The ever-growing carbon foot-print and the scarcity of raw materials urge societies towards sustainability, recycling, and the circular economy. Bio-based substituents are thus favourable to supersede conventional materials. Natural fibre reinforced composites especially with biobased and thermoplastic matrices are one of the leading eco-friendly materials with the potential to replace glass fibre reinforced plastics (GFRP).
Natural ﬁbres are complex hierarchical structures made of biopolymers. Natural ﬁbres used in composite materials often refer to plant ﬁbres extracted from lignocellulosic biomass such as wood ﬁbres and other plant ﬁbres from the stem (termed as bast fibre), leaf, fruit and seed. Flax bast fibre is the most established natural reinforcing fibre in the composite industry. Motivations for the integration of plant-based reinforcements and specifically flax in composite designs as an alternative for GFRP are:
The specific stiffness of bast fibres is in the range of glass fibre. Flax has among the highest specific mechanical properties among bast fibres. As a reinforcement, flax has good damping properties and specific strength approaching glass fibre.
Natural fibres capture CO2 during growth. When the material is incinerated or bio-degraded at end-of-life, the captured carbon is released back into the environment. Because the material needs limited energy during cultivation, harvest and extraction, the overall Carbon footprint is relatively small. In case of a thermoplastic matrix, mechanical recycling by re-melting is a clear option as natural fibres show limited abrasion and can be recycled easily.
One hectare of flax absorbs more than 3.7 metric tons of carbon dioxide and converts it into oxygen. This means 3000 hectares of flax can absorb the emissions of one thousand cars driving around the globe. Compared to glass fibre, flax has nearly zero human toxicity and almost one-third of the greenhouse gas emissions.
Flax plants can flourish in the humid climate of Europe without irrigation and pesticides. The operation is near zero waste as all by-products can be converted to an added-value product.
Although novel flax fibre preforms recently introduced to the market, command high prices, the basic flax fibres, certainly when scutched-only flax is used, can be available at prices at or below the level of glass fibre.
Regardless of the stated merits, the application of flax in composites is limited in comparison to the glass reinforced plastics, which comprise almost 85% of the global composite market. Researchers focus on the following areas to encourage large-scale integration of plant fibres into composite designs:
Natural fibres are quite hydrophilic, and their affinity with high-performance resins should be tailored due to the hydrophobic chemistry of polymeric matrices. Tuning matrix chemistry or surface functionalization/ treatment of fibres are methods to develop a stronger interphase.
Natural fibres are sensitive to humidity and tend to swell due to their hydrophilic nature. Moisture absorption can thus lead to damage inside the composite and can also complicate composite processing. Research is done to limit these problems, e.g. by fibre treatment.
Fibre retting and separation approaches are being developed to achieve continuous plant fibres as long as possible without harming the fibre structure. Innovative roving and fabric architectures add value to the final product. For instance check products like “ampliTex” and “powerRibs”: http://www.bcomp.ch/en/products
Considering the chemical heterogeneity and surface texture (roughness, porosity, the density of yarns), natural fibres are far from being ideal materials. Accordingly, established characterisation methods such as contact angle measurement and micromechanical tests should be altered to take inherent complexities of natural fibres into account. For instance check: https://www.tut.fi/fibrobotics/fibrobond/
ESR1 - Tampere University