Evaluation of fiber surface treatment and toughening of thermoset matrix on the interfacial behaviour of carbon fiber-reinforced cyanate matrix composites

https://doi.org/10.1016/j.compscitech.2005.05.008Get rights and content

Abstract

Composites of carbon fiber and cyanate ester matrix have been investigated. The research has been focused on the interfacial behaviour. The influence of carbon fiber surface treatments, sizing and oxidation, and also modification of thermoset matrix by the addition of a thermoplastic, has been analysed. In order to relate the interfacial behaviour with the characteristics of the components, a physico-chemical characterisation of the carbon fibers and of the cyanate ester resins has been carried out. Thereafter, the apparent interfacial shear strength, the interlaminar shear strength and the Mode II delamination fracture toughness, GIIC, have been measured by pull-out test, short beam-shear test, and end-notched flexure test, respectively. For fractographic analysis the surfaces of laminates fractured under Mode II deformation have also been examined by scanning electron microscopy.

Introduction

High performance polymer matrix composites based on carbon fiber and cyanate ester resins have gained a great deal of attention in the aerospace industry because of their excellent thermal and dielectric properties also having similar processability to conventional epoxy based composites [1], [2], [3]. Nevertheless, although cyanate ester resins are known to be relatively tough compared with other thermosetting matrices, some applications require improved fracture resistance [4]. Thus, several attempts have been made to improve the toughness of cyanate ester by adding elastomers or thermoplastics [5], [6], [7], [8], [9], [10], [11], [12]. Traditionally, the most successful method has been the addition of a suitable elastomer to the uncured resin [2], [5], [6], [7], [8]. However, in the last years, the modification with high-performance thermoplastics has been more investigated because of the advantage, as compared to rubber modification, that there is no reduction in thermal and mechanical properties of the cyanate matrix [5], [6], [7], [9], [10], [11], [12]. The properties of these types of materials are closely related to the generated morphologies, which are controlled by the thermoplastic content and the curing conditions [6], [7], [8], [9], [10], [11], [12].

On the other side, for taking advantage of the excellent mechanical properties of carbon fibers in composite materials an optimal interfacial adhesion between fiber and matrix is required. Due to stress transfer from one fiber to another goes through the matrix the interphase/interface plays a main role in the overall mechanical performance of carbon fiber-reinforced materials. The interfacial behaviour depends in a great extent on the carbon fiber surface. As the carbon fiber is extremely inert, usually untreated carbon fiber composites exhibit a weak bonding between fiber and matrix, giving as result composites with relatively low interlaminar shear strength. This problem has been overcome to a large extent by the development of fiber surface treatments, e.g., oxidative etching, polymer coating (sizing) or plasma activation, which improve the bond strength between the carbon fiber and the polymeric matrix [13], [14], [15], [16], [17].

The main objective of the present work was to analyse the influence on the interfacial behaviour of the composites, on the one hand, of the thermoplastic toughening of cyanate ester matrix and on the other hand, of carbon fiber surface treatments, such as sizing and oxidation. In order to evaluate the quality of the fiber/matrix interphase/interface, the apparent interfacial shear strength (IFSS) by pull-out measurements and the interlaminar shear strength (ILSS) by short beam-shear test (SBS) were measured. The resistance to delamination was also characterised by analysis of fracture toughness under mode II with end–notched flexure test (ENF).

Section snippets

Materials and specimens

The dicyanate ester resin used in this study was a bisphenol A dicyanate, DCBA, with the trade name AroCy B10, 99.5% purity, and with a cyanate equivalent of 139 g/eq. The catalyst system used was a mixture of 360 ppm of copper (II) acetylacetonate, Cu(AcAc)2, with a 2% of nonylphenol, NP, both from Aldrich [18]. For toughening of DCBA matrix a polyether imide, PEI, Ultem 1000, from General Electric, with the following properties: M¯n=12,000andM¯w=30,000, ρ25 °C = 1.27 g/cm3, and Tg = 220 °C was

Materials characterization

The objective of using two oxidation ways and sizing of carbon fibers was to optimise the interfacial behaviour. Chemical characterization of the different carbon fiber surfaces was carried out with several techniques.

The commercial sizing was analysed by 1H NMR. This sizing was obtained by evaporation of the CH2Cl2 solution used for “washing” the commercial carbon fiber. Fig. 1(a) shows the obtained spectrum. Fig. 1(b) shows the proposed structure: a diglicidil ether of bisfenol A, DGEBA, with

Conclusions

In this work, composites of carbon fiber and cyanate ester matrix have been investigated. The research has been focused on the interfacial behaviour by analysing the influence of carbon fiber surface treatments, sizing and oxidation, and also thermoplastic-modification of thermoset matrix on shear properties and delamination fracture toughness.

Characterization of carbon fiber and DCBA matrix has shown that the unsized and oxidised carbon fibers do not seem to have a clear effect on curing of

Acknowledgements

The authors acknowledge the Ministerio de Ciencia y Tecnología (Acción Integrada HA01-8) y CICYT (MAT98-0656) for the financial support for the development of this work.

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