Cardo poly (ether sulfone) toughened E51/DETDA epoxy resin and its carbon fiber composites

Abstract

A toughener that can effectively improve the interlaminar toughness in carbon fiber composites is crucial for various applications. We investigated, the toughening effects of phenolphthalein-based cardo poly (ether sulfone) (PES-C) on E51/ DETDA epoxy and its carbon fiber composites (CFCs). Scanning electron microscopy showed that the phase structures of PES-C/epoxy blends change from island (of dispersed phase) structures to bi-continuous structures (of the matrix) as the PES-C content increased, which is associated with reaction-induced phase separation. After adding 15 phr PES-C, the glass transition temperature (Tg) of the blends increased by 51.5 °C, and the flexural strength, impact strength and fracture toughness of the blends were improved by 41.1%, 186.2% and 42.7%, respectively. These improvements could be attributed to the phase separation structure of the PES-C/epoxy system. A PES-C film was used to improve the mode-II fracture toughness (GIIC) of CFCs. The GIIC value of the 7 μm PES-C film toughened laminate was improved by 80.3% compared to that of the control laminate. The increase in GIIC was attributed to cohesive failure and plastic deformation in the interleaving region.

1. Introduction

Carbon fiber reinforced plastics (CFRPs) are widely used in industries such as aerospace, high-end sport equipment and other structural applications, mainly due to their high specific strength and excellent mechanical properties[1,2]. However, the drawback of poor interlaminar toughness of carbon fiber composites (CFCs), which is related to the properties of the resin matrix, cannot be ignored[3–6]. Therefore, enhancing the interlaminar fracture toughness of CFCs by improving the toughness of epoxy resin (EP) has received extensive attention.

It is well known that the brittleness of epoxy resin gets directly inherited into CFCs[7] and a tough matrix can effectively absorb impact energy and increase energy dissipation for crack extension[8]. In recent years, some high-performance thermoplastic resins have been widely used as tougheners of EP and CFCs, including polyetherimide (PEI)[9–11], polyethersulfone (PES)[12–14], polysulfone (PSF)[8,15,16] and polyetherketone cardo (PEK-C)[1,17,18]. However, there still exist some considerable disadvantages in the aforementioned tougheners, such as the requirements of high dosage of toughener and reduction of the Tg, which are not conducive to reducing cost or application at high temperatures. For example, Girard et al.[19] reported that the fracture toughness of EP containing 20 phr PEI was enhanced by 85% due to the bicontinuous structure, but the fracture toughness of EP with 10 phr PEI addition was remained nearly unchanged. Investigation by Sun et al.[8]suggested that the impact strength and flexural strength of 15 phr PSF/EP blends were improved by 65.3% and 22.3%, respectively. However, the Tg of PSF/EP blends decreased, which affected the range of operational temperature of the EP and its CFCs. Jiang et al.[20] demonstrated that adding 15 phr PES into EP could increase the fracture toughness and impact strength by 24.4% and 6.4%, respectively. Cheng et al.[21] used a PES porous film to toughen carbon fiber composites and the GIIC was 55.1% higher than the control sample when the added film thickness was 90 μm. Although the toughening effect was good, thicker PES films would require higher economical investments, making them commercially unviable. Li et al.[22] reported that the improvement of 48.4% in the GIIC of CFRP was achieved by adding PSF hybrid nanofibers with 10% MWNTs-EP. However, the GIIC of solely PSF nanofibers interleaved CFRP was increased by only 10%. Zhou et al.[23] prepared PES/PEK-C hybrid nanofiber veils as interleaves for CFRP, and the GIIC value was increased by 46.9% when the interlayer areal density reached 19.2 g m−2.

Hence, it is important to find tougheners that can be employed in low dosage without sacrificing Tg of EP. The thermoplastic PES-C is a potential toughening material for EP due to its specific fracture toughness[24].