Polyurethane for Composite Matrix Applications

[Bloefeld]

I spent the better part of an hour speaking with my main contact at Huntsman for polyurethane resin systems for composites. This guy is one of the world authorities on the topic.

If you don't want to read the gory details, I can give you a very short answer as to their suitability for non-production ski makers. Forgetaboutit, they won't work.

The long answer is related to a whole bunch of things. Among them are;

1) Visco-elastic properties. The more glassy the resin the more issues surround handling and cure heats. For composites 400,000 plus modulus, a glassy resin is needed. This is why most epoxies work pretty well;

2) Moisture. 0.2% moisture content in the materials will cause the polyurethane to foam. By foam I mean like house insulation foam. It takes a great deal of care and heat to drive the moisture from the materials. In the case of the wood core, it is next to impossible to do;

3) Mixing. As I pointed out earlier, mixing must be done by relatively sophisticated plural component mixing systems. The polyol and ISO should be kept under a nitrogen blanket in the tanks and the tanks should have a desicant filled dryer attached to it. ISO is great at dissolving gasses, so it sucks up water vapour like crazy. So even if you to everything more or less perfectly, you can still have a pile of foam as the ISO picks up air as it is mixed at the gun head;

4) Heat. To eliminate many of these problems, the cure should be initiated with correct levels of catalyst and a bunch of heat. This however cures the resin so fast that it will not flow into the fibre. Urethanes have been used successfully in resin transfer moulding (really tricky set-up) and I have used them with great success in pultrusion.

5) Post cure heat. This is a topic for both epoxy and urethane. Unless the resin is cured to nearly its glass transition temperature (Tg) it will not properly consolidate and achieve its full properties. Usually this requires a specific ramp up of temperature over time. For example 1 degree C per minute from ambient to 180C and hold for two hours. Then cool at half the rate. This gets complicated, but the most important thing if nothing else is done is to bring the resin to a temperature close to its Tg. Even if only for a few minutes you will get better results than just picking some random number to heat to and leaving it.

These are issues that can and have been sorted out in many instances, but for things like wet lay-up or vacuum infusion the technical issues would require many hundreds of thousands of dollars to resolve and as much more in testing.

My recommendation is to stick to epoxy or epoxy vinyl-ester products and give urethanes a pass unless you want to go head to head with Head.

Cheers,

Bloefeld.