The inventor of the process, Dr. Layman, has said you can put all sorts of PP into the process and still purify it, but that from an economic perspective, you need high levels of purity (90%+). No mention of lower purities causing thermodynamic problems.

(see comments below)

https://www.purecycle.com/blog/frequently-asked-questions-about-pcts-purification-technology

Actually, from a technical point of view, the process and the technology can handle, you know, feedstocks with 1% polypropylene. It's just a function of techno-economics that we have to keep the purity high. As I mentioned earlier, we've taken a used dirty diaper and when I say used, I mean, used, all of the glory in it, and we can purify that and that has a lot of—diapers have a lot of extra stuff beyond the bio-burden. There's a lot of different materials, there's glues, adhesives so from a technology point of view, we’re extremely robust. The reason we have a specification of 85%, 90% plus is if you put these materials into the process, and let's say, we had a 50% feedstock, you're putting a lot of energy and effort into melting, starting to do steps to the process, but that material is ultimately not generating revenue. And so we want to ensure that if we can mechanically enrich materials from our mechanical steps in advance of the solvent process, that we do that so that we are left with the best likelihood of, you know, high-profit process, where you know every material that's going in is as efficient as possible in terms of feedstock contamination. If, however, a material does end up being, let's say a truck shows up, it's expected to have 90% but instead it has 85%. The process and the technology can cope with those higher amounts of contamination. Again, it will just ultimately reduce the throughput for that moment in time at which the higher, more taxing feedstock is being run through the process and the technology.