Polymers 2023 , 15 , 1393
9of 21
Immersion in NaOH aqueous solution removes hemicellulose, lignin, pectin and other impurities from the fibre surface. This results in a rougher surface, which in turn improves the mechanical interlocking of the fibres with the matrix. Borah et al. [106] found that alkali treatment of betel nut fibres before composite formation improved tension strength by 18%, elongation at break by 6%, bending strength by 11% and impact strength by 18%. The reaction between the fibre and alkali solution can be represented by the equation below [107]. Fibre − OH + NaOH → Fibre − O − Na + + H 2 O The utilisation of oxidising agents such as hydrogen peroxide (H 2 O 2 ) in natural fibres can also eliminate the cementing substances from surface of the fibres, which hinder adhesion with the polymer matrix. For natural fibres, alkali treatment results in the formation of an alkali-resistant linkage between lignin and hemicellulose that may impede the removal of lignin. Using H 2 O 2 breaks these bonds and delignifies lignocellulosic fibre, which enhances the interfacial adhesion with the polymer matrices [108,109]. The acetylation of fibre also improves hydrophobicity, which enhances interfacial adhesion by reducing the moisture absorption of the cellulose components. This includes treatments of acetic or propionic acid at elevated temperatures with or without the combination of an acid catalyst [110]. Acetylation of the cellulose components substitutes hydroxyl groups of the cell wall, which increases the hydrophobicity of the natural fibres. This method improves compatibility with the polymer matrix by decreasing water absorption. Finally, coupling agents also reduce inherent incompatibility between the polymer matrix and natural fibres, which enhances the interfacial adhesion. Polymers consist of bifunctional groups which effectively react with both the fibre and the matrix. Organofunc- tional silane coupling agents form covalent bonds with the hydroxyl groups of cellulose. Alkoxy groups are hydrolysable. Moisture facilitates hydrolysis and forms silanols which further react with the hydroxyl groups of the fibre. Consequently, stable covalent bonds are formed with the cell wall that are chemisorbed onto the fibre surface [110–112]. This chemisorption commonly improves the degree of cross linking at interface, which improves affinity of organophilic polymers [110]. Adding hydrocarbon chains by the modification of natural fibre with silanes modifies their wettability and reduces water uptake as covalent bonding forms cross-linking between the fibre and the matrix. However, the use of alkaline or any other chemical treatments adversely impacts product sustainability, with green techniques preferred for natural fibre modification. Smith et al. [113] prepared a sustainable green composite based on agave fibre ( Agave tequilana ) modified with poly(3-hydroxybutyrate) (PHB) in the presence of a small quantity (0.1 phr) of organic peroxide through one-step reactive extrusion processing. Results showed that 25 wt% agave fibre with 0.1 phr peroxide improved flexural strength by 46%, impact strength by 45% and heat deflection temperature (HDT) by 39% compared with neat PHB. These findings suggested that the presence of peroxide provides a cost-effective and sustainable alternative to petroleum-based conventional plastics for food packaging. Mohanty et al. [114] chemically modified date palm leaf (DPL) using acrylic acid and tested the dispersion and compatibility with polyvinylpyrrolidone composites for packaging applications. Prepared biocomposites reinforced with 26 wt% DPL fibre loading showed promise for use as water- and chemical-resistant hydrophobic packaging materials. Nazrin et al. [115] studied the incorporation of nanocellulose to enhance the properties of thermoplastic starch (TPS), polylactic acid (PLA) and polybutylene succinate (PBS) for food packaging. They reported that the addition of nanocellulose in TPS improved the low water barrier and tensile properties, while the addition of nanocellulose into PBS and PLA enhanced the oxygen barrier properties and mechanical strength. Natural jute fibre incorporated with a red grape pomace extract (RGPE) has been developed for active packaging. The RGPE was derived from pressurised liquid extraction (PLE) and enhanced solvent extraction (ESE) techniques [116]. The packaging showed excellent antibacterial activities against E. coli , S. aureus and Pseudomonas aeruginosa . The RGPE extract from PLE using C 2 H 5 OH:H 2 O (as a solvent) had 11 major phenolic com-
Made with FlippingBook Digital Proposal Creator