Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-09-07
2001-09-18
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S240000, C428S474400, C428S475800, C428S476100, C428S480000, C428S483000, C428S500000, C428S507000, C428S511000, C428S532000, C428S535000, C428S537500, C428S688000
Reexamination Certificate
active
06291590
ABSTRACT:
This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/FI98/00014 which has an International filing date of Jan. 7, 1998, which designated the United States of America.
The invention relates to a multimodal ethylene polymer based extrusion coating composition and its use for coating a paper or board substrate.
The purpose of this invention is to provide a polyethylene composition for extrusion coating having good processability and good mechanical properties.
The normal polyethylene extrusion coating grades currently in the market, are low density polyethylenes obtained from high pressure process. Typically, such low density polyethylenes have good processability, but inferior environmental stress crack resistance (ESCR) and seal strength as well as poor hot tack when compared with linear polyethylenes.
Also some linear polyethylenes, high density, medium density and linear low density polyethylenes are used in extrusion coating applications. However, the extrusion coating compositions are normally prepared by blending linear materials with high pressure low density polyethylene in order to achieve sufficient processability. The problem with linear materials is their poor processability. That has been overcome to some extent by blending the linear material with high pressure low density polyethylene. The benefit of linear polyethylenes is their better mechanical strength allowing thinner coatings.
The addition of high pressure low density polyethyllene (PE-LD) to linear high density or low density polyethylene (PE-HD or PE-LLD) usually leads to a loss in mechanical and barrier properties. Another drawback of blends is the extra cost caused due to the additional compounding step, where the blend of PE-HD or PE-LLD and high pressure PE-LD is made.
The idea of the present invention is to use a bimodal or multimodal linear polyethylene material in extrusion coating without adding any high pressure PE-LD. The bimodality or multimodality of the ethylene polymer of this invention ensures a sufficient throughput while the superior mechanical properties of linear polyethylenes are maintained. The material is especially suitable for coextrusion.
The purpose of the invention has been achieved with a polymer composition comprising a multimodal ethylene polymer, which contains from 80 to 100% by weight of ethylene repeating units and from 0 to 20% by weight of C
3
-C
10
alpha-olefin repeating units, has a density of between 0.920 and 0.960 g/cm
3
, and which is a blend of at least two different ethylene polymers.
By a multimodal ethylene polymer is in connection with the present invention meant an ethylene polymer having broad molecular weight distribution produced by blending two or more ethylene polymer components with different molecular weights or by polymerizing ethylene to different molecular weights in a process with two or more reactors in series. By contrast, a unimodal polyethylene, like those conventionally used in extrusion coating, is obtained from only one ethylene polymer component produced in only one step.
The extrusion coating composition according to the present invention is a multimodal ethylene polymer. The multimodal ethylene polymer is by definition a blend of at least two ethylene polymers having different molecular weights. According to an important embodiment of the present invention , said blend is the product of polymerization process comprising at least two steps. In the process, the first ethylene polymer is prepared by polymerizing ethylene in the presence of a catalyst system in the first step and the second polymer is prepared by (co)polymerizing ethylene and optionally a higher alpha olefin in the presence of a catalyst system in the second step. Said steps can be performed in any order.
The idea of the present invention can be realized with any kind of ethylene polymerization catalyst, such as chromium catalyst, a Ziegler-Natta catalyst or a metallocene catalyst. Typical catalyst systems are e.g prepared according to WO91/12182 and WO95/35323 which are herewith included by reference. A preferential single site polymerization catalyst system is that based on a group 4 metal metallocene and alumoxane.
When performing said polymerization process comprising at least two steps, one or more catalyst systems, which may be the same or different, can be used. It is preferential, if the blend of ethylene polymers is the product of the polymerization process, that the same catalyst system is used in the subsequent steps.
The most convenient way to control the molecular weight during the multistep polymerization is to use hydrogen, which acts as a chain-transfer agent by intervening in the insertion step of the polymerization mechanism. Hydrogen may be added in suitable amounts to any step of the multistep polymerization.
It is prior known to prepare multimodal and especially bimodal olefin polymers in two or more polymerization reactors in serie. Such processes are exemplified by EP 040992, EP041796, EP 022376 and WO92/12182 which are hereby included as reference concerning the preparation of multimodal ethylene polymers for the claimed extrusion coating material. According to these references each of said polymerization step can be performed in liquid phase, slurry or gas phase.
According to the present invention, it is preferential to perform said polymerization steps as a combination of slurry polymrization and gas phase polymerization. Preferentially the first step is a slurry polymerization and the second step a gas phase polymerization.
The slurry polymerization is preferentially performed in a so called loop reactor. The gas phase polymerization is performed in a gas phase reactor. The polymerization steps can optionally be preceded by a prepolymerization, whereby up to 20% by weight and preferentially 1-10% by weight of the total ethylene polymer amount is formed.
According to another important embodyment of the invention, the multimodal ethylene polymer can be produced by mixing at least two ethylene polymers having different average molecular weights.
A suitable melt flow rate MFR
2
of the ethylene polymer blend is between 1 and 30 g/10 min.
The molecular weight distribution curve of a multimodal ethylene polymer shows either several peaks or a broad peak lacking small fractions of extremely low and extremely high molecular weight fractions.
Of course, the finished polymerization or mixing product can be further treated to modify its average molecular weight and molecular weight distribution according to the application to which the material is used.
The polymer composition of this invention is suitable for extrusion coating of fiber based materials like paper and paperboard. The grammage of paper or paperboard substrate is typically 20 g/m
2
-400 g/m
2
.
The substrate used in the extrusion coating can also be a plastic film made of polyester, polyamide, cellophane, polypropylene and oriented polypropylene. When using plastic film substrate, the thickness is typically 10 &mgr;m-80 &mgr;m.
In addition, also aluminium subtrate can be used and then the thickness of substrate is from 6 &mgr;m to 300 &mgr;m.
The polymer composition according to this invention can be used as the only extrusion coating layer over the substrate or it can be used in multilayer products as one layer or several layers.
Depending on the final product, it is, of course, possible to use extrusion coating structures in which the multimodal ethylene polymer blend is used together with certain other polyethylene. That other ethylene polymer can be a high pressure low density ethylne polymer or copolymer or a unimodal linear ethylene polymer. The multimodal polymer and the other polymer are mixed before the extrusion coating.
REFERENCES:
patent: 5338589 (1994-08-01), Bohm et al.
patent: 5371145 (1994-12-01), Daniell et al.
patent: 5494965 (1996-02-01), Harlin et al.
patent: 5631069 (1997-05-01), Wooster et al.
patent: 5674342 (1997-10-01), Obijeski et al.
patent: 6090893 (2000-07-01), Harlin et al.
patent: 9212182A1 (1992-07-01), None
paten
Laiho Erkki
Sainio Markku
Salminen Hannu
Vähälä Martti
Birch & Stewart Kolasch & Birch, LLP
Borealis Technology Oy
Nutter Nathan M.
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