Compositions that photocure

Hybrid systems are understood to be photocurable compositions that cure by both ionic and free radical mechanisms. As can be seen in Chapter 2, section 2.3.4. photoinitiators were actually designed for use with systems that contain both type of monomers, or prepolymers, and both to cure by both mechanisms. One such example are combinations of phosphonium and sulfonium salts described in that chapter ... 

A photocurable ink composition that contains a dendrimer has been described. The dendritic polymer has a molecule structure in which a functional group is dense on the surface with high density in comparison to a linear polymer. 

An advantage of this type of photopolymerizations is that as they are non-radical chain polymerizations, they are insensitive to oxygen. In addition, as the cation is relatively stable, the reaction is able to continue in the dark. Applications of this chemistry may be found in the fields of coatings, adhesives, printing inks, and also for photocurable composites and microelectronic photoresists. 

Trifunctional (meth)acrylate fluorine-containing cyclic and acyclic silane compounds have been prepared that form photocurable resin compositions. Coatings from these resins are anti-fouling and resist organic stains from oil mist and fingerprints without detracting from surface mar resistance. 

Diacrylate monomers have been prepared that are photocurable in visible light and that have small polymerization shrinkage and high X-ray contrast properties. When polymerized with 0.01 to 0.04 pm glass powder, these dental composites were easily machined into artificial teeth. 

Typical physical properties obtainable with UV cured silicones are provided In Table I. Incorporation of reactive unsaturation into the silicone polymer backbone In combination with a photosensitization system provided the photocure capability. Properties of a standard heat-cured encapsulant developed for use on semiconductor devices, Dow Corning HIPEC R-6103, are provided for comparative purposes. Clearly, introduction of a photocrosslinking mechanism into a siloxane type composition has afforded the desired result. The one-part, solventless, UV curable silicone composition cured rapidly upon exposure to UV radiation, providing a cured composition which has retained the typical properties that make silicones so attractive for protection of semiconductor devices. 

While the photocurable polymers used for S-FlL avoid the viscosity and thermal concerns associated with the thermoplastics used in NIL, they are not without then-problems. During polymerization, the photocurable polymers undergo a volumetric shrinkage or densification due to chemical bond formation. Consequently, the feature size, shape, and position could be affected. Studies have shown, however, that by controlling the resist composition, the shrinkage is limited to the z-direction, resulting in a reduced aspect ratio [131,137],... 

Many compounds can fit the definition of Iree-radial photoinitiators. They can be peroxides, disulfides, azo compounds, ketones, aldehydes and other. One example is a diphenyldisulfide that has been utilized in photocurable systems based on styrene-unsaturated polyester compositions ... 

Several publications suggest that an optimum concentration of a pbotoinitiator should be such that the optical density or absorbance of the reacting composition should be 0.434 to yield an optical density at which maximum radiation is absorbed. This number has meaning, however, only for ideal systems, cured by monochromatic radiation. With commercial sources of radiation that consist of 20 to 40 emission lines this does not apply. Also, it is important to know that variations in the concentrations of a photoinitiator within various locations of the film that is being photocured, dramatically affect flic locations where the radiation is absorbed, such as at the surface, flnoughout the material, or at some other location. 

In this section a few typical photocurable compositions were assembled to illustration the formulations that are used commercially as photocurable coatings. There is no attempt, however, to present here descriptions of various coating formulations. The descriptions of the compositions listed below appeared in chemical and patent literature. How well these materials actually perform as coatings is not known to this writer. 

Few new observations in the photocurable hyperbranched polyester/eutectic nematic liquid crystals (E7) system was reported by Kim et al. [163] Higher order mesophase is induced at some specific compositions, which is found absent in their individual components and possible escape of some ingredients observed during prolonged annealing. It is observed that in the eutectic mixture some components is expelled due to its weak affinity with the hyperbranched polyester. The induced smectic phase at 63-93 wt % E7 in the blend is influenced by the shift of eutectic nematic point. The presence of isotropic (I), isotropic + nematic (I + N), pure nematic (N), nematic +smectic A (N + SmA), and induced smectic A (SmA) phase is dependent on the composition and temperature. 

Glass fiber reinforced composites based on epoxy-acrylate modified UPRs were studied [228]. The authors showed that UPRs, endcapped with acrylate groups and diluted with reactive multifunctional acrylic and allylic monomers in the presence of a photoinitiator, can be photocrosslinked with UV radiation as glass fiber laminates in a rapid process. It was found that the physical properties of the photo-crosslinked laminates are well correlated with the molecular weight of the polyester, the amount of multifunctional monomer added, and the glass fiber content. A greater improvement of the tensile and flexural properties of the photocured products was observed for multifunctional acrylate or acrylether monomers added to the UPR (Table 31) than for allylic monomers. 

ED paints may be thermally or photochemically cured for improved performance. None the less, it was some time before serious attempts were made to use ED photocurable films as resists for metal patterning. It had been foreseen that dry-film photoresists, which have been the mainstay of the printed circuit board inner-layer fabrication process for the last two decades, would soon reach their resolution limit and that a process that coated much thinner layers of resist would take over. ED resists that were capable of coating layers up to five times thinner than dry film seemed the natural successors. In 1986 the Rohm and Haas Co. [2] issued a patent describing a photoresist composition for cataphoretic deposition onto copper during the process of forming a printed circuit board. Many other patents in this field, describing both cataphoretic and anaphoretic deposition of a wide variety of resins, have been issued since then. 

Other synthetic fibers, as well as natural fibers, were employed in epoxy-based multicomponent systems. Thus, a comparative study between polymeric fibers reinforced epoxy resins and natural fibers reinforced ones, all obtained by UV curing, was performed [186]. Isotactic PP (iPP) fibers modified with 20 wt% EVA and hemp fibers were employed, while the matrices used were epoxy acrylate and epoxy methacrylate. Data indicated that relatively regular distribution of fibers was achieved the addition of fibers caused an increase in Shore hardness of the epoxy methacrylate based composites the epoxy acrylated composites showed a decrease in hardness when EVA-modified iPP fibers were used, whereas hemp fibers caused an opposite effect. Even more, the iPP fibers reinforced photocurable composites displayed a brittle to ductile fracture transition. 

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