Re: why do PVC molds break down in the presence of oils and fats?

Sorry for the delayed answer. It took some time to find it. I may not even 
have the correct one. But it looks like the breakdown of PVC in the 
presence of fats and oils is due to the leaching out of the plasticizers 
inside PVC that have been added to it to improve its processability or 
flexibility. When in contact with oils (not mineral oils - PVC are 
extremely in mineral oils) especially cooking oils and fats, the 
plasticizers may be drawn out from the PVC resulting in breakdown of the 
PVC soles. SOmetimes polymeric plasticizers are added to PVC to make them 
oil-resistant. Applications such as geomembranes use such oil-resistant 
plasticizers. I also posted this question in sci.polymers and I quote 
below a part of the response.

"We went on to test the PVC in the presence of a number of other systems 
that were not hydrocarbon in nature, including synthetic esters, and 
vegetable oils of a variety of types, and we found that the same effect 
was occurring.  It did appear that the oils were leaching out the 
plasticisers, an effect later confirmed using quantitative FTIR (infrared 
spectroscopy). As yet, we have not found any oil that does not have this 
effect with PVC."

So there goes your answer.
Have fun