The interaction of epoxy resin and an additional electrolyte with non-oxidised carbon black in colloidal dispersions

Dedicated to Professor Hans-Georg von Schnering on the occasion of his 70th birthday
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Abstract

The chemical interaction of non-oxidised carbon black (CB) and an epoxy resin in the non-cured state has been examined. Evidence for the influence of the amine hardener-epoxy content on the charge transfer to the CB is found by investigation of the resulting electrically induced agglomeration of the particles. The stability of the dispersion can be altered by the addition of copper chloride as an electrolyte. A description for the charging process is given based on a proton transfer from surface sites on the CB to the polymer.

Introduction

There exists a wide variety of technical applications for carbon black (CB)-filled polymers, either in the liquid state (e.g. paints and inks) or in the solid state (e.g. conductive polymers). For the description of the particle arrangement and the resulting physical properties, various models have been proposed. Among them the statistical percolation theory is well established in the literature [1], [2]. However, for the network formation of CB particles with spatial dimensions in the sub-micron range we have to take into account the diffusion process and the particle–particle as well as the particle–matrix interactions. Thus, CB-polymer dispersions can be treated as colloids with resulting steric and electrostatic stabilisation mechanisms [3], [4].

The influence of oxidised CB on the curing reaction of an epoxy resin was already described by Nakahara et al. [5]. In general, the addition of oxidised CB accelerates the initial cure reaction of the epoxy system by a catalytic action of the surface groups. A similar influence of oxidised carbon particles in polysulphide-epoxy composites was found by Babenko et al. [6]. Harbour et al. have shown the generation of electron accepting surface sites on the CB by acid oxidation [7]. A further electrochemical reduction of these sites results in an electron transfer to the carbon, which was confirmed by electron spin resonance measurements.

There is much less published on the interaction of polymers with non-oxidised CB. Even with this CB in an epoxy matrix it is possible to observe the charging of the particles [8] and effects of electrostatic stabilisation [4]. FTIR spectra taken by Manna et al. [9] revealed that in non-oxidised CB at least phenolic surface groups are present. For the interaction of their CB and epoxy groups in epoxidised natural rubber, a phenolic ether-type linkage is proposed.

With our experimental results we show the influence of the composition of the matrix polymer on the efficiency of the chemical reduction of surface groups and by this on the resulting amount of excess charges on the CB. Furthermore, the saturation of these charges by the addition of copper chloride is experimentally evidenced.

Section snippets

Materials and experimental methods

The matrix used in this study is an epoxy polymer based on a bisphenol-A resin (ARALDITE LY556, CIBA GEIGY) and an aromatic amine hardener (ARALDITE HY932). The standard curing mixture contains 77% epoxy resin and 23% amine hardener, both components are delivered in the liquid state. The conductive filler is a non-oxidised CB (PRINTEX XE2, DEGUSSA-HÜLS AG) with a primary particle diameter of about 30 nm. Due to the manufacturing technique the particles have a high BET surface area of 950 m2 g−1

Results and discussion

Fig. 1 shows two optical micrographs of the CB dispersion taken 5 and 180 min after the application of an electric field between the electrodes (black stripes on the left- and right-hand side of the micrographs). Immediately after the mixing of the components the CB aggregates are evenly distributed and no cluster formation appears equivalent to the right-hand side of the micrograph (a). Flandin et al. [11] show that for this polymeric system excess charges on the particles cause the

Acknowledgements

We gratefully acknowledge the discussions with M. Kupke and J. Sandler of the Polymer Composites Section, Technical University Hamburg-Harburg and Prof. M. Bredol, Department of Chemical Engineering at the University of Technical Sciences Münster. This work was supported by the Deutsche Forschungsgemeinschaft.

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