Microcapsule for heat-storing material

A microcapsule for heat-storing material which encapsulates a compound capable of undergoing phase transitions, said microcapsule containing a high-melting compound having a melting point 20.degree.-110.degree. C. higher than that of the compound capable of undergoing phase transitions.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a microcapsule for heat-storing material 
which is used for cooling or heating a substance or for maintaining a 
substance at a constant temperature. More particularly, the present 
invention relates to a microcapsule for heat-storing material which has a 
good fluidity under any temperature condition and a high stability over an 
extensive period of time, and can retain latent heat at a high density. 
The microcapsule for heat-storing material of the present. invention can 
be utilized as a heating and cooling medium for air conditioning, or it 
can be utilized as a portable high-temperature insulation material or 
low-temperature insulation material by holding in any of various packaging 
materials and containers. 
2. Related Art 
A heat-storing material most generally used every day is water. Water is 
used for high-temperature or low-temperature insulation usually in the 
form of hot water or ice and can be said to be the most inexpensive 
heat-storing material in daily life. 
In general, as compared with a method utilizing only sensible heat which 
does not accompany the phase transitions of a substance, a method of heat 
storage by utilization of latent heat accompanying the phase transitions 
has the following advantage. In this method, since a large quantity of 
thermal energy can be stored at a high density in a narrow temperature 
range including the melting point, the volume of a heat-storing material 
can be reduced, and moreover the heat loss can be kept small because no 
large temperature difference develops, considering the large quantity of 
heat stored. 
As a heat-storing material in which there is utilized latent heat 
accompanying phase transitions, in particular, phase transitions between 
liquid and solid, any heat-storing material can be used so long as it has 
a melting point or freezing point. There is preferably used a heat-storing 
material which is physicochemically stable and absorbs heat of fusion of 
20 kcal/kg or more in practical application. The following materials are 
generally known as typical heat-storing materials. 
(1) Inorganic compounds containing a large amount of water of 
crystallization, for example, calcium chloride hexahydrate, sodium sulfate 
decahydrate, sodium hydrogenphosphate dodecahydrate, sodium thiosulfate 
pentahydrate, and nickel nitrate hexahydrate. 
(2) Organic compounds, for example, aliphatic hydrocarbons such as 
tetradecane, pentadecane, cyclohexane, etc.; aromatic hydrocarbons such as 
benzene, naphthalene, etc.; fatty acids such as lauric acid, stearic acid, 
etc.; alcohols such as lauryl alcohol, stearyl alcohol; and ester 
compounds such as methyl stearate, methyl cinnamate, etc. 
For increasing the heat exchange efficiency of these various heat-storing 
materials, there have been proposed means for encapsulating the 
heat-storing materials in microcapsules [for example, Jap. Pat. Appln. 
Kokai (Laid-Open) Nos. SHO 62 (1987)-1452, SHO 62 (1987)-45680, SHO 62 
(1987)-149334, SHO 62 (1987)-225241, SHO 63 (1988)-115718, SHO 63 
(1988)-217196, and HEI 2 (1990)-258052]. 
All of the methods for encapsulation in microcapsules disclosed in the 
above references are capsulation methods in which water or any of the 
inorganic compounds belonging to the above group (1) is encapsulated in 
microcapsules. Also in the case of the organic compounds belonging to the 
group (2), i.e., the organic compounds which undergo phase transitions, 
namely, which have a melting point, the employment of a conventional 
capsulation method permits production of an oil-in-water type microcapsule 
dispersion which is seemingly rich in capsule solidness and fluidity. 
When there was produced an oil-in-water type dispersion of microcapsules 
encapsulating the aforesaid organic compound capable of undergoing phase 
transitions and a heat-storing operation was carried out, the following 
problem was found to be caused. The dispersion of microcapsules 
encapsulating the compound capable of undergoing phase transitions which 
is usable in the present invention, repeats heat absorption or heat 
dissipation on heating or cooling, respectively, and can be used for 
various purposes. It was found that in this case, there is caused a 
phenomenon that the melting point and freezing point of the compound 
capable of undergoing phase transitions which has been encapsulated in the 
microcapsules are different from each other, namely, a remarkable 
supercooling phenomenon. 
It is known that the supercooling phenomenon is usually caused in greater 
or lesser degree when a compound capable of undergoing phase transitions 
is cooled to fall into a solid state. In the case of a microcapsule 
encapsulating the compound capable of undergoing phase transitions, like 
the microcapsule of the present invention, the supercooling phenomenon is 
markedly accelerated, so that high energy has been necessary for the phase 
transitions. 
In general, as a method for preventing the supercooling phenomenon, there 
are known a method using a nucleating agent such as metal powder or clay 
powder, and a mechanical method in which agitation, slight vibration, 
application of an electric shock, or the like is carried out. When either 
of these methods is applied to the microcapsule of the present invention, 
the former method is disadvantageous in that the encapsulation of the 
powder in the microcapsule results in the deterioration of wall of the 
microcapsule and the restriction of particle size of the microcapsule by 
the particle size of the powder. The latter method, i.e., the mechanical 
method is hardly effective probably because the compound capable of 
undergoing phase transitions is completely segregated from the outside of 
the microcapsule. 
OBJECT AND SUMMARY OF THE INVENTION 
An object of the present invention is to provide a microcapsule for 
heat-storing material which encapsulates a compound capable of undergoing 
phase transitions and in which the supercooling phenomenon of the compound 
capable of undergoing phase transitions is prevented, namely, the 
difference between the melting point and freezing point of the compound is 
very small when heating and cooling are conducted. 
The present inventor investigated for achieving the above object and 
consequently made it possible to obtain a microcapsule for heat-storing 
material which contains a compound capable of undergoing phase transitions 
and a compound having a melting point higher than that of the compound 
capable undergoing phase transitions (hereinafter referred to as 
"high-melting compound"), and in which the supercooling of the compound 
capable of undergoing phase transitions is prevented by the presence of 
the high-melting compound, namely, the difference between the melting 
point and freezing point of the compound becomes very small, when heating 
and cooling are conducted, due to the presence of the high-melting 
compound. Thus, the present invention has been accomplished. That is, the 
present invention provides a microcapsule for heat-storing material which 
encapsulates a compound capable of undergoing phase transitions, said 
microcapsule containing a high-melting compound having a melting point 
20.degree.-110.degree. C. higher than that of the compound capable of 
undergoing phase transitions. Said microcapsule is explained below in 
detail. 
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
As the high-melting compound used in the present invention, a compound 
having a melting point 20.degree.-110.degree. C., preferably 
30.degree.-100.degree. C. higher than that of the compound capable of 
undergoing phase transitions is suitable. A compound having a melting 
point below the above temperature range is not preferable because the 
supercooling phenomenon cannot be sufficiently prevented. A compound 
having a melting point beyond the above range is also not preferable 
because for example, when the temperature drops to some extent in a 
capsulation step, such a compound separates out because of its 
insufficient miscibility with the compound capable of undergoing phase 
transitions. As to the content of the high-melting compound, this compound 
is contained in an amount of 0.5 to 40 (wt/wt) %, preferably 1 to 35 
(wt/wt) %, relative to the amount of the compound capable of undergoing 
phase transitions. When the content is below the above content range, the 
effect of the present invention is not sufficient. When the content is 
beyond the above content range, an undesirable influence on capsule wall 
formation reaction is brought about, and the quantity of heat stored is 
decreased in proportion to the decrease of the proportion of the compound 
capable of undergoing phase transitions. Therefore, both of such contents 
are not desirable. 
As the compound capable of undergoing phase transitions which is used in 
the present invention, any compound can be used so long as it has a 
melting point or a freezing point. Specifically, there can be used the 
above-exemplified inorganic compounds containing a large amount of water 
of crystallization and organic compounds such as aliphatic hydrocarbons, 
aromatic hydrocarbons, alcohols, fatty acids, ester compounds, etc. 
Preferable examples of the compound capable of undergoing phase 
transitions are straight-chain aliphatic hydrocarbons having 10 or more 
carbon atoms, such as tetradecane, pentadecane, hexadecane, eicosane, 
docosane, etc,; and ester compounds such as alkyl myristate, alkyl 
palmitate, alkyl stearate, etc. These compounds capable of undergoing 
phase transitions may be used as a mixture of two or more thereof for 
producing a heat-storing material having a melting point fit for a 
purpose. 
Specific examples of the high-melting compound used in the present 
invention are aliphatic hydrocarbon compounds, aromatic compounds, esters 
(including fats and oils), fatty acids, alcohols and amides. When the 
compound capable of undergoing phase transitions, i.e., the heat-storing 
material is a nonpolar compound such as an aliphatic hydrocarbon or an 
aromatic hydrocarbon, preferable examples of the high-melting compound are 
fatty acids, alcohols and amides which have a higher polarity than does 
the nonpolar compound. For example, when tetradecane (m.p. 6.degree. C.) 
is used as the compound capable of undergoing phase transitions, specific 
examples of the high-melting compound are cetyl alcohol, stearyl alcohol, 
eicosanol, myristic acid, palmitic acid, behenic acid, stearic acid amide, 
ethylenebisoleic acid amide, methylolbehenic acid amide and 
N-phenyl-N'-stearylurea. These compounds may be used singly or in 
combination of two or more thereof. 
A compound capable of undergoing phase transitions and a high-melting 
compound are subjected to capsulation by a suitable method for 
encapsulation in microcapsules. As a method for encapsulating a liquid 
nonmiscible with water in microcapsules, there can be employed, for 
example, a coacervation method using gelatin and an anionic macromolecule, 
an interfacial polymerization method, an in-situ method and a method using 
yeast (Jap. Pat. Appln. Kokai (Laid-Open) No. SHO 63 (1988)-88033, etc.). 
For the use of the capsulation product as a heat-storing material for a 
long period of time, the microcapsules are required to have a good 
physical durability during a thermal cycling from a low temperature region 
to a high temperature region and a high chemical stability to the compound 
capable of undergoing phase transitions and a medium for cooling or 
heating. For satisfying this requirement, it is most preferable to use 
microcapsules each composed of a membrane of an aminoplast resin which are 
formed by an in-situ method. 
The aminoplast resin is a resin obtained by polymerization reaction of an 
amino compound with formaldehyde. Specific examples of the aminoplast 
resin are urea-formaldehyde resins, melamine-formaldehyde resins and 
benzoguanamine-formaldehyde resins. The melamine-formaldehyde resins are 
the most preferable as the aminoplast resin. 
Encapsulation in microcapsules by the use of any of the above aminoplast 
resins can be carried out usually by the following procedure. 
1. A step of preparing an aminoplast resin precondensate. 
2. A step of emulsifying and dispersing the compound capable of undergoing 
phase transitions and the high-melting compound in an aqueous dispersant 
solution. 
3. A step of adding the precondensate prepared in the step 1 to the 
emulsion prepared in the step 2, and then stirring the resulting mixture 
with heating to form a membrane around each particle of a mixture of the 
compound capable of undergoing phase transitions and the high-melting 
compound. 
A specific example of process for the preparation of the aminoplast resin 
precondensate in the step 1 is given below by taking the case where a 
melamine resin is used. A water-soluble melamine-formaldehyde 
precondensate can be obtained by mixing melamine powder and formalin (a 
37% aqueous formaldehyde solution) in a molar ratio of 1:1 to 1:4 and 
heating the mixture at about 60.degree. C. or higher under weakly alkaline 
conditions. 
The amount of the aminoplast resin added is approximately 1-30 (wt/wt) %, 
preferably 5-20 (wt/wt) %, relative to the compound capable of undergoing 
phase transitions. When the amount is below the above range, the strength 
of the microcapsules is not sufficient. When the amount is beyond the 
above range, the content of the membrane material in a coldness-storing 
material becomes too high, so that the efficiency of heat storage is 
decreased. Therefore, both of such amounts are not desirable. 
Specific examples of the dispersant used in the step 2 are acrylic acid 
copolymers, ethylene-maleic anhydride copolymers, methyl vinyl 
ether-maleic anhydride copolymers, styrene-maleic anhydride copolymers, 
butadiene-maleic anhydride copolymers, vinyl acetatemaleic anhydride 
copolymers, and sodium salts thereof. These dispersants are added in an 
amount of 1.0 to 20.0 (wt/wt) % relative to the aqueous dispersant 
solution. 
The pH of the aqueous dispersant solution is adjusted to a pH at which the 
membrane formation reaction of the aminoplast resin proceeds most 
efficiently. Usually, the pH is adjusted to an acidic pH of 2 to 7, 
preferably 3 to 6. 
The step of emulsifying the compound capable of undergoing phase 
transitions and the high-melting compound is carried out by adding said 
two compounds to the aqueous dispersant solution, and stirring the 
resulting mixture by means of a commercially available emulsifying and 
dispersing apparatus or the like until the particle size of emulsified 
particles becomes approximately 1-50 .mu.m, preferably 1-5 .mu.m. 
Subsequently, the previously prepared aminoplast resin precondensate is 
mixed with the emulsion of the compound capable of undergoing phase 
transitions and the high-melting compound, and the resulting mixture is 
stirred with heating to polymerize the precondensate around the emulsified 
particles, whereby a water-insoluble resin is produced. Thus, 
microcapsules encapsulating said two compounds are obtained. The 
temperature at heating in the capsulation is 40.degree.-100.degree. C., 
preferably 60.degree.-80.degree. C. The stirring is conducted in the above 
temperature range for 30 minutes to 4 hours. 
Although the thus obtained dispersion of the microcapsules encapsulating 
the compound capable of undergoing phase transitions and the high-melting 
compound can achieve the object of the present invention as it is, there 
is, if necessary, obtained a desired heat-storing material in the form of 
an aqueous liquid by adding ethylene glycol, propylene glycol, various 
inorganic salts, antiseptics, various stabilizers, thickeners, colorants, 
dispersion assistants, specific gravity adjustors, wetting agents, etc. 
The higher the content of the microcapsules in the heat-storing material, 
the larger the quantity of latent heat. Therefore, the higher the content 
becomes, the more preferable it is. For maintaining a good fluidity, it is 
preferable to adjust the content to 20 to 70 (wt/wt) %, preferably 40 to 
60 (wt/wt) %. When the content is beyond the above range, the heat-storing 
material is increased in viscosity and hence becomes poor in fluidity. The 
content is below the above range, the heat-storing material is poor in 
coldness-storing effect. Therefore, both of such contents are not 
desirable.

The present invention is illustrated in detail with the following examples, 
which should not be construed as limiting the scope of. the invention. The 
melting point, freezing point and heat of fusion described in the examples 
were measured by means of a differential calorimeter (Model DSC-7, mfd. by 
Perkin Elmer Corp., 25 U.S.A.). 
EXAMPLE 1 
To 5 g of melamine powder were added 6.5 g of a 37% aqueous formaldehyde 
solution and 10 g of water, and the resulting mixture was adjusted to pH 8 
and then heated to about 70.degree. C. to obtain an aqueous 
melamine-formaldehyde precondensate solution. 
In 100 g of a 5% aqueous solution of sodium salt of a styrene-maleic 
anhydride copolymer which had been adjusted to pH 4.5 were dissolved 76 g 
of n-tetradecane (m.p. about 6.degree. C., heat of fusion 50.8 kcal/kg) as 
compound capable of undergoing phase transitions and 4 g of stearyl 
alcohol (m.p. 58.degree. C.) as high-melting compound. The resulting mixed 
Solution was added to the same aqueous sodium salt solution as above with 
vigorous stirring and emulsification was carried out until the particle 
size became 2.6 .mu.m. 
The whole of the aforesaid aqueous melamine-formaldehyde precondensate 
solution was added to the resulting emulsion and stirred at 70.degree. C. 
for 2 hours. Then, the pH of the resulting mixture was adjusted to 9 to 
terminate the capsulation. A rigid-polyethylene bag was packed with a 
mixture of 100 parts of the thus obtained microcapsule dispersion and 30 
parts of ethylene glycol to obtain a portable coldness-storing material. 
When the coldness-storing material was allowed to stand in a household 
freezer for about 1 hour, the coldness-storing material was not solidified 
and its coldness-storing effect lasted for a long period of time. 
EXAMPLE 2 
Capsulation was carried out in the same manner as in Example 1 except for 
using a mixed solution obtained by dissolving at about 90.degree. C., 79 g 
of n-eicosane (m.p. 37.degree. C., heat of fusion 59 kcal/kg) as compound 
capable of undergoing phase transitions, in place of tetradecane and 1 g 
of behenic acid (m.p. about 77.degree. C.) as high-melting compound. The 
microcapsule dispersion thus obtained was packed into a rigid-polyethylene 
bag to obtain a portable high-temperature insulation material. 
EXAMPLE 3 
Capsulation was carried out in the same manner as in Example 1 except for 
using a mixed solution obtained by dissolving 60 g of n-pentadecane (m.p. 
9.degree. C., heat of fusion 38 kcal/kg) as compound capable of undergoing 
phase transitions, in place of tetradecane and 20 g of stearic acid amide 
(m.p. about 100.degree. C.) as high-melting compound. The microcapsule 
dispersion thus obtained was packed into a rigid-polyethylene bag to 
obtain a portable low-temperature insulation material. 
Comparative Example 1 
Capsulation was carried out in the same manner as in Example 1, except that 
80 g of tetradecane was used alone. 
Comparative Example 2 
Capsulation was carried out in the same manner as in Example 1 except for 
using a mixed solution obtained by dissolving 76 g of tetradecane, i.e., 
the same compound as used in Example 1 and 4 g of lauryl alcohol (m.p. 
24.degree. C.) as high-melting compound. 
Comparative Example 3 
Capsulation was carried out in the same manner as in Example 1 except for 
using a mixed solution obtained by dissolving 76 g of tetradecane, i.e., 
the same compound as used in Example 1 and 4 g of ethylene-bisoleic acid 
amide (m.p. 118.degree. C.) as high-melting compound. Consequently, a 
large portion of the ethylene-bisoleic acid amide separated out on the 
surface of the resulting microcapsule dispersion. Thus, the high-melting 
compound could not be encapsulated in microcapsules. 
In Table 1, the degree of supercooling of the microcapsules obtained in 
Examples 1 to 6 and Comparative Examples 1 to 3 is expressed in terms of 
the difference (.DELTA.T) between the melting point and the freezing point 
measured by means of a differential calorimeter. The smaller .DELTA.T 
value means the lower degree of supercooling. 
TABLE 1 
__________________________________________________________________________ 
Compound capable of under- 
going phase transitions 
High-melting compound 
Melting Content* 
Melting 
point .degree.C. 
% point .degree.C. 
.DELTA.T 
__________________________________________________________________________ 
Example 1 
Tetradecane 
6 Stearyl alcohol 
5.3 58 1.8 
Example 2 
Eicosane 37 Behenic acid 
1.3 77 2.5 
Example 3 
Pentadecane 
9 Stearic acid amide 
33.3 100 2.8 
Example 4 
n-Hexadecane 
11 Lauric acid amide 
1.0 84 4.9 
Methyl myristate 
Example 5 
Methyl palmitate 
15 Lauric acid amide 
1.0 84 2.6 
n-Hexadecane 
Example 6 
Undecanol 
10 Lauric acid amide 
1.0 84 4.8 
n-Hexadecane 
Comparative 
Tetradecane 
6 None -- -- 25 
Example 1 
Comparative 
Tetradecane 
6 Lauryl alcohol 
5.3 24 16 
Example 2 
Comparative 
Tetradecane 
6 Ethylenebisoleic 
5.3 118 21 
Example 3 acid amide 
__________________________________________________________________________ 
*Note: The content is given in (wt/wt) percents, relative to the amount? 
of the compound capable of undergoing phase transitions. 
EXAMPLE 4 
Capsulation was carried out in the same manner as in Example 1 except for 
using a mixed solution (.DELTA.T=4.9.degree. C.) obtained by dissolving at 
90.degree. C. a mixture (m.p. 1.degree. C., heat of fusion 38kcal/kg) of 
50 g of methyl myristate (m.p. 18.degree. C.) and 50 g of n-hexadecane 
(m.p. 18.degree. C.), as compound capable of undergoing phase transitions, 
in place of tetradecane and 1 g of lauric acid amide (m.p. about 
84.degree. C.) as high-melting compound. A desirable result was obtained 
as in Example 1. 
EXAMPLE 5 
Capsulation was carried out in the same manner as in Example 1 except for 
using a mixed solution (.DELTA.T=2.6.degree. C.) obtained by dissolving at 
90.degree. C. a mixture (m.p. 15.degree. C., heat of fusion 28 kcal/kg) of 
50 g of methyl palmitate (m.p. 28.degree. C.) and 50 g of n-hexadecane 
(m.p. 18.degree. C.), as compound capable of undergoing phase transitions, 
in place of tetradecane and 1 g of lauric acid amide (m.p. about 
84.degree. C.) as high-melting compound. A desirable result was obtained 
as in Example 1. 
EXAMPLE 6 
Capsulation was carried out in the same manner as in Example 1 except for 
using a mixed solution (.DELTA.T=4.8.degree. C.) obtained by dissolving at 
90.degree. C. a mixture (m.p. 10.degree. C., heat of fusion 40 kcal/kg) of 
50 g of undecanol (m.p. about 16.5.degree. C.) and 50 g of n-hexadecane 
(m.p. 18.degree. C.), as compound capable of undergoing phase transitions, 
in place of tetradecane and 1 g of lauric acid amide (m.p. about 
84.degree. C.) as high-melting compound. A desirable result was obtained 
as in Example 1. 
In the examples, the .DELTA.T values shown in Table 1 indicate the degree 
of supercooling of microcapsules encapsulating each compound capable of 
undergoing phase transitions. It is clear that the degree of supercooling 
of the microcapsule obtained by the use of a mixture of the compound 
capable of undergoing phase transitions and a high-melting compound is 
lower than that of the microcapsule containing no high-melting compound. 
In the case of the microcapsule obtained by the use of a high-melting 
compound having too high a melting point, a large portion of this compound 
separated out in the capsulation step and could not be encapsulated in the 
microcapsule.