Microvoids in Glass-Resin Composites. Their Origin and Effect on Composite Strength

Abstract

Microvoids are trapped in the resin between individual filaments in glass-resin composites to the extent of 10 to 109 voids per cubic inch. These air voids result from slow and unequal capillary penetration of resin into spaces between filaments during the impregnation process. The poor resin wettability of commercially finished filaments aggravates the situation. Reducing the contact angle to zero and/or causing the tension to oscillate as the strand passed through the resin markedly reduced the number of voids. Composite test rings having void contents of 0.2 volume % or less had interlaminar shear strengths 40 to 100% greater than the conventional rings with high void contents (5%).Qlass-resin composite materials have attracted increasing interest wherever there is a need for materials having high strength-to-weight ratios. Usually, these composites are fabricated by the resin impregnation of glass fiber mat, glass cloth, or continuous glass filaments.

Characterization of voids formed during liquid impregnation of non-woven multifilament glass networks as related to composite processing

“…This suggests a critical transition between a zone of entrapment to that of no entrapment as the surface wettability of the filaments composing the mat structure decreases. This result along with the observations of Bascom and Romans [6] suggests that entrapment is high under conditions of extremely low as well as extremely high wettabilities; and is minimal at intermediate contact angles that are close to 90°. Local nonuniformities are one cause of entrapment.…”

“…Studies have indicated that poor resin wettability of commercial finished filaments resulted in an increase in the formation of voids [6]. Reducing the contact angle to zero and/or oscillating the tension in the strand as it passed through the resin markedly reduced the number of voids.…”

“…An investigation on the origin of voids, especially interstitial voids, has been reported by Bascom and Romans [6]. Researchers have used techniques such as density measurements, water absorption, micmgraphy, ultrasonic scan, and radiography to determine void content.…”

Solvent effect of water on S-glass

“…Most experimental results and microscopic and submicroscopic observations reported in the literature [2][3][4]10,11] agree that the water molecule is the major transgressor in causing debonding between the fibres and the resin matrix. The mutual strengthening effect of the major components in the composite is thus adversely affected.…”

Techniques of Measuring Contact Angles

“…(59) In this method, the drop is held in a small ring in the microscope field, and the fiber, which is suspended between support posts, is passed (advancing or retreating) through the center of the ring. If the ring is small enough and the fiber homogeneous, the liquid surface will be axially symmetrical about the fiber.…”