Plastic Inorganic Fillers: Types and Characteristics Analysis

In the field of plastic processing, various inorganic powder fillers have become key auxiliary materials for optimizing product quality and reducing production energy consumption due to their unique properties and cost advantages. Among them, calcium carbonate, as the most widely used powdered inorganic filler, holds an important position in many types of plastics owing to its affordable price, abundant availability, and non-toxic and odorless nature. Its whiteness can reach up to 96%, allowing for free coloring, and it exhibits excellent chemical stability and ease of drying. From the perspective of filler classification, it mainly includes three categories: heavy calcium carbonate, light calcium carbonate, and activated calcium carbonate, each with distinct focuses in preparation processes, physical characteristics, and applicable scenarios.

Ground calcium carbonate (commonly referred to as GCC) is made from limestone and other raw materials through mechanical crushing, screening classification, and surface treatment. Based on the crushing method, it can be further divided into dry ground calcium carbonate (commercially known as double fly powder) and wet ground calcium carbonate. Due to the mechanical crushing process, GCC particles have irregular shapes and uneven sizes, with an average particle size between 1 to 10μm, where particles below 3μm account for 50%. Its density is 2.7 to 2.9g/cm³ and it is almost insoluble in water. In recent years, with advancements in air pulverization, classification, and other technologies, finer GCC products have been mass-produced, and even ultrafine GCC with a particle size as small as 0.1μm can be prepared. Such products are often used in PVC plastics to effectively reduce production costs, improve product dimensional stability, and when used in conjunction with clay, they can also enhance the elasticity and heat distortion resistance of plastics. Precipitated calcium carbonate (commonly referred to as PCC) is produced by chemical methods and is also known as precipitated calcium carbonate. The particles are mostly spindle-shaped, needle-shaped, or columnar, with similar particle sizes of 1 to 10μm, but particles below 3μm account for a higher proportion, reaching 80%. Its density is 2.65g/cm³, and it is widely used in polyolefin plastics. Besides having similar functions to GCC, it also offers superior reinforcing effects and acid resistance. Activated calcium carbonate (also known as colloidal calcium carbonate or modified calcium carbonate, referred to as "Hakuenka" in Japan) is made by surface modification and activation of PCC or GCC powder using surface treatment agents like stearic acid (about 3% by mass). It appears as a white, fine, soft powder with a density of 1.99 to 2.01g/cm³. Plastics filled with it not only meet strength requirements and have smooth surfaces but also exhibit good lubricity and significantly improved processability. Additionally, calcium carbonate is categorized by particle size into four grades: microparticles (1 to 5μm), microfine (0.1 to 1μm), ultrafine (0.02 to 0.1μm), and superfine (less than 0.02μm). Currently, ultrafine calcium carbonate is mainly produced using a "double spray process" combining continuous spray carbonation and spray drying. This process can achieve fine agglomeration of calcium carbonate particles and uniform surface activation. When the particle size is between 0.005 to 0.02μm, its reinforcing effect can rival that of white carbon black.

According to the Color Masterbatch Industry Network, besides calcium carbonate, talc is the second most used plastic filler. Its main component is hydrated magnesium silicate (chemical formula 3MgO・4SiO2・H2O), made from natural talc through crushing and selection, and it is a typical lamellar filler. Talc powder has stable chemical properties, a slippery feel, and a density of 2.7 to 2.8g/cm³. When used as a plastic filler, it can enhance the hardness, flame retardancy, acid and alkali resistance, electrical insulation, dimensional stability, and creep resistance of products. It also has a lubricating effect, which can reduce wear on processing machinery and molds, but excessive use may affect the welding performance of the product. Thanks to its lamellar structure, plastics filled with talc powder have strong rigidity and low anisotropy, making them suitable for processing large and flat products. In PP plastics, it is commonly used as a crystallization nucleating agent, which can refine PP spherulites, increase crystallinity, and enhance rigidity. It can also be applied to PVC, PE, PA, and PC resins, with an addition amount typically ranging from 10% to 40%. The molding process conditions and equipment are similar to those of calcium carbonate-filled plastics, and various plastic products can be produced through extrusion, pressing, injection, calendering, and other methods. Due to its non-toxic properties, it can be used in products that come into contact with food.

Wollastonite (chemical formula CaSiO3) is the third largest plastic filler, following calcium carbonate and talc. Natural wollastonite has a β-type calcium silicate chemical structure, with needle-like particles being predominant, along with rod-like and granular shapes. It appears as a white crystal, contains no crystalline water, has low hygroscopicity, is non-toxic, and has a low thermal expansion coefficient and high thermal stability. It is also resistant to chemical corrosion and weather aging, and exhibits excellent mechanical and electrical properties. Its refractive index is 1.62, which is close to that of PVC mixtures, making it the preferred filler for transparent PVC products. Moreover, due to its needle-like structure, wollastonite provides some reinforcement to plastics and can partially replace more expensive glass fibers. In terms of application scenarios, wollastonite can be used with PVC, PP, PE, PA, polyester, epoxy resin, and phenolic resins, with the amount typically not exceeding 40%. When used, it is common to treat the surface with silane coupling agents to enhance compatibility with resins.

Kaolin (chemical formula Al2O3・2SiO2・2H2O) is a type of clay mineral powder, also known as china clay, white clay, or porcelain clay. Its main component is hydrated aluminum silicate, featuring a layered structure, with a soft texture and a slippery feel. It has a density of 2.2–2.6 g/cm³ and a pH value of 5–6. It is non-toxic and exhibits flame-retardant properties. Compared to calcium carbonate, kaolin has better forming and processing characteristics. It can be molded normally even when the filling amount reaches three times that of the resin, whereas calcium carbonate becomes difficult to mold when the filling amount reaches the same level as the resin. As a plastic filler, kaolin is notable for its excellent electrical insulation properties and can be used in PVC insulated wire sheaths, PE and PP cables, film composites, etc. For instance, adding 10% by mass of kaolin to PVC can enhance electrical insulation performance by 5–10 times. Additionally, kaolin can be incorporated into polystyrene-based film composites for manufacturing printing paper, and in polyester and epoxy resins to adjust resin viscosity, optimize forming and processing performance, and improve wear resistance. Furthermore, it can serve as a nucleating agent for PP, providing some flame-retardant effects. The typical addition amount of kaolin in plastics ranges from 5% to 60%, with a particle size requirement of over 100 mesh. However, due to its strong hygroscopicity, it must be stored to prevent moisture and caking, and must be dried before use to avoid affecting the quality of plastic products. In some cases, its surface may need to be treated to enhance its affinity with plastics.

Mica, as a type of layered hydrous aluminosilicate mineral, has a complex composition that includes aluminum, potassium, lithium, magnesium, iron, and other elements. Mica filler is made from natural mica fragments that are crushed and processed into mica powder, typically with a particle size of 8–10 μm and an aspect ratio of about 30. It features a typical flaky structure, glassy luster, and a water content of 1.0%–4.2%. Since it is non-toxic, it can be used in products that come into contact with food. In terms of application, mica powder can be used to fill thermoplastic plastics such as PE, PP, PVC, PA, ABS, and polyester, as well as thermosetting plastics such as EP (phenolic resin) and PF (epoxy resin). The typical addition amount ranges from 10% to 40%, with particle sizes between 100 and 325 mesh. As a plastic filler, mica powder can significantly improve the tensile modulus and flexural modulus of plastic products, endow products with excellent electrical insulation, heat resistance, and dimensional stability, and also enhance moisture and corrosion resistance. However, its relatively high price to some extent limits the expansion of its application scenarios.

Diatomaceous earth is formed by the deposition of unicellular algae on the seabed or lakebed, and is composed mainly of SiO2. It has a porous structure, is lightweight, and can be easily ground into powder. It appears as a white or light yellow powder with a particle size of 25-40 μm and a density of 1.6-2.3 g/cm³. As a high-quality lightweight filler, diatomaceous earth can be used in PVC, PO (polyolefin), and thermosetting plastics. It can also serve as an anti-caking agent and is applied in lightweight, soundproof, and thermal insulation building composite materials. However, it also has notable drawbacks, such as high oil absorption and resin absorption. Therefore, its usage should be reasonably controlled based on actual needs.

Carbon black is a product generated by the incomplete combustion of hydrocarbon compounds under controlled conditions, with a wide variety of types. According to the preparation method, it can be divided into channel black, furnace black, and thermal decomposition black, among others. When carbon black is added to polymers, it not only protects plastics from photodegradation and thermal oxidation, but also improves the rigidity of plastic products. The fineness of carbon black significantly affects product performance: the finer the particles, the higher the blackness of the product, the stronger the ultraviolet shielding effect, the better the aging resistance, and the lower the surface resistivity; however, it also increases the difficulty of dispersion. When used as a filler, furnace black with relatively large particle size is often selected, generally ranging from 25 to 75 μm.

Silicon dioxide (chemical formula SiO2) is classified as rock powder, mainly divided into quartz powder and silica powder. Natural SiO2 powder is mostly used in thermosetting resins, such as unsaturated polyester and EP (epoxy resin), for the production of artificial marble and artificial stone. Silicon dioxide can also be prepared synthetically; these synthetic products are usually called white carbon black, which is a reinforcing filler second only to carbon black. It has a matting effect and can improve the electrical insulation and hardness of plastics. However, it has poor flowability and high viscosity, so process optimization or the use of other additives is required during processing to improve its performance.