Effects of Processing on Natural Fibres in Thermoplastic Composites

Transcription

1 1 Effects of Processing on Natural Fibres in Thermoplastic Composites Kalle Nättinen, Heidi Peltola, VTT Technical Research Centre of Finland Bo Madsen, Danish Technical University/Risoe Roberts Joffe, Luleå Technical University COST FA0904 Meeting Espoo,

2 2 Process and R&D chain Materials Tensile and impact properties Fibre size vs. processing Fibre orientation Property diagram Conclusions Content

3 3 Target: Alternatives for current thermoplastics Material parameters: Composition: Modified Starch + plasticiser Natural fibre: Hemp or Flax Fibre content: +30 % Biogenic and biodegradable Performance: Good mechanical properties: Stiffness and strength impact strength, storage stability Easy processing Economically competitive

4 4 Starch acetate composites Process chain Extraction & acetylation Starch acetate Plastisation Melt processable starch acetate Pelletising Compounding with fibres Natural fibres Pellets Corn (starch) Injection molding & extrusion

5 5 Project R&D chain Acetylation of starch: VTT Fibres production Bafa, Ekotex Mechanical properties VTT Melt processing of starch acetates & composites VTT Sample object production Haidlmair, Medop, UAR, Conenor Uniformity & porosity DTU/Risoe Fire retardancy and water uptake WUT Micromechanical modelling & microstructural characterisation LTU

6 6 Materials Matrix: Starch Acetate, a melt processible starch derivative Acetylation of starch to...reduce hygroscopicity reduce intermolecular hydrogen bonding to lower the T g to enable melt processing Plasticising of the starch acetate to obtain ductile material Xu et al. 2004

7 7 Native and Acetylated Starch Properties Starch polymer DS ACET Mw (g/mol) T g ( C) T deg ( C) Viscosity (cp) Native not determined Hylon VII Acetylated 2.6 not soluble not soluble Native not determined CereStar Acetylated 2.6 not soluble not soluble

8 8 Plasticiser volume fraction Effect on neat matrix stiffness and strength Stiffness (GPa) y = -9.0x R 2 = Stress at max. load (MPa) y = x R 2 = Plasticiser volume fraction Plasticiser volume fraction Linear decrease of tensile properties by increasing plasticiser % Minimum % set by processing limitations & ductile properties

9 9 Materials: Fibres Two kinds of bast fibres mm hemp fibre 4 mm flax fibre Fibres pelletised using Kahl sieve-squeezer (custom sieve matrix) and water as processing aid

10 10 Plasticiser content Effect on composite stiffness, strength and impact strength Normalised mechanical properties Plasticiser content (w-%) Plasticiser content (w-%) 10% Fibre 40% Fibre Nättinen et al., Polymer Composites, 31(3) 2010,

11 11 Fibre content Effect on composite stiffness, strength and impact strength 6 Normalised mechanical properties Fibre content (w%) Nättinen et al., Polymer Composites, 31(3) 2010,

12 12 (Open symbols) Density (g/cm 3 ) Composite uniformity Density and fibre W f Fibre weight fraction (Closed symbols) Distance along tensile specimen (mm) Nättinen et al., Polymer Composites, 31(3) 2010,

13 13 Fibre-matrix compatibility Fracture surfaces (SEM) Good wetting of the fibres Uniform distribution L. Wallström, LTU

14 14 Comparison: 40% Wood fibres + PP vs 40% Flax + Starch acetate Property Wood + PP * Flax + starch acetate Tensile strength (MPa) Stiffness (GPa) Impact strength (kj/m 2 ) * Ichazo et al, Composite structures 2001

15 15 Conclusions: Melt processing and mechanical properties Best tensile properties at low plasticiser and high fibre content Best impact properties at high plasticiser and medium fibre content Best composite properties: Tensile modulus: 8.3 GPa Tensile strength: 51 MPa Impact strength: 5.4 kj/m 2 with 40% flax in matrix with 20% plasticiser Similar performance with both flax and hemp fibres Processibility in compounding and molding confirmed by analyses of Uniformity of injection molded samples Porosity Reproducibility Production of demonstrator objects by injection molding and extrusion

16 16 Process and R&D chain Materials Tensile and impact properties Fibre size vs. processing Fibre orientation Property diagram Content Effect of processing on fibres

17 17 25 Unprocessed The effect of processing on fibre length Flax fibre length histogram 20 Pelletised Fully processed Starch acetate Melt processable starch acetate Natural fibres Frequency Pelletizing Pellets Compounding with fibres Molding Fibre length (mm) Peltola et. al Advances in Mat. Sci & Eng 2011

18 18 The effect of processing Hemp vs. Flax Peltola et. al Advances in Mat. Sci & Eng 2011

19 19 Fibre histograms Hemp vs. Flax, full processing Hemp Flax 50 Frequency Fibre length (mm) Peltola et. al Advances in Mat. Sci & Eng 2011

20 20 Fibre dimensions Width, aspect ratio Unprocessed Fully processed Fiber Fiber Length (mm) Fiber Width (mm) Aspect Ratio Fiber Length (mm) Fiber Width (mm) Aspect Ratio Hemp ± ± ± ± Flax ± ± ± ± Peltola et. al Advances in Mat. Sci & Eng 2011

21 21 Viscosity and shear Effect on fibre length 0.6 Fibre length (mm) wt% flax 40 wt% flax Linear ( Plasticiser content (wt%)

22 22 Fibre orientation 1.0 Perpendicular Cumulative frequency degrees I 0 degrees II 30 degrees 60 degrees 90 degrees I 90 degrees II Aspect ratio of fiber cross-sections Flow direction Median aspect ratio of fiber cross-sections Peltola et. al Advances in Mat. Sci & Eng 2011 Angle of composite cross-section

23 23 Stiffness Modeling vs. experimental

24 24 Property diagram Youngs modulus as a function of fibre and plasticiser content Input Mechanical performance data Fibre dimension changes Fibre orientation Porosity Modelling parameters Output Possibility to predict composite properties or design composites with desired properties Young's modulus (GPa) Fiber weight fraction Plasticizer content (%) Madsen et al, J. Comp. Mat 2011

25 25 Conclusions: Processing effects on fibre dimensions and orientation Bast fibre dimensions heavily reduced by processing: pelletising and compounding Different effect on the two bast fibre types: hemp and flax Largest effect with high viscosity compound (high fibre content and/or low plasticiser content) Fibre orientation along flow direction Realistic prediction/design of composites with desired properties possible

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27 27 VTT creates business from technology