Refractory Binders

In general, a binder gels refractory materials and improves the green strength of the formed shape during drying and firing. Much like how they are used in cement, binders are the glue that holds the formed cast refractory together. Organic binders, which get burned out during firing, may be used. Inorganic binders, however, will remain within the material and play an integral part in the formed shape. Thus, selecting the appropriate inorganic binder is critical to the performance of castable refractories.

Selecting the ideal binder should be based on its ability to optimize 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 to produce formed thermal ceramics and refractories. When introduced to ceramic slurries, LUDOX^® exists as individual, nonaggregated silica particles. These particles integrate themselves between organic binders used to coat the ceramic fibers, forming a tight aggregate of fibers vacuum formed into shapes. On drying, the silica particles concentrate to react with themselves, the organic binders, and the alumina, silica, aluminosilicate, and other ceramic materials in the slurry. The strength of the refractory increases with subsequent heating as the organic binders are burned off and the surface of LUDOX^® is dehydrated, forming permanent chemical bonds within the ceramic material that are inert, insensitive to moisture, and stable at high temperatures. The benefits of a refractory composite material formed using LUDOX^® colloidal silica are as follows:
 

• Resistance to vibration and mechanical shock
• Low thermal expansion
• Low density
• Improved thermal shock resistance; useful at temperatures up to 3000°F
• Easy control of properties
• Reduced refractory failure

Recommended LUDOX^® Colloidal Silica Grades for Refractory Materials

LUDOX^® SM, HS-40, and AS-40 are the most commonly used colloidal silicas in refractory applications. LUDOX^® SM has the most surface area per gram of colloidal silica and is, therefore, the most efficient binder of these grades. In extremely high-temperature applications, however, the sodium content in LUDOX^® SM makes it unacceptable. In such applications, LUDOX^® HS-40 may be used, as it offers a lower sodium content. Alternatively, LUDOX^® AS-40 can be used if sodium must be avoided completely, as it is most used in materials that require the highest temperature resistance.

LUDOX^® TMA and CL are colloidal silicas that are less frequently used but offer distinctive properties that make them useful in specific applications. LUDOX^® TMA has been modified to maintain a high charge density independent of pH and is preferred in slurries that are acidic or neutral in pH. LUDOX^® CL is unique because it carries a positive surface charge and 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. In some cases, colloidal silica used in the slurry binding process may migrate to the surface of the refractory when drying, yielding a harder exterior yet softer interior. Using LUDOX^® to post treat will mitigate this effect and yield a more uniform strength. This process is also useful to add strength when post firing a composite made with organic binders, where the colloidal silica will replace itself in place of the organic binder that has been burned off, increasing the strength and hardness of the composite.

Vacuum-Forming Processes with LUDOX^®

Listed in the table below are some examples of vacuum-forming processes for manufacturing refractory composites and the typical physical properties of the resulting composite.
 

Summary & Conclusion

Castable refractory materials are made commercially from a slurry of ceramic fibers, binders, and fillers. 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 the three components mentioned above. Therefore, selecting 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 as an inorganic binder to produce incredibly heat-resistant and chemically inert refractory materials. Several grades are available to suit various slurry chemistries. Contact us below to discuss your refractory manufacturing process and explore what LUDOX^® colloidal silica can do for you.