Hierarchical Composites Made Entirely from Renewable Resources

Recent interest in the utilisation of greener materials has reinitiated the interest in natural fibres and/or fibrils as reinforcement for polymers. However, such bio-based composites often exhibit properties that fall short of expectations due to (i) inadequate processing conditions, resulting in filler agglomeration and poor filler dispersion within the matrix, (ii) variations in natural fibre properties, often due to geographical and seasonal variability, (iii) anisotropy of the natural fibres themselves, (iv) high linear coefficient of thermal expansion for natural fibres and (v) the incompatibility between typically hydrophilic natural fibres and hydrophobic polymer matrices resulting in poor interfacial adhesion between the phases. Chemical modification of natural fibres is often performed to enhance the fibre-matrix interface. A new type of modification, which involves depositing a coating of nanosized cellulose onto natural fibres or dispersing nano-sized cellulose in natural fibre reinforced composites, has been shown to improve the fibre-matrix interface and the overall mechanical performances of such composites, which we term hierarchical (nano)composites. Such composites are also known as multiscale, nanoengineered or nanostructured composites. This paper reviews the current progress of green hierarchical (nano)composites made entirely from renewable materials. As a backdrop, here we look at how nature organises structures across different length scales. We discuss techniques to achieve percolated nanofiller networks within the matrix, at low-medium loading fractions (typically 6–10 vol.%) and processing routes to achieve high loading fractions, then focus on those used to produce truly hierarchical structures in terms of their processing and resultant properties. By creating hierarchical structures within bio-based composite materials we expect to match and improve upon non-renewable polymers.