JPS62185149A - Discoloring time managing layer - Google Patents

Abstract

PURPOSE:To enable the improvement of wetting characteristic without using no wetting agent, by using a film produced from a multi-layer structure particle emulsion comprising a core and a shell for a discoloring time managing layer. CONSTITUTION:A discoloring time managing layer is interposed between an [A] layer containing a microcapsule of an oily substance and [B] layer containing a substance which will cause a discoloring when in contact with the oily substance and controls the required time for the oily substance which reaches [B] layer to cause a discoloring after breaking the microcapsule and flowing out. Then, the managing layer has a core made of (metha) ester acrylate copolymer using stylene as requisite component while having a shell made of (metha) ester acrylate copolymer using methyl methacrylate as requisite component and it is a continuous film obtained by drying a multi-layer structure particle emulsion comprising the core and shell. The time to the discoloring can be controlled according to purposes by changing the rate of the stylene and methyl methacrylate as respective requisite components of the core and shell.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a color change time management layer, and more particularly, a layer (A) comprising microcapsules of a liquid substance and a layer (A) comprising microcapsules of a liquid substance, It is located between the C83 layers containing a substance that causes discoloration, and controls the time until the liquid substance that ruptures the microcapsules and flows out reaches [B]H and causes discoloration. , relates to a color change time control layer for temperature control indicator labels for articles that require strict temperature control such as refrigerated or frozen foods.

[Conventional technology]

For products that require temperature control, it has been desired to develop a storage temperature control label that irreversibly changes color when the temperature exceeds a predetermined temperature. There are foods, refrigerated foods, fresh flowers, medicines, etc., and at present, no effective means has been found to detect whether or not these items are kept at normal controlled temperatures during storage or transportation. or,
Although it is necessary to strictly control the temperature of products such as frozen and refrigerated foods, it is difficult to know whether the product will be stored under the necessary temperature control and delivered to the user after it is manufactured. Therefore, a specific temperature management system is required in related industries.

In order to solve these problems, the present inventor conducted extensive research and applied an irreversible coloring system using a methine dye precursor and an oxidizing dye to the temperature display unit. proposed a characteristic and effective unit (Japanese Patent Application Laid-open No. 6O-53984), that is, the unit is (1
) A hydrophobic organic substance of any melting point, (2) a methine dye precursor, and (3) an oxidizing material represented by a benzoquinone derivative are used as essential constituent materials. The temperature control label used is the above fl.
+ microcapsules, (2) and (3), or microcapsules of (2) dissolved in il+, and (3) held on a support as essential components.

Although such units are extremely effective in performing strict temperature control, they still have the following problems in actual distribution processes. In other words, when the target item is exposed to a temperature higher than the specified temperature, spoilage and deterioration occur due to the propagation of various bacteria, etc., but the target time for rot and deterioration differs depending on the nature of the target item. Needless to say, there is therefore a need for an indicator that manages the discoloration time in accordance with the target time of deterioration and deterioration of the target article. Indicators (tags) used to simplify temperature control are affixed to the surface of the package where the target food, such as meat or fish, is packaged with polymeric materials, for example, when considering refrigerated foods. It is common to be attached. In other words, the indicator is immediately exposed to the storage environment temperature, but the meat, fish, etc. that are originally subject to control are not able to immediately follow the environmental temperature due to their own specific heat or the heat shielding effect of the packaging container, and it takes time. causing delays. In addition, even if the target item follows the environmental temperature, it will not only rot or deteriorate immediately and become unedible, but will also deteriorate depending on the environmental conditions over the course of several hours or tens of hours, such as the proliferation of various bacteria, etc. There are a wide variety of products that are subject to this, ranging from those that are inedible to those that are not edible, depending on the degree of food. In other words, there are different allowable storage times for edible food depending on the food.

Generally, refrigerated foods are required by law to be stored at temperatures below 10°C. However, not all things become inedible in a short period of time when exposed to environmental temperatures of 10°C or higher; on the other hand, for example, certain pharmaceuticals and catalysts may undergo reactions or deterioration to a certain degree. Therefore, a system that must maintain a strict storage temperature and that rapidly discolors when the storage temperature is exceeded is useful. In other words, each object has its own allowable storage time,
This is why we need a system that suits each individual. Therefore, it is desired to develop a system that can freely control the time required for discoloration depending on the purpose, such as the decomposition behavior of the target article.

The present inventors discovered that the above-mentioned problems could be solved by a color change time management layer made of a film made from a polymer emulsion containing a specific amount of methyl methacrylate or styrene, and filed a patent application earlier. (Special application 1986)
No. 0-205375, Japanese Patent Application No. 1983-205376).

[Problem that the invention seeks to solve]

As described above, the present inventors have succeeded in developing a system that allows the time until discoloration to be freely controlled depending on the objective such as the decomposition behavior of the target article. However, these polymer emulsions, etc., contain, for example, microcapsules of an oily substance [A] layer and CB) a layer containing an article that causes discoloration upon contact with the core oily substance, by coating with a roll bar or air knife. However, in order to apply it uniformly, wetting characteristics become a problem, and the addition of a wetting agent is essential. However, many wetting agents cause a depression of the melting point of oily substances with a given melting point. In addition, wetting agents that do not lower the melting point have insufficient wetting properties or have many problems.

[Means for solving problems]

As a result of intensive studies to solve these problems, the inventors of the present invention have developed a material that is manufactured from a multilayer structure particle emulsion consisting of a core and a shell for the discoloration time management layer. The present invention has been achieved based on the discovery that the above can be improved.

That is, the present invention is interposed between a layer (A) containing microcapsules of an oily substance and a layer CB) containing a substance that causes discoloration upon contact with the cedar oily substance, and destroys the microcapsules. In the discoloration time management layer, which controls the time required for the oily substance flowing out to reach the CB layer and cause discoloration, the control layer has a core (hereinafter abbreviated as core) made of styrene. It is a (meth)acrylic ester copolymer that has an essential component, and the shell (hereinafter referred to as shell) is a (meth)acrylic ester copolymer that has methyl methacrylate as an essential component.It consists of a core and a shell. This color change time management layer is characterized by being a continuous film obtained by drying a multilayer structure particle emulsion.

In the present invention, by changing the ratio of styrene and methyl methacrylate, which are essential components of each of the core and shell, it is possible to freely control the time until discoloration depending on the objective such as the decay and deterioration behavior of the target article. I can do it. Further, the color change time control layer of the present invention exhibits an excellent effect in preserving products that slowly rot and deteriorate, such as dairy products.

As mentioned above, the color change time management layer of the present invention is a continuous film obtained by drying a core-shell emulsion, and the core-shell emulsion used for this purpose can be prepared by various known polymerization methods, such as photopolymerization. , a thermal polymerization method, a radiation polymerization method, a ring-opening one-bath method, a radical polymerization method, etc. Among various polymerization methods, a radical polymerization method is preferably used, and a core-shell emulsion polymerization method using only a redox catalyst or a reducing agent in an aqueous medium is most preferably used.

In the core-shell emulsion silane used in the color change/time control layer of the present invention, the core is a (meth)acrylic acid ester copolymer containing styrene as an essential component, and the shell is a copolymer containing methyl methacrylate as an essential component. It is a (meth)acrylic acid ester copolymer. In this case, the components to be copolymerized are (a) ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, and crotonic acid; (b) aromatic substances such as vinyltoluene and vinylbenzene. (C) Methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, ethyl methacrylate, butyl methacrylate, 2-methacrylate
- alkyl esters of acrylic or methacrylic acid, such as ethylhexyl (containing methyl methacrylate in the core); (d) hydroxyalkyl acrylic or methacrylic acids, such as hydroxyethyl acrylate, hydroxyethyl methacrylate; ester, (e) acrylamide or methacrylamide derivatives such as acrylamide, methacrylamide, N-methylol acrylamide, N-methylol methacrylamide, further diacetone acrylamide, vinylpyrrolidone,
Various monomer components include glycidyl acrylate, glycidyl methacrylate, vinyl acetate, vinyl chloride, vinylidene chloride, acrylonitrile, and methacrylonitrile. Furthermore, in addition to the above-mentioned nonionic or anionic monomers, cationic vinyl monomers such as trimethylaminoethyl methacrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, and triethylaminoethyl methacrylate may be mentioned. Of course, the components used are not limited to these exemplified compounds.

Of course, there is no problem in using monomers with a fi of 34 fi or higher. Further, two or more types of copolymers may be used in combination.

These various components to be subjected to copolymerization may be polymerized in a premixed state, monomers may be added stepwise to perform block copolymerization, or graft copolymerization may be performed.

A continuous film obtained by drying the core-shell emulsion thus obtained is used as the color change time management layer of the present invention.

The core-shell emulsion in the present invention has an average particle diameter of 0.01 to 2.0μ, preferably 0.05 to 0.8μ.
White water with microscopic particles of μ? It is preferable to use an R4 liquid which has the property of causing polymer particles to fuse together to form a uniform film when the water evaporates upon drying. In addition, core-shell emulsions are applied to a support by coating, etc., and become a color change time control layer by drying and forming a film, but the layer formed by coating or printing is continuously formed by forming a film under general drying conditions. In order to effectively form a film, the glass transition point of the copolymer is preferably 80°C or lower, preferably 60°C or lower.

Specifically, the color development time management layer of the present invention comprises microcapsules of an oily substance provided on a support (A)
and a layer [81] comprising a substance that causes discoloration upon contact with the components in the microcapsules. As the support, a flexible film-like material such as silver or polymer is generally used, and (A)! Or [B]
The layers are generally coated by various coating methods such as air knife coating, bar coating, blade coach ink, clavia coating, roll coating, etc. or by various printing methods such as gravure printing, flexography, offset printing, screen printing, etc. Set it up by the method.

Then, the core-shell emulsion of the present invention is coated or printed by the method described above, and generally a coating method is used.

In a preferred embodiment, when the color change time management layer of the present invention is used, since it is obtained as a low viscosity aqueous suspension, a coating method suitable for applying a low viscosity coating liquid such as an air knife coater or a bar coater is generally preferred. It is used in

The color change time management layer of the present invention has a drying time of 0. O1~20
It is in the form of an extremely thin continuous film of μ, preferably about 0205 to 10 μ, and is formed by applying the coating method described above and then drying at a temperature raised to a temperature at which a continuous film can be sufficiently formed. Furthermore, on top of that there is a CB) layer or (
A) Provide a layer. These layers are also provided by coating or printing. In this way (A) [
and CB) l1ffl, the color change time management layer of the present invention can be provided.

In label-like products provided with the discoloration time management layer of the present invention, the time-controlled discoloration portion is often required to be in the form of letters or pictures, so the printing method includes, among other things, microcapsules. When printing the layers, screen printing is most preferably used, as printing methods that can cause microcapsules to break due to pressure are inconvenient.

Core-shell emulsion is a composition that is the main component of the color change time control layer provided between the microcapsule layer of the oily substance and the color developer layer, and has properties that play a role in controlling swelling, diffusion, penetration, etc. of the oily substance. It must have the following characteristics. Incidentally, a resin film with a large barrier effect itself does not allow oily substances to pass through, so it is not suitable as the discoloration time management layer of the present invention, and a resin film with a too small barrier effect allows oily substances to pass through. , which is still unsuitable as the color change time management layer of the present invention. In other words, in order to become the discoloration time control layer required in the present invention, it must have both appropriate barrier properties and appropriate carrier properties. This objective will be achieved. Furthermore, when the proportions of styrene and methyl methacrylate, which are preferred monomers, are used in the core and shell of the core-shell emulsion of the present invention are changed, there is no adverse effect on the leakage characteristics, and only the discoloration behavior changes. . In this case, the ratio of styrene in the (meth)acrylic ester copolymer serving as the core to methyl methacrylate in the (meth)acrylic ester copolymer serving as the shell is preferably 1:99 to 1:99 in terms of molar ratio. 70:30, even better Ma L < Ha1
0:90-50:50 (DJ'Q range).

It goes without saying that one or more other polymer emulsions, latexes, water-based polymers, etc. can be blended into the core-shell emulsion depending on the purpose of modification.

In the present invention, as means for causing discoloration, for example, color development using a methine dye precursor and a benzoquinone derivative as proposed in JP-A No. 60-53984, or color development using phthalide AM 8 and an organic or inorganic solid acid. can be used. For example, one of these color-forming components is dissolved in a hydrophobic organic compound having a melting point near the temperature that requires control to form an oily substance, microencapsulated by a known method, and contained in the (A) layer, and the other may be included in the CB) layer. Alternatively, the above hydrophobic organic compound may be microencapsulated and contained in the (A) layer, and both coloring components may be contained in the [B] layer. Variations of these may also be used. Various hydrophobic organic compounds are used depending on the purpose, but for example, n-butyl caprate, n-butyl caprate, which is a nonvolatile hydrophobic organic compound with a sharp melting point in the range of -30°C to +50°C. -Ethyl caprate, n-dodecyl acetate, ethyl n-undecanoate, n-decylhensen, undecyl caproate, heptyl caprate, n-butyl laurate, 〇-butyl myristate, n-dodecylbenzene, lauric acid Examples include methyl, ethyl myristate, isobutyl valmitate, n-butyl palmitate, methyl myristate, and the like.

In addition to the microcapsules and substances that cause discoloration, the layers (A) and CB) contain natural or synthetic polymeric binders, solvents, and inorganic or organic fillers for the purpose of improving coating and printability. It is common to include

When the temperature indicating layer provided with the color change time management layer of the present invention is used, the microcapsules are destroyed by roll pressure, impact pressure, etc. When the temperature reaches or exceeds the control temperature, the substance within the microcapsules melts and begins to change color after passing through the color change time management layer of the present invention by dissolution, permeation, diffusion, etc. The time required for the start of discoloration varies depending on the viscosity, polarity, molecular weight, etc. of the oily substance within the microcapsules, so the monomer composition, particle size, molecular weight, and film of the time management layer used will depend on the required time required for the start of discoloration. Correspond by changing the thickness, etc.

〔Example〕

The present invention will be explained below with reference to Synthesis Examples, Examples, and Comparative Examples. Note that % and parts are by weight unless otherwise specified.

Samples (A) to (F) were prepared by the method described below.

Synthesis Example 1 A stirrer was attached to a No. 11 round bottom flask equipped with a heating device,
Charge 513 g of water, IQg of sodium dioctyl sulfosuccinate as an emulsifier, and 1.0 g of Rongalisoto as a reducing agent. After heating to 90°C with slow stirring, 33.1 g of styrene monomer as a core material, 81.6 g of butyl acrylate monomer, and methacrylic acid. Monomer9.
A mixture of 1 g was added dropwise over 2 hours using a charge pump to obtain a white moist core material with an average particle size of 0.12 μm. In addition to this, 180.4 g of methyl methacrylate monomer and 163.2 g of butyl acrylate monomer were added as shell materials.

Methacrylic acid monomer 5.5g, acrylic acid monomer 3
, 1g mixed with 3.5g using a charge pump.
The mixture was added dropwise over a period of time, and the reaction was continued for an additional 2 hours to obtain 1000 g of a white cloudy emulsion. The average particle diameter of this product was 0.26μ, and the concentration was 47.6%.

The molar ratio of styrene to methyl methacrylate in the core and shell of the obtained core-shell emulsion was 15:85. This is designated as sample (1).

Synthesis Example 2 Into the same apparatus as in Synthesis Example 1, 483.8g of water, 12g of sodium dioctyl sulfosuccinate as an emulsifier, 1.2g of longalisotho as a reducing agent were charged, and after heating to 90°C while stirring slowly, styrene monomer was added as a core material. 1
15.9g, butyl acrylate monomer 85.7g-
A mixture of 9.6 g of methacrylic acid monomer was added dropwise using a charge pump over 2.5 hours to obtain a cloudy white core material with an average particle size of 0.11 μm. In addition to this, methyl methacrylate 111.
5g, butyl acrylate monomer 171.4g, methacrylic acid monomer 5.8g, acrylic acid monomer 3.
A mixture of 2 g was added dropwise over 4 hours using a charge pump, and the reaction was continued for an additional 2 hours to obtain 1000 g of a white cloudy emulsion. The average particle size of this product was 0.3μ, and the concentration was 50.3%. The molar ratio of styrene to methyl methacrylate in each of the obtained core-shell emulsions was 50:50. This is designated as a sample (If).

Synthesis Example 3 Into the same apparatus as in Synthesis Example 1, 483.6 g of water, 14 g of sodium dioctyl sulfosuccinate as an emulsifier, 1.4 g of Rongalite as a reducing agent were charged, and after heating to 90°C while stirring slowly, styrene monomer was added as a core material. 1
83.7g, butyl acrylate monomer 84.9g
A mixture of 9.5 g of methacrylic acid monomer was added dropwise using a charge pump over a period of 3 hours to obtain a cloudy white core material with an average particle size of 0.09 μm. To this thing,
Additionally, 44.2 g of methacrylic acid monomer was added as a shell material.
, butyl acrylate monomer 169.8g, methacrylic acid monomer 5.7g, acrylic acid monomer 3.2
A mixture of g was added dropwise using a charge pump over 4.5 hours, and the reaction was continued for an additional 2 hours to obtain 1000 g of a white cloudy emulsion. The average particle size of this product was 0.28μ, and the concentration was 50.1%. The molar ratio of styrene to methacrylic acid in each of the resulting core-shell emulsions was 80:20. This is designated as sample (III).

Synthesis Example 4 Into the same apparatus as Synthesis Example 1, 516.7 g of water and 10 g of sodium dodecylbenzenesulfonate were charged, and 0.9 g of potassium persulfate and 0 of sodium bisulfite were added as polymerization catalysts.
．． After adding 4g and raising the temperature to 75°C while stirring slowly,
Styrene monomer 110.7g, butyl acrylate monomer 218.2g, methacrylic acid monomer 29.
A mixture of 4 g of methyl methacrylate, 106.8 g of acrylic acid monomer, and 6.2 g of acrylic acid monomer was added dropwise at a rate of 5 g per minute using a charge pump, and 5 hours after charging the monomers, 1000 g of a white cloudy emulsion was obtained. The average particle size of this product is 0.26μ, and the concentration is 47.2%.
Met. The molar ratio of methyl methacrylate to styrene in the obtained copolymer was 50:50. This is designated as a sample (rV).

Synthesis Example 5 100g of lO% polyvinyl alcohol was placed in a 200ml stainless steel beaker, and 2.5-di-n
-butoxycarbonyl-3,6-siphenylsulfonyl-1,4-benzoquinone Ig was added and stirred at high speed (1
00ORPM) for 3 hours to obtain a color developer dispersion.

In a separate container, 50g water, 20g calcium carbonate, 10%
20 g of starch was added, 10 g of a color developer dispersion was added, and the mixture was stirred and mixed to obtain a coating liquid with a solid content of 23% and a viscosity of 150 centipoise. This is designated as sample (V).

Synthesis Example 6 Myristic acid ethyl ester in which 2% of 4'-bis-dimethylamino-3-methyl-4-ethoxy-triphenylmethane was dissolved was coated with a melamine resin film by the known In-3itu ff1 method. A microcapsule slurry was obtained (average particle size 4.0μ, solid content 50%).

A capsule-containing coating liquid was obtained by mixing 100 parts of the microcapsule slurry, 20 parts of styrene-butadiene latex, and 100 parts of water. This is designated as a sample (VT).

Core-shell emulsions (sample) obtained in Synthesis Examples 1 to 3
1) to (III)), styrene-methyl methacrylate copolymer emulsion obtained in Synthesis Example 4 (Sample (IV))
, the color developer obtained in Synthesis Example 5 (sample [■]), and the microcapsules of the dye precursor obtained in Synthesis Example 6 (sample [■]) were used in combination, and Examples 1 to 3 and Comparative Example 1 and Comparative Example 2 using 5% carboxymethylcellulose as the color change time management layer and Comparative Example 3 without the color change time management layer were conducted.

Example 1 Sample (V) was placed on general high-quality paper (tsubo ff180g/n ())
tJJl colorant 'fr No. 10 was coated using a coating roll bar to give a solid content of 5 g/rd, and after drying at 80° C. for 1 minute, a developer layer was obtained.

Sample this thing! ] was coated using a No. 10 coating roll bar to a solid content of 2 g/rd, and dried at 80°C for 1 minute.

Next, sample (Vl) microcapsules were added with a solid content of 6g/
No. 10 coating roll bar to make it td
It was coated using the following method and dried at 80°C for 1 minute. Through the above steps, a sample consisting of three layers, a color developer layer, a color change time control layer, and a microcapsule layer, was obtained. This will be referred to as sample (A).

Example 2 Sample [B] was obtained in the same manner as in Example 1, except that sample [■] was used as the core-shell emulsion.

Example 3 Sample [C] was obtained in the same manner as in Example 1 except that sample [[l] was used as a core-shell emulsion.

Comparative Example 1 Sample CD) was obtained in the same manner as in Example 1, except that sample (rV) was used instead of sample 4 [I].

Comparative Example 2 5% instead of core-shell emulsion of sample [Summer]
Sample (E) was obtained in the same manner as in Example 1, except that the carboxymethyl cellulose solution of 1 was used.

Comparative Example 3 Sample CF) was obtained in the same manner as in Example 1 except that the core-shell emulsion was not entirely coated. .

The above samples (1) to (rV) and the 5% carboxymethyl cellulose solution were tested for swelling characteristics, and the sample (
A discoloration test was conducted on A) to (F).

(Wetting property test conditions) l) Prepare 5 cleaned glass pieces of length 10 (J, width 5 co +, J! J size 21) and keep them horizontal. Next, samples (1) -
(rV) and 5% strength carboxymethylcellulose solution A was applied once on each glass piece to a thickness of 2μ using an applicator roll.

Dry at room temperature, observe the repellency per JJ with the naked eye, and express it as the number of repellents.

2) Place the sample (
V) Color developer was applied using a No. 10 coating roll bar so that the solid content was 5 g/rrr.
After drying for 1 minute at °C, a developer layer was obtained.

Prepare 5 sheets of this material, each with samples (1) to (IV
) and he 5z carpoxymethylcererose? The liquid was coated using a No. 10 coating roll bar so that the solid content was 2 g/rd, dried at 80°C for 1 minute, and then coated with a No. 10 coating roll bar.
Observe the hit repellents with the naked eye and express the number of repellents.

(Discoloration test conditions) ■) Microcapsule breaking machine varnish-per-calender (linear pressure 100 kg/am) 2) Microcapsule breaking temperature; normal temperature 3) Temperature history: After microcapsule breaking, leave at room temperature for 5 minutes, then 0°C After putting it in the freezer for 2 hours, the temperature was raised to 15°C, and then maintained at 15°C.

4) Color measurement: Visual observation (checked at any time) Next, the results of the crumbling characteristic test are shown in Table 1, and the results of the color change test are shown in Table 2.

〔Effect of the invention〕

The wetting property results shown in Table 1 and the discoloration test results shown in Table 2 will be explained above. By coating the core-shell emulsion of the present invention and then providing a color change time management layer consisting of a film obtained by drying, it is possible to avoid melting and melting with oil having a predetermined melting point without using a wetting agent. Wetting properties are improved without causing point drop, and discoloration becomes uniform.Furthermore, by changing the ratio of styrene, the core component, and methyl methacrylate, the shell component, it fully demonstrates its function as a discoloration time management layer. It became clear (Examples 1 to 3)
. Incidentally, in the case of the styrene-methyl methacrylate copolymer of Comparative Example 1, if no wetting agent was used, it would cause blurring, causing uneven coloring, and the coloring density could not be reached.

Further, as a result of examining the carboxymethyl cellulose of Comparative Example 2 as a color change time control layer, it was found to be eluted by moisture in the refrigerator and did not function as a time control layer at all. Furthermore, in Comparative Example 3, after the microcapsules were broken, the color changed before being placed in a refrigerator at 0° C., and the color change density immediately increased, making it impossible to control the time at all.

From the above, by using the color change time control layer obtained by coating and drying the core-shell emulsion of the present invention, it is possible to freely control the induction time to suit the allowable storage time of each article, and the temperature It is clear that it is extremely useful for management indicators.

Claims (1)

[Claims] [A] layer comprising microcapsules of an oily substance and a substance that causes discoloration upon contact with the oily substance.
In the discoloration time control layer that is interposed between the B] layers and controls the time required for the oily substance that breaks the microcapsules and flows out to reach the [B] layer and cause discoloration. , the management layer has a core made of a (meth)acrylic ester copolymer containing styrene as an essential component, and a shell made of a (meth)acrylic ester copolymer containing methyl methacrylate as an essential component. A color change time control layer characterized by being a continuous film obtained by drying a multilayer structure particle emulsion consisting of shells.