Thickener used in water-borne painting

With the enhancement of people's awareness of environmental protection policy and the vigorous promotion of national environmental policies, it is imperative to develop water-based coatings without VOC emissions, which has become the focus of paint manufacturers.
Because of its own characteristics, water-borne coatings or water soluble alcohol ether as solvent, its coatings have low viscosity and poor rheological properties. Thickening agents are often needed to improve their rheological properties.

Waterborne coatings with thickening agent can significantly improve the rheological properties of the coating, helps to improve the pseudoplastic paint, when used at high shear rate, the coating can easily stop shear thinning, when or low shear stress, but also can paint thickening, these characteristics, one can improve the storage stability of coatings and avoid the fillers of settlement coatings, while in the spraying process, the atomization is helpful to water paint, on the other hand, in the construction process, to prevent the sagging of paint coating, ensure good performance of construction.

Classification of thickener

Thickeners in waterborne coatings can be classified into cellulose, acrylates, polyurethane associating, polyamide wax and inorganic thickeners according to their different kinds.
- cellulose -

Cellulose thickeners are the most widely used thickeners in the history of development. According to their molecular structure, cellulose thickeners are divided into methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose and hydroxypropyl cellulose, among which the most commonly used hydroxyethyl cellulose (HEC) is thickening agent.

Cellulose thickener as thickening mechanism is the main use of the structure of the hydrophobic backbone with water to form hydrogen bonds, and the structure of the other polar group interaction, build a three-dimensional mesh structure, improve the rheological volume of polymer, polymer free mobile space limit, so as to improve the viscosity of coatings. When the applied shear stress, three-dimensional network structure was destroyed, intermolecular hydrogen bonds disappeared, the viscosity decrease, when the force is removed, re formed hydrogen bonds, three-dimensional network architecture is re established, so as to ensure that the coating can have good rheological performance.

The cellulose thickener structure contains abundant hydroxyl groups and hydrophobic segments, the thickening efficiency is very high, not sensitive to PH, but because of its waterproof effect is not good, and the influence of film leveling, susceptible to microbial degradation and other shortcomings, the main application is mainly used in latex paint.
- polyacrylate -

Alkaline swelling acrylic acid thickener is a cross-linked copolymer emulsion. The copolymer is in the form of acid and very small particle state. Its shape is milky white, its viscosity is relatively low, it has good stability at low pH value, and it is insoluble in water. When the alkali agent is added, it is converted into a clear high swelling dispersion. Its thickening is produced by neutralization of hydroxides and carboxylic acids. When the addition of alkali agent, almost immediately, not easily make changes for the ionization of carboxylic group ionized carboxylic acid or ammonium salts form along the copolymer chain anion generated in the center of the electrostatic repulsion effect of the copolymer crosslinking the rapid expansion and extension. The results of local dissolution and swelling make the original particles increase by a lot of times, and the viscosity increases significantly. Because the crosslinking can not be truly dissolved, the copolymer in the form of salt can be regarded as a dispersion of the copolymer which has been greatly increased.

Polyacrylic thickener has high thickening efficiency, fast thickening speed, good biological stability, but sensitive to pH value, poor water resistance, low gloss and so on.

- Polyurethane Association class -

The associative polyurethane thickener is destroyed under the action of shear force, and the viscosity is reduced. When the shear force is gone, the viscosity can be resumed, which prevents the hang up phenomenon in the construction process. And the viscosity recovery has a certain lag, which is beneficial to the film flow leveling. The relative molecular mass of polyurethane thickeners (thousands to tens of thousands) is much lower than that of the first two kinds of thickeners. The relative molecular mass (hundreds of thousands to millions) is much lower than that of the thickeners. The high water solubility of cellulose thickeners will affect the water resistance of coatings. However, polyurethane thickeners have hydrophilic and hydrophobic groups on the molecule, hydrophobic groups have strong affinity with the substrate of the coating, and can enhance the water resistance of the coating. Because the latex particles are involved in the association, it does not produce flocculation and has good leveling, which makes the coating smooth and has high gloss.

- polyamide wax -

Waterborne polyamide wax is an amide salt thickening agent, acrylic and polyurethane thickener compared, thixotropic index high transparency and gloss does not affect the film, while the aqueous silver paint and waterborne wood paint (water-based paint with antisettling matte powder) arrangement, directional very good, has been widely used in aqueous silver paint and preparation of waterborne wood paint. The mechanism is mainly like hydrogen bonding and macromolecular entanglement. The former is the main one. At present, the domestic work on this aspect is relatively few. It is mainly monopolized abroad, and its price is relatively high, which needs domestic research and development.

- inorganic thickener -

Inorganic thickeners mainly include modified bentonite, attapulgite and so on. Thickener has the advantages of thickening, thixotropic, pH value wide and good stability. But because bentonite is a kind of inorganic powder, it has good absorbability and can obviously reduce the gloss of the coating surface, which is similar to the effect of the extinction agent. Therefore, in the use of bentonite in latex coatings, we should pay attention to the control of the amount. Nanotechnology has realized the nanoscale of inorganic particles and some new properties of inorganic thickeners.

The thickening mechanism of the inorganic thickener is complex, and it is generally considered to be internal electricity.

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