Toughening mechanisms in modified syntactic foams

Lead Research Organisation: Imperial College London
Department Name: Dept of Mechanical Engineering

Abstract

Syntactic foams are stiff but lightweight materials which comprise hollow glass microspheres embedded in an epoxy polymer matrix. This makes them excellent materials for structural applications, such as the core in sandwich panels with carbon fibre reinforced polymer skins. However, as they are based on highly crosslinked thermosetting polymer they can be brittle, and hence be susceptible to damage and cracking whilst in service. Their performance can be improved by increasing their toughness by the addition of modifiers such as rubber, polymeric or ceramic particles to the epoxy matrix. Thus this project will investigate the morphology, mechanical properties, fracture toughness and toughening mechanisms of syntactic foams with the addition of various modifiers. The toughening mechanisms will be identified, and the knowledge gained will be used to produce higher performance syntactic foams.

Publications

10 25 50

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509486/1 01/10/2016 31/03/2022
1817974 Studentship EP/N509486/1 03/10/2016 30/09/2020 Sammy Chee He
 
Description The research focuses on syntactic foams comprising hollow glass microspheres embedded in an epoxy matrix. Syntactic foams are critical materials in lightweight structures, being extensively used in the marine and aerospace industries. This is due to its low density and high specific strength. However, the use of this type of syntactic foam is limited due to its brittleness. This research has successfully toughened syntactic foams with the addition of milled carbon fibres. Milled carbon fibres successfully increased the tensile strength and fracture energy of syntactic foams, by 52% and 183% respectively. Analytical modelling for these properties show excellent agreement to the experimental data, with a mechanism describing the tensile behaviour being proposed for the first time. A journal article about this research has been published: https://doi.org/10.1016/j.matdes.2019.107654

Syntactic foams have also been modified with carboxyl-terminated butadiene (CTBN) rubber. CTBN rubber has been commonly used as a toughening agent for bulk epoxy polymers with great effectiveness, and the toughening mechanisms responsible have been well-established and modelled. However, whether these increases in toughness can be achieved when CTBN modified epoxy is used as the matrix in syntactic foams is not well understood. CTBN modified bulk epoxy polymers and syntactic foams were made to compare the toughness of the two types of materials. It is found that adding 12 weight percentage (wt%) of CTBN into the epoxy matrix of the syntactic foams increased the fracture energy by 54%, but is significantly less than that seen for the bulk epoxy polymer where there was a 990% increase for the same CTBN concentration. It is concluded that there is little transferability in toughness when the CTBN modified epoxies are used as the matrix for syntactic foams. Under scanning electron microscopy, it was revealed that the CTBN rubber phase-separated into complex co-continuous microstructures in the syntactic foams at higher concentrations, in contrast to the bulk epoxy polymers where all the rubber particles were spherical. The presence of the co-continuous structures corresponds to the increase in fracture energy of the syntactic foams at higher CTBN concentrations. It is postulated that the geometric boundaries introduced by the hollow glass microspheres affect the flow processes during phase separation of the CTBN rubber, leading to the difference in rubber morphology compared to the bulk epoxy polymer. Numerical modelling of the phase separation of CTBN near geometric boundaries is currently underway. A journal paper for the findings is currently in draft.
Exploitation Route The significant increases in tensile strength and fracture energy achieved in this research will increase the overall usefulness of syntactic foams in structural applications in the aerospace and marine industries, enabling the design of stronger, lighter, and more fuel-efficient vehicles.

Work can be done to confirm the newly proposed mechanism for the tensile behaviour of milled carbon fibre modified syntactic foams. If confirmed to be correct, it can be greatly useful in Composites research.

Modelling of the phase separation of rubber in rubber-modified epoxy polymers can be greatly beneficial in the selection of materials, design, and manufacturing process of rubber-toughened composites.
Sectors Aerospace, Defence and Marine,Construction,Manufacturing, including Industrial Biotechology,Transport

URL https://doi.org/10.1016/j.matdes.2019.107654