The influence of monomer and polymer properties on the removal of organic vehicle from ceramic and metal moldings
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J. R. G. Evans and M. J. Edirisinghe Department of Materials Technology, Brunei University, Uxbridge, Middlesex UBS 3PH, England
E. H. Twizell Department of Mathematics and Statistics, Brunei University, Uxbridge, Middlesex UB8 3PH, England (Received 28 June 1993; accepted 6 March 1995)
This paper describes the effects of monomer and polymer properties on the competition between degradation of organic vehicle and transport of degradation products in ceramic moldings during pyrolysis. An experimentally tested model is studied systematically for ranges of material and process parameters characteristic of known polymers and their degradation products. The work highlights the properties having the greatest influence on the successful removal of organic vehicle from molded ceramics. The polymer properties controlling the diffusion constant are linked to the temperature dependence of viscosity of the molten suspension. Enthalpy of vaporization of the organic vehicle and the activation energy for the diffusion coefficient have a commanding influence on the critical heating rate for avoidance of defects. Preliminary guidelines emerge for the design of polymers for plastic forming of ceramic suspensions.
I. INTRODUCTION The ability to manufacture artifacts of complex shape from engineering ceramic or metal powders has been enhanced by the transfer of techniques used in polymer processing, principally injection molding. Powder is incorporated at volume fractions in the 0.5-0.75 range to produce a suspension in an organic vehicle which can be shaped by taking advantage of the shear and extensional flow properties of high polymers. The organic phase is then removed to leave a particle assembly of comparable prefired density to that achieved by conventional compaction and capable of being sintered using standard methods. Common to these processes is the difficult and time-consuming stage of removing the organic phase from the polymer-ceramic composite without introducing defects. A slow controlled pyrolysis of the organic phase is the most popular way of achieving this. The critical stage occurs early in pyrolysis before continuous porosity has developed. Among the several mechanisms for weight loss, thermal degradation is the most likely to affect the integrity of the particle assembly because it occurs throughout large sections, liberating degradation products into solution in the "parent" polymer. These must diffuse in the molten polymer throughout the section to the surface where evaporation takes place, and this path involves low mass transport coefficients. Previous work has therefore focused on this mechanism, selecting it as a priority for quantitative analysis.1 The model so obtained describes the competition be2060 http://journals.cambridge.org
J. Mater. Res., Vol. 10, No. 8, Aug 1995 Downloaded: 16 Mar 2015
tween the rate of generation of degradation product and its diffusion to the surface of the body in solution in the parent polymer, at a stage where the latter fills the pore space. If the vapor pressure over
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