Abstract
The biodegradability of polymers depends on several factors. However, the most critical aspects are the accessibility of the structure for moisture and enzyme diffusion and the capacity of the microbes in the environment to assimilate the final monomers. The accessibility of the polymer structure to enzymes and water depends primarily on crystallinity, hydrophobicity, and the steric effects of the side groups in the polymer backbone. In general, biologically synthesized polymers are readily biodegradable in natural environments but synthetic polymers are either less biodegradable or degrade very slowly. However, such generalizations should be avoided. To understand the compatibility of biomaterials and the environment, both the disintegration step of the biodegradation process and the assimilation and mineralization of these fragments by microorganisms must be investigated. Mineralization occurs when the oligomers and monomers assimilated within the cells are converted to CO2 and H2O (aerobic), and CO2, CH4, and H2O (anaerobic). Although the disintegration of the polymeric structure limits the biodegradation rate and is most easily detected, the final pieces may accumulate in the environment if they are not fully mineralized. Such accumulation could contribute to an issue with microplastics that may be much more difficult to address than the removal of macroscopic, large polymer-based debris.
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Effects of Chemical and Morphological Structure on Biodegradability of Fibers, Fabrics, and Other Polymeric Materials
Marielis C. Zambrano, Joel J. Pawlak, and Richard A. Venditti *
The biodegradability of polymers depends on several factors. However, the most critical aspects are the accessibility of the structure for moisture and enzyme diffusion and the capacity of the microbes in the environment to assimilate the final monomers. The accessibility of the polymer structure to enzymes and water depends primarily on crystallinity, hydrophobicity, and the steric effects of the side groups in the polymer backbone. In general, biologically synthesized polymers are readily biodegradable in natural environments but synthetic polymers are either less biodegradable or degrade very slowly. However, such generalizations should be avoided. To understand the compatibility of biomaterials and the environment, both the disintegration step of the biodegradation process and the assimilation and mineralization of these fragments by microorganisms must be investigated. Mineralization occurs when the oligomers and monomers assimilated within the cells are converted to CO2 and H2O (aerobic), and CO2, CH4, and H2O (anaerobic). Although the disintegration of the polymeric structure limits the biodegradation rate and is most easily detected, the final pieces may accumulate in the environment if they are not fully mineralized. Such accumulation could contribute to an issue with microplastics that may be much more difficult to address than the removal of macroscopic, large polymer-based debris.
Keywords: Biodegradable; Fiber; Chemical structure; Morphology; Fabric; Textile; Cellulose
Contact information: Department of Forest Biomaterials, College of Natural Resources, North Carolina State University, Campus Box 8005, Raleigh NC 27695-8005 USA;
* Corresponding author: richardv@ncsu.edu
GRAPHICAL ABSTRACT
INTRODUCTION
In the last century, the utilization and production of plastic materials have increased exponentially due to the boom of the petrochemical industry (Browne et al. 2010; Thevenon et al. 2011; Geyer et al. 2017). Plastic materials have excellent mechanical properties, can resist water and other environmental damage, and their versatility makes them suitable for multiple applications (Browne et al. 2010; Thevenon et al. 2011). Their contributions to society in health, food, housing, and beyond have been immense. The petrochemical industry is fully developed, and its products have very competitive prices in the market. Therefore, the production and demand of plastics continue to grow (Browne et al. 2010; Thevenon et al. 2011). However, less than half of this plastic ends up in landfills or being recycled, and the rest is still in use or littering the continents and oceans (Rochman et al. 2013; Geyer et al. 2017).
The accumulation of plastics in the environment has become important in the last 50 years, especially in water bodies (Thompson et al. 2004). In the last decade, attention has been focused on microplastics, which are particles smaller than 5 mm in size that represent a hidden threat to the environment (Browne et al. 2011). Researchers have estimated that a minimum of 5.25 trillion plastic particles weighing 270,000 tons are floating in the world’s oceans (Eriksen et al. 2014). This constitutes only 0.1% of the world’s annual plastic production, which was 380 million metric tons in 2015 (Eriksen et al. 2014; Geyer et al. 2017).
In the textile industry, global clothing sales have doubled in the last 15 years (Cooper n.d.; Ellen Macarthur Foundation 2017). In 2015, the global consumption of apparel and footwear was 62 million tons and it is expected to increase by 63% by the year 2030, which implies a 62% rise in waste (Textile Exchange n.d.; Global Fashion Agenda & The Boston Consulting Group 2017). This represents an increase in solid waste of 57 million tons of waste generated annually (Global Fashion Agenda & The Boston Consulting Group 2017). Globally, only around 18% of clothing is collected for reuse or recycling and 57% is going to landfills (Fig. 1) (Global Fashion Agenda & The Boston Consulting Group 2017; Gwozdz et al. 2017). This is mainly due to the fast fashion phenomenon that involves quick style changes and low prices/quality, a combination that results in a reduction of the times that a garment is worn before it is discarded (Cooper n.d.; Eco Watch n.d.; Siegle n.d.). There are several options to reuse and recycling textile materials: reuse, fabric recycling, fiber recycling, polymer/monomer recycling, and energy recovery (Johnson et al. 2020). Nevertheless, these processes are still not fully implemented due to technical constraints (complex separation processes and low quality products) and limitations in the supply chain (cost, volume, collection, sorting, and transportation) (Johnson et al. 2020).
In addition, Boucher and Friot (2017) reported that synthetic textiles are the primary source of microplastics. Even if this portion only represents ~ 1% of the textile waste (Fig. 1), these small particles are ingested by aquatic fauna and transferred to the human food chain, potentially causing problems for the human health (Thevenon et al. 2011; Wagner et al. 2014; Rochman et al. 2015; Miranda and de Carvalho-Souza 2016; Kim et al. 2018; Liebmann et al. 2018). Despite being relatively inert, due their large surface-to-volume ratio and chemical composition, these particles can adsorb pollutants and pathogens and transfer them via ingestion to other trophic levels (Wagner et al. 2014; GESAMP 2015; Rummel et al. 2017; Egbeocha et al. 2018; Wang et al. 2018).
There is enough evidence supporting that synthetic and natural textile fibers and other plastics do not fully degrade in wastewater treatment systems, landfills, and the environment. Microplastics, mainly particles and fibers smaller than 100 µm, have been observed in wastewater effluents at low concentration (Browne et al. 2011; Magnusson and Norén 2014; McCormick et al. 2014; Talvitie et al. 2015, 2017a; b; Mintenig et al. 2017; Lares et al. 2018; Wolff et al. 2018). In the wastewater treatment process, more than 98% of microplastics are retained in the sewage sludge and they are transferred to the environment when used for soil amendment/fertilizer (Nizzetto et al. 2016; Mintenig et al. 2017; Talvitie et al. 2017b; Lares et al. 2018). The fibers identified in the effluents are typically from the textile industry. Mainly polyethylene terephthalate (PET) has been observed, but important quantities of cellulosic fibers such as cotton and rayon are also present (Talvitie et al. 2015, 2017a; b; Mintenig et al. 2017; Ziajahromi et al. 2017; Lares et al. 2018; Wolff et al. 2018). In the environment, the average size of plastic particles seems to be decreasing due to fragmentation (Barnes et al. 2009). In addition, Suaria et al. (2020) compiled a global dataset of oceanic water samples and observed that 8.2% of oceanic fibers are synthetic, 79.5% cellulosic, and 12.3% of animal origin.
Fig. 1. Global material flow of clothing (Ellen Macarthur Foundation 2017; Global Fashion Agenda & The Boston Consulting Group 2017).
Moreover, microplastic/fiber fragments have been observed in landfill leachates of active (young and old) and closed facilities, which is also an indication that even under anaerobic conditions plastics are fragmentated but not fully mineralized (Kilponen 2016; Praagh et al. 2018; He et al. 2019; Su et al. 2019). Therefore, it is essential to understand the biodegradability of plastics, especially polymers used in the textile industry.
In early plastic development, to guarantee the durability and longevity of these materials, the production of plastics mainly focused on preventing or reducing degradation. Today, due to the concern about the fate of plastics in the environment after their intended use, the promotion of degradation is seen as a positive attribute (Krzan et al. 2006).
In general, petroleum-based plastics are not biodegradable, and their accumulation in the environment represents a social concern that impacts human health and the normal behavior of natural ecosystems. As a response to the problems of plastic accumulation and petroleum dependence, the production of plastics based on renewable resources is growing (Ashter 2016). Bio-based plastics can replace harmful conventional plastics (Ashter 2016). Biopolymers are produced from renewable natural sources such as chitin, gluten, corn, starch, or vegetable oil (Ashter 2016; Karamanlioglu et al. 2017). Bioplastics are often biodegradable, but this is not always true (Ashter 2016; Karamanlioglu et al. 2017). Bio-based refers to the feedstock, and biodegradable describes the end of life of the material. The biodegradability of bio-based plastic is not related to its bio-based content. Even fossil-based plastics can be designed to be biodegradable, as biodegradability depends on the polymer structure and physical properties (Ashter 2016).
The effects of chemical and morphological structure on biodegradability of fibers, fabrics, and other polymeric materials are reviewed herein to summarize and identify which aspects could be manipulated in different stages of the life cycle of materials, especially textiles, to promote or delay biodegradation.
MECHANISMS OF POLYMER BIODEGRADATION
According to the American Society for Testing and Materials (ASTM) and the International Organization for Standardization (ISO), degradation is ‘‘an irreversible process leading to a significant change of the structure of a material, typically characterized by a loss of properties (e.g., integrity, molecular weight, structure, or mechanical strength) and fragmentation. Degradation is affected by environmental conditions and proceeds over a period of time comprising one or more steps” (Krzan et al. 2006; ASTM D6691 − 09 2009; ISO 14851:2019 2019). In the environment, degradation occurs due to a combination of various mechanisms and factors (Krzan et al. 2006) (Fig. 2).
Abiotic Degradation
Abiotic degradation factors do not involve microorganisms, but they are necessary to fragment the polymeric material and produce smaller units that can be biodegraded (Mochizuki and Hirami 1997; Krzan et al. 2006; Lucas et al. 2008). The action of mechanical stress, temperature, humidity, solar light exposure, rain, and wind, etc. can weaken polymeric materials. Many transformations occur, such as changes in mechanical, physical, and chemical properties (Krzan et al. 2006; Lucas et al. 2008). The most important abiotic processes are oxidation and hydrolysis.
In synergy with light degradation, the O2 or O3 present in the atmosphere produce free radicals that can attack covalent bonds in polymers; they can also generate crosslinking or chain scissions (Lucas et al. 2008). Likewise, hydrolysis depends on water activity, temperature, pH, time, and the presence of hydrolyzable covalent bonds (ester, anhydride, amide, carbamine, or ester amine). Diffusion of water or oxygen inside the material structure is limited by its crystallinity and the molecular architecture, polar nature, and molecular mobility properties of the polymer (Mochizuki and Hirami 1997; Lucas et al. 2008).
Biotic Degradation
Biodegradation, or biotic degradation, is the breaking of polymeric bonds associated with the action of enzymes in living organisms (Krzan et al. 2006). Microorganisms, such as bacteria, protozoa, algae, and fungi, can grow on the surface or inside the polymeric material, form biofilms, and secrete slime matter, acids, and enzymes that can penetrate the surface of the material and disrupt its pore structure (Lucas et al. 2008).
Of all these factors, enzyme action is most often the determinant step during biodegradation. Enzymes are proteins that act as catalysts decreasing the activation energy of some chemical reactions (Lucas et al. 2008). There are different types of enzymes, such as endo-enzymes (catalytic action within the polymer chain), exo-enzymes (catalytic reactions that occur mainly at the ends of the polymer), constitutive enzymes (non-substrate specific), and inductive enzymes (substrate-specific enzymes) (Lucas et al. 2008). Enzymes are too molecularly large to be involved in bulk erosion and have poor diffusion characteristics in bulk materials; thus, they are only responsible for surface deterioration (Lucas et al. 2008).
After the polymer backbone is reduced to oligomers and monomers by depolymerization under the action of the factors mentioned above, these simpler compounds can penetrate the cell wall of microorganisms, providing the energy and elements necessary for living, growing, and reproduction, which is called assimilation (Lucas et al. 2008). After assimilation, the complete degradation of a substance occurs within the cell, which is called mineralization (Pagga 1997). The mineralization catabolic pathway depends on the environment where the microorganism can grow (Lucas et al. 2008). Microorganisms digest the organic products of plastic degradation under aerobic or anaerobic conditions (Krzan et al. 2006). During aerobic biodegradation O2 is available, and aerobic microorganisms control the process and form CO2, H2O, and biomass as final products (Fig. 2) (Gu 2003). Anaerobic biodegradation occurs in the absence of O2, and anaerobic consortia of microorganisms are responsible for polymer deterioration (Gu 2003). The primary products are microbial biomass, CO2, CH4, and H2O under methanogenic conditions or H2S, CO2, and H2O under sulfonic conditions (Fig. 2) (Gu 2003). During both processes, aerobic or anaerobic, microorganisms need a carbon source for growth and reproduction.
More details about the specific mechanisms of disintegration and mineralization of plastics and other polymeric materials are presented in several published reviews (Pagga 1997; Gu 2003; Krzan et al. 2006; Lucas et al. 2008; Gewert et al. 2015; Tiwari and Maurya 2018)
Fig. 2. Schematic representation of plastics degradation processes in the environment, adapted from Krzan et al. (2006)
FACTORS THAT AFFECT POLYMER DEGRADATION
The biodegradation of polymeric materials depends on the characteristics and properties of the material, such as molecular composition of the polymer, presence of functional groups, the intermolecular interactions, balance between hydrophobicity and hydrophilicity, crystallinity, level of orientation, morphology, configuration, surface structure, and molecular weight (Mochizuki and Hirami 1997; Gu 2003; Eyerer et al. 2010). Biologically synthesized polymers are generally readily biodegradable in natural environments, whereas synthetic polymers are either less biodegradable or degrade very slowly (Gu 2003). In addition to the polymer characteristics, biodegradation also depends on the environmental conditions that affect microbial growth and the presence of microbial communities able to generate the appropriate enzymes, assimilate, and metabolize the polymer (Gu 2003).
This review aims to summarize how the properties of textile materials affect the biodegradability of the polymers in the structure. The literature reviewed contains textile studies and studies on films and composites and how their properties influence degradation, as these studies also have implications for fiber and fabric behavior.
Crystallinity, Solid-State Morphology, and Moisture Diffusion
Crystallinity is one of the most critical factors during degradation (Mochizuki and Hirami 1997). In general, crystallinity is measured by X-Ray Diffraction (XRD), solid-state 13C NMR, infrared (IR) and Raman spectroscopies, and differential scanning calorimetry (DSC) (Szcześniak et al. 2008; Park et al. 2010; Linares et al. 2019). The amorphous regions of the polymers are easier to degrade, chemically and enzymatically, than crystalline regions. This is related to the primary chemical structure of the polymer, which controls how the molecules pack in the crystalline matrix. However, this is not the only factor; the kind of process involved in the manufacturing of materials defines the orientation, packing, and crystalline structure of the final good.
Table 1. Tensile Tenacity and Ultimate Elongation of PCL Fibers at Different Draw Ratios and Effect of Enzymatic Degradation in the Crystallinity and Orientation of PCL Fibers (Adapted from Mochizuki et al. (1995))
One of the main methods to enhance the mechanical properties of fibers is the drawing process. The influence on the enzymatic hydrolysis by lipase of polycaprolactone (PCL) fibers at different draw ratios was evaluated (Mochizuki et al. 1995). At higher draw ratios, there was an increase in the orientation of the polymers within the fiber; therefore, there was an increase in crystallinity and tensile tenacity, and a decrease in ultimate elongation (Table 1). This study showed that the draw ratio was inversely related to the extent of the enzymatic hydrolysis of the fibers, which can be seen in Fig. 3 by the decrease in total organic carbon (TOC) formation (measured by combustion catalytic oxidation) and weight loss. It was also suggested that the enzymatic attack occurred preferentially in the amorphous or less ordered regions because the enzymes had more free space to move and bind to the polymer. Crystalline regions seem to be susceptible to degradation after the amorphous part of the material is consumed (Mochizuki et al. 1995).
Fig. 3. The TOC formation and weight loss of PCL fibers with different draw ratios after 16 h in the aqueous solution containing lipase of Rhizopus arrhizus at 30 °C (adapted from Mochizuki et al. (1995))
According to the SEM images shown in Fig. 4, the degradation occurs from the surface and then proceeds from erosion of the surface to the inside of the fiber structure. The undrawn fibers showed deterioration on the surface and an important decrease in diameter after enzymatic hydrolysis in comparison to the drawn fibers. The presence of spherulites are an indication of the low orientation in the undrawn fibers, which made them susceptible to the enzymatic attack of the endo-enzyme lipase. In drawn fibers, the existence of highly-oriented fibrillar stripes parallel to the fiber axis indicated that the spherulites were extended and modified during the drawing process, which generated the highly crystalline structure that is resistant to lipase hydrolysis.
The effect of structure on the enzymatic hydrolysis of poly(butylene succinate-co-ethylene succinate)s (P(BS-co-ES)s) was studied using lipases obtained from several microorganisms to degrade hot-pressed co-polyester films (Mochizuki et al. 1997). The degradation was monitored by water-soluble total organic carbon (TOC) formation. Figure 4 shows the relation between crystallinity, chemical structure, and enzymatic degradation.
Fig. 4. The SEM images of PCL fibers with different draw ratios before and after enzymatic degradation (adapted from Mochizuki et al. (1995))
At 53 mol% of ES, there was a minimum in crystallinity, which coincided with the maximum rates of enzymatic hydrolysis for two lipases from R. arrhizus (not shown) and P. nitens (Fig. 5). The optimum biodegradation point was attributed to the presence of mostly amorphous or less ordered structures in films. This study concluded that both the chemical primary structure of the polymer and the decrease in crystallinity that impacts the accessibility of the polymer to microbial attack critically affect the rate of degradation.