(1) Volatility

Liquid paraffin, natural paraffin, stearic acid, butyl stearate, etc. with low melting point are more volatile, so they should not be used more as initial lubricants. PE wax and OPE wax with higher melting points (usually above 100 ℃) have less volatility and can be used as lubricants in the middle and later stages. Only after melting does the lubricant form a lubricating film (outer lubricant) and is more likely to penetrate into the particles of PVC layers (inner lubricant). The initial lubricant has a low melting point and is prone to volatilization, and its volatile parts are prone to condensation, which can clog the vacuum system or condense in the injection mold, affecting the mechanical properties of the weld line.

(2) Changes in chemical structure

Stearate is a commonly used heat stabilizer and lubricant, which may transform into metal chlorides and stearates in the processing of hard PVC. As commonly used in the past, basic lead stearate and lead stearate may transform into PbCl2 and HSt during processing.The lubricity and volatility of HSt, lead stearate, and basic lead stearate are different. As the original lubrication system had a balanced internal and external lubrication effect, in the later stage of processing, when it was transformed into HSt, due to changes in chemical structure, the internal and external lubrication effect became unbalanced. In lubrication systems, use basic lead stearate and lead stearate with caution, and use them in conjunction with paraffin and PE wax.

Through the analysis of the molecular structure of lead stearate and HSt, it can be seen that the lead atom and hydrogen atom show positive electricity after bonding with other groups with strong electronegativity, and have affinity for the metal atoms with free electrons. However, Pb element is the fourth A element in the sixth cycle, with multiple layers (even 4f layers) of external electrons. The mutual repulsion of electrons makes it less attractive to the external electrons and has relatively poor bonding ability. However, hydrogen atoms only have one extranuclear electron, and after losing electrons during bonding, they become positively charged exposed proton nuclei, which are easily combined with free electrons to form compounds. So the ability of lead stearate to form complex bonds with metal surfaces is poorer than that of HSt. The carboxyl part of HSt forms strong chemical adsorption bonds with the metal surface containing free electrons, while the long chain alkyl part dissolves with alkanes (such as paraffin and polyethylene wax), forming a lubricating film reinforced by stearic acid and carboxyl groups with strong "riveting effect". The commonly used external lubricants for hard PVC, paraffin wax and Polyethylene wax, are non-polar alkyl compounds that can only form a physical adsorption layer with minimal interaction force with the metal surface of PVC and processing equipment. Under the action of 180 ℃ -200 ℃ and strong shear force, this adsorption layer - the lubricating film - is easily broken. Therefore, when used in combination with HSt, the adhesion strength and tear strength of paraffin and PE wax lubricating film on PVC and metal surfaces can be improved, and the external lubrication effect of paraffin and PE wax can be enhanced.

In the formulation of rigid PVC, comparing the activity of barium stearate, calcium stearate and lead stearate heat stabilizers, it can be seen that barium soap and calcium soap are the least active, and they are heat stabilizers in the middle and later stages. Calcium stearate is a commonly used internal lubricant for hard PVC. Even if its dosage is one, its mass fraction will not affect the lubrication balance in the early, middle, and later stages of the lubrication system. Zinc soap, cadmium soap, and lead soap are much more active than barium soap and calcium soap, so they all have the effect of initial heat stabilizers.

Generally, a sufficient amount of initial heat stabilizer is added to the hard PVC formula to ensure that the product is white or colorless. Calcium stearate is a mid to late stage heat stabilizer. Calcium stearate, as a lubricant in these formulations, does not participate in thermal stabilization reactions and still exists in the system in the form of calcium stearate. If calcium stearate is used as both a lubricant and a heat stabilizer in the later stages of processing. Calcium stearate will partially convert into HSt, which will seriously affect lubrication balance. In the presence of calcium stearate, even a small amount of HSt can greatly promote the external lubrication of non-polar alkane lubricants such as paraffin.

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