Thermal Aging of PVC The process is relatively complex, and there can often be several chemical reaction processes at the same time. The most important one is the decomposition and dehydrogenation reaction, which is the main cause of aging of PVC. Some people think that: In the polyvinyl chloride molecular chain, due to the electronegativity of the chlorine atom, the hydrogen atom on the ortho-methylene group has a partial positive charge. charge. Such a structure can remove a molecule of hydrogen chloride once heated. And form the structure of “allyl chloride”.
Some people also think. The regular structure of polyvinyl chloride is relatively stable, but during the polymerization process, due to chain transfer and other relationships, the polyvinyl chloride molecules originally contain a small amount of allyl chloride.
Due to the particularity of the allyl structure, when forming positive ions orH radicals, they are relatively stable due to co-talking. Therefore, the chlorine atoms on allyl star are quite active, and both ionic reactions and radical reactions can be carried out easily.
Due to the reactivity of allyl chloride, when heated, it is easy to remove a molecule of hydrogen chloride together with the hydrogen atom on the adjacent methylene group. In this way, common double bonds will appear on the macromolecular chain of polyvinyl chloride, and this reaction can continue. The result is a conjugated“polyolefin structure”.
HC1 decomposed from polyvinyl chlorideHC1 has a catalytic effect on the de-HC1 reaction. Therefore, once the decomposition of polyvinyl chloride and dehydrogenation begins, it will continue to be autocatalyzed. If the average number of double bonds in the resulting polyene structure exceeds 10, PVC will begin to turn yellow. At the same time, it is exposed to heat. The polyolefin structure is easily oxidized, producing radical compounds that absorb ultraviolet light and lead to further oxidative degradation. The color of PVC becomes darker and the physical and mechanical properties decrease.
In addition, due to the reactivity of allyl chlorine atoms, it is easy to produce C- under the action of light and heat The C1 bond is homolytically broken to generate free radicals. During the processing and use of polyvinyl chloride, it will inevitably come into contact with oxygen, which will cause free radical reactions and promote the degradation and cross-linking of polyvinyl chloride, that is, it will promote the aging of polyvinyl chloride.
Based on Based on the above mechanism, the heat stabilizer should have the following functions:
① Can bind to detached HC1 and terminate its autocatalytic effect;
② Can replace active chlorine atoms (i.e. allyl chlorine atoms) in the molecule and inhibit deoxidation of hydrogen Reaction;
③Can form double bonds with polyene structure Addition reaction to eliminate or reduce the discoloration and color deepening of products;
④ It can prevent the oxidation of polyolefin structure.
In fact, not all heat stabilizers can do the above Therefore, antioxidants and light stabilizers must be added to the stabilizing system of polyvinyl chloride.
It is clear that basic lead salts can capture hydrogen chloride. The function of fatty acid soaps may be to replace the active hydrogen atoms on the polyvinyl chloride molecular chain to form a relatively stable ester, thus Inhibits the dehydrogenation reaction.
The zinc chloride and cadmium chloride generated after replacement can also be activatedC-C1 bond promotes the degradation of polyvinyl chloride, but in the presence of phosphorus soap and calcium soap, zinc chloride and cadmium chloride react with them to regenerate fusoap. and zinc soap, the last remaining residues are chelium chloride and calcium chloride. These two are harmless to polyvinyl chloride.
The mechanism of action of organotin stabilizers has been studied using tracer atoms. According to the research, the organic tin compound first coordinates with the chlorine on the PVC molecular chain. In the ligand electric field, the chlorine on the molecular chain Active chlorine atoms are exchanged with Y groups, leaving Y groups on the PVC molecular chain, thereby inhibiting the dehydrochlorination reaction.
Maleate-type organotin compounds can also perform a diene addition reaction with the diene structure of the common urethane to form Cutting off polyene chains, the result
represses co-rolling The progressive development of the system,
It is generally believed that the polyene chromophoric group is formed byabout 10 conjugated double bonds arranged together . Therefore, breaking the polyene into several short bond segments with less than 10 conjugated double bonds is probably the best way to prevent discoloration.
The role of epoxide, concave energy is that epoxide first reacts with hydrogen chloride to form chlorohydrins, and nitrogen alcohols can then react withThe double bond effect of PVC molecular chain kernel.
There are also a large number of reactions between phosphite andPVC’ and its decomposition products, and metal soap stabilizers. According to the research report, the artistic reaction is more complicated and the explanation is unclear. , which will not be introduced here,
Currently, the development of foreign heat stabilizers is developing in the direction of non-toxic, efficient, multi-functional, non-metallic, etc., especially in the field of environmental pollutionH is becoming more and more serious today, and it seems that there will be some changes in the use of metal stabilizers.
