Effect of material and process parameters on the mechanical properties of unidirectional and multidirectional flax/polypropylene composites

doi:10.1016/j.compstruct.2003.09.018

Composite Structures

Volume 62, Issues 3–4, 2003, Pages 443-448

https://doi.org/10.1016/j.compstruct.2003.09.018 Get rights and content

Abstract

Unidirectional (UD) and multidirectional (MD) flax/polypropylene composites were studied. Flax with varying retting degree and boiled flax was used as reinforcement for the UD composites and unmodified and maleic acid anhydride modified polypropylene (MAA-PP) was used as matrix. Multidirectional flax/polypropylene composites were manufactured on laboratory scale and on pilot scale. They were made from needle-punched hybrid flax/PP non-wovens. Normally retted flax as well as boiled flax was used. For the specimens made on pilot scale, a third kind of flax, namely bleached flax was also studied. The influence of different process times and temperatures on the mechanical properties of the composites was analysed. Generally, the composites have adequate good mechanical properties. The unidirectional composites of boiled flax combined with MAA-PP show the best mechanical properties. Contrary to the UD composites, flax treatment did not lead to the expected property improvements for MD composites.

Introduction

Due to ecological concerns, there is still interest in the use of natural fibres as reinforcement for composites. Natural fibres, such as flax, are renewable raw materials and fully combustible. They have good specific mechanical properties. Natural fibre reinforced composites used in automobile applications have recently gained in importance [1], [2], [3], [4], [5]. Typical applications range from door panels, rear-window shelves and car roofs, to spare-wheel covers and noise absorber panels [3]. Multidirectional flax/polypropylene composites were made on laboratory scale and on pilot scale in order to develop a car rear-window shelf that could compete with the conventional used ones. Different kinds of flax/polypropylene non-wovens were used as starting material and pressed under varying process parameters. The effects on the mechanical and physical properties were studied. Unidirectional composites made of different flax fibres and polypropylene matrices were also analysed.

Section snippets

Materials

For the unidirectional composites, dew retted hackled long flax fibres varying in retting degree, namely green (VC008), under-retted (VC009) and normally retted flax (VC010) was used as reinforcement. Also boiled flax fibres (VANN) were studied. Polypropylene (PP) (with MFR 8 g/10 min) and maleic acid anhydride modified polypropylene (MAA-PP) (0.6% maleic acid anhydride graft, MW 108800) were selected as matrix material.

The multidirectional composites were manufactured from flax/polypropylene

Production of composite samples

Pressing on laboratory scale was done with the use of a heated press and in a mold. After 12 min pre-heating, the samples were pressed at ±220 °C (205 °C for MAA-PP) during 2.5 min and then cooled for 6 min.

On pilot scale, tests with three different temperatures (190, 200 and 210 °C) in combination with two pressing times (3.5 and 5 min) were carried out. Thereafter the composites were cold pressed during 5 min. The 200 °C–3.5 min combination was taken as a basis for further production of

Results and discussion

An overview of the results for the UD composites is given in Table 1 and Fig. 1.

The stress efficiency factors (see Table 2) for the different composites were calculated as follows: $ε_{S} =(σ_{C} −σ_{P} φ_{P})/(φ_{F} ×σ_{F})$ with σ as the stress and φ as the volume fraction for composite (C), polymer matrix (P), and fibre (F).

It has to be remarked that the strength values for the fibres are tensile values since no flexural tests could be carried out and flexural tests of composites lead partly to tensile tensions.

Conclusions

Flax/polypropylene composites varying in material and process parameters were studied. The influence of the process parameters is described in the results. For the material variations it is concluded that fibre treatment, such as boiling of flax, leads to improved mechanical properties for unidirectional and almost completely compacted multidirectional flax/polypropylene composites. The use of maleic acid modified polypropylene increase the properties of all studied composites due to an

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Effect of material and process parameters on the mechanical properties of unidirectional and multidirectional flax/polypropylene composites

Abstract

Introduction

Section snippets

Materials

Production of composite samples

Results and discussion

Conclusions

Entwicklung und Eigenschaften von Nadelvliesstoffen aus Flachs für Kunststoffverbunden

Tech. Text.

Faserhaltige Bauteile für die Automobilausstattung Zur Leistungs fähigkeit von Naturfasern

Textil Prax. Int.