Water Vapor Permeability of Polyethylene and Other Plastic Materials

Low-density polyethylene sheathing materials have water vapor permeabilities on the order of 10−8at 22°C. High-density polyethylenes have permeabilities about one-third to one-sixth that of the low-density polyethylene. Copolymers of polyethylene have higher permeabilities than the homopolymer. As an example, 15 percent ethyl acrylate comonomer increased the permeability by a factor of 10 over that of straight low-density polyethylene. The nonolefinic polymers tested have higher permeabilities. For example, polyurethane plug compound has a permeability more than 80 times higher than low-density polyethylene. Finally, it was found that the addition of carbon black decreases the water vapor permeability roughly in proportion to the amount of carbon black, and that the permeability of these materials increases with increasing porosity. To make these measurements, two types of laboratory apparatus have been constructed. The first of these makes the permeability measurement on a tubular sample of the material, and the other on films. Both methods used an electrolytic moisture monitor, which is commercially available, to make necessary determinations of water transfer rate through the plastic.