Process for preparing a simulated in situ polyethylene blend

Details Process for preparing a simulated in situ polyethylene blend Process for preparing a simulated in situ polyethylene blend

1996-12-03

2002-11-26

Eashoo, Mark (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Forming continuous or indefinite length work

With prevention of equipment fouling accumulations or deposits

C525S240000

Reexamination Certificate

active

06485662

ABSTRACT:

TECHNICAL FIELD
This invention relates to a process for preparing a simulated in situ polyethylene blend, which can be converted into film having a small number of or essentially no gels (or fish-eyes).
BACKGROUND INFORMATION
Polyethylenes of various densities have been prepared and converted into film characterized by excellent tensile strength, high ultimate elongation, good impact strength, and excellent puncture resistance. These properties together with toughness are enhanced when the polyethylene is of high molecular weight. However, as the molecular weight of the polyethylene increases, the processability of the resin usually decreases. By providing a blend of polymers of high molecular weight and low molecular weight, the properties characteristic of high molecular weight resins can be retained and processability, particularly extrudability (a characteristic of the lower molecular weight component) can be improved.
The blending of these polymers is successfully achieved in a staged reactor process similar to those described in U.S. Pat. Nos. 5,047,468 and 5,149,738. Briefly, the process is one for the in situ blending of polymers wherein a high molecular weight ethylene copolymer is prepared in one reactor and a low molecular weight ethylene copolymer is prepared in another reactor. The process typically comprises continuously contacting, under polymerization conditions, a mixture of ethylene and one or more alpha-olefins with a catalyst system in two gas phase, fluidized bed reactors connected in series, said catalyst system comprising: (i) a supported magnesium/titanium based catalyst precursor; (ii) one or more aluminum containing activator compounds; and (iii) a hydrocarbyl aluminum cocatalyst, the polymerization conditions being such that an ethylene copolymer having a melt index in the range of about 0.1 to about 1000 grams per 10 minutes is formed in the high melt index (low molecular weight) reactor and an ethylene copolymer having a melt index in the range of about 0.001 to about 1 gram per 10 minutes is formed in the low melt index (high molecular weight) reactor, each copolymer having a density of about 0.860 to about 0.965 gram per cubic centimeter and a melt flow ratio in the range of about 22 to about 70, with the provisos that:
(a) the mixture of ethylene copolymer matrix and active catalyst precursor formed in the first reactor in the series is transferred to the second reactor in the series;
(b) other than the active catalyst precursor referred to in proviso (a), no additional catalyst is introduced into the second reactor.
While the in situ blends prepared as above and the films produced therefrom are found to have the advantageous characteristics heretofore mentioned, the commercial application of these granular bimodal polymers for high clarity film applications is frequently limited by the level of gels obtained. Particle size distribution and flow characteristics studies indicate that the gas phase resins having an average particle size (APS) of about 400 to about 600 microns exhibit significant compositional, molecular, and rheological heterogeneities. When such a granular resin is compounded, for example, with a conventional twin screw mixer in a single pass, and the resulting pellets are fabricated into film, the film exhibits a high level of gels ranging in size from less than about 100 microns to greater than about 500 microns. These gels adversely affect the aesthetic appearance of the product. The gel characteristics of a film product are usually designated by a subjective scale of Film Appearance Rating (FAR) varying from minus 50 (very poor; these films have a large number of large gels) to plus 50/plus 60 (very good; these films have a small amount of, or essentially no, gels). The FAR of the single pass film product mentioned above is generally in the range of about minus 50 to about minus 10/0. For commercial acceptability, the FAR should be plus 20 or better.
In addition to the FAR problem, in situ blends, which meet the film manufacturer's requirements, have to be custom made in two reactors connected in series under defined conditions. It would, of course, simplify matters if the high melt index polymer and the low melt index polymer could be purchased separately since they more readily fall within the off-the-shelf spectrum of available polymers.
This option has been explored, and blends of various polymers have been found to provide desirable bimodal characteristics. The blending has been traditionally carried out by dry blending and/or melt blending. These blends, however, have never been able to overcome the FAR problem.
DISCLOSURE OF THE INVENTION
An object of this invention, therefore, is to provide a process for preparing a simulated in situ blend from two independently prepared gas phase polymers, which blend not only has desirable bimodal characteristics, but can be extruded into a film having a relatively high FAR. Other objects and advantages will become apparent hereinafter.
According to the present invention such a process has been discovered. The process comprises:
(i) providing a first polyethylene, prepared independently, having a melt index in the range of about 5 to about 3000 grams per 10 minutes and a density in the range of about 0.900 to about 0.975 gram per cubic centimeter and a second polyethylene, prepared independently, having a flow index in the range of about 0.01 to about 30 grams per 10 minutes and a density in the range of about 0.860 to about 0.940 gram per cubic centimeter, the weight ratio of the first polyethylene to the second polyethylene being in the range of about 75:25 to about 25:75;
(ii) blending the first polyethylene with the second polyethylene;
(iii) melting the blend; and, prior to extrusion or pelletizing,
(iv) passing the molten blend through one or more active screens, in the case of two or more active screens) positioned in series, each active screen having a micron retention size in the range of about 2 to about 70, at a mass flux of about 5 to about 100 pounds per hour per square inch.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The first and second polyethylenes can be homopolymers of ethylene or copolymers of ethylene and at least one alpha-olefin comonomer having 3 to 8 carbon atoms, preferably one or two alpha-olefin comonomers provided that they have the necessary melt indices, flow indices, and densities. The alpha-olefins can be, for example, propylene. 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene. The preferred comonomer combination is 1-butene and 1-hexene.
It will be understood that the simulated in situ blend can be characterized as a bimodal resin. The properties of bimodal resins are strongly dependent on the proportion of the high molecular weight component, i.e., the low melt index component.
Each of the resins can be produced in the gas phase using a magnesium/titanium based catalyst system, which can be exemplified by the catalyst system described in U.S. Pat. No. 4,302,565. These resins can also be produced in a slurry or solution phase. Further, the catalyst system can be a vanadium based catalyst system such as that described in U.S. Pat. No. 4,508,842; a chromium based catalyst system such as that described in U.S. Pat. No. 4,101,445; a metallocene catalyst system such as that described in U.S. Pat. No. 5,317, 036; or other transition metal catalyst systems. Many of these catalyst systems are often referred to as Ziegler-Natta catalyst systems. Catalyst systems, which use chromium or molybdenum oxides on silica-alumina supports, are also useful. Subject process should be operative with all of the various catalyst systems useful in preparing polyethylene.
A typical magnesium/titanium catalyst system can be described as follows:
The solid particulate precursor can be supported or unsupported. Another catalyst system is one where the precursor is formed by spray drying and used in slurry form. Such a catalyst precursor, for example, contains titanium, magnesium, and aluminum halides, an electron donor, and an inert filler. The precursor is then intr

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