Staged reactor process

Details Staged reactor process Staged reactor process

1999-03-31

2001-02-06

Nutter, Nathan M. (Department: 1711)

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

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S243000, C525S245000, C525S249000

Reexamination Certificate

active

06184299

ABSTRACT:

TECHNICAL FIELD
This invention relates to a process for preparing an in situ polyethylene blend in which process improved reactor operability and/or product enhancement is achieved.
BACKGROUND INFORMATION
There has been a rapid growth in the market for linear low density polyethylene (LLDPE), particularly resin made under mild operating conditions; typically at pressures of 100 to 400 psi and reaction temperatures of less than 120° C. This low pressure process provides a broad range of LLDPE products for blown and cast film, injection molding, rotational molding, blow molding, pipe, tubing, and wire and cable applications. LLDPE has essentially a linear backbone with only short chain branches, about 2 to 6 carbon atoms in length. In LLDPE, the length and frequency of branching, and, consequently, the density, is controlled by the type and amount of comonomer used in the polymerization. Although the majority of the LLDPE resins on the market today have a narrow molecular weight distribution, LLDPE resins with a broad molecular weight distribution are available for a number of non-film applications.
LLDPE resins designed for commodity type applications typically incorporate 1-butene as the comonomer. The use of a higher molecular weight alpha-olefin comonomer produces resins with significant strength advantages relative to those of ethylene/1-butene copolymers. The predominant higher alpha-olefin comonomers in commercial use are 1-hexene, 4-methyl-1-pentene, and 1-octene. The bulk of the LLDPE is used in film products where the excellent physical properties and drawdown characteristics of LLDPE film makes this film well suited for a broad spectrum of applications. Fabrication of LLDPE film is generally effected by the blown film and slot casting processes. The resulting film is 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, the properties characteristic of high molecular weight resins can be retained and processability, particularly the extrudability (from 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 higher density ethylene copolymer is prepared in a high melt index reactor and a lower density ethylene copolymer is prepared in a low melt index 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) 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 1000 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 22 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 in situ blends prepared as above and the films produced therefrom are found to have the advantageous characteristics heretofore mentioned, it is found that reactor operability is affected by the choice of the cocatalyst, particularly trimethylaluminum.
DISCLOSURE OF THE INVENTION
An object of this invention, therefore, is to provide a process for preparing an in situ blend in which catalyst productivity is substantially increased over prior art in situ processes. Other objects and advantages will become apparent hereinafter.
According to the present invention such a process has been discovered. The process comprises contacting ethylene and at least one alpha-olefin comonomer having 3 to 8 carbon atoms with a transition metal based catalyst system in two fluidized bed reactors connected in series, in the gas phase, under polymerization conditions, with the provisos that:
(a) ethylene is introduced into each reactor;
(b) 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;
(c) other than the active catalyst referred to in proviso (b), no additional catalyst is introduced into the second reactor;
(d) a hydrocarbyl aluminum cocatalyst is introduced into the first reactor and a different hydrocarbyl aluminum cocatalyst or no cocatalyst is introduced into the second reactor;
(e) in the reactor in which a low melt or flow index copolymer is made:
(1) the alpha-olefin is present in a ratio of about 0.01 to about 0.8 mol of alpha-olefin per mol of ethylene; and
(2) optionally, hydrogen is present in a ratio of about 0.001 to about 0.3 mol of hydrogen per mol of ethylene; and
(f) in the reactor in which a high melt or flow index polymer is made:
(1) optionally, alpha-olefin is present in a ratio of about 0.005 to about 0.6 mol of alpha-olefin per mol of ethylene; and
(2) optionally. hydrogen is present in a ratio of about 0.2 to about 3 mols of hydrogen per mol of ethylene.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
As noted, the blend is produced in two staged reactors connected in series wherein a mixture of resin and active catalyst is transferred from the first reactor to the second reactor in which another polymer is prepared and blends in situ with the copolymer from the first reactor.
The gaseous materials used in the process can be introduced into the reactors via a recycle gas. The recycle gas is defined as a mixture of gases including ethylene and one or more alpha-olefins having 3 to 8 carbon atoms, preferably one or two alpha-olefins, as comonomers (alpha-olefin is required in the first reactor recycle gas and is optional in the second reactor recycle gas), and, optionally, one or more inert gases such as nitrogen (to make up the desired reactor pressure), inert hydrocarbons, and hydrogen. The alpha-olefins can be, for example, propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene. The recycle gas can also be referred to as the inlet gas or cycle gas.
Preferred comonomer combinations are:
first reactor
second reactor
1-hexene
1-hexene
1-butene
1-hexene
1-butene
1-butene
1-hexene
1-butene
The 1-hexene/1-hexene combination is found to give the best film properties. It is noted that an ethylene homopolym

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