Wednesday, March 17, 2010

Why I hate PVC

If PVC had only been invented yesterday, the inventors would have published a nice paper about it and that would be the end of it. Maybe they would have saved a few samples for posterity, but the rest should have gone right into the dumpster and that would have been it. Done. Gone. Good riddance.

"PVC, how do I hate thee? Let me count the ways."

I don't even know where to start, although I know not to start with any arguments about the negative environmental impact of the chlorine and how it creates dioxin when burned and how plasticizers are killing us all or...

Instead, let's look at it from the point of view of an engineer. How about all the endless additives that are needed? The first of these are of course plasticizers, although they certainly are not a necessity as unplasticized PVC is widely used, but when plasticizers are used, they can often be added in very high loadings ( > 100 phr). And what about the internal and external lubricants? And the various heat stabilizers (lead, tin, cadmium...)? There can be still more additive too such as pigments, impact modifiers, UV absorbers, flame retardants, fillers... [1] Face it, without additives, PVC is nothing.

And then once added, the plasticizers won't stay put [2]. They come oozing out over time, gumming up adhesive labels on the surface or anything in contact with the plastic.

Proceesing PVC is a real treat too. We run quite a bit here of it here at Aspen Research (including some with wood flour added) and need special fire training because if an extruder overheats, HCl is one of the products. A room with high levels of HCl is just not something you want to run into without a partner to drag you out if you go down and an independent air supply.

Running rheology tests on the material is a joy too. I can't heat the material too high (see above) and have to work quickly because it has less ability to handle even moderate heat for long periods of time. In short, dynamic mechanical analysis of the stuff is not my favorite way to spend a week.

The one amusingly redeeming quality of PVC is the color change that sometimes occurs in white PVC. Instead of the oxidation --> chain breaking reaction that occurs in olefins, in the pinking reaction H + Cl is removed from the repeat units and a double bond forms along the backbone. When the double bonds are alternate with single bonds, conjugation occurs. If the conjugation is long enough, the material will absorb visible light, starting with the blue end of the spectra and moving towards green and yellow, leaving on the red. Given the initial white color, white + red --> pink. Pink!

But PVC is here to stay (despite the efforts of Greenpeace). I have no problem with it or products made from it. But you can make me happy if you ask someone else to work with it instead of me. It's just too much of a struggle without an appropriate reward.

[1]To be fair, this last list of additives are often added to polymers in general and are not unique to PVC.

[2] I've always suspected but have never been able to confirm that this occurs because of a thermodynamic incompatibility between the PVC and the plasticizers. It is not simply because of the high loading per se. Pressure-sensitive adhesives can have similarly high levels of tackifiers added and yet I am not familiar with any similar migration of the tackifiers. Can anybody confirm my suspicions?

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