There are two defects in the direct use of nanometer calcium carbonate in organic polymer matrix:
First, intermolecular forces, electrostatic interactions, hydrogen bonds, oxygen bridges, etc. will cause the agglomeration of calcium carbonate powder;
The second is that the surface of nano calcium carbonate has a strong hydrophilic and strong basic hydroxyl group, which will make its affinity with the polymer worse, and easily form agglomerates, resulting in uneven dispersion in the polymer, resulting in two The interface defects between these materials cannot reflect the nanometer effect of nanometer calcium carbonate.
With the continuous expansion of the application fields of nano calcium carbonate, these shortcomings become more obvious. In order to give full play to the advantages of nano calcium carbonate and improve the properties of nano calcium carbonate, different surface modification methods and modifiers must be used for surface modification treatment.
The commonly used modifiers of nanometer calcium carbonate mainly include coupling agents, surfactants, polymers, inorganics, etc.
1. Titanate coupling agent
Titanate coupling agent can be divided into monoalkoxy type, coordination type and chelating type. In order to improve the uniformity of the reaction, it needs to be dissolved and diluted with an inert solvent, and then added to the surface modifier in the form of spray. Better to disperse and mix with calcium carbonate particles and chemically coat the surface.
Titanate has a good modification effect and has been widely used, but the brown color of titanate affects the whiteness of the modified product, is more expensive, and may endanger human health (causing liver cancer).
2. Aluminate coupling agent
The alkoxy group at one end of aluminate can react with free protons on the surface of calcium carbonate, and the other end can produce entanglement or cross-linking with polymer molecular chain, and its performance is better than titanate.
The alumina modified calcium carbonate can be better dispersed in the organic medium, the mechanical properties are improved, and the physical and mechanical properties and processing properties of the product can be improved. It is widely used in filled plastics and other products.
The advantages of aluminate are: lighter color, non-toxic, high thermal decomposition temperature, low price, convenient packaging and transportation, so it has a wide range of applications. The disadvantage of aluminate is that it is easy to hydrolyze and cannot be used in wet surface modification process.
3. Borate coupling agent
The borate coupling agent used as a modifier has many advantages: non-toxic and antibacterial, excellent coupling function, good thermal stability, and good hydrolysis resistance. Therefore, borate as a nano-calcium carbonate modifier can be used not only for dry surface modification process, but also for wet modification treatment.
4. Fatty acid (salt)
The mechanism of action of fatty acid (salt) is to use calcium carbonate with a large number of hydrophilic hydroxyl groups distributed on the surface, presenting a strong alkaline characteristic. Its RCOO- reacts with Ca2+, CaHCO3+, CaOH+ and other components in the calcium carbonate slurry to form fatty acid calcium The precipitate is coated on the surface of calcium carbonate particles. The hydrocarbon group in the calcium fatty acid makes the surface properties of calcium carbonate change from hydrophilic to lipophilic.
After the nanometer calcium carbonate is modified by fatty acid, the dispersibility is improved, and it has better compatibility with organic polymer materials. Calcium carbonate modified with fatty acid (salt) is mainly used to fill PVC plastics, cable materials, adhesives, inks, coatings, etc. Among them, stearic acid (salt) is the most commonly used and very cheap surface modifier for calcium carbonate.
5. Phosphate (salt) and condensed phosphoric acid
Phosphate (salt) modifiers are also anionic surfactants. They mainly corrode the surface of calcium carbonate by phosphate to generate calcium phosphate and calcium hydrogen phosphate, which are tightly coated on the surface of nano-calcium carbonate particles with long-chain alkyl groups. Align outwards, making nano calcium carbonate hydrophobic
The application of phosphate modified nano-calcium carbonate in composite materials can not only improve the processing performance and mechanical properties of the material, but also improve acid resistance and flame retardancy.
Using condensed phosphoric acid (metaphosphoric acid or pyrophosphoric acid) to modify the surface of calcium carbonate powder can overcome the disadvantages of poor acid resistance and high surface pH of calcium carbonate powder. The pH of the modified product is 5.0～8.0 (the pH before modification is 9.0～10.5), it is hardly soluble in weak acids such as acetic acid, and has good acid resistance.
In addition, zinc sulfate and water glass are added in the calcium carbonate carbonization process for surface modification. When the resulting product is applied to styrene butadiene rubber, its elongation at break and tear strength can be improved.
6. Quaternary ammonium salts
Quaternary ammonium salt is a cationic surfactant. One end of its molecule can be cross-linked with polymer materials, and the other end is positively charged to cause electrostatic adsorption on the surface of calcium carbonate.
7. Reactive monomers, active macromolecules
The small molecule carboxylic acid with unsaturated bond in the reactive monomer can interact with nano calcium carbonate to disperse nano calcium carbonate; using its reactivity (unsaturated bond) can be grafted with polyolefin to form grafts and strengthen The interface between nano calcium carbonate and polymer.
The macromolecules in the active macromolecules can act on the surface of calcium carbonate to improve the affinity and dispersibility between it and the organic polymer materials.
The polymer can be adsorbed on the surface of calcium carbonate directionally, so that calcium carbonate has charge characteristics and forms a physical or chemical adsorption layer on the surface of calcium carbonate particles, increasing the distance between particles, preventing adhesion and agglomeration between calcium carbonate particles, and improving dispersibility. Commonly used polymers mainly include oligomers, high polymers and water-soluble polymers.
Hyperdispersants are different from traditional surfactants and are mainly composed of a solvent section and an anchor section. The anchoring section is generally a polar group, which can be tightly bonded to the particle surface in the form of single-point anchoring or multi-point anchoring; the solvent section has different polarities and is suitable for the modification of polymers with different polarities.
Theoretically speaking, by adjusting the relative molecular mass and functional groups of the two phases, a surface treatment agent that meets almost all requirements can be obtained.
Inorganic modifiers generally cannot change calcium carbonate from hydrophilic to hydrophobic, but can improve the dispersibility and acid resistance of nano-calcium carbonate. If used in combination with other modifiers, the effect is better. Commonly used are metaphosphoric acid (salt), polyphosphoric acid (salt), aluminate (salt), alum, barium salt, etc.
Inorganic electrolyte is adsorbed on the surface of calcium carbonate particles, on the one hand, it can increase the surface potential value and induce steric hindrance, resulting in electrostatic repulsion of the electric double layer, thus improving the dispersion of particles; on the other hand, due to steric hindrance This prevents hydrogen ions from contacting the inner calcium carbonate particles, significantly improving its acid resistance.
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