Molecular sieve abstract
An improvement in sealed insulating glass having an adsorbent disposed
about all or part of the interior periphery of the glass is described.
The improvement lies in employing a molecular sieve zeolite capable
of adsorbing water and incapable of or having a very limited capacity
for adsorbing nitrogen and oxygen as the adsorbent. This capability
is obtained by using a molecular sieve zeolite whose pore size of
3 angstrom units does not adsorb oxygen and nitrogen as well as
employing a molecular sieve zeolite whose effective pore diameter
of about 4 angstrom units or larger permit the entry of oxygen and
nitrogen but which has acquired an ability to suppress such adsorbtion
by being pretreated by preadsorbing a minor amount of a low molecular
weight polar material such as water vapor, ammonia, methanol etc.
Preventing the adsorbtion and desorbtion of oxygen and nitrogen
in a double glazed window eliminates pressure variations within
the window space created by such gas movement.
Molecular sieve claims
What is claimed is:
1. In a double glazed window having two parallel panes of glass
spaced apart to leave a rectangular parallelepiped space between
the panes, having the peripheries of the two panes sealed to enclose
said air space and having a molecular sieve zeolite adsorbent disposed
along all or part of the interior periphery of the enclosed air
space, the improvement which comprises employing as the adsorbent
a molecular sieve zeolite having effective pore apertures of about
4 Angstrom units or larger and having preadsorbed on it a low molecular
weight polar material in minor amount sufficient to exhaust up to
about 15 percent of the capacity of the zeolite for adsorption of
the polar material.
2. In a double glazed window having two parallel panes of glass
spaced apart to leave a rectangular parallelepiped air space between
the panes, having the peripheries of the two panes sealed with a
flexible resin to enclose said air space and having an adsorbent
adapted to prevent condensation on the interior surfaces of the
panes disposed along all or part of the periphery of the enclosed
air space, the improvement which comprises employing as the adsorbent
a mixture of a particulate molecular sieve zeolite having an effective
pore diameter about 4 Angstrom units or larger and having preadsorbed
on it a low molecular weight polar material in minor amount sufficient
to exhaust from about 3 to 15 percent of the capacity of the zeolite
for adsorption of the polar material, and a particulate non-zeolite
adsorbent having a strong affinity for hydrocarbon adsorption and
an average pore diameter which permits entry of benzene molecules
into the pore space.
3. In a double glazed window having two parallel panes of glass
spaced apart to leave a rectangular parallelepiped space between
the panes, having the peripheries of the two panes sealed to enclose
said air space and having a molecular sieve zeolite adsorbent disposed
along all or part of the interior periphery of the enclosed air
space, the improvement which comprises employing as the adsorbent
a molecular sieve zeolite having effective pore apertures of about
4 Angstrom units or larger and having water vapor preadsorbed on
it in quantity sufficient to give the zeolite a total water content
in excess of 1.5 percent and up to about 4 percent by weight.
4. In a double glazed window having two parallel panes of glass
spaced apart to leave a rectangular parallelepiped air space between
the panes, having the peripheries of the two panes sealed with a
flexible resin to enclose said air space and having an adsorbent
adapted to prevent condensation on the interior surfaces of the
panes disposed along all or part of the periphery of the enclosed
air space, the improvement which comprises employing as the adsorbent
a mixture of a particulate molecular sieve zeolite having an effective
pore diameter about 4 Angstrom units or larger and having water
vapor preadsorbed on it in amount sufficient to give the molecular
sieve zeolite a total water content in the range greater than 1.5
percent and up to about 5 percent by weight, and a particulate non-zeolite
adsorbent having a strong affinity for hydrocarbon adsorption and
an average pore diamter which permits entry of benzene molecules
into the pore space.
5. In a double glazed window having two parallel panes of glass
spaced apart to leave a rectangular parallelepiped space between
the panes, having the peripheries of the two panes sealed to close
said air space and having a molecular sieve zeolite adsorbent disposed
along all or part of the interior periphery of the enclosed air
space, the improvement which comprises employing as the adsorbent
a molecular sieve zeolite having effective pore apertures large
enough to adsorb benzene vapor and having water vapor preadsorbed
on it in quantity sufficient to give the zeolite a total water content
in excess of 1.5 percent and up to about 4 percent by weight.
6. The improvement defined in claim 5 wherein the molecular sieve
zeolite disposed along the periphery of the enclosed space is Molecular
Sieve Type X.
7. The improvement defined in claim 1 wherein the preadsorbed polar
material is water.
8. In a double glazed window having two parallel panes of glass
spaced apart to leave a rectangular parallelepiped air space between
the panes, having the peripheries of the two panes sealed to close
said air space and having a molecular sieve zeolite adsorbent disposed
along all or part of the interior periphery of the enclosed air
space, the improvement which comprises employing as the adsorbent
a molecular sieve zeolite having pore diameters of about 4 Angstrom
units or larger and having a minor amount of water preadsorbed on
it, the quantity of preadsorbed water being a quantity of water
which increases the weight of the adsorbent by from 1.5 to 4 percent
of its weight determined just after maintaining the adsorbent at
600.degree. F. for four hours.
Molecular sieve description
BACKGROUND OF THE INVENTION
Double glazed windows have been in use for some time as described
in "Windows--Performance, Design and Installation" by
Beckett and Godfrey, Van Nostrand Reinhold, New York (1974). A double
glazed window consists of two parallel panes of glass which are
spaced apart to leave an air space between the two panes and having
the periphery of the space between the two panes closed by a moderately
flexible sealant which extends between the two panes along their
peripheries, holding them apart and enclosing a generally rectangular
parallelepiped body of air between the two panes. Polybutene resins
and polysulfide resins are commonly used as sealants in the construction
of the double glazed windows.
The purpose of a double glazed window is to provide thermal insulation
and insulation against noise. At the time of their writing, Beckett
and Godfrey noted the problem of condensation of water vapor contained
in the air in the space between the two panes when the temperature
of the air space drops below its dew point and noted that, "In
the context of windows, condensation can occur both on the surface
of the glass and on the frame facing the room and with double windows,
additionally within the cavity between the two glazings. Whenever
it occurs, the results can be very troublesome, impairing the view
out and leading to the deterioration of the paint work and window
frames." They note also that dehydrating agents and desiccants
such as silica gel may be placed in the cavity to adsorb moisture
from the entrapped air and so contribute to the suppression of condensation.
Double glazed windows, commonly referred to as sealed insulating
glass, commonly have a narrow body of solid adsorbent positioned
in the space between the two panes and lying in close proximity
to the sealing resin which both holds the two panes together and
apart. The solid adsorbent is commonly held in a generally rectangular
aluminum tube which is either perforated or not completely sealed
so that the enclosed air may have contact with the adsorbent and
this adsorbent may lie along all or part of the interior periphery
of the sealed insulating glass.
Passage of time and acquisition of experience has shown that condensation
of water vapor is not the sole condensation problem attending the
use of double glazed windows but that additionally over a period
of time some decomposition of organic sealants occurs releasing
condensible vapors such as hydrocarbon vapors or organic sulfide
vapors which may also condense on the interior surface of the glass
panes. It is current practice to use as the adsorbent to suppress
condensation, a synthetic zeolite, sometimes referred to as a molecular
sieve, or silica gel, or activated alumina, or a mixture of synthetic
zeolite and a second adsorbent such as silica gel. The use of a
second adsorbent to supplement large pore molecular sieve adsorbents
was based on the observation that the rapid adsorption of water
vapor by the molecular sieve reduces its capacity for adsorption
of hydrocarbon vapors or organic sulfides. The molecular sieves
which have been employed have all had pore diameters of such size
that nitrogen molecules and oxygen molecules as well as water vapor
molecules were able to penetrate the pores of the adsorbent.
The use of molecular sieve zeolites of this character has given
rise to a problem which appears not to have been recognized heretofore,
but if it has been recognized, either it has been ignored or no
solution for it has been proposed so far as is now known.
The relatively recent discovery of the "energy problem"
portends a great increase in the use of double glazed windows going
far beyond current use in predominantly glass covered skyscrapers
and extending to extensive use in dwelling houses and apartments.
The seemingly certain large increase in the use of double glazed
windows suggests that they be constructed to provide maximum efficiency
and life and suggests that the problem which attends the use of
adsorbents which adsorb not only water vapor but also nitrogen and
oxygen can no longer be ignored.
The problem may be defined as follows. In the northern part of
the temperate zone the temperature of the air enclosed between the
two panes of a double glazed window may easily rise to 110.degree.
F. or above on a warm summer day and may fall to 0.degree. F. or
below on a cold winter night. At the lower temperatures in this
range, the molecular sieve zeolites currently used adsorb not only
water vapor but also adsorb substantial amounts of oxygen and nitrogen.
At higher temperatures adsorbed oxygen and nitrogen tend to be released
from the adsorbent and migrate back into the gas space enclosed
between the two panes. The resultant cycles of adsorption and desorption
with temperature variation, both day-night variation and seasonal
variation, results in significant changes in the pressure of the
air enclosed between the two panes. The pressure of the enclosed
air may commonly vary by 6% or more merely as a result of adsorption
or desorption of oxygen and nitrogen. This pressure variation is,
of course, amplified by the affect of temperature. For example,
with rising temperature, not only are nitrogen and oxygen desorbed
from the molecular sieve zeolites now in use, but in addition the
rise in temperature itself causes an increase in the pressure of
the gas enclosed between the two relatively rigid panes. Conversely,
with falling temperature, the adsorption of nitrogen and oxygen
increases with a resultant lowering of the pressure of the gas in
the space enclosed between the two panes and in addition, the lowering
of the temperature itself causes a further reduction in the pressure
of the enclosed gas. These continuing fluctuations in pressure cause
some distortion of view through the double glazed windows and, further,
these fluctuations cause a backward and forward movement of the
panes themselves with a resultant tendency to weaken the seals between
the two panes formed by the resins and ultimately to permit openings
between the exterior air and the enclosed air through the sealing
resin which permits the enclosed space to more or less breathe with
the result that over a period of time capacity of the adsorbent
to take up additional water vapor introduced through such breathing
is exhausted.
BRIEF DESCRIPTION OF THE INVENTION
Pursuant to the present invention, the adsorbent which is disposed
along the periphery of the space enclosed by the two panes of a
double glazed window is a mixture of two adsorbent components. One
adsorbent suitable as a first component is a molecular sieve zeolite
which strongly adsorbs water vapor and which is either incapable
of adsorbing nitrogen and oxygen molecules or has been preconditioned
so that its capacity to adsorb oxygen and nitrogen is greatly reduced.
One specific adsorbent meeting these requirements is the 3 A molecular
sieve manufactured and sold by Union Carbide Corporation and by
W. R. Grace & Co. This material has an average pore diameter
in the range about 3 angstrom units, strongly and readily adsorbs
water vapor and it does not adsorb either oxygen or nitrogen.
The chemical composition of this particular molecular sieve is
indicated by the following formula:
the water content of the composition varies with the degree of
dryness or activation of the zeolite but in the desired activated
state should not exceed about 1.5% of the weight of the total composition.
Other adsorbents suitable for use as the first component and which
do not adsorb nitrogen or oxygen because of small average pore diameter
may be obtained by starting with a sodium zeolite having average
pore diameter size about 4 angstrom units and displacing a substantial
part of the sodium with potassium. The resultant potassium or partly
potassium sieve has a reduced average pore diameter which permits
entry of water vapor molecules into the pores and excludes oxygen
and nitrogen molecules from the pores.
Still another type of adsorbent which may be used as the first
component of the mixture is a molecular sieve zeolite which has
effective pore diameters which permit entry of oxygen and nitrogen
into the pores, e.g., effective pore diameters of about 4 A or larger
but which has been pretreated by preadsorbing a minor amount of
a low molecular weight polar material such as water vapor, ammonia,
methanol, ethanol, methyl amine and the like. Preadsorption of such
materials has been reported to suppress oxygen and nitrogen adsorption
but the mechanism of the suppression has not been explained nor
has use of such pretreated adsorbents in double glazed windows been
suggested. [Breck, et al., Journal of the American Chemical Society,
785963 (1956)].
The second component of the adsorbent is either silica gel or activated
alumina having average pore diameters which permit the adsorption
of benzene vapor. Silica gel or activated alumina is placed in the
air space between the panes of the double glazed window for the
purpose of adsorbing hydrocarbon and/or organic sulfide vapors which
get into the space enclosed between the two panes as a result of
slow decomposition of the polysulfide or polyolefin resins which
are commonly used to seal the periphery of the double glazed window
and which cause staining or discoloration of the interior surfaces
of the panes unless they are promptly removed from the enclosed
air space. Activated carbon will also function efficiently as a
second adsorbent but because of its color more than usual care must
be taken to confine it to the periphery of the interior space in
the double glazed window. Mixtures of two or more of silica gel,
activated alumina and activated carbon may be used as the second
adsorbent if desired.
DETAILED DESCRIPTION OF THE INVENTION
Molecular sieve zeolites now generally referred to in the art as
Type A molecular sieve zeolites are described in U.S. Pat. No. 2882243.
Type A zeolites are described as truncated cube octahedrons having
an internal central cavity or cage of 11 A.degree. diameter. The
central cavities are entered through circular apertures of much
smaller diameter, the diameter being determined by the specific
cations contained. For instance, the Type 4 A molecular zeolite
has the formula Na.sub.12 [(AlO.sub.2).sub.12 (SiO.sub.2).sub.12
].XH.sub.2 O. When fully hydrated X is 27 but the sieve is activated
to give it adsorbent capability by heating to drive the water of
crystallization off until the water content of the total composition
is reduced to 1.5% by weight or below. The Type 4 A sieve has an
aperture opening about 4 A in diameter. When a substantial proportion
of the sodium content of the 4 A sieve is replaced by potassium,
the aperture diameter is reduced to about 3 A. For example, the
Type 3 A molecular sieve is formed by displacing sodium from the
Type 4 A sieve with potassium to reach the formula K.sub.9 Na.sub.3
[AlO.sub.2).sub.12 (SiO.sub.2).sub.12 ].XH.sub.2 O. The Type 3 A
molecular sieve has aperture openings of 3 A diameter. Other molecular
sieves such as Type 5 A, Type 10X, Type 13X, etc. have larger aperture
openings.
Directionally, the diameter of the aperture opening determines
which molecules will be able to pass through the aperture opening
into the central cage of the zeolite and so be adsorbed. It might
be expected that the molecular sieve having aperture openings of
4 A would permit entry of molecules having a kinetic diameter less
than 4 A and exclude from entry into the central cavity molecules
having kinetic diameters greater than 4 A. The matter of entry and
exclusion, however, is not quite that simple. Breck and Smith writing
in Scientific American, January 1959 note, "One might expect
that molecules more than a 3.5 angstrom in diameter would be unable
to enter the crystals (of a Type A sieve having aperture diameters
of 3.5 angstroms) but the reality is not quite so simple. We find,
for example, that ethane molecules with a diameter of 4 angstrom
units readily pass through the 3.5 angstrom apertures at normal
temperatures; propane molecules 4.9 angstrom units in diameter do
not. The reason becomes clear enough when we recall that atoms are
not rigid bodies. They more nearly resemble pulsating rubber balls.
The pulsations of both the aperture atoms and the incoming molecules
combine to make the effective diameter of the aperture considerably
larger than its free diameter of 3.5 angstroms. Moreover, the kinetic
energy of the incoming molecules helps them to `shoulder their way`
through the opening. We have found in general that at ordinary temperatures
molecules up to 0.5 angstroms wider than the free diameter of the
aperture can pass through it easily. Larger molecules enter the
crystal with greater and greater difficulty; molecules 1 angstrom
wider cannot enter at all."
The quoted material above indicates the difficulty of defining
a molecular sieve zeolite which will admit certain molecules and
exclude others in terms of aperture diameter and kinetic diameter
of the molecules. In order to know whether a molecular sieve having
a given aperture diameter will admit or exclude molecules having
a kinetic diameter greater than the aperture opening but not more
than 1 angstrom greater, it is necessary to make a simple test by
exposing the molecular sieve to the materials with which it may
be hoped will be excluded and determine whether or not they are
admitted or excluded.
The Type 3 A molecular sieve admits and adsorbs water molecules
and excludes oxygen molecules and nitrogen molecules. The minimum
kinetic diameter of a water molecule has been reported at 2.65 A
and the minimum kinetic diameters of oxygen and nitrogen molecules,
respectively, at 3.46 and 3.64 A. To determine whether a molecular
sieve prepared by displacing part of the sodium from a 4 A sieve
with potassium will admit or exclude nitrogen and oxygen requires
a simple test of this sort if less than half of the sodium has been
displaced.
As noted above, the essential property of a molecular sieve zeolite
which can be used to solve the problem of pressure swings in the
space between the panes of double glazed windows is that the zeolite
be capable of adsorbing water vapor and incapable of or having only
a limited capacity for adsorbing oxygen and nitrogen. 3 A molecular
sieve has this essential property because its average pore diameters
are too small to permit entry of oxygen or nitrogen molecules into
the pores. Molecular sieves such as Types 4 A, 5 A, 10X, 13X and
the like have average pore diameters which permit entry of oxygen
and nitrogen molecules into the pores, but if they are pretreated
by preadsorbing a small amount of a low molecular weight polar material
such as water vapor, ammonia, methanol, ethanol, methyl amine and
the like, then their adsorption of oxygen and nitrogen is greatly
reduced, i.e., to less than 20 percent of the adsorption that would
occur absent the pretreating. For convenience, molecular sieve zeolite
adsorbents having average pore diameters about 4 A or larger on
which minor amounts of low molecular weight polar materials have
been preadsorbed will be referred to hereinafter as pretreated zeolites.
The quantity of low molecular weight polar material preadsorbed
on the pretreated zeolites consists of minor amounts up to about
0.05 ml. per gram of zeolite, preferably in the range 0.0125 to
0.05 of normal density liquid polar material per gram of activated
zeolite. Expressed in another way, the quantity of adsorbed polar
material involves minor amounts up to about 4 percent of the weight
of the zeolite, preferably in the range 1 to 4 percent.
The case of water which is a preferred polar material requires
special comment. The zeolites freshly prepared have a high water
content and in order to impart adsorptive activity to the zeolite,
it is dehydrated at 350.degree. C. or higher, the water content
being reduced below 1.5 percent by weight as a maximum and usually
to a level about 1 percent by weight. The residual water content
is probably not adsorbed water but probably water of crystallization.
At all events, when water is used as the preadsorbed polar liquid,
the preadsorption is carried to the point where the total water
content of the zeolite is from above 1.5 to about 4 percent by weight.
It should be noted that one could reach water content levels in
this range by controlling the activation by dehydration to leave
about 1.5 to about 4 percent by weight of water in the zeolite.
Water in amounts above 1.5 percent by weight has the same effect
as preadsorbed water and for present purposes such excess water
is considered preadsorbed.
The desired level of preadsorbed water can be accurately fixed
by heating the adsorbent to a temperature of 600.degree. F. and
maintaining it at such temperature for a period of four hours. A
stream of dry air is passed over the adsorbent during the four-hour
period. At the end of the four-hour period the adsorbent is fully
activated. The resulting fully activated adsorbent is cooled and
then exposed to water vapor until it shows a weight increase of
1.5 to 4 percent. It is then ready for use in a double glazed window
where it will adsorb water vapor from the air filling the space
between the panes but will not adsorb oxygen or nitrogen.
The use of water as the polar material preadsorbed presents an
apparent difference between water and the other polar adsorbents
in the sense of total polar material present in the adsorbent because
of the fact that the activated zeolites have a water content as
water of crystallization. In the case of water, the preadsorption
must be carried to the point where the total water content of the
adsorbent is generally in the range above 1.5 up to 4 percent by
weight. In the case of ammonia and the other polar adsorbents, preadsorption
to give the adsorbent a polar material content in minor amounts
up to about 4 percent adequately suppresses oxygen and nitrogen
adsorption, preferably amounts in the range 0.75 to 4 percent.
The effective quantity of preadsorbed polar material may also be
expressed in terms of percent of capacity of the adsorbent to adsorb
the polar material, so expressed the quantity of polar material
is sufficient to exhaust from about 3 to 15 percent of the adsorptive
capacity of the zeolite for the particular polar material adsorbed.
Adsorbents for use in double glazed windows to control condensation
of water vapor and of hydrocarbons or organic sulfides on the interior
surfaces of the panes may be prepared by mixing Type 3 A molecular
sieve zeolite with either a silica gel adsorbent or an activated
alumina adsorbent having pore diameters sufficiently large to permit
the adsorption of benzene molecules.
These adsorbent mixtures should contain a minimum of about 15 percent
by weight of the Type 3 A molecular sieve zeolite or pretreated
zeolite and a minimum of about 25 percent by weight of silica gel
or activated alumina. Both adsorbents are in the form of small particles
having a mesh size generally in the range 10 to 30. The mesh size
of the particles is not critical but sizes in this range facilitate
filling the perforated aluminum tubes which are laid along the interior
periphery of the double glazed window.
An alternate heretofore unrecognized solution to the problems associated
with the adsorption and desorption of oxygen and nitrogen is one
in which a second adsorbent component is not required. It involves
the use of a molecular sieve with pores sufficiently large to permit
the adsorption of benzene vapor, i.e., having effective pore diameters
above 6 A, preferably 6 A to 13 A, but which has been pretreated
by preadsorbing a minor amount of a low molecular weight polar material
such as water vapor, ammonia, methanol, ethanol, methyl amine and
the like. When disposed along the periphery of the space enclosed
by the two panes of a double glazed window, these pretreated larger
pore molecular sieves are capable of coadsorbing hydrocarbon and
organic sulfide vapors and additional water vapor, but pressure
fluctuations due to the adsorption and desorption of oxygen and
nitrogen would be eliminated or greatly reduced.
The quantity of the adsorbent mixture theoretically required to
control water vapor condensation and hydrocarbon condensation is
quite small being somewhat less than 7 grams for a 3 foot by 5 foot
double glazed window having a one-half inch space between the panes.
Because, however, minor imperfections in the sealing of the two
panes of double glazed windows are unavoidable in a fair proportion
of them which permits migration of water vapor from the outside
air into the interior space, because hydrocarbon or organic sulfide
release is more rapid during the curing of the resin and prompt
removal of these vapors is necessary to avoid staining of the interior
surface, and because consumers are demanding extended warranties
on the life of double glazed windows, the quantity of adsorbent
disposed along the periphery of the interior space should be a quantity
in the range about 0.01 gram to 1.0 gram of adsorbent for each cubic
inch of space enclosed between the two panes, larger amounts may
be used if desired but ordinarily no benefit attends the use of
larger amounts. In the event that more than two panes of glass are
used, i.e., a triple glazed window is produced the same adsorbent
loading would be used in the spaced between adjacent panes.
While it is preferred to use a mixture of particulate molecular
sieve zeolite with particulate silica gel, activated alumina or
activated carbon, effective suppression of condensation with simultaneous
avoidance of pressure fluctuations due to nitrogen and oxygen adsorption
and desorption may be achieved by filling some rectangular aluminum
tubes with the molecular sieve zeolite and others with the second
adsorbent and then placing zeolite filled tubes along one or more
peripheral sides of the space enclosed between the two panes and
tubes filled with the second adsorbent along one or more of the
remaining peripheral sides. Additionally, the filling of the rectangular
aluminum tubes may be carried out not only by pouring granular adsorbent
into the tubes but also, if desired, the adsorbents may be compressed
into rod-like shape sized to slide into the aluminum tubes.
While the greater proportion of the double glazed windows now manufactured
employ the combination of polyolefin or polysulfide resins and adsorbent
filled aluminum tubes to maintain spacing between the two panes
and seal the periphery of the space enclosed between the panes,
some double glazed windows are manufactured using lead strips and
an adhesive to close the space between the panes and maintain the
spacing between them. In such windows, the second adsorbent is not
required because there are no resin decomposition products to contend
with, only a zeolite molecular sieve adsorbent capable of adsorbing
water vapor and incapable of adsorbing nitrogen and oxygen need
be used. In this type of double glazed window, from about 0.01 to
0.6 grams of adsorbent per cubic inch of enclosed space adequately
suppress water vapor condensation. |