Control of Free-Radical Technical Paper Technical Reactivity in Photopolymerization of Acrylates By Patrick Kramer, Leonard n practical use, it is the rate of is observed, paired with an increase in Davis and Richard Jones crosslinked network formation that ultimate rub resistance. This optimal is of most interest in the free-radical concentration depends strongly on This paper was selected “Best Student Icuring of acrylates. One of the simplest nitroxide structure, but in all cases Paper” at the RadTech 2012 Technology indicators of the vitrification point is it occurs at nitroxide concentrations Conference and Exhibition. For the sharp rise in methyl ethyl ketone much smaller than the estimated free- the full original article, please visit double rubs (MEKR) that occurs as a radical concentration at the point-of- www.radtech.org. function of radiation dose applied. It cure. Both observations are consistent is well known that oxygen inhibition with a colloidal model of vitrification and radical-radical recombination where first-formed cure domains are limit efficiency, delaying the onset of cemented together in a slower second network formation to higher dose or step. Finally, we show that latent cure longer exposure time. is accentuated by these longer-lived This paper explores means to alter radicals and oxidation products the reactivity of the intermediate free formed from them, allowing a post- radicals—in one case by chain transfer heat to significantly advance cure by to thiol and in another by complexation re-initiation of free radicals. with stable nitroxide radicals, thereby altering the population of chain- Introduction carrying species to favor longer-lived The ultraviolet (UV) and electron radicals or radical precursors which, beam (EB) polymerization of by their nature, are less affected by the acrylated materials proceeds by a normal termination processes. By chain radical addition mechanism. During transfer formation of thiyl radicals, a photocure event, a large number a simple titration of reactive radicals of highly reactive radical species is possible as observed by detection are produced with a short, intense of the point of maximum MEKR as a exposure to radiation.1 The production function of dose for different starting and consumption of these radical thiol concentrations. By this means, we species is typically organized into four can demonstrate that approximately major steps—initiation, propagation, seven times more radicals are created chain transfer and termination, as in a typical UV-cure formula following illustrated in the following:2,3 600 mJ/cm2 exposure than are created Initiation depends on the in an EB-cure formula using the same formation of free-radical species. monomer following 30 kGy exposure. This step generally begins with the At low concentrations of organic fragmentation of a photoinitiator (UV) nitroxides, a surprising decrease in or acrylated monomer (EB) upon radiation dose at the vitrification point exposure to radiation: ISSUE 4 2012 RADTECH REPORT 33 termination pathway is radical-radical mechanical properties—all while Reaction 1 IA I + A recombination: curing at faster web speeds, with lower lamp power or less inerting. Reaction 5 Included in the initiation step is the In this work, we investigate R1 + R2 R1 –R2 Technical Paper Technical addition of an initiating radical I to a several methods of controlling monomer molecule: R1 and R2 are growing radical chains photopolymerization by changing the or initiating radicals. A second type nature of the propagating radical. The Reaction 2 of termination is the reaction of a strategies employed are chain transfer I + CH =CHX I–CH – CHX 2 2 propagating radical with triplet oxygen to produce thiyl radicals from carbon It is our convention that I is to form a peroxy radical species: radicals using multifunctional thiols; reversible complexation of carbon an initiating radical with correct Reaction 6 electronic structure to efficiently add radicals by stable nitroxide radicals; R + O2 ROO to the monomer, while A is a radical and thermal post-cure of a radiation- The alkyl peroxy radical does not species with lower probability to cured system. undergo further addition to a monomer;7 initiate chain growth.2-6 The group X We report here the results of the oxygen inhibition, the common name denotes the remainder of the monomer first strategy for control of radical for this termination reaction, is a major molecule. As this paper focuses on reactivity—chain transfer using drawback to photopolymerizations crosslinking systems, X generally multifunctional thiols. The use conducted in air.8,9 contains multiple acrylate groups. of nitroxide radicals to reversibly The rate of double-bond conversion occurs when the complex propagating radicals and Propagation (Reaction 3) is of primary importance product of a previous radical addition, improve cure efficiency, and the only in the early stages of cure, prior possibility of thermal re-initiation in a R, encounters and adds to an to gelation. We refer to gelation as the system photocured in the presence of unsaturated group: point at which liquid flow of the solution nitroxides or alkoxyamines is detailed Reaction 3 ceases. After gelation occurs, great in the full paper. R + CH2 =CHX R–CH2– CHX differences in mechanical properties can be evident with only minor Experimental A radical species of greater differences in overall conversion.10 Materials molecular weight is produced, which Thus, instead of monitoring cure All coating solutions were based may continue in further addition kinetics in terms of conversion, we on the monomer, ethoxylated reactions. study the final network properties as a trimethylolpropane triacrylate Alternatively, when a radical function of total exposure to radiation. (EOTMPTA, Photomer 4149, IGM species R1 approaches a growing The strength of a radiation-cured, Resins) (Figure 1). This monomer was or terminated chain R2-H with an acrylate-polymer film depends on the used as received, without removal of accessible hydrogen, abstraction of this rate of vitrification, as characterized by inhibitor. The base stock solution for atom may occur: stronger polymer network formation EB curing consisted of 0.5% (w/w) Reaction 4 at lower exposure. Here vitrification BYK-345 (BYK-Chemie), a proprietary R1 + R2 –H R1 –H + R2 refers to the formation of an infinite leveling agent, in EOTMPTA. polymer network through crosslinking. To prepare the stock coating solution This chain transfer event results in The evolution of this network is for UV application, 2% (w/w) of the 1 2 the termination of R ’s growth, but R hindered by termination reactions such photoinitiator, 1-[4-(2-hydroxyethoxy) may now act as a propagating species. as oxygen inhibition and radical-radical phenyl]-2-hydroxy-2-methyl-1-propan- Radical species are eliminated from recombination. If a significant number 1-one (Irgacure 2959, Ciba, BASF), propagation by a number of rapid of the propagating radical species can 15% (w/w) of a proprietary amine termination reactions. As carbon- be protected from such unproductive acrylate, CN373 (Sartomer Company centered radicals are highly reactive, termination reactions, favorable Inc.), and 0.5% (w/w) BYK-345 were their combination with other radicals network properties can develop with added to EOTMPA. to produce a closed-shell species is shorter irradiation times or with less Thiol-containing stock solutions favored and this reaction is considered input of energy. Through more efficient for UV and EB were prepared with irreversible on the timescale of radiation use of initiating radicals, we can 4% (w/w) pentaerythritol tetra- curing. One form of termination produce the same or even improved 3-mercaptopropionate (PETMP, 34 RADTECH REPORT ISSUE 4 2012 below 200 ppm O2, unless otherwise Figure 1 noted. Applied dose was varied by adjusting beam current at 100 keV, Structure of monomer used in this work—EOTMPTA with a constant web speed of 75 fpm. The parameter of greatest interest Paper Technical from a practical perspective, and the one we measured directly to characterize extent of cure, is the toughness of the final polymerized film. The technique used to ascertain cured film toughness was MEKR. A cotton- tipped swab was dipped in MEK, pressed against the film over the black (inked) portion of the Leneta card, Hampshire Chemical Corporation) Company) using a U.S. #3 Meyer rod and an up-and-down rubbing motion (Figure 2) added to the respective for a constant coating thickness. was performed with a standard, hard base stock solution. An equal mass UV curing was conducted with a pressure against the card. The number of EOTMPTA to the added PETMP single 300 watts-per-inch, medium- of double rubs (one rub downward is removed in the preparation of pressure mercury lamp under air. followed by another back up to the thiol stock solutions, so that the Web speed was varied and the starting position) necessary to break concentrations of other additives exposure was monitored by a UV through the film and reveal the white (photoinitiator, acrylated amine, Power Puck (EIT). The recorded paper beneath the black ink, uniformly leveling agent) do not differ from the exposure is the sum of UV-A, UV-B, over the rubbed area, was recorded base stock solutions. Samples with and UV-C energy densities. as the MEKR resistance for the film intermediate PETMP concentrations An Advanced Electron Beam in question. were prepared by blending the laboratory processor with nitrogen Solvent rubs, in general, test the original stock solutions with thiol- inerting was used for EB curing. For resistance of a cured thin film to containing stock solutions. The PETMP typical sample exposure, the chamber solvent swelling, as well as physical concentrations are reported as mmol was flushed with nitrogen until oxygen strain from the rubbing action. PETMP per kilogram solution. All thiol- concentration was below 200 ppm. A film’s susceptibility to these containing solutions were used for cure To investigate the sensitivity of processes decreases with the extent trials within one week of mixing.