Sustainability in Natural-Fiber-Reinforced Polymers

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This entry is adapted from the peer-reviewed paper 10.3390/su16031223

Fiber-reinforced polymer composites (FRCs) from renewable and biodegradable fiber and sustainable polymer resins have gained substantial attention for their potential to mitigate environmental impacts. The limitations of these composites become evident when considered in the context of high-performance engineering applications, where synthetic fiber composites like glass or carbon FRCs typically dominate. A balance between the performance of the composite and biodegradability is imperative in the pursuit of what may be termed an environmentally conscious composite.

biocomposite mechanical properties sustainability recycling

1. Fiber Reinforcement and Its Sustainability

Generally, the composites for engineering applications are dominated by glass and carbon-fiber composites owing to their excellent mechanical performance, being lightweight and desirable for heavy-duty applications ^[1]. The most used synthetic polymers are epoxy, vinyl ester, and unsaturated polyester which serve as adhesives, coatings, and casting materials derived from petroleum-based sources. Natural-fiber composites compared to their synthetic-fiber competitors have relatively inferior mechanical, chemical, and thermal properties that limit them to specific applications. However, the environmental issues connected to the recyclability and biodegradability of these thermoset resins and synthetic reinforcements has caused a shift toward more environmentally friendly materials.

Natural fibers are a rising reinforcement used for automotive composite components to replace conventional glass-fiber composites ^[2]. The benefits encompass cost-effectiveness, good thermal and acoustical insulation properties, availability, enhanced CO₂ sequestration, energy recovery, reduced dermal and respiratory irritation, and reduced tool wear during machining ^[2]^[3]. The strength of these composites also depends on the fiber–matrix interaction, the compatibility and competitiveness of the reinforcement with the polymer resin, and the fiber–matrix interfacial adhesion. There is a lot of interesting research around building the interfacial adhesion of natural-fiber polymer composites and reducing the drawbacks of the water-absorption behavior of these composites. The majority of the research is centered on improving the mechanical properties, surface treatment through chemical modifications to improve the interfacial adhesion properties, better manufacturing processes, etc. ^[2]^[4]^[5]^[6]. Before selecting natural-fiber composites for engineering applications, it is mandatory to evaluate the properties of both the fiber and resin and the fiber–matrix interaction. Figure 1 schematically explains the levels of the criteria influencing the selection of natural-fiber composites for engineering applications. Here, the specific composite performance directly relates to how well the composite performs based on the mechanical properties, thermal conductivity, etc., whereas the natural-fiber properties focus on the intrinsic characteristics of the natural fibers used, which are the fiber type, length, orientation, etc. The polymer-base properties deal with the attributes of the polymer matrix, which include the type, composition, and compatibility. The composite characteristics encompass broader aspects like processing methods and the overall structure of the composite material. The general composite performance serves as a comprehensive evaluation level, ensuring that the selected natural-fiber composite not only meets the individual criteria but also excels in delivering desired outcomes across a range of performance indicators. The schematic diagram helps to illustrate how these levels of criteria collectively influence the decision-making process for selecting natural-fiber composites in engineering applications. For example, with the same polymer-resin system, different natural-fiber reinforcements can significantly change the properties and behavior of the final composite product ^[3]^[7]. Comparing these values gives an indication of the difference in the strength of these natural fibers with the synthetic reinforcement, helping people to choose the appropriate fiber type for applications. Nanocelluloses, derived from downsized plant-cellulose fibers produced industrially from renewable wood biomass, are gaining attention for their potential to replace petroleum-based materials and contribute to a more sustainable society, as reflected in the increasing scientific publications and patents. Cellulose nanonetworks (CNNeWs), cellulose nanofibrils (CNFs), and cellulose nanocrystals (CNCs) hold significant potential for contributing to a sustainable society ^[8]. A study by Thanga et al. ^[9] reported that silane treatment enhanced cellulose-nanofibers’ properties and their incorporation into epoxy nanocomposites showed promising physical, mechanical, and thermal characteristics, making them suitable for lightweight structural applications.

Figure 1. Schematic illustration of the levels of criteria influencing the selection of natural-fiber composites ^[10].

Currently various bio-based resins, both thermosets and thermoplastics, are studied for their recyclability using both natural- and synthetic-fiber reinforcements. The environmentally friendly resins formulated also exhibit improved mechanical performance through the incorporation of diverse natural fibers. The biodegradability of NFRCs is assigned to easiness in the breakdown of individual constituents within the composites. Biodegradability alongside recyclability, renewability, and sustainability hold benefits for present and future climatic deliberations ^[11]. There is an escalating focus on the demand for eco-friendly materials due to the continuous elevation of standards and regulations against harmful substances. Researchers are thus advocating the production of bio-based materials, particularly NFRCs, in this context. NFRCs, due to their eco-friendly nature and lower energy-consumption value of 9.5 MJ/kg compared to conventional synthetic-fiber composites that require 54.7 MJ/kg, have significantly contributed to the choice of selecting sustainable materials for engineering applications ^[12]. NFRCs are gaining market traction due to their lower environmental impacts compared to synthetic composites, driven by their reduced climate effects. Products with economic features such as biodegradability and renewability are experiencing a rise in market volume. Figure 2 presents the various characteristics related to sustainability. Moreover, the cultivation of natural fibers is an excellent way of revenue generation, and the waste residues during their production can again be used for landfills. The land used for farming can be utilized; moreover, crops such as hemp and flax yield seeds, substances, and oils for diverse applications, ranging from food and textiles to industrial and medicinal purposes ^[13]. Importantly, the mass produced by these materials is biodegradable at the end of their life cycle.

Figure 2. Various characteristics related to sustainability ^[10].

NFRCs have a volume fraction in the range of 60–70% with the rest of the polymer matrix. The sustainability aspects of these composites are derived from most of the constituents in these composites being obtained from living plant and animal sources. The market share of the United States stated that the composite market share was GBP 2.7 billion in 2006, rising to GBP 3.3 billion in 2012, with an expected annual increase of 3.3%. Between 1994 and 2004, there was a significant increase in market share, with a growth rate of 13%, equivalent to 275 million kilograms. ^[14]. The global annual market growth for NFRCs averaged 38% from 2003 to 2007, with Europe holding the highest annual growth rate of 48%. Industries based on composite materials have established their success worldwide, and currently, NFRCs are making effective contributions to these sectors. The life-cycle-assessment studies support the ecological benefits of composite materials over the aluminum-based structural components. The aircraft industries have supported the reduction in CO₂ contents by 15–20% with the inclusion of composite materials. The lignin, cellulose, and hemicellulose constituents in natural fibers make them environmentally friendly compared to traditional composites ^[15]. The abundance of cellulose helps in decomposing the material naturally without any requirement for additional energy. Some of the biodegradable natural-fiber composites with the polylactic-acid-based-resin system degrade easily ^[16]. The energy associated with the burning of China reed strands is estimated at 14 MJ/kg. The incineration process does not release CO₂ into the atmosphere. The burning of these natural fibers results in positive carbon credits and reduces risks to the environment. The gross energy requirement (GER) for natural fibers and residues utilized for energy generation at the end of their life-cycle assessment is illustrated in Figure 3.

Figure 3. Gross energy requirement (GER) for some of the natural fibers (MJ/kg of field plant) ^[10].

2. Thermoset Matrices and Their Sustainability

The resin systems bind the fibers together, transfer the load within the reinforcements, and protect them from environmental factors such as moisture, chemicals, and abrasion along with providing chemical and environmental resistance. Generally, synthetic or petroleum-based polymers are used for the manufacturing of fiber-reinforced composite materials, and these polymers are produced from petroleum-based resources. Synthetic polymers are of two types: thermosets and thermoplastics. The thermoset polymers are irreversible and cannot be remolded by heating once they are cured during the composite fabrication. The thermoset polymers having unlinked molecules when added with curing agents with the support of heat initiate the chemical reaction for the curing of the polymer. During this process, the molecules cross-link and form long molecular chains with cross-linked structures transferring them to solid form. The thermoset matrix material has better modulus, creep, and resistance to thermal and chemical environments when compared to thermoplastic composites. These resin systems have the drawback of being brittle and possessing lower fracture toughness at room temperature ^[17]^[18]. Commonly used thermosets to produce fiber-reinforced composites are epoxy, vinyl ester, unsaturated polyester, and phenolics.

With greater interest in sustainable composites, thermoset resin faces great challenges in terms of reuse and recycling. The lack of recycling in turn causes environmental concerns after the end of the life cycle. The permanent irreversible chemical chains make them unfit to be remolded, reprocessed, and recycled. Some recent research works ^[19]^[20]^[21]^[22]^[23] have focused on the recycling of these thermoset composites and explain the ways of developing thermosets that are less inert and receptive with the inclusion of dynamic covalent bonds into the thermosetting material. A research work by Lorena et al. ^[24] performed the recycling of a flax-fiber bio-based thermosetting composite under mild recycling conditions extracting the fiber reinforcement and epoxy resin. The epoxy composite’s disposal approach involved employing a chemical-recycling process, enabling the retrieval of the original material with a substantial yield in recovery.

The global market for thermosets, specifically epoxy, dominates with a 70% share ^[25] due to their excellent mechanical properties, dimensional stability, and adhesive properties. Among these epoxy thermosets, 90% are sourced from the petroleum monomer diglycidyl ether of bisphenol A (DGEBA), which causes health hazards to humans and is not renewable ^[26]. Both bisphenol A (BPA) and epichlorohydrin pose hazards to living organisms ^[27]^[28]. The alternative means of developing these thermoset polymers from renewable sources and developing their mechanical properties are of greater importance in the current research. Sources such as plant oil, itaconic acid, rosin, lignin, furan, etc. are some of the examples ^[29]. He et al. ^[17] presented an eco-friendly method of producing thermoset composites with the inherent characteristics of polyimine-based covalent adaptable networks (CANs). The initial step involves the creation of polyimine thin films through the recycling of composite scraps, serving as the raw material for subsequent composite manufacturing. Another work by Monteserin et al. ^[30] presented the development of environmentally friendly epoxy vitrimers that are formulated using a Schiff base derived from the reaction between a petroleum-derived diamine (4,4′-diamine diphenyl methane, DDM) and two key components—a bio-based compound, vanillin, and an epoxy resin derived from linseed oil (LO). Notably, these vitrimers exhibit the ability to be reprocessed and recycled under mild conditions.

The recent research focus on vitrimer-based natural-fiber composites is because of the self-healing ability of the vitrimers. Mingen et al. ^[31] studied hemp-based, recyclable, bio-based matrix composites by developing a dual-network vitrimer matrix from hempseed oil and limonene derivatives. The fiber–matrix adhesion of the composites was further enhanced by the introduction of amino silane into the vitrimer matrix, which increased the cross-link density and the toughness of the matrix. The imine and hydroxy-ester dynamic bonds of the matrix allow the recycling of the composites with a mild and low-cost aminolysis process. A research study by Ali et al. ^[32] developed discontinuous flax-fiber, vitrimer-based composites that are sustainable and evaluated the repair performance. The interfacial shear strength (IFSS) properties of the composites were compared with a standard epoxy and achieved a high level of adhesion compared to the standard epoxy. The IFSS value of the epoxy-based composites was 11.8 MPa, whereas the vitrimer-based composites registered the IFSS value as 20.0 MPa. The low-temperature repair of the vitrimer-based composites was achieved while placing the sample between the heated plates in a 50 kN load cell UTM, while applying a pressure of 0.69 MPa for 5 min at 120 °C. The repair capabilities of the composites were observed using the end-to-end and single-patch repair methods. The research work by Li et al. ^[33] studied recyclable, high-performance, ramie-yarn-reinforced, polyimine vitrimer composites and observed self-healing, moldable, and good water-barrier properties. The experimental findings observed that the tensile strength values of the vitrimer-based composites were superior when compared to most of the natural-fiber polymer composites with the same fiber and volume fraction. Moreover, the vitrimer-based natural fibers were capable of undergoing physical recycling at least nine times without any degradation in performance. The tensile strength and modulus values of the recycled composites (even after the ninth time of recycling) were still comparable with the properties of the initial samples.

3. Thermoplastic Matrices and Their Sustainability

Numerous industrial polymer-composite products find applications across diverse fields, including aerospace, marine, sports industries, and beyond. The demand and use for polymer composites are increasing day by day ^[6]^[34]. Consequently, environmental issues have surged over the past years. The use of thermoplastic polymers as the matrix has been tremendously increasing in the area of natural- and synthetic-fiber-reinforced polymer composites due to their high performance and recyclability ^[35]. The management of waste and waste disposal plays a pivotal role in sustainability. Moreover, the need and awareness for sustainable materials are growing among composite industries, leading to the use of thermoplastic polymers instead of traditional materials.

Thermoplastic polymers are composed of linear molecular chains that soften and harden due to the application of heating and cooling. These polymer chains are linked to each other via intermolecular entanglement such as van der Waals forces or dipole–dipole interactions. As a result, when these polymers are reheated such weak entanglement breaks down to a molten stage and is easily reformed into a solid by cooling ^[36]. The nondegradation nature of thermoplastic carbon chains during heating makes this polymer more vulnerable to recycling. Figure 4 shows the molecular structure of thermoplastic and thermosetting polymers. Thermoplastic polymers are distinguished into crystalline, semi-crystalline, and amorphous polymers on the basis of their polymer arrangements. Crystalline polymers are polymer chains arranged in ordered, repeating, and three-dimensional patterns with a higher impact performance. High- and low-density polyethylene (HDPE and LDPE) and polypropylene (PP) come under this category because the glass transition state of these polymers is at negative degree Celsius. The random arrangement of molecular chains in polymers is referred to as amorphous thermoplastic polymers, for example, polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), polyphenyl sulphone (PPSU), polycarbonate (PC), and acrylonitrile butadiene styrene (ABS) ^[37]^[38]. Lastly, in the case of polybutylene terephthalate (PBT) and polyethylene terephthalate (PET), these polymers contain both amorphous and crystalline structures and are known as semi-crystalline polymers. Figure 5 shows the different types of thermoplastic polymers classified based on their performance and structure ^[39]. These thermoplastic polymers are compatible with natural fibers as matrices for building structures and construction sectors due to their economical, durability, damage-tolerance, and flame- and chemical-resistance properties ^[40].

Figure 4. Structure of thermoplastic and thermosetting resins ^[41].

Figure 5. Polymer pyramid illustrated based on structure, performance, application, and cost ^[39].

Thermoplastic polymers can be recycled by sustainable means such as mechanical, thermal, and chemical processes, making them prone to reuse ^[42]. The process of mechanical recycling involves the granulating, squashing, grinding, and eventual milling of clean polymers and their composites. Only thermoplastic materials like PE, PP, PET, and PVC may be recycled by this method. Mechanical recycling is a low-cost procedure, although it requires a larger initial machinery investment. The crucial drawback of mechanical recycling is the lowering of molecular weight due to the breakage of molecular chain links under the action of environmental moisture and acid. Moreover, the involvement of nonhomogeneous plastic wastes affects the performance of recycled composite parts ^[43]. When considering factors like time, carbon footprint, environmental effect, and cost management, this recycling procedure is the most efficient and sustainable one.

Chemical recycling is considered as supplementary to mechanical recycling. By the process of chemical recycling, thermoplastic polymers are chemically converted into monomers. Once again, these monomers are subjected to new polymerization to obtain similar polymers or compatible polymer components. The requirement of high expertise for handling the process and high investment make this less vulnerable for industries ^[44]^[45], although chemical recycling, on the other hand, resulted in savings of 3.5 billion barrels of oil for the processing of the global production submission warrant (PSW) with a savings of USD 176 billion US ^[46]. In the thermal-recycling process (thermochemical), thermoplastic polymers are decomposed into solid mixtures such as gas (CO, CH₄, CO₂), oils (toluene, phenols, benzene), and polyaromatic chars. These compounds have a higher calorific value and vary in molecular weight, making them more suitable as fuels and sources for different manufacturing processes ^[47]^[48]. This method is preferably suggested for those polymers that cannot be recycled using mechanical recycling methods like fiber-reinforced composites, multilayered packaging materials, and mixtures of PS/PE/PP. The major advantage of the pyrolysis process is the high temperature resulting in the breakage of molecular bonds that rely on random fragmentation, depolymerization, and polymer type.

Apart from synthetic thermoplastic polymers, there are bio-based thermoplastic polymers, in which the monomers are derived from renewable or organic matter including starch or plants. If a polymer composite has at least one bio-based or biodegradable component, it is considered a biocomposite. The sustainable way of reducing environmental impacts caused by synthetic or oil-derived polymers is to replace them with biopolymers. Apart from reducing carbon emissions, this also imparts several advantages, such as biodegradability, biocompatibility, reduction in global warming, and carbon dioxide sequestration ^[49]. Polylactic acid (PLA), polyhydroxyalkanoates (PHAs), chitosan, and starch are biopolymers obtained from renewable raw materials and are degradable, while nonbiodegradable biopolymers like PE, PP, PVC, and PET are also made from renewable sources. On the contrary, there are several fossil resources derived from biodegradable biopolymers, for example, poly (butylene adipate-co-terephthalate), polycaprolactone, and poly (butylene succinate) ^[50]. Figure 6 depicts (a) various types of biopolymers, their origin and degradability, and (b) the classification of biopolymers based on monomeric units ^[51]. The use of biopolymers has significantly reduced the environmental impacts of products during their entire life cycle. However, this effect can be further reduced by considering the recyclability of bio-based polymer systems. Recycled biopolymers are currently used for several applications in industries by considering the importance of sustainability ^[52]^[53]. Numerous researchers have examined the feasibility of repurposing biodegradable polymers to mitigate the ecological consequences associated with their life cycle ^[54]^[55]^[56]. A study conducted by Badia et al. ^[57] on the mechanical recycling of PLA found that mechanical recycling is a cost-effective and simple approach. Similar conclusions were also made in other studies ^[58]^[59].

Figure 6. The classification of biopolymers is based on (a) origin and degradability and (b) monomer units ^[51].

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Subjects: Engineering, Mechanical

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Vishnu Prasad

Amal Alliyankal Vijayakumar

Thomasukutty Jose

Soney C. George

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Update Date: 08 Feb 2024

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1. Fiber Reinforcement and Its Sustainability

2. Thermoset Matrices and Their Sustainability

3. Thermoplastic Matrices and Their Sustainability

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