Dr. Nilmini PJ Dissanayake, National Physical Laboratory
Flax (grown for fibre) and linseed (grown for seed oil) are varieties of the same plant species Linum usitatissimum bred with an emphasis on the required product, Figures 1-2. Linen is a commonly known textile made from flax fibres, Figure 3. In the UK, the flax is normally sown in March-May and may grow to one-metre high dependent on the variety.
Technical Fibre Extraction The diameter of flax stem at the base varies between 1 and 2 mm, and the height is about 80 cm (Figure 4 – Stem). The bark (the outer layer of the stem) and the xylem (the woody body/shive) are eliminated during the flax processing stages of retting and scutching. The fibre bundles are made of several elementary fibres glued together by pectin cement (Figure 4 – Bundle). Initially, the technical fibres are extracted by partial separation of these fibre bundles (for example during hackling). The fibres can be as the same length as the flax stem. Elementary fibres have two types of cell walls; the outer primary and the inner secondary cell walls (Figure 4 – Elementary fibre). These cell walls can be divided into three sections in terms of thickness and structure. The fibre also has a hollow space (lumen) which is filled by cytoplasm during cell life and disappears when the plant dies (i.e. during retting). Each cell wall consists of concentric lamella in which the cellulose fibrils are embedded in an amorphous matrix composed of pectin and hemicelluloses (Figure 4 – Lamella).
The hierarchy of technical fibre extraction from the stem by mechanical decortication (breaking, scutching, hackling) in micro and nano scale is shown in Figure 5.
Flax in composites Bast fibres (fibres extracted from the stem) such as flax, hemp, kenaf, jute, and ramie are more likely to be adopted as reinforcement in composites. These bast fibres are advantageous over the other cellulose based fibres (seed fibre, leaf fibre or fruit fibre) due to high modulus, tensile strength and low specific gravity i.e. stiffness and strength to weight ratios. Typical mechanical properties of flax fibres compared to glass fibres are shown in Table 1.
Table 1. Typical mechanical properties of flax fibres compared to glass fibres (Zhu et al, Review-Recent Development of Flax Fibres and Their Reinforced Composites Based on Different Polymeric Matrices, Materials 2013, 6, 5171-5198)
Fibres like flax/linseed and hemp are currently grown commercially in UK/Europe and used in composites as shown in Figure 6 (top) and (middle) for a wide range of automotive applications such as interior panels of passenger cars (Figure 6 (bottom)) and truck cabins, door panels and cabin linings due to their advantages such as low cost, light weight, renewability, biodegradability in addition to their eco-friendly aspect. The biodegradability contributes towards a healthy ecosystem and low cost and the performance of the fibres satisfy the economic interest of industry. These fibres are less abrasive than synthetic fibres adding an advantage in processing and recycling of the composite material in general.
The Future Presently, the applications of natural fibre-reinforced composites are limited to interior and non-structural applications mainly due to poor moisture resistance caused by the hydroxyl and other polar groups in natural fibres which makes them hydrophilic in nature. Their mechanical and physical properties are strongly dependent on the climate, location and weather which make it difficult to predict their respective composite properties and failure mechanisms. Their long-term durability under mechanical and environmental loading also limits their widespread use in many other practical engineering applications. Moisture uptake, long-term durability under different environmental conditions (e.g. elevated temperature, humidity) and their use in load bearing applications are worth investigating further to expand the use of natural fibre composites industries such as civil and structural.
Summary The application of natural fibres such as flax and hemp in composites is growing in many sectors such as automotive, sports, furniture, packaging and is constantly in development mainly due to their advantages compared to synthetic fibre reinforced composites. This includes low weight, low cost, high specific properties in addition to their eco-friendly aspect. These materials are considered as a promising alternative to the synthetic materials, however a widespread use of these materials is curbed by technical difficulties. Extensive research needs to be conducted to overcome the main drawbacks of natural fibre reinforcement in composites (e.g. sensitivity for moisture, temperature etc). Wider adoption of natural fibre composites will contribute towards reducing the global carbon footprint and will provide renewable, sustainable and less toxic alternative to synthetic fibre-reinforced composites in many industrial applications.
Title image: Vero Manrique on Unsplash
