ABSTRACT:The precursor fiber quality has a large impact on carbon fiber processing in terms of its performance, production yield, and cost. Polyacrylonitrile precursor fibers have been used commercially to produce strong carbon fibers with average tensile strength of 6.6 GPa. There is a scope to improve the average tensile strength of carbon fibers, since only 10% of their theoretical strength has been achieved thus far. Most attempts to increase the tensile strength of carbon fibers have been made during the conversion of precursor fiber to carbon fiber. This review highlights the potential opportunities to enhance the quality of the polyacrylonitrile-based precursor fiber during polymer synthesis, spinning, and postspinning. These high-quality precursor fibers can lead to new generation carbon fibers with improved tensile strength for high-performance applications.

The yellowing of wool exposed to sunlight is a serious commercial shortcoming compared to cotton and synthetic fibres, particularly when photostable brilliant whites and bright pastel shades are required. Part 1 of this review discusses the effects of light on wool keratin and the factors that affect the rate of photoyellowing, including oxidative bleaching, fluorescent whitening and the presence of moisture. The effect of variation of the wavelength of light, particularly attenuation of the ultraviolet wavelengths in sunlight by window glass that can result in photobleaching of wool is described. The experimental techniques that have been used to study the complex photochemistry involved in yellowing, and to identify the nature of the yellow chromophores formed, are also discussed. Part 2 of the review will focus on the photochemical mechanisms involved, and discusses potential ways for improving wool's photostability.

Part 1 of this review discussed the effects of light on wool keratin and the factors that affect the rate of photoyellowing. Part 2 describes the various photochemical mechanisms that have been proposed to account for photoyellowing of wool by sunlight, and focuses in particular on their strengths and weaknesses. The mechanisms involved in the rapid photoyellowing of wool treated with a fluorescent whitening agent are described in some detail since this remains a serious commercial problem. Existing methods for reducing the rate of wool photoyellowing are reviewed, together with a discussion of the remaining scientific and technical challenges to achieve bright, white wool fabrics and garments of high photostability.Scheme 1

A silk cocoon protects a silkworm during its pupal stage from various threats. We systematically investigated the role of fiber, sericin, and embedded crystals in the UV protection of a silk cocoon. Diffuse reflectance and UV absorbance were measured and free radicals generated during exposure to UV radiation were quantified using photoinduced chemiluminescence (PICL). We identified the response to both UV-A and UV-B radiations by silk materials and found that sericin was primarily responsible for UV-A absorption. When sericin was removed, the photoinduced chemiluminescence intensity increased significantly, indicating higher UV-A-induced reactions of cocoons in the absence of sericin. There is progressively higher sericin content toward the outer part of the cocoon shell that allows an effective shield to pupae from UV radiation and resists photodegradation of silk fibers. The study will inspire development of advanced organic photoprotective materials and designing silk-based, free-radical-scavenging antioxidants.

Generation of hydroxyl radicals in the presence of buffered terephthalate solution produces flourescent 2‐hydroxyterephthalate. Wool, nylon, cotton and polyester all generate hydroxyl radicals when irradiated with UVA light, and to a lesser extent with blue light, in terephthalate solution. Trace metal ions (particularly iron and copper) contribute to hydroxyl radical generation in wool and cotton. UVA irradation of wool in deuterated solution does not affect the concentration of hydroxyl radicals or the amount of yellowing observed relative to undeuterated solution. This suggests that 1O2 is not involved in the wet photoyellowing of wool. Although the photoinitiation stages are clearly different, it is likely that similar free radical processes are responsible for both hydroxyl radical production and wet photoyellowing in all four fibre types. This straight forward fluorescence technique has potential to correlate the free radical photodegeneration of fibres in the presence of various additives designed to improve performance.