Emulsion-type phenolic resin composition, adhesive, and brake pad

Abstract

To provide an emulsion-type phenolic resin composition suitable for adhesives which exhibits superior adhesive strength and yields a cured product with enhanced heat resistance.SOLUTION: An emulsion-type phenolic resin composition contains water, phenolic resin particles, polyvalent phenol, and hexamethylene tetramine, where the phenolic resin particles are dispersed in the water.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an emulsion-type phenolic resin composition, an adhesive containing the same, and a brake pad produced using the adhesive. [Background technology]

[0002] Phenolic resins are widely used as molding materials for various substrates, binders for friction materials, binders for abrasives, adhesives for wood, binders for laminated materials, binders for molds, coating agents, epoxy resin curing agents, etc., taking advantage of their excellent heat resistance, adhesiveness, mechanical properties, electrical properties, and price advantages. Phenolic resins are used, for example, by impregnating or coating various substrates, or as binders for various organic and inorganic substrates. Phenolic resins are broadly classified into novolac-type phenolic resins that are heat-cured by adding a curing agent such as hexamethylenetetramine, and resol-type phenolic resins that are heat-cured alone, and are used differently depending on the properties, applications, and purposes.

[0003] In recent years, from the viewpoints of reducing the environmental load and occupational safety and health, there has been a demand for phenolic resin materials that do not use organic solvents or that use reduced amounts of organic solvents. As an alternative, there has been a demand for phenolic resins that can be provided as water-soluble resins using water as a solvent or water-dispersible resins using an aqueous dispersant.

[0004] From the viewpoint of making thermosetting resins water-based, resol-type phenolic resins can be made water-based relatively easily by adjusting the molecular weight and the number of functional groups due to their inherent properties. For example, Patent Document 1 discloses an aqueous adhesive containing a polyester resin, a resol-type phenolic resin, and a basic compound. Patent Document 1 describes that the use of a metal hydroxide such as NaOH or an organic amine compound such as triethylamine as the basic compound prevents aggregation of the resulting aqueous adhesive and ensures stability.

[0005] On the other hand, since novolak phenolic resins are usually solid resins at room temperature, it is difficult to make them aqueous-based. Even if an aqueous dispersion of the resin is prepared, the average particle size of the dispersed resin particles is in the extremely large range of 10 μm to 100 μm, and the resin particles may settle or aggregate, making it difficult to obtain long-term dispersion stability.

[0006] In order to solve the above problems, Patent Document 2, for example, describes a technique for dispersing a novolac phenolic resin in an aqueous medium, in which an acid group-containing aliphatic hydrocarbon group having 3 to 30 carbon atoms is introduced as a substituent on an aromatic nucleus into the molecular structure of a novolac phenolic resin dispersed in an aqueous medium, and the pH of the aqueous medium is adjusted to a range of 7.0 to 9.0, thereby making it possible to reduce the resin particle size of the novolac phenolic resin to less than 1 μm without using a large amount of dispersant, and to obtain an aqueous dispersion of novolac phenolic resin having excellent storage stability.

[0007] As yet another technique for obtaining an aqueous phenolic resin dispersion, Patent Document 3 proposes a technique for obtaining a hydrophilic phenolic resin by introducing an ionic moiety into the main chain structure of a phenolic resin. More specifically, Patent Document 3 proposes that a resol-type or novolac-type phenolic resin precursor, a modifier containing at least one functional moiety and at least one ionic moiety capable of reacting with the phenolic resin precursor, and a multihydroxyphenolic compound are reacted in the presence of water to obtain a phenolic resin containing a pendant ionic moiety in the main chain structure. By having the pendant ionic moiety, this phenolic resin can be stably dispersed in an aqueous medium in the form of water-insoluble particles. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2007-302786 A [Patent Document 2] JP 2008-150449 A [Patent Document 3] Patent No. 4005311 Summary of the Invention [Problem to be solved by the invention]

[0009] However, when a metal hydroxide is blended with the aqueous adhesive containing the aqueous resol type phenolic resin of Patent Document 1, the water resistance of the cured product of the aqueous adhesive may be poor due to the presence of the metal hydroxide. Also, when an organic amine compound is blended with the aqueous adhesive containing the aqueous resol type phenolic resin of Patent Document 1, the water solubility of the resulting adhesive may be poor.

[0010] In addition, since the novolac phenolic resin aqueous dispersion of Patent Document 2 has a dispersed phase of water, when it is applied to a substrate as an adherend during use, it may have poor wettability to the substrate. Specifically, the novolac phenolic resin aqueous dispersion of Patent Document 2 does not adhere well to the adherend because the resin component does not fully blend with the adherend, and as a result, the desired adhesive strength may not be obtained.

[0011] Furthermore, when the aqueous phenolic resin dispersion of Patent Document 3 is applied to a substrate as an adherend during use, the phenolic resin present as water-insoluble particles does not adhere sufficiently to the substrate, and as a result, the desired adhesive strength may not be obtained. [Means for solving the problem]

[0012] The present invention has been made in consideration of the above problems, and has an object to provide an emulsion-type phenolic resin composition that can be used as an adhesive, which is water-based yet has excellent adhesion to adherends, and therefore has excellent adhesive strength, and the heat resistance of the cured product is improved.

[0013] According to the present invention, there are provided an emulsion-type phenolic resin composition, an adhesive, and a brake pad as described below. [1] Water, Phenolic resin particles; Polyphenols, and hexamethylenetetramine, The phenolic resin particles are dispersed in the water. Emulsion-type phenolic resin composition. [2] The emulsion-type phenolic resin composition according to item [1], wherein the phenolic resin particles are composed of a phenolic resin and an emulsifier. [3] The emulsion-type phenolic resin composition according to item [1] or [2], wherein the polyhydric phenol and the hexamethylenetetramine are dissolved in the water. [4] The emulsion-type phenolic resin composition according to any one of items [1] to [3], wherein the phenolic resin comprises a novolac-type phenolic resin, a resol-type phenolic resin, or a combination thereof. [5] The emulsion-type phenolic resin composition according to any one of items [1] to [4], wherein the polyhydric phenol includes at least one selected from the group consisting of dihydroxybenzenes and trihydroxybenzenes. [6] The emulsion-type phenolic resin composition according to any one of items [1] to [5], wherein the polyhydric phenol includes at least one selected from resorcinol, catechol, and pyrogallol. [7] The emulsion-type phenolic resin composition according to item [2], wherein the emulsifier comprises at least one selected from polyacrylamide, casein, hydroxyethyl cellulose, and polyvinyl alcohol. [8] The emulsion-type phenolic resin composition according to any one of items [1] to [7], wherein the phenolic resin has a weight-average molecular weight of 100 or more and 15,000 or less. [9] The emulsion-type phenolic resin composition according to any one of items [1] to [8], wherein the content of the polyhydric phenol is 0.1% by mass or more and 10% by mass or less with respect to the phenolic resin particles.

[10] The emulsion-type phenolic resin composition according to any one of items [1] to [9], wherein the content of the hexamethylenetetramine is 1% by mass or more and 20% by mass or less with respect to the phenolic resin particles.

[11] An adhesive comprising the emulsion-type phenolic resin composition according to any one of items [1] to

[10] .

[12] The adhesive according to item

[11] , which is used for manufacturing a brake pad.

[13] A metal substrate; A brake pad comprising: a friction material adhered to the metal substrate via the adhesive according to item

[11] or

[12] . Effect of the Invention

[0014] According to the present invention, there is provided an emulsion-type phenolic resin composition that can be used as an adhesive, which has excellent adhesion to adherends, and therefore has excellent adhesive strength, and the heat resistance of the cured product is improved. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, an embodiment of the present invention will be described. In this specification, the expression "a to b" in the explanation of a numerical range means "a or more and b or less" unless otherwise specified. For example, "5 to 90%" means "5% or more and 90% or less."

[0016] The resin composition of this embodiment is an oil-in-water type aqueous emulsion in which the resin component of the dispersion is dispersed in water, and contains water, phenolic resin particles, polyhydric phenol, and hexamethylenetetramine. In the resin composition of this embodiment, the phenolic resin of the dispersion is dispersed in water in the form of particles. Furthermore, the resin composition of this embodiment contains polyhydric phenol and hexamethylenetetramine, which are present in a dissolved state in water. In other words, the resin composition of this embodiment is an aqueous emulsion in which phenolic resin particles are dispersed in an aqueous solution of polyhydric phenol and hexamethylenetetramine.

[0017] The emulsion-type phenolic resin composition of the present embodiment (sometimes simply referred to as "resin composition" in this specification) is provided in the form of an aqueous emulsion in which a phenolic resin is dispersed in an aqueous medium. The resin composition of the present embodiment does not use an organic solvent as a solvent / dispersion medium, and therefore reduces the environmental load.

[0018] The emulsion-type phenolic resin composition of the present embodiment does not contain any or almost no metal hydroxides or organic amine compounds that are blended in conventional aqueous solutions of resol-type phenolic resins, and therefore does not suffer from problems such as insufficient water resistance due to metal hydroxides or insufficient water solubility due to organic amine compounds.

[0019] The emulsion type phenolic resin composition of this embodiment contains phenolic resin particles as a resin phase, and polyhydric phenol and hexamethylenetetramine in an aqueous phase. When the emulsion type phenolic resin composition of this embodiment is applied to an adherend, the polyhydric phenol in the aqueous phase migrates to the surface of the adherend and is present on the surface at high density. Even if the adherend is a metal plate having fine surface irregularities, the polyhydric phenol can penetrate into the surface irregularities of the metal plate. This is thought to be because the polyhydric phenol is a low molecular weight molecule and has a high degree of freedom. Following the application / coating of the emulsion type phenolic resin composition to the adherend, a heat treatment is performed, whereby the polyhydric phenol and the phenolic resin particles are cured by hexamethylenetetramine to form a crosslinked body. The crosslinked body (cured product) obtained in this manner is highly adherent to the adherend, and therefore the coating film made of the crosslinked body has high adhesive strength.

[0020] Adhesives such as those described in Patent Document 3, which are used by applying an aqueous phenolic resin, which is a reaction product of a phenolic resin, a modifier, and a multihydroxyphenol compound, to an adherend, may have insufficient adhesion to the surface of the adherend due to the large molecular weight of the aqueous phenolic resin, and therefore may not provide sufficient adhesive strength. On the other hand, in the resin composition of the present embodiment, the polyhydric phenol and the phenolic resin each exist as a single substance before use (before being applied to the adherend), and when heated during use (after being applied to the adherend), the polyhydric phenol and the phenolic resin form a crosslinked body and exhibit adhesiveness. By employing the above mechanism, the resin composition of the present embodiment realizes high adhesion to the adherend and high adhesion between the adherends.

[0021] In one embodiment, the phenolic resin particles of the dispersion may be dispersed in water by an emulsifier. This allows the phenolic resin in the emulsion to maintain the state immediately after production without interacting with the polyhydric phenol and hexamethylenetetramine. Therefore, the curing reaction between the phenolic resin and the polyhydric phenol does not progress to cause the phenomenon of high molecular weight and high viscosity, and the state immediately after production is maintained until use, so that the resin composition of this embodiment is excellent in handleability.

[0022] The emulsion-type phenolic resin composition of the present embodiment is produced by the following steps. Step 1: A step of reacting a phenol with an aldehyde to obtain a phenolic resin. Step 2: A step of adding the phenolic resin obtained in step 1 and, if necessary, an emulsifier to water and stirring to disperse the phenolic resin in water. Step 3: A step of adding a polyhydric phenol and hexamethylenetetramine to the aqueous dispersion of the phenolic resin obtained in step 2.

[0023] In step 1, a phenolic resin is obtained by condensation polymerization of a phenol and an aldehyde. The phenolic resin obtained in step 1 may be a novolac type phenolic resin or a resol type phenolic resin as long as it is water-insoluble.

[0024] When the emulsion-type phenolic resin composition of the present embodiment contains a novolac-type phenolic resin as the dispersed phenolic resin particles, step 1 is a step of obtaining a novolac-type phenolic resin by condensation polymerization of phenols and aldehydes in the presence of an acid catalyst ("step 1-1").

[0025] Examples of phenols that can be used in the synthesis of the novolak-type phenolic resin in step 1-1 include phenol; cresols such as o-cresol, m-cresol, and p-cresol; ethylphenols such as o-ethylphenol, m-ethylphenol, and p-ethylphenol; butylphenols such as isopropylphenol, butylphenol, and p-tert-butylphenol; alkylphenols such as p-tert-amylphenol, p-octylphenol, p-nonylphenol, and p-cumylphenol; halogenated phenols such as fluorophenol, chlorophenol, bromophenol, and iodophenol; monohydric phenol substitutes such as p-phenylphenol, aminophenol, nitrophenol, dinitrophenol, and trinitrophenol; monohydric phenols such as 1-naphthol and 2-naphthol; and polyhydric phenols such as resorcin, alkylresorcin, pyrogallol, catechol, alkylcatechol, hydroquinone, alkylhydroquinone, phloroglucin, bisphenol A, bisphenol F, bisphenol S, and dihydroxynaphthalene. These may be used alone or in combination of two or more. Among them, it is preferable to use phenol from the viewpoint of production costs.

[0026] Aldehydes that can be used in the production of the novolac-type phenolic resin in step 1-1 include formaldehyde, paraformaldehyde, trioxane, acetaldehyde, propionaldehyde, polyoxymethylene, chloral, hexamethylenetetramine, furfural, glyoxal, n-butylaldehyde, caproaldehyde, allylaldehyde, benzaldehyde, crotonaldehyde, acrolein, tetraoxymethylene, phenylacetaldehyde, o-tolualdehyde, salicylaldehyde, and the like. These may be used alone or in combination of two or more. In addition, precursors of these aldehydes or solutions of these aldehydes may also be used. Among these, from the viewpoint of production cost, it is preferable to use an aqueous formaldehyde solution.

[0027] Examples of the acid catalyst that can be used in the production of the novolak phenolic resin in step 1-1 include organic acids such as acetic acid, formic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, benzoic acid, salicylic acid, sulfonic acid, phenolsulfonic acid, and p-toluenesulfonic acid; and inorganic acids such as hydrochloric acid, sulfuric acid, sulfates, phosphoric acid, and sulfates.

[0028] In the production of the novolac phenolic resin in step 1-1, the molar ratio of the aldehydes to the phenols (F / P) is, for example, 0.5 or more, preferably 0.55 or more, and more preferably 0.6 or more. The upper limit of the molar ratio of the aldehydes to the phenols (F / P) is, for example, 1.2 or less, preferably 1.1 or less, and more preferably 1.0 or less. By carrying out the reaction under conditions where the molar ratio of the aldehydes to the phenols (F / P) is within the above range, a novolac phenolic resin having a desired weight average molecular weight can be obtained.

[0029] In the production of the novolac phenolic resin in step 1-1, the step of reacting the phenols with the aldehydes in the presence of an acid catalyst is preferably carried out at a temperature of, for example, 60°C to 120°C, preferably at a temperature of 80°C to 100°C, for example, for a reaction time of 10 minutes to 100 minutes. This allows the reaction to proceed efficiently and sufficiently. In addition, by carrying out the reaction under heating, the starting materials are mixed uniformly, and the molecular weight of the resulting novolac phenolic resin can be made uniform by intermolecular entanglement and intermolecular action. The reaction time is not particularly limited, and may be appropriately determined depending on the type of starting materials, the blending molar ratio, the amount and type of catalyst used, and the reaction conditions.

[0030] The reaction solvent used for producing the novolac phenolic resin in step 1-1 is generally water, but an organic solvent may also be used. Specific examples of such organic solvents include alcohols, ketones, aromatics, etc. Specific examples of alcohols include methanol, ethanol, propyl alcohol, ethylene glycol, diethylene glycol, triethylene glycol, glycerin, etc. Specific examples of ketones include acetone and methyl ethyl ketone. Specific examples of aromatics include toluene and xylene, etc.

[0031] The weight average molecular weight of the novolac phenolic resin obtained in step 1-1 is, for example, 100 to 15,000, preferably 400 to 12,000, more preferably 500 to 10,000. The novolac phenolic resin having a weight average molecular weight in the above range is preferred because it is easy to emulsify. In addition, the cured product obtained by curing the novolac phenolic resin has high mechanical strength and heat resistance.

[0032] The novolac phenolic resin produced by the above method may be subjected to post-treatment to remove unreacted free phenol contained therein, if necessary. As the post-treatment, a method of performing normal pressure distillation to 150°C, followed by vacuum distillation at 500 Pa to 250°C can be used. The novolac phenolic resin obtained by the above method has a content of unreacted free phenols reduced to 1.0% by mass or less, preferably 0.8% by mass or less, more preferably 0.6% by mass or less. The novolac phenolic resin obtained by the above method also has a content of unreacted free aldehydes reduced to 1.0% by mass or less, preferably 0.8% by mass or less, more preferably 0.6% by mass or less.

[0033] When the emulsion-type phenolic resin composition of this embodiment contains resol-type phenolic resin particles as a dispersion, step 1 is a step of condensation polymerizing phenols and aldehydes in the presence of a basic catalyst to obtain a water-insoluble resol-type phenolic resin ("step 1-2").

[0034] Examples of phenols that can be used in the synthesis of the water-insoluble resol-type phenolic resin in step 1-2 include phenol; cresols such as o-cresol, m-cresol, and p-cresol; ethylphenols such as o-ethylphenol, m-ethylphenol, and p-ethylphenol; butylphenols such as isopropylphenol, butylphenol, and p-tert-butylphenol; alkylphenols such as p-tert-amylphenol, p-octylphenol, p-nonylphenol, and p-cumylphenol; halogenated phenols such as fluorophenol, chlorophenol, bromophenol, and iodophenol; monohydric phenol substitutes such as p-phenylphenol, aminophenol, nitrophenol, dinitrophenol, and trinitrophenol; monohydric phenols such as 1-naphthol and 2-naphthol; and polyhydric phenols such as resorcin, alkylresorcin, pyrogallol, catechol, alkylcatechol, hydroquinone, alkylhydroquinone, phloroglucin, bisphenol A, bisphenol F, bisphenol S, and dihydroxynaphthalene. These may be used alone or in combination of two or more. Of these, cresols, ethylphenols and alkylphenols are preferably used from the viewpoint of making the resulting resol type phenolic resin water-insoluble.

[0035] Aldehydes that can be used in the production of the water-insoluble resol-type phenolic resin in step 1-2 include formaldehyde, paraformaldehyde, trioxane, acetaldehyde, propionaldehyde, polyoxymethylene, chloral, hexamethylenetetramine, furfural, glyoxal, n-butylaldehyde, caproaldehyde, allylaldehyde, benzaldehyde, crotonaldehyde, acrolein, tetraoxymethylene, phenylacetaldehyde, o-tolualdehyde, salicylaldehyde, and the like. These may be used alone or in combination of two or more. In addition, precursors of these aldehydes or solutions of these aldehydes may also be used. Among these, from the viewpoint of production cost, it is preferable to use an aqueous formaldehyde solution.

[0036] Examples of basic catalysts that can be used in the production of the water-insoluble resol-type phenolic resin in step 1-2 include hydroxides of alkali metals or alkaline earth metals such as sodium hydroxide, potassium hydroxide, calcium hydroxide, etc.; carbonates such as sodium carbonate, calcium carbonate, etc.; oxides such as lime; sulfites such as sodium sulfite; phosphates such as sodium phosphate; amines such as ammonia, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, hexamethylenetetramine, pyridine, etc.

[0037] In the production of the water-insoluble resol-type phenolic resin in step 1-2, the molar ratio of aldehydes to phenols (F / P) is, for example, 1.5 or more, preferably 1.5 to 2.0, more preferably 1.6 to 1.9, and the reaction is carried out by adding the above-mentioned basic catalyst as a polymerization catalyst and refluxing for an appropriate time. By carrying out the reaction under conditions where the molar ratio of aldehydes to phenols (F / P) is within the above range, a resol-type phenolic resin having a desired weight average molecular weight can be obtained.

[0038] In the production of the water-insoluble resol type phenolic resin in step 1-2, the step of reacting phenols with aldehydes in the presence of a basic catalyst is preferably carried out at a temperature of, for example, 60°C to 120°C, preferably at a temperature of 80°C to 100°C, for example, for a reaction time of 10 minutes to 100 minutes. This allows the reaction to proceed efficiently and sufficiently. In addition, by carrying out the reaction under heating, the starting materials are uniformly mixed, and the molecular weight of the resulting resol type phenolic resin can be made uniform due to intermolecular entanglement and intermolecular action. The reaction time is not particularly limited, and may be appropriately determined depending on the type of starting materials, the blending molar ratio, the amount and type of catalyst used, and the reaction conditions.

[0039] The reaction solvent used for the synthesis of the water-insoluble resol-type phenolic resin in step 1-2 is generally water, but an organic solvent may also be used. Specific examples of such organic solvents include alcohols, ketones, aromatics, etc. Specific examples of alcohols include methanol, ethanol, propyl alcohol, ethylene glycol, diethylene glycol, triethylene glycol, glycerin, etc. Specific examples of ketones include acetone and methyl ethyl ketone. Specific examples of aromatics include toluene and xylene, etc.

[0040] By appropriately setting the reaction conditions in step 1-2, a water-insoluble resol-type phenolic resin can be obtained. The reaction conditions for obtaining a water-insoluble resol-type phenolic resin include (i) obtaining a phenolic resin containing a structural unit derived from an alkylphenol by using the above-mentioned cresols, ethylphenols, and alkylphenols as phenols, (ii) obtaining a high molecular weight phenolic resin by reducing the amount of the basic catalyst used or neutralizing with an acid after the reaction is completed to make the pH of the reaction mixture weakly alkaline, and (iii) obtaining a high molecular weight phenolic resin by reacting at a relatively high reaction temperature of about 100°C for a relatively long time of about 60 to 100 minutes, or a combination thereof.

[0041] The weight average molecular weight of the water-insoluble resol type phenolic resin obtained in step 1-2 is, for example, 100 to 15,000, preferably 400 to 12,000, more preferably 500 to 10,000. The water-insoluble resol type phenolic resin having a weight average molecular weight in the above range is preferred because it is easy to emulsify. In addition, the cured product obtained by curing the resin has high mechanical strength and heat resistance.

[0042] The resol type phenolic resin obtained by the above method has a content of unreacted free phenols reduced to 1.0 mass % or less, preferably 0.8 mass % or less, more preferably 0.6 mass % or less, and the novolac type phenolic resin obtained by the above method has a content of unreacted free aldehydes reduced to 1.0 mass % or less, preferably 0.8 mass % or less, more preferably 0.6 mass % or less.

[0043] Following step 1, which is either step 1-1 or step 1-2, in step 2, the phenolic resin obtained in step 1 is mixed with an emulsifier in water and stirred to emulsify and disperse the phenolic resin in water. This results in a phenolic resin aqueous dispersion in which the phenolic resin is dispersed in water in the form of fine particles. In step 2, the phenolic resin is mixed with water in an amount of, for example, 3% by mass or more and 50% by mass or less, preferably 5% by mass or more and 45% by mass or less, more preferably 10% by mass or more and 40% by mass or less, based on the entire water dispersion obtained.

[0044] The average particle size D50 of the phenolic resin particles in the aqueous dispersion is, for example, 0.1 μm to 30 μm, preferably 0.5 μm to 20 μm, more preferably 1 μm to 15 μm. The phenolic resin particles having an average particle size within the above range are uniformly dispersed in water, do not precipitate during storage, and are easy to handle. The average particle size of the phenolic resin particles can be controlled by adjusting the stirring conditions, such as the stirring speed and stirring time. As the stirring conditions, for example, a stirring speed of 200 rpm to 400 rpm and a stirring time of 1 to 10 hours can be adopted.

[0045] Examples of the emulsifier used in step 2 include polyacrylamide, casein, hydroxyethyl cellulose, and polyvinyl alcohol. These may be used alone or in combination of two or more. These emulsifiers are used in an amount of, for example, 10% by mass or more and 80% by mass or less, preferably 20% by mass or more and 70% by mass or less, and more preferably 30% by mass or more and 60% by mass or less, based on the phenolic resin. By using the emulsifier in an amount within the above range, the phenolic resin can be uniformly dispersed in water.

[0046] Following step 2, in step 3, polyhydric phenol and hexamethylenetetramine are added to the phenolic resin aqueous dispersion obtained in step 2. Examples of polyhydric phenols used in step 3 include dihydroxybenzene and trihydroxybenzene. Examples of dihydroxybenzenes include resorcinol and catechol, and examples of trihydroxybenzenes include, but are not limited to, pyrogallol. Among them, pyrogallol is preferably used from the viewpoint of the adhesive strength of the crosslinked body obtained. These polyhydric phenols may be used alone or in combination of two or more. The polyhydric phenol is preferably 0.5% by mass or more and 20% by mass or less based on the total solid content of the resin composition. By using the polyhydric phenol in an amount within the above range, a resin composition having excellent adhesiveness and excellent handleability can be obtained.

[0047] In step 3, hexamethylenetetramine is added in addition to the polyhydric phenol. Hexamethylenetetramine has high water solubility, so it dissolves in the aqueous phase of the phenolic resin dispersion obtained in step 2. Hexamethylenetetramine is added to the phenolic resin dispersion obtained in step 2 so that the amount of hexamethylenetetramine relative to the solid content of the resulting resin composition is, for example, 1% by mass or more and 20% by mass or less, preferably 2% by mass or more and 15% by mass or less, more preferably 3% by mass or more and 10% by mass or less. By using hexamethylenetetramine in the amount within the above range, the curability of the resulting resin composition is improved, and the mechanical strength of the cured product of the resin composition can be improved.

[0048] The resin composition of the present embodiment obtained through the above-mentioned steps 1 to 3 can be stored in a uniformly dispersed state without aggregation or precipitation of the phenolic resin particles. Furthermore, the resin composition of the present embodiment has an unreacted phenol content of 0.1 mass% or less based on the entire resin composition, and an unreacted aldehyde content of 0.1 mass% or less based on the entire resin composition.

[0049] The resin composition of the present embodiment may further contain additives depending on the application. Examples of additives that can be used include a thixotropic agent, a thickener, a dispersant, a curing accelerator, and an antifreeze agent. Examples of thixotropic agents or thickeners include fumed silica, precipitated silica, bentonite, diatomaceous earth, sepiolite, calcium carbonate, polyacrylates, and the like. Examples of dispersants include anionic dispersants such as carboxylates, sulfonates, sulfates, and phosphates; cationic dispersants such as quaternary ammonium salts, sulfonium salts, and pyridinium salts; amphoteric dispersants such as betaine-type dispersants and sulfonates; and nonionic surfactants such as polyhydric alcohols and polyoxyethylene. Examples of the hardening accelerator include organic acids such as benzoic acid, salicylic acid, sulfonic acid, phenolsulfonic acid, and paratoluenesulfonic acid, and resorcinol. Antifreezing agents include those that lower the freezing temperature, such as methanol, ethylene glycol, and glycerin. These may be used alone or in combination of two or more.

[0050] (Application) The emulsion-type phenolic resin composition of the present embodiment can be used, for example, as an adhesive for friction materials, an adhesive for abrasives, an adhesive for wood, an adhesive for laminated materials, and an adhesive for molds. For example, when the resin composition of the present embodiment is used as an adhesive for friction materials, the emulsion-type phenolic resin composition of the present embodiment is applied to a metal substrate for a brake pad. As the application method, a spray application method, a roll coater method, etc. can be used. Next, the applied resin composition is heated, for example, at 100°C for 20 minutes to dry and remove the water, which is the solvent. Then, a friction plate for a brake pad is placed on the side of the dried resin composition layer on this metal substrate, and heated and pressed, for example, at a temperature of 150°C. After heating and pressing, the phenolic resin particles and the polyhydric phenol are baked, for example, at a temperature of 200°C, and the phenolic resin particles and the polyhydric phenol are completely cured with a hexamethylenetetramine curing agent. This makes it possible to manufacture a brake pad in which the metal substrate and the friction plate are firmly bonded. Since the emulsion-type phenolic resin composition of the present embodiment does not contain an organic solvent, the organic solvent does not volatilize during heating and drying. Therefore, it is easy to handle and has a small environmental impact. Furthermore, since the emulsion-type phenolic resin composition of the present embodiment produces a cured product having high heat resistance, the composition can be suitably used as an adhesive for producing articles such as brake pads and abrasives that are expected to be used in high-temperature environments.

[0051] Although the embodiments of the present invention have been described above, these are merely examples of the present invention, and various configurations other than those described above can also be adopted. EXAMPLES

[0052] EXAMPLES Hereinafter, the present invention will be described with reference to examples and comparative examples, but the present invention is not limited to these.

[0053] [Preparation of emulsion-type phenolic resin composition] (Preparation Example 1) In a three-neck flask equipped with a stirrer and a thermometer, 1000 parts by mass of phenol and 10 parts by mass of oxalic acid were charged and heated to 100°C, after which 500 parts by mass of 37% formaldehyde (molar ratio F / P=0.58) was gradually added over 2 hours. After that, a reflux reaction was carried out for 2 hours, followed by normal pressure distillation to 150°C, and further reduced pressure distillation at 500 Pa to 250°C. The content of unreacted phenol in the obtained novolak type phenolic resin, as measured by gas chromatography, was below the detection limit (0.1%), and the content of unreacted aldehyde was below the detection limit (0.1%). After the reaction, the mixture was cooled to below 110°C, and 650 parts by mass of polyacrylamide (Arakawa Chemical Industries, Ltd., "Polystron 117", 15% aqueous solution) and 1,500 parts by mass of water were added and stirred for 1 hour. 48 parts of hexamine were then added to obtain 3,000 parts by mass of a phenolic resin dispersion.

[0054] (Examples 1 to 3, Comparative Example 1) The phenolic resin dispersion obtained in Preparation Example 1 above was mixed with a polyhydric phenol shown in Table 1 to obtain an emulsion-type phenolic resin composition.

[0055] [Performance evaluation of emulsion-type phenolic resin composition] In Examples 1 to 3 and Comparative Example 1, the emulsion-type phenolic resin compositions obtained were measured for the physical properties shown below, and their performance as adhesives was evaluated. (Adhesive strength) After degreasing a 25mm x 150mm pickled steel plate with acetone, the above-mentioned emulsion type phenolic resin composition was applied to the plate so that the film thickness after drying would be 8 to 15μm, and the plate was dried at 80°C for 30 minutes. The surfaces coated with the resin composition were laminated together at a thickness of 10mm, clamped under a pressure of 5MPa, and cured at 200°C for 1 hour to prepare a test piece, and the adhesive strength was calculated by a tensile test. The results are shown in Table 1. A higher adhesive strength (MPa) value indicates a higher adhesive strength.

[0056] (Heat resistance) The emulsion-type phenolic resin composition obtained in each of the Examples and Comparative Examples was cured at 200°C for 1 hour and then pulverized in a small pulverizer. The pulverized material was heated from 30 to 300°C at a rate of 10°C / min by TG / DTA, and then held at 300°C for 3 hours, after which the weight residual ratio was measured. A higher residual ratio (%) indicates less thermal decomposition and higher heat resistance.

[0057] [Table 1]

[0058] The emulsion-type phenolic resin composition of the embodiment has both improved adhesive strength and heat resistance, and can be suitably used as an adhesive for producing articles such as brake pads used in high-temperature environments.

Claims

1. Water, Phenol resin particles; Polyphenols, and hexamethylenetetramine, The phenolic resin particles are dispersed in the water. Emulsion-type phenolic resin composition.

2. 2. The emulsion-type phenolic resin composition according to claim 1, wherein the phenolic resin particles are composed of a phenolic resin and an emulsifier.

3. 2. The emulsion-type phenolic resin composition according to claim 1, wherein the polyhydric phenol and the hexamethylenetetramine are dissolved in the water.

4. 2. The emulsion-type phenolic resin composition according to claim 1, wherein the phenolic resin comprises a novolac-type phenolic resin or a resol-type phenolic resin, or a combination thereof.

5. 2. The emulsion-type phenolic resin composition according to claim 1, wherein the polyhydric phenol comprises at least one selected from the group consisting of dihydroxybenzenes and trihydroxybenzenes.

6. 2. The emulsion-type phenolic resin composition according to claim 1, wherein the polyhydric phenol comprises at least one selected from the group consisting of resorcinol, catechol, and pyrogallol.

7. 3. The emulsion-type phenolic resin composition according to claim 2, wherein the emulsifier comprises at least one selected from the group consisting of polyacrylamide, casein, hydroxyethyl cellulose, and polyvinyl alcohol.

8. 2. The emulsion-type phenolic resin composition according to claim 1, wherein the phenolic resin has a weight average molecular weight of 100 or more and 15,000 or less.

9. 2. The emulsion-type phenolic resin composition according to claim 1, wherein the content of the polyhydric phenol is 0.1% by mass or more and 10% by mass or less with respect to the phenolic resin particles.

10. 2. The emulsion-type phenolic resin composition according to claim 1, wherein the content of the hexamethylenetetramine is 1% by mass or more and 20% by mass or less with respect to the phenolic resin particles.

11. An adhesive comprising the emulsion-type phenolic resin composition according to any one of claims 1 to 10.

12. The adhesive of claim 11 used to manufacture a brake pad.

13. A metal substrate; A brake pad comprising: a friction material adhered to the metal substrate via the adhesive according to claim 11.

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