The polymer chains in thermoplastics are linear or branched and do not become cross-linked as in the case of thermosets. While there are many different chemical types of thermoplastics, those made from only four monomers ethylene, propylene, styrene, and vinyl chloride account for about 90 percent of all thermoplastics produced in the United States Figure 3. Of these four types, polypropylene has grown most rapidly in recent years—production has increased eightfold over the past two decades. Thermoplastic polyesters, primarily PET, are growing even more rapidly at the present time driven mainly by.
Future activities will focus strongly on recycling.
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In the case of PET, recycling can be accomplished by chemical depolymerization to monomers or oligomers followed by repolymerization to PET or other products. Such processes are currently in use for products that come into contact with food, while simple reprocessing is used for less critical products. The so-called engineering thermoplastics, which include the higher-performance, more expensive polymers such as the polyacetals, polycarbonates, nylons, polyesters, polysulfones, polyetherimides, some acrylonitrile butadiene styrene ABS materials, and so on, have generally exhibited stronger growth than the commodity plastics see Table 3.
These materials generally have greater heat resistance and better mechanical properties than the less expensive commodity thermoplastics and, therefore, are used in more demanding applications, such as aircraft, automobiles, and appliances. A major area of development is.
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TABLE 3. The area of blends and alloys is reviewed separately below. New products and advances in processes have resulted from the ring-opening polymerization of cyclic oligomers; for example, new developments in polycarbonates are particularly noteworthy. Other new products can be expected based on copolymers, and entirely new polymers are under development.
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A further category sometimes referred to as high-performance engineering thermoplastics commands even higher prices for yet higher levels of performance. These include highly aromatic polymers such as poly phenylene sulfide , several new polyamides, polysulfones, and polyetherketones. Development of new molecular structures has dominated this sector. Polymer chains with quite rigid backbones have liquid crystalline order, which offers unique structural properties as described below. Figure 3. Approximately one-third are used in packaging, primarily containers and film. The data in Figure 3.
To understand the diversity of products and opportunities that is possible, it is useful to review developments that have occurred in thermoplastics based on ethylene, one of the simplest monomers possible. Commercial production of polyethylene commenced in England during the early s using a free radical process operating at very high pressures 30, to 50, psi. The structure proved to be far more complex than the simple textbook repeat unit, —CH 2 CH 2 —, would suggest Figure 3. The backbone has short-and long-chain branches. The short-chain branches, typically four carbons long, interfere with the ability.
Because the short-chain branches reduce crystallinity and, thus, density, this material is called low-density polyethylene LDPE.
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In the late s, a linear or unbranched form of polyethylene was developed as a result of advances in coordination polymerization catalysis. An accidental finding by K. Ziegler in the early s at the Max Planck Institute of Mulheim, Germany, resulted in a fundamentally new approach to polyolefins. It was found that transition metal complexes could catalyze the polymerization of ethylene under mild conditions to produce linear chains with more controlled structures.
As a result, this polymer was more crystalline with higher density, and it became known as high-density polyethylene HDPE. Similar catalytic procedures were used by G. Natta to produce crystalline polypropylene. The properties of this polymer are a result of unprecedented control of the stereochemistry of polymerization. The newer material did not replace the older one; it was used for different purposes.
The cost factor plus innovations in. It is a copolymer of ethylene and an alpha-olefin like butene-1, hexene Thus, short-chain branches of controlled length and number are introduced into the chain without any long-chain branches, and the material is called linear low-density polyethylene LLDPE; see Figure 3. As a result, the production of LDPE initially declined, but its production has been growing again since Construction of new high-pressure production facilities may be required in the next decade to meet demands.
Currently this is the only process by which copolymers can be made with polar monomers such as vinyl acetate or acrylic acid.
HDPE is fabricated primarily by molding. Blow-molded food bottles and auto gasoline tanks constitute major markets. Very large containers made by rotational molding represent a specialized growth area. A process known as "gel spinning" has been commercialized, which produces fibers of ultrahigh-molecular-weight polyethylene. New technology based on single-site metallocenes holds promise for the production of a new range of products. This brief review of the history and future prospects for olefin polymers illustrates the need for research of all types e. These materials are complex in terms of molecular structure, and so there are many ways to tailor their behavior provided the basic knowledge and tools for structural determination are available and are integrated with innovative process technology.
Much of the present research is directed toward the design of catalysts that yield materials that are easier to process. Rapid progress has resulted from an integration of catalyst synthesis and reactor and process design. As a recent example, a new polyolefin alloy product has been developed by exposing a designed catalyst to a series of different olefin monomer feeds to produce a polymer particle that is composed of polymers with different properties.
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Extrusion of those particles results directly in a polymer alloy. Structural thermoplastics are a vital part of the national economy, and considerable opportunity remains for economic growth and scientific inquiry. New specialized materials will continue to offer rewards in the marketplace. At the high-performance end, several entirely new polymer structures are likely to emerge over the next decade. A major part of the growth in "new" materials will be in the area of blends or alloys.
The vitality of thermoplastics cannot be judged only on the basis of the introduction of what might be called "new materials. 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.
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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.
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.
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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.
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