1 SCIENTIFIC REPORTS | 7: 11939 | DOI:10.1038/s41598-017-11919-1 www.nature.com/scientificreports Enhancing the Mechanical Toughness of Epoxy-Resin Composites Using Natural Silk Reinforcements Kang Yang 1 , Sujun Wu 1 , Juan Guan 1 , Zhengzhong Shao 2 & Robert O. Ritchie 1,3 Strong and tough epoxy composites are developed using a less-studied fibre reinforcement, that of natural silk. Two common but structurally distinct silks from the domestic B. mori/Bm and the wild A. pernyi/Ap silkworms are selected in fabric forms. We show that the toughening effects on silk-epoxy composites or SFRPs are dependent on the silk species and the volume fraction of silk. Both silks enhance the room-temperature tensile and flexural mechanical properties of the composite, whereas the more resilient Ap silk shows a more pronounced toughening effect and a lower critical reinforcement volume for the brittle-ductile transition. Specifically, our 60 vol.% Ap-SFRP displays a three-fold elevation in tensile and flexural strength, as compared to pure epoxy resin, with an order of magnitude higher breaking energy via a distinct, ductile failure mode. Importantly, the 60 vol.% Ap-SFRP remains ductile with 7% flexural elongation at lower temperatures (−50 °C). Under impact, these SFRPs show significantly improved energy absorption, and the 60 vol.% Ap-SFRP has an impact strength some eight times that of pure epoxy resin. The findings demonstrate both marked toughening and strengthening effects for epoxy composites from natural silk reinforcements, which presents opportunities for mechanically superior and “green” structural composites. Epoxy resin is used extensively as the matrix in high-performance polymer composites for aeronautical and astro- nautical applications. Carbon fibre (CF), as one of the stiffest and strongest fibre reinforcements, is commonly selected to reinforce such epoxy matrices. However, one critical problem for these safety-critical applications is the brittle mechanical performance and poor toughness of CF reinforced epoxy composites, especially under high-rate/impact loading and at low temperatures 1–4 . Accordingly, there has been a concerted effort in recent years to improve the impact and low-temperature toughness of such epoxy-based composites 5–7 . “Hard” nano-fillers such as graphene nano-sheets carbon nanotubes are commonly used to enhance the low-temperature toughness of polymer composites 8–12 . Alternatively, “soſt” rubbery materials have been used to improve the low-temperature toughness of epoxy-resin composites 13,14 . However, to date there have been few attempts to improve the impact and low-temperature toughness of these composites through modification of the fibre reinforcement. Impact strength is one important measure of material’s mechanical toughness or resistance to fracture 15–17 . As polymer composites are increasingly finding applications, much work has focused on identifying their fun- damental toughening mechanisms 18,19 . Figure 1 presents the impact strength, measured with Charpy impact tests, of a wide range of popular, representative polymer composites as a function of the reinforcement volume fraction. What is striking about this comparison is that many widely-used high-performance synthetic fibre rein- forcements, such as UHMWPE (ultra-high molecular weight polyethylene) and natural fibres, e.g., flax fibre, do not impart high impact strength at all, despite their high strength and stiffness 20–23 . Carbon, glass and silk fibres can lead to impact strengths greater than 100 kJ m −2 in epoxy-resin matrix composites but of these fibres, silk fibres are the only natural reinforcements to provide such high impact strength. In this regard, the impact 1 Intl. Research Center for Advanced Structural and Biomaterials, School of Materials Science and Engineering, Beihang University, Beijing, 100191, China. 2 State Key Laboratory of Molecular Engineering of Polymers, Laboratory of Advanced Materials, Department of Macromolecular Science, Fudan University, Shanghai, 200433, China. 3 Materials Sciences Division, Lawrence Berkeley National Laboratory and Department of Materials Science & Engineering, University of California, Berkeley, CA94720, USA. Correspondence and requests for materials should be addressed to J.G. (email: [email protected]) or R.O.R. (email: [email protected]) Received: 27 June 2017 Accepted: 1 September 2017 Published: xx xx xxxx OPEN
Enhancing the Mechanical Toughness of Epoxy-Resin Composites Using Natural Silk Reinforcements
Jun 17, 2023
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