CN114426693A

CN114426693A - Preparation method of colorless transparent polyimide film with low yellowness and high mechanical property

Info

Publication number: CN114426693A
Application number: CN202111650662.XA
Authority: CN
Inventors: 张涛; 宋明雄; 熊斐; 杨帆; 张文浩
Original assignee: Ningbo Solartron Technology Co Ltd
Current assignee: Ningbo Solartron Technology Co Ltd
Priority date: 2021-12-29
Filing date: 2021-12-29
Publication date: 2022-05-03
Anticipated expiration: 2041-12-29
Also published as: CN114426693B

Abstract

The invention relates to the field of high polymer materials, and discloses a preparation method of a colorless transparent polyimide film with low yellowness and high mechanical property. The finally prepared polyimide film still has higher mechanical and heat-resistant properties when the light transmittance is more than 90 percent.

Description

Preparation method of colorless transparent polyimide film with low yellowness and high mechanical property

Technical Field

The invention relates to the field of high polymer materials, in particular to a preparation method of a colorless transparent polyimide film with low yellowness and high mechanical property.

Background

Polyimide is a high molecular polymer containing imide units on a main chain, has excellent heat resistance, mechanical property, chemical stability and dielectric property, and is widely applied to the fields of aerospace, semiconductor packaging, solar cells and the like.

In recent years, with the vigorous development of flexible display products, particularly flexible mobile phones and flexible wearable devices, higher requirements are put on the bending performance of materials, the traditional glass substrate is difficult to meet the requirements, and polyimide films have excellent heat resistance and mechanical properties, are receiving more and more attention, and are regarded as one of the best materials for developing flexible displays.

The traditional wholly aromatic polyimide film is easy to generate electron transfer complexation in molecules and among molecules to form an electron transfer complex (CTC), so that the film generally presents light yellow or even dark brown, and the light transmittance of the film is lower. Semi-aromatic, full-aliphatic and alicyclic polyimide films have poor heat resistance and mechanical properties although they are nearly colorless. Therefore, in order to apply the polyimide film to the field of flexible display, it is necessary to solve the contradiction between the heat resistance, optical transparency and mechanical properties.

CN201810705651.9 discloses a cellulose modified polyimide corona-resistant film and a preparation method thereof, wherein pretreated cellulose is added with aluminum isopropoxide, a silane coupling agent and an emulsifier, and is dispersed at high speed in a high-pressure emulsifying machine to form uniform emulsion; mixing and dissolving 4, 4' -diaminodiphenyl ether and dimethylacetamide to form an amine-containing solution, stirring and mixing the emulsion and the amine-containing solution, and then adding pyromellitic dianhydride to the mixed solution in steps to prepare polyamic acid resin; the obtained polyamic acid resin forms a film through a tape casting process, and the film is heated to obtain the cellulose modified polyimide film. The cellulose forms a net structure in the film, so that the strength of the film is improved; the introduction of the cellulose can also improve the dielectric property and corona resistance of the film. In the patent, the mechanical property of the film is improved by introducing cellulose into the polyimide material, but in the scheme, the cellulose is loaded in nano alumina, and the cellulose is only dispersed in the polyimide substrate in a physical blending mode, so that the compatibility is limited, the stability is poor, the cellulose is easy to precipitate after later aging, and the cellulose is not suitable for being added in a large amount.

In addition, the traditional method for reducing the yellowness of the polyimide film is mainly completed by adding a fluorine-containing monomer, introducing a bulky substituent, a main chain bending structure, an asymmetric structure, reducing a conjugated double bond structure and the like, and the purpose of the method is mainly to reduce the intramolecular or intermolecular charge transfer effect, so that the formation of an electron transfer complex (CTC) is reduced, and the light transmittance and the transparency of the polyimide film are improved.

In summary, in order to meet the higher requirements of the flexible display device on the polyimide film, it is a problem to be urgently solved by researchers at present to further optimize the light transmittance, haze, yellowing index, mechanical properties and heat resistance of the colorless transparent polyimide film.

Disclosure of Invention

In order to solve the technical problems, the invention provides a preparation method of a colorless transparent polyimide film with low yellowness and high mechanical property, and the film obtained by the invention has lower yellowness index, higher mechanical property and heat resistance while keeping the light transmittance more than 90 percent, and can meet the higher requirements of flexible display devices on the polyimide film.

The specific technical scheme of the invention is as follows: a preparation method of a colorless transparent polyimide film with low yellowness and high mechanical property comprises the following steps:

(1) dissolving alicyclic diamine in a polar aprotic organic solvent A, adding aminated nanocellulose, continuously stirring, adding alicyclic dianhydride in batches, and continuously stirring at 0-10 ℃ under the protection of inert gas to obtain a polyamic acid solution; the molar ratio of the alicyclic diamine to the alicyclic dianhydride is 1.1-1: 1-1.1, and the mass of the aminated nanocellulose is 1-10% of that of the alicyclic dianhydride.

(2) Respectively adding carboxylated nanocellulose, a catalyst and a dehydrating agent into the polyamic acid solution, continuously stirring at 100-200 ℃ under the protection of inertia to obtain a polyimide solution, soaking, washing and drying to obtain polyimide resin; the mass of the carboxylated nano-cellulose is 1-10% of that of the alicyclic dianhydride.

(3) And dissolving the polyimide resin in a polar aprotic organic solvent B to obtain a polyimide resin solution, coating the polyimide resin solution on a substrate, and sequentially baking in low-temperature, medium-temperature and high-temperature environments to obtain the polyimide film.

In the method, the aminated nano-cellulose and the carboxylated nano-cellulose are introduced into the full-alicyclic polyimide film, and the aminated nano-cellulose and the carboxylated nano-cellulose can participate in the reaction of alicyclic diamine and alicyclic dianhydride as diamine monomers and end capping agents respectively and are doped into a polyimide polymer main chain in a covalent bond form, so that the stability is good, the mechanical property and the heat resistance of the polyimide film can be improved, and the high-temperature yellowing effect caused by amino groups on the diamine can be inhibited. The finally prepared polyimide film still has higher mechanical and heat-resistant properties when the light transmittance is more than 90 percent. Specifically, the method comprises the following steps:

aiming at the improvement of mechanical property and heat resistance: in the step (1), the aminated nanocellulose is added into the raw material, and due to the existence of amino functional groups on the aminated nanocellulose, the aminated nanocellulose can be regarded as a diamine monomer and can be subjected to an imide reaction with dianhydride to participate in a polyamic acid synthesis process, so that the aminated nanocellulose is doped into a polyimide main chain in a covalent bond combination mode, and compared with physical blending, the stability is better, the mechanical property and the heat resistance of the material can be greatly improved, and the optical transparency of a polyimide film cannot be influenced due to the self-transparency characteristic.

For improvement of transparency and yellowing: first, in the step (1), the present invention selects alicyclic diamine and alicyclic dianhydride as monomers, and can effectively reduce the occurrence of intramolecular and intermolecular electron transfer complexes (CTCs) without breaking the conjugated system in the polyimide film, thereby reducing the yellowing phenomenon caused by the electron transfer complexes and improving the optical transparency of the film. In the step (2), the carboxylated nanocellulose is added into the polyamic acid solution after the polymerization reaction is finished to serve as a terminal group of the polyimide, the carboxyl group of the carboxylated nanocellulose and the amino group on the alicyclic diamine at the end of the polyimide molecule chain are subjected to dehydration reaction under the action of the catalyst and the dehydrating agent to form the carboxylated nanocellulose terminal-terminated polyimide resin, and simultaneously, the carboxyl group of the carboxylated nanocellulose and the amino group on the alicyclic diamine at the end of the polyimide molecule chain can also be subjected to dehydration reaction with partial unreacted alicyclic diamine residues, so that the content of the unreacted amino groups on the diamine at two ends of the main chain of the polyimide polymer and the alicyclic diamine is reduced, the yellowing phenomenon caused by the high-temperature oxidation of the amino group is eliminated, the optical transparency of the polyimide film is further improved, and the problem that the yellow index of the colorless transparent polyimide film is higher is fundamentally solved.

Preferably, in the step (1), the amino group content in the aminated nano-cellulose is 1.3-1.8mmol/g, the molecular weight is 5000-8000, the diameter is 10-20nm, and the length is 100-300 nm.

Preferably, in the step (2), the carboxyl content in the carboxylated nanocellulose is 1.2 to 3mmol/g, the molecular weight is 5000 to 8000, the diameter is 10 to 20nm, and the length is 100-300 nm.

By adding the two modified nanocelluloses, the mechanical property, the heat resistance and the yellowing resistance of the polyimide film can be improved on the premise of not influencing the transparency. However, in the research process, the two types of nanocellulose are self-crosslinked in the reaction process, so that the actual improvement effect is influenced. In order to prevent the reaction between the two types of nanocellulose, the invention limits the feeding sequence of the raw materials, and particularly follows the sequence of diamine, aminated nanocellulose, dianhydride, carboxylated nanocellulose, catalyst and dehydrating agent, and the materials are fully stirred so as to ensure that the reaction is more complete. In addition, in order to make the two types of nanocellulose more easily participate in the reaction and further improve the improvement effect, the specifications of the two types of nanocellulose are also strictly limited, and finally, the nanocellulose with the diameter, the length, the molecular weight and the content of the functional groups is found to have the best effect. If the molecular weight is too high, the nanocellulose may agglomerate in the reaction process, which is not favorable for the reaction, and the agglomeration may cause uneven internal stress of the film and thus the mechanical properties of the film may be reduced.

Preferably, in step (1), the alicyclic diamine includes one or more of 1, 3-cyclohexanediamine, 1, 4-cyclohexanediamine, 1, 3-cyclobutanediamine, 4-diaminodicyclohexylmethane (PACM), and 3, 3-dimethyl-4, 4-diaminodicyclohexylmethane (DMDC).

Preferably, in the step (1), the alicyclic dianhydrides include 1,2,3, 4-cyclobutanetetracarboxylic dianhydride (CBDA), bicyclo [2.2.2] oct-7-ene-2, 3,5, 6-tetracarboxylic dianhydride (BCDA), 1,2,3, 4-cyclohexanetetracarboxylic dianhydride (1, 2,3, 4-CHDA), 1,2,3, 4-cyclopentanetetracarboxylic dianhydride (CPDA), 3-carboxymethyl-1, 2, 4-tricarboxylic cyclopentane dianhydride (TCPDA), cyclooctadienetetracarboxylic dianhydride (CODA), norbornane-2-spiro- α -cyclopentanone- α '-spiro-2' -norbornane-5, 5',6,6' -tetracarboxylic dianhydride (CPODA), hydrogenated pyromellitic dianhydride (HPMDA), and hydrogenated 3, one or more of 3 ', 4, 4' -biphenyltetracarboxylic dianhydride (HBPDA).

Preferably, in the step (1), the polar aprotic organic solvent a is one or more of N, N-Dimethylacetamide (DMAC), N-Dimethylformamide (DMF), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), Propylene Glycol Methyl Ether Acetate (PGMEA), tetramethyl sulfoxide, N '-dimethyl-N, N' -propylene urea (DMPU), dichloromethane, chloroform, tetrahydrofuran, m-cresol, and γ -butyrolactone.

Further, the polar aprotic organic solvent a is one of N, N-Dimethylacetamide (DMAC), N-Dimethylformamide (DMF), N-methylpyrrolidone (NMP), and Dimethylsulfoxide (DMSO).

Preferably, the step (1) specifically comprises: alicyclic diamine is dissolved in a polar aprotic organic solvent A, amination nano-cellulose is added, stirring is continuously carried out for 5-20 min at the speed of 100-1500 r/min, alicyclic dianhydride is added for 2-10 times, and stirring is continuously carried out for 5-24 h at the temperature of 0-10 ℃ under the protection of inert gas, so that a polyamic acid solution is obtained.

Preferably, in the step (2), the catalyst is a tertiary amine compound including one or more of triethylamine, pyridine, isoquinoline and picoline, and further preferably triethylamine or pyridine.

Preferably, in the step (2), the dehydrating agent is one or more of acetic anhydride, propionic anhydride, butyric anhydride, phthalic anhydride, trimellitic anhydride and hexahydrophthalic anhydride, and further preferably acetic anhydride, propionic anhydride or butyric anhydride.

Preferably, in the step (2), the mass of the catalyst and the dehydrating agent is 0.1 to 1% of the total mass of the alicyclic diamine and the alicyclic dianhydride.

Preferably, the step (2) specifically comprises: respectively adding carboxylated nanocellulose, a catalyst and a dehydrating agent into the polyamic acid solution, continuously stirring for 5-24 hours at 100-200 ℃ under the protection of inertia at 100-1500 r/min to obtain a polyimide solution, soaking and washing for 1-5 times by using an ethanol water solution (ethanol: water is 1: 3), and drying to obtain the polyimide resin.

Preferably, in the step (3), the polar aprotic organic solvent B is one or more of N, N-Dimethylacetamide (DMAC), N-Dimethylformamide (DMF), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), Propylene Glycol Methyl Ether Acetate (PGMEA), tetramethyl sulfoxide, N '-dimethyl-N, N' -propylene urea (DMPU). Preferably a mixed solution of N, N-Dimethylacetamide (DMAC) and Propylene Glycol Methyl Ether Acetate (PGMEA). Further, the mass ratio of N, N-Dimethylacetamide (DMAC) to Propylene Glycol Methyl Ether Acetate (PGMEA) in the mixed solution is 2: 8-5: 5.

Preferably, the alicyclic diamine, the alicyclic dianhydride, the polar aprotic organic solvent A and the polar aprotic organic solvent B are used after being purified, wherein the moisture content in the polar aprotic organic solvent A and the polar aprotic organic solvent B is 0-0.05%.

Preferably, in the step (3), the low temperature is specifically 40-80 ℃, and preferably 50-70 ℃; the baking time is 5-30 min, preferably 10-20 min; the temperature of the medium temperature is specifically 140-180 ℃, and is preferably 150-170 ℃; the baking time is 5-30 min, preferably 10-20 min. The high temperature is specifically 220-270 ℃, and preferably 240-270 ℃; the baking time is 5-30 min, preferably 10-20 min.

In the research process, the team of the invention finds that the baking process is very important, and if the process is improper, the performance of the film is seriously influenced, so that the invention needs to deeply research to obtain a reasonable baking process. Specifically, in the three-stage baking process of the present invention, the low-temperature baking is performed first to level the polyimide solution and slowly volatilize the solvent therein, and if the high-temperature treatment is performed directly, the bubbling phenomenon due to the violent volatilization of the solvent in a short time is likely to occur.

Firstly, baking at low temperature and then baking at medium temperature, and aims to further dry the solvent and promote the arrangement of polymer molecular chains so that the polyimide film can be formed and stripped; in the middle temperature section, if the temperature is too low, the solvent is not volatilized enough, so that the solvent content in the film is too high to be stripped during stripping; if the temperature is too high, the substrate may curl, resulting in uneven film appearance and reduced performance. And finally, heating for high-temperature baking, wherein the aim is to thoroughly dry the solvent, further promote the ordered arrangement of molecular chains and eliminate the internal stress of the polyimide film, so that the mechanical property of the polyimide film is improved and the thermal shrinkage rate of the polyimide film is reduced.

Further preferably, the polyimide film on the substrate needs to be peeled off and stuck on the needle plate with a hollow in the middle between the middle temperature stage and the high temperature stage because the substrate cannot endure the temperature of the high temperature stage. Secondly, the polyimide film can be fixed in a needle plate binding mode, excessive shrinkage of the polyimide film when the polyimide film is heated at a high-temperature stage is prevented, and straightening and flattening of the polyimide film are promoted. In addition, both sides of the film can be fully contacted with air in a needle plate pricking mode, so that the film is heated uniformly, and the curling phenomenon caused by uneven heating after the film is taken out from a high-temperature section and peeled is prevented.

Preferably, in the step (3), the polyimide resin solution has a solid content of 5 to 20% and a viscosity of 2000 to 8000 cP.

Preferably, in the step (3), the substrate is a high temperature resistant substrate including one of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), Polyethyleneimine (PEI) and polyphenylene sulfide (PPS).

Compared with the prior art, the invention has the following technical effects:

(1) according to the invention, the aminated nano-cellulose is taken as diamine, and reacts with dianhydride to generate imide reaction, and is doped into the polymer main chain of the polyimide in a covalent bond mode, so that the high transparency characteristic of the film can be ensured, and the mechanical property and the heat resistance of the film can be greatly improved, thereby overcoming the problem of low mechanical and heat resistance of the alicyclic polyimide.

(2) According to the invention, the carboxylated nano-cellulose is used as the end capping group of the polyimide, so that the amino content in the polyimide resin can be reduced, the yellowing phenomenon caused by high-temperature oxidation of amino is eliminated, and the optical transparency of the polyimide film is further improved.

(3) Aiming at the characteristics of the polyimide polymer after the amination nanocellulose and the carboxylation nanocellulose are introduced, the baking process is optimized in a targeted manner, and a film with a smooth surface can be obtained; and the internal stress can be effectively eliminated, so that the mechanical property of the polyimide film is improved, and the thermal shrinkage rate of the polyimide film is reduced.

(4) According to the invention, the addition sequence of the two types of nano-cellulose is further optimized, so that the two types of nano-cellulose can be effectively prevented from self-crosslinking. In addition, the invention also strictly limits the specifications of the two types of nanocellulose, and can ensure that the two types of nanocellulose can participate in the reaction more easily and further improve the improvement effect.

(5) The polyimide film prepared by the invention has higher optical transparency, mechanical property and heat resistance, the thickness is 10-50 mu m, the light transmittance is more than or equal to 90.0%, the B value of CIE Lab is less than or equal to 1.0, the Tg is more than or equal to 300.0 ℃, the haze is less than or equal to 1%, the tensile strength is more than or equal to 150MPa, the elongation is more than or equal to 20%, the elastic modulus is more than 2.50GPa, and the heat shrinkage rates in TD and MD directions are less than 0.1%; can meet the performance requirement of the flexible display device on the polyimide film.

Detailed Description

The present invention will be further described with reference to the following examples.

General examples

A preparation method of a colorless transparent polyimide film with low yellowness and high mechanical property comprises the following steps:

(1) alicyclic diamine is dissolved in a polar aprotic organic solvent A, aminated nano-cellulose (with the amino content of 1.3-1.8mmol/g, the molecular weight of 5000-8000, the diameter of 10-20nm and the length of 100-300nm) is added, the mixture is continuously stirred for 5-20 min at the speed of 100-1500 r/min, alicyclic dianhydride is added for 2-10 times, and the mixture is continuously stirred for 5-24 h at the temperature of 0-10 ℃ under the protection of nitrogen, so that a polyamic acid solution is obtained. The molar ratio of the alicyclic diamine to the alicyclic dianhydride is 1.1-1: 1-1.1, and the mass of the aminated nanocellulose is 1-10% of that of the alicyclic dianhydride. Wherein:

the cycloaliphatic diamine comprises one or more of 1, 3-cyclohexanediamine, 1, 4-cyclohexanediamine, 1, 3-cyclobutanediamine, 4-diaminodicyclohexylmethane (PACM), and 3, 3-dimethyl-4, 4-diaminodicyclohexylmethane (DMDC).

The alicyclic dianhydride includes l, 2,3, 4-cyclobutanetetracarboxylic dianhydride (CBDA), bicyclo [2.2.2] oct-7-ene-2, 3,5, 6-tetracarboxylic dianhydride (BCDA), 1,2,3, 4-cyclohexanetetracarboxylic dianhydride (1, 2,3, 4-CHDA), 1,2,3, 4-cyclopentanetetracarboxylic dianhydride (CPDA), one or more of 3-carboxymethyl-1, 2, 4-tricarboxylic acid cyclopentane dianhydride (TCPDA), cyclooctadiene tetracarboxylic dianhydride (CODA), norbornane-2-spiro- α -cyclopentanone- α '-spiro-2' -norbornane-5, 5',6,6' -tetracarboxylic dianhydride (CPODA), hydrogenated pyromellitic dianhydride (HPMDA), and hydrogenated 3,3 ', 4, 4' -biphenyltetracarboxylic dianhydride (HBPDA).

The polar aprotic organic solvent A is one or more of N, N-Dimethylacetamide (DMAC), N-Dimethylformamide (DMF), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), Propylene Glycol Methyl Ether Acetate (PGMEA), tetramethyl sulfoxide, N '-dimethyl-N, N' -propylene urea (DMPU), dichloromethane, chloroform, tetrahydrofuran, m-cresol and gamma-butyrolactone. Further, the polar aprotic organic solvent a is one of N, N-Dimethylacetamide (DMAC), N-Dimethylformamide (DMF), N-methylpyrrolidone (NMP), and Dimethylsulfoxide (DMSO).

(2) Respectively adding carboxylated nanocellulose (the content of carboxyl is 1.2-3 mmol/g, the molecular weight is 5000-8000, the diameter is 10-20nm, and the length is 100-300nm), a catalyst and a dehydrating agent into the polyamic acid solution, continuously stirring for 5-24 h at 100-200 ℃ and under the protection of inertia at 100-1500 r/min to obtain a polyimide solution, soaking and washing for 1-5 times by using an ethanol water solution (ethanol: water is 1: 3), and drying to obtain the polyimide resin. The mass of the carboxylated nanocellulose is 1-10% of that of the alicyclic dianhydride, and the mass of the catalyst and the dehydrating agent is 0.1-1% of the total mass of the alicyclic diamine and the alicyclic dianhydride. Wherein:

the catalyst is a tertiary amine compound, and comprises one or more of triethylamine, pyridine, isoquinoline and picoline, and further preferably triethylamine or pyridine.

The dehydrating agent is one or more of acetic anhydride, propionic anhydride, butyric anhydride, phthalic anhydride, trimellitic anhydride and hexahydrophthalic anhydride, and further preferably acetic anhydride, propionic anhydride or butyric anhydride.

(3) And dissolving the polyimide resin in a polar aprotic organic solvent B to obtain a polyimide resin solution with the solid content of 5-20% and the viscosity of 2000 cP-8000 cP. Coating the polyimide resin solution on a substrate, and sequentially baking in low-temperature, medium-temperature and high-temperature environments, wherein the specific steps are as follows: the low temperature is 40-80 ℃, preferably 50-70 ℃; the baking time is 5-30 min, preferably 10-20 min; the medium temperature is 140-180 ℃, and preferably 150-170 ℃; the baking time is 5-30 min, preferably 10-20 min. And after the medium-temperature stage, peeling off the polyimide film on the substrate, binding the polyimide film on a needle plate with a hollow middle part, and entering the high-temperature stage. The high temperature is 220-270 ℃, preferably 240-270 ℃; the baking time is 5-30 min, preferably 10-20 min. And baking to obtain the polyimide film. Wherein:

the polar aprotic organic solvent B is one or more of N, N-Dimethylacetamide (DMAC), N-Dimethylformamide (DMF), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), Propylene Glycol Methyl Ether Acetate (PGMEA), tetramethyl sulfoxide and N, N '-dimethyl-N, N' -propylene urea (DMPU). Preferably a mixed solution of N, N-Dimethylacetamide (DMAC) and Propylene Glycol Methyl Ether Acetate (PGMEA). Further, the mass ratio of N, N-Dimethylacetamide (DMAC) to Propylene Glycol Methyl Ether Acetate (PGMEA) in the mixed solution is 2: 8-5: 5.

The base material is a high-temperature-resistant base material and comprises one of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), Polyethyleneimine (PEI) and polyphenylene sulfide (PPS).

In the steps, the alicyclic diamine, the alicyclic dianhydride, the polar aprotic organic solvent A and the polar aprotic organic solvent B are used after being purified, wherein the water content in the polar aprotic organic solvent A and the polar aprotic organic solvent B is 0-0.05%.

Example 1

A500 mL three-necked flask equipped with a mechanical stirrer, a thermometer, and a reflux condenser was charged with 11.4189g (0.1mol) of 1, 4-cyclohexanediamine and 150g N, N-Dimethylacetamide (DMAC). Nitrogen was passed through and the solution was mechanically stirred at 500r/min for 20min to obtain a clear solution. 1.1209g of aminated nanocellulose (with amino content of 1.3-1.8mmol/g, molecular weight of 5000-8000, diameter of 10-20nm and length of 100-300nm) is added, and after stirring for 20min at 500r/min, 22.4170g (0.1mol) of hydrogenated pyromellitic dianhydride (HPMDA) is added in 5 times. Stirring is continued for 6h at 500r/min under the reaction condition of 5 ℃ to obtain polyamic acid solution. 1.1209g of carboxylated nanocellulose (with carboxyl content of 1.2-3 mmol/g, molecular weight of 5000-8000, diameter of 10-20nm and length of 100-300nm), 0.5g of pyridine and 0.5g of acetic anhydride are sequentially added into the polyamic acid solution, and the mixture is continuously stirred for 6 hours at 500r/min under the conditions of nitrogen protection and 120 ℃ reaction to obtain viscous polyimide solution. Slowly pouring the polyimide solution into an ethanol water solution (the mass ratio of ethanol to water is 1: 3) to obtain white filaments, replacing the ethanol solution every 8 hours for 1 time, and soaking and washing the filaments for 3 times in total. Then drying for 24h at 80 ℃ to obtain white filiform polyimide resin.

10g of the polyimide resin obtained above was dissolved in 40g N, a mixed solution of N-dimethylacetamide (DMAc) and Propylene Glycol Methyl Ether Acetate (PGMEA) (the mass ratio of DMAc to PGMEA is 3: 7), to obtain a polyimide resin solution having a solid content of 20% and a viscosity of 2000cP to 8000 cP. The polyimide resin solution is coated on a PET substrate with the thickness of 250 mu m by an automatic coating machine, and the PET substrate is firstly put into an oven with the temperature of 50 ℃ for baking for 10min and then is transferred to an oven with the temperature of 150 ℃ for baking for 10 min. After baking, the film is peeled off from the PET substrate and is bundled on a needle plate with a hollow middle part, and then the PET substrate is placed into a 240 ℃ oven to be baked for 10 min. And finally, taking down the film from the needle plate to obtain the polyimide film.

Example 2

The specific operation and conditions of example 2 are the same as in example 1 except that the alicyclic diamine and the alicyclic dianhydride are replaced with 1, 3-cyclobutanediamine and 1,2,3, 4-cyclobutanetetracarboxylic dianhydride (CBDA), respectively.

Example 3

Example 3 the specific procedure and conditions were the same as in example 1 except that the alicyclic diamine and the alicyclic dianhydride used were replaced with 3, 3-dimethyl-4, 4-diaminodicyclohexylmethane (DMDC) and hydrogenated 3,3 ', 4, 4' -biphenyltetracarboxylic dianhydride (HBPDA), respectively.

Example 4

The specific operation and conditions of example 4 were the same as in example 1, except that the mass of the aminated nanocellulose and the carboxylated nanocellulose was changed from 1.1209g to 2.2417 g.

Example 5

The specific operation and conditions of example 5 are the same as those of example 1, except that: the solvent is changed from N, N-Dimethylacetamide (DMAC) to N-methylpyrrolidone (NMP), and the mass ratio of DMAc to PGMEA in the mixed solution of N, N-dimethylacetamide (DMAc) and Propylene Glycol Methyl Ether Acetate (PGMEA) is 5: 5.

Example 6

The specific operation and conditions of example 6 were the same as those of example 1 except that the baking temperature and time were changed to 70 deg.C, 170 deg.C and 260 deg.C for 30min each.

Comparative example 1

The specific operation and conditions of comparative example 1 were the same as in example 1 except that no aminated nanocellulose and carboxylated nanocellulose were added in comparative example 1.

Comparative example 2

A500 mL three-necked flask equipped with a mechanical stirrer, a thermometer, and a reflux condenser was charged with 11.4189g (0.1mol) of 1, 4-cyclohexanediamine and 150g N, N-Dimethylacetamide (DMAC). Nitrogen was passed through and the solution was mechanically stirred at 500r/min for 20min to obtain a clear solution. 22.4170g (0.1mol) of hydrogenated pyromellitic dianhydride (HPMDA) were added in 5 portions. Stirring is continued for 6h at 500r/min under the reaction condition of 5 ℃ to obtain polyamic acid solution. 1.1209 nano cellulose crystal (CMC) is added into the PAA solution, and ultrasonic dispersion is carried out for 10min, thus obtaining the CNC/PAA mixed solution. The mixed solution was dropped onto a glass plate for spin coating. After spin coating, the coated film was placed in a drying oven for programmed temperature rise to thermal imidize the film. A heating step: 50 ℃/10min, 150 ℃/10min and 250 ℃/10 min. And after naturally cooling to room temperature, peeling the film in deionized water to obtain the CNC/CPI composite film.

Comparative example 3

The specific operation and conditions of comparative example 3 are the same as those of example 1, except that only aminated nanocellulose and no carboxylated nanocellulose were added in comparative example 3.

Comparative example 4

The specific operation and conditions of comparative example 4 were the same as those of example 1, except that only the carboxylated nanocellulose and no aminated nanocellulose were added in comparative example 4.

Comparative example 5

The operation and conditions of comparative example 5 are the same as those of example 1, except that the substrate is glass and the whole imidization process is completed in one oven without transferring and needle plate binding, and the specific temperature and time are 80 ℃ and kept for 2 h; heating to 120 ℃, and keeping for 1 h; heating to 150 ℃, and keeping for 1 h; heating to 200 ℃, and keeping for 1 h; the temperature is increased to 250 ℃ and kept for 1h, and the temperature rising rate is 5 ℃/min.

Results and discussion:

the polyimide films prepared in the above examples and comparative examples were tested for various properties according to the following test standards:

mechanical properties were as per GB/T13542.4-2009 part 2 of film for electrical insulation: test method, the test instrument is a CTM universal tensile machine.

The light transmittance is tested according to GB/T2410-2008, the haze is tested according to ASTM D1003-6, and the testing instrument is a HunterLab spectrocolorimeter.

Color space (CIE Lab) b value testing the test was performed according to ASTM E313 with the test instrument being a HunterLab spectrocolorimeter.

Heat shrinkage (dimensional stability) according to GB/T13542.2-2009 part 2 of film for Electrical insulation: test methods the test was carried out (150 ℃ C.) with a 2.5 Xelement (CNC video tester) model E04501.

The glass transition temperature (Tg) was analyzed using a static thermomechanical analyzer (TMA).

The test results are shown in table 1 below:

TABLE 1

From the comparison of the data in the table above, it can be seen that: compared with the comparative example 1, the polyimide film prepared in the example 1 has higher tensile strength, elongation and elastic modulus, which shows that the aminated nanocellulose is regarded as diamine, participates in the synthesis reaction of the polyimide, and is embedded into the polymer main chain of the polyimide in a covalent bond mode, so that the mechanical property of the polyimide film can be enhanced, and the aminated nanocellulose has excellent mechanical strength. In addition, because the aminated nanocellulose has a large number of hydroxyl groups, a large number of hydrogen bonds can be formed between the aminated nanocellulose and the polyimide, so that the crosslinking density of the polymer is increased, and the thermal performance of the polyimide film is enhanced. In addition, due to the existence of the carboxylated nanocellulose, the amino group on the diamine at the molecular chain end of the polyimide and the amino group on the diamine which is not reacted in the solution are converted into structures which are not easy to yellow, so that the polyimide film terminated by the carboxylated nanocellulose has a lower b value, namely a lower yellow index.

Comparing the various examples, it is clear that the properties of the polyimide film can be influenced by the kinds of alicyclic diamine and dianhydride, the concentrations of aminated and carboxylated nanocellulose, the baking temperature and time during film formation, and the like. With the increase of the content of the aminated nanocellulose and the carboxylated nanocellulose, the mechanical property and the thermal property of the polyimide film are enhanced, and the b value is further reduced. The main reason is that more aminated nano-cellulose brings more hydrogen bonds and covalent bonds, so that the cross-linked network of the polymer is more compact, and the mechanical property and the heat resistance of the polymer are greatly enhanced. In addition, more carboxylated nanocellulose can react more amino groups on the diamine into other structures, thereby reducing the high-temperature yellowing phenomenon caused by the amino groups.

Compared with the comparative example 2, the polyimide film prepared in the embodiment has various performances superior to those of the comparative example 2, because the nano cellulose crystals in the comparative example 2 are physically mixed and are connected with the polyimide film only through hydrogen bonds, and the aminated nano cellulose and the carboxylated nano cellulose in the embodiment are connected with the polymer main chain of the polyimide film through two modes of covalent bonds and hydrogen bonds, so that the bond energy is stronger, and the reinforcing effect on the polyimide film is more obvious. Secondly, in comparative example 2, the diamine at the end of the polymer chain was not treated and therefore the yellowness was also greater than in the examples.

The performance of each example is better than that of comparative examples 1-2 compared to comparative examples 3 and 4, mainly because comparative examples 1-2 each incorporate only one modified nanocellulose, which only partially strengthens the film and therefore has weaker overall performance than the examples.

Compared with the comparative example 5, the mechanical property and the heat shrinkage property of the example are higher than those of the comparative example 5, mainly because in the comparative example 5, the mechanical property and the heat shrinkage property are weaker than those of the example because the film is not heated uniformly without a needle plate, molecules cannot be arranged orderly, and the internal stress of the film cannot be eliminated.

The raw materials and equipment used in the invention are common raw materials and equipment in the field if not specified; the methods used in the present invention are conventional in the art unless otherwise specified.

The above description is only a preferred embodiment of the present invention, and is not intended to limit the present invention, and all simple modifications, alterations and equivalents of the above embodiments according to the technical spirit of the present invention are still within the protection scope of the technical solution of the present invention.

Claims

1. A preparation method of a colorless transparent polyimide film with low yellowness and high mechanical property is characterized by comprising the following steps:

(1) dissolving alicyclic diamine in a polar aprotic organic solvent A, adding aminated nano-cellulose, continuously stirring, adding alicyclic dianhydride in batches, and continuously stirring at 0-10 ℃ under the protection of nitrogen to obtain a polyamic acid solution; the molar ratio of the alicyclic diamine to the alicyclic dianhydride is 1.1-1: 1-1.1, and the mass of the aminated nanocellulose is 1-10% of that of the alicyclic dianhydride;

(2) respectively adding carboxylated nanocellulose, a catalyst and a dehydrating agent into the polyamic acid solution, continuously stirring at 100-200 ℃ under the protection of nitrogen to obtain a polyimide solution, soaking, washing and drying to obtain polyimide resin; the mass of the carboxylated nano-cellulose is 1-10% of that of the alicyclic dianhydride;

2. The method of claim 1, wherein:

in the step (1), the amino content in the aminated nano-cellulose is 1.3-1.8mmol/g, the molecular weight is 5000-8000, the diameter is 10-20nm, and the length is 100-300 nm;

in the step (2), the carboxyl content in the carboxylated nano-cellulose is 1.2-3 mmol/g, the molecular weight is 5000-8000, the diameter is 10-20nm, and the length is 100-300 nm.

3. The method of claim 1 or 2, wherein: in the step (1), the step (c),

the alicyclic diamine comprises one or more of 1, 3-cyclohexanediamine, 1, 4-cyclohexanediamine, 1, 3-cyclobutanediamine, 4-diaminodicyclohexylmethane and 3, 3-dimethyl-4, 4-diaminodicyclohexylmethane;

the alicyclic dianhydride comprises one or more of 1,2,3, 4-cyclobutanetetracarboxylic dianhydride, bicyclo [2.2.2] oct-7-ene-2, 3,5, 6-tetracarboxylic dianhydride, 1,2,3, 4-cyclohexanetetracarboxylic dianhydride, 1,2,3, 4-cyclopentanetetracarboxylic dianhydride, 3-carboxymethyl-1, 2, 4-tricarboxylic cyclopentane dianhydride, cyclooctadienetetracarboxylic dianhydride, norbornane-2-spiro-alpha-cyclopentanone-alpha '-spiro-2' -norbornane-5, 5',6,6' -tetracarboxylic dianhydride, hydrogenated pyromellitic dianhydride, and hydrogenated 3,3 ', 4, 4' -biphenyltetracarboxylic dianhydride;

the polar aprotic organic solvent A is one or more of N, N-dimethylacetamide, N-dimethylformamide, N-methylpyrrolidone, dimethyl sulfoxide, propylene glycol methyl ether acetate, tetramethyl sulfoxide, N '-dimethyl-N, N' -propylene urea, dichloromethane, chloroform, tetrahydrofuran, m-cresol and gamma-butyrolactone.

4. The method of claim 1 or 2, wherein: the step (1) specifically comprises the following steps: alicyclic diamine is dissolved in a polar aprotic organic solvent A, amination nano-cellulose is added, the mixture is continuously stirred for 5-20 min at the speed of 100-1500 r/min, alicyclic dianhydride is added for 2-10 times, and the mixture is continuously stirred for 5-24 h at the temperature of 0-10 ℃ under the protection of nitrogen gas to obtain a polyamic acid solution.

5. The method of claim 1 or 2, wherein: in the step (2),

the catalyst is a tertiary amine compound, and comprises one or more of triethylamine, pyridine, isoquinoline and picoline;

the dehydrating agent is one or more of acetic anhydride, propionic anhydride, butyric anhydride, phthalic anhydride, trimellitic anhydride and hexahydrophthalic anhydride;

the mass of the catalyst and the dehydrating agent is 0.1-1% of the total mass of the alicyclic diamine and the alicyclic dianhydride.

6. The method of claim 1 or 2, wherein: the step (2) specifically comprises the following steps: respectively adding carboxylated nanocellulose, a catalyst and a dehydrating agent into the polyamic acid solution, continuously stirring for 5-24 hours at 100-200 ℃ under the protection of nitrogen at 100-1500 r/min to obtain a polyimide solution, soaking and washing the polyimide solution with an ethanol water solution, and drying to obtain the polyimide resin.

7. The method of claim 1 or 2, wherein: in the step (3), the polar aprotic organic solvent B is one or more of N, N-dimethylacetamide, N-dimethylformamide, N-methylpyrrolidone, dimethyl sulfoxide, propylene glycol methyl ether acetate, tetramethyl sulfoxide and N, N '-dimethyl-N, N' -propylene urea.

8. The production method according to claim 1 or 2, characterized in that: in the step (3), the step (c),

the low temperature, the medium temperature and the high temperature are respectively 40-80 ℃, 140-180 ℃ and 220-270 ℃, and the baking time is 5-30 min each time;

after the medium-temperature baking, the formed polyimide film is peeled off from the base material, is pricked on a needle plate with a hollow middle part, and enters the high-temperature baking.

9. The method of claim 1 or 2, wherein: in the step (3), the solid content of the polyimide resin solution is 5-20%, and the viscosity is 2000 cP-8000 cP.

10. The method of claim 1 or 2, wherein: in the step (3), the base material is a high-temperature-resistant base material and comprises one of polyethylene terephthalate, polyethylene naphthalate, polyethyleneimine and polyphenylene sulfide.