Filling Silicone Rubber: TCE Variation

[bcavender]

thread335-506939

GoGreen’s blending question raises a question about modifying a rubber’s coefficient of thermal expansion by a filler.

For an example, let’s take SiliconeRubber and Graphite.

TCEs:
Silicone Rubber 190 (10-6in/in-degF)

Graphite ~ 2.2 (10-6in/in-degF)

If I create a homogenous mixture of the two in a 50/50 proportion, would the resultant TCE of the mixture exhibit a TCE of the average of the two or ~94?

I couldn’t find any research on quantitatively predicting the TCE variation of mixtures … ie would this process essentially be reasonably proportional to the component ratios or are there other more esoteric factors that enter into mixture performances?

Best regards,
B

 

[Compositepro]

Yes, it follows the rule of mixtures, using volume percent. If you use a fiber filler the fiber will restrain the expansion in the fiber direction, which then causes the rubber to expand more in the orthogonal direction. Silica is generally a more suitable filler for silicone than graphite. Fumed silica is what is used to keep silicone sealant from sagging.

 

[bcavender]

Silica is a great moderator for TCE, but it also has a very low thermal conductivity which in this case (that I didn't mention up front) conducting heat to and from the cured final shape is important.

So I need a filler where the physical expansion can be fine tuned, conduct heat handily while maintaining a high electric resistivity (in case of an adjacent wire failure would occur and touch it-trying to think of all the possible worst case situations).

Thanks! I do appreciate your time and knowledge!
B

 

[Compositepro]

Graphite is electrically conductive. Hexagonal boron nitride is an non-electrically conductive filler with high thermal conductivity. The thermal conductivity of BN may be a thousand times higher than silicone but adding it may only double or triple the poor thermal conductivity of silicone. The filler will also make the silicone stiffer and more prone to tear

 

[bcavender]

It sounds like fillers for SiRubber need to be kept very low to keep it strong … nothing like 30-40%?