Causes of fading of blow molded products

Blow molded products, also known as hollow blow molded products, is a rapidly developing plastic processing method. The tubular plastic parison obtained by extrusion or injection molding of a 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 passed into the parison to blow the plastic parison. It swells and closely adheres to the inner wall of the mold. After cooling and demolding, various hollow products are obtained. The manufacturing process of blown film is very similar in principle to blow molding of hollow products, but it does not use a mold. From the perspective of plastic processing technology classification, the molding process of blown film is usually included in extrusion.

We often find that, after long-term storage, blow-molded products will more or less fade, so why will blow-molded products fade? Let ’s talk about the reasons for the discoloration of blow-molded products under the popular science.

Light resistance

The light resistance of the colorant directly affects the discoloration of the product. For outdoor products exposed to strong light, the light resistance (lightfastness) level of the colorant used is an important indicator. The light resistance level is poor, and the product will fade quickly in use. The weather resistance of the weathering products should be no less than six, preferably seven or eight, and indoor products can be four or five. The light resistance of the carrier resin also has a large effect on the change of color, and the molecular structure of the resin caused by ultraviolet radiation changes and fades. Adding light stabilizers such as ultraviolet absorbers to the masterbatch can improve the light resistance of colorants and colored blow molded products.

2. Acid and alkali resistance

The discoloration of colored blow molded products is related to the chemical resistance (acid and alkali resistance, redox resistance) of the colorant. For example, molybdenum chromium red is resistant to dilute acids, but sensitive to alkalis, and cadmium yellow is not resistant to acids. These two pigments and phenolic resins have a strong reducing effect on some colorants, which seriously affects the heat resistance and weather resistance of the colorants and discoloration occurs.

3. Antioxidant

Some organic pigments undergo gradual degradation or other changes after oxidation and gradually fade. This process is one of high temperature oxidation during processing, and the other is oxidation in the presence of strong oxidants (such as chromate in chrome yellow). After the use of lakes, azo pigments and chrome yellow, the red will gradually decrease.

4. Heat resistance

The thermal stability of a pigment refers to the degree of thermal weight loss, discoloration, and discoloration of the pigment at the processing temperature. The composition of inorganic pigments is metal oxides and salts with good thermal stability and high heat resistance. The pigments of organic compounds undergo molecular structure changes and a small amount of decomposition at a certain temperature. Especially for PP, PA, PET products, when choosing a colorant, we must pay attention to the heat resistance of the pigment on the one hand, and consider the heat resistance time of the pigment on the other.