Commercial High-Density Polyethylene, Properties, and Manufacture

High-density polyethylene (0.94–0.97 g/cm³) is produced commercially with two types of catalysts:

1. Ziegler–Natta type catalysts

 2. Transition metal oxides on various supports

The two catalytic systems are used at different conditions. Both types have undergone evolution from earlier development. The original practices are summarized in Table 6.1. The Ziegler process yields polyethylene as low as 0.94/cm3 in density, but process modifications can result in products with a density of 0.965 g/cm3. The transition metal oxide catalysts on support, on the other hand, yield products in the density range of 0.960–0.970 g/cm3. The original development by Ziegler led to what appears to be an almost endless number of patents for various coordination-type catalysts and processes. As described in Chapter 4, such catalysts have been vastly improved. Progress was made toward enhanced efficiency and selectivity. The amount of polymer produced per gram of the transition metal has been increased manyfold. In addition, new catalysts, based on zirconium compounds complexed with methylaluminoxane oligomers(sometimes called Kominsky catalysts), were developed. They yield very high quantities of polyethylene per gram of the catalyst. For instance, a catalyst, bis(cyclopentadienyl)-zirconium dichloride combined with methylaluminoxane, is claimed to yield 5,000 kg of linear polyethylene per gram of zirconium per hour [14]. An important factor in the catalysts activity is the degree of oligomerization of the aluminoxane moiety. The catalytic effect is enhanced by increase in the number of alternating aluminum and oxygen atoms. These catalysts have long storage life and offer such high activity that they need not be removed from the product, because the amount present is negligible [14, 15]. This makes the work-up of the product simple. The continuous solution processes are usually carried out between 120 and 160C at 400–500 lb/in.² pressure. The diluents may be cyclohexane or isooctane. In one zone reactors, the solid catalyst is evenly dispersed throughout the reactor. In the two zone reactors (specially constructed), the polymerizations are conducted with stirring in the lower zone where the catalysts are present in concentrations of 0.2–0.6% of the diluent. Purified ethylene is fed into the bottom portions of the reactors. The polymers that form are carried with small portions of the catalyst to the top and removed. To compensate for the loss, additional catalysts are added intermittently to the upper “quiescent” zones. In suspension or slurry polymerizations, various suspending agents, like diesel oil, lower petroleum fractions, heptane, toluene, mineral oil, chlorobenzene, or others, are used. The polymerization temperatures are kept between 50 and 75C at only slightly elevated pressures, like 25 lb/in.2. If these are batch reactions, they last between 1 and 4 h. The slurry reactor is illustrated in Fig. 6.2. Polymerizations catalyzed by transition metal oxides on support were described variously as employing solid/liquid suspensions, fixed beds, and solid/gas-phase operations. It appears, however, that the industrial practices are mainly confined to use of solid/liquid suspension processes. The polymerization is carried out at the surface of the catalyst suspended in a hydrocarbon diluent.

Fig. 6.2 Commercial flow reactor for slurry polymerization of ethylene with Ziegler–Natta catalysts as illustrated in a British patent In continuous slurry processes, the temperatures are kept between 90 and 100C and pressures between 400 and 450 lb/in.2. The catalyst concentrations range between 0.004 and 0.03% and typical diluents are n-pentane and n-hexane. Individual catalyst particles become imbedded in polymer granules as the reaction proceeds. The granules are removed as slurry containing 20–40% solids. There are variations in the individual processes. In some procedures, the temperature is kept high enough to keep the polymer in solution. In others, it is kept deliberately low to maintain the polymer in slurry. The products are separated from the monomer that is recycled. They are cooled, precipitated (if in solution), and collected by filtration or centrifugation. Various reactors were developed to handle different slurry polymerization processes. The slurry is maintained in suspension by ethylene gas. The gas rises to the top and maintains agitation while the polymer particles settle to the bottom where they are collected. Several companies adopted loop reactors. These are arranged so that the flowing reactants and diluents continuously pass the entrance to a receiving zone. The heavier particles gravitate from the f lowing into the receiving zone while the lighter diluents and reactants are recycled. To accommodate that, the settling area must be large enoughfor the heavy polymerparticles to becollectedand separated. In addition to suspension, a gas-phase process was developed. No diluent is used in the polymeri zation step. Highly purified ethylene gas is combined continuously with a dry-powdery catalyst and then fed into a vertical fluidized bed reactor. The reaction is carried out at 270 psi and 85–100C. The circulating ethylene gas fluidizes the bed of growing granular polymer and serves to remove the heat [15]. Formed polymer particles are removed intermittently from the lower sections of the vertical reactor. The product contains 5% monomer that is recovered and recycled. Control of polymer density is achieved by copolymerization with a-olefins. Molecular weights and molecular weight distributions are controlled by catalyst modifications, by varying operating conditions, and/or use of chain transferring agents [15], such as hydrogen [16]. This is illustrated in Fig. 6.3.

Fig. 6.3 Illustration of a gas-phase process (from Burdett, by permission of the American Chemical Society). The reactors for the fluidized gas-phase process are simple in design. There are no mechanical agitators, and they rely upon blowers to keep the bed fluidized and well mixed. Catalysts and cocatalysts are fed directly into the reactor [25]. Rieger and coworkers [26] investigated the gas-phase polymerization of ethylene using supported α-diimine nickel catalysts. The reaction of 2,5-, 2,6-, and 1,4-dithiane ligands with Ni(acac)₂ and trityl tetrakis(pentafluorophenyl)borate yielded the corresponding Ni(II) complexes in high yields. These complexes were supported on silica without a chemical tether and were used as catalysts for ethylene polymerization reactions in the gas phase. Furthermore, ethylene was polymerized with the unsupported 2,5-complexes in homogeneous solution for comparison. The influence of ligand structure, hydrogen, and temperature on polymerization performance was investigated. The supported catalysts exhibited moderate to high activities and produced polyethylenes ranging from high-density polyethylene to linear low-density polyethylene, without the further addition of an α-olefin comonomer.

The weight average molecular weights of most commercial low-and high-density polyethylenes range between 5,000 and 300,000. Very low molecular weight polyethylene waxes and very high molecular weight materials are also available. The molecular weight distributions for high-density polyethylene vary between 4 and 15. The product generally has fewer than three branches per thousand carbon atoms[9].

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Manufacturing Techniques

Polypropylene

Materials Similar to Polyethylene

Preparation of Polyethylene by Coordination Mechanism

Preparation of Polyethylene by a Free-Radical 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