Increasing the tensile properties of a unidirectional/mat dry reinforcement for composite applications via the addition of nanocellulose combined with click chemistry modifications.
Topic(s) : Material science
Co-authors :
Abdelhadi BLAL (CANADA), Gilbert LEBRUN , Eric LORANGER (CANADA), François BROUILLETTE (CANADA)Abstract :
The mechanical properties of natural fiber composite materials are closely linked to the fiber-matrix and fiber-fiber interface properties [1-2]. Physical or chemical treatments can be applied individually or jointly to the fibers and resin to improve these properties. Among them, the Huisgen 1,3-Dipolar Cycloaddition click chemistry reaction has been recently considered [3-4]. It is particularly interesting as it leads to chemically stable and strong covalent bonds, compared with the much weaker usual fiber-fiber bonds of the Van Der Waals type. This study focuses on the click chemistry modification of flax fibers in unidirectional-mat (UD-Mat) flax reinforcements. Our objective is to develop improved reinforcements for composite applications. In addition, the integration of nanocellulose was considered to strengthen further the dry reinforcement for its use in laminated composite materials [5].
The flax fibers selected for this study were functionalized in three stages. First, the surface fibrillation (FIB) of the short fibers (used for the mat binder) was performed to increase the exposed fiber surface area. Second, a chemical (cleaning) pre-treatment was performed to remove unwanted compounds present at the surface of fibers such as pectin and waxes. These first two treatments make the alcohol groups of cellulose accessible to the nucleophilic substitution reaction of the final click chemistry treatment reaction. For this latter reaction to occur, the propargylation of unidirectional flax fiber yarns and the tosylation-azidation of the short flax mat fibers must be performed in aqueous media. Next, crosslinking is carried out without a catalyst using a papermaking process with water as a solvent to form improved UD-Mat flax fiber reinforcements. FTIR and EDX analyses of fibers treated at intermediate stages (alkaline pretreated, propargylated, tosylated, azidated, and crosslinked) confirmed the presence of various chemical functions on the surface of modified fibers with a very high degree of substitution. The tensile properties of UD-Mat reinforcements showed increases in the elongation at break, force per unit width at break, and tensile stiffness of respectively 140%, 239%, and 185% for reinforcements containing the covalent fiber-fiber bonds obtained from click chemistry. Additionally, reinforcements containing 3% and 6% of nanocellulose (UDM-3 and UDM-6) showed even higher increases in strength and stiffness, while a higher percentage of nanocellulose resulted in a decrease in these properties. Overall, the combined use of surface fibrillation, click chemistry modifications, and nanocellulose adding (the UDM-FIB-CLICK-3 combination) resulted in the highest increase in strength compared to using click chemistry alone. These results bring new opportunities for the strengthening of dry flax fiber preforms and their composites obtained from liquid composite molding processes (like RTM) or from prepregs.
The flax fibers selected for this study were functionalized in three stages. First, the surface fibrillation (FIB) of the short fibers (used for the mat binder) was performed to increase the exposed fiber surface area. Second, a chemical (cleaning) pre-treatment was performed to remove unwanted compounds present at the surface of fibers such as pectin and waxes. These first two treatments make the alcohol groups of cellulose accessible to the nucleophilic substitution reaction of the final click chemistry treatment reaction. For this latter reaction to occur, the propargylation of unidirectional flax fiber yarns and the tosylation-azidation of the short flax mat fibers must be performed in aqueous media. Next, crosslinking is carried out without a catalyst using a papermaking process with water as a solvent to form improved UD-Mat flax fiber reinforcements. FTIR and EDX analyses of fibers treated at intermediate stages (alkaline pretreated, propargylated, tosylated, azidated, and crosslinked) confirmed the presence of various chemical functions on the surface of modified fibers with a very high degree of substitution. The tensile properties of UD-Mat reinforcements showed increases in the elongation at break, force per unit width at break, and tensile stiffness of respectively 140%, 239%, and 185% for reinforcements containing the covalent fiber-fiber bonds obtained from click chemistry. Additionally, reinforcements containing 3% and 6% of nanocellulose (UDM-3 and UDM-6) showed even higher increases in strength and stiffness, while a higher percentage of nanocellulose resulted in a decrease in these properties. Overall, the combined use of surface fibrillation, click chemistry modifications, and nanocellulose adding (the UDM-FIB-CLICK-3 combination) resulted in the highest increase in strength compared to using click chemistry alone. These results bring new opportunities for the strengthening of dry flax fiber preforms and their composites obtained from liquid composite molding processes (like RTM) or from prepregs.