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Emerging trends in plastics technology.

Polymers are the backbone of plastics. The giants of the molecular world, they can be built from simple molecules (monomers) into stars, chains, brushes and trees to generate desired application-specific qualities. The objective of this review is to highlight some examples of exciting, emerging developmental trends in plastics and in their technologies. The scope is not limited to documenting a series of developments in plastics technology but also to inform readers where these events are taking place.

Plastic nanocomposites

As nanocomposite technology slowly matures, plastic nanocomposites (PNC) are finding commercial applications in automotive parts, packaging films, appliances, fire-retardant electrical enclosures, and housings. These are materials where nanometer (nano comes from Greek word for midget; nanometer means one billionth of a meter) particles are dispersed in a polymeric matrix. The matrix can be single or multiphase. As for definition, a nanoscale particle is a material with at least one dimension in the nanometer range. The critical reinforcing effects of nanosized particles come from its aspect ratio (ratio of the length or thickness to that of the diameter), very large specific surface area (viz. 800[m.sup.2]/g), and the particle-matrix interactions. If the aspect ratio of a nanoparticle is over 500, the reinforcing effect is similar to an infinitely large particle.

Natural clays (mainly montmorillonite) and synthetic clays (fluorohectorite) are mostly used as nanoparticles in PNCs. The clay is any material (natural or synthesized) having a cation-exchange capacity of 50 to 200 milliequivalent/100g and a large surface area. A typical example is smectite type of clays. However, mixing polymers and clays is not a simple process. There are two reasons for this: immiscibility (like mixing water and oil) and very tight packing of individual clay layers. When clays are treated with an organic intercalant, the space between clay platelets expands and interactions are enhanced so the dispersion of organoclay may take place. Intercalant is an oligomer or polymer that is sorbed between platelets of the layered material and complexes with the platelet surfaces to form an intercalate. (1)

The original concepts for PNC sprang from the invention of polyamide-clay composites in the Toyota Research Corporation in 1985. (2) At that time, the objective was to making under-the-hood heat resistant automotive parts lighter than metal. However, recent commercialization of newly developed thermoplastic olefin (TPO) nanocomposite by General Motors has attracted producers from every sector of the polymers industry to reconsider their polymer systems. To include rubber in a thermoplastic matrix is well known. That is what Basell Polyolefins' (Previously Montell) Catalloy process does with higher levels of rubber particles in a polyolefin matrix (PP). By incorporating 2.5 wt% of exfoliated smectite type nanoclay particles in TPO, Basell Polyolefins and GM developed a PNC that replaced 15% talc-filled PP, translating into 7% to 8% weight savings. Exfoliation is done to delaminate the clay platelets via intercalation.

The process of exfoliation is primarily done via in-reactor polymerization or by melt compounding. When particles are dispersed into plastic resins, the resulting PNC behaves as a single phase and a single component material. The property improvements to PNC include stiffness, barrier to liquids and gases, flame retardancy, electrical conductivity, etc. Several application specific examples of PNC involving PET, PP, TPE and nylon were recently presented by Lan et al., (3) while Utracki and Kamal (1) have detailed clay-containing polymeric nanocomposites. The clay's structure gives it a unique position over other particles. Owing to nanoscale dimension of exfoliated clay platelets, PNC may replace any polymeric matrix (thermoplastic, thermoset or rubbery) in multiphase applications such as alloys, blends, composites, or foams.

Exfoliated montmorillonite platelets are about 1 nanometer thick, and hence below the visible light wavelength, and therefore yield transparent particles--a critical demand for packaging applications. Furthermore, a gas barrier is easily achieved in PNC due to the platy nature of the clay particle, which enhances polymer crystallization, leading to a more tortuous path. (4) Another attractive property of PNC is its ability to form char. This has opened up several possibilities of creating fire-retardant (FR) polyolefin nanocomposites. (3) From the standpoint of cost, PNC reduces the amount of FR additive package required, while maintaining equivalent fire ratings. As macro-reinforcements (e.g., glass fibers) the nano-reinforcing clay platelets improve rigidity and strength and significantly reduce shrinkage. For example, the latter property is of particular concern to dentists--shrinkage of the acrylic filling results in short lifetime of the repairs. Incorporation of 1 wt% of clay may reduce this shrinkage by a factor of 10.

Another nanoscale particle is hollow carbon nanotubes (CNT) that are thousands of times smaller in diameter than carbon fibers. CNT can be compounded into thermoplastics to produce nanotube composites with uniform surface resistivity of [10.sup.4] to [10.sup.9] ohms/sq. Uniformity reduces the "hotspots" encountered with carbon fiber-filled composites. Nanotube composites are excellent for automotive applications such as fuel system components and for electrostatic painting. Johnson et al. inserted single wall CNT to epoxy to make a composite that is 3.5 times harder than original epoxy. Only 1% of CNT showed 125% increase in thermal conductivity at room temperature. (5)

Studies on plastic nanocomposites have taken different unique, application driven, directions such as nanoporous-nanocomposites (6) and foamed nanocomposites (7). The future of plastic nanocomposites depends not only on volume based markets such as PP in automotive applications and PVC in pipe or siding, but also on speciality areas like aerospace, electronics, and biomedical devices. These materials can be manufactured by established processes including extrusion, injection molding, reaction injection molding, or resin transfer molding.

Plastic electronics

Conductive plastics is another area that can change modem life. Plastics are known to be insulators. This notion has changed since polyacetylene was doped in 1977. (8) A thin polyacetylene film could be oxidized with iodine vapor to increase its electrical conductivity a billion times. It is well known that conductivity depends on the electronic structures of materials. Because of the large density of free electrons in metals, they conduct electricity. This is not the case with polymers. However, a polymer containing conjugated double bonds along its backbone can behave like a conductor if it is well treated. When polyacetylenes or polyanilines are doped, they can be made to have conductivity similar to metallic copper. The list of applications for conductive polymers is growing. For instance, polypyrrole has been tested as microwave-absorbing "stealth" (radar invisible) screen coatings. (9)

As an alternate to doping, Schon et al. (10) have developed a solution cast process by which polythiophene film becomes superconducting at 235[degrees]C. This study shows the possibility of tuning the electrical properties of conjugated polymers from insulating to superconducting. Essentially, conductive polymer molecules at controlled micron length scales can be engineered to behave as Light Emitting Polymers (LEP), photodiodes, or as switch, thin film transistors (TFTs) that are changing the way we use cell phones, monitors, and TV. Although organic TFTs are the future, inorganic TFTs on a flexible substrate have already been demonstrated by Rollotronics via "roll-to-roll" manufacturing. In this technique, a continuous sheet of plastic is unrolled from one spool, covered with circuit-board-like patterns of silicon, and collected on another spool providing flexible transistors on a roll. (11) This makes plastic electronics an exciting area. (12,13) Mixing inorganic nano-rods with semiconducting plastics, Ali visatos et al. (14) have produced a new flexible plastic solar cell.

This flexible nature of plastics has overwhelmed its inorganic counterpart along with low cost manufacturing. Utilizing a vacuum coating process (in a single processing sequence), flexible Organic Light Emitting Diodes (OLED) can be produced in rapid-throughput, roll-coating systems. However, challenges are critical to the success of plastic electronics. OLED are known to break up in presence of water and oxygen. With diffusion barrier materials and substrates based on vacuum polymer deposition, flexible and transparent substrates for display are today's reality. (15) Current specifications for permeation rates of substrates for OLED are on the order of [10.sup.-3] [cm.sup.3]/[m.sup.2]/day at standard temperature and pressure for oxygen and [10.sup.-6] g/[m.sup.2]/day for water vapor. (16) A multilayered organic-inorganic barrier can be engineered to the specific performance requirements into a plastic substrate for display applications. It is claimed that engineered barrier substrates would be comparable in price to the glass substrates currently purchased for use in flat panel displays. (17,18)

Another field of interest in display technology is electronic ink. This is an electronic paper that appears as a plastic film and looks like a printed paper. Until now, electronic ink has been used with standard, rigid backplanes designed for liquid crystal displays. Researchers at Cambridge, Massachusetts--based E-Ink have completed the first prototype, a functional electronic ink display on a plastic substrate that can be twisted without disturbing the print image. This has demonstrated the possibility of a fully flexible electronic paper-like display. (19)

Self-assembly process

Self-assembly is the creation of parts capable of fusing together into a complex system without an outside builder. The process is simple but can have an enormous impact on how we think, live, and work. This is how nature works. Double helical strands of DNA molecule, where chemical bases are attracted selectively to each other ("T" with an "A" or "C" with a "G" and never otherwise), are a case in point. In a cell, this process is very complex. However, the underlying principle is "recognition". Recognition can be of different types, such as the chemical recognition at the molecular level (example: chemical bases of DNA), or the geometrical recognition at the micro level.

Expanding the latter approach, scientists at Alien Technology deposit integrated circuits across a plastic substrate to manufacture a flexible display for smart cards. In this fluid self-assembly (FSA) process, (20) transistors are floated into place across a large surface area that has transistors shaped as holes. As the circuit approaches a hole, it fits into the hole perfectly because it fits only one way. In this manner, a thin, low cost, and light plastic film is used instead of costly and brittle glass to continuously produce smart cards. Whitesides et al. recently used a patterned assembly of integrated semiconductor devices (LED) suspended in water to fabricate flexible (using transparent polyimide) cylindrical displays that light up in any specified pattern. (21) The study showed the feasibility of assembling 1500 silicon cubes on an area of 5 square centimeters in less than 3 minutes with a defect rate of ~2%. The power of the self-assembly concept has also been demonstrated by creating complex stru ctures in minutes, for example, 15 layers of bubbles of uniform sizes (0.2-2.0 microns in diameter), in 30- to 40-micron-thick polymer films. (22)

Polystyrene is an inexpensive plastic and is used for a variety of applications. However, it is brittle. The technique of self-assembly can change the structure of polystyrene, providing enhanced properties. When very small amounts of designed molecules, known as dendron rodcoils, are dissolved in styrene monomers, the molecules interact with one another, forming weak bonds and assembling into ribbon-like structures. (23) Rigid ribbons guide polymer chains line up closely alongside them, improving mechanical properties. Furthermore, the presence of thin ribbons makes polystyrene strongly birefringent, a property that moves light in specific directions. In other words, the modified polystyrene also can reflect and transmit certain wavelengths of light and could become a material for advanced photonics.

Another promising development comes from a group of researchers at Cornell University who used the self-assembly technique to produce a flexible creamic. The group of Ulrich Weisner (24) used a di-block copolymer with a silica type material to produce a hybrid material of cubic bicontinuous structure. The resulting hybrid material not only of has the component properties have other behaviors at different structural levels from battery electrolytes to fuel cells to separating live proteins.

Fuel cells

The fuel cell is another new technology development area. The technology offers not only efficiency and cleanliness but also abundant hydrogen and renewable energy. In principle, fuel cells run on hydrogen and convert chemical energy into electrical energy without using a combustion process. Hydrogen combines with oxygen, from the air, to produce electricity, water, and heat without any emission (ideally).

Many cells are combined into a fuel cell stack to produce large amounts of electricity. Bipolar plates separate neighboring cells and serve as the anode for one fuel cell and the cathode for the adjacent one. In a proton exchange membrane (PEM) fuel cell, a thin plastic film (membrane), coated with a catalyst (platinum and/or ruthenium) on both sides generates electricity. When hydrogen or hydrogen-rich gas from the reformer gets to the coated plastic membrane, hydrogen molecules are broken into protons and electrons. The protons penetrate the membrane and combine with oxygen to produce water and heat. However, the membrane does not allow the electrons to pass through. When electrons get around the membrane, they generate direct current (DC). A power conditioner then transforms the DC power to AC power, reducing voltage spikes and thereby completing the PEM fuel cell structure.

Today's commercially available plastic membrane is a sulfonated fluoropolymer called Nafion is manufactured by DuPont. The problem with Nafion is that above 80[degrees]C, its conductivity is greatly reduced. This happens because of the lack of humidity. A similar problem faces sulfonated ethylene styrene interpolymer. Researchers are working on composites, aromatic polyimide, and sulfonated styrene ethylene membranes to address this problem. (25-27).

The membrane is not the only involvement of plastic in a REM fuel cell. End plates holding the PEM fuel cell stack are also made of plastics. Plug Power Inc., producer of PEM fuel cells for home power generation, uses highly graphite-loaded vinyl ester polymeric bulk compounds (BMC 940) for its end plates. Effective lightweight storage of hydrogen in a minimum space provides plastics further application opportunities. The potentials for plastic usage in making fuel cells represent future challenges for scientists and engineers. Developing lower-cost membranes, greater operating temperature ranges, without wet environment, injection moldable end plates in bulk, and hydrogen storage tanks are among those challenges.

Tissue engineering

Interestingly, plastics are not limited to material applications. Soon they may provide spare human body parts. Plastics, as biomaterials, are at the forefront of current tissue engineering activity. Motivations stem from the unique properties of polymers and the current need for custom-made materials for specific medical applications. Although biodegradable polymeric biomaterials have gained acceptance in clinical use including regulatory approval for controlled drug delivery, only a modest success has been recorded in sustained protein release--a key to tissue engineering.

The major drawback in designing implantable organs is in providing a blood supply to them. An effective solution to this problem could be by encouraging surrounding cells and blood vessels to grow into the new organ. Structural versatility and attractive physico-chemical properties make plastic an excellent candidate for tissue engineering scaffolds, and have become the subject of intense study. Injecting plastics with proteins known as growth factors could guide and support tissue in-growth. In the case of a damaged bone, for example, as plastic continues to degrade inside the body, proteins are released. These growth factors attract blood vessels from the healthy surrounding bone that flood the damaged area, bringing nutrients that the new tissue requires to survive. That is how periodontists today can utilize biodegradable plastics for tissue regeneration. Tissue growth is facilitated by a three-dimensional framework with properties that encourage favorable cell responses.

Essentially, these 3-D biological interfaces enable specific types of cells to attach themselves to the scaffold, grow, and organize into functional tissue. (28) The idea may sound simple, but in reality, designing polymeric biomaterials for specific applications offers diverse challenges. For example, bones at different body locations (a vertebra, a jawbone, or a thighbone) are subject to different structural loads; and once healed (healthy tissues having been grown), the material should disappear at a predetermined rate without any toxic effect to the body. Tuning these polymers to disintegrate at the same rate as the new bone is developed is equally important.

It is not surprising that no single plastic material can be ideal for tissue engineering scaffolds. Several polymeric systems have been designed as scaffolds for engineering functional tissues. (29-32) However, plastics as biomaterials have developed considerably more than expected. Polymers are now being developed with built-in adhesion sites that act as cell hosts in giving shapes that mimic different organs. It should be possible in the future to use multilayered polymers that release a series of growth factors necessary for healing an ailing part at predetermined time intervals.

As well, stem cells could be combined to the growth factors in seeding the plastic scaffolds. Professor Robert S. Langer's group at MIT is actively pursuing three-dimensional polymer scaffolds for growing human tissues such as skin, cartilage, blood vessels, and nerves. Likewise, different types of poly[phosphoester(s)] have been developed by Professor Kam Leong's group at John Hopkins University that have the attributes of elastomeric, gel-like, and crystalline materials for construction of 3-D scaffolds with controlled porous architecture. Notwithstanding regulatory worries or legal concerns, plastic biomaterials are poised to increase the effectiveness and longevity of our body parts.

Combinatorial approach and high-throughput technique

The push for a speedy discovery of new materials for applications ranging from nanocomposites to biotechnology has made researchers pursue the combinatorial approach. In simple terms, the "combinatorial" approach provides a way to find promising compounds or materials via a route that is faster, better, and cheaper.

To conduct a study in the traditional sense, we implement a few measurements or a few samples at a time. In the combinatorial approach, we are talking about a scale of hundreds and thousands of measurements at a time. These high-throughput solutions are due to the combinations of tools and techniques such as robotics, computation, and miniaturization.

High-throughput technology (HTPT) has been gestating and growing for over three decades. In fact, Hanak reported (33) the first example of a combinatorial approach with increased efficiency for the discovery of new materials in 1970. Only recently has the technology started to mature for the efficient synthesis and characterization of complex polymeric systems. (34-35) Amis et al. have reviewed current advances in combinatorial methods for polymer materials science. (36) An important step towards the technique is to create a library of compounds (using robots or automated tools) based on physical and structural properties that are likely to yield the desired polymer molecule. Many different variables can be inserted, e.g., temperature, pressure, time, thickness, composition. This library of compounds is then screened for "lead" candidates that could produce the target polymer. Examples of the many benefits of HTPT are the design of a new catalyst, the optimization of reaction conditions for the controlled pre paration of nanoscale materials (advanced materials), or the production of biodegradable polymer blends with optimum microstructure (tissue engineering).

The technique can be used to synthesize polymers that can give functional attributes such as wettability, lubricity, and anti-adhesion properties or enhance the activity of a functional molecule. Symyx Technologies Inc. as constructed discovery platforms as a part of HTPT aiming at materials that show high affmity to specific surfaces. Recently, Symyx has provided a second module of Discovery Tools[TM] to GIRSA that performs an average of 48 experiments a day for the development discovery, and optimization of different polymers. (37) Soon combinatorial and high throughput technique will be the driving force behind many discoveries.


Applications of plastics hold enormous promise for the future, ranging from nanotechnology to biotechnology. The passive nature of plastics is slowly changing. Development is taking place where plastics are responding to their changing surroundings., be it water, temperature, light, or electricity. It is their molecular uniqueness that gives plastics the strength to bring science and technology together. No other material but plastics represents an open book of new ideas and technologies for a brighter tomorrow.


The author wishes to thank L.A. Utracki and G. Czeremuszkin for valuable comments; and helpful suggestions from P.J. Cook and V. Flaris are also appreciated.

This article has been peer reviewed and recommended for publication by SPE's New Technology Committee.


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Author:Mukhopadhyay, Prithu
Publication:Plastics Engineering
Geographic Code:1USA
Date:Sep 1, 2002
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