What causes that oily film between the keypad membrane and the circuit board on cordless phones?

Answer 1

Not being a chemist, or in the electronics manufacturing industry, I can only guess, so this is just a temporary answer UNTIL an expert joins in with a better answer. I SUSPECT that the oily film which develops on and between the kepad membrane and the circuit board in cordless phones, and basically any such keypad, is the result of "bleed out" [seepage] of "PLASTICISORS" [spelling?]in the membrane. Plasticisors are compounds added to plastics to preserve flexibility and inhibit or prevent the "drying out" and cracking of most plastics over time. I think I have been told, or read, that Silicone is one of these compounds. In some cases, like the keypad membrane, the compound can bleed out as a liquid, while in some cases, it "outgasses" in the form of a vapor without leaving an oily residue. This also happens with automobile tires, dash board covers, vinyl upholstery, etc. We often correct this problem by using one or more spray or wipe-on "rejuvinator" products to replace the "lost" compounds.