TECHNICAL FIELD

The present invention relates to: a stretchable wiring film which has high stretchability, high strength, and high water repellency, as well as less decrease in electric conductivity when the stretchable wiring film is stretched; and a method for forming the stretchable wiring film.

BACKGROUND ART

A recent growing popularity of Internet of Things (IoT) has accelerated the development of such major wearable devices as watches and glasses that allow for Internet access. Even in the fields of medicine and sports, wearable devices for constantly monitoring the user's physical state are increasingly demanded, and such technological development is expected to be further encouraged.

One typical wearable device is attached to the body of a user to constantly monitor the state of physical conditions. The wearable device normally includes a bio-electrode for detecting an electric signal transmitted from a body, wires for sending the electric signal to a sensor, a semiconductor chip serving as a sensor, and a battery, as well as an adhesive pad to be attached to the skin. Patent Document 1 describes detailed structures of a bio-electrode, a wiring part surrounding the bio-electrode, and an adhesive pad. The wearable device disclosed in Patent Document 1 includes a bio-electrode, a silicone-based adhesive film disposed around the bio-electrode, a sensor device, and a meandering-shaped stretchable silver wiring part coated with a stretchable urethane film between the bio-electrode and the sensor device, which are connected by the wiring part. Even if such a metal wire does not have stretchability by itself, the electric conductivity of the metal wire to be stretched can be secured by forming the wire in such a meandering shape that horseshoes are aligned, or by preparing a wrinkled substrate.

If a wire not in a meandering shape but in a straight shape is electrically conductive when stretched also, the wire area is more compact and the design will be more attractive. Hence, stretchable electro-conductive pastes and electrically conductive inks have been actively developed. Examples thereof include stretchable wirings using galinstan consisting of gallium-indium-tin or a liquid metal consisting of gallium-indium, an electro-conductive paste containing silicone mixed with silver particles (Patent Document 2), a mixture of a polyester resin and/or a polyurethane resin with metal particles (Patent Document 3), a mixture containing silver nanowires as a metal additive (Patent Document 4), a stretchable wiring containing a fluorine rubber, a surfactant, and a silver filler in combination in which silver nanoparticles are generated during annealing (Patent Document 5), and the like. Like these, there have been many proposals and applications most of which are electro-conductive pastes obtained by mixing stretchable resins with metal fillers such as silver.

An electro-conductive wire using an electro-conductive paste in which a resin is mixed with a metal filler is electrically conductive by electron percolation phenomenon among the electro-conductive particles. The electric conductivity of such an electro-conductive wire is decreased when the applied wire is stretched together with a substrate. When the wire is stretched, the distances of the metal particles in the wire are increased, thereby increasing the insulation distance. As a result, the percolation phenomenon hardly occurs, so that the electric conductivity is decreased.

Stretchable substrates for mounting stretchable wirings have also been developed. As the stretchable substrates, cloths using stretchable fibers and stretchable sheets have been examined. Examples of the stretchable sheets include silicone sheets and urethane sheets.

A urethane film has high stretchability and strength, and excellent mechanical properties as a film coated on a stretchable substrate or a stretchable wiring. Unfortunately, the hydrolysis inherent in the urethane film lowers its stretchability and strength. Meanwhile, the silicone film has no such hydrolytic nature, but the strength inherently remains low.

Hence, the use of silicone urethane polymers, whose main chain has both a urethane bond and a siloxane bond, has been examined. Advantageously, cured products of the silicone urethane polymer are characterized by higher strength than single silicone and less hydrolytic nature than single polyurethane. Such cured products unfortunately fail to achieve the strength equivalent to single polyurethane and the water repellency equivalent to single silicone, and the strength and water repellency are in-betweens of those inherent in silicone and polyurethane.

Highly stretchable urethane films tend to have a sticky top surface to the touch. The sticky top surface causes difficulty in separating films that are put together, and failure in printing since the film is stuck to a printing plate when screen printing is performed on this film. On the other hand, silicone films have high release characteristics and are prevented from sticking with each other thereby. However, due to the lower strength of silicone, thin silicone films easily break in stretching. When screen printing is performed on a silicone film, failure in printing due to sticking with a printing plate can be avoided, but the lower adhesion to paste causes peeling off of the cured paste. This comes from high release characteristics of a silicone top surface. On the other hand, urethane films have higher adhesion to paste and are prevented from peeling off of the cured paste.

Accordingly, it is desirable to develop a stretchable film which has higher stretchability, strength, and water repellency, and which enables printing of a stretchable electro-conductive paste on the stretchable film without peeling off of the printed paste.

CITATION LIST

Patent Literature

Patent Document 1: Japanese Patent Laid-Open Publication No. 2004-033468Patent Document 2: WO2016/204162Patent Document 3: Japanese Patent No. 6343903Patent Document 4: WO2017/217509 A1Patent Document 5: WO2018/110632 A1

SUMMARY OF INVENTION

Technical Problem

Due to these backgrounds, it has been demanded to develop: an electro-conductive paste, a printing pattern, a printing method, and a stretchable substrate which prevent the electric conductivity from decreasing in stretching.

In view of the circumstances, the present invention aims to provide: a stretchable wiring film having less decrease in electric conductivity in stretching and excellent water repellency on the film top surface; and a method for forming the stretchable wiring film.

Solution to Problem

To achieve the object, the present invention provides a stretchable wiring film comprising:

(A) a stretchable film comprising, at least as a top surface of the stretchable film, a cured product of a stretchable film material comprising a silicone polyurethane resin, wherein the top surface of the stretchable film has a repeated uneven pattern formed with depths of 0.1 μm to 5 mm and pitches of 0.1 μm to 10 mm; and

(B) a stretchable wiring, wherein

the stretchable wiring is formed on the top surface of the stretchable film where the repeated uneven pattern is formed.

Such a stretchable wiring film has less decrease in electric conductivity in stretching and has excellent water repellency on the film top surface.

The stretchable wiring preferably contains an electro-conductive powder made of carbon or a metal selected from gold, silver, platinum, copper, tin, titanium, nickel, aluminum, tungsten, molybdenum, ruthenium, chromium, and indium.

In the inventive stretchable wiring film, such a stretchable wiring is favorably used.

Moreover, the silicone polyurethane resin is preferably a silicone-pendant type polyurethane resin having a structure shown by the following general formula (1):


wherein R1, R2, and R3may be identical to or different from each other and each represent a linear, branched, or cyclic alkyl group having 1 to 6 carbon atoms, a phenyl group, or a 3,3,3-trifluoropropyl group; R4's may be identical to or different from each other and each represent a linear, branched, or cyclic alkyl group having 1 to 6 carbon atoms, a phenyl group, a 3,3,3-trifluoropropyl group, or a —(OSiR1R2)s—OSiR1R2R3group; R5represents a hydrogen atom or a linear or branched alkyl group having 1 to 4 carbon atoms; R6represents a single bond, a methylene group, or an ethylene group; R7represents a hydrogen atom or a methyl group; X represents a linear or branched alkylene group having 3 to 7 carbon atoms and optionally containing an ether group; “q”, “r”, and “s” each represent an integer in a range of 0 to 20; and a1 and a2 represent proportions of repeating units and satisfy ranges of 0≤a1<1.0, 0≤a2<1.0, and 0<a1+a2≤1.0.

Such a silicone polyurethane resin enables the stretchable wiring film with more excellent strength.

Further, the silicone-pendant type polyurethane resin preferably has a structure shown by the following general formula (2):


wherein R1to R7, X, “q”, “r”, a1, and a2 are as defined above; R8to R17may be identical to or different from each other and each represent a linear, branched, or cyclic alkylene group having 2 to 12 carbon atoms, or an arylene group having 6 to 12 carbon atoms; “m”s may be identical to or different from each other and each represent 1 to 200; and b1, b2, b3, and b4 represent proportions of repeating units and satisfy ranges of 0≤b1<1.0, 0≤b2<1.0, 0≤b3<1.0, 0≤b4<1.0, and 0<b1+b2+b3+b4<1.0.

Such a silicone-pendant type polyurethane resin makes the stretchable wiring film have further improved strength and the film top surface cause less sticking.

Furthermore, the silicone-pendant type polyurethane resin preferably has a structure shown by the following general formula (3):


wherein R1to R17, X, “m”, “q”, “r”, a1, a2, b1, b2, b3, and b4 are as defined above; R18represents a linear, branched, or cyclic alkylene group having 1 to 20 carbon atoms and optionally having an ether group or an ester group; R19represents a hydrogen atom or a methyl group; and “c” represents the number of unit in one molecule and satisfies a range of 1≤c≤4.

Such a silicone-pendant type polyurethane resin is particularly suitably cured by heating and/or light irradiation.

The stretchable wiring film preferably has an elongation percentage in a range of 5 to 500%.

Such a stretchable wiring film can be favorably used for a bio-electrode, for example.

The stretchable wiring film preferably comprises a stretchable film configured to cover a stretchable wiring.

The inventive stretchable wiring film may have such a configuration.

The stretchable film configured to cover a stretchable wiring is preferably a cured product of a stretchable film material comprising the silicone-pendant type polyurethane resin having a structure shown by the general formula (1).

In such a stretchable wiring film, the stretchable wiring is covered with the stretchable film having higher strength.

The present invention also provides a method for forming a stretchable wiring film, comprising the steps of:

(1) applying a stretchable film material comprising a silicone-pendant type polyurethane resin having a structure shown by the following general formula (1) onto a substrate having a repeated uneven pattern formed with depths of 0.1 μm to 5 mm and pitches of 0.1 μm to 10 mm;

(2) curing the stretchable film material by heating and/or light irradiation;

(3) peeling a cured product of the stretchable film material from the substrate to form a stretchable film having a top surface with the repeated uneven pattern; and

(4) applying a stretchable electro-conductive paste onto the top surface of the stretchable film where the repeated uneven pattern is formed to form a stretchable wiring,


wherein R1, R2, and R3may be identical to or different from each other and each represent a linear, branched, or cyclic alkyl group having 1 to 6 carbon atoms, a phenyl group, or a 3,3,3-trifluoropropyl group; R4's may be identical to or different from each other and each represent a linear, branched, or cyclic alkyl group having 1 to 6 carbon atoms, a phenyl group, a 3,3,3-trifluoropropyl group, or a —(OSiR1R2)s—OSiR1R2R3group; R5represents a hydrogen atom or a linear or branched alkyl group having 1 to 4 carbon atoms; R6represents a single bond, a methylene group, or an ethylene group; R7represents a hydrogen atom or a methyl group; X represents a linear or branched alkylene group having 3 to 7 carbon atoms and optionally containing an ether group; “q”, “r”, and “s” each represent an integer in a range of 0 to 20; and a1 and a2 represent proportions of repeating units and satisfy ranges of 0≤a1<1.0, 0≤a2<1.0, and 0<a1+a2≤1.0.

Such a method for forming a stretchable wiring film makes it possible to easily form a stretchable wiring film which has less decrease in electric conductivity in stretching and excellent water repellency on the film top surface.

In this event, the inventive method may comprise, between the step (1) and the step (2), (1′) a step of pressure-bonding a polyurethane film onto the stretchable film material.

Alternatively, the inventive method may comprise, between the step (2) and the step (3),

(2′-1) a step of coating the cured product of the stretchable film material with a stretchable film material containing a polyurethane resin, and

(2′-2) a step of curing the stretchable film material containing the polyurethane resin by heating and/or light irradiation to form a polyurethane film.

Such methods for forming a stretchable wiring film make it possible to easily produce a stretchable wiring film including a stretchable film which is a laminate of: the top surface having the repeated uneven pattern; and the other portion having a different composition from that of the top surface.

The stretchable electro-conductive paste preferably contains an electro-conductive powder made of carbon or a metal selected from gold, silver, platinum, copper, tin, titanium, nickel, aluminum, tungsten, molybdenum, ruthenium, chromium, and indium.

The inventive method for forming a stretchable wiring film makes it possible to form a stretchable wiring film in which the stretchable wiring containing such an electro-conductive powder is formed.

Advantageous Effects of Invention

As described above, the inventive stretchable wiring film has less decrease in electric conductivity when stretched. The stretchable film on which the conductive wiring is formed has excellent stretchability and strength equivalent to those of polyurethane, and the film top surface is free from sticking when touched and super water repellent so that water drops can roll on the film top surface. Moreover, in the present invention, the presence of the unevenness on the top surface of the layer based on the silicone pendant polyurethane in the stretchable film causes super water repellent phenomenon. The inventive stretchable wiring film not only makes water droplets hardly adhere thereto but also can reduce the adhesion of foreign matters, and the top surface of the inventive stretchable wiring film is free from sticking and has favorable texture. Further, when a conductive wiring is formed on this uneven substrate, a fine meandering-shaped structure is formed in a depth direction of the film. This makes it possible to prevent the electric conductivity from decreasing in stretching. Accordingly, the inventive stretchable wiring film is particularly favorably usable as a stretchable film including a wiring part for connecting a bio-electrode to a sensor for a wearable device. Furthermore, the inventive method for forming a stretchable wiring film makes it possible to easily form a stretchable wiring film as described above.

