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Issue 01/2015

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Automotive Biobased

Automotive Biobased compositesandwiches for automotive applications By: Jovana Džalto, Luisa A. Medina, Peter Mitschang Institut für Verbundwerkstoffe GmbH Kaiserslautern, Germany In many industries it is discussed whether fossil raw materials can be replaced by renewable raw materials without any loss of mechanical properties or economic benefits. In Europe, Natural fibre reinforced composites (NFC) have been applied for decades in the automotive industry due to their environmental and economic benefits. Usually, technical natural fibres are processed to non-wovens like needle mats, subsequently mixed with a polymer (both thermoplastics and thermosets) and afterwards processed to semi-structural components such as door panels, roof stiffenings, and back-rests. The specific mechanical properties of NFC are almost as high as the properties of glass fibre reinforced composites (GFC), but do not reach their level entirely [1]. In order to expand the application area of biocomposites to structural components, it is necessary to improve their performance. Therefore, the main objective of this research is the investigation and optimisation of the processing of biocomposites made of bi-directional flax textiles with biobased polyfurfuryl alcohol resin (PFA), which is gained from the waste of the sugar industry and is therefore not in competition with the food industry. Production of high performance bio-composites The mechanical performance of composites is mainly determined by the orientation of the reinforcing fibre in the component. On this account, the standard natural fibre nonwovens with random fibre orientation are replaced in this work by natural fibre 2x2 twill textiles (Biotex Flax and Biotex Jute) made of flax and jute. The thermoset bio-resin (BioRez 080101), which is based on polyfurfuryl alcohol, is applied as polymer matrix. So far, PFA resin was mainly known in foundry engineering as a binder for sand due to its non-flammability and was considered as eco-friendly and harmless alternative to formaldehyde resins [2]. PFA resin is gained from pentosecontaining biomass, in particular from bagasse, the waste of the sugar cane industry. The natural fibre textiles are impregnated by dipping into a resin solution. Excess resin is removed by running the textiles through foulard rollers with a defined gap, which can be varied to control the amount of resin pick-up. In a further step, the pre-impregnated textiles (pre-pregs) can be processed to 100 % biobased composites. This is performed by using a hot press technique, which is also the most common technique for processing thermoset NFC in the automotive industry [3]. PFA resin cures at relatively low temperatures (for natural fibres a suitable temperature of about 155 °C at a pressure of 10 bar). The cure is accomplished by a polycondensation reaction in which water steam is produced. This can lead to undesirable delamination or porosity in the composite. In order to avoid this, the pressing tool is lifted repeatedly to release the water steam. The labscale pressing time is 370 seconds and can be optimised for industrial applications by using an open tool which reduces the ventilation cycles. Fig. 1: Bio-sandwich made of natural fibre reinforced PFA resin and cork core Natural fibre reinforced PFA resin Cork core 16 bioplastics MAGAZINE [01/15] Vol. 10

Automotive Production of high performance bio-sandwiches For applications in which the materials are subjected to high bending loads, the bending stiffness can be increased by the production of a bio-sandwich composite (Fig. 1). The sandwich construction method aims to manufacture a product which combines the properties of its components with the goal to optimise the product for the intended use. Basically, a sandwich consists of two outer layers and a low density core material. In this work, flax fibre reinforced PFA resin is used as a thin skin layer which absorbs tensile and compression forces and further prevents buckling of the composite. A cork composite (NL 20) provides the core with low density. It consists of cork granules, the waste product of cork stopper manufacturing. The cork has a density of 0.25 g/cm 3 and as a core in the compound it absorbs thrust forces and preserves the skin layers from deformation. The processing of bio-sandwiches with cork composite as the core material is analogous to the production of biocomposites in the hot press technique. In this work, two layers of pre-preg at each side of the cork composite are forming the cover layers. Different initial thicknesses of cork composite (5 mm, 6 mm, and 10 mm) were used as the core. Due to the low density of cork core, the material consists mostly of air, which expands at high temperatures. In addition, the applied pressure of 10 bar increases the boiling point of the water produced from the PFA curing. Even if this water is still liquid at a temperature of 155 °C, it will expand as water steam at the end of the process when the pressure is released. This could lead to a delamination between the core and the skin layers. On that account the cork sandwiches are cooled under pressure within the production process. This extends the process time to 2100 seconds in lab-scale. Mechanical Properties of Biocomposites The biocomposites as well as the bio sandwich parts have been investigated for their mechanical and physical properties in order to perform a subsequent evaluation for their suitability as structural components. The bending properties of the natural fibre/PFA resin panels have been determined according to DIN EN ISO 178 and have been compared to standard NFC materials known from the automotive industry. Fig. 2 shows the flexural modulus of a standard natural fibre reinforced polypropylene (NF/PP) with 50 wt.% fibre content and a standard natural fibre reinforced thermoset (NF/TS) with 65 wt.% fibre content in comparison with the materials described above. Although the fibre contents of flax/PFA and jute/PFA are 47 wt.%, respectively 36 wt.%, and thus lower than in conventional NFC materials, an increased modulus between 60 % and 200 % can be observed. The flexural stress (Fig. 3) is about 35 % to 160 % higher than the flexural stress of conventional materials. In both cases the developed flax composite has slightly higher mechanical properties than the jute composite, because of the higher fibre weight fraction and the naturally higher mechanical properties of flax fibres compared to jute fibres [4]. In addition, these high mechanical properties can be increased by combining the developed material with cork composites. In this context three different cork thicknesses Flexural modulus [MPa] Effective bending stiffness [kN/mm²] 10,000 8,000 6,000 4,000 2,000 0 Standard NF/PP approx. 50 % NF Standard NF/TS approx. 65 % NF Flax / PFA 47 % NF Fig. 2: Flexural modulus of NF/PFA composites compared to standard NFC materials used in the automotive industry Flexural stress [MPa] 140 120 100 80 60 40 20 0 Standard NF/PP approx. 50 % NF Standard NF/TS approx. 65 % NF Flax / PFA 47 % NF 47 % NF Jute / PFA 36 % NF Jute / PFA 36 % NF 36 % NF Fig. 3: Flexural stress of NF/PFA composites compared to standard NFC materials used in the automotive industry 4,000 3,000 2,000 1,000 0 5 mm cork 6 mm cork 10 mm cork Reference (no cork) Fig. 4: Effective bending stiffness of flax/PFA bio-sandwiches compared bioplastics MAGAZINE [01/15] Vol. 10 17

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