At last! A non-PE polymer that I can rotomold!

henwood

Dr. Nick Henwood

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.

Our legal counsel notes: Dr. Henwood speaks for himself and not for ARM as a whole. We encourage feedback on this project but comments should be on technical matters raised. Dr. Henwood’s responses are solely his responsibility and not ARM’s.

I hope that regular readers to this blog series are not getting too depressed.  So far, I’ve looked at two polymers (ABS and PC) that both have – let’s be kind – “challenges” when you come to mold them.

This time I’ll talk about two related polymers that you definitely CAN rotomold.  

Polyamide “aka nylon” (PA) 11 and 12 are pretty similar to each other and all the remarks that follow relate to both variants.  As it happens, rotograde PA11 is made in the US and rotograde PA12 is made in Europe, so you tend to see more of the each type in its area of origin.

In my experience, PA 11 and 12 are pretty much as easy to rotomould as every rotomoulder’s favorite polymer, polyethylene (PE).  I’ve even come across molders who mold nylons and PE on the same arm, to the same cycle.

Another “second cousin”, nylon 6, presents the rotomoulder with a few more challenges, so I will cover this material is a separate blog.

PA has a great combination of properties compared to PE.  It’s considerably stiffer and harder and it resists the tendency to get floppy at high temperatures much better than PE does.  So, for example, it would be a suitable material for a rotomoulded hydraulic oil tank; these days the oil in hydraulic systems can exceed 200°F, at which point PE would be pretty much a blob.

At Room Temperature, PA has good impact strength.  When the ambient temperature drops below freezing, you do get a drop-off in impact properties.

In addition to all of the above, PA can provide a good bond to paint systems and it also provides an excellent barrier to permeation by hydrocarbon fluids like gasoline.

So … what’s not to like?

Most of you probably know what’s coming – the price.  There’s no doubt that PA 11 and 12 powders for rotomoulding are eye-wateringly expensive.  Added to which they have a substantially higher density than PE, which means that you will need more pounds of shot weight with PA in order to create a part with the same thickness as PE.

Polyamide is an engineering material and these types of polymers do tend to be more pricey than commodity grades like PE and PP.  In addition, they generally need to be ground cryogenically to form a suitable powder, which adds cost.  There are special packaging needs also.

It all adds up I guess, but there is no doubt that the very high unit price of PA 11 & 12 has stunted its growth as a polymer used in the roto industry.  Years ago, my company sold the only rotograde PA 11 & 12 available in Europe, so I had a unique opportunity to know the size of the market.  I was selling lots of rotograde PE too, so I knew what size that market was as well.  I can tell you that the market for rotograde PA12 was only 0.1 percent of the equivalent PE market.  I’ll bet the US market is similar.

Of course, there were applications where only PA 12 would do and there were customers who were willing to pay the price for the properties that the polymer offered.  Unfortunately, there were just not that many.

One thing to watch with PA 11 / 12 grades is their reaction with water

Both grades are hygroscopic and will absorb moisture from the atmosphere.  The rate of moisture absorption is much less than some other polymers (eg PA6), but it is still considered sensible to supply the powder packed in containers that have a moisture barrier.  Even when the bag is opened, moisture uptake is pretty slow, so you don’t get much deterioration as long as you close the bag up after each use.

Prolonged exposure to hot water will cause polyamides to degrade by hydrolysis, so it’s not considered to be suitable for hot water or steam applications.

One way of mitigating the high cost of the polymer is to use it as part of a multi-layer structure.  Personally, I think that this may be the way to go with a number of the polymers I will be describing in this blog series.  Because of its importance, I’m going to save multi-layers as a blog on its own.  So watch this space!

As always, please do respond to my blog and give me some feedback.

It would be great to get comments from rotomoulders who are using PA11 (or PA12) for roto applications.  Did you find it as easy to mold as I have described?

It would also be great to get comments from rotomolders who are making multi-layer parts on a commercial scale, from any combination of polymers (including PE).

How do molders out there feel about multi-layer moldings?  Whilst it’s admittedly not that easy to do on a conventional three-arm carousel, some of the new machine concepts out there can make multi-layers a realistic prospect.  I’m sure we would all be interested to hear from some of our forward-thinking machinery suppliers.  How about favouring us with your wisdom guys?

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.

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