Enhancing the Mechanical Toughness of Epoxy-Resin Composites Using Natural Silk Reinforcements

Abstract

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.

Figure 1

Comparison of the impact strength at room temperature of polymer composites reinforced by glass fibres, carbon fibres, plant fibres and silk fibres. The corresponding composites are respectively abbreviated as GFRP (Glass Fibre Reinforced Plastics), CFRP (Carbon Fibre Reinforced Plastics), PFRP (Plant Fibre Reinforced Plastics) and SFRP (Silk Fibre Reinforced Plastics).

Figure 2

Scanning electron microscopy (SEM) images of the microstructure of (a) the plain woven Ap silk fabric, (b) pure epoxy resin, (c) the fabricated silk fibre-reinforced composites (SFRP) and (d) an enlarged view of the local region in (c). Comparison of stress-strain curves of pure epoxy resin, 60 vol.%-Bm-SFRP composite and 50/60 vol.%-Ap-SFRP composite at quasi-static strain rates under ambient conditions (temperature ~20 °C, humidity ~50%) under uniaxial tensile mode (e) and flexural mode (f). The inset in (e) shows the fracture morphology of 50 vol.% Ap-SFRP with arrows denoting the fractured and pulled-out fibres. (g) and (h) show derived mechanical properties at room temperature under quasi-static strain rates including specific modulus (refer to left axis), specific strength (refer to right inside axis) and breaking energy as a measure of toughness (refer to right outside axis).

Figure 3

Repeated stress-strain curves of pure epoxy resin and SFRPs from Bm and Ap silk reinforcements (at quasi-static strain rate under flexural deformation) at temperatures: (a) 20 °C; (b) −50 °C; (c) −100 °C and (d) −150 °C. Comparisons of flexural mechanical properties including specific flexural modulus (e), specific flexural strength (f) and breaking energy (g) of pure epoxy resin and SFRPs at the four temperatures.

Figure 4

(a) High-speed camera images of the impact process of the pure epoxy resin, and Bm- and Ap-SFRPs with 30 vol.% or 60 vol.% silk reinforcement; (b) Schematics of the fracture morphologies after impact tests and photographs of the fractured samples from top and side views. (i) matrix resin’s brittle fracture; (ii) 30 vol.% Bm-SFRP’s semi-ductile fracture; ductile fracture of (iii) 60 vol.% Bm-SFRP and (iv) 30 vol.% Ap-SFRP; (v) ductile response of 60 vol.% Ap-SFRP. (c) Sketch map of mechanisms for ductile responses to impact on the left, including epoxy resin fracture, fibre pull-out and fracture, interfacial debonding; SEM images on the right showing contributions on a fracture surface: epoxy fracture, fibre fracture and pull-out are highlighted contributions in 30 vol.% Bm-SFRP; and interfacial debonding is highlighted as a significant contribution in 30 vol.% Ap-SFRP.