Polyethylene

The stuff in your picture is a foam, if you just pour in a liquid you won't get a foam. Two part expanding foams from liquids are available, the concern would be to get an even distribution around the wheel, so that it is balanced, and without soft spots

Commercially there are foams put in tyres (or tires, depending where you are) to prevent flats, but they tend to be heavy, which I realize you are trying to stay awat from. I have used forklifts where this as been done. The people that do it put a low, slow expanding liquid in the tire, and have to keep it rotating for a number of hours while it expands and sets. Even so, the tires do not end up being balanced, but for a slow speed forklift, that is not a problem

If you have a foam that is light enough for your application, will it be strong enough? The pipe insulation you show tends to have a poor compressive strength, and my concern would be that it won't stop the tire from flattening.

I would be very interested to know what you end up using and how it works!

Thanks for your input. Think about this too, I figured on having a duel pump as to vacuum out all the air in the tube then pump in liquid to expand the after cure I could pump up with air and foam would actually be floating inside. It would not have to hold weight just prevent the tube from pinching when hitting sizable rock with great force. sort of just a cushion from the sidewall collapsing towards the rim.

There is a comercially available tube which can be brought through motor cycle accessory stores. It is called a Moose Tube.

I've seen this product but you don't use a tube with it and it tends to break down with heat. I am trying to get something to pump in and then inflate tube afterwards. this way you control the pressure with air and the foam just acts as a cushion to prevent the sidewall collapsing to the rim and pinching the tube.

Never done this, but it looks to me as you will have to first feel the tire with the foaming agent, and mount it later once set. If not I believe you will get a barrier built which will prevent further injection, that is if you agent is not totally liquid when injected, and if liquid it will not foam. using filler as used in the packing or building industries, Poly Styrofoam or a like is durable and cheap.

Wangito.

Bicycle tyres can be fitted with a polyurethane tape between the inner tube and the inside of the tread of the tyre. "Tuffy Tape" is/was a UK brand name <usual disclaimer> - run over a nail or a bunch of keys in the road inadvertently and it will give a measure of protection against punctures.

Have you considered using atactic polyalphaolefin (APAO)? I believe that a properly placed band of this stuff (perhaps, itself, inside a thin plastic tube) between the tire and the tube might effectively seal a puncture.

Is this foam really polyethylene basd?

I can only go by the label and it reads polyethylene so I guess thats right