Hot melt adhesive

Abstract

This invention provides a hot melt adhesive that is excellent in adhesion, heat resistance, and blocking properties, and is useful for bonding 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. [Solution] A hot melt adhesive comprising an ethylene-vinyl acetate thermoplastic resin composition containing 100 parts by weight of an ethylene-vinyl acetate thermoplastic resin (A) having a vinyl acetate content of 5 to 40% by weight, and 0.5 to 30 parts by weight of cellulose nanofibers (B) having an average fiber diameter of 1 to 1,000 nm.

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 widely used as a representative hot melt adhesive because they offer easy adjustment of adhesive strength, excellent cold resistance, flexibility, moldability, compatibility with various additives, and low cost. While the main purpose of hot melt adhesives has traditionally been to provide adhesion, in recent years, new functions such as gas barrier properties, heat shielding properties, ultraviolet shielding properties, near-infrared shielding properties, thermal conductivity, and electromagnetic wave shielding properties are being investigated.

[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.

[0004] 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.

[0005] 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).

[0006] Furthermore, a bonding method has been proposed (see, for example, Patent Document 6) characterized by comprising the steps of: placing both the main component of the hot melt adhesive and a solution containing cellulose nanofibers and cellulose nanofibers 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. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Patent No. 7248988 [Patent Document 2] Patent No. 7303015 Publication [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 [Overview of the project] [Problems that the invention aims to solve]

[0008] The compositions containing cellulose nanofibers proposed in Patent Documents 1-3 do not describe anything regarding hot-melt adhesives. Furthermore, while Patent Document 4 describes a hot-melt adhesive in which an aqueous dispersion of oxidized cellulose nanofibers is blended with a thermoplastic resin, the low affinity with thermoplastic resins such as ethylene-vinyl acetate resins, which have low hydrophilicity, and the insufficient dispersibility of cellulose nanofibers in the thermoplastic resin, result in a problem where the characteristics of cellulose nanofibers are not fully exhibited. In addition, the hot-melt adhesive described in Patent Document 5 uses cellulose nanofiber powder, and the low fiber length / fiber diameter (aspect ratio) results in problems such as insufficient heat resistance and strength. Furthermore, the hot-melt adhesive described in Patent Document 6 is a mixture of water-soluble carboxymethylcellulose and cellulose nanofibers in a synthetic rubber such as polyurethane or an ethylene-vinyl acetate copolymer as the main component, but the low affinity between cellulose nanofibers and thermoplastic resins such as ethylene-vinyl acetate resins, which have low hydrophilicity, and the insufficient dispersibility of cellulose nanofibers in the thermoplastic resin, result in a problem where the characteristics of cellulose nanofibers are not fully exhibited.

[0009] Therefore, the present invention aims to provide a hot melt adhesive that is excellent in terms of adhesion, heat resistance, blocking resistance, and the like. [Means for solving the problem]

[0010] As a result of diligent research to solve the aforementioned problems, the inventors of the present invention have found that a hot-melt adhesive comprising an ethylene-vinyl acetate resin and an ethylene-vinyl acetate resin composition containing specific cellulose nanofibers exhibits excellent adhesion, heat resistance, blocking resistance, etc., and have completed the present invention.

[0011] 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 thermoplastic resin composition containing 0.5 to 30 parts by weight of cellulose nanofibers (B) with an average fiber diameter of 1 to 1,000 nm per 100 parts by weight of an ethylene-vinyl acetate thermoplastic resin (A) having a vinyl acetate content of 5 to 40% by weight. [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]

[0012] 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 adhesive properties, heat resistance, blockage resistance, etc., and its industrial value is extremely high. [Modes for carrying out the invention]

[0013] The present invention will be described in detail below.

[0014] The ethylene-vinyl acetate thermoplastic resin (A) can be any copolymer of ethylene and vinyl acetate, such as ethylene-vinyl acetate copolymer, ethylene-vinyl acetate-maleic anhydride terpolymer, ethylene-vinyl acetate-maleic acid terpolymer, ethylene-vinyl acetate-α,β-unsaturated carboxylic acid alkyl ester terpolymer, ethylene-vinyl acetate copolymer saponified, or ethylene-vinyl acetate-α,β-unsaturated carboxylic acid glycidyl ester terpolymer. Furthermore, two or more of these resins may be mixed and used.

[0015] In addition, 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.

[0016] The density of the ethylene-vinyl acetate copolymer-based thermoplastic resin is not particularly limited, but is usually 920 to 980 g / m 3 Since it is easy to obtain a hot melt adhesive excellent in the balance of adhesiveness, heat resistance and blocking resistance, it is 925 to 970 g / m 3 It is more preferably. These ethylene-vinyl acetate-based thermoplastic resins can be used alone, mixed or compounded.

[0017] 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 is usually preferably 10 to 3,500 g / 10 min, and more preferably 70 to 2,500 g / 10 min because of good molding processability.

[0018] Also, the raw material of the cellulose nanofiber (B) may be any material containing cellulose nanofiber, and those derived from plants such as wood, bamboo, hemp, rice, jute, kenaf, cotton, beet, oil palm, cloth, pulp, recycled pulp, waste paper and agricultural waste such as olive oil pressing residue, or those derived from algae, microorganisms (e.g., acetic acid bacteria) or animals (e.g., sea squirts) are generally known, and any of them can be used in the present invention. Preferably, it is cellulose derived from plants or microorganisms, and more preferably cellulose derived from plants. And these cellulose nanofibers can be used alone or in combination. Furthermore, the cellulose nanofibers can also be used as a masterbatch previously contained in an ethylene-vinyl acetate-based resin or the like.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] An ethylene-vinyl acetate thermoplastic resin composition containing an ethylene-vinyl acetate thermoplastic resin (A) and cellulose nanofibers (B) may also contain a biomass resin (C) useful for carbon neutrality. These biomass resins are generally classified into types produced within microorganisms, types obtained by polymerizing monomers acquired 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 (C) 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.

[0025] The ethylene-vinyl acetate thermoplastic resin composition may be used with a mixture of various additives, without departing from the objectives of the present invention. For example, it may contain 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. Furthermore, it may be a mixture of 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.

[0026] As a method for producing an ethylene-vinyl acetate thermoplastic resin composition, polyethylene thermoplastic resin, cellulose nanofibers, and optionally any additives or thermoplastic resins can be fed into a twin-screw extruder or tandem extruder and produced by melt extrusion molding.

[0027] The hot-melt adhesive of the present invention is made from the above-mentioned ethylene-vinyl acetate thermoplastic resin composition and can be molded into a film or sheet shape by air cooling, water cooling inflation, T-die method, calendering method, injection molding, or compression molding, depending on the purpose of 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 (for example, 80 to 140°C). As for other bonding methods, a method can also be used in which the hot-melt adhesive is sprinkled on the adherends in powder form and the other adherend is heat-pressed onto it. Furthermore, 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.

[0028] Hot melt adhesives can be suitably used for bonding containers, packaging materials, mobile 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]

[0029] 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.

[0030] The materials used in the examples and comparative examples are shown below.

[0031] 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.

[0032] 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 3 MFR 60g / 10 minutes.

[0033] 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.

[0034] 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.

[0035] Polylactic acid (hereinafter referred to as PLA(C)); manufactured by NatureWorks, (product name) Ingeo(registered trademark) 4060D, density 1.25 g / m² 3 MFR 6.0g / 10 minutes.

[0036] Low-density polyethylene (hereinafter referred to as PE(D)); manufactured by Tosoh Corporation, (product name) Petrocene (registered trademark) 248, density 917 g / m³ 3 MFR 58g / 10 minutes.

[0037] Microcellulose fiber (hereinafter referred to as MCF(B-2)); manufactured by Rettenmeyer Co., Ltd., (product name) ARBOCEL (registered trademark) BC200, average fiber length 300 μm, number average fiber diameter 20 μm.

[0038] Examples of pulp preparation 8 kg (5 kg of solid content) of unbleached softwood kraft pulp (manufactured by Fletcher Challenge Canada, trade name Machenzie) (hereinafter referred to as NUKP) was subjected to refining treatment, and defibration treatment by repeated refining treatment was carried out until its freeness (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. The degree of substitution of acetyl groups in the obtained acetylated NUKP was 0.85.

[0039] Example of sheet preparation of hot melt adhesive Pellets of a composition composed of an ethylene-vinyl acetate copolymer resin and cellulose nanofibers 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 and a pressure of 10 kg / cm 2 for preheating treatment for 5 minutes, heating treatment at a temperature of 140 °C and a pressure of 100 kg / cm 2 for 3 minutes, and then cooling at a temperature of 30 °C and a pressure of 100 kg / cm 2 for 5 minutes to produce a sheet of hot melt adhesive. <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.

[0040] Example 1 100 parts by weight of ethylene-vinyl acetate copolymer (A-1) was uniformly mixed with 5 parts by weight of acetylated NUKP beforehand, and the mixture was 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. The mixture was then melt-kneaded under conditions where the cylinder temperature of the kneading zones was heated to 210°C to produce a polyethylene-based thermoplastic resin composition in which acetylated NUKP was dispersed. The obtained ethylene-vinyl acetate copolymer resin composition was then supplied to a film molding machine with a screw diameter of 15 mmφ equipped with a T-die with an outlet width of 100 mm, melt-kneaded at a temperature of 140°C, and the extruded strands were cut to obtain hot-melt adhesive pellets. Furthermore, the average fiber diameter of acetylated cellulose nanofibers (hereinafter referred to as CNF(B-1)) measured by observation with a scanning electron microscope (SEM) was 78 nm, and the aspect ratio was 8,500. Next, the obtained pellets were used for measurement and evaluation. The evaluation results are shown in Table 1. The obtained hot-melt adhesive exhibited excellent adhesion, heat resistance, and blocking resistance.

[0041] Examples 2-6 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), and polylactic acid (C) as shown in Table 1. These pellets were then evaluated using the same method as in Example 1. The evaluation results are shown in Table 1. All obtained hot melt adhesives exhibited excellent adhesion, heat resistance, and blocking resistance.

[0042] Comparative Examples 1-6 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), polylactic acid (C), 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, 5, and 6 exhibited poor adhesive properties. The hot melt adhesives obtained from Comparative Examples 1, 4, and 5 exhibited poor heat resistance, while the hot melt adhesives obtained from Comparative Examples 1 and 3 exhibited poor blocking resistance.

[0043] [Table 1]

[0044] [Table 2] [Industrial applicability]

[0045] This invention provides a hot melt adhesive that is excellent in adhesion, heat resistance, and blocking properties, and is useful for bonding 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.

Claims

1. A hot melt adhesive comprising an ethylene-vinyl acetate thermoplastic resin composition containing 0.5 to 30 parts by weight of cellulose nanofibers (B) with an average fiber diameter of 1 to 1,000 nm, per 100 parts by weight of an ethylene-vinyl acetate thermoplastic resin (A) having a vinyl acetate content of 5 to 40% by weight.

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 composition further comprises 10 to 100 parts by weight of biomass resin (C).

Images