Natural-fibre thermoset composite prepregs

(Published on March – April 2008 – JEC Magazine #39)

LUISA MEDINA, SCIENTIST, MANUFACTURING SCIENCE

RALF SCHLEDJEWSKI, GROUP LEADER, MANUFACTURING SCIENCE INSTITUT FÜR VERBUNDWERKSTOFFE GMBH

WDue 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.

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.

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).

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 (Figure 2).

Component manufacturing and results

In the scope of this work, vacuum press technology was used instead of the conventional press process commonly used for natural-fibre composites. Press plates were manufactured without additional conditioning in a plant at Wickert Maschinenbau GmbH. The prepregs were pressed at 20 bar in a mould preheated to 200°C. Due to the residual moisture content in the prepregs and the condensation products from the curing reaction, the cavity had to be ventilated briefly during the press cycle.

The first step consisted in determining the pressing parameters under usual press conditions without vacuum. After several trials, a press time of 100 seconds was established. The consolidated plates were taken directly from the heated mould and cooled down at room temperature, without any deformation. When press time was reduced, bubbles were observed on the surface of the cured materials.

Using the same parameters, several test plates were pressed in the vacuum chamber. It took just a few seconds to reach a pressure of approximately -1mbar, and the total press time of 100 seconds was maintained. These plates exhibited a better surface with more regular colour. In addition, the working conditions were notably improved, since the direct evacuation of the evaporating moisture reduced the noise and odour level during the pressing process. The process was further optimized though a systematic reduction of the total press time. In the end, a 50% reduction was achieved without impairing the mechanical properties of the composite.

Several tests were carried out to characterize the mechanical properties of the components. Despite the low matrix content, all the natural-fibre-reinforced composites produced under all press conditions showed high mechanical properties, meeting the automotive industry’s requirements for interior parts. These good mechanical characteristics can be attributed to the very good distribution of the matrix particles within the prepreg and to the very good fibre/matrix adhesion. SEM analyses of the composites showed the regular appearance of fibre defibrillation in the fracture surface (Figure 3).

Additionally, no air gap was found in the boundary surface, indicating very good fibre-surface wet-out by the matrix (Figure 4).

The material was developed primarily for the automotive industry. In cooperation with system suppliers, different parts were manufactured in mass production moulds under standard conditions (Figure 5). The moulded components did not show any warpage or deformation after demoulding and cooling.

Natural-fibre-reinforced composite materials are not limited to automotive applications, so the new fibre composite was also used to manufacture a variety of components such as executive cases, trays, and a sport seat (Figure 6).

Conclusion

One of the procedure’s main advantages is that, unlike thermoplastic-matrix prepregs, no additional pre-heating step is required, giving a very short cycle time (under a minute). Due to the easy handling compared to alternative procedures, form pressing of natural-fibre-reinforced thermoset-matrix prepregs is a very promising method for manufacturing high-strength, high-rigidity structural components with minimum component weight.