JEC COMPOSITES MAGAZINE - Issue #39 - March/April 2008 - 46

FEATURE “Environment” Natural-fibre thermoset composite prepregs A new high-fibre-content prepreg system using an aqueous acrylic resin has been developed. A hot press equipped with a vacuum chamber is used to cure the material, resulting in a very short processing cycle and excellent component performance. LUISA MEDINA, SCIENTIST, MANUFACTURING SCIENCE RALF SCHLEDJEWSKI, GROUP LEADER, MANUFACTURING SCIENCE INSTITUT FÜR VERBUNDWERKSTOFFE GMBH Prepreg manufacture Needle-punched mats of hemp (Cannabis sativa L.) and kenaf (Hibiscus cannabinus L.) fibres (processed by Dittrich und Söhne GmbH) were impregnated with the acrylic resin. The first goal was to develop a suitable technology for impregnating the natural fibre mats with the matrix system. Several impregnation techniques were tested. The best results were reached with a foulard system (Figure 1). W Due to their good mechanical properties and low production costs, natural fibres are becoming a feasible choice as reinforcement for polymer matrices. Their low density also affords very good weight-saving potential (approximately 1.5 g/cm3). Natural-fibre composites have good processing and acoustic properties, as well as other advantages over glass-fibre materials, such as good life-cycle performance and easier processability. In Europe, the use of natural-fibre-reinforced composites is almost exclusively limited to passenger car applications, although they also have potential applications in trucks, buses and trains, as well as in new fields such as design parts (book shelves, chairs, cases, etc.). Natural fibres for technical applications are usually processed as a needled mat, impregnated with a polymeric matrix, and press formed. Fig. 1: Scheme of he impregnation process using a foulard system Matrix system A matrix screening step was carried out to develop a natural-fibrereinforced prepreg with very low matrix content. Thermoplastic resin properties allow a fibre content in the composite of no more than 50-60 wt.%. We tested different thermoset polymer systems from several raw-material suppliers as matrix, then evaluated the systems’ processing conditions and mechanical characteristics. For the research work, we decided on Acrodur®, an aqueous acrylic resin developed by BASF Aktiengesellschaft. This phenoland formaldehyde-free resin is a combination of polycarboxylic acid and polyalcohol. At temperatures above 130°C, the polycarboxylic acid reacts with the polyalcohol, forming polyester. Below this temperature, the resin exhibits thermoplastic properties and gives the prepreg material very good storage stability, compared to other thermoset resins. Following cure, the material exhibits thermoset properties, becoming hydrophobic, tough, and wear-resistant. This dipping process was used to apply the matrix to the nonwovens by binder dispersion. The matrix content in the semi-finished material can be determined by reducing the binder concentration, since this technique allows unlimited dilution of the matrix in water, or by adjusting the pressure of the rollers in the foulard system. After impregnation, the semi-finished material was dried to achieve approximately 15% residual moisture. Scanning electron microscope (SEM) investigations showed how homogeneously the matrix particles cover the fibres. This impregnation process ensures that the matrix is evenly distributed over and around the prepreg fibres Fig. 2: SEM picture of the impregnated fibres (Figure 2). JEC Composites Magazine / No39 March - April 2008

JEC COMPOSITES MAGAZINE - Issue #39 - March/April 2008

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #39 - March/April 2008