PVC vs HDPE for water piping

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Hello Chemistry Community!

I basically have to choose which polymer is better for water piping, though there is some chemistry I am not understanding associated with the relations of tacticity, crystallinity and strength of the polymers. (PVC and HDPE)

I did some research, and found that PVC is atactic (or heterotactic) because of the Chlorine units on the backbone (yet it doesn't say why the chlorine makes it like that?). This makes PVC more amorphous making it harder to arrange into layers , making it less crystalline, unlike HPDE (which is very crystalline because it's non branched and syndiotactic).PVC is known for its strength and durability, apparently it's 'Semi-crystalline' and has much more compression strength than HDPE (9500 psi compared to 3500-3700 psi for HDPE). Is it for this reason ? :

HDPE is far more crystalline, since it's linear and syndiotactic, though is it less stronger than PVC because of it only harnessing intermolecular forces within its chains, is this why? And PVC, which has covalent bonds (cross links?) between its chains because of its chlorine, which makes it stronger than HDPE regardless of
how less crystalline it is? I doubt that is why though... I think it may be the molar mass distribution or something along the lines...maybe?

2) I also was wondering if the reason for HDPE's glass transition temperature being 120 to 130 °C (248 to 266 °F) compared to 80°C for PVC, was because HDPE is more crystalline? But aren't PVC's covalent bonds holding chains together? Because apparently glass transition allows 'molecular chains to slide past each other when a force is applied', shouldn't HDPE have a lower Tg temperature than PVC and have its molecules 'slide' easier when it reaches its Tg?

This is really bugging me, I apologize for the rather lengthy question, though help would be much appreciated

Thank you for your time.

dikutip dari: http://www.chemicalforums.com/index.php?topic=51528.0


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