With most resin and plastic emulsions freezing causes breakdown, which may take the form of partial or complete coagulation. Generally speaking, the coagulation can seriously mar the properties of paints or adhesives in which the emulsion might subsequently be used. This dispersed coagulum could be removed by filtration. Coagulation starts from the walls of the container and works inwards with progressive drop in temperature or repetition of the freeze/thaw process. Some emulsions are more sensitive to freezing than others; for example, all the polystyrene emulsions, because of their fine particle size and low colloidal protection, tend to be unstable to freeze/thaw conditions and will generally break down on a single freezing.
Polyvinyl acetate emulsions vary considerably in their freeze/thaw stability. The large particle type is much more stable than the fine and will stand repeated freezing and thawing without showing any coagulation. The same emulsion plasticized with dibutyl phthalate is rather more sensitive, but even so will withstand several cycles of freezing and thawing without coagulation. Although by no means universally true, it is often the case that the thicker looking emulsions have the greater storage stability under cold conditions. This is because large particle size is usually associated with high viscosity and stability is enhanced by increase in particle size. The nature of the protective colloid, too, is critical, and some colloids, such as polyvinyl alcohol, are more effective than others, which include the ethylene oxide alcohol derivatives.
If the emulsifying content is inadequate or unsuitable, or in the unlikely event of the water in the emulsion having too high a salt content, the emulsion may break up so that solidification occurs in the form of lumps. Some emulsions are prepared to withstand long periods of storage, others, owing to such inherent susceptibilities, may commence to break up within comparatively short periods.
Plastic materials which become contaminated with dust, dirt, etc., may become useless for the production of high-grade articles. When this material is in dried form (molding powders, etc.) and is packed in paper bags, it is particularly liable to depreciation by reason of the bags coming into contact with foreign matters. Care is necessary in handling damaged bags to avoid contamination of the unexposed portion of their contents.
The following are individual comments on various plastic materials for guidance in assessing the cause and extent of damage, and in the handling and treatment of plastic materials.
1. The effect of damage by
(a) High humidity
(b) Salt water
(c) Fresh water
(d) Treatment, if any
2. (a) Effect of damage by heat
(b) Treatment, if any
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