Rayon -Based Fibers

Fig. 2. Young s modulus corrected for porosity as a function of preferred orientation curve is based on theoretical model where = rayon-based fibers Q — PAN-based fibers and A = pitch-based fibers (2). To convert GPa to psi, multiply by 145,000.

For cellulose, or rayon-based fibers production involves three distinct steps heat treatment to 350°C to form a thermally stable char, carbonization at 1000-2000°C,... 

The majority of commercial carbon fibers are produced from polyacrylonitrile (PAN) fibers. In fact, HTA-12K PAN-based carbon fibers are the most commonly used commercial carbon fiber (15). PAN-based fibers are the strongest commercially available carbon fibers and dominate structural applications. Mesophase pitch-based carbon fibers represent a smaller but significant market niche. These fibers develop exceptional moduli and excel in lattice-based properties, including stiffness and thermal conductivity (1). Rayon-based fibers are used in heat shielding and in missile nosecones (16). Carbon fibers made from high performance pol5oners (17-19) or from chemical vapor deposition of hydrocarbons, such as benzene or methane, display imique properties that make them potentially attractive futime alternatives (20-22). 

The first carbonization of cellulose-based fibers dates back to Thomas Edison, who carbonized a natural cellulose filament for use as an incandescent lamp filament. In the mid-1950s, the Carbon Wool Corporation introduced the first commercial carbonized rayon fibers (79). PAN- and pitch-based carbon fibers have replaced rayon-based fibers in most high performance applications however, they continue to find use as ablative materials in missile nosecones and heat shielding (16). Additionally, the combination of low cost, ease of handling, and high natural porosity makes rayon an attractive precursor for activated carbon fibers (see CELLULOSE Fibers, Regenerated). 

Each of the three major precursor processes has its own special place in history and will be discussed in turn. Rayon-based fibers were first in commercial production (in 1959) and led the way to earliest applications, which were primarily military. PAN-based fibers have proven to be superior to rayon-based fibers in several respects, notably in tensile strength, and have largely dominated the explosive growth of the industry since 1970. Pitch-based fibers, however, are uniquely capable of achieving extremely high axial Young s modulus and thermal conductivity and, therefore, have an assured place in critical military and space applications. 

These hlgh-modulus rayon-based fibers were used extensively In the U.S. as well as In France and Germany In the development of epoxy-matrix composites for space structures. They were also used to develop a new generation of high-strength and stiffness carbon-carbon coiqtosltes for rocket nozzle throats and missile nose-tips. In recent years, the less costly hlgh-modulus PAN- and pitch-based carbon fibers have been substituted for the rayon-based materials. 

Rayon-based fibers are not as strong as PAN-based fibers. They are used in insulation and some Ccirbon-carbon and ablative applications because of a good match of properties with the carbonized matrix. 

Rayon-based fibers were the first carbon fibers produced commercially. They were developed in the 1960 s specifically for the reinforcement of ablative components for rockets and missiles. However, they are difficult to process into high-strength, high-modulus fibers and have been replaced in most structural applications by PAN or pitch-based fibers. 

The low-modulus rayon-based fibers are the only ones now produced in the form of carbon cloth or felt (Thornel WCA, VCL, VCK, eind VCX from Amoco Performance Products). Primary uses are in carbon-carbon composites and high-temperature insulation. 

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