Sizing effects on the interfacial shear strength of a carbon fibre reinforced two-component thermoplastic polymer

Abstract

Here carbon fibre surface treatment conditions (current, bath conductivity, and sizing) gave a comprehensive set of 27 unique carbon fibre samples. Examination of the interfacial shear strength (IFSS) in a two-component polymethyl methacrylate resin via the V-notch shear test. Epoxy-sized fibres show a strong dependence on surface treatments and were widely variable (25.6 ± 0.72 to 38.5 ± 2.69 MPa). We suggest the epoxy sizing is soluble in the methylmethacrylate monomer and is removed from the surface before complete polymerisation. Adhesion in epoxy-sized fibres seemed linked to the ratio of oxygen and nitrogen on the fibre surface (determined by XPS). Polyamide-sized fibres possessed a smaller data variation (37.3 ± 3.34 to 33.4 ± 3.18 MPa), suggesting the polyamide-sizing masks the underlying surface chemistry of the fibre induced by the variations in surface treatment. A hybrid of these two outcomes was observed with the polyurethane sized fibres (36.1 ± 2.19 to 28.8 ± 2.82 MPa), suggesting a mixture of effects observed above.

Introduction

Composite materials, consisting of at least two dissimilar materials, have been consistently present throughout history and nature for thousands of years. In recent times carbon fibre reinforced composites (CFRC) have had an intense focus as a structural material in a variety of applications. These materials offer large benefits in light weighting, as they possess a very high strength-to-weight ratio and can provide directional reinforcement rather than isotropic strength as per an analogous metallic component possessing strength in all directions. As with all composites, the interface between the dissimilar materials is of critical importance to the overall performance of the composite material [1], [2], [3], [4], [5], [6].

The modulation of fibre-to-polymer compatibility for commercially available fibres is usually provided in the form of a ‘sizing agent’ applied to the fibres in the final stage of manufacture. These sizing agents are often emulsions of proprietary constituents, which are classed according to ‘compatibility’ with a supporting resin. Unfortunately, the secrecy behind the manufacturing conditions of the carbon fibres themselves and the constituents of the sizing agents, has led to a large degree of conflicting information in the scientific literature regarding the role and impact of sizing on fibre-to-matrix adhesion [7], [8], [9], [10], [11], [12], [13].

With respect to supporting matrices, a large volume of work to optimise the surface and sizing of carbon fibres has been carried out with thermoset resins [7], [12], [14], [15]. Thermoplastics, on the other hand remain a persistent challenge for high interfacial adhesion. These polymers offer high fracture toughness, chemical resistance, the ability to be remoulded and being reshaped without negatively affecting the material properties. Most importantly, while thermosets (e.g. epoxy) at the end of their life are put in either landfill or the fibres are reclaimed by pyrolysis of the resin, thermoplastics can be reused by melting at a given temperature (e.g. 110 °C for low density polyethylene) [16].

Due to the increasing importance of component recycling, and the prevalence of thermoplastics in everyday items, it is worth investigating the interaction between carbon fibre and thermoplastics to achieve a better fibre to matrix adhesion [17], [18], [19].

As with thermoset resins, there is no consensus on what factors (sizing, roughness, and/or chemical interaction) govern fibre-to-matrix adhesion. Even though a significant amount of research was undertaken to investigate the interfacial adhesion between thermosets and carbon fibre, only a limited amount has been conducted with thermoplastics [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]. However, most of these studies conducted with thermoplastics focus on ultimate part performance and not specifically on fibre-to-matrix adhesion between the fibre and the resin. The most common methods for the sample preparation are hot pressing and injection moulding [38], [39], though a few tests with thermoplastics were carried out to investigate IFSS. For example, single fibre fragmentation test (SFFT) with polyphenylene sulphide (PPS), a microbond test with PPS, and a (multi-fibre fragmentation test) MFFT with polycarbonate were undertaken [30], [40], [41], [42]. In a study done by Stojcevski et al. interfacial adhesion was investigated with an V-notch shear test (Iosipescu test) [43], which can be easily conducted and reproduced.

Processing problems of thermoplastics are a hurdle to enable further investigations in terms of adhesion to carbon fibre, due to their high melting point and viscosity. In this work, this problem is circumvented by using Elium®, a two-component resin that uses in-situ polymerisation to give a poly methyl methacrylate (PMMA) supporting resin. Due to its low viscosity at room temperature we were able to manufacture V-notch shear test samples in a similar way to traditional thermosets.