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Increasing hardness of physically foamed PS profiles

Increasing hardness of physically foamed PS profiles

Increasing hardness of physically foamed PS profiles

(OP)
I'm currently searching for a way to make our profiles harder and (if possible) cheaper. Current density is about 250 kg/m³, durometer hardness 22-25 Sh.D.

The obvious way is to use some kind of mineral filler. Preliminary tests have shown that both talcum and calcium carbonate can be dosed up to about 20% without deteriorating the quality too much. Oddly enough we see no increase in hardness. On the contrary, the hardness significantly decreases with added mineral filler.

I suspect this has everything to do with the structure and size of the foam cells, but exactly how the filler affects this is not known to me.

So, two questions: does anyone have experience with a similar process?

Are there any other possibly useful filler materials? I will be testing with wollastonite soon, mica is ruled out because of the glitter.

RE: Increasing hardness of physically foamed PS profiles

(OP)
Yes, we are experimenting with coextrusion as well. Forgot to mention that. We've tried extruding the coex layer together with the body of the profile but we haven't seen a significant increase in hardness from this. Possibly because of velocity discrepancies or because the blowing agent diffuses into the coex layer.

RE: Increasing hardness of physically foamed PS profiles

It may help to increase the temperature of the extrusion die (or exit portion of the die) to the point that the polymer in contact with the die surface drops to a low viscosity so the bubbles can breakdown and form a solid surface film on the profile. Foam that is hot wire cut has this effect.  

RE: Increasing hardness of physically foamed PS profiles

(OP)
Bubble coalescence/collapse on the surface means the surface will become rougher. This would be bad for market perception and volume of paint needed to apply.

RE: Increasing hardness of physically foamed PS profiles

(OP)
IRstuff: I think you are talking about EPS foam, which generally has much lower density (30-60 kg/m³ i think, ours are about 200). As such, the profiles we make are a lot harder. Target hardness is that of MDF, though I doubt we'll get that far.

Pat: Coextrusion has not yielded demonstrable results yet but the top layer should make our profiles much, much harder. I am more looking for a way to make them temporarily harder until the coextrusion route works out.  

RE: Increasing hardness of physically foamed PS profiles

Would coating with a thick laver of polyester two pot thermoset resin be viable short term. There might be quite a bit of sanding and polishing involved. That is how surfboards are traditionally made, well with glass fibre as well as polyester resin.

Regards
Pat
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RE: Increasing hardness of physically foamed PS profiles

But, EPS density range covers your stipulated density, and there are a number of coatings that can be applied, like Styrocrete Styropoxy.

I don't know what your Durometer hardness corresponds to, but this: http://benchmarkfoam.com/eps-properties/ shows EPS able to take 40 psi loads witn 10% deflection.  With an additional coating, it might get close to MDF, but given the overall density requirement, that sounds like a difficult challenge, since the relatively low density region is bound to collapse under load.

EPS supposedly can be made up to 640 kg/m^3, which should be quite hard, but will probably not be as hard as MDF.

TTFN
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RE: Increasing hardness of physically foamed PS profiles

Part of the reason that your hardness may be decreasing with increasing fill is that the particles can act as nucleation sites. Overnucleation leads to poorer cell structure and a greater percentage of open cells. So there may be an optimum percentage of talc or calcium carbonate that you're exceeding. The coex idea seems best, it's just a matter of finding the right stuff to skin it and get the process conditions nailed down. The nice thing is that because the skin will be a relatively low percentage of product weight, you can use more expensive engineering plastics.

RE: Increasing hardness of physically foamed PS profiles

(OP)
PCNJ50a: thanks for that! Overnucleation would fit my observations perfectly: higher open-cell content, ridiculous densities and high moisture absorption.

IRstuff: I did not know EPS came in such high densities; in that case we are talking about the same product as mine. I doubt whether TV casings really go as high as 600 kg/m³ but I might be wrong :)

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