Discoloration of PVC products
1) It is found that the color is not uniform in the process (slight deviation of color is allowed);
2) Yellow changing after heating deformation;
3)After heating, the condition is good. it turns grey after the making of the window section.
In the production process of the profile, when the basic condition of raw materials and mould without changing,while the plasticizing section and the homogenization section of the temperature increased by about 5 ℃, the profiles produced yellowing tendency.It also shows that the thermal stability of the lack of efficiency, should increase the amount of heat stabilizer added, especially in the formula by adding fluorescence Whitening agents such as whitening additives, heat stabilizers, whitening effect is not obvious.
The relationship between molecular structure and color of PVC
(1)Since PVC resin is a kind of heat-sensitive plastic, its light stability is poor. Under the action of heat and light, the branch undergoes HCI reaction. The polyene structure molecule is generated when the number of conjugated double bonds in the main chain is not too much Trace of color, hydrogen chloride will first react with the surrounding potential of acid-reactive substances, and conjugated double bonds become the new active site within the PVC molecular chain, the photo-induced into a large molecule free radicals, PVC is vulnerable to oxidation, resulting Color change.
(2)There is a certain amount of low molecular weight components in the PVC resin, which reduces the thermal stability of the polymer. The mechanism of PVC decomposition includes free radical mechanism, ionic mechanism, single molecular mechanism, and the like. Besides PVC stabilizers, May be affected by the quality of the PVC resin itself, such as whether there is residual PVC resin too much initiator.
(3)The presence of certain impurities in the polymer, such as the addition of initiator, catalyst, acid, alkali, etc. in the polymerization process, or the absorption of moisture during storage will reduce the stability of the polymer. Because these substances can cause molecular - ion degradation reaction, CPE contains more Cl2, HCl and other low molecular weight substances, easy to accelerate the thermal decomposition of the resin. Therefore, poor stability of the PVC + CPE system, by increasing the amount of stabilizer can also eliminate some profiles yellowing problem.
)Hard PVC structural profile, which is a mixed system consisting of PVC as a continuous phase heterogeneous polymer and mixture kneaded heat stabilizer, light stabilizer lubricant, filler and pigment, when the PVC profile is exposed to the natural environment , Once the PVC component is dehydrochlorinated, the primary component of calcium carbonate is converted to a new component of calcium chloride which is dispersed in the layer of oxidative degradation products and forms a potential water absorption site, which is one of the reasons for the yellowing of the outdoor profile .
(5) In addition to the temperature and the stress, the plastics are subjected to thermal stress and shear stress, resulting in thermal degradation in the process of high stirring, cold stirring and extrusion.
When solving the color difference, the single method can not deal with the problem better, if you follow the below 4 aspects, to solve more quickly:
(1) Reduce the processing temperature than the processing temperature does not reduce the color white;
(2) The effect of adding single lead salt in compound stabilizer is not better than adding compound stabilizer.
(3) Simply add stabilizer and not as good as both the selection of some stabilizers, but also the choice of whitening masterbatch or high whiteness filler to increase the effect of titanium dioxide fast;
(4) Simply add titanium white powder and then increase the amount of some lubricants better.
It is generally believed that low molecular weight 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, so that the movement of macromolecular segments is much easier.
The above measures will prevent PVC from decomposing to some extent and reduce inconsistencies in the profile processing. In my opinion, the most effective approach is to optimize the design of the stabilization system to fundamentally inhibit the PVC conjugate dilatancy structure.
Production practice with homemade UV lamp on the profile UV aging 12 hours, there is a clear color change. It is undoubtedly necessary to test the natural weather exposure of the product in time in order to screen out the formula with excellent weatherability from the small material.
Lead and sulfur pollution discoloration It is reported: "PVC profiles in outdoor rain shower and strong sunlight, profiles partial surface discoloration, and some are the same window with a profile on the remnants of the raindrops more discoloration, the surface color is light gray Or light gray. " Among other reasons, ultramarines are one of the reasons for their dispersal as a light cyan color. Ultramarine blue silicic acid-containing sulfur compounds, which are alkali-resistant and acid-resistant, are used in PVC profiles in excessive or large amounts, In theory, with the role of sulfur compounds may lead to black lead sulfide, discoloration phenomenon has appeared in many profiles doors and windows, which is the use of lead salt series stabilizer at the same time join the lower quality titanium dioxide and ultramarine profile factory worth Attention questions.