Environmental and Agricultural Microbiology. Группа авторов
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4.2 Polyamides
Definition
NylonTM is a necessary term that represents an important class of PAs. PA with amide linkage exhibits high thermomechanical properties and higher softening temperature because of hydrogen bonding, which provides chain symmetry [9]. Presence of amide linkages in PA used as engineering thermoplastics as a film or fibers form. Nylon 6-6 is the most commercialized polymer widely used because of its high thermomechanical properties [11].
The Nylon-P,Q (Figure 4.1) refers to the number of carbon atom used in the monomeric chain, which was commercialized as NylonTM is petroleum-derived nylon-6,6 and nylon-6 [6, 9]. PAs are generally a non-biodegradable polymer, although amide linkages were degraded by disrupting the hydrogen bonding [6]. Petroleum-based polymers are posing a significant threat to the environment and its sustainability. Bio-based materials can be alternatives to these petroleum-based polymers. Bio-derived PAs are very much sustainable. The establishment of these sustainable bio-based PAs reduces the use of petroleum-based polymers and reduces ecological problems. PAs, on the other hand, are one of the most consumed polymers as it consumed globally around 7.4 million tons/ annum for the year 2016. Total consumption of PA in 2016 is divided into two parts: PA fibers and film shares 55% globally but 45% share textiles industries used as seats, carpet, sportswear, and different clothes. Some of the bio based, especially fatty acids in vegetable oils, are the source of various monomers to achieve PA.
Figure 4.1 Structures of polyamide with trade name NylonTM (Nylon-P, Q).
4.2.1 Bioavailability and Production
Nondegradable polymers are one of the big issues and create lots of stress over the environment because of PA dumping. A sustainable polymer helps in reducing this stress over the environment. Variety of bio-PAs are derived from renewable raw materials such as PA 4,6; PA 4,4; PA 4,10; PA 4; PA 6,10; PA, 10,10; PA 10,12; and PA 11 [6, 9]. Five decades back, European company Arkema first developed Rilsan, which is 100% castor oil-based PA (PA-11) [6]. Many bio-based polymers were synthesized from castor oil-based with 60% sebacic acid, which exhibits superior performances than petroleum-derived PA 6 and PA 6,6. Nylon 4 is synthesized after ring-opening polymerization of 2-pyrrolidone [2, 5a, 7]. Recently, itaconic acid-based heterocyclic PA has been introduced environmentally degradable, which can reduce the burden of polymer waste [8].
4.2.2 Biodegradability of Polyamides
PAs containing amide linkage have strong hydrogen bonding that is less susceptible to the degradation, but some bacteria can attack their low molecular chain [6, 11, 12]. It should be kept in mind that according to IUPAC terminology, the biodegradable polymer is able to undergo chain scissions, resulting in a decrease in molar mass due to enzymatic process from the action of cells; however, in vitro activity of isolated enzymes cannot be considered as biological activity [12, 13]. Even though bio-based material is composed or derived in whole or natural products issued from the biomass, it does not mean that the material is biodegradable. Certain aliphatic PAs are susceptible to biodegradation by microorganisms (fungi or bacterium) [2]. Some of the thermophilic bacteria isolated from the soil favor the degradability of PA 12 and PA 66 in the culture medium [2]. Some of the white-rot fungal strains have these three kinds of enzyme which are able to degrade the nylon [5b, 12, 13]. Some of the marine bacteria degraded the PA 6 and PA 66, such as Bacillus sphericus, Vibrio furnisii and Brevundimonas vesicularis. The PA can be degraded due to the endogenous enzymatic hydrolysis of an amide linkage [3]. The 14C-labeled nylon-6,6 exposed to various enzyme solutions in vitro, but it was unaffected by some of the enzyme-like esterases, but it degraded after exposure of chymotrypsin, trypsin, and papain.
4.2.3 Degradation of Nylon 4 Under the Soil
Nylon 4 is synthesized from 2-pyrrolidone, which means it is lactam of γ-aminobutyric acid (GABA). It has been reported that nylon 4 is different from other nylon because it degrades under the soil in the activated sludge [7, 12, 14]. Further, nylon 4 was blended with nylon 6, and its degradability was investigated, only nylon 4 part was degraded. Further, Yamano et al. found to degrade the nylon 4 inside the activated sludge further isolated Pseudomonas sp. with the strain ND-10 and ND-11 and GABA as a byproduct [14].
4.2.4 Fungal Degradation of Nylon 6 and Nylon 66 (Synthetic Polyamide)
Nylon 6 is synthesized via ring-opening polymerization of the ε-caprolactam (Figure 4.2) and comes with various commercial names such as perlon, nylon, and steelon. A semi-crystalline linear PA is obtained from ring-opening polymerization of ε-caprolactam in the presence of a tin octoate catalyst [12–14]. In the presence of carbon and nitrogen loving fungi, ε-caprolactam is also degraded. Due to the strong interaction of hydrogen bonding, the degradation rate is slow, but some microorganisms can be degraded PAs, including the bacterial genera Pseudomonas, Achromobactor, and Corynebacterium and Bjerkandera adusta [12–15]. However, some of the authors reported the two kinds of fungal genera, which are lignolytic fungus Phanerochaete chrysosporium NCIM 1073 and Tarmetes versicolor NCIM 1086 in submerged cultivation using nitrogenous nutrient as a stimulator.
Figure 4.2 Ring-opening polymerization of caprolactam.
Both white-rot fungi are well known for lignolytic activity, which attacks the lignin. PA sheets were exposed through submerges cultivation process for microbial degradation, and nylon sheets decreased its thickness and molecular weight. Jozefa Friedrich et al. (2007) had tested 58 fungi for their degradation ability; out of these fungal strain, two fungi were more labile toward the degradation. The white-rot fungi, B. adusta and P. chrysosporium can degrade the polymer, but especially the Bjerkandera adusta disintegrated the fibers. U. Klun et al. (2003) were also tested the same kind of fungus P. chrysosporium that is well known for its lignolytic activity [12–15]. Abiotic (PA-6 placed without fungus) showed a partially weight loss, which means lesser than biotic (PA-6/fungus). Degradation of nylon-6 was observed in the culture of the basidiomycete B. adusta inside the submerged medium, initially break the surface part of the PA. Nylon 6 and Nylon 66 are also degraded in the presence of bacteria Pseudomonas aeruginosa NCIM 2242, which only targets the chain which contains an amide linkage.
4.2.5 Itaconic Acid-Based Heterocyclic Polyamide
Itaconic acid-based PA (Figure 4.3) kept inside the soil for 1 year results in decreased shape, size, and color of polymer resins and photo-solubilization behavior under UV light, which favors the ring-opening phenomenon which reduces the threats of waste disposal [8].
Figure 4.3 Itaconic acid-based heterocyclic polyamide.
4.2.6 Summary and Future Development