Particle size and shape 511 MECHANICAL PROPERTIES 9 Tensile

Large quantities of clay minerals (and other micron-sized mineral powders) are used widely in materials science. Mineral fillers are major components, for example, of paints, rubber, plastics and paper and board. While the original object of such incorporation was to reduce costs, performance is now being "engineered" by design or choice of the relevant mineral particles. This paper concentrates on the use of mineral powders in polymer and paper science. In both application areas, the size and shape of the mineral particles can affect mechanical properties such as stiffness and tensile and impact strength. However, other characteristics are also modified. For example, the drainage rate in paper formation and the rheology of a polymer melt, properties which are critically important to the rate of processing of the relevant materials, can be greatly affected. Finally, the size and shape of the mineral particles also affect appearance, i.e. the smoothness, uniformity, optical properties etc. Mechanical and rheological data are presented for mineral-filled polypropylene, as are mechanical and optical data for filled and coated paper. There has been much interest in recent times in measuring the size and shape of small, micron or sub-micron, particles. The priori ty assigned to the topic results from the fact that the subject is of more than academic importance. Many of the most technologically impor tant fluids are colloidal in nature; their behaviour depends on interactions between sub-micron particles and their surrounding fluids; interactions which depend on surface charge and chemistry, but also very critically on the size and shape of the particles themselves. The rheological behaviour of toothpastes , paints and lubricating oils, for example, are all "engineered" by suitable design or choice of the relevant colloidal particles. Powdered materials of micron size are used widely in materials science. Mineral fillers are major components , for example, of paints, rubbers , plastics of various kinds (both thermoplast ics and thermosets) and paper and board . The original driving force for incorporat ion of fillers was to reduce overall costs. However , in every case, addi t ion of the filler modifies the proper t ies of the material . Indeed, considerat ion of commercial practice in the manufacture of each of the mater ia ls ment ioned above makes it obvious that powdered fillers are now selected carefully to enhance performance in some way, whether it be in terms of mechanical proper t ies , surface smoothness or optical proper t ies such as brightness or opacity. In this paper , emphasis has been placed on polymers and papers. In each case, an a t tempt has been made to show how the size and shape of the mineral particles determines proper t ies of technical and commercial importance. 9 1993 The Mineralogical Society