**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 transitionmetal catalysts to produce Medium Density Polyethylene (MDPE) and linear lowdensity 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 [8–10].

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

#### *Polyethylenes: A Vital Recyclable Polymer DOI: http://dx.doi.org/10.5772/intechopen.102836*

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 silicaalumina 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 can also be produced from other routes rather than from high and lowpressure processes. Linear polyethylene with the repeat unit of ᷈↭(CH2)n↭ by


#### **Table 1.**

*Manufacturing & processing development of polyethylene.*

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].
