Table of contents (click on the titles to go to the chapters directly)

• Introduction

• Glass fibres versus natural fibres

• The compounding process (watch the video!)

• References

In the past decade industry has followed the academic world in realising that natural fibres like flax, hemp and jute can be an excellent renewable and sustainable substitute for glass fibres as reinforcement in thermoplastic and thermoset composite materials. Natural fibres have good intrinsic mechanical properties, a low density compared to glass fibres as well as a lower price. Research publications nearly all show a composite stiffness for flax fiber reinforced composites close to or even higher than that of commercial glass mat reinforced thermoplastic composites (GMT) and thermoset sheet moulding compound (SMC) [1,2,3]. The strength of natural fibre composites, however, in most cases was low compared to the strength of glass fibre reinforced composites, even under optimised fibre-matrix interaction [2,4]. This is basically due to the composite-like structure of natural fibers [5]; they are generally not single filaments as most manmade fibres but they can have several physical forms, which depend on the degree of fibre isolation. The physical form of natural fibres should be taken into account when evaluating natural fibre-based composites.

The physical flax fibre form being present in composite materials ranges from fibre bundles to elementary fibres, or to even further opened-up shapes (figure 1). The mechanical properties of these different fibre forms differ strongly. Flax fibre bundles are being obtained after the first isolation processes called breaking and scutching. These fiber bundles have an acceptable price-performance ratio and are often commercially used in natural fibre mat reinforced thermoplastic (NMT) and thermoset composites. Their lateral strength is rather poor compared with their axial strength, mainly due to the weak pectin bonds between the so-called ‘technical fibres’. The really strong fibres are the elementary fibres, which have an average tensile strength up to 1500 MPa.

The combination of cellulose content per weight unit of an natural fibre source and their fibre form/aspect ratio largely determines the amount of reinforcement of extrusion compounded composite granules. For example, cellulose-rich flax bast fibres, which can also be further opened-up to relatively thin elementary fibres, can have a dramatic different reinforcing effect on the thermoplastic matrix compared with softwood flower. The latter has a lower cellulose content and a lower aspect ratio. As a result wood flower will act more like a filler in extrusion compounded granules, whereas flax fibres will act more like a true reinforcer.

These intrinsic natural fibre properties should be taken into account when designing a process and product route, which is an alternative to the traditional (f.i. chalk-)filled or glass fiber reinforced composite materials. In order to technically compete with the properties of glass fibre reinforced extrusion compounded granules it seems essential to pay as much attention the fibre-matrix dispersion as to its distribution. The latter determines to what extent fibres are homogeneously mixed into the matrix, the former determines the form/aspect ratio or in other words: the extent to which the natural fibre is opened-up to a smaller fibre dimensions. Nowadays, by using our patented extrusion compounding technology, annual bast fibres can be both homogeneously mixed through the thermoplastic matrix, and opened up to elementary fibres which have a preferred high aspect ratio. The resulting mechanical properties of the compounded granules is such that they can compete with glass fibre reinforced thermoplastics.