Polyethylene (PE) is one of the widely utilized plastics globally, valued for its durability but unsustainable due to its resistance to biodegradation in a natural environment, leading to severe environmental accumulation. Recent studies have identified microorganisms, insects, and potential PE-degrading enzymes (PEases) capable of breaking down PE, suggesting a possible route for biorecycling. However, research in this area remains in its early stages, with limited understanding of the enzymatic mechanisms involved and the degradation products formed. A major barrier lies in the chemically inert nature of PE’s carbon–carbon and carbon–hydrogen bonds, which makes enzymatic degradation particularly challenging and unlikely to occur through a single enzyme. Overcoming these limitations requires the discovery and engineering of complex enzymatic pathways, supported by emerging tools such as omics technologies, structure-guided design, and computer-aided enzyme engineering. In parallel, the biotechnological upcycling of PE waste into value-added chemicals, by first breaking down PE into smaller products and then using them as microbial feedstocks, holds significant potential but is currently underexplored. To date, polyhydroxyalkanoate (PHA) remains the most studied PE waste upcycled biopolymer product, with only a few other studies showing production of diacids, protein, wax esters, and lipids. This highlights the need for expanded research into microbial metabolism and metabolic engineering to enable more diverse and efficient PE waste bioconversion routes. This review summarizes the current state as an integrated effort for biorecycling of PE, including PE pretreatment technologies, enzymatic PE degradation, microbial PE degradation, and PE upcycling into value-added chemicals via metabolic engineering. This review also highlights key scientific challenges and outlines future directions for PE degradation and transforming PE waste into valuable and sustainable products.

Omega-3 polyunsaturated fatty acids (PUFAs), especially eicosapentaenoic acid (EPA, C20:5), are crucial dietary fats known for their numerous health benefits. However, traditional sources of EPA, like fish oil, raise sustainability and environmental concerns, underscoring the need for alternative production methods. The engineered oleaginous yeast Yarrowia lipolytica has emerged as a promising candidate for sustainable production of EPA. This study explores the efficient production of EPA with an earlier engineeredY. lipolytica strain Y8412, utilizing waste cooking oil (WCO) as an alternative carbon source. While cofeeding WCO resulted in increased total lipid content, it also caused an increase in intracellular free fatty acid (FFA) levels, which can be toxic to cells and reduce EPA synthesis. To solve this issue, we first overexpressed FAA1 and GPD1 genes converting excess FFAs into triglycerides (TAGs). Additionally, we knocked out TGL3/4 genes, which encode lipases linked to lipid bodies, to minimize the degradation of TAGs back into FFAs. The modified strains significantly reduced intracellular FFA levels and improved EPA production. Notably, the TGL4 knockout strain Y8412T4- showed 57% increase in EPA production titer and nearly 50% increase in carbon conversion yield compared to the parental strain Y8412 fed with glucose only. These findings suggest that preventing TAG degradation by knocking out TGL4 is an effective approach for enhanced EPA production when WCO is used to partially replace glucose as the carbon source. This study offers an effective engineering strategy for low-cost, high-yield, and sustainable production of omega-3 fatty acids from waste feedstocks.

Poly(ethylene terephthalate) (PET) is widely used for its high strength-to-weight ratio, gas barrier properties, and chemical resistance. The growing PET use highlights the demand for a better recycling system. Enzymatic recycling, alongside mechanical and chemical methods, is eco-friendly and yields properties similar to virgin PET. Substrate properties (T-g, crystallinity, and specific surface area [SSA]) and enzyme stability significantly impact conversion efficiency. Higher SSA and lower crystallinity tend to yield improved depolymerization when employing leaf compost-cutinase (LCC-ICCG) enzymes. This study explored melt extrusion and foaming as pretreatment techniques to modify PET structural properties, using a low-cost chemical foaming agent (CFA). The monomer conversion rate and efficiency during depolymerization were measured and related to the processing, extrudate micro- and meso-structure, and polyester type. Pretreated PET substrates showed reduced Tg, crystallinity, density, and enhanced SSA, resulting in a 90% mass loss for foamed RPET and VPET substrates within 2 days. In contrast, PET with similar to 30% of cyclohexanedimethanol comonomer exhibited a nearly 50% lower depolymerization rate, with zero BHET production. It indicates that the combination of low crystallinity, low Tg, and high SSA leads to improved monomer conversion. These findings emphasize the significance of amorphization and foaming in enhancing PET enzymatic depolymerization.

Poly(ethylene terephthalate) (PET) is a common single-use plastic and a major contributor to plastic waste. PET upcycling through enzymatic depolymerization has drawn significant interests, but lack of robust enzymes in acidic environments remains a challenge. This study investigates in-situ product removal (ISPR) of protons and monomers from enzymatic PET depolymerization via a membrane reactor, focusing on the ICCG variant of leaf branch compost cutinase. More than two-fold improvements in overall PET depolymerization and terephthalic acid yields were achieved employing ISPR for an initial PET loading of 10 mgPET mlbuffer -1. The benefit of ISPR was reduced for a lower initial loading of 1 mgPET mlbuffer -1 due to decreased need for pH stabilization of the enzyme-containing solutions. A back-of-envelop analysis suggests that at a modest dilution ratio, ISPR could help achieve savings on caustic base solutions used for pH control in a bioreactor. Our study provides valuable insights for future ISPR developments for enzymatic PET depolymerization, addressing the pressing need for more sustainable solutions towards plastic recycling and environmental conservation.

Polyesters are an omnipresent material used for a variety of applications (e.g., bottles, packaging, textile, windshield), roughly comprising 8% of plastics produced worldwide. Enzymatic recycling is an emerging solution to deal with the increasingly diverse polyesters that are not suitable for mechanical recycling. However, enzyme activity and efficiency are still the limiting factors impeding enzymatic recycling for different plastic waste forms. The effects of thermal and structural properties (e.g., glass transition temperature, crystallinity, specific surface area), which are determined by chemical composition and preprocessing, directly influence enzyme recycling efficiency. This work investigates two extrusion methods (single screw and twin-screw extrusion) to pretreat a range of copolyesters (RPET, PETG, Ecozen, Tritan and PBT) and modify their properties (i.e., glass transition temperature (T g ), crystallinity (%), and molecular weight (M n )). A PET-specific enzyme, leaf-branch compost cutinase (LCC ICCG ), produced from a fed-batch fermentation of Escherichia coli BL21(DE3), was used for the enzymatic depolymerization of different polyesters. Several copolyesters showed improved depolymerization after pretreatment, as measured by rate and amount of monomers produced. Those that did not depolymerize were found to have exceptionally high glass transition temperature or percent crystallinity, highlighting the importance of these physical parameters on conversion efficiency.

The complex relationship between kinetics and substrate morphology during enzymatic depolymerization of melt processed poly(ethylene terephthalate) (PET) is explored and the effects of competing transformations are analyzed. Extruded PET substrates from post-consumer recycled PET (RPET) bottles flakes subjected to enzymatic depolymerization are examined to reveal increases in crystallinity from similar to 10% post-extrusion to > 30% after 3 days of depolymerization as well as increases in glass transition temperature (T-g) from similar to 66 degrees C to > 80 degrees C. Further investigation into this behavior shows that post-extrusion RPET substrates do not exhibit changes in crystallinity when subjected to dry annealing at 65 degrees C in an oven over 7 days, but they do experience annealing in depolymerization buffer solution with no enzymes at 65 degrees C within 3 days. This difference may be attributed to plasticization of PET in the presence of water, also known as solvent induced crystallization. The impact of this plasticized annealing behavior is demonstrated by subjecting RPET substrates to increasing enzyme-to-substrate loads. As enzyme load increases, overall conversion of substrates increases despite crystallinity also increasing to similar levels regardless of the initial enzyme loading. The competition between depolymerization and crystallization suggests that the rate at which PET substrates are depolymerized at the operational temperature is integral to achieving high conversion to monomeric product. This, in turn, also suggests that savings from lowered enzyme loadings may be more detrimental than helpful in pursuing the most cost-effective recycling systems.

Millions of tons of waste polyester plastics, including polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), and polybutylene terephthalate (PBT), end up in the environment as soil and water contaminants. Recent advances in enzymatic polyester degradation have motivated researchers toward the biorecycling of plastic wastes. Leaf-branch compost cutinase (LCC) enzymes have been proven to be effective in the biodegradation of PET. This study focuses on enzymatic depolymerization of waste PET, PTT, and PBT materials by using an ICCG variant of LCC (LCCICCG) produced from Escherichia coli BL21(DE3). The degradation efficiency of the polyesters was determined by the monomer terephthalic acid (TPA) released from the depolymerization reaction. It was found that the most efficient depolymerization was achieved for PET, followed by PTT and PBT. A kinetic model based on Langmuir adsorption and the Michaelis-Menten equation was developed to describe the enzymatic depolymerization of PET, PTT, and PBT with various enzyme and substrate loadings. The model simulation results revealed that the LCCICCG enzyme loading should be linearly increased as the work capacity of the polyester substrate increases. A specific enzyme loading of 0.91 mg/g PET is suggested to achieve 90% depolymerization of PET within three days. The experimental data and model simulation results can be used to help further engineer the enzyme and process to achieve a complete biodegradation of polyester wastes at a large scale.

Poly(ethylene terephthalate) (PET) is one of the world's most widely used polyester plastics. Due to its chemical stability, PET is extremely difficult to hydrolyze in a natural environment. Recent discoveries in new polyester hydrolases and breakthroughs in enzyme engineering strategies have inspired enormous research on biorecycling of PET. This study summarizes our research efforts toward large-scale, efficient, and economical biodegradation of post-consumer waste PET, including PET hydrolase selection and optimization, high-yield enzyme production, and high-capacity enzymatic degradation of post-consumer waste PET. First, genes encoding PETase and MHETase from Ideonella sakaiensis and the ICCG variant of leaf-branch compost cutinase (LCCICCG) were codon-optimized and expressed in Escherichia coli BL21(DE3) for high-yield production. To further lower the enzyme production cost, a pelB leader sequence was fused to LCCICCG so that the enzyme can be secreted into the medium to facilitate recovery. To help bind the enzyme on the hydrophobic surface of PET, a substrate-binding module in a polyhydroxyalkanoate depolymerase from Alcaligenes faecalis (PBM) was fused to the C-terminus of LCCICCG. The resulting four different LCCICCG variants (LCC, PelB-LCC, LCC-PBM, and PelB-LCC-PBM), together with PETase and MHETase, were compared for PET degradation efficiency. A fed-batch fermentation process was developed to produce the target enzymes up to 1.2 g L-1. Finally, the best enzyme, PelB-LCC, was selected and used for the efficient degradation of 200 g L-1 recycled PET in a well-controlled, stirred-tank reactor. The results will help develop an economical and scalable biorecycling process toward a circular PET economy.

In recent years, graphene has been subjected to intense scientific research interests owing to the remarkable properties of this class of materials. An appropriate loading of this youngest 2-D or 3-D honeycomb structured carbon based allotropic nanoscale material in the host polymer can considerably advance the physical, chemical, structural, mechanical, and electrical features of polymeric materials. Graphene filled polymer nanocomposites are attractive material as a functional component in solar energy devices and fuel cells. Graphene/polymer nanocomposites are noted to beneficial in absorption of photon, charge carrier transportation, and separation of charges when utilized as photoanode or counter electrode material in solar energy devices. In fact, the use of graphene/polymer nanocomposites particularly provides prolonged lifetime of fuel cells and solar energy related devices along with high productivity and liberty of repeatability. The remarkably high physical and chemical features of graphene/polymer nanocomposites are highly valued in fuel cell applications for the boosting of conduction of proton. This chapter will chiefly focus on the remarkable applications and potential use of graphene/polymer nanocomposites in solar energy and fuel cells. It will mainly be comprised of five sections: Section 6.1 will discuss a comprehensive introduction of graphene along with its synthesis techniques, its remarkable features, and major factors contributing in the enhancement of its properties. Section 6.2 will illustrate different types of graphene/polymer nanocomposites along with their commercially and economically acceptable fabrication methods and performance effecting parameters. The potential use of these nanocomposites in fuel cell and solar energy applications will be highlighted in Section 6.3 and 6.4, respectively. Lastly, in Section 6.5, the commercialization and property advancement challenges and future of graphene/polymer nanocomposites will be discussed.

This research presents an investigation on the incorporation of atactic polypropylene (APP) as a modifier in bitumen at weight concentrations from 0 to 30 wt%. An APP-modified bitumen (APPMB) was prepared by conventional, hot, and in situ blending and characterized for mechanical, morphological, thermal, structural, and physical properties. APPMB specimens using maleic anhydride (MA) functionalized polymer at its optimized concentration of 20 weight percentage (wt%), along at several other concentrations, were prepared using in situ and hot blending methods using an overhead stirrer mixer at a blending temperature of 160 degrees C for 2 h. Fourier transform infrared revealed a strong interaction of APP or APP-g-MA with bitumen. The ultimate compressive strength, acquired in Mega Pascal (MPa) units, was increased from 0.24 MPa at 5 wt% APP to 0.70 MPa at 20 wt% APP loading. Scanning electron microscopy micrographs manifested dispersed phases of APP in bitumen due to the high temperature and shearing application with overhead stirring. In situ and hot blended specimens exhibited better mechanical, thermal, and structure-property relations. This research highlights the novelty in terms of property optimization and advancements at a high-weight percentage of the modifier (i.e., 20 wt%) along with emphasizing the comparison of three different fabrication techniques; however, the previous research manifested the advancements in the properties of polymer and bitumen blends at low concentrations of polymers.

An intermediate stage in the net casting that can potentially limit the features of the final metallic product, called dewaxing, is highly dependent on features of pattern wax. The general process of dewaxing, involved in investment casting for the reuse of the wax, requires a high amount of energy and entails high processing time through conventional heating methods thus resulting in a lower rate of processing at a bulk scale production. In this research, the pattern wax blends, containing polyethylene, ethylene–vinyl acetate, bitumen, and paraffin wax, were investigated for their microwave processing time in the presence of susceptor material (polyaniline). Polyaniline in wax blends was incorporated as a microwave susceptor in concentrations of 0.15, 0.45, 0.75, 1.5, 3, 5, and 10 wt%. Advancements in processing properties of pattern wax blends, using microwave dewaxing technology, were observed along with the improvement in melting behavior of pattern wax blends owing to the creation of heat-sensitive sites due to the addition of a susceptor. The decrease in complete flow time was observed from 202 to 55 s by the addition of 10 wt% of polyaniline in wax. A ceramic die and stand to hold the die at a hotspot in microwaves were designed for experiments. FTIR-ATR, rheological, TGA, DSC, mechanical (UTM), and morphological analysis (SEM and OM) were performed to investigate the behavior of pattern wax blends under the application of microwave heating. An inverse trend was observed in processing time with susceptor loading in microwaves.

Wood polymer composites (WPCs) are considered as one of the highly competent classes of hybrid composite materials having potential applications in automotive, furniture, and construction industry. In this present study, fabrication of PP-g-MA compatibilized PP-wood flour composites was accomplished utilizing melt blending extrusion. The extruder was operated at a temperature profile of 180–210 °C. Wood flour, prior to its incorporation in polymeric material, was sieved to get fine wood flour particles. Compression-molded specimens of composites were characterized for their morphological attributes and for the determination of their chemical makeup, mechanical features, and thermal stability. FTIR analysis determined the chemical makeup of WPCs. Thermogravimetric analysis revealed the thermal stability of composites at temperature higher than 280 °C. Composites were rated as V2 grade according to their flammability performance. SEM analysis manifested the dispersed state of wood flour in the PP matrix. Mechanical properties manifested the increased stiffness of WPCs owing to increase in loading of wood flour which was associated with the restricted motion of polymer chains imparted by the wood flour particles. Surface topology study of in-hole drilled surface was also carried out by performing machining of WPCs for the analysis of surface roughness as a function of drill speed. Furthermore, the environmental sustainability of WPCs machining was demonstrated, and the reduction in waste generation through drilling of composites was observed.

Healthcare waste management is considered one of the biggest challenges that the world is going to face in the future. This threat is becoming reality owing to the worldwide sharp rise in healthcare waste generation particularly during the current COVID-19 pandemic. Like many other environmental crises, hospital plastic waste management is an area that got very little attention despite being highlighted in the literature, local media, as well as in international electronic and print media. This mini-review was conducted to assess the overall prevailing situation regarding hospital plastic waste management in Pakistan. Several illegal and unethical activities have been observed regarding hospital plastic waste management in Pakistan which includes unhygienic recycling, repacking of used hospital plastic items, open dumping on land, and disposal of hospital plastic waste in the ocean. To improve these conditions, suggestions have been made regarding the better management of hospital plastic waste.

Photovoltaic (PV) technology is considered as a major drive for achieving exigent energy targets. All around the world, huge photovoltaic power plants for the purposes of energy generation at extremely competitive production cost are being constructed in the arid and desert areas. But this advantage of cost effectiveness seems to be disappeared due to the soiling issue of PV modules. The cleaned PV modules are therefore required in the areas where the panels are more likely to become dirty. The major concern related to the cleaning of PV modules is the strategy utilized for cleaning of extremely competitive PV modules. However, manually cleaning the surface of panels, traditionally, is an economically time exhaustive procedure. Moreover, tiny particles are difficult to remove through manual cleaning. Recently, various researches have been endeavoured towards development of water repelling surfaces with novel structures and viable applicability in self-cleaning solar panels. This chapter will discuss the recent developments in organic superhydrophobic coatings for PV modules. It will be comprised of four parts: Section 1 will highlight the basic concepts and principles of superhydrophobic phenomenon. Section 2 will describe the superhydrophobic coating methods. Organic coating materials and their applications for the self-cleaning of PV modules will be discussed in Sect. 3. Finally, the commercialization challenges and future prospects of self-cleaning coatings will be discussed in Sect. 4.

Dewaxing is an intermediate step in investment casting which can determine the properties of final metallic product and is strongly dependent on the properties of pattern wax. Processing of pattern wax composites with the conventionally available heating processes is energy intensive and requires high processing time, thus resulting in the lower processing rate at larger scale. Use of microwave technology for the dewaxing study is highly attractive alternative owing to the advantage of energy-efficient processing. The processing time of pattern wax composite of paraffin wax, bitumen, polyethylene and EVA in microwaves oven as a function of susceptor has been investigated. Carbon black was used as microwave susceptor in wax composites at several weight percentages. The efficient processing and melting behavior of pattern wax composite utilizing microwave dewaxing, thanks to the creation of heat sensitive sites within the pattern wax composite by the susceptor material, were analyzed in this study. FTIR-ATR, rheometer, TGA, DSC, UTM, SEM and OM were used to study the behavior of the wax composite. With the increase in the susceptor (CB) loading from 0.15 to 0.75%, an increase in the thermal stability of 6.86% was observed as depicted by the increase in the on-set temperature of the specimens. 71.9% decrease in the melt flow time of pattern wax composite was observed with the incorporation of 1.5% of CB as compared to reference specimen which manifested an inverse relationship between susceptor loading and processing time in the presence of microwave.

Renewable resources including polycarbohydrates, lignin, proteins, and polyacids are the intrinsically valuable class of materials that are naturally available in great quantities. Their utilization as green additives and reinforcing bio-fillers, in substitution of environmentally perilous petroleum-based fillers, for developing high-performance green rubber blends and composites is presently a highly tempting option. Blending of these renewable materials with elastomers is not straight-forward and research needs to exploit the high functionality of carbohydrates and other natural materials as proper physicochemical interactions are essential. Correlating and understanding the structural properties of lignin, carbohydrates, polyacids, and other biopolymers, before their incorporation in elastomers, is a potential approach towards the development of green elastomers for value-added applications. Promising properties i.e., biodegradability, biocompatibility, morphological characteristics, high mechanical properties, thermal stability, sustainability, and various other characteristics along with recent advancements in the development of green elastomers are reviewed in this paper. Structures, viability, interactions, properties, and use of most common natural polycarbohydrates (chitosan and starch), lignin, and proteins (collagen and gelatin) for elastomer modification are extensively reviewed. Challenges in commercialization, applications, and future perspectives of green elastomers are also discussed. Sustainability analysis of green elastomers is accomplished to elaborate their cost-effectiveness and environmental friendliness.

The barrier property enhancement of polymers is presently a matter of great concern for the manufacturing of food packaging and films with excellency in moisture and gas resistance. The objective of this work was to enhance the barrier performance of Nylon12/kaolin clay nanocomposites against water vapors and oxygen. Kaolin clays of different aspect ratios were utilized for nanocomposites manufacturing. Nanocomposites were prepared in twin screw extruder operating at 160-200 degrees C, with an increment of 10 degrees C, and at 110 rev/min. The loading of clay was varied from 1 to 5 wt%. Scanning electron microscopy and X-ray diffraction characterizations were used to investigate the morphological properties of nanocomposites. The transmission electron microscopy analysis was used to confirm the dispersion of clays in Nylon12 matrix. Enhancement in barrier performance of nanocomposite was noticed at 5 wt% clay loading. Oxygen barrier of nanocomposites was observed to be more prominent than water vapors owing to the presence of hydrogen bonding in Nylon12 structure which restricted oxygen passage. Experimental barrier values of nanocomposites were also fitted on barrier models namely Nielsen, Cussler model, and Gusev-Lutsi model.

The approach of upgrading barrier properties of polymer with nanofillers is effectual and well established now-a-days to manufacture films and quality packaging. This research deals with enhancement of barrier attributes of clay-based high density polyethylene nanocomposites against oxygen and water permeability. Na-Montmorillonite and two grades of kaolin clays were used for HDPE/clay nanocomposites preparation in twin-screw extruder with temperature profile of 160, 170, 180, 190 and 200 °C along the extruder at 110 rpm with clay loading up to 10 wt%. The extent of maximum reduction in water and oxygen vapors permeation was more prominent at 5 wt% sample of each clay. XRD analysis revealed no exfoliation in kaolin clay samples but indicated presence of exfoliation in Na-MMT samples. SEM and TEM micrographs revealed nano-level dispersion for both kaolin clays and Na-MMT and agglomerate formation at high wt% of clays. Experimental data for oxygen and water vapors permeation through nanocomposites were fitted on Nielsen, Cussler and Gusev–Lutsi models. Modelling of barrier performance was utilized for acquiring aspect ratio of filler for nanocomposites. Maximum and average aspect ratio of fillers and third-degree polynomial equation for nanocomposite, through curve fitting, were determined to predict improved barrier properties of nanocomposites.