ReviewRecycling of natural fiber composites: Challenges and opportunities
Graphical abstract
Introduction
Natural fiber reinforced polymer composites (NFRPCs) are a subset of fiber reinforced polymer composites. This subset includes various polymer matrices, both thermosetting and thermoplastic, reinforced by a variety of natural fiber sources arising from wood, plant stalks (bast and core fibers), grasses, seed hairs, and so on (Bongarde and Shinde, 2014). NFRPCs are typically processed using common polymer processing techniques, such as compression and injection molding, as well as extrusion and pultrusion. Common benefits of utilizing natural fibers for reinforcing polymers include their “green” and sustainable nature, biodegradability, excellent stiffness to weight ratio, low abrasion, and low cost compared with synthetic fibers such as glass or carbon fibers (Liu et al., 2020; Zhao et al., 2020b).
NFRPCs have been used for at least 100 years, with most commercial activity occurring during the past 30 to 40 years, especially in thermoplastic composite technology (Gardner et al., 2015). Common applications of NFRPCs include automotive parts, such as interior panels, and building materials, such as architectural moldings, decking, and railing components (Holbery and Houston, 2006; Klyosov, 2007). Recently, NFRPCs have been applied in additive manufacturing (Zhao et al., 2019). The most common thermoplastics utilized for NFRPCs are polyolefins (e.g., polypropylene [PP] and polyethylene [PE]), polyvinyl chloride (PVC), polyamides (e.g., nylon), and, recently, biobased polylactic acid (PLA). Thermoplastic polymer matrices are promising in NFRPCs because of their low cost, wide availability, and low processing temperatures (typically < 225 °C), which are suitable for processing natural fibers without experiencing thermal degradation (Wang et al., 2020; Yao et al., 2008; Zhao et al., 2020b).
Natural fiber feedstocks for fiber reinforced composites can often be obtained from residues from primary processing of wood or agricultural materials. These feedstocks are obtained as both powders (e.g., wood flours) and chopped bast fibers. Wood flour is typically manufactured from sawdust, planer shavings, and molding residues and obtained from secondary processing of wood lumber. However, bast fibers are obtained from the processing of plant stalks such as jute, kenaf, and hemp. Once collected, natural fibers are appropriately comminuted to a particular size, dried to remove moisture, and then compounded with plastics and/or other additives (e.g., processing aids, colorants, UV protectants) that will comprise the final composite. Feedstock suppliers and composite producers utilize virgin polymers and recycled polymers (post-consumer or post-industrial), depending on the availability and end use application of the material.
NFRPCs are often regarded as green, sustainable, and recyclable materials. Mechanical and thermal (e.g., gasification) (Parparita et al., 2015) recycling methods have been used to recycle NFRPCs. Unfortunately, commercial recycling and reuse activity for these materials is limited in North America, although recycling activity is becoming more common in the European Union, per legislative policies. Recycling is becoming more important for materials relative to the circular economy (Moazzem et al., 2021) in which future manufacturing processes will be designed to minimize and eventually remove waste out of the system (Zhao et al., 2022). Fig. 1 shows a flow diagram of recycling of natural fiber composites. In the future, composite materials will be expected to be reused and/or repurposed rather than relegated to landfills. There are challenges as well as opportunities in the recycling of NFRPCs. For example, obtaining NFRPCs at the end of life is currently neither feasible nor cost-effective. In the automobile industry, cars at the end of their useful lifetime are often sent through a shredder. During this process, polymeric materials have to be separated from metal and glass. This is certainly possible, but the resulting polymeric mixtures often cannot be recycled without additional separation steps. In the building materials area, there are currently no commercial recycling programs for decking, railings, and architectural moldings at the end of life, so these materials end up in the landfill or are combusted for energy.
When manufacturers utilizing NFRPCs in the production of materials are in control of the feedstock supply chain, there are some excellent examples of recycling and reprocessing of materials for NFRPCs commercially. For example, when Trex began manufacturing decking in the 1990s, their primary feedstocks were recycled low density PE (LDPE) plastic grocery bags and wood flour (Trex, 2020). Anderson Windows took advantage of their residue streams for wood and vinyl window production, and they created PVC-wood composites (Fibrex®) for new window applications (Andersen, 2020). Producers of wood plastic composite decking reprocess their off-spec material in home construction to use as feedstocks for new decking, usually in the core of co-extruded profiles. These are just a few examples of recycling and reprocessing practices currently applied in the NFRPC industry.
The terms “recycling” and “reprocessing” are often used interchangeably to refer to reuse of processed material. The terms “external recycling” and “internal recycling” are also sometimes used to indicate if the material is being used after its service life (external) or reprocessed internally in the same production line as is done for waste of defective products. Finally, the terms “chemical recycling” and “mechanical recycling” are used to differentiate processes in which the main components of the composite material are broken down to their chemical constituents or are retained in their original form but reprocessed into a similar product, respectively. Recycling provides different benefits attributable to the good performance of the composites after certain processing cycles and the cost savings of an additional production phase (Bourmaud et al., 2016). Depending on the final application for recycled material, composites may have different recycling potential (Faraca and Astrup, 2019). A primary recycling method for post-consumer plastic waste is mechanical recycling, in which the plastic waste is ground, washed, and reprocessed to form new secondary plastic materials that maintain their original chemical structure. However, for composite recycling, the composite type and application play a critical role in the recycling efficiency (Faraca and Astrup, 2019). Some composites such as wood/PP and flax/PP maintain good stability regarding mechanical properties after recycling (Bourmaud et al., 2016).
As manufacturers transition toward circular economy practices (Huysman et al., 2017), it will be important to have a complete understanding of the impact of recycling and reprocessing on material properties of NFRPCs as they are reused in subsequent product applications. Research on recycling and reprocessing of NFRPCs has been reported over the past several decades and is the focus of this review. The scope of the paper focuses on thermoplastic matrices, and the effects of reprocessing/recycling on the material properties (e.g., mechanical, thermal, hygroscopic, viscoelasticity) of NFRPCs. The mechanisms behind these property changes are discussed. Mitigation approaches to overcome degradation of material properties during recycling are discussed in detail, and the main challenges in these approaches are discussed. An effort is made to provide a rationale for reprocessing/recyclability assessment from practical considerations to obtain materials, as well as from a use perspective for subsequent product applications and outlook.
Section snippets
Mechanical properties
The mechanical performance of polymer composites is one of the most important factors examined in determining their recyclability. The effect of reprocessing on the properties of neat polymers depends on many factors, including the method of processing (e.g., thermo-mechanical, chemical, aging), processing parameters (e.g., temperature, time, speed and shear rate, humidity, UV exposure), and, of course, the molecular makeup of the polymer itself. Fig. 2 shows the chemical structures of some
Functional additives
During initial processing of virgin plastic and its first service life, irreversible mechano-chemical, chemical, or irradiative changes take place in the polymer chain and natural fiber filler (Pfaendner, 2010). In these chemical degradation processes, often induced by radical mechanisms, new chemical groups are often formed in the polymer structure, changing the polymer composition. The concentration of these new structures increases with the service time and the environment of exposure, but
Emerging applications
The global market for biocomposites is expected to reach $41 billion net worth by 2025 (Zwawi, 2021). NFRPCs have been widely used and have potential for a variety of applications such as 3D printing (Bhagia et al., 2021), automotive (Keskisaari and Kärki, 2018), tribological, packaging, and biomedical applications. For example, Li et al. (Li et al., 2019) investigated the cellulose nanocrystal reinforced poly(ethylene glycol) diacrylate composites for 3D printing use. Cellulose nanocrystal has
Challenges and future directions
There is a research gap regarding the safety, durability, and especially recyclability of NFRPCs. Safety hazards might exist when recycling composites that contain formaldehyde-based adhesives that might give off volatile organic compounds. Durability will be an issue if the composites are used in exterior applications where they are not protected from weather, biological attack, and so on. Natural fiber composites, by their nature, are recyclable. The major technical challenges for recycling
Conclusions
Natural fibers have been increasingly used to reinforce polymers. Compared with synthetic fibers, such as glass or carbon fibers, NFRPCs are a sustainable, cost-effective alternative in many applications with low to moderate strength and stiffness requirements. Repetitive reprocessing using combinations of granulation, grinding, shredding, mixing, melt compounding, extrusion, pelletization, compression molding, and injection molding has been used to recycle NFRPCs. However, the commercial
Declaration of Competing Interest
The authors declare no competing financial interest.
Acknowledgments
The authors acknowledge the support from the US Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office under CPS Agreement 35714, and the US DOE FY 2021 BETO Project under Contract 2.5.6.105 with UT-Battelle LLC. This manuscript was authored in part by UT-Battelle LLC under contract DE-AC05–00OR22725 with DOE. The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a
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