Are you a valve manufacturer? If you are (no doubt), why should anyone give you a free answer? The reason you are in business is to develop your own products. So, DEVELOP YOUR OWN PRODUCTS!!!!!!!!!
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"Reason is not automatic. Those who deny it cannot be conquered by it. Do not count on them. Leave them alone." - Ayn Rand
For LPG which contains Butane, natural rubber should not be used, as it will be attacked and perish.
You will need a special plastic compound as butane is extremely 'hard' on plastics, I think Viton is the material of choice but please check with a pneumatic supplier of valves for use with butane and propane.
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A little knowledge is a dangerous thing - Googling is far worse!
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