# CO2 and rubber and silicone...



## ian_m (7 Mar 2012)

Been niggling in my mind for ages about CO2 and rubber and silicone as a general no no in aquatic world, but not seen any real evidence of why not. (OK someone in US tried 25m of silicone tubing as tank CO2 diffuser and got nothing and others report issues with clear vinyl tubing cracking).

Well something cropped up at work, to do with gasses and rubber, and in this case oxygen, rubber is a complete no no with pressurised O2 (obvious fire risk). I mentioned "what about CO2 and rubber/silicone".

Well the external engineer we were working with (on plant control) stated

- CO2 dissolves in rubber and CO2 permeates (passes through) rubber. 
- CO2 dissolves in silicone less than rubber, but CO2 has a much higher permeability in silcone.
- The amount of CO2 depends on type of rubber/silicone and partial pressure.
- Silicone membranes are being researched as "CO2 extractors" to remove CO2 from boiler gases by as CO2 passes quite easily through the silicone.
- Silicone tube for CO2 is available, but it has a barrier layer to stop it escaping.
- Fill a balloon with CO2 and it will "leak out" considerable quicker than air and eveb helium. Argon is best non leaking gas to fill balloons with. This is an experiment we could try with our CO2 setups.
- Google for "why does CO2 escape from a container more easily than air?" for some numbers and why CO2 passes through rubber.
- Generally, he said, the CO2 would cause the rubber/silcone to expand and change properties. Even worse if CO2 is turned on and off as seal would possibly expand and contract as CO2 came and went and seal would creep and overtime may compromise the seal.
- CO2 does cause some plastics issues, change in physical size, hardening etc, but he wasn't sure which ones.
- As for 2 bar CO2 for fish tank usage, probably wouldn't have any issues, but generally best to avoid rubber/silicone and CO2 if at all possible.

So there you go. CO2 and rubber and silicone maybe a no no.


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## bluemoon280 (7 Mar 2012)

Hmmmm. So what impact will running co2 in a tank over time have on the silicon holding the glass together?

I would hope little else we would have lots of failing tanks. 

Your thoughts would be useful though

Regards


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## ian_m (7 Mar 2012)

bluemoon280 said:
			
		

> Hmmmm. So what impact will running co2 in a tank over time have on the silicon holding the glass together?


All the reports I read were talking about gaseous CO2, not CO2 disolved in water.

I suspect solubility of CO2 is far greater in water than silicone, thus will preferentially stay in water rather than the silicone.

Not too sure about rubber as that has a far greater CO2 solubility than silicone, so might pull the CO2 out of the water.

But a good point.


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