Polyimide foams

Polyimide foams have outstanding advantages of high-temperature resistance, oxidation resistance, and low smoke evolution. They can be prepared by two routes, as shown below. One method consists of the reaction of aromatic diamines with carboxylic dianhydrides (99-102), as shown in model reaction (1) Another synthetic route is the reaction of 

In this section, isocyanate-based imide foams, according to model reaction (2) will be described, and other imide foams prepared by model reaction (1) will be described in a separate section. 

An example of the jH eparation method at room temperature is as follows (106). A mixture of 161 parts by weight (1.00 equivalent) of 3,3, 4,4 -benzophenone tetracarboxylic dianhydride and 132 parts by weight (1.00 equivalent weight) of polymeric isocyanate was prepared by mechanical blending. 

To this liquid mixture was added a mixture of 50 parts by weight (0.473 equivalent) of a polyol of equivalent weight 105.6 (the adduct of propylene oxide and a mixture of polyamines containing 50% by weight of methylenedianiline obtained by the acid condensation of aniline and formaldehyde) 50 parts by weight of dimethyl sulfoxide and 10 parts by weight of a silicone surfactant. 

These mixtures were mechanically blended for 10 seconds at room temperature and poured rapidly into a wooden mold (14 x 6 x 4 ) and allowed to expand freely at room temperature. After approximately 10 to 15 minutes, the resultant foam was very rigid, posessing fine cells. The residual solvent was removed from the resultant foam by placing it in a lOO C oven for 4 days. The foam had the following physical properties density, 2.82 pcf compressive strength (parallel to rise), 30.2 psi and flame test (ASTM D 1692), total length burned, 0.1 inch. 

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Polyimides and related materials have also been used in a number of specialist applications. Polyimide foams (Skybond by Monsanto) have been used for the sound deadening of jet engines. Polyimide fibres have been produced by Rhone-Poulenc (Kermel) and by Upjohn. 

The polyimide foams are flexible and have a very low density (7 kg/m ), associated with good fire behaviour, a broad service temperature range and good soundproofing and thermal insulation qualities. These materials are sensitive to diluted strong bases, concentrated salts and acids. Other foams have densities varying from 15-250 kg/m. ... 

An alternative means of reducing the dielectric constant of polyimides is to have a low dielectric constant component dispersed as a second phase within the rigid polyimide matrix. Two key approaches have been pursued to this end. The first involves the preparation of polyimide block copolymers with highly fluorinated coblocks and the second involves the generation of a polyimide foam. The main drawback to the first approach is the solubility of highly fluorinated blocks in organic media which will permit copolymerization with polyimides. This led to the investigation of new semi-fluorinated polymers derived from polyfaryl ethers). 

An alternative means of generating a polyimide foam with pore sizes in the nanometer regime has been developed [80-90]. This approach involves the use of block copolymers composed of a high temperature, high Tg polymer and a second component which can undergo clean thermal decomposition with the evolution of gaseous by-products to foam a closed-cell, porous structure (Fig. 7). 

Williams, M. K., Holland, D. B., Melendez, O., Weiser, E. S., Brenner, J. R., and Nelson, G. L. Aromatic polyimide foams Factors that lead to high fire performance, Polym. Degrad. Stabil. 2005, 88, 20-27. 

Similar studies on foam morphology were reported by Williams et al.12,13 for polyimide foams with different densities or surface area also two different chemical formulations were used. Comparing foams with the same chemical composition, it was shown that no consistent correlation could be found between PHRR and foam density or open cell content while greater correlation is proved between the surface area and PHRR because they showed the same trend. Foams having the same density but different surface area and chemical composition show great variation of PHRR (up to 50%) both at 75 and 50kW/m2. 

An alternate process to produce polyimide foam is based mixture containing diester of BTDA (BTDE) and diamines is heated in a clo mold. The imidization takes place with formation of methanol and water vapor which serve as foaming gas [108, 109]. 

Thermosetting foams can be defined as foams having no thermoplastic properties. Accordingly, thermosetting foams include not only cross-linked polymer foams, but also some linear polymeric foams having no thermoplastic properties, e.g., carbodiimide foams and polyimide foams. These foams do not melt and turn to char by heating. 

Methods for making isocyanate-based polyimide foams include a one-shot process by admixing carboxylic dianhydrides with an organic polyisocyanate at room temperature in the presence of a dipolar aprotic organic solvent (103, 106, 142, 251). The resulting foams from this method exhibited outstanding thermal resistance, as shown in Figure 47. 

Polyimide foams can be prepared at high temperatures without any catalyst. An example is shown below (105). 4,4 -Diphenylmethane diisocyanate in an amount of 44 grams was melted gently in a beaker with continuous agitation. After 16 min when the temperature of the melt was 390 C, 24 grams of trimeUitic anhydride was added. After 4 min, the temperature was 485°F and shortly thereafter agitation was stopped. As a yellow foam cake developed, the beaker was covered with a metal plate... 

MISCELLANEOUS AND SPECIALTY FOAMS (Epoxy Foams, Polyester Foams, Silicone Foams, Urea-Formaldehyde Foams, Polybenzimidazole, Foams, Polyimide Foams, Polyphosphazene Foams, and Syntactic Foams)... 

PBI foams have the best thermal stability of the three foams— polyisocyanate, PBI and PI—but generally are less resistant to oxidation at elevated temperatures than are the PI (polyimide) foams. The PBI foams are the most expensive of the three types. So far their fabrication technology appears to be limited to the molding of small- to moderate- sized items (8). 

In the case of using aromatic polyisocyanate in making polyimide foams, crosslinking is increased by adding a highly functional polyol to react with part of the isocyanate so that the foam contains some urethane structure. A typical foam prepared in this way would have a density of 3.97 Ib/ft (63.5 kg/m ) and a K-factor of 0.26 Btu/(h) (fP) ( F/in). Foams of this type have been prepared in a range of densities of 2.5 to 18.5 Ib/ft (40 to 296 kg/m ) with compressive strengths of 25 to 1340 psi (172 to 9239 kPa) (8). 

MIL-T-24708(SH) ThermallAcoustic Insulation Barrier Material Polyimide Foam, 29 July 1988, 10 pp (FSC 5640) (SH)... 

Covers lightweight, flexible polyimide foam as a layered thermal-insulation-barrier material developed for use within a specific frequency range on submarines. Cunently there is only one Type, with three Gasses, 1, 2, and 3, with single, double and triple layers, respectively. The layers of polyimide foam are bonded to a polyj osphazene/barium sulfate sheet. 

Heat resistant Intractable thermoset polylmides (Vespel, Kinel, Kapton) have been supplemented by injection moldable polyamlde-lmides (Torlon) and modified polylmides (Kanox, Toramid).( Thermoplastic polyimide adhesives were cited as one of the top 100 Inventions in 1981. Polyimide foam does not Ignite at temperatures below 4300C and is being considered as a cushioning material in public conveyances. 

Fig. 10. a TEM micrograph of polyimide foam derived from imide/propylene oxide triblocl copolymer, b TEM micrograph of polyimide foam derived from imide/propylene oxide graf copolymer... 

Fig. 14. Stress vs temperature plots for polyimide and polyimide foam...

Solimide Foam. [Ethyl] Flexible polyimide foam for thermal and acoustical insulation applies. 

J. M. Vazquez, R. J. Cano, B. J. Jensen, and E. S. Weiser. Polyimide foams. US Patent 6 956 066, assigned to The United States of America as represented by the Administrator of the National Aeronautics and Space Administration (Washington, DC), October 18, 2005. 

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