Post curing of epoxy

Brought forward from a previous post but different thread:

[COLOR=blue]Something I didn’t know. I just read on Sails Etc that if epoxy cures at say 20C then later it can soften (warp) at any temp above that. If it cures at 40C it doesn’t soften until the temp is above 40C. What this meant to me is that if you build a fin(or hull) in the winter when your shop is cool it will be more vulnerable to summer heat. I’m thinking we should all be making an oven so that all our epoxy parts will be cured at 35 or 40C. That way we don’t have to worry about the sun so much. Apparently you can heat treat it after to raise the softening temp[/COLOR]

The response received:

Hi Dick,

Thank you for submitting this questions and posting this answer in your RC Sailing forum. The physical characteristics that you are referring to is a an epoxy’s Glass Transition Temperature (Tg). The Tg is the temperature in which cured (vitrified) epoxy becomes molecularly active after the initial curing. When epoxy is formulated, the specific chemical compounds that used will determine the epoxy’s overall attributes. This includes the compression yield, hardness, tensile strength, tensile elongation, flexural modulus, flexural strength, tensile modulus, annular shear, izod impact and the Glass Transition Temperature.

Certain epoxy formulations are room temperature cure, where the epoxy formula reaches its maximum physical characteristics at room temperature. If these room temperature cured epoxies are heated significantly beyond their Tg, they can start to soften as a result. In the case of 105 / 205 system, the Ultimate Tg is 142 degrees Fahrenheit. Now this is not to say that at 143 deg F that 105/205 turns to soup. This attribute is determined by a specific test procedure on an instrument called a Differential Scanning Calorimeter or DSC. The machine can measure subtle changes in the test sample as a result of elevated heat. At 200 deg F a sample of 105/205 will become malleable, but when cooled, becomes just as a hard as it used to be.

There are epoxy formulation that are used in more technical applications that benefit from a Post Cure. A post cure is additional heat that is applied to an epoxy part to help it reach its full physical characteristics. These formulations are such that there is a significant amount of energy that is in the epoxy in the form of chemical compounds that create higher physical properties. However, there is a problem for these more technical epoxy systems to reach these higher physical properties. When the epoxy is reacted, and the material starts to plasticize (get hard), the ability for the molecules of resin and hardener to find each other and crosslink becomes increasingly difficult. In this case there are partial cross-links formed and in some cases un reacted material. These initial room temperature cures of these epoxy systems give way to a material that is quite brittle, hence why we call this the ‘B’ Stage. These epoxy systems require additional heat beyond the initial Tg to allow the material to become chemically active again and aid the continuation of the reaction and cross-linking.

Now achieving these high physical properties is a little tricky, if you heat the part to fast and too high of temperature, one can actually stunt the development of the physical properties. This is known as over shooting the Tg. Let me provide and example, if a technical epoxy system room temperature cures to a ‘B’ Stage and while at that ‘B’ Stage, the epoxy has an initial Tg of 135 deg F, If that epoxy system is then placed into a 200 F oven the material may heat up too quickly beyond its ultimate Tg and there will be additional reactions and cross linking but because of the rapid heating molecules that could have cross linked become entrapped and can not cross link thus creating a condition where the ultimate physical properties can never be reached. Instead one should ‘chase’ the Tg, where the part is slowly ramped up to temperature. During this ramp time, as the part approaches and exceeds the initial Tg, additional reaction and cross linking is occurring and the physical properties are being increasingly developed, including the Tg. So at as the part reaches and exceeds 135 F, the Tg is no longer 135 F, it may be 145 F, but as the part reaches 145 F, now the material has had a chance to additionally cross link and the Tg may increase to 150 F and so on until the material reaches it highest possible physical characteristics.

There is however a ceiling on how high these properties can be developed, this is based on the chemicals used and the formulation. [COLOR=darkorange]In the case of room temperature cured epoxies, there is no significant advantage to laying up a part at 20 deg C and then post curing at 40 deg C. The physical characteristics will be achieved regardless of that additional heat. Now heating a part up to a slightly elevated temperature with room temperature cured systems, will help the epoxy system reach their ultimate properties more quickly.

I hope this helps to answer your questions, please feel free to contact me directly for additional answers to your questions.

Best Regards,

John M Thomas
Technical Advisor

My sincere thanks to John Thomas and the Gougeon Brothers, for the technical response to my original question. I took the liberty of bolding one portion of the response, and highlighting another in ORANGE that I felt directly addresses the question/concern. Also, This is based on the WEST SYSTEM products specifically, but may vary depending on the actual epoxy formulation of the product/brand you may be using.

Glad I could help and again - a tip of the hat to the guys at Gougeon Brothers.


Thanks Dick
That sheds a little more light.

Hi all, thanks for starting this thread.

While I was browsing the web, looking for a epoxy resin and carbon fibre supplier in Switzerland I stumbled over this documentation:

it’s in german and english. 228 pages of information about composite technology. Also covers the topics of curing (tempering) epoxy.


PS: Their product catalogue is quite impressive too, and they deliver abroad… (in german and french only)

Hi Everyone,

Post curing of Epoxies is very important and should be carried out as good general practice. It is up to the builder to do the best they can with the tools they have, and heat is available to all!

West 105 system is okay for basic jobs but I recommend that you try west Pro-set or Ampreg 26 for stiffer lighter components. These resins must be post cured for 16 hours @ 50 degrees before full cure is achieved.

Its vital that you follow some rules for successful results:

1 If you are not vacuum bagging then all post curing must take place after initial cure at room temp. If you are bagging then you can put heat on after the resin has ‘gelled’ a bit… this helps with the flow and will remove more resin giving a better fibre/resin ratio for lighter components.

2 Pre cure all tooling to the temperature you want to eventually cure the final product - this avoids shrinkage and you can easily check for distortion before you make your hull etc.

3 Cure all products in the mould and not after demoulding.

4 Once you have assembled the yacht, run another lower temp but for longer as this will cure all the bonding nicely inside the yacht. not too hot as the fillets and glue joins can shrink and cause distortions in the hull/deck.

5 Use a temperature gauge!! Very important. Tape the sensor to the laminate to get a better idea of the conditions the laminate is being cured under. Just measuring Air temp is not okay.

Ovens can be easily made from thick cardboard boxes and a hairdryer, axial fan heater or a variable heat gun. generally the more air flowing around the more evenly the heat is distributed. be careful with the heat settings, I regularly cure things up to 100 degrees like this!
If you want to get flash then you could build a permanent oven under a bench using styrene insulation sheets tape the joins etc to seal it. We call these “Buvens” in our trade…

There are a lot more tips etc but I don’t want this post to drag on…
Happy building and remember that with epoxies… heat is our friend!

Informative post. I wonder what the heat properties of the usual epoxies used for gluing are? Also, on something like West, where is the transition between a nice hot day that cures things fast, and a premature post cure? If the post cure is done before the room temperature cure, what happens to the properties of the epoxy? (I’ll admit to pulling this stunt quite a few times when in a hurry.)

A few points:

-Better to say 50C. Here in the USA, it’s very easy to forget that the rest of the world uses more rational measurements.

-Someone I knew who molded a lot of wings made a point of using similar materials for the mold (in this case carbon fiber) to match the thermal expansion characteristics. If you’re using unidirectional fabrics, I wonder if this wouldn’t have to include parallel fiber orientation, since epoxy and, say, carbon fiber have different thermal expansion characteristics.

-If you’re molding parts around a male mold, which is tapered, think about whether the part will clamp on if it shrinks. If so, pull it off after room temperature cure. I’m thinking you could probably put it back on the mold for post cure, but I know what happens if you don’t! This was a carbon fiber tail boom for an RC glider over a tapered form. I think it may have been because all the carbon was lengthwise, only glass diagonally.

-If you’re using a hair dryer or space heater and a cardboard box, better watch things like a hawk and have a fire extinguisher handy. Also, some of these devices may not hold up very long, especially if the flow is restricted a bit. I seem to recall burning out a cheap heat gun this way. You can also do various things with light bulbs and a fan (cover the bulbs with some sort of metal screen), an oil filled heater and a fan (at least somewhat safer), heating pads (i.e. for sore muscles), and electric blankets. It might be a good idea, if available, to use a metal box and fiberglass insulation. A car on a sunny day makes a pretty good curing oven if you keep your piece in the shade. Where I live, you can get 140 F at the height of summer, and I’m sure it’s more further south. Fire risk is very low and you don’t have to watch it, as long as you’ve arranged things so you won’t epoxy your upholstery. Obviously, if the car is that hot you don’t want to store boats made from room temperature epoxy in there, or anything that hasn’t been post cured yet.

Black, polyethelyne garbage bags. 30 gallon size will fit most classes of boats - 50 gallon for the bigger one and use sun. Epoxy doesn’t stick to the bag, and you can add a thermometer to watch internal temps - opening/closing neck of bag to regulate. A “poor man’s” oven - but for replacing a chink in a leading edge of daggerboard at a regatta (big boat) it worked while I had lunch. In an hour WEST “Fast” had cured enough to scrape the repair flat and hit the water.

Process much like your car “oven” :wink:

For relatively small parts, use large plastic soda bottles filled with hot water from the tap for a heat source. Wrap the part in with the bottles in a towel, blanket or something. Make sure the bottles are well sealed and dry on the outside. They stay warm for quite a while. More than two hours, anyway.

For extra speed and excitement, use a microwave to heat up the filled bottles. But figure out how long to get a reasonable temperature and do NOT exceed that amount of time in the microwave. I hate to think what a pressurized steam explosion would do to a house.

Hi again,

Thanks for pointing out that I was referring to the Celcius scale… My bad!

I would have to agree on using similar materials for moulds as this is the ‘ultimate’ way to ensure you get a perfect part. We build 25 meter female moulds over pre cured plugs in low temp carbon pre-preg for AC yachts and Volvo’s… Once released, no filler is used and just paint (2 coats!) is applied and then wet sanded, but for model yachting this a little excessive and way too expensive for one off parts. In the group we are all trying to keep the cost down whilst maximizing success by sharing some tips & knowledge.

Another thing to watch out for - There are also materials that expand and deform slightly when heated. I am referring to styrene foam commonly used for insulating houses.( Not white polystyrene) It is fantastic to shape and cut with a hot wire, but will expand, or “blow” when you get to 45C or more and should be avoided for high temperature mould/plug applications…

Please be careful as the topic might be steering off course a little. Post curing should be a measured and monitored process for the optimum end result and not a fast way to get you back on the water…

JEEZ!! But they’re thick coats right? Please give us that.

Bit off topic… but nope… Just enough to cover… paint is heavy. We re-paint as and when necessary. Thats part of shore crew life.

Okay “Boatbuilders”
Here is a great and cheap temperature sensor from IKEA.

So the next time some one is going that way - get them to grab you one or two. They are found in the culinary section with the pots and pans.

I have 4 of these now and I think that they are a great addition to any home builders tool kit and at €6 each they are an absolute bargain.

They even have a temperature alarm which is programmable to go off at any temp… Outstanding!

The only modification I have done is to take off the stainless meat skewer that is on the probe to reveal the sensor on the end. This is then covered with heat shrink. I feel that the temperature response is better when its attached to the surface you are monitoring.

It also doubles as a workshop thermometer for when you are laminating too.

I just thought this might be useful for people wanting a good budget temp sensor. the accuracy is perfect as I have checked them against a K-type thermocouple I have as well… not bad at all for the money, Thanks IKEA!