Calcium carbonate is widely used in plastics, which can not only reduce costs, but also improve the performance of some aspects of plastic materials. For example, activated light calcium carbonate can improve the impact toughness of PVC plastics and achieve a certain fineness. After interface regulation treatment, heavy calcium can significantly improve the impact toughness of polyethylene plastics and polypropylene plastics, and polypropylene woven bags with ordinary fineness heavy calcium carbonate have improved surface slip resistance and printability.
1. Calcium carbonate requirements for plastic films
At present, 1250 mesh heavy calcium is used in polyethylene plastic film. 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 the plastic film is concerned that the maximum particle size cannot exceed 10μm, because there are many film products (such as mulch), and the thickness itself is less than 10μm, even if the large particles only occupy 1/10,000, its absolute number (number of particles) will also be very large, and each large particle may cause a quality accident of the plastic film.
It must be noted that it is not that heavy calcium thicker than 1250 mesh cannot be used, nor does it mean that the finer than 1250 mesh is 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 with the same filling amount.
Considering that the finer the heavy calcium, the higher the price, and the technical conditions that mainly rely on the high-speed mixer for the surface treatment of the heavy calcium, the heavy calcium used for polyethylene plastic film is preferably 1250 mesh, and the fineness is too fine. and decentralization problems that are difficult to solve.
Can light calcium be used in polyethylene plastic film? Generally no one uses light calcium, but it does not mean that it cannot be used. Ordinary light calcium is mostly agglomerates of particles of several microns, 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 the PE film.
Nano-calcium carbonate is essentially light calcium carbonate, and there is also the problem of particle agglomeration, and because the price is much higher than ordinary light calcium, so far no one cares.
2. Technical points of calcium carbonate masterbatch production process
(1) Reunion problem
In order to uniformly disperse the calcium carbonate particles in the filler masterbatch into the matrix plastic, the calcium carbonate particles must be completely coated first. If the calcium carbonate has agglomerated during the production of the filler masterbatch, the agglomerated particles cannot be opened in the extruder of the blown film, which is bound to form numerous white spots on the film. Even white hard particles may form “clouds”.
To solve the problem of agglomeration of calcium carbonate particles, the first is to prevent excessive friction during surface treatment. Once static electricity is generated due to friction, it is very easy to agglomerate. Secondly, the amount of surface treatment agent should be sufficient. When the particle surface is changed to be lipophilic by the action of the coupling agent, its surface energy is greatly reduced, and it is not easy to agglomerate with each other.
(2) Filling ratio problem
The higher the proportion of calcium carbonate in the filler masterbatch, the lower the raw material cost of the product and the more competitive the market. Due to the small particle size of calcium carbonate particles, the large number of particles, and the large total surface area, more carrier resin is required to coat it, not only the melt viscosity of the carrier resin is required to be small, but also a sufficient amount. In order to obtain a film-grade filler 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 problem
There are different opinions on what form the material in the high-speed mixer should be stirred into. Some people have tried to mix it into a complete paste and then discharge it or just mechanically mix it evenly, and the particles of the carrier resin still keep the original particle shape and then discharge.
Our experience is that the best carrier resin has begun to melt, and it is fully contacted with the surface-treated heavy calcium to form a dough-like shape but not into a paste. At this time, the material is discharged immediately. If it becomes a paste, it will be very difficult to discharge the material, and the time delay will reduce the production efficiency. If the material remains in the state of granules and powder, not only the composition cannot be uniform, but also when the material is fed into the twin-screw extruder, the granulated powder will be regenerated again. Separation will cause some products to have more heavy calcium and some resins, which will cause uneven material when blowing the film, which is prone to “clouds” or white spots.
(4) Extrusion equipment
The extruder used for material mixing and granulation is also very important. The co-rotating parallel twin-screw extruder is far superior to the single-screw extruder with a mixing section. The advantages of mixing and granulating equipment can make up for the lack of surface treatment of calcium carbonate in the high-speed mixer and initial mixing with the carrier resin. Therefore, the selection of artificial extrusion mixing and granulating equipment and the selection and arrangement of screw sleeves are also very important. of.
(5) Granulation problem
In the granulation part, the operation mode of water cooling of the strip should be avoided as much as possible, because of the level of manual operation and the sense of 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 hot 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. The main reason is that it is easy to mix with the particles of the matrix resin when blowing the film. Rapid melting in a short period of time facilitates intermixing 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 do a good job of the air-cooled die face eagerly cutting the head, the temperature is not easy to control, the particles cannot be cooled down, and it is easy to stick. At this time, it is appropriate to use the conveyor belt cooling method, and this method is suitable for the production of various filler masterbatches or modified special materials, but requires a longer distance for cooling and requires a larger site.