Clothdiaperman

You ask the ultimate question. Great wealth  awaits one can answer it

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The answer is that you can't. Plastic pants are like any other article of clothing; they wear out.
The specific reason they get crackly: Plasticized pvc such as plastic panties are made of is a mixture of polyvinyl chloride, a hard substance that's used to make water pipes, window frames, and such, and various plasticizers. The key word there is "mixture" as opposed to, say, "compound". So, in use what happens is that the plastic in your pants is likely to lose plasticizer with use. Fortunately it's a slow process in that contact with body oils, urine, feces, hot soapy water etc. will slowly cause plasticizer to be lost. Plasticizers are oil-based compounds, so any contact of oils with plastic pants will rapidly remove plasticizers.