Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in tubular or loop reactor
Reexamination Certificate
2000-03-29
2001-09-18
Teskin, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in tubular or loop reactor
C526S065000, C526S073000, C526S088000, C526S116000, C526S348500, C526S905000
Reexamination Certificate
active
06291601
ABSTRACT:
The present invention relates to a process for the production of polyethylene in particular high density polyethylene (HDPE) having a bimodal molecular weight distribution.
For polyethylene, and for high density polyethylene (HDPE) in particular, the molecular weight distribution (MWD) is a fundamental property which determines the properties of the polymer, and thus its applications. It is generally recognised in the art that the molecular weight distribution of a polyethylene resin can principally determine the physical, and in particular the mechanical, properties of the resin and that the provision of different molecular weight polyethylene molecules can significantly affect the rheological properties of the polyethylene as a whole.
Since an increase in the molecular weight normally improves the physical properties of polyethylene resins, there is a strong demand for polyethylene having high molecular weight. However, it is the high molecular weight molecules which render the polymers more difficult to process. On the other hand, a broadening in the molecular weight distribution tends to improve the flow of the polymer when it is being processed at high rates of shear. Accordingly, in applications requiring a rapid transformation employing quite high inflation of the material through a die, for example in blowing and extrusion techniques, the broadening of the molecular weight distribution permits an improvement in the processing of polyethylene at high molecular weight (this being equivalent to a low melt index, as is known in the art). It is known that when the polyethylene has a high molecular weight and also a wide molecular weight distribution, the processing of the polyethylene is made easier as a result of the low molecular weight portion and also the high molecular weight portion contributes to a good impact resistance for the polyethylene film. A polyethylene of this type may be processed utilising less energy with higher processing yields.
The molecular weight distribution can be completely defined by means of a curve obtained by gel permeation chromatography. Generally, the molecular weight distribution is defined by a parameter, known as the dispersion index D, which is the ratio between the average molecular weight by weight (Mw) and the average molecular weight by number (Mn). The dispersion index constitutes a measure of the width of the molecular weight distribution. For most applications, the dispersion index varies between 10 and 30.
It is known in the art that it is not possible to prepare a polyethylene having a broad molecular weight distribution and the required properties simply by mixing polyethylenes having different molecular weights.
As discussed above, high density polyethylene consists of high and low molecular weight fractions. The high molecular weight fraction provides good mechanical properties to the high density polyethylene and the low molecular weight fraction is required to give good processability to the high density polyethylene, the high molecular weight fraction having relatively high viscosity which can lead to difficulties in processing such a high molecular weight fraction. In a bimodal high density polyethylene, the mixture of the high and low melting weight fractions is adjusted as compared to a monomodal distribution so as to increase the proportion of high molecular weight species in the polymer. This can provide improved mechanical properties.
It is accordingly recognised in the art that it is desirable to have a bimodal distribution of molecular weight in the high density polyethylene. For a bimodal distribution a graph of the molecular weight distribution as determined for example by gel permeation chromatography, may for example include in the curve a “shoulder” on the high molecular weight side of the peak of the molecular weight distribution.
The manufacture of bimodal polyethylene is known in the art. It is known in the art that in order to achieve a bimodal distribution, which reflects the production of two polymer fractions, having different molecular weights, two catalysts are required which provide two different catalytic properties and establish two different active sites. Those two sites in turn catalyse two reactions for the production of the two polymers to enable the bimodal distribution to be achieved. Currently, as has been known for many years, as exemplified by EP-A-0057420, the commercial production of bimodal high density polyethylene is carried out by a two step process, using two reactors in series. In the two step process, the process conditions and the catalyst can be optimised in order to provide a high efficiency and yield for each step in the overall process.
In the applicant's earlier WO-A-95/10548 and WO-A-95/11930, it was proposed to use a Ziegler-Natta catalyst to produce polyethylene having a bimodal molecular weight distribution in a two stage polymerisation process in two liquid full loop reactors in series. In the polymerisation process, the comonomer is fed into the first reactor and the high and low molecular weight polymers are produced in the first and second reactors respectively. The introduction of comonomer into the first reactor leads to the incorporation of the comonomer into the polymer chains in turn leading to the relatively high molecular weight fraction being formed in the first reactor. In contrast, no comonomer is deliberately introduced into the second reactor and instead a higher concentration of hydrogen is present in the second reactor to enable the low molecular weight fraction to be formed therein.
These prior processes suffer from the technical disadvantage that some unreacted comonomer can pass through from the first reactor to the second reactor thereby to react with the ethylene monomer therein leading to an increase in the molecular weight of the fraction produced in the second reactor. This in turn can deteriorate the bimodality of the molecular weight distribution of the combined high and low molecular weight polymers leading to a reduction in mechanical properties.
WO-A-98/58001 discloses an olefin polymerisation process comprising at least two polymerisation stages without the need for certain interstage reaction mixture treatment steps wherein in an earlier stage an olefin polymerisation is effected in the presence of hydrogen and a rapidly hydrogen consuming catalyst to produce a relatively lower molecular weight (higher melt flow) polymer and in a later stage an olefin polymerisation is effected whereby to produce a relatively lower melt flow rate polymer. It is disclosed that the process allows comonomer incorporation even when hydrogen is used and moreover the problem of removal of unreacted hydrogen or comonomer between the earlier and later polymerisation stages can be avoided.
EP-A-0881237 discloses a process for producing bimodal polyolefins with metallocene catalysts using two reaction zones.
U.S. Pat. No. 5276115 discloses a process for the polymerisation of olefins in the presence of hydrogen comprising contacting the olefin, a cyclopentadienyl compound and a catalyst comprising a transmission metal compound at polymerisation conditions. The polymerisation may be carried out in series reactors. A cyclopentadienyl compound can be introduced into the second reactor without prior removal of remaining hydrogen which was introduced into the first reactor. The polymerisation catalyst comprises a Ziegler-Natta catalyst.
EP-A-0288226 discloses a process for adjusting the melt flow of olefin polymer products in which a hydrogenation catalyst is employed to reduce the melt flow of the polymer products.
U.S. Pat. No. 5,739,220 discloses an olefin polymerisation process in which at least two introductions of hydrogen are made during the olefin polymerisation reaction. Suitable catalysts include metallocenes and conventional Ziegler-Natta catalysts blended with or modified by such metallocenes. It is disclosed that the catalyst system is bifunctional in that it functions both as a polymerisation catalyst and has a hydrogenation catalyst.
The present in
Fina Research S.A.
Jackson William D.
Teskin Fred
LandOfFree
Production of polyethylene having a broad molecular weight... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Production of polyethylene having a broad molecular weight..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Production of polyethylene having a broad molecular weight... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2457205