Thermoset Urea closures are known for providing the widest range of chemical compatibility and for being the most temperature tolerant of plastic closures. Thermosetting plastics (thermosets) such as urea resins are polymers that irreversibly cure; thus urea caps and closures cannot be remelted after they are formed (although some liners may melt). Thermoset and phenolic caps also provide the most consistent adherence to close dimensional tolerances. Ideal for pharmaceutical applications. Common resins include urea, phenolic, and melamine.