KR20180104490A - Transparent cover-lay film having low elestic modulus in which both side adhesion is possible and flexible printed circuit board including the same - Google Patents

Abstract

The present invention relates to a transparent cover-lay film having low elasticity and to a manufacturing method thereof, wherein an adhesive having low elasticity properties is included in the transparent cover-lay film, so that flexibility is ensured because the modulus of elasticity is lower than the modulus of elasticity of conventional cover-lay films and a colorless transparent cover-lay film and a flexible printed circuit board (FPCB) using the same can be provided.

Description

[0001] The present invention relates to a low-elasticity transparent coverlay film capable of double-sided adhesion and a flexible cover-lay film having the same,

The present invention relates to a transparent coverlay film and a FPCB having the same. More particularly, the present invention relates to a coverlay film which can secure both transparency and flexibility while ensuring flexibility because of low elastic modulus of a conventional coverlay film, And an FPCB having the same.

2. Description of the Related Art In recent years, there has been an increasing need for printed circuit boards (PCBs) that are easy to be embedded even in a narrower space due to integration, miniaturization, thinning, high density and high bending of electronic products. Accordingly, a Flexible Printed Circuit Board (FPCB) capable of miniaturization and high density and having repeated flexing has been developed. Such a flexible printed circuit board is being used more rapidly due to the technological development of smart phones, portable mobile electronic devices (smart watches, smart glasses, etc.), and the demand thereof is increasing.

Generally, in order to manufacture a flexible printed circuit board, a flexible film is formed on a flexible copper clad laminate in which a copper foil layer is formed on both sides or an end face of an insulating base film such as polyimide having high heat resistance and high flexibility, a dry film is laminated on the surface of the substrate and then a circuit pattern is formed by exposure, development and etching in sequence, and then a coverlay film is bonded to the outside of the circuit pattern and hot- A circuit board is manufactured.

The flexible copper-clad laminate is composed of a three-layer type composed of three layers of a copper foil, a polyimide base film and an epoxy thermosetting adhesive layer, a two-layer type having a polyimide-based adhesive which is homogeneous with the polyimide base film, There is also a two-layer type that does not have an adhesive layer. In addition, the coverlay is laminated for the purpose of protecting the copper wiring of the copper-clad laminate, and is generally composed of a polyimide base film and an epoxy thermosetting adhesive layer. At this time, in order to exhibit a stable performance, an adhesive composition mainly composed of an epoxy resin having good heat resistance and chemical resistance has been widely used as an adhesive for use in an electronic material such as a semiconductor sealing material or an epoxy-based FPCB.

Korean Patent Application No. 2014-0084415 of the prior art discloses a non-halogen fast curing adhesive composition and a coverlay film using the same, and Korean Patent Application No. 2012-0117438 discloses a method of producing a flexible printed circuit board .

However, since the coverlay film including such an adhesive composition is not ensured in flexibility due to excessive rigidity, the FPCB including the coverlay is manufactured, and the fairness and reliability of parts mounting on a wearable device, I had a problem.

In addition, the polyimide layer and the adhesive layer constituting the conventional coverlay film are opaque yellow. Recently, display-equipped products and / or electronic parts have been increasingly required to have transparency for internal parts together with flexible and aesthetic reasons, It is expected that devices will create high value-added markets in various fields such as home, industrial, and education. FPCB should also be developed as a transparent base for marketable transparent displays. Especially, Resistor wiring is required. However, even in a technology-leading country, transparent FPCB is being developed, but the formation of low resistance transparent wiring using Cu has not yet been attempted.

Transparent FPCB is essential for high transparency, but it requires high-temperature solder reflow process when it is connected with various devices. Therefore, it is necessary to secure high heat resistance of transparent material, and it is necessary to manufacture technology compatible with PCB process and roll- ) Mass production is absolutely necessary to reduce the price of transparent FPCB.

In order to manufacture such a transparent FPCB, a fine circuit forming technique or the like is applied. Therefore, there is a growing need to develop a colorless transparent cover film material while satisfying rigidity and flexibility at the same time.

Korean Patent Publication No. 10-2014-0084415 (published July 2014.07)

Disclosure of Invention Technical Problem [8] Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a coverlay film capable of double- .

In order to solve the above-described problems, the low-elasticity transparent coverlay film of the present invention has transparent adhesive layers (transparent first adhesive layer and transparent second adhesive layer) formed on both sides of the transparent substrate layer.

According to a preferred embodiment of the present invention, the thickness of the transparent base layer is 5 占 퐉 to 25 占 퐉, the thickness of the transparent first adhesive layer and the thickness of the transparent second adhesive layer are 5 占 퐉 to 50 Lt; / RTI >

According to a preferred embodiment of the present invention, the low elastic clear coverlay film of the present invention may have a metal foil layer, a transparent first adhesive layer, a transparent substrate layer, and a transparent second adhesive layer sequentially laminated.

According to a preferred embodiment of the present invention, the transparent first adhesive layer and the transparent second adhesive layer may be semi-cured.

According to a preferred embodiment of the present invention, the low elastic transparent cover lay film is the transparent average thickness of the first adhesive layer of the invention (P _1), the average thickness of the transparent substrate layer (P ₂₎ and the transparent second the average thickness of the adhesive layer (P ₃₎ to the equation (1) or a satisfies the equation (2), wherein the transparent first adhesive layer and the transparent first at least one layer of the second adhesive layer is the elastic modulus of 0.1 ~ 2.0Gpa upon curing, And may have a modulus of elasticity of preferably 0.5 to 1.5Gpa.

[Equation 1]

0.1? (P ₁ + P ₂ ) / P ₃ ? 10.0

In the above equation (1), P ₃ is a rational number satisfying 5 μm ≦ P ₃ ≦ 50 μm.

&Quot; (2) "

0.7? (P ₁ + P ₂ ) / P ₃ ? 4.0

In the above equation (2), P ₃ is a rational number satisfying 5 μm ≦ P ₃ ≦ 50 μm.

According to a preferred embodiment of the present invention, the transparent substrate layer may include at least one selected from transparent polyimide (PI) resin, transparent polyethylene terephthalate (PET) resin and transparent polyethylene naphthalate (PEN) resin .

According to a preferred embodiment of the present invention, the transparent PI resin is a dianhydride including at least one selected from the group consisting of dianhydrides represented by the following formula (1); And diamines represented by the following formula (2): ## STR2 ## wherein R 1 is a hydrogen atom or a methyl group;

[Chemical Formula 1]

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또는

이며,In the formula 1, wherein R ₁ is

,

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,

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or

Lt;

(2)

H ₂ NR ₂ -NH ₂

,

,

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,

또는

이다.In formula (2), R ₂ is

,

,

,

,

,

,

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or

to be.

According to a preferred embodiment of the present invention, the transparent adhesive layer (transparent first adhesive layer and / or second adhesive layer) comprises an epoxy thermosetting resin, an acrylic binder resin, a curing agent, a catalyst, a filler and a silane coupling agent And a transparent acrylic adhesive.

According to a preferred embodiment of the present invention, the epoxy thermosetting resin may include a cycloaliphatic epoxy resin, and the cycloaliphatic epoxy resin may include bisphenol-A type epoxy, bisphenol F Bisphenol-F type epoxy, bisphenol-S type epoxy, bisphenol-AD type epoxy, phenol novolac epoxy, cresol novolac epoxy, Novolac Epoxy, bisphenol-A Novolac type epoxy, dicycolpentadiene type epoxy and multi-functional novolac type epoxy. . ≪ / RTI >

According to a preferred embodiment of the present invention, the acrylic binder resin may include a cyanoacrylate binder resin represented by the following formula (3).

(3)

In Formula (3), R ₁ is an alkyl group, an alkoxyalkyl group, a cycloalkyl group, an alkenyl group, an aralkyl group, an aryl group, an allyl group or a haloalkyl group of C ₁ to C 15.

According to a preferred embodiment of the present invention, the curing agent may include at least one selected from a phenol-based curing agent, an acid anhydride-based curing agent, an amine-based curing agent and an imidazole-based curing agent.

According to a preferred embodiment of the present invention, the filler is an inorganic filler comprising at least one selected from aluminum oxide, magnesium hydroxide and calcium carbonate; And phosphine type fillers; And the like.

According to a preferred embodiment of the present invention, the catalyst may be an imidazole compound such as a 2-alkyl-4-methylimidazole compound, a 2-alkyl-4-ethylimidazole compound or a 2-phenylimidazole compound; Metal boron fluoride compounds consisting of tin boron fluoride and zinc boron fluoride; And metal octoate compounds consisting of boron trifluoride, tin octoate and zinc octoate; And the like.

According to a preferred embodiment of the present invention, the silane coupling agent is selected from the group consisting of an epoxy group-containing silane coupling agent, an amino group-containing silane coupling agent, a vinyl tri (C1 to C3 alkoxy) silane-containing silane coupling agent, 3-methacryloxypropylmethyldimet Containing silane coupling agent and 3-methacryloxypropyltrimethoxysilane-containing silane coupling agent.

According to a preferred embodiment of the present invention, the transparent adhesive layer (the transparent first adhesive layer and / or the second adhesive layer) has a light transparency of at least 91%, preferably at least 93% The temperature (Tg) may be 120 deg. C or higher, preferably 140 deg. C to 200 deg.

According to a preferred embodiment of the present invention, the transparent adhesive layer (the transparent first adhesive layer and / or the second adhesive layer) may have an adhesive strength of 0.6 kg / cm ² or more, preferably 0.7 kg / cm ^{2 or} more.

According to another preferred embodiment of the present invention, the metal foil layer of the low elasticity coverlay film of the present invention includes a copper foil or an aluminum foil, and the thickness of the metal foil layer may be 2 to 40 mu m.

According to a preferred embodiment of the present invention, the low elastic clear coverlay film of the present invention further comprises a protective substrate layer on one side of the transparent first adhesive layer and / or the transparent second adhesive layer, May be 6 to 200 mu m.

According to a preferred embodiment of the present invention, the protective substrate layer may include at least one of a paper film and a plastic resin film.

(1) forming a transparent base film; And (2) coating a transparent adhesive on both surfaces of the transparent base film, thereby providing a method of producing a low-elastic transparent coverlay film.

(1) forming a transparent base film; (2) forming a transparent first adhesive layer on one side of the transparent substrate film, and then sequentially laminating the metal foil layers; And (3) forming a transparent second adhesive layer on the other side of the transparent base film. The present invention also provides a method for producing a low elastic clear coverlay film.

According to one preferred embodiment of this invention, the average thickness of the first adhesive layer (P _1), the average thickness of the transparent substrate layer (P ₂₎ and the second average thickness of the adhesive layer (P ₃₎ has the formula (1) or (2), and at least one of the transparent first adhesive layer and the transparent second adhesive layer may have an elastic modulus of 0.1 to 2.0Gpa, and preferably an elastic modulus of 0.5 to 1.5Gpa when cured.

According to a preferred embodiment of the present invention, the transparent substrate film is formed of at least one resin selected from transparent polyimide (PI) resin, transparent polyethylene terephthalate (PET) resin and transparent polyethylene naphthalate (PEN) resin .

According to a preferred embodiment of the present invention, the transparent adhesive layer (transparent first adhesive layer and / or second adhesive layer) of the low elastic clear coverlay film producing method of the present invention is in a semi-cured state, The average thickness (P ₁ ) of the first adhesive layer is 5 탆 to 50 탆, the average thickness (P ₂ ) of the transparent base layer is 5 탆 to 25 탆, the average of the second adhesive layer The thickness (P ₃ ) may be 5 탆 to 50 탆.

Further, in order to solve the above-mentioned problems, the present invention provides a low-elasticity FPCB comprising the above-mentioned transparent coverlay film.

According to another preferred embodiment of the present invention, the low elastic FPCB of the present invention can be used in at least one of a cellular phone, a camera, a notebook, and a wearable device.

INDUSTRIAL APPLICABILITY The low-elasticity transparent coverlay film of the present invention and the method for producing the same are excellent in flexibility because they have adhesiveness with a low elasticity property and are lower in elasticity than conventional coverlay films, and they can satisfy rigidity, It is possible to provide a cover-lay film which can be transparently adhered on both sides.

1 and 2 are sectional views of a coverlay film according to a preferred embodiment of the present invention.

As used herein, the term " transparent " means colorless transparent.

Hereinafter, the present invention will be described in more detail.

The low elastic clear coverlay film of the present invention comprises a transparent substrate layer 30 and transparent adhesive layers 20 and 40 on both sides of the transparent substrate layer as shown in Fig. ; And (2) coating a transparent adhesive on both surfaces of the transparent base film.

Further, the method may further include forming a protective film on at least one of the transparent adhesive layers formed on both sides of the protective film.

The transparent base layer is formed by filming a resin containing at least one selected from a transparent polyimide (PI) resin, a transparent polyethylene terephthalate (PET) resin and a transparent polyethylene naphthalate (PEN) resin, (PI) resin into a film.

Wherein the transparent PI resin is a dianhydride containing at least one selected from the group consisting of dianhydrides represented by the following formula (1); And a diamine represented by the following formula (2): ## STR2 ## wherein R is a hydrogen atom or a methyl group;

[Chemical Formula 1]

,

,

,

,

,

,

,

,

또는

이며, 바람직하게는

,

,

,

또는

이다.In the formula 1, wherein R ₁ is

,

,

,

,

,

,

,

,

or

, And preferably

,

,

,

or

to be.

(2)

H ₂ NR ₂ -NH ₂

,

,

,

,

,

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,

또는

이며, 바람직하게는

,

,

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또는

이다.In formula (2), R ₂ is

,

,

,

,

,

,

,

or

, And preferably

,

,

,

or

to be.

The thickness of the transparent base layer may be 5 占 퐉 to 25 占 퐉, and the thickness of the transparent adhesive layer formed on both sides of the transparent base layer may be 5 占 퐉 to 50 占 퐉.

Further, the low elastic clear coverlay film of the present invention comprises the steps of: (1) forming a transparent base film; (2) forming a transparent first adhesive layer on one side of the transparent substrate film, and then sequentially laminating the metal foil layers; And (3) forming a transparent second adhesive layer on the other surface of the transparent base film, thereby forming a metal foil layer 10, a transparent first adhesive layer 20, a transparent substrate layer 30) and the transparent second adhesive layer 40 are laminated in this order. Here, the transparent base layer is the same as the transparent base layer described above.

In the step (2), the transparent first adhesive layer is coated on one side of the transparent substrate layer, and then the transparent first adhesive layer is dried by heat treatment at 30 to 190 ° C for 1 to 10 minutes. Specifically, a composition of the transparent first adhesive layer is uniformly applied to one side of the transparent substrate layer using a reverse roll coater, a die coater, and a comma coater, And then heat-treated for 2 to 4 minutes through an in-line drying oven at 50 to 170 ° C to dry the transparent first adhesive layer laminated on one surface of the transparent substrate layer.

Next, a metal foil layer is laminated on the surface of the transparent first adhesive layer through a heating roller at 30 ° C to 100 ° C by applying a spinning of 1 to 10 kg / cm 2. Specifically, the metal foil layer is preferably laminated on the surface of the dried transparent first adhesive layer on the transparent base layer via a heating roller at a linear pressure of preferably 4 to 7 kg / cm, preferably at 50 to 70 ° C.

Finally, the first adhesive layer is semi-cured by heat treatment at 40 ° C to 80 ° C for 6 to 100 hours. Specifically, for further effective semi-curing of the second adhesive layer, additional heat treatment can be performed at 40 ° C to 80 ° C, preferably 50 ° C to 70 ° C for 12 to 72 hours.

Next, in the step (3), a second adhesive layer is coated on one surface of the transparent substrate layer, and then the transparent second adhesive layer is dried by heat treatment at 30 ° C to 190 ° C for 1 to 10 minutes. Specifically, a composition of the transparent second adhesive layer is uniformly applied to one side of the transparent substrate layer using a reverse roll coater, a die coater, and a comma coater on one surface of the transparent substrate layer , In an in-line drying oven at 30 to 190 ° C, preferably 50 to 170 ° C for 2 to 4 minutes to laminate and dry the transparent second adhesive layer. Thereafter, a second adhesive layer is laminated on the other surface of the transparent base layer and a linear pressure of 5 to 7 kg / cm is applied, and the second adhesive layer can be bonded to the other surface of the transparent base layer through a heating roller at 50 ° C to 80 ° C. Finally, the second adhesive layer is semi-cured by heat treatment at 40 ° C to 80 ° C for 6 to 100 hours. Specifically, for further effective semi-curing of the second adhesive layer, additional heat treatment can be performed at 40 ° C to 80 ° C, preferably 50 ° C to 70 ° C for 12 to 72 hours.

Thus prepared, the low elastic transparent coverlay film (100, 200) for in, the average of the transparent first adhesive layer average thickness (P _1), a transparent substrate layer average thickness (P ₂₎ and a transparent second adhesive layer of the The thickness (P ₃ ) satisfies the following formula (1), preferably, the following formula (2), and the transparent second adhesive layer may have an elastic modulus of 0.1 to 2.0Gpa when cured, preferably 0.5 to 1.5Gpa The elastic modulus of the coverlay film is lower than that of the conventional coverlay film, so that flexibility can be ensured and the rigidity can be satisfied.

[Equation 1]

0.1? (P ₁ + P ₂ ) / P ₃ ? 10.0

In the above equation (1), P ₃ is a rational number satisfying 5 μm ≦ P ₃ ≦ 50 μm.

&Quot; (2) "

0.7? (P ₁ + P ₂ ) / P ₃ ? 4.0

In the above equation (2), P ₃ is a rational number satisfying 5 μm ≦ P ₃ ≦ 50 μm.

If the coverlay films 100 and 200 of the present invention are out of the range of Equation (1), the rigidity and flexibility required by the coverlay film of the present invention can not be satisfied. In other words, if the coverlay film of the present invention has a P ₁ + P ₂ / P ₃ of less than 0.1, the problem of stiffness and handling may occur, and P ₁ + P ₂ / P ₃ If it exceeds 10.0, a problem of ductility occurs.

Further, if at least one of the transparent first adhesive layer 20 and the second adhesive layer 40 is out of the elastic modulus of 0.1 to 2.0 Gpa during curing, the rigidity and flexibility required by the coverlay film of the present invention can be satisfied I will not. In other words, when the modulus of elasticity is less than 0.1 GPa, the rigidity of the coverlay film is low, so that it is easily broken by external impact. When the modulus of elasticity exceeds 2.0 GPa, the rigidity of the coverlay film is excellent, May occur. Therefore, the coverlay film of the present invention not only satisfies the above-described formula (1), but also because the transparent first adhesive layer 20 and / or the second adhesive layer 40 satisfy the elastic modulus of 0.1 to 2.0 Gpa , Not only rigidity but also flexibility can be satisfied at the same time.

2, the coverlay film 200 of the present invention includes a metal foil layer 10, a transparent first adhesive layer 20, a transparent substrate layer 30, and a transparent second adhesive layer 40, do.

The metal foil layer 10 is a metal having conductivity and ductility and is laminated on one side of the first adhesive layer 20 and may be provided with a circuit pattern. And may be formed by patterning in a desired shape. The metal foil layer 10 may include at least one of a thin copper foil, an aluminum foil, a tungsten foil foil and an iron foil foil and preferably has excellent flexibility and high electrical conductivity Or an aluminum foil having an aluminum foil.

The thickness of the metal foil layer 10 may be 2 to 40 μm, preferably 5 to 35 μm, and if the thickness of the metal foil layer 10 is less than 2 μm, the productivity of the FPCB of the present invention may decrease If it exceeds 40 탆, it may be difficult to impart ductility to the coverlay film 200 to be manufactured.

The transparent first adhesive layer 20 is formed between the metal foil layer 10 and the transparent base layer 30 in a semi-cured state and serves to adhere the metal foil layer 10 and the transparent base layer 30 to each other.

The transparent substrate layer 30 is formed between the first adhesive layer 20 and the second adhesive layer 40 and is formed of a transparent polyimide (PI) resin, a transparent polyethylene terephthalate (PET) resin and a transparent polyethylene naphthalate (PEN) resin, preferably a resin containing a transparent polyimide (PI) resin, into a film. As described above, the transparent polyimide resin may include dianhydride including at least one selected from the group consisting of the dianhydrides represented by Formula 1; And a diamine represented by the following formula (2): ## STR2 ## wherein R is a hydrogen atom or a methyl group;

The thickness of the transparent base layer 30 is not particularly limited, but may be 5 탆 to 25 탆, preferably 5 탆 to 15 탆, in consideration of the thickness of the coverlay films 100 and 200 to be manufactured. If the thickness of the transparent substrate layer 30 is less than 5 mu m, the workability of the flexible circuit board may be lowered. If the thickness of the transparent substrate layer 30 is more than 25 mu m, the ductility may be deteriorated.

The transparent adhesive layer 40 shown in Fig. 1 and / or the second adhesive layer 40 shown in Fig. 2 are laminated on one side of the transparent substrate layer 30 in a semi-cured state.

The transparent second adhesive layer 40 and / or the transparent first adhesive layer 20 may include a transparent adhesive including an epoxy thermosetting resin, an acrylic binder resin, a curing agent, a catalyst, a filler, and a silane coupling agent.

The epoxy-based thermosetting resin is converted into a strong molecular bonding structure having a net structure as the chemical bonding reaction between the epoxy resin components proceeds after curing, thereby improving the heat resistance and moisture resistance of the final bonded body, It plays a role. The epoxy-based thermosetting resin may include a cycloaliphatic epoxy resin. Examples of the cycloaliphatic epoxy resin include bisphenol-A type epoxy, bisphenol-F type epoxy, Bisphenol-S type epoxy, bisphenol-AD type epoxy, phenol novolac epoxy, cresol novolac epoxy, bisphenol-A novolak type epoxy resin, Low-chlorine type epoxy, rubber-modified epoxy, fatty-modified epoxy, urethane-modified epoxy, epoxy-modified epoxy, A silane-modified epoxy, a dicycolpentadiene type epoxy and a multi-functional novolac type epoxy, wherein the silane- Preferred are bisphenol-A type epoxy, bisphenol-A novolac type epoxy, O-cresol novolac type epoxy, dicyclopentadiene, A dicycolpentadiene type epoxy and a multi-functional novolac type epoxy.

The acrylic binder resin may include a cyanoacrylate binder resin represented by the following formula (3).

(3)

In Formula (3), R ₁ is an alkyl group, an alkoxyalkyl group, a cycloalkyl group, an alkenyl group, an aralkyl group, an aryl group, an allyl group or a haloalkyl group of C ₁ to C 15.

The acrylic binder resin may be used in an amount of 30 to 80 parts by weight, preferably 30 to 60 parts by weight, based on 100 parts by weight of the epoxy thermosetting resin. When the amount of the acrylic binder resin used is less than 30 parts by weight, There may be a problem that it is not ensured, and if it exceeds 80 parts by weight, there may be a problem that the curing of the adhesive proceeds too quickly.

The filler serves to improve the cost reduction and the elastic modulus of the transparent adhesive layer (including the first adhesive layer 20 and / or the second adhesive layer), and the filler may be a metal such as aluminum hydroxide, magnesium hydroxide, calcium carbonate An inorganic filler, and a phosphine type filler, and may preferably include an aluminum hydroxide type filler or a phosphorous type filler. The filler may be used in an amount of 20 to 100 parts by weight, preferably 30 to 80 parts by weight, based on 100 parts by weight of the epoxy thermosetting resin. If the amount of the filler used is less than 20 parts by weight, If it exceeds 100 parts by weight, there may be a problem of deterioration of adhesion.

The hardener serves to improve the heat resistance of the transparent adhesive layer (including the first adhesive layer 20 and / or the second adhesive layer). The hardener may be an amine type hardener, a phenol type hardener, a hardener, an acid anhydride type hardener, an imidazole compound, a polyamine compound, a hydrazine compound, and a dicyandiamide compound , Preferably at least one of an amine type hardener, a phenol type hardener and an acid anhydride type curing agent. The curing agent may be used in an amount of 10 to 100 parts by weight, preferably 15 to 50 parts by weight, based on 100 parts by weight of the epoxy thermosetting resin. If the amount of the curing agent is less than 10 parts by weight, If it exceeds the weight part, there may be a problem of deterioration of the adhesive strength.

In addition, the catalyst serves to accelerate the curing of the transparent adhesive layer (including the first adhesive layer 20 and / or the second adhesive layer), and it is possible to improve filling property, adhesive force, heat resistance, and the like in the metal foil layer circuit portion. Examples of the catalyst include a 2-alkyl-4-methylimidazole compound, an imidazole compound such as a 2-alkyl-4-ethylimidazole compound and a 2-phenylimidazole compound, boron tin fluoride and zinc boron fluoride A metal boron fluoride compound; A metal octoate compound consisting of boron trifluoride and tin octoate and zinc octoate, and may include at least one of the group consisting of an imidazole compound and at least one of boron trifluoride . The catalyst may contain 0.1 to 2.0 parts by weight, preferably 0.3 to 1.0 part by weight, based on 100 parts by weight of the epoxy thermosetting resin. If the amount of the catalyst is less than 0.1 parts by weight, the problem of heat resistance may occur. If the amount of the catalyst is more than 2.0 parts by weight, a problem of adhesion may occur.

The silane coupling agent may be an epoxy group-containing silane coupling agent, an amino group-containing silane coupling agent, a vinyl tri (C1 to C3 alkoxy) silane containing silane coupling agent, a 3-methacryloxypropylmethyldimethoxysilane- - methacryloxypropyltrimethoxysilane-containing silane coupling agent. The amount to be used may be 10 to 35 parts by weight, preferably 12 to 25 parts by weight, based on 100 parts by weight of the epoxy thermosetting resin. If the amount of the silane coupling agent is less than 10 parts by weight, the effect of increasing the adhesive strength may not be exhibited. If the amount exceeds 35 parts by weight, other properties such as heat resistance may be deteriorated.

As the above-mentioned additives, an ultraviolet protective agent, a heat resistance enhancer, and the like may be further used.

The thickness of the transparent adhesive layer (including the first adhesive layer and / or the second adhesive layer) may be 5 탆 to 50 탆, preferably 15 탆 to 35 탆. If the thickness of the transparent adhesive layer is less than 5 mu m, there may arise a problem of insufficient filling of the step between the metal foil layer and the circuit. If the thickness exceeds 50 mu m, there may be a problem of unnecessary increase in thickness of the flexible circuit board of the present invention, there is a problem that a burr occurs during the drilling process.

The protective cover layer 50 may further include a transparent adhesive layer 20 on both sides of the coverlay film 100 of the present invention and a protective substrate layer 50 laminated on one side of the transparent second adhesive layer 40 of the coverlay film 200 .

In the protective substrate layer 50, the transparent adhesive layer (including the second adhesive layer) is semi-cured, and is laminated on one side of the transparent adhesive layer so that the transparent adhesive layer as a peelable release film can prevent foreign matters from the external environment, The surface of the transparent adhesive layer in the pressing step can be protected.

The protective substrate layer 50 may include at least one of a paper film or a plastic resin film. In other words, the protective substrate layer 50 may be a paper film coated with a polyethylene, polypropylene or methylpentene copolymer resin or a polyethylene, a polypropylene, a methylpentene copolymer resin, a polyethylene terephthalate A plastic resin film such as a phthalate resin, a polyethylene naphthalate resin, or the like.

The thickness of the protective substrate layer 50 is not particularly limited, but may be in the range of 6 탆 to 200 탆 in consideration of the processability and manufacturing qualities of the FPCB, and preferably 12 탆 to 200 탆 based on a paper film, It may be 6 mu m to 75 mu m. If the thickness of the protective substrate layer 50 is less than 6 mu m, there may arise a problem that processability due to the thin film is deteriorated. If the thickness exceeds 200 mu m, unnecessary increase in the price and thickness and weight of the coverlay films 100, There may be a problem that the uplink is upward.

In the transparent coverlay film of the present invention, the transparent adhesive layer (including the first adhesive layer and / or the second adhesive layer) has a light transparency of 91% or more, preferably 93% or more, The temperature (Tg) may be 120 deg. C or higher, preferably 140 deg. C to 200 deg.

In the transparent coverlay film of the present invention, the adhesive strength of the transparent adhesive layer (including the first adhesive layer and / or the second adhesive layer) may be 0.6 kg / cm ² or more, preferably 0.7 kg / cm ^{2 or} more.

The transparent coverlay film 100 of the present invention composed of the transparent base layer and the transparent adhesive layer has a light transparency of not less than 90%, preferably not less than 91%, more preferably not less than 92% .

Next, the present invention provides an FPCB including the coverlay film.

The FPCB (Flexible printed circuit board (FPCB)) is an electronic component developed by reducing the size and weight of an electronic product. The FPCB including the coverlay film of the present invention has low elasticity and is excellent in rigidity as well as flexibility.

The FPCB of the present invention is used as a core component of an electronic product such as a mobile phone, a camera, a notebook, a wearable device, a computer and a peripheral device, a mobile communication terminal, a video and audio device, a camcorder, a printer, a DVD player, Equipment, medical equipment, and preferably at least one of a mobile phone, a camera, a notebook, and a wearable device.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. It will be understood that various changes and modifications may be made without departing from the scope of the present invention. For example, each component specifically illustrated in the embodiments of the present invention can be modified and implemented. It is to be understood that all changes and modifications that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

The present invention will now be described more specifically with reference to the following examples. However, the following examples should not be construed as limiting the scope of the present invention, and should be construed to facilitate understanding of the present invention.

[ Example ]

Preparation Example  1: Preparation of polyimide film

200 mL of N-methyl-2-pyrrolidone (NMP) was placed in a 500 mL two-necked round bottom flask substituted with nitrogen gas, and the dianhydride represented by the following formula 1-1 and the diamine represented by the following formula 2-1 At a molar ratio of 1: 1 and reacted at 24 to 25 ° C for 24 hours to prepare a polyamic acid solution of polyamic acid.

Next, 0.25 mol of anhydrous acetic acid and 0.25 mol of pyridine were added to the solution, and the mixture was coated on a glass substrate. Imidization reaction was then carried out to obtain 93.5 Polyimide having a light transmittance of ~94% was synthesized.

[Formula 1-1]

이다.In Formula 1-1, R < ₁ >

to be.

[Formula 2-1]

H ₂ NR ₂ -NH ₂

이다.In formula (2-1), R ₂ is

to be.

Preparation Example  2-1 to 2-4: Preparation of transparent adhesive

An epoxy-based thermosetting resin, an acrylic binder resin, a curing agent, a catalyst, a filler and a silane coupling agent were blended and dissolved at a temperature of from room temperature to 40 ° C or less to prepare a transparent adhesive.

The optical transparency, the glass transition temperature, and the adhesive strength of each of the produced transparent adhesives are shown in Table 1 below.

Classification (parts by weight) Preparation Example 2-1 Preparation Example 2-2 Comparative Preparation Example 2-1 Comparative Preparation Example 2-2 Epoxy thermosetting resin 100 100 100 100 Acrylic binder resin 47 60 47 47 Silane coupling agent 15 15 40 15 filler 40 40 40 85 Hardener 20 20 20 20 catalyst 0.5 0.5 0.5 0.5 Optical transparency
(Based on 500 nm wavelength) 93.3% 92.5% 90.6% 87.4% Glass transition temperature 142 ° C 140 ° C 123 ℃ 146 ° C Adhesive strength 0.72 kg / cm ² 0.75 kg / cm ² 0.74 kg / cm ² 0.63 kg / cm ²

, 여기서, R₁은 메톡시에틸기이다.
*실란커플링제 : 3-메타크릴옥시프로필트리메톡시실란 함유 실란커플링제
*필러 : 인 타입 필러(phosphine type filler)(클라리언트, OP930)
*경화제 : 아민 타입 경화제(amine type hardener)(CTN, DDS)
*촉매(경화촉진제) : 경화촉진제(시코쿠화학, 2MA-OK)* Epoxy thermosetting resin: O-cresol novolac type epoxy (Kukdo Chemical, YDCN 7P)
* Acrylic binder resin:

, Wherein R < ₁ > is a methoxyethyl group.
Silane coupling agent: 3-methacryloxypropyltrimethoxysilane-containing silane coupling agent
* Filler: phosphine type filler (Clariant, OP930)
* Hardener: amine type hardener (CTN, DDS)
* Catalyst (curing accelerator): Curing accelerator (Shikoku Chemical, 2MA-OK)

Example 1

The transparent adhesive prepared in Preparative Example 2-1 was uniformly coated on one surface of the transparent polyimide film (transparent substrate layer) prepared in Preparative Example 1 using a comma coater, And then heat-treated through an in-line drying oven to form a transparent adhesive layer (transparent first adhesive layer) and to be semi-cured. A polyethylene terephthalate (PET) film (SKC, SG31) used as a protective substrate layer having a thickness of 36 mu m was bonded to the surface of the transparent adhesive layer semi-cured at 65 DEG C through a heating roller at a linear pressure of 6 kg / cm, Layer.

Next, a transparent second adhesive layer was formed in the same manner as in the case of the transparent first adhesive layer using the transparent adhesive prepared in Preparation Example 2-1 on the other surface of the transparent base layer on which the transparent first adhesive layer was laminated, , And then a protective film layer was formed in the same manner as above to produce a low elastic clear coverlay film.

The optical transparency of the low-elastic transparent coverlay film (excluding the protective film layer) prepared was 91.4%.

Example 2

(1) The transparent joining agent prepared in Preparative Example 2-1 was uniformly coated on one side of the transparent polyimide film (transparent substrate layer) prepared in Preparation Example 1 using a comma coater, And then heat-treated for 3 minutes through an in-line drying oven to form a transparent first adhesive layer and semi-cured. At this time, the thickness (P ₂ ) of the transparent base layer is 50 탆.

(2) A metal foil layer made of a copper foil (Mitsui, 3EC-3 1/3 oz) was applied to the surface of the semi-cured transparent first adhesive layer on the transparent substrate layer through a heating roller at 60 캜 under a linear pressure of 6 kg / , And then heat-treated at 100 DEG C for 200 minutes to cure the transparent first adhesive layer. At this time, the thickness (P ₁ ) of the first adhesive layer is 5 탆.

(3) The transparent adhesive prepared in Preparative Example 2-1 was uniformly coated on the other surface of the transparent base layer and heat-treated at 110 ° C for 3 minutes through an in-line drying oven to form a second adhesive layer And semi-cured. At this time, the thickness (P ₃ ) of the second adhesive layer is 50 탆.

(4) A linear pressure of 6 kg / cm was applied to a polyethylene terephthalate (PET) film (SKC, SG31) used as a protective substrate layer having a thickness of 36 탆, 2 adhesive layer to produce a coverlay film.

Example 3

A coverlay film was produced in the same manner as in Example 2. [ However, the transparent adhesive prepared in Preparation Example 2-2 was used instead of the transparent adhesive prepared in Preparation Example 2-1 in the step (3).

Example 4

A coverlay film was produced in the same manner as in Example 2. [ However, the thickness (P ₂ ) of the transparent base layer in step (1) is 25 μm, the thickness (P ₁ ) of the cured first adhesive layer in step (2) is 50 μm, The thickness (P ₃ ) of the adhesive layer is 5 탆.

Example 5

A coverlay film was produced in the same manner as in Example 4. [ However, the transparent adhesive prepared in Preparation Example 2-2 was used instead of the transparent adhesive prepared in Preparation Example 2-1 in the step (3).

Example 6

A coverlay film was produced in the same manner as in Example 2. [ However, the thickness (P ₂ ) of the transparent base layer in step (1) is 25 μm, the thickness (P ₁ ) of the cured transparent first adhesive layer in step (2) is 20 μm, 2 The thickness (P ₃ ) of the adhesive layer is 30 탆.

Example 7

A coverlay film was produced in the same manner as in Example 6. [ However, the transparent adhesive prepared in Preparation Example 2-2 was used instead of the transparent adhesive prepared in Preparation Example 2-1 in the step (3).

Example 8

A coverlay film was produced in the same manner as in Example 2. [ However, the thickness (P ₂ ) of the transparent base layer in step (1) is 5 μm, the thickness (P ₁ ) of the cured first adhesive layer in step (2) is 40 μm, The thickness (P ₃ ) of the adhesive layer is 10 탆.

Example 9

A coverlay film was produced in the same manner as in Example 8. [ However, the transparent adhesive prepared in Preparation Example 2-2 was used instead of the transparent adhesive prepared in Preparation Example 2-1 in the step (3).

Example 10

A coverlay film was produced in the same manner as in Example 9. [ However, the thickness (P ₂ ) of the transparent base layer in step (1) is 5 μm, the thickness (P ₁ ) of the cured first adhesive layer in step (2) is 25 μm, The thickness (P ₃ ) of the adhesive layer is 25 탆.

Example 11

A coverlay film was produced in the same manner as in Example 10. [ However, the transparent adhesive prepared in Preparation Example 2-2 was used instead of the transparent adhesive prepared in Preparation Example 2-1 in the step (3).

Comparative Example  One

A coverlay film was produced in the same manner as in Example 2. [ However, in the step (3), the transparent adhesive prepared in Comparative Preparation Example 2-1 was used in place of the transparent adhesive prepared in Preparation Example 2-1.

Comparative Example  2

A coverlay film was produced in the same manner as in Example 2. [ However, in step (3), the transparent adhesive prepared in Comparative Preparation Example 2-2 was used in place of the transparent adhesive prepared in Preparation Example 2-1.

Comparative Example  3

A coverlay film was produced in the same manner as in Example 4. [ However, in the step (3), the transparent adhesive prepared in Comparative Preparation Example 2-1 was used in place of the transparent adhesive prepared in Preparation Example 2-1.

Comparative Example  4

A coverlay film was produced in the same manner as in Example 4. [ However, in step (3), the transparent adhesive prepared in Comparative Preparation Example 2-2 was used in place of the transparent adhesive prepared in Preparation Example 2-1.

Comparative Example  5

A coverlay film was produced in the same manner as in Example 2. [ However, the thickness (P ₂ ) of the transparent base layer in step (1) is 25 μm, the thickness (P ₁ ) of the cured first adhesive layer in step (2) is 70 μm, The thickness (P ₃ ) of the adhesive layer is 5 탆.

Comparative Example  6

A coverlay film was prepared in the same manner as in Comparative Example 5. [ However, the transparent adhesive prepared in Preparation Example 2-2 was used instead of the transparent adhesive prepared in Preparation Example 2-1 in the step (3).

Comparative Example  7

A coverlay film was produced in the same manner as in Example 2. [ The thickness (P ₂ ) of the transparent base layer in step (1) is 5 μm, the thickness (P ₁ ) of the cured first adhesive layer in step (2) is 5 μm, The thickness (P ₃ ) of the adhesive layer is 70 μm.

Comparative Example  8

A coverlay film was produced in the same manner as in Comparative Example 7. [ However, the solution prepared in Preparation Example 2-2 was used instead of the transparent adhesive prepared in Preparation Example 2-1 in the step (3).

Experimental Example

The coverlay films prepared in the above Examples and Comparative Examples were evaluated on the basis of the following physical property evaluation methods, and the results are shown in Table 2.

(1) Modulus of elasticity

After curing the second adhesive layer prepared in each of the 10 cm x 1 cm sized examples and comparative examples, the elasticity at the time of tensile was measured by applying an adhesive tensile tester at a tensile speed of 50 mm / min.

(2) Peel strength

After removing the protective substrate layer of the coverlay film prepared in each of the Examples and Comparative Examples, a 1oz thick copper foil (Mitsui, 3EC-3) was laminated on the surface of the second adhesive layer and laminated on the surface of the second adhesive layer at 160 DEG C for 60 minutes x 30 kgf , And the adhesive properties of the copper foil and the coverlay film were evaluated by using a peeling test machine.

(3) Soldering heat resistance

After removing the protective substrate layer of the coverlay film prepared in each of the examples and the comparative examples, a 1oz thick copper foil (Mitsui, 3EC-3) was laminated on the surface of the second adhesive layer, (Hot press) under the condition of 30 kgf, and the sample was placed in a molten bath for 10 seconds at a temperature of 260 ° C or higher to observe the appearance of the sample. At this time, the evaluation criteria of soldering heat resistance are as follows.

?: 300 占 폚 or higher /?: 280 to 300 占 폚 /?: 260 to 280 占 폚 / 占: 260 占 폚 or less

(4) Flexural characteristics

The coverlay film prepared in each of the examples and the comparative examples was laminated on a pattern circuit in the form of a 2L type FCCL (Flexible Copper Clad Laminate) (Doosan Electronics, product name: 122000) in accordance with JIS C 6471 8.2, The number of times until the coverlay film was broken after repeated bending was measured at a rate of 175 rpm per minute under a condition of a radius of 0.38 mm.

Coverlay film Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Thickness ratio
((P ₁ + P ₂ ) / P ₃ ) 1.1 1.1 15 15 1.5 1.5 4.5 The first and second adhesive layers
Modulus of elasticity (Gpa) 0.6 0.5 0.6 0.4 0.6 0.4 0.6 Adhesion (gf) 1,070 1,095 1,725 1,830 1,220 1,370 1,566 High Temperature Adhesion (gf) 857 986 1,521 1,684 1,104 1,211 1,324 Soldering heat resistance ◎ ○ ◎ ○ ◎ ○ ◎ Restoring force (gf) 53 40 57 46 42 36 47 Example 9 Example 10 Example 11 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Thickness ratio
((P ₁ + P ₂ ) / P ₃ ) 4.5 1.2 1.2 1.1 1.1 15 15 The first and second adhesive layers
Modulus of elasticity (Gpa) 0.4 0.6 0.4 2.0 0.08 2.0 0.08 Adhesion (gf) 1,630 1,320 1,440 430 480 570 670 High Temperature Adhesion (gf) 890 978 1,510 1,640 1,100 1,230 1,370 Soldering heat resistance ○ ◎ ○ △ X △ X Restoring force (gf) 45 36 24 173 21 183 25 Comparative Example 5 Comparative Example 6 Comparative Example 7 Comparative Example 8 Thickness ratio
((P ₁ + P ₂ ) / P ₃ ) 19 19 0.14 0.14 The second adhesive layer
Modulus of elasticity (Gpa) 0.6 0.4 0.6 0.4 Adhesion (gf) 1,440 1,520 810 920 High Temperature Adhesion (gf) 1,360 1,420 780 830 Soldering heat resistance △ △ △ △ Restoring force (gf) 72 63 67 58

As shown in Table 2, it was confirmed that the coverlay film of the example according to the present invention showed lower elasticity and excellent bending property than the coverlay film of the comparative example.

Specifically, Example 2 in which the thickness ratio described in Table 2 was 1.1, the first adhesive layer and the second adhesive layer were formed from the solution prepared in Preparative Example 2-1, and Example 2 in which the thickness ratio was 1.1 and the solution prepared in Preparation Example 2-2 It is confirmed that Example 3 in which the first adhesive layer and the second adhesive layer were formed had better adhesive force, brazing heat resistance and bending property than Comparative Examples 1 and 2. It is also confirmed that Examples 2 and 3 have better solder heat resistance and flexural characteristics than Comparative Examples 5 and 6. Further, Example 2 and Example 3 are superior to Comparative Examples 7 and 8 in adhesive strength and solder heat resistance.

It can be confirmed that Examples 4 and 5 are superior to Comparative Examples 3 and 4 in adhesive strength, solder heat resistance and bending property.

 Therefore, the coverlay film of the present invention has flexibility lower than that of the conventional coverlay film and thus has flexibility, can have a certain rigidity, can be bonded on both sides while having transparency, Can also satisfy rigidity, flexibility and transparency at the same time.

Claims (17)

A transparent first adhesive layer on one side of the transparent base layer and the transparent base layer, and a transparent second adhesive layer on the other side,
Wherein the transparent base layer comprises at least one selected from a transparent polyimide (PI) resin, a transparent polyethylene terephthalate (PET) resin and a transparent polyethylene naphthalate (PEN) resin,
Wherein the transparent first adhesive layer and the transparent second adhesive layer comprise an acrylic adhesive.
The low-elasticity transparent coverlay film according to claim 1, wherein the transparent first adhesive layer and the transparent second adhesive layer are semi-cured.
The transparent polyimide resin according to claim 1, wherein the transparent polyimide resin
A dianhydride comprising at least one selected from the group consisting of dianhydrides represented by the following formula (1); And a diamine containing at least one selected from the group consisting of a diamine represented by the following formula (2): embedded image and a colorless transparent polyimide resin obtained by imidizing a polyamic acid solution ;
[Chemical Formula 1]

In the formula 1, wherein R ₁ is

,

,

,

,

,

,

,

,

or

Lt;
(2)
H ₂ NR ₂ -NH ₂
In formula (2), R ₂ is

,

,

,

,

,

,

,

or

to be.
The transparent first adhesive layer according to claim 1, further comprising a metal foil layer on one side of the transparent first adhesive layer,
Wherein the transparent first adhesive layer, the metal foil layer, the transparent first adhesive layer, the transparent base layer and the transparent second adhesive layer are laminated in this order.
5. The method of claim 4, shall satisfy the following equation 1, the transparent first (P ₁₎ the average thickness of the first adhesive layer, the average thickness of the transparent substrate layer (P ₂₎ and a transparent second average thickness of the second adhesive layer (P _3),
Wherein at least one of the transparent first adhesive layer and the transparent second adhesive layer has a modulus of elasticity of 0.1 to 2.0Gpa when cured.
[Equation 1]
0.1? (P ₁ + P ₂ ) / P ₃ ? 10.0
In the above equation (1), P ₃ is a rational number satisfying 5 μm ≦ P ₃ ≦ 50 μm.
5. The method of claim 4,
The average thickness of the transparent first adhesive layer (P _1), the average thickness of the transparent substrate layer (P ₂₎ and the average thickness of the transparent adhesive layer 2 (P ₃₎ is to satisfies the equation 2,
Wherein at least one of the transparent first adhesive layer and the transparent second adhesive layer has an elastic modulus of 0.5 to 1.5Gpa when cured.
[Equation 2]
0.7? (P ₁ + P ₂ ) / P ₃ ? 4.0
In the above equation (2), P ₃ is a rational number satisfying 5 탆? P ₃ ? 50 탆.
5. The method of claim 4,
The thickness (P ₁ ) of the transparent first adhesive layer is 5 μm to 50 μm, the thickness (P ₂ ) of the transparent base layer is 5 μm to 25 μm, the thickness (P ₃ ) of the transparent second adhesive layer is 5 μm to 50 Lt; RTI ID = 0.0 > g / m. ≪ / RTI >
5. The method of claim 4,
Wherein the metal foil layer comprises a copper foil or an aluminum foil,
Wherein the metal foil layer has a thickness of 2 to 40 占 퐉.
The method according to claim 1,
And a protective substrate layer laminated on one surface of at least one of the transparent first adhesive layer and the transparent second adhesive layer,
Wherein the protective substrate layer has a thickness of 6 to 200 占 퐉.
10. The method of claim 9,
Wherein the protective substrate layer comprises at least one of a paper film and a plastic resin film.
A low-elasticity flexible printed circuit board (FPCB) comprising the transparent coverlay film of any one of claims 1 to 10.
12. The method of claim 11,
Wherein the FPCB is used for at least one of a cellular phone, a camera, a notebook, and a wearable device.
(1) forming a transparent base film; And
(2) coating a transparent adhesive on both surfaces of the transparent base film,
The transparent base film is formed of at least one resin selected from transparent polyimide (PI) resin, transparent polyethylene terephthalate (PET) resin and transparent polyethylene naphthalate (PEN) resin,
Wherein the transparent adhesive includes an acrylic adhesive and is in a semi-cured state.
(1) forming a transparent base film;
(2) forming a transparent first adhesive layer on one side of the transparent substrate film, and then sequentially laminating the metal foil layers; And
(3) forming a transparent second adhesive layer on the other surface of the transparent base film,
Wherein the transparent first adhesive layer and the second adhesive layer comprise an acrylic adhesive and are semi-cured,
Wherein the transparent substrate layer is formed of at least one resin selected from a transparent polyimide (PI) resin, a transparent polyethylene terephthalate (PET) resin and a transparent polyethylene naphthalate (PEN) resin. Way.
The method of claim 14, wherein the transparent and the average thickness of the first adhesive layer (P _1), the average thickness of the transparent substrate layer (P ₂₎ and the average thickness of the transparent adhesive layer 2 (P ₃₎ is satisfied with the following equation 1,
Wherein at least one of the transparent first adhesive layer and the transparent second adhesive layer has a modulus of elasticity of 0.1 to 2.0Gpa when cured.
[Equation 1]
0.1? (P ₁ + P ₂ ) / P ₃ ? 9.0
In the above equation (1), P ₃ is a rational number satisfying 5 탆? P ₃ ? 40 탆.
15. The method of claim 14, the transparent second, and the average thickness of the first adhesive layer (P _1), the average thickness of the transparent substrate layer (P ₂₎ and a transparent second average thickness of the second adhesive layer (P ₃₎ is to satisfy the equation 2,
Wherein at least one of the transparent first adhesive layer and the transparent second adhesive layer has an elastic modulus of 0.5 to 1.5Gpa when cured.
[Equation 2]
0.7? (P ₁ + P ₂ ) / P ₃ ? 4.0
In the above equation (2), P ₃ is a rational number satisfying 5 탆? P ₃ ? 50 탆.
15. The method of claim 14, the transparent thickness of the first adhesive layer (P ₁₎ is 5㎛ ~ 50㎛, the thickness of the transparent substrate layer (P ₂₎ is 5㎛ ~ 25㎛, the second transparent adhesive layer thickness (P ₃ ) Is from 5 탆 to 50 탆.

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