Selection of thickening agents in waterborne coatings

The amount of coating additives in the coating is very small, but it can significantly improve the performance of the coating, and has become an indispensable part of the coating. The commonly used auxiliaries of waterborne coatings are film auxiliaries, thickening agents, dispersants, wetting agents, defoaming agents, plasticizers, anti-mildew fungicides and so on. Thickener is a rheological agent, which can not only thicken the coating, prevent the phenomenon of flow hanging in the construction, but also give the coating excellent mechanical properties and storage stability. It is a very important additive for water-based coatings with low viscosity.

Classification of thickeners for waterborne coatings

At present, there are many varieties of thickeners available on the market, mainly inorganic thickeners, cellulose, polyacrylates and associated polyurethane thickeners. Cellulose thickeners have a long history of use, many varieties, methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl methyl cellulose, etc., used to be the mainstream of thickeners, of which the most commonly used is hydroxyethyl cellulose. Polyacrylate thickener can be basically divided into two kinds: one is water-soluble polyacrylate; The other is acrylic acid, methacrylic acid homopolymer or copolymer emulsion thickener, this thickener itself is acidic, must be neutralized with alkali or ammonia water to pH8~9 to achieve the thickening effect, also known as acrylic acid alkali swelling thickener. Polyurethane thickener is a new association thickener developed in recent years. Inorganic thickener is a kind of thixotropic gel mineral formed by water absorption and expansion. There are mainly bentonite, attapulgite, aluminum silicate and so on, of which bentonite is the most commonly used.

Thickening mechanism

Cellulose thickener The thickening mechanism of cellulose thickener is that the hydrophobic main chain and the surrounding water molecules are associated by hydrogen bonds, which increases the fluid volume of the polymer itself and reduces the space for free movement of particles, thus improving the viscosity of the system. The viscosity can also be improved through the winding of molecular chains, showing that there is high viscosity at static and low shear, and low viscosity at high shear. This is because at static or low shear rate, the cellulose molecular chain is in a disordered state, which makes the system show high viscosity. However, at high shear velocity, the molecules are arranged in an orderly manner parallel to the flow direction, which is easy to slide each other, so the viscosity of the system decreases.

The thickening mechanism of polyacrylic acid thickener is that the thickener dissolves in water, and through the same electrostatic repulsion of carboxylic acid ions, the molecular chain is stretched from spiral to rod, thus improving the viscosity of the water phase. In addition, it also increases the viscosity of the system by forming a network structure between the latex particles and the pigment.

The thickening mechanism of associated polyurethane thickeners was studied in detail by A.J. Reuvers. Hydrophilic groups and hydrophobic groups are introduced into the molecular structure of this kind of thickener, which makes it show the properties of surfactants. When the concentration of its aqueous solution exceeds a certain concentration, micelles are formed, and the micelles and polymer particles are associated to form a network structure, which increases the viscosity of the system. On the other hand, one molecule has several micelles, which reduces the mobility of water molecules and increases the viscosity of water phase. Such thickeners not only affect the rheology of the coating, but also interact with adjacent latex particles, and if the effect is too strong, it is easy to cause the stratification of latex.

Inorganic thickener

The thickener bentonite is a kind of layered silicate, which expands to form a flocculent substance after absorbing water. It has good suspension and dispersion, and combines with the right amount of water to form a colloid, in which charged particles can be released to increase the viscosity of the system.

The characteristics and selection of various thickening agents

Cellulose thickener Cellulose thickener has high thickening efficiency, especially for water phase thickening; Limited to the coating formula, widely used; Wide range of pH available. However, there are some disadvantages such as poor leveling, more spatter phenomenon, poor stability and easy to be degraded by microorganisms. Because it has low viscosity at high shear and high viscosity at static and low shear, the viscosity increases rapidly after coating is completed, which can prevent flow hanging, but on the other hand, it causes poor leveling. Some studies have shown that the relative molecular weight of the thickener increases, and the spatter property of the latex coating also increases. Cellulose thickeners are prone to spatter due to their large relative molecular weight. This kind of thickener is through the "fixed water" to achieve the thickening effect, the pigment and latex particles are very little adsorption, the volume expansion of the thickener filled with the entire water phase, the suspended pigment and latex particles squeezed to one side, easy to produce flocculation, and therefore poor stability. Because it is a natural polymer, it is vulnerable to microbial attack.

Polyacrylic acid thickener Polyacrylic acid thickener has strong thickening and good leveling, good biological stability, but it is sensitive to pH value and poor water resistance.

The associated polyurethane thickener is damaged under the action of shear force, and the viscosity is reduced. When the shear force disappears, the viscosity can be restored, which can prevent the phenomenon of flow hanging in the construction process. And its viscosity recovery has a certain lag, which is conducive to the film leveling. The relative molecular mass of polyurethane thickeners (thousands to tens of thousands) is much lower than that of the first two types of thickeners (hundreds of thousands to millions) and will not contribute to spatter. The high water solubility of cellulose thickener will affect the water resistance of the coating film, but the molecules of polyurethane thickener have both hydrophilic and hydrophobic groups, and the hydrophobic groups have strong affinity with the substrate of the coating film, which can enhance the water resistance of the coating film. Because the latex particles participate in the association, there is no flocculation, so it can make the film smooth and have a high gloss. Many properties of associated polyurethane thickeners are better than other thickeners, but because of their unique micellar thickening mechanism, the components that affect micelles in the coating formulation will inevitably affect the thickening properties. When using such thickeners, the influence of various factors on the thickening performance should be fully considered, and the emulsion, defoamer, dispersant and film-forming additives used in the coating should not be easily replaced.

Inorganic thickener Water-based bentonite thickener has the advantages of strong thickening, good thixotropy, wide range of pH value adaptation and good stability. However, since bentonite is an inorganic powder with good light absorption, it can significantly reduce the surface gloss of the coating film and play a similar role as a matting agent. Therefore, when using bentonite in luminous latex coatings, attention should be paid to controlling the amount. Nano technology has realized the nano formation of inorganic particles, and has also given some new properties to inorganic thickeners.

Selection of thickening agents in waterborne coatings

The high viscosity hydroxyethyl cellulose with large thixotropy and the carboxyl-containing polyethylene thickener with Bingham flow should be selected in the thick paste coating. In the flat latex coating with high and medium pigment volume concentration (PVC), the high viscosity type and low viscosity type hydroxyethyl cellulose can be used together, and the acrylic emulsion thickener can be selected. For glossy latex coatings, choose thickeners that do not affect the sheen of the film, such as ALLied Coliod's VG 2 and Rohm and Hass's acrylic emulsion thickener TT-935, and polyurethane thickeners Exp300 or QR-708. The mixture of thickeners is better than the single use. Styrene propylene, pure acrylic emulsion for flat, bright high-grade latex paint, can be used with two rheological characteristics of thickening agents, such as 612 and 621N, 1550 and 2000, to achieve the balance of high, middle and low shear viscosity, or with HASE, HN-HEC. Vinyl acetate copolymers, such as vinegar C, vinegar tertiary emulsion, usually the emulsion particle size is larger, low and medium PVC latex paint, can be used with HEUR(612, 1550) with high shear thickening efficiency and HEC, HASE.

Research progress of thickeners for waterborne coatings

L.M. Zhang gave a comprehensive introduction to the synthesis method and hydrophobic modification of cellulose thickeners, and discussed their solubility characteristics, viscosity behavior, surface activity and other properties in aqueous solution. Jiang Qibin et al. developed a new modified association cellulose thickener by grafting appropriate association bonds on the basis of hydroxyethyl cellulose. Its relative molecular weight is low, spatter ability is small, and can be associated with the pigment filler in the latex coating to form a three-dimensional network structure to ensure the stability of the latex coating. The properties of water-soluble polyacrylate thickener can be further improved by modifying it. For example, SN-Thickener series products SN-603, SN-607 and SN-612 of Diamond Shamrock Company in the United States are polyether acrylic thickeners modified with ethyl carbamic acid. Recently, due to the higher requirements for water resistance and construction performance of latex coatings, hydrophobic thickeners have been developed. For example, Rheovis CR from Allied Colliod in the UK is one of these thickeners. It is a kind of reactive surfactant connected to the branch chain of the traditional alkali activated acrylic emulsion thickener molecule. The longer hydrophobic end groups form micelles with each other in water, so that the thickening form changes from a linear structure to a network structure. The study of Luo Hong, et al. showed that the construction property, covering power, leveling property, anti-current hanging property, electrolyte resistance and post-thickening property of acrylic acid association thickener were superior to those of vinyl acetate acrylic acid and non-association thickener. The chemical structure of reactive surfactants has a significant effect on the rheological properties of associated thickeners. The longer the hydrophilic group of the reactive surfactant, the smaller the amount of thickener. The hydrophobic group is about 12 carbon, and the electrolyte resistance is the best. There is no hydrophobic group of polyether type reactive surfactant, stability is very poor. J. Edward Glass et al have discussed in detail the interaction between association thickener and surfactant, both of which act by adsorbing to the surface of the particles of the coating components. Therefore, excessive amount of surfactant in the formulation will cause the association thickener to displace from the surface of the latex particles into a continuous phase, thus inhibiting the association ability of the association thickener. Resulting in paint leveling, gloss and cover power decreased.