Techniques

A high Tg resin may be cured in two ways: It can be cured initially at elevated temperatures or it can be partially cured (becomes solid) at room temperature and then heated (post-cured). The two routes get one to the same place if sufficient time is allowed for both. The first route can be tricky because the epoxy reaction is exothermic and may excessively spike the temperature. Most boats are laid up at room temperature - hand lay up, vacuum bagged or infused - and post cured later simply because this route is considerably easier. This is the one we'll concentrate on.

Recall the reverse "S" shaped curve of Part 2 - the one you drew out, right? If a high Tg resin is room temperature hardened at 70F it will have a curve slightly to the left of the curve for the same resin hardened at 90F. In the case of a high Tg resin system both of these curves are to the left of the ultimate curve. The object of post curing, therefore, is to PUSH the curve to the right as opposed to PULLING it to the right. When it is pushed one stays in the glass-like region of the curve. When it is pulled then one is in the rubber-like region. Since you want to keep the laminates stiff while post-curing stay in the glass-like region. You do this by starting the post curing process at room temperature and ramping up slowly - maybe 10-15 degrees per hour depending upon the size, shape and mass of the part. You can never go too slowly but you can go too quickly. If you go too quickly you will still post cure the part but it may warp in the rubber-like area and take on a permanent set once it has fully cured. Post curing in a mold may help avoid warping. Waiting for several days at room temperate before post curing helps, also.

There are many ways to create a post cure "oven" and none need to be complex since all you have to do is get to 140-150F and hold things there for several hours. On a hot July day the back of your minivan will do it for small parts. Ramp up by slowly closing the windows as the morning wears on. Put a thermometer somewhere near the curing part and you'll control things about as well as the engineers operating the autoclave at the Boeing plant. For larger parts you can rig some black polyethylene sheeting on some ropes over than hull set out in the sun. Put the pointy end of a cheap digital meet thermometer into some kid's play clay stuck on the laminate. Use a fan to circulate the air in the tent. If you're inside the barn, tent with clear polyethylene and use a couple of space heaters. Electric is best. Defeat the temperature control and use fans to circulate the air. If you must use direct fired heaters chose propane. Kerosene will fill the tent with unburned hydrocarbons, which may affect secondary bonding. Remember that these heaters use oxygen so you must provide a source. They will create carbon monoxide if there is not enough oxygen so stay out of the tent and provide adequate room ventilation. In larger communities it may be possible to rent these heaters with indirect fire capabilities. These keep burnt gases out of the tent.

We once post cured a repair on a thirty-foot spinnaker pole by running the pole through a big cardboard box so that the repair area was inside the box (the pole was on sawhorses). We cut a small hole in the box bottom and inserted a hot air gun nozzle. A lab thermometer stuck in the top provided temperature control. We sat around monitoring things while quaffing a beer or two and talking about the importance of removing the sub forestay before jibing.

The points to remember are this. Heat is heat and the epoxy doesn't give a twit about the source. Monitor the process in some way and stick around to make sure you don't catch anything on fire. Oh, have an extinguisher handy just in case.