Causes of Color Fading of Colored Plastic Products
Colored plastic products will fade due to many factors. The fading of colored plastic products is related to light resistance, antioxidant activity, heat resistance and acid and alkali resistance of colourment and characteristics of resin. The fading causes are analyzed as follows:
1. The light resistance of colorant directly impacts on the fading of products. For outdoor products subject to strong light exposure, the light resistance (sunproofing) level is an important index of colorant. In case of poor light resistance level, the products will fade soon during use. The light resistance level selected for the weather-resistance product shall not be lower than level VI, and level VII and VIII are preferred. Level IV and V are optional for indoor products. The light resistance of carrier resin has a great impact on the color change. Due to the resin exposure incurred by ultraviolet light, the molecular structure will be changed, causing fade. The light resistance of colorant and colored plastic products can be improved by adding light stabilizer, such as ultraviolet light absorber in the master batch.
2. The heat stability of heat resistance pigment refers to the degree of thermal weight loss, discoloring and fading of pigment under processing temperature. The composition of inorganic pigment is metallic oxide and salt, so it has good heat stability and high heat resistance. For the pigment of organic compounds, its molecular structure will be changed and decompose in a small amount under certain temperature, particularly, PP, PA and PET products. In case of processing temperature of 280℃ above, selection of colorant shall be taken into account of heat resistance and heat resistance duration of pigment. Generally, the heat resistance duration is required as 4-10min.
3. Oxidation resistance. After oxidation, some organic pigments will have macromolecular degradation or other changes and then fade accordingly. Such process includes high-temperature oxidation in processing and oxidation caused by strong oxidant (such as chromate in chrome yellow). Red will faded gradually after mixture of color lake, azo pigment and chrome yellow.
4. Acid and alkali resistance. The fading of colored plastic products is related to the chemical resistance of colorant (acid and alkali resistance, oxidoreduction resistance). For example, the molybdate red is resistant to dilute acid but sensitive to alkali. The cadmium yellow is not resistant to acid. These two pigments and phenolic resin have strong reducing action on some colorants, seriously impacting on the heat resistance and weather resistance of colorant and causing fade accordingly.
With respect to the fading of colored plastic products, plastic products shall be selected as per the processing conditions and using requirements after comprehensive assessment for properties above of required pigment, dye, surface active agent, dispersing agent, carrier resin and anti-aging assistant.