Influence of natural fiber treatment on tensile properties of composite flax/jute/polypropylene

Nowadays, everything is focused on circular economy and reuse of raw materials. In the agricultural sector, there are a lot of waste fibers that need to be disposed of in some way, so accordingly, a lot of emphasis is placed on mixing natural fibers into polymer matrices. But the big problem is that natural fibers are polar/hydrophilic (attract water) while many polymers are non-polar/hydrophobic, leading to poor bonding, weak interfaces, moisture absorption, and reduced mechanical strength, plus issues like flammability, variability in fiber quality, and potential biodegradation. Addressing these challenges often requires surface treatments (like alkali, silane) to improve adhesion and make fibers more hydrophobic. In this research, an analysis was made of how different ways of treating natural fibers (flax/jute mix) affect the tensile properties of flax/jute composites and polypropylene as a matrix. Natural fibers were treated with NaOH along with a control group of test specimens in which the fibers were not treated. In addition to the use of NaOH, in order to further increase stability at elevated temperatures and improve the compatibility of the fibers and the matrix, the fibers were additionally treated with colloidal silica (SiO2) 40 wt. % in H2O. The fiber treatment procedure was as follows: fibers were soaked in 5% NaOH for 24 h, then they were rinsed with distilled water, after that fibers were soaked in 5% SiO2 for 24 h and rinsed with distilled water. After treatment fibers were dried for 24 h at a temperature of 60 C and crushed and ground in mill. Mixing was carried out on a compounder Brabender MetaStation 4E at a temperature of 180 C and a speed of 60 min-1 for 15 min. From the conducted tests it can be concluded that treating the fibers in NaOH alone as well as in both NaOH and SiO2 solutions increases the tensile and breaking strength, breaking strain and strain at yield, while the tensile modulus remains unchanged compared to untreated fibers. Furthermore, it can be observed that the tensile modulus increases by mixing the fibers into polypropylene, while the tensile strength and strain at yield remain similar to pure polypropylene. From the conducted analysis it is concluded that to obtain better tensile properties it is necessary to treat natural fibers, and the combination of both solutions will give better properties.
The work was created as part of the project of the company Funda d.o.o. Development of an innovative structural support for industrial flat roofs (IP.1.1.03.0064), within the application Strengthening strategic partnerships for innovation in the process of industrial transition (call reference IP1.1.03.). The authors would like to thank the European Union for funding the project.