Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR RECOVE:RING
SY~THETIC RESINOUS LATEX SOLIDS
Synthetic resinous materials are prepared in
a variety of ways including mass polymerization, suspension
polymeri~ation, solution polymerization and emulsion
polymerization~ For many rssinous materials it is de-
sirable that they be prepared by emulsion pol~ner-
ization since the desired particle size, molecular
weight or graf-ting reaction is more readily obtained by
: emulsion polymerization than by other polymerization
methods.
Latex solids have been recovered most frequently
by adding an electrolyte to the latex, usually with
heating and agitation to cause the latex particles to
: agglomerate into macro particles which are readily
filtered, washed and dried. Typical processes are
discussed in U.S. 3,248,455; 3,345,430; and 3,438,923.
For 50m~ purposes the use of electrolyte in coagulation
results in undesired retention of the emulsifier ernployed
in the emulsion polymerization and often retention of
at least some of the coagulation electrolyte.
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To overcome the problem of electrolyte
retention, nitrile polymer latexes have been coagulated
by shear coagulation. In this process a latex is
subjected to mechanical shear un'cil at least a major
portion of the latex particles have agglomerated. With
a latex solids content of about 20 to 30 weight percent,
the shear coagulated product is a more or less grainy
paste. U.S. 3,821,345 discloses a shear coagulation
process wherein the resultant paste of a nitrile polymer
latex is extruded, placed in hot water for a period of
time and then washed and dried.
It would be desirable if there were available
an impxoved process for the recovery of latex solids
which required minimal energy and ~uantities of water
and steam.
These advantages are achieved by ~he present
invention in a process for the recovery of synthetic
resinous thermoplastic latex solids ~rom a latex char-
acterized by (A) providing a latex of a synthetic
resinous thermoplastic polymer containing about 10 to
50 weight percent solids, (B) subjecting the latex to
mechanical shear sufficient to transform the latex into
a paste-like mass, (C) admixing the paste-like mass
with steam under pressure with mechanical shear provided
by the admixture of steam with said mass to thereby
heat the paste-like mass above the softening point of
the polymer and form a plurality of macro particles of
which at least 90 weight percent are retained on an 80
mesh U.S. Sie-ve Size screen, and subsequently (D) subjecting
said macro particles to mechanical working to expel at
least a majority of water associated therewith.
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The process of khe present invention is
operable with any synthetic resinous thermoplastic
latex having solids content by weight of from about 10
to about 50 weight percent. Typically, latexes which
are useful in the present process include latexes of
polystyrene, polymethyl methacrylate, polybutadiene,
polyisoprene, polyvinylacetate, polyvinylchloride as
well as various copolymer latexes including styrene-
-butadiene latexes, vinylchloride-vinylacetate copolymers,
vinylidene chloride-vinylchloride latexes, polymethyl-
methacrylate latexes-polymethylacrylate latexes.
Latexes which particularly benefit from with the present
invention are styrene-acrylonitrile-rubber latexes
wherein styrene-acrylonitrile copolymer is grafted to a
diene rubber substrate such as polybutadiene.
The only component in addition to the latex
that is required is process stPam. Steam of commercial
purity under pressures of about 25 to 400 pounds per
square inch guage (170-2760 kPa) are generally satisfactory.
During the heating of the paste-like mass prepared by
shear coagulation, the temperature of the solids should
be raised at least to the softening point of the polymer
to permit desired agglomera~ion. Therefore, the steam
pressure for a particular latex must be sufficiently
high to raise the polymer to its softening point.
If it is desired to dilute the latex prior to
shear coagulation to provide a paste of a ~ore flowable
consistency, water is employedO Usually it is desirable
that such water be deionized to minimize possible
introduction of ma~erials which might affect the thermal
stability of the desired end product.
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Furth~r features and ad~antages of the present
invention will become more apparent from the Drawing
wherein:
Figure 1 is a simplified schematic represen-
tation of the process in accordance with the present
invention.
Figure 2 i5 a representation of a s~eam paste
mixing device such as is employed ln Figure 1.
Figure 3 is a schematic sectional represen-
tation of the steam paste mixing inlet portion of thedevice of Figure 2.
In Figure 1 there is schematically depicted
an apparatus 10 suitable for the practice of the process
of the present invention. The apparatus 10 comprises
in cooperative combination a shear coagulator 11. The
shear coagulator 11 has in association therewith a
latex carrying conduit 12 which discharges a synthetic
resinou~ thermoplastic latex into coagulator 11. The
conduit 12 has associated therewith a steam supply
conduit 13 attached to supply skeam to latex in the
conduit 12 and raise the temperature to a desirable
coagulating temperature; for example, 40 - 90C. The
shear coagulator 11 discharges a stream of paste-like
mass 14 to a mixing and forwarding apparatus 16. The
mixing and forwarding apparatus 16 beneficially can be
a rotary type mixer with blade~ affixed to a shaft, the
blades being inclined at an angle to the shaft to
provide a ~orwarding action. The mixer 16 has an inlet
17 and a discharge 18. The discharge 18 of the mixer
30 16 is in communication with a pump 19. The pump 19
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beneficially is a screw type pump such as is commercially
available under the trade name o~ Moyno. The pump 19
has an inlet in operative communication with the discharge
18, mixing and forwarding device 16, and a discharge
conduit 21 in operative communication with a steam
mixing and shearing device 22. The mixing and shearing
device 22 has a steam inlet 23 and an outlet 24. The
mixing and shearing device 22 ~orms the paste-like mass
from the pump 19 into a wet granular mass. The wet
granular mass passes through the discharge 24 into a
mechanical dewatering apparatus 26 having an inlet
communicating with the discharge 24. The mechanical
dewatering apparatus 26 has a first or solids discharge
27 and a liquid discharge line 28. The liquid discharge
line 28 is in communication with a filter or screen
assembly 29. The filter assembly 29 has a liquid
discharge 31 and a solids discharge 32. The solids
discharge 32 discharges to the inlet 17 of the mixing
and forwarding ~evice 16. The solids discharge 27
passes to a grinder 33 which com~utes the solid material
from the mechanical dewatering devicP 26. The particulated
solids from the grinder 33 are passed through conduit
35 to a cooler such as a rotary cooler 36 having a
cooling water inlet 37 and a cooling water discharge
38~ Particulate material from the cooler 36 is discharged
via line 39 into a storage hopper 40 and subsequently
passed from the hopper 40 through line 41 for packaged
shipment and final use.
In Figure 2 there is a schematic representation
of a steam-paste mixing apparatus generally designated
by the reference numeral 50. The apparatus 50 is
generally equivalent to the mixer designated by the
reerence numeral 22 in Figure 1. The mixer 50 comprises
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an inlet mixing assembly generally designated by the
reference numeral 51 which comprises steam valve 52
having a steam inlet 53 and a discharge region 54. The
discharge region 54 of the valve 52 is in communication
with a paste inlet mixing and shearing assembly 55
having a paste inlet 56 and a high shear region 57.
The high shear region 57 has a discharge end 58 which
is in full communication with a pipe section 59. The
pipe section 59 remote from the high shear region 57 is
connected to a reducer 61. The discharge of the reducer
61 is in communication with a backpressure valve 62.
Beneficially the valve 62 is a fluid operated pinch
valve. By fluid operated pinch valve is meant a valve
that comprises a housiny, a flexible tube is disposed
within the housing and serves to convey fluids there-
through. Space between the tube and the housing is in
communication with a source of a pressurized fluid
whcih can be selectively applied thereto to collapse
the flexible tube and thereby close the valve or to
remove at least a portion of the pressurized fluid to
thereby open the valve. The valve 62 remote from the
reducer 61 is in communication with conduit 63. The
conduit 63 remote from the valve 62 terminates in a
disentrainment chamber 64. The chamber 64 has an
overhead vent 65 through which steam may escape and a
bottom discharge 66 through which the solid wet par-
ticulate product is withdrawn.
Figure 3 is a schematic sectional represen-
tation of the steam pas~e mixing section 51 of Figure
2. Disposed within the high shear region 57 is a tube
68. The tube 68 has a first inlet end 69 and a second or
discharge end 71. The tube 68 is adjustably mounted
within the mixing section in such a manner that the
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location of the inlet end 69 can be axially positioned
toward or away from the valve 52 and thereby vary the
shearing and agitating effect o~ the steam on the latex
paste-like mass provided from inlet 56.
In the practice of the process of the present
invention with particular reference to the Drawing,
latex is passed through conduit 12 wh~ere it is heated
by steam introduced from conduit 13. The shear coagu-
lator 11 is adjus~ed until a desired paste-like con-
figuration is obtained. For example, a suitable shear
coagulator is a butter churn of the generally horizontal
cylindrical drum variety having internal blades wh:ich
rotate about the axis of the drum, the blades having a
clearance from the drum of about one-eighth of an :inch
(3.2 mm). When temperakure of the incoming latex and
rotational speed of the shear coagulator 11 have been
adjusted to provide desired paste-like effluent, the
paste-like mixture is passed into inlet 17 of the mixer
16. The mixer 16 provides the dual function of forwarding
the paste toward the pump 14 as well as mix into the
paste any solids which are returned through line 32.
In the event that the paste consistency is thicker than
desired, the mixer 16 can be employed to optionally
dilute the paste with water to provide a more flowable
stream. The pump 19 beneficially forwards the paste
through the line 21 into the s~eam mixing and shearing
device 22. Such a mixing and shearing device is
schematically depicted in Figures 2 and 3. When paste
starts to flow, for example, through inlet 56, steam is
introduced through the opening 53 and controlled by the
valve 52. Pressure within the pipe section S9 is
controlled in part by the appropriate opening and
closing of the valve 62 and adjustment of the tube 69
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until the desired crumb is obtain~d. Such a crumb
con~ains a plurality of macro particles of which at
least 90 weight percent are retained on an 80 mesh U.S.
Sieve size screen having 0.177 mm sieve openings, 0.119
mm diameter wire, and 34 mesh/cm. The resultant slurry
preferably at a temperature below the softening temperature
of the latex polymer passes from the mixer 50 through
opening 66 into mechanical dewatering device such as
device 26. A suitable dewatering device is a so-called
expeller or expressing apparatus which basically is a
screw extruder having longitudinal slots formed in the
barrel thereof of width sufficient to permit water or
like liquids to flow therethrough and yet sufficiently
narrow to prevent solids from passing through. Roller
mills and like expressing apparatus are also suitable
and may be u~ed alone or in combination. The solids
material discharged from the expeller is ground to a
desirable size, collected if necessary by a collector
such as hopper 40 and stored for future use.
For many applications it is not necessary to
remove all of the water. Typically the water content
of the ma~erial emerging from the mechanical dewatering
device such as device 26 is about 10 to 20 weight
percent. Also, it is generally desirable to prepare as
large a batch charge of latex as possible since individual
adjustment o~ the apparatus is usually necessary for
each batch.
To illustrate further the present invention,
co~eniently, mixer such as is depicted in Figure 2 for
a throughput sli~htly in excess of 2,000 pounds (908
kg) o~ latex per hour employs as pipe 59 three-inch
(7.6 cm)diameter stainless steel Schedule 40 pipe. A
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tube, such as tube 69, is about one inch (2.54 cm~ in
diameter. The conduit 61 is a stainless steel reducer
from three-inch (7.6 cm) to two-inch (5.1 cm) pipe.
The valve 62 is a nominal t~o-inch (5.1 cm) pipe size
and the disentrainment chamber 64 is about eighteen
inches (45.7 cm) in diameter, and operated at about
atmospheric pressure.
Example 1
A plurality of latex batches were prepared
containing 41 weight percent styrene, 20 weight percent
acrylonitril~ and 39 percent butadiene based on latex
solids. The latex particle size was about 1600 angstroms
(0.16 microns) in diameter and the latexes were about
31 weight percent solids.
The range of operating conditions for the
shear coagulator and average values for about fifty
batches o~ latex are set forth in Table I.
TABLE I
OPERATING CON~ITIONS
MEAN
PARAMETER UNITS VALUE RANGE
Latex Feed ~ate lb/hr 2350 1200-4400
[kg/hr)(1067)(545-1998)
Coagulation Temp. C 65 ~3-60
25 Hydroset Backpressure psig 60 55-75
(kPa) (415) (380-520)
Hydroset Temp. C 119 86-148
~xpeller Output lb/hr 727 250-850
Pressed Cake (kg/hr)(330) (113-386)
30 Outlet Moisture wt % 19 10-20
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Example 2
Another series of latexes were prepared
containing 46 weight percent butadiene, 17 percent
acrylonitrile and 37 percent styrene by preparing a
butadiene latex and gxafting thereon styrene-acrylonitrile
to provide latexes having about 37 weight percent
solids and a particle size of about 1400 angstroms
(0.14 microns). The latex was coagulated at 37 percent
solids and diluted in the mixing and forwarding apparatus
to about 26 to 32 percent solids in order to provide a
more flowable paste. The range of operating conditions
and ~he mean values are set forth in Table II.
TABLE II
OPERATING CONDITIONS
MEAN
PARAMETER UNITS VALUE RANGE
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Latex Feed Rate* Ib/hr 2210 900-3700
(kg/hr~(1003)(409-1680)
Coagulation Temp. C 46 38-66
20 Hydroset Backpressure psig 35 10-80
(kPa) (240) (70-550)
Hydroset Temp. C 95 70-115
Expeller Output lb/hr 500 280-580
Pressed Cake (kg/hr)(227) (127-263)
25 Outlet Moisture wt % 11.8% 9-14
*Coagulated paste diluted to approximately 26 to 32%
solids by weight prior to mixing and hdyrosetting.
In a manner similar to the foregoing, other
synthetic resinous thermoplastic latexes are readily
coagulated and dewatered.
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