Elsevier

Carbon

Volume 42, Issue 1, 2004, Pages 5-9
Carbon

Production and characterisation of vapour grown carbon fiber/polypropylene composites

https://doi.org/10.1016/j.carbon.2003.09.012Get rights and content

Abstract

Polymeric composites are widely used in the aircraft and automotive industries. Their high strength-to-weight ratio makes significant weight reduction possible. Beside this advantage, the polymer materials also offer a good corrosion resistance but the mechanical and electrical properties are not satisfactory. In order to increase these properties, vapour grown carbon fibers (VGCF) with high strength and metal-like electrical conductivity can be embedded in the polymeric matrix. To ensure a good adhesion between the fibers and the polymer matrix a functionalization of the chemically inert surface of the fibers is necessary.

In the present research work oxygen-containing functional groups were introduced on the fiber surface through cold plasma treatment. Measurements of the fiber surface energy after plasma functionalization showed an enhancement of at least 50% of the initial value. The VGCF/PP composites with different amounts of VGCF were made through extrusion and injection molding. The results show that the degree of fiber surface functionalization and the fiber distribution and orientation in the polypropylene (PP) matrix may strongly influence the mechanical properties of the composite.

Introduction

Vapour grown carbon fibers are a relatively new type of carbon fiber which are produced from the pyrolysis of a hydrocarbon gas, such as benzene and methane, in the presence of hydrogen at temperatures around 950–1200 °C [1], [2], [3], [4], [5], [6]. The fiber growth is initiated by ultra-fine transition metal catalyst particles, usually Fe, Co, Ni, deposited on a substrate (seeded catalyst method) or directly injected into the gas (floating catalyst method) [1]. Depending on the preparation conditions, VGCFs can be made with diameters between several tens of nanometers up to tens of microns and length from several microns up to many centimeters [4], [5], [6]. These fibers have been characterized in terms of the highly preferred orientation of their graphitic basal planes parallel to the fiber axis, with an annular ring texture in the cross section. This structure gives rise to excellent mechanical properties, very high electrical and thermal conductivity for a high graphitisation degree of the fibers [2], [3]. Due to their physical properties and their potentially low cost of production this fibers are interesting for their possible applications, principally as reinforcement in composite materials [1], [7]. Therefore, polymer matrix composites are the main application of carbon fibers.

For application of the VGCFs as reinforcement, fibers must possess a good functionalization of the outer surface (low contact angle and high surface energy). The modification of fibers surface can be made by gas treatments or liquid treatments [8]. Plasma treatment at room temperature and low pressure is an interesting and relatively new method between all gas treatments. Oxygen plasma represents an alternative to conventional oxidation treatments aimed at modifying the surface properties of carbon fibers and does not lead to substantial changes in fiber texture [9], [10]. Measurements of the contact angle in different liquids, especially water, indicated that plasma oxidation of activated carbon fibers increased its hydrophilicity, which suggests chemical addition of oxygen containing functional groups to the carbon fibers’ outer graphite layers.

Section snippets

Experimental

In the present work the morphology of carbon fibers before and after the treatment by oxygen plasma as well as the composite interface between the reinforcement material (VGCNFs) and polymer matrix (PP) were characterized and the mechanical properties of the composites were measured.

Vapour grown carbon nano-fibers (VGCNFs) are PYROGRAF III from Applied Sciences, Inc. with diameters of 50–200 nm and length of 100–200 μm. The morphology of the VGCNFs was studied by SEM (scanning electron

Results and discussion

SEM micrographs of the untreated carbon nano-fibers studied in this work are presented in Fig. 1. This fibers are in the as-grown state, without any heat treatment which could be done in order to increase the graphitisation degree. It is well known that the VGCNFs have hollow cores and are in fact carbon nano-tubes. The nano-fibers are generally composed of concentric cylindrical graphene tubules, each with the structure of role-up sheet and have a smooth surface. This kind of morphology is

Conclusions

The adequacy of oxygen plasma treatment to improve the adhesion in carbon fiber/polypropylene composites has been demonstrated in the present work. VGCNFs were treated by RF oxygen plasma in a rotating barrel and fluidized bed plasma reactor at different plasma parameters. The oxygen plasma treatment increased the surface energy of the fibers and decreased its water contact angle leading to an improved wettability of the fibers by the PP-matrix. The introduction of oxygen containing functional

Acknowledgements

Financial support by the German Federal Ministry for Education and Research (contract number 13N7741) in gratefully acknowledged.

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