Abstract:

Objective In order to tackle the low fluorescence intensity and photobleaching resistance of organic fluorescent anti-counterfeiting printing coatings commonly used in textile printing and dyeing industry, aggregation-induced emission (AIE) fluorophore-tetraphenyl ethylene (TPE) labeled chitosan (CS) fluorescent anti-counterfeiting coatings were prepared and their application properties were investigated.

Method The typical AIE monomer-tetraphenyl ethylene-isothiocyanate (TPE-ITC) was synthesized initially. The TPE-ITC product was characterized by Fourier transform infrared spectroscopy (FT-IR) and MALDI-TOF mass spectrometry (MS). The study took CS as the representative of bio-based polymer paints, labeled different amounts of TPE fluorophores onto CS using the TPE-ITC as fluorescent monomers, and prepared a series of TPE-CS fluorescent paints with different degrees of labeling (DL). The series of TPE-CS products were characterized by FT-IR and ¹H-nuclear magnetic resonance (¹H-NMR) to confirm the labeling of TPE fluorophores. Main application properties of the TPE-CS paints, such as fluorescence intensity, photobleaching resistance and thermal stability, were evaluated. Anti-counterfeit printing on cotton fabric was carried out using the TPE-CS coatings and the common color fastness of the printed fabric was tested systematically according to the national standards.

Results FT-IR and MS spectra confirmed that the synthetic product was the expected AIE fluorescent monomer TPE-ITC. FT-IR and ¹H-NMR spectra proved that TPE fluorophores were successfully grafted onto the molecular chain of CS. The higher was the feed concentration of TPE-ITC monomers, the higher was the DL of TPE-CS. TGA thermograms illustrated that TPE-CS (DL of 2.56%) had nearly the same thermal stability with unlabeled CS. With the increase in the DL, the fluorescence intensity of TPE-CS showed a gradual increase, reflecting its unique AIE advantage. The TPE-CS coating was colorless and thus its printing pattern on the fabric had good concealment in the sunlight. On the contrary, because of the high fluorescence quantum yield of the AIE fluorophores, the TPE-CS coating was found to emit blue fluorescence under ultraviolet light, and the printing pattern on the fabric could be easily recognized with the naked eye. With the increase in the DL of the TPE-CS, the fluorescent intensity of the printing patterns on the fabric kept increasing. Compared with aggregation caused quenching (ACQ) fluorophore labeled CS, the anti-counterfeiting TPE-CS coatings exhibited superior fluorescence intensity and photobleaching resistance. Even if the DL of TPE-CS was only 1.43%, the relative fluorescence intensity of the anti-counterfeit coating solution (1 000 mg/L) could exceed 1 090. After exposure to high-intensity ultraviolet light for 1 h, the relative fluorescence intensity of the solution still reached 94.9% of that before photobleaching. The common color fastness ratings for the fabric printed by TPE-CS 3^# were all above level 3.

Conclusion Labeling AIE fluorophore TPE onto CS macromolecule is one of the effective methods to prepare high-performance organic polymer fluorescent anti-counterfeiting printing paint. The high heat stability of CS is not affected by the labeling of TPE group. The TPE-CS anti-counterfeiting coating within the appropriate range of the DL is sufficient to withstand the baking temperature during printing. In addition, the fluorescence emission intensity of TPE-CS is much higher than that of F-CS after UV photobleaching with the same time and intensity. The use of TPE-CS fluorescent anti-counterfeiting coatings can solve the problems of the ones labeled by ACQ fluorophore, such as low fluorescence emission intensity and weak photobleaching resistance. Taking the application properties and preparation cost into serious consideration, TPE-CS with the DL of 1.43% shows good fluorescence emission performance and high photobleaching resistance and thus the DL value is appropriate. The fabric printed by the TPE-CS coating possesses high fastness to sunlight. Therefore, it can compensate for the lack of sunlight resistance of ACQ fluorescent paints and is suitable for long-term outdoor use. The AIE fluorophore labeled bio-based polymer has great potential to be widely used in the field of textile fluorescent anti-counterfeiting printing as a kind of environment-friendly coating with extensive sources and excellent performance. The preparation and application of TPE-CS fluorescent printing coating can overcome the defects of commonly used fluorescent paints and can be used as an important reference for the preparation of other kinds of AIE fluorophores labeled bio-based fluorescent coatings, such as starch, cellulose, and protein.

Key words: tetraphenyl ethylene, chitosan, fluorescent coating, printing coating, fluorescence emission property, photobleaching resistance, thermal stability, fluorescent anti-counterfeiting printing