Abstrict A desiccant composition having a moderate Langmuir Type 1 moisture
adsorption isotherm with a separation factor of from 0.05 to 0.13
is disclosed. The composition contains silica gel, a modified 13.times.
molecular sieve (modified by replacing at least 20% of the sodium
cations with other metallic cations, e.g., potassium cations), and
desirably a hydrophobic adsorbent (desirably a hydrophobic molecular
sieve). The composition may be used as the desiccant in a rotary
regeneratable dehumidification wheel, thereby significantly enhancing
dehumidification performance and simultaneously removing a significant
proportion of the airborne pollutants typically contained in indoor
and outdoor air.
Claims We claim:
1. A desiccant composition having a Type 1M moisture adsorption
isotherm comprising (a) silica gel and (b) modified 13.times. molecular
sieve in which at least 20 percent of the sodium cations have been
replaced by one or more metallic cations.
2. The desiccant composition of claim 1 further comprising (c)
hydrophobic adsorbent.
3. The desiccant composition of claim 2 in which the hydrophobic
adsorbent is a hydrophobic silica gel.
4. The desiccant composition of claim 2 in which the hydrophobic
adsorbent is a hydrophobic molecular sieve.
5. The desiccant composition of claim 4 in which the hydrophobic
molecular sieve is a high silica Y-type molecular sieve.
6. The desiccant composition of claim 2 in which the silica gel
on a dry basis comprises 13 to 26 percent by weight of the composition,
the modified 13.times. molecular sieve on a dry basis comprises
74 to 82 percent by weight of the composition, and the hydrophobic
adsorbent on a dry basis comprises 0.1 to 5 percent by weight of
the composition.
7. A desiccant-coated substrate comprising the desiccant composition
of claim 6.
8. A device for gas treatment comprising the desiccant composition
of claim 6 designed so that the gas to be treated can be brought
into contact with the desiccant composition.
9. The device for gas treatment of claim 8 which is a rotary dehumidification
wheel.
10. A device for air treatment comprising the desiccant composition
of claim 6 designed so that the air to be treated can be brought
into contact with the desiccant composition.
11. The desiccant composition of claim 2 in which the silica gel
on a dry basis comprises 17 to 20 percent by weight of the composition,
the modified 13.times. molecular sieve on a dry basis comprises
78 to 80 percent by weight of the composition, and the hydrophobic
adsorbent on a dry basis comprises 2 to 3 percent by weight of the
composition.
12. The desiccant composition of claim 1 in which the silica gel
on a dry basis comprises 13 to 26 percent by weight of the composition
and the modified molecular sieve on a dry basis comprises 74 to
82 percent by weight of the composition.
13. The desiccant composition of claim 1 in which the silica gel
on a dry basis comprises 17 to 20 percent by weight of the composition
and the modified molecular sieve on a dry basis comprises 78 to
80 percent by weight of the composition.
14. The desiccant composition of claim 1 in which at least some
of the one or more metallic cations are potassium cations.
15. The desiccant composition of claim 1 in which substantially
all of the one or more metallic cations are potassium cations.
16. The desiccant composition of claim 1 having a separation factor
of from 0.06 to 0.12.
17. A desiccant-coated substrate comprising the desiccant composition
of claim 1.
18. A device for gas treatment comprising the desiccant composition
of claim 1 designed so that the gas to be treated can be brought
into contact with the desiccant composition.
19. The device for gas treatment of claim 18 which is a rotary
dehumidification wheel.
20. A device for air treatment comprising the desiccant composition
of claim 1 designed so that the air to be treated can be brought
into contact with the desiccant composition.
21. A desiccant composition having a Type 1M moisture adsorption
isotherm comprising (a) 13 to 26 percent by weight silica gel on
a dry basis, (b) 74 to 82 percent by weight modified 13.times. molecular
sieve on a dry basis in which at least 20 percent of the sodium
cations have been replaced by one or more metallic cations, and
(c) 0 to 5 percent by wight hydrophobic adsorbent on a dry basis.
22. The desiccant composition of claim 21 in which the hydrophobic
adsorbent is selected from the group consisting of hydrophobic silica
gel and hydrophobic molecular sieve.
23. The desiccant composition of claim 22 in which the hydrophobic
molecular sieve is a high silica Y-type molecular sieve.
24. The desiccant composition of claim 21 in which at least some
of the one or more metallic cations are potassium cations.
25. The desiccant composition of claim 21 in which substantially
all of the one or more metallic cations are potassium cations.
26. The desiccant composition of claim 21 in which at least 30
percent of the sodium cations in the 13.times. molecular sieve have
been replaced by one or more metallic cations.
27. The desiccant composition of claim 26 in which at least some
of the one or more metallic cations are potassium cations.
28. The desiccant composition of claim 26 in which substantially
all of the one or more metallic cations are potassium cations.
29. The desiccant composition of claim 21 having a separation factor
of from 0.06 to 0.12.
30. The desiccant composition of claim 21 having a separation factor
of 0.07 to 0.11.
31. The desiccant composition of claim 21 having a separation factor
of 0.08 to 0.10.
32. The desiccant composition of claim 21 in which the silica gel
on a dry basis comprises 17 to 20 percent by weight of the composition,
the modified 13.times. molecular sieve on a dry basis comprises
78 to 80 percent by weight of the composition, and the hydrophobic
adsorbent on a dry basis comprises 2 to 3 parts by weight of the
composition.
33. A desiccant-coated substrate comprising the desiccant composition
of claim 21.
34. A device for gas treatment comprising the desiccant composition
of claim 21 designed so that the gas to be treated can be brought
into contact with the desiccant composition.
35. The device for gas treatment of claim 34 which is a rotary
dehumidification wheel.
36. A device for air treatment comprising the desiccant composition
of claim 21 designed so that the air to be treated can be brought
into contact with the desiccant composition.
37. A desiccant composition having a Type 1M moisture adsorption
isotherm with a separation factor of from 0.06 to 0.12 comprising
(a) 13 to 26 percent by weight silica gel on a dry basis, (b) 74
to 82 percent by weight modified 13.times. molecular sieve on a
dry basis in which at least 20 percent of the sodium cations have
been replaced by potassium cations, and (c) 0 to 5 percent by weight
hydrophobic adsorbent on a dry basis.
38. The desiccant composition of claim 37 in which the hydrophobic
adsorbent is selected from the group consisting of hydrophobic silica
gel and hydrophobic molecular sieve.
39. The desiccant composition of claim 38 in which the hydrophobic
molecular sieve is a high silica Y-type molecular sieve.
40. The desiccant composition of claim 37 in which at least 30
percent of the sodium cations in the 13.times. molecular sieve have
been replaced by potassium cations.
41. The desiccant composition of claim 37 having a separation factor
of 0.07 to 0.11.
42. The desiccant composition of claim 37 having a separation factor
of 0.08 to 0.10.
43. The desiccant composition of claim 37 in which the silica gel
on a dry basis comprises 17 to 20 percent by weight of the composition,
the modified 13.times. molecular sieve on a dry basis comprises
78 to 80 percent by weight of the composition, and the hydrophobic
adsorbent on a dry basis comprises 2 to 3 percent by weight of the
composition.
44. The desiccant composition of claim 43 in which at least 30
percent of the sodium cations in the 13.times. molecular sieve have
been replaced by potassium cations.
45. A desiccant-coated substrate comprising the desiccant composition
of claim 44.
46. A device for gas treatment comprising the desiccant composition
of claim 44 designed so that the gas to be treated can be brought
into contact with the desiccant composition.
47. The device for gas treatment of claim 46 which is a rotary
dehumidification wheel.
48. A device for air treatment comprising the desiccant composition
of claim 44 designed so that the air to be treated can be brought
into contact with the desiccant composition.
49. A desiccant-coated substrate comprising the desiccant composition
of claim 43.
50. A device for gas treatment comprising the desiccant composition
of claim 43 designed so that the gas to be treated can be brought
into contact with the desiccant composition.
51. The device for gas treatment of claim 50 which is a rotary
dehumidification wheel.
52. A device for air treatment comprising the desiccant composition
of claim 43 designed so that the air to be treated can be brought
into contact with the desiccant composition.
53. A desiccant-coated substrate comprising the desiccant composition
of claim 37.
54. A device for gas treatment comprising the desiccant composition
of claim 37 designed so that the gas to be treated can be brought
into contact with the desiccant composition.
55. The device for gas treatment of claim 54 which is a rotary
dehumidification wheel.
56. A device for air treatment comprising the desiccant composition
of claim 37 designed so that the air to be treated can be brought
into contact with the desiccant composition.
Description BACKGROUND OF THE INVENTION
This invention concerns desiccant mixtures, particularly desiccant
mixtures that are useful in gas (e.g., air) treatment systems, for
example, in heating, ventilation, and air conditioning ("HVAC")
systems, and most particularly desiccant mixtures that are useful
in dehumidification systems.
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 a 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 solution, 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.
The use of desiccant-based drying for, e.g., air conditioning would
significantly increase if the cost of such drying could be reduced.
Thus, if rotary desiccant-based dehumidification wheels could remove
more moisture more efficiently from, e.g., make-up (atmospheric
or supply) air from outside a building and transfer it more efficiently
to the exhaust air leaving the building and being returned to the
atmosphere, the cost of such desiccant-based drying wheels and the
cost of operating systems using such wheels would significantly
decrease. The Gas Research Institute ("GRI") estimated
that a 75 to 80% decrease in the cost of state of the art desiccant-based
dehumidification wheels would be required to allow open cycle desiccant-based
cooling systems to be mass produced and cost competitive with conventional
air conditioning systems.
Research sponsored by GRI and conducted by Enerscope, Inc. concluded
that a desiccant material having an adsorption isotherm that differed
from the isotherm for currently available desiccant materials could
provide the significantly better performance that would help reduce
the cost of desiccant-based dehumidification wheels. Specifically,
modeling by Enerscope indicated that optimum performance would be
provided by a desiccant having a moderate Langmuir Type 1 moisture
adsorption isotherm ("Type 1") with a separation factor
of approximately 0.1. (U.S. Pat No. 5052188 which is assigned
to GRI and is discussed above, concerns zeolite materials having
an isotherm with a separation factor within the range of about 0.07
to about 0.1 that are said to be ideal desiccants for gas-fired
air conditioning and dehumidification systems.)
The modeling suggested about a 30% increase in cooling performance
achieved by substituting a Type 1 desiccant (i.e., a desiccant having
the above-referenced moderate Langmuir Type 1 moisture adsorption
isotherm) for the silica gel desiccant in current dehumidification
wheels, all else being equal. That would tend to reduce the fraction
of the wheel area for dehumidifying the incoming process air, all
else being equal. More importantly, the modeling suggested that
because the steep heat and mass transfer wave fronts could be substantially
better contained with such a Type 1 desiccant wheel, the Type 1
wheel could maintain a lower moisture level for a longer operating
time, all other design parameters being equal. That in turn was
predicted to reduce the fraction of the wheel area required for
regeneration. Thus, both sections of the dehumidification wheel
assembly (the process or drying section, where a lower moisture
portion of the wheel dries incoming air and becomes moisture laden,
and the regeneration section, where the moisture laden portion of
the wheel is heated by the hot air being exhausted to the atmosphere
to dry that portion of the wheel) would be reduced in size and allow
overall wheel area to be reduced by up to 60%.
In fact, calculations predicted that as compared to a state-of-the-art
silica gel dehumidification wheel, at one set of typical conditions
a Type 1 desiccant dehumidification wheel needed to be only about
half as large in area. That would reduce the cost of the wheel quite
substantially if the cost of the desiccant per se and the process
for making the wheel containing the desiccant were not significantly
greater than for state-of-the-art silica gel wheels. Such a reduction
in the size of the wheel would also reduce the size and therefore
the cost of other components of the system. It was predicted that
the net result of using a Type 1 desiccant would make a Type 1 desiccant-based
air conditioning system less expensive than state-of-the art systems
using silica gel, lithium chloride, or molecular sieve wheels and
would tend to make such a Type 1 air conditioning system cost competitive
with conventional air conditioning systems, which use chilled water
or vapor compression.
The higher performance of a Type 1 desiccant and its potential
for reducing the size of a Type 1 desiccant-based dehumidification
wheel would also counteract another factor tending to require future
wheels (and systems using them) to be larger in size for a given
building than they have had to be. That factor is the recently recognized
need to increase the amount of outside air brought into a building
per unit time per person to reduce the concentration of contaminants
inside the building and to help prevent so-called sick building
syndrome.
The only known Type 1 desiccant known to applicants is that of
the GRI patent discussed above (U.S. Pat. No. 5052188). Unfortunately,
the process for making that material requires numerous costly steps,
at least on a laboratory scale and, to the best knowledge of the
present applicants, has not been commercialized.
Thus, there is a continuing need for a Type 1 desiccant, particularly
one that is cost effective and can be made easily. There is also
a need for Type 1 desiccant based dehumidification wheels that can
be easily and economically produced using environmentally lower-impact
production techniques (e.g., without organic solvents) and for Type
1 desiccant-coated substrates that can be used to make those wheels.
There is also a need for Type 1 desiccant-coated substrates in general
in which the desiccant particles in the coating have a high percentage
of their original adsorption capacity, in which the Type 1 desiccant
particles in the coating have a high percentage of their original
ability to adsorb and desorb, in which the binder matrix has good
breathability, and in which the Type 1 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 Type
1 desiccant-coated substrates that have thick coatings (i.e., coatings
over 2 mil thick per side) and in which the desiccant particles
constitute a high percentage by weight of the coating. There is
also a need for Type 1 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
Type 1 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 Type I desiccants and for substrates,
wheels, and gas (e.g., air) treatment devices incorporating such
desiccants that can remove contaminants from the air being treated.
SUMMARY OF THE INVENTION
"Type 1M" desiccants, "Type 1M" desiccant-coated
substrates, and "Type 1M" desiccant-coated dehumidification
wheels and other gas (e.g., air) treatment devices utilizing those
desiccants and substrates that have those features and satisfy those
needs have now been developed. As used herein, "Type 1M"
refers to a desiccant having a moderate Langmuir Type 1 moisture
adsorption isotherm with a separation factor of from 0.05 to 0.13
desirably from 0.06 to 0.12 preferably from 0.07 to 0.11 and most
preferably from 0.08 to 0.10. Separation factor is defined by the
following equation in which "SC" is the loading fraction
of water in dry desiccant, "FC" is the relative vapor
pressure of water (P/P.sub.o, where P is the partial pressure of
water and P.sub.o is the partial pressure of water at saturation),
and "R" is the separation factor:
Broadly, the desiccant composition of this invention is a desiccant
composition having a Type 1M moisture adsorption isotherm comprising
(a) silica gel and (b) modified 13.times. molecular sieve in which
at least 20 percent of the sodium cations have been replaced by
one or more metallic cations.
In another aspect the desiccant composition of this invention is
a desiccant composition having a Type 1M moisture adsorption isotherm
comprising (a) 13 to 26 percent by weight silica gel, (b) 74 to
82 percent by weight modified 13.times. molecular sieve in which
at least 20 percent of the sodium cations have been replaced by
one or more metallic cations, and (c) 0 to 5 percent by weight hydrophobic
adsorbent.
In another aspect the desiccant composition of this invention is
a desiccant composition having a Type 1M moisture adsorption isotherm
with a separation factor of from 0.06 to 0.12 comprising (a) I3
to 26 percent by weight silica gel, (b) 74 to 82 percent by weight
modified 13.times. molecular sieve in which at least 20 percent
of the sodium cations have been replaced by potassium cations, and
(c) 0 to 5 percent by weight hydrophobic adsorbent.
Other aspects of the invention concern substrates comprising the
Type 1M desiccant, devices for gas (e.g., air) treatment comprising
the Type 1M desiccant composition designed so that the gas to be
treated may be brought into contact with the desiccant composition,
and such devices for gas (e.g., air) treatment that are rotary dehumidification
wheels.
In some preferred embodiments the hydrophobic adsorbent is a hydrophobic
silica gel or a hydrophobic molecular sieve, and more preferably
a high silica Y-type molecular sieve. In other preferred embodiments
at least 30 percent of the sodium cations in the 13.times. molecular
sieve have been replaced by one or more metallic cations, and more
preferably by potassium cations. In other preferred embodiments
the moisture adsorption isotherm has a separation factor of 0.07-0.11
and more preferably 0.08-0.10.
The desiccant composition of this invention has a nearly ideal
isotherm shape and is relatively low cost. It has high moisture
adsorption capacity, relatively low heat of adsorption, high chemical
stability, high heat stability, and the ability to co-sorb (i.e.,
concurrently adsorb during moisture adsorption) a wide range of
materials considered to be pollutants in indoor and outdoor air.
The desiccant composition of this invention can dry air to be treated
for a long cycle period, and the use of this desiccant composition
on a rotary dehumidification wheel allows both the process and regeneration
portions of the rotary dehumidification wheel to be smaller than
they would otherwise be if, for example, silica gel were used by
itself as the desiccant on the wheel. The desiccant composition
of this invention can be applied to a substrate with a relatively
simple process that uses water and not an organic solvent, the desiccant
retains a high percentage of its original adsorption capacity and
the binder matrix has sufficient breathability in the desiccant-coated
substrate even in thick coatings, and the coating containing the
desiccant has excellent adherence to the substrate, which allows
the desiccant-coated substrate to be formed into shapes having abrupt
radii.
Most unexpectedly, although Enerscope's theoretical modeling had
indicated the advantage of Type 1 behavior at high regeneration
temperatures and had indicated the performance advantage at lower
regeneration temperatures to be minimal at best, with the present
invention a significant performance advantage was found even with
regeneration temperatures below 200 to 250 degrees Fahrenheit (about
93.degree. to 121.degree. C.). As a result, a desiccant-based cooling
system of this invention has a higher COP (coefficient of performance),
has reduced energy consumption, and makes it possible to utilize
lower temperature waste or surplus heat that may be available for
regeneration. Other features and advantages of the invention will
be apparent from this disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
To facilitate further discussion of the invention, the following
drawings are provided in which:
FIG. 1 is a graph showing the moisture adsorption isotherms for
Type 1M desiccants having different separation factors; and
FIG. 2 is a graph showing the relative moisture adsorption isotherms
for various desiccant materials and for a Type 1M desiccant.
These drawings are provided for illustrative purposes only and
should not be used to unduly limit the scope of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The desiccant composition of this invention may be used for any
purpose concerning moisture removal and in any device and in any
field; however, it finds particular use in the field of gas (and
most particularly air) treatment. In that field, it finds particular
use in heating, ventilation, and air conditioning ("HVAC").
In that specific area, the desiccant composition finds particular
use in devices that remove moisture from one air stream and transfer
the moisture to a second air stream. Most preferably, the desiccant
composition of this invention is used in regeneratable rotary wheels
that are used in dehumidification and air conditioning systems.
The desiccant composition is particularly useful for such wheels
because it possesses a moisture adsorption isotherm that is most
advantageous for such systems. The desiccant of this invention is
referred to herein as a "Type 1M" desiccant, by which
is meant a desiccant having a moderate Langmuir Type 1 moisture
adsorption isotherm with a separation factor of from 0.05 to 0.13
desirably from 0.06 to 0.12 preferably from 0.07 to 0.11 and most
preferably from 0.08 to 0.10. Separation factor is defined by the
following equation in which "SC" is the loading fraction
of water in dry desiccant, "FC" is the relative vapor
pressure of water (P/P.sub.o, where P is the partial pressure of
water and P.sub.o is the partial pressure of water at saturation),
and "R" is the separation factor:
FIG. 1 is a graph showing adsorption isotherms for desiccant compositions
of this invention having separation factors ("R") of 0.06
0.08 0.10 and 0.12. Thus, the x-axis value is "FC" and
the y-axis value is "SC" expressed as a percentage. The
shapes of these curves make the respective desiccants ideal for
their intended uses, e.g., in a rotary regeneratable dehumidification
wheel. The separation factor for any particular desiccant of this
invention may vary within the ranges specified. In other words,
the separation for a Type 1M desiccant of this invention may he,
for example, 0.08 for part of the moisture adsorption curve and
0.09 for another part of the curve.
The desiccant composition of this invention comprises at least
two and preferably three different desiccants. The two essential
components of the desiccant composition of this invention are silica
gel and modified 13.times. molecular sieve. The preferred desiccant
composition of this invention includes a third component in addition
to the first two essential component and comprises silica gel, modified
13.times. molecular sieve, and hydrophobic adsorbent (preferably
a hydrophobic molecular sieve). The relative amounts of the three
components in the desiccant composition of this invention are as
follows (based on dry weight):
______________________________________ Component Weight Percent
______________________________________ Silica gel 13-26 preferably
17-20 most preferably 19 Modified 13X molecular sieve 74-82 preferably
78-80 most preferably 79 Hydrophobic adsorbent 0-5 preferably
2-3 most preferably 2 ______________________________________
The silica gel used may be a single silica gel or a mixture or
two or more different silica gels. The silica gel has pore sizes
ranging from about 10 to 100 Angstroms and can adsorb larger molecules
of materials (such as those considered to be pollutants) at the
same time water is being adsorbed. Preferably the silica gel used
is a single silica gel, and the preferred silica gel is a normal
density (about 1.05 grams/cc) synthetic silica having an average
pore diameter of about 25 Angstroms and a high surface area of about
675 square meters/gram. Preferably the size of the silica gel particles
is uniform (although it need not be) and is about 10 microns, although
sizes within the range of 3 to 100 microns, more usually 3 to 50
microns, and desirably 3 to 20 microns may be used. Any silica gel
may be used provided it can be used in accordance with this invention
to provide its benefits. Syloid Silica, grades 63 and 64 most preferably
grade 63 marketed by W. R. Grace have been found to be particularly
suitable.
The modified 13.times. molecular sieve is a conventional 13.times.
molecular sieve in which at least 20%, preferably at least 30%,
and most preferably about 35%, of the sodium cations have been replaced
by one or more metallic cations. Higher percentages of sodium replacement
may be used, but typically little improvement is observed beyond
a 50 percent replacement of the sodium cations. Thus, the percentage
of the sodium cations replaced by the one or more cations will generally
be in the range of from 20 to 60 percent, more often from 30 to
50 percent, and most often from 35 to 40 percent. Desirably at least
some of the one or more metallic cations replacing the sodium cations
are potassium cations, preferably substantially all of the one or
more metallic cations replacing the sodium cations are potassium
cations, and most preferably all of the one or more cations replacing
the sodium cations are potassium cations.
Replacement of sodium by potassium in the 13.times. molecular sieve
results in pore sizes of about 8 Angstroms. One advantage of this
invention is that in addition to adsorbing moisture from gas (e.g.,
air) being treated using a desiccant mixture of this invention,
materials that are generally considered to be pollutants can be
adsorbed from the gas at the same time. A pore size of about 8 Angstroms
and the cation adsorption effect are advantageous for adsorbing
many of the materials considered to be pollutants in indoor and
outdoor air (e.g., carbon monoxide, carbon dioxide, formaldehyde).
This complements the adsorption capabilities of the silica gel relative
to pollutants.
Molecular sieves of the 13.times. type are well-known to those
skilled in the art. Methods for exchanging sodium cations for metallic
cations in molecular sieves are also well-known to those skilled
in the art and, specifically, methods for exchanging sodium cations
for potassium cations in molecular sieves are well-known to those
skilled in the art (e.g., the method used to produce potassium 3A
molecular sieve from the basic sodium 4A molecular sieve). On a
laboratory scale, small quantities of potassium-modified 13.times.
molecular sieve were made by (1) mixing 100 grams of 13.times. molecular
sieve with 730 milliliters of water and 55 grams of potassium chloride
at 65.degree. C. and holding the mixture at temperature for 30 minutes
with stirring, (2) filtering to recover the molecular sieve particles,
(3) mixing the recovered particles with the same quantities of water
and potassium chloride at the same temperature and holding at temperature
for 30 minutes with stirring, (4) repeating steps 2 and 3 and (5)
filtering to recover the thrice-exchanged particles, washing with
water, and filtering.
Modification of the 13.times. molecular sieve is important if the
desired Type 1M moisture adsorption isotherm for the desiccant mixture
is to be obtained. Use of conventional 13.times., 5A, 4A, or 3A
molecular sieves in place of the modified 13.times. material does
not allow one to obtain the desired isotherm.
Any 13.times. molecular sieve can be used as the starting material
for making the preferred modified 13.times. molecular sieve, any
modifications technique may be used, and any one or more replacement
cations may be used provided the resulting modified 13.times. molecular
sieve can be used in accordance with this invention to provide its
benefits. Conventional 13.times. molecular sieves marketed by W.
R. Grace and U.O.P. have been found to be particularly suitable.
Preferably the size of the modified 13.times. molecular sieve particles
is uniform (although it need not be) and is about 10 microns, although
sizes within the range of 3 to 100 microns, more usually 3 to 50
microns, and desirably 3 to 20 microns may be used.
The hydrophobic adsorbent can be any material that in combination
with the two essential components of the preferred desiccant (the
silica gel and the modified molecular sieve) provides the desired
properties, including the Langmuir Type 1M moisture adsorption isotherm.
An example of a suitable hydrophobic adsorbent that can be used
is a high silica Y-type molecular sieve marketed by U.O.P. under
the name Purasiv-173 (or MHSZ-173). Alternatively, a hydrophobic
silica gel may be used, e.g., a hydrophobic silica gel marketed
by Cabot Corporation under the name Cab-O-Sil TS-610; however, the
Purasiv-173 material is most preferred. Preferably the size of the
hydrophobic adsorbent particles is uniform (although it need not
be) and is about 10 microns, although sizes within the range of
3 to 100 microns, more usually 3 to 50 microns, and desirably 3
to 20 microns may be used. When used for, e.g., air treatment, the
hydrophobic adsorbent complements the two essential components (the
silica gel and the modified 13.times. molecular sieve) by preferentially
adsorbing organic materials (as opposed to water) even at the low
concentrations typically encountered in indoor and outdoor air and
even at high humidities. Such pollutants may include alcohols, aldehydes,
ketones, aliphatics, and aromatics (e.g., chlorinated hydrocarbons).
FIG. 2 is a graph showing the relative shapes of moisture adsorption
isotherms for various desiccants. The x-axis is the relative water
vapor pressure and the y-axis is the relative moisture loading as
a percent of the maximum value. The uppermost curve is a typical
curve for types 4A and 13.times. molecular sieves, and the lowest
curve (a straight line) is a typical curve for silica gel. The middle
curve is representative of the desiccant compositions of this invention
(also see FIG. 1).
Preferably the desiccant composition of this invention will be
held in a binder matrix adhered to a substrate although this invention
does not require the Type 1M desiccant composition to be used in
any particular form, or with a binder, or with a substrate, or in
any particular device. The binder connects the desiccant particles
to each other and to the substrate and the substrate provides structural
integrity and strength. Together the substrate, binder, and desiccant
particles comprise a monolith. The monolith 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 (e.g., a dehumidification wheel) and the desiccant
particles will be on the surface of the passageways for gas (e.g.,
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 1.2 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 1.2
mils thick that is coated on both major faces and is thereafter
formed into a honeycomb as 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, for water adsorption, 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 pas 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. When using the preferred binder
with the Type 1M desiccant composition of this invention, a pH-adjusting
agent is desirably added to bring the pH 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, the preferred binder, and the desiccant
composition of this invention, when making 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
(as with the desiccant composition of this invention), 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 desiccant 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, 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 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 water
and other materials (if any) to be adsorbed to reach the desiccant
particles and to allow the water and other materials 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. |