Powder materials are usually used as fillers in the form of particles. The shape of the particles is not very regular, but for the performance of plastics, the geometric shape of the filler particles has an important influence on the physical and mechanical properties of the filled system. Therefore, the particle shape of the powder material is the first thing to consider when using it.

1. Geometric Characteristics

　　Powder materials are usually used as fillers in the form of particles. The shape of the particles is not very regular, but for the performance of plastics, the geometric shape of the filler particles has an important impact on the physical and mechanical properties of the filled system. Therefore, the particle shape of the powder material is the first thing to consider when using it.

　　For flaky particles, the concept of aspect ratio is often used, which is the ratio of the planar size (longitudinal or transverse) to the thickness of the flaky particle; for fibrous particles, the concept of aspect ratio is often used, which is the ratio of the length to the diameter of the fibrous particle.

　　 Guangxi Longhua The particle shapes of calcium carbonate are mostly square, hexagonal, multifaceted, and irregular square particles. Its shape is very important in plastic processing and affects the fluidity and physical properties of the products.

　　2. Particle Size

　　An important point of filler modification technology is to uniformly disperse the powder particles into the plastic matrix as much as possible, one by one, like islands of various sizes in the sea, which is called the sea-island structure. Generally speaking, the smaller the particle size of the filler, if it can be dispersed uniformly, the better the mechanical properties of the filled system; however, 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 the powder particles and select them according to actual needs.

　　Currently, there are many ways to describe the particle size and distribution of powder particles. Before there is a unified naming method and regulation, the plastics industry often uses the mesh number method, that is, the particle size is expressed by the number of meshes of the sieve that the powder particles can pass through. In fact, the mesh number measured by this method refers to the largest size in the three-dimensional direction of the largest particle in this specification of powder particles.

　　In plastic products, it is required that the particle size distribution of calcium powder is as narrow as possible, that is, the minimum and maximum particle sizes are graded, and products with particle sizes within a certain range are collected according to usage requirements to ensure the dispersibility, transparency, mechanical properties, and oil absorption rate of the products.

　　3. Specific Surface Area

　　The surface roughness of filler particles varies. That is, for particles of the same volume, the surface area is not only related to the geometric shape of the particles (the spherical surface area is the smallest), but also to the roughness of their surface. The specific surface area is the surface area of a unit mass of filler, and its size is directly related to the affinity between the filler and the resin, and the difficulty and cost of surface activation treatment of the filler.

　　4. Surface Free Energy

　　The magnitude of the surface free energy of filler particles is related to the ease of dispersion of the filler in the matrix resin. When the specific surface area is constant, the larger the surface free energy, the easier it is for the particles to agglomerate and the more difficult it is to disperse. When treating the surface of the filler, reducing its surface free energy is one of the main goals.

　　5. Density

　　The density of the filler is related to the stacking state of the filler particles. Because the particles of light calcium carbonate are spindle-shaped, while the particles of heavy calcium carbonate are broken stone-shaped, there are gaps between the particles during stacking, and the volume of the former is significantly larger than the latter, so the apparent density of light calcium carbonate is smaller 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 for individual particles, the former is 2.4～2.7g/cm3, and the latter is 2.7～2.9g/cm3. In the field of plastic filling modification, what really affects the overall density of the filled system is the density of the individual filler particles and their existence in the plastic matrix—whether they are agglomerated together, and whether there are gaps between them and the matrix plastic molecules, etc.

　　6. Oil Absorption

　　The amount of plasticizer dioctyl phthalate (DOP) that a unit mass of filler can absorb is called oil absorption. In plastic products using plasticizers, if the filler has a high oil absorption value, it will increase the consumption of plasticizers. The oil absorption value of the filler is related to the particle size, distribution, and surface structure of the filler particles. The oil absorption value of light calcium carbonate is often several times that of heavy calcium carbonate. Therefore, under the condition of achieving the same plasticizing effect on the resin, the use of heavy calcium can reduce the amount of plasticizer. Generally, heavy calcium requires an oil absorption value of less than 35mL/100g.

　　7. Hardness

　　The hardness of the filler particles themselves has duality. On the one hand, fillers with high hardness can improve the wear resistance of the filled plastic material. On the other hand, due to the addition of fillers, especially fillers with high hardness, the filled system is easy to cause serious wear and tear on the surface of the processing equipment and molds that the material comes into contact with during processing. When this wear is serious, the economic losses far exceed the benefits brought by the use of fillers, which will affect the application of this powder material in plastics.

　　Mohs hardness is a relative comparison of the scratching ability between materials. The Mohs hardness of a person's fingernail is 2, which can scratch talc, but it is powerless on calcite.

　　Of course, the wear of processing equipment is different for fillers with different hardness. On the other hand, for fillers of a certain hardness, the wear intensity of the metal surface of the processing equipment increases with the increase of filler particle size, and after a certain particle size, its wear intensity tends to be stable. In addition, the difference in hardness between the two relatively ground materials is also related to the magnitude of the wear intensity. It is generally believed that when the metal strength is higher than 1.25 times the abrasive hardness, it belongs to low wear; when the metal strength is 0.8～1.25 times the abrasive hardness, it belongs to medium wear; when the metal strength is lower than 0.8 times the abrasive hardness, it belongs to high wear.

　　For example, the metal material commonly used for the barrel and screw of plastic extruders is 38CrMoAl alloy steel, which is nitrided, and its Vickers hardness is 800～900, while the Vickers hardness of heavy calcium is about 140. Therefore, the extrusion machine processing of plastics filled with calcium carbonate, although there is wear, is not particularly significant, at least it is tolerable; while fly ash glass microspheres or quartz sand, their Vickers hardness is above 1000, and these materials filled with plastics cause extremely serious wear on nitrided steel. After processing tens of tons of materials, the nitriding layer of the screw no longer exists (the nitriding layer is about 0.4mm thick). If ordinary 45# steel is subjected to boronizing treatment, its Vickers hardness can reach about 2000. At this time, the wear of the screw caused by the same glass microspheres or quartz sand filled materials is very slight, only equivalent to the wear of heavy calcium on nitrided steel.

　　8. Whiteness

　　The whiteness of fillers has a crucial impact on the color and appearance of filled plastic materials and products. Generally, the higher the whiteness, the less the impact on the coloring of the filled plastic, only affecting the vibrancy of the color. Since there are currently no completely transparent fillers, filled plastics are often opaque. If the filler's whiteness is low or has other colors, only black or dark-colored plastic products can be made.

　　9. Refractive Index

　　Plastic materials themselves have a large difference in refractive index; the refractive index of most general-purpose plastics is between 1.50 and 1.60. When the refractive index of the powder filler is the same as or close to that of the plastic matrix, their addition to the matrix plastic has a small impact on light occlusion. Conversely, the filled plastic has a strong light occlusion effect.

　　For most minerals, their refractive index is more than one. Only crystals with a cubic lattice structure and isotropic amorphous substances have a unique refractive index. For example, salt is a typical isometric (cubic) crystal, while glass is a typical isotropic amorphous non-crystalline substance. Crystals such as calcite and quartz have two equal short axes perpendicular to the third axis (long axis). When light travels along the long axis, its propagation speed is unchanged. When light travels in other directions, it is decomposed into two light rays with different speeds, producing two refractive indices. The two refractive indices of calcite are 1.658 and 1.486, respectively, and those of quartz are 1.553 and 1.554, respectively.

　　10. Light Absorption and Reflection

　　Ultraviolet light can cause the degradation of polymer macromolecules. The wavelength range of ultraviolet light is 0.01-0.4μm. Carbon black and graphite, when used as fillers, can absorb light waves in this wavelength range, thus protecting the filled polymer from degradation caused by ultraviolet irradiation. Some substances can not only absorb ultraviolet light but also convert ultraviolet light with shorter wavelengths into visible light with longer wavelengths through re-emission. If 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 light is a light wave with a wavelength range above 0.7μm. Some fillers can absorb or reflect light waves in this wavelength range. Using fillers such as mica, kaolin, and talc in agricultural greenhouse films can effectively reduce the transmittance of infrared light, thereby significantly improving the heat preservation effect of agricultural greenhouse films.

　　11. Electrical Properties

　　Metals are excellent conductors of electricity; therefore, metal powders used as fillers can affect the electrical properties of filled plastics. However, as long as the filling amount is not large and the resin matrix wraps each metal filler particle, the change in electrical properties will not be abrupt. Only when the filler amount increases to the point where the metal filler particles come into contact with each other will the electrical properties of the filled plastic change abruptly, and the volume resistivity will decrease significantly.

　　Fillers made of minerals are all electrical insulators; theoretically, they will not affect the electrical properties of the plastic matrix. It should be noted that due to the influence of the surrounding environment, a layer of water molecules will condense on the surface of the filler particles. Depending on the properties of the filler surface, the form and strength of the combination of this layer of water molecules with the filler surface are different. Therefore, after the filler is dispersed into the resin matrix, the electrical properties it exhibits may be different from those reflected when it exists alone. In addition, during the crushing and grinding process, due to the breakage of valence bonds, it is very likely to carry static electricity, forming adsorbed aggregates, which is more likely to occur when making ultra-fine fillers with extremely high fineness.

　　12. Moisture

　　Calcium carbonate itself is not easy to absorb water and does not contain structural water or crystalline water, but in the usual mineral collection, storage, processing, and warehousing processes, due to the extremely small powder particles, it is easy to absorb moisture. Plastics have extremely high requirements for moisture content during use. The powder standard requires ≤0.5%, and in actual application, the powder moisture content should be ≤0.3%; the smaller the moisture content, the less the impact on plastic products.