PVC heat stabilizers and mechanism

PVC is the first ideal structure - tail structure-CH2-CHCl-CH2-CHCl-, is quite stable. But so far the PVC synthesis method can not be like synthetic butadiene rubber, as the role of catalyst in Jigger so there are rules for orientation cis butadiene polymerization is a free radical polymerization of vinyl chloride random polymerization, in addition to its stable First - tail structure, as well as the first - the first structure-CH2
-CHCl-CHCl-CH-, tail - tail structure-CHCl-CH2-CH2-CHCl-; generate a coupling disproportionation vinyl Results CH2 = CH-CHCl
-CH2-structure and allyl chloride-CH2-CH = CH-CHCl-CH2 and so on.
Generated in the PVC synthetic allyl chloride, chlorine and tertiary carbon double bond in its molecular chain structure instability, unstable cis
Sequence is: PVC within the molecular chain allyl chloride> tertiary carbon Chlorine> terminal allyl chloride> sec chlorine. PVC processing is easily degraded but because
The PVC molecular chain structure, there vegetable stearic acid  is instability factors, such modification not be stabilized, and its decomposition temperature is about 130 ℃, but
PVC resin to be processed into useful articles, molding temperature to above 190 ℃. Therefore, you must add to its junction heat stabilizer
Improved structure for stabilization. Four types of heat stabilizers heat discoloration Comment
Ideal thermal stabilizer should also absorb HCL, remove the active site, the addition of a conjugated polyene chain, from the destruction of carbenium
Sub-salt, to prevent the auto-oxidation and other functions, without generating a catalytic degradation of PVC products. The plastic processing aid  actual heat stabilizer due to
Has a different function and behave in a different thermal stability characteristics, can be divided into early-type, long type, intermediate type and versatile
Four categories.
1) cadmium, zinc soap is typical of early-type heat stabilizers, can quickly absorb HCL, and Cd, Zn in the catalyst effectively carboxylic
Replace PVC chain radical unstable chlorine atoms, thereby effectively suppressing the initial degradation and coloring, but because of consumption of fast transformation products CdC12, ZnC12, but also off the HCL PVC efficient catalysts, which will lead to the aluminium distearate  degradation of PVC material suddenly becomes a vicious black, so
Long-term thermal stability. 2) barium, calcium soap is a typical long-term thermal stabilizers, absorption HCL function only and therefore can not effectively suppress coloring PVC,
But the transformation products BaC12, CaC12 does not have catalytic activity, does not cause a sudden black PVC, long-term thermal stability. 3) fatty acids, organic tin are intermediate, both absorbs HCL, can effectively replace PVC with chain carboxylate unstable chlorine
Atoms, and conversion products do not have catalytic activity.