One question per week: Basics of polymers – why is PE classified as a plastic?
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This post was last edited by QXZ-1966 on 2009-9-19 at 10:09. From the perspective of cohesive energy density, the PE chain exhibits characteristics typical of rubber; why is it classified as a plastic rather than a rubber?
The properties of PE are like those of plastics rather than rubber. If only the flexibility of a single molecular chain is considered, PE is the best choice, as it can serve as an excellent rubbery elastic material. However, this potential is undermined by the crystallinity of PE molecules, as crystallization turns PE into a crystalline plastic.
This post was last edited by scp501 on 2009-9-19 12:13. Polyethylene chains have good symmetry, no side chains, and low steric hindrance, which makes it easy for them to crystallize; sliding between molecular chains is unlikely to occur. Polyethylene has a very high crystallization rate, and under normal conditions its amorphous form cannot be obtained; I recall that the amorphous form of polyethylene can only be achieved in liquid nitrogen. Due to its high crystallinity, around 98%, it possesses good mechanical properties, and it is also resistant to acids and alkalis, reflecting the characteristics of plastics.
Is PE not as good as PVC when under pressure? The day before yesterday, when I went to pick up samples, I saw PE pipes; could some expert please explain this to me?
PE, especially low-pressure PE, has good molecular linearity and no branches. Therefore, polymer crystals are easily formed. As a result, the apparent elasticity does not reach that of rubber. For this reason, the copolymerization of ethylene and propylene results in the uniform introduction of short side chains into the chain, which disrupts crystallinity and yields EPDM.
It’s hard to say; among the commonly used piping materials today, PE pipes generally have a lower pressure resistance compared to PVC. This is probably because PE cannot withstand long-term static water pressure. 4# zixuanliu