Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Polyethylene Film Composition
Technical Field
This invention relates to a blend of polyethylene copolymers
such that a film extruded therefrom has 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 United States
patents 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
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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,
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
broad molecular weight distribution polymers for high quality film
applications is frequently limited by the level of gels obtained. These
gels adversely effect the aesthetic appearance, extrudability, and
physical properties 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 (very good; these
films have a small amount of, or essentially no, gels). For commercial
acceptability, the FAR should be plus 20 or better.
Disclosure of the Invention
An object of this invention, therefore, is to provide an in situ
blend, which can be converted into a film having an exceptionally high
FAR. Other objects and advantages will become apparent hereinafter.
According to the present invention such a blend has been
discovered. The in situ blend of this invention comprises a mixture of
first and second ethylene/alpha-olefin copolymers wherein the alpha-
olefin has 3 to 8 carbon atoms, the first copolymer having a density in
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the range of 0.9035 to 0.908 gram per cubic centimeter and a roll-
milled flow index in the range of about 1.4 to about 2.6 grams per 10
minutes and the second copolymer having a density in the range of
.925 to .945 gram per cubic centimeter and a melt index in the range of
about 200 to about 400 grams per 10 minutes; the weight ratio of the
first copolymer to the second copolymer being in the range of about
45:55 to about 60:40, and the in situ blend having a density in the
range of 0.919 to 0.924 gram per cubic centimeter, a flow index in the
range of 65 to 90 grams per 10 minutes and a melt flow ratio in the
range of about 85 to about 115.
Description of the Preferred Embodiments)
The film is generally formed by extrusion. The extruder is a
conventional one using a die, which will provide the desired gauge. The
gauge of the films of interest here can be in the range of about 0.4 to
about 10 mils, and is preferably in the range of about 0.5 to about 6
mils. An example of an extruder, which can be used in forming the
film, is a single screw type modified with a blown film die and air ring
and continuous take off equipment. A typical single screw type
extruder can be described as one having a hopper at its upstream end
and a die at its downstream end. The hopper feeds into a barrel, which
contains a screw. At the downstream end, between the end of the
screw and the die, is a screen pack and a breaker plate. The screw
portion of the extruder is considered to be divided up into three
sections, the feed section, the compression section, and the metering
section, and multiple heating zones from the rear heating zone to the
front heating zone, the multiple sections and zones running from
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upstream to downstream. If it has more than one barrel, the barrels
are connected in series. The length to diameter ratio of each barrel is
in the range of about 16:1 to about 30:1. The extrusion can take place
at temperatures in the range of about 160 to about 270 degrees C, and
is preferably carried out at temperatures in the range of about 180 to
about 240 degrees C.
The blend is produced by the in situ blending of two polymers.
The flow index of the first copolymer can be in the range of about 1.4 to
about 2.6 grams per 10 minutes, and is preferably in the range of about
1.7 to about 2.4 grams per 10 minutes. The melt index of the second
copolymer can be in the range of about 200 to about 400 grams per 10
minutes, and is preferably in the range of about 250 to about 350
grams per 10 minutes.
The copolymers are copolymers of ethylene and one alpha-olefin
comonomer having 3 to 8 carbon atoms, for example, propylene, 1-
butene, 1-hexene, 4-methyl-1-pentene, or 1-octene. The preferred
comonomers are 1-butene and 1-hexene.
The linear polyethylene blend components can be produced using
various transition metal catalysts. The polyethylene blend of subject
invention is preferably produced in the gas phase by a low pressure
process. The blend can also be produced in the liquid phase in
solutions or slurries by conventional techniques, again at low
pressures. Low pressure processes are typically run at pressures below
1000 psi whereas high pressure processes are typically run at
pressures above 15,000 psi. Typical transition metal catalyst systems,
which can be used to prepare the blend, are magnesium/titanium based
catalyst systems, which can be exemplified by the catalyst system
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described in United States patent 4,302,565; vanadium based catalyst
systems such as those described in United States patents 4,508,842;
5,332,793; 5,342,907; and 5,410,003; a chromium based catalyst system
such as that described in United States patent 4,101,445; and
metallocene catalyst systems such as those described in United States
patents 4,937,299; 5,317,036; and 5,527,752. 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. Preferred catalyst systems for
preparing the components for the blends of this invention are
magnesium/titanium catalyst systems and metallocene catalyst
systems.
The magnesium/titanium based catalyst system will be used to
exemplify the process, e.g., a catalyst system in which the precursor is
formed by spray drying and is used in slurry form. Such a catalyst
precursor, for example, contains titanium, magnesium, an electron
donor, and, optionally, an aluminum halide. The precursor is then
introduced into a hydrocarbon medium such as mineral oil to provide
the slurry form. This catalyst system is described in United States
patent 5,290,745.
The electron donor is an organic Lewis base, liquid at
temperatures in the range of about 0 degrees C to about 200 degrees C,
in which the magnesium and titanium compounds are soluble. The
electron donor can be an alkyl ester of an aliphatic or aromatic
carboxylic acid, an aliphatic ketone, an aliphatic amine, an aliphatic
alcohol, an alkyl or cycloalkyl ether, or mixtures thereof, each electron
donor having 2 to 20 carbon atoms. Among these electron donors, the
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preferred are alkyl and cycloalkyl ethers having 2 to 20 carbon atoms;
dialkyl, diaryl, and alkylaryl ketones having 3 to 20 carbon atoms; and
alkyl, alkoxy, and alkylalkoxy esters of alkyl and aryl carboxylic acids
having 2 to 20 carbon atoms. The most preferred electron donor is
tetrahydrofuran. Other examples of suitable electron donors are
methyl formate, ethyl acetate, butyl acetate, ethyl ether, dioxane, di-n-
propyl ether, dibutyl ether, ethyl formate, methyl acetate, ethyl
anisate, ethylene carbonate, tetrahydropyran, and ethyl propionate.
While an excess of electron donor is used initially to provide the
reaction product of titanium compound and electron donor, the reaction
product finally contains about 1 to about 20 moles of electron donor per
mole of titanium compound and preferably about 1 to about 10 moles of
electron donor per mole of titanium compound.
An activator compound, which is generally used with any of the
titanium based catalyst precursors, can have the formula AlRaXbHc
wherein each X is independently chlorine, bromine, iodine, or OR'; each
R and R' is independently a saturated aliphatic hydrocarbon radical
having 1 to 14 carbon atoms; b is 0 to 1.5; c is 0 or 1; and a+b+c = 3.
Preferred activators include alkylaluminum mono- and dichlorides
wherein each alkyl radical has 1 to 6 carbon atoms and the
trialkylaluminums. Examples are diethylaluminum chloride and tri-n-
hexylaluminum. About 0.10 to about 10 moles, and preferably about
0.15 to about 2.5 moles, of activator are used per mole of electron
donor . The molar ratio of activator to titanium is in the range of about
1:1 to about 10:1, and is preferably in the range of about 2:1 to about
5:1.
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The hydrocarbyl aluminum cocatalyst can be represented by the
formula R3A1 or R2AlX wherein each R is independently alkyl,
cycloalkyl, aryl, or hydrogen; at least one R is hydrocarbyl; and two or
three R radicals can be joined to form a heterocyclic structure. Each R,
which is a hydrocarbyl radical, can have 1 to 20 carbon atoms, and
prefer ably has 1 to 10 carbon atoms. X is a halogen, preferably
chlorine, bromine, or iodine. Examples of hydrocarbyl aluminum
compounds are as follows: triisobutylaluminum, tri-n-hexylaluminum,
di-isobutyl-aluminum hydride, dihexylaluminum hydride, di-isobutyl-
hexylaluminum, isobutyl dihexylaluminum, trimethylaluminum,
triethylaluminum, tripropylaluminum, triisopropylaluminum, tri-n-
butylaluminum, trioctylaluminum, tridecylaluminum,
tridodecylaluminum, tribenzylaluminum, triphenylaluminum,
trinaphthylaluminum, tritolylaluminum, dibutylaluminum chloride,
diethylaluminum chloride, and ethylaluminum sesquichloride. The
preferred cocatalyst is trimethylaluminum. The cocatalyst compounds
can also serve as activators and modifiers.
It is preferred not to use a support. However, in those cases
where it is desired to support the precursor, silica is the preferred
support. Other suitable supports are inorganic oxides such as
aluminum phosphate, alumina, silicalalumina mixtures, silica modified
with an organoaluminum compound such as triethylaluminum, and
silica modified with diethyl zinc. A typical support is a solid,
particulate, porous material essentially inert to the polymerization. It
is used as a dry powder having an average particle size of about 10 to
about 250 microns and preferably about 30 to about 100 microns; a
surface area of at least 200 square meters per gram and preferably at
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least about 250 square meters per gram; and a pore size of at least
about 100 angstroms and preferably at least about 200 angstroms.
Generally, the amount of support used is that which will provide about
0.1 to about 1.0 millimole of titanium per gram of support and
preferably about 0.4 to about 0.9 millimole of titanium per gram of
support. Impregnation of the above mentioned catalyst precursor into
a silica support can be accomplished by mixing the precursor and silica
gel in the electron donor solvent or other solvent followed by solvent
removal under reduced pressure. When a support is not desired, the
catalyst precursor is generally used in liquid form.
Activators can be added to the precursor either before and/or
during polymerization. In one procedure, the precursor is fully
activated before polymerization. In another procedure, the precursor is
partially activated before polymerization, and activation is completed
in the reactor. Where a modifier is used instead of an activator, the
modifiers are usually dissolved in an organic solvent such as
isopentane and, where a support is used, impregnated into the support
following impregnation of the titanium compound or complex, after
which the supported catalyst precursor is dried. Otherwise, the
modifier solution is added by itself directly to the reactor. Modifiers
are similar in chemical structure and function to the activators as are
cocatalysts. For variations, see, for example, United States patent
5,106,926. The cocatalyst is preferably added separately neat or as a
solution in an inert solvent, such as isopentane, to the polymerization
reactor at the same time as the flow of ethylene is initiated.
The polymerization is, preferably, conducted in the gas phase
using a continuous fluidized process.
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A relatively low density copolymer is prepared in the first
reactor (the first copolymer) and a relatively high density copolymer in
the second reactor (the second copolymer). The first copolymer has a
relatively high molecular weight and the second copolymer has a
relatively low molecular weight. The weight ratio of the first copolymer
to the second copolymer can be in the range of about 45:55 to about
60:40.
The low density copolymer:
The flow index can be in the range of about 1.4 to about 2.6
grams per 10 minutes, and is preferably in the range of about 1.7 to
about 2.4 grams per 10 minutes. Note that this is a roll-milled flow
index, which provides a more accurate flow index. This is accomplished
by taking a sample from the reactor in which the low density
copolymer is made, and roll-milling it before the flow index is
measured. The molecular weight of this polymer is, generally, in the
range of about 275,000 to about 230,000 Daltons. The density of the
copolymer can be in the range of 0.9035 to 0.908 gram per cubic
centimeter, and is preferably in the range of 0.9035 to 0.908 gram per
cubic centimeter. The Mw/Mn ratio can be in the range of about 3.5 to
about 8, and is preferably in the range of about 3.5 to about 5.5.
Melt index is determined under ASTM D-1238, Condition E (also
referred to as 190/2.16). It is measured at 190 degrees C and 2.16
kilograms and reported as grams per 10 minutes. Flow index is
determined under ASTM D-1238, Condition F (also referred to as
190/21.6). It is measured at 190 degrees C and 10 times the weight
used in determining the melt index, and reported as grams per 10
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minutes. The roll-milled technique is described above. Melt flow ratio
is the ratio of flow index to melt index.
The high density component:
The melt index can be in the range of about 200 to about 400
grams per 10 minutes, and is preferably in the range of about 250 to
about 350 grams per 10 minutes. The molecular weight of the high
density copolymer is, generally, in the range of about 25,000 to about
20,000 Daltons. The density of the copolymer can be in the range of
0.925 to 0.945 gram per cubic centimeter, and is preferably in the
range of 0.930 to 0.940 gram per cubic centimeter. The Mw/Mn ratio
can be in the range of about 3.5 to about 8, and is preferably in the
range of about 3.5 to about 5.5.
The in situ blend or final product can have a flow index in the
range of about 65 to about 90 grams per 10 minutes. The molecular
weight of the final product is, generally, in the range of about 160,000
to about 200,000. The density of the blend can be in the range of 0.919
to 0.924 gram per cubic centimeter, and is preferably in the range of
0.919 to 0.923 gram per cubic centimeter. The melt flow ratio of the
blend can be in the range of about 85 to about 115, and is preferably in
the range of about 90 to about 110. The blend can have an MwfMn
ratio of about 12 to about 18, and preferably has an Mw/Mn ratio of
about 12 to about 17. Mw is the weight average molecular weight; Mn
is the number average molecular weight; and the Mw/Mn ratio can be
referred to as the polydispersity index, which is a measure of the
breadth of the molecular weight distribution.
The low density component (process conditions):
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The mole ratio of alpha-olefin to ethylene can be in the range of
about 0.12:1 to about 0.18:1, and is preferably in the range of about
0.14:1 to about 0.17:1. The mole ratio of hydrogen (if used) to ethylene
can be in the range of about 0.02:1 to about 0.06:1, and is preferably in
the range of about 0.03:1 to about 0.05:1. The operating temperature
is generally in the range of about 65 degrees C to about 75 degrees C.
The ethylene partial pressure in the low density (high molecular
weight) reactor can be in the range of about 25 to about 50 psi, but it is
found that high FARs are more easily obtained when the ethylene
partial pressure is in a preferred range of about 40 to about 50 psi.
The total pressure is generally in the range of about 250 to 320 psig.
The high density component (process conditions):
The mole ratio of alpha-olefin to ethylene can be in the range
of about 0.2:1 to about 0.4:1, and is preferably in the range of about
0.25:1 to about 0.35:1. The mole ratio of hydrogen to ethylene can be in
the range of about 1.4:1 to about 2.5:1, and is preferably in the range of
about 1.6:1 to about 2.0:1. The operating temperature is generally in
the range of about 80 degrees C to about 90 degrees C. The ethylene
partial pressure can be in the range of about 75 to about 150 psi and is
preferably in the range of about 90 to about 120 psi. The total pressure
is generally in the range of 400 to 450 psig.
A typical fluidized bed reactor is exemplified in United States
patent 4,482,687, and can be described as follows:
The bed is usually made up of the same granular resin that is to
be produced in the reactor. Thus, during the course of the
polymerization, the bed comprises formed polymer particles, growing
polymer particles, and catalyst particles fluidized by polymerization
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and modifying gaseous components introduced at a flow rate or velocity
sufficient to cause the particles to separate and act as a fluid. The
fluidizing gas is made up of the initial feed, make-up feed, and cycle
(recycle) gas, i.e., comonomers and, if desired, modifiers and/or an inert
carrier gas.
The essential parts of the reaction system are the vessel, the
bed, the gas distribution plate, inlet and outlet piping, a compressor,
cycle gas cooler, and a product discharge system. In the vessel, above
the bed, there is a velocity reduction zone, and, in the bed, a reaction
zone. Both are above the gas distribution plate.
Conventional additives, which can be introduced into the blend,
are exemplified by antioxidants, ultraviolet absorbers, antistatic
agents, pigments, dyes, nucleating agents, fillers, slip agents, fire
retardants, plasticizers, processing aids, lubricants, stabilizers, smoke
inhibitors, viscosity control agents, crosslinking agents, catalysts, and
boosters, tackifiers, and anti-blocking agents. Aside from the fillers,
the additives can be present in the blend in amounts of about 0.1 to
about 10 parts by weight of additive for each 100 parts by weight of
polymer blend. Fillers can be added in amounts of up to 200 parts by
weight and more for each 100 parts by weight of the blend.
The advantage of the film prepared from the in situ blend of this
invention is the consistently high FAR.
All molecular weights mentioned in this specification are weight
average molecular weights unless otherwise designated.
Patents mentioned in this specification are incorporated by
reference herein.
The invention is illustrated by the following examples.
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Examples 1 and 2
Example 1 is an embodiment of the invention and example 2 is a
comparative example. Both examples use the same steps and
conditions except as set forth in the Table.
The preferred catalyst system is one where the precursor is
formed by spray drying and is used in slurry form. Such a catalyst
precursor, for example, contains titanium, magnesium, aluminum
halides, and an electron donor. The precursor is then introduced into a
hydrocarbon medium such as mineral oil to provide the slurry form.
See United States patent 5,290,745. The catalyst composition and
method of preparing same used in this example is of the same
composition and preparation method as example 1 of 5,290,745 except
that 0.25 mol of tri-n-hexylaluminum per mol of tetrahydrofuran is
used instead of 0.2 mol.
Polyethylene is produced using the following standard
procedure.
Ethylene is copolymerized with 1-hexene in the first reactor, and
the addition of 1-butene in the second reactor. Trimethylaluminum
(TMA) cocatalyst is added to each reactor during polymerization as a
50 weight percent solution in hexane. The temperature in the first
reactor is 70 degrees C and the temperature in the second reactor is 85
degrees C. Each polymerization is continuously conducted after
equilibrium is reached under the conditions set forth here and in the
Table.
Polymerization is initiated in the first reactor by continuously
feeding the above catalyst precursor and cocatalyst into a fluidized bed
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of polyethylene granules together with ethylene, 1-hexene, and
hydrogen. The resulting copolymer mixed with active catalyst is
withdrawn from the first reactor and transferred to the second reactor
using second reactor gas as a transfer medium. The second reactor
also contains a fluidized bed of polyethylene granules. Ethylene, 1-
butene, and hydrogen are introduced into the second reactor where
they come into contact with the copolymer and catalyst from the first
reactor. Additional cocatalyst is also introduced. The product blend is
continuously removed.
Variable polymerization conditions, resin properties, film
extrusion conditions, and film properties are set forth in the Table and
Notes.
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Table
Example 1 2
Reactor I II I II
pressure 296 431 298 433
(psig)
C2 PP(psi) 44.5 112.5 42.4 118.8
H2/C2 0.043 1.79 0.033 1.78
(mole ratio)
C4/C2 0 0.248 0 0.320
(mole ratio)
C6/C2 0.152 0.018 0.143 0.011
(mole ratio)
N2 (%) 74.1 19.4 76.5 14.5
H2 (%) 0.62 45.09 0.45 47.11
C2H4 (%) 14.33 25.23 13.59 26.53
C4H8 (/) 0 6.26 0 8.48
IC5 (%) 6.81 2.58 7.00 2.02
C6H12 (%) 2.18 0.46 1.94 0.28
TMA flow 9.72 4.83 10.05 3.35
(lbs/hr)
production 36.1 41.3 35.9 47.6
rate (1000
lbs/hr)
catalyst feed17.5 0 16.9 0
(lbs/hr)
C2 feed 30.0 38.5 30.5 43.6
(1000 lbs/hr)
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Example 1 (continued) 2 (continued)
Reactor I II I II
C4 feed 0 4.00 0 5.88
( 1000 lbs/hr)
C6 feed 6141 0 5382 0
(lbs/hr)
H2 feed 0.47 177 0.33 233
(lb s/hr)
N2 feed 504 0 626 0
(lbs/hr) .
IC5 feed 1730 0 1599 0
(lbs/hr)
Bed weight 95 173 94 172
(1000 lbs)
Residence 2.6 2.2 2.6 2.1
Time (hrs)
Split 0.47 0.53 0.43 0.57
(weight
fraction)
Resin
Analysis
Ti (ppm) 3.49 1.90 3.37 1.69
Al/Ti (molar22.4 33.1 28.9 34.6
ratio)
Melt index ----- 0.80 ----- 0.63
(g/10 min)
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Example 1
Resin
Analysis
(continued)
Flow index 1.85 83.7 1.23 81.4
(g/10 min) (roll-milled) (roll-milled)
MFR ----- 104 ----- 130
Density 0.9057 0.9216 0.9067 0.9216
(g/cc)
Film
FAR ----- plus 50 ----- minus 30
Notes to Table:
psig = pounds per square inch gauge.
C2 PP = partial pressure of ethylene reported in psi (pounds per square
inch).
H2/C2, C4/C2, and C6/C2 are mole ratios of hydrogen, 1-butene, and 1-
hexene, respectively, to ethylene.
N2, H2, C2H4, C4H8, ICS, C6H12, are nitrogen, hydrogen, ethylene, 1-
butene, isopentane, and 1-hexene, respectively. °/ is percent by mole .
TMA = trimethylaluminum.
Split = weight fraction of individual component
Ti ppm = parts per million by weight of titanium in the resin.
Al/Ti = molar ratio of aluminum to titanium in the resin.
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Density is measured by producing a plaque in accordance with ASTM
D-1928, procedure C, and then testing as is via ASTM D-1505. It is
reported in gram per cubic centimeter.
FAR = film appearance rating as explained above. The films are
prepared by extrusion in a 3.5 inch GloucesterT"" blown tubular film
extruder having a length to diameter ratio of 24:1; a linear low density
polyethylene screw; a 6 inch die; and a die gap of 60 and 120 mils. FAR
is determined for each film.
Roll Milling Procedure: Granular resin from the reactors is placed on a
two roll mill that is commonly available in the industry. The rolls are
set at a temp of 148 C and initially set at the closet spacing. The resin
is placed on the rolls for about 5 minutes and then roll milled at low
RPM's (< 5) for about 5 minutes with a gap of 0.008 inches. The
milled crepe should be removed and refed approximately 3 times
during the period. The sample is then removed and the flow properties
measured.
