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Issue 01/2018

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Automotive NATURTRUCK High temperature reinforced PLA for automotive applications By: Miguel Ángel Valera Gómez Senior Compounding researcher Luis Roca Blay Head of Compounding AIMPLAS Paterna (Valencia), Spain Currently the main thermoplastic materials used in cabin trucks are petrol derived standard ABS, and PC/ABS materials. However, as in many other sectors, there is an increasing demand of materials derived from renewable resources, so called biocomposites, due to their higher sustainability. Nevertheless, the biocomposites must meet same stringent mechanical, thermal and flame retardancy requirements to be used in automotive applications. Neat PLA is a well known biopolymer with good mechanical properties It is a stiff material with good tensile and flexural properties but at the same time is very brittle and has a low temperature of use. Its heat deflection temperature (HDT/B) is 55 – 56 °C. Which is the glass transition temperature (Tg) of the amorphous part of the PLA. For this reason, the use of conventional PLA biocomposites is limited to low temperature applications, and therefore it is not suitable for the automotive sector, where HDT above 100 °C is usually required. One approach to improve the HDT of PLA is to increase the part crystallinity. Following this approach, within the EU- Project NATURTRUCK injected parts for the interior cabin of trucks based on PLA and natural fibres with improved thermal and flame retardancy properties to substitute standard ABS and PC/ABS materials were developed. To develop the biocomposites, an injection moulding grade of PLA with low D-isomer content (< 0.5 %) was used to allow fast PLA crystallization rates [1]. The crystallization rate was increased with the use of nucleating agents and bioplasticizers as crystallization promoter. The natural fibres were supplied by the project partner BAFA (Germany), and the Institute of natural fibres and medicinal plants (IWNIRZ) (Poland) carried out the fibre modification. Different natural fibres were tested, flax, kenaf and hemp fibres, at 15 wt %. Hemp fibres were selected as the most suitable fibres for the biocomposites. Both cut and pelletized hemp fibres were tested in the compounding process. But the use of hemp pellets improved the fibre handling and feeding to the extruder, and finally gave the best results. A Commercial halogen free flame retardant, ammonium polyphosphate (APP), was used to improve the flame retardancy of the biocomposites. The amount of APP was kept to the minimum (10 to 14 %wt) that allowed us to achieve Renault truck standards (HB classification according to UL94). Other additives like impact modifiers and antioxidants were also used. In this area, the project partner Químicas del Vinalopó developed natural antioxidants from the Cistaceae plant family that were evaluated as natural antioxidants in PLA and PLA biocomposites. The extracts, rich in polyphenols, improved drastically the thermal stability of non-plasticized PLA/natural fibre composites (the onset of degradation temperature was improved by 45 °C). The PLA/natural fibre biocomposites were prepared by AIMPLAS (Plastics Technology Centre in Spain) and the project partner re8 Bioplastics (Sweden) using a pilot plant co-rotating twin screw extruder (TSE). The obtained biocomposites were injection molded into test specimens for characterization purposes. The crystallization or annealing treatment of the parts was carried out after injection moulding and using a laboratory microwave oven (Mw) by Aimplas, or using an Infra-Red (IR) oven by Aimplas and the project partner Proac (Spain). The mechanical and thermal properties, and flammability were determined in Aimplas laboratories before and after the annealing of the parts. The prototype design was carried out by CTAG and Renault and the final truck parts were obtained by CTAG, Plascam and Polycom. The biocomposites were first injected in a cold mould (20 °C), the process was fast but the crystallinity and therefore HDT were low (

Automotive NATURTRUCK Footrest mould NATURTRUCK Treated Hemp fibres NATURTRUCK Ref. Hemp fibres Table 1: Properties of PLA/hemp biocomposite obtained with hemp fibres PLA+Ref.Hemp(15%)+FR Bio content=79wt% Property As injected Annealed in mould Annealed with IR Standard Tensile modulus (MPa) 2360 ± 38 2820 ± 123 2760 ± 71 UNE-EN ISO 527-2 Tensile strength (MPa) 37.5 ± 0.3 38.2 ± 0.2 39.5 ± 0.4 UNE-EN ISO 527-2 Elongation (%) 3.3 ± 0.1 4.0 ± 0.1 4.5 ± 0.2 UNE-EN ISO 527-2 Flexural modulus (MPa) 2800 ± 87 2980 ± 108 2820 ± 116 UNE-EN ISO 178 Flexural strength (MPa) 46.3 ± 0.5 55.2 ± 1.0 50.5 ± 1.1 UNE-EN ISO 178 Charpy Unnotched impact strength (KJ/m 2 ) At 23°C At -40°C 17 ± 2 17 ± 2 15 ± 1 14 ± 1 20 ± 1 15 ± 1 UNE-EN ISO 179-1 HDT (B) (0.45MPa) (°C) 42.1 ± 0.4 143.6 ± 0.2 132.7 UNE-EN ISO 75-2 Melt flow index (190°C, 5kg) 19.8 ± 0.6 UNE-EN ISO 1133-2 Speed of flame spread (mm/min) - 54.2 ± 3.6 - UL94, HB crystallized parts with HDT(B)>130 °C were obtained using injection moulding cycles below 75s. Finally, the annealing of the parts after being injected in a cold mould (fast injection process) was studied using MW or IR ovens. In this case the crystallinity of the PLA was increased from 55 % with only a heating step of 3 – 4,5min and then cooling at room temperature. This way, the HDT(B) of the biocomposites was increased from 40 °C for plasticized formulation or 57 °C for non-plasticized formulations, to 100 °C and 150 °C respectively. Formulated PLA/hemp biocomposites fulfilled the mechanical, fire and thermal resistance requirements of one of the proposed truck parts called bracket part. However, the impact strength at room temperature required for the footrest part (35 kJ/m 2 ) was more difficult to be achieved while maintaining one of the requirements (at least 80 % of raw materials from renewable resources). The properties of one of the developed biocomposite formulations are showed in table 1. The final formulations contained almost 80 % of raw materials from renewable resources, and can be recycled at typical levels used in the automotive industry (up to 10 wt %) without significant loss in material properties. In addition, NATURTRUCK biocomposites are biodegradable. However, the biodegradation rate depends on the final crystallinity of the part. If treated properly it can be also considered as compostable according the standard EN 13432:2000. PLA/hemp fibre biocomposites with very good tensile properties (stiff and resistant materials) were developed. The HDT(B) of the biocomposites and therefore, their maximum temperature in use was drastically improved after annealing. Therefore, fire and temperature resistant PLA/ hemp fibre biocomposites were successfully developed for interior parts in the automotive sector, and can be also used in durable applications were high temperature resistance is required. However, further improvements in the impact strength of the biocomposites should be made, but generally comes with a reduction of the biobased content. ACKNOWLEDGMENTS NATURTRUCK project ( ) has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration (GA 605658). Aimplas also like to show their gratitude to the project partners IWNIRZ, BaFa, Renault, CTAG, Proac, Químicas del Vinalopó, Polycom, Plascam, and re8 Bioplastics. References: [1] J.J Kolstad, J. Appl. Polym. Sci., 1996, 62, 1079. bioplastics MAGAZINE [01/18] Vol. 13 23

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