Delrin (polyformaldehyde) toilet valve failures and exploding toilets

The sustained case of the lecture. DuPont's Delrin (introduced 1958) was tested against 200 solvents but not against water. Half-a-ppm chlorine causes decomposition and cracking; ten years before this lecture, Hoechst Celanese paid a $900M class-action settlement and DuPont paid a (confidential, >$1B) settlement. Toilet float valves cracked under 60 psi water pressure and blew the ceramic lid off. Tom: "I made a lot of money off that."

I don't have a piece of Delrin here, but DuPont — did I tell you the Delrin story? Polyacetal, probably — but it's basically a very simple polymer. DuPont came out with polyacetal, which they called Delrin as their trade name, D-E-L-R-I-N, in 1958. There's a paper that has 200 different solvents, and they said this is the plumbing material of the future. They had 200 different solvents that it was resistant to. You could pour any organic you wanted, any solvent you wanted down the sink, and nothing in your food was going to touch it. The one solvent they didn't check — water. I'm not kidding. You read this paper, they never checked water. It turns out, if it's very pure water — in a nuclear reactor, water would be great, Delrin would last forever. But if it has half a part per million chlorine, it will start to decompose, and it will crack.

The lecture's center case. DuPont's 1958 paper tested Delrin against 200 solvents — but never against water. 1962 German study found chlorine-induced stress corrosion cracking. Hoechst paid $900M settlement; DuPont paid more than $1B (confidential). Toilet float valves made of Delrin became brittle after 2–3 years and failed under 60 psi water pressure, blowing off ceramic tank tops. Teaching point: plastics corrode, and "corrosion resistant" specifications depend critically on what was tested.

One of the best stories about plastics corroding was DuPont came out with something called Delrin, which is polyformaldehyde — it's called acetal, but basically it's polyformaldehyde. Formaldehyde's one of the simplest hydrocarbons. I think it's CH₂O, you can Google it. If you polymerize this, you can make a ten thousand psi plastic which is very strong, easy to injection mold. DuPont came out with a paper in like 1958 that tested polyformaldehyde in two hundred different solvents, and it was resistant to all these chemicals, and this was the plumbing material of the future.