APPENDIX F GLOSSARY[29]

APPENDIX F GLOSSARY[29]

Alkyd resin.—Any condensation product involving a polybasic acid and a polyhydric alcohol. Typical examples are phthalic glyceride and its modifications containing combined fatty acids or rosin. Representative examples are Rezyls and Glyptal.

Aminoplast.—General terms for synthetic resins from amino or amido compounds. A typical example is urea-formaldehyde.

Amorphous.—Devoid of crystalline structure. This condition is rare. Many substances which are apparently amorphous show microcrystallinity, particularly under X-ray examination.

A-stage resins.—Thermosetting resins reacted only to the initial stage where they are soluble and fusible. The normal stage of a resin used for impregnation.

Bonding strength.—The amount of adhesion between a binder and filler. More specifically, the measure of the extent to which the composite layers of a laminated product are bonded together.

Brittleness.—Liability to break, generally to a conchoidal fracture.

B-stage resins.—Thermosetting resins reacted to a stage where they soften when heated and swell in contact with liquids but do not entirely fuse or dissolve. This is the preferred stage for the resin in molding compositions.

Casting.—Forming a material into a shape by pouring it when liquid into a mold. The product from the mold is used as such or mechanically worked in various ways to the final articles, as by sewing, cutting, blanking, turning, drilling, forming, swaging, grinding, polishing, sanding, or routing.

Compressive strength.—Resistance to deformation under applied pressure.

Condensation.—A chemical reaction in which two or more molecules combine with a separation of water or some other simple substance. Applied to synthetic resins it means the formation of a resin by combination of a number of molecules with elimination of water, ammonia, hydrogen chloride, or other simple substance. Examples of condensation resins are alkyd, phenol-aldehyde, and urea-formaldehyde resins. The final products are also called condensation-polymers. (See Polymerization.)

Copolymerization.—The term applied when two or more substances polymerize at the same time to yield a product which is not a mixture of separate polymers but a complex having properties different from either polymer alone. For example, vinylite is produced by polymerization of a mixture of vinyl acetate and vinyl chloride.

C-stage resins.—Thermosetting resins in the final stage in which they are infusible and insoluble. The state of the resin in the final molded article.

Curing.—The change of a binder from the soluble-fusible condition to the substantially insoluble-infusible form by chemical action. The heat-setting of a resinoid. Action is analogous to vulcanization of rubber.

Dielectric strength.—Voltage gradient at which a continuous electrical discharge will take place between two electrodes when the material in question is placed between the electrodes and a potential difference is applied to them.

Elastic.—A substance which exhibits rubberlike properties or “high elasticity” over a wide range of applied forces.

Elastic deformation.—When a substance reverts to its original dimensions on release of an applied stress.

Elastic limit.—The point at which a body begins to yield under a stress; that is, when the stress is equal to or greater than the internal friction.

Elasticity.—The property by virtue of which a body reverts to its normal bulk or shape after deformation by an applied force.

Extrusion molding.—A molding procedure for extended shapes of uniform cross section, whereby a heat-softened substance is forced through an orifice of form coinciding with the cross section of the article.

Flexibility.—Capability of bending without breaking.

Gums.—Viscous vegetable secretions which harden but, unlike resins, are water soluble. The name is often applied, particularly in the varnish industry, to natural resins such as copals.

Hardness.—Property of substances determined by their ability to abrade or indent one another. Often measured by the extent or depth of indentation produced by a standard substance under a predetermined load.

Impact strength.—The measure of toughness of a material. Generally determined by the energy required to break a specimen in one blow.

Injection molding.—A molding procedure whereby a heat-softened plastic material is forced from a receptacle into a cavity which gives the article of desired shape. Used particularly for thermoplastics since the scrap can be reused. As soon as the composition in the mold cools sufficiently to be rigid, the mold is opened and the molded article removed. An analogy of injection molding in another field is shown by the linotype machine.

Inserts.—Parts of a finished molded article which are of different material from the molding composition but are set in place or positioned by the molding operation.

Laminated products.—Sheets of material united by a binder. For example, sheets of paper or wood coated and/or impregnated with a resinous composition and subjected to pressure, generally with heat.

Monomer.—The simplest repeating structural unit of a polymer. For addition polymers this represents the originally unpolymerized compound.

Phenoplast.—A general term for phenol-aldehyde resins. Synonymous with popular term “phenolics.”

Plastics.—All substances that can be molded. In general a plastic is a substance which behaves as a solid at stresses less than a certain amount known as the yield value and as a viscous liquid at stresses greater than this. The name is also applied to substances which originally but not ultimately fulfill this condition. For example, it is applied to thermoset compositions or resinoids in the final stages.

Plasticity.—Susceptibility to and the retention of deformation. Capacity of taking and retaining the form of a mold. The property of solids by virtue of which they hold their shape permanently under the action of small shearing stresses but are readily deformed, worked, or molded under larger stresses.

Polymerization.—A chemical change resulting in the formation of a new compound whose molecular weight is a multiple of that of the original substances. The products of the reaction are called polymers. To distinguish from those resulting from condensation (q. v.), they are often designated addition polymers, since the reaction is that of successive addition of a large number of relatively small molecules (monomers) to form the final polymer.

Power factor.—In an insulating material, the ratio of total power loss (watts) in the material to the product of voltage and current in a capacitor in which that material is a dielectric.

Preforms.—Molding powders converted by pressure and without heat into a denser coherent form which approximates the shape of the final hot-pressed article. Molding material converted to preforms has about half the bulk factor of the original powder. Other forms of densified composition which do not necessarily approximate the shape of the final molding are tablets, briquettes, pellets, pills, and balls.

Resin.—A term generally referring to a physical condition at room temperature approximating the physical properties of natural resins. However, the temperature of reference should not be limited to room temperature and the term is here intended to embrace all substances which within a certain temperature range show these, properties. For example, many oil-modified alkyd resins are viscous liquids at room temperature but not at lower temperatures; polystyrene is a resin at room temperature but rubberlike when warmed.

Resinoids.—The class name applied to thermosetting resins. Temporary thermoplastics. The name is also often applied to the final cured resins.

Softening point.—Resins have no sharp melting point. Application of heat causes gradual change from a brittle or exceedingly thick and slow flowing material to a softer and less viscous liquid. The softening point is the temperature at which the material flows at a definite rate or to a definite distance.

Synthetic resin.—A complex, substantially amorphous, organic semisolid or solid material (usually a mixture of substances) built up by chemical reaction of comparatively simple compounds and, depending upon the temperature at which the examination is made, approximating the natural resins in various physical properties: namely, luster, fracture, comparative brittleness, insolubility in water, fusibility or plasticity when heated or exposed to heat and pressure, and, at a certain more or less narrow temperature range before fusion, showing a degree of rubberlike extensibility; but commonly deviating widely from natural resins in chemical constitution and behavior with reagents.

Synthetic rubber.—Caoutchouc synthesized in the laboratory. The term is a misnomer and most probably represents an impossibility.

Tensile strength.—The greatest internal force per unit of cross section which a material develops before failure under tension.[30]

Thermoplastic.—The property of softening under heat. All molding materials are thermoplastic at the initial application of heat. One class (the so-called thermoplastics) remains soft permanently under heat; the other (thermosetting), after first softening, sets or cures more or less quickly to a more solid form. A practical distinction is that with the first class the mold must be cooled before the molded article is removed, but not with the second. A thermoplastic substance is adequately rigid at normal temperatures and under ordinary conditions of stress but is capable of deformation under heat and pressure.

Thermosetting.—The property of undergoing a chemical change when heated whereby a hardened product is obtained. Property most pronounced in phenol and urea formaldehyde resins and less so with alkyds. A thermosetting substance possesses initially the properties of a thermoplastic but under the influence of heat undergoes chemical change so that it is no longer thermoplastic but becomes permanently infusible.

Viscosity.—Internal friction or resistance to change of form of a liquid. The constant ratio of shearing stress to rate of shear.

Water-absorption.—Amount of water taken up when exposed to humid conditions or when immersed. Both rate of absorption and total absorption are important, also change in dimensions. A certain amount of absorbed water may improve mechanical properties but usually weakens electrical characteristics.