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Palanisamy, S., Murugesan, T. M., Palaniappan, M., Santulli, C., Ayrilmis, N., and Alavudeen, A. (2024). “Selection and processing of natural fibers and nanocellulose for biocomposite applications: A brief review,” BioResources 19(1), 1789-1813.

Abstract

In this study the recent developments in raw materials, manufacturing processes, and applications of natural fiber composites (NFCs) were reviewed. Natural fibers can represent a substitute for man-made fibers (including glass, aramid, and carbon) in a variety of biocomposite applications. Physical and chemical properties of the natural fibers are given and compared with the synthetic fibers. Advantages and disadvantages of NFCs in comparison with synthetic fibers such as glass and carbon fibers have been proposed. Criteria are described for the selection and processing of natural fibers for polymer composites used in different sectors such as automotive and building industries. The nanocellulose production methods, unique properties, and its recent industrial application in various sectors are given. This short review on NFCs considers their chemical, physical, and mechanical characteristics, as well as their various applications.


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Selection and Processing of Natural Fibers and Nanocellulose for Biocomposite Applications: A Brief Review

Sivasubramanian Palanisamy,a,* Thulasi Mani Murugesan,b Murugesan Palaniappan,c Carlo Santulli,d Nadir Ayrilmis,e and Azeez Alavudeen f

In this study the recent developments in raw materials, manufacturing processes, and applications of natural fiber composites (NFCs) were reviewed. Natural fibers can represent a substitute for man-made fibers (including glass, aramid, and carbon) in a variety of biocomposite applications. Physical and chemical properties of the natural fibers are given and compared with the synthetic fibers. Advantages and disadvantages of NFCs in comparison with synthetic fibers such as glass and carbon fibers have been proposed. Criteria are described for the selection and processing of natural fibers for polymer composites used in different sectors such as automotive and building industries. The nanocellulose production methods, unique properties, and its recent industrial application in various sectors are given. This short review on NFCs considers their chemical, physical, and mechanical characteristics, as well as their various applications.

DOI: 10.15376/biores.19.1.Palanisamy

Keywords: Natural fiber composites (NFCs); Density; Chemical properties; Mechanical properties; Applications

Contact information: a: Department of Mechanical Engineering, P T R College of Engineering & Technology, Thanapandian Nagar, Madurai – Tirumangalam Road, Madurai, 625008, Tamilnadu, India; b: Department of Mechanical and Aerospace Engineering, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada; c: Department of Mechanical Engineering, College of Engineering, Imam Mohammad Ibn Saud Islamic University, Riyadh 11432, Kingdom of Saudi Arabia; d: School of Science and Technology, Università di Camerino, Camerino, Italy; e: Department of Wood Mechanics and Technology, Forestry Faculty, Istanbul University-Cerrahpasa, Bahcekoy, Sariyer, 34473, Istanbul, Turkey; f: Department of Mechanical Engineering, Kalasalingam Academy of Research and Education, Krishnankovil, Srivilliputhur, Virudhunagar, Tamilnadu. India;

* Corresponding author: sivaresearch948@gmail.com

INTRODUCTION

In the last decades, natural fibers have gradually been proposed as a potential alternative to man-made fibers in polymer composites in a wide range of engineering applications. These initiatives are motivated by their low costs and acceptable mechanical properties, sustainable and abundant natures, and lessening of the carbon footprint (Wambua et al. 2003; Alhijazi et al. 2020). The goal of making natural fiber composites (NFCs) with limited environmental impact and tending to carbon dioxide neutrality has been gaining attention, as regards to both the use of thermosets and thermoplastics (Mohanty et al. 2002). From these early studies, the growth in the market of natural fibers aimed at the biocomposite sector has been steadily increasing throughout the world. It can be easily suggested that in any region in the world, there will be one or more natural fibers having potential to be exploited in composites. According to some recent studies, the output of natural fiber composites manufacturers was expected to increase by 10% to 400% globally within the span of several years (Uddin 2013; Akter et al. 2022). Further developments specific to their application in composites depend on the tailoring of hybridization processes between carbon, glass, aramid, and natural fibers, and the use of different, more focused treatments, even moving away from the technologies that have become established in textile products (Khalid et al. 2021). Geographic distribution of the commercially important fibers for biocomposite manufacture is presented in Fig. 1. China is the leading country in the production of plant fibers followed by Bangladesh, France, India, and others. Flax, hemp, kenaf, sisal, jute, ramie, and abaca fibers are the most abundant fibers all over the world.

  a)                                                                                     b)

Fig. 1. a) Geographic distribution of the commercial fibers for biocomposite manufacture. b) World production of plant fibers (excluding cotton) by country. Reproduced with permission from Gardner Business Media, Inc. Source: https://www.compositesworld.com/articles/natural-fiber-composites-whats-holding-them-back

Integrating, in a sound and durable way, tough and lightweight natural fibers into polymeric materials can produce composites having high specific strength, both in the case of thermoplastics and thermosets. Table 1 lists some natural fibers that are both widely used and commercially viable worldwide, and the total amount of fiber produced globally. The use of eco-friendly composites in the production of automobile parts is also becoming increasingly widespread (Dahlke et al. 1998).

Table 1. The World’s Natural Fibers and Their Global Production

* Data from: (Faruk et al. 2012; Sathishkumar et al. 2022)

This review concentrates on the processing techniques for NFCs and on their mechanical properties and prospective applications. The materials considered are obtained with different fibers and using industrial (not purposely synthesized) matrices, both oil-based and bio-based. This brief review generally does not consider the question of natural fiber selection, which would involve their full characterization and considerations on their market/availability. However, some indications on the chemical composition of the relevant natural fibers (lignin, cellulose, hemicellulose content) are offered as a means to give some suggestions on their more or less hydrophilic/hydrophobic character. The classification of natural fibers, their physical and mechanical properties, NFC manufacturing methods, and recent developments are given. Moreover, the significance of nanocellulose, its classification, its unique properties, and some recent industrial applications of nanocellulose in biocomposites are reviewed.

Natural Fibers

Natural fibers can originate from plants, animals, or minerals (Célino et al. 2014). Notable among animal fibers are protein fibers, such as wool, silk, and even human hair (Suman et al. 2016). Figure 2 shows some examples of fiber classification. It is possible to classify some plant-based fibers according to their origin: stalk, grass, wood, leaf, fruit, bast, and seed. In plant fibers, the prevalently crystalline cellulose is held together in cell walls exploiting the random orientation of hemicellulose and lignin (Fig. 2).

Fig. 2. Fiber Classification. (Source: Ahmad et al. 2019. Advances in Civil Engineering, article ID: 5185806, Creative Commons BY 4.0)

In natural fibers, the structural assembly formed by cellulose often contains waxy substances. The natural matrix material can include hemicellulose or pectin, or mixtures of organic polymers, naturally synthesized from phenol-propane units, giving rise to lignin (Chen 2014). The coverage provided by amorphous lignin adds strength to the network of hemicellulose and cellulose, and is likely to create a protective layer within the fibrous structure (Mohanty et al. 2005). Hemicellulose is a cementing element in the cell wall, which forms a matrix embedding the microfibrils of crystalline cellulose (Fig. 3). The secondary walls of the crystalline cellulose microfibrils, constituted by three layers, of which the middle one is the thickest, as effectively described e.g., by Rahman et al. (2015), determine the mechanical performance of the fiber. In the case the crystalline fraction of cellulose is sufficiently high, the fiber may offer excellent tensile strength, in a micrometric or even nanometric form (Ibrahim et al. 2010). This arrangement has been found to have a positive impact, e.g., on the mechanical characteristics of polypropylene after surface modification (Joonobi et al. 2010), which appears to be accountable for the effective bonding and reduced proneness to degradation of natural fibers in composites than other layers, which is the approximately 80% of total thickness and the main load bearing layer (Kalia et al. 2014).

Fig. 3. Representation of structures of cellulose, lignin, and hemicellulose. (Source: Haq et al. 2021, Catalyst 11, article 309, Creative Commons BY 4.0)

The strength of wood fibre is determined by the amount of and orientation degree of microfibrils in these layers, as well as their polymerization degree and crystallization. Thus, the orientation degree of microfibrils at S2 layer determines the physical and mechanical properties of the wood fibre (Fig. 4). The modulus of the fibre improves with decreasing microfibril angle. In general, fibers with a high cellulose content and lower microfibril angle possess a good strength with a lower elongation at break.

Fig. 4. Structure of wood cell of lignocellulosic plant fiber. (Source: Zhang et al. 2022, Forests 13, 439. Creative Commons BY 4.0)

Among the natural fibers, flax, jute, hemp, sisal, kenaf, oil and date palm, coir, banana, abaca, areca nut, and cotton are the most important plant fibers from a commercial and technological standpoint. In particular, three natural fibers used in composites from the early period of experimentation are hemp, jute, and flax, though over time the field of exploration has extended to a very large number of species. Jute is a fiber with important mechanical qualities for use in ropes and sacks, grown in Bangladesh, India, Thailand, and some Latin American countries (Mwaikambo 2006). Flax is Europe’s most essential bast fiber, though some cultivars of it are also cropped in China (Wang et al. 2018). Some of the commercial natural fibers are presented in Figure 5.

Fig. 5. Some industrially used plant fibers. (Source: Nurazzi et al. 2021, Polymers 13, 646. Creative Commons BY 4.0)

A comparison of the natural fibers and man-made fibers in terms of cost and per weight ($/kg) is displayed Fig. 6. The man-made fibers, especially carbon fibers, are quite expensive in comparison to natural fibers. Furthermore, the carbon footprint of the natural fibers (4 GJ/ton) is greatly lower than synthetic fibers such as glass (30 GJ/ton) and carbon fibers (130 GJ/ton) due to the fact that lower energy requirement and lower CO2 emission energy as compared to the synthetic fibers (Maiti et al. 2022).

Fig. 6. Comparison of natural fibers and man-made fibers in terms of cost/weight. (Source: Ahmad et al. 2015, Macromolecular Materials and Engineering 300, 10-24. Creative Commons BY 4.0)

A complex mix of agronomical factors, resulting in chemical, physical, and mechanical characteristics of the fibers, is encountered when dealing with the performance of plant fibers. The species and cultivars from which fibers are extracted, their harvesting time, and their chemical composition and cellulose crystallinity are obviously determinant for the characteristics of the composite. Other inherent characteristics of the fibers are their microfibrillar angle, i.e., the angle within the dominant S2 sublayer by which the fibrils are wound together to form the technical (extractable) fiber, the dimension of lumens and porosity, and the cell length and diameter, which influence the relevant fiber’s aspect ratio (Ku et al. 2011; Thakur and Thakur 2014). All the aforementioned characteristics are able to influence the fibers’ mechanical characteristics. To offer some elements for comparison, the mechanical properties of a number of natural fibers are compared with E-glass fibers in Table 2. When passing to the composite, fiber extraction, matrix selection, interfacial strength, fiber dispersion, composite production process, orientation of fiber, and porosity are the primary elements impacting mechanical performance (Pickering et al. 2016). Also, the modulus of the natural fiber has been reported to diminish as its diameter increases (Van den Oever et al. 2000). This can be attributed to the possible separation of fibrils, normally referred to as “fibrillation” and to the subsequent higher percent of porosity within the fibers, which is typical of cellulose, especially when stiffened by applying chemical treatments (Sharma et al. 2015). Due to their chemical composition, natural fibers have however a most unfavorable property for their use, which is hygroscopicity. Absorbing moisture has negative consequences on their characteristics and, as a result, their performance over a long service term (Wang et al. 2006). According to Table 2, among the commercial natural fibers, the flax fiber has the highest tensile strength, which makes it attractive for polymer composites used in automotive industry, followed by hemp, ramie, sisal, and jute fibers.

Table 2. Mechanical Characteristics of Some Fibers*

  1. * Adapted from Pickering et al. (2016).
  2. Manufacturing Techniques of NFCs

Depending on the geometry of the component to be manufactured, fiber-reinforced plastics can be obtained in various ways, all of which are based on the polymerization concept. Table 3 provides a list of works on natural fiber composites obtained with different manufacturing methods and processes.

Table 3. Different Types of Manufacturing Methods and Processes of Composites

Hand Layup

This method of moulding involves placing fiber reinforcements by hand and pouring polymer resin on top of them. A secondary layer of fiber reinforcements is placed on the top of the polymer matrix surface, where a slightly pressurized roller is moved on the reinforcement fibers to avoid air penetrating among the layers and remaining among them (Summerscales et al. 2013; Fiore et al. 2015). After that, the process is repeated for each polymeric matrix and fiber layer until all necessary layers are stacked, and cure can be allowed for the time necessary. This method is best suited for smaller batches of composites. The viscosity of a resin must be low enough for it to be worked manually due to high diluent/styrene levels; this necessary step decreases somewhat the mechanical characteristics though. When hand layup is used on one side only, this makes a smooth, high-quality finish. In addition, it has a greater degree of material design flexibility, though requiring a longer cycle for curing, up to 24 to 48 h (Mishra and Biswas 2013; Ramesh et al. 2014).

Spray Layup

Similar to hand layup, spray layup is a hand moulding technique that extends the hand layup method. This method has gained considerable attention also on natural fiber composites (Srinivas et al. 2017). In this procedure, spraying pressurized resin and reinforcement into a chopped fibers’ geometry is accomplished by using a spray gun. Matrix material, as well as reinforcement, could be sprayed at the same time or at different moments in time, in a sequential manner. A roller is passed over the sprayed surface with a small amount of strain to remove any air that may have been trapped in the layups. Immediately after spraying up to the desired thickness, the material is allowed to cure at room temperature for some time before being removed from the mould (Ku et al. 2011). Low viscosity matrices are preferably used with this method, which might affect their mechanical properties, so that this manufacturing process is best suited for low volume production to offer a low cost composite with good surface finish on only one side (Omrani et al. 2016).

Filament Winding

Filament winding is primarily used to generate closed and open-end structures, depending on the application. In this system, the composite is obtained by winding tended filaments on a mandrel rotating around a spindle, while a shipping eye is carried in axis with the rotating mandrel, laying down fibers as per request. Convex shapes can be created using this method. Low viscosity resins are typically preferred (Belaadi et al. 2014). In natural fiber composites, tailored types of epoxies have been used (Lehtiniemi et al. 2011).

Compression Molding

When it comes to thermoplastic matrices, compression molding is most commonly used with unfastened chopped fiber, short or long fiber mats, which are either highly irregular or aligned, though it might be also applied in the case of thermosetting matrices. Before applying stress and heat, the fibers are typically stacked variously with thermoplastic resin sheets to create a more substantial structure. The molding material is usually preheated, then placed in an open hollow space, which is more substantially heated (Rong et al. 2001; de Andrade Silva et al. 2008) before being pressed into the mold. An example of hot-press molding process for automotive doors is presented in Fig. 7.

Fig. 7. Hot press molding of non-woven hemp-pp composites used in manufacutre of the door of the car. (Source: https://www.globalhemp.com/blog/automotive-composites/). Used with permission from Global Hemp Company

Injection Molding

In injection molding, warming bands, and the frictional motion of an oscillating screw barrel combine to melt the material in a heated barrel. Plastic is then introduced into the mold cavity through an injection nozzle, where it hardens during cooling down phase to conform to the geometry of the cavity’s internal structure. When the parts are solidified, the plate is opened, and the component is thrown out using executor pins. The mildew tool is installed on a movable container. This procedure results in a smooth surface finish and is also appropriate for higher volume applications. The tensile strength of this process is slower than that of most thermoset systems (Zou et al. 2011).

Fig. 8. Injection molded biocomposite. (Source: https://www.arburg.com/en/solutions/by-technology/special-injection-moulding-processes/). Used with permission from Arburg Company Press

Density

The maximum theoretical density for natural lignocellulosic fiber is related to that of pure microcrystalline cellulose, which is just below 1.6 g/cm3 (Sun 2005). Of course, natural fibers always have a lower density than that because they contain not only cellulose, but they have significant levels of porosity. Even lower is the density of natural fiber composites, where the matrices are normally lighter than the fibers. To provide examples, Table 4 shows the density of natural fiber enhanced polymer composites (NFRPs).

Table 4. Different Natural Fibers and their Densities

Chemical Characteristics of Natural Fibers

The chemical characteristics of natural fibers depend on the respective amounts of cellulose, hemicellulose, and lignin. Beyond that, the amount of crystallinity also affects the structural potential of the fibers. Pectin and waxes generally are present often in negligible quantities in ordinary fiber, whereas remaining ash does not normally exceed a few percent. Fibers defined as lignocellulosic and used as such in natural fiber composites contain less than 50% lignin, in the latter case being defined as “wood”. A selected number of studies that report the values of cellulose, hemicellulose, and lignin are reported in Tables 5, 6 and 7, respectively.

Table 5. Cellulose Content of Some Natural Fibers