Natural fibres in Composites: Lightweight Construction with Flax

 3 min

Technical textiles and fibre-reinforced composites are undergoing a materials revolution: materials made from renewable raw materials are once again gaining significant importance in this sector. The growing demand for high-performance materials for lightweight construction and technical applications is coupled with increasing requirements for product sustainability. Natural fibres such as flax, hemp and cotton combine both of these requirements: they offer functional properties and a lower environmental impact. The basic principle of a composite material is simple: two or more individual materials firmly bonded together complement one another in such a way that the overall system fulfils its purpose better than any single component alone. The fibres provide strength and stiffness, whilst the matrix – usually a plastic – determines the shape and holds the fibres in position. A simple experiment demonstrates just how effective this principle is: a wet towel hung up in frosty conditions becomes so stiff due to the frozen water that it can briefly serve as a serving tray.

Natural fibres as engineering materials: from Egypt to the Trabant

Natural fibres are among the oldest engineering materials. Even straw-reinforced mud bricks follow the composite principle. In ancient Egypt, flax served as a raw material for ropes, nets, sails and clothing, thereby forming the basis for transport and trade. Disused sails found a second life as mummy wrappings – an early example of circular thinking, which is regaining significance today in the context of sustainability.

Industrial mass-production applications of natural fibres are nothing new either. The bodywork of the Trabant P601 consisted of phenolic resin-bonded cotton fibres and thus represented an early fibre-reinforced composite material for high-volume automotive exterior applications. The material achieved high flexural and tensile strengths, good impact resistance and high long-term durability. This example refutes the widespread assumption that natural fibres are only suitable for technically undemanding tasks.

The properties of a natural fibre are closely linked to its biological function within the plant: this differentiation enables the targeted selection of fibres depending on the application. Bast fibres are particularly suitable for lightweight and rigid structural components made from fibre-reinforced composites.

Bast fibres (flax, hemp) fulfil a structural function and therefore exhibit high strength and stiffness.
Seed fibres (cotton) serve a dispersal function and are characterised by their length and elongation.
Fruit fibres (coconut) fulfil a protective function and absorb a great deal of energy.

Bast fibres in lightweight construction: assessment, semi-finished products and applications

A robust sustainability assessment of technical materials requires more than simply comparing emissions per kilogram. For composite materials, the respective load case and the specific mechanical properties must form the basis for comparison, as flax, glass and carbon fibres differ considerably in this respect. Ashby’s material indices serve as a benchmark – key figures that, for example, express stiffness in relation to density. According to this benchmark, bast fibres achieve high specific stiffness. Regenerated cellulose fibres, due to their low density, perform similarly to glass fibres, particularly under bending loads, whilst causing less environmental impact. Flax and hemp therefore demonstrate their strengths particularly in plate-like components subjected to surface loads.

Over the past two decades, a wide range of semi-finished textile products made from natural fibres has emerged in Europe. This ranges from cost-effective, easily drapable non-wovens and felts, through unidirectional textiles – in which the fibres run in a single direction – to low-twist yarns and rovings. Added to these are hybrid fabrics, lightweight 0/90 lattice structures as well as semi-finished products with minimised fibre undulation – that is, minimal waviness of the fibres within the fabric. This diversity allows the mechanical properties of natural-fibre-reinforced composites to be specifically tailored to the component in question.

The GreenBox project at Bremen University of Applied Sciences demonstrates how these semi-finished products can be used in lightweight construction. The team led by Jörg Müssig developed sustainable sandwich structures for mobile applications and used them to produce two prototypes: a market sales van and an off-road camper van. High-quality flax fabrics were used for the camper van’s cabin, which is subject to heavy wear and tear. For the sandwich panels of the market van, which are subject to lower stresses, hemp needle-punched felts were sufficient as face layers, combined with suitable core materials. Both systems achieve medium to high specific stiffness with low mass and a good environmental footprint – and demonstrate that natural fibres represent a viable alternative even in demanding structural components.

Source: Trade journal ‘Melliand Textilberichte’

Photo: Arthur