Polyimide Foams Research Articles

Lightweight and Ultrastrong Polymer Foams with Unusually Superior Flame Retardancy.

High-performance flame-retardant materials are urgently needed to address outstanding issues that pertain to safety. Traditional flame retardants are toxic to the environment and/or lack the physical properties required for use in many contemporary applications. Here, we show that isocyanate-based polyimide (PI) foam, a flammable material, can exhibit unusually superior flame retardancy as well as other excellent properties, such as being lightweight and displaying high mechanical strength, by incorporating red phosphorus (RP)-hybridized graphene. The covalent bonds formed between the graphene platelets and the PI matrix provide the resultant PI foam with a specific Young's modulus (83 kNm kg-1) that is comparable to or even higher than those displayed by state-of-the-art foams, including silica aerogels, polystyrene foams, and polyurethane foams. In addition, even a low content of the RP-hybridized graphene (2.2 wt %) results in an exceptionally higher limiting oxygen index (39.4) than those of traditional flame-retardant polymer-based materials (typically 20-30). The resultant PI foam also exhibits thermal insulation properties that are similar to that of air. Moreover, the RP-hybridized graphene is prepared using a one-step ball milling process in 100% yield, and does not require solvent or produce waste. The preparation of the flame-retardant PI foams can be scaled as the starting materials are commercially available and the techniques employed are industrially compatible.

Thermal degradation and flame retardant properties of isocyanate-based flexible polyimide foams with different isocyanate indices

A series of isocyanate-based flexible polyimide foams were prepared with different isocyanate indices. The structure and properties of these polyimide foams were characterized in detail. The results indicated that with the increase of isocyanate index, the density decreased from 8.7kg/m3 to 5.7kg/m3. The open cell ratio was in the range of 94.9%–96.4% and less influenced by the content of isocyanate. The compression strength increased first with the increase of isocyanate index, and reached the maximum value at an isocyanate index of 1.04. All the polyimide foams possessed excellent thermal stability, with temperature at weight loss of 5% and 10% being in the range of 288.5–320.0°C and 337.7–369.0°C, respectively. The limiting oxygen index (LOI) values for all the samples were above 25%, indicating the high flame resistance character of polyimide. The heat release peak values, the total heat release and mean heat release rate exhibited similar trends of little changes with isocyanate index varying from 0.89 to 1.04 and increasing rapidly with the index more than 1.11, indicating the side products of amino or uramido produced by excess isocyanate would damage the flame resistance properties of polyimide foams severely.

Effects of aramid honeycomb core on the flame retardance and mechanical property for isocyanate‐based polyimide foams

ABSTRACTIn this work, composite materials containing aramid honeycomb core (ARHC) reinforced isocyanate‐based polyimide foams (IBPIFs) were prepared by a simple free‐foaming process. The flame retardance, smoke performance, and mechanical property of the composite materials along the transverse (W) and longitude (L) directions of ARHC were studied. Results of limiting oxygen index and cone calorimeter reveal that the space partition action of ARHC on IBPIFs, barrier action of honeycomb structure on fire, and generation of straight, flurry and porous char layer in the honeycomb cells during combustion effectively enhanced the flame retardance of IBPIFs. Mechanical property test shows that IBPIFs and ARHC presented a positive synergetic effect, and ARHC increased the compression strength of IBPIFs along the W and L directions of ARHC by approximately 300%. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45041.

Improved thermal stability and flame resistance of flexible polyimide foams by vermiculite reinforcement

ABSTRACTIn this work, a series of flexible polyimide composite foams with dianhydride and isocyanate as the starting materials were prepared by pre‐dispersing vermiculite in isocyanate. The experimental results revealed that the cellular diameter of foams obviously decreased with the addition of vermiculite. The open cell content decreased with the increase of vermiculite, and foams containing vermiculite possessed higher apparent densities of about 10 kg/m3when compared with the neat polyimide foams. The compressive strength was dramatically enhanced by vermiculite incorporation, and reached maximum when the filler content was 2%, indicating dramatically reinforcing effect of vermiculite. With the addition of vermiculite, the thermal stability and flame resistance of polyimide foams were improved. The limiting oxygen index of polyimide foams showed an increase from 31 to 33.5% with increase of vermiculite from 0 to 8% and the peak heat release rate decreased, indicating high flame retardant properties. Vermiculites possess excellent heat insulation and high temperature stability. When conditioned at high temperature, the filler would block the heat transfer during the bulk foam. Therefore, it is feasible to further increase the mechanical and thermal properties of polyimide foams by vermiculite filling. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci.2017,134, 44828.

Fabrication of isocyanate‐based polyimide foam by a postgrafting method

ABSTRACTDensity and flame retardancy controlled isocyanate‐based polyimide foam was prepared by a postgrafting method. The first solution containing prepolymer that was synthesized by dianhydride and overdose isocyanates was added into the second solution containing dianhydride derivatives, water, catalysts, and surfactants. The possible reactions during preparation are discussed. The obtained Fourier transform infrared spectra indicate that an increased amount of imide rings was generated with increasing molar ratio of the anhydride/isocyanate groups. The size and walls of the cells became smaller and thinner with less carbon dioxide (CO2) escaping into the air during the first solution preparation process, as shown in scanning electron microscopy images. The thermogravimetric analysis curves demonstrated that the 5% weight loss temperature (T5%) was greater than 289 °C, and the residual weight retention at 800 °C was more than 45%. In addition, differential thermogravimetry curves demonstrated that the thermal stability decreased with more byproducts in polyimide foams. The limiting oxygen index increased gradually from 30.63% ± 0.56 to 48% ± 0.50 with increasing molar ratio of the anhydride/isocyanate groups. Meanwhile, the density of obtained polyimide foams ranged from 38.31 kg/m3 ± 0.90 to 99.53 kg/m3 ± 10.85. When the molar ratio of anhydride/isocyanate groups ranged from 0.4 to 0.8, the prepared isocyanate‐based polyimide foams all exhibit both great flame retardancy and lower density. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44240.

Two-layer anti-reflection coating with mullite and polyimide foam for large-diameter cryogenic infrared filters.

We have developed a novel two-layer anti-reflection (AR) coating method for large-diameter infrared (IR) filters made of alumina, for use at cryogenic temperatures in millimeter wave measurements. Thermally sprayed mullite and polyimide foam (Skybond Foam) are used as the AR material. An advantage of the Skybond Foam is that the index of refraction is chosen between 1.1 and 1.7 by changing the filling factor. Combination with mullite is suitable for wide-band millimeter wave measurements with sufficient IR cutoff capability. We present the material properties, fabrication of a large-diameter IR filter made of alumina with this AR coating method, and characterizations at cryogenic temperatures. This technology can be applied to a low-temperature receiver system with a large-diameter focal plane for next-generation cosmic microwave background polarization measurements, such as POLARBEAR-2 (PB-2).

Preparation and properties of polyimide/chopped carbon fiber composite foams

In this study, a series of reinforced polyimide (PI)/carbon fiber (CF) composite foams were fabricated through thermal foaming of polyester ammonium salt (PEAS) precursor powders. The PEAS precursor powders containing different contents of chopped CF were synthesized from benzophenone‐3,3′,4,4′‐tetracarboxylic dianhydride (BTDA) and 4,4′‐diaminodiphenyl ether (ODA). The effects of different CF loadings on foaming behavior of PEAS/CF composite precursor powders, final cellular morphology, and physical properties of PI composite foams were investigated. The results revealed that the chopped CF acted as nucleation agent in the foaming process. The dispersion of CF can be evaluated using digital microscope. It is interesting to find that the chopped CF were highly oriented along the direction of cell arrises. As a result, the mechanical properties of PI foams were significantly enhanced owing to the incorporation of chopped CF. Furthermore, the thermal stability of PI composite foams were also slightly improved owing to fine dispersion of CF. In addition, the PI/CF composite foam shows uniform cell size distribution and the best comprehensive physical properties as chopped CF loading at around 6 wt%. Copyright © 2016 John Wiley & Sons, Ltd.

A novel vacuum-assisted method for fabricating flexible polyimide foams from 3,3′,4,4′-oxydiphthalic anhydride/4,4′-oxydianiline

In this research, a novel method for fabricating flexible polyimide foams was developed. The foam was derived from the precursor synthesized via esterification of 3,3′,4,4′-oxydiphthalic anhydride and 4,4′-oxydianiline. After drying process, the remaining solvent acted as blowing agent. By adjusting the synthesis condition, low-molecular weight precursor was obtained and can be foamed under environment temperature. With this novel type of precursor and vacuum-assisted foaming process, we could easily obtain any shape of foams, which was easy to process under relatively low temperature. The foam could be imidized later in order to improve mechanical strength. During the imidization process, in situ Fourier transform infrared (FTIR) analysis was applied to analyze the variation of the imidization percentage with time. The density of the foams could be adjusted from 12 to 30 kg m−3 with uniform cell structure. Other properties of foams were also investigated. The glass transition temperature of the precursor was 70°C, which is obviously lower than that of the usual previously reported. The 5% and 10% weight loss temperatures of the finally imidized foam can reach up to 567°C and 593°C, respectively. The foams belong to open cell structure with 98% open cell content. The limiting oxygen index of the foams varied from 38.6 to 41.9, depending on foam densities. The compressive strength of the foam ranged from 30.6 to 90.6 kPa when the density changed from 15 to 28 kg m−3. In addition, the in situ FTIR analysis was applied to analyze the imidization percentage of the foam during imidization process.

Synthesis and properties of polyimide foams containing benzimidazole units

A series of novel polyimide foams containing benzimidazole units were prepared from polyester ammonium salt (PEAS) precursor powders, which were synthesized by co-polymerization of BTDA with two diamines, BIA and ODA, in various molar ratios.

Effects of strain-rate and temperature on mechanical behavior of polyimide foam in compression

Effects of strain-rate and temperature on mechanical behavior of polyimide foam in compression

Thermal design of spacecraft solar arrays using a polyimide foam

The design of the Thermal Control System (TCS) of spacecraft solar arrays plays a fundamental role. Indeed, the spacecraft components must operate within a certain range of temperature. If this doesn't occur, their performance is reduced and they may even break. Solar arrays, which are employed to recharge batteries, are directly exposed to the solar heat flux, and they need to be insulated from the earth's surface irradiation. Insulation is currently provided either with a white paint coating or with a Multi Layer Insulation (MLI) system [1]. A configuration based on an open-cell polyimide foam has also been recently proposed [2]. Using polyimide foams in TCSs looks very attractive in terms of costs, weight and assembling. An innovative thermal analysis of the above cited TCS configurations is carried out in this paper, by solving the porous media energy equation, under the assumption of Local Thermal Equilibrium (LTE) between the two phases. Radiation effects through the solar array are also considered by using the Rosseland approximation. Under a stationary daylight condition, temperature profiles are obtained by means of the finite-element based code COMSOL Multiphysics®. Finally, since the weight plays an important role in aerospace applications, weights of the three TCS configurations are compared.

Sound transmission through double cylindrical shells lined with porous material under turbulent boundary layer excitation

This paper investigates sound transmission through double-walled cylindrical shell lined with poroelastic material in the core, excited by pressure fluctuations due to the exterior turbulent boundary layer (TBL). Biot׳s model is used to describe the sound wave propagating in the porous material. Three types of constructions, bonded–bonded, bonded–unbonded and unbonded–unbonded, are considered in this study. The power spectral density (PSD) of the inner shell kinetic energy is predicted for two turbulent boundary layer models, different air gap depths and three types of polyimide foams, respectively. The peaks of the inner shell kinetic energy due to shell resonance, hydrodynamic coincidence and acoustic coincidence are discussed. The results show that if the frequency band over the ring frequency is of interest, an air gap, even if very thin, should exist between the two elastic shells for better sound insulation. And if small density foam has a high flow resistance, a superior sound insulation can still be maintained.

Graphene Oxide: A Versatile Agent for Polyimide Foams with Improved Foaming Capability and Enhanced Flexibility

Close-celled aromatic polyimide (PI)/graphene foams with low density and improved flexibility were fabricated by thermal foaming of poly(amic ester)/graphene oxide (PAE/GO) precursor powders. The PAE/GO precursor powders were prepared by grafting GO nanosheets with PAE chains, which led to efficient dispersion of the GO nanosheets in PAE matrix. Incorporation of GO resulted in an enhanced foaming capability of the precursor, i.e., enlarged cell size and decreased foam density. Notably, a decrease of 50% in the foam density was obtained via the addition of only 2 wt % GO in the precursor. In the foaming process, the GO nanosheets functioned as a versatile agent that not only provided heterogeneous nucleation sites but also produced gaseous molecules. By analyzing the foaming mechanism, the excellent features of GO in heat transfer, gas barrier, and strength reinforcement also facilitated to obtain large and uniform cells in the foams. In addition, the PI/graphene foams exhibited a prominent flexibility and...

Structure and dielectric properties of polyimide/silica nanocomposite nanofoam prepared by solid‐state foaming

ABSTRACTIn this article, polyimide (PI)/silica nanocomposite nanofoams were prepared by solid‐state foaming using supercritical CO2 as foaming agent. To control the cell size and morphology of the PI/silica foam, the silica nanoparticles as nucleating agent were in situ formation from TEOS via sol‐gel process, which make the silica nanoparticles homogeneously dispersed in PI matrix. The resulting PI/silica nanocomposite nanofoams were characterized by scanning electron microscopy (SEM), the image analysis system attached to the SEM and dielectric properties measurements. In PI/silica nanocomposite nanofoams, one type of novel morphology was shown that each cell contained one silica nanoparticle and many smaller holes about 20–50 nm uniformly located in the cell wall. This special structure could visually prove that the nucleation sites during foaming were formed on the surface of nucleating agents. Compared with those of neat PI foam, the cell size of PI/silica nanocomposite nanofoams was smaller and its distribution was narrower. The dielectric constant of PI/silica nanocomposite nanofoams was decreased because of the incorporation of the air voids into the PI/silica nanofoams. While the porosity of PI/silica nanocomposite nanofoam film was 0.45, the dielectric constant of the film (at 1 MHz) was reduced from 3.8 to about 2.6. Furthermore, the dielectric constant of PI/silica nanofoam films remained stable across the frequency range of 1×102∼1×107 HZ. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42355.

Effects of hydrotalcites and tris (1-chloro-2-propyl) phosphate on thermal stability, cellular structure and fire resistance of isocyanate-based polyimide foams

Isocyanate-based polyimide foams (PIFs) with different dosages of liquid tri (1-chloro-2-propyl) phosphate (TCPP) and micro-sized hydrotalcites (LDHs) particles alone, as well as different mixing ratios of TCPP and LDHs, were prepared via a one-step process in this work. Limiting oxygen index (LOI) and cone calorimeter test (CCT) results indicated that TCPP exhibited more pronounced flame retardant efficiency than LDHs for isocyanate-based PIFs. However, scanning electron microscopy images and digital macrostructural images results showed that, in contrast to LDHs, when the dosage of TCPP exceeded 10% it caused a clear cracking effects on the macro-cellular structure and opening cell effects on the micro-cellular structure for isocyanate-based PIFs. Because the dramatically volatilization of TCPP during the postcuring process caused obvious cellular contraction in the isocyanate-based PIFs. Meanwhile, the use of TCPP also obviously decreased the thermal stability of isocyanate-based PIFs unlike LDHs. However, when these two flame retardants were used in combination, they could effectively enhance the fire resistance and ensure macro- and micro-cellular structures of the isocyanate-based PIFs, unlike standalone use of TCPP and LDHs. When 10% TCPP was simultaneously used with 10% LDHs, the macro- and micro-cellular structures of the resultant foams were clearly improved compared with foams prepared using TCPP only. These results were believed to be attributable to LDHs dispersion in the foams, which enhanced the strength of cellular windows and skeletons, then restrained the cell contraction. Compared with foams without flame retardants, the fire resistance of the isocyanate-based PIFs prepared with 10% TCPP and 10% LDHs was obviously enhanced; specially, the LOI was enhanced by 29.4%, and the peak of heat release rate (PHRR) decreased by 36.1%. Thus, the use of liquid and solid flame retardants in combination may effectively yield isocyanate-based PIFs with high quality cellular structures and excellent fire resistance.

The compressive behaviour and constitutive equation of polyimide foam in wide strain rate and temperature

These days, polymer foams, such as polyurethane foam and polystyrene foam, are used in various situations as a thermal insulator or shock absorber. In general, however, their strength is insufficient in high temperature environments because of their low glass transition temperature. Polyimide is a polymer which has a higher glass transition temperature and high strength. Its mechanical properties do not vary greatly, even in low temperature environments. Therefore, polyimide foam is expected to be used in the aerospace industry. Thus, the constitutive equation of polyimide foam that can be applied across a wide range of strain rates and ambient temperature is very useful. In this study, a series of compression tests at various strain rates, from 10 −3 to 10 3 s −1 were carried out in order to examine the effect of strain rate on the compressive properties of polyimide foam. The flow stress of polyimide foam increased rapidly at dynamic strain rates. The effect of ambient temperature on the properties of polyimide foam was also investigated at temperature from − 190 ° C to 270° ∘ C. The flow stress decreased with increasing temperature.

A comparative study of structure and electromagnetic interference shielding performance for silver nanostructure hybrid polyimide foams

Comparison of structure and electromagnetic interference shielding performance for silver nanostructures hybrid polyimide foams.

The Production of Polyimide Foam Materials Based on Acrylonitrile and (Meth)Acrylic Acid

A study was made of the influence of the structure and ratio of the principal monomers, and also the microstructure of obtained block copolymers of (meth) acrylonitrile and (meth)acrylic acid, on the formation, in the process of heat treatment, of poly(meth)acrylimide foam materials.

Enhanced polyimide proportion effects on fire behavior of isocyanate-based polyimide foams by refilled aromatic dianhydride method

Lightweight and fire-resistant isocyanate-based polyimide foams (PIFs) were rapidly formed via a simple one-pot process; polymethylene polyphenylene isocyanate (PAPI) was added into a solution containing refilled aromatic dianhydride. The obtained Fourier transform infrared spectra (FT-IR) showed that the refilled 3,3′,4,4′-benzophenone tetracarboxylic acid dianhydride (BTDA) reacted with isocyanate and amino groups in PIFs during the forming and postcuring process. These reactions increased the percentage of polyimide in the foams. Meanwhile, the reaction between refilled BTDA which has a higher reactivity than its derivatives and amino groups, reduced the reaction between isocyanate and amino groups. Subsequently, the generation of ureido was restrained. Thermal gravimetric analysis (TGA) and differential thermogravimetry (DTG) results presented that when the refilled BTDA dosage was increased, the percentage of polyimide and ureido were increased and reduced, respectively. However, the proportion of different ingredients in the PIFs prepared by different formulations was basically the same when the refilled BTDA dosage exceeded 50%. As such, the refilled dosage of BTDA between 50% and 60% was sufficient for this system, the refilled BTDA had extreme effects on enhancing polyimide proportion and reducing the generation of ureido. Fire resistance was characterized by limiting oxygen index (LOI) and cone calorimeter (CCT). Results showed that with the gradual increasing in polyimide proportion of the PIFs resulting from the increase in refilled BTDA dosage, their fire resistance was remarkably improved, which was mainly reflected by the following: gradual increase in LOI value, decrease in heat release rate (HRR), decrease in the peak of HRR (PHRR), decrease in total smoke production (TSP), and decrease in average specific extinction area (ASEA). Compared with the foam prepared without refilled BTDA, the foam prepared with 60% BTDA refilled dosage exhibited increased LOI value from 20.8% to 30.1%, PHRR reduction from 181.91 kW/m2 to 41.54 kW/m2, and significant reduction of HRR. The reduction in TSP and ASEA reached up to 78.2% and 83.1%, respectively. Meanwhile, when the refilled dosage of BTDA exceeded 60%, the LOI value also showed a slower increase. This change was possibly due to the residual BTDA in foams, which acted as a retardant packing role. The density of foams also increased when the post-additive dosage of BTDA was increased. Possible chemical reactions involved in this preparation method of isocyanate-based PIFs were also discussed in this paper.

Dynamic and Quasi-Static Compressive Deformation Behaviour of Polyimide Foam at Various Elevated Temperature

In order to clarify the effect of strain rate and test temperature on the compressive strength and energy absorption of polyimide foam, a series of compression tests for the polyimide foam with two different densities were carried out. By using three testing devices, i.e. universal testing machine, dropping weight machine and sprit Hopkinson pressure bar apparatus, we performed a series of compression tests at various strain rates (10-3~103s-1) and at several test temperatures in the range of room temperature to 280 ̊C. At over 100 s-1, the remarkable increase of flow stress was observed. The negative temperature dependence of strength was also observed.