High activity catalyst prepared with alkoxysilanes

Details High activity catalyst prepared with alkoxysilanes High activity catalyst prepared with alkoxysilanes

1997-07-28

2001-09-18

Wu, David W. (Department: 1713)

Catalyst, solid sorbent, or support therefor: product or process

Catalyst or precursor therefor

Plural component system comprising a - group i to iv metal...

C502S104000, C502S118000, C502S120000, C502S125000, C526S124300, C526S124500, C526S124600

Reexamination Certificate

active

06291384

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to catalyst compositions for the production of polyethylene resins. The greatest benefits realized from the catalysts of this invention are attributable to the effect of the catalyst composition on the molecular weight distribution (MWD) of the polyolefin resin produced in a polymerization reaction in the presence of these catalysts.
BACKGROUND OF THE INVENTION
A narrow MWD of linear low density polymers is desirable as the tear strength of films blown from these resins will be much improved. The exact MWD is influenced by catalyst composition whereas the actual molecular weight is more usually altered by altering process conditions in (co) polymerization reactions.
SUMMARY OF THE INVENTION
The invention relates to controlling the MWD of polyolefin resins by controlling catalyst synthesis variables. The interaction of silica calcined at elevated temperatures, sequentially, with a dialkylmagnesium compound, for example, dibutylmagnesium (DBM), an alkoxysilane reagent preferably tetraalkyl orthosilicate, e.g., tetraethyl orthosilicate (TEOS), and TiCl
4
produces catalyst precursors which exhibit high activity in olefin polymerization reactions in the presence of trialkylaluminum cocatalysts.
Unexpectedly, it has been found that the calcination temperature of a silica support for these catalysts can materially affect the ratio, I
21.6
/1
2.16
sometimes denoted simply by the acronym MFR. [I
21.6
and I
2.16
are measured according to ASTM D-1238, conditions F and E, respectively.]
One of the measures of the MWD of a linear low density polyethylene (LLDPE) or a high density polyethylene (HDPE) resin is the melt flow ratio (MFR), which is the ratio of the high-load melt index or flow index (I
21.6
) to the melt index (I
2.16
) of a given resin:
MFR=
I
21.6
/I
2.16
The MFR is an indication of the MWD of the polymer: the higher the MFR value, the broader the MWD. Resins having relatively low MFR values, e.g., of about 20 to about 30, have relatively narrow MWDs. LLDPE resins having such relatively low MFR values produce warpage-free injection-molded articles and film with better strength properties, for example improved dart drop strength, compared to those of resins with high MFR values.
A decrease in the MFR value coincides with tear strength improvement in the machine direction of linear low density polymers blown into film. At constant tetraalkyl orthosilicate levels in the catalyst precursor, there is a large decrease in resin MFR units, of 3 to 4 units, with an increase in calcination temperature of the support between 600° and 700° C. All other factors remaining constant, increasing the tetraalkyl orthosilicate levels, e.g., tetraethyl orthosilicate, in the catalyst presursor will also produce resins with lower MFR values.
DETAILED DESCRIPTION OF THE INVENTION
The invention is directed to forming certain catalysts for ethylene-olefin copolymerization reactions. In accordance with preferred embodiments of the invention, the proportions of the components of the catalyst precursor of the invention satisfy the equation wherein K value is a ratio of (transition metal)/{(Mg)+4(Si)},
wherein (transition metal) is the concentration of transition metal in units of mmol/gram of silica;
wherein (Mg) is the concentration of Mg provided by the dialkylmagnesium compound, in units of mmol/gram of silica; and
wherein (Si) is the concentration of Si provided by the tetraalkyl orthosilicate in units of mmol/gram of silica, preferably equation K=[Ti]/([Mg]+4[Si]) which is less than 0.5 usually less than 0.4. The “[Ti]”, “[Mg]” and “[Si]” in the formula refer to the concentrations of Ti (provided by the transition metal compound, e.g. TiCl
4
); the concentration of Mg provided by the organomagnesium compound and the concentration of Si provided by the silane compound. The concentration of each is calculated in units of mmol/gram of silica support; outside of this K range, the toughness of the resins produced in polymerization reactions catalyzed by the catalysts of the invention and the strength of the films fabricated therefrom decline.
Suitable carrier materials for the catalyst precursors include solid, porous materials such as silica, alumina and combinations thereof. Such carrier materials may be amorphous or crystalline. These carriers may be in the form of particles having a particle size of from 0.1 micron to 250 microns, preferably from 10 to 200 microns, and most preferably from 10 to 80 microns. Preferably, the carrier is shaped in the form of spherical particles, e.g., spray-dried silica. The carrier material should be porous. The internal porosity of these carriers may be larger than 0.2 cm
3
/g. The specific surface area of these materials is at least 3 m
2
/g, preferably at least 50 m
2
/g, and more preferably from, 150 to 1500 m
2
/g.
It is desirable to remove physically bound water from the carrier material prior to contacting it with the catalyst ingredients. This water removal may be accomplished by heating the carrier material.
If the chosen carrier is porous silica, it may contain silanol groups. Silanol groups in silica may be present in an amount from about 0.5 to about 5 mmol of OH groups per gram of carrier; but the amount will vary inversely with heating (or dehydration) temperatures. That is, a relatively small number of OH (silanol) groups may be removed by heating the carrier from about 1500 to about 250° C., whereas a relatively large number of OH groups may be removed by heating at 500° to 800° C. The duration of the heating may be from 16 to at least 4 hours.
In a most preferred embodiment, the carrier is silica which, prior to the use thereof in the first catalyst precursor synthesis step, has been dehydrated by fluidizing it with nitrogen or air and heating to at least 600° C. for 4-16 hours to achieve a surface hydroxyl group concentration of about 0.7 mmol per gram. In preferred embodiments, herein the temperature of calcination is greater than 600° and up to 870° C. The surface hydroxyl concentration of silica may be determined according to J. B. Peri and A. L. Hensley, Jr.,
J. Phys. Chem.,
72 (8), 2926 (1968). Internal porosity of carriers can be determined by a method termed the BET-technique, described by S. Brunauer, P. Emmett and E. Teller in
Journal of the American Chemical Society,
60, pp. 209-319 (1938). Specific surface areas of carriers can be measured in accordance with the above-mentioned BET-technique with use of the standardized method as described in
British Standards
BS 4359, Volume 1, (1969). The silica of the most preferred embodiment is a high surface area, amorphous silica with the surface area=300 m
2
/g and pore volume of 1.65 cm
3
/g. It is a material marketed under the tradenames of Davison 952 by the Davison Chemical Division of W. R. Grace and Company, or Crosfield ES 70 by Crosfield Limited.
In fact, it has been discovered that heating the silica support at temperatures of greater than 600° C. narrows the MWD of the copolymers produced with the catalysts of the invention.
The carrier material is slurried in a non-polar solvent. Preferably, all subsequent steps for catalyst precursor preparation are conducted at temperatures of about 25° to about 80° C., preferably to about 40° to about 65° C. to ensure maximum catalyst activity.
Suitable non-polar solvents are materials which are liquid at reaction temperatures and in which all of the reactants used herein, e.g., the organomagnesium compound, the silane compound, and the transition metal compound, are at least partially soluble. Preferred non-polar solvents are alkanes, such as isopentane, n-hexane, isohexane, n-heptane, octane, nonane, and decane, although a variety of other materials including cycloalkanes, such as cyclohexane, aromatics, such as toluene and ethylbenzene, may also be employed. The most preferred non-polar solvents are isopentane, isohexane and heptane. As indicated above, the solvent should be free of e

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