Relevant technical characteristics of calcium carbonate in plastic applications

1. Geometric features

Usually powder materials are used as fillers in granular form. The shape of the particles is not very regular, but for the performance of the plastic, the geometry of the filler particles has an important influence on the physical and mechanical properties of the filling system, so the particle shape of the powder material is the first thing to pay attention to when using it.

For flaky particles, the concept of aspect ratio is often used, that is, the ratio of the plane size (longitudinal or transverse) of the flaky particles to the thickness; for fibrous particles, the concept of aspect ratio is often used, that is, the length of the fibrous particle is equal to ratio of diameters.

Most of the particle shapes of calcium carbonate are tetragonal, hexagonal, multi-cube and irregular square particles, and its shape is very important for the fluidity and the physical properties of products in plastic processing.

2. Particle size

The important point of the filling modification technology is to disperse the powder particles into the plastic matrix evenly and one by one as much as possible, like the 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 they can be dispersed uniformly; 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 select them according to actual needs.

At present, there are many expressions for the particle size and distribution of powder particles. Before there is no unified naming method and regulations, the plastic industry often uses the mesh number method, that is, how many objects can the particles of the powder material pass through? The mesh number of the sieve indicates its particle size. In fact, the mesh number measured by this method refers to the largest size in the three-dimensional direction of the largest particle in the powder particles of this specification.

In plastic products, the narrower the particle size distribution of calcium powder is, the better, that is, the smallest particle size and the largest particle size are classified. Mechanical properties and oil absorption.

3. Specific surface area

The surface roughness of filler particles varies. That is, the surface area of ​​particles of the same volume is not only related to the geometric shape of the particle (the spherical surface area is the smallest), but also to the roughness of its 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 filler surface activation treatment.

4. Surface free energy

The surface free energy of filler particles is related to the ease of dispersion of fillers in the matrix resin. When the specific surface area is constant, the surface free energy is large, the particles are easier to agglomerate with each other, and the more difficult to disperse. In the surface treatment of fillers, reducing its surface free energy is one of the main goals.

5. Density

The density of the filler is related to the packing state of the filler particles. Since the particles of light calcium carbonate are in the form of spindles, while the particles of heavy calcium carbonate are in the shape of broken stones, there are gaps between the particles when they are stacked, and the volume of the former is significantly larger than that of the latter. The apparent density is lower 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 their single particles, the former is 2.4 ~ 2.7g/cm3, the latter Which is 2.7 ~ 2.9g/cm3. In the field of plastic filling and modification, what really affects the overall density of the filling system is the density of individual filler particles and their existing forms in the plastic matrix—whether they are agglomerated together, and whether there are voids between them and the matrix plastic molecules.

6. Oil absorption value

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

7. Hardness

The hardness of the filler particles itself has a dual nature. On the one hand, the filler with high hardness can improve the wear resistance of the filled plastic material. The contacting processing equipment and the surface of the mold cause serious wear, and when the wear is serious, the economic loss caused by the use of fillers far exceeds 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 scoring ability between materials. Human fingernails have a Mohs hardness of 2 and can scratch talc, but not calcite.

Of course, fillers with different hardnesses have different wear on processing equipment. On the other hand, for fillers with a certain hardness, the wear intensity of the metal surface of the processing equipment increases with the increase of the particle size of the filler. After a certain particle size, the wear and tear strength tends to stabilize. In addition, the difference in the hardness of the two relatively ground materials is also related to the wear strength. 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 to 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 Happening.

For example, the metal material usually used for the barrel and screw of the plastic extruder is 38CrMoAl alloy steel. After nitriding, 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 and tear, it is not particularly significant, and at least tolerable; and fly ash glass beads or quartz sand, whose Vickers hardness is above 1000, these materials are filled with The wear of plastic on nitrided steel 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). Boronizing the ordinary No. 45 steel, its Vickers hardness can reach about 2000. At this time, the same glass beads or quartz sand filled materials have very slight wear on the screw, which is only equivalent to heavy calcium to nitriding Wear of steel.

8. Whiteness

The whiteness of the filler has a crucial influence on the color and even 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 the vividness of the color. Since there is no fully transparent filler at present, the filled plastic is often opaque. If the color of the filler is not high in whiteness or has other colors, it can only be used for black or dark plastic products.

9. Refractive index

The refractive index of plastic material itself to light is very different, and the refractive index of most general-purpose plastics is 1.50 to 1.60. When the refractive index of the powder filler is the same or similar to that of the plastic matrix, they have little effect on the light-shielding property after they are added to the matrix plastic.

For most minerals there is more than one refractive index. Only crystals with cubic lattice structure and isotropic amorphous substances have a unique refractive index. For example, table salt is a typical equiaxed (cubic) crystal, while glass is a typical isotropic amorphous amorphous substance. Crystals such as calcite and quartz have two equal minor axes and are perpendicular to the third axis (the major axis). When the light propagates along the long axis, its propagation speed does not change, and when the light propagates in other directions, it is decomposed into two light rays with different speeds, resulting in two refractive indices. The two refractive indices of calcite are 1.658 and 1.486, respectively, and the two refractive indices of quartz are 1.553 and 1.554, respectively.

10. Absorption and reflection of light

Ultraviolet light can degrade polymer macromolecules. The wavelength range of UV light is 0.01 to 0.4 μm. Carbon black and graphite are used as fillers. Since they can absorb light waves in this wavelength range, they can protect the filled polymer from degradation caused by UV radiation. Some substances can not only absorb ultraviolet light, but also convert ultraviolet light with a shorter wavelength into visible light with a longer wavelength by re-emitting light. If it is used as a filler, it can not only avoid the damaging effect of ultraviolet light, but also increase the energy of visible light radiation. .

Infrared rays are light waves in the wavelength range of 0.7 μm or more. Some fillers can absorb or reflect light in this wavelength range. The use of mica, kaolin, talc and other fillers in the agricultural greenhouse film can effectively reduce the transmittance of infrared rays, thereby significantly improving the thermal insulation effect of the agricultural greenhouse film.

11. Electrical properties

Metals are good 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 particle of the metal filler, and the change of its electrical properties will not change abruptly. Only when the amount of filler increases and the particles of the metal filler reach the level of contact with each other, the electrical properties of the filled plastic will change abruptly, and the volume resistivity will drop significantly.

Fillers made of minerals are electrical insulators, and 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. After being added to the resin matrix, the electrical properties exhibited may be different from those reflected when they exist alone. In addition, in the process of pulverization and grinding, due to the breaking of valence bonds, the filler is likely to be electrostatically charged to form adsorbed aggregates, which is more likely to occur when making extremely fine fillers.

12. Moisture

Calcium carbonate itself is not easy to absorb water and does not contain structural water and crystal water, but in the process of ore collection, storage, processing and storage, the powder particles are extremely small and easy to absorb water. Plastics have extremely high requirements for moisture content in use. The powder standard requires ≤0.5%. In practical application, the powder moisture content should be ≤0.3%. The smaller the moisture content, the less the impact on plastic products.