Preparation and Characterisation of Binder-Free All-Cellulose Composites

Abstract

A recent emerging concept of all-cellulose composites within the field of environmentally friendly materials has received increasing attention. The main advantage of these materials is the lack of using additional bonding agents such as polymer resins as in the case of e.g. phenolic resin based panel products or natural fibre reinforced plastics that increase their environmental impact. Two different routes for the production of all-cellulose composites have been followed. The obtained materials were characterised by scanning electron microscopy, X-ray diffraction, flexure and tensile mechanical tests, thermogravimetric analysis, pycnometry and water absorption tests. The first strategy makes use of the selective dissolution method where the cellulose fibre skins are partially dissolved to form a matrix phase that bonds the fibres together, while the strong core fibres are maintained and impart a reinforcing effect to the composites. The influence of the dissolution time, activation time and the fibre source were assessed. It was found that a dissolution time of 18 h led to materials with the best overall mechanical performance (5.5 GPa and 145 MPa for Young’s modulus and tensile strength, respectively), as this time allowed for the dissolution of a sufficient amount of fibre surface to obtain good interfacial bonding between fibres, while keeping a considerable amount of remaining fibre cores that provide a strong reinforcement to the composite, leading to materials that outperform natural fibres reinforced polypropylene composites. Still, the previous methodology has the drawback of using chemical substances of high environmental impact (solvents). In order to overcome this, a new concept in the production of all-cellulose composites is proposed in this work, which makes use of the intrinsic bonding capability between cellulose fibres to enhance the hydrogen bond network in order to produce materials of good mechanical performance. A new experimental procedure was developed, based on the refinement of cellulose fibres in order to increase their specific surface area, thus increasing the interfibre bonding capability, and achieving materials with excellent mechanical properties, up to 17 GPa and 119 MPa for flexural modulus and strength, respectively, and low water absorption. These new high-performing environmentally friendly materials are based on renewable resources and are 100% recyclable and biodegradable.