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The promotion and application of thickeners in the latex paint industry and the development of this industry: Latex paint is a mixture of pigments, fillers, and polymer dispersions; additives are necessary to adjust its viscosity so that it possesses the rheological properties required at various stages of production, storage, and application. Such additives are generally referred to as thickeners; they are substances that can increase the viscosity of coatings and improve their rheological properties, which is why they are also called rheological thickeners. Regarding the requirements for the rheological properties of latex paint, it should have a high viscosity during storage to prevent stratification or sedimentation ； During the construction phase, latex paint is subjected to high shear rates; therefore, it requires a low viscosity so as not to drag the brush during application, to mist well when sprayed, and to avoid splattering when applied with rollers. After coating, shear is eliminated; although the viscosity is restored, it should not do so too quickly, to provide sufficient time for leveling. Although various thickeners can increase the viscosity of coatings, the shear rates to which the coatings are subjected during production, storage, and application vary, and coatings are required to exhibit specific properties; as a result, different viscosities are needed. This requires the use of thickeners with different properties to meet the viscosity requirements of the coating at various stages. Thickeners are generally classified into organic and inorganic categories based on their chemical composition. The inorganic thickeners currently in use are mainly bentonite ； Organic thickeners mainly include cellulose-based, acrylic-based, and polyurethane-based types. Below are only the main properties of common rheology thickeners and their applications in latex paints. 1. Bentonite: The main mineral component of bentonite is montmorillonite. The layers between the montmorillonite particles carry negative charges, acting as large anions with negative charges that can adsorb cations, water, and polar organic molecules. Bentonite has a very strong water-absorbing capacity, and can absorb up to 5 times its own weight in water ； After absorbing water, its volume increases, expanding to several times to over a dozen times its dry volume and forming a gel-like substance. Bentonite can release charged particles in water, acting as a dispersant; it improves the suspension properties of water-based coatings and enhances their stability during storage. Polar groups such as —OH, —COOH, and —CCOH in organic molecules like polyvinyl alcohol, acrylic acid, and hydroxyethyl cellulose can be attracted by the negative charges between the layers of montmorillonite, causing one end of these polar groups to insert into the interlayer spaces and forming a montmorillonite-organic complex. Once such a complex is formed, it becomes irreversible; upon dehydration, it turns into a hydrophobic substance. Therefore, bentonite, as a thickening agent, not only does not affect the water resistance of the coating, but it can also improve it ； Montmorillonite interacts with organic substances to form a networked three-dimensional structure, thereby improving the suspension properties of the coating ； After drying, it remains in the coating film where it acts as a filler to improve coverage. Due to its low whiteness and adverse effect on the leveling properties of coatings, its use in coatings is limited; it can only be applied in mid-to-low-grade interior and exterior wall coatings as well as thick-suspension latex paints. In exterior wall coatings, the use of bentonite improves the water resistance of the coating film and is inexpensive; therefore, its appropriate use offers good technical and economic benefits. Expansive clays are widely used in cost-effective interior wall latex paints, ordinary exterior wall latex paints, and thick-sealant latex paints with a pigment volume concentration higher than the critical value ； It has almost no application in glossy or semi-gloss latex paints with low pigment volume concentrations. Given that bentonite affects the leveling properties of coatings and has an adverse effect on color, its use is restricted. 2. Cellulose thickeners: Cellulose-based materials that can be used in coatings include methyl cellulose, hydroxyethyl cellulose, and hydroxypropyl methyl cellulose, among others. The most notable feature of cellulose thickeners is their significant thickening effect; they also confer a certain water-retaining capacity on coatings, allowing them to delay the drying time during application to some extent. Cellulose thickeners also endow the coating with a certain thixotropy, preventing sedimentation and stratification during storage. However, such thickeners can also cause poor leveling properties in coatings, especially when products with high viscosity are used. Cellulose is a nutrient for microorganisms; therefore, measures to prevent mold should be strengthened when using it. Cellulose thickeners can only thicken the aqueous phase; they have no thickening effect on other components in water-based coatings, nor can they cause any significant interaction between the pigments and emulsion particles in the coating, thus failing to modify the rheology of the coating. Cellulose thickeners generally can only increase the viscosity of coatings at low and moderate shear rates (commonly referred to as KU viscosity). 2.1 Hydroxyethyl cellulose: The specifications and types of hydroxyethyl cellulose products are primarily distinguished based on the degree of substitution and viscosity. Companies in our country such as Wuxi Sanyou Chemical Co., Shijiazhuang Jinhua Cellulose Company, Luzhou Tianpu Fine Chemical Factory, as well as American companies like Agualon and Dow Chemica1, all supply hydroxyethyl cellulose in various viscosity grades and substitution levels to the market. In addition to differences in viscosity, hydroxyethyl cellulose varieties can also be classified into normally soluble types, rapidly dispersing types, and bio-stable types through modifications during the production process. In terms of usage, hydroxyethyl cellulose can be added at different stages of the coating production process. For the rapid-dispersing type, it can be added directly in powder form, but the pH value of the system should be below 7 before addition. This is because hydroxyethyl cellulose dissolves more slowly at low pH levels, allowing sufficient time for water to penetrate into the interior of the particles; afterward, the pH value is increased to facilitate rapid dissolution. It is also possible to follow the appropriate steps to prepare a glue of a certain concentration and add it to the coating system. 2.2 Hydroxypropyl methylcellulose: The thickening effect of hydroxypropyl methylcellulose is essentially the same as that of hydroxyethyl cellulose, that is, it increases the viscosity of coatings at low and moderate shear rates. Hydroxypropyl methylcellulose is resistant to enzymatic degradation, but its water solubility is lower than that of hydroxyethyl cellululose; it also has the drawback of gelling when heated. For the surface-treated hydroxypropyl methylcellulose, it can be directly added to water; after stirring to disperse it, alkaline substances such as ammonia water are added to adjust the pH value to 8–9, and stirring continues until complete dissolution is achieved. For hydroxypropyl methylcellulose that has not been surface-treated, it can be soaked in hot water at a temperature of over 85°C to cause swelling; after cooling to room temperature, cold water or ice water can be added along with stirring to ensure complete dissolution. 2.3 Methyl cellulose: The properties of methyl cellulose are similar to those of hydroxypropyl methyl cellulose, but its viscosity stability varies less with temperature. Hydroxyethyl cellulose is the most widely used thickener in latex paints, and it is employed in high-end, mid-range, and low-end latex paints as well as in thick-bodied latex paints. It is widely used for thickening ordinary latex paints, lime calcium powder latex paints, etc. Next is hydroxypropyl methylcellulose, which also sees a certain level of use driven by manufacturers. Cellulose methylate has almost no application in latex paints, but it is widely used in powdered interior and exterior wall putties, owing to its rapid solubility and good water-retention properties. High-viscosity methyl cellulose can endow the putty with excellent thixotropy and water retention, enabling it to have good spreading properties. 3. Synthetic polymer thickeners: The two types of synthetic polymer thickeners that are most widely used are acrylic and polyurethane. 3.1 Acrylics Acrylic thickeners include two categories: polyacrylates and acrylate copolymers (alkali-swellable type). One of the thickening mechanisms of acrylate thickeners is that their particles can adsorb onto the surface of the latex particles in the coating; after swelling in alkaline conditions, a coating layer is formed, which increases the volume of the latex particles. This hinders the Brownian motion of the particles, thereby increasing the viscosity of the coating system ； Second, the swelling of the thickener increases, raising the viscosity of the aqueous phase. In water with a pH of 8–10, the carboxyl groups dissociate, resulting in a swollen state ； When the pH value is greater than 10, it dissolves in water and loses its thickening effect; therefore, its thickening property is very sensitive to pH value. Similar to cellulose ether thickeners, alkali-swollen acrylic thickeners do not have a thickening effect on other components in the paint, nor can they induce significant interactions between the pigments and emulsion particles in the paint; as a result, they are unable to adjust the rheology of the paint. These thickeners are characterized by a significant thickening effect and low cost, but they have an adverse impact on both the leveling property of the coating and the water resistance of the film formed. They only increase the viscosity at low to moderate shear rates, thereby improving the coating’s resistance to sagging and settling. The use of acrylate thickeners affects the water resistance of the coating film ； It reduces the leveling property of the coating due to the strong thixotropy it imparts to the coating. Therefore, such thickeners are generally used only in low-quality interior wall latex paints, or as auxiliary thickeners in mid-quality interior wall latex paints; their use in exterior wall coatings is rare. However, in mid- and low-grade interior wall latex paints, acrylate thickeners are almost always essential components, mainly due to their low cost and significant thickening effect. With the advancement of research on the synthesis and application techniques of rheology-thickening agents, some of today’s acrylate-based thickeners, after being modified, are able to increase the viscosity of coatings at high shear rates and also endow them with excellent rheological properties. 3.2 Polyurethanes One of the thickening mechanisms of polyurethane thickeners is that their molecules can undergo hydration and swelling, thereby thickening the aqueous phase. Another mechanism is their possession of properties similar to those of surfactant molecules; they are polymer compounds with linear hydrophilic chains to which lipophilic groups are attached at both ends, meaning they contain both hydrophilic and hydrophobic groups in their structure. In this way, micelles are formed when its concentration in aqueous solution exceeds a certain value. Micelles can associate with the polymer particles in the emulsion and with pigment particles that have absorbed dispersants to form a three-dimensional network structure; by interconnecting and entangling with each other, they increase the viscosity of the system. Therefore, polyurethane thickeners are also known as associative thickeners. The association structures formed by polyurethane thickeners in coatings break apart at high shear rates and re-form under low shear rates, maintaining a dynamic equilibrium; this allows for the regulation of the coating’s rheology and enables it to achieve leveling properties. Furthermore, since one molecule carries several micelles, this structure reduces the tendency for water to migrate, thereby effectively preventing stratification and precipitation during the storage of the coating. These thickeners have hydrophobic properties and have almost no effect on the water resistance of the coating film. These thickeners have a low relative molecular mass, resulting in limited intermolecular entanglement in the aqueous phase, and thus they do not provide a significant thickening effect on the aqueous phase. Furthermore, within the range of low shear rates, the formation of intermolecular associations exceeds the breakdown of such associations, allowing the entire system to maintain its inherent suspended dispersion state, with a viscosity close to that of the dispersion medium (water). Thus, it causes the latex paint system to exhibit a lower apparent viscosity at low shear rates ； Furthermore, since such thickeners increase the intermolecular potential energy due to aggregation between particles in the dispersed phase, more energy is required at high shear rates to break this aggregation; moreover, a greater shear force is needed to achieve the same shear strain, resulting in a higher apparent viscosity of the system at high shear rates. In other words, polyurethane thickeners significantly increase the viscosity of coatings at high shear rates (commonly referred to as ICI viscosity), thereby improving the leveling properties of the coatings. However, with the advances in materials science, some polyurethane thickeners can now also significantly increase the viscosity of coatings at low and medium shear rates. Compared with acrylate and cellulose thickeners, polyurethane thickeners are mainly used in exterior latex paints and mid-to-high-end interior latex paints. The higher high-shear viscosity and lower low-shear viscosity can effectively compensate for the shortcomings of conventional thickeners (such as acrylic and cellulose-based ones) in terms of the rheological properties of coatings. By using these two types of thickeners together, it is possible to adjust the rheological properties of latex paints to meet various requirements, including the ability to apply thick coats and ensure uniform film thickness. It is worth noting that for a good coating formula, one thickener will never be used to increase the viscosity of the coating, nor can a single thickener meet all the performance requirements of the coating. Therefore, the best approach is to use several thickeners to increase the viscosity of the coating at different shear rates. For example, hydroxyethyl cellulose or associative thickeners (such as Rohm and Haas’ DR-72) are used to increase the KU viscosity of coatings, while polyurethane thickeners are used to raise the ICI viscosity and improve the rheological properties of the coatings. Here, it is only conceptually explained how different thickeners are used together in coatings; as for how to optimize this combination, it needs to be determined based on experiments and factors such as compatibility with the dispersant used. 4. Development and Prospects of Rheological Thickeners 4.1 Development of Rheological Thickeners As a component of latex paint, rheological thickeners have inevitably evolved alongside the development of latex paint. Since rheological thickeners can improve the rheological properties of coatings, the development of various types of rheological thickeners and related application technologies has resulted in latex paints possessing improved performance, which in turn promotes their use and further development. Before synthetic resin emulsion building coatings became the dominant type in our country, the thickeners used in water-based coatings were basically limited to bentonite, cellulose, etc., and their usage amounts were also very small. In recent years, as thickeners have seen widespread use, attention has also been paid to their varieties, quality, and application technologies. As the number of varieties increases, consumption also rises rapidly. The alkali-swelling thickeners and associative thickeners currently in use are mostly new types that have been developed in recent years or have had their properties improved. In particular, associative thickeners can improve the leveling property, anti-settling property, consistency in the container, coverage, and touch feel of latex paints, thereby yielding more satisfactory results for exterior wall coatings as well as high-performance interior wall coatings. Due to its good effects, research on its application technologies has also received attention. Another development in thickeners in recent years has been the creation of rheological thickeners based on entirely new concepts. Examples include those sold in the market as additives for coatings, as well as rheological modifiers developed by certain manufacturers and used in coating production; these are what are known as \"modified colloids\", and they represent a new type of rheological thickener. On the other hand, our country is rich in mineral resources; utilizing natural mineral products as thickening rheology agents is also one of the areas of research. For example, further studies on bentonite, as well as research on sepiolite as a thickening rheology agent, represent new advancements in thickening agents in recent years. In addition to developing new varieties, improvements have also been made to the performance of existing thickeners to meet new requirements. For example, hydroxyethyl cellulose and hydroxypropyl methyl cellulose are products that have been in use traditionally; not only have they not been phased out, but their usage has even increased. One reason for this is that manufacturers continuously improve the performance of these thickeners to meet new requirements. For example, developing products that are easy to disperse, rapidly soluble, resistant to biodegradation, and have better viscosity stability over a wide temperature range. 4.2 Future Prospects The rheological properties of latex paints are a very complex issue; most professionals in this field still lack a sufficient understanding of these properties as well as the skills needed to utilize them in coatings. Improving their application in coatings is therefore a matter that requires further research, dissemination of knowledge, and enhancement of relevant skills. The lower-end versions of some branded products offer excellent properties such as a good appearance inside the container, an optimal opening experience when the container is opened, good coating leveling, and a pleasant feel to the coating, all at reasonable prices. One of the key technical aspects involved is the use of rheology-thickening agents. Therefore, as the requirements for coating performance evolve, the application technology of thickeners will receive greater attention and develop accordingly. Improving the performance of existing varieties and thereby developing new types of thickeners represents another direction for progress in this field. This is because there are many types of thickeners currently in use, but all of them have various performance shortcomings; there is therefore room for improvement based on requirements such as their ability to enhance the properties of coatings and their ease of use. Furthermore, developing new thickeners that meet the ever-increasing requirements for coating performance and ease of use will also remain a constant goal in the pursuit of technological advancement and economic benefits.