Keyword

1 Introduction

Plastic polymers constitute a significant amount of waste that is produced across the world. 10% of the total household waste consists of plastic, whereas plastic makes up of 60 to 80% of the beaches and oceans waste [1]. The particles of the plastic found in the marine water could release a variety of organic pollutants such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), nonylphenol, polybrominated diphenyl ethers (PBDEs), dichlorodiphenyltrichloroethane (DDT), and bisphenol A (BPA). These chemicals are not only harmful to the aquatic life, but due to the ingestion and biomagnification, they eventually become a serious threat to humans [2,3,4] A potential route for the conversion of plastic to the microplastics and its absorption by different life forms is depicted in Fig. 1 [5]. Plastic does not undergo degradation and remains a threat to all life forms for several years [6]. Naturally, plastic can be degraded in four different ways, i.e., photodegradation, thermooxidative degradation, hydrolytic degradation, and biodegradation. The process of degradation starts with photodegradation leading to thermooxidative and hydrolytic degradation, eventually completing with biodegradation. The whole process of degradation is prolonged and can take up to 50 or more years for plastic to completely degrade [7, 8].

Fig. 1
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Copyright 2016, Elsevier

Potential pathway of plastic debris transportation and its biological interactions. Adapted with permission from Ref. [5]

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For several years, scientists have been searching for different ways to minimize the waste plastic polymers. The most commonly used methods are landfills and incineration [2]. Both methods are undesirable in terms of cost, environmental safety, and availability of the space. Only a fraction of the plastic waste is treated by these methods due to economic viability [9].

There is another approach for the reduction of waste plastic from the earth. This approach involves the production of nanomaterials from the waste plastic polymers [10]. Nanomaterials include a variety of substances like nanoparticles, nanotubes, quantum dots, nanosheets, etc. [11,12,13,14]. Nanomaterials have multiple applications in industry, food, agriculture, transportation, medicine, housing, communications, etc. [15,16,17,18,19]. All the nanomaterials have attracted a prevalent interest in nearly all domains of science and engineering due to their astonishing physical and chemical properties [20]. Most of the materials and procedures required for the fabrication of the nanomaterials are expensive. The production cost can be reduced by using waste materials for the synthesis of nanomaterials [21,22,23]. Several waste materials like fruit’s peels, spent battery, automotive shredder, crop residue, and building concretes are now used for the synthesis of a variety of nanomaterials [24,25,26,27,28]. Waste plastic can also serve the purpose. It provides important raw material (such as carbonaceous feed) for the synthesis of different nanomaterials. Fabrication of the nanomaterials from waste plastic can serve two important purposes. The plastic pollution can be eliminated, and a useful substance can be obtained from literally the trash. Several researchers have prepared nanoelectrocatalysts by using waste plastic one way or another for the production of sustainable energy [29]. The plastic waste consists of different polymers such as polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polystyrene [30], polyurethane (PU), and polyethylene terephthalate [31, 32]. The present chapter will explain the use of waste plastic polymers for the fabrication of different nanomaterials and the applications of the synthesized nanomaterials because this selective information will be helpful to explore further ventures in the field of recycling of plastic and production of low-cost nanomaterials.

2 Types of Recovered Nanomaterials from Waste Plastic

2.1 Nanoparticles from Waste Plastic and Their Applications

Small scale particles below 100 nm in diameter are referred to as nanoparticles. Based on their characteristics, structural appearance, and shape, they can be categories into various varieties like ceramic nanoparticles, metal nanoparticles, polymeric nanoparticles, and fullerenes. Nanoparticles have peculiar physical and chemical characteristics because of their minimal size and large surface area. Because of absorptivity in the visible region, these nanoparticles show various colors and are considered to be optically active due to their size. The size, structure, and shape of nanoparticles make them very reactive and tough. Nanoparticles are thought to be feasible for widespread applications such as their involvement in catalytic action, energy projects, tomography, therapeutic, and environmental applications [33]. A precise outlook on the widespread applications of nanoparticles is depicted in Fig. 2. Different materials with a variety of origin like biological, organic, inorganic, and the combination of organic and inorganic materials have been used for the synthesis of nanoparticles. Nanoparticles from inorganic materials have been fabricated with variant techniques such as vapor deposition, ball milling, electro-spraying, metallic salts reduction, sol–gel processes, thermos-analysis, and coprecipitation. Organic nanoparticles are usually synthesized by using approaches like dialysis, microemulsion, nanoprecipitation, and via rapid expansion of materials in supercritical solutions. Moreover, biological nanoparticles like viruses, lipoproteins, ferritin, and exosomes are naturally obtained, and the biological nanoparticles can also be prepared from polysaccharides, peptides, and proteins. Similarly, hybrid nanoparticles are fabricated from the organic and inorganic substrates. The coulomb energy, rigidity, superparamagnetism, surface-to-volume ratio, and catalytic action of nanoparticles are usually higher than the respective bulk substances [34]. Tremendous efforts have been made for the fabrication of low-cost, highly effective nanoparticles for various purposes. The use of the plastics for the synthesis of nanoparticles cannot only reduce the cost of processing but will also eradicate the problem of the pollution. Several studies have reported the use of waste plastic for the fabrication of different nanoparticles.

Fig. 2
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Applications of nanoparticles

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In an investigation, photoluminescent carbon nanoparticles were synthesized from waste plastic bags. The used plastic bags were mainly composed of PP, PVC, or polyvinylidene chloride. These nanoparticles were fabricated in four stages, i.e., thermooxidative degradation, polymerization, carbonization, and passivation. The thermal oxidation of the particles could occur in hydrogen H2O2. The oxidized material could accumulate via hydrogen bonding, followed by the subsequent polymerization via aldol condensation or intermolecular dehydration. Intermolecular dehydration led to the formation of hydrophobic carbon cores, and the non-polymerized species in the solution led to the hydrophilic surface passivation, resulting in the formation of oxygen functionalized photoluminescent carbon nanoparticles [35]. The photoluminescent carbon nanoparticles have several applications. They are involved in bioimaging, sensors, drug delivery, optoelectronics, and photocatalysis [36,37,38,39].

Expanded PS waste was converted into useful nanoparticles via nanoprecipitation. The method employed for the conversion of PS to nanoparticles was cost-effective and straightforward. In this process, expanded PS was dissolved in THF, which was followed by the addition of water in order to achieve precipitation of PS. Nanoparticles (diameter < 500 nm) thus obtained were later on kept at 50 °C for the evaporation of the excessive solvent. Centrifugation and re‐dispersion of the nanoparticles were performed in Milli‐Q water in order to achieve the particle separation [40]. PS nanoparticles have several uses, including separation of chemical substances by electrophoresis [41], perturbing of lipid membranes by significantly reducing molecular diffusion in the membranes, hence, serving in the study of cholesterol management, [42] and affecting the coagulation of blood by reducing thrombin generation [43], etc.

Waste PE was employed in an investigation for the fabrication of hafnium carbide (HfC) nanoparticles. The nanoparticles were prepared via the thermal-reduction route in a stainless steel autoclave with waste PE, metallic Li, and hafnium dioxide at 700 °C. XRD analysis of the prepared sample indicated that the nanoparticles of HfC are face-centered cubic. A similar method of preparation was employed for the preparation of HfC with waste PVC and polytetrafluoroethylene (PTFE). The waste plastic was used as the carbonaceous feed in the preparation of the concerned nanoparticles [44]. The HfC has very high melting points and is resistant to corrosion, high phase stability, and low oxygen diffusion coefficient hence is used in aerospace devices [45]. They are also used in spark plasma sintering, which is needed for compacting ceramics [46].

Multifunctionalized fluorescent carbon dots can be prepared from PS waste. Ramanan et al. have synthesized water-soluble nitrogen-doped carbon dots from the waste expanded PS by a single-step solvothermal process. The fabricated carbon dots have shown remarkably small photoluminescence lifetime, low cytotoxicity, promising photostability, and stable luminescence quantum yield in different solutions. The prepared carbon dots were also utilized as fluorescence probes for selective detection of Au3+ ions, and 53 n M limit of detection was attained [47]. Carbon dots are frequently used in bioimaging, metallic ion detection (Ag, Hg), optical imaging, and biological works [48,49,50,51,52].

MgCNi3 nanoparticles fixed in carbon nanosheets have been effectively prepared from waste PE in a single-step reaction. For the preparation, these nanoparticles metallic sodium, magnesium chloride, and waste PE were placed in stainless steel autoclave at 800 °C for 10 h. Afterward, the autoclave was cooled to room temperature, and the black product was obtained. TEM and XRD analysis have shown that the MgCNi3 nanoparticles are integrated into the nanosheets, and MgCNi3 has an antiperovskite structure. Magnetization analysis depicts 6.8 K superconducting transition of the synthesized materials [53]. Ternary carbides and nitrides with the antiperovskite structure have several useful properties. They have nearly zero temperature resistivity coefficient, negative thermal expansion, phase separation, and magnetostriction. Owing to these and several other properties, these materials are extensively studied [54, 55].

In a recent study, nanoparticles of molybdenum carbide (Mo2C) have been prepared with waste PVC. The waste polymer provided the carbonaceous feed as a carbon source, whereas molybdenum sulfide was used for the metallic component. The nanoparticles were prepared at 600 °C in stainless steel autoclave. XRD characterization depicted that the Mo2C nanoparticles have an orthorhombic crystal structure. SEM and TEM analysis showed the average size of Mo2C crystalline nanoparticles of 50 nm [56]. Mo2C are highly active electrocatalysts that are involved in the production of sustainable hydrogen via electrochemical splitting of water. They are also capable of catalyzing hydrodeoxygenation of vanillin and for the production of γ-valerolactone from levulinic acid hydrogenation [57,58,59].

2.2 Carbon Nanotubes from Waste Plastic and Their Applications

Carbon nanotubes are a newly found carbon form consisting of a coiled layer of hexagonally arranged graphite showing as a smooth cylinder. They have a size range between 0.4 and 100 nm while the length is up to 1 mm. Carbon nanotubes are available as single-walled nanotubes (monolayer of the atoms in the above-said arrangement) and as multi-walled nanotubes (multiple layers embedded axially within one another, in the form of matryoshka doll) as shown in Fig. 3. Carbon nanotubes have used a wide range of applications. Particularly, more enormous current bearing potential and mechanical strength of nanotubes as compared to the metals allow them to be used as interconnects in microelectronics. While the considerably high bandwidth of thin single-walled nanotubes proposes their application as small-sized transistor components, besides this, they are considered to be perfectly adapted to low-voltage emitting devices like flat-panel displays because of the small radius curvature at the tips [60].

Fig. 3
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Graphical comparison of multi-walled carbon nanotubes and single-walled carbon nanotubes. Single-walled carbon nanotubes are monolayered rolled-up graphene, whereas multi-walled carbon nanotubes are several rolled-up graphene layers concentrically

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Carbon nanotubes have enjoyed a prime spot in the different areas of science and engineering since their discovery owing to their remarkable properties such as high tensile strength (100 times more than that of stainless steel), high modulus, low density, high electrical, and thermal conductivities comparable to copper and diamond, respectively [61]. In Fig. 4, the uses of carbon nanotubes in the various field of science are exhibited. Waste plastic of different types has been used for the fabrication of carbon nanotubes. Carbon nanotube has been prepared with vacuum pyrolysis of PP with Fe nanoparticles, where PP, Fe catalyst, and p-xylene were mixed for the formation of the precursor. Precursors were coated with Si wafers, which resulted in the formation of film-shaped species of ~600 nm. Afterward, the prepared specimens were preheated and pyrolyzed in the quartz tube furnace at 200 °C. The product was collected on formvar covered Cu grids. TEM was used for the characterization of the carbon nanotubes [62].

Fig. 4
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Some applications and properties of carbon nanotubes (CNT)

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Thermolytic decomposition of PP and PE produces olefins as the major products, whereas the thermolytic decomposition of PS and PVA yields aromatic hydrocarbons. The chemical reaction between the Fe catalysts and olefins produced from PP and PE is resulted in the formation of long carbon nanotubes. In contrast, the chemical reaction between the Fe catalyst and aromatic hydrocarbons is released from decomposing PS and PVA and resulted in the congealing of the walls of the carbon nanotubes due to the secondary pyrolytic accumulation [62].

Carbon nanotubes could be generated from different waste plastics such as PS, PE, and PP by heating different waste plastics at very high temperatures (700–900 °C) in an inert nitrogen atmosphere. Afterward, the obtained effluents (both aliphatic and aromatic) were burned with air, but PS residues were not combusted in the air as they have already enriched with oxygen. The burning produced CO and CO2, which can be termed as the possible carbonaceous sources for the carbon nanotubes. In the end, stainless steel meshes were employed to develop the nanotubes by chemical vapor deposition [63].

Multi-walled carbon nanotubes and sustainable hydrogen were produced from waste (car bumper plastic) as well as virgin PP in a two-step process without the generation of carbon dioxide. In this process, PP was first pyrolyzed in a screw kiln over montmorillonite and zeolites. The effluents thus obtained were decomposed in a moving-bed kiln with Ni catalyst (produced in situ from NiO) for the simultaneous generation of producing multi-walled carbon nanotubes and sustainable hydrogen. An increase in the yield of multi-walled carbon nanotubes was observed with the increase in temperature (from 550 to 750 ℃) required for pyrolysis. Also, a significant increase in the yield was observed from the doping of the Ni catalyst with Cu. This revealed that the coupled-catalysts are more efficient for use in the generation of these nanotubes [64].

Bajad et al. reported a process for the synthesis of carbon nanotubes and fuel oil by using waste plastic as a precursor. The nanotubes and the fuel oil were prepared from waste plastic in a locally prepared batch pyrolysis reactor separately. For the preparation of the fuel oil, waste PS granules and mixed waste plastic granules were loaded in the reactor in vacuum at 400 ℃, and the effluents were condensed in water condenser at 280 ℃. The oil obtained from mixed plastic was greater in quantity as compared to that obtained from waste PS. For the synthesis of nanotubes, the same reactor was employed as an autoclave, and PP bottles were used as precursors. A six trays stainless steel catalyst holder was placed inside the reactor for catalyst holding as well as for the deposition of the carbon nanotubes. The PP and catalyst were placed inside the reactor for 30 min at 800 ℃ before cooling to the room temperature. Calorific values, FTIR, and density analysis were used to confirm the formation of the fuel oil, whereas the formation of the nanotubes was confirmed with SEM and TEM analysis [65].

Carbon nanotubes have wide-ranging applications, where they are used as a substrate for neuronal growth, and they are involved in the functioning of thin-film transistors. They are conjugated with carbohydrates, proteins, and nucleic acids for various biomedical applications. They are clinically used as biomaterials. They are used in a mixture with metals such as Al for metallurgical purposes and as electrodes in fuel cells [66,67,68,69,70,71].

2.3 Nanocomposites from Waste Plastic and Their Applications

The nanocomposite is a hard material with multiple phases, where one, two, or three dimensions of one of these phases is below 100 nm [72,73,74,75,76,77,78]. They are the substances that integrate nanosized species into some standard material matrix [72,73,74,75,76,77]. Their structure is a combination of matrix and filler in which nanofillers like fibrils, pieces, and particles encircled and connected as a distinct group in the material matrix. The mixing of the two materials has a drastic enhancement in properties of the substrate materials such as toughness, mechanical strength, thermal, or electrical conductivity. Majority of the nanocomposites with remarkable technical abilities are consisted of two phases and can be categorized into three types, i.e., nano-filamentary composites (consist of specific model nested with nanosized filaments), nanolayered composites (consist of changeable coverings of nanosized dimensions), and nanoparticulate composites (consist of specific form in which nanosized particles are placed) [79, 80]. Figure 5 shows some of the prominent applications of polymeric nanocomposites in the field of medical sciences. Numerous waste plastic polymers have been used for the development of a variety of nanocomposites.

Fig. 5
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Applications of polymer nanocomposites in medical sciences

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In an environmentally friendly process, sponge-like, Fe/carbon nanotube nanocomposites were prepared by a solvent-free autogenic technique. Waste PP was pyrolyzed at 600 °C in stainless steel autoclave along with equal amounts of ferrocene and NaN3 for 12 h. Afterward, the mixture was cooled to room temperature, and the product was obtained in the form of black precipitates. The analysis of the nanocomposites by different techniques revealed that both the carbon nanotubes and Fe nanoparticles in the carbon nanotubes were 30 nm in diameter [81]. This type of carbon-encapsulated magnetic nanoparticles has remarkable applications. They are involved in magnetic resonance imaging (MRI), integrated diagnosis and therapeutics, drug delivery, ultra-high-density magnetic recording media, and electromagnetic (EM) wave absorption [82,83,84,85,86].

Waste PVC is also used for the development of useful nanocomposites. One-step conversion of PVC to tantalum carbide/carbon nanocomposite was achieved [87]. Wang et al. used small pieces of waste PVC from the hose and put them in autoclave along with the metallic Li and Ta2O5 at 700 °C for 10 h. The raw product thus obtained was washed with hydrochloric acid, distilled water, and absolute alcohol for the removal of residual substances. The product was later vacuum dried and characterized with various techniques including XRD, TEM, thermogravimetric analysis (TGA), and XPS [88]. TaC nanomaterials are considered a remarkable material for cutting tools, rocket nozzles, and other aerospace uses [87].

Fang et al. used waste packaging PE for the synthesis of organic montmorillonite/PE nanocomposites. The nanocomposites were prepared by coextrusion of PE fresh milk packaging bags and nanosized organic montmorillonite. The thermal properties of the waste PE bags and synthesized nanocomposites were also compared. It was found that the melting range of nanocomposites was lowered, whereas the thermal stability was improved. The prepared material was used as the successful asphalt-modifying agent [89]. Asphalt is a common organic binding substance that is used for corrosion resistance, waterproofing, and moisture resistance. Asphalt requires the addition of synthetic polymers such as styrene-butadiene rubber (SBR), styrene–butadiene–styrene (SBS), and PE as the modifiers for proper functioning, and this significantly increases the cost of the material and renders its use [90, 91]. However, the materials mentioned above provide a low-cost solution to the problem, along with the mitigation of undesirable waste.

Nanocomposite photocatalyst consisting of TiO2 reduced graphene oxide (rGO) and g-C3N4 restrained on waste PS were prepared [92]. The nanocomposite was developed in powder as well as in the film form. The facile calcination method was used for the preparation of the nanocomposites. All the raw materials (TiO2, rGO, g-C3N4) other than PS were kept in the furnace at 520 °C for 4 h. The waste PS in the form of local thermocol was dissolved in acetone and dried to result in the transformation of the solid form into small lumps. Afterward, the nanocomposites were immobilized on the PS lumps by the solvent casting method. The mixture of chloroform and ethanol was used as the solvents. The prepared nanocomposites were used in the multiphase airlift reactor for the degradation of remazol turquoise blue (RTB) dye (Cu-phthalocyanine complex). This was done for the evaluation of the photocatalytic behavior of the prepared nanocomposites. 60% and 93% decolorization was achieved with the film and powdered nanocomposites catalysts, respectively, under sunlight irradiation [92]. In the treatment of wastewater, complex dyes are not easily removed. If dyes are left unattended and discharged to the natural water sources, they become a serious threat to aquatic life as well as to humans [93].

Thermoplastic polyurethane (PU) is not only the cause of environmental pollution, but also it is a fire hazard. Ultrathin β-Co(OH)2 nanosheets were fabricated via the surfactant self-assembly process and integrated into thermoplastic PU which reduces the flammability of the material. The integration of ultrathin β-Co(OH)2 nanosheets into the PU matrix efficiently reduced the release of heat and controlled volatiles toxicity [94]. In the same study, the prepared β-Co(OH)2/PU nanocomposites were converted by a green autocatalytic method into high value-added carbon-based materials with 85% yield. This innovative idea helps in the reduction of plastic polymeric waste by its conversion into useful materials via an environmentally friendly process [94].

An investigation has converted waste PET to unsaturated polyester nanocomposites were developed and has also analyzed its technological and economic aspects [95]. In another investigation, PET waste was first depolymerized via diethylene glycol and 1,4-butanediol in the presence of zinc acetate. This resulted in the formation of glycolysis product, which was employed in the synthesis of unsaturated polyester. The unsaturated polyester prepared from the glycolysis products was used for the fabrication of nanocomposites. The unsaturated polyester was melted, and styrene was added to it with vigorous shaking at 60 °C, afterward, different nanofillers such as cloister and modified montmorillonite clay were added to the mixture.

In contrast, hydrogen peroxide and cobalt naphthenate were added as initiators and accelerators, respectively. The obtained nanocomposites were left for curing overnight at room temperature, followed by post-curing for 2 h at 80 °C [96]. The unsaturated polyester resins are the most widely employed thermoset resins because of their proficient physical properties. They are also employed in thermosetting composite polymers as matrix resins [97].

2.4 Graphene-Based Nanomaterials from Waste Plastic and Their Applications

Graphene has appeared as a favorable nanomaterial due to its remarkable characteristics [15, 98,99,100,101,102]. A single two-dimensional sheet of hexagonally arranged carbon is referred to as graphene. In graphene, each carbon is sp2 hybridized. Of these, the carbon sigma bond that exists in-plane is observed to be the most durable bond, whereas the out of the plane, the π bond, is responsible for delocalized electrons. These delocalized electrons make the graphene active for conductivity and offer weak nuclear forces between graphene and its substrate or among the various layers of graphene. This remarkably unique structure of graphene is responsible for astounding physical properties of the material, making it the epitome of research in the various fields of science and engineering. Among several other properties, graphene also has a promising charge carrying capacity, due to which it acts as light relativist tiny particle or Dirac fermions. Under optimized parameters, they can proceed with excellent dispersion. This property enabled graphene to exhibit a remarkable phenomenon. Graphene is a two-dimensional semiconductor with zero-bandgap. It is also known to have zero band energy and can exhibit shows strong bipolar electric field effect. It also depicts a rare half-integer quantum hall effect for both hole and electron holder when the chemical potential is adjusted via the electric field. Furthermore, graphene exhibits absorptivity of about 2.3% in the visible region and appeared extremely translucent [103]. The heat-conducting power, k, of graphene’s monolayer, is about 5000 WmK−1 at room temperature. Graphene has excellent mechanical strength. The necessary mechanical qualities of graphene membranes consisting of the independent single sheets were observed by the nano-indentation technique by AFM. Young’s modulus and breaking strength of the material are found to be 1.0 T Pa and 42 N m–1, respectively, that make it the strongest materials ever [104]. Graphene has many useful applications; some of them are exhibited in Fig. 6. Several useful graphene-based nanomaterials have been fabricated using waste polymeric plastics.

Fig. 6
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Applications of graphene in different areas of science and engineering

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Graphene foil was synthesized from various plastic waste products, including plastic bottles, i.e., mineral water and cleaning agent bottles, plastic bags, valve bags, protective plastic wrappers, and lunch boxes [105]. These used plastic products are primarily composed of PS, PVC, PP, PET, polymethyl methacrylate (PMMA), and PE. Here, graphene foil has been prepared by solid-state chemical vapor deposition (CVD) technique. In this process, Ni foil was cleaned by using solvents like acetone and alcohol through sonicator and washed with water. Cleaned Ni foil is then subjected to annealing in the presence of Ar/H2 atmosphere at 1050 °C for 30 min to discharge oxides from the surface of the metal. Afterward, all the chopped plastics were subjected to heat, and the melt was shifted into the CVD system. The growth action has occurred at 1050 °C for 120 min. The graphene foil is obtained after the separation of Ni foil through etching in the presence of FeCl3/HCl. Conductivity analysis showed that fabricated graphene foil has a conductance power of 3824 S cm−1. This property enables the material for several practical applications. The prepared foil depicted ultra-low-voltage responsivity and hence was employed in an electro-thermal heater, which heats up to 322.6 °C for the voltage as low as 5 V. Due to the remarkable flexibility of the material, it was also employed as the bendable electrode lithium-ion battery resulting into the high functioning battery [106].

In a recent study, molted salt from PET waste bottles was used for the fabrication of graphene nanostructures. For the preparation of the nanostructures, the plastic was cut into small pieces and was subjected to pyrolysis with sodium chloride into an alumina crucible via an oxidation resistance furnace at 10 °C min−1 to 1300 °C, followed by the immediate cooling at room temperature. This led to the formation of solidified salt and the graphene product. The graphene product was separated from the salt by dissolving the mixture in distilled water. The salt gets dissolved in the water, and the graphene formed the suspension, which was separated by vacuum filtration. The formed product was characterized by TEM, XRD, Raman spectroscopy, and differential scanning calorimetry (DSC) [107]. The nanostructured is obtained in the form of crystalline nanosheets of graphite having a thickness of less than 10 nm with a very high electrical conductivity of 1150 S m-1. Graphene-based nanostructure has a wide range of applications because of its high conductivity and surface area. They are used as conductive composites in electronic devices, in energy storage systems, to harvest solar energy, inks conduction, and have other environmental purposes [31, 108,109,110,111].

In another study, graphene nanoplatelets were synthesized by using a waste tire as a carbon source. The said nanomaterial was obtained via the chemical treatment of the tires. When the waste tires were subjected to the pyrolysis, carbon black was obtained, which was transformed into graphene. The graphene thus obtained was cheaper in manufacturing than the commercial graphene. TEM and SEM were used for the determination of structure, morphology, and dispersion ability of the prepared nanomaterials. The fabricated nanomaterial was used in the synthesis of polyamide 66-based locomotive pieces. The prepared material was proposed as a novel composite based on thermoplastic for the potential applications in the locomotive industry [112].

In another investigation, PET from the waste bottle was used for the synthesis of graphene via thermal decomposition. The applied method for the conversion of PET to graphene nanoparticles was proven to be facile, reproducible, and cost-effective. In this process, raw PET was torn to pieces and screened to get the appropriate size range between 1 and 3 mm by use of conventional sieve shaker. After that, PET waste was put into a stainless steel autoclave reactor and placed into the furnace at the temperature of about 800 °C for 1 h. The product thus obtained was left to cool for the whole night. Synthesized graphene nanoparticles were analyzed with various characterization techniques, including TEM, SEM, Raman spectroscopy, BET, TGA, and FTIR [113].

2.5 Other Nanomaterials from Waste Plastic and Their Applications

Other than the nanomaterials mentioned above, numerous other nanoscale materials are fabricated with a variety of plastics. A brief account is given in Table 1.

Table 1 Nanomaterials from waste plastic and their applications
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3 Conclusion

Plastic waste is one of the biggest modern world problems. Microplastic has adverse effects on both terrestrial and aquatic biomes. In all the ecosystems, the level of microplastic is continuously increasing. The non-biodegradability is allowing the spread of the plastics from municipal sewage to the depths of the oceans. There are several procedures to recycle them, but each has its limitations and disadvantages.

Several attempts have been made to use the waste plastics for the fabrication of a variety of useful nanomaterials. This chapter has elaborated on the synthesis, applications, and characterization approaches of various nanomaterials like nanoparticles, nanotubes, nanocomposites, and graphene-based substances from different waste plastic polymers such as polyvinyl chloride, polyethylene, and polypropylene. The conversion of the waste plastic to the useful nanoparticles is an optimistic approach that cannot only mitigate the pollution but can also provide useful materials at cheaper rates.

4 Future Perspectives

Nanomaterials are still evolving and have deep-rooted applications in modern-day problems. Using the plastic-based nanomaterials to cope with modern problems is a favorable option both in terms of environmental and economic aspects. However, the use of waste plastics has some areas which need the attention of researchers and scientists in future for better productivity and sustainability. For instance, the development of the benchmark process is needed for the conversion of plastics into the nanomaterials as the existing methods are not universal for the wide-ranging polymer of plastic. Different polymers can be converted to the nanomaterials via different methods, which are not feasible on a large scale, as we need separating units as well as the separate treatment plants for variable polymers. A formal protocol of optimized parameters must be developed so that the size and shape of the nanomaterials (particularly carbon nanotubes) can be controlled. Methods for lowering the decomposition temperature of the feed plastic should be determined to cut the energy costs. Several processes of the waste plastic conversion to the nanomaterials involve the generation of undesirable chemicals like dioxins and furans; studies must be made to seek their mitigation or eradication if possible.