Estimation of the elastic constants of highly porous cellular plastics reinforced with fibres embedded in foam struts

In order to enhance the mechanical properties of polymer foams, fillers of different materials and sizes are being applied. Fibrous fillers shorter than the characteristic foam cell dimensions have the potential of efficient reinforcement, due to their high aspect ratio, without detrimentally interfering with foam structure. A theoretical model is developed for evaluation of the effect of filler on foam stiffness. The elastic response of rigid polymer foams filled with short fibres, of length commensurable with that of foam struts, is modelled by using the orientational averaging technique. Explicit expressions for components of the stiffness tensor of composite foams are derived in terms of elasticity characteristics of the polymer and reinforcing fibres, fibre content, foam density, and parameters of the orientational distribution of foam struts. The model is applied to describing the foam stiffness as a function of fibre loading of polyurethane foams filled with milled carbon fibres.