Encapsulating a building in popcorn may sound like a crazy way to boost energy efficiency... But researchers at Göttingen University’s Faculty of Forest Sciences and Forest Ecology in Germany have proven its suitability.
The material meets thermal insulation and fire-resistance parameters, and is water repellent, the team says. Available commercially, the university’s quirky insulating material is part of a range that includes a popcorn-based binder for wood materials.
However novel they become, composites are not new. Egyptians used mud and straw mixtures in construction many thousands of years ago. However, as the methods and tools to analyse the structure–property and processing relationships of molecules evolve, so too do the materials under investigation.
Whether corn kernel-based composites will prove economically viable under real-world economy dynamics remains to be seen. But, for a number of reasons, composite manufacturing is gaining traction.
Composites are multiphase materials formed using a combination of different compounds bonded together. Blended, the materials provide properties unattainable from one component alone.
They comprise fibres and/or particles/flakes embedded in a matrix or layers of materials – or any combination thereof.
The weight-saving race to the minimum
Light-weighting is a key driver for composite materials innovation. Compared with average car weights of almost 1860 kg, the 1500 kg BMW i8 Coupé is exceptionally lightweight. Built up from an aluminium chassis, the vehicle’s passenger compartment is made of a high-strength carbon-fibre reinforced composite (CFRP).
Boeing revolutionised its 787 jet airliner range with a composite fuselage and wings. The 787 model is reportedly 80% composite by component volume and 50% composite by weight overall. And carbon fibre makes up 65% of the weight of each aeroplane’s 35 tons of CFRP.
Long before BMW and Boeing began innovating to reduce weight, the composite material sheet moulding compound (SMC) was commercialised. Developed for exterior car body parts, the material comprised both glass and carbon fibres. Since using SMC instead of steel reduced weight by 60%, and by 30% when replacing aluminium parts, it offered a way for the automotive industry to reduce the CO2 emissions of vehicle usage.
Polymer matrix composites
The most widely known composites comprise a synergic combination of high performance fibres and a polymer matrix. In this blend, the fibre provides the high strength and the polymer spreads the load to provides resistance to weathering and corrosion. Thermosetting polymers allow for the formation of the reinforced article with optimum distribution of strength and stress resistance.
Maximum strength reinforced composite materials are achieved with a base of a mat of glass fibres, typically 5–10 mm thick. After lining or covering a model shape with the mat, it is sprayed with a liquid polymer, to which catalyst has just been added, so that it begins to polymerise. A low viscosity liquid polymer helps penetration of fibre mats. This method of fabrication is possible at room temperature. The curing phase (which involves the cross linking of molecules) can alternatively initiated by heat at around 150ºC rather than a catalyst.
The three best known thermosetting resins used in composites are unsaturated polyester resins (UPR), vinyl esters and epoxy resins. They find use in a range of applications including wind turbine blades, microwave cooker components as well as automotives and medical equipment.
The charts below demonstrate the diverse end uses for UPR in China and Europe. A large percentage of China's UPR usage is goes to infrastructure. In Europe, however, this category is missing, probably due to a difference of classification. UPR use in transport is much greater in European countries than in China.
Source: Cefic European UP/VE Resin Association / Chinese Society for Composite Materials
Breakthrough in thermoplastic composites
Extending the scope of traditional fibre reinforced thermosetting compounds is a relatively new matrix polymer, namely long fibre reinforced thermoplastic (LFRT). Easily processed using standard moulding equipment, LFRTs can deliver much higher mechanical performance than traditional short fibre compounds.
LFRTs are available as pellets for use in injection moulding and compression moulding techniques. This not only brings enhanced mechanical properties and weight reduction to the moulded article, it permits moulding at speed. The under two minutes moulding cycle times sought by automobile manufacturers is possible with thermoplastics.
Thermosets, however, are not able to so readily meet such moulding speeds. And, unlike thermosets, thermoplastic parts can be combined together by heat, sonic or induction welding. Adhesives and fasteners needed in thermosetting applications are eliminated with thermoplastic composites.
Common thermoplastic and fibre composites
As the above table shows, fine glass fibres are commonly used in composite thermoplastics made with PET, polybutylene terephthalate (PBT), polylactide (PLA) and high density polyethylene (HDPE), among others.
Acrylonitrile butadiene styrene (ABS) and polycarbonate-based composites are also commonly reinforced with glass – or steel – fibres. Glass fibres are also used in thermoplastic polyurethane (TPU) and polypropylene (PP) composites, and these plastics-based composite structures utilise carbon fibre reinforcement for the highest performance.
Composites made using polyamides – PA 6 or PA 66 – may feature carbon, steel or glass fibre. Polyamide-based composites however, are particularly suitable for a continuous filament-reinforced thermoplastic solution.
'Continuous filament-reinforced thermoplastic composites are as strong as metal
but offer the design freedom of plastics'
Charles Fryer, Senior Advisor, Tecnon OrbiChem
Continuous filament reinforced thermoplastics (CFRT) come in the form of unidirectional tapes. Filament is also used for woven sheets, filament winding and pultrusion. These techniques permit more complex designs than with thermosets or metals.
Speaking on the sidelines of the JEC World trade show in Paris in 2019, independent consultant Michel Jansen highlighted engineered thermoplastics’ recent growth trajectory. He said thermoplastics applications could be counted 'on the fingers of one hand’ in the early 2000s. 'In the years following, you could see thermoplastics composites growing.'
Thermoplastic unidirectional tapes allow for stronger, lighter, smoother and therefore better and 'totally non crimp - or near-zero crimp - products', Jansen said. 'That's very beneficial for performance, while smoother surfaces mean better aesthetics.
'Every step in the supply chain from unidirectional tape to end product is mass scalable, so you can really scale it up to mass production which is crucial for the industry,' he added.
Out of this world
Aromatic polyamide fibre – also known as Aramid – is ideal for PA 6 and 66 composite applications. A high-performance fibre, its molecules comprise relatively rigid polymer chains. Aramid is a high strength material with good resistance to abrasion and organic solvents – qualities that also make it suitable for aerospace and military application.
Polyetheretherketone (PEEK) and polyetherimide (PEI) composites are preferred for use in the aerospace and aviation industry. PEI has high heat and radiation resistance. It has been used in aerospace antennae equipment as well as for Airbus and jet plane floor panelling. PEEK's mechanical performance and thermal resistance set it apart, as well as its chemical and hydrolysis resistance.
With the performance capabilities of materials in as constant a state of flux as the boundaries of molecular science, the future of composites is strong. Today popcorn keeping you warmer, tomorrow... Who knows? The only limit is in the imagination.
Tecnon OrbiChem's business information platform OC360 covers most of the plastics for composite structures.