Measurement of Opacity in Fibers

Abstract

A light-transmission measurement was developed to compare opacities of experimental nylon 6-T fibers. A fixed concentration of chopped-up fibers was suspended in a mixture of organic liquids, which matched the average refractive index and density of the fibers, to suppress surface scattering and to prevent settling. The transmission of any given sample showed a spectral dependence, i.e., decrease of transmission with decreasing wavelength, indicating that the size of the scattering unit was of the order of magnitude of the wavelength of visible liglit. This dependence, as measured with a Beckman spectrophotometer, was analyzed by a semi-theoretical ap proach to yield values of n, the wavelength exponent appearing in the scattering law. The values of n obtained lay between about 0.9 and 2.3 and decreased on boiling, which was to be expected theoretically if the size of the scatterer was increasing. There was a rough correlation, with all the samples, between the n values and the percent transmission at a fixed wavelength, indicating that the differences in "voidiness" among the different fibers were a function of the size of the light scattering unit. The transmission showed a good inverse correlation with V, the void volume, as calculated from density measurements, assuming a melt-spun sample to be void-free. This suggested that the opacity was indeed caused by voids and not by other kinds of light scatterers—spherulites, gels, etc. There was an excellent correlation between transmission and the quantity V/n, which emphasized the dual dependence of opacity on total void volume and void size.