Control of Free-Radical Reactivity in Photopolymerization of Acrylates

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. results for the inerting level, we used of crosslinking. Thus, we used MEKR Methods short inerting times to achieve up as a measure of the global extent of Solutions were applied to unsealed to about 700 ppm O2 at the time of Leneta cards (form N2A, Leneta irradiation. Cures were conducted crosslinking, which develops during the final stages of cure with very small changes in double-bond conversion.10 Figure 2 Cure Enhancement by Structure of tetrafunctional thiol used in this work Chain Transfer —PETMP Thiols are well-known chain transfer agents in radical polymerization.3,11,12 The chain transfer constant, or the ratio of chain transfer rate constant to propagation rate constant, for n-butyl mercaptan in styrene (a typical thiol in a monomer analogous to acrylates) 3 is CS = 21. That is, for a propagating radical, hydrogen abstraction to form thiyl radicals occurs 21 times more rapidly than addition to monomer

ISSUE 4 2012 RADTECH REPORT 35 Propagation of cure by thiyl radicals Figure 3 results in the formation of a large 1 2 number of thioether (R CH2-S-CH2R ) 0.0 mmol/kg PETMP linkages in the crosslinked network.

Technical Paper Technical This is a flexible bond18 that can increase the strain tolerated by the polymer film without breakage. Even more added structural stability can result if multifunctional thiols are used. In this case, thiol molecules can lead to additional crosslinks in the acrylate network once their functional ends connect initially separated chains.

Results We investigated the effects of formulating UV- and EB-curable coating solutions with small amounts double bonds. Such speed is necessary recombination and reaction with of the tetrathiol PETMP. A series of 16,17 in radiation-cured polymerizations oxygen, as compared to their carbon UV-coating formulae with 0.0, 15.3, where consumption of the monomer analogues. Thus, we can expect a 20.5 and 45.8 mmol/kg PETMP was is very rapid due to the high initial higher concentration of thiyl radicals in prepared, coated on Leneta and cured 1 concentration of radicals. the late stages of cure than is possible with UV exposure varying between 146 By including thiol species into with carbon radicals (R). Development and 882 mJ/cm2. MEK double rubs were a formula for radiation curing, we of favorable mechanical properties performed, and the results of these add several reactions to the general is strongly linked to the extent of evaluations are shown in Figures 3-6. scheme presented earlier. Let Y-SH crosslinking, which occurs during this In Figure 3 is the cure response of represent a (possibly multifunctional) period. Hence, the replacement of R a control EOTMPTA coating with no thiol. Any radical that comes into by Y-S will result in a more efficient added thiol. It conforms to the typical contact with the thiol group is likely to use of the radical “potential” built up sigmoid shape. Following the onset of abstract hydrogen: during initiation and, therefore, a more gelation (at which point MEKR can be thoroughly cured film. measured), there is a “gel” plateau at Reaction 7  I, A, RCH2 CHX, ROO + Y–SH  IH, AH, RCH CH X, ROOH + Y–S 2 2 Figure 4 Thus, normally unproductive 15.3 mmol/kg PETMP radicals such as A and ROO are replaced by thiyl radicals (Y-S), “soft” radical species which efficiently add to acrylate double bonds.13,14 Chain transfer between carbon radicals (Reaction 4) can lead to sterically hindered radical sites which cannot add to acrylate double bonds.15 The conversion of these radicals to thiyl species through hydrogen abstraction can be viewed as a restoration of their propagating ability. The thiyl radicals formed during cure are resistant to both

36 RADTECH REPORT ISSUE 4 2012 6.1 mmol/kg PETMP. These were Figure 5 applied to Leneta cards and cured with EB dose varying from 10 kGy to 20.5 mmol/kg PETMP 35 kGy. MEKR results for these cured

films are presented in Figures 7-10. Paper Technical These EB cure response curves mirror the ones obtained for the UV-cured system—the character of the curve and the MEKR values obtained depends on the amount of PETMP added. Figure 7 is a control sample with no PETMP. The vitrified plateau begins at a dose of about 25 kGy, and 11 MEKR are obtained in this final regime. In the cure response plots Figures 8 and 9, there are peaked upper plateau regions. The doses for this peak range from 22 to 28 kGy low exposure with lower MEKR values. Figure 3. However, while the lower and 22 to 33 kGY, respectively, and Once the critical dose is approached plateau region is at similar MEKR about 15 MEKR are obtained. Once for the onset of vitrification, mechanical values between the two, the control 6.1 mmol/kg PETMP is added, as properties develop rapidly with curve (Figure 3) provides 33 MEKR at in Figure 10, the sigmoid shape is exposure. The plot ends with the the upper plateau but the high PETMP restored. Additionally, a much greater upper (or “vitrified”) plateau at high exposure. The plots at intermediate Figure 6 PETMP concentration in Figures 4 and 5 show a peak in MEKR values, 45.8 mmol/kg PETMP which begins after 500 mJ/cm2 UV exposure. Note that the points following the peak exposure domain belong to the unmodified upper plateau region as detected in Figure 3. Increasing the concentration of PETMP from 15.3 mmol/kg in Figure 4 to 20.5 mmol/kg in Figure 5 results in an increase in the height of this peak, from 42 to 51 MEKR. Furthermore, the length of the peak region (range of UV exposure values corresponding to large MEKR) increases in the direction of higher irradiation with greater concentrations of PETMP. While curve (Figure 6) has an upper plateau vitrified plateau MEKR value, over 20 600 mJ/cm2 gives only 30 MEKR in at 50 MEKR. Thus, hugely improved (double that of the control), is Figure 4, this exposure corresponds mechanical properties are obtained at obtained for these cure conditions. to the maximum MEKR value of 51 in the vitrified plateau with a sufficient Figure 5. formulated level of PETMP. Colloidal Model for Acrylate Finally, as shown in Figure 6, a An analogous experiment was Polymer Growth concentration of 45.8 mmol/kg PETMP conducted with EB curing. We To explain the shape of the cure results in a cure response curve that prepared a series of EB coating response curves for UV- and EB- has returned to the sigmoid shape of formulae containing 0.0, 2.0, 4.1 and coating solutions (as the concentration

ISSUE 4 2012 RADTECH REPORT 37 constant of thiols, it is reasonable Figure 7 to assume that carbon radicals are converted to thiyl radicals as rapidly 0.0 mmol/kg PETMP as they form. Therefore, the early

Technical Paper Technical stages of cure (building of colloidal domains of active propagation) always involve thiyl addition to C=C bonds. However, with greater applied doses of radiation, the number of initiating radicals increases. The final stage of cure (vitrification through the fusion of colloidal domains) will take place predominantly by the propagation mechanism corresponding to the radical species of greatest concentration. If the number of available thiol groups exceeds the number of

of PETMP is varied) we introduce Figure 8 a qualitative, two-stage colloidal model for polymer growth during 2.0 mmol/kg PETMP radiation cure. In the first stage of curing, radicals are formed through initiation processes and begin to propagate by adding to the plentiful monomer double bonds. These growing chains are distributed stochastically throughout the liquid monomer solution. Chain addition propagates outward from these sites, radiating into the free surrounding monomer. Once most of the eventual double- bond conversion is complete, the film will contain tightly crosslinked, highly branched domains (corresponding to the beginning sites of initiation) (at low MEKR) to vitrified plateau initiating radicals, the final stages separated by a more weakly connected (at high MEKR) corresponds to the of cure will also be dominated by polymer with fewer crosslinks. fusion of colloidal domains into a more the thioether-forming addition of 19 While there are domains of dense, rigid global network. We find that Y-S to CH2=CHX. However, if the nearly infinite molecular weight the propagation chemistry occurring concentration of R (carbon centered polymer of high modulus, these are during this transition is key to radical) is greater than Y-S at the end connected by a much lower modulus determining the polymer strength. of cure, the more rigidly crosslinked material. The low MEKR obtained As evidenced by the sustained high all-acrylate polymer will be the at the early, gel plateau in a cure MEKR values at the vitrified plateau material which solidifies the overall response curve corresponds to the in Figures 6 and 10, crosslinked network. MEKR results show this material failing at the weak regions polymer produced in the presence of brittle linking material to be much connecting the colloidal, spherical multifunctional thiols is stronger than less rub-resistant than the flexible domains of high crosslink density. that produced without a thiol. Due thioether-containing polymer produced The transition between gel plateau to the extremely high chain transfer through thiyl addition.

38 RADTECH REPORT ISSUE 4 2012 associate a number [SH] of thiol Figure 9 groups such that the given exposure is the crossover point between peak 4.1 mmol/kg PETMP and low plateau MEKR values. This

concentration of thiol groups is not Paper Technical expected to equal the total number of radicals formed at initiation. Rather, it is the number of propagating radicals contributing to the development of mechanical properties and available for chain transfer to yield thiyl radicals. The crossover points for the UV-cured data series in Figures 3-6 were calculated by taking the midpoint exposure between the end of the peak region and the start of the unmodified upper plateau region. These crossover exposure values are given in Table 1. We could only determine a lower Figure 10 bound for the crossover point with [SH] = 183.2 mmol/kg, as the (thiyl 6.1 mmol/kg PETMP addition) peak domain extends throughout all the vitrified plateau points. That is, no crossover is observed. Analogous calculations were performed for the EB-cured series in Figures 7-10 and can be found in Table 2. Again, only a lower bound for the crossover dose with 24.4 mmol/ kg thiol groups could be found in EB. By varying [SH] and determining the crossover point in this way, we titrate the number of radicals formed at each measured crossover exposure. A plot of MEKR at a fixed exposure against [SH] will have the form of a titration curve.

Thiol Groups Titrate Radicals Table 1 Based on the colloidal model of acrylate photopolymerization, we have Crossover exposure values for UV-cured EOTMPTA inferred a quantitative link between coatings with PETMP the total concentration of thiol groups The crossover is obtained by linear interpolation in the abscissa of [SH] and the exposure at which the the cure response curve, between the final peaked point and the rate of carbon radical propagation first nonpeaked plateau point. exceeds the rate of thiyl radical [PETMP] (mmol/kg) [SH] (mmol/kg) Exposure (mJ/cm2) propagation. For convenience, we refer 15.3 61.2 574 to this as the crossover point for the 20.5 82.0 635 given SH concentration. Equivalently, 45.8 183.2 >882 for each level of radiation exposure within the vitrified plateau, we can

ISSUE 4 2012 RADTECH REPORT 39 competition between thiyl and acrylate Table 2 addition, we were able to estimate that seven times more radicals are produced Crossover dose values for EB-cured EOTMPTA in UV cure than in the EB process.

Technical Paper Technical coatings with PETMP In the full paper, we make use of Crossover is defined and calculated in the same manner as for UV the colloidal model developed here to results in Table 1. explain the results of two additional [PETMP] (mmol/kg) [SH] (mmol/kg) Dose (kGy) investigations into the control of free- 2.0 8.0 28.8 radical reactivity. 4.1 16.4 33.8 Within a narrow range of 6.1 24.4 > 35.0 concentrations, stable nitroxide radicals afford benefits in cure efficiency, as measured by a lowering of the onset By linear interpolation of the to the 70 mmol/kg concentration of point or dose required for the beginning crossover data, we found that a propagating radicals, as measured by of vitrification, along with an increase 600 mJ/cm2 UV exposure required a titration with thiol groups, shows that in the eventual MEKR obtained. The thiol group concentration of 70 mmol/kg fewer radicals than the theoretical reversible complexation of radicals (in for equivalence with propagating maximum actually contribute to cure. the form of alkoxyamines) grants them radicals. For an EB dose of 30 kGy, an effectively longer lifetime. Radicals the necessary value of [SH] was Conclusions that would be terminated in the early 10 mmol/kg. Hence, we have measured The photoinitiated free-radical stages of cure are complexed rapidly; the ratio of propagating radicals in polymerization of acrylates in modern and these complexes dissociate UV curing to that in EB as 7-to-1. applications requires extremely rapid during late cure; when the total The fact that far fewer radicals are conversion using high concentrations radical concentration has been greatly produced in EB cure, as compared of initiating radicals. The molecular reduced and a small increase in radical to UV, is not surprising as there are architecture of the polymer generated concentration leads to a much greater clearly pronounced differences in the is, thus, largely uncontrolled. As extent of vitrification. mechanical properties of UV- and EB- reported in the full paper, we have It was found that the presence cured coatings. The UV control cure explored several distinct means of alkoxyamines during photocure response in Figure 3 has a vitrified of altering photopolymerization (generated in situ from nitroxides or plateau with more than 30 MEKR, dynamics and, hence, final mechanical added at the time of formulation) leads while the EB control curve (Figure 7) properties, through the intelligent to what can be called stored radical shows only 12 MEKR at this plateau. control of free-radical reactivity. potential. Post-heating of a cured Formulated concentration of Partial replacement of carbon- film that contains these materials photoinitiator in our UV-cured samples centered radicals with sulfur-centered shows a significant increase in MEKR. was 2.0 % (w/w) Irgacure 2959 or thiyl radicals (from chain transfer The apparent activation energies of [PI] = 89.17 mmol/kg. The published to a thiol) results in a propagating post-curing; and the dependence on quantum yield for dissociation, Φ, of species that is resistant to radical- oxygen concentration during cure; Irgacure photoinitiators ranges from radical recombination and oxygen suggests that oxidized species, rather 0.2 to 0.6.20–22 Thus, using Φ = 0.6, the inhibition. Furthermore, the thioether than alkoxyamines themselves, are maximum concentration of radical bonds produced in the cured film add responsible for releasing radicals pairs produced is: strength and flexibility. during thermal initiation. w Competition between this thiyl Reaction 8 addition and standard acrylate Φ  [PI] = 53.5 mmol/kg. Acknowledgements propagation provided an interesting Patrick Kramer acknowledges the Doubling this value, since there are means of probing the growth of cured support of Sun Chemical Corporation two free radicals originating from domains through space and led to for employment through the summer each radical pair, we find the predicted the development of a colloidal model internship program in 2009, 2010 maximum concentration of initiating for acrylate polymerization. Based and 2011. radicals to be 107 mmol/kg. Comparison on this model and the data showing

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