Rheology:
Rheology refers to the deformation and flow properties of a substance under the action of external force, mainly referring to the relationship between stress, deformation, deformation rate and viscosity during processing. Rheology is an important property of coatings, which directly affects the appearance, construction performance and storage stability of coatings. Thickener is a kind of rheological additive, which can not only thicken the coating and prevent sagging during construction, but also endow the coating with excellent mechanical properties and storage stability. It is a very important additive for water-based paints with low viscosity and slow surface drying. Thickeners currently used in water-based coatings include cellulose ether thickeners, inorganic thickeners, acrylic thickeners, and water-based polyurethane associative thickeners.

1. Cellulose ethers
The mechanism of action of cellulose ether thickeners is mainly due to the macromolecular chains with light groups, which can not only have strong hydration with water but also produce intertwining between molecular chains, thus increasing the water content. phase viscosity. The repeating anhydroglucose unit in the cellulose molecular chain makes the molecular chain straight and relatively rigid. This form makes HEC with the same molecular weight occupy a larger volume than polyethylene oxide and polyacrylamide, so it is beneficial to increase The viscosity of the aqueous phase is particularly effective. For a given type of cellulose ether, the relative molecular mass is the determining factor in obtaining thickening efficiency and rheological properties. HEC with high relative molecular weight has more hydrogen bonding and stronger van der Waals interaction, and the increase of intermolecular winding increases the viscosity. The use of HEC will lead to low viscosity of the coating at high shear rate, which will affect the fullness of the coating film, and the rapid recovery of viscosity at low speed will affect the leveling property. The hydrophobically modified cellulose ethers (HMHEC) can not only hydrogen bond with water but also have a certain degree of hydrophobic association, which improves the performance of HEC.

2. Polyurethane thickener (HEUR)

Polyurethane thickener HEUR is a non-ionic hydrophobic modified ethylene oxide polyurethane block copolymer. HEUR is very similar to a macromolecular surfactant in the water phase of latex paint, and can form micelles. The hydrophilic end is associated with water molecules through hydrogen bonds, and the hydrophobic end is associated with latex particles, surfactants and other hydrophobic structures through intermolecular alignment. Adsorb together to form a three-dimensional network structure in water.

The relative molecular mass of HEUR thickener (thousands to tens of thousands) is lower than that of cellulose thickeners (hundreds of thousands to millions), and the effective volume after hydration increases less. Limited entanglement and thus insufficient thickening of the aqueous phase. The association between the thickener and the particles of the dispersed phase can increase the intermolecular potential energy, which shows a higher apparent viscosity at a high shear rate, which is conducive to the fullness of the coating film: with the disappearance of the shear force, its three-dimensional network The structure gradually recovers, which facilitates the leveling of the paint.

3. Fumed silica thickeners
are very easy to form a uniform three-dimensional network structure through the association of hydrogen bonds between them in oily systems, which can increase the viscosity of coatings. And this three-dimensional network structure will be destroyed when it is subjected to mechanical shear force, so that the viscosity of the coating will drop, and good fluidity will be restored: when the shear force is eliminated, the three-dimensional structure (hydrogen bond) will recover by itself, and the viscosity of the coating will increase. This characteristic of fumed silica endows the coating with very good storage performance and construction performance. In particular, high-build coatings (such as marine paints) can not only ensure good fluidity of the coating under a certain construction shear force, but also ensure the thickness of the coating film in one construction. During the construction process, because the solvent at the edge of the coating volatilizes quickly, resulting in uneven surface tension, it is easy for the paint to move to the edge. The fumed silica network can effectively prevent the movement of the paint and form a thick edge, and at the same time prevent the paint from Sag phenomenon during film formation.

4. Water-based bentonite thickener

Water-based bentonite thickening rheology additive is purified and modified silicate clay (montmorillonite attapulgite, sepiolite), which is off-white or beige non-toxic, tasteless, soft powder. It has excellent expansion, colloid dispersion, suspension, thickening and thixotropy in water, and is an excellent anti-sedimentation agent, latex stabilizer and rheological additive for water-based systems. The water-based bentonite thickening rheology additive also has good adsorption properties and ion exchange properties. It has a synergistic effect with most thickeners. When using water-based thickening rheological additives, especially high-viscosity water-based thickening rheological additives, the amount of thickener can be greatly reduced.

Greesol R19, R39, and R59 are hydrophobically modified polyurethane thickeners, which do not contain organotin APEO. R19 provides excellent high-shear thickening efficiency and leveling; R39 provides excellent medium-shear thickening efficiency and good colorant stability; R59 provides excellent low-shear thickening efficiency and good thixotropy. Splash resistance, tank stability and thermal stability, and water resistance of the coating film.