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62 _8 FEW YEARS AGO, who would have dreamed that Man aircraft could cir- cumnavigate the earth without landing or refueling? Yet in 1986 the novel aircraft Voyager did just that (Figure 5.11. The secret of Voyager's long flight lies in advanced materials that did not exist a few years ago. Much of the airframe was constructed from strong, lightweight polymer-fiber composite sections assembled with durable, high-strength ad- hesives; the engine was lubri- cated with a synthetic multicom- ponent liquid designed to maintain lubricity for a long time under continuous operation. These special materials typify the ad- vances being made by scientists and engineers to meet the de- mands of modern society. The future of industries such as transportation, communications, electronics, and energy conversion hinges on new and im- proved materials and the processing technolo- gies required to produce them. Recent years have seen rapid advances in our understanding of how to combine substances into materials with special, high-performance properties and how to best use these materials in sophisticated designs. Chemical engineers have long been involved in materials science and engineering and will become increasingly important in the future. Their contributions will fall in two categories. For commodity materials, which are nonpro- prietary formulations with well-established chemical compositions and property standards, chemical engineers will help maintain U.S.'com- petitiveness by creating and improving pro- cesses to make these chemicals as pure as possible and in high yields at the lowest possible investment and operating costs. For advanced materials, which are generally multicomponent, often proprietary, compositions designed to have very specific performance properties in specific uses, the competitive edge will come from chemical engineers who excel in controlling FRONTIERS IN CHEUICAL ENGINEERING FIGURE 5.1 The first airplane to circle the globe without refueling was Voyager, which accomplished this feat in 1986. This novel aircraft was made possible by high-performance lightweight materials and adhesives that were used in its construction. Chemical engineering research is crucial to the design of such new materials and their large-scale, efficient production. Copyright 1986 by Doug Shane Visions. molecular conformation, microscopic and mac- roscopic structure, and methods of combining the components in a way that will maximize product performance. Chapter 4 discussed chemical engineering challenges presented by materials and chemi- cally processed devices for information storage and handling. In this chapter, five additional classes of materials are covered: polymers, polymer composites, advanced ceramics, ce- ramic composites, and composite liquids. CHALLENGES TO CHEMICAL ENGINEERS The revolution in materials science and en- gineering presents both opportunities and chal- lenges to chemical engineers. With their basic background in chemistry, physics, and mathe- matics and their understanding of transport phenomena, thermodynamics, reaction engi- neering, and process design, chemical engineers can bring innovative solutions to the problems of modern materials technologies. But it is imperative that they depart from the traditional "think big" philosophy of the profession; to participate effectively in modern materials sci
POLYMERS, CERAMICS, AND COMPOSITES ence and engineering they must learn to "think small." The crucial phenomena in making mod- ern advanced materials occur at the molecular and microscale levels, and chemical engineers must understand and learn to control such phenomena if they are to engineer the new products and processes for making them. This crucial challenge is illustrated in the selected materials areas described in the following sec- tions. Polymers The modern era of polymer science belongs to the chemical engineer. Over the years, poly- mer chemists have invented a wealth of novel macromolecules and polymers. Yet understand- ing how these molecules can be synthesized and processed to exhibit their maximum theoretical properties is still a frontier for research. Only recently has modern instrumentation been de- veloped to help us understand the fundamental interactions of macromolecules with them- selves, with particulate solids, with organic and inorganic fibers, and with other surfaces. Chem- ical engineers are using these tools to probe the microscale dynamics of macromolecules. Using the insight gained from these techniques, they are manipulating macromolecular interactions both to develop improved processes and to create new materials. The power of chemical processing for con- trolling materials structure on the microscale is illustrated by the current generation of high- strength polymer fibers, some of which have strength-to-weight ratios an order of magnitude greater than steel. The best known example of these fibers, Kevlar~, is prepared by spinning an aramid polymer from an anisotropic phase (a liquid phase in which molecules are sponta- neously oriented over microscopic dimensions). This spontaneous orientation is the result of both the processing conditions chosen and the highly rigid linear molecular structure of the aramid polymer. During spinning, the oriented regions in the liquid phase align with the fiber axis to give the resulting fiber high strength and rigidity. The concept of spinning fibers from anisotropic phases has been extended to both solutions and melts of newer polymers, such as 63 polybenzothiazole, as well as traditional poly- mers such as polyethylene. Ultrahigh-strength fibers of polyethylene have been prepared by gel spinning. The same concept, controlling the molecular orientation of polymers to produce high strength, is also being achieved through other processes, such as fiber-stretching carried out under precise conditions. In addition to processes that result in mate- rials with specific high-performance properties, chemical engineers continue to design new pro- cesses for the low-cost manufacture of poly- mers. The UNIPOL process for the manufacture of polyethylene is a good example of the con- tributions of engineering research to polymer processing. Polyethylene is probably the quin- tessential commodity polymer. It has been man- ufactured worldwide for decades, and current U.S. production exceeds 15 billion pounds per year. Considering the global capital investment in existing plants for making polyethylene, it could be argued that inventing a new process for its manufacture is a waste of time and money. Not so. Chemical engineers at Union Carbide designed a proprietary catalyst that allowed polyethylene to be made in a fluidized-bed, gas- phase reactor operating at low temperature and pressure (below 100°C and 21 Bar). The resulting process produces a polymer with exceptional uniformity and can precisely control the molec- ular weight and density of the product. The advantages of the process (including a low safety hazard from the mild operating conditions and minimal environmental impact since there are no liquid effluents and unreacted gases are recirculated) are such that, in 1986, UNIPOL process licensees had a combined capacity suf- ficient to supply 25 percent of the world's demand for polyethylene. This is remarkable market penetration for a new process technol- ogy for a mature commodity, particularly in light of the tremendous existing (and fully am- ortized) worldwide capacity for polyethylene. In 1985, Union Carbide and Shell Chemical successfully extended the UNIPOL process to the manufacture of polypropylene, another ma- jor polymer commodity. Interestingly, the first two licensees for the new polypropylene process were a Japanese chemical company and a Ko- rean petroleum company.
64 Polymer Composites Polymer composites consist of high-strength or high-modulus fibers embedded in and bonded to a continuous polymer matrix (Figure 5.21. These fibers may be short, long, or continuous. They may be randomly oriented so that they impart greater strength or stiffness in all direc- tions to the composite (isotropic composites), or they may be oriented in a specific direction so that the high-performance characteristics of the composite are exhibited preferentially along one axis of the material (anisotropic compos- ites). These latter fiber composites are based on the principle of one-dimensional microstruc- tural reinforcement by disconnected, tension- bearing"cables" or"rods." To achieve a material with improved prop- erties (e.g., strength, stiffness, or toughness) in more than one dimension, composite laminates can be formed by bonding individual sheets of anisotropic composite in alternating orienta- tions. Alternatively, two-dimensional reinforce- ment can be achieved in a single sheet by using fabrics of high-performance fibers that have been woven with enough bonding in the cross- overs that the reinforcing structure acts as a connected net or trusswork. One can imagine that an interdisciplinary collaboration between WHISKERS OR SHORT FIBERS FABRIC ,,, / v LONG FIBERS ~ ,' ,' ,' ,''~ O () () Oo/ LAMINATE FIGURE 5.2 Fibers that are either very strong or very stiff can be used to reinforce polymers and ceramics. The resulting materials, known as composites, usually have one of the structures depicted in this figure. Clockwise from the upper left, reinforcement may be accomplished by embed- ding randomly oriented fibers, by orienting fibers along a particular axis, by assembling reinforced layers into lami- nates, or by embedding fabrics of reinforcing fibers in the material. FRONTIERS IN CHEMICAL ENGlNEERl\G chemical engineers and textile engineers might lead to ways of selecting the warp, woof, and weave in fabrics of high-strength fibers to end up with trussworks for composites with highly tailored dimensional distributions of properties. First-generation polymer composites (e.g., fiberglass) used thermosetting epoxy polymers reinforced with randomly oriented short glass fibers. The filled epoxy resin could be cured into a permanent shape in a mold to give lightweight, moderately strong shapes. The current generation of composites is being made by hand laying woven glass fabric onto a mold or preform, impregnating it with resin, and curing to shape. Use of these composites was pioneered for certain types of military aircraft because the lighter airframes provided greater cruising range. Today, major compo- nents for aircraft and spacecraft are manufac- tured in this manner, as are an increasing number of automobile components. The current generation of composites are being used in automotive and truck parts such as body panels, hoods, trunk lids, ducts, drive shafts, and fuel tanks. In such applications, they exhibit a better strength-to-weight ratio than metals, as well as improved corrosion resistance. For example, a polymer composite automobile hood is slightly lighter than one of aluminum and more than twice as light as one of steel. The level of energy required to manufacture this hood is slightly lower than that required for steel and about 20 percent of that for aluminum; molding and tooling costs are lower and permit more rapid model changeover to accommodate new de- signs. Polymer composite hoods and trunk lids are commercial on the 1987 models of one major U.S. automobile line, and the early problems of higher manufacturing cost and of achieving adequate production have been largely over come. The mechanical strength exhibited by these composites is essentially that of the reinforcing glass fibers, although this is often compromised by structural defects. Engineering studies are yielding important information about how the properties of these structures are influenced by the nature of the glass-resin interface and by structural voids and similar defects and how microdefects can propagate into structural fail
POLYMERS, CERAMICS, AND COMPOSITES ure. These composites and the information gained from studying them have set the stage for the next generation of polymer composites, based on high-strength fibers such as the aramids. Advanced Ceramics For most people, the word "ceramics" con- jures up the notion of things like china, pottery, tiles, and bricks. Advanced ceramics differ from these conventional ceramics by their composi- tion, processing, and microstructure. For ex- ample: · Conventional ceramics are made from nat- ural raw materials such as clay or silica; ad- vanced ceramics require extremely pure man- made starting materials such as silicon carbide, silicon nitride, zirconium oxide, or aluminum oxide and may also incorporate sophisticated additives to produce specific microstructures. · Conventional ceramics initially take shape on a potter's wheel or by slip casting and are fired (sintered) in kilns; advanced ceramics are formed and sintered in more complex processes such as hot isostatic pressing. · The microstructure of conventional ce- ramics contains flaws readily visible under op- tical microscopes; the microstructure of ad- vanced ceramics is far more uniform and typically is examined for defects under electron micro- scopes capable of magnifications of 50,000 times or more. Advanced ceramics have a wide range of application (Figure 5.3~. In many cases, they do not constitute a final product in themselves, but are assembled into components critical to the successful performance of some other com- plex system. Commercial applications of ad- vanced ceramics can be seen in cutting tools, engine nozzles, components of turbines and turbochargers, tiles for space vehicles, cylinders to store atomic and chemical waste, gas and oil drilling valves, motor plates and shields, and electrodes for corrosive liquids. Because advanced ceramics provide key com- ponents to other technologies for major im- provements in performance, their impact on the U.S. economy is much greater than is indicated by their sales figures. Ceramic components used 65 in turbines permit the construction of engines that operate at much higher temperatures than metallic engines, thus greatly increasing their thermodynamic efficiency and compactness. Ceramic liners and other ceramic components in diesel engines provide added benefits, such as the elimination of the need for water cooling and the prompter ignition of the fuel. An in- vestment in wear-resistant ceramic cutting tools can be more than repaid by the decrease in downtime for sharpening or replacing a dulled or worn metallic tool. Given these advantages, it is not surprising that market forecasts for advanced ceramics (including ceramic composites) are optimistic; in fact, sales in the year 2000 are predicted to be $20 billion. The market for advanced ce- ramics in heat engines is slated to grow by 40 percent per year to a total of $1 billion in 2000. The use of advanced ceramics is predicted to grow 16 percent per year over the next 5 years, and sales for automotive applications are fore- cast to increase from $53 million per year in 1986 to $6 billion per year by the end of the century. Uniform microstructure is crucial to the su- perior performance of advanced ceramics. In a ceramic material, atoms are held in place by strong chemical bonds that are impervious to attack by corrosive materials or heat. At the same time, these bonds are not capable of much "give." When a ceramic material is subjected to mechanical stresses, these stresses concen- trate at minute imperfections in the microstruc- ture, initiating a crack. The stresses at the top of the crack exceed the threshold for breaking the adjacent atomic bonds, and the crack prop- agates throughout the material causing a cata- strophic brittle failure of the ceramic body. The reliability of a ceramic component is directly related to the number and type of imperfections in its microstructure. As the requirements for greater homogeneity in ceramics become more stringent, and the scale at which imperfections occur becomes smaller, the need for chemical processing of ceramics becomes more compelling. Traditional approaches to controlling ceramic microstruc- ture, such as the grinding of powders, are reaching the limits of their utility for microstruc
66 wee, resistance Low thermal cxDension Ret rac~ori ness I nsula~ion ..:r ~ f ~ Thermal is' ' \conductivitV H igh \ strength A /''.. ."..''~..'.'.~..',,',2. #. , . ~ ~ . ~ .... ~ Hi i. ~ Advanced ceramics Biological compatibility tural control. Chemical engineers have an un- paralleled opportunity to contribute their ex- pertise in reaction engineering to problems that are in need of new analytical, synthetic, and processing tools. These include sol-gel process- ing and the use of chemical additives in ceramic processing. FRONTIERS IV Cl~iE.~L 3ElYGIlYEERING strength / Optical condensing F I uorescence Transl ucence ., ~ .,.,./ ,' Optical , ~ cow conductivity .,.,.,.~.,..,y. ~ ~ Electrical \ Electrical conductivity Semiconductivity Piezoelectric Dielectric Magnetic am FIGURE 5.3 The myriad functions, properties, and applications of advanced ceramics. Reprinted from High-Technology Ceramics in Japan, National Materials Advisory Board, National Research Council, 1984. Sol-Gel Processing The use of sol-gel techniques to prepare ceramic powders has recently attracted much interest in academia and industry. Sol-gel tech- niques involve dissolving a ceramic precursor (e.g., tetramethyl orthosilicate) in a solvent and
POLYMERS, CERAMICS' AND COMPOSITES 67 FIGURE 5.4 Stages in sol-gel processing are captured by a new electron microscopy technique. (1) Spherical particles tens of nanometers across can be seen in a colloidal silica sol. (2) Addition of a concentrated salt solution initiates "elation. (3) The gelled sample, after drying under the electron beam of the microscope' shows a highly porous structure. Courtesy, J. R. Bellare, J. K. Bailey, and M. L. Mecartney, University of Minnesota. subjecting it to a carefully controlled chemical reaction, hydrolysis (Figure 5.41. When the hydrolysis products first appear as a separate phase, they are fine colloids consisting of small particles, some with radii as small as a few nanometers. This colloidal suspension (the sol) further reacts and polymerizes to form a porous high-molecular-weight solid (the gel) that con- tains the solvent as a highly dispersed fluid component in its internal network structure. Removal of the solvent leaves behind solids with a wide variety of macrostructures depend- ing on the solvent and the way in which it was removed. These macrostructures can be sin- tered to convert them to dense ceramics. Sol-gel techniques are of interest because they can be used to prepare powders with a narrow distribution of particle size. These small particles undergo sintering to high density at temperatures lower by several hundred degrees centigrade than those used in conventional ce- ramic processing. Sol-gel processes may also be used to prepare novel glasses and ceramics such as · ceramics with novel microstructures and distributions of phases, ~ amorphous powders and dried gels that can be processed without crystallization to fully dense amorphous materials whose synthesis might not otherwise be possible, · materials with controlled degrees of poros- ity and possibly tailored surfaces within pores, and · ceramics with surfaces modified to alter their response to mechanical forces or to pro- mote their adhesion to other materials. Sol-gel processes also allow the manufacture of preforms that, upon sintering, collapse to a final product with the proper shape. There are many unresolved problems in sol- gel processing, many of which revolve around the poorly characterized chemistry of the pro- cess. Understanding and controlling the poly- merization reactions that produce the gel are key challenges, as are characterizing and opti- mizing both the removal of fluid from the gel and the subsequent sintering of the porous solid to a fully dense ceramic body. Solving these problems will make sol-gel processing the pro- cess of choice for the synthesis of a wide variety of ceramics, glasses, and coatings. Chemical Additives in Ceramic Processing Another area to which chemical engineers can contribute is the use of chemical additives
68 to improve the properties of ce- ramic materials. For example, zirconium oxide can form a meta- stable state in ceramic bodies that is denser than its normal state. The incorporation of suit- able chemical additives stabilizes the metastable state sufficiently to allow the fabrication of parts containing it. When a crack forms in such a ceramic part, the zir- conium oxide region at the crack tip changes to the less dense form. The resulting expansion blunts the crack tip and stops its propagation (Figure 5.51. This strategy for using a chemical ad- ditive to improve ceramic resis- tance to cracking is called trans- formation toughening. Ceramic Composites Like polymer composites, ce- ramic composites consist of high- strength or high-modulus fibers embedded in a continuous ma- trix. Fibers may be in the form of "whiskers" of substances such as silicon carbide or aluminum oxide that are grown as single crystals and that therefore have fewer defects than the same substances in a bulk ceramic (Figure 5.6~. Fibers in a ceramic composite serve to block crack propagation; a growing crack may be deflected to a fiber or might pull the fiber from the matrix. Both processes absorb energy, slowing the propagation of the crack. The strength, stiffness, and toughness of a ceramic composite is prin- cipally a function of the reinforcing fibers, but the matrix makes its own contribution to these properties. The ability of the composite material to conduct heat and current is strongly influ- enced by the conductivity of the matrix. The interaction between the fiber and the matrix is also important to the mechanical properties of the composite material and is mediated by the chemical compatibility between fiber and matrix at the fiber surface. A prerequisite for adhesion between these two materials is that the matrix, FRONTIERS IN CHEMICAL ENGINEERING FIGURE 5.5 Zirconia ceramics can be made stronger and less brittle by using chemical additives to stabilize a more compact tetragonal structure that does not naturally occur at room temerature. When such a phase is subjected to stress, it can change phases, expanding to the monoclinic structure. This expansion fills any stress-initiated crack and prevents it from moving. This micrograph shows a zirconia ceramic composed of lozenge-shaped grains. pore with grain boundaries radiating from its top and base dominates the picture. Courtesy, National Physical Laboratory (United Kingdom). in its fluid form, be capable of wetting the fibers. Chemical bonding between the two components can then take place. Ceramic matrix composites are produced by one of several methods. Short fibers and whis- kers can be mixed with a ceramic powder before the body is sintered. Long fibers and yarns can be impregnated with a slurry of ceramic particles and, after drying, be sintered. Metals (e.g., aluminum, magnesium, and titanium) are fre- quently used as matrixes for ceramic composites as well. Ceramic metal-matrix composites are fabricated by infiltrating arrays of fibers with molten metal so that a chemical reaction be- tween the fiber and the metal can take place in a thin layer surrounding the fiber. As with advanced ceramics, chemical reac- tions play a crucial role in the fabrication of ceramic composites. Both defect-free ceramic fibers and optimal chemical bonds between fiber
POLYMERS, CERAMICS, ANID COMPOSITES FIGURE 5.6 This is a fractured sample of a ceramic composite (alumina with 30 volume-percent silicon carbide whiskers). The lighter regions of circular or cylindrical shape are randomly oriented whiskers protruding from the fractured surface. The rod-like depressions in the surface mark places where whiskers nearly parallel with the fracture were pulled out. Courtesy, Roy W. Rice, W. R. Grace and Company. and matrix are required for these composites to exhibit the desired mechanical properties in use. Engineering these chemical reactions in reliable manufacturing processes requires the expertise of chemical engineers. Composite Liquids A final important class of composite materials is the composite liquids. Composite liquids are highly structured fluids based either on particles or droplets in suspension, surfactants, liquid crystalline phases, or other macromolecules. A number of composite liquids are essential to the needs of modern industry and society because they exhibit properties important to special end uses. Examples include lubricants, hydraulic traction fluids, cutting fluids, and oil-drilling muds. Paints, coatings, and adhesives may also be composite liquids. Indeed, composite liquids are valuable in any case where a well-designed liquid state is absolutely essential for proper delivery and action. All composite liquids are produced by the 69 chemical processing industries, and chemical engineers face continuing challenges in tailoring their end-use properties. Some of these chal- lenges are illustrated in the following examples: · Motor lubricants are complex liquid com- posites in which components provide different performance characteristics. The basic com- ponent is a hydrocarbon oil with a fixed boiling range. It must have sufficient viscosity at engine operating temperatures to prevent the friction and wear of moving surfaces, but must be fluid enough below freezing temperatures for winter start-up. Viscosity modifiers are high-molecu- lar-weight polymers that reduce the temperature coefficient of viscosity (viscosity index). Sus- pended colloidal particles of calcium or mag- nesium carbonate are added to neutralize engine acids and are stabilized by adsorbed polymers and surfactants to prevent coalescence. Solids dispersants are low-molecular-weight polymers with functional groups that pick up carbon particles generated in combustion and maintain them in suspension. At low temperatures, the waxes (straight-chain paraffin hydrocarbons) in the lubricant form long crystals to set up a solid gel. To prevent this, low-molecular-weight poly- mers, called pour point depressants, are added to co-crystallize with the wax; the resulting smaller crystals do not gel. Finally, there are antiwear additives and antioxidants to reduce engine wear and deposits. Lubricants with out- standing viscosity indexes enable an engine to start when the lubricant temperature is as low as -40°C and yet operate well when the lubri- cant temperature is as high as 200°C. Other additives allow broadening the temperature range further by providing increased thermal and ox- idative stability. The use of synthetic base oils allows still broader ranges of operating temper- atures, up to 500°C. ~ Advanced adhesives are composite liquids that can be used, for example, to join aircraft parts, thus avoiding the use of some 30,000 rivets that are heavy, are labor-intensive to install, and pose quality-control problems. Ad- hesives research has not involved many chem- ical engineers, but the generic problems include surface science, polymer rheology and ther- modynamics, and molecular modeling of ma
70 serials near or at interfaces. The scientific and engineering skills needed are very similar to those needed for polymer composites and mul- ticomponent polymer blends. The time-tested mechanical methods developed for joining met- als are not satisfactory for composite and other advanced materials, and chemical engineers skilled in interracial science are well qualified to contribute to this area. · Another class of liquid composites is that of coating compositions used to deposit thin films on a substrate or other films; these com- posites have evolved from typical paints and varnishes into multilayer films in which each layer contributes specific properties to the en- semble. Such films may be paints used for sealing and decorative purposes, films used for printing or packaging purposes, or multilayer products used in recording tapes and photo- graphic products. All are based on generic scientific principles that include many common elements from thermodynamics, polymer sci- ence, rheology, and fluid mechanics. Liquid composites seldom behave as New- tonian fluids. These complex mixtures usually contain macromolecules, suspended particles, and surfactants. They are frequently multi- phase, and changes in phase composition or formation of new liquid phases may occur over the range of operating conditions. Phase com- position may be shifted by chemical reaction, by shear forces, or simply by changes in tem- perature or pressure. Liquid-liquid and liquid- solid equilibria are crucial. Detailed molecular understanding of the interactions among such components as surfactants, polymers, and par- ticles is essential for the rational design of liquid composites. Much of this design is now accom- plished by informed empiricism, which is useful for the incremental improvement of current products but inadequate for major changes and innovation. INTERNATIONAL COMPETITION The potential markets for the advanced ma- terials discussed in this chapter are lucrative, and most nations that possess the technological infrastructure needed to invent, develop, and FRONTIERS l^\ C~EMICAL EAGINEERING understand these materials are mounting major efforts to exploit these developing markets. Polymers and Polymer Composites The Panel on Advanced Materials of the NSF Japanese Technology Evaluation Program (JTECH) issued a report in 1986 assessing the status and direction of Japanese research and development efforts in several high-technology polymer areas.' The panel noted that the major Japanese chemical companies already manufac- ture most of the commercially available poly- mers and that "since 1970, there has been an increasing flow of upgraded technology from Japan to the United States." For engineering plastics and resins, the panel judged the United States to be ahead in basic research (although the lead is diminishing), on a par with Japan in advanced development, and behind Japan in product implementation. The JTECH panel also compared the U.S./ Japanese position in high-strength/high-modulus polymer research and development. Its conclu- sions substantially agree with the following statements, drawn from a recent report of the NAS/NAE/IOM Committee on Science, Engi- neering, and Public Policy.2 The United States has a strong position in the development of high-strength polymers, but comparable activity in this area exists in Japan. The Japanese Ministry of International Trade and Industry (MITI) has designated the development of a "third-generation" fiber as a government-sub- sidized project, beginning in 1983 and targeted for practical application by 1988. Because of the overwhelming importance of the load-bear- ing fibers in composites, the field is sensitive to breakthroughs in stronger fibers. Such a break- through could come from Japan; for example, a Japanese group was first to patent a process for making high-strength fibers from polytethylene terephthalate) . Much of the technology used for manufac- turing carbon fibers in the United States is licensed from Japanese companies. The high level of Japanese carbon-fiber technology sug- gests that Japanese companies may produce many of the expected future advances in these materials.
POLYMERS, CERAMICS, A\D COMPOSITES The United States currently has a strong position in composite manufacturing and pro- cessing technology and leads the world in de- veloping major applications in aircraft, sporting goods, and automotive components. There is growing overseas activity in composites tech- nology, however, particularly among European aircraft companies. Ceramics and Ceramic Composites Japan is the United States' chief competitor in ceramics. There is a widespread but false -perception that Japan leads the United States in ceramics in general. Nevertheless, it is clear that the Japanese effort in ceramics is compre- hensive and long term and that several Japanese companies lead their U.S. industrial counter- parts in specific technologies. There is tremen- dous enthusiasm in Japan for the potential of ceramics, and a recent report of a U.S. visiting team to Japan3 reached the following conclu- sions: · The Japanese are committed to vigorously developing and dominating the field of advanced ceramics. They have put in place a well-inte- grated national effort primarily based on gov- ernment-industry interactions. · Industrial management commitment to long- term research and development appears to be more solid in Japan than in the United States. · Japanese research is focusing increasingly on fundamental research issues. Government- inspired basic research programs are being put in place, and Japan's position as a net consumer of basic ceramics research may change in the coming years. Given the potential future importance of ce- ramics in areas as diverse as electronics (see Chapter 4), machine tools, heat engines, and superconductors (see Chapter 4), the United States can ill afford to surrender technical lead- ership to its competitors. The dominant trend in the field is toward materials with finer micro- structures, fewer defects, and better interac- tions at interfaces (particularly in composites). Chemical processes provide important tools to capture the promise of ceramics for the benefit 71 of our society and to maintain our international competitive position in technology. Composite Liquids The field of composite liquids has not received much attention outside the industries associated with specific liquid products (e.g., the petroleum industry). In areas such as lubrication, the United States has clear technological leader- ship. The situation is less clear for liquid crystals and adhesives, where there is greater compe- tition from Europe and Japan. INTELLECTUAL FRONTIERS A wide variety of chemical engineering re- search frontiers involve advanced materials and belong in the mainstream of academic chemical engineering departments. The following list of frontiers ranges from the molecular level to the systems level. Microscale Structures and Processes The study of materials has traditionally cen- tered on the influence of molecular composition and microstructure on mechanical, electrical, optical, and chemical properties. At the molec- ular level are a variety of research frontiers that can profitably draw chemical engineers into close collaboration with physical and theoretical chemists. They include the following research areas. New Concepts in Molecular Design of Composite Materials The toughest challenge and the greatest op- portunity in chemical engineering for high-per- formance materials lie in the development of wholly new designs for composite solids. Such materials are typified by composites reinforced by three-dimensional networks and truss- works microstructures that are multiply con- nected and that interpenetrate the multiply con- nected matrix in which they are embedded. In such materials, both reinforcement and matrix are continuous in three dimensions; the com- posite is bicontinuous. Geometric prototypes of
72 such structures are found in certain liquid crys- tals and colloidal gels and in bones and shells. The challenge is to break away from today's technology, to go beyond today's research in two-dimensional fabric-like reinforcement, and to determine how to create truly three-dimen- sional microstructures and design, construct, and process them so as to control the properties of the composite. A potentially promising area is "molecular composites," in which the fiber and its sur- rounding matrix have the same composition and differ only in molecular structure or morphol- ogy. This might involve forming the composite from very stiff, linear polymer molecules, some of which are aligned during the forming step as reinforcing crystallites in the amorphous re- gions the matrix. An analogous ceramic com- posite may be envisioned. There are difficult engineering problems to be solved in learning how to control the orientation of the crystalline regions and the ratio between crystalline and amorphous regions in the material. The Role of Interfaces in Materials Chemistry Two general problems relate to the role of interfaces in advanced materials. The first is simply that we do not have the theory or the computational or experimental ability to under- stand the interatomic and microscopic interac- tions at the interfaces between components of an advanced material, on which its properties are critically dependent. There is a general need for research on processes at interfaces and on the structure-property-performance relation- ships of interfaces. The second problem relates to the role that interfaces play in mediating chemical reactions in the synthesis of composite materials. This problem has three parts, which are illustrated here for polymeric composites. · First, in composites with high fiber con- centrations, there is little matrix in the system that is not near a fiber surface. Inasmuch as polymerization processes are influenced by the diffusion of free radicals from initiators and from reactive sites, and because free radicals FRONTIERS iN CHEMICAL ENGINEERING can be deactivated when they are intercepted at solid boundaries, the high interracial area of a prepolymerized composite represents a radi- cally different environment from a conventional bulk polymerization reactor, where solid bound- aries are few and very distant from the regions in which most of the polymerization takes place. The polymer molecular weight distribution and cross-link density produced under such diffu- sion--controlled conditions will differ apprecia- bly from those in bulk polymerizations. · Second, the molecular orientation of the fiber and the prepolymer matrix is important. The rate of crystal nucleation at the fiber-matrix interface depends on the orientation of matrix molecules just prior to their change of phase from liquid to solid. Thus, surface-nucleated morphologies are likely to dominate the matrix structure. ~ Third, the ultimate mechanical properties of a composite will be strongly influenced by the degree to which the matrix wets the fiber surface and by the degree of adhesion between the two after curing. Both phenomena depend on intimate details of the surface science of the two phases, about which little is known. Molecular modeling techniques, augmented by careful measurements of the structure of the interracial regions, hold promise for elucidating details of these three aspects of interracial control of matrix polymerization. Understanding the Molecular Behavior of Complex Liquids Basic understanding of the liquid state of matter still lags behind that of the solid and gaseous states. Our knowledge of interactions in multicomponent liquids containing macro- molecules and suspended solids is extremely limited. Thus, the study of complex liquids, including polymer solutions, sots, gels, and composite liquids, is a significant challenge for chemical engineers. The ability to predict liquid-liquid and liquid- solid equilibria in complex systems is still rather undeveloped, in part because of the lack of systematic and molecularly interpreted experi- mental information. Considerable research has
POLYMERS, CERAMICS, AND COMPOSITES been conducted on the behavior of liquids near their critical points, on lower critical solution phenomena, on spinodal decomposition, and on related dynamics such as the growth and mor- phology of new phases, but generalized corre- lations and connections of theory to practice are few. Molecularly motivated empiricisms, such as the solubility parameter concept, have been valuable in dealing with mixtures of weakly interacting small molecules where surface forces are small. However, they are completely inad- equate for mixtures that involve macromole- cules, associating entities like surfactants, and rod-like or plate-like species that can form ordered phases. New theories and models are needed to describe and understand these sys- tems. This is an active research area where advances could lead to better understanding of the dynamics of polymers and colloids in so- lution, the theological and mechanical proper- ties of these solutions, and, more generally, the fluid mechanics of non-Newtonian liquids. Chemical Dynamics and Modeling of Molecular Processes Chemical dynamics and modeling were iden- tified as important research frontiers in Chapter 4. They are critically important to the materials discussed in this chapter as well. At the molec- ular scale, important areas of investigation in- clude studies of statistical mechanics, molecular and particle dynamics, dependence of molecular motion on intermolecular and interracial forces, and kinetics of chemical processes and phase changes. Mechanistic studies are particularly needed for the hydrolysis and polymerization reactions that occur in sol-gel processing. Currently, little is known about these reactions, even in simple systems. A short list of needs includes such rudimentary data as the kinetics of hydrolysis and polymerization of single alkoxide sol-gel systems and identification of the species present at various stages of gel polymerization. A study of the kinetics of hydrolysis and polymerization of double alkoxide sol-gel systems might lead to the production of more homogeneous ce- ramics by sol-gel routes. Another major area 73 for exploration is the chemistry of sol-gel sys- tems that might lead to nonoxide ceramics. The Intimate Connection Between Materials Synthesis and Processing Materials synthesis and materials processing have classically been thought of as separate activities, and in the days of simple, homoge- neous materials, they were. But today's com- plex materials are bringing these two areas closer together in research and in practice. Four outstanding intellectual challenges demonstrat- ing this connection are described in this section. Processing of Complex Liquids Complex liquids are ubiquitous in materials manufacture. In some cases, they are formed and must be handled at intermediate steps in the manufacture of materials (e.g., sots and gels in the making of ceramics, mixtures of monomer and polymer in reactive processing of poly- mers). In other cases (e.g., composite liquids), they are the actual products. Understanding the properties of complex fluids and the implications of fluid properties for the design of materials processes or end uses presents a formidable intellectual challenge. Complex liquids seldom behave as classical Newtonian fluids; thus, analysis of their behav- ior requires a thorough understanding of non- Newtonian rheology. The importance of this knowledge is illustrated by the following two examples: · The problem of processing complex liquids while they are undergoing rapid polymerization is an important challenge in reactive polymer processing (e.g., reactive injection molding and reactive extrusion). In these processes, the viscosity of a reaction mixture, as it proceeds from a feed of monomers to a polymer melt product, may change by 7 decades or more in magnitude. Fluid mixtures flowing into a mold of complicated geometry may exhibit large tem- perature gradients from the highly exothermic chemical reactions taking place and significant spatial variations in viscosity and molecular weight distribution.
· Rheology is especially important to the understanding of composite liquids in their many applications as products (e.g., lubricants, sur- face coating agents, and additives for enhanced oil recovery and drag reduction). Such liquids usually contain polymers, and their behavior is frequently viscoelastic under use conditions. While data on linear response and relatively mild shear flows are available for nonassociating polymer solutions in the relevant ranges of molecular size and concentration, far fewer data are available on liquid systems that contain particles or micelles, particularly those in which there are strong interparticle interactions. Knowledge of the fluid mechanics of ordered liquids is similarly sparse. Information on the response to rapid shear flows and extensional flows, even in simple polymer solutions, is very limited. Thus, we are far from having depend- able equations from which models of such fluids could be developed and farther still from a generalized molecular understanding of the structure-property relations of these fluids and from extrapolations of the flow patterns and stress distributions in such fluids in geometries close to those in which they are used. For example, there is significant divergence between theoretical prediction and empirical observation of the flow of lubricants in journal bearings. Even if satisfactory equations of state and constitutive equations can be developed for complex fluids, large-scale computation will still be required to predict flow fields and stress distributions in complex fluids in vessels with complicated geometries. A major obstacle is that even simple equations of state that have been proposed for fluids do not always converge to a solution. It is not known whether this difficulty stems from the oversimplified nature of the equations, from problems with numerical mathematics, or from the absence of a laminar steady-state solution to the equations. Processing of Powders One route to better ceramic powders, sol-gel processing, has already been described in this chapter. There are, however, many other pos- sible routes to improved ceramic powders. These FRONTIERS IN CHE10~AL ENGINEERi\G routes include refinements of older processes, such as precipitation and thermal decomposi- tion, as well as newer processes, such as plasma processing and chemical vapor deposition. The nucleation and particle-growth processes in such systems need to be described quantitatively to enable better process development and scale- up. Chemical engineering frontiers include the development of new chemical processes for producing ceramic raw materials, such as sub- micron, spherical, uniform powders, and high- strength fibers and whiskers. Chemical engineers could also work to devise processes to improve the flow characteristics of powders after they are formed. Such research would help control agglomeration of particles in subsequent processing steps as well as facil- itate the production of compacted ceramic pre- forms. For example, gas-solid chemical reac- tions might be used to tailor the chemical composition of powders. As another example, better methods of compounding powders with binders might be achieved by processes that mix powders with suitable binders in a liquid and then spray dry the resulting suspension. Powder processing is also one element in the engineering of grain boundaries in large, com- plex parts. Such engineering would allow sin- tering ceramics to full density without degrading oxidation resistance and long-term strength. Processing of Polymers Other important research challenges confront chemical engineers in the area of polymer pro- cessing. One concerns the interactions of poly- mers with their environment. For example, contacting a glassy polymer with a solvent or swelling agent may lead to unusual diffusion characteristics in the polymer, stress formation, crazing, or cracking. Such phenomena are poorly understood because glassy polymers may ex- hibit complex viscoelastic behavior in the pres- ence of a liquid or during their second-order (glass) transitions. The study of diffusion in glassy polymers is a virgin research area for chemical engineers. A better understanding of polymer-solvent interactions could have impor- tent payoffs in the development of positive resists for microcircuit manufacture (see Chap
POLYMERS, CERAMICS, AND COMPOSITES ter 4), because the dissolution characteristics of polymeric resists are crucial to their appli- cation and removal during microlithography. The focus of research on engineering ther- moplastics with enhanced mechanical, thermal, electrical, and chemical properties has shifted away from synthesizing novel polymers toward combining existing polymers. Multicomponent polymer blends pose interesting and challenging new problems for chemical engineers. Many multicomponent polymeric melts are homoge- neous at processing temperatures but separate during cooling. Judicious choice of stress levels during cooling and of the cooling rate can effect changes in the structure and morphology of the end product and hence in its properties. The fluid prepolymer in which the load-bear- ing fibers of a polymer composite are placed undergoes further polymerization and cross- linking during the thermal curing of the com- posite. The chemical reactions that occur during curing are exothermic and are difficult to con- trol. Some regions in the composite material react adiabatically while others lose heat by conduction to their surroundings. The resulting point-to-point variations in polymer matrix mo- lecular weight and cross-link density result in changes in the composite's properties and qual- ity. We need to better understand and control these variations in well-characterized processes and to deduce how to change the geometry of the finished object or the distribution of fibers within it to compensate for the variations in polymer structure that might inherently arise . . during processing. Process Design and Control Because processing conditions and history have such an important influence on the con- formation and properties of materials, there is a need to develop models and systems for the measurement and control of materials manufac- turing processes so that processes can be better designed, more precisely controlled, and auto- mated. Opportunities for chemical engineers in process design and control, including advanced mathematical modeling of polymer processing, are explored in depth in Chapter 8. ~5 It is particularly important to study process phenomena under dynamic (rather than static) conditions. Most current analytical techniques are designed to determine the initial and final states of a material or process. Instruments must be designed for the analysis of materials processing in real time, so that the crucial chemical reactions in materials synthesis and processing can be monitored as they occur. Recent advances in nuclear magnetic resonance and laser probes indicate valuable lines of de- velopment for new techniques and comparable instrumentation for the study of interfaces, complex liquids, microstructures, and hierar- chical assemblies of materials. Instrumentation needs for the study of microstructured materials are discussed in Chapter 9. Fabrication and Repair of Materials Systems Advanced materials systems based on poly- mers, ceramics, and composites are constructed by assembling components to create structures whose properties and performance are deter- mined by the form, orientation, and complexity of the composite structure. The properties of these assemblages are determined not by the sum of weighted averages of the components but rather by synergistic effects in intercon- nected phases. For this reason, the study of fabrication of hierarchical assemblages of ma- terials, as well as the study of mechanisms for repairing defects in assembled structures, must be supported by fundamental research. Designing Systems from the Molecules on Up Successful systems design and fabrication depend on understanding the connections be- tween microscale phenomena and macroscale behavior of materials. For example, with suf- ficient insight into intermolecular interactions, appropriate models, and the computational power of supercomputers, it may be possible to predict changes in macromolecular configurations when loads are imposed on polymers or changes in the properties of a material as a result of
76 branching or cross-linking the material's ma- cromolecular structure. A related problem in composites is the need to design optimal fiber orientations for a com- posite part given the set of stress vectors and levels to which the part will be subjected. These design considerations would be useful in de- signing airframe components such as parts for the tail, wing, or fuselage. A similar problem is assessment of the performance penalties that might result from imperfections in manufacture. Solutions to these problems lie in the realm of computer-aided design and manufacturing (CAD/CAM). This area of technology is being developed rapidly by mechanical engineers, but the problems encountered include many that are logical extensions of polymer process en- gineering. Interdisciplinary collaborations be- tween mechanical and chemical engineers should be fostered for problems where chemical ex- pertise would be valuable. Just as chemical engineers of a previous era contributed exten- sively to the knowledge of heat and mass trans- fer by collaborating with mechanical engineers, so are they now well positioned to contribute to composites CAD/CAM and to the education of students who may one day use and oversee these processes in industry. Chemical Processing in the Fabrication of Materials Systems One might imagine that the fabrication of materials systems involving polymers, ce- ramics, and composites would be principally a concern of mechanically oriented materials en- gineers. This is not true. For example, the mechanical attachment of composites to other materials (e.,g., metal parts) by drilling holes in the composite and attaching mechanical fas- teners can alter and degrade the performance of the composite. In a number of situations, joining and fabrication processes involving chemical reactions with the material will be needed in systems fabrication. Fundamental research to support materials assembly and fabrication probably centers on the science and technology of adhesion, al- though research on mechanical assembly driven by chemical action, such as the self-assembly FRONTIERS IN CHE.~ICAL ENGINEERING of large molecules or particles, also holds prom- ise for solving some fabrication problems. Detection and Repair of Flaws in Materials Systems A central problem in complex materials sys- tems of any kind involves testing to detect flaws, analysis to predict their effect on re- maining service life of the system, and repair strategies to overcome them. For the structural materials discussed in this chapter, these prob- lems are uncharted territory in need of explo- ration by chemical engineers. There is a general need for nondestructive test methods capable of determining whether the manufacturing process for a polymer, ce- ramic, or composite has achieved the desired microstructure. Chemical engineers can profit- ably contribute to interdisciplinary efforts to develop such test methods. For example, one type of defect in a composite arises because the placement of the fibers is different from what was intended. This may reflect perturbations in the filament-winding operations on a mandrel or fiber movement during curing in response to differential stresses. The processing expertise of chemical engineers could be useful in devel- oping instrumentation to detect such flaws dur- ing the manufacturing process so that automatic control and correction of the process can be invoked to avoid or compensate for flaws. There is also a need for methods to predict the effects of flaws and the remaining service life of a flawed or degraded part in use For example, because of limited basic knowledge about composites, structures based on them are now overdesigned for considerably greater mar- gins for error than those required for metal structures, thereby losing some of the inherent superiority of composites. Finally, attempts to repair composite struc- tures will become increasingly common in future years as the use of composites spreads. At this point, a fundamental repair science for com- posites is completely lacking. Since such strat- egies are likely to depend heavily on chemical reactions to heal breaks and flaws, chemical engineers should be at the forefront of this emerging field.
POLYMERS, CERAMICS, AND COMPOSITES IMPLICATIONS OF RESEARCH FRONTIERS Chemical engineers are already equipped to pursue the frontiers outlined in this chapter. The core undergraduate curriculum provides both a science base and an engineering knowl edge base for approaching problems in materials phenomena and processing. At the same time, undergraduate chemical engineering students would benefit from a broader exposure to prob lems in materials science and engineering. What is needed is a better integration of such problems into the curriculum at all levels. This is best achieved by developing better instructional ma terial and example problems for existing courses in thermodynamics, transport, and reaction en gineering. What is not needed is a proliferation of general, encyclopedic materials courses for undergraduates. All the scientific and engineering disciplines involved in materials research are in need of better instrumentation and facilities. Suitable equipment for chemical engineers interested in materials questions might include the following: solid-state NMR spectrometry; spin-echo NMR spectrometry; Raman spectroscopy; secondary ion mass spectrometry; X-ray photoelectron spectroscopy; laser light scattering; advanced dynamic rheometers; computer-controlled, fully equipped poly- 2 mer~zation reactors; · directional irradiation devices; and · dynamic mechanical property measure ment equipment. Special efforts to help academic institutions acquire these instruments are needed. Future chemical engineers will be required to under stand the design and operation of sophisticated equipment in the analysis of materials proper ties. An early exposure to these techniques is highly desirable, and is probably indispensable to quality research at the graduate level. The Matenals Research Laboratones (MRLs), sponsored by the NSF, have been one mecha nism for providing instrumentation and facilities support to small groups of principal investiga 77 tors with interests in materials. There is a perception in the chemical engineering com- munity that MRLs are more physics directed, and probably not open to significant participa- tion by chemical engineers. One way of ad- dressing this problem would be for NSF to target more funds to the Division of Materials Research with an emphasis on interdisciplinary and process studies. Another way might be to develop mechanisms intermediate between MRLs and ERCs that would promote engineering re- search on materials. A final goal for improving the chemical en- gineenng contribution to materials research would be to develop focused continuing ec~ucat~on programs to help qualified chemical engineers move aggressively into materials-related areas. Such courses might take a number of forms. The AIChE might take the lead in sponsoring short courses within the context of its existing continuing education program. Universities might provide complementary, more intense exposure to the problems and opportunities in materials research by initiating special workshops, mas- ters degree programs, or sabbaticals for indus- trial researchers. NOTES JTECH Panel Report on Advanced Materials in Japan. La Jolla, Calif.: Science Applications In- ternational Corp., 1986. National Academy of Sciences-National Academy of Engineering-Institute of Medicine, Committee on Science, Engineering, and Public Policy. "Re- port of the Research Briefing Panel on High- Performance Polymer Composites," in Research Briefings 1984. Washington, D.C.: National Acad- emy Press, 1984. 3. National Research Council, National Materials Advisory Board. High-Technology Ceramics in Japan (NMAB-418~. Washington, D.C.: National Academy Press, 1984. SUGGESTED READING C. G. Gogos, Z. Tadmor, D. M. Kalyon, P. Hold, and J. A. Biesenberger. "Polymer Processing: An Overview." Chem. Eng. Prog., 83 (6), June 1987, 49. National Research Council, Engineering Research Board. "Materials Systems Research in the United
States," in Directions in Engineering Research. Washington, D.C.: National Academy Press, 1987. D. R. Uhlmann, B. J. J. Zelinski, and G. E. Wnek. "The Ceramist as Chemist Opportunities for New Materials." Mat. Res. Soc. Symp. Proc., 32, 1984, 59. iFlR0NTIFIRS IN AL ERG U.S. Congress, Office of Technology Assessment. New Structural Materials Technologies: Oppor- tunities for the Use of Advanced Ceramics and Composites A Technical Memorandum (OTA- TM-E-321. Washington, D.C.: U.S. Government Printing Office, 1986.
