Study On Integrated Photosensitizers Used For Visible Light Curable Pigment-based Inkjet Printing Of Textiles

Textile pigment-based inkjet printing is suitable for all kinds of textile fibres, that is, there is no selectivity for fibres, and possesses relatively low energy and water consumptions, suggesting a promising application potential. However, the polymeric binder component in ink is easy to cause nozzle clogging, hindering the processing process. For solving this problem, developing photo-curing pigment-based ink is poposed, where photopolymerization system composed of oligomers, monomers, is used to displace the polymer binder in ink to overcome the nozzle clogging problem. And when this ink is jetted onto fibres, followed by irradiation under visible light with certain intensity, photopolymerization in situ will occur and form a tough film on the fibres surface to fix pigment particles, achieving an expected color fastness. Besides, photo-curing technology is characterized by high efficiency and co-friendly. So, this idea will bring a new direction for the development of pigment-based digital printing technology. For this purpose, the development of photo-curing ink is the key technology, in which the photo-initiating system is its important component, determining the rate and efficiency of photo-curing. So, the study on visible light initiating system for photo-curing ink of textile digital printing has important academic significance and application value.Based on the characteristic of pigment-based ink system, and referring to recent theoretical research about the visible photo-curing, erythrosine B(a kind of food dyestuff) with high absorbance in visible light region, and relative low price is chosen as sensitizer to sensitize tertiary amine for generating active centers for polymerization. By linking erythrosine B with the classic tertiary amine, a series of integrated senstizers are synthesized to improve the compatibility with photopolymerization system and avoid the migration of sensitizer component from polymer. And the promoted photoinitiating efficiency can be expected. Subsequently, the synthesized derivatives are combined with the well-known coinitiator and the synergist to be used to initiate photopolymerization. Their photoinitiating ability, photo-bleaching behavior and the characteristic of photo-curing films initiated by them are investigated to obtain the relationship between structure and property. Based on experimental results, the synergistic mechanism of three-component system is proposed. And the optimal photoinitiaing system is chosen for pigment-based photo-curing ink. Some beneficial theories and technological achievements have been obtained to provide some useful information for the development of photo-curing ink.1. Six erythrosine B derivatives with a linked co-initiator on the carboxylate group separated by the methylene untits with different length, or phenolic group are synthesized. And the corresponding derivatives without linked coinitiator as reference compounds are prepared. These derivatives exhibit high absorbance in UV-visible region, and good compatibility with polymerization system. And compared to the derivatives with modified carboxyl group, the derivatives with modified both carboxyl and phenolic groups have poorer visible light absorption and are great blue-shifted in visible spectrum, but possess higher spectrum stabilization especially in acid medium. Fluorescence emission spectrum and laser flash photolysis measurement suggest that intramolecular photochemical reaction rate is higher than the intermolecular one.2. The kinetics studies of free radical photopolymerization show that:(1) in viscous resin, with the decrease of the linkage distance between sensitizer and coinitiator moiety in linked derivatives, the radical photo-initiating efficiency increases in turn, suggesting the advantage of adjacent and intramolecular reaction;(2) the derivatives with a linked coinitiator on carboxyl group display higher photoinitiating efficiency than with a linked one on phenolic group, because the spatial location between the photo-active group of sensitizer and the coinitiator moiety in the latter is unfavorable for photochemical reaction between them, resulting in low efficiency of generating the tertiary amine radical for polymerization;(3) The derivative with the two linked co-initiators shows excellent photoinitiating efficiency, due to higher local concentration of co-initiator in molecular;(4) when the equal-molar iodonium salt is combined with the above initiators, the initiating efficiency is enhanced, and this enhancement for the initiators containing the linked sensitizer is more remarkable;(5) for the initiating systems composed of sensitizer, coinitiator and iodonium salt, the initiating efficiency of the systems containing the linked sensitizers is higher than of containing the separated counterparts, suggesting that it is effective for a coinitiator linked to the carboxyl group of erythrosine B to promote initiating efficiency.3. The kinetics studies of free radical photopolymerization show the following results. The initiating systems containing linked sensitizers exhibit poorer initiating ability than containing no linked one, and with shortening of linkage distance between the sensitizer and the coinitiator moiety in linked sensitizers, the photoinitiating efficiency descends in turn, being different from the above radical photopolymerization. It is because that the cationic active centers formed on coinitiator moiety of linked sensitizers possess poorer mobility in polymer network than the free one, and the closer distance between sensitizer and coinitiator moiety in linked sensitizers results in the greater steric hindrance of active centers on coinitiator moiety for cationic photopolymerization. These results suggest that the linked sensitizers are unsuitable to be used as sensitizers in cationic photopolymerization.4. The synergistic mechanisms of the systems composed of sensitizer, coinitiator and iodonium salt for radical/cationic photopolymerization are suggested:(1) under irradiation, the fast photochemical reaction between sensitizer and iodonium salt will result in photobleaching of the former and photolysis of the latter. Subsequently, the phenyl radical generated by photolysis, abstracts the hydrogen from tertiary amine(coinitiator) to generate radical active center for radical polymerization, enhancing photoinitiating efficiency;(2) the phenyl radical generated by the above three component system oxidizes the formed tertiary amine radical to carbon cationic center for cationic polymerization, enhancing cationic photoinitiating efficiency. These proposed mechanisms further improve the classic photochemical reaction theory of three component system.5. The performance testing of films show that:(1) Young’s modulus of films initiated by the derivatives/tertiary amine/iodonium salt systems positively correlate to photoinitiating ability of the systems in radical photopolymerization. But the film initiated by the system containing the sensitizer without a linked coinitiator is relatively tougher, due to excellent mobility of this system resulting in uniform polymerization;(2) films show different color, suggesting that different initiating systems have different photobleaching efficiency. The photobleaching testing of the systems in solution and polymerization condition show that photobleaching of sensitizers is mainly caused by the photochemical reaction between the carbonyl group on xanthene ring and tertiary amine. So the sensitizers with a linked coinitiator on carboxyl group are more easily photo-bleached than the corresponding counterparts, due to fast intramolecular reaction of the former. So, for a practical application, the systems containing sensitizers with a linked coinitiator on carboxyl group are suitable to be used as the visible light initiating systems for digital printing of textiles.6. EB6/EDB/IOD system with efficient photobleaching is chosen as visible light initiating system for photo-curing ink. The pigment-based photo-curing ink containing EB6/EDB/IOD and CQ/EDB/IOD, respectively, are performed a digital printing on cotton fabric, followed by photo-cured under visible light. Dry-wet rubbing fastness of the printed samples all reach 4-5 grade, and the apparent color of the samples is similar. So, EB6/EDB/IOD is expected to displace CQ/EDB/IOD system to be used as the visible light initiating system for pigment-based photo-curing ink of textile digital printing.