Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Effecting a change in a polymerization process in response...
Reexamination Certificate
1998-12-18
2002-08-27
Teskin, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Effecting a change in a polymerization process in response...
C526S153000, C526S164000, C526S085000, C526S901000, C524S856000
Reexamination Certificate
active
06441107
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the gas phase production of polybutadiene. More particularly, the invention relates to a method for controlling the molecular weight and/or molecular weight distribution of high cis-1,4-polybutadiene as it polymerized from 1,3-butadiene in the presence of a rare earth catalyst in a gas phase process.
BACKGROUND OF THE INVENTION
Commercially, polybutadiene (BR), which is highly amorphous, has long been produced using solution, slurry/bulk suspension, or emulsion processes in which the reaction medium is liquid, not gaseous. There is little or no gas present in these processes such that a gas is never the fluidizing medium or mechanism. Some of these processes are batch or plug flow in which all the catalyst is in contact with monomer for a uniform time and the liquid monomor(s) are consumed without being replenished. Hence, in these processes polymer molecular weight is correlated and plotted as a function of percent conversion of the monomer. Monomer conversions of 70-90% per pass through the reactor system are typically sought in order to achieve the desired molecular weight of the polymer. Others of these processes are operated in a continuous manner, but not as a single continuous stirred tank reactor, rather as a series of such reactors, for example four to nine. Fresh monomer is only added to the first reactor in the series and monomer is allowed to be consumed without replenishment in the subsequent reactors. It is well known in the art that a series of continuous stirred tank reactors operated this way approximates a plug flow reactor in its kinetics and so molecular weight is also correlated with monomer conversion for these multi-reactor processes and high monomer conversions per pass are again sought. Finally, some processes use two different types of reactors in series—an initial continuous stirred tank reactor followed by a plug flow reactor in which high monomer conversions and thus high molecular weight can be achieved. See, for example, U.S. Pat. No. 4,710,553. A common feature in all of these processes is that the monomer concentration around the catalyst is decreasing substantially as the catalyst travels through the continuous process toward the polymer exit or as residence time increases in a batch reactor. Another common feature is that catalyst residence time in the exiting product is uniform for batch and plug flow reactors and somewhat uniform for a series of continuous stirred reactors.
All of these commercial BR processes are energy and labor intensive, multi-step processes and involve liquids of viscosities that increase greatly during the course of the polymerization. A major concern in all these processes is that the mixture remain sufficiently low in viscosity so that it can be processed in the equipment such as mechanically stirred vessels and/or extruders. Because of the generally high viscosity involved in such conventional liquid BR processes, the addition of solids in these processes is generally avoided, as their presence makes processing all the more difficult since they increase the viscosity of the mix. In addition, in these processes, when molecular weight regulators are employed to control the molecular weight of the polymer as it is being produced, chemical compounds that are liquid or solid are generally chosen rather than hydrogen gas. See, for example, AU 595,291 and U.S. Pat. Nos. 5,637,661 and 4,663,405. The final product of these processes is a solid mass or bale. The solid mass or bale is not granular or powdery or even small strips without additional pulverization, grinding or chopping steps that are undertaken by the end-user.
Alpha olefin homopolymers and copolymers (e.g., polyethylene, polypropylene, ethylene-hexene polymers, and the like) have long been produced using gas phase processes. They are highly crystalline as compared to other polymers, such as polybutadiene (BR), ethylene propylene copolymer (EPR) and ethylene propylene diene terpolymer (EPDM), which have only more recently been produced in gas phase processes. Typically, alpha olefin homopolymers and copolymers are produced in gas phase processes at higher temperatures relative to polybutadiene, EPR and EPDM. In the gas phase production of alpha olefin homopolymers and copolymers and of BR, EPR and EPDM, the gaseous monomer(s) and catalyst are fed continuously. This means the monomers are continually replenished. Monomer conversion per pass through a fluidized bed is typically only about 1-4% of the monomer concentration. This low conversion per pass, continual monomer replenishment, and backmixed characteristic of the gas phase process result in monomer concentration around the catalyst particle being quite constant throughout the polymerization. Nevertheless, total monomer efficiency in a gas phase polymerization is typically 95% or higher by means of returning the unreacted gas mixture to the reacting bed. The catalyst is removed in and with the product, so catalyst present in the reactor has a wide distribution of residence times. For single reactor systems, such as those commonly employed in gas phase polymerization processes, such residence times completely cover the range from catalyst that is only present in the reactor for seconds before it exited with product to catalyst that has been present in the reactor for many, many hours before exiting. For alpha olefin homopolymers and copolymers the fluidizing gas medium is gaseous unreacted monomer plus some inert gas, such as nitrogen. Hydrogen is typically employed as a chain transfer agent rather than a liquid or solid chemical compound since these compounds may not distribute as well and can interfere with end-use properties and/or fluidization. Historically, in these polymerizations, liquid in significant quantities was shunned prior to condensing mode operation. In condensing mode, liquid which is immediately vaporized is used to cool the exothermic polymerization, thereby increasing production. Because of the crystalline nature of the forming polymers, the feeding of liquid monomers, which are not vaporized and cannot be adsorbed or absorbed in the crystalline polymer, was thought to cause the collapse of the fluidized bed. Solids, other than that employed in a support for a catalyst, were also avoided since they can adversely affect properties of end-use products (e.g., films). The final polymer is dry, granular and/or powdery upon exiting the reactor.
More recently, it has been discovered that BR, as well as other similar amorphous polymers such as EPR, EPDM, polyisoprene and the like, can be produced in gas phase processes such as those taught in U.S. Pat. Nos. 4,994,534 and 5,453,471; and WO 96/04323. WO 96/04323 teaches the gas phase production of BR using a rare earth catalyst (e.g., neodymium) in which product in the reactor is not “dry” like crystalline alpha olefin polymers. Significant amounts of liquid monomer are typically present in gas phase BR processes. However, after purging to remove monomer the product is in granular, free-flowing form, thereby eliminating end-user grinding, pulverization and chopping. Hence, these gas phase processes for amorphous polymers differ significantly from both conventional liquid BR processes and from gas phase alpha olefin polymerization processes.
In these gas phase processes such as for BR, in order to produce polymers above the softening or sticking temperature, most of these gas phase processes prefer to employ a solid inert particulate material which serves to prevent agglomeration of the bed of forming polymer. Because this fluidization aid reacts with the cocatalyst, a total amount of cocatalyst must be fed that is higher than that employed in crystalline alpha olefin polymerizations, where no fluidization aid is used, in order to have the desired amount of cocatalyst available to react with the catalyst and monomer. This balance in cocatalyst level between that needed to react with the fluidization aid and that needed for proper functioning of the polymerization catalyst is difficult to maint
Apecetche Maria Angelica
Cann Kevin Joseph
Moorhouse John Henry
Muruganandam Natarajan
Teskin Fred
Union Carbide Chemicals & Plastics Technology Corporation
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