Curing of Buna-N

[Janman]

My company is trying to heat cure our "Buna-N" rubber washers. We start off with a 90-95 durometer (shore A) Buna, and we try to heat-cure it to 95-100 durometer reading. We typically accomplish this by baking it at 350 degrees for 1 hour. Typically this process creates smoke.

This last batch of material that we got in from our supplier is around 90 durometer to start, and we baked it for 1 hour at 350 (without any smoke or stink), but it didn't have much of an effect at all. (93 durometer and still very flexible).

My questions are:

1. Is there a better method for curing Buna-N?
2. In the off-chance that our supplier supplied some other variation of rubber, is there a method to determine whether we received Buna vs an EPDM?

Forgive my ignorance in rubber characteristics. Any help would be greatly appreciated.

 

[polym3]

Why not just have your supplier make the washers to the hardness you desire?

If you are baking these in an oven, the smoke is probably as a result of the oxidation of the product.

The best way to cure these would be in an enclosed mold. The cure cycle needs to be completed in the mold, i.e. in the absence of oxygen. Post cure operations really add more expense to something that could be made to spec in the first place.

 

[Janman]

Long story short is that we used to buy them like that until one supplier told us that we should just bake them in the oven because that is what they would do anyways. I've only been with this company a little over a year, so this is the extent of the knowledge that I have regarding this matter.

We are currently trying to find material that is in that durometer range, but the samples that we received are still too soft even though the durometer reading is in the 95 range. Under our use of the material it deformed too much and stayed deformed.

 

[polym3]

Sometimes modulus is a better spec than hardness. It is a better way of describing stiffness so you don't end up with hard, but flexible product.

Ask the vendor to make the product with a low sulfur (EV - efficient vulcanization) cure system. This will give you the best compression set characteristics (and if you still have to post-cure, the oxidation probably won't be as bad).

Nitrile can also be cured with peroxides for higher hardness and stiffness. The tear and tensile strength is not as good as the EV system, but I think the compression set should still be quite good.

Both these systems are more expensive that conventional sulfur...so be prepared.

 

[Janman]

polym3 - thanks for the help thus far.

I have this nagging feeling that the material that we have may not be Nitrile Buna-N, is there any way to tell whether it is or not?

 

[polym3]

The only rough test for identifying cured rubber is the "burn" test.

THe following are the "subjective" ways some people describe the smell, etc:

NITRILE:
1) Burns with sooty flame leaving untolerable smell.
2) Burns with a characteristic smell which leaves a nasty taste in the mouth. Somewhat similar to burning fat.

EPDM:
1) Burns with a yellowish flame similar to a candle.
2) Burns readily with sooty flame leaving dry and waxy odor.

I hope these help.