Methods to prevent discoloration of plastic profiles

Thermo-photooxidation reaction occurs in the production and application of plastic profiles. Polymer macromolecules are oxidized by trace impurities such as water, acid, alkali and air under the action of heat and pressure, and the molecular weight decreases while the macromolecular structure changes. Its performance is most intuitive is the surface yellowing, gloss gradually disappeared, serious or even gray.

The discoloration of profiles leads to customers'doubts about the properties of PVC plastics, and its consequences are serious. Many people think that to solve the problem of inconsistent color in profile processing, as long as the yellowing profile production line reduces the processing temperature, it can reduce the color difference.

However, some people believe that the greatest impact of yellowing is PVC, CPE, stabilizer, and other main and auxiliary raw materials in different batches of PVC, CPE, stabilizer, there will be different color difference in processing. In this case, even if a certain amount of whitening agent and anti-photoaging agent are added alone, they will not play a very important role.

The main manifestation of discoloration of heat, oxygen and light aging is yellowing of profiles. When promoting their products, some stabilizer manufacturers claim that using their products can reduce the number of parts and cost compared with similar manufacturers, although the results can also produce profiles with satisfactory appearance.

However, in the recycling of waste products and in the strict process temperature requirements, due to the insufficient thermal stability efficiency, the damage to the molecular structure of PVC is not visible to the naked eye, so there are some situations in production as follows:

(1) When the profile is processed, it is found that the color is not uniform (for example, slight color deviation is allowed).

(2) The profile yellows obviously after heating deformation.

(3) After heating, the profile is in good condition, but it will turn grey in a short time when it is made into doors and windows. Profiles do not yellowing in normal production, but yellowing in later window products.

In the production process of plastic profile products, when the basic conditions such as raw materials and moulds remain unchanged, if the temperature of plasticizing and homogenizing sections is raised by about 5 C, the yellowing tendency of the profiles will occur, which also indicates that the thermal stabilization efficiency is insufficient. The addition of heat stabilizers, especially fluorescent whitening, should be increased, in the formulation. When additives such as whitening agents are used, the heat stabilizer is insufficient and the whitening effect is not obvious.

When solving the problem of chromatic aberration, a single method can not deal with the problem better. For example, considering the following four aspects, it can solve the problem more quickly.

(1) The color becomes whiter when the processing temperature is lowered than when the processing temperature is not lowered.

(2) The effect of adding single lead salt in composite stabilizer is not better than that of adding compound stabilizer.

(3) The effect of adding stabilizer is faster than choosing partial stabilizer, whitening masterbatch or high whiteness filler.

(4) It is better to add some internal lubricants only with titanium dioxide. It is generally believed that low molecular weight substances have plasticizing effect on polymers. One of the mechanisms is that the interaction between macromolecules and small molecules replaces the interaction between macromolecules, which makes the chain movement of macromolecules much easier.