Puzzling polypropylene properties

Factual Questions
1

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.

2

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

The other one was probably made by injection molding. Look for a protrusion in the center where the plastic was injected.