Debinding

The organic additives and other pressing aids, also called binders, must be removed from the green body before sintering. This process of removing the binder from ceramic green bodies is called debinding. Ideally, the binder is removed completely without affecting the order of the particle packing or causing any new defects in the green body.

However, residual impurities such as carbon and organic ions and defects such as cracks and large pores can have a negative influence on the microstructure development during sintering. The properties of the produced component suffer from this. Debinding is, therefore, a critical step in the processing of ceramics, especially when shaping methods are used that require a high proportion of binders in the green body.

Methods of debinding

There are three possible methods for debinding ceramic green bodies:

  1. Extraction by capillary flow into the pores of the surrounding medium
  2. Solvent extraction
  3. Thermal decomposition

The thermal debinding

By far the most commonly used method is thermal decomposition, also called thermal debinding. There are two ways to remove the binder: By heating at ambient pressure in an oxidative or non-oxidative atmosphere or under vacuum, in the form of steam. The process of thermal decomposition is influenced by both chemical and physical factors. Chemical factors include the composition of the binder. This determines the decomposition temperature, which is normally between 100 and 400 °C. Physically, the removal of the binder is controlled by the heat transfer into the component and the mass transport of the degraded decomposition products.

The following points must be observed during debinding

The heat treatment must be carried out slowly during thermal debinding, as pressure can be generated in the component due to the decomposition of the binder and the associated steam generation. A sufficient amount of oxygen or air should also be supplied to enable binder removal. Special precautions must be taken when debinding large products or products with high binder content (e.g. products produced by injection moulding, extrusion, slip casting or other plastic forming methods). In some cases, debinding is performed in a separate low-temperature furnace followed by firing in a high-temperature furnace. The maximum heating rate is determined by the product size and shape. High heating rates can cause a temperature gradient in the product, which leads to uneven shrinkage in the inner layers of the product. This can lead to cracking.

Temperature homogeneity, as well as a constant and homogeneous air or oxygen flow within the furnace during each debinding process, is also of great importance to avoid temperature gradients in the product. Temperature and air flow should, therefore, be set, measured and controlled very carefully. It is also very important to continue heating after debinding and to sinter the products before transport. Ceramic products are very brittle and sensitive to mechanical stress in the phase between debinding and sintering. Transporting the goods before sintering may, therefore, lead to cracking or major damage.

The waste gases produced during debinding contain hazardous organic materials and must be carefully extracted from the furnace and treated in a separate chamber in accordance with the relevant safety and environmental regulations. In practice, binder systems consist of a mixture of at least two components that differ in volatility and chemical decomposition. This makes it even more complicated to control the process of debinding.

 

References:
Concise Encyclopedia of Advanced Ceramic Materials, ed. by R.J. Brook, Pergamon, 1991, Oxford
M.N. Rahaman, "Ceramic Processing and Sintering" 2nd edition, CRC Press, 2003, New York

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