Inorganic mineral fillers

 

Calcium carbonate (CaCO3) Calcium carbonate is the most commonly used powdered inorganic filler at present. It is cheap, rich in sources, non-toxic and tasteless, with a whiteness of up to 96%, free coloring, good chemical stability, and easy drying. Therefore, it is widely used in many plastics.
From perspective of fillers, calcium carbonate can be divided into three categories: heavy calcium carbonate, light calcium carbonate and activated calcium carbonate.
Heavy calcium carbonate, referred to as heavy calcium, is made from limestone and other materials through mechanical crushing, screening, grading, and surface treatment. According to crushing method, it is divided into dry heavy calcium carbonate (trade name double fly powder) and wet heavy calcium carbonate.
Because it is mechanically crushed, shape is irregular and particle size is also different. Average particle size is 1 to 10μm, of which 50% are below 3μm, density is 2.7 to 2.9g/cm3, and it is almost insoluble in water. In recent years, due to progress of crushing (such as air flow crushing) and classification technology, finer products can be produced, and even 0.1μm ultrafine heavy calcium carbonate can be produced. Heavy calcium carbonate is often used in PVC. Its function is to reduce costs and improve dimensional stability. It can improve elasticity and thermal deformation when used with clay.
Light calcium carbonate, referred to as light calcium, is calcium carbonate produced by chemical methods, also known as precipitated calcium carbonate. Its particle shape is mostly spindle-shaped, needle-shaped or columnar, with a particle size of 1 to 10μm, of which 80% are below 3μm, and density is 2.65g/cm3. Light calcium carbonate is often used in polyolefins. Its function is same as that of heavy calcium carbonate, with good reinforcing effect and better acid resistance.
Activated calcium carbonate, also known as colloidal calcium carbonate and modified calcium carbonate, is called Baiyanhua in Japan. It is made by modifying and activating surface of light calcium carbonate or heavy calcium carbonate powder with a surface treatment agent (stearic acid, about 3% by mass). It has a white, delicate, soft powder with a density of 1.99-2.01g/cm3. Plastic products filled with activated calcium carbonate have certain strength and smooth surface, good lubricity and are easy to process.
Calcium carbonate is classified according to particle size: when particle size is 1-5μm, it is micro-particle calcium carbonate; when particle size is 0.1-1μm, it is fine calcium carbonate; when particle size is 0.02-0.1μm, it is ultrafine calcium carbonate; when particle size is less than 0.02μm, it is ultrafine calcium carbonate.
At present, production of ultrafine calcium carbonate mostly adopts continuous spray carbonization and spray drying process (i.e. double spray process), which can make apparent agglomerates of calcium carbonate finer and surface of particles activated evenly. When particle size is 0.005-0.02μm, its reinforcing effect is equivalent to that of white carbon black.

 

Talc (3MgO·4SiO2·H2O) is mainly composed of hydrated magnesium silicate, which is obtained by crushing and selecting natural talc. It is a typical flaky filler with inactive chemical properties and a greasy feel. Density is 2.7-2.8g/cm3. Talc is a filler with a dosage second only to CaCO3. As a plastic filler, it can improve hardness, flame retardancy, acid and alkali resistance, electrical insulation, dimensional stability, creep resistance, and lubricity of product. It can reduce wear on machinery and molds; but excessive dosage will affect weldability of product.
Since talcum powder is a flaky filler, rigidity of product when filled with plastic is large and anisotropy is small. It can be used to process large and flat products. Talc powder is mainly used as a crystallization nucleating agent for PP to make spherulites of PP fine, improve crystallinity, and increase rigidity. It can also be used in resins such as PVC, PE, PA and PC, and addition amount is generally 10% to 40%. Its molding process conditions and equipment are similar to those of calcium carbonate filled plastics. Various types of plastic products can be manufactured by extrusion, pressing, injection, calendering and other methods. Talc powder is non-toxic and can be used for products in contact with food.

Wollastonite (CaSiO3) Wollastonite is the third largest filler after CaCO3 and talcum powder. Natural wollastonite has a β-type calcium silicate chemical structure and is a mixture of needles, rods and granules, with needles being main one. Wollastonite is white crystal without crystal water, low hygroscopicity, non-toxic, low thermal expansion coefficient, high thermal stability, chemical corrosion resistance, weathering resistance, excellent mechanical and electrical properties.
Refractive index of wollastonite is 1.62, which is close to that of PVC mixture, and is preferred filler for PVC transparent products. Due to needle-like structure of wollastonite, it has a certain reinforcing effect on plastics and can partially replace expensive glass fiber. Wollastonite can be used in PVC, PP, PE, PA, polyester, epoxy resin and phenolic resins, addition amount is generally less than 40%, and silane coupling agents are often used for surface treatment.
Inorganic mineral fillers

Kaolin (A12O3·2SiO2·2H2O) Kaolin is a kind of clay, that is, powder of clay minerals, also known as pottery clay, white clay, porcelain clay, is a silicate with hydrated aluminum silicate as main component. It has a layered structure, is soft and greasy, has a density of 2.2-2.6 g/cm3, a pH value of 5-6, is non-toxic, and has flame retardancy. Kaolin molding processability is better than CaCO3, and it can still be molded when filling amount reaches 3 times that of resin, while CaCO3 filling amount reaches 1 times and molding is difficult.
As a plastic filler, kaolin has excellent electrical insulation properties and can be used as PVC insulated wire sheath, PE, PP cables, film composite materials, etc. For example, adding 10% kaolin by mass to PVC can improve electrical insulation performance by 5-10 times. Kaolin can also be incorporated into polystyrene-based film composite materials to make printed paper. In polyester and epoxy resin, viscosity and molding performance of resin can be adjusted, and wear resistance can be improved. Kaolin can be used as a nucleating agent for PP and has a certain flame retardant effect.
Amount of kaolin added to plastics is generally 5% to 60%, and particle size is above 100 mesh. It should be noted that kaolin has a large hygroscopicity. It should be noted that it should be prevented from caking due to moisture during storage. Kaolin must be dried before use to avoid affecting quality of plastic products. Sometimes surface of kaolin needs to be treated with lipophilicity to improve its affinity with plastics.

Composition of mica is very complex. It is a layered hydrous aluminum silicate mineral of elements such as aluminum, potassium, lithium, magnesium, and iron. Mica filler is mica powder made by crushing natural mica fragments. Its particle size is usually 8-10μm, and its aspect ratio is usually about 30. It is a typical flake structure with glass luster and a water content of 1.0%-4.2%. It is non-toxic and can be used for products in contact with food.
Mica powder can be filled with thermoplastic plastics such as PE, PP, PVC, PA, ABS, polyester, and thermosetting plastics such as EP (phenolic resin) and PF (epoxy resin). Addition amount is generally 10%-40%, and particle size is 100-325 mesh.
Mica as a plastic filler can greatly improve tensile elastic modulus and bending elastic modulus of plastic products, give products excellent electrical insulation, heat resistance and dimensional stability, improve moisture resistance and corrosion resistance, but disadvantage is that price is relatively high.

 

Diatomaceous earth is a fossil formed by accumulation of single-cell algae on seabed or lake bottom. Its main component is SiO2. It is porous, light, and very easy to grind into powder. It looks like white or light yellow powder with a particle size of 25 to 40 μm and a density of 1.6 to 2.3 g/cm3.
Diatomaceous earth is a high-quality lightweight filler that can be used in PVC, PO and thermosetting plastics; it can also be used as an anti-adhesion agent for lightweight, sound insulation, heat insulation and other building composite materials. Disadvantage is that it absorbs a large amount of oil and resin.

Carbon black is a substance generated by incomplete combustion of hydrocarbon compounds under controlled conditions. There are many varieties. According to preparation method, it can be divided into channel carbon black, furnace carbon black and thermal cracking carbon black. Carbon black added to polymers can not only protect against light degradation and thermal oxidation, but also improve rigidity of plastic products. Fineness of carbon black affects performance of product. The finer particles, the higher blackness, the stronger UV shielding effect, the better aging resistance, and the lower surface resistivity of product, but it is difficult to disperse to some extent.
As a filler, carbon black with a larger particle size can be used, generally 25-75μm.

(SiO2) Silicon dioxide is a type of rock powder, which can be divided into quartz powder and silica powder. Natural SiO2 powder is mainly used in thermosetting resins, such as unsaturated polyester and EP, to produce artificial agate and artificial marble. SiO2 can also be synthesized, generally called white carbon black, which is a filler with reinforcing effect second only to carbon black. It has a matting effect, can improve electrical insulation and hardness of plastics. Disadvantage is poor fluidity and high viscosity.