Laminate

Abstract

To provide a laminate of a metal substrate and a polyimide layer which shows little decrease in adhesion (peel strength) and high reliability even when used at a high temperature for a long time.SOLUTION: A laminate comprising a metal substrate and a polyimide layer laminated on at least one surface of the metal substrate, wherein the metal substrate has a volume resistivity of 10 μ Ω·cm or more and contains a nickel and / or chromium element, and the polyimide layer has a thickness of 3 to 100 μ m, is composed of two or more layers, contains 3 to 90 mol% of a structure represented by the following formula (1) in each of the two or more layers, and has a 0.1% thermal weight loss temperature of 350 °C. or higher.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a laminate having a metal substrate and a polyimide layer laminated on at least one surface of the metal substrate, and more particularly to a laminate suitable as a flexible planar heating element. [Background technology]

[0002] Polyimide is a heat-resistant resin obtained by ring-closing polyamic acid, which is synthesized by the condensation reaction of tetracarboxylic anhydride and diamine as raw materials. It has excellent resistance to thermal decomposition due to the rigidity of its molecular chain, resonance stabilization, and strong chemical bonds. It has high durability against chemical changes such as oxidation and hydrolysis, and is flexible, with excellent mechanical and electrical properties. Polyimide is widely used in the insulating resin layer of flexible printed circuits (FPCs), which are commonly used in electronic devices.

[0003] Because of these properties of polyimide, polyimide resins have traditionally been used favorably as materials for constructing planar heating elements such as flexible heaters, and for example, laminates have been proposed in which a polyimide layer (film) containing or not containing a filler is formed on a metal foil such as a stainless steel foil or a copper foil. The applicant of the present application has provided a laminate of a metal substrate and a polyimide layer, which, in addition to the previously proposed laminates suitable for flexible heaters and planar heating elements, has excellent heat uniformity in the planar direction, as well as excellent toughness and flexibility, and is efficient in terms of power consumption and heat generation (Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2023-149143 Summary of the Invention [Problem to be solved by the invention]

[0005] Although sheet heating elements having a polyimide layer (film) have been known for some time, as mentioned in Patent Document 1, sheet heating elements are not only intended to heat piping, pipes, and irregularly shaped objects, but are also used in many applications where heat resistance is required, such as heaters for cigarettes and automobiles, heaters for fuel cells, and for thermal fixing of toner in copiers and laser printers, and there are many applications where heat resistance of over 200°C is required.

[0006] In such applications, the polyimide layer is often used for a relatively long period of time, and even if it is used intermittently, it is often used for a long period of time. When the polyimide layer is used for a relatively long period of time at a high temperature of about 200°C, the polyimide layer and the metal substrate are likely to deteriorate due to oxidation. In particular, if the polyimide layer deteriorates due to oxidation, the adhesive strength (peel strength) will decrease, causing the polyimide layer to peel off from the metal substrate, making the polyimide layer difficult to use. Furthermore, since the polyimide layer is likely to be used in everyday life, there is a risk of fire or explosion in some cases, and therefore a highly reliable polyimide layer is required. However, the above-mentioned Patent Document 1 merely proposes excellent properties such as the above-mentioned uniformity of heat in the surface direction as an issue, but does not propose any solution for ensuring adhesion between the polyimide layer and the metal substrate, nor does it mention the heat resistance of the polyimide itself. The same points are true of the prior art documents described in Patent Document 1, and even in technologies older than Patent Document 1, sufficient consideration has not been given to concerns such as oxidation degradation and the resulting decrease in adhesive strength (peel strength) when used for a relatively long time at a high temperature of about 200°C.

[0007] Therefore, in laminates suitable for flexible heaters and planar heating elements using polyimide layers, in addition to the demand for conformability to complex shapes and precise heating, there was room for improvement overall in terms of reliability and the reduction in adhesiveness (peel strength) due to oxidative degradation when used at high temperatures for a relatively long period of time, as well as use at high temperatures.

[0008] The present invention was made based on the above-mentioned findings, and its object is to provide a laminate of a metal substrate and a polyimide layer that exhibits little deterioration in adhesion (peel strength) even when used for a relatively long period of time at high temperatures and is highly reliable, and in particular to provide a laminate that is suitable for use as a flexible sheet heating element. [Means for solving the problem]

[0009] That is, the present invention is as follows. [1] A laminate having a metal substrate and a polyimide layer laminated on at least one surface of the metal substrate, the metal substrate has a volume resistivity of 10 μΩ cm or more and contains nickel and / or chromium elements; The polyimide layer has a thickness of 3 to 100 μm, is composed of two or more layers, and each of the two or more layers contains 3 to 90 mol % of a structure represented by the following formula (1), and has a 0.1% thermal weight loss temperature of 350°C or higher. [ka] [In general formula (1), Z is a group represented by the following (1)-1, (1)-2, (1)-3, or (1)-4, and each of the polyimide layers may contain two or more structures with different Zs.] [ka] [2] The laminate according to [1], wherein the polyimide layer contains 50 to 100 mol % of the structure represented by the formula (1) and the structure represented by the following formula (2) in each of the two or more layers: [ka] [In general formula (2), the linking group X is a single bond or -CO-NH-, the substituents Y each independently represent an alkyl or alkoxy group having 1 to 3 carbon atoms which may be substituted with a fluorine atom, and m and n each independently represent an integer of 0 to 4.] [3] The laminate according to [1] or [2], wherein the polyimide layer contains 50 to 97 mol % of the structure represented by formula (2) in the thickest layer of the two or more layers. [4] The laminate according to [1] or [2], which is used as a flexible planar heating element. [Effects of the Invention]

[0010] According to the present invention, a laminate of a metal substrate and a polyimide layer can be obtained that exhibits little deterioration in adhesion (peel strength) even when used for a relatively long time at high temperatures and has high reliability. Such a laminate is suitable for use as a flexible planar heating element, for example, for heating piping, pipes, and irregularly shaped objects, as well as for use as a heater for cigarettes or vehicles, a heater for fuel cells, and a heating part for thermally fixing toner in copiers, laser printers, etc. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the present invention will be described.

[0012] <Laminate> The laminate according to the present embodiment includes a metal substrate and a polyimide layer laminated on at least one surface of the metal substrate, the metal substrate has a volume resistivity of 10 μΩ cm or more and contains nickel and / or chromium elements; The polyimide layer has a thickness of 3 to 100 μm, is composed of two or more layers, and contains 3 to 90 mol % of a structure represented by the following formula (1) in each of the two or more layers, and has a 0.1% thermal weight loss temperature of 350°C or higher. [ka] [In general formula (1), Z is a group represented by the following (1)-1, (1)-2, (1)-3, or (1)-4, and each of the polyimide layers may contain two or more structures with different Zs.] [ka]

[0013] (metal substrate) The metal substrate may be one that generates heat by resistive heating when current is applied, but it must have a high resistance per unit length and be highly efficient at generating heat. In order to achieve these characteristics and facilitate easy adjustment of heating output and current flow, a metal with a volume resistivity of 10 μΩ·cm or higher is used. Preferably, the volume resistivity is 30 μΩ·cm or higher, and more preferably 50 μΩ·cm or higher. While there are no particular limitations on the upper limit, a higher volume resistivity increases heat generation, but the required power also increases depending on the wiring length due to the high resistivity, so a value of 200 μΩ·cm or lower is preferred. The volume resistivity value preferably satisfies the above at a temperature of 25°C.

[0014] Metals with such specific volume resistivity are preferably those containing nickel and / or chromium elements to satisfy the above-mentioned specific volume resistivity range and to suppress oxidation degradation of the metal at high temperatures by being covered with a very dense and adhesive film on the surface where oxygen and hydroxyl groups bond. Alloys are also suitable. Examples include, but are not limited to, stainless steel alloys, iron-nickel alloys, nickel-chromium alloys, iron-chromium-aluminum alloys, and composites thereof. Among these, stainless steel alloys and iron-chromium-aluminum alloys are preferred for their mechanical strength, heat resistance, and workability, with stainless steel alloys being more preferred. As stainless steel alloys, foils of any stainless steel alloy, such as austenitic, austenitic-ferritic, ferritic, martensitic, and composites thereof, can be used. Among these, SUS304 and iron-chromium-aluminum alloys are preferred for their mechanical strength, heat resistance, workability, heat generation, and price.

[0015] The thickness of the metal substrate is not particularly limited, but from the viewpoints of flexibility, processability, etc., it is preferable that the thickness of the metal foil is 10 to 100 μm, and more preferably 10 to 50 μm.

[0016] The shape of the metal substrate can be appropriately selected depending on the application of the laminate, and may be any of sheet, wire, string, tape, etc. Furthermore, it may be appropriately processed by a known method, for example, may be processed into a precise pattern by an etching method, as long as it does not impair the object of the present invention. Furthermore, it may be surface-treated by a known method.

[0017] (Polyimide layer) As described above, the polyimide layer constituting the laminate of the present embodiment has a thickness of 3 to 100 μm, is composed of two or more layers, and each of the two or more layers contains 3 to 90 mol % of the structure represented by formula (1), and has a 0.1% thermal weight loss temperature of 350° C. or higher.

[0018] Here, the polyimide layer may be laminated on one side (single surface) of the metal substrate, or may be laminated on both sides of the metal substrate. When the metal substrate has a polyimide layer on only one side, the side of the metal substrate not having the polyimide layer (the opposite surface) may not have any layer laminated thereon depending on the application or purpose, or a layer made of a material with good heat insulating properties may be laminated thereon to prevent heat from escaping from the metal substrate.

[0019] Furthermore, the polyimide layer used in this embodiment is composed of two or more polyimide layers per side. Composing the polyimide layer with two or more layers is preferable because each layer can have a different function. However, the two or more layers may be composed of the same polyimide layer. For example, an embodiment can be adopted in which a polyimide having excellent adhesive properties is used as the polyimide layer in contact with the metal substrate, and a polyimide having excellent heat resistance and low thermal expansion properties is used as the layer in contact with this polyimide layer. This allows the polyimide layer as a whole to have excellent adhesive properties, little warping, and a laminate having excellent heat resistance and low thermal expansion properties. Therefore, in this embodiment, a polyimide layer consisting of at least two polyimide layers is used. Furthermore, a third or more layers that are not directly laminated on the metal substrate may be laminated. Examples of layer configurations will be described later. In this embodiment, the inclusion of a layer of a resin other than the polyimide layer is not excluded, and the layer of the other resin can be appropriately selected and used within a range that does not impair the effects and objects of the present invention, or taking into consideration the application, etc. The layer of the other resin is not limited, and examples thereof include a resin layer of silicone, epoxy, etc.

[0020] The polyimide layer has a thickness of 3 to 100 μm per side of the metal substrate (total thickness of two or more layers). When polyimide layers are provided on both sides of the metal substrate, each polyimide layer has a thickness within the above range. By setting the thickness of the polyimide layer per side within this range, it becomes possible to provide thermal conductivity, toughness, flexibility, processability, and support for the film itself while preventing breakage. The thickness can be adjusted depending on the application and properties of the laminate, but a preferred thickness is within the range of 10 to 70 μm, and more preferably 10 to 40 μm.

[0021] As is well known, the basic structure of polyimide has a structural unit (hereinafter sometimes referred to as "structural unit A") derived from a tetracarboxylic dianhydride (hereinafter sometimes simply referred to as "acid anhydride") component and a structural unit (hereinafter sometimes referred to as "structural unit B") derived from a diamine component, and when one structural unit A and one structural unit B linked together are considered as one repeating unit, polyimide is composed of a polymer of these repeating units. The repeating unit of polyimide can be represented by the following general formula:

[0022] [ka] (In the above formula, R 1 is a tetravalent acid anhydride residue, and R 2 is a divalent diamine residue.

[0023] The polyimide layer of the laminate of this embodiment is characterized in that each of the two or more layers essentially contains the structure represented by formula (1). The structure represented by formula (1) has multiple aromatic rings containing the groups (1)-1 to (1)-4. Compared to structures with relatively low heat resistance, such as aliphatic chain functional groups, the structure is heat-resistant and less susceptible to decomposition even at high temperatures, making it suitable for use in high-temperature environments. Furthermore, the presence of ether groups between these aromatic rings provides excellent flexibility, facilitating the formation of intermolecular entanglements, resulting in excellent interlayer adhesion, film breaking elongation, tear resistance, and toughness. Therefore, by incorporating this structure as a common structure in each of the two or more layers, it is possible to improve not only the high-temperature adhesion between the metal substrate and polyimide layer but also the high-temperature adhesion between the polyimide layers, thereby improving the high-temperature adhesion of the entire laminate. Furthermore, as described above, it is also possible to improve the breaking elongation and other properties of the resulting film. Therefore, although the effect of the structure represented by formula (2) described later is also thought to contribute in part, by having the structure of formula (1) present in each of the two or more layers, not only can the adhesiveness (high-temperature adhesiveness) of the entire laminate be improved even when used at high temperatures for a relatively long period of time (for example, at least about 100 hours at 150°C), but also the adhesiveness can be prevented from being impaired even when used for a relatively long period of time, thereby generally improving the high-temperature adhesiveness of the entire laminate, and also making it possible to provide excellent elongation at break when made into a film.

[0024] The content of the structure represented by formula (1) is set to be within a range of 3 to 90 mol % in each of the two or more layers (each layer). That is, in each layer (each layer), the structure represented by formula (1) is contained within a range of 3 to 90 mol % in the repeating units per mole of polyimide. If the content of the structure represented by formula (1) is less than 3 mol %, the adhesion, elongation, and toughness of the polyimide layer may be reduced. On the other hand, if it exceeds 90 mol %, the content of other structures will be relatively low, and therefore, the properties based on other structures (e.g., heat resistance and low thermal expansion) described below may not be sufficiently obtained.

[0025] The structure represented by formula (1) may be contained in either or both of the structural unit A and the structural unit B in the repeating unit, and in one of the repeating units, the structural unit A (acid anhydride residue R 1 ) and structural unit B (diamine residue R 2 ) or both. If one repeating unit contains one structure of formula (1), the content will be 100 mol %, and if two repeating units contain two, the content will be 200 mol %. Therefore, it is preferable to adjust the content in the above range by adjusting the amount contained in either or both of the structural unit A and the structural unit B. In this regard, in the present embodiment, as will be described later, the amount of the structural unit A (acid anhydride residue R 1 ) side is a preferred embodiment in which a component having excellent heat resistance and low thermal expansion is used. In addition, since it can exhibit heat resistance, adhesiveness, and high-temperature adhesiveness, it is preferable to use the structural unit B (diamine residue R 2 ) side contains the structure represented by formula (1). However, the polyimide layer in this embodiment is not limited to this embodiment, and the composition of each layer can be adjusted appropriately depending on the required properties (for example, adhesiveness, heat resistance, low thermal expansion, etc.) and applications.

[0026] The polyimide layer in this embodiment preferably includes at least the following two layers as the two or more polyimide layers. That is, as described above, it is preferable to adopt an embodiment in which a layer having excellent adhesion to a metal substrate is laminated as the polyimide layer (P1) in contact with the metal substrate, and a polyimide layer (P2) having excellent heat resistance and low thermal expansion properties is used as the layer in contact with this polyimide layer (P1). In such an embodiment, it is preferable that the polyimide layer (P1) contains a relatively large amount of the structure represented by formula (1), and the content is preferably within a range of 50 to 90 mol %, more preferably 60 to 90 mol %, even more preferably 70 to 90 mol %, and even more preferably 80 to 90 mol %.

[0027] On the other hand, in the polyimide layer (P2) in such a preferred embodiment, the content of the structure represented by the formula (1) is preferably adjusted taking into consideration other structures (for example, the structure represented by the formula (2) described later), and the preferred content is in the range of 3 to 50 mol %.

[0028] Components having such a structure represented by formula (1) include compounds having the groups (1)-1 to (1)-4 as Z. Examples of the diamine component include 1,3-bis(3-aminophenoxy)benzene (APB), 1,3-bis(4-aminophenoxy)benzene (TPE-R), 1,4-bis(4-aminophenoxy)benzene (TPE-Q), 2,7-bis(4-aminophenoxy)naphthalene, 4,4'-bis(4-aminophenoxy)biphenyl, and bis[4-(4-aminophenoxy)phenyl]ether. However, the diamine component is not limited to these, and compounds having the groups (1)-1 to (1)-4 as Z can be used. Examples of the acid anhydride component having the structure represented by formula (1) include, but are not limited to, the following compounds having the groups (1)-1 to (1)-4 as Z. When an acid anhydride component having the structure represented by formula (1) is used, both ends of the structure represented by formula (1) each have two single bonds as an acid anhydride residue. [ka]

[0029] In the polyimide layer of the present embodiment, it is more preferred that each of the two or more polyimide layers contains the structure represented by formula (1) and the structure represented by formula (2) below in a total amount of 50 to 100 mol %. That is, it is more preferred that each of the two or more layers contains the structures represented by formulas (1) and (2) within this range as a diamine component. The structure represented by formula (2) below is a rigid structure in which two benzene rings are linked by a single bond or an amide bond (-CO-NH-) as the linking group X, and is excellent in heat resistance, a low thermal expansion coefficient, and dimensional stability. Furthermore, when a side chain Y group is present, the structure has higher planarity. [ka] [In general formula (2), the linking group X is a single bond or -CO-NH-, the substituents Y each independently represent an alkyl or alkoxy group having 1 to 3 carbon atoms which may be substituted with a fluorine atom, and m and n each independently represent an integer of 0 to 4.]

[0030] Examples of such a structure of formula (2) when X is a direct bond include 4,4'-diaminobiphenyl, 2,2'-dimethyl-4,4'-diaminobiphenyl (m-TB, also referred to as m-tolidine), 3,3'-dimethyl-4,4'-diaminobiphenyl (also referred to as o-tolidine), and 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFMB). Furthermore, when X is an amide bond, examples include 4,4'-diaminobenzanilide (DABA), 4,4'-diamino-2'-methoxybenzanilide (MABA), 3,5-diamino-3'-trifluoromethylbenzanilide, and 3,5-diamino-4'-trifluoromethylbenzanilide. Among these, m-TB or DABA is preferably used for its heat resistance, dimensional stability, and thermal diffusivity. That is, in a preferred embodiment, each layer contains the structure represented by formula (1) (e.g., APB or TPE-R) and the m-TB or DABA represented by formula (2) in a total amount within the range of 50 to 100 mol %.

[0031] Here, in the preferred embodiment as described above, when a polyimide layer formed by laminating at least a polyimide layer (P1) and a polyimide layer (P2) is used, the content of the structure represented by formula (2) in the polyimide layer (P1) is preferably in the range of 10 to 50 mol %, more preferably 10 to 40 mol %, even more preferably 10 to 30 mol %, and still more preferably 10 to 20 mol %.

[0032] On the other hand, in the polyimide layer (P2) in such a preferred embodiment, the content of the structure represented by the formula (2) is in the range of 50 to 97 mol %, more preferably in the range of 70 to 97 mol %. In such a preferred embodiment, the polyimide layer (P2) preferably constitutes the main layer, and typically constitutes the thickest layer among the polyimide layers composed of two or more layers. By setting the content represented by the formula (2) in the main polyimide layer (P2) within the above range, the polyimide layer and the laminate as a whole can be provided with heat resistance, dimensional stability, and low thermal expansion, which is preferable.

[0033] Here, although not limited thereto, when the thickness of the polyimide layer (P1) in contact with the metal substrate is T1 and the thickness of the main polyimide layer is T2, the thickness of T1 is preferably in the range of 1 μm to 4 μm, and the thickness of T2 is preferably in the range of 4 μm to 30 μm. From another perspective, the thickness of T1 is preferably 20% or less of the total thickness of the polyimide layer. In this case, it is a preferred embodiment that the polyimide layer (P2) constituting the main layer has a thickness of 60% or more, more preferably 70% or more, and even more preferably 80% or more of the total thickness of the polyimide layer. The main polyimide layer (P2) is preferably composed of a non-thermoplastic polyimide, as described below.

[0034] Specific configurations of two or more polyimide layers have been partially described above, but may also be, for example, a two-layer structure consisting of a polyimide layer (P1) laminated directly on the metal substrate and a polyimide layer (P2) laminated on the polyimide layer (P1) without being directly laminated on the metal substrate. While not limited to the following configurations 1 to 5, a two-layer or three-layer structure is preferred, and a three-layer structure in which a third polyimide layer (P3) is laminated in the order M1 / (P1) / (P2) / (P3) is more preferred. M1 and M2 represent metal substrates, and M1 and M2 may be the same or different. The polyimide layer (P1) laminated directly on the metal substrate and the third polyimide layer (P3) may have the same composition. For example, when multiple polyimide layers are formed by a casting method, a two-layer structure may be formed in which a polyimide layer (P1) is laminated directly onto the metal substrate from the cast side, and a polyimide layer (P2) is laminated in that order. Alternatively, a three-layer structure may be formed in which a polyimide layer (P1) is laminated directly onto the metal substrate from the cast side, a polyimide layer (P2) is laminated in that order from the cast side, and a third polyimide layer (P3). The term "cast side" here refers to the surface of the support (metal substrate) when forming the polyimide layer. The surface of the multiple polyimide layers opposite the cast side is referred to as the "laminate side." Unless otherwise specified, a metal substrate may or may not be laminated on the laminate side.

[0035] Configuration 1;M1 / P1 / P2 Configuration 2: M1 / P1 / P2 / P1 (or P3) Configuration 3: M1 / P1 / P2 / P1 (or P3) / M2 (or M1) Configuration 4: M1 / P1 / P2 / P1 (or P3) / P2 / P1 (or P3) / M2 (or M1) Configuration 5;P2 / P1 / M1 / P1 / P2

[0036] In this case, the polyimide constituting the polyimide layer (P1) and the polyimide layer (P3) is preferably a thermoplastic polyimide, which improves the adhesiveness of the polyimide layer as a whole and makes it suitable for use as an adhesive layer for a metal substrate.

[0037] A preferred embodiment has at least a thermoplastic polyimide layer (P1) and a non-thermoplastic polyimide layer (P2) made of a non-thermoplastic polyimide, and at least one of the non-thermoplastic polyimide layers (P2) has a polyimide layer (P1) that will become a thermoplastic polyimide layer. That is, the polyimide layer (P1) is preferably provided on one or both sides of the non-thermoplastic polyimide layer.

[0038] The non-thermoplastic polyimide layer constitutes a low-thermal expansion polyimide layer, and the thermoplastic polyimide layer constitutes a high-thermal expansion polyimide layer. Here, the low-thermal expansion polyimide layer refers to a polyimide layer having a coefficient of thermal expansion (CTE) preferably in the range of 1 ppm / K to 25 ppm / K, more preferably in the range of 3 ppm / K to 25 ppm / K. The high-thermal expansion polyimide layer refers to a polyimide layer having a CTE preferably in the range of 35 ppm / K or more, more preferably in the range of 35 ppm / K to 80 ppm / K, and even more preferably in the range of 35 ppm / K to 70 ppm / K. The polyimide layer can be made to have a desired CTE by appropriately changing the combination of raw materials used, the thickness, and the drying and curing conditions.

[0039] The CTE of the entire polyimide layer is preferably within the range of 10 to 30 ppm / K. By controlling it within this range, deformation such as curling can be suppressed and high dimensional stability can be ensured. Here, CTE is the average value of the thermal expansion coefficients in the MD and TD directions of the polyimide layer.

[0040] Here, non-thermoplastic polyimide generally refers to polyimide that does not soften or exhibit adhesiveness even when heated. In the present invention, however, it refers to a polyimide having a storage modulus of 1.0×10 at 30°C measured using a dynamic viscoelasticity measuring device (DMA). 9 Pa or more, and the storage modulus at 350°C is 1.0 × 10 9The term "thermoplastic polyimide" refers to a polyimide having a storage modulus of 1.0×10 Pa or more at 30° C., as measured by DMA. In addition, thermoplastic polyimide (also referred to as "TPI") generally refers to a polyimide having a clearly identifiable glass transition temperature (Tg). In this embodiment, however, the term "TPI" refers to a polyimide having a storage modulus of 1.0×10 Pa or more at 30° C. 9 Pa or more, and the storage modulus at 300°C is 1.0 × 10 8 Polyimides with a modulus of less than 1 Pa are also referred to as polyimides.

[0041] The polyimide layer used is not limited, and commercially available polyimide films can be used as they are. However, examples include those formed by [1] a method in which a solution of polyamic acid, a polyimide precursor, is repeatedly applied to a metal substrate as a supporting substrate and dried, followed by imidization to form a polyimide layer (hereinafter referred to as a sequential coating method); and [2] a method in which a laminated structure of polyamic acid is simultaneously applied to a metal substrate as a supporting substrate by multilayer extrusion, dried, and then imidized to form a polyimide layer (hereinafter referred to as a multilayer extrusion method). Alternatively, a film may be formed by heat-pressing the film with or without an adhesive. From the viewpoints of dimensional stability and adhesion to the metal substrate, a film formed by a sequential coating method is preferred. The method for applying a polyimide solution (or polyamic acid solution) to a substrate is not particularly limited, and application can be performed using a coater such as a comma, die, knife, or lip. When forming two or more polyimide layers, for example, a method in which the above-mentioned coating, drying, and heating operations are repeated is preferred.

[0042] Furthermore, as heat resistance characteristics, the polyimide layer in this embodiment preferably has a 0.1% thermal weight loss temperature (Td0.1) of 350° C. or higher, more preferably 360° C. or higher, and even more preferably 370° C. or higher. Controlling the polyimide layer properties within such a range makes it difficult for even a slight thermal weight loss of 0.1% by mass to occur even under severe heating conditions assumed to be high temperatures and long-term use in actual use, and is therefore preferable because it makes it possible to suppress a decrease in adhesive strength (peel strength).

[0043] Furthermore, when the polyimide layer of this embodiment is applied to the use of a flexible planar heating element, it is preferable that the thermal diffusivity, i.e., the ease with which heat spreads, is neither too small nor too large. The thermal diffusivity can also be appropriately adjusted by adjusting the composition of the polyimide layer (e.g., by selecting one with high thermal conductivity) and the layer structure (e.g., arrangement, thickness, etc.). In this embodiment, since the polyimide layer is made up of two or more layers, it is preferable that the thermal diffusivity in the thickness direction is adjusted, and the thermal diffusivity in the thickness direction is 0.05 to 2.0 mm. 2 / s. More preferably, it is 0.09 to 2.0 mm 2 / s. It is also preferable that the thermal diffusivity in the plane direction is within the above range.

[0044] (Other diamine components, diamine residues) In the polyimide layer of the present embodiment, the diamine components other than the diamine components (diamine residues) having the structures represented by the above formulas (1) and (2) may be selected from any diamine components generally used in the synthesis of polyimides, as long as they do not impair the effects and purposes of the present invention. However, aromatic diamine compounds are preferred. Diamine compounds having an aliphatic skeleton may also be used. When these other diamine components are used, they are preferably used in an amount of 50 mol % or less, more preferably 30 mol % or less, and even more preferably 10 mol % or less of the total diamine components.

[0045] (Other acid anhydride components, acid anhydride residues) In the polyimide layer of the present embodiment, the acid anhydride component (acid anhydride residue) other than the structure represented by formula (1) can be selected from any tetracarboxylic acid anhydride component generally used in the production of polyimides, as long as it does not impair the effects and purposes of the present invention. However, a preferred embodiment uses pyromellitic dianhydride (PMDA) and / or 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA). PMDA and BPDA have the properties of controlling the molecular orientation in polyimides, suppressing increases in the coefficient of thermal expansion (CTE), and improving heat resistance, thereby enabling polyimides and resin films formed using the polyamic acid of the present embodiment to have a low CTE and improved heat resistance. Therefore, a more preferred embodiment of the polyimide layer, as described above, is one in which the structure represented by formula (1) is contained in the diamine residue side and PMDA and / or BPDA are used as the main component in the acid anhydride residue side.

[0046] The content of PMDA and / or BPDA is preferably 50 mol% or more, based on 100 mol% of all acid anhydride components. It is more preferably 60 to 100 mol%, and even more preferably 80 to 100 mol%. That is, the acid anhydride residues derived from these acid anhydride components are preferably 50 mol% or more, based on 100 mol% of all acid anhydride residues in the polyamic acid and polyimide to be produced, more preferably 60 to 100 mol%, and even more preferably 80 to 100 mol%. Among these, the use of PMDA is more preferred. The use of PMDA results in a rigid structure, which not only lowers the CTE but also increases the glass transition temperature, making this a more preferred embodiment. When PMDA is used, it is also preferably 50 mol% or more, based on 100 mol% of all acid anhydride components, more preferably 60 to 100 mol%, and even more preferably 80 to 100 mol%.

[0047] (Other ingredients) The polyimide layer of the present embodiment may contain or be mixed with a thermally conductive filler as needed, as long as it does not impair the object of the present invention. Examples of the thermally conductive filler include silicon dioxide, aluminum oxide, boron nitride, magnesium oxide, beryllium oxide, aluminum nitride, silicon nitride, aluminum fluoride, calcium fluoride, and metal salts of organic phosphinic acid, as well as other components. These components may be used alone or in combination of two or more. The shape of the filler may include, but is not limited to, spherical, powdery, fibrous, needle-like, and scaly shapes.

[0048] In this embodiment, the polyimide layer mainly serves to cover and insulate the metal substrate as a heating element and to uniformize the temperature within the surface, but it is preferable that the polyimide layer has good heat conduction from the metal substrate when used as a sheet heating element, and preferably has high thermal conductivity. In addition, it is also preferable that the polyimide layer has good flexibility and toughness so that it can be used as a flexible sheet heating element.

[0049] The polyamic acid used in the polyimide layer is composed of a tetracarboxylic acid residue, which is a tetravalent group derived from a tetracarboxylic dianhydride component, and a diamine residue, which is a divalent group derived from a diamine component. When the combination of these components is considered as one repeating unit, the polyamic acid is composed of a polymer of this repeating unit.

[0050] For example, polyimide acid, a polyimide precursor, is typically obtained by dissolving a predetermined tetracarboxylic dianhydride component and a diamine component in approximately equimolar amounts in an organic solvent and then polymerizing the mixture at a temperature typically ranging from 0 to 100°C for 30 minutes to 24 hours with stirring. During the reaction, the reactants are dissolved so that the resulting precursor is in a concentration of 5 to 30 wt %, preferably 10 to 20 wt %, in the organic solvent. Examples of organic solvents used in the polymerization reaction include N,N-dimethylformamide, N,N-dimethylacetamide (DMAC), N-methyl-2-pyrrolidone, 2-butanone, dimethyl sulfoxide, dimethyl sulfate, cyclohexanone, dioxane, tetrahydrofuran, diglyme, triglyme, and γ-butyrolactone. Two or more of these solvents can also be used in combination, and aromatic hydrocarbons such as xylene and toluene can also be used in combination.

[0051] Although there are no limitations, the viscosity of the polyamic acid is preferably adjusted to a range of 1,000 to 50,000 cP by adjusting the concentration and Mw. If the viscosity is too high, it may be diluted by adding a solvent.

[0052] The synthesized polyamic acid is usually advantageously used as a reaction solvent solution, but it can be concentrated, diluted, or replaced with another organic solvent as necessary. Furthermore, polyamic acid is advantageously used because it generally has excellent solvent solubility. The method for imidizing polyamic acid is not particularly limited, and a suitable method is, for example, heat treatment in the solvent at a temperature in the range of 80 to 400°C for 1 to 24 hours. [Example]

[0053] The present invention will be specifically described below based on examples, but the present invention is not limited to the scope of these examples.

[0054] The abbreviations used in the examples represent the following compounds. PMDA: Pyromellitic dianhydride BPDA: 3,3',4,4'-biphenyltetracarboxylic dianhydride m-TB: 2,2'-dimethyl-4,4'-diaminobiphenyl TPE-R: 1,3-bis(4-aminophenoxy)benzene DABA: 4,4'-diaminobenzanilide BAPP: 2,2-bis(4-aminophenoxyphenyl)propane DMAc: N,N-dimethylacetamide

[0055] The properties evaluated in the examples were evaluated according to the following evaluation methods.

[0056] [Viscosity measurement] The viscosity of the polyamic acid solution was measured using a cone-plate viscometer equipped with a thermostatic water bath (manufactured by Tokimec Co., Ltd.). The measurements were taken at 25°C.

[0057] [Weight average molecular weight (Mw)] Measurement was performed by gel permeation chromatography (manufactured by Tosoh Corporation, product name: HLC-8220GPC). Polystyrene was used as a standard substance, and N,N-dimethylacetamide was used as a developing solvent.

[0058] Coefficient of Thermal Expansion (CTE) A polyimide film (3 mm × 15 mm) was heated from 30°C to 280°C at a heating rate of 10°C / min while applying a load of 5.0 g in a thermomechanical analyzer (TMA), and then cooled from 250°C to 100°C. The thermal expansion coefficient was measured from the elongation (linear expansion) of the polyimide film during cooling.

[0059] [Thermogravimetric reduction temperature (Td0.1)] A polyimide film weighing 10 to 20 mg was heated at a constant rate from 30°C to 550°C in a nitrogen atmosphere using a thermogravimetric analyzer (TG) TG / DTA6200 manufactured by Seiko Corporation, and the change in weight was measured. The weight at 200°C was set to zero, and the temperature at which the weight loss rate was 0.1% was defined as the thermal weight loss temperature (Td0.1, unit: °C).

[0060] [Curl occurrence] A laminate sample of metal foil and polyimide layer was cut into a size of 16 cm x 26 cm and left in a constant temperature and humidity environment (23±3°C, 50±5% RH) for 24 hours, after which the amount of warping at the four corners was measured using a vernier caliper. If there was warping toward the resin side or the metal side, the point with the greatest amount of warping was recorded as the laminate curl. A laminate curl of 10 mm or less was recorded as ◯, and a laminate curl of 10 mm or more was recorded as ×.

[0061] [Peel Strength] A test sample with a 1 mm-wide circuit obtained from a laminate of a metal foil support and a polyimide layer was held in an oven at a predetermined temperature for a predetermined time, then removed from the oven. Using a tension tester, the polyimide layer side of the test sample was fixed to an aluminum plate with double-sided tape, and the metal foil was peeled off in a 180° direction at a rate of 50 mm / min to determine the peel strength between the metal foil and the polyimide layer. Peel strengths of 0.5 kN / m or more were evaluated as good, and those less than 0.5 kN / m were evaluated as bad.

[0062] [Thermal diffusivity through thickness] The resin film (polyimide layer) was cut into a size of 20 mm x 20 mm, and the thermal diffusivity in the thickness direction (mm 2 / s) was measured.

[0063] [Thermal diffusivity in the plane] The resin film (polyimide layer) was cut into a size of 5 mm × 30 mm, and the thermal diffusivity (mm) in the surface direction was measured using a measuring device (manufactured by Advance Riko Co., Ltd., product name: Laser PIT device (optical AC method)). 2 / s) was measured.

[0064] Synthesis Examples 1-6 To synthesize polyamic acid solutions A to F, DMAc as a solvent was added to a 500 ml separable flask under a nitrogen stream so as to obtain the solid content concentration shown in Table 1, and the diamine component and acid anhydride component (molar parts) shown in Table 1 were added. The mixture was stirred at room temperature for 36 hours to carry out a polymerization reaction, and viscous polyamic acid solutions A to F were prepared.

[0065] [Table 1]

[0066] [Example 1] Polyamic acid solution D was uniformly applied to a metal substrate (SUS304, manufactured by Nippon Steel Chemical & Material Co., Ltd., thickness: 30 μm, volume resistivity: 72 μΩ·cm) to a thickness of 2 μm after curing. The solution was then stepwise heated and dried up to 120°C to remove the solvent. Polyamic acid solution A was then uniformly applied to the resulting thickness of 21 μm after curing. The solution was then stepwise heated and dried up to 120°C to remove the solvent. Polyamic acid solution D was then uniformly applied to the resulting thickness of 2 μm after curing. The solution was then stepwise heated and dried up to 120°C to remove the solvent. After forming three polyamic acid layers in this way, the resulting layer was subjected to stepwise heat treatment from 130°C to 360°C to complete the imidization. This formed polyimide layer 1, consisting of polyimide layer D / polyimide layer A / polyimide layer D, with a thickness of 25 μm, and prepared laminate 1.

[0067] To evaluate the properties of the polyimide layer 1 in the obtained laminate 1, the SUS foil was removed by etching to produce the polyimide layer 1, and the CTE, Td0.1, and thermal diffusivity were measured. In addition, the curl occurrence and peel strength of the obtained laminate 1 were measured. The evaluation results are shown in Table 2.

[0068] [Examples 2 to 3, Comparative Examples 1 and 4] Laminates 2 to 4 and 7 according to Examples 2 to 3 and Comparative Examples 1 and 4 were obtained in the same manner as in Example 1, except that the type of polyamic acid solution used was changed, and polyimide layers 2 to 4 and 7 were similarly obtained and evaluated in the same manner as in Example 1. The evaluation results are shown in Table 2.

[0069] Comparative Example 2 The metal substrate used was changed to copper foil (electrolytic copper foil, manufactured by Fukuda Metal Foil & Powder Co., Ltd., product name: CF-T4MDS-HD-35, thickness: 35 μm, Rz=1.4 μm, specific volume resistivity: 3 μΩ cm), the type of polyamic acid solution was changed as shown in Table 2, and a laminate 5 according to Comparative Example 2 was obtained in the same manner as in Example 1, a polyimide layer 5 was obtained in the same manner, and evaluated in the same manner as in Example 1. The evaluation results are shown in Table 2.

[0070] Comparative Example 3 Polyamic acid solution B was uniformly applied to the same SUS foil as above as the metal substrate so that the thickness after curing would be 27 μm, and then the solution was dried by stepwise heating in a temperature range up to 120°C to remove the solvent. After forming one polyamic acid layer in this way, stepwise heat treatment was performed from 130°C to 360°C to complete the imidization, forming polyimide layer 6 consisting of polyimide layer B and having a thickness of 27 μm, thereby preparing laminate 6. As in Example 1, the SUS foil was etched away to produce polyimide layer 6, which was then evaluated in the same manner. The evaluation results are shown in Table 2.

[0071] [Table 2]

[0072] Although the embodiments of the present invention have been described in detail above for the purpose of illustration, the present invention is not limited to the above-described embodiments and various modifications are possible.

Claims

1. A laminate having a metal substrate and a polyimide layer laminated on at least one surface of the metal substrate, the metal substrate has a volume resistivity of 10 μΩ cm or more and contains nickel and / or chromium elements; The polyimide layer has a thickness of 3 to 100 μm and is composed of two or more layers, each of which contains 3 to 90 mol % of a structure represented by the following formula (1), and the laminate has a 0.1% thermal weight loss temperature of 350° C. or higher. 【Chemistry 1】 [In general formula (1), Z is a group represented by the following (1)-1, (1)-2, (1)-3, or (1)-4, and each of the polyimide layers may contain two or more structures with different Z.] 【Chemistry 2】

2. The laminate according to claim 1, wherein the polyimide layer contains 50 to 100 mol % of the structure represented by formula (1) and the structure represented by formula (2) below in each of the two or more layers: 【Transformation 3】 [In general formula (2), the linking group X is a single bond or —CO—NH—, the substituents Y each independently represent an alkyl group or alkoxy group having 1 to 3 carbon atoms which may be substituted with a fluorine atom, and m and n each independently represent an integer of 0 to 4.]

3. 3. The laminate according to claim 1, wherein the polyimide layer contains 50 to 97 mol % of the structure represented by formula (2) in the thickest layer of the two or more layers.

4. 3. The laminate according to claim 1, which is used as a flexible sheet heating element.

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