Surface modification method and fluororesin

Abstract

To provide a surface modification method that is excellent in improving the adhesion of the surface of a fluororesin, and a fluororesin with excellent surface adhesion. [Solution] A surface modification method comprising reacting a radical-reactive compound having an acid anhydride group with a partially fluorinated fluororesin on the surface of a fluororesin containing a partially fluorinated fluororesin in the presence of an organometallic compound. The fluororesin contains a partially fluorinated fluororesin, and the structure having an acid anhydride group exists only on the surface.

Description

Technical Field

[0001] The present invention relates to a surface modification method and a fluororesin.

Background Art

[0002] Fluororesins are excellent in chemical stability and thermal stability and have the characteristic of a low coefficient of friction. Due to these characteristics, fluororesins are expected to be applied in various fields such as the electronics field, the biotechnology field, and the medical field.

[0003] Fluororesins may be used in combination with other materials. However, since fluororesins have a low surface energy and are chemically stable, their adhesiveness to other materials is low. In order to maximize the performance of fluororesins, the development of a surface modification technique suitable for the intended use is required.

[0004] In Patent Document 1, a surface modification method is proposed in which a fluororesin is reacted with a radical reactive compound in the presence of an organometallic compound. According to this surface modification method, graft polymerization is possible at a lower temperature than before. Further, Patent Document 1 describes that in the examples, the hydrophilicity and water absorption of the surface were improved by the surface modification.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The fluororesin surface-modified by the surface modification method of Patent Document 1 has improved adhesiveness because the hydrophilicity of the surface is improved compared to before the surface modification. However, depending on the application, more excellent adhesiveness may be required.

[0007] The present invention provides a surface modification method that is excellent in improving the adhesion of the surface of a fluororesin, and a fluororesin with excellent surface adhesion. [Means for solving the problem]

[0008] The present invention has the following aspects. [1] A surface modification method comprising reacting a radical-reactive compound having an acid anhydride group with a partially fluorinated fluororesin on the surface of a fluororesin containing a partially fluorinated fluororesin in the presence of an organometallic compound. [2] The surface modification method according to [1], wherein the radical-reactive compound is at least one selected from the group consisting of maleic anhydride, itaconic anhydride, citraconic anhydride, and 5-norbornene-2,3-dicarboxylic acid anhydride. [3] The surface modification method according to [1] or [2], wherein the radical-reactive compound is reacted such that the graft rate calculated by the following formula is 0.1 to 3.0%. Graft rate (%) = {(Mass of fluororesin after surface modification - Mass of fluororesin before surface modification) / Mass of fluororesin before surface modification} × 100 (%) [4] A surface modification method according to any of the above [1] to [3], wherein the reaction temperature when reacting the radical-reactive compound is 0 to 100°C. [5] A surface modification method according to any of the above [1] to [4], wherein the partially fluorinated fluororesin does not have acid anhydride groups. [6] Any of the surface modification methods [1] to [5] above, wherein the partially fluorinated fluororesin is at least one selected from the group consisting of ethylene-tetrafluoroethylene copolymer and vinylidene fluoride polymer. [7] A surface modification method according to any of the above [1] to [6], wherein the fluororesin is in the form of a film. [8] A fluororesin that includes a partially fluorinated fluororesin, A fluoropolymer in which a structure containing acid anhydride groups exists only on the surface. [9] The fluororesin of [8] having a structure having units based on a radical-reactive compound having an acid anhydride group.

[10] The fluororesin according to [9], wherein the radical-reactive compound is at least one selected from the group consisting of maleic anhydride, itaconic anhydride, citraconic anhydride, and 5-norbornene-2,3-dicarboxylic acid anhydride.

[11] The partially fluorinated fluororesin is any of the fluororesins described in [8] to

[10] above, wherein the fluororesin is at least one selected from the group consisting of ethylene-tetrafluoroethylene copolymers and vinylidene fluoride polymers.

[12] A fluororesin in the form of a film, one of the above [8] to

[11] . [Effects of the Invention]

[0009] According to the present invention, a surface modification method that is excellent in improving the adhesion of the surface of a fluororesin, and a fluororesin with excellent surface adhesion can be provided. [Modes for carrying out the invention]

[0010] The meanings and definitions of terms used in this invention are as follows: A numerical range represented using "~" includes the numbers on both sides of the "~". The term "acid anhydride group" refers to a group represented by -C(=O)-OC(=O)-. A "radical-reactive compound" is a compound in which polymerization proceeds via a radical chain reaction.

[0011] [Surface modification method] A surface modification method according to one embodiment of the present invention involves reacting a partially fluorinated fluororesin with a radical-reactive compound having an acid anhydride group (hereinafter also referred to as "radical-reactive acid anhydride") on the surface of a fluororesin containing a partially fluorinated fluororesin, in the presence of an organometallic compound. As a result, radical-reactive acid anhydrides are grafted onto the partially fluorinated fluororesin on the surface of the fluororesin, forming a structure having acid anhydride groups.

[0012] <Fluororesin> Partially fluorinated fluororesins have hydrogen atoms bonded to carbon atoms. By having at least a portion of the fluororesin constituting the fluororesin be a partially fluorinated fluororesin, radical-reactive acid anhydrides can be grafted onto the surface of the fluororesin.

[0013] Examples of partially fluorinated fluororesins include ethylene-tetrafluoroethylene copolymer (ETFE), vinylidene fluoride polymer (PVDF), vinyl fluoride polymer, vinylidene fluoride-hexafluoropropylene copolymer, tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymer, tetrafluoroethylene-propylene copolymer, tetrafluoroethylene-vinylidene fluoride-propylene copolymer, ethylene-hexafluoropropylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene copolymer, and propylene-chlorotrifluoroethylene copolymer. ETFE may further have units based on monomers other than ethylene and tetrafluoroethylene. The same applies to other polymers. The partially fluorinated fluororesin in the fluororesin may be one or more types. As the partially fluorinated fluororesin, at least one selected from the group consisting of ETFE and PVDF is preferred. ETFE and PVDF have excellent mechanical strength, heat resistance, and moldability, and can be used in a wide range of applications. Among them, ETFE is a well-balanced fluororesin that combines excellent chemical resistance and electrical properties with mechanical properties and moldability. It is preferable that the partially fluorinated fluororesin in the fluororesin before surface modification does not contain acid anhydride groups.

[0014] The fluororesin may further contain other fluororesins besides the partially fluorinated fluororesin. Examples of other fluororesins include perfluoro(alkyl vinyl ether)-tetrafluoroethylene copolymer (PFA), hexafluoropropylene-tetrafluoroethylene copolymer, chlorotrifluoroethylene polymer, and polytetrafluoroethylene (PTFE). When the fluororesin contains other fluororesins, there may be one or more types of other fluororesins in the fluororesin.

[0015] The ratio of the partially fluorinated fluororesin to all the fluororesins (the total of the partially fluorinated fluororesin and other fluororesins) in the fluororesin is preferably 50% by mass or more, more preferably 70% by mass or more, and may be 100% by mass. The higher the ratio of the partially fluorinated fluororesin, the more sites react with the radical-reactive acid anhydride on the surface of the fluororesin, and the more excellent the adhesion improving effect.

[0016] Additives may be added to the fluororesin. As the additives, known additives can be appropriately selected according to the use. The addition amount of the additives is, for example, 0 to 10% by mass based on the total mass of the fluororesin.

[0017] The fluororesin may be pre-formed into an arbitrary shape before surface modification. Examples of the shape of the fluororesin include film shape and particle shape. When the fluororesin is in film shape, the film thickness is preferably 10 to 1000 μm, more preferably 12 to 500 μm, and even more preferably 15 to 200 μm. When the film thickness is within this range, the balance between the strength and flexibility of the film is good, so it is easy to handle and the characteristics of the fluororesin are more likely to be exhibited. Here, the film thickness is a measured value by a micrometer.

[0018] When the fluororesin is in particle shape, the average particle diameter of the particles is preferably 0.1 to 1000 μm, more preferably 0.5 to 500 μm, and even more preferably 1 to 200 μm. When the average particle diameter of the particles is within this range, the processability and ease of handling are excellent, and the characteristics of the fluororesin are more likely to be exhibited. Here, the average particle diameter of the fluororesin particles is measured by a laser diffraction / scattering method or the like.

[0019] <Radical-reactive compound> Examples of radical-reactive acid anhydrides include compounds having polymerizable unsaturated bonds and acid anhydride groups. Examples of polymerizable unsaturated bonds include polymerizable carbon-carbon double bonds and polymerizable carbon-carbon triple bonds. As the radical-reactive acid anhydride, at least one selected from the group consisting of maleic anhydride, itaconic anhydride, citraconic anhydride, and 5-norbornene-2,3-dicarboxylic acid anhydride is preferred due to its excellent adhesion-enhancing effect. These radical-reactive acid anhydrides have low homopolymerizability. Therefore, the acid anhydride group structure formed solely from these radical-reactive acid anhydrides is thought to be a short-chain structure with about one or two molecules of the radical-reactive acid anhydride grafted onto it. This is considered to be one of the reasons for its excellent adhesion-enhancing effect.

[0020] Radical-reactive acid anhydrides may be used in combination with other radical-reactive compounds. Examples of other radical-reactive compounds are those described in International Publication No. 2021 / 060511. In terms of improving adhesion, a higher proportion of radical-reactive acid anhydrides to the total of all radical-reactive compounds is preferable, and it is particularly preferable that the radical-reactive compounds consist solely of radical-reactive acid anhydrides.

[0021] The amount of radical-reactive compound used is calculated based on the surface area of ​​the treated surface (1 m²) when the fluororesin is in film form. 2 Preferably, the amount is 0.001 to 1000 moles, more preferably 0.01 to 500 moles, and more preferably 0.05 to 400 moles. However, if the scale of production is large, then preferably 0.001 to 50 moles, more preferably 0.01 to 20 moles, even more preferably 0.05 to 10 moles, and particularly preferably 0.03 to 5 moles. If the fluororesin is in particulate form, for example, if the average particle size is 1 to 100 μm, then the surface area of ​​the treated surface is 1 m². 2 Preferably, the amount is 0.0001 to 100 moles, more preferably 0.001 to 50 moles, and more preferably 0.01 to 10 moles. When the amount of radical-reactive compound used is within this range, graft polymerization can be carried out in a shorter time.

[0022] <Organometallic compounds> In the surface modification method of this embodiment, organometallic compounds function as radical initiators. By using organometallic compounds, radicals can be generated at low temperatures, for example, below 70°C. The organometallic compound is preferably one that contains at least one element selected from the group consisting of Group 12, Group 13, and Group 15 elements of the IUPAC (International Union of Pure and Applied Chemistry) periodic table. Examples of Group 12 elements are zinc, cadmium, and mercury; examples of Group 13 elements are boron and aluminum; and an example of Group 15 elements is antimony.

[0023] As the organometallic compound, at least one selected from the group consisting of dialkylzinc complexes and trialkylborons is more preferred. As the alkyl group of the dialkylzinc complex and trialkylboron, alkyl groups having 1 to 6 carbon atoms are preferred, and among these, ethyl groups and butyl groups are preferred. As the dialkylzinc complex and trialkylboron, at least one selected from the group consisting of diethylzinc 1,10-phenanthroline complex, diethylzinc 2,2'-bipyridine complex, and tributylboron is even more preferred, and diethylzinc 1,10-phenanthroline complex is particularly preferred. Dialkylzinc complexes and trialkylborons exhibit different states depending on the number of carbon atoms in the alkyl group and the temperature range. For example, within the reaction temperature range, diethylzinc 1,10-phenanthroline complex is a solid, while tributylboron is a liquid. If liquid, it may be used directly in the reaction or diluted with a solvent. If solid, it may be diluted with a solvent before use, or if the radical-reactive compound is liquid, it may be used directly in the reaction. When used directly, the reaction proceeds more easily because it is not diluted. Dialkylzinc complexes and trialkylborons can undergo graft polymerization in a short time of 24 hours or less, even at low temperatures of 50°C or below. Furthermore, trialkylboron is preferred as a radical initiator because it facilitates graft polymerization even when the type of radical-reactive compound or fluororesin is changed.

[0024] Organometallic compounds can be purchased commercially or synthesized using conventionally known synthesis methods. For example, diethylzinc 1,10-phenanthroline complex (Phen-DEZ) can be synthesized by the method described in the examples below in International Publication No. 2021 / 060511.

[0025] The amount of organometallic compound used is preferably 0.001 to 3 moles, more preferably 0.001 to 2 moles, and even more preferably 0.002 to 1 mole, per mole of the radical-reactive compound. Organometallic compounds may be added to the reaction system all at once or added in stages.

[0026] <Auxiliary catalyst> In the surface modification method of this embodiment, a radical-reactive acid anhydride may be reacted with a partially fluorinated fluororesin in the presence of a co-catalyst. Using a co-catalyst can promote or regulate radical generation from organometallic compounds. Examples of co-catalysts include metal oxides, alcohols, quinones, hydroquinones, oxygen, carbon disulfide, amines, water, and hydrogen peroxide. As a co-catalyst, at least one selected from the group consisting of these compounds is preferred, oxygen is more preferred, and molecular oxygen (O2) is even more preferred. When oxygen is used as a co-catalyst, the radical generation reaction proceeds more efficiently, and the graft reaction can be carried out in a shorter time even at low temperatures of 65°C or below. The amount of co-catalyst used is, for example, 0.1 to 1.2 moles per mole of organometallic compound. If the amount of co-catalyst used is below the above upper limit, the radicals generated from the organometallic compound are less likely to be deactivated.

[0027] <Liquid media> In the surface modification method of this embodiment, a radical-reactive acid anhydride may be reacted with a partially fluorinated fluororesin in the presence of a liquid medium. The liquid medium is not particularly limited as long as it can dissolve or disperse organometallic compounds, radical-reactive compounds, and co-catalysts, if used. Examples of liquid media include organic solvents, water, and mixtures thereof. The liquid medium is preferably one that can dissolve or disperse organometallic compounds, and can be appropriately selected depending on the type of organometallic compound.

[0028] For example, when the organometallic compound is a dialkylzinc complex or trialkylboron, a nonpolar organic solvent such as toluene, xylene, or hexane is preferred. When using a liquid medium, the concentration of the dialkylzinc complex is not particularly limited, but is preferably 0.01 to 5.0 M, more preferably 0.02 to 3.0 M, and even more preferably 0.05 to 2.0 M. When using a liquid medium, the concentration of trialkylboron is not particularly limited, but is preferably 0.01 to 5.0 M, more preferably 0.02 to 3.0 M, and even more preferably 0.05 to 2.0 M.

[0029] The amount of liquid medium used is not particularly limited, but using too much is undesirable because it reduces the concentration of organometallic compounds and radical-reactive anhydrides, thus slowing down the reaction rate. In the surface modification method of this embodiment, if the organometallic compound is in a liquid state within the reaction temperature range, it is preferable not to use a liquid medium.

[0030] <Reaction conditions> The reaction temperature is preferably 0 to 100°C, more preferably 20 to 80°C, and even more preferably 30 to 70°C. If the reaction temperature is above the lower limit, the radical-reactive acid anhydride reacts easily. If the reaction temperature is below the upper limit, the fluororesin is less likely to deform.

[0031] The reaction time is preferably 0.1 to 36 hours, more preferably 0.2 to 24 hours, even more preferably 0.5 to 18 hours, and particularly preferably 1 to 12 hours. The reaction time can be appropriately set depending on the type and amount of organometallic compound and radical-reactive anhydride, the reaction temperature, etc.

[0032] A non-oxidizing atmosphere is preferred for the reaction. A non-oxidizing atmosphere is preferably an argon gas atmosphere, a nitrogen gas atmosphere, or a hydrogen gas atmosphere. By carrying out the graft reaction in a non-oxidizing atmosphere, the formation of by-products can be further suppressed.

[0033] <Graft rate> The graft rate achieved by the surface modification method of this embodiment is preferably 0.1 to 3.0%, more preferably 0.2 to 2.0%, and even more preferably 0.3 to 1.0%. When the graft rate is above the lower limit, acid anhydride groups are sufficiently introduced to the surface, resulting in better adhesion. When the graft rate is below the upper limit, the heat resistance is better.

[0034] The graft rate is calculated using the following formula. Graft rate (%) = {(Mass of fluororesin after surface modification - Mass of fluororesin before surface modification) / Mass of fluororesin before surface modification} × 100 (%) The grafting rate can be adjusted by the amount of radical-reactive compound used, the reaction conditions, etc. Reaction conditions include temperature, pressure, time, and the amount of initiator used.

[0035] [Fluororesin] A fluororesin according to one embodiment of the present invention includes a partially fluorinated fluororesin, and a structure having acid anhydride groups (hereinafter also referred to as "acid anhydride group structure") exists only on the surface. Typically, multiple acid anhydride group structures are dispersed on the surface of the fluororesin. Typically, acid anhydride group structures are bonded to the partially fluorinated fluororesin on the surface of the fluororesin.

[0036] The fluororesin of this embodiment is the same as the fluororesin before surface modification in the surface modification method described above, except that an acid anhydride group structure is present on its surface.

[0037] The acid anhydride group structure is formed, for example, by the surface modification method described above. In this case, the acid anhydride group structure has units based on radical-reactive acid anhydrides. A single acid anhydride group structure may have one or more units based on radical-reactive acid anhydrides. The acid anhydride group structure may further have units based on radical-reactive compounds other than radical-reactive acid anhydrides. In terms of improving adhesion, it is preferable that the proportion of units based on radical-reactive acid anhydride to the total number of units constituting the acid anhydride group structure is high, and it is particularly preferable that the acid anhydride group structure consists only of units based on radical-reactive acid anhydride.

[0038] The fluororesin of this embodiment can be laminated with other materials to form a laminate. Other materials include metals such as copper, iron, stainless steel, and aluminum, as well as synthetic resins such as polyamide, polyimide, and polyvinyl alcohol. [Examples]

[0039] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following examples. Examples 1 to 12 are examples, and Examples 13 to 14 are comparative examples.

[0040] [Abbreviation] The abbreviations used in the examples are as follows: ETFE ···Ethylene-tetrafluoroethylene copolymer TBB... Tributylboron MAH ···Maleic anhydride IAH ···Itaconic Acid AA ···Acrylic acid MMA · Methyl methacrylate MeOH ··· methanol

[0041] [Evaluation Method] (FT-IR) Fourier transform infrared spectroscopy (FT-IR) measurements were performed using the FT / IR-6600 instrument manufactured by JASCO Corporation, employing the ATR-FTIR method with a resolution of 4 cm. -1 (Total number of measurements: 32), Measurement range: 4000-600 cm -1 The measurement was performed using [this method].

[0042] (Graft rate) The graft rate was calculated using the following formula after confirming the C=O absorption peak derived from the radical-reactive compound by FT-IR measurement. Graft rate (%) = {(Mass of fluororesin after surface modification - Mass of fluororesin before surface modification) / Mass of fluororesin before surface modification} × 100 (%) In this embodiment, ETFE film was used as the "fluororesin".

[0043] (Adhesiveness) Surface-modified ETFE film and copper foil were layered and hot-pressed. The interlayer peel strength of the resulting laminate was measured. The hot-pressing conditions were a temperature of 300°C and a pressure of 1 MPa, held for 10 seconds. Peel strength was measured by the degree of peel when the laminate was fixed to the chuck of a tensile testing machine and peeled 180°.

[0044] [Example 1] The surface of the ETFE film was modified using the following procedure. A cylindrical separable flask with a capacity of 300 mL (mouth diameter × body diameter × height: φ75 × φ80 × 120 ± 3 mm) was placed with a stirring bar, a 50 mm × 50 mm ETFE film (Fluon® ETFE, manufactured by AGC Inc.; thickness 250 μm), and 5 g (50 mmol) of MAH. The inside was then purged with argon gas, and 30 mL of deoxygenated and dehydrated toluene was added as the liquid medium. After heating to 60 °C, 2 mL of a 50 mass% toluene solution of TBB (4.4 mmol of TBB) was added dropwise. While stirring, equimolar (4.4 mmol) of molecular oxygen (O2) was added to the TBB, and stirring was continued at 60 °C for 2 hours. After that, the ETFE film was removed, and the TBB adhering to the surface was washed off with hexane. To further remove any ungrafted MAH, the ETFE film was washed with acetone, methanol, and water, respectively, and then dried under vacuum for 6 hours. After surface modification, FT-IR measurements revealed a C=O absorption peak originating from MAH. The graft rate and adhesion evaluation results are shown in Table 1.

[0045] [Example 5] The surface of the ETFE film was modified using the following procedure. A cylindrical separable flask with a capacity of 300 mL (mouth diameter × body diameter × height: φ75 × φ80 × 120 ± 3 mm) was placed with a stirring bar, a 50 mm × 50 mm ETFE film (Fluon® ETFE, manufactured by AGC Inc.; thickness 250 μm), and 1 g (10 mmol) of MAH. The inside was then purged with argon gas, and 30 mL of deoxygenated and dehydrated toluene was poured in as the liquid medium. After heating to 60 °C, 1 mL of a 50 mass% toluene solution of TBB (2.3 mmol of TBB) was added dropwise. While stirring, an equimolar amount (2.3 mmol) of molecular oxygen (O2) with TBB was added, and after 60 minutes, another 2.3 mmol of TBB was added dropwise. Then, an equimolar amount (2.3 mmol) of molecular oxygen (O2) with the added TBB was added, and stirring was continued at 60 °C for 2 hours. Subsequently, the ETFE film was lifted, and the TBB adhering to the surface was washed off with hexane. To further remove any ungrafted MAH, the ETFE film was washed with acetone, methanol, and water, respectively, and then dried under vacuum for 6 hours. After surface modification, FT-IR measurements revealed a C=O absorption peak originating from MAH. The graft rate and adhesion evaluation results are shown in Table 1.

[0046] [Examples 2-4, 6-12] Surface modification of the ETFE film was carried out in the same manner as in Example 1, except that the type and amount of the medium, the type and amount of the radical-reactive compound, the amount of 50% by mass toluene solution of TBB added, and the reaction conditions (temperature, time) were as shown in Table 1. The graft rate and the results of the adhesion evaluation are shown in Table 1.

[0047] [Example 13] The surface of the ETFE film was modified using the following procedure. A cylindrical separable flask with a capacity of 300 mL (mouth diameter × body diameter × height: φ75 × φ80 × 120 ± 3 mm) was placed with a stirring bar, a 50 mm × 50 mm ETFE film (Fluon® ETFE, manufactured by AGC Inc.; thickness 250 μm), and 10 mL (146 mmol) of acrylic acid (AA). The flask was then purged with argon gas, and 100 mL of deoxygenated water was added as the liquid medium. 10 mL of a 10% by mass TL solution of TBB (6 mmol TBB) was added dropwise while stirring at 23°C. Molecular oxygen (O2) in an equimolar amount to TBB was then added, and stirring continued at 23°C for 1 hour. After that, the ETFE film was removed, and the film was washed with acetone, methanol, and water, respectively, to remove the TBB adhering to the surface and the ungrafted AA. The film was then dried under vacuum for 6 hours. After surface modification, FT-IR measurements revealed a C=O absorption peak originating from AA. The graft content was 1.1%. In the evaluation of adhesion, the ETFE film and copper foil did not adhere under the above conditions of hot pressing, and therefore the peel strength could not be measured.

[0048] [Example 14] Surface modification of an ETFE film was performed in the same manner as in Example 1, except that a hexane solution of TBB was used in the dropwise amounts shown in Table 1 instead of a 50% by mass toluene solution of TBB, toluene was not injected as a liquid medium, and the type and amount of radical-reactive compound and reaction conditions (temperature, time) were as shown in Table 1. The graft rate and adhesion evaluation results are shown in Table 1.

[0049] In Table 1, in the TBB / TL column, the number with mL indicates the volume of 50% by mass toluene solution of TBB added (however, Example 13 is the volume of 10% by mass toluene solution of TBB added, and Example 14 is the volume of hexane solution of TBB added), and the value in parentheses after the volume indicates the molar amount of TBB.

[0050] [Table 1]

[0051] In Examples 13-14, where a radical-reactive compound without an acid anhydride group (AA or MMA) was used, the surface-modified ETFE film and the copper foil did not adhere. In contrast, in Examples 1-12, the surface-modified ETFE film and the copper foil adhered with sufficient adhesive strength.

Claims

1. A surface modification method comprising reacting a radical-reactive compound having an acid anhydride group with a partially fluorinated fluororesin on the surface of a fluororesin containing a partially fluorinated fluororesin, in the presence of an organometallic compound.

2. The surface modification method according to claim 1, wherein the radical-reactive compound is at least one selected from the group consisting of maleic anhydride, itaconic anhydride, citraconic anhydride, and 5-norbornene-2,3-dicarboxylic acid anhydride.

3. A surface modification method according to claim 1 or 2, wherein the radical-reactive compound is reacted such that the graft rate calculated by the following formula is 0.1 to 3.0%. Graft rate (%) = {(Mass of fluororesin after surface modification - Mass of fluororesin before surface modification) / Mass of fluororesin before surface modification} × 100 (%)

4. The surface modification method according to claim 1 or 2, wherein the reaction temperature when reacting the radical-reactive compound is 0 to 100°C.

5. The surface modification method according to claim 1 or 2, wherein the partially fluorinated fluororesin does not have acid anhydride groups.

6. The surface modification method according to claim 1 or 2, wherein the partially fluorinated fluororesin is at least one selected from the group consisting of ethylene-tetrafluoroethylene copolymers and vinylidene fluoride polymers.

7. The surface modification method according to claim 1 or 2, wherein the fluororesin is in the form of a film.

8. This is a fluororesin that includes partially fluorinated fluororesins. A fluoropolymer in which a structure containing acid anhydride groups exists only on the surface.

9. The fluororesin according to claim 8, wherein the structure has units based on a radical-reactive compound having an acid anhydride group.

10. The fluororesin according to claim 9, wherein the radical-reactive compound is at least one selected from the group consisting of maleic anhydride, itaconic anhydride, citraconic anhydride, and 5-norbornene-2,3-dicarboxylic acid anhydride.

11. The fluororesin according to any one of claims 8 to 10, wherein the partially fluorinated fluororesin is at least one selected from the group consisting of ethylene-tetrafluoroethylene copolymers and vinylidene fluoride polymers.

12. A fluororesin according to any one of claims 8 to 10, which is in the form of a film.

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