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GAMMA TRANSITION; GLASSY TRANSITION. The change in an amorphous polymer or in an amorphous region of a partially crystalline polymer from (or to) a viscous or rubbery condition to (or from) a hard and relatively brittle one. This transition generally occurs over a relatively narrow temperature region and is similar to the solidification of a liquid to a glassy state; it is not like a first order phase transformation where discontinuities occur. Not only do hardness and brittleness undergo rapid changes in this temperature region, but other properties such as thermal expansibility and specific heat also change rapidly. This phenomenon has been called second-order transition, glass transition, rubber transition, and rubbery transition. The word transformation has also been used instead of transition.
GAMMA-TRANSITION TEMPERATURE; GLASSY-TRANSITION TEMPERATURE. The temperature region in which the gamma or glassy transition occurs. The measured value of gamma- or glassy-transition temperature depends to some extent on the details of the method of test. (See GAMMA TRANSITION.)
GATE. In injection and transfer molding, the gate is the orifice through which the melt enters the cavity.
GEL. (1) A semisolid system consisting of a network of solid aggregates, in which liquid is held. (2) The initial jelly-like solid phase which develops during the formation of a resin from a liquid. Both types of gel have very low strengths and do not flow like a liquid. They are soft and flexible, and will rupture under their own weight unless supported externally.
GELATION. (1) Formation of a gel. (2) In vinyl dispersions, formation of gel in the early stages of fusion.
GEL POINT. The stage at which a liquid begins to exhibit pseudoelastic properties. This stage may be conveniently observed from the inflection point on a viscosity-time plot. (See GEL.)
GLASS. An inorganic product of fusion which has cooled to a rigid condition without crystallizing. Glass is typically hard and relatively brittle, and has a conchoidal fracture.
GLASS FINISH. A material applied to the surface of a glass reinforcement to improve its effect upon the physical properties of the reinforced plastic.
GLASS TRANSITION. (See GAMMA TRANSITION.)
GLASS TRANSITION TEMPERATURE. A reversible change that occurs in an amorphous polymer when it reaches a certain temperature range in which the material undergoes a transition from a hard, brittle, glassy state to a flexible condition. At the glass transition temperature, the polymer chains become free to rotate and to slide past each other. Above the so-called "glass temperature,'' the material acts like a viscous liquid; below the glass temperature, it behaves like a solid.
GLUE. Originally, a hard gelatin obtained from hides, tendons, cartilage, bones, etc., of animals. Also an adhesive prepared from this substance by heating with water. Through general use the term is now synonymous with the term adhesive. (See ADHESIVE, SIZING.)
GLUE JOINT. That part of an aggregated product which comprises the adhesive (or glue) and the parts in contact therewith.
GRANULAR STRUCTURE. Apparent incomplete fusion of, and at least partial retention of, their original form by the particles from which a plastic is formed.
GRID. Channel shaped mold-supporting members.
GRINDING-TYPE RESIN. A vinyl resin which requires grinding to effect dispersion in plastisols or organosols.
GUIDE PIN. Pin which assures alignment of the mold halves.
GUIDE-PIN BUSHING. A guiding bushing through which the leader pin moves.
GUM. Any of a class of colloidal substances, exuded by or prepared from plants sticky when moist, composed of complex carbohydrates and organic acids, which are soluble or swell in water. The term gum is sometimes used loosely to denote various materials that exhibit gummy characteristics under certain conditions, for example, gum balata, gum benzion, and gum asphaltum. Gums are included by some in the category of natural resins. (See ADHESIVE, GLUE, RESIN.)
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