Improved curing and reduced moisture sensitivity of structural composites based on active cellulose nanofibril fibres and epoxy resin
Topic(s) : Material science
Co-authors :
Sabrina ASSENHEIMER (SWEDEN), Anna WIBERG , Mats JOHANSSON , Malin ÅKERMO (SWEDEN)Abstract :
One of the main drivers for replacing structural components with lightweight materials such as fibre-reinforced polymers (FRP), is the positive impact on sustainability during the use phase. However, the production of conventional fibres is very energy intensive and is mainly based on fossil fuels, resulting in carbon dioxide emissions. This has led to an increased interest in natural fibres. However, there are also challenges associated with these types of fibres, such as a finite length and a relatively high degree of variability in their properties. They also have a relatively large fibre diameter and a hydrophilic surface, which results in poor adhesion to conventional resins and attracts moisture.
A new type of man-made natural fibre based on cellulose nanofibrils (CNF) has been developed by using a microfluidic flow-focusing technique, a continuous gel thread is spun with very high control of the internal structure from a nanocellulose dispersion. This gel thread is subsequently dried into a continuous small and controllable diameter CNF fibre with specific properties in the range of conventional advanced fibre systems. In addition, the spinning technique allows for the modification of properties, such as making them more stretchable and flexible, in a way that differs from current solutions.
In comparison to glass and carbon fibres, CNF fibres provide an inherent chemically active surface with plenty hydroxide groups which form a strong covalent bond with the epoxy resin. Previous research has shown that already 15 vol% of CNFs in the form of a porous network, lower the activation energy and increase the curing rate of epoxy resin. This leads to excellent fibre-matrix adhesion, resulting in improved composite strength and an increase in the glass transition temperature. The surface hydroxide groups absorb moisture, but it has been shown that this moisture is re-emitted through the formation of a covalent bond with the epoxy resin. This makes the material less sensitive to moisture. Compared to the pure CNFs, CNF fibres have a reduced surface area, which results in a reduced number of hydroxide groups contributing to strong bonding with the epoxy resin. This study aims to confirm that the observations on porous network CNFs can be transferred to composites with high volume fraction of CNF fibres. Tests are performed on pure epoxy resin, CNF fibres and composites with approximately 55 vol% of CNF fibres. Since the fibres takes an active part in the resin curing in the future this enables tailoring of the epoxy resin towards less added reactants.
A new type of man-made natural fibre based on cellulose nanofibrils (CNF) has been developed by using a microfluidic flow-focusing technique, a continuous gel thread is spun with very high control of the internal structure from a nanocellulose dispersion. This gel thread is subsequently dried into a continuous small and controllable diameter CNF fibre with specific properties in the range of conventional advanced fibre systems. In addition, the spinning technique allows for the modification of properties, such as making them more stretchable and flexible, in a way that differs from current solutions.
In comparison to glass and carbon fibres, CNF fibres provide an inherent chemically active surface with plenty hydroxide groups which form a strong covalent bond with the epoxy resin. Previous research has shown that already 15 vol% of CNFs in the form of a porous network, lower the activation energy and increase the curing rate of epoxy resin. This leads to excellent fibre-matrix adhesion, resulting in improved composite strength and an increase in the glass transition temperature. The surface hydroxide groups absorb moisture, but it has been shown that this moisture is re-emitted through the formation of a covalent bond with the epoxy resin. This makes the material less sensitive to moisture. Compared to the pure CNFs, CNF fibres have a reduced surface area, which results in a reduced number of hydroxide groups contributing to strong bonding with the epoxy resin. This study aims to confirm that the observations on porous network CNFs can be transferred to composites with high volume fraction of CNF fibres. Tests are performed on pure epoxy resin, CNF fibres and composites with approximately 55 vol% of CNF fibres. Since the fibres takes an active part in the resin curing in the future this enables tailoring of the epoxy resin towards less added reactants.