MVTR resin produced with post-titanated Cr/Si/Ti catalyst

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...

Reexamination Certificate

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C526S104000, C526S106000, C526S107000, C526S348100

Reexamination Certificate

active

06531565

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the production of polyethylene resins which are suitable for forming film having low MVTR (moisture vapor transmission rate). The present invention especially relates to such processes for producing polyethylene resin using a heterogeneous catalyst comprising chromium and titanium on a solid support.
BACKGROUND OF THE INVENTION
The polymerization of ethylene to produce polyethylene using a catalyst comprising chromium or chromium and titanium on solid support, such as silica, has been known for several decades. An early reference in this area is U.S. Pat. No. 2,825,721 to Hogan and Banks. A second group of catalysts for ethylene polymerization are Ziegler catalysts, comprising titanium halide and aluminum alkyl, also have been extensively used to produce polyethylene. A third group of catalysts for ethylene polymerization are catalysts comprising molybdenum, as described, for example, in U.S. Pat. No. 2,692,257. However, molybdenum catalysts have not been extensively used for ethylene polymerization. More recently, a fourth group of catalysts, metallocenes, comprising a cyclopentadienyl substituted transition metal, also have come into use.
Of these four groups of catalysts, the present invention is concerned with ethylene polymerization processes using a catalyst comprising chromium on a solid support, such as a silica.
Numerous references have been published concerning use of chromium-silica catalysts for polyethylene production, including references wherein the chromium component is impregnated onto a support such as silica and references where chromium and silica are formed as a cogel. Example references wherein the catalyst is formed by an impregnation procedure include U.S. Pat. No. 2,825,721. Examples of forming the catalyst by cogelation include U.S. Pat. No. 5,115,053 (cogel of chromium and silica) and U.S. Pat. No. 5,183,792 (tergel of chromium and titanium and silica).
For the chromium-silica catalysts which include titanium as a component, the titanium is generally introduced or added to the chromium-silica catalyst in one step. Thus, for instance, U.S. Pat. No. 4,294,724 states that it is known that titanium affects the polymerization activity of silica supported chromium catalysts in a way that is of special importance in slurry polymerization. However, when titanium is co-precipitated with the silica, it produces a hydrogel which does not have sufficient strength to resist serious collapse of the pores during simple drying, such as spray drying. Accordingly, in order to take full advantage of the improvement which can be imparted to the melt index capability through the use of titanium in accordance with the prior art, the titanium had to be co-precipitated with the silica and the resulting hydrogel (cogel) dried by a more expensive azeotrope distillation or washing with a liquid oxygen-containing water soluble organic compound.
References such as McDaniel's “Supported Chromium Catalysts for Ethylene Polymerization”,
Advances in Catalysis
(1985) pages 47-98, teach that the amount of titanium in the catalyst should not be high, as high titanium levels lead to sintering and reduce surface area. Thus, McDaniel states at page 78: “That titania increases the termination rate also can be seen in FIG. 14. Here the melt index, which reflects the termination rate of some co-precipitated sample, is plotted against the titania concentration. At 650° C. and 760° C. calcining temperatures, the melt index increases with titania content, but at 870° C., a peak in melt index is obtained, followed by a sharp drop. This is due to sintering, which can be considered as the earliest stages of melting. Sintering destroys the surface area and porosity of the catalyst. Although Cr/silica itself does not sinter at 870° C., the added titania does promote sintering, as impurities often lower the melting point of solids. Both activity and MI potential are diminished by sintering, and the more titania added, the more easily the catalyst sinters.”
Turning now to MVTR, as pointed out in WO 96/19527 to Davis, one of the most common polyolefin polymers valued for its low moisture or water vapor transmission rate (MVTR) is high density polyethylene (HDPE). Generally, HDPE's are those which have densities at or above about 0.940 g/cc. Generally, the higher the density, the better a resin's MVTR for a given package thickness. The Davis reference refers to use of a metallocene catalyst, citing Davis' co-pending patent application U.S. Ser. No. 08/093,901 which discloses metallocene catalyst systems that can be used to produce polymers having not only excellent strength, sealing, and optical properties, but having superior water vapor transmission rates. The polymers are disclosed to have use in the packaging industry. A film is disclosed having at least one layer having a density less than about 0.935 g/cm
3
, a M
w
/M
n
less than about 3, a CDBI greater than about 80%. The layer includes a resin having a density about 0.90 g/cm
3
and an MVTR of less than about 2.25 g·mil/100 in
2
/day. Thus, Davis is directed to improved (lower) MVTR through use of a resin having a relatively narrow MWD. Davis points out that in certain embodiments of his invention, the resin produced has a density in the range of from about 0.935 to about 0.965 g/cm
3
, a M
w
/M
n
less than about 3, and an article made using the resin has a water vapor transmission rate less than 0.54 g·mil/100 in
2
/day, preferably less than 0.4 g·mil/100 in
2
/day. Also, in
Plastics Technology,
August 1999, in an article by J. Krohn et al. titled “Keep It Dry, Optimize Moisture Barrier in PE Films”, at pages 60-61, the authors state “Thus, structure 3 excelled in barrier because it was the only one to have a skin layer of higher MI resin with narrower MWD, both of which contribute inherently to better barrier.”
SUMMARY OF THE INVENTION
According to the present invention, a process is provided for polymerizing ethylene to form a polyethylene homopolymer having an MWD greater than 4 and suitable for forming a film having an MVTR less than 0.3 g·mil/100 in
2
/day, which process comprises contacting the ethylene under slurry or gas phase reaction conditions with a chromium-silica cogel catalyst containing titanium, wherein the catalyst contains titanium added in at least two steps: (1) titanium added as part of a first cogel formation, and (2) titanium added in a post-titanation step after the first cogel catalyst is formed and dried.
Preferably, the polyethylene polymer produced in accordance with the present invention has an MVTR below 0.25 g·mil/100 in
2
/day, most preferably less than 0.2 g·mil/100 in
2
/day.
Also, preferably the polyethylene polymer produced in accordance with the process of the present invention has an MWD greater than 4, more preferably greater than 5, and most preferably greater than 6. The MWD may be as high as 10; more preferably the high range of MWD is about 8.
The term “cogel” as used in connection with the present invention is used to embrace cogellation of two components, such as the chromium and silica, as well as cogellation of three (more precisely, a “tergel”) components, such as chromium-titanium and silica.
According to a preferred embodiment of the present invention, the amount of titanium in the first cogel formation is sufficient to produce a first cogel containing between 1 wt. % and 5 wt. % titanium, more preferably 1.5 wt. % to 4 wt. % titanium, and most preferably 2 wt. % to 3 wt. % titanium, based on the dried first cogel. The amount of titanium added in the post-titanation step in accordance with the present invention preferably is sufficient so that the post-titanated catalyst contains between 5 wt. % and 15 wt. % titanium, more preferably between 6 wt. % and 12 wt. % titanium, and most preferably between 7.0 wt. % and 9.5 wt. %, based on the calcined catalyst.
For the catalyst used in the process of the present invention, preferably the amount of titanium added by post titanation and preferably also the calcination co

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