Manufacture of HDPE

AH technologies employed for catalytic polymerization processes in general are widely used for the manufacture of HDPE. The two most often used technologies are slurry polymerization and gas-phase polymerization. Catalysts are usuaHy fine-tuned for a particular process. 

The current and projected HDPE capacities are shown in Table 3, and producers of resins in Table 4. In most cases, an accurate estimation of the total HDPE volume is compHcated by the fact that a large number of plants also use the same reactors for manufacture of HDPE or LLDPE. UHMWPE is produced in the United States (Himont and American Hoechst), in Japan (Asahi), and in Germany (Hoechst) worldwide capacity is approximately 45,000 tons. The use of post-consumer (recycled) HDPE is gradually increasing in volume. The growth of recycling programs is driven principally by economics (110,114) it has increased from a mere 60,000 tons in 1989 to 350,000 tons in 1994 and is expected to increase to 1.4 million t in the year 2000 (115). 

Chromium Oxide-Based Catalysts. Chromium oxide-based catalysts were originally developed by Phillips Petroleum Company for the manufacture of HDPE resins subsequendy, they have been modified for ethylene—a-olefin copolymerisation reactions (10). These catalysts use a mixed sihca—titania support containing from 2 to 20 wt % of Ti. After the deposition of chromium species onto the support, the catalyst is first oxidised by an oxygen—air mixture and then reduced at increased temperatures with carbon monoxide. The catalyst systems used for ethylene copolymerisation consist of sohd catalysts and co-catalysts, ie, triaLkylboron or trialkyl aluminum compounds. Ethylene—a-olefin copolymers produced with these catalysts have very broad molecular weight distributions, characterised by M.Jin the 12—35 and MER in the 80—200 range. 

Chromium oxide-based catalysts, which were originally developed for the manufacture of HDPE resins, have been modified for cthylcnc-< -olcfin copolymerization reactions. These catalysts use. a mixed silica-titania support containing from 2 to 20 wt % of Ti. 

Complex 6.37 has been shown to be an active catalyst for the manufacture of HDPE. Note that 6.37 is a 15-electron complex with chromium in the formal oxidation state of 3 +. The mechanism of polymerization involves generation of coordinative unsaturation through the dissociation of a THF molecule from 6.37. The evidence for an oxidation state of 3+ in the commercial catalyst comes from the fact that complex 6.38 is active for polymerization. However, complex 6.39, identical to 6.38 in every respect except the oxidation state of the metal ion, is inactive. Note that the oxidation state of chromium in 6.39 is 2+. 

Ziegler catalysts are also used for the manufacture of HDPE at low (40-150 psi) or medium (290-580 psi) pressures. The polymerization is carried out in a stirred reactor at about 80-90°C and 145 psi, or in a loop reactor at about 450 psi and temperatures between 65 and 110°C. Since the catalyst is very efficient, its concentration is kept very low, obviating the need for catalyst deactivation and removal steps. Conversions are very high. The solvent is removed by centrifugation, while the polymer is dried in a fluidized bed drier. 

In 1954 Hoechst was the first manufacturer of HDPE by Ziegler process, followed by Hiils, Riihrchemie, Montecatini, Shell, Mitsui, Dow, Esso, Gulf, Hercules, Koppers, Monsanto, and Union Carbide. In 1957, Hercules introduced... 

Table 2.5. Manufacturers of HDPE geomembranes certified for landfill liners in Germany ...

The Nobel Prize in Chemistry was awarded to Karl Ziegler and Giulio Natta in 1963 for their research in developing olefin polymerization catalysts primarily based on titanium compounds and aluminum alkyl compounds required for the catalyst initiation process. However, by 1963 the details on the discovery of the Cr-based catalyst system in which the chromium compound was supported on amorphous silica were widely published and commercially important for the manufacture of HDPE. In the view of this author, the 1963 Nobel Prize awarded in chemistry should also have included two additional scientists, John P. Hogan and Robert L. Banks from Phillips Petroleum. 

In the 1960s, scientists at the Union Carbide Corporation developed two additional silica-supported, chromium-based catalysts that are used in a gas-phase process for the manufacture of HDPE. One catalyst is based on... 

Successful pilot plant results allowed for the design of a commercial continuous process for the manufacture of HDPE... 

Union Carbide/UNIPOL Union Carbide introduced the gas-phase, fluid-bed process under the tradename of UNIPOL commercially in the late 1960s using a G-1250 reactor (reactor volume 1250 cubic feet) for the manufacture of HDPE. The LLDPE was added to the product mix in about 1977. Union Carbide formed a joint venture with Exxon in 1996 to pursue the addition of single-site catalysts to the... 

High density polyethylene (HDPE) is defined by ASTM D1248-84 as a product of ethylene polymerisation with a density of 0.940 g/cm or higher. This range includes both homopolymers of ethylene and its copolymers with small amounts of a-olefins. The first commercial processes for HDPE manufacture were developed in the early 1950s and utilised a variety of transition-metal polymerisation catalysts based on molybdenum (1), chromium (2,3), and titanium (4). Commercial production of HDPE was started in 1956 in the United States by Phillips Petroleum Company and in Europe by Hoechst (5). HDPE is one of the largest volume commodity plastics produced in the world, with a worldwide capacity in 1994 of over 14 x 10 t/yr and a 32% share of the total polyethylene production. 

Orientation. Most articles made of HDPE, including film, fiber, pipes, and injection-molded articles, exhibit some degree of molecular and crystal orientation (21). In some cases, orientation develops spontaneously for example, during melt flow into a mold and its subsequent crystallisation. When blown HDPE film and fiber are manufactured, orientation can be introduced dehberately by stretching. 

Solution Polymerization. Two solution polymerization technologies ate practiced. Processes of the first type utilize heavy solvents those of the second use molten PE as the polymerization medium (57). Polyethylene becomes soluble ia saturated C —hydrocarbons above 120—130°C. Because the viscosity of HDPE solutions rapidly iacrease with molecular weight, solution polymerization is employed primarily for the production of low mol wt resias. Solution process plants were first constmcted for the low pressure manufacture of PE resias ia the late 1950s they were later exteasively modified to make their operatioa economically competitive. 

HDPE is one of the largest commodity plastics manufactured worldwide. Dynamics of HDPE production is represented by the following data indicating both the existing and projected demand (t/yr) (114) ... 

Countries produciug commodity LLDPE and their capacities, as well as production volumes of some U.S. companies, are Hsted iu Table 5. Iu most cases, an accurate estimate of the total LLDPE production capacity is compHcated by the fact that a large number of plants are used, iu turn, for the manufacture of either HDPE or LLDPE iu the same reactors. VLDPE and LLDPE resius with a uniform branching distribution were initially produced in the United States by Exxon Chemical Company and Dow Chemical Company. However, since several other companies around the world have also aimounced their entry into this market, the worldwide capacity of uniformly branched LLDPE resins in 1995 is expected to reach a million tons. Special grades of LLDPE resins with broad MWD are produced by Phillips Petroleum Co. under the trade name Low Density Linear Polyethylenes or LDLPE. 

Manufacture of milk containers using postconsumer HDPE has the potential to significantly lower the GHG emissions and energy due to the... 

HDPE is used in the manufacture of toys and other household articles. Because of its high tensile strength and stiffness polyethylene is also for many other purposes. 

Mixed C4 olefins (primarily iC4) are isolated from a mixed C olefin and paraffin stream. Two different liquid adsorption high-purity C olefin processes exist the C4 Olex process for producing isobutylene (iCf ) and the Sorbutene process for producing butene-1. Isobutylene has been used in alcohol synthesis and the production of methyl tert-butyl ether (MTBE) and isooctane, both of which improve octane of gasoHne. Commercial 1-butene is used in the manufacture of both hnear low-density polyethylene (LLDPE) and high-density polyethylene (HDPE)., polypropylene, polybutene, butylene oxide and the C4 solvents secondary butyl alcohol (SBA) and methyl ethyl ketone (MEK). While the C4 Olex process has been commercially demonstrated, the Sorbutene process has only been demonstrated on a pilot scale. 

A review is presented of the nitrogen autoclave process for the manufacture of crosslinked polyolefin foams. Process and product developments over the last few years are summarised and future possibilities are described. Process developments include use of higher temperatures and pressures to produce foams having densities as low as 10 kg/cub.m. Product developments include foams based on HDPE/LDPE blends, propylene copolymers and metallocene-catalysed ethylene copolymers. The structure and properties of these foams are compared with those of foams produced by alternative processes. 5 refs. 

LDPE and PTFE are manufactured by processes involving little contact with metals, and should not be a major source of aluminum. On the other hand, HDPE and PP have similar manufacturing processes, based on catalysis, that involve aluminum, among other metals. The contamination risk may therefore be very high. In addition, the manufacture of the container itself can lead to aluminum contamination, considering the use of lubricants, stabilizers and other additives. For example, metal soaps, such as aluminum, magnesium, sodium, and tin, were sanctioned by the FDA to be used as stabilizers in plastic containers for parenteral nutrition solutions [76]. 

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