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Preview this item Preview this item. Series: Advanced structured materials , v. Elastomers having immense structural possibilities for chemical and mechanical modifications to generate novel properties, functions and applications especially in tire and engineering areas. The books discuss the various attempts reported on solving these problems from the point of view of the chemistry and the structure of elastomers, highlighting the drawbacks and advantages of each method.
Highly cross-linked thermosets which are susceptible to brittle failure can be effectively toughened by blending them with rubbers. However, if the materials are already cross-linked, then blending with rubber as it is done in the conventional way with thermoplastics, is virtually impossible. Thus it asks for an altogether different method to accomplish successful blending. Initially, miscible liquid rubbers in small amounts or preformed rubber particles are incorporated in the matrix of curing agent incorporated precured thermosets resins and then the whole mass is subjected to curing.
The phase separation, in case of liquid rubber toughening depends upon the formulation, processing and curing conditions and incomplete phase separation may occur resulting in unwanted lowering of glass transition temperature. The phase separation in case of liquid rubber is based upon nucleation and growth. In case of preformed rubber particles, these difficulties are not encountered and the resulting morphology can be better controlled.
However, the problem of proper dispersion of these particles in the themoset resins limits the use of this method. The improvement in fracture resistance occurs in either case due to dissipation of mechanical energy by cavitation of rubber particles followed by shear yielding of the matrix. Rubber particle size plays an important role in improving toughening and very small or very large sizes are undesirable.
The toughenability increases with increase in inherent ductility of the matrix. Blending of two or more elastomers is carried out for several purposes. The properties of an elastomer blend depend strongly on its state of compatibility and miscibility. In this chapter, recent advances on development of interphase modification and compatibilization of rubber-based blends are summarized.
Finally, new challenges and opportunities of rubber-based blends are given. IPNs are based on combinations of two or more polymers and are younger cousins to polymer blends, blocks and grafts. All these are members of a larger class of multicomponent polymeric systems, where as in IPNs, the polymers are crosslinked, thus providing a mechanism for controlling the domain sizes and reducing creep and flow.
Though the idea behind IPN synthesis is to effect molecular level interpenetration of the polymer networks, most IPNs form immiscible systems with phase separation during some stage of synthesis. The literature review shows that Sperling and coworkers at Lehigh university, USA followed by Frisch from University of Detroit and Frisch from Suny, Albany have made the most contributions to this research area. The current review on IPNs summarises the processing, properties and applications of IPNs, with special focus on some recent developments and trends. As a most general definition, filler is a finely divided solid that is used to modify the properties of a material in which it is dispersed.
From the inception of the rubber industry, fillers have a crucial role in either providing durability and performance or in reducing the price by decreasing the rubber partition in the compound. The fillers used in rubbers can be divided into two main groups such as black and non-black fillers. Besides the conventional micron size fillers, nanofillers recently have gained both academic and industrial importance. In this chapter, it is aimed to introduce the fillers used in rubbers.
Their characteristics and their impact on properties of rubbers are discussed by giving examples from the recently published literature. In addition to the conventional ones, the new emerging nano-fillers and their added value to the rubbers are given in detail by highlighting some selected studies. Magnetorheological elastomers MRE are smart materials whose modulus or mechanical performances can be controlled by an external magnetic field.
In this chapter, the current research on the MRE materials fabrication, performance characterisation, modelling and applications is reviewed and discussed. Either anistropic or isotropic or MRE materials are fabricated by different curing conditions where magnetic field is applied or not. Both steady-state and dynamic performances were studied through both experimental and theoretical approaches. The modelling approaches were developed to predict mechanical performances of MREs with both simple and complex structures.
The sensing capabilities of MREs under different loading conditions were also investigated.
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The review also includes recent representative MRE applications such as adaptive tuned vibration absorbers and novel force sensors. 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.
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. 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. 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.
Advances in Elastomers I : P. M. Visakh :
They have gained significant importance in. 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. Aryl-phosphine-oxide-containing materials show considerable promise for producing advanced organic materials with significantly improved flammability resistance. A new development is the possibility of bridging organic and inorganic materials to produce organic-inorganic composite networks. Elastomers, or rubbers, are soft and compliant polymers that are able to experience large, reversible deformations.
Only long-chain polymers are capable of this type of elasticity. Elastomers are typically amorphous, network polymers with lower cross-link density than thermoset plastics. Most thermosets can be made to function as elastomers above their glass transition temperatures. Historically, elastomers have played an important role in the industrialization, prosperity, and security of the United States.
Synthetic elastomers were born of necessity during World War II, when the United States was cut off from most of its supplies of natural rubber in Southeast Asia. Low-temperature emulsion polymerizations were developed to produce highly successful synthetic rubbers, particularly styrene-butadiene copolymers. In one of the most remarkable success stories in modern industry, a production capacity of 1.
This industry continues in the United States see Table 3. Annual production figures have declined in recent years owing to a number of factors, including advances in the use-life of tires. Originally, all elastomers were thermosets or chemically cross-linked materials, and so their flexibility in processing, especially reprocessing or recycling, was severely limited. Thermoplastic elastomers represent a current major growth area that comprises a growing number of chemical concepts. The first materials were styrene-based block copolymers that phase separate at the molecular level to produce relatively hard polystyrene domains, which act as temporary, physical.
The resulting elastomer is thermoplastic, and it is possible to reprocess it by simply heating it to above the glass transition temperature of polystyrene. It is thus a reprocessible elastomer. These materials are the result of the development of anionic polymerization methods, which are now practiced on a large scale in spite of the tremendous experimental difficulties associated with the organometallic initiators used in this process. Similar concepts have been implemented commercially for polyurethanes, polyesters, polyether-amides, and so on. Other versions are the so-called dynamically vulcanized blends of plastic and rubber that can be molded or extruded like thermoplastics.
Current characterization techniques do not permit probing all of the potentially critical structural issues of such complex materials. With regard to theory, there is a need for a better understanding of the topology of the network structure that is required for the recoverability exhibited by elastomers.
More specifically, we need to know how to characterize entanglements and their effects on mechanical properties. Such topological features would be expected to have large effects on both equilibrium and dynamic properties, and their control could help greatly in the design of more competitive elastomers.
Although the deepest insights into rubberlike elasticity will almost certainly come from molecular theories, phenomenological approaches are also frequently useful, particularly for practical purposes. These theories attempt to fit stress-strain data using a minimal number of parameters, which are then used to predict other mechanical properties. There is also a need for more experimental data on deformations other than simple elongation and swelling, which because of their simplicity are the ones used in the overwhelming majority of elasticity studies.
One benefit would be additional, discriminating data for evaluating elasticity theories. Another would be the better understanding of the properties of elastomers for conditions under which they are frequently used. An understanding of segmental orientation of chains in deformed networks is essential for an understanding of strain-induced crystallization. Such crystallization greatly enhances the mechanical properties of an elastomer, and its control could be of considerable competitive advantage.
Advances in theory, as well as additional experiments, are required for progress in this area. There is increasing interest in the study of elastomers that also exhibit mesomorphic behavior, from liquid crystalline entities either in the chain backbone or in the side chains. These materials combine some of the most intriguing properties of liquid crystalline molecules of low molecular weight with the elastomeric properties of polymeric networks.
Materials with this unique behavior should be exploited.
An example is the orientation of an anisotropic phase by a mechanical force, in analogy to the use of electric or magnetic fields on low-molecular-weight mesogens. A new subject in the area of rubberlike elasticity is the phenomenon of gel collapse, in which a swollen network abruptly deswells shrinks in response to a relatively small change in its environment.
The collapse can be triggered by. In films and fibers the collapse is rapid enough for these systems to have potential applications as mechanical switches, artificial muscle, mechanochemical engines, and so on. Exploiting this new development will require advances in theory, as well as additional experiments in actual functioning devices. A particularly challenging problem is the development of a more quantitative molecular understanding of the effects of filler particles, in particular carbon black in natural rubber and silica in siloxane polymers.
Such fillers provide tremendous reinforcement in elastomers in general, and how they do this is still poorly comprehended. Certainly the bonding between the reinforcing phase and the elastomeric matrix is critical. Investigation of the bonding between the biopolymer elastin and the collagen fibers that are threaded through it for reinforcement could provide valuable insights into this problem.
A related problem exists in the hybrid organic-inorganic composites. Finally, there is a need for more high-performance elastomers, which remain elastomeric to very low temperatures but are relatively stable at very high temperatures and resist hostile environments. The elastomeric ethylene-propylene-diene monomer EPDM rubbers, made by copolymerization of ethylene, propylene, and a diene using Ziegler catalysts, are particularly resistant to ozone.
The polysiloxanes are one of the most important classes of high-performance elastomers and are being developed and improved in most industrialized countries. The fluoroelastomers are another class that is under intense development. Polyphosphazenes have rather low glass transition temperatures in spite of the fact that the skeletal bonds of the chains are thought to have some double-bond character.
There are, thus, a number of interesting problems related to the elastomeric behavior of these unusual semi-inorganic polymers. A fiber may be defined as a structure whose length is much greater than its cross-sectional dimension. The diameter of fibers is characterized in dTex, a unit of linear density corresponding to the weight in grams of a 10,meter length of the fiber the same units are used to describe individual filaments and multifilament yarns.
Typical filament dTex values run from 1 to One gram of a 1-dTex filament is over 5 miles in length. The value of the U. Some typical applications of fiber are listed in Table 3. Over the past several decades, a number of trends have become evident in the commodity fiber business:. Synthetic fiber volumes have grown at the expense of natural fibers.
The drivers are lower costs and technical improvements, which allow the synthetics to emulate desirable natural fiber aesthetics while exhibiting superior in-use performance. The commodity markets are divided primarily among nylon, polyester, and polyolefin, with polyester emerging as the largest. Cost-performance and environmental considerations have led to a diminution in the use of cellulosics and acrylics. This same time period has seen the rapid growth of high-performance fiber technologies.
These technologies fall into three classes:. High-modulus, high-strength fibers based on rodlike, liquid crystalline nematogenic polymers.
Polyurethane/Epoxy Interpenetrating Polymer Network
The most common examples are the lyotropic aramids and the thermotropic polyesters. These fibers are characterized by tensile moduli greater than 70 gigapascals GPa , tensile strengths on the order of 3 to 4 GPa, and low properties in compression or shear. Morphological manipulation of conventional polymers, such as high-molecular-weight.
Polymeric precursor fibers that can be converted to other chemical forms after spinning. The most common examples are acrylic fibers that can be converted to carbon fibers and a variety of silicon-containing polymeric fibers that can be converted to silicon carbide or silicon nitride fibers. Typical applications of high-performance fibers are composite reinforcement, ropes and cables, and antiballistic clothing. As a group, these fibers represent successful technical developments, but they have proved less commercially attractive than once believed for a variety of reasons.
The spinning process can be described as follows. A polymer is first converted to a liquid through melting or dissolution, and the liquid is then continuously forced through a spinnerette a plate with many of small holes to form filaments. Most polymeric fibers are semicrystalline. If the polymer forms a stable melt, the process is called melt spinning. For polymers that degrade prior to melting, the polymer is spun from a solution; if the solvent is evaporated, the process is termed dry spinning; if the solution is coagulated in a nonsolvent bath, the process is termed wet spinning.
Removal of the spinnerette from the wet spinning coagulation bath is the innovation known as dry-jet wet spinning. The ratio of final filament velocity to the initial filament velocity is termed the drawdown ratio. The principal parameters controlling the as-spun structure and, hence, properties of the as-spun filament are the rate of cooling and the applied stress. Crystallinity once formed can be further oriented by stretching and perfected through annealing.
Key structural elements are the amount and orientation of crystalline regions, the orientation of noncrystalline regions, and connectivity between regions, tie molecules, and so on. Careful control of the sequence in which chains are oriented and crystallized has a profound effect on the microstructure produced. Such controlled processing allows, for example, the decoupling of crystalline and noncrystalline orientation, enabling fibers with high tensile modulus correlated with high crystalline orientation and low thermal shrinkage correlated with low noncrystalline orientation to be produced.
Typical spinning speeds are thousands of meters per minute, typical melt drawdowns are on the order of , and typical solid-state draw ratios range from about 2 to 6 in conventional processing to greater than 50 in the production of certain high-performance products. High-performance fiber processing is characterized by maximizing axial chain orientation and minimizing.
To control friction and static behavior in subsequent processing, a variety of oils or other surface treatments are applied to the fibers prior to take-up. The many complex processing steps of fibers add to the stress-temperature history of the fiber and hence significantly modify the end-use properties of the material.
To a large extent, the conditions employed in spinning, in addition to the particular chemistry of the polymer being spun, determine the end-use performance of a fiber. Work on future fibers will focus on producing cost-performance improvements and product variants through morphological control rather than new chemistries.
With the huge lengths of fibers produced, process robustness and property uniformity have always been major issues; future products will make more use of advanced computerized process control and will operate in areas of property response that are less sensitive to minor process variation.
Elimination of downstream process steps will lead to additional cost-performance improvements, for example, on-line texturing and surface modifications to meet specific friction or adhesion requirements. Environmental considerations will influence future fiber developments in a number of areas. The elimination of solvent-based processing will be driven by stricter emissions standards, as will the elimination of heavy metal catalysis.
Novel processes based on very fast melting techniques e. The reduction of off-specification production will become more important as the cost of waste disposal increases and as easy-to-reclaim fibers grow in importance e. The future of high-performance fibers lies in the reduction of costs and the improvement of utilization. The former is best influenced by lower-cost monomers, and the latter through the development of manufacturing technologies that allow cost-effective part production from fiber-reinforced composites.
High-performance fiber development will cease to be solely performance driven and will, as in the case of all other fibers, become driven by cost and performance. Silks, produced by worms and spiders, have attracted attention because they possess tensile properties similar to those of high-performance synthetic fibers but with much higher toughness. The use of recombinant DNA techniques allows silks of specific molecular architectures to be produced and their performance to be correlated with specific chemical and physical features.
The increased structure-property insights gained from these studies should allow the definition of biomimetic fibers, based on other than naturally occurring amino acids, with greatly improved performance characteristics. An adhesive is a material that, by means of surface attachment, can hold together solid materials. Adhesives have been used for most of recorded history. They are mentioned in Egyptian hieroglyphics, in the Bible, and in the writings of the early natural philosophers.
The physical strength of an assembly made by the use of adhesives, known as an adhesive joint, is due partly to the forces of adhesion, but primarily to the cohesive strength of the polymeric materials used to formulate the adhesive. Thus, the range of strengths available in adhesive joints is limited to the strengths of the polymers useful in the formulation of adhesives.
Indeed, the technology of adhesives tracks well with the technology of polymers. As new polymers were synthesized, new adhesives were developed that used those polymers. Adhesives are typically classified by their use or application. Thus structural adhesives are those materials used to join engineering materials such as metals, wood, and composites. Usually, it is expected that an adhesive joint made with a structural adhesive is capable of sustaining a stress load of 1, psi 6.
Hot melt adhesives are those adhesives that are applied from the melt and whose properties are attained when the adhesive solidifies. Pressure-sensitive adhesives provide adherence and strength with only finger pressure during application. Adhesive tapes are manufactured by applying a pressure-sensitive adhesive to a backing. Rubber-based adhesives are, as the name implies, based on elastomers and are usually applied as a mastic or spray applied from solvent or water. Pressure-sensitive adhesives can be considered to be a subset of rubber-based adhesives. The ease of application of pressure-sensitive adhesives is superior to all other types of adhesives except possibly hot melt adhesives.
Responsivity to finger pressure alone forming a bond is a desirable property, and pressure-sensitive adhesives of sufficient strength to perform structural tasks have been developed recently. One of the major uses of these double-coated foam tapes is to fasten most of the exterior and interior decorative and semistructural materials to the body of an automobile. The use of these foam tapes allows faster assembly and eliminates mechanical fasteners, which are a source of corrosion.
Each of the major classes of adhesives described above can be further classified by its chemistry. Thus, the majority of structural adhesives are based on one or more of the following chemistries: phenolic, epoxy, acrylic, bismaleimide, imide, and protein derived from blood, soybean, casein, and so on. The majority of hot melt adhesives are based on one or more of the following chemistries: waxy hydrocarbons, polyethylene, polypropylene, ethylene-vinyl acetate, polyamides, and polyesters. Rubber-based adhesives are, for the most part, formulated using neoprene, nitrile, and natural rubbers.
Pressure-sensitive adhesives are based on natural rubber, vinyl ethers, acrylics, silicones, and isoprene-styrene block co-polymers. Many paper-binding adhesives are based on dextrin or other. Adhesives have several advantages over other joining technologies. In general, adhesives have a lower density than mechanical fasteners, and so weight savings can be realized. Polymer-based adhesives have viscoelastic character and are thus capable of energy absorption.
The energy absorption manifests itself in the form of dampening of vibrations and in the increase of fatigue resistance of a joint. Adhesives can be used to join electrochemically dissimilar materials and provide a corrosion-resistant joint. Adhesive joining is limited by the fact that an engineering database is unavailable for most adhesive materials.
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The strength and durability of an adhesive bond are subject to the nature of the surfaces to be joined. Part of the reason industrial adhesives have been so successful is that methods have been found to clean and treat surfaces to form good bonds. A better understanding of proper surface preparation for adhesives is needed. The major limitations to the broader use of adhesives in industry are the extreme sensitivity of adhesive bonding to surface conditions and the lack of a nondestructive quality control method.
Adhesive technology can be solidly advanced by the synthesis of new monomers and polymers that extend the range of applicability of adhesive bonding. Thus, new materials should allow adhesives to be more flexible at cryogenic temperatures, more oxidation resistant at high temperatures, stronger at elevated temperatures, and more tolerant of an ill-prepared or low-surface-energy adherent. The engineering aspects of adhesive technology can be solidly advanced by including adhesive technology in university engineering courses and establishing an engineering database.
In addition, an easy, nondestructive method of predicting the strength of a joint would be a major advance in the applicability of adhesives. Two drivers of advances in adhesive technology in the near future are economics and the environment. To be environmentally acceptable, new adhesive formulations should contain a minimum of solvent and in some applications should be biodegradable. To be economically attractive, adhesives should be easy to use and should provide a value-added feature to the customer that outweighs the disadvantages cited above.
The time scale for introducing totally new polymers is increasing because the simplest monomers and the processes for converting them into polymers have already been identified and introduced into the marketplace. Furthermore, with increasing regulatory obstacles and the high cost of research, the economic stakes for introducing generically new polymers based on previously unknown chemistry and manufacturing processes have been raised considerably.
Because this field was initially dominated by the ready opportunities for chemical innovation, serious development based on the more physical approach of alloying or. Now, the area of polymer blends is one of the routes to new materials that is most actively pursued by the polymer industry. There are several driving forces for blending two or more existing polymers. Quite often, the goal is to achieve a material having a combination of the properties unique to each of the components, such as chemical resistance and toughness.
Another issue is cost reduction; a high-performance material can be blended with a lower-cost polymer to expand market opportunities. A third driving force for blending polymers of different types is addition of elastomeric materials to rigid and brittle polymers for the purpose of toughening. Such blends were the first commercial example of polymer blend technology and, even today, probably account for the largest volume of manufacturing of multicomponent polymer systems.
The main problem is that frequently when polymers are blended, many critical properties are severely depressed because of incompatibility. On the other hand, some blends yield more or less additive property responses, and others display certain levels of synergism. The problem is knowing how to predict in advance which will occur and how to remedy deficiencies.
From a fundamental point of view, one of the most interesting questions to ask about a blend of two polymers is whether they form a miscible mixture or solution. The thermodynamics of polymer blends is quite different from that of mixtures of low-molecular-weight materials, owing to their molecular size and the greater importance of compressibility effects. Because of these, miscibility of two polymers generally is driven by energetic rather than the usual entropy considerations that cause most low-molecular-weight materials to be soluble in one another. The simple theories predict that miscibility of blends is unlikely; however, recent research has shown that by carefully selecting or designing the component polymers there are many exceptions to this forecast.
The phase diagram for polymer blends is often opposite of what is found for solutions of low-molecular-weight compounds.
Advances in elastomers I : blends and interpenetrating networks
Polymers often phase separate on heating rather than on cooling as expected for compounds of low molecular weight. Theories to explain the behavior of miscible polymer blends have emerged, but theoretical guidance for predicting the responsible interactions is primitive. With the advent of modern computing power and software development, molecular mechanics calculations of this type are being attempted.
Neutron scattering has provided considerable insight about the thermodynamic behavior of blends and the processes of phase separation. One of the earliest blend products was a miscible mixture of poly phenylene oxide and polystyrene. The former is relatively expensive and rather difficult to process. The addition of polystyrene lowers the cost and makes processing easier. Numerous other commercial products are now based on miscible or partially miscible polymer pairs, including polycarbonate-polyester blends and high-performance ABS materials.
Frequently, the unfavorable polymer-polymer interactions that lead to immiscibility cause an unstable and uncontrolled morphology and a weak interface. These features translate into poor mechanical properties and low-value products, that is, incompatibility. When this is the case, strategies for achieving compatibility are sought, generally employing block or graft copolymers to be located at the interface, much like surfactants.
These copolymers can be formed separately and added to the blend or formed in situ by reactive coupling at the interface during processing. The former route has, for example, made it possible to make blends of polyethylene and polystyrene useful for certain packaging applications by addition of block copolymers formed via anionic synthesis. However, viable synthetic routes to block copolymers needed for most commercially interesting combinations of polymer pairs are not available. For this reason, the route of reactive compatibilization is especially attractive and is receiving a great deal of attention for development of commercial products.
It involves forming block or graft copolymers in situ during melt processing by reaction of functional groups. Extensive opportunities exist for developing schemes for compatibilization and for fundamental understanding of their mechanisms. A better understanding of polymer-polymer interactions and interfaces e.
Especially important is the development of experimental techniques and better theories for exploring the physics of block and graft copolymers at such interfaces. This knowledge must be integrated with a better understanding of the rheology and processing of multiphase polymeric materials so that the morphology and interfacial behavior of these materials can be controlled. A wide variety of compatibilized polymer alloys have been commercialized, and the area is experiencing a high rate of growth.
A product based on poly phenylene oxide , a polyamide, and an elastomer has been introduced for use in forming injection-molded automobile fenders and is currently being placed on several models of U. The polyamide confers toughness and chemical resistance, the poly phenylene oxide contributes resistance to the harsh thermal environment of automotive paint ovens, while the elastomer provides toughening. Another automotive application is the formation of plastic bumpers by injection molding of ternary blends of polycarbonate, poly butylene terephthalate , and a core shell emulsion-made elastomeric impact modifier Figure 3.
In this blend, the polycarbonate brings toughness, which is augmented at low temperatures by the impact modifier, while the poly butylene terephthalate brings the needed chemical resistance to survive contact with gasoline, oils, and greases. In the first example, the poly phenylene oxide and polyamide are very incompatible, and reactive coupling of the phases is required for morphology control and for interfacial strengthening. In the second example, the polycarbonate and polyester apparently interact well enough that no compatibilizer is needed.
They are chosen for their class-A surface, dimensional stability, impact strength, and corrosion and chemical resistance. The side claddings on these vehicles are molded of a resin that is a polyester-polycarbonate alloy, chosen for its cold temperature impact strength, chemical resistance, and quality surface. More than 60 pounds of engineering thermoplastics can be found on many of the vehicles. Toughening by the addition of rubber was first practiced for commodity polymers, such as polystyrene, poly vinyl chloride , polypropylene, and poly methyl methacrylate PMMA.
Widely different processes and product designs were required to achieve optimal products. Now this approach is being applied to engineering thermoplastics and thermosets in order to move these materials into applications that require stringent mechanical performance under demanding conditions.
This ensures an excellent growth opportunity for a variety of toughening agents.