1 Influence of the chemical structure of lubricants
1.1 The lubricity of cyclic molecules is worse than that of chain molecules.
1.2 In homologous chain molecules, the larger the molecular weight and the longer the carbon chain, the better the external lubricity.
1.3 The polar atoms and double bonds of chain molecules increase their compatibility with polymers and enhance their internal lubricity characteristics.
1.4 The polar group of the chain molecule is more lubricious when it is located at the end of the molecular chain than at other positions.
1.5 Among chain molecules, the presence of branched chains in molecules with the same carbon number will increase the characteristics of external lubrication.
2 Influence of resin
Different resins have different compatibility with the specified lubricant and show different lubricity. For example, stearic acid, stearic acid amide, stearic acid ester, etc. have internal lubricating effect on polar PVC and polyamide, but show external lubricating effect on non-polar polyethylene and polypropylene.
Lubricity is also related to the molecular weight, molecular weight distribution and polymerization method of PVC. pvc internal lubricant, This is related to the rate at which the lubricant is absorbed by the resin.
3 Influence of processing conditions
When the temperature increases, part of the lubricant volatilizes and reduces the lubricating effect, but at the same time, increasing the temperature promotes the compatibility of the lubricant and the polymer, resulting in the original external lubricant becoming an internal lubricant. For example, in the early stage of calendering or rolling, metal soap and stearic acid mainly show external lubrication, but with the extension of heating time, the external lubrication is small, and the phenomenon of sticking rolls occurs.
Improve the shearing effect of the processing process, forcing the compatibility to increase and the external lubricity to decrease.
4 Influence of Lubricant Melting Point and Viscosity
The lubricity in plastic molding is related to the fastness of the lubricant film. The formation and collapse of the lubricant film in turn depends on its melting point and the viscosity of the lubricant melt at the processing temperature.
The film layer of the lubricant with the lowest viscosity at the molding temperature is easily broken.
5 Effects of polymer blends
ABS, CPE, MBS are polymer modifiers that improve the impact of PVC. The compatibility with lubricants is large, and the butadiene component reduces the fluidity, so the amount of lubricants must be increased.
In order to improve the processing fluidity of PVC and the gloss of the product, a certain amount of vinyl chloride-vinyl acetate copolymer is often added. Due to its greater compatibility with lubricants, a larger amount of lubricant needs to be used. Otherwise, the system gels prematurely, becomes sticky and loses processability, or the surface gloss of the product deteriorates.
6 The influence of metal materials
The rollers of the calender are machined from chilled cast iron, which tends to stick to the polymer. Chrome-plated calender rolls are less likely to stick.
In the calendering process of rubber and PVC, stearic acid is often used as a lubricant, and its lubricating effect is related to the lubricity of metal soap film formed by chemical action of metal and fatty acid.
For extrusion, nitrided steel, chrome steel and chrome-plated steel are commonly used for the screw and the sleeve in the barrel, which have less adhesion to the plastic melt.
7 Influence of other additives
In the case of adding a large amount of fillers such as calcium carbonate, talc, clay, etc., the friction of the system is large, and the fillers can absorb the lubricants greatly. It is necessary to increase the amount of internal and external lubricants for normal processing.
Among heat stabilizers, metal soaps are themselves lubricants, while lead salts and most organotins have poor lubricity.
Generally, the lubricity of the heat stabilizer is:
Metal soap>liquid composite metal soap>lead salt>tin laurate>organotin maleate～=organotin thiol
In semi-rigid and soft PVC formulations containing plasticizers, internal lubricants can generally not be used in such formulations due to the internal lubricating effect of the plasticizer itself.
Other additives such as antistatic agents, antifogging agents, coupling agents, flame retardants, etc. will have a certain impact on the lubricity of plastics.
If the amount of lubricant is too large, there is a danger of over-lubrication, so that it changes from internal lubrication to external lubrication, delaying the plasticization of PVC resin.
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