5/7: Plastigels, Newton, and Conditioning

So far, you’ve learned that polymer clay is a vinyl plastigel made of plasticizer and PVC, that flows when pressure is applied.

When we’re talking about flow, we have to mention good old Isaac Newton and a property called viscosity. That’s the term for how thick and pourable something is. Honey is more viscous than water. Newton noticed that when you squeeze water, it doesn’t change viscosity. (Which is why hydraulics work so nicely to transfer force.)

But not everything flows according to the same rules. There are materials, called non-Newtonian fluids, that change their viscosity when pressure is applied. Some non-Newtonian materials flow faster (become thinner and runnier) when you apply pressure to them. Others become thicker, less runny, and flow more slowly when you apply pressure. And some of these fluids will crack apart when you apply force too quickly (yes, rate matters). You’ve undoubtedly played with oobleck, which is a cornstarch and water slurry? That’s non-Newtonian and it gets thicker when you apply pressure to it (and cracks apart when you move it too quickly). Quicksand is also non-Newtonian. Take a moment to watch this video. (Turn your volume down til 2:48 due to wind noise.)

What determines how a non-Newtonian material flows? It’s a complex relationship between the particles that make it up and the liquids that they’re suspended in. Some particles will interlock in ways that mean they don’t slide past each other as easily. It’s a topic that physicists have argued about for years, so I won’t get into the nitty-gritty of it. Just know that particle size and shape can affect how a non-Newtonian material flows.

As you’ve probably guessed, polymer clay is non-Newtonian. But…and this should not be a surprise by now…it’s not simple. Particle size matters. And polymer clay is made of a whole range of particles. The PVC resin often comes in a range of sizes (and different brands use different PVC resin types, each with different particle sizes). Fillers such as bentonite or calcium carbonate have their own particle sizes. Plus pigments are particles. And the specific plasticizer used also matters. So each brand (and color) of clay can have a different response to pressure. It can even have a situation where a single polymer clay mass can have contradictory responses to pressure, getting both thicker and thinner in response to different amounts of pressure. Yikes, this is getting complicated!

And now…conditioning.

Just as you saw in the video where the quicksand became more fluid when it was stomped upon, polymer clay changes viscosity when it’s disturbed. Conditioning disturbs the interaction between the particles in the clay, allowing them to flow more freely past each other. We condition polymer clay to affect its flow and behavior (similar to stomping on quicksand). And because it’s the specific integration between particle sizes and fluids (plasticizer) that affects the flow of a non-Newtonian material, different brands or colors of polymer clay will require different amounts of “disturbance” to become more fluid. Temperature, as we already know, adds another fun twist to the equation and is why warm clay often requires very little conditioning.

And now you see how the polymer chains within the PVC particles have nothing to do with conditioning. That’s just a misunderstanding. Also, conditioning doesn’t have anything to do with mixing. It has to do with pressure and Newton and flow. We don’t need to mix polymer clay to condition it. We need to disturb it by applying force. This is why you can condition canes and sheets without mixing them. You just “disturb” them.

And just like the woman in the video pulling her leg out of the quicksand quickly, moving polymer clay TOO quickly will give you a lot of resistance. What happens then? Well, that’s for my next article.