Originally Posted by
combatpigg
MJD, did you just say that nitrocellulose dope can be improved with urethane hardener..? I'd like to do some tests if that is feasible.
Yes CP - but I've never got around to trying it out yet. The cellulose molecule has three-OH (hydroxyl) groups on it, those are the guys that react with isocyanates. When it is nitrated, the -OH groups are converted into -O-NO2.. but not all of them, esp in coating grades. In highly nitrated cellulose, i.e. as used in explosives & pyrotechnics, more of them are converted, whereas in lacquer/film grades the nitration is lower (level of nitration is expressed in % N) and as a result there are more functional OH groups available. (oh yeah, and they are less dangerous too..)
So yes, addition of an isocyanate should result in some degree of crosslinking. Crosslinking will harden, toughen the film, and increase chemical resistance to some degree. I can't qualify the amounts off the top, I'd have to know the curative side formulation (% of isocyanate and which one) and do some chicken scratching to come up with a starting point.
To do it by experimentation I'd use uncut nitrate dope, and make a sample matrix with 10, 20, 30, 40% by volume of your chosen curative, paint them out and also retain a small sample in a shallow dish, as in 1/8" deep, to flash off and form a small disc that you can fool with to see the properties., i.e. bend, flex, hit, chew. Use the materials neat when you mix them, then if you have to add solvent after do it then, to make it easier to maintain control of the amounts. By doing this, you should get a feel for the % that might be of benefit. Adding excess curative will result in weakening of the film too.. the unreacted
And with all isocyanate cured systems, moisture is your enemy. Water, yes is H20, but can be thought of as H-OH (aha! A reactive -OH group), and isocyanates tend to react preferentially with it to form CO2 and urea, rather than react with the (typically) less reactive prepolymer. That's the essence of the bad news with water and two part PU's - the water steals the curative and makes spoodge in the process.
You could find a guy that sells propellant binder ingredients to amateur rocketeers, and buy a bit of curative.. if you're ambitious that is.
PM coming.