Ask Dr. Nick: Incorporating Recycle in Rotomolded Parts

Could you comment on the potential for using recycled PE in rotomolded water tanks?

Dr. Nick: The incorporation of recycled PE into tank materials is a relatively new concept and is not covered by any of the normal standards (eg ASTM D1998).  Some general comments on the subject are given below:

1. We need to be specific when we use the term “recycle”.  As a minimum, we should differentiate between post-industrial and post-consumer sources.  The former, eg utilizing molding scrap as a feed for material, is a common and established practice in rotomolding.  The latter, ie generating a feed for rotomolding from recycled consumer waste, is not yet generally practised.  I will concentrate my other comments on post-industrial recycle sources.
2. One practice, that is unfortunately all too common, is to turn roto scrap straight back into a powder, by granulation followed by pulverization.  This is a low-cost method, but it will result in a significant loss of material properties.  The property loss will be significant, even if the recycled material is added at a relatively low percentage.
3. The loss in properties is particularly apparent in relation to impact strength, especially at low temperatures.  Typically, we express this as Mean Failure Energy (MFE), measured in ft-lbs or Joules; there is an ARM Procedure¹ on how to measure this property.
4. In my own labs, I recently conducted a study where powder recycled from scrap was dry blended with “virgin” natural powder of the same specification.  Above 40% recycle content, the MFE dropped completely, to zero.  Even at low addition levels, the MFE was greatly reduced, by over 80%.  In addition, the failure mode changed from ductile (this was only for 100% virgin) to totally brittle (at any recycle addition rate, even very low levels).
5. In my personal opinion, using recycled material that has simply been reground and dry blended back into virgin is a bad idea.  Others may think differently, but at least I have data to back up my opinion!
6. Good roto PE grades will have a substantial additive package compounded in at source.  This is designed to protect the PE from the aggressive heat environment prevalent in the rotomolding process.  The package incorporated into grades destined for other processes (eg extrusion or injection molding) will be an order of magnitude weaker than for roto.  The additives work sacrificially; they are consumed during the process.
7. Several years ago, I was part of a research project² that tracked the consumption of additives during the rotomolding process; this required the employment of sophisticated analytical techniques.  We found that additive consumption varied, depending on a range of contributory factors.  As a general rule of thumb, at least 40% of additives were consumed during the rotomolding of relatively thin-walled parts; for thicker walled parts, this consumption increased.  The roto grades we used in the study all contained very robust additive packages to start with.
8. Bottom line: a LOT of additives will be lost in the first pass of rotomolding.  If these additives are not replenished in recycled PE, the resultant material will be inadequately stabilized against heat degradation.  If the original virgin grade was less than adequately stabilized in the first place, there may be almost nothing left!
9. In the absence of heat stabilizers, any UV stabilizers will start to be consumed for heat protection.  Therefore, you also need to be aware of possible reductions in long-term UV protection.
10. Adding stabilizers by dry blending is not satisfactory; for full effect, they should be melt compounded into the PE via an extrusion process.
11. With all of the above in mind, the sensible way of using recycled materials is to compound it and to top-up the stabilization package.  As part of the study described in Point 4, I incorporated recycle at various high percentages (including 100%) into virgin, by compounding, and boosted the additive package back to the original level.  With this treatment, the MFE was the same as for virgin material.  The failure mode tended towards brittle at very high levels of recycle, although the ductile failure mode still predominated.
12. One other important aspect of using recycle: you may find that, even with compounding, there is a visible color shift in the material.  There may also be implications for smell and taste aspects, and this could be pertinent specifically to potable water storage tanks.  Significantly more work needs to be done in this area.
13. If the recycled material is incorporated as a separate layer, in a multi-shot tank, a number of the issues described above may be mitigated.  However, this will add another layer of complexity to the production process.

Hopefully you find the above comments to be useful.  There is an element of my personal opinion here, so treat this as a guide, not a set of rules!

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.

1. ARM Low Temperature Impact Test. ↩︎
2. HENWOOD, NG et al: Removing the Mystery from Rotomoulding, New insights into the physiochemical processes involved leading to improved quality control; Proc. ANTEC, 2005
   SHARIFI, P et al:  Studies of Degradation Effects during Rotational Molding; Proc. ANTEC, 2012 ↩︎