Preparation of Polyethylene by a Free-Radical Mechanism

Up to the late 1960s, most low-density polyethylene was produced commercially by high-pressure free-radical polymerization. Much of this has now been replaced by preparation of copolymers of ethylene with a-olefins by coordination polymerization. These preparations are discussed further in this chapter. High-pressure polymerizations of ethylene, however, might still practiced in some places and it is, therefore, discussed here. The reaction requires a minimum pressure of 500 atm [1] to proceed. The branched products contain long and short branches as well as vinylidene groups. With an increase in pressure and temperature of polymerization, there is a decrease in the degree of branching and in the amount of vinylidene groups [2, 3]. Free-radical commercial polymerizations are conducted at 1,000–3,000 atm pressure and 80–300C. The reaction has two peculiar characteristics: (1) a high exotherm and (2) a critical dependence on the monomer concentration. In addition, at these high pressures oxygen acts as an initiator. At 2,000 atm pressure and 165C temperature, however, the maximum safe level of oxygen is 0.075% of ethylene gas in the reaction mixture. Any amount of oxygen beyond that level can cause explosive decompositions. History of polyethylene manufacture contains stories of workers being killed by explosions. Yet, the oxygen concentration in the monomer is directly proportional to the percent conversion of monomer to polymer, though inversely proportional to the polymer’s molecular weight. This limits many industrial practices to conducting the reactions below 2,000 atm and below 200C. These reactions were done, therefore, between 1,000 and 2,000 atm pressures. Small quantities of oxygen, limited to 0.2% of ethylene, are accurately metered in [4, 5]. The conversion per each pass in continuous reactors is usually low, about 15–20%. There is an induction period that varies inversely with the oxygen concentration to the power of 0.23. During this period oxygen is consumed autocatalytically. This is not accompanied by any significant decrease in pressure. A high concentration of ethylene is necessary for a fast rate of chain growth, relative to the rate of termination. Also, high temperatures are required for practical rates of initiation. If oxygen is completely excluded and the pressure is raised to between 3,500 and 7,750 atm, while using relatively low temperatures of 50–80C, linear polyethylene forms [6]. The reactions take about 20 h. Various solvents can be used, like benzene, isooctane, methyl, or ethyl alcohols. Higher ethyl alcohol concentrations and low concentrations of the initiator result in higher molecular weights. The products range from 2,000 for wax-like polymers to 4,000,000 for nearly intractable materials. Favorite free-radical initiators for this reaction are benzoyl peroxide, azobisisobutyroni trile, di-t-butylperoxydicarbonate, di-t-butyl peroxide, and dodecanoyl peroxide. Above conditions differ, however, from typical commercial ones, because such high pressures and long reaction times are not practical. The actual commercial conditions vary, depending upon location and individual technology of each company. Often, tubular and multiple-tray autoclaves are used [7]. Good reactor design must permit dissipation of the heat of polymerization (800–1,000 cal/g), with good control over other parameters of the reaction. Tubular reactors are judged as having an advantage over stirred autoclaves in offering greater surface-to-volume ratios and better control over residence time [7]. On the other hand, the stirred autoclaves offer a more uniform temperature distribution throughout the reactor. The tubular reactors have been described as consisting of stainless steel tubes between 0.5 and 1 in. in internal and about 2 in. in the external diameters. The residence time in these tubes is from 3 to 5 min, and they can be equipped with pistons for pressure regulation. Pressure might also be controlled by flow pulses to the reactor [8]. For the oxygen-initiated reactions, the optimum conditions are [7] 0.03–0.1% oxygen at 190–210C and 1,500 atm pressure. At this pressure, the density of ethylene is 0.46 g/cm3. This compares favorably with the critical density of ethylene that is 0.22 g/cm3. Once the polymerization is initiated, the liquid monomer acts as a solvent for the polymer. Impurities, such as acetylene or hydrogen cause chain transferring and must be carefully removed. In some processes, hindered phenols are added in small quantities (between 10 and 1,000 ppm). This has the effect of reducing long-chain branching and yields film grade resins with better clarity, lower haze, and a reduced amount of microgels. Also, diluents are used in some practices. Their main purpose is to act as heat-exchanging mediums, but they can also help remove the polymer from the reactor. Such diluents are water, benzene, and ethyl or methyl alcohols. Sometimes, chain transferring agents like carbon tetrachloride, ketones, aldehydes, or cyclohexane might also be added to control molecular weight. The finished product (polymer–monomer mixture) is conveyed to a separator where almost all of the unreacted ethylene is removed under high pressure (3,500–5,000 psi) and recycled. The polymer is extruded and palletized. Ethylene conversion per pass is a limiting factor on the economics. A tubular reactor is illustrated in Fig. 6.1.

Fig. 6.1 Illustration of a tubular reactor Polyethylene prepared in this way may have as many as 20–30 short branches per 10,000 carbon atoms in the chain [9] and one or two long-chain branches per molecule, due to “backbiting”

The reaction results in predominantly ethyl and butyl branches. The ratio of ethyl to butyl groups is roughly 2:1 [11, 12]. Chain transferring to the tertiary hydrogens at the location of the short branches causes elimination reactions and formation of vinylidene groups [13, 14]. This mechanism also accounts for formation of low molecular weight species.

Commercial grades of low-density polyethylene vary widely in the number of short and long branches, average molecular weights, and molecular weight distributions. M_w/M_n is between 20 and 50 for commercial low-density materials. The short branches control the degree of crystallinity, stiffness, and polymer density. They also influence the flow properties of the molten material.

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مواضيع ذات صلة

Manufacturing Techniques

Polypropylene

Materials Similar to Polyethylene

Commercial High-Density Polyethylene, Properties, and Manufacture

Preparation of Polyethylene by Coordination Mechanism

Polyethylene and Related Polymers

Thermodynamics of Ring-Opening Polymerization

Ring-Opening Polymerizations by a Free Radical Mechanism

Polymerization of Trioxane

Ring-Opening Polymerizations of Cyclic Acetals

Polymerization of Oxepanes

The Propagation Reaction