resins, common and uncommon
Polyurethane, polyester, and epoxy resins are the most common. I hate polyester. I hate the smell and I hate the surface finish and feel. And I hate how the smell never seems to go away even after the resin has cured... I mostly use epoxy, but I also like polyurethane. Expoy tends to be slow. I am not aware of any accelerant additives that can be used. Polyurethane is fast. Curing starts in about 5 to 10 minutes, so you have to be quick. A thick bucket will cure faster. Keep it cool and thing to slow curing. Even thin, cool sections will cure faster than most resins. Some can demold in 10 or 15 minutes. Full strength in half hour is typical. The resin does get warm and thermal runaway is possible, but doesn't seem as sensitive as epoxy. Vinylester is becoming more popular (sort of a polyester and epoxy hybrid -- it still sucks the way polyester does.)
- cyanate ester
- I have not used these resins. apparently similar to, but superior to epoxide. Tougher and does not absorb moisture like epoxy.
- epoxy (epoxide)
- These are my favorite, resins. People commonly think epoxy is water-proof, but it has poor water resistance. But some grades may be water-proof-enough -- some can be used to make boats, after all. But don't count on epoxy being water-proof until you have tested it (a good rule for all resins). Some epoxy resins actually turn white and get rubbery if exposed to water for a long time. They may or may not recover when allowed to dry.
- I don't know much about these resins. They don't shrink much. They sound similar to epoxy in properties. They might have a higher thermal resistance.
- bismaleimide (bis-Maleimide, BMI)
- I don't know much about these resins. Apparently used in high-performance composites like the F35 jet fighter.
- bismaleimide-triazine resin (BT)
- this is a blend of BMI and Cyanate ester resins.
- BT-epoxy (bismaleimide-triazine-epoxy)
- this is a blend of epoxy and BT (which is a blend of bismaleimide and cyanate resins). I believe this is the material most commonly used for PC boards (printed circuit).
- phenol formaldehyde (PF)
- phenolic resin
- unsaturated polyester (UP)
- I hate polyester. It smells and it is difficult to get a good cure. It doesn't like to get too hot or too cold. If it gets too hot it can get crumbly or rubbery. If it gets too cold it can get sticky. It tends to get sticky anyway. The sticky surface never cures. There are special additives and after coatings that address this problem, but they never seem to work very well.
- I always thought it was a thermoplastic resin, but apparently, some are known as pseudo-thermoplastic resins. Thermoset resins of this polymer are available.
- polyurethane (PUR)
- vinyl ester
- super glue and crazy glue. This is the glue that cements your fingers together instantly. Other materials such as baking soda and wool will also nearly instantly cure these resins. Wool can actually get hot enough to give off smoke because the resin cures so quickly. This polymer cures from water in the air or on the surface. Note that the water should be residual surface moisture on most surfaces which is normally invisible. The surface should appear "dry", not actually wet. Cyanoacrylates include methyl 2-cyanoacrylate, ethyl-2-cyanoacrylate, n-butyl cyanoacrylate, and 2-octyl cyanoacrylate.
- I believe all of these are rubber. I am not aware of any hard silicone resins. These are nearly the best rubber resins, except that they can be permeable to water and air. They may be fine for continuous duty pumping, but should not be used to hold a seal against static pressure. One exception often found is when used with glass for aquariums.
Many resins after the curing agent has been mixed may be stored in a freezer for extended periods of time. The temperature will affect the rate of cure. A normal home fridge at 0F/-18C will almost halt the curing process, but not completely. The resin may feel hard and frozen, but it still continues to cure. This can be an excellent way to cast very thick sections that would otherwise overheat due to run-away thermal acceleration of the curing process. Thick epoxy casting that might actually get hot enough to ignite in a room at room temperature can be stored in a freezer for several days to a week or more. Then the casting may be removed and allowed to warm up. At this stage, the casting will likely not be totally cured. It may have the texture of a heavy putty or hard ice cream. It may hold its shape somewhat or it may sag a bit. At this stage, enough of the resin should have cured that it is no longer in danger of thermal run-away. It may get a little warm as the rest of the resin finishes curing, but it should be able to dissipate the heat.
This is also a good way to store extra resin or to get different flow properties that would otherwise be impossible. Because the resin will cure more evenly the partially cured resin will be thicker, but not lumpy. It behaves more predictably than resins that have had thixotropic compounds added. It has properties you don't otherwise see in resins that cure normally at room temperature (I mean the room's actual ambient temperature. The resin obviously gets hot while curing. Even resin stored in a freezer should be a tiny bit warmer than the freezer because the chemical reaction curing the resin continues, albeit more very slowly)
I have not tested but this may also be a good way to cast a resin seal and limit shrinkage of the resin which might cause pulling away from the seal. I am thinking of epoxy on glass seals in particular. Be sure to use a low-outgassing resin if working with a vacuum. If shrinkage is a concern then investigate a thermoset resin named benzoxazine which apparently has near zero shrinkage. I have not researched its outgassing properties to see if it is suitable for vacuum seals.
adhesion with silanes
Silane treatment of glass to improve epoxy bond adhesion. Silanes are a broad group of chemicals when deposited on a glass will change the surface chemistry to be more or less favorable to certain types of bonds. In the biological sciences, silanization is often used to prevent live samples from sticking to the glass walls of containers or microscope slides. When silanization is used in polymer sciences it is most often used to improve the adhesion strength of glass and resin. This can be critical for vacuum seals. There are many silane agents. Coupling type: agent:
- universal epoxy-silane
- Dow Corning Z-6032 vinylbenzylaminoethylaminopropyltrimethoxysilane
- hydrochloride, cationic-silane
See also #freezing for a possible method to cure resin without stress from shrinkage. I believe that because many resins cure at a higher temperature they shrink when they cool back to ambient temperature. If this is the cause then partially curing in a freezer may prevent shrinkage stress. A thermoset resin named benzoxazine apparently has near zero shrinkage. I have not researched its outgassing properties to see if it is suitable for vacuum seals.
A well known test method is defined by the IPC (Institute for Interconnecting and Packaging Electronic Circuits), IPC TM-650 2.6.16 Pressure Vessel Method for Glass Epoxy Laminate. This test method exposing epoxy-glass laminates to a pressure cooker.
heat transfer -- thermal conductivity
- Epoxy Technology -- See Tech Tips
- Making Polymers Terrific site that gives a good polymer foundation.