KR20130053063A

KR20130053063A - Composite sheet, flexible substrate comprising the same and display apparatus comprising the same

Info

Publication number: KR20130053063A
Application number: KR1020110118553A
Authority: KR
Inventors: 정은환; 이우진; 임성한; 김성국; 김영권
Original assignee: 제일모직주식회사
Priority date: 2011-11-14
Filing date: 2011-11-14
Publication date: 2013-05-23

Abstract

The present invention relates to a composite sheet, a composition for a composite sheet, a flexible substrate comprising the same and a display device including the same. More specifically, the present invention relates to a composite sheet prepared by adjusting the viscosity of the silicone resin contained in the matrix to a specific range, a composition for composite sheets, a flexible substrate including the same, and a display device including the same. The present invention improves the surface flatness while maintaining the mechanical properties of the composite sheet.

Description

Composite sheet, flexible substrate comprising the same and a display device including the same {Composite sheet, flexible substrate comprising the same and display apparatus comprising the same}

The present invention relates to a composite sheet, a composition for a composite sheet, a flexible substrate comprising the same and a display device including the same. More specifically, the present invention relates to a composite sheet prepared by adjusting the viscosity of the rubber compound contained in the composition for the matrix to a specific range, a composition for composite sheet, a flexible substrate comprising the same and a display device including the same. The present invention improves the surface flatness while maintaining the mechanical properties of the composite sheet.

The glass substrate is excellent in heat resistance and transparency, and has a low coefficient of linear expansion. Therefore, organic substrates have been widely used as liquid crystal display elements, substrates for organic EL display elements, color filter substrates, solar cell substrates, and the like. However, the glass substrate is limited in thickness and weight reduction of the liquid crystal display due to the thick thickness and heavy weight, and has a problem in that it is vulnerable to impact resistance. In addition, the brittleness of the glass material makes it unsuitable for use as a substrate for display.

Accordingly, a flexible substrate made of a plastic optical film material has been spotlighted as a material to replace a conventional glass substrate. The flexible substrate has characteristics suitable for liquid crystal displays, next generation display devices such as organic EL, electronic paper, and the like.

However, the flexible substrate made of only the plastic composite sheet has a problem of high thermal expansion coefficient and poor rigidity. Accordingly, a method of manufacturing a composite sheet by supplementing rigidity by impregnating a reinforcement material including glass fiber or glass cloth in a matrix resin as a polymer material is used.

Meanwhile, in order to use the composite sheet as a flexible substrate, minimum surface flatness must be secured in order to provide surface performance of the display in addition to rigidity, transparency, heat resistance, and flexibility. However, the surface flatness may be lowered due to factors such as low surface properties of the reinforcement material including glass fiber cloth, consequent flow of the conformal matrix, shrinkage upon curing, and the like.

An object of the present invention is to provide a composite sheet having a high surface flatness while maintaining mechanical properties.

Another object of the present invention to provide a composite sheet that can solve the problem of lowering the surface flatness due to curing shrinkage during curing.

Still another object of the present invention is to provide a composite sheet capable of solving the problem of deterioration of surface flatness in the process of compounding the reinforcing material in the matrix.

Still another object of the present invention is to provide a composition for composite sheets.

Still another object of the present invention is to provide a method for producing the composite sheet.

Still another object of the present invention is to provide a flexible substrate and a display device including the composite sheet.

Composite sheet in one aspect of the invention is a matrix; And a reinforcing material included in the matrix, and the surface flatness Ra of the composite sheet may be greater than 50 nm and less than 300 nm.

In one embodiment, the matrix may comprise a crosslinked rubber compound.

In one embodiment, the rubber compound is styrene-butadiene rubber (SBR), butadiene rubber, isoprene rubber, chloroprene rubber, neoprene rubber, ethylene-propylene-diene terpolymer, styrene-ethylene-butylene-styrene ( SEBS) block copolymers, styrene-ethylene-propylene-styrene (SEPS) block copolymers, acrylonitrile-butadiene rubber (NBR), hydrogenated nitrile rubber (NBR), florinated It may include one or more selected from the group consisting of rubber (fluorinated rubber), and plasticized polyvinyl chloride rubber.

In one embodiment, the matrix may further comprise an inorganic filler.

Another composition of the composite sheet of the present invention is a composition for a matrix; And a reinforcing material impregnated in the composition for the matrix, wherein the composition for the matrix may have a viscosity of 3,500 cps to 7,000 cps at 25 ° C.

In one embodiment, the composition for the matrix of the polyorganosiloxane resin (A) having a viscosity of 1,500-3,500cps at 25 ℃ and the polyorganosiloxane resin (B) having a viscosity of 9,000-12,000cps at 25 ℃ Mixtures may be included.

Another method of manufacturing a composite sheet of the present invention comprises the step of impregnating and curing the reinforcing material in the composition for the matrix comprising a rubber-based compound, a crosslinking agent, a catalyst and an inhibitor, the composition for the matrix is 3,500cps to 25 ℃ It can have a viscosity of 7,000 cps.

In one embodiment, the rubber-based compound may include a mixture of a polyorganosiloxane resin having a viscosity of 1,500-3,500 cps at 25 ° C. and a polyorganosiloxane resin having a viscosity of 9,000-12,000 cps at 25 ° C. .

In one embodiment, the composition for the matrix may further include an inorganic filler.

Another aspect of the present invention, the flexible substrate and the display device may include the composite sheet.

The present invention provides a composite sheet having a high surface flatness. The present invention provides a composite sheet that can solve the problem of lowering the surface flatness due to curing shrinkage during curing. The present invention provides a composite sheet that can solve the problem of lowering the surface flatness in the composite process of the reinforcing material in the matrix.

1 is a cross-sectional view of a composite sheet according to an embodiment of the present invention.
1: matrix, 2: reinforcement, 10: composite sheet

Composite sheet according to an aspect of the present invention may have a surface flatness (Ra) of more than 50nm to less than 300nm. Within this range, sufficient display performance can be ensured when used as a flexible substrate. Preferably the surface flatness may be 51 nm-299 nm, more preferably 51 nm-150 nm, most preferably 51-100 nm.

Surface flatness can be measured contactlessly using an optical profiler. Although a method of measuring by scraping the surface of the composite sheet with a tip may be selected, the non-contact measurement is more advantageous due to the surface properties since the matrix used in the present invention includes a rubber-based compound.

The composite sheet of the present invention may have a coefficient of thermal expansion of 0 ppm / ° C-400 ppm / ° C, preferably 0 to 10 ppm / ° C, more preferably 3 to 7 ppm / ° C. Within this range, thermal deformation can be suppressed in manufacturing the flexible substrate. The coefficient of thermal expansion is an ASTM E 831 method. The dimensional change with temperature can be measured using a thermo-mechanical analysizer (expansion mode, force 0.05N) and then measured from the change curve of the sample length with temperature. It doesn't work.

The thickness of the composite sheet of the present invention may be 15㎛-200㎛. Within this range, it can be used as a composite sheet for a flexible substrate.

Composite sheet of the present invention is a matrix; And it may be composed of a reinforcing material contained in the matrix. Figure 1 shows one embodiment of the composite sheet of the present invention. According to FIG. 1, the composite sheet 10 has a structure in which a reinforcing material 2 is included in the matrix 1. The reinforcement may be included in a dispersed, layered structure. The reinforcing material may be present inside the impregnated rubber-based compound as a support.

In the composite sheet of the present invention, the matrix: reinforcing material may be included in a weight ratio of 70:30 to 95: 5, preferably in a weight ratio of 80:20 to 90:10. In the above range, it may have physical properties of the composite sheet that can be used as a flexible substrate.

In the composite sheet of the present invention, the matrix may include a crosslinked rubber compound.

The rubber compound may include silicone rubber, styrene-butadiene rubber (SBR), butadiene rubber, isoprene rubber, chloroprene rubber, neoprene rubber, ethylene-propylene-diene terpolymer, styrene-ethylene-butylene Styrene (SEBS) block copolymers, styrene-ethylene-propylene-styrene (SEPS) block copolymers, acrylonitrile-butadiene rubber (NBR), hydrogenated nitrile rubber (NBR), And at least one member selected from the group consisting of fluorinated rubber, and plasticized polyvinylchloride rubber. Preferably, silicone rubber can be used.

The silicone rubber may include a polyorganosiloxane resin including a unit of Formula 1 below.

&Lt; Formula 1 >

(Wherein R and R 'are the same as or different from each other, a hydrogen atom, a substituted or unsubstituted C1-C20 alkyl group, a substituted or unsubstituted C2-C20 alkenyl group, a substituted or unsubstituted C2-C20 alkynyl group, Substituted or unsubstituted C1-C20 alkoxy group, substituted or unsubstituted C3-C30 cycloalkyl group, substituted or unsubstituted C3-C30 cycloalkenyl group, substituted or unsubstituted C3-C30 cycloalkynyl group, substituted or unsubstituted C6-C30 aryl group, substituted or unsubstituted C6-C30 aryloxy group, s may be an integer from 2 to 1000)

Preferably, R and R 'may be a C1-C10 alkyl group, a C2-C10 alkenyl group, or a C6-C10 aryl group.

The weight average molecular weight of the silicone rubber may be 5,000 to 150,000 g / mol. Within the above range, the ease of synthesis and mechanical properties after curing of the matrix resin in the composite sheet can be expected.

Glass transition temperature of the silicone rubber may be -150 ℃ ~ 30 ℃. Preferably it is -100 degreeC-20 degreeC, More preferably, it may be -80 degreeC-0 degreeC. In the above range, it has the advantage of excellent flexibility and rigidity and a small coefficient of thermal expansion.

Styrene-butadiene rubber (SBR), butadiene rubber, isoprene rubber, chloroprene rubber, neoprene rubber, ethylene-propylene-diene terpolymer, styrene-ethylene-butylene when the rubber compound contains silicone rubber Styrene (SEBS) block copolymers, styrene-ethylene-propylene-styrene (SEPS) block copolymers, acrylonitrile-butadiene rubber (NBR), hydrogenated nitrile rubber (NBR), It may further comprise one or more selected from the group consisting of fluorinated rubber, and plasticized polyvinylchloride rubber.

The rubber compound may have a viscosity of 3,500 cps to 7,000 cps at 25 ° C.

Such a composite sheet of the present invention comprises a composition for a matrix comprising a rubber-based compound, a crosslinking agent, a catalyst and an inhibitor; And curing of the reinforcing material impregnated in the composition.

The matrix may further comprise an inorganic filler. The inorganic filler can cause the matrix to have a viscosity of 3,500 cps-7,000 cps at 25 ° C. when added to the rubber compound.

The inorganic filler may use particles that can be used as a conventional filler. For example, the inorganic filler may use silica, alumina, titania, or a mixture thereof, but is not limited thereto.

The inorganic filler may have a particle radius of 1 nm-20 nm, preferably 1 nm-10 nm. In the above range, the dispersibility of the filler and may have mechanical properties as a composite sheet after curing of the matrix resin.

The inorganic filler may be included in an amount of 1-20% by weight, preferably 1-10% by weight, based on the rubber compound.

Matrix in the composite sheet may be included in 5 to 30% by weight, more preferably 10 to 20% by weight. Within this range, flexibility can be provided to the composite sheet.

The reinforcement is embedded in the matrix. The reinforcement may have a refractive index difference of 0.01 or less from the matrix. Within this range, it may have excellent transparency and light transmittance. Preferably, the difference between the matrix and the refractive index may be 0.0001 to 0.007.

The reinforcing material is from the group consisting of glass fibers, glass fiber cloth, glass fibric, glass nonwovens, glass mesh, glass beads, glass flakes, silica particles and colloidal silica. It may include one or more selected. Preferably, a glass fiber cloth can be used.

Reinforcing material in the composite sheet may be included in 70 to 95% by weight, preferably 80 to 90% by weight. Within this range, it is possible to ensure high heat resistance and mechanical properties of the flexible substrate, and to improve transparency, flexibility and light weight.

Curing may be carried out at 50 ° C.-100 ° C., preferably at 60 ° C.-100 ° C., for 0 minutes-5 hours, preferably 30 minutes-1 hour 30 minutes.

The composition for a matrix contains a rubber compound, a crosslinking agent, a catalyst and an inhibitor. The content of the rubber compound and the reinforcing material is as described above.

The crosslinking agent can use the crosslinking agent normally used in manufacture of the composite sheet for flexible substrates. For example, polyorganosiloxanes having Si—CH 3 and Si—H can be used. The crosslinking agent may be included such that the molar equivalent ratio of the number of moles of Si—H of the crosslinking agent to the number of moles of the C 2 -C 20 alkenyl group of the silicone rubber is 1.0 or more, preferably 1.0-1.3.

The catalyst may be a catalyst commonly used in the production of a composite sheet for a flexible substrate. For example, as the platinum-based or rhodium-based catalyst, a complex of platinum and an organic compound, a platinum and vinylated organosiloxane complex, a rhodium and an olefin complex, and the like can be used. Specifically, a vinylalkylsilane platinum complex containing Karstedt catalyst, platinum black, chloroplatinic acid, chloroplatinic acid-olefin complex, chloroplatinic acid-alcohol coordination compound, or a mixture thereof can be used. The catalyst may be included in the weight of the metal, 2 to 2000 ppm, preferably 5 to 500 ppm relative to the rubber compound.

The inhibitor can cure the silicone resin at a high temperature by inhibiting the action of the catalyst at 25 ° C. and not inhibiting the action at high temperatures.

Inhibitors may be used conventionally used in the manufacture of a composite sheet for a flexible substrate. For example, inhibitors include acetylenic alcohols including dimethyl-1-hexyn-3-ol, pyridine, phosphines, organic phosphites, unsaturatedamides, dialkylcarboxylates, dialkylacetylenedicarboxylates, alkylated Maleate, diallyl maleate, or mixtures thereof. Inhibitors may be included at 100 to 2500 ppm relative to the rubber compound.

The mixture of the catalyst and the inhibitor may be included in an amount of 0.01-1% by weight based on the rubber compound. Within this range, the catalytic action can be sufficiently exhibited. Preferably, it may be included in 0.05-0.2% by weight.

In the present specification, 'impregnation' may include a structure in which the reinforcing material is dispersed or included in the matrix composition or formed in a single layer or a multilayer structure in the matrix composition.

The composition for the matrix may have a viscosity of 3,500 cps-7,000 cps at 25 ° C. before curing. When the viscosity is less than 3,500 cps, the flowability of the matrix is high, and the surface flatness after curing is lowered. If the viscosity is higher than 7,000 cps, it may cause problems of impregnation and fairness during the compounding process of the reinforcement. Preferably, the viscosity may be 4,000 cps-6,000 cps at 25 ° C.

The rubber compound may include one or two or more kinds, and may have a viscosity of 3,500 cps-7,000 cps at 25 ° C.

The rubber compound may be a mixture of a polyorganosiloxane resin (A) having a viscosity of 1,500-3,500 cps at 25 ° C. and a polyorganosiloxane resin (B) having a viscosity of 9,000-12,000 cps at 25 ° C. Polyorganosiloxane resin (B) is a resin having a higher viscosity than the polyorganosiloxane resin (A), the weight average molecular weight may be 50,000-150,000g / mol.

The polyorganosiloxane resin (B) may be included at 5-60% by weight, preferably 10-50% by weight of the polyorganosiloxane resin (A). Within this range, the impregnability is good and the surface flatness of the composite sheet after curing may have an excellent effect.

Another method of producing a composite sheet of the present invention may include the step of impregnating and curing the reinforcing material in the composition for the matrix having a viscosity of 3500-7000cps at 25 ℃.

The composition for a matrix contains a rubber compound, a crosslinking agent, a catalyst and an inhibitor. The content of the matrix and the reinforcing material is as described above.

The composition for the matrix may further include an inorganic filler. The content of the inorganic filler is as described above.

Curing may be carried out at 50 ° C.-100 ° C., preferably 60-100 ° C., for 0 minutes −1 hour 30 minutes and preferably less than 30 minutes.

Specifically, the composition for a matrix may be impregnated with a reinforcing material, then sandwiched between the release-treated films and then laminated and cured.

The composite sheet manufactured by the manufacturing method may have a surface flatness (Ra) of more than 50 nm to less than 300 nm.

A flexible substrate and a display device including the flexible substrate according to another aspect of the present invention may include the composite sheet. Flexible substrates are used for displays or optical devices such as substrates for liquid crystal display devices (LCDs), substrates for color filters, substrates for organic EL display devices, substrates for solar cells, substrates for touch screen panels, and the like. It is available.

Hereinafter, the configuration and operation of the present invention through the preferred embodiment of the present invention will be described in more detail. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

Details that are not described herein will be omitted since those skilled in the art can sufficiently infer technically.

Manufacturing example One

Synthesis was performed using phenyldimethyl dimethoxy siloxane (PMDMS), dimethyl dimethoxy siloxane (DMDMS) and vinyl trimethoxy siloxane (VTMS). The weight ratio of (PMDMS + DMDMS): VTMS was maintained at 95: 5. 900 g of PMDMS, 615 g of DMDMS, and 60 g of VTMS were weighed and then hydrolyzed under KOH at 70 ° C. for 1 hour. The reaction was continued at 90 ° C., and toluene and water were added to lower the temperature to 25 ° C. and washed with water. Add 300 g of Vi-MM (1,1,3,3, -tetramethyl-1,3-divinyl disiloxane) and endcap at 50 ° C for 5 hours, wash at 25 ° C, remove solvent and remove PDMS resin as silicone rubber (Polyorganosiloxane resin having a phenyl group, a methyl group, and a vinyl group) was prepared. The viscosity in 25 degreeC of the manufactured PDMS resin was 3,000 cps. Viscosity was measured by Viscometer (Brookfield) at 25 ° C.

Manufacturing example 2

The reaction time before proceeding to the end capping in Preparation Example 1 was prolonged, to prepare a PDMS resin with a viscosity of 10,000cps at 25 ℃.

Manufacturing example 3

The reaction time was shortened in Preparation Example 1 to prepare a PDMS resin having a viscosity of 100 cps at 25 ° C.

The specifications of the specific components used in the following examples and comparative examples are as follows.

1.With rubber compound

(a1) PDMS resin prepared in Preparation Example 1

(a2) PDMS resin prepared in Preparation Example 2

(a3) PDMS resin prepared in Preparation Example 3 was used.

2. Glass fiber cloth (D-glass cloth, Owen corning) was used as the reinforcing material.

3. HMS-991 (Gelest) was used as a crosslinking agent.

4. Karstedt catalyst (PT-CS-1.8CS, Umicore) was used as a catalyst.

5. Surfynol was used as inhibitor.

6. As the inorganic filler, nano silica having a particle radius of 10 nm was used.

Example One

50 wt% of PDMS resin (a2) was added to the PDMS resin (a1) and stirred. To the obtained mixture, a crosslinking agent (HMS-991, Gelest) was added so that the molar equivalent ratio of the number of moles of Si-H of the crosslinking agent was 1.1 to the number of moles of vinyl group of the PDMS resin. 0.05 wt% of the mixture of catalyst and inhibitor was added to the total weight of PDMS resin and stirred to prepare a composition for the matrix. As a result of measuring the viscosity in the same manner as above, the viscosity at 25 ° C of the composition for matrix was 6,000 cps. 10 g of the prepared matrix composition was poured into 2 g of glass fiber cloth on a release-treated film, and degassed under vacuum. Placed between the release-treated film and laminated at a pressure of 0.1Mpa, and cured for 1 hour at 80 ℃ to prepare a composite sheet.

Example 2

10 wt% of the PDMS resin (a2) was added in Example 1, except that the viscosity of 4,000 cps at 25 ℃ of the composition for the matrix was carried out in the same manner to prepare a composite sheet.

Example 3

In Example 1, 5 wt% of the inorganic filler was added instead of the PDMS resin (a2) to prepare a composite sheet by performing the same method except that the viscosity was 5,000 cps at 25 ° C of the matrix composition.

Comparative example One

A composite sheet was prepared in the same manner as in Example 1, except that the PDMS resin (a2) was not used and the viscosity was 3,000 cps at 25 ° C. of the composition for the matrix.

Comparative example 2

10 wt% of PDMS resin (a3) was added instead of PDMS resin (a2) in Example 1, except that the viscosity was 1,500 cps at 25 ° C. of the composition for a matrix. .

Comparative example 3

A composite sheet was prepared in the same manner as in Example 1, except that 50 wt% of PDMS resin (a3) was added instead of PDMS resin (a2), except that the viscosity was 500 cps at 25 ° C. of the composition for a matrix.

Comparative example 4

Using the PDMS resin (a2) instead of the PDMS resin (a1) in Example 1, except that the viscosity of 10,000cps at 25 ℃ of the composition for the matrix was carried out in the same manner to prepare a composite sheet.

Experimental Example

With respect to the composite sheet prepared in Examples and Comparative Examples, the following physical properties were evaluated and the results are shown in Table 1.

Property evaluation method

(1) Surface flatness (Ra, nm): It was measured by non-contact using an optical profiler. The composite sheet was measured 5 points using NT1100 (Veeco) in an area of 1.2mm x 9mm and the average value was taken.

(2) Light transmittance (%): The light transmittance was measured at 550 nm using a UV spectrometer (Lambda 35, PerkinElmer) for the composite sheet.

(3) Matrix resin impregnation: (circle) the case where glass fiber cloth is well impregnated with the composition for matrices before hardening, and the case where it is not impregnated are represented by x.

Viscosity of the Composition for Matrix
(25 ℃, cps) Surface flatness
(Ra, nm) Light transmittance (%) Matrix Resin Impregnation Example 1 6000 90 92 ○ Example 2 4000 150 91 ○ Example 3 5000 110 91 ○ Comparative Example 1 3000 300 90 ○ Comparative Example 2 1500 400 90 ○ Comparative Example 3 500 550 90 ○ Comparative Example 4 10,000 - - ×

As shown in Table 1, the composite sheet prepared by adjusting the viscosity at 25 ° C. of the matrix before curing as in the present invention can be seen that the surface flatness is very low while maintaining a high light transmittance. On the other hand, the composite sheet of Comparative Examples 1-3 out of the viscosity has a significantly higher flatness than the present invention, the Comparative Example 4 can not produce a composite sheet due to poor impregnation to measure the surface flatness and transmittance I could not.

Claims

matrix; And a reinforcing material contained in the matrix, wherein the surface flatness (Ra) of the composite sheet is greater than 50 nm and less than 300 nm.

The composite sheet of claim 1, wherein the matrix comprises a crosslinked rubber compound.

The rubber compound of claim 2, wherein the rubber compound is silicone rubber, styrene-butadiene rubber (SBR), butadiene rubber, isoprene rubber, chloroprene rubber, neoprene rubber, ethylene-propylene-diene terpolymer, styrene -Ethylene-butylene-styrene (SEBS) block copolymer, styrene-ethylene-propylene-styrene (SEPS) block copolymer, acrylonitrile-butadiene rubber (NBR), hydrogenated nitrile rubber, NBR), fluorinated rubber, and a plastic sheet comprising at least one member selected from the group consisting of plasticized polyvinyl chloride rubber.

The method of claim 1, wherein the reinforcing material is glass fiber, glass fiber cloth, glass fibric, glass nonwoven, glass mesh, glass beads, glass flake, silica particles and Composite sheet characterized in that it comprises one or more selected from the group consisting of colloidal silica.

The composite sheet of claim 1, wherein the matrix further comprises an inorganic filler.

A composition for a matrix; And a reinforcing material impregnated in the composition for the matrix, wherein the composition for the matrix has a viscosity of 3,500 cps to 7,000 cps at 25 ° C.

According to claim 6, wherein the matrix composition is a polyorganosiloxane resin (A) having a viscosity of 1,500-3,500cps at 25 ℃ and polyorganosiloxane resin (B) having a viscosity of 9,000-12,000cps at 25 ℃ Composite sheet composition comprising a mixture of.

The composite sheet composition according to claim 7, wherein the polyorganosiloxane resin (B) is contained in an amount of 5-60% by weight based on the polyorganosiloxane resin (A).

The composition for composite sheets according to claim 6, wherein the matrix composition further comprises an inorganic filler.

Impregnating and curing the reinforcing material in the composition for the matrix including the rubber compound, the crosslinking agent, the catalyst and the inhibitor,
The matrix composition has a viscosity of 3,500cps to 7,000cps at 25 ℃ manufacturing method of the composite sheet.

The rubber compound is a polyorganosiloxane resin (A) having a viscosity of 1,500-3,500 cps at 25 ° C and a polyorganosiloxane resin (B) having a viscosity of 9,000-12,000 cps at 25 ° C. Production process characterized in that the mixture.

The method according to claim 11, wherein the polyorganosiloxane resin (B) is contained at 5-60% by weight based on the polyorganosiloxane resin (A).

The method of claim 10, wherein the matrix composition further comprises an inorganic filler.

A composite sheet prepared by the manufacturing method of claim 10 and having a surface flatness Ra of greater than 50 nm and less than 300 nm.

A flexible substrate comprising a composite sheet of claim 1 or a composite sheet manufactured by the manufacturing method of claim 10.

A display device comprising the flexible substrate of claim 15.