A Review of Permeability and Flow Simulation for Liquid Composite Moulding of Plant Fibre Composites

Abstract

Liquid composite moulding (LCM) of plant fibre composites has gained much attention for the development of structural biobased composites. To produce quality composites, better understanding of the resin impregnation process and flow behaviour in plant fibre reinforcements is vital. By reviewing the literature, we aim to identify key plant fibre reinforcement-specific factors that influence, if not govern, the mould filling stage during LCM of plant fibre composites. In particular, the differences in structure (physical and biochemical) for plant and synthetic fibres, their semi-products (i.e., yarns and rovings), and their mats and textiles are shown to have a perceptible effect on their compaction, in-plane permeability, and processing via LCM. In addition to examining the effects of dual-scale flow, resin absorption, (subsequent) fibre swelling, capillarity, and time-dependent saturated and unsaturated permeability that are specific to plant fibre reinforcements, we also review the various models utilised to predict and simulate resin impregnation during LCM of plant fibre composites.

Keywords: biocomposites; flow modelling; liquid composite moulding (LCM); natural fibres; permeability; polymer matrix composites (PMCs); resin transfer moulding (RTM).

Figure 1

SEM images of plant fibres. High-quality (a) and poorly retted (b) flax fibres and in planta cross-section (c) of flax fibres. (d,e) Wood fibres. One can notice the difference in lumen sizes between flax and wood. The presence of kink bands in flax is also visible. Images by the author A.B.

Figure 2

Compaction mechanisms in various plant fibre preforms: (a) yarn cross-section deformation, void consolidation, and nesting–packing in a non-woven unidirectional flax composite [14,41,42]. Image by the author DUS. (b) Fibre bending and flattening and void consolidation in a random mat wood fibre composite [40]; image reproduced with permission from © 2004 Wiley. (c) Yarn cross-section deformation and flattening and nesting–packing in a woven flax composite [43].

Figure 3

(a): Increased compaction pressure on a wood pulp random mat increases the fibre volume fraction, with partial contribution from cumulative lumen collapse-associated fibre deformation [40]; image reproduced with permission from © 2004 Wiley. Inset SEM depicts a collapsed fibre. (b): Lumen collapse observed in a woven jute fabric due to increase in compaction pressure at higher fibre volume fractions [13]. Reproduced with permission from © 2011 Sage.

Figure 4

Comparison of the (unsaturated) permeability of sisal, jute, and E-glass reinforcements impregnated with glycerin solution (glycerin/water ratio of 0.88:0.12) at ambient temperature and viscosity of 1.2 Pa∙s. Carman–Kozeny constants (C and n, Equation (5)) are also provided. Adapted from [23], with permission © 2004 Sage.

Figure 5

Permeability measurement and flow visualisation setups: (a,b) 1D linear flow through line gate injection [57]; (c,d) 2D radial flow through central injection [19,57]; (a,c) reproduced from [57], with permission from © 1995 Wiley; (d) reproduced from [19], with permission from © 2011 Taylor and Francis.

Figure 6

Permeability testing of woven jute fabric with glycerin solution at ament conditions. Adapted from [24], with permission from © 2010, Elsevier. (a) Plot of square of flow front position with times for different infusion conditions (related to Darcy’s law). (b) Plot of saturated and unsaturated permeability against porosity (= 1 – fibre volume fraction) (related to the Carman–Kozeny equation).

Figure 7

Catalysed phenolic resin flow at elevated temperature (60 °C) in 2-layer random mat hemp reinforcements before (a) and after (b) fibre washing and edge flow problems were resolved. The flow front is smoother, more uniform, and quasi-1D in (b). Flow front isochrones are shown, where each isochrone represents 20 s. The mould filling direction is from left to right. Reproduced from [25], with permission from © 2000 Elsevier.

Figure 8

The complex dual-scale flow of resin in fibrous preforms can generate voids. Macro-scale flow relates to the advance of resin between yarns or tows (i.e., inter-yarn flow), while micro-scale flow relates to the penetration of resin into a yarn (i.e., intra-yarn flow). Note that permeability is also different at the two scales. For instance, (a) for low fibre content, due to low yarn permeability but high overall permeability, the yarn is not properly impregnated, and thus intra-yarn voids may form, while (b) for high fibre content, although the yarn and overall permeability are similar, capillary flow in the yarn dominates, and therefore inter-yarn voids are formed. Images reproduced by author D.U.S. from [41,63], showing jute and flax unidirectional fabrics impregnated by epoxy and polyester resins at ambient conditions.

Figure 9

Resin flow simulation conducted in RTM-Worx software for the vacuum-assisted, light-RTM manufacture of the lower part of a flax–vinylester composite agricultural chemical storage tank. Images show progression of the flow front at time intervals as the part is 10%, 45%, 70% and 100% filled, with an image of the final fabricated component. Adapted from [64], with permission from © 2014 Prof. C Kong.