Ethylene polymerization

Details Ethylene polymerization Ethylene polymerization

1999-11-29

2003-10-14

Brooks, J. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S105000, C526S106000, C526S113000, C526S114000, C526S118000, C526S119000, C526S126000, C526S131000, C526S348200, C526S348600, C526S348500

Reexamination Certificate

active

06632896

ABSTRACT:

This invention relates to polymer resins, their production and their use, in particular polyethylene resins suitable for blow moulding applications.
Household and industrial containers (HIC) have been produced for over 30 years by blow moulding of high density polyethylene (HDPE) resins produced using chromium (Cr) catalysts which are commercially available from catalyst producers such as Grace, Crosfield and PQ. The HDPE resins produced using Cr catalysts have excellent extrudability and thus allow high output for blow moulding machines. Indeed blow moulding machines are often specifically constructed for optimum performance using Cr catalyst produced HDPE (Cr-HDPE), for example in terms of screw configuration, and die and forming tool construction. As a result Cr-HDPE resins are essentially the industry standard for HIC blow moulding worldwide.
The most important properties for a good Cr-HDPE grade for HIC blow moulding are high environmental stress crack resistance (ESCR) and high stiffness in the moulded product, good extrudability of the molten resin (to permit high output by the blow moulding machine) and swelling and sagging properties of the molten resin which match the configuration of the blow moulding machine and result in uniform wall thickness in the moulded product.
Improvements in these properties for Cr-HDPE have resulted in HIC produced by blow moulding of Cr-HDPE becoming increasingly competitive relative to HIC (e.g. bottles or containers) produced by other techniques or using other materials.
For containers, e.g. household and industrial containers such as bottles, barrels, tubs, jars, vats, etc. and container closures, e.g. lids, caps etc., and in particular containers for liquids containing organic solvents or detergents, one especially important property is ESCR. This is measured by standard tests and is a measure of the ability of the container to remain viable in use, to be stacked, to be left exposed, etc.
However, while Cr-HDPE has become an industry standard for blow moulding of HIC, the ESCR values achievable using commercially available Cr catalysts is less than is desirable for various HIC and there remains a need for improved HDPEs for blow moulding of HIC. Furthermore, using commercially available Cr catalysts, in order to produce Cr-HDPE having appropriately high extrudability for blow moulding of HIC it has been necessary to operate polymerization reactors at temperatures which are so high as to be close to the level where reactor fouling occurs. Under these conditions, the HDPE production rate has been found to be lower than the production rates for other polymers using the same reactors. Thus there is also a need for Cr-HDPEs which can be produced more efficiently.
We have now surprisingly found that these problems may be addressed by the use of a blend of supported Cr catalysts for Cr-HDPE production, where a first Cr catalyst is supported on an alumina-silica carrier and a second Cr catalyst is supported on a titania-silica carrier or co-precipitated with titania-silica as a tergel.
Viewed from one aspect the invention thus provides a high density polyethylene, e.g. having a density of at least 0.957 g/mL, more preferably 0.958 to 0.965 g/mL, produced using a chromium-catalysed polymerization, wherein the catalyst comprises a first silica-supported chromium catalyst having a pore volume of at least 2 mL/g, a surface area of at least 350 m
2
/g (preferably at least 400 m
2
/g, more preferably at least 450 m
2
/g) and a chromium content of 0.1 to 1.0% by weight and a second silica-supported chromium catalyst having a pore volume of at least 2 mL/g, a surface area of at least 450 m
2
/g (preferably at least 500 m
2
/g, especially up to 600 m
2
/g) and a chromium content of 0.1 to 1.0% by weight, wherein the silica support of said first catalyst also comprises alumina and the support of said second catalyst also comprises titania.
Thus the second catalyst may for example be a co-gel (i.e. Cr on TiO
2
/SiO
2
) or a tergel (i.e. Cr/TiO
2
/SiO
2
).
The chromium component of the catalysts may, as in conventional catalysts, be a chromium oxide (e.g. Cr
2
O
3
, CrO
3
or CrO) or a precursor compound convertible in use to a chromium oxide. In general Cr
3
O
3
is preferred as this can be transformed by calcination to CrO
3
. In the reactor, the chromium is transformed to the active form CrO.
Viewed from an alternative aspect the invention comprises a high density polyethylene containing catalyst residues of titanium, chromium, aluminium, silicon and optionally also boron, having an MFR
2
of at least 0.2 g/10 min, an MFR
21
of at least 20 g/10 min and a bottle ESCR F
50
of at least 280 hours.
Viewed from a further aspect, the invention provides a process for the preparation of a polyethylene, in particular an HDPE suitable for blow moulding of RIC, which comprises polymerizing ethylene and, optionally an ethylenically unsaturated comonomer copolymerizable therewith, in the presence of a catalyst comprising a first silica-supported chromium catalyst having a pore volume of at least 2 mL/g, a surface area of at least 350 m
2
/g (preferably at least 400 m
2
/g, more prferably at least 450 m
2
/g) and a chromium content of 0.1 to 1.0% by weight and a second silica-supported chromium catalyst having a pore volume of at least 2 mL/g, a surface area of at least 450 m
2
/g (preferably at least 500 m
2
/g, especially up to 600 m
2
/g) and a chromium content of 0.1 to 1.0% by weight, wherein the silica support of said first catalyst also comprises alumina and the silica support of said second catalyst also comprises titania, and preferably a co-catalyst, e.g. a trialkylboron.
Viewed from a still further aspect the invention provides a catalyst system for ethylene polymerization, said system comprising a first silica-supported chromium catalyst having a pore volume of at least 2 mL/g, a surface area of at least 350 m
2
/g (preferably at least 400 m
2
/g, more prferably at least 450 m
2
/g) and a chromium content of 0.1 to 1.0% by weight and a second silica-supported chromium catalyst having a pore volume of at least 2 mL/g, a surface area of at least 450 m
2
/g (preferably at least 500 m
2
/g, especially up to 600 m
2
/g) and a chromium content of 0.1 to 1.0% by weight, wherein the silica support of said first catalyst also comprises alumina and the silica support of said second catalyst also comprises titania, and preferably a co-catalyst, e.g. a trialkylboron. The components of such a system may be mixed in the polymerization reactor or before insertion into the polymerization reactor.
In the catalyst systems of the invention, the silica supports preferably have substantially the same pore volumes, e.g. the titania-silica pore volume is from 70 to 130% of the alumina-silica pore volume, more preferably 80 to 120%, especially 90 to 110%.
Viewed from a yet still further aspect the invention also provides the use of a high density polyethylene according to the invention for blow moulding, particularly of HIC, especially bottles, in particular having an internal volume of 0.1 to 25L.
Viewed from another aspect the invention provides a blow moulded polyethylene container formed from a high density polyethylene according to the invention.
Viewed from a still further aspect the invention provides a high density polyethylene container (e.g. a bottle) having a bottle ESCR F
50
value of at least 280 hours, preferably at least 300 hours, e.g. 300 to 340 hours.
In the alumina:silica and titania:silica supports in the chromium catalysts, the silica content is preferably 80 to 99 wt %.
The pore volume in the catalyst supports is preferably 2 to 3 mL/g, especially 2.1 to 2.9 mL/g. The surface area is preferably 450 to 600 m
3
/g. Such supported chromium catalysts are generally referred to as high pore volume catalysts.
For use in gas phase polymerization, the chromium content of the catalysts may be at the low end of the specified ranges, e.g. 0.1 to 0.2% wt. For slurry polymerization, higher chromium contents, e.g. 0.5 to 10% wt will generally be preferred.
Chromium c

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