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Calcium carbonate has been used as an inorganic filler in plastic filling for many years. In the past, calcium carbonate was generally used as a filler for the main purpose of reducing costs, and achieved good results. In recent years, with the widespread use in production and a large number of research findings, filling a large amount of calcium carbonate can not significantly reduce the performance of the product, and even greatly improve some aspects, such as mechanical properties, thermal properties, etc. 1 Overview of calcium carbonate There are two types of calcium carbonate used as fillers in plastics: heavy calcium (referred to as heavy calcium) and light (referred to as light calcium). Due to different preparation methods, light calcium has a large accumulation volume and appears light. In fact, there is little difference in density between the two. (1) Light calcium carbonate. Generally speaking, light calcium carbonate refers to ordinary products that meet the national standard GB4794-84. The density of light calcium carbonate is 2.4~2.7g/cm3, its long diameter is 5~12μm, its short diameter is 1~3μm, and its average particle size is 2~3μm. The dominant method for producing light calcium in industry is the carbonization method, namely: CaCO3 limestone-△-CaO quicklimestone-H2O-Ca(OH)2 slaked lime-CO2-CaCO3 light calcium (2) heavy calcium carbonate. It can be made by grinding and classifying natural calcium carbonate minerals such as calcite, marble, and chalk. The heavy calcium carbonate used in plastics today mostly uses calcite as a raw material. The physical properties of calcite: density 2.60~2.75g/cm3, hardness (Mohs) 3, solubility (18℃) 0.0013g/100g water, decomposition temperature 900℃. Heavy calcium carbonate used to be called single fly powder (200 mesh), double fly powder (320 mesh), four fly powder (400 mesh) and calcite powder. There is currently no heavy calcium suitable for plastic filling. * * standards or industry standards, but each company has its own product enterprise standards or control indicators. my country has abundant calcium carbonate resources, which are distributed almost all over the country, with Sichuan and Guangxi having the largest reserves. According to statistics, there are currently more than 500 companies producing calcium carbonate in my country. In order to meet the market demand for calcium carbonate in the plastics, papermaking and coating industries, many new equipment and new production lines have been introduced or independently developed in recent years to produce micro, ultrafine and nanoscale calcium carbonate. my country currently has 16 nano-calcium carbonate production lines with an annual output of more than 3,000 tons. In actual use, calcium carbonate is generally not added directly to plastics. In order for calcium carbonate to be evenly dispersed in the plastic and optimize its performance, the calcium carbonate must first be surface activated. According to the molding process and performance requirements of the final plastic product, calcium carbonate of a certain particle size is selected, activated with coupling agents, dispersants, lubricants and other additives, then a certain amount of carrier resin is added and mixed evenly, and then extruded and granulated with a twin-screw extruder to obtain calcium carbonate film masterbatch. Under normal circumstances, the calcium carbonate content in the masterbatch is 80wt%, the total content of various additives is about 5wt%, and the carrier resin is 15wt%. 2 The addition of calcium carbonate can * * Reduce plastic costs. Calcium carbonate is extremely abundant in reserves and very simple to prepare, so the price is very cheap. For example, the prices of calcium carbonate of several particle sizes most commonly used in plastic fillers (special white heavy calcium carbonate, whiteness 95, calcium content 99): (325~400 mesh 120 yuan) - (600 mesh 280 yuan) - (800 mesh 320 yuan) - (1250 mesh 650 yuan)/t, factory delivery. After surface activation treatment, the price is also around 2,000 yuan/t. Plastic particles (polyethylene) are relatively expensive. In terms of special pipe materials, the prices of domestic and foreign polyethylene (with carbon black) are above 9,000 yuan/t. There is a big price difference between the two. The more calcium carbonate is added to the plastic, the lower the cost will be. Of course, calcium carbonate cannot be added indefinitely. Considering the toughness of plastic products, the filling amount of calcium carbonate is generally controlled within 50wt% (data provided by calcium carbonate filler manufacturers). For the production of plastic and steel-plastic composite pipes, plastic is the main raw material. * * Reducing the cost of plastics will undoubtedly greatly reduce production costs and help increase profits. 3 Modification effect of calcium carbonate Heavy calcium carbonate can increase the volume of plastic products, reduce costs, increase hardness and stiffness, reduce the shrinkage of plastic products, and improve dimensional stability; improve the processing performance of plastics, improve their heat resistance, improve the astigmatism, scratch resistance, and smoothness of plastics; at the same time, it has obvious effects on the toughening effect of notch impact strength and viscosity flow during the mixing process. 3.1 Mechanical properties After filling with calcium carbonate, due to the high hardness of calcium carbonate, the hardness and stiffness of plastic products will be increased, and the mechanical properties will be enhanced. The tensile strength and flexural strength of the product are improved, and the elastic modulus of plastic products is significantly increased. Compared with FRP, its tensile strength, flexural strength and flexural modulus are roughly the same as FRP. The thermal deformation temperature is generally higher than FRP. The only thing inferior to FRP is that its notch impact strength is lower, but this shortcoming can be overcome by adding a small amount of short glass fibers. For pipes, filling calcium carbonate can improve several of its indicators, such as tensile strength, steel ball indentation strength, notch impact strength, viscosity flow, heat resistance, etc.; but at the same time, it will reduce several of its toughness indicators, such as fracture elongation, rapid cracking, simply supported beam impact strength, etc. 3.2 Thermal properties After adding fillers, due to the good thermal stability of calcium carbonate, the thermal expansion coefficient and shrinkage rate of the product can be reduced in all aspects. Unlike glass fiber reinforced thermoplastics, which have different shrinkage rates in different aspects, adding fillers can make the warpage and curvature of the product smaller. This is the biggest feature compared with fiber fillers. The thermal deformation temperature of the product increases with the increase in fillers. 3.3 Radioactive fillers have a certain ability to absorb rays and can generally absorb 30% to 80% of incident ultraviolet rays to prevent the aging of plastic products. 3.4 The special modification effect of ultrafine calcium carbonate. The particle size of calcium carbonate can also be made into various sizes. Calcium carbonate with a particle size of 0.1 to 1 μm is called micro, calcium carbonate with a particle size of 0.1 to 0.02 μm is called ultrafine, and particle size ≤ 0.02 μm is called ultrafine. Plastics are filled with ultra-fine grade or finer calcium carbonate, which has special effects in changing the performance of the product. Rigidity and toughness are two important performance indicators of plastic products. How to ensure that plastic products have good rigidity and toughness at the same time has been one of the important topics in materials science research for a long time. In order to improve the toughness of plastic products, the method of adding rubber or elastomer is generally used to achieve the purpose of toughening and modification, but it damages the valuable rigidity of the material, and the processing performance and heat resistance of the material will be reduced. In the 1990s, people discovered through a large number of experiments that after filling a larger amount of ultra-fine calcium carbonate particles in plastic, not only the rigidity of the plastic was not damaged, but the toughness was also greatly improved, up to 2 to 3 times. This changes the situation in the past that filling modified plastics must be at the expense of certain mechanical properties, and the mechanical properties of modified plastics decrease as the filler filling amount increases. In the past, coupling agents such as aluminate, titanate, silane and acid phosphite were generally used for surface activation treatment of inorganic fillers. After being mixed with carrier resin and lubricant, surface activation treatment was performed in the molten state. In addition to coupling agents, the new surface activation treatment process also selects to add a certain amount of plasticizers, compatibilizers, dispersants, etc. based on the molding process and performance requirements of the final plastic product, and cold coating is performed in a high mixer at room temperature. 4 Actual use results Anyuan Pipeline Industrial Co., Ltd. is a large enterprise that produces steel-framed polyethylene plastic composite pipes. It has an annual output of 300km of pipes of various specifications, which requires about 3000t of polyethylene plastic raw materials. In order to save production costs, the company tried adding different proportions of fillers (granulated ordinary heavy calcium carbonate) to polyethylene plastic to produce steel skeleton composite pipes. After various tests on the trial production products, except for the pipe's elongation at break, which was greatly reduced, other properties still met the product requirements, and its steel ball indentation strength and heat resistance were greatly improved. Through trial production, it was found that the maximum addition ratio is 40wt%. At this time, the product looks good and the main performance is basically not affected. This can save about 1,200 yuan per ton of plastic. If fully used, the annual production cost can be saved by 3.6 million yuan, which is a very considerable benefit.