Increasing hardness of physically foamed PS profiles 1

[Arcodes]

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.

 

[patprimmer]

Is there any way to increase the density significantly on the surface only.

Regards
Pat
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[Arcodes]

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.

 

[Compositepro]

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.

 

[Arcodes]

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.

 

[IRstuff]

Most PS foam used for TV packaging is quite hard; are you talking about harder than that?

TTFN
faq731-376

 

[patprimmer]

If you do the co-extrusion right, there will be no bubbles near the surface to collapse so the surface should in fact be higher gloss.

Regards
Pat
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[Arcodes]

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.

 

[patprimmer]

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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[IRstuff]

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
faq731-376

 

[pcnj50a]

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.

 

[Arcodes]

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