As the temperature increases, the molecular bonds of the matrix and the interface will weaken and the kinetic energy will increase, which may cause some bonds to break under a certain load.
Microcrack theory
The matrix has a certain inhibitory effect on fiber defects, which can limit the expansion of the fiber damage area, and as the temperature rises, the matrix toughness becomes larger, the inhibition of the fiber is weakened, the damage area will expand, and the strength will decrease accordingly.
Combined with the above heat resistance mechanism, what are the ways to improve the heat resistance of FRP?
1. Increase the glass transition temperature Tg, which is the characteristic temperature for measuring the operation of polymer chain segments. To increase the glass transition temperature is to increase the degree of crosslinking of the resin to make its structure more stable; the usual method is to introduce a heat-resistant skeleton and add naphthalene rings, aromatic rings, heterocycles, etc. to the resin to improve thermal stability; Performance curing agent; post-curing treatment, etc.
2. Strictly control the production and curing process, pay attention to details, and try to avoid defects and cracks.
3. Moisture-proof treatment, such as fibers and fabrics and other reinforcing materials, keep them as dry as possible during use, and dehumidify them when necessary. On the one hand, moisture absorption will increase the thermal expansion coefficient of the resin matrix and have a plasticizing effect on the resin, which will lower the glass transition temperature of the resin matrix and reduce the modulus; on the other hand, it will easily diffuse to the interface through the resin matrix, making the interface The adhesion is weakened, so the substrate can use a resin with a small moisture absorption, or add a water-repellent agent to the resin to improve its heat resistance.
4. To ensure a good bonding interface, choose matching fibers and resins. For thermoplastic composites, the crystallinity of crystalline composites can be increased to improve heat resistance.
5. Add heat-resistant resins such as polyimide and bismaleimide to the resin, or add thermoplastic high-temperature resistant resins such as PEEK, but the compatibility with the original resin should be considered to improve the composite material. heat resistance.
Heat resistance is only one of the performances of FRP, often in the design of materials and processes, other performances must also be taken into account, which may conflict with each other, and comprehensive consideration is required to select the material system and the corresponding molding process.