On the dispersion of fine ceramic powders in polymers

Abstract

The dispersion of fine (sub-micrometre size) ceramic powders in polymers which act as a vehicle for their processing has been investigated extensively during the past decade [1-5]. Several novel ceramic fabrication processes, such as injection moulding [6], blow moulding [7], vacuum forming [8] and film blowing [9], are dependent crucially on dispersing homogeneously several chemical types of these fine ceramic powders in a variety of both low and high molecular weight polymers. Dispersion is dependent on several types of ceramic-polymer interactions and these have been reviewed recently by Evans [10]. Good dispersion can be accompanied by a strong attachment of the polymer to the surface of the ceramic powder, and although this could be beneficial in obtaining a homogeneous as-formed microstructure, the subsequent removal of the polymer prior to sintering of the ceramic can be made more difficult [11]. Most ceramic-polymer suspensions used in the forming process mentioned above are prepared by melt processing [12]. Therefore, dispersion of the fine ceramic in the polymer is dependent on shear mixing at a temperature above the softening point of the polymer, but often microstructural investigation of the formulations prepared in this way shows the presence of a variety of defects, including undispersed polymer and relics of agglomerated powder [1]. In addition, demands placed by the necessity to use ultra-fine (nano-sized) powders, which are very agglomerated, to achieve high sintered densities have placed limitations on eren high shear melt processing methods such as twin roll milling and twin screw extrusion [13]. It may be possible to overcome these difficulties if a high volume loading of ceramic powder can be dispersed homogeneously in a suspension of polymer particles in aqueous or nonaqueous medium prior to melt processing. This letter sets the scene for the exploitation of this method in the forming of engineering ceramics. Previously, related studies have been limited to monodisperse model systems, where heterocoagulation in colloidal dispersions containing more than one type of particle has been studied in polymer and paint [14], mineral [15] food and pharmaceutical [16] and composite [17, 18] systems. A major drawback in the assessment of dispersion of fine ceramic powders in polymers is the lack of suitable experimental techniques. A ceramic-polymer suspension can be ashed to remove the polymer and the ceramic powder can be examined microscopically. The polymer can also be dissolved in a suitable solvent and the resulting suspension can be 'studied using light scattering or Coulter counter