold Shrinkage ASTM D955 Electrical Properties Dielectric Strength ASTM D149 (IEC 243) Volume Resistivity (dry) ASTM D257 (IEC 93) Tracking Resistance (CTI) ASTM D495 High Voltage Arc Track Rate UL 746 Permitivity Relative ASTM D150 (Dielectric Constant) (IEC 250) at 60 Hz at [10.sub.6] Hz at [10.sub.4] Hz at [10.sub.3] Hz Loss Tangent ASTM D150 (IEC 250) at 60 Hz at [10.sub.4]Hz Continuous Service Estimated for Temperature typical applications Polyether-Sulfone Properties (PES) General Properties Specific Gravity 1.37 Water Absorption 24 hour immersion -0.006 Taber Abrasion (1kg load 6-8 CS17 wheel) Mechanical Properties Tensile Strength 83-140 at 20[degrees]C/68[degrees]F Elongation at Break 3-80 Flexural Modulus 2.6-8.4 at 20[degrees]C/68[degrees]F Rockwell Hardness (M scale) 88-98 Izod Impact Strength (6.4 mm specimen) Notched 76-120 Un-notched 430-540 (some grades no break unnotched) Flexural Strength 111-190 Melting Temperature Glass Transition Temperature Thermal Properties Heat Distortion Temperature 203-216 at 1.82 MPa (397-421) Vicat Softening Point at Load 1 Kg 222-226 (432-439) 5 Kg Coefficient of Linear Thermal (2.3-5.5) Expansion X [10.sup.-5] (per [degrees]C) Mold Shrinkage 0.2-0.6 Electrical Properties Dielectric Strength 15-20 Volume Resistivity (dry) 1.7 x [10.sup.15] -1.0 x [10.sup.18] Tracking Resistance (CTI) 20-120 High Voltage Arc Track Rate 0.8-5.5 Permitivity Relative (Dielectric Constant) at 60 Hz 3.51-4.0 at [10.sub.6] Hz at [10.sub.4] Hz at [10.sub.3] Hz 3.5 Loss Tangent at 60 Hz 0.001-0.003 at [10.sub.4]Hz 0.003-0.004 Continuous Service Temperature Poly-phenylene Sulfide Polyetherimide Properties (PPS) (PEI) General Properties Specific Gravity 1.350-1.365 1.27-1.29 Water Absorption 24 hour immersion 0.02 0.16-0.25 Taber Abrasion (1kg load 20-45 CS17 wheel) Mechanical Properties Tensile Strength 33-90 95-115 at 20[degrees]C/68[degrees]F Elongation at Break 3-8 50-150 Flexural Modulus 3.75-4.2 2.8-2.85 at 20[degrees]C/68[degrees]F Rockwell Hardness (M scale) 90-100 Izod Impact Strength (6.4 mm specimen) Notched 25-75 1.0 Un-notched 8.0-45.0 26 Flexural Strength 125-145 125-130 Melting 280-290 Temperature (536-554) Glass Transition 95 217-225 Temperature (173) (393-407) Thermal Properties Heat Distortion Temperature 110-120 210-227 at 1.82 MPa (230-248) (400-408) Vicat Softening Point at Load 1 Kg 219 (426) 5 Kg Coefficient of Linear Thermal (4.0-5.5) Expansion x [10.sup.-5] (per [degrees]C) Mold Shrinkage 0.3-0.6 0.18-0.35 Electrical Properties Dielectric Strength 17-18.5 18-26 Volume Resistivity (dry) (1.1-4) 1.0 x [10.sup.11] x [10.sup.17] Tracking Resistance (CTI) 101 High Voltage Arc Track Rate Permitivity Relative (Dielectric Constant) at 60 Hz 3.2 at [10.sub.6] Hz at [10.sub.4] Hz 2.7-3.3 1.7-3.12 at [10.sub.3] Hz 2.8-3.2 Loss Tangent at 60 Hz 0.0021 at [10.sub.4]Hz 180 170 Continuous Service (356) (338) Temperature Polyether-Ketone-Ketone Properties (PEKK) General Properties Specific Gravity 1.28-1.31 Water Absorption 24 hour immersion <0.30 Taber Abrasion (1kg load CS17 wheel) Mechanical Properties Tensile Strength 105-115 at 20[degrees]C/68[degrees]F Elongation at Break 10-15 Flexural Modulus 4.5-5.0 at 20[degrees]C/68[degrees]F Rockwell Hardness (M scale) 99 Izod Impact Strength (6.4 mm specimen) Notched 1.3 Un-notched no break Flexural Strength 190-195 Melting 354-360 Temperature (670-680) Glass Transition 163 Temperature (325) Thermal Properties Heat Distortion Temperature at 1.82 MPa Vicat Softening Point at Load 1 Kg 5 Kg Coefficient of Linear Thermal (2.2-3.8) Expansion x [10.sup.-5] (per [degrees]C) Mold Shrinkage 0.014-0.02 Electrical Properties Dielectric Strength 22-25 Volume Resistivity (dry) 1.2 x [10.sup.16] Tracking Resistance (CTI) High Voltage Arc Track Rate Permitivity Relative (Dielectric Constant) at 60 Hz at [10.sub.6] Hz at [10.sub.4] Hz at [10.sub.3] Hz 3.3 Loss Tangent at 60 Hz at [10.sub.4]Hz 0.04 250 Continuous Service (482) Temperature Polyether-ether-Ketone Properties (PEEK) General Properties Specific Gravity 1.30-1.32 Water Absorption 24 hour immersion 0.1-0.4 Taber Abrasion (1kg load CS17 wheel) Mechanical Properties Tensile Strength 90-130 at 20[degrees]C/68[degrees]F Elongation at Break 20-150 Flexural Modulus 3.0 at 20[degrees]C/68[degrees]F Rockwell Hardness (M scale) 99-100 Izod Impact Strength (6.4 mm specimen) Notched 0.8-1.55 Un-notched no break Flexural Strength 170-180 Melting 334-340 Temperature (633-644) Glass Transition 143-171 Temperature (289-340) Thermal Properties Heat Distortion Temperature 160-170 at 1.82 MPa (320-340) Vicat Softening Point at Load 1 Kg 5 Kg Coefficient of Linear Thermal (4.1-4.7) Expansion x [10.sup.-5] (per [degrees]C) Mold Shrinkage 0.1-0.3 Electrical Properties Dielectric Strength 14-19 Volume Resistivity (dry) 1 x [10.sup.15] -4.9 x [10.sup.16] Tracking Resistance (CTI) 40 High Voltage Arc Track Rate Permitivity Relative (Dielectric Constant) at 60 Hz 3.2-3.3 at [10.sub.6] Hz 3.4 at [10.sub.4] Hz 3.2-3.3 at [10.sub.3] Hz Loss Tangent at 60 Hz 0.004 at [10.sub.4]Hz 220-250 Continuous Service (398-480) Temperature Polyarylether-ketone Properties (PAEK) General Properties Specific Gravity 1.3 Water Absorption 24 hour immersion 0.1-0.8 Taber Abrasion (1kg load CS17 wheel) Mechanical Properties Tensile Strength 93-118 at 20[degrees]C/68[degrees]F Elongation at Break 2.8-50 Flexural Modulus 3.65 at 20[degrees]C/68[degrees]F Rockwell Hardness (M scale) 86-100 Izod Impact Strength (6.4 mm specimen) Notched 0.8-6.2 Un-notched no break Flexural Strength 130 Melting 323-381 Temperature -688 Glass Transition 170 Temperature (338) Thermal Properties Heat Distortion Temperature 162-170 at 1.82 MPa (294-338) Vicat Softening Point at Load 1 Kg >240 (>434) 5 Kg Coefficient of Linear Thermal (4.1-4.4) Expansion x [10.sup.-5] (per [degrees]C) Mold Shrinkage 0.8-1.45 Electrical Properties Dielectric Strength 14.2-25 Volume Resistivity (dry) > [10.sup.16] Tracking Resistance (CTI) High Voltage Arc Track Rate Permitivity Relative (Dielectric Constant) at 60 Hz at [10.sub.6] Hz at [10.sub.4] Hz at [10.sub.3] Hz Loss Tangent at 60 Hz at [10.sub.4]Hz > 300 Continuous Service (542) Temperature Poly-phenylene-Sulfone ("Ceramer"/"Ceramer) Plus") Properties (PPS/PPSO2) General Properties Specific Gravity 1.31-1.41 Water Absorption 24 hour immersion 0.1-0.3 Taber Abrasion (1kg load CS17 wheel) Mechanical Properties Tensile Strength 42-45 at 20[degrees]C/68[degrees]F Elongation at Break 1.2-2.1 Flexural Modulus 3.3-3.5 at 20[degrees]C/68[degrees]F Rockwell Hardness (M scale) Izod Impact Strength (6.4 mm specimen) Notched Un-notched Flexural Strength 89-91 Melting Temperature Glass Transition >330 Temperature Thermal Properties Heat Distortion Temperature 250-295 at 1.82 MPa (482-563) Vicat Softening Point at Load 1 Kg 5 Kg Coefficient of Linear Thermal (3.8-4.8) Expansion x [10.sup.-5] (per [degrees]C) Mold Shrinkage Electrical Properties Dielectric Strength 12.4 Volume Resistivity (dry) [10.sup.15] Tracking Resistance (CTI) High Voltage Arc Track Rate Permitivity Relative (Dielectric Constant) at 60 Hz at [10.sub.6] Hz at [10.sub.4] Hz at [10.sub.3] Hz Loss Tangent at 60 Hz at [10.sub.4]Hz Continuous Service Temperature * Values represent data from different publications. Table 10 Applications & Properties of LCPs Typical Applications Key Properties Electrical/Electronic Sockets Excellent Processability Bobbins, Switches Ability to fill thin wall parts Connectors - Electrical, Fiberoptic Excellent dimensional stability Fiberoptic Cables Ability to withstand IR & vapor phase soldering temperatures Chip Carriers Very high relative thermal indexes Printed Circuit Boards Excellent flame resistance Surface Mount Technology Parts UL ratings-50% regrind approval Health Care Sterilization Trays Very high strength & stiffness Dental Tools Excellent flowability Surgical Instruments Smooth surfaces Excellent creep resistance Excellent radiation, chemical & steam resistance: USP Class VI Industrial/Consumer Printers, Copiers, Fax Machines High strength to weight ratio Business Machine Housings Excellent flammability ratings Low smoke generation Conductive grades with electrostatic dissipation & shielding characteristics Telecom Shields/Frames Excellent flowability, processability Chip Card Reader Excellent temperature, dimensional stability Antenna Parts Excellent mechanical strength & stiffness Platability Cryogenics Food Packaging Vessels Very high strength & stiffness Waste Containers Excellent low-temperature properties Excellent radiation, chemical & steam resistance Packaging & Film Food & Liquid Packaging Excellent barrier properties Coatings Excellent stiffness & strength Fuel Tanks Excellent dimensional stability Printed Circuit Boards Fibers & Yarns Cables, Ropes, Fishing Poles Very high stiffness & strength Sailcloth Low moisture absorption Cut-Resistant Gloves Good flex strength Table 11 Polymer Alloys & Blends Made by Japanese Companies Minor Component ** PBT Main Component PPE PC PA PET Polyphenylene Ether (PPE) @ @ Polycarbonate (PC) @ # Polyamide (PA) # @ Polyester (PBT, PET) @ @ Polyoxymethylene (POM) @ Polyphenylene Sulfide (PPS) @ * @ * ABS & Polystyrene (PS) @ * @ * @ * Polyolefin (PO) @ * * Polyvinyl Chloride (PVC) Liquid Crystal Polymer (LCP) * * Thermosetting Plastics Minor Component ** ABS Main Component PPS PS PO PVC Polyphenylene Ether (PPE) @ # @ * Polycarbonate (PC) # * * Polyamide (PA) @ @ @ * Polyester (PBT, PET) @ @ * Polyoxymethylene (POM) @ Polyphenylene Sulfide (PPS) * ABS & Polystyrene (PS) @ # Polyolefin (PO) * # * Polyvinyl Chloride (PVC) Liquid Crystal Polymer (LCP) Thermosetting Plastics Minor Component ** Main Component LCP SPS Elastomer Polyphenylene Ether (PPE) * @ Polycarbonate (PC) @ * @ Polyamide (PA) @ Polyester (PBT, PET) * @ Polyoxymethylene (POM) @ * Polyphenylene Sulfide (PPS) * @ ABS & Polystyrene (PS) @ * # Polyolefin (PO) # * Polyvinyl Chloride (PVC) # Liquid Crystal Polymer (LCP) * Thermosetting Plastics # # Consumption over 5000 tons per year @ Consumption less than 5000 tons per year * Under intense research & development * Includes copolymer & modified polymer Table 12 Polymer Alloys & Blends Recently Commercialized of Developed in Japan Polymer Company Compatibilizer, Modifiers TPE graft copolymers NOF Block copolymers: for surface modification NOF for decreasing curing shrinkage NOF SEBS for PP/PS blends JSR Terminal functional PP & PP Tonen Chemical macromonomers Modifiers for recycled PET Toyobo/Toyoda Gosei Modifiers for recycled ABS Nippon A&L Comb-shaped functional copolymers Mitsubishi Chemical Commodity Plastics Blends PS/syndiotactic PS blends Idemitsu Petrochemical Reversibly crosslinkable polyolefin Mitsubishi Chemical blends Engineering Plastics Alloys ABS/PA6 alloy new grade for Honda's Nippon A&L Insight PPE/PAielastomer alloy with nano- Mitsubishi Chemical network structure Super Engineered Plastics Alloys PEEK/PEI blend film Mitsubishi Plastics Polymer/Silica Hybrids (Sol-Gel Method) Epoxy resin/silica Arakawa Chemical Polyurethane/silica Arakawa Chemical Polyamideimide/silica Arakawa Chemical/Furukawa Electric Thermoplastic Elastomers In-reactor TPO Idemitsu Petrochemical Polymer Product Compatibilizer, Modifiers TPE graft copolymers Nofally Block copolymers: for surface modification Modiper F, FS for decreasing curing shrinkage Modiper SV, MS SEBS for PP/PS blends Dynaron new grades * Terminal functional PP & PP (under development) macromonomers Modifiers for recycled PET Ecoviropet Modifiers for recycled ABS Repelle System Comb-shaped functional copolymers (under development) Commodity Plastics Blends PS/syndiotactic PS blends Zarec E, D Reversibly crosslinkable polyolefin TRC Polymer * blends Engineering Plastics Alloys ABS/PA6 alloy new grade for Honda's Techniace Insight PPE/PAielastomer alloy with nano- (under development) network structure Super Engineered Plastics Alloys PEEK/PEI blend film Ibuki Polymer/Silica Hybrids (Sol-Gel Method) Epoxy resin/silica Compoceran E * Polyurethane/silica Compoceran U * Polyamideimide/silica (under development) Thermoplastic Elastomers In-reactor TPO (under development) * Commercialization in near future
The authors would like to acknowledge the strong contribution of Donna Del Giuclice in assisting during the preparation of this paper in all stages including information research, obtaining references, and preparation of the manuscript.
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|Author:||Friedman, Michael; Walsh, Gerard|
|Publication:||Polymer Engineering and Science|
|Date:||Aug 1, 2002|
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