Plastics (polymers, more precisely) possess the characteristic that they do no change directly from solid to liquid like many other materials. Rather, they go through a “glass transition.” Below the glass transition temperature they are relatively brittle, but above it they are ductile and formable.
PVC’s glass transition temperature is ~80 °C. If you have worked with PVC you may have noticed it is not typically not brittle even though it is often used well below that temperature. In order to make PVC more flexible, plasticizers are added. Plasticizers lower the glass transition temperature to keep the component flexible, even at low temperature. This allows the many applications shown below.
The concern with plasticizers is that they can evaporate or leach out of the PVC or other plastics. These chemicals, often phthalates, have been found to pose health hazards such as endocrine disruption. Bisphenol-A (BPA) is a good example of a plasticizer which has received a good share of negative publicity recently.
Fortunately, although engineering can lead to unexpected problems, it can also address those problems. One such solution is the use of Click chemistry. This method uses quick, modular chemical reactions to combine small chemical units.
This technique was used to provide a different method for plasticizing where the additive is covalently bonded to the PVC. The replacement chemistry is very similar to phthalates in structure and achieves the same end, but with a more permanent connection to the PVC. Traditional plasticizers are embedded between polymer chains but not covalently bonded. This allows them to evaporate or leach out over time.
There is some disagreement as to the rate of leaching or need for concern. Somewhat predictably, the European Council for Plasticisers and Intermediates (ECPI) claims that, “Phthalates do not readily migrate and leach into the environment…” Depending on the interpretation of “readily,” it may or may not be cause for concern over the “miniscule” amounts that are released.
So if you’ve ever wondered what that new car smell really is, you can now enjoy it with a new appreciation. It is the outgassing of the many additives used to improve the look, mechanical properties, and longevity of plastics.
Initially, the rate of evaporation of additives in plastics is much higher, so the scent is stronger. Not as appealing when you think about it like that. Innovative methods to mimic plasticizers with covalently bonded alternatives can lead to a cleaner, fresher smelling future. Now about those pants…
DISCLAIMER: The video below is put out by ECPI and may not be entirely objective. The video is informative on the role and variety of phthalates as plasticizers.
Images courtesy of pvc.org and truewellpipes.com