Heat-expanded foam sheet and bonding method

Abstract

The objective is to provide a heat-foamed sheet capable of properly bonding gaps between adherends. Furthermore, the objective is to provide a bonding method utilizing this heat-foamed sheet. [Solution] The heat-foamed sheet comprises a sheet-like substrate and a foamable adhesive layer provided on one or both sides of the sheet-like substrate. Furthermore, the foamable adhesive layer is at least one outermost layer of the heat-foamed sheet and contains an epoxy resin, a curing agent, a thermoplastic resin, and a foaming agent, and the arithmetic mean roughness Ra of the outermost surface of the foamable adhesive layer is 0.4 μm or more.

Description

Technical Field

[0001] The present invention relates to a heat-expandable sheet. The present invention also relates to an adhesion method using this heat-expandable sheet.

Background Art

[0002] Conventionally, an adhesive can be cured between a plurality of adherends to adhere those adherends and is used in various locations.

[0003] Among adherends, there are those with distortion, those with a non-smooth surface and deep irregularities, and porous ones. By using a liquid adhesive, it is possible to fill the space between the adherends and adhere them. Depending on the adhesion location, the gap for filling the adhesive may be very narrow and it is difficult for the adhesive to enter, so it may be difficult to completely fill the gap with the adhesive. In such a case, for example, it is possible to fill the gap by using a larger amount of adhesive than the original required amount, but this requires the work of removing the unnecessarily used adhesive, increasing the work load, and there is also a risk of contaminating the adherend and the surrounding work area. In addition, there are also problems such as the liquid adhesive flowing before curing and flowing out or overflowing outside the adhesion part.

[0004] Thus, in the case of a liquid adhesive, due to the difficulty of accurately controlling the application amount and the application location, filling the gap is likely to be insufficient, and as a result, there are many problems in workability during use, such as a decrease in the adhesion area and a decrease in the adhesion strength.

[0005] To solve the aforementioned problems, it is known to use sheet-type adhesives that are solid at room temperature (Patent Documents 1-4). With sheet-type adhesives, it is possible to apply a specified amount of adhesive to a specified location on the adherend, and contamination of the adherend and the surrounding work area can be eliminated. Sheet-type adhesives have the problem of poor conformability to objects with uneven surfaces or distortions, but conformability can be improved by applying pressure with 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 excellent because their melt viscosity decreases significantly when heated and they have high heat resistance, and they are generally widely used. [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 Publication No. 2019-203062 [Overview of the project] [Problems that the invention aims to solve]

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

[0008] For example, if the sheet adhesive is thicker than the gap, it will be too tight to insert. Forcing the sheet adhesive in may cause peeling or wrinkling of the adhesive, resulting in unevenness of the bonded surface, reduced and inconsistent bond strength. On the other hand, if the sheet adhesive is made thinner than the gap to make it easier to insert, it may not be able to fill the gap, leading to problems such as no bond at all or extremely weak bond strength.

[0009] Furthermore, the problems described above are not limited to cases where multiple parts are connected by fitting, but can similarly occur when bonding gaps between adherends.

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

[0011] The present invention has been made in view of the above problems, and aims to provide a heat-foamed sheet that can properly bond gaps between adherends. The present invention also aims to provide a bonding method using the heat-foamed sheet described above. [Means for solving the problem]

[0012] The above problem was solved by providing the sheet-like adhesive with an adhesive layer that expands (foams) when heated. Specifically, the following methods were used. <1> Therefore, preferably <2> The above problems were resolved by the following measures. <1> A heat-foamed sheet comprising a sheet-like substrate and a foaming adhesive layer provided on one or both sides of the sheet-like substrate, The foamed adhesive layer is the outermost layer of at least one of the heat-foamed sheets. The foamed adhesive layer comprises epoxy resin, curing agent, thermoplastic resin, and foaming agent. A heat-foamed sheet having an arithmetic mean roughness Ra of 0.4 μm or more at the outermost surface of the foamed adhesive layer. <2> The arithmetic mean roughness Ra on the surface of the sheet-like substrate is 0.4 μm or greater. <1> The heat-expanding sheet described above. <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-expanding sheet described above. <4> The static friction coefficient at the outermost surface of the foamed adhesive layer is 0.5 or less. <1> ~ <3> A heat-expanding sheet as described in one of the following. <5> The hardening agent contains an amide-based hardening agent. The equivalent ratio of the curing agent to the equivalent amount of epoxy resin is 0.8 to 1.2. <1> ~ <4> A heat-expanding sheet as described in one of the following. <6> Thermoplastic resin contains elastomer, The elastomer content is 3 to 30% by mass relative to the total mass of the resin composition constituting the foamed adhesive layer. <1> ~ <5> A heat-expanding sheet as described in one of the following. <7> The thermoplastic resin includes a high softening point elastomer with a Tg in the range of 100-120°C. <1> ~ <6> A heat-expanding sheet as described in one of the following. <8> The foaming agent is either organic or inorganic. <1> ~ <7> A heat-expanding sheet as described in one of the following. <9> The foaming agent contains thermally expandable microcapsules. Thermally expandable microcapsules are dispersed in epoxy resin, curing agent, and thermoplastic resin. <1> ~ <8> A heat-expanding sheet as described in one of the following. <10> The content of thermally expandable microcapsules is 3 to 19% by mass relative to the total mass of the resin composition constituting the foamed adhesive layer. <9> The heat-expanding sheet described above. <11> The foaming initiation temperature of the foamed adhesive layer is below the curing activity temperature in the curing reaction of the foamed adhesive layer. <1> ~ <10> A heat-expanding sheet as described in one of the following. <12> In the curing process of the foaming adhesive layer, there is a minimum viscosity temperature at which the viscosity of the epoxy resin is the lowest in the temperature range from the foaming start temperature of the foaming adhesive layer to the curing active temperature in the curing behavior of the foaming adhesive layer. The heat-expandable sheet according to any one of <1> to <11>. <13> The heat-expandable sheet according to any one of <1> to <12>, which is for an insulating sheet. <14> The heat-expandable sheet according to any one of <1> to <13>, which is disposed between the first adherend and the second adherend and is used to adhere the first adherend and the second adherend by filling the gap between both adherends by foaming. <15> Dispose the heat-expandable sheet according to any one of <1> to <13> between the first adherend and the second adherend, An adhesion method for adhering the first adherend and the second adherend by filling the gap between both adherends by foaming. <16> The adhesion method according to <15>, wherein the first adherend has a structure for accommodating the second adherend.

Advantages of the Invention

[0013] With the heat-expandable sheet of the present invention, it becomes possible to appropriately adhere the gap between adherends.

Brief Description of the Drawings

[0014] [Figure 1] It is a schematic cross-sectional view showing the configuration of the heat-expandable sheet of the present invention. [Figure 2] It is a graph showing the relationship between the foaming start temperature, the minimum viscosity temperature, and the curing active temperature for a predetermined foaming adhesive layer.

Embodiments for Carrying Out the Invention

[0015] <Heat-expandable sheet> Figure 1 is a schematic cross-sectional view showing the structure of the heat-foamed sheet 1 of the present invention. The heat-foamed sheet 1 of the present invention includes a sheet-like substrate 10 and a foamable adhesive layer 20 provided on one or both sides of the substrate. Furthermore, the foamable adhesive layer 20 is at least one outermost layer of the heat-foamed 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] The present invention, having the above configuration, makes it possible to properly bond gaps between adherends.

[0017] The heat-foaming sheet of the present invention has a foamable adhesive layer 20 that expands (foams) when heated, making it possible to make the thickness of the foamable adhesive layer thinner than the gap between the adherends in the initial design. Furthermore, the heat-foaming sheet of the present invention has a surface roughness Ra of 0.4 μm or more on its outermost surface, so that contact between the surface and the object becomes point contact, reducing frictional resistance. Therefore, when the heat-foaming sheet is inserted into a gap between adherends or when the heat-foaming sheet slides along the surface of the adherends during bonding work, the opportunities for the heat-foaming sheet to come into contact with the adherends are reduced, and even if contact occurs, the external force (frictional resistance) experienced by the heat-foaming sheet is reduced, thus suppressing the occurrence of wrinkles and variations in the heat-foaming sheet (especially the foamable adhesive layer), and improving workability. When the adherends are bonded, the gap between them is filled by expansion due to heating. At this time, the bulk volume of the adhesive layer increases as foaming begins, but once foaming exceeds its peak, the bulk volume begins to decrease, and sufficient adhesive strength may not be obtained. In this invention, high filling performance is ensured by using a curing agent to cure the resin at an appropriate time after foaming when the bulk volume has increased.

[0018] For example, when connecting two pipes of different sizes by fitting, the heat-foaming sheet of the present invention can be attached to the inner surface of the larger pipe or the outer surface of the smaller pipe, the pipes can be fitted together, and then heated to foam the adhesive layer and fill the gap between the pipes. In this specification, "fitting" broadly includes a form in which one adherend is housed in the other adherend and connected.

[0019] Furthermore, the heat-foamed sheet of the present invention, by containing a thermoplastic resin, is highly flexible, and combined with its foaming properties, it can ensure high filling performance even on surfaces in locations where pressure cannot be applied by pressing or other means, or on surfaces with complex shapes. In addition, because epoxy resin is used, it has excellent heat resistance, and its adhesive strength does not easily decrease even at high temperatures.

[0020] Thus, the heat-foamed sheet of the present invention overcomes the conflicting challenges of foaming and adhesive strength by balancing workability, fillability, rapid curing, flexibility, and heat resistance, achieving both sufficient adhesive strength and fillability. This enables highly reliable adhesion between adherends and stable fixed support, and provides an adhesive sheet that is also excellent in various phenomena such as thermal conductivity and vibration stress.

[0021] For example, Patent Document 1 describes a laminate comprising a thermoplastic resin film and a nonwoven fabric as an insulating material with excellent heat resistance and electrical properties, and a method for manufacturing the above laminate by heat laminating the surface-treated film / nonwoven fabric (paragraph 0012). However, when using laminated films or laminates as components, there are problems such as delamination occurring at the film / film interface or film / nonwoven fabric interface, or the heat resistance of the laminated film or laminate being impaired by the heat generated during processing. Furthermore, the components of a laminated film only have an insulating function and do not have an adhesive function to other substrates.

[0022] Patent Document 2 states that the main objective is to improve adhesion and workability, and that by using a crystalline resin, fluidity during heating can be increased (melt viscosity can be reduced), which is advantageous for thermal expansion (paragraph 0029). Furthermore, while it is necessary to use crystalline epoxy to maintain low tackiness, if non-tackiness (low tackiness) is to be imparted, the volume of crystalline epoxy in the resin must be increased, which has a drawback in terms of flexibility. In addition, the degree of freedom in approaching heat resistance and reactivity tends to be low.

[0023] Patent Document 3 describes an adhesive sheet in which a thermally expandable adhesive layer is formed on the surface of a substrate, and a release agent layer is further formed on top of the adhesive layer (Claim 1, etc.). This adhesive sheet is tack-free and a highly workable thermally expandable adhesive sheet. When used, the adhesive sheet is placed in a predetermined position and heated, which breaks down the release agent layer and incorporates it into the adhesive, causing the adhesive to appear on the surface. However, there is a problem that the adhesive strength tends to decrease due to the dispersion of the release agent. In addition, it is necessary to go through a release layer coating process, which is disadvantageous in terms of cost.

[0024] Patent Document 4 also describes a tack-free, highly workable, heat-expandable adhesive sheet (abbreviated). The outer layer is provided with an adhesive permeable layer, the glass transition temperature of this adhesive permeable layer is set higher than the curing start temperature of the adhesive constituting the adhesive layer, and the adhesive permeable layer is preferably a nonwoven fabric. However, rapid curing properties and heat resistance (suppression of decrease in adhesive strength at high temperatures) have not been considered. Furthermore, because a nonwoven fabric is used, it is difficult to adjust the amount of expanding adhesive that permeates, which is disadvantageous in terms of adhesive stability.

[0025] The structure of the heat-foamed sheet of the present invention will be described in detail below. <<Sheet-like substrate>> The sheet-like substrate 10 is a structural component of the heat-foamed sheet 1. The layer structure of the sheet-like substrate 10 may be a single layer 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 either an inorganic or organic material. For example, a metal film can be used when electrical conductivity is required for the heat-foamed sheet, and a resin film can be used when electrical insulation is required for the heat-foamed sheet.

[0027] The metal film is not particularly limited, but for example, copper foil and aluminum foil can be used. 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 is not particularly limited, but may include polyester resins such as polyethylene terephthalate (PET), polybutylene terephthalate, polyethylene naphthalate (PEN), and aromatic polyester; polycarbonate; polyarylate; polyurethane; polyamide resins such as polyamide and polyetheramide; polyimide resins such as polyimide (PI), polyetherimide, and polyamideimide; polysulfone resins such as polysulfone and polyethersulfone; polyetherketone resins such as polyetherketone and polyetheretherketone; polyphenylene sulfide (PPS); and modified polyphenylene oxide. The resin film may consist of one of these resins or a mixture of two or more resins. From the viewpoint 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 and 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 a sheet-like substrate, the surface roughness Ra of the outer surface is preferably 0.4 μm or more. Having the above surface roughness Ra of the sheet-like substrate ensures that the foamed adhesive layer formed on the sheet-like substrate by coating it with a liquid adhesive composition has a similar surface roughness. 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 undesirable for the roughness on the sheet-like substrate to exceed that thickness. Moreover, 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 Figure 1, the sheet-like substrate 10 preferably 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 Figure 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. Using a nonwoven fabric layer makes it easier to produce a surface with a high surface roughness Ra.

[0031] The film substrate 11 is preferably composed of the metal film or resin film described above. The preferred materials for each film are the same as those described 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 aramid fiber nonwoven fabric, glass fiber nonwoven fabric, polyphenylene sulfide fiber nonwoven fabric, heat-resistant nylon fiber nonwoven fabric, and 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, acrylic, preferably epoxy) that has excellent heat resistance and insulation properties.

[0034] <<Foam adhesive layer>> As described above, the foamed adhesive layer 20 in the heat-foamed sheet 1 is the outermost layer of at least one of the heat-foamed sheets and contains epoxy resin, curing agent, thermoplastic resin, and foaming agent, and the surface roughness Ra of the outermost surface of the foamed adhesive layer 20 is 0.4 μm or more. The foamed adhesive layer 20 may also contain other additives such as curing accelerators and fillers as needed. The foamed adhesive layer 20 is the outermost layer of the heat-foamed sheet. In Figure 1, the foamed adhesive layer 20 is formed on both sides of the sheet-like substrate 10, but the foamed adhesive layer 20 may be formed on only one side of the sheet-like substrate 10. No further layers are formed on the outer surface of the foamed adhesive layer 20. Also, even if a nonwoven fabric layer 13 is formed, the foamed 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 foamed adhesive layer 20 may be formed on only one side. The thickness of the foamed 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 type epoxy resin, bisphenol F type epoxy resin, bisphenol S type epoxy resin, hindertoin type epoxy resin, biphenyl type epoxy resin, alicyclic epoxy resin, triphenylmethane type epoxy resin, phenol novolac type epoxy resin, cresol novolac type epoxy resin, naphthol novolac type epoxy resin, dicyclopentadiene / phenol epoxy resin, alicyclic amine epoxy resin, aliphatic amine epoxy resin, and epoxy resins modified in various ways. The epoxy resin may consist of a single compound of these or a mixture of two or more. From the viewpoint of heat resistance and electrical insulation, the epoxy resin is preferably a novolac type epoxy resin.

[0036] The epoxy resin content is preferably 30 to 65% by mass of the total mass of the resin composition constituting the foamed adhesive layer. A content of 30% by mass or more provides advantages in heat resistance and insulation. A content of 65% by mass or less provides advantages in flexibility. A content of 40 to 62% by mass is more preferable, and 50 to 60% by mass is even more preferable. "Resin composition constituting the foamed adhesive layer" refers to the composition consisting of all the materials contained in the foamed adhesive layer, and includes components obtained by removing volatile matter from the liquid adhesive composition.

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

[0038] In the curing agent, in order to cause a proper curing reaction and ensure sufficient adhesive strength, the equivalent ratio of the curing agent to the equivalent amount of epoxy resin (equivalent amount of curing agent / equivalent amount of epoxy resin) 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 that the above equivalent ratio is 0.8 to 1.2.

[0039] In this specification, epoxy equivalent is the value obtained by dividing the molecular weight of the epoxy compound by the number of epoxy groups in one molecule. 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. Curing agent equivalent is the value obtained by dividing the molecular weight of the curing agent by the number of active hydrogens (sites that react with epoxy groups) in one molecule. Curing agent equivalent can be determined by the acetyl chloride-potassium hydroxide titration method. Furthermore, it can be calculated from the results of component analysis performed using nuclear magnetic resonance (NMR), gas chromatography (GC), and gel permeation chromatography (GPC).

[0040] The thermoplastic resin preferably comprises 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 resin, and even more preferably comprises at least one of acrylic thermoplastic elastomer and urethane thermoplastic elastomer. The thermoplastic resin may consist of a single compound of these, or a mixture of two or more. The glass transition temperature Tg of the elastomer is preferably 100 to 120°C.

[0041] The thermoplastic resin content is preferably 3 to 30% by mass relative to the total mass of the resin composition constituting the foamed adhesive layer. A content of 3% by mass or more allows the foamed adhesive layer to acquire flexibility and improve foaming properties. A content of 30% by mass or less maintains the flexibility of the foamed adhesive layer within an appropriate range, ensuring sufficient adhesive strength. A content of 4 to 25% by mass is more preferable, and 5 to 20% by mass is even more preferable. In particular, it is preferable that the thermoplastic resin is an elastomer and that its content is 3 to 30% by mass.

[0042] The blowing agent is not particularly limited, and either inorganic or organic blowing agents may be used. Specifically, the blowing agent preferably includes at least one of the following: inorganic blowing 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 p-toluenesulfonyl hydrazide; semicarbazide compounds such as p-toluenesulfonyl semicarbazide; triazole compounds such as 5-morpholyl-1,2,3,4-thiatriazole; N-nitroso compounds such as N,N'-dinitrosoterephthalamide; and microencapsulated blowing agents in which hydrocarbon solvents are microencapsulated. The blowing agent may consist of one of these compounds or a mixture of two or more. Among these, the blowing agent is preferably a thermally expandable microcapsule because it does not inhibit the curing of the foamed adhesive layer and minimizes adverse effects on the physical properties of the epoxy resin.

[0043] A thermally expandable microcapsule is a microcapsule that has a thermoplastic resin with gas barrier properties as its shell, and a thermal expander encapsulated inside the shell. When a thermally expandable microcapsule is heated, the thermoplastic resin of the shell softens, and the volume of the thermal expander increases, causing the capsule to expand. For example, the vaporization of low-boiling-point hydrocarbon compounds can be used to expand the capsule.

[0044] The foaming initiation temperature of the foamed adhesive layer containing thermally expandable microcapsules is preferably above the softening point of the epoxy resin. The foaming initiation temperature of the foamed adhesive layer can be determined, for example, by examining 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 expansion of the foamed adhesive layer begins in conjunction with the foaming of the thermally expandable microcapsules. If this foaming initiation temperature is above the softening point of the epoxy resin, the thermal expander can expand sufficiently within the softened epoxy resin, and the thickness of the foamed adhesive layer after foaming can be made uniform. Furthermore, the foaming initiation temperature of the foamed adhesive layer is preferably below the curing activation temperature in the curing behavior of the foamed adhesive layer. The curing activation temperature in the curing behavior of the foamed adhesive layer can be determined, for example, by examining the relationship between the temperature measured by a differential scanning calorimetry (DSC) device and the exothermic energy of the curing reaction, as the temperature at which the exothermic peak is observed. If this foaming initiation temperature is below the curing activation temperature, it is possible to prevent the epoxy resin from curing before foaming. Furthermore, by setting the softening point of the epoxy resin below the curing activity temperature, it is possible to prevent the epoxy resin from gelling during the coating process and the subsequent drying process when the manufacturing process of the heat-foamed sheet includes a melting or solution coating process.

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

[0046] Furthermore, in the curing process of the foamed adhesive layer, it is preferable that there is a minimum viscosity temperature within the temperature range from the foaming initiation temperature of the foamed adhesive layer to the curing activity temperature in the curing behavior of the foamed adhesive layer, where the viscosity of the epoxy resin is at its lowest. The minimum viscosity temperature where the viscosity of the epoxy resin is at its lowest can be determined, for example, by looking at the relationship between the temperature measured by a viscoelasticity measuring device and the melt viscosity (complex viscosity) of the resin sample, and finding the temperature at which the off-peak occurs on the complex viscosity curve. For example, Figure 2 is a graph showing the relationship between the foaming initiation temperature, the minimum viscosity temperature, and the curing activity temperature for a given foamed adhesive layer. In Figure 2, temperature a is the foaming initiation temperature (135°C) obtained from the TMA curve of the sample (foamed adhesive layer), temperature b is the minimum viscosity temperature (192°C) obtained from the complex viscosity curve of the sample (epoxy resin in the foamed adhesive layer), and temperature c is the curing activity temperature (210°C) obtained from the DSC curve of a similarly prepared sample (foamed adhesive layer). Because the minimum viscosity temperature falls within the temperature range from the foaming initiation temperature to the curing activation temperature, the viscosity of the epoxy resin is sufficiently reduced after the foaming of the foamed adhesive layer begins but before the curing of the foamed adhesive layer is activated. As a result, the epoxy resin does not inhibit the foaming of the foamed adhesive layer, and a good foaming ratio can be obtained.

[0047] The foaming agent content is preferably 3 to 19% by mass relative to the total mass of the resin composition constituting the foamed adhesive layer. A content of 3% by mass or more ensures sufficient foaming and improves adhesive strength. A content of 19% by mass or less suppresses the reduction in adhesive strength caused by excessive foaming. 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 that its content is 3 to 19% by mass.

[0048] In the foamed adhesive layer, as described above, the surface roughness Ra of the outermost surface is 0.4 μm or more. A surface roughness Ra of 0.4 μm or more results in point contact with the adherend, reducing the external force (frictional resistance) on the heat-foamed sheet even when contact occurs. This suppresses wrinkles and variations in the heat-foamed sheet (especially the foamed adhesive layer), improving workability. The surface roughness Ra of the foamed 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 foamed adhesive layer is preferably 50 μm or less. This ensures consistent foaming of the foamed adhesive layer and sufficient adhesion to the adherend. Moreover, the surface roughness Ra of the foamed adhesive layer is more preferably 30 μm or less, even more preferably 20 μm or less, and particularly preferably 10 μm or less. Furthermore, 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 at the outermost surface of the foamed adhesive layer is preferably 0.5 or less. This reduces frictional resistance to the adherend, thereby suppressing wrinkles and variations in the heat-foamed sheet (especially the foamed adhesive layer), and improving workability. The static friction coefficient is preferably 0.24 to 0.5.

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

[0051] <<Other additives>> The foamed adhesive layer in the heat-foamed sheet may contain a curing accelerator. The curing accelerator preferably contains at least one of the following: 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 of these, or a mixture of two or more. The content of the curing accelerator is preferably 0.1 to 5% by mass, more preferably 0.5 to 4% by mass, and may also be 1 to 3% by mass, based on the total mass of the resin composition constituting the foamed adhesive layer.

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

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

[0054] <Bonding method using heat-expanded foam sheets> The bonding method of the present invention involves placing the above-mentioned heat-foamed sheet between a first adherend and a second adherend, and bonding the first adherend and the second adherend by filling the gap between the two adherends with foam.

[0055] The method for arranging the heat-expandable sheet between the first and second adherends is not particularly limited. For example, one method is to insert the heat-expandable sheet into the gap between the first and second adherends, or to attach the heat-expandable sheet to the second adherend and then fit the portion to which the sheet is attached to the first adherend.

[0056] The bonding method of the present invention makes it possible to properly bond gaps between adherends. In particular, the present invention is useful for bonding when the gap between adherends is narrow, or when bonding adherends involve relative sliding movement, such as when the first adherend has a structure that accommodates the second adherend.

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

[0058] The present invention will be described in more detail below with reference to examples. The materials, amounts used, proportions, processing content, and processing procedures shown in the examples can be modified as appropriate, as long as they do not depart 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 following evaluations were performed using the heat-expanded sheets and adhesive sheets prepared as described below for individual evaluation.

[0060] <<Preparation of heat-expanded foam sheets>> A sheet-like substrate with a laminated structure was prepared by laminating a nonwoven fabric (DuPont Teijin Advanced Papers, product name Nomex464, 1.5 mil) onto one side of a PEN (polyethylene naphthalate) film (manufactured by Teijin Film Solutions, Ltd., 25 μm) using an acrylic resin adhesive. Hereinafter, this sheet-like substrate will be referred to as the "NPN sheet" or simply "NPN". Next, adhesive compositions prepared with the formulations shown in the tables below were applied to the nonwoven fabric surface of the NPN sheet using a baker-type applicator so that the thickness after drying was 35 μm. This was dried at 110°C for 90 seconds to remove the solvent, thereby forming a foamed adhesive layer. Through the above process, a heat-foamed sheet having a foamed adhesive layer on one side of the NPN sheet was obtained.

[0061] <<Preparation of adhesive sheets for individual unit evaluation>> 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 to a dry thickness of 50 μm. This was then dried at 110°C for 90 seconds to remove the solvent, forming a foamed adhesive layer. Through these steps, an adhesive sheet for individual evaluation was obtained, having a foamed adhesive layer on one side of the release film. The release film used was HY-NS70, manufactured by Higashiyama Film Co., Ltd.

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

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

[0064] <Evaluation Method> The evaluation method for each evaluation item shown in the table is as follows:

[0065] <<Surface Roughness>> A 15 x 40 mm sheet was cut from the heat-foamed sheet before foaming and curing, and the arithmetic mean surface roughness Ra was measured using a surface roughness measuring instrument (Kosaka Laboratory Co., Ltd., SurfCorder SD-40D).

[0066] <<Static friction coefficient (frictional resistance)>> The static friction coefficient on the surface of the foamed adhesive layer of a heat-foamed sheet before foaming and curing was measured, referencing JIS K7125. A smooth SUS plate was used as the mating material for rubbing against the heat-foamed sheet. The measurement speed was 100 mm / min, and the travel distance was 130 mm.

[0067] <<Foaming ratio>> The heat-expandable sheet was cut into 50 x 50 mm pieces and placed in a heating furnace set to 200°C for 10 minutes to expand and harden. The expansion ratio was calculated as [film thickness after hardening - film thickness of the sheet substrate] / [film thickness before heating furnace - film thickness of the sheet substrate]. In the evaluation items, "lifting" indicates that the foamed adhesive layer lifted away from the substrate after hardening, resulting in a defective product.

[0068] <<Flexibility>> A 10 x 100 mm sheet was cut from an adhesive sheet used for individual evaluation before foaming and curing. The release film was peeled off to separate the foamed adhesive layer. This was folded 180 degrees, and a 2 kg weight was applied to the folded portion for 5 seconds. After releasing the load, the surface condition of the foamed adhesive layer was observed, and its flexibility was evaluated according to the following criteria. ×: The sheet shape could not be maintained. △: Cracks appeared, but the sheet shape was maintained. ○: The sheet shape was maintained.

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

[0070] <<Peel Strength>> Peel strength was measured using IPC-TM-650 as a reference. The conductor width used in the measurement was 5 mm. The sample for measurement was prepared by using a foamed adhesive layer taken from an adhesive sheet used for individual evaluation to create a laminate consisting of a PI (polyimide) film (thickness 50 μm) / foamed adhesive layer / copper foil (GTS-MP 1 oz, bonded to the S side). The laminate was then molded and cured using a press machine under the conditions of 160°C, 4 MPa, and 20 minutes.

[0071] <<Shear Adhesion>> A 12.5 x 25 mm sheet was cut from the heat-foamed sheet before foaming and curing, and measurements were taken using this sheet according to the following procedure. Samples were prepared by fixing a heat-expanded foam sheet to an SPCC board using a 110°C rubber roll, placing 0.4 mm thick spacers on both sides of the heat-expanded foam sheet, stacking another SPCC board on top, and securing the gaps between the SPCC boards using a fixing jig (clamp, etc.). The above sample was heated to 200°C at a heating rate of 50°C / min, and then heated at 200°C for 7 minutes to cause the sheet in the sample to foam (four times foam) and harden. At this time, the gap between the two SPCC plates was fixed at 0.4 mm. Subsequently, the shear adhesion strength of the above samples was measured using a universal testing machine at a test speed of 5 mm / min.

[0072] <<Foaming initiation temperature T1_TMA>> Using a thermomechanical analyzer (Hitachi High-Tech Science Corporation, TMA7100), the foamed adhesive layer removed from the adhesive sheet was placed in an aluminum cell and measured under conditions of 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 resulting TMA curve was defined as the foaming start temperature T1 of the foamed adhesive layer.

[0073] <<Curing activation temperature T2_DSC>> Using a differential scanning calorimetry system (DSC7000X, Hitachi High-Tech Science Corporation), the foamed adhesive layer removed from the adhesive sheet was used as a sample. Measurements were taken under conditions of a heating rate of 50°C / min and a temperature range of 0°C to 300°C. The top temperature of the exothermic peak in the resulting DSC curve was defined as the curing activity temperature T2.

[0074] <<Minimum viscosity temperature Tα_Rheometer>> The samples were prepared as follows. Samples containing a foaming agent expand during measurement, making accurate measurement difficult. Therefore, using the method described in "<<Preparation of adhesive sheets for individual evaluation>>", the foaming agent was removed from the adhesive composition, and the adhesive sheets were prepared in the same manner otherwise. Subsequently, the adhesive layer was laminated using a rubber roll heated to 110°C to prepare samples with a thickness of approximately 600 μm. A viscoelasticity measuring device (TA Instruments, ARES-G2) was used to perform measurements under the following conditions: heating rate of 50°C / min, temperature range of 25°C to 220°C (held for 10 minutes after reaching 220°C), deformation mode "shear", and frequency of 1 Hz. The temperature at which the obtained complex viscosity η* value was smallest was defined as the minimum viscosity temperature Tα.

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

[0076] <<Mirror finish treatment>> The release film was placed on the foamed adhesive layer so that the side of the release film that was to be released was in contact with the foamed adhesive layer. 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 foamed adhesive layer a mirror finish. The release film used was HY-NS70, manufactured by 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. In the table, "α" represents the equivalent ratio of the curing agent to the equivalent of the epoxy resin. 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) constituting the foamed adhesive layer. The same applies to the other tables.

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

[0079] <Rating 2> This section evaluates the effect of using phenolic resin as a curing agent on the properties. In Reference Example 3, phenolic resin was used as the curing agent.

[0080] [Table 4]

[0081] <Rating 3> This section evaluates the effect of elastomer content on sheet properties. In Comparative Example 5, foaming occurred during the drying of the adhesive composition applied to the NPN sheet, making it impossible to produce a heat-foamed sheet. [Table 5]

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

[0083] <Rating 5> This section evaluates the effect of foaming agent content on sheet properties. [Table 7]

[0084] <Rating 6> This section evaluates the effect of different types of foaming agents (differences in foaming initiation temperature) on sheet properties. It was found that a small difference between the foaming initiation temperature T1 and the minimum viscosity temperature Tα leads to a decrease in shear adhesion. Furthermore, it was confirmed that the relationship between T1, T2, and Tα changes when the heating rate decreases. [Table 8] [Explanation of Symbols]

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

Claims

1. A heat-foamed sheet comprising a sheet-like substrate and a foaming adhesive layer provided on one or both sides of the sheet-like substrate, The foamed adhesive layer is at least one outermost layer of the heat-foamed sheet, The foamed adhesive layer comprises an epoxy resin, a curing agent, a thermoplastic resin, and a foaming agent. The content of the thermoplastic resin is 15 to 30% by mass relative to the total mass of the resin composition constituting the foamed adhesive layer. The foaming agent comprises thermally expandable microcapsules, A heat-foamed sheet wherein the static friction coefficient of the outermost surface of the foamed adhesive layer with respect to the SUS plate is 0.5 or less.

2. A heat-foamed sheet comprising a sheet-like substrate and a foaming adhesive layer provided on one or both sides of the sheet-like substrate, The foamed adhesive layer is at least one outermost layer of the heat-foamed sheet, The foamed adhesive layer comprises an epoxy resin, a curing agent, a foaming agent, and an inorganic filler. The foaming agent comprises thermally expandable microcapsules, The inorganic filler content is 3 to 20% by mass relative to the total mass of the resin composition constituting the foamed adhesive layer. A heat-foamed sheet wherein the static friction coefficient of the outermost surface of the foamed adhesive layer with respect to the SUS plate is 0.5 or less.

3. A heat-foamed sheet comprising a sheet-like substrate and a foaming adhesive layer provided on one or both sides of the sheet-like substrate, The foamed adhesive layer is at least one outermost layer of the heat-foamed sheet, The foamed adhesive layer comprises an epoxy resin, a curing agent, a thermoplastic resin, and a foaming agent. The content of the thermoplastic resin is 15 to 30% by mass relative to the total mass of the resin composition constituting the foamed adhesive layer. The foaming agent comprises thermally expandable microcapsules with a particle size of 9 to 12 μm. A heat-foamed sheet wherein the static friction coefficient of the outermost surface of the foamed adhesive layer with respect to the SUS plate is 0.5 or less.

4. A heat-foamed sheet comprising a sheet-like substrate and a foaming adhesive layer provided on one or both sides of the sheet-like substrate, The foamed adhesive layer is at least one outermost layer of the heat-foamed sheet, The foamed adhesive layer comprises an epoxy resin, a curing agent, and a foaming agent. The foaming agent comprises thermally expandable microcapsules, The arithmetic mean roughness Ra of the outermost surface of the foamed adhesive layer is 1.0 μm or more. A heat-foamed sheet wherein the static friction coefficient of the outermost surface of the foamed adhesive layer with respect to the SUS plate is 0.5 or less.

5. The heat-foamed sheet according to any one of claims 1 to 3, wherein the arithmetic mean roughness Ra on the surface of the sheet-like substrate is 0.4 μm or more.

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

2.

7. The heat-expandable sheet according to any one of claims 1 to 6, wherein the content of the heat-expandable microcapsules is 3 to 19% by mass relative to the total mass of the resin composition constituting the foamed adhesive layer.

8. The heat-foamed sheet according to any one of claims 1 to 7 is placed between the first adherend and the second adherend. A bonding method for adhering a first adherend and a second adherend by filling the gap between the two adherends with foam.

9. The bonding method according to claim 8, wherein the first adherend has a structure for accommodating the second adherend.

Images