The particle shape of calcium carbonate is mostly tetragonal, hexagonal, multi-faceted and irregular square particles. Its shape is very important for the fluidity and physical properties of products in plastic processing.
2 Particle size
The important point of filling modification technology is that the powder particles are evenly dispersed into the plastic matrix as much as possible, like large and small islands in the sea, which is called the island structure. Generally speaking, the smaller the particle size of the filler particles, the better the mechanical properties of the filling system if it can be dispersed evenly; but the smaller the particle size, the higher the processing cost, and the more difficult it is to achieve uniform dispersion. Therefore, it is very important to understand the particle size and distribution of powder particles and to choose them according to actual needs.
At present, there are many kinds of powder particle size and its distribution. Before there is no unified naming method and regulation, the plastic industry often uses the mesh number method, that is, the particle size of the powder material can pass through the mesh number of the sieve. In fact, the mesh number measured by this method refers to the maximum size of the largest particle in the three-dimensional direction in the powder particles of this specification.
In plastic products, it is required that the particle size distribution of calcium powder is as narrow as possible, that is, the minimum particle size and the maximum particle size are classified. According to the use requirements, products with particle size within a certain range are collected to ensure the dispersion, transparency, mechanical properties and oil absorption rate of the products.
3 Specific surface area
The surface roughness of the filler particles varies. That is, the surface area of particles of the same volume is not only related to the geometry of the particles (the smallest spherical surface area), but also to the roughness of the surface. The specific surface area is the surface area of the filler per unit mass, and its size is directly related to the affinity between the filler and the resin, the difficulty and cost of the surface activation treatment of the filler.
4 Surface free energy
The size of the surface free energy of the filler particles is related to the ease of dispersion of the filler in the matrix resin. When the specific surface area is constant, the surface free energy is large, and the particles are more likely to agglomerate with each other and more difficult to disperse. In the surface treatment of the filler, reducing its surface free energy is one of the main objectives.
The density of the filler is related to the packing state of the filler particles. Because the particles of light calcium carbonate are spindle-shaped, while the particles of heavy calcium carbonate are in the shape of broken stones, when piled up, there are gaps between the particles, and the volume of the former is significantly larger than that of the latter, so the apparent density of light calcium carbonate is less than that of heavy calcium carbonate, but this does not mean that light calcium is 'light' and heavy calcium is 'heavy', because the difference between them is very small in terms of individual particles, the former is 2.4 to 2.7g/cm3, and the latter is 2.7 to 2.9g/cm3. In the field of plastic filling modification, what really affects the overall density of the filling system is the density of the individual particles of the filler and their presence in the plastic matrix-whether they are condensed together, and whether there are gaps between the plastic molecules and the matrix.
6 Oil absorption value
The amount of plasticizer dioctyl ester (DOP) that can be absorbed per unit mass of filler is referred to as the oil absorption number. In a plastic product using a plasticizer, if the oil absorption value of the filler is high, the consumption of the plasticizer is increased. The oil absorption value of the filler is related to the particle size, distribution and surface structure of the filler. The oil absorption value of light calcium carbonate is often several times that of heavy calcium carbonate. Therefore, in the case of achieving the same effect on resin plasticization, the use of heavy calcium can reduce the amount of plasticizer. Generally, heavy calcium requires an oil absorption value of less than 35mL/100g.
The hardness of the filler particles itself has duality. On the one hand, the filler with high hardness can improve the wear resistance of the filled plastic material. On the other hand, due to the addition of the filler, especially the filler with high hardness, the filling system is easy to cause serious wear on the surface of the processing equipment and the mold contacted by the material during the processing. When the wear is serious, the economic loss caused by the use of the filler far exceeds the benefits, it will affect the application of this powder material in plastics.
Mohs hardness is a relative comparison of the ability to score between materials. Human fingernails have a Mohs hardness of 2 and can be scored on talc, but nothing can be done on calcite.
Of course, the hardness of different sizes of filler on the processing equipment wear is different, on the other hand, for a certain hardness of the filler, the metal surface of the processing equipment wear strength with the increase of filler particle size and rise, to a certain particle size after the wear strength tends to be stable. In addition, the difference in hardness between the two materials that are relatively ground is also related to the size of the wear strength. It is generally believed that when the metal strength is higher than 1.25 times the abrasive hardness, it is a low wear condition; when the metal strength is 0.8 to 1.25 times the abrasive hardness, it is a medium wear condition; when the metal strength is lower than 0.8 times the abrasive hardness, it is a high wear condition.
For example, the metal material usually used for the screw barrel and screw of the plastic extruder is 38CrMoAl alloy steel. After nitriding treatment, its Vickers hardness is 800-900, while the Vickers hardness of heavy calcium is about 140. Therefore, the plastic filled with calcium carbonate is processed by an extruder, although there is wear, but it is not particularly significant, at least it can be tolerated. However, the Vickers hardness of fly ash glass beads or quartz sand is above 1000. The wear of nitrided steel filled with plastic is extremely serious. After processing dozens of tons of materials, the nitrided layer of the screw does not exist (the nitrided layer is about 0.4mm thick). The ordinary 45 steel boronizing treatment, its Vickers hardness can reach about 2000, at this time, the same glass beads or quartz sand filling material on the screw wear is very slight, only equivalent to heavy calcium on nitrided steel wear.
The whiteness of the filler has a crucial influence on the color and appearance of the filled plastic materials and products. Generally, the higher the whiteness, the smaller the effect on the coloring of the filled plastic, only affecting the vividness of the color. Since there is no completely transparent filler, the filled plastic is often opaque. If the color whiteness of the filler is not high or other colors, it can only be made of black or dark plastic products.
The plastic material itself has a great difference in the refractive index of light, and the refractive index of most common plastics is between 1.50 and 1.60. When the refractive index of the powder filler is the same or similar to the refractive index of the plastic matrix, they are added to the matrix plastic after the light covering effect is small, on the contrary, the filling plastic on the light covering effect is strong.
For most minerals, the refractive index is more than one. Crystals with a cubic lattice structure and isotropic amorphous substances have the only refractive index. For example, salt is a typical equiaxed (cubic) crystal, while glass is a typical isotropic amorphous non-crystalline substance. Crystals such as calcite and quartz have two equal minor axes and are perpendicular to the third (major) axis. When the light propagates along the long axis, its propagation speed is unchanged, and when the light propagates along other directions, it is decomposed into two rays with different speeds, resulting in two refractive indexes. The two refractive indices of calcite are 1.658 and 1.486, and the two refractive indices of quartz are 1.553 and 1.554.
10 Absorption and reflection of light
Ultraviolet light can degrade the macromolecules of the polymer. Ultraviolet light has a wavelength in the range of 0.01 to 0.4 μm, and carbon black and graphite are used as fillers to protect the filled polymer from degradation induced by ultraviolet radiation because they absorb light in this wavelength range. Some substances can not only absorb ultraviolet light, but also convert shorter wavelength ultraviolet light into longer wavelength visible light by re-emitting. If it is used as a filler, it can not only avoid the destructive effect of ultraviolet light, but also increase the energy of visible light radiation.
Infrared rays are light waves in a wavelength range of 0.7 μm or more. Some fillers can absorb or reflect light waves in this wavelength range. The use of mica, kaolin, talcum powder and other fillers in agricultural greenhouse film can effectively reduce the transmittance of infrared rays, thereby significantly improving the thermal insulation effect of agricultural greenhouse film.
11 Electrical performance
Metals are excellent conductors of electricity, so the use of metal powders as fillers can affect the electrical properties of filled plastics. However, as long as the filling amount is not large, the resin matrix wraps each metal filler particle, and its electrical properties will not change abruptly. Only when the amount of filler increases to the extent that the particles of the metal filler come into contact with each other, the electrical properties of the filled plastic will change abruptly and the volume resistivity will decrease significantly.
Fillers made of minerals are electrical insulators and, in theory, have no effect on the electrical properties of the plastic matrix. It should be noted that due to the influence of the surrounding environment, the particle surface of the filler will condense a layer of water molecules. Depending on the surface properties of the filler, the form and strength of the combination of the water molecules and the filler surface are different. Therefore, after the filler is dispersed into the resin matrix, the electrical properties exhibited may be different from the electrical properties reflected when it exists alone. In addition, the filler is likely to be charged with static electricity due to the breakage of the valence bond during the pulverization and grinding process to form adsorbed aggregates, which is more likely to occur when a fine filler with extremely high fineness is produced.
Calcium carbonate itself is not easy to absorb water, does not contain structural water and crystal water, but in the process of ore collection, storage, processing and storage, because the powder particles are very small, it is easy to absorb water. Plastic in the use of moisture content requirements are extremely high, powder standard requirements ≤ 0.5%, the actual application, the powder water content should be ≤ 0.3%, the smaller the water content of plastic products, the smaller the impact.