Plastic dictionary

Plastic Materials -
Plastic materials, often called resins, are made up of many repeating groups of atoms or molecules linked in long chains (called polymers) that combine elements such as oxygen, hydrogen, nitrogen, carbon, silicon, fluorine, and sulfur. Both the lengths of the chains and the mechanisms that bond the links of the chains are related directly to the mechanical and physical properties of the materials.

There are two main groups: thermoplastics and thermosets.

Thermoplastic materials become soft and moldable when heated and change back to solid when allowed to cool. Examples of thermoplastics are acetal, acrylic, cellulose acetate, nylon, polyethylene, polystyrene, vinyl, and nylon. Thermoplastic materials that are flexible even when cool are known as thermoplastic elastomers or TPEs. When thermoplastic materials are heated, the linked chains of molecules can move relative to each other, allowing the mass to flow into a different shape. Cooling prevents further flow. Although the heating/cooling cycle can be repeated, recycling reduces mechanical properties and appearance.

Thermoset plastics such as amino, epoxy, phenolic, and unsaturated polyesters, are so named because they experience a chemical change during processing and become hard solids. Although the structures of thermoset materials are similar to those of thermoplastic materials, processing develops permanent cross-links between adjacent molecules, forming complex networks that prevent relative movement between the chains at any temperature. Many rubbers that are processed by vulcanizing, such as butyl, latex, neoprene, nitrile, polyurethane, and silicone, also are classified as thermosets. Heating a thermoset degrades the material so that it cannot be reprocessed satisfactorily.

Elastomers are flexible materials that can be stretched up to about double their length at room temperature and can return to their original length when released. Thermoplastic elastomers are often used in place of rubber, and may also be used as additives to improve the impact strength of rigid thermoplastics.

Thermoplastics can be classified by their structures into categories such as amorphous (noncrystalline), crystalline, and liquid crystalline polymers (LCP).

Amorphous thermoplastics include polycarbonate, polystyrene, ABS (acrylonitrile-butadiene-styrene), SAN (styrene-acrylonitrile), and PVC (polyvinylchloride).

Crystalline thermoplastics have polymer chains that are packed together in an organized way, unlike the unorganized structures of amorphous plastics, they include acetal, nylon, polyethylene, polypropylene, and polyester. The organized regions in crystalline thermoplastics are joined by noncrystalline (amorphous) zones, and the structure is such that the materials are stronger and stiffer, though less impact resistant than completely noncrystalline materials. Crystalline thermoplastics have higher melting temperatures, higher shrinkage and warpage factors than amorphous plastics.

Liquid crystalline plastics (LCP's) are polymers with highly ordered rod-like structures and posses high mechanical property values, good dimensional stability, good chemical resistance and are easy to process.  The melting temperatures are similar to those of crystalline plastics. Unlike amorphous and crystalline plastics, liquid crystalline plastics retain significant order in the melt phase. As a result, they have the lowest shrinkage and warpage of the three types of thermoplastics.

Mixtures -  Characteristics of plastics materials can be changed by mixing or combining different types of polymers and by adding nonplastics materials. Particulate fillers such as wood, flour, silica, sand, ceramic, carbon powder, tiny glass balls, and powdered metal are added to increase modulus and electrical conductivity, to improve resistance to heat or ultraviolet light and to reduce cost.  Plasticizers are added to decrease modulus and increase flexibility. Other additives may be used to increase resistance to ultraviolet light and heat or to prevent oxidation.

Reinforcing fibers of glass, carbon, or Aramid (aromatic polyamide fibers having high tensile strength, a range of moduli, good toughness, and stress-strain behavior similar to that of metals) are added to improve mechanical properties. Careful design and selection must be used to position the fibers so that they will provide the required strength where it is needed. Continuous fiber may be positioned carefully in either a thermoplastics or thermoset matrix to produce basic parts generally called composites.  These products have the highest mechanical properties and cost of the reinforced plastics.

Copolymers embody two or more different polymers and may have properties that are completely different from those of the individual polymers (homopolymers) from which they are made. An approach known as alloying consists of pure mechanical blending of two or more different polymers, often with special additives to make them compatible. These "alloys" are compounded so as to retain the most desirable characteristics of each constituent, especially in impact strength and flame resistance. However, properties usually are intermediate between those of the constituent materials.