In addition to the sulfonyl group-containing diamine, an aromatic diamine capable of providing a high heat resistance and a low thermal expansion coefficient to a polyimide film may be used as the aromatic diamine component for forming the polyamic acid resin. Such aromatic diamine may be 4,4'-diaminodiphenyl propane, 4,4'-diaminodiphenyl methane, benzidine, 4,4'-diaminodiphenyl ether, 3,3'-dichlorobenzidine, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl ether, 3₅4'-diaminodiphenyl ether, 1,5-diamino naphthalene, 2,6-diamino naphthalene, 4,4'-diaminodiphenyl diethyl silane, 4,4'- diaminodiphenyl silane, 4,4'-diaminodiphenyl ethyl phosphine oxide, p- phenylene diamine, m-phenylene diamine, a derivative thereof, or a mixture thereof. In particular, p-phenylene diamine is preferred owing to a low cost and a low thermal expansion coefficient.

An acid dianhydride component for forming the polyamic acid resin as used herein is an aromatic acid dianhydride. The aromatic acid dianhydride may be pyromellitic acid dianhydride, 4,4'-oxydiphthalic acid dianhydride, 3,4,3',4'-biphenyl tetracarboxylic acid dianhydride, 3,4,3',4'-benzophenone tetracarboxylic acid dianhydride, 3,4,3 ',4'-diphenyl sulfone tetracarboxylic acid dianhydride, 2,2'-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride, 2,3,6,7-naphthalene tetracarboxylic acid dianhydride, 1,4,5,8-naphthalene tetracarboxylic acid dianhydride, 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl] propane dianhydride, 3,4,9, 10-perylene tetracarboxylic acid dianhydride, or a mixture thereof. In particular, 3,4,3',4'-biphenyl tetracarboxylic acid dianhydride is preferred since it can provide a good moisture stability to a polyimide film.

The polyimide film of the present invention can be manufactured by a method known in the art. For example, the polyimide film can be manufactured by a method comprising: (a) reacting an aromatic acid dianhydride and an aromatic diamine in the presence of a polar organic solvent to obtain a polyamic acid solution, (b) casting the polyamic acid solution on a support, a rotary drum, or a steel belt, followed by primary curing to obtain a gel film, and (c) heating the gel film to complete imidization.

In the above method, step (a) for obtaining the polyamic acid solution which is a precursor of a polyimide resin may be performed by one of the following known methods.

(1) An aromatic tetracarboxylic acid dianhydride is reacted with a deficient molar amount of an aromatic diamine in the presence of a polar organic solvent to obtain a prepolymer having an acid anhydride group at both ends, and the prepolymer is then polymerized with the aromatic diamine so that the molar amount of the aromatic tetracarboxylic acid dianhydride equals to that of the aromatic diamine; (2) An aromatic tetracarboxylic acid dianhydride is reacted with an excessive molar amount of an aromatic diamine in the presence of a polar organic solvent to obtain a prepolymer having an amino group at both ends, and the prepolymer is then polymerized with the aromatic tetracarboxylic acid dianhydride so that the molar amount of the aromatic tetracarboxylic acid dianhydride equals to that of the aromatic diamine;

(3) An aromatic diamine is dissolved in a polar organic solvent, and the resultant is then incubated with a substantially equal molar amount of an aromatic tetracarboxylic acid dianhydride so that polymerization occurs;

(4) An aromatic tetracarboxylic acid dianhydride is dissolved and/or dispersed in a polar organic solvent, and the resultant is then incubated with a substantially equal molar amount of an aromatic diamine so that polymerization occurs; or

(5) A mixture of substantially equal molar amounts of an aromatic tetracarboxylic acid dianhydride and an aromatic diamine is incubated in a polar organic solvent so that polymerization occurs.

A solvent that can be used in the synthesis of a polyamic acid may be a polar aprotic solvent. For example, the solvent may be N₅N- dimethylfqrmamide, N-methyl-2-pyrrolidone, N,N-dimethylacetamide or a combination thereof. The polyamic acid thus obtained is imidized into polyimide by a conventional thermal or chemical curing method. According to the thermal curing method, imidization is achieved by a heat treatment in the absence of a dehydrating agent and an imidization catalyst. According to the chemical curing method, a dehydrating agent (e.g., an acid anhydride such as acetic anhydride) and an imidization catalyst (e.g., tertiary amines such as isoquinoline, β-picoline, pyridine) are added to a solution of polyamic acid in an organic solvent. For example, a polyamic acid solution is mixed with a dehydrating agent and an imidization catalyst at a low temperature, and the resultant mixture is coated or cast on a substrate such as a support, a rotary drum or a steel belt, and heated at 50 to 200⁰C, preferably at 70 to 150^°C to induce partial curing and drying by activation of the dehydrating agent and the imidization catalyst to thereby obtain a self-supporting gel film.

Subsequently, the ends of the gel film thus obtained are fixed on a support, and the gel film is heated. At this time, the gel film is heated at 200 to 600^°C for 3 to 30 minutes to completely imidize the residual polyamic acid into polyimide through a dehydrating and ring-closing reaction. If the heating temperature is higher than 600^°C or the heating time is longer than 30 minutes, film degradation may occur. On the other hand, if the heating temperature is less than 200^°C or the heating time is shorter than 3 minutes, desired effects may not be achieved.

The polyimide film thus obtained has an average thickness of 5 to 125 μm.

The polyimide film of the present invention has a Young's modulus of 4.5 to 9.5 GPa, preferably 5.5 to 7.5 GPa, a tensile strength at breakage of 15 to 30 kgf/mnf, preferably 17 to 25 kgf/mnf, and a tensile elongation at breakage of 25 to 60%, preferably 35 to 50%. If the Young's modulus of the polyimide film is less than 4.5 GPa, the mechanical dimensional stability of the polyimide film may be insufficient, and during punching, burrs or chips may be easily formed. On the other hand, if the Young's modulus of the polyimide film exceeds 9.5 GPa, much energy may be required for initial cutting during punching. Meanwhile, if the tensile strength at breakage of the polyimide film is less than 15 kgf/mnf or the tensile elongation at breakage is less than 25%, film breakage may occur during punching, and thus, fine burrs or chips may be left around the holes. The burrs and chips may adversely affect the quality of the circuits manufactured. On the other hand, if the tensile strength at breakage of the polyimide film exceeds 30 kgf/nuif or the tensile elongation at breakage exceeds 60%, fibrous burrs or chips may be formed during punching.

The polyimide film according to the present invention is characterized in that a ratio of the tensile strength at breakage to the yield strength satisfies Equation 1 below. The yield strength is defined as a tensile strength at a first point at which the slope of the tensile strength-elongation curve of a film rapidly decreases.

Tensile strength at breakage / Yield strength < 1.35

If the ratio of the tensile strength at breakage to the yield strength is greater than or equal to 1.35, too much energy is required between initial cutting and final cutting during punching with a higher likelihood that burrs or chips are formed. The lower limit of the ratio of the tensile strength at breakage to the yield strength is not particularly defined since there are few polymer films having the ratio of the tensile strength at breakage to the yield strength of 1.0 or less.

The inventive polyimide film satisfying the above physical properties exhibits improved adhesive strength with metal, flexural endurance, and punching characteristics, and thus, can be used in electronic devices such as a flexible circuit board or the like. For example, when a polyimide film of the present invention is used in manufacturing a two-layered flexible copper clad laminate (FCCL), the polyimide film has good punching characteristics, an adhesive strength with metal of 0.6 kgf/nmf or more, preferably 0.7 kgf/mπf or more, more preferably 0.8 kgf/mnf or more, and 20,000 cycles or more of bending, preferably 35,000 cycles or more, more preferably 50,000 cycles or more.

As described above, a polyimide film according to the present invention exhibits improved adhesive strength with metal, flexural endurance and punching characteristics, and thus, can be efficiently used as a film for electronic devices.

Hereinafter, the present invention will be described more specifically by Examples. However, the following Examples are provided only for illustrations, and thus the present invention is not limited to or by them.

Example 1

Dimethylacetamide (DMAc) (468.97 g) was introduced in a reactor, and the reaction temperature was set to 40^°C . Then, p-phenylene diamine (PPD) (18.51 g, 75 mole % based on the total mole of diamine components) and 4,4'- bis(4-aminophenoxy)diphenyl sulfone (BAPS) (24.71 g, 25 mole % based on the total mole of the diamine components) were added thereto and the reaction mixture was stirred until the components were completely dissolved. Then, pyromellitic acid dianhydride (PMDA) (28.40 g, 60 mole % based on the total mole of acid dianhydride components) and 3,4,3',4'-biphenyl tetracarboxylic acid dianhydride (BPDA) (26.88 g, 40 mole % based on the acid dianhydride components) were gradually added thereto so that the molar amount of the acid dianhydride components was substantially equal to that of the diamine components, and the reaction mixture was then stirred for one hour to obtain a solution of polyamic acid in DMAc.

The polyamic acid solution was mixed with 5 mole eq. (based on the polyamic acid) of acetic anhydride (AA) and 1 mole eq. (based on the polyamic acid) of isoquinoline (IQ), and the reaction mixture was uniformly coated on a glass plate, dried at 110 "C for 10 minutes, and delaminated to obtain a gel film. Then, the gel film was fixed to a support frame using pins, heated at 250^°C for 5 minutes, and at 450^°C for 5 minutes to induce a dehydrating and ring-opening reaction (imidization reaction) to thereby obtain a polyimide film with a thickness of 38 μm.

Example 2

The procedure of Example 1 was repeated except for using DMAc

(465.2 g); PPD (27.53 g, 95 mole %) and BAPS (5.80 g, 5 mole %) as diamine components; and PMDA (33.35 g, 60 mole %) and BPDA (31.56 g, 40 mole %) as acid dianhydride components.

Example 3

The procedure of Example 1 was repeated except for using DMAc

(472.3 g); PPD (9.09 g, 45 mole %) and BAPS (44.47 g, 55 mole %) as diamine components; and PMDA (23.23 g, 60 mole %) and BPDA (21.99 g, 40 mole %) as acid dianhydride components.

Example 4

The procedure of Example 1 was repeated except for using DMAc (471.89 g); PPD (13.62 g, 65 mole %) and BAPS (29.36 g, 35 mole %) as diamine components; and BPDA (57.02 g, 100 mole %) as an acid dianhydride component.

Example 5

The procedure of Example 1 was repeated except for using DMAc (465.7 g); PPD (21.66 g, 75 mole %) and 4,4-diaminodiphenyl sulfone (ASN) (11.9 g, 25 mole %) as diamine components; and PMDA (34.98 g, 60 mole %) and BPDA (31.45 g, 40 mole %) as acid dianhydride components.

Comparative Example 1

The procedure of Example 1 was repeated except for using DMAc (465.55 g); PPD (29.06 g, 100 mole %) as a diamine component; and PMDA (23.47 g, 40 mole %) and BPDA (47.47 g, 60 mole %) as acid dianhydride components. Comparative Example 2

The procedure of Example 1 was repeated except for using DMAc (473.79 g); PPD (5.55 g, 30 mole %) and BAPS (51.88 g, 70 mole %) as diamine components; and PMDA (22.41 g, 60 mole %) and BPDA (20.15 g, 40 mole %) as acid dianhydride components.

Comparative Example 3

The procedure of Example 1 was repeated except for using DMAc

(476.13 g); BAPS (62.13 g, 100 mole %) as a diamine component; and PMDA (12.53 g, 40 mole %) and BPDA (24.34 g, 60 mole %) as acid dianhydride components.

The polyimide films manufactured in Examples 1 to 5 and Comparative

Examples 1 to 3 were evaluated for the following properties, and the results are summarized in Table 1 below.

(1) Tensile strength at breakage, tensile elongation at breakage, yield strength and Young's modulus

The tensile strength at breakage, tensile elongation at breakage, and

Young's modulus of the polyimide films were measured using a UTM machine

(Instron) according to ASTM D882. The yield strength was defined as a tensile strength corresponding to the elongation value at the intersection of an initial slope and a final slope of a tensile strength-elongation curve.

(2) Adhesive strength with metal

In order to measure the adhesive strength of each polyimide film to metal, two-layered FCCLs were manufactured as follows. First, the polyimide films were surface-treated with plasma under a reduced pressure under an atmosphere of oxygen and argon, and an alloy of nickel and chromium was vacuum-deposited on the polyimide films. Then, a copper target was vacuum- deposited to a thickness of 2,500 A on the resultant structures. The resultant films were subjected to electrolysis plating to obtain two-layered FCCLs having a copper layer with a thickness of 8 μm ± 1 μm.

The 90° peeling strength of the two-layered FCCLs thus obtained were measured ten times according to IPC TM 650 2.4.9. An average of the peeling strengths was calculated.

(3) Flexural endurance

The cycles of bending of the FCCLs were ten times measured according to IPC TM 650 2.4.3, and an average of the cycles of bending was calculated.

(4) Punching characteristics

Holes with a diameter of 4 mm were formed in the FCCLs by means of a punch. The degree of burrs or chips formed was observed with the naked eye. The criteria of the judgment were as follows: Good: no breakage in punched sections, no burrs and chips, and clear surface

Normal: partial breakage in punched sections or occurrence of burrs or chips with a size of 1 mm or less

Bad: severe breakage in punched sections or occurrence of burrs or chips with a size of greater than 1 mm

Table 1

Composition of film Physical properties of film

Acid dianhydπde Diamine (a) (b) (c)⁽A) (e) rø (g) (h)

PMDA BPDA PPD BAPS ASN (kgf/ (%) (GPa) (kgf/ (kgf/ (cycles)

(mol%) (mol%) (mol%) (mol%) (mol%) mnf) ■if) cm)

Exam 1 60 40 75 25 - 21 45 7 1 17 1 24 076 46,300 Good

Exam 2 60 40 95 5 - 28 52 8 3 21 1 32 0 69 52,100 Good

Exam 3 60 40 45 55 - 16 32 6 1 14 1 17 0 84 31,040 Good

Exam 4 - 100 65 35 - 18 40 6 2 15 1 22 0 81 42,600 Good

Exam 5 60 40 75 - 25 26 28 7 9 20 1 27 0 71 39,100 Good

Comp 1 40 60 100 - - 38 42 10 2 19 1 96 0 52 18,200 Bad

Comp 2 60 40 30 70 - 15 24 7 0 13 1 19 0 85 19,400 Normal

Comp 3 40 60 - 100 - 14 22 5 8 12 1 14 0 91 14,800 Bad

Exam.: Example Comp.: Comparative Example

PMDA: pyromellitic acid dianhydride BPDA: biphenyl tetracarboxylic acid dianhydride

PPD: p-phenylene diamine BAPS: bisaminophenoxy sulfone

ASN: diaminodiphenylsulfone

(a): tensile strength (b): elongation

(c): Young's modulus (d): yield strength

(e): tensile strength / yield strength (f): adhesive strength

(g): flexural endurance (h): punching characteristics

As shown in Table 1, the inventive polyimide films had a Young's modulus of 4.5 to 9.5 GPa, a tensile strength at breakage of 15 to 30 kgf/W, a tensile elongation at breakage of 25 to 60%, and a ratio of the tensile strength at breakage to the yield strength of less than 1.35. Therefore, when used in electronic devices, the inventive polyimide films can exhibit improved adhesive strength with metal, flexural endurance, and punching characteristics, as compared with those of Comparative Examples.

While the invention has been described with respect to the above specific embodiments, it should be recognized that various modifications and changes may be made to the invention by those skilled in the art which also fall within the scope of the invention as defined by the appended claims.

Claims

WHAT IS CLAIMED IS

1. A polyimide film derived from a polyamic acid resin obtained by copolymerization of an aromatic acid dianhydride component and an aromatic diamine component, wherein the aromatic diamine component comprises 5 to 60 mole % of a diamine having a sulfonyl group in its main chain based on the total mole of the aromatic diamine component.

2. The polyimide film of claim 1, wherein the sulfonyl group-containing diamine is at least one selected from the group consisting of 3,3'- diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone, 4,4'-bis(4- aminophenoxy)diphenyl sulfone, 4,4'-bis(3-aminophenoxy)diphenyl sulfone and 3,6-thioxanthenediamine-10,10-dioxide.

3. The polyimide film of claim 2, wherein the sulfonyl group-containing diamine is 4,4'-bis(4-aminophenoxy)diphenyl sulfone represented by Formula 1 below: <Formula 1>

4. The polyimide film of claim 1, wherein the aromatic diamine component further comprises at least one selected from the group consisting of 4,4'-diaminodiphenyl propane, 4,4'-diaminodiphenyl methane, benzidine, 4,4'- diaminodiphenyl ether, 3,3'-dichlorobenzidine, 4_s4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 1,5-diamino naphthalene, 2,6-diaminonaphthalene, 4,4'-diaminodiphenyl diethyl silane, 4,4'- diaminodiphenyl silane, 4,4'-diaminodiρhenyl ethyl phosphine oxide, p- phenylene diamine, m-phenylene diamine and derivatives thereof.

5. The polyimide film of claim 1, wherein the aromatic acid dianhydride component is at least one selected from the group consisting of pyromellitic acid dianhydride, 4,4'-oxydiphthalic acid dianhydride, 3,4,3',4'-biphenyl tetracarboxylic acid dianhydride, 3,4,3',4'-benzophenone tetracarboxylic acid dianhydride, 3,4,3',4'-diphenyl sulfone tetracarboxylic acid dianhydride, 2,2'- bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride, 2,3,6,7-naphthalene tetracarboxylic acid dianhydride, 1,4, 5, 8 -naphthalene tetracarboxylic acid dianhydride, 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl] propane dianhydride, and 3,4,9,10-perylene tetracarboxylic acid dianhydride.

6. The polyimide film of claim 1, which has a Young's modulus of 4.5 to 9.5 GPa, a tensile strength at breakage of 15 to 30 kgf/mnf, and a tensile elongation at breakage of 25 to 60%.

7. The polyimide film of claim 1, wherein the tensile strength at breakage and the yield strength of the polyimide film satisfy Equation 1 below: <Equation 1> Tensile strength at breakage / Yield strength < 1.35

8. An electronic device comprising the polyimide film of any one of claims 1 through 7.