Ind. Eng. Chem. Res. 1999, 38, 3353-3359
3353
Kinetics of Photopolymerization of Acrylates with Functionality of 1-6 Igor V. Khudyakov,* Jodi C. Legg, Michael B. Purvis, and Bob J. Overton Alcatel Telecommunications Cable, Claremont, North Carolina 28610
The photopolymerization of a number of neat acrylate monomers with acrylate functionality from 1-6 was studied with photoDSC (DSC ) differential scanning calorimetry) and with a cure monitor using a fluorescent probe. PhotoDSC results show that conversion of monomers ranges from 40 to 100%, depending upon the functionality and structure of a monomer. Kinetics of a heat flow was well described as an autoaccelearted reaction for all monomers. Kinetics of the hardening of a sample under light at an ambient temperature was nicely fit into the twoexponential law; rate constants k1 and k2 in this empirical analysis were in the range of 0.5-35 min-1. The effect of an inhibitor and of air oxygen on kinetics of photopolymerization was also studied. The dependence of the rate of polymerization and conversion upon the functionality of a monomer is discussed. Introduction The photopolymerization of acrylates has been intensively studied during the past 2 decades; see for review refs 1-4. There is a permanent interest in polymerization of vinyl monomers (oligomers) from the standpoint of polymer chemistry and physics. Photopolymerization has an increasing relevance in industrial applications such as UV-cure of acrylate coatings. Acrylate urethane protective coatings are widely used in the manufacture of optical fiber.5,6 The present work is devoted to kinetics of photopolymerization of acrylates of different functionality (from 1-6). Most of these materials are widely used in the coating industry. Acrylate protective coatings are blends of different monomers or oligomers, photoinitator(s), and some additives. Usually no solvent is added to coatings, which are 100% on solids. Highly viscous mixtures are diluted by reactive diluents, which are acylates as well. UV-cure of such coatings is not accompanied by the release of a solvent or any other volatile organic compounds (VOC). Polymerization of acrylate coatings results in their hardening, or vitrification. Evidently, homopolymerization to high degrees of conversion is a very complex process. The kinetics of photopolymerization has been extensively studied with differential scanning calorimetry (DSC),7-18 real-time IR,3,4,7 and fluorescent probes.19-21 A material which is initially a (viscous) liquid undergoes conversion into a solid polymer. Formal kinetic treatment of polymerization is hardly possible, and the attempts of rigorous analysis of polymerization are promising but they are impractical so far; cf. e.g., ref 22. Most of the publications on homopolymerization present kinetic curves without even an attempt of kinetic treatment in a wide span of conversion ξ. At the same time, modeling of polymerization is promising for the quantitative analysis of polymerization in different reactors.2 The goal of the present work * To whom correspondence should be addressed. Phone: 828-459-8526. Fax: 828-459-9346. E-mail igor.khudyakov@ cable.alcatel.com.
was to find a practical relation of ξ ) ξ(t) and to study a dependence of the rate of polymerization and final conversion upon functionality (N) in a series of monomers of similar structure. The same solutions were studied with photoDSC and with a fluorescent probe. Experimental Section Reagents and Solutions. Acrylate monomers (oligomers) were received from Aldrich, Sartomer, and UCB Radcure. The manufacturer adds to all acrylates 100500 ppm of the inhibitor 4-methoxyphenol (MEHQ) or hydroquinone (HQ); the exact amount can be found in catalogues. Isobornyl acrylate (IBOA) and 2-hydroxyethyl acrylate (2-HEA) were from Aldrich. 1,6-Hexanediol diacrylate (abbreviated as HDDA) was either from Aldrich (designated here as HDDAa) or of Sartomer, viz., SR238 (which is designated here as HDDAs). Other reagents include Sartomer SR351, which is 1,1,1-trimethylolpropane triacrylate (abbreviated as TMPTA), SR355, which is 1,1,1′,1′-tetramethyloldipropyl ether tetraacrylate (abbreviated as DTMPTA), and SR399, which is (1,1-(diacryloyloxymethyl)-1-(hydroxymethyl))methyl (2′,2′,2′-triacryloyloxymethyl)ethyl ether or dipentaerythritol pentaacrylate (abbreviated as DPEPA). Chart 1 presents the chemical structures of acrylates. Table 1 lists the monomers and one oligomer, their functionality, and their molecular weight (MW). We used two UCB materials, namely, Ebecryl CL 1039, which is a urethane monoacrylate (abbreviated here as EB1039), and Ebecryl 8301, which is a hexafunctional aliphatic urethane acrylate oligomer (abbreviated here as EB8301). The manufacturer does not present chemical structures of EB1039 and EB8301.23 We will use the term “monomer” below to denote a monomer and EB8301. Monomers were purified by passing them through 20 cm of a column filled with basic alumina.8,9,24 The lack of an inhibitor (MEHQ or HQ) in purified materials was confirmed with UV spectroscopy.24 Photoinitiator Darocur 4265 (Ciba Specialties) was used as received. Darocur is a liquid at room temperature and a 50:50 wt % mixture of (2,4,6-trimethylben-
10.1021/ie990306i CCC: $18.00 © 1999 American Chemical Society Published on Web 07/30/1999
3354 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 Chart 1
zoyl)diphenylphosphine oxide and 2-hydroxy-2-methyl1-phenylpropan-1-one. A fluorescent probe was 4-(dimethylamino)-4′-nitrostilbene (DMANS) from Acros. All solutions contained 3 wt % of a photoinitiator and (25) × 10-4% of DMANS. In some experiments we used 2,6-di-tert-butyl-4-methylphenol (ionol) from Aldrich as an inhibitor of free-radical polymerization. Techniques. (a) DSC. We used a DuPont 930 differential photocalorimeter (TA Instruments). The radiation source was a 200 W medium-pressure mercury lamp. Samples were continuously irradiated with the full light of the lamp during 1 min at 313 K. DSC traces or thermograms were recorded under isothermal conditions. Results were analyzed using DPC Standard Data Analysis software V4.1. Samples had a mass of 0.91.3 mg, and they were weighted in air. Samples were placed into a standard aluminum DSC pan, and the reference aluminum pan was empty. PhotoDSC experiments were run either in the air or in inert atmosphere. In the latter case samples were flushed with nitrogen or argon (50 mm3/min) for 15 min prior to irradiation and during the irradiation of a sample. The measured heat Q of the polymerization of a monomer (Table 1) was an average value of the measurements of threefive samples. (b) Spectrofluorimeter. We used a CM 1000 Cure Monitor of Spectra Group Ltd. The basic principles of the method are described in refs 19 and 20 and in the manual for the device. Experiments were run at an ambient temperature, and the temperature of a sample was not controlled. Polymerization was initiated by excitation light of the spectrofluorimeter, which was selected as λ 380 nm in all experiments. (This is not the best wavelength to induce fluorescence of DMANS, but irradiation at λ ) 380 nm allows one to initiate polymerization). In a number of cases we monitored the
dark polymerization (or “postpolymerization”, “postcure”, or “after cure”) of a monomer. In such cases a limb of a monochromator was in a few seconds turned in such a way that visible light was emitted (λ ) 430-470 nm). Visible light does not initiate polymerization, but it does induce the fluorescence of DMANS and allows one to monitor postcure. To monitor photopolymerization of monomers, samples were prepared as recommended in the device manual. Namely, a “resin sandwich” was made between two standard microscope slides. Adhesive tape of a thickness of approximately 0.1 mm was used as a spacer. Slides were pressed to each other with spring clips. Samples were prepared in the air or under an intensive flush of nitrogen or argon prior to laminating a sample between glass plates. We will be naming polymerization occurring between slides as polymerization in the air, in the nitrogen, or in the argon atmosphere, respectively. The thickness of a polymerizing layer is large enough in order to consider polymerization as bulk polymerization. In some experiments quartz slides of the same size as microscope slides were used. Cured or polymerized samples, which are necessary for selection of the most sensitive wavelengths for monitoring fluorescence, were prepared by cure with an Iwasaki processor of liquid samples between glass slides. A cured sample demonstrates a blue-shifted emission maximum of DMANS. The head of a cure monitor was positioned over the sample in the same way in all experiments in order to keep the incident light intensity constant. The device monitors variation of fluorescence intensity ratio at two wavelengths in real time. The monitor demonstrates a cure profile, i.e., an intensity ratio or degree of cure measured in arbitrary units vs time. Kinetic data were retrieved as ASCII files and were analyzed with Igor 3.12 software. Three to five experiments per each composition were run. Results DSC. Figure 1 demonstrates a typical DSC trace. Table 1 presents heat Q released during the photopolymerization of different monomers. DSC data analysis software suggests a convenient treatment of experimental data in terms of autocatalytic or autoaccelerated reaction:
-dC/dt ) kauCm(1 - C)n
(1)
Here, C is the relative amount of the remaining monomer, and C ) 1 - ξ. The dimension of kau is 1/t. Heat flow (cf. Figure 1) is usually considered in DSC experiments as a rate of polymerization (eq 1).13 It is assumed that the total Q released during polymerization corresponds to ξ ) 1.0. Software prompts one to select certain limits of applicability of eq 1. We simulated DSC traces within limits ξ ) 0.05-0.95. All DSC traces were well fitted assuming the validity of eq 1; cf. Figure 1 as an example. The “best” values of kau, m, and n are presented in Table 1. Spectrofluorimetry. Figures 2-6 show kinetic plots of photopolymerization of different monomers. Kinetics of polymerization is the kinetics of multiple bond consumption. Strictly speaking, the cure monitor measures the kinetics of the hardening of a sample. We use the common assumption that the kinetics of hardening is very similar or identical to the kinetics of
Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3355 Table 1. Thermodynamic and Kinetic Parameters of Photopolymerization of Acrylates of Different Functionalitya monomer
N
MW
IBOA IBOA 2-HEA 2-HEA EB1039 EB1039 EB1039 EB1039 EB1039 HDDAs HDDAs HDDAs HDDAs HDDAa HDDAa HDDAa HDDAad HDDAad TMPTA TMPTA TMPTA TMPTA DTMPTA DTMPTA DTMPTA DTMPTA DPEPA DPEPA DPEPA DPEPA DPEPA EB8301 EB8301 EB8301 EB8301 EB8301
1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 3 3 3 3 4 4 4 4 5 5 5 5 5 6 6 6 6 6
208 208 116 116 215 215 215 215 215 226 226 226 226 226 226 226 226 226 296 296 296 296 438 438 438 438 524 524 524 524 524 1100 1100 1100 1100 1100
purified
inert atmosphere N2 N2 Ar N2
Y Y
N2 N2
Y Y
N2 N2 Ar N2 N2
Y Y
N2 N2
Y Y
N2 N2
Y Y Y
N2 Ar N2 Ar
Y Y
N2
Q, J/g
Qtheor,b J/g
ξmax × 100
kau, min-1
m
n
280 285 525 570 380
385 385 690 690 372 372 372 372 372 708 708 708 708 708 708 708 708 708 810 810 810 810 730 730 730 730 763 763 763 763 763 436 436 436 436 436
73 74 76 82 100c
33 24 25 30 26
0.53 0.43 0.45 0.55 0.42
1.66 1.21 1.12 1.26 1.31
94 96 95 76 79 74 85 77 81 69 65 69 57 52 56 51 51 51 52 50 41 41 41 41
29 27 29 26 28 25 28 25 27 25 26 27 24 25 24 23 24 26 25 27 18 19 22 25
0.44 0.41 0.42 0.42 0.47 0.38 0.45 0.40 0.43 0.38 0.38 0.38 0.38 0.43 0.39 0.38 0.38 0.40 0.39 0.35 0.38 0.40 0.37 0.42
1.35 1.34 1.32 1.25 1.26 1.26 1.25 1.26 1.26 1.26 1.26 1.30 1.37 1.43 1.38 1.35 1.45 1.43 1.44 1.29 1.45 1.43 1.41 1.39
f f
24 23
0.41 0.39
1.53 1.52
f f
25 26
0.42 0.41
1.54 1.46
350 360 355 540 565 525 605 545 575 488 462 493 465 420 450 415 375 375 380 365 315 315 316 313 350 350 352 348
k1, min-1
k2, min-1
3.5 2.5
0.35 0.22
15 24 11 19 20 4.0 6.0 4.5 3.7 4.7 4.3 4.7 5.2e
2.0 23 1.2 19 19 0.4 0.3 0.5 0.5 0.5 0.8 0.8 0.8e
4.0 6.8 23 28 5.0 7.0 23 32 2.5 2.9
0.6 0.8 5.6 3.0 0.5 0.6 5.5 3.5 0.5 0.5
4.4 3.1 4.8 2.8 4.8 2.5 2.6
0.5 0.3 0.6 0.3 1.3 1.0 1.2
a Determination errors are as follows: Q, 10%; k , (1 min-1; m, (0.02; n, (0.02; k b au 1(2), 10%. Qtheor was calculated according to eq 4. Most probably ξmax of EB1039 should be accepted as 1.0, if we take into account an uncertainty in estimation of Qtheor and a determination error of Q. d Monomer contained 0.04 M of ionol. e The first 10 s of reaction were excluded from simulation. f An estimation is not possible because EB8301 has acrylate polyol of lower functionality in its formulation.23
c
Figure 1. DCS trace of photopolymerization of EB1039 in the nitrogen atmosphere. Irradiation lasted from 0 to 180 s. Insert: linear fit of eq 1 to the experimental data.
Figure 2. Kinetics of photopolymerization of EB1039 in the nitrogen atmosphere (solid line) and a fit of eq 3 to the experimental data (dashed line).
polymerization.19,20 The kinetics of hardening is even more important than the kinetics of double bond consumption from a practical standpoint of the cure of coatings. Many curves have an initial period of relatively slow reaction which lasts from several seconds to ∼10 s (“induction period”25); cf. Figures 3-5. We added ionol to HDDAa in a high concentration of 0.04 M or 8000
ppm. Figure 6 demonstrates the kinetic curve of polymerization of HDDAa in the presence of ionol. An evident induction period of ∼10 s is observed due to the presence of an inhibitor of free-radical polymerization.26,27 There was no noticeable difference in polymerization rates of the same monomers prepared with quartz or glass slides.
3356 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999
Figure 3. Kinetics of polymerization of HDDAa in air. Photoinitiation of polymerization lasts during the following: (a) whole period of measurements (solid line); (b) during first ∼170 s. Termination of photoinitiation and the beginning of monitoring of dark polymerization are marked with arrows. Dashed line represents a fit of eq 3 to the experimental data (a).
Figure 5. Kinetics of polymerization of EB8301 in air. Photoinitiation of polymerization lasts during the following: (a) whole period of measurements (solid line); (b) the first ∼25 s. Termination of photoinitiation and the beginning of monitoring of dark polymerization are marked with arrows. Dashed line represents a fit of eq 3 to the experimental data (a).
Figure 4. Kinetics of polymerization of IBOA in air. Photoinitiation of polymerization lasts during the following: (a) whole period of measurements; (b) first ∼25 s. Termination of photoinitiation and the beginning of monitoring of dark polymerization are marked with arrows.
Figure 6. Kinetics of polymerization of HDDA in air in the absence of ionol in the presence of 0.04 M of ionol (solid line). Dashed line represents a fit of eq 3 to the experimental data; the first 10 s of polymerization were excluded from the simulation.
The hardening of a sample during polymerization of 2-HEA was not enough to get reliable kinetic traces. This observation may be related to a relatively low Tg ) 258 K of the corresponding polymer. Discussion 1. Formal Kinetics Approach. The following wellknown equation is often used as a start for description of the chain reaction of polymerization:1-4,26
ξ/ξmax ) 1- exp(-kpolt)
(2)
Here, the final conversion of a monomer into polymer ξmax ) 1.0, and the rate constant of polymerization kpol
) kp[ωin/(2kt)]1/2, where ωin is the rate of (photo)initiation of polymerization, 2kt is the rate constant of bimolecular termination of macroradicals, and kp is the rate constant of propagation of polymerization. The derivation of eq 2 implies that polymerization occurs under steady-state conditions, and the initial (ξ ≈ 0) and the final (ξ ≈ 1) stages of reactions, i.e., the onset and decay of steadystate regime, are ignored. Steady-state conditions mean that reaction is characterized by long kinetic chains (ν .1) and by a duration of polymerization of t . [2ktωin]-1/2. ωin should be constant during polymerization or allowed to vary slowly compared to the time of onset of steady-state conditions. Application of eq 2 to real systems requires the fulfillment of other assumptions: kp and kt do not depend on the mass (length) of a radical and do not
Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3357
depend on ξ, bimolecular reactions between macroradicals are the only way of chain decay, macroradicals do not interact with radicals of the initiator, and a polymer chain may have only one radical center during polymerization. Dramatic changes of the properties of media take place in our systems. Polymerization is accompanied by the formation of a polymeric network, especially in the case of multifunctional acrylates, by the shrinkage of volume, and by the hardening of a sample.3,4,15,19 Polymerization at conversions of a monomer of ξ ) 0.2-0.4 is often accompanied by acceleration. This is due to the onset of a gel or the Trommsdorff-Norrish effect, a phenomenon recognized for 60 years.28 Polymerization decelerates at later stages due to the vitrification of a sample and due to the decrease of mobility (and thus, reactivity) of all components in a sample.29 The rate of photoinitiation decreases with the increase in viscosity of media as well.30 The situation becomes more complex in vitrified media, because elementary reactions in a solid polymer follow more complicated kinetic laws compared to reactions in the liquid phase.31 2. Empirical Kinetic Relations. Evidently one does not expect that the kinetics of homopolymerization, especially of a multifunctional monomer, will be described by eq 2 with a “constant constant” kpol within a large span of conversion. An ascending S-shape kinetic curve reaching a plateau is frequently observed in the experiment (cf. Figures 3-5 and figures presented in review articles3,4). PhotoDSC kinetic measurements are well described as autoaccelerated reactions; cf. eq 1 and Figure 1. The conditions of photoDSC experiments are isothermal. Autoacceleration occurs even in isothermal conditions due to the hindrance of termination between macroradicals;28 cf. section 1 above. Equation 1 does not account for the expected deceleration at high conversions; however, three fitting parameters of eq 1 allow one to get a good fit within selected limits of ξ. The initial part of the S-shape kinetic curve of slow polymerization is not necessarily an induction period of a chain reaction;26,27 see section 4 below. Almost all kinetic curves obtained in the present work with a cure monitor were nicely fitted into a twoexponential kinetic relation:
ξ/ξmax ) 1 - A1 exp(-k1t) - A2 exp(-k2t)
(3)
where Ai and ki were fitting parameters. Curves fitting results in the values of A1 + A2 ≈ 1.0 in accordance with expectations. The “best” values of k1 and k2 are presented in the Table 1. Polymerization was observed during 100-300 s; k1(2) (Table 1) are average values obtained by the fitting of three to five kinetic curves. A slightly worse fit was obtained for S-shape curves, or curves with an “induction period”; cf. Figures 3a and 5a.25 Equation 3 fits the experimental data better if the first initial 5-10 s out of 100-300 s of a kinetic curve are ignored. Equation 3 is an empirical relation. We observed that fitted k1(2) values decrease slightly with an increase of the time of observation of polymerization. Photopolymerization curves reach a plateau in 10-20 min after the beginning of irradiation, or polymerization proceeds with an extremely low rate for ca. 1 h in our experimental conditions. Figures 3-5 demonstrate that dark photopolymerization of UV-irradiated monomers occurs. The rate of dark polymerization is lower than the rate
of photopolymerization of the same monomer at the same time t after the start of a reaction (cf. Figures 3-5). Dark polymerization can last for a very long time with a very low rate.15 It was shown with solid-state NMR that dark polymerization of certain multifunctional acrylates results in an additional ∼2% conversion of a monomer during 1 week at room temperature.15 Usually the storage of a UV-irradiated sample of a multifunctional acrylate at elevated temperatures results in faster dark polymerization.9,16 Kinetics of polymerization was well described even by one exponent in a number of cases studied in the present work. These are the results of data fitting with close k1 and k2 values; cf. an example of EB1039 in Table 1. In such curious cases eq 2 holds true, but the validity of eq 2 does not have much physical meaning. There is no correlation between kau and k1(2). Obviously, the faster the polymerization, the higher are k1 and k2. A similar statement does not hold true for kau due to variation of two other fitting parameters m and n under simulation of kinetic curves (cf. Table 1). We will also add that kau and k1(2) refer to different temperatures: kau is obtained at 313 K, whereas samples studied by a cure monitor are initially at room temperature. The temperature of a thin sample in a cure monitor probably increases slightly in the course of polymerization. We did not observe a correlation between the rate of polymerization reflected by k1(2) and the functionality of a monomer N. We noted that EB1039 and DTMPTA have the fastest rates of polymerization among monofunctional and multifunctional acrylates, respectively; cf. Table 1. 3. Conversion of a Monomer. kau values alone do not allow the comparison of the reactivity of different monomers. At the same time, photoDSC experiments allow us to estimate the value of a monomer conversion ξmax, whereas simulation of kinetic curves with eq 3 does not lead to that value. It is known that complete polymerization of 1 mol of acrylate results in a release of QA of ca. 80 kJ of heat.7,9,13 We will use the value of QA ) 80 kJ/mol in our estimations. Another assumption is that complete polymerization (ξmax ) 1.0) of acrylate of functionality N results in the release of 80N kJ of heat.33 The expected maximum or theoretical value Qtheor of heat released under complete polymerization of 1 g of a multifunctional acrylate is
Qtheor ) NQA/(MW) ) 80000 N/(MW)
J/g (4)
The values of Qtheor for our monomers are presented in Table 1.34 A ratio of Q determined in DSC experiments (Table 1) to Qtheor can be considered to be ξmax (Table 1). It was found that Q only slightly depends on temperature,10 and estimations of ξmax at 313 K (Table 1) can be used for polymerization of monomers at an ambient temperature studied by a cure monitor. ξmax values vary from 0.4 (N ) 5) to 1.0 (N ) 1; EB1039); cf. Table 1. These results corroborate the observations of other papers that ξmax for multifunctional acrylates is essentially less than 1.0.3,4,8-10,14-19 In a series of similarly structured monomers HDDA, TMPTA, DTMPTA, DPEPA (Chart 1), ξmax decreases with an increase of N from 2 to 5; cf. Table 1. It was observed that ξmax depends on the functionality and structure of a monomer, relative rates of polymerization and volume contraction, and Tg of a poly-
3358 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999
mer.15,16,18 In a series of similarly structured monomers, the lower Tg, the higher is ξmax.18 The properties of samples vitrified under irradiation by a cure monitor will be reported, as well as the room temperature IR study of polymerization of monomers used in this work. 4. Effects of Air Oxygen and Inhibitor. In our experiments run with photoDSC and a cure monitor we did not observe any reliable effect of air oxygen vs inert atmosphere and an effect of purification of a monomer. Purification was the removal of MEHQ (HQ) and most likely the removal of impurities. Table 1 demonstrates that the values of Q and k1(2) do not depend on conditions of reaction mentioned above. In a couple of cases (EB1039 and EB8301, Table 1), we observed faster reactions in the argon atmosphere than in the air. The common expectation is that in an inert atmosphere the constants k1(2) and the amount of heat released in polymerization Q would be higher than the corresponding values for air. As follows from the data of Table 1, this is not always the case. We do not have a consistent explanation of this important experimental fact at present. We have started experiments on photopolymerization in different atmospheres with real-time IR. One possible effect of air oxygen in photopolymerization is a quenching of the excited state of a photoinitiator. However, photoinitiators used in this work (cf. Experimental Section) have very short-lived excited states, and the air oxygen should not affect ωin.37 The role of air oxygen in free-radical polymerization in solution is well-understood: poorly reactive in propagation of polymerization peroxyl radicals are formed in the presence of air, and polymerization occurs slower that expected, characterized by an induction period τind. During τind a consumption of molecular oxygen takes place. Peroxyl radicals are formed in a reaction of alkyl radicals of a monomer or macroradicals. Such antioxidants as MEHQ, HQ, or ionol behave as effective inhibitors of dark polymerization in the presence of air oxygen. Antioxidants interact with peroxyl radicals and terminate kinetic chains.26,35 However, we observed S-shape photopolymerization kinetic cures with IBOA and HDDA (both of Aldrich), both in the presence and the absence of an inhibitor or air oxygen. The reason for such a poor sensitivity to air oxygen may be tentatively assigned to a rapid consumption of dissolved oxygen during the initial stage of photopolymerization, a rapid growth of viscosity of a sample, and a deceleration of diffusion of oxygen to the reaction zone. Dissolution of a large amount of ionol in HDDAa in the presence of air oxygen results in a deceleration of the initial period of the reaction (10 s); cf. Figures 6 and 3. The following simple formula allows us to estimate the rate of initiation ωin of polymerization at low conversions:26,27
ωinτind ) fCion
(5)
Here f is the stoichiometric coefficient of inhibition, which is approximately 2 for phenols,26,27 Cion is the concentration of dissolved ionol. The straightforward application of eq 5 to our experimental results (Figure 6) leads to enormously high ωin ) 8 × 10-3 M/s.36 Such a huge rate of photoinitiation with the monochromatic light of a 75 W lamp of a cure monitor cannot be achieved.36 The obtained result means that eq 5 is not
applicable, and that only a small fraction of ionol was consumed in the area of photoirradiation and in the close vicinity to this area. We consider homopolymerization of acrylates, i.e., the situation where acrylate groups have a concentration of 1-10 M. Free radicals of an initiator and macroradicals have a higher rate of reaction with the surrounding acrylate groups rather than with ionol in a hardening sample. The extent to which homopolymerization is sensitive to oxygen and an inhibitor depends on many factors: the structure of a monomer, the solubility of oxygen in a monomer, the concentration of an initiator, the rate constants of reactions, and the rate of vitrification of a sample. We did not observe any induction period in our DSC experiments. Polymerization starts immediately at the beginning of irradiation at 313 K; cf. Figure 1. (The time resolution of the device is 0.2 s.) A photoDSC experiment with ionol in the air atmosphere results in a slightly lower Q value for HDDA with ionol compared to other experiments with HDDA, but that difference does not exceed the determination error of Q (Table 1). After the consumption of a fraction of ionol, the photopolymerization of HDDA occurs with the same rate as that without the inhibitor; cf. Table 1. Such a performance is expected for inhibited chain reactions.26,27 We have presented above a tentative explanation of our observations of inhibitor and air oxygen effects in photopolymerization of acrylates. There are interesting reports in current literature on the absence of the effect of air oxygen on acrylate polymerization.11,12 Estimations of ref 12 lead to the conclusion that the concentration of oxygen drops several orders of magnitude during the first 0.1 s of polymerization. It was mentioned above that many kinetic curves of photopolymerization have an S-shape; cf. Figures 3-5 and refs 4 and 5. It is not evident if the initial period of polymerization, for example, the time of consumption of 1% of a monomer, is an induction period of a chain reaction. Carefully purified monomers in an inert atmosphere also often have S-shape kinetic curves and such a curve may be quite expectable for a given monomer with rate constants of elementary reactions, which strongly depend on ξ. At the same time we cannot ignore a possible reason that in some cases a cure monitor does not measure an initial small change of viscosity (polarity) of a polymerizing monomer. Conclusions The photopolymerization of eight monomers was studied with photoDSC and with a cure monitor. The kinetics of photopolymerization was successfully treated as an autoaccelerated reaction (eq 1) and with empirical eq 3. The fitting of an experimental kinetics of polymerization into two-exponential equation 3 allows us to obtain quantitative parameters k1 and k2 ascribed to the reactivity of photohardenable resins. k1(2) values allow one to rank monomers and oligomers in their ability to undergo photopolymerization and to select materials capable for fast polymerization. Estimated in DSC experiments, the degree of polymerization ranges from ξmax ∼ 1.0 (monofunctional acrylate) to ξmax ∼ 0.4 (pentafunctional acrylate). There is a tendency of ξmax to decrease in the series of similarly structured di-, tri-, tetra-, and pentafunctional acrylates. Results of the present work show that the rate of polymerization of studied acrylates and their conversions are not much affected by the purity of a material and the presence of
Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3359
air oxygen. The properties of obtained polymers, the effect of light intensity, and the concentration of a photoinitiator require further study. Acknowledgment The authors thank J. Barker for his support of this work. I.V.K. is grateful to Professor D. Neckers for valuable comments on photopolymerization. Literature Cited (1) Fouassier, J.-P. Photoinitiation, Photopolymerization, and Photocuring: Fundamentals and Applications; Hanser: Munich, 1995. (2) Dotson, N. A.; Galva´n, R.; Laurence, R. L.; Tirrell, M. Polymerization Process Modeling; VCH Publishers: New York, 1996. (3) Decker, C. The Use of UV Irradiation in Polymerization. Polym. Int. 1998, 45, 133. (4) Decker, C. Photopolymerization and Ultraviolet Curing of Multifunctional Monomers. Mater. Sci. Technol. 1997, 18, 615. (5) Rayss, J.; Widomski, J.; Luzinov, I.; Voronov, A.; Minko, S. Effect of Polyacrylate Binding Layers on Adhesion of UV-Cured Epoxyacrylate Protective Coatings on Optical Fibers. J. Appl. Polym. Sci. 1998, 67, 1913. (6) Barraud, J.-Y.; Gervat, S.; Ratovelomanana, V.; Boutevin, B.; Parisi, J.-P.; Cahuzac, A.; Jocteur, R. Polymer Material of the Polyurethane Acrylate Type for Coating an Optical Fiber or for an Optical Fiber Tape. U.S. Patent 5,567,794, 1996. (7) Jansen, J. F. G. A.; Dias, A. A.; Hartwig, H. Astramol Polypropeleneimine Dendrimers as Norrish Type II Amine Synergists. In Proceedings of the RadTech North American UV/EB Conference; 1998; p 207. (8) Jo¨nsson, S.; Sundell, P.-E.; Hultgren, J.; Sheng, D.; Hoyle, C. E. Radiation Chemistry Aspects of Polymerization and Crosslinking. A Review and Future Environmental Trends in “Non-Acrylate” Chemistry. Prog. Org. Coat. 1996, 27, 107. (9) Klosterboer, J. G.; van de Hei, G. M. M.; Gossink, R. G.; Dortant, G. C. M. The Effect of Volume relaxation and Thermal Mobilization of Trapped Radicals on the Final Conversion of Photopolymerized Diacrylates. Polym. Commun. 1984, 25, 322. (10) Tryson, G. R.; Shultz, A. R. A Calorimetric Study of Acrylate Photopolymerization. J. Polym. Sci., Polym. Phys. Ed. 1979, 17, 2059. (11) Jo¨nsson, S.; Hasselgren, C.; Ericsson, J. S.; Johansson, M.; Clark, S.; Miller, C.; Hoyle, C. E. Oxygen Accelerating Effects in Copolymerization of Donor-Acceptor Pairs. Proceedings of RadTech North American UV/EB Conference; 1998; p 189. (12) Costela, A.; Garcia-Moreno, I.; Dabrio, J.; Sastre, R. Photochemistry and Photopolymerization Activity of p-Nitroaniline in the presence of N,N-Dimethylaniline as a Bimolecular Photoinitiator System. J. Polym. Sci., Polym. Chem. Ed. 1997, 35, 3801. (13) Doornkamp, A. T.; Tan, Y. Y. Kinetic Study of the Ultraviolet-Initiated Polymerization of a Polyester Urethane Diacrylate by Differential Scanning Calorimetry. Polym. Commun. 1990, 31, 362. (14) Dietz, J. E.; Cowans, B. A.; Scott, R. A.; Peppas, N. A. SolidState NMR Spectroscopy for Chracterization of Acrylate Reactions. In Photopolymerization: Fundamentals and Applications; ACS Symposium Series 673; American Chemical Society: Washington, DC, 1996; p 28. (15) Dietz, J. E.; Peppas, N. A. Reaction Kinetics and Chemical Changes During Polymerization of Multifuctional (Meth)acrylates for the Production of Highly Crosslinked Polymers Used in Information Storage Systems. Polymer 1997, 38, 3767. (16) Klosterboer, J. G.; van de Hei, G. M. M.; Boots, H. M. J. Inhomogeneity During the Photopolymerization of Diacrylates: D.S.C. Experiments and Percolation Theory. Polym. Commun. 1984, 25, 354. (17) Culberston, B. M.; Wan, Q.; Tong, Y. J. Preparation and Evaluation of Visible Light-Cured Multi-Methacrylates for Dental Composities. Macromol. Sci. 1997, A34, 2405.
(18) Gunduz, N.; Shultz, A. R.; Shobha, H. K.; Sankarapandian, M.; McGrath, J. E. Photopolymerization Kinetics of New Dimethacrylates by Photo DSC. Polym. Prepr. 1998, 39, 646. (19) Jager, W. F.; Lungu, A.; Chen, D. Y.; Neckers, D. C. Photopolymerization of Polyfunctional Acrylates and Methacrylates Mixtures: Characterization of Polymeric Networks by a Combination of Fluorescence Spectroscopy, and Solid State Nuclear Magnetic Resonance. Macromolecules 1997, 30, 780. (20) Popielarz, R.; Neckers, D. C. Real Time Monitoring of Degree of Cure and Coat-Weight Photocurable Coatings by Fluorescence Probe Technique and Instrumentation. Proc. Rad. Technol. 1996, 1, 271. (21) Pekcan, O ¨ .; Yilmaz, Y.; Okay, O. Real Time Monitoring of Polymerization Rate of Methyl Methacrylate Using Fluorescence Probe. Polymer 1997, 38, 1693. (22) Goodner, M. D.; Lee, H. R.; Bowman, C. N. Method for Determining the Kinetic Parameters in Diffusion-Controlled FreeRadical Homopolymerization. Ind. Eng. Chem. Res. 1997, 36, 1247. (23) We used molecular weights of EB1039 and EB8301 reported by UCB Radcure (Table 1). UCB reports MW of 1100 for hexafunctional urethane acrylate EB8301. However, EB8301 has an essential amount of polyol acrylates of lower functionality in its formulation. (24) Schaeken, T. C.; Warman, J. M. Radiation-Induced Polymerization of a Mono- and Diacrylate Studied Using Fluorescent Molecular Probe. J. Phys. Chem. 1995, 99, 6145. (25) “Induction period” is often arbitrarily considered as the time required for consumption of 1% of a monomer; cf., e.g., ref 15. (26) Denisov, E. T.; Azatyan, V. V. Inhibition of Chain Reactions; Russian Academy of Sciences: Chernogolovka, 1997 (in Russian). (27) Denisov, E. T.; Khudyakov, I. V. Mechanisms of Action and Reactivity of the Free Radicals of Inhibitors. Chem. Rev. 1987, 87, 1313. (28) For an excellent discussion of problems pertinent to the gel effect, see: O’Neil, G. A.; Wisnudel, M. B.; Torkelson, J. M. A Critical Experimental Examination of the Gel Effect in Free Radical Polymerization: Do Entanglements Cause Autoaccelration? Macromolecules 1996, 29, 7477. (29) The rate of polymerization decreases also due to a trivial reason, namely, the consumption of a monomer. (30) Khudyakov, I. V.; Yakobson, B. I. Influence of Solvent Viscosity on Cage Effect. Russ. J. Gen. Chem. 1984, 54, 3. (31) See for example: Caspar, J. V.; Khudyakov, I. V.; Turro, N. J.; Weed, G. C. ESR Study of Lophyl Free Radicals in Dry Films. Macromolecules 1995, 28, 636. (32) We found that many kinetic curves with an “induction period” can be quite satisfactorily simulated with ξ ) A erfc(k/t) or ξ ) A erfc(kt-1/2) functions. Such fitting may be useful for practice. (33) This assumption undermines that the reactivity of any acrylate group in a multifunctional acrylate is independent of the presence of other acrylate groups. (34) There are a number of such estimates for common acrylates, and, fortunately, they do not differ dramatically. For example, for TMPTA we found reports of Qtheor ) 820, 826, and 873 J/g.10,14,15 (35) Kurland, J. J. Quantitative Aspects of Synergistic Inhibition of Oxygen and p-Methoxyphenol in Acrylic Acid Polymerization. J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 1139. (36) Usually ωin for thermo- and photoinitiation which lasts minutes and hours are in the range of 10-6-10-8 M/s.26 (37) Turro, N. J.; Khudyakov, I. V. Single-Phase Primary Electron Spin Polarization Transfer in Spin-Trapping Reactions. Chem. Phys. Lett. 1992, 193, 546. (b) Turro, N. J.; Khudyakov, I. V.; Dwyer, D. W. An Electron Spin Polarization (CIDEP) Investigation of Reactive Free Radicals with Polynitroxyl Stable Free Radicals. J. Phys. Chem. 1993, 97, 10530.
Received for review April 26, 1999 Revised manuscript received June 21, 1999 Accepted June 21, 1999 IE990306I
