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Plastics Definitions and Terms

All definitions are given only within the context of the field of polymers.

ADDITIVE - A compound or substance added to a polymer during the final synthesis stages or in subsequent processing to improve or alter some characteristic of the polymer. Additives as a class of materials are not intended to increase strength properties. Examples: Pigments, lubricants, anti-stats, flame retardants, and plasticizers. (See REINFORCEMENT and FILLER)

ALLOY (also BLEND) - A mixture of two chemically different polymers to form a homogenous substance having properties different but often a combination of the two original polymers. An alloy differs from a copolymer in that any polymer molecule in the mixture is representative of only one monomer. In a copolymer, all molecules have units of both monomers. Alloys are often referred to as blends or hybrids.

AROMATIC - A Description used for chemicals that have at least one ring structure derived from benzene in their chemical structure. Benzene rings are made by six carbon atoms forming a hexagonal structure with alternating single and double bonds (three of each). The description is very general and covers a wide range of chemicals. The word "aromatic" is used because of the strong offensive smell of benzene it its raw state. Many of the chemicals classified as aromatic have a very different smell or no smell at all. A benzene ring structure with one bonding site is a "phenyl" ring or group. (See BENZENE RING)

ASPECT RATIO - The relative comparison of one dimension of an object to another. For fibers, the aspect ratio is the length divided by the diameter. For mica, it is the the shorter of the length and width of a platelet to its thickness. For complex objects like a particle of clay, it is a relative number approximating the ratio of the longer two dimensions to the shorter. This ratio is key in how effective a reinforcement is within a matrix of polymer molecules. Given uniform composition and coupling agents, a higher aspect ratio reinforcement will result in a higher increase in strength. There is a limit after which the effect is insignificant, but it varies with type or reinforcement and polymer. Aspect ratio is a variable in determining how much stress can be transferred to the fibers or platelets and diluted before being transferred back into the weaker polymer matrix.

ATOM - The most basic compositional unit of the elements composed of protons, electrons, and neutrons. Elements are any substance composed solely of chemically identical atoms. Carbon, oxygen, sulfur, nitrogen, iron, aluminum, silver and gold, are but a few of over 100 known elements. New elements are still being discovered.


BENZENE RING - A chemical structure composed of six carbon atoms arranged in a stable cyclic structure. Each carbon atom is single bonded to the next carbon atom on one side, and double bonded to the carbon atom on the other side. Each also has a hydrogen atom bonded to it. Phenyl groups are benzene rings where one of the carbon atoms is bonded to another molecule, making the entire cyclic structure a substituent or side group of that molecule. Phenyl groups are relatively large and bulky, and generally have a significant influence on the properties of the polymer they are present in.

BONDS or BONDING - Forces between atoms which hold them in relative proximity to each other resulting in larger structures called molecules (organic materials only). Primary bonds are between atoms of the same molecule and are strongest. They result from the sharing of two electrons by both atoms. Secondary bonds are between atoms of different molecules or remote sections of the same molecule. They are the result of attractions due to polarity, induced polarity due to displaced electrons, and temporary polarity due to vibration and spinning. These bond forces are weak in comparison to primary bonds.


COPOLYMER - A polymer composed of two different monomers where the repeating structural units of both are present within each molecule. The frequency and order of the units can vary as in the three most common types: random, block, and alternating. The end product generally has properties intermittent between those of polymers of the two composing monomers. The two materials must have compatibility to be copolymerized.

COVALENT BOND - A bond where one or pairs of electrons are equally shared between two atoms producing a stable electron configuration and a very stable molecule. Covalent are the strongest of the molecular bonds.

CRAZING - A series of or the forming of very fine cracks in the surface of a material, usually a polymeric substance. Crazing is generally caused by chemical attack or other degrading agents such as ultraviolet radiation. (See "STRESS CRACKING")


DEGREE OF POLYMERIZATION (DP) - The number of repeat units in the chain of a molecule. In a condensation polymer a repeating unit is composed of a monomer group from each reactive species.

DIELECTRIC CONSTANT - The comparison of the capacitance of an insulating material to that of air. Capacitance is the ability of a material to store electrical charge when exposed to electrical current. A low dielectric constant is desired for plastic components used to insulate and isolate electrical components from each other. High dielectric constant materials are desirable for use as the insulator portion of capacitors, so that the electrical energy can be stored in as small a volume of material as possible. FILLER (also EXTENDER) - A compound or substance added to a polymer during the initial synthesis process or in subsequent processing to decrease the volume of resin needed to produce a given product. Fillers are generally much lower in cost than the resins they are used in, thus reducing resin cost per part. Fillers or extenders are generally not used with engineering resins. (See ADDITIVE and REINFORCEMENT)


GLASS TRANSITION TEMPERATURE (Tg) - The temperature at which a material's characteristics change from that of a glass to that of rubber. This temperature varies widely for thermoplastics. Some may have Tg below zero such as polyethylene and polypropylene but still have plastic characteristics due to other factors such as crystallinity. Below their Tg these polymers become quite brittle. (See Section Five, "Viscoelasticity of Polymers")

GRADES - Refers to polymers which belong to the same chemical family, and are produced by the same manufacturer. They may vary in processing or performance due to differences is molecular weight, additives, or other structural features. For example, a supplier of polycarbonate may have flame resistant grades, glass fiber reinforced grades, a conductive grade, and easy flowing grades.


MOLECULAR WEIGHT DISTRIBUTION - Statistically describes the sizes and frequency of occurance of different molecular chain lengths within a given sample or lot of polymer. Wide and skewed distributions result significant variance in properties. Narrow distributions are more consistent.

MOLECULE - A group of atoms bonded together which forms the fundamental structural unit of most organic substances. The number of atoms can range from two to millions. A molecule is the smallest unit of a substance which still retains the properties of that substance.

MONOMER - Small molecules of an organic substance which are the most basic structural unit of polymers. Monomers are generally gases or liguids. When bonded together in long chains they form solid materials or polymers.


ORGANIC - Refers to a general class of substances whose composition is based on the element carbon. Organic infers some relationship to materials which at some point in time were alive.


PHENYL GROUP - (See BENZENE RING or AROMATIC)

PLASTIC - A synthetic or naturally occuring organic substance generally characterized by being formable or pliable at some stage during its formation or subsequent manufacturing process. Many materials, such as glass, become plastic under the right conditions.

POLYMER - A substance formed by a chemical reaction in which two or more small organic units join to form large units composed of repeating small units. This term is often used interchangably with "plastic". (See - PLASTIC) POLYMERIZATION - The process or chemical reaction by which low molecular weight monomers are converted to high molecular weight polymers. Most polymerization processes are classified as condensation (step) reactions or addition (chain) reactions.


REINFORCEMENT - A substance or material added to a polymer during the final synthesis stages or in subsequent processing to improve the strength properties of the polymer. Examples: Glass fibers, carbon fibers, glass beads, mica, clay, and organic fibers. (See ADDITIVE and FILLER)

RESIN - Any of a large class of synthetic substances that have some of the properties of natural resin (or rosin) but differ chemically. "Resin" is often used as a general term for polymers or plastics, and denotes a class of material.

STRESS CRACKING - A series of or the process of cracking under induced mechanical stress. Stress cracking generally initiates with microscopic surface cracks causes by chemical attack or other degrading agent such as ultraviolet radiation. Under mechanical stress, the microcracks propagate eventually producing a localized failure.


TERPOLYMER - A polymer composed of three different monomers where the repeating structural units of all three are present within each molecule. The influence of all three types of monomer are evident in the property profile of the polymer. Common terpolymers include ABS and ASA (Acrylonitrile/Styrene/Acrylic)


VISCOSITY - A measure of a material's resistence to flow when a mechanical stress is applied. Viscosity is quantitatively defined in terms of shear stress and shear rate.



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