Thickeners come in a wide variety, and their preparation methods vary depending on the type. Generally, low-molecular-weight thickeners are relatively simple to prepare; for example, low-molecular-weight inorganic thickeners are combined with surfactants for thickening; ether/amine oxide thickeners are prepared through oxidation reactions; and ester thickeners can be obtained through direct esterification. High-molecular-weight thickeners occupy a larger market share. Besides inorganic and natural high-molecular-weight thickeners, most are prepared through emulsion polymerization and reverse emulsion polymerization, with a few using solution polymerization, bulk polymerization, and precipitation polymerization.
Solution Polymerization
Solution polymerization refers to the polymerization process of monomers dissolved in a solvent and initiators, etc. Its components are typically polymeric monomers, oil-soluble/water-soluble initiators, solvent/water.
Research on solution polymerization has mainly focused on the preparation of polyacrylic acid thickeners. Its characteristic is that a large amount of solvent is required to dissolve the polymer during the preparation process. Most of these solvents are insoluble in water, and subsequent recycling is necessary. Therefore, the cost is high, and it is also detrimental to environmental protection. Bulk Polymerization
Bulk polymerization is the process of monomer self-polymerization initiated or accelerated by a heat source (light, heat, radiation energy, etc.) with little or no initiator/catalyst. This method has low requirements for monomers, does not require solvent dissolution, and yields products with few impurities and high purity. In recent years, some researchers have begun to use bulk polymerization (a two-step method) to prepare associative polyurethane thickeners, first bulk polymerizing a polyurethane prepolymer and then finally end-capping it with a long-chain fatty alcohol to obtain the product.
Emulsion Polymerization
Emulsion polymerization refers to the process where monomers are uniformly dispersed in water under mechanical stirring with the help of an emulsifier to form an emulsion, and then an initiator is added to initiate monomer polymerization.
Emulsion polymerization can accommodate higher reaction rates, yields polymers with higher molecular weights, is easy to control in production, and allows for easy removal of residual monomers. Based on these advantages, research and development of this preparation method has been rapid. Acrylic thickeners have excellent thickening and suspending properties, and are compatible with almost all nonionic, anionic, amphoteric surfactants, and many cationic polymers, thus attracting the attention of researchers.
Reverse Emulsion Polymerization
Reverse emulsion polymerization refers to polymerization in which a water-insoluble organic solvent and a water-soluble monomer form a water-in-oil emulsion in water under the action of an emulsifier.
This method is fast, mild, and yields high molecular weight and relatively pure products. Both its thickening effect and electrolyte resistance are superior to those of emulsion polymerization.
The types of thickeners prepared by reverse emulsion polymerization are similar to those prepared by emulsion polymerization, mainly polyacrylic acid thickeners. Compared to emulsion polymerization, reverse emulsion polymerization is more suitable for preparing electrolyte-resistant thickeners. New technologies can be introduced based on reverse emulsion polymerization, such as radiation polymerization to prepare printing thickeners, allowing for artificial control of the polymerization rate and avoiding excessively rapid reactions.
Precipitation Polymerization
Research on the preparation of thickeners by precipitation polymerization is relatively limited. It typically involves adding a precipitant to a mixture of an organic solvent (benzene, toluene, or alkanes, etc.) and acrylic acid monomers to prepare a precursor precipitate, which is then dried or calcined.
Compared with reverse emulsion polymerization, precipitation polymerization produces products with poor thickening properties and sensitivity to electrolytes. Introducing some comonomers (such as octadecyl methacrylate) into the polymer can improve its electrolyte resistance.
