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

transverse section of the small hemp yarn fibres and P. radiata defibrated hemp fibres as
shown in Figure 4c-d. The larger raw hemp fibres and water retted hemp fibres had a
more irregular structure and could therefore not be packed so tightly (Figure 4a-b).
The porosity content in the composites was 2 – 6% v/v with hemp yarn, 2 – 5% v/v with
fungal defibrated hemp fibres and 5 – 9% v/v with water retted hemp (Table 4). The data
for detailed measurements of Vf and Vp are shown in Figure 5. The porosity content
generally increased with increasing fibre content, due to the fibre lumens and to voids at
the fibre-matrix interface. Porosity was present to the highest extent in the composites
reinforced with raw hemp bast and in the composites with water retted hemp fibres, due
to the long distance of epoxy penetration through the epidermis on the fibre bundle
surface (Figure 4a-b). This resulted in incomplete impregnation of the fibre bundles
resulting in gaps between the matrix and the fibres and within the fibre bundles. Smaller
and fewer gaps were found in the composites with hemp yarn and with fungal defibrated
hemp fibres for which the epidermis had been removed, as seen by scanning electron
microscopy in Figure 4c-d. The epoxy seemed therefore to impregnate these fibres
better, resulting in lower porosity content.
Table 4. Physical and mechanical properties of the laminates reinforced with the
defibrated hemp fibres and hemp yarn. Consideration of porosity was done with n =2.1
for the composite tensile strength σcu and nE=1.0 for the composite stiffness Ec.
Fibre type and
defibration
V f-obt
% v/v
V p-range
% v/v
ε c-average
%
σ m-average
MPa
σ fu 3
MPa
Raw hemp bast Lay-up 26 6 – 7 0.9 23 535 ± 117 78 ± 12
Water retted hemp Lay-up 31 5 – 9 0.7 19 586 ± 108 88 ± 12
C. sub. defibrated Lay-up short 28 3 – 5 0.7 18 434 ± 63 77 ± 15
hemp Lay-up 2
536 88
P. rad. defibrated Lay-up short 2 – 4 0.8 22 521 ± 80 82 ± 13
hemp Lay-up 32 3 – 5 0.9 25 643 ± 111 94 ± 15
Hemp yarn
Wound
Lay-up
35 2 – 6
3 – 4
1.8
1.6
43
39
728 ± 44
677 ± 92
60 ± 4
61 ± 7
Norway spruce 1
Mature wood 26 74 340 41
1: Mechanical properties for dry Norway spruce (Picea abies), with a bulk density of 0.40 g/cm 3 and a cell wall
density of 1.50 g/cm 3 after correction of the bulk tensile strength (88 MPa) and stiffness (11 GPa) for porosity
(Boutelje & Rydell, 1986; Klinke et al., 2001).
2: Values for “Lay-up short” are multiplied by 1.234 for strength and multiplied by 1.144 for stiffness to consider
the effect of short fibre length, see text for details.
3: Standard deviations are determined for the results obtained for the individual test specimens (6 – 8
specimens/laminate) and 3 – 4 laminates/fibre type.
Risø-PhD-11 131
Ef 3
GPa