Polyester vs epoxy resin

I'm sure this has been covered extensively in the past, but I could not find the answer when searching. Other than strength, what are the advantages/disadvantages of epoxy vs. polyester resin. I'm building a 1 inch foam core hardtop. I want to be able to walk on it for repairs etc. I will not stand on it when running, but it will support a 4 ft. radar antenna. The top is 8 ft wide and 9 ft long. There is a 1 ft wide radar arch under the back of it that is very stable. I'll add 2 supporting aluminum triangulated supports to the front. I'm thinking 1810 glass with 5 lb divincell foam with polyester resin. Do I need epoxy? Any suggestions are appreciated.

 

Epoxy adheres better, both to existing structure and to new dry fiber. Epoxy is truly waterproof even after many years of immersion. Polyester can sometimes take in water and form little blisters. There are a very wide variety of curing agents for epoxy, leading to widely varied cured state properties, so the resin can be tailored to your specific needs. For the same reason, cure times can be varied according to need. Epoxy is dramatically more solvent resistant in the cured state than polyester.

Polyester cures fast, even in cold conditions and is a bit cheaper.

Jimbo

 

Polyester is usually more brittle than epoxy...isn't as waterproof and doesn't play well with foams (usually melts them). Epoxy is stronger, relatively more flexible and less likely to crack. It adheres to most anything and is resistant to most chemicals (as stated above). Unfortunately it is more expensive than Polyester. An in the middle resin is Vinylester resin...but I don't know it's reactivity with various foams.

Steve

Steve

 

Thanks for the replies. I live in South Florida and ambient temp is 80 F plus. From what I've read I'll need to go with epoxy to get enough working time. I'm thinking I need about 1 hr to get the glass down and wetted out, then the foam and possible vacuum. Any suggestions on type of epoxy and working times would be appreciated.

 

This is case where the tailorability epoxy resin can be of use. There are low reactivity curing agents that can easily give you more than 1 hour @ 80F. If you need a lot of time, consider an elevated temp curing agent like EMI-24. Unlike epoxy curing agents which are mostly either amines or polyamides, EMI-24 is actually a catalyst for epoxy. With a catalyst, the mix ratio is variable and less critical. It's also very low, 1-5%. The advantage is you get over 1 week of work time @ ISO standard (25C or ~77F). But with elevated temp of only about 120F, it cures in about 6 hours. I did a short run (4 units) boat hull this way. It used foam cores and vacuum bagging and all the stringer and gussets were installed in the primary layup which took several hours. After bagging we took a spray can of flat black lacquer and painted the back of the vac bag and wheeled the whole mold out into the sunny parking lot. It got quite hot and cured. For the next three we made a light bulb array that lowered from the roof and ditched the flat black lacquer. Uncured resin left on the shop floor did take over 1 week to harden up, but did fully harden with no additional heat.

Jimbo

 

I use a proprietary mixture for hot weather, which provides over an hour at this time of the year and just about an hour in the dog days of summer. West System 209 hardener will give you about 50 minutes in 90 degree heat, assuming you spread it out into a very thin skin after mixed, but before applied, so heat doesn't build up. You also can store and mix your epoxy indoors where room temperatures will slow the reaction, then spread it out real thin (about a 1/16") and bring it to the work outside. I have an A/C section of my shop that I use for this (and painting) reason, to control the epoxy reaction (the key to successful epoxy use). On 80 degree days you'll have over an hour and it will not cure well at all, below 70.

Often I do "hot on hot" and then bring the work into the A/C section to cool and cure. Epoxy viscosity and its ability to penetrate porous surfaces is greatly enhanced by heating it slightly (120 degrees max). The work can also be heated too. I'll leave a part in the sun for a few hours to warm it and heat the epoxy (unmixed) in the microwave. I then move the part into a cooler area and apply the warm epoxy to the now cooling part. The contracting air and fibers of the piece will draw in epoxy much better thne if I applied at room temperature. It also has the side benefit of preventing "out gassing" which can create bubbles on the surface during the cure process. Out gassing is a result of the air and wood fibers warming (and expanding) during the cure, because of the thermolidic reaction, which is part of the epoxy cure process. The gases have to go somewhere so they appear as bubbles on the surface. You can pop them with a heat gun or a torch, but often you'll have remaining pin holes and broken bubbles everywhere. Insuring the work is cooling down during the cure (by using preheated goo and parts) will stop this common problem.

 

Par,
That's a very cool little trick, and one that I have not heard of before.


Jimbo

 

Try it Jimbo, you'll find better penetration. I've done side by side tests with the same pieces of lumber and have an average of 30% more penetration with the hot on hot method. When embalming wood with epoxy, this can add a considerable margin. Imagine a slight ding in a plank at the local dock. If more of the fibers have goo in their cellular structure, at a deeper level, then a minor ding can survive longer before repairs need to be addressed. It would be nice if repairs were made quickly and in a timely fashion, but most aren't and are left to fester into something else. In the current state of unseaman like, custodial upkeep of our vessels, any little bit helps. I consider it cheap insurance on my work.

 

All excellent points. One more idea, I have had many great results by building panels in polyesters resin and cloth then encasing then in epoxy. I get stiffness of polyesters and strength and flexibility of epoxy. Also reduces costs 30%. Try it.

 

Under elongation, the polyester resin will break at 2%, which is well below the epoxy and fabric used, so you're using the polyester as a "bulking" agent of less then sufficient strength. A more flexible sheathing of epoxy just serves to contain the busted up poly laminate underneath, which seems counter productive to me. Vinylester would be a much better choice, but the cost differences aren't that big, particularly on a small job like this hardtop.

 

Since everybody is on this subject, I have a question about epoxy. When mixing, can you add a coloring agent to this like with gelcoat? You'll have to forgive me, I am new at all this. I just built a 10 wooden jon boat from the Glen L plans and I was wanting to coat the bottom of it with something a little thick and also have it colored. I used a epoxy spray paint because I had no idea of what I was doing, but it turned out pretty nice, until I had to drag the boat along the sand and of course, it chipped some of it off. Any help would be greatly appreciated.

 

Polyester & epoxy

Your almost right, except when I am making a part like waste tank or floor board, it is easier and faster to work with polyester part. The epoxy bonds very well to completed part. Honestly I haven't work much with vinylester. I pay $15 a gallon for poly resin and $30 for Epoxy. I imagine Vinylester would be very close to what I pay for epoxy.

On coloring epoxy, you can use pigments. I used colored talcs or other coloring for layers. However remember epoxy don't like UV. They will yellow. I paint over it with epoxy paint that has uv for final coat.