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This review describes the development, special features and applications of silica hydride-based stationary phases for HPLC. The unique surface of this material is in contrast to ordinary, standard silica, which is the material most frequently used in modern HPLC stationary phases. The standard silica surface contains mainly silanol (Si-OH) groups, while the silica hydride surface is instead composed of silicon-hydrogen groups, which is much more stable, less reactive and delivers different chromatographic and chemical characteristics. Other aspects of this material are described for each of the different bonded moieties available commercially. Some applications for each of these column types are also presented as well as a generic model for method development on silica hydride-based stationary phases.
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It has been more than 30 years since the concept of fabricating a silica-based HPLC stationary phase having a surface composed of silicon-hydride (Si-H) moieties instead of silanols (Si-OH) was first formulated [ 1 , 2 , 3 ]. The original approach proposed has been modified extensively over the years so that a proprietary process is now used in a commercially available product [ 4 ]. A substantial number of research articles and reviews have been published over the years documenting the evolution of silica hydride-based stationary phases from a hypothesis to a proven experimental concept that can function in a manner different from existing HPLC materials [ 4 ]. This review will cover three different areas that are of importance to users of HPLC that can be used for column selection: a) the unique features of silica hydride-based phases that distinguish them from other commercially available materials, b) various applications by column type that can serve as illustrations of the usefulness of these materials, and c) typical steps used in the development of methods on silica hydride stationary phases.
The ability to retain polar and nonpolar compounds applies to all stationary phases created on a silica hydride surface. A plot of retention as a function of the percentage of organic component in the mobile phase results in a U-shaped curve ( ). On the left-hand side of the graph, at low organic or high water content, reversed-phase retention is observed. On the right-hand side, at high organic or low water content, normal-phase retention is obtained. At intermediate mobile phase compositions, it is possible to have both reversed-phase and normal-phase retention operating simultaneously. For compounds with significant polar and nonpolar components in its structure, retention can be achieved in either mode, thus giving the analyst more options in developing a suitable method. This is a hypothetical graph, and the exact shape depends on both the analyte and the stationary phase. Other phases, such as certain fluorinated bonded compounds, also display this behavior to a limited extent in comparison to the broad range of retention properties exhibited by silica hydride-based materials. It has also been demonstrated that silica hydride can be prepared under supercritical fluid conditions but only in very small quantities [ 8 ].
The retention for most nonpolar compounds on silica hydride columns is similar to that for ordinary silica-based stationary phases, i.e., hydrophobic interactions between the analyte and a bonded organic moiety such as C18 or C8. Thus, typical reversed-phase HPLC analyses can be done on these types of silica hydride phases. However, the mechanism of retention for polar compounds on silica hydride stationary phases was an issue for many years, since the surface is hydrophobic, yet strong retention of hydrophilic analytes was observed for a broad range of compounds in the aqueous normal phase mode. It was determined [ 7 ] that in mobile phases with a high content of an organic solvent such as acetonitrile, auto-dissociation of water occurs on silica hydride with hydroxyl ions prevalent on the surface of the material, giving it a negative overall charge. This phenomenon is similar to what happens to oil droplets in mixed solutions of organic solvent and water. Thus, retention of compounds having a positive charge or those having a positively polar component such as an amine group occurs by charge attraction and negatively charged or polarized molecules by displacement.
There are numerous features that distinguish silica hydride from the ordinary silica used to fabricate the vast majority of commercially available HPLC stationary phases. The most obvious of these differences are the surface properties of these two materials. The surface of ordinary silica is populated by polar silanol groups (Si-OH) while the surface of silica hydride is composed of nonpolar silicon hydride moieties (Si-H). This profound difference has significant ramifications when comparing the two materials with respect to physical properties and chromatographic behavior. Because of the polar nature of silanols, many analytes can be adsorbed on the surface of stationary phases fabricated on ordinary silica. Even extensive endcapping leaves a significant number of silanols on the surface. However, silica hydride materials are composed of almost entirely Si-H groups that precludes the adsorption of most analytes on the surface. Another consequence of this difference in surfaces is that ordinary silica strongly adsorbs water leading to an aqueous layer of at least 4–10 monolayers in contrast to silica hydride having less than a monolayer of water under most chromatographic conditions [ 5 , 6 ]. This is especially significant when silica-based materials are used in the hydrophilic interaction liquid chromatography (HILIC) mode. Most HILIC analyses depend on a partitioning of analytes from the mobile phase to this adsorbed water layer as a means of chromatographic retention. Since such a layer does not exist on silica hydride materials, these phases utilize other mechanisms for polar compound retention (see discussion below). Another consequence of the lack of a water layer is the rapid equilibration between runs in gradient analysis or rapid equilibration when mobile phase conditions are changed for different analyses or method development. Another ramification of the absence of a significant water layer on the silica hydride surface is evident in organic normal phase chromatography. Since ordinary silica readily absorbs water, it must be carefully removed from the mobile phase; otherwise, the results can vary significantly from run-to-run on both an intraday and interday basis. However, the low affinity for water on a silica hydride surface eliminates the need to scrumptiously dry mobile phase solvents for organic normal phase chromatography.
The bonding technology used to make these materials, hydrosilation, is completely different than that used for columns based on ordinary silica, organosilanization. The latter utilizes an organoilane reagent that reacts with the silanols on the surface to bond an organic moiety. Hydrosilation involves reacting an olefin or alkyne compound with the Si-H group to attach the organic species to the surface. The production of silica hydride results in approximately the same number of Si-H groups on the surface as ordinary silica has silanols. However, at the end of the bonding reactions, the remaining groups on the stationary phase are polar silanols for ordinary silica but nonpolar Si-H for the silica hydride phases. For silica hydride phases, the organic moiety is attached via a direct Si-C, while for ordinary silica, the linkage is Si-O-Si-C. The direct silicon-carbon bond provides much higher stability than the siloxane linkage obtained with organisilanization. In addition, the hydrosilation reactions have proven to be robust and reproducible ( ). For the same analysis on different lots of the same column material, variations in retention times or capacity factors (k) are no more than a few tenths of a percent RSD.
While all silica hydride columns can operate in both the reversed-phase and normal-phase modes, the bonded moiety usually determines what retention properties will predominate. Thus, a nonpolar bonded moiety such as C18 will generally be used for reversed-phase applications, but it still has normal-phase properties in contrast to most other stationary phase with this type of modification. More polar bonded groups, such as diol or amide, would generally be used in normal-phase applications, but it still retains some reversed-phase properties. The following sections contain applications for each type of commercially available silica hydride column, which at this time happens to be produced by only one company. Columns types that are not available include chiral, size exclusion, anion-exchange, gel permeation, sub two-micron particle size and core shell.
This is the most widely used of the silica hydride-based columns. It was the first stationary phase to display capabilities for analyzing a broad range of polar compounds. Even today, it has features that either are not available or less versatile than most HILIC phases. The ability of equilibrate rapidly in normal-phase applications under a variety of mobile phase conditions (usually two to three column volumes) is in most cases vastly superior to HILIC columns. Another aspect of the Diamond Hydride column is its use of low buffer concentrations in the mobile phase. Typical additive concentrations are in the 5 to 15 mM range, while many HILIC applications can be significantly higher (30 to 100 mM is not unusual). This can be particularly important when using mass spectrometry for detection. High additive concentrations can leave deposits on the ion source, thus lowering sensitivity and requiring frequently cleaning. While acetonitrile is the most common organic solvent used in ANP analyses on the Diamond Hydride column, acetone [9] and methanol [10] have been shown to be applicable as well. Another feature of this stationary phase is its ability to retain certain strongly polar compounds in the normal-phase mode at very low concentrations of organic in the mobile. This often occurs at 20 to 30% organic content and has been referred to as “super ANP”.
The analysis of polar compounds is an important component that occurs in a variety of different application areas, such as pharmaceuticals, metabolites, food, forensics, environment, and biotechnology. Many examples describing a variety of analyses of hydrophilic compounds can be found in the literature as well as on the Microsolv Technology website [11]. Some examples of the approximately 200 listed application notes through the website are the analysis of hydrophobic and hydrophilic peptides in a single run [12], benzodiazepines in urine [13], acrylamide [14], the pharmaceutical Xanax [15], anatoxin, which is a neurotoxin implicated in many poisoning incidents [16], and the common household product ingredient cetylpyridinium chloride [17]. Some examples of hydrophilic compounds analyzed on the Diamond Hydride in the literature are common metabolites [9], cathinones [18], collagen and elastin crosslinks [10], bactericidal targets [19], sugars [20], lipids [21], thiopurines [22], dietary supplements [23], juices and cereals [24], peptides [25], nucleotides [26], drug levels in human serum [27], and disease pathways [28,29,30]. In another report, the Diamond Hydride was combined with an RP column for a comprehensive survey of both polar and nonpolar metabolites [31]. One study compared the Diamond Hydride and three other Type-C stationary phases with respect to the relative retention of small molecules [32]. This range of applications displays the versatility of the Diamond Hydride column for polar compound analysis that goes beyond the capability of typical HILIC phases.
The phenyl phase having a hydrophobic moiety bonded to the silica hydride surface is generally used for reversed-phase applications, particularly those where the analytes contain aromatic or other types of unsaturated sites. Retention is enhanced for these types of analytes though π-π interactions with the bonded moiety.
An example of an application for the phenyl column in the literature is the analysis of 16 common drugs of abuse by LCMS in under 8 min [33]. Under a different set of experimental conditions, the THC-delta-9-COOH can also be analyzed on the phenyl column. Additional articles citing the use of phenyl hydride columns involve the analysis of rice [34], mycotoxins in grains [35], and jaboticaba fruit [36]. However, there are more than 30 examples of analyses using the phenyl hydride column on the Microsolv website [37]. Included among the applications presented are the analysis of 10 phenolic acids in rice, methylenedioxymethamphetamine (MDMA) in plasma and the pharmaceutical compounds coricidin, fluoxetine, and ketorolac. An interesting example is shown in for the analysis of common components found in cough syrup [37]. This is a gradient analysis that is an overlay of five consecutive runs with an equilibration time of 3 min between runs. The run-to-run reproducibility of Cogent columns is one of their essential features, as well as rapid column equilibration between gradient runs.
Open in a separate windowA more recently developed silica hydride column is the amide stationary phase. The amide phase focuses primarily on hydrophilic molecules and is especially applicable to the analysis of sugars and various carbohydrates. Both a fundamental description of this phase and some examples of applications can be found in a published article [38].
Additional information on applications of the silica hydride-based amide column can be found online. A range of analyses are presented, relating to compounds that are not carbohydrates. An interesting example is the determination of the antidepressant fluoxetine (Prozac) in a capsule [39]. The compound is polar with a secondary amine, an ether linkage, and a trifluoromethyl group. The lot-to-lot reproducibility of this phase is also demonstrated in the application with three synthetic batches showing virtually identical retention times for the analyte. Nitrogen-containing compounds can often be difficult to analyze in reversed-phase due to adsorption on residual silanols and is often challenging in the HILIC mode. The separation of pyrilamine and 4-amino-3-chloropyridine with good peak shape demonstrates the ability of the silica hydride-based amide column for these types of compounds [39]. Another application involves the analysis of the pharmaceutical compound tizanidine in tablet form used to treat muscle spasms and cramps [39]. A particularly interesting application is shown for the separation of glucose and fructose in cola. These structurally similar compounds are a challenging separation, but can be done in the ANP mode under isocratic conditions using the amide silica hydride column [39].
Open in a separate windowThis is a unique stationary phase that was first made with conventional bonding techniques on ordinary silica before it was subsequently adapted to a silica hydride matrix [40]. Cholesterol is a liquid crystal (solid with an ordered structure) in the native state. It was postulated and later proved that some of this ordered arrangement of the molecules exists even when one end is attached to a surface. In addition to hydrophobic interactions between the analyte and the bonded cholesterol moiety, there was discrimination based on the morphology of the stationary phase, which had a slot-like configuration. Thus, analytes that were more linear and could penetrate into the slots were retained better than bulkier molecules that were preferentially excluded from this restricted environment. The same exclusion phenomenon was also confirmed on the silica hydride cholesterol phase [41]. The phase was also tested for its effectiveness in small molecule separations [32].
Over the years, since its initial commercialization, a number of interesting applications have been developed. Among these are the separation of six serum corticosterones that can be used as disease markers in various clinical analyses [42], the drugs atropine [42] and doxcycline and methacycline [42], and the fruit juice component limonin [42]. A good example of the shape selectivity of the UDC cholesterol column is the separation of the monounsaturated C18 fatty acids shown in [42]. The two compounds, oleic acid (cis isomer) and elaidic acid (trans isomer), are separated on the UDC column with the cis isomer eluting first. This is due to the fact that the trans isomer is a linear molecule and penetrates deeper into the ordered stationary phase than the cis isomer, which is partially excluded due to its bent configuration.
Open in a separate windowWhile aqueous normal-phase chromatography is based on the presence of the silica hydride surface, the bonded moiety, as demonstrated with the amide phase, can also provide additional interactions that can facilitate the separation of hydrophilic compounds. In this case, the bonded moiety is a carboxcyclic acid compound. This functional group is the basis of weak cation-exchange properties for HPLC stationary phases. Thus, it can be surmised that this bonded phase would provide additional interactions for amines or other nitrogen containing compounds. An example of the effective use of this phase was demonstrated for the analysis of nucleotides related to clinical analysis [26]. Another interesting analysis successfully developed for the UDA phase was for the determination of ethyl glucuronide and ethyl sulfate [43]. These compounds are useful long-term markers for alcohol abuse and are often tested for in people in certain critical jobs, such as airline pilots or public safety personnel.
Additional examples of the use of this column can be found in online application notes. Pharmaceutical compounds containing nitrogen are another potential use of the UDA column. The antibiotic tobramycin was analyzed using a water/acetonitrile isocratic mobile phase with 0.5% formic acid [44]. Tablets containing vardenafil (Levitra) were extracted and then analyzed by HPLC using the UDA column [44]. A gradient method was developed in the ANP using a mobile phase containing DI water/acetonitrile with 0.1% formic acid. Excellent run-to-run repeatability was achieved with a 0.2% RSD for the retention time. A challenging analysis is the separation of the adenine nucleotides, AMP, ADP, and ATP, as shown in [44]. The chromatogram shown was obtained by gradient analysis in the ANP mode using a mobile phase A of water with ammonium formate buffer and mobile phase B of acetonitrile with ammonium acetate buffer. Excellent peak shape was obtained for these compounds, which often produce significant tailing on other types of stationary phases.
Open in a separate windowThis stationary phase is the silica hydride version of the standard octadecyl column used in a large variety of reversed-phase applications. In addition to having a hydride surface, its other distinguishing feature is that it has a double attachment to the surface. Thus, the designation bidentate to denote this particular structural feature. As expected, the Bidentate C18 (BD C18) is amenable to a broad range of reversed-phase applications. Mycotoxins have a high toxicity and are a potential problem for human health. A number of these compounds have been analyzed in grain samples using the BD C18 column [35]. The nutritional supplement resveratrol in capsule form and in wine has been analyzed for both the cis and trans isomers [45]. Limonin, a bitter substance that can affect juice quality, has been analyzed on the BD C18 column [46]. Both reversed-phase and aqueous normal-phase performance was evaluated for a set test samples [32]. These are just some examples of analyses found in the literature for this stationary phase.
Online application notes provide additional examples of the usefulness of this column. A significant number of protocols involve pharmaceutical compounds. Among these are information for the separation of the isobaric opioid drugs morphine and hydromorphine [47], atorvastatin [47], the laxative bisacodyl [47], and the muscle relaxant dantrolene sodium [47].
While most reversed-phase applications are done with a C18 column, there are some instances when the analytes are so hydrophobic that these compounds take an unusually long time to elute or may not elute at all. Therefore, the way to reduce these interactions between the bonded moiety and the analyte is to make the stationary phase less hydrophobic. The simplest way to accomplish this is to shorten the bonded alkyl change. Hence, using a C8 (octyl) moiety accomplishes this goal. The C8 bonded phase on silica hydride has the same advantage as the C18 column with the ligand having a double attachment to the surface. An interesting application using the BD C8 column for the analysis of bisphenol A on carbonless paper often used as receipts for copying has been published [48]. Two extraction methods, one via digestion and another via migration, both gave satisfactory results with reproducible retention times.
There are numerous applications which can be viewed online for this phase. Many involve applications to pharmaceutical analysis. A protocol has been developed for the over-the-counter antihistamine chlorpheniramine maleate to identify the principal ingredient as well as a number of potential impurities [49]. Others include the antimalaria agent Clindamycin [49], the analgesic tramadol with data from two column lots demonstrating reproducibility [49], and prednisone, which is often used to treat arthritis or lupus, with the data showing excellent run-to-run reproducibility [49]. Another example of a pharmaceutical application is shown in . This analysis involves the prodrug sulfisoxazole acetyl and two potential preservatives often used in the formulation [49]. The figure contains overlaid chromatograms from two different lots of material, demonstrating the reproducibility of the manufacturing process.
Open in a separate windowFor very large molecules, particles with pore sizes around 110 Å are too small and may be excluded from interaction with the bonded organic moiety. Therefore, a silica hydride-based stationary phase with a bidentate-attached C8 group has been developed using a particle with a 300 Å pore size. A number of potential applications are documented to illustrate the usefulness of this particular stationary phase. The resolving power of this phase is demonstrated by the separation of two very similar cytochrome c proteins; one from horse heart and the other from bovine heart [50]. Another potential application is the separation of peptides [50]. This technique would be applicable to the characterization and purification of synthetic peptides. There is growing interest in the analysis of glycoproteins. It has been shown that the BD C8 phase can successfully retain these types of molecules with good peak shape [50].
The diol phase involves the attachment of polar organic moiety to the silica hydride surface. Therefore, it is generally used in the aqueous normal-phase mode for the analysis of hydrophilic compounds. Because the modified surface is different than the Diamond Hydride, the amide, and UDA phases, it has different selectivity and provides another option for trying to optimizing a particular type of separation. One study demonstrated how this column could be used to analyze a number of drugs of abuse [33]. Another investigation involved separating and analyzing uric acid cycle metabolites in the ANP mode where pre-column derivatization is not necessary as in many reversed-phase methods [51]. The diol phase was also included in a comparative study four different silica hydride phases [32].
Application notes for a number of other analyses are available that demonstrate some additional uses for this phase. The selectivity of this phase was highlighted in a study that separated ascorbic acid, niacin, riboflavin, folic acid, pyridoxine, metformin, and thiamine using a gradient ANP protocol [52]. In a similar type of experiment, the compounds warfarin, hydroxybupropion, and codeine in blood serum samples were analyzed with a simple liner ANP gradient. [52]. A good example of the dual retention capabilities of the silica hydride stationary phases is presented in separation of benzodiazepines on the diol column. The urine sample was analyzed with gradients in both the reversed-phase and aqueous normal-phase modes [52]. The chromatograms of both modes are shown in .
Open in a separate windowSilica C is the name used for an unmodified silica hydride particle. Because it has a low level of water adsorption (less than 0.5 of a monolayer), it should be an excellent choice for organic normal-phase chromatography. This feature is in direct contrast to ordinary silica, which strongly adsorbs water. Thus, when using ordinary silica in organic normal-phase chromatography, the mobile phase must the carefully dried to obtain reproducible retention times. Such precautions are not necessary for Silica C due to its low affinity for water. This concept was proven for the evaluation of phenolic compounds using traditional organic normal-phase chromatography on the Silica-C stationary phase [53]. Another example of an organic normal-phase application is the analysis of nonylphenol, a compound used in the synthesis of surfactants [54].
The Silica-C stationary phase has also been proven to be successful in the aqueous normal phase mode [55,56]. A number of pharmaceutical compounds have been analyzed using this stationary phase by ANP. A protocol was developed for the antihistamine diphenhydramine using an isocratic mobile phase of 50:50 water/acetonitrile with 5 mM ammonium acetate [57]. This particular example demonstrates the strong normal-phase retention properties of the silica hydride phases considering the large percentage of water in the mobile phase. In addition, this method was tested on four different lots of the Silica-C material, giving excellent reproducibility among columns (see ). Another drug, ketotifen, used for the treatment of asthma, was analyzed by a gradient ANP method [57]. This tertiary amine compound had strong retention on the Silica-C and the protocol developed gave excellent run-to run reproducibility. Phenylglycine used in the synthesis of lactam antibiotics was analyzed with an isocratic mobile phase consisting of 80:20 acetonitrile/water with 0.5% formic acid [57].
Open in a separate windowBy 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.
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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 that remain visible in the final coating, reduced thickening efficiency, poor thixotropic stability over time, lower gloss and transparency, and possibly film defects.
Fumed silica has excellent thermal stability, but as the temperature of the coating environment increases, particularly upon shear, the wetting properties of the coating typically improve. This can lead to over-dispersion, whereby the aggregates are reduced past their optimum association level. Once this occurs, reduced thickening efficiency results, sometimes to the point where it appears as if no thickener was added. Fumed silica is one of the smallest particle size materials added to a coating formulation. This component should be added early in the formulation, preferably to the resin or binder, rather than into non-film-forming components like solvents for optimal effect. Caution should be used when post-adjusting batches with powder, as only minimal shear that is inadequate for homogeneous incorporation can often be used at this stage. Post addition with low shear may bring formulations to their desired rheology, but this can deteriorate over time, as larger agglomerates slowly wet-out.
Lab evaluations have demonstrated that multiple performance attributes can also 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. These features are achieved without compromising gloss and other appearance attributes. An example is shown in Figure 6, where a pre-made aqueous fumed silica dispersion improves the film formation in combination with reduced coalescent solvent levels. These improvements have been seen with many different film-forming resins and using less or no coalescing solvents. This can help reduce the overall volatile organic compounds of the formulation, in addition to providing the other properties referenced above. This enhanced film formation is a result of reduced stress propagation due to the reinforcing effects of the very finely dispersed fumed silica.9
Anti-Corrosion/Water Repellency
Hydrophobically modified grades of fumed silica have also been used together with anticorrosive pigments to improve corrosion performance and water repellency of coatings. These grades are not considered to be anti-corrosion pigments, but they 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 are used to ensure that there are enough particles in the coating matrix to support a hydrophobic barrier, improve the mechanical properties of the film, 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. Two examples of this effect are shown in Figure 7.
Proper dispersion is again needed to homogeneously distribute the silica throughout the coating matrix for maximum effect. It is suggested that the treated fumed silica be dispersed together with the anti-corrosion pigments to ensure optimal dispersion. The best results have been obtained using DDS, TMOS, and HMDS treatments, although the impact on formulation rheology must also be considered.
Scratch Resistance
The development of silica particles for the specific purpose of improving scratch resistance came from the creation of particles for high reinforcement of elastomers and composites. The critical factor needed to achieve high reinforcement is to be able to fill the polymer matrices with significantly higher levels of fumed silica without increasing the viscosity to unworkable levels. This is achieved by structure modification through a proprietary post-processing to achieve a highly reduced structure and a low level of aggregation. This results in a material that can be used as a reinforcing filler to increase the mechanical strength of the coating and impart scratch resistance. The corresponding physical property observed is a significant increase in the bulk density (Figure 8) and a dramatic reduction in the ability of silica to increase viscosity. When compared to other reinforcing fillers, such as alumina, silica has the advantage of a lower refractive index of 1.46, more closely aligned with many polymer systems, which results in improved transparency and clarity. These materials can also 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. This can be seen in the scanning electron microscopy (SEM) analysis of a cross-section of a high-solids coating (Figure 9), which shows a homogenous distribution through the film with no surface enrichment resulting from higher particle density.
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 using both scratching methods, and reduced haze was observed after the panels were subjected to Elcometer testing (Figure 10). The addition of 5% silica did slightly reduce gloss, but the silica significantly improved scratch resistance.
Results in Figure 10 show three variants of DDS-treated structure-modified silica. Variant 1 is the original that requires milling, variant 2 the same DDS structure modified silica pre-dispersed in methoxypropyl acetate (MPA), and variant 3 is the newest version, which is easy-to-disperse.
Free Flow, Fluidization, Transfer Efficiency
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, and high-transfer efficiency as well as reduced Faraday cage effects for even film thickness and optimized appearance during application. Hydrophilic- and hydrophobic-treated fumed silica and alumina can be used to improve flow, storage stability with reduced moisture pick-up, and improved fluidization and transfer efficiency.
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 they are charged into the hopper to help them feed more consistently and homogeneously into the extruder. The typical loading of flow additives used 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 are used to influence the final powder coating properties, they must be added after extrusion and be oriented on the outside of the powder coating particles. There are typically two places where silica or alumina (or a combination of the two) can be added into the process to achieve this: 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 out of the powder and reduce the final dose remaining in the powder coating. The typical dosage level used in the chipping or as post add is also 0.1–0.3% by weight.
A study was organized with the University of Western Ontario to assess specific 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. The additive dosage level was adjusted based on the particle size of the powder. A 0.3% dosage level was used for the coarse powder and a 0.5% dosage level for the fine powder. A third powder was a tribo-applied polyester powder coating. The attributes tested 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 a more consistent level of jetness than the disk applied with powder containing no additive. This test measures how much powder is transferred and the consistency of coverage of 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 applications conditions. After application, these 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 was effective in reducing Faraday cage effects in the coarse, black powder coating.
Fluidization efficiency was also assessed in this study. The results in coarse and fine powder show that particle size of the powder coating significantly affects the type of additive most effective for improving fluidization. Alumina was more effective in improving fluidization in the coarse powder coating as measured by lower air velocities needed to obtain 20% bed expansion, while silica was more effective in the fine powder coating (Figure 12). This trend suggests that additive packages may need to be adjusted based on their particle sizes.
Gloss Control
Gloss is defined, according to DIN EN ISO 4618, as 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 of the light in a range of angles. Gloss level can be characterized by the angular distribution of light scattered from a surface, measured with a glossmeter or reflectometer, and it is dependent upon the viewing angle (Figure 13).
There is no common or globally accepted definition of the term “matte.” It is always measured based on a comparative measurement of the gloss against a standard.2-5 For coating surfaces, the term “gloss” means almost complete reflection in the sense that 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).6
Lin and Biesiada demonstrated that matting is a function of both the silica particle size and degree of coating shrinkage during drying (either through solvent evaporation, chemical reaction, or coalescence).7 Larger particles are more efficient at reducing gloss as a function of silica dosage, but the larger particles can lead to a rough surface and increased dirt pickup over time. Most silica grades used for matting coatings are produced via wet and gas phase processes and are classically larger than the grades used for rheology. Some of these grades can influence thickening to a lesser or greater extent. Surface treatment, either with wax or reactive oligomers, can help reduce viscosity build-up, prevent hard settling, and improve transparency and stability. Recent technology developments have produced silicas with improved haptic effects like soft-feel.
Recent Developments in Silica Technology
While considered a mature technology, the use of silica in coatings continues to benefit through innovation. Romer described a new process for producing precipitated silica that allows greater control of particle morphology during the precipitation process to produce highly spherical particles with narrow-sized distributions (Figure 15).8 The spherical shape imparts high apparent hardness to improve scrub, abrasion, and burnish resistance of the formulated coating, with low binder demand and minimal impact on coating rheology. These spherical silica particles also can provide matting properties, depending on particle size, and they have excellent transparency for use in deep colors and clear coats.
Both hydrophilic (200 m2/g) and some hydrophobically treated grades (e.g., DDS, HDS, HMDS, 130–300 m2/g) of fumed silica can be used in water-based coatings when it is possible to adequately disperse the powder into water dispersible resins and solutions. However, the low viscosity and high dielectric constant makes water a poor grinding medium for fumed silica, and it is difficult to achieve the degree of de-aggregation and dispersion of particles needed to achieve optimum benefits in water-based coatings. Incorporation of hydrophobically treated grades can also be difficult due to the poor wetting properties of water-based formulations, especially when resin solids drop below 35% non-volatile content. Additives, such as acetylenic diols, can help to improve the wetting and dispersion of fumed silica into water, but care must be taken to ensure that the additives do not disrupt the silica network formation, reducing rheology control.
When adding the silica directly into water, or when using a shear-sensitive, film-forming resin, it is recommended to use a pre-dispersed form of fumed silica to overcome these challenges. A new aqueous dispersion of a functionalized fumed silica has been developed using a new production process and carefully selected additives. The silica is already dispersed, so it can be easily stirred into water-based formulations without requiring high shear dispersion.
The new dispersion WF 7620, contains 20% functionalized silica with a high surface area of 300 m2/g and demonstrates outstanding rheological effectiveness in waterborne coatings, especially those applied via spray application where anti-sagging properties are critical while maintaining excellent flow and levelling properties.
This is demonstrated in the jump-curve rheology graph shown in Figure 16. The jump curve simulates a spray application where the coating is sheared at high shear (500s-1) continuously to simulate the spraying process and then the shear is suddenly reduced to 0.1s-1 simulating the coating on the substrate after application. A fast build-up of viscosity is desired to prevent sagging after application onto vertical surfaces. This enables the formulator designing perfect finishes for three-dimensional parts including general industrial coatings, transportation coatings, plastic coatings, and wood coatings. The jump curve demonstrates the new dispersion WF 7620 and gives a significant increase and improvement in rheological efficiency, compared to an existing water-based dispersion of fumed silica based on a fumed silica core of 130 m2/g. This improved performance is due to the use of a higher surface area silica, combined with a novel functionalization in-situ. The use of a higher surface area fumed silica in the dispersion also helps to improve clarity and transparency and helps to reduce haze in the final coating.
Conclusion
Modified grades of silica have been used for many years to improve a variety of performance attributes in many different coating applications. These include rheology, film formation, and mechanical properties as well as surface appearance. The morphology of the silica particles, size distribution, and surface treatment are critical to the broad range of properties that can be attained using these materials. Recent advances in the manufacturing and post-treatment processing of silica have continued to develop new grades of silica that offer new and or improved performance for the coatings industry.
References
CoatingsTech | Vol. 17, No. 6 | June 2020
Suggested reading:This review describes the development, special features and applications of silica hydride-based stationary phases for HPLC. The unique surface of this material is in contrast to ordinary, standard silica, which is the material most frequently used in modern HPLC stationary phases. The standard silica surface contains mainly silanol (Si-OH) groups, while the silica hydride surface is instead composed of silicon-hydrogen groups, which is much more stable, less reactive and delivers different chromatographic and chemical characteristics. Other aspects of this material are described for each of the different bonded moieties available commercially. Some applications for each of these column types are also presented as well as a generic model for method development on silica hydride-based stationary phases.
It has been more than 30 years since the concept of fabricating a silica-based HPLC stationary phase having a surface composed of silicon-hydride (Si-H) moieties instead of silanols (Si-OH) was first formulated [ 1 , 2 , 3 ]. The original approach proposed has been modified extensively over the years so that a proprietary process is now used in a commercially available product [ 4 ]. A substantial number of research articles and reviews have been published over the years documenting the evolution of silica hydride-based stationary phases from a hypothesis to a proven experimental concept that can function in a manner different from existing HPLC materials [ 4 ]. This review will cover three different areas that are of importance to users of HPLC that can be used for column selection: a) the unique features of silica hydride-based phases that distinguish them from other commercially available materials, b) various applications by column type that can serve as illustrations of the usefulness of these materials, and c) typical steps used in the development of methods on silica hydride stationary phases.
The ability to retain polar and nonpolar compounds applies to all stationary phases created on a silica hydride surface. A plot of retention as a function of the percentage of organic component in the mobile phase results in a U-shaped curve ( ). On the left-hand side of the graph, at low organic or high water content, reversed-phase retention is observed. On the right-hand side, at high organic or low water content, normal-phase retention is obtained. At intermediate mobile phase compositions, it is possible to have both reversed-phase and normal-phase retention operating simultaneously. For compounds with significant polar and nonpolar components in its structure, retention can be achieved in either mode, thus giving the analyst more options in developing a suitable method. This is a hypothetical graph, and the exact shape depends on both the analyte and the stationary phase. Other phases, such as certain fluorinated bonded compounds, also display this behavior to a limited extent in comparison to the broad range of retention properties exhibited by silica hydride-based materials. It has also been demonstrated that silica hydride can be prepared under supercritical fluid conditions but only in very small quantities [ 8 ].
The retention for most nonpolar compounds on silica hydride columns is similar to that for ordinary silica-based stationary phases, i.e., hydrophobic interactions between the analyte and a bonded organic moiety such as C18 or C8. Thus, typical reversed-phase HPLC analyses can be done on these types of silica hydride phases. However, the mechanism of retention for polar compounds on silica hydride stationary phases was an issue for many years, since the surface is hydrophobic, yet strong retention of hydrophilic analytes was observed for a broad range of compounds in the aqueous normal phase mode. It was determined [ 7 ] that in mobile phases with a high content of an organic solvent such as acetonitrile, auto-dissociation of water occurs on silica hydride with hydroxyl ions prevalent on the surface of the material, giving it a negative overall charge. This phenomenon is similar to what happens to oil droplets in mixed solutions of organic solvent and water. Thus, retention of compounds having a positive charge or those having a positively polar component such as an amine group occurs by charge attraction and negatively charged or polarized molecules by displacement.
There are numerous features that distinguish silica hydride from the ordinary silica used to fabricate the vast majority of commercially available HPLC stationary phases. The most obvious of these differences are the surface properties of these two materials. The surface of ordinary silica is populated by polar silanol groups (Si-OH) while the surface of silica hydride is composed of nonpolar silicon hydride moieties (Si-H). This profound difference has significant ramifications when comparing the two materials with respect to physical properties and chromatographic behavior. Because of the polar nature of silanols, many analytes can be adsorbed on the surface of stationary phases fabricated on ordinary silica. Even extensive endcapping leaves a significant number of silanols on the surface. However, silica hydride materials are composed of almost entirely Si-H groups that precludes the adsorption of most analytes on the surface. Another consequence of this difference in surfaces is that ordinary silica strongly adsorbs water leading to an aqueous layer of at least 4–10 monolayers in contrast to silica hydride having less than a monolayer of water under most chromatographic conditions [ 5 , 6 ]. This is especially significant when silica-based materials are used in the hydrophilic interaction liquid chromatography (HILIC) mode. Most HILIC analyses depend on a partitioning of analytes from the mobile phase to this adsorbed water layer as a means of chromatographic retention. Since such a layer does not exist on silica hydride materials, these phases utilize other mechanisms for polar compound retention (see discussion below). Another consequence of the lack of a water layer is the rapid equilibration between runs in gradient analysis or rapid equilibration when mobile phase conditions are changed for different analyses or method development. Another ramification of the absence of a significant water layer on the silica hydride surface is evident in organic normal phase chromatography. Since ordinary silica readily absorbs water, it must be carefully removed from the mobile phase; otherwise, the results can vary significantly from run-to-run on both an intraday and interday basis. However, the low affinity for water on a silica hydride surface eliminates the need to scrumptiously dry mobile phase solvents for organic normal phase chromatography.
The bonding technology used to make these materials, hydrosilation, is completely different than that used for columns based on ordinary silica, organosilanization. The latter utilizes an organoilane reagent that reacts with the silanols on the surface to bond an organic moiety. Hydrosilation involves reacting an olefin or alkyne compound with the Si-H group to attach the organic species to the surface. The production of silica hydride results in approximately the same number of Si-H groups on the surface as ordinary silica has silanols. However, at the end of the bonding reactions, the remaining groups on the stationary phase are polar silanols for ordinary silica but nonpolar Si-H for the silica hydride phases. For silica hydride phases, the organic moiety is attached via a direct Si-C, while for ordinary silica, the linkage is Si-O-Si-C. The direct silicon-carbon bond provides much higher stability than the siloxane linkage obtained with organisilanization. In addition, the hydrosilation reactions have proven to be robust and reproducible ( ). For the same analysis on different lots of the same column material, variations in retention times or capacity factors (k) are no more than a few tenths of a percent RSD.
While all silica hydride columns can operate in both the reversed-phase and normal-phase modes, the bonded moiety usually determines what retention properties will predominate. Thus, a nonpolar bonded moiety such as C18 will generally be used for reversed-phase applications, but it still has normal-phase properties in contrast to most other stationary phase with this type of modification. More polar bonded groups, such as diol or amide, would generally be used in normal-phase applications, but it still retains some reversed-phase properties. The following sections contain applications for each type of commercially available silica hydride column, which at this time happens to be produced by only one company. Columns types that are not available include chiral, size exclusion, anion-exchange, gel permeation, sub two-micron particle size and core shell.
This is the most widely used of the silica hydride-based columns. It was the first stationary phase to display capabilities for analyzing a broad range of polar compounds. Even today, it has features that either are not available or less versatile than most HILIC phases. The ability of equilibrate rapidly in normal-phase applications under a variety of mobile phase conditions (usually two to three column volumes) is in most cases vastly superior to HILIC columns. Another aspect of the Diamond Hydride column is its use of low buffer concentrations in the mobile phase. Typical additive concentrations are in the 5 to 15 mM range, while many HILIC applications can be significantly higher (30 to 100 mM is not unusual). This can be particularly important when using mass spectrometry for detection. High additive concentrations can leave deposits on the ion source, thus lowering sensitivity and requiring frequently cleaning. While acetonitrile is the most common organic solvent used in ANP analyses on the Diamond Hydride column, acetone [9] and methanol [10] have been shown to be applicable as well. Another feature of this stationary phase is its ability to retain certain strongly polar compounds in the normal-phase mode at very low concentrations of organic in the mobile. This often occurs at 20 to 30% organic content and has been referred to as “super ANP”.
The analysis of polar compounds is an important component that occurs in a variety of different application areas, such as pharmaceuticals, metabolites, food, forensics, environment, and biotechnology. Many examples describing a variety of analyses of hydrophilic compounds can be found in the literature as well as on the Microsolv Technology website [11]. Some examples of the approximately 200 listed application notes through the website are the analysis of hydrophobic and hydrophilic peptides in a single run [12], benzodiazepines in urine [13], acrylamide [14], the pharmaceutical Xanax [15], anatoxin, which is a neurotoxin implicated in many poisoning incidents [16], and the common household product ingredient cetylpyridinium chloride [17]. Some examples of hydrophilic compounds analyzed on the Diamond Hydride in the literature are common metabolites [9], cathinones [18], collagen and elastin crosslinks [10], bactericidal targets [19], sugars [20], lipids [21], thiopurines [22], dietary supplements [23], juices and cereals [24], peptides [25], nucleotides [26], drug levels in human serum [27], and disease pathways [28,29,30]. In another report, the Diamond Hydride was combined with an RP column for a comprehensive survey of both polar and nonpolar metabolites [31]. One study compared the Diamond Hydride and three other Type-C stationary phases with respect to the relative retention of small molecules [32]. This range of applications displays the versatility of the Diamond Hydride column for polar compound analysis that goes beyond the capability of typical HILIC phases.
The phenyl phase having a hydrophobic moiety bonded to the silica hydride surface is generally used for reversed-phase applications, particularly those where the analytes contain aromatic or other types of unsaturated sites. Retention is enhanced for these types of analytes though π-π interactions with the bonded moiety.
An example of an application for the phenyl column in the literature is the analysis of 16 common drugs of abuse by LCMS in under 8 min [33]. Under a different set of experimental conditions, the THC-delta-9-COOH can also be analyzed on the phenyl column. Additional articles citing the use of phenyl hydride columns involve the analysis of rice [34], mycotoxins in grains [35], and jaboticaba fruit [36]. However, there are more than 30 examples of analyses using the phenyl hydride column on the Microsolv website [37]. Included among the applications presented are the analysis of 10 phenolic acids in rice, methylenedioxymethamphetamine (MDMA) in plasma and the pharmaceutical compounds coricidin, fluoxetine, and ketorolac. An interesting example is shown in for the analysis of common components found in cough syrup [37]. This is a gradient analysis that is an overlay of five consecutive runs with an equilibration time of 3 min between runs. The run-to-run reproducibility of Cogent columns is one of their essential features, as well as rapid column equilibration between gradient runs.
Open in a separate windowA more recently developed silica hydride column is the amide stationary phase. The amide phase focuses primarily on hydrophilic molecules and is especially applicable to the analysis of sugars and various carbohydrates. Both a fundamental description of this phase and some examples of applications can be found in a published article [38].
Additional information on applications of the silica hydride-based amide column can be found online. A range of analyses are presented, relating to compounds that are not carbohydrates. An interesting example is the determination of the antidepressant fluoxetine (Prozac) in a capsule [39]. The compound is polar with a secondary amine, an ether linkage, and a trifluoromethyl group. The lot-to-lot reproducibility of this phase is also demonstrated in the application with three synthetic batches showing virtually identical retention times for the analyte. Nitrogen-containing compounds can often be difficult to analyze in reversed-phase due to adsorption on residual silanols and is often challenging in the HILIC mode. The separation of pyrilamine and 4-amino-3-chloropyridine with good peak shape demonstrates the ability of the silica hydride-based amide column for these types of compounds [39]. Another application involves the analysis of the pharmaceutical compound tizanidine in tablet form used to treat muscle spasms and cramps [39]. A particularly interesting application is shown for the separation of glucose and fructose in cola. These structurally similar compounds are a challenging separation, but can be done in the ANP mode under isocratic conditions using the amide silica hydride column [39].
Open in a separate windowThis is a unique stationary phase that was first made with conventional bonding techniques on ordinary silica before it was subsequently adapted to a silica hydride matrix [40]. Cholesterol is a liquid crystal (solid with an ordered structure) in the native state. It was postulated and later proved that some of this ordered arrangement of the molecules exists even when one end is attached to a surface. In addition to hydrophobic interactions between the analyte and the bonded cholesterol moiety, there was discrimination based on the morphology of the stationary phase, which had a slot-like configuration. Thus, analytes that were more linear and could penetrate into the slots were retained better than bulkier molecules that were preferentially excluded from this restricted environment. The same exclusion phenomenon was also confirmed on the silica hydride cholesterol phase [41]. The phase was also tested for its effectiveness in small molecule separations [32].
Over the years, since its initial commercialization, a number of interesting applications have been developed. Among these are the separation of six serum corticosterones that can be used as disease markers in various clinical analyses [42], the drugs atropine [42] and doxcycline and methacycline [42], and the fruit juice component limonin [42]. A good example of the shape selectivity of the UDC cholesterol column is the separation of the monounsaturated C18 fatty acids shown in [42]. The two compounds, oleic acid (cis isomer) and elaidic acid (trans isomer), are separated on the UDC column with the cis isomer eluting first. This is due to the fact that the trans isomer is a linear molecule and penetrates deeper into the ordered stationary phase than the cis isomer, which is partially excluded due to its bent configuration.
Open in a separate windowWhile aqueous normal-phase chromatography is based on the presence of the silica hydride surface, the bonded moiety, as demonstrated with the amide phase, can also provide additional interactions that can facilitate the separation of hydrophilic compounds. In this case, the bonded moiety is a carboxcyclic acid compound. This functional group is the basis of weak cation-exchange properties for HPLC stationary phases. Thus, it can be surmised that this bonded phase would provide additional interactions for amines or other nitrogen containing compounds. An example of the effective use of this phase was demonstrated for the analysis of nucleotides related to clinical analysis [26]. Another interesting analysis successfully developed for the UDA phase was for the determination of ethyl glucuronide and ethyl sulfate [43]. These compounds are useful long-term markers for alcohol abuse and are often tested for in people in certain critical jobs, such as airline pilots or public safety personnel.
Additional examples of the use of this column can be found in online application notes. Pharmaceutical compounds containing nitrogen are another potential use of the UDA column. The antibiotic tobramycin was analyzed using a water/acetonitrile isocratic mobile phase with 0.5% formic acid [44]. Tablets containing vardenafil (Levitra) were extracted and then analyzed by HPLC using the UDA column [44]. A gradient method was developed in the ANP using a mobile phase containing DI water/acetonitrile with 0.1% formic acid. Excellent run-to-run repeatability was achieved with a 0.2% RSD for the retention time. A challenging analysis is the separation of the adenine nucleotides, AMP, ADP, and ATP, as shown in [44]. The chromatogram shown was obtained by gradient analysis in the ANP mode using a mobile phase A of water with ammonium formate buffer and mobile phase B of acetonitrile with ammonium acetate buffer. Excellent peak shape was obtained for these compounds, which often produce significant tailing on other types of stationary phases.
Open in a separate windowThis stationary phase is the silica hydride version of the standard octadecyl column used in a large variety of reversed-phase applications. In addition to having a hydride surface, its other distinguishing feature is that it has a double attachment to the surface. Thus, the designation bidentate to denote this particular structural feature. As expected, the Bidentate C18 (BD C18) is amenable to a broad range of reversed-phase applications. Mycotoxins have a high toxicity and are a potential problem for human health. A number of these compounds have been analyzed in grain samples using the BD C18 column [35]. The nutritional supplement resveratrol in capsule form and in wine has been analyzed for both the cis and trans isomers [45]. Limonin, a bitter substance that can affect juice quality, has been analyzed on the BD C18 column [46]. Both reversed-phase and aqueous normal-phase performance was evaluated for a set test samples [32]. These are just some examples of analyses found in the literature for this stationary phase.
Online application notes provide additional examples of the usefulness of this column. A significant number of protocols involve pharmaceutical compounds. Among these are information for the separation of the isobaric opioid drugs morphine and hydromorphine [47], atorvastatin [47], the laxative bisacodyl [47], and the muscle relaxant dantrolene sodium [47].
While most reversed-phase applications are done with a C18 column, there are some instances when the analytes are so hydrophobic that these compounds take an unusually long time to elute or may not elute at all. Therefore, the way to reduce these interactions between the bonded moiety and the analyte is to make the stationary phase less hydrophobic. The simplest way to accomplish this is to shorten the bonded alkyl change. Hence, using a C8 (octyl) moiety accomplishes this goal. The C8 bonded phase on silica hydride has the same advantage as the C18 column with the ligand having a double attachment to the surface. An interesting application using the BD C8 column for the analysis of bisphenol A on carbonless paper often used as receipts for copying has been published [48]. Two extraction methods, one via digestion and another via migration, both gave satisfactory results with reproducible retention times.
There are numerous applications which can be viewed online for this phase. Many involve applications to pharmaceutical analysis. A protocol has been developed for the over-the-counter antihistamine chlorpheniramine maleate to identify the principal ingredient as well as a number of potential impurities [49]. Others include the antimalaria agent Clindamycin [49], the analgesic tramadol with data from two column lots demonstrating reproducibility [49], and prednisone, which is often used to treat arthritis or lupus, with the data showing excellent run-to-run reproducibility [49]. Another example of a pharmaceutical application is shown in . This analysis involves the prodrug sulfisoxazole acetyl and two potential preservatives often used in the formulation [49]. The figure contains overlaid chromatograms from two different lots of material, demonstrating the reproducibility of the manufacturing process.
Open in a separate windowFor very large molecules, particles with pore sizes around 110 Å are too small and may be excluded from interaction with the bonded organic moiety. Therefore, a silica hydride-based stationary phase with a bidentate-attached C8 group has been developed using a particle with a 300 Å pore size. A number of potential applications are documented to illustrate the usefulness of this particular stationary phase. The resolving power of this phase is demonstrated by the separation of two very similar cytochrome c proteins; one from horse heart and the other from bovine heart [50]. Another potential application is the separation of peptides [50]. This technique would be applicable to the characterization and purification of synthetic peptides. There is growing interest in the analysis of glycoproteins. It has been shown that the BD C8 phase can successfully retain these types of molecules with good peak shape [50].
The diol phase involves the attachment of polar organic moiety to the silica hydride surface. Therefore, it is generally used in the aqueous normal-phase mode for the analysis of hydrophilic compounds. Because the modified surface is different than the Diamond Hydride, the amide, and UDA phases, it has different selectivity and provides another option for trying to optimizing a particular type of separation. One study demonstrated how this column could be used to analyze a number of drugs of abuse [33]. Another investigation involved separating and analyzing uric acid cycle metabolites in the ANP mode where pre-column derivatization is not necessary as in many reversed-phase methods [51]. The diol phase was also included in a comparative study four different silica hydride phases [32].
Application notes for a number of other analyses are available that demonstrate some additional uses for this phase. The selectivity of this phase was highlighted in a study that separated ascorbic acid, niacin, riboflavin, folic acid, pyridoxine, metformin, and thiamine using a gradient ANP protocol [52]. In a similar type of experiment, the compounds warfarin, hydroxybupropion, and codeine in blood serum samples were analyzed with a simple liner ANP gradient. [52]. A good example of the dual retention capabilities of the silica hydride stationary phases is presented in separation of benzodiazepines on the diol column. The urine sample was analyzed with gradients in both the reversed-phase and aqueous normal-phase modes [52]. The chromatograms of both modes are shown in .
Open in a separate windowSilica C is the name used for an unmodified silica hydride particle. Because it has a low level of water adsorption (less than 0.5 of a monolayer), it should be an excellent choice for organic normal-phase chromatography. This feature is in direct contrast to ordinary silica, which strongly adsorbs water. Thus, when using ordinary silica in organic normal-phase chromatography, the mobile phase must the carefully dried to obtain reproducible retention times. Such precautions are not necessary for Silica C due to its low affinity for water. This concept was proven for the evaluation of phenolic compounds using traditional organic normal-phase chromatography on the Silica-C stationary phase [53]. Another example of an organic normal-phase application is the analysis of nonylphenol, a compound used in the synthesis of surfactants [54].
The Silica-C stationary phase has also been proven to be successful in the aqueous normal phase mode [55,56]. A number of pharmaceutical compounds have been analyzed using this stationary phase by ANP. A protocol was developed for the antihistamine diphenhydramine using an isocratic mobile phase of 50:50 water/acetonitrile with 5 mM ammonium acetate [57]. This particular example demonstrates the strong normal-phase retention properties of the silica hydride phases considering the large percentage of water in the mobile phase. In addition, this method was tested on four different lots of the Silica-C material, giving excellent reproducibility among columns (see ). Another drug, ketotifen, used for the treatment of asthma, was analyzed by a gradient ANP method [57]. This tertiary amine compound had strong retention on the Silica-C and the protocol developed gave excellent run-to run reproducibility. Phenylglycine used in the synthesis of lactam antibiotics was analyzed with an isocratic mobile phase consisting of 80:20 acetonitrile/water with 0.5% formic acid [57].
Open in a separate windowBy 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 that remain visible in the final coating, reduced thickening efficiency, poor thixotropic stability over time, lower gloss and transparency, and possibly film defects.
Fumed silica has excellent thermal stability, but as the temperature of the coating environment increases, particularly upon shear, the wetting properties of the coating typically improve. This can lead to over-dispersion, whereby the aggregates are reduced past their optimum association level. Once this occurs, reduced thickening efficiency results, sometimes to the point where it appears as if no thickener was added. Fumed silica is one of the smallest particle size materials added to a coating formulation. This component should be added early in the formulation, preferably to the resin or binder, rather than into non-film-forming components like solvents for optimal effect. Caution should be used when post-adjusting batches with powder, as only minimal shear that is inadequate for homogeneous incorporation can often be used at this stage. Post addition with low shear may bring formulations to their desired rheology, but this can deteriorate over time, as larger agglomerates slowly wet-out.
Lab evaluations have demonstrated that multiple performance attributes can also 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. These features are achieved without compromising gloss and other appearance attributes. An example is shown in Figure 6, where a pre-made aqueous fumed silica dispersion improves the film formation in combination with reduced coalescent solvent levels. These improvements have been seen with many different film-forming resins and using less or no coalescing solvents. This can help reduce the overall volatile organic compounds of the formulation, in addition to providing the other properties referenced above. This enhanced film formation is a result of reduced stress propagation due to the reinforcing effects of the very finely dispersed fumed silica.9
Anti-Corrosion/Water Repellency
Hydrophobically modified grades of fumed silica have also been used together with anticorrosive pigments to improve corrosion performance and water repellency of coatings. These grades are not considered to be anti-corrosion pigments, but they 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 are used to ensure that there are enough particles in the coating matrix to support a hydrophobic barrier, improve the mechanical properties of the film, 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. Two examples of this effect are shown in Figure 7.
Proper dispersion is again needed to homogeneously distribute the silica throughout the coating matrix for maximum effect. It is suggested that the treated fumed silica be dispersed together with the anti-corrosion pigments to ensure optimal dispersion. The best results have been obtained using DDS, TMOS, and HMDS treatments, although the impact on formulation rheology must also be considered.
Scratch Resistance
The development of silica particles for the specific purpose of improving scratch resistance came from the creation of particles for high reinforcement of elastomers and composites. The critical factor needed to achieve high reinforcement is to be able to fill the polymer matrices with significantly higher levels of fumed silica without increasing the viscosity to unworkable levels. This is achieved by structure modification through a proprietary post-processing to achieve a highly reduced structure and a low level of aggregation. This results in a material that can be used as a reinforcing filler to increase the mechanical strength of the coating and impart scratch resistance. The corresponding physical property observed is a significant increase in the bulk density (Figure 8) and a dramatic reduction in the ability of silica to increase viscosity. When compared to other reinforcing fillers, such as alumina, silica has the advantage of a lower refractive index of 1.46, more closely aligned with many polymer systems, which results in improved transparency and clarity. These materials can also 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. This can be seen in the scanning electron microscopy (SEM) analysis of a cross-section of a high-solids coating (Figure 9), which shows a homogenous distribution through the film with no surface enrichment resulting from higher particle density.
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 using both scratching methods, and reduced haze was observed after the panels were subjected to Elcometer testing (Figure 10). The addition of 5% silica did slightly reduce gloss, but the silica significantly improved scratch resistance.
Results in Figure 10 show three variants of DDS-treated structure-modified silica. Variant 1 is the original that requires milling, variant 2 the same DDS structure modified silica pre-dispersed in methoxypropyl acetate (MPA), and variant 3 is the newest version, which is easy-to-disperse.
Free Flow, Fluidization, Transfer Efficiency
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, and high-transfer efficiency as well as reduced Faraday cage effects for even film thickness and optimized appearance during application. Hydrophilic- and hydrophobic-treated fumed silica and alumina can be used to improve flow, storage stability with reduced moisture pick-up, and improved fluidization and transfer efficiency.
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 they are charged into the hopper to help them feed more consistently and homogeneously into the extruder. The typical loading of flow additives used 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 are used to influence the final powder coating properties, they must be added after extrusion and be oriented on the outside of the powder coating particles. There are typically two places where silica or alumina (or a combination of the two) can be added into the process to achieve this: 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 out of the powder and reduce the final dose remaining in the powder coating. The typical dosage level used in the chipping or as post add is also 0.1–0.3% by weight.
A study was organized with the University of Western Ontario to assess specific 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. The additive dosage level was adjusted based on the particle size of the powder. A 0.3% dosage level was used for the coarse powder and a 0.5% dosage level for the fine powder. A third powder was a tribo-applied polyester powder coating. The attributes tested 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 silicahydrophilic 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 a more consistent level of jetness than the disk applied with powder containing no additive. This test measures how much powder is transferred and the consistency of coverage of 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 applications conditions. After application, these 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 was effective in reducing Faraday cage effects in the coarse, black powder coating.
Fluidization efficiency was also assessed in this study. The results in coarse and fine powder show that particle size of the powder coating significantly affects the type of additive most effective for improving fluidization. Alumina was more effective in improving fluidization in the coarse powder coating as measured by lower air velocities needed to obtain 20% bed expansion, while silica was more effective in the fine powder coating (Figure 12). This trend suggests that additive packages may need to be adjusted based on their particle sizes.
Gloss Control
Gloss is defined, according to DIN EN ISO 4618, as 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 of the light in a range of angles. Gloss level can be characterized by the angular distribution of light scattered from a surface, measured with a glossmeter or reflectometer, and it is dependent upon the viewing angle (Figure 13).
There is no common or globally accepted definition of the term “matte.” It is always measured based on a comparative measurement of the gloss against a standard.2-5 For coating surfaces, the term “gloss” means almost complete reflection in the sense that 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).6
Lin and Biesiada demonstrated that matting is a function of both the silica particle size and degree of coating shrinkage during drying (either through solvent evaporation, chemical reaction, or coalescence).7 Larger particles are more efficient at reducing gloss as a function of silica dosage, but the larger particles can lead to a rough surface and increased dirt pickup over time. Most silica grades used for matting coatings are produced via wet and gas phase processes and are classically larger than the grades used for rheology. Some of these grades can influence thickening to a lesser or greater extent. Surface treatment, either with wax or reactive oligomers, can help reduce viscosity build-up, prevent hard settling, and improve transparency and stability. Recent technology developments have produced silicas with improved haptic effects like soft-feel.
Recent Developments in Silica Technology
While considered a mature technology, the use of silica in coatings continues to benefit through innovation. Romer described a new process for producing precipitated silica that allows greater control of particle morphology during the precipitation process to produce highly spherical particles with narrow-sized distributions (Figure 15).8 The spherical shape imparts high apparent hardness to improve scrub, abrasion, and burnish resistance of the formulated coating, with low binder demand and minimal impact on coating rheology. These spherical silica particles also can provide matting properties, depending on particle size, and they have excellent transparency for use in deep colors and clear coats.
Both hydrophilic (200 m2/g) and some hydrophobically treated grades (e.g., DDS, HDS, HMDS, 130–300 m2/g) of fumed silica can be used in water-based coatings when it is possible to adequately disperse the powder into water dispersible resins and solutions. However, the low viscosity and high dielectric constant makes water a poor grinding medium for fumed silica, and it is difficult to achieve the degree of de-aggregation and dispersion of particles needed to achieve optimum benefits in water-based coatings. Incorporation of hydrophobically treated grades can also be difficult due to the poor wetting properties of water-based formulations, especially when resin solids drop below 35% non-volatile content. Additives, such as acetylenic diols, can help to improve the wetting and dispersion of fumed silica into water, but care must be taken to ensure that the additives do not disrupt the silica network formation, reducing rheology control.
When adding the silica directly into water, or when using a shear-sensitive, film-forming resin, it is recommended to use a pre-dispersed form of fumed silica to overcome these challenges. A new aqueous dispersion of a functionalized fumed silica has been developed using a new production process and carefully selected additives. The silica is already dispersed, so it can be easily stirred into water-based formulations without requiring high shear dispersion.
The new dispersion WF 7620, contains 20% functionalized silica with a high surface area of 300 m2/g and demonstrates outstanding rheological effectiveness in waterborne coatings, especially those applied via spray application where anti-sagging properties are critical while maintaining excellent flow and levelling properties.
This is demonstrated in the jump-curve rheology graph shown in Figure 16. The jump curve simulates a spray application where the coating is sheared at high shear (500s-1) continuously to simulate the spraying process and then the shear is suddenly reduced to 0.1s-1 simulating the coating on the substrate after application. A fast build-up of viscosity is desired to prevent sagging after application onto vertical surfaces. This enables the formulator designing perfect finishes for three-dimensional parts including general industrial coatings, transportation coatings, plastic coatings, and wood coatings. The jump curve demonstrates the new dispersion WF 7620 and gives a significant increase and improvement in rheological efficiency, compared to an existing water-based dispersion of fumed silica based on a fumed silica core of 130 m2/g. This improved performance is due to the use of a higher surface area silica, combined with a novel functionalization in-situ. The use of a higher surface area fumed silica in the dispersion also helps to improve clarity and transparency and helps to reduce haze in the final coating.
Conclusion
Modified grades of silica have been used for many years to improve a variety of performance attributes in many different coating applications. These include rheology, film formation, and mechanical properties as well as surface appearance. The morphology of the silica particles, size distribution, and surface treatment are critical to the broad range of properties that can be attained using these materials. Recent advances in the manufacturing and post-treatment processing of silica have continued to develop new grades of silica that offer new and or improved performance for the coatings industry.
References
CoatingsTech | Vol. 17, No. 6 | June 2020
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