Factors affecting the weather resistance of PVC plastic windows and measures to prevent aging
Polyvinyl chloride (PVC) plastic window has many advantages and characteristics, it is more resistant to corrosion than wooden windows and steel windows; it has better sealing, sound insulation and thermal insulation properties than aluminum windows and steel windows, and it has exquisite appearance and easy maintenance. It is versatile and can be coordinated with various buildings. However, when rigid PVC is used as a window frame, especially for exterior window profiles, it needs to be exposed to the outdoors for a long time. It is subject to atmospheric environmental erosion such as wind, sun, rain, environmental pollution, etc., and the environmental damage factors are extremely complicated. The problems in the selection of resins, additives, and molding processes of raw materials make weather resistance an important indicator for people to evaluate the quality of PVC plastic windows.
1 Factors affecting the weather resistance of PVC plastic windows
The weather resistance of PVC mainly refers to the problem of aging. PVC is a synthetic polymer material. The causes of aging of polymer materials are due to both internal and external factors. The internal factor mainly refers to the molecular structure state (including chemical structure and physical structure) of the polymer material itself and the nature and proportion of each component in the polymer material system. The structural state of the polymer material and its composition formula largely determine the pros and cons of the aging resistance of the material. External factors refer to external environmental factors.
1.1 The internal factors affecting the weather resistance of PVC plastic windows
The weakness of the molecular structure of PVC itself is the main internal factor that affects its weatherability. Under normal conditions, the molecular structure of PVC is a regular linear molecular chain formed by connecting vinyl chloride monomers in a head-to-tail manner. Chlorine and hydrogen atoms are only combined with secondary carbon atoms, which is supposed to be relatively stable. However, in actual production, especially when the polymerization temperature is high, irregular connections such as "head" and "tail" may be generated, as well as double bonds, branches, etc., and the presence of initiators, impurities, etc. may also affect the molecules of PVC. Structure and purity.
The "head" and "tail", the connected irregular structure lead to poor thermal stability of PVC, and easy to remove hydrogen chloride (HCL).
The presence of double bonds, especially the unsaturated double bonds formed at the ends of the molecular chain, is also a structural weakness, they are easily oxidized, broken, etc., and the unstable allyl chloride contained therein is also easily released.
When a branched chain is present, the tertiary or tertiary hydrogen atom at the branching point is a site susceptible to attack during the aging reaction because their chain energy is low, and in particular, the tertiary chlorine chain is more susceptible to activation. The tertiary chlorine at the branching point can react with the adjacent H to remove HCl even during polymerization, and then generate a double bond structure in the molecular chain.
When a peroxide is used as an initiator or aerobic in the polymerization process of PVC, a group such as -OH or -0 can be contained in the molecular chain, which is an inducing factor for subsequent aging and performance degradation.
1.2 External factors affecting the weather resistance of PVC plastic windows
The external factors affecting the weather resistance of PVC mainly include energy factors such as light (wavelength, intensity), radiation, heat, force and electricity, as well as air, reactive gas, oxygen, ozone, H2S, SO2, HCI, water, organic solvents, and microorganisms. Environmental factors such as hazards and insect damage are the main factors that cause aging of PVC and other polymer materials during storage or use. From the main mechanism of PVC aging, the aging caused by light, heat and oxygen is the main factor affecting the weatherability of PVC.
(1) The effect of ultraviolet radiation
The weathering problem mainly refers to the destructive effect caused by light. Photoaging is mainly the destruction of polymer materials by ultraviolet rays (UV) with shorter wavelengths and higher energy. UV irradiation has a great influence on the weather resistance of PVC plastic windows. The UV wavelength is 200-380 nm, which has enough energy to cause chemical bonds of PVC molecules to break and generate free radicals. Table 1 lists the energy of the bond energy of the C-H bond and the C-CI bond in the PVC molecule compared to the UV wavelength.
As can be seen from Table 1, UV with a wavelength of 290 nm has an energy of 418 kJ, which is sufficient to break the C-H bond and the C-CI bond in the PVC. The UV with a wavelength of 300 nm, which has an energy of 397 kJ, is also sufficient to break the C-CI bond. It can be seen that UV with a wavelength of 290-300 nm is extremely destructive to the bulk structure of PVC, and in fact, UV with a wavelength of 310 nm can cause the C-CI bond in the PVC molecule to break out of the HCI to form a polyene conjugated double bond. Lead to aging, especially in the presence of oxygen, water, heat, PVC plastic window will appear crack, embrittlement, whitening, etc., and lead to a significant drop in impact strength, tensile strength and electrical insulation properties.
(2) Temperature, humidity and oxygen effects
Temperature also plays an important role in the aging process of PVC, which is related to the temperature at which it is used. The surface temperature of the PVC plastic window used outdoors is not high (close to the ground temperature), which is not enough to cause the PVC to immediately decompose. However, since the PVC plastic window absorbs the infrared rays in the sunlight and converts it into heat energy, the surface temperature is high, especially the mutual influence of light, oxygen and the like, so that the heat accelerates the aging of the PVC plastic window, and the higher the temperature, The faster the photoaging degradation rate. Tests have shown that for every 10 °C increase in temperature, the degradation rate is twice as fast. This involves the problem of light and thermal stability of colored, especially brown, dark profiles. Different color plastic windows have different surface temperatures at the same temperature, and the light and thermal stability will be significantly different. Table 2 lists the surface temperatures of black and white PVC plastic windows under sunlight. It can be seen from Table 2 that the surface temperature of the white PVC plastic window is small compared with the air temperature. Therefore, it is recommended to use a white PVC plastic window in the higher temperature zone or a colored composite window on the outside. In addition, the alternating thermal and cold temperatures that occur with the seasons and day and night also have a certain impact on the aging of PVC plastic windows.
PVC is easily decomposed by heat and is more intense if oxygen is present. Table 3 compares the speed of thermal oxidative degradation of PVC in oxygen, air, and nitrogen. As can be seen from Table 3, PVC is the fastest to remove HCI under an oxygen atmosphere.
Humidity is also an important factor affecting the aging of PVC plastic windows. At the same temperature, the temperature is much faster in very humid areas than in dry areas. When the climate is dry, the ozone disperses and disappears. In a humid climate, ozone adheres to water droplets and becomes temporary oxidized water. Contact with PVC plastic windows can also oxidize PVC plastic windows.
In addition, geographical location and other conditions such as wind, dust and atmospheric pollution have a certain impact on the aging properties of PVC plastic windows.
2 Measures to prevent aging of PVC plastic windows
2.1 Selection of impact modifiers
At present, there are three main types of impact modifiers suitable for PVC window (door) profiles, namely chlorinated polyethylene (PE-C), (ethylene/vinyl acetate) copolymer (E/VAC), Acrylate copolymer (ACR).
PE-C modified PVC is the earliest application, but the narrow processing range of this product will cause the product to turn yellow and other phenomena, showing a significant reduction trend in the market.
The annual consumption of E/VAC is the largest among the three impact modifiers, accounting for more than 55%. However, due to the narrower extrusion temperature range and lowering the Vicat softening temperature, the amount of use begins to decrease.
ACR has the advantages of low melting temperature, good processing fluidity, wide processing temperature range, etc. The appearance of the product is white, smooth and fine, and has excellent impact strength and weather resistance. The use of ACR modifiers will be an effective way to improve the weatherability of PVC plastic windows.
2.2 Selection of stability system
In addition to the thermal stability, the stabilizer in the PVC plastic window profile must have both weather resistance and processability. The most commonly used thermal stabilization systems are lead salts, cadmium-cadmium and organotin. They are ideal heat stabilizers for PVC, and some of them have strong light and heat stabilization effects.
The lead salt stabilization system has the best effect and low price. It has good processing fluidity, thermal stability and aging resistance when used in combination with metal soaps and lubricants, and it has a large proportion in PVC processing at home and abroad. It has a good stabilizing effect on PE-C modified PVC.
The thermal stability of bismuth is good, the light stability of cadmium is good, and the bismuth-cadmium stabilization system has both light and heat stability, and is suitable for outdoor use. If lead is combined with a lanthanum-cadmium stabilizer, the photostability and weather resistance can be further improved, and the degree of whitening of the product can be reduced, but the toxicity of cadmium is large.
Among the organotin stabilizers, tin mercaptide has good thermal stability but poor weather resistance. The best weathering resistance in tin is dibutyltin dilaurate and tin maleate.
2.3 Selection of colorants
Years of experience at home and abroad have shown that white plastic doors and windows can be used as durable building materials. The rutile TiO has a cubic structure, has a high density, a stable crystal structure, a large refractive index, can effectively scatter light, has high optical stability, and has a remarkable protective effect on materials, and is suitable for outdoor use. Rutile TiO2 is the most ideal colorant for white plastic windows, and it also has the function of shielding and scattering ultraviolet rays.
Under the general climatic conditions in China, 4-6 parts of TiO2 can be used in the manufacture of PVC plastic window profiles, and the dosage can be appropriately increased in areas with high light radiation and high temperature. If an organotin stabilization system is used, 12 to 15 parts of TiO 2 are required.
For colored PVC plastic window profiles, pigments with a temperature resistance of 200 ° C or higher, light resistance (7 to 8 grades), acid and alkali resistance (> grade 4), and migration resistance (grade 5 or higher) are used as colorants. It is also possible to use a composite co-extrusion process, which is extruded on the outer surface of the white profile with a thin layer or film of polymethyl methacrylate, and a post-processing method can be used.
2.4 Selection of UV absorbers and antioxidants
In the PVC plastic window profile formulation, if a system with good light and heat stability, such as lead dibasic phosphite in lead stability system, not only has good thermal stability, but also has good ultraviolet shielding effect. It also has the function of antioxidants, and the system also has TiO2 which can shield and scatter ultraviolet rays, so it is generally unnecessary to add UV absorbers and antioxidants. However, in areas where the climate is hot and the light radiation is strong, in order to further improve the weather resistance, an ultraviolet absorber such as UV-531 or UV-9 may be added in an appropriate amount. However, it is worth noting that the matching of such auxiliaries with other auxiliaries has a strong selectivity.
There are many factors affecting the weather resistance of PVC plastic windows. The weather resistance of PVC plastic windows depends on the influence of the atmosphere when the quality of the products is guaranteed. The atmospheric environment factor is a variable that changes with the seasons and time, and also varies with geographical conditions. Among them, ultraviolet light is the main factor for aging PVC materials, and the combination of heat and moisture will also degrade PVC materials. At the same time, it is known from the aging mechanism of PVC materials that the aging process of PVC window frame profiles is mainly the process of cross-linking and breaking the polymer chains under the combined action of light, heat and oxygen to cause discoloration and strength drop. In the process, the selection of good modifiers, stabilizers, colorants, UV absorbers and antioxidants is the most important measure to prevent aging of PVC plastic windows and improve the weather resistance of PVC plastic windows.