Foams ready for take-off

The commercial aviation industry is continuously looking into new and innovative material solutions to increase part commonality, save weight and costs and improve operational efficiency without compromising on key properties in use such as structural strength and resistance and tolerance to impact, fire and aggressive liquids such as Skydrol or cleaning agents. Light metal or composite parts undergo impact damage throughout their service life and as a result require maintenance and repair, which drives up costs.

Solvay recently introduced a new generation of foams for aerospace applications made of Radel PPSU polymer and branded Tegracore. It is set to become a game changer due to the multiple technical attributes the core integrates, including compliance with the most demanding FST requirements.

Tegracore is a component of Tegralite, a new family of high performance light-weighting materials solutions from Solvay that address the needs of the aeronautic industry in improving fuel efficiency, reducing waste and in speeding up the production, refurbishment and maintenance of planes at a lower cost.

A proprietary foam manufacturing process industrialised by Solvay's alliance partner 3A Composites ensures the required high strength-to-weight ratio at a typical density of 50kg/m3, characterised by a closed cell morphology that takes advantage of the physical properties of the PPSU polymer. Radel PPSU has a proven track record of more than 25 years in aerospace applications. Tegracore PPSU offers excellent thermoforming capabilities without excessive spring-back or shrinkage and displays both a low coefficient of thermal expansion and thermal conductivity. It also exhibits intrinsically very low moisture absorption and high resistance to aggressive aerospace liquids like jet fuel and Skydrol. This property profile gives it an immediate fit as a universally applicable core material in sandwich structures for a wide range of high volume aerospace applications, including sidewall, ceiling, floor and privacy panels as well as seat shells, luggage bins, galleys, cargo holds and radomes.

Resistance to impact

In order to substantiate the mechanical impact behaviour of appropriate sandwich solutions, Solvay has performed various comparative tests on compression moulded flat panels with cores made of PPSU foam vs. classical meta-aramid fibre-based honeycomb. Two types of glass fibre reinforced skins were used: a thermosetting phenolic matrix and a formulated sulfone-based thermoplastic resin matrix from Aonix Materials Corporation. In falling dart tests at different energy levels, the PPSU foam core structures, and particularly those with the thermoplastic prepreg skin, showed up to 100% higher impact damage resistance compared to the honeycomb structures. Further trials also revealed excellent damage tolerance after impact testing, whereas the honeycomb panels displayed delamination and collapsing of the vertical walls leading to an irreversible failure.

In addition, the PPSU foam-based structures provide outstanding peel strength achieved by means of chemically compatibilising the core and skin materials, eliminating the need of a bonding agent between core and skin. Working with Tegracore avoids the need for edge filling and surface finishing, i.e. typical labour intensive operations, when compared with honeycomb structures. And, due to the ductile behaviour of the PPSU polymer, Tegracore is not friable - an important additional attribute for work safety.

Furthermore, the high-performance PPSU foam can withstand prolonged exposure to temperatures from -40°C to +200°C exhibiting excellent thermal insulation properties. One typical application is a new generation of extremely lightweight, low-maintenance and thermally insulating passenger service trolleys developed and commercialised by AeroCat.

The first AeroCat trolleys were originally conceived in 2008 to replace standard aluminium with an all-plastics design that would save up to 10kg per trolley and with up 12 trolleys on-board a wide body aircraft at all times, that's a saving of nearly 120kg. The first generation, equipped with injection moulded and extruded Radel PPSU components, received major attention among airlines and catering companies. In a lifecycle assessment study conducted by the Netherlands Organisation for Applied Scientific Research (TNO), it was found that the all-plastics trolley undercuts the carbon footprint of comparable aluminium counterparts by an overall factor of 1.9 from the cradle to recycling.

A large part of this is due to the weight savings and the superior thermal insulation provided by the thermoplastic material. If only 10% of an estimated two million of airline catering trolleys in use were switched to the AeroCat design, this would reduce CO2 emissions by more than 300 thousand tons each year, equivalent to the annual CO2 release accounted for by the electrical power needed to light the homes of 1.7 million people. At NPE 2012, the AeroCat trolley won the Innovation in Plastics Award as well as the Project Award in the Automotive & Transportation category and was commercially implemented by several airlines operating global fleets of Boeing and Airbus aircraft.

Further development since then has focused on maximising the safety and security characteristics, the insulating properties and optimising the material base for high quality, high volume production. The latest AeroCat trolley features structural components using aerospace compliant UltraMaterials skins from Aonix with Tegracore.

Insulate and innovate

Beyond providing additional weight savings, the foam also outperforms other material solutions in terms of thermal insulation. Airline catering trolleys are cooled with dry ice that can suffer quick sublimation due to the surrounding ambient temperature and the absorption of its cooling energy by the body of aluminium trolleys. The excellent insulation capacity of Tegracore results in significantly improved cooling efficiency and provides a step-change enhancement offering airlines more flexibility in managing flight turnaround without compromising on-board food quality. It opened up a path towards achieving the ‘non-electric fridge on wheels', reducing the carbon footprint even further. At the end of the trolley's lifetime, its all-thermoplastic parts can easily be recycled and thermally processed into new applications.

Tegracore PPSU takes the aviation industry's approach of sustainability to yet another level beyond the barriers of traditional composite materials for rugged as well as light-weighting components. While the structural foam is already orbiting the planet in various applications such as the self-efficient Solar impulse 2 aircraft and Airbus A350 XWB, its use in the new catering trolley from AeroCat demonstrates the foam's versatility for a broad range of high volume aviation products to improve the operational efficiency of OEMs and airlines through lower weight, reduced labour costs, better function integration and superior maintenance performance.

Authors: Dr Armin Klesing, global business development manager aerospace & composites, Solvay Specialty Polymers; Dr Guy Van Meulebeke CAE senior specialist, Solvay Specialty Polymers; Theo Alsemgeest, CEO AeroCat.

www.solvayplastics.com