Absolute Viscosity - The tangential force on unit area of either of two parallel planes at unit distance apart when the space between the planes is filled with the fluid in question and one of the planes moves with unit differential velocity in its own plane. The C.G.S. unit for absolute (or dynamic) viscosity is poise (dyne-sec./sq. cm.). Centipoise (0.01) is often used.

Accelerator - A substance that hastens a reaction, particularly one which speeds up the vulcanization of rubber. Also known as Promoter.

Accumulator - A term used mainly with reference to blow molding equipment which designates an auxiliary ream extruder which is used to provide extremely fast parison delivery. The accumulator cylinder is filled with plasticated melt coming from the extruder between parison deliveries or "shots" and is stored or "accumulated" until the plunger is required to deliver the next parison.

Acrylic Resin - A synthetic resin prepared from acrylic acid or from a derivative of acrylic acid.

Acrylonitrile - A monomer with the structure (CH2:CHCN). It is most useful in copolymers. Its copolymer with butadiene is nitrile rubber, and several copolymers with styrene exist that are tougher than polystyrene. It is also used as a synthetic fiber and as a chemical intermediate.

Acrylontrile Butadiene Styrene (abbreviated ABS) - Acrylonytrile and styrene liquids and butadiene gas are polymerized together in a variety of ratios to produce the family of ABS resins.

Adhesion Promoter - A coating which is applied to the substrate before it is extrusion coated with the plastic and which improves the adhesion of the plastic to the substrate.

Adhesive - A substance which applied as an intermediate is capable of holding materials together by surface attachment.

Affinity - The attraction for another substance.

Aging - The change of a material with time under defined environmental conditions, leading to improvement or deterioration of properties.

Air Gap - In extrusion coating, the distance from the die opening to the tip formed by the pressure roll and chill roll.

Air Ring - A circular manifold used to distribute an even flow of the cooling medium, air, onto a hollow tubular form passing through the center of the ring. In blown tubing, the air cools the tubing uniformly to provide uniform film thickness.

Air-assist Forming - A method of thermoforming in which a flow of air pressure is employed to partially pre-form the sheet immediately prior to the final pull down onto the mold using vacuum.

Alkyd Resin - Polyester resins made with some fatty acid as a modifier.

Alkyl - A general term for monovalent aliphatic hydrocarbon radicals.

Alloy - Composite material made up by blending polymers or copolymers with other polymers or elastomers under selected conditions, e.g., styrene-acrylonitryle copolymer resins blended with butadiene-acrylonitrile rubbers.

Allyl Resin - A synthetic resin formed by the polymerization of chemical compounds containing the group CH2=CH-CH2-. The principal commercial allyl resin is a casting material that yields allyl carbonate polymer.

Alpha-cellulose - A very pure cellulose prepared by special chemical treatment.

Amorphous Phase - Devoid of crystallinity - no definite order. At processing temperatures, the plastic is normally in the amorphous state.

Angle Press - A hydraulic molding press equipped with horizontal and vertical rams, and specially designed for the production of complex moldings containing deep undercuts.

Annealing - A process of holding a material at a temperature mean, but below its melting point, the objective being to permit stress relaxation without distortion of shape. It is often used on molded articles to relieve stresses set up by flow into the molds.

Anti-friction Compounds - Materials specifically formulated to reduce or eliminate friction.

Antioxidant - Substance which prevents or slows down oxidation of material exposed to air.

Antistatic Agents - Methods of minimising static electricity in plastic materials. Such agents are of two basic types: (1) metallic devices which come into contact with the plastics and conduct the static to earth. Such devices give complete neutralization at the time, but because they do not modify the surface of the material it can become prone to further static during subsequent handling; (2) chemical additives which, mixed with the compound during processing, give a reasonable degree of protection to the finished products.

Arc Resistance - Time required for a given electrical current to render the surface of a material conductive because of carbonization by an arc flame.

Aromatic Hydrocarbons - Hydrocarbons derived from or characterized by presence of unsaturated resonant ring structures.

Asbestos - A gray, non-burning, non-conductive and chemical resistant material occurring in long fibers or fibrous masses, sometimes used as a filler for reinforcement.

Autoclave - Closed strong vessel for conducting chemical reactions under high pressure; (2) in low-pressure laminating, a round or cylindrical container in which heat and gas pressure can be applied to resin-impregnated paper or fabric positioned in layers over a mold.

Autoclave Molding - Modification of the pressure bag method for molding reinforced plastics. After lay-up, entire assembly is placed in steam autoclave at 50 to 100 psi. Additional pressure achieves higher reinforcement loadings and improved removal of air.

Automatic mold - A mold for injection or compression molding that repeatedly goes through the entire cycle, including ejection, without human assistance.

Average Molecular Weight (viscosity method) - The molecular weight of polymeric materials determined by the viscosity of the polymer in solution at a specific temperature. This gives an average molecular weight of the molecular chains in the polymer independent of specific chain length. Falls between weight average and number average molecular weight.