| Photo-curing technology is an environmental friendly technology featured with low consumption, low pollution, high efficiency, fast room temperature curing and complete curing. It has been widely used in inks, coatings, photoresistor, adhesive, printed circuit board (PCB),3D printing and other fields. As one of the important components in the photopolymerization systems, photoinitiators not only have a critical effect on the polymerization rate and curing degree, but also have an impact on the performance of the photo-curing materials. UV-curing technology has achieved great success with the development of UV light with high irradiation property. The needs in safety, environmental protection and some special applications such as dental filling materials, especially the recent development of visible LED with high intensity, promote the development of visible photo-curing technology. Therefore, it is of great significance to develop visible light photoinitiators. Photoinitiators could be classified as free radical, cationic and anion photoinitiators. Among them, the free radical photoinitiator is the most popular one. In comparison to UV light, visible light is not strong enough to make Type I photoinitiator cleavage. So, visible light photoinitiators are almost Type II. Typical Type II photoinitiators could yield radicals only in the presence of hydrogen donors such as tertiary amines, alcohols, thiols and ethers, which would lead to the problems such as odor, toxicity, yellowing and migration. An effective way to solve these problems is to develop one-component visible light photoinitiators, which not only helps to extend the scope of applications of photo-curing technology but also helps to explore novel photo-curing materials.Thioxanthone is one of the most intensively investigated Type II photoinitiators in UV curing coatings because of its cheap raw materials, mature preparation technology and easy storage. Thioxanthone has a broad absorption peak which is located at 380 nm with a high molar extinction coefficient. The main peak could be adjusted to the visible light region by the introduction of donor-acceptor structures or the extension of the conjugation length. In this thesis, the introduction of aminoalkyl group to thioxanthone molecule makes the main absorption wavelength of thioxanthone shift to the visible light region. Meanwhile, aminoalkyl groups in thioxanthone molecule could be used as hydrogen donors so as to form one-component thioxanthone photoinitiators, such as polymerizable small molecular, hydrophilic and functional photoinitiators. Their photoinitiation activities and mechanisms are studied systematically. The main contents are described as follows.Chapter 1 introduces the photo-curing system briefly, reviews the initiation mechanism, classification of photoinitiators and oxygen inhibition in free radical polymerization and focuses on the recent development of visible light photoinitiators. The design idea and main contents of this thesis are put forward according to the current development trend of photoinitiators.In chapter 2, three aminoalkyl thioxanthone photoinitiators Ⅱ-PIs (TX-A, TX-B and TX-C) with allyl, benzyl and butyl groups are prepared by the nucleophilic substitution reaction, respectively. Their structures are confirmed by nuclear magnetic resonance (NMR) and mass spectrometry (MS). UV-vis spectrum indicates that the dominant absorption wavelength of Ⅱ-PIs is located around 440 nm. Visible light photolysis of Ⅱ-PIs in solution is studied by UV-vis spectrophotometer ’H NMR and electron paramagnetic resonance (EPR). Ⅱ-PIs could yield a-aminoalkyl radicals and perform visible light photolysis. The mechanism is proposed according to these results. The photopolymerization kinetics of Ⅱ-PIs is carried out by real time infrared spectroscopy (RT-IR). Ⅱ-PIs could initiate polymerization of 1,6-hexanedioldiacrylate (HDDA) and their initiation activities follow the order of TX-B> TX-A> TX-C. TX-B (1.0 wt%) initiates the polymerization of trimethylolpropane triacrylate (TMPTA) and the double bond conversion reaches to 33% at 10 min (I= 28 mW cm-2). Therefore, Ⅱ-PIs are efficient one-component visible light photoinitiators. Particularly, TX-B with benzyl group is the most efficient one.In chapter 3, three polymerizable acrylated thioxanthone photoinitiators Ⅲ-PIs (TX-PA, TX-EA and TX-BDA) are synthesized and characterized by NMR and MS. The interaction mechanism of Ⅲ-PIs is proposed according to the results of visible light photolysis. The synergistic effect of aminoalkyl and acryloxy groups in Ⅲ-PIs is of great importance in the process of photolysis. The results of photopolymerization kinetic indicate that Ⅲ-PIs are one-component visible light photoinitiators with high efficiency. All of them could initiate the polymerization of HDDA, TMPTA and pentaerythritol triacrylate (PETA). In Ⅲ-PIs/HDDA systems, the photoinitiation activies of Ⅲ-PIs follow the order of TX-BDA> TX-EA> TX-PA. In TX-PA (3 × 10"5 mol/g)/TMPTA system, the double bond conversion of TMPTA reaches to 51% after 15 min irradiation (I= 28 mW cm-2). The migration study indicates that III-PIs have excellent migration stability. The residual TX-BDA in polymer film prepared from the photopolymerization of HDDA is only 90 ppm. This feature gives III-PIs a great potential in food packing and biomedical fields. Therefore, the synergistic effect of amino thioxanthone and acryloxy groups in photoinitiators improves the photolysis rate, photoinitiation activity and migration stability of III-PIs. More acryloxy and aminoalkyl group in III-PIs would lead to their higher photoinitiation activities and migration stability.In chapter 4, three hydrophilic thioxanthone-polyether visible light photoinitiators TX-MPEGs are synthesized through the nucleophilic substitution reaction of polyethylene glycol monomethylether (MPEGs) and 2-((4-(chloromethyl)benzyl)(methyl)amino)-9H-thioxanthen-9-one) (III-6). Their structures are characterized by NMR and UV-vis spectrophotometer. The polyether segment of TX-MPEGs brings them about the excellent dispersion properties. The results of dynamic light scattering (DLS) imply that TX-MPEGs could form a micelle in aqueous solution with thioxanthone as a core and polyether as a shell. The chain length of polyether has an impact on the size of micelle. The interaction mechanism of TX-MPEGs upon irradiation is proposed according to the photolysis results of TX-MPEGs in THF and H2O. The results of photopolymerization kinetics demonstrate that TX-MPEGs could be used as one-component visible light photoinitiators for the water-borne photopolymerization. The addition of hydrogen donor N-methyldiethanolamine (MDEA) improves their activities to initiate polymerization of acrylamide and 2-hydroxyethyl methacrylate.In chapter 5, a thioxanthone-polysiloxane visible light photoinitiator (TX-PSO) is prepared by the nucleophilic substitution reaction of 2-[(4-hydroxybenzyl)(methyl)amino]-9H-thioxanthen-9-one (Ⅲ-5) and y-chloropropylmethylpolysiloxane-co-dimethyl-polysiloxane (PSO-Cl). TX-PSO is characterized by NMR, FT-IR, gel permeation chromatography (GPC) and UV-vis spectrophotometer. The results of visible light photolysis, the photopolymerization kinetic and migration stability indicate that the polymeric and intramolecular synergistic effect make TX-PSO higher photolysis rate, photoinitiation activity and migration stability in cured polymer films in comparison to TX-B. TX-PSO could not only initiate polymerization of polyurethane diacrylate prepolymer, but also change the thermal stability and the surface property of the cured film from hydrophilic to hydrophobic. Therefore, TX-PSO is a functional macromolecular visible light photoinitiator with high efficiency, migration stability and the ability to change the surface energy and thermal stability of photo-curing materials.In chapter 6, thioxanthone-polysiloxane with silicon-hydrogen bond visible light photoinitiators (VI-PIs) are synthesized and characterized by NMR, FT-IR and GPC. The results of EPR display that VI-PIs could yield a-aminoalkyl radicals, silyl radicals and hydrogen radicals upon the exposure of visible light. The study of photopolymerization kinetics indicates that VI-PIs are efficient one-component visible light photoinitiators with the ability to eliminate oxygen inhibition. VI-PIs could initiate polymerization of HDDA in air (I= 15 mW cm-2). The silicon-hydrogen bond has a contribution to the photoinitiation activity of VI-PIs and its existing state and content have an effect on the initiation efficiency of VI-PIs. Higher silicon-hydrogen content of VI-PIs would lead to higher initiation activity and better performance to eliminate oxygen inhibition. In comparison to the photopolymerization system with physical addition of H-PSO, VI-PIs have better compatibility and could be used in wide systems. The possible interaction process of VI-PIs upon visible light exposure is proposed according to the results of visible light photolysis, EPR and photopolymerization kinetics. A higher polysiloxane content of VI-PIs would lead to the increased water contact angle and a higher thermal stability of polymer films prepared from polyurethane diacrylate prepolymer. The results indicate that VI-PIs are not only the efficient one-component visible light photoinitiators with the ability to eliminate oxygen inhibition, but also could improve the surface performance and thermal stability of photo-curing materials. These features imply that they have great potential in the future multifunctionalized materials applications. |
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