epoxy using flexible agents tests with in

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epoxy using flexible agents tests with in

Postby saltydawg » Thu Sep 17, 2020 5:19 pm

Okay we all know epoxy, esp when thick build ups is brittle. Well reading over at expo portal in the sawtooth ulnimited thread that he wishes the epoxy he bought was a little less brittle ( maybe a little sever of a statement ). And I bought uscomposites thicker epoxy and I have been doing adhesion tests with the foam ( dow 25 psi exp ) to the fiberglass skin I will be using trying to figure out a repeatable way to put just enough epoxy in the sandwich. My thicker spots at the edges where I got squeeze out are brittle but over 1/16 inch thick.

So I have a couple of samples of the epoxy to foam test pieces for a base line. I then started reading about flexible epoxys and what can be used to make them flexible. I found about one item from a guy in new hampshire, and talked to him. His stuff he uses for a few things, one to stop epoxy from curing on his spatulas and stuff. It can be used to make epoxy more flexible in low doses. Smooth on makes one they sell, they say it makes "some" of their epoxys cure flexible and give ratios, based on weight. Well that would work with their epoxys, but does not roll over to others. We have to test.

So I bought some of smooth ons stuff, its a commercial plasticizer not cheap but it seems like it will go a long ways. My first tests where 3 samples. Using a large syringe while this worked fine for measuring the epoxy and hardener it was a little too gross of a measure for the additive it turns out. My first 3 batches where 15 ml of epoxy 7.5 ml of hardener ( from here out the mix ) and from about 1.5 ml to 3 or so to 5 or so of the additive. As I said its was tough to get an exact ratio. Plus not very repeatable. My epoxy I bought is mixed by volume only.

Each mix was used to glue a piece of fiber glass sheet ( frp ) to the foam, spread out on plastic sheet and the cup turned over on non stick al foil. Smooth on says it extends pot life esp at higher ratios, I put each sample in front of my 500 watts quartz light to keep the temp up as my garage is dropping in to the 60s at night and I am not turning the heat on for tests. After a few hours they where all gelling nicely at probably 80 degree surface temp as they should.

23 hours later I looked at everything. All samples cured, the epoxy cured from a stiff but flexible ( but did crack when folded 180 degrees and squeezed ) to stiff fruit roll up depending on how much additive. So obviously much more flexible than needed. Then I move to the plastic sheet, they all seems to bond pretty well ( it was an amazon bubble shipping bag ) to it, but remained attached and flexible as I played with it. Next the important test, adhesion. Well I ripped every piece of the fiberglass off the foam, the joint held, the foam failed. It actually seemed to take a thicker layer of the foam ripped off then a straight epoxy test. I did not sand either the foam or fiberglass sheet ( I want worse case tests ) . I wonder if the plasticizer made it penetrate further into the foam, and I mean chemically, they do state it thins the epoxy.

So it seems the stuff did what I want, but way more than needed at the approx ratios I tried. None of which where exact, or very repeatable for small mixes. So I have another test going right now 15 ml epoxy 7.5 ml of hardener and 15 grams of the additive. 32 grams was about 1.5 ml of the hardener, so I am around .5 or .6 of a ml. Kind of hard to measure in a 100 ml syringe.

Either way based on what the current test does I will be using some of the additive when I make my panels. If this last test still seems too flexible I will just move down another 1/2 of what my ratio is. I will be making the panel that does my floor this weekend, the least structurally important panel. So I might use the 15 grams per 15 ml of epoxy mix or it might be 7.5 grams. I will see where the last test winds up. Either way I will in effect have one more test, the left over epoxy from the floor panel to decide on my final ratio for the wall and roof panels.

Over all I am happy with the additive, as it makes the epoxy not as brittle, still bonds well. And its not like I am depending the stiffness of the epoxy for strength. The two .055 thick skins and the foam when bonded is plenty strong enough.
Last edited by saltydawg on Thu Sep 17, 2020 8:10 pm, edited 2 times in total.
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Re: epoxy using flexible agents tests with in

Postby saltydawg » Thu Sep 17, 2020 5:48 pm

Before anyone says buy a different epoxy I bought many gallons of this, and unless I tried hundreds of dollars of different epoxies I might never find one I liked 100 %. At 50 to 100 a gallon for epoxy even trying 5 or 6 half gallon bottles of epoxies would get expensive.
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Re: epoxy using flexible agents tests with in

Postby John61CT » Thu Sep 17, 2020 8:09 pm

What is the reason for using epoxy in the first place?

There are many very strong adhesives on the market designed specifically to remain flexible.

3M and Sika are very well respected for marine applications, 3M 5200 is famous for being "too permanent", their 4200 and Sikaflex 291 are not quite as strong but very flexible

Dozens of other choices out there, tech support at either company can advise.

Also Dow.
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Re: epoxy using flexible agents tests with in

Postby saltydawg » Thu Sep 17, 2020 8:22 pm

John61CT wrote:What is the reason for using epoxy in the first place?

There are many very strong adhesives on the market designed specifically to remain flexible.

3M and Sika are very well respected for marine applications, 3M 5200 is famous for being "too permanent", their 4200 and Sikaflex 291 are not quite as strong but very flexible

Dozens of other choices out there, tech support at either company can advise.

Also Dow.


Well sika and a few other are moisture cure. Kind of hard to get moisture into the center of a 5.5 x 12 foot side wall panel that is made out of closed cell foam and frp sheet. 4200 and 5200 are in the category as well.

3m does some great stuff, but the cost would be insane. Heck I will probably use between 8 and 10 gallons of epoxy, what you do think 10 gallons of 5200 would cost?

I looked in to polyurethane glue, the issue is again the moisture cure. I did find one that is designed for SIP panels for houses. They spray the second surface with water. A very controlled amount of water, and then work time is just a few mins until it has to be in a press. It is mostly used with osb skins, so the water can get out if there is too muck. The nice thing is panel turn around time is less than an hour. The glue costs 200 bucks for 5 gallons, so comparable to cheaper epoxy.

It will take 45 mins to an hour per panel to glue both skins to the core then in to a vacuum bag for clamping also not suitable for a moisture cure. Not enough time for most glues.

There are some pretty good contact adhesives but they are not near as structural as epoxy is.
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Re: epoxy using flexible agents tests with in

Postby saltydawg » Thu Sep 17, 2020 8:26 pm

I like free post count bumps.

I am using torosen 939 as the glue to mount the panels to the frame and each other. its 21 bucks for a sausage tube pack. If I had to guess if I tried to use it i would spends 200 bucks a panel, and again moisture cure.

Edit so I dont get another free post count.

There are some 2 part polyurethane glues but they dont have enough open time before they skin over, and they take very special equipment to spray.

Epoxy is the best choice esp for a DIY job.

If you look back I have been looking at this stuff for 8 years, posts from 2012. I am pretty knowledgeable on whats out there
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Re: epoxy using flexible agents tests with in

Postby saltydawg » Thu Sep 17, 2020 8:34 pm

John61CT wrote:What is the reason for using epoxy in the first place?

There are many very strong adhesives on the market designed specifically to remain flexible.

3M and Sika are very well respected for marine applications, 3M 5200 is famous for being "too permanent", their 4200 and Sikaflex 291 are not quite as strong but very flexible

Dozens of other choices out there, tech support at either company can advise.

Also Dow.


Second quote because I can. Go look at howards build thread. he had issues with sika opening up due to temp changes ( he thinks ) moving the al skins vs the frame

viewtopic.php?f=50&t=62581&start=375
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Re: epoxy using flexible agents tests with in

Postby pchast » Thu Sep 17, 2020 9:11 pm

I'd like to suggest that you use weights to mix your samples for those small sizes. It would be much more precise a ratio. I have had trouble getting such small samples to work uniformly from a syringe or dropper.
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Re: epoxy using flexible agents tests with in

Postby saltydawg » Fri Sep 18, 2020 2:21 pm

First when I mix the large batches for making panels I will have an easier time getting ratios correct, or closer. The small mixes where just for tests.

Now for the important stuff. the 15 grams in the epoxy 22.5 ml mixed worked perfectly. Puddles that cured can be bent about 20 to 30 degrees then they snap, I expect it to get a little stiffer as the epoxy cures more. As for adhesion the joint held the foam is what fails and it takes a little over a 1/16 inch of foam with it, vs straight epoxy it was just a little under a 1/16 inch. I am assuming that it is due to the epoxy giving more and the foam flexing more so it pulls more off.

No what is perfect is multiplying the ratios by by 10 gives me 32 oz of resin, 16 hardener and 150 grams of the flexer. so nice easy math. edit corrected epoxy amounts

Now I dont want anyone usingmy math or ratios. every epoxy is going to be different or not work with the flexer. So if you use any of the above info its all on you.
Last edited by saltydawg on Thu Oct 08, 2020 9:20 pm, edited 1 time in total.
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Re: epoxy using flexible agents tests with in

Postby tony.latham » Fri Sep 18, 2020 8:49 pm

I find your testing interesting. I didn’t realize there was a way to do this.

It sounds like you’re planning on using this for your panels and am wondering why you think it may need to flex a bit?

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Re: epoxy using flexible agents tests with in

Postby saltydawg » Fri Sep 18, 2020 11:13 pm

Well epoxy can be brittle. The real issue is once a crack starts it does not stop ie it shatters. The way the polymer chains form in epoxy there is really no end to the chain. When used for fiberglass cloth a crack starts and eventually runs in to a glass fiber which stops it. So when used for bonding, esp when the joint is thick, if you get flex enough to crack the epoxy, the crack keeps running. Yes the bond still holds, but now it becomes more like a roll top desk lid if taken to the extreme.

if the epoxy is too brittle it can cause cracking just from something as simple as a frame that flexed or a big wind gust. 99.9 % of the time the cracks will happen and you will never know. When you do know its because you get delamination and bubbles on the skin. Epoxy has 2 weakness, shock and heat. it gets weaker when hot, and shock causes the cracks but more resistant to cracks when hot. The other issue is air bubbles under the skin, when they heat up the air expands. This now puts more stress on the bond.

The fun part is getting that little bit of flex, which is done by limiting the length of the polymer chains solves almost all of the above issues. The best way is not to limit polymer chain length but is to give the crack something to stop against. be it glass fiber or even better is whats called core shell rubber. Its microscopic rubber balls coated with a polymer that the epoxy can bond to. So any cracks that start run in to one of the balls at a microscopic level stop before they even really get started. Just like drilling a hole at the end of a crack in metal, it releases the stress.

Why I did it, I expect micro cracking. Its going to happen. I will have flex, I will have bumps and bruises. why not give the panels some flex, and I mean a basically un measurable amount to flex if it helps prevent a problem. it just the "glue" will have a little give. I would bet my floor panel will weigh about 50 lbs for a 10 by 5.5 panel. If I where to pick one up by one end and use a brittle epoxy I bet I would be able to hear the cracks, just like in a frozen lake. So the total for every panel will be some where just north of 250 lbs maybe as much as 300 lbs.

What does a piece of 3/4 ply wood weight for a 4x8 about 40 to 50 lbs? So my cabin weight for a 12 by 5,5 by 5.5 trailer should be the same as a 4x8 plywood skeleton frame covered with 2 layers of luan. And be stronger. with no rot potential.
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Re: epoxy using flexible agents tests with in

Postby saltydawg » Fri Sep 18, 2020 11:21 pm

The above post, is my way of saying its gonna flex, the epoxy if brittle will crack a little. So I made the glue have some give, now less cracking.

The rest is why I am building this way. And I planned on it in 2012. Jess, aka sawtooth just took all the info on how to do it and put it in his sawtooth unlimited build. I am just doing my own spin on it, and doing some material differences so avoid some of the issues/concerns he has brought up on his build
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Re: epoxy using flexible agents tests with in

Postby aggie79 » Tue Oct 13, 2020 11:49 am

So what is the actual brand and name of the plasticizer?
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Re: epoxy using flexible agents tests with in

Postby saltydawg » Tue Oct 13, 2020 9:23 pm

aggie79 wrote:So what is the actual brand and name of the plasticizer?


I have avoided giving that out because I dont want someone using my ratios, or with an epoxy it does not work with and blaming me. I have 3/4 gallon of it left if anyone wants a small sample to test or play with.

I have put the name and who makes it in this thread just not in one sentence or even one post
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Re: epoxy using flexible agents tests with in

Postby John61CT » Wed Oct 14, 2020 1:36 am

Which foam panels are you using, how much flex in them?
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Re: epoxy using flexible agents tests with in

Postby saltydawg » Wed Oct 14, 2020 4:51 pm

In the first post, Dow 25 psi eps
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