The mechanism and application of PVC stabilizer
The mechanism and application of PVC stabilizer
Thermal stabilizer is one of the essential additives for PVC processing, and PVC thermal stabilizer is used in a small number, but its role is huge. Thermal stabilizer can be used in PVC processing to ensure that PVC is not easy to degrade and stable. The commonly used thermal stabilizers in PVC processing are alkaline lead salt stabilizers, metal soap stabilizers, organic tin stabilizers, rare earth stabilizers, epoxides and so on. PVC degradation mechanism is complex, the mechanism of different stabilizers is different, and the stable effect achieved is different. 1. PVC thermal degradation mechanism of PVC in 100 ~ 150 ℃ apparent decomposition, ultraviolet light, mechanical force, oxygen, ozone, hydrogen chloride and some active metal salt and metal oxide etc will greatly accelerate the decomposition of PVC. The thermal oxygen aging of PVC is more complex. Some literatures report that the thermal degradation process of PVC is divided into two steps. (1) dehydrochlorination: the active chlorine atoms are removed from PVC polymer chains to produce hydrogen chloride, and the conjugate polyolefins are generated. (2) the formation of much longer chain olefins and aromatic ring: with the further of degradation, the chlorine atoms on the allyl unstable is easy to take off, generating a longer chain of conjugated olefins, referred to as the "chain" of the dehydrogenation, at the same time there is a small amount of C - C key fracture, cyclization, produce a small amount of aromatic compounds. Decomposition of hydrogen chloride is the main cause of PVC aging. The mechanism of degradation of PVC is complicated and there is no unified conclusion, and the researchers have proposed  free radical mechanism, ion mechanism and single molecular mechanism. 2. The thermal stability of PVC mechanism In the process of machining, the thermal decomposition of PVC for the other properties change little, mainly affect the color of the finished product, adding heat stabilizer can inhibit the early stages of the product color. When the quality score of the removed HCl reaches 0.1%, the color of PVC begins to change. Depending on the number of conjugated double bonds, PVC presents different colors (yellow, orange, red, brown, black). If there is oxygen in the pyrolysis process, there will be the formation of colloidal carbon, peroxide, carbonyl and ester base compounds. However, the thermal degradation of PVC has a great influence on the properties of the materials for a long time. Adding heat stabilizer can delay the degradation of PVC or reduce the degradation of PVC. Adding heat stabilizer in PVC processing process can inhibit the degradation of PVC, so heat stabilizer plays the main role: by replacing unstable chlorine atoms, the absorption of hydrogen chloride, addition reaction with unsaturated parts such as way to suppress the degradation of PVC molecules. The ideal heat stabilizer should have a variety of functions: (1) replace the active and unstable substituents, such as the chlorine atom or the allyl chloride on the tertiary carbon atom, to produce a stable structure; (2) to absorb and neutralize the HCl in the process of PVC processing and eliminate the automatic catalytic degradation of HCl; (3) neutralization or passivation of metal ions and other harmful impurities that catalyze the degradation; (4) the continuous growth of unsaturated bonds can be blocked by a variety of chemical reactions, which can inhibit degradation coloring; (5) it is best to protect the uv light. 3. PVC stabilizer, action mechanism and purpose 3.1 lead salt stabilizer  can be divided into three categories: (1) simple lead salt stabilizers, which are mostly salt-base salts containing PbO; (2) thermal stabilizer with lubricating effect, mainly neutral and salt base salts of fatty acids; (3) compound lead salt stabilizers, and solid and liquid complex stabilizers containing a synergistic mixture of lead and other stabilizers and components. The thermal stability of lead salt stabilizer is stronger, has good dielectric properties, and the price is low, and the lubricant reasonable ratio can make the PVC resin processing temperature range wide, processing and after processing product quality is stable, is by far the most commonly used stabilizers. Lead salt stabilizer is mainly used in hard products. Lead salt stabilizer is characterized by high thermal stabilizer, excellent electrical performance and low price. But lead salts are toxic and cannot be used for food products or transparent products, and are easily produced by sulphide pollution to produce black lead sulphide. 3.2 metal soap stabilizer stearic acid soap hot stabilizer is generally alkaline earth metals (calcium, cadmium, zinc, barium etc.) and stearic acid, lauric acid and other saponification. There are many kinds of products, each has its own characteristics. In general, the lubricating stearic acid is better than the lauric acid, and the cinnamic acid with PVC is better than the stearic acid.
Metallic soap due to absorption of HCl, some varieties can also through its root of metal ions in the catalysis of fatty acids instead of the active site of Cl atom, so you can have different levels of heat stable effect of PVC. The PVC industry rarely has a single metal soap compound, usually several metal soap compounds. Regular calcium and zinc soap stabilizers. Based on the mechanism of Frye - horst and stability mechanism of calcium/zinc compound stabilizer can think: first of all, allyl chloride zinc soap and PVC chain reaction, and then zinc calcium soap, soap chlorine and chloride reaction of the unsteady metal chloride. At this time, as the intermediate media of the auxiliary stabilizer, the chlorine atoms are transferred to the calcium soap to regenerate the zinc soap, delaying the generation of zinc chloride with the effect of promoting dehydrochlorination. Calcium and zinc stabilizer can be used as a non-toxic stabilizer, used in food packaging and medical equipment, medicine packaging, but its stability is relatively low, calcium stabilizer dosage is large in the opacity, easy to spray frost. Calcium and zinc stabilizers generally use polyols and antioxidants to improve their performance. There are already transparent calcium and zinc composite stabilizers used in hard tube materials in China. 3.3 organic tin stabilizer organic tin is usually methyl, n-butyl and octyl. Most of the production in Japan is butyl tin, which is more common in Europe, which is the standard non-toxic stabilizer approved by Europe, while the United States USES more of the methyl tin. There are three main types of organic tin stabilizers: (1) fatty acid salts, which mainly refer to dibutyl tin dibutyl, dilaurate, and diocenric acid. (2) maleic acid salts, mainly maleic acid dibutyl tin, dibutyl tin (maleic acid monobutyl), dibutyl tin, maleic acid, etc. (3) sulfide salts, of which two (sulphate carboxylic acid) ester is the most used. Organic tin heat stabilizer, with good performance are used in rigid PVC products and transparent products of good varieties, especially dotl almost become non-toxic packaging products indispensable stabilizer, but its price is more expensive. The organic tin thermal stabilizer (thiol) has a good stabilizing effect on PVC. In particular, the liquid organic tin stabilizer, which is a solid thermal stabilizer, can be better mixed with PVC resin than liquid organic tin stabilizer. Organic tin stabilizer (tin) can replace unstable Cl atoms in polymer and make PVC resin have long-term stability and initial color retention. In this paper, the stable mechanism of thioglycolic acid tin is proposed: (1) S atom can replace the unstable Cl atom, thus inhibiting the generation of conjugate polyolefins. (2) HCl is the product of thermal degradation of PVC and can accelerate the generation of conjugate polyolefins. The HCl can be absorbed by thiothioacetate. 3.4 rare earth stabilizer rare earth heat stabilizers mainly include the rich light rare earth lanthanum, cerium, neodymium organic weak acid salt and inorganic salt. The types of organic weak acids are stearic acid rare earth, fatty acid rare earth, salicylic acid rare earth, citric acid rare earth, lauric acid rare earth, rare earth, etc. A preliminary study on the mechanism of rare earth stabilizer is: (1) the special electronic structure of rare earth lanthanides (the outermost layer 2, outer eight electronic structure, there are many empty orbit), the empty orbital energy level difference is very small, under the effect of external heat oxygen or under the effect of polar group, the outer layer or outer electrons were intensified, with unstable Cl PVC chain coordination, and can be resolved and PVC processing of hydrogen chloride ligand complex formation, and rare earth elements with strong attraction between chlorine, can have the effect of control of free chlorine, which can prevent or delay the autooxidation chain reaction of hydrogen chloride, thermal stability. (2) rare earth multifunctional stabilizers for PVC processing PVC itself contains the oxygen and physical adsorption of ionic impurities, and into the lattice point of rare-earth multifunctional stabilizers, avoid them to maternal C - Cl key impact vibration. Therefore, the activation energy of PVC dehcl can be enhanced by the use of the multifunction stabilizer of rare earth, thus delaying the thermal degradation of PVC plastic. (3) the suitable anionic groups in rare earth compounds can be used to replace the allyl chloride atoms in PVC large molecules and eliminate this degradation weakness, which can also achieve a stable goal. Rare earth stabilizer domestic research is more. In general, the stability of rare earth heat stabilizer is better than metal soap stabilizer, has good long-term heat stability, and with other species have a wide range of synergies between stabilizer, has good tolerance, not polluted by sulfur, storage stability, the advantages of non-toxic environmental protection. In addition, the rare earth elements have a unique coupling effect with CaCO3 and promote the plasticization effect of PVC. Therefore, the use of CaCO3 can be increased, and the use of ACR of processing AIDS can be reduced effectively. The stable effect of rare earth on polyvinyl chloride is characterized by its unique synergistic effect. Due to the appropriate coordination of some metals, ligand and stabilizers, rare earth can greatly improve the stabilizing effect. 3.5 other stabilizers
3.5.1 track of epoxy epoxy soybean oil, flaxseed oil, epoxy epoxy can tall oil, epoxy butyl stearate, dioctyl epoxy compounds is polyvinyl chloride (PVC) commonly used heat stabilizer, they are used with the stabilizer have higher synergy, light stability and the advantages of non-toxic, suitable for soft, especially soft FVC products are exposed to the sun, usually is not used for rigid PVC products, its shortcoming is easy. Studies have pointed out that the epoxy sunflower oil added to contain different metallic soap salt (Ba/Cd and Ca/zinc) PVC, through the determination of the thermal stability of the material, found that sunflower oil and metal soap salt has the very good synergy, can enhance the thermal stability of PVC material, analyzes the causes of synergy: the degradation of HCl are absorbed by sunflower oil and metal soap salt, HCl concentration decreases at the same time reduce the PVC of HCl speed (HCl has catalytic effect to PVC degradation), improve the thermal stability of PVC. 3.5.2 hydroxyl groups, such as pentaerythritol and xylitol, have a certain thermal stabilizing effect on PVC, which is a commonly used auxiliary heat stabilizer for PVC. Through the dehydrochlorination rate and thermal stability experiment, found that do not contain heavy metals and zinc heat stabilizer of PVC/thermal stability of polyol extended to 200 ℃, its stable effect related to the types of polyol and hydroxyl number, especially containing the end hydroxyl polyol promote PVC thermal stability for a long time, produced by the absorption degradation HCl. 3.5.3 other phosphatates, beta-diones, dihydropyrimidine, etc. can be used as auxiliary heat stabilizers for PVC, absorbing the resulting HCl and delaying the color change of PVC. 4 at present situation and development trend of PVC heat stabilizer after entering the 21st century, due to the global demand for environmental protection is becoming more and more strict, growing regulations limit of heavy metals stabilizer, make further heat stabilizer production and consumption to non-toxic, low toxicity, composite efficient direction, lead-free, no cadmium has been paid attention to by the developed countries, substitute products appear constantly, and application of lead, cadmium cadmium (especially) the application of stabilizers has have been gradually declining trend, there have been some non-toxic or low toxic heat stabilizer (e.g., organotin compounds, calcium/zinc soap salt and rare earth stabilizer, etc.). Although in recent years, our country complex, non-toxic and low toxicity of heat stabilizer production and development has made considerable achievement, but compared with the world's advanced level there is a lot of deficiencies and more gap (such as varieties, small production scale, etc.). The production and application of new heat stabilizer in China can not meet the development of domestic PVC industry. Some of the thermal stabilizers needed for high-grade PVC products are mainly imported. The rapid development of China's PVC industry has provided a good market guarantee and broad development space for the development of the thermal stabilizer industry, and also higher requirements for the hot stabilizer industry. In order to strengthen the research and development of new-type thermal stabilizer in China, we should pay attention to the following points: (1) strengthen the research and improvement of the original lead-free and cadmium-free zinc stabilizer and improve the quality of the original products; (2) gradually establish a large group of large-scale auxiliary production plants based on the source of raw materials and market distribution; (3) cooperate with other PVC auxiliaries in the development and production of multi-compound products, further reduce waste of resources and environmental pollution, and promote the sustainable development of the "green" auxiliary industry