Homogeneous polyethylene

Thin films based on cellulose esters treated with an organic solvent were used for separation of isomeric xylenes, however, low values of factor jSp were achieved [130]. Modifications of hydrophyhc membranes were used for separation of Cg-aromatic isomers. Separation factor for p- to m-xylene was 1.69 [131]. Commercial polyvinyl alcohol membrane in the presence of CBf4 as a selective feed complexing agent was used to separate p- to m-xylene [133]. Dense homogenous polyethylene membranes were used for separation of aromatic Cg-isomers [132]. The rate of mass transport across the membrane increased... 

In other words, when a homogeneous polyethylene solution was cooled at a constant rate, the point of beginning crystallization was reached. This point was indicated by the first visual turbidity. At all concentrations, homopolymer A had a point of beginning crystallization about 10°C higher than that of copolymer B. This finding seemed reasonable since the crystallization temperature depends on the comonomer content and on the degree of short-chain branching of the polyethylene. From the curve we concluded that the temperature at which styrene diffused into LDPE A was about 10°C above that for diffusion into LDPE B at a comparable rate of diffusion. 

Another breakthrough obtained by Lai et al. of The Dow Chemical Company is described in a new composition of matter patent and its continuation in part patent pertaining to homogeneous polyethylene resins containing small and controlled amounts of long chain branching. These resins... 

A review of the most cited U.S. patents in the polyolefins field as of August 2006, gives an indication of the most important patents in this fast-developing field. The two patents by Lai et al. (U.S. 5,272,236 and 5,278,272) are the two most cited patents in the field of polyolefins (553 and 445 citations, respectively) and these two patents survived opposition worldwide. The third most cited patent (389 citations) in the polyolefins area is by Stevens and Neithamer of The Dow Chemical Company, describing constrained geometry catalyst systems for producing homogeneous polyethylene. The fourth and fifth most cited U.S. patents in this field are ExxonMobil Chemical s U.S. 

Hindryckx, R, Dubois, R, Jerome, R., Teyssie, R, and Garcia Marti, M. 1997. Rolymeiization-filled composites prepared with highly active filler-supported Al/Ti/Mg catalysts. 1. Synthesis of homogeneous polyethylene-based composites. Journal of Applied Polymer Science 64 423-438. 

Great interest has been focused on the soluble metallocene / MAO catalyst system since the announcement that a soluble biscyclopentadiertylziruconiumdichloride / MAO catalyst system provides very homogeneous polyethylene with extremely high act i V i ty. ... 

One of the early popular low power research and training reactors was the AGN-201, suppHed by Aerojet General Nuclear. This is a homogeneous sohd fuel reactor, consisting of a mixture of polyethylene and uranium at 20% enrichment in The core 235U... 

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

In the Institut Fransais du Petrc le process (62), ethylene is dimerized into polymer-grade 1-butene (99.5% purity) suitable for the manufacture of linear low density polyethylene. It uses a homogeneous catalyst system that eliminates some of the drawbacks of heterogeneous catalysts. It also inhibits the isomerization of 1-butene to 2-butene, thus eliminating the need for superfractionation of the product (63,64). The process also uses low operating temperatures, 50—60°C, and pressures (65). 

Polymerization of olefins such as styrene is promoted by acid or base or sodium catalysts, and polyethylene is made with homogeneous peroxides. Condensation polymerization is catalyzed by acid-type catalysts such as metal oxides and sulfonic acids. Addition polymerization is used mainly for olefins, diolefins, and some carbonyl compounds. For these processes, initiators are coordination compounds such as Ziegler-type catalysts, of which halides of transition metals Ti, V, Mo, and W are important examples. 

F. Polyethylene polymer autoclav e type reactors usually contain 8 to 120 impellers of the same or different circulation designs on a single shaft to ensure rapid total homogeneous mixing in the reactor, w hich contains a gas at about 30,000 psi and, hence, the fluid is neither a gas or a liquid because the densities are about the same. 

The purity of the product was checked by vapor phase chromatography on a polyethylene glycol on Teflon column at 72°, 15 p.s.i., and a flow rate of 102 ml. of helium per minute. The sample appeared to be homogeneous, but, since the amine tails badly on the column, it is not possible to detect the presence of a small amount of water (less than 3%). 

Reactive compatibilization is also carried out by adding a monomer which in the presence of a catalyst can react with one or both phases providing a graft copolymer in situ that acts as a compatibilizer. Beaty and coworkers added methyl methacrylate and peroxide to waste plastics (containing polyethylene [PE], polypropylene [PP], PS, and poly(ethylene terephthalate) [PET]). The graft copolymer formed in situ homogenized the blend very effectively [19]. 

Terephthalic acid (p-TA or TA), a raw material for polyethylene terephthalate (PET) production, is one of the most important chemicals in petrochemical industry. Crude terephthalic acid (CTA), commonly produced by homogeneous liquid phase p-xylene oxidation, contains impurities such as 4-carboxybenzaldehyde (4-CBA, 2000-5000 ppm) and several colored polyaromatics that should be removed to obtain purified terephthalic acid (PTA). PTA is manufactured by hydropurification of CTA over carbon supported palladium catalyst (Pd/C) in current industry [1]. 

Fig. 1. DSC thermogram of polymers (a) polyethylene with homogeneous catalyst (b) polyethylene with embedded catalyst (c) catalyst embedded polystyrene.

On the other hand, the polymer prepared by the embedded catalyst shows T around 130 °C, which is a typical melting temperature of high density polyethylene. There was little activity difference between the polyethylene produced by embedded particles and those by homogeneous catalysts. The results of ethylene polymerization using embedded catalyst and homogeneous catalyst are summarized in Table 1 and Fig. 2,... 

Fig. 3. SEM image of polyethylene particles (a) with homogeneous catalyst (b) with embedded...

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