The threshold pressure of infiltration into fibrous preforms normal to the fibers’ axes

The equation for the threshold pressure of infiltration of liquids into fibrous preforms is revisited in this paper. It is shown that the commonly accepted equation to calculate the threshold pressure of infiltration is suitable only when the liquid is infiltrated along the axes of the fibers. A set of new equations is proposed to describe the threshold pressure of infiltration normal to the fibers’ axes. The threshold pressure of non-wetting liquids is found to be inversely proportional to the smallest distance between the fibers, while it is commonly believed to be inversely proportional to the diameter of the fibers. The difference between the results of these two equations can be 1–2 orders of magnitude. The new equations are suggested to be used to ensure filling small spaces between fibers, and thus to improve the final physical properties of the composites. The threshold contact angle of spontaneous infiltration was found between 0° and 43° (i.e. much below the commonly believed 90°) as a function of the ratio of the separation between the fibers and their diameters.