PVC MIXING FACTS: FACT #1
A Fast Heating Process does not Mean a Fast Cooling Process
It might seem obvious that the mixing processes of heating and cooling are different. However, to prove it, we came up with some mixing trials. After several tests we found that there is a correlation between the cooling process and the ingredients used.
General differences in heating and cooling mixing processes
Of course, temperature makes a big difference, but this might not count. In the heating mixing process, friction needs to be created to put energy and heat inside the PVC blend. Most of the friction comes through the mixing tool design, but also from the friction of the PVC particle itself and from the blend ingredients.
In the cooling mixing process, the low temperature is present and just needs to be exchanged to the PVC blend.
Several parameters influence the cooling time.
Cooling surface: The cooling surface of the cooling mixer is very important. The bigger the effective cooling surface of the cooling mixer is, the faster the material is cooling down.
Tip speed and design of mixing tools: The mixing tools in the cooling mixer are designed to put no heat in the mixture, in contrast to the heating mixer. The tip speed is chosen so high that the material is moving but as low that it does not create any heat.
Ingredients: Several ingredients do have a different effect on the cooling mixer and on the cooling time.
How do ingredients affect the cooling mixing time?
For example, CaCO3: Mixtures with a lot of CaCO3 have a much shorter mixing time, but why is this the case? Since in the cooling mixer the task is to exchange and to transfer heat as soon as possible, the thermal conductivity is decisive. The thermal conductivity of PVC is about 0.15 W/(m*K) whereas the thermal conductivity of CaCO3 is more than 35 times higher i.e. about 5.5 W/(m*K). That means that the CaCO3 accelerates the cooling progress.
Another example are plasticizers: Plasticizers delay the cooling mixing process, but why is this the case? The thermal conductivity of plasticizers is very close to the one of PVC. However, plasticizers have double of the thermal capacity. That means that plasticizers safes more energy / heat inside than other materials. Therefore, the heat exchange process (cooling) takes longer than the other PVC blends.