A two-step carbon fiber surface treatment and its effect on the interfacial properties of CF/EP composites: The electrochemical oxidation followed by grafting of silane coupling agent

Highlights

• The grafting of KH550 onto fiber surfaces had little influence on surface crystal structure.

• The introduction of KH550 resulted in decreased contact angle and increased surface energy.

• The tensile strength of KH550 modified fibers also increased.

• The IFSS and ILSS values of CF/EP composites increased by 73.1% and 61.2%, respectively, after surface treatment.

Abstract

A two-step surface treatment was applied on the surfaces of polyacrylonitrile(PAN)-based carbon fibers (CFs) including the electrochemical oxidation process followed by grafting of silane coupling agent KH550. The evolution of the physiochemical structure of the fiber surface during the surface treatment was investigated using Infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and surface tension/dynamic contact angle measurements. The results showed that the CF surface exhibited a significant increase in the relative content of oxygen-containing functional groups after surface treatment while its crystal structure had little change after the grafting of KH550 agents. Effects of surface treatment on the mechanical properties of CFs and the interfacial properties, including the interfacial shear strength (IFSS) and interlaminar shear strength (ILSS) of carbon fiber reinforced epoxy (CF/EP) composites were evaluated in detail. Both the electrochemical oxidation and the introduction of KH550 agents onto CF surfaces improved the IFSS and ILSS values of CF/EP composites by comparison with untreated fibers. In particular, the maximum interfacial properties of CF/EP composites were obtained by the electrochemical oxidation together with the grafting of KH550 agents onto CF surfaces.

Introduction

Carbon fiber reinforced epoxy (CF/EP) composites have been widely used in the aerospace, automotive, and chemical industries during the past decades [1,2]. The mechanical properties of CF/EP composites mainly depend on their constituents as well as the interfaces between the CFs and epoxy matrix. The CF/EP interface plays an important role in transferring external stress from the matrix to the reinforcing fibers, which could reduce stress concentration and improve the final mechanical properties of CF/EP composites [[3], [4], [5]].

Generally speaking, the manufacturing processes of CFs are composed of preparation of PAN-precursors, stabilization, low-temperature carbonization and high-temperature carbonization. In the process of high-temperature carbonization, the fibers undergo a heat treatment at a temperature almost up to 1600 °C, which results in the elimination of polar elements and the enrichment of carbon element on CF surfaces [6,7]. The interfacial bonding strength between CFs and epoxy matrix also becomes relatively low due to the chemical inertness and poor wettability of the CF surfaces [[8], [9], [10]]. Therefore, the inert surfaces of CFs become an important factor which could directly affect the final properties of CF/EP composites [[11], [12], [13]]. In order to improve the interfacial properties of CF/EP composites, the chemical bonding, van der Waals attraction and hydrogen bond force between CFs and the epoxy matrix are necessary [[14], [15], [16]].

Different types of surface modification have been utilized to improve CF surface activity, including plasma [[17], [18], [19]], electrochemical oxidation [[20], [21], [22]], liquid phase oxidation [23] and thermal treatment [24,25]. Among these treatments, electrochemical oxidation is usually preferred for commercial uses. It is well known that CF surfaces experience significant changes in the electrochemical oxidation, e.g. surface roughness increase which can improve the mechanical interlock. The introduction of oxygen-containing functional groups during the electrochemical oxidation can also improve the chemical bonding between CFs and the resin matrix. As a result, the electrochemical oxidation improves the interfacial properties of the corresponding composites to a certain extent. However, in the meantime, the mechanical properties of CFs significantly decrease due to the chemical oxidation and etching in the treatment. In order to effectively reduce the damage to mechanical properties of CFs during electrochemical oxidation, graphene oxide (GO) has been introduced onto CF surfaces in our previous research [26]. The mixed GO and co-monomer solution has been added into the electrolyte solution during the surface electrochemical oxidations of CFs, and GO-comonomers are successfully electrografted onto CF surfaces by one-step method. The introduction of GO leads to increases both in the relative content of functional groups on CF surfaces and the mechanical properties of CFs [26].

The grafting of silane coupling agent is also considered effective on CF surface modification. The silane coupling agent has been widely used as the sizing agents after fiber surface oxidation, or as the reactive sites in the modification of resin matrix. The introduction of coupling agents onto CF surfaces has two advantages. For one thing, silane coupling agents provide a large number of functional groups which can react with CF surfaces. The abundant functional groups of coupling agents can effectively improve surface wettability of CFs and thus enhance the compatibility between carbon fibers and resin matrix. Secondly, the introduction of coupling agents into the interface of composites can produce a chemical bridge between CFs and the resin matrix. The functional groups of resin matrix can form strong covalent bonds with the functional groups on the surface of silane-treated CFs. As a consequence, a wonderful compatibility between the reinforcing fibers and the matrix can be ensured and the interfacial bonding strength of CF/EP composites are also improved [27,28].

Coating with coupling agent is one of the most important methods for the surface treatment of high performance fibers [29], e.g. silane coupling agents have been widely used in the surface modification of glass fiber. But the direct introduction of silane coupling agents onto untreated CF is not effective due to high chemical inertness and poor wettability of untreated CFs [[30], [31], [32], [33]]. Choi et al. treated CFs by oxidization with nitric acid before coating with coupling agent glutaric dialdehyde. The results showed that the coupling agent improved adhesion between CFs and the phenolic resin by forming a chemical bond between fiber and resin. The mechanical properties of composites reinforced by CFs after surface oxidation together with coupling agent coating process were significantly better than those untreated CFs [34].

Owing to the excellent properties of coupling agents, a two-step method was employed in the surface treatment of CFs in the present investigation. The CFs were treated through electrochemical oxidation followed by grafting of silane coupling agent KH550 onto CF surfaces. The functional groups of KH550 agents mainly consist of amino groups and the hydrophilic amino group has strong polarity which can react with the functional groups of CF surfaces and the methoxy groups of the resin. In the present investigation, the evolution of surface microstructure during surface treatment was inspected and the effect of the two-step surface treatment on the interfacial properties of CF/EP composites was evaluated in detail. The interfacial reaction mechanism of CF/EP composites was also discussed.