Photoinitiating Systems Research Articles

Recent Advances on Furan-Based Visible Light Photoinitiators of Polymerization

Photopolymerization is an active research field enabling to polymerize in greener conditions than that performed with traditional thermal polymerization. At present, a great deal of effort is devoted to developing visible light photoinitiating systems. Indeed, the traditional UV photoinitiating systems are currently the focus of numerous safety concerns so alternatives to UV light are being actively researched. However, visible light photons are less energetic than UV photons so the reactivity of the photoinitiating systems should be improved to address this issue. In this field, furane constitutes an interesting candidate for the design of photocatalysts of polymerization due to its low cost and its easy chemical modification. In this review, an overview concerning the design of furane-based photoinitiators is provided. Comparisons with reference systems are also established to demonstrate evidence of the interest of these photoinitiators in innovative structures.

Latest Advances in Highly Efficient Dye-Based Photoinitiating Systems for Radical Polymerization.

Light-activated polymerization is one of the most important and powerful strategies for fabrication of various types of advanced polymer materials. Because of many advantages, such as economy, efficiency, energy saving and being environmentally friendly, etc., photopolymerization is commonly used in different fields of science and technology. Generally, the initiation of polymerization reactions requires not only light energy but also the presence of a suitable photoinitiator (PI) in the photocurable composition. In recent years, dye-based photoinitiating systems have revolutionized and conquered the global market of innovative PIs. Since then, numerous photoinitiators for radical polymerization containing different organic dyes as light absorbers have been proposed. However, despite the large number of initiators designed, this topic is still relevant today. The interest towards dye-based photoinitiating systems continues to gain in importance, which is related to the need for new initiators capable of effectively initiating chain reactions under mild conditions. In this paper we present the most important information about photoinitiated radical polymerization. We describe the main directions for the application of this technique in various areas. Attention is mainly focused on the review of high-performance radical photoinitiators containing different sensitizers. Moreover, we present our latest achievements in the field of modern dye-based photoinitiating systems for the radical polymerization of acrylates.

Influence of Eugenol and Its Novel Methacrylated Derivative on the Polymerization Degree of Resin-Based Composites.

The aim of this work was to assess the limiting rate of eugenol (Eg) and eugenyl-glycidyl methacrylate (EgGMA) at which the ideal degree of conversion (DC) of resin composites is achieved. For this, two series of experimental composites, containing, besides reinforcing silica and a photo-initiator system, either EgGMA or Eg molecules at 0-6.8 wt% per resin matrix, principally consisting of urethane dimethacrylate (50 wt% per composite), were prepared and denoted as UGx and UEx, where x refers to the EgGMA or Eg wt% in the composite, respectively. Disc-shaped specimens (5 × 1 mm) were fabricated, photocured for 60 s, and analyzed for their Fourier transform infrared spectra before and after curing. The results revealed concentration-dependent DC, increased from 56.70% (control; UG0 = UE0) to 63.87% and 65.06% for UG3.4 and UE0.4, respectively, then dramatically decreased with the concentration increase. The insufficiency in DC due to EgGMA and Eg incorporation, i.e., DC below the suggested clinical limit (>55%), was observed beyond UG3.4 and UE0.8. The mechanism behind such inhibition is still not fully determined; however, radicals generated by Eg may drive its free radical polymerization inhibitory activity, while the steric hindrance and reactivity of EgGMA express its traced effect at high percentages. Therefore, while Eg is a severe inhibitor for radical polymerization, EgGMA is safer and can be used to benefit resin-based composites when used at a low percentage per resin.

Inclusion Complexes of Photosensitizers with Cyclodextrins for Enhancing the Fabrication of Volume Grating

A supramolecule photosensitizer (supra-photosensitizer) based on the host-guest complexation of cyclodextrins and a bis-chalcone dye (BDEA) was prepared. Methyl-β-cyclodextrin (Me-β-CD), with the highest inclusion rate, was confirmed as the best host among the four cyclodextrins. The host-guest properties of Me-β-CD and BDEA were characterized by FTIR, XRD, 1H NMR, PLQY, SEM, Job's plot, Benesi-Hildebrand plot, and others. Compared to the conventional photosensitizers, the supra-photosensitizers showed higher fluorescence emission and longer fluorescence lifetime whether as a powder or distributed in a film. The improvement in fluorescence coincided with improvement in photoinitiation efficiency and was proven to enhance the generation of volume grating. It is expected that the supra-photosensitizer may open a new avenue for the design of high-performance photoinitiation systems.

Curing profile and marginal gap formation using a liner containing long-wavelength-absorbing photoinitiator: an in vitro study.

The aim was to evaluate the marginal-gap formation and curing profile of a new restorative technique using a liner with long-wavelength-absorbing photoinitiator (LWAP). Box-shaped preparations (6mm × 4mm × 4mm) were made in third molars. All samples were treated with Clearfill SE Bond and divided into 4 groups (n = 5), according to restorative technique used: (1) incremental technique (INC-Technique); (2) camphorquinone-based liner (CQ-Liner) + bulk-fill resin composite; (3) LWAP-based liner (LWAP-Liner) + bulk-fill resin composite; and (4) bulk-fill technique without liner (BF-Technique). The marginal gaps (%) for all the samples were measured using micro-computed tomography. The restorations were cross-sectioned, and the degree of conversion (DC) and Knoop microhardness were evaluated at different depths (0.3, 1, 2, 3, and 4mm). INC-Technique, CQ-Liner, and LWAP-Liner groups showed significantly fewer marginal gaps than those from the BF-Technique group. The BF-Technique specimens had the lowest DC and microhardness in depth. All the other techniques presented similar degree of conversion and microhardness at all the depths. The use of liners, regardless of the photoinitiator system, decreased the marginal-gap formation and improved the curing profile of bulk-filling restoration technique.

The future of visible light photoinitiators of polymerization for photocrosslinking applications

Photopolymerization under visible light is an active research field whose intense research activity is supported by the recent advances in 3D and 4D printing. Within this perspective article, emerging trends in visible light photopolymerization for photocrosslinking applications will be presented. Notably, the performant multicomponent photoinitiating systems extensively used in the past are now progressively discarded in favor of mono-component systems easier to handle. The reduction of side-products formed during polymerization, the reduction of oxygen inhibition, the formation of dyes by intermolecular charge transfer interactions or the development of more ecological polymerization processes using sunlight as the irradiation source have recently been the focus of intense research efforts. Furthermore, emerging trends in processes favoring the elongation of the polymer lifetime such as self-healing will be presented and this perspective article aims at presenting the most recent breakthroughs in these attempts. To end, the future vision of visible light photopolymerization for photocrosslinking applications will be discussed.

The Blue‐LED‐Sensitive Naphthoquinone‐Imidazolyl Derivatives as Type II Photoinitiators of Free Radical Photopolymerization

AbstractIn this work, new naphthoquinone‐based photoinitiators are synthesized and applied for the first time in free radical photopolymerization. In the presence of acrylate monomers, these PIs display excellent photoinitiation capabilities under blue‐light LED@405 nm irradiation in a two‐component photoinitiating system in which the additive is a common iodonium salt (bis‐(4‐tert‐butylphenyl)iodonium hexafluorophosphate ‐ Iod) or a tertiary amine (ethyl 4‐dimethylaminobenzoate ‐ EDB). In controlled tests, the polymerization performance of dye4/Iod is equal to or better than that of common commercial Type II photoinitiators (2‐isopropylthioxanthone and benzophenone). Meanwhile, only a trace amount of photoinitiator is required (as low as 0.05%) to initiate the photopolymerization. It is worth noting that some of these compounds can also be used in one‐component photoinitiating system (monocomponent Type II behavior) and exhibit high photoinitiation ability. A series of complementary characterization analyses of two new naphthoquinone dyes (i.e. dye 1 and dye 4) with very similar chemical structures are carried out to investigate the relevant photochemical mechanism. Finally, 3D printing experiments are carried out by a simple and convenient Direct Laser Write (DLW) technique.

Study on New Dental Materials Containing Quinoxaline-Based Photoinitiators in Terms of Exothermicity of the Photopolymerization Process.

Modern dentistry places great demands on the dental composites used for filling tooth cavities or treating cavitated tooth decay. The aim of the work was to modify the properties of composites by changing the initiators and co-initiators. This was achieved by using initiators based on a quinoxaline skeleton and co-initiators that are derivatives of acetic acid, which is an advantage of these photoinitiating systems due to the elimination of aromatic amines from the photocurable composition. The composites also differed in dental fillers. The effect of the compounds on the exothermicity of the photopolymerization process, the surface morphology of the obtained materials and the maximum compressive strength were determined. The photoinitiating capacity of the two-component systems was tested by the microcalorimetric method using the multifunctional monomer TMPTA, typical for dental filler compositions. The new photoinitiating systems show particularly good efficiency of free radical polymerization initiation, which occurs by the photoinduced intermolecular electron transfer (PET) mechanism. The comparison of the tested systems with camphorquinone, a photoinitiator traditionally used in dentistry, made it possible to observe a decrease in temperature during photopolymerization without a significant decrease in the polymerization rate or increase in photocuring time, as well as a better homogeneity of the surface of the obtained polymeric materials. This indicates that dye-acetic acid derivative systems may be useful in dental applications.

Light-Assisted Synthesis of Silver and Gold Nanoparticles by New Benzophenone Derivatives.

Benzophenone derivatives were evaluated as new photoinitiators in combination with triethylamine (TEA) and iodonium salt (Iod) for very rapid and efficient formation of metal nanoparticles in an organic solvent, by which silver and gold ions were reduced under light at 419 nm (photoreactor) with an irradiation intensity of 250 microwatts/cm2. The new benzophenone derivatives combined with TEA/Iod salt showed good production of metal nanoparticles (Au0 and Ag0) and a small size of nanoparticles of around 4-13 nm. The photochemical mechanisms for the production of initiating radicals were studied using cyclic voltammetry, where a negative ΔG of around -1.96 eV was obtained, which made the process favorable. The obtained results proved the formation of amine and phenyl radicals, which led to the reduction of gold III chloride or silver ions to the gold and silver NPs. The UV-vis spectroscopy technique was used as a very beneficial tool for the surface plasmon resonance band detection of metal nanoparticles. To sum up the results, we have observed that nanoparticles (NPs) were distributed differently in different photoinitiator systems and the particle size also changed by changing the system of initiation. In comparison to the system alone, not only were the nanoparticles smaller but they were also generated within a shorter period of irradiation time for the system BP\Iod\TEA. Finally, the quenching process of benzophenone fluorescence by the gold and silver nanoparticles was investigated.

Free-radical photoring-opening polymerization of cyclic allylic sulfide: From photocleavable photoinitiators to three components photoredox systems

Photopolymers form a family of materials that have been developed to meet specific demands in fields such as coatings, varnishes, semiconductors, optical fibers, printing plates, holographic recording, etc. However, the multifunctional acrylate monomers currently used present an important volume contraction. A drawback for applications where little shrinkage during polymerization is mandatory to keep shapes and properties of the material. To overcome this problem, monomers reacting by ring opening polymerization can be used. Among possible monomers, cyclic allylic sulfides are very interesting candidates: they are highly reactive and their polymerization mechanism is almost not affected by the presence of oxygen. In this work the effects of the photoinitiating systems on the free radical ring opening photopolymerization kinetics of an eight-member cyclic allylic sulfide is studied. It is shown that cleavable type I, bimolecular type II and three components redox photoinitiating systems allow the formation of linear photopolymers with high polymerization rates and final conversions. The photopolymerization kinetics obtained show that the photopolymerization is not sensitive to oxygen which leads to reduced exposure time and higher sensitivity. This remarkable property should also extend the usage of such monomers in the coating applications where oxygen inhibition is a drawback. Finally, the benefit of ring opening polymerization process is demonstrated trough the volume contraction measurement which was found to be lower than 1.5%.

Photo-activated polymerization inhibition process in photoinitiator systems for high-throughput 3D nanoprinting

Abstract The systems for multiphoton 3D nanoprinting are rapidly increasing in print speed for larger throughput and scale, unfortunately without also improvement in resolution. Separately, the process of photoinhibition lithography has been demonstrated to enhance the resolution of multiphoton printing through the introduction of a secondary laser source. The photo-chemical dynamics and interactions for achieving photoinhibition in the various multiphoton photoinitiator systems are complex and still not well understood. Here, we examine the photoinhibition process of the common photoinitiator 7-diethylamino 3-thenoylcoumarin (DETC) with inhibition lasers near or at the multiphoton printing laser wavelength in typical low peak intensity, high repetition rate 3D nanoprinting processes. We demonstrate the clear inhibition of the polymerization process consistent with a triplet absorption deactivation mechanism for a DETC photoresist as well as show inhibition for several other photoresist systems. Additionally, we explore options to recover the photoinhibition process when printing with high intensity, low repetition rate lasers. Finally, we demonstrate photoinhibition in a projection multiphoton printing system. This investigation of photoinhibition lithography with common photoinitiators elucidates the possibility for photoinhibition occurring in many resist systems with typical high repetition rate multiphoton printing lasers as well as for high-speed projection multiphoton printing.

High-Performance Photoinitiating Systems for LED-Induced Photopolymerization

Currently, increasing attention has been focused on light-emitting diodes (LEDs)-induced photopolymerization. The common LEDs (e.g., LED at 365 nm and LED at 405 nm) possess narrow emission bands. Due to their light absorption properties, most commercial photoinitiators are sensitive to UV light and cannot be optimally activated under visible LED irradiation. Although many photoinitiators have been designed for LED-induced free radical polymerization and cationic polymerization, there is still the issue of the mating between photoinitiators and LEDs. Therefore, the development of novel photoinitiators, which could be applied under LED irradiation, is significant. Many photoinitiating systems have been reported in the past decade. In this review, some recently developed photoinitiators used in LED-induced photopolymerization, mainly in the past 5 years, are summarized and categorized as Type Ⅰ photoinitiators, Type Ⅱ photoinitiators, and dye-based photoinitiating systems. In addition, their light absorption properties and photoinitiation efficiencies are discussed.

Cyanine/iodonium salt as a broad-absorbing photoinitiating system for radical photopolymerization under near-UV, visible and NIR light

Cyanine/iodonium salt as a broad-absorbing photoinitiating system for radical photopolymerization under exposure to near-UV, visible and NIR light.

Fluorescent core-shell magnetic nanoparticles by type II photoinitiated polymerisation.

We report a fluorescent monomer-free method for the synthesis of fluorescent and stable magnetic nanocomposites using a benzophenone/rhodamine B bimolecular photoinitiator system. The method allows the synthesis of a fluorescent polymer shell layer around magnetic nanoparticles in one step by UV irradiation at ambient temperature.

An anthraquinone-based oxime ester as a visible-light photoinitiator for 3D photoprinting applications

An anthraquinone-based oxime ester photoinitiating system as a new Type I photoinitiator for 3D photoprinting under LEDs@405 nm irradiation.

102 - Long-term evaluation of dentin bonding properties of the photo-initiator system contained in universal adhesives used in fiber post luting procedure

102 - Long-term evaluation of dentin bonding properties of the photo-initiator system contained in universal adhesives used in fiber post luting procedure

High potential of new dyeing photoinitiators for fast curing of (meth)acrylate compositions under low intensity UV–Vis light

The present work describes novel two-component photoinitiating systems for radical polymerization of (meth)acrylate-based compositions.

Highly efficient carbon dot-based photoinitiating systems for 3D-VAT printing

In this work, different types of carbon dots (CDs) based on citric acid as a precursor were synthesized using an efficient procedure to purify these materials from low molecular by-products and fluorophores.

Novel multi-material photo-curable resins containing high-performance photoinitiating systems and nano additives dedicated to 3D-VAT printing

The suitability of new 9-[(E)-2-phenyl]anthracene derivatives with iodonium salt as high-performance photoinitiating systems for obtaining novel multi-material photo-curable nanocomposites resins containing dedicated to 3D-VAT printing.

Photoinitiation abilities of indeno- and indoloquinoxaline derivatives and mechanical properties of dental fillings based on multifunctional acrylic monomers and glass ionomer

Photopolymerization is a rapidly expanding technology with a wide variety of applications, from imaging to biomedical uses. Here, we present enhanced photosensitization of polymerization by photoredox pairs based on indene- and indoloquinoxaline dyes and thiophenoxyacetic acid for dental applications. The photoinitiating ability of the two-component systems was tested using several multifunctional monomers of various structures typical for dental fillings. The new photoinitiating systems show excellent efficiency in initiating free radical polymerization which occurs by photoinduced intermolecular electron transfer (PET) mechanism. Modification of the dye structure by extending the chain of conjugated π bonds, the arrangement of aromatic rings and the type of heteroatom, allowed to change the spectroscopic properties of the tested compounds and use them as absorbers of light emitted by dental lamps. The improved mechanical properties of the cured polymers, i.e., lowering the temperature during photopolymerization and an increase in polymer hardness were observed for the proposed photoinitiating systems compared to the camphorquinone – photoinitiator traditionally used in dentistry. This indicates that the dye – thiophenoxyacetic acid systems may be useful as photoinitiators in dental applications. The undoubted advantage of these photoinitiating systems is the elimination of the toxic aromatic amines traditionally used as co-initiators from the dental composition. It was possible thanks to the use of thiophenoxyacetic acid. Its application does not reduce the polymerization rate and does not extend the photocuring time.