Color Changes in Plastic Products Caused by High-temperature Molding

When using color masterbatch to color plastic products, color changes may occur, thus affecting the quality of the product.
 
Analyzing the mechanism of discoloration caused by the following factors provides a reference for plastic part manufacturers to correctly select raw materials and produce accepted plastic products.
 
Color changes caused by the molding of the plastic
Oxidized, degraded and discolored matrix resin in the high-temperature molding
When the heating ring or heating plate of the plastic molding processing equipment is always in a heating state due to the loss of control, it is easy to cause the partial temperature to be too high, causing the resin to oxidize and degrade at high temperatures. For those heat-sensitive plastics, for example, PVC, color changes easily happen in the molding process. In severe cases, it will burn and turn yellow or even black, and a large number of low-molecular volatiles will escape. This degradation includes reactions such as depolymerization, random chain scission, removal of side groups and low molecular.
 
(1) Depolymerization
The depolymerization reaction system first breaks the macromolecular end, and then quickly removes the monomer according to the chain mechanism, which is especially easy to carry out above the polymerization upper limit temperature.
 
(2) Random chain scission (degradation)
For polymers like PE, when they are molded at high temperatures, any position in the main chain may be broken, and the molecular weight decreases rapidly. However, the monomer yield is very small. This kind of reaction is called random chain scission. Sometimes, it is also called degradation. The free radicals formed after chain scission of polyethylene are very active, and there is a lot of secondary hydrogen around them, which are prone to chain transfer reactions; almost no monomers generate.
 
(3) Removal of the substituent group
When polyvinyl chloride, polyvinyl acetate, polyacrylonitrile and polyvinyl fluoride are heated, the substituent group will be removed. Taking polyvinyl chloride (PVC) as an example, PVC is processed at a temperature below 180 to 200°C. However, it begins to dehydrogenate (HCl) at a lower temperature (for example, 100 to 120°C). It loses HCl fast at about 200°C. The polymer becomes darker and the strength becomes lower.
 
Free HCl has a catalytic effect on dehydrochlorination, and metal chlorides, such as ferric chloride formed by hydrogen chloride and processing equipment promote catalysis: 3HCl+Fe→FeCl3+3HCl
 
During thermal processing of PVC, a few percent of acid absorbents, such as barium stearate, organic tin and lead compounds must be added to improve its stability.
 
(4) Oxidative degradation
Polymers are exposed to oxygen in the air during processing and use, which accelerates oxidative degradation when heated. The thermal oxidation of polyolefin belongs to the mechanism of free radical chain reaction and has autocatalytic behavior, which can be divided into three steps: initiation, growth and termination. The cleavage of the chain caused by the hydroperoxide group leads to a decrease in molecular weight, and the main products of the homolytic cleavage are alcohols, aldehydes, and ketones, which are finally oxidized to carboxylic acids. Carboxylic acids play a major role in metal-catalyzed oxidation.