Injection moulding of natural reinforced plastics

Transcription

1 Injection moulding of natural reinforced plastics Dipl.-Ing. Dipl.-Wirtsch.-Ing Andreas Marek; Prof. Dr. Hartmut Widdecke Institut für Recycling; Robert-Koch-Platz 8a; 3844 Wolfsburg ABSTRACT Typical fibres used in reinforced plastics are glass- and carbon-firbres. Recently natural fibres have found more and more interest. In this study completely biodegradable reinforced plastics were obtained by injection moulding. KEYWORDS: natural fibres; reinforced plastics; injection moulding; biodegradable plastics 1 BIODEGRADABLE PLASTICS AS MATRIX SYSTEMS Biodegradable polymers (Schroeder 1998) which can be used as matrix systems for reinforced plastics are either starch (e.g. blends) and cellulose (e.g. cellulose acetate) or polyesters (e.g. polylactide). A detailed study of the different properties of these materials was recently carried out in our laboratories (Raschke, 2). An example is given in table 1. The cellulose derivate and the natural polyester show similar properties as polystyrene. To improve the mechanical properties different additives or natural fibres can be used. Table 1: Mechanical properties of different plastics polystyrene polylactide cellulose acetate density [g/cm³] 1,5 1,26 1,27 breaking t [N/mm²] tensile-e-modulus [N/mm²] impact [kj/m²] 6 59,25 34, ,5 4 1

2 2 NATURAL FIBRES Natural fibres from plants can be divided into four groups: Bast fibre (flax, jute, hemp) Leaf fibre (sisal, palmfibre) Seed fibre (cotton, capoc) Fruit fibre (cocos, pineapple) All of them are biodegradable but also other advantages in comparison with glass or carbon fibre can be listed: Lower density (e.g. sisal: 1,2 g/cm3, glass: 2,5 g/cm3): This is important for the construction of low-weight components. Minor abrasion: That means less stress for the tools. Higher elasticity than glass fibres: The biocomposites do not splitt so heavyly, which may be important for applications in the interior of cars (Hanselka 1997). No support of hothouse effect: CO 2 -recycling However there may exist also some disadvantages. Fibre quality and availability could be a problem. Table 2: Mechanical properties of plant and glass fibres [Höck 1994, Gassan 1997, Hanselka 1998; Niederstad 1997] density [g/cm³] tensile [N/mm²] spec. tensile [kn/mm² x cm³/g] E-modulus [GPa] elongation [%] jute 1, ,13, ,6 3,1 ramie 1,5 85 9,57,6-2,4 hemp 1, ,13, flax 1,4 1, ,17, ,3 2,8 sisal 1, ,42, glass 2,6 1748, ,1 The natural fibres show different mechanical properties (tab. 2). Especially interesting is the specific tensile demonstrating that natural fibres may be used alternatively in comparison to glass fibres. 2

3 3 COMPOSITES The development of biocomposites started in the 8s. Duromeric and thermoplastic material are used (e.g. Keller 1999), but in case of the injection moulding process of biocomposites only very few studies are available. Therefore the influence of natural fibre on the properties on biodegradable plastics was investigated. Sisal fibre was used because of low density and a high specific tensile. Also it is commercially available and easy to handle. For the matrix material cellulose acetat was chosen. The composites were generated in two different ways. First in cooperation with the MIT Mischtechnik GmbH, Detmold the matrix and the fibre were mixed into a heating-cooling system to get agglomerate, which was hackled in a granulator. At the end the granulate was used for injection moulding. Samples of the new composite were characterised. In another process both materials were mixed in a twinscrewextruder folloed by a granulation process. Then the injection moulding process was repeated to determine the properties. Figure 1: Mechanical properties of sisal reinforced cellulose acetate (mixer) [%] tensile tensile E- modulus flexural flexural E- Modulus % fibres 1 % fibres 2 % fibres Figure 1 shows the mechanical properties of the reinforced plastic which was produced in the heating-cooling mixer. The impact, the tensile and flexural E-modulus was improved by the use of fibres. In a second project we used a twinscrewextruder and a gravimetric dosage unit. We investigated biocompostits with 1 up to 4 % of fibres. Figure 2 shows the mechanical properties of these composits. 3

4 Figure 2: Mechanical properties of sisal reinforced cellulose acetate (extruder) [%] tensile tensile E- modulus flexural flexural E- Modulus % fibre 1 % fibres 2 % fibres Similar results were obtained using this process. Only the tensile of the extruder material is higher than the tensile of the mixer material, although the fibres became shorter during the extrusion process. This effect could be minimized with another screw configuration. These results are confirmed in by investigation of sisal reinforced polypropylene. Figure 3 shows the tensile-e-modulus of reinforced polypropylene. The tensile-e-modulus increased with increasing fibre content. 4

5 Figure 3: Tensile-E-modulus of sisal reinforced polypropylene tensile-e-modulus [MPa] tensile-e-modulus [%] fibre [%] Tensile-E-Modulus [MPa] Tensile-E-Modulus [%] 4 SUMMARY The results show the potential of reinforced plastic for an injection moulding application. The continuous production by extrusion is an interesting alternative for the synthesis of biocomposites. The use of the mixer could be suggested for small charges. Natural fibres in comparison to classic fibres show similar mechanical properties. Fibres can also be used to minimize the cocts of biodegradble plastics. LIST OF REFERENCES Schroeter, J.: Biologisch abbaubare Werkstoffe. Kunststoffe 88 (1998) 7, S.1-14 Raschke, M.; Marek, A.; Otten, A.; Widdecke H.: Kunststoffe aus nachwachsenden Rohstoffen Ein Vergleich; Tagungsband Narossa 2 6. Int. Fachkongress für nachwachsende Rohstoffe; Juni 2 Hanselka, H. Naturfassern verstärken die Werkstoffe kommender Produktgenerationen, Industrieanzeiger 1/2,1997, S28ff Höck, P. Verstärkung von Polypropylen durch Flachsfasern auf Gleichdralldoppelschneckenextrudern, Dissertation an der RWTH Aachen, 1994 Gassan, J. Naturfaserverstärkte Kunststoffe Korrelation zwischen Struktur und Eigenschaften der Fasern und deren Composites, Dissertation an der GH Kassel,

6 Hanselka, H. Faserverbundwerkstoffe aus nachwachsenden Rohstoffen für den ökologischen Leichtbau, Materialwissenschaft und Werkstofftechnik 29, Willey-VCH Verlag, 1998, S Niederstad, G., Hanselka, H., u.a. Ökonomischer und ökologischer Leichtbau mit faserverstärkten Polymeren,, 2. Aufl., Expert Verlag, 1997, S Keller, A. et al.: Bastfaserverstärkte, biologisch abbaubare Polyester- Einfluss der Faserverfeinerung durch Dampfaufschluss: 2nd International Wood and Natural Fibre Composites Symposium, Kassel 1999 The authors are thankful to the Ministry for Science and Culture of Lower Saxony (MWK) / Arbeitsgruppe Innovative Projekte (AGIP) for financial support 6