Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2000-09-11
2002-09-24
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...
C526S133000, C526S159000, C526S131000, C526S165000, C526S172000
Reexamination Certificate
active
06455660
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to novel polyethylene compositions.
Many different grades of polyethylene are manufactured for different applications, and equally there is a wide variety of physical properties of polyethylene which are important in each case. Generally it is not just one property which is important for a particular application, but several: finding a polyethylene which possesses the right combination of those properties is the major objective of much research. For example, in pipe and moulding applications properties such as density, viscosity (i.e. melt flow rate), impact strength and rigidity are all important.
BACKGROUND
The use of certain transition metal compounds to polymerise 1-olefins, for example, ethylene, is well established in the prior art. Silica-supported chromium catalysts using the Phillips process have been known for several decades. The use of Ziegler-Natta catalysts, for example those catalysts produced by activating titanium halides with organometallic compounds such as triethylaluminium, is fundamental to many commercial processes for manufacturing polyolefins. In recent years the use of certain metallocene catalysts (for example biscyclopentadienylzirconiumdichloride activated with alumoxane) has provided catalysts with potentially high activity. These different catalyst systems provide polymeric products with a variety of properties.
Commodity polyethylenes are produced commercially in a variety of different types and grades. Homopolymerisation of ethylene with transition metal based catalysts leads to the production of so-called “high density” grades of polyethylene. These polymers have relatively high stiffness and are useful for making articles where inherent rigidity is required, such as pipe and moulded products.
WO98/27124, published after the earliest priority date of this invention, discloses that ethylene may be polymerized by contacting it with certain iron or cobalt complexes of selected 2,6-pyridinecarboxaldehydebis(imines) and 2,6-diacylpyridinebis(imines). There is no disclosure regarding the properties of polyethylene produced by such catalysts and because the polyethylene produced in those Examples where the molecular weight is higher than oligomeric mostly shows extremely broad molecular weight distributions, it would not have the range of properties considered in this application.
SUMMARY OF THE INVENTION
We have discovered a class of novel homopolymers of ethylene which have a combination of properties that make them particularly suitable for use in pipe, film and moulded products. Accordingly a first aspect of the invention provides a homopolymer of ethylene which has:
an annealed density D/weight average molecular weight M
W
relationship defined by the equation equation D>1104.5M
W
−0.0116
; and
either a Charpy Impact I/High Load Melt Index H relationship defined by the equation I>35.0H
−0.4
,
or a dynamic storage modulus G′ of 2.9 or less.
Weight average molecular weight M
W
is measured by GPC. Annealed density is measured to specification ISO 1872-1:1993 using test method ISO 1183:1987. Charpy impact is measured according to ISO 179-1982/2/A on sheets compression moulded according to specification BS EN ISO 1872-2:1997. High Load Melt Index (HLMI) is a commonly used measure, which like MFR gives an indication of melt viscosity and hence molecular weight. It is determined by a melt indexer in terms of the melt output (g/10 minutes) under a given high load (21.6 kg) through a standard die orifice. In this application HLMI is measured according to ASTM D 1238 condition F, 21.6 kg at 190° C.
Dynamic storage modulus G′ is formally defined as the storage modulus measured at a loss modulus (G″) of 5 kPa. It is essentially the modulus of the melt measured “in phase” with the imposed oscillation in a dynamic test, and can be considered to quantify the elasticity of the melt. The steady state compliance (J
s
o
) is a viscoelastic property of polymers. Methods for measuring J
s
o
, and a discussion of its utility, can be found in a number of text books (see for example Chapters 2 and 10 of “Melt Rheology and its Role in Plastics Processing, Theory and Applications”, by John M. Dealy and Kurt F. Wissbrun, published by Van Nostrand Reinhold, New York, 1990). J
s
o
is recognised as a useful property for polymer characterisation, and has been found to be independent of a polymer's average molecular weight but strongly affected by its molecular weight distribution, particularly by the fraction of very high molecular weight polymer present. Measurement of J
s
o
or some associated melt viscoelastic property is a far more sensitive method for characterising polymers for subtle differences in molecular weight distribution than are dilute solution measurements. However J
s
o
is difficult to measure directly for high molecular weight polyethylenes, and therefore an indirect method is used: it can be related to the storage modulus. (G′), measured in a dynamic test at low frequency (&ohgr;), by the relationship
G
′(&ohgr;)=J
s
o
[G
″(&ohgr;)]
2
for &ohgr;→0
where G″ is the loss modulus, also measured at low frequency. In practice therefore, it is possible to measure G′ at a low reference value of G″, and to use this parameter as an indication of the fraction of very high molecular weight polymer present. The method for measuring G′ is described in the Examples below.
In a second aspect the invention provides a homopolymer of ethylene which has a polydispersity M
W
/M
n
of 16 or less, and and wherein the width of its molecular weight distribution at half the peak height is at least 1.6. The width of the molecular weight distribution is measured on a logarithmic scale.
Preferably the polydispersity M
W
/W
n
is between 7 and 16. Number average molecular weight M
n
like M
W
is measured by GPC according to NAMAS method MT/GPC/02. At such relatively low polydispersities we have found that the homopolymers of the invention have a distinctive molecular weight distribution which can be expressed mathematically in the above manner. It is believed that this may at least partly account for some of the novel properties recited below.
Preferably the annealed density/molecular weight relationship is defined by the equation D>1105.5M
W
−0.0116
. The Charpy Impact/HLMI relationship is preferably defined by the equation I>37.0H
−0.42
, more preferably I>38.8H
−0.42
.
Whilst polyethylene homopolymers are known which have properties defined by at least one of the above relationships, none has properties defined by both the density and Charpy Impact relationships. This unique combination of properties makes the polyethylene of the invention particularly suitable for a number of applications. For example, the improved density: melt mass-flow rate (MFR) performance (MFR being inversely proportional to molecular weight) of the compounds of the invention means that for a given MS it is possible to produce articles such as bottles or drums with a higher rigidity: weight ratio. This is particularly advantageous for the production of fast-cycling thin-walled bottles. The higher impact strength: MFR ratio is advantageous for drum or large container applications either to improve the impact strength of a container for a given weight, or to reduce the weight for a given impact strength. Thus the compounds of the invention enable containers to be made with reduced weight whilst maintaining both rigidity and impact strength.
It is also preferred that the homopolymer has an MFR drop on compounding of 20% or less when the HLMI is less than 10. By “MFR drop on compounding” is meant the difference between the Melt Flow Ratio of compounded pellets of the homopolymer and the MFR of the powder before compounding. The MFR of polyethylene generally drops upon compounding: the smaller the drop, the smaller the change in processing and viscosity properties upon compounding from powder into pellets. Thus the relatively s
Clutton Edward Quentin
Hope Philip Stephen
Partington Stephen Roy
Reid Samson John Norman
BP Chemicals Limited
Finnegan, Henderson Farabow, Garrett and Dunner L.L.P.
Lu Caixia
Wu David W.
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