Castable materials that are made from the oxides of aluminum, silicon, calcium, and zirconium that retain their form and strength at high temperatures are known as refractory materials, or simply refractories. Most often refractory composites are cast using ceramic fibers made of alumina, silica, or aluminosilicate. These heat-resistant materials are formed into various shapes and used as refractory products in a wide variety of industrial applications such as kilns, glass casting, metal casting, furnaces, incinerators, and ovens. The properties of refractory products are dependent on the type and amount of refractory material, binder and filler material as well as the manufacturing process.
In general, the purpose of a binder is to gel the refractory materials and improve the green strength of the formed shape during drying and firing. Much like cement, binders are the glue that hold the formed cast refractory together. Organic binders can be burnt out during firing but inorganic binders are more common and will remain within the formed shape. Because they remain an integral part of the formed shape, selection of the appropriate inorganic binder is critical to the performance of castable refractories.
There are many types of binders that are used commercially and selection should be based on optimizing the efficiency of the manufacturing process and the performance of the cast refractory in the end application.
LUDOX® Colloidal Silica Binders
The high surface area and chemical reactiveness of LUDOX® Colloidal Silica make them excellent binders for inorganic refractory fibers used in the production of formed thermal ceramics and refractories. In the ceramic slurry, LUDOX® exists as individual, non-aggregated particles. On drying, the colloidal silica particles concentrate to react with themselves and the alumina, silica, aluminosilicate, and other ceramic materials in the ceramic slurry. Strength of the refractory increases with subsequent heat steps as the surface of LUDOX® is dehydrated and bonds that are inert, insensitive to moisture, and stable at high temperatures are formed. The benefits of a refractory composite formed using LUDOX® are listed below:
Resistance to vibration and mechanical shock
Low thermal expansion
Improved thermal shock resistance; Useful at temperatures up to 3000°F
Easy control of properties
Reduced refractory failure
Recommended LUDOX® Grades for Refractory Applications
LUDOX® SM, HS-40, and AS-40 are the most commonly used grades in refractory applications. SM has the most surface area per gram and is therefore the most efficient binder of the LUDOX® grades. In some applications the sodium content of LUDOX® SM makes it unacceptable. In these applications LUDOX® HS-40 offers a lower sodium content. LUDOX® AS-40 can be used if sodium must be avoided completely and is most commonly used in applications that require the highest temperature resistance. LUDOX® TMA and CL are used less frequently but offer unique properties that make them useful in specific applications. LUDOX® TMA has been modified to maintaining a high charge density that is independent of pH and should therefore be used in systems that are acidic or neutral pH. LUDOX® CL is unique in that it carries a positive surface charge. It can be used to flocculate and coagulate anionic solids in a slurry.
Post-treatment with LUDOX®
In addition to being used as a binder in the ceramic slurry, LUDOX® colloidal silica can be applied to the surface of bonded refractory materials to improve the strength and hardness of the composite. This process is useful to harden the exterior of a composite made with starch or to provide more uniform strength to castable refractories made with other inorganic binders.
Vacuum-Forming Processes with LUDOX®
Some example vacuum-forming processes for manufacturing refractory composites and the typical physical properties of the resulting composite are listed in the table below:
Summary & Conclusion
Castable refractory materials are used commercially as a slurry of ceramic fibers, binder, and filler. These slurries are cast into many different shapes and are used in a variety of industrial applications. The properties of the formed refractory can vary greatly depending on the amount and composition of three components mentioned above so selection of the appropriate raw materials is critical to the heat-resistance and overall performance of the refractory in the end application. LUDOX™ colloidal silica is ideally suited for use as an inorganic binder for the production of incredibly heat resistant and chemically inert refractory materials. Contact us today to discuss your current refractory manufacturing process and what LUDOX™ colloidal silica can do for you.