I. Forward
In 2021, China’s PA6 production capacity is 5.715 million tons, and it is expected to reach 6.145 million tons in 2022, with a growth rate of 7.5%. China’s PA6 has a high degree of localization. From a global perspective, about 55% of PA6 chips are used for fibers, and about 45% of chips are used for engineering plastics and films for automobiles, electronics, railways, etc. The total consumption of PA6 in China will reach 4.127 million tons in 2021, about 20% of which will be used for engineering plastics.
Nylon 6, also known as polyamide 6 and PA 6, has good mechanical strength and crystallinity, and has the characteristics of corrosion resistance and wear resistance. Although its comprehensive performance is excellent, it also has a series of disadvantages, such as PA6 does not have strong acid and alkali resistance, low impact strength at low temperature and dry state, and its own hydrophilic group will cause high water absorption, then elastic modulus, creep resistance, impact strength, etc. will be greatly reduced after absorbing water, thus affecting the dimensional stability of the product, and also affecting the electrical properties of the product. Therefore, it is necessary to do research on the modification of PA6.
II. PA6 performance
The raw materials of PA have a wide range of sources, which is the basis for its large-scale industrial production. Due to the regular arrangement of PA molecular structure, many hydrogen bonds can be formed between macromolecules, so it has high crystallinity and has outstanding characteristics in terms of mechanical properties, chemical properties, thermal properties, etc., specifically including:
(1) It has high tensile strength and bending strength;
(2) It has good impact resistance;
(3) It has high heat resistance;
(4) It has the characteristics of wear resistance and self-lubrication, which is a metal material Incomparable characteristics;
(5) Good swelling resistance and corrosion resistance to chemical solvents and drugs;
(6) Good flow processability, and products can be processed by injection molding, extrusion, blow molding and other methods;
(7) ) has excellent barrier properties;
(8) has high chemical activity, and the polar groups in it can react with monomers and polymers containing polar groups to form new polymer compounds.
In order to give PA6 stronger mechanical properties, various modifiers are often added, the most common of which is glass fiber; in order to give PA6 stronger impact resistance, elastic materials such as POE, SBR or EPDM and synthetic rubber are usually added. If there is no additive in the PA6 product, the plastic raw material has a shrinkage rate of 1%~1.5%, and a product with a shrinkage rate of 0.3% can be obtained after adding glass fiber. Among them, the hygroscopicity and crystallinity of materials are the main factors that determine the shrinkage rate of molding assembly, and there is also a functional relationship between process parameters such as plastic part design and wall thickness and actual shrinkage rate.
The drying treatment of PA6 for injection molding is easy to absorb water, so the drying treatment must be paid great attention to before actual processing. If the supplied material is wrapped with waterproof material, keep the container in a closed state. When the humidity is greater than 0.2%, you should choose hot air not lower than 80°C for 16 hours; if the material is exposed to the air for not less than 8 hours, then It should be vacuum dried at 105°C for more than 8 hours.
III. Production process of PA6
1. Two-stage polymerization method
The two-stage polymerization method is mainly divided into two stages of pre-polymerization and post-polymerization, and is generally suitable for the production of high-viscosity products such as industrial cord fabrics. The two-stage polymerization method mainly includes three methods, which are pre- and post-atmospheric pressure polymerization, pre-pressurized polymerization followed by reduced-pressure polymerization, pre-high pressure polymerization and post-atmospheric pressure polymerization. Among them, the reduced-pressure polymerization method involves relatively large investment and requires high cost; the pre-high pressure and post-atmospheric pressure are the second; the pre- and post-polymerization are both operated at normal pressure, which is low in cost and does not need to consume too much investment.
2. Normal pressure continuous polymerization method
Atmospheric pressure continuous polymerization method is applicable in the production of PA6 civil yarn, among which the production process of NOY company in Italy is the most representative. The characteristics of this method are continuous large-scale continuous polymerization for 20 hours under the condition of 260°C; slices are obtained in the hot water countercurrent stage; the residual oligomers are dried by nitrogen flow, and the monomers are extracted and recovered, and continuously evaporated and concentrated, and intermittent distillation is introduced at the same time. Concentration process. This method has outstanding continuous production performance, can obtain higher quality products, higher output, and does not occupy too much area in practical application, and is a typical civilian silk production process.
3. Batch hydrolysis polymerization method
The batch hydrolysis polymerization method uses a pressure-resistant polymerization kettle. The method is applicable to the production of multi-variety and small-batch engineering plastic grade slices. One-time feed, after the reaction (one-time discharge), use nitrogen pressure to discharge the pellets, extract and dry to obtain PA6. The batch polymerization process is divided into three stages: the first stage is hydrolysis ring-opening polycondensation; the second stage is vacuum polymerization; the third stage is equilibrium reaction.
Batch polymerization is suitable for the production of many varieties and small batches of products, and can produce products with different viscosities and copolymerized PA, but the consumption of raw materials is higher than that of continuous polymerization, the production cycle is long, and the repeatability of product quality is poor.
4. Twin-screw extrusion continuous polymerization process
The twin-screw extrusion continuous polymerization process is a new process developed in recent years. It adopts anionic catalytic polymerization, and caprolactam is dehydrated and activated and then continuously enters the twin-screw extruder. In twin-screw extrusion, the reaction material moves along the axial direction with the rotation of the screw, and its relative molecular weight increases continuously. The low molecular weight is drawn out of the twin-screw extruder by the vacuum system, and the polymer is cooled and diced, dried and packaged.
The process has the characteristics of short process and simple production process, and the unreacted monomer with low relative molecular weight can be directly recycled after being extracted from the reaction system, and the monomer content of the product is very low without extraction. The slices have low moisture and short drying time, which can greatly reduce energy consumption. At the same time, the control of the relative molecular weight of the product can be adjusted by the residence time of the material in the twin-screw extruder.
IV. Research on PA6 Modification
1. Enhanced modification
Due to the presence of hydrogen bonds in PA6 molecules, it will inevitably affect its flexibility and strength. With the increase of hydrogen bond density, the mechanical strength of PA6 will be increased accordingly. And the more carbon atoms exist, the flexible chain will be further extended, so that it has better toughness. Adding glass fibers can enhance the mechanical properties of PA6 composites. The regularity of tetragonal ZnO whiskers is extremely high. Based on this, the results of the research on its promotion of the reinforcement effect of cast PA show that when the whisker content is 5%, the cast PA composite has the highest tensile strength. The content will reduce the heat resistance and water absorption of the material. After the fly ash was treated with silane coupling agent, it was filled and modified into cast PA6 products, and the final products had better thermal stability, shrinkage rate and water absorption.
2. Flame retardant modification
The oxygen index of PA6 is 26.4, which is a flammable material. National laws and regulations clearly require the flame retardancy of polymer materials, so when PA6 is used in electrical related products, it is necessary to attach great importance to its flame retardant modification. Among the materials prepared by blending various metal hypophosphites and PA6, the flame retardancy of aluminum hypophosphite is relatively good. When its content is 18%, the loss on ignition of the material can reach 25, and UL94 can reach V-0 level. .
Melamine cyanurate (MCA) modified with red phosphorus can be used as a flame retardant for PA6. Red phosphorus can hinder the formation of large planar hydrogen bond networks between melamine and cyanuric acid, thereby refining MCA, and MCA can be charred under the action of red phosphorus, so MCA can play a flame-retardant role in the condensed phase and gas phase after modification. It is conducive to the improvement of PA6 flame retardancy. The limiting oxygen index (LOI) of the composite material can be improved by adding guanidine sulfamate into the PA6 matrix through the melt blending method. The vertical combustion test found that when the addition amount is 3%, the amount of droplet generation is higher than that of pure PA6. Significantly reduced, when its addition is not less than 5%, the UL94 grade of the material is raised to V-0 grade.
3. Toughening modification
The toughened resin or elastomer is added to the PA resin, and the toughened and modified PA can be obtained after blending and extrusion.
When the toughening agent is polarized SBS, the toughened blend system of polarized SBS and PA6 is obtained by mechanical melt blending method. When the amount of polarized SBS is increased, the notched impact strength of the system increases, and the flexibility of the material increases. Sex will also improve. Compared with PA6 and EPDM composite materials, the toughened blend system of maleic anhydride grafted EPDM rubber and PA6 has better rubber-plastic compatibility and higher toughness. When the amount of maleic anhydride grafted EPDM rubber is 15%, the blended material has a notched impact strength 9 times higher than that of PA6 material.
4. Filling modification
The economical and affordable fillers are added to the PA resin, and the filled and modified composite PA material can be obtained after blending and extruding. Using silicon carbide as thermal conductive filler, treating the filler surface with coupling agent KH560 and epoxy resin E51, and adopting twin-screw extrusion blending process, the thermal conductive PA composite material prepared has excellent performance. When the filling amount of thermally conductive filler, PA6 chain extension and surface treatment are changed, the crystallization, heat resistance, mechanical and thermal conductivity properties of the composite will also change accordingly.
The composite material product obtained after processing PA6 and organic montmorillonite by melt blending injection molding method has excellent friction and wear, heat resistance and mechanical properties. The filler is metal aluminum powder, the substrate is copolymerized PA6 and PA66, and the composite material can be prepared by melt blending. When the content of aluminum powder increases, the tensile strength of the composite increases first and then decreases, and the flexural modulus increases gradually, while the impact strength decreases. After PA6 is filled with fly ash microbeads, the hardness, impact and tensile strength of the material can be greatly improved, and the product can be endowed with better stability.
5. PA alloy
PA6 alloy belongs to multi-component system, most of which are composed of not less than two kinds of polymers, among which blended polymers, graft copolymers and block copolymers are widely used. The composite material prepared by blending PA6 and maleic anhydride grafted polypropylene (PP-g-MAH) has much lower water absorption than PA6, and has a much higher impact strength than PA6.
Mix low-density polyethylene (LDPE), maleic anhydride (MAH) and initiator dicumyl peroxide (DCP) in proportion to prepare grafted low-density polyethylene (LDPE-g-MAH), and then melt The blend of LDPE-g-MAH and PA6 can be obtained after the blending method combined with a small amount of PA6. Maintaining a fixed amount of DCP, when the amount of maleic anhydride is 1.0 parts, the blend with the best tensile strength can be obtained; when the amount of maleic anhydride is maintained at 1.0 parts, the change of the amount of DCP will not cause excessive changes in the properties of the blend When the amount of DCP is 0.6 phr, the blend with the best tensile strength can be obtained.
V. Concludes
Representative companies in PA6 polymerization technology include Switzerland’s Inventa, Italy’s NOY, Germany’s Kart Fischer and Zimmer. After efforts of actively learning, absorbing and introducing a large number of modern equipment (such as VK tube and other core technologies), China has greatly improved the PA6 production technology and process, and gradually approached the direction of international development.
China’s PA6 polymerization capacity maintains a rapid expansion trend, with a production capacity far exceeding that of PA66. At this stage, the research on the modification of PA6 is basically based on reinforcement, toughening, flame retardancy, filling, and antifouling (by introducing a strongly electronegative group on the PA6 macromolecular chain to shield its combination with acid dyes). , so as to achieve antifouling), etc. Although currently such modifications are basically carried out by blending special materials, the modification methods of extrusion and reaction are also applicable. With the further development of modern technology, nanomaterials can be introduced to modify PA6 to obtain modified PA6 materials with high hardness, high strength, high toughness, high temperature resistance, and electroplating, so as to effectively meet the needs of various fields.
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