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

structure, which made much higher fibre contents possible (>55% v/v; Figure 33f;
Unpublished data).
Vf, max [% v/v]
60
50
40
30
20
10
0
0.1 1 10 100 1000
Fibre area [10 3 μm 2 ]
Figure 34. Obtainable fibre content with the applied fibre types versus the fibres
transverse section area.
9.2.2 Modelling of porosity content and fibre content
The porosity content in the composites is presented in Paper IV and modelled versus
volume fraction of fibres in Appendix B. The porosity content increased with increasing
fibre content, due to fibre lumens and to voids at the fibre-matrix interface (Figure 35). It
seemed reasonable to assume a linear relationship between the porosity content and Vf
and Vm. The porosity constant αf could be determined by linear regression of Vp versus
Vf.
Wf
ρm
V f =
Wf
ρm
( 1+
α f ) + ( 1−
Wf
) ρ f ( 1+
αm
)
( 1−
W f ) ρ f
( 1−
W f ) ρ f
Vm
= V f =
W f ρm
W f ρm
( 1+
α f ) + ( 1−
Wf
) ρ f ( 1+
αm
)
V = α V + α V
p
f
f
m
m
Expression for the regression line for determination of αf:
= a ⋅V
+ b
Vp f
The hemp fibres produced by fungal treatment were found to fit with αf similar to αf for
hemp yarn (0.12-0.16 v/v), and the raw hemp bast with αf similar to αf for water retted
hemp (0.26-0.28 v/v; Table 7; unpublished data). For hemp yarn and fungal defibrated
hemp fibres, αf was larger than for the glass fibres (0.03 v/v) due to fibre lumens in
hemp fibres. In water retted hemp and raw hemp bast, αf was even larger due to
epidermis cells with large lumens and poor impregnation of these very large fibre
bundles. It seemed reasonable to use αm = 0.005 v/v since there is high uncertainty on Vp
for fibre volume fractions below 10% and since small air bubbles were observed in the
epoxy matrix (Figure 35; unpublished data).
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