Behaviour of natural fibres during composite processing

The research work of Anne Le Duc deals with the development of composites made from polypropylene and natural fibres.

     To reinforce polyolefins usually glass fibres are incorporated. Yet, the interest in natural fibres as reinforcing agents in polymer-based composites has grown drastically during the past decades years. Several reasons can explain this trend, in particular the renewable aspect of natural fibres. They are also less abrasive than glass fibres and they have many other advantages, such as good mechanical properties, low density, good insulation (thermal, electric, acoustic), easy availability and thus a low cost.
     However, a few drawbacks such as a high variability, incompatibility with hydrophobic polymers, tendency to form aggregates and poor resistance to moisture reduce their potential as reinforcing material.
One part of this work will concern composites processing and understanding of the reasons of fibre length reduction.

In this context the aim of this research is as follows:

  • to understand how and why fibres are breaking during processing,
  • to investigate how the composition and structure of selected natural fibres influence the properties of composites,
  • to correlate fibre type and behaviour during processing with composite properties.

In this work we are interested in composites made of polypropylene reinforced with natural fibres (Tencel® and flax). Polypropylene graft maleic anhydride was used as a compatibilizer to improve the affinity of fibres with matrix and the adhesion properties. We can observe in Fig.1 that the flax fibres are coated and trapped in the matrix after a fracture in liquid nitrogen.

Interface of polypropylene/flax fibres composite (20 wt %)
 Fig. 1. Interface of polypropylene/flax fibres composite (20 wt %) observed by scanning electron microscopy

     Optical microscopy analysis of fibre dimensions after compounding allowed to build fibre size distributions. Initially long fibres were cut in more times than the short ones. They had also broader size distributions, and they were more difficult to disperse.
     For example, the length of flax fibres 9 mm was reduced 90 times whereas the length of Tencel® 2 mm was reduced 15 times during compounding (Fig.2.). And man made fibres Tencel® were not cut in their width contrary to the flax bundles

Fibres observed by optical microscopy
Fig. 2.  Fibres observed by optical microscopy: initial flax 9 mm fibres (a1) and Tencel® 2 mm fibres (b1); the same fibres in the composite after processing: flax (a2) and Tencel® (b2)

     In order to understand the mechanisms of rupture of these natural fibres, their behaviour was observed by rheo-optics during flow in a non-Newtonian fluid. The fibres were placed in a highly viscous matrix to reproduce the stress conditions during compounding and to make correlations with size distributions.
     The break-up of fibres was observed and different behaviours were qualitatively evidenced. Generally, all natural fibres studied are breaking by fatigue after an accumulation of strain, contrarily to what is known for glass fibres. Flax fibres break around the kink bands (Fig.3. a2, a3) while cellulose II fibres (Tencel®) seem to break after numerous bending (Fig.3. b2). And after a while in the flow, all fibres have tendency to tangle and to turn around themselves in the vorticity direction (Fig.3. b3).

The obtained results will be then correlated to the mechanical properties of the composites.

Fibre behaviour under shear flow
Fig. 3. Fibre behaviour under shear flow: (a) Elementary flax fibres, (b) Man made fibres (Tencel®) (x: flow direction, y: vorticity direction, z: observation direction)

>> To know more about rheo-optics at CEMEF:
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