Polyethylene silica-titania

Keywords fumed silica alumina/silica, titania/silica alumina/silica/titania Ni(II) Cd(II) Pb(II) polyethylene glycol) poly(vinyl alcohol) adsorption potentiometric titration surface charge density... 

Reaction temperature seems to have little effect on the LCB levels in the polymer, which is convenient because reaction temperature is a primary variable used to control MW in the manufacture of polyethylene. Figure 78 shows another Arnett plot representing polymers made from Cr/silica-titania activated at four different temperatures [407]. Each catalyst was tested at a series of reaction temperatures, ranging from 98 up to 110 °C, in order to vary the polymer MW. The polymers were analyzed by rheology, and when plotted on an Arnett graph, the points representing each activation temperature formed a unique line. That is, the variation of reactor temperature resulted in points distributed along a line, and each activation temperature produced a different line. 

Although titania is very useful as a promoter for Cr/silica catalysts, it is a poor catalyst support itself. It does not have the porosity necessary for polymerization, but, more significantly, it also does not seem to provide an adequate chemical environment for the chromium. In one experiment, a high-surface-area (100 m2 g 1) TiC>2 carrier was impregnated with the usual 0.4 Cr atoms nm 2, followed by calcination at 400 and at 500 °C. A small amount of Cr(VI) was stabilized, and when tested for polymerization activity the catalyst provided very low yields of polyethylene. Whereas Cr/silica-titania produces polymers of lower MW than those made by Cr/silica, Cr/titania yielded polymers of higher MW. Indeed, it produced ultrahigh-MW polymer (UHMW PE). This finding is consistent with the view that it is the acidity created when titania is combined with silica that is important, and not the titania itself. 

This tendency is seemingly an advantage to polyethylene manufacturers. However, the titania tends to absorb the fluoride, perhaps selectively, to form Ti-F surface groups. Consequently, the fluoride displaces chromium from the titania. It converts the Cr/silica-titania catalyst back to one resembling Cr/silica, which is known to concentrate the branching mostly in the low-MW side of the distribution (see Figure 103). 

The original recipe involved the aqueous impregnation of chromic acid on silica, although nowadays less-poisonous chromium(III) salts are used. Over the years, a family of Phillips-type catalysts has emerged producing no less than 50 different types of polyethylene, and this versatility is the reason for the commercial success of the Phillips ethylene polymerization process. The properties of the desired polymer product can be tailored by varying parameters such as calcination temperature, polymerization temperature and pressure, by adding titania as... 

Polydimethylsiloxane and Zr oxo species have been reacted to give hybrid materials which were characterized by solid-state NMR.149 TEOS/PEG (polyethylene glycol) materials are biphasic systems. The materials were studied by 13C NMR, EPR, and thermal analysis.150 Other systems studied include titania/polyvinylacetate,151 titania/PEG,152 silica/polyacrylates,153 polyimide/ silica,154"156 linseed oil alkyds/titania,157 and PVC/titania and vanadia/sulfonated polyaniline... 

Selective oxide formation within one domain follows similar methods. Kim et al. demonstrated titania and silica hybrid materials formed in self-assembled perpendicular cylinders of poly (ethylene oxide) in PS-b-PEO upon exposure to titanium tetrachloride or silicon tetrachloride vapors. Pai et al. used humidified supercritical carbon dioxide to infiltrate metal alkox-ides such as tetraethylorthosilicate into poly(ethylene oxide)-btocfe-poly(propylene oxide)-btocfe-poly(ethylene oxide) and polyethylene-btod -poly(ethylene oxide) containing trace... 

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