Epoxy vs Polyester Resin

Guys I need some advice. I bought five litres of Polyester fibreglass resin today ($50 with hardener) as compared to the epoxy resin they had (four litres for $110 with resin).
I can't really find a straight answer about the polyester resin. Apparently it shrinks, is a little brittle, etc.. compared to epoxy.
For modelling or non-structural purposes, is polyester ok? Should I just spend the money and buy epoxy?

Another question is do I need to use gelcoat. The gelcoat that the shop sold was only compatible with the polyester resin and not the epoxy.

Ive done aircraft parts and automotive parts and models. I only use Epoxy. Its stronger the shrink rate is minimal compared to polyester. And theres no smell to deal with.

I always use Gel coat. Some guys dont if they want clear parts for LEDs.


The best thing to remember whether polyester or epoxy is to not flood the part with resin. The drier the part the stronger it is. Just use enough resin to wet the material.

Any questions dont hesitate to ask.

Al

Thanks for the advice Al. I think I will return the polyester and buy epoxy. Polyester may work for what I want, but the epoxy will give me peace of mind.

So basically to make a fibreglass mould from what I understand:

1. Wax whatever you are moulding (some youtube vids say that mold release wax is bad and only use pure Carnauba Wax thats used to wax cars. This doesn't sound right to me, surely a mold release wax is specifically made to do the job right)

2. Spray on a few layers of PVA.

3. Lay up your fibreglass and epoxy.

I might have to find another material supplier methinks. Struth if they can't even supply gel coat for their own epoxy resin...

BTW the mould I plan on making will form around half of a pilates ball with MDF as a flange, as its about the only thing I can think of that is a sphere of around 700mm. I assume I won't have to wax the ball as I can just deflate it to get it out of the mould.

PJgearhead said:

Thanks for the advice Al. I think I will return the polyester and buy epoxy. Polyester may work for what I want, but the epoxy will give me peace of mind.

So basically to make a fibreglass mould from what I understand:

1. Wax whatever you are moulding (some youtube vids say that mold release wax is bad and only use pure Carnauba Wax thats used to wax cars. This doesn't sound right to me, surely a mold release wax is specifically made to do the job right)

2. Spray on a few layers of PVA.

3. Lay up your fibreglass and epoxy.

I might have to find another material supplier methinks. Struth if they can't even supply gel coat for their own epoxy resin...

BTW the mould I plan on making will form around half of a pilates ball with MDF as a flange, as its about the only thing I can think of that is a sphere of around 700mm. I assume I won't have to wax the ball as I can just deflate it to get it out of the mould.

Click to expand...

For Wax i use Carnuba. its a good paste wax and ive never have any issues with it. some waxs have to be thermally rated for parts that are cured in an oven. but you dont need to worry about that.

I would still wax the ball. and if your making a mold i would definetly use Gel Coat. if you were just laying up parts i would say you could get away with not using it. but for a mold i would recommend it.

A trick with PVA is to spray a light coat and take a heat gun over the top to help dry it, and it will also make it lay down flat. and i would do a minimum of 3 coats.

then go ahead and do your glass layup. ive used the same Gelcoat for polyester and Epoxy so not sure what the deal is with that brand.

Good luck. and post pics. also when doing fiberglass its good to have all the material cut and ready to go before you start. i always have extra gloves and mixing cups standing by. it really sucks when the resin starts to thicken and you have to look for stuff.


hope that helps Al

Polyester resin - cheapest to purchase, produces toxic fumes, mixing ratios don't have to be exact but prone to linear shrinkage

Epoxy - more expensive, non-toxic fumes and mixing ratio is critical for strength. Less chance of warping during cure

Vinylester - a mid point between the other two. Cheaper than epoxy with better curing properties than polyester (but not as good as epoxy). Can be hard to find in smaller quantities.

Also, if you want to colour your job without gelcoat, use polyurethane as it flexes more readily with the composite structure.

Personally, don't skimp and go the epoxy. If its non-structural use the West system. If its structural, use a Hysol product.

Hey guys...
Im deciding whether to buy epoxy resin in bulk. I just have one question is epoxy flooring resin suitable for laminating my art work or photos.
I know it is possible to do I was just wondering if the industrial flooring grade would be different.

Thanks for the replys fellas.This is what confuses me though, conflicting advice.
From what I read, its the polyester that has toxic fumes, not the epoxy. Plus the mixing ratios with epoxy apparently you can adjust them pretty far to suit conditions such as temperature and working time, is this right?

West Systems is a bloody good product but its expensive. While I don't mind paying for quality, what I'm doing with it is not structural or have to be relied upon with my life like in a boat. The cheaper product should be ok?

Both products are toxic. But polyester really stinks. You should always do a layup in a well ventilated area.

The epoxies i use come with a set hardener. And mixing ratio. Theres slow, medium and fast. Always follow mixing directions. As for temp. Here in Texas i dont care what the bottle says. In 105 degrees, slow set is still 12-15 minutes. So i only mix small batches. After you do it a few times youll get the hang of mixing certain amounts and see how long before it cooks off on you.

I dont care for West products. There ok. But not as reliable as some others ive used. I would buy a small amount and try it out. Best way to learn.


Al

Sorry, you are correct, I wrote that late at night and didn't proof read it before posting. Swap them around.....epoxy non-toxic, polyester is the toxic one.

The toxic component of polyester resins is the styrene monomer which is also the source of polyesters strong smell.

The mixing ratio of polyesters can vary widely, epoxies not so much.

They're both toxic, but most room-temp cure epoxies are now made with no smell, so while the fumes still damage your brain they won't tick off the neighbors on both sides of you and smell up your house by leaking from your garage into the house. Also, when using polyester, the methyl ethyl ketone peroxide used as a catalyst is really really toxic—worse than the styrene monomer in the resin itself.

Most "gel-coats" are the barrier/outside coats for polyester-based systems, and are simply polyester resins with thickeners and coloring added. When epoxy-based resin systems are used, "surface coat" is what we call that initial layer of thickened resin used on the inside of the mold. Do NOT try to use polyester-based gel-coats if you plan to laminate with epoxy-resin, as many are not compatible. Get an epoxy-based "surface coat."

With polyester systems, a gel-coat is often used BOTH on the surface of the mold (a "tooling" gel-coat) AND on the surface of the part. When using epoxy, though, weight savings is often paramount, meaning that while a surface coat is used for the mold, the parts themselves do not have a thickened/heavy surface coat. This is why you can actually SEE the carbon fiber fabric often used with epoxy resin. Instead, a couple layers of clear, unthickened eopxy resin is brushed into the mold first. Some folks spray in some 2K urethane primer as a thin surface layer for the part. Or you can do what I do, mix a thin surface coat from scratch using West Systems 404 filler and graphite powder mixed with a batch of epoxy resin—thick for a mold surface coat, thinner for a part surface coat.

It's not a matter of using mold-release wax OR canauba wax. GOOD mold release waxes ARE carnauba-based. PVA is extra protection that some like to use in addition to the wax.

Thanks for that info. Helps me understand a bit more.

This is what I have bought as of yesterday, plus a 10metre roll of 450 chop and 10metre of 225 chop.

Polyester always gives me a massive hangover, avoid if you're sensitive to fumes.

I would buy some Acetone too. so you can clean any equipment like the rollers.

and I would do a small test before jumping into the actual part.


Al

Can't see the image, so I don't know if you went with epoxy vs. fiberglass resin.

I can say that chopped-strand mat is usually used as a reinforcement with polyester resins, as the binder used to hold the strands together is made to be dissolved by polyester resin's styrene monomer. When using epoxy, woven fabrics are usually used as reinforcement. That said, some chopped-strand mats work ok with some epoxies, and some companies offer special chopped-strand mats with a binder that dissolves in epoxy.