The present invention relates to a method for drying substrate.
BACKGROUND OF THE INVENTION
Drying coated substrates, such as webs, requires supplying energy
to the coating and then removing the evaporated liquid. The liquid to be evaporated
from the coating can be any liquid including solvents such as organic solvent systems
and inorganic systems which include water-based solvent systems. Convection, conduction,
radiation, and microwave energy are used to supply energy to coated webs. Applied
convection or forced gas flow is used to remove the evaporated liquid. Applied convection
is defined as convection produced by the input of power and caused intentionally.
It excludes convection caused merely by web movement, natural convection, and other,
unavoidable, forces. In some instances where the vapors are non-toxic, such as water
evaporation, the vapor is removed by flashing off into the ambient atmosphere.
In conventional drying technology, large volumes of gas, inert or
not, are required to remove evaporated liquid from the gas/liquid interface. These
dryers require large spaces between the coated web being dried and the top of the
drying enclosure to accommodate the large gas flows. Drying is governed at the gas/liquid
interface by diffusion, convection, boundary layer air from the moving web and impinging
air streams, vapor concentrations, and liquid to vapor change-of-state convection,
among other factors. These phenomena occur immediately above the coated web, typically
within 15 cm of the surface. Because conventional dryers have a large space above
the coated web, and they can only control the average velocity and temperature of
the bulk gas stream, they have limited ability to control these phenomena near the
For organic solvent systems, the vapor concentrations in these bulk
gas streams are kept low, typically 1-2%, to remain below the flammable limits for
the vapor/gas mixture. These large gas flows are intended to remove the evaporated
liquid from the process. The expense to enclose, heat, pressurize, and control these
gas flows is a major part of the dryer cost. It would be advantageous to eliminate
the need for these large gas flows.
These gas streams can be directed to condensation systems to separate
the vapors before exhausting, using large heat exchangers or chilled rolls with
wiping blades. These condensation systems are located relatively far from the coated
web in the bulk gas flow stream. Due to the low vapor concentration in this gas
stream, these systems are large, expensive, and must operate at low temperatures.
It would be advantageous to locate the condensation systems close
to the coated substrate where the vapor concentrations are high. However, conventional
heat exchangers would drain the condensed liquid by gravity back onto the coating
surface and affect product quality unless they were tilted or had a collection pan.
If they had a collection pan they would be isolated from the high concentration
web surface. If they were tilted dripping would probably still be a problem. Also,
conventional heat exchangers are not planar to follow the web path and control the
U.S. Patent No. 4,365,423 describes a drying system which uses a foraminous
surface above the web being dried to shield the coating from turbulence produced
by the large gas flows to prevent mottle. However, this system does not eliminate
applied convection, requires using secondary, low efficiency solvent recovery, and
has reduced drying rates. Also, because of the reduced drying rates, this patent
teaches using this shield for only 5-25% of the dryer length.
German offenlegungeschrift No. 4009797 describes a solvent recovery
system located within a drying enclosure to remove evaporated liquid. A chilled
roll with a scraping blade is placed above the web surface and removes the vapors
in liquid form. No applied convection removes the evaporated liquid. However, the
roll is only in the high vapor concentration near the surface for a short section
of the dryer length. This does not provide optimal control of the conditions at
the gas/liquid interface. In fact as the roll rotates it can create turbulence near
the web surface. Also, this system can not adapt its shape to the series of planar
surfaces of the coated web as it travels through the dryer. Therefore, the system
can not operate with a small, planar gap to control drying conditions and can not
achieve optimum condensing efficiency.
U.K. patent No. 1 401 041 describes a solvent recovery system that
operates without the large gas flows required for conventional drying by using heating
and condensing plates near the coated substrate. The solvent condenses on the condensing
plate and then the condensed liquid drains by gravity to a collection device. This
apparatus uses only gravity to remove the liquid from the condensing surface. Accordingly,
the condensing surface can not be located above the coated substrate since gravity
will carry the condensed liquid back onto the coated substrate. In the drawings
and discussion (page 3, lines 89-92) the condensing surface is described as vertical
or with the coated substrate, coated side facing down, above the condensing surface.
Applying a coating to the bottom side of the substrate or inverting the substrate
after application of the coating is not the preferred method in industry. Coating
in an inverted position and inverting a coated substrate before drying can create
coating defects. These limitations greatly reduce the flexibility of the method
and entail significant costs to adapt it to standard manufacturing methods. This
requirement for vertical or inverted drying is very likely the reason this method
has not been adopted or discussed in the industry.
U.K. patent No. 1 401 041 also describes, on page 2, line 126 to page
3 line 20, the problems of this method with growth of the liquid film layer on the
condensing surface and droplet formation. Because "the resulting liquid film 14
may increase in thickness towards the lower end of the condenser," the length of
the condensing surface is limited by the buildup and stability of this film layer.
Limiting the length of the condensing surface will limit the dryer length or require
exiting the drying system with the coating not dried. This has the undesirable effect
of losing some of the solvent vapors to the atmosphere, losing control of the drying
phenomena, and creating defects. Another limitation is that the distance of the
condensing surface from the coated substrate "can hardly fall below about 5 millimetres"
to prevent contacting the condensing liquid film with the substrate, and to prevent
droplets from contacting the substrate.
The limitations of this system to vertical or inverted drying, limits in the length
of the dryer, and the inability to operate at desired distances from the coated
substrate render it inadequate to achieve the desired drying benefits.
U.S. Patent No. 4,112,586 discloses a method for drying cardboard
or paper. The cardboard or paper is in web form and is dried by passing the wet
web, supported by a drying band, into contact with a heated drying surface. The
lower surface of the drying band opposite the web is in contact with a cooled surface.
The water in the web is evaporated and then condensed and captured in the drying
band. The water is subsequently removed from the drying band by mechanical force
(suction) after the drying band and the web are mechanically separated. During the
evaporation process, the drying band is in contact with the web. Direct contact
with the web can be undesirable when producing performance coatings with stringent
surface quality requirements.
There is a need for a method for drying coated substrates which provides
improved control of the conditions near the gas/liquid interface, which eliminates
the need for applied convection to transport the evaporated liquid, and which improves
the efficiency of the condensation vapor recovery systems.
SUMMARY OF THE INVENTION
There is also a need for a method that can operate with small gaps adjacent the
The present invention is specified in claim 1.
A film layer of condensate can be created on the condensing surface
to prevent formation of droplets of condensate and prevent bridging of the condensate
to the substrate.
The condensing surface can be spaced less than 5 mm from the substrate. In another
embodiment, the condensing surface can be located above the substrate.
In another embodiment the condensed liquid is transported toward a condensing surface
The condensed liquid can be removed, at least in part by using capillary
forces. Additionally, gravity may also assist in removing the condensate from the
condensing surface. For example, the condensing surface can be tilted to at least
one transverse side of the coated substrate. A plurality of condensing surfaces
could be used.
One could be a condensing platen located above and tilted to at least one transverse
side of the coated substrate, and others could be sheets, having upper and lower
surfaces. The sheets can be located below the condensing platen such that they are
slanted away from the horizontal with their lower edge facing the lower edge of
the condensing platen. The sheets can overlap each other and be spaced apart in
the overlap region.
The rate of drying can be controlled by controlling the height of
the gap and the temperature differential between the coated substrate and the condensing
The condensing surface can be formed on a stationary or rotating belt.
Alternatively, the condensing surface can be formed of flat or grooved plates of
any type, tubes, fins, or other shapes. The condensing surface can be formed of
a foraminous plate, which uses Young-Laplace surface tension forces to retain and
capillary forces to transport the condensate.
If the condensing surface causes the condensed liquid to flow longitudinally,
a collection system can be used to collect the liquid or structure on the condensing
surface can direct the liquid. Structure, such as ribs, on the condensing surface,
can limit the buildup of condensate and prevent the formation of droplets.
BRIEF DESCRIPTION OF THE DRAWINGS
- Figure 1 is a perspective view of the drying apparatus for carrying out the
- Figure 2 is an end view of the apparatus of Figure 1.
- Figure 3 is a cross-sectional view taken along line 3-3 of Figure 1.
- Figure 4 is a perspective view of the drying apparatus according to another
embodiment for carrying out the invention.
- Figure 5 is an end view of the apparatus of Figure 4.
- Figure 6 is a cross-sectional view of the drying apparatus according to another
embodiment for carrying out the invention.
- Figure 7 is a cross-sectional view of the drying apparatus according to another
embodiment for carrying out the invention.
- Figure 8 is a cross-sectional view of the drying apparatus according to another
embodiment for carrying out the invention.
- Figure 9 is a schematic side view of the drying apparatus according to another
embodiment for carrying out the invention.
- Figure 10 is a cross-sectional view of the drying apparatus according to another
embodiment for carrying out the invention.
- Figure 11 is a side view of the drying apparatus according to another embodiment
for carrying out the invention.
- Figure 12 is a schematic side view of an apparatus showing process variables.
The present invention provides a method for controlling the transport
of mass and energy and for drying coatings on a coated substrate, such as a moving
web, with a condensing surface creating a small, controlled-environment gap above
the coating surface.
Other physical and chemical phenomena that occur during the drying process, such
as chemical reactions, curing, and phase changes, which can also be affected by
In the embodiment of Figures 1, 2, and 3, drying (heating the liquid
to evaporate it to a vapor, transporting the vapor away from the web, condensing
the vapor, and transporting the condensed vapor (also known as condensate) away
from the web) occurs without requiring the applied gas convection associated with
conventional drying methods. This reduces mottle formation associated with many
precision coatings and enables drying at increased drying rates. In the embodiment
of Figures 4-12, at least the removal of the evaporated liquid from the web occurs
without requiring applied gas convection. All versions of this system attain improved
control of the phenomena occurring near the gas/liquid interface and attain high
liquid recovery efficiencies.
All versions use condensation to remove evaporated liquid in a gap
which can be substantially planar without requiring applied convection forces, and
where ambient and boundary layer convection forces are minimized. The drying system
has numerous advantages over the conventional drying technology by creating a small,
controlled-environment gap adjacent the coating surface, and by eliminating the
requirement for applied convection from the drying mechanism. In some products a
chemical reaction or other physical and chemical processes occur in the coating
during drying. The drying system functions whether or not these processes are proceeding
within the process. The drying system can affect these processes during drying.
One example is of moisture-cured polymers dispersed or dissolved in a solvent that
can be adversely affected during the drying process due to the presence of humidity
in the drying atmosphere. Because the invention can create a small, controlled environment
gap above the coating surface, it is substantially simpler to provide a controlled
humidity drying atmosphere to improve the curing of these polymers. By improving
control of the drying phenomena and creating a small, controlled environment gap
above the coated surface, there are many other applications where other physical
and chemical processes occurring during the drying process can benefit.
In an embodiment of the method the drying system can be combined with
applied convection, and the applied convection can be produced by forcing gas across
the coating, either longitudinally, transversely, or in any other direction. This
can provide additional mass transfer or other modification to the atmosphere above
the coated surface. This method could be used where applied convection is not a
detriment to product properties.
The inventors have found that in drying substrates, significant drying
improvements and increased drying rates occur when the distance from the condensing
surface to the coated substrate is below 5 millimeters. The system of U.K. patent
No. 1 401 041 is not practically operable in the range where significant drying
control improvements can be made.
Many kinds of condensing structures can be used, such as plates of
any type, whether flat or not, porous or not, structured or not, or other shapes
such as tubes or fins. The condensing surface structure can combine macro, meso,
and micro scale geometries and dimensions. The condensing structure can be parallel
to the web or angled with the web, and can have planar or curved surfaces.
The condensing surface must satisfy three criteria. First, it must
be capable of sufficient energy transfer to remove the latent heat of condensation.
Second, the condensate must at least partially wet the condensing surface. Third,
the condensing surface must prevent the condensed vapor (the condensate) from returning
to the coated surface of the web. Associated with a condensing surface is an effective
critical condensate film thickness which marks the onset of film nonuniformities.
This thickness is a function of the condensing surface material, geometry, dimensions,
topology, orientation, configuration, and other factors, as well as the physical
properties of the condensate (such as surface tension, density, and viscosity).
Another feature of the system is condensate transport and removal. This maintains
the condensate film thickness less than the effective critical thickness and can
be accomplished by capillary forces, gravitational forces, mechanical forces, or
various combinations of these forces.
Capillary force, or capillary pressure, can be described as the resultant
of surface tension acting in curved menisci and is governed by the fundamental equation
of capillarity known as the Young-LaPlace equation. The Young-LaPlace equation is
Δp = σ(1/R1 + 1/R2), where ΔP is the pressure
drop across the interface, σ is the surface tension, and R1 and
R2 are the principal radii of curvature of the interface.
Capillarity is discussed in detail in Adamson, A. W. "Physical Chemistry of Surfaces,
4th ed.11, John Wiley & Sons, Inc. (1982). Figures 1, 2, 4, 5, 8, 9, and 10
show examples of using capillary forces, along with other forces, to move the condensate
on the condensing surface.
Gravitational forces result from the position of the fluid mass in
a gravitational field, which is the hydrostatic head. Figures 6, 7, and 9 show examples
that use gravitational forces, along with other forces, to move the condensate on
the condensing surface.
Other mechanisms can be used in conjunction with capillary forces
to remove the condensed liquid from the condensing surface to prevent the condensed
liquid from returning to the substrate.
For example, mechanical devices, such as pumping systems, can be used to remove
the condensed liquid from the condensing surface. Figure 11 shows an example that
use mechanical forces, along with capillary forces, to remove the condensate from
the condensing surface.
Figures 1 and 2 show an apparatus using two platens. Figures 4 and
5 show an apparatus using one platen. In both versions, one platen has a condensing,
liquid-transport surface located a short distance from the coated surface of the
web. Distances of less than 15-20 cm are preferred. Distances less than 5 mm yield
more advantages. Distances less than 0.5 mm and even distances as low as 0.1 mm
and less are attainable.
In Figures 1 and 2, the apparatus 10 includes a condensing platen
12, which can be chilled, spaced from a heated platen 14. The condensing platen
12 is set to a temperature T1, which can be above or below ambient temperature,
and the heated platen 14 is set to a temperature T2, which can be above
or below ambient temperature. The coated web 16 temperature is T3. The
web position is defined by h1, and h2, the distances between
the respective facing surfaces of the web 16 and the condensing and heated platens.
Figure 12 shows the relative locations of these variables. The total gap between
the condensing platen and any heating platen, h, is the total of h1,
h2, and the coated web thickness. The web 16, having a coating 18, travels
at any speed between the two platens. Alternatively, the web can be stationary and
the entire apparatus 10 moves or both the web and apparatus move. The platens are
stationary within the apparatus. The heated platen 14 is located on the non-coated
side of the web 16, either in contact with the web or with a small gap h2
between the web and the platen. The condensing platen 12 is located on the coated
side of the web 16, with a small gap h1, between the web and the platen.
The condensing platen 12 and the heated platen 14 eliminate the requirement for
applied convection forces both above and below the web 16. Drying is controlled
by adjusting the temperatures T1, T2, and distances h1,
The condensing platen 12, which can be stationary or mobile, is placed near the
coated surface (such as 5 cm away, or closer). The arrangement of the platens creates
a small gap adjacent the coated web. The gap is substantially constant, which permits
small amounts of convergence or divergence. Also, the gap is substantially constant
notwithstanding any grooves (discussed below) on the condensing surface. The orientation
of the platens is not critical. The condensing platen 12 can be above the web (as
shown in Figures 1, 2, 4-8, and 11-12), below the web (with the coating on the bottom
surface of the web), and the system can operate with the web vertical or at any
other angle, including being tilted around the axis of the direction of web travel.
The heated platen 14 supplies energy without requiring applied convection
through the web 16 to the coating 18 to evaporate liquid from the coating 18 to
dry the coating. Energy is transferred by a combination of conduction, radiation,
and convection achieving high heat transfer rates. This evaporates the liquid in
the coating 18 on the web 16. The evaporated liquid from the coating 18 then is
transported (using diffusion and convection) across the gap h1, between
the web 16 and the condensing platen 12 and condenses on the bottom surface of the
condensing platen 12.
As shown in Figure 3, the bottom surface of the condensing platen
12 is the condensing surface 22 and has transverse open channels or grooves 24 which
use capillary forces to prevent the condensed liquid from returning to the coating
by gravity and to move the condensed liquid laterally to edge plates 26. The grooves
can be triangular, rectangular, circular, or other more complex shapes or combinations
The groove material, geometry, and dimensions are designed to accommodate the required
mass flow and the physical properties of the condensate, such as surface tension,
viscosity, and density.
A specific type of condensing surface is one, which has open channels
or grooves with corners. This type of capillary condensing surface, shown for example
in Figure 3, is a geometrically specific surface, which can be designed with the
aid of the Concus-Finn Inequality (Concus P. and Finn R. "On the Behavior of a Capillary
Surface in a Wedge," Proceeding of the National Academy of Science, vol. 63, 292-299
(1969) which is:
α + &thetas;s < 90°, where α is half the included angle
of any corner and &thetas;s is the gas/liquid/solid static contact angle.
The static contact angle is governed by the surface tension of the liquid for a
given surface material in gas. If the inequality is not satisfied, the interface
is bounded; if the inequality is satisfied, the interface does not have a finite
equilibrium position and the meniscus is unbounded. In this latter case, the liquid
will advance by capillarity indefinitely or to the end of the channel or groove.
Cornered grooved surfaces are helpful when the coating liquid has a high surface
tension, such as water. Capillary surfaces with corners are discussed in great detail
in Lopez de Ramos, A. L., "Capillary Enhanced Diffusion Of CO2 in Porous
Media," Ph.D. Dissertation, University of Tulsa (1993).
The grooves 24 also can be longitudinal or in any other direction.
If the grooves are in the longitudinal direction, a suitable collection system can
be placed at the ends of the grooves to prevent the condensed liquid from falling
back to the coated surface 18. This embodiment limits the length of a condensing
plate 12 and also limits the minimum gap h1.
When the liquid reaches the end of the grooves 24 it intersects with
the angle between the edge plates 26 and the condensing surface 22. A liquid meniscus
forms and creates a low pressure region which draws the condensate from the condensing
surface to at least one edge plate. Gravity overcomes the capillary force in the
meniscus and the liquid flows as a film or droplets 28 down the face of the edge
plates 26. The edge plates 26 can be used with any condensing surface, not just
one having grooves. The droplets 28 fall from each edge plate 26 and can be collected
in a collecting device (not shown). For example, a slotted pipe can be placed around
the bottom edge of each edge plate 26 to collect the liquid and direct it to a container.
The edge plates 26 are shown throughout the application as contacting the ends of
the condensing surface of the condensing platens. However, the edge plates can be
adjacent the condensing platens without contacting them as long as they are functionally
close enough to receive the condensed liquid.
Alternatively, the condensed liquid need not be removed from the platen
at all, as long as it is removed from the condensing surface 22, or at least prevented
from returning to the web 16. Also, the edge plates 26 are shown as perpendicular
to the condensing surface 14, although they can be at other angles with it, and
the edge plates 26 can be smooth, grooved, porous, or other materials.
The heated platen 14 and the condensing platen 12 can include internal
passageways, such as channels. A heat transfer fluid can be heated by an external
heating system and circulated through the passageways to set the temperature T2
of the heated platen 14. The same or a different heat transfer fluid can be cooled
by an external chiller and circulated through the passageways to set the temperature
T1 of the condensing platen 12. Other mechanisms for heating the platen
14 and cooling the platen 12 can be used.
The apparatus 30 of Figures 4 and 5 is similar to that of Figures
1-3 except there is no heating platen. In the apparatus 30, the web 16 is heated
to evaporate the liquid from the coating by any heating method or combination of
heating methods, whether conduction, radiation, microwave, convection, or ambient
energy, using any type of heater. This can include but is not limited to a heated
drum, radiant heating devices, or forced gas flows. This system can even operate
without any applied energy, even outside the dryer, using only ambient energy to
evaporate the liquid. The apparatus otherwise operates the same as that of Figures
1-3, without requiring applied convection for transport of the evaporated liquid
from the web 16 to the condensing surface 22 on the condensing platen 12. The gap
h, between the coated web 16 and the condensing surface 22 is isolated from the
heating devices by any combination of the web 16 and web supports or other barriers.
This can isolate the area from any applied convection.
Capillarity can be combined with gravity. Figures 6 and 7 show embodiments
of the apparatus where gravity is used in conjunction with capillarity to move the
liquid solvent on the condensing surface. The condensing surface 22 is on a plate
42 which is tilted to one transverse side of the web 16 in Figure 6 and the condensing
surface 22 is on one or two plates 44 which are tilted from the center to both transverse
sides of the web 16 in Figure 7. In both cases capillarity and gravity are used
to move the liquid away from the condensing surface. The angle could be centered
on the longitudinal centerline of the web or it can be off-center.
Figure 8 is another embodiment where capillary forces move the liquid
on the condensing surface.
In this embodiment the condensing plate 46 is a porous or wicking material, such
as sintered metal or sponge, which uses capillary forces to transport the liquid
solvent. The solvent condenses on the condensing surface 22 and is distributed throughout
the condensing plate 46 due to capillary forces. The edge plates 26 adjacent the
condensing plate 46 form a capillary surface. A liquid meniscus forms and creates
a low pressure region which draws the condensate from the condensing surface to
at least one edge plate. Gravity overcomes the capillary force and the liquid flows
as a film or droplets down the surface of the edge plate 26.
Figure 9 shows another embodiment where capillary and gravity forces
are used to transport the condensed liquid on the condensing surfaces 22. As shown,
condensing surfaces 22 are formed on many surfaces. A condensing platen 50 is tilted
to one side or from the center to both sides above the web 16. Thin sheets 48 of
material are suspended below the condensing platen 50 and located such that they
are slanted away from the horizontal with their lower edge facing the lower edge
of the condensing platen 50. As shown, the sheets 48 of material overlap by at least
0.05 cm and are spaced apart in the overlap region by a 0.01-0.25 cm slot.
Vapor that condenses on the condensing surfaces 22 will be retained on the surfaces
by surface tension.
Gravity carries the condensed liquid down each upper surface of the sheets 48 in
a cascade effect until the liquid is beyond the edge of the web 16. Liquid that
is condensed on the lower surface of the thin sheets 48 will transport to the overlap
region and capillary forces created by the slot will draw the liquid into the slot.
The liquid will then be transferred to the upper surface of the next sheet 48 and
gravity will carry it in a cascade manner to the edge of the substrate. Thus, liquid
condensing on the lower surface of the sheets will not form droplets that fall back
to the coated substrate. In some cases it is desirable for the liquid to completely
fill the slot between the sheets 48 and the condensing platen 50.
Figure 10 is another embodiment which can combine gravity and capillary
forces to transport the liquid on the condensing surface. In this embodiment a porous,
slotted, sponge, honeycomb, screened, or otherwise foraminous material 52 is attached
to and located below a condensing platen 54. The spacing between the condensing
platen 54 and the foraminous material 52, the dimensions of the foramina in the
material 52, and the ratio of open area to solid area on the foraminous material
52 are all designed to cause the surface tension forces to retain the liquid on
the three condensing surfaces 22. The apparatus is located adjacent to the web 16.
Vapor condensing on the condensing surfaces 22 will be retained as liquid in the
voids of the foraminous material and in the plate spacing region 56. As liquid is
removed from the plate spacing region 56, liquid on the side of the foraminous material
52 facing the web 16 will be transported by capillary forces to fill the void in
the plate spacing region 56. Liquid can be removed from the plate spacing region
56 either by gravity, capillary, or mechanical forces. By sloping the condensing
platen 54 away from the horizontal in any direction, gravitational forces will remove
liquid from the plate spacing region 56 to a point beyond the edge of the web 16.
Alternatively, the liquid can be removed from the plate spacing region 56 by positioning
at least one edge plate 26 at the edge of the condensing platen 54.
The edge plate 26 contacts the condensing platen 54 to form a capillary surface.
The edge plates can, in some uses, contact the foraminous material 22. A liquid
meniscus forms and creates a low pressure region which draws the condensate toward
at least one edge plate.
Gravity overcomes the capillary force and the liquid flows as a film or droplets
down the surface of the edge plate 26. Also, the condensate can be mechanically
pumped out of the plate spacing region 56.
Figure 11 schematically shows an embodiment, which uses a pump 80
to remove the condensed liquid from the condensing surface. The pump can be any
type of pump, and any other device for creating negative pressure can be used. As
also shown in Figure 11, the condensed liquid can be driven toward the transverse
center of the condensing surface before removal, such as by capillarity and gravity.
In another use, the system can first remove fluid from a coated substrate.
Then, the system, at a downweb location from the drying location, can be used "in
reverse" to add some small portion of moisture or additional reactant to the substrate
to modify the coating.
The apparatus can operate outside of a dryer configuration without
any applied energy, and with only ambient heat to evaporate the liquid. By controlling
the temperature of the condensing surface 22 to be at or near the ambient temperature,
the liquid evaporation will only occur until the vapor concentration in the gap
h, between the condensing surface and the web 16 is at the saturated concentration
as defined by the condensing surface 22 and web 16 temperatures. The liquid that
has evaporated will be contained and carried by the viscous drag of the web through
the gap h, to the exit of the system. Undesirable drying can be reduced and vapor
emissions can be isolated from ambient conditions.
The drying method of the invention can be used to reduce or virtually
stop the drying of the coating.
The rate of drying is a function of the gap height and vapor concentration gradient
between the coated surface 18 of the web 16 and the condensing surface 22.
For a given gap h1, the temperature differential between the web 16 and the condensing
surface 22 defines the vapor concentration gradient. The higher the coated surface
18 temperature relative to the condensing surface 22, the greater the rate of drying.
As the temperature of the condensing surface 22 approaches the coated surface 18
temperature, the drying rate will tend to zero. In conventional drying the vapor
concentration gradient cannot be controlled without using an expensive inert gas
Some liquid coatings have multiple solvents where one or more of the solvents function
to slow down the rate of drying for optimum product properties. By adjusting the
coated surface 18 and condensing surface 22 temperatures, the invention can reduce
the drying rate and possibly eliminate the requirement of using solvents to retard
the drying rate.
The rate of drying is controlled by the height of the gap h1,
and the temperature differential between the coated surface 18 and the condensing
Therefore for a given temperature differential, the rate of drying can be controlled
by the position of the condensing plate which defines the gap h1. Thus
by changing the dimensions of the drying system, such as by changing the relative
gaps, it is possible to control the rate of drying. Conventional dryers do not have
Drying some coated webs using applied convection can create mottle
patterns in the coatings. Mottle patterns are defects in film coatings that are
formed by vapor concentration or gas velocity gradients above the coating, which
cause non-uniform drying at the liquid surface. Normal room air currents are often
sufficient to create these defects. The invention can be used to reduce and control
natural convection induced defects, such as mottle, at locations outside the desired
drying position. In locations where the coated surface is not in the drying region
and would otherwise be exposed to convection from either ambient air currents or
from a turbulent boundary layer air due to web movement, the apparatus, with grooves
or other liquid transport and removal features, devices, structures or without,
can be located adjacent to the coated web 16 separated by a gap h1. The location
of the condensing plate 12 adjacent the coated web 16 can isolate the ambient air
currents from the coating surface. It can also prevent the boundary layer air above
the coated surface from becoming turbulent.
Accordingly, defects due to convection outside the drying position, such as mottle,
can be reduced or eliminated. The apparatus can be operated with condensation and
solvent removal similar to Figures 4-12, or it can even operate without condensation
and solvent removal by raising the condensing surface 22 temperature above the dew
point of the vapors in the gap h1.
In all embodiments it may be desirable to provide multiple zones of
heating and condensing components using multiple pairs. The temperatures and gaps
of each pair of heating and condensing components can be controlled independently
of the other pairs. The zones can be spaced from each other or not.
The systems of all of the embodiments use condensation close to the
coated web 16 with a small gap between the coating on the web 16 and the condensing
surface 22. There is no requirement for applied convection and there is very little
vapor volume. The vapor concentration and convection forces can be controlled by
adjusting the web temperature, the gap, and the condensing surface temperature.
This provides improved control of the conditions near the gas/liquid interface.
Because the plate temperatures and gap can be continuous and constant throughout
the drying system, heat and mass transfer rates are more uniformly controlled than
with conventional drying systems. All of these factors contribute to improved drying
performance. It also improves the efficiency of the condensation vapor recovery
systems, providing for liquid recovery at high efficiencies at no additional cost
compared to known expensive methods of burning, adsorption, or condensation in a
secondary gas stream.
Also, there is less of a concern about the ambient air above the web
exploding or being above the flammability limit. In fact, where the gap is very
small, such as less than 1 cm, flammability concerns may be eliminated because the
entire space above the web has insufficient oxygen to support flammability.
Additionally, this system eliminates the need for large gas flows. The mechanical
equipment and control system is only 20% of the cost of a conventional air flotation
Experiments were conducted with 30.5 cm wide platens having transverse
grooves. The bottom platen was heated to temperatures in the range of 15°C through
190°C with a heat transfer fluid circulated through passageways in the platens.
As the heat is transferred to the coating, the liquid in the coating evaporates.
The temperature of the condensing platen was controlled by any suitable method
in the range of -10°C through 65°C to provide the driving force for vapor transport
and condensation. An effective range of the gap h1, is 0.15-5 cm. Mottle-free
coatings were obtained.
In one example, a mottle-prone polymer/MEK solution at 11.5% solids, 2 centipoise,
7.6 micron wet thickness, and 20.3 cm wide was coated. The web was 21.6 cm wide
and traveled at a speed of 0.635 m/s. The temperature of the heated platen used
to heat the web was controlled at 82°C. The condensing platen temperature was controlled
at 27°C. The overall length of the platens was 1.68 m and they were mounted at a
3.4° angle from horizontal with the inlet side at a lower elevation. The inlet to
the platens was located 76 cm from the coating application point. The heated platen
was separated from the web by a gap of approximately 0.076 cm. The gap h1,
was set at 0.32 cm.
The capillary grooves were 0.0381 cm deep with a .076 cm peak-to-peak distance,
an angle α of 30°, and 0.013 cm land at the top of the grooves. The web was
dried mottle-free in the 1.68 m length of the platens although there was some residual
solvent in the coating when it left the platens. A conventional dryer would require
approximately 9 m to reach the same drying point, requiring the dryer to be more
than five times larger.
Other applications for this system include drying adhesives where
blister defects are common. Blister defects may be caused by the coating surface
forming a dried skin before the rest of the coating has dried, trapping solvent
below this skin. With conventional drying, the solvent vapor concentration in the
bulk gas is very low because of flammability limits. If too much heat is applied
to the coating, the solvent at the surface will flash very quickly into the low
vapor concentration gas stream and will form the skin on the surface. The system
of this invention creates a controlled vapor concentration in the space above the
web which can reduce the tendency to form a skin on the surface. Other applications
are in areas where dryers are run at high solvent concentrations to obtain specific
The system provides advantages beyond solvent recovery and drying
performance. Another advantage involves a simplified process for subjecting the
coating fluid to a magnetic field. Rather than positioning a magnetic field generator
within a known dryer, with the present invention the magnetic field generator can
be positioned outside of the dryer (i.e., outside of apparatus 10, 30). This is
enabled by the compact nature of the apparatus. This is especially suitable when
coating a metal particulate-loaded fluid onto a substrate to make such products
as video and audio recording tape, computer and data storage tape, computer diskettes,
and the like. Being outside of the apparatus, the magnetic field generators are
easily adjustable and maintained.
This setup also improves particle orientation.
Magnetic output is improved if the particles are physically oriented in the direction
Conventionally the orienting device is contained within the dryer and the particles
are oriented at a single point or multiple points as the solvent is removed.
One advantage here is that because the magnetic orienting device is outside of
the dryer and is nonintrusive (conventional orienting devices inside the dryer disrupt
the convection heat and mass transfer), it will not affect the solvent removal rates
in any way. This allows uniform solvent removal. The magnetic particles are easily
oriented when the fluid is less viscous at the early stages of drying with this
invention. As the particles leave a conventional orienting device in the early stages
of drying, any components of the magnetic field which are not in the plane of the
coating will reorient the particles in a nonpreferred direction, such as tipping
them vertically. As the solvent is removed, the viscosity increases, making it difficult
for the orienting device to rotate the particles. The particles will not be reoriented
when leaving the field or by interparticle forces.
Another advantage is that because of its small size and increased
solvent removal rates, the invention allows orienting particles at the beginning
of the dryer and orienting device. The uniform field holds the particles in the
preferred direction as the solvent is removed in a uniform drying environment to
such a level that the viscosity is increased to the point that the viscous forces
dominate. This prevents undesirable particle disorientation as it leaves the orienting
device or from interparticle forces. Drying in conventional dryers causes the surface
of the product to roughen. Removing the solvent in the controlled environment of
the dryer of this invention appears to create smoother surfaces at elevated solvent
removal rates. This also improves magnetic output as, for example, the resulting
tape will ride closer to the recording head.