What are test-tube stoppers made of?

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 “hollow”, translucent, pliable, thin plastic type of test-tube stoppers are made of Polyethylene2 or Low-Density Polyethylene3, the latter being known by the trade name “Dynalon” The “vent”/”cap”, clear, hard, brittle,…

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