# Puzzling polypropylene properties **URL:** \ **Category:** Factual Questions\ **Created:** [August 29, 2011, 5:03pm UTC](https://boards.straightdope.com/t/puzzling-polypropylene-properties/594513 "2011-08-29T17:03:21Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [August 29, 2011, 5:03pm UTC](https://boards.straightdope.com/t/puzzling-polypropylene-properties/594513/1 "2011-08-29T17:03:21Z")
I have two food containers both labeled as polypropylene. One can tolerate heat, while the other shrinks, twists and hardens when exposed to boiling water. My guess is that the non-heat resistant container was formed in a process involving stretching and has a lot of strain that’s released when it’s heated, but as a total non-expert on plastics I’m turning to the teeming millions for a definite answer. The heat resistant one came with a helping of lasagna and instructions to microwave, while the other was filled with minced meat and probably should be kept out of the microwave. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [August 30, 2011, 2:21pm UTC](https://boards.straightdope.com/t/puzzling-polypropylene-properties/594513/2 "2011-08-30T14:21:27Z")
I’m guessing the shrinky one is like the ones used to sell meat in supermarkets. It was probably made as a sheet, heated to soften, then draped over a mold, like a bedsheet. Once solidified it was removed. Disposable plastic plates are also made like that. It’s called thermoforming [Thermoforming - Wikipedia](http://en.wikipedia.org/wiki/Thermoforming) The other one was probably made by injection molding. Look for a protrusion in the center where the plastic was injected.