From the Cozy Newsletter #61
All epoxy systems have a minimum and a maximum curing temperature. At the minimum curing temperature, for example, room temperature, an epoxy system achieves a good degree of curing, that is, over 80%, and produces a hard, shiny, tack free surface, with good properties, but the properties, including glass transition temperature, are not maximized. For both L335 and L285, the properties achieved from room temperature cures are quite acceptable for aircraft use. The glass transition temperature of L335 after a room temperature cure, would be about 130 deg. F (the glass transition temperature is the temperature at which the epoxy begins to soften and become rubbery). To obtain 100% of the available properties and maximum glass transition temperature requires either post curing or curing at the maximum curing temperature. For L335, the maximum curing temperature or post curing temperature would be about 160 deg. F and results in a glass transition temperature of about 185 deg. F. But to achieve maximum cure and glass transition temperature, the layup would have to be held at the maximum curing temperature for about 16 hours. For L285 the maximum curing or post curing temperature would be about 170 deg. F and result in a maximum glass transition temperature of about 240 deg. F. Again, it would have to be held at the high temperature for about 16 hours.
So is it necessary to post cure your airplane? Actually, no. As either parts of your airplane or the entire airplane sit at temperatures above 70 deg. F, some post curing occurs. Even if your airplane sits outside in the sun, and the ambient temperature reaches 100 deg. F, it will do so gradually, and the post curing will proceed gradually so that glass transition temperature will always be considerably higher than ambient. The wings will not sag. Remember, you are going to paint your airplane white, and the surface temperature (only the top surface in the sun) will only get about 10 degrees higher than ambient. So on a 100 degree day, some post curing will already have occurred, so the glass transition temperature will already be higher than 130 deg. F and approaching the maximum of 185 deg. Now there is a possible exception to this rule. If you are going to paint your airplane black, or some other dark color, a fully post-cured L335 might not have a high enough glass transition temperature (185 deg. F might not be high enough). So, if you intend to paint your airplane black or some other dark color, you probably better use an epoxy like L285, and do some post curing before you park it out in the sun on a hot summer day. Remember, with an epoxy like L335, the glass transition temperature will always be higher than ambient and the epoxy will not soften unless or until the surface temperature reaches the maximum glass transition temperature, which in the case of L335 is about 185 deg. F. If you paint your airplane white, at least on the top, the surface should never reach this temperature even outside in Saudi Arabia in the summer. Even if you are not going to paint your airplane a dark color, but are willing to spend a little extra money on the best epoxy available, consider using L285. That would be our choice. If you do not understand, or have further questions, please contact us.
Last updated: 2022-03-26 21:49:07 -0700