Continuous processing of pre-pregs using radio frequency heating

Abstract

Commercial carbon fiber composites are fabricated using pre-impregnated fibers with epoxy or “pre-pregs”, that are B-Staged (partially cured) using large ovens to ensure partial cross-linking for easy handling. The impetus for this work came from the exciting discovery that carbon fiber itself may show rapid heating in response to applied RF fields; previously, this was known only for select nanomaterials. In this investigation, we demonstrate the radio frequency (RF) response of three pre-pregs – unidirectional CF/epoxy, [0/90] weave CF/epoxy and CNT/epoxy. Heating rates up to 6 °C/s can be achieved at input powers as low as 5 W, and as high as 70 °C/s at 25 W. Such high heating rates are difficult to achieve in a conventional oven. Finite element simulations also showed similar heating trends to the experimental RF response of carbon fiber/epoxy pre-preg. Further, it was found that exposure to RF energy for specified timespan can achieve the desired degree of cross-linking in carbon fiber/epoxy and CNT/epoxy pre-pregs. A continuous pre-preg manufacturing method is also demonstrated, where controlled translational speed over RF applicators is used to achieve desired temperatures.