Discoloration of PVC environmentally friendly products - Guangyuan Ruifeng New Materials

Category: PVC products

Release Time: 2020-06-09

Summary: Non-uniform color was found during processing (minor color deviation is acceptable); the profile showed significant yellowing after heat deformation; the state was good after heating, but the profile turned gray shortly after being made into doors and windows. The profile did not yellow during normal production, but yellowed later when made into window products.

During processing, inconsistencies in color were observed (minor color deviations are acceptable); after heating and deformation, the profile showed significant yellowing; after heating, the state was good, but the profile turned gray shortly after being made into doors and windows. The profile does not yellow during normal production, but yellows later when made into window products. During profile production, when the raw materials and molds, etc., remain unchanged, if the temperature of the plasticizing and homogenizing sections is increased by about 5℃, the resulting profile shows a tendency to yellow, indicating insufficient thermal stability. The amount of heat stabilizer should be increased, especially when fluorescent whitening agents and other whitening aids are added to the formulation; insufficient heat stabilizer results in poor whitening effect. (PVC environmentally friendly products, environmentally friendly crosslinking agents, environmentally friendly plasticizers)

I. Relationship between PVC molecular structure and color

1. Because PVC resin is a heat-sensitive plastic with poor light stability, under the action of heat and light, the side chains undergo a de-HCI reaction, forming polyene structures. When the number of conjugated double bonds in the main chain is not too large, a small amount of color difference is produced. Hydrogen chloride will first react with surrounding potentially acid-active substances, and the conjugated double bonds become new active sites within the PVC molecular chain. After being initiated by light into macromolecular free radicals, PVC is easily oxidized, resulting in color change.

2. PVC resin contains a certain amount of low-molecular-weight components, which reduces the thermal stability of the polymer. The mechanism of PVC decomposition includes free radical mechanism, ionic mechanism, and unimolecular mechanism. PVC decomposition, in addition to stabilizers, may also be affected by the quality of the PVC resin itself, such as whether excessive initiator remains in the PVC resin.

3. If certain impurities exist in the polymer, such as initiators, catalysts, acids, and alkalis added during polymerization that are not completely removed, or moisture absorption during storage and transportation, the stability of the polymer will be reduced. Because these substances can cause molecular-ionic degradation reactions, CPE contains a large amount of low-molecular substances such as Cl2 and HCl, which easily accelerate the thermal decomposition of the resin. Therefore, for PVC+CPE systems with poor stability, increasing the amount of stabilizer can also eliminate some profile yellowing problems. (PVC environmentally friendly products, environmentally friendly crosslinking agents, environmentally friendly plasticizers)

4. Hard PVC structural profiles are composed of a multiphase polymer with PVC as the continuous phase, kneaded with heat stabilizers, light stabilizers, lubricants, fillers, and pigments. When PVC profiles are exposed to the natural environment, once the PVC component dehydrochlorinates, the original calcium carbonate component will be converted into a new calcium chloride component, which is dispersed in the oxidation degradation layer and forms potential water absorption sites. This is one of the reasons for the yellowing of outdoor profiles.

5. In addition to temperature, there is also stress. Before molding, plastics are subjected to shear stress and tensile stress during high-speed mixing, cold mixing, and extrusion, causing thermal degradation. (PVC environmentally friendly products, environmentally friendly crosslinking agents, environmentally friendly plasticizers)

When solving color difference problems, a single method cannot ideally solve the problem. If considered comprehensively from the following four aspects, the solution will be faster:

1. Lowering the processing temperature results in a whiter color than not lowering the processing temperature;

2. Simply adding lead salts to the compound stabilizer does not have a better effect than increasing the compound stabilizer;

3. Simply adding stabilizers is not as effective as selecting some stabilizers and whitening masterbatches or high-whiteness fillers, and adding titanium dioxide is faster;

4. Simply adding titanium dioxide and moderately increasing some internal lubricants has a better effect.

It is generally believed that low-molecular substances have a plasticizing effect on polymers. One mechanism is that the interaction between macromolecules and small molecules replaces the interaction between macromolecules, making the movement of macromolecular segments much easier.

The above measures will prevent PVC decomposition to varying degrees and reduce the inconsistent color phenomenon during profile processing. The author believes that the most effective approach is to optimize the design of the stabilization system and fundamentally inhibit the formation of PVC conjugated polyene chromophores. (PVC environmentally friendly products, environmentally friendly crosslinking agents, environmentally friendly plasticizers)

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Keywords: Discoloration of PVC environmentally friendly products - Guangyuan Ruifeng New Materials

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