Views: 2 Author: Site Editor Publish Time: 2024-08-26 Origin: Site
1. Calcium content
Calcium content is an important quality technical indicator of calcium carbonate. The modified masterbatch has a high requirement for calcium content of calcium carbonate, generally above 98%. The higher the calcium content, the more stable the processing performance of calcium carbonate. When the calcium content is low, it will increase the content of heavy metals and other non-metallic impurities, which will not only affect the use of calcium carbonate, but also affect the processing technology, affect product quality or cause harm to processing equipment.
2. Whiteness and hue
The whiteness of calcium carbonate directly affects the whiteness and colorability of the modified masterbatch, and the modified masterbatch has very strict requirements on the whiteness of calcium carbonate.
At present, the whiteness of 400-mesh calcium carbonate is required to reach more than 95%, and the whiteness of some ore-processed calcium carbonate can reach about 97%. In order to improve the whiteness of the product, some modified masterbatch manufacturers often add fluorescent brighteners to achieve the effect of increasing whiteness.
However, since the chemical components of fluorescent whitening agents are prone to decomposition reactions under high temperature and high shear, if the type of fluorescent whitening agent is improperly selected or excessive amounts are added, not only will the product decompose and turn yellow, but it will also become non-dispersible or condense and agglomerate.
Fluorescent brighteners can cause certain irritation to plastic products that involve hygiene and safety and come into contact with the human body, and should not be used.
Hue is also a very important technical indicator of calcium carbonate. The determining factors of temperature and hue are the structure and source of the ore. These factors will directly affect the colorability of plastic products, and affect the color and gloss of plastic products. Therefore, the whiteness and hue of calcium carbonate are important technical indicators that cannot be ignored.
3. Water and volatile matter content
The raw materials of calcium carbonate, such as natural marble and calcite, generally do not contain structural water. After being processed into ultrafine powder, it is easy to absorb moisture during processing, storage and transportation. Calcium carbonate sometimes contains trace amounts of volatile substances. If the content of these substances is too high, it will affect the modified masterbatch and plastic products.
The moisture and volatile content in calcium carbonate should generally be controlled at around 0.3%. In actual application, when the moisture content in the product is ≥0.3%, it is easy to affect the product quality, making the product surface rough, and causing bubbles, burning, decomposition and other phenomena. Therefore, it is very important to control the moisture content for the quality of the modified masterbatch. The commonly used method is to use a high-speed mixer to heat and dry to eliminate moisture.
In the production of calcium carbonate high-content filled modified masterbatch, if conditions permit, try to avoid using water tank soaking strand pelletizing and water ring granulation processes. The crawler drive net dry pelletizing process and grinding surface hot cutting granulation process can minimize the moisture and volatile content of the modified masterbatch while increasing production output, and can ensure the product quality of the modified masterbatch.
4. Particle size distribution
The particle size distribution of calcium carbonate is an important quality control indicator for modified masterbatch. The particle size analysis of inorganic mineral powders usually uses a relatively simple screening method, and the commonly used specifications in the screening method are expressed by "mesh". The so-called mesh number is the number of sieve holes on each inch of the sieve. However, a more accurate and scientific particle size analysis method is to measure the actual size of the particle size, expressed in microns (μm).
Sieving is the most traditional particle size analysis method, which involves sieving ultrafine powders with good dispersion through a sieve with a certain mesh number. The weight ratio of the powder passing through the sieve to the powder passing through the sieve is the sieving rate. The finest commonly used standard sieve is 500 mesh (equivalent to about 25μm). The new electrodeposition sieve can sieve powders as small as 5μm (2500 mesh), but the sieving time is long and it is easy to get clogged. For ultrafine powders smaller than 10μm (1250 mesh), the sieving method is difficult to use for particle size analysis and detection.
In practical applications, the particle size distribution range of granular materials such as calcium carbonate has large fluctuations. The narrower the particle size distribution range, the more stable the material performance, and the wider the particle size distribution range, the worse the material performance. For example, the particle size of 1250 mesh calcium carbonate should be around 10μm. If the particle size distribution is 5-38μm, although the theoretical average particle size can be close to 10μm, it is easy to condense into agglomerates due to its small particle size and large specific surface area; and a large particle size will cause the surface of the product to be rough, so its processing performance and mechanical properties will be affected.
5. Particle shape
The particle shape of calcium carbonate can be divided into polygons, flat bodies, multi-faceted bodies, rectangular bodies, long rods, and other irregular particle shapes according to different mineral structures.
The particle shape of calcium carbonate has a great influence on the processing technology, product quality, melt fluidity and mechanical properties of modified masterbatch. Calcium carbonate in the shape of cubes, polygons and cuboids has good melt fluidity in the processing of modified masterbatch, is easy to be coated and cross-linked with coupling agents, and has relatively small wear on processing equipment. The disadvantage is that after the modified masterbatch is applied to plastic products, it is easy to affect the mechanical properties of plastic products (such as tensile strength, bending modulus, etc.).
Flat and long rod-shaped calcium carbonate particles have a relatively large specific surface area, and the amount of coupling agent, plasticizer and other additives in the modified masterbatch needs to be appropriately increased. Otherwise, it is easy to cause uneven coating, poor melt fluidity, increased host power, increased head resistance, and overheating decomposition. However, this type of material is beneficial to the physical properties of plastic products, which can improve tensile strength, bending strength, and reduce the shrinkage rate of plastic products.
6. Content of other components
The chemical composition, metal impurities and other components in calcium carbonate also have a certain impact on the quality of calcium carbonate, and will have a certain impact on the application in plastic modified masterbatch and plastic products. Calcium carbonate generally contains a small amount of magnesium oxide (MgO), silicon dioxide (SiO2), ferric oxide (Fe2O3), aluminum oxide and trace heavy metals (aluminum, arsenic, mercury, etc.). The content of other components varies with the origin and ore.
MgO in calcium carbonate has low hardness and slightly lower whiteness than calcium carbonate. Its chemical stability and processing performance are similar to those of calcium carbonate and it generally does not have a significant impact on calcium carbonate.
SiO2 is a natural quartzite with a small specific surface area and good fluidity. However, SiO2 has a high hardness (Mohs hardness 7), which causes serious wear on equipment, especially when the coating is uneven or the amount of plasticizer is small. If the filled modified masterbatch is used in the production and processing of polypropylene flat wire, it is easy to collide with the high-speed slitting tool and often damage the blade, affecting production and processing.
Although the content of Fe2O3 in calcium carbonate is very small, it is easy to undergo chemical changes when iron is mixed with chemical additives. Fe2O3 is easy to decompose and turn yellow after heating, affecting the whiteness and appearance of the modified masterbatch.
The lower the content of SiO2 and Fe2O3 in calcium carbonate, the smaller the impact on the quality of the modified masterbatch. Generally, a 5% hydrochloric acid dilution solution can be added to calcium carbonate in a glass dish for chemical analysis, and the amount of precipitate in the solution can be observed to determine the SiO2 content; the solution color is slightly yellow, indicating that the Fe2O3 content is high.
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