28
2024
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05
The reasons for fading of blow molded products
Author:
Also known as hollow blow molded products, a rapidly developing plastic processing method. The tubular plastic preform obtained by extrusion or injection molding of thermoplastic resin is placed in a split mold while it is hot (or heated to a softened state). After the mold is closed, compressed air is immediately introduced into the preform to blow and expand the plastic preform tightly against the inner wall of the mold. After cooling and demolding, various hollow products are obtained. The manufacturing process of blow molded film is very similar to that of hollow products in principle, but it does not use molds. From the perspective of plastic processing technology classification, the forming process of blow molded film is usually included in extrusion. We often find that blow molded products may fade more or less after long-term storage, so why do blow molded products fade? Let's explain to everyone the reasons for the fading of blow molded products.
1. Light resistance
The light resistance of colorants directly affects the fading of products. For outdoor products exposed to strong light, the light resistance (sun resistance) level of the colorants used is an important indicator. If the light resistance level is poor, the products will quickly fade during use. The light resistance level selected for weather resistant products should not be lower than level six, and it is best to choose levels seven or eight. Indoor products can choose levels four or five. The light resistance of the carrier resin also has a significant impact on the color change, and the molecular structure of the resin changes and fades after exposure to ultraviolet radiation. Adding UV absorbers and other light stabilizers to the masterbatch can improve the light resistance of colorants and blow molded products.
2. Acid and alkaline resistance
The fading of colored blow molded products is related to the chemical resistance (acid and alkali resistance, redox resistance) of the coloring agent. Molybdenum chromium red is resistant to dilute acids but sensitive to alkalis, while cadmium yellow is not acid resistant. These two pigments and phenolic resins have a strong reducing effect on certain colorants, seriously affecting their heat resistance, weather resistance, and fading.
3. Antioxidant properties
Some organic pigments undergo degradation of large molecules or other changes after oxidation, leading to gradual fading. This process involves either high-temperature oxidation during processing or oxidation when exposed to strong oxidants such as chromate ions in chrome yellow. After mixing lake, azo pigment and chrome yellow, the red color will gradually fade.
4. Heat resistance
The thermal stability of pigments refers to the degree of thermal weight loss, discoloration, and fading of pigments at processing temperature. The components of inorganic pigments are metal oxides and salts, with good thermal stability and high heat resistance. Organic compound pigments, on the other hand, undergo changes in molecular structure and slight decomposition at a certain temperature. Especially PP When selecting colorants for Pp and PET products, attention should be paid to both the heat resistance of the pigment and the heat resistance time of the pigment.
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