Reasons for yellowing of PVC profiles and countermeasures

Discoloration of PVC profiles:

Heat, oxygen, photoaging

Most people think that solving the problem of inconsistent color processing of profiles can reduce the chromatic aberration as long as the yellowing profile production line is lowered in processing temperature. However, some people believe that the biggest yellowing effect is PVC, CPE, stabilizers, different batches of PVC, CPE, stabilizers and other main auxiliary materials, there will be different color differences in processing. In this case, even if a certain amount of brightener or anti-photoaging agent is added in a single manner, it does not play much role.

The main manifestation of heat, oxygen and photoaging discoloration is the yellowing of profiles. Some stabilizer manufacturers claim to use their products compared with similar manufacturers when promoting their products, which can reduce the number of copies and reduce costs. It can produce apparently satisfactory profiles, but in the recovery of waste products, in the harsh process temperature requirements, due to the lack of thermal stability efficiency, the damage to the molecular structure of PVC is not observed by the naked eye, so during production The following situations occur:

(1) When the profile is processed, it is found that the color is not uniform (if slight deviation of color is allowed);

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

(3) The state is good after heating, but the profile is made gray when the door and window are not long. The profile does not yellow during normal production, but yellows in later windowed products.

In the profile production process, when the basic conditions such as raw materials and molds are unchanged, if the temperature of the plasticizing section and the homogenization section is increased by about 5 °C, the resulting profile will have a yellowing tendency, indicating that the thermal stability efficiency is insufficient. The amount of heat stabilizer added should be increased, especially when a whitening aid such as a fluorescent whitening agent is added to the formulation, the heat stabilizer is insufficient, and the whitening effect is not obvious.

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 branch undergoes de-HCI reaction and polyene structure molecules. When the number of conjugated double bonds in the main chain is not too large, it is produced. Trace color difference, hydrogen chloride will first react with the surrounding potential acid-active substances, and its yoke double bond becomes a new active site in the PVC molecular chain. After being induced by light into a macromolecular radical, PVC is easily oxidized. Color change.

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

(3) If there is some impurity in the polymer, for example, the initiator, catalyst, acid, alkali, etc. added during the polymerization process are not completely removed, or the water is absorbed during storage and transportation, the stability of the polymer is lowered. Because these substances can cause molecular-ion degradation reaction, CPE contains more low-molecular substances such as Cl2 and HCl, which is easy to accelerate the thermal decomposition of the resin. Therefore, for the PVC CPE system which is not stable, the yellowing problem of some profiles can be eliminated by increasing the amount of stabilizer.

(4) Hard PVC structural profile, which is a hybrid system composed of PVC as a continuous phase of multiphase polymer and mixture kneading heat stabilizer, light stabilizer lubricant, filler and pigment. When PVC profile is exposed to nature In the environment, once the PVC component is dehydrochlorinated, the original component of calcium carbonate is converted into a new component of calcium chloride, which is dispersed in the oxidative degradation layer and forms a potential water absorption position, which is the reason for the yellowing of the outdoor profile. one.

(5) In addition to the temperature and stress, the plastic is subjected to thermal stress degradation by shear stress and tensile stress during high-stirring, cold-stirring, extrusion, etc. before molding.

When solving the chromatic aberration, the single method is not ideal for dealing with the problem. For example, from the following four aspects, the solution is faster:

(1) lowering the processing temperature and whitening without lowering the processing temperature;

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

(3) It is better to use some stabilizers instead of stabilizers, and to use whitening masterbatch or high whiteness filler to increase the effect of titanium dioxide.

(4) Simply add titanium dioxide and then moderately increase the effect of some internal lubricants. 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, so that the movement of macromolecular segments is much easier.

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

In the production practice, the profile was UV-aged for 12 hours with a self-made UV lamp tube, which showed obvious color change. Timely testing of the product for natural weather exposure aging is undoubtedly necessary in order to screen out formulations with excellent weatherability from small materials.

Lead sulfur pollution

After the PVC profile is exposed to rain and strong sunlight outdoors, the surface of the profile is discolored. Some of the profiles on the same window are more discolored in the place where the raindrops remain, and the surface color is light gray or light gray. For other reasons. In addition, the uneven distribution of ultramarine as a light cyan colorant is one of the reasons. The ultramarine--aluminum silicate-containing sulfur compound is resistant to alkali and acid, and is used in PVC profiles in large amounts or in large amounts. Theoretically, It is said that the phenomenon of discoloration may be caused by the action of sulfur compounds, and the phenomenon of discoloration has appeared in many profile door and window factories. This is a noteworthy problem in the use of lead salt series stabilizers and the addition of lower quality titanium dioxide and ultramarine profiles. .

Some manufacturers have reported that the profiles in contact with the rubber sealing strips also have obvious discoloration, and the discoloration is extended to the periphery. Except for the inferior sealing strips, which contain easily deposited oil-like materials, the other reason is the residual sulfur and lead in the rubber sealing strip. The result of producing lead sulfide.

Whitening of profiled materials

Titanium dioxide is the best white pigment and is widely used in plastic profiles. It is also a very good light shielding agent. Rutile type (R type) titanium dioxide has a relative density of 4.26 and a refractive index of 2.72. It can reflect or refract most of visible light, and can completely absorb ultraviolet light with a wavelength of less than 410 nm. It has good heat resistance and weather resistance, and has a strong effect of shielding ultraviolet rays. One of the important raw materials for weather resistance of profiles, as well as inhibiting the dehydrochlorination of PVC, delaying the decomposition of PVC, but due to its high market price, some profilers have reduced the cost of titanium dioxide to 3.6 parts, or even more Less, this greatly reduces the weatherability of the profile.

When the content of titanium dioxide in the product is 8%, the hiding rate tends to be balanced, and it is unhelpful to exceed the range. Since the addition of titanium dioxide to the polymer material increases the viscosity, the fluidity decreases, and the torque increases, which makes processing difficult. Generally, about 5 parts can be added, and a small amount of fluorescent whitening agent can be added to the whiteness.

Fluorescent whitening agent is a special kind of powdery organic matter, which can absorb ultraviolet light below 400nm, convert the absorbed energy, radiate purple or blue fluorescence of 400-500nm, and compensate for the loss of matrix reflection. Purple or blue wavelength. Therefore, it can be said that the fluorescent whitening agent not only whitens but also has a certain ultraviolet absorption effect. A small amount of blue pigment is added to the white product in order to achieve the purpose of "shading". The fluorescent whitening agent plays a role of "adding light" by increasing the amount of reflected light, and is different from the "dimmering" effect of adding blue whitening. The brightness of the surface of the product is increased, so that the color of the product is clearer and more vivid, and the whitening effect of "rejugation" is achieved.

The selection of the fluorescent whitening agent should take into account the melting point and decomposition temperature, light fastness, solubility and maximum absorption wavelength. Domestic general brighteners such as PF type have low decomposition temperature, initial decomposition temperature of 178 ° C, maximum absorption wavelength of 363 nm, and sublimation phenomenon, although the price is low, but the effect is not good. For example, type 0B, melting point 196-203 ° C, decomposition temperature greater than 220 ° C, maximum absorption wavelength 375 nm, "0B-1" type, melting point 353-359 ° C, maximum absorption wavelength 374 nm, maximum emission fluorescence wavelength 434 nm; of which "127" Especially suitable for PVC products.

Selection of processing aid ACR

Improper selection of processing aid ACR will also cause yellowing of the color. The profile should be selected with good plasticity, good fluidity, low viscosity, good oxidation resistance (such as HF-401/402/100) as processing aid, because viscosity The high plasticization is slow, the material flowability is poor, and the residence time is long, causing the product to be yellowish in color.

Pigment selection

Many manufacturers choose inorganic pigments to brighten white and color, and believe that inorganic pigments are good in light fastness, but they do not know that their fastness is only 1-2. Although the heat resistance of the ultramarine is very good, its acid resistance is very poor. Because PVC will decompose hydrogen chloride during processing, once the mixing is uneven, it is easy to disintegrate part of the high molecular material with high ultramarine content and generate free Sulfur, which in turn reacts with the lead salt of the stabilizer, produces lead sulfide, which causes the white profile to be dark.

In the PVC profile, the use of fluorescent whitening agent and phthalocyanine blue whitening and coloring, weather resistance is better than ultramarine. However, the amount of phthalocyanine blue is not well controlled. The coloring rate per 100 kilograms of PVC/1.5g phthalocyanine blue pigment is 20 times larger than that of ultramarine blue, which is easy to produce chromatic aberration, and because the general phthalocyanine blue produces green light, the sensory effect is not good.