The surface modifiers of mineral fillers, such as barium sulphate and calcium carbonate, can be divided into four major categories: surfactants, coupling agents, organic polymers, and inorganic substances according to the structure and characteristics of the fillers.
Although using just a few amount, surfactants can significantly change the surface or interfacial properties of the fillers. Their molecular structure is characterized by the presence of a long non-polar hydrocarbon group as the hydrophobic group, and the long chain structure is similar to that of polymers, which assures the compatibility of the surfactants.
The other end is a short polar group as the hydrophilic group, usually carboxyl and ether, that can physically adsorb or chemically react with the nanoparticles. It forms a coating on the surface to improve the performance of nanoparticles.
Surfactants are classified into non-ionic and ionic types (cationic and anionic). Common anionic surfactants include stearates, phosphates, etc. Cationic surfactants include cetyl trimethyl ammonium bromide salts, dodecylpyridine hydrochloride, etc. Amphoteric ionic surfactants include amino acids, betaine and imidazoline. Non-ionic surfactants include fatty acid polyoxyethylene glycol esters, polyols, etc.
Coupling agents are low molecular compounds with two chemically different functional groups that can react with both inorganic and organic substances. There are many types of coupling agents, which can be classified according to their chemical structures: silane coupling agents, phosphorus-containing compounds, titanate esters, amines, and complexes, etc.
Surfactants and coupling agents are basically small-molecule substances. However, as the amount of filler increases, the problem that small molecule modifiers cannot get rid of the deterioration of product performance will come out. Using organic polymers for surface treatment can overcome such problem. Currently there are six types of organic polymers used as surface treatment agents:
- Oligomers or polymers with low melting point or in liquid state, such as low-density polyethylene, random polypropylene, oxidized polyethylene,various ethers, etc.
- Linear condensation prepolymerwith low melting point or in liquid state, such as polyester, phenolic resin, epoxy resin, unsaturated resin, etc.
- Linear or comb-type polymeric hyperdispersants.
- Polymer solubilizers with polar group grafting and block chains.
- Polymer solutions or emulsions.
- Polymers with low melting point and high melt flow plastics.
In plastic filling and modification, inorganic substances are generally not used as surface treatment agents for inorganic fillers. But if the filler is organic such as starch and wood flour, some inorganic substances will be used as a surface treatment agent.
At present, adding different surface modifiers in the production process of barium sulphate and calcium carbonate can achieve some special requirements of the products and make the performance of the products more outstanding.