| Photoluminescent labels have become one of the mainstream technologies for packaging anti-counterfeiting at present.In light of their unique properties,rare-earth up-conversion（UC）NaYF4 particles,as the most potential photoluminescent materials,have been integrated into ink systems and surfaced in the areas of packaging anti-counterfeiting including medicine,gift boxes,tobacco,and wine,to name a few.With increasingly severe environmental problems,there has been a growing surge of interesting to develop high-performance waterborne fluorescent inks that can address the market requirements.Nevertheless,the UC emission efficiency and water dispersion of NaYF4 crystals are far from satisfactory due to the restricts of energy transfer mechanisms and synthesis conditions.And the deficient performances of waterborne binders limit ulteriorly the wide application of the UC materials in the field of packaging anti-counterfeiting.Furthermore,it is noteworthy to highlight that the fluorescent anti-counterfeiting systems,with single color or monotone form,can no longer be qualified for high-end brand protection and anti-counterfeiting traceability.On the condition,the three-primary-color fluorescent WPU inks,with excellent comprehensive properties,were prepared for gravure printing on the basis of the optimized syntheses of the three-primary-colorβ-NaYF4 crystals and high-performance waterborne polyurethane（WPU）dispersion.Then high-resolution UC fluorescent full-color imaging was realized,a triple-tiered anti-counterfeiting system for a core with UC fluorescent anti-counterfeiting was constructed,as well as a scientific evaluation model for the system was proposed based on plentiful works,which broke through the limitation of monochrome,solid,and single fluorescent anti-counterfeiting.The primary contents and results are summarized as follows:1.The preparation of three-primary-colorβ-NaYF4 crystals capped with WPU prepolymer/TSC was optimally conducted via one-step hydrothermal synthesis method.It was found that chelating ligands,reaction conditions,and ion doping concentration affected the crystal structure and properties of NaYF4 crystals.On the one hand,the chelation and termination effects from the WPU prepolymer would lead to increased crystal quality.Both of a relatively low molecular weight and a suitable number of chelating anchors were beneficial to the emission enhancement.And the processes of nucleation and crystal growth could be adjusted synthetically as the WPU prepolymer was compounded with TSC.On the other hand,the emission intensity enhanced with the increase of reaction temperature,NH3·H2O dosages or the doping ions concentration at first and then exhibited a gradual decrease tendency upon the further increase,while decreased sharply with the addition of ethanol.The co-doping with a certain concentration of K+or Mg2+resulted in the emission enhancement.The optimizedβ-NaYF4 crystals following orthogonal experimental design did offer several advantages,such as strong luminescence,good dispersion,uniform size,excellent quality,and high spectral purity.2.ZIF-8 and NH2-ZIF-8 crystals were introduced for the preparation of modified WPU dispersion in combination with ketone hydrazine crosslinking while promoting enhancements of the water resistance and heat resistance to already existing one,and then the three-primary-color fluorescent WPU inks were further prepared for gravure printing.It was found that with the addition of a certain amount of ZIF-8 or NH2-ZIF-8crystals,the contact angles of WPU coating films were obviously enhanced,the water absorption was reduced in turn,the initial decomposition temperature and the temperature at the maximum rate of weight loss was increased simultaneously.The compatibility between the NH2-ZIF-8 and WPU further enhanced the water resistance and thermal stability of the film.The optimized three-primary-color inks exhibiting shear thinning behaviors were suitable for gravure printing.Interestingly,the prints could present three-primary-color fluorescence with the same visual brightness under the 980 nm excitation.3.The chromatic characteristic and color reproduction rules of the three-primary-color inks were discussed,as well as a prediction model for UC fluorescent spectra was constructed for achieving the high-resolution UC fluorescent full-color imaging.It was found that tone reproduction properties of the primary-color ink were significantly different from each other.The brightness levels and gradient compression were restricted by the color of substrates.Both of dot surface coverage and paper types made a difference to the fluorescence emission.In a process of multicolor reproduction based on the three-primary-color inks,the chromaticity was garnered in according to the additive color synthesis.It was noted that dot distribution affected the comprehensive expression of multicolor emission.High emission intensity and wide color gamut could be achieved with the screening processes of amplitude modulation or local dot juxtaposition.Moreover,the spectral prediction model based on the Neugebauer equation was suitable for UC spectral prediction in the multicolor reproduction.4.Focusing on UC fluorescent anti-counterfeiting,the triple-tiered anti-counterfeiting system was constructed with smooth integration of several prepress security technologies.And the corresponding evaluation model was proposed.It was found that the limitation of pure high-tech anti-counterfeiting was broken.And the detections and recognition of security features manifested that the system featured omnidirectional,hard to forge,low cost,and easy promotion.The evaluation model based on the analytic hierarchy process（AHP）was conducive to the efficient optimization of system quality.A perfect blend of security elements with the packaging did offer a high-end product multiple functions spring from the coexistence and complementarity of first-line,second-line,and third-line anti-counterfeiting,promoting the application of UC fluorescent materials in the area of high-end packaging anti-counterfeiting. |
