1. Definition and function of thickener

Additives that can significantly increase the viscosity of water-based paints are called thickeners.

Thickeners play an important role in the production, storage and construction of coatings.

The main function of the thickener is to increase the viscosity of the coating to meet the requirements of different stages of use. However, the viscosity required by the coating at different stages is different. E.g:

During the storage process, it is desirable to have a high viscosity to prevent the pigment from settling;

During the construction process, it is desirable to have a moderate viscosity to ensure that the paint has good brushability without excessive paint staining;

After construction, it is hoped that the viscosity can quickly return to a high viscosity after a short time lag (leveling process) to prevent sagging.

The fluidity of waterborne coatings is non-Newtonian.

When the viscosity of the paint decreases with the increase of the shear force, it is called a pseudoplastic fluid, and most of the paint is a pseudoplastic fluid.

When the flow behavior of a pseudoplastic fluid is related to its history, that is, it is time-dependent, it is called a thixotropic fluid.

When manufacturing coatings, we often consciously try to make the coatings thixotropic, such as adding additives.

When the thixotropy of the coating is appropriate, it can solve the contradictions of the various stages of the coating, and meet the technical needs of the different viscosity of the coating in the storage, construction leveling, and drying stages.

Some thickeners can endow the paint with high thixotropy, so that it has a higher viscosity at rest or at a low shear rate (such as storage or transportation), so as to prevent the pigment in the paint from settling. And under high shear rate (such as coating process), it has low viscosity, so that the coating has sufficient flow and leveling.

Thixotropy is represented by thixotropic index TI and measured by Brookfield viscometer.

TI=viscosity (measured at 6r/min)/viscosity (measured at 60r/min)

2. Types of thickeners and their effects on coating properties

(1) Types In terms of chemical composition, thickeners are divided into two categories: organic and inorganic.

Inorganic types include bentonite, attapulgite, aluminum magnesium silicate, lithium magnesium silicate, etc., organic types such as methyl cellulose, hydroxyethyl cellulose, polyacrylate, polymethacrylate, acrylic acid or methyl Acrylic homopolymer or copolymer and polyurethane etc.

From the perspective of the influence on the rheological properties of coatings, thickeners are divided into thixotropic thickeners and associative thickeners. In terms of performance requirements, the amount of thickener should be less and the thickening effect is good; it is not easy to be eroded by enzymes; when the temperature or pH value of the system changes, the viscosity of the coating will not be significantly reduced, and the pigment and filler will not be flocculated. ; Good storage stability; good water retention, no obvious foaming phenomenon and no adverse effects on the performance of the coating film.

①Cellulose thickener

The cellulose thickeners used in coatings are mainly methylcellulose, hydroxyethylcellulose and hydroxypropylmethylcellulose, and the latter two are more commonly used.

Hydroxyethyl cellulose is a product obtained by replacing the hydroxyl groups on the glucose units of natural cellulose with hydroxyethyl groups. The specifications and models of the products are mainly distinguished according to the degree of substitution and viscosity.

The varieties of hydroxyethyl cellulose are also divided into normal dissolution type, rapid dispersion type and biological stability type. As far as the method of use is concerned, hydroxyethyl cellulose can be added at different stages in the coating production process. The fast-dispersing type can be added directly in the form of dry powder. However, the pH value of the system before adding should be less than 7, mainly because hydroxyethyl cellulose dissolves slowly at low pH value, and there is sufficient time for water to infiltrate into the inside of the particles, and then the pH value is increased to make it Dissolves quickly. Corresponding steps can also be used to prepare a certain concentration of glue solution and add it to the coating system.

Hydroxypropyl methylcellulose is a product obtained by replacing the hydroxyl group on the glucose unit of natural cellulose with a methoxy group, while the other part is replaced by a hydroxypropyl group. Its thickening effect is basically the same as that of hydroxyethyl cellulose. And it is resistant to enzymatic degradation, but its water solubility is not as good as that of hydroxyethyl cellulose, and it has the disadvantage of gelling when heated. For the surface-treated hydroxypropyl methylcellulose, it can be directly added to water when used. After stirring and dispersing, add alkaline substances such as ammonia water to adjust the pH value to 8-9, and stir until fully dissolved. For hydroxypropyl methylcellulose without surface treatment, it can be soaked and swelled with hot water above 85°C before use, and then cooled to room temperature, then stirred with cold water or ice water to fully dissolve it.

②Inorganic thickener

This kind of thickener is mainly some activated clay products, such as bentonite, magnesium aluminum silicate clay, etc. It is characterized in that in addition to the thickening effect, it also has a good suspension effect, can prevent sinking, and will not affect the water resistance of the coating. After the coating is dried and formed into a film, it acts as a filler in the coating film, etc. The unfavorable factor is that it will significantly affect the leveling of the coating.

③ Synthetic polymer thickener

Synthetic polymer thickeners are mostly used in acrylic and polyurethane (associative thickeners). Acrylic thickeners are mostly acrylic polymers containing carboxyl groups. In water with a pH value of 8-10, the carboxyl group dissociates and becomes swollen; when the pH value is greater than 10, it dissolves in water and loses the thickening effect, so the thickening effect is very sensitive to the pH value.

The thickening mechanism of the acrylate thickener is that its particles can be adsorbed on the surface of the latex particles in the paint, and form a coating layer after alkali swelling, which increases the volume of the latex particles, hinders the Brownian motion of the particles, and increases the viscosity of the paint system. ; Second, the swelling of the thickener increases the viscosity of the water phase.

(2) Influence of thickener on coating properties

The effect of the type of thickener on the rheological properties of the coating is as follows:

When the amount of thickener increases, the static viscosity of the paint increases significantly, and the viscosity change trend is basically consistent when subjected to an external shear force.

With the effect of thickener, the viscosity of the paint drops rapidly when it is subjected to shear force, showing pseudoplasticity.

Using a hydrophobically modified cellulose thickener (such as EBS451FQ), at high shear rates, the viscosity is still high when the amount is large.

Using associative polyurethane thickeners (such as WT105A), at high shear rates, the viscosity is still high when the amount is large.

Using acrylic thickeners (such as ASE60), although the static viscosity rises rapidly when the amount is large, the viscosity decreases rapidly at a higher shear rate.

3. Associative thickener

(1) thickening mechanism

Cellulose ether and alkali-swellable acrylic thickeners can only thicken the water phase, but have no thickening effect on other components in the water-based paint, nor can they cause significant interaction between the pigments in the paint and the particles of the emulsion, so The rheology of the paint cannot be adjusted.

Associative thickeners are characterized in that in addition to thickening through hydration, they also thicken through associations between themselves, with dispersed particles, and with other components in the system. This association disassociates at high shear rates and re-associates at low shear rates, allowing the rheology of the coating to be adjusted.

The thickening mechanism of the associative thickener is that its molecule is a linear hydrophilic chain, a polymer compound with lipophilic groups at both ends, that is, it has hydrophilic and hydrophobic groups in the structure, so it has the characteristics of surfactant molecules. nature. Such thickener molecules can not only hydrate and swell to thicken the water phase, but also form micelles when the concentration of its aqueous solution exceeds a certain value. The micelles can associate with the polymer particles of the emulsion and the pigment particles that have adsorbed the dispersant to form a three-dimensional network structure, and are interconnected and entangled to increase the viscosity of the system.

What’s more important is that these associations are in a state of dynamic balance, and those associated micelles can adjust their positions when subjected to external forces, so that the coating has leveling properties. In addition, since the molecule has several micelles, this structure reduces the tendency of water molecules to migrate and thus increases the viscosity of the aqueous phase.

(2) The role in coatings

Most of the associative thickeners are polyurethanes, and their relative molecular weights are between 103-104 orders of magnitude, two orders of magnitude lower than ordinary polyacrylic acid and cellulose thickeners with relative molecular weights between 105-106. Due to the low molecular weight, the effective volume increase after hydration is less, so its viscosity curve is flatter than that of non-associative thickeners.

Due to the low molecular weight of the associative thickener, its intermolecular entanglement in the water phase is limited, so its thickening effect on the water phase is not significant. In the low shear rate range, the association conversion between molecules is more than the association destruction between molecules, the whole system maintains an inherent suspension and dispersion state, and the viscosity is close to the viscosity of the dispersion medium (water). Therefore, the associative thickener makes the water-based paint system exhibit lower apparent viscosity when it is in the low shear rate region.

Associative thickeners increase the potential energy between molecules due to the association between particles in the dispersed phase. In this way, more energy is needed to break the association between molecules at high shear rates, and the shear force required to achieve the same shear strain is also greater, so that the system exhibits a higher shear rate at high shear rates. Apparent viscosity. The higher high-shear viscosity and lower low-shear viscosity can just make up for the lack of common thickeners in the rheological properties of the paint, that is, the two thickeners can be used in combination to adjust the fluidity of the latex paint. Variable performance, to meet the comprehensive requirements of coating into thick film and coating film flow.