Thursday, June 24, 2010

Environmental Stress Cracking

Something that I’ve never felt comfortable with is “environmental stress cracking” (ESC) [1]. Certain plastics, when exposed to nonsolvents and stress, will show crazing and cracking. Take away the stress or the nonsolvents and the problem goes away too.

The strangest and most common cases are those involving polyethylene and surfactants. That’s right, surfactants. Keep in mind that polyethylene can laugh at just about any liquid you throw at it, strong acids, bases… Even its solvents don’t cause many problems at room temperature. So how can a surfactant be such a thorn in the side?

Common rationalizations [2] are that the surfactant is able to interrupt the secondary bonding within the material. That’s fine and nice, but then how come ESC doesn’t occur in polypropylene?

[1] Another of those terms that is not quite unique enough to avoid confusion with what an untrained person would think it is, much like “good manufacturing practices”. Everybody thinks they follow good manufacturing practices, but the FDA has a huge document that will tell you otherwise, because to them, it means something very specific.

[2] opposed to explanations...

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