Ive heard that resin shrinkage can lead to twisted boards and more rocker than originally shaped, due to extra glass and resin on the deck.
Does anyone know if UV cure resins shrink more or less than regular catalized resin?
If there is more shrinkage using UV cure, what things can be done to better control or minimize it?
read about foam shrinkage but never heard of any kind of cured resin shrinking… ?
C&P:
Predicting Shrinkage in Polyester Reinforced by Glass Fabrics
T. Vu-Khanh
University de Sherbrooke, Faculté de Génie /Département Mécanique, 2500 bout, de l’Université, Sherbrooke, Québec, Canada, J1K 2R1,
V. Do-Thanh
University de Sherbrooke, Faculté de Génie /Département Mécanique, 2500 bout, de l’Université, Sherbrooke, Québec, Canada, J1K 2R1
Polyester is one of the most common resins used in contact lay-up method because of its low cost, room-temperature curing, wide availability, ease of handing, etc. However, the main disadvantage of this resin is the large volumetric shrinkage after curing (up to about 0.5%). This represents a major problem because it can cause unexpected defects in the molded composite parts such as warpage, distortion, rippled surface, etc. The effect of resin shrinkage on composite deformations is very complex because of the anisotropic properties induced by the fibers, especially in woven fabric composites with interlacing yarns. Moreover, in many applications, when the part geometry has a double curvature, the forming process usually results in significant in-plane shear deformation of the interlaced yams. The angle between the fill and the warp threads is no longer orthogonal because the fabric must follow the shape of the mold. In this work, an approach to measure the shrinkage coefficients of the interlaced yarns of fabric structure has been developed. The recently proposed sub-plies model has been used to predict deformations due to resin shrinkage in woven fabric composite. Resin shrinkage can lead to an expansion in the laminates with specific angles between undulated yams, due probably to a straightening effect on the fibers. Expansions due to matrix shrinkage were verified on several woven laminates. Prediction of deformations due to matrix shrinkage by the sub-plies model is in good agreement with experimental measurements.
Didja get all that?
That cut and paste doesn’t really address whether there’s a difference between UV or catalyzed (I suspect not).
This link, though brings up a question that might warrant its own thread, namely, vinylester resins. http://www.redrockstore.com/resin.htm
Has anyone ever used them?
Hey craftee,
Had to check with the composites gurus before I could answer your question, here’s what they told me:
UV cure polyester shrinks less than MEKP catalyzed because it cures faster and less styrene monomer escapes
during the cure.
Rocker changes during cure are usually non-existant to minimal, but there are cases of significant distortion; this is
almost always in direct proportion to how important the board is. For example if I shape a board for a guy who surfs twice
per year and can barely stand up, the rocker will turn out identical to the shaped blank. If I shape a board for Joe Pro who gave
me detailed measurements and/or supplied a rocker template, the thing will gain a half an inch in tail rocker and a quarter inch twist
after glassing.
I’ve got some good stories about this if you want to hear them.
Makes me glad we work off rocker tables now and deviations and twist are in my past.
Mike
"UV cure polyester shrinks less than MEKP catalyzed because it cures faster and less styrene monomer escapes
during the cure."
Does this mean it’s weaker as well, because there is less styrene evaporation?
JSS
Does this mean it’s weaker as well, because there is less styrene evaporation?
Actually UV cure polyester is ‘‘stronger’’ than MEKP cat, mainly because all the little polyols get organized better.
I’m sure this has been discussed on here somewhere, right? If not, there are others on here more qualified than me to
explain it.
If I remember correctly, you don’t want to lose styrene, either. It’s the ‘‘glue’’ that polymer chains are set in.
Resin chemistry isn’t my area, I probably know just enough to get myself in trouble. I’m more of a shapy, designy kind of guy.
Mike
Hi Allan,
I have used vinylester in the past and it is very good, the only problems were the price and the terrible colour.
Regards
Daren
Hey Guys
I have used to with goods result.
But only to put in fin boxes and plugs.
I ordered it from fibreglass australia I think they are called.
Pre-mixed for me with milled fibre and white pigment.
Apparently it has a better bond than polyester.
Turned out quite good.
Cheers
a little birdy once mentioned it might be poorly design or poorly utilized light boxes that are typically used in production shops.
I sure if parts of the UV laminate kick at different times due to their location to the fixed light bulbs used, that could cause twisting especially if the board isn’t supported properly which is why the use of rocker tables like Mike mentioned is a plus.
but you never know
I’m sure 80% of those manufacturing boards don’t care whether their worked is ruined by a glasser because 80% of their clients won’t notice it either. Kind of like how certain folks look at auto paint jobs I guess…
Here’s a little animation showing why you don’t want to lose styrene. Styrene is not only a solvent, but also a monomer in the system. In other words, what doesn’t evaporate is crosslinked into the cured solid.
polyester resin/styrene reaction
click on pic in link to add peroxide and start the reaction
so it does make sense that if you lose less styrene that there will be less shrinkage.
Love the animation, do you know if there is a similar one for epoxies?
Rik
Very cool, thanks!
JSS
Yeah some good info being contributed here.
I wonder about the results of the following experiment.
Cut two 4 inch wide strips of thin 1/8" luan ply, say 6 feet long each.
On a flat surface, glass some 6oz surfboard cloth on one side,
one sample gets regular MEKP treatment,
the other UV cured in the sun or light box.
The one that shrinks the most should be easily identifiable with more bend.
I would do this myself but I dont have any poly resin…anyone want to volunteer?
This knowledge could make your poly boards better and it would take less than one hour of your time.
Do it in the spirit of old Saint Nick?
So since minimization of styrene loss will improve strength, do you have any recommendations to reduce the loss? Is it better to set off batches hotter so they cure more quickly, and therefore there’s less time for the styrene to evaporate?
Thanks for the cool links
there is such a thing as kicking polyester too hot or too cold, resin manufacturer’s recommend certain catalyst levels to give maximum cured resin properties. There are also different catalyst (or even different grades of the common MEKP) that affect gel and cure times.
look at this for different catalyst grades
So, it really is an intersection of art and science sometimes. Craftee’s experiment is a good place to start. And if you are really serious you could vary catalyst grades and amounts and then look for shrinkage and strength differences. Then you could write a technical paper like the one in the earlier post or keep the info to yourself and build stronger boards that leave the imports scratching their heads.
One extra benefit of UV cure is that the first layer of the resin to start curing is at the top where the light hits first. This tends to trap styrene in the resin while the rest of it cures giving less shrinkage etc.
MEKP cures on the other hand tend to be fastest near the foam because off the heat retention there. This tends to heat the bottom of the resin faster and drives styrene to the surface where it can escape.
One extra benefit of UV cure is that the first layer of the resin to start curing is at the top where the light hits first. This tends to trap styrene in the resin while the rest of it cures giving less shrinkage etc.
MEKP cures on the other hand tend to be fastest near the foam because off the heat retention there. This tends to heat the bottom of the resin faster and drives styrene to the surface where it can escape.
Now THAT’S dandy!
Thanks Rob.
This issue came up a lot with setting fin boxes.
A lot of times the boxes would rise. Resin shrinks with curing, it does not expand. So what is going on?
A hot batch will heat the nearby gases and cause a predictable pressure and volume increase. And this has the capability of screwing up a finbox set or a lam if the resin is set off hot enough to cause variable expansion of the foam. Of course the foam expands when the box is setting, and then contracts and cracks as it cools, so the entire process results in a very weak set.
I’m nearly certain that the specified shrinkage in resins when curing is insignificant to cause any change in shape. Its the heat that gets ya.
HTH.
In addition to using UV, theoretically it would seem beneficial, that when using a UV light box, placing the lights off to the sides rather than over the deck and bottom, would allow to rails to cure a bit faster than the panels. This might stiffen the rails a bit first and help to control or limit rocker distortions during in box curing. Just a theory of course.
I could envision using an adjustable light box to better suit the board being laminated. One could have a sliver of side light approx following the profile curve controlled by a special partition. Then pulling back the partition or curtain to get full side lighting, followed by deck or bottom lighting. Hmm…