Thermal debinding

the most commonly used debinding technique is thermal debinding, which is based on the pyrolytic degradation of organic additives. 

During the rise in temperature of the thermal cycle, as the orgaitic compounds in the pores of the green part become liquid, they are subjected to the action of capillary forces. This capillary ntigration of the orgaitic compotmds, from the large diameter pores towards the low diameter pores, will lead to a redistribution of the organic phase within the porosity. 

When the debinding temperature reaches higher values, generally beyond 180°C, the polymeric chains of the binders are snbjected to chemical degradation reactions. To the displacement of the binder in the liquid state is added the diffusion of the gas species within the porosity of the sample. The volatile species formed with low molecular mass (HjO, CO, CO2, CH4, light unsaturated hydrocarbons, etc.) are ehminated by diffusion and surface evaporation. 

The reaction products resulting from these three types of mechanisms have a structure generally close to that of hydrocarbons. These hydrocarbons, except for methane and ethane, are thermally unstable beyond 400°C compared to their basic elements (C, H2) and so the thermal activation generates the rupture of C-C and C-H bonds. As the energy of the C-C bonds (345 kJ moT ) is weaker than that of the C-H bonds (413 kJ moT ), the rupture of C-C bonds is more frequent. 

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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. 

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)]. 

Figure 1. Model for thermal debinding by oxidative degradation where the binder-vapor interface is at a distance L from the surface of the compact. (After Ref. 2.)...

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

Log90] Lograsso BL, German RM (1990) Thermal debinding of injection molded powder compacts. J Powder Metall Int vol 22 No 1 pp 17-22... 

German, R.M. (1987) Theory of thermal debinding, Int. J. Powder Met. 23, 237. Classic article on this... 

During thermal debinding, the feedstocks are heated until the binder is removed via thermal decomposition. Hence, it is important to study the thermal decomposition of the binder in the presence of the powder. The TGA (thermogravimetric analysis) is usually used for this study. The samples would be heated at programmed heating rate with a stream of nitrogen gas purging the furnace s interior to reduce oxidation of the metal powder in the feedstocks. 

In order to eliminate flaws inherent to thermal debinding and to shorten the duration of this stage, other techniques for the extraction of organic shaping additives have been developed. These techniques are based on an under- or overpressure of the treatment atmosphere, on microwave heating, capillary migration of molten binder, sublimation of a binder in aqueous phase, or solubilization by catalytic reaction or solvents. 

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