Reasons and solutions for yellowing of pvc profiles

June 3, 2019

Reasons and solutions for yellowing of pvc profiles

 

In the production of pvc profile and its application, thermal photooxidation reaction will occur. The polymer macromolecule will be oxidized by trace moisture, acid, alkali and other impurities under the action of heat and pressure, and the molecular weight will decrease. Molecular structure changes. This chemical change is called degradation, and the most intuitive performance is that the surface is yellowed, the gloss gradually disappears, and even the gray is changed. The discoloration of the profile causes the customer to have doubts about the performance of the PVC material, and the consequences are serious.

 

Discoloration of pvc profile

 

Heat, oxygen, photoaging
 

Many 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 of raw materials and molds are not changed, 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 assistant such as a fluorescent whitening agent is added to the formulation, the heat stabilizer is insufficient, and the whitening effect is not obvious.


  

Relationship between molecular structure and color of PVC
  
(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 potential acid-active substances around it, 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 includes 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 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 profile structure, which is of PVC polymer for the continuous phase of the multiphase mixture is kneaded with heat stabilizers, light stabilizers, lubricants, fillers and pigment "mixed system composed of PVC profile when exposed to the natural 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 by shear stress and tensile stress during high-stirring, cold-stirring, extrusion, etc. before molding.

When solving chromatic aberrations, a single method is not ideal for dealing with problems. For example, from the following four aspects, it is faster to solve:
(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 plasticizing effects 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, and there was a significant 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, etc., 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 reduce 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, which is unhelpful beyond its scope. 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 "target="_blank"> pigment is added to the white product to achieve the purpose of "shading the yellow". The fluorescent whitening agent increases the amount of reflected light to "add light" and whitens with blue. The effect of "dimmering" is different, so that the surface brightness 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.

 

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 (as confirmed by Guangzhou aging). Although the heat resistance of the ultramarine blue 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.

 

Fluorescent whitening agent and phthalocyanine blue are used for whitening and color grading in PVC profiles, and weather resistance is better than ultramarine. However, the amount of phthalocyanine blue is not well controlled. The coloring rate per 100 kg of PVC/1.5 g phthalocyanine blue pigment is 20 times larger than that of ultramarine blue, which is easy to produce chromatic aberration, and the general sensory effect is not good due to the general green color of phthalocyanine blue. Huaxing Company has been tested and screened by a large number of experiments. It uses a fluorescent whitening agent such as 0B to add a trace of “high temperature blue” to produce a concentrated whitening masterbatch. After more than 20 profiles, it has achieved satisfactory whitening, brightening and improved profiles. The effect of weather resistance. Some of the dispersants in the latest grades of pigment dispersants are similar to the KD-105 products of Chengdu Zhongzhu Chemical Co., Ltd. of Polymer Auxiliary Research Institute of Sichuan University. Considering that the lubricants have been coordinated by the general manufacturers in the profile formulation, the pigments are dispersed. The agent is helpful for brightening the hard PVC product. The pigment dispersant is an ester compound dispersant, which has good compatibility with PVC, so the migration is small, no precipitation, the welding angle strength and the low temperature. The impact is improved, the clogging of the vacuum eye of the mold is avoided, and the brightness and the reinforcing effect of the profile production are ideal.

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