Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2000-10-26
2003-11-04
Choi, Ling-Siu (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S352000, C526S161000, C526S159000, C526S127000
Reexamination Certificate
active
06642339
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to copolymers of ethylene and alpha-olefins in particular to low density copolymers and also to films produced from said copolymers.
In recent years there have been many advances in the production of polyolefin copolymers due to the introduction of metallocene catalysts. Metallocene catalysts offer the advantage of generally higher activity than traditional Ziegler catalysts and are usually described as catalysts which are single-site in nature. Because of their single-site nature the polyolefin copolymers produced by metallocene catalysts often are quite uniform in their molecular structure. For example, in comparison to traditional Ziegler produced materials, they have relatively narrow molecular weight distributions (MWD) and narrow Short Chain Branching Distribution (SCBD).
Although certain properties of metallocene products are enhanced by narrow MWD, difficulties are often encountered in the processing of these materials into useful articles and films relative to Ziegler produced materials. In addition, the uniform nature of the SCBD of metallocene produced materials does not readily permit certain structures to be obtained.
Recently a number of patents have published directed to the preparation of films based on low density polyethylenes prepared using metallocene catalyst compositions.
WO 94/14855 discloses linear low density polyethylene (LLDPE) films prepared using a metallocene, alumoxane and a carrier. The metallocene component is typically a bis (cyclopentadienyl) zirconium complex exemplified by bis (n-butylcyclopentadienyl) zirconium dichloride and is used together with methyl alumoxane supported on silica. The LLDPE's are described in the patent as having a narrow Mw/Mn of 2.5-3.0, a melt flow ratio (MFR) of 15-25 and low zirconium residues.
WO 94/26816 also discloses films prepared from ethylene copolymers having a narrow composition distribution. The copolymers are also prepared from traditional metallocenes (eg bis (1-methyl, 3-n-butylcyclopentadienyl) zirconium dichloride and methylalumoxane deposited on silica) and are also characterised in the patent as having [a] narrow Mw/Mn values typically in the range 3-4 and in addition by a value of Mz/Mw of less than 2.0.
However, it is recognised that the polymers produced from these types of catalyst system have deficiencies in processability due to their narrow Mw/Mn. Various approaches have been proposed in order to overcome this deficiency. An effective method to regain processability in polymers of narrow Mw/Mn is by the use of certain catalysts which have the ability to incorporate long chain branching (LCB) into the polymer molecular structure. Such catalysts have been well described in the literature, illustrative examples being given in WO 93/08221 and EP-A-676421.
Furthermore, WO 97/44371 discloses polymers and films where long chain branching is present, and the products have a particularly advantageous placement of the comonomer within the polymer structure. Polymers are exemplified having both narrow and broad Mw/Mn, for example from 2.19 up to 6.0, and activation energy of flow, which is an indicator of LCB, from 7.39 to 19.2 kcal/mol (31.1 to 80.8 kJ/mol). However, there are no examples of polymers of narrow Mw/Mn, for example less than 3.4, which also have a low or moderate amount of LCB, as indicated by an activation energy of flow less than 11.1 kcal/mol (46.7 kJ/mol).
We have now found that it is possible to prepare copolymers of ethylene and alpha-olefins having narrow Mw/Mn and low or moderate amounts of LCB. These polymers are suitable for many applications which will be known to those skilled in the art, but in particular are advantageous for preparing films with an excellent balance of processing, optical and mechanical properties.
SUMMARY OF THE INVENTION
Thus, according to the present invention there is provided a copolymer of ethylene and an alpha olefin having 3 to 10 carbon atoms, said polymer having
(a) a density in the range 0.900 to 0.940
(b) an apparent Mw/Mn of 2-3.4
(c) I
21
/I
2
from 16 to 24
(d) activation energy of flow from 28 to 45 kJ/mol
(e) a ratio Ea(HMW)/Ea(LMW)>1.1, and
(f) a ratio g′(HMW)/g′(LMW) from 0.85 to 0.95.
Preferred copolymers are those having
(a) a density in the range 0.900 to 0.940
(b) an apparent Mw/Mn in the range 2 to 3
(c) I
21
/I
2
from 18-24
(d) activation energy of flow from 28 to 45 kJ/mol
(e) a ratio Ea(HMW)/Ea(LMW)>1.1, and
(f) a ratio g′(HMW)/g′(LMW) from 0.85 to 0.95.
Most preferred copolymers are those having
(a) a density in the range 0.900 to 0.940
(b) an apparent Mw/Mn in the range 2.5 to 3
(c) I
21
/I
2
from 18-24
(d) activation energy of flow from 30 to 35 kJ/mol
(e) a ratio Ea(HMW)/Ea(LMW)>1.2, and
(f) a ratio g′(HMW)/g′(LMW) from 0.85 to 0.95.
By apparent Mw/Mn is meant a value of Mw/Mn uncorrected for long chain branching.
The significance of the parameters Ea(HMW)/Ea(LMW) and g′(HMW)/g′(LMW) is described below. The experimental procedures for their measurements are described later in the text.
DETAILED DESCRIPTION OF THE INVENTION
The polymers contain an amount of LCB which is clearly visible by techniques such as GPC/viscometry and flow activation energy. The content of LCB is lower than reported in many earlier publications, but is still sufficient, when coupled with broadened Mw/Mn, to give improved processability compared to linear polymers of narrow MWD (Mw/Mn less than about 3), which do not contain LCB.
For the measurement of LCB, we have found that the most useful techniques are those which have a particular sensitivity to the presence of LCB in the high molecular weight chains. For these high molecular weight molecules, the physical effects of LCB on the solution and melt properties of the polymer are maximised. Hence detection of LCB using methods based upon solution and melt properties is facilitated.
Activation energy of flow is commonly used as an indicator of the presence of LCB in polyethylenes as summarised in the aforementioned WO 97/44371. For lower amounts of LCB, for which the global activation energy is of the order of 28 to 45 kJ/mol, it is found that the LCB has a strong effect upon the activation energy as measured at low test rates (ie the region in which the rheology is dominated by the high molecular weight (HMW species). Therefore, the ratio of activation energy derived from the low rate data Ea(HMW) tends to exceed that derived from the high rate data, Ea(LMW). Hence polymers containing LCB predominantly in the high molecular weight chains tend to show the ratio EA(HMW)/Ea(LMW) greater than unity.
A further well established method indicating the presence of LCB is gel permeation chromatography with on-line detection of viscosity (GPC/OLV). By combining the data from 2 detectors, the ratio g′ can be derived as a function of molecular weight; g′ is the ratio of the measured intrinsic viscosity [&eegr;] divided by the intrinsic viscosity [&eegr;]
linear
of a linear polymer having the same molecular weight. In polymers containing LCB, the g′ measured at high molecular weights tends to be less than that measured at low molecular weights. To quantify this effect, we have used a simple ratio g′(HMW)/g′(LMW). g′(HMW) is the weighted mean value of g′ calculated for the 30% of the polymer having the highest molecular weight, while g′(LMW) is the weighted mean value of g′ calculated for the 30% of the polymer having lowest molecular weight. For linear polymers, g′ is equal to 1 at all molecular weights, and so g′(HMW)/g′(LMW) is also equal to 1 when there is no LCB present. For polymers containing LCB, g′(HMW)/g′(LMW) is less than 1. It should be noted that the g′ data can be corrected for the effect of short chain branching (SCB). This would normally be done using a mean value of SCB content, the correction being applied uniformly at all molecular weights. Such a correction h
Chai Choon Kooi
Frye Christopher James
BP Chemicals Limited
Choi Ling-Siu
Finnegan, Henderson Farabow, Garrett and Dunner L.L.P.
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