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N-Vinylamides and Reduction of Oxygen Inhibition in Photopolymerization of Simple Acrylate Formulations 1
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ChrisW.Miller ,C.E.Hoyle ,S.Jönsson ,C.Nason ,T.Y.Lee , W. F. Kuang , andK.Viswanathan 2
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UCB Chemicals Corporation, 2000 Lake Park Drive, Smyrna, GA 30080 School of Polymers and High Performance Materials, The University of Southern Mississippi, Box 10076 Southern Station, Hattiesburg, MS 39406 Fusion UV-Curing Systems, 910 Clopper Road, Gaithersburg, MD 20878-1357 Becker-Acroma, Inc., Brantford, Ontario, Canada 2
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Cyclic N-vinylamides were found to increase the maximum relative rates of photopolymerization of hexanedioldiacrylate formulations at low light intensities under nitrogen and dramatically under air, while acyclic N-vinylamides and cyclic N-alkylamides were found to have much smaller, though still significant, effects in air. The combination of the cyclic aliphatic amide and N-vinylamide functionalities generally resulted in the maximum benefit for reduction of oxygen inhibition resulting in rapid rates of polymerization in air at low light intensities. While the complete mechanistic explanation of the rate enhancement is still unclear, several possible mechanistic pathways are discussed.
Introduction Molecular oxygen exists at ambient conditions as a diatomic molecule with a triplet diradical electronic ground state (7,2). Due to its triplet and radical characteristics, oxygen has high chemical reactivity towards radicals and towards molecules in their excited triplet states. Thus, free-radical photopolymerizations can be inhibited significantly by ambient molecular oxygen, especially under curing conditions such as very low film thickness, low light
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© 2003 American Chemical Society
In Photoinitiated Polymerization; Belfield, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
Downloaded by UNIV OF MISSISSIPPI on June 19, 2015 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch001
3 intensity, or low cure dosage. Several mechanisms are responsible for inhibition including quenching of photoinitiators and scavenging of initiator and polymer radicals. These reactions result in decreased rates of initiation, increased rates of termination, and inclusion of oxygenated species in the surface layers of the cured films. The efficiency of the quenching and radical scavenging reactions depends upon the concentrations of oxygen, radicals, and double bonds. A discussion of the kinetics offree-radicalpolymerization and competing reactions in the context of oxygen inhibition was given in a previous report (3). A number of techniques have been utilized to combat oxygen inhibition, including use of high intensity lamps/high cure dosages (4-8), inert atmospheres/sandwiching materials (9,10% and additives such as amines, which have been shown to consume oxygen by a photo-oxidation process (4,11-17). In our initial studies (3,18) we confirmed earlier reports (19,20) that NVP dramatically increases the relative rates of acrylate polymerization in air at low light intensities and that a reversible complexation occurs between oxygen and NVP as observed in the UV absorption spectrum (21). In the current work, we have extended our study to include other cyclic and acyclic N-vinylamides and N-alkylamides and to inquiries into the chemical phenomena involved in the observed rate effects.
Experimental Results Conditions and Compounds Photo-differential scanning calorimetry (Photo-DSC) was performed on a Perkin-Elmer DSC-7 modified with quartz windows and a medium pressure mercury lamp. Samples were typically 2 - 3μΙ, injected into specially crimped aluminum sample pans giving a film thickness of approximately 250μπι. Real time Fourier-transform infrared spectroscopy (RTFTIR) was performed on a Bruker FTIR/FTR system modified with a nitrogen-cooled MCT detector and a shuttered xenon lamp. The RTFTIR samples were sandwiched between two salt plates to simulate polymerization under inert atmospheres and placed on the surface of a single salt plate to allow curing in air. Ultraviolet-visible spectroscopy was performed on a Cary 500 UV-visible/NIR spectrophotometer by Varian using standard 1cm quartz sample cells. Unless specifically noted otherwise, all samples for Photo-DSC and RTFTIR were formulated to contain 1 weight-percent 2,2-dimethoxy-2-phenylacetophenone (DMPA) as photoinitiator. All monomers were obtained from Aldrich Chemical Company with the exceptions of 1,6-hexanedioldiacrylate (HDDA), which was also obtained from UCB Chemicals Corporation, and N-vinylformamide (NVF), which was obtained from Air Products Corporation. All monomers were used as received, and polymerization inhibitors were not removed. Names, structures, and acronyms of N-substituted amides used in this study are given in Figure 1.
In Photoinitiated Polymerization; Belfield, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
Downloaded by UNIV OF MISSISSIPPI on June 19, 2015 | http://pubs.acs.org Publication Date: March 3, 2003 | doi: 10.1021/bk-2003-0847.ch001
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