Starting with billions of molecules of monomer in a reactor, heat and pressure are applied in the presence of catalysts, causing one of the monomer double bonds to rearrange into two "half-bonds," one at each end. These half-bonds combine with half bonds of other rearranged monomer molecules, forming stable "whole bonds" between them. As each monomer joins with others, the chain length grows until it meets a stray hydrogen, which combines with the reactive end, stopping chain growth at that point.
During the polymerization reaction, millions of separate polymer chains grow in length simultaneously, until all of the monomer is exhausted. By adding predetermined amounts of hydrogen (or other chain-stoppers), chemists can produce polymers having a fairly consistent average chain length. Chain length (molecular weight) is important because it determines many properties of a plastic, and it also affects its processing characteristics. The major effects of increasing chain length are increased toughness, creep resistance, stress-crack resistance, melt temperature, melt viscosity, and processing difficulty.
However, not all polymer molecules can be manufactured to an exact, specified length, so each batch will have an average molecular-weight distribution. There can be either a broad or a narrow spread between molecular weights of the largest and smallest molecules, and the polymer still could have the same average. A narrow distribution provides more uniform properties; a broad distribution makes a plastic easier to process.
After polymerization is completed, the finished polymer chains resemble long, intertwined bundles of spaghetti, with no physical connections between chains. Such a polymer is called a thermoplastic (heat-moldable) polymer.
Although there is no direct physical connection between individual thermoplastic chains, there is a weak electrostatic attraction (van der Waals force) between polymer chains that lie very close together. This intermolecular force, which tends to prevent chain movement, is heat sensitive, becoming stronger when the plastic is cold and weaker when it is hot. Heating a thermoplastic, therefore, weakens the intermolecular forces, allowing the polymer molecules to slide over each other freely during the molding process. Upon cooling, the forces become strong again and "freeze" the molecules together in the new shape.
Molding a thermoplastic is similar to molding candlewax in this respect. But if too much heat is applied or if the plastic is heated for too long a time, the molecular chains break, causing permanent property degradation -- particularly toughness. Continuous pressure (bending or deforming) on a molded part also causes the chains to slide over each other, resulting in creep, or cold flow, which can seriously affect part dimensions.
Strength of the intermolecular attractive force varies inversely with the sixth power of the distance between chains. Thus, as the distance is halved, the attractive force increases by a factor of 64. For this reason, chain shape is as important as chain length. If a polymer molecule has a symmetrical shape that can pack closely, the intermolecular forces are very large compared to those of a nonsymmetrical shape. Therefore, two kinds of polyethylene can have different physical properties because of the difference in their density, which depends on their ability to pack together. The molecules of high-density polyethylene have very few side branches to upset their symmetry, so they can approach adjacent molecules quite closely, resulting in high intermolecular attractive forces. Low-density polyethylene, on the other hand, contains many more side branches, which create nonsymmetrical areas of low density and, therefore, low intermolecular attraction.
Another consequence of denser molecular packing is higher crystallinity. As symmetrical molecules approach within a critical distance, crystals begin to form in the areas of densest packing. A crystallized area is stiffer and stronger; a noncrystallized (amorphous) area is tougher and more flexible. Other effects of increased crystallinity in a polyethylene polymer are increased resistance to creep, heat, and stress cracking, and increased mold shrinkage.
In general, crystalline polymers are more difficult to process, have higher melt temperatures and melt viscosities, and tend to shrink and warp more than amorphous polymers. They have a relatively sharp melting point; that is, they do not soften gradually with an increase in temperature. Rather, they remain hard until a given quantity of heat is absorbed, then change rapidly into a low-viscosity liquid. Reinforcement of crystalline polymers with fibers of glass or other materials improves their load-bearing capabilities significantly.
Amorphous polymers, on the other hand, soften gradually as they are heated, but they do not flow as easily (in a molding process) as do crystalline materials. Reinforcing fibers do not significantly improve the strength of amorphous materials at higher temperatures. Examples of amorphous thermoplastics are ABS, polystyrene, polycarbonate, polysulfone, and polyetherimide. Crystalline plastics include polyethylene, polypropylene, nylon, acetal, polyethersulfone, and polyetheretherketone.
Another method for altering molecular symmetry is to combine two different monomers in the polymerization reaction so that each polymer chain is composed partly of monomer A and partly of monomer B. A polymer made from two different monomers is called a copolymer; one made from three different monomers is called a terpolymer. All long, repeating chains are polymers, regardless of how many monomers are used. But when a polymer family includes copolymers, the term "homopolymer" is used to identify the single monomer type. An example is the acetal family; acetal resins are available both in homopolymer and copolymer types.
Final properties of a copolymer depend on the percentage of monomer A to monomer B, the properties of each, and on how they are arranged along the chain. The arrangement may alternate equally between the two monomers, producing a symmetrical shape capable of a high degree of crystallization. Or the arrangement may be random, creating areas of high crystallinity separated by flexible, amorphous areas. Such a copolymer usually has good rigidity and impact strength. Block copolymers have large areas of polymerized monomer A alternating with large areas of polymerized monomer B. In general, a block copolymer is similar to an alternating copolymer except that is has stronger crystalline areas and tougher amorphous areas. If both types of blocks are crystalline, or both amorphous, a wide variety of end products is possible, having characteristics ranging from hard, brittle plastics to soft, flexible elastomers. A graft copolymer is made by attaching side groups of monomer B to a main chain of monomer A. A copolymer having a flexible polymer for the main chain and grafted rigid side chains is very stiff, yet has excellent resistance to impact -- a combination of properties not usually found in the same plastic. Copolymers always have different properties from those of a homopolymer made from either monomer.
Compounders of plastics modify properties of a thermoplastic material by many other methods as well. For example, fibers are added to increase strength and stiffness, plasticizers for flexibility, lubricants for easier molding or for increasing lubricity of the molded parts, antioxidants for higher temperature stability, UV stabilizers for resistance to sunlight, and fillers for economy. Other additives such as flame retardants, smoke suppressants, and conductive fibers or flakes provide special properties for certain applications. Plastic compounds can be varied widely as to type and amount of these additives, and every modification produces a compound with different properties. Examples of thermoplastic products are polyethylene squeeze bottles, nylon gears and rollers, acrylic lenses, ABS (acrylonitrile-butadiene-styrene) business-machine and appliance housings, polystyrene-foam cups, polycarbonate safety helmets and glazing sheet for bus windows, and polyphenylene sulfide chemical pumps and automotive underhood components.
Thermoset plastics are made quite differently from thermoplastics. Polymerization (curing) of thermoset plastics is done in two stages, partly by the material supplier and partly by the molder. For example, phenolic (a typical thermoset plastic) is first partially polymerized by reacting phenol with formaldehyde under heat and pressure. The reaction is stopped at the point where mostly linear chains have been formed. The linear chains still contain unreacted portions, which are capable of flowing under heat and pressure.
The final stage of polymerization is completed in the molding press, where the partially reacted phenolic is liquefied under pressure, producing a crosslinking reaction between molecular chains. Unlike a thermoplastic monomer, which has only two reactive ends for linear chain growth, a thermoset monomer must have three or more reactive ends so that its molecular chains crosslink in three dimensions. Rigid thermosets have short chains with many crosslinks; flexible thermosets have longer chains with fewer crosslinks.
After it has been molded, a thermoset plastic has virtually all of its molecules interconnected with strong, permanent, physical bonds, which are not heat reversible. Theoretically, the entire molded thermoset part could be a single giant molecule. In a sense, curing a thermoset is like cooking an egg. Once it is cooked, reheating does not cause remelting, so it cannot be remolded. But if a thermoset is heated too much or too long, the chains break and properties are degraded.
Phenolic, urea, and melamine thermoset plastics are polymerized by a "condensation" reaction, wherein a by-product (water, for example) is created during the reaction in the mold. Such volatile by-products cause dimensional instability and low part strength unless they are removed during molding.
Other thermoset plastics, such as epoxy and polyester, cure by an "addition" reaction, resulting in no volatile by-products and fewer molding problems. Most addition-cured thermoset plastics are liquid at room temperature; the two ingredients can simply be mixed and poured into molds where they crosslink (cure) at room temperature into permanent form -- much like casting concrete. Molds are often heated, however, to speed the curing process.
In general, thermoset plastics, because of their tightly crosslinked structure, resist higher temperatures and provide greater dimensional stability than do most thermoplastics. Examples of thermoset plastic products include glass-reinforced-polyester boat hulls and circuit-breaker components, epoxy printed-circuit boards, and melamine dinnerware.
Alloys and blends
Yet another way to create more variations in plastics is alloying -- an effective and economical method to improve a weak property in a base resin. Plastic alloys, also called blends or polyblends, are usually designed to retain the best characteristics of each material. Properties that have been found to be most responsive to improvement are impact strength, heat-deflection temperature, flame retardance, chemical and weather resistance, and processibility. A study by Battelle reports that research on polymer blending generates about 1,000 patents per year.
Although no totally accepted definition exists, most engineers and chemists in the plastics industry agree that a plastic alloy is identified by most of these characteristics:
- The combination of polymers does not depend on chemical bonds; the mixture is entirely mechanical. Thus, copolymers (some acetals and polyolefins) and terpolymers are not alloys. Nor are epoxy compounds that copolymerize with hardeners that contribute to the properties of the cured resins.
- The mixture has a single melt-transition temperature.
- At least one property or characteristic of the base polymer is improved synergistically by the addition of the other polymer(s). The property may be physical or mechanical, or the improvement may be in processibility or cost. If synergistic improvement is not achieved, at least the best properties of all constituents are retained.
- Each minor component of a plastic alloy constitutes at least 5% of the mix. Many are nearer the 50:50 range. This "requirement" differs considerably from those involving metal alloys. There, only enough of an alloying element need be present to effect a change in a mechanical or physical property. The magnitude of such a change is not important.
A new technology that combines incompatible polymers to form blends called interpenetrating polymer networks (IPNs) promises to provide cost/performance benefits not previously available in engineering plastics. Several companies are working on IPN development, but only a few have made developmental IPN materials available.
IPNs consist of an interwoven matrix of two polymers. A typical method for producing these alloys involves crosslinking one of the monomers in the presence of the other polymer. The need for chemical similarity between the two types of molecules is thus reduced because the crosslinking physically traps one phase within the other. The result is a structure composed of two different materials intertwined together, each retaining its own physical characteristics. The relationship is similar to that between small blood vessels and the surrounding tissue in the human body.
Patented IPN technology by Shell Chemical Co. is based on the capability of the company's Kraton G thermoplastic elastomer (styrene-butadiene-block copolymer) to form stable and reproducible structures when properly mixed in the melt stage. The blends provide properties of the individual phases of the mixture and have few or no property losses that might be expected from combining incompatible materials. The results are materials having the best performance features of both an engineering thermoplastic and a thermoplastic rubber.
Although Shell was the first to come out with injection-moldable IPNs, Allied and Du Pont are also working on similar materials. Research at Allied-Signal Corp., for example, is focused on combining the toughness of thermoplastics with the solvent resistance, heat resistance, and dimensional stability of thermoset resins. Allied researchers feel that, when the IPNs become fully commercial, they will likely be processed by reaction-injection molding (RIM), by injection molding with a postmolding oven cure, or by a sheet-molding process followed by a cure cycle.
Other current thermoplastic IPN technology is based on crystalline resins such as nylon 6, 6/6, and 6/10, PBT, acetal, and polypropylene, with silicone as the IPN. These IPNs can be further modified with reinforcements and lubricants such as glass or carbon fibers and PTFE. The silicone IPN functions as a nonmigratory silicone lubricant, release agent, and flow modifier. It also plays an important role in controlling shrinkage and warpage of resins and composites. IPNs provide excellent wear performance in gears and bearings and superior dimensional control in parts such as keyboard frames for electronic typewriters.
Another way to improve the compatibility of two dissimilar polymers often involved a third material. The "compatibilizer" material is a grafted copolymer consisting of one of the principal components and a material similar to the other principal component. The functional groups in segment A of the third material will have an affinity for the polymer produced from monomer A, and the functional groups in segment B an affinity for polymer B. The mechanism is similar to that of soap improving the solubility of a greasy substance in water. The soap contains components that are compatible with both substances.