polyisobutylene characteristics

[ADavis]

Has research been done regarding the porosity characteristics of butyl, rubber, or other materials similar to polyisobutylene (PIB)?

I am looking to test the effectiveness of various PIB thicknesses as it relates to adhesion and moisture transmittance for glass sealing applications.

Thanks in advance for any help!

[Thomas]

Has research been done regarding the porosity characteristics of butyl, rubber, or other materials similar to polyisobutylene (PIB)?

I'm not a chemist, but I think these materials don't have any porosity in the sense used by WUFI. If they can take up water, I guess it's not contained in any pore spaces but instead goes into 'solution' in the polymer matrix. This is different from what WUFI assumes, but as long as the laws describing the moisture transport in these polymers are of the same type as the diffusion formulas used by WUFI and you know the effective permeabilities, you should be able to simulate these systems as well.

In WUFI, the porosity is only used to describe the total volume of the pore spaces: the maximum water content is (porosity * 1000 kg/m3). The maximum water content is reached when all pores are completely filled with water (e.g. by condensation), while 'free saturation' (the end point of the moisture storage function) is the result of normal capillary water absorption, with some air bubbles trapped in dead-end pores. All relative humidities between 0% and 100% correspond to equilibrium water contents between 0 kg/m3 and free saturation. Free saturation is usually not exceeded but when condensation occurs, the moisture range between free and maximum saturation may become relevant for the simulation. So the porosity is needed to tell WUFI by how much the condensing water content can exeed free saturation.

This moisture range is probably not relevant to your simulations (and presumably doesn't even exist in those polymers), so you don't need to describe it. In this case you are basically free to use any number for the porosity; WUFI only requires that the maximum water content be higher than free saturation, that is, (porosity*1000) must be higher than the free saturation you defined via the moisture storage function.

It's probably advisable to choose a porosity in such a way that maximum saturation only slightly exceeds free saturation, in order to exclude unexpected and unwanted spurious 'moisture buffering capacity' in case condensation conditions should occur in your simulations.

Regards,
Thomas