Thermal Stabilizers
Introduction
Polyvinyl chloride (PVC) is extremely sensitive to heat, poor thermal stability, the processing temperature is greater than 90 degrees Celsius decomposition occurs, so PVC processing must add an appropriate amount of thermal stabilizers to inhibit its decomposition.
Product Composition
Thermal stabilizers mainly include lead salts, metal soaps, organotin, complex thermal stabilizers, phosphites, monoalcohols and polyalcohols, epoxies and nitrogenous compounds. The first four types account for most of the amount of thermal stabilizers.The high efficiency, low cost, low coloration and electrical insulation of thermal stabilizers have always been the research, development and production direction.Since the 1980s, due to the increasing awareness of environmental protection worldwide, non-toxic and pollution-free thermal stabilizers have become a hot spot of development.
Mechanism of action
(1) Neutralize the HCL escaping from PVC to inhibit its autocatalysis.Such as lead salts, organic acid metal soaps, organotin compounds, epoxides, amines, metal alcohols, phenols and metal mercaptan salts, etc., can react with HC1 and inhibit the HC1 dehydrogenation of PVC.Me (RCOO) 2 + 2 hci – > Me rcooh C12 + 2
(2) Replace or eliminate unstable factors such as allyl chlorine atoms or tertiary carbon chlorine atoms in PVC molecules, and eliminate the starting point of HCI removal.Such as organic tin stabilizer of tin atom with unstable chlorine atom PVC molecular ligand, while the sulfur atom of organotin, along with the corresponding carbon atoms in PVC complexation, sulfur atoms in the ligand and unstable chlorine atom displacement occurs, when the presence of HC1, coordination bond split occurred, thiol and PVC molecular carbon atoms to bond tightly to inhibit further HCI formation of double bond.Among the metal soaps, the substitutions of zinc and cadmium soaps with unstable chlorine atoms were the fastest, barium and calcium soaps were the slowest, and lead soaps were in the middle.At the same time, the metal chloride generated has different degrees of catalytic effect on dehydrochlorination, the strength of which is in turn as followsZnCl2> CdCl2>> BaCl2 , CaCl2 > R2SnCl2
(3) Add with double bond and conjugate double bond to prevent the development of polyene structure and reduce coloring.The salt or complex of unsaturated acid has a double bond, and the conjugated double bond in PVC molecule has a diene addition reaction,Thus, the conjugated structure is destroyed and discoloration is inhibited.In addition, metal soap substituted allyl chloride with double bond transfer, which destroyed the polyene structure and inhibited the discoloration.
(4)Capture free radicals to prevent automatic oxidation.For example, adding phenolic heat stabilizer can arrest the removal of HCL because hydrogen atom free radicals given by phenol can be coupled with degraded PVC macromolecular free radicals to form substances that cannot react with oxygen, and thus have a thermal stabilization effect.The heat stabilizer may have one or both functions.
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