Polyethylenes: A Vital Recyclable Polymer

Abstract

Polyethylene (PE) is a synthetic polymer made from the polymerization of ethylene. It is the most widely used plastic in the world. Its production, processing, usage, applications, and disposal system had made the study of this plastic very significant. The role played by this plastic in the world has made the knowledge of its usage, disposal system, processing, recycling, and applications inevitable. The chapter discussed the general overview of this plastic product, the production, properties, and disposal systems. The processing of recycled polyethylene is vital in its end-use through collecting, sorting, cleaning, separation, and compounding, and this was extensively treated. We also discussed the opportunities, applications, and limitations of polyethylene recycling. At the end of this chapter, one will know the production, processing, recycling, and applications of polyethylene plastic and the dangers posed by this plastic if a proper disposal system is not followed.

1. Introduction

Polyethylene (PE) is a synthetic resin made from the polymerization of ethylene [1]. It is an ethylene polymer with the structural formula of (-CH2-CH2-)_n. This is generated at high pressures and temperatures in the presence of a catalyst, based on the desired characteristics and properties for the final product. It is manufactured as branched low-density polyethylene (LDPE), linear high-density polyethylene (HDPE), and many other variants. This olefin plastic can be combined with other elements, compounds, and monomers to form other polyethylene brands and co-polymers. The primary production processes employed are the Ziegler-Natta, metallocene, and chromium/silica catalysts in their manufacture, and this production pathway affects the final products’ mechanical and end-use properties. Water, acids, alkalis, most solvents, and chemicals do not affect polyethylene. Polyethylene offers superior low-temperature resistance, excellent chemical resistance, excellent power insulation, intense pressure, and high radiation resistance. Polyethylene is highly susceptible to environmental stress (both chemical and mechanical) and has low heat-aging resistance. Polyethylene characteristics and properties vary based on the molecular structure and density of the polymer [2]. PE is used in various items and packaging, including milk jugs, drinking straws, bottle caps, detergent bottles, cream tubs, waste bins, water pipes, children’s toys, and films, as well as plastic bags. Most unwanted items are disposed of in landfills, burned, and recycled, while some are inappropriately disposed of and strewn on streets and highways, wreaking havoc on our environment and marine life. Improperly disposed polyethylene waste poses many public health concerns and harms flora and fauna and the environment, mainly when it is not collected and disposed of properly [3]. Efforts are made to convert or reuse polyethylene products through recycling. Polyethylene waste disposal, reduction, and recycling generate many benefits if handled well [4]. This chapter will discuss polyethylene production and processing, properties, application and usage, disposal systems, and recycling.

2. Production and processing

Polyethylene is composed of carbon (C) and hydrogen (H), and these elements can be combined in several ways to make different types of polymer [5]. It is produced by modifying natural gas (methane, ethane, and propane blend), fermentation, ethanol dehydration via acetaldehyde hydrogenation, pyrolysis, and catalytic cracking of crude petroleum product or its distillation into gasoline. Branched low-density polyethylene (LDPE) was discovered first through free radical polymerization under high pressure and temperature by ICI Laboratories UK in 1933 [6]. Linear High-Density Polyethylene (HDPE) is manufactured at low pressures using Ziegler-Natta catalysts in slurry or gas-phase processes. Metallocene linear low-density polyethylene (mLLDPE) is also produced using low-pressure polymerization technology, which copolymerizes ethylene with another monomer, such as butene-1 or hexene-1, with the help of metallocene catalyst. The metallocene catalyst leads in resins with very consistent and specific properties, such as superior toughness and stiffness balance. Low-pressure polymerization technology employs transition-metal catalysts to produce Medium Density Polyethylene (MDPE) and linear low-density polyethylene (LLDPE) products. However, comonomers are introduced into the reaction to form small short-chain branches on the linear molecule, causing the density of the polymer to decrease [7]. The production process of PE involves the polymerization of ethylene into polyethylene. This process takes five routes from ethylene to the preparation and production of polyethylene, namely high pressure, metallocene, Ziegler-Natta, Standard Oil, and the Phillips processes [8].

High-Pressure polymerization is usually done at high pressures (0.1–0.3KN/mm²) and temperatures from 80 to 300°C, using free-radical initiators like azo-diisobutyronitrile. This is accomplished by reducing the exotherm using flowing or running water through a jacketed reactor by utilizing a high cooling surface volume ratio in the right portion of the continuous reactor. In this process, 10–30% of the monomer is converted to polymer, which is then extruded as granules [8, 9, 10].

PEs with short-chain branching are produced through metallocene manufacturing routes using metallocene catalysts. Metallocene catalysts are made by infusing zirconium or titanium, or other transition compounds into a cyclopentadiene-based structure. In this technique, a monomer in gaseous form and a metallocene catalyst are loaded into a fluidized bed reactor at pressures less than 24 MPa and temperatures just under 100°C. The process is very adaptable, and different kinds of PE can be produced by modifying the reaction conditions using the catalysts. Short branches of PE are formed by adding small amounts of propene, butene, hexene, or octene into the monomer feed. They are designed using existing polymerization processes, giving way for metallocene-PEs having different properties and grades [8].

Ziegler Processes are the result of Ziegler’s, Natta’s, and co-workers’ efforts. This polymerization allows the generation of a coordination complex due to the reaction between the initiator and the catalysts. This complex controls how the monomer approaches the growing chain. In this process, ethylene is supplied under low pressure into a reactor containing a liquid hydrocarbon that serves as a diluent and the catalyst. The catalyst is composed of titanium tetrachloride and aluminum triethyl. The catalyst complex can be produced ahead of time and then fed into the vessel, or it can be prepared in situ by feeding the components directly into the main reactor. In the absence of oxygen and water, the reaction can reach temperatures of up to 70°C. The polymer precipitates from the solution, forming a slurry, and the reactants are emptied into a catalyst decomposition tank [9].

The Standard Oil Company Process utilizes a transition metal oxide in combination with a promoter. Temperatures and pressures in the reactant vary from 230 to 270°C and 4 MPa–8 MPa, respectively. As a catalyst, molybdenum oxide is utilized to fast track the reaction, while sodium or calcium as metals or hydrides are used as promoters. The reaction occurs within a reactor in a hydrocarbon solvents [8, 10].

The Phillips Process includes dissolving ethylene in a liquid hydrocarbon solvent and then polymerizing it at 130-160°C and 1.4–3.5 MPa pressure with a supported 5 percent chromium oxide catalyst on a finely split silica-alumina catalyst. The major role of the solvent is to dissolve the polymer as it develops while simultaneously serving as a heat transfer medium. The recommended catalyst is a finely split silica-alumina catalyst that has been activated by heating to about 250°C and includes 5% chromium oxides, principally CrO₃. The combination is then transported through a gas–liquid separator, where the ethylene is flashed off, the catalyst is removed from the separator’s liquid product, and the polymer is removed from the solvent [11].

These processes of polymerization reaction involve three stages: pre-treatment stage, reaction stage, and separation stage, and the process is sensitive to a catalyst that initiates the reaction to produce free radicals. The production process takes place in a continuous operation that requires a source of pure ethylene, suitable compression equipment at high pressure, and a high-pressure reactor to perform quick and high exothermic polymerization control. In the pre-treatment stage, the ethylene stream is pressurized into a compressor and heat exchangers, and this is maintained at 2000 bar and 150°C with the initiator before entering the reaction zone. The reaction zone is modeled in high-pressure tubular reactors with a water vapor stream to ensure steady temperature conditions in the reactor. At this stage, the initiators generate free radicals that link up with different ethylene chains to form a vinyl polymer. Then, other radical and monomer transfers occur at propagation to increase the number of chains formed gradually. Finally, the polymerization process is completed at termination by temperature conditions that disrupt the chemical bond formed, producing low-density polyethylene. The final stage involves the removal of the polymer and its separation. Also, Flash equipment is used to evaporate ethylene that does not react and then reused. LDPE is polymerized, forming large chains and generating enough entropy in the order of their bonds between monomers, causing a reduction of their density.

Three processes are used in the manufacture of high-density polyethylene. The first is the solution process, where the catalyst, initiator, and monomer are dissolved in a solvent in a continuously stirred tank reactor [12]. The solvent is removed when the polymerization is completed while isolating the polymer. The second is the slurry process, where the catalyst and polymer are suspended in a liquid medium in a continuously stirred tank reactor or tubular reactor where polymerization takes place. The catalyst and polymer are not dissolved in the medium and are separated after polymerization. The final and third process is the gas phase process, where no solvent is involved. The monomer and the catalyst are blown into the fluidized bed reactor for polymerization to take place. The processes of PE production are cataloged in Table 1.

PE type	Year	Company	Process	Catalyst
LDPE	1933	ICI Laboratory, UK	High-pressure free-radical polymerization; 2000–3000 bar, 80-300°C	Low quantity of oxygen
LDPE	1943	Union Carbide & Du Pont	Film extrusion and injection molding	Free radical catalysts using initiators (peroxides)
HDPE	1953	Philips Petroleum, Ziegler	Gas-phase processes in solution or slurry	Ziegler-Natta organo-metallic catalysts
HDPE	1955	Hoechst A G in Germany	Low-pressure polymerization	Organo-metallic catalysts
MDPE			Low-pressure polymerization	Transition-metal catalysts
LLDPE	1950s	Du-Pont	Low-pressure solution process	Transition-metal catalysts
HDPE	1973	Union Carbide	Their fluidized bed gas-phase process	Ziegler-Natta
LLDPE	1977	Union Carbide	Low-pressure gas-phase process	Free radical catalysts using initiators (peroxides)
LLDPE; VLPE	1980s	Dow Chemicals	Solution process	Ziegler/Natta, Cr/Mo oxide
mLLPE	1990	EXXOM MOBIL	Gas-phase PE Process	Metallocene catalysts

Table 1.

Manufacturing & processing development of polyethylene.

PE can also be produced from other routes rather than from high and low-pressure processes. Linear polyethylene with the repeat unit of ᷈↭(CH₂)n↭ by condensation polymerization of an ethereal solution of diazomethane [13]. Also, Linear PE with a molar mass of up to 1.3 kg/mol is produced by the reaction of decamethylene dibromide with sodium in the Wutz reaction. Other processes include reducing carbon monoxide by modified Fischer Tropsch process, reducing polyvinyl chloride (PVC) with lithium aluminum hydride, and hydrogenation of polybutadiene [8].

3. Properties

Polyethylene as a polymeric material possesses excellent properties, which makes it useful as an engineering material. These properties depend on the density, molecular weight, molecular weight distribution, degree of long-chain branching, and short branching. It possesses low water absorption, moderate to low gas permeability, good toughness, flexibility at low temperatures, relatively low heat resistance, and many chemicals. The use of PE as an engineering material is due to these excellent properties greatly affected by density, molecular weight, and molecular weight distribution, as enumerated in Table 2 [15].

Property	Increase in density	Increase in molecular weight	Increase in molecular weight distribution
Tensile strength	Increases	Increases	—
Impact strength	Decreases	Increases	Decreases
Stiffness	Increases	Increases	Decreases
Stress crack resistance	Decreases	Increases	Increases
Abrasion resistance	Increases	Increases	—
Chemical resistance	Increases	Increases	—
Melt strength	—	Increases	Increases
Softening point	Increases	—	Increases
Low-temperature brittleness	Increases	Decreases	Decreases

Table 2.

Effects of density, molecular weight and molecular weight distribution on some properties of polyethylene [14].

Polyethylene type	Mol. weight (g/mol)	Density (g/cm^3)	Degree of crystallinity (%)	Melting temp (°C)	Enthalpy of melting (j/g)	Yield strength (MPa)	Melt index rate (g/10 min)
HDPE	>250,000	0.940–0.970	70–90	120–130	203	20–40	0.1–150
LDPE	10,000–50,000	0.915–0.940	45–55	105–115	103	4–16	0.1–150
LLDPE	50,000–200,000	0.915–0.926	35–45	112–124	160	8–45	0.1–150
MDPE	100,000–200,000	0.930–0.940	30–60	115–130	141.9	15–40	0.1–150

Table 3.

Important PE grades and properties.

Polyethylene has the following engineering properties in near-absolute terms. The crystallinity of LDPE is between 40 and 60%, depending on the degree of branching and thermal history, while that of HDPE is between 60 and 90%, depending on the cooling rate and thermal history. It melts in the absence of air at 300°C to form transparent liquid except for its cross-linked polymer, which does not melt. Its specific heat at 20°C is 1330–2400 JKg⁻¹ K⁻¹ for HDPE, and 1900–2300 JKg⁻¹ K⁻¹ for LDPE, coefficient of linear expansion is 0.00017–0.00022 K⁻¹ for LDPE and 0.00013–0.00020 K⁻¹ for HDPE, Thermal conductivity at 23°C is 0.33Wm⁻¹ K⁻¹, melting point (MP) 109–125°C for low-density polyethylene and 130–135°C for high-density polyethylene. The specific gravity of LDPE is between 0.915–0.94, while that of HDPE is 0.94–0.97. PE has a refractive index of 1.51–1.52. PE also has excellent electrical properties. Its electrical resistivity is 1017–1019 ohm-m, dielectric strength 20 kV–160 kV/mm, and dielectric constant of 2.28. Polyethylenes, when exposed to moisture for 1 year, increase their weight by 0.2%. This shows excellent water resistance. PE’s average elastic modulus is between 0.565–1.500GPa for HDPE and 0.190–0.520GPa for LDPE, flexural modulus is between 0.28–1.86GPa for HDPE and 0.152–2.200GPa for LDPE. It also has Flexural yield strength of 13.8–75.8 MPa, compressive yield strength of 4 MPa-23 MPa for HDPE, and Tensile strength (TS) at yield is 7 MPa-16 MPa and that at the break at 23°C is 32 MPa-60 MPa for LDPE. LDPE has a hardness of 50–60 while its HDPE counterpart is 65–70 with an impact of 0.92 for LDPE and 0.96 for HDPE [16].

3.1 Mechanical properties

Some of the significant mechanical properties include Tensile strength, compressive strength, flexural strength, impact strength, Viscoelasticity, and abrasion resistance.

3.1.1 Tensile strength

Tensile strength is the ability of a material to resist a force that tends to pull it apart. It is a basic provision for classifying the properties of given polymer materials at a specified loading rate and temperature. The ultimate tensile strength of PE at 0-70°C is 11 MPa–25 MPa, while its yield strength at the same temperature range is 6 MPa-30 MPa. This polymer property depends on percentage crystallinity, the thermodynamic stability of the primary PE chain, chain orientation, and packing density of the crystalline chain [17]. Polyethylene’s strength, rigidity, friction, and hardness are low but have high impact strength and ductility. Three main tensile strengths are essential in the study of the properties of a polymer. The yield strength is the stress the PE can withstand without permanent deformation. The ultimate tensile strength is the maximum stress the PE can withstand, while the breaking strength is the stress coordinate on the strain–stress curve at the point of rupture.

Polyethylene shows excellent creep under an applied force. Elastic strain is the strain in the region of stress/strain curve of material under deformation, which recovers its shape on the release of applied stress. Elastic strain is reversible. Young’s modulus is the slope of the graph ab covering from a being the origin of the stress–strain graph to point b, representing the reversible region of the stress–strain in Figure 1. After point b, strain is no longer proportional to stress, and the slope of the stress–strain graph changes at an increasing rate, and the strain is irreversible. The materials continue to deform after point c until it breaks at point d [18]. The percentage elongation we get during a tensile experiment is significant because it provides information on the ductility of the polyethylene under investigation. Materials with a high degree of elongation will exhibit high ductility. This is because the force necessary to sustain sample elongation and finally break the specimen changes very little at the yield point, and this makes the value of the yield strength and the breaking strength quite close [14].

Figure 1.

Stress vs. strain curve of a typical PE.

3.1.2 Compressive strength

Compressive strength is the ability of a polymer material to resist the direct pressure of applied compressive force [19]. It is the ability of the polyethylene material under test to resist loads applied, thereby shortening the length of the material under compression. Its force acts in the opposite direction to the tensile force applied to the load. Compression is a force that pushes the particles of material closer together, thereby reducing the size of the materials. The compressive modulus is equal to the elastic modulus at minor strains and gives a reliable compressive stress and strain ratio. When a compressive force is applied to a PE material, it yields slowly and hardly fails.

3.1.3 Flexural strength

Flexural strength is defined as a material’s resistance to distortion on the application of load, while flexural modulus measures the capacity of the test sample to bend. Flexural strength represents the amount of force necessary to break a test sample with a specific diameter.

3.1.4 Impact strength

Impact strength can be defined as the capacity of a test polymer sample to withstand fracture when a sudden force is applied to it. It is the energy absorbed by this sample without breaking. The most frequent tests for plastic materials are the drop-weight test, the Izod Impact Test, and the Charpy Impact Test. Both approaches assess a PE test sample’s capacity to absorb energy upon failure. PE’s Izod impact resistance values range from 0.534 to 0641kj/m at ordinary room temperature. It determines how brittle or rigid material will be when subjected to a suddenly applied load. It is affected by the volume of the specimen, the presence of a notch, cold working, and water absorption in a polymer [20].

3.1.5 Viscoelasticity

PE is a viscoelastic material. It exhibits elastic and viscous behavior when stress is applied. The application of this force results in an instantaneous elastic strain followed by a viscous, time-dependent strain [21]. This makes the material display and behave partly as a crystalline metal and partly a very high viscosity fluid. This viscoelastic behavior of polyethylene polymer makes the curve of the stress–strain of any test sample of polyethylene be divided into three essential segments.

The viscoelastic nature of PE provides for creep and stress relaxation, which are two unique engineering characteristics employed in the HDPE design [22]. This property of the polymer materials is fundamental because the functionality and applicability of the plastic can change after a while and may lead to defects and loss of functionality over sometime [23].

3.1.6 Abrasion resistance

The capacity of a polymer to withstand the wear caused by contact with another surface is referred to as abrasion resistance. PE has excellent abrasion resistance in a variety of end-use situations. Furthermore, the abrasion resistance of this material has led to its extensive usage in engineering and technical applications. Generally, the abrasive wear resistance of polymers correlates with the reciprocal product of their ultimate tensile stress and the elongation at break [24]. It increases with increasing molecular weight, the molecular weight distribution, and the degree of crystallinity of polymeric materials [25]. Table 3 shows the common properties of some grades of polyethylene.

3.2 Electrical properties

PE is an excellent electrical insulator with good tracking-resistant properties. It is easily electrostatically charged. Its arc resistance is 200–250 seconds, with surface resistivity greater than 10¹³ohm. PE has a dielectric strength between 450 and 1000 volts/mil and a dielectric constant of 2.25–2.35 @ 60 Hz. Also, PE’s Volume resistivity is greater than 10¹⁶ohm-m with a Dissipation factor greater than 0.0005 @ 60 Hz [26]. These properties make the polymer be used in any engineering application that requires high insulation requirements.

3.3 Optical properties

PE can vary between almost transparent as in LDPE, translucent as in LLDPE, and opaque as in HDPE. The ability of polyethylene films to scatter light is determined by the quenching conditions and sample’s thermal history. Based on recent quantitative studies, the light scattered by a thin polyethylene sample quenched to 0°C from 125°C is much lower than that scattered by a heat-treated sample [27]. Because the refractive index along the tangent to the PE spherulites is lower than that along the radius of the spherulites, extruded polyethylene films have a slight positive birefringence. As a result, the clarity of polyethylene film is determined by the light scattered by it [28].

3.4 Thermal properties

Polyethylene exhibits a low thermal conductivity. It has a thermal expansion coefficient of 0.26 mm/°C and thermal conductivity of 0.4 W/m per °C. The specific heat of PE depends on temperature. Its low-density form has a specific heat capacity of 2.3 J/g at room temperature and 2.9 J/g at 120–140°C. The higher the molar mass and the more the branching, the lower the brittle point. Polyethylene is sensitive to surface imperfections. The standard commercial grade of high-density polyethylene’s melting point is between 120 and 180°C, and LDPE is 105–115°C [29]. The zero-shear rate apparent viscosity of linear PE is related to the weight-average molar mass by Eq. (1) for polymers with a molar mass over 5 kg/mol.

$$\left(\eta a;0\right)=KM{w}^{3.4}$$E1

Polymers with long branches do not fit in the above equation, and Eq. (2) represents a relationship between polymers of different degrees of long branching. In many cases

$$log\left(\eta a;0\right)=A+BM{n}^{1/2}$$E2

where $log\left(\eta a;0\right)$ = zero shear rate apparent viscosity, K and A are constants, M_w = weight average molecular weight, and Mn = number average molecular weight [30].

Generally, as the branching and molecular weight distribution increases, the viscosity of the PE increases, and its shear rate decreases. The increase in the molecular weight of the polymer increases its viscosity, decreases its melt flow index, and decreases its critical shear rate. Finally, an increase in the molecular weight distribution decreases the PE flow behavior index [11, 13, 31, 32].

4. Applications and usage

Polyethylene has been used extensively in food and beverage packaging because of its excellent properties and cheapness. It is the most used engineering material finding application in food packaging, construction, industrial and chemical industries, automobiles, and other allied companies. It can be used in the film, container, and tubing forms produced by extrusion, blow molding, injection, thermoforming cast, and other-oriented processes. It can be combined with other polymers to form plastics with improved performance characteristics and properties such as polyvinyl chloride (PVC), polypropylene (PP), polyethylene terephthalate (PET), and ethylene vinyl alcohol (EVOH), low-density polyethylenes are mainly used for films production. In contrast, high-density polyethylenes are used for containers and pipings [4, 33].

Different production methods and processes are employed in the manufacture of polyethylene products. They include film-making methods, injection molding, blow molding, and extrusion methods to produce various products. HDPE can be used in packaging and film applications, consumer goods, fibers, textiles, pipes, and fittings. Applications of HDPE in packaging include jerry can, domestic and industrial containers, chemical containers, drums, crates, dustbins, detergent containers, garbage containers, housewares, iceboxes, toys, sports nets, ropes, fishing nets, different types of pipes for gas, water, sewage, drainage, sea outfalls, industrial application, cable protection, steel pipe coating, industrial and decorative fabrics, fuel tanks and sheeting for telecommunication and energy cables due to its excellent chemical resistance and good mechanical and physical properties [34].

LDPE’s most popular application is in plastic bags. It is with LDPE that containers, various types of laboratory equipment are formed. Also, it finds application in dispensing bottles, tubings, wash bottles, plastic bags for computer parts and components. LDPE is used for packaging pharmaceutical and squeezes bottles, closures and caps, trash bags, liners, and films for packaging frozen and dry food items and laminations. Other applications include housewares, agricultural bags and films, hoses for water pipes, sub conductor insulators, and cable jackets due to their excellent plasticity, strength, and low water absorption properties. It is used to manufacture milk carton linings, bowls, buckets, squeezable bottles, and cling films [35].

Other applications of polyethylene, including LLPDE, are its application in general purpose films, garment packaging, agricultural films, stretch films, food boxes, and coating cables. It can be used for heat-sealed overwrapping film and container liners for bulk transport, etc.

5. Disposal systems for polyethylene

Polyethylene usage poses a significant threat to public hygiene and the aquatic environment. PE gets accumulated at dumping sites, waterways, gutters, drains, agricultural fields, residences, and roadsides, resulting in a refuse stockpile. The littering of the environment with polyethylene has made the disposal system very difficult. Governments, corporate organizations, and individuals are looking for a cleaner environment by developing different ways of disposing of the waste generated through polyethylene. Some of the proposed disposal systems include (1) recycling, (2) Composting, (3) Landfills, and (5) incineration [36].

5.1 Recycling

Recycling is one of the methods of reducing polyethylene waste littering in our environment. Polyethylene recycling is a process by which polyethylene waste is collected, recovered, and converted into valuable products. These materials can be found in agriculture, packaging, food, pipes, beverage bags, toys, electronics construction, electronic and allied sectors [37, 38]. The most straightforward plastic recycling processes involve collecting, sorting, washing, shredding, melting, and pelletizing. The actual process varies, and this is based on the type of recycled product to be formed. The waste materials collection involves picking all the plastic bags and bottles from dumpsites, waste bins, riversides, homes, businesses, and institutions and gathering them at collection points, recycling facilities for reuse. These collected plastics are sorted into each polyethylene group, separated manually or with machines, and washed to remove impurities. Washing removes the impurities such as adhesives, labels, oil that affect operation. Then they are shredded into fine particles and are fed into various component machines for use. Also, shredded plastic pieces can be used for other applications, such as a binder for paving stones and block making and additive within the asphalt. Recycling involves three processes, namely mechanical, chemical and energy recovery processes [39].

The mechanical recycling process involves using cleaned, sorted, and shredded plastic granules or pellets to form new products like trash cans, toys, bottles, bags, and other reusable products. The primary recycling process involves the conversion of the cleaned shredded plastic waste into similar products using injection molding and extrusion principles. The quality of the manufactured product can be improved by the introduction of virgin raw materials, reinforcers, stabilizers and master batches to the scrap or plastic waste. The difference between the primary recycled product and that gotten from the virgin raw materials is in the quality of the manufactured products. Severino et al. [40] studied the effect of nanofillers and compatibilizers on the mechanical properties of extruded low-density polyethylene waste and the results showed enhanced properties when compared to ordinary LDPE waste. The primary and secondary types of recycling involve the reuse of the products in their original form and structure. Figures 2–5 show the mechanical processing of used polyethylene films taking place at the University of Nigeria Nsukka, Enugu State, Nigeria. These films are used water sachets were used to make paving stones for the construction industry. Virgin polyethylene granules can be added here to improve the desired mechanical properties. This mechanical processing technique also involves the collection of plastic waste, sorting, cleaning, drying and reusing of plastic waste. The cleaned waste plastic can be cut into size, agglomerated and then extruded into pellets before being used to manufacture the desired products [41]. The pellets or granules can be fed into an injection molding machine, blow molding machine or an extruder to form different products. Also, the melt can be mixed with other materials to form varieties of composites.

Figure 2.

Typical dumpsite where waste collection is being carried.

Figure 3.

Sorting of the polyethylene film waste collected.

Figure 4.

Drying of sorted waste.

Figure 5.

Waste conversion.

It is in the secondary mechanical process that plastic blocks, paving stones and varieties of products are formed for construction and structural purposes. It can be used as partition walls in buildings for non-load bearing applications. Plastic waste has been used as aggregate in asphalt to improve the skid and crack resistance of pavements [42]. Kumi- Larbi and associates studied the effect of particle size of sand on some properties of water sachet/sand composites used as paving stones/bricks and the results showed that the LDPE (water sachet) bonded sand showed an improvement in the durability, compressive strength, specific heat and thermal diffusivity of the bricks and can be used to as a substitute for cement in some building applications [43].

The third method or tertiary recycling of polyethylene products involves a chemical recycling process in which polymers are chemically converted to monomers or depolymerized to monomers and oligomers through a chemical reaction. The chemical inertness of the polymer and its variable structural nature has limited most studies on polyethylene to pyrolysis. The chemical recycling process is a process that reduces a polymer to its original monomeric form. The technology uses chemical reactions, heat or both to break down used plastics into raw materials for other chemicals, fuel or new plastics. PE can be processed through thermal or catalytic pyrolysis to produce monomers and small organic molecules. Thermal pyrolysis uses heat under high pressure to break down the PE to different smaller monomeric molecules while catalytic pyrolysis uses a catalyst to reduce both the reaction time and temperature required to break down the PE into its component monomers [44]. Major research is being carried out on the chemical recycling of the world’s biggest plastic waste from PE and the results have been impressive and when concluded will help to reduce the menace associated with PE disposal. Ha’jekova and his colleagues investigated the recycling of LDPE using co-pyrolysis with naphtha at the temperature range of 740°C -820°C and found a high yield of alkenes as its primary products and coke [45].

The energy recovery process is the fourth method of polyethylene recycling, also known as quaternary recycling. This is the recovery of the plastic’s energy content on the application of external heat. Incineration is an energy recovery method that reduces a large volume of organic materials in an enclosed system. This method generates a high amount of energy from the polyethylene fed inside the incinerator but poses a higher health risk due to the high production of toxic substances that are carcinogenic to human health.

5.2 Plastic composting

This is the breakdown of waste plastics into small natural substances such as carbon dioxide, water, and methane by microorganisms. Composting can be done through aerobic or anaerobic methods. Aerobic composting is the breakdown of the polyethylene wastes by microorganisms in the presence of oxygen or air, while anaerobic composting is the decomposition or degradation of plastic waste substances by microorganisms in the absence of oxygen. Generally, microorganisms bind themselves to the plastics’ surface, colonize the exposed surface, catalyze enzymatic degradation of the plastic into lower molecular weight monomers, dimers, and oligomers, and finally form carbon dioxide and water as its bye products [46]. Some of the microorganisms that biodegrade polyethylene include, Acinetobacter sp., Ideonella sakaiensis, Bacillus sp., Staphylococcus sp., Streptococcus sp., Diplococcus sp., Micrococcus sp., Pseudomonas sp., and Moraxella sp. ss bacteria and, Aspergillus niger, A. ornatus, A. cremeus, A. flavus, A. candidus, A. ochraceus, A. nidulans, and A. Glaucus as fungi [47].

5.3 Landfills

Plastic waste constitutes a more significant proportion of municipal solid waste. Most of these solid wastes end in landfills. Research has it that it takes more than 300 years for polyethylene to degrade entirely, and this means that plastic waste disposal is a problem that needs to be talked about globally. Landfills remain the safest way of polyethylene disposal, and the waste is subjected characteristically to mechanical stress, decomposition, leachate, and chemical reactions both by aerobic and anaerobic conditions. Landfills have become a breeding ground for dangerous reptiles and insects. It poses environmental pollution, facilitating the release of harmful pollutants in the air when the landfill is set on fire. Carcinogenic and estrogenic compounds are leased, which are dangerous to life. These compounds harm human and aquatic health, causing diseases like breast and ovarian cancer [48]. The policy on solid waste disposal hinges on the reduction, reuse, and recycling of plastic waste to reduce landfill waste [49].

5.4 Incineration

Incineration is a waste treatment process that involves burning waste materials through the application of heat. Incineration is a widely used disposal method because it takes little space but is not generally used because of concerns about producing toxic gases as the plastics are burnt. The process of incineration involves waste collection, sorting, storage, handling, waste combustion, pollution control, to residue collection and handling. The incinerator is designed to capture the component gases for industrial purposes and remove the feedstock produced. The main product of incineration is carbon dioxide, and this is captured through modern technology to avoid associated problems of CO₂ like ozone layer depletion, which causes global warming. However, only a tiny fraction of the petroleum supply is used to produce polymers, out of which less than 2% of the used products are incinerated. Carbon dioxide and carbon monoxide emissions from incinerated sources are not significant compared to carbon dioxide production arising from the burning of fossil fuels. Incineration and gasification are some of the technologies which convert waste to wealth. Incineration produces high-temperature heat, while combustible gasses are a product of gasification [50].

5.5 Bio- and photo-degradation

The biodegradation of PE is a process whereby microorganisms are used to modify and consume the polymer as a primary source of energy and, by so doing, change the physical and chemical properties leading to structural deterioration, weight loss, and eventual gas evolution. Polyethylene biodegrades naturally over a long period. It takes polyethylene films up to 300 years to biodegrade and polyethylene terephthalate more than 450 years to do the same too. Biodegradation of this polymer can be enhanced by introducing microbes, sunlight, moisture and increasing the hydrophilic properties through modification during production. The Physico-chemical method of PE degradation includes UV and thermal treatment, oxidation of PE with nitric acid followed by the microbial attack, and catalytic enzyme degradation. All these cause reductions of polymer chain size, modification of the structure of the polymer, and eventual weight loss.

6. Conclusion

Polyethylene achieved its dominant position because it has excellent mechanical and physical properties, the ease with which it can be manufactured and converted into various packaging forms, and its relatively low cost. These excellent characteristic properties have made the polymer the materials of the future. One of the problems associated with polyethylene is that it leaches additives. This leaching is increased by acids, how long the polyethylene stays in contact with food items, and the increase in heat applied during service conditions. However, meeting the challenges posed by plastics is not simple, and there exists a lack of awareness surrounding the plastic waste problem. Also, the world is putting efforts and investing in cheaper and more biodegradable polymers, better ways of disposal, and a cleaner environment. Polyethylene is a commodity plastic. Its widespread usage has made disposal of end-use products a problem. Knowledge of the production and processing pathways and their properties will significantly aid businesses and consumers in the proper application and best disposal systems. This will help get a world that is clean, safe, and habitable.

Conflict of interest

The authors declare no conflict of interest.