Recently, due to environmental issues and increasing concerns about global energy, natural plant fibres that can replace traditional synthetic fibres as reinforcements in degradable composite materials have received increased attention. Compared to synthetic fibres, plant fibres are more abundant and with greater output. Moreover, as reinforcements in composites, they have a high specific strength, high modulus and are lightweight, cost-effective, as well as environmentally friendly.
Studies have shown that hemp fiber can be a suitable natural reinforcing material for composite application due to its vast availability, price stability, low density, delicate fineness, high toughness and good heat resistance. Furthermore, PLA is a biodegradable material that has wide applications and demonstrates the highest strength among all currently available degradable polymers. Natural fibres reinforced with biodegradable polymers are truly ‘green’ composites. The advantages of using such composites include lower power consumption and lower greenhouse gas emissions. Life cycle assessment (LCA) studies have revealed the potentially significant benefits of green hemp fibre composites compared to glass fibre composites, especially in automotive and transportation products. Therefore, composites fabricated from hemp fibres and PLA are truly ‘green’ composites that fundamentally solve the shortage of raw materials caused by petroleum issues and address environmental problems caused by garbage pollution.
Recent study assessed the fire resistance characterization of hemp fiber reinforced polyesters and reported that the increase in the composite hemp fiber volume contributed to the formation of an effective thermally insulating char layer. However, based on current knowledge, there is no recent study about the fire behavior of hemp fiber reinforced PLA composites and the effect of surface treatments on the composite fire reaction. The characterization of the fire resistance would increase the consideration of natural fiber reinforced composites in construction and demanding transportation (automobile/ aeronautical) applications.
Thus, Alao and colleagues investigated the effects of surface pretreatment (water and alkali) and modification with silane on moisture sorption, water resistance, and reaction to fire of hemp fiber reinforced polylactic acid (PLA) composites at two fiber loading contents (30 and 50 wt.%).
They evaluated moisture adsorption at 30, 50, 75 and 95% relative humidity, and determined water resistance after a 28-day immersion period. In addition, they used the cone calorimetry technique to investigate response to fire.
They showed that, moisture behaviour and reaction to fire of hemp fiber reinforced PLA composites were found to largely depend on the PLA/fiber content and fiber surface (pre)treatment.
They also found that, alkali pretreatment and silane modification not only improves the fiber dispersion and homogeneity within the composites, but also decrease the composite’s hydrophilic characteristics. Additionally, it has been shown that the water pretreatment has little effect on the composite studied characteristics; however, the further silane modification produced significantly higher moisture resistance and better reaction to fire, which may be attributed to the extra removal of the amorphous cell wall content and silane coupling at the hemp fiber surface. The Oswin model accurately predicted the adsorption isotherm for all composites.
“Overall, increasing the fiber amount from 30 to 50 wt.% increased the composite sensitivity to moisture/water, mainly due to the availability of more hydroxyl groups and to the development of a higher pore volume, but fire protection improved due to a reduction in the rate of thermal degradation induced by the reduced PLA content.”
Moreover, with the help of the new Oswin’s model they predicted the composite adsorption isotherm. They found that, 30 wt.% alkali and silane treated hemp fiber composite had the lowest overall adsorption (9%) while the 50 wt.% variant produced the highest ignition temperature (181 ± 18 °C).
Finally, they demonstrated that, the combined surface treatments of the fibers primarily improved the composite fire protection qualities. Overall, the alkali pretreatment of hemp fibers and surface modification with silane led to the most promising results for the use of hemp reinforced PLA composites. However, considering the expense of silane agents, a simple alkali treatment may be sufficient to efficiently increase the considered properties.
“Our study shows that adequate hemp fiber surface treatment allows the improvement of the composite durability during service, which opens opportunities for the use of sustainable composites in the transportation and construction sectors.”— they concluded.
Featured image: Schematic representation of the research objective. © Authors
Reference: Alao, P.F.; Marrot, L.; Kallakas, H.; Just, A.; Poltimäe, T.; Kers, J. Effect of Hemp Fiber Surface Treatment on the Moisture/Water Resistance and Reaction to Fire of Reinforced PLA Composites. Materials 2021, 14, 4332. https://doi.org/10.3390/ma14154332
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