Monday, February 08, 2010

Plasticizers that Stay Put

This new article in Macromolecules (open access!) seems like it will be the hot topic of the week, with some already claiming that it is a "safer" plastic.

Short and sweet, the researchers added a thiol group to the benzene ring in DOP,which then later reacts in a solvent (cyclohexanone) with the PVC via an unidentified reaction. There is loss of plasticization-strength-per-unit-mass, but the plasticizer does seem to stay put looking at what a hexane extraction process can pull out. The authors are so bold as to say that the extact is "zero"[1]. This certainly is an aggressive test to look for plasticizer movement, but it would also be useful to look at the surface over time (heat can certainly be used to accelerate the diffusion.)

I can't that I'm excited about the use of solvent in the DOP + PVC reaction step. Solvents are certainly something that industry is avoiding more and more each week. And the synthesis uses more than just the cyclohexanone I noted above. An H2O/methanol wash is used to stop the reaction, and then THF/hexane is used to precipitate the PVC. That's three different solvents used to modify a commodity plastic. Certainly some industries can consider affording this (I'm thinking medical tubing), but it would certainly require many additional steps of testing to ensure that there are acceptable levels of residual solvent in the final PVC.

Despite the hype, I don't think you'll find this commmercialized anytime soon.

One last thought (added on 2/9/2010): The hexane extraction doesn't even have a control to show how well hexane could extract unreacted plasticizer. Until that is done, there should be serious doubts about the validity of this study.

[1]Note that this is incorrect - it should be state that the extracted plasticizer was below the detection limit. Zero means zero - that there was not a single molecule present. Worse, the detection limit of the technique was never determined, so it it not clear at all how small amount they can detect. The first data point in Figure #5 for the non-modified DOP is at 1.5 mg/ml (1.5 ppm), which is nice but certainly not zero. I'm dissapointed that the reviewers let this go by.

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