In the waterborne wood coating system, rheological additives play a very important role and affect all aspects of the coating. A satisfactory and effective rheological curve needs to have two functions:
(1) Thixotropy. Good thixotropy can solve the effect of anti-settling and anti-sagging. Remember: Good thixotropy can keep the emulsion particles in a state of parallel arrangement when the silver powder is sprayed onto the board, which is especially important for water-based wood coatings.
(2) Late liquidity. After spraying onto the board, the coating needs some thixotropy to prevent sagging and maintain particle alignment. After this window period, the shrinkage of the coating film leads to further alignment of the emulsion particles. At this time, the lowest possible viscosity is conducive to further alignment and leveling of the coating film.
There are three main types of rheological additives commonly used in waterborne industrial coating systems:
(1) Alkali swelling type. For example, cellulose, acrylic acid (ASE), which thicken the aqueous phase and are very sensitive to pH.
(2) Association PU type (HEUR). It is generally divided into 3 categories, Newton type (leveling agent often said in the field of architectural coatings), pseudoplasticity and strong pseudoplasticity (water-based systems need to use strong pseudoplasticity). These are particle thickening, sensitive to surfactants, and insensitive to pH.
(3) Fumed silica, organic bentonite, waxes (hydrogen bonding type).
