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
2002-03-21
2004-03-30
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...
C525S193000, C525S240000
Reexamination Certificate
active
06713561
ABSTRACT:
The present invention relates to a polyethylene moulding compound having a multimodal molecular weight distribution and to a method for the production of this moulding compound in the presence of a catalytic system comprising a Ziegler catalyst and co-catalyst via a multistep reaction sequence consisting of successive liquid-phase polymerizations, and to hollow articles produced from the moulding compound by extrusion blow moulding.
Polyethylene is widely used for the production of mouldings and containers since it is a material having a low inherent weight which nevertheless has particularly high mechanical strength, high corrosion resistance to moisture and water in combination with atmospheric oxygen and absolutely reliable long-term resistance and since polyethylene has good chemical resistance and in particular can easily be processed for bottles, canisters and fuel tanks in motor vehicles.
EP-A-603,935 has already described a moulding compound based on polyethylene which has a bimodal molecular weight distribution and which is also suitable, inter alia, for the production of pipes.
A raw material having an even broader molecular weight distribution is described in U.S. Pat. No. 5,338,589 and is prepared using a highly active catalyst disclosed in WO 91/18934 in which magnesium alkoxide is employed in the form of a gelatinous suspension. Surprisingly, it has been found that the use of this material in mouldings, in particular in pipes, facilitates a simultaneous improvement in the properties of stiffness and creep tendency, which are usually contradictory in partially crystalline thermoplastics, on the one hand, and stress cracking resistance and toughness on the other hand.
The known bimodal products are distinguished, in particular, by good processing properties at the same time as an outstanding stress cracking/stiffness ratio. This combination of properties is of particular importance in the production of hollow articles from plastics, such as bottles, canisters and fuel tanks in motor vehicles. In addition to this property combination, however, the production of plastic hollow articles requires the highest possible swelling rate of the plastic melt, since the swelling rate is directly responsible for enabling the optimum setting of wall thickness control, the formation of the weld line and the weldability in industrial production in extrusion blow moulding.
It is known that having high swelling rates can be produced well using so-called Phillips catalysts, i.e. polymerization catalysts based on chromium compounds. However, the plastics produced in this way have an unfavourable stress cracking/stiffness ratio compared with the known plastics having a bimodal molecular weight distribution.
EP-A-0 797 599 discloses a process which even gives a polyethylene having a trimodal molecular weight distribution in successive gas-phase and liquid-phase polymerizations. Although this polyethylene is already very highly suitable for the production of hollow articles in extrusion blow moulding plants, it is, however, still in need of further improvement in its processing behaviour owing to the plastic melt swelling rate, which is still too low.
The object of the present invention was the development of a polyethylene moulding compound by means of which an even better ratio of stiffness to stress cracking resistance compared with all known materials can be achieved and which, in addition, has a high swelling rate of its melt, which, in the production of hollow articles by the extrusion blow moulding process, not only enables optimum wall thickness control, but at the same time also facilitates excellent weld line formation and wall thickness distribution.
This object is achieved by a moulding compound of the generic type mentioned at the outset, whose characterizing features are to be regarded as being that it comprises from 30 to 60% by weight of a low-molecular-weight ethylene homopolymer A, from 65 to 30% by weight of a high-molecular-weight copolymer B comprising ethylene and another olefin having from 4 to 10 carbon atoms, and from 1 to 30% by weight of an ultrahigh-molecular-weight ethylene homopolymer or copolymer C, where all percentages are based on the total weight of the moulding compound.
The invention furthermore also relates to a method for the production of this moulding compound in cascaded suspension polymerization, and to hollow articles made from this moulding compound with very excellent mechanical strength properties.
The polyethylene moulding compound according to the invention has a density in the range ≧0.940 g/cm
3
at a temperature of 23° C. and has a broad trimodal molecular weight distribution. The high-molecular-weight copolymer B comprises small proportions of up to 5% by weight of further olefin monomer units having from 4 to 10 carbon atoms. Examples of comonomers of this type are 1-butene, 1-pentene, 1-hexene, 1-octene or 4-methyl-1-pentene. The ultrahigh-molecular-weight ethylene homopolymer or copolymer C may optionally also comprise an amount of from 0 to 10% by weight of one or more of the above-mentioned comonomers.
The moulding compound according to the invention furthermore has a melt flow index, in accordance with ISO 1133, expressed as MFI
190/5
, in the range from 0.01 to 10 dg/min and a viscosity number VN
tot
, measured in accordance with ISO/R 1191 in decalin at a temperature of 135° C., in the range from 190 to 700 cm
3
/g, preferably from 250 to 500 cm
3
/g.
The trimodality can be described as a measure of the position of the centres of the three individual molecular weight distributions with the aid of the viscosity numbers VN in accordance with ISO/R 1191 of the polymers formed in the successive polymerization steps. The following band widths of the polymers formed in the individual reaction steps should be taken into account here:
The viscosity number VN
1
measured on the polymer after the first polymerization step is identical with the viscosity number VN
A
of the low-molecular-weight polyethylene A and is in accordance with the invention in the range from 40 to 180 cm
3
/g.
VN
B
of the relatively high-molecular-weight polyethylene B formed in the second polymerization step can be calculated from the following mathematical formula:
VN
B
=
VN
2
-
w
1
·
VN
1
1
-
w
1
where w
1
represents the proportion by weight of the low-molecular-weight polyethylene formed in the first step, measured in % by weight, based on the total weight of the polyethylene having a bimodal molecular weight distribution formed in the first two steps, and VN
2
represents the viscosity number measured on the polymer after the second polymerization step. The value calculated for VN
B
is normally in the range from 150 to 800 cm
3
/g.
VN
C
for the ultrahigh-molecular-weight homopolymer or copolymer C formed in the third polymerization step is calculated from the following mathematical formula:
VN
C
=
VN
3
-
w
2
·
VN
2
1
-
w
2
where w
2
represents the proportion by weight of the polyethylene having a bimodal molecular weight distribution formed in the first two steps, measured in % by weight, based on the total weight of the polyethylene having a trimodal molecular weight distribution formed in all three steps, and VN
3
represents the viscosity number which is measured on the polymer after the third polymerization step and is identical with the VN
tot
already mentioned above. The value calculated for VN
C
is in accordance with the invention in the range from 900 to 3000 cm
3
/g.
The polyethylene is obtained by polymerization of the monomers in suspension or at temperatures in the range from 20 to 120° C., a pressure in the range from 2 to 60 bar and in the presence of a highly active Ziegler catalyst composed of a transition-metal compound and an organoaluminium compound. The polymerization is carried out in three steps, i.e. in three successive steps, with the molecular weight in each case being regulated with the aid of metered-in hydrogen.
The polymerization catalyst's long-term activity, which is necessary for the cascade
Berthold Joachim
Böhm Ludwig
Enderle Johannes-Friedrich
Schubbach Reinhard
Basell Polyolefine GmbH
Connolly Bove & Lodge & Hutz LLP
Nutter Nathan M.
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