Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR CONTINUOUSLY PREPARING
CROSSLINKED, SOLUTION-CAST POLYMER FILM
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent
Application
Serial No. 60/728,138 filed October 19, 2005, is hereby claimed.
BACKGROUND
Field of the Disclosure
[0002] The disclosure relates generally to a method and apparatus for
continuous
solution casting of a polymer to create a film. More particularly, the
disclosure relates
to a method and apparatus for injecting a secondary agent, such as a cross-
linking
agent, into a stream of polymer solution just upstream of a casting die, for
continuously producing a polymer film, such as polyvinyl alcohol, that is
crosslinked.
Brief Description of Related Technology
[0003] Though the general technology for producing plastic materials has been
used for decades, solvent-film casting is attracting increasing interest. One
of the
reasons is that specific requirements in the fields of water-soluble packaging
and other
related applications can only be met by this technology.
[0004] The development of a continuous process to manufacture thin plastic
films
was closely linked to the emerging photographic industry starting from the end
of the
19th Century. In those times, no other technology was available for industrial
film
forming, and polymer science was also still in its infancy. Two different
technologies
were soon developed: (1) casting on wheels or large drums; and (2) casting
onto
endless flexible metal belts. Surprisingly, both are still in use today,
together with a
third technology, casting onto moving plastic films. However, since the
development
of extrusion technologies for the production of thermoplastic polymer films,
the
importance of solvent casting methods has declined. Today, solvent casting is
a
specific manufacturing method which is used for niche markets and films with
specific and high quality requirements.
[0005] Typical solvent casting systems utilize an organic solvent such as
acetone,
aniline, dimethyl sulfoxide (DMSO), benzene, dimethyl formamide (DMF), methyl
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ethyl ketone (MEK), ethyl acetate, ethylene dichloride, toluene,
tetrahydrofuran, and
the like. Such solvents usually necessitate a complex solvent vapor recovery
and
rehabilitation system. Further, human and environmental exposure to these
solvents
is most undesirable. Use of water as the primary solvent can overcome these
disadvantages. No recovery and rehabilitation system is therefore necessary,
and
environmental and human exposure is not an issue.
[0006] There are many other processes for the formation of films, including
calendering, extrusion, plastisol cast systems, and organosol cast systems.
Extrusion
and calendering are processes which melt the polymer and shape the plastic
prior to
freezing. Plastisol and organosol casting processes involve the melting af the
polymer in a plasticizer matrix, after which the solvent action of the
plasticizer forms
a film.
[0007] In prior methods and apparatus, the solution that is eventually cast
onto a
moving surface, containing the base polymer and secondary components such as
plasticizers, fillers, surfactants, actives, and colorants, is prepared by
combining the
base polymer and secondary components with water in a tank and then mixing.
The
homogeneous solution or suspension is then pumped through one or more
operations
including de-aeration and filtering and then fed to a solution casting die for
casting
onto the moving surface, such as a traveling belt.
[0008] Polyvinyl alcohol (PVOH) membranes have been proven useful in
desalination, separation of organic solvents and phenols from water, ion
exchange, as
battery separators, and in biomedical applications. PVOH is a good candidate
for a
membrane because it is easily processable, exhibits high mechanical stability,
and is
non-toxic. Since PVOH is highly hydrophilic, unmodified membranes become
highly
swollen in water. Previously, PVOH membranes have been modified by various
methods, such as batch chemical methods, irradiation, and heat-treatment. On
the
macro scale, thickness, pore structure and crosslinking type of the PVOH
membrane
are found to influence solvent transport.
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SUMMARY
[0009] One aspect of the disclosure provides a method for continuously
preparing a
solvent cast film having a secondary component, including continuously
providing a
pressurized stream of polymer solution, combining a fluid stream including a
crosslinking agent with the pressurized stream of polymer solution,, mixing
the
combination of polymer solution and fluid stream in-line, continuously
applying the
resulting homogeneous mixture of polymer solution and crosslinking agent to a
moving surface, and then evaporating solvent from the mixture.
[0010] Another aspect of the disclosure provides an improved method of casting
a
polymer solution including a crosslinking agent onto a substrate for
evaporating off a
solvent and forming a crosslinked film, the improvement including continuously
injecting the crosslinking agent into a stream of polymer solution, mixing the
resulting stream of polymer solution with the crosslinking agent in-line, and
then
casting the resulting polymer solution onto a moving substrate to continuously
produce crosslinked film.
[0011] Further aspects and advantages will be apparent to those of ordinary
skill in
the art from a review of the following detailed description, taken in
conjunction with
the drawings. While the method, system, and improvement are susceptible of
embodiments in various forms, the description hereafter includes specific
embodiments with the understanding that the disclosure is illustrative, and is
not
intended to limit the invention to the specific embodiments described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] For further facilitating the understanding of the present invention,
five
drawing figures are appended hereto, wherein:
[0013] Figure 1 shows and example of a system for solvent casting according to
the
disclosure;
[0014] Figure 2 shows an embodiment of an adjustable sheeting die for casting
polymer solution;
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[0015] Figure 3 shows an example of the relationship between drums and a band
in
a basic band casting machine;
[0016] Figures 4 (elevation view) and 5 (plan view) show an embodiment of an
injection system and associated feed and in-line mixing components.
DETAILED DESCRIPTION
[0017] The invention generally relates to a method and apparatus for solution
casting to continuously form a polymer film that includes one or more
secondary
components.
[0018] The solution cast process offers several unique features which
conventional
fusion processes lack. In solvent casting, film formation depends upon
solubility, not
melting. Tlius, a wide range of polymeric alloys can be produced by solvent
casting.
Because the flowability to form a film is provided by the solvent, a pure
resin tihn can
be manufactured without adulteration by heat, stabilizers, plasticizers, or
lubricants.
Only additives which are beneficial to the finished product need to be
incorporated
with the polymer.
[0019] Solvent casting can provide a film which has excellent dimensional
stability
as well as reduction in or freedom from pinholes, gels and other
imperfections. Due
to the very low heat history which is inherent in a film produced by solvent
casting
processing, the process can also provide an extended service life to the film.
[0020] The method generally involves the steps of continuously pumping a feed
of
polymer solution towards a casting surface, continuously combining one or more
secondary components with the feed of polymer solution, mixing the
combination,
and then depositing the combined solution onto the casting surface.
Preferably, the
secondary component includes a crosslinking agent.
[0021] Prior methods and systems for producing crosslinked films, particularly
crosslinked PVOH, were performed batchwise. In the method described herein,
one
or more crosslinking agents are continuously injected into a stream including
the base
polymer in solution prior to film formation and crosslinking. Among the
benefits
which can be achieved by various embodiments of the method and system is the
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benefit of flexibility and efficiency in creating different types of film by
more
efficiently changing the film formulation and preparing the system for
production of a
new formulation, in continuous production of crosslinked film, and in
consistency of
product quality.
[0022] In one embodiment, the method involves continuously providing a
pressurized stream of polymer solution, continuously combining a fluid stream
including a secondary component with the pressurized feed of polymer solution,
homogeneously mixing the combination of polymer solution and fluid stream in-
line,
continuously applying the resulting homogeneous mixture of polymer solution
and
secondary component to a continuously moving surface, and then evaporating
solvent
from the mixture to form a polymeric film. When the secondary component
includes
a crosslinking agent, the crosslinking begins upon introduction of the
crosslinking
agent into the polymer solution, and may continue after film formation and
even after
the product film is removed from the casting surface and collected (e.g.,
wound onto a
roll).
[00231 The polymer solution is any substantially homogeneous mixture of a
polymer in a suitable solvent. The term "polymer solution" is used herein to
refer to
such a solution prior to continuous injection of a secondary component as
described
herein, except when stated otherwise. The disclosed method and system is
ideally
suited for a water-soluble polymer, such as polyvinyl alcohol (PVOH),
dissolved in
water. The water content of the PVOH solution is preferably within the range
of from
about 60% by weight to about 85% by weight. Suitable water-soluble materials
include, but are not limited to polymers, copolymers and derivatives thereof.
[0024] For example, the water-soluble material can include a polymer selected
from the following group, including water-soluble copolymers and other
derivatives
thereof, and mixtures thereof: polyvinyl alcohols, polyethylene oxides,
dextrans,
starches, cellulose derivatives (eg., hydroxyethyl cellulose, hydroxypropyl
cellulose,
and other cellulose ethers), polyvinylpyrrolidone, polyacrylamide, polyacrylic
acid,
polyacrylates, pectin, alginates, proteins and derivatized proteins (e.g.,
gelatin, corn
zein, whey protein).
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[0025] While other polymer solutions are suitable for use with the disclosed
system, the description of the embodiments herein is made with specific
reference to
the manufacture of crosslinked PVOH film.
[0026] If polyvinyl alcohol or a copolymer thereof is used, then the PVOH can
be
partially or fully hydrolyzed. Polyvinyl alcohol (PVOH) is a synthetic resin
generally
prepared by the alcoholysis, usually termed hydrolysis or saponification, of
polyvinyl
acetate.
[0027] Fully hydrolyzed PVOH, where virtually all the acetate groups have been
converted to alcohol groups (e.g., 98% or greater degree of hydrolysis), is a
strongly
hydrogen-bonded, highly crystalline polymer which dissolves only in hot water-
e.g., rapid dissolution at temperatures of about 60 C and greater.
[0028] If a sufficient number of acetate groups are allowed to remain after
the
hydrolysis of polyvinyl acetate, the PVOH polymer then being known as
partially
hydrolyzed, it is more weakly hydrogen-bonded and less crystalline and is
soluble in
cold water-e.g., rapid dissolution at temperatures of about 10 C and greater.
[0029] Both fully and partially hydrolyzed PVOH types are commonly referred to
as PVOH homopolymers although the partially hydrolyzed type is technically a
vinyl
alcohol-vinyl acetate copolymer.
[0030] Because there are so many chemically different types of products that
can
be made from water-soluble films, the polymer solutions must be formulated in
different ways. That is, a PVOH resin, crosslinking agent, plasticizer system
and
other ingredients can vary and can provide a range of films with different
product
characteristics, from a highly water-swellable hydrogel-type film to a rigid
membrane
film.
[0031] "Water soluble" refers to a film which, when exposed to water, begins
to
dissolve or disintegrate to its smallest components. Polyvinyl alcohol (PVOH)
is a
hydrophilic polymer and the plasticizers typically used in its manufacture
also have an
affinity for water. PVOH will absorb moisture from a wet atmosphere and give
up
moisture to a dry atmosphere. As moisture content increases (even with
humidity), a
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PVOH film will tend to quickly become softer and more elastic, losing tensile
properties and increasing in ultimate elongation. Also, the coefficient of
friction of a
PVOH film will increase with increased moisture content.
[0032] The polymer solution can consist of or consist essentially of
solvent(s) and
base polymer resin(s), with one or more crosslinking agents and optional
secondary
agents injected into a stream of the polymer solution before application to
the surface
on which the film is formed. In another embodiment, the polymer solution can
include common processing aids that would find utility in a wide variety of
formulations, such as plasticizers, lubricants, release agents, fillers,
extenders,
antiblocking agents, detackifying agents, antifoams and other functional or
decorative
ingredients, in amounts suitable for their intended purpose, and one or more
crosslinking agents are injected into a polymer solution feed stream and then
mixed,
before application to the surface on which the film is formed.
[0033] For PVOH as the water-soluble polymer, the crosslinking agent may be
any
chemical agent that can form chemical bonds with the hydroxyl groups of PVOH.
[0034] Such crosslinking agents include, but are not limited to, monoaldehydes
(e.g., formaldehydes, hydroxyacetaldehydes, and hydroxyadipaldehydes),
dialdehydes
(e.g., glyoxal, glutaraldehyde and succinic dialdehyde), aldehyde-containing
resins
(e.g., trimethylol melamine, melamine formaledehyde), polyfunctional
carboxylic
acids (e.g., dicarboxylic acids such as maleic, oxalic, malonic and succinic
acids),
citric acid, glycidyl and other difunctional methacrylates, N-lactam
carboxylates,
dithiols (e.g., m-benzodithiol), urea-formaldehyde and melamineformaldehyde,
dimethyl urea, di-isocyanates, boric acid and borates, salts of niultivalent
anions (e.g.,
ammonium zirconium carbonate), inorganic polyions (e.g., molybdate and
tungstate),
cupric salts and other Group lB salts, polyamide-epichlorohydrin resin
(polyazetidine
prepolymer), and combinations of any of the foregoing. In one embodiment of
the
method, a dialdehyde (e.g., glyoxal, glutaraldehyde, or both) is preferred.
[0035] Some crosslinking agents undergo direct condensation reactions with
hydroxyl groups to form covalent bonds (such as esterification and
acetalization
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reactions with carboxylic acids and aldehydes, respectively). Other
crosslinking
agents can have one or more of the following functionalities: those that form
complexes via labile polar covalent interactions, those that crosslink via
ionic
interactions, those that crosslink via hydrogen bonding interactions, and
combinatioiis
of such crosslinlcing agents. All such types of crosslinking agents are
contemplated
for use in the method described herein. An example of a water-soluble
polyamide-
epichlorohydrin is available under the trade name POLYCUP 172 by Hercules,
Inc. of
Wilmington, Delaware.
[0036] The foregoing crosslinking agents are particularly suitable for
polyvinyl
alcohol and many of the other water-soluble polymers described herein, but
other
crosslinking agents may be more appropriate or convenient for others of the
described
water-soluble polymers, and still other crosslinking agents may be more
appropriate
or convenient for other polymers which can be solution-cast and crosslinked.
For
example, alginates are very conveniently crosslinked by simple calcium salts.
Furthermore, as it is known in the art, various crosslinking agents are used
with a
catalyst, such as an acid catalyst with an aldehyde crosslinker.
[0037] The crosslinking agent preferably is present in an amount up to about
10 %
by weight, for example about 1 % to about 10 % by weight, or 5 % to about 10 %
by
weight, based on the weight of the water-soluble polymer. For example, water-
soluble polyamide-epichlorohydrin preferably is used in an amount of about 7-
10 %
by weight of a PVOH polymer. As another example, boric acid is preferably used
in
an amount of about 5 % by weight of a PVOH polymer.
[0038] Other secondary components can be colorants, such as those soluble in
the
polymer solution (e.g., an acid dye, direct dye, basic dye, other water-
soluble dye, or
any combination thereof) and/or those insoluble in the polymer solution.
[0039] In another embodiment, it is contemplated that the secondary component
will be an insoluble particulate. For example, a particulate can be used to
impart a
desired decorative appearance to the resulting film. Accordingly, particulates
that
have one or more properties such as coloration, reflectivity, fluorescence,
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translucence, opalescence, pearlescence, and the like, are suitable. Insoluble
pigments
are one type of particulate matter contemplated. The particulates can have any
morphology, including spherical, crystalline, irregular, and planar.
[0040] In another embodiment, it is contemplated that the secondary component
will be a soluble particulate. For example, a shaving or flake of colored
water-soluble
material (e.g., PVOH or gum arabic) can be used, and can be completely
dissolved by
the time of casting to provide the resulting film with a tinted or colored
film, or it can
be partially dissolved by the time of casting to provide the film with a non-
homogenous appearance. In another example, microcapsules can be used as a
secondary component. Thus, a microcapsule with a relatively soft shell (e.g.,
gelatin)
can be introduced and sheared in the mixing step to release color or another
agent
(e.g., scent, crosslinking agent) into the polymer mixture. The degree and
timing of
shear before casting can be used to control the characteristics of the
resulting fihn. A
color-containing microcapsule with a relatively rigid shell (e.g., gelatin
with a degree
of crosslinking) can be used to introduce color into the polymer solution and
provide
the resulting film with a non-homogeneous appearance. Water-soluble
microcapsules
or microspheres are preferably slurried in a non-aqueous carrier (e.g., a
glycol) prior
to injection into an aqueous polymer solution. -
[0041] In one embodiment, the particulates will have an average particle size
of 1
micron to 100 microns. In another embodiment, the particulates will have an
average
particle size of 4 microns to 25 microns. The solids content of the insoluble
particulate secondary component in a fluid injected into the polymer solution
preferably is in a range of about 3 1o by weight, based on the total weight of
the
polymer solution (wt.%) to about 10 wt.%. Other secondary agents can be
selected
from among plasticizers, lubricants, release agents, fillers, extenders,
antiblocking
agents, detackifying agents, antifoams and other functional or decorative
ingredients,
and combinations of any of the foregoing.
[0042] The fluid stream which includes the secondary component can take any
desired form, such as, but not limited to, a solution, a suspension, an
emulsion, a sol,
and a gel.
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[0043] The secondary component will typically be present in the fluid in a
much
greater concentration than will be desired in the ultimate film product,
resulting in a
relatively low flow rate of fluid injection. Accordingly, the fluid including
the
secondary component will typically be injected into the polymer solution
stream in a
relatively low ratio of secondary component to polymer solution. For example,
the
ratio can be about 1:10 to about 1:100 by volume of secondary component to
polymer
solution, such as with a water-soluble dye. For a crosslinking agent, the
ratio of
crosslinking agent to polymer solution can be about 0.3:10 to about 0.3:100,
for
example. In one type of embodiment, the flow rate of fluid containing the
secondary
component will be on the order of liters or tens of liters per hour whereas
the flow rate
of polymer solution is on the order of hundreds of liters per hour. For
example, the
flow rate of fluid containing a soluble dye as a secondary component can be in
a range
of about 0.5 gal./hr (2 Uhr) to about 5 gal./hr (191/hr) when the flow rate of
polymer
solution is about 100 gal/hr (3791/hr).
[0044] The fluid stream preferably has a sufficient viscosity such that its
volumetric flow rate can be accurately measured. In one embodiment, it is
contemplated that the viscosity of the fluid streain containing the secondary
component will be at least 30 cps at 185 F (85 C), for example about 70 cps
to about
80 cps at 185 F (85 C). It is contemplated that the fluid can include a
glycal, such
as propylene glycol, to adjust the viscosity to the desired range, for example
when the
secondary component is water-soluble, such as a water-soluble dye.
[0045] In contrast to the fluid containing the secondary component, the
polymer
solution will typically have a relatively high viscosity and solids content.
For
example, the polymer solution can have a solids content of at least about 20
wt.%, or
about 25 wt.% to about 40 wt.%. The viscosity can be, for example, at least
30,000
cps at 185 F (85 C), for example about 40,000 cps to about 50,000 cps at 185
F
(85 C).
[0046] In a die casting method, the pressure of the supplied polymer solution
will
typically be relatively high, such as at least 100 psi (0.7 MPa), or about 100
psi to
about 200 psi (about 0.7 MPa to about 1.4 MPa). In one embodiment of the
method
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and system described herein, the fluid containing the secondary component is
pressurized to exceed the polymer solution stream pressure at the point of
injection, in
order to accurately and reliably inject the secondary component into such a
pressurized polymer solution stream. The degree of overpressure is preferably
at least
120% (e.g., 120 psi (0.8 MPa) for a polymer solution pressure of 100 psi (0.7
MPa)).
[0047] The method can be performed by any suitable apparatus, such as a band
casting system, a particular embodiment of which will now be described in
connection with the figures. A preferred embodiment of a band casting system
includes a mixing system, a band casting machine comprising at least first and
second
rotating drums about which a continuous metal band is tensioned and travels
with the
rotation of the drums, a polymer applicator such as an adjustable sheeting die
or other
device used for applying the polymer solution from the tank to the metal band,
and a
drying chamber enclosing a least a portion of the metal band downline of the
sheeting
die.
[0048] The overall solvent casting system is generally referenced by the
number
"10." Other components are similarly and consistently numbered th.roughout the
specification and drawings. While some embodiments disclosed herein are
described
for use with a particular continuous'band casting machine, such as, for
example, those
designed and manufactured by Berndorf Belt Systems, Inc. of Carpentersville,
Illinois, other such band casting macliines are be capable of adaptation for
implementation of the described method and apparatus.
[0049] The general components of a system for solvent casting according to the
disclosure can be described with reference to Figure 1. The embodiment of a
solvent
band casting system 10 begins with a mixing system 12 for mixing and storing a
polymer solution. The mixing system 12 can be a single tank, or in a preferred
embodiment may comprise a plurality of tanks and attendant piping, pumps, and
valves to control the flow of the polymer solution among the tanks. Proximate
the
mixing system 12, a band casting machine 14 is shown including first and
second
rotating drums 16 and 18, respectively, about which a continuous loop of metal
band
20 is tensioned and travels with the rotation of the drums 16, 18. Between the
mixing
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system 12 and casting machine 14 is shown an injection system 82 (shown
schematically) coupled to a polymer solution feed line 13 by a conduit
junction 84.
Downstream of the junction 84 is an in-line mixer 86 (shown schematically in
Fig. 1
and in cut-away view as a static mixer in Fig. 4), disposed just prior to the
casting die
22. The conduit junction 84 can be a manifold, with a plurality of associated
injection
systems 82 for a plurality of secondary components or component mixtures, each
of
such components or component mixtures independently added to the polyrner
solution
feed. A plurality of secondary components can also be injected at various
points
along the polymer solution feed system (e.g., as measured by distance from the
applicator such as a casting die), due to potential effects on viscosity,
component
interactions, and targeted characteristics of the product film desired.
[0050] A coating device such as a casting die 22 (e.g., a sheeting die) is
used to
apply the polymer solution to the metal band 20 of the casting machine 14. A
feed
line 13 connects the mixing system 12 and injection system 82 to the die 22
and is
used to feed the polymer solution from the mixing system 12, through the
various
optional components and operations, and to the die 22. The die 22 (see Fig. 2)
comprises an internal chamber (not shown) and a slot-shaped orifice 11
extending
across the width of the die 22. The gap (e.g., determined by an adjustable
vertical
dimension) of the orifice 11 is variable across the width of the die 22 and is
used to
assist in controlling the thickness of the film produced by the casting system
10. The
gap can be monitored and/or adjusted.
[00511 A drying chamber 24 is shown enclosing a portion of the loop of metal
band
20 downline of the sheeting die 22. The drying chamber 24 of the embodiment
shown
comprises an upline zone 26 and a downline zone 28. Each zone 26, 28 includes
a
heater (burner) 30 located near an air inlet 32 and an exhaust blower 34
located near
an air outlet 38. The portion of the metal band 20 within the drying chamber
24 at
any given time, travels over and is supported by a series of support rollers
or idlers 40.
The embodiment shown in Figure 1 includes a series of idlers 40 representing
the
combination of idlers and associated sensors for monitoring rotation of the
idlers.
[0052]. Film is removed at the end drum (tail drum) 18.
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[0053] In the embodiment shown, the base polymer solution is first mixed in a
batch operation. The mixing takes place in the mixing system 12 (see Fig. 1).
In the
embodiment shown, the mixing system 12 includes a bulk handling station 44, a
mixer 46 having a mix tank 72, a hold tank 48 and a run tank 50. The bulk
handling
station 44 (shown schematically in Fig. 1) is used for holding at least the
polymer raw
ingredient for the desired solution, and may include other, secondary
components.
These ingredients can include various resins, polymers, plasticizers, and
other
additives. Accordingly, the bulk handling station 44 can include a number of
vessels
or tanks, each corresponding to one or more different ingredients. Each of the
tanks
or vessels is in flow communication with the mixer 46 for transporting the
desired
ingredients into the mix tank 72. Additionally, the various ingredients may be
manually fed into the mix tank 72.
[0054] The mixer 46 includes a jacketed mix tank 72. The mixer 46 also
includes
a mix motor 78, a mixer shaft 74 and a plurality of mixing blades 76. The
various
mixing blades 76 on the mix shaft 74 provide a combination of high shear
mixing and
vertical movement of the solution to promote mixing. The mix shaft 74 and
blades 76
are centrally located within the housing and are operably connected to the mix
motor
78. Preferably, the motor 78 is a powerful one of at least about 150
horsepower. A
suitable motor can be obtained from Morehouse-Cowles of Fullerton, California.
The
means of delivering the ingredients to the mix tank 72 and means of delivery
of the
solution can include conduits such as piping 80 and 13, respectively, between
source
and destination in combination with various pumps, as will be apparent to
those of
ordinary skill in the art.
[0055] The batch mixing process begins by filling or charging the mix tank 72
with
water and a variety of components that can include plasticizers, flatting
agents,
surfactants, and the like. These ingredients may need to be added at different
moments of the mixing process due to their potential affect on viscosity,
interactions,
and targeted characteristic of the product desired. The quantity of water can
affect
both the mixing process as well as the quality of the product produced.
Temperature
of the solution or suspension is maintained within a controlled range to
promote
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efficient dispersion of the resin. The polymer resin is then added under rapid
agitation effected by the mixing blades 76. Varying amounts of water may be
added
throughout the resin addition to assist in the mixing process.
[00561 After the resin has been added, the tank temperature set point is
adjusted to
accelerate dissolution of the resin. As the resin dissolves, viscosity will
typically
increase, making it necessary to increase the speed of the mix motor 78 to
maintain
adequate solution movement without causing damage to the solution or mix. tank
72.
[0057] The amount of time required to produce a batch of mixed solution
depends
on the size of the batch and the type oÃresin. The mixed batch of polymer
solution is
then pumped out of the mix tank 72 to a hold tank 48 or a run tank 50, but
typically to
a hold tank 48.
[0058] The hold tank 48 is typically used to hold the solution to allow
bubbles
(e.g., air bubbles) and other imperfections (such as gels or affects due to
temperature
variation) to rise to the top and be separated from the solution. This
preferably occurs
while the solution is undergoing mild agitation to maintain the solution.
Typically,
the hold tank 48 is maintained at a temperature of 185 F (85 C) through use
of a
water or steam jacket to prevent coagulation. Other heating methods are
acceptable.
An agitator or stirrer (not shown) may also help minimize coagulation of the
solution
and maintain uniform temperature throughout the tank. Both the temperature and
the
agitation preferably are monitored and controlled by the controller 36. A feed
line 13
runs from the hold tank 48 to the run tank 50, from where solution is
continuously
pumped to the die 22 for casting onto the band 20. One or more filters 47 may
be
placed between the hold tank 48 and the run tank 50, between the run tank 50
and the
junction 84, between the junction 84 and the die 22 or in a plurality of such
locations.
When the secondary component includes an insoluble particulate, then
preferably a
filter is not disposed between the junction 84 and the die 22.
[0059] An embodiment of the injection system and associated feed and mixing
components is shown in Figs. 4 (elevation view) and 5 (plan view). The
injection
system includes a reservoir tank 90 for holding a supply of fluid containing
secondary
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component. When the fluid containing the secondary component would tend to
become non-homogeneous in the reservoir tank 90 (e.g., pigment settling or
separation of an emulsion), then the tank can include an associated agitator
or mixer
(e.g., a stirrer, in-tank eductor, or any other suitable mixer; not
illustrated).
[0060] A positive displacement gear pump 92 and associated A/C motor 94 with a
variable frequency drive (not illustrated) feed the fluid from the tank 90 via
feed line
conduit 96. In embodiments wherein accurate control over the fluid injection
rate is
not required, other types of pumps can be used, such as a peristaltic pump. A
needle
valve 98 is disposed in the fluid path between the pump 92 and the junction 84
with
the polymer solution feed line conduit 13 to control the fluid pressure.
[0061] The illustrated injection system 82 also includes various optional
components. Fluid pressure is monitored with gauge 100. The embodiment of the
system shown includes a volumetric gear flow meter 102 and a check valve 104
disposed between the needle valve 98 and the junction 84.
[0062] With certain embodiments, it may be desirable to heat the fluid
containing
the secondary component before injection and combination with the polymer
solution.
Accordingly, the injection system 82 is shown with a heater 106 disposed
between the
pressure gauge 100 and the needle valve 98.
[0063] Polymer solution pressure at the point of injection is monitored by
gauge
110, and downstream pressure after the in-line mixer 86 is monitored by gauge
112.
The in-line mixer 86 preferably is a static mixer, and can be of any desired
length to
provide homogeneous mixing of the polymer solution and secondary component.
[0064] In a preferred control scheme, the pressure of the fluid supply is
adjusted by
manually adjusting the needle valve 98 to an amount of approximately 120% of
the
polymer solution pressure, and then a controller (e.g., a proportional,
integral,
derivative controller) is used to regulate the fluid flow rate to a desired
setpoint. For
example, a PID feedback loop can be established by monitoring fluid volumetric
flow
rate with the gear flow meter 102 and controlling the speed of the pump motor
94 to
achieve a desired fluid volumetric flow rate setpoint.
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[00651 In an alternative control scheme, the volumetric flow rate of the
polymer
stream can be measured upstream and downstream of the junction 84, and the
pump
motor 94 speed can be adjusted to achieve the desired difference between the
flow
rates.
[0066] The band casting machine 14 is further understood with reference to
Fig. 3.
. The casting machine 14 is comprised of a first or lead drum 16 and a second
or end
drum 18. Extending about lead drum 16 and end drum 18 is a continuous loop of
metal band 20. The drums 16 and 18 travel in the direction indicated by the
arrows,
imposing a similar revolution of the band 20. In a preferred embodiment, the
drums
are approximately 65 inches wide and 48 inches in diameter, and the band 20 is
approximately 61 inches wide with a circumference of approximately 325 feet. A
suitable band casting machine is available from Berndorf Belt Systems, Inc. of
Carpentersville, Illinois.
[0067] The first or lead drum 16 is preferably hollow to allow for pre-heating
the
band 20 prior to coating with or casting.the polymer solution. The second or
end
drum 18 is preferably cooled to assist removal of the final film product.
[0068] As shown in Fig. 3, the loop of metal band 20 has a production or upper
portion 21 and a return or under portion 23. The outer surface 25 of the band
is used
to support the applied polymer solution during drying. A plurality of idlers
40 (see
Fig. 1) may be spaced along the underside of upper portion of the band 20 to
provide
support of the band 20. The idlers 40 may also be monitored (e.g., by position
sensors
for monitoring rotation), to determine movement of the band 20. As the band 20
can
be a very expensive piece of equipment, any complications of production which
might
tend to damage the band 20, such as an idler that stops rotating (e.g.,
resulting in the
band being dragged across the idler or guiding the band off the edge of the
drums 16,
18) can be avoided by monitoring and taking appropriate control action.
[0069] For casting a PVOH solution, the band 20 will typically travel from a
temperature of about 125 F (52 C) at the lead drum 16 to a temperature of
about 215
F (102 C) at the end drum 18. These temperature changes can affect the
tracking of
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the band 20 on drums 16 and 18. As the dimensions of the band 20 change --
even
incrementally due to heating or cooling -- the band 20 can begin to run off
one end of
a drum. Accordingly, the band preferably is made of stainless steel to address
the
varying thermal gradient of the system existing between the lead drum 16 and
the end
drum 18. Other metals, alloys, plastics, or rubbers, having desired thermal
expansion
parameters may also be suitable for construction of a casting band 20.
[0070] The process of solvent casting occurs with application of a layer of
polymer
solution onto the band surface 25. This is accomplished by the use of polymer
solution applicator such as. a sheeting die 22 or other coating device. A
suitable die
22 is commercially available from Extrusion Dies Inc. of Chippewa, Falls,
Wisconsin
or Cloeren Incorporated of Orange, Texas. The sheeting die 22 coats (deposits)
a
continuous curtain of polymer solution across the width of the band 20. The
die 22
(see Fig. 2) includes an internal channel (not shown) through which the
solution
flows. At the end of the channel is a slot-shaped orifice 11 which extends
across the
width of the die 22. An upper surface of the slot is formed by a lip 53 and is
deformable with respect to a lower surface 55 of the slot to allow for changes
to be
made to the dimensions of the slot opening 11. A series of threaded bolts 52
across
the width of the die are used to vary the dimensions of the slot opening
depending
upon the direction of rotation of the bolts. Additionally, the bolts 52 may be
heated or
cooled to control the thickness of the slot 11. The controlled expansion and
contraction of the bolts can vary the dimensions of the slot 11. Some of the
parameters which affect the film quality and thickness can be addressed at the
die,
including the die gap, die pressure, and angle of incidence to the band
surface. Those
skilled in the art are readily able to make the proper adjustments to achieve
a desired
film quality and thickness. A sheeting die is the preferred embodiment,
however
other devices may be used to apply the polymer solution to the band surface.
[0071] The foregoing description is given for clearness of understanding only,
and
no unnecessary limitations should be understood therefrom, as modifications
within
the scope of the invention may be apparent to those having ordinary skill in
the art.
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[0072] Throughout the specification, where compositions are described as
including components or materials, it is contemplated that the compositions
can also
consist essentially of, or consist of, any combination of the recited
components or
materials, unless described otherwise.
[0073] The practice of a method disclosed herein, and individual steps
thereof, can
be performed manually and/or with the aid of electronic equipment. Although
processes have been described with reference to particular embodiments, a
person of
ordinary skill in the art will readily appreciate that other ways of
performing the acts
associated with the methods may be used. For example, the order of various of
the
steps may be changed without departing from the scope or spirit of the method.
In
addition, some of the individual steps can be combined, omitted, or further
subdivided
into additional steps.
