Colored plastic PVC products will fade due to many factors. The discoloration of colored plastic products is related to the light resistance, oxygen resistance, heat resistance, acid resistance of color powder as well as the properties of the resin used.
The following is a detailed analysis of the fading factors in plastic coloring:
1. Light resistance of colorants
The light resistance of colorants has a direct effect on the color fading of the products. For outdoor products exposed to strong light, the light resistance (sun resistance) grade of the colorant used is an important index. The light resistance grade is poor, and the product will fade quickly in use. Weather-resistant products should be selected for light resistance of not less than 6 grade, the best choice is 7 and 8 grades , indoor products can be selected for 4, 5 grades.
The light resistance of the carrier resin also has a great influence on the color change, and the molecular structure of the resin will change after uv exposion, which causes color fading. The light resistance of colorants and colored plastic products can be improved by adding ultraviolet absorbent and other light stabilizers in masterbatch.
2. Heat resistance
Thermal stability of heat-resistant pigments refers to the degree of heat loss, discoloration and fading of pigments at processing temperature.
The composition of inorganic pigment is metal oxide, salt, good thermal stability and high heat resistance. And the pigment of organic compounds will have molecular structure changes and a small amount of decomposition at a certain temperature. Especially, PP, PA, PET products, the processing temperature is above 280 degrees C. On one hand, we should pay attention to the heat resistance of the pigments. On the other hand, we should consider the heat resistance of the pigment. The heat resistance time of pigments is usually 4-10min..
3. Oxidation resistance
Some organic pigments gradually fade after oxidation due to degradation of macromolecular degradation and other changes. During the process, on one head, there is high - temperature oxidation in processing; on the other head, there is the oxidation of a strong oxidizing agent ( such as chromates in chrome yellow ) . After the color lake , the azo pigments and the chrome yellow are mixed , the red color gradually decreases .
4. Acid and alkali resistance
The fading of colored plastic products is related to the chemical resistance of coloring agent (acid and alkali resistance, redox resistance). For example, molybdenum chrome red is sensitive to alkali, and cadmium yellow is not acid resistant. These two pigments and phenolic resins play a strong role in reducing action of some colorants, which seriously affect the heat resistance, weathering resistance and color fading.
In view of the discoloration of colored plastics products, the above properties of pigments, dyes, surfactants, dispersants, carrier resins and anti-aging aids should be comprehensively evaluated according to the processing conditions and application requirements .