Shrinkage photoinitiated

The nature and type of initiation scheme plays an important role in the performance of the adhesive [194,202-204]. Stresses due to polymerization shrinkage lead to the creation of a gap between the adhesive and tooth material. In the case of bulk chemical initiation, shrinkage stresses tend to create gaps at all interfaces, drawing material inward isotropically. With a photoinitiation scheme, polymerization begins at the free surface and pulls the material away from the dentin towards the free surface [194]. Thus the gap is created at the... 

The materials studied displayed relatively low glass transition temperatures and therefore poor orientational stability. Future studies should be directed to solving this problem. Additional research is required to improve the efficiency of photoinitiation for materials with strong absorptions in the visible region of the spectrum. Other issues such as strain caused by shrinkage during crosslinking need to be addressed. 

A liquid preparation with solid polystyrene (0.6 g) dissolved in liquid styrene monomer (1.5 mL) was cast against a mold. Polymerization was accomplished with UV irradiation (21°C, 18 h). Solid PS was included to reduce the degree of shrinkage that occurred when monomeric styrene was photopolymerized [85]. In a similar manner, PMMA dissolved in MMA was cast against a Si master. Upon UV polymerization (with BME as the photoinitiator), a PMMA chip is formed. Nearly 100 PMMA chips can be replicated using a single Si master [223]. 

Cationic cure mechanisms are an alternative approach to uv curing. This involves the photogeneration of ions, which initiate ionic polymerization. This process is not subject to oxygen inhibition, as are some of the free radical mechanisms. Cationic cure mechanisms generally also provide less shrinkage and improved adhesion. The disadvantages are that the photoinitiators are sensitive to moisture and other basic materials. The acidic species can also promote corrosion. As a result, the vast majority of uv formulations are acrylate-based and cure by a free radical mechanism. 

These results point to two processes, premature radical chain termination and film shrinkage, which compete in determining the ultimate polymerization conversion efficient of multifunctional acrylates. It is obvious that critical attention must be paid to the pulse repetition rate, photoinitiator concentration, and acrylate functionality in developing any photopolymerizable system for laser-initiated polymerization. Future publications on laser-initiated polymerization of multifunctional acrylates will deal with monomer extraction of partially polymerized films, mechanical properties of laser polymerized films, and the Idnetics of single-pulsed systems. 

The advantages of a cationic adhesive formulation over that of a free radical one are the lack of oxygen sensitivity, less shrinkage on curing, and better adhesion. The disadvantages are that the photoinitiators are sensitive to moisture and basic materials and that the addic species can promote corrosion. Consequently, the majority of UV formulations used in industry are acrylate based and cured by a free radical mechanism. 

The systems are designed in order to improve the reaction rate of the mixture and the physical properties of the photopoljmier. The flexibility of the two photoinitiation schemes in one system allows for numerons possibilities in achieving greater control of viscosity, conversion, shrinkage, adhesion, and ultimate strength. The kinetics of hybrid photopolymerization systems are more difficult because two reactive systems (free-radical and cationic) mnst be resolved from one another. Cationic photopolymerization kinetics are more difficnlt to analyze than free-radical kinetics because the pseudo-steady-state assumption is often not valid for the cationic active center concentration, and the natnre and concentration of the cationic active centers is difficult to determine (p. 376 of Ref 33, see also Photopolymerization, Cationic). 

Low shrinkage and good adhesion was also achieved by using waterbased UV-curable acrylate resins, like Laromer PE-55W from BASF, associated to Irgacure 2959 from Ciba SC as photoinitiator. After drying at 80°C, the tacky film obtained proved to be an excellent adhesive to assemble glass plates or transparent films by a short exposure to UV-radiation, which transforms the tacky resin into a low-modulus elastomer. The multiple-step processing can be represented schematically as follows ... 

The laser irradiation induced the formation of polymer cylinders in the bulk of the film. Acrylic monomers underwent a volume decrease of 10 to 15% when polymerized. The underlying reason was that the gradient of chemical composition resulting from the spatially controlled conversion of monomer into polymer immediately induced the flowing of monomer and photoinitiator molecules to the illuminated areas. Hollows created by volume shrinkage were almost immediately filled by reactive species. 

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