Polymers 2023 , 15 , 1393
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as herbicides, fungicides and insecticides and other pollutants in the environments such as polycyclic aromatic hydrocarbons might be present in natural fibres. Special additives such as plasticisers, including phthalates, are also added in formulations of packaging materials. These unnatural substances might migrate into food during storage causing spoilage. Temperature, activation energy and microstructures of the packaging films can also restrict diffusion of unwanted substances into food [99,100]. Thus, it is necessary to decontaminate natural fibres during extraction before composite preparation. Finally, the biodegradability/compostability of the packaging materials is also of interest to consumers. Using full bioplastics as matrices and natural fibres produces eco-friendly packaging with low carbon footprints. Biodegradation rates of natural fibre composites depend on the nature of the fillers, reinforcements and the matrix as well as the composite ratios.
Table2. Properties of packaging materials.Reprinted with permission from Ref. [81] Copyright 2008 Elsevier Ltd.
Property
Examples
Tensile strength, tear properties, compression properties, bending stiffness, edge crush resistance, burst strength, puncture resistance, folding endurance, wet strength and delamination Oxygen permeability (OP), water vapour permeability (WVP), Volatile permeability and water absorption capacity
Structural properties
Barrier and absorption properties
Manufacturability and manufacturing quality
Uniformity of thickness, density and moisture content Toxicology parameters and migration studies Abrasion resistance and static and kinetic friction Compostability in biodegradation tests and disintegration tests
Migration into food
Non-structural functionality Degradability/compostability
3. Physical and Chemical Modifications of Fibres for Food Packaging Numerous studies have investigated natural fibre-reinforced polymers because of their improved strength and stiffness with the low cost, biodegradability, renewability and abundancy of natural fibres [101,102]. However, mixing natural fibre with a polymer matrix commonly causes poor mechanical composite properties because of (a) poor compatibility between the polar hydrophilic natural fibre and the non-polar hydrophobic polymer matrix and (b) non-homogeneous dispersion of fibre and wood powder in the PP matrix [103,104]. The modification of natural fibres can reduce their hydrophilicity and, thereby, increase their compatibility. The removal of unwanted wax and increasing surface roughness increases the contact surface area of the fibres, which in turn increases the transfer of stress uniformly into the matrix. Several chemical and physical treatments to improve the compatibility of natural fibres are as follows. 3.1. Chemical Modification Techniques Composites reinforced with chemically treated natural fibres generally show enhanced mechanical properties because of improved interfacial adhesion between the fibre and the matrix [82]. Numerous surface treatment methods are available for the modification of natural fibres. Alkaline (NaOH) treatment/mercerisation is the simplest, most effective and commonly used chemical treatment. Traditionally, mercerisation is a technique to modify surface of cotton. Strong caustic soda solutions are used to treat materials for 1–3 min under tension and low temperatures, followed by washing. Holding cotton fabric under tension in the caustic solution helps to maintain its original dimensions. As a result, the fibres have more a rounded structure in the cross section, reflecting light to improve lustre. Mercerised cotton also involves a change in crystalline structure and degree of crystallinity, thereby reducing stresses and increasing the strength of the weak points in the fibre [105].
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