Plastics Definitions and TermsAll 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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