Ask Dr. Nick: Is it Possible to Rotomold TPE?

Question: What can you tell me about rotomolding TPE?

Dr. Nick: The term “thermoplastic elastomers”, or TPE’s, covers a range of different materials, using a number of different chemistries.  According to ISO 18064, there are six generic classes of commercial TPE’s, together with one unclassified category:

• Styrenic block copolymers, TPS
• Thermoplastic polyolefin elastomers, TPO
• Thermoplastic vulcanizates, TPV
• Thermoplastic polyurethanes, TPU
• Thermoplastic copolyesters, TPC
• Thermoplastic polyamides, TPA
• Unclassified thermoplastic elastomers, TPZ

Some TPE’s will be made directly in a reactor (“copolymers”) and some will be made from a physical mix of polymers (usually a plastic and a rubber).

Probably, some types of TPE will be rotomoldable and some will not.

I have had success rotomolding TPO’s (thermoplastic polyolefin elastomers), which are essentially extremely low density polyethylene.  They can be used to make a part with reasonable stiffness and fantastic recovery properties; the part can be subjected to massive deflection, but substantially recover its shape once released.  Some commercial applications are road barriers, boat fenders and furniture.

In terms of whether a particular TPE will be rotomoldable, here are the main issues that will need to be considered:

1. The inherent flexibility of the material will most likely make it difficult to pulverize into a powder.  Using the normal type of ambient system is unlikely to be successful; the material may need to be cryogenically ground.  This can be done, but it’s significantly more expensive.
2. A key factor with any material is whether it has acceptable viscosity characteristics when melted.  Unlike other processes, rotomolding takes place under zero shear conditions.  We rely of the melt viscosity being sufficiently high to allow melting powder particles to fuse together, without us applying additional force or pressure.  Initially this can be assessed by modified MI measurements.  The other way is to simply try to rotomold a sample.
3. Watch out for unusually high melting points.  Linear branched PE’s, which we commonly use in roto, melt at around 250°F and need to be raised to approx. 400°F to cure properly.  If your new material melts at a substantially higher temperature, you may start degrading it before it can be correctly cured.
4. In any event, heat degradation should be carefully considered.  Roto grades need substantially more antioxidants to protect them, because of the relatively long cook times that we use.  I’ve found that the usual phosphite / hindered phenolic additives we use in PE are often effective for other types of polymer, so this may be just a question of getting some advice from an additive supplier or compound formulator.
5. TPE’s that incorporate a rubber component may have an extra susceptibility to heat degradation, especially if they utilize polybutadiene rubbers.
6. Whilst considering additives, you will also need to be mindful of UV stability, especially for outdoor applications.  A practical alternative could be to start off with a compounded black material.
7. My experience with TPO’s was that, unsurprisingly, they exhibited much lower shrinkage than standard PE’s.  Whilst this is potentially an attractive feature, your part design should take account of this and the mold may need to be multi-sectional in order to be able to extract the part at the end of the cycle.  You may also need to experiment with different mold releases; some types of TPE can be very sticky and difficult to release compared to polyethene.
8. Finally, don’t expect the TPE material supplier to know much, if anything, about rotomolding requirements.  As an industry, we suffer from being a niche market!

Please don’t be discouraged from pursuing your customer’s request.  Finding new material options that we can rotomold is essential to sustaining growth in our industry.  One day we may run out of products to make from polyethylene, much as we all love it as a material!

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.