hi, I am doing a projectof polyurethane-glass fiber composite. Although
I dryed the glass fiber before it was used, however, when I transfer
the glass fiber from the oven to the processing machine, it absorbs
water. The water absorbed by glass fiber react with PU and produce
bubble in the composite. Does any one know how to get rid of the water
in the composite? Thanks.
Obviously avoiding the water in the fibers is always the best step if
it can be done without excessive inconvienence and cost. Can you use
p-toluenesulphonyl isocyanate (PTSI) to scavenge the water?
Aspen Research -
"Turning Questions into Answers"
Opinions expressed herein are my own and may not represent those of my
In addition to John's recommendations, you might look at changing your glass
sizing. Common dry glass sizings (on roving, tow, etc) can readsorb water
pretty quickly, but you should be able to locate a glass vendor offering a
more hydrophobic(lipophilic) system, which can give you a lower liklihood of
significant water pickup.
Also, how you handle the dry glass will matter. Is the glass moving from a
high temperature oven to a significantly lower temperature area quickly?
Your problem may be simple condensation, and can be fixed by a variety of
different ways, like getting the ambient temperature up (and humidity down).
If you cannot change the ambient characteristics of the transfer area,
longer time at a lower temperature in your drying oven is a little bit
better, as would a ramp-down of temperature in the oven before transfer.
R. David Zopf
Bomar Specialties. Co.
You might have water in your resin to start with in which case a moisture
scavenger is your solution (although I would use powdered molecular sieve
from somebody like Grace Ind. rather than tosyl Isocyanate - it's very
effective but it's a bit nasty).
Secondly, you could also try running the urethane at a reduced ratio (i.e.
less isocyanate than you would normally use) of 4-6% or so. Less isocyanate
available for reaction with moisture = less outgassing, although this will
depend on your catalyst package. Properties will suffer a bit though so
don't go too far.
Thirdly, Isocyanate, particularly if it's old, can get saturated with CO2
gas and sometimes this comes out during cure. Often a good idea to degas
your iso prior to mixing asa well in case you have this problem.
Finally you could try degassing whilst curing as if you've got everything
else right, this will give you virtually bubble free castings.
Hope this helps,
If you find the PTSI a bit nasty (which it is, and can add an unpleasant
odour if used in higher quantities) you could look at an oxazolidine as a
moisture scavenger instead. These can also act as a cure aid as they ring
open in the presence of moisture and provide amine groups, so moisture
curing of urethanes occurs without carbon dioxide production.