Recent Developments in Soluble Silicate Based Binders and Coatings Mike McDonald, Janice Hamilton, National Silicates, Toronto, ON, and Judy Thompson, PQ Corporation, Valley Forge, PA ABSTRACT While soluble silicate binders have been used effectively for decades in various end uses, recent developments and discoveries may improve and expand the use of silicates. This paper reviews the traditional use and chemistry of sodium silicate binders and also describes how surfactants, viscosity modifiers, lubricants and non-traditional setting agents can provide new characteristics and flexibility in the use of silicate binders. The discussion also includes specialty uses for potassium and lithium silicates. These developments should further broaden the range of materials that can be agglomerated using soluble silicate based binders, thus providing more formulation options. 1. INTRODUCTION Soluble silicates have been used as binders for many years and in many applications1,2. In fact, a 1949 reference paper from the PQ library lists 28 ores or fuels and 10 patents where sodium silicate was used as the binding agent. These “classic” silicate-based binder formulations either utilize a high SiO2/Na2O sodium silicate by itself or exploit the setting reaction of sodium silicate with a common calcium salt or cementitious based material. Iron ore, silica, detergents and roofing granules are a few examples of materials that have used silicate based binders and coatings for decades. Recent trends have tended to favor the traditional advantages of silicate based binders. As inorganic binders, soluble silicates do not face the handling, safety and environmental issues associated with NOX and VOC’s. Furthermore, the pricing of soluble silicates is not subject to the same market fluctuations as petroleum based or sugar based binders. Another trend that has favored sodium silicate is the agglomeration of waste or low value materials into value added products. Typically, these value added products require a higher level of binder performance such as strength, durability and water resistance vs. agglomerated material that is to be recycled. An example of such application is the use of a modified sodium silicate to make light-weight porous spheres out of recycled container glass3 Recent developments and ideas summarized in this paper address some improvements that can be made to better utilize soluble silicates including sodium, potassium and lithium silicates. 2. SILICATE CHEMISTRY REVIEW Soluble silicates are manufactured by fusing high purity quartz sand (SiO2) with sodium or potassium carbonate (Na2CO3 or K2CO3) in an open hearth furnace at 1100 – 1200°C. The resulting glass is then dissolved using high pressure steam to form liquid silicate or “waterglass” which is clear and slightly viscous. The key parameter that determines the properties of soluble silicate solutions is the weight ratio of SiO2/Na2O. For example, a “3.2” ratio sodium silicate has 3.2 kg of SiO2 for every 1kg of Na2O. The typical range of commercially available ratios is 1.6 to 3.2. At the molecular level, the fundamental building block of silicate species is the silica tetrahedron consisting of the silicon atom at the center of an oxygencornered pyramid. Each oxygen atom may be associated with a hydrogen atom an alkali metal (Na, K, Li), or it may be linked to another silica tetrahedron. The silica can link to form chains, cyclic and larger polymeric structures. (Figure 1) The species typically carry an overall negative charge having the monovalent alkali atoms in loose association. Monomer Linear Trimer Cyclic Trimer Figure 1. Monomer Silica Tetrahedron, Linear and Planar Cyclic Silica Species Soluble silicates then are inorganic, polymeric, alkaline materials. They are also moderately strong buffers and can be involved in four basic types of chemical reactions, each of which can play a role in binder applications. The chemical reactions are: Gelation Metal Ion Reactions / Precipitation Hydration / Dehydration Surface Charge Modification 3. SODIUM SILICATE AS A BINDER Agglomerated materials require a binder in order to achieve acceptable strength. In general, binders can be divided into three groups: matrix, film and chemical.4 Sodium silicate is unique in that it can serve in all three of these capacities. For example, as a matrix binder, sodium silicate would be used in conjunction with Portland cement or pozzolan blended cement binders. Further discussion on film forming and chemical binding attributes follows. 3.1 SODIUM SILICATE AS A FILM FORMER Film forming binders are like glues and function by the evaporation of water or other solvent. Commercially available sodium silicates contain 45-65% water by weight. Loss of a small portion of this water, even under ambient conditions, will result in a strong, rigid, glassy film. Rate of drying will depend on ratio, concentration, viscosity, film thickness as well as temperature and relative humidity. The silicate binder may be subject to dissolution depending on use conditions; however some moisture resistance can be obtained by simply drying the silicate more completely through the addition of heat. 3.1.1 Surface Tension Sodium silicate has a naturally high surface tension that is close to that of water i.e. ~ 76 dynes. The penetration and adhesive quality of soluble silicates can be improved with surfactant/wetting aids. With reduced surface tension, sodium silicate can penetrate through smaller openings and spread over a greater area. Table 1 provides a summary of the physical properties with the addition of an anionic surfactant. Wetting agents appear to strengthen the bond by increasing the spread, although if the spread is too great, decrease in bond strength can result. Table 1. % Surfactant vs. Surface Tension, Wetting and Drop Shape % Surfactant Surface tension* 0 76.7 0.05 46.6 0.1 39.9 0.2 34.7 0.4 35.3 Seconds to wet (darken) fiber >600 >420 <180 <30 <15 Drop shape Beads Beads, some spreading Slightly flat Flat Flat *Du Noüy Ring method 3.1.2 Viscosity and Texture An increase in the viscosity of the silicate will decrease the wetting coefficient, the amount of water absorbed and rate of absorption. Aside from adding water, the viscosity of sodium silicate can be decreased by the addition of a small amount of potassium hydroxide or by gently warming the silicate. (Figure 2) 250 sodium silicate sodium silicate + 2% KOH viscosity (cps) 200 150 100 50 0 21 30 38 43 50 temperature C Sodium silicate – 3.2 ratio KOH ~45% solids Figure 2. Addition of 45% KOH reduces the viscosity of 3.2 ratio sodium silicate. Decrease in viscosity as a function of temperature is also shown. The presence of clay in sodium silicate does not influence the total water loss but does serve to give a tougher, more coherent film, free of bubbles and drying cracks. Glycerin and other plasticizers such as sorbitol may also aid by retaining moisture to produce a smooth film. 3.1.3 Lubrication Good lubrication properties are important in briquetting because reducing the particulate friction and improving the flow onto the rolls provide for better and more efficient compaction. Almost all liquid binders will also act as lubricants; however some chemicals are better at performing this dual task. High quality, unadulterated silicate has a very low coefficient of friction (CoF). Regrettably, this low CoF is lost with the addition of solids and/or water. Other industries using silicate (the oilfield industry in particular) have expressed a need for soluble silicate with a lower CoF. Consequently, research was conducted to try to lower the CoF by either adding a lubricant to the silicate or by chemically modifying the silicate with respect to how it interacts with metal surfaces. In developing a lubricant for alkali silicate, the basic criteria included: effective, compatible with silicate, environmentally friendly and relatively low cost. A wide range of chemical lubricants was investigated using a block and ring lubricity tester. These materials included: glycols, olefins, phosphates, surfactants, glucosides, asphaltenes and esters. Of these, the most effective were ester based, and to a lesser extent, glycols and certain glucosides. (Table 2) Also investigated and found effective was the modification of sodium silicate by the addition of tetralkylammonium compounds such as tetramethyl ammonium hydroxide (TMAH). It is thought that TMAH reacts with sodium silicate to form tetramethylammonium silicate with improved lubricity coming via the methyl groups. Table 2. Effect of Lubricants on Coefficient of Friction Water Sodium silicate Sodium silicate + water + 3% glycol +2% ester + 1% TMAH 3.2 CoF 0.36 0.05 0.40 0.29 0.18 0.30 SODIUM SILICATE AS A CHEMICAL BINDER Chemical binders function by reacting with the material being agglomerated or by formulating with multiple components that will react with each other. Sodium silicate has a long history of being used as a chemical binder. The best example is the use of sodium silicate with a soluble source of calcium (i.e. the Midrex process). The reaction of calcium salts with silicate forms calcium silicate hydrate. Other traditional setting aids used with sodium silicate are shown in Table 3. Table 3: Traditional Setting Agents Category Example Inorganic Salts CaCl2, Ca(OH)2, Mg(OH)2, NaH2BO3, Na2CO3 Mineral Acids H2SO4, HCl Organic Acids CO2, acetic acid, citric acid Inorganic Oxides ZnO, CaO, MgO A study was carried out to demonstrate the strength and durability of materials bound with sodium silicate. Figure 3 shows the benefits of time in developing strength properties at 3% and 10% of binder matrix. Compressive Strength kPa 900 800 700 600 500 400 300 200 100 0 3% of Mix 10% of Mix 3Days 3 Months Figure 3. Compressive Strength of Sodium Silicate Binder in Limestone Gravel Matrix Recently, development efforts have focused on non-traditional setting agents such as glycolic acid, sodium acid pyrophosphate (SAPP), and calcium lignosulfonate. The goal was to achieve one or more of the following attributes: Higher final strength Better control of set Longer set Environmental benefits Figure 4 provides a compressive strength comparison of the non-traditional versus other setting agents with 30% concentration sodium silicate in a limestone gravel matrix at 25°C. 4 2S O H 4 aS O C co lic G ly SA PP g aL i C et ic Ac Cl H l2 C aC Compressive Strength kPa 180 160 140 120 100 80 60 40 20 0 Figure 4. Compressive Strength Comparison of Setting Agents at 25°C. Lignin products are used in many applications that are similar to silicate end uses5. Traditionally, the two technologies have been viewed as competitive. For example, lignins and silicates are used as dispersants and can sequester metals. However, recent work has shown that there may be some complementary applications, particularly for calcium lignosulfonate. A laboratory study was designed to investigate the interaction between calcium lignosulfonate and silicates over a range of concentrations. The interactive effects were observed with both sodium and potassium silicates. The results of this work showed that the setting characteristics of these systems were not typical of the rapid interaction seen with other calcium salts and allowed for much more flexibility in the binding process. Table 5 shows the setting effects of mixing calcium lignosulfonate with sodium silicate. Calcium lignosulfonate is available as a solid or liquid (~50% solids) and either form can be used with silicates. Table 5: Setting Times of Sodium Silicate with Calcium Lignosulfonate Sodium Silicate N Calcium Lignosulfonate Setting Time (3.2 ratio) %w/w %w/w 5 5 3hr 5 10 4-5 min 10 25 1-2 min At present, sodium silicate remains the workhorse of silicates for binder applications primarily because it is the least expensive of the alkali silicates. For specialty applications, however, there may be justification to use potassium or lithium silicate as highlighted below in the following paragraphs. 4. POTASSIUM SILICATE Potassium silicate is typically used when sodium is undesirable for a particular end use. For example, potassium silicate is preferred for the clays and fluxing materials used in welding rod applications. It is also used to bind vanadium pentoxide catalysts for sulfuric acid manufacture and in acid-resistant cements. More recently potassium silicate is being considered as a binder for fertilizer applications. In this case, the more soluble potassium silicate can strengthen prills or pellets but then dissolve in use to release the nutrients of the fertilizer while providing additional potassium and silicon for plants. Potassium is a primary plant nutrient, and silicon has gained significant recognition as a beneficial plant nutrient that helps to physically strengthen plant tissue as well as improve nutrient uptake and alleviate salt and drought stress. 5. LITHIUM SILICATE Lithium silicate is a specialty silicate that is now being manufactured in the US. The properties of lithium silicate are intermediate between those of sodium silicate and organic ammonium silicates. The difference in properties between lithium and sodium silicate can be ascribed to the fact that hydrated lithium ion is larger than the hydrated sodium ion and therefore can stabilize more silica in the colloidal state. Table 6 provides a comparison between high ratio lithium, potassium and sodium silicate. Table 6: Property Comparison of Various High Ratio Alkali Silicates Wt % Alkali Wt % SiO2 Weight ratio Molar ratio pH Sp. Gr. Viscosity (cp) Cation radius of hydration (nm)6 Lithisil 25 2.5% Li2O 20.5 8.2 4.1 10.8 1.2 20 34.0 Kasil 1 8.3% K2O 20.8 2.5 4.1 11.3 1.26 40 27.6 N 8.9% Na2O 28.7 3.2 3.2 11.2 1.38 180 23.2 Compared to sodium and potassium silicate binders, certain surfaces can achieve better adhesion with lithium silicate. As a binder or coating, lithium silicate can also impart high levels of strength and water resistance. While lithium silicate is considerably more expensive than sodium silicate, improvements in adhesion can also be seen when a small amount of lithium silicate is blended with sodium or potassium silicate7. Lithium silicates are somewhat compatible with water-miscible organic solvents and slightly compatible with non-polar solvents (Table 7). This behavior is different from that of sodium silicate solutions which are not compatible with organic solvents to any extent at comparable SiO2 concentrations. Lithium silicate would allow for better bonding of material covered with organic solvents. Table 7. Compatibility of Lithium Silicate with Water-Miscible Organic Solvents Composition Final Mixture % SiO2 % Solvent 5.06 Mole Ratio Lithium Silicate Methanol 18.5 7.1 17.9 10.0 13.7* 19.7 Ethanol 19.0 4.4 Acetone 17.9 10.2 Dioxane 19.9 -----Tetrahydrofura 19.2 3.5 n 19.0 4.2 7.52 Mole Ratio Lithium Silicate Methanol 20.3 11.3 19.2 15.7 13.4* 18.0 Ethanol 19.6 8.8 15.5* 5.7 Acetone 18.8 12.3 Acetone 13.3* 19.0 Dioxane 19.9 7.4 Tetrahydrofura 20.0 6.7 n Solvent Stability 7 Days <10 min. 3 hours Gel 40 min. Gel >7 days >7 days 50 min. 15 min. 45 hrs <10 min >7 days 3 days 45 min. Gel 3 days * From stock lithium silicate solutions containing about 17% SiO2. All others from stock solutions containing about 20% SiO2 ACKNOWLEDGEMENT The authors would like to thank PQ Corporation and National Silicates for permission to publish this paper. BIBLIOGRAPHY 1. W.L. Schleyer, “Sodium Silicate As A Briquetting Binder”, Proceedings of the 9th Biennial Briquetting Conference, 1965 2. R.E. Wright, “Smelter Dust Agglomeration with Sodium Silicate”, Proceedings of the 15th Biennial Conference of IBA, 1977 3. United States Patent 4430107 4. “Selecting Binders and Lubricants for Agglomeration Processes” by Karl R. Komarek (Chemical Engineering Magazine, December, 1967) 5. SRI Consulting, 2005 Chemical Economics Handbook 6. .“Synthetic Inorganic Silicates,” Encyclopedia of Chemical Technology, Vol. 18, 2nd ed., 1969 by John Wiley & Sons, Inc., p150. 7. US Patent 4,347,890; Allyn-Pyzik & Falcone, PQ Corp
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