Plastic and nonmetallic article shaping or treating: processes – Utilizing special inert gaseous atmosphere or flushing mold...
Reexamination Certificate
1999-05-03
2002-09-24
Eashoo, Mark (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Utilizing special inert gaseous atmosphere or flushing mold...
C264S141000, C264S211240
Reexamination Certificate
active
06454976
ABSTRACT:
TECHNICAL FIELD
This invention relates to a process for the extrusion of polyethylene into pellets useful in the production of blown film.
BACKGROUND INFORMATION
Ziegler-type catalysts have undergone development over the years to improve the economy and quality of various polyethylene products. This development has tended towards narrowing the molecular weight distribution of the resin; however, narrow molecular weight distributions are not desirable for blown film resins, i.e., the narrow molecular weight distribution does not improve the processability of the resins into blown film in terms of bubble stability. Rather, bubble stability is a characteristic of broad molecular weight distribution resins.
A particularly good technique for producing broad molecular weight distribution polyethylenes is through the use of a two stage polymerization process similar to those mentioned in U.S. Pat. Nos. 5,047,468 and 5,149,738. Briefly, this process is one for the in situ blending of polymers wherein an ethylene copolymer is prepared in a high melt index reactor and an ethylene copolymer is prepared in a low melt index reactor, the two reactors being connected in series. 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) an aluminum containing activator compound; 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 3000 grams per 10 minutes is formed in the high melt index 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 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 14 to about 70, with the provisos that:
(a) the mixture of ethylene copolymer matrix and active catalyst formed in the first reactor in the series is transferred to the second reactor in the series;
(b) other than the active catalyst referred to in proviso (a) and the cocatalyst referred to in proviso (e), no additional catalyst is introduced into the second reactor;
(c) in the high melt index reactor:
(1) the alpha-olefin is present in a ratio of about 0.02 to about 3.5 moles of alpha-olefin per mole of ethylene; and
(2) hydrogen is present in a ratio of about 0.05 to about 3 moles of hydrogen per mole of combined ethylene and alpha-olefin;
(d) in the low melt index reactor:
(1) the alpha-olefin is present in a ratio of about 0.02 to about 3.5 moles of alpha-olefin per mole of ethylene; and
(2) hydrogen is, optionally, present in a ratio of about 0.0001 to about 0.5 mole of hydrogen per mole of combined ethylene and alpha-olefin; and
(e) additional hydrocarbyl aluminum cocatalyst is introduced into the second reactor in an amount sufficient to restore the level of activity of the catalyst transferred from the first reactor to about the initial level of activity in the first reactor.
While the resins produced by these two stage processes can be turned into products having superior mechanical strength and other advantageous physical and chemical characteristics, the resins generally do not achieve a high level of bubble stability.
It is well known that one way to achieve this characteristic is to tailor the broad molecular weight distribution resin after it is produced. Tailoring is simply controlled light crosslinking, which can be used in the fabrication of fibers, films, molded products, and the like to provide products having excellent physical and chemical properties for particular applications. It is often achieved by the use of additives, which are homogenized, alloyed and/or combined with the resin through various extrusion/mixing/pelletizing systems. In many cases, the additive is a critical factor in the commercial success of the final product.
Useful tailoring additives are free radical generators such as organic peroxides and oxygen. Unfortunately, excessive amounts of free radical generators can cause chain scission, which is characterized by a rupture of chemical bonds in the backbone and side chains of the polymer. The result is a decrease in the solid state strength of the resin product. Thus, the amount of free radical generator must be carefully controlled. In addition, organic peroxides increase operating costs because of the cost of the peroxides; additional equipment needed to safely handle the peroxides; and the presence of undesirable by-products, particularly when the tailored polymer will be used in FDA applications. In the case of oxygen tailoring, when melt temperatures become to high, the solid state strength of the blown film becomes unacceptably low. Also, high amounts of oxygen, e.g., 21 percent based on the volume of the gases used in extrusion systems, are often required due to the design of the equipment. But too much oxygen in the presence of unmelted polymer in the form of granules or powders can lead to dust explosions.
DISCLOSURE OF THE INVENTION
An object of this invention, therefore, is to provide a process for the tailoring of broad molecular weight distribution polyethylene whereby processability is enhanced in terms of bubble stability and high solid state strength is achieved in blown film while avoiding the drawbacks of the organic peroxides, and the high melt temperature and dust explosion problems of oxygen. Other objects and advantages will become apparent hereinafter.
According to the present invention such a process has been discovered. The process is one for the extrusion of polyethylene having a broad molecular weight distribution wherein the polyethylene is prepared in pellet form, said extrusion taking place in a pelletizing extruder having one or more zones essentially filled with polyethylene and two or more zones partially filled with polyethylene including a feed zone comprising (i) introducing the polyethylene into the extruder at a temperature sufficient to melt the polyethylene; (ii) introducing a mixture of an inert gas and oxygen into at least one of the partially filled zones other than the feed zone, said mixture containing about 1 to about 21 percent by volume oxygen based on the volume of the gaseous mixture; (iii) passing the molten polyethylene through each zone at melt temperature; and (iv) extruding the polyethylene into pellets and cooling same.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
As noted, the polyethylene is preferably produced in two staged reactors connected in series wherein a mixture of resin and catalyst precursor is transferred from the first reactor to the second reactor in which another copolymer is prepared and blends in situ with the copolymer from the first reactor. The polyethylene can also be produced in one reactor as described, for example, in U.S. Pat. No. 4,302,565, or in three or more reactors provided that a broad molecular weight distribution polyethylene is made by the process.
The resin can be extruded into pellets in a conventional extruder adapted for that purpose. Extruders and processes for extrusion are described in U.S. Pat. Nos. 4,814,135; 4,857,600; 5,076,988; and 5,153,382. Examples of various extruders, which can be used in forming the pellets are single screw and multiscrew types. A typical pelletizing extruder can be illustrated by a two stage twin screw melter/mixer with a feed section and a vent section, a gear pump, a pelletizing device, and various other sections. Another typical pelletizing extruder can be illustrated by a two stage single screw extruder. Thus, the term “extruder”, as used in this specification, is considered to include conventional extruders and mixers, both of which are adapted to form pellets. A typical single screw type extruder can be described as one having a hopper at its upstream end and a die at its downstream
Eashoo Mark
Union Carbide Chemicals & Plastics Technology Corporation
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