Clay Crafting, Final Project, Science

The Chemistry of Polymer Clay

Since polymer clay is synthetically made, what is it made out of? After doing some research, I learned that polymer clay is made out of two major components: PVC and plasticizers. Let’s go through each of these separately.

PVC, also known as polyvinyl chloride, is a rigid plastic that’s used in a variety of applications, namely construction (do PVC pipes ring any bells?). PVC is synthesized in a chemical reaction called radical chain polymerization from its monomer (basic building block), vinyl chloride.

pvc

The little “n” indicates that there are a bunch of vinyl chloride molecules (which are monomers) connected together in order to make the larger polymer, polyvinyl chloride (PVC).

The mechanism of this reaction is based in organic chemistry, which will be in my presentation. (It’s the best part, right? Gotta save it for last! Unless of course, you’d rather learn about it before-hand. If so, check out that link!)

With that established, what are plasticizers?

A plasticizer is a substance that’s added to PVC to make it soft (you can’t have soft polymer clay without it!). There are two main kinds of plasticizers — phthalates and adipates — and the former is most commonly used in polymer clay. There have been some medical concerns and debates about the safety of adding phthalates specifically in polymer clay, and polymer clay brands have been known to reformulate their clays to reduce the amount of phthalates (such as Premo clay in 2008). However, if you follow the directions on the packaging and take care to wash your hands after handling the clay, there shouldn’t be any health consequences. (Here’s a really thorough article about polymer clay safety!)

The reason why plasticizers make polymer clay soft is simple, and has to do with how plasticizer molecules interact with PVC molecules. Between molecules, attractive and repulsive forces exist. The attractive forces bring the molecules closer together, and the repulsive forces push them apart. The forces between molecules are called intermolecular forces. The reason why plasticizers make PVC soft is because the plasticizer molecules fit in between the PVC molecules, preventing them from having strong attractions to one another. (These attractions are based on charges! Remember that opposite charges attract, and same charges repel.)

Here’s a little diagram that shows this. (It’s a larger version of this diagram.)

plasticizer diagram

The circle represents the plasticizer molecule, and the rectangles represent PVC molecules. The pluses and minuses represent electrostatic charges, which attract and repel each other (the negative and positive charges attract each other, and “like charges” repel each other).

The plasticizer molecule interferes between the strong electrostatic forces between the individual PVC molecules, which means that the PVC molecules are less packed together. As a result, the overall PVC material is more flexible!

Here’s a cool video that shows what happens when you heat a PVC pipe. (This pipe is being installed for a swimming pool.)

So what happens when you heat polymer clay? Unlike air-dry clays, polymer clay requires heating in order to harden (cure). The reason for this can be explained by the properties of PVC and plasticizers. As you saw in the above video, after the pipe was heated, it turned into spaghetti. It became extremely flexible, which is how the plumbers were able to install the pipe around the pool so easily. After the pipe cools down, however, it becomes rigid again. This flexibility is due to the plasticizers, and the rigidity due to the PVC molecules. However, there’s an important distinction to make note of here: heating a PVC pipe does not get rid of the plasticizer molecules, but heating clay does. (I’m not sure why this is. I think it’s because polymer clay contains a specific plasticizer that is permanently removed by exposure to high temperatures?)

Anyways! I hope you enjoyed this post! Stay tuned for “part three” of this polymer clay series — all about baking polymer clay!

References

http://beadnerd.com/?p=590

http://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Polymer_clay

http://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Polyvinyl_chloride

http://www.eng.buffalo.edu/Courses/ce435/2001ZGu/Phthalate_Plasticizers/PhthalatePlasticizersReport.htm

http://www.innovateus.net/content/polyvinyl-chloridepvc

http://www.pvc.org/en/p/how-is-pvc-made

http://www.pvc.org/en/p/plasticisers

http://higheredbcs.wiley.com/legacy/college/solomons/0470401419/spe_tops/special_topic_b.pdf

http://www.clays.org/journal/archive/volume%2012/12-1-603.pdf

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