Co-authors​ :

 Nina GRAUPNER (GERMANY), Christian GAUSS (NEW ZEALAND), Kim L. PICKERING , Stefan SCHMIDT , Jörg MÜSSIG (GERMANY) 

Using regenerated cellulose fibres, especially lyocell fibres, in composite applications is not new. Lyocell fibres are particularly characterised by their high toughness compared to plant bast fibres (see, e.g., [1]). Improving the adhesion between the generally hydrophobic matrix and the hydrophilic fibre is quite important. Besides the use of adhesion promoters [2], alternative approaches involve modifying the fibre surface, e.g. by plasma treatment, enzymatic processes, ultrasound or chemical treatment methods. One method for significantly increasing the specific surface area and, thus, the bonding surface between the matrix and the fibre is fibrillation. While fibrillation is unwanted in textile applications and makes processing more difficult, this process can offer distinct advantages in fibre-reinforced composites. Similar to a plant root system, the fibrillated nanofibrils can intertwine with the matrix, making them more resistant to detaching from the matrix when force is applied [3]. In this study, fibrillation was deliberately induced to improve the reinforcing effect of short and randomly oriented lyocell fibres in a PLA and PP matrix [4]. It was shown that fibrillation slightly reduces the tensile strength and elongation at break of the fibres, while it does not affect Young´s modulus. However, the interfacial shear strength could be increased from 3.3 to 4.4 MPa in a PP matrix and from 7.1 to 8.2 MPa in a PLA matrix. Fig. 1 shows that fibrillation only takes place on the fibre surface, and the core of the fibre is hardly affected. Despite the lower tensile strength of the fibrillated lyocell fibres, the strength of the composites was increased by a factor of 1.15 for PP and 1.62 for PLA compared to the composites produced with untreated fibres (Tab. 1). The Young´s modulus of the composites was increased by a factor of 1.41 for PP and 1.20 for PLA with fibrillated fibres. The impact strength was also improved by a factor of 1.38 for PP-composites and 1.23 for PLA-composites by using fibrillated fibres. As improved fibre/matrix adhesion has a more significant effect on the mechanical properties of short fibre-reinforced materials compared to long fibre-reinforced composites, fibrillated lyocell fibres were used to produce 3D-printed PLA composites [5]. The tensile properties were significantly improved by using 30 mass% of the fibrillated fibres compared to the materials with untreated lyocell fibres (Tab. 1). By using PLA modified with maleic anhydride (MA-PLA) and heat treatment, the tensile properties of the 3D-printed material with fibrillated lyocell fibres were further increased, achieving a tensile strength of 85 MPa, Young´s modulus of 7.2 GPa and an elongation at break of 3.2 %. The treatments described herein can significantly improve the properties of short-fibre-reinforced composites, expanding the potential applications for bio-based composite materials.