CA2000723A1 - Coating material for proofing substrate against dew condensation - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A coating material for proofing a substrate against dew condensation, comprising [A] a hydroscopic polymer possessing a hygroscopicity of said polymer in the air kept at the temperature of 20°C and relative humidity of 90% of not less than 30% by weight and a water absorption capacity in the range of 1 to 30 times to the own weight and [B] a synthetic resin emulsion as essential components.

Description

COATING MATERIAL FOR PROOFING SUBSTRATE
AGAINST DEW CONDENSATION
BACKGROUND OF THE INVENTION
Field of the Invention:
This invention relates to a coating material for proofing a substrate against dew condensation.
Particularly, it relates to a coating material for proofing a substrate against dew condensation which has high water absorption rate as well as being capable of forming a coating free from damage due to repeating of water absorption and moisture release and, hence, which material can be suitably used for coating on portions where dew condensation apts to break out including wall surfaces of architecture facing north, ceilings, attics and piping.
Description of the Prior Art: ;
As a coating material for proofing a given substrate against dew condensation, there has been proposed a product which is vested with a water-absorbing property by the addition of an absorbent body pigment such as diatomaceous earth, pearlite, or zeolite besides such ordinary paint components as synthetic resin emulsion, coloring pigment, dispersant, tackifier, fungicide, and antiseptic (Japanese Patent Laid-Open SHO 57(1982)-151,661). This coating b material is deficient in ability to prevent dew condensation because it has no satis~actory ability to absorb water. If the absorbent body pigment is used in a large amount for the purpose of enhancing the ability to absorb water, the coating material is no longer capable of forming a film rich in strength.
For the elimination of the drawbacks suffered as described above by the coating material capable of proofing a substrate against dew condensation owing to the use of an absorbent body pigment, there has been proposed a coating material adapted to proof a substrate against dew condensation by the incorporation of a highly absorbent resin (Japanese Patent Laid-Open SHO 62(1987)-205,171 and -:

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SHO 62(1987)-265,364). This coating material for proofing substrates against dew condensation owing to the incorporation of a highly absorbent resin enjoys an improved ability to absorb water and consequently exhibits an improved ability to preclude dew condensation. The highly absorbent resin has a capacity for taking up water approximately to 50 to 1,000 times to its own weight. Since it absorbs water excessively, it requires a long time in releasing the absorbed moisture.
Since it is swelled greatly with absorbed water, the film formed with the coating material is deprived of surface smoothness by addition of only a small amount of water. The film sustains cracks because the difference of swelling and contraction of the film during the repeated cycles of admission and release of water is large.
In the case of an aqueous paint, if this paint incorporates therein the highly absorbent resin in a required amount, it acquires unduly high viscosity or undergoes heavy gelation to a point where the produced paint is effectively applied to a given surface only with difficulty.
An object of this invention is, therefore, to provide an aqueous coating material for proofing a substrate against dew condensation having a sufficient property for proofing against dew condensation and being capable of releasing the moisture immediately, maintaining the surface smoothness of the coating during the course of absorbing waterl and forming a coating free from occurrence of cracks even by repeated cycle of absorbing water and releasing the moisture and, what is more, being free from bringing about environmental pollution.
SUMMARY OF THE INVENTION
The object can be accomplished by a coating material for proofing a substrate against dew condensation, comprising a [A] a hygroscopic polymer possessing a hygroscopicity of the polymer in the air kept at the ... . .

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20007i~3 temperature of 20C and relative humidity of 90 % of not less than 30 % by weight and a water absorption capacity in the range of 1 to 30 times to the own weight and [B] a synthetic resin emulsion as essential components.
The dew-preventing coating material of this invention is characterized by the fact that it exhibits an outstanding ability to prevent condensation when it is in the form of a film and also by the fact that the film of this coating material releases absorbed moisture quickly, retains high surface smoothness during the course of water absorption, and avoids sustaining any crack during repeated use through cycles of absorption and release of moisture.
Since the coating material of this invention contains a polymer of high hygroscopicity, it exhibits a highly satisfactory moisture-controlling function when it is used in the linings of warehouses, closets, and other containers such as clothes boxes which require relatively high airtightness.
EXPLANATION OF THE PREFERRED EMBODIMENT
The synthetic emulsion to be used in the present invention is an O/W type emulsion. Examples of the O/W type emulsion include the emulsions of acryl resin, urethane resin, chloroprene rubber, ethylene-vinyl acetate copolymer, styrene-butadiene rubber, and epoxy resin. These emulsions may be used singly or jointly as a blend of two or more members.
The hygroscopic polymer [A] to be used in the present invention, in an atmosphere having a temperature of 20~C and a relative humidity of 90 %, exhibits a hygroscopicity of not less than 30 ~ by weight, preferably not less than 50 % by weight, and a water absorption capacity in the range of 1 to 30 times, preferably 2 to 20 times, to its own weight. If the hygroscopicity is less than 30 ~ by weight, the film formed of the coating material absorbs water at an unduly low rate and exhibits no satisfactory ability to prevent dew condensation by a :.................. - , , -. . ~ , ~ : . :, .
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. ~)007Z3 certain unknown mechanism. Though the hygroscopicity of the hygroscopic polymer [A] depends upon a kind of the monomer, composition and others, it generally ranges from 50 to 130 %
by weight.
The hygroscopic polymer [A] in accordance with the present invention is not specifically limited but it is required that the hygroscopicity and the water absorption capacity fulfill the definitions mentioned above. In general, it is a cross-linked polymer obtained from the radical polymerization of the monomer components containing the hydrophilic, preferably water soluble monomer as an essential component. Typical examples of method for producing such hygroscopic polymer [A] are as follows:
(1) a method which comprises preparing a monomer component comprising (a) 2 to 25 mol% of a hydroxyl group-- containing a,~-ethylenically unsaturated monomer, (b) 5 to mol% of a carboxyl group-containing a,~-ethylenically unsaturated monomer, and (c) 93 to 45 mol% of a carboxylate group-containing a,~-ethylenically unsaturated monomer, providing that the total of (b) and (c) is in the range of 98 to 75 mol~, radically polymerizing said monomer component, and heat-treating the resultant polymer thereby causing the hydroxyl group and carboxyl group possessed by the polymer to react with each other and form a crosslinked structure.

(2) a method which comprises radically polymerizing at lea~t one monomer selected from the group consisting of unsaturated acids, unsaturated sulfonic acids, unsaturated damides, monomers containing a hydroxyl group and monomers containing an amino group in the presence of a cross-linking monomer.
Of these methods, the method (2) is advantageous over the method (2), because the method (2) requires a large amount of the cross-linkable monomer in order to provide the hygroscopic polymer [A] which has the hygroscopicity and the water absorption capacity within the definitions mentioned ', ~
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above. In the method (1), when suitable amount of the cross-linking monomer is jointly used during the radical polymerization, the above-defined hygroscopic polymer [A] can be preferably produced with ease and convenience.
The hydroxyl group-containing a,~-ethylenically unsaturated monomer (a) [hereinafter referred to as "monomer (a)"] to be used in the present invention may be a water-soluble monofunctional monomer containing a sole hydroxyl group in the molecular unit thereof or a polyfunctional monomer containing two or more hydroxyl groups in the molecular unit thereof. As typical examples of the monomer (a), allyl alcohol; hydroxyalkyl esters of vinyl carboxylic acid monomers such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 3-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate, and 4-hydroxybutyl methacrylate; di- and tri-alkylene glycol mono(meth)acrylates such as diethylene glycol monoacrylate, triethylene glycol monoacrylate, dipropylene glycol monoacrylate, tripropylene glycol monoacrylate, diethyelene glycol monomethacrylate, triethylene glycol monomethacrylate, dipropylene glycol monomethacrylate, polyalkylene glycol mono(meth)acrylates (providing that the number of repeating units of alkylene is in the range of 4 to 50); mono(meth)acrylates of polyhydric alcohols such as glycerol mono(meth)acrylate, neopentyl glycol mono(meth)acrylates, and pentaerythritol mono(meth)acrylates may be mentioned. These monomers (a) may be used either singly or jointly in the form of a mixture of two or more members. Of course, a monomer such as vinyl alcohol which, though incapable of existing by itself, is enabled to form a hydroxyl group by being polymerized with vinyl acetate and subsequently saponified can be used as a monomer (a).
The carboxyl group-containing a,~-ethylenically unsaturated monomer (b) [hereinafter referred to as "monomer (b)"] is soluble in water. Typical examples of the monomer ., ~ .

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~000723 (b) include acrylic acid, methacrylic acid, maleic acid, and itaconic acid. Among other monomers (b) mentioned above, acrylic acid and methacrylic acid prove to be particularly preferable. One member or a mixture of two or more members selected from the monomers (b) cited above may be used.
The carboxylate group-containing ~,~-ethylenically unsaturated monomer (c) [hereinafter referred to as "monomer (c)"] is soluble in water. As typioal examples of the monomer (c), alkali metal salts and ammonium salts of the carboxyl group-containing a,~-ethylenically unsaturated monomers (b) may be cited. One member or a mixture of two or more members selected from the monomers (c) mentioned above amy be used.
The monomer component of the present invention may additionally incorporate therein other a,~-ethylenically unsaturated monomer copolymerization with the monomer component in a ratio not so high as to impair the ester crosslinking density or the water absorption ratio. This additional monomer is preferable to be soluble in water.
Examples of this additional monomer include 2-acrylamide-2-methylpropane sulfonic acid, sulfoethyl (meth)acrylates, 2-(meth)acryloylethane sulfonic acid, acrylamide, N-methylol acrylamide, acrylonitrile, methyl(meth)acrylates, ethyl acrylate, isopropyl acrylate, butyl acrylate, dimethyl maleate, diethyl maleate, dibutyl maleate, and vinyl acetate. The amount of this additional monomer to be used is preferable to be less than 29 mol%, preferably to be in the range of O to 20 mol%, per the total amount of the monomers mentioned above.
Further, the monomer component may incorporate therein a crosslinking monomer containing two or more unsaturated groups in the molecular unit thereof. This crosslinking monomer is preferable to be soluble in water.
Typical examples of this crosslinking monomer include ethylene glycol diacylate, ethylene glycol dimethacrylate, diethylene glycol diacrylate, diethylene glycol , .

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2~;)Q723 dimethacrylate, triethylene glycol diacylate, triethylene glycol dimethacrylate, polyethylene glycol di(meth)acrylates, polypropylene glycol di(meth)acrylates, glycerol di(meth)acrylates, glycerol tri(meth)acrylates, trimethylol propane diacrylate, trimethylolpropane dimethacrylate, trimethylol propane triacrylate, trimethylol propane trimethacrylate, neopentyl glycol di(meth)acrylates, N,N'-methylene bisacrylamide, and N,N'-methylene bismethacrylamide. When the crosslinking monomer is used in the monomer component, it lends itself to shorten the time required for the heat treatment performed for the ester crosslinking of the monomer component, preclude the otherwise possible degradation of the polymer due to a protracted heating at an elevated temperature, ensure formation of a polymer of high quality. A gel-like polymer obtained without crosslinking monomer has markedly adherence of the surface thereof and, hence, such handling as finely cutting for the sake of making dry easy or filtering suspended substances, is difficult. For this reason, use of the crosslinking agent in a small amount makes the adherence of the gel polymer surface lower and handling thereof easy, so that it is preferable. The amount of this crosslinking monomer is in the range of 0.001 to 0.5 mol %, preferably 0.01 to 0.3 mol ~, based on the monomer component.
For the radical polymerization of the monomer component, the method of aqueous polymerization heretofore employed for the polymerization of a water-absorbing polymer can be used in its unmodified form. This radical polymerization can easily be carried out, for example, by a method which comprises forming a plurality of cavities by superposing compressively a plurality of horizontal frames adapted to form tightly closed cavities on being joined by superposition and provided with heat transfer surfaces and bult-in heat medium passages adapted to manifest the function of a polymerization reaction temperature regulator, chargirig the cavities with at least one monomer or a monomer '. -', ' ' . ' ' ', ' ~ ' ,:: ' ' '' ' . ',, -:

Z0C:~0723 solution, passing a heat medium through the plurality of heat medium passages thereby keeping the temperature of the polymerization reaction system through the medium of the heat transfer surfaces at a leave within a desired range and polymerizing the monomer, and on completion of the polymerization, relieving the joined horizontal frames of the pressure and removing the produce polymer therefrom as disclosed in Japanese Patent Publication SHO 48(1973)-42,466, a method which effects radical aqueous solution polymerization of a monomer destined to form a hydrated polymer by continuing the polymerization in a container provided with a plurality of rotary stirring shafts and, with the advance of the polymerization, finely dividing the polymer as formed by virtue of the shearing force generated by the rotation of the stirring shafts as disclosed in Japanese Patent Laid-Open SHO 57(1982)-34,101 and U.S.P. No.
4,652,001, or a reverse phase suspension polymerization method as disclosed in Japanese Patent Publication SHO
59(1984)-37,003.
The polymerization initiator to be used in the present invention may be any of the water-soluble radical polymerization initiators heretofore known in the art. As examples of the polymerization initiator, persulfates such as potassium persulfate, sodium persulfate, and ammonium persulfate, and water-soluble azo compounds such as 2,2'-azobis (2-amidinopropane) hydrochloride (produced by Wako Junyaku K.K. and marketed under product code of "V-50") may be cited. A redox type initiator is also available for the same purpose. Examples of the redox type initiator include combinations of persulfates mentioned above as examples with such reducing agents a~ hydrogen sulfites like sodium hydrogen sulfite and potassium hydrogen sulfite, sulfite and potassium sulfite, thiosulfates like sodium thiosulfate and potassium thiosulfate, L-ascorbic acid, and ferrous salts.
The amount of the polymerization initiator to be used is in the range of 0.001 to 0.5 ~ by weight, preferably 0.002 to . .

0.3 % by weight, based on the total amount of the monomers.
The temperature of the aqueous solution polymerization is in the range of 10 to 120C, preferably 30 to 100C. The concentration of the monomer component in the aqueous solution is 10 to 70 % by weight, preferably 20 to 60 % by weight.
The polymer which is obtained as described above is subjected, either during or after the course of drying to a heat treatment at a material temperature in the range of 130 to 250C, preferably 150 to 250C, for a period in the range of 10 minutes to 20 hours, preferably 10 minutes to 10 hours, so that the polymer is enabled to induce ester crosslinking of the hydroxyl group and carboxyl group contained in the molecular unit thereof. As the result, there is obtained a crosslinked hydrophilic polymer possessing a desired water absorption capacity the range of 1 to 30 times to its own weight. By the method of the present invention, since the monomer component containing no crosslinking monomer or containing such a monomer in an irreducibly minimum amount required is subjected to the aqueous solution polymerization and the produced polymer is heated for crosslinking reaction, the aqueous solution polymerization can be carried out at a high monomer concentration and the degree of crosslinking can be controlled as desired by suitable selection of the conditions of the heating to permit easy manufacture of a hydrophilic polymer possessing a water absorption capacity of 1 to 30 times to its own weight. In this casej the temperature conditions for the drying or the heat treatment are as described above. If the temperature is lower than 130C, since the reaction between the hydroxyl group and the carboxyl group does not easily proceed, it is difficult to obtain a hygroscopic polymer possessing a water absorption capacity of 1 to 30 times to its own weight. Conversely, if the temperature exceed 250C, the produced hygroscopic polymer is poor in quality. Since the method of the . ~ .

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production in accordance with the present invention, which has caused the carboxylic group-containing monomer to neutralize during the state of monomer, does not involves such complicated steps as step for post-neutralization of the polymer after polymerization and after heat treatment, it is excellent method capable of producing the product with good productablility.
The hygroscopicity in the present invention measured as follows:
Hygroscopicity: A powdery sample hygroscopic polymer, 0.2 g, was placed in an aluminum cup 50 mm in diameter and 10 mm in height and left standing for one week in a constant temperature constant-humidity container adjusted to a temperature of 20C and a relative humidity of 90~. The powder thus allowed to absorb moisture was weighed. The hygroscopicity of the sample was calculated by the following formula.
Weight (g) of sample after absorption of moisture - weight (g) of sample Hygroscopicity =before moisture absorption X 100 (%) Weight (g) of sample before moisture absorption If the water absorption capacity of the hygroscopic polymer is less than 1 time to its own weight, the hydrophilic polymer suffers from poor economy because it has a small capaeity for water absorption and, therefore, must be used in an unduly large amount. If the water absorption capacity exceeds 30 times to its own weight, the film formed of the coating material possibly sustains cracks or loses surface smoothness by the impacts of repeated use through eyeles of swelling and eontraetion. The film also has a disadvantage that it absorbs water excessively, takes much time in releasing absorbed moisture, and gathers mold.
Depending on the water absorption capacity of the hydrophilic polymer and the amount of the polymer to be added, the synthetic resin emulsion coating material using . .

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2aoo723 this hydrophilic polymer may suffer from aggravation of viscosity and consequent gelation.
The particle diameter of the hygroscopic polymer [A]
is not specifically restricted. For the sake of surface smoothness of the film to be formed of the coating material, however, the particle diameter is desired to be no more than 200 microns, preferably no more than 100 microns. The ratio of the synthetic resin emulsion [B] to the hygroscopic polymer [A] in the coating material contemplated by the present invention may be suitably selected to fit the conditions of the intended use and the physical properties to be expected. It is generally such that the proportion of the hygroscopic polymer [A] is in the range of 10 to 200 parts by weight, preferably 20 to 100 parts by weight, based on 100 parts by weight of the solids of the synthetic resin emulsion [B].
The dew-preventing coating material can be prepared without requiring any special process. It is produced by sequentially adding the ingredients of the coating material to a suitable despersing machine such as, for example, a ball mill, a sand mill, or a high speed mill and homogeneously mixing and dispersing them in the machine. On the surface of a substrate, the produced dew-preventing coating material can be applied in the form of a film by any of the known coating methods such as, for example, spraying, brushing, roller coating, trowel coating, flow coating, application with a flow coater, and application with a roller coater. Otherwise, it can be incorporated in the substrate by impregnation.
The dew-preventing coating material of the present invention can be used in various types of paper plastics, woven fabrics, non-woven fabrics, inorganic departing from the spirit of the invention disclosed herein. All the proportions in the working examples are based on weight.
Method for production of hygroscopic polymer:
Production Example 1 : ,. . .
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~ ~0007X3 A jacketed stainless steel twin-arm kneader having an inner volume of 2.5 liters, an opening 150 mm x 150 mm in area, and a depth of 150 mm, and provided with two sigma type vanes 90 mm in diameter of rotation was fitted with a lid. In this kneader, 1,200 g of an aqueous solution of a monomer component comprising 65.0 mol% of sodium acrylate, 20.0 mol% of acrylic acid, 15.0 mol~ of hydroxyethyl acrylate, and 0.05 mol% of N,N'-methylene bisacrylamide (monomer concentration 15% by weight) was placed and nitrogen gas was blown into displace the internal gas of the reaction system. Then, the two sigma type vanes were rotated at speeds of 67 and 56 rpm, hot water at 35C was passed through the jacket to heat the interior of the reaction system, and polymerization initiators were added.
After 5 minutes following the addition of the polymerization initiators, the monomer component began to polymerize. The temperature of the interior of the reaction system reached 83C after 20 minutes following the addition of the polymerization initiators. The hydrated gel polymer containing water was divided into minute particles about 5 mm in diameter. The stirring of the contents of the reaction system was further continued. After 60 minutes following the start of the polymerization, the lid was removed form the kneader and the hydrated gel polymer was removed from the kneader. The hydrated gel polymer was dried in an ordinary hot air drier at 130C for 2 hours and then comminuted to obtain a powdered hygroscopic polymer ( 1 ) .
The resultant hygroscopic polymer (1) was found to possess the water absorption ratio of 17 times to the own weight and hygroscopicity of 105% by weight. The water absorption ratio was measured as follows:

Water absorption ratio: A tea bag-like pouch (40 mm x 60 mm) made of non-woven fabric was packed evenly with 0.5 g of a powdery sample polymer was impregnated with deionized , .

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~0007Z3 water and, after 60 minutes, dried on 10 sheets of tissue ~ .
paper 120 x 200 mm in area (produced by Jujo Kimbary and marketed under trademark designation of "Kimwipe Wiper") to drain excess water, and weighed. The water absorption ratio was calculated by the following formula.

Water Weight (g) after absorption absorption = - weight (~) before absorPtion ratio Weight (g) before absorption Production Example 2 A hygroscopic polymer (II) was obtained by preparing a hydrated gel by following the procedure of Production Example 1, except that an aqueous solution of a monomer component composed of 67.5 mol% of sodium acrylate, 22.5 mol% of acrylic acid, 9.95 mol% of 2-hydroxyethyl acrylate, and 0.05 mol% of N,N'-methylene bisacrylamide was used in the form of an aqueous solution containing the monomer component in a concentration of 37% by weight, drying the hydrated gel in an ordinary hot air drier at 100C for 2 hours, comminuting the dried gel, and heat treating the ..
polymer powder in a still drier at 180C for 3 hours, the polymer (II) was found to possess a water absorption ratio of 5 times to its own weight and a moisture absorption ratio of 95% by weight. -Example 1 and 2 and Controls 1 to 3 :
Coating materials of working examples and controls were prepared by mixing varying raw materials in varying proportions indicated in Table 1.
They were obtained by sequentially adding the raw materials to a high-speed mill set to rotation from the .
beginning.
The coating materials and the films formed of the coating materials were tested for varying physical -`
properties indicated in Table 1.

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Table 1 Example Example Control Cortrol Control cryl emulsion 1)100 100 100 100 100 ispersant 2)0.5 0.5 0.51.0 1.0 .
ygroscopic polymer II 20 _ _ _ _ ygroscopic polymer I _ 12 _ _ _ ighly absorbent resin A 3) _ _ 4 _ _ " B 4) _ _ _ _ 10 iatomaceous earth 5) _ _ _ 100 .eavy calcium carbonate 6) 50 50 50 30 50 ~itanium dioxide 7)20 20 20 20 20 'ackifier 8) 2 2 1 2 2 `ilm-forming auxiliary 9) 5 5 5 5 5 :
)efoaming agent 10)1 1 1 1 1 later 83 104 1265 110 152 Concentration (%) 52 47 9 56 40 Viscosity (CPS) 11)38,00042,000167,00041,00032,000 )ew-preventing property O O O X X
late of water absorption O O O X X
~ater absorption raito O O O O
qoisture-releasing ratio O O X O . O
~urability O O X O X
~urface smoothnes~ of film O O X O O

~00723 Notes:
1) Acryl emulsion: Product of Nippon Shokubai Kagaku Kogyo Co., Ltd. marked under trademark designation of "Acryset 202E"
2) Dispersant: Product of Nippon Shokubai Kagaku Kogyo Co., Ltd. marketed under trademark designation of "Aqualic DL-40S"

3) Highly absorbent resin A: Product of Nippon Shokubai Kagaku Kogyo Co., Ltd. marketed under trademark designation of "Aqualic CA" having a water absorption ratio of 250 times to its own weight and a hygroscopicity of 110%
by weight 4) Highly absorbent resin B: Product of Sumitomo Chemical Industries Co., Ltd. marketed under trademark designation of "Sumicagel R-30" having a water absorption -ratio of 25 times to its own weight and a hygroscopicity of 6% by weight 5) Diatomaceous earth: Marketed by Tokyo Kogyo Boeki Shokai under trademark designation of "Sellaite 281"

6) Heavy calcium carbonate: Product of Nitto Funka Kogyo K.K. marketed under product code of "NS #30"

7) Titanium dioxide: Product of Teikoku Kako K.K.
marketed under product code of "JR-701"

8) Tackifier: Product of Union Carbide marketed under trademark designation of "Cellosize OP-4400" in the form of an aqueous 2.5% solution.

9) Film-forming auxiliary: Marketed by Nagase Sangyo K. K. under trademark designation of "Texanol"

10) Defoaming agent: Product of Shinetsu Silicon K.K.
marked under trademark designation of "Silicon KM-72"

11) Viscosity: BL type viscosimeter, No. 4 roller, 6 rpm at 25C
The times of test involved and the procedures therefor are as shown below.
(A) Dew-preventing property ... . .

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~000723 ~ sample was applied on an aluminum juice can in an amount calculated to produce a dry film 1 mm in thickness and dired at room temperature for 7 days. Then, the juice can was cooled to about 5C by filling the can with water and ice. The coated juice can was kept standing in a constant temperature and constant humidity bath kept at a temperature of 20C and a relative humidity of 90~ and held under continued visual observation as to the occurrence of dew on the surface of the film. The formation of dew was rated on a three-point scale, wherein:
O stands for total absence of dew condensation in 30 minutes.
O stands for slight dew condensation in 30 minutes.
X stands for dew condensation and consequent fall of water drops in 30 minutes.
(B) Speed of water absorption A dry sheet (2 mm in thickness) was prepared from a sample coating material. One drop (about 0.05 cc) of water was dropped through a pipet onto the dry sheet. The time was clocked between the landing of the water drop on the dry sheet and the disappearance of the water drop due to absorption by the dry sheet. The speed of water absorption was rated by a three-point scale, wherein:
O stands for no more than 2 minutes.
O stands for 2 to 9 minutes.
X stands for no less than 9 minutes.
The dry sheet was produced by setting a frame adapted to mold a dry film 2 mm in thickness on a mold release paper, casing a given sample of coating material in the cavity of the frame, smoothening the surface of the cast sample with a glass rod, and allowing the cast sample to dry at room temperature for 7 days.
(C) Water absorption ratio A dry sheet (2.0 mm thick x 50 mm x 100 mm) was prepared from a given sample coating material, immersed in water at 20C for 3 hours. The wet sheet was then tested , .. . .

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O stands for no less than 80% by weight of absorption ratio.
O stands for 50 to 80% by weight of absorption ratio.
X stands for no more than 50% by weight of absorption ratio.
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Water Sheet weight (g) after immersion -absorption = sheet wei~ht (~) before immersion X 100 ratio (%) Sheet weight (g) before immersion (D) Moisture releasing ratio A sample sheet which had undergone the test for water absorption ratio was left standing in a constant temperature constant-humidity bath kept at a temperature of 20C and a relative humidity of 40%. The sheet was then tested for water content. The moisture releasing ratio was rated on a three-point scale, wherein:
O stands for no more than 2% by weight of water content.
O stands for 2 to 10% by weight of water content.
X stands for no less than 10% by weight of water content.
Sheet weight (g) before moisture Moisture release - sheet weight ~g) after release = moisture release X 100 ratio (%) Sheet weight (g) before moisture release (E) Durability A film of a given sample polymer was subjected to 10 cycles of the alternative absorption and moisture release treatment. After each cycle, the film surface was visually examined for possible abnormal phenomenon before it was -: : , ..: ~ , , : : .
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subjected to the subsequent cycle. The durability was rated on a three-point scale, wherein:
O stands for absence of abnormal phenomenon after 10 cycles.
O stands for softening of film and slight occurrence of crack after 10 cycles.
X stands for separation of polymer during immersion in water after the second cycle.
(F) Surface smootheness of film A given sample coating material was applied on a slate plate in an amount calculated to produce a dry film 1 mm in thickness. The applied coating material was dried at room temperature for 7 hours. Then, the produced film was immersed in water at 20C for 24 hours. The film removed from the water was visually examined for sur~ace smoothness.
The surface smoothness was rated on a three-point scale, wherein:
O stands for satisfactory surface smoothness free from crack.
O stands for slightly poor surface smoothness accompanied by slight occurrence of cracks.
X stands for inferior surface smoothness accompanied by occurrence of cracks and separation of polymer.

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Claims (8)

1. A coating material for proofing a substrate against dew condensation, comprising [A] a hygroscopic polymer possessing a hygroscopicity of not less than 30% by weight of said polymer in the air kept at the temperature of 20°C
and relative humidity of 90% and a water absorption capacity in the range of 1 to 30 times to the own weight and [B] a synthetic resin emulsion as essential components.

2. A coating material according to Claim 1, wherein said hygroscopic polymer [A] has the water absorption capacity in the range of 2 to 20 times to the own weight.

3. A coating material according to Claim 1, wherein said hygroscopic polymer [A] having the water absorption capacity thereof 1 to 30 times to its own weight by a process of manufacture which comprises preparing a monomer component comprising (a) 2 to 25 mol% of a hydroxyl group-containing .alpha.,.beta.-ethylenically unsaturated monomer, (b) 5 to mol% of a carboxyl group-containing .alpha.,.beta.-ethylenically unsaturated monomer, and (c) 93 to 45 mol% of a carboxylate group-containing .alpha.,.beta.-ethylenically unsaturated monomer, providing that the total of (b) and (c) is in the range of 98 to 75 mol%, radically polymerizing said monomer component, and heat-treating the resultant polymer thereby causing the hydroxyl group and carboxyl group possessed by the polymer to react with each other and form a crosslinked structure.

4. A coating material according to Claim 3, wherein hydroxyethyl acrylate and/or hydroxyethyl methacrylate are essentially used as said hydroxyl group-containing .alpha.,.beta.-ethylenically unsaturated monomer (a).

5. A coating material for according to Claim 3, wherein at least one member selected from the group consisting of acrylic acid, methacrylic acid and maleic acid is essentially used as said carboxyl group-containing .alpha.,.beta.-ethylenically unsaturated monomer (b).

6. A coating material according to Claim 3, wherein said carboxylate group-containing .alpha.,.beta.-ethylenically unsaturated monomer (c) is an alkali metal salt or ammonium salt of (meth)acrylic acid.

7. A coating material according to Claim 3, wherein said monomer components further comprises a cross-linkable monomer.

8. A coating material according to Claim 1, wherein the hygroscopicity of said hygroscopic polymer [A] is not less than 50% by weight.