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02 | 2008

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Natural Fibres 40 Charpy

Natural Fibres 40 Charpy A-notched impact strength +23°C 30 [kJ/m 2 ] 20 10 0 40 30 PP Composites PHA Composites PLA Composites Figure 6: Charpy A-notched impact strength at 23°C. Charpy A-notched impact strength -30°C With the addition of abaca fibres the stiffness of all composites increased significantly. The level is much higher for stiff PLA than for PHA and PP. PHAs have similar stiffness to PP. It can also be seen that the fibres significantly increase the tensile strength. Especially with man-made cellulose fibres a significant improvement can be achieved. The PHA blends and composites, however, did not meet the researcher’s expectations. Probably, the polymer decomposed during processing, along the lines of the following formula: [kJ/m 2 ] 20 10 H O O CR CHR CH 2 O O 0 PP Composites PHA Composites PLA Composites T>180° Figure 7: Charpy A-notched impact strength at -30°C. CHR O OH H 2 C CR O O unreinforced polymer abaca composite cellulose composite All of the latest news will be presented during the upcoming 7th Global WPC and Natural Fibre Composites Congress to be held in Kassel on June 18th 2008. Besides Wood Plastic Composites (WPC), the Congress focus will be on biofibres and biocomposites. Specialists from all over the world (approx. 45 lectures) will cover a wide scope, from material development to the newest applications in the field of Natural Fibre Composites (NFC). Besides the lecture programme, an interactive poster presentation and an exhibition will take place in the historical location of the ‘Stadthalle Kassel’. For more details please visit www.wpcnfk.de. Proceedings from previous conferences can also be ordered at www.kutech-kassel.de. This is due to poor thermal stability at higher temperatures. This effect can be avoided by using multifunctional polymers as chain extenders and compatibilisers. For example by the addition of the reactive chain extender Joncryl ® (BASF), a significant increase in strength and stiffness can be achieved. For instance the addition of 1% by weight of Joncryl 4368 S raises the strength from 27.3 to 34.6 MPa. Other properties can be similarly enhanced. Thanks to the use of a coupling agent in PP composites, a higher tensile strength could be achieved. The major improvement in mechanical properties is noticeable in the impact properties (Charpy A - notched impact test, figs. 6 and 7). Because of its special fibre structure, reinforcing with man-made cellulose leads to the best effect: an enhancement up to factor 6 (for PHA composites, fig. 6). This is due to favourable fibre geometry and roughness. A pull-out mechanism occurs more often with man-made cellulose composites then with abaca. The pull-out of fibres absorbs a high amount of fracture energy and therefore an improvement of impact strength can be observed. It is obvious that 14 bioplastics MAGAZINE [02/08] Vol. 3

PHA/cellulose composite shows the highest impact strength of all the materials tested. Reinforcing with abaca improves the impact strength only for biocomposites, whereas using man-made cellulose the overall results are much better compared with unreinforced virgin polymers. Probably this is due to the fracture mechanics at the fibre/matrix interface, which has been mentioned. The abaca fibres that were used are low priced natural fibres with relatively good mechanical properties. Reinforcing with manmade cellulose showed a significant improvement in all tested values, but the fibre price is not competitive with abaca. This means that the choice of fibres should be made with regard to the fibre cost and the requirements of the endproduct properties. In the tests described here it was shown that some mechanical properties of the analysed biocomposites are very similar to ‘common’ PP composites. The PP used here was impactmodified. For that reason some results, where the ductility is the major factor, are better, for example compared to stiff PLA composites. However, by modifying (blending) biopolymers like PHAs it is possible to achieve a comparable level of toughness. Biocomposites based on PLA can be applied where the stiffness and strength are the ‘key factors’. Further research should be focused on compatibilisation of highly hydrophilic and polar natural fibres with non-polar polymer chains. Also a study of functional additives (e.g. the chain extender Joncryl) should be undertaken with a view to melt stabilisation and molecular weight improvement. Acknowledgements: The authors are grateful to Rieter Automotive AG (Mr. B. Scherübl) for the supply of abaca fibres. www.kutech-kassel.de bioplastics MAGAZINE [02/08] Vol. 3 15

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