Advanced natural fibre composites for civil engineering and other applications

Long-term durability and aging studies would also be beneficial for comprehending the potential of these ZnO NPs-reinforced natural fibre composites to withstand various environmental stressors, such as UV exposure, humidity, and thermal cycling.

Material scientists are focusing on creating polymer matrix composites with natural or synthetic fibres and fillers for their strength, durability, and lightweight nature in various applications. Notably, in recent years, there has been a surge in the development of environmentally friendly composite materials, with natural fibres being a popular choice owing to their low weight, cost, improved mechanical properties, and biodegradability. Despite the various benefits associated with natural fibre-reinforced composites, they have certain drawbacks including inadequate interfacial adhesion, limited moisture resistance, dimensional and thermal instability, poor durability, and poor mechanical properties. Consequently, these inherent drawbacks impede the widespread use of natural fibre-reinforced composites in high-performance and enduring applications.

Hence, researchers have developed multiple methods to improve the properties of natural-fibre-reinforced composites for diverse applications. One approach involves reinforcement of polymers with organic and inorganic fillers to enhance the toughness of the matrix material. Another method focuses on chemically modifying fibres to improve the fibre-matrix interface using alkali treatment, coupling agents, and nano particles in surface treatment. Among these methods, the incorporation of nano fillers through advanced manufacturing techniques has significantly improved natural fibre-reinforced composite materials by modifying the interaction between the fibre and matrix.

Nano fillers have the potential to improve the mechanical and functional performances of composites, thereby expanding their applications in various fields. In the realm of nano-based natural fibre-reinforced composites, a larger aspect ratio of nano fillers provides better reinforcement, leading researchers to become more interested in this material. These nano fillers are inorganic or organic materials that are used in the production of composites. They can be inorganic, such as alumina, magnesia, silica, zinc oxide, titanium dioxide, and calcium carbonate, or naturally organic, such as carbon black and cellulosic fibres. The incorporation of nano fillers results in the reduction of available free spaces, increases the stiffness of the laminates, and facilitates bonding between the matrix and fibres, thereby promoting improved interaction. Natural fibre-reinforced composites (NFRCs) containing nano particles (NPs) are characterised by their eco-friendly nature, reduced water absorption, and enhanced mechanical properties. They are used in the construction, transportation, aerospace, and other consumer products. Furthermore, incorporating specific nano materials into NFRCs can impart additional functionalities such as antibacterial, anti-odor, UV protection, and highly hydrophobic properties.

Zinc oxide nano particles (ZnO NPs), along with other types of nano particles, play a significant role in enhancing the properties of the composites. The incorporation of ZnO NPs in composites has been found to improve composite properties, including barrier, mechanical, and antimicrobial activities. They have been widely studied for their versatile applications, offering advantages such as antibacterial activity, piezoelectricity, corrosion resistance, thermal conductivity, and their potential as structural super capacitors. Despite the existing body of literature on composite materials, a notable gap remains in the literature concerning the effects of incorporating ZnO nano fillers into NF-based composites. Given the growing demand for natural-fibre-reinforced composites with enhanced properties, this gap represents a crucial area for further investigation. Therefore, the author is motivated to address this research gap and to present a comprehensive review based on significant findings in the field.

Natural fibres in composite materials: Properties and issues

Composition and properties of natural fibre

Natural fibres are extracted from various renewable resources, including animals, vegetable plants, and minerals. From a composite standpoint, plant fibres, such as basts, leaves, and fruits exhibit the most desirable properties for bio composites. Plant fibres typically include cellulose (60–80 per cent), hemicellulose (20–30 per cent), and lignin (5–20 per cent), with the remainder consisting of wax, pectin, moisture, and water-soluble organic components. The chemical composition of natural fibres, including their crystallinity, microfibrillar angle, defects, and physical properties, significantly influence their performance. These fibres are susceptible to biological, chemical, mechanical, thermal, photochemical, and aqueous degradation, depending on their constituents.

Physical properties of plant fibres: Plant fibres, including leaves, fruits, stems, grasses, and wood, have physical properties, such as dimensions, density, moisture absorption, and physical structure. They are suitable as fillers in engineering and non-engineering applications because of their lightweight nature, compatibility, porous structure, and affordability. However, as the fibre type varied, the cellulose content and crystallinity also changed.

Mechanical properties of plant fibre: The natural fibre structure and dimensions, including the density and microfibril angle, directly affect the mechanical properties of NFRCs, which depend on the inherent properties of natural fibres. Figure 1a illustrates that high-density fibres frequently exhibit higher strength and stiffness than low-density fibres do. Similarly, the elastic modulus of natural fibres strongly depends on the micro fibrillar angle (MFA), with a low MFA resulting in stiffer fibres (Figure 1 (b,c)). This is because a low MFA enables cellulose fibrils to be almost parallel to the loading axis and can support more load, thereby increasing the stiffness. However, owing to their high stiffness, these low-MFA fibres typically exhibit brittle behavior. However, natural fibres with a high MFA typically exhibit a large plastic deformation and high toughness. The diameter of the fibres and the level of cellulose polymerisation are additional factors that affect the mechanical characteristics of the natural fibres. The chemical constituents and physical and mechanical properties of plant-based natural fibres have been summarised.

Figure 1: Mechanical properties of plant fibre

Polymer/matrix materials

A matrix connects the reinforcing components in a composite, and plant fibres are combined with a polymeric matrix to create eco-friendly composites. The matrix holds the fibres in place and prevents cracks and damage. For polymeric matrices, thermosets or thermoplastics may be used because of their advantageous properties such as low density, low electrical and thermal conductivity, and good corrosion resistance. However, fillers have also been added to improve the performance (mechanical characteristics) of polymers and reduce the cost of materials. The properties of most commonly used polymeric matrices have been determined.

Thermoplastics, such as polyethylene, polypropylene, and polyvinyl chloride, are commonly utilised as matrices for natural fibre composites because of their low melting temperatures, whereas thermosets, including phenolic, polyester, and epoxy resins, are most frequently used in NFRCs. When heated, thermoplastics soften and can be remolded without significant degradation into a variety of products. Therefore, recyclability was the most important characteristic. However, thermoset matrices cannot be recycled or their shape cannot be changed once polymerisation (curing) is completed. The main thermoset resins used for natural fibre composites are epoxy and unsaturated polyesters because of their good interactions and better wettability with natural fibres, which attain good strength during the application of the developed composite material. Epoxy resin, a polymeric matrix, offers chemical, corrosive, and strong adhesive properties. However, they exhibit brittleness, shrinkage, and low-impact properties, making them unsuitable for high-performance aircrafts and automobiles. The demand for particulate fillers and polymer materials for industrial and structural applications has increased. Therefore, researchers are exploring nano sized fillers such as ZnO NPs, carbon black, clay, and nano fibres to expand their use in high-tech fields. Nano composites of polymer matrices reinforced with semiconductor nano particles are gaining interest because of their improved optical and electrical properties. Zinc oxide nanostructures are becoming popular in scientific and industrial fields because of their physical properties, particularly their electrical, optical, and piezoelectric properties.

Natural fibre reinforced polymeric composites (NFRPCs)

Fibre-reinforced polymer composites (FRPC) are lightweight, strong materials with enhanced mechanical properties. NFRCs are gaining popularity owing to their low environmental impact and low cost. Plant fibres are a promising research topic because of their low price, low carbon emissions, abundant availability, low density, bio-renewable traits, and low energy consumption. They are used in construction, aerospace, ballistics, wind energy, and automotive. However, natural fibres have limitations, such as low thermal stability, high flammability, high moisture absorption, and variation in mechanical properties. To overcome these limitations, studies have focused on combining natural fibres with other fillers such as nanofillers. The choice of nanofiller depends on the intended application to overcome the shortcomings of natural fibres. Akter et al. highlighted the growing trend of polymer composites reinforced with natural fibres using keywords such as natural fibres, plant fibres, reinforced polymers, and composites, as shown in Figure 2.

Figure 2: Growing trend of polymer composites reinforced with natural fibres 

Strategies to enhance NFRPCs properties

Natural fibres, which are hydrophilic, have poor interfacial interactions with hydrophobic polymeric materials, limiting the stress transfer between composite components. Chemical and physical modifications have been explored to improve interfacial adhesion and overall composite properties. The treatment of natural fibres can improve the biodegradation stability and mechanical performance. However, these methods do not always yield the desired performance, thermal stability, or barrier resistance. Incorporating nanofillers into NFRCs can eliminate these drawbacks. Combining organic natural fibres with inorganic or organic polymers and nano particles has the potential to improve the mechanical performance and expand applications. Researchers have found that adding nanofillers to a composite matrix enhances fibre-matrix interfacial interactions, improves matrix toughening properties, and reduces weight simultaneously.

Nano-based natural fibre reinforced polymeric composites

Composites often contain natural or artificial fillers in various forms such as particles, fragments, fibres, sheets, and whiskers. Nano particles or nano fillers have the potential to improve the thermal and mechanical properties of polymer composites, flame retardancy, and reduce moisture adsorption. They also offer exclusive flexible functionalities and better mechanical strengths than pure materials. Researchers are increasingly using nano materials in composite preparation owing to their large relative surface area and quantum effect, which enhance material properties, such as chemical reactivity, heat resistance, strength, and optical, electrical, and magnetic behaviors. Nano materials can exist in single- or multi-stage forms, with single-phase materials typically having at least one of the dimensions smaller than 100 nm. Multiphase nano composites can be engineered by adding nano materials to the matrix, forming the basis of these composites.

Nano particle-embedded natural-fibre-reinforced composites perform well at high temperatures without altering the processing conditions or melting temperatures. Some thermoset polymers become fragile owing to crystallisation; however, this issue can be eliminated by adding bio fibres and nano fillers (nano TiO₂, SiO₂, carbon nano tubes, ZnO, and graphene oxides). Adding a nano filler to the polymer matrix enhances the composite density and hardness of NFRCs. Natural fibres have better specific properties and are lighter than synthetic fibres, which, when combined with another reinforcing agent (nano filler), enhances the performance of nano-bio composites. Various nano particle applications are currently being investigated for their use as fillers in natural fibre-reinforced polymer composites. ZnO NPs are an interesting candidate due to their large surface area, non-toxicity, availability, low cost, stability, high ultraviolet absorption capacity, and strong antimicrobial activity. This review is necessary because of the extensive use of ZnO NPs fillers in NFRCs by numerous researchers.

Why ZnO nanoparticles?

Metal-oxide nano composites are of special interest because of their distinctive thermal, mechanical, optical, magnetic, electronic, and catalytic properties. Zinc oxide (ZnO), one of the many different types of metal oxide semiconductors, has attracted interest because of its exceptional qualities, such as high optical and thermal stability, low toxicity, excellent physical and chemical stability, and widespread availability. ZnO is an inorganic substance that is insoluble in water and is typically found as a white powder. It is used as an additive in a variety of substances and goods such as lubricants, cosmetics, plastics, rubbers, ointments, food supplements, paper, pigments, batteries, ceramics, fire retardants, first-aid tapes, and cement.

ZnO is a binary compound with a wide bandgap (3.37 eV) and wurtzite structure. Because of their piezoelectric properties, they have potential applications in sensors and micromechanical systems. ZnO NPs have also been extensively studied for their applications in biomaterials, medicine, wastewater treatment, and electronics. A study on ZnO nano material biocompatibility, antimicrobial performance, and multiphase morphology of bio mimetic nano composites materials with ZnO/sodium alginate/hydroxyapatite-oriented granules and ZnO/hydroxyapatite hydrogels demonstrated the Zn+ behavior of different composite materials.

The formation reaction of ZnO was described by the following authors:

Z⁢n2++ 2⁢O⁢H−→ 2⁢O⁢H2−4and Zn (OH)2−4→ ZnO + 2⁢H2⁢O + 2⁢H⁢O−(1)

In a previous study, the average particle size of ZnO ranged from 12 nm to 30 nm. The distribution of the nanoparticle was homogeneous, along with a very good compatibility in terms of the thermal property.

Applications and future prospects

NFRCs with enhanced properties incorporating ZnO NPs as fillers can be utilised in various industries, offering innovative and sustainable solutions. The benefits of incorporating ZnO NPs as matrix fillers in natural-fibre-reinforced composites have been summarised. Devaraju et al. studied the tensile, impact, and flexural properties of palm fibre composites and found them to be harmless to the environment, low-cost, and easily available, making them a potential wood substitute for indoor applications and automobile components. The study examines nano-ZnO content’s impact on moisture absorption and dimensional stability in composites made from wood flour and polypropylene with a coupling agent, potentially serving as value-added materials. Torres et al. studied the mechanical properties and fracture behavior of agave fibre bio-based epoxy laminates reinforced with zinc oxide. The results highlight the need to continue evaluating the potential applications of these green composites in the construction and automotive industries.

Murshid et al. reported the effect of ZnO NPs alone and in combination with nanoclay reinforced with jute fabric and glutaraldehyde-crosslinked soy flour “‘green’” nano composites. Consequently, the ZnO NPs and nano clay-filled SF/J composites are eco-friendly and can be used for applications in new fields. Areca fibre-reinforced epoxy nano composites exhibit exceptional adhesion, excellent mechanical properties, resistance to water and antimicrobial activity, and are suitable for various engineering applications as protective polymer materials, enhancing performance and durability. Numerous researchers have investigated the use of ZnO NPs as matrix fillers in food packaging applications to reduce the migration of fibres/packaging materials. The use of ZnO NPs in NFRCs presents promising opportunities for improved mechanical, thermal, antimicrobial, and sustainable properties, thereby enhancing durability and environmental sustainability. A compelling review conducted by Dejene explored the application of ZnO NPs for the surface treatment of natural fibres prior to composite manufacturing, and revealed interesting results on the mechanical, thermal, and water resistance properties of their composites. Therefore, there is a clear need for further investigation to evaluate the effectiveness of utilising ZnO NPs for fibre treatment compared to their incorporation as fillers in the matrix for creating NFRCs. Furthermore, the incorporation of ZnO NPs as filler, combined with cellulosic fibreas sourced from agro-waste, in the naturally extracted thermoplastic starch matrix, is recommended for active packaging applications, as proposed by Dejene and Geletaw.

Conclusion

The use of ZnO NPs as fillers in NFRCs can potentially improve their mechanical strength, water repellence, thermal stability, and antimicrobial properties. This could lead to new applications in automotive, construction, and packaging industries. Uniform dispersion of nanoparticles is crucial for quality composite fabrication, and surface modification can reduce the nanoparticle surface energy and prevent agglomeration. This uniform distribution provides stiffness to the matrix material and enhances the mechanical and thermal properties of composites. However, further research is needed to optimise the dispersion and alignment of ZnO NPs, investigate their long-term durability, and assess their environmental impacts. Overall, this approach contributes to the development of high-performance, sustainable materials with improved properties. In the future, it may be possible to work on hybridisation by incorporating several types of NPs and analysing the combined benefits of such mixing. Long-term durability and aging studies would also be beneficial for comprehending the potential of these ZnO NPs-reinforced natural fibre composites to withstand various environmental stressors, such as UV exposure, humidity, and thermal cycling. From a sustainability standpoint, additional research is necessary to investigate the biodegradability and recyclability of these composite materials as well as their potential for circular economy applications. The development of scalable manufacturing processes to produce larger quantities is also an important step towards commercialisation. Finally, application-specific studies and computational modeling can provide in-depth insights into the structure-property relationships and failure mechanisms, ultimately guiding the design of ZnO NPs-reinforced natural fibre composites for specific end-use requirements.

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About the author:

• Dr KGopalakrishnan and T Jayapraba are from the Department of Physics, S&H, SSM College of Engineering, Komarapalayam, Tamil Nadu.
• Sona M Anton, R Sasirekha and Z Shahanaz Department of Fashion Design, Hindustan Institute of Technology and Science, Chennai, Tamil Nadu.
• Dr G K Manikandan and K KHema are from the Department of Mechanical Engineering, SSM College of Engineering, Komarapalayam, Tamil Nadu.
• M Ezhilarasi and M Senthilraja are from the Department of Civil Engineering, KSR Institute of Technology, Thiruchengode, Tamil Nadu.
• Dr N Gokarneshan are from the Department of Textile Chemistry, SSM College of Engineering, Komarapalayam, Tamil Nadu.
• L Reena is from the Department of Civil Engineering Builders Engineering College

Kangeyam, Tirupur, Tamil Nadu

• M Manoj Prabagar is from the Department of Costume Design and Fashion Vivekananda College of Arts and Science Tiruchengode, Tamil Nadu