1. Heat stabilizer
The heat stabilizer must be able to capture the autocatalytic HCL released by the PVC resin, or be able to react with the unstable polyolefin structure produced by the PVC resin to prevent or reduce the decomposition of the PVC resin. Generally, the characteristics, functions and product requirements of the heat stabilizer selected in the formula are considered. E.g:
Lead salt stabilizers are mainly used in hard products. Lead salt stabilizers have the characteristics of good thermal stabilizer, excellent electrical properties and low price. However, its toxicity is high, and it is easy to contaminate products, so it can only produce opaque products. In recent years, a large number of composite stabilizers have appeared, and the single-component stabilizer has been in danger of being replaced. The characteristics of the composite stabilizer are strong specificity, low pollution, and simple ingredients for processing enterprises. However, since there is no uniform standard, the composite stabilizers of various companies are very different.
Barium and cadmium stabilizers are a kind of heat stabilizers with better performance. It is widely used in PVC agricultural film. Usually barium cadmium zinc and organic phosphites and antioxidants are used together.
Calcium and zinc stabilizers can be used as non-toxic stabilizers in food packaging, medical equipment and pharmaceutical packaging, but their stability is relatively low. When the amount of calcium stabilizers is large, the transparency is poor, and it is easy to spray. Calcium-zinc stabilizers generally use polyols and antioxidants to improve their performance. Recently, calcium-zinc composite stabilizers for hard pipes have appeared in China.
Organotin heat stabilizers have better performance and are better varieties for PVC rigid products and transparent products. In particular, octyl tin has almost become an indispensable stabilizer for non-toxic packaging products, but its price is relatively expensive.
Epoxy stabilizers are usually used as auxiliary stabilizers. This kind of stabilizer can improve the stability of light and heat when used together with barium-cadmium-calcium-zinc stabilizer, but its disadvantage is that it is easy to exude. Also used as auxiliary stabilizers are polyols and organic phosphites.
In recent years, rare earth stabilizers and hydrotalcite-based stabilizers have also appeared. The main characteristics of rare earth stabilizers are excellent processing performance, while hydrotalcite is a non-toxic stabilizer.
2. Antioxidants
During the processing and use of PVC products, oxidation occurs due to the action of heat and ultraviolet rays, and its oxidative degradation is related to the generation of free radicals. The main antioxidant is a chain scission terminator or free radical scavenger. Its main function is to combine with free radicals to form stable compounds and terminate the chain reaction. The main antioxidant for PVC is generally bisphenol A. There are also auxiliary antioxidants or hydrogen peroxide decomposers, PVC auxiliary antioxidants are triphenyl phosphite and phenyl diisooctyl phosphite. The main and auxiliary antioxidants can play a synergistic effect when used together.
3. UV Absorber
PVC products used outdoors are exposed to ultraviolet rays in its sensitive wavelength range, and the PVC molecules become excited, or their chemical bonds are destroyed, causing free radical chain reaction, which promotes the degradation and aging of PVC. In order to improve the ability of anti-ultraviolet, ultraviolet absorbers are often added. Commonly used UV absorbers for PVC include triazine-5, UV-9, UV-326, TBS, BAD, and OBS. The effect of triazine-5 is good, but the film is slightly yellow due to the yellow color, which can be improved by adding a small amount of phthalocyanine blue. UV-9 is commonly used in PVC agricultural film, and the general dosage is 0.2~0.5 parts. TBS, BAD and OBS, which are salicylic acids, have mild effects. When used in combination with antioxidants, they will have a good anti-aging effect. For non-transparent products, the weather resistance is generally improved by adding light-shielding rutile titanium dioxide. At this time, if an ultraviolet absorber is added, a large amount is required, which is not very cost-effective.
4. Chelating agents
In the PVC plastic stabilization system, the commonly added phosphites are not only auxiliary antioxidants, but also act as chelating agents. It can form metal complexes with harmful metal ions that promote the removal of HCL from PVC. Commonly used phosphites are triphenyl phosphite, phenyl diisooctyl phosphite and diphenyl octyl phosphite. In PVC agricultural film, the general dosage is 0.5~1 part. When used alone, it is easy to color at the initial stage and has poor thermal stability. Generally, it is used together with metal soaps.
