It's the Rheo Thing: Curses! Environmental Stress Cracking

Tuesday, September 06, 2016

Curses! Environmental Stress Cracking

We've had a lot of aphids attacking plants in our garden this year. My usual approach is to use an insecticidal soap that I make by diluting down from a concentrate. "Normal" soap is the sodium salt of a fatty acid, while an insecticidal soap is just the potassium salt, which means the soap is relatively safe. It also works pretty well against the critters, and clean-up is easy. But they have gotten so out of hand that instead of making it up as needed, I made up a larger amount in a spray bottle and kept it in a cabinet.

Over the weekend, I knocked the bottle to the garage floor. No biggie; I picked it up and put it back in its place. But yesterday, I noticed that the soap had leaked out of the container. I picked up the bottle and saw this:

Environmental Stress Cracking of Polyethylene due to Soap

What an idiot I was. It was a polyethylene bottle with dilute soap in it and it had been stressed by the fall to the cement - a situation just begging for environmental stress cracking and that was what I got.

Environmental stress cracking (ESC) is where polyethylene and other plastics, when exposed to non-solvents and stress, break down unexpectedly. Soap and polyethylene. How can there by any interactions, especially at room temperature, especially when PE can withstand strong acids and bases, concentrated soaps and surfactants, bleach, hydrogen peroxide and more? And if you think you have an idea, then be prepared to explain why polypropylene doesn't experience ESC.

As you can imagine, I'll be on the lookout for a PP-based spray bottle for next year.