JP7285085B2 - Insulation material and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a heat insulating material using a porous structure such as airgel and a method for producing the same.

Silica airgel is a porous material having a pore structure with a size of about 10 to 50 nm in which silica fine particles are connected to form a skeleton. The thermal conductivity of silica airgel is less than that of air. For this reason, the development of heat insulating materials that take advantage of the high heat insulating properties of silica airgel is progressing.

For example, Patent Literature 1 describes, as a heat insulating layer constituting a heat insulating sound absorbing material, a heat insulating layer containing airgel particles and a binder that binds the airgel particles together. The heat insulating layer described in the same document is produced by press-molding a mixture of airgel particles and a binder. Paragraph [0020] of the same document describes that the heat insulation is improved when the number of pores between airgel particles is small and the filling rate of the airgel particles is high.

Patent Literature 2 describes a heat insulating material in which a plurality of airgel particles are bonded with an adhesive. In paragraph [0041] of the same document, since the heat insulating property is reduced when voids are formed inside the heat insulating material, in order to prevent the generation of voids, the average particle size of the airgel particles should be 500 μm or more. Are listed. Paragraphs [0074] to [0077] describe that the airgel particles to which the adhesive has been attached are placed in a press and are heat-pressed to produce a heat insulating material.

Patent Literature 3 describes a heat insulating material containing airgel particles having a water-soluble silane coupling agent layer on their surfaces and a binder that bonds the airgel particles together. In paragraphs [0035] and [0036] of the same document, it is stated that when the density of the heat insulating material is low, the gaps between the airgel particles become large and the heat insulating properties deteriorate. It is stated to be 5 g/cm ³ . Paragraph [0041] describes that a mixture of airgel particles and a binder is heat-pressed to produce a heat insulating material.

Patent Document 4 describes a heat insulating sheet comprising an airgel layer (heat insulating layer) formed by binding airgel particles with a rubber-based binder and having a density of 0.1 to 0.5 g/cm ³ . Paragraphs [0038] and [0039] of the same document describe that the ratio of gaps between airgel particles is not so large when viewed from the entire airgel layer, and the volume ratio is about 10% at maximum. In addition, it is described that when the density of the airgel layer is low, the gaps between the airgel particles become large and the amount of air increases, resulting in a decrease in heat insulating properties. Paragraphs [0071] to [0076] of the same document describe the production of an airgel layer by heating and pressurizing a mixture of airgel particles and a rubber-based binder.

JP 2015-197662 A WO2014/024413 JP 2018-100679 A WO2015/045211 Japanese translation of PCT publication No. 2013-534958

As described in Patent Documents 1 to 4 above, heat insulating materials using airgel particles are produced by press-molding a mixture of airgel particles and a binder. In order to improve the heat insulating property, the development is proceeding based on the idea that it is effective to fill the airgel particles as high as possible and reduce the voids between the airgel particles to increase the density. However, when the airgel particles are highly filled, the heat insulating material becomes brittle, so cracks occur on the surface, and the airgel particles are likely to fall off (so-called powder falling off).

On the other hand, as described in Patent Document 5, instead of press molding, a method of preparing a paint by dispersing airgel particles in a binder liquid and drying the paint to produce a heat insulating material is also known. According to this method, since the paint shrinks and warps when it dries, there is a problem that the obtained heat insulating material is likely to crack or fall off.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a heat insulating material which is excellent in heat insulating properties and is less likely to crack or fall off, and a method for manufacturing the same.

(1) In order to solve the above problems, the heat insulating material of the present invention includes a plurality of porous structures having a skeleton formed by connecting a plurality of particles and having pores inside, and the porous structures being bonded to each other. and a binder, voids exist between the porous structures, and the volume ratio of the voids in the heat insulating material is 10% or more and 55% or less. .

(2) The method for producing a heat insulating material of the present invention includes a paint preparation step of preparing a paint in which a porous structure is dispersed in a binder liquid, an adjustment step of adjusting the state of gas contained in the paint, and the paint is applied to the surface of the base material and dried.

(1) The porous structure constituting the heat insulating material of the present invention has pores inside which form a skeleton formed by connecting a plurality of particles. The size of pores formed between skeletons of the porous structure is about 10 to 50 nm, and most of the pores are so-called mesopores of 50 nm or less. Since the mesopores are smaller than the mean free path of air, heat transfer is impeded. Therefore, the heat insulating material of the present invention has excellent heat insulating properties.

In the heat insulating material of the present invention, voids exist between the porous structures. The volume ratio of voids in the heat insulating material is 10% or more and 55% or less. As described above, conventionally, the idea was to reduce the gaps between the porous structures as much as possible and to highly fill the porous structures to improve the heat insulation. However, in the present invention, voids are positively present between the porous structures based on the opposite idea. As a result, when adopting a method of preparing a paint by dispersing a porous structure in a binder liquid as a method of manufacturing a heat insulating material, applying the paint to a base material, and drying the paint, shrinkage strain during drying causes voids. It is possible to suppress the occurrence of cracks and the accompanying falling off of the porous structure (powder falling off). Further, according to the studies of the present inventors, it has been confirmed that if the volume ratio of the voids is 10% or more and 55% or less, the change in thermal conductivity is small and the desired heat insulating properties can be secured.

As the voids increase, the binder can be efficiently used for binding the porous structures together, so that the bonding strength between the porous structures increases, which is effective in preventing the porous structures from coming off. Further, according to the heat insulating material of the present invention, it is possible to secure desired heat insulating properties without highly filling the porous structure, so that cost reduction can be achieved.

(2) The method for producing a heat insulating material of the present invention has an adjustment step of adjusting the state of the gas contained in the paint after preparing the paint. In the adjustment process, the amount of gas such as air contained in the paint is adjusted, the size of air bubbles is adjusted, and the size of air bubbles is made uniform. As a result, the paint can contain an appropriate amount of gas, and a uniform coating film with little unevenness can be formed in the next coating step. When the coating film dries, the voids formed between the porous structures cushion the distortion caused by shrinkage, thereby suppressing the occurrence of cracks.

Further, according to the method for producing a heat insulating material of the present invention, since the surface of the coating film is not pressurized, minute irregularities are likely to be generated on the surface of the heat insulating material to be obtained. The presence of air in the unevenness increases the thermal resistance of the interface. Therefore, it is advantageous in improving the heat insulating properties of the heat insulating material, compared with the hot pressure molding method such as press molding.

BRIEF DESCRIPTION OF THE DRAWINGS It is a cross-sectional schematic diagram in one Embodiment of the heat insulating material of this invention. 4 is a top view photograph of the heat insulating material of Example 1. FIG. It is a SEM photograph of the cross section in the thickness direction of the same heat insulating material (magnification: 50 times). 4 is a top view photograph of the heat insulating material of Comparative Example 1. FIG. It is a SEM photograph of the cross section in the thickness direction of the same heat insulating material (magnification: 50 times).

First, one embodiment of the heat insulating material of the present invention will be described. In FIG. 1, the cross-sectional schematic diagram of the heat insulating material of this embodiment is shown. As shown in FIG. 1, the heat insulating material 1 has a plurality of porous structures 10 and a binder 11. As shown in FIG. The porous structure 10 is a silica airgel having a skeleton formed by connecting a plurality of silica particles and having pores inside. The binder 11 is composed of an aqueous emulsion binder containing styrene-butadiene rubber as a binder component. The binder 11 binds the porous structures 10 together. A void 12 exists between the porous structures 10 . The volume ratio of the voids 12 in the heat insulating material 1 is 36.5%.

Since the heat insulating material 1 has the porous structure 10 (silica airgel), it has excellent heat insulating properties. Silica airgel has hydrophobic sites on the surface and inside. Therefore, the hydrophilic aqueous emulsion binder hardly penetrates into the pores of the silica airgel and does not inhibit the heat insulating effect of the silica airgel. Voids 12 exist between the porous structures 10 at a volume ratio of 36.5%. Even if the voids 12 are present, the desired thermal insulation is maintained because the volume ratio is 10% or more and 55% or less. When the heat insulating material 1 is manufactured by applying and drying a paint in which the porous structure 10 is dispersed in a binder liquid, shrinkage strain during drying is buffered by the voids 12 . This suppresses the occurrence of cracks, and further suppresses falling off (powder falling off) of the porous structure.

Although one embodiment of the heat insulating material of the present invention has been described above, the heat insulating material of the present invention is not limited to the above-described form, and modifications that can be made by those skilled in the art without departing from the gist of the present invention. It can be implemented in various forms with improvements and the like. Next, preferred embodiments of the heat insulating material of the present invention and the method for manufacturing the same will be described.

<Insulation material>
The heat insulating material of the present invention has a plurality of porous structures and a binder that binds the porous structures together.

[Porous structure]
The porous structure has a skeleton formed by connecting a plurality of particles and has pores inside. It is desirable that the diameter of the particles (primary particles) forming the skeleton is about 2 to 5 nm, and the size of the pores formed between the skeletons is about 10 to 50 nm. Many of the pores are so-called mesopores of 50 nm or less. The shape of the porous structure is not particularly limited, and may be a spherical shape, an irregular block shape, or the like. When the maximum length of the porous structure is defined as the particle size, the average particle size of the porous structure is desirably about 1 to 200 μm. The larger the particle diameter of the porous structure, the smaller the surface area and the larger the pore volume. For example, those having an average particle size of 10 μm or more are suitable. On the other hand, considering the stability of the paint in which the porous structure is dispersed in the binder liquid, the ease of coating, and the prevention of falling off of the porous structure in the heat insulating material, the average particle size is 100 μm or less. is preferred. In addition, when two or more types with different particle diameters are used together, the small-diameter porous structure enters the gap between the large-diameter porous structures, so the filling amount can be increased while maintaining the voids, resulting in heat insulation. The effect of increasing the

Porous structures include hydrophilic ones having hydrophilic sites on the surface and inside, and hydrophobic ones having hydrophobic sites. Among these, the hydrophilic porous structure is fragile and easily crumbles. In addition, there is a possibility that moisture or the like may enter the inside and the pores may be crushed. Therefore, the heat insulating material of the present invention is preferably a hydrophobic one having a hydrophobic site on at least the surface of the surface and the inside. When a hydrophobic porous structure is used, when an aqueous binder containing water as a solvent is used, the binder is less likely to enter the pores of the porous structure, and thus the heat insulating properties are less likely to be impaired. Moreover, the surface of the porous structure may be treated with a silane coupling agent or the like. Surface treatment can impart hydrophilicity or hydrophobicity to the surface of the porous structure.
The type of porous structure is not particularly limited. Examples of primary particles include silica, alumina, zirconia, and titania. Among them, from the viewpoint of excellent chemical stability, a silica airgel in which the primary particles are silica, that is, a silica airgel in which a plurality of silica particles are linked to form a skeleton is desirable. Silica airgel is white and reflects infrared rays. Therefore, the use of silica airgel can impart a heat shielding effect to the heat insulating material.

Silica airgel having a hydrophobic site on at least the surface of the surface and the interior can be produced by subjecting it to a hydrophobizing treatment such as imparting a hydrophobic group during the production process. If at least the surface has a hydrophobic site, it is possible to suppress penetration of moisture and the like, so that the pore structure is maintained and the heat insulating property is less likely to be impaired. The method for producing silica airgel is not particularly limited, and the drying process may be carried out under normal pressure or under supercritical conditions. For example, if the hydrophobizing treatment is performed before the drying step, the need for supercritical drying is eliminated, that is, the drying can be performed under normal pressure, which facilitates production at low cost. Silica airgel may be produced, for example, by the method described in Japanese Patent No. 5250900.

The content of the porous structure in the heat insulating material may be appropriately determined in consideration of the thermal conductivity, flexibility, mechanical strength, etc. of the heat insulating material. For example, from the viewpoint of reducing the thermal conductivity (increasing the heat insulating property), the content of the porous structure is preferably 40% by mass or more when the total mass of the heat insulating material is 100% by mass. . More preferably, it is 50% by mass or more and 65% by mass or more. On the other hand, if there are too many porous structures, the heat insulating material may easily fall off, or the heat insulating material may become hard, resulting in a decrease in mechanical strength. Therefore, the content of the porous structure is desirably 75% by mass or less when the total mass of the heat insulating material is 100% by mass.

[binder]
The type of binder is not limited as long as it can bind the porous structures together. When the porous structure is hydrophobic, an aqueous binder using water as a solvent is suitable because it is difficult to penetrate into the pores of the porous structure. As the water-based binder, there are water-soluble binders and emulsion-like binders. Among them, emulsion-like binders (aqueous emulsion-based binders) are preferable. Aqueous emulsion binders are emulsified by introducing surfactants or hydrophilic groups. According to the aqueous emulsion type binder, the surfactant and hydrophilic group volatilize during drying, which reduces the hydrophilicity and makes it difficult to dissolve in water. The emulsification method may be a forced emulsification type using a surfactant as an emulsifier or a self-emulsification type in which a hydrophilic group is introduced.

The binder component may be resin or rubber. The glass transition temperature (Tg) of the binder is preferably −5° C. or lower, more preferably −20° C. or lower, from the viewpoint of high adhesiveness to the porous structure and softening of the heat insulating material to make it difficult to crack. For example, in the case of an aqueous emulsion binder, it may be a resin emulsion or a rubber emulsion. Examples of resins include acrylic resins, urethane resins, mixtures of acrylic resins and urethane resins, and the like. Examples of rubber include styrene-butadiene rubber (SBR), nitrile rubber, silicone rubber, urethane rubber, and acrylic rubber. Urethane resin, styrene-butadiene rubber, and the like are suitable from the viewpoint of making the heat insulating material flexible. From the viewpoint of increasing the strength of the binder portion and improving the strength of the heat insulating material, a cross-linking agent or the like may be used in combination to cross-link the binder component.

The content of the binder may be appropriately determined in consideration of the bondability between the porous structures, the thermal conductivity of the heat insulating material, and the like. For example, from the viewpoint of increasing the bonding strength between porous structures, the content of the binder is preferably 10% by mass or more when the total mass of the heat insulating material is 100% by mass. It is more suitable in it being 15 mass % or more. On the other hand, from the viewpoint of reducing the thermal conductivity (increasing the heat insulating property), the content of the binder is preferably 25% by mass or less when the total mass of the heat insulating material is 100% by mass. It is more preferable that it is 20% by mass or less.

[Other ingredients]
A porous structure having hydrophobic sites on its surface or inside does not easily absorb water. Among them, silica airgel has a low specific gravity, so it easily floats on water. Therefore, it is difficult to disperse silica airgel in a binder liquid using water as a solvent, and the dispersing process takes time. Moreover, even if the paint is prepared, there is a problem that the silica airgel separates from water and tends to float. Moreover, when pressure is applied to the paint, there is a risk of separation, making it difficult to apply with a coating machine and unsuitable for continuous production.

In order to solve such problems, the heat insulating material of the present invention desirably contains a polysaccharide in addition to the porous structure and the binder. Polysaccharides are glycosidic bonds of one or more monosaccharides and have high viscosity. As will be described later, when a paint is prepared by adding a porous structure to a binder liquid, adding a polysaccharide increases the viscosity of the paint, making it difficult to separate the porous structure from the binder liquid. Therefore, the dispersibility of the porous structure is improved, and the porous structure can be stably retained in the paint. Also, when the viscosity of the paint is high, it becomes difficult to drip, so the paint can be easily applied to the base material. Polysaccharide suppresses the separation of the porous structure by thickening due to the entanglement of molecular chains. Therefore, even if a polysaccharide is blended, a heat transfer path is unlikely to be formed, and the heat insulating properties are unlikely to deteriorate.

For example, when the porous structure is hydrophobic, if a polysaccharide having both a hydrophilic site and a hydrophobic site is blended, the hydrophobic site selectively binds to the hydrophobic site of the porous structure, and the hydrophilic site becomes the porous structure. By being placed around the body, it becomes like a protective colloid. This action also suppresses the separation of the binder liquid and the porous structure, and improves the dispersibility of the porous structure. As a result, the time required for dispersion can be shortened, making it easier to make a paint. In addition, polysaccharides having hydrophilic moieties are less likely to enter pores of a hydrophobic porous structure.

Polysaccharides include carboxymethylcellulose, carboxyethylcellulose, carboxypropylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, xanthan gum, agarose, carrageenan and the like. In particular, those with a long main chain and no or short side chains have more entanglement of the molecular chains. As a result, since the retention of the porous structure is enhanced, it is possible to suppress the falling off of the porous structure from the heat insulating material. For example, carboxymethylcellulose is suitable.

As described above, the heat insulating material of the present invention may contain other components (additives) such as a cross-linking agent and polysaccharide in addition to the porous structure and the binder.

[Gap]
The heat insulating material of the present invention has voids between porous structures. There are a plurality of voids, and considering the mechanical strength and thickness of the heat insulating material, the size of each void is preferably less than the thickness of the coating film, for example, 500 μm or less, more preferably 300 μm or less. As used herein, the size of the void means the maximum length of the void.

The volume ratio of voids in the heat insulating material is 10% or more and 55% or less. If the volume ratio of the voids is less than 10%, when a heat insulating material is produced by applying a paint in which the porous structure is dispersed in a binder liquid and drying it, shrinkage strain during drying cannot be buffered. , cracks are more likely to occur. A preferred void volume fraction is greater than 10%, or even greater than 25%. Conversely, if the volume ratio of the voids exceeds 55%, the mechanical strength of the heat insulating material is reduced and the heat insulating material becomes brittle. A preferable void volume ratio is 40% or less.

The volume ratio of voids can be calculated by the following formula (I), or can be measured by observing the cross section of the heat insulating material with an electron microscope and analyzing the image of the obtained cross section photograph. The calculated value of the formula (I) and the image analysis value of the cross-sectional photograph substantially match. For example, if the measured density of the heat insulating material is unknown, the image analysis value based on the cross-sectional photograph may be adopted, and if the cross-sectional photograph is not clear, the calculated value of formula (I) may be adopted.
Volume ratio of voids (%) = [Volume of air] / [Volume of solid content * 1 total calculated from the measured density of insulation material] x 100 (I)
In the formula (I), [volume of air] is calculated by the following formula (Ia).
[Volume of air] = [Total volume of solid content *1 calculated from the measured density of the insulating material] - [Sum of calculated values of individual volumes of solid content *1] (Ia)
*1 Solid content includes porous structures, binders and additives.

In this specification, the density of the heat insulating material is measured by the "liquid weighing method" defined in JIS Z 8807:2012, and the value is defined as the "measured density of the heat insulating material". Note that the measured density of the heat insulating material is a value in which not only the voids between the porous structures but also the pores in the porous structures contribute. From the viewpoint of containing desired voids, low thermal conductivity, and mechanical strength that can withstand practical use, the measured density of the heat insulating material is less than 0.15 g/cm ³ , further less than 0.1 g/cm ³ is desirable.

[others]
The thickness of the heat insulating material of the present invention may be appropriately determined in consideration of heat insulating properties, durability, and the like. If the heat insulating material is too thin, the desired heat insulating effect cannot be obtained. Therefore, it is desirable that the thickness of the heat insulating material is 0.2 mm or more. Conversely, if the insulation is too thick, it is not only costly but also weak and brittle. Therefore, it is preferable that the thickness of the heat insulating material is 1 mm or less, more preferably 0.5 mm or less.

As will be described later, the heat insulating material of the present invention is desirably produced by coating and drying the coating. For example, press molding makes the surface of the insulating material uniform by applying pressure. On the other hand, when a coating is applied and dried to manufacture, since no pressure is applied, fine irregularities are likely to be generated on the surface of the coating film (heat insulating material). When air intervenes in the unevenness, the thermal resistance of the interface increases, which is advantageous for improving heat insulation. For example, the arithmetic mean roughness (Ra) of the surface of the heat insulating material is desirably 0.1 μm or more and 20 μm or less.

For example, the heat insulating material of the present invention is desirably flexible from the viewpoint that it can maintain its shape without cracking even when it is arranged on a curved surface. For example, the elastic modulus is desirably 0.01 MPa or more, 0.02 MPa or more, 100 MPa or less, and 50 MPa or less. The elongation at break is desirably 1.5% or more, 2.0% or more, and 10% or less. The tensile strength at break is desirably 0.10 MPa or more and 1 MPa or less. For the elastic modulus, elongation at break, and tensile strength at break, values measured by performing a tensile test specified in JIS K 6251:2010 are adopted. The tensile test is performed using a dumbbell-shaped No. 3 test piece with a distance between chucks of 20 mm and a tensile speed of 20 mm/min. The elastic modulus is calculated from the slope of the stress-strain (SS) curve between 0.1 and 0.4 mm of tensile distance.

<Method for manufacturing heat insulating material>
The method for manufacturing the heat insulating material of the present invention is not particularly limited, but the manufacturing method of the present invention described below is suitable from the viewpoint of containing voids at a desired volume ratio. The method for manufacturing a heat insulating material of the present invention has a paint preparation process, an adjustment process, and a coating process.

[Paint preparation process]
This step is a step of preparing a paint in which a porous structure is dispersed in a binder liquid. The binder liquid is a liquid in which a binder is dissolved or dispersed in a solvent such as water. In this step, a coating material may be prepared by adding additives such as a porous structure, a binder, and optionally a polysaccharide and a cross-linking agent to a solvent such as water. For example, a porous structure having hydrophobic sites on its surface or inside does not easily absorb water. On the other hand, when the specific gravity is small, it tends to float in water and is difficult to disperse. Therefore, when adding a polysaccharide in consideration of the dispersibility of the porous structure, it is preferable to add the binder and the polysaccharide to water to increase the viscosity of the liquid before adding the porous structure.

[Adjustment process]
This step is a step of adjusting the state of the gas contained in the paint prepared in the previous step. Preparation of the paint in the previous step is often carried out while stirring. Agitation of paint in air entraps air in the paint and increases the amount of gas it contains. In this step, the amount of gas such as air contained in the paint is adjusted, the size of air bubbles is adjusted, and the size of air bubbles is made uniform. For example, the stirring speed, stirring time, etc. may be adjusted to continue stirring the paint, defoaming the paint, or blowing gas into the paint. Examples of the defoaming treatment include stirring treatment using a rotating and revolving mixer, standing still, and the like.

From the viewpoint that the gas state can be easily adjusted, it is desirable that this step includes a stirring step of stirring the paint. Furthermore, it is desirable to have a standing step of allowing the paint to stand after the stirring step. The higher the stirring speed and the longer the stirring time, the greater the amount of gas contained in the paint. Conversely, the longer the standing time after stirring, the less the amount of gas contained in the paint due to defoaming. The agitation may be impeller agitation, or positive shearing force or ultrasonic waves may be applied. A rotation/revolution stirring device or a media-type stirring device may be used.

[Coating process]
This step is a step of applying the paint whose gas state has been adjusted in the previous step to the surface of the substrate and drying it. Examples of the material of the base material include cloth such as nonwoven fabric and resin. The shape of the substrate is not particularly limited, and may be film-like or molded. For coating, a coating machine such as a blade coater, bar coater, die coater, comma coater (registered trademark), roll coater, or the like, or a spray may be used. Alternatively, the substrate may be dipped in paint. When the base material is made of cloth or porous material, part of the applied paint impregnates the inside of the base material. Drying may be performed at a temperature of 80 to 120° C. for several minutes to several tens of minutes.

EXAMPLES Next, the present invention will be described more specifically with reference to examples.

<Manufacture of heat insulating material>
[Example 1]
First, in water, styrene-butadiene latex A ("A7609" manufactured by Asahi Kasei Co., Ltd., solid content 52.2% by mass, binder component Tg 2 ° C.) as an aqueous emulsion binder, and carboxyl methyl cellulose (CMC: (molecular weight 380,000, density 0.90 g/cm ³ ), and while stirring with a stirring blade, silica airgel having hydrophobic sites on the surface and inside (average particle size 90 μm, central pore size 30 nm, density 0.90 g/cm 3 ). 11 g/cm ³ ) was added (paint preparation step). After continuing to stir with the stirring blade for 30 minutes, the mixture was allowed to stand still for 60 minutes to produce a paint (stirring step, standing step). The rotation speed of the stirring blade in the stirring process was 1000 rpm. Silica airgel was produced as follows according to the method described in paragraphs [0102] to [0106] of Japanese Patent No. 5250900.

First, a surfactant and urea were added and dissolved in an acetic acid aqueous solution. Methyltrimethoxysilane (MTMS) was then added with stirring. The molar ratio at this time was MTMS:water:acetic acid:urea=1:15.9:0.0860 (5 mM):1.43. Subsequently, the mixed solution was stirred at room temperature for 30 minutes, placed in a sealed container, and allowed to stand at 60° C. for 3 days for gelation and aging. After that, the resulting wet gel was immersed in methanol at 60° C. for 8 hours and washed. A total of three washes were performed. Subsequently, the washed wet gel was immersed in hexane at 55° C. for 8 hours for solvent exchange. A total of two solvent exchanges were performed. The solvent-exchanged wet gel was then air-dried overnight and dried in an oven at 100° C. until no weight change occurred. Thus, a silica airgel having hydrophobic sites on the surface and inside was obtained. The obtained silica airgel was pulverized with a pulverizer until the particle diameter became about 90 μm before use.

Next, a polyethylene terephthalate (PET) film was blade-coated with the paint after standing. And it was made to dry under 100 degreeC for 1 hour, and the 5-mm-thick sheet-like heat insulating material was manufactured (coating process). The insulation produced is referred to as Example 1 insulation. The content of silica airgel in the heat insulating material of Example 1 is 82.8% by mass, and the content of binder is 14.6% by mass. The blending amounts of the raw materials used are summarized in Table 1 below.

When the density of the heat insulating material of Example 1 was measured by the "liquid weighing method" defined in JIS Z 8807:2012, it was 0.093 g/cm ³ . Using this density value (measured density), the volume ratio of the voids in the heat insulating material of Example 1 was calculated according to the above formula (I), and it was 36.5%.

FIG. 2 shows a top view photograph of the heat insulating material of Example 1. As shown in FIG. FIG. 3 shows a photograph taken with a scanning electron microscope (SEM) of a cross section in the thickness direction of the heat insulating material (magnification: 50 times). As shown in FIG. 2, no cracks were observed in the heat insulating material of Example 1. Further, as shown in FIG. 3, a dark portion was observed between the porous structures, and it was confirmed that voids existed between the porous structures. The size of each void was 300 to 1000 μm, and the average was about 500 μm. In the SEM photograph of FIG. 3, many voids were seen in the upper left.

[Example 2]
A heat insulating material of Example 2 was produced in the same manner as in Example 1, except that the stirring conditions in the stirring step and the standing time in the standing step were changed. In the manufacturing method of the heat insulating material of Example 2, the rotational speed of the stirring blade was 500 rpm, the stirring time was 10 minutes, and the standing time was 180 minutes. When the density of the heat insulating material of Example 2 was measured in the same manner as in Example 1, it was 0.136 g/cm ³ . Using this density value, the volume ratio of voids in the heat insulating material of Example 2 was calculated according to the above formula (I), and was 10.8%.

[Example 3]
A heat insulating material of Example 3 was produced in the same manner as in Example 1, except that the stirring conditions in the stirring step and the standing time in the standing step were changed. In the manufacturing method of the heat insulating material of Example 3, the rotation speed of the stirring blade was 1200 rpm, the stirring time was 15 minutes, and the standing time was 10 minutes. When the density of the heat insulating material of Example 3 was measured in the same manner as in Example 1, it was 0.069 g/cm ³ . Using this density value, the volume ratio of voids in the heat insulating material of Example 3 was calculated according to the above formula (I), and it was 54.9%.

[Example 4]
In the same manner as in Example 1, except that the type of binder was changed to styrene-butadiene latex B (“JSR-0545” manufactured by JSR Corporation, solid content 54% by mass, Tg of binder component −31° C.). The insulation of Example 4 was produced. When the density of the heat insulating material of Example 4 was measured in the same manner as in Example 1, it was 0.091 g/cm ³ . Using this density value, the volume ratio of the voids in the heat insulating material of Example 4 was calculated according to the above formula (I) and found to be 33.0%.

[Comparative Example 1]
A heat insulating material of Comparative Example 1 was produced in the same manner as in Example 1, except that the stirring conditions in the stirring step and the standing time in the standing step were changed. In the method of manufacturing the heat insulating material of Comparative Example 1, the stirring time was set to 5 minutes and the standing time was set to 300 minutes. When the density of the heat insulating material of Comparative Example 1 was measured in the same manner as in Example 1, it was 0.156 g/cm ³ . Using this density value, the volume ratio of voids in the heat insulating material of Comparative Example 1 was calculated according to the above formula (I), and was 4%.

FIG. 4 shows a top view photograph of the heat insulating material of Comparative Example 1. As shown in FIG. FIG. 5 shows a SEM photograph of a cross section in the thickness direction of the heat insulating material (magnification: 50 times). As shown in FIG. 4, the heat insulating material of Comparative Example 1 had cracks throughout. Further, as shown in FIG. 5, a slightly dark portion was observed between the porous structures, confirming the presence of very small voids.

[Comparative Example 2]
A heat insulating material of Comparative Example 2 was produced in the same manner as in Example 1, except that the blending amount of each raw material and the standing time in the standing step were changed. In the method of manufacturing the heat insulating material of Comparative Example 2, the blending ratio of the binder was increased from that of Example 1, and the stationary time was set to 300 minutes. The content of silica airgel in the heat insulating material of Comparative Example 2 was 68.6% by mass, and the content of binder was 29.3% by mass. When the density of the heat insulating material of Comparative Example 2 was measured in the same manner as in Example 1, it was 0.2 g/cm ³ . Using this density value, the volume ratio of voids in the heat insulating material of Comparative Example 2 was calculated according to the above formula (I), and it was 2%.

[Comparative Example 3]
A heat insulating material of Comparative Example 3 was manufactured by changing the blending amount of each raw material and the manufacturing method. In the method of manufacturing the heat insulating material of Comparative Example 3, the blending ratio of the binder was increased more than that of Example 1, and press molding was performed instead of coating with paint. The content of silica airgel in the heat insulating material of Comparative Example 3 is 88.7% by mass, and the content of binder is 10.3% by mass. Press molding was performed as follows. First, a release paper was placed on the bottom of the mold, and the paint was filled on it. Then, the mold was clamped and pressed at a pressure of 0.98 MPa for 30 minutes to form a sheet having a thickness of 5 mm. When the density of the heat insulating material of Comparative Example 3 was measured in the same manner as in Example 1, it was 0.2 g/cm ³ . Using this density value, the volume ratio of voids in the heat insulating material of Comparative Example 3 was calculated according to the above formula (I), and it was 2%.

<Measurement of physical properties of heat insulating material>
The elastic modulus, elongation at break (E _b ), and tensile strength at break (T _b ) of each manufactured heat insulating material were measured by performing a tensile test specified in JIS K 6251:2010. The tensile test was performed using a dumbbell-shaped No. 3 test piece at a distance between chucks of 20 mm and a tensile speed of 20 mm/min. The elastic modulus was calculated from the slope of the stress-strain (SS) curve between 0.1 and 0.4 mm tensile distance. The measurement results are summarized in Table 1 below.

<Evaluation of heat insulating material>
Each of the manufactured heat insulating materials was evaluated for heat insulating properties, presence or absence of cracks, and detachability of silica airgel (degree of falling powder).

[Thermal insulation properties]
The thermal conductivity of the heat insulating material was measured using a heat flux meter "HC-074" manufactured by Eiko Seiki Co., Ltd. in accordance with the heat flow meter method of JIS A1412-2:1999. If the thermal conductivity is 0.024 W / m · K or less, the thermal insulation is good (indicated by a circle in Table 1 below), and if the thermal conductivity exceeds 0.024 W / m · K, the thermal insulation is poor ( Indicated by x in Table 1 below).

[Presence or absence of cracks]
A test piece was prepared by cutting the heat insulating material into a strip having a width of 20 mm and a length of 100 mm. The prepared test piece was wound around a cylindrical member having a diameter of 40 mm so that the long side was along the circumferential direction, and the states of the test piece before and after winding were compared. If cracks do not occur on the surface of the test piece after winding, there are no cracks (indicated by ○ in Table 1 below), and if cracks occur, there are cracks (indicated by × in Table 1 below). evaluated.

[Removability of silica airgel]
Weakly adhesive tape (manufactured by 3M "Scotch (registered trademark) peelable tape" (product number: 811-3-12)) is attached to the surface of the heat insulating material, and silica airgel adheres when it is peeled off. I checked whether Then, when silica airgel did not adhere, it was evaluated as no dropout (indicated by ◯ in Table 1 below), and when a large amount adhered, it was evaluated as dropout (indicated by x in the same table).

Table 1 shows the compounding amounts of the raw materials used, the manufacturing conditions, the physical properties of the heat insulating material, and the evaluation results.

As shown in Table 1, in the heat insulating materials of Examples 1 to 4 in which the volume ratio of voids is 10% or more and 55% or less, no cracks were observed, and almost no silica airgel fell off. It is considered that this is because voids are formed between the silica airgels by forming the film with the paint containing an appropriate amount of air, and the shrinkage strain generated during drying can be buffered by the voids. In addition, the fact that the heat insulating materials of Examples 1 to 4 are flexible can also be seen from the fact that their elongation at break is 1.5% or more. Moreover, in the heat insulating materials of Examples 1 to 4, the thermal conductivity was small in spite of the presence of voids. For example, the thermal conductivity of air at 40° C. is about 0.0272 W/m·K. In other words, it can be seen that the heat insulating materials of Examples 1 to 4 have higher heat insulating properties than the air.

On the other hand, the heat insulating materials of Comparative Examples 1 and 3 have very few voids. For this reason, the heat insulating material of Comparative Example 1 cracked during film formation, and the heat insulating material of Comparative Example 3 was manufactured by press molding, but had poor flexibility, so cracks occurred when bending. has arisen. In addition, since the heat insulating material of Comparative Example 3 was produced without blending polysaccharides, the retention of the silica airgel was poor, resulting in frequent falling off. Since the heat insulating material of Comparative Example 2 contained a large amount of binder, no cracks occurred even though the material had only 2% voids. However, in addition to the fact that the amount of silica airgel is small, the binder serves as a heat transfer path, resulting in a high thermal conductivity and poor heat insulation.

The heat insulating material of the present invention is suitable for heat insulating interior materials for automobiles, heat insulating materials for houses, heat insulating materials for home appliances, heat insulating materials for electronic parts, heat insulating materials for heat and cold insulation containers, and the like.

1: heat insulating material, 10: porous structure, 11: binder, 12: voids.

Claims (1)

A plurality of porous structures having a skeleton formed by connecting a plurality of particles and having pores therein, and a binder that binds the porous structures together;
voids exist between the porous structures,
A method for manufacturing a heat insulating material, wherein the volume ratio of the voids in the heat insulating material is 10% or more and 55% or less,
a coating preparation step of preparing a coating in which the porous structure is dispersed in a binder liquid;
an adjustment step of adjusting the state of the gas contained in the paint;
A coating step of applying the coating material to the surface of the base material and drying it;
has
A method for producing a heat insulating material, wherein the adjustment step includes a stirring step of stirring the paint, and a standing step of leaving the paint still after the stirring step.

Images