Impact of Alkali and Silane Treatment on Hemp/PLA Composites’ Performance: From Micro to Macro Scale

Abstract

This study investigated the effect of hemp fiber pretreatments (water and sodium hydroxide) combined with silane treatment, first on the fiber properties (microscale) and then on polylactide (PLA) composite properties (macroscale). At the microscale, Fourier transform infrared, thermogravimetric analysis, and scanning electron microscopy investigations highlighted structural alterations in the fibers, with the removal of targeted components and rearrangement in the cell wall. These structural changes influenced unitary fiber properties. At the macroscale, both pretreatments increased the composites’ tensile properties, despite their negative impact on fiber performance. Additionally, silane treatment improved composite performance thanks to higher performance of the fibers themselves and improved fiber compatibility with the PLA matrix brought on by the silane couplings. PLA composites reinforced by 30 wt.% alkali and silane treated hemp fibers exhibited the highest tensile strength (62 MPa), flexural strength (113 MPa), and Young’s modulus (7.6 GPa). Overall, the paper demonstrates the applicability of locally grown, frost-retted hemp fibers for the development of bio-based composites with low density (1.13 to 1.23 g cm−3).

Effect of Hemp Fiber Surface Treatment on the Moisture/Water Resistance and Reaction to Fire of Reinforced PLA Composites

“…For both pretreatments, hemp fibers showed a reduction in weight. The 4% weight reduction measured on the water pretreated hemp fibers is the result of the removal of water-soluble, fiber cell wall contents, as it was qualitatively characterized by Fourier transform infrared (FTIR) in a previous work [ 10 ] and also observed by Bourmoud et al [ 22 ]. The higher 14% drop in fiber mass due to alkali pretreatment is attributable to the extraction of significant non-cellulosic components (hemicelluloses, lignin, and some quantities of solubles (wax)) [ 10 ].…”

“…The 4% weight reduction measured on the water pretreated hemp fibers is the result of the removal of water-soluble, fiber cell wall contents, as it was qualitatively characterized by Fourier transform infrared (FTIR) in a previous work [ 10 ] and also observed by Bourmoud et al [ 22 ]. The higher 14% drop in fiber mass due to alkali pretreatment is attributable to the extraction of significant non-cellulosic components (hemicelluloses, lignin, and some quantities of solubles (wax)) [ 10 ]. Indeed, in a previous study, FTIR peaks attributed to conjugated carboxylic esters in hemicelluloses and wax, acetyl groups in lignin and conjugated carbonyl groups in lignin disappeared after alkali pretreatment [ 10 ].…”

“…The hemp fiber pretreatments (water, alkali) and modification with silane were carried out according to Alao et al [ 10 ] ( Figure 2 ). In all cases, the hemp fibers were oven-dried at 80 °C before and after treatment.…”

“…PLA is produced from annual bioresources, mainly through the fermentation of corn, and display a cost-effective production compared to other polymers developed from renewable resources such as soya oil-based epoxy, lignin and starch [ 9 ]. Hemp PLA composites have shown promising results [ 10 ] and can be regarded as appropriate materials for a vast range of applications and minimize the environmental threats associated with petroleum-based products.…”

“…According to Pejic et al [ 14 ] the alkali treatment of hemp fibers results in the gradual removal of hemicellulose and lignin that causes the rearrangement of fibrils and the formation of new capillary spaces between completely or partially separated fibers, which increases the roughness of the hemp fiber surface and enhances wetting with the polymer matrix. Additionally, the modification of the hemp fiber surface by alkali treatment brought improvements of the composite tensile and flexural properties [ 10 ]. Due to their elemental composition (carbon, hydrogen and oxygen) associated with cellulose, hemicelluloses and lignin main components, lignocellulosic fibers are flammable and can easily decompose in the event of a fire hazard [ 15 ].…”