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A novel composite based on a natural-fibre-reinforced acrylic thermoset for use in the automotive industry was developed. The main features of the material are utilization of high proportions of renewable resources, improved mechanical properties and simplified manufacturing.
(Published on December 2007 – JEC Magazine #37)
GÜNTER WUZELLA, DIPL.ING AND
DR. ANDREAS KANDELBAUER, WOOD CARINTHIAN COMPETENCE CENTER
The development of moulded parts based on materials other than metals includes the challenging decision on which raw materials to use. Sometimes only a combination or a mixture of more than one raw material fulfils all defined objectives of the target specification. Composite materials and especially fibre-reinforced composites (FRC) are developed for this purpose. FRC are widely used in technical applications, where lightweight structures of high strength are demanded. Nowadays, many FRC are used that contain polymeric thermoplastic or thermosetting matrix materials and synthetic fibres such as carbon, glass or other as reinforcement. Sheet moulding compound (SMC) and bulk moulding compound (BMC) composites are types of FRC based on thermosetting resins that have long been used in automotive applications. Recent developments have led to formulations with excellent mechanical, creep and thermal resistance properties. Thus, SMC/BMC composites are gaining interest for automotive applications where tight tolerances and dimensional stability over a temperature range are required.
Kompetenzzentrum Holz GmbH is a partnership of different companies and research institutes constituting the leading Austrian research institute in the areas of wood, wood processing and chemistry. It consists of four divisions:
Natural fibre reinforcement
As reinforcement, natural fibres have become common in some applications. Major reasons for the use of natural fibres in such composites are (1) reduced costs, (2) reduced weight (10 to 30%), (3) good performance in accidents (no splintering), (4) superior environmental balance and (5) the possibility to manufacture complex structural elements. Although moulded parts based on natural fibres offer a significant reduction in weight, they are normally only used for applications where minor mechanical properties are demanded. Typical uses in the automotive industry are indoor panels (Figure 1a), seatback panels (Figure 1b), hat racks, etc.
Depending on the synthetic polymer used (thermoset or thermoplastic), two production technologies are employed commercially. A blend of natural and polypropylene (PP) fibres (“thermoplastic matrix”) is processed into a nonwoven (process A). The mats are heated to melt the thermoplastic fibres and pressed into the desired shape in a cold press. Alternatively, non-wovens based on natural fibres are coated with thermosets (“thermosetting matrix”), such as epoxy or polyurethane resins (process B). The ultimate material is generated by curing the resin under heat in a hot press. Both materials have limitations. With process A, a PP content of 50% and more is needed for a sufficient impregnation of the natural fibres with thermoplastic matrix, and the parts are sensitive to temperature. With process B, on the other hand, the mats have to be moulded immediately after impregnation because of the high reactivity of the thermoset. Furthermore, the manufacturing process relies on safety equipment, because harmful volatiles can be released due to the chemistry of commonly used thermosets.
The goal of our project was to develop a new automotive composite that contains renewable materials in proportions as high as possible for lightweight structures with high mechanical properties, thermal stability and easy-to-use fabrication (high storage stability and no release of harmful volatiles).
In the current contribution, nonwovens based on a mixture of kenaf and flax at the ratio of 50:50 are discussed. Mixtures of finer flax fibres with coarser fibres like kenaf lead to an optimum balance in material properties of the nonwovens (Figure 2). The finer flax fibres lead to improved stability. At the same time, complete saturation with binder is achieved since kenaf has no wax film on the fibre surface to act as a barrier against impregnation. The matrix system used was a reactive aqueous acrylic resin, which is free of phenol and formaldehyde. In contrast to composites made of the usual formaldehyde resin-based thermoset binders, the fabricator does not need to apply the resin immediately before moulding the parts, since curing takes place at temperatures above 100°C. At lower temperatures, the binder is thermoplastic, which allows the production of impregnated mats as semi-finished products with excellent storage stability. Impregnation of the non-wovens was carried out using two different resin contents, 15% and 25% (Figure 3).
Moulded parts of different densities were pressed (Figure 4) and their technological performance was tested: flexural strength (FS), flexural modulus of elasticity (MOE), water uptake (WU), impact strength, thickness swell and weatherability in a climate chamber, and sound absorbance. The results for FS, MOE and WU are discussed below in detail. As a result of the laying and needling processing steps, the properties of the resulting non-wovens and the final moulded parts are highly anisotropic. Thus, flexural strength and flexural modulus of elasticity were tested in parallel and perpendicular to the processing direction. Samples in production direction generally displayed higher mechanical strength. Both flexural properties depend primarily on the density of the moulded composite (Figure 5 and Figure 6).
The higher the density of the moulded part, the better the flexural properties. The binder content has only a small effect. Please note the high values of flexural properties for pressed parts composed of this material that have a density of 0.9 g/cm3 and above: samples in production direction have a flexural MOE between 7-8 GPa and a flexural strength as high as 70-85 MPa. Samples perpendicular to this direction have a flexural MOE about 4.5 GPa and a flexural strength of about 50 MPa.
Water uptake also depends on the density of the moulded composite (Figure 7).
The water uptake decreases with increasing density of the moulded part. By choosing higher binder contents, water uptake is further reduced, especially at lower composite densities. Moisture-induced thickness swell of moulded composites depends only on the resin content, and not on the density of the moulded part. A higher binder content renders a composite more rigid against dimensional changes, and the fibres are better protected against water.
Thus, despite a high water uptake, the thickness swell is not significantly affected. Since water uptake and thickness swell are critical, these properties were further improved by 50% upon application of 1.5% of a hydrophobizing agent.
The advantages for the manufacturer of this composite material are (1) preparation of prepregs instead of impregnating the mats immediately before moulding, (2) the good storage stability of impregnated mats and (3) no risk of releasing harmful volatiles during processing.