Journal of Materials Synthesis and Processing, Vol. 7, No. 4, 1999 Foaming Power, Bubble Nature, and Sample Density Related to the Expansion Regime in Polyurethane Foams Dario T. Beruto,1,2 Massimo Baiardo,1 and Stefano A. Mezzasalma1 Foamed polyurethane products were obtained from mixtures of polyolic and isocyanate compounds to which different amounts of water were added. The foaming process was carried out in a special homemade reactor that allows us to record continuously the change in volume and total pressure produced by the foam during its expansion period. From these measurements, the associated foaming work and power were derived. It is shown that the foaming power is joined with important internal system parameters such as surface tension, viscosity, bubble and water mole numbers, average bubble dimension, and coalescence kinetics. The closed/open nature of the bubbles during the expansion foaming period was experimentally investigated. The effect of the amount of water added on the foaming power and on the final apparent foam density was evaluated. The apparent density, which decreases as the amount of added water increases, reaches a critical value where the structural collapse occurs independently of the foaming temperature. A link between collapse criticality and the correspondent kinetic rate is suggested and discussed. KEY WORDS: Polyurethane foam expansion regimes; foaming power; open/closed bubble kinetics; apparent critical density value. 1. INTRODUCTION Biomimetic processes have been recently proposed to obtain new materials that are useful in many fields of materials science and medicine [1-4]. In orthopedics, in seeking nature imitation in bone-substitute products, it is mandatory to produce a solid matrix with an open cellular structure [5-7]. The cells should have dimen- sions ranging between 100 and 200 um and their struts and walls should be formed by bioactive surfaces. Under these conditions, when osteocity cells will be the host in these cavities, it is believed that bone formation is stim- ulated [8,9]. In many clinical applications, the bone-substitute product needs to have high mechanical properties [10]. In such cases, the products usually consist of ceramic 1 Dipartimento di Edilizia, Urbanistica, Ingegneria dei Materiali, Uni- versity of Genoa, P. le J. F. Kennedy, Pad. D, 16129 Genova, Italy. 2 To whom correspondence should be addressed. materials belonging to the apatite family [hydroxyapatite (HAP) and calcium-deficient carbonated apatite, i.e., dahllite] [11]. These products are obtained through clas- sical high-temperature processing so that their mechani- cal properties are satisfactory [12,13]. Unfortunately, as the high-temperature treatment may change the chemical nature of apatite powders, their osteoconductivity can be poor. In other clinical applications [14-16], where high mechanical properties are not determinant, a promising way to obtain bone-substitute compounds consists, first, of producing a bioinert polymeric cellular solid and, then, of trying to cover it through bioactive ceramics [17]. In the framework of a wide research project [39], polyurethane foams have been proposed for use as a potentially useful matrix for bone-substitute compounds. Polyurethane has been chosen as a material of study because of its application in biomedical fields [16,18]. Furthermore, quite recently [19-21], it has been possible 229 1064-7562/99/0700-0229$16.00/0 © 1999 Plenum Publishing Corporation