Makromol. Chem. 180,2027-2030 (1979) 2027

Short Communication

A Novel Method for Consuming Oxygen Instantaneously in Photopolymerizable Films

Christian Decker

Laboratoire de Photochimie Generale, Equipe de Recherche Associee au CNRS, Ecole Nationale Supkrieure de Chimie, 68093 Mulhouse Cedex, France

(Date of receipt: May 22, 1979)

Light-induced polymerizations are known to be strongly inhibited by atmospheric oxygen which acts both as a quencher of excited states and as scavenger of initiator and growing po- lymer radicals. This inhibition effect is even more pronounced than in the common free-radical polymerizations since photopolymerizable systems are usually irradiated in thin films which have a large surface through which oxygen from air can diffuse.

In the course of our studies on UV curable printing inks, we developed recently a new meth- od for consuming rapidly the oxygen dissolved in the monomer, before initiating the polymeri- zation'). The basic idea was to convert molecular oxygen into singlet oxygen (lo2) by red-light irradiation in the presence of a dye sensitizer and to scavenge the resulting lo2 by an adequate acceptor. The methylene-blue (MB) photosensitized oxidation of 1,3 diphenylisobenzofuran (1) was chosen as the system to develop for several reasons. Its kinetics, being known in some detail'), is extremely fast, which is an important criterion for a system for which oxygen diffu- sion is expected to have a prominent but negative effect. Furthermore, it is one of the few rapid singlet oxygen reactions that yields only one major, well characterized product: 1,2-dibenzoyl- benzene (2)3). Finally, this oxidation product can be used as photo-initiator since we have shown that it initiates the photopolymerization of acrylic monomers nearly as efficiently as benzophenone'). The reaction scheme for this dye-sensitized photo-oxidation is the follow- ing:

hufred) 30 '(MB) - '(MB)* - '(Me)* - '(MB) + '0;

1 2

According to the stoechiometry of these reactions, each molecule of acceptor consumes one half molecule of oxygen; in order to consume completely the dissolved oxygen, the initial con- centration of 1 must thus be adjusted to at least twice the concentration of oxygen in the air-sa- turated solution, i. e. = 5 mmol.1- I . Under the selected conditions of reactants concentrations and intensity of light, essentially all the oxygen was consumed within 20 s, thus allowing the photopolymerization of acrylamide to proceed in air nearly as fast as in an inert atmosphere').

2028 C. Decker

It should be mentioned that these experiments were carried out in 2-propanol solution, in py- rex cells which have only a small surface area through which atmospheric oxygen could diffuse (surface/volume=0,5). The rate of oxygen consumption was then fast enough to offset the ad- ditional amounts of oxygen diffusing into the solution during the photolysis.

The present paper describes how this method of oxygen elimination can be extended to the photopolymerization in films, a technique which is increasingly used for UV curable coatings and imaging systems. Since the viscosity of the film is relatively high and increases as polymer- ization proceeds, air diffusion within the film is expected to re reduced4); but this positive fac- tor appears to be more than compensated by the large surface through which 0, diffuses in the upper layers of the film, thus leading to an undercured tacky surface. In order to remove the dissolved oxygen faster than new oxygen diffuses inward the film, we decided to use high in- tensity flashes, expecting a large increase in the rate of 0, consumption.

Due to the difficulty of measuring the oxygen concentration in thin films, the kinetics of the reaction was followed by simply monitoring the decrease of the absorption of 1 at 410 nm, as- suming similar consumption laws for both 0, and 1. Such an approximation seems reasonable since 1 was shown to disappear exclusively by reaction with singlet oxygen, no consumption of 1 being observed, in the time scale investigated, by red-light photolysis of a nitrogen-saturated solution of MB, 1, and acrylamide. A direct relationship can thus be established between the quantum yields of consumption of oxygen and 1:

The close to unity value of indicates that scavenging by 1 is the major route of deactiva- tion of singlet oxygen.

Solutions of acrylamide in 2-propanol (10-30 weight-%) containing MB (0,3 mmol .l-’) and 1 ( 5 mmol. 1-‘) were poured on a glass plate to obtain films with surface to volume ratios in the range of 20 to 200. These films were irradiated by using a common camera flash (Agfatronic 401-CBS) which delivers =lo-’ Einstein.crn-’ *) per flash at a distance of 5 cm, as deter- mined by fern oxalate actinometry. Duration of the flash could be adjusted between 20 and 1200 ps.. The light was passed through an aqueous solution of potassium bichromate (1 weight- %) to isolate the useful 500-800 nm wavelength region. The combination of MB and 1 appears to be well suited for our investigation since MB exhibits a strong absorption above 500 nm in a region where 1 is transparent, thus avoiding any direct excitation of this acceptor, while the bleaching of 1 can be easily followed at 410 nm where MB is nearly transparent.

Fig. 1 shows how the concentration of 1 decreases stepwise after each of the repetitive flashes. When the photon flux density is increased, it is even possible by a single flash to con- sume 1 completely, and therefore, to eliminate within 1 ms all the oxygen dissolved in a 0,2 mm thick film. For thin films (< 0,Ol mm), this can be achieved by using lower photon flux densi- ties, thus allowing larger areas to be cured. Since the re-oxygenation process occurs rapidly in thin films, it is imperative that the photopolymerization of the monomer be initiated just after the red flash, either by continuous UV irradiation or by using a second flash light which emits in the wavelength range where 2 and most of the usual photo-initiators absorb (300400 nm).

’’ 1 Einstein=l mol ofphotons; energie=L.h.~=6,02.10~~~6,63.10-~~. u ( J . s ) , here ~=8,16.10’~ s-I.

A Novel Method for Consuming Oxygen Instantaneously in Photopolymerizable Films 2029

Fig. 1. eral photon flux densities. For the definition of Einstein (E) see footnote on page 2028

Consumption of 1.3 diphenylisobenzofuran (1) in the dye-sensitized flash photo-oxidation, for sev-

Under those conditions and by taking &,a-dimethoxy-a-phenylacetophenone (DMPA) as photo-initiator, the quantum yield of acrylamide polymerization in air is significantly higher in the pre-irradiated film (G2.30) than in the untreated control sample ( @ = 6 ) , and not far from the value observed in a pure nitrogen atmosphere (@=45). The propagation reaction appears to be much less sensitive to the oxygen effect, since the weight average molecular weight (Ew) drops from 13000 in nitrogen to 8500 in air, as determined from intrinsic viscosities in water: [q] = 6,3. 1 0 - 3 ~ w * 8 0 ' s 6 ) . Assuming a random distribution ( M w / M , = 2), the number of polym- er molecules produced per quantum absorbed was calculated to be 0,5 in nitrogen and 0,l in air. This signifies that, if benzoyl radicals and benzyl ether radicals, resulting from photocleav- age of DMPA, are equally effective as initiators of the polymerization of acrylamide (as was demonstrated for benzoin ether in methyl methacrylate')), 25% of the absorbed light results in polymer formation in a nitrogen atmosphere. This value is comparable to the one obtained by Pappas et a1.') in their study of the photopolymerization of methyl methacrylate by using a similar concentration of photo-initiator. In the presence of air, the quantum efficiency drops to 5%, indicating that initiator radicals are effectively scavenged by oxygen (DMPA triplets are too short-living to be quenched')), whereas the growing polymer radicals seem to be much less reactive towards oxygen. By flashing the monomer system with red light in the presence of 1 and MB just before initiating the UV polymerization in air, the quantum efficiency increased to 20%.

The dye-sensitized photo-oxidation of 1,3-diphenylisobenzofuran appears thus as a conve- nient method to consume instantaneously most of the oxygen dissolved in photopolymerizable films, thus reducing effectively the oxygen inhibition effect in UV curable systems.

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2030 C. Decker

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(1975)

C. Decker, SPSE Symposium, “Photopolymer Systems-Imaging Science and Technology”, Washington, 1978; C. Decker, J. Faure, M. Fizet, L. Rychla, Photogr. Sci. Eng. 23, 137 (1979)