I've noticed that with 12 min and 30 min epoxies they are still a bit tacky even after 24 hours, but feel really solid after two or three days.

Does this mean that its not at full strength even though the bottle might say full cure time is 8 hrs?

I'm not sure. I have noticed lately that some dries really hard and some remains tacky. I attribute that to inconsistent ratios of hardener to epoxy. I just finished off a bottle of epoxy and I had hardener left over (about 2 teaspoons). I never payed too much attention to the tic marks on the side of the bottle because it takes so long to run down the sides and with 5 min epoxy you need to hurry if you want to get it wiped off the covering.

The time listed on epoxies is the working time, not the curing time. Some manufacturers confuse things by putting the cure time on the bottles, but they are in the minority.

30 minute epoxy has a 30 minute working time and a set time of about 4 hours. All epoxies take months to fully cure - the chemical process never really stops, but 24 hours is good for almost any epoxy 5 minute to one hour.

You are correct in assuming that even 5 minute epoxy take 24 hours or more to reach full strength. It is also correct that any epoxy that has less than 30 minutes set up time is much weaker than the epoxies with slower set up times. Humidity also plays a part, that is why sometimes when people thin with alcohol they wind up with a permanently sticky mess as the alcohol contained to much water.

Getting the ratio right is paramount. If you put in too little, or too much, hardenere it takes a long time to fully cure, or it just won't cure.
Dennis-

Don't forget temperature.
If you work in a cool basement or garage epoxy takes a bit longer, too.
If the temp in the shop is cool AND you happen to get a bit less hardener than resin, it can take even more time.

Phil in MN
[8D]

A good trick I learned from another builder.

Whenever gluing critical components (especially where it's hard to see), leave some epoxy in your mixing cup with the stir-stick.

After the allocated cure time, check your sample epoxy to make sure it hardened correctly. Sometimes by incorrectly mixing, or from old epoxy, etc., it just doesn't cure right. I've at least one occasion in which the epoxy in my cup was still gooey the next day. I knew I had to take the joint apart and re-glue. Most parts are obvious if the epoxy doesn't cure. But for some parts, like joining two layers for a firewall, it's not as obvious.

To follow up on this topic, I am dealing with the cure time issue also, and having problems with the epoxy setting up too quickly. I have been using DHP 30 minute stuff which has been mixed accurately @ 1:1 in a 70 degree environment. If I'm lucky I get 7 minutes before it goes off. Using it on non critical areas, but need a little more working time. I have a general question for all epoxy types, 5 min, 30 min, 1-hr etc : if I try to slow down the set up time by mixing with a little less hardener say 1 part resin to 3/4 part hardener ( which does slow it down), does it compromise the strength of the epoxy? Anyone?

Thanks, Joe

If you are going to thin epoxy with alcohol use "denatured alcohol", available at Home Depot I believe, as it contains no water and use sparingly. Keep alcohol tightly sealed when not in use as it is highly "hygroscopic" like your fuel is.

garch 22 - time to switch epoxies. Heat will significantly speed up cure time, but 7 minutes is ridiculous in a 70 degree environment. I've had 30 minute epoxies I was lucky to get 15 minutes out of, but I live in Florida and the heat down here in the summer makes epoxy set up in 1/3 the listed time.

Another consideration is the overall quantity mixed. The higher the volume, the quicker the set-up time, as well as a higher exotherm.

Higher temperatures will greatly accelerate curing time. In addition, the chemical reaction is exothermic (produces heat). If you mix a fairly large quantity in a fairly narrow/deep container, there is less surface area to radiate heat so the heat of reaction cannot dissipate resulting in a rapid "cure" - often at somewhat less time than the published cure time. After thorough mixing, the epoxy will set up much more slowly in a wide, shallow container (or flat surface) because the heat does not build up and accelerate the reaction. Placing the flat tray of mixed epoxy on a cool surface (i.e. a cool, wet cloth) will significantly extend your working time. Just keep moisture out of the mix. I also keep the mixing stick/container around to judge when the mix is cured.

Ross

Joe,

Using slightly less hardner will slow down the cure rate. In fact, when I'm mixing supposedly equal lines of resin and hardner, I err for less harnder than resin. I wouldn't go so far as to say 1 part resin and 3/4 part hardner, but slightly less harnder will not compromise the final cure.

Joe,

Using slightly less hardner will slow down the cure rate. In fact, when I'm mixing supposedly equal lines of resin and hardner, I err for less hardner than resin. I wouldn't go so far as to say 1 part resin and 3/4 part hardner, but slightly less hardner will not compromise the final cure.

Ed, that's the question I was trying to address. I've found it's possible to vary the ratio, and think the hardener is only a catalyst, and does not affect the strength of the cured epoxy, just can't find any published material to confirm this. Think I'll contact one of the manufacturers and try to get their final word, and will report back to the forum on this.

The tic marks tend to be a very inaccurate method of measuring. I mix my epoxy in 1 ounce dosage cups to be sure of the ratios. The cups are disposable, cheap and readily available at your local hobby shop or drugstore. (they might be a little cheaper at the drugstore)

I read on at least one epoxy manufacturer's web site that you should not do this. Can I find the link now? Of course not. I always keep a variety of epoxies in the shop and choose one with the appropriate cure time.

Edit: Found it http://www.wessex-resins.com/westsystem/wsmixing.html

I remember this thread. I was just getting started in the hobby. I pour out both at the same time making sure the size of the stream is the same and the hardener puddle is the same size as the epoxy puddle. Never had a problem.

me too zetor. Just pour out equal sized pudles and go for it. Only times I have had problems was when I had too much resin in the mix or when it was cool with high humidity.

BTW, We use 91% Alcohol you can get at Walgreens. Not the regular stuff it has too much water in it. Get the 91% , stick your finger in it and wipe a joint to make it slick as glass or to form a perfect fillet. Just be aware that if you have any nicks in your cuticle its going to burn like the dickens,,, but it sure does slick out the surface of an epoxy job if you dont overdo it. Obviously its perfect for cleanign epoxy off your hands, but once again its oging to burn in any little cut..