Calcium carbonate is widely used in plastics. It can not only reduce costs, but also improve certain aspects of the performance of plastic materials. For example, activated lightweight calcium carbonate can improve the impact toughness of polyvinyl chloride plastics to a certain fineness. Heavy calcium can significantly improve the impact toughness of polyethylene plastics and polypropylene plastics through interface control treatment, and the surface slip resistance and printability of polypropylene woven bags with ordinary fine heavy calcium carbonate can be improved.
1. Requirements for calcium carbonate used in plastic films
At present, 1250 mesh heavy calcium is used in polyethylene plastic films. The 1250 mesh mentioned here means that the maximum particle size does not exceed 10 μm. Some companies use d97 as the number of microns to indicate the particle size of heavy calcium, but plastic films are concerned that the maximum particle size cannot exceed 10 μm, because there are many film products (such as mulch films) whose thickness is less than 10 μm, even if large particles only account for One in 10,000, the absolute number (number of particles) will be very large, and each large particle may cause a quality accident of the plastic film.
It must be noted that it does not mean that heavy calcium thicker than 1250 mesh cannot be used, nor does it mean that finer than 1250 mesh is so bad. In the case of good dispersion, the smaller the particle size of heavy calcium, the better the mechanical properties and appearance of the plastic film will be at the same filling amount.
Considering that the finer the heavy calcium, the higher the price, and the current technical conditions that mainly rely on high-speed mixers for surface treatment of heavy calcium, the heavy calcium used for polyethylene plastic films is preferably 1250 mesh. If the fineness is too fine, surface treatment will occur. and dispersion problems that are difficult to solve.
Can light calcium be used in polyethylene plastic film? So far, no one has used light calcium, but it does not mean that it cannot be used. Ordinary light calcium is mostly an agglomeration of several micron particles, and it is more difficult to disperse them. In addition, the crystal form of light calcium particles is very different from that of calcite, and the difference in refractive index has a more significant impact on the light transmittance of PE films.
Nano calcium carbonate is essentially light calcium carbonate, which also has the problem of particle agglomeration, and because the price is much higher than ordinary light calcium, no one is paying attention to it so far.
2. Technical key points of calcium carbonate masterbatch production process
(1) Reunion issue
In order for the calcium carbonate particles in the filling masterbatch to be evenly dispersed into the matrix plastic, the calcium carbonate particles must be completely coated first. If the calcium carbonate has been agglomerated during the production of filled masterbatch, the agglomerated particles cannot be opened in the extruder for blown film, and many white spots will inevitably form on the film. Even white hard particles may form “clouds”.
To solve the problem of agglomeration of calcium carbonate particles, we must first prevent excessive friction during surface treatment. Once static electricity is generated due to friction, agglomeration will easily occur. Secondly, the amount of surface treatment agent must be sufficient. When the particle surface is changed to lipophilicity by the coupling agent, its surface energy is greatly reduced, and it is not easy for them to agglomerate with each other.
(2) Filling ratio problem
The higher the proportion of calcium carbonate in the filling masterbatch, the lower the raw material cost of the product and the more competitive it is in the market. Since the calcium carbonate particles have a small particle size, a large number of particles, and a large total surface area, more carrier resin is required to cover them. Not only is the melt viscosity of the carrier resin required to be small, but also a sufficient amount is required. In order to obtain a film-grade filling masterbatch with good performance, the weight percentage of calcium carbonate is generally not more than 80%, and the weight percentage of carrier resin is generally not less than 13%.
(3) Material form issues
There are different opinions on what form the materials in the high mixer should be mixed into. Someone once experimented with mixing it into a complete paste and then discharging the material, and just mechanically mixing it evenly, and then discharging the carrier resin particles while still maintaining the original particle shape. There is no obvious difference in the application effect of the filled masterbatch made in the film in the two extreme cases.
(4) Extrusion equipment
The extruder used for mixing and granulating materials is also very important. A co-rotating parallel twin-screw extruder is far superior to a single-screw extruder with a mixing section. The advantages of mixing and granulating equipment can make up for the deficiencies in surface treatment of calcium carbonate in high mixers and initial mixing with carrier resin. Therefore, the selection of manual extrusion mixing and granulating equipment and the selection and arrangement of thread sleeves are also crucial. of.
(5) Granulation problem
In the granulation part, water-cooling operation of strips should be avoided as much as possible. Because of the problem of manual operation level and responsibility, once the material contains water, it will bring major hidden dangers to the blown film. If polyethylene resin is used as the carrier resin, it is appropriate to use the air-cooled die surface cutting process.
The shape and size of the particles are very particular. Generally, a disc with a diameter of 3 to 5 mm is suitable, and its thickness is about 1 mm. This is mainly because it is easy to mix with the particles of the matrix resin when blowing the film. It melts quickly in a short period of time to facilitate blending with the matrix resin material.
When the diameter of the extruder is larger (for example, the diameter is above Φ72mm) and the output is higher than 300kg/h, it is difficult to make a cutting head with an air-cooled die surface, the temperature is difficult to control, and the particles cannot be cooled down and are prone to adhesion. At this time, it is appropriate to use the conveyor belt cooling method, and this method is suitable for the production of various filling masterbatch or modified special materials, but it requires a longer distance for cooling and requires a larger site.