Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1999-04-22
2001-12-18
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...
C526S130000, C526S134000, C526S154000, C526S348500, C526S348600, C502S235000, C502S256000
Reexamination Certificate
active
06331599
ABSTRACT:
The present invention relates to copolymers of ethylene and of at least one alpha-olefin exhibiting an advantageous combination of characteristics which renders them particularly suited to processing by extrusion and extrusion-blow moulding and more particularly to the manufacture of articles exhibiting good resistance to stress cracking. It also relates to a process for producing these copolymers.
It is known that the resistance to cracking of a polyethylene increases by incorporating an alpha-olefin therein. However, the maximum amount of alpha-olefin which can be incorporated in polyethylene is limited because its incorporation results in a decrease in the standard density of polyethylene and consequently a decline in the mechanical properties, such as the stiffness. Furthermore, when, in the presence of a chromium catalyst, a small amount of alpha-olefin is incorporated in polyethylene the incorporation does not always occur homogeneously, in particular with regard to chains of high molecular mass.
U.S. Pat. No. 5,236,998 provides a solution to this problem by preparing an ethylene copolymer comprising three polyethylene fractions, of which only the fraction of high molecular weight comprises the alpha-olefin, by means of a Ziegler-Natta catalyst in two reactors arranged in parallel, the first of which only comprises ethylene, at high temperature, and the second of which comprises an ethylene/alpha-olefin mixture, at lower temperature, and then the polymerization is continued in a third reactor, in which the polymers resulting from the two reactors are combined. Such a process is difficult to carry out industrially and produces a heterogeneous mixture of resins comprising residual chlorine originating from the catalyst.
Furthermore, it is known that copolymers comprising fewer catalytic residues and more particularly comprising very little chlorine can be obtained by means of catalysts based on chromium oxide deposited on a support (generally known as chromium catalysts). It is also known that ethylene polymers obtained with catalysts of this type have a better melt strength than polymers obtained by means of Ziegler-Natta catalysts. However, the use of conventional chromium catalysts in a process for the copolymerization of ethylene and of at least one alpha-olefin in a single stage does not make it possible to incorporate the alpha-olefin homogeneously in the copolymer and more particularly in the chains of higher molecular weight. In addition, the copolymers produced by means of conventional chromium catalysts do not have, for a given melt flow index, both a high standard density and a high alpha-olefin content. Furthermore, the copolymers produced by means of conventional chromium catalysts in single-stage polymerization processes have a relatively narrow distribution of molecular masses and a ratio of the dynamic viscosities &mgr;
0
/&mgr;
2
which are only adjustable within narrow limits.
The present invention is targeted at overcoming the abovementioned disadvantages by providing ethylene copolymers, obtained by means of a catalytic solid based on chromium oxide, which exhibit, for a given melt flow index, a better compromise between the standard density and the alpha-olefin content and which also exhibit a relatively broad distribution of the molecular masses.
The invention consequently relates to a copolymer of ethylene and of at least one alpha-olefin obtained by means of a catalytic solid based on chromium oxide, the said copolymer having a melt flow index HLMI, expressed in g/10 min, an amount of alpha-olefin Q, expressed in g of alpha-olefin per kg of copolymer, a standard density SD, expressed in kg/m
3
, and dynamic viscosities &mgr;
0
and &mgr;
2
measured at 190° C., respectively at rate gradients of 1 and 100 s
−1
, corresponding to the relationships
SD
≧(952.75+5.40×log
HLMI
−0.79
×Q
),
and
&mgr;
0
/&mgr;
2
≧(23.67−6.67×log
HLMI
).
For the purposes of the present invention, the term “copolymers of ethylene and of at least one alpha-olefin” is understood to denote copolymers comprising monomer units derived from ethylene and monomer units derived from one or more alpha-olefins and comprising at least 90%, in particular at least 95%, by weight of monomer units derived from ethylene. The copolymers according to the invention preferably comprise at least 97% by weight of monomer units derived from ethylene. The copolymers composed essentially of monomer units derived from ethylene and of monomer units derived from one or more alpha-olefins are particularly preferred.
The alpha-olefins are generally chosen from those comprising from 3 to 12 carbon atoms, more particularly from those comprising from 3 to 8 carbon atoms. Good results have been obtained with 1-butene and/or 1-hexene. The copolymers of ethylene and of 1-hexene are very particularly preferred.
For the purposes of the present invention, the alpha-olefin content Q is expressed in g of alpha-olefin per kg of copolymer. It is measured by
13
C NMR according to the method described in J. C. Randall, JMS-Rev. Macromol. Chem. Phys., C29(2&3), p. 201-317 (1989), that is to say that the content of units derived from the alpha-olefin is calculated from the measurements of the integrals of the lines characteristic of the alpha-olefin with respect to the integral of the line characteristic of the units derived from ethylene (30 ppm).
The alpha-olefin content in the copolymer according to the invention is generally at least 1 g per kg of polymer, in particular at least 4 g/kg, values of at least 6 g/kg being favourable. The alpha-olefin content is usually at most 100 g/kg, preferably at most 50 g/kg, of polymer. An alpha-olefin content which does not exceed 30 g/kg is particularly preferred.
One of the essential characteristics of the copolymer according to the invention is therefore that it exhibits, for a given melt flow index HLMI and a given alpha-olefin content Q, a higher SD than known ethylene copolymers. In the context of the present invention, the standard density SD is measured according to ASTM Standard D 792. The SD is measured on a sample prepared according to ASTM Standard D 1928, Procedure C. The SD of the copolymer according to the invention is preferably at least equal to (952.75+5.40×log HLMI−0.70×Q). Copolymers exhibiting an SD at least equal to (952.75+5.40×log HLMI−0.63×Q) are particularly preferred.
The copolymers according to the invention usually exhibit an SD of greater than 935 kg/m
3
. Copolymers which have given good results are those in which the SD is at least equal to 940 kg/M
3
, more particularly those in which the SD is at least equal to 945 kg/m
3
. The SD of the copolymers according to the invention generally does not exceed 965 kg/M
3
and it preferably does not exceed 962 kg/m
3
. Copolymers in which the SD does not exceed 959 kg/M
3
are particularly preferred.
The copolymers according to the invention generally exhibit a melt flow index HLMI, measured at 190° C. under a load of 21.6 kg according to ASTM Standard D 1238, Condition F (1986), which does not exceed 100 g/10 min and generally does not exceed 50 g/10 min. The HLMI is generally at least 0.1 g/10 min, in particular at least 0.5 g/10 min.
Another essential characteristic of the copolymers according to the invention is their relatively broad distribution of molecular masses, characterized by a ratio &mgr;
0
/&mgr;
2
, between the dynamic alpha-olefin and process for its production viscosities &mgr;
0
and &mgr;
2
measured at 190° C., respectively at rate gradients of 1 and 100 s
−1
, of greater than or equal to (23.67−6.67×log HLMI). In the context of the present invention, the dynamic viscosity &mgr;
2
is determined by extrusion of the polymer at 190° C. through a die with a length of 15 mm and a diameter of 1 mm at a constant rate corresponding to a rate gradient of 100 s
−1
and by measuring the force transmitted by the piston during its descent. The dynamic viscosity &mgr;
2
is the
Harlan R.
Solvay Pololefins Europe-Belcium
Wu David W.
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