In the research the impact of different concentrations of optical brightening agent (Ultraphor CK) on the photocolouration of woven cotton fabric, dyed by pad-dry-cure method with the commercial microencapsulated photochromic dye (Itofinish UV blue) and binder (Itobinder AG) was studied. The letter was added to improve binding of microcapsules onto fabric. After padding, followed the drying and curing of the samples. The mass per surface area, stiffness, breaking strength and elongation, air permeability, colour fastness to rubbing, colour fastness to domestic washing and colour fastness to light was measured. The colour of the samples before and after the one-minute illumination with UV light source was spectrophotometrically evaluated by the use of reflectance spectrophotometer. Higher values of mass per unit area, stiffness, breaking strength and elongation were obtained on dyed samples regardless of the dyebath composition in comparison with the undyed cotton fabric. The air permeability was lower on dyed samples. The increase of optical brightening agent concentration in the dyebath resulted in lower embedment of microencapsulated photochromic dye onto cotton fabric and thus a reduction of the mass per unit area and an increase of air permeability of the fabric. The dyed samples have higher colour fastness to wet than to dry rubbing. With the increasing number of washing cycle the colour fastness of dyed samples to domestic washing is decreased. Addition of the binder into the dyebath enable binding of microcapsules of the photochromic dye onto cotton fabric. Optical brightening agent decreases photocolouration of the cotton fabric and therefore increases the colour fastness to light at prolonged exposure (longer than 12 hours) of samples to Xenotest apparatus.
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