After the unsaturated polyester resin is cured, it will age under long-term exposure. The reasons for photoaging come from two aspects: on the one hand, the energy of light breaks the covalent bond of the resin; on the other hand, the impureness of the resin itself causes a breach in the damage. As a result, the resin accelerates degradation.
The sunlight after passing through the atmosphere has different wavelengths, and the light of different wavelengths has different energy. Various covalent bonds in the resin have different bond energies. When the energy of a certain wavelength of light exceeds a certain covalent bond energy, it will be broken. The wavelength of ultraviolet light is 30~40 m, and if fully absorbed by the resin, the energy can reach 299~399kj/mol. Some covalent bonds require energy of 167~418kj/mol, and the corresponding wavelength is 71~29m. Visible ultraviolet light is sufficient to open these covalent bonds. The energy of this part of light accounts for about 12% of the total solar energy. It first harms the 0-0 bond, C-Br bond, C-CL bond, C-O bond in the resin. Therefore, halogen-containing flame retardant resins are liable to turn yellow. The ester bond in the resin becomes a weak point under attack. As for the C—C bond, the light energy that can interrupt it accounts for only 5%. For the C—H, O—N, C—C, and C—O bonds, the bond energy is greater than 410 kj/mol, so it is not subject to damage.
The purity of the resin itself is another important factor in light aging resistance. When the purity is high, it absorbs light larger than 30m, so it is not easily destroyed. In fact, the resin contains a small amount of impurities, and the impurities are oxidized after absorbing ultraviolet light to form carbon groups. The carbon-based absorbs ultraviolet light with a wavelength of 28~33m, and transmits the light energy to the entire molecular chain, which degrades at weak points. If other factors simultaneously age the resin, the photodegradation will be accelerated.
In order to prevent photoaging, generally ultraviolet absorbers can be added to the resin, also known as light stabilizers. This ultraviolet absorber can be dissolved in the resin and has strong absorption capacity for ultraviolet light. After absorbing the light energy, it can be converted into other energy that is not harmful to the resin structure such as secondary radiation energy and vibration energy.