JP2005037017A

JP2005037017A - Drying method

Info

Publication number: JP2005037017A
Application number: JP2003198799A
Authority: JP
Inventors: Eiji Aizawa; 栄次相澤
Original assignee: Dai Nippon Printing Co Ltd
Current assignee: Dai Nippon Printing Co Ltd
Priority date: 2003-07-17
Filing date: 2003-07-17
Publication date: 2005-02-10

Abstract

<P>PROBLEM TO BE SOLVED: To provide a drying method capable of preventing the disorder of a coating film surface, and obtaining a precise coating surface. <P>SOLUTION: The temperature difference between a face side and a back side of a coating film is suppressed to be small by heating a base material 1 not only from a coating surface side but also from a back surface side, and the coating film is dried under a condition that the Marangoni number Ma and the Rayleigh number Ra in the coating film do not exceed respective critical numbers. Disorder of the coating film surface is prevented to obtain a precise coating surface by suppressing generation of Marangoni convection and Benard convection in the coating film. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、長尺フィルム、枚葉の薄板などの基材の表面に塗られた塗膜を乾燥する方法に関し、特に、エレクトロニクス製品、光学製品、ディスプレイ製品などのように、塗膜の精密乾燥が必要な分野で用いるのに好適な乾燥方法に関する。
【０００２】
【従来の技術】
基材に塗布した塗布液の乾燥には、図２に示すように、基材１を、その塗布面を上にしてガイドロール２によって水平に走行するように案内し、基材１の走行経路の上方に配置した複数の熱風ノズル３から熱風を塗膜表面に吹き付ける方法が広く使用されている。しかしながら、この方法では、熱風が塗膜に直接吹き付けられるため、熱風によって塗膜表面に凹凸が生じ、面質が悪くなることがあり、カラーフィルタや液晶のように塗膜表面をきわめて平滑な状態に乾燥する必要のある用途には適さない。そこで、塗膜の精密乾燥を行う方法として、図３、図４に示すように、給気装置４と排気装置５によって熱風を基材１の進行方向と平行に並流又は向流で吹く方法が用いられている。また、基材１の下面側から赤外線ヒータによって基材を加熱することによって塗膜を乾燥させる方法も知られている。
【０００３】
【発明が解決しようとする課題】
しかしながら、これらの乾燥方法では、塗膜の面質をかなり向上させることができたが、やはり、図５（ｃ）に誇張して示すように、乾燥後の塗膜６の表面面質が悪くなり、膜厚が不均一となることがあった。
【０００４】
本発明者らは乾燥時に塗膜に生じる膜厚不均一の原因を鋭意検討の結果、次の事項を見出した。すなわち、塗膜表面を熱風で加熱、乾燥させる方法では、図５（ａ）に示すように、乾燥すべき塗膜７の表面のみに熱を与えているため、塗膜７の表面７ａの温度が上昇し、表面７ａと底面７ｂで温度差が発生する。液体は、温度が上昇するにつれ、表面張力が低下するので、乾燥中に発生する塗膜７の表面７ａと底面７ｂとの間の温度差により、塗膜７の表面７ａと底面７ｂとの間で表面張力差が発生する。これにより、塗膜７内に対流（マランゴニ対流）が発生し、乾燥後の塗膜表面に悪影響を与え、膜厚の不均一を生じていた。一方、図５（ｂ）に示すように、基材７を裏面側から赤外線等によって加熱した場合は、塗膜７の裏面７ｂの温度上昇により表面張力の他に密度が低下する。そのため、マランゴニ対流以外に、塗膜７内には密度差による対流（ベナール対流）が発生し、乾燥後の塗膜表面に悪影響を与え、膜厚の不均一を生じていた。
【０００５】
本発明はかかる知見に基づいてなされたもので、乾燥中の塗膜内に生じる恐れのあるマランゴニ対流及びベナール対流を防止して、塗膜の精密乾燥を行うことを課題とする。
【０００６】
【課題を解決するための手段】
本発明は上記課題を解決するため、マランゴニ対流の発生しやすさを示す値であるマランゴニ数Ｍａ及びベナール対流の発生しやすさを示す値であるレイリー数Ｒａの両方が、各々の臨界数を越えない条件を満たす温度条件で乾燥を行うという構成としたものであり、この構成により、乾燥中に塗膜内でマランゴニ対流もベナール対流も発生せず、精密な乾燥が可能となる。
【０００７】
上記した温度条件を確保するための加熱方法には種々な方法を取り得るが、好ましい方法としては、基材を塗布面側から熱風で加熱し、裏面側から赤外線ヒータで加熱する方法を挙げることができる。この方法を用いることで、塗布面を熱風で加熱すると共に塗布面から発生した溶剤蒸気を熱風で敏速に除去でき、また、裏面側からは赤外線ヒータによって敏速に加熱することができ、塗膜の乾燥速度を高めることができる。
【０００８】
【発明の実施の形態】
本発明の乾燥方法の実施に用いる乾燥装置も、基本的な形態は、図１に示すように、基材１を、その塗布面を上にしてガイドロール２によって水平に走行するように案内し、走行中の基材１を加熱して塗膜の乾燥を行うものである。この乾燥方式において、塗膜におけるマランゴニ数Ｍａは以下の式で表される。
【０００９】
【数１】

【００１０】
臨界マランゴニ数は８０であることから、塗膜の表面と裏面の温度差ΔＴは以下のように求められる。
【００１１】
【数２】

【００１２】
レイリー数Ｒａは以下の式で表される。
【００１３】
【数３】

【００１４】
臨界レイリー数Ｒａは１７００であることから、ΔＴは以下のように求められる。
【００１５】
【数４】

【００１６】
本発明では、基材１の塗膜に対する乾燥条件の設定に当たって、乾燥速度を考慮して塗膜の加熱温度を定めるのみならず、上記式（１）、（２）を満たすように、塗膜の表面と裏面の温度差ΔＴを設定し、その乾燥条件で乾燥を行う。これにより、乾燥中、塗膜内におけるマランゴニ数Ｍａ及びレイリー数Ｒａの両方が、各々の臨界数を越えることがなく、従って、塗膜内でマランゴニ対流もベナール対流も発生せず、精密な乾燥を行うことができる。
【００１７】
ここで、上記式（１）、（２）を満たす温度差ΔＴの設定は次のように行えば良い。すなわち、式（１）、（２）における臨界温度差ΔＴｍ、ΔＴｒは、塗布液の物性、塗膜の膜厚、塗膜の温度等によって定まるので、あらかじめ、塗布液の表面張力、粘度の温度依存性、濃度依存性を動的表面張力計、レオメータ等にてあらかじめ測定しておき、それを基にして、塗膜膜厚、乾燥時の塗膜温度等の条件を当てはめて、上記式（１）、（２）から臨界温度差ΔＴｍ、ΔＴｒを計算する。そして、塗膜の表面と裏面の温度差ΔＴを、求めた臨界温度差ΔＴｍ、ΔＴｒよりも小さい値に設定すればよい。
【００１８】
ところで、上記式（１）、（２）に規定する臨界温度差ΔＴｍ、ΔＴｒには、粘度μ、動粘度νが入っており、この物性は塗膜の乾燥程度に応じて大きく変動する。従って、上記式（１）、（２）で求める臨界温度差ΔＴｍ、ΔＴｒは、乾燥装置を通過中の基材の位置に応じて異なっている。具体的には、乾燥開始領域では乾燥が進んでいないため、粘度μ、動粘度νがきわめて小さく、このため臨界温度差ΔＴｍ、ΔＴｒが小さくなり、乾燥が進むにつれ、臨界温度差ΔＴｍ、ΔＴｒは大きくなる。乾燥条件の設定に当たっては、塗膜の表面と裏面の温度差ΔＴが乾燥装置内の全域において、上記した臨界温度差ΔＴｍ、ΔＴｒを越えないように設定するが、この際、この温度差ΔＴは、乾燥装置内の全域において一定となるように設定してもよいし、乾燥装置内での位置によって異なるように設定してもよい。例えば、温度差ΔＴを、乾燥装置内の全域において一定となるように設定する場合には、乾燥開始領域における臨界温度差ΔＴｍ、ΔＴｒを求め、その値を越えないように、温度差ΔＴを設定し、その温度差ΔＴを乾燥装置内の全域に適用すればよい。また、温度差ΔＴを乾燥装置内の位置によって異なるように設定する場合には、例えば乾燥装置内を、図１に示すように前半領域、中盤領域、後半領域に分け、各領域において、それぞれの入口部分における臨界温度差ΔＴｍ、ΔＴｒを求め、その値を越えないように、温度差ΔＴを設定し、その温度差ΔＴを各領域内に適用すればよい。
【００１９】
上記したように、本発明では、塗膜の表面と裏面の温度差ΔＴを上記式（１）、（２）を満たすように設定して乾燥を行う。この乾燥条件を確保するには、塗膜を基材の一方の面からの加熱のみでは困難である。そこで、本発明の実施に当たっては、水平に走行している基材１に対して、その上下面から加熱を行う。基材の上下面から加熱に用いる加熱手段は任意であるが、基材上面の加熱は、図１に示すように、基材１を塗布面側から熱風で加熱することが、塗布面の加熱のみならず塗布面から発生した溶剤を敏速に除去できるので、好ましい。この際、熱風が塗膜表面の面質を低下させないよう、熱風は極力基材１に対して平行に流すことが望ましい。熱風の方向は基材１に対して並流でも向流でもよい。また、乾燥装置内を、例えば、前半領域、中盤領域、後半領域に分けて温度条件をそれぞれ別個に設定可能としたい場合には、乾燥装置内を各領域に分けると共に各領域にそれぞれ給気装置１２、排気装置１３を設ければ良い。一方、基材１の下面側からの加熱は赤外線ヒータ１５によって行うことが、装置が簡単で且つ加熱効率が良いので、好ましい。
【００２０】
【実施例】
図１に示す乾燥装置を用い、次の乾燥条件で乾燥テストを行い、乾燥後について塗膜表面を検査した。その結果を表１に示す。乾燥装置内の前半領域、中盤領域、後半領域の温度条件は同一とした。また、乾燥の初期領域におけるマランゴニ数Ｍａ、レイリー数Ｒａを計算により求めた。これも表１に示す。
［乾燥条件］
・基材：ＰＥＴフィルム（厚さ１００μｍ）
・塗布液：インキ（主溶剤はＭＥＫ）
・塗布厚：塗布直後１００μｍ
・基材走行速度：３ｍ／分
・熱風温度（塗布面の表面温度に等しいと想定される）：５０°Ｃ
・赤外線ヒータ１０による基材加熱温度：７０°Ｃ、６０°Ｃ、５５°Ｃ
【００２１】
【表１】

【００２２】
表１から分かるように、テスト３では、マランゴニ数Ｍａ、レイリー数Ｒａが共に臨界数よりも小さくなっており、塗布面の品質は良好であった。
【００２３】
【発明の効果】
以上のように、本発明は、乾燥中における塗膜のマランゴニ数Ｍａ及びレイリー数Ｒａが共に臨界数を越えない温度条件で乾燥を行うことにより、塗膜内でのマランゴニ対流及びベナール対流の発生を抑制でき、塗膜内での対流による塗膜表面の乱れを防ぎ、精密な塗工面が得られるという効果を有している。
【図面の簡単な説明】
【図１】本発明方法の実施に用いる乾燥装置の１例を示す概略側面図
【図２】従来の乾燥装置の１例を示す概略側面図
【図３】従来の乾燥装置の他の例を示す概略側面図
【図４】従来の乾燥装置の更に他の例を示す概略側面図
【図５】（ａ）、（ｂ）は塗膜に生じる現象を説明する概略断面図、（ｃ）は乾燥後の塗膜表面に生じる凹凸を誇張して示す概略断面図
【符号の説明】
１基材
２ガイドロール
３熱風ノズル
４給気装置
５排気装置
６、７塗膜
１２給気装置
１３排気装置
１５赤外線ヒータ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for drying a coating film applied to the surface of a substrate such as a long film or a sheet of thin sheet, and in particular, precise drying of a coating film, such as an electronic product, an optical product, or a display product. The present invention relates to a drying method suitable for use in a field where the
[0002]
[Prior art]
As shown in FIG. 2, the coating liquid applied to the base material is dried by guiding the base material 1 so that the base material 1 travels horizontally by the guide roll 2 with its application surface facing upward. A method of spraying hot air from a plurality of hot air nozzles 3 disposed on the surface of the coating film is widely used. However, in this method, since hot air is blown directly onto the coating film, the hot air may cause irregularities on the surface of the coating film, resulting in poor surface quality, and the surface of the coating film may be extremely smooth like a color filter or liquid crystal. It is not suitable for applications that require drying. Therefore, as a method for precisely drying the coating film, as shown in FIGS. 3 and 4, a method in which hot air is blown in parallel or countercurrent with the air supply device 4 and the exhaust device 5 in parallel with the traveling direction of the substrate 1. Is used. Also known is a method of drying a coating film by heating the substrate with an infrared heater from the lower surface side of the substrate 1.
[0003]
[Problems to be solved by the invention]
However, these drying methods can considerably improve the surface quality of the coating film, but the surface quality of the coating film 6 after drying is poor as shown in FIG. 5C. Thus, the film thickness may be non-uniform.
[0004]
As a result of intensive studies on the cause of film thickness non-uniformity occurring in the coating film during drying, the present inventors have found the following matters. That is, in the method of heating and drying the coating film surface with hot air, as shown in FIG. 5A, only the surface of the coating film 7 to be dried is heated, so the temperature of the surface 7a of the coating film 7 Rises and a temperature difference occurs between the front surface 7a and the bottom surface 7b. Since the surface tension of the liquid decreases as the temperature rises, the temperature difference between the surface 7a and the bottom surface 7b of the coating film 7 generated during drying causes a difference between the surface 7a and the bottom surface 7b of the coating film 7. The surface tension difference occurs. As a result, convection (Marangoni convection) was generated in the coating film 7, adversely affecting the surface of the coating film after drying, and non-uniform film thickness. On the other hand, as shown in FIG. 5B, when the base material 7 is heated from the back surface side by infrared rays or the like, the density decreases in addition to the surface tension due to the temperature increase of the back surface 7b of the coating film 7. Therefore, in addition to Marangoni convection, convection (Benard convection) due to density difference occurs in the coating film 7, adversely affects the coating film surface after drying, and causes non-uniform film thickness.
[0005]
The present invention has been made based on such knowledge, and an object of the present invention is to prevent Marangoni convection and Benard convection that may occur in a coating film during drying and to precisely dry the coating film.
[0006]
[Means for Solving the Problems]
In order to solve the above-described problems, the present invention has both the Marangoni number Ma, which is a value indicating the likelihood of occurrence of Marangoni convection, and the Rayleigh number Ra, which is a value indicating the likelihood of occurrence of Benard convection, of each critical number. The drying is performed under a temperature condition that does not exceed the temperature. With this configuration, neither Marangoni convection nor Benard convection occurs in the coating film during drying, and precise drying is possible.
[0007]
Although various methods can be taken as the heating method for ensuring the above-described temperature condition, a preferable method is to heat the substrate with hot air from the coating surface side and with an infrared heater from the back surface side. Can do. By using this method, the coated surface can be heated with hot air and the solvent vapor generated from the coated surface can be quickly removed with hot air, and from the back side, it can be quickly heated with an infrared heater. The drying rate can be increased.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
The drying apparatus used for carrying out the drying method of the present invention, as shown in FIG. 1, is basically guided so that the substrate 1 travels horizontally by the guide roll 2 with its application surface facing up. The coated substrate is dried by heating the traveling substrate 1. In this drying method, the Marangoni number Ma in the coating film is expressed by the following equation.
[0009]
[Expression 1]

[0010]
Since the critical Marangoni number is 80, the temperature difference ΔT between the front surface and the back surface of the coating film is obtained as follows.
[0011]
[Expression 2]

[0012]
The Rayleigh number Ra is expressed by the following equation.
[0013]
[Equation 3]

[0014]
Since the critical Rayleigh number Ra is 1700, ΔT is obtained as follows.
[0015]
[Expression 4]

[0016]
In the present invention, in setting the drying conditions for the coating film of the substrate 1, not only the heating temperature of the coating film is determined in consideration of the drying speed, but also the coating film so as to satisfy the above formulas (1) and (2). A temperature difference ΔT between the front surface and the back surface is set, and drying is performed under the drying conditions. Thereby, during drying, both the Marangoni number Ma and the Rayleigh number Ra in the coating film do not exceed the respective critical numbers. Therefore, neither Marangoni convection nor Benard convection occurs in the coating film, and precise drying is performed. It can be performed.
[0017]
Here, the temperature difference ΔT satisfying the above equations (1) and (2) may be set as follows. That is, since the critical temperature differences ΔTm and ΔTr in the equations (1) and (2) are determined by the physical properties of the coating liquid, the film thickness of the coating film, the temperature of the coating film, etc., the surface tension and viscosity temperature of the coating liquid are determined in advance. Dependence, concentration dependence is measured in advance with a dynamic surface tension meter, rheometer, etc., and based on this, the conditions such as coating film thickness, coating film temperature during drying are applied, and the above formula ( The critical temperature differences ΔTm and ΔTr are calculated from 1) and (2). And what is necessary is just to set the temperature difference (DELTA) T of the surface of a coating film and a back surface to a value smaller than calculated | required critical temperature difference (DELTA) Tm and (DELTA) Tr.
[0018]
By the way, the critical temperature differences ΔTm and ΔTr defined in the above formulas (1) and (2) contain a viscosity μ and a kinematic viscosity ν, and their physical properties vary greatly depending on the degree of drying of the coating film. Therefore, the critical temperature differences ΔTm and ΔTr obtained by the above formulas (1) and (2) differ depending on the position of the base material passing through the drying apparatus. Specifically, since the drying has not progressed in the drying start region, the viscosity μ and the kinematic viscosity ν are extremely small. Therefore, the critical temperature differences ΔTm and ΔTr become small, and as the drying proceeds, the critical temperature differences ΔTm and ΔTr are growing. In setting the drying conditions, the temperature difference ΔT between the front surface and the back surface of the coating film is set so as not to exceed the above-described critical temperature differences ΔTm, ΔTr in the entire area of the drying apparatus. At this time, the temperature difference ΔT is It may be set so as to be constant throughout the entire area of the drying apparatus, or may be set differently depending on the position within the drying apparatus. For example, when the temperature difference ΔT is set to be constant throughout the entire area of the drying apparatus, the critical temperature differences ΔTm and ΔTr in the drying start region are obtained, and the temperature difference ΔT is set so as not to exceed the value. The temperature difference ΔT may be applied to the entire area in the drying apparatus. When the temperature difference ΔT is set differently depending on the position in the drying apparatus, for example, the interior of the drying apparatus is divided into a first half area, a middle board area, and a second half area as shown in FIG. The critical temperature differences ΔTm and ΔTr at the inlet portion are obtained, the temperature difference ΔT is set so as not to exceed the values, and the temperature difference ΔT is applied to each region.
[0019]
As described above, in the present invention, drying is performed by setting the temperature difference ΔT between the front surface and the back surface of the coating film so as to satisfy the above formulas (1) and (2). In order to ensure this drying condition, it is difficult to heat the coating film only from one side of the substrate. Therefore, in carrying out the present invention, the base material 1 traveling horizontally is heated from its upper and lower surfaces. The heating means used for heating from the upper and lower surfaces of the base material is arbitrary, but the heating of the upper surface of the base material can be performed by heating the base material 1 with hot air from the application surface side as shown in FIG. In addition, the solvent generated from the coated surface can be removed quickly, which is preferable. At this time, it is desirable that the hot air flow as parallel to the substrate 1 as possible so that the hot air does not deteriorate the surface quality of the coating film surface. The direction of the hot air may be parallel or countercurrent to the substrate 1. In addition, when it is desired to separately set the temperature conditions by dividing the inside of the drying device into, for example, the first half region, the middle plate region, and the second half region, the inside of the drying device is divided into each region and the air supply device is provided in each region 12, Exhaust device 13 may be provided. On the other hand, heating from the lower surface side of the substrate 1 is preferably performed by the infrared heater 15 because the apparatus is simple and the heating efficiency is good.
[0020]
【Example】
Using the drying apparatus shown in FIG. 1, a drying test was performed under the following drying conditions, and the surface of the coating film was inspected after drying. The results are shown in Table 1. The temperature conditions of the first half area, the middle board area, and the second half area in the drying apparatus were the same. Further, the Marangoni number Ma and the Rayleigh number Ra in the initial dry region were obtained by calculation. This is also shown in Table 1.
[Drying conditions]
・ Base material: PET film (thickness 100 μm)
・ Coating liquid: Ink (Main solvent is MEK)
・ Application thickness: 100μm immediately after application
-Substrate traveling speed: 3 m / min-Hot air temperature (assumed to be equal to the surface temperature of the coated surface): 50 ° C
-Substrate heating temperature by infrared heater 10: 70 ° C, 60 ° C, 55 ° C
[0021]
[Table 1]

[0022]
As can be seen from Table 1, in Test 3, the Marangoni number Ma and the Rayleigh number Ra were both smaller than the critical number, and the quality of the coated surface was good.
[0023]
【The invention's effect】
As described above, the present invention generates Marangoni convection and Benard convection in a coating film by performing drying at a temperature condition in which both the Marangoni number Ma and the Rayleigh number Ra of the coating film do not exceed the critical number during drying. It is possible to suppress turbulence of the coating film surface due to convection in the coating film and to obtain a precise coating surface.
[Brief description of the drawings]
FIG. 1 is a schematic side view showing an example of a drying apparatus used for carrying out the method of the present invention. FIG. 2 is a schematic side view showing an example of a conventional drying apparatus. FIG. 3 is another example of a conventional drying apparatus. FIG. 4 is a schematic side view showing still another example of a conventional drying apparatus. FIGS. 5A and 5B are schematic cross-sectional views for explaining a phenomenon occurring in a coating film, and FIG. Schematic sectional view exaggerating the unevenness that occurs on the coating surface after drying [Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Base material 2 Guide roll 3 Hot air nozzle 4 Air supply apparatus 5

Exhaust apparatus

6, 7 Coating film 12 Air supply apparatus 13 Exhaust apparatus 15 Infrared heater

Claims

基材に塗られた塗布液の乾燥において、乾燥中の塗膜におけるマランゴニ数及びレイリー数の各々が臨界値を越えない条件を満たす温度条件で乾燥を行うことを特徴とする乾燥方法。A drying method comprising drying a coating solution applied to a substrate under a temperature condition that satisfies a condition that each of a Marangoni number and a Rayleigh number in a coating film being dried does not exceed a critical value.

基材を塗布面側から熱風で加熱し、裏面側から赤外線ヒータで加熱することを特徴とする請求項１記載の乾燥方法。The drying method according to claim 1, wherein the substrate is heated with hot air from the coated surface side and heated with an infrared heater from the back surface side.