(1) volatile
Low melting point of liquid paraffin, natural paraffin, stearic acid, stearic acid butyl ester and other volatile, for the initial lubricant, dosage should not be too much.
PE wax and OPE wax with high melting point (generally more than 100℃) have less volatilization and can be used as medium and late lubricants.
Only after melting, lubricant formed lubrication film (outer lubricant), also more easily infiltrate into the PVC layer particles (inner lubricant).
The initial lubricant has a low melting point and is volatile, and its volatile part is easy to condense and block the vacuum system, or condense in the injection mold, affecting the mechanical properties of weld marks.
(2) change of chemical structure.
Stearate is a common heat stabilizer and lubricant, which may be converted into metal chloride and stearate in hard PVC processing.
Such as the usual basic lead stearate, lead stearate, they may be converted to PbCl2 and HSt in the process.
The lubricity and volatility of HSt, lead stearate and basic lead stearate are different.
For example, the internal and external lubrication of the original lubrication system is balanced. In the later stage of processing, after the transformation into HSt, the internal and external lubrication is unbalanced due to the change of chemical structure.
In the lubrication system, alkali lead stearate and lead stearate should be used with caution.
Through the analysis of the molecular structure of lead stearate and HSt, it can be seen that after bonding with other groups with strong electronegativity, both lead atoms and hydrogen atoms show positive electromagnetism and have affinity for atoms of metal with free electrons.
But Pb element is the first Ⅳ cycle 6 A element, A multi-layer (even with 4 f layer) extranuclear electron, electronic mutually exclusive effect, making it the attraction of the outer electrons is weak, the bonding is relatively poor.
Hydrogen, on the other hand, has only one exonuclear electron, and when it bonds and loses its electron, it becomes an exposed, positively charged proton core, easily bound to free electrons to form a compound.
Therefore, the ability of lead stearate to form complex bond with metal surface is worse than that of HSt.
The carboxyl group part of HSt forms an intensified adsorption bond with the metal surface with free electrons, and the long-chain alkyl part is mutually soluble with alkanes (such as paraffin, polyethylene wax, etc.) to form a "riveting" lubricating film reinforced by stearic acid and carboxyl group.
Hard PVC commonly used external lubricant paraffin, PE wax non-polar alkyl compounds, and PVC and processing equipment on the metal surface can only form a very small force of physical adsorption layer, in 180℃ -- 200℃ and strong shear force, this layer of adsorption layer -- lubrication film -- is easy to be destroyed.
Therefore, when used in combination with HSt, the adhesive strength and tearing strength of paraffin and PE wax lubrication film on PVC and metal surfaces can be improved, and the external lubrication effect of paraffin and PE wax can be enhanced.
(3) influence of electronegativity and standard electrode potential.
Lead stabilizer has good thermal stability for a long time, which can keep the color of PVC, inhibit the formation of conjugated double bond of PVC, and prevent the product from changing color.
Metal soaps with initial thermal stability need strong coordination ability of metal elements, which can replace unstable chlorine atoms and inhibit PVC color change, such as soaps containing Zn and Cd elements.
Different metal atoms or ions have different ability to attract electrons, which determines the stability of metal soap in PVC.
The greater the electronegativity of the metal atom or the higher the potential of the standard electrode, the greater the ability of the atom or the corresponding metal ion to attract electrons, the easier it is to form complex bond intermediates with the elements with high electronegativity, and then form new compounds.
Table 1 shows the electronegativity and standard electrode potentials of some metal elements.
Table 1 electronegativity and standard electrode potential of some elements
|
元素 |
Ba |
Ca |
Zn |
Cd |
Pb |
|
电负性 标准电极电位/V |
0.89 -2.90 |
1.00 -2.87 |
1.65 -0.763 |
1.69 -0.403 |
2.33 +0.83 |
In the metal elements listed in table 1, Pb, Cd and Zn have higher electronegative property and electrode potential, so it can be considered that Pb, Cd and Zn atoms or ions are more likely than Ba, Ca atoms or ions to form complexation intermediates with negatively charged chlorine atoms in the unstable allyl chloride in PVC resin, and then form chloride.
In the hard PVC formula, by comparing the activity of barium stearate, calcium stearate and lead stearate heat stabilizer, it can be seen that barium soap and calcium soap are the least active, and they are the middle and late heat stabilizer.
Calcium stearate is a commonly used inner lubricant of hard PVC. Even if the amount of calcium stearate is l mass, it will not affect the lubrication balance of the initial, 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 stabilizer.
When the elements Zn and Cd in zinc soap and cadmium soap collide with the chlorine atoms in the negatively charged allyl chloride, they combine to form chloride, while the stearic acid radical replaces the original position of chlorine atoms, which eliminates the unstable group of PVC and improves the stability of PVC.
We're talking about perfect ideal.
After the formation of chloride, the stearic acid root replaces the position of allyl chloride on the premise that: (1) the distance between the stearic acid root and the corresponding positive carbon ion of allyl chloride is less than 0.1 nm;
An effective collision can occur.
If the distance is large (the resins are constantly subject to strong shear forces as the plastics are processed, and their relative positions are constantly changing), there is no effective collision to replace the chlorine atoms.
In thick PVC melt, the molecular thermal motion of stearic acid radical can be neglected, even if the stearic acid radical ions with carbon is carbon (anchor) distance in the distance of the bonding, under the effect of heat and strong shear force, it is impossible to replace 100% chlorine atom, most of the oxygen is PVC thermal degradation properties of activated within 0.1 nm does not necessarily just the presence of calcium stearate, lead stearate.
At this time, PVC resin is first thermal degradation to produce HC1, and then the reaction is
2 HCl + PbSt2 - PbCl2 + 2 HST
One molecule of lead stearate produces two molecules of HSt.
Therefore, a considerable amount of HSt must be generated after lead stearate participates in the thermal stabilization reaction to absorb HC1.
In general hard PVC formula is added enough initial heat stabilizer to ensure that the product is white or colorless.
Calcium stearate is a medium and late heat stabilizer.
Calcium stearate, which is used as a lubricant in these formulations, does not participate in the thermal stabilization reaction and is still present in the system as calcium stearate.
If calcium stearate is used both as a lubricant and as a heat stabilizer in the later stages of processing.
Calcium stearate will partially convert to HSt, which will seriously affect the lubrication balance.
In the presence of calcium stearate, even a very small amount of HSt will greatly promote the external lubrication of paraffin and other non-polar paraffin lubricants.
