Properties of hemp fibre polymer composites -An optimisation of ...

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9.4 Effect of cellulose structure on fibre mechanical properties Even though the hemp fibres were handled with mild methods like hand peeling and fungal defibration, higher fibre strength than 643 MPa was not obtained, which is similar to the strength of traditionally produced hemp yarn (677 MPa). The similar results are surprising and indicate that the final fibre strength is not very dependent on the defibration method applied (Table 8). The hemp fibres and Norway spruce had tensile strength approximately linear increasing versus the cellulose content showing that pure cellulose has 850 MPa in tensile strength (Figure 40a). The determined cellulose strength was about 10% of the obtainable strength in agreement with Lilholt (2002). Therefore, strength reduction seems to occur from the molecular level to the single fibre level (8000→1200±400 MPa; Madsen et al., 2003) and from the single fibre level to the fibre assembly level (1200±400 → 643±111 MPa). Single fibres are presumably weakened due to flaws like kinks inside the fibres (Bos et al., 2002), weak interface between the fibre wall concentric lamellae (Paper II) and insufficient binding strength between the reinforcing cellulose, hemicellulose, lignin and pectin (Morvan et al., 1990). Fibre assemblies are weakened due to variations in single fibre strength as explained by Weibull analysis (Lilholt, 2002) describing the fact that the fibre bundle strength is always lower than the average strength of the same fibres (Coleman, 1958). For flax fibres and cotton fibres, the bundle efficiency has been found as 0.46 – 0.60 indicating that the effective strength of many fibres in for example a composite is roughly half the single fibre strength (Bos et al., 2002; Kompella and Lambros, 2002). It has also been suggested that mild handling and defibration result in high single fibre strength but with larger scattering counteracting the higher fibre strength. These facts explain the similar strength in the composites with traditionally produced hemp yarn and mildly defibrated hemp fibres. The fibre stiffness increased with the cellulose content in the fibres obtained by the fungal defibration and water retting. A linear dependence on the crystalline cellulose content could be established (Figure 40b). The hemp yarn had lower stiffness than implied from the cellulose content, which may be due to the high twisting angle introduced during the spinning process. It has been stated, that increasing twisting angles decrease fibre stiffness (Page et al., 1977). The wood fibres had lower stiffness, which can be explained by the low cellulose crystallinity (60 – 70%) compared with the hemp fibres (90 – 100%; Figure 14). Amorphous cellulose, hemicellulose, lignin and pectin are expected to have lower stiffness than crystalline cellulose, which are linear molecules orientated in the test direction resulting in high stiffness. In contrast, the plant fibre stiffness appeared to increase linearly versus the cellulose content to 125 GPa for pure crystalline cellulose. 64 Risø-PhD-11
Fibre strength σ f [MPa] Fibre stiffness E f [GPa] 1000 800 600 400 200 150 120 90 60 30 Low cellulose crystallinity Twisted yarn 0 0 20 40 60 80 100 Cellulose content [% w/w] 0 Low cellulose crystallinity 0 20 40 60 80 100 Cellulose content [% w/w] Twisted yarn Figure 40. Fibre tensile strength (a) and stiffness (b) determined on porosity corrected composite data versus cellulose content for treated hemp fibres, hemp yarn, barley straw and Norway spruce. The effects of cellulose crystallinity and twisting angle are indicated. Risø-PhD-11 65
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Risø-PhD-11(EN) Properties of hemp
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Contents Preface 6 1 Resumé 7 2 Li
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PhD thesis: Properties of hemp fibr
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1 Resumé Karakterisering af hampef
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2.4 Oral presentations Anders Thyge
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4 Introduction Hemp (Cannabis sativ
Page 13 and 14: 4.2 The outline of the thesis The f
Page 15 and 16: The only fertilizer used was 360/48
Page 17 and 18: Temperature [ o C] 35 30 25 20 15 1
Page 19 and 20: A B Figure 7. A: Model of transvers
Page 21 and 22: Figure 8. Lignin (a), pectin (b) an
Page 23 and 24: Figure 11. Microfibril angles (MFA)
Page 25 and 26: Teleman, 1997). The crystal structu
Page 27 and 28: to the computing effort and the sub
Page 29 and 30: Figure 15. The structure of hemicel
Page 31 and 32: fibre mats, which are made by air-d
Page 33 and 34: Figure 18. From fibre filament to a
Page 35 and 36: A B C D Figure 20. Chemical structu
Page 37 and 38: water vapour and air are removed fr
Page 39 and 40: 7 Defibration methods Defibration m
Page 41 and 42: Table 5. Fibre yield and cellulose
Page 43 and 44: a b c Composition [% w/w] 100 Compo
Page 45 and 46: 8 Fibre strength in hemp The streng
Page 47 and 48: 8.2 Effect of pre-treatment of hemp
Page 49 and 50: E-modulus [GPa] 90 80 70 60 50 40 3
Page 51 and 52: Second mode: Failure is controlled
Page 53 and 54: structure, which made much higher f
Page 55 and 56: Table 7. Porosity factors determine
Page 57 and 58: Thereby, the effective fibre streng
Page 59 and 60: a b Composite tensile strength σcu
Page 61 and 62: Table 9. Mechanical properties in a
Page 63: a σ c/ρ c [10 3 m 2 /s 2 ] b Ec/
Page 67 and 68: 11 References Andersen TL, Lilholt
Page 69 and 70: Kaar WE, Cool LG, Merriman MM, Brin
Page 71 and 72: Stowell EZ, Liu TS, 1961. On the Me
Page 73 and 74: Appendix A: Comprehensive composite
Page 75 and 76: The porosity constants can now be d
Page 77 and 78: Appendix D: Density and mechanical
Page 79 and 80: Paper I Changes in chemical composi
Page 81 and 82: Risø-PhD-11 81
Page 89 and 90: Paper II Hemp fiber microstructure
Page 91 and 92: ABSTRACT Characterization of hemp f
Page 93 and 94: METHODS AND TECHNIQUES Treatment of
Page 95 and 96: 2000) and their orientation gave th
Page 97 and 98: By TEM microscopy, the single fiber
Page 99 and 100: Table 1. Chemical composition of th
Page 101 and 102: Table 2. Tensile strength (σ) and
Page 103 and 104: Morvan, C., Jauneau, A., Flaman, A.
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Risø-PhD-11 115
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Risø-PhD-11 117
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Paper IV Comparison of composites m
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ABSTRACT Aligned epoxy-matrix compo
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the calculation of volume fractions
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solution (pH 4) for 4 h at 85°C. L
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E ⎛ dσ c ⎞ = ⎜ ⎟ ⎝ dε
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Table 2. Chemical composition, stru
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transverse section of the small hem
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Long fibres that pass through the e
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Figure 6. Effect of composite fabri
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DISCUSSION The defibration of hemp
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This investigation showed that the
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Bos, H.L., Molenveld, K., Teunissen
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Thygesen, A., 2006. Properties of h
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Risø-PhD-11 145
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Risø’s research is aimed at solv
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