epoxy + heat = trouble!

[Jeff Bartley]

Hi Folks,
While working on a bed recently I had need to epoxy a cracked panel. My design for this bed entailed a 'frame and panel' headboard except that the 'panels' are simply live-edged cherry boards oriented vertically. After getting into the project I realized that several of the cherry boards had some questionable cracking. Two of these boards received a series of butterfly keys and another (in the shorter foot-board) wasn't cracked all the way through so I decided to drill a couple holes in from the ends along the crack and then inject epoxy into the holes so that it would spread into the crack.
And that's where my troubles started. I had heard somewhere that you could encourage the flowability of epoxy with heat so I set the 30-min epoxy in front of a halogen light for a few minutes, then mixed it, then sucked it up with one of those disposable syringes. The idea was to put the syringe into each hole and inject the epoxy.
It worked great for the first three holes but by the fourth the epoxy was set up...in under five minutes! This epoxy was suppose to have a 30-min working time.
So, the question to the epoxy experts: does heat (or excessive heat) always reduce the working time? And the more important question: did I compromise the strength of the epoxy?
Jeff

[johnny means]

Heat does speed up drying time. And yes some strength is lost when curing is done to fast, though I wouldn't worry. Not as many molecular bonds are given a chance to form. I'm sure though you lost some working time the curing process slowed down to a normal pace soon after application. Next time, if cold is actually a problem, use a warming blanket to keep the piece warm as it sets up. Also larger amounts mixed up at once will cure faster than smaller amounts. The epoxy creates heat while curing and the greater mass produces more heat with less heat loss.

BTW be careful when using epoxy it can get itself hot enough to catch fire

[Howard Acheson]

Heat will greatly decrease the set time of epoxy. In the boatyard we used 1 hour epoxy and flash cured it with an infrared heat lamp. We could set it in 5 minutes.

[Jon van der Linden]

Epoxy cure times are temperature related. If it's too cold then the epoxy won't cure. There should be a chart with temp/cure times. If it's too cold you can microwave the resin for a few seconds - not the hardener, and certainly not mixed. Microwaving resin is something that you'd need to experiment with, unless you're doing cups+ we're probably talking less than 10 seconds. That slight amount of heat is all it takes to encourage flow, but it also decreases cure time.

Usually epoxy only exotherms significantly when there's a lot of it. A small quantity like you'd use to fill a crack won't do that.

[James Baker SD]

IF I remember my chemistry from many years ago, 10 degree C (18 degree F) approximately doubles the speed of a chemical reaction, or in this case, cuts the working time of the epoxy in half.

James

[Larry Edgerton]

A better way if you do this again is to warm the object. That way it is where it needs to be when the curing process is initiated. I did this yesterday. I had to epoxy some threaded rod in a sign, so I dropped the rod in the holes filled with epoxy, then warmed the rods with a torch. Slow West System was cured in three minutes.

I use a heat gun to gently heat up the area around a mortice and tenon for example if I want a little more speed.

Keep in mind the faster an epoxy cures, generally the less strength it will have. this not only applys to the hardner, but also artificial means of speeding up curing. Sometimes the extra strength in not necessary, but if it is a critical joint, that is something to keep in mind.

Patience is a good thing with epoxy, when things go wrong, they can go wrong in a hurry.

Humidity also affects cure time, so if you are working on a hot humid summer day, have all your ducks in a row before mixing. I put the resin/hardner in the fridge unmixed for a bit to lengthen my working time. The 30 minute is 30 minutes at a given temp and relitive humidity level, any changes from that either lengthen or shorten the reaction time.

[Richard Jones]

Heat will greatly decrease the set time of epoxy. In the boatyard we used 1 hour epoxy and flash cured it with an infrared heat lamp. We could set it in 5 minutes.

Geez, I never really looked at your location, Howard. In another life, I used to live and work up at the boatyard at Masonboro........

To the OP's ? : Warm, not hot, helps it flow, but there can be a fine line. Humidity also affects how epoxy works, at least for me. I used to bed "a lot" of varmint and BR rifles. Would always do them in my basement shop, but then took the whole rig upstairs and suspended it from my mantle over my gas logs. Warm, not hot, and cure time was greatly reduced.

Also agree on warming the piece as well, if possible.

Rich

[Jon van der Linden]

Humidity was mentioned in a previous post. Humidity is universally bad with all epoxies I've used. It can prevent a cure and cause finish problems and reduce strength. Personally I wouldn't put epoxy resin or hardener in a fridge because of the potential for condensation... which would ruin the epoxy.

With heat, the heat from a heat gun is way too hot for most epoxies. Common epoxies that set in an hour or so and cure overnight don't really complete their cure for days. Consequently they are often placed in special "ovens" to complete the cure and increase their strength. All epoxies are different, but the ones I use would generally have an oven temperature of about 130 degrees F. Higher temperatures, up to a point, actually increase strength. The idea that heat decreases strength is in my experience not the case. Going over these relatively low temperatures, i.e. 130F, can cause other problems.

If you need significantly different performance than your epoxy is giving, you really need a different epoxy that is formulated for your purpose, there are many many epoxies out there.

[Alan Schaffter]

Epoxy curing is an exothermic reaction- it generates heat. Epoxy with hardener added and left in a large cup or other container, can get hot enough to cause a fire!!

Years ago I built an experimental aircraft from foam, fiberglass and VERY SLOW SETTING (12 hrs +) epoxy. The epoxy started out the consistency of molasses so to get it to flow and wet out the glass fabric more easily we would heat it. The typical method was to put the epoxy proportioning pump with its reservoirs of resin and hardener in a box with a 100W light bulb. The epoxy and hardener were not mixed until after they exited the separate pump spouts. The warm epoxy would flow much easier but had less working time and full cure time was greatly reduced.

[Stephen Cherry]

WEST system has lots of good books and info on using epoxy.

[Forrest Bonner]

Heat really does thin epoxy as Alan says. Back in my younger days of winding motors for slot cars I would put the thick epoxy on the windings then twirl the motor over a match. Made the epoxy get thin and flow down into the windings and set quickly.
Forrest

[Jeff Bartley]

Thanks for all the suggestions!
Alan, what type of plane did you build? The advice I received about heat also came with the suggestion to use the very slow setting type of epoxy. This advice came from my Dad who built a Rutan Long-EZ. For some reason I keep using the cheap stuff found at the local hardware but I don't think the slow-set type can be purchased in small amounts.