PVC Molecular Structure and Color of PVC Sheets
In the production and application of board materials, not only the oxidation reaction of hot light can be generated, especially under the effect of heat and pressure of polymer macromolecules, it can be oxidized by trace amounts of water and impurities such as acid and alkali in the air, resulting in lower molecular weight. And the structure of macromolecules changes. This chemical change is called degradation, and the most intuitive way to express it is that the surface appears yellow and the gloss is constantly disappearing, and severe graying may occur. So, what is the relationship between PVC molecular structure and color?
(1) Because PVC resin is a kind of heat-sensitive plastics, the photostability among them is relatively poor, so under the action of heat and light, it causes the branched chain to generate de-HCI reaction and structural molecules of polyenes, so when the main chain is conjugated When the number of double bonds is small, color differences will occur. At this time, hydrogen chloride will react with the surrounding acid-active substances, and the conjugated double bonds will become new active sites within the PVC molecular chain, resulting in photoinitiation. After the free radicals of macromolecules, PVC is easily oxidized and discolored.
(2) There is a low molecular weight component in the PVC resin, which reduces the thermal stability of the polymer. The mechanism of the PVC decomposition has a free radical mechanism, an ion mechanism, and a single molecular mechanism. The PVC decomposition is not only Stabilizer, it may also be affected by the quality of PVC resin, such as: PVC resin there is no residual initiator.
(3) Some impurities still exist in the polymer. For example, the initiators, catalysts, acids, alkalis, etc. added during the polymerization process cannot be removed, or the moisture is absorbed during storage and transportation, which may reduce the polymerization. The stability of the material. Because these substances can cause the degradation reactions of molecules and ions, CPE also contains many low-molecular substances such as Cl2 and HCl, which accelerates the decomposition of the resin. Therefore, for the stable PVC system, we can eliminate some yellowing problems by increasing the amount of stabilizer.
(4) For hard PVC structural profiles, it mainly uses a mixture of heat stabilizers, light stabilizers, fillers, pigments, etc. that are combined with a heterogeneous polymer and mixture inside the PVC, so when the PVC When the board is exposed to the natural environment, when the dehydrochlorination of the PVC component is encountered, the calcium carbonate component is converted into a new component of calcium chloride and is also dispersed in the oxidatively degraded layer to form a potential water absorption. Location, resulting in yellowing of outdoor profiles.
(5) In addition to temperature, there is stress. When the plastic is formed before or during the process of high-stirring, cold-stirring, extruding, etc., it is thermally degraded if it is subjected to shear stress and tensile stress. Therefore, when we are dealing with color differences, if a single method is not an ideal solution, it can be more quickly resolved by considering the following aspects:
a, reduce the processing temperature than the processing temperature does not reduce the color of white;
b. The effect of simply adding a single lead salt in the composite stabilizer is not better than the effect of increasing the composite stabilizer.
c. Simply adding stabilizers is not as good as selecting some stabilizers, but also using whitening masterbatch or high whiteness filler.