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Old 09-29-2017, 09:20 PM   #1
mattman

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Default Understanding polyester resin...

So we are trying to use a polyester resin for a thin gelcoat (in-mold) in a small area on a part that is backed filled with expanding foam. Having limited experience with polyester resins, I was hoping some people might be able to shed some light on the tackiness and bondability of various cured polyester layers. Its my understanding that laminating resin cures tacky while finish resin, which has a wax content that deprives the surface of air, cures tack-free.
  • Am I correct that, once fully cured, the laminating resin can be bonded to since the surface remains tacky, but the finish resin would require styrene monomer or a tie coat to become bondable again?
  • Will laminating resin be tack-free on the mold side since it was not exposed to air?
  • Finally, we have used Duratec Sunshield as in in-mold coating with epoxy before. It is allowed to fully cure (as directed by Duratec) and is completely tack-free, yet bonds great with epoxy without any kind of treatment. Is this product (Sunshield) unique among polyesters for this combo of tack-free and bondable?
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Old 09-30-2017, 07:14 AM   #2
f1rob

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Normal gelcoat remains tacky so it adheres to the following laminate
You would use was addative if you wanted a gel Finnish on the inside of a panel to improve the b surface appearance
Normal laminating resin dries hard
Hard and shine on it's own
If your laminating a mould for example an you put 4oz down when you come to do the next 4 you would hit the surface with 80 grit to de nib it and aid adhesion
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Old 09-30-2017, 08:14 AM   #3
mattman

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f1rob, thanks for the response. Just to clarify a few things:

-you are saying that normal laminating resin dries tack free? When suppliers warn of it leaving a tacky finish (example), how is this different?

-what do you mean by "an additive for a gel finish on the inside of a panel to improve the b surface appearance"? Do you mean an additive is added to laminating resin in order to keep the non-mold side tacky? If yes, what is the additive? How does this improve the mold-side (b side?) appearance?
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Old 10-01-2017, 02:16 AM   #4
Hanaldo

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This depends a little bit what you are referring to - you don't want to confuse gelcoat/flowcoat with laminating resin. Not that you have, but for the sake of the discussion - gelcoat is the surface coat that is designed to be used in-mould; it cures tacky to allow chemical bonding with the subsequent laminate. Flowcoat is what some might refer to as a finishing coat -
it is simply gelcoat with 5% wax-in-styrene solution added to it so that it cures completely - it is designed to be used as a post-mould surface cost, or as a 'filler' for repairs/resurfacing.

When speaking of laminating resins, you need to ask your supplier if it is waxed or unwaxed. Generally speaking, most typical GP/vinyl ester laminating resins will be unwaxed. This does not mean that they cure 'tacky', they generally don't - they will normally feel completely hard to the touch. The difference is when you try to sand or grind it. Unwaxed resin will clog the paper/grinding discs very very quickly, waxed resin won't. This is especially important for pool builders because they need to do so much grinding. It's also important to know that a waxed resin will need to be cleaned well prior to attempting any secondary bonding, whilst an unwaxed resin will remain easily bond-able for several days in a clean environment.
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Old 10-01-2017, 06:41 AM   #5
mattman

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Hanaldo, so for an in-mold application, if you do not need the high viscosity and opacity, laminating resins can perform the same function as gelcoats of being able to bond to the subsequent layer without any sort of treatment - even though gelcoat is tacky and laminating resin is not? Finally, is "bondable" for both limited to polyesters and vinyl esters as apposed to polyurethane?
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Old 10-01-2017, 07:26 AM   #6
Hanaldo

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That depends entirely on what you actually need. For most purposes, no, a laminating resin wouldn't perform the same function as a gelcoat - it is far more brittle, more susceptible to osmosis and weathering, and does not tend to have good gloss retention or clarity.

That said - yes, an unwaxed laminating resin will bond to the subsequent laminate just as well as a gelcoat would. That has a lot to do with surface energy of similar chemicals, which is why I would not attempt to bond directly to a polyester or vinyl ester laminate with anything but polyester or vinyl ester without pepping the surface. I wouldn't advise trying polyurethane or epoxy or anything else. This is where products like Duratec and Scott Bader Crystic top coats are quite unique.
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Old 10-01-2017, 07:48 AM   #7
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I work almost exclusively with polyester (PE) and vinylester (VE) resins. There are quite a few variables in the questions you've asked.

PE and VE gelcoats that you use on a mold surface are designed for continuous lamination. You spray it, let it cure, and then laminate over it. They do have "open" windows - the time between initial cure and when the surface becomes too cured for reliable bonding. My suppliers have always said from the time it is solidly gelled up to about 12 hours.

PE and VE resins have different open windows depending on the specific chemistry. DCPD poyester resins have an open time of 24 hours whereas some other PE's and VE's can have 72 hours or even more open time.

Don't get too caught up with the term tacky. With both gelcoat and resin, tackiness is not the important measure. Each resin can have more or less tackiness when compared with another and still be bondable. What is important is the chemical makeup. Obviously, you can't "feel" the chemistry but that is really what determines the available open time.

On your question about the gelcoat against the mold, yes, that surface will be fully cured. The cure of PE's and VE's, unless modified by additives, are air inhibited. That means a surface exposed to air won't fully cure. This allows for good chemical bonding in secondary operations. Surfaces against molds or covered by impermeable films fully cure. Likewise, additives can "seal" the surface and provide full cure.

Your question about Duratec is related to additives. There are multiple types of additives that can be used to achieve cure. The most well known additive is probably wax (paraffin dissolved in styrene). There are also additives that are more chemical in nature. Duratec uses these chemical additives. Wax additives need to be removed completely or else your bond suffers. The chemical additives vary in secondary bond issues - some don't affect bond and some do. It's been my experience that secondary bonding resin is also a factor. Epoxies may need less surface prep than PE and VE. I would thoroughly test your materials before jumping in with both feet.

Finally, I note that you said you are trying to use polyester resin as a gelcoat. If I'm reading this right, you might find coating the mold surface with straight resin will be hard to achieve. Resin, being rather thin, doesn't always stay where you put it. It runs, it fish-eyes, it separates. It's not made for coating mold surfaces.
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