Acrylic fibers modulus

Carbon and Graphite Fibers. Carbon and graphite fibers (qv) are valued for their unique combination of extremely high modulus and very low specific gravity. Acrylic precursors are made by standard spinning conditions, except that increased stretch orientation is required to produce precursors with higher tenacity and modulus. The first commercially feasible process was developed at the Royal Aircraft Fstablishment (RAF) in collaboration with the acrylic fiber producer, Courtaulds (88). In the RAF process the acrylic precursor is converted to carbon fiber in a two-step process. The use of PAN as a carbon fiber precursor has been reviewed (89,90). 

Carbon fiber is made from acrylic fiber by pyrolysis. Around 1960, Dr. Shindo of "the Governmental Industrial Research Institute, Osaka" found that polyacrylonitrile fiber was carbonized under suitable conditions and yielded carbon fiber with high-strength and high-modulus. At that time, advanced composite materials reinforced with such carbon fibers were unclear, but In 1965, Toray came to be involved in the development of carbon fiber. 

We close this section with a few comments regarding the ultrahigh-modulus acrylic fibers. It is estimated that the theoretical crystalline modulus for polyethylene is 240 GPa hence there has been an intense effort over the last 10-15 years to develop spinning processes to exploit the high-modulus potential. This goal has been achieved by gel-spinning techniques [177]. Allen et al. [178] have estimated the theoretical modulus that might be obtained for... 

PAN by use of gel-spinning or other techniques to create a completely extended chain. Their estimate, based on a comparison of the rod-like PAN conformation with helical chain conformations of other polymers, is that the maximum tensile modulus of atactic PAN would be about 55 GPa. They conclude that PAN with ultrahigh modulus cannot be made by gel spinning, or by any other means, due to the intrinsic chain properties. It is proposed that the strong intramolecular nitrile repulsions that cause the PAN to adopt a rod-like, semiextended conformation do not allow the chain to unravel completely from its semiex-tended conformation. Attempts to develop gel-spinning processes in acrylic fibers are discussed in Section 12.5.4. 

The relationship between fiber structure, cross-sectional shape, and bending modulus (stiffness) was discussed in Section 12.5.2 and is also described by Morton and Hearle [353]. The influence of the cross-sectional shape is particularly important for acrylics. Fibers that have high aspect ratio cross sections, such as the dry-spun fibers with a dog-bone-shaped cross section, tend to be more compliant and softer in comparison with the wet-spun fibers, which have a more rounded cross section. 

The low hot-wet strength of acrylics can be attributed to the maimer in which water plasticizes the unique laterally bonded acrylic fiber structure. The water lowers the Tg to approximately 70°C, but this is not sufficient to account for the extremely low modulus near the boil, since other fibers including nylon are also highly plasticized by water. However, these fibers contain a well-defined, stable, three-dimensional crystalline phase that is thought not to be penetrated by the water. Therefore, they can act to reinforce the fiber and limit the drop in modulus at temperatures above the Tg, where the amorphous phase has become rubbery. The crystalline phase in the acrylic fiber is highly imperfect (as was discussed in Section 12.4) and can probably be easily penetrated and plasticized by the water. 

Figure 3 shows the other example of the dynamic mechanical measurements by the AutovibronTM, The dynamic modulus and the loss tangent of the acrylic fiber are obtained at very small temperature intervals (< 1.5 C). The glass transition temperature, maximum tan 6 value of the a peak, and dynamic moduli - temperature curve show similar results in the published paper. These parameters and length changes (d ) at the glass transition are summarized in Table I. 

As mentioned earlier, UV-curable resin formulations are very attractive for fiber coating because of the rapid cross-linking rates that are achievable. Most commonly, epoxy- or urethane-acrylate resins are employed (18-22), and viscosity and cross-link density are controlled through the addition of reactive diluents. With these systems work has focused on producing low modulus, low T properties (20-22) through the incorporation of appropriate chemical constituents to enhance higher chain flexibility, for example, ether linkages. 

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