Synergistic effect of MWCNTs functionalization on interfacial and mechanical properties of multi-scale UHMWPE fibre reinforced epoxy composites

Abstract

Multi-walled carbon nanotubes (MWCNTs) as a secondary reinforcement were incorporated to an epoxy matrix reinforced with ultra-high molecular weight polyethylene (UHMWPE) fibre. To achieve a high level of compatibility with epoxy matrix, both the UHMWPE fibre and MWCNTs were chemically treated with glycidyl methacrylate (GMA) and amino-thiol end radicals via free radical polymerization and click chemistry, respectively. The effects of these treatments on properties of the multi-scale composites, i.e. epoxy consisting of micro (UHMWPE) and nano (MWCNT) size reinforcements, were studied through investigation of micromechanical and macromechanical properties of the composites. Results showed both fibre surface treatment and matrix modification improved fibre-matrix wettability, work of adhesion, interfacial shear strength (IFSS), and tensile properties. Compared with untreated composites e.g., UHMWPE fibre/epoxy, the combination of a low loading of aminated MWCNTs (a-MWCNTs) and GMA-treated UHMWPE fibre in an epoxy resin enhanced work of adhesion, IFSS, tensile strength, and tensile modulus by ∼26.0%, ∼336.1%, ∼67.3%, and ∼35.5%, respectively. Analysis of surface morphology of micro and macro composites were performed by SEM observations, demonstrating that the mechanical interlocking effect along with the covalent bonding at interface are synergistically playing a major role in such significant improvements in composite properties.

Introduction

Ultra-high molecular weight polyethylene (UHMWPE) fibre reinforced composites have received great attentions due to their extraordinary properties such as light weight, high specific strengths and moduli, ease of fabrication, bio-compatibility, and bio-stability and ballistic performance arising from fibre phase. However, the non-polar and chemical inertness of UHMWPE fibres surface inherently result in poor adhesion properties with most polymers [1], [2], [3], [4]. Improved interfacial adhesion could enhance various properties of UHMWPE composites including mechanical properties and even open up new applications for this family of composite materials [5]. As mentioned, in spite of the aforementioned drawback related to UHMWPE fibre, this fibre is still increasingly used in different applications including aerospace, military, and sporting goods [6], [7], [8]. There are two general approaches to overcome the issues with adhesion and improve the interfacial properties. This is either by increasing the surface energy of the fibre through surface treatment or decreasing the surface energy of the matrix through inclusion of nano-reinforcement [8], [9], [10]. The polar functional groups have been introduced onto the fibre surface via various treatment techniques; for instance, plasma treatment, chemical etching, corona treatment, radiation-initiated graft polymerization and chemical grafting [4], [11], [12], [13], [14]. In addition to this, the polymer matrix can be modified through incorporation of different nano-reinforcements, such as graphene nanoplatelets (GnPs), carbon nanofibres (CNFs), multi-walled carbon nanotubes (MWCNTs), and nanoclay [15], [16], [17], [18], [19], [20]. theoretical and experimental results found in literature, have revealed that carbon nanotubes possess outstandingly high elastic modulus, greater than 1 TPa (the elastic modulus of diamond is 1.2 TPa) and stated strengths 10–100 times higher than the strongest steel at a fraction of the weight [21]. These modification and surface treatment approaches cause changes in surface energy, wettability, roughness, and interfacial interactions leading to enhancement of the fibre-matrix adhesion [14], [22], [23]. However, both fibre treatment and matrix modifications are usually accompanied by some undesirable problems. For example, among different fibre surface treatments, chemical grafting is known as an elegant approach to provide composite with a strong interface through chemical bonding. However, chemical grafting, particularly at high density of grafting, could deteriorate fibre properties such as tensile properties and consequently affects composite ultimate properties adversely [9], [11], [24], [25], [26], [27]. On the other hand, matrix modifications resulting in best mechanical properties do not normally occur at very low nanofiller contents. In fact incorporation of higher contents into polymer matrices often requires proper approaches including high power of ultrasonication, surface modification and use of surfactant and dispersing medium to achieve uniform dispersions and prevent nanofillers from being aggregated [28], [29], [30], [31], [32].

Despite many reports on the effect of UHMWPE fibre treatments on composites properties, there are few studies related to improvement of interfacial adhesion between UHMWPE fibre and epoxy through modifications of both fibre and epoxy matrix. In our previous study [11], compared with UHMWPE fibre/epoxy, inclusion of 1 wt% of pure CNFs alongside glycidyl methacrylate (GMA)-treated UHMWPE fibre (5 wt%) in epoxy increased tensile strength and modulus ∼52.3% and ∼26.2%, respectively. Sadeghi et al. [33] showed that the interfacial shear strength (IFSS) between epoxy and UHMWPE fibre grafted with ∼11%, ∼25%, and ∼40% GMA showed ∼126%, ∼195%, and ∼220% enhancements, correspondingly. Additionally, they exhibited that tensile strength of ∼11% GMA-grafted UHMWPE fibre-epoxy composite revealed only ∼10% enhancement, compared with untreated one. According to Jiang et al. [34], an atmospheric pressure plasma jet treatment on UHMWPE fibre could increase IFSS of UHMWPE reinforced epoxy composites by ∼330%. Li et al. [27] reported that the highest amount of improvement in IFSS (∼271%) of UHMWPE fibre/epoxy was observed for the samples reinforced with UHMWPE fibre grafted with methacrylic acid and acryl amide (UHMWPE-co-pMAA-pAM fibres). Zhang et al. [35] showed that the incorporation of ∼1 wt% nano-SiO₂ during gel-spinning process could improve IFSS of UHMWPE/SiO₂ composite fibres by ∼148% compared with untreated one. Zhamu et al. [36] showed that inclusion of 0.3 wt% reactive graphitic nanofibres to UHMWPE fibre/epoxy composite result in ∼34% and ∼27% increase in critical load (initial debonding load) and peak load, respectively. In addition to this, ∼116% and ∼107% increases in critical debonding energy and pull-out energy were observed, respectively. Among various nanofillers, carbon nanotubes are repeatedly used to improve properties of composites through fibre treatment or matrix modification [10], [37], [38], [39], [40].

In this work, simultaneously, both UHMWPE fibre and MWCNTs were chemically modified so that the compatibility of both fibre and matrix with MWCNTs is improved. It was hypothesized that aminated MWCNTs could react with epoxy functional groups of both GMA-treated UHMWPE fibre at interface and epoxy matrix, as well. Furthermore, as mentioned earlier, it was argued that adverse effects associated with fibre and matrix modifications could be minimized by employing simultaneous modifications but at less intensive levels. Therefore, herein, we have used a minimum amount of chemical grafting of fibre e.g., 5 wt% combined with a low loading of MWCNTs e.g., 0.2 wt% to ensure that the final properties of composites would not be affected by negative consequences of both fibre and matrix modifications. For this purpose, effect of the functional groups attached on UHMWPE fibres and MWCNTs on wettability, interfacial adhesion, mechanical properties and morphology of multi-scale epoxy composites were examined using dynamic contact angle, microbond, tensile test, and scanning electron microscopy techniques.