Note: Descriptions are shown in the official language in which they were submitted.
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METHOD TO CONTROL MELT ELASTICITY IN LDPE
FIELD OF THE INVENTION
The present invention relates to the free radical initiated higher pressure
polymerization of ethylene. More specifically, the present invention relates
to a
process for controlling the melt elasticity of low density polyethylene
produced in
tubular high pressure reactors by altering the relative levels of mono-
functional versus
di-functional initiators added at each injection point without significantly
altering the
peak temperature for each individual injection point.
BACKGROUND OF THE INVENTION
Free-radical initiated high pressure polymerization of ethylene was the first
process discovered for producing polyethylene, and resulted in a low density
polyethylene (LDPE resin) useful for a variety of applications, including
film, coating,
molding, and cable and wire insulation. Depending on the type of reactor used
and
the reaction conditions, LDPE resins produced in a free-radical initiated high
pressure
process can vary widely with respect to polymer properties, particularly those
properties that impact processability. Autoclave reactors provide a more
dispersed
residence time distribution, providing a wide molecular weight distribution
indicative of
the presence of long chain branching. Conversely, residence times in tubular
reactors
are more uniform and generally result in a narrower molecular weight
distribution.
Tubular reactors provide an advantage over autoclave reactors, however, in
that they
provide higher conversion levels, are more amenable to commercial scale up,
and are
less capital and energy intensive.
The degree of long chain branching impacts the melt elasticity of a polymer,
which can be described as the tendency of a molten polymer to flex or distort.
A
higher melt elasticity corresponds to a molten polymer that better resists
deformation
when stretched, which is why LDPE resins with increased levels of long chain
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branching, and therefore increased melt elasticity, are useful in extrusion
coating
applications. The resistance to deformation provides reduced neck-in
properties
suitable for this type of application. Conversely, LDPE resins with reduced
long chain
branching are more suitable for blown films, which aren't associated with
strict neck-in
requirements but demand lower haze to be commercially attractive. A process
that
would allow commercial producers to switch, with minimal process changes,
between
producing in a tubular reactor, high and low melt elasticity LDPE resin would
be
economically advantageous as it would provide a single platform that can be
used to
produce a selection of LDPE resin polymers that can be used in different
applications,
differentiated mainly on the degree of long chain branching.
Altering process conditions using a tubular reactor to increase the level of
long
chain branching is known in the art. U.S. Patent 3,293,233, issued December
20,
1966 in the name of Erchak Jr., et. al., assigned to Rexall Drug and Chemical
Company, teaches the use of two injection points for introduction of oxygen or
a
peroxide initiator into a tubular reactor for high pressure production of low
density
polyethylene. The use of two injection points as opposed to just one
corresponded
with an increase in the conversion rate and in the long chain branching index,
measured as a function of extrudate swelling. The authors also noted that haze
improved after adding a second injection point, which conflicts with the
notion that
increased long chain branching, while increasing melt elasticity, has a
detrimental
effect on haze. Also, the patent did not teach the use of di-functional
initiators, nor the
variation of the ratio of di-functional initiators to mono-functional
initiators in order to
impact the degree of long chain branching. Furthermore, the patent did not
address
maintenance of peak temperature following each injection point after varying
the
initiator component. In fact, the effect seen can be attributed to variations
in the peak
temperature profile seen with the addition of a second injection point.
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Long chain branching can be introduced into LDPE resin when produced in a
high pressure tubular reactor by using di-functional initiators, as opposed to
using the
traditional mono-functional initiators that include only a single 0-0 peroxide
bond. A
discussion of the mechanism of action for di-functional initiators, which
contain at least
two 0-0 bonds, can be found in an article by P.K.F. Khazaeai and R. Dhib in
the
Journal of Applied Polymer Science, Volume 109, pages 3908-3922 (2008). The
article teaches that the use of di-functional initiators in high pressure
polymerization of
ethylene accelerates the polymerization rate, produces branching, and modifies
rheological properties. The polymers produced show a much wide molecular
weight
distribution. The article does not teach the addition of mixture of mono-
functional and
di-functional initiators into a high pressure process using a tubular reactor
with
multiple injection points. Furthermore, there is no discussion on how altering
the
relative levels of the initiators added at each injection point while
maintaining the peak
temperature for each injection point can be used to alter the long chain
branching
without significantly effecting other properties.
U.S. Patent 9,238,700, issued January 19, 2016 in the name of Littmann et.
al.,
assigned to Basell Polyolefine GmBH, teaches the use of bi-functional
comonomers
as a way to incorporate higher levels of long chain branching into LDPE resin.
The bi-
functional comonomers described contain two different functional groups,
including an
unsaturated group and a group capable of acting as a chain transfer agent. The
bi-
functional comonomers are distinct from the initiators used to start the
reaction. Also,
the patent includes di-functional initiators in its list of possible
initiators but does not
teach that altering the ratio of di-functional initiator to mono-functional
initiator, while
maintaining a particular peak temperature profile, can influence the melt
elasticity of
the resulting LDPE resin.
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What is needed is a process where the long chain branching, and ultimately the
melt elasticity, can be tailored to suit the desired end use application,
while at the
same showing little to no variation in other properties, for example melt
index and
density. Ideally, being able to switch between various types of LDPE resin
based
primarily on melt elasticity when in possession of a single tubular reactor
would give
producers an effective tool for producing particular LDPE resins that vary
mainly on
melt elasticity and can be used in a variety of applications.
SUMMARY OF THE INVENTION
In one embodiment of the present invention there is provided a process for
controlling the melt elasticity of low density polyethylene produced in a free-
radical
initiated process in a high pressure tubular reactor wherein at least one of
one or more
mono-functional initiators and one or more di-functional initiators are
injected into said
tubular reactor via at least one of two or more injection ports at flow rates
independently controlled for each mono-functional and di-functional initiator,
said
process comprising the steps of:
i) raising or lowering the ratio of the flow rate of the one or
more di-
functional initiators relative to flow rate of the one or more mono-functional
initiators injected into said tubular reactor via at least one of said
injection ports;
and
ii) maintaining the peak temperature control for each of said injection
ports,
wherein the variation of peak temperature control for each of said injection
ports is less than 5%.
In another embodiment of the invention, the alteration of the ratio of the
flow
rates of the one or more di-functional initiator flow rates relative to the
flow rates of the
one or more mono-functional initiators injected into said tubular reactor via
at least
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one of said injection ports has no significant effect on the conversion rate,
such that
the conversion rate varies less than 3%.
In another embodiment of the invention, the alteration of the ratio of the
flow
rates of the one or more di-functional initiator flow rates relative to the
flow rates of the
one or more mono-functional initiators injected into said tubular reactor via
at least
one of said injections ports results in a variation of the melt index of the
resulting low
density polyethylene that is less than 0.30 dg/10min.
In another embodiment of the invention, the alteration of the ratio of the
flow
rates of the one or more di-functional initiator flow rates relative to the
flow rates of the
one or more mono-functional initiators injected into said tubular reactor via
at least
one of said injection ports results in a variation of the density of the
resulting low
density polyethylene that is less than 0.005 g/cc.
In another embodiment of the invention the free-radical initiated process
further
comprises the introduction of a chain transfer agent into said tubular reactor
via at
least one of said injection ports. The chain transfer agent is selected from
the group
comprising olefins, aldehydes, ketones, alcohols, saturated hydrocarbons,
ethers,
thiols, phosphines, amines, amides, esters, and isocyanates, and combinations
thereof.
In another embodiment of the invention the chain transfer agent is isopropyl
alcohol and is introduced into said tubular reactor at a flow rate from 8
kg/hr to 40
kg/hr, preferably from 20 kg/hr to 38 kg/hr, most preferably from 25 kg/hr to
35 kg/hr.
In another embodiment of the invention the chain transfer agent is heptane and
is introduced into said tubular reactor at a flow rate from 8 kg/hr to 40
kg/hr, preferably
from 20 kg/hr to 38 kg/hr, most preferably from 25 kg/hr to 35 kg/hr.
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In another embodiment of the invention wherein the mono-functional and di-
functional initiators are chosen from the group consisting of organic
peroxides,
hydroperoxides, peresters, and azo compounds.
In an embodiment of the invention at least one of the mono-functional
initiators
is t-butyl peroxyneodecanoate.
In an embodiment of the invention at least one of the mono-functional
initiators
is t-butyl peroxy-2-ethylhexanoate.
In another embodiment of the invention at least one of the mono-functional
initiators is di-t-butyl peroxide.
In another embodiment of the invention at least one of the di-functional
initiators is 1,1-di(t-butylperoxy)cyclohexane.
In an embodiment of the invention the one or more mono-functional initiators
are injected into said tubular reactor at a flow rate from 0.1 kg/hr to 10
kg/hr,
preferably from 0.15 kg/hr to 8.5 kg/hr, most preferably from 0.2 kg/hr to 7.5
kg/hr.
In an embodiment of the invention said tubular reactor comprises three
injection ports.
In another embodiment of the invention said tubular reactor comprises four
injection ports.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Free-radical initiated high pressure process
The free-radical initiated high pressure process used for producing low
density
polyethylene (also referred to herein as LDPE resin) is well known to those
skilled in
the art. Two types of reactors are used for this process: tubular reactors and
autoclave reactors. The present invention is relevant only with respect to
tubular
reactors. These reactors are characterized by their length, configured in a
way that
reactants are fed into the reactor at one end and flow through, usually at
near steady
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state, to an output end where LDPE resin is captured. The properties of the
LDPE
resin produced can vary widely, depending on the flow rates, pressure,
temperature,
and the types of initiator or initiators used to initiate the polymerization
of ethylene.
Furthermore, tubular reactors frequently are designed such that initiators can
be
injected not only at a single injection point located at the front end of the
reactor but
also at predetermined injection points throughout the length of the reactor.
In an
embodiment of the invention the tubular reactor comprises a single injection
point. In
another embodiment the tubular reactor comprises two injection points. In
another
embodiment of the present invention the tubular reactor comprises three
injection
points. In yet another embodiment of the present invention the tubular reactor
comprises four or more injection points.
The flow rate and pressure used in the process of the present invention may be
selected by the person skilled in the art choosing settings appropriate
considering the
specific reactor to be used and the desired polymer to be produced. Ranges for
both
flow rate and pressure would fall within limits normally used and well known
in the art.
In an embodiment of the invention mono-functional initiators and di-functional
initiators are injected into the tubular reactor via at least one of the
injection ports at a
flow rates from 0.1 kg/hr to 10 kg/hr, preferably from 0.15 kg/hr to 8.5
kg/hr, most
preferably from 0.2 kg/hr to 7.5 kg/hr.
Initiators
In addition to choosing flow rates and pressure a skilled worker must consider
the temperature distribution profile along the length of the reactor. In their
paper
entitled "Free radical polymerization engineering ¨ I. A new method for
modeling free
radical homogeneous polymerization reactions", Chemical Engineering Science,
39(1), 87-99 (1984), Villermaux and Blavier declare that temperature
distribution
"plays the central role in determining the properties of the polymer
products." The
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free-radical initiated polymerization of ethylene is exothermic, which means
that
temperature distribution along the length of the reactor is a function of the
polymerization reaction and the capacity to remove excess heat. Removing
excess
heat from tubular reactors is known to those skilled in the art. Known methods
are
compatible with the present invention.
Choice of initiator or mixture of initiators injected into each injection site
will also
have an impact on the temperature distribution profile. A large selection of
initiators
are well known, along with the optimal temperature profile. Choosing an
appropriate
temperature profile to produce a polymer with desired characteristics is known
within
the art, and includes the ability to identify initiators that can provide the
temperature
profile, in combination with the particular reactor and its temperature
control abilities.
The skilled worker would understand that choosing initiator or initiators one
must
consider the rate of decomposition and half-life of the initiators.
Mono-functional initiators include compounds having a peroxide functional
group, or single 0-0 bond, having a formula of R1-00-R2. Examples of mono-
functional initiators suitable for use with the present invention include
organic
peroxides, hydroperoxides, peresters, and azo compounds. Specific examples
include t-butyl peroxyneodecanoate, t-butyl peroxy-2-ethylhexanoate, and di-t-
butyl
peroxide.
Di-functional initiators include compounds that include two peroxide
functional
groups having a formula of R1-00-R2-00-R3. Examples of di-functional
initiators
suitable for use with the present invention include organic peroxides,
hydroperoxides,
peresters, and azo compounds. In an embodiment of the present invention the di-
functional initiator is 1,1-di-(t-butylperoxy)cyclohexane.
Chain transfer agents
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Another common addition to tubular reactors during free-radical initiated
polymerization of ethylene are chain transfer agents. During propagation the
free-
radical at the end of the growing chain is transferred to the newly added
ethylene
molecule providing a reactive end for addition of the next ethylene molecule.
Propagation can continue provided the final ethylene retains the free radical.
Chain
transfer agents allow termination of the growing chain by transferring the
free radical
to the chain transfer agent, without reforming a free radical at the end of
the growing
polyethylene chain. The chain transfer agent having a free radical is now a
target for
propagation through addition of ethylene monomers. Chain transfer agents in
effect
control the length or average molecular weight of the polyethylene produced.
Without
a chain transfer agent the molecular weight is normally controlled by altering
reaction
conditions.
Chain transfer agents commonly used in the art include olefins, aldehydes,
ketones, alcohols, saturated hydrocarbons, ethers, thiols, phosphines, amines,
amides, esters, and isocyanates, and combinations thereof. Any of these are
appropriate for use with an embodiment of the present invention.
In embodiment of the present invention the chain transfer agent is isopropyl
alcohol. In yet another embodiment of the invention the chain transfer agent
is
heptane.
Flow rates of chain transfer agent used also fall within the scope of
knowledge
possessed by the skilled person. The rate may vary between injection points
when a
reactor with more than one injection point is used.
In an embodiment of the present invention the chain transfer agent introduced
into the reactor via at least one of the injection ports is injected at a flow
rate from 8
kg/hr to 40 kg/hr, preferably from 20 kg/hr to 38 kg/hr, most preferably from
25 kg/hr to
kg/hr.
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Peak temperature
The temperature distribution profile refers to the temperature within and
along
the length of the reactor. Ethylene entering the reactor at the inlet is
preheated so as
to promote decomposition of the free-radical initiator, forming free-radicals
that can
start polymerization. The preheat temperature is chosen to align with the
decomposition temperature of the initiator or initiators injected into the
reactor at the
upstream end. The peak temperature within the reactor at the upstream end and
following initiator injection points is a function of the quantity of
initiator injected into
the reactor. The temperature peaks at a point where the initiator levels drop
off and
polymerization reduces due to lower levels of free-radicals. The release of
heat by
polymerization increases the temperature within the reactor, an effect that
can be
partially minimized by cooling means in the form of a water cooled jacket
surrounding
the reactor. Increasing the quantity of initiator at any point has the effect
of increasing
the peak temperature. Conversely, decreasing the amount of initiator lowers
the peak
temperature.
The temperature within and along the length of the reactor can be plotted
versus position along the length of the reactor to give a temperature
distribution
profile. The peak temperatures for each injection point are clearly seen in
the profile
as the temperature peaks immediately downstream of each injection point, then
gradually cools before the next injection point or at the end of the reactor.
The
experienced operator understands that the temperature distribution profile can
be
associated with variousµproperties found in the LDPE resin produced.
Alteration of the
profile by altering peak temperature of one or more of the injection points
can have
effects on the molecular structure and hence the properties of the resulting
polymer.
Furthermore, changing the peak temperatures may also impact the conversion
rate of
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ethylene, or ethylene and one or more comonomers, into polyethylene. For the
present invention the peak temperature for each injection point may vary.
Melt Elasticity
The present invention seeks to provide a way of altering the melt elasticity
of a
low density polyethylene produced in a free-radical initiated process. The
present
invention is useful for modifying melt elasticity while attempting to maintain
other
properties of the LDPE resin as to provide consistency among produced resins.
The
process seeks to provide a way to produce resins that differ mainly in their
degree of
long chain branching.
Melt elasticity can be assessed in a number of ways, including the use of
rheological measurements. For the purposes of the present invention melt
elasticity is
quantified by G' at G" = 500 kPa. The higher the G' at G" = 500 kPa the higher
the
melt elasticity. The melt elasticity is dependent upon the degree of long
chain
branching within the polymer. Long chain branching is commonly present in LDPE
resin produced in a free-radical initiated high pressure process, but the
levels are
enhanced when di-functional initiators are used in the process.
The present invention begins with an LDPE resin that satisfies particular
properties with the exception of melt elasticity, and acts to alter the
process conditions
to produce a similar LDPE resin but with an altered melt elasticity. For
example, an
operator may be using process conditions that result in an LDPE resin within a
desired
density range, but not with the required melt elasticity. In that instance the
present
invention may provide a solution that increases melt elasticity but does
significantly
alter density. In an embodiment of the invention the peak temperature for each
injection point does not vary by more than 5 C.
The change in the ratio of di-functional initiator to mono-functional
initiator is
performed with the melt elasticity in mind. For a higher melt elasticity, an
increase in
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the amount of di-functional initiator added to at least one of the injection
points is
required. In order to prevent a significant change in the peak temperature a
corresponding decrease in the mono-functional initiators at the same injection
point
would be required. The present invention also contemplates adding di-
functional
initiator to one or more injection points where previously no di-functional
initiator was
added. As before, the skilled operator would understand that in that instance
the
amount of mono-functional initiator added at the same injection point would
need to be
reduced to keep the peak temperature within a desired range.
Applications that would benefit from the present invention include producing
resins for foaming and extrusion coating applications. In that instance the
ratio of di-
functional initiator to mono-functional initiator would be increased for at
least one of
the injection points. The greater the desired increase in melt elasticity
means allowing
for a greater the change in the ratio or for increasing the number of
injection points
where the ratio is increased.
Applications where reducing the melt elasticity is desired include film
applications. In film, a lower haze is desirable because it provides more
clarity. In a
process where LDPE resin with higher melt elasticity is produced, the operator
may
reduce the ratio of di-functional initiator to mono-functional initiator added
to one or
more of the injection points. In order to maintain the peak temperature an
alteration in
the amount of mono-functional initiator, or chain transfer agent, added would
be
required. In addition, the present invention contemplates removing di-
functional
initiators entirely so as to reduce the melt elasticity to its lowest possible
in the
prevailing conditions, including the temperature distribution profile. By
maintaining the
temperature profile the production or conversion rate is not significantly
lowered, an
effect that would be expected when di-functional initiators are removed from
the
process.
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EXAMPLES
Four experimental LDPE resins were produced in a tubular reactor with four
injection points. The peak temperature controls for each injection point,
which
determine the temperature distribution profile, are shown in Table 1. Resins A
through C share a similar temperature distribution profile, with peak
temperature
control at each injection not varying by more than 2 C. The fourth resin, a
comparative, used a unique temperature distribution profile where peak
temperature
control for all four injection points were higher than those used for the
experimental
resins A through C.
TABLE 1
Peak temperature control for each injection point
Values are in C.
Comp. Resin A Resin B Resin C
1st injection point 267 255 254 252
2nd injection point 267 250 250 249
3rd Injection point 266 240 240 239
4th Injection point 267 238 241 239
Table 2 lists the flow rates of initiators and chain transfer agents added at
each
injection point for the resins produced. The mono-functional initiators
employed
include t-butyl peroxyneodecanoate (M Fl-I), t-butyl peroxy-2-ethylhexanoate
(M Fl-2),
and di-t-butyl peroxide (MFI-3). The di-functional initiator employed was 1,1-
di-(t-
butylperoxy)cyclohexane (DFI-1) and the chain transfer agent employed was
isopropyl
alcohol (IPA). The sample resins A through C differed in the flow rates for
the di-
functional initiator (DTBC) added at the 2" and 3rd injection points. In
resin A the flow
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rate for DTBC at the 2nd injection point was doubled, and at the 3rd injection
point
DTBC was included whereas it was absent for resins B and C. Additionally, to
maintain peak control temperature Resin A was produced without the use of the
mono-functional initiator DTBP. The comparative resin employed a unique
combination of initiators, maintaining a higher peak control temperature for
each of the
injection points mainly due to higher flow rates for the DTBP mono-functional
initiator.
TABLE 2
Flow rates (kg/hr) of mono-functional and di-functional initiators, and
chain transfer agent added at each injection point.
Comp. Resin A Resin B Resin C
1st Injection Point
MFI-1 5.00 7.00 7.00 7.00
MFI-2 4.00 4.00 4.00 4.00
MFI-3 0.28 0.00 0.05 0.04
DFI-1 0.66 0.49 0.25 0.24
IPA 21.00 24.02 20.00 20.00
2' Injection Point
MFI-1 5.50 7.00 5.50 5.50
MFI-2 4.50 6.00 4.50 4.50
MFI-3 1.04 0.00 0.34 0.37
DFI-1 2.00 4.10 2.00 2.00
IPA 20.01 21.03 20.01 20.01
3rd a Injection Point
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MFI-1 0.00 0.00 0.00 0.00
MFI-2 0.01 2.70 3.00 3.00
MFI-3 1.40 0.00 0.15 0.16
DFI-1 1.00 2.00 0.00 0.00
IPA 19.00 20.00 19.99 20.01
A 4th
Injection Point
MFI-1 0.00 0.00 0.00 0.00
MFI-2 0.00 4.00 0.00 0.00
MFI-3 0.983 0.00 0.251 0.227
DFI-1 0.00 0.00 0.00 0.00
IPA 13.99 16.04 15.99 15.00
The properties of the resins produced demonstrate the usefulness of the
present invention. By increasing the amount of di-functional initiator added
at one or
more of the injection points a change in the melt elasticity, measured as G' @
G" =
500 kPa, was seen. The change in melt elasticity did not result in a
significant change
in the density, MI, or polydispersity index (PDI). By maintaining a
temperature
distribution profile, through non-variance of peak temperature control at each
injection
point, the melt elasticity of an free-radical initiated LDPE resin can be
manipulated by
changing the amount of di-functional to mono-functional initiator added at one
or more
of the injection points. Resin A comprises the highest G' value and would
suitable for
use in foaming type applications. Resins B and C comprise lower G' values but
are
improved in their haze %, making them suitable for use in film applications.
These
results show that when starting with a particular resin, for example Resin B,
the
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amount of initiators added may be altered while keep the peak temperature
control
consistent to produce a resin that varies mostly in respect to the G' value.
TABLE 3
Product properties
Comp. Resin A Resin B Resin C
MI (dg/min) 2.37 2.30 2.28 2.00
Density (g/cc) 0.9212 0.9234 0.9237 0.9235
G' @ G"=500kPa (kPa) 73.00 80.00 73.00 77.00
Mn (g/mol) 20028 20931 23123 21277
Mw (g/mol) 65820 61509 64513 61652
Mz (g/mol) 144951 129110 131169 1206636
PDI 3.29 2.90 2.79 2.90
Haze (%) 5.10 5.20 4.50 4.80
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