Stock material or miscellaneous articles – Hollow or container type article – Polymer or resin containing
Reexamination Certificate
2000-07-24
2003-11-11
Wilson, D. R. (Department: 1713)
Stock material or miscellaneous articles
Hollow or container type article
Polymer or resin containing
C428S035900, C138S145000, C138S146000, C138S143000
Reexamination Certificate
active
06645588
ABSTRACT:
The invention relates to a coating composition and its use for coating a solid substrate. Coatings are applied to surfaces of all types to provide protection and decoration. Protection may be required against corrosion, oxidative aging, weathering, or against mechanical damage.
When coating without solvent, the coating material should have good processing properties, i.e. the material should be easily melt coatable within a wide temperature interval, low shrinking, high mechanical strength, high surface finish and high environmental stress cracking resistance (ESCR). Since all these requirements have been difficult to fulfill, prior known coating materials have meant compromises whereby good properties in one sense have been achieved at the expense of good properties in another sense.
It would mean considerable advantages if the above mentioned compromise regarding the properties of a coating composition could be avoided. It would be particularly desirable to improve the coatability such as the melt flow properties when coating and the shrinkability of the coating material as well as the environmental stress cracking resistance of a product made from the coating material.
The purpose of the invention is to provide a coating material having good melt coating processability, low shrinking, high service temperature range and good environmental stress cracking resistance. The invention also strives for efficient coating speed expressed as high winding speed of the extruded material.
These purposes of the invention have now been achieved by means of a coating composition, which is substantially characterized in that it comprises a multimodal ethylene polymer, which contains from 80 to 100% by weight of ethylene repeating units and from 0 to 200% by weight of C
3
-C
10
&agr;-olefin repeating units, has a density of between 0.915 g/cm
3
and 0.955 g/cm
3
, and is a blend of at least a fast ethylene polymer having a first average molecular weight and a first molecular weight distribution and a second ethylene polymer having a second molecular weight, which is higher than said first molecular weight, and a second molecular weight distribution, said blend having a third molecular weight and a third molecular weight distribution.
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 ethylene polymer is obtained from only one ethylene polymer component produced in only one step.
The average molecular weights and the molecular weight distributions can be measured and expressed by any conventional method applied to ethylene polymer products. In this connection, it is convenient that the average molecular weights are measured and expressed as melt flow rates MFR
i
m
, where i refers to said first, second and third average molecular weights and m refers to the load of the piston used for measuring the MFRs, which load in the following examples generally is 5.0 kg (m=5, see ISO 1133). The molecular weight distributions are conveniently expressed as flow rate ratios, FRR
i
m
1/
m
2, i.e. the ratios between high load MFR
i
s and low load MFR
i
s, where i refers to said first, second and third molecular weight distributions, and m
1
and m
2
refer to the high load, generally 21.6 kg (m=21), and low load, generally 5.0 kg (m=5) or 2.16 kg (m=2), respectively.
By Melt Flow Rate (MFR) is meant the weight of a polymer pressed through a standard cylindrical die at a standard temperature in a laboratory rheometer carrying a standard piston and load. Thus MFR is a measure of the melt viscosity of a polymer and hence also of its average molecular weight. The smaller the MFR, the larger is the average molecular weight. It is frequently used for characterizing a polyolefin, especially polyethylene, when the standard conditions MFR
m
are: temperature 190° C.; die dimensions 9.00 cm in length and 2.095 cm in diameter; load of the piston, 2.16 kg (m=2), 5.0 kg (m=5), 10.0 kg (m=10), 21.6 kg (m=21). See Alger, M. S. M., Polymer Science Dictionary, Elsevier 1990, p. 257.
By Flow Rau Ratio (FRR
m
1/
m
2) is meant the ratio between the melt flow rate (MFR
m
1) measured at a standard temperature and with standard die dimensions using a heavy load (
m
1) and the melt flow rate (MFR
m
2) measured at the same temperature with the same die dimensions using a light load (
m
2). Usually, for ethylene polymers, the heavy load m
1
is 21.6 kg (m
1
=21) and the light load m
2
is 5.0 kg (m=5) or 2.16 kg (m
2
=2) (ISO 1133). The larger the value of the FRR
m
1/
m
2, the broader is the molecular weight distribution.
The present invention is based on the finding that multimodal ethylene polymer has excellent coating application properties such as good processability and low shrinkage as well as superior environmental stress cracking resistance.
The 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 a polymerization process comprising at least two steps. In the process, said first ethylene polymer is prepared by polymerizing ethylene in the presence of a catalyst system in a first step and said second polymer is prepared by polymerizing ethylene in the presence of a catalyst system in a second step. Said steps can be performed in any order, whereby the ethylene polymer of each step is present in the following step or steps. However, it is preferential that said blend is the product of said polymerization process, wherein said first step is performed before said second step. This means, that first, an ethylene polymer having a lower average molecular weight is produced and then, in the presence of the lower molecular weight ethylene polymer, an ethylene polymer having a higher average molecular weight is produced.
The idea of the present invention can be realized with any kind of ethylene polymerization catalyst, such as a chromium catalyst, a Ziegler-Natta catalyst or a group 4 transition metallocene catalyst. According to one embodiment of the present invention, said blend forming the multimodal ethylene polymer is the product of a polymerization process in which said first step and/or said second step is performed in the presence of a catalyst system comprising a procatalyst based on a tetravalent titanium compound, such as a TiCl
4
/MgCl
2
/optional inert carrier/optional internal electron donor procatalyst, and a cocatalyst based on an organoaluminum compound, preferentially a R
3
Al/optional external electron donor procatalyst wherein R is a C
1
-C
10
alkyl. Typical catalyst systems art: 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 (IUPAC 1990) 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 said blend is the product of said polymerization process, in which said catalyst system is added to said first step and the same catalyst system is used at least in said second step.
The most convenient way to regulate the molecular weight during the multistep polymerization of the present invention 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. However, it is preferential that in said first step a hydrogen amount is used, leading to
Aarila Jari
Asumalahti Markku
Hagstrom Bengt
Leiden Leif
Martinsson Hans-Bertil
Birch & Stewart Kolasch & Birch, LLP
Borealis Technology Oy
Wilson D. R.
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