Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1999-04-02
2002-01-01
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S065000, C526S066000, C526S114000, C526S124200, C526S125100, C526S113000, C526S348000
Reexamination Certificate
active
06335411
ABSTRACT:
The present invention relates to ethylene polymers having an advantageous combination of characteristics which makes them particularly suitable for use by extrusion and by extrusion blow-moulding in order to manufacture articles (for example tubes) which have excellent mechanical properties and, in particular, high resistance to crazing under stress. The invention also relates to various processes for obtaining these ethylene polymers.
It is generally known that resins having a high elongational viscosity (which is reflected by a high blow-up ratio) lend themselves particularly well to use by extrusion and by extrusion blow-moulding. For example, Belgium patent BE 84,0378 (Solvay & Co.) describes polyethylenes obtained by polymerization in a single reactor in the presence of a catalytic solid which is prepared by reacting an oxygen-containing organomagnesium compound with an oxygen-containing organotitanium compound and an oxygen-containing organozirconium compound, and by subsequently treating the reaction product thus obtained with an aluminium halide. The known polyethylenes have a high blow-up ratio. However, their mechanical properties are such that the resistance to crazing under stress of tubes extruded from these polyethylenes is low.
Moreover, polyethylenes of improved mechanical properties, and in particular of high resistance to crazing under stress, are known. For example, patent application EP 603935 (Solvay) discloses ethylene polymers obtained by polymerization in at least two reactors in series, in the presence of a titanium catalyst. The ethylene polymers thus obtained have good mechanical properties (high resistance to crazing under stress). However, the ethylene polymers have a low blow-up ratio.
The present invention is directed towards overcoming the abovementioned drawbacks by providing a novel ethylene polymer having both a high blow-up ratio and a high resistance to crazing under stress, which is particularly suitable for use by extrusion and by extrusion blow-moulding.
Consequently, the invention relates to an ethylene polymer having a blow-up ratio (RB) of at least 1.4, a resistance to crazing under stress (ESCR) of at least 55 h and a melt index (MI
5
) of at least 0.2 g/10 min.
One of the essential characteristics of the ethylene polymer according to the invention thus lies in the combination of a high blow-up ratio with a high resistance to crazing under stress.
The blow-up ratio of the ethylene polymer according to the invention is measured by extruding, at 190° C. and at a rate gradient of 100 s
−1
, the ethylene polymer through a die 30 mm in length and 2 mm in diameter at a constant rate of extrusion, and by measuring how far it is necessary to displace the piston in order to extrude a rod 70 mm in length. The blow-up ratio is defined by the relationship R
B
=0.5707 e, in which e represents the displacement of the piston expressed in mm. The cylinder and the piston of the rheometer used for this measurement satisfy the criteria of that used to measure the melt index according to ASTM standard D1238 (1986).
The resistance to crazing under stress of the ethylene polymer is measured according to the following procedure. Ten plates 125 mm×12.7 mm×3.2 mm in size are pressed from a sheet of ethylene polymer. Two notches are made therein, the first 60 mm from one end of the plate and the second 15 mm from the other end of the plate. The notched plates are subjected to a constant flexural force of 7.36 N, corresponding to a stress less than the stress at the plastic flow threshold, and are simultaneously immersed in a surfactant solution comprising 3 ml of nonylphenoxy-poly(ethyleneoxy)ethanol per liter of water at a temperature of 60° C. The time after which the test samples break is noted and the average time corresponding to the breaking of 50% of the test samples is calculated.
For the purposes of the present invention, the expression “ethylene polymers” is understood to denote ethylene homopolymers as well as copolymers of ethylene with at least one co-monomer. Ethylene copolymers are the most advantageous. Co-monomers which may be mentioned are alpha-olefins containing from 3 to 8 carbon atoms. Butene, hexene and mixtures thereof are preferred. The co-monomer content in the ethylene copolymer is generally at least 0.1% by weight, in particular at least 0.5% by weight, values of at least 1% by weight being favourable. The co-monomer content is usually not more than 10% by weight, more precisely not more than 8% by weight, values of not more than 5% by weight being the most common.
The ethylene polymers according to the invention usually have a melt index, measured at 190° C. and at a 5 kg load according to ASTM standard D 1238—Condition P (1986) (referred to hereinbelow as MI
5
) of at least 0.3 g/10 min, in particular of at least 0.6 g/10 min. The MI
5
values generally do not exceed 10 g/10 min, usually do not exceed 5 g/10 min and more especially do not exceed 2 g/10 min.
Ethylene polymers according to the invention which are preferred are, in addition, characterized by a dynamic viscosity &eegr;, expressed in dPa s and measured at a rate gradient of 100 s
−1
at 190° C., such that the ratio
log
(
177470
/
MI
5
)
-
log
η
2
-
log
(
2.53
×
MI
5
)
is at least 0.55. Preferably, this ratio is at least 0.59, values of at least 0.61 being particularly advantageous. In most cases, this ratio is not more than 0.73 and usually not more than 0.70.
The ethylene polymers according to the invention usually have a standard density, measured according to ISO standard 1183 (1987), of at least 945 kg/m
3
, in particular of at least 950 kg/m
3
, values of at least 952 kg/m
3
being preferred. The standard density generally does not exceed 965 kg
3
, and more precisely does not exceed 960 kg/m
3
, values of not more than 958 kg/m
3
being the most preferred.
The invention also relates to various processes for the preparation of the ethylene polymer described above.
In a first process for the preparation of ethylene polymer according to the invention, a single catalytic solid containing titanium and zirconium as active elements is used in a polymerization in two reactors arranged in series.
The first preparation process consists more particularly in polymerizing the ethylene optionally with one or more co-monomers in two reactors in series in the presence of a catalytic solid comprising titanium and zirconium in a Zr/Ti molar ratio of at least 2, and in the presence of a co-catalyst, the first reactor being fed with ethylene, optionally with co-monomer and/or with hydrogen, with catalytic solid and with co-catalyst, the reaction medium of the first reactor being transferred into the second reactor, and the second reactor also being fed with ethylene and optionally with co-monomer. Preferably, hydrogen is introduced into at least one of the two reactors.
The catalytic solid used in the first process according to the invention advantageously comprises from 0.5 to 10% by weight of titanium (preferably from 1 to 6% by weight), from 5 to 40% by weight of zirconium (preferably from 10 to 25% by weight), the Zr/Ti molar ratio being at least 2, from 20 to 80% by weight of halogen (preferably from 40 to 60% by weight), from 1 to 30% by weight of magnesium (preferably from 5 to 15% by weight), and from 0.5 to 10% by weight of aluminium (preferably from 1 to 3% by weight). The remainder consists of residual organic groups derived from the reactants used, in particular alkoxy and alkyl groups. The halogen is preferably chlorine.
The Zr/Ti ratio in the catalytic solid is preferably at least 2.5, values of at least 3 being particularly preferred. The Zr/Ti ratio usually does not exceed 10 and more precisely does not exceed 8, values of not more 6 being preferred.
In a second process for the preparation of ethylene polymer according to the invention, a mixture of two catalytic solids is used, the first containing a single active element, namely titanium, and the second containing two a
Bian Jiang
Koch Benoît
Promel Michel
Choi Ling-Siu
Schneller Marina V.
Solvay Polyolefins - Europe Belgium
Venable
Wu David W.
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