Kathleen Van de Velde

Thermoplastic pultrusion of natural fibre reinforced composites

Thanks to the good mechanical properties and the ecological and environmental advantages, the natural fibre flax offers good opportunities as a reinforcement material for composites, especially thermoplastic ones. One technique for the manufacture of composites on a continuous basis is pultrusion. This paper describes the possibility of the use of flax as reinforcement in thermoplastic pultruded composites.

Thermal and mechanical properties of flax fibres as potential composite reinforcement

Scutched and hackled long flax fibres varying in retting degree, namely green, under-retted and normally retted flax, and elementary fibres were tested for their mechanical and thermal properties. Tensile tests were performed on individual fibres and on bundles. The thermal stability is evaluated with thermogravimetric analyses (TGA) and determination of the mechanical properties after thermal exposure. For the investigated samples, no significant influence of the retting degree on tensile strength can be determined. Exposure to different temperatures results in a weight decrease and affects the mechanical properties of the fibres.

Influence of Fiber Surface Characteristics on the Flax/Polypropylene Interface

Dew retted hackled long flax was treated with propyltrimethoxysilane, phenylisocyanate, and maleic acid anhydride modified polypropylene (MAA-PP). The wetting parameters of the untreated and treated flax fibers were obtained with dynamic contact angle measurements. The results showed that the treated flax fibers are less polar than the untreated flax fiber. The interfacial adhesion between the flax fibers and poly-propylene, respectively, maleic acid anhydride modified polypropylene, was studied with fiber pull-out tests. The highest increase in apparent shear stress was obtained by treatment of the flax fiber with MAA-PP.

Wettability of natural fibres used as reinforcement for composites.

Wetting parameters e. g. contact angles, work of adhesion, fibre surface energy and interfacial energy of different flax and glass fibres in different liquids were determined. A polypropylene film was also studied by the Wilhelmy technique. The surface of the fibres was analysed with scanning electron microscopy. Results pointed out that the flax fibres are partially wetted out by polar solvents. A baseline shift hysteresis was observed, possibly due to hydratation of the surface. The wettability of the glass fibres with a finish for thermosets is higher in polar solvents, compared with glass fibres used as reinforcement for thermoplastics. It is observed that the wettability is independent of the fibre diameter for the glass fibres with the same cross-sectional shape. The difference in contact angle hysteresis between the fibres can be explained by the difference in presence of higher surface energy components. By comparing the wettability of the fibres and polypropylene, some fibres could be expected to have a better adhesion with polypropylene than the others.

Influence of fibre and matrix modifications on mechanical and physical properties of flax fibre reinforced poly(propylene).

In this work several types of flax were used as reinforcement in poly(propylene) based unidirectional composites. These flax types included non-treated as well as treated (boiled) samples. On the other hand, two types of poly(propylene) were used as matrix: non-modified poly(propylene) (PP), and maleic anhydride modified poly(propylene) (MAAP-PP). The influence of both fibre and matrix modification was studied through mechanical (flexural) and physical (density, sorption, and drying) tests. Combination of boiled flax with MAAP-PP proved to yield the best mechanical properties, combined with good physical properties. A 100% stress transfer between fibre and matrix could be calculated in this optimised case. Interlaminar shear strentgh tests were done in order to confirm this improved fibre-matrix adhesion.

Development of a Flax/Polypropylene Composite with Optimal Mechanical Characteristics by Fiber and Matrix Modification

Dew retted hackled long flax was treated with propyltrimethoxysilane, phenylisocyanate and maleic acid anhydride modified polypropylene (MAA-PP). The results of these treatments on fiber wetting and interfacial properties have already been reported in a previous article [Van de Velde, K. and Kiekens, P. (2001). J. Thermoplastic Com. Mat., 14: 244-245]. Further characterization of the treated fibers by sorption tests, roughness estimation, and tensile testing was done in order to provide additional information on the fiber properties. Based on these fiber and interface properties, a selection of fibers and matrices was made and some tests were done on the resulting composites. These composites consisted of MAA-PP treated flax fibers or untreated fibers as reinforcement, and MAA-PP or unmodified PP as matrix material. The tests on the composites included flexural, interlaminar shear strength (ILSS) and sorption tests, as well as determination of composite compositions. As could be expected from the earlier test results on the fibers and interface, the composites based on MAA-PP treated fibers, as well as those based on an MAA-PP matrix, generally had improved composite properties. The composites made with an MAA-PP matrix were not only the easiest ones to produce, they also had the best properties.

Effect of Flax/PP panel process parameters on resulting composite properties

When producing (pressing) flax/PP composite panels, several processing parameters have to be considered. This paper describes how these parameters affect some properties of the resulting panel. The multidirectional flax/PP composite panels were made from needle-punched hybrid nonwovens. First the most appropriate process times and temperatures were determined, using unmodified flax/PP nonwovens. Once these parameters were optimized, also panels consisting of modified flax/PP (boiled and bleached flax) were pressed. Degree of needling and test direction were other parameters that were considered. Density, thickness, relative surface weight and flexural properties were determined. Extra needling only seemed useful at low process temperatures while test direction had no clear effect on results. Flax treatments did not lead to the expected property improvements but it is believed that combining a mild flax treatment with modified PP might lead to improved results, and possibly to a reduction in the odor that is released during and after production.