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According to data and practical verification, there are many factors affecting the discoloration of PC lamp covers. The fading of colored plastic products is related to the light fastness, oxidation resistance, heat resistance, acid and alkali resistance of pigments and dyes, as well as the properties of the resins used. During the production of color masterbatches, pigments, dyes, surfactants, dispersants, carrier resins, and anti-aging additives can only be selected after a comprehensive evaluation of the above properties. The following are common causes of product discoloration and related performance characteristics, provided for reference.
1. Light fastness

The light fastness of colorants directly affects product fading. For outdoor products exposed to strong sunlight, the light fastness (weather resistance) rating of the colorant used is a critical indicator. Poor light fastness will cause the product to fade quickly during use. This is why items such as highway anti-glare panels and water-filled barriers gradually lose color after several years of sun exposure. Generally, a certain amount of UV-resistant components is added during blow molding to ensure long-term color retention.

2. Heat resistance

The thermal stability of pigments refers to the degree of thermal weight loss, discoloration, and fading at processing temperatures. Inorganic pigments, which are composed of metal oxides and salts, have good thermal stability and high heat resistance. Organic pigments, however, may undergo molecular structure changes and slight decomposition at certain temperatures. In actual production, if PC lamp covers require high-temperature processing, pigments with better heat resistance must be selected to prevent discoloration and fading, ensuring the overall appearance of the product.

3. Oxidation resistance

Some organic pigments gradually fade due to macromolecular degradation or other changes after oxidation. This process occurs due to high-temperature oxidation during processing, or oxidation caused by strong oxidizing agents (such as chromate ions in chrome yellow). When lakes or azo pigments are mixed with chrome yellow, the red color will gradually diminish.

4. Acid and alkali resistance

The fading of colored plastic products is related to the chemical resistance of colorants (acid and alkali resistance, redox resistance). For example, molybdenum chrome red is resistant to dilute acids but sensitive to alkalis, while cadmium yellow is not acid-resistant. These two pigments, along with phenolic resins, can exert a strong reducing effect on certain colorants, severely affecting their heat resistance and weather resistance, leading to fading.
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