Heat foaming sheet, and bonding method

Abstract

To provide a heat foaming sheet which enables appropriate bonding of a gap between adherends, and a bonding method using the heat foaming sheet.SOLUTION: A heat foaming sheet contains a sheet-like base material, and a foamable adhesive layer provided on one surface or both surfaces of the sheet-like base material. The foamable adhesive layer is at least one outermost layer of the heat foaming sheet, contains an epoxy resin, a curing agent, a thermoplastic resin and a foaming agent, and has arithmetic average roughness Ra on the outermost surface of the foamble adhesive layer of 0.4 μm or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a heat-foamable sheet and a bonding method using the heat-foamable sheet. [Background technology]

[0002] BACKGROUND ART Conventionally, adhesives are used in a variety of places, and are capable of bonding a plurality of adherends together by curing between the adherends.

[0003] Adherends include those with distortion, those with uneven surfaces with deep concavities and convexities, and porous objects. Liquid adhesives can be used to fill and bond the gaps between the adherends. Depending on the bonding location, the gaps to be filled with adhesive may be very narrow, making it difficult for the adhesive to penetrate, making it difficult to completely fill the gaps. In such cases, for example, it may be possible to fill the gaps using a larger amount of adhesive than originally required. However, this requires the unnecessary removal of adhesive, increasing the workload and potentially contaminating the adherends and the surrounding work area. Furthermore, liquid adhesives can flow before curing, resulting in problems such as spillage or overflowing outside the bonded area.

[0004] As described above, with liquid adhesives, it is difficult to accurately control the amount and location of application, which can lead to insufficient filling of gaps, resulting in a reduction in the adhesive area and adhesive strength, and other problems with workability during use.

[0005] To solve the above problems, it is known to use a sheet-type adhesive that is solid at room temperature (Patent Documents 1 to 4). With a sheet-type adhesive, it is possible to apply a specified amount of adhesive to a specified location on an adherend, and it is also possible to prevent contamination of the adherend and the surrounding work area. Sheet-type adhesives have the problem of poor conformability to objects with irregularities or distortion, but conformability can be ensured by applying pressure using a press or the like. In this case, it is known to improve conformability by using an adhesive that melts when heated. When using thermosetting resins, epoxy adhesives in particular are widely used because they are superior in that they have a large decrease in melt viscosity when heated and have high heat resistance. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 5695937 [Patent Document 2] Patent No. 6067967 [Patent Document 3] Patent No. 6220100 [Patent Document 4] Japanese Patent Application Publication No. 2019-203062 Summary of the Invention [Problem to be solved by the invention]

[0007] However, in the case of adherends in which multiple parts are connected by fitting, such as pipes or dowels, it is difficult to completely fill the gaps between the adherends using a sheet adhesive.

[0008] For example, if the thickness of the sheet adhesive is thicker than the gap, it will be too cramped to insert the sheet adhesive into the gap. Forcing the sheet adhesive into the gap can cause the adhesive to peel or wrinkle, potentially resulting in uneven bonding surfaces and reduced or variable adhesive strength. On the other hand, if the sheet adhesive is made thinner than the gap to make it easier to insert, it will be unable to fill the gap, potentially resulting in problems such as no adhesion or extremely weak adhesive strength.

[0009] Furthermore, the above-described problems are not limited to the case of adherends in which a plurality of parts are connected by fitting, but can also occur when bonding gaps between adherends.

[0010] Therefore, a method for properly bonding gaps between adherends is desired.

[0011] The present invention has been made in view of the above-mentioned problems, and has an object to provide a heat-foamable sheet that can properly bond gaps between adherends, and a bonding method that uses the heat-foamable sheet. [Means for solving the problem]

[0012] The above problem was solved by providing the sheet adhesive with an adhesive layer that expands (foams) when heated. Specifically, the following measures were taken: <1> Preferably, <2> The above problem was solved by the following means. <1> A heat-foamable sheet comprising a sheet-like substrate and a foamable adhesive layer provided on one or both sides of the sheet-like substrate, the foamable adhesive layer is at least one outermost layer of the heat-foamable sheet, the foamable adhesive layer comprises an epoxy resin, a curing agent, a thermoplastic resin, and a foaming agent; A heat-foamable sheet, wherein the arithmetic mean roughness Ra of the outermost surface of the foamable adhesive layer is 0.4 μm or more. <2> The arithmetic mean roughness Ra on the surface of the sheet-like substrate is 0.4 μm or more; <1> The heat-foamable sheet according to claim 1. <3> The sheet-like substrate includes a film substrate and a nonwoven fabric layer provided on one or both sides of the film substrate via an adhesive layer. <1> or <2> The heat-foamable sheet according to claim 1. <4> The static friction coefficient on the outermost surface of the foamable adhesive layer is 0.5 or less; <1> ~ <3> 10. The heat-foamable sheet according to any one of the above items. <5> the curing agent includes an amide-based curing agent; The equivalent ratio of the curing agent to the epoxy resin is 0.8 to 1.2. <1> ~ <4> 10. The heat-foamable sheet according to any one of the above items. <6> the thermoplastic resin includes an elastomer; The content of the elastomer is 3 to 30% by mass based on the total mass of the resin composition constituting the foamable adhesive layer. <1> ~ <5> 10. The heat-foamable sheet according to any one of the above items. <7> The thermoplastic resin contains a high-softening point elastomer having a Tg in the range of 100 to 120°C. <1> ~ <6> 10. The heat-foamable sheet according to any one of the above items. <8> The blowing agent is organic or inorganic. <1> ~ <7> 10. The heat-foamable sheet according to any one of the above items. <9> the blowing agent includes thermally expandable microcapsules; The thermally expandable microcapsules are dispersed in an epoxy resin, a hardener, and a thermoplastic resin. <1> ~ <8> 10. The heat-foamable sheet according to any one of the above items. <10> The content of the thermally expandable microcapsules is 3 to 19% by mass relative to the total mass of the resin composition constituting the foamable adhesive layer. <9> The heat-foamable sheet according to claim 1. <11> The foaming initiation temperature of the foamable adhesive layer is equal to or lower than the curing activation temperature in the curing reaction of the foamable adhesive layer; <1> ~ <10> 10. The heat-foamable sheet according to any one of the above items. <12> During the curing process of the foamable adhesive layer, there is a minimum viscosity temperature at which the viscosity of the epoxy resin is at its lowest within the temperature range from the foaming initiation temperature of the foamable adhesive layer to the curing activation temperature in the curing behavior of the foamable adhesive layer. <1> ~ <11> 10. The heat-foamable sheet according to any one of the above items. <13> For insulating sheets, <1> ~ <12> 10. The heat-foamable sheet according to any one of the above items. <14> The adhesive is placed between a first adherend and a second adherend, and is used to bond the first adherend and the second adherend by filling the gap between the two adherends with foam. <1> ~ <13> 10. The heat-foamable sheet according to any one of the above items. <15> Between the first and second adherends <1> ~ <13> and placing the heat-foamable sheet according to any one of the above. A bonding method for bonding a first adherend and a second adherend by filling the gap between the two adherends with foam. <16> The first adherend has a structure for accommodating the second adherend. <15> The bonding method described in [Effects of the Invention]

[0013] The heat-foamable sheet of the present invention makes it possible to properly bond gaps between adherends. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a schematic cross-sectional view showing the configuration of a heat-foamable sheet of the present invention. [Figure 2] 1 is a graph showing the relationship between foaming initiation temperature, minimum viscosity temperature, and cure activation temperature for a given foamable adhesive layer. DETAILED DESCRIPTION OF THE INVENTION

[0015] <Heat-foaming sheet> Fig. 1 is a schematic cross-sectional view showing the configuration of a heat-foamable sheet 1 of the present invention. The heat-foamable sheet 1 of the present invention comprises a sheet-like substrate 10 and a foamable adhesive layer 20 provided on one or both sides of the substrate. The foamable adhesive layer 20 is at least one outermost layer of the heat-foamable sheet and contains an epoxy resin, a curing agent, a thermoplastic resin, and a foaming agent, and the arithmetic mean roughness Ra (hereinafter also simply referred to as "surface roughness") of the outermost surface of the foamable adhesive layer 20 is 0.4 µm or more.

[0016] By virtue of the above-described configuration, the present invention makes it possible to properly bond gaps between adherends.

[0017] The heat-foamable sheet of the present invention has a foamable adhesive layer 20 that expands (foams) upon heating, allowing the thickness of the foamable adhesive layer to be thinner than the gap between the adherends in the initial design. Furthermore, the heat-foamable sheet of the present invention has a surface roughness Ra of 0.4 μm or greater on its outermost surface, allowing point contact between the surface and the object, thereby reducing frictional resistance. Therefore, when the heat-foamable sheet slides along the surface of the adherend during bonding, such as when the heat-foamable sheet is inserted into a gap between the adherends, the heat-foamable sheet is less likely to come into contact with the adherend. Even if contact does occur, the external force (frictional resistance) applied to the heat-foamable sheet is reduced, thereby preventing wrinkles and unevenness in the heat-foamable sheet (particularly the foamable adhesive layer), improving workability. Furthermore, when bonding the adherends, the heat-foamable sheet expands due to heating, filling the gap between the adherends. During this process, the adhesive layer's bulk volume increases as foaming begins, but once foaming peaks, the bulk volume begins to decrease, potentially resulting in insufficient adhesive strength. In the present invention, a curing agent is used to cure the resin at an appropriate timing after foaming when the bulk volume has increased, thereby ensuring high filling properties.

[0018] For example, when two pipes, one large and one small, are connected by fitting, a bonding method is possible in which the heat-foamable sheet of the present invention is attached to the inner surface of the larger pipe or the outer surface of the smaller pipe, the pipes are fitted together, and the pipes are heated to foam the foamable adhesive layer, thereby filling the gap between the pipes. In this specification, "fitting" broadly includes a form in which one adherend is housed within the other adherend and connected.

[0019] Furthermore, the heat-foamable sheet of the present invention contains a thermoplastic resin, which provides excellent flexibility, and this, combined with its foaming properties, enables it to ensure high filling properties even on surfaces that are in locations where pressure cannot be applied using a press or that have complex shapes. Furthermore, the use of an epoxy resin provides excellent heat resistance, and the adhesive strength is unlikely to decrease even at high temperatures.

[0020] Thus, the heat-foamable sheet of the present invention overcomes the conflicting issues of foaming and adhesive strength by achieving a good balance between workability, fillability, fast curing, flexibility, and heat resistance, thereby achieving both sufficient adhesive strength and fillability. This enables adhesion with high adhesive reliability between adherends and stable fixation and support, and makes it possible to provide an adhesive sheet that is also excellent in terms of thermal conductivity, vibration stress, and other phenomena.

[0021] For example, Patent Document 1 describes a laminated body composed of a thermoplastic resin film and a nonwoven fabric as an insulating material with excellent heat resistance and electrical properties, and describes a method for producing the laminated body by thermally laminating a surface-treated film and a nonwoven fabric (paragraph 0012). However, when using the laminated film or laminated body as a component, there are problems such as peeling at the film / film interface or film / nonwoven fabric interface, and the heat associated with the lamination process can impair the heat resistance of the laminated film or laminated body. Furthermore, the laminated film component only has an insulating function and does not have an adhesive function to other adherends.

[0022] Patent Document 2, which aims primarily at improving adhesiveness and workability, states that the use of crystalline resins can increase fluidity when heated (reducing melt viscosity), which is advantageous for thermal expansion (paragraph 0029). Furthermore, while the use of crystalline epoxy is necessary to maintain low tackiness, imparting non-stickiness (low tackiness) requires increasing the volume of crystalline epoxy within the resin, which has drawbacks in terms of flexibility. Furthermore, this tends to limit the degree of freedom in approaching heat resistance and reactivity.

[0023] Patent Document 3 describes an adhesive sheet in which a heat-expandable adhesive layer is formed on the surface of a substrate, and a release agent layer is further formed on the adhesive layer (Claim 1, etc.). This adhesive sheet is a heat-expandable adhesive sheet that is tack-free and easy to work with; when used, the adhesive sheet is placed in a predetermined position and heated, causing the release agent layer to break down and be absorbed into the adhesive, exposing the adhesive to the surface. However, there is a problem in that adhesive strength is easily reduced due to release agent dispersion. In addition, a release layer application step is required, which is disadvantageous in terms of cost.

[0024] Patent Document 4 also describes a tack-free, heat-expandable adhesive sheet with high workability (abstract). An adhesive-permeable layer is provided on the outer layer, and the glass transition temperature of this adhesive-permeable layer is set higher than the curing initiation temperature of the adhesive that constitutes the adhesive layer. The adhesive-permeable layer is preferably made of nonwoven fabric. However, rapid curing and heat resistance (suppression of reduction in adhesive strength at high temperatures) have not been investigated. Furthermore, because nonwoven fabric is used, it is difficult to adjust the amount of expanding adhesive that penetrates, which is disadvantageous in terms of adhesive stability.

[0025] The configuration of the heat-foamable sheet of the present invention will be described in detail below. <<Sheet-shaped substrate>> The sheet-like substrate 10 is a member that forms the skeleton of the heat-foamable sheet 1. The layer structure of the sheet-like substrate 10 may be a single-layer structure or a laminated structure. The overall thickness of the sheet-like substrate 10 is, for example, 1 to 300 μm, preferably 5 to 200 μm, and more preferably 25 to 120 μm.

[0026] The material of the sheet-like substrate is not particularly limited, and may be an inorganic material or an organic material. For example, a metal film can be used when the heat-foamable sheet is required to have electrical conductivity, and a resin film can be used when the heat-foamable sheet is required to have electrical insulation.

[0027] The metal film is not particularly limited, but examples thereof include copper foil and aluminum foil. The thickness of the metal film is, for example, 1 to 100 μm, preferably 10 to 70 μm, and more preferably 15 to 50 μm.

[0028] The resin film may include, but is not limited to, polyester-based resins such as polyethylene terephthalate (PET), polybutylene terephthalate, polyethylene naphthalate (PEN), and aromatic polyester; polycarbonate; polyarylate; polyurethane; polyamide-based resins such as polyamide and polyetheramide; polyimide-based resins such as polyimide (PI), polyetherimide, and polyamideimide; polysulfone-based resins such as polysulfone and polyethersulfone; polyetherketone-based resins such as polyetherketone and polyetheretherketone; polyphenylene sulfide (PPS); and modified polyphenylene oxide. The resin film may be composed of a single resin or a mixture of two or more of these resins. From the viewpoints of heat resistance and electrical insulation, the resin film is preferably a PEN film, a PET film, or a PI film, more preferably a PEN film or a PET film, and even more preferably a PEN film. The thickness of the resin film is, for example, 1 to 100 μm, preferably 10 to 70 μm, and more preferably 15 to 50 μm.

[0029] In the sheet-like substrate, the surface roughness Ra of the outer surface is preferably 0.4 μm or more. When the sheet-like substrate has the above surface roughness Ra, the foamable adhesive layer formed on the sheet-like substrate by coating the liquid adhesive composition has a surface roughness of the same order. The surface roughness Ra of the sheet-like substrate is more preferably 1.0 μm or more, even more preferably 1.5 μm or more, and particularly preferably 1.8 μm or more. Furthermore, the surface roughness Ra of the sheet-like substrate is preferably 50 μm or less. Generally, the thickness of the adhesive layer that can be coated at one time is about 50 μm, so it is not preferable for the roughness on the sheet-like substrate to be thicker than that thickness. Furthermore, the surface roughness Ra of the sheet-like substrate is more preferably 30 μm or less, even more preferably 20 μm or less, and particularly preferably 10 μm or less.

[0030] In particular, as shown in Fig. 1, it is preferable that the sheet-like substrate 10 includes a film substrate 11 and a nonwoven fabric layer 13 provided on one or both sides of the film substrate 11 via an adhesive layer 12. In Fig. 1, the adhesive layer 12 and the nonwoven fabric layer 13 are formed on both sides of the film substrate 11, but the adhesive layer 12 and the nonwoven fabric layer 13 may be formed on only one side of the film substrate 11. The use of a nonwoven fabric layer makes it easier to create a surface with a high surface roughness Ra.

[0031] The film substrate 11 is preferably made of the above-mentioned metal film or resin film. The preferred materials for each film are the same as those mentioned above.

[0032] The nonwoven fabric layer 13 is formed from a nonwoven fabric containing fibers such as cellulose fibers, polyester fibers, nylon fibers, aramid fibers, polyphenylene sulfide fibers, liquid crystal polymer fibers, glass fibers, metal fibers, and carbon fibers. The nonwoven fabric layer may also contain two or more types of fibers. Among these, the nonwoven fabric layer preferably contains an aramid fiber nonwoven fabric, a glass fiber nonwoven fabric, a polyphenylene sulfide fiber nonwoven fabric, a heat-resistant nylon fiber nonwoven fabric, or a heat-resistant polyester fiber nonwoven fabric. The thickness of the nonwoven fabric layer is, for example, 1 to 50 μm, preferably 5 to 40 μm, and more preferably 8 to 25 μm.

[0033] The material of the adhesive layer 12 for bonding the nonwoven fabric layer 13 to the film substrate 11 is not particularly limited, and may be a thermoplastic resin-based, thermosetting resin-based, or elastomer-based adhesive. In particular, the adhesive layer 12 is preferably made of a resin (epoxy or acrylic, preferably epoxy) that has excellent heat resistance and insulating properties.

[0034] <<Foamable adhesive layer>> As described above, the foamable adhesive layer 20 in the heat-foamable sheet 1 is at least one outermost layer of the heat-foamable sheet. It contains an epoxy resin, a curing agent, a thermoplastic resin, and a foaming agent. The surface roughness Ra of the outermost surface of the foamable adhesive layer 20 is 0.4 μm or greater. The foamable adhesive layer 20 may also contain other additives, such as a curing accelerator and a filler, as needed. The foamable adhesive layer 20 is the outermost layer of the heat-foamable sheet. While FIG. 1 shows the foamable adhesive layer 20 formed on both sides of the sheet-like substrate 10, the foamable adhesive layer 20 may be formed on only one side of the sheet-like substrate 10. No additional layer is formed on the outer surface of the foamable adhesive layer 20. Even when a nonwoven fabric layer 13 is formed, the foamable adhesive layer 20 may not be formed on the nonwoven fabric layer 13. For example, the nonwoven fabric layer 13 may be formed on both sides of the film substrate 11, but the foamable adhesive layer 20 may be formed on only one side. The thickness of the foamable adhesive layer is, for example, 10 to 100 μm, preferably 15 to 70 μm, and more preferably 20 to 50 μm.

[0035] The epoxy resin preferably includes bisphenol A epoxy resin, bisphenol F epoxy resin, bisphenol S epoxy resin, hinderedoin epoxy resin, biphenyl epoxy resin, alicyclic epoxy resin, triphenylmethane epoxy resin, phenol novolac epoxy resin, cresol novolac epoxy resin, naphthol novolac epoxy resin, dicyclopentadiene / phenol epoxy resin, alicyclic amine epoxy resin, aliphatic amine epoxy resin, and epoxy resins modified with these. The epoxy resin may consist of a single compound of these or a mixture of two or more types. From the viewpoints of heat resistance, electrical insulation, etc., the epoxy resin is preferably a novolac epoxy resin.

[0036] The content of the epoxy resin is preferably 30 to 65% by mass relative to the total mass of the resin composition constituting the foamable adhesive layer. A content of 30% by mass or more results in superior heat resistance and insulating properties. Furthermore, a content of 65% by mass or less results in superior flexibility. The content is more preferably 40 to 62% by mass, and even more preferably 50 to 60% by mass. The "resin composition constituting the foamable adhesive layer" refers to a composition consisting of all materials contained in the foamable adhesive layer, and includes components obtained by excluding volatile components from a liquid adhesive composition.

[0037] The curing agent preferably includes at least one of the following: an amide-based curing agent such as dicyandiamide or aliphatic polyamide; an amine-based curing agent such as diaminodiphenylmethane, metaphenylenediamine, ammonia, triethylamine, or diethylamine; a phenol-based curing agent such as bisphenol A, bisphenol F, phenol novolac resin, cresol novolac resin, or p-xylene novolac resin; or an acid anhydride-based curing agent. The curing agent may consist of a single compound or a mixture of two or more of these. Furthermore, the use of a latent curing agent is preferred because it can extend storage stability (pot life). A latent curing agent is generally a curing agent whose curing is accelerated by external stimuli such as heat or light. In the present invention, the use of a latent curing agent allows the foamed epoxy resin to be rapidly cured at the desired timing. Among the latent curing agents, from the viewpoint of curing reaction speed, it is more preferred that the curing agent include at least one of an amide-based curing agent and an amine-based curing agent, and even more preferred is dicyandiamide.

[0038] In order to cause an appropriate curing reaction in the curing agent and ensure sufficient adhesive strength, the equivalent ratio of the curing agent to the epoxy resin equivalent (curing agent equivalent / epoxy resin equivalent) is preferably 0.8 to 1.2, more preferably 0.85 to 1.15, and even more preferably 0.9 to 1.1. In particular, it is preferable that the curing agent contains an amide-based curing agent and the equivalent ratio is 0.8 to 1.2.

[0039] In this specification, the term "epoxy equivalent" refers to the molecular weight of an epoxy compound divided by the number of epoxy groups in one molecule. The epoxy equivalent can be determined by potentiometric measurement using a 0.1 mol / L perchloric acid / acetic acid standard solution in accordance with JIS K7236. The curing agent equivalent is the molecular weight of the curing agent divided by the number of active hydrogens (sites that react with epoxy groups) in one molecule. The curing agent equivalent can be determined by the acetyl chloride-potassium hydroxide titration method. Furthermore, component analysis can be performed using nuclear magnetic resonance (NMR), gas chromatography (GC), and gel permeation chromatography (GPC), and the curing agent equivalent can be calculated from the analytical results.

[0040] The thermoplastic resin preferably contains at least one of polyester resin, butyral resin, urethane resin, acrylic resin, carboxyl-terminated butadiene nitrile rubber (CTBN), and epoxy-modified butadiene. In particular, the thermoplastic resin is more preferably an elastomer-based resin, and more preferably contains at least one of an acrylic thermoplastic elastomer and a urethane thermoplastic elastomer. The thermoplastic resin may be composed of a single compound of these, or a mixture of two or more of these. The glass transition temperature Tg of the elastomer is preferably 100 to 120°C.

[0041] The content of the thermoplastic resin is preferably 3 to 30% by mass relative to the total mass of the resin composition constituting the foamable adhesive layer. When the content is 3% by mass or more, the foamable adhesive layer acquires flexibility and improves foamability. Furthermore, when the content is 30% by mass or less, the flexibility of the foamable adhesive layer is maintained within an appropriate range, and sufficient adhesive strength can be ensured. The content is more preferably 4 to 25% by mass, and even more preferably 5 to 20% by mass. In particular, it is preferable that the thermoplastic resin is an elastomer and the content is 3 to 30% by mass.

[0042] The foaming agent is not particularly limited, and both inorganic and organic foaming agents may be used. Specifically, the foaming agent preferably includes at least one of the following: inorganic foaming agents such as ammonium carbonate, ammonium bicarbonate, ammonium nitrite, ammonium borohydride, and azides; fluorinated alkanes such as trichloromonofluoromethane; azo compounds such as azobisisobutyronitrile; hydrazine compounds such as paratoluenesulfonylhydrazide; semicarbazide compounds such as p-toluenesulfonylsemicarbazide; triazole compounds such as 5-morpholyl-1,2,3,4-thiatriazole; N-nitroso compounds such as N,N'-dinitrosoterephthalamide; and microencapsulated foaming agents obtained by microencapsulating hydrocarbon solvents. The foaming agent may consist of a single compound or a mixture of two or more of these. Among these, thermally expandable microcapsules are preferred because they do not inhibit the curing of the foamable adhesive layer and minimize adverse effects on the physical properties of the epoxy resin.

[0043] Thermally expandable microcapsules are microcapsules with a shell made of a thermoplastic resin with gas barrier properties and a thermal expansion agent encapsulated inside the shell. When thermally expandable microcapsules are heated, the thermoplastic resin shell softens, and the volume of the thermal expansion agent increases, causing the capsule to expand. For example, the vaporization of a low-boiling-point hydrocarbon compound can be used to expand the capsule.

[0044] The foaming initiation temperature of the foamable adhesive layer containing thermally expandable microcapsules is preferably equal to or higher than the softening point of the epoxy resin. The foaming initiation temperature of the foamable adhesive layer can be determined, for example, from the relationship between the temperature measured by a thermomechanical analyzer (TMA) and the amount of expansion of the sample, as the temperature at which the foamable adhesive layer begins to expand due to the expansion of the thermally expandable microcapsules. If this foaming initiation temperature is equal to or higher than the softening point of the epoxy resin, the thermal expansion agent can be sufficiently expanded in the softened epoxy resin, thereby achieving a uniform thickness for the foamable adhesive layer after foaming. Furthermore, the foaming initiation temperature of the foamable adhesive layer is preferably equal to or lower than the curing activation temperature in the curing behavior of the foamable adhesive layer. The curing activation temperature in the curing behavior of the foamable adhesive layer can be determined, for example, as the temperature at which the exothermic peak occurs, from the relationship between the temperature measured by a differential scanning calorimetry (DSC) and the exothermic energy of the curing reaction. If this foaming initiation temperature is equal to or lower than the curing activation temperature, curing of the epoxy resin before foaming can be prevented. Furthermore, by setting the softening point of the epoxy resin to be equal to or lower than the curing activation temperature, when the production process for the heat-foamable sheet includes a melting or solution coating step, gelation of the epoxy resin can be prevented during the coating step and the accompanying drying step.

[0045] The softening point of the epoxy resin can be measured using the ring and ball softening point test method specified in JIS K 2207. The foaming initiation temperature of the foamable adhesive layer is, for example, in the range of 70 to 200°C, preferably 100 to 180°C.

[0046] Furthermore, during the curing process of the foamable adhesive layer, it is preferable that there is a minimum viscosity temperature at which the epoxy resin has the lowest viscosity within the temperature range from the foaming initiation temperature of the foamable adhesive layer to the curing activation temperature of the foamable adhesive layer. The minimum viscosity temperature at which the epoxy resin has the lowest viscosity can be determined, for example, from the relationship between the temperature measured with a viscoelasticity measuring device and the melt viscosity (complex viscosity) of the resin sample as the temperature showing the off-peak of the complex viscosity curve. For example, Figure 2 is a graph showing the relationship between the foaming initiation temperature, minimum viscosity temperature, and curing activation temperature for a given foamable adhesive layer. In Figure 2, temperature a is the foaming initiation temperature (135°C) determined from the TMA curve of the sample (foamable adhesive layer), temperature b is the minimum viscosity temperature (192°C) determined from the complex viscosity curve of the sample (epoxy resin in the foamable adhesive layer), and temperature c is the curing activation temperature (210°C) determined from the DSC curve of a similarly prepared sample (foamable adhesive layer). Since the minimum viscosity temperature is within the temperature range from the foaming start temperature to the curing activation temperature, the viscosity of the epoxy resin is sufficiently reduced after the foaming of the foamable adhesive layer begins but before the curing of the foamable adhesive layer is activated, so that the epoxy resin does not inhibit the foaming of the foamable adhesive layer and a good expansion ratio is obtained.

[0047] The content of the foaming agent is preferably 3 to 19% by mass relative to the total mass of the resin composition constituting the foamable adhesive layer. When the content is 3% by mass or more, sufficient foaming can be ensured, improving the adhesive strength. Furthermore, when the content is 19% by mass or less, a decrease in adhesive strength due to excessive foaming can be suppressed. The content is more preferably 4 to 17% by mass, even more preferably 5 to 15% by mass, and particularly preferably 7 to 12% by mass. In particular, it is preferable that the foaming agent contains thermally expandable microcapsules, and the content is 3 to 19% by mass.

[0048] As described above, the foamable adhesive layer has a surface roughness Ra of 0.4 μm or more on its outermost surface. A surface roughness Ra of 0.4 μm or more ensures point contact with the adherend, reducing the external force (frictional resistance) the heat-foamable sheet experiences even when contact occurs. This reduces the occurrence of wrinkles and unevenness in the heat-foamable sheet (particularly the foamable adhesive layer), improving workability. The surface roughness Ra of the foamable adhesive layer is more preferably 1.0 μm or more, even more preferably 1.5 μm or more, and particularly preferably 1.8 μm or more. Furthermore, the surface roughness Ra of the foamable adhesive layer is preferably 50 μm or less. This allows the foamable adhesive layer to foam evenly, ensuring sufficient adhesion to the adherend. Furthermore, the surface roughness Ra of the foamable adhesive layer is more preferably 30 μm or less, even more preferably 20 μm or less, and particularly preferably 10 μm or less. When a liquid adhesive composition is applied to a sheet-like substrate, the surface roughness of the foamed adhesive layer may be approximately the same as the surface roughness of the sheet-like substrate.

[0049] The static friction coefficient of the outermost surface of the foamable adhesive layer is preferably 0.5 or less. This reduces frictional resistance against the adherend, thereby preventing wrinkles and unevenness in the heat-foamable sheet (particularly the foamable adhesive layer), improving workability. The static friction coefficient is preferably 0.24 to 0.5.

[0050] The means for forming the surface roughness of the foamable adhesive layer is not particularly limited. For example, by applying a liquid adhesive composition to a sheet-like substrate having a desired surface roughness and drying it, the surface roughness of the sheet-like substrate can be reflected on the surface of the foamable adhesive layer. Alternatively, a method may be used in which a foamable adhesive layer is formed on a sheet-like substrate, and then a surface roughness (unevenness) is formed on the surface of the foamable adhesive layer by physical or chemical treatment. Alternatively, a method may be used in which a foamable adhesive layer having a surface roughness (unevenness) formed in advance by physical or chemical treatment is attached to the sheet-like substrate. In the present invention, from the viewpoint of workability, a method in which a liquid adhesive composition is applied to a sheet-like substrate having a desired surface roughness is preferred. When forming a desired surface roughness on a sheet-like substrate, it is preferable to use the nonwoven fabric described above.

[0051] <<Other additives>> The foamable adhesive layer in the heat-foamable sheet may contain a curing accelerator. The curing accelerator preferably includes at least one of imidazoles such as 2-methylimidazole, 2-methyl-4-ethylimidazole, and 2-phenylimidazole; tertiary amines such as 1,8-diazabicyclo[5.4.0]undecene-7, triethylenediamine, and benzyldimethylamine; and organic phosphines such as tributylphosphine and triphenylphosphine. The curing accelerator may consist of a single compound or a mixture of two or more of these. The content of the curing accelerator is preferably 0.1 to 5% by mass, more preferably 0.5 to 4% by mass, and may be 1 to 3% by mass, based on the total mass of the resin composition constituting the foamable adhesive layer.

[0052] The foamable adhesive layer in the heat-foamable sheet may contain a filler. The filler preferably contains at least one inorganic filler such as aluminum oxide, magnesium oxide, calcium oxide, aluminum hydroxide, magnesium hydroxide, boron nitride, silicon nitride, silicon oxide, or talc (magnesium silicate). The filler may consist of a single compound of these or a mixture of two or more compounds. The content of the filler is preferably 3 to 20% by mass, more preferably 5 to 17% by mass, and may be 7 to 15% by mass, based on the total mass of the resin composition constituting the foamable adhesive layer.

[0053] The heat-foamable sheet of the present invention can be used for various purposes depending on its physical properties. For example, if the heat-foamable sheet has electrical conductivity, it can be used as a conductive sheet for electronic equipment or electronic devices, and if the heat-foamable sheet has electrical insulation, it can be used as an insulating sheet for electronic equipment or electronic devices.

[0054] <Adhesion method using heat-foaming sheets> The bonding method of the present invention bonds the first and second adherends by placing the above-mentioned heat-foamable sheet between the first and second adherends and filling the gap between the two adherends by foaming.

[0055] The method for disposing the heat-foamable sheet between the first and second adherends is not particularly limited, and examples include inserting the heat-foamable sheet into the gap between the first and second adherends, or attaching the heat-foamable sheet to the second adherend and fitting the attached portion of the sheet into the first adherend.

[0056] The bonding method of the present invention enables appropriate bonding across a gap between adherends. The present invention is particularly useful for bonding when the gap between the adherends is narrow or when the bonding between the adherends involves relative sliding movement, such as when a first adherend has a structure that accommodates a second adherend.

[0057] The heating conditions for foaming the heat-foamable sheet are adjusted as appropriate, for example, a maximum temperature in the range of 150 to 200°C and a rate of 35 to 70°C / min. [Example]

[0058] The present invention will be explained in more detail below with reference to examples. The materials, amounts used, ratios, treatment contents, treatment procedures, etc. shown in the examples can be appropriately changed without departing from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the specific examples shown below.

[0059] <Sample Preparation> The heat-foamable sheets and adhesive sheets for standalone evaluation prepared as follows were used to carry out the following evaluations.

[0060] <<Making heat-foamable sheets>> A sheet-like substrate with a laminated structure was prepared by laminating a nonwoven fabric (DuPont Teijin Advanced Paper, product name Nomex 464, 1.5 mil) to one side of a PEN (polyethylene naphthalate) film (Teijin Film Solutions, 25 μm) using an acrylic resin adhesive. Hereinafter, this sheet-like substrate will be referred to as "NPN sheet" or simply "NPN." Next, adhesive compositions prepared according to the formulations shown in the tables below were applied to the nonwoven fabric side of the NPN sheet using a Baker-type applicator to a dry thickness of 35 μm. This was dried at 110°C for 90 seconds to remove the solvent, forming a foamable adhesive layer. This process resulted in a heat-foamable sheet with a foamable adhesive layer on one side of the NPN sheet.

[0061] <<Creating an adhesive sheet for standalone evaluation>> The adhesive compositions prepared according to the formulations shown in the tables below were applied to the release-treated surface of a release film (38 μm) using a Baker-type applicator so that the dry thickness was 50 μm. This was then dried at 110°C for 90 seconds to remove the solvent, forming a foamable adhesive layer. This process yielded an adhesive sheet for standalone evaluation, with a foamable adhesive layer on one side of the release film. The release film used was HY-NS70, a product of Higashiyama Film Co., Ltd.

[0062] <Ingredients> The structure and type of each raw material used in the following examples are as follows: [Table 1] [Table 2]

[0063] The following products were used as raw materials: A1: "EPICLON N-690" epoxy equivalent weight 225g / eq (DIC Corporation) A2: "EPICLON N-890" epoxy equivalent 210g / eq (manufactured by DIC) B1: "Dicy7" amine equivalent 21g / eq (Mitsubishi Chemical Corporation) B2: "Phenolite LF-7911" hydroxide equivalent 118g / eq (DIC Corporation) C1: S-LEC KS-6Z (manufactured by Sekisui Chemical Co., Ltd.) C2: Vamac GLS (DuPont) C3: Byron UR-3500 (manufactured by Toyobo Co., Ltd.) E: EH-5046S (ADEKA) F:CT-76 (Asada Flour Milling Co., Ltd.) D1 to D3: Commercially available thermally expandable microcapsules.

[0064] <Evaluation method> The evaluation methods for each evaluation item shown in the table are as follows.

[0065] <<Surface roughness>> A 15×40 mm sheet was cut out from the heat-foamable sheet before foaming and curing, and the arithmetic mean surface roughness Ra was measured using a surface roughness measuring device (Surfcorder SD-40D, manufactured by Kosaka Laboratory Co., Ltd.).

[0066] <<Static friction coefficient (friction resistance)>> The static friction coefficient of the foamable adhesive layer surface of the heat-foamable sheet before foaming and curing was measured with reference to JIS K7125. A smooth stainless steel plate was used as the mating material against which the heat-foamable sheet was rubbed. The measurement speed was 100 mm / min, and the travel distance was 130 mm.

[0067] <<Expansion ratio>> The heat-foamable sheet was cut into 50 x 50 mm pieces and placed in a heating furnace set at 200°C for 10 minutes to foam and cure. The expansion ratio was calculated by [film thickness after curing - film thickness of sheet-like substrate] / [film thickness before placing in heating furnace - film thickness of sheet-like substrate]. The "floating" evaluation indicates that the foamable adhesive layer floated from the substrate after curing, resulting in a defective result.

[0068] <<Flexibility>> A 10 x 100 mm sheet was cut from the adhesive sheet for standalone evaluation before foaming and curing, and the release film was peeled off to leave the foamable adhesive layer as a standalone. This was folded 180 degrees, and a 2 kg weight was applied to the folded part for 5 seconds. After the load was released, the surface condition of the foamable adhesive layer was observed and its flexibility was evaluated according to the following criteria. ×: The sheet shape could not be maintained. △: Cracks occurred but the sheet shape was maintained. ◯: The sheet shape was maintained.

[0069] <<Tackiness (lamination)>> A 10 x 25 mm sheet was cut out from the adhesive sheet for individual evaluation before foaming and curing, and tackiness (lamination ability) was evaluated based on the number of times it could be transferred onto an SPCC plate (manufactured by Nippon Test Panel Co., Ltd., product name SPCC-SB, 1.6 mm thick) using a rubber roll at 100°C. ×: 1 time △: 2 to 3 times 〇: 4 or more times

[0070] <<Peel strength>> Peel strength was measured with reference to IPC-TM-650. The conductor width used in the measurement was 5 mm. The measurement sample was prepared by using a foamable adhesive layer removed from an adhesive sheet for standalone evaluation to create a laminate with a PI (polyimide) film (50 μm thick) / foamable adhesive layer / copper foil (GTS-MP 1 oz, adhered to the S-side). The laminate was then molded and cured using a press at 160°C, 4 MPa, and for 20 minutes.

[0071] <<Shear adhesive strength>> A 12.5 x 25 mm sheet was cut out from the heat-foamable sheet before foaming and curing, and this was used for measurement according to the following procedure. A sample was prepared by fixing the heat-foamable sheet to an SPCC plate using a 110°C rubber roll, placing 0.4 mm thick spacers on both sides of the heat-foamable sheet, placing another SPCC plate on top of it, and fixing the gap between the SPCC plates using a fixing jig (e.g., clamp). The sample was heated to 200°C at a rate of 50°C / min and then heated at 200°C for 7 minutes to expand (4 times the original size) and harden the sheet in the sample. At this time, the gap between the two SPCC plates was fixed at 0.4 mm. After that, the shear adhesive strength of the above samples was measured using a universal testing machine at a test speed of 5 mm / min.

[0072] <<Bubbling start temperature T1_TMA>> Using a thermomechanical analyzer (TMA7100, manufactured by Hitachi High-Tech Science Corporation), the foamable adhesive layer removed from the adhesive sheet was placed in an aluminum cell and measured at a heating rate of 50°C / min, a temperature range of 30°C to 300°C, and a load of -49mN.The temperature at which expansion behavior began in the obtained TMA curve was defined as the foaming initiation temperature T1 of the foamable adhesive layer.

[0073] <<Curing activation temperature T2_DSC>> Using a differential scanning calorimeter (DSC7000X, manufactured by Hitachi High-Tech Science Corporation), the foamable adhesive layer removed from the adhesive sheet was used as a sample and measured at a heating rate of 50°C / min over a temperature range of 0°C to 300°C.The top temperature of the exothermic peak in the obtained DSC curve was determined as the curing activation temperature T2.

[0074] <<Minimum viscosity temperature Tα_rheometer>> Samples were prepared as follows. Samples containing a foaming agent expand during measurement, making accurate measurements difficult. Therefore, adhesive sheets were prepared in the same manner as described in "Preparation of Adhesive Sheets for Single-Phase Evaluation," except for removing the foaming agent from the adhesive composition. The adhesive layer was then laminated using a rubber roll heated to 110°C to prepare samples approximately 600 μm thick. Measurements were performed using a viscoelasticity measuring device (TA Instruments, ARES-G2) at a heating rate of 50°C / min, a temperature range of 25°C to 220°C (held for 10 minutes after reaching 220°C), a shear deformation mode, and a frequency of 1 Hz. The temperature at which the complex viscosity η* was lowest was defined as the minimum viscosity temperature (Tα).

[0075] <Rating 1> In this section, the influence of the arithmetic mean roughness Ra of the adhesive sheet surface on the sheet properties was evaluated. In Comparative Example 1, a foamable adhesive layer was formed on one side of an NPN sheet, and then the foamable adhesive layer was subjected to the following mirror finish treatment. In Comparative Example 2, instead of the NPN sheet, a PET film was used as the substrate, and an adhesive composition was applied to one side of the PET film to form a foamable adhesive layer.

[0076] <<Mirror finish treatment>> A release film was placed on the foamable adhesive layer so that the release-treated surface of the release film was in contact with the foamable adhesive layer, and in this state, it was passed between two rubber rolls heated to 110°C at a speed of 0.5 m / min, and then the release film was peeled off to give the outermost surface of the foamable adhesive layer a mirror finish. The release film used was HY-NS70, a product of Higashiyama Film Co., Ltd.

[0077] The results of Evaluation 1 are shown in the table below. In the table, "A1" and "A2" represent parts by mass in the epoxy resin. "α" in the table represents the equivalent ratio of the curing agent equivalent to the epoxy resin equivalent. In the table, "curing accelerator," "elastomer," "foaming agent," and "filler" represent the content (mass%) relative to the total mass of the resin composition (epoxy resin, thermoplastic resin (elastomer), curing agent, curing accelerator, foaming agent, and filler) that constitutes the foamable adhesive layer. The same applies to the other tables.

[0078] The results of Evaluation 1 show that a high surface roughness Ra of the adhesive sheet reduces the static friction coefficient of the adhesive sheet. Therefore, by using an adhesive sheet of the present invention with a high surface roughness Ra, the adhesive sheet can be inserted into gaps between adherends with little resistance, preventing peeling and wrinkling of the foamed adhesive layer. The surface roughness Ra of the NPN sheet itself (without a mirror finish) used in this evaluation was 3.0, and the static friction coefficient was 0.24. [Table 3]

[0079] <Rating 2> In this section, the influence on the properties when a phenolic resin is used as a curing agent is evaluated. In Reference Example 3, a phenolic resin was used as the curing agent.

[0080] [Table 4]

[0081] <Rating 3> In this section, the effect of the elastomer content on the sheet properties was evaluated. In Comparative Example 5, foaming occurred during drying of the adhesive composition applied to the NPN sheet, and a heat-foamable sheet could not be produced. [Table 5]

[0082] <Rating 4> In this section, we evaluated the effect of the type of elastomer on the sheet properties. Here, the Tg of C1 is 100-110°C, that of C2 is -24°C, and that of C3 is 10°C (values ​​provided by the manufacturer). [Table 6]

[0083] <Rating 5> In this section, the effect of the foaming agent content on the sheet properties was evaluated. [Table 7]

[0084] <Rating 6> In this section, we evaluated the effect of the type of foaming agent (differences in foaming start temperature) on sheet properties. We found that when the difference between the foaming start temperature T1 and the minimum viscosity temperature Tα is small, the shear adhesive strength decreases. We also confirmed that the relationship between T1, T2, and Tα changes when the heating rate decreases. [Table 8] [Explanation of symbols]

[0085] 1 Heat-foaming sheet 10 Sheet-shaped substrate 11 Film substrate 12 Adhesive layer 13 Non-woven layer 20 Foaming adhesive layer

Claims

1. A heat-foamable sheet comprising a sheet-like substrate and a foamable adhesive layer provided on one or both sides of the sheet-like substrate, the foamable adhesive layer is at least one outermost layer of the heat-foamable sheet, the foamable adhesive layer comprises an epoxy resin, a curing agent, a thermoplastic resin, and a foaming agent; The heat-foamable sheet, wherein the foamable adhesive layer has an arithmetic mean roughness Ra of 0.4 μm or more on its outermost surface.

2. The heat-foamable sheet according to claim 1, wherein the surface of the sheet-like substrate has an arithmetic mean roughness Ra of 0.4 μm or more.

3. 3. The heat-foamable sheet according to claim 1, wherein the sheet-like substrate comprises a film substrate and a nonwoven fabric layer provided on one or both sides of the film substrate via an adhesive layer.

4. The heat-foamable sheet according to any one of claims 1 to 3, wherein the foamable adhesive layer has an outermost surface having a static friction coefficient of 0.5 or less.

5. the curing agent includes an amide-based curing agent, The heat-foamable sheet according to any one of claims 1 to 4, wherein an equivalent ratio of the curing agent to the epoxy resin is 0.8 to 1.

2.

6. the thermoplastic resin includes an elastomer, The heat-foamable sheet according to any one of claims 1 to 5, wherein the content of the elastomer is 3 to 30 mass% relative to the total mass of the resin composition constituting the foamable adhesive layer.

7. The heat-foamable sheet according to any one of claims 1 to 6, wherein the thermoplastic resin comprises a high-softening point elastomer having a Tg in the range of 100 to 120°C.

8. The heat-foamable sheet according to any one of claims 1 to 7, wherein the foaming agent is organic or inorganic.

9. the foaming agent includes thermally expandable microcapsules; The heat-foamable sheet according to any one of claims 1 to 8, wherein the thermally expandable microcapsules are dispersed in an epoxy resin, a curing agent, and a thermoplastic resin.

10. The heat-foamable sheet according to claim 9, wherein the content of the thermally expandable microcapsules is 3 to 19% by mass relative to the total mass of the resin composition constituting the foamable adhesive layer.

11. The heat-foamable sheet according to any one of claims 1 to 10, wherein a foaming initiation temperature of the foamable adhesive layer is equal to or lower than a curing activation temperature in a curing reaction of the foamable adhesive layer.

12. 12. The heat-foamable sheet according to claim 1, wherein, in a curing process of the foamable adhesive layer, there is a minimum viscosity temperature at which the viscosity of the epoxy resin is minimum within a temperature range from the foaming initiation temperature of the foamable adhesive layer to a curing activation temperature in the curing behavior of the foamable adhesive layer.

13. The heat-foamable sheet according to any one of claims 1 to 12, which is used as an insulating sheet.

14. The heat-foamable sheet according to any one of claims 1 to 13, which is disposed between a first adherend and a second adherend, and is used to bond the first adherend and the second adherend by foaming to fill a gap between the two adherends.

15. The heat-foamable sheet according to any one of claims 1 to 13 is placed between a first adherend and a second adherend, A bonding method for bonding a first adherend and a second adherend by filling a gap between the two adherends with foam.

16. The bonding method according to claim 15, wherein the first adherend has a structure for accommodating the second adherend.

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