Received an email from Fiberglass Supply. Some interesting info about how cure rates are affected by temperature

Quote:

Heat and Epoxy

We all want to know: How long do I have to work with the epoxy? Usually the next question is how fast will it set up?

For a given hardener geltime and cure speed are directly affected by temperature.
Let me say it again, geltime and cure speed are directly affected by temperature.

So if we want to know how much time we have to work with the material and how quickly it will begin to gel or set up we begin with the manufacturer's data sheet. Epoxy resin datasheets all will give you the gel time or pot life time at a given temperature for a specific hardener (usually 77f or 25c, sometimes other temperatures), and gel time sample size, (usually around 100ml or 3 oz). Sometimes they will also give you a tack free or set time, which can be very helpful.

A couple of things to know about those times, usually to calculate them they mix up a batch in a cup and monitor that cup. A 100cc mass of epoxy is going to develop heat differently than a thin film of epoxy or a thin layer in a laminate, the thin layer will gel slower since it is not retaining heat the same way the mass in the cup does, conversely should you mix up a 1500cc batch and leave it in the bucket it will develop heat differently too, in that case it will develop it faster and gel sooner.

The rule of thumb with epoxy resin systems is that for every 10 degrees C ( or 18 F) you raise or lower the temperature the geltime/cure time will be either cut in half (if the temperature rises) or double (if it falls).

This is important because it helps us determine what our working conditions need to be and what hardener speed to pick if we are unable to change our conditions. It also helps us understand what is happening. A couple of examples:

We laid up a cored panel in a demo using wet layup vacuum bagging techniques, in order to have time to get everything in the bag before the resin began to kick we used a slow hardener. The next morning (approx 12 hours after it was bagged) we came in and the person building the part removed it from the bag and found that it was still flexible. Temperatures in the shop dipped down into the 50’s that night. The resin system they used was Resin Research 2000CE with slow hardener, the slow hardener at 77F has a 5 hour set time, which means at 59-60F it would have a 10 hour set time, with the temperatures dropping even below that and with it being a thin laminate, it is reasonable that even 12 hours after layup the resin was still pliable. Had they done the calculations before de-bagging they would have realized that perhaps it would be a good idea to wait a little longer or to have added heat to the project overnight.

We have a motorcycle side cover part that we build in workshops and classes and wanted to see if we could demold a part in under 1 hour from the time of injection. So we calculated out what our working time needed to be and determined that with the fast hardener at 90-100F we would have enough time to get part wetout, then we looked at set time. Set time for this resin (Resin Research 2050) with fast hardener is 2.5 hours at 77F. So at 95F it would be 1.25 hours, at 113 F it would be 37.5 minutes, at 131 it would be 18 minutes. In this case we determined to run the mold at around 140 F for 40 minutes and then drop it below 120F for demold. Running with that setup we were successful in hitting our goal of demolding an epoxy infused part in under 1 hour from the time of injection.

Using this rule of thumb, that for every 18 F in temperature change the gel time will double or be cut in half you can easily figure out what hardener to pick for the conditions you will work in or what the conditions are that you need to create to keep your project or process moving at the proper rate. We have developed a calculator and put it up on our website that allows you to enter your resin gel time or set time and datasheet temperature and it will calculate and graph out a times for a temperature range around the data you enter. You can find that calculator here.

I could end this here but there are a few more things I want to throw out:

One trick that we will often do with infusion where we have a bucket of resin mixed up that we are feeding out of that is just sitting there generating heat is to put it into a heat sink, which often times is simply a bucket of cold water. As the resin kicks off and generates heat that heat is absorbed by the heat sink and keeps the bucket at a stable temperature extending your geltime while the part wets out.

Another issue we have run into is trying to laminate in direct sunlight. The sun can heat surfaces up to around and sometimes past 180F. Think about that, if you are putting a 20 minute geltime resin onto a surface that is say 160F that means you will have less than a minute to work with it. In practice you’ll have a little longer than a minute, about 5 actually but don’t ask how we know.

We have been doing a lot of infusion lately on a variety of surfaces and at different temperatures, one thing that we have noticed as we have done that is that when we run the resin at higher temperatures (which gives us a lower viscosity) is that pin hole and surface pitting issues go way down, to the point of getting perfect surfaces (direct contact molding with no surface coat). What the magic temperature is will depend on the resin system, with Pro-Set we like to run around 100F, some of the other resins we have run other resins as high as 230F.

And finally speaking of infusion, take a look at this Pro-Set data sheet, they provide a graph of viscosity vs temperature that illustrates clearly why we like to run our epoxy infusions at higher temperatures. At room temp it has a viscosity of around 300 cps, bump that temp up to 90-100 you are looking at a viscosity range of 150-100 cps (which is the range for most polyester or vinylester infusion systems). Manipulating that viscosity swing can make the difference between a successful infusion or a painstaking disaster.