Polymers binder burnout

A small amount of polymer can produce an enormous volume of gas, which must be removed from the porous green body. The flow of this gas in the porous network of the ceramic green body can create a pressure build-up which puts stress on the green body. The stress induced by flow and temperature gradients in the green body are also discussed, so that binder burnout conditions can be selected to prevent cracking of the green body. 

A complete picture of polymer thermal degradation is clouded because multiple reaction mechanisms can be operable for a single polymer at a single temperature, leading to a host of volatile products and residues. Therefore, one has to consider the relative rates of these competing reactions to establish an optimized and controlled binder burnout. 

P is the number of polymer molecules of degree of polymerization n, R is the number of radicals found in a volume V, R is the number of polymer radicals with degree of polymerization n found in a volume, V. For other definitions, please use the nomenclature associated with Table 15.2. Noting equation 15.14, the kinetics of polymer degradation are very complex. Only the most simple mechanisms have been thoroughly researched. These simplified reactions presented in Table 15.2 are sometimes zero order, more frequently first order, and infiiequently second order in polymer mass. These simplified rate expressions are typically used to model binder burnout. 

To discuss binder burnout in detail, let us consider a spherical green body of radius R with a polymer completely filling its pores undergoing an oxidative thermal decomposition. Figure 15.9 is a schematic of this process. We will use the case where the number of moles of oxygen, n, is 1 that is,... 

Accounting for these rate determining steps the kinetics of binder burnout can be established for simple decompositions of polymers, such as depolymerization and the oxidation of carbon. For polymers with... 

The major problem confronting extrusion and injection moulding is the removal of the binder. Binder burnout must proceed at a slow rate (taking up to several days) so as to avoid problems with slumping and crack formation. The polymer removal time increases drastically when the size of the green body increases, thus making it difficult, if not impossible, to produce parts with thick cross-sections. 

This process is more popular in polymers. For the injection molding of ceramics, a thermoplastic polymer is required to the extent of 40 vol.%. This is called the binder. For example, to injection mold silicon carbide, ethyl cellulose is one such binder that is used. During the firing process, a binder removal soak, called binder burnout, is given to eliminate the polymer from the final product. 

Consequently, preferred preceramic polymers are those that are curable liquids or soluble, thermoplastic solids. When us in powder processing, preceramic polymer binders obviate extended binder burnout cycles since the polymer "bums in" to the consolidated ceramic or composite phase. 

Gelcasting is a recently introduced process that is based on ideas taken from the traditional ceramics industry and the polymer industry [167-173]. In summary, a slurry of ceramic particles dispersed in a monomer solution is poured into a mold, and the monomer is polymerized to immobilize the particles and to form a gel-like bonding phase (i.e., a binder). The system is removed from the mold while still wet, dried by evaporation of the hquid, heated to burnout the organic additives, and finally sintered. 

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