May. 13, 2024
Agriculture
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By C. Jim Reader and Maria Nargiello, Evonik Corporation
The field of coatings technology has utilized many forms of silica-based particles over the last 70 years. This large, varied class of fillers is divided into two categories: crystalline and amorphous morphology. With increasing scrutiny and sensitivity in the coatings industry to reduce workplace hazards, there is greater emphasis on amorphous technology to replace crystalline silica. Amorphous silica is highly adaptable and can be modified in both powder and pre-dispersed forms. Numerous engineered technologies have been developed to solve many coatings problems.
Amorphous silica technology addresses functionalities such as rheological control, suspension of pigments and fillers, reinforcement of coatings film, scratch resistance, hydrophobicity, anti-corrosion benefits, oleophobicity, and gloss reduction. It also acts as a carrier of trace actives for homogeneous distribution and enhances flow control, charge, and fluidization of powdered coatings. Various types of tailor-made modifications will be highlighted for their performance attributes. The importance of proper dispersion and homogeneous distribution within a coating matrix will be reviewed.
This article will cover how amorphous silica technology is differentiated and engineered to create specially tailored solutions to enhance the performance of coatings, along with the latest technical developments in this field.
Silica, or silicon dioxide, is one of the most abundant minerals on earth. It is estimated that quartz, the most stable form of this family of materials, makes up more than 10% of the earth’s crust and is a key raw material for producing glass and silicon, widely used in the construction industry.
Silica is also crucial for the coatings industry, offering functionalities such as rheological control, enhanced film formation, improved mechanical properties of the final coating film, free flow and fluidization enhancement of powders, and gloss control. It is also used in defoamers. Coatings industry silica grades are synthetically produced, meeting higher quality control standards, and often having tighter physical-chemical requirements. Adjusting particle size and morphology during production, alongside surface treatment and densification, achieves the variety of performance properties.
A summary of the main methods for producing synthetic silica is shown in Figure 1. The most common types of silica used in modern coatings are produced either by a liquid phase process of precipitation or a gas phase process of flame hydrolysis. Precipitated silica is produced through the controlled reaction of sodium silicate ("water glass") and sulfuric acid, similar to silica gel production. The silica is precipitated, filtered, washed, and dried before milling and classification.
The production of fumed silica began with the discovery of the flame hydrolysis of silicon tetrachloride by Harry Klöpfer in 1943. This wartime discovery aimed to produce silica that could act as a white reinforcing filler to modify rubber, essential for tire production to replace oil used to make carbon black. A simple diagram of the process is shown in Figure 2. The overall chemistry of the process is efficient and versatile. A vaporizable metal precursor is fed into a hydrogen/air flame, and the hydrolysis product, silicic acid, rapidly condenses to the metal oxide. Multiple pathways to particle formation are possible, such as particle growth through deposition, particle evaporation, aggregation, and aggregate coagulation. The efficiency of the overall chemistry makes the process amenable to variation. A diverse array of metal oxides beyond silica has been produced, including mixed metal systems and surface-modified and doped particles for various industries and applications.
Fumed silica consists of three conceptual levels of structure (Figure 3). The primary particle exists only briefly in the flame. Primary particles fuse to form an aggregate, the secondary particle structure. Isolated primary particles are rare. The tertiary structure is an agglomeration of the secondary structures. This collection of particle aggregates can be disrupted by applying shear, reforming over time after shear removal. This mechanism is how fumed silica imparts pseudo-plastic rheological properties to formulations.
A comparison of the different physical properties of synthetic silica is shown in Table 1. Notably, all synthetic silica types are amorphous and do not contain crystalline silica, as confirmed by X-ray diffraction.
The second element of particle design is surface modification to render hydrophilic particles hydrophobic. This is achieved by reacting surface silanol groups with different silanes. These treatments create various grades of fumed silicas that differ in hydrophobicity, tribo-electrostatic charge, and thickening efficiency. A summary of typical surface treatments with corresponding attributes is shown in Table 2. The level of treatment, measured by carbon content and methanol wettability (Figure 4), indicates the consistency of treatment and the balance of hydrophilic to hydrophobic surface.
The Multipoint Methanol Wettability method is a quantitative test for measuring the level and consistency of hydrophobic treatment. A 0.2 g sample of treated silica is added to a series of graduated test tubes containing 8 ml of methanol in water dilutions made in 5% increments, from 100% water to 100% methanol. The silica/solution mixtures are shaken and then centrifuged under controlled conditions. Depending on hydrophobicity and surface treatment consistency, the silica wets differently into each water. The amount of wetted silica in each solution is recorded and plotted to a curve known as the methanol wettability fingerprint. Silicas needing higher methanol amounts for wetting are more hydrophobic. Consistently treated silica shows a steep rise in wetting, while a gradual curve indicates a wider range in the consistency of treatment. Precipitated silicas can also be surface treated, typically with waxes and reactive oligomers, to improve product and formulation stability and reduce viscosity impact.
The third element of particle design is structure modification via several proprietary processes. Granulation results in larger, individual spherical particles ranging from 20–30 µm that are porous, acting as free-flowing carriers of liquid-based actives and oils. Other chemical and mechanical post-processes reduce structure (i.e., the level of aggregation or agglomeration). Products from post-processing can have significantly higher bulk densities and dramatically reduced thickening efficiency due to reduced levels of aggregation at the primary aggregate level. The functional benefit resulting from such grades is enhanced scratch and abrasion resistance, as higher loading can be achieved with minimal impact on formulation viscosity. This higher loading results in reinforced domains, driving the scratch and abrasion resistance.
Fumed silica, in various grades and modifications, has been used for decades in coating formulations to impart thixotropy, anti-settling, and anti-sag properties. Proper selection and adequate dispersion to homogenously distribute aggregates throughout the coating matrix are essential for good performance. Proper grade selection depends loosely on dosage, particle size, structure, and surface treatment. Untreated, hydrophilic fumed silica grades perform best in non-polar environments, while hydrophobically modified grades, such as those treated with DDS, TMOS, and HMDS (Table 2), are more efficient as polarity increases, as shown in Figure 5.
Grades treated with TMOS and HMDS are highly effective for high solids and radiation-cure systems. Polydimethylsiloxane-treated grades are the most hydrophobic, ideal for high solids and 100% solids systems, though care must be taken as migration of free PDMS may cause surface defects or adhesion problems.
Proper dispersion of fumed silica is crucial. When optimizing a dispersion for thickening efficiency and rheological enhancement, parameters such as shear rate, dispersion time, temperature control, and sequence of addition are important. High-speed dispersion using a saw-type blade at a shear rate >10 m/s is recommended. Longer dispersion time will not compensate for inadequate shear rate. Poor dispersion typically results in larger agglomerates, reduced thickening efficiency, poor thixotropic stability, lower gloss and transparency, and possibly film defects.
Fumed silica has excellent thermal stability, but increasing temperature during shear improves the wetting properties of the coating, potentially leading to over-dispersion. This reduces thickening efficiency, sometimes to the point of appearing as if no thickener was added. Fumed silica, as one of the smallest particle size materials, should be added early in the formulation, preferably to the resin or binder. Caution is advised when post-adjusting batches with powder, as minimal shear may lead to inadequate incorporation. Post addition with low shear may achieve desired rheology initially, but this can deteriorate over time as larger agglomerates slowly wet out.
Lab evaluations have demonstrated that multiple performance attributes can be enhanced using fumed silica dispersions. These include improved suspension of pigments, fillers, and matting agents, reduced tack, improved dirt pick-up resistance, enhanced film strength, and even improved film formation without compromising gloss and other appearance attributes. An example shown in Figure 6 demonstrates how a pre-made aqueous fumed silica dispersion improves film formation and reduces coalescent solvent levels. These improvements have been seen with many different film-forming resins, using less or no coalescing solvents, helping reduce overall volatile organic compounds of the formulation. Enhanced film formation results from reduced stress propagation due to the reinforcing effects of finely dispersed fumed silica.
Hydrophobically modified grades of fumed silica enhance the corrosion performance and water repellency of coatings when used with anticorrosive pigments. These grades are not anti-corrosion pigments but work effectively with many classes of anti-corrosive pigments, such as modified barium metaborate, calcium phosphosilicate, and zinc dust. Loadings between 1.0% and 3.0% by weight of total formulation ensure sufficient particles in the coating matrix to support a hydrophobic barrier, improve mechanical properties, and increase hydrophobicity. Water repellency, measured by improved blister resistance, can be improved at lower loading levels starting at 0.5% by total formulation weight. Examples of this effect are shown in Figure 7.
Proper dispersion is again needed for homogenously distributing the silica throughout the coating matrix. It is suggested that treated fumed silica be dispersed with the anti-corrosion pigments for optimal dispersion. Best results have been obtained using DDS, TMOS, and HMDS treatments, although the impact on formulation rheology must also be considered.
The development of silica particles for improving scratch resistance emerged from the creation of particles for high reinforcement of elastomers and composites. High reinforcement is achieved by filling polymer matrices with higher levels of fumed silica without increasing viscosity to unworkable levels. This is done by structure modification through proprietary post-processing, resulting in highly reduced structure and low aggregation. This material can be used as a reinforcing filler to increase mechanical strength and impart scratch resistance. A significant increase in bulk density and a dramatic reduction in viscosity are observed. Silica has the advantage of a lower refractive index of 1.46, more closely aligned with many polymer systems, resulting in improved transparency and clarity. These materials can be hydrophobically modified with DDS, HMDS, and TMOS for improved water resistance.
The main consideration for successful use of surface-treated, structure-modified fumed silica particles is adequate loading level. Optimum loading levels start at 5% by weight on total formulation and can approach 15%. Inorganic particle load must be high enough to attain a homogenous density through the polymer to achieve a consistent, reinforced matrix, as seen in the SEM analysis of a high-solids coating cross-section (Figure 9).
Five percent loading of an easy-to-disperse (E2D) structured modified silica particle treated with DDS achieved improved scratch resistance in a high-solids system, tested by a dry scratch method using a Crock meter (abrasive paper) and wet scratching using an Elcometer (40 double strokes, bristle brush, and 0.15% quartz in water slurry). Improved scratch resistance and higher gloss retention of the coating was observed after both scratching methods, and reduced haze was observed after Elcometer testing (Figure 10). The addition of 5% silica slightly reduced gloss, but significantly improved scratch resistance.
Figure 10 shows three variants of DDS-treated structure-modified silica. Variant 1 is the original requiring milling, variant 2 is the same DDS structure-modified silica pre-dispersed in methoxypropyl acetate (MPA), and variant 3 is the newest version, which is easy to disperse.
Powder coatings, whether conventional, fine, thermosetting, thermoplastic, tribo, or UV-cured, all require good flow, reduced moisture pick-up, good package stability (no caking), efficient fluidization, high transfer efficiency, and reduced Faraday cage effects for even film thickness and optimized appearance during application. Hydrophilic and hydrophobic-treated fumed silica and alumina improve flow, storage stability with reduced moisture pick-up, and improved fluidization and transfer efficiency.
For more information, please visit Hebei Silicon Research Electronic Materials Co., Ltd..
In practice, flow additives used in powder coatings can be added in one of three places during the powder coating manufacturing process: 1) directly in the hopper, 2) dosed into the powder during chipping, or 3) post-added after pulverization. Flow additives can be dry blended into problematic powdered components before charging them into the hopper, ensuring consistent feeding into the extruder. Typical loading of flow additives in this step is 0.1–0.3%. Flow additives used to pre-treat ingredients are extruded into the powder matrix and do not influence the bulk flow properties after compounding and pulverization.
When additives influence final powder coating properties, they must be added after extrusion and be oriented outside the powder coating particles. There are typically two places where silica or alumina (or a combination) can be added into the process: 1) prior to chipping where the additive is cut into the powder coating particle, or 2) after pulverization and classification. Care should be taken when dosing the additive before pulverization, as classification systems can remove the additive, reducing the final dose in the powder coating. Typical dosage level used in chipping or post-adding is also 0.1–0.3% by weight.
A study with the University of Western Ontario assessed performance attributes associated with four classes of additives in different powder coatings. The first two powder coatings were corona-applied. The first was a conventional polyester with d50 of 31.5 µm, and the second was a finer particle size polyester with d50 of 21.5 µm. Additive dosage level was adjusted based on powder particle size: 0.3% for coarse powder and 0.5% for fine powder. A third powder was a tribo-applied polyester powder coating. Tested attributes were angle of repose (flowability), bed expansion (flowability and fluidity), transfer efficiency, Faraday cage effects, gloss, and gel time. Four types of silica were tested: untreated hydrophilic silica with a surface area of 200 m2/g, HDMS- and aminosilane-treated silica with a surface area of 200 m2/g, DDS-treated silica with a surface area of 130 m2/g, and HDMS-treated silica with a surface area of 300 m2/g.
Surface-treated alumina was most effective at improving transfer efficiency and reducing Faraday cage effects due to its neutral to slightly positive electrostatic charge character. This is shown in Figure 11 where a disk applied with coarse powder containing 0.3% high surface area alumina (130 m2/g) has more consistent jetness than a disk applied with powder containing no additive. This test measures the amount and consistency of powder transferred to the disk (by weight) under controlled application conditions.
Faraday cage effects were measured by determining how much coating is deposited in the inner trough of a test specimen. The interior parts of the trough have three removable panels under controlled application conditions. After application, the inner panels are removed and weighed. Reduced Faraday cage effects (improvement) are associated with higher, more consistent weights of powder deposited on these inner removable panels. The 0.3% alumina treated with TMOS effectively reduced Faraday cage effects in the coarse, black powder coating.
Fluidization efficiency was also assessed. Results showed that the particle size of the powder coating significantly affects the additive's effectiveness in improving fluidization. Alumina was more effective in improving fluidization in the coarse powder coating as measured by lower air velocities needed for 20% bed expansion, while silica was more effective in the fine powder coating (Figure 12). This trend suggests that additive packages may need adjustment based on particle sizes.
Gloss, defined by DIN EN ISO 4618, is the human perception of the more-or-less directed reflection of light rays from a surface. Glossy surfaces appear shiny and reflect most light in the specular (mirror-like) direction, while matte surfaces diffuse most light in a range of angles. Gloss level is characterized by the angular distribution of light scattered from a surface, measured with a glossmeter or reflectometer, and it depends on the viewing angle (Figure 13).
There is no globally accepted definition of "matte"; it is measured based on comparative gloss against a standard. For coating surfaces, "gloss" means almost complete reflection, where the surface reflects and scatters incident light in a wide-angle cone. The greater the cone angle, the less gloss is generally observed (Figure 14).
Lin and Biesiada showed that matting is a function of both silica particle size and coating shrinkage during drying (through solvent evaporation, chemical reaction, or coalescence
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