A novel surface modification of carbon fiber for high-performance thermoplastic polyurethane composites
Graphical abstract
Introduction
Carbon fibers (CF) present exceptional properties, such as high stiffness, high specific strength, and outstanding wear resistance, which have attracted considerable research interest in exploiting them as excellent reinforcements for thermoplastic resin composites [1], [2], [3]. The polymer usually acts as the continuous phase (the matrix) while the carbon fiber is the discontinuous phase, improving both the strength and toughness of the composites [4].
Thermoplastic polyurethane (TPU), a kind of thermoplastics with the mechanical performance characteristics of rubber, is described as “bridging the gap between rubber and plastics”. TPU is widely applied for daily use ranging from ski boots and footwear to gaskets, hoses, and seals because of its high elasticity combined with high abrasion resistance [5]. However, the strength and abrasion resistance of TPU yet to be further improved to deal with more demanding conditions. Short carbon fiber can be used to reinforce the abrasion resistance of TPU as well as other mechanical properties [4]. However, short carbon fiber reinforced TPU shows less effective in properties such as stress-strength, tear resistance, hardness compared to other reinforcing materials such as aromatic polyamide due to the less intensive matrix-fiber interaction [4], [6], [7]. Thus, researchers devote to modify the surface of CF to improve adhesion to the resin since the inadequate interphase between fibers and matrix would be a drawback [8], [9].
Compared with some traditional modification methods, such as acid oxidation treatment [10], plasma treatment [11], [12], gas phase oxidation [13], heat treatment [14], [15], and so on, polymerization treatment of carbon fiber demonstrates many advantages. Graft polymerization treatment is to graft macromolecules on the carbon fiber surface, so as to rough the fiber-matrix interaction area and introduce functional groups. For different resin matrix, the suitable surface functional groups are needed [16] according to compatibility, cost, production conditions, and so on. Before grafting process, people usually need to do an oxidation process so that reactive functional groups could form on the surface of fiber. Electrochemical oxidation is the most prevalent surface treatment of the carbon fiber, by which the surface chemistry of fiber would be improved, so as to improve its application potential on materials such as carbon fiber reinforced composites [17]. A previous study [18] used toluene-2,4-diisocyanate (TDI) to bind isocyanate functional groups on carbon nanotudes, which produced polyurethane (PU) composite coatings and improved its wear properties. However, TDI molecules have small molecular weight compared to 4,4′-diphenylmethane diisocyanate (MDI) molecules, which makes them more volatile with high vapor-pressure. Besides, TDI has certain toxicity, which may even be carcinogenic. On the other side, the two –NCO groups of MDI have similar activities, while that of TDI are different: the activity of the o-methyl –NCO group is 25 times smaller than p-methyl –NCO group. So in this study, MDI was used as a replacement of TDI due to its better properties. MDI is commonly used as a raw material to produce polyurethane, and it can be used to modify the surface of pristine carbon fibers to improve the interfacial characteristics between CF and TPU matrix. Furthermore, the electrochemical method we use is quick and easy, which is more suitable for industrialized continuous production compared to the former method, needing CF to be immersed into mixed acid solution for 2 h.
In this paper, the effects of MDI surface treatment on CF, which was electrochemically oxidized in advance, were investigated by fourier-transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, atomic force microscope and scanning electron microscope. We revealed the potential of MDI grafting polymerization to be developed as an effective surface modification of carbon fibers. To evaluate the possible applications of this surface modification carbon fiber, we compounded the MDI modified CF with TPU matrix and presented a series comparison between CF and MDI-CF reinforced TPU composites, using methods of tensile properties test, dynamic mechanical performance test, tribological test, and hardness test. It is proved that MDI grafting polymerization is an effective CF surface treatment to raise the interfacial adhesion between fiber and matrix, which gives TPU composites outstanding mechanical properties and excellent wear resistance for applications.
Section snippets
Materials
The carbon fibers used in this study were T700SC PAN-based 12 K tow fibers purchased from Toray Industries, Inc. Granular TPU (Polyester Type, AVALON® 85 AE) and MDI with purity of 99% 4,4′-isomer were provided by Huntsman Polyurethanes Shanghai Ltd. and used as received. Sulfuric acid (AR grade, 98%), acetone were supplied by Sinopharm Chemical Reagent Co., Ltd.
Surface modification of carbon fiber
The detailed scheme for the MDI modified CF is illustrated in Fig. 1. Carbon fibers, which took positive power supply, were bundled by
SEM
The SEM images of CF, eCF, and MDI-CF are shown in Fig. 2. to observe differences in morphological microstructure of those fibers. The surface of CF was clean and smooth due to the existence of sizing agent. After electrochemical oxidation, the roughness of eCF increased significantly, showing some small paralleled, different levels deep ridges and striations along the fiber axis on the surface of fiber. The surface of MDI-CF was wrapped by grafting polymerization of MDI. MDI molecules have
Conclusion
In this study, a novel and simple route using MDI as the modifier was developed to modify CF surface, and the properties of TPU/MDI-CF composites were evaluated. The fiber surface properties had obviously changed after MDI grafting and coating treatment. The roughness of the surface and disorder of carbon atoms increased significantly. Reactive functional groups were introduced to the fiber surface, and contents of oxygen, nitrogen and oxygen containing functional groups were increased, which
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