due to insufficient fiber/matrix adhesion, and the porosity due to poor fiber impregnation. The voids in the matrix, which are identified to be air-filled cavities, are the matrix correlated porosity [10]. Both types of porosity can neither be influenced by optimizing the process nor by ideal fiber/matrix content in the composite, because they are assumed to be intrinsic material properties, which are linearly correlated with the absolute fiber and matrix volume. They are schematically described in Figure 3.
The porosity which can be minimized by the optimal combination of fiber and matrix weight fraction and the optimal process is the structural porosity which is mainly induced by the low packing ability of cellulose fibers [11] and in case of furan resin by the foaming characteristic of the matrix. It is assumed that the lower the amount of resin (at a given amount of textile layers), the more the resin is free to expand to fill the mold volume, and the higher is the resulting porosity content in the produced plates (Figure 4).
This means that the plate which consists of 3 prepreg layers should have the highest porosity content, whereas the plates which consist of 5 prepreg layers should have the lowest porosity content of the plates. In order to validate this assumption and to apply a model for conversion between the constituents, the volumetric interaction of the constituent volumes in the 7 plates can be quantified as follows. The weight fraction of the textile ȥtextile is measured by the ratio of the area weight of the composite plates AWcomposite and the area weight of
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Figure 4: Schematic illustration of reducing structural porosity (vps).
the applied textile layers AWtextile layers (equation (2)). Since the air-filled voids do not contribute to the composite weight, the weight fraction of the resin ȥresin is the remaining weight fraction in the composite (equation (3)).
(2) ȥresin = 1 - ȥtextile (3)
The volume fraction of the textile ijtextile as well as the volume fraction of the resin ijresin can be calculated as shown in equations (4) and (5) [12].
(4)
(5)
ȡcomposite is the density of the composite and it is determined in each plate by the ratio of the composite plate and its volume. The density of the total cured furan resin ȡfuran was measured in a previous work as 1.334 g/cm3 [13]. The density of the textile ȡtextile can be calculated according to the linear mixing rule with the textile weight fraction (18.5 wt.-%) and density of PES fibers ȡPES 1.415 g/cm3 (measured by project partner Riso DTU, Denmark) and the weight fraction and density of flax fibers ȡflax 1.5 g/cm3 [14] as shown in equation (6).
ȡtextile ÂȡPESÂȡflax
1.48 g/cm3