Development of Constituents for Multi-functional Composites Reinforced with Cellulosic Fibers

University dissertation from Luleå : Luleå tekniska universitet

Abstract: Bio-basedcomposites are being increasingly used in applications where weight saving,and environmental friendliness is as important as structural performance. Obviously, bio-based materials have their limitations regarding durability and stability of the properties,but their potential in use for advanced applications can be expanded if they were functionalized and considered beyond their structural performance.Multifunctionalityincomposites can be achieved by modifyingeither of the composite constituents at different levelsso that they can perform energy-associated roles besides their structural reinforcement in the system. For the fibers, this can be done at the microscale by altering theirmicrostructure during spinning process or by applying functional coatings. As for the matrix, it is usually done by incorporating additives that can impart the required characteristics to the matrix. The nano-sized additives that mightbe considered for this objective are graphene and carbon nano-tubes. A big challenge with such materials is the difficulty to reachthe dispersionstate necessary for formation ofstable network to overcome the percolation threshold for conductivity. However, once the network is formed, the composite can have improved mechanical performance together with one or more of the added functionalities such as barrier capabilities,thermal and/or electrical conductivities or electromagnetic interference ability.Enormous work has been done to achieve the functionality incomposites produced with special care in laboratories. However, when it comes to mass production, it is both cost and energy inefficient to use tedious,complex methods for the manufacturing. Hence there is a need to investigate the potential of using scalable and industrial-relevant techniques and materials with acceptable compromise between cost and properties.The work presented in this thesis is performedwithin two projects aiming to achieve functional composites based on natural and man-made cellulosic fibers suitable for industrial upscaling. Conductive Regenerated Cellulose Fibers (RCFs) were produced by coating them with copper by electroless coating process using commercial materials. On the other hand, commercial masterbatches based on Graphene Nano-Platelets (GNPs) were used to produce wood polymer composites (WPC) with added multifunctionality by melt extrusion process. The process is one of the conventional methods used inpolymerproductionand needsno modifications for processingfunctional composites. Both materials together can be used to produce hybrid functional composites.The incorporation of the GNP into HDPE has resulted in improvement in the mechanical propertiesof polymer as well as composite reinforced with wood fibers. Stiffness has increased to a large extent while effect on the strength was less pronounced(>100% and 18% for stiffness and strength at 15%GNP loading). The enhancement of thermal conductivityat higher graphene loadingswas also observed. Moreover, time-dependent response of the polymer has also been affected and the addition of GNP has resulted in reduced viscoplastic strains and improved creep behavior.The copper-coated cellulose fibers showed a significant increasein electrical conductivity(<1Ω/50mm of coated samples) and a potential in use as sensor materials. However, these results come with the cost of reduction in mechanical properties of fibers (10% and 70% for tensile stiffness and strength, respectively) due to theeffect ofchemicals involved in the process.