Most rubber grommets are exposed to light (including natural light and artificial light) during storage and use. In the long-term illumination, their aging process will be accelerated (compared to when they are not exposed to light), and the surface will gradually age. This aging is characterized by a progressively brittle surface with cracks and a decrease in elasticity. Similar to heat aging, ozone aging, etc., photoaging is also caused by oxygen aging, and under the conditions of mutual light and superposition of light and oxygen, the macromolecular structure of the rubber is destroyed, the performance is degraded, and the use value is eventually lost. This aging process formed by the combination of light and oxygen is called “photoaging”.

(1) Formation mechanism of photoaging

Light is the energy sourced from the sun in nature. Light waves have different energies due to the unequal wavelengths of the bands.

The shorter the wavelength of light, the greater the energy contained, and the stronger the damage to the object being irradiated. Therefore, the ultraviolet light has the greatest damage, and they will cause damage to polymer materials (including rubber). Sunlight is the total source of natural light. Although it has been filtered through the atmosphere before reaching the ground, the remaining ultraviolet rays are only 5% to 6% of the original, but still have considerable destructive power, especially at high altitudes because of the air. Thin, more harmful, and more damaging to rubber.

(2) Characteristics of light aging of rubber grommets

According to the above description, the characteristics of photoaging of rubber grommet are summarized into the following three points:

1 Photoaging is based on the premise of light, taking oxygen aging as the trigger, starting from the surface of the rubber, first forming a film on the rubber surface, and then further cracking and developing in depth;

2 Because the surface of the rubber grommet is first in contact with light and oxygen, the illumination is a necessary condition. After the surface forms a crack, it develops from the surface to the inside.

3 The light aging of light-colored glue is faster than that of black and dark glue. This is because carbon black can form a shielding effect after entering the rubber matrix, which absorbs and filters light, thus delaying the transmission of ultraviolet light.

(3) Preventive measures against photoaging

To prevent photoaging, you can start with rubber grommets main material selection and formulation design improvements.

1) Select a host material with high saturation. Light is theoretically possible, and it lacks practical significance.

2 Add a large amount of carbon black (especially channel black) to shield and filter, effectively absorb visible light and reduce the transmission of ultraviolet light, but only for black rubber grommets. Of course, some light-colored fillers can also perform the same functions, such as titanium dioxide and lithopone. However, their mechanism of action is different from that of carbon black. It is not absorbed but reflects ultraviolet light, so it has the taste of “rejecting the enemy outside the country”.

3 Adding a UV absorber to convert light energy into heat energy through “energy conversion”, consuming energy by releasing heat, and also preventing photoaging. The ultraviolet absorbing agent has a group which absorbs ultraviolet rays such as C==N, -N=N.N==O, etc., and a UV stabilizer such as benzophenone has such a group. They can effectively prevent the photoaging of rubber and prolong the service life, and the required amount is very small, only 0.1 to 0.5 parts. The most widely used UV absorbers are UV-9 and UV-P.