Plastic Spontaneoulsy Shatters?

Why do objects made from Lucite (clear plastic) develop cracks and eventually shatter? I have noticed that lots of objects made from clear plastic (Lucite ?) start out clear, but over time, internal cracks develop. I had a pepper mill which did this-eventually the thing disintegrated (it broke into small bits). We had plastic cupboard handles that did this also. Since the items were all inside, ultraviolet light wasn’t the cause.
I’ve noticed this kind of disintegration is common with all clear plastics, but opaque plastics don’t seem to have this problem.
What causes this?

Not sure but have some useful clues.

It doesn’t happen with all clear plastics. Polycarbonate, like the Lexan brand, seems to last very well - it’s tough enough that they make boat propellers out of it, and I used to make things by bending window-grade polycarbonate in sheet metal machinery. But it’s expensive. Acrylic, like the Lucite brand, is not nearly this strong and tough. It shatters if you hammer it and it’s difficult to drill and machine without cracking.

Many plastics are by nature somewhat brittle and prone to cracking. Many things made of plastic are actually made of “filled” polymers, meaning that some kind of powder has been added. Fillers often help prevent cracks from spreading. Fillers generally make a plastic opaque, though.

The most common polymer is polyethylene, which is very similar to candle wax, the molecules being longer. It is never transparent, because it is birefringent (it has a different index of refraction for waves of different polarization, and this varies randomly throughout). But it is pretty tough, and doesn’t crack easily. Many of the nontransparent plastic things you see that don’t crack are polyethylene. I don’t think its noncracking nature and transparency are related, though.

Bakelite is a very old kind of plastic, and it’s not transparent. It also tends to crack pretty easily.

PVC is transparent, though not crystal clear, if it has no fillers. However, without plasticizer, it is quite brittle. With a lot of plasticizer, it is vinyl. So, in its case, plasticizer is the important difference. A plasticizer is an oil or other compound that the polymer is blended with, forming an oil version of gelatin.

You probably have some ultraviolet light inside, and for that matter blue light is a little harmful. I don’t know what role if any that played, but things being inside doesn’t protect them completely.

Residual stresses after molding can cause these type of fractures. Filled plastics (never clear) are much stronger and more resistant to stress cracking. Softer plastics are also less likely to develop cracks as opposed to more brittle plastics like Acrylics or Polycarbonates.