ARM Technical Director, Nick Henwood, is drafting a publication for the ARM Education Committee on rotomoldable polymers other than PE, PP, and PVC. Early drafts of each chapter will be published here every two weeks.
If you’re looking for a polymer that’s superior to polyethylene (PE) in terms of properties, ABS (acrylonitrile butadiene styrene terpolymer) is a good candidate. It’s a very popular choice made by injection molders who want to make great looking parts that are heat resistant and tough.
So can you rotomold it?
Given that nobody does at the moment, you can probably already sense that there’s a great big “BUT” looming; several “buts”, in fact!
The “BUT” that seems to beset rotomolding ABS is butadiene, the “B” in ABS. This “B” component (aka comonomer) creates issues for rotomoulders in two critical areas.
Firstly, the butadiene comonomer is not very heat stable. In the relatively harsh conditions that prevail in rotomolding, especially the long cycle times, the butadiene becomes unstable and degrades. You could think of this as a similar effect as when you over-cook PE; eventually the material degrades and loses its ductility and toughness. Bad news, in either case.
It’s quite likely that the degradation problem could be overcome by using an inert atmosphere (i.e. nitrogen) inside the mold, which will ensure that the level of oxygen is sufficiently low to inhibit the typical hydrocarbon degradation reactions that occur with high temperatures and long cycles. Personally, I’ve always struggled with using inert gases in practical roto situations; your typical steel or aluminum mould is sufficiently leaky that it’s hard to maintain any pressure inside it. However, with the right parting line design and (maybe) some type of gasket, this could be a practical option.
The second issue with the butadiene component is the way it is distributed within the matrix formed by the other two comonomers (styrene and acrylonitrile). I’ll try not to descend into techno-babble, but this story does get a bit complicated.
There are several different ABS production processes that combine the three components (acrylonitrile, butadiene and styrene) together and each of these processes result in a different distribution of butadiene within the polymer matrix.
The suspension process produces quite a poor dispersion of butadiene but, counter-intuitively, this is far better for rotomoulding than another ABS production process, the emulsion process.
Why is the dispersion of the butadiene component so important? It’s well known that finer dispersions of one component in another can act to significantly increase the viscosity of the melted polymer mix. When injection molders encounter a problem like this, they just ramp up the mold pressure to speed up polymer flow. Unfortunately, we rotomoulders don’t have that luxury, because we mold at ambient pressure. I’ve encountered exactly this same problem when trying to make filled PE blends; you can easily turn a runny PE with a 50MI into an unmoldadable 0.5MI, just by incorporating a fine filler. Very frustrating…
Going back to the suspension process, another significant benefit is that the product that comes out of the reactor is in the form of small pearl-shaped beads that are apparently rotomoldable, eliminating the need for a grinding step.
Unfortunately – wouldn’t you know it – the emulsion process is far more common amongst ABS producers than the suspension process; emulsion does have significant other benefits over suspension for injection molders.
In the mid-1970’s, a Japanese company, Diacel, introduced a grade called ABSROM; this material was claimed to be rotomoldable, but it never progressed to full commercial availability. A subsequent study by the University of Utah, in the late-1990’s, suggested that some other polymer companies might have the right technology (ie the suspension process) to be able to produce a “roto” grade. After this, the trail seems to go cold….
So what’s the bottom line of this ABS story?
In terms of the “size of the prize”, it’s an attractive option. If our industry could get its hands on rotomoldable ABS, there are a number of important new application areas that could open up. Not least in the automotive sector; they love ABS as a material for injection molded parts.
In terms of the technical barriers, the heat instability of the butadiene might be manageable, with some process modification. The problem with butadiene dispersion is about (step 1) locating an ABS producer with the right manufacturing process and (step 2) persuading them to make relatively small quantities for us. Big polymer companies usually won’t get out of bed for less than two million pounds of resin, but who knows, we might get lucky!
My conclusion: despite the barriers, this seems to be a polymer where some targeted effort might yet yield a real dividend.
I hope this has given you an overview of the issues. Please do respond to the blog and give me some feedback. Once again, anyone out there with practical experience of rotomoulding ABS would be especially welcome to comment.
Here’s some questions, to get you all started: Have any of you ever rotomolded ABS? “How was it for you?” Is my rendition of the history of ABS accurate? Does anyone have anything to add? Do any of our super mold makers have any tips on sealing a mold for inert gas flushing? Are any of our specialist material suppliers interested in doing some work on ABS? Which of our suppliers are already investing some of your profits in R&D related to ABS? Please leave a reply with your feedback.
Read the introduction to Dr. Nick’s Guide to Alternative Polymers
Dr Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has 25 years-plus experience in rotomolding, specializing in the fields of materials development and process control. He operates as a consultant, researcher and educator through his own company, Rotomotive Limited, based in UK.