PVC resin has a special molecular chain structure, which determines its low resistance to heat, light and oxygen, and is particularly easy to decompose and change color. Theoretically, it is generally believed that the de-He1 reaction of the PVC molecular chain under the action of heat, light, oxygen, etc., to form a long conjugated polyene structure is the main reason for the degradation and discoloration (yellowing) of PVC. Therefore, the degradation mechanism and stabilization technology of PVC resin in the processing and application process has always been a research focus in the polymer materials industry.

The weather resistance of PVC profiles, especially the color stability, has always been one of the most important manifestations of the quality of the profiles, and it is also one of the focus issues of the profile and plastic steel doors and windows industry. The discoloration of PVC profiles is related to the in-depth promotion and development of PVC plastic steel doors and windows. For this reason, controlling various factors that affect the discoloration of PVC profiles, at least ensuring that the profiles meet the national standard 4 000/6 000 h aging standard, and maintain a bright luster for a long time, which has become the high-end pursuit of the plastic steel industry. The discoloration phenomenon of PVC profile includes its aging and discoloration (yellowing, graying, etc.) during extrusion, storage and use, and the loss of gloss and chalking of the profile after aging (white profile). Among them, the white profiles are all products with bright luster, and the white profiles whose gloss has disappeared and chalked are still discolored profiles. The discoloration of PVC profiles depends on the combination of external and internal conditions. Among them, the external conditions include ultraviolet radiation, certain temperature, humidity and oxygen conditions, air pollution (such as dust, sulfur, acid rain, etc.); while the internal conditions are the degradation of PVC molecules themselves to remove HC1. The author analyzed the influence of external weather conditions on PVC degradation and discoloration, and from the profile formula, discussed the influence of heat stabilizers, titanium dioxide and other pigments on the discoloration of PVC profiles and their control factors. This is the analysis and analysis of the discoloration of PVC profiles. Prevention provides relevant theoretical basis for the development of high-grade PVC profiles.

1. External factors that affect the discoloration of PVC profiles
1.1 UV irradiation
The light irradiated by the sun to the outer air layer is a continuous spectrum with a wavelength of 0.7 to 3000 nm, of which the ultraviolet radiation of 300 to 400 nm is the main cause of polymer degradation. Table 1 compares the energy of different wavelengths of radiation and some typical polymer bond energies. It can be seen from Table 1 that the energy of photons in the ultraviolet range of 290-40()nm is 300-419 kJ/E, which is higher than the bond energy of some typical chemical bonds in polymers. The different latitudes of the earth have different ultraviolet radiation intensity, which determines that different areas have different requirements for the weather resistance of PVC profiles.

Table 1 Energy of different wavelength radiation and some typical polymer bond energy
Wavelength/nm Energy/(kJ&E) Chemical bond Bond energy/(kJ&mol)
29() 419 C H 38(1～42()
300 398 C～C 34(1～35(1
32(1 375 C 0 32()～38()
35() 339 C Cl 3(1()～ 34(1
40() 3()() C— N 32()～33(1

1.2 Temperature and humidity
In addition to ultraviolet radiation, the effects of temperature, humidity and regional climate on the discoloration of PVC profiles should also be considered. After the temperature rises, thermal degradation may overlap with light degradation, or even dominate. This is mainly because the PVC profile absorbs the infrared rays in the sunlight wave into heat energy, which increases the surface temperature and accelerates the aging and discoloration. The moisture (humidity) in the air is also one of the inducements of PVC degradation. Water can affect the degradation of PVC through extraction, hydrolysis, and photochemical reactions of certain pigments or titanium dioxide. In addition, due to the temperature difference between indoor and outdoor, moisture in the air condenses on the surface of the profile to form condensation (including oxygen), which can accelerate the oxidative degradation of the polymer.

1.3 Air pollution
The radiation that causes plastic aging, that is, the radiation that reaches the surface of the earth, consists of two parts: direct light from the sun ("solar radiation") and scattered light ("sky radiation"). The light scattering ability is inversely proportional to the fourth power of the wavelength (Rui Lee's Law). Suspended particles (dust) in the air can scatter sunlight, which means that ultraviolet rays can reach the ground more easily. Therefore, suspended particles in the air may increase the ultraviolet rays reaching the surface and accelerate the degradation of PVC. According to the optical properties of the atmosphere, attention should also be paid to the various organic substances in the atmosphere, including photochemical products that can cause the generation of oxidant smoke. These organic substances can even destroy the ozone layer in the stratosphere, thereby increasing ultraviolet radiation and accelerating the degradation of PVC. For PVC profiles of lead stabilized system, under the condition of relatively high sulfur content in the air, sulfur pollution reaction of lead stabilizer may occur, resulting in black PbS. When the PVC profile is exposed to acid rain, under strong sunlight, the residual acid rain on the profile will generate "HS". This ion is soluble in water, reacts with the lead stabilizer in the PVC profile, and can also form a precipitate to make the PVC Discoloration of the surface of the profile. Therefore, climatic conditions are the primary external condition that affects the discoloration of the PVC profile. When designing a high weather-resistant PVC profile formula, the first consideration should be the climatic conditions of the environment where the profile is used.
Related tests have shown that for the same set of PVC profile samples (A～E, standard), the Ab during the 36-month aging test in Phoenix, USA is significantly greater than that in Miami. This fully shows that hot and dry weather conditions have an accelerating effect on PVC degradation and discoloration. From the production and use practice of PVC profiles in my country, it is known that profiles are particularly prone to aging/discoloration in areas with dry and strong solar radiation, which is consistent with the above research results.

two. Internal factors affecting the discoloration of PVC profiles
The degradation of PVC resin and the removal of HC1 are the main internal factors for the discoloration of profiles. The schematic diagram of the degradation mechanism is shown in Figure 5.

Particle size and distribution, molecular mass size and distribution, appearance color, thermal stability, etc. The main inducing factors for PVC degradation are: allyl chloride, tertiary carbon chloride and other defective structures; whether the PVC molecule is a head-to-head structure or a head-to-tail structure; a certain amount of low-molecular-weight components; and residual initiators, Impurities such as catalysts, acids and bases. These inducing factors cause PVC to rapidly induce the removal of HC1 under the action of external light, oxygen and trace moisture to form a conjugated polyene sequence in the PVC molecular structure. The polyene structure accumulated during the photodegradation process is likely to quickly become the main structure that absorbs light. As the degree of degradation deepens and the micro-molecular structure continues to change, its light absorption also changes continuously, resulting in changes in the appearance, color and gloss of PVC profiles. At the same time, the mechanical properties and impact strength of the profile are reduced, and the surface may also produce powder.

three. Influence of additives on discoloration of PVC profile
3.1 The influence of heat stabilizer
The light stability of PVC is related to heat stabilizers. At present, the heat stabilizers used in PVC profiles mainly include lead salts, organic tins, Ca/Zn and Ba/Cd metal soaps, metal soaps and epoxy compounds synergistic systems, and rare earth stabilizers. The stabilization mechanism of various stabilizers is not the same: ① Absorb HC1 produced by PVC degradation to prevent its catalysis and promotion of PVC degradation, thereby preventing and delaying PVC degradation;
② It reacts with the active allyl chloride on the PVC molecular chain to prevent continuous de-HC1 reaction and effectively inhibit the formation of conjugated polyene structure;
③ It reacts with the double bonds produced by the degradation of PVC and blocks the effect of conjugated double bonds. Therefore, the heat stabilizer can effectively reduce the formation of "polyene color bodies" or the unstable structure of "polyene color bodies" in the PVC molecular structure under the action of heat and mechanical shear. In this way, during the light and oxygen aging process, the formation of "polyene color bodies" in the molecular structure of PVC will also be controlled. This is the control effect of heat stabilizers on the aging and discoloration of PVC.

3.2 The influence of antioxidant system
Currently. The stabilizers used by PVC profile manufacturers are mainly composite stabilizers, except for the main compound that stabilizes PVC degradation. It also contains lubricating system, antioxidant system and other stabilizing systems. The antioxidant system may include phenolic stabilizers and phosphite co-stabilizers. These compounds may contain easily discolored organic impurities, and in addition, they themselves may undergo discoloration under the action of light, oxygen, and water. Some reports point out that some phosphite products may contain color pollution ingredients. Commonly used phosphite products include triphenyl phosphite (TPP) and phenyldiisooctanol phosphite (PDLOP), in which TPP is made by the reaction of phenol and phosphorus trioxide. Phenol is a chemically unstable substance, it will undergo tautomerism under light, and is reddish brown. If the TPP production process is not strictly controlled, the hydroxyl value of the product will be too large, that is, more phenol monomers remain. Such products may cause discoloration of the profile during outdoor storage or application. Even if TPP has no residual phenol monomer, because of its poor hydrolytic stability, it will hydrolyze to form phenol monomer under humid, light and heat conditions, which will also cause the profile to change color. Some phenolic antioxidants with low steric hindrance (such as BHT) may undergo coupling reactions under the conditions of ultraviolet radiation, certain humidity and oxygen (as shown in Figure 7).

The coupling reaction can produce a quinone substance with a conjugated double bond structure. The decomposition product of the substance after exposure to light can absorb the long-wavelength part of visible light and become yellow, resulting in yellowing of the PVC profile. Therefore, the type and quality of antioxidants in composite stabilizers are also factors that affect the discoloration of PVC profiles.

3.3 The influence of titanium dioxide
The most effective method for white PVC profiles to resist UV damage is to add rutile titanium dioxide with high weather resistance specifications. Titanium dioxide is not only a white pigment in the PVC profile formula, but also an important light shielding agent.
The effect of titanium dioxide on the weather resistance of PVC profiles can be summarized in three aspects.
①It can absorb ultraviolet rays to reduce the discoloration caused by light and oxygen aging of the resin inside the profile;
②The discoloration of PVC can be covered by scattering and refraction of visible light;
③As a semiconductor material, the photocatalytic effect of TiO2 can cause negative effects such as loss of light, powdering and lead ash of the profile. The loss of gloss and chalking of white PVC profiles after aging should also belong to the category of discoloration. The tarnishing and chalking are closely related to the photocatalytic effect of TiO2. In the PVC matrix with rutile titanium dioxide added, the penetration of ultraviolet rays is mainly concentrated on the surface of the material. TiO2 particles can accelerate the aging and degradation of the surface resin due to their photocatalytic effect. In the initial stage of aging degradation, due to the "in-situ" degradation of the PVC resin around the TiO2 particles, the surface of the resin becomes rough and the surface of the profile loses its luster. With the degradation of the resin gradually increasing, TiO2 combined with the matrix. And filler particles gradually fall off, and then pulverization occurs, and the mechanical properties of the profile will also decrease. In the lead-stabilized system, TiO2 photocatalysis may also induce the reduction reaction of lead salt and make the profile black. Therefore, for the titanium dioxide used in PVC profiles, the key is to suppress its photocatalytic activity and prevent the aforementioned discoloration through weather-resistant surface coating modification. The coated modified titanium dioxide is coated with a special dense inorganic compound. It can effectively reduce the occurrence of surface degradation phenomena such as loss of gloss and chalking of the profile. At the same time, mechanical properties sensitive to surface defects (such as tensile strength, impact strength, elongation at break, etc.) can also provide good retention. The use of titanium dioxide should consider the cooperation with the thermal stability system and the use conditions of the profile. Thiol-based organotin stabilizers provide limited light stability, so in the formulation system dominated by mercaptan-based organotin stabilizers, the amount of titanium dioxide must be higher than other stabilizer systems. In the United States, in the profile formulations based on organotin stabilizers, the general amount of titanium dioxide is 8-10 parts.

3.4 The influence of pigments
Most of the domestic profile manufacturers add ultramarine blue and fluorescent whitening agent for color matching. Ultramarine is an aluminum silicate with a sodium polysulfide group and a special structure. Its crystal structure is shown in Figure 8.
It contains sodium polysulfide group, the ultramarine blue without special surface treatment has poor acid resistance and heat resistance, and it is easy to decompose even under weak acid conditions to form "HS." and gradually fade. The acid system in the processing process (PVC thermal processing Discoloration may occur in the system), which reacts with lead or tin-containing stabilizers to form PbS (black) and SnS (yellow), which will cause the PVC profile to change color. When the profile is exposed to acid rain, under strong sunlight, the profile The acid rain remaining on the surface and the action of ultramarine blue will also generate "HS-". This ion dissolves in water and reacts with the Pb present in the profile to form a precipitate, discoloring the surface of the profile, that is, "vulcanization" pollution. Therefore, if ultramarine blue is used in the profiles using the lead salt system, the possibility of discoloration under certain climatic conditions (acid rain, sunlight) is relatively high, and acid-resistant ultramarine should be used as much as possible to prevent "vulcanization" pollution happened. Fluorescent brightener is a special organic substance that can absorb ultraviolet light below 400 rim, convert the absorbed energy, and radiate 400-500 rim purple or blue fluorescence. But the structure of different fluorescent whitening agents is different, and their weather resistance is also different. Fluorescent brightener is also one of the discoloration factors in the PVC profile formula system. The selection of fluorescent whitening agent should take into account the influence of melting point, decomposition temperature, light fastness, solubility and maximum absorption wavelength on the discoloration of PVC profiles. The domestic fluorescent whitening agent is generally PF type, its decomposition temperature is relatively low, the general initial decomposition temperature is 178℃, the maximum absorption wavelength is 363 rim, and there is sublimation phenomenon, although the price is low, the effect is not good; OB type fluorescent whitening The melting point is 196～203℃, the decomposition temperature is greater than 220℃, and the maximum absorption wavelength is 375rim; OB-1 type optical brightener, the melting point is 353～359℃, the maximum absorption wavelength is 374 rim, and the maximum emission fluorescence wavelength is 434rim . The weather resistance of the latter two fluorescent whitening agents is correspondingly higher.

3.5 The influence of CaCO3
The main purpose of adding CaCO in the PVC profile formula is to increase the filling and reduce the cost. The addition of CaCO3 as an inorganic rigid component can increase the hardness and heat distortion temperature of the product to a certain extent, while reducing the molding shrinkage rate of the product. However, the particle size and particle size distribution of CaCO and its compatibility with the matrix are limited. In the case of a large amount of it, agglomeration will inevitably occur in the matrix, thereby reducing the toughness of the material and affecting the gloss and color of the profile. Some reports point out that the moisture absorption capacity of CaCO3 is significantly reduced after organic surface treatment. However, if PVC is degraded and HCl is removed, CaC03 may be converted into CaCl2, which is dispersed in the photo-oxidation degradation product layer of PVC to form potential water absorption sites, which will easily cause the profile to change color.
CaCO3 is closely related to the pulverization phenomenon in the aging process of PVC profiles. Profiles with higher CaCO3 filling content are prone to "pulverization", which makes new doors and windows poorer in gloss. Chalking is the result of changes in surface morphology due to chain scission or cross-linking during the degradation of PVC. After the photo-oxidation degradation of the PVC resin, its surface becomes rough and the gloss of the profile decreases significantly. At the same time, due to the scission or degradation of the PVC resin, the matrix of the profile produces voids or microcracks, which are connected to the matrix.

CaCO particles will fall off the surface. In the case of CaCO, when the filling amount is higher, the CaCO and agglomerates in the matrix will increase, and the filler particles shed from the matrix will increase, causing more serious powdering.

3.6 Other
The discoloration of PVC profile is a structure of multiple factors. In addition to the internal and external influencing factors and additives discussed above, the factors that cause discoloration of the profile in the production process and equipment mold should also be considered. The main purpose is to minimize thermal oxidative degradation of PVC under thermal processing, mechanical shearing and other conditions to ensure that the profile has better stability under conditions of light, oxygen, and weather aging. This includes the control of thermal oxygen degradation by various processes such as mixing, extrusion, plasticizing, shaping, and cooling.

4 Conclusion
The discoloration of PVC profiles is the result of a combination of internal and external factors. The external conditions include ultraviolet radiation, certain temperature, humidity and oxygen conditions, and air pollution (dust, sulfur, acid rain, etc.). Hot and dry weather conditions can accelerate the degradation and discoloration of PVC. The internal factor is that the PVC resin molecule itself degrades and discolors HC1 under light and oxygen conditions. Therefore, controlling the quality of PVC resin, especially controlling the degradation of PVC resin and the formation of polyene structure, is the primary problem to solve the discoloration of profiles. In addition, various additives in the formulation of PVC profile production, including stabilizers, antioxidant systems, titanium dioxide, pigments and fillers, also have discoloration factors due to their different qualities. By optimizing the stable system in the formula, and at the same time with the necessary weather-resistant specifications of rutile titanium dioxide, controlling the quality of the resin and various additives in the formula and its own possible discoloration factors are necessary conditions to reduce the discoloration of the profile.

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