Review of the Application of Kaolin in the Production of Various Adhesives and Sealants

Introduction: What is Kaolin and Why is it Important in the Adhesive and Sealant Industry?

Kaolin, also known as "china clay" in everyday language, is one of the oldest, most well-known, and most widely used non-metallic minerals in the world. This material, essentially composed of hydrous aluminum silicate (kaolinite), is used in dozens of industries—from tiles and ceramics to papermaking, paint manufacturing, and rubber production—due to its light color, layered structure, chemical neutrality, and economical price. However, one of the most important yet least known markets for this mineral is the adhesive and sealant manufacturing industry; an industry that is inconceivable in its current form without kaolin as a filler and rheology modifier.

In the formulation of a mode adhesive or sealant, a large portion of the product's volume consists not of expensive polymeric materials, but of mineral fillers. Kaolin plays multiple roles in these formulations: it increases the bulk volume of the material and reduces the final cost, controls the flow behavior (rheology) of the mixture during production, packaging, and application, and in some cases improves the mechanical, thermal, and chemical resistance of the final product. This is why manufacturers of construction adhesives, industrial adhesives, and silicone, polyurethane, and acrylic sealants always place kaolin at the top of their raw materials list.

The Kani Sang Amiran Project, as one of the mineral materials producers in the country, offers its kaolin product in the mesh range of 450 to 1500, aimed at applications in the tile, ceramic, glaze, paper, and paint industries. This same wide particle size range enables the use of this product in a broad range of adhesive and sealant formulations—from ceramic and construction adhesives with coarser particles to delicate silicone sealants with very fine meshes.

In this article, we attempt to comprehensively and practically examine the application of kaolin in the production of various adhesives and sealants. In the first step, we will become familiar with the structure and inherent characteristics of this mineral to understand why the adhesive industry depends on it. Then we will explore specialized applications in construction adhesives, wood and packaging adhesives, and silicone and polyurethane sealants, and analyze the effect of particle size (mesh) on the performance of the final product. Finally, we will review the technical and economic benefits of using kaolin, selection and storage tips, and the outlook for this mineral in Iran's adhesive and sealant market.

There is a close relationship between the quality of the kaolin used and the quality of the final product; a silicone sealant made with inappropriate kaolin may sag after exiting the tube, crack on the surface, or develop a non-uniform color after drying. For this reason, precise knowledge of this mineral is of strategic importance not only for adhesive and sealant manufacturers, but for all practitioners in industries dealing with mineral fillers.

Structure and Characteristics of Kaolin that Make it Suitable for Adhesive Manufacturing

To understand why kaolin is effective in the adhesive and sealant industry, we must look at its molecular structure and physical characteristics. Kaolin belongs to the layered silicate group, and its main structural unit, kaolinite, is composed of a tetrahedral silica layer (SiO₄) and an octahedral alumina layer (Al(OH)₆). These two layers are coected to each other by shared oxygens and hydrogen bonds, forming a distinct layered structure. This same layered structure is the source of many of the properties that distinguish kaolin in the adhesive industry.

One of the most important characteristics of kaolin is its particle shape. Kaolin particles are flat, hexagonal plates that extend in one direction. This sheet-like shape allows the particles to align next to one another and build a layer-by-layer structure. The result of this sheet-like arrangement is desirable rheological behavior in the formulation: the adhesive or sealant mixture remains thick and stable at rest, but under shear stress (such as stirring in a tank or pressure during application) it becomes more fluid and is easily pumped or applied. This characteristic, referred to as "thixotropy" and "shear properties," directly affects the quality of cartridge packaging, sag resistance on walls, and the leveling capability of the sealant.

Another prominent feature is the hydroxyl (OH) groups on the surface of kaolin particles. These groups enable the formation of hydrogen bonds and weak chemical interactions with the polymeric matrix, resins, and even the substrate surface. Although kaolin does not create adhesion by itself, through these surface groups it can disperse well in the polymeric network and increase the mechanical stability of the system. This same mechanism is widely used in other industries such as rubber production; as noted in the article Review of the Application of Kaolin in the Automotive Tire Manufacturing Industry, this mineral plays a filler role in improving mechanical and process properties.

The chemical neutrality of kaolin is one of the main reasons it is preferred over other fillers. This material is stable over a wide pH range, does not undergo unwanted reactions with the organic and inorganic additives of the formulation, and does not cause color change, gassing, or quality loss during storage. In addition, the low hardness of kaolin (about 2 to 2.5 on the Mohs scale) minimizes equipment wear during stirring and pumping processes—an important economic advantage for adhesive factories that work with high toages of filler.

Light color and high whiteness are also fundamental characteristics of high-quality kaolin, enabling its use in light and colored formulations. To understand the importance of this topic in assessing product quality, you can refer to the guide Evaluation of Appearance Quality and Color of Kaolin Powder. Alongside all these points, kaolin's appropriate oil and water absorption capacity, its medium specific gravity, and its compatibility with a wide range of water-based, solvent-based, and even solvent-free (Hot Melt) systems have tued this mineral into a versatile material for adhesive and sealant formulators.

Application of Kaolin in the Production of Construction, Tile, and Ceramic Adhesives

The highest volume of kaolin consumption in the adhesive industry belongs to construction adhesives, and especially tile, ceramic, and stone adhesives. These adhesives, which are typically produced based on cement, water-redispersible resins (such as VAE/VAM), or a combination of both, require a filler that reduces costs while maintaining the workability of the mixture and, after drying, provides adequate mechanical strength alongside the binder base. Kaolin performs exactly this role.

In tile and ceramic adhesives, kaolin is known as a functional filler. The presence of this material's sheet-like particles gives the adhesive mixture a smooth, pasty consistency, helps it adhere well to the trowel, and prevents sagging when spread on a wall. This characteristic, called "slip resistance," is vital for wall and ceiling tiling. In large construction projects, if the adhesive pulls the tiles downward and causes them to overlap after application, the cost of rework and execution time increases dramatically.

In cement-based adhesives, kaolin plays another role as well: as a processing aid, it facilitates the uniform distribution of cement, sand, and other additives and prevents water bleeding during application. As a result, after hardening, a more cohesive adhesive with lower porosity and higher adhesion to both the substrate and the back of the tile is produced. The presence of kaolin also reduces capillary absorption and rapid water uptake by the surface, which is beneficial for a longer open time of the ready-to-use adhesive and for moisture resistance.

The application of kaolin is not limited to cementitious adhesives. In wood and paper adhesives, in the formulation of industrial polyester and epoxy adhesives, and in hot melt adhesives used in the packaging and cork industries, kaolin serves as an inexpensive and compatible filler. In coating and roller-applied adhesives, the presence of kaolin ensures that the adhesive layer, once dry, has uniform color, a smooth surface, and high paintability. In many of these cases, the choice of kaolin particle size (mesh) varies depending on the transparency or covering power desired by the formulator.

Also, in tiling and stonework adhesives used with dark stones or colored tiles, kaolin works alongside other fillers to help create a consistent texture and prevent surface staining. Alongside kaolin, other minerals such as talc are also used in the construction industry and in the production of standardized mineral products; to lea about the standardization path for these materials, you can read the article Review of National and Inteational Standards in White Talc Production. In general, in all these applications, the consistency of quality, color, and particle size distribution of kaolin are variables that directly determine the quality of the final construction adhesive.

The Role of Kaolin in the Production of Various Sealants (Silicone, Polyurethane, and Acrylic)

The sealant industry is one of the largest markets for kaolin consumption worldwide. Sealants are pasty materials used to fill joints, expansion joints, building coections, glazing, sanitary fixtures, and for waterproofing. Unlike adhesives, whose primary purpose is joining two surfaces, sealants must provide adhesion along with the ability to absorb energy, accommodate joint movement, and seal. Among these, kaolin plays a vital role in the structure and performance of these products.

In the production of silicone sealants, kaolin is one of the main fillers, sometimes making up 30 to 40 percent of the formulation's weight. In fact, in a silicone sealant, a significant portion of the volume of the tube in your hand is mineral material, not silicone polymer. The reason for this is kaolin's remarkable ability to control rheology: the mixture must be fluid enough to exit the cartridge under hand pressure or a dispensing gun, yet thick and stable enough to remain in the joint after extrusion, resist sagging, and hold its shape until fully cured. With its layered structure, kaolin delivers this behavior in the best possible way.

One of the key issues in silicone sealants is the transparency or semi-transparency of the product. To produce transparent sealants, kaolin with very fine particle size (high mesh, near 1500 mesh) and very light color is needed to minimize light scattering so that the final product remains transparent or semi-transparent. In contrast, for colored and white sealants, lower mesh sizes can be used, since transparency is not important in these products. Additionally, the surface of kaolin particles in silicone sealants is modified through surface treatment so that it remains compatible with the silicone base and oil does not separate from the mixture over time—otherwise the sealant would become hard, brittle, or sticky after a few months.

In polyurethane (PU) sealants and acrylic sealants, kaolin plays a similar role in rheology control and cost reduction, but adds other features as well: in polyurethane sealants used in the automotive, marine, and industrial flooring industries, kaolin acts as a reinforcing filler and helps improve abrasion, oil, and heat resistance. In acrylic building sealants and wood sealants, kaolin improves covering power, paintability, and reduces shrinkage during drying. As a result, filled joints do not crack and require no rework.

Moreover, in the production of specialized sealants such as double-sided adhesive tapes, sealants for facade projects and structural glazing, antibacterial sanitary sealants, and even electronic sealants, kaolin is present as a multi-purpose material. Ultimately, it should be noted that correctly selecting the kaolin mesh range, controlling the moisture level, and monitoring impurities directly affect the service life, appearance, and performance of the final sealant, and this selection must be made according to the sealant's chemical base and the project's requirements.

The Effect of Particle Size (Mesh) on Adhesive and Sealant Performance

One of the most important technical parameters in selecting kaolin for the adhesive and sealant industry is its particle size, i.e. its "mesh." The word mesh refers to the number of openings per inch of sieve; the higher the mesh number, the finer the particles. The kaolin product of the Kani Sang Amiran Project is offered in the 450 to 1500 mesh range—a range that covers a complete spectrum from relatively coarse to very fine particles and enables precise selection based on formulation requirements.

Effect of particle size on rheology: finer kaolin particles have a larger contact surface with the surrounding liquid (water, solvent, or resin), and as a result, at a constant weight percentage, they thicken the mixture and increase its viscosity. In other words, if a sealant formulation is produced with 1500-mesh kaolin, the final mixture may be much thicker than when 450-mesh kaolin is used. This feature allows the formulator to adjust the viscosity, shear rate, and thixotropic behavior of the product by choosing the right mesh, without the need for expensive chemical additives.

Effect on transparency and appearance: in transparent and semi-transparent silicone sealants, particle size plays a decisive role. The finer the particles (higher mesh), the less light is scattered and the more transparent the product appears. This is why high-mesh kaolin (near 1500 mesh) is used to produce high-quality transparent sealants. Conversely, in construction adhesives, wood adhesives, and colored sealants, transparency is not important, and lower-mesh kaolin (450 to 600 mesh) can be used, which is less costly and, in many cases, due to its coarser particles, exhibits better filling behavior in high-volume formulations.

Effect on mechanical strength and final surface finish: particle size also affects the quality of the dried product's surface. Finer particles produce a smoother, more uniform surface with lower porosity, which is important for decorative sealants and adhesives with high visual quality standards. Coarser particles may create a slightly rougher surface, but in retu, in some systems they can deliver better abrasion resistance and mechanical density. The choice between these two options depends on the type of final product and its appearance standards.

Effect on sedimentation and stability: in thier formulations, such as low-viscosity water-based adhesives, coarser kaolin particles tend to settle more quickly. In such cases, using higher-mesh kaolin is advantageous due to its slower settling rate and the formation of a more stable suspension structure. It should also be noted that besides particle size, the particle size distribution is important; a powder with a uniform distribution provides more predictable behavior during production. This is why sourcing kaolin from a supplier that continuously performs particle size quality control is of vital importance for adhesive and sealant manufacturers.

Technical and Economic Benefits of Using Kaolin in Adhesive Formulations

Using kaolin in the production of adhesives and sealants is not merely a conventional choice, but a technically and economically justified decision. Below, we examine the most important benefits of this mineral from both technical and economic perspectives.

Reduced production costs: mineral fillers are always more economical than polymers and resins. By replacing a portion of the expensive resin or polymer with kaolin, production costs are significantly reduced without compromising the quality of the final product. This is of great importance in competitive markets such as construction adhesives and general-purpose sealants, where profit margins are low. Thanks to its relatively abundant reserves and simple processing, kaolin is one of the most cost-effective mineral fillers available on the market.

Rheology and shear behavior control: as previously mentioned, kaolin's sheet-like structure imparts desirable rheological behavior to the mixture. This advantage allows the producer to manufacture a product that is easy to pump, fills cartridges without air entrapment, applies smoothly onto surfaces, and resists sagging. As a result, both the end user's experience improves and product loss during production and packaging decreases.

Increased thermal and chemical stability: kaolin is a stable mineral that does not decompose at high temperatures and is chemically neutral to most substances. Its presence in industrial sealants and adhesives enhances the product's resistance to heat, moisture, oil, and chemicals. For example, in sealants used in the automotive and marine industries, this thermal and chemical stability is vital.

Reduced shrinkage and cracking: one of the common problems in solvent- or water-containing sealants and adhesives is volumetric shrinkage during drying, which leads to cracking. The presence of kaolin as a non-volatile solid maintains the material's volume during drying, reduces shrinkage, and prevents the formation of fine cracks on the surface. This feature is especially important in acrylic sealants and water-based adhesives that are exposed to moisture and temperature changes.

Improved color and paintability: the whiteness and light color of high-quality kaolin enable the production of light and colored adhesives and sealants. Also, kaolin particle surfaces are compatible with dyes and pigments and facilitate uniform color distribution in colored formulations. For further reading on this topic, you can consult the guide Evaluation of Appearance Quality and Color of Kaolin Powder.

Other benefits: in addition to the above, kaolin's low hardness causes less wear on production equipment (mixers, pumps, and nozzles). Its proper packing density reduces the separation of water and organic materials during storage, and in certain applications, kaolin even acts as a curing aid and improves fire resistance. Taken together, these benefits have made kaolin the default filler in most commercial adhesive and sealant formulations around the world.

Selection, Quality Control, and Storage Tips for Kaolin for Adhesive Manufacturers

Selecting the right kaolin for an adhesive or sealant production line is a decision that directly affects the quality of the final product and production costs. In this section, we review the most important criteria to consider when purchasing, testing, and storing this mineral.

Kaolin selection criteria: the first parameter is particle size (mesh), which should be chosen based on the product type and rheological requirements. The second parameter is kaolin's color and whiteness, which is critically important for light and transparent products and must also be uniform in colored products. The third criterion is moisture content; high moisture in kaolin can cause bubbles, quality loss, and even prevent complete curing in sensitive formulations (especially silicone sealants, which cure through polymerization reactions). The fourth criterion is purity and the level of impurities; organic impurities, iron oxide, titanium, and other heavy metals can cause color changes, unwanted reactions with additives, and even interfere with silicone catalysts.

Quality control upon receipt at the production line: it is recommended that every batch of kaolin be tested for color uniformity, particle size distribution, and moisture content before entering the production process. The simplest tests include visual inspection, moisture determination using a rapid moisture meter, and, where possible, particle size analysis. Conducting these tests on a regular, ongoing basis ensures that any potential quality variations in the ore body are quickly detected and corrected in the formulation. To lea about the standardized approach in the mineral industry, read the article Review of National and Inteational Standards in White Talc Production.

Storage and handling tips: kaolin should be stored in a dry, covered environment protected from direct moisture. Contact with water or high humidity, besides raising the initial moisture content, can cause particle caking and the formation of hard masses at the bottom of the storage pile, which create serious problems during transfer and weighing in production. Storage near reactive chemicals, acids, and volatile organic substances should also be avoided. When using big bags, it is recommended to place them on pallets, elevated from the ground, and covered with a protective cover.

Formulation adjustment: if the kaolin supply source or its specifications change, the formulation should be reviewed and adjusted accordingly. A change in particle size may require readjusting viscosity-control additives, lubricants, or even the amount of water/solvent. Professional producers typically prepare a laboratory sample of the final formulation with the new kaolin and evaluate parameters such as viscosity, sag resistance, drying time, adhesion, and mechanical strength after complete curing.

Ultimately, sourcing kaolin from a reliable and consistent supplier is the key to success in continuously producing high-quality products. The Kani Sang Amiran Project offers its kaolin product with particle size specifications of 450 to 1500 mesh and primary applications in the tile, ceramic, glaze, paper, and paint industries; this broad range also makes it adaptable to various adhesive and sealant formulations.

Conclusion: The Position of Kaolin in the Future of the Adhesive and Sealant Industry

Throughout this article, we have examined the irreplaceable role of kaolin in the production of various adhesives and sealants from different angles. From its molecular structure and sheet-like particle shape that create desirable rheological behavior, to its broad applications in construction adhesives, tile and ceramic adhesives, and silicone, polyurethane, and acrylic sealants, and ultimately the importance of choosing the right mesh. All of these points demonstrate that kaolin is not merely a "cheap filler," but a functional mineral that directly shapes the quality, price, and performance of the adhesive and sealant industry's end products.

The adhesive and sealant market in Iran and the region is growing, driven by the growth of the construction sector, infrastructure development, the expansion of the automotive industry, and rising demand for industrial products—and this growth brings with it increased demand for mineral fillers, including kaolin. In this environment, producers who can offer their products at optimized costs and consistent quality by correctly selecting kaolin quality and mesh will gain a better competitive position. Access to domestically produced kaolin with a wide mesh range (450 to 1500 mesh), such as the product offered by the Kani Sang Amiran Project, enables Iranian manufacturers to meet their needs at scale without relying on imports.

Future trends: in recent years, interesting developments have taken place in the mineral filler industry. One such trend is the development of surface-treated kaolins, which, through an organic coating on the particle surfaces, achieve better compatibility with various polymer bases and are used in high-quality sealants and advanced industrial adhesives. Another trend is the development of calcined kaolin (metakaolin), which imparts pozzolanic properties and high mechanical strength in specific systems. Although these are specialized products, they point the way to the industry's future development.

Also, with growing attention to environmental sustainability and the circular economy, the position of minerals as relatively eco-friendly alteatives to expensive, processed chemicals is being strengthened. Kaolin, as an abundant and low-hazard mineral, holds a special position in this approach. Alongside this, the standardization of mineral production processes and quality control in Iran is also developing, which can help integrate the market and increase buyer confidence.

Final recommendation: if you are active in the adhesive, sealant, tile and ceramic, or related industries, we suggest that, before making a purchasing decision, you determine your formulation's exact requirements (in terms of viscosity, transparency, drying speed, and final strength), and then select a kaolin whose particle size and quality specifications align with those needs. For more information about product specifications, you can visit the kaolin product page on the Kani Sang Amiran Project website. We hope this article has provided you with a clear and practical perspective on the position of this mineral in the adhesive and sealant industry.

The information in this article is based on the specifications of the kaolin product from the Kani Sang Amiran Project (https://ksamiran.ir/products/kaolin/) and general sources from the adhesive and sealant industry.