Abstrict Desiccant-coated substrates, regeneratable rotary dehumidification
wheels and other devices for gas (e.g., air) treatment using those
substrates, and processes for making them are disclosed. The substrates
may have coatings in thicknesses of from about 2 to about 10 mils
containing particles of one or more adsorbent desiccants and an
organic water-based binder. The desiccant particles retain a high
fraction of their original adsorption capacity because the pores
of the desiccant particles contain a pore-clearing agent prior to
the binder setting and the pore-clearing agent leaves the pores
during the manufacturing process to prevent the binder from blocking
the pores. In preferred embodiments a mixture of different desiccants
is used and a particle suspending agent keeps the particles well-mixed
so that the desiccant particles in the coated substrate will be
as well-mixed as possible. The suspending agent may also function
as the pore-clearing agent. A pH-adjusting agent may be used to
control the pH of the coating if the binder or another constituent
of the coating is pH-sensitive.
Claims I claim:
1. A process for making a desiccant-coated substrate capable of
being used at temperatures over 150 degrees Fahrenheit, the desiccant
being in the form of particles and the particles having pores and
being adhered to the substrate by a binder, the coated substrate
being sufficiently flexible and the coating having sufficient adherence
to the substrate so that the coated substrate can be formed into
corrugated shapes, the desiccant particles in the coated substrate
having at least 60% of their original adsorption capacity and the
binder having good breathability; said process comprising the steps:
(a) forming an aqueous suspension comprising particles of one or
more desiccants, a water-based organic binder, a suspending agent
to help maintain the desiccant particles in suspension, and an organic
pore-clearing agent at least some of which enters at least some
of the pores of the desiccant particles;
(b) depositing the suspension on the substrate; and
(c) causing the binder of the deposited suspension to set so that
the deposited desiccant particles adhere to the substrate and causing
at least some of the pore-clearing agent to leave the pores of the
desiccant particles to prevent the binder from occluding at least
some of the pores of the adhered desiccant particles, thereby to
form a desiccant-coated substrate capable of being used at temperatures
over 150 degrees Fahrenheit and of sufficient flexibility and having
a coating having sufficient adherence to the substrate so that the
desiccant-coated substrate can be formed into corrugated shapes
and in which the desiccant particles in the coated substrate have
at least 60% of their original adsorption capacity and in which
the binder has good breathability.
2. The process of claim 1 wherein the suspending agent is also
the pore-clearing agent.
3. The process of claim 1 wherein the suspending agent is also
the pore-clearing agent and is N-methyl-2-pyrrolidone.
4. The process of claim 1 wherein the binder is a polyurethane
binder.
5. The process of claim 1 wherein the adhered desiccant coating
on the substrate is at least 2 mils thick.
6. The process of claim 1 wherein the adhered desiccant coating
on the substrate is at least 3 mils thick.
7. The process of claim 1 wherein the one or more desiccants are
selected from the group consisting of molecular sieves and silica
gels.
8. The process of claim 1 wherein the one or more desiccants comprise
at least two desiccants.
9. The process of claim 1 wherein step (a) comprises forming an
aqueous suspension comprising the particles of one or more desiccants,
a suspending agent to help maintain the desiccant particles in suspension,
and an organic pore-clearing agent at least some of which enters
at least some of the pores of the desiccant particles and thereafter
adding to the suspension a water-based organic binder.
10. The process of claim 1 wherein step (a) comprises forming an
aqueous suspension comprising the particles of at least two desiccants;
a suspending agent to help maintain the desiccant particles in suspension;
a pH-adjusting agent to adjust the pH; and an organic pore-clearing
agent at least some of which enters at least some of the pores of
the desiccant particles; and thereafter adding to the suspension
a water-based organic binder.
11. The process of claim 10 wherein the pH-adjusting agent is ammonium
hydroxide.
12. The process of claim 1 wherein step (b) comprises using a double
helically wound rod to deposit suspension on the substrate.
13. The process of claim 1 wherein step (c) comprises heating the
suspension deposited on the substrate to cause the binder to set
and to cause at least some of the pore-clearing agent to leave the
pores of the desiccant particles to prevent the binder from occluding
at least some of the pores of the adhered desiccant particles.
14. The process of claim 1 wherein the substrate has two major
faces and steps (b) and (c) are carried out for each major face
of the substrate, thereby adhering a desiccant coating to each face.
15. The process of claim 1 wherein step (c) comprises causing the
pore-clearing agent to leave the pores of the desiccant particles
to prevent the binder from occluding pores of the adhered desiccant
particles so that the desiccant particles in the coated substrate
have at least 75% of their original adsorption capacity.
16. The process of claim 1 wherein step (c) comprises causing the
pore-clearing agent to leave the pores of the desiccant particles
to prevent the binder from occluding pores of the adhered desiccant
particles so that the desiccant particles in the coated substrate
have at least 90% of their original adsorption capacity.
17. The process of claim 1 further comprising preloading the particles
of desiccant with water before step (a) so that the water in the
particles will be approximately in equilibrium with respect to water
in the suspension.
18. The process of claim 1 wherein step (a) comprises forming an
aqueous mixture of some of the particles of the one or more desiccants,
thereafter adding the suspending agent and the organic pore-clearing
agent, thereafter adding more of the particles of the one or more
desiccants, and thereafter adding the water-based organic binder.
19. A process for making a desiccant-coated substrate capable of
being used at temperatures over 150 degrees Fahrenheit, the desiccant
being in the form of particles and the particles having pores and
being adhered to the substrate by a binder, the coated substrate
being sufficiently flexible and the coating having sufficient adherence
to the substrate so that the coated substrate can be formed into
corrugated shapes, the desiccant particles in the coated substrate
having at least 75% of their original adsorption capacity and the
binder having good breathability; said process comprising the steps:
(a) forming an aqueous suspension comprising particles of one or
more desiccants, a suspending agent to help maintain the desiccant
particles in suspension, and an organic pore-clearing agent at least
some of which enters at least some of the pores of the desiccant
particles;
(b) adding a water-based organic binder to the suspension;
(c) depositing the suspension on the substrate in a thickness of
at least 2 mils; and
(d) causing the binder of the deposited suspension to set so that
the deposited desiccant particles adhere to the substrate and causing
at least some of the pore-clearing agent to leave the pores of the
desiccant particles to prevent the binder from occluding at least
some of the pores of the adhered desiccant particles, thereby to
form a desiccant-coated substrate capable of being used at temperatures
over 150 degrees Fahrenheit and of sufficient flexibility and having
a coating having sufficient adherence to the substrate so that the
desiccant-coated substrate can be formed into corrugated shapes
and in which the desiccant particles in the coated substrate have
at least 75% of their original adsorption capacity and the binder
has good breathability.
20. The process of claim 19 wherein the suspending agent is also
the pore-clearing agent.
21. The process of claim 19 wherein the suspending agent is also
the pore-clearing agent and is N-methyl-2-pyrrolidone.
22. The process of claim 19 wherein the binder is a polyurethane
binder.
23. The process of claim 19 wherein the one or more desiccants
comprise at least two desiccants, the desiccants being selected
from the group consisting of silica gels and molecular sieves.
24. The process of claim 19 wherein the water-based organic binder
is pH-sensitive and step (a) further comprises adding a pH-adjusting
agent to adjust the pH of the suspension.
25. The process of claim 24 wherein the pH-adjusting agent is ammonium
hydroxide.
26. The process of claim 19 wherein step (d) comprises heating
the suspension deposited on the substrate to cause the binder to
set and to cause at least some of the pore-clearing agent to leave
the pores of the desiccant particles to prevent the binder from
occluding at least some of the pores of the adhered desiccant particles.
27. The process of claim 19 further comprising preloading the particles
of desiccant with water before step (a) so that the water in the
particles will be approximately in equilibrium with respect to water
in the suspension.
28. The process of claim 1 wherein the corrugated shapes result
from a corrugation process using corrugation gears having pressure
angles of from about 7 to about 60 degrees.
29. The process of claim 1 wherein the corrugated shapes have a
bend angle measured at the interior of the apex of the bend of at
least about 14 degrees.
30. The process of claim 1 wherein the corrugated shapes have hydraulic
diameters of at least 0.5 millimeters.
31. The process of claim 19 wherein the corrugated shapes result
from a corrugation process using corrugation gears having pressure
angles of from about 7 to about 60 degrees.
32. The process of claim 19 wherein the corrugated shapes have
a bend angle measured at the interior of the apex of the bend of
at least about 14 degrees.
33. The process of claim 19 wherein the corrugated shapes have
hydraulic diameters of at least 0.5 millimeters.
34. A desiccant-coated substrate comprising a coating layer and
a substrate, the coating layer being from about 2 mils to about
10 mils thick and comprising a water-based organic binder and at
least 75% by weight desiccant, the desiccant being in the form of
particles and the particles having pores, the coated substrate being
sufficiently flexible and the coating layer having sufficient adherence
to the substrate so that the coated substrate can be formed into
corrugated shapes, the desiccant particles in the desiccant-coated
substrate having at least 75% of their original adsorption capacity,
the binder having good breathability, and the desiccant-coated substrate
being able to be used at temperatures over 150 degrees Fahrenheit.
35. The desiccant-coated substrate of claims 34 wherein the desiccant
particles in the coated substrate have at least 90% of their original
adsorption capacity.
36. The desiccant-coated substrate of claim 35 wherein the desiccant
comprises at least 85% by weight of the coating layer.
37. The desiccant-coated substrate of claim 34 wherein the coating
layer is from about 3 mils to 8 mils thick.
38. The desiccant-coated substrate of claim 37 wherein the desiccant
comprises at least 85% by weight of the coating layer.
39. The desiccant-coated substrate of claim 38 wherein the desiccant
particles in the coated substrate have at least 90% of their original
adsorption capacity.
40. The desiccant-coated substrate of claim 34 wherein the binder
is polyurethane.
41. The desiccant-coated substrate of claim 40 wherein the desiccant
comprises at least 85% by weight of the coating layer.
42. The desiccant-coated substrate of claim 41 wherein the desiccant
particles in the coated substrate have at least 90% of their original
adsorption capacity.
43. The desiccant-coated substrate of claim 34 wherein it is capable
of being used at temperatures over 200 degrees Fahrenheit.
44. A desiccant-coated substrate made by the process of claim 1.
45. A desiccant-coated substrate made by the process of claim 19.
46. The desiccant-coated substrate of claim 34 wherein the corrugated
shapes result from a corrugation process using corrugation gears
having pressure angles of from about 7 to about 60 degrees.
47. The desiccant-coated substrate of claim 34 wherein the corrugated
shapes have a bend angle measured at the interior of the apex of
the bend of at least about 14 degrees.
48. The desiccant-coated substrate of claim 34 wherein the corrugated
shapes have hydraulic diameters of at least 0.5 millimeters.
Description Methods are known for adhering particles of desiccant (e.g., molecular
sieve particles, silica gel particles) to substrates to form desiccant-coated
substrates used for air treatment, for example, heat and/or moisture
recovery wheels that can be used in HVAC systems. Such wheels include
total energy recovery (or enthalpy) wheels, which remove heat and
moisture from one airstream and transfer them to another airstream,
and dehumidification wheels, which transfer a significant amount
of moisture while attempting to minimize heat transfer from one
airstream to another. For example, it is known to make an enthalpy
wheel, which has a thin (one-thousandth of an inch, i.e., 1 mil)
layer of desiccant coating on each of the two major faces of its
foil-like substrate, by saturating molecular sieve particles with
water, dispersing them in an organic solvent containing a polyurethane
binder composition to form a slurry, coating the slurry onto one
major face of an aluminum foil substrate using a Rotogravure printing-type
process, heating the composite sufficiently to set the binder to
adhere the particles to the substrate and to cause the water to
vaporize to prevent the binder from occluding the pore openings
of the desiccant particles, repeating those steps to adhere a layer
of desiccant particles to the other major face of the substrate,
and then forming the wheel from the final composite. See also U.S.
Pat. Nos. 3338034; 4036360; 4769053; 5052188; and 5120694.
U.S. Pat. No. 3338034 concerns adsorbent-coated thermal panels,
specifically non-porous panels coated with thin layers of gas adsorbent
adapted for rapid heating and cooling. The panels may be made of
metal, preferably aluminum, stainless steel, or copper, and zeolite
molecular sieves are preferred (column 2 lines 12-41). Preferably
the adsorbent is bonded to the panel wall using an inorganic binder
(e.g., clays) substantially free of any organic binder (column 3
lines 9-49). After the adsorbent-binder mixture has been applied
to the surface of the panel wall, desirably the adsorbent is heated
sufficiently to set or cure the binder and thereby bind the adsorbent
to the panel. If the adsorbent is a zeolite, the heating also serves
to liberate water adsorbed by the zeolite molecular sieve. See column
3 lines 50-62. The adsorbent may be mixed with the binder to form
an aqueous slurry (e.g., column 4 lines 35-38). Gases that can
be adsorbed include water, carbon dioxide, and vaporized organic
liquids (column 5 lines 1-4).
U.S. Pat. No. 4036360 concerns a package having a desiccant composition.
This patent refers to prior art packages at column 1 lines 22-41
including one that uses microporous polyurethane bonding a nylon
mesh to form a sheet material (U.S. Pat No. 3326810). This patent
uses prepolymerized polyurethanes to bind large quantities of desiccants
such as zeolites (column 2 lines 7-39). Other organic resin can
be mixed with the polyurethane (column 3 lines 10-18). Example
1 shows tetrahydrofuran mixed with polyurethane and silica gel and
then coated onto polyester film.
U.S. Pat. No. 4769053 (assigned to Semco) concerns total enthalpy
air-to-air rotary energy exchangers, also known as total heat wheels,
and total heat exchange media employed in those wheels. A layer
of coating composition comprising a molecular sieve material is
applied to at least a portion of the surface of the sensible heat
exchange material. The substrate may be a foil material of, e.g.,
aluminum, stainless steel, kraft paper, nylon fiber paper, mineral
fiber paper, asbestos, or plastic (column 4 lines 56-61). The heat
exchange media (molecular sieve material) adsorbs water but not
contaminants, such as hydrocarbons, carbon monoxide, nitrogen dioxide,
and sulfur dioxide (column 3 lines 18-30). Suitable molecular sieve
materials are described at column 5 line 4 to column 6 line 41
and preferably have a pore diameter of about 3 Angstroms. Suitable
binders are set forth at column 6 lines 41-58 and include polyurethanes,
nitrile-phenolics, water-based binders, and alkyd-based resins.
The binder composition preferably includes a solvent such as toluene
(column 6 lines 58-61). Methods of making the heat exchange media
are set forth at column 6 line 42 to column 7 line 19. The binder
and molecular sieve material should be applied so that the binder
does not block the pores of the molecular sieve, which would destroy
the ability of the molecular sieve to function (Id.).
U.S. Pat. No. 5052188 concerns a process for reducing the polarity
on the internal surfaces of various zeolites having an SiO.sub.2
to Al.sub.2 O.sub.3 ratio of at least about 3 and an average pore
diameter size within the range of from about 4 to about 10 Angstroms.
The modified zeolites are prepared by heating the starting zeolite
in an aqueous medium also containing an acid or a source of ammonium
ions to at least partially dealuminize the zeolite and thereby increase
the ratio of silicon to aluminum present in the tetrahedral structure.
The process also provides for the hydrogen ion exchange with respect
to those zeolites that contain significant amounts of metallic cations
in the structure, thereby replacing the bulky metallic cations with
less bulky hydrogen ions, which in turn increases the water adsorptive
capacity of the zeolite. Achievement of the appropriate equilibrium
between reduced surface polarity and increased sorptive capacity
is said to yield zeolite materials having an isotherm with a separation
factor within the range of from about 0.07 to about 0.1. Those modified
zeolites are said to be ideal desiccants for gas-fired air conditioning
and dehumidification systems, for example, systems using regeneratable
rotary desiccant wheels.
U.S. Pat. No. 5120694 concerns a method of coating an aluminum
substrate (e.g., a foil) with a solid adsorbent (e.g., silica gel
or a molecular sieve) comprising heating the surface of the substrate,
contacting the surface with a slurry containing the adsorbent and
a binder, and heating the coating to form a hardened surface. Suitable
binders include clay (column 5 lines 8-30). The slurry may contain
a dispersing agent or surfactant to aid in suspending the particles
or to vary the slurry viscosity, e.g., a polymeric carboxylic acid
or tetrasodium pyrophosphate (column 6 lines 5-15). The suspending
liquid for the slurry is preferably water (column 6 lines 16-43).
The coated product may be used in a desiccant wheel for cooling,
refrigeration, and dehumidification (column 9 lines 20-29).
Rotary air-to-air total energy exchangers may be used in the HVAC
field to recover both sensible energy (from a temperature change)
and latent energy (from adsorbing water) from an exhaust air stream
and then exchange these with an incoming air supply stream. The
ability to recover the latent energy is of significant interest
because such recovery occurs when, and as a result of, dehumidifying
the outdoor air during a cooling cycle and from humidifying the
outdoor air during a heating cycle, thereby reducing the energy
demands required to condition outdoor air during those cycles.
The rotary wheel in such a total energy recovery system typically
rotates at about 20 revolutions per minute and is commonly a thin
substrate (e.g., a 2-mil thick aluminum foil) coated on both sides
with a particulate desiccant in a binder matrix (typical coating
thickness of about 1 mil on each side). Because the primary function
of such a wheel is to recover both energy and moisture, because
the desiccant readily picks up moisture and has a relatively low
heat capacity, and because the substrate readily picks up heat but
not moisture, the mass of desiccant in such a wheel is relatively
low (about 15-30% of the total wheel mass) and the mass of the substrate
(e.g., aluminum) is relatively high (about 70-85% of the total wheel
mass). Additionally, the speed of revolution is necessarily high
relative to the flow of air being processed to increase the rate
at which heat and mass can be transferred from one air stream to
the other air stream.
In contrast, a rotary wheel used for dehumidification only and
not for total energy recovery has relatively less substrate mass
(40-50%), relatively more desiccant mass (50-60%), and rotates more
slowly (e.g., 0.25 revolutions per minute). That increases the amount
of water that can be adsorbed and reduces the amount of carry-over
heat that is transferred to the cooler air stream. A desiccant used
for such a wheel desirably has as high a water adsorption capacity
as possible and as much desiccant mass on the wheel as is consistent
with technical and economic constraints (desirably, coating thicknesses
of more than 1 mil). Furthermore, although some non-desiccant mass
must be used to carry and support the desiccant (i.e., the substrate
and the binder), the wheel should have as little non-desiccant mass
as possible because such mass is dead weight and reduces the wheel's
dehumidification efficiency and increases the energy required for
regeneration.
Regardless of the type of wheel or other desiccant monolith (i.e.,
structural unit comprising the substrate carrying the desiccant
particles) used or desiccant-based system in question, the binder
holding the desiccant particles to the substrate should not significantly
interfere with the functioning of the desiccant (e.g., should not
occlude the pores of the desiccant or otherwise adversely affect
its adsorptive or desorptive capabilities), should facilitate formation
of the monolith (e.g., make coating the surface of the substrate
with desiccant easy), should adhere to the desiccant tightly (to
prevent loss of desiccant from the binder-desiccant coating layer,
for example, by dusting), should present a readily cleanable surface,
and should adhere the binder-desiccant coating layer tightly to
the substrate. The binder must also function under the specified
operating conditions, e.g., in the specified thermal and chemical
environment. For example, a desiccant-coated total heat wheel is
required to operate at temperatures of up to only about 100 degrees
Fahrenheit (about 38.degree. C.). In contrast, a desiccant-coated
dehumidification wheel should not be adversely affected by temperatures
up to about 350 degrees Fahrenheit (about 177.degree. C.) and must
be able to be repeatedly cycled between first temperatures in the
range of 50 to 100 degrees Fahrenheit (about 10.degree. to 38.degree.
C.) and second temperatures in the range of 300 to 350 degrees Fahrenheit
(about 149.degree. to 177.degree. C.) without any adverse consequences,
e.g., delamination of the binder-desiccant coating from the substrate.
Some early dehumidification wheels utilized a honeycomb paper impregnated
with sodium silicate to form a backbone, which was then impregnated
with a desiccant. Because absorbent desiccants such as lithium chloride,
calcium chloride, and lithium bromide deliquesce and change from
solid to liquid upon saturation, this type of desiccant could be
easily deposited into the paper backbone by dipping the honeycomb
wheel into a solution of the desiccant.
However, a significant problem with this type of desiccant was
its loss from the wheel if the desiccant was allowed to reach saturation,
although that usually could be avoided because of the high absorption
capacity of such compounds (they can hold up to twice their own
weight in water). Even so, problems occurred when such wheels became
wet, came into contact with high humidity, or came into contact
with pollutants such as sulfur dioxide and nitrogen dioxide. Also,
manufacturing such wheels required numerous steps, including forming
the special paper, winding and corrugating the paper to form the
honeycomb, forming a silicon dioxide backbone by dipping the honeycomb
into an aqueous sodium silicate solutions heating to drive off the
water, impregnating with desiccant (e.g., LiCl) in a water bath,
heating to drive off the water, grinding the wheel surface flat
to open plugged flutes of the honeycomb, and hardening the surface.
Use of that manufacturing procedure made mass production difficult
and increased cost.
An advance over wheels utilizing absorbent desiccants is the use
of solid adsorbents such as silica gel, activated alumina, and molecular
sieves because they are chemically stable and do not deliquesce.
Because solid adsorbents adsorb water in an amount equal to only
a fraction of the their own weight, wheels using such desiccants
must carry significantly more adsorbent mass than the earlier wheels
(e.g., four to six times as much desiccant mass). To accommodate
this much higher desiccant mass, some current dehumidification wheels
are made from sheets formed using papermaking equipment from a mixture
of pulp, desiccant, and binder in which the desiccant becomes an
integral part of each sheet. However, sheets containing 50% or more
desiccant (a desiccant wheel having acceptable performance needs
at least 50% of its mass to be active desiccant) are difficult to
form into honeycomb media and must be handled carefully because
of decreased web strength resulting from the high desiccant loading.
This makes mass production difficult and increases costs.
Other current dehumidification wheels utilizing solid adsorbents
are made by preparing special paper, winding and corrugating the
paper to form the honeycomb wheel, impregnating with sodium or ethyl
silicate, converting the silica to silica gel using an acid or base,
heating to dry the silica gel backbone and eliminate organic materials,
grinding the wheel surface flat to open plugged flutes of the honeycomb,
and hardening the surface. However, the dipping steps result in
uneven film coatings and limit the amount of active desiccant that
can be deposited on the wheel. Furthermore, the multi-step process
is complex and makes the wheels costly to prepare.
Thus, there is a continuing need for efficient desiccant-based
dehumidification wheels that can be easily and economically produced
using environmentally lower-impact production techniques (e.g.,
without organic solvents), for desiccant-coated substrates that
can be used to make those wheels, and for desiccant-coated substrates
in general in which the binder matrix of the coating is sufficiently
breathable so that the material(s) to be adsorbed can reach the
desiccant particles quickly enough, in which the desiccant particles
in the coating have a high percentage of their original capacity
to adsorb and desorb, and in which the desiccant-coated substrate
has sufficient flexibility and the coating has sufficient adherence
to the substrate so that the desiccant-coated substrate can be formed
into shapes having abrupt radii without the coating losing its integrity
or its adherence to the substrate. There is also a need for desiccant-coated
substrates that have thick coatings (i.e., coatings 2 mils thick
or more per side) and in which the desiccant particles constitute
a high percentage by weight of the coating. There is also a need
for desiccant-coated substrates that have thick even coatings, i.e.,
a coating that does not vary significantly in its thickness along
a given substrate. There is also a need for desiccant-coated substrates
that can be used at temperatures above 150 degrees Fahrenheit (about
66.degree. C.), preferably above 200 degrees Fahrenheit (about 93.degree.
C.), and particularly for substrates that can be repeatedly cycled
during use between first temperatures in the range of 50 to 100
degrees Fahrenheit (about 10.degree. to 38.degree. C.) and second
temperatures in the range of 300 to 350 degrees Fahrenheit (about
149.degree. to 177.degree. C.). There is also a need for desiccant-coated
substrates in which the desiccant comprises two or more different
desiccants that desirably are as well-intermixed (i.e., homogeneous)
as possible in the coating layer. There is also a need for devices
for air treatment besides dehumidification wheels that utilize such
desiccant-coated substrates. Finally, there is a need for cost-effective
methods for making such substrates and dehumidification wheels and
other air treatment devices.
SUMMARY OF THE INVENTION
Desiccant-coated substrates, dehumidification wheels and other
gas (e.g., air) treatment devices utilizing those substrates, and
methods of manufacture that have those features and satisfy those
needs have now been developed.
Broadly, the desiccant-coated substrate of this invention comprises
a coating layer and a substrate, the coating layer being from about
2 mils to about 10 mils thick and comprising a water-based organic
binder and at least 75% by weight desiccant, the desiccant being
in the form of particles and the particles having pores, the coated
substrate being sufficiently flexible and the coating layer having
sufficient adherence to the substrate so that the coated substrate
can be formed into shapes having abrupt radii, the desiccant particles
in the desiccant-coated substrate having at least 75% of their original
adsorption capacity, the binder having good breathability, and the
desiccant-coated substrate being able to be used at temperatures
over 150 degrees Fahrenheit (about 66.degree. C.).
The gas (e.g., air) treatment device of this invention comprises
a desiccant-coated substrate capable of being used at temperatures
over 150 degrees Fahrenheit (about 66.degree. C.), the device having
passageways for gas to flow through and contact the desiccant, the
desiccant being in the form of particles and the particles having
pores, the desiccant-coated substrate comprising a coating layer
and a substrate, the coating layer being from about 2 mils to about
10 mils thick and comprising a water-based organic binder and at
least 75% by weight dry desiccant, the coated substrate being sufficiently
flexible and the coating layer having sufficient adherence to the
substrate so that the coated substrate can be formed into shapes
having abrupt radii, the desiccant particles in the coated substrate
having at least 75% of their original adsorption capacity and the
binder having good breathability.
One process of this invention is a process for making a desiccant-coated
substrate capable of being used at temperatures over 150 degrees
Fahrenheit (about 66.degree. C.), the desiccant being in the form
of particles and the particles having pores and being adhered to
the substrate by a binder, the coated substrate being sufficiently
flexible and the coating having sufficient adherence to the substrate
so that the coated substrate can be formed into shapes having abrupt
radii, the desiccant particles in the coated substrate having at
least 60% of their original adsorption capacity and the binder having
good breathability; said process comprising the steps:
(a) forming an aqueous suspension comprising particles of one or
more desiccants, a water-based organic binder, a suspending agent
to help maintain the desiccant particles in suspension, and an organic
pore-clearing agent at least some of which enters at least some
of the pores of the desiccant particles;
(b) depositing the suspension on the substrate; and
(c) causing the binder of the deposited suspension to set to adhere
the deposited desiccant particles to the substrate and causing at
least some of the pore-clearing agent to leave the pores of the
desiccant particles to prevent the binder from occluding at least
some of the pores of the adhered desiccant particles, thereby to
form a desiccant-coated substrate capable of being used at temperatures
over 150 degrees Fahrenheit (about 66.degree. C.) and of sufficient
flexibility and a coating having sufficient adherence to the substrate
so that the desiccant-coated substrate can be formed into shapes
having abrupt radii and in which the desiccant particles in the
coated substrate have at least 60% of their original adsorption
capacity and the binder has good breathability.
Another process of this invention is a process for making a device
for gas treatment comprising a desiccant-coated substrate capable
of being used at temperatures over 150 degrees Fahrenheit (about
66.degree. C.), the device having passageways for gas to flow through
and contact the desiccant, the desiccant being in the form of particles
and the particles having pores and being adhered to the substrate
by a binder, the coated substrate being sufficiently flexible and
the coating having sufficient adherence to the substrate so that
the coated substrate can be formed into shapes having abrupt radii,
the desiccant particles in the coated substrate having at least
60% of their original adsorption capacity and the binder having
good breathability; said process comprising the steps:
(a) forming an aqueous suspension comprising particles of one or
more desiccants, a water-based organic binder, a suspending agent
to help maintain the desiccant particles in suspension, and an organic
pore-clearing agent at least some of which enters at least some
of the pores of the desiccant particles;
(b) depositing the suspension on the substrate;
(c) causing the binder of the deposited suspension to set to adhere
the deposited desiccant particles to the substrate and causing at
least some of the pore-clearing agent to leave the pores of the
desiccant particles to prevent the binder from occluding at least
some of the pores of the adhered desiccant particles, thereby to
form a desiccant-coated substrate of sufficient flexibility and
a coating having sufficient adherence to the substrate so that the
desiccant-coated substrate can be formed into shapes having abrupt
radii and in which the desiccant particles in the coated substrate
have at least 60% of their original adsorption capacity and the
binder having good breathability; and
(d) forming the desiccant-coated substrate into shapes having abrupt
radii and having passageways for the flow of gas arranged so that
gas flowing through the passageways comes into contact with the
desiccant.
In preferred embodiments the desiccant-coated substrate is used
in a gas treatment device that is a regeneratable dehumidification
wheel, the suspending agent is also the pore-clearing agent and
is N-methyl-2-pyrrolidone, the water-based organic binder is polyurethane,
the coating is from 2 to 10 mils thick, more preferably from 3 to
8 mils thick, the desiccant particles in the coating on the substrate
have at least 75% of the adsorption capacity of the original particles,
more preferably at least 90% of the capacity of the original particles,
the desiccant is at least 85% by weight of the coating layer, the
one or more desiccants are chosen from the group consisting of molecular
sieves and silica gels, and the binder is pH-sensitive and a pH-adjusting
agent is used to adjust the pH of the mixture containing the desiccant
and binder that is applied to the substrate so that the binder will
function properly. Other features and advantages of the invention
will be apparent from this disclosure.
DETAILED DESCRIPTION OF THE INVENTION
The desiccant-coated substrates of this invention may be used for
any purpose concerning removal of moisture and/or other gaseous
substances in any device and in any field; however, they find particular
use in the field of gas treatment and most particularly in the field
of air treatment. In that field, they find particular use in heating,
ventilation, and air conditioning ("HVAC"). In that specific
area, the substrates find particular use in devices that remove
moisture from one air stream and transfer the moisture to a second
air stream. Most preferably, the substrates of this invention are
used in regeneratable rotary wheels that are used in dehumidification
systems.
The monolith, which comprises the substrate carrying the desiccant,
may have any shape or size and may contain the desiccant particles
on its inner surface or surfaces, on its outer surface or surfaces,
or on both its inner and outer surfaces. A particularly useful monolith
will be in the shape of a wheel, for example, a total energy recovery
wheel or a dehumidification wheel, and the desiccant particles will
be on the surface of the passageways for air flow that run from
one major face of the wheel to the other major face. Desirably,
such a wheel will have a honeycomb structure, and its manufacture
is further described below.
The choice of substrate is not critical and can be any substrate
that can function under the conditions of intended use in accordance
with this invention. Thus, the substrate may have any size or shape
and be of any material that has the required physical and chemical
properties. Desirable properties include good strength (e.g., tensile),
temperature resistance, durability, and the appropriate degree of
rigidity (the substrate must be both sufficiently stiff but yet
flexible enough to be bent for certain applications). If the substrate
is to be bent or otherwise formed into a non-planar shape (e.g.,
corrugated with triangular, sinusoidal, or square flutes), the substrate
should have sufficient formability and memory. Suitable substrates
include planar and non-planar (e.g., corrugated) substrates made
of metal, natural and synthetic polymers, and inorganics (e.g.,
ceramics). The substrates may be formed from fibers. Thus, the substrates
may be of aluminum, stainless steel, polyester, PETG (polyethylene
terephthalate glycol), polypropylene, polytetrafluoroethylene, and/or
fibrous webs incorporating polymer fibers, metal fibers, ceramic
fibers, and/or cellulose fibers. The preferred materials include
aluminum and polymer films of polyester (e.g., Mylar polyester)
or of PETG, of which aluminum is most preferred because it is relatively
low cost, easily coated and formed, has a high maximum working temperature,
and is non-inflammable.
Generally, thinner substantially planar substrates prior to coating
are preferred and suitable thicknesses range from about 0.5 mils
to 5 mils, usually 0.6 mils to 4 mils, and preferably 0.8 to 2 mils,
of which about 1.3 mils is most preferred for a dehumidification
wheel. Two or more different substrates may be used together, e.g.,
in the same device for the treatment of air or other gases. Thus,
for example, a formable coated substrate may be corrugated and joined
to a relatively less formable coated substrate, which composite
article is then rolled to form a honeycomb for a dehumidification
wheel.
The substrate may be coated on only one side or one more than one
side. If the substrate has two major faces, e.g., a foil, both major
faces may be coated with the desiccant-binder coating. If the substrate
has more than two major sides or faces, e.g., a parallelepiped,
all or fewer than all of the faces may be coated. A preferred substrate
for a dehumidification wheel is an aluminum foil approximately 1
mil thick that is coated on both major faces and is thereafter formed
into a honeycomb as described below.
The desiccant can be any particulate desiccant and may be a mixture
of particles of different sizes of the same desiccant or a mixture
of particles of different desiccants of the same or different sizes.
The choice of desiccant is not critical and may be any desiccant
particles that can function under the conditions of intended use
in accordance with this invention, i.e., the desiccant particles
should have the required physical and chemical properties. For example,
the desiccant particles should have sufficient mechanical strength
and chemical resistance. Thus, permissible desiccants include naturally
occurring, modified, and synthetic aluminosilicates, aluminas, silica
gels, molecular sieves or zeolites, activated carbon, and activated
alumina. Although the particle size is not critical, the size of
the desiccant particles generally ranges from 3 to 100 microns,
usually from 3 to 25 microns, and preferably from 3 to 10 microns.
A particularly preferred desiccant for dehumidification wheels comprises
particles of three different desiccants, namely, silica gel, a modified
13X molecular sieve, and a hydrophobic adsorbent, and is further
described below.
The binder forming the matrix of the coating layer in which the
desiccant particles reside can be any binder that can function under
the conditions of intended use in accordance with this invention.
Thus, the binder must be compatible with the substrate, the desiccant,
and the other components of the desiccant-coated substrate and must
have the required chemical and physical properties. For a dehumidification
wheel, the binder should be able to function under temperatures
of up to about 350 degrees Fahrenheit (about 177.degree. C.). For
other applications, the binder need not function at temperatures
as high. Desirably, the coating mixture, which contains binder,
desiccant particles, and other components, is relatively easy to
apply to the substrate.
The binder should have sufficient flexibility, adhesion to the
desiccant particles and substrate, durability, breathability, and
strength. The binder desirably is readily cleanable and should retard
loss of desiccant from the coating layer (e.g., by dusting). For
a substrate that is corrugated to form, for example, a dehumidification
wheel, the binder must adhere strongly to the substrate and the
desiccant particles because it is preferred that the substrate be
coated and then corrugated rather than being corrugated and thereafter
coated.
The binder should permit the desiccant particles in the final desiccant-coated
substrate to have sufficient adsorption capacity. Solid desiccants
adsorb materials into their pores, and thus in the final desiccant-coated
substrate the binder should not block or occlude the pores of the
desiccant particles. That means desirably that neither the pore
openings on the surface of the particles or the internal pore volume
inside the particles should be occluded. If the pores are plugged
or the particles are completely encapsulated, overall adsorptive
capability is reduced.
The binder network (or matrix) connecting the desiccant particles
to one another and to the substrate should be sufficiently porous
to allow the materials that are to be adsorbed (e.g., water vapor)
to pass through the binder matrix and reach the contained desiccant
particles, that is, the binder matrix should have good breathability.
Even if the binder does not occlude the pores of the desiccant particles,
if the mass transport of material to be adsorbed is unduly hindered
by the binder (that is, the binder matrix lacks good breathability),
the adsorptive capability and adsorption rate of the desiccant-coated
substrate will be too low. For example, if water is the material
to be adsorbed, the water should reach all of the available desiccant
within a period of from about 1.5 to 4 minutes for a typical rotating
dehumidification wheel. Using a binder that does not unduly hinder
mass transport is particularly important if thick coatings are used
because as the coating thickness increases, any significant retarding
effect by the binder on mass transport through it becomes more noticeable.
For example, with a thick coating (e.g., 4 mils), water vapor needs
to pass through only about 1 mil of binder to move from the surface
of the coating to a desiccant particle that is 1 mil below the surface
of the coating but needs to pass through about 4 mils of binder
to reach a desiccant particle that is at the bottom of the coating
and near the substrate surface.
Although the binder can be water-based or solvent-based, desirably
the binder is a water-based material so that organic solvents are
not needed and the carrier or slurry medium of the coating composition
can be water. That has obvious environmental, cost, and other advantages.
The binder desirably is an organic material (e.g., a carbon-containing
material such as a polymer) as opposed to an inorganic material
(e.g., clay).
The preferred binders are solvent-based polyurethane, nitrile/phenolic-based,
water-based acrylics, and water-based polyurethane. The most preferred
binder is a water-based polyurethane sold by Roymal Coatings &
Chemical Co., Inc. (Newport, N.H.) under the name Polyurethane Aqueous
Dispersion #42823. This material is a polyurethane emulsion containing
about 37% solids and comprises aliphatic or aromatic isocyanate
plus polyester resin.
The desiccant coating mixture that is applied to the substrate
thus contains desiccant and binder and will generally also contain
a solvent or slurry medium. For example, along with the preferred
water-based polyurethane binder and desiccant particles, the coating
mixture will desirably also contain additional water as the slurry
medium. Because different desiccants may have different pH values
in water and because the binder may be pH-sensitive (e.g., it may
not adhere sufficiently to the substrate above or below certain
pH values), it may also be necessary to use a pH-adjusting agent
to control the pH of the coating mixture to bring it to within a
suitable range or to a particular value that permits the coating
process of this invention to be used.
For example, most molecular sieve desiccants are quite basic in
solution and silica gels are typically quite acidic. The most preferred
water-based polyurethane binder desirably is used in this invention
with a neutral to mildly basic pH. Thus, if a silica gel were to
be used as the desiccant along with the preferred binder, a pH-adjusting
agent would desirably be added to bring the pH from highly acid
(silica gel in water) to neutral or mildly basic (the preferred
pH for that binder).
Whether or not a pH adjustment because of the desiccant should
be made to maximize binder properties, it may be necessary or desirable
to adjust the pH because of other components in the coating composition.
Additionally, it may be desirable to adjust the pH because of the
substrate used. For example, if aluminum is used as the substrate,
adhesion of the coating layer to the aluminum will generally be
improved if the pH of the coating mixture is from about 7.5 to about
9.5.
The pH-adjusting agent may be any material that can adjust the
pH of the coating mixture to the desired value so that the benefits
of this invention can be obtained. Usually the pH-adjusting agent
will be a single compound but it may also comprise one or more compounds.
With water as a slurry medium and the preferred binder and desiccant
mixture for a dehumidification wheel, ammonium hydroxide has been
found to be a suitable pH-adjusting agent. Although the pH-adjusting
agent may be added to the coating mixture at any point in its preparation,
it is desirable to add the agent prior to addition of the binder.
Furthermore, with water as the slurry medium and the preferred binder
and desiccant mixture, it is desirable to add the pH-adjusting agent
to the water prior to the addition of the desiccant.
It may be desirable for the coating mixture to contain a suspending
agent to help maintain the desiccant particles in suspension so
that the desiccant particles will not settle out and are evenly
distributed in the coating mixture. For example, the coating mixture
will generally be applied to the substrate from a reservoir of coating
mixture. If the slurry first leaving the reservoir to coat the beginning
of a particular section of substrate does not have as high a concentration
of desiccant as the slurry leaving the reservoir to coat the end
of that particular section of substrate, the beginning of that section
of the substrate will contain less desiccant than the end of that
section. In most application such uneven distribution of the desiccant
would be undesirable.
Furthermore, even if the coating composition in the reservoir were
kept well-mixed so that the mixture applied to the substrate was
homogeneous, the desiccant particles might tend to settle after
application and before setting of the binder. In other words, the
particles might tend to fall to the bottom of the coating layer,
which would result in the top of the coating layer being relatively
poorer in desiccant and richer in binder and the bottom of the layer
(near the substrate) being relatively richer in desiccant and poorer
in binder. That in turn would tend to reduce the adsorptive capacity
of the coated substrate and also tend to reduce the adhesion of
the coating layer to the substrate because more of the desiccant
would be farther from the top of the coating layer, resulting in
more of the material to be adsorbed (e.g., water vapor) having to
travel through more of the binder.
With a coating composition containing two or more different desiccants,
the problem of maldistribution of the desiccant particles may be
exacerbated if the different desiccants tend to remain in suspension
to different degrees. For example, if the desiccant comprises desiccant
S and desiccant T and desiccant S tends to settle out of suspension
more than desiccant T does, in the absence of any suspending agent
to counteract that tendency, the coating mixture removed from the
reservoir for coating would tend to have a lower ratio of S to T
as compared to the original bulk ratio of S to T in the entire coating
mixture. Furthermore, even if the coating composition were kept
well-mixed in the reservoir so that the ratio of S to T in the slurry
applied to the substrate was the same as the original bulk ratio
of S to T, the vertical cross-section of the coating on the substrate
would tend to have an uneven distribution of S and T. That is because
after the coating mixture was applied and before the binder had
set to lock the particles in position, desiccant S would tend to
settle to the bottom of the coating layer (towards the substrate)
more than desiccant T would. This would be particularly apparent
in a thick coating where the S particles might tend to be in the
middle and bottom of the coating layer and the T particles might
tend to be in the top and middle of the coating layer. This problem
is further aggravated if three or more different desiccants are
used, as in the preferred composition.
A suspending agent may also be desirable for maintaining the homogeneity
of the coating mixture with respect to its other components. The
suspending agent for the desiccant particles may be the same as
or different from the suspending agent for the other constituents
of the coating composition. Thus, the coating composition may contain
one, two, or even more suspending agents.
Neither the suspending agent or agents or any other component of
the coating mixture should interfere with the functioning of the
desiccant (e.g., none of the components should occlude the pores
of the desiccant in the final coated substrate or otherwise significantly
reduce its capacity) or interfere with the breathability of the
binder matrix or with the coating process (e.g., none of the components
of the coating mixture should cause the binder to set improperly).
For example, isopropyl alcohol was found to be suitable for use
as a suspending agent under certain conditions with the preferred
desiccant and binder, but under other conditions the isopropyl alcohol
apparently reduced the breathability of the coated substrate and
the adhesion of the coating layer to the aluminum substrate to undesirable
levels.
Any suspending agent may be used that allows the benefits of this
invention to be achieved. A particularly preferred suspending agent
is N-methyl-2-pyrrolidone. Use of that compound with the preferred
binder, desiccant, and substrate results in desiccant-coated substrates
having good properties, including good adhesion of the coating layer
to the substrate, good desiccant adsorption capacity, good binder
matrix breathability, good flexibility, and good durability, and
helps maintain homogeneity or well-mixing of the coating composition
for extended periods of time. The quantity of suspending agent used
should desirably be the minimum amount needed to achieve the desired
effect. The preferred suspending agent can be used in low enough
amounts (typically no more than about 15% by weight of the solid
desiccant particle weight in the composition) so that the coating
composition can be classified as a water-based system.
The coating composition desirably also contains an organic pore-clearing
agent. "Organic" includes carbon-containing compounds
as opposed to inorganic compounds such as water. The function of
the pore-clearing agent is to prevent occlusion or blockage of the
pores, which may result from encapsulation of the desiccant particle
by the binder. Without being bound by any theory, the pore-clearing
agent may prevent occlusion by breaking through the setting binder
or by breaking through the set binder. Pore-clearing agents that
prevent other types of occlusion or that function in other ways
are all included within the term "pore-clearing agent"
as used herein.
If the pore-clearing agent is to function by being placed in the
pores of the desiccant particles prior to setting of the binder,
the pore-clearing agent may be placed in the pores prior to addition
of the particles to the coating composition or after the particles
have been added to the coating composition. The pore-clearing agent
may then be expelled from the pores during or after setting so as
to punch holes in the binder that would otherwise occlude the pore
openings. In addition, the presence of the pore-clearing agent inside
the pores may also prevent the binder and any other potentially
occluding substances from entering the pores. With such an agent,
it is desirable that the kinetic diameter of the pore-clearing agent
be less than the pore diameter of the desiccant(s) utilized so that
at least a portion of the pore-clearing agent can be co-sorbed into
the desiccant along with the water that enters during mixing of
the components to form the coating composition (when water is the
solvent or suspension medium).
If the solvent (preferably water) and the pore-clearing agent are
to be removed by heating the "wet" coating after it has
been applied to the substrate, it is desirable that the pore-clearing
agent be less volatile (have a higher boiling point) than the solvent
(preferably water) so that the binder will be set to some extent
when the pore-clearing agent first starts and then continues to
be driven out of the pores of the desiccant. (The bulk of the adsorbed
water will have left the desiccant pores before the bulk of the
pore-clearing agent starts to leave.) In this case, the pore-clearing
agent will force its way through the binder matrix, thereby creating
porosity in the binder matrix. If the binder matrix is sufficiently
set at that time, some or all of that porosity will become permanent,
thereby imparting breathability to the final coated substrate. As
noted above, sufficient binder matrix breathability (i.e., "good
breathability") is needed during operation to allow the materials
to be adsorbed to reach the desiccant particles and to allow the
materials to be adsorbed to reach the desiccant particles quickly
enough.
Most desirably, the pore-clearing agent is the last component of
the coating mixture to be removed from the coating mixture during
the coating process. Accordingly, if heat is used to set the binder
and remove the solvent, pore-clearing agent, suspending agent, and
any other volatile components, the pore-clearing agent should also
have a lower volatility (i.e., a higher boiling point) than any
of those other components (unless, for example, the pore-clearing
agent is also the suspending agent).
An additional desirable function of the pore-clearing agent in
that case results from its final slow release throughout the coating
layer. Specifically, it helps "stabilize" the coalescing
and setting of the binder so that the binder sets evenly throughout
the thickness of the coating layer and prevents "skinning over"
of the outer surface of the coating (i.e., formation of an undesirable
outer skin). For example, if a pore-clearing agent and suspending
agent (desirably the same material) are not utilized, the solvent
might be driven off unevenly, which would tend to cause the upper
portion of a coating thicker than 1 to 2 mils to cure or set completely
while the lower portion remained uncured. As a result, the solvent
from the lower portion would have to try to break through the upper
set portion. That in turn would tend to cause formation of blisters
and holes on the outer upper surface of the coating and also tend
to cause portions of the coating to blow off of the substrate ("flaking").
If the suspending agent is not also the pore-clearing agent, it
is preferred that the suspending agent have a volatility (boiling
point) between that of the solvent and that of the pore-clearing
agent and, most preferably, closer to that of the pore-clearing
agent. If the volatility of the suspending agent is not closer to
that of the pore-clearing agent, the suspending agent may be driven
off too quickly in the coating process, which might tend to cause
the desiccant particles to undesirably settle out (towards the substrate)
before the binder had set sufficiently.
The pore-clearing agent may be any substance that can perform the
desired function and is compatible with the other constituents of
the coating composition and allows the advantages of this invention
to be achieved. Desirably, the pore-clearing agent is also another
component of the coating composition. For example, it is preferred
that the pore-clearing agent also be the suspending agent. Most
unexpectedly, it has been found that N-methyl-2-pyrrolidone can
function in the coating composition as both the suspending agent
and the pore-clearing agent and thus that compound is preferred.
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