Application of heat stabilizers in PVC plastics
Polyvinyl chloride (PVC) is extremely sensitive to heat, poor thermal stability, processing temperature greater than 90 degrees Celsius, decomposition occurs, so PVC processing must be added to the appropriate amount of heat stabilizer to inhibit its decomposition.
Heat stabilizers mainly include lead salts, metal soaps, organotin, composite heat stabilizers, phosphite, monohydric alcohols and polyols, epoxy and nitrogen-containing compounds, the first four categories accounted for the majority of heat stabilizers dosage. The high efficiency and durability of heat stabilizers, low cost, low coloring and electrical insulation has been the direction of research, development and production. since the 1980s, due to the growing worldwide awareness of environmental protection, non-toxic and environmentally friendly heat stabilizers have become a hot spot for development.
During the thermal degradation of PVC, almost no monomer is produced, but a large amount of HC1 is generated.The thermal degradation mechanism of PVC is very complex, and similarly, the action mechanism of heat stabilizers is also very complex. Comprehensive research results, the role of heat stabilizers can be summarized as preventive (neutralize HC1, replace unstable chlorine atoms and prevent auto-oxidation) and remedial (react with the unsaturated parts of PVC to destroy the positive carbon ions) two kinds.
(1) Neutralization of HCL derived from PVC to inhibit its autocatalytic effect. Such as lead salts, organic acids, metal soaps, organotin compounds, epoxy compounds, amines, metal alcohol salts and phenol salts and metal thiol salts, etc., can be reacted with HC1 to inhibit the dehydrogenation of PVC HC1 reaction.
Me(RCOO)2+2HCI→Me C12 +2RCOOH
(2) Displace or eliminate the unstable factors such as allyl chlorine atom or tertiary carbon chlorine atom in PVC molecule to eliminate the initiation point of de-HCI. Such as organotin stabilizers of tin atoms and PVC molecules of unstable chlorine atoms in the coordination bond, at the same time the sulfur atoms in the organotin and PVC in the corresponding carbon atoms complex, the sulfur atoms in the ligand body and the unstable chlorine atoms in the replacement, when there is the presence of HC1, the coordination bond splitting, the sulfhydryl group and the PVC molecules in the carbon atoms firmly bonded to inhibit the further dehydrocation of the reaction of the formation of a double bond. Among the metal soaps, zinc and cadmium soaps have the fastest substitution reaction with unstable chlorine atoms, barium soap is the slowest, calcium soap is slower, and lead soap is in the middle. At the same time, the resulting metal chlorides have different degrees of catalytic effect on the dehydrogen chloride in the following order of strength
ZnCl2> CdCl2>> BaCl2 , CaCl2 > R2SnCl2
(3) Addition to double bonds and conjugated double bonds prevents the development of polyene structures and reduces coloration. Salts or complexes of unsaturated acids have double bonds and undergo diene addition reaction with conjugated double bonds in PVC molecule.
Thus destroying its conjugated structure and inhibiting color change. In addition, metal soap replaces allyl chloride with double bond transfer, which destroys the polyene structure and inhibits color change.
(4) Capturing free radicals and preventing auto-oxidation. Such as the addition of phenolic heat stabilizers can block the dehydrogenation of HCL is due to the phenol given hydrogen atom radicals can be coupled with the degradation of PVC macromolecular free radicals, the formation of substances that can not react with oxygen, and has a heat stabilizing effect. This heat stabilizer can have one or both of several effects.
There is synergy between different heat stabilizers. This is the case with calcium-zinc compound stabilizers, which are generally regarded as safe and non-toxic by the medical profession at home and abroad. The U.S. Health Administration and the Japanese Health Bureau consider the calcium-zinc composite series of stabilizers to be a non-toxic additive for plastic products. It is foreseeable that calcium-zinc composite stabilizers will eventually build the future non-toxic PVC stabilizer system in various countries.
Zinc carboxylic acid and PVC allyl chloride reaction is calcium and zinc composite stabilizers stabilizing effect of the main reaction, the presence of calcium carboxylic acid will be zinc soap and PVC unstable chlorine atoms generated by the harmful ZnCl2 regeneration into zinc soap, thus activating the zinc soap on the one hand, on the other hand, greatly reducing its catalytic ability, and its own transformation into harmless calcium chloride, thereby inhibiting the destructive effect of ZnCl2.
The rapid development of the PVC industry has greatly driven the development and progress of heat stabilizers. The development and development of heat stabilizers show new weather, continue to low-toxicity, non-toxicity development, lead, cadmium heat stabilizers accounted for the proportion of the year-on-year decline in the proportion of organotin development momentum slowed down; zinc heat stabilizers to speed up the pace of development; new heat stabilizers hydrotalcite series, rare earth series, organic antimony series, perchlorate series of development and application of the momentum of the development and application of the positive development; multifunctional composite “The development and application of new heat stabilizers such as hydrotalcite series, rare earth series, organic antimony series, etc. show positive development momentum. Between different heat stabilizers, heat stabilizers and plasticizers, lubricants, antioxidants and other additives, sometimes there is a synergistic effect. In order to achieve the desired stabilizing effect, they will be in accordance with the appropriate ratio and method.
