Smectites as thickeners for latex paints

Artur Palasz, Ph.D., Spektrochem, looks at smectites minerals, used mainly in coatings due to their sorption properties. Artur discusses the influence of smectites on the properties of waterborne paints, especially latex paints, as well as the usefulness of these additives as stabilisers in pigment concentrate formulations.

Smectites are minerals that are often used due to their sorption properties, i.e. their ability to absorb substances. The most common smectites are dioctahedral smectites, such as montmorillonite, bentonite, nontronite or beidellite and trioctahedral smectites, such as saponite, hectorite or sauconite. The most popular montmorillonite is a type of clay that is exceptionally capable of absorbing water and other substances. It is widely used in the cosmetics, pharmaceutical and food industries as a thickener and stabiliser. All smectites are a type of phyllosilicates (layered silicates) that swell and therefore have the ability to absorb substances, which makes them useful in various industrial and utility applications, e.g. in drilling, for absorbing liquid leaks, etc. Some types of smectites are also used for absorbing toxins in the digestive system and are used as an oral suspension in the case of food poisoning.

Figure 1. Smectites in solid form can be in the form of powder of various colours and shades

Origin and general characteristics of smectites

Smectites get their name from the Ancient Greek σμηκτός (smēktós) which means “lubricated”; from σμηκτρίς (smēktrís) “walker’s earth, fuller’s earth”; earth that has the property of cleaning. They are an inorganic mixture of various swelling sheet silicates (phyllosilicates), which have a three-layer 2:1 structure and belong to the clay minerals. Smectites are a group of clay minerals with a complex chemical composition. The main chemical components of smectites include:

• Silica (SiO₂): This is the main chemical component of smectites, although its content may vary depending on the specific type of smectite. Silica occurs in the layered structure of smectites.
• Aluminum (Al₂O₃): Another important chemical component of smectites that also forms a layered structure with silica.
• Water (H₂O): Water is a component important for the structure of smectites, occurring between their layers. The water content in smectites can be significant and affects their physical properties, such as the ability to absorb substances.

In addition, smectites may contain various amounts of admixtures and impurities, such as iron, magnesium, calcium, potassium, sodium and other elements. The exact chemical composition of smectites may vary depending on their specific type and the geological conditions in which they were formed.

Smectites are formed mainly as a result of geological processes related to chemical and mechanical weathering of rocks. These processes may include:

• Chemical weathering: Water containing various chemical compounds can react with rock minerals, dissolving and converting these minerals into new compounds. In the case of smectites, silicon-containing minerals, such as clay minerals, often react with water and various chemicals to form smectites.
• Hydrothermal mineralisation processes: Smectites can also be formed as a result of hydrothermal activity, where high temperature and pressure promote mineral changes in rocks, leading to the formation of smectites.
• Deposition and diagenesis: Smectites may be the product of sedimentation processes where fine mineral particles are transported by water and deposited at the bottom of a body of water. Then, under the influence of pressure and time, diagenesis processes occur, which lead to the transformation of sediments into rocks, including smectites.
• Metasomatic reactions: Smectites can also be formed by metasomatic reactions, which occur when hot mineral solutions flow through rocks, causing an exchange of elements between the solution and the rock, which can lead to the formation of new minerals, including smectites.

These geological processes can occur in a variety of environments, such as soils, river sediments, lakes, seas, and within the contact zone between igneous rocks and surrounding sedimentary rocks. Smectite deposits are widespread throughout the world and occur in various geological regions. Some of the largest and most famous areas where smectite deposits occur include:

• The United States is one of the main producers of smectites in the world. Smectite deposits can be found in various states, including Texas, Wyoming, Montana, Georgia, California, and Nevada.
• Russia is another important producer of smectites. Significant deposits occur mainly in Siberia and the Urals.
• China is also a large producer of smectites. The deposits are mainly located in Jiangsu, Zhejiang, Guangdong and Hubei provinces.
• Greece has significant deposits of smectites, especially on the island of Milos, where smectites have been mined since ancient times.
• Turkey is also an important producer of smectites, especially around the city of Balikesir.
• Italy has deposits of smectites, especially in the area of Sardinia.

Smectite deposits also occur in Australia, especially in Queensland and Kangaroo Island. Additionally, smectites are also mined in other countries around the world, including Canada, Germany, Ukraine, Morocco, Mexico, Brazil and India. Their occurrence is related to various geological processes that have occurred in these regions for millions of years

Absorption properties and building a spatial network creating a gel with good rheological properties in water mean that smectites are used in the paint industry as thickeners (rheology modifiers). The diversity of composition and specific properties resulting from the deposit and the process of formation of smectites means that it is impossible to define one group of all smectites as rheological additives for paints in terms of their impact on the properties and their performance in formulations. Each smectite will influence and build rheological properties in a different way depending on its effectiveness in the formulation. Therefore, it is very important to characterise them in formulations and assess performance for specific types of smectites, and not to theoretically attribute to them the properties they can influence. In this article, I discuss, using examples, the influence of smectites on the properties of waterborne paints, especially latex paints, as well as the usefulness of these additives as stabilisers in pigment concentrate formulations.

Smectites in waterborne paints

Smectites are extremely interesting rheological additives that can introduce a pseudoplastic and thixotropic profile to the properties of latex paints. However, it is not possible to unambiguously treat all types of smectites or, more specifically, bentonites, in terms of unifying their impact on rheological properties. The multitude of factors influencing their effectiveness in formulations depending on the deposit, chemical composition, ability to swell and build a rheological structure mean that these thickeners, despite one group, can differ greatly in their effectiveness [1]. Generally, their use as rheological modifiers comes down to building the basic viscosity structure in the area of low- and mid-shear forces measured using a Brookfield and Stomer viscometer, and their participation in the formulation is usually supported by additional thickeners modifying the viscosity, e.g. cellulose ethers in an appropriate ratio. with smectites or HEUR thickeners, especially to increase efficiency in the high-shear forces area (measurements using cone-plate viscometers) [2].

The viscosity given to latex paints effectively in the low- and mid-shear forces does not mean that all rheological properties will be ideal. Therefore, it is very important to consciously and extensively build a rheological structure verified by measuring secondary properties, such as sagging resistance. Figure 2 shows photos after Anti-Sag Index ASTM D4400 tests of latex paint with three different smectites in doses ensuring a viscosity measured with a Stormer viscometer in the range of 100-110 KU (measured according to ASTM D562 method B). As you can see, despite setting the initial viscosity at a similar level, the samples show different resistance to sagging, expressed in mils (thousandth of an inch).

Figure 2. Differences in sagging resistance of paints PVC 34% with different smectites

The sagging resistance of paints when smectites are used as thickeners depends on many factors. Firstly, from the method of hydrating them and introducing them into the paint, as discussed in the second part of the article, and also from the effectiveness of the impact of the “house of cards” created by the three-dimensional structure of the gel and its ability to hold a wet, thick layer of paint on vertical surfaces. These effects are closely related to PVC (Pigment Volume Concentration), the latex binder and surfactants present in the formulation. Therefore, there is no single answer as to whether smectite helps with sagging resistance. The answer is yes, but depending on its type, degree of hydration, and the ingredients present in the formulation.

Another parameter that smectites as thickeners influence in latex paints is Spattering Resistance. The spattering resistance test conducted in accordance with ASTM D4707 eliminates the influence of the type of paint roller, which may have a key impact on the repeatability of the test, because the spattering is induced using a standardized notched-spool roller and repetitive movements during the test make it possible to assess the actual impact of the rheological additive on spattering. Figure 3 shows a comparison of spattering resistance for paints with two different smectites in the formulation. The photos of catch papers after catching drops from the spattering test show a clear difference in the results of paint splash resistance when painting with a roller. The ASTM test, assessed on a scale from 0 to 10, where 0 is the worst result and 10 means no spattering, showed for sample samples that smectites providing the same initial viscosity do not have the same effect on the rheological property of spattering resistance. The sample on the left received a score of 5, which clearly shows that there are visible drops of paint on the catch paper. The sample on the right has a score of 9, which means that only a few small dots of paint are visible. These results show how paints based on various smectites prepared exclusively with smectites as thickeners in the formulation (for the purposes of such a test) can obtain very different spattering resistances, which means that this type of thickeners can also effectively build viscosity in the high-shear forces area and constitute a basis for further modification of rheological properties with other types of thickeners.

Figure 3. Comparison of spattering resistance of paints with two different smectites

Paints with varying degrees of gloss constitute an extremely important part of the market, especially in relation to wood paints and wall paints. Rheological additives such as smectites are usually identified only by their effect on rheological properties. Figure 4 shows the effect of three smectites used in the formulation of waterborne acrylic paint (PVC 16%) at a dose of 0.5% of active ingredients in the entire formulation on the gloss of the coatings. These paints were prepared with smectites and HEUR thickener. Gloss grading was made using MPI’s Detailed Performance Standards criteria.

Figure 4. Comparison of the influence of smectites as thickeners on the gloss of coatings

As can be seen for the prepared formulation, one can notice the influence of smectites on obtaining coatings with a gloss that can be classified as gloss (above 70% at angle 60°) for Smectite X, while samples Y and Z obtained a gloss below 70% classified as semi-gloss. This means that smectite as a thickener may affect the gloss of coatings and its use in glossy coatings can be taken into account provided that specific types of smectites that do not reduce the gloss are selected. In turn, smectites, which cause a slight reduction in gloss, can be successfully dedicated to applications in semi-gloss, satin-like, velvet or eggshell coatings, as an additional factor supporting gloss reduction, even though it is a rheological additive that does not appear at first glance. taken into account in terms of the gloss parameter.

Smectite thickeners are also an interesting rheology stabilizer in pigment concentrates used in tinting systems. Figure 5 shows photos from a stability test of a pigment concentrate prepared from red iron oxide pigment after 6 months. In test tubes, there is a clear effect on the reduction of syneresis, while on spatulas there is no settling in the case of the sample with smectite used as a stabiliser.

Figure 5. Stability of pigment concentrates

Use of smectites in formulations

As you can see, smectites can introduce extremely interesting rheological properties and influence the quality of formulated latex paints, however, in order for them to be able to boost rheological properties, they must be properly introduced into the system. However, these are extremely difficult ingredients to introduce into the formulation of waterborne paints, just like all phyllosilicates used as thickeners. The whole difficulty lies in the appropriate separation of the silicate layers and building a rheological structure from them that will effectively ensure rheological properties and stabilisation of the paint. As an expert in phyllosilicates thickeners, I will present in a nutshell the most important issues related to their incorporation into latex paint formulations and the most optimal methods of dosing paints.

First of all, you should start with the fact that smectite thickeners are delivered in the form of off-white, cream, yellow, sometimes gray powder in the form of fluffy powder to a compact form of loose clay. This form is not completely dry, because smectites contain moisture on average from 5 to even 13%, which should be deducted from the material used to prepare the formulation and the moisture content should be determined appropriately with each delivery using a moisture analyser (Figure 6). Such moisture content and its fluctuations in different deliveries resulting from the natural nature of smectites mean that the amount of thickeners in formulations from deliveries of different batches must be adjusted. For this purpose, in order to maintain a constant dosage level in terms of active substances, it is necessary to correct the amount of smectite in the formulation each time based on the moisture content data performed during QC of smectite supplies.

Figure 6. Determination of moisture content in smectite sample

Hydration

Smectite thickeners are composed of silicate layers, which swell under the influence of shear forces and water, separating into individual layers. This process is called hydration of silicate layers. As a result, a gel is created in water with the structure of separated layers of sheets, forming the previously mentioned “house of cards”. It can be destroyed under the influence of mixing and shear forces, after which it returns to its original form, building viscosity again [1][3]. Thanks to this property, it is possible to build the rheological structure of paints using smectites.

The process of hydration and gel preparation takes place as a result of combining the four most important parameters: smectite concentration in water depending on the individual characteristics of the deposit and the ability to build viscosity, shear rate, time of exposure of shear forces and the quality of water used for the hydration process (water hardness is important for the ability to to swelling and affecting the viscosity of paints).

Figure 7. Smectite thickener during hydration in water using cowles dissolver

The hydration process is carried out at high shear forces, usually using a cowles dissolver with a serrated dispersing disk (Figure 7) at a shear rate of up to 15 m/s (3,000 fpm) and for a time of 10 to 30 minutes. The gel concentration is selected in such a way as to obtain the appropriate level of pourability with the maximum possible loading of smectite concentration into the gel. The selection of this concentration is determined experimentally to ensure good manipulation of the gel during further operations (pouring, pumping and introducing into the paint).

Figure 8. Comparison of pourability of two smectites at different gel concentrations in water

The concentration of the gel prepared for further use in the paint production process is strongly dependent on the viscosity of smectite obtained in water after its hydration. Figure 8 shows a pourability comparison to determine the maximum loading concentration of two smectites, which shows how different these minerals from two different deposits can be. These concentrations are selected experimentally in the laboratory by analyzing viscosity (Figure 9), peripheral speed needed to prepare the gel, gel stability, temperature during hydration, type of water used for hydration and other variables affecting the final gel concentration.

Figure 9. Measurement of viscosity of hydrated samples of phyllosilicate thickeners

The pre-gels obtained in this way can be stored and used for the production of latex paints. Properly hydrated smectites can be used after preparing gels for up to several months. Preventively, it is necessary to add preservatives in-cans to protect against microbial contamination, which smectites in pre-gel form tend to cause (Figure 10).

Figure 10. Microbiological contamination of bentonite gel several days after preparation

Incorporation

Smectite thickeners introduced into paints should be in the form of previously prepared hydrated pre-gels, and the dosing should take place in the let-down process in the equaliser. A properly prepared fully hydrated pre-gel can then be dosed together with other thickeners at the end of the production process and leveled to obtain the appropriate viscosity. Some types of phyllosilicates thickeners, including smectites, can be added to the grinding process, especially when their role is to stabilise mill-base slurries, however, dosing in the form of powder to the grinding process is not always recommended due to the risk of insufficient hydration of silicate sheets.

Summary 

Smectites are a group of minerals that can be successfully used in the production of latex paints as thickeners, especially as an additional modifier to ensure optimal rheological properties of low and medium PVC formulations. The obtained rheological properties, which are briefly described in this article, show that there is a large variation depending on the characteristics of a given smectite and it cannot be clearly assumed that every smectite will work well as a rheological modifier for spattering or sagging. In general, almost any well-hydrated smectite will be suitable as thickeners for modification in the low- and mid-shear forces area, however, the boosting of rheological parameters and the impact on other coating properties will typically depend on the deposit and chemical characteristics, as well as the interaction of smectite with individual formulation raw materials.

Author: Artur Palasz, Ph.D., Spektrochem – Technical Center of Raw Materials for Architectural Paints, Poland

Email: artur.palasz@spektrochem.pl

References:

[1] A. Palasz, Phyllosilicate Thickeners as One Family of Clays in Each of Them is Different, Waterborne Symposium 2024, New Orleans, LA, US

[2] A. Palasz, Focus on architectural: Correct use of viscometers and side tests to characterize the rheology of latex paints, PPCJ, April 04, 2024

[3] A. Palasz, The Influence of Water on the Hydration of Mineral Thickeners, Coatings World, March 2022