Note: Descriptions are shown in the official language in which they were submitted.
13218~
LAB-148
PROCESS FOR REMOVING THE SOLVENT FROM A
POLYMER SOLUTION
BACKGROUND OF THE INVENTION
The present invention relates to a process for
removing the solvent from a polymer solution. In
particular, the present invention relates to a process
for removing solvent from an elastomeric polymer solution
containing at least about 20% by weight polymer.
Prior processes of removing the solvent from polymer
solutions to obtain polymer granules or pellets are
generally conducted in two stages (i.e. two separate
systems). The first stage is needed to remove the
majority of the solvent to obtain a relatively viscous
solution which is then introduced into the second stage,
a degassing apparatus containing one or two worm screws
having a plurality of openings arranged at the periphery
of the screw(s) that allow the vaporized solvent to
escape.
The first stage is generally carried out either in a
reactor or a worm screw. In either case, problems arise
in controlling the flow and in transferring the polymer
solution between the two systems, particularly when
highly viscous solutions of elastomeric polymer are
treated.
Another problem observed with the two-stage systems
is that the resulting polymer has poor heat stability.
This poor heat stability may be due in part to the many
dead zones present between the screws in sequence and to
the long residence time of the polymer in both apparatus.
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~32~2
Moreover when solutions of elastomeric polymers of
the rubbery type are treated in accordance with
conventional processes in conventional apparatus using
under water pelletization, crosslinking of the rubber is
observed at the die location. This crosslin~ing is due
at least in part to the high temperature resulting from
the pressure buildup behind t:he cooled die face in this
type of pelletization apparatus.
After removing solvent from the polymer solucion,
the resulting polymer should have as low a residual
volatile content as possible; however, the polymer
resulting from the standard two-stage processes has a
residual volatile content of about 0.5% by weight. This
may be due at least in part to the small size of the
screw employed in the second stage needed to handle the
viscous solution.
It would, therefore, be very desirable to have a
single stage process of removing essentially all of the
solvent from polymer solutions, especially elastomeric
polymer solutions.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide
an improved process for removing solvent from a polymer
solution.
It is a further object of the present invention to
provide a single-stage process of removing solvent from
elastomeric polymer solutions.
It is yet a further object of the present invention
to provide a process of removing solvent from polymer
solutions that obtains a polymer having a residual
volatile content less than about 0.1 weight %.
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SUMMARY OF THE PRESENT INVENTION
The process of the present invention comprises a
single system entailing the following steps;
- introducing a heated polymer solution, preferably
elastomeric, into a hopper feeding an extrusion system
at a pressure higher than the boiling pressure of the
solvent;
- depressurizing the solution thereby removing a major
portion of the solvent via a ventilation zone situated
so as to not interfere with the solution while forcing
the solution forward;
- reheating and repressurizing the solution then
removing the remainder of the solvent in the
ventilation zones forward of the feed hopper;
- introducing the resulting solvent free elastomeric
polymer through a die; and
- cutting the polymer in a pelletizer.
BRIEF DESCRIPTION OF THE DRAWING
The drawing is a diagram of the preferred solvent
removal system of the present invention.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
The present invention is a new single-system process
for removing solvent from an elastomeric polymer
solution. This single-system process pro~ides good
control of polymer flow and avoids the problems resulting
from the transfer of viscous polymer solutions between
two systems.
When employed in the present invention, the term
"elastomeric polymer" means a polymer at least 15% by
weight of which consists of an elastomer phase, any
residue consisting of a thermoplastic phase.
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The proces5 of the present invention generally
removes the solvent ~rom the elastomeric polymer until
the content of residual volatiles is less than about 0.1
by weight.
The preferred process of the present invention for
removing, in a single system, the solvent from a solution
of elastomeric polymer containing at least 20~ polymer is
characterized in that it comprises the following steps:
- introducing into the hopper feeding a worm screw the
solution of elastomeric polymer, heated beforehand to
a temperature between about 150 and 200C, under a
controlled pressure higher than the boiling pressure
of the solvent;
- creating a depressurization in the feed 7one
sufficient to remove about 80 to 95% of the solvent
via the ventilation zone situated to the rear of the
feed hopper, while forcing the solution of elastomeric
polymer forward;
- reheating the polymer solution to a temperature
between about 150 to 200C;
- removing the remainder of the solvent in the
ventilation zones situated forward of the feed hopper;
- optionally injecting, if appropriate, between the
various ventilation zones a quantity of water between
about 0.5 and 2% by weight of polymer, so as to
promote the removal of solvent;
- introducing the solvent-free elastomeric polymer melt
through a die; and
- cutting the polymer in the pelletizer situated
immediately behind the die.
The elastomeric polymers which are treated in the
process of the present invention can be of a rubbery or
thermoplastic nature. To this end, in most cases it is
preferred to apply this treatment to copolymers of a
rubbery nature made from vinylaromatic monomers and
13~8~2
conjugated diene monomers since these copolymers are
widely used and the removal oE solvent from solutions
cont~aining these copolymers by conventional processes is
difficult. Examples of such include copolymers of
styrene and butadiene or isoprene, in which the
proportion of conjugated diene monomer is at least 15% by
weight.
When solutions of elastomeric polymers of a
thermoplastic nature are treated according to the process
of the present invention, the polymers are preferably
block copolymers of styrene and a conjugated diene, and
have a high styrene content. It is understood that the
process of ~he present invention also applies to
elastomeric polymers of the vinylaromatic/conjugated
diene block copolymer type in which the conjugated diene
fraction is partially or completely hydrogenated.
The polymers that are treated according to the
process of the present invention are produced by the
polymerization of the corresponding starting monomers in
solution. The resulting polymers or copolymers are then
in the form of a solution in the polymerization solvent,
which is, in most cases, a paraffinic, cycloparaffinic or
aromatic hydrocarbon. Examples of suitable solvents
particularly include cyclohexane, pentane, hexane,
cyclopentane, isooctane, benzene, toluene, the like, and
mixtures thereof such as mixtures of hexane with
cyclohexane. The polymer content in the solution is
preferably between about 20 and 60% by weight. However,
the single-system process of the present invention is
most useful when treating solutions with a polymer
content of about 40 weight %.
Prior to the present invention processes of removing
solvent from polymer solutions could only reduce the
residual volatile content of the polymer down to abou-t
0.5 weight %. The applicant has unexpectedly discovered
that by treating the polymer solution in a single-system
process according to the present invention it is now
possible to reduce the residual volatile content down to
about 0.1 we:ight ~ and below.
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When removing solvent Erom the polymer solutions
according to the present invention, it was unexpected
that a major portion of the solvent would be removed via
a ven-t situated to the rear of the feed hopper. The
removal of a major portion of the solvent in this manner
is an important feature of the present invention if the
resulting polymer is to have a low residual volatile
content. It was found that about 80 to 95% of the
solvent can be removed in this manner.
It was expected that the removal of a major portion
of the solvent from the rear of the screw would cause
polymer to block the vent. It was unexpectedly found,
however, that this removal is quite effortless and allows
the polymer melt to continue to move freely within the
worm screw.
This rear ventilation is produced by preheating the
solutions, preferably to about 150-200C and by creating
a depressurization in the feed zone of the worm screw.
This depressurization can be regulated by means of the
control valve situated in the line feeding the worm
screw.
The removal of solvent has the effect of
considerably reducing the temperature of the polymer
solution. The applicant has found that, in order to
obtain a good efficient removal of the solvent in the
subsequent ventilation zones, it is necessary to reheat
the polymer solution, preferably to a temperature between
about 150 and 200C. The heat can come from either a
heated screw barrel (maintained at a temperature between
about 180 and 250C~ or by providing the screw with
components that introduce shearing forces or, yet again,
by combining the various means. If the polymer solution
is not reheated to a sufficient temperature, then levels
of residual volatiles cannot be reduced down to about 0.1
weight %.
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The residual solvent is progressively removed
through the various forward ventilation zones in step
with the forward movement of the polymer melt in the
screw.
These zones are preferably at a pressure at or below
atmospheric pressure and the pressure is reduced in step
with the progress of the polymer solution in the screw.
The pressure changes from about atmospheric pressure to
approximately 5 mbars absolute at the screw end. There
are preferably from 3 to 5 ventilation or degassing
zones, depending upon the quantity of solvent to be
removed.
A fluid such as water can be supplied between each
of the degassing zones to further promote the removal of
the solvent. Any fluid that would aid in the removal of
solvent from the solution is considered useful in the
present invention, howeverr water is most preferred. The
preferred amount of water introduced into the polymer
melt between each degassing zone is between about 0.5 and
2% by weight based on the polymer since this amount will
aid in the removal of solvent but yet not interfere with
the polymer melt.
As specified above, the degassing zones are at
different pressures. In order to avoid suction,
sealing members are preferably introduced into the
extruder screw between the degassing zones. These
generally consist of left-handed screw components if the
worm screw is right-handed. It is also preferred that a
component of this type be placed after the areas where
water is introduced, since these left-handed screw
components cause intimate back-mixing.
Generally under water pelletizers are required when
pelletizing rubbery or sticky polymers to avoid pellets
clumping. However, in the usual under water pelletizers
crosslinking has been observed with these polymers. This
crosslinking produces insoluble polymer gels lowering the
quality of the resulting polymer product thereby making
the polymer unsuitable for further applications.
8 ~ 3 ~
The applicant has unexpectedly found that the
pellets produced according to the process of the present
invention do not clump together if properly handled after
pelletizing (i.e. not allowing hot polymer pellets to
touch), this avoids the requirement of under water
pelletizing. Since under water pelletizing is not a
required condition of the present invention, it is
possible to avoid the crosslinking problems.
The applicant has found that by employing an
eccentric pelletizer which permits the die to be
installed immediately downstream of the screw, it is
possible to reduce the dead zones and to reduce the
pressure drop across the die so that the temperature of
the product remains below the crosslinking temperature of
the polymer.
The speed of the screw is one method of controlling
the devolatization of the present invention. Operating
within the temperatures and pressures disclosed herein
the applicant has found that industrial size screws have
to be operated at a speed of about 50 to 300 RPM for a
good devolatilization to be obtained. This speed is
preferably between about 100 and 200 RPM.
According to the single-system process of the
present invention, almost all of the solvent is
successfully removed from the elastomeric polymer
solutions, without giving rise to any screw locking or
blocking problems.
It is understood that the present invention relates
in particular to the treatment of elastomeric polymer
solutions, however, the present invention can also be
applied to any type of polymer that is in the form of a
solution in solvent.
The process of the present invention is described
with the aid of the attached figure which shows an
extruder for removing the solvent from a polymer solution
containing at least 20 weight ~ polymer. However, this
figure is not intended to limit the reasonable scope of
the present invention.
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The polymer solution is introduced into the feed
hopper (10) of an extruder (12) incorporating a twin worm
screw (14).
The pressure in the extruder and the ~eed rate are
controlled by means of the control valve (16).
The extruder contains a rear ventilation zone (18)
through which the majority of the solvent is removed.
The polymer melt is driven forward by means of the worm
screws (14). To remove the remainder of the solvent, a
number of ventilation zones (20) have been arranged on
the extruder (12). Between the ventilation zones,
provision has also been made for a means (22) of
introducing a fluid, generally water, to promote further
the removal of the solvent.
The polymer melt, freed from its solvent in this
manner, passes through the die (24) and is cut into
pellets by means of the pelletizer (26).
The following examples are given in order to
illustrate the present invention, but are not intended to
limit the reasonable scope thereof.
Example 1
After the solution polymerization of butadiene and
styrene has been carried out, a cyclohexane solution
containing 38% radial styrene-butadiene copolymer
containing 40% styrene was recovered. -
This solution was introduced at a temperature of
170C into the feed hopper of a twin-screw extruder
(constructed with Werner-Pfleiderer (WP) type ZSK-57
modules) at a rate of 75 kg/hour. The pressure was
regulated by means of the valve situated in the feed line.
This extruder has a ventilation zone situated to the rear
of the feed hopper and three zones situated forward of
this hopper. The die and the pelletizer have been
arranged immediately downstream of the screw.
1 o
The following operating conditions were applied:
Vacuum in the ventilation zones:
- rear zone800 mbar
1st forward zone 800 mbar
2nd forward zone 80 mbar
3rd forward zone 10 mbar
Speed of rotation: 230 RPM
Water was injected before the last two ventilation
zones, in an amount of 1~ before each zone based on the
weight of polymer.
Ninety-two percent of the solvent was recovered in
the rear zone, 3% in each of the 1st and 2nd forward
zones, and 2% in the 3rd and final forward zone. The
final content of volatile material was 0.1% by weight.
Before pelletization the pressure was 15 bars and
the temperature of the material was 195C.
The polymer was pelletized perfectly, without the
need of under water cutting.
'
Example 2
After the solution polymerization of butadiene and
styrene has been carried out, a cyclohexane solution
containing 38% styrene-butadiene block copolymer
containing 75% styrene was recovered.
This solution was introduced at a temperature of
165C into the feed hopper of a twin-screw extruder (WP
type ZSK-57) at a rate of 80 kg/hour. The pressure was
regulated by means of the valve situated in the feed line.
This extruder has a ventilation zone situated to the rear
of the feed hopper and three zones situated forward of
this hopper. The die and the pelletizer have been
arranged immediately down-stream of the screw.
The following operating conditions were applied:
Vacuum in the ventilation zones:
1, ~ 3 ~
- rear zone 800 mbar
1st forward zone 800 mbar
2nd forward zone 80 mbar
3rd forward zone 7 mbar
Speed of rotation: 270 RPM
Water was injected before the last two ventilation
zones, in an amount of 1% before each zone based on the
weight of the polymer~
Eighty-nine percent of the solvent was recovered in
the rear zone, 3~ in the 1st forward zone, 6% in the 2nd
forward zone, and 2~ in the 3rd and final forward zone.
The final content of volatile material was 0.1~ by
weight.
Before pelletization the pressure was 15 bars and
the temperature of the material was 192C.
The polymer was pelletized perfectly, without the
need of under water cutting.
Example 3
After the solution polymerization of isoprene and
styrene has been carried out, a cyclohexane solution
containing 38% styrene-isoprene block copolymer
containing 15% styrene was recovered.
This solution was introduced at a temperature of
160C into the feed hopper of a twin-screw extruder (WP
type ZSK-57) at a rate of 50 kg/hour. The pressure was
regulated by means of the valve situated in the feed line.
This extruder has a ventilation zone situated to the rear
of the feed hopper and three zones situated forward of
this hopper. The die and the pelletizer have been
arranged immediately downstream of the screw.
The following operating conditions were applied:
Vacuum in the ventilation zones:
12 ~ L8~
- rear zone800 mbar
1st forward zone 600 mbar
2nd forward zone 60 mbar
3rd forward zone 9 mbar
Speed of rotation: 220 RPM
~ ater was injected before the last two ventilation
zones, in a total amount of 1% based on the weight of
polymer. t
Eighty-nine percent of the solvent was recovered in
the rear zone, 4% in the 1st forward zone, 6% in the 2nd
forward zone, and 1% in the 3rd and final forward zone.
The final content of volatile material was 0.1% by
weight.
Before pelletization the pressure was 8 bar and the
temperature of the material was 180C.
The polymer was pelletized perfectly, without the
need of under water cutting.
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