Stiff rubber types of test-tube stoppers (of high chemical resistance) are made of M35 Green Neoprene1 and are known by the trade name "Plasticoid"
The "vent"/"cap", clear, hard, brittle, plastic type of test-tube stoppers are made of Polypropylene4, an example are "Greiner Bio-One"
- https://www.thomassci.com/Laboratory-Supplies/Stoppers/_/Rubber-Stoppers2 ↩
- https://www.thomassci.com/Laboratory-Supplies/Caps/_/Polyethylene-Caps ↩
- https://www.thomassci.com/Laboratory-Supplies/Stoppers/_/Stoppers1 ↩
- https://www.thomassci.com/Cell-Culture/Culture-Tubes/_/Two-Position-Vent-Stopper-Polypropylene-Tubes ↩