Ask Dr. Nick: Why Am I Observing Brittle Failures in LLDPE Parts?

Question: I’ve been using the Low Temperature Impact Test to evaluate our LLDPE cooked parts and consistently observing brittle failures. I suspect LLDPE might not be suitable for this test, but I lack supporting data. I’m reaching out to determine if LLDPE is the issue, prompting a material switch, or if our cooking parameters require refinement.

Dr. Nick: The ARM Low Temperature Impact Test certainly IS suitable to assess most LLDPEs.

If you’re getting brittle failures, there are several possible reasons:

1. Your parts are undercooked.  If you haven’t removed most of the bubbles in the wall, by sufficient cooking, they will act as crack initiation sites and likely cause brittle failures.
2. Your parts are overcooked.  You have heated the material past a temperature where heat degradation of the polymer occurs.  If the stabilization package in the LLDPE is inadequate for roto, you may find that you simply can’t locate an optimum temperature at which to mold.  I’ve seen this in grades made by PE suppliers with limited experience of the needs of rotomoulders; of all the processes, rotomolding is by far the most heat aggressive and stabilizer levels need to be orders of magnitude greater than for injection and blow molding.
3. If you can measure the Inner Air Temperature inside your mold, that will give you a good guide to whether you are at an optimum processing condition.  Devices like K-Paq, Rotolog, DataPaq all do this.  Typically, aim for a Peak IAT (PIAT) of 375-425 degF, but consult your PE supplier and see what they recommend.   If you don’t have a device, strap a heat-sensitive label to your vent; this will give you an idea of your PIAT.  These are sacrificial products, but they are relatively inexpensive.  There may be other suppliers, but I know that you can get them from the www.493K.com online shop.
4. Your material is contaminated in some way.  As soon as you introduce even tiny levels of solid fillers or pigments, the failure mode of the natural material goes from ductile to brittle.
5. Pigment, dry blended, counts as a contaminant.  It immediately changes failure mode in natural material from ductile to brittle, as well as often reducing Mean Failure Energy (MFE) by 50% or more.  I don’t believe that this can be changed by changing the mixing method from low-speed to high-speed blending; I’ve done extensive testing of this and, however you dry blend, you get brittle failures.  There are people out there who claim differently, but none of them have ever been able to provide me with documented evidence.  Having said this, high speed blending does have some other benefits, although production costs tend to be higher than slow speed blending (higher power consumption and more difficult mixer clean-downs).
6. You have mixed two dissimilar powders together.  I’ve seen molders mix two different grades, in an attempt to achieve a specific density.  This is never a good idea.  Neither is the (fairly common) practice of dry blending color compound (aka “precolor”) with natural, to reduce material cost.  Sometimes this works OK, but sometimes it really doesn’t.
7. You use a very high MI resin (>10 g/10 min); these resins do not always exhibit ductile failure mode.  Sometimes a combination of relatively high density (>0.940 g/cm³) and higher MI (>4 g/10min) affects failure mode, although there are some grades out there, at this spec, that work well.
8. As sample plaques get thicker, brittle failures become more common when using a dart with a 1-inch tip.  That is why, for really thick-walled samples, we recommend you use the 30# dart, which has a ½ inch tip.  A 1-inch tip is specified for the 10#, 15#, and 20# dart types, which will be good for thicknesses up to approx. ³/₈ inch thick (depending on material type).
9. The ARM procedure specifies a maximum drop height of 10 feet.  Dropping the dart from higher will increase the velocity on impact above an acceptable range.
10. It’s not uncommon to get a few brittle failures in the ARM test, even with a good material processed under favorable conditions.  My rule of thumb is that if less than 20% of my failures are brittle, I can assume that the overall failure mode is ductile.  The test specifies a variation of no more than ±10% in the thickness of sample plaques.  Thickness variations beyond that limit can be the cause of increased brittle failures.

I think that’s a reasonably comprehensive list.  If I was going to start somewhere, I would start with Point 2; that’s the most common cause of brittleness problems, in my experience.  I test a lot of grades for all sorts of people, and I find materials with inadequate stabilization surprisingly frequently.  Most local grades made locally in North America and Europe are pretty good in this respect, but offshore resins sometimes prove to be deficient.

Anyway, I hope that sheds some light on the subject.  Don’t despair!

Dr. Nick Henwood, Technical Director of the Association of Rotational Molders, is a 30-year expert in materials and process control. He operates Rotomotive Limited as a consultant, researcher, and educator in the UK and was inducted into the Rotational Molding Hall of Fame in 2022.