The Process of Thermal Debinding

In practice, binder system may consist of three or four additives that differ in their volatility and chemical decomposition. Furthermore, interactions between the binder and the particle surfaces may alter the decomposition kinetics of the pure polymer. In view of its complex nature, a detailed analysis of thermal debinding is not useful. Instead, we consider the basic features of the process for a simplified system consisting of a powder compact with a single binder [e.g., a high molecular weight thermoplastic polymer such as poly(methyl methacrylate), poly(propylene), or poly(vinyl butyral)]. 

Table 1. Effect of Process Variables on the Time for Thermal Debinding by Oxidative Degradation...

Binder removal can be accomplished by thermal decomposition or by dissolutiion. In ceramics, the thermal decomposition method is commonly used and will be considered here. The process is referred to as thermal debinding or, more simply, as binder burnout. In thermal debinding of ceramic green bodies, both chemical and physical factors are important. Chemically, composition of the binder determines the decomposition temperature and the decomposition products. Physically, the removal of the binder is controlled by heat transfer into the body and mass transport of the decomposition products out of the body. 

Thermoplastic based binders are by far the most widely used. These forms of binder usually contain a wax as a major component and a thermoplastic as the minor component. Additives are usually added for lubrication, viscosity control, wetting and improving powder-binder interaction. Debinding of such binders is normally achieved via thermal degradation, wicking, solvent extraction or even photo-degradation. Thermoplastics commonly used include polyethylene, polystyrene, polypropylene and ethylene vinyl acetate. Table 2 highlights some of the common thermoplastics used in PIM or MIM processes. 

---