Rubber composites in engineering

Materials World magazine
1 Feb 2016

Rubber has found many applications working in tandem with complementary materials, from large steel-cored oil and gas hoses to thin, oil-resistant glass cords. James Perkins reports from the recent Rubber in Engineering Group meeting on rubber composites, in London.

Insulated cross arm has the power to transform energy infrastructure

The Arago insulating cross arm (ICA) for supporting electrical conductors on overhead transmission lines has performed well in more than two years of service on the SHETL network in Scotland, and the business is now looking to partner with established insulator manufacturers.

The technology, which won an IET Innovation Prize in 2012, was installed in the snowy, mountainous terrain of the Cairngorm National Park, where winds often reach 150 mph, and is still operated at 400 kV in a dedicated coastal test facility. 'There has been no loss in performance.' said Gerry Boyce, from Haydale Composite Solutions (formerly EPL Composite Solutions), a partner in the Arago venture. '[The ICA] has stood up to all required electrical tests at 400 kV including a 1,425 kV lightning test.'

The material construction is a liquid silicone rubber exterior over-moulded on a glass fibre reinforced epoxy resin structural core, with integrated metal end fittings. The silicone rubber is formulated to resist high electrical field stresses and repel water, even in highly polluted environments. Boyce said that the specialist polymers are created with a superhydrophobic surface that reduces the contact angle between water and the compound, allowing it to run off easily.

The design raises the power line higher off the ground, so it can be run at higher voltages and currents, offering a net capacity increase of up to 250%.

The arms were designed to be retrofitted on existing pylons, but can also be used to build new pylons up to 25% shorter. ‘An option to build smaller lines can alleviate significant planning constraints, offering big financial savings to transmission system operators’, Boyce said.

The risk-averse energy industry will be a tough one to crack, but Arago, which was incorporated in 2010 after several years of joint research and development between EPL and the School of Electrical and Electronic Engineering at the University of Manchester, has the power to do just that.

Timing belt changes should be a thing of the past, said Dr Christopher Stevens, Technical Director at NGF Europe, a St Helens-based manufacturer of oil-resistant rubber-reinforced glass cord for automotive applications such as car door seals and timing belts.

'It is up to the car manufacturer to impose a timing belt change on you,' said Stevens, 'It doesn't have to be changed.'

The company won a Queen's Award for its cord, which is used in timing belts across the automotive industry, including in the Ford EcoBoost engine used in the Focus and Fiesta, and is proven to last the lifetime of an engine.

To make the cord, the glass matrix – a bundle of fibres each just seven micrometres thick – is pulled through a bath of resorcinol formaldehyde latex (RFL), before being crosslinked.

The glass-rubber composite has some remarkable properties – it is stiff in tension (20 GPa), while soft in bending (10 MPa) and doesn't stretch, has no creep flow and its performance increases under a high load. 

The material was difficult to test, however, as the cord continually broke where it was clamped to the testing machine. This was a hurdle that had to be overcome to test the full lifetime of the product.

The company came up with a novel way of testing the cord in order to overcome this challenge. The cord was wound around bollards before clamping, which allowed NGFE to test the cord and demonstrate an improvement in fatigue life.

Making big, reinforced hoses

There were two talks on large, rubber-reinforced oil-and-gas hoses, one from an established player in the industry and another from a newcomer with disruptive technology.

Dunlop Oil and Marine, based in Grimsby, has been a leading manufacturer of oil and gas hoses for around 50 years, while TANIQ, from Rotterdam in the Netherlands, was incorporated as a spinout from the Delft University of Technology in 2010.

The hoses that these two businesses make are wound with many layers of rubber reinforcement in order to stand up to the tough conditions of life at sea connecting oil tankers to buoys, and buoys to the pipelines on the ocean floor.

John Davis, Compounder at Dunlop Oil and Marine, described the process behind making one of the company's two-tonne, 10–12 metre-long sections of hose.

A sheet of nitrile rubber is wound onto a steel core rotated on a mandrel, with steel wire applied to each end to bind the rubber and another spiral helical wire to act as a spring and provide a smooth bend. For a floating section of pipe, a layer of a buoyant elastomeric compound is added, before a final weather- and chemical-resistant rubber layer is added to the section of hose.

According to the video Davis showed alongside his presentation, the rubber is wound onto the hose by two workers operating a machine that moves lengthways along the hose to apply the rubber sheet evenly as the mandrel spins.

Later, TANIQ's Koen Spaanderman described a new manufacturing technology for the rubber industry – bringing together a high-precision robot with custom-made design and analysis software for mandrel-built products, such as expansion joints and large diameter hoses.

'The workflow and link between the design software for optimised designs, finite element analysis, and the way the robot precisely manufactures these designs is unique,' said Spaanderman.

Manufacturers can analyse the product as it is made and designed. Thin and flexible rubber tape, combined with other reinforcement, is placed on geodesic paths with uniform pressure and placement. It precisely controls the angle of winding as the tape is applied.

The business is a spinout from the Delft University of Technology Aerospace Faculty that aims to bring technology developed for aerospace to the rubber industry.

Diaphragms for composite forming

Diaphragm forming has emerged as an efficient method of preforming parts for resin transfer moulding manufacture of carbon fibre reinforced plastic parts. Engineers at the University of Nottingham are testing this forming method, as Research Fellow Oliver McGregor explained. 

The characterisation of the rubber diaphragm is an area of focus for the team, which wants to better understand its behaviour and reaction to friction and fatigue, hole propagation, temperature resistance and the impact of the thickness of the material on the composite part.

Using a diaphragm-forming machine developed within the university, the rubber was found to last for more than 200 forms before failure, but it is susceptible to the sharp edges of the composite material. To predict and prevent this damage, the team is using biaxial testing alongside its own finite element analysis software.

There were six candidate materials tested – Stretchlon 350, Mosite, three types of silicones and one latex. Three load cases were used, at room temperature and 85˚C and two loading regimes – monotonic and cyclic. Only the Stretchlon was significantly affected by the change in temperature, while the Mosite and silicones all exhibited similar stress-strain behaviour, but the stiffness of latex is lower than silicones and the silicones were not always isotropic. The team's experimental data was matched by the predictions made by its FE model.

If better understood, the diaphragm could be used for more forms, while also providing better-moulded parts, with less wrinkling, buckling and fibre slipping in the composite's fibres.