Extrusion processors’ problems meet their match

All answers are courtesy of Allan Griff, extrusion expert and the host of our webinar series. Don't miss the rest of the series; the next webinar, scheduled for February 24, will have Allan discussing what happens between the end of the screw and the die: screening, mixing, pumping, and a few other things. The registration link is already active here.

Both of today's questions deal with processing of high-density polyethylene (HDPE).

1. What is the best way to reduce gel levels in HDPE?
AG: First, make sure you are talking about gels by examining them in a low-power microscope. Gels are uncolored, even if the resin is colored, and roundish but not perfectly round in shape. They won't dissolve in solvents for the base resin, but that's academic for HDPE as anything that dissolves HDPE is something you won't want to work with.

If you are sure these are "classic" gels, the next question is whether they are coming in with the resin or are they created in the process. The best way to find this out is to run the virgin resin through a clean small extruder. It's virtually impossible for gels to form from clean resin in just a few minutes. Thus, if the product comes out with gels from the beginning, and the gel content stays more or less the same for a while, they are in the resin and you have a good case for rebate or refund if it was sold to you as clean. There is even a machine on the market which blows film and counts "discontinuities per square meter of surface" to quantify gel content.

You can also get the same answer on production machinery but it is less precise. If the gels don't appear for a while, maybe a few hours or even a few shifts, and the material and conditions stay the same, it's likely that they are forming in the system, most likely the adapter and die in places where the melt is moving most slowly (large diameters, sharp bends). Formulation matters: some processing aids coat the inside of the flow paths and discourage gel formation, as well as reduce back pressure and thus allow lower melt temperature. Resins vary in thermal stability, too, and less-stable materials will degrade faster into gels and (eventually) black or brown particles. Running a small diameter (low mass rate) in a large machine will contribute to this, as the melt must move more slowly and stay hot longer. If the gels start as soon as the suspected resin is used and stay relatively constant, they are in the feed. You can, of course, have both things going on at once, where there are plenty of gels in the incoming resin but after a while you are adding to them by degrading materials in the head/die.

 
2. How and why does overheating HDPE make it stronger?
AG: When any polymer is heated there is some chain breakage. As might be imagined, the hotter the temperature and the longer the time at that temperature, the more the degradation. However, there is also a cross-linking and chain-growing reaction that takes place where the loose, broken ends of a molecule look for and find places on some other chain to attach. This makes that molecule larger and stronger and compensates for the weakening effect of chain breakage. In the case of some HDPE grades, the viscosity actually increases (equivalent of melt index decrease). But this can't go on indefinitely, because the breakage is normally inhibited by antioxidant in the formulation, which is consumed as the melt remains at the high temperature, and eventually is used up. After that, the breakage occurs faster than the crosslinking and the viscosity starts to fall again, representing a weakening effect.
 
I was involved in these experiments many years ago, when we were trying to show that the use of "scrap" HDPE pipe did not necessarily weaken the pressure pipe made from such a mixture. We ran several HDPEs in a torque rheometer for up to an hour, far longer than the usual residence time in an extruder, and sure enough, the torque (measure of viscosity) which had slowly increased for the first half-hour or so, started to fall off as expected from the above explanation.
 
However, before we all go out and buy recycled milk bottles to strengthen our HDPE film and pipe, we must remember two things:
(a) recycle already has some of the antioxidant used up (two meltings if it's been pelletized, one if flake), and
(b) there is contamination inherent in reuse of scrap, and particles may act as stress concentrators which lead to failure even if the base resin is still strong. What's important here is fine screening and selection of resins with good resistance to crack propagation. People who run pressure pipe will know what I mean, or if they don't, they should. —[email protected]