Elastomers permeation rates

At constant conditions, different fluids will diffuse at different rates into a particular elastomer (with their rates raised proportionally by increasing the exposed area), and each will reach the far elastomer-sample surface proportionally more rapidly with decreasing specimen thickness. Small molecules usually diffuse through an elastomer more readily than larger molecules, so that, as viscosity rises, diffusion rate decreases. One fluid is likely to diffuse at different rates through different elastomers. Permeation rates are generally fast for gases and slow for liquids (and fast for elastomers and slow for thermoplastics and thermosets). 

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. 

All materials tend to be somewhat permeable to chemical molecules, but the permeability rate of some elastomers tends to be an order of magnitude greater than that of metals. Though permeation is a factor closely related to absorption, factors that influence the permeation rate are diffusion and temperature rather than concentration and temperature. Permeation can pose a serious problem in elastomer-lined equipment. When the corrodent permeates the elastomer, it comes into contact with the metal substrate that is then subject to chemical attack. 

Experimentation at temps, up to 130C and pressures up to 700 atmospheres was carried out to assess the explosive decompression performance of a number of elastomeric compounds. Permeation measurements showed that permeation rate decreased from proportionality against pressure at high pressures, and at 700 atm. and 27C for carbon dioxide (now a liquid) permeation apparently was effectively halted reasons suggested include the forced close packing of molecular chains and the dependency of D on concentration. These observations are discussed together with aspects of nucleation, bubble growth and elastomer fracture an optimised balance of properties related to these factors is presumably necessary for an elastomer to withstand explosive decompression. 10 refs. UK... 

A more distinct way to look at fluoroelastomers is to compare their fuel permeation resistance, particularly when the fuel has been modified by an oxidizing agent [28]. Figure 4.6 illustrates typical permeation rates achieved with fluoroelastomers compared with other elastomers. 

It might be thought as a consequence of measurements such as these that leakage factors are the main issues in fuel containment. However, although obviously important, in some cases a leak might occur only at intermittent intervals, and the associated problem might well be easily resolvable by component replacement. In contrast, the relevance of permeation to fluid containment is its continuous nature—its rate may be low, but it occurs all the time that fluid is contacting elastomer. Hence, this phenomenon is now considered in association with related processes absorption, adsorption, and diffusion. 

Increasing temperature permits greater thermal motion of diffusant and elastomer chains, thereby easing the passage of diffusant, and increasing rates Arrhenius-type expressions apply to the diffusion coefficient applying at each temperature," so that plots of the logarithm of D versus reciprocal temperature (K) are linear. A similar linear relationship also exists for solubUity coefficient s at different temperatures because Q = Ds, the same approach applies to permeation coefficient Q as well. 

Diffusion of a gas or liquid through a semi-permeable material. The permeability of elastomers to gases varies with the elastomer type and with the gas. Butyl rubber is much less permeable to air than is natural rubber hence its use in tyre inner tubes and similar apphcations. The rate of permeation is generally related to the size of gas molecule, i.e., the smaller the molecule the higher the rate. The exception is C02 which has a rate 10 to 100 times greater than that of nitrogen. 

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. 

A study was made of gas decompression failures in elastomeric seals using a fracture mechanics approach with considerations of gas permeation. An equation is proposed for the tearing energy associated with crack growth from internal gas bubbles in a finite thickness elastic media. The effects of gas pressure, temperature, rate of decompression and mechanical strain were studied for a range of elastomers used in oil and gas sealing applications. A theoretical treatment is presented based on a fracture mechanics criterion for fracture from an internal disc shaped flaw in a thick elastic medium. Permeation theory provides a quantification of the amount of gas available internally to initiate failures. 21 refs. 

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