Articles comprising a substrate and a polymeric desiccant either
impregnated therein or coated thereon are disclosed as well as processes
for their manufacture. The invention also contemplates a process
for synthesizing a polymeric dessicant in particulate form for use
as such or for use as a coating material for desiccant articles.
1. A process for making a desiccant article for repeated cycles
of water vapour absorption and desorption comprising the steps of:
(a) preparing a polymerizable organic solution containing a polymerizable
monomer selected from the group consisting of acrylic acid, methacrylic
acid and itaconic acid in which up to 50% of the carboxyl groups
are neutralized by treatment with a base, a homolytic reaction initiator,
at least 0.1% by weight of a cross-polymerization agent and an organic
solvent, said polymerizable organic solution containing less than
35% by weight of water;
(b) impregnating a cellulose fibre substrate with the solution
(c) heating the the impregnated substrate in a substantially oxygen-free
atmosphere to initiate polymerization of the monomer;
(d) treating the polymerized substrate in an alkaline solution
to transform the polymer into a salt; and
(e) drying the article.
2. The process of claim 1 wherein the cellulose fibre substrate
is paper or cardboard.
3. The process of claim 2 wherein the cellulose fibre substrate
is in flat or corrugated form.
4. The process of claim 1 wherein the monomer is acrylic acid which
is present in the solution in a concentration between 2.5M to 4.0M.
5. The process of claim 4 wherein the homolytic reaction initiator
is peroxide, sodium persulphate or azabisisobutyronitrile.
6. The process of claim 1 wherein the amount of cross-polymerization
agent in the solution is between 0.1-2% by weight.
7. The process of claim 1 wherein the organic solvent is acetone
or a glycol.
8. The process of claim 1 wherein the cross-polymerization agent
is trimethylolpropane triacrylate.
9. The process of claim 6 wherein the amount of cross-polymerization
agent in the solution is between 1-2% by weight of the amount of
10. The process of claim 1 wherein the cross-polymerization agent
is trimethylolpropane ethoxylate triacrylate or divinyl benzene.
11. The process of claim 1 wherein the polymerizable organic solution
is heated to a temperature of from 60.degree. C. (140.degree. F.)
to 80.degree. C. (176.degree. F.) to initiate polymerization.
12. The process of claim 1 wherein the alkaline solution contains
a monovalent cation of potassium, sodium, lithium or ammonium.
13. The process of claim 1 wherein the alkaline solution contains
potassium hydroxide or sodium hydroxide dissolved in methanol.
14. The process of claim 1 wherein about 20-50% of the carboxyl
groups in the monomer are neutralized.
FIELD OF THE INVENTION
The present invention concerns the manufacture of desiccant articles
consisting of a substrate or other material onto which is synthesized
a polymeric desiccant and, in particular, a dessicant article capable
of multiple cycles of absorption and desorption of gases such as
water vapour in the air or in any gaseous stream.
This article has possible applications in the field of air treatment,
such as dehumidification, in systems for the transfer of moisture
and heat between two air streams, in HVAC systems and in other applications
involving moisture control and recovery.
The solid desiccants used in air treatment or liquid absorption
systems are primarily inorganic (silica gel, molecular sieves, etc).
They take the form of fine powders which must be bonded to a rigid
substrate. There are a number of techniques for depositing these
desiccants, some of which have been patented. Examples include patents
filed in the United States under U.S. Pat. Nos. 3338034; 4769053;
5052188; 5120694; 5496397; and 5542968.
U.S. Pat. No. 5542968 describes a method which involves mixing
the desiccant powder with fibres in a solution containing a binder
and fire retardants, among other ingredients. A manufacturing process
borrowed from the paper industry is then used to produce sheets
of this compound. Canadian Patent No. 1285931 uses a technique
which consists of coating a metallic substrate with a mixture consisting
primarily of an inorganic desiccant and a heat-curable binder or
adhesive in a solvent. The powder is then bonded to the substrate
by heating the article. U.S. Pat. No. 4172164 describes the use
of a solvent to partially dissolve the thermoplastic substrate,
leaving the polymer particles imbedded in it following evaporation
of the solvent. These techniques have the disadvantage of inhibiting
to some extent the absorption of water by the desiccant powder,
which may deliquesce and become detached under conditions of actual
Another category of articles relates to water absorption in diapers
or sanitary napkins. The absorbent materials used in these applications
are polymers capable of absorbing up to hundreds of times their
own weight in water and are hence referred to as superabsorbents.
U.S. Pat. Nos. 3699103 and 3810468 describe a method which consists
of spreading a copolymer of acrylic acid and acrylamide, in powder
form, on a fibrous material. This material is exposed to steam to
swell the particles of powder, then compressed and dried to bond
the polymer to the fibres. One of the disadvantages of this method
is the fact that the polymer detaches from the fibres after absorbing
the liquid and swelling. Another method described in U.S. Pat. No.
3005456 consists of treating the fibres with chloroacetic acid
to permit the attachment of carboxymethyl groups for absorbency.
This technique uses chloroacetic acid, a very expensive product,
in a propanol solution. In addition, the absorptive capacity of
the fibres is considered insufficient. Finally, another method described
in U.S. Pat. No. 5026596 consists of producing a water absorbent
polymer coated article which has excellent water absorption and
swelling properties. This patent is the result of continual improvement
of the techniques described in Japanese Patents Nos. 50-82143/1975
55-84304/1980 and 58-84804/1983. These applications involve large
quantities of polymer and, once it is swollen by the absorbed liquid,
the structure of the article deteriorates and disintegrates.
SUMMARY OF THE INVENTION
The article of the present invention is intended for the absorption
of water in gaseous form (i.e. water vapour or humidity) and not
water in liquid form, as is the case with superabsorbents. With
superabsorbents, the theory is to attempt to maximize the water-absorptive
capacity of the polymer, which causes it to swell considerably.
Absorptive capacities on the order of several tens to several hundreds
of times the dry weight of the material are thereby obtained. In
order not to restrict swelling, the polymer is weakly cross-linked.
To that end, the cross-polymerizing agent (CPA) used during the
synthesis to give a minimum of structure to the superabsorbent product
is introduced in very small amounts. The proportion of CPA is typically
on the order of 0.1% of the quantity of acrylic acid. Most cross-polymerizing
agents dissolve readily in aqueous solution when used in such proportions.
In the case of the present invention, however, applicants try to
control the swelling of the material when it absorbs water vapour
or other gas and also to limit the absorption of liquid water. For
this reason, the polymer of the present invention is strongly cross-linked.
This is achieved by using a greater quantity of CPA than in the
case of the superabsorbent materials. The proportion of CPA is typically
on the order of 1% to 2% of the quantity of acrylic acid. For quantities
of this magnitude, a solubility problem can arise with certain CPAs
such as trimethylolpropane triacrylate. To solve this problem, it
is necessary to replace part of the water used as solvent with an
organic solvent such as acetone, in which the CPA dissolves more
readily and more uniformly. The presence of organic solvents thus
makes it possible to obtain a polymeric gel that is uniform and
three-dimensional. Typically, the maximum quantity of water recommended
is 35% by volume of the total volume of solution. Another advantage
of the use of compatible organic solvents is that they do not alter
the structure of certain supports such as cardboard or paper substrates
containing a glue.
As an illustration, the maximum absorption capacity of the article
of the present invention for deionized water is 7 times the weight
of the polymer by itself or 2.5 times that of an article made of
cardboard, for example, treated with 20% polymer by weight. A higher
absorption capacity has the effect of destroying the structure of
the article, which would make it difficult to obtain a product that
is sufficiently rigid for some applications such a humidity exchanger.
In addition, this article is designed to withstand a very large
number of absorption/desorption or dehumidification/regeneration
cycles, which is not the case with the superabsorbent materials.
These are generally designed for single-use applications (diapers,
paper towels, toilet paper, etc.).
In this application, applicants are also seeking a high rate of
absorption and desorption, so that the article reacts rapidly to
a sudden variation in the concentration of water vapour or other
gases in the flow of air in contact with the desiccant. Deposition
of the polymer in a thin layer on the walls of the substrate makes
it possible to obtain very rapid sorption kinetics.
In general, the process of the present invention consists of impregnating
a substrate of a given shape with a monomer solution and heating
it to initiate polymerization. This process does not alter the desiccant's
absorptive properties. In addition, it gives the substrate a number
of other interesting properties, including mechanical rigidity and
fire resistance. Due to the thermal properties of the polymer, the
desiccant article is also capable of transferring heat from a warm
air (gas) stream to a cool air (gas) stream within a recovery system.
The bond obtained between the desiccant and the substrate is very
strong and permits it to withstand a large number of absorption
and desorption cycles without any deterioration in absorptive properties
or physical characteristics. A further advantage of this technique
is the fact that these properties can be controlled by adjusting
the composition and quantity of polymer. Most supports made from
natural or synthetic cellulose fibres are permeable to air, which
can pose a contamination problem in certain applications such as
air exchangers. Treatment of this type of support with the polymer
makes it much more air-tight and also more rigid, even using amounts
of the polymer on the order of only 10% by weight. Cellulosic-based
substrates are preferred due to their chemical affinity for polymer
and low cost. Preferably, the support is made from corrugated cardboard
or paper due to the high quality/price ratio. However, other types
of substrate made from natural or synthetic fibres, woven or non-woven,
be used. The polymer has also been successfully deposited on silica-gel
powder in order to fix it by some other technique, such as gluing
on to substrates made of metal or plastic material. Other inorganic
powders (talc, etc.) or organic powders (skeletons of micro-organisms,
etc.) can also be used.
The monomer solution consists primarily of a member of carboxylic
acid family such as acrylic acid or methacrylic acid, a homolytic
reaction initiator such as peroxide and a cross-polymerization agent
such as trimethylolpropane triacrylate. The mixture is soluble in
water and/or a solvent such as acetone.
Following impregnation with the solution, the substrate is heated
in a chamber with low oxygen to a temperature of between 60.degree.
C. and 80.degree. C. (140.degree. F. and 176.degree. F.) to effect
The acidic polymer is then placed in contact with an alkaline solution
of sodium, potassium or other hydroxide, to transform it into a
salt. This operation gives the polymer its absorptive properties,
the cations being linked to the polymer chain in an ionic manner.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The technique first presented here consists of manufacturing a
desiccant article from a cellulose-based substrate and a polymerizable
monomer solution, to permit the absorption of gaseous products such
as water vapour. The product must retain its physical structure
and sorptive properties even after repeated use. Polymer-based desiccants
have the advantage of being readily modified to obtain the desired
absorptive properties, as well as other properties of interest for
certain applications. They can also be obtained in a number of geometric
forms. Some have a certain affinity for structural products used
as substrates, which can facilitate bonding.
For one of the potential applications of the article developed,
a potassium salt of polyacrylic acid has been polymerized on a paper
or cardboard substrate. The water vapour absorption properties of
the desiccant are in no way affected by the presence of the substrate,
even when the proportion of polymer by mass is relatively low. In
addition, the article possesses good fire resistance and acceptable
The process consists of preparing a monomer solution with a base
of acrylic, methacrylic or itaconic acid or a mixture thereof. The
concentration of the base monomer in the solution can be adjusted
on the basis of the desired proportion of desiccant by mass to be
obtained. In the preferred embodiment, acrylic acid is used. The
quantity of acrylic acid should be between 2.5M and 4.0M; at less
than 2.5M, the gel obtained will be insufficiently rigid, and above
4.0M, there is a risk of the reaction being too violent (exothermic)
and thus difficult to control. Between 20% and 90% of the carboxyl
groups must be neutralized by the addition of potassium hydroxide
(KOH) or another base. Preferably, 50% of the carboxyl groups must
be neutralized by adding a solution of KOH dissolved in water. The
total quantity of water in the final solution must not exceed 35%
of the overall volume. A greater volume of water would risk destroying
the structure of the cellulosic fibre support and would limit the
maximum quantity of dissolved cross-polymerizing agent.
The monomer is then mixed with a sufficient quantity of a homolytic
reaction initiator such as a peroxide, azabisisobutyronitrile or
other initiator, in water, acetone and/or other solvents. The amount
of reaction-initiating agent must be sufficient to start the reaction,
that is, about 1% of the total solution volume, although an excess
of this substance would have no impact on the polymerized product.
A cross-polymerization agent such as trimethylolpropane ethoxylate
triacrylate, divinylbenzene or other cross-polymerization agent
is added to the solution in a quantity corresponding to the desired
density of cross-linkages to be obtained in the polymer. To obtain
an article capable of absorbing enough water vapour without excessive
swelling, it is necessary to use 0.1% to 2.0% by volume of a cross-polymerizing
agent, preferably trimethylolpropane triacrylate. The increase in
volume (or swelling) of the desiccant material as a result of the
absorption of water vapour can be controlled by the proportion of
cross-polymerization agent used to synthesize the polymer.
A quantity of organic solvents (acetone, for example) must be added
to bring about complete solution. Other solvents may be used or
mixed with the acetone. In order to minimize loss of acrylic acid
during the heating phase, it is possible to use propylene glycol,
ethylene glycol or other solvents compatible with acetone and having
a high boiling point.
The solution is well mixed, then applied evenly to the cellulose-based
substrate. The article, impregnated with the solution, is placed
in a closed chamber containing minimum oxygen and heated to a temperature
sufficient to initiate polymerization. The polymerization reaction
thus initiated should be completed within a few minutes, depending
on the rate of thermal exchange in the chamber. Since polymerization
is a radical reaction which is blocked in the presence of oxygen,
it is therefore preferable to minimize the amount of oxygen in the
solution in order to avoid the formation of short-chain molecules
or a poor polymerization yield. The presence of minute quantities
of oxygen should have no perceptible effect on the quality of polymerization.
In practice, purging with a flow of nitrogen or argon is usually
sufficient to displace any oxygen dissolved in the solution or present
in the dead space around the article.
Heating temperature must be sufficient to initiate polymerization,
but must not lead to excessive evaporation of the acrylic acid.
A temperature of 80.degree. C. to 120.degree. C. is suggested. Preferably,
the heating equipment will be sufficiently powerful to minimize
the heating time. High-frequency or microwave ovens are especially
recommended, but a sufficiently powerful conventional oven can be
used effectively and can reduce manufacturing costs.
The extent of cross-linkage is fixed by the amount of cross-polymerizing
agent which has actually reacted during the polymerization. To that
end, it is important to ensure that it is uniformly and completely
dissolved in the monomer solution. In the present case, where the
preferred agent is trimethylolpropane triacrylate which has limited
aqueous solubility, it is necessary to use organic solvents such
as acetone, propylene glycol as well as other compatible solvents.
The organic solvents promote improved solubility of the cross-polymerizing
agent, which makes it possible to obtain a polymeric gel with a
three-dimensional structure. It is preferable to limit the volume
of water to 35% of the total volume of monomer solution.
Once the polymerized solution is bonded to the substrate, the polymer
is placed in contact with a hydroxide solution of sodium, potassium,
lithium, ammonium or other monovalent or bivalent cations. The polymer
is transformed into a salt of the cation corresponding to the alkaline
solution used to give the polymer its absorptive properties. In
the preferred embodiment, the acrylic acid based polymer is converted
to a polyacrylic acid salt by wetting the article with a solution
of potassium hydroxide or sodium hydroxide dissolved in methanol.
Potassium hydroxide is preferred as it gives the polymer better
The substrate and the desiccant are then dried to form a rigid
It has also been found that the polymeric desiccant can, in general,
be similarly synthesized onto other materials or particles such
as silica gel or other organic or inorganic powders. Finely powdered
silica gel, for example can be added to a mixture containing the
homolytic reaction initiator solution and the acrylic acid solution.
The mixture is then permitted to polymerize under suitable temperature
and atmospheric (low oxygen) conditions. The resultant acidic polymer
containing the silica gel is ground into small particles and then
transformed into its salt by wetting with a suitable alkaline solution
to give the polymer its absorptive properties. This material is
dried and may then be further crushed or pulverized to obtain a
desiccant powder. This desiccant powder may then be used as such
or may be applied using known techniques, such as with adhesive,
to a variety of substrates like metals and plastics.
Illustrations of the principles of the present invention are provided
by way of the following examples which are not to be considered
as limiting. The production method may be modified and other chemicals
may be used in various quantities as will be understood by those
skilled in the art.
Polyacrylic Acid Potassium Salt on Cellulose
The desiccant polymer is synthesized directly on a cellulose support
in a series of steps. A commercially available corrugated cardboard
is provided to be used as a support for the samples. First of all,
the support is moistened with an aqueous homolytic reaction initiator
solution (solution A1) mixed with an acrylic acid solution (solution
B1) in ratios of 1/10 and 9/10 respectively. The wetted cardboard
is then purged with argon and heated in the drying oven to 80.degree.
C. to permit polymerization. Finally, the acid polymer is transformed
into a potassium salt by immersion in a solution of potassium hydroxide.
After drying in the drying oven, the product resembles plasticized
Commercially available corrugated cardboard
2) Aqueous homolytic reaction initiator solution (solution A1)
5 grams of sodium persulphate are dissolved in 100 ml of deionized
3) Acrylic acid solution (solution B1)
274 ml of acrylic acid (4 M) and 275 ml of 12-propanediol are
placed in a 2 l flask mounted with a septum and a bubbler. The acrylic
acid solution is cooled to under 10.degree. C. in an ice and water
bath. 132 g of potassium hydroxide (85%, 2M) are transferred into
a 500 ml beaker together with enough deionized water to make 250
ml. The heat released by the KOH as it dissolves is dissipated by
placing the beaker in an ice and water bath. This solution is then
added slowly to the acrylic acid solution; the temperature of the
mixture should not exceed 30.degree. C. Following this addition,
27 ml of trimethylolpropane ethoxylate triacrylate (7/3) and 200
ml of acetone are added. The solution is stirred with a magnetic
stir bar for one hour at room temperature. This produces 1 liter
of a polymerizable solution containing a 4M concentration of acrylic
acid, half of which has been neutralized as a potassium salt.
4) The corrugated cardboard is immersed in the acrylic acid solution
mixed with the aqueous solution in ratios of 9/10 and 1/10 respectively.
The excess solution is removed by means of a paper towel. The cardboard
is then placed in a glass desiccator. A mechanical pump is used
to create a vacuum in the desiccator, and argon is then introduced
into the desiccator to establish atmospheric pressure. The purging
operation is repeated to remove as much oxygen as possible from
the acrylic acid solution and the desiccator. The desiccator is
then placed in the drying oven at 70 to 80.degree. C. for two hours
to permit polymerization.
5) Finally, the corrugated cardboard is immersed in a solution
of potassium hydroxide (KOH) for approximately twenty minutes. The
solution is prepared by adding 5.0 g to 100 ml of methanol and 50
ml of deionized water. The cardboard is then dried in the drying
oven. The initial weight of the corrugated cardboard was 0.758 g.
After synthesis of the desiccant polymer, its weight is 0.953 g,
a difference of 0.195 g, representing the quantity deposited on
the cardboard, or 20% by weight of polymer deposited on the corrugated
cardboard. It is then washed again with methanol and dried to ensure
quantitative polymerization. Microgravimetric measurements have
given the following absorption capacities (percentage dry mass of
polymer) as a function of the relative humidity of the air:
TABLE 1 ______________________________________ Relative humidity
of the air (%) 30 60 90 Absorption capacity (%) 35 45 90 ______________________________________
The product is fire resistant as well and does not support the
development of airborne bacteria. The article allows very little
air permeability. Note that the concentration of acrylic acid (2.5M
in this example) may be increased to obtain a larger deposit of
polymer and would thus absorb more water vapour or other gases;
for example, a 4M concentration of acrylic acid would produce a
deposit of approximately 32% by weight.
Powdered Polyacrylic Acid Potassium Salt on Silica Gel
A desiccant powder is obtained when acrylic acid is polymerized
on silica gel. Synthesis is performed by mixing the acrylic acid
solution (solution B2) with the homolytic reaction initiator solution
(solution A2) and adding this mixture to finely powdered silica
gel. The mixture is then purged with argon and polymerized by heating
in the drying oven. The acid polymer is ground into small particles
and transformed into a potassium salt by immersion in a solution
of potassium carbonate. The material is then dried and crushed to
a fine powder.
1) Base Material
Aldrich silica gel (5.0 g, 2-25 .mu.m, 500 m.sup.2 /g).
2) Aqueous homolytic reaction initiator solution (solution A2)
200 mg of sodium persulphate dissolved in 10 ml of water.
3) Acrylic acid solution (solution B2)
Same as solution B in Example 1
4) The Aldrich silica gel is transferred into a 250 ml flask mounted
with a vacuum head and septum. The silica gel is purged twice with
argon. One milliliter of the initiator solution (solution A2) is
aspirated into a 10 ml syringe, then 9 ml of the acrylic acid solution
is aspirated into the same syringe and mixed with the initiator
solution. The resulting solution is then added to the silica gel
and the system is purged twice more with argon. The silica gel/aqueous
solution mixture is heated in the drying oven for 2 hours at 60.degree.
C. The product recovered after heating resembles a flexible plastic
completely enveloping the silica gel.
5) This product is ground into small particles and dispersed in
a water-methanol solution (50 ml of each) containing 5.0 g of potassium
carbonate. It is allowed to stand for two hours in suspension in
the alkaline solution; the particles are then filtered and dried
in the drying oven. The white product recovered after drying is
crushed into a fine powder using a mortar and pestle.
The weight of the white product is 7.97 g. This material consists
of 5.0 g of silica gel together with a deposit of 2.97 of desiccant
polymer. This procedure is merely an example of a deposit performed;
the protocol is not necessarily optimal and certain chemicals may
be replaced by other compatible products.
It will be understood from the foregoing that the examples and
embodiments referred to herein are intended to be illustrative of
the principles of the invention and should not be construed as limiting.
Those skilled in the art will appreciate that various modifications
and/or substitutions in both the materials and the process can be
effected without departing from the spirit and scope of the invention
as defined in the appended claims.