Lightfastness
The lightfastness of colorants directly affects the fading of products. For outdoor products exposed to intense sunlight, the lightfastness (sunfastness) grade of the colorants used is a crucial indicator. A lower lightfastness grade leads to rapid fading of the products during use. The lightfastness grade for weather-resistant products should not be lower than grade six, while grade seven or eight can be chosen for indoor products. The lightfastness performance of the carrier resin also significantly influences color changes, with the molecular structure altering and causing fading after UV radiation. Adding UV absorbers and other light stabilizers to the masterbatch can enhance the lightfastness of colorants and colored blow-molded products.
2. Acid and alkali resistance
The fading of colored blow-molded products and the chemical resistance of the coloring agents (alkaline and acid resistance, oxidation-reduction resistance) are related. For instance, molybdenum chrome red is resistant to dilute acid but sensitive to alkalis, while cadmium yellow is not acid-resistant. These two pigments, along with phenolic resins, have a strong reducing effect on certain coloring agents, severely affecting their heat and weather resistance, and causing fading.
3. Antioxidant properties
Certain organic pigments fade gradually after oxidation, either through the degradation of large molecules or other changes. This process is due to high-temperature oxidation during processing and oxidation caused by strong oxidizing agents, such as the chromate ion in chrome yellow. When lake pigments, azo pigments, and chrome yellow are mixed, the red color will gradually fade.
4. Heat resistance
The heat stability of pigments refers to the degree of thermal weight loss, color change, and fading under processing temperatures. Inorganic pigments are composed of metal oxides and salts, which have good heat stability and high thermal resistance. On the other hand, pigments made from organic compounds may undergo molecular structural changes and a small amount of decomposition at certain temperatures. Particularly for PP and PET products, when selecting colorants, one must pay attention to both the heat resistance and the heat stability duration of the pigments.
