Olefin polymerization process

Details Olefin polymerization process Olefin polymerization process

2001-08-20

2003-09-09

Choi, Ling-Siu (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S348000, C526S123100, C526S130000, C526S133000, C502S103000, C502S117000, C502S202000

Reexamination Certificate

active

06617403

ABSTRACT:

BACKGROUND
This invention relates to the copolymerization of a mono-1-olefin monomer, such as ethylene, with a higher alpha-olefin comonomer.
It is well known that mono-1-olefins, such as ethylene, can be polymerized with catalyst systems employing vanadium, chromium or other metals on supports such as alumina, silica, aluminophosphate, titania, zirconia, magnesia and other refractory metals. Initially, such catalyst systems were used primarily to form homopolymers of ethylene. It soon developed, however, comonomers such as propylene, 1-butene, 1-hexene or other higher, mono-1-olefins were copolymerized with ethylene to provide resins tailored to specific end uses. Often, high density and/or high molecular weight copolymers can be used for blow molding applications and the blow molding process enables rapid processing into a desired molded product. Unfortunately, these copolymers often are plagued by various types of surface roughness as a result of a constant desire to increase processing rates.
This surface roughness has been described loosely in the past as having “melt fracture instabilities” or “worms”. Worms, or melt fracture instabilities, can be defined broadly as irregularities and instabilities, such as anomalous, ridge-like structures, that are formed during melt processing and are clearly observed on the inside of an otherwise smooth, blow molded article. Worms can occur randomly and intermittently on either the interior or exterior surface of the molded article and can detach from the surface, causing unacceptable contamination of the contents or even structural degradation of the molded article. Generally, melt fracture instabilities are observed only on the interior of the molded article because the heat of the die, or mold, can cause smoothing of the exterior surface of the molded article.
Variance of the shear rates (extruder screw RPMs) for each type of copolymer can affect the melt fracture instabilities. At low shear rates, the extrudate usually is smooth and exhibits no melt fracture instabilities. As shear rates are increased, the extrudate can have a matte, or sharkskin-type, finish which is characterized by fine scale irregularities on the extrudate surface. At even higher shear rates, slip-stick, spurt, or cyclic melt fracture can be observed. At the slip-stick point, the pressure in the extruder periodically oscillates between high and low pressure. Worms are formed and can always be seen at the slip-stick point of an extrusion process, herein defined as the critical shear rate. Finally, as screw speed in increased even further, the copolymer can enter a period of continuous slip. Another way to describe critical shear rate is the overall velocity over the cross section of a channel in which molten polymer layers are gliding along each other or along the wall in laminar flow.
Most polymer processing operations occur within a limited window of extrusion (shear), or production, rates. Obviously, one way to avoid melt fracture instabilities is to limit, i.e., decrease, production rates and use very low extrusion rates. Thus, an improved polymer is one which either does not exhibit melt fracture instabilities at higher shear rates, i.e. has a higher critical shear rate. However, while it is possible to increase the critical shear rate by increasing polymer melt index and/or decreasing polymer molecular weight distribution, other polymer properties will be negatively affected. Therefore, it is very desirable to produce a polymer that does not encounter melt fracture instability, i.e., a polymer that has high critical shear rates. Furthermore, increasing polymer production rates into articles of manufacture while minimizing melt fracture instabilities is an efficient use of polymer product and processing equipment.
SUMMARY OF THE INVENTION
Therefore, it is an object of this invention to provide an improved olefin polymerization process.
It is another object of this invention to provide a process to produce copolymers of ethylene and mono-1-olefins that can be processed at increased production rates and have increased critical shear rates.
It is still another object of this invention to provide a process to produce copolymers of ethylene and mono-1-olefins that have a broadened melt processing window.
It is yet another object of this invention to provide a process to produce copolymers of ethylene and mono-1-olefins that have increased critical shear rates without the loss of other polymer physical properties.
It is still another object of this invention to provide a composition comprising copolymers of ethylene and mono-1-olefins having higher critical shear rates that can be processed at high production rates into articles of manufacture.
In accordance with this invention, herein is provided a polymerization process comprising contacting:
a) ethylene monomer;
b) at least one mono-1-olefin comonomer having from about 2 to about 8 carbon atoms per molecule;
c) a catalyst system comprising chromium supported on a silica-titania support, wherein said support comprises less than about 5 weight percent titanium, based on the weight of the support, and wherein said catalyst system has been activated at a temperature within a range of about 900° F. to about 1050° F.; and
d) a trialkyl boron compound,
wherein said contacting occurs in a reaction zone in the absence of hydrogen, at a temperature within a range of about 180° F. to about 215° F.,
and recovering an ethylene/mono-1-olefin copolymer.
In accordance with another embodiment of this invention, a copolymer comprising ethylene and a mono-1-olefin having from about 3 to about 8 carbon atoms per molecule is provided, wherein said copolymer has a density within a range of about 0.935 g/cc to about 0.96 g/cc; a high load melt index (HLMI) within a range of about 0.5 g/10 minutes to about 30 g/10 minutes; and a critical shear rate for the onset of slip-stick melt fracture of greater or equal to about 1000 sec
−1
.
In accordance with this invention, there is provided a polymerization process consisting essentially of contacting:
a) ethylene monomer;
b) at least one mono-1-olefin comonomer having from about 2 to about 8 carbon atoms per molecule;
c) a catalyst system comprising chromium supported on a silica-titania support, wherein said support comprises less than about 5 weight percent titanium, based on the weight of the support, and wherein said catalyst system has been activated at a temperature within a range of about 900° F. to about 1050° F.; and
d) a trialkyl boron compound,
wherein said contacting occurs in a reaction zone in the absence of hydrogen, at a temperature within a range of about 180° F. to about 215° F.,
and recovering an ethylene copolymer.
In accordance with another embodiment of this invention, a copolymer consisting essentially of ethylene and a mono-1-olefin having from about 3 to about 8 carbon atoms per molecule is provided, wherein said copolymer has a density within a range of about 0.935 g/cc to about 0.96 g/cc; a high load melt index (HLMI) within a range of about 0.5 g/10 minutes to about 30 g/10 minutes; and a critical shear rate for the onset of slip-stick melt fracture of greater or equal to about 1000 sec
−1
.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The terms “polymer” and “copolymer” are used interchangeably in this disclosure. Both terms include a polymer product resulting from polymerizing ethylene monomer and a mono-1-olefin, or higher alpha-olefin, comonomer, selected from the group consisting of propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, and/or 4-methyl-1-pentene.
Catalyst Systems
As used in this disclosure, the term “support” refers to a carrier for another catalytic component. However, by no means, is a support necessarily an inert material; it is possible that a support can contribute to catalytic activity and selectivity.
The catalyst system support used in this invention must be a silica-titania support. As used in this disclosure, references to “silica” mean a silica-containing material generally composed of 80 to 100 weight percent silica, the

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