LDPE stands for low desity poly(ethene), where as HDPE stands for high density poly(ethene). Basically, HDPE is stronger than LDPE.
To form LDPE, a trace of oxygen needs to be present, in addition with high pressure. The poly(ethene) chains are randomly branched, and cannot pack closely together.
To form HDPE, however, we'd use a catalyst at 50 Celcius, and slightly higher pressure than LDPE. The poly(ethene) chains are straighter and can pack closely together.
Thermosoftening polymers are plastics that, when heated, can be reshaped. This is because weak intermolecular forces are present, only. Thermosoftening plastics consist of tangled polymer chains.
Thermosetting polymers are plastics that, when heated, cannot be reshaped, but char instead. This is due to the strong covalent cross links (it also has weak intermolecular forces). Thermosetting plastics are bonded together, not tangled together.
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