Mass Transfer Processes During Multicomponent Bender Thermolysis
- PDF / 558,455 Bytes
- 8 Pages / 420.48 x 639 pts Page_size
- 29 Downloads / 231 Views
MASS TRANSFER PROCESSES DURING MULTICOMPONENT BENDER THERMOLYSIS JENNIFER A. LEWIS* AND MICHAEL J. CIMA.A *University of Illinois at Urbana-Champaign, Dept. of Materials Science and Engineering, Urbana, IL 61801, (217) 244-4973. "*Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, Cambridge, MA 02139, (617) 253-6877. ABSTRACT Both the binder viscosity and the % saturation will change during the removal of multicomponent binder systems. The effects of these changes were evaluated for the polyvinyl butyral (PVB) - dibutyl phthlate (DBP) system as the plasticizing constituent was selectively removed at isothermal conditions (Tio < 1700C). Experiments were performed to determine the viscosities of the PVB-DBP system at various isotherms and at different concentrations of DBP. A scaling model was developed to determine the relative importance of capillary forces on the distribution of binder within ceramic compacts during thermolysis. This model was modified to account for the influence of fluid saturation on both the capillary driving force and the permeability of the wetting phase (i.e., binder). Two important results are derived from this analysis: (1) the change in binder viscosity affects the length scale (h) over which capillary forces act to redistribute the PVB-DBP to a greater degree than the change in saturation, and (2) the value of h can be significantly lower than the representative macroscopic dimension of the green compact and still permit a capillary-controlled distribution process for multicomponent binders. ISTRODUCTION Multicomponent binder systems are widely-used processing aids for fabricating advanced ceramics. These aids are temporary additives which must be removed prior to densification. The removal process (or thermolysis) is time consuming, and is often the rate-limiting step in the manufacture of tape-cast and injection-molded ceramics. In previous work, capillary forces were shown to influence the distribution of binder in alumina-based green tapes during thermolysis resulting in enhanced removal kinetics [1-31. These tape-cast sheets contained a multicomponent binder system; polyvinyl butyral (PVB) and dibutyl phthalate (DBP). A theoretical scaling model was also developed in previous work to distinguish the relative importance of capillary forces and diffusion on binder distribution processes in ceramic green bodies. The primary objective of this study is to modify this scaling model to account for changing binder saturation and viscosity during the removal process. Secondly, the model will be applied to binder thermolysis from A12 0 3 -PVB-DBP tapes to illustrate the impact of these changes on the mass transfer processes that occur for multicomponent binder systems. The experimental portion of this research was performed to determine quantitatively how binder viscosity changes as both a function of temperature and binder composition for PVB-DBP system. These values in combination with previous results will be used for the theoretical calculations.
Mat. Res
Data Loading...
