Poly imide Foams

For poly(imide) foams suitable blowing agents include water, methanol, ethanol, acetone, 2-butoxyethanol, ethyl glycol butyl ether, ethylene glycol, halogen substituted organic compound, and ether (9). 

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Soundproofing and thermal insulation of missiles, planes and helicopters cryogenic protection on satellites, piping insulation, protection for embarked equipment in poly-imide foams. 

Originally three general routes for the production of poly(imide) foams have been known... 

Pores can also be classified on the basis of their state, either open or clo.sed. In order to identify the pores by gas adsorption (a method which has frequently been used for activated carbons), they must be exposed to the adsorbate gas. If some pores are too small to accept gas molecules they cannot be recognized as pores by the adsorbate gas molecules in other words, these pores are closed pores for the gas used. These pores are called latent pores and include closed pores. Closed pores are not necessarily in small size. Thus, when carbon foam was prepared by the impregnation of poly(imide) into a poly(uTethatie) foam followed by carbonization, large macropores, few millimeters in size, were formed in the center of a block of foam, which gave the advantage that the loam can float on water [2]. 

Carbon foams were also prepared by impregnating poly(amide acid) into poly(urethane) foams used as template, followed by imidization and carbcmization [2]. The foams were tested as adsorbents for atmospheric humidity and also as supfmrts for anatase-type TiOi photocatalysts. In Fig. 47, clmnges in pore morphology with impregnation of poly(imide) and carbonization at 1273 K are shown. 

Nonflame-retardant systems are polymeric systems that inherently have some level of flame retardance and therefore do not require additive or reactive flame retardants. Examples include PVC and its compounds, poly(vinylidene chloride) films and compounds, phenolic foams, amide-imides, polysul-fones, and poly(aryl sulfides). 

Acetic anhydride and pyridine can be added as reaction catalysts for imidization of the poly(anude amic acid) if desired. The glass transition temperature of the PAIs is 270-320°C. The density of the foams is 0.1-0.5 gcm . Table 14.3 shows the properties of the final materials dependent on the degree of imidization. 

Foaming of PAI will result in heat-resistant foams. There are such foams made from other types of polymers. Advantages and drawbacks of typical high heat-resistant foams are compared in Table 14.2. PI resin foams are regarded as one of the most applicable materials by virtue of their excellent heat stability and flame retardancy. Research has been targeted to improve the physical properties and to simplify the process of preparation. On the other hand, PAI resins exhibit a better heat resistance than poly(ether imide) resins. In addition, they can be more easily processed in the melt. 

One physical foaming process involving glassy poly(ether imide)s and poly(ether sulfone)s using CO2 has been investigated. Two types of porosities were observed, closed microcellular and bicontinuous. In this work, the foaming behaviors of thin (-TSftM) extruded poly(ether Itnide) (PEI) and poly(ether sulfone) (PES) films were studied (Table I). 

Another unique approach toward low 6 is to disperse fine foams in PI films, since the e of air is unity. This technique developed by Hedrick et al. [208] typically involves the preparation of PS-PAA-PS (PS polystyrene) triblock copolymer, imidization, and finally higher temperature annealing where thermally labile PS block undergoes thermolysis (depolymerization) to form submicron pores. They utilized a variety of other thermally unstable block such as poly(a-methylstyrene), poly(propylene oxide), PMMA, poly(e-caprolactone), and aliphatic polyesters and examined the effects of chemical structure, fraction, and molecular weight of the block on the resultant morphology (pore size, shape, porosity) and dielectric and thermal, and mechanical properties. In this case, the resulting porous structure depends on the initial microphase separation domain structure of the thermally labile triblock. For example, nano-foamed PI (19% porosity) prepared from triblock consisting of PMDA-3F [3F = l,l-bis(4-amino-phenyl)-l-phenyl-2,2,2-trifluoroethane] (see Fig. 62 for its structure) and poly(propylene oxide) showed a considerably lower e = 2.3) than that of the non-porous homo PMDA-3F e = 2.9) [209]. 

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