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The Use of Engineered Silica to Enhance Coatings

By C. Jim Reader and Maria Nargiello, Evonik Corporation The field of coatings technology has utilized many forms of silica-based particles in the last 70 years. This large, varied class of fillers is generically broken into two categories of crystalline and amorphous morphology. With ongoing scrutiny and sensitivity in the coatings industry to move towards less hazards in the workplace, greater emphasis is placed on suitable amorphous technology to replace crystalline silica technology. Amorphous silica is highly adaptable and flexible to be modified in both powder and pre-dispersed forms, and numerous engineered types of technologies have been developed to provide functional solutions to many coatings problems. Amorphous silica technology has been developed to address functionalities including: rheological control, suspension of pigments and fillers, and reinforcement of coatings film; to impart scratch resistance, hydrophobicity / anti-corrosion benefits, and oleophobicity; as a carrier of trace actives into coatings for homogenous distribution; for flow control, charge, and fluidization enhancement of powdered coatings; and gloss reduction of liquid systems. Particle technology and modification will be addressed along with performance attributes highlighted for each of the types of tailor-made modifications. The importance of proper dispersion and homogenous distribution within a coating matrix will be reviewed. This article will address how amorphous silica technology is differentiated and engineered to create specially tailored solutions to enhance the performance of coatings and will highlight the latest technical developments in this field. Introduction Silica, or silicon dioxide, is one of the most abundant minerals present on earth. It is estimated that quartz, the most stable form of this complex family of materials, makes up more than 10% of the earth’s crust and, as a major component of the natural sands widely used in the construction industry, is a key raw material to produce glass and silicon.1 Silica is also an important raw material for the coatings industry, as it can provide a wide range of functionalities and benefits. These include rheological control, enhanced film formation, improved mechanical properties of the final coating film, free flow and fluidization enhancement of powders, and control of gloss. Silica is also an important raw material for the formulation and production of defoamers. The silica grades used in the coatings industry are produced synthetically and typically meet greater quality control standards, often having tighter physical-chemical requirements, such as color and brightness. The enormous variety of performance properties is achieved by adjusting the particle size and morphology during production as well as via surface treatment and densification of the silica particles in downstream processes. 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 gas phase process of flame hydrolysis. Precipitated silica is produced by the controlled reaction of sodium silicate (“water glass”) and sulfuric acid similar to the production of silica gels. The silica is precipitated, filtered, washed, and dried before milling and classification (Figure 1). The production of fumed silica began with the discovery of the flame hydrolysis of silicon tetrachloride by Harry Klöpfer in 1943. This discovery was part of a wartime effort to produce silica that could act as a white reinforcing filler to modify rubber, which was then much needed 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 for instance, 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 elegant efficiency of the overall chemistry makes the process very amenable to variation. A diverse array of metal oxides beyond silica has been produced, including mixed metal systems and surface modified and doped particles that can be used for a wide variety of industries and applications. Fumed silica consists of three conceptual levels of structure (Figure 3). The primary particle only exists for a short time in the flame. Primary particles fuse together to form an aggregate, which is the secondary particle structure. Isolated primary particles, in this model, are rare. The tertiary structure is an agglomeration of the secondary structures. This collection of particle aggregates can be disrupted by the introduction of shear, and then reform over time after the shear is removed from the system. This mechanism is the means by which fumed silica imparts pseudo-plastic rheological properties to formulations. A comparison of the different physical properties of synthetic silica is shown in Table 1. It is important to note that all of these synthetic silica types are amorphous and do not contain crystalline silica. This has been confirmed by X-ray diffraction. The second element of particle design is surface modification to render the hydrophilic particles hydrophobic in character. This is achieved by the reaction of the surface silanol groups with different silanes. These treatments create different grades of fumed silicas that vary in hydrophobicity, tribo-electrostatic charge, and thickening efficiency. A summary of the typical surface treatments with corresponding attributes is shown in Table 2. The level of treatment, which can be 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 method to measure the level and consistency of hydrophobic treatment. The 0.2 g of the treated silica is added to a series of graduated test tubes containing 8 ml of dilutions of methanol in water made in 5% increments, starting with 100% water, 95% water, and 5% methanol up to 100% methanol. The silica/solutions mixtures are shaken and then centrifuged under controlled and defined conditions. Depending on the level of hydrophobicity and consistency of surface treatment, the silica will wet differently into each water. Methanol mixtures and the amount of wetted silica in each solution mixture is recorded and plotted to a curve known as the methanol wettability fingerprint. Silicas requiring higher methanol amounts for wetting are more hydrophobic. Consistently treated silica shows a steep rise in wet-in, whereas a more 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 one of several proprietary processes. Granulation results in larger, individual spherical particles in the range of 20–30 μm that are porous; their main function is to act 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 resulting 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 are enhanced scratch and abrasion resistance, as higher loading can be achieved with minimal impact to formulation viscosity. This higher loading results in reinforced domains, which drives the scratch and abrasion resistance. Rheology and Film Formation Fumed silica, in various grades and modifications, has been used for decades in coating formulations to impart thixotropy, anti-settling, and anti-sag properties. The main requirements for good performance are proper selection and adequate dispersion to homogenously distribute aggregates throughout the coating matrix. Proper grade selection can be loosely correlated to dosage, particle size, structure, and surface treatment. Untreated, hydrophilic fumed silica grades give the best performance in non-polar environments, whereas hydrophobically modified grades, such as those treated with DDS, TMOS, and HMDS (Table 2) are more efficient as polarity increases. This trend is 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. This technology can also be considered in high solids and 100% solids systems, where it is very effective. However, care must be taken, as this surface modification is not fully reacted to the surface, and migration of the free PDMS may cause surface defects or adhesion problems. Proper dispersion of the fumed silica is critical to good performance. When optimizing a dispersion for thickening efficiency and rheological enhancement, several parameters should be considered including shear rate. Dispersion time, temperature control, and sequence of addition are all 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. The consequences of poor dispersion are typically larger agglomerates […]

Crosslinking Waterborne Coatings With Bipodal Silanes for Improved Corrosion Protection Performance

By Jacob D. Shevrin and Sheba D. Bergman,  Evonik Corporation As global environmental concerns continue to overshadow the use of well-established metal surface pretreatment processes such as chromate treatment and phosphatization, the need for environmentally friendly corrosion protection systems has never been greater. A promising solution to this worldwide regulatory issue is waterborne silane technology, which can offer a heavy metal-free, volatile organic compound (VOC)-free alternative to protecting metals from corrosion. The mechanism behind this corrosion protection can best be explained by the passivation of a metal surface with a waterborne silane film, which acts as a barrier to water, salts, and other corroding materials in the surrounding environment. It is important to note that the waterborne silane technology investigated in this work can be viewed as a type of conversion coating or pretreatment to the metal surface, rather than a conventional  waterborne coating or primer. Certain waterborne silane technology requires high-temperature curing procedures for optimal results, which can be difficult to achieve in certain applications or industries. With the use of bipodal silanes, the additional crosslinking introduced into the system can alleviate the need for this high-temperature curing procedure. In this novel work, we demonstrate that the incorporation of a bipodal silane into waterborne silane systems improves the surface passivation of the metal surface, enhances the hydrophobicity of the system, and increases the crosslinking density of the system, leading to significant improvements in the corrosion resistance of waterborne silane technology. INTRODUCTION Whether it be for a bridge, a tunnel, an automobile, an electronic component, or a building, corrosion protection technology plays one of the most important roles in maintaining the integrity and longevity of the world around us. There are many well-established methods for protecting metals from corroding over time, including chromate treatment and phosphatization, which have been widely used for corrosion protection across the globe for decades.1,2 While these processes are inexpensive and well-known, governmental regulations and overall awareness of the hazards associated with these methods are growing. In particular, hexavalent chromium, a key material used in chromate treatment for the past 90 years, has recently been subject to new far-reaching restrictions. After the European Union classified hexavalent chromium as a carcinogen and mutagen in 2013, Europe’s Registration, Evaluation, Authorisation & Restriction of Chemicals (REACH) regulations have forced hexavalent chromium to be phased out of most industry applications across Europe. While most industries had to stop using hexavalent chromium by January 2019, some industries, such as the aerospace industry, have been allowed to continue using hexavalent chromium through 2026. However, corrosion protection technology for aerospace applications takes several years of research, development, and qualification, which is why the time for investigating chromium-free corrosion protection technology is now. One viable alternative to these hazardous corrosion protection systems is silane technology. While organofunctional silanes have been widely used as adhesion promoters for several decades now, the use of these materials in corrosion resistant coatings is a more recent development. When properly prepared and applied, organofunctional silane coatings have the ability to form protective barriers on metal substrates, which subsequently protect the metals from corroding over time.3 Previous studies have shown that a only a small amount of active silane content, ranging from 0.2–2.0 wt% solids, is necessary for improving the adhesion of a coating system.4 For this reason, the incorporation of an organofunctional silane into a coating system can provide adhesion promotion or corrosion resistance without significantly increasing the volatile organic content (VOC) of the system. This is one of the many reasons why waterborne silane technology offers excellent corrosion resistance performance without the need for hazardous pretreatments, volatile solvents, or heavy metals. The mechanism behind an organofunctional silane adhering to a metal surface is an important process to understand before investigating the corrosion resistance performance of waterborne silane technology. Over the past several decades, organofunctional silanes have been used as coupling agents for organic and inorganic materials across many different industries. Organofunctional silanes contain a hydrolyzable alkoxysilane (Si–OR) functional group that can bond with inorganic surfaces. In this work, the organofunctional silanes to be investigated have silicon functional groups comprising of alkoxy groups, specifically methoxy and ethoxy groups. Organofunctional silanes also consist of an organofunctional group that can react with organic systems. The simultaneous reaction of the silicon functional groups and organofunctional groups allow organofunctional silanes to act as an adhesion promoter between inorganic and organic materials. For an organofunctional silane-based system to adhere to an inorganic substrate, hydrolysis must first take place at the alkoxy sites to form silanol groups. When the hydrolyzed organofunctional silane comes into contact with an inorganic surface, the silanol groups can initially form hydrogen bonds with the hydroxyl groups on the inorganic surface. Upon removal of moisture from the system, these hydrogen bonds can form siloxane bonds between the organofunctional silane and inorganic surface. These siloxane bonds provide the strong adhesion characteristics for which organofunctional silanes are well known.5 With proper surface preparation and material selection, organofunctional silane-based coatings can form siloxane bonds to many sites on the inorganic surface, forming an organofunctional siloxane network in the process (Figure 1). This organofunctional silane film can passivate the surface of a metal substrate, providing a barrier to keep water and salts from coming in contact with the metal surface. Furthermore, the organofunctional groups can provide additional hydrophobicity and adhesion promotion of any subsequent organic topcoats that may be applied for further corrosion protection. As previously mentioned, an elevated temperature curing procedure is typically required to drive off all the moisture in an organofunctional silane coating. This thermal curing procedure is not always feasible depending on the specific application or industry, and this has led to the exploration of alternative curing methods for organofunctional silane coatings. With the use of organofunctional bipodal silanes, the additional crosslinking density introduced into the system may alleviate the need for this elevated temperature curing procedure. This additional crosslinking density stems from the influx of alkoxy groups from the organofunctional bipodal silane. While it is possible for these additional alkoxy groups to undergo crosslinking at room temperature, the condensation of silanol groups is significantly accelerated at elevated temperatures.6 Additionally, the rate of crosslinking depends on several other factors, including pH, presence of solvents, and the concentration of silanes in the system.7 Although organofunctional trialkoxy silanes are commonly used in a wide variety of coating applications, organofunctional bipodal silanes, such as 1,2-bis(triethoxysilyl)ethane (Figure 2), can have six or more alkoxy groups. As these alkoxy groups undergo hydrolysis and condensation in the system, the additional siloxane bonds formed can accelerate the curing process of the system.8 It is important to note that a two-carbon spacer links the six alkoxy groups on each side of 1,2-bis(triethoxysilyl)ethane. These alkyl chains are responsible for the hydrophobic nature of this organofunctional bipodal silane. For this reason, 1,2-bis(triethoxysilyl)ethane is commonly used in solvent-based systems, where the hydrophobic nature of this organofunctional bipodal silane does not interfere with its solubility in alcohol-based systems.9 Although it is rather difficult for hydrophobic organofunctional silanes to exhibit good stability in waterborne systems, optimizing the pH of the system to a slightly acidic value (pH 4–5) can maximize the hydrolysis rate and minimize the condensation rate of organofunctional bipodal silanes.10,11 This allows for improved solubility and hydrolytic stability of organofunctional bipodal silanes in waterborne systems. The two waterborne systems that are presented in this work include a waterborne organofunctional silanol system with functionalized colloidal silica and a waterborne organofunctional silanol system without functionalized colloidal silica. Both these waterborne systems do not contain any volatile organic compounds, which is why they are commonly used as environmentally friendly alternatives to harmful corrosion resistance technology. The waterborne organofunctional silanol system with functionalized colloidal silica can also be used as a transparent sol-gel topcoat, while the waterborne organofunctional silanol system without functionalized colloidal silica can be used as a surface modifier for organic materials or as an adhesion-promoting additive into waterborne polymer systems. While these waterborne systems provide excellent corrosion resistance on their own, organofunctional bipodal silanes will be explored as performance-enhancing additives to these waterborne systems, in hopes of better understanding how to improve this new technology. EXPERIMENTAL METHODS Materials 1,2-Bis(triethoxy)silylethane (Dynasylan® * BTSE), waterborne organofunctional silanol system with functionalized colloidal silica (Dynasylan® SIVO 110), and waterborne organofunctional silanol system without functionalized colloidal silica (Dynasylan® HYDROSIL 2926) are available from Evonik Industries AG. Sodium hydroxide (99.99% pure) and ethyl alcohol (99.5% pure) were purchased from Sigma Aldrich. Bulk Kleen® * 737G (a proprietary alkaline powder cleaner) was purchased from Bulk Chemicals. Deionized (DI) water was obtained with a water purification system (WaterPro®  † Plus) originally purchased from LabConco Corporation. Aluminum 6061T6® ‡ substrates were purchased from ACT Test Panels, LLC. Formulation Preparation The waterborne coatings evaluated in this article were formulated in 150 mL glass beakers (see Table 1 for ingredient breakdown) and allowed to mix for 96 h before use. This extensive mixing time is preferred to allow adequate time for the silane molecules in the formulation to hydrolyze and condense in the presence of water. After enough time, the condensation of silanol groups in the formulations will start to have a considerable impact on the viscosity of the coating, eventually leading to decreased film formation properties. This condensation rate is particularly low at pH 4–5 for the waterborne coatings evaluated in this article, allowing for approximately three weeks of sufficient stability before the onset of minor visual changes to the solutions. These visual changes could include precipitation, haziness, and increases in the viscosity of the system while mixing. 1,2-Bis(triethoxysilyl)ethane is 100% active solids, while the waterborne organofunctional silanol system with functionalized colloidal silica is 36% active solids, and the waterborne organofunctional silanol system without functionalized colloidal silica is 30% active solids. The final wt% solids of the waterborne coating formulations in this article were chosen to obtain transparent films that do not cause any negative optical characteristics to the metal surfaces. It is also important to note that the optimal silane concentration in a waterborne protective coating directly depends on the surface roughness of the metal substrate.12 Cleaning and Application Procedures Metal Surface Cleaning Procedure Before applying the waterborne formulations described above, it is crucial that the metal substrates are properly cleaned for optimal surface wetting properties. The metal substrates were first wiped with an ethyl alcohol-soaked paper towel. Following this solvent wipe, the metal substrates were dried with an air gun and placed in an alkaline washing solution (prepared by adding 15 g of Bulk Kleen® 737G to one liter of DI water and stirring for several hours before use) for 3 min at 140–150°F. The aluminum substrates were rinsed with DI water and dried with an air gun following the alkaline washing procedure. Coating Application Procedure After the metal substrates were properly cleaned and the waterborne coating formulations were allowed to fully hydrolyze, the coatings were applied via a dip coating procedure. The metal substrates were fully immersed in the waterborne silane formulations for 60 sec at room temperature, removed from the solution, and hung vertically in a fume hood for 10 min to allow the excess liquid to drip off the metal surface. Although some of the waterborne coating formulations were milky white, all the coatings evaluated in this article formed transparent films upon application. Curing Procedure After being allowed to dry at room temperature for 10 min following the dip coating procedure, the coated metal substrates were either left in the fume hood at room temperature to dry for an additional 48 h, or placed in an oven for 30 min at either 80°C (formulation WB1) or 180°C (formulations WB2, WB3, WB4, and WB5). It is important to note that after curing, the dry-film thicknesses of the waterborne organofunctional silanol system with functionalized colloidal silica and the waterborne organofunctional silanol system without functionalized colloidal silica were less than 1 mm. Testing Procedures Contact Angle Measurement Procedure Once the coatings were fully cured, a goniometer (Ramé-Hart, Inc.) was used to measure the contact angle of DI water on the coated substrates. Each measurement reported in this article is the average of 10 contact angle measurements to ensure the accuracy of this method. The standard deviation of each set of 10 contact angle measurements represents the statistical error reported in this data. It is important to note that although the metal substrates were slightly bent during the production process, all measurements were done on aluminum substrates of the same production batch and at the same locations on each substrate. Neutral Salt Spray Testing Procedure Before evaluating the coated metal substrates in a neutral salt spray test, wax (IGI 1334 paraffin wax supplied by Lone Star Candle Making Co.) was used to coat the edges of the metal substrates. Corrosion resistance was evaluated in a Q-Fog® § Cyclic Corrosion Tester (The Q-Panel Company) according to ASTM B117. Alkaline Resistance Testing Procedure A solution containing 10% sodium hydroxide (NaOH) and 90% DI water was prepared by stirring at room temperature until all the NaOH pellets were fully dissolved. After properly applying and curing the waterborne coating formulations on the aluminum substrates, the panels were immersed in the alkaline solution for 10 min at room temperature. The panels were then removed, rinsed with DI water, observed visually, and then placed in a neutral salt spray test as described above. Electrochemical Impedance Spectroscopy (EIS) Testing Procedure EIS testing was performed by Matergenics, Inc. Gamry PCI4/750™* potentiostats were used to record the impedance spectra at frequencies of 0.1–100,000 cycles/sec. The coated metal panels were immersed in an aqueous conductive 3.5% NaCl solution during testing. To achieve a relatively stable open circuit potential for EIS measurements, the coated metal panels were immersed in the conductive 3.5% NaCl solution for 20 min before collecting impedance data. All measurements were performed in a grounded Faraday cage at room temperature. RESULTS AND DISCUSSION Surface Contact Angle Analysis As mentioned previously, incorporation of an organofunctional bipodal silane into a waterborne system is expected to increase the hydrophobicity, surface passivation, and crosslinking density of the system, leading to improved corrosion resistance performance. In particular, 1,2-bis(triethoxysilyl)ethane was chosen for investigation in waterborne systems because of the two-carbon spacer group between the silicon functional groups on either side of the organofunctional bipodal silane. This two-carbon alkyl chain not only provides significant hydrophobicity, but is also a short enough chain to allow for sufficient solubility in waterborne systems. While there are many ways of characterizing the hydrophobicity of a metal coating, valuable insight can be gained by visually observing the behavior of water on the coated surface. Water tends to spread out when placed on an uncoated aluminum surface, indicating a hydrophilic surface. On an aluminum surface that has been coated with a waterborne organofunctional bipodal silane coating, the water tends to bead up (Figure 3). While visually observing the behavior of water on a metal surface is typically a good indication of whether the metal surface is hydrophilic or hydrophobic, contact angle measurements of water droplets on a metal surface can better quantify this behavior. The average contact angle of a DI water droplet on uncoated aluminum is 44° ± 1.5°, while the average contact angle of a DI water droplet on WB1-coated aluminum was 72° ± 1.6° (Figure 4). Thus, coating the aluminum surface with a waterborne organofunctional bipodal silane coating increased the contact angle of DI water by ~64%. Because a surface is typically considered hydrophobic when the contact angle of water on the surface is > 90°, it can be said that the this waterborne organofunctional bipodal silane system made the surface less hydrophilic. While the average contact […]

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