Overcoming the oxygen inhibition in the photopolymerization of acrylates: A study of the beneficial effect of triphenylphosphine

Triphenylphosphine (TPP) was used in free-radical UV-curable resins to reduce oxygen inhibition effect. The relative influence of concentration, monomer viscosity, light intensity and sample thickness on TPP efficiency was investi- gated by real time infrared spectroscopy. It is shown that TPP is an effective oxygen scavenger. The mechanism was investi- gated by means of Laser Flash Photolysis. V C 2010 Wiley Peri- odicals, Inc. J Polym Sci Part A: Polym Chem 48: 2462-2469, 2010 INTRODUCTION The inhibition effect of molecular oxygen in free-radical photopolymerization reactions has always been a key issue. 1-5 Indeed, the radicals formed during the initiation of the reaction are scavenged by oxygen leading to peroxy radicals that can hardly initiate or contribute to the polymer- ization reaction. Moreover, oxygen is also capable to quench the photoinitiator excited states. Consequently, detrimental effects are observed such as the presence of an inhibition time and the decrease in both polymerization rate and final conversion ratio, resulting in tacky surfaces. These features were nicely confirmed recently by modeling the effect of oxy- gen inhibition on the photopolymerization kinetics, providing a good description of the different mechanisms involved. 6 Physical and chemical methods that may be used to over- come oxygen inhibition have been recently reviewed. 5 Increasing light intensity or photoinitiator concentration is a possible way to achieve this goal. Thereby, a large concentra- tion of radicals is generated upon irradiation, consuming the dissolved oxygen during the light curing process. Use of high-viscosity monomers or increase in sample thickness is also an alternative way to limit the oxygen diffusion from the surrounding atmosphere to the bulk. Another effective method is to purge the sample with an inert gas (nitrogen or carbon dioxide) before the polymerization process, 7,8 the drawback being the important additional costs. The addition of compounds containing easily abstractable hydrogen atoms such as amines or thiols is a different way to limit the oxy- gen inhibition effect: the peroxy radicals formed from the reaction of the initiating radicals or the growing chain with oxygen react with a hydrogen donor and create a new ini- tiating radical and a hydroperoxide. The latter can lead to further photodecomposition. 9-11 Similarly, use of monomer containing some abstractable hydrogen (i.e., N-vinylamides, acrylate containing ether groups) has been shown to be ben- eficial. 12 Besides these different ways, photochemical meth- ods may also be used to consume dissolved oxygen before polymerization. One method is based on light-absorbing spe- cies that sensitize singlet oxygen from dissolved molecular oxygen. Once it is produced, singlet oxygen can react with a scavenger such as benzofuran derivatives to form hydroper- oxides or endoperoxides; these compounds are subject to further decomposition upon irradiation, thereby, removing the molecular oxygen from the system. 13