Glass Transition Temperature
The Glass Transition Value/Temperature - also known as Tg or Glass-Liquid Transition - is the temperature at which a solid or amorphous material transforms from a solid (glassy) state to a soft, rubbery state. As the temperature increases to a certain point, so does the bendability of the material. The temperature at which this transition occurs is different from material to material and can even vary within certain material types. Epoxies, for example, can have very different Tg values (commonly ranging from 50°C to above 90°C) depending on the type of epoxy and intended application. Many polyurethane resins, once properly cured, will have a Tg value around 50°C. PVC, which many of our finboxes are made from commonly has a Tg of 80°C (176°F). This means that the softness of the material will increase beyond this point and the material will begin to warp. *It should be pointed out that no data has been published on the Tg of the attachment components we commonly install in our surfboards.
The importance of a value like Glass Transition for surfboards should be immediately apparent. Our surfboards spend a lot of time in the sun where a material’s temperature (especially if it has a dark color) could exceed 50°C (122°F). For this reason, I have chosen (and tested!) a polymer that, once cured at high temperature, achieves a Tg of around 80°C, a comfortable temperature for the wide range of climates a surfboard may be used in. However, it should be stated that no surfboard products should ever be left in the direct sunlight or heated areas for extended periods. UV and/or extra heat degrades and permanently damages your materials (just think of what the sun can do to your skin!)
Achieving the optimal Tg of a thermoset polymer (like polyester, epoxy or polyurethane) cannot always be accomplished by simply curing the material at room temperature, but rather by slowly/gradually curing the material while increasing the temperatures over time. My components (after an initial 24h cure at room temp) undergo 7 additional days of high temperature post cure to make sure that the plastic sets/cures/bonds well. After this they are ready for QC and installation.
Tg is also very important to consider when laminating and finish coating a board, especially if you’re using an epoxy resin that wasn’t designed for surfboard purposes or isn’t being sold by a surfboard materials distributor. The question could also be posed as to whether or not the board’s post-cure is sufficient enough to arrive at the resin’s potential Tg in the first place. If not, you may, over time, have a weak board on your hands.
In the photo below, I am testing thermoset components at 100°C/212°F at which point they are slightly pliable and a bit easier to warp.
