Glass filled HDPE extrusion - much higher density than theoretical values.

[Nereth1]

Hi all,

I am by no means a materials scientist or materials engineer so I hope that this is not a silly question.

We are reverse engineering our own product, as the material (a glass filled HDPE) was blended in another country by a company that wouldn't give us much by way of datasheets.

The (presumably E-glass) fill in the HDPE was measured by burnout test to be 25.6%. This is the average of 5 total tests with results ranging from 23.6% to 26.1%.

The density of the complete composite was measured by gas pycnometer to be 1.245g/cm³.

The density of the HDPE fraction therefore comes out, theoretically, at 1.056g/cm³.

To my knowledge, raw HDPE only gets up to around 0.98g/cm³. I have been told that residual compressive strain from extruding the product could cause the higher density, and thus it is not unexpected. But that would be a hell of a lot of strain (keeping in mind the poisson's ratio would not allow volume to drop in direct proportion to strain), and I could not find any reference to how large such an effect could be when I searched for it. Does that sound like a reasonable explanation, or is something more unexpected going on?

 

[Demon3]

I checked the numbers at my favorite density calculator:

http://www.netcomposites.com/calculators/volume-weight-fractions

You are right that something is off. If you need to deformulate this I may be able to help or send you to someone who can.

Chris DeArmitt PhD
President - Phantom Plastics LLC

Trusted adviser to leading companies around the world

http://www.phantomplastics.com

 

[Nereth1]

Yes, that is the same calc I did more or less. Personally I think the most likely reason for it is an error in the density reading.

Have you ever heard of HDPE showing significantly elevated densities under any conditions, e.g. internally compressively stressed and held there by the fiber fill? Could such an effect reasonably account for a roughly 10% higher density?

I have no real experience in the area of extrusions and composites, so I am not sure if this elevated density is an understood physical effect, or if something is simply wrong in either the measurements or the material.