Opto-thermal properties of fibres: 3-effect of anisotropic optical parameters in polypropylene fibres as a function of annealing process

Abstract

This study deals with the changes in the optical properties of polypropylene fibres (4:1 draw ratio 515 tex polypropylene, Bolton, UK) due to the effect of thermal annealing on its structural characteristics. Changes in the structure of annealed polypropylene fibres at a constant time of 2 h and different temperatures (60–140 °C) were studied interferometrically. The diffraction of a He-Ne laser beam was used to measure the dimensional parameters transverse sectional shape and the area of the polypropylene fibres sections. An optical microscope was used to confirm the geometrical parameters of the cross-sections. The diffraction technique was utilized to study the effect of annealing on swelling factors of these fibres for some different liquids at room temperature 19 ± 1 °C. Two methods were applied for the measurement of the refractive indices and birefringence of the polypropylene annealed and unannealed fibres. The first was concerned with the application of the Becke-line method for determining the refractive indices and the birefringence of the skin layer. The second method used multiple-beam Fizeau fringes in transmission to observe the changes in the values of the refractive indices due to the annealing process in the skin and core layers. Also, the mean refractive indices and their profiles were determined, taking the refraction and different temperatures into consideration. The resulting data were used to calculate the polarizabilities per unit volume and the isotropic refractive index.