CAPILLARY IMPREGNATION OF ALIGNED FIBROUS BEDS - EXPERIMENTS AND MODEL

Capillary flow affects the amount of air entrapped between fiber reinf orced composites during the impregnation. This paper investigates the permeability of aligned fiber bundles by capillary flow in both longit udinal and transverse directions. Experiments measure capillary flow b y the rate of weight gain of fiber-filled tubes having one end in cont act with a fluid at atmospheric pressure. The driving force for this f low is capillary pressure only; no external driving pressure is applie d during impregnation. Permeabilities obtained under these conditions are slightly lower than those measured under conditions of positive ex ternal pressure. The Kozeny constant for longitudinal flow are found t o be 1.06, and for transverse flaw to be about 8. A simple model is de rived to illustrate the role of capillary Bow along fiber reinforcemen ts during wetting. While capillary flow is insignificant with high inj ection sates, it is significant at low injection rates. The model indi cates that for a specified fiber volume fraction in the fiber bundle, there exists a unique injection speed in which the bulk fluid flow fro nt matches the capillary flow front inside the bundle. Injection at th at speed is usually too slow to be practical, but it would substantial ly reduce the void content in fiber bundles after impregnation. At hig h injection rates, the model indicates the possible trapping of air in side fiber tows due to the capillary flow lagging behind the bulk flow .