Water Vapor Permeability of Polyethylene and Other Plastic Materials

The post war years have seen a phenomenal proliferation of plastic materials throughout industry, and the Bell System has been no exception. At least part of the reason for this widespread and ever increasing use is the attractive ease of fabrication of plastic materials and their relatively low cost. Their inertness to certain environmental factors and their chemical and physical stability also add to their value in a great number of applications, including environmental protection. Not only are these plastics used to enclose and isolate an entire apparatus or structure from its environment, they are also used 391 392 T H E BELL SYSTEM TECHNICAL J O U R N A L , FEBRUARY 1967 as protective coatings and as seals such as O-rings in metal containers. One of the undesirable and detrimental factors in the environment is water, and consequently, a need developed for information on the water resistant characteristics of the many plastics in use. This water resistant characteristic is the water vapor permeability (WVP) of the material, and has been measured for several materials of interest. This paper discusses the need for the measurements, the. method of making and correlating measurements, and the significance of the results. Several problems have been documented involving moisture transfer rates in complicated, composite plastic materials. Water in small amounts, particularly in pulp insulated cable, has detrimental effects on the electrical characteristics of the core. Splicing an Alpeth cable into a paper insulated cable results in moisture accumulation in the latter cable, presumably as a result of moisture diffusion through the Alpeth sheath and subsequent migration into the paper core.