How to improve the high temperature resistance of bidirectional polyimide film
The core of improving the high temperature resistance of bidirectional polyimide (BPI) films is to enhance the thermal stability of material molecular chains, prevent thermal oxidative aging and decomposition at high temperatures, and improve the high-temperature integrity of the microstructure through molecular structure optimization, preparation process improvement, composite modification, and o
The core of improving the high temperature resistance of bidirectional polyimide (BPI) films is to enhance the thermal stability of material molecular chains, prevent thermal oxidative aging and decomposition at high temperatures, and improve the high-temperature integrity of the microstructure through molecular structure optimization, preparation process improvement, composite modification, and other means. The following provides a detailed explanation from technical principles to specific methods, covering key areas such as chemical modification, process control, and composite reinforcement:
1、 Chemical modification: Optimize molecular chain structure and enhance intrinsic thermal stability
The high temperature resistance of polyimide is essentially determined by the chemical structure of its molecular chains. By adjusting monomer selection, introducing special functional groups, or changing the chain connection method, its thermal stability can be improved from the source. This is the most core and fundamental path of modification.
1. Choose high rigidity/high heat resistance monomers to enhance the intermolecular forces
Polyimide is polymerized from dianhydride (anhydride monomer) and diamine (amine monomer), and the monomer structure directly affects the rigidity, conjugation, and intermolecular forces of the molecular chain
Introducing aromatic fused/heterocyclic monomers: selecting monomers with rigid structures such as naphthalene ring, phenanthrene ring, fluorene ring, and carbazole ring (such as derivatives of 1,4,5,8-naphthalene tetracarboxylic acid dianhydride NTCDA and 4,4 '- diaminodiphenyl ether ODA) can enhance the conjugated system and steric hindrance of the molecular chain, reduce segment motion at high temperatures, and increase the thermal decomposition temperature (Td). For example, replacing traditional phthalic anhydride (PMDA) with NTCDA can increase the Td of polyimide by 20-50 ℃ and increase the upper limit of long-term use temperature by 10-30 ℃.
Introducing fluorine/silicon containing monomers: Introducing fluorine atoms (such as hexafluorodianhydride 6FDA) or siloxane segments (such as copolymerization of diaminodiphenyl sulfone and siloxane diamine) into the molecular chain can reduce intermolecular forces and improve oxidation resistance - the strong electronegativity of fluorine atoms can prevent free radical oxidation reactions at high temperatures, while siloxane segments can form a dense SiO ₂ protective layer at high temperatures, delaying material decomposition.
Avoid weak bond structures: reduce the easily decomposable ester bonds, ether bonds (excess), methylene groups, etc. in the molecular chain, and prioritize strong chemical bonds such as C-C, C-N (aromatic amine), C=O (imide ring), etc., to reduce the probability of chain breakage at high temperatures.
2. Molecular chain cross-linking: constructing a three-dimensional network structure
By chemically crosslinking linear polyimide molecular chains into a three-dimensional network structure, it can significantly prevent segment slip and thermal decomposition at high temperatures, improve the heat distortion temperature (HDT) and long-term heat resistance:
Crosslinking agent modification: Adding multi-functional crosslinking agents (such as triamines, monomers containing alkynyl/azide groups) during the polymerization process, forming C-C or C-N crosslinking bonds during high-temperature curing. For example, the introduction of 4,4 '- diaminodiphenylacetylene (DAPE), whose alkynyl group undergoes cyclic crosslinking at 250-300 ℃, increases the tensile strength retention rate of the film from 60% to over 85% at 300 ℃.
Radiation crosslinking: Through radiation treatment such as gamma rays and electron beams, polyimide molecular chains undergo free radical crosslinking to form a stable three-dimensional network. This method does not require additional crosslinking agents and is suitable for scenarios with high purity requirements (such as the field of electronic insulation), but requires control of radiation dose (excessive amounts can easily cause chain breakage).
2、 Process control: Optimize the microstructure of thin films and reduce defects
The preparation process directly affects the crystallinity, orientation, density, and defects (such as bubbles and grain boundary voids) of biaxially stretched polyimide films, and these microstructural defects can become the "breakthrough point" for thermal oxidative aging at high temperatures, reducing the actual heat resistance performance.
1. Optimize the polymerization and film-forming process (pre polymer stage)
Control the degree of polymerization and molecular weight distribution: By adjusting the molar ratio of dianhydride to diamine (usually 1:1.02 to avoid excessive monomer residue), reaction temperature (low-temperature condensation, such as forming polyamide acid PAA at -5-0 ℃), and reaction time, ensure the generation of high molecular weight, narrow distribution polyamide acid (PAA) solution - high molecular weight chains are more likely to form dense structures, reducing defects caused by small molecule volatilization at high temperatures.
Reduce impurities and bubbles in PAA solution: Remove small particles and bubbles from the solution by filtration (using a 0.22 μ m pore size filter membrane) and vacuum defoaming (standing at 25-40 ℃ for 1-2 hours), avoiding the formation of pores after film formation (pores will accelerate the permeation of oxygen at high temperatures, causing thermal oxygen aging).
2. Correctly control the biaxial stretching and imidization processes (core forming stage)
Bidirectional stretching (longitudinal MD+transverse TD) and imidization (dehydration cyclization) are key steps that determine the orientation and crystallinity of BPI films, directly affecting their high temperature resistance performance
Bidirectional stretching parameters: Control the stretching temperature (usually 20-30 ℃ below the glass transition temperature Tg, such as 180-220 ℃), stretching ratio (MD/TD ratio is usually 2-3 times and needs to be symmetrically matched), and stretching rate (slow and uniform to avoid internal stress). Reasonable stretching can arrange molecular chains in an orderly manner along the stretching direction, improve crystallinity (from amorphous or low crystalline to partially crystalline), and the molecular chain forces in the crystalline region are stronger, with better high temperature resistance than those in the amorphous region.
Imitization process: Adopting "gradient heating" imidization (such as 150 ℃/1h → 250 ℃/1h → 350 ℃/0.5h) to avoid rapid heating causing PAA dehydration, bubble formation or cracking. Adequate imidization (degree of imidization>98%) can reduce residual carboxyl groups (- COOH) and amide groups (- CONH -) - these groups are easily decomposed at high temperatures, releasing small molecules such as CO ₂ and H ₂ O, and damaging the film structure.
3、 Composite modification: Introducing high-temperature resistant fillers to construct a "reinforcement protection" system
By introducing high-temperature resistant inorganic/organic fillers into the polyimide matrix, the physical barrier and thermal stability enhancement effects of the fillers can be utilized to further enhance the high-temperature resistance of the film (especially suitable for scenarios that require breaking through the upper limit of pure polyimide heat resistance).
1. Inorganic nano filler composite (mainstream direction)
Select inorganic nanoparticles with high temperature resistance and high dispersibility, and achieve synergistic effects with the matrix through "nanoscale dispersion" to avoid macroscopic agglomeration and performance degradation:
Carbon based fillers, such as graphene (GO/rGO) and carbon nanotubes (CNT), have high thermal conductivity (graphene thermal conductivity ≈ 5000 W/(m · K)) and thermal stability (Td>600 ℃ in an inert atmosphere). They not only enhance the thermal stability of the film, but also accelerate heat conduction at high temperatures, avoiding local overheating. For example, adding 0.5wt% reduced graphene oxide (rGO) can increase the long-term use temperature of BPI films from 260 ℃ to 290 ℃, and after aging at 290 ℃ for 1000 hours, the retention rate of electrical insulation strength can be increased from 55% to 78%.
Ceramic/oxide fillers: such as aluminum oxide (Al ₂ O3), silicon dioxide (SiO ₂), boron nitride (BN) - these fillers are resistant to high temperatures (Td>1000 ℃) and have good chemical stability. They can form a "physical barrier layer" in the polyimide matrix, preventing the penetration of oxygen and heat and delaying thermal oxidative aging. For example, adding 5wt% nano BN can reduce the quality loss rate of BPI film from 8% to 3% at 300 ℃ (aging for 1000 hours).
Rare earth oxide fillers, such as cerium oxide (CeO ₂) and lanthanum oxide (La ₂ O3), have excellent antioxidant properties and can capture free radicals (such as · OH, · O ₂) generated at high temperatures, preventing chain decomposition reactions caused by free radicals and improving thermal oxygen stability.
2. Organic inorganic hybrid modification
The inorganic phase (such as siloxane and titanoxane) and polyimide molecular chain are chemically combined (rather than simply physically mixed) by means of sol gel method, in-situ polymerization and other methods to improve the interface compatibility and high-temperature synergistic effect. For example, by mixing tetraethyl orthosilicate (TEOS) with PAA solution and hydrolyzing it in situ to generate SiO ₂, SiO ₂ forms hydrogen bonds or covalent bonds with the carboxyl groups of PAA. The Td of the hybrid film can be increased by 30-60 ℃, and its moisture and heat resistance (electrical stability under high temperature and high humidity) is also significantly improved.
4、 Surface modification: construction of high-temperature protective coating
For BPI films that need to be applied in extreme high temperature scenarios (such as short-term use above 350 ℃ or long-term use above 300 ℃), a "surface protective barrier" can be formed by coating the substrate with a high-temperature resistant coating, reducing direct contact between the substrate and high-temperature environments (oxygen, corrosive gases):
Coating material selection: Priority should be given to high-temperature resistant ceramic coatings (such as Al ₂ O3, ZrO ₂), metal coatings (such as gold and silver, suitable for scenarios with both conductivity requirements), or high-temperature resistant polymer coatings (such as modified derivatives of polyetheretherketone PEEK and polytetrafluoroethylene PTFE).
Coating process: magnetron sputtering (metal/ceramic coating), sol gel coating (ceramic coating) or solution scraping (polymer coating) are used to ensure uniform coating thickness (usually 1-5 μ m) and tight bonding with the substrate (to avoid falling off at high temperature). For example, sputtering a 500nm thick Al ₂ O ∝ coating on the surface can extend the aging life of BPI films from 500 hours to over 1200 hours at 350 ℃.
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