Correct o-ring materials?

[jirushi]

Hi,

What is the best o-ring material to use in CO2 airguns?

I have come across: nitril, silicone, urethane.

For instance, TR Robb sells QB78 seals that seem to be NBR 90 (nitril). For tube cap and/or valve body.
They seem to work well (as in, they don't leak) but someone told me that I should be using urethane.

I often come across white urethane seals in tube caps and valve bodies.
A few times, after discharging all CO2 and disassembling the gun thereafter, a white seals has made crackling noises.
I guess this meant that it had absorbed CO2 and wasn't made of the right material?
But perhaps white o-rings can be made of other material than urethane.

Then I have seen red and blue tube cap o-rings.
The red ones can be silicon. An American gentleman has told me that this is not compatible with CO2.
No idea what the blue one can be. Perhaps urethane with a colour.

I thought it would be good to do a post about this, as perhaps other tinkerers are confused by these o-ring materials too?
In pcp's it's easy: nitril. NBR 70 or 90.

Many thanks,
Louis

[Hartcliffer]

I always used nitrile seals in my 78's and 79, and never had a problem with them, although I always used to soak them in silicone grease overnight (don't know why, or even where I heard it from, but I did it). I used to change them every 12 months or so, and I never had a rifle leak in all the years I had them (still have them, in bits).

[the logun-ator]

I've not done a lot with co2 stuff but they type used also depends on the location in the gun and what the seal does.
I know co2 seals can freeze which is totally opposite to say a pcp inlet orings where it can get overheated /crack a lot quicker that any other orings in the gun.
It also depends on if its a static orings or a dynamic one.the latter is better in viton as they wear a lot less than std nbr stuff.
Cracking sounds heard can actually just be grease/lubricant on the seals/internals moving when the gun is pressured up in both co2 and pcp.
Poly seals can be hard wearing but can only be used in certain places.
Not sure if this has helped but there you go

[--ped--]

pu90 doesn't absorb co2 so when it is exposed to atmosphere it won't swell like all the other seals do
silicone only absorbs it a little but is very soft and will be damaged easily
epdm if smothered in silicone oil is ok but will swell then go down
viton will swell and i find will them break easily
nbr will swell
the interesting thing with compatibility charts is they don't take into account exposing the o ring to atmosphere after it has been in contact with co2
these pick show before and after being exposed to liquid co2 for 24hrs

[Benelli B76]

The colour in polyurethane seals is irrelevant, it is just a dye, I worked is a urethane factory for 20 years, we used red, blue or green to suit the customer. Plain urethane can be white or toffee colour and either TDI, NDI, or MDI mix. It is the best all round material to use but sometimes too hard to easily seal in certain applications (high Shore hardness) which can be measured with a Durometer if you buy one.

Baz

[flogger]

Go on line and put in. an 0 ring material selection guide 19/05/2016-Martins Rubber.Tells about all types including neoprene.