Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1999-06-24
2001-06-12
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S129000, C526S169000, C502S170000, C502S117000, C502S256000
Reexamination Certificate
active
06245869
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process for producing polyethylene, and in particular a process producing medium density polyethylene using a chromium-based catalyst.
Polyethylene is known for use in the manufacture of a wide variety of articles. The polyethylene polymerisation process can be varied in a number of respects to produce a wide variety of resultant polyethylene resins having different physical properties which render the various resins suitable for use in different applications. Medium density polyethylene resins are known for use in making films. Such medium density films are known to have good resin processability due to the presence of long chain branching in the polyethylene polymer molecules. It is known to produce such resins using chromium-based catalysts, which have been known for some time. Unfortunately, some medium density resins produced by such catalysts suffer from the problem that when the film is to be used for the packaging of foods for human consumption, it is required that the level of extractable or volatile compounds in the polyethylene resin is low so that the food is not inadvertently contaminated. For example, in the United States the Food and Drug Administration has set maximum limits for the amount of extractable or volatile compounds in polyethylene resins for food applications.
It is also known to produce polyethylene using metallocene catalysts, typically unbridged metallocene catalysts. Such catalysts are known to produce polyethylene resin films having low values of extractable components due to the narrow molecular weight distribution in the polyethylene resin, which yields a very low proportion of components in the resin at low molecular weight. Unfortunately, such films suffer from the technical problem that the combination of narrow molecular weight distribution and the absence of long chain branching are responsible for the resins having poor processability.
U.S. Pat. No. 5,284,926 discloses an ethylene polymerisation catalyst system and process. It is disclosed that the catalyst may be a chromium-based catalyst which may be subjected to fluorination and subsequent reduction by carbon monoxide. The polymerization process may be carried out in the presence of a trialkyl aluminium co-catalyst. However, there is no disclosure of the production of medium density polyethylene resins, or in particular the production of such resins suitable for food packaging having a low level of extractable or volatile compounds in the resin.
U.S. Pat. No. 5,352,658 discloses a similar catalyst system and polymerization process. This specification addresses the problem of providing excellent low temperature impact strength and preventing swelling in the blow moulding process on emergence from the extruder die. The specification does not address the problem of the production of medium density polyethylene films having suitability for food packaging by having low levels of extractable or volatile compounds.
EP-A-0548805 discloses polymerization catalysts and processes for the production of high density polyethylene homopolymers. There is no disclosure of the production of medium density polyethylene resins suitable for use in making film packaging for food.
U.S. Pat. No. 5,071,927 discloses high-temperature slurry polymerization of ethylene using a chromium-based catalyst and a trialkyl boron or polyalkyl silane co-catalyst. Although the production of a medium density polyethylene resin is disclosed, there is no disclosure of such a resin having low levels of extractable or volatile compounds to render the resin suitable for food packaging.
SUMMARY OF THE INVENTION
The present invention aims to provide a process which at least partially overcomes these problems of the prior art.
Accordingly, the present invention provides a process for producing medium density polyethylene by copolymerising ethylene and an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms in the presence of a chromium-based catalyst having a support comprising silica and titania, the catalyst being fluorinated and having been chemically reduced by carbon monoxide, and in the presence of a co-catalyst selected from at least one of an aluminium alkyl and a zinc alkyl to produce a polyethylene copolymer having a density of from 0.930 to 0.945 g/cc and a dispersion index (D) of from 9 to 13.
The present invention further provides the use for reducing the fraction of low molecular weight volatile compounds of a medium density polyethylene having a density of from 0.930 to 0.945 g/cc, of a chromium-based catalyst, having a silica and titania support and which is fluorinated and has been chemically reduced by carbon monoxide, together with at least one of an aluminium alkyl and a zinc alkyl co-catalyst in a process for copolymerising ethylene and an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms.
The present invention is predicated on the surprising discovery by the present inventor that by treating a particular chromium-based catalyst with fluorine and chemically reducing the catalyst with carbon monoxide, and by using a particular co-catalyst it is possible strongly to reduce the level of extractable or volatile compounds of a medium density polyethylene resin without reducing the processability of the catalyst. The chromium-based catalyst continues to provide processability due to the presence of long chain branching in the polyethylene resin. Such a resin having a low level of extractable or volatile compounds may be used to produce food packaging, e.g. as films.
REFERENCES:
patent: 4564660 (1986-01-01), Williams et al.
patent: 5071927 (1991-12-01), Benham et al.
patent: 5284926 (1994-02-01), Benham et al.
patent: 5352658 (1994-10-01), Evertz et al.
patent: 5641842 (1997-06-01), McDaniel et al.
patent: 6015866 (2000-01-01), Debras et al.
patent: 0548805 (1993-06-01), None
Bodart Philippe
Debras Guy
Fina Research S.A.
Rabago R.
Wheelington Jim D.
Wu David W.
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