High performance polymers: Composites for Extreme Engineering Environments

The composites industry (civil aerospace, wind turbines) relies heavily on the use of epoxy resins reinforced with carbon fibres (CFRPs) and polyesters reinforced with glass fibres (GFRPs), which make up a large part of the commercial market, due to low cost and wide versatility. However, increasingly the physical characteristics associated with epoxy resins (limited glass transition temperature (Tg), limited fire or heat resistance, brittleness, high moisture content, and poor hot/wet performance somewhat limit the range of technological applications in which they can be used. For example, when selecting materials for the next generation of jet propulsion engines, for nuclear power stations, for satellites in low Earth orbit (LEO), or for low observable (stealthy) composite structures, commercial epoxy resins may be considered to have limited capability. In these instances, materials selectors turn to higher performance thermoset polymers (such as cyanate esters, bismaleimides, and polybenzoxazines) or engineering thermoplastics (based on highly aromatic polymers, such as polyaryletherketones or polyimides). Development of a nanophase, through introduction of nanomaterials (carbon nanotubes, graphene, nanosilica, nanoclays, or nanoalumina), leads to improvements in fracture toughness or electrical or thermal conductivity. These functional nanomaterials form the basis of multifunctional composites, with additional properties to complement their lightweight and high specific strength. We are also interested in the effect of integration of the materials to improve processing characteristics, leading to faster reaction, reduced processing times, or reduced viscosity, making it easier to manufacture large, complex composite structures. We research the fabrication, characterisation, simulation, manufacture, and testing of these advanced composites. Active projects include the development of: new polymer composites to withstand the effects of atomic oxygen and the rigours of satellite operation in low Earth orbit (with Oxford space Systems and the Mexican Government), composites to withstand extended use at elevated temperatures and the improvement of interlaminar toughness (sponsored by Rolls-Royce and Hexcel Composites); low observable composites for stealth applications.

Contact: Professor Ian Hamerton

Multifunctional smart materials and nanocomposites

Electro-Bonded Composites for adaptive adhesion and morphing structures: New lightweight, multifunctional structures with directional electrical conductivity, directional magnetic permeability, directional dynamic force reaction and good heat extraction.

Electro-magnetic Composites Aluminium, glass and Kevlar, infused with epoxy, for use in novel configuration, lightweight, high power density, electrical drives/actuators

Contact: Professor Ian Bond

Nanoporous materials

Nanoporous materials such as nanostructured carbons, metal-organic frameworks and zeolites are extremely efficient at capturing and trapping molecules. This makes them very useful for carbon dioxide capture (for mitigating global warming), water purification, and drug delivery in medical applications. Creating composites will immobilise these nanoporous materials in a matrix, which allows control of the overall porous structure, and easy recovery and regeneration, allowing them to be used safely and efficiently. We research the fabrication, characterisation and testing of these nanoporous material composites, and are also interested in the effect of integration of functional nanomaterials to improve curing rates and remove unwanted voids in composite manufacture.

Example of a polymer-clay scaffold

Contact: Professor Valeska Ting

Energy materials

Development of new composite materials can be used to address challenges in sustainable energies. We research the design, development and testing of composites for supremely lightweight and aerodynamic wind turbine blades for efficient electrical energy generation, high capacity supercapacitor electrode materials that are capable of storing and quickly releasing electrical energy, as well as new composite nanomaterials that can store hydrogen (a promising low-carbon, renewable replacement for fossil fuels) to allow its safe and efficient storage on-board next-generation hydrogen fuel cell electric vehicles.

Hydrogen fuel cell vehicles currently employ costly cryogenic hydrogen storage or very high pressure gas tanks, leading to safety concerns

Contact: Professor Valeska Ting

Innovative Multi-materials manufacturing

Through the advent of dynamic alignment of the reinforcement phase and the application of smart multi-materials within additive manufacturing, we have created 3D printed composite materials which have: (1) unique functionally graded architectures by instantaneously changing the reinforcement alignment at the point of construction; and, (2) unique hydrophobic/ hydrophilic architectures which morph, adapt and evolve over time in response to their environment.

In the first example, we have developed a technique for in situ manipulation of discontinuous fibrous structure mid-print, within a 3D printed polymeric composite architecture (Figure 1). In the second example, we have 3D printed hydrophilic-hydrophobic polyurethane tri-layers to create robust hydration responsive actuated hinges, enabling complex origami tessellations to be made constructed and deployed with commercially available materials and printers (Figure 2).

Figure 1: Schematic representation of printer and ultrasonic manipulation rig. (a) Switchable laser module is attached to the print head carriage, and traces out the shape of the printed part. The laser can be deliberately defocused to cure large regions of the epoxy resin slowly by increasing the height of the laser module.

Figure 2: Top: multi-material trilayers construction of hydrophobic polyurethane top and bottom skins (pink), with a hydrophilic polyurethane core (white). The amount of bending can be controlled by changing the gap width G and the skin and core thickness S and C. Lower: Actuation of Miura-ori origami fold pattern, containing 10 vertices, from dehydrated (left) to hydrated (right).

Contact: Professor Richard Trask

Materials variability in processing

Reinforcing fibre materials may exhibit local variability in material properties which can influence the resin infusion, leading to defects, and lowering mechanical properties. We are developing in-situ monitoring methods to identify variability and adjust manufacturing processes to mitigate the occurrence of variability induced defects.

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Surface pressure distribution of a composite preform placed in a moulding tool

Contact: Dr James Kratz

Lignocellulosic materials and natural fibres

Cellulose nanofibres have potential to replace conventional fibres as they possess high mechanical stiffness and strength. These fibres can also be used in functional materials for applications in energy storage, photonics, biomedical materials and adaptive composites.

Myoblasts (muscle cells) cultured in cellulose nanocrystals

Contact: Professor Steve Eichhorn

Cellulose nanomaterials

Cellulose is one of the most utilised materials on the planet, and plants produce the polymer on an enormous scale. The plant cell walls are comprised of hierarchical structures, with very small fibres on length scales of 10-100nm. We extract these structures from plants and use them as reinforcement in composites as they are known to possess very high mechanical stiffness and strength (>100 GPa stiffness, 2-4 GPa strength). These cellulose nanomaterials can be used in a variety of applications, not just composites, including as the basis for carbon electrodes in supercapacitors, for tissue engineering substrates and biomedical implants and for photonic (liquid crystals) and plasmonic structures.

Carbon nanotubes growing on a carbon nanofibre produced from electrospun cellulose

Contact: Professor Steve Eichhorn

Auxetics

Auxetics are synonymous of materials with negative Poisson’s ratio. Auxetic solids expands when subjected to a tensile loading, opposite to ‘classical’ material systems. We develop here in Bristol classes of laminated composites, honeycombs, architected multi scale structures and foams with auxetic characteristics. We also develop smart auxetics - i.e., the development of architected auxetics with different smart materials as fundamental constituents (shape memory alloys, polymers, piezoelectrics).These material systems are all designed from first principles, manufactured and characterised in-house. We use a variry of manufacturing strategies to produce these peculiar solids, from 3D printing and patented foam thermoforming, to Kirigami (Origami plus cuts) techniques that make auxetics available through the use of mainstream composites manufacturing processes.

Contact: Professor Fabrizio Scarpa

Self-healing materials

Self-healing composites via intrinsic polymers, embedded microcapsules, and vascular networks.

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Contact: Professor Ian Bond