Abstract:

Objective In order to address the complexity in preparation and high cost for current fabric based anti-counterfeiting technology, carboxylated polystyrene fluorescent microspheres were synthesized and loaded on fabric for anti-counterfeiting application. The fluorescent microspheres prepared by this paper have great anti-counterfeiting effect on different types of fabrics, offering obvious practical value for curbing fake products and for improving the economic value of fabric and clothing brands.
Method Carboxylated polystyrene fluorescent microspheres with negative surface charges were prepared at 80 ℃ by copolymerization of styrene and acrylic acid as reactive monomer, polyvinylpyrrolidone as stabilizer, azo diisobutyronitrile as initiator, deionized water and ethanol as solvent. The cationic surfactant octagyltrimethyl ammonium bromide was used as the modifier, and the fluorescent dye fluorescein isothiocyanate was adsorbed by electrostatic self-assembly method to prepare carboxylated polystyrene fluorescent microspheres with yellow-green fluorescence. The microspheres were loaded on different fabrics to produce fabrics with anti-counterfeiting function.
Results Fluorescent microspheres with different contents of fluorescent dyes were successfully prepared by electrostatic self-assembly method. The fluorescent microspheres were measured by Fourier infrared spectrometer and all contained isothiocyanate characteristic functional groups at 2 036 cm^-1(Fig. 3). With the increase of the content of fluorescent dye isothiocyanate, the surface negative charge value and particle size was gradually increased. When the content of fluorescent dye isothiocyanate was 10% of the carboxylated polystyrene microspheres, the surface charge was -22.1 mV and the particle size was about 1 406.0 nm(Fig. 2 and Fig. 6). The surface morphology of fluorescent microspheres with different contents of fluorescent dyes was observed by scanning electron microscopy, which showed that the surface of the microspheres was smooth, monodisperse and uniform in size, as shown in Fig. 5. The emission spectral wavelengths of fluorescent microspheres with different contents of fluorescent dyes measured by fluorescence spectrophotometer were all about 517 nm(Fig.4(b)), which was consistent with the emission wavelength of isothiocyanate fluorescent dyes(Fig.4(a)). The fluorescent microspheres were measured at different times (1-9 d) and pH values (3-11), which indicated that the prepared fluorescent microspheres had great fluorescence stability(Fig.4 (c) and (d)). The fluorescent fabric prepared by treating the fluorescent microspheres on silk, cotton and cotton/spandex fabric had no obvious phenomenon under ordinary light, but had bright yellow-green fluorescence under ultraviolet light (365 nm)(Fig.8). It was found that adding a small amount of water-based polyurethane aqueous solution could solve the problem that fluorescent microspheres were easy to fall off from the fabric. After hundreds of times of friction and a long time of washing, great fluorescence intensity was maintained(Fig. 9). After 400 times of friction and 30 min of washing, the fluorescence intensity still maintained at 81.8% and 85.7% of the original fluorescence intensity, demonstrating satisfactory color fastness to meet the requirement arising from transportation and storage in practical occasions.
Conclusion The preparation of carboxylated polystyrene fluorescent microspheres by electrostatic self-assembly method were easy to operate. The surface of the fluorescent microspheres was smooth, the particle size was relatively uniform, the fluorescence intensity was high, and the wavelength had an obvious emission peak at 517 nm, which was consistent with the emission wavelength of isothiocyanate fluorescent dye. Under different times and different pH value, the fluorescence stability was great, respectively treated in silk, cotton and cotton/spandex fabrics under ordinary light had no obvious change, but under ultraviolet light (365 nm) it was bright yellow-green, with obvious fluorescence anti-counterfeiting effect. Adding a small amount of aqueous polyurethane solution can improve the fluorescent microspheres treatment on the fabric easy to fall off the problem, providing a great performance. In conclusion, the carboxylated polystyrene fluorescent microspheres prepared in this research can meet the role of fluorescence pseudo-detection under different environments and different fabrics, and have broad application prospects and practical application value for fiber and fabric anti-counterfeit detection.

Key words: fabric anti-counterfeiting, carboxylated polystyrene microsphere, fluorescein isothiocyanate, cationic surfactant, electrostatic self-assembly, fluorescent microsphere