PVC vs HDPE piping material

[Doze]

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.

[hypervalent_iodine]

It's been a little while since I've had to deal with polymer chemistry, but I'll see if I can help you as best I can.

 

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?).

 

Though I am sure you know this, tacticity is a term that refers to the relative stereochemistry of adjacent chiral centres within a polymer chain. Achieving a defined sequence of chiral centres in a polymer (i.e. with iso- and syndiotactic polymers) requires stereoselective modes of polymerization. PVC, however, is generally made via radical polymerization. This type of mechanism, coupled with the fact that the vinyl chloride monomer is planar, means that there is really nothing that would promote formation of one stereocentre over another. Thus, you get a random sequence of both along the polymer chain.

 

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 ? :

 

Firstly, a correction. To my knowledge, PVC is an amorphous solid, rather than a semi-crystalline one.

 

I'm not entirely sure what your question is here, but I'll try to explain why PVC is less crystalline that HDPE for you. Crystallinity of polymers comes down to how tightly they pack together. A good analogy to this would be comparing beeswax and olive oil; at room temperature, beeswax is fairly solid whereas olive oil is a liquid. The reason behind this is due to how they pack. Olive oil contains a large portion of fatty acid chains that have cis double bonds in them, such as oleic acid:

 

 

 

The result of this is that the fatty acid chains become kinked (as in the diagram) and as such, the fatty acids can't pack in tightly together causing the olive oil to therefore exist as a liquid. Beeswax on the other hand does not contain as many of these and instead consists of more saturated fats (i.e. fatty acids without double bonds), such as palmitic acid:

 

 

 

 

These type of straight chain hydrocarbons are not kinked, unlike in the oleic acid. As such, these fatty acid chains can pack very close to one another, which in turn causes the beeswax to solidify at a lower temperature than olive oil.

 

A long analogy, but the case is very similar when we then talk about crystallinity in that it all comes down to the packing. Consider the following pictures:

 

 

 

I used polystyrene as it gives a much clearer and more apparent picture of what I'm trying to say. It should be immediately obvious that the polymer on the right (the isotactic one) will pack a lot tighter and into a lot more of an ordered structure than will the one on the left. Simply, my point is this: a higher degree of structural order and symmetry on a molecular level will lead to a higher degree of crystallinity on a macro level. HDPE contains little to no branch chains and is completely without stereocentres, which means that it will pack much more tightly and much more orderly than PVC, which is atactic. In fact, HDPE is one of the few polymers whose individual chains lie in an almost completely straight line, compared to the spaghetti-like mess you see in other polymers like PVC.

 

The compression strength you mentioned I think would come down to how brittle each polymer is. If you have a look at the stress-strain plots of each of these, I suspect what you'll find is that PVC will undergo what's called 'necking', which refers to the way some polymer materials stretch and deform before they snap (such as in this picture). HDPE on the other hand will act somewhat like glass in the sense that it will withstand a certain amount of pressure before it simply snaps. This has obvious importance in terms of its use as material for pipes, etc. This is a guess though, so perhaps you might want to look into it (or someone else here can correct me).

 

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

 

Its high degree of crystallinity is what makes it not as strong as PVC. It's extremely brittle because of how linear it is. PVC, however, is more like a spaghetti matrix; there are polymer chains everywhere and they're all interwoven and just generally being crazy. I don't think that PVC is cross linked simply because mechanistically, it is not likely to happen.

 

 

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',

 

I would think it was because of the high crystallinty of HDPE compared to PVC, yes.

 

shouldn't HDPE have a lower Tg temperature than PVC and have its molecules 'slide' easier when it reaches its Tg?

 

No. The T_g is a type of phase change that occurs before, but approaches, the melting temperature, T_m. A highly crystalline solid isn't going to melt at a lower temperature than something that's an amorphous solid, now is it? It's the same with the T_g. The spaghetti analogy I keep mentioning is in fact a commonly encountered one for amorphous solids of this type. Like spaghetti, individual polymer strands are flexible and can easily slide over one another. It is not the same for HDPE, which has a more restricted molecular motion, hence the difference.

 

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

 

That's ok, sorry for the rather lengthy answers.

Edited February 6, 2012 by hypervalent_iodine

[Doze]

Thank you very much for your reply, only if there was some kind of rewarding system here that I would be able to express my thanks better .

 

There is another question that is getting to me, you don't have to answer it if you don't want to (you've helped me greatly enough anyway) How does HDPE become Isostatic or Syndiostatic through Ziegler-Natta catalytic polymerization OR anionic polymerization? All I know is the Electron transfer in initiation and that allows for the addition of monomers, however shouldn't the areas of attack be random like free radical polym. ?

 

 

Thank you again.

[hypervalent_iodine]

The Ziegler-Natta catalyst(s) would likely promote facial bias during the addition of monomers, hence giving you syndiotactic or isotactic polymer chains. Essentially what that means is that there is a component of the catalyst that sterically blocks one face of the growing chain, which means that a monomer unit can only be added to the other face. This means that you preferentially get one stereoisomer forming over the other. I'm not terribly familiar with anionic polymerisation, but I imagine it would be the same. The anion itself will be flat, so I presume that there is something else in the reaction that is promoting facial bias.

 

If you're interested, it might be worth your while to look up the concepts of prochirality and how various things can effect the chiral outcome of a reaction. This wiki article gives a brief overview of it, though an organic chemistry text book will probably give you a better insight. Alternatively, I can always go through it with you

 

PS. Sorry for the delay, I know I said I'd do this on Saturday but I got a little caught up with other things and then forgot about it.

[Doze]

Thank you very much for the help.