The high-pressure process relied on large and complex plants that required careful process control. Therefore, the discovery in 1953 of the appropriate catalysts that allowed the process to be carried under low pressure (~500 psi) was welcomed by the industry . Three types of catalysts were developed about that time: the Ziegler-type catalysts typically obtained by reacting alkyl aluminum compounds with titanium chloride6; metal oxide catalyst systems, developed by Phillips Petroleum in the United States, typically made of chromium oxide supported on a silicaceous carrier ) and a different type of oxide catalyst developed by Standard Oil Company. The first plants based on the Ziegler catalyst went on line in Germany by 1955 and a plant based on the Phillips catalyst in Texas opened in 1957. The third catalyst system developed much slower and was picked up by the Japanese plastics industry in a plant opened in 1961.
The first LLDPE resin was marketed by Du Pont in 1960 followed shortly thereafter by Union Carbide. The polymerization was effected by organo-metallic catalysts (Ziegler-Natta catalysts) and carried out at a relatively low pressure. Moderate levels of a-olefin co-monomers are used to control the density of the resin product. The inclusion of suitable co-monomers in the polymer chains disrupts crystallinity and reduces the density and rigidity of the resin. Despite its superior properties the LLDPE leaves much to be desired at the molecular level. The co-monomer is not homogeneously distributed across the different molecular weights and the molecular weight distribution is rather broad (narrower distributions are believed to yield better properties). Efforts to produce a well-defined narrow molecular weight distribution recently led to the development of yet another catalyst system, a class of single-site catalysts called metallocenes. The catalyst is an organometallic compound of a group IV metal atom attached to two cyclopetadienyl groups and two alkyl halides (or to methyl alumoxane moieties).
6 R2AlCl + TiCU
Gas-phase polymerization represents an important advance in the manufacturing technology for polyolefins. In the Unipol (gas-phase) process ethylene and any co-monomers (usually other olefins such as oct-1-ene, hex-1-ene) are fed continuously into a fluidized-bed reactor at a pressure of about 0.7-2.0. MPa and at a temperature of less than 100oC. The catalyst is added directly into the bed. The gas-phase process is not constrained by problems associated with pumping or handling viscous polymer solutions and the solubility of the resin product. The solid polyethylene is directly removed from the reactor with any residual monomer being purged and returned to the bed. The gas-phase reactors are able to take advantage of the new metallocene catalysts with little engineering modification. A schematic diagram of a Unipol-type reactor is shown in Figure 2.6.
Most importantly the development of gas-phase processes allowed the energy-intensive high-pressure polymerization process to be replaced by the far more energy-efficient low-pressure process. The gas-phase processes offer the greatest versatility of products in terms of resin density and melt index of polyethylenes. From an environmental standpoint the gas-phase reaction is of particular interest and offers several advantages over the conventional technology as recently summarized by Joyce .
1. The gas-phase polyethylene plants7 introduced by Union Carbide in 1968 require only about half the capital investment of the high-pressure plants and far fewer workers to operate them. The wastage of ethylene during the polymerization process is also lower in the gas-phase process, reducing cost of the product and conserving the monomer resources.
2. The energy costs of solvent recovery associated with the older processes are saved in the gas-phase process. By the mid-1970s the process refinements had reduced the energy costs of polyethylene manufacture by about a half, compared to that of the conventional high-pressure processes. Figure 2.7, based on reference , illustrates the record of the polyethylene industry over the years in energy conservation via process improvement.
3. A major advantage in moving away from the solvent and slurry processes is the elimination of the emission problems associated with the use of hexane and butane solvent (or suspension agents in the case of slurries). Minimum release of volatile organic compounds (VOCs) into the atmosphere is desirable during manufacture.
4. The number of workers needed to operate a modern polyethylene plant is minimal. These labor savings do translate into very real energy and resource savings, particularly in a manufacturing setting. As opposed to the 84 individuals needed to produce a million pounds of the resin in the days the industry started in the 1940s, only a single operator in a modern plant can produce the same volume of even a better grade of resin, today.
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