Crystallization

[uncle ken]

Does anyone know what causes the old pickguards to crystallize and decompose? Also what causes the old plastic tuning keys from the 30s to crumble and decompose?

[Dave Cohen]

The answer is the same for both. The plastics are made by polymerization in a solvent, followed by forming by extrusion, or rolling, etc. The plastic is actually more than just the polymer. Plastics consist of polymer(s), plus some solvent, plus some small to intermediate sized molecules called plasticizers. The "new-car-smell" is the smell of plasticizers. The plasticizer molecules fill the voids between the long-chain polymer molecules and keep them from collapsing onto each other. That keeps the plastic structure flexible, and, well, "plastic". As the plastic ages, both remaining solvent molecules and plasticizer molecules diffuse from the polymer structure, allowing the polymer molecules to collapse closer to each other, forming local crystallites. As the number of crystallites increases, the plastic structure becomes brittle, and has more voids. When the number of voids increases sufficiently, the plastic gradually self-destructs. Does that explain it sufficiently? Good, b/c there will be a quiz tomorrow <g>.

[uncle ken]

Thanks, that's one heck of an explanation. I guess the next question is why do some pieces stay so well preserved while others don't. I'm assuming there's something in the environment that is causing this to happen. I had the tuning keys and pickguard decompose on one of my mandolins from the late 30s. It seemed like it happened quickly some years ago. I don't know if it's from the bad air in Southern California or if I exposed them to a solvent from a polish or cleaner that I may have used. I think I read that the plastic back then was made from laquer which isn't as stable as what was used in later times.

Also, is there anything than can be done to help preserve these materials?

[Michael Lewis]

Local environments can effect plastics but more probably what you are experiencing is due to the particular batch of plastic. I have seen celluloid parts from the teens that are still strong and solid, while we all have seen celluloid from more recent batches that have crumbled to dust. I believe this is the case as more than one guitar has had only some pieces of binding crumble while adjacent pieces remain solid.