Ligno-cellulose fibre poly(lactic acid) interfaces in biocomposites

Thermoplastic composite materials based on renewable raw materials are promoted globally in various ways. They can be prepared as durable, but biodegradable materials as poly(lactic acid) (PLA) ligno-cellulose composites are. Their advantages are their absence of fossil carbon, material recyclability, conversion to products using normal thermoplastic converting methods, absence of microplastic formation due to their biodegradability and haptic, unplastic-like feeling. The most important issue in tailoring PLA ligno-cellulosic fibre composites is to control the interfacial connections between fibre and polymer and porosity. The main components affecting the interface are fibre length, fibre aspect ratio, fibre surface roughness and surface chemistry, fibre modification and additives used in composite such as plasticisers and coupling agents. In injection-moulded products and especially with short wood fibres, the fibre length is usually below the critical fibre length needed for fibre reinforcement. This is due to fibre degradation occurring in typical thermoplastic processing at high temperatures. Compounding followed by injection moulding were the main processing methods also used in this study. The main aims of this research are to prove that regardless of the fibre dimensions of ligno-cellulosic wood fibre PLA composites, there is a need to couple between fibre and polymer to obtain optimal properties for the composite. It is also suggested that especially suitable coupling agents are oils from renewable origin and especially from modified oils such as epoxidised linseed oil (ELO) which can provide coupling between fibre and a polymer matrix. The conclusions of this research are the following: The fibre-polymer bonding was improved and the porosity of the material reduced in PLA composites with cellulose nanofibres (CNF), high consistency nanocellulose (HefCel), micro-cellulose (MC) and bleached softwood kraft pulp (BSKP) with ELO addition on fibre. The improved fibre dispersion with nanocelluloses, especially with HefCel was found due to ELO addition. The coupling of fibre and polymer was proved by FTIR analysis, by increased melt viscosity and by improved mechanical performance due to ELO addition to compounds.