Exel and Diversified Structural Composites: Leading the field in carbon fibre pultrusion

EXEL COMPOSITES NEWSLETTER MAY 2018

in May 2018, Exel Composites announced a further significant step in its growth strategy with the acquisition of Diversified Structural Composites (DSC), marking its expansion into the substantial US market. As an experienced technical pultruder, particularly known for high-level carbon fibre applications, DSC strengthens and extends both Exel's technological know-how and its product portfolio.
In this first part of a two-part article, we introduce DSC and outline two projects which showcase its carbon fibre technologies.

Technical focus

DSC's roots lie in specialised epoxy resin technology. Established in 1978 as Diversified Fabricators Inc, the company's founder, a polymer chemist, developed sophisticated epoxy resin systems. These proved a natural partner for carbon fibre in high performance applications and the company soon became known as the premier carbon pultruder in the US market. DSC subsequently diversified and expanded its business and now supplies a broad range of both carbon and glass fibre products from its facility in Erlanger, Kentucky.

As David Lewis, DSC's Director of Technology & Business Development, explains, one of the company's key strengths is its proven understanding of fibre and resin technology, enabling it to create the optimal solution for each application.

"We are experienced in running multiple carbon fibre types, from standard and intermediate modulus PAN-based fibres, right through to exotic, pitch-based carbon fibres of up to 640 GPa tensile modulus," he explains. "Over the years we have also successfully made parts using S-glass, aramid, quartz and other speciality fibres as well as standard E-glass. We offer proprietary polyester, vinyl ester, urethane and other resins, as well as epoxy systems ranging from aerospace-grade down to the commodity epoxies required for the wind energy market."

Aerospace showcase

DSC’s carbon spar technologies have found a role in pioneering aerospace projects.

In the mid to late 2000s, DSC delivered carbon fibre wing spar reinforcements for WhiteKnightOne and WhiteKnightTwo, the carrier aircraft for Virgin Galactic's commercial spacecraft enterprise. The capabilities displayed on this programme led to DSC's selection for the Stratolaunch reusable air-launch platform, which aims to reduce the cost of putting satellites into orbit. The twin-fuselage Stratolaunch carrier aircraft will take off from a runway and fly to the approximate cruising altitude of a commercial airliner before releasing a satellite-bearing launch vehicle. The aircraft will then return to Earth for reloading, refuelling and reuse. Reusable air launch to orbit systems such as Stratolaunch offer considerable advantages over conventional vertical rocket launches, including significant cost savings and more flexible operations.
Stratolaunch is the world’s largest aircraft by wingspan at 117 m and the largest all-composite plane ever built. During 2012-2013, DSC manufactured multiple carbon fibre pultrusions for the wing spars as well as profiles for the longerons.

"To attain the stiffness and strength required these parts were manufactured using intermediate modulus carbon fibre and a proprietary epoxy resin," notes Lewis. "Our epoxy system delivered the hot/wet strength performance that other resins could not, which was an important factor in our selection for this project. We also met the extremely challenging straightness tolerance specified due to the spar mould design."

Stratolaunch is under construction at the Mojave Air & Space Port in California and is currently being prepared for its first flight. It could be operational by the end of this decade.

Exel has been active in the aerospace sector for around 25 years and is a major supplier of pultruded products such as carbon fibre strips and plate, T-stiffeners and other custom profiles.

"We are approved by a number of aerospace suppliers, including Aero Composites and Israel Aerospace Industries," reports Kari Loukola, Exel Composites' SVP Sales and Marketing, who will transition to the newly-created role of SVP Exel Composites Americas and President of DSC in July 2018. "To meet the stringent quality requirements for this industry we have a dedicated factory space for aerospace products and we have developed on-line cure monitoring and NDT systems."

Challenging oil industry application

A further showcase for DSC's technology lies in an innovative intervention/monitoring system for the shale oil industry. For this application the company supplies a specialised 15 mm diameter carbon fibre/epoxy rod equipped with a fibre optic core. The 6.2 km long rod is deployed into a producing or injecting well where it gathers temperature, acoustics and other downhole data required for optimal well and reservoir management. Carbon fibre rods outperform steel alternatives in this application since the lightweight composite rod can be 'pushed' from the surface into a well's horizontal sections, which it not possible with heavier steel rods. The composite rod is also extremely strong and inert to the majority of corrosive substances found in the hostile downhole environment.

Lewis points out that this product places particularly challenging demands on the pultrusion process, since the resin employed is a tough epoxy system which requires a very slow cure to attain its optimal properties. This means that to produce each rod, the pultrusion line must run continuously, without fault, for 23 days.

"Because of the cost of the carbon fibre and the fibre optics, the process risk is very high," Lewis explains. "We've therefore performed extensive work to develop a pultrusion process that is extremely robust and foolproof, and exceptional process controls are in place to ensure there are no failures."

DSC has also developed spooling technology to enable the delivery of long lengths of product. This enables the stiff carbon rod to be wound onto 4.5 m reels for shipping to the customer.
Exel is also highly experienced in developing carbon fibre rods for multiple applications. These include high voltage cables, where the low weight and high stiffness of carbon fibre combined with low thermal expansion can be exploited to improve the performance and efficiency in power transmission and distribution. For offshore applications such as moorings of tension leg platforms and strength members in umbilicals, carbon composite offers the required combination of strength, stiffness and excellent corrosion resistance.

For challenging industrial applications such as these, and many others, Exel and DSC will leverage their combined capabilities to provide innovative composite solutions.

Read more in June 2018 Newsletter, where we take a look at the company’s strengths in the wind energy sector and its glass fibre activities.