Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in two or more physically distinct zones
Reexamination Certificate
1998-09-26
2001-06-26
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in two or more physically distinct zones
C526S068000, C526S069000, C526S106000, C526S073000, C526S160000, C526S943000, C526S348600
Reexamination Certificate
active
06252017
ABSTRACT:
BACKGROUND TO THE INVENTION
The present invention relates to a process for producing polyethylene having improved crack and/or impact resistance.
DESCRIPTION OF THE PRIOR ART
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 product a wide variety of resultant polyethylene resins having different physical properties which render the various resins suitable for use in different applications. In particular, it is known to use polyethylene for use in applications where the polyethylene is required to have crack resistance, both resistance to rapid and to slow crack growth. For example, polyethylene is known for use in the manufacture of pipes where it is required that the material of the pipe has sufficient crack resistance so as to avoid inadvertent fracture in service. It is also known to use medium density polyethylene for the manufacture of films where the polyethylene is required to have impact resistance.
Chromium-based catalysts used for the production of polyethylene have been known for some time. Typically, for the manufacture of polyethylene use as a pipe resin, the polyethylene is polymerised in a reactor using a chromium-based catalyst to product a monomodal molecular weight distribution of the resultant polyethylene resin.
It is known in the art that the physical properties, in particular the mechanical properties, of a polyethylene product can vary depending on what catalytic system was employed to make the polyethylene. This is because different catalyst systems tend to yield different molecular weight distributions in the polyethylene produced.
SUMMARY OF THE INVENTION
The present invention aims in one aspect to provide a process for producing polyethylene, in particular a polyethylene having a monomodal molecular weight distribution, having improved crack and/or impact resistance.
Accordingly, the present invention provides a process for producing polyethylene having improved crack and/or impact resistance, the process comprising copolymerising ethylene and an alpha-olefinic comonomer comprising from 3 to 8 carbon atoms, in the presence of a chromium-based catalyst in a first reactor, feeding the polyethylene copolymer thereby produced and the chromium-based catalyst to a second reactor, and in the second reactor copolymerising ethylene and an alpha-olefinic comonomer comprising from 3 to 8 carbon atoms, in the presence of the chromium-based catalyst thereby to produce a second polyethylene product, the first and second polyethylene products forming a combined polyethylene having a monomodal molecular weight distribution.
Preferably, a greater amount of comonomer is incorporated into the second polyethylene product than in the first polyethylene product.
Preferably, the combined polyethylene comprises from 50 to 70% by weight of the first polyethylene product and from 30 to 50% by weight of the second polyethylene product.
To achieve such a monomodal molecular weight distribution, the first and second polyethylene products individually have substantially the same molecular weight distribution and melt index.
The present inventor has discovered surprisingly that by producing a polyethylene pipe resin having a monomodal distribution using two reactors in series, as opposed to using a single reactor, the resultant pipe resin exhibits better resistance to slow crack growth and to rapid crack propagation.
The present inventor has further discovered surprisingly that by producing a medium density polyethylene resin (MDPE) having a monomodal distribution using two reactors in series, the resultant resin can be used to produce films having improved impact resistance and/or also tear strength.
REFERENCES:
patent: 5478898 (1995-12-01), Standert et al.
patent: 50698/96 (1996-11-01), None
Debras Guy
Vandaele Hugo
Fina Research S.A.
Harlan R.
Wheelington Jim D.
Wu David W.
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