Weak interfacial regions imply that the filler particles cannot carry any of the load applied to the composites and the en- tire stress has to be carried solely by the matrix material. Because of poor wetting, the filler particles and agglomerates retain air in their hollow spaces, introducing porosity to the internal structure of the composites. Agglomeration of the filler particles and dewetting of the polymer at the interphase aggravate the situation by creating stress concentration points.