Efficient Photoinitiators Research Articles

Spatial and temporal evolution of the photoinitiation rate for thick polymer systems illuminated by polychromatic light: selection of efficient photoinitiators for LED or mercury lamps

AbstractBACKGROUND: Four common free radical photoinitiators were evaluated for use in thick photopolymerizations illuminated with a medium‐pressure 200 W mercury–xenon arc lamp and a high‐intensity 400 nm light‐emitting diode (LED) lamp. For each photoinitiator/lamp combination, the spatial and temporal evolution of the photoinitiation rate profile was analyzed by solving the set of differential equations that govern the light intensity gradient and initiator concentration gradient for polychromatic illumination.RESULTS: The simulation results revealed that two of the four photoinitiators evaluated were ineffective for photoinitiating thick polymer systems. The photoinitiator bis(2,4,6‐trimethylbenzoyl)‐phenylphosphine oxide, in combination with the 400 nm LED lamp, was shown to be the most efficient photoinitiator/light source combination for photoinitiation of thick systems.CONCLUSION: The results show that some photoinitiators commonly used for photopolymerization of thin coatings are ineffective for curing thick systems. LED light sources provide advantages over traditional mercury lamps, and may have tremendous potential in the effective photoinitiation of thick polymer systems. Copyright © 2008 Society of Chemical Industry

Synthesis and photoinitiation activity of macroinitiators comprising benzophenone derivatives

Macrophotoinitiators bearing 4-substituted benzophenones bound to an N-isopropylacrylamide chain were synthesized in one-step reaction. Triethanolamine was used as coinitiator. These systems were evaluated as photoinitiators of the N-isopropylacrylamide polymerization in different solvents. They present a high photoinitiation efficiency that depends on the structure of the initiator and on the medium properties. In all solvents, the macroinitiators were more efficient than the corresponding low-molecular-weight analog. The photophysics of ketones was studied in the different solvents. The triplet state of the ketone was deactivated by the amine with a diffusional controlled rate. The characterization of the transients produced in this process explained the influence of the 4-substituent and of the solvent on the yield of the active radical and the polymerization efficiency.

Photopolymerisable formulations for enhanced properties of pigmented films

Ultraviolet (UV) curing is an important and widely used surface technology for coatings. The curable formulations containing: monomer: epoxydiacrylate, photoinitiating system: 4-methoxybenzoyldiphenylphosphine oxide/2-hydroxy-2-methyl-1-phenyl-propan-1-one, additive reactive: tertiary amine, and inorganic thermoresistant pigments: white (TiO2) and blue (Al2O3–Cr2O3), were cured like films by UV exposure. A series of experiments was carried out in order to optimise the photoinitiator ratios in photoinitiating system, and pigment content in the photocurable formulation. Films with enhanced mechanical properties (pendulum hardness, scratch resistance), high resistance at solvents and good appearance were developed. In order to provide adequate hiding power of the pigmented formulation, the pigment content has been chosen in the range 5–15%. Up to 15% pigment content may affect the efficiency of photoinitiator. The polymerisation efficiency (conversion and polymerisation rates) of photoinitiating systems in pigmented formulations was performed by differential scanning photocalorimetry (photo DSC) in comparison with the original systems without pigments.

New Photoinitiators Based on the Silyl Radical Chemistry: Polymerization Ability, ESR Spin Trapping, and Laser Flash Photolysis Investigation

New highly efficient photoinitiators (PI) based on silyl radicals chemistry are proposed. The ability of these structures to initiate both free radical polymerization and free radical promoted cationic polymerizations is investigated. These compounds (4-tris(trimethylsilyl)silyloxybenzophenone, 2-(1-adamantylcarbonyl)-1,1,1,3,3,3-hexamethyl-2-(trimethylsilyl)trisilane, di(9H-fluoren-9-yl)(dimethyl)silane, and 4-(bromophenylthio)dimethyl-tert-butylsilane) directly generate silyl radicals under light irradiation as characterized by ESR spin-trapping and laser flash photolysis experiments. High rates of polymerization and high final conversions are obtained. A low oxygen inhibition in the photopolymerization processes is outlined. The present results evidence the high potential of this new class of PI based on the silyl radical chemistry.

Triaryl(1-pyrenyl)bismuthonium Salts: Efficient Photoinitiators for Cationic Polymerization of Oxiranes and a Vinyl Ether

Photoirradiation of triaryl(1-pyrenyl)bismuthonium salts in acetonitrile afforded triarylbismuthanes and pyrene, accompanied by the generation of protic acids. Triaryl(1-pyrenyl)bismuthonium hexafluoroantimonates have proven to behave as efficient photoinitiators for cationic polymerization of oxiranes and a vinyl ether, affording the corresponding polymers in good yields within 1 min.

Effect of coinitiator type on initiation efficiency of two‐component photoinitiator systems based on Eosin

AbstractEffect of coinitiator structure on relative initiation efficiency of two‐component Eosin/coinitiator systems has been evaluated quantitatively in polymerization of poly(ethylene glycol) diacrylate as an example monomer. The initiation efficiency has been measured by the Fluorescence Probe Technique (FPT), using Eosin both as a photoinitiator component and as a fluorescent probe. A LED/fiber optic‐based measurement system has been developed and applied in this study. It has been found that from among 17 compounds tested, the following coinitiators form most efficient photoinitiating systems in combination with Eosin, when exposed to visible light: coinitiator, relative efficiency = triethanolamine, 1.0; 2,6‐diisopropyl‐N,N‐dimethylaniline, 0.70; 2‐benzyl‐2‐(dimethylamino)‐4′‐morpholinobutyrophenone (Irgacure 369), 0.91; carbon tetrabromide, 2.1; [4‐[(2‐hydroxytetradecyl)oxy]phenyl]phenyliodonium hexafluoroantimonate (SarCat 1012), 28. These relative efficiencies refer to the following component concentrations: [Eosin] = 8.6 × 10−4 M, (0.05% by weight); [coinitiator] = 4.3 × 10−2 M. The factors affecting the initiation efficiency of the systems studied are discussed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3519–3532, 2008

Solubility of Irgacure ® 2959 photoinitiator in supercritical carbon dioxide: Experimental determination and correlation

CIBA photoinitiator Irgacure ® 2959 is a highly efficient radical photoinitiator for UV curing systems. Furthermore, and due to its proven low cellular toxicity, this photoinitiator can also be employed in the photopolymerization of polymers and copolymers intended for biomedical and tissue engineering applications, not only in conventional pharmaceutically accepted liquid solvents, but also in supercritical CO 2 (scCO 2). In this work we report the experimental measurement and correlation of the solubility of Irgacure ® 2959 in scCO 2. Results were obtained using a static analytical method, at 308.2, 318.2 and 328.2 K, and in a pressure range from 10.0 up to 26.0 MPa. Experimental data were correlated with three density-based models (Chrastil, Bartle and Méndez-Santiago–Teja models) and with an equation-of-state (EOS) model: the Peng–Robinson cubic equation of state (PR-EOS) together with the conventional van der Waals mixing and combining rules. Several different sets of solid properties, estimated by different methods available in literature, were used in EOS correlations. The importance of the estimation methods used for the determination of solid properties (namely sublimation pressure) was discussed in terms of correlation results quality. To the best of our knowledge, these are the first experimental scCO 2 solubility results for this class of initiators and the obtained solubility results indicate the feasibility of using this photoinitiator in free radical polymerization reactions carried out in scCO 2 like, for example, dispersion and precipitation polymerizations, or in the development of interpenetrating polymer networks (IPN's).

EFFECTS OF ORGANIC COLORANTS ON PHOTO-INITIATED CROSSLINKING AND PHOTO-OXIDATION DEGRADATION OF POLYETHYLENE AND RELATED MECHANISM

The effects of three organic colorants on photo-initiated crosslinking and photo-oxidation degradation of polyethylene (PE) samples irradiated by microwave excited (MWE) UV lamp in the melt and the related mechanism have been studied by gel content and thermal extension rate determinations, X-ray photoelectron spectroscopy (XPS), mechanical property tests, UV spectroscopy, and light microscope. The data from the gel content and thermal extension rate determinations of photo-crosslinked polyethylene (XLPE) samples show that the three colorants can decrease the efficiency of photo-initiated crosslinking of polyethylene to some different degree, in which the effect of red colorant is the largest among the three colorants. The colorized samples of 1 mm thickness are easily to be crosslinked to a satisfactory gel content of about 70% by the MWE lamp and optimized reaction conditions, such as the concentration of colorant, irradiation time, and so on. The XPS results give the evidence that the colorants can accelerate the surface photo-oxidation during the photo-crosslinking of polyethylene. The photo-oxidation products such as -CH2-O- and -C(C=O)- groups on the surface of XLPE samples with the colorants apparently increase with increasing the irradiation time. The data from the mechanical tests show that the colorants reduce the tensile strength and improve the elongation at break of XLPE samples. All the above results show that the effects of the three colorants on photo-initiated crosslinking and photo-oxidative degradation decrease with the order of red > blue > green colorants. The light microscope photos show that the colorant can disperse well in PE resin. The mechanism of the colorant effects can be elucidated by comparison of the UV absorption spectra of photo-initiator and colorants. This is because the colorants absorb the same UV wavelength regions as photo-initiator, and thus decrease the photo-crosslinking efficiency of photo-initiator and accelerate the surface photo-oxidation of XLPE materials.

Development of new heterobicationic monomethine dyes as effective photoinitiator of free radical polymerization in visible-light region

The dye initiators consisting of N-[3-(p-alkylpyridine)propyl]-2-[N-(3-bromopropyl)quinoline]-2-methylenebenzotiazolium diiodides as chromophores and n-butyltriphenylborate anion as electron donor were prepared to achieve an efficient photoinitiator of free radical polymerization in a visible-light region. The relative photoinitiating efficiencies of novel photoinitiators of vinyl monomers polymerization were evaluated. The results obtained clearly documented that the bicationic photoinitiators exhibit a marked increase in the photoinitiation ability compared to the initiators consisting of a single charged hemicyanine dye. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Two-photon 3D lithography: A Versatile Fabrication Method for Complex 3D Shapes and Optical Interconnects within the Scope of Innovative Industrial Applications

Laser material processing using intense laser pulses, which are provided from ultrafast laser systems, enable sophisticated structuring methods such as the two-photon 3D lithography. This method is based on the simultaneous absorption of two photons in a photosensitive material that is transparent for the laser wavelength. The non linear laser-matter interaction induces material changes that are tightly confined around the laser focus and build up a structure in the volume of the material. These intrinsic 3D capabilities allow easy fabrication of a physical structure from a CAD model. In combination with suitable materials, it is possible to realize complex 3D structures and photonic microand nano-systems. We introduce our experimental setup comprising laser source, three axes sample stage and 3D registration of the sample. We present two distinct applications of the versatile two-photon 3D lithography on organic photosensitive materials: first, the fabrication of complex arbitrarily shaped 3D micro-structures and second, the fabrication of direct laser-written, embedded multimode waveguides that are aligned relative to preconfigured printed wiring boards (PWBs). In the first case, the high resolution aspect of the method is addressed. We could fabricate small structures with feature sizes less than 1μm in a material with a very efficient photo initiator at a low concentration of only 0.025 wt%. In the case of waveguide fabrication, the lithographic method is combined with astigmatic beam focusing for a larger interaction volume as required for writing multimode waveguides. Due to the sample registration prior to the waveguide fabrication, the alignment of the waveguides becomes an intrinsic part of the fabrication process itself. A single organic-inorganic hybrid material is used for both, the waveguide core and its cladding, because the material exhibits a sufficiently large increase of the refractive index upon laser irradiation. The function of such waveguides is demonstrated by monitoring transmitted light at the waveguide exit and the optical loss of the fabricated waveguides is found to be approx. 0.64 dB/cm from cut back measurements. The total optical loss of a 12 cm optical interconnect on the PWB (including coupling losses at both waveguide caps and propagation loss) is in the range 11-20 dB. The ultimate goal of this approach is the large scale fabrication of leading-edge PWBs with an integrated optical communication layer.

Reactivity and efficiency of difunctional radical photoinitiators

AbstractThree difunctional cleavable photoinitiators based on the α‐hydroxyketone structure were tested in photopolymerization reactions with Fourier transform infrared spectroscopy. The excited‐state processes were investigated with time‐resolved laser absorption spectroscopy and laser‐induced photocalorimetry, which allowed the cleavage quantum yields to be determined and estimates of the triplet‐state lifetimes and interaction rate constants with a methacrylate monomer to be given. Modeling calculations helped to describe the ground‐state properties and showed that the difunctional photoinitiators exhibited different chromophoric groups than the corresponding monofunctional photoinitiator. An examination of the photochemical reactivity versus the practical efficiency observed in the polymerization showed that these difunctional photoinitiators exhibited better efficiency than the parent molecule, mainly because of their increased absorption properties. Their practical efficiency appeared to be better because of better light absorption when standard polychromatic light sources were used. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Poly(propylene imine) dendrimers as hydrogen donor in Type II photoinitiated free radical polymerization

Three generations of poly(propylene imine) dendrimers, namely (PPI-16, PPI-32 and PPI-64; 16, 32 and 64 for generations 3, 4 and 5, respectively) were used as hydrogen donors in photoinitiated free radical polymerization of methyl methacrylate by using one of the following photosensitizers; benzophenone and thioxanthone. The effect of generation number of the dendrimer on photoinitiation efficiency and molecular weight of the resulting polymers was investigated. Glass transition temperatures and particle size measurements of the resulting polymers indicated the presence of nearly stretched polymer chains around the dendrimers. The location of hydrogen donating sites was evaluated by photolysis studies in the absence of monomer by using a stable radical namely, 2,2,6,6-tetramethylpiperidine- N-oxyl free radical (TEMPO) and showed that hydrogen abstraction occurs from the inner tertiary amino groups. The TEMPO attached dendrimers were further used in the nitroxide mediated radical polymerization (NMP) of styrene to yield star polymers.

Kinetic elucidation of a novel photoproduct via irradiation of isopropylthioxanthone with maleic anhydride

Isopropylthioxanthone is a popular cosynergist for Type II and three-component photoinitiators used to photopolymerize ultraviolet (UV) curable coatings and can be used in the presence of maleic anhydride derivatives to efficiently cure coatings used in numerous applications. Isopropylthioxanthone (ITX) was shown via ultraviolet–visible (UV–vis) spectroscopy to yield a single photoproduct with maleic anhydride (MA) when irradiated (λ ≥ 350 nm). Following irradiation of ITX and MA at set time intervals, high-performance liquid chromatography (HPLC) was used to observe the formation of the photoproduct by monitoring the UV–vis absorbance spectra of the photoproduct and the reactants. Based on the consumption of ITX shown in the HPLC plots, the complete rate law was calculated based on the initial rates. Finally, a possible mechanism is proposed for the formation of the photoproduct which may provide evidence for the reduced initiation efficiency of photoinitiators incorporating ITX and a MA derivative when compared to a similar photoinitiator substituting an N-substituted maleimide for the MA derivative.

A water‐soluble supramolecular‐structured photoinitiator between methylated β‐cyclodextrin and 2,2‐dimethoxy‐2‐phenylacetophenone

AbstractA water‐soluble supramolecular‐structured photoinitiator (SSPI) was synthesized by supramolecular self‐assembling between methylated β‐cyclodextrin (MβCD) and hydrophobic 2,2‐dimethoxy‐2‐phenylacetophenone (DMPA). The structure of SSPI was characterized by X‐ray diffraction, FTIR, 1H NMR, UV–vis, and fluorescence spectra. The results indicated that MβCD and DMPA had formed 1 : 1 inclusion complex in methanol solution. The binding constant (K) for the complex was 7.51 × 102M−1. SSPI could be dissolved in water easily and its water‐solubility was 15.3 g/100 mL. SSPI was the more efficient photoinitiator than DMPA for the photopolymerization of acrylamide (AM) in homogeneous aqueous system. The conversion for photopolymerization of trimethylolpropane triacrylate system initiated by SSPI was similar to that initiated by DMPA. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Thioxanthone−Anthracene: A New Photoinitiator for Free Radical Polymerization in the Presence of Oxygen

A novel thioxanthone−anthracene (TX-A) photoinitiator, namely 5-thia-pentacene-14-one, possessing the respective photochromic groups was synthesized. TX-A is an efficient photoinitiator for free radical polymerization of acrylic and styrenic type monomers in the presence of oxygen. UV−vis, FT-IR, and fluorescence spectroscopic and polymerization studies revealed that photoinitiation occurs through anthracene chromophore. In contrast to thioxanthone-based photoinitiators, TX-A does not require an additional hydrogen donor for the initiation.

Improving spatial resolution of two-photon microfabrication by using photoinitiator with high initiating efficiency

The lateral spatial resolution (LSR) in two-photon induced polymerization was improved to 80nm by using an anthracene derivative (9,10-bis-pentyloxy-2,7-bis[2-(4-dimethylamino-phenyl)-vinyl]anthracene (BPDPA)) as a highly sensitive and efficient photoinitiator. Photocurable resin containing 0.18mol% BPDPA exhibited a low polymerization threshold of 0.64mW at 800nm. Theoretical calculations showed that the LSR can be increased by reducing the laser power, indicating that the LSR could be improved using more sensitive initiators in the future.

Novel Photosensitive Thio-Containing Polyurethane as Macrophotoinitiator Comprising Side-Chain Benzophenone and Co-Initiator Amine for Photopolymerization

A novel thio-containing polyurethane (PU) as macrophotoinitiator (PUIA) was synthesized through polycondensation of 3,5-diamino-4‘-thiophenylbenzophenone (DATBP), hexamethylene-1, 6-diissocyanate (HDI), and N-methyldiethanolamine (MDEA). The beznophenone and co-initiator amine structures were successfully introduced into the backbones of PUIA. A macrophotoinitiator without co-initiator amine in polymer chain (PUI) and a macroamine (PUA) were also synthesized for comparison. FT−IR, 1H NMR and GPC analyses confirmed the structures of all polymers. The UV−vis spectra of PUIA and PUI are similar and both exhibit the maximal absorption near 320 nm. ESR spectra indicate PUIA can efficiently generate free radicals. The photopolymerization of trimethylolpropane triacrylate (TMPTA) and PU prepolymer, initiated by PUIA, PUI/MDEA, PUI/PUA, DATBP/MDEA, and DATBP/PUA, was studied by photo-DSC. The results show that PUIA is the most efficient photoinitiator for both TMPTA and PU prepolymer. As for the polymerization of PU prepolymer, the final conversion is higher than 97%.

The role of the photoinitiating systems towards the inhibiting effect of phenolic compounds in the photopolymerization of acrylates

Photopolymerization reactions of wood coatings under UV and visible light have been carried out. The influence of model phenolic derivatives found in wood extractives on the polymerization kinetics as well as the coating properties of clear coating formulations exposed to light in laboratory and industrial type conditions has been discussed as a function of various UV/visible or visible photoinitiating systems based on ketones and dyes. The properties of suitable formulations are discussed. Several kinds of efficient photoinitiating systems are particularly investigated such as isopropylthioxanthone/amine; bis-acylphosphine oxide derivative/isopropylthioxanthone/amine and Rose Bengal/amine/additive.

Quinolineimidazopyridinium derivatives as visible-light photoinitiators of free radical polymerization

Several dyes containing Quinolineimidazopyridinium moiety (QIPD) have been synthesized and evaluated as photoinitiators for free radical polymerization induced with the visible emission of a commercial dental lamp. The tested dyes were tested as photoinitiators in the presence of selected electron donors. Different substituents introduced into both the pyridine ring and quinoline moieties of the dyes influence photophysical properties of the investigated systems. Several different groups were tested including heavy atoms (–Cl, –Br, –I) and electron accepting (–NO 2) group. Analysis of the properties of the tested dyes allows one to conclude that there is a significant heavy atom effect on their photoinitiation efficiency. The type of the applied counter-ion has no effect on the overall performance of the photoinitiating system. QIPDs possess broad structured spectra with long-wavelength part located at the boundary of visible and UV parts of light spectrum. This makes QIPDs good candidates for the use as photoinitiating systems for dental restorative materials.

A Highly Efficient Polyurethane‐Type Polymeric Photoinitiator Containing In‐chain Benzophenone and Coinitiator Amine for Photopolymerization of PU Prepolymers

AbstractSummary: A novel polyurethane‐type polymeric photoinitiator (PUIOA) was synthesized through polycondensation of a novel diamine AAPBP, TDI and MDEA. The BP and coinitiator amine structures were successfully introduced into the backbones of PUIOA. A polymeric photoinitiator without the coinitiator amine in the polymer chain (PUIO) was also synthesized for comparison. FT‐IR, 1H NMR and GPC analyses confirmed the structures of polymeric photoinitiators. The UV‐vis spectra of PUIOA and PUIO are similar to the parent AAPBP, and both exhibit high red‐shifted maximal absorption as compared with BP. ESR spectra indicate that PUIOA can generate free radicals most efficiently. Photopolymerization of the Polyurethane prepolymer, initiated by PUIOA, PUIO/MDEA, AAPBP/MDEA and BP/MDEA, was studied by photo‐DSC. The results show that PUIOA is the most efficient photoinitiator for PU prepolymer.Synthesis routes for polymeric photoinitiators.magnified imageSynthesis routes for polymeric photoinitiators.