Stock material or miscellaneous articles – Hollow or container type article – Polymer or resin containing
Reexamination Certificate
2001-12-17
2004-08-03
Nolan, Sandra M. (Department: 1772)
Stock material or miscellaneous articles
Hollow or container type article
Polymer or resin containing
C138S137000, C428S036920
Reexamination Certificate
active
06770341
ABSTRACT:
The present invention relates to a polyethylene molding material having a bimodal molecular weight distribution, and to a high-strength pipe produced from this molding material.
Polyethylene is widely used for the production of pipes, for example for gas- and water-transport systems, since a material having particularly high mechanical strength, high corrosion resistance and absolutely reliable long-term stability is required for such pipes. Numerous publications describe materials having an extremely wide variety of properties and processes for their production.
EP-A-603 935 has already described a molding material based on polyethylene which has a bimodal molecular weight distribution and is also said to be suitable, inter alia, for the production of pipes. However, pipes produced from the molding materials in accordance with this reference are unsatisfactory with respect to their long-term resistance to internal pressure, their stress cracking resistance, their low-temperature notched impact strength and their resistance to rapid crack growth.
In order to achieve pipes having balanced mechanical properties and thus an optimum property combination, it is necessary to employ a raw material which has an even broader molecular weight distribution. A raw material of this type is described in U.S. Pat. No. 5,338,589 and is produced using a highly active catalyst which is disclosed in WO 91/18934 and in which the magnesium alkoxide is employed as a gel-form suspension. Surprisingly, it has been found that the use of this material in moldings, in particular in pipes, enables a simultaneous improvement on the one hand in the properties of rigidity and creep tendency, which are usually contradictory in partially crystalline thermo-plastics, and on the other hand stress cracking resistance and toughness.
EP-A-0 739 937 has already disclosed a pipe which has mechanical properties which meet the very highest demands made by consumer associations and have resulted in classification of this pipe in quality class “PE 100” in accordance with ISO/DIS 9080.
The object of the present invention was the development of a polyethylene molding material with which even better strength of the pipes produced therewith can be achieved compared with the known pipe material in strength class PE 100 in accordance with ISO/DIS 9080.
This object is achieved by a molding material according to claim
1
. The invention furthermore also relates to a pipe produced from this molding material having really excellent mechanical properties, and to its use for the construction of gas and water lines.
The polyethylene molding material according to the invention has a density at a temperature of 23° C. in the range ≧0.948 as a natural product, i.e. without addition of dye, and ≧0.959 g/cm
3
as a black-colored product having a carbon black content in the range from 2 to 5% by weight, based on the total weight of the black-colored product, and has a broad bimodal molecular weight distribution, in which the ratio of the weight of the low-molecular-weight fraction to the weight of the relatively high-molecular-weight fraction is in the range from 0.5 to 2.0, preferably from 0.8 to 1.8. The polyethylene may comprise small proportions of up to 5% by weight of further monomer units having from 4 to 10 carbon atoms. Examples of such comonomers are 1-butene, 1-pentene, 1-hexene or 4-methyl-1-pentene.
The bimodality can be described as a measure of the position of the centers of the two individual molecular weight distributions with the aid of the viscosity numbers (VN) in accordance with ISO/R 1191 of the polymers formed in two successive polymerization steps. The VN
1
of the low-molecular-weight polyethylene formed in the first polymerization step is from 40 to 90 cm
3
/g, while VN
total
of the end product is in the range from 300 to 450 cm
3
/g. VN
2
of the relatively high-molecular-weight polyethylene formed in the second polymerization step can be calculated from the following mathematical formula:
VN
2
=
VN
total
-
w
1
·
VN
1
1
-
w
1
where w
1
stands for 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 two steps. The value calculated for VN
2
is normally in the range from 500 to 2000 cm
3
/g.
The molding material according to the invention has long-term properties which even go beyond the requirement for quality class PE 100 in accordance with ISO/DIS 9080 of 10.0 MPa after 50 a as LCL (lower critical confidence limit), determined by the extrapolation method. Surprisingly, the polyethylene molding material according to the invention achieves an extremely high resistance to slow crack growth with the requisite relatively high rigidity and at relatively high yield stress. This high stress cracking resistance is evident from the fact that brittle fracture was not observed in the LTHS (long term hydrostatic strength) test within a time interval of 33,000 h at a temperature for 80° C. on pipes produced from the molding material according to the invention.
In the LTHS test, the service life of the pipes without brittle fracture is determined by extrapolation to 50 years. Owing to the extremely high stress cracking resistance achieved by the molding material according to the invention, the ductile line in the creep diagram, with the aid of which the service life is extrapolated to 50 years, is very flat. Consequently, an internal pressure of 12.5 MPa arises in accordance with this test method for pipes produced from the molding material according to the invention at a test temperature of 23° C. and a service life of 50 years, which accordingly results in a new quality class, PE 125.
The extrapolation curve at 23° C. can be described mathematically by the following equation:
&sgr;=
K·t
failure
−&eegr;
For the standard values K=15.6 and &eegr;=−0.017, the following values arise through extrapolation for pipes produced from the molding material according to the invention:
t
failure
10 h
10,000 h
50 years
&sgr;
15.0
13.3
12.5
The polyethylene is obtained by polymerization of the monomers in suspension, in solution or in the gas phase 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 Ziegler catalyst composed of a transition-metal compound and an organoaluminium compound. The polymerization is carried out in two successive steps, the molecular weight of the polyethylene being regulated in each step with the aid of hydrogen.
The polyethylene molding material according to the invention may, besides the polyethylene, also comprise further additives. Such additives are, for example, heat stabilizers, antioxidants, UV absorbers, light stabilizers, metal deactivators, peroxide-destroying compounds, basic costabilizers in amounts of from 0 to 10% by weight, preferably from 0 to 5% by weight, but also fillers, reinforcing agents, plasticizers, lubricants, emulsifiers, pigments, optical brighteners, flame retardants, antistatics, blowing agents or combinations of these in total amounts of from 0 to 50% by weight.
The pipe according to the invention is produced by firstly plasticating the polyethylene molding material in an extruder at temperatures in the range from 200 to 250° C. and then extruding the molding material through an annular die and cooling it. Pipes of the type according to the invention are generally suitable for all pressure classes in accordance with DIN 8074.
For conversion into pipes, it is possible to employ both conventional single-screw extruders having a smooth feed zone and also high-performance extruders having a finely grooved barrel and forced-conveying feed. The screws are typically designed as decompression screws with a length of from 25 to 30 D (D=Ø). The decompression screws have a metering zone in which temperature differences in the melt are compensated and in which the aim is for the relaxation stresses produce
Berthold Joachim
Böhm Ludwig
Damm Elke
Enderle Johannes-Friedrich
Schulte Ulrich
Basell Polyolefine GmbH
Connolly Bove & Lodge & Hutz LLP
Nolan Sandra M.
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