One-step amino-functionalization of milled carbon fibre for enhancement of thermo-physical properties of epoxy composites
Introduction
For decades, polyacrylonitrile-based carbon fibres have been widely employed in carbon fibre reinforced epoxy (CFRE) composites because of owning a unique combination of excellent mechanical, physical and chemical properties, such as high strength, high modulus and low density (1.65–2.00 g/cm3) [1]. CFRE composites with their promising strength-to-weight and stiffness-to-weight ratios are considered ideal for low-weight high performance materials in different applications, such as sports goods, automotive and aerospace industries. In spite of all merits related to composites reinforced with continuous carbon fibre, the increasing demand leads to an increasing amount of waste associated with fabrication of these composites [2], [3], [4], [5]. Moreover, carbon fibres are costly products, both in terms of energy consumed during manufacturing (up to 165 kW h/kg) and raw material price (up to 40 £/kg). Manufacturing waste is around 40% of all such composite waste caused, whereas woven trimmings hold more than 60% to this number [6], [7]. Carbon fibre waste recycle are often accompanied with different stages including surface heat treatment, mechanical grinding, sieving, milling, and drying. The resulting material will be milled carbon fibre (MCF) which has the advantage of being inexpensive and free from any sizing and binder [8], [9].
The mechanical performance and thermal characteristics of carbon-reinforced composite materials are highly governed by interfacial characteristics and physicochemical interactions between reinforcements and resin matrix, because a strong interfacial adhesion and good wettability ensure effective transition of stress across filler-matrix interface [10], [11], [12]. However, a large number of studies have shown that the low surface energy as well as the inertness characteristics of carbon fibre surface can limit its performance in CFRE composite due to its poor adhesion to most organic resin matrices [13], [14], [15], [16], [17], [18]. Hence, resin modification and carbon fibre surface treatment are the well-known approaches to improve the interfacial adhesion via different mechanisms such as chemical bonding, Van der Waals bonding, mechanical interlocking and surface wetting [19], [20], [21], [22], [23].
Carbon fibre surface treatment mainly is categorized into two groups; oxidative and non-oxidative treatments. Oxidation treatments consist of liquid-phase, gas-phase, and plasma oxidation. On the other hand, non-oxidative approaches involve either deposition of more active forms of nanomaterials on carbon fibre surface i.e. CNTs, graphene oxide, and POSS [24], [25], [26] or organic reactions e.g. in-situ generated diazonium salts [13], [27], [28]. Most of these approaches for introduction of organic molecules on carbon fibre surface are limited and hindered either by side effects of oxidation process or several chemical steps with difficult purifications. Most recently, development of non-oxidative organic reactions having high reactive intermediates for chemical functionalization of carbon fibre surface under mild conditions have been extensively examined. Among these chemical approaches, addition of free radicals having different functional groups to carbon based structures were extensively investigated for functionalization of fullerene, CNT, and graphene nanomaterials [29], [30], [31], [32].
In this work, for the first time, the free radicals produced from azo compounds was utilized to functionalize the graphitic structures of milled carbon fibre (MCF) surface which is inherently more stable than other type of carbon based materials e.g., CNT and graphene. This method was then applied for preparation of an amine rich MCF surface which could be heralded for its ability to reinforce epoxy-based composites through improvement of interfacial adhesion. For this purpose, the epoxy composites containing surface-functionalized MCF were prepared and its features, such as thermal, mechanical, and morphological properties, were systematically studied.
Section snippets
Materials
The epoxy system used was diglycidyl ether bisphenol A, trade name: D.E.R. 332, with an epoxy equivalent weight of 175 g/eq and the hardener was m-phenylenediamine (PDA) which were obtained from Sigma-Aldrich. Polyacrylonitrile-based milled carbon fibres with density of 1.65–1.75 g/cm3, diameter of 7 μm, mesh of 50–800, and carbon content of ⩾98% was supplied by Fibertech Korea Company. The microscopy image of the milled carbon fibre used in this work is presented in Fig. 1. Azobisisobutyronitrile
Characterization of functionalized MCF
Scheme 1 presents the mechanism of radical addition on MCF surface and its further amination. According to the mechanism, thermal decomposition of AIBN produces two isobutyronitrile radicals which are enough active to be anchored to the aromatic structures of carbon fibre and from the surface possessing nitrile groups (n-MCF). However, microwave irradiation could induce readily anchoring isobutyronitrile radicals by providing sufficient energy. For our purpose, enhancement of interfacial
Conclusion
Chemical functionalization of milled carbon fibres was successfully performed using isobutyronitrile radicals thermally produced by decomposition of 2,2′-azobisisobutyronitrile (AIBN), followed by direct nucleophile attack of tetraethylenepentamine on nitrile groups to have an amine-rich MCF surface. The detection of CN and NH2 absorption peaks in FTIR spectrums as well as increase of ID/IG ratios in Raman spectrums along with direct SEM observations confirmed successful grafting of
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