polyurethane composite bubble problem

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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?
John Aspen Research - www.aspenresearch.com "Turning Questions into Answers"
Opinions expressed herein are my own and may not represent those of my employer.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Good Luck!
R. David Zopf Bomar Specialties. Co.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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,
Simon

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

isocyanate
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.
Colin
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@gmail.com wrote:

Hopper mounted drying system. -40DP minimum (Desiccant, CDA, Vacuum). Another way you can go is nitrogen purge on your hopper.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.