Composites with high temperature resistance, also known as refractories, are used in a wide variety of industrial applications that require extraordinary physical and thermal properties. These composites can be formed into complex shapes that are useful commercially in industries ranging from aerospace to consumer goods. The properties of a refractory composite is dependent on the type and amount of refractory fiber and binder as well as the manufacturing process.
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 refractory particles. 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 resistance; Useful at temperatures up to 3000°F
Easy control of properties
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 composites 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 composites 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:
In general, the purpose of a refractory binder is to gel materials and improve the green strength of a formed refractory shape in order to reduce defects that occur prior to firing. There are many types of binders that are used in refractories and selection should be based on optimizing the efficiency of the manufacturing process and performance of the formed refractory in the end application. ChemPoint has product experts standing by to help answer your questions. Please feel free to contact us today to discuss your current refractory manufacturing process.