Ton Peijs

Effects of environmental conditions on mechanical and physical properties of flax fibers

The environmental degradation behaviour of flax fibers and their mechanical properties were investigated. Upgraded Duralin flax fibers, which have been treated by a novel treatment process for improved moisture and rot sensitivity, were studied. Results showed that upgraded Duralin flax fibers absorbed less moisture than untreated Green flax fibers, whereas the mechanical properties of the upgraded fibers were retained with moisture absorption, if not improved. In addition electrochemical studies were conducted on these fibers. These data agreed well with conventional moisture absorption data. Zeta (ζ)-potential measurements at different pH-levels showed differences for Duralin fibers, which can be attributed to differences in morphological features.

Mechanical Properties of Natural-Fibre-Mat- Reinforced Thermoplastics based on Flax Fibres and Polypropylene

Thermoplastic composites based on flax fibres and a polypropylene (PP) matrix were manufactured using (i) a film-stacking method based on random fibre mats and (ii) a paper making process based on chopped fibres. The influence of fibre length and fibre content on stiffness, strength and impact strength of these so-called natural-fibre-mat-reinforced thermoplastics (NMTs) is reported and compared with data for glass-mat-reinforced thermoplastics (GMTs), including the influence of the use of maleic-anhydride grafted PP for improved interfacial adhesion. In addition some preliminary data on the influence of fibre diameter on composite stiffness and strength is reported. The data is compared with the existing micro-mechanical models for strength and stiffness. A good agreement was found between theory and experiment in case of stiffness whereas in the case of strength the experimental values fall well below the theoretical predictions. Results indicated that NMTs are of interest for low-cost engineering applications and can compete with commercial GMTs when a high stiffness per unit weight is desirable. Results also indicated that future research towards significant improvements in tensile and impact strength of these types of composites should focus on the optimisation of fibre strength rather than interfacial bond strength.

Environmental durability of flax fibres and their composites based on polypropylene matrix

The environmental degradation behaviour of flax fibres and their polymer composites are explored. New upgraded Duralin flax fibres, which have been treated by a novel treatment process for improved moisture and rot sensitivity were studied. Environmental studies showed that these upgraded Duralin flax fibres absorb less moisture than untreated Green flax fibres, whereas the mechanical properties of the treated fibres were retained, if not improved. The effect of this novel flax fibre treatment on the environmental behaviour of natural-fibre-mat-reinforced thermoplastics (NMTs) is investigated by monitoring the moisture absorption and swelling, and measuring the residual mechanical properties of the flax/polypropylene composites at different moisture levels. The moisture absorption and swelling of the upgraded flax fibre composites is approximately 30% lower than that of composites based on Green flax fibres.

On the mechanical properties, deformation and fracture of a natural fibre/recycled polymer composite

Abstract A composite laminate based on natural flax fibre and recycled high density polyethylene was manufactured by a hand lay-up and compression moulding technique. The mechanical properties of the composite were assessed under tensile and impact loading. Changes in the stress–strain characteristics, of yield stress, tensile strength, and tensile (Youngs) modulus, of ductility and toughness, all as a function of fibre content were determined experimentally. A significant enhancement of toughness of the composite can be qualitatively explained in terms of the principal deformation and failure mechanisms identified by optical microscopy and scanning electron microscopy. These mechanisms were dominated by delamination cracking, by crack bridging processes, and by extensive plastic flow of polymer-rich layers and matrix deformation around fibres. Improvements in strength and stiffness combined with high toughness can be achieved by varying the fibre volume fraction and controlling the bonding between layers of the composite.

Morphological investigations of polypropylene single-fibre reinforced polypropylene model composites

Melt-spun isotactic polypropylene (iPP) fibres have been prepared with moduli of about 12 GPa and strength of 730 MPa. Single-fibre model composites are prepared by embedding constrained high-modulus iPP fibres in thin films of a matrix material based on the same isotactic polypropylene grade. The morphology of these composites has been investigated by optical microscopy and low-voltage scanning electron microscopy techniques. After isothermal crystallisation from the melt a transcrystalline layer was found having lamellar crystals grown perpendicular to the fibre axis. The work illustrates that the processing of polypropylene fibre reinforced polypropylene composites as self-reinforced single-polymer composite systems is feasible and that these composites may fulfil the demands for fully recyclable engineering composites.

The extraordinary reinforcing efficiency of single-walled carbon nanotubes in oriented poly(vinyl alcohol) tapes

This paper reports on oriented poly(vinyl alcohol)/single-walled carbon nanotube (PVA/SWNT) tapes that were prepared by a mild processing route, involving the use of dimethyl sulfoxide (DMSO) as a solvent. Composite films with homogeneously dispersed SWNTs were cast from solution and drawn into oriented tapes using solid-state drawing. The obtained tapes showed the extraordinary reinforcing effects of the SWNTs, as the addition of 1.0 wt% SWNTs tripled the tensile strength of the PVA tapes. Micromechanical analysis showed that the nanotube contribution to the composite strength was as high as 88 GPa, which is very high when compared to other data reported in the literature, and for the first time begins to exploit the theoretical strength of nanotubes.

Fabrication and property prediction of conductive and strain sensing TPU/CNT nanocomposite fibres

In this study, thermoplastic polyurethane (TPU) fibres containing multi-walled carbon nanotubes (MWNTs) and fabricated via an extrusion process were demonstrated to possess a tuneable level of electrical conductivity. A simple approach based on the time–temperature superposition applied to the electrical conductivity of carbon nanotube (CNT) percolating in a thermoplastic polyurethane (TPU) melt was also developed to predict the conductivity of the nanocomposite fibres. The observation of Arrhenius dependence of zero-shear viscosity and the assumption of simple inverse proportionality between the variation of conductivity, due to network formation, and viscosity allow a universal plot of time variation of conductivity to be composed, which is able to predict the conductivity of the extruded fibres. The same TPU/CNT fibres were also demonstrated to possess good strain sensing abilities, which makes them good candidates for applications in smart textiles.

Fatigue Properties of Highly Oriented Polypropylene Tapes and All-Polypropylene Composites

This paper describes the fatigue behaviour of newly developed all-polypropylene (all-PP) tapes and composites, with reference to the composite processing conditions, testing temperature and making a comparison with commercial alternatives. All-PP tapes are highly oriented and their failure behaviour follows that of other highly oriented polymers. All-PP woven composites fail ultimately due to PP tape failure. However, this failure mode is accompanied by delamination of fabrics in the woven structure. Consolidation pressure plays a decisive role in controlling the interlaminar properties and hence the delamination resistance and furthermore the fatigue limit of the composite. Comparison of all-PP woven composites with commercial alternatives based on glass and natural fibres reveals the excellent relative performance of all-PP composites under fatigue loads. Fatigue properties of all-PP composites are however sensitive to the testing temperature, and elevated temperatures can lead to a rapid reduction of the fatigue resistance of these all-polymer systems.

Continuous-glass-fibre-reinforced polypropylene composites II. Influence of maleic-anhydride modified polypropylene on fatigue behaviour

This study investigates the fatigue behaviour of continuous-glass-fibre-reinforced polypropylene composites, under longitudinal, shear and transverse loadings. In order to study the tension-tension fatigue behaviour under different failure modes, specimens are used with fibres oriented at 0, 10, ± 45 and 90° with the loading direction, respectively. The experiments are carried out on composites based on an isotactic-polypropylene (PP) and a blend of this homopolymer and maleic-anhydride modified polypropylene (MA-PP), the latter showing better interfacial bonding with the silanized E-glass fibres. Since the fatigue performance of composite materials is strongly determined by the matrix, the fatigue behaviour of pure matrix material is also investigated. Results indicated that improved adhesion in the glass/MA-PP composites has a positive effect on the damage development as measured by stiffness reduction during fatigue life. The fatigue sensitivity, i.e. the degree of reduction of fatigue strength with increasing life, is however not significantly affected by the matrix/interphase modification.

Effective reinforcement in carbon nanotube–polymer composites

Carbon nanotubes have mechanical properties that are far in excess of conventional fibrous materials used in engineering polymer composites. Effective reinforcement of polymers using carbon nanotubes is difficult due to poor dispersion and alignment of the nanotubes along the same axis as the applied force during composite loading. This paper reviews the mechanical properties of carbon nanotubes and their polymer composites to highlight how many previously prepared composites do not effectively use the excellent mechanical behaviour of the reinforcement. Nanomechanical tests using atomic force microscopy are carried out on simple uniaxially aligned carbon nanotube-reinforced polyvinyl alcohol (PVA) fibres prepared using electrospinning processes. Dispersion of the carbon nanotubes within the polymer is achieved using a surfactant. Youngs modulus of these simple composites is shown to approach theoretically predicted values, indicating that the carbon nanotubes are effective reinforcements. However, the use of dispersant is also shown to lower Youngs modulus of the electrospun PVA fibres.