The three major problems of PVC coloring

Polyvinyl chloride is poor in thermal stability and light resistance. Decomposition of hydrogen chloride begins at 140°C, so the use of colorants cannot cause adverse reactions. In addition, the effect of 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. 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.

First, processing stability Some of the 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 chloride and lithopone white pigments will reduce the thermal stability of PVC resins. 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.

Second, migration mobility occurs only in plasticized PVC products, and when dyes or organic pigments are used. The so-called migration refers to the presence of partially soluble dyes or organic pigments in the surrounding solvent. The plasticizer penetrates the surface of the PVC article. Those dissolved dye particles are also carried on the surface of the product. This leads to solutions. Bleeding, solvent bleeding or blooming.

Another problem is "fouling." Refers to the coloring agent in the coloring process, because the coloring material is poor or not completely compatible with the free from the system, deposited on the surface of the processing equipment (such as the extruder barrel wall, die hole wall) .

Third, weather resistance refers to the ability of pigments to withstand various climates. These include visible and ultraviolet light, moisture, temperature, atmospheric chlorination, and chemicals encountered during use of the product. The most important weather resistance, including colorfastness, resistance to chalking, and durability of physical properties. Organic pigments, on the other hand, differ in their structure. In addition, in the formulation containing white pigments, the weatherability of the pigment may be more severely affected.

The fading, darkening or change of color of the pigment is generally caused by the reactive gene of the pigment. These reactive genes can interact with atmospheric moisture or chemicals—acids and bases. For example, cadmium yellow will fade under moisture and sunlight. Litholol has better lightfastness and is suitable for most indoor applications. It will be severely discolored when used outdoors with acids and alkalis.

Dehydrochlorination was measured according to JIS-K-6723 and the temperature was measured at 180°C. Based on the time of dechlorination of the unpigmented polyvinyl chloride complex, the prolongation or retardation time was measured in 5% and 10% intervals. A negative value indicates accelerated decomposition.

Table 1

As can be seen in Table 1, rutile titanium dioxide has a better stabilizing effect on polyvinyl chloride than anatase-type white powder; calcium carbonate, especially light calcium carbonate, has a greater stabilizing effect; especially chrome yellow and tin When stabilizing agents are used together, the stabilizing effect is better; the ultramarine blue has the effect of promoting the dehydrochlorination of polyvinyl chloride. Deuterium oxide does not have a noticeable effect on the dehydrochlorination of PVC.

1. Effect of Metal Ions in Pigments Certain metal ions in pigments 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 differences, such as containing calcium, the hue difference is small; containing manganese is a large hue difference, which is due to manganese and other metals promote PVC dehydrochlorination caused.

* Substance type colorants (such as cadmium red, yellow, etc.) are used for the coloration of polyvinyl chloride, and the coloring agent may liberate the hydrogen. Such colorants should not be mixed with lead stabilizers to avoid the formation of black lead.

2. Influence of Pigments on the Electrical Insulation of Polyvinyl Chloride As polyvinyl chloride as a cable material is the same as polyethylene, the electrical properties after coloring should be considered. In particular, polyvinyl chloride is inferior to polyethylene because of its inherent insulation, so the influence of pigments is even greater. Note that the choice of inorganic pigments for coloring PVC is better than organic pigments (except furnace black and anatase titanium dioxide).