Hello,
Im having trouble understanding why I am seeing increases in viscosity at low frequency and high temperature for some neat polymers. Ive attached an example with neat LDPE. Temperatures 110C - 170C behave like you would expect for a polymer but a 190C and 210C, the visosity starts to increase as the frequency reduces.
We've seen this in some composites, and Ive always assumed it has to do with some rheological percolation. But here you can see we have a neat polymer with no filler material, which shouldnt have any percolation effects.
Dear Matthew Eaton, this is attributed to the branching length and density. Please have a look at the attached file. My Regards
Is this behavior reversible? Is it not possible that some crosslinking occurs?
It is kind a strange behavior, but might be related to effects of additives or non-polymeric species e.g. catalyst residue, antioxidant, stabilizer etc. on the rheological behavior. It looks like a something induces crosslinking at these temperatures, possibly by abstracting hydrogen from the backbone. You would make a deep search about additives introduced into the PE. You should also experimentally work on the issue more in detail. You can re-check this behavior by applying time sweep tests at 190 and 210.
Probably cross-linking of LDPE started from high temparure that is very usual during usual PE processing when termal stabilizers (phenols, phosphites) are at low level or just consumed by a previous processing step
You can have a time sweep at a constant F (1 HZ) to see if the LDPE melt is stable under high T. Your frequency sweep may take 20 min, which is too long to not have PE degraded/crosslinked under high temperatures.
- Badges
- Science topic
- Similar topics
- Mathematics