Coating method and sheet material

Abstract

Proposed is a coating method capable of suppressing coating film unevenness and suppressing coating film thickness variation when a coating film is formed on the surface of a substrate by using a coating liquid in which the content ratio of a filler is increased. A coating method including: using a coating apparatus (1) including a conveying roller (2) that feeds a substrate (W); a knife coater (3) that is disposed with a gap from the conveying roller (2) and provided with a knife portion (3b); and a liquid-reserving member (4) having a one end portion (4a) positioned on a side of the conveying roller (2) and partitioning a liquid-reserving space (S) together with the conveying roller (2), to form a coating film (C) on a surface of the substrate (W), wherein the coating liquid (L) contains a resin, a solvent, and a filler, and the filler is contained in a content ratio of 65 to 95 parts by mass with respect to 100 parts by mass of a mixture of the resin and the filler, an outer diameter D1 of the conveying roller (2) is 50 to 270 mm, an outer diameter D2 of the knife coater (3) is 50 to 170 mm, and a distance D3 from the gap to the one end portion (4a) is 1 to 120 mm.

Description

Coating method and sheet material

[0001] The present invention relates to a coating method and a sheet material.

[0002] Various types of circuit boards have been used for some time. For example, Patent Documents 1 and 2 disclose circuit boards in which a circuit pattern is laminated on a base substrate via an insulating layer. The insulating layer serves to ensure a withstand voltage between the base substrate and the circuit pattern. Furthermore, by incorporating a filler into the insulating layer, the filler can ensure the thermal conductivity of the insulating layer. With such a circuit board, heat generated by electronic components mounted on the circuit pattern and transferred to the circuit pattern can be transferred to the base substrate via the insulating layer and dissipated from the base substrate to the outside.

[0003] The insulating layer can be obtained by preparing a sheet material having a coating film of an insulating layer-forming composition formed on the surface of a substrate such as a PET film, and thermally transferring the coating film of the sheet material to a base substrate or a circuit pattern. A coating device and a coating method for forming a coating film on the surface of a substrate are disclosed in, for example, Patent Document 3.

[0004] JP 2009-246079 A JP 2013-254921 A Japanese Patent No. 4857217 A

[0005] In recent years, as electronic devices have become more powerful and smaller, the heat generated by electronic components has become increasingly greater, making the heat dissipation properties of insulating layers more important. The present inventors have conducted extensive research into this issue and discovered that an insulating layer with excellent heat dissipation properties can be obtained by significantly increasing the filler content of an insulating layer-forming composition compared to conventional methods. However, when a coating film is formed on the surface of a substrate using a coating liquid with such an increased filler content, conventional coating methods such as those described in Patent Document 3 have resulted in uneven appearance of the coating film and large variations in thickness, making mass production difficult.

[0006] In view of these problems, the present invention aims to provide a coating method that can suppress coating unevenness and variation in coating thickness when forming a coating film on the surface of a substrate using a coating liquid with an increased filler content, and a sheet material obtainable by this coating method.

[0007] The present invention provides a coating method for forming a coating film of a coating liquid contained in a liquid reservoir member on the surface of a substrate using a coating device having a conveying roller that feeds out a wound long substrate, a columnar or cylindrical knife coater that is positioned above the conveying roller with a gap between it and the conveying roller and that is provided with a knife portion, and a liquid reservoir member that has one end located to the side of the conveying roller and that defines a liquid reservoir space between the conveying roller and the knife coater, wherein the coating liquid contains a resin, a solvent, and a filler, the content of the filler being 65 to 95 parts by mass relative to 100 parts by mass of a mixture of the resin and the filler, the outer diameter of the conveying roller being 50 to 270 mm, the outer diameter of the knife coater being 50 to 170 mm, and the distance from the gap to the one end of the liquid reservoir member being 1 to 120 mm.

[0008] In the above-described coating method, the content of the solvent is preferably 10 to 50 parts by mass with respect to 100 parts by mass of the mixture of the resin and the filler.

[0009] In the above-mentioned coating method, the viscosity of the coating liquid is preferably 100 to 20,000 cP at a temperature of 25°C.

[0010] In the above-described coating method, it is preferable that the filler is a mixture of boron nitride and aluminum oxide, and the content of the boron nitride is 60 to 90 parts by mass per 100 parts by mass of the mixture of the boron nitride and aluminum oxide.

[0011] The present invention also provides a sheet material comprising the above-mentioned substrate and the above-mentioned coating film.

[0012] According to the coating method of the present invention, when a coating liquid with an increased filler content is used, it is possible to suppress unevenness in the coating film formed on the surface of the substrate and also to suppress variations in the thickness of the coating film.

[0013] FIG. 1 is a diagram showing one embodiment of a coating device used in a coating method according to the present invention; FIG. 2 is a diagram (grayscale) showing unevenness in a coating film; FIG. 3 is a diagram (grayscale) showing unevenness in a coating film; FIG. 2A is a diagram (binarized version of FIG. 2B) showing unevenness in a coating film; FIG. 3B is a diagram (grayscale) showing a cross section of a coating film analyzed by SEM; FIG. 4 is a diagram (binarized version of FIG. 4A) showing a cross section of a coating film analyzed by SEM; FIG. 4C is a diagram (binarized version of FIG. 4B) showing a cross section of a coating film analyzed by SEM;

[0014] Hereinafter, one embodiment of the coating method according to the present invention will be described with reference to the accompanying drawings. Note that the drawings are schematic, and the thickness, width, and ratio of each part may differ from those actually implemented.

[0015] First, the coating device used in the coating method according to the present invention will be described with reference to Figure 1. The coating device 1 of this embodiment is equipped with a transport roller 2, a knife coater 3, and a liquid reservoir member 4, and is used to form a sheet material comprising the substrate W and the coating film C by applying a coating film C to the surface of a long substrate W.

[0016] The transport roller 2 has a columnar or cylindrical shape and is configured to rotate in the direction of the arrow shown in the figure by a motor or the like (not shown). The substrate W is wound around the transport roller 2, and the substrate W can be fed as the transport roller 2 rotates. The transport roller 2 in this embodiment has a length sufficient to allow the use of a substrate W with a maximum width of approximately 1 m. The outer diameter D1 of the transport roller 2 will be described later.

[0017] The knife coater 3 includes a columnar or cylindrical main body 3a and a knife portion 3b provided on the outer peripheral surface of the main body 3a. As shown in the figure, the knife portion 3b has an edge-like (sharp) cross section and extends longitudinally along the axial direction of the main body 3a. The length of the knife coater 3 is set to be approximately the same as that of the conveying roller 2. The outer diameter D2 of the knife coater 3 will be described later.

[0018] As shown in the figure, the knife coater 3 is positioned above the transport roller 2 with the knife portion 3b closest to the transport roller 2 and is held with a gap between the tip of the knife portion 3b and the outer circumferential surface of the transport roller 2. This gap is set to a level that allows a coating film C of a predetermined thickness to be formed on the surface of the substrate W in a state where the substrate W is wound around the transport roller 2 (for example, the thickness of the coating film C is about 200 μm).

[0019] As shown in the figure, the reservoir member 4 is disposed tilted with respect to the vertical direction, and one end 4a of the reservoir member 4 is located to the side of the conveying roller 2. Note that the symbol D3 shown in Fig. 1 indicates the distance from the gap to the one end 4a (the length of the chord connecting the part of the outer circumferential surface of the conveying roller 2 closest to the knife portion 3b to the part closest to the one end 4a).

[0020] As shown in the figure, the coating device 1 has a liquid reservoir space S defined by a conveying roller 2 and a liquid reservoir member 4. The liquid reservoir space S contains a coating liquid L for forming a coating film C. When the coating liquid L is contained in the liquid reservoir space S, the lower part of the knife coater 3 is immersed in the coating liquid L.

[0021] The substrate W is formed of, for example, a PET film having releasability.

[0022] The coating liquid L also contains a resin, a solvent, and a filler. Examples of resins include thermosetting resins such as epoxy resin, phenolic resin, melamine resin, urea resin, unsaturated polyester resin, alkyd resin, and cyanate resin. The resin may also be a liquid crystal polymer, a resin with a mesogenic skeleton, engineering plastics such as polycarbonate, nylon, and polyamide, or other high thermal conductivity resins. One type of resin may be used alone, or two or more types may be used in combination. The solvent may be, for example, a glycol ether-based solvent (e.g., ethyl carbitol, methyl carbitol, butyl carbitol, etc.). The filler preferably has excellent insulating properties and high thermal conductivity, such as aluminum oxide, boron nitride, silica, aluminum nitride, silicon nitride, and magnesium oxide. Other materials contained in the coating liquid L include, for example, an additive (curing agent) that cures the resin, which is the main component. The curing agent is selected depending on the type of resin, and is not particularly limited as long as it reacts with the resin. For example, when an epoxy resin is used, examples of the curing agent include an amine-based curing agent, an imidazole-based curing agent, and a phenol-based curing agent. A catalyst may also be added to promote the curing of the resin. For example, when an epoxy resin is used, a phosphorus-based, thiol-based, tertiary amine-based, or imidazole-based catalyst may be used. Regarding the resin content in the coating liquid L, when a curing agent or a catalyst (additive) is used together with the resin (main component), the resin content is calculated including the curing agent and the catalyst.

[0023] The filler content in the coating solution L of this embodiment is significantly higher than the content in conventional insulating layers formed from coating film C. Specifically, the filler content is 65 to 95 parts by mass per 100 parts by mass of the mixture of resin and filler contained in the coating solution L. By significantly increasing the filler content in this manner, when an insulating layer of a circuit board is formed using the coating film C obtained from the coating solution L, the thermal conductivity of the insulating layer can be, for example, 15 W / mK or more, thereby imparting high thermal conductivity to the insulating layer. Note that, considering the quality and productivity of forming the coating film C from the coating solution L while ensuring high thermal conductivity as an insulating layer, it is more effective for the filler content to be 70 to 90 parts by mass per 100 parts by mass of the mixture of resin and filler, and even more effective to be 75 to 85 parts by mass. Furthermore, from the standpoint of the quality of the coating film C and the thermal conductivity of the insulating layer, it is preferable to use a mixture of boron nitride and aluminum oxide, for example. In this case, the content of boron nitride is effectively 60 to 90 parts by mass, more effectively 65 to 85 parts by mass, and even more effectively 70 to 80 parts by mass relative to 100 parts by mass of the mixture of boron nitride and aluminum oxide.

[0024] Furthermore, in consideration of the quality and productivity of the coating film C, the content of the solvent is preferably 10 to 50 parts by mass, more preferably 20 to 45 parts by mass, and even more preferably 30 to 40 parts by mass, per 100 parts by mass of the mixture of the resin and the filler.

[0025] Next, a coating method for forming a coating film C on the surface of a substrate W using a coating device 1 will be described. To carry out this coating method, as shown in FIG. 1, the substrate W is wrapped around a transport roller 2, and a coating liquid L is stored in a liquid storage space S. Then, a motor (not shown) or the like is driven to rotate the transport roller 2 in the direction of the arrow shown in the figure. As a result, the substrate W is fed in the direction of the arrow shown in the figure, and the coating liquid L passes through the gap between the transport roller 2 and the knife coater 3, forming a coating film C on the surface of the substrate W.

[0026] In a conventional coating apparatus 1, as shown in the comparative example described below, the diameter D1 of the transport roller 2 is approximately 300 mm, the diameter D2 of the knife coater 3 is approximately 200 mm, and the distance D3 from the gap between the transport roller 2 and the knife coater 3 to one end 4a is approximately 140 mm. When a coating film C was formed on the surface of a substrate W using such a coating apparatus 1 and a coating liquid L with an increased filler content, the coating film C exhibited uneven appearance and large film thickness variations. The unevenness of the coating film C makes it difficult to perform automatic foreign matter inspection using a visual inspection machine or the like to automatically detect foreign matter contained in the coating film C. Furthermore, if the coating film C exhibits large film thickness variations, for example, when the coating film C is used as an insulating layer of a circuit board, uniform contact with the base substrate or circuit pattern may not be ensured.

[0027] After extensive investigation into the unevenness and film thickness variation of the coating film C, it was determined that the resin content of the coating liquid L used decreased as the filler content increased, which was inferred to be one of the contributing factors. Specifically, as the substrate W was fed, the coating liquid L was continuously supplied to the gap between the conveying roller 2 and the knife coater 3. Therefore, a strong compressive force acts in this gap due to the coating liquid L being supplied successively. The inventors of the present application inferred that, because the resin has a higher fluidity than the filler, this compressive force acts to squeeze the resin out of the coating liquid L, leading to the occurrence of unevenness and film thickness variation. Another contributing factor is that the increased filler content makes the filler more likely to settle in the liquid reservoir space S where it is contained.

[0028] For this reason, the inventors of the present application considered that the compressive force would be reduced by minimizing the amount of coating liquid L supplied to the gap between the transport roller 2 and the knife coater 3, thereby making it less likely that the mixed state of the resin and filler would be disturbed when the coating liquid L passes through the gap. That is, they considered that it would be effective to reduce the amount of coating liquid L supplied to the gap between the transport roller 2 and the knife coater 3 by reducing the outer diameters of the transport roller 2 and the knife coater 3. They also considered that it would be effective to reduce the volume of the liquid reservoir space S by moving the liquid reservoir member 4 closer to the transport roller 2, in order to reduce the amount of coating liquid L and suppress settling of the filler in the liquid reservoir space S.

[0029] Based on these considerations, further investigations revealed that when the outer diameter D1 of the conveying roller 2 is 50 to 270 mm, the outer diameter D2 of the knife coater 3 is 50 to 170 mm, and the distance D3 from the gap to the one end 4a is 1 to 120 mm, the unevenness occurring in the coating film C is suppressed and the film thickness variation of the coating film C is also suppressed. Further investigations revealed that to suppress the occurrence of such unevenness and reduce film thickness variation in the coating film C, the outer diameter D1 of the conveying roller 2 is more preferably 75 to 250 mm, and even more preferably 100 to 200 mm. Furthermore, it was found that the outer diameter D2 of the knife coater 3 is more preferably 75 to 170 mm, and even more preferably 100 to 150 mm. It was also found that the distance D3 from the gap to the one end 4a is more preferably 1 to 100 mm, and even more preferably 1 to 50 mm.

[0030] The present inventors also investigated the viscosity of the coating liquid L, in addition to examining the diameter D1 of the conveying roller 2 in the coating device 1. This was because it was believed that reducing the viscosity of the coating liquid L could potentially suppress the occurrence of uneven appearance in the coating film C. However, when the viscosity of the coating liquid L was reduced, filler settling was observed in the formed coating film C. Therefore, further investigation was conducted into a viscosity that could suppress both the occurrence of unevenness and filler settling, and it was discovered that if the viscosity of the coating liquid L was 100 to 20,000 cP at a temperature of 25°C, the occurrence of unevenness and filler settling could be suppressed. Further investigation into this point revealed that a viscosity of 2,000 to 10,000 cP is more preferable, and 4,000 to 6,000 cP is even more preferable. The viscosity is a value measured using a Brookfield rotational viscometer (a digital viscometer DV2T (manufactured by Eiko Seiki Co., Ltd.)) at a shear rate of 8 (1 / s).

[0031] Examples of the present invention will be described below, but the present invention is not limited to these examples.

[0032] Example A PET film was prepared as the substrate W. A composition containing a resin (main agent and additive (curing agent)), a solvent, and a filler was prepared as the coating liquid L. The content of each component relative to the coating liquid L (parts by mass of each component when the coating liquid L is 100 parts by mass) is also shown. [Base] Bisphenol A type epoxy resin (trade name EXA-850CRP; manufactured by DIC Corporation), 11.2 parts by mass [Curing agent] 4,4'-diaminodiphenyl sulfone (trade name SEIKACURE S; manufactured by Wakayama Seika Kogyo Co., Ltd.), 4.2 parts by mass [Solvent] Ethyl carbitol (trade name Ethyl Carbitol; manufactured by Sankyo Chemical Co., Ltd.), 27.6 parts by mass [Filler (A)] Boron nitride (BN) (trade name HP-40, particle shape is agglomerated, average particle diameter (d50) is 36 μm; manufactured by Mizushima Ferroalloy Co., Ltd.), 43 parts by mass [Filler (B)] Aluminum oxide (trade name AS20, particle shape is rounded, average particle diameter (d50) is 22 μm; manufactured by Showa Denko K.K.), 14 parts by mass That is, in the prepared coating liquid L, the contents of filler (A) and filler (B) were 78.7 parts by mass relative to 100 parts by mass of the mixture of resin (base resin and curing agent), filler (A), and filler (B), the content of solvent was 38.1 parts by mass relative to 100 parts by mass of the mixture of resin (base resin and curing agent), filler (A), and filler (B), and the content of filler (A) was 75.4 parts by mass relative to 100 parts by mass of the mixture of filler (A) and filler (B).

[0033] The viscosity of the coating liquid L is 4300 cP as measured at a shear rate of 8 (1 / s) using a Brookfield rotational viscometer (a digital viscometer DV2T manufactured by Eiko Seiki Co., Ltd.).

[0034] The diameter D1 of the transport roller 2, the diameter D2 of the knife coater 3, and the distance D3 from the gap between the transport roller 2 and the knife coater 3 to one end 4a in the coating device 1 are as shown in the following Table 1 for the comparative example and examples 1 to 3. When a coating film C was formed using the above-mentioned substrate W and coating liquid L using the coating device 1 of the comparative example and examples 1 to 3, the results are as shown in Table 1.

[0035] The "appearance" column in Table 1 was evaluated as follows: ⊚: Almost no unevenness was observed in the appearance of the coating film C. ○: Some unevenness was observed in the appearance of the coating film C. ×: Much unevenness was observed in the appearance of the coating film C.

[0036] As shown in Table 1, the coating film C obtained with the coating apparatus 1 of the comparative example exhibited significant variations in appearance. Figure 2A is a photograph of the surface of the coating film C of the comparative example. Note that Figure 2A shows this photograph in grayscale, and Figure 2C shows the same photograph as Figure 2A but with binarization. The coating film C of the comparative example also exhibited significant variations in film thickness. On the other hand, the coating film C obtained with the coating apparatus 1 of Examples 1 and 2 exhibited only slight variations in appearance. The film thickness variations of the coating film C were also sufficiently small. The coating film C obtained with the coating apparatus 1 of Example 3 exhibited almost no variations in appearance (see Figure 2B; Figure 2B shows a grayscale photograph of the surface of the coating film C, and Figure 2D shows the same photograph as Figure 2B but with binarization). The film thickness variations of the coating film C were even smaller than those of Examples 1 and 2.

[0037] As mentioned above, depending on the viscosity of Coating Liquid L, filler settling may occur in the resulting Coating Film C. Here, Coating Film C was formed using Coating Liquid L with reduced viscosity (3300 cP, with an increased amount of the solvent described above, while leaving the other components unchanged). SEM observation of the cross-section of this coating film C revealed filler (aluminum oxide) settling (see FIG. 3A , where FIG. 3A is a grayscale image of the SEM observation, and FIG. 3C is a binarized version of the same image as FIG. 3A ). In contrast, SEM observation of the cross-section of Coating Film C obtained with Coating Liquid L (viscosity 4300 cP) using Coating Liquid L described above using Coating Liquid L with Coating Device 1 of Example 3 revealed no filler settling, resulting in favorable results (see FIG. 3B , where FIG. 3B is a grayscale image of the SEM observation, and FIG. 3D is a binarized version of the same image as FIG. 3B ).

[0038] Although one embodiment of the present invention has been described above, the present invention is not limited to the specific embodiment, and various modifications and changes are possible within the scope of the spirit of the present invention as set forth in the claims unless otherwise specifically limited in the above description. Furthermore, the effects of the above embodiment are merely examples of the effects resulting from the present invention, and do not mean that the effects of the present invention are limited to the above effects.

[0039] For example, if the outer diameter D1 of the transport roller 2 and the outer diameter D2 of the knife coater 3 are reduced, the rigidity of the transport roller 2 and the knife coater 3 may decrease, causing them to bend. In such a case, for example, a reinforcing roller for suppressing the bending of the transport roller 2 and the knife coater 3 may be provided on the bending side, and the coating film C may be formed by this coating device 1.

[0040] 1: Coating device 2: Conveying roller 3: Knife coater 3b: Knife section 4: Liquid reservoir member 4a: One end C: Coating film L: Coating liquid S: Liquid reservoir space W: Substrate

Claims

1. A coating method for forming a coating film of a coating liquid contained in a liquid reservoir member on the surface of a substrate using a coating device having a conveying roller that delivers a wound long substrate, a columnar or cylindrical knife coater that is positioned above the conveying roller with a gap from the conveying roller and is provided with a knife section, and a liquid reservoir member whose one end is located to the side of the conveying roller and defines a liquid reservoir space between the conveying roller and the coating device, wherein the coating liquid contains a resin, a solvent, and a filler, and the filler content is 65 to 95 parts by mass per 100 parts by mass of a mixture of the resin and the filler, the outer diameter of the conveying roller is 50 to 270 mm, the outer diameter of the knife coater is 50 to 170 mm, and the distance from the gap to the one end of the liquid reservoir member is 1 to 120 mm.

2. The coating method according to claim 1, wherein the content of the solvent is 10 to 50 parts by mass per 100 parts by mass of the mixture of the resin and the filler.

3. The coating method according to claim 1 or 2, wherein the viscosity of the coating liquid is 100 to 20,000 cP at a temperature of 25°C.

4. A coating method according to any one of claims 1 to 3, wherein the filler is a mixture of boron nitride and aluminum oxide, and the content of the boron nitride is 60 to 90 parts by mass per 100 parts by mass of the mixture of the boron nitride and aluminum oxide.

5. A sheet material comprising the substrate and the coating film according to any one of claims 1 to 4.