Crosslinked resin foam

Abstract

[Problem] To provide crosslinked resin foam being excellent in terms of foam property and shape stability, while having suppressed amount of harmful gas, e.g., ammonia, generated. [Solution] This crosslinked resin foam is obtained by crosslinking and foaming a resin composition comprising (A) a thermoplastic olefin resin and (B) a blowing agent. The crosslinked resin foam has a gel content of 35-85% and satisfies the following (i) and / or (ii). (i) The crosslinked resin foam has a concentration of ammonium ions detected by ion chromatography of less than 1,000 μg per g of the crosslinked resin foam. (ii) The resin composition has an azodicarbonamide content of 0.01 mass% or less.

Description

Cross-linked resin foam

[0001] The present invention relates to a crosslinked resin foam.

[0002] Crosslinked resin foams obtained by forming a crosslinked structure (three-dimensional network structure) in a resin composition and foaming the resulting crosslinked structure have excellent properties such as light weight, heat insulation, and compressive stress due to their cellular structure, and can also be provided with desired water resistance, chemical resistance, etc. by selecting the type of resin and the foaming agent. For these reasons, crosslinked resin foams are widely used in fields such as building materials, electronic products, automobiles, energy equipment, and medicine.

[0003] Azodicarbonamide (ADCA) is widely known as a blowing agent used in the production of crosslinked resin foams (see, for example, Patent Document 1). ADCA is a thermal decomposition type blowing agent that decomposes at around 200°C and generates a large amount of nitrogen gas, thereby forming a cellular structure in a resin composition. While ADCA has excellent foaming properties, its thermal decomposition generates gases such as carbon monoxide and ammonia in addition to nitrogen gas, and also produces sublimable decomposition residues such as cyanuric acid, which can easily cause problems such as environmental pollution, metal corrosion, foaming furnace contamination, and mold contamination. Therefore, blowing agent compositions have been proposed that reduce the amount of ADCA used by combining them with other blowing agents, while also incorporating a higher fatty acid salt to reduce corrosiveness. For example, Patent Document 2 discloses a blowing agent composition containing specific amounts of ADCA, p,p'-oxybis(benzenesulfonohydrazide) (OBSH), sodium bicarbonate or potassium bicarbonate, citric acid or a salt thereof, and a higher fatty acid salt, and an expandable thermoplastic polymer composition containing 0.01 to 0.2% by weight of this blowing agent composition. Patent Document 2 also discloses that a low-foam sheet having an expansion ratio of 1.01 to 3 times can be obtained by extruding this expandable thermoplastic polymer composition from a kneading extruder, and that this expandable thermoplastic polymer composition may contain a crosslinking agent or a crosslinking aid. Patent Document 2 also discloses crosslinked resin foams that do not use ADCA as the blowing agent but use other blowing agents. For example, Patent Document 3 discloses an invention of a method for producing a polyolefin resin foam, which comprises mixing raw materials containing a polyolefin resin, a crosslinking agent, sodium bicarbonate that decomposes upon heating to generate carbon dioxide gas and water vapor, and a water-absorbent resin, the content of the water-absorbent resin being set so that the water absorption capacity, determined based on the product of the water absorption capacity and the content of the water-absorbent resin, is 20 to 1,300 parts by mass per 100 parts by mass of the polyolefin resin, and then heating the mixture; and foaming the polyolefin resin with carbon dioxide gas generated by the decomposition of the sodium bicarbonate.

[0004] JP 2013-245251 A JP 11-021364 A JP 2007-217639 A

[0005] If the amount of ADCA is reduced, as in the case of the expandable thermoplastic polymer composition described in Patent Document 2, the foamability of the composition is low, and the resulting foam has a poor expansion ratio, as described above. Furthermore, the technology described in Patent Document 2 assumes the use of a specific small amount of ADCA, which may cause problems such as environmental pollution in the long term. Furthermore, the resin foam described in Patent Document 3 uses sodium bicarbonate as a blowing agent, as described above. Sodium bicarbonate generates carbon dioxide gas and water vapor upon thermal decomposition. After foaming, the water vapor cools and turns into water, reducing the volume. This reduces the pressure within the foam cells, causing shrinkage and deformation of the foam. To address this issue, the resin foam described in Patent Document 3 incorporates a water-absorbing resin, which absorbs the water vapor generated by the thermal decomposition of sodium bicarbonate, thereby suppressing foam shrinkage. However, water-absorbing resins absorb rainwater and moisture, which can cause dewdrops on the foam surface and reduce insulation properties.

[0006] An object of the present invention is to provide a crosslinked resin foam that is excellent in foaming properties and shape stability while suppressing the generation of harmful gases such as ammonia gas.

[0007] As a result of intensive studies aimed at solving the above-mentioned problems, the present inventors have found that, even without using ADCA as a blowing agent, shrinkage, deformation, and the like of a crosslinked resin foam after production can be effectively suppressed by increasing the gel fraction of the crosslinked resin foam within a specific range. Based on these findings, the present inventors have conducted further studies and have now completed the present invention.

[0008] That is, the above-mentioned problems have been solved by the following means. [1] A crosslinked resin foam obtained by crosslinking and foaming a resin composition containing (A) a thermoplastic olefin resin and (B) a blowing agent, wherein the crosslinked resin foam has a gel fraction of 35 to 85%, and satisfies the following (i) and / or (ii): (i) The crosslinked resin foam has an ammonium ion concentration of less than 1000 μg / g of crosslinked resin foam as detected by ion chromatography. (ii) The resin composition has an azodicarbonamide content of 0.01% by mass or less. [2] The crosslinked resin foam according to [1] above, which has an expansion ratio of 7 times or more. [3] The crosslinked resin foam according to [1] or [2] above, wherein the crosslinked structure of the crosslinked resin foam is formed by the action of an organic peroxide in the resin composition and / or the action of electron beam irradiation. [4] The crosslinked resin foam according to any one of [1] to [3], wherein a siloxane compound is not included in the five types of outgassing with the highest extraction amounts detected by gas chromatography mass spectrometry of the crosslinked resin foam. [5] The crosslinked resin foam according to any one of [1] to [4], wherein the (B) blowing agent contains an inorganic carbonate.

[0009] The crosslinked resin foam of the present invention is excellent in foaming properties and shape stability while suppressing the generation of harmful gases such as ammonia gas.

[0010] The crosslinked resin foam of the present invention is a crosslinked resin foam obtained by crosslinking and foaming a resin composition containing (A) a thermoplastic olefin-based resin and (B) a blowing agent. The crosslinked resin foam has a gel fraction of 35 to 85% and satisfies the following (i) and / or (ii): (i) the ammonium ion concentration as detected by ion chromatography is less than 1000 μg / g of crosslinked resin foam. (ii) the content of ADCA in the resin composition is 0.01% by mass or less. Preferred embodiments of the crosslinked resin foam of the present invention will now be described.

[0011] [Resin Composition] The resin composition used to form the crosslinked resin foam of the present invention (hereinafter also referred to as the "resin composition used in the present invention") contains (A) a thermoplastic olefin resin (also referred to as component (A)) and (B) a blowing agent (also referred to as component (B)). Components (A) and (B), as well as the optional components described below, may each be used alone or in combination of two or more. It is preferable that the resin composition is substantially free of ADCA as a blowing agent. That is, the content of ADCA in the resin composition is preferably 0.1% by mass or less, more preferably 0.01% by mass or less, even more preferably 0.001% by mass or less, even more preferably 0.0001% by mass or less, and even more preferably 0.00001% by mass or less, and it is particularly preferable that the resin composition is free of ADCA. The components contained in the resin composition used in the present invention are described below.

[0012] ((A) Thermoplastic olefin resin) The resin composition used in the present invention contains (A) a thermoplastic olefin resin as a base resin. The thermoplastic olefin resin may be an olefin homopolymer or an olefin copolymer. An olefin homopolymer is a polymer of one type of olefin. An olefin copolymer is a copolymer of a certain olefin with a different olefin, or a copolymer of an olefin with a compound other than an olefin having a carbon-carbon double bond (for example, a vinyl compound or a styrene compound). In other words, any polymer containing an olefin component as a constituent component is encompassed by component (A).

[0013] Component (A) preferably contains (A-1) a resin selected from polyethylene and polypropylene (also referred to as component (A-1)). It is also preferable that component (A) is component (A-1). Component (A) may contain, together with component (A-1), a resin other than component (A-1). As the resin other than component (A-1), (A-2) a resin having a structure that stabilizes radicals (also referred to as component (A-2)) is preferred.

[0014] Component (A-1) is preferably a polyethylene resin. Examples of this polyethylene resin include low-density polyethylene and high-density polyethylene. Of these, low-density polyethylene is preferred. In the present invention, low-density polyethylene is polyethylene having a density of 860 kg / m ３ More than 940kg / m ３ The high-density polyethylene refers to polyethylene having a density greater than that of the low-density polyethylene. The low-density polyethylene may be what is known as "low-density polyethylene" or "ultra-low-density polyethylene" having long chain branches, or may be linear low-density polyethylene obtained by copolymerizing ethylene with a small amount of an α-olefin monomer.

[0015] Examples of the "radical-stabilizing structure" in component (A-2) include a double bond or a polar group. This polar group is a group containing a heteroatom (such as an oxygen atom, sulfur atom, or nitrogen atom), such as an ester group, an ether group, a thioester group, a thioether group, a carboxy group, a hydroxy group, an amino group, or an imino group. Preferred examples of component (A-2) include ethylene-acrylic acid copolymers, ethylene-ethyl acrylate copolymers, ethylene-vinyl acetate copolymers, polybutadiene compounds (preferably, butadiene-based elastomers), and styrene-based elastomers. Of these, ethylene-vinyl acetate copolymers or butadiene-based elastomers are preferred. A butadiene-based elastomer is an elastomer containing a component derived from a butadiene compound. A butadiene compound includes not only butadiene itself, but also compounds in which one or more hydrogen atoms of butadiene have been substituted. For example, isoprene (2-methyl-1,3-butadiene) is a type of butadiene compound. The compound in which one or more hydrogen atoms of butadiene are substituted is preferably a hydrocarbon. A preferred example of the butadiene-based elastomer is 1,3-butadiene polymer, more preferably 1,2-polybutadiene.

[0016] When component (A) contains both component (A-1) and component (A-2), the content ratio of component (A-1) to component (A-2) is, on a mass basis, preferably component (A-1):component (A-2)=95:5 to 55:45, more preferably component (A-1):component (A-2)=90:10 to 60:40, and even more preferably component (A-1):component (A-2)=85:15 to 70:30.

[0017] The thermoplastic olefin resin can be synthesized by a conventional method, or a commercially available product can be used.

[0018] In the resin composition, the content of component (A) is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more. The content is preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 70% by mass or less. The preferred range of the content of component (A) is 10 to 90% by mass, more preferably 20 to 80% by mass, and even more preferably 30 to 70% by mass.

[0019] (B) Foaming Agent The resin composition used in the present invention contains (B) a foaming agent. The foaming agent is preferably a thermally decomposable foaming agent. A thermally decomposable foaming agent is a foaming agent that decomposes upon heating to generate gas. As the thermally decomposable foaming agent, a foaming agent that does not generate ammonia or cyanuric acid upon thermal decomposition is preferred from the viewpoint of environmental safety, etc., and an inorganic foaming agent (preferably an inorganic carbonate) is more preferred from the viewpoint of not inhibiting the crosslinking reaction. Examples of inorganic carbonates include potassium carbonate, sodium carbonate, sodium bicarbonate (baking soda), barium carbonate, magnesium carbonate, lithium carbonate, iron(II) carbonate, and silver(I) carbonate, with sodium bicarbonate being preferred. The foaming agent is commercially available. An example of a commercially available inorganic carbonate is Uniform AZ P-4 (trade name) manufactured by Otsuka Chemical Co., Ltd.

[0020] From the viewpoint of enhancing the foamability of the resulting crosslinked resin foam, the resin composition preferably contains 10 parts by mass or more of component (B) per 100 parts by mass of component (A), more preferably 20 parts by mass or more, even more preferably 30 parts by mass or more, and even more preferably 60 parts by mass or more. Furthermore, from the viewpoint of suppressing a decrease in the expansion ratio due to cell breakage, the content is preferably 200 parts by mass or less, more preferably 180 parts by mass or less, even more preferably 150 parts by mass or less, and even more preferably 120 parts by mass or less. The content of component (B) is preferably within a preferred range, from 10 to 200 parts by mass, more preferably from 20 to 180 parts by mass, even more preferably from 30 to 150 parts by mass, and even more preferably from 60 to 120 parts by mass, per 100 parts by mass of component (A).

[0021] ((C) Foaming Aid) The resin composition used in the present invention can contain (C) a foaming aid (also referred to as component (C)) to the extent that the effects of the present invention are not impaired. By blending the foaming aid into the resin composition, for example, it is possible to adjust the decomposition temperature of the foaming agent within an appropriate range and promote the decomposition of the foaming agent. Examples of the foaming aid include metal oxides such as zinc oxide, metal fatty acid salts such as zinc stearate, urea, citric acid, p,p'-oxybisbenzenesulfonylhydrazide (OBSH), carbon black, antimony trioxide, decabromodiphenylene oxide, trimellitic anhydride, maleic anhydride, and benzotriazole. Among these, OBSH or citric acid is preferred as the foaming aid, as they do not inhibit the decomposition of component (B) and are safe for the environment and humans. The content of the component (C) is preferably 100 parts by mass or less, more preferably 70 parts by mass or less, even more preferably 40 parts by mass or less, and also preferably 20 parts by mass or less, preferably 10 parts by mass or less, and may be 5 parts by mass or less, relative to 100 parts by mass of the component (A). When the resin composition contains component (C), the content of the component (C) relative to 100 parts by mass of the component (A) is usually 0.1 parts by mass or more, preferably 0.2 parts by mass or more, and more preferably 0.3 parts by mass or more. The content of the component (C) relative to 100 parts by mass of the component (A) is preferably in the range of 0 to 100 parts by mass, more preferably 0.1 to 100 parts by mass, even more preferably 0.2 to 70 parts by mass, and even more preferably 0.3 to 40 parts by mass, and may be 0.3 to 20 parts by mass, 0.3 to 10 parts by mass, or 0.3 to 5 parts by mass.

[0022] The resin composition used in the present invention can be crosslinked and foamed to obtain the crosslinked resin foam of the present invention. The crosslinked structure can be formed using a crosslinking agent or by irradiation with an electron beam. Crosslinking by a crosslinking agent and electron beam crosslinking can also be combined. That is, the crosslinked structure of the crosslinked resin foam of the present invention can be formed by the action of an organic peroxide in the resin composition and / or the action of electron beam irradiation. The resin composition can also contain a crosslinking aid, a crosslinking-accelerating resin, etc.

[0023] (D) Crosslinking Agent The resin composition used in the present invention can contain (D) a crosslinking agent (also referred to as component (D)) to the extent that the effects of the present invention are not impaired. Examples of the crosslinking agent include radical generators, and organic peroxides can be suitably used. The radical generator thermally decomposes to generate radicals, and the action of these radicals can abstract hydrogen from resin components, etc., to create reactive sites and form a crosslinked structure. Furthermore, by using it in combination with a crosslinking aid described below, the generated radicals can cause an addition reaction of carbon-carbon unsaturated bonds, forming a crosslinked structure. As the radical generator, it is preferable to use an organic peroxide such as 1,1-di(t-butylperoxy)cyclohexane, dicumyl peroxide (DCP), 2,5-dimethyl-2,5-di-(t-butylperoxy)-3-hexyne, α,α'-bis(t-butylperoxydiisopropyl)benzene, t-butylperoxycumene, 4,4'-di(t-butylperoxy)valeric acid n-butyl ester, or 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane. Among these, when an inorganic carbonate that thermally decomposes at low temperatures is used as component (B), it is preferable to use an organic peroxide whose 1-minute half-life temperature (the temperature at which 50% of the organic peroxide decomposes when held at a predetermined temperature for 1 minute) is 165°C or less, and it is more preferable to use an organic peroxide whose 1-minute half-life temperature is 130°C or more and 165°C or less. When a blowing agent that thermally decomposes at low temperatures is used as component (B), by setting the one-minute half-life temperature of the crosslinking agent within the above-mentioned preferred temperature range, both the foamability and the degree of crosslinking (crosslinking point density) of the resulting crosslinked resin foam can be achieved within a preferred range. For example, when a crosslinking agent with a one-minute half-life temperature above 165°C is used, the decomposition temperature of the blowing agent is lower than the crosslinking initiation temperature, making it difficult to achieve a high expansion ratio for the resulting crosslinked resin foam. Therefore, by controlling the one-minute half-life temperature to 165°C or below, even when the decomposition temperature of the blowing agent is low, the crosslinking agent decomposes before the blowing agent decomposes, forming a crosslinked structure, and then foaming by the blowing agent proceeds, thereby obtaining a high-expansion foam. Furthermore, by setting the one-minute half-life of the crosslinking agent to 130°C or higher, the progress of the crosslinking reaction during processing (before the crosslinking treatment) can be suppressed.Examples of such organic peroxides include 1,1-di(t-butylperoxy)cyclohexane (1-minute half-life temperature: 153.8°C). The content of component (D) is preferably 5.0 parts by mass or less, more preferably 4.0 parts by mass or less, and even more preferably 3.0 parts by mass or less, per 100 parts by mass of component (A). When the resin composition contains component (D), the content of component (D) per 100 parts by mass of component (A) is typically 0.1 parts by mass or more, preferably 0.2 parts by mass or more, and even more preferably 0.3 parts by mass or more. The preferred range of the content of component (D) per 100 parts by mass of component (A) is preferably 0 to 5.0 parts by mass, more preferably 0.1 to 5.0 parts by mass, even more preferably 0.2 to 4.0 parts by mass, and even more preferably 0.3 to 3.0 parts by mass.

[0024] (E) Crosslinking Auxiliary Agent The resin composition used in the present invention may contain (E) a crosslinking auxiliary (also referred to as component (E)) to the extent that the effects of the present invention are not impaired. Examples of the crosslinking auxiliary include allyl compounds and polyfunctional vinyl compounds, and among these, polyfunctional acrylate compounds and / or polyfunctional methacrylate compounds and / or polyfunctional vinyl ether compounds are preferred. Specific examples include trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, and divinylbenzene, with trimethylolpropane trimethacrylate being preferred. The polyfunctional vinyl compound preferably has two or more vinyl groups (including cases where the vinyl groups are contained as acryloyl groups or methacryloyl groups) per molecule, more preferably three or more, and even more preferably three to six. The content of component (E) is preferably 5.0 parts by mass or less, more preferably 4.0 parts by mass or less, more preferably 3.0 parts by mass or less, and even more preferably 2.0 parts by mass or less, per 100 parts by mass of component (A). When the resin composition contains component (E), the content of component (E) per 100 parts by mass of component (A) is usually 0.1 parts by mass or more, preferably 0.2 parts by mass or more, more preferably 0.3 parts by mass or more, and even more preferably 0.5 parts by mass or more. A preferred range for the content of component (E) per 100 parts by mass of component (A) is preferably 0 to 5.0 parts by mass, more preferably 0.1 to 5.0 parts by mass, even more preferably 0.2 to 4.0 parts by mass, even more preferably 0.3 to 3.0 parts by mass, and even more preferably 0.5 to 2.0 parts by mass.

[0025] (Other Components) In addition to the components (A) and (B), the resin composition used in the present invention may contain other components such as dispersants, fillers, pigments, light stabilizers, heat stabilizers, lubricants, compatibilizers, antioxidants, flame retardants, and flame retardant aids, as long as the effects of the present invention are not impaired. Furthermore, the resin composition used in the present invention preferably has a low content of resins formed by graft polymerization of silane compounds, such as silane-grafted polyolefins, and the content of resins formed by graft polymerization of silane compounds in the resin composition is preferably 1% by mass or less, more preferably 0.1% by mass or less, and even more preferably 0.01% by mass or less, and it is even more preferable that the resin composition does not contain resins formed by graft polymerization of silane compounds.

[0026] [Method for producing resin composition] The resin composition used in the present invention can be obtained by melt-kneading and homogenizing the above-mentioned components (A) and (B), and further, if necessary, the above-mentioned optional components, using a commonly used kneading device such as a batch kneader, a roll, a kneader, a Banbury mixer, or the like, or a twin-screw extruder.

[0027] The resin composition used in the present invention may be a molded article obtained by molding the melt-kneaded product, which can be molded by various molding methods such as extrusion molding and press molding.

[0028] [Crosslinked Resin Foam] The crosslinked resin foam of the present invention is a crosslinked resin foam obtained by crosslinking and foaming the resin composition, wherein the crosslinked resin foam has a gel fraction of 35 to 85%, (i) an ammonium ion concentration of less than 1000 μg / g of crosslinked resin foam as measured by ion chromatography, and / or (ii) an ADCA content of 0.01 mass% or less in the resin composition. "Less than 1000 μg / g of crosslinked resin foam" refers to a concentration of less than 1000 μg per gram of crosslinked resin foam. In the present invention and this specification, "by crosslinking and foaming" refers to forming a crosslinked structure in the resin composition and then foaming the crosslinked product either simultaneously with the formation of the crosslinked structure or after the formation of the crosslinked structure. Furthermore, in the present invention, the crosslinked resin foam has the specific feature of being obtained by crosslinking and foaming the resin composition, as described above, but this simply specifies the state of the crosslinked resin foam. In other words, this specific feature further clarifies the structure and properties of the crosslinked resin foam.

[0029] The thermoplastic olefin-based resin contained in the crosslinked resin foam of the present invention is basically the same as that described in the resin composition above, and the content is also substantially the same. On the other hand, it differs from the resin composition in that the resin etc. forms a crosslinked structure. That is, in the crosslinked resin foam of the present invention, the term "thermoplastic olefin-based resin" is used to mean a state in which the resin (polymer) of the above-mentioned component (A) is incorporated as a constituent component of the crosslinked body formed by the crosslinking reaction.

[0030] (Gel Fraction) The crosslinked resin foam of the present invention has a gel fraction of 35 to 85%. Specifically, the crosslinked resin foam of the present invention is a foam containing a solvent-insoluble polymer with a three-dimensional network structure and its swollen form (gel), with countless polymer chains connected in a network-like structure. The gel fraction refers to the ratio of the mass of the gel portion, which remains unextracted when a sample is extracted with xylene, to the mass before extraction with xylene. In the present invention, the gel fraction can be measured by the following method. Each foam sample is cut out to a weight of 1.00±0.050 g, and its mass is measured (Mass A). The sample is placed in a flask, 100 ml of xylene is added, and the solvent is boiled at 120°C for 24 hours±5 minutes. The sample is then removed from the boiling solvent, and the remaining sample is dried in a vacuum oven at 80°C±2°C under a negative pressure of at least 85 kPa for 24 hours. After cooling, the sample is measured to a sensitivity of 1 mg (Mass B). The gel fraction (%) is calculated from the measured masses A and B using the following formula (1): Gel fraction (%) = {(mass B) / (mass A)} × 100 (Formula 1) When the gel fraction of the crosslinked resin foam of the present invention is less than 35%, the degree of crosslinking is too low to form a foam structure, or even if the degree of crosslinking is sufficient to form a foam structure, shrinkage is likely to occur after production. In particular, when sodium carbonate, an inorganic carbonate, is used as component (B), significant shrinkage may occur due to cooling of the water vapor generated by decomposition. In the crosslinked resin foam of the present invention, the gel fraction is preferably 40% or more, more preferably 50% or more, and even more preferably 60% or more. Furthermore, the gel fraction is preferably 82% or less, and more preferably 80% or less. Preferred ranges for the gel fraction are 40 to 85%, more preferably 50 to 85%, even more preferably 50 to 82%, even more preferably 50 to 80%, and even more preferably 60 to 80%. By controlling the gel fraction within the above preferred range, it is possible to further suppress deformation and shrinkage of the crosslinked resin foam after foaming while maintaining a high expansion ratio.Examples of methods for increasing the gel fraction include using an organic peroxide having a one-minute half-life temperature of 165°C or less to cause a crosslinking reaction at a high density, or controlling the amount of electron beam irradiation to cause crosslinking at a high density.

[0031] (Apparent Density) The shape of the crosslinked resin foam of the present invention is not particularly limited, and for example, the thickness is 1 to 20 mm and the apparent density is 200 kg / m ３ The following sheet forms are available: Apparent density: 25 to 200 kg / m ３ More preferably, it is 30 to 175 kg / m ３ The "apparent density" of the crosslinked resin foam of the present invention is a value calculated by dividing the mass of a test piece of the foam cut into a predetermined size (e.g., 10 cm × 10 cm) by the volume of the foam (volume including the cellular portion).

[0032] (Expansion Ratio) In the crosslinked resin foam of the present invention, the expansion ratio is preferably 7 times or more, more preferably 10 times or more, more preferably 15 times or more, and even more preferably 20 times or more. From the viewpoint of preventing the foam from breaking during use or processing, the expansion ratio is preferably 50 times or less, and more preferably 40 times or less. Therefore, the preferred range of the expansion ratio is preferably 7 to 50 times, more preferably 10 to 50 times, even more preferably 15 to 40 times, and even more preferably 20 to 40 times. The expansion ratio can be calculated using samples before and after the foaming treatment according to the following formula (Formula 2). For example, the resin composition can be used as the unfoamed sample, and the crosslinked resin foam can be used as the foamed sample. Expansion Ratio = Density of Unfoamed Sample / Apparent Density of Foamed Sample (Formula 2)

[0033] (Ammonium Ion Concentration) In one embodiment of the crosslinked resin foam of the present invention, the ammonium ion concentration detected by ion chromatography is less than 1,000 μg / g of crosslinked resin foam. For example, when ADCA is used as a blowing agent, not only nitrogen gas but also ammonia gas is generated during thermal decomposition, and ammonia remains in the resulting crosslinked resin foam. Harmful substances remaining in the foam in this way can cause metal corrosion, etc. The ammonium ion concentration of the crosslinked resin foam of the present invention, detected by ion chromatography, is preferably 500 μg or less / g of crosslinked resin foam, more preferably 200 μg or less / g of crosslinked resin foam, and even more preferably 100 μg or less / g of crosslinked resin foam. It is also more preferable that the ammonium ions are suppressed to below the detection limit.

[0034] The ammonium ion concentration can be measured by the ion chromatography method specifically as follows: Approximately 1.0 g of sample is weighed into a polypropylene container, 40 mL of ultrapure water is added, and then the container is weighted down with plastic wrap to prevent the sample from floating, and the lid is sealed. Extraction is performed at 90°C for 24 hours, and the container is allowed to cool. After that, the sample is filtered and diluted to prepare a sample solution. The amount of ammonium ions in the obtained sample solution is measured by ion chromatography using a Thermo Scientific ICS-6000. The amount of ammonium ions per 1 g of crosslinked resin foam can be calculated by dividing the measured amount of ammonium ions (μg) by the weight of the sample (g).

[0035] (Siloxane Compound Concentration) The crosslinked resin foam of the present invention preferably contains substantially no siloxane compounds. For example, when a silane-grafted polyolefin is used as the base resin of a resin composition and a crosslinked resin foam is produced using silane crosslinking, the resulting material having siloxane bonds volatilizes and deposits on the substrate of electronic components, potentially causing poor contact or conduction problems. Therefore, it is preferable that the five compounds extracted most abundantly (the first to fifth compounds in descending order of extraction amount, excluding unknown substances) in the outgassing detected by gas chromatography-mass spectrometry of the crosslinked resin foam do not contain siloxane compounds. The detected siloxane compound content may be 100 μg or less per 1 g of crosslinked resin foam, 50 μg or less per 1 g of crosslinked resin foam, or 10 μg or less per 1 g of crosslinked resin foam, and the foam preferably contains substantially no siloxane compounds.

[0036] The siloxane compounds can be measured, for example, by Headspace (HS)-GC / MS analysis under the following conditions. <Sample Preparation> 100 mg of sample is sealed in a glass vial, and the sealed sample is heated under the following HS conditions to recover volatile components. The recovered volatile components are then measured and detected under the following GC / MS conditions. <HS conditions> Apparatus: TELEDYNE TEKMAR HT3 HS autosampler Heating conditions: 200°C, 60 minutes Measurement mode: Loop mode Carrier gas: He, original pressure 40 kPa GC transfer line: 220°C Injection port temperature: 220°C Injection method: Split 10 mL / min <GC / MS conditions> Apparatus: Thermo TRACE GC ULTRA / TRACE DSQ Column: Thermo TR-5MS, 30 m, 0.25 mm ID, 0.25 μm Heating conditions: Heat at 40°C for 4 minutes, heat at 20°C / min, heat at 250°C for 10 minutes 30 seconds Ionization method: EI method Ion source temperature: 250°C MS transfer line temperature: 250°C Detection method: Scan (m / Z = 29 to 600)

[0037] [Method for Producing Crosslinked Resin Foam] Examples of methods for producing the crosslinked resin foam of the present invention include a method in which the formation of a crosslinked structure in a resin composition and a foaming treatment are carried out simultaneously (in parallel), and a method in which a crosslinked structure is formed in a resin composition and then the foaming treatment is carried out. Examples of these methods are described below. Each of the methods described below may be used alone, or two or more of them may be used in combination. In either method, the above-mentioned (D) crosslinking agent or (E) crosslinking aid may be blended into the resin composition as needed to form a stronger crosslinked structure.

[0038] <Method of Simultaneously Forming a Crosslinked Structure and Foaming Treatment> A resin composition of the present invention is prepared by blending the above-mentioned (D) crosslinking agent and, if necessary, (E) crosslinking aid. Kneading during this preparation is performed at a temperature (approximately 100 to 130°C) at which the (D) crosslinking agent and (B) foaming agent are unlikely to decompose. The resin composition can be in the form of pellets. The resulting resin composition is fed to an extruder and extrusion-molded at a resin temperature of approximately 100 to 130°C to obtain an unfoamed sheet having the desired thickness and width. This unfoamed sheet is placed in a heating and foaming furnace adjusted to approximately 180 to 230°C to cause a crosslinking reaction and foaming, thereby producing a sheet of the crosslinked resin foam of the present invention.

[0039] <Method of Foaming After Formation of Crosslinked Structure> The crosslinked resin foam of the present invention can be obtained by irradiating a resin composition with ionizing radiation to form a crosslinked structure, followed by heating to decompose the blowing agent and generate gas. Examples of ionizing radiation that can be used include α-, β-, and γ-rays, electron beams, and neutron beams. The electron beam irradiation intensity should be such that the gel fraction of the resulting foam falls within the range of 35 to 85%, and the acceleration voltage is preferably 100 kV to 1000 kV, more preferably 400 kV to 1000 kV. The absorbed dose is preferably 10 kGy to 300 kGy, more preferably 10 kGy to 100 kGy.

[0040] The present invention will be described in more detail based on the following examples and comparative examples, but the present invention is not limited to these.

[0041] [Preparation Examples of Resin Foam] Materials used to prepare the resin foams of Examples 1 to 14 and Comparative Examples 1 to 6 are shown in Table 1 below. Details of the materials used are as follows.

[0042] <Raw materials used> (thermoplastic olefin resin) LDPE (low-density polyethylene, manufactured by Japan Polyethylene Corporation: Novatec (registered trademark) LD LC600A (product name), MFR 7.0 g / 10 min, density 0.918 g / cm ３ ) Polybutadiene-based thermoplastic elastomer (manufactured by ENEOS Materials Corporation: RB810 (trade name), MFR 3.0 g / 10 min, density 0.901 g / cm ３ Ethylene-vinyl acetate copolymer (Mitsui Dow Polychemicals: V523 (trade name), MFR 14 g / 10 min, density 0.960 g / cm ３ ) (Foaming agent) Inorganic carbonate (baking soda, manufactured by Otsuka Chemical Co., Ltd.: AZ P-4 (trade name)) ADCA (azodicarbonamide, manufactured by Otsuka Chemical Co., Ltd.: Uniform AZ VI50ST (trade name)) (Crosslinking agent) 1,1-di(t-butylperoxy)cyclohexane (manufactured by NOF Corporation: Perhexa (registered trademark) C (trade name), 1-minute half-life: 153.8°C) Dicumyl peroxide (manufactured by NOF Corporation: Percumyl (registered trademark) D (trade name), 1-minute half-life: 175.2°C) (Crosslinking aid) TMPT: (trimethylolpropane triacrylate, manufactured by Shin-Nakamura Chemical Co., Ltd.: NK Ester TMPT (trade name))

[0043] <Method for Producing Foam> (Examples 1 to 11, Comparative Examples 1 to 5) The raw materials were mixed according to the composition shown in Table 1 below and melt-kneaded at 110°C using a 1-liter kneader. The kneaded mixture was sheeted using an open roll, cooled, and then cut and granulated using a pelletizer to obtain a resin composition (pellets). The resulting pellets were fed into an extruder set at a temperature of 100 to 120°C and molded into a 2-mm-thick sheet. The pellets were then cut into 10-cm squares to produce unfoamed sheets. These unfoamed sheets were then introduced into a hot air oven adjusted to 190°C, where crosslinking and foaming were carried out simultaneously at this high temperature to produce the foams of Examples 1 to 11 and Comparative Examples 1 to 5.

[0044] (Examples 12-14, Comparative Example 6) The raw materials were mixed according to the composition shown in Table 1 below, melt-kneaded at 110°C using a 1-liter kneader, and the kneaded mixture was sheeted using an open roll. After cooling, it was cut and granulated using a pelletizer to obtain a resin composition (pellets). The resulting pellets were fed into an extruder set at 100-120°C and molded into a 2-mm thick sheet. The sheet was then cut into 10 cm square pieces to produce unfoamed sheets. These unfoamed sheets were then irradiated with electron beams (Condition 1: Acceleration voltage 750 kV, Exposure dose 20 kGy, Irradiation method: both sides, Irradiation atmosphere: air; Condition 2: Acceleration voltage 500 kV, Exposure dose 10 kGy, Irradiation method: both sides, Irradiation atmosphere: air) to promote crosslinking, and then introduced into a hot air oven adjusted to 200°C to promote foaming. The foams of Examples 12-14 and Comparative Example 6 were produced.

[0045] [Performance Evaluation] The following performance evaluations were carried out using each of the foams (Examples 1 to 14 and Comparative Examples 1 to 6) produced as described above. The results of each test are shown in Table 1 below. The blend amounts in the table below are in parts by mass.

[0046] (Measurement of deformation and shrinkage rate of crosslinked foam) Each foam was left at room temperature for 24 hours, and the presence or absence of deformation was visually confirmed. The shrinkage rate (%) of each foam was calculated using the following (Equation 3). The thickness of each foam was determined as the arithmetic mean value of 10 measurements taken at 10 random locations. Shrinkage rate (%) = {(thickness of crosslinked foam immediately after foaming - thickness of crosslinked foam after 24 hours) / thickness of crosslinked foam immediately after foaming} x 100 (Equation 3) - Evaluation criteria - ◯: Shrinkage rate is 0% or more but less than 3% △: Shrinkage rate is 3% or more but less than 8% ×: Shrinkage rate is 8% or more

[0047] (Gel Fraction Measurement) Each foam sample was cut to 1.00±0.050 g and its mass was measured (Mass A). Each foam sample was placed in a flask, and 100 ml of xylene was added to the flask to completely immerse the sample. The solvent was boiled at 120°C for 24 hours±5 minutes. The sample was then removed from the boiling solvent, and the residue of each foam sample was dried for 24 hours in a vacuum oven at 80°C±2°C under a negative pressure of at least 85 kPa. After cooling, the sample was measured in 1 mg increments (Mass B). The gel fraction (%) was calculated from the measured masses A and B using the following formula (1): Gel Fraction (%) = {(Mass B) / (Mass A)} × 100 (Formula 1)

[0048] (Foaming test) The density of each of the unfoamed sheets of Examples 1 to 14 and Comparative Examples 1 to 6, and each of the foams obtained by crosslinking and foaming the sheets (which were left at room temperature for 24 hours after production), was measured, and the expansion ratio was calculated using the following formula (2): Expansion ratio = Density of unfoamed sheet / Apparent density of foam (Formula 2) The apparent density of each of the foams of Examples 1 to 14 was 50 to 175 kg / m ３ was within the range.

[0049] (Quantitative Analysis of Ammonium Ions) Approximately 1.0 g of each foam sample was weighed into a polypropylene container, and 40 mL of ultrapure water was added. The container was then weighted down with plastic wrap to prevent the sample from floating, and the lid was then sealed. Extraction was performed at 90°C for 24 hours, and the container was allowed to cool. After cooling, the container was filtered and diluted to prepare a sample solution. The amount of ammonium ions in the resulting sample solution was measured by ion chromatography using a Thermo Scientific ICS-6000. The amount of ammonium ions per 1 g of crosslinked resin foam was calculated by dividing the measured amount of ammonium ions (μg) by the weight of the sample (g). A detected ammonium ion concentration of less than 1000 μg / g of foam was evaluated as "Good," and a detected ammonium ion concentration of 1000 μg or more / g of foam was evaluated as "Poor."

[0050] (Quantitative analysis of siloxane compounds) 100 mg of sample was placed in a glass vial and heated under the following HS conditions to recover the volatile components. The recovered volatile components were then analyzed under the following GC / MS conditions. No siloxane compounds were detected in any of the foams. <HS conditions> Apparatus: TELEDYNE TEKMAR HT3 HS autosampler Heating conditions: 200°C, 60 minutes Measurement mode: Loop mode Carrier gas: He, original pressure 40 kPa GC transfer line: 220°C Injection port temperature: 220°C Injection method: Split 10 mL / min <GC / MS conditions> Apparatus: Thermo TRACE GC ULTRA / TRACE DSQ Column: Thermo TR-5MS, 30 m, 0.25 mm ID, 0.25 μm Heating conditions: Heat at 40°C for 4 minutes, heat at 20°C / min, heat at 250°C for 10 minutes 30 seconds Ionization method: EI method Ion source temperature: 250°C MS transfer line temperature: 250°C Detection method: Scan (m / Z = 29 to 600)

[0051]

[0052]

[0053] As is clear from Table 1, the foams of Comparative Examples 1, 2, and 4, which had a gel fraction of 30% or less, did not form a foamed structure due to insufficient crosslinking, and all had poor expansion ratios. The foam of Comparative Example 6, which had a gel fraction of more than 30% but less than 35%, was able to form a normal foamed structure due to the crosslinking structure, but shrunk immediately after production. The foam of Comparative Example 3, which had a gel fraction of more than 85%, did not form a normal foamed structure. Furthermore, ammonium ions were detected in the foam of Comparative Example 5, which used ADCA as a blowing agent, by ion chromatography analysis. In contrast, no harmful gases such as ammonia gas were detected in any of the foams (Examples 1 to 14) that satisfied all of the requirements of the present invention, and they also had excellent foamability and shape stability.

[0054] While the present invention has been described in connection with embodiments thereof, we do not intend to limit our invention to any of the details of the description unless otherwise specified, and believe that the claims should be construed broadly without departing from the spirit and scope of the invention as set forth in the appended claims.

[0055] This application claims priority based on Japanese Patent Application No. 2024-040366, filed on March 14, 2024, the contents of which are incorporated herein by reference as part of the present specification.

Claims

1. A crosslinked resin foam obtained by crosslinking and foaming a resin composition containing (A) a thermoplastic olefin resin and (B) a blowing agent, wherein the crosslinked resin foam has a gel fraction of 35 to 85% and satisfies the following (i) and / or (ii): (i) the ammonium ion concentration of the crosslinked resin foam detected by ion chromatography is less than 1000 μg / g of crosslinked resin foam, and (ii) the content of azodicarbonamide in the resin composition is 0.01% by mass or less.

2. The crosslinked resin foam according to claim 1, having an expansion ratio of 7 times or more.

3. A crosslinked resin foam according to claim 1 or 2, wherein the crosslinked structure of the crosslinked resin foam is formed by the action of an organic peroxide in the resin composition and / or by electron beam irradiation.

4. The crosslinked resin foam according to any one of claims 1 to 3, wherein the five outgases extracted in the largest amounts from the crosslinked resin foam detected by gas chromatography mass spectrometry do not include a siloxane compound.

5. The crosslinked resin foam according to any one of claims 1 to 4, wherein the (B) blowing agent comprises an inorganic carbonate.

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