Hot melt adhesive

Abstract

The present invention provides a hot melt adhesive that is excellent in thermal conductivity, toughness, heat resistance, and dimensional stability, and is useful for bonding and heat dissipation of portable devices such as smartphones, electrical equipment such as home appliances and car stereos, electronic devices such as laptop computers, interior and exterior materials for automobiles, interior and exterior materials for buildings and building materials, furniture, window glass, etc. [Solution] A hot melt adhesive comprising an ethylene-vinyl acetate-based thermoplastic resin composition comprising 100 parts by weight of an ethylene-vinyl acetate-based thermoplastic resin (A) having a vinyl acetate content of 5 to 40% by weight, 0.5 to 30 parts by weight of cellulose nanofibers (B) with an average fiber diameter of 1 to 1,000 nm, and 5 to 150 parts by weight of at least one thermally conductive substance (C) selected from metal particles, metal oxide particles, metal nitride particles, metal carbide particles, carbon nanotube particles, graphite particles, graphene particles, metal fibers, metal oxide fibers, metal nitride fibers, metal carbide fibers, carbon nanotube fibers, graphite fibers, graphene fibers, and carbon fibers, wherein the ratio of the thermal conductivity of the hot melt adhesive comprising the ethylene-vinyl acetate-based thermoplastic resin composition (thickness direction) / (planar direction) in percent is 60 to 110%.

Description

[Technical Field]

[0001] This invention relates to a hot-melt adhesive. [Background technology]

[0002] Hot melt adhesives are adhesives that bond materials by softening or melting a resin that is solid at room temperature through heating. Hot melt adhesives are classified into reactive and non-reactive types and are used to bond dissimilar materials such as plastics to plastics, or metals or wood to plastics. Among hot melt adhesives, ethylene-vinyl acetate copolymer hot melt adhesives are generally popular as representative hot melt adhesives because they offer easy adjustment of adhesive strength, excellent cold resistance, flexibility, moldability, compatibility with various additives, and low cost. The main purpose of hot melt adhesives has traditionally been to provide adhesion, but in recent years, new functions such as heat shielding, ultraviolet shielding, near-infrared shielding, thermal conductivity, and electromagnetic wave shielding are being considered. In particular, regarding heat dissipation, with the miniaturization and increased power output of electrical and electronic equipment, semiconductors, and batteries, as well as the increased power output of power devices and power generation equipment, the amount of heat generated by components, equipment, and devices is increasing, and there is a growing demand for excellent heat conductive materials and heat conductive adhesives that use heat conductive materials as adherends in order to suppress ignition and failure. Furthermore, the demand for thermally conductive materials and thermally conductive adhesives that use thermally conductive materials as substrates is increasing, partly to mitigate the impact of rising temperatures due to recent climate change on the heat generation of components, equipment, and devices.

[0003] On the other hand, conventional adhesives using solvents have been criticized for several issues, including low yield, high carbon dioxide emissions and energy consumption during the solvent drying process, the need to comply with VOC (volatile organic compound) emission standards, and the need for measures to prevent health damage to workers. Furthermore, while water-based paints do not emit VOCs, they have problems such as high energy consumption during the drying process and the need for wastewater treatment. In addition, UV-curing adhesives that do not use solvents shrink significantly during curing and require UV irradiation equipment, necessitating space allocation for manufacturing facilities and measures to prevent UV light leakage. On the other hand, hot-melt adhesives do not use solvents and have features such as being solvent-free, bonding in a short time, enabling automated application, high yield, high productivity, and a clean and safe working environment. For these reasons, they have been attracting increasing attention in recent years as a solvent-free bonding method, particularly in the automotive sector. Cellulose nanofibers are plant-derived biomass materials that possess characteristics such as being lightweight, high-strength, having a low dimensional change rate, and exhibiting thixotropy. As a result, numerous composite materials and molded products using cellulose nanofibers with inks, adhesives, plastics, etc., have been proposed (see, for example, Patent Documents 1-3). Furthermore, a box (see, for example, Patent Document 4) is made by laminating a paper substrate with a hot-melt adhesive containing a thermoplastic resin and cellulose nanofibers; a hot-melt adhesive containing cellulose nanofibers comprising a resin that melts in the range of 70°C to 160°C and powdered cellulose nanofibers (see, for example, Patent Document 5); a bonding method (see, for example, Patent Document 6) is proposed, characterized by the steps of: placing both the main component of the hot-melt adhesive and a solution containing cellulose nanofibers and carboxymethylcellulose between a first object and a second object so that they overlap; and applying pressure to the overlapping portion of the first object, the second object, the main component, and the solution. In addition, resin compositions containing a thermoplastic resin, cellulose nanofibers, and a thermally conductive filler have been proposed (see, for example, Patent Documents 7 to 11). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 7248988 [Patent Document 2] Patent No. 7303015 [Patent Document 3] Japanese Patent Publication No. 2021-109911 [Patent Document 4] Japanese Patent Publication No. 2023-37114 [Patent Document 5] Japanese Patent Publication No. 2022-44861 [Patent Document 6] Patent No. 7038529 [Patent Document 7] Patent No. 6870400 [Patent Document 8] Patent No. 6913307 [Patent Document 9] Patent No. 6795555 [Patent Document 10] Japanese Patent Publication No. 2021-143249 [Patent Document 11] Japanese Patent Publication No. 2021-50327 [Overview of the project] [Problems that the invention aims to solve]

[0005] The cellulose nanofiber compositions proposed in Patent Documents 1-3 do not mention anything about hot melting. Furthermore, the compositions described in Patent Documents 4-6 have problems such as not having sufficiently satisfactory thermal conductivity. In addition, the resin compositions described in Patent Documents 7-11 have problems such as not having sufficiently satisfactory adhesive properties.

[0006] Therefore, the present invention aims to provide a hot-melt adhesive that is excellent in thermal conductivity, adhesion, heat resistance, and blocking resistance. [Means for solving the problem]

[0007] As a result of diligent research to solve the aforementioned problems, the inventors have discovered that a hot-melt adhesive comprising an ethylene-vinyl acetate thermoplastic resin composition containing an ethylene-vinyl acetate thermoplastic resin, specific cellulose nanofibers, and a specific thermally conductive substance exhibits excellent thermal conductivity, adhesion, heat resistance, and blocking resistance, thus completing the present invention.

[0008] In other words, the embodiments of the present invention are as follows [1] to [3]. [1] A hot melt adhesive comprising an ethylene-vinyl acetate-based thermoplastic resin composition comprising 100 parts by weight of an ethylene-vinyl acetate-based thermoplastic resin (A) having a vinyl acetate content of 5 to 40% by weight, 0.5 to 30 parts by weight of cellulose nanofibers (B) with an average fiber diameter of 1 to 1,000 nm, and 5 to 150 parts by weight of at least one thermally conductive substance (C) selected from metal particles, metal oxide particles, metal nitride particles, metal carbide particles, carbon nanotube particles, graphite particles, graphene particles, metal fibers, metal oxide fibers, metal nitride fibers, metal carbide fibers, calcium titanate fibers, strontium titanate fibers, carbon nanotube fibers, graphite fibers, graphene fibers, and carbon fibers, wherein the ratio of the thermal conductivity of the hot melt adhesive comprising the ethylene-vinyl acetate-based thermoplastic resin composition (thickness direction) / (planar direction) in percent is 60 to 110%. [2] The hot melt adhesive according to [1], wherein the cellulose nanofiber (B) is a cellulose nanofiber (B) in which the hydroxyl groups of cellulose are acetylated or acylated. [3] The hot melt adhesive according to [1] or [2], wherein the ethylene-vinyl acetate thermoplastic resin composition further comprises 10 to 100 parts by weight of biomass resin (C). [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a hot-melt adhesive that does not emit VOCs, has high productivity and yield, and is excellent in thermal conductivity, heat resistance, and blocking resistance, and its industrial value is extremely high. [Modes for carrying out the invention]

[0010] Hereinafter, the present invention will be described in detail.

[0011] The ethylene-vinyl acetate-based thermoplastic resin (A) may be any resin in which ethylene and vinyl acetate are copolymerized. For example, ethylene-vinyl acetate copolymers, ethylene-vinyl acetate-maleic anhydride terpolymers, ethylene-vinyl acetate-maleic acid terpolymers, ethylene-vinyl acetate-α,β-unsaturated carboxylic acid alkyl ester terpolymers, saponified ethylene-vinyl acetate copolymers, ethylene-vinyl acetate-α,β-unsaturated carboxylic acid glycidyl ester terpolymers, and other ethylene-vinyl acetate-based thermoplastic resins can be mentioned. Also, two or more of these resins may be mixed and used.

[0012] Further, the ethylene-vinyl acetate-based thermoplastic resin (A) has a vinyl acetate content of 5 to 40% by weight. When the vinyl acetate content is less than 5% by weight, the adhesiveness of the hot melt adhesive becomes poor. On the other hand, when the vinyl acetate content exceeds 40% by weight, the blocking resistance of the hot melt adhesive becomes poor.

[0013] The density of the ethylene-vinyl acetate copolymer-based thermoplastic resin is not particularly limited, but usually, it is 920 to 980 g / m 3 and it is easy to obtain a hot melt adhesive excellent in the balance of adhesiveness, heat resistance, and blocking resistance, so it is more preferably 925 to 970 g / m 3 It is more preferable that these ethylene-vinyl acetate-based thermoplastic resins can be used alone, mixed, or compounded.

[0014] Furthermore, the melt flow rate (MFR) of the ethylene-vinyl acetate-based thermoplastic resin at 190°C and a load of 2.16 kg is not particularly limited, but usually, it is preferably 10 to 3,500 g / 10 minutes, and more preferably 70 to 2,500 g / 10 minutes because the molding processability is good.

[0015] Furthermore, the raw material for cellulose nanofiber (B) can be anything containing cellulose nanofibers, and is generally known to be derived from plants such as wood, bamboo, hemp, rice, jute, kenaf, cotton, beet, oil palm, cloth, pulp, recycled pulp, waste paper, and agricultural residues such as olive oil residue, or from algae, microorganisms (e.g., acetic acid bacteria) or animals (e.g., sea squirts), and any of these can be used in the present invention. Preferably, it is cellulose derived from plants or microorganisms, and more preferably, it is cellulose derived from plants. These cellulose nanofibers can be used individually or in mixtures. Moreover, the cellulose nanofibers can also be used as a masterbatch that has been pre-contained in an ethylene-vinyl acetate resin or the like.

[0016] Cellulose nanofibers (B) have an average fiber diameter of 1 to 1,000 nm. If the average fiber diameter of the cellulose nanofibers is less than 1 nm, the productivity of the cellulose nanofibers will be low and production costs will increase. On the other hand, if the average fiber diameter of the cellulose nanofibers exceeds 1,000 nm, the effect of improving heat resistance by adding cellulose nanofibers will be poor, and the adhesive properties will also be poor. Cellulose nanofibers may be used after being defibrated to the above average fiber diameter, or they may be defibrated in the extruder when forming the ethylene-vinyl acetate thermoplastic resin composition to achieve the above average fiber diameter.

[0017] Furthermore, while there are no particular restrictions on the fiber length of the cellulose nanofiber (B), it is preferable that the fiber length / fiber diameter (aspect ratio) be between 10 and 10,000, as this results in particularly excellent heat resistance of the hot melt adhesive. The average fiber diameter and average fiber length of the cellulose nanofiber were determined by measuring the fiber diameter using a scanning electron microscope (SEM) and taking the average value of the diameters of 50 or more fibers.

[0018] Furthermore, known and publicly available methods can be used to produce the cellulose nanofiber (B). For example, it can be produced by mechanically opening the fibers using a twin-screw extruder, tandem extruder, Banbury mixer, pressure kneader, homogenizer, media stirring mill, vibrating mill, grinder, ball mill, high-pressure water jet, ultrasonic dispersion machine, beater, disc refiner, conical refiner, double disc refiner, etc. A method of melt kneading using a twin-screw extruder or tandem extruder is preferred because it allows for efficient fiber defibration with particularly high productivity.

[0019] Furthermore, it is preferable that the cellulose nanofiber (B) is a cellulose nanofiber in which the hydroxyl groups of the cellulose nanofiber are acetylated or acylated. By using acetylated or acylated cellulose nanofibers, excellent dispersibility in ethylene-vinyl acetate thermoplastic resins can be obtained, resulting in a hot-melt adhesive with excellent adhesion, heat resistance, etc. As a method for acetylating or acyling the hydroxyl groups of the cellulose nanofiber, publicly known and publicly used methods can be used. For example, a method can be used in which the hydroxyl groups of cellulose are replaced with acid anhydrides such as acetic anhydride, propionic anhydride, butyric anhydride, pentanoic anhydride, hexanoic anhydride, decanoic anhydride, benzoic anhydride, stearic anhydride, maleic anhydride, succinic anhydride, phthalic anhydride, maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified diene polymer, and polybasic acid anhydrides. The degree of substitution of the hydroxyl groups of the cellulose nanofibers with acetyl or acyl groups is optimized as appropriate depending on the type of polyethylene resin constituting the present invention and the target physical properties, but it is particularly preferable that it be 0.5 to 1, as this results in a hot melt adhesive with particularly excellent adhesion, heat resistance, etc.

[0020] Furthermore, the amount of cellulose nanofibers blended is 0.5 to 30 parts by weight per 100 parts by weight of the ethylene-vinyl acetate thermoplastic resin. If the amount of cellulose nanofibers blended is less than 0.5 parts by weight, the heat resistance of the hot melt adhesive will be poor. On the other hand, if the amount of cellulose nanofibers blended exceeds 30 parts by weight, the adhesive properties of the hot melt adhesive will be poor.

[0021] The thermally conductive material (C) constituting the hot-melt adhesive of the present invention is at least one thermally conductive material (C) selected from metal particles, metal oxide particles, metal nitride particles, metal carbide particles, carbon nanotube particles, graphite particles, graphene particles, metal fibers, metal oxide fibers, metal nitride fibers, metal carbide fibers, carbon nanotube fibers, graphite fibers, graphene fibers, and carbon fibers. In this specification, materials with a length-to-width ratio (aspect ratio) of 3 or more are defined as fibers, and those with a ratio of 3 or less are defined as particles. Examples of metals include silicon, copper, and aluminum; examples of metal oxides include aluminum oxide, silicon dioxide, magnesium oxide, iron oxide, beryllium oxide, titanium dioxide, zinc oxide, calcium titanate, and strontium titanate; examples of metal nitrides include aluminum nitride, silicon nitride, boron nitride, and gallium nitride; and examples of metal carbides include silicon carbide, titanium carbide, boron carbide, and tungsten carbide. Thermally conductive materials are classified into conductive and insulating types, and the appropriate type is selected depending on the application of the hot-melt adhesive, such as for parts, equipment, or devices.

[0022] Furthermore, the amount of the thermally conductive substance (C) to be blended is 5 to 150 parts by weight per 100 parts by weight of the ethylene-vinyl acetate thermoplastic resin. If the amount of the thermally conductive substance (C) is less than 5 parts by weight, the thermal conductivity, i.e., heat dissipation, of the hot melt adhesive will be poor. On the other hand, if the amount of the thermally conductive substance (C) exceeds 150 parts by weight, the adhesiveness of the hot melt adhesive will be poor. The hot melt adhesive of the present invention is characterized by the combined use of the cellulose nanofiber and the thermally conductive substance (C). Interaction occurs so that the thermally conductive substance (C) is supported on the cellulose nanofiber dispersed in the ethylene-vinyl acetate thermoplastic resin, resulting in uniform dispersibility and orientation of the thermally conductive substance (C) in the ethylene-vinyl acetate thermoplastic resin. As a result, heat conduction pathways are efficiently formed in the ethylene-vinyl acetate thermoplastic resin, and excellent thermal conductivity, i.e., heat dissipation, is achieved with the addition of a smaller amount of the thermally conductive substance (C). Furthermore, since thermal conductive particles and thermal conductive fibers generally align in the orientation direction of the film, the thermal conductivity in the thickness direction of the film is significantly inferior to the thermal conductivity in the planar direction of the film. On the other hand, in the hot melt adhesive made of the ethylene-vinyl acetate thermoplastic resin that constitutes the present invention, the ratio of (thickness direction) to (planar direction) of the thermal conductivity of the hot melt adhesive made of the ethylene-vinyl acetate thermoplastic resin composition is 60-110%. Since the cellulose nanofibers are interwoven three-dimensionally in the polyethylene thermoplastic resin, the thermal conductive particles and thermal conductive fibers that interact with the cellulose nanofibers are also dispersed and oriented three-dimensionally, resulting in a smaller difference between the thermal conductivity in the thickness direction and the planar direction of the film made of the ethylene-vinyl acetate thermoplastic resin composition. The smaller difference between the thermal conductivity in the thickness direction and the planar direction of the film allows it to be used in a wide variety of applications where thermal conductivity in the thickness direction is important, applications where thermal conductivity in the planar direction is important, and applications where both thermal conductivity are important.

[0023] The hot-melt adhesive of the present invention may contain a biomass resin (D) that is useful for carbon neutrality. Biomass resins are generally classified into types produced in microorganisms, types obtained by polymerizing monomers obtained by fermenting, decomposing, and modifying biomass such as starch and oils, and types obtained by chemically modifying natural products such as polysaccharides. Examples include polylactic acid, polyhydroxyalkanoate, polybutylene succinate, biomass polyethylene, biomass polypropylene, biomass polyethylene terephthalate, biomass polytrimethylene terephthalate, and biomass polyamide. The amount of biomass resin (D) blended with 100 parts by weight of the ethylene-vinyl acetate thermoplastic resin is preferably 10 to 100 parts by weight, as this is effective for carbon neutrality.

[0024] The ethylene-vinyl acetate thermoplastic resin may be used in combination with various additives without departing from the objectives of the present invention. For example, one or more conventional additives such as plasticizers, slip agents, antiblocking agents, antioxidants, heat stabilizers, ultraviolet absorbers, light stabilizers, waxes, rosin, and terpenes, which are conventionally known, may be added. Furthermore, it may be used in combination with one or more of various thermoplastic resins, such as polyethylene resins like high-density polyethylene, low-density polyethylene, and linear low-density polyethylene, polyester resins like polyethylene terephthalate, polyamide resins like polyamide 6, polyolefin resins like polypropylene, polystyrene resins like polystyrene, and polyurethane.

[0025] The ethylene-vinyl acetate thermoplastic resin can be manufactured by melt extrusion molding, which involves feeding the ethylene-vinyl acetate thermoplastic resin (A), the cellulose nanofiber (B), the thermally conductive substance (C), and any optional additives or thermoplastic resins into a twin-screw extruder or tandem extruder.

[0026] The hot melt adhesive consists of the above-mentioned ethylene-vinyl acetate thermoplastic resin composition and can be molded into film or sheet-like forms by air cooling, water cooling inflation, T-die method, calendering method, injection molding, or compression molding, depending on the intended use, and then bonded to various adherends. In this case, these molded bodies can be sandwiched between adherends and then bonded by heating at a predetermined temperature (e.g., 80-140°C). Other bonding methods include sprinkling the hot melt adhesive in powder form onto the adherends and then heat-pressing the other adherend. Methods such as coating one adherend with the hot melt adhesive by extrusion coating and then heat-pressing the other adherend, or laminating at least two types of adherends by extrusion lamination can also be used.

[0027] Hot melt adhesives can be suitably used for bonding and heat dissipation purposes for containers, packaging materials, portable devices such as smartphones, electrical equipment such as home appliances and car stereos, electronic devices such as laptop computers, interior and exterior materials for automobiles, interior and exterior materials for buildings and building materials, furniture, window glass, and the like. [Examples]

[0028] The present invention will be described in detail below with reference to examples, but the present invention is not limited in any way by these examples. It is not something that should be done.

[0029] The materials used in the examples and comparative examples are shown below.

[0030] Ethylene-vinyl acetate copolymer (hereinafter referred to as A-1); manufactured by Tosoh Corporation, product name UltraCen (registered trademark) 720, vinyl acetate content 28% by weight, density 957 g / m³ 3 , MFR150g / 10min.

[0031] Ethylene-vinyl acetate copolymer (hereinafter referred to as A-2); manufactured by Tosoh Corporation, product name UltraCen (registered trademark) 752, vinyl acetate content 32% by weight, density 955 g / m³ 3MFR 60g / 10 minutes.

[0032] Ethylene-vinyl acetate copolymer (hereinafter referred to as A-3); manufactured by Mitsui Dow Polychemical Co., Ltd., (product name) EV45X, vinyl acetate content 46% by weight, density 970 g / m³ 3 , MFR100g / min.

[0033] This is referred to as ethylene-vinyl acetate copolymer saponified product (hereinafter referred to as A-4). Manufactured by Tosoh Corporation, (product name) Mersen (registered trademark) H6820, vinyl acetate content 6% by weight, density 960 g / m³ 3 , MFR5.5g / 10min.

[0034] Polylactic acid (hereinafter referred to as PLA(D)); manufactured by NatureWorks, (product name) Ingeo(registered trademark) 4060D, density 1.25 g / m² 3 MFR 6.0g / 10 minutes.

[0035] Low-density polyethylene (hereinafter referred to as PE(E)); manufactured by Tosoh Corporation, (product name) Petrocene (registered trademark) 248, density 917 g / m³ 3 MFR 58g / 10 minutes.

[0036] Microcellulose fiber (hereinafter referred to as MCF(B-2)); manufactured by Rettenmeyer Co., Ltd., (product name) ARBOCEL BC200, average fiber length 300 μm, number average fiber diameter 20 μm.

[0037] Thermal conductive particles, thermal conductive fibers Metallic silicon particles (hereinafter referred to as C-1); manufactured by Kinsei Matec Co., Ltd., (product name) Metallic Silicon #600; silicon content 98.5% by weight, average particle size 6 μm, irregularly shaped powder, aspect ratio 1.

[0038] Carbon fiber (hereinafter referred to as C-2); manufactured by Mitsubishi Plastics, Inc., (product name) DiaLead (registered trademark) K223HE; fiber length 6 mm, fiber diameter 10 μm, aspect ratio 600.

[0039] Scaly boron nitride particles (hereinafter referred to as C-3); manufactured by Denki Kagaku Kogyo Co., Ltd., (trade name) Denka Boron Nitride (registered trademark) SGP; average particle diameter 18 μm, aspect ratio 1.

[0040] Carbon nanotube fibers (hereinafter referred to as C-4); manufactured by Hodogaya Chemical Co., Ltd., (trade name) MWNT-7; average single fiber diameter 65 nm, average single fiber length 40 μm, aspect ratio 615, multi-walled carbon nanotubes.

[0041] Example of pulp adjustment 8 kg (solid content 5 kg) of hydrous softwood unbleached kraft pulp (manufactured by Fletcher Challenge Canada, trade name Machenzie) (hereinafter referred to as NUKP) was subjected to refiner treatment, and defibration treatment by repeated refiner treatment was performed until its drainage degree (CSF) reached 50 ml. Next, 5,000 ml of acetic anhydride was added to the refined NUKP, and after reacting at 80 °C for 4 hours, the reaction product was cooled to 40 °C, separated from the liquid, and then acetic anhydride and acetic acid were removed at 70 °C under reduced pressure. Then, it was dried under reduced pressure at 60 °C for 20 hours to obtain acetylated NUKP. Also, the degree of acetyl group substitution of the obtained acetylated NUKP was 0.85.

[0042] Example of producing a hot melt adhesive sheet Pellets of a composition consisting of an ethylene-vinyl acetate copolymer resin, cellulose nanofibers, and a heat conductive substance (C), etc. were placed in a press mold with a thickness of 1 mm × length of 150 mm × width of 150 mm, and using a compression molding machine (manufactured by Shindo Metal Industry Co., Ltd., (trade name) AWFA.50), at a temperature of 140 °C, a pressure of 10 kg / cm 2 for 5 minutes of preheating treatment, and at a temperature of 140 °C, a pressure of 100 kg / cm 2 [[ID=(以下省略)]] for 3 minutes of heat treatment, and then cooled at a temperature of 30 °C, a pressure of 100 kg / cm 2 for 5 minutes to produce a hot melt adhesive sheet. [[ID=(以下省略)]] <Measurement of thermal conductivity> The thermal conductivity was measured using a thermal conductivity measuring device (ULVAC, Inc., product name TC7000; ruby ​​laser) under conditions of 23°C, employing the laser flash method. For the thermal conductivity in the thickness direction, the heat capacity Cp and thermal diffusivity α in the thickness direction were determined by a one-dimensional method, and for the thermal conductivity in the plane direction, the thermal diffusivity α' in the plane direction was determined by a two-dimensional method. The thermal conductivity was then calculated using the following formula. Thermal conductivity in the thickness direction = ρ × Cp × α Thermal conductivity in the plane = ρ × Cp × α' Here, density ρ was measured according to ASTM D-792 A method (water displacement method). A decorative film was judged to have excellent thermal conductivity if its thermal conductivity in both the thickness and planar directions was 1.5 W / mK or higher. Furthermore, to evaluate the anisotropy of thermal conductivity, the ratio of (thickness direction) to (planar direction) thermal conductivity was calculated as a percentage. A value close to 100% indicates low anisotropy, while a value close to 0% or significantly exceeding 100% indicates high anisotropy. Therefore, films with a value between 60% and 110% were judged to have low thermal conductivity anisotropy. <Measurement of adhesive strength of hot melt adhesive> A 1mm thick hot melt adhesive sheet was cut to 80mm x 100mm and placed on each of the substrates cut to 100mm x 100mm (aluminum plate with a thickness of 0.2mm, stainless steel plate with a thickness of 0.3mm, and polycarbonate plate with a thickness of 3mm). A laminate film consisting of 100μm thick polyethylene terephthalate laminated with 30μm polyethylene was used as a support, and the polyethylene side was placed in contact with the hot melt adhesive sheet. Next, using a bonding test machine (far-infrared heating furnace manufactured by Tapi Thermal Engineering Co., Ltd., (product name) UC-3), the samples were heated at 80°C for 20 minutes to obtain test pieces in which the hot melt adhesive sheet and each substrate were bonded without the inclusion of air bubbles. The adhesive strength of the hot melt adhesive was measured using an Autograph (ORIENTEC, product name RTE-1210) under the conditions of a peeling speed of 300 mm / min, a peeling angle of 180 degrees, and a sample width of 15 mm. The adherends used in the test were as follows: Aluminum and stainless steel plates were commercially available products from general retail stores. For the polycarbonate sheet, Mitsubishi Engineering Co., Ltd., product name Yupiron Sheet (registered trademark) NF20000, thickness: 3 mm was used. <Measurement of heat shrinkage rate of hot melt adhesive> A sheet of hot-melt adhesive, prepared using pellets of an ethylene-vinyl acetate copolymer resin composition, was marked with a 50 mm square mark in the center and placed on a 3 mm thick Teflon® sheet. It was then placed in a gear oven (YASUDA SEIKI No. 102, (product name) SHF-77 gear aging tester) and subjected to heat treatment at 80°C for 20 minutes. The dimensions of the mark were measured before and after heat treatment, and the heat shrinkage rate was evaluated by the formula: average dimension after heating / average dimension before heating × 100 (%). If the heat shrinkage rate was less than 10%, the hot-melt adhesive was judged to have excellent heat resistance. <Evaluation of Blocking Resistance of Hot Melt Adhesives> Ten grams of hot-melt adhesive pellets were placed in a 20 mL disposable polypropylene cup and left undisturbed in a gear oven (YASUDA SEIKI No. 102, (product name) SHF-77 gear aging tester) and heated at 40°C for two days. The pellets were removed from the cup and the blocking state between the pellets was observed. A state where the blocking between pellets was strong and required crushing to separate them was judged to have poor blocking resistance, while a state where there was no blocking between pellets and no crushing was required was judged to have excellent blocking resistance.

[0043] Example 1 100 parts by weight of ethylene-vinyl acetate copolymer (A-1), 5 parts by weight of acetylated NUKP, 15 parts by weight of metallic silicon particles (C-1), and 10 parts by weight of carbon nanotubes (C-4) were pre-mixed uniformly and fed into the hopper of a twin-screw extruder (manufactured by Japan Steel Works Ltd., product name TEX-25αIII, L / D=55) having four kneading zones. Meanwhile, 15 parts by weight of carbon fiber (C-2) was fed into the hopper of the side feeder of the same twin-screw extruder, and the mixture was melt-kneaded under conditions where the cylinder temperature of the kneading zone was heated to 220°C to produce an ethylene-vinyl acetate copolymer resin composition in which acetylated NUKP, metallic silicon particles (C-1), carbon fiber (C-2), and carbon nanotubes (C-4) were dispersed. The obtained ethylene-vinyl acetate copolymer resin composition was then supplied to a film molding machine with a screw diameter of 15 mmφ and equipped with a T-die with an outlet width of 100 mm. The mixture was melt-kneaded at a temperature of 140°C, and the extruded strands were cut to obtain hot-melt adhesive pellets. The average fiber diameter of the acetylated cellulose nanofibers (hereinafter referred to as CNF(B-1)) measured by scanning electron microscopy (SEM) was 58 nm, and the aspect ratio was 9,500. Next, the obtained pellets were used for various measurements and evaluations. The evaluation results are shown in Table 1. The obtained hot-melt adhesive exhibited excellent adhesion, thermal conductivity, heat resistance, and blocking resistance.

[0044] Examples 2-8 Hot melt adhesive pellets were obtained using the same method as in Example 1, with the blending ratios of ethylene-vinyl acetate copolymer resin (A), acetylated cellulose nanofibers (B-1), metallic silicon particles (C-1), carbon fibers (C-2), flaky boron nitride particles (C-3), carbon nanotubes (C-4), and polylactic acid (D) as shown in Table 1. The obtained pellets were then used for measurement and evaluation. The evaluation results are shown in Table 1. The obtained hot melt adhesive exhibited excellent adhesion, thermal conductivity, heat resistance, and blocking resistance.

[0045] Comparative Examples 1-7 Hot melt adhesive pellets were obtained using the same method as in Example 1, with the blending ratios of ethylene-vinyl acetate copolymer resin (A), acetylated cellulose nanofiber (B-1), microcellulose fiber (B-2), metallic silicon particles (C-1), carbon fiber (C-2), flaky boron nitride particles (C-3), carbon nanotubes (C-4), polylactic acid (D), and low-density polyethylene (D) as shown in Table 2. These pellets were then evaluated using the same method as in Example 1. The evaluation results are shown in Table 2. The hot melt adhesives obtained from Comparative Examples 2, 4, 6, and 7 exhibited poor adhesive properties. The hot melt adhesives obtained from Comparative Examples 1, 5, and 7 exhibited high anisotropy in thermal conductivity. The hot melt adhesive obtained from Comparative Example 6 exhibited poor thermal conductivity. The hot melt adhesives obtained from Comparative Examples 1, 3, and 6 exhibited poor heat resistance, and the hot melt adhesives obtained from Comparative Examples 1 and 3 exhibited poor blocking resistance.

[0046] [Table 1]

[0047] [Table 2] [Industrial applicability]

[0048] The present invention provides a hot melt adhesive that is excellent in thermal conductivity, adhesion, heat resistance, and dimensional stability, and is useful for bonding and heat dissipation of portable devices such as smartphones, electrical equipment such as home appliances and car stereos, electronic devices such as laptop computers, interior and exterior materials for automobiles, interior and exterior materials for buildings and building materials, furniture, window glass, etc.

Claims

1. A hot melt adhesive comprising an ethylene-vinyl acetate-based thermoplastic resin composition comprising 100 parts by weight of an ethylene-vinyl acetate-based thermoplastic resin (A) having a vinyl acetate content of 5 to 40% by weight, 0.5 to 30 parts by weight of cellulose nanofibers (B) having an average fiber diameter of 1 to 1,000 nm, and 5 to 150 parts by weight of at least one thermally conductive substance (C) selected from metal particles, metal oxide particles, metal nitride particles, metal carbide particles, carbon nanotube particles, graphite particles, graphene particles, metal fibers, metal oxide fibers, metal nitride fibers, metal carbide fibers, carbon nanotube fibers, graphite fibers, graphene fibers, and carbon fibers, wherein the ratio of the thermal conductivity of the hot melt adhesive comprising the ethylene-vinyl acetate-based thermoplastic resin composition (thickness direction) / (planar direction) in percent is 60 to 110%.

2. The hot melt adhesive according to claim 1, wherein the cellulose nanofiber (B) is a cellulose nanofiber (B) in which the hydroxyl groups of cellulose are acetylated or acylated.

3. The hot melt adhesive according to claim 1 or 2, wherein the ethylene-vinyl acetate thermoplastic resin further comprises 10 to 100 parts by weight of biomass resin (D).

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