Influence of rare earth treatment on interfacial properties of carbon fiber/epoxy composites

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Abstract

In this work, the effect of soaking and irradiating in praseodymium nitrate solution on the surface physicochemical properties of carbon fibers and interfacial properties of composites has been investigated, and the interfacial adhesion mechanism of treated carbon fiber/epoxy composite was analyzed. It was found that both of immersion and irradiation lead to an increase of fiber surface roughness, improvement of oxygen-containing groups, enhancement of disorder degree and introduction of praseodymium element on carbon fiber surface. As a result, the coordination linkage between fibers, praseodymium ion and matrix is formed and the interlaminar shear strength (ILSS) of composites is increased, due to the improvement of interfacial adhesion between fibers and matrix resin. Moreover, γ-ray irradiation treatment is superior to immersion treatment in promoting interfacial properties owing to the increase of carboxyl and carbonyl.

Introduction

Polymer matrix composites are the main application of carbon fibers. The interface between carbon fibers and resin matrix plays a critical role in controlling the overall properties of the composites, such as off-axis strength, fracture toughness and environment stabilities. Interfacial characteristics determine the way loads can be transferred from the polymer to the fiber, and are often quantified in terms of the so-called interlaminar shear strength (ILSS). Below a certain critical strength, the surface of the fibers is not sufficient for the transfer to occur adequately. However, the inertness characteristics of carbon fiber surface usually result in inferior wettability and weak adhesion between the fibers and resin [1], [2], [3]. As a result, it is necessary to treat or modify the surface of carbon fibers in their application.

A literature survey shows that a range of experimental techniques have been applied to modify the surface of carbon fibers, including anodization, plasma treatment and solution and gas phase oxidation [1], [4], [5], [6], [7]. Most modifications on the interfacial properties of carbon composites have been focused on increasing the surface free energy or introducing organic groups on the fibers, and the interfacial property was improved at all levels after modification.

Rare earth compounds have been widely used in optics, electronics, metallurgy and chemical engineering because of their special characteristics [8], [9], [10], [11], [12]. During the past several decades, most investigations were focused on the effect and application of rare earth elements for the metal surfaces, and a few were related to using rare earth elements as non-metal material surface modifier [13], [14], [15]. In particular, less information is available about applying the rare earth elements for the surface treatment of carbon fiber, as well as the effect of rare earth elements on the adhesive property of carbon fiber composites.

The purpose of the present work is to evaluate the surface treatment of carbon fibers by means of Pr(NO3)3 solution, investigate the effect of different surface treatment conditions on interfacial properties of carbon/epoxy composites and relate them with the interfacial behavior in polymer matrix composites.

Section snippets

Preparation

The polyacrylonitrile (PAN) based carbon fibers with kidney-type cross-section investigated in current studies were supplied by Institute of Coal Chemistry, CAS (the average width is 9 μm and the thickness is 3 μm, linear mass is 0.0627 g−1). Praseodymium nitrate was prepared by dissolving praseodymium oxide (purchased from Shanghai Chemical Reagent Co.) in nitric acid. The matrix system used was E-618 epoxy resin system consisting of diglycidyl ether of bisphenol-A (supplied from Yue-Yang Chem.

Surface topography of fibers

The SEM images of the original, immersion treated and irradiation treated carbon fibers are shown in Fig. 1a–c, respectively. Compared to the original carbon fiber, the treated carbon fiber surface is rougher, some granules are formed and the grooves of fiber surface become wider and deeper. Moreover, the immersed carbon fiber shows a more distinct increase in roughening and depth of grooves than the irradiated fiber. It is indicated that the praseodymium ion etches the carbon fiber surface

Conclusions

The treated carbon fiber surface was rougher and some granules were formed, compared to the original carbon fiber. The element of praseodymium was detected, the carbonyl carbon in ketones and quinines (Cdouble bondO) and carboxyl or ester (COOH/COOR) functional groups were increased, and the graphitic carbon (Csingle bondC) and phenolic or ether oxygen (Csingle bondOH/Csingle bondOsingle bondC) functional groups were decreased after treatment. Rare earth treatments also brought about an increase in the degree of disorder on fiber surface. ILSS of

Acknowledgement

The authors would like to thank Dr. Sun Liang from School of Materials Science and Engineering for his insightful discussions and provision of praseodymium oxide.

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