There are two basic types of plastic closures used for scientific applications: Thermoset (Bakelite) and thermoplastic (polypropylene).
Thermoset closures (urea, phenolic, melamine, etc.)
Thermoset closures cannot be remelted after they are formed. They provide the widest range of chemical compatibility and they exhibit the most tolerance to temperature of all plastic closures. Because thermoset closures are rigid, they provide the most consistent adherence to close dimensional tolerances.
Phenolic is short for Phenol Formaldehyde Resin (PF). At 220°C (428°F) or above, Phenolic caps will start to decompose. However, due to their non-chemically reactive nature, when exposed to corrosive or harmful chemicals over time, phenolic caps will not give out the way other polymers might. They're very tough when used in the correct temperature range.
Thermoplastic closures (Polypropylene, Polyethylene, etc.)
Thermoplastics can be remelted after initially formed. Polypropylene caps are known for good impact strength, cost effectiveness, and pliability. Polypropylene is autoclavable and polyethylene is not. Polypropylene can withstand high temperatures (it's melting point is 130°C), but is less suitable for freezing temperatures where it can become brittle. The melting point for polyethylene is 115–135 °C and it is best for option freezing temperatures.
Metal Closures offer the widest range of temperature tolerances and are very resistant to fracture from impact. A metal closure is manufactured from either steel (coated with anti-corrosive coating of either chromeplate or tinplate) or aluminum.
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