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With China Plastic Machinery
With China Plastic Machinery
PVC heat stabilizers are mainly used for PVC and other chlorine-containing polymers. People have found that PVC plastics can only be processed and molded at temperatures above 160¡ãC, and it begins to thermally decompose at 120-130¡ãC, releasing HCl gas. If the production of HCl is not suppressed, the decomposition will be further aggravated. This problem has been the main problem that has plagued the development and application of PVC plastics.
Studies have found that if PVC plastics contain a small amount of impurities such as lead salts, metal soaps, phenols, aromatic amines, etc., it will not affect its processing and application, and can also delay its thermal decomposition to a certain extent. The above problems have been solved, which has led to the establishment and continuous development of the field of heat stabilizer research.
In a broad sense, any additive that can improve the thermal stability of polymers is called a heat stabilizer. Due to the poor thermal stability of polyvinyl chloride, most of the PVC stabilizers in the world are used for polyvinyl chloride. Therefore, the PVC stabilizer usually refers to the heat stabilizer of polyvinyl chloride and its copolymers.
Generally, the heat stabilizer is classified according to its chemical composition, which can be basic lead salt, metal soap, organic tin, epoxy compound, phosphite, polyol, etc. PVC stabilizers can be divided into main stabilizers and auxiliary stabilizers according to the size of the effect. The auxiliary stabilizer itself has only a small stabilizing effect or no heat stabilizing effect, but it has a synergistic effect when used with the main stabilizer; the main stabilizer is generally a heat stabilizer containing metal. Pure organic compounds such as epoxy compounds, phosphites, polyols, etc. are generally used as auxiliary stabilizers.
Because it is often necessary to use multiple PVC heat stabilizers at the same time to achieve good stability of polyvinyl chloride, some commercial PVC stabilizers are compounded with multiple ingredients to become composite stabilizers, such as barium cadmium stabilizers, barium zinc stabilizers, etc. These composite stabilizers usually have added lubricants and other additives required for polyvinyl chloride processing to facilitate user use. There are many varieties of composite stabilizers, which are in three forms: powder, paste, and liquid.
Therefore, there are several types of heat stabilizers:
1. Lead salt composite stabilizer
2. OBS organic stabilizer
3. Organic tin stabilizer
4. Liquid composite stabilizer
5. Calcium zinc composite stabilizer
6. Barium zinc composite stabilizer
7. Potassium zinc composite stabilizer
(1) Absorption of hydrogen chloride
Me(OOCR)2+ 2HCl===MeCl2+ 2HOOCR
Where Me is: Pb, Ba, Cd, Ca, Zn, Sn, Sb, Mg, Sr, etc.
(2) Elimination of unstable chlorine atoms Replacement or elimination of unstable chlorine atoms.
(3) Preventing auto-oxidation Polyvinyl chloride is easily oxidized and degraded by O2 under the action of heat, oxygen and shear force.
(4) Adding metal salts of maleic acid can inhibit or eliminate double bonds or shorten or reduce them.
People always hope that PVC stabilizers can be easily dispersed in the compound species and can be used for a long time without precipitation after being made into products. However, when the compatibility of the stabilizer is poor, it will move from the inside of the product to the external surface and finally remain on the surface of the product. If the sprayed material is a powdery solid substance, it is called blooming; if the sprayed material is liquid, it is called "sweating"
Generally speaking, some metal soaps, lubricants and maleic acid esters of organic tin are prone to blooming; PVC stabilizers such as phosphites and phosphates will also increase blooming. For metal soaps, metals with low electronegativity have less blooming; aromatic acid radicals have less blooming than fatty acid radicals; the longer the carbon chain in the fatty acid radical, the more serious the blooming.
1. Transparency of PVC stabilizers and products
Organotin, especially maleate esters and thiol organotin, is a PVC stabilizer with good transparency, mainly used in hard transparent products; metal soap is transparent and is mostly used in translucent products; inorganic lead salts are opaque and can only be used in opaque products.
2. Whitening phenomenon
Transparent plastic products will whiten and lose transparency if they are immersed in water, exposed to the sun outdoors, or bent or stretched. Water immersion whitening refers to the phenomenon that transparent plastic plates appear turbid and opaque after being immersed in water for a certain period of time; exposure whitening refers to the phenomenon that plastic hard plates are placed in the air due to the effects of moisture, carbon dioxide, sulfur dioxide and light in the atmosphere; stress whitening refers to the phenomenon that plastic products appear white at the crease when they are subjected to mechanical external forces.
3. Fish-eye bubbles
When a transparent or translucent plastic sheet is observed against the light, small round particles similar to fish eyes can be seen in the sheet. These are "fish eyes". "Bubbles" are caused by the volatile components in the PVC stabilizer or the imbalance of processing properties
(1) Cause
PVC is made by the polymerization of vinyl chloride monomer initiated by free radicals. During the reaction, chain transfer reactions will occur during the growth of the molecular chain to generate tertiary carbon atoms. The chlorine atoms and hydrogen atoms connected to the tertiary carbon atoms become active atoms due to their low electron cloud distribution density and low bond energy. They can easily remove a portion of HCl from the adjacent H and Cl. The molecular structure of PVC resin is arranged end to end as shown in the following formula:
If the PVC resin is purely a linear structure as mentioned above, and all of them are secondary carbon atoms combined with chlorine atoms, then its stability is relatively good.
But in fact, even PVC resin with high purity will start to decompose HCl at above 100¡æ, which means that there are still unstable factors in its molecular structure.
It is believed that the unstable structure of PVC is caused by the free radical polymerization of vinyl chloride, and the molecule may contain the following structure.
In the above structure, one end contains secondary or tertiary chlorine, the other end has a ¦Â-unstable group, and there is oxygen in the structure. The reason may be that part of it is generated by the peroxidation reduction of the initiator, and the other oxygen-containing structure may be formed by the polymerization reaction in the presence of trace oxygen or by post-oxidation of the polymer.
(2) Solution
Due to the poor thermal stability of PVC, it is necessary to add corresponding stabilizers to repair the defects of the PVC chain and absorb the HCL produced by the dechlorination of PVC in time. At the same time, due to the processing characteristics of PVC with high viscosity and large shear, it is necessary to add corresponding lubricants such as polyethylene wax when adding stabilizers.
Polyethylene wax, or PE wax, is low molecular weight polyethylene, which is directly polymerized from ethylene. Products synthesized by different synthesis processes and catalytic systems differ in molecular weight, molecular weight distribution and molecular chain structure, and the performance of the corresponding products will also be significantly different. PE wax is usually a white powder with an average molecular weight of 1500-5000 and a melting point of 100-120 degrees. It has an excellent external lubrication effect in PVC processing, and can effectively improve the fluidity, yield, dispersion, surface brightness and demolding of PVC processing. Due to its large molecular weight, high melting point, and very good high temperature stability, it shows a strong external lubrication effect under high temperature and high shear conditions.
Polyethylene wax products can effectively improve the dynamic thermal stability of PVC and the fluidity of the processing process, increase the extrusion efficiency of PVC products, improve the surface gloss of the products, and effectively reduce the precipitation phenomenon in the processing of PVC products.
PE waxes on the Chinese market are mostly divided into several types:
PE wax produced by homopolymerization of ethylene. The PE wax produced by this method has good external lubrication performance, high gloss, narrow molecular weight distribution and very stable quality. Representative products include the AC series of Honeywell in the United States;
Low molecular weight polyethylene cracking products, commonly known as cracking wax, the cracking process is simple and easy to operate, but the molecular weight distribution of the product fluctuates with the production process. The quality is relatively good, and there will still be a small number of low melting point components;
The by-product in the ethylene polymerization process, commonly known as secondary brand wax, is purified through a refining process to obtain PE wax. The product has low viscosity, high melting point, excellent external lubrication performance and low price, but the quality fluctuates with the changes in raw materials and processes. Due to the characteristics of the refining process, it is inevitable that there will be more low melting point components in the product.
The development of PVC stabilizers has brought many conveniences to our lives. The research and development of each additive will derive PVC products that affect our lives. PVC material is polyvinyl chloride, which can be seen everywhere in life. It is used to make anti-corrosion pipes, pipe fittings, oil pipelines, centrifugal pumps and blowers. PVC hardboard is widely used in the chemical industry to make linings of various storage tanks, corrugated boards of buildings, door and window structures, wall decorations and other building materials. Due to its excellent electrical insulation performance, it can be used in the electrical and electronic industries to make plugs, sockets, switches and cables. In daily life, polyvinyl chloride is used to make sandals, raincoats, toys and artificial leather. These PVC products affect all aspects of our lives.
The increase in demand for PVC products has driven the development of the PVC stabilizer industry. PVC products affect our lives, so the development of PVC stabilizers is getting closer and closer to life and affects life. The development of industry PVC stabilizer industry is also developing rapidly. The development trend of additives is also towards non-toxicity and multifunctionality. Compounding. Molecular weight, rapid progress in compounding additives and containerization technology.
The development and research of PVC stabilizers tends to be non-toxic. The development and research of additives tends to be non-toxic and pollution-free. The development and research of substitutes for Cd and Pb stabilizers are very active. Ba-Zn, Ca-Zn and other composite metal heat stabilizers and organic tin and organic antimony stabilizers are developing rapidly and have large consumption. Vitamin E has been put into practical use as a "green" antioxidant for polyolefins and is expected to become a substitute for BHT in the future. The non-toxic and environmentally friendly Ba-Zn and Ca-Zn stabilizers pioneered by Tianyueguo have been put into the market and received a good market response.
Multifunctional trend of additives
Great progress has been made in the development of multifunctional additives. Antistatic plasticizers, flame retardant plasticizers, and multifunctional stabilizers have all been launched.
PVC stabilizer compound additives and containerization trend
Based on the principle of synergistic effects between various additives, compounding additives into a container helps facilitate plastic processing and fully automated operations. Compound products often appear in the system of hindered phenol main antioxidants and phosphite auxiliary antioxidants, while concentrated additives are mainly used in the field of polyvinyl chloride processing.
PVC stabilizer high molecular weight trend
The development progress of stabilizing additives, plasticizers, flame retardants and other varieties reflects this trend. In terms of stabilizing additives, high 150 molecular weight hindered amine light stabilizers also show anti-thermal oxidation effects, high molecular weight plasticizers have extraction resistance; high molecular weight flame retardants have improved heat resistance and compatibility with resins.
PVC stabilizers will definitely develop towards green, healthy, environmentally friendly and diversified development in the future. Integrate into our lives.
Reference formula of calcium zinc stabilizer
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