As promised, here is the first of a set of entries on different aspects of flow-induced crystallization.
The picture below shows the tremendous impact that flow can have on crystallization rates. It's a little confusing, but it will become clear shortly. This plot is for polypropylene, but you will find similar results for HDPE and others. Look first at the left-hand side. This shows the number of crystal nuclei formed (Nc on a log scale) under non-flow conditions and at different temperatures. The cooler the temperature, the more crystals there are - pretty standard stuff. Any higher than 130 oC and the number of crystals is too small to measure (or at least wait around for them to form!).Now look at the right-hand side. This data is all taken at 140 or 150 oC, higher than all the data on the left-hand-side and look at what is going on - the number of crystal nuclei is in the same range as under the quiescent conditions, but the temperatures are 50 degrees or more higher.
Considering that most polymer processes have some form of flow occurring in them, the applications for this can be extremely profound across a wide range of products and processes.
As I mentioned before, things have changed in this area of research in the last 20 years, but data such as this has not. In the next post, I'll start talking about the explanations for why this behavior occurs - what we use to think and what is the current thinking.