Evolution of mechano-chemistry and microstructure of a calcium aluminate-polymer composite: Part II. Mixing rate effects
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Evolution of mechano-chemistry and microstructure of a calcium aluminate-polymer composite: Part II. Mixing rate effects L. S. Tan and A. J. McHugh Department of Chemical Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
M. A. G¨ulg¨un and W. M. Kriven Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 (Received 12 June 1995; accepted 11 March 1996)
Microstructure, microchemistry and mechanical properties of hardened macrodefect-free (MDF) composites processed at various rotor rates in a Banbury mixer were investigated. A quiescently formed calcium aluminate-polyvinyl alcohol composite served as a substitute for an unmixed system. Results from the Banbury studies in conjunction with microchemical analysis of the unmixed composite showed evidence that the polymer-particle interaction is a mechanically induced crosslinking reaction. The rate of the mechano-chemistry increases with mixing speeds. Scanning electron micrographs (SEM) and transmission electron micrographs (TEM) of hardened composites mixed for 15 min at 30, 50, 100, and 200 rpm indicate that much of the mechanical strength of MDF is due to the crosslinked interphase zones that blanket the cement grains. Stresses in the paste due to mixing can destroy the interphase layer, leading to a weaker hardened composite. Microchemical analysis revealed that the mechano-chemistry of the system did not vary with changes in the mixing conditions studied.
I. INTRODUCTION
High strength calcium aluminate-polymer composites, better known as macro-defect-free cement (MDF), consist of a densely packed network of cement grains embedded in a continuous polymer matrix. This polymer-cement composite is produced via shearcompression of a standard formulation first reported by Birchall et al.1,2 MDF has been traditionally processed on a two-roll mill; however, McHugh and Tan3,4 recently provided an alternative method. Their technique employed a Banbury mixer coupled with a torque rheometer and was successful in forming an MDF paste, the precursor to the hardened composite. The new technique also yielded quantitative information pertaining to the network formation during MDF processing. Previous studies5,6 have shown that processing history affects the mechanical and morphological characteristics of the hardened MDF composite. In particular, the mechanical work during mixing induces a chemical reaction between the organic and inorganic phases. This mechano-chemistry is responsible for the evolution of the MDF paste during processing. Since the polymer-particle matrix governs the final microstructure and mechanical properties of the hardened material, quantitative understanding of MDF paste development is vital to successful processing of this J. Mater. Res., Vol. 11, No. 7, Jul 1996
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composite. Following the protocol established in Part I of this series,6 this publication shows the i
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