In the production and processing formula of hard PVC, comparing the activity of heat stabilizer barium stearate, calcium stearate and lead stearate, it can be seen that barium soap and calcium soap are not active. They are mid-to-late heat stabilizers. Calcium stearate is a commonly used internal lubricant for hard PVC. Even if the amount of calcium stearate is l mass, it will not affect the lubrication balance of the lubrication system in the early, middle and late stages. Zinc soap, cadmium soap and lead soap are much more active than barium soap and calcium soap, so they all have the role of initial heat stabilizer. When the Zn and Cd elements in zinc soap and cadmium soap collide with the chlorine atom in the electronegative allyl chloride, they combine to form chloride, while the stearate root replaces the original position of the chlorine atom, eliminating the unstable groups of PVC and improving the stability of PVC.
This is the perfectly ideal state. After the formation of chloride, the preconditions for stearate to replace allyl chloride are:
① The distance between stearate and allyl chloride is less than 0.1 nm.
② Effective collision can occur.
If the distance is large (the resins are continuously subjected to strong shear forces during plastic processing, and their relative positions change from time to time), the substitution of chlorine atoms is not possible without effective collision. In the thick PVC melt, the molecular thermal motion of stearate is negligible. Even if stearate is within the bonding distance from the positive carbon ion (carbon anchor), it is impossible to displace chlorine atom 100% under the action of heat and strong shear force. In most cases, calcium stearate and lead stearate are not necessarily present within 0.1nm of the activation point of PVC thermal oxygen degradation. At this time, the PVC resin is first thermally degraded to produce HC1, and then reacts into 1 molecule of lead stearate to produce 2 molecules of HSt. Therefore, a considerable amount of HSt must be generated after the absorption of HC1 by lead stearate in the thermal stabilization reaction.
General hard PVC formula are added in sufficient amount of initial heat stabilizer, to ensure that the product is white or colorless. Calcium stearate is the medium and late heat stabilizer. Calcium stearate as a lubricant in such formulations does not participate in the thermal stabilization reaction and remains in the system as calcium stearate. If calcium stearate is used both as a lubricant and as a heat stabilizer, at the later stage of processing. Calcium stearate will be partially converted into HSt, which will seriously affect the lubrication balance. In the presence of calcium stearate, even a small amount of HSt greatly enhances the external lubrication of non-polar paraffin lubricants such as paraffin.