Particle and pore size distributions of investments

The particle-size distributions of investment powders and the pore-size distributions of set-fired investments were investigated by electrozone size analysis, and by mercury intrusion scanning porosimetry, respectively, for a number of different investments, representing all three types available, namely: the gypsum-, phosphate- and silicate-bonded. The pore volume (V), the pore surface area (S), and their first derivatives with respect to radius, r, dV/dr and dS/dr, were obtained with pressures up to 414 MPa corresponding to spherical pores of 0.0018 micron radius if the Washburn equation is to apply. Results distinguished pores with a range of different sizes. Besides the porosity due to the matrix binder, porosity was also attributable to the refractory components. This latter type of porosity occurred with pore radii below about 0.03 micron, whereas matrix porosity occurred above this pore size. Matrix porosity for the phosphate materials was smallest at about 0.03 to 0.5 micron in radius, followed by the gypsum materials at about 0.3 to 0.6 micron, and by the silicate materials at about 1 to 10 micron. The particle-size distribution of the investment powders may affect both types of porosities, whereas the setting chemistries of the particular investment types may affect matrix porosity, by the generation of gaseous by-products. No major distinctions in pore size and distributions occurred between air-set and hygroscopic-set materials. This study emphasizes that at present, knowledge of the total pore volume only is inadequate for characterizing porosity distribution and hence investment permeability.