Chromium-containing polymers

A typical coating contains iron oxides (sometimes doped with cobalt or chromium), a polymer system that acts as a binder, various additives (including up to 1.5% lecithin on a dry solids basis), and solvents such as methyl ethyl ketone and cyclohexanone (454 58). This material is milled into a dispersion and deposited onto the tape and dried. 

After 1958, many other new applications were found to take advantage of the properties offered by HDPE made with chromium-containing catalysts. In 1958, ethylene-butene copolymers were introduced [18] and other polymer grades were also soon developed. 

In 1968, new copolymers were introduced containing 1-hexene instead of 1-butene. This change provided improved physical properties of polymers made with Cr/silica catalysts. A new process to produce LLDPE was announced by Phillips in 1969 [22,23]. Polymers with densities as low as 0.925 g mL-1 were produced in a modified PF process with chromium-containing catalysts. Nevertheless, polymers with densities <0.93 g mL 1 were not common among Phillips licensees because of the tendency of low-density polymer to swell, and this swelling limited reactor output. 

Kosyanchuk, L. E, Babkina, N. V., Yarovaya, N. V, Kozak, N. V, and Lipatov, Y. S., 2008. Phase separation in semi-interpenetrating polymer networks based on crosslinked poly(urethane) and linear p>oly(methyl methacrylate) containing iron, copper, and chromium chelates. Polymer Science Series A 50(4) 434-433. 

Taylor M.M., Diefendorf E.J., Thompson C.J., Brown E.M, and Marmer W.N. (1994) Extraction and Characterization of Chrome Free Protein from Chromium-Containing Collagenous Waste Generated in the Leather Industry, In Polymer from Agricultural Coproducts, M.L.Fishman, R.B. Friedman, S.J.Huang (eds.) ACS Symp.Ser. 575, pp.171-187. 

Second, in the early 1950s, Hogan and Bank at Phillips Petroleum Company, discovered (3,4) that ethylene could be catalyticaHy polymerized into a sohd plastic under more moderate conditions at a pressure of 3—4 MPa (435—580 psi) and temperature of 70—100°C, with a catalyst containing chromium oxide supported on siUca (Phillips catalysts). PE resins prepared with these catalysts are linear, highly crystalline polymers of a much higher density of 0.960—0.970 g/cnr (as opposed to 0.920—0.930 g/cnf for LDPE). These resins, or HDPE, are currentiy produced on a large scale, (see Olefin polymers, HIGH DENSITY POLYETHYLENE). 

Processes for HDPE with Broad MWD. Synthesis of HDPE with a relatively high molecular weight and a very broad MWD (broader than that of HDPE prepared with chromium oxide catalysts) can be achieved by two separate approaches. The first is to use mixed catalysts containing two types of active centers with widely different properties (50—55) the second is to employ two or more polymerization reactors in a series. In the second approach, polymerization conditions in each reactor are set drastically differendy in order to produce, within each polymer particle, an essential mixture of macromolecules with vasdy different molecular weights. Special plants, both slurry and gas-phase, can produce such resins (74,91—94). 

Union Carbide Corp. also uses a siUca-supported chromium catalyst in their extremely low cost Unipol gas-phase linear low density ethylene copolymer process, which revolutionized the industry when it was introduced in 1977 (86—88). The productivity of this catalyst is 10 —10 kg polymer/kg transition metal contained in the catalyst. By 1990, the capacity of Unipol linear low density polyethylene reactors was sufficient to supply 25% of the world s total demand for polyethylene. 

The compositions of the conversion baths are proprietary and vary greatly. They may contain either hexavalent or trivalent chromium (179,180), but baths containing both Cr(III) and Cr(VI) are rare. The mechanism of film formation for hexavalent baths has been studied (181,182), and it appears that the strength of the acid and its identity, as well as time and temperature, influences the film s thickness and its final properties, eg, color. The newly prepared film is a very soft, easily damaged gel, but when allowed to age, the film slowly hardens, assumes a hydrophobic character and becomes resistant to abrasion. The film s stmcture can be described as a cross-linked Cr(III) polymer, that uses anion species to link chromium centers. These anions may be hydroxide, chromate, fluoride, and/or others, depending on the composition of the bath (183). 

The preferred catalyst is one which contains 5% of chromium oxides, mainly Cr03, on a finely divided silica-alumina catalyst (75-90% silica) which has been activated by heating to about 250°C. After reaction the mixture is passed to a gas-liquid separator where the ethylene is flashed off, catalyst is then removed from the liquid product of the separator and the polymer separated from the solvent by either flashing off the solvent or precipitating the polymer by cooling. 

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