resin hardens

g-twizzl wrote:

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Try mixing resin in smaller batches like a quart or pint at a time,

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A Quart At a time??!! Holy crap that's way too much at one time! I live in Florida and my batches are limited to about 6 or 7 oz . I work outside in the sun, as I don't have a garage. The resin I use asks for 8 drops MEKP per oz of resin. I usually only use 6 drops and still have a small amount  that hardens before I can use it all.

Another thing to consider is what brand resin you are using. When I first started fiberglassing I used West Marine resin (which sucks by the way) and only got about 10 minutes work time out of it. But Us Composites B-440 resin will get you about 30 -45 min work time. Again it depends on how much MEKP is used, outside temps etc.

slange22 wrote:

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Contrary to what most believe, resin hardens with the help of UV light and NOT heat. If you are glassing in direct sunlight in the dead of summer, your resin will gel and harden faster that you can occomplish anything. Using too much hardener doen't help either.

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And how do you think the UV light makes the resin go hard? Its because UV light can be used as a heating source.  The resulting reaction is exothermic

When you mix you MEKP hardener with a polyester resin, it lowers the activation energy of the styrene monomer in the resin.  This then means that the resin is able to set at lower temperatures.  If you expose a resin mix to an isolated heat source it will eventually cure (will be very brittle thou becuase you are forcing the monomers to bond).  When your working in the sunlight you are working in the heat.  Hotter conditons means that the resin sets faster as there is more free engery at elevateed temperatures.  Colder wokring conditions require more hardener to further lower the activation engery.

I appreciate your knowledge of physics, but...

a black light is a UV light source. Have you ever felt a black light get hot? Probably not. I understand the whole MEKP (methyl ethyl keytone peroxide) concept. I will however contradict what you said. Experiment. Mix a batch of resin and spread it over two different projects. Place BOTH in the dark and use a black light over one. UV from a black light does not travel far, so the two can be in the same room (garage). They both will cure, but I bet my left one the UV assisted project cures first.

I know that they will both cure, and that the one exposed to blacklight will cure faster.  I could given the thermodynamic propeties of a certian resin mix tell you how quickly it would cure as a realtion to distance away from a UV light source.  The same would apply for a spurce such as sunlight thou

I think you are totally missing the point here mate.  UV light is a source of electromagnetic radiation.  That means that its a source of heat.  While a UV light never appers to get hot, on an atomic level each wave of UV light transmitts engery(heat) to the surafce that its pointed at.  The excitement of particles at the bonding zone results in the catalyst helping the breaking and forming of new bonds withhin the particle network.

Your orignial argument is wrong.  UV light has no special properties that makes fibreglass harden.  It can speed up the drying process becuase it is a heat source.  What is sunlight??  Sunlight is a stronger and more concentrated form of UV light.  There will be a relationship between curing time of a resin and light intensity that its exposed to.

Please before you put forawrd an argument fully understand what you are talking about.  Go and do your experiment and use sunlight instead of UV light.  When the item placed in sunlight cures quicker, come back and undersantd what iam saying an correct yourself.

I think even my 14yr old brother could tell me that UV light is a source of heat and it will speed up reactions.  The intensity will however vary this time.

I dont think curing in the sunlight will greatly or noticably (by a person) increase the brittleness of the resin.  I would say there would be a change in the stress strain curve of the material and it could be measured or calcualted experimentally.  But i dont think you could physically tell the differnce.  The amount of catalyst added will determine the maximum speed at which the reation can occur.  If you add more catalyst then is recommened then you could in theory speed up the reation to a point were either the chemical balance of the product is upset, or the bonds are formed too fastto be as stable as slower forming bonds.

If you use the correct amount of catalyst. letting and item cure in the sunlight should be no worries at all.

its very simple follow manufacturers instructions and mix '''working batches" batches you can apply before hardening occures.