The polymer usually has a high viscosity after melting. During the processing, when the molten polymer passes through a narrow slit, a gate, etc., the polymer melt must be rubbed against the surface of the processing machine. Friction is very detrimental to the processing of the polymer, which reduces the melt flow, while severe friction can make the surface of the film rough, lack of luster or flow pattern. For this reason, it is necessary to add an auxiliary agent for the purpose of improving lubricity, reducing friction, and reducing interfacial adhesion. This is the lubricant. In addition to improving fluidity, the lubricant can also function as a melt accelerator, anti-blocking and antistatic agent, and a slip agent.
Lubricants can be divided into external lubricants and internal lubricants. The role of external lubricants is mainly to improve the friction between the polymer melt and the hot metal surface of the processing equipment. It is less compatible with the polymer and easily migrates out of the melt, so it forms a thin layer of lubrication at the interface between the plastic melt and the metal. The internal lubricant has good compatibility with the polymer, and it acts to reduce the cohesive force between the polymer molecules inside the polymer, thereby improving the internal friction heat generation and melt flowability of the plastic melt. Commonly used external lubricants are stearic acid and its salts; internal lubricants are low molecular weight polymers. Some lubricants have other functions. In fact, every kind of lubricant has the effect of fulfilling certain requirements. It always works together with internal and external lubrication, but it is more prominent in one aspect. The same lubricant will exhibit different lubrication effects in different polymers or under different processing conditions. For example, under high temperature and high pressure, the internal lubricant will be extruded to become an external lubricant.
In the production of plastic film, we also encounter some adhesion phenomena. For example, in the production of plastic film, the two films are not easy to separate, which brings difficulties to automatic high-speed packaging. To overcome this, a small amount of surface lubricity-enhancing aid may be added to the resin to increase external lubricity, commonly referred to as an anti-blocking agent or slip agent. Generally, in the molecular structure of a lubricant, there are long-chain non-polar groups and polar groups, and their compatibility in different polymers is different, thereby showing different internal and external lubrication effects. According to chemical components, commonly used lubricants can be classified into the following categories: fatty acids and their esters, fatty acid amides, metal soaps, hydrocarbons, organosilicon compounds, and the like.
Lubricants have a variety of properties in the actual processing of plastics, such as in the mixing and calendering process, to prevent the polymer from sticking to the barrel, to suppress frictional heat generation, to reduce the mixing torque and load, and to prevent the polymer material. Thermal degradation. At the time of extrusion molding, the fluidity is improved, the adhesion of the polymer material to the cylinder and the mold is improved, and the retentate is prevented and reduced. It also improves the appearance and gloss of the film.
From the point of view of processing machinery, external lubricants play an important role in molding processes such as kneading, calendering, and slush molding. In extrusion and injection molding, internal lubricants are more effective.
The amount of lubricant is generally 0.5%~1%. When selecting, pay attention to:
1. The flow properties of the polymer have met the needs of the molding process, and the effect of external lubrication is mainly considered to ensure internal and external balance;
2. Whether the external lubrication is effective, whether it can form a complete liquid film on the plastic surface layer at the molding temperature, so the melting point of the external lubricant should be close to the molding temperature, but the difference is 10 ° C ~ 30 ° C To form a complete film;
3. Does not reduce the mechanical strength of the polymer and other physical properties.
When selecting a lubricant in production, it should be made to meet the following requirements:
4. Lubrication efficiency is high and lasting;
5. The compatibility with the resin is moderate, the balance of internal and external lubrication;
Do not spray frost, not easy to scale;
6. The surface gravity is small, the viscosity is small, the expansion at the interface is good, and the interface layer is easy to form;
7. Try not to reduce the various excellent properties of the polymer, and does not affect the secondary processing properties of the plastic;
8. It has excellent heat resistance and chemical stability, and does not decompose or volatilize during processing;
9. Does not corrode equipment, does not pollute the film, and has no toxicity.
However, the simple use of a lubricant is often difficult to achieve, and several lubricants are required to be used in combination. In recent years, composite lubrication has developed rapidly, and the effect of the lubricant can be viewed from multiple angles when selected.
Commonly used lubricants are stearic acid, butyl stearate, oleamide, ethylene bis stearamide, and the like.
Many paraffinic substances can be used as lubricants, such as natural paraffin, liquid paraffin (white oil), microcrystalline paraffin, etc., but the effects are different. Natural paraffin wax is used for external lubrication. It can be used as a lubricant and release agent for various plastics. The general dosage is 0.2~1phr, but its compatibility, thermal stability and dispersibility are not very good. The dosage should not be too large. The internal lubricant is used in combination; while the white oil is mostly used as the internal lubricant of PVC and PS, the lubricating performance is good and the thermal stability is also good, and the general dosage is 0.5 phr. They are all drug-free and can be used for food packaging. There is also a kind of microcrystalline paraffin: it is also used as a lubricant in plastic processing. The dosage is 1~2Phr, and the thermal stability and lubricity are better than ordinary paraffin.
Low molecular weight polymers are also widely used as lubricants, such as polyethylene waxes and low molecular weight polypropylenes, which have good internal and external lubricity and are non-toxic. Polyethylene wax is suitable for extrusion and calendering of PVC and other materials. The dosage is generally 0.1~1phr, which can improve processing efficiency, prevent film adhesion, improve the dispersion of filler or pigment, and the compatibility and transparency are not very good; irregularity The structure of low molecular weight polypropylene can be used as a rigid PVC, PE lubricant, excellent performance, can improve the dispersion of other additives, the dosage is 0.05~0.5phr.
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