Polyvinyl chloride is poor in thermal stability and light resistance. Hydrogen chloride starts to decompose at 150°C and an adverse reaction occurs with the amount of plasticizer. In addition, the effect of the pigment on PVC is reflected in whether the pigment reacts with PVC and other components that make up the PVC product and the pigment itself is resistant to migration and heat resistance. Some ingredients in the colorant may promote degradation of the resin. For example, iron ions and zinc ions are catalysts for the degradation reaction of PVC resin. Therefore, the use of iron oxide (red, yellow, brown and black) pigments or zinc oxide, zinc sulfide and lithopone white pigments will reduce the thermal stability of the PVC resin. Some colorants may interact with the degradation products of PVC resins. If the ultramarine pigment is poor in acid resistance, it will interact with the hydrogen chloride generated by PVC decomposition and lose its proper color during the PVC coloring process. Therefore, regarding the coloring of PVC, the characteristics of the resin and related additives are taken into consideration, and the characteristics of the pigment are combined. The following issues should be noted when selecting colorants.
1, some of the metal ions in the pigment will promote thermal oxygen decomposition of polyvinyl chloride resin as shown in Figure 1.
The determination method is the hue change when pigmented polyethylene is heated to 180°C. Because the pigment contains metal ions to accelerate the decomposition of PVC, resulting in hue changes. At the same time, it should be noted that the same color shade red can make PVC produce different color, such as calcium, the color difference is small; containing manganese is a large color difference, which is due to manganese and other metals promote PVC dehydrochlorination caused.
Sulfide-based colorants (such as cadmium red, yellow, etc.) are used for the coloration of polyvinyl chloride, and hydrogen sulfide may be released due to the colorant. Such colorants should not be mixed with lead stabilizers to prevent the formation of black lead sulfide.