_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.