Polyester resin curing oddness

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alexhibbert

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Hi all,

I'm far more experienced with epoxy resins, but am using polyester laminating resin (not flow or gelcoat), due to costs, to make a sheet with a core and carbon laminates either side.

I've done some tests with small offcuts, and am getting odd behaviour. I've tried various MEKP %s between 1 and 2%, and both standard and summer grade MEKP. Excess resin in the pot is curing fully, with zero tack by the next morning. Same for on overspill left open to the air. But, the laminate itself is curing on one side and not the other. The underside is against a polypropylene sheet, and this seems to cure ok. On the top side, I'm using release film and then a flat chipboard with weights on top - and it's not curing. It's still either tacky or actually wet the next day. I uncover it, and over the next couple of days it hardens up.

Isn't this the opposite of what you'd expect? I thought laminating resin is supposed to remain tacky if cured whilst exposed to the air, and so needs to be covered to get a hard tack-free surface. Am I thinking about air inhibited cure that's only an epoxy thing? Do I need to allow for stryene to evaporate, so shouldn't totally cover both sides?

Temperatures are a little on the low side, but since some resin is curing, I assume it's not too low.

Thanks
 
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in these temperatures and humidity you will have a hard time getting resin to cure if it is in thin sheets.
making polyester gel coated panels at this time of year i would be using about 4% hardner to get the first layer of gelcoat to become tacky,then maybe 3% for the next layer of gelcoat then resin and glass once that kicks off at about 2%,
also found using an electric blanket to warm the laminating table helped as well as lighting the wood burner to get the air temp up.

if your cloth is at all damp,which it will be if it has not been stored in a warm dry environment you will never get resin to cure properly

i have also found that if temps are low letting mixed resin stand for a few minutes in the pot once it has been mixed will also improve the cure time for thin layers
 
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Gelcoat with set tacky if exposed to air.
Laminating resin normally not so.
To do what you are doing I would use a common polythene sheet or bag as the top release 'film'

Regarding the temperature, I would perhaps warm the carbon with a hot air gun, hairdryer, fan heater, not only warm it up but also ensure it's dry.
TBH, I'd probably use epoxy, the cost of the carbon normally puts the resin in the shade, but if it's 'to hand' so be it.
 
Ah thank you. I actually got the summer grade, as the size of the laminate is fairly large, and I had a problem a few months ago when the cure began before I'd finished applying the resin. I think I've overcompensated and gone too low on the MEKP umpf.

The working area is heated, so it's not outdoors, but not fully up to room temperature, and I don't think damp is an issue.

I'll try and find the MEKP level that gives me enough working time, and then use a heater to kickstart the cure once I'm happy it's ready to be compressed and cured.
 
I'm sure a rule of thumb is...5°c and below and you are going to run into problems.

I did some transom reinforcement in October and ambient temp was 8°c and with MEKP and 4% catalyst it took 2 days to fully cure. Even then i had to keep it warm with the wife's hairdryer. It eventually fully dried hard not tacky...

This time of year is definitely not the time to be using resin unless its in a heated unit or garage with a constant warm temp. Or atleast constantly above 5°c

I've read some proper horror stories where people have pushed their luck only to find they have to spent twice the money and time to rectify a job they should have waited to do.

If you're going to heat BE VERY CAREFUL not to overheat with a heat gun as can cause cracking.
 
TernVI said:
Gelcoat with set tacky if exposed to air.
Laminating resin normally not so.
To do what you are doing I would use a common polythene sheet or bag as the top release 'film'

Regarding the temperature, I would perhaps warm the carbon with a hot air gun, hairdryer, fan heater, not only warm it up but also ensure it's dry.
TBH, I'd probably use epoxy, the cost of the carbon normally puts the resin in the shade, but if it's 'to hand' so be it.
Click to expand...

Is this different fundamentally to epoxy? Epoxy laminating resin remains tacky unless air is removed throughout the cure.

Yes, the top film is a purpose made composites film, so should't be the issue.

Thanks for heating advice.

Re: costs, I get carbon very cheaply, so it's worth it for the tensile strength and stiffness.
 
alexhibbert said:
Is this different fundamentally to epoxy? Epoxy laminating resin remains tacky unless air is removed throughout the cure.

Yes, the top film is a purpose made composites film, so should't be the issue.

Thanks for heating advice.

Re: costs, I get carbon very cheaply, so it's worth it for the tensile strength and stiffness.
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you need to add wax 1-2% if you want a non tacky top surface
 
The stuff I used (cant remember the correct name) had 2% wax styrene added to isolate from the air to allow the surface to fully cure.
 
atol said:
you need to add wax 1-2% if you want a non tacky surface
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Not wanting to be difficult, but to add wax to a gelcoat surely just gives you a flow/top coat. Gelcoat is, I thought (whether poly or epoxy) just normal resin with filler and pigments.

I've made tons of parts with no tack, and all I needed to do is exclude air during the cure. Never added wax.
 
alexhibbert said:
Not wanting to be difficult, but to add wax to a gelcoat surely just gives you a flow/top coat. Gelcoat is, I thought (whether poly or epoxy) just normal resin with filler and pigments.

I've made tons of parts with no tack, and all I needed to do is exclude air during the cure. Never added wax.
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I maybe wrong and someone please correct me if i am but as far as I'm aware,

I bought a loyds marine poly resin and catalyst which had the 2% styrene already mixed.

If your NOT using a wax styrene substance to isolate the air from the job to allow to cure you need to finish off with a "flow coat", That top coat will become the shield and allow the job to harden and cure fully...

If you ARE using a 2% styrene then the finish will cure hard and fully on its own without the need for a "flow/Top coat".

Please correct if im wrong.
 
alexhibbert said:
Not wanting to be difficult, but to add wax to a gelcoat surely just gives you a flow/top coat. Gelcoat is, I thought (whether poly or epoxy) just normal resin with filler and pigments.

I've made tons of parts with no tack, and all I needed to do is exclude air during the cure. Never added wax.
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correct gelcoat has no wax,flocoat is just gelcoat with wax added

peel ply should act like the inside of a mold so no wax needed,but you could be having surface curing problems due to temperature and humidity or lack of sufficient catalyst leaving it tacky
 
Gelcoat polyester resin is chemically different from layup Resin. Resin A, Resin B.
Layup resin should generally set non-tacky, exposed to air or not as shown by the spare stuff in the OP's pot.
Gelcoat needs to be covered or wax added if you don't want it tacky.

Layup resin setting tacky is usually too little hardener, old resin, contamination, dew/humidity before it's cured, damp glass or some kind of bad luck.

Styrene is a solvent, present in the resin and also used to add the wax to gelcoat to make flowcoat. Wax is sold as 'wax in styrene'.
 
TernVI said:
Gelcoat polyester resin is chemically different from layup Resin. Resin A, Resin B.
Layup resin should generally set non-tacky, exposed to air or not as shown by the spare stuff in the OP's pot.
Gelcoat needs to be covered or wax added if you don't want it tacky.

Layup resin setting tacky is usually too little hardener, old resin, contamination, dew/humidity before it's cured, damp glass or some kind of bad luck.

Styrene is a solvent, present in the resin and also used to add the wax to gelcoat to make flowcoat. Wax is sold as 'wax in styrene'.
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Thanks for the input everyone. I think keeping an eye on temperature, and being more generous with the MEKP should solve it.

(That's a notable difference from epoxy layup resin then: in that the surface of uncovered layup epoxy resin is tacky once cured, and needs to be flowcoated, have another laminate layer added, or have a compatible skinning resin painted on. I've only ever managed to get epoxy resin (laminating / layup) to cure in open air without a tack in a pot with quite a lot of resin depth - the exotherm got it seriously hot and for whatever reason it cured rock solid with no tack.)
 
alexhibbert said:
Thanks for the input everyone. I think keeping an eye on temperature, and being more generous with the MEKP should solve it.

(That's a notable difference from epoxy layup resin then: in that the surface of uncovered layup epoxy resin is tacky once cured, and needs to be flowcoated, have another laminate layer added, or have a compatible skinning resin painted on. I've only ever managed to get epoxy resin (laminating / layup) to cure in open air without a tack in a pot with quite a lot of resin depth - the exotherm got it seriously hot and for whatever reason it cured rock solid with no tack.)
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Yeah thats another thing to keep in mind too, the thinner the layer is the less heat will be generated by form of chemical reaction so the layer might never cure fully in colder temps, thats probably why it went rock hard in the pot as opposed the work piece. Slowly warming with a hairdryer and a top coat should sort it.
 
There are many different epoxy resins, many should set as a coating without being tacky, others are specifically intended to be tacky to bond to another layer.
I tend to cover epoxy if I'm laminatingwith glass or carbon, because I'm keen to use the minimum resin. And If I'm just coating something I will often heat the resin with a hot airgun to make it really runny and penetrate the wood. This tends to encourage fast, non tacky setting at least with general purpose SP and WEST that I use.
With some epoxy you can get 'amine bloom' a cloudy surface gunge if conditions are wrong or mixing ratios are not exact. This wants removing if it's sticky.
 
TernVI said:
There are many different epoxy resins, many should set as a coating without being tacky, others are specifically intended to be tacky to bond to another layer.
I tend to cover epoxy if I'm laminatingwith glass or carbon, because I'm keen to use the minimum resin. And If I'm just coating something I will often heat the resin with a hot airgun to make it really runny and penetrate the wood. This tends to encourage fast, non tacky setting at least with general purpose SP and WEST that I use.
With some epoxy you can get 'amine bloom' a cloudy surface gunge if conditions are wrong or mixing ratios are not exact. This wants removing if it's sticky.
Click to expand...
Interesting I am sure but the OP is using polyester resin
 
TernVI said:
Gelcoat polyester resin is chemically different from layup Resin. Resin A, Resin B.
Layup resin should generally set non-tacky, exposed to air or not as shown by the spare stuff in the OP's pot.
Gelcoat needs to be covered or wax added if you don't want it tacky.

Layup resin setting tacky is usually too little hardener, old resin, contamination, dew/humidity before it's cured, damp glass or some kind of bad luck.

Styrene is a solvent, present in the resin and also used to add the wax to gelcoat to make flowcoat. Wax is sold as 'wax in styrene'.
Click to expand...

Absolutely spot on, saved me some typing (y)
 
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