What are the reasons for the yellowing of PVC plastic?

What are the reasons for the yellowing of PVC plastic?

The yellowing of plastics refers to the chain reaction of the chain-like polymer molecular chain breaking under high temperature, oxidizing environment, or under the action of ultraviolet, infrared, and microorganisms, which triggers free radical detachment.

The common yellowing of plastics mainly includes the following:

1. Ultraviolet radiation

The light irradiated by the sun to the outer air layer is a continuous spectrum with a wavelength of 0.7-3000nm, of which 300-400nm ultraviolet radiation is the main cause of polymer degradation.And the energy of photons in the ultraviolet range of 290-400nm is 300-419kJ/E, which is higher than the bond energy of some typical chemical bonds in polymers.

2. The influence of heat stabilizer

Different heat stabilizers have different effects on the light stability of PVC.

Ba/Cd and organotin carboxylate compounds (such as maleic anhydride ester organotin stabilizers) can provide a certain degree of UV stability to PVC profiles used outdoors, while sulfur-containing organotin compounds (such as sulfur Alcohol organic tin stabilizers) provide limited light stability.The light stability of the di-salt lead stabilizer is higher than that of the tri-salt lead stabilizer.

3. The influence of antioxidant system

At present, the stabilizers used by PVC profile manufacturers are mainly composite stabilizers. In addition to the main compound that stabilizes the degradation of PVC, they also include lubricating systems, antioxidant systems and other stabilizing systems.

The antioxidant system may include phenolic stabilizers and phosphite co-stabilizers. These compounds may contain easily discolored organic impurities, and in addition, they may undergo discoloration reactions under the action of light, oxygen, and water.