Thermally Stable Polymer Fibers

Thermally-stable polymer fibers also can be obtained directly from polymers that have high decomposition temperatnres. Table 17.6 shows a few examples of such polymer fibers. The aromatic and/or heterocyclic stmctures on the polymer main chains make these fibers thermally stable. The maxumnn use temperatures of these thermally stable polymer fibers are greater than 300°C, or even 400°C. Some of these fibers only lose 20 to 30% of their original weight at temperatures up to 900°C. 

THERMAL PROPERTIES OF FIBERS Table 17.6. Chemical structures of thermally stable polymer fibers. 

What are the common features of the thermally-stable polymer fibers shown in Table 17.6 ... 

Polyimides (PI) were among the eadiest candidates in the field of thermally stable polymers. In addition to high temperature property retention, these materials also exhibit chemical resistance and relative ease of synthesis and use. This has led to numerous innovations in the chemistry of synthesis and cure mechanisms, stmcture variations, and ultimately products and appHcations. Polyimides (qv) are available as films, fibers, enamels or varnishes, adhesives, matrix resins for composites, and mol ding powders. They are used in numerous commercial and military aircraft as stmctural composites, eg, over a ton of polyimide film is presently used on the NASA shuttle orbiter. Work continues on these materials, including the more recent electronic apphcations. 

Production of New Pol3miers. The polybenzimidazole developed as a raw material for specially thermal-stable textile fibers by the Celanese Research Company proved to be a potential membrane polymer due to the exceptionally high water absorption (17). 

PBTs have received attention in recent years as a result of their unique physical properties. PBTs are among the most thermally stable polymers reported so far. The high degree of molecular rigidity in their backbone produces high-modulus polymers, which are applied as fibers and multilayer circuit boards [50,71,72]. 

Aromatic polybenzimidazoles were synthesized by H. Vogel and C. S. Marvel in 1951 with anticipation, later justified, that the polymers would have exceptional thermal and oxidative stability. Subsequently, NASA and the Air Force Materials Laboratory (AFML) sponsored considerable work with polybenzimidazoles for aerospace and defense applications as a non-flammable and thermally stable textile fiber and as high temperature matrix resins, adhesives and foams. The route to fiber used solutions of high molecular weight polymer. Structural applications used low temperature melting pre-polymers that were cured (polymerized) in place. Applications of polybenzimidazoles were not implemented in the 60 s and 70 s since the polymers tetraamine precursors were not commercially available. 

In a most recent paper [418] the preparation of corn fiber arabinoxylan esters by reaction of the polymer with C2-C4 anhydrides using methanesul-fonic acid as a catalyst is described. The water-insoluble derivatives with high molecular weight showed glass-transition temperatures from 61 to 138 °C, depending on the DS and substituent type. The products were thermally stable up to 200 °C. Above this temperature their stability rapidly decreased. 

The first widely used synthetic polymer was phenol formaldehyde (Bakelite). It is made by heating phenol (C6H5OH—hydroxybenzene) together with formalde-hyde (H2CO).These react to yield a three-dimensionally cross-linked polymer. To reduce the brittleness of Bakelite, it is usually filled with fibers or platelets of an inert solid. It is a good electrical insulator, relatively hard, and thermally stable to a few hundred degrees Centigrade. Its hardness is 50-60 kg/ mm2 (Mott, 1956). 

Polymer cross-linking has also been achieved with l-arylsulfonyl-4-butyltetrazolin-5-ones (255) and with bistetrazolyl-m -phenylene molecules of this type (72GEP2226525). Tetrazole polymers of type (256) were prepared from thermal reactions of diazides with dinitriles. They are constituted as flexible fibers and have been claimed for use as thermally stable propellant binders and as plastics (68USP3386968). 

We already have reported on the replacement of the terephthalic acid with kinked diphenylether dicarboxylic acids (4). 3,4 - and 4,4 -Dicarboxydiphenylether (3,4 -0 and 4,4 -0) were synthesized and all-aromatic polyesters were prepared represented by structure 1. These polyesters were thermotropic with melt transitions decreasing to about 200°C with increasing replacement of the terephthalic acid with the kinked monomers. The polymers generally were thermally stable without measurable weight loss until well over 400°C. We wish here to supplement our previous studies with rheological measurements and fiber spinning of the polymers, including some measurements of fiber properties. 

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