Novel one-component polymeric benzophenone photoinitiator

Materials Chemistry and Physics 143 (2014) 1391e1395

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Novel one-component polymeric benzophenone photoinitiator containing poly (ethylene glycol) as hydrogen donor Kemin Wang, Yuhui Lu, Penghui Chen, Jingsong Shi, Hongning Wang, Qiang Yu* School of Materials Science and Engineering, Changzhou University, Changzhou 213164, PR China

h i g h l i g h t s Novel one-component polymeric benzophenone photoinitiator was synthesized. This photoinitiator contained poly (ethylene glycol) as hydrogen donor. This photoinitiator was the elimination of amine based hydrogen donors.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 August 2013 Received in revised form 14 November 2013 Accepted 24 November 2013

Benzophenone (BP) is a common initiator which is often used in the UV-curing production and related fields. However, the shortcomings such as toxicity, odor, and migration always limit the development of the traditional BP photoinitiator. Polymeric benzophenone photoinitiator (PEG-BP) was synthesized basing on polyethylene glycol (PEG), succinic anhydride, 4-hydroxybenzophenone and epichlorohydrin. The structure of PEG-BP was characterized by IR and 1H NMR. The radiation absorption of PEG-BP was detected by UV spectrophotometer with the maximum absorption wavelength at 283 nm in acetonitrile solvent, undergone significant redshift corresponding to small molecule photoinitiator BP, thus enhanced the photosensitive efficiency of UV-curing system in the long wavelength region. Besides, PEG-BP has better water solubility than BP, thus could be used in both oil and aqueous solution. The obvious advantage of this initiator was the elimination of amine based hydrogen donors and to provide alternative hydrogen donors with easily availability and non-toxicity. The effects of molecular weights of PEGBP, photoinitiator concentration, UV-radiation intensity and different monomers on photopolymerization kinetics were investigated in detail. The synthesized polymeric photoinitiators are attractive to be used as type II photoinitiators. Ó 2013 Elsevier B.V. All rights reserved.

Keywords: Fourier transform infrared spectroscopy (FTIR) Polymers Photoemission Irradiation effects

1. Introduction Photoinitiated polymerization forms the basis of numerous applications in manufacture of printed circuits, encapsulation of electronic components, coatings, and printing inks [1e3]. In these systems, a key factor to control the photopolymerization process is photoinitiator, which absorbs UV radiation and generates active radicals to initiate the polymerization [4]. Photoinitiated free radical polymerizations are usually carried out in the presence of photoinitiators of the ‘‘cleavage’’ type (type I) or hydrogen abstraction type (type II) [5]. Type II photoinitiators are a second class of photoinitiators based on compounds which are able to react by direct hydrogen abstraction reaction (e.g. from ethers) or by * Corresponding author. E-mail addresses: [email protected] (K. Wang), [email protected] (Q. Yu). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.11.051

electron/proton transfer (e.g. from amines), thereby producing initiating radicals [6e8]. Benzophenone is by far one of the most widely used conventional type II photoinitiators. The photoreduction of triplet-state benzophenone by tertiary amines has been extensively investigated [9e11]. Compared with low-molecular weight analogs, polymeric photoinitiators have obtained much attention recently due to several advantages, such as low odor, nontoxicity, and compatibility improved with formulation components [12]. However, these polymeric photoinitiators were limited to be used only in one type of UV-curing system: oil or aqueous. Because of its good hydrophility and hydrophobility, PEG chain is of our interest in the preparation of polymeric photoinitiators with good compatibility in both oil and aqueous photo-curing systems. Moreover, PEG chain can act as hydrogen donor due to its structure of ether [13], which could avoid the use of amine coinitiator, and eliminate the shortcomings resulted from amines.

1392

K. Wang et al. / Materials Chemistry and Physics 143 (2014) 1391e1395

Scheme 1. Synthesis route of PEG-BP.

In this text, we successfully developed a series of novel onecomponent polymeric photoinitiator based on benzophenone with different molecular weight. These photoinitiators can readily initiate polymerization of acrylate/methacrylate monomers very efficiently without the requirement of additional hydrogen donor. 2. Experiment part 2.1. Materials 4-hydroxybenzophenone, benzophenone (BP), ethyl-4dimethylaminobenzoate (EDAB, 99%) (Aldrich) were used as received. tripropylene glycol diacrylate (TPGDA), Tetrahydrofurfuryl acrylate (THFA), Tetraethyleneglycol dimethacrylate (TEGDMA), and Trimethylolpropane triacrylate (TMPTA) were obtained from Sartomer Company (Warrington, PA, USA). Epichlorohydrin, Succinic Anhydride, Tetrabutylammoniumbromide, Polyethylene Glycol (PEG) with different molecular weight were obtained from Aladdin Reagent Co., Ltd. (Shanghai, China). 4-(2,3Epoxypropyloxy) benzophenone (EBP) was synthesized from 4hydroxybenzophenone with the corresponding epichlorohydrin by using alkali as catalyzer according to references [14].

optical fiber, Canada) and an IR analyzing radiation beam at room temperature. The radiation intensity on the surface of samples was measured by UV radiometer. The spectrometer was operated in the absorbance mode and the absorbance change of ]CeH peak area from 6101 to 6219 cm1 in the near IR range was correlated to the extent of polymerization. The conversion of the functional groups could be calculated by measuring the peak area at each time of the reaction and determined as following:

DC% ¼ ðA0 At Þ 100=A0 where DC is the degree of double bond conversion at t time, A0 is the initial peak area before radiation, and At is the peak area of the double bonds at t time. 2.4. Instrumentation FT-IR spectra were recorded on a Nicolet iS5 instrument (Nicolet Instrument, Thermo Company, USA). The 1H NMR spectra were recorded on a Bruker AV500 unity spectrometer, with C D3Cl as solvent and tertamethysilane (TMS) as the internal standard. The radiation intensity was detected by UV Radiometer (Beijing Normal University, China).

2.2. Synthesis of PEG-BP The one-component polymeric Photoinitiators were synthesized according to Scheme 1. PEG (0.01 mol) and Succinic Anhydride (0.02 mol) were added to 100 mL three-necked round bottom flask. The mixtures were stirred at 80 C for 3 h, and then pale yellow viscous liquid was obtained. EBP (0.01 mol) and Tetrabutylammoniumbromide (0.03 g) were added to the flask and were kept at 38 C for about 8 h. FTIR was used to monitor the process of the reaction. When the epoxy peak at about 910 cm1 disappeared, the reaction was cooled to room temperature. The obtained product was PEG-BP. 2.3. Real-time infrared spectroscopy Real-time infrared spectroscopy (RT-FTIR) was recorded on a Nicolet iS5 instrument (Nicolet Instrument, Thermo Company, USA). Uniform mixture samples of photoinitiator and monomer were placed in a mold made from glass slide and spacers with 15 1 mm in diameter and 1.2 0.1 mm in thickness. The samples were placed in the compartment of a Fourier transform infrared spectrometer and were simultaneously exposed to a UV-radiation source (EFOS Lite, 50-W miniature arc lamp, with 5-mm crystal

Fig. 1. FTIR spectra of PEG-BP and EBP.


1393

UVeVis absorption spectra were recorded in acetonitrile and water solution by a Hitachi U-3010 UVeVis spectrophotometer (Hitachi High-Technologies Corporation, Tokyo, Japan). A cell path length of 1 cm was employed. 3. Result and discussion One-component polymeric benzophenone photoinitiator (PEGBP) was synthesized according to Scheme 1. Through the introduction of PEG segments into the polymeric main chain, both hydrophility and hydrophobility were obtained in PEG-BP. The FT-IR spectra of EBP and one-component polymeric photoinitiators were shown in Fig. 1. Compared the spectrum of PEG-BP with EBP, it can be seen that the final product PEG-BP was obtained, which was confirmed by the appearance of peak at 1730 cm1 related to ester bond and the disappearance of peak at 912 cm1 related to epoxy group. The 1H NMR spectra of EBP and PEG-BP were shown in Fig. 2. In both spectra, the characteristic signals at 7.90e7.40 ppm, and 6.98 ppm are assigned to the hydrogen atoms of phenyl ring. In the spectrum of EBP, the chiral NMR splitting signals at 4.35 (dd), 4.29 (d), 3.59e3.24 (m), 3.18e2.91 (m), and 2.81 (dd) ppm are assigned to the hydrogen atoms of epoxy group, indicating the existence of epoxy group in EBP. While in the spectrum of PEG-BP, the peaks related to the protons of polymeric photoinitiator chains shifted to wider ranges compared with EBP. The peaks related to the hydrogen atoms of epoxy group almost disappeared, indicating the completion of addition reaction between epoxy group and remaining trace amount of unreacted EBP. The peaks at 3.5e 3.7 ppm related to the hydrogen atoms of PEG can be obviously observed, whereas which are absent in the EBP spectrum. UVeVis spectra of PEG-BP with different molecular and BP in acetonitrile and water were shown in Fig. 3. As shown in Fig. 3a andb, compared with BP, the absorption wavelength of polymeric photoinitiators produced significant red-shift. This can be explained as the influence of macromolecular structure and substitutional group. The absorption in the range of 300e400 nm is attributed to the pep* type transition for BP moiety possessing a low extinction coefficient for the spin forbidden transition [15]. The absorption behavior of the PEG-BP with different molecular weight can be related to the solubility. The molar extinction coefficient of PEG600-BP and PEG1000-BP were higher than that of PEG200-BP in aqueous solution.

Fig. 2. 1H NMR spectra of EBP and PEG-BP.

Fig. 3. UVeVis absorption spectra of PEG-BP with different molecular weight and BP in acetonitrile (a) and in aqueous solution (b) (concentration ¼ 2.5 * 104 mol L1).

The kinetics of photopolymerization of the photoinitiator in different conditions was studied by real time infrared spectroscopy. The real-time IR (RTIR) technology has been widely used to measure the double bond conversion (DC) of (meth)acrylate monomers under radiation [16,17]. Upon radiation, the extent of polymerization as a function of time was accurately reflected by measuring the decrease of the ]CeH absorbance peak area. The rate of polymerization (Rp) could be calculated by the time derivative of the DC curve [18]. To understand the efficiency of PEG-BPs in photoinitiation of commercial acrylate crosslinkers, we studied the photopolymerization of TPGDA initiated by PEG-BPs with different molecular weight, as well as low-molecular weight analogs BP/EDAB initiator system. Simple mixtures of BP and PEG oligomers with different molecular weight were also examined for comparison. The polymerization behavior of TPGDA, initiated by PEG-BPs with different molecular weight, BP/EDAB and BP/PEG with different molecular weight were shown in Fig. 4. Compared to the system of BP/EDAB, PEG600-BP possessed almost the same polymerization rate and final double bond conversion. Although the BP/EDAB system is one of the most efficient photoinitiaor, there are many serious disadvantages of using amines (e.g. their mutagenicity and their tendency to induce substrate corrosion). Therefore, PEG600BP could serve as a substitute for the conventional initiator BP/ EDAB in a variety of practical UV curing applications, which could avoid the use of large numbers of amines in the system and

1394


Fig. 4. Photopolymerization of TPGDA initiated by different photoinitiators. Radiation intensity ¼ 30 mW cm2 [photoinitiator] ¼ 2 105 mol g1.

overcome some shortcomings of the use of amine. Furthermore, compared with results obtained from BP/PEG initiating system with PEG molecular weight of 600 and 1000, the photopolymerization rate was much faster and final conversion was much higher for onecomponent polymeric photoinitiator PEG-BP (for PEG600-BP and PEG1000-BP). Hydrogen abstraction from one of the methyl groups adjacent to the amino group by a photoexcited BP molecule [19,20] leads to the formation of macroinimer and ketyl radicals, where the latter radical is known to undergo radical coupling and is thus, ineffective in initiating the polymerization reactions. It became evident that the hydrogen abstraction also occurs from PEG itself without the

Fig. 5. Photopolymerization of TPGDA with different concentration of PEG600-BP as initiator. Radiation intensity ¼ 30 mW cm2.

requirement of additional hydrogen donors. The chain length of PEG could affect the conversion of TPGDA for both BP/PEG and PEGBP initiators. When BP/PEG as photoinitiating system, the molecular weight of PEG increased from 600 to 1000, the double bond conversion increased, which may be attributed to the increased hydrogen donor. When PEG-BP as photoinitiator, the molecular weight of PEG increased from 200 to 600, the rate of photopolymerization significantly increased; however, further increase of PEG molecular weight form 600 to 1000, that is, the molecular weight of PEG-BP increased from 1276 to 1676, the conversion decreased, which may be attributed to the limited mobility according to reference [13]. Similar conclusion was reported in this

Scheme 2. Possible photoreaction mechanism of PEG-BP.


1395

THFA, TPGDA, TEGDMA and TMPTA were used to show the photoinitiating activity of PEG-BP initiator. Fig. 7 displayed the conversion vs. time plots of different monomers. We could find that the polymerization rate for TPGDA was higher than that of TEGDMA, indicating the efficiency of PEG-BP for diacrylates was higher than that for dimethacrylates. Besides, the functionality of acrylates had a strong influence on both the polymerization rate and final conversion. With the increase of acrylate functionality, the content of residual unsaturations rose. For monoacrylate THFA, the lower viscosity resulted in higher final conversion compared with diacrylate TPGDA and triacrylate TMPTA. As the functionality increase, the viscosity of the resin increased, resulting in gel-effect and the higher cross-link density, which set a limit to the extent of conversion [21]. 4. Conclusion

Fig. 6. Photopolymerization of TPGDA with different UV-radiation intensities. [PEG600-BP] ¼ 1 wt%.

The benzophenone-based type II polymeric photoinitiators, PEG-BP with different molecular weight, were synthesized. The photoinitiating activity was examined based on difunctional monomer TPGDA. The photoinitiating efficiency was affected by the molecular weight, photoinitiator concentration, UV-radiation intensity and different monomers. The PEG600-BP showed the same initiating activity as the BP/EDAB system. The obvious advantage of this initiator is the elimination of amine based hydrogen donors and to provide alternative hydrogen donors with easily availability and non-toxicity. Moreover, the water solubility of PEG provides the use of initiating system in water-borne formulations. Therefore, it is expected that the synthesized polymeric photoinitiators are attractive to be used as type II photoinitiators. Acknowledgments The author would like to thank the National Natural Science Foundation of China (21304011, 21101017) and a project funded by the priority academic program development of Jiangsu Higher Education Institutions for its financial support. References

Fig. 7. Plots of double bond conversion of different monomers vs irradiation time. [PEG600-BP] ¼ 1 wt%, radiation intensity ¼ 30 mW cm2.

reference for PEO as co-initiator. The interaction of photoexcited sensitizer with hydrogen donating PEG is suppressed at more viscous media. Hydrogen abstraction by the excited PEG-BP can proceed either on the same molecule (intramolecular process) to generate a molecule with three radical sites or on a closed-by PEG-BP molecule (intermolecular process), as shown by Scheme 2. The alkyl radicals initiate polymerization while the ketyl radical BPH is inefficient in initiating free-radical polymerization. The effect of different PEG600-BP concentrations on the photopolymerization of TPGDA was shown in Fig. 5. When the concentration of PEG600-BP increased from 0.1 to 5 wt%, the final double bond conversion increased, because the higher the PEG600BP concentration, the more the free radical could produce during UV radiation, resulting in the higher double bond conversion. Fig. 6 showed the conversion versus time plots of TPGDA initiated by 1 wt% PEG600-BP at different UV-radiation intensities. The polymerization rate and final DC increased with increase of UVradiation intensity. This was because the higher UV-radiation intensity could yield more radicals which led to the increase in the polymerization rate and final double bond conversion.

[1] J.P. Fouassier, Photoinitiation, Photopolymerization, and Photocuring Fundamentals and Applications, Hanser, New York, 1995. [2] C. Roffy, Photogeneration of Reactive Species for UV-curing, Wiley, New York, 1997. [3] C. Decker, Prog. Polym. Sci. 21 (1996) 593e650. }nssonb, F. Morel, C. Decker, Polymer 40 (1999) [4] S.C. Clarka, C.E. Hoyle, S. Jo 5063e5072. [5] J.P. Fouassier, J.F. Rabek, in: Radiation Curing in Polymer Science and Technology, vols. IeIV, Elsevier Applied Science, London, 1993. [6] A. Ledwith, M.D. Purbrich, Polymer 14 (1973) 521e522. [7] R.S. Davidson, in: D. Bethel, V. Gold (Eds.), Advances in Physical Organic Chemistry, Academic Press, London, 1983. [8] A. Ledwith, J.A. Bosley, M.D. Purbrich, J. Oil Colour Chem. Assoc. 61 (1978) 95e 104. [9] S.G. Cohen, A. Parola, G.H. Parsons, Chem. Rev. 73 (1973) 141e161. [10] J.C. Scaiano, J. Photochem. 2 (1973) 81e118. [11] M.V. Raumer, P. Suppan, E. Haselbach, Chem. Phys. Lett. 252 (1996) 263e266. [12] X. Jiang, J. Luo, J. Yin, Polymer 50 (2009) 37e41. [13] M. Atilla Tasdelen, N.t. Moszner, Y. Yagci, Polym. Bull. 63 (2009) 173e183. [14] L. Cheng, W. Shi, Prog. Org. Coat. 71 (2011) 355e361. [15] S. Jauk, R. Liska, Macromol. Rapid Commun. 26 (2005) 1687e1692. [16] T. Scherzer, S. Müller, R. Mehnert, A. Volland, H. Lucht, Polymer 46 (2005) 7072. [17] J.W. Stansbury, S.H. Dickens, Dent. Mater. 17 (2001) 71e79. [18] C. Decker, K. Moussa, Macromolecules 22 (1989) 4455e4462. [19] F. Minto, M. Gleria, A. Pegoretti, L. Fambri, Macromolecules 33 (2000) 1173e 1180. [20] J. Du, Y. Murakami, T. Senyo, Adam, K. Ito, Y. Yagci, Macromol. Chem. Phys. 205 (2004) 1471e1478. [21] R. Mehnert, A. Pincus, I. Janorsky, R. Stowe, A. Berejka, in: Chemistry & Technology of UV & EB Formulation for Coatings, Inks & Paints, vol. I, SITA Technology Ltd., London, 1997, p. 217 (Chapter VIII).