Study On Preparation And Properties Of Heat-Resistant And Heat-Insulating Polyimide Films

With the rapid development of aerospace,national defense,high-temperature industry,and construction,the working temperature,mechanical properties,and thermal insulation of thermal insulation materials have posed new challenges.Polyimide（PI）,as a special engineering plastic,has the characteristics of low density,good thermal stability,a low expansion coefficient,and excellent mechanical properties.It is often used to prepare polymer matrix composites with different properties.Aerogels have a unique three-dimensional network structure,which gives them low density,high porosity,and a high specific surface area but also makes the mechanical properties of aerogels very poor.Therefore,polyimide was used as the composite matrix,and SiO₂aerogel（SA）was used to improve the thermal insulation performance to prepare polymer matrix composites with excellent heat resistance and thermal insulation performance.At present,there are few reports about SA/PI composites.In this paper,polyimide and aerogel composites are discussed.On this basis,a breakthrough is made by utilizing the low thermal conductivity of hollow glass microspheres（HGM）sprayed on the surface of the PI film to form a sandwich structure in order to improve the thermal insulation performance further.In this paper,biphenyltetracarboxylic dianhydride and p-phenylenediamine were used as monomers to polymerize and then compounded with SiO₂aerogel to prepare heat-resistant and heat-insulating polyimide films by thermal imidization.The main contents of this paper include the effect of aerogel content on SA/PI composite films and the effect of heat treatment temperature on SA/PI composite films.Then the effects of the adhesive,hollow glass beads,and the thickness of the film substrate on the structure and properties of the composite film were discussed.The following aspects are the main conclusions of this paper:（1）Polyimide（PI）composite films were prepared by a two-step method.SA/PI composite films were prepared by the thermal imidization of polyamic acid（PAA）solution and SiO₂aerogel.It was found that the mechanical properties and thermal stability of the composite films decreased with an increase in SA content.At the same time,the aerogel still retains its unique three-dimensional network structure in the interior of PI,and the thermal insulation performance of the composite film is obviously improved.20 wt%SA/PI composite film has the best comprehensive properties;the tensile strength is 126.58 MPa,the elongation at break is 15.5%,and the mechanical properties are still high.The 5%thermal decomposition temperature is 548.1°C,close to 550°C,and the water absorption is0.83%at 12 h,which can be used in most fields.Compared with pure PI film,the temperature difference of the surface stable temperature of the film under the same heat source is 12.3°C,and the heat insulation efficiency is improved by 7.8%.（2）In this paper,high-performance 20 wt%SA/PI composite films were prepared by thermal imidization.With the increase in thermal imidization temperature,the rigid segments of PI molecular chains increased,and the orientation and order of PI molecular chains also increased.A more regular arrangement was formed among the PI molecular chains.When the heat treatment temperature is increased to 450°C,the tensile strength of the 20 wt%SA/PI composite film is161.71 MPa,and the elongation at break is 17.7%.Compared with the 20 wt%SA/PI composite film obtained by heat treatment at 300°C,the tensile strength is increased by 43.8%,and the properties are greatly improved.Therefore,450°C was used as the heat treatment temperature of 20 wt%SA/PI composite films in the subsequent experiments.（3）The effects of vacuum defoaming conditions on PAA solution on the properties of 20 wt%SA/PI composite films were investigated.The tensile strength and thermal insulation properties of the PI film without defoaming are slightly lower than those of 20 wt%SA/PI composite film treated by vacuum defoaming,and the tensile strength is 149.21 MPa and 161.71 MPa,respectively.The surface stability temperatures of the films were 147.4℃and 144.7℃,respectively.However,the elongation at break of the pure PI film without defoaming is much lower than that of20 wt%SA/PI composite film,which is 3.3%and 17.7%,respectively.It can be seen that the bubbles greatly influence the mechanical properties of the PI film and have a general improvement effect on heat insulation,so the vacuum defoaming process is adopted for the subsequently prepared PI composite film.（4）The effect of spraying a coating on the surface of the HGM/SA/PI composite film was discussed.It was found that when the spraying amount of HGM was 0.2 g/m L and the content of P84 was 0.02 g/m L,the dispersion effect of HGM on the surface of the PI film was the best,and the heat insulation effect was the best.The stable temperature of the HGM/SA/PI composite film decreased from 148.2°C to 120.2°C,and the temperature difference with a light source was 79.8℃.The tensile strength is 34.5 MPa,and the elongation at break is 3.6%.The mechanical strength of the HGM/SA/PI composite film is still maintained.Since the heat insulation performance of the HGM-embedded SA/PI composite film is best when the spraying amount of HGM is 0.2 g/m L,the HGM-embedded SA/PI composite film with a spraying amount of HGM of 0.2 g/m L and a P84 of 0.02 g/m L is adopted later.（5）The effect of the coating thickness of PAA on the HGM/SA/PI composite film was investigated.It was found that the HGM/SA/PI composite film changed from ductile fracture to brittle fracture with the increase in thickness of the PI base film.When the coating thickness was 2000μm,a large number of cracks appeared in the PI base film.When the film thickness was 1500μm,the mechanical properties of PI were the best;the tensile strength and elongation at break were 69.48 MPa and8.7%,respectively.Finally,the high-performance HGM sandwich SA/PI composite film with heat resistance and insulation was obtained.