Rubber hardness calculation

[Halilcan]

Hi all,

How can I calculate the hardness (ShoreA) of a silicone rubber compound? I have 70 Sh and 40 Sh base rubber, peroxide catalyst, and quartz as filler. I would like to obtain 65 Sh rubber. Is there any equation to estimate the hardness before producing?
İf i would mix the same amount of 70Sh and 40Sh base rubber (let’s say 10 kg per each base) how much filler am i going to need to reach the 65 shore value?

Thanks,

 

[berti]

Hi,

that is no rocket science.

Most silicone bases are additive concerning hardness, chemists call it mixing cross --> 5 parts of 40 Shore and 25 parts of 70 Shore.

(5x40 + 25*70)/30 = 65

Ground quartz has quite low influence on hardness (appr. +2 pts. per 10 parts of filler). Quartz is not so healthy stuff, look for a ready batch from your silicone supplier.
Quartz reduces Tensile strength , so check your specification before.

Remark: If you mix 40 and 70 1:1, result will be 55, hence you need appr. 50 parts of quartz. It is possible, it is cheap, but it is very crumbly stuff with low mechanicals.

Peroxide: Some silicones show very different Shore A values with different peroxides and all nominal hardness values are +/-5 usually, so better test hardness of your "40" and "70" with the intended peroxide type first before calculating final hardness.

Or simply do trial and error ;-(( Let us know your results.

Good luck.

Berti