Method utilizing ultrasonically induced cavitation to impregnate porous sheet passing through a resin bath

A method of impregnating a flexible, porous, thick, high basis weight, cellulosic sheet material comprising the steps:

• (1) continuously passing a flexible, porous cellulosic sheet material, having a basis weight of over about 150 lbs./3,000 sq. ft. and a thickness of over about 10 mils, at a rate of speed of from about 350 feet/minute to about 800 feet/minute, through a bath of liquid, organic, "B"-stageable impregnating resin, having a viscosity of from about 10 cps. to about 750 cps. at 25.degree. C., and through a cavitated zone in close proximity to the resonant vibrating surface of at least one ultrasonic wave generator, said cellulosic sheet material passing from about 1/4 inch to about 6 inches away from the surface of the ultrasonic wave generator, where a differential length of the cellulosic sheet has a dwell time in the resin bath of from about 0.2 second to about 1 second, said ultrasonic wave generator being completely immersed in the resin and operating at a frequency of over about 10,000 Hz and at a radiated power level effective to provide a combination vibratory pressure on the resin molecules, and cavitation effect causing degassing and microstreaming of the resin, and heating of the resin within the cavitated zone along the width of the passing sheet, said heating causing a lowering of the resin viscosity, said combination of vibratory pressure and cavitation degassing and microstreaming effect causing resin impregnation throughout the void volume of the cellulosic sheet;
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• (2) passing the resin impregnated sheet through a drying means to "B"-stage the resin.