Discoloration of uPVC Profiles

 

In the production and application of pvc profiles, thermo-photooxidation reaction occurs. Under the action of heat and pressure, polymer macromolecules are oxidized by trace moisture, acid, alkali and other impurities and air, and the molecular weight decreases and the macromolecules are oxidized. Structural changes. This kind of chemical change is called degradation, and its most intuitive manifestation is the yellowing of the surface, the gradual loss of luster, and the serious and even graying. The discoloration of the profile causes customers to doubt the performance of the PVC material, and the consequences are serious.

1. Discoloration of upvc window profile materials

1, heat, oxygen, light aging and discoloration

 Many people think that to solve the problem of color inconsistency in profile processing, as long as the yellowing profile production line lowers the processing temperature, the color difference can be reduced. However, some people believe that PVC, CPE, and stabilizers have the greatest impact on yellowing. Different batches of PVC, CPE, stabilizers and other main and auxiliary raw materials will have different color differences during processing. In this case, even a single addition of a certain amount of brightener and anti-photoaging agent will not have much effect.

The main manifestation of heat, oxygen, and light aging discoloration is the yellowing of the profile. Some stabilizer manufacturers claim that their products can reduce the number of copies and reduce the cost compared with similar manufacturers when they promote their products. It is possible to produce profiles with satisfactory appearance, but in the recycling of waste products and in the strict process temperature requirements, due to its insufficient thermal stability efficiency, the damage to the molecular structure of PVC is not observable by the naked eye, so during production Some of the following situations occur:

(1) The color of the pvc profile is found to be inconsistent during processing (such as a slight deviation in color is allowed);

(2) The profile is obviously yellowed after heating and deformation;

(3) The state is good after heating, but the profile will turn gray after a short time when it is made into pvc doors and windows. The upvc profile does not turn yellow in normal production, but turns yellow in the subsequent windowed products.

In the process of profile production, when the basic conditions such as raw materials and molds are unchanged, if the temperature of the plasticization section and homogenization section is increased by about 5°C, the resulting profile tends to yellow, which also indicates that its thermal stability efficiency is insufficient , The amount of heat stabilizer should be increased, especially when the whitening aids such as fluorescent whitening agent are added to the formula, the heat stabilizer is insufficient and the whitening effect is not obvious.

2, the relationship between PVC molecular structure and color

(1) Because PVC resin is a kind of heat-sensitive plastic, its light stability is poor. Under the action of heat and light, the branched chain will undergo de-HCI reaction and polyene structure molecules. When the number of conjugated double bonds in the main chain is not too much, it will be produced. With slight chromatic aberration, hydrogen chloride will first react with the surrounding potential acid-reactive substances, and its conjugated double bond becomes a new active position in the PVC molecular chain. After being triggered by light into macromolecular free radicals, PVC is prone to oxidation, resulting in Discoloration.

(2) There are a certain amount of low molecular weight components in the PVC resin, which reduces the thermal stability of the polymer. The decomposition mechanism of PVC includes free radical mechanism, ion mechanism, single molecule mechanism, etc. In addition to PVC decomposing stabilizers, there are It may be affected by the quality of the PVC resin itself, such as whether there is too much residual initiator in the PVC resin.

(3) If there are certain impurities in the polymer, such as initiators, catalysts, acids, alkalis added in the polymerization process, etc., which can not be removed completely, or absorption of water during storage and transportation, the stability of the polymer will be reduced. Because these substances can cause molecular-ion degradation reactions, CPE contains more low-molecular substances such as Cl2 and HCl, which can easily accelerate the thermal decomposition of the resin. Therefore, for the poorly stable PVC+CPE system, the yellowing problem of some profiles can also be eliminated by increasing the amount of stabilizer.

(4) Rigid PVC profiles which is a mixed system composed of polyphase polymer with PVC as continuous phase and mixture kneading heat stabilizer, light stabilizer lubricant, filler and pigment. When PVC profile is exposed to natural environment When the PVC component is dehydrochlorinated, the original calcium carbonate component will be converted into a new calcium chloride component, which is dispersed in the oxidative degradation product layer and forms potential water absorption sites, which is one of the reasons for the yellowing of outdoor profiles .

(5) In addition to temperature and stress, plastics are subjected to shear stress and tensile stress in the processes of high stirring, cold stirring, and extrusion before being formed, which will cause thermal degradation.

 When solving the color difference, a single method is not an ideal solution to the problem. For example, considering the following four aspects, it is faster to solve:

(1) The color becomes whiter when lowering the processing temperature than without lowering the processing temperature;

 (2) The effect of simply adding a single lead salt to the composite stabilizer is not better than adding the composite stabilizer;

(3) Simply adding stabilizers is not as good as using some stabilizers, but also using whitening master batch or high whiteness fillers to increase the effect of titanium dioxide quickly;

(4) It is better to add titanium dioxide only for upvc profiles and moderately add some internal lubricant. It is generally believed that low-molecular substances have a 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 movement of macromolecular segments much easier.

 The above measures will prevent the decomposition of PVC to varying degrees and reduce the inconsistency of color in profile processing. The author believes that the most effective way is to optimize the design from a stable system and fundamentally inhibit the conjugated polychromatic structure of PVC profile.

In the production practice, the self-made ultraviolet lamp is used to carry out the ultraviolet aging of the profile for 12 hours, and there is a significant color change. It is undoubtedly necessary to carry out the natural weather exposure and aging test on the product in time, in order to screen out the formula with excellent weather resistance from the small material aspect.