Emerging applications for neat LCPs.
Thermotropic liquid crystalline polymers (LCPs) are remarkable for their unique combination of properties. These high-performance materials have a rigid rod-like crystalline structure that gives them a number of significant performance advantages over conventional amorphous and semicrystalline polymers.[(1)] LCPs have been used in applications as commonplace as electronic connectors and as unique as fibers in an airbag that cushioned Pathfinder's landing on Mars.
The majority of current LCP applications involve the injection molding of fiber-filled and mineral-filled resins. A combination of excellent strength, rigidity, and dimensional stability, along with the ability to flow easily into mold cavities where thin walls are formed, allows these resins to be used in electrical, electronic, telecommunication, and aerospace applications. Resistance to sterilizing radiation has also enabled these materials to be used in medical devices.
Yet LCPs are not limited to applications that must be injection molded. Their use in such processes as extrusion, lamination, and blow molding; for example, is currently being explored. Film, sheet, and laminate made from unfilled LCPs have excellent barrier properties - of major interest today in medical, chemical, and food packaging applications. Moreover, new LCP grades and alternate processing technologies may lead to the development of novel products for new markets. One promising area of research is the use of LCPs as flow enhancers in blends with other polymers.
This article describes emerging applications and challenges for LCPs and LCP blends. It also describes how, with judicious choices of comonomer, one can vary the processing temperatures and provide specific chemical functional groups to promote mixing with other thermoplastics.
Chemistry and Microstructure
Thermotropic copolyesters and polyesteramides are among the polymers most amenable to melt fabrication. Early R&D efforts concentrated on high-performance fibers, but researchers soon began to focus on melt processable resins for injection molding. Although the development of LCPs began in the mid-1970s, only a handful of companies currently manufacture LCP resins for commercial purposes. Principal global suppliers are Ticona (formerly Celanese) and its affiliate, Polyplastics (Vectra LCP); Amoco Performance Polymers (Xydar); and DuPont (Zenite).
The critical factor in the design of such polymers is balancing the molecular perfection that preserves liquid crystallinity with the imperfection that permits processing at conventional melt temperatures. The rod-like molecules of LCPs remain aligned in both the melt and solid states. In the molten state, they readily slide over one another, giving the resin very high flow trader shear, especially during high-speed injection molding. The processing of LCPs in elongational flow fields results in a highly oriented extended chain structure in the solid state. These structures form because their nematic character in the melt dominates the flow characteristics of the liquid crystals.
As a result of the long relaxation times, the orientation of LCPs in the melt "freezes" into the solid state. During injection molding, as resin fills the mold, surface molecules align with the elongational flow and form two highly oriented outer "skin" layers. The combination of fountain flow and complex flow results in a relatively unoriented core. Because of this "skin-core" morphology and the resultant properties, LCPs have been described as self-reinforced composites.[(2)] Thinner parts have a greater proportion of this skin relative to the core of the part, generally resulting in higher strength and stiffness than in thicker articles. Tensile and flexural moduli of unfilled injection molded test bars are in the 5 to 20 GPa range - typical of glass-fiber-filled semicrystalline polymers.
The morphology of highly oriented extruded rods and fibers resembles the "skin" structure in molded parts and results in exceptional strength and stiffness. The high modulus of LCP fibers with diameters of [approximately]25 [[micro]meter] - 50 to 90 GPa - is directly related to their high chain orientation, measured by X-ray diffraction to be about 0.98 to 0.995 (1.0 denotes perfect orientation). Fiber properties improve when the fibers are subjected to a high temperature ([approximately]10 [degrees] C to 20 [degrees] C below the melting temperature) in an inert atmosphere for several minutes to several hours. This process increases their tensile strength from about 1.5 GPa to [greater than]3.0 to 5.0 GPa. The molecular origin of this improvement is the solid state polymerization of the polyesters and the increased molecular orientation. The low moisture absorption of the LCPs results in stable properties in moist environments, making them ideal fibers for cables and ropes and ideal fabrics for sail-cloth and cut-resistant gloves.
Properties and Applications
LCPs offer a balance of properties unmatched by most other resins. They are generally selected for a specific application based on three or more key properties,[(3)] such as those in Table 1. For instance, in the molding of electrical connectors, high flow in thin walls, dimensional stability at high temperatures, and inherent flame retardance are the main reasons for choosing an LCE Such properties as high flow, stiffness, and resistance to sterilizing radiation and gases may make them candidates for surgical instruments. Their electrical and thermal properties are relevant to interconnect devices, while low mold shrinkage and low moisture absorption make LCPs excellent candidates for injection molding in a range of environments.
Although it is well known that LCP film, sheet, and laminates exhibit excellent dimensional stability for films for printed circuit boards, and exceptional barrier properties, equal to or better than any other melt processable polymer, few commercial applications have existed in these areas. The challenges have been the price of LCPs, the processing into films, and the lack of wholly aromatic LCPs that can be processed at temperatures below 280 [degrees] C.
All three challenges have been met within the past 10 years. As volumes have increased and new compositions have been developed, LCPs have become less costly. Experience has improved our understanding of LCP processing. Biaxial orientation during flow in extrusion, injection blow molding, or blown film processes can reduce property imbalances and ameliorate fibrillation. Also, new processing methods are being developed.[(4)] In addition, products that will provide the impetus for further development are now commercially available.
Polymer blends are a potentially inexpensive mute to the formulation of new products without the need for exhaustive R&D costs associated with the development of new base polymers.[(5)] Interest has grown as blends with improved physical properties, better processability, and lower cost have been developed. Typically, polymers are blended to improve cost-performance profiles - tailoring the properties to fit given applications. For instance, the addition of polycarbonate or polyphenylene oxide to commodity polymers, such as polystyrene or ABS, can improve properties vs. the commodity polymer Blends are often formed from a combination of a crystalline and an amorphous polymer. Demand continues to grow for these conventional polymer blends.
For the most part, blends are formed from polymers that are at least partially incompatible and thus consist of two or more phases. The morphology and properties of the blend depend on the chemistry of the polymers and additives, the mixing process, the rheology, and, very important, the final forming process, such as injection molding or extrusion. If there is some compatibility between the polymers, or if a compatibilizer is added, the morphology and the properties of the blend will be affected. In such a case, the polymers may react with one another, such as by transesterification, resulting in a modification of the blend morphology and properties.
The purpose of the compatibilizer is to impart more useful physical or chemical properties to the blend. This often results in changes in the viscosity of one polymer or the viscosity ratio, which is a key factor contributing to blend morphology. Changes in dispersion or interfacial adhesion also occur Very large dispersed phase domains, several microns in diameter, can act as flaws, resulting in poor impact properties. Agents to improve compatibility are used to enhance adhesion between the different polymers. Polymers that are miscible, or those that react or transesterify during processing, might require an additive to reduce this chemical reaction. For optimum properties, the blend phases should have some adhesion, but too much can result in properties no better than those of the matrix polymer.
Table 1. Applications and Properties of LCPs. Typical Applications Key Properties Electrical/Electronic * Sockets * Excellent processability * Bobbins, switches * Ability to fill thin wall * Connectors-electrical, fiberoptic parts * Fiberoptic cables * Excellent dimensional * Chip carriers stability * Printed circuit boards * Ability to withstand IR and * Surface mount technology parts vapor phase soldering temperatures * Very high relative thermal indexes * Excellent flame resistance * UL ratings - 50% regrind approval Health Care * Sterilization trays * Very high strength and * Dental tools stiffness * Surgical instruments * Excellent flowability * Smooth surfaces * Excellent creep resistance * Excellent radiation, chemical, and steam resistance; USP Class VI Industrial/Consumer * Printers, copiers, fax machines * High strength to weight * Business machine housings ratio * Excellent flammability ratings * Low smoke generation * Conductive grades with electrostatic dissipation and shielding characteristics Telecom * Shields/frames * Excellent flowability, * Chip card reader processability * Antenna parts * Excellent temperature, dimensional stability * Excellent mechanical strength and stiffness * Platability Cryogenics * Food packaging vessels * Very high strength and * Waste containers stiffness * Excellent low-temperature properties * Excellent radiation, chemical, and steam resistance Packaging and Film * Food and liquid packaging * Excellent barrier properties * Coatings * Excellent stiffness and * Fuel tanks strength * Printed circuit boards * Excellent dimensional stability Fibers and Yarns * Cables, ropes, fishing poles * Very high stiffness and * Sailcloth strength * Cut-resistant gloves * Low moisture absorption * Good flex strength
Block copolymers, graft copolymers, and plasticizers can all function as compatibilizers. Maleic anhydride-based copolymers are often used with polyolefins to produce a product with a superior balance of properties. Examples of alloys and blends are PPO/PS, PC/ABS, PBT/PET - some of which are partially miscible and thus easier to blend - and many polymer/elastomer systems. In the best cases, the desirable properties of each component are retained, while the blend itself exhibits some exceptional synergistic characteristics.
Blends With LCPs
Some of the earliest LCPs contained both aromatic and aliphatic moleties, thought to offer a compromise between conventional commodity resins and wholly aromatic LCPs requiring high processing temperatures. They were positioned as blend partners for such resins as polyolefins, polyesters and polycarbonate. However, these mixed aliphatic-aromatic LCPs, and their blends, generally have inferior thermal and hydrolytic properties without a significant cost incentive to drive their use; thus they have never lived up to their expected potential. On the other hand, wholly aromatic LCP copolyesters and polyester-amides have exhibited remarkable growth and decreased prices. These resins offer a combination of exceptional performance and ease of processing.
What is known today about LCP blends? A review is outside the scope of this paper, but several general references are cited as examples (See Refs. 4 through 13). Table 2 provides a list of the common factors affecting blend properties, along with reasons for using LCP blends.
Neat LCP Polymers
LCPs with fiber and mineral fillers have been developed as high performance engineering resins for injection molding applications. These LCPs include grades that can be processed at temperatures as low as 220 [degrees] C and as high as 400 [degrees] C. Different compositions can provide processing advantages for these polymers in blends with thermoplastics, thermosets, and elastomers.
Vectra A950, a typical standard grade LCP, is a wholly aromatic copolyester. The introduction of an amide-containing comonomer produces an LCP (B950) with a similar process range but with a functionality quite useful for blending with polyolefins, polyamides, and polyesters. A950 and B950 are typical of most standard LCPs in which the process temperatures are in the range of 280 [degrees] C to 320 [degrees] C, depending on the specific comonomer composition.
LCPs are used for moldings, thin films, and laminates, and in blends with both high and low levels of thermoplastics.[(6-11)] Applications that can benefit from either lower or higher temperature LCPs and functional LCPs are the subject of the next section. For comparison, all the unfilled grades discussed were injection molded into standard test bars, and typical mechanical and thermal properties were measured (Table 3). The mechanical properties of the untilled LCPs are clearly better than those of many glass-fiber-reinforced thermoplastics. Other LCPs are also widely available for use in higher temperature applications, such as in printed wiring boards, and for blending with higher temperature or less tractable polymers, such as polyimides.
Low Temperature LCPs
Copolyesters (LCP1) and copolyester-amide LCPs (LCP 2) combine the consistency, stability, dimensional precision, and barrier properties of traditional wholly aromatic LCPs with processability at lower temperatures ranging from 220 [degrees] C to 280 [degrees] C. These lower-temperature resins are formulated with higher levels of monomers, such as naphthoates, isophthalic acid, and other diacids, which can modify the thermal properties. The combination of properties of these LCPs makes them suitable for use unfilled in moldings or laminates, and in blends with polyolefins, polycarbonate, and polyesters.
Some of the early R&D resins, in addition to these newer LCPs, such as LCP 1, are aromatic copolyesters with thermal transitions at about 220 [degrees] C. Several laboratories have succeeded in blending these polymers with polyolefins, polycarbonate, and polyesters. For such blends to be useful, it is typical to address the various factors listed in Table 2 to control the structure-property relations of the blend. For instance, Harrison[(11)] made blends of polyolefins with 5% to 15% of a low-temperature LCP and processed them into blown films, using a counter-rotating annular die to control the LCP molecular orientation. These films have several hundred times the stiffness of the polyolefin matrix. Compatibilizers and antioxidants, such as ethylene-methacrylic acid copolymer; were used in the blends. These self-reinforced blends have enhanced stiffness due to their well-known fibrillar LCP microstructure.[(12)]
LCP2 is a wholly aromatic ester-amide, also with a lower process temperature profile, The process temperatures of LCP1 and LCP 2 are shown in Fig. 1 to be lower than those for standard LCP grades. Both of these polymers are nematic LCPs, with a fine domain texture similar to that typical of LCPs, as seen in polarized light [ILLUSTRATION FOR FIGURE 2 OMITTED].
Research has indicated that the chemical functionality of LCPs can often moderate [TABULAR DATA FOR TABLE 3 OMITTED] the adhesion of a blend's components, control morphology, and result in improved properties - especially impact resistance. These materials include two types of functional LCPs. First are the ester-amide LCPs, such as LCP 2, described above. Copolyesteramides have been blended with many polymers, ranging from polyolefins and polycarbonate to intractable, high-temperature resins, such as polyarnideimide, to take advantage of their exceptional barrier and stiffness properties. For instance, blends with [greater than]60% polyester-amide LCP with a polyolefin and a compatibilizer provide good barrier properties, in addition to some adhesion with polyolefins when used in a multi-layer laminate. Of course, as with copolyesters, blends with polyester-amides can also benefit from a broad range of compatibilizers, block copolymers, and other additives to modify melt rheology and interfacial tension.
The second type of functional polymers, ionomers, are well known in the field of conventional polymers. Polyethylene-based ionomers [e.g., poly(ethylene-co-sodium methacrylate)] are used in many conventional blends. Ionic interactions are known to increase compressive strength because of the enhanced interactions between the polymer chains. Blends with ionomers have improved compatibility as a result of the ionic interactions between two or more polymers.
LCPs with ionomeric monomers in their backbone are not so well known as conventional polymers, but they permit similar ionic interactions.[(13)] Introduction of ionic groups, such as metal sulfonate groups, into the polymer backbone of standard copolyesters, for instance, provides ionic bonds between the polymer chains and ionic linkages for interaction with other materials. The addition of low levels of ionic moieties in the wholly aromatic structure has been shown to improve the adhesion between the polymer and metals, such as aluminum.
Researchers have characterized modified LCPs with 0.1% to 15% ionomeric content and have shown proof of the incorporation of the kinked ionic group.[(13)] Such LCPs exhibit reduced rigidity of the polymer chain and a decrease of the melt transition temperature - the temperature at which the solid structure becomes a fluid nematic melt. The optically anisotropic nematic texture is still apparent in these LCPs (similar to Fig. 2). The melt viscosity of such a modified ionomeric LCP, LCP 3, is shown in Fig. 1 to be similar to that of a standard LCE Such grades can be used for unfilled moldings and extrudates, and may provide adhesion in multilayer laminates or in blends with either conventional or ionomeric polymers, along with adhesion to metals. Unfilled LCP 3 exhibits high melt strength, making it an excellent candidate for blow molding.
Standard LCPs, with and without ionomeric functionality, have been assessed as blend partners with polycarbonate (PC). In both cases, grades with 5% to 30% LCP result in a reduction in viscosity vs. PC [ILLUSTRATION FOR FIGURE 3 OMITTED]. Compatibilizers and other additives can control the degree of fibrillation in the final part and maximize stiffness without delamination occurring. The advantage of the ionomeric LCP may be an increase in tensile and flex modulus, along with enhanced toughness. The addition of LCP to PC results in easier flow in thin wall parts and increased modulus with decreasing wall thickness [ILLUSTRATION FOR FIGURE 4 OMITTED], vs. the lower modulus of PC.
Knowledge of blend morphology is very important in understanding the properties of blends with LCPs. Figure 5 shows the typical morphology observed in a scanning electron microscope (SEM) image of a blend of PC/LCP ([approximately]85/15 wt %). The PC forms the matrix with rounded LCP "domains" dispersed in the core [ILLUSTRATION FOR FIGURE 5A OMITTED] of the test bar and more elongated domains in the oriented skin [ILLUSTRATION FOR FIGURE 5B OMITTED]. These elongated LCP domains, or fibrils, are more obvious when the sample is tilted in the SEM [ILLUSTRATION FOR FIGURE 5C OMITTED]. Morphology and properties of the blends can vary as a function of the processing method,[(5-9)] the specific formulation of the blend, and of course, part thickness.
The exceptional barrier properties of neat LCPs [ILLUSTRATION FOR FIGURE 6 OMITTED] make them well suited for use in packaging applications and also in barrier layers. Some of the newer grades, with a range of functionality and process temperatures, also have similar low permeability to oxygen, water vapor, carbon dioxide, and other gases. The low gas solubility and very low permeability properties are consistent with their highly ordered microstructure, high degree of orientation, and the microfibrillar organization of LCPs.
These superior barrier properties of LCPs allow their use as much thinner barrier layers than those made of EVOH or PVDC, with performance equivalent to, or better than, barriers made of these materials. Thus, even though their cost per pound may be higher, LCPs are cost effective barrier layers for numerous applications, including film, sheet, and multi-layer laminates in bottles, trays, and jars, in addition to liners for metal, plastic, and composite tanks.
A wide range of market segments - food, beverage, medical, industrial, and electronics - utilize LCPs. Many of the applications benefit not only from the barrier properties of the LCPs but also from to their low CTE, chemical resistance, high stiffness, and strength.
LCPs can be processed into moldings, thin films, and multilayer articles by conventional means, although some process development may be required. Some applications have been limited by the types of LCPs available. Recently developed lower-temperature and functional LCPs provide a broader array of unfilled, filled, and blended polymers for high-volume applications, which are expected to drive this technology into the next century.
Table 2. LCP Blends: Effect of LCPs on Properties.
Factors Affecting Blend Properties
* Any factor affecting the morphology of the blends can influence the properties.
* The chemistry of the polymers, especially the potential of miscibility.
* Additives or compatibilizers.[(10)]
* The mixing process itself has a limited effect compared with the other factors.
* The rheology of the blend, specifically the viscosity ratio of the components, has a major effect on morphology.
* The final process used to make the article, i.e., molding or extrusion, can affect the morphology and properties.
Effect of LCPs in Blends
* At low levels, LCPs can act as a process aid and reduce the viscosity of the polymer matrix.
* LCPs can improve the stiffness of the matrix polymer and provide reinforcement for thin wall parts as a result of the formation of in situ composites.
* Blends in which the LCP is the matrix can have enhanced thermal, barrier, and physical properties vs. non-LCP blends; LCP blends display improved adhesion in laminates.
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|Title Annotation:||liquid crystalline polymers|
|Author:||Sawyer, L.C.; Linstid, H.C.; Romer, M.|
|Date:||Dec 1, 1998|
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