Functional Roles of Silica Particles in Industrial Coatings

This study investigates the role of various amorphous silica types as colloidal silica, fumed silica, silica gel and precipitated silica in paint and coating formulations. Each type exhibits unique structural and surface characteristics that contribute distinctly to coating performance. The influence on key functional properties such as matting efficiency, scratch resistance, rheological behavior, and corrosion protection is discussed here. Through comparative evaluation, the paper highlights how the careful selection and incorporation of these silica types can optimize both the aesthetic and protective functions of modern paints, offering valuable guidance for formulators aiming to enhance coating performance through tailored silica technologies.

Keywords:Silica; Nanoparticles; Ammonia; Gloss; Coatings

Silica is one of the most extensively researched chemicals in science and engineering, sometimes known as silicon, which is a compound composed of silicon and oxygen (SiO2). Over %95 of the earth’s crust is made of minerals containing silica. Silica compounds are found naturally in the environment and can also be produced synthetically. It naturally occurs in both crystalline and amorphous forms and both are possessing distinct properties. Frequently experienced crystalline silica forms are quartz, tridymite, cristobalite and part of a production of glass and concrete and as well as high-temperature applications like ceramics and refractory materials [1]. The precision in the coating industry on less hazards in workplace push the investigations on amorphous technology rather than crystalline silica technology. Amorphous silica can be produced both powder and pre-dispersed forms easily [2].

Synthetically produced Silica Nanoparticles (SNPs), one of the most widely utilized materials in modern science, and used in paints and coatings industry for a long time. Although it has been worked on for many years, the interest in the studies still continues with a greater potential [3]. Silica is one of the important raw materials for the coatings industry since it carries a wide range of features. It can be utilized to manage several properties such as gloss, mechanical properties of the final coated film, film formation, and mostly rheology. Silica can effectively prevent sagging, improve scratch resistance and bonding strength between coating and substrate, improve the leveling performance of the coating. The huge variety of performance requirements can be achieved not only by adjusting the particle size and morphology during production but also via surface treatments. Particle size and morphlology are strongly depend on the production method which are mentioned in following paragraphs. Silica has an adjustable surface including hydrophilic silanol groups (SiOH) and slightly hydrophobic siloxane groups that can be altered through thermal and chemical treatments. While siloxane bridges only weakly accept hydrogen bonds, silanol groups are hydrophilic due to their capacity to both donate and accept hydrogen bonds. Types of silanol groups and siloxane bridges on the surface of amorphous silica are shown in Figure 1. By modifying the functional groups on the surface, viscosity and antisettling behavior can be optimized. Silicone is an also important raw material for defoamers [4,5]. Silicone defoamers are highly regarded since it has a surface tension of about 20-21mN/m which is significantly lower than water (72mN/m) and many other liquids. Because of this, they quickly reduce the surface tension of a system and achieve effective defoaming effect. Synthetic amorphous silica can be existed in several forms as colloidal silica, fumed silica, precipitated silica, and silica gel as shown in Figure 2. Each type of silica product provides unique functionalities that make them ideal for specific applications [6].

Figure 1:Surface of amorphous silica.

Figure 2:Different forms of silica [7].

Colloidal silica

An electrostatically stabilized dispersion of amorphous silica particles (aqueous suspension) in water is known as colloidal silica and it is unique that it is a liquid, whereas the majority of silica materials are in solid forms that are challenging to treat. Particle collision, bonding, and aggregation into long-chain networks are decrease the colloidal silica stability. This can be triggered by adding salts or cations into the solution or by adjusting the pH of the solution. Particles have large surface charge density when the colloids are far from their isoelectric point in charge stabilization [7]. The silica particles in colloid are extremely small since the colloidal state in the colloidal state particles are large enough (>1nm) to exhibit noticeable departures from the characteristics of real solutions yet small enough (≤1μm) to be unaffected by gravity [8]. Colloidal silica is among the simplest forms of silica to include in formulations. In the colloidal range, ratio of surface to volume is very high. Thus, colloidal silica particles possess extremely large surface areas that enhance their reactivity through silanol groups located on the particle surface. Colloidal silica adhere well to not only many surfaces but also polymer functional groups, metal ions, and even other silica particles by ionic and hydrogen bonds through silanol groups. By this way colloidal silica is an excellent surface modifier, cross-linker, binder, and reinforcing agent in various industrial applications [8,9].

A number of different synthesis routes of colloidal silica have been developed. Synthetic silica can be produced on an industrial scale via two main processes categorized by the phase of the silica feedstock which is, gaseous and liquid routes are presented in Figure 3. Within these two major categories are a number of distinct synthesis methods each having advantages and disadvantages in the final usage [9]. To produce colloidal silica different methods have been suggested but sol-gel method is mostly used method with well-defined and uniform morphologies as well as preferable purity and homogeneity on a molecular level. Of particular interest, the Stöber process, which allows production of spherical, monodisperse silica particles with final mean particle sizes between 0.05 and 2μm, is one of the most well-used sol-gel synthesis routes. The method is based on reacting a silica source (mainly tetraethyl orthosilicate, TEOS) with a water-solvent mixture in the presence of a base catalyst (ammonia, NH4OH) and alcohol as the solvent. Sop [10] synthesized SNPs by Stöber method successfully with a particle size of 500nm with narrow distribution as shown in Figure 4. In this method, the particle growth rate is limited by the hydrolysis of alkoxysilane molecules and proceeds through the surface condensation of hydrolyzed monomers or small oligomers [10]. Colloidal silica can also be produced from sodium silicate solution by ion exchange method without using starting materials which is generally expensive. However, the regeneration of ion exchange resins is seen as a significant problem in financial and environmental terms. Furthermore, Na ions remaining in the product could diminish the purity and stability of the colloid [11].

Figure 3:Colloidal silica synthesis methods.

Figure 4:a) SEM photograph of the SNPs synthesized by Stöber method; b) number frequency size distribution obtained from the SEM image.

Fumed silica

Fumed silica is a light, fluffy powder made of amorphous silica particles produced by flame hydrolysis process which is efficient and versatile. The reaction is done in the hydrogen/air flame which a vaporizable metal precursor is fed and the hydrolysis product, silicic acid for example, rapidly condenses to the metal oxide. The particle sizes of the fumed silicas are usually on the order of tens of nanometers and they possess a large surface area and significant reactivity. Silanol groups present on the surface enhance hydrogen bonding and Van der Waals interactions among different components in a chemical formulation. Fumed silica particles can be modified to gain hydrophobic character which is necessary for specific applications. In the incorporation step, careful mixing must provide. Fumed silica is frequently used as a rheology modifier in formulations [2,12,13].

Precipitated silica

Precipitated silica is produced from sodium silicate by the reaction with sulfuric and hydrochloric acids which forms amorphous silica particles. The particle size of precipitated silica is greater than that of fumed silica, which is beneficial for producing an abrasive effect for polishing or cleaning surfaces. Controlling both the particle size and porosity of precipitated silica is useful for specific applications which is related to process conditions on the precipitation reaction takes place. Temperature, concentration of the solutions used, and the ratio of the components are the main factors that affect the physical properties of the silica particles. It can be used to rheology modifier in coating applications. But careful attention must be done to incorporate into formulations to create a suspension [2,6,13].

Silica gel

Silica gel is a highly porous synthetic amorphous silica defined by coarse granules or beads. It is produced by acidifying a solution of sodium silicate, resulting in a gel. Silica gel features the broadest particle size variation within the silica family, with each separate bead potentially measuring from a few micrometers to several millimeters in diameter. The material is made up of a network of tiny pores that are coupled each other, giving it a large surface area in a comparatively small volume. Silica gel is highly porous which enables moisture by uptake physical adsorption of water molecules onto the surfaces of these pores. Hence the importance and difference of silica gel in various industries is its ability to control moisture [2,13]. A comparison of the main physical properties of synthetic silica is shown in Table 1. The distribution of Particle Sizes (PSD) is another important characteristic of SNPs that must be precisely measured for effective application [2,3].

Table 1:Main properties of different type of silica.

The untreated silica particles possess a hydrophilic surface due to the presence of silanol groups which are very weak acids and are hardly reactive. Because of this reason, it is possible to chemically convert them. Treatments with chlorosilanes, alkoxysilanes, silazanes and siloxanes have all proven successful. The main aim for surface modification is to make the hydrophilic particles more hydrophobic. Hydrophobic character improves the viscosity, stability, and flow properties of the paint which is necessary especially for in high-quality coatings. For waterbased coatings, although the hydrophobic silicas‘ density is greater than water, hydrophobic types are not wetted by water, they remain on the surface of water. Typical surface treatments is shown in Table 2 and Figure 5 shows the final surface after reaction with the silica surface resulting in the –O-Si bridges to the functional groups. Aerosil R 972, as the first hydrophobic product, was the initial “aftertreated” product developed for commercial use and launched in early 1962. The product’s hydrophobic properties are attributed to the dimethylsilyl groups present on its surface. Both hydrophilic and some hydrophobically modified grades (e.g., DDS, HDS, HMDS) of fumed silica can be used in water-based coatings if adequate dispersion is provided for homogenous incorporation [2,14,15]. The DDS, HMDS, and TMOS modifications can also improve water resistance.

Table 2:Common surface treatments for fumed silicas..

Figure 5:Several fumed silica surface treatments and resulting surface groups [15].

One of the important parameters to introduce fumed silica particles in coatings is the proper dispersion to get best performance. Shear rate, dispersion time, temperature control, and addition level are main factors that must be deal with for adequate dispersion. Generally, addition under high-speed mixing using a saw-type blade at a shear rate >10m/s is recommended. When shear rate is inadequate, dispersion time alone is ineffective. The consequences of poor dispersion are typically visible larger particles (agglomerates) formed in final coating, reduced thickening efficiency, poor thixotropic stability over time, lower gloss and transparency, and several film defects. On other point of view, paints and coatings manufacturers have been looking for ways of making dispersion processes less complex and reduced production costs, while retaining performance. This need has been met by fumed silica technology by minimizing the number of production steps required for processing and the necessary batch cycle duration. It is applicable in both waterbased and solventbased coatings, but, the low viscosity and high dielectric constant makes water a poor continuous phase for fumed silica, making it challenging to attain a uniform dispersion of particles for optimal advantages in water-based coatings. So, pre-dispersed form of fumed silica is recommended to overcome these throwbacks [2].

One of the important parameters to introduce fumed silica particles in coatings is the proper dispersion to get best performance. Shear rate, dispersion time, temperature control, and addition level are main factors that must be deal with for adequate dispersion. Generally, addition under high-speed mixing using a saw-type blade at a shear rate >10m/s is recommended. When shear rate is inadequate, dispersion time alone is ineffective. The consequences of poor dispersion are typically visible larger particles (agglomerates) formed in final coating, reduced thickening efficiency, poor thixotropic stability over time, lower gloss and transparency, and several film defects. On other point of view, paints and coatings manufacturers have been looking for ways of making dispersion processes less complex and reduced production costs, while retaining performance. This need has been met by fumed silica technology by minimizing the number of production steps required for processing and the necessary batch cycle duration. It is applicable in both waterbased and solventbased coatings, but, the low viscosity and high dielectric constant makes water a poor continuous phase for fumed silica, making it challenging to attain a uniform dispersion of particles for optimal advantages in water-based coatings. So, pre-dispersed form of fumed silica is recommended to overcome these throwbacks [2].

Fumed silica with different modifications has been utilized for many years in coating formulations to give thixotropy, anti-settling and anti-sagging properties. The preferred surface modification allows the formulator to adjust mainly the rheological properties for the specific end use application. Untreated, hydrophilic fumed types perform better in non-polar settings, while hydrophobically modified types, like those modified with DDS, TMOS, and HMDS, are more effective as polarity rises. According to the literature the fumed silica with the PDMS surface treatment provides the most effective thickening in epoxy coating related to its high degree of hydrophobicity and the long polydimethylsiloxane chains. The PDMS chains engage with the siloxane chains of neighbouring particles, facilitating the creation of the three- dimensional fumed silica network. The interactions between particles in the three-dimensional network enhance viscosity at low shear rates. Trimethoxy-Octyl-Silane (TMOS) has the next most effective thickener due to the relatively long hydrophobic eight-carbon chain extending from the silica surface. Other surface treatments provide lesser thickening efficiency. The short chain Dimethyl-Dichloro- Silane (DDS) provides the least amount of viscosity modification [2,15].

To enhance the corrosion resistance and water-repellent qualities of coatings, hydrophobic modified types of fumed silica have been utilized alongside anticorrosive pigments [2]. They function efficiently with various kinds of anti-corrosive pigments, including modified barium metaborate, calcium phosphosilicate, and zinc dust. Incorporating 1-3% by weight of total formulation particles is essential for anticorrosive formulations to guarantee sufficient particles in the coating matrix, thereby creating a hydrophobic barrier, enhancing the film’s mechanical properties, and boosting hydrophobicity. According to the literature, the best results have been obtained using DDS, TMOS, and HMDS treatments nut meanwhile the impact on formulation rheology must also be considered to be sure that there no additional viscosity increase. Ionexchanged silica anticorrosive pigments alone are also alternatives to traditional chromate- based pigments anticorrosive pigments. They are non-toxic and it is noteworthy that addition levels are often about one-third to one-half that of conventional anticorrosive pigments in weight. Figure 6 shows the representation of Ca/Silica pigments surface [2,16].

Figure 6:Surface of Ca/Silica pigments.

Scratch resistance is considered as one of the most important criteria in paint and coating industry. Traditionally the scratch resistance of an organic coating can be improved by the addition of high content of inorganic filler by creating high reinforcement of elastomers and composites. The polymer matrices need to be loaded with much greater amounts of fumed silica without altering the viscosity. This is accomplished through structural alterations while maintaining a low level of aggregation, leading to enhanced mechanical strength and scratch resistance of the coating. Additionally, there was a notable rise in bulk density and a sharp decline in the viscosity increase. When compared to other reinforcing fillers like alumina, silica has the advantage of a lower refractive index (1.46), ability to closely align with many polymer systems, which results in improved transparency and clarity [2]. As a consequence, coating formulations containing colloidal silica resist abrasions and scratches in two principal ways:
a. Colloidal silica enhances the density of cross-linking reactive groups in organic resins due to its abundant hydroxyl surface groups.
b. Colloidal silica particles are quite hard (5.5 on the Mohs scale of mineral hardness) and greatly enhance coating hardness [17].

The gloss is an optical property of paints which is a measure of the paint finish when light reflects off it. The gloss level is characterized by the angular distribution of light reflected from a surface, which can be measured using a glossmeter or reflectometer, and it changes with the angle of view. The greater the light a surface reflects, the glossier it looks. It another words, the smoother the coating surface, the glossier it appears. Achieving the right gloss plays a vital role for the specific end use applications. Roughening the coating film’s smoothness and elevating its surface roughness reduced the gloss. Low gloss also can be achieved by introducing structures or groups that can absorb light. Matting occurs due to the size of silica particles and the degree of coating contraction that takes place during drying, whether through solvent evaporation, chemical reactions, or coalescence. Larger particles are more effective at reducing gloss, but they can lead to a rough surface and increased dirt buildup over time. The majority of silica types employed for matting coatings are created through wet and gas phase methods and are traditionally larger than those utilized for rheology. Certain grades may affect thickening to varying degrees. Surface treatment, using either wax or reactive oligomers, can help minimize viscosity accumulation, avoid hard settling, and enhance clarity and stability. Recent technological advancements have created silica with enhanced haptic features such as a soft touch [2,18].

The application of silica in coatings remains advantageous due to ongoing innovation. Producing silica particles with regulated particle shape and narrow size distributions is essential. The round shape gives a high apparent hardness, enhancing scrub, abrasion, and burnish resistance of the formulated coating, while requiring less binder and having a minimal effect on coating rheology. These spherical silica particles can also offer matting characteristics, depending on their size, and they possess superb transparency suitable for use in deep hues and clear finishes. Surface treatments are crucial in numerous applications, including the paint and coating industry. For many years, modified silica grades have been utilized to enhance rheology, film formation, mechanical properties, and surface appearance.

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