Polyimide film having uniform thickness and method for preparing the same

Abstract

This invention relates to the field of thermally conductive material preparation technology, specifically to a polyimide film with uniform thickness and its preparation method, comprising the following steps: preparation and controlled thickening of a prepolymer solution; isothermal and pressure degassing and multi-stage filtration; slot extrusion coating and multi-segment gradient curing; room temperature self-peeling and online winding; thereby obtaining the polyimide film with uniform thickness. This invention achieves better control of molecular weight distribution through a two-step polymerization method, and, combined with multi-stage gradient drying, completely avoids defects such as bubbles, pinholes, and microcracks caused by rapid solvent evaporation or rapid temperature rise. Through controllable micro-lateral stretching and final high-temperature relaxation treatment, the shrinkage stress during drying and imidization processes is effectively offset, significantly reducing film warpage and improving planar isotropy and dimensional stability.

Description

Technical Field

[0001] This invention relates to the field of thermal conductive material preparation technology, specifically to a polyimide film with uniform thickness and its preparation method. Background Technology

[0002] Polyimide is a high-performance resin material widely used in engineering plastics, microelectronics, communication technology, and optoelectronic display technology due to its excellent heat resistance, mechanical properties, chemical corrosion resistance, high insulation properties, and low coefficient of thermal expansion. With the rapid development of the information industry and portable electronic products, the integration scale of microelectronic components and power equipment is becoming increasingly smaller, and the high current and high heat flux density operating environment will place higher demands on the heat dissipation performance of thermally conductive materials, becoming a technological bottleneck for the continued development of the electronics industry. Graphite film materials have excellent electrical and thermal conductivity. The thermal conductivity of microcrystalline graphite sheets can reach over 2000 W / (m·K).

[0003] Currently, the highest-performing high thermal conductivity graphite films are mainly made from polyimide films as precursor raw materials, which are then processed through ultra-high temperature graphitization. However, polyimide film precursor products generally suffer from defects such as uneven thickness and incomplete thermal imidization. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a polyimide film with uniform thickness and its preparation method, thereby solving the problems of uneven thickness and incomplete thermal imidization that are common in existing polyimide film products based on precursors.

[0005] This invention provides a method for preparing a polyimide film with uniform thickness, comprising the following steps: Step 1: Under the protection of an inert gas, the diamine monomer is completely dissolved in a portion of an aprotic polar solvent to form a diamine solution. At a temperature of -5℃ to 5℃, dianhydride monomers with a molar ratio of 1:(0.95 to 0.99) to the diamine monomer are added in batches to the diamine solution to carry out the first-stage polymerization reaction and obtain a low-viscosity prepolymer solution. Step 2: Heat the prepolymer solution obtained in Step 1 to 15℃~30℃, stir and add the remaining (0.01~0.05 molar equivalent) dianhydride monomer in batches to carry out the second stage of polymerization to obtain a polyamic acid solution; Step 3: Transfer the polyamic acid solution obtained in Step 2 to a constant temperature and sealed degassing tank, and perform continuous circulation degassing treatment for 4-8 hours under constant micro negative pressure; pass the degassed solution through a three-stage filtration system consisting of a 20μm metal sintered filter, a 5μm polypropylene filter bag, and a 1μm absolute precision filter element to obtain a clear and transparent coating liquid. Step 4: The coating liquid obtained in Step 3 is continuously and stably coated onto a flexible stainless steel foil carrier with a fluorosilane release agent on a uniformly moving surface through a slit extrusion coating head equipped with a precision metering pump and a die lip temperature control device, forming a wet film. The wet film then enters a multi-segment curing channel with programmed temperature control, and is processed according to the following gradient: Section 1: Temperature 80℃~120℃, wind speed 0.5~2.0m / s, relative humidity <30%, time 20~40min; Section 2: Temperature 150℃~180℃, time 20~40min, at the end of this section, 0.5%~1.5% transverse stretching is applied to the film; Section 3: Programmed temperature rise, sequentially passing through 200℃ (10 min), 250℃ (10 min), 300℃ (20-30 min), and 350℃ (5-10 min); Section 4: Process at a temperature of 280℃~300℃ for 5~10 minutes.

[0006] Step 5: The fully imidized polyimide film from Step 4 is carried to the cooling roller along with the carrier foil. After cooling to below 50°C, the extremely low adhesion between the polyimide film and the fluorosilane-treated carrier foil is used to achieve automatic curling and peeling at the peeling roller. After peeling, the film is inspected for thickness and scanned for defects online, and then wound up under constant tension control to obtain the polyimide film.

[0007] As a preferred embodiment of the present invention, in step one, the diamine monomer is a mixture of 4,4'-biphenyldiamine and 4,4'-diaminobenzoylaniline, with a molar ratio of (8:2) to (6:4).

[0008] As a preferred technical solution of the present invention, in step one, the dianhydride monomer is a mixture of biphenyltetracarboxylic dianhydride and pyromellitic dianhydride, with a molar ratio of (9:1) to (7:3).

[0009] As a preferred technical solution of the present invention, in step one, the aprotic polar solvent is a mixed solvent of N-methylpyrrolidone and γ-butyrolactone, with a volume ratio of (7:3) to (8:2).

[0010] As a preferred embodiment of the present invention, in step two, the solid content of the polyamic acid solution is 14% to 18%.

[0011] The present invention also provides a polyimide film with uniform thickness, including the polyimide film prepared by the preparation method described above.

[0012] As a preferred embodiment of the present invention, the thickness of the polyimide film is 40–80 μm.

[0013] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention achieves better control of molecular weight distribution through a two-step polymerization process, and, combined with multi-stage gradient drying, completely avoids defects such as bubbles, pinholes, and microcracks caused by rapid solvent evaporation or rapid temperature rise.

[0014] 2. This invention effectively counteracts the shrinkage stress during drying and imidization by controlling minute lateral stretching and final high-temperature relaxation treatment, significantly reducing the warpage of the film and improving planar isotropy and dimensional stability. Detailed Implementation

[0015] The preferred embodiments of the present invention will now be described in detail so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby providing a clearer and more explicit definition of the scope of protection of the present invention.

[0016] In the description of this invention, unless otherwise stated, "a plurality of" means two or more; the terms "upper," "lower," "left," "right," "inner," "outer," "front end," "rear end," "head," "tail," etc., indicating orientation are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the terms "first," "second," "third," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Example 1

[0017] Step 1: In a four-necked flask equipped with a stirrer, a nitrogen inlet tube, and a low-temperature bath, add 20.024 g (0.1 mol) of 4,4'-biphenyldiamine, 4.406 g (0.02 mol) of 4,4'-diaminobenzoylaniline (molar ratio 8:2), and 260 g of mixed solvent (N-methylpyrrolidone to γ-butyrolactone volume ratio 7.5:2.5). Stir at room temperature until completely dissolved.

[0018] Step 2: Cool the system to 0±2°C. Under vigorous stirring, slowly add a total of 27.636 g (0.117 mol) of biphenyltetracarboxylic dianhydride (BPDA, molar ratio to total diamine 0.975:1) in five batches, controlling the reaction temperature to not exceed 5°C. After the addition is complete, continue stirring at 0°C for 2 hours to obtain a low-viscosity prepolymer solution.

[0019] Step 3: Heat the system to 25°C, and add 2.844 g (0.012 mol) of BPDA in two portions (to achieve a total dianhydride to total diamine molar ratio of 1.075:1), stirring for 3 hours. Finally, add 36 g of the above mixed solvent to adjust the solid content to approximately 16%, obtaining a homogeneous, amber-colored polyamic acid solution with an intrinsic viscosity of approximately 2.8 dL / g.

[0020] Step 4: The polyamic acid solution obtained in Step 3 was continuously circulated for degassing at 25°C and -0.03 MPa micro-negative pressure for 6 hours. Then, it was sequentially passed through a 20μm metal filter, a 5μm polypropylene filter bag, and a 1μm absolute precision filter element to obtain a clear and transparent coating solution.

[0021] Step 5: Using a slot extrusion coating machine, with the die temperature set to 30°C, apply the coating liquid at a speed of 2.0 m / min onto a flexible stainless steel foil with a width of 500 mm that is coated with fluorosilane release agent. The wet film thickness is approximately 320 μm.

[0022] The wet film enters the multi-zone curing oven, as detailed below: Section 1: Temperature 100°C, wind speed 1.0 m / s, time 30 min.

[0023] Section 2: Temperature 165°C, time 30 min, apply 1.0% transverse stretch to the film at the end of this section.

[0024] Section 3: Programmed temperature increase, 200°C (10 min) → 250°C (10 min) → 300°C (25 min).

[0025] Section 4: Temperature 290°C, treated for 8 min at -0.2% tension (slight relaxation).

[0026] Step Six: After the film cools to 40°C, it is automatically and smoothly peeled off from the steel foil at the peeling roller. After online inspection, it is wound up to obtain a polyimide film with a thickness of 50±1.5μm.

[0027] Performance test results: Thickness uniformity (CV value): 2.7%.

[0028] Tensile strength (MD / TD): 420 MPa / 410 MPa.

[0029] Elongation at break (MD / TD): 45% / 43%.

[0030] Warpage (100mm×100mm sample): <2mm. Example 2

[0031] The difference from Example 1 is as follows: Diamine: 4,4'-Biphenyldiamine (0.1 mol) was used exclusively.

[0032] Dihydride: All biphenyl dianhydride (BPDA) was used, employing a two-step method, with a total molar ratio of 1.06:1.

[0033] Solvent: NMP to γ-butyrolactone volume ratio 7:3.

[0034] Final solids content: 14%, intrinsic viscosity: 2.5 dL / g.

[0035] Same as step four in Example 1, with a degassing time of 4 hours.

[0036] Same as step five in Example 1, with a coating speed of 2.5 m / min and a wet film thickness of approximately 285 μm.

[0037] The wet film enters the multi-zone curing oven, as detailed below: The coating speed is 2.5 m / min, and the wet film thickness is approximately 285 μm.

[0038] Curing process: Section 1: Temperature 90°C, wind speed 1.0 m / s, time 25 min.

[0039] Section 2: Temperature 160°C, time 25 min, 0.8% transverse stretch is applied to the film at the end of this section.

[0040] Section 3: Programmed temperature rise, 190°C (10 min) → 240°C (10 min) → 290°C (25 min).

[0041] Section 4: Temperature 285°C, treated for 5 min at -0.2% tension (slight relaxation).

[0042] Following step six in Example 1, a thin film with a thickness of 40±1.0 μm was obtained.

[0043] Performance test results: Thickness uniformity (CV value): 2.0%.

[0044] Tensile strength anisotropy ratio (MD / TD): 1.02.

[0045] Heat shrinkage rate (200°C, 2h): <0.05%.

[0046] Warpage (100mm×100mm sample): < 1mm.

[0047] Comparative Examples

[0048] Step 1: Mix equimolar amounts of 4,4'-biphenyldiamine and biphenyltetracarboxylic dianhydride (molar ratio 1:1) rapidly in a single step at -15°C in NMP solvent, and stir for 6 hours until the solid content is approximately 10%.

[0049] Step 2: The solution is left to stand at -15°C for 36 hours, and then degassed under vacuum at room temperature for 2 hours.

[0050] Step 3: Manually coat the solution onto a glass plate, with a wet film thickness of approximately 1000 μm. Place it in an oven and dry it at 80°C for 20 minutes with forced air, then place it directly into an oven preheated to 300°C and heat for 4 hours.

[0051] Step 4: After cooling the glass plate with the film, immerse it in 90°C hot water to peel off the film.

[0052] Test Results Film thickness: ~65μm, but the thickness is not uniform, with significant differences between the edge and the center.

[0053] Appearance: The film has visible cloud-like patterns, microcracks at the edges, and scattered bubbles on the lower surface (the side in contact with the glass).

[0054] Warping: Severe, spontaneously curls into a cylindrical shape and cannot be placed flat.

[0055] Large dispersion in mechanical properties: Tensile strength fluctuates by more than ±15% when tested at different locations.

[0056] Difficult to peel off: In some areas, the film adheres to the glass plate, and forced peeling causes local damage.

[0057] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "connected" and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0058] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a polyimide film with uniform thickness, characterized in that, Includes the following steps: Step 1: Under the protection of an inert gas, the diamine monomer is completely dissolved in a portion of an aprotic polar solvent to form a diamine solution. Under the temperature conditions of -5℃ to 5℃, dianhydride monomers with a molar ratio of 1:(0.95 to 0.99) to the diamine monomer are added in batches to the diamine solution to carry out the first-stage polymerization reaction and obtain a low-viscosity prepolymer solution. Step 2: Heat the prepolymer solution obtained in Step 1 to 15℃~30℃, stir and add the remaining (0.01~0.05 molar equivalent) dianhydride monomer in batches to carry out the second stage of polymerization to obtain a polyamic acid solution; Step 3: Transfer the polyamic acid solution obtained in Step 2 to a constant temperature and sealed degassing tank, and perform continuous circulation degassing treatment for 4-8 hours under constant micro negative pressure; pass the degassed solution through a three-stage filtration system consisting of a 20μm metal sintered filter, a 5μm polypropylene filter bag, and a 1μm absolute precision filter element to obtain a clear and transparent coating liquid. Step 4: The coating liquid obtained in Step 3 is continuously and stably coated onto a flexible stainless steel foil carrier with a fluorosilane release agent on a uniformly moving surface through a slit extrusion coating head equipped with a precision metering pump and a die lip temperature control device, forming a wet film. The wet film then enters a multi-segment curing channel with programmed temperature control, and is processed according to the following gradient: Section 1: Temperature 80℃~120℃, wind speed 0.5~2.0m / s, relative humidity <30%, time 20~40min; Section 2: Temperature 150℃~180℃, time 20~40min, at the end of this section, 0.5%~1.5% transverse stretching is applied to the film; Section 3: Programmed temperature rise, sequentially passing through 200℃ (10 min), 250℃ (10 min), 300℃ (20-30 min), and 350℃ (5-10 min); Section 4: Process at a temperature of 280℃~300℃ for 5~10 minutes. Step 5: The fully imidized polyimide film from Step 4 is carried to the cooling roller along with the carrier foil. After cooling to below 50°C, the extremely low adhesion between the polyimide film and the fluorosilane-treated carrier foil is used to achieve automatic curling and peeling at the peeling roller. After peeling, the film is inspected for thickness and scanned for defects online, and then wound up under constant tension control to obtain the polyimide film.

2. The method for preparing a polyimide film with uniform thickness according to claim 1, characterized in that: In step one, the diamine monomer is a mixture of 4,4'-biphenyldiamine and 4,4'-diaminobenzoylaniline, with a molar ratio of (8:2) to (6:4).

3. The method for preparing a polyimide film with uniform thickness according to claim 1, characterized in that: In step one, the dianhydride monomer is a mixture of biphenyltetracarboxylic dianhydride and pyromellitic dianhydride, with a molar ratio of (9:1) to (7:3).

4. The method for preparing a polyimide film with uniform thickness according to claim 1, characterized in that: In step one, the aprotic polar solvent is a mixed solvent of N-methylpyrrolidone and γ-butyrolactone, with a volume ratio of (7:3) to (8:2).

5. The method for preparing a polyimide film with uniform thickness according to claim 1, characterized in that: In step two, the solid content of the polyamic acid solution is 14% to 18%.

6. A polyimide film of uniform thickness, characterized in that, This includes polyimide films prepared by the preparation method according to any one of claims 1-5.

7. A polyimide film with uniform thickness according to claim 6, characterized in that: The thickness of the polyimide film is 40–80 μm.