A dry process has been invented as an improved means of manufacturing composite prepreg tapes that consist of high- temperature thermoplastic polyimide resin matrices reinforced with carbon and boron fibers. Such tapes are used (especially in the aircraft industry) to fabricate strong, lightweight composite- material structural components. The inclusion of boron fibers results in compression strengths greater than can be achieved by use of carbon fibers alone.
Until now, polyimide/carbon-and-boron-fiber tapes have been made in a wet process: Boron fibers are calendered onto a wet prepreg tape comprising carbon fibers coated with a polyimide resin in solution. In the calendering step, the boron fibers, which typically have relatively large diameters, are pushed only part way into the wet prepreg. As a result, the boron fibers are not fully encapsulated with resin. In addition, the presence of solvent in the prepreg contributes significantly to the cost of the finished product in two ways: (1) the tackiness and other handling qualities are such that the prepreg tape must be laid up in a labor-intensive process and (2) the solvent must be removed and recovered before and/or during the final cure of the polyimide.
The present dry process is intended to enable the manufacture of prepreg tapes (1) that contain little or no solvent; (2) that have the desired dimensions, fiber areal weight, and resin content; and (3) in which all of the fibers are adequately wetted by resin and the boron fibers are fully encapsulated and evenly dispersed. Prepreg tapes must have these properties to be useable in the manufacture of high-quality composites by automated tape placement. The elimination of solvent and the use of automated tape placement would reduce the overall costs of manufacturing.
In this process, a layer of parallel boron fibers is formed and sandwiched between two layers of parallel carbon-fiber tows coated with a powdered polyimide resin. The layers are then heated and pressed together to form a composite tape. As shown in the figure, the boron fibers and the powder-coated carbon-fiber tows are pulled off reels and through combs that form the groups of fibers into the various layers with the lateral spacings consistent with the desired areal densities of carbon and boron fibers. The three layers are pulled through a furnace and maintained parallel until they reach a position where each layer of coated carbon-fiber tows slides against a set of impregnation/spreader bars. The temperature zones in the furnace are set to provide enough heat to melt the polyimide before arrival at the bars. The bars are heated to promote the flow of the resin system while facilitating the spreading of the tows as they slide over and under the bars.
After passing the bars, the layers are brought out of the furnace and pressed together between two forming die rollers. The speed of pulling of the tape and its fiber constituents is controlled by means of a pair of drive rollers downstream of the forming die rollers. The speed is chosen such that the time at temperature is adequate for the required melt flow. After passing through the drive rollers, the finished tape is wound on a takeup reel.
This work was done by Harry L. Belvin, Roberto J. Cano, and Norman J. Johnston of Langley Research Center and Joseph M. Marchello of Old Dominion University. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Manufacturing category.