Binder removal

Extraction by capillary flow is also known as wicking, in which the green body is heated in a packed powder bed or on a porous substrate that can absorb the molten binder. The net time t used to remove the binder through wicking is given by [225]  

Solvent extraction involves immersing the component in a liquid in which at least one binder can be dissolved, leaving an open porous structure for subsequent binder burnout. Full debinding is desirable because the resulting powder compaction will have almost no strength. The time t for debinding is given by [225]  

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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. 

Thermal degradation, as outlined above, leads to production of volatile, low molecular weight products throughout the binder phase and the removal of the binder by evaporation of a liquid. Binder removal by this mechanism m.ay be quite similar to the drying of a moist granular material considered above (17.2.3.3.1). Considerable redistribution of the liquid occurs, and the evaporation front does not move uniformaly into the bodyT Instead, pore channels first develop deep in the body as liquid from the larger pores is drawn into the smaller pores. 

One solution of the conflict might involve the use of a sintering aid to enhance densification during firing. Table 1 also indicates that a low ambient pressure or a vacuum serves to reduce the time for binder removal. A vacuum, however, does not lead to oxidative degradation. Furthermore, temperature control and transport of heat are... 

Pellet pressing Binder removal 138-207 MPa, 53% theoretical density green density... 

To create an initial porous structure before thermal binder removal, it is also possible to extract parts of the binder system by solvent extraction. In this technique the ceramic green bodies are immersed in a liquid which solvates one of the components in the binder used. A prerequisite of this process is that the binder component to be removed first has a high solubility in the solvent to ensure rapid dissolution. Furthermore the viscosity of the resulting polymer solution should be low, and diffusion of the liquid inside the body should be fast. Swelling of the binder components during solvent extraction can cause defects to be formed inside the ceramic body because of the internal stresses generated. Solvent extraction is a relatively fast and the cost of the equipment relatively low. 

The process of binder removal is kept slow to reduce the possibility of macrodefects being produced. Figure 23.15 shows a plot of a binder removal cycle. In this plot a pressurized gas, called a sweep gas, has been passed over the part to help sweep away the vapor. The cycle time also depends on the size of the part. Thin sections take much shorter times than thick sections. The debinding time is proportional to the square of the section thickness of the compact—the familiar parabolic kinetics seen in our discussion of reactions in Chapter 25. 

FIGURE 23.15 Pressure-induced binder removal cycle. 

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