Abstract
This paper aims to investigate the ability of an eco-friendly and cheap treatment based on sodium acetate solutions to improve the mechanical properties of flax fiber-reinforced composites. Flax fibers were treated for 5 days (i.e., 120 h) at 25 °C with mildly alkaline solutions at 5%, 10% and 20% weight content of the sodium salt. Quasi-static tensile and flexural tests, Charpy impact tests and dynamical mechanical thermal (DMTA) tests were carried out to evaluate the mechanical properties of the resulting composites. Fourier transform infrared analysis (FTIR) was used to evaluate the chemical modification on the fibers surface due to the proposed treatment, whereas scanning electron microscope (SEM) and helium pycnometry were used to get useful information about the morphology of composites. It was found that the treatment with 5% solution of sodium acetate leads to the best mechanical performance and morphology of flax fiber-reinforced composites. SEM analysis confirmed these findings highlighting that composites reinforced with flax fibers treated in 5% sodium acetate solution show an improved morphology compared to the untreated ones. On the contrary, detrimental effects on the morphology as well as on the mechanical performance of composites were achieved by increasing the salt concentration of the treating solution.
Highlights
In the last decades, due to improved sustainability, competitive properties and lower cost, natural fiber-reinforced polymer composites (NFRPCs) have attracted the attention of researchers as a potential substitute of synthetic fiber-reinforced polymer composites.For these reasons, they have found an application in several engineering fields such as the automotive, marine and sports equipment fields
The mercerization or “alkalization” is one of the most widely used chemical treatments, which consists in the soaking of natural fibers in an alkaline solution to reduce the contents of lignin, pectin, waxy substances and natural and artificial impurities covering the external surface of the cell wall of the fiber
Composites reinforced with flax fibers soaked in 5% sodium acetate solution show tensile strength and modulus equal to 73.7 MPa (i.e., +7.5%) and 6.94 GPa (+26%) in comparison to untreated composites with values of 68.5 MPa and 5.50 GPa, respectively
Summary
Due to improved sustainability, competitive properties and lower cost, natural fiber-reinforced polymer composites (NFRPCs) have attracted the attention of researchers as a potential substitute of synthetic fiber-reinforced polymer composites.For these reasons, they have found an application in several engineering fields such as the automotive, marine and sports equipment fields. It is widely known that hydrophilic natural fibers are covered with pectin and waxy substances, which prevent the hydroxyl groups from reacting with polar and hydrophobic organic matrices This fact leads to early aging and low mechanical strength of the composites. Suitable interfacial bonding allows proper load transfer from the matrix to the reinforcement, which helps to reduce stress concentrations as well as improving the overall mechanical properties [2,3] To overcome this issue, several physical [4,5,6] and chemical treatments [7,8,9,10] of natural fibers have been widely carried out with the main aim of enhancing the adhesion with hydrophobic polymeric matrices. The most commonly used chemical is sodium hydroxide (NaOH), and the reaction can be represented by the following equation: Fiber-OH + NaOH → Fiber-O-Na+ + H2 O (1)
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