Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2000-05-31
2003-11-18
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...
C526S105000, C526S106000, C526S348200, C526S348500, C526S348600
Reexamination Certificate
active
06649709
ABSTRACT:
DESCRIPTION
The present invention relates to a continuous vapor-phase fluidized-bed process for the preparation of ethylene homopolymers and copolymers having a density of from 0.87 to 0.97 g/cm
3
in which ethylene or mixtures of ethylene and C
3
-C
8
&agr;-monoolefins are (co)polymerized in the presence of a supported chromium catalyst in the polymerization zone of a vapor-phase fluidized-bed reactor under pressures ranging from 1 to 100 bar and at temperatures ranging from 30° to 125° C. in the vapor phase in an agitated bed of bulk material comprising particulate polymer, the resultant heat of polymerization is removed by cooling the recirculated reactor gas and the resulting (co)polymer is removed from the vapor-phase fluidized-bed reactor.
The present invention also relates to ethylene homopolymers and copolymers produced by this process, to the use of such ethylene copolymers for the preparation of films and to films produced using these ethylene copolymers.
The properties of ethylene homopolymers and copolymers concerning the processibility and mechanical stability thereof are substantially governed by the density thereof, the average molar mass thereof, the molecular mass distribution thereof, the nature of the comonomer and the distribution of comonomer in terms of molar mass. These properties are correlated in a complex manner with the manufacturing conditions of the homopolymers and copolymers and can be influenced both by physical process parameters such as pressure and temperature and by the choice of certain catalysts.
A parameter that is of particular significance for the processibility of ethylene homopolymers and copolymers is the meltflow rate. In addition to the nature and distribution of the comonomer, another factor primarily governing the melt flow rate is the average molar mass of the polymer.
Processes for the preparation of ethylene copolymers in vapor-phase fluidized beds using supported chromium catalysts are revealed in EP-A-1-0175532 and EP-A-1-0475603, for example. In order to avoid coagulation of particles of polymer these polymerization processes are carried out at various temperatures depending on the density and thus on the softening temperature of the polymer but always at temperatures well below the softening temperature.
EP-B-0571826 describes a continuous vapor-phase fluidized-bed process for the preparation of ethylene homopolymers and copolymers which is carried out at temperatures only slightly below the softening temperature of the particles of polymer. The catalyst used is in this case a Ziegler's catalyst containing titanium and magnesium.
The polymers prepared by the known vapor-phase fluidized-bed processes are still unsatisfactory as regards processibility.
It was thus the object of the present invention to provide a process for the preparation of ethylene homopolymers and copolymers using a supported chromium catalyst giving products having improved processing properties.
Accordingly, there has been found a continuous vapor-phase fluidized-bed process for the preparation of ethylene homopolymers and copolymers having a density d of from 0.87 to 0.97 g/cm
3
, in which ethylene or mixtures of ethylene and C
3
-C
8
&agr;-monoolefins are (co)polymerized in the presence of a supported chromium catalyst in the polymerization zone of a vapor-phase fluidized-bed reactor under pressures ranging from 1 to 100 bar and at temperatures ranging from 30° to 125° C. in the vapor phase in an agitated bed of bulk material comprising particulate polymer, the resultant heat of polymerization is removed by cooling the recirculated reactor gas and the resulting (co)polymer is removed from the vapor-phase fluidized-bed reactor, wherein, for the preparation of a (co)polymer of a specific density d, the (co)polymerization is carried out at a temperature in a range which is restricted by an upper envelope define by equation I
T
H
=
171
+
6
d
′
0.84
-
d
′
(
I
)
and a lower envelope defined by equation II
T
L
=
173
+
7.3
d
′
0.837
-
d
′
,
(
II
)
in which the variables have the following meanings:
T
H
is the highest reaction temperature in ° C.;
T
L
is the lowest reaction temperature in ° C.;
d′ is the numerical value of the density d of the (co)polymer to be synthesized.
There have also been found novel ethylene homopolymers and copolymers which have improved properties and can be produced by this process, the use of such ethylene copolymers for the preparation of films, and also films prepared from these ethylene copolymers.
An essential feature of the process of the invention is the combination of a high polymerization temperature with a specific catalyst, namely a supported chromium catalyst. Both factors are known to have an influence on the properties of the polymers to be synthesized. Thus high reactor temperatures have a preferential influence on the chain terminating reaction as against chain growth. The higher the reactor temperature, the lower the average molar mass M
W
and, consequently, the higher the melt flow rate. However, upper limits are set to the reactor temperature by the softening temperature of the polymer that is formed.
On the other hand, the catalyst also has a strong influence on the properties of the polymers to be synthesized. Thus in the case of chromium catalysts there is a marked correlation between the porosity of the support and the average molar mass of the polymer produced. The greater the pore volume of the support, the lower the average molar mass M
W
and consequently the higher the melt flow rate (M-P. McDaniel, J. Polym. Sci., Polym. Chem. E. 21, 1217 (1983)).
Even the temperature at which a chromium catalyst is activated influences the properties of the polymers. Below the sintering temperature of the support material used the following association is true: the higher the activating temperature of the chromium catalyst, the lower the average molar mass M
W
and consequently the higher the melt flow rate of the polymer produced.
It has now been found, surprisingly, that polymers which scarcely differ from conventional polymers as regards the comonomer configuration thereof, the density thereof and the melt flow indices thereof but which have been polymerized at a higher temperature, show different processing properties and in this respect are superior to the conventional polymers, in some cases distinctly so.
For the process of the invention to be efficacious it is important, when preparing a (co)polymer of a specific density d, to carry out (co)polymerization at a temperature T in a range restricted by the upper envelope defined by the above equation I and the lower envelope defined by the above equation II. This means that temperatures T which are outside this range may not be used during the process of the invention, as the process will not otherwise be successful. In other words, equations I and II indicate the highest reaction temperature T
H
and the lowest reaction temperature T
L
at which a (co)polymer having a certain desired density d can just be prepared using the process of the invention.
The process of the invention is carried out in a vapor-phase fluidized-bed reactor, as described in detail in, for example, EP-A 0,004,645, EP-A 0,089,691, EP-A 0,120,503 or EP-A 0,241,947. The vapor-phase fluidized-bed reactor is generally a more or less long tube through which there flows recirculated reactor gas. The recirculated reactor gas is generally fed to the lower end of the vapor-phase fluidized-bed reactor and is withdrawn at the upper end thereof. The recirculated reactor gas is usually a mixture of ethylene, if desired a molecular weight modifier such as hydrogen, and inert gases such as nitrogen and/or saturated hydrocarbons such as methane, butane or hexane. Furthermore, the reactor gas can contain C
3
-C
8
&agr;-monoolefins such as propylene, but-1-ene, pent-1-ene, hex-1-ene, hept-1-ene and oct-1-ene. Preference is given to a process in which ethylene is copolymerized with 1-hexene. The velocity of the reactor gas, measured as void tube velocity, mus
Bauer Peter
Evertz Kaspar
Feindt Hans-Jacob
Hecker Manfred
Karer Rainer
Basell Polyolefine GmbH
Cheung William
Keil & Weinkauf
Wu David W.
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