Preparation and Properties of Polymer Mixtures

A useful and generally accepted method of increasing the available selection of plastics is the manufacture, by blending existing polymers, of mixed materials with new properties. Properties of the blend can be expected to correspond to the additive properties of the plastics making up the mixture, in accordance with their proportions. This is very often the sole purpose of the process, e.g., when plasticizing a brittle polymer with a more plastic one. It can however also be expected that some properties of the blend may be better than what would be given by pure additivity, as is observed with some mixed solvents or alloys. Polymers are mixed industrially on processing machinery similar to that used for processing plastics, or mixing in additives, i.e., on mills, in internal mixers, mixing extruders, Ko-Kneaders, etc. Adequate mixing is often only possible at the processing temperature of the material which requires the highest temperature. Experience has shown that in the usual processing conditions only certain polymers are miscible. Nonmiscible polymers separate in the course of or at the end of the mixing process. Such material disintegrates into thin layers or fibers on cooling. The practical compatibility of polymers is not easy to explain theoretically. We shall see that in some systems the mixtures are mutual solutions of their components, while in others the possibility of forming a true mixture must be excluded, yet blends with valuable properties are easily obtained in practice.