In ceramic manufacturing, binders are additives mixed with ceramic powders to increase the mechanical and cohesive strength of pre-fired green ceramics. The increased strength provided by the binder in a ceramic system allows ceramic parts to pass through the production process and make it to the firing step without breaking or falling apart. Ceramic binders come in many varieties, but the ideal ones accomplish the following characteristics.
Increase the mechanical strength of formed ceramic parts
Do not cause the ceramic paste to stick to processing equipment
Burn out with low ash content upon firing
Do not interfere with glazing
Ceramic Binder Types
Most ceramic binders fall into one of two categories: inorganic or organic.
Inorganic Binders
Inorganic binders are mineral-based materials often based on silicate chemistry and increase mechanical strength at incredibly high temperatures. While inorganic binders like sodium silicate and bentonite are relatively inexpensive, they often require a higher usage rate than organic binders and sometimes cause issues with viscosity that lead to ceramic defects.
Organic Binders
Organic binders cover all polymeric-based materials used in ceramics that are not mineral based. This type of binder forms hydrogen bonds with individual ceramic powder particles during drying. Organic binders are typically more expensive than inorganic binders but can be used at a lower rate and usually offer additional benefits to ceramic processing that inorganic binders cannot.
METHOCEL™: The Ideal Ceramic Binder
METHOCEL™ is a line of methylcellulose and hydroxypropyl methylcellulose polymers that are excellent organic binders and processing aid additives for producing thin-walled or complex-shaped ceramics. These water-soluble polymers are unique from other organic binders in that they form a three-dimensional gel structure at elevated temperatures, providing extra stability to green ceramic bodies during drying and early-stage firing. This feature is referred to as thermal gelation and also prevents binder migration and reduces the potential for stress cracks and blisters during firing. The graph below illustrates the increase in green strength as a function of thermal gelation using a torque rheometer to measure the viscosity of an aluminum oxide ceramic mixture containing METHOCEL™ A4M.
The Effect of Thermal Gelation on Green Strength
As depicted, when ceramic mixes containing METHOCEL™ are held above the thermal gelation temperature, the result is a sharp increase in green strength and cohesiveness.
Improving Ceramic Processes with METHOCEL™
As shown above, METHOCEL™ is exceptional at improving the mechanical strength of green ceramic bodies. Perhaps equally impressive is how METHOCEL™ can simplify ceramic manufacturing processes and improve their efficiency. Thermal gelation also changes ceramic mixes from having adhesive properties to having better cohesive properties at the gelation temperature.
Below, a ceramic mix containing METHOCEL™ was observed in a torque rheometer below and above the thermal gelation point. Below the thermal gelation point, the ceramic mix sticks to the rheometer. However, above the thermal gelation point, the ceramic mix separates cleanly from metal surfaces. This is an incredibly valuable characteristic in the extrusion, injection molding, and tape casting of intricate ceramics.
Ceramic Mix in a Torque Rheometer
In addition to the thermal gelation benefits, METHOCEL™ increases the lubricity of the ceramic mix. Ceramic processing equipment, such as molds and dies, are susceptible to wear due to the friction that ceramic particles impart. By increasing lubricity, METHOCEL™ helps reduce particle wear to increase the service life of equipment and reduce energy costs associated with molding, casting, and extruding ceramic parts. Furthermore, METHOCEL™ burns out cleanly during firing, ensuring no residue is left that may impact the structural integrity or shape of the ceramic part.
Preventing Binder Migration
Many organic binders will migrate to the surface of drying ceramic bodies along with the evaporating liquid. This can cause a layer of binder to form at the ceramic surface, inhibiting drying. Upon firing, the migration and resultant uneven binder distribution can lead to stress cracks, surface blisters, and other surface defects. METHOCEL™ binders remain fixed in the three-dimensional network formed during thermal gelation and will not migrate to the surface. Therefore, ceramic bodies containing METHOCEL™ as a binder are more consistent and far less likely to show surface defects.
Gelation Variation of METHOCEL™ Binders
The thermal gelation temperature, gel firmness, and dynamic viscosity of a ceramic mix containing a METHOCEL™ additive vary based on the type and concentration of METHOCEL™ in the system. In particular, thermal gelation causes a significant increase in the apparent viscosity of ceramic mixes at varying concentrations and temperatures based on the grade used. This makes it easy to obtain the right balance of properties for each type of ceramic being manufactured.
Apart from the considerations above, selecting the appropriate METHOCEL™ grade for a ceramic application depends on the formulation and processing requirements of processing techniques. The grades listed in the table below are starting point recommendations based on these techniques and applications.
Other METHOCEL™ grades and concentrations may be recommended depending on your requirements. Our METHOCEL™ specialists are happy to assist you in finding the right grade and getting you a sample. Click below to get in touch.
