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Polymer Science and Engineering: The Shifting Research Frontiers 3 Manufacturing: Materials and Processing Materials as a field is most commonly represented by ceramics, metals, and polymers. While noted improvements have taken place in the area of ceramics and metals, it is the field of polymers that has experienced an explosion in progress. Polymers have gone from being cheap substitutes for natural products to providing high-quality options for a wide variety of applications. Further advances and breakthroughs supporting the economy can be expected in the coming years. Polymers are derived from petroleum, and their low cost has its roots in the abundance of the feedstock, in the ingenuity of the chemical engineers who devised the processes of manufacture, and in the economies of scale that have come with increased usage. Less than 5 percent of the petroleum barrel is used for polymers, and thus petroleum is likely to remain as the principal raw material for the indefinite future. Polymers constitute a high-value-added part of the petroleum customer base and have led to increasing international competition in the manufacture of commodity materials as well as engineering thermoplastics and specialty polymers. Polymers are now produced in great quantity and variety. Polymers are used as film packaging, solid molded forms for automobile body parts and TV cabinets, composites for golf clubs and aircraft parts (airframe as well as interior), foams for coffee cups and refrigerator insulation, fibers for clothing and carpets, adhesives for attaching anything to anything, rubber for tires and tubing, paints and other coatings to beautify and prolong the life of other materials, and a myriad of other uses. It would be impossible to conceive of our modern world without the ubiquitous presence of polymeric materials. Polymers have become
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Polymer Science and Engineering: The Shifting Research Frontiers an integral part of our society, serving sophisticated functions that improve the quality of our life. The unique and valuable properties of polymers have their origins in the molecular composition of their long chains and in the processing that is performed in producing products. Both composition (including chemical makeup, molecular size, branching and cross-linking) and processing (affected by flow and orientation) are critical to the estimated properties of the final product. This chapter considers the various classes of polymeric materials and the technical factors that contribute to their usefulness. In spite of the impressive advances that have been made in recent years, there are still opportunities for further progress, and failure to participate in this development will consign the United States to second-class status as a nation. MATERIALS Structural Polymers The familiar categories of materials called plastics, fibers, rubbers, and adhesives consist of a diverse array of synthetic and natural polymers. It is useful to consider these types of materials together under the general rubric of structural polymers because macroscopic mechanical behavior is at least a part of their function. Compared with classical structural materials like metals, the present usage represents a considerable broadening of the term. As shown in Table 3.1, man-made plastics, fibers, and rubber accounted for U.S. production of about 71 billion pounds in 1992 (Chemical & Engineering News, 1993), and production has tripled over the last 20 years. The price received by the original manufacturer ranges from roughly $0.50 to several dollars per pound, depending on the material. At $20 per barrel, crude oil costs about $0.06 per pound, and so conversion to polymers represents considerable value added. Because these materials go through several manufacturing steps before reaching the final consumer, the ultimate impact on the national economy is measured in the hundreds of billions of dollars each year. TABLE 3.1 U.S. Production of Some Man-Made Structural Polymers, 1992 Pounds (billions) Plastics 57.6 Fibers 9.1 Rubber 4.2 SOURCE: Data from Chemical & Engineering News (1993), p. 44.
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Polymer Science and Engineering: The Shifting Research Frontiers These materials have many different chemical and physical forms, such as cross-linked versus non-cross-linked, crystalline versus amorphous, and rubbery versus glassy. More recently, structural polymers having liquid crystalline order have become important. Structural polymers are rarely used in the pure form but often contain additives in small quantities, such as antioxidants, stabilizers, lubricants, processing aids, nucleating agents, colorants, and antistatic agents or, in larger quantities, plasticizers or fillers. There is rapid growth in the areas of blends and composites. In composites, a material of fixed shape, such as particles (filler) or fibers, is dispersed in a polymer matrix. The filler or fiber may be an inorganic material or another organic polymer. Blends (or alloys) on the other hand consist of two or more polymers mixed together and differ from composites in that the geometry of the phases is not predetermined prior to processing. Some polymers are used for many different purposes. A good example is poly(ethylene terephthalate), or PET, which was originally developed as a textile fiber. It is now used in film and tape (virtually all magnetic recording tape is based on PET), as a molding material, and as the matrix for glass-filled composites. One of its largest uses is for making bottles, especially for soft drinks. PET is also used in blends with other polymers, such as polycarbonate. Plastics The word "plastic" is frequently used loosely as a synonym for "polymer," but the meaning of "polymer'' is based on molecular size while "plastic" is defined in terms of deformability. Plastics are polymeric materials that are formed into a variety of three-dimensional shapes, often by molding or melt extrusion processes. They retain their shape when the deforming forces are removed, unlike some other polymers such as the elastomers, which return to their original shape. Plastics are usually categorized as thermoplastics or thermosets, depending on their thermal processing behavior. Thermoplastics Thermoplastics are polymers that soften and flow upon heating and become hard again when cooled. This cycle can be repeated many times, which makes reprocessing during manufacturing or recycling after consumer use possible using heat fabrication techniques such as extrusion or molding. The polymer chains in thermoplastics are linear or branched and do not become cross-linked as in the case of thermosets. While there are many different chemical types of thermoplastics, those made from only four monomers (ethylene, propylene, styrene, and vinyl chloride) account for about 90 percent of all thermoplastics produced in the United States (Figure 3.1). Of these four types, polypropylene has grown most rapidly in recent years—production has increased eightfold over the past two decades. Thermoplastic polyesters, primarily PET, are growing even more rapidly at the present time (driven mainly by
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Polymer Science and Engineering: The Shifting Research Frontiers FIGURE 3.1 U.S. production of thermoplastics by type, 1990. SOURCE: Reprinted with permission from Chemical & Engineering News (1991), p. 54. Copyright© 1991 by the American Chemical Society. packaging applications), with current sales nearly one-quarter of those for polypropylene. For the long term, the majority of commodity thermoplastics are expected to follow their traditional growth (Chemical & Engineering News, 1992), with continued opportunities for both process and product innovation. Future activities will focus strongly on recycling. In the case of PET, recycling can be accomplished by chemical depolymerization to monomers or oligomers followed by repolymerization to PET or other products. Such processes are currently in use for products that come into contact with food, while simple reprocessing is used for less critical products. The so-called engineering thermoplastics, which include the higher-performance, more expensive polymers such as the polyacetals, polycarbonates, nylons, polyesters, polysulfones, polyetherimides, some acrylonitrile butadiene styrene (ABS) materials, and so on, have generally exhibited stronger growth than the commodity plastics (see Table 3.2). These materials generally have greater heat resistance and better mechanical properties than the less expensive commodity thermoplastics and, therefore, are used in more demanding applications, such as aircraft, automobiles, and appliances. A major area of development is
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Polymer Science and Engineering: The Shifting Research Frontiers TABLE 3.2 Pounds of Selected High-volume Engineering Thermoplastics Sold in the United States, 1981 and 1991 Pounds (millions) Percentage Increase 1981 1991 Since 1981 Thermoplastic polyesters 1,230 2,550 107 Acrylonitrile butadiene styrene 968 1,130 17 Nylon 286 556 94 Polycarbonate 242 601 148 Poly(phenylene oxide)-based alloys 132 195 48 Polyacetal 88 140 59 SOURCE: Data from Modern Plastics (1982, 1992). new blends or alloys of engineering plastics that are designed for specialty market products and are usually quite tough and chemical resistant. (The area of blends and alloys is reviewed separately below.) New products and advances in processes have resulted from the ring-opening polymerization of cyclic oligomers; for example, new developments in polycarbonates are particularly noteworthy. Other new products can be expected based on copolymers, and entirely new polymers are under development. A further category sometimes referred to as high-performance engineering thermoplastics commands even higher prices for yet higher levels of performance. These include highly aromatic polymers such as poly(phenylene sulfide), several new polyamides, polysulfones, and polyetherketones. Development of new molecular structures has dominated this sector. Polymer chains with quite rigid backbones have liquid crystalline order, which offers unique structural properties as described below. Figure 3.2 shows the major categories of use for thermoplastics. Approximately one-third are used in packaging, primarily containers and film. The data in Figure 3.2 are dominated by the huge volume of the five or so commodity thermoplastics; hence, the products with greater value based on engineering or advanced thermoplastics do not emerge in true proportion to their contribution to the national economy. To understand the diversity of products and opportunities that is possible, it is useful to review developments that have occurred in thermoplastics based on ethylene, one of the simplest monomers possible. Commercial production of polyethylene commenced in England during the early 1940s using a free radical process operating at very high pressures (30,000 to 50,000 psi). The structure proved to be far more complex than the simple textbook repeat unit, –CH2 CH2–, would suggest (Figure 3.3). The backbone has short-and long-chain branches. The short-chain branches, typically four carbons long, interfere with the ability
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Polymer Science and Engineering: The Shifting Research Frontiers FIGURE 3.2 Categories of uses for thermoplastics in the United States, 1990. SOURCE: Reprinted with permission from Chemical & Engineering News (1991), p. 56. Copyright© 1991 by the American Chemical Society. of the chain to crystallize, thus affecting solid-state properties, while the long-chain branches (comparable in length to the backbone itself) mainly affect melt rheological or flow properties that influence processing behavior. Because the short-chain branches reduce crystallinity and, thus, density, this material is called low-density polyethylene (LDPE). In the late 1950s, a linear or unbranched form of polyethylene was developed as a result of advances in coordination polymerization catalysis. An accidental finding by K. Ziegler in the early 1950s at the Max Planck Institute of Mulheim, Germany, resulted in a fundamentally new approach to polyolefins. It was found that transition metal complexes could catalyze the polymerization of ethylene under mild conditions to produce linear chains with more controlled structures. As a result, this polymer was more crystalline with higher density, and it became known as high-density polyethylene (HDPE). Similar catalytic procedures were used by G. Natta to produce crystalline polypropylene. The properties of this polymer are a result of unprecedented control of the stereochemistry of polymerization. Because of the effects of molecular structure on crystallinity, HDPE is as much as 5 times stronger and 1 order of magnitude stiffer than LDPE. The newer material did not replace the older one; it was used for different purposes. In the 1970s, the high-pressure LDPE process became increasingly expensive relative to the lower-pressure HDPE process. The cost factor plus innovations in
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Polymer Science and Engineering: The Shifting Research Frontiers FIGURE 3.3 Schematic of the structure of high-density polyethylene, low-density polyethylene, and linear low-density polyethylene.
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Polymer Science and Engineering: The Shifting Research Frontiers catalysts and process technology led to a new material that had most of the attributes of LDPE but was produced by a more economical low-pressure process similar to that used for HDPE. It is a copolymer of ethylene and an alpha-olefin (like butene-1, hexene-1). Thus, short-chain branches of controlled length and number are introduced into the chain without any long-chain branches, and the material is called linear low-density polyethylene (LLDPE; see Figure 3.3). Production of this material grew at a rate of about 20 percent per year during the 1980s to current usage of about 5 × 109 pounds per year. As a result, the production of LDPE initially declined, but its production has been growing again since 1986. Construction of new high-pressure production facilities may be required in the next decade to meet demands. Currently this is the only process by which copolymers can be made with polar monomers such as vinyl acetate or acrylic acid. HDPE is fabricated primarily by molding. Blow-molded food bottles and auto gasoline tanks constitute major markets. Very large containers made by rotational molding represent a specialized growth area. A process known as "gel spinning" has been commercialized, which produces fibers of ultrahigh-molecular-weight polyethylene. The less crystalline LDPE and LLDPE are primarily extruded into film products, with each having specialized uses. New technology based on single-site metallocenes holds promise for the production of a new range of products. This brief review of the history and future prospects for olefin polymers illustrates the need for research of all types (e.g., catalysis, process, and structural characterization) in order to capitalize on economic opportunities. These materials are complex in terms of molecular structure, and so there are many ways to tailor their behavior provided the basic knowledge and tools for structural determination are available and are integrated with innovative process technology. Much of the present research is directed toward the design of catalysts that yield materials that are easier to process. Rapid progress has resulted from an integration of catalyst synthesis and reactor and process design. As a recent example, a new polyolefin alloy product has been developed by exposing a designed catalyst to a series of different olefin monomer feeds to produce a polymer particle that is composed of polymers with different properties. Extrusion of those particles results directly in a polymer alloy. Structural thermoplastics are a vital part of the national economy, and considerable opportunity remains for economic growth and scientific inquiry. New specialized materials will continue to offer rewards in the marketplace. At the high-performance end, several entirely new polymer structures are likely to emerge over the next decade. A major part of the growth in "new" materials will be in the area of blends or alloys. The vitality of thermoplastics cannot be judged only on the basis of the introduction of what might be called "new materials." Continuous improvement and diversification of existing polymers constitute another measure. One source estimates that the number of "grades'' of existing polymers tripled during the 1980s (Chemical & Engineering News,
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Polymer Science and Engineering: The Shifting Research Frontiers 1991). This trend is expected to continue but will require greater sophistication in terms of process technology, characterization, and structure-property relationships (especially modeling) than has been required in the past. Thermosets Thermoset materials are broadly defined as three-dimensional, chemically resistant networks, which in various technologies are referred to as gels, vulcanizates, or "cured" materials. Applications as diverse as coatings, contact lenses, and epoxy adhesives can be cited. Thermosets are defined here as rigid network materials, that is, as materials below their glass transition temperature. Thermosets are formed when polyfunctional reactants generate three-dimensional network structures via the progression of linear growth, branching, gelation, and postgelation reactions. The starting monomers must include at least some reactive functionality greater than two, which will ensure that as the reaction proceeds, the number of chain ends will increase. They will eventually interconnect to produce a gelled network material. This process may be followed by observing the viscosity increase as a function of time or from the percent reaction completed. In many cases, this can be predicted mathematically. As the gel begins to form, the soluble fraction decreases and eventually is eliminated altogether. An important consideration with respect to rigid thermosetting networks is the extensively studied interrelationship between reactivity, gelation, and vitrification. As the reaction proceeds, the glass transition temperature rises to meet the reaction temperature, and the system vitrifies; that is, the motion of the main chain stops. At this point, the reaction essentially stops for all practical purposes. This has been conveniently described in terms of a time-temperature-transformation cure diagram. Thermosetting systems can be formed either by chain or step polymerization reactions. The chemistry of thermoset materials is even now only partially understood, because they become difficult to characterize once they reach the three-dimensional insoluble network stage. Thermal and dynamic mechanical methods have been widely used to characterize these materials, and solid-state nuclear magnetic resonance (NMR) has begun to have some impact on this problem. Thermoset materials make up approximately 15 percent of the plastics produced in the United States. Figure 3.4 shows recent data on the production of the various types of thermosets and their uses. Phenolics make up the largest class of thermoset materials. Some polyurethanes are classified as thermosets, although many urethane and urea materials can be produced in linear thermoplastic or soluble forms, such as the well-known elastomeric spandex fibers. Urea-formaldehyde-based materials continue to be significant and, in fact, were the systems used in the first "carbonless" paper. Unsaturated polyesters are derived from maleic anhydride and propylene glycol, which are then dissolved in styrene and cross-linked into a network. They have gained significant importance in
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Polymer Science and Engineering: The Shifting Research Frontiers FIGURE 3.4 U.S. production of thermosets by type for 1990 (top) and their areas of use in 1989 (bottom). SOURCE: Reprinted with permission from Chemical & Engineering News (1991), p. 39. Copyright© 1991 by the American Chemical Society.
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Polymer Science and Engineering: The Shifting Research Frontiers automobiles and construction. The resulting glass-reinforced composites are frequently called sheet molding compounds (SMC). Thermoset materials, although smaller in total volume than the thermoplastics, are used in a number of very high performance applications, such as matrix resins or structural adhesives in composite systems such as those used for aerospace applications. These composites are normally reinforced with glass, aramid, or carbon fibers. Important examples of such matrix materials include the epoxies, bismaleimides, cyanates, acetylenes, and more recently, benzocyclobutene systems. The existing database for matrix resins and structural adhesives is much more established for thermosets than it is for high-performance thermoplastics such as the poly(arylene ether ketones), certain polyaryl imides, and poly(phenylene sulfide). Major research needs in the area of polymer-based composites include better ways to improve the toughness of thermosetting systems and better techniques for processing those formed from high-performance thermoplastics. Advances in processing and toughening thermosets are occurring on several fronts. Methods for generating the network have been investigated by many organizations. The most conventional methods involve use of a thermal-convection-oven-type curing, often in autoclaves. However, recently there has been considerable effort in electromagnetic (or microwave) processing of high-performance polymeric matrix resins, particularly for structural adhesives and composite structures. An approach for "toughening" that has been investigated over the last 10 years involves the incorporation of either rubbers or reactive engineering thermoplastics into networks, such as epoxies, to develop a complex morphology. Here the added material is dispersed as isolated domains or forms co-continuous morphologies. Most of the original studies focus on rubber toughening, and an extensive body of literature deals with utilization of carboxyl functional nitrile rubbers to toughen epoxy adhesives. More recently, advantages associated with the utilization of engineering thermoplastics have been realized. These include, for example, the ability to retain stiffness and thermo-oxidative stability, as well as in some cases, chemical resistance. These properties are often severely diminished with rubber-toughened thermosetting systems. Fracture toughness can be significantly improved. This is significant in terms of improving the durability of advanced organic materials utilized in structural adhesives and composites. The interfacial adhesion between the separate polymer phases, as well as their proportions, morphology, and molecular characteristics, is of prime significance in improving fracture toughness. Other forefront areas include the development of new chemistries and, in particular, better characterization of leading candidate materials. The bismaleimides are considered to be somewhat more thermally stable than the epoxy materials and are being seriously considered for various applications, such as the high-speed civil transport airplane, which is planned for commercialization in the next 10 years. Aspects of the flammability of these materials are also crucial.
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Polymer Science and Engineering: The Shifting Research Frontiers functional uses such as coatings for surface protection, films for a wide variety of uses, fibers for fabrics and carpeting, and an enormous variety of molded shapes. Melt Processing Melt processing is the most widely used and generally the preferred processing method. It is used for polymers that become liquid at elevated temperatures so that they can be extruded into fibers, films, tubes, or other linear shapes or molded into parts of complex shape. Such processes involve much more than simply changing the physical shape of the polymer; they also influence phase morphology, molecular conformations, and so on and ultimately have an important role in the performance of the product. Molding A mold is a hollow form that imparts to the material its final shape in the finished article. The term "molding" is employed for processes involving thermosets and thermoplastics and includes injection, transfer, compression, and blow molding. The injection molding process is the most common method of making plastic parts. In that process, thermoplastic pellets are melted and pumped toward a melt reservoir by a rotating screw. When enough molten plastic has accumulated, the screw plunges forward to push the melt into a steel mold. The plastic solidifies on cooling, and the mold is opened for removal of the part. Injection molding cycle times vary from a few seconds to minutes, depending on the plastic and the part size. Molding machines have become very sophisticated, and they are capable of turning out large numbers of molded articles with little or no operator attention. The heated plastic conforms intimately to the polished mold surface, which may be of complex shape, and the part produced usually requires little or no further machining or polishing. The mold and the machine that delivers plastic to the mold can be quite costly; therefore, the technology is suited only to parts needed in large numbers. Even so, injection molding is a process capable of exceptionally low cost in comparison with production processes for metal or ceramic parts. Some of the current challenges in polymer processing include developing new materials, achieving greater precision, pursuing process modeling and development, and recycling. Some examples of new materials are special blends of existing polymers, polymer composites with fiber reinforcement, and liquid crystalline polymers. Some of these new materials are expensive and may be difficult to form into desired shapes; however, they are of value to the defense and aerospace industries in applications in which weight and performance are more important than cost and processibility. In contrast, automotive and appliance industries use materials that are less expensive, readily molded, and dimensionally
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Polymer Science and Engineering: The Shifting Research Frontiers accurate. Yet there are limitations to what can be done, even with common materials. Molten plastics are viscous, and making thin parts may require high pressures. Further, the plastic shrinks as it cools, and this tendency must be compensated for by using oversized mold cavities. Molds are expensive, from several thousands to millions of dollars each. Plastic materials are rheologically complex, and as a result many factors can affect the properties and dimensional accuracy of parts made from them. There are variations in operation of the molding machine, small temperature fluctuations, and differences in molecular orientation caused by flow into the mold. However, injection molding has been brought to levels that allow tolerances on small parts in the micrometer range. Among the high-performance plastics that have been introduced to meet the demands of the high-precision market are the thermotropic liquid crystalline polymers and low-viscosity versions of high-temperature materials such as polyetherimides and polyaryl sulfones. Advances in processing are occurring at a rapid rate as on-line sensing, computing, and process feedback allow control and optimization of the molding process that were undreamed of only a few years ago. Parameters of importance include injection speed, peak pressure, hold pressure, and mold temperature, along with less obvious factors such as "cushion length" and position-or pressure-dependent cutoff. As the processing industry learns to take advantage of the capabilities of the new machines and materials, precision injection molding can be expected to make further inroads into the domain of machined metal parts. Process dynamics and the properties of the finished article are critically dependent on the conditions of flow and solidification, down to the molecular level. As the mold is filling, the molten polymer solidifies first along the walls. The material that is farthest from the wall flows more rapidly, leading to a shearing and molecular elongation in the wall area. After the flow front has passed and the mold is full, the central regions solidify under conditions in which shear elongation is not a major factor. This solidification process leads to a morphology characterized by a skin of highly oriented polymer around a core of less oriented material. The two layers are mechanically and optically distinct. Control of these components through polymer composition and processing technology is a central issue in the production of precision, high-performance parts. Skin effects are most obvious in parts made from polymers that crystallize. Amorphous polymers are much less influenced. These morphological factors have important consequences for the failure mode and fracture mechanics of the finished part. Additional processing techniques, such as gas-assisted molding and injection-compression molding, are gaining industrial acceptance. Materials suppliers are developing new plastics with enhanced flow characteristics and better
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Polymer Science and Engineering: The Shifting Research Frontiers physical properties. Compact disks can be made because of high flow grades of polycarbonate and the injection-compression molding process. The importance of injection molding, and of precision injection molding in particular, can hardly be overstated. The economies that can be realized in the production of mechanically complex parts can contribute to the feasibility of large-scale manufactured products. For example, communications systems of the future, such as fiber optics to the home providing broad-band information, depend on many manufactured details that must be reduced in cost if the concepts are to succeed. Polymer-based solutions are essential to the realization of the promise of progress in diverse areas. Extensive progress has also been made recently in blow molding, especially to form bottles for various packaging applications. Special grades of polymers with uniquely tailored rheological properties, via broad molecular weight distribution and chain branching, have been developed for this market. Stretch blow molding processes allow control of the development of chain orientation and crystalline structure for materials such as poly(ethylene terephthalate) to gain better barrier properties. Extrusion Melt extrusion processes are usually the most convenient, economical, and environmentally favorable for film and sheet manufacturing. Screw extruders, in which a rotating screw transports material through a heated barrel and a shape-forming die, are the heart of such processes. Extruder screws, which can be very sophisticated, are designed with the help of extensive computer-assisted modeling. Frequently, mixing, compounding, and devolatilization are also involved to process formulations that include special additives, such as antioxidants, plasticizers, flame retardants, lubricants, pigments, fillers, and other polymers. Films are formed through film blowing of thin-walled tubes or drawing and tentering of cast films. Optimization of the process requires fundamental understanding of material properties and processing characteristics. The properties of the fabricated product are strongly dictated by the details of the fabrication process. Influential variables include uniaxial or biaxial orientation, degree of crystallinity, morphology of amorphous and/or crystalline regions, internal stress, and dimensional control. The complete system—from material design, synthesis, and formulation to product design and processing—must be addressed to achieve optimal material selection for a specific application. All elements of the system are important. Materials scientists and engineers, who understand structure-property relationships and can manipulate molecular design, work closely with process engineers and product designers in a systems approach to meet the growing demand for extrudable polymers that have specific characteristics.
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Polymer Science and Engineering: The Shifting Research Frontiers Solid-State Forming Solid-state forming is one of the newest polymer processing methods and is especially useful for increasing strength and modulus of polymeric materials. The latter involves achieving morphologies with well-aligned, extended, and closely packed chains, which can be done by synthesizing rigid rodlike polymers containing parasubstituted aromatic structures in the chain backbone or by processing conventional flexible chain polymers in ways that lead to similar results. In this processing approach, a highly oriented and extended chain conformation with substantially increased tensile modulus may be achieved by solid-state deformation of thermoplastics using the extrusion or alternate drawing techniques, such as extrusion of supercooled melts, and by drawing from gel or dilute flowing solutions. High-density polyethylene (HDPE) has been widely studied, in order to understand the morphological transformations and because it can now be drawn into one of the highest specific tensile moduli and strengths in both fiber and film form. Solution Processing Not all polymers can be fabricated by the convenient and economical melt processing techniques; nor is this desirable in some instances. Certain polymers have such strong interchain bonding that they do not melt or flow when heated until they reach temperatures at which chemical degradation occurs. These intractable, nonmoldable polymers are often fabricated, typically into fibers or films, by solution methods. Some well-known examples include certain cellulosics and polyacrylonitrile materials, plus specialty polymers for high-performance fibers like Nomex®, Kevlar®, and polybenzimidazoles (see the vignette "Polymers Stronger Than Steel"). Often such materials are soluble only in aggressive solvents such as sulfuric acid. In the usual case, the polymer is dissolved to relatively high concentrations, and these solutions are extruded into the form of fibers or film, or solvent cast into films. The polymer is solidified by removal of the solvent by evaporation through optimized drying regimes (dry spinning) or by coagulation and extraction with a nonsolvent (wet spinning). Cellulose is not generally soluble without chemical modification, and so rayon fiber manufacture involves a series of reactions, first to make the polymer soluble and then to harden it by regeneration of the original cellulosic structure. These types of solution processes are less well understood than melt processing operations, owing to their added complexity and the relatively little attention paid to this area because of the small quantity of products produced in this way. Because of the necessary solvent recovery steps, these processes tend to be expensive. The products produced by them often have complex morphological structures associated with the manner in which the solvent is removed that can leave residual porosity or other structural features not typical of melt-fabricated materials. In
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Polymer Science and Engineering: The Shifting Research Frontiers POLYMERS STRONGER THAN STEEL One often thinks of polymers as soft plastic. However, when certain polymers are spun into fibers, the resulting materials are truly amazing. All polymers are long, chain-like molecules made up of many smaller molecules linked together. In flexible fibers, such as polyester, the chains are partially relaxed; that is, as we follow a polymer chain along a fiber, it passes alternately through regions that are well ordered and rigid and regions that are disordered and softer. However, the chains in these new rigid polymers are fully extended and lie parallel to each other, like a fistful of uncooked spaghetti. These fibers are unexpectedly strong for their weight. One such fiber, poly(paraphenylene terephthalamide), was found to have a tensile strength higher than that of a steel fiber of the same dimensions, yet it weighs one-fifth as much. The commercial development of this product was a long and very expensive process. More than 12 capital-intensive steps are required to convert the basic aromatic feedstocks into a strong polymer. The key to realizing the outstanding inherent strength of these materials is the process by which the polymer is spun into fibers. The first attempts using traditional techniques involving injecting a stream of the polymer directly into a cooling bath did not result in an unusual material. However, it was found that raising the spinner head above the cooling water bath that the still-molten spun fiber falls into imparted unprecedented strength to the fiber. As the molten fiber falls toward the cooling bath, the polymer molecules become stretched and aligned. The oriented polymers can then form hydrogen bonds between molecules in adjoining chains, further strengthening the fiber. Because the molecules are fully aligned along their axis, forces on the fiber are absorbed by strong chemical bonds, not by the weak, loosely intertwined chains in the flexible polymers. One of the aramid polymers, Kevlar®, has saved the lives of many soldiers and police officers. It has such high impact resistance that an aramid vest thin enough to be worn comfortably under the shirt will stop a bullet as effectively as steel plate. The helmets worn during Operation Desert Storm were also made of this remarkable material. This strong, lightweight material is now taking the place of steel belts in radial tires and is being exploited in a myriad of other applications that require a high strength-to-weight ratio or resistance to corrosive environments, such as the cables that anchor oil-drilling platforms at sea. The molecular structure of a polymer is important, but, as shown above, the orientation of polymer molecules in relation to each other also plays a major role in determining the properties of the final product. The molecules' orientation can be greatly affected by the chemical engineers' choice of the way(s) in which the material is processed, allowing many more ways to customize polymers for their applications than are possible through chemistry alone. addition, the potential solvent emissions and exposure of workers raise numerous environmental and health concerns. Nevertheless, such fabrication methods are the only option available for certain materials of critical importance. Interest in polymers with very high heat resistance and other special attributes is likely to increase the need for solution processing technology, because by the nature of their properties such materials cannot be melt fabricated.
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Polymer Science and Engineering: The Shifting Research Frontiers In other cases, solution processing is required because of the nature of the product being fabricated rather than the nature of the polymer. Application of certain types of paint or coatings is most conveniently done by making the polymer fluid by dissolving it in a solvent and hardening by solvent removal. Melt fabrication cannot be used to economically perform certain operations like tablet coating (for controlled delivery) in the pharmaceutical industry. Formation of polymer membranes with complex structures consisting of extremely thin skins overlaying a porous support layer generally involves a variety of solution-processing protocols that can hardly be accomplished in any other way. Prepregs, used to make continuous-fiber-reinforced composites, often involve a solution processing step. "Molecular composites" based on rigid rod polymers dispersed in a matrix of a random coil chain polymer have attracted enthusiasm recently, and most techniques for forming them will invariably involve solution technology. The state of scientific knowledge about the thermodynamics, rheology, diffusion, and morphology development in multicomponent polymer-solvent systems will need to be advanced in order to place these types of processes on a solid technical footing. For example, recent attempts to mathematically model the formation of asymmetric membranes have failed to give some of the insight critically needed simply because the fundamental elements of such models are not currently well-enough understood. Environmental and health concerns associated with solution processing must be dealt with at several levels. There are opportunities for innovation in both the processes and the materials used. One avenue of interest is systems that use more benign solvents. Ongoing work in the area of supercritical fluid technology appears promising for some applications. Dispersion Processing Dispersion processing—the generation of particulate forms of polymers and their conversion into products—is a key technique that is likely to grow in importance, driven by both environmental and materials considerations. The polymer may be in the form of a dry powder, an aqueous dispersion (e.g., a latex), or a nonaqueous dispersion (NAD). A number of examples illustrate the current state of this technology and future directions. Polytetrafluoroethylene, or Teflon®, is a powder after the polymerization process, and its melting point is so high that melt fabrication is usually not practical. However, by combinations of heat and pressure it is possible to fabricate solid forms in much the same way as used in powder metallurgy. Powder coatings are applied to various substrates by a number of techniques, including fluidized beds and flame heating. Powdered liquid crystalline polymers can be "extruded" below their melting points in much the same way as metal powders
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Polymer Science and Engineering: The Shifting Research Frontiers are formed. Technology is being developed to convert polymer powders into three-dimensional prototype parts by using a computer-driven laser-sintering technique. There is much to be learned about the physics of sintering polymer powders. Emulsion polymerization is widely practiced because it is a convenient process for producing high-molecular-weight polymers using free radical mechanisms at high rates while controlling the exothermic nature of the reaction. In some cases, the resulting aqueous dispersion of the very fine polymer particles (called a latex) can be used directly for convenient fabrication of products. For example, the ubiquitous rubber gloves employed nowadays to protect against the spread of AIDS are made by dipping forms into a rubber latex followed by vulcanization. The popular water-based latex paints are made by emulsion polymerization. The paint film is formed by the evaporation of water, as opposed to organic solvents for oil-based paints. Painters also like cleaning up using water rather than paint thinner or solvents. The tiny emulsion polymer particles fuse into a continuous film driven by surface tension forces as the water evaporates. The rheological characteristics of the polymer must be designed such that fusion can occur without macroscopic flow of the coating. Environmental considerations strongly favor formation of coatings using latexes rather than solutions that "cure" by evaporation of organic solvents. Nonaqueous dispersion technology has emerged as a means of applying high-performance coatings (e.g., automotive paints) while minimizing solvent emissions. These are sophisticated materials in which fine polymer particles are formed by dispersion polymerization in a nonaqueous environment in which dispersant polymer chains prevent coalescence of the particles through steric stabilization. By this route, high-solids fluids of acceptable viscosity can be rapidly applied to metal substrates to form high-performance coatings with reduced emission of organic solvents. Inverse emulsion homo-or co-polymerization of water-soluble monomers such as acrylamide or acrylamide with co-monomers, such as cationic quaternary acrylates, represents a new commercially practiced method for the generation of materials that are suitable as flocculants. The materials as generated are extremely effective for purifying water, even at a level of 500 parts per million. When activated in water via modification of the surfactant or stabilizer, they can induce large flocs to be formed, which can be centrifuged from waste systems like sewage. The resulting flocs can be certrifuged, dried, and used as fertilizers. The resulting water is, of course, in a much purer state than before this treatment. Such operations are already in wide commercial use in many of the large cities of Europe and North America and are an example of how polymers can help clean the environment. Producing such polymers is already a multimillion dollar business.
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Polymer Science and Engineering: The Shifting Research Frontiers Process Models Extensive efforts have been devoted to the development of mathematical modeling or simulation of polymer processes to develop sophisticated manufacturing methods; however, much remains to be done. Most of these efforts have been applied to polymerization processes or to fabrication processes. The most essential ingredient of the former has been kinetic modeling of the reactions involved, while rheological behavior of the fluid polymer is usually the center-piece of fabrication models. Each type may be coupled with mass and energy balances, along with accounting for heat, mass, and momentum transfer in varying levels of detail. Major issues in all cases are the level of sophistication of the models used and the accuracy of the input data. In the past, the sophistication or level of detail was limited by the computational power that could be brought to bear on the problem; however, this no longer seems to be the limitation. Future work in this area is expected to be extensive and more limited by the nature of the physical models available as the trend moves from simple process questions that can be answered by simple phenomenological models to complex product questions that require more detailed molecular models. The most common driving force for developing process models is to aid in design. This function is often referred to as computer-aided design (CAD). For example, at the most elementary level a model of a polymerization process must be able to size the reactor and ancillary hardware and, in fact, to aid in making choices about the type of reactor system that would be most advantageous from a process point of view. Once a process configuration has been selected, computer-based models are generally able to predict temperature, pressure, and conversion profiles in time or space. More sophisticated versions are able to predict certain product qualities, such as molecular weight averages, copolymer composition, and branching frequencies. Models of this kind are valuable for modern computer-based process control schemes. They can also be of great value for evaluation of process safety issues. The future lies in going several steps beyond this to predict intrinsic polymer properties such as processing and end-use performance behavior. Such computer-based methods can be of value in the formulation, at least as a first estimate, and setting of process conditions to produce new grades of existing products. Extensive efforts have been devoted to modeling fabrication operations such as extrusion, mixing, and molding operations. The sophistication employed depends on the objective to be achieved. Again, the most common motivation for model development is to aid design. For example, because molds are expensive to build, it is valuable to have models that are able to calculate whether a particular design will function as intended (e.g., fill up in the available time) for possible operating variables. Most companies currently use externally or internally available software, combined with phenomenological rheological and heat transfer characterization
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Polymer Science and Engineering: The Shifting Research Frontiers of the polymer, to assess a proposed design before cutting metal. In spite of their computational complexity, such models answer only fairly simple process questions. Molecular-based models of flow behavior are emerging, which if properly combined with a process model are likely to be able to answer more subtle questions about both the process and the product. For example, in addition to simply determining whether the mold will fill properly, it would be useful to predict the crystallinity and molecular orientation (and thus the mechanical properties) of the product. A good example of the progress being made in the development of molecular theories is in the field of rheology, which is one of the basic sciences needed for the understanding of polymer processing. Rheology involves macroscopic shape changes of a polymeric fluid in complex transient stress and temperature fields. In rheological studies, well-defined stresses or strains are applied in order to measure and to predict the mechanical behavior of complex materials. The most important recent advances in polymer rheology came about in the 1970s, when P.G. de Gennes proposed a model for macromolecular motion that was then explored by M. Doi and S.F. Edwards to derive a rheological constitutive equation for the stress in macromolecular fluids. Many rheological phenomena have since been explained in terms of molecular parameters such as molecular weight, branching, chain stiffness, surface interaction, and polydispersity. Many of the current efforts in rheology focus on the basic understanding of the effects of phase transitions and anisotropy. Other major efforts concentrate on new continuum models that expand and fine-tune the predictions of Doi and Edwards. These initiatives are supported by advances in experimental rheology that give direction to the development of its theory. CONCLUSIONS Chemical products, in general, represent one of the few areas of the U.S. economy where the value of exports exceeds that of imports. Polymers contribute substantially to this positive balance of trade in chemicals. For example, plastics in both primary and nonprimary forms contributed $6.0B, or 37 percent, to the net positive trade balance in chemicals during 1992 (Chemical & Engineering News, 1993). Plastics manufacturing is an important part of the national economy. While the commodity polymers represent a maturing industry, significant process and product innovations continue to be introduced and appear likely to lead to healthy growth rates for the foreseeable future. A great deal of the current research and development on polymeric materials and associated processes is being driven by environmental considerations. Research on the recycling of polymers has been ongoing for more than two decades, and considerable progress has been made in some areas. A great deal of effort has been devoted to developing automated processes for segregation
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Polymer Science and Engineering: The Shifting Research Frontiers according to polymer type, and a uniform identification system has been agreed to by the industry to aid segregation by the consumer. Reprocessing of comingled waste leads to poor physical properties. Compatibilization may be technically possible, but it is quite expensive in relation to the value of the products produced. On the other hand, some plastic products appear to be easily recycled. For example, poly(ethylene terephthalate), which is used for soft drink bottles, is easily identified and is usually relatively free of contamination. A number of ways of reusing these materials have been identified, and some are in use. Perhaps the most significant are those that cause the polymer to revert chemically to oligomeric species that can be repolymerized into poly(ethylene terephthalate) or other polymeric products. While innovations in polymer recycling are needed, other options must also be pursued. Opportunities include source reduction and design of products with recycling in mind. The potential value of biodegradable polymers as part of the solution to the solid waste and litter problem needs to be better understood. Such materials are likely to be much more expensive than the relatively inexpensive polymers currently used, and their performance may not be as good. It is not clear that any materials biodegrade in landfills. In any case, the release of the low-molecular-weight degradation products into the environment could lead to more serious air or water contamination concerns. Incineration for fuel value is another, and perhaps the ultimate, form of recycling of polymers. Most polymers are derived from oil, and about 95 percent of all the oil produced is burned for its energy value; thus oil converted into polymers is simply being borrowed for a while to be used as a material prior to returning to its ultimate fate of being burned for its energy. Of course, concerns about the impact of incineration on health and the environment need to be resolved. Polymer interfaces are key to the performance of composites, blends or alloys, lubricants, adhesives, coatings, and thin films. Advances in the fundamental understanding of these interfaces and methods to engineer desired performance of these surfaces will no doubt lead to improved products and a competitive edge. New engineering plastics, including some blends or alloys, with ever-increasing performance characteristics continue to be introduced and in many cases are being used for structural applications traditionally dominated by other materials, mainly metals. Ease of fabrication into dimensionally precise parts with high-quality surface finish is one driving force. For polymer-based materials being used in highly critical structural applications, there is need for a better understanding of the mechanical, chemical, and environmental factors that affect their useful lifetimes and for methodologies to predict lifetimes in complex situations. Fracture mechanics techniques are not yet used to the same extent for polymers as for other structural materials. There are opportunities for new polymer systems with controlled permeability properties for use in food packaging, medicine, clothing, agriculture, industrial
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Polymer Science and Engineering: The Shifting Research Frontiers manufacturing, pollution control, water purification, and other applications. Innovative synthesis, knowledge of structure-property relations, and fabrication technologies will need to be integrated to achieve functional products that are cost-effective. Instrumentation is emerging that makes possible real-time determination of chemical and physical structures during processing, allowing instantaneous comparison with process simulation and control models and quality assurance. Rapid progress in the development of production processes is being made through implementation of a ''systems approach" wherein the materials, their composition, and the process are all considered variables in an iterative process. The systems approach, in comparison with the traditional "compartmentalized" approach, facilitates rapid identification of the critical material and processing parameters and aids manufacturing procedures. The relationship between processing and properties, particularly for complex polymer systems such as blends, composites, and liquid crystalline polymers, is not fully developed. The role of rheology in the development of structure is at a particularly rudimentary stage, and it appears that significant advances could be possible. The development of real-time probes of structure during processing would be of immense value. Of course, a linkage must be made between the structure generated during processing and the performance of the product. REFERENCES Barbero., E., and H.V.F. Gangarao. 1991. "Structural Applications of Composites in Infrastructure." SAMPE Journal 27 (No. 6, Nov./Dec.):9. Chemical & Engineering News. 1991. Vol. 69, No. 23, June 10, p. 39. Chemical & Engineering News. 1992. "Facts & Figures for the Chemical Industry." Vol. 70, No. 26, June 29, pp. 62-63. Chemical & Engineering News. 1993. "Facts & Figures for the Chemical Industry." Vol. 71, No. 26, June 28, pp. 38-83. McDermott, J. 1993. Advanced Composites: 1993 Blue Book. Cleveland, Ohio: Advanstar Communications. Modern Plastics. 1982. "Materials 1982: A Modern Plastics Special Report." Vol. 59, No. 1, January, pp. 55-87. Modern Plastics. 1992. "Resins 1992: Supply Patterns Are Changing." Vol. 69, No. 1, January, pp. 53-96. Pasztor, A. 1992. "Composites Makers Take Aim at Nondefense Markets; As Pentagon Budget Shrinks, Firms Seek to Dispel Cost, Safety Concerns." Wall Street Journal. August 26, pp. B3-B4. Strathman, H. 1991. Effective Industrial Membrane Processes: Benefits and Opportunities, M.K. Turner, ed. New York: Elsevier.
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