Elastomers water permeation

Plastics and elastomers are generally thought to be resistant to or unaffected by mild environments such as water Immersion. Commonly, an organic coating polymer or seal Is accepted as providing a water-tl t barrier even when the device Is Immersed In water. But water will permeate the barrier. If only slowly. The phenomenon of water permeation Is reviewed In (1,2). 

A third possible explanation Is that absorption of water In elastomers lAilch contain water soluble fillers has been shown to create pockets of solution which continue to grow until the osmotic pressure Is balanced by the containment pressure of the rubber. This Introduces a very complex picture for water permeation there are pockets of permeant, which cause mechanical stresses In the rubber matrix, creating thereby channels for water diffusion. 

It Is reasonable to assume that a mechanical stress would supply energy to the elastomer thereby Increasing the rates of any processes. For example. In aging, plastics or elastomers which are highly elongated will decompose more rapidly than unstressed controls. There Is, however, no published Information on the effects of ultrasonic radiation on water permeation through elastomers. 

G 1. Gamble, L., L. Westerman, and E. Knipp The molecular weight distribution of elastomers. Comparison of gel permeation chromatography with other techniques. Waters Ass. Werkssdirift. 

Time, temperature, and mechanical stress must all be considered when an elastomer or plastic Is to be used as a barrier to water or some other permeant. Long time exposures can cause changes In composition of a barrier which lead to Increased solubility of the permeant and to higher permeation rates. 

Temperature effects are important, not only in high temperature applications, but also during accelerated aging of a material for laboratory study and subsequent evaluation of in-service life-time. Solubility of water In Neoprene Increases greatly with temperature as does permeation rate. The relative permeation rates of 3.5% saltwater and deionized water were shown to depend on elastomer composition. 

Permeability. Blon elastomer has much less diffusion of silicon oil and water than silicon rubber under the same testing conditions (ASTM D814). Comparative permeation rates are listed on Table VI. Applications demanding low permeable materials Include Implantation of encapsulated electronic devices and silicon oll-fllled breast prostheses. 

The environment in which an article is used may influence bond durability (see also Durability fundamentals). Atmospheric ozone can cause time-dependent crack growth in vulcanized elastomers in addition, ozone can induce failure at a bond with certain bonding agents. Although water is only slightly soluble, it can permeate elastomers by an osmotic mechanism induced by salt-Uke impurities. As a result, the uptake in salt water is generally less than that in pure water. Rubber to metal bond failure has been found to occur in a time-dependent manner under salt water in the presence of electrochemical activity but much more slowly, if at all, in its absence (see also Cathodic disbondment). In the absence of imposed electrochemical activity, effects are likely to depend particularly on the metal used and its corrosion resistance. Provision of a bonded rubber cover layer over all metal surfaces subject to immersion is likely to enhance bond durability. 

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