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Determination of photoinitiated polymerization of multifunctional acrylates with acetic acid derivatives of thioxanthone by RT-FTIR
Progress in Organic Coatings 64 (2009) 1–4
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Determination of photoinitiated polymerization of multifunctional acrylates with acetic acid derivatives of thioxanthone by RT-FTIR Feyza Karasu, Meral Aydin, M. Arif Kaya, Demet Karaca Balta, Nergis Arsu ∗ Department of Chemistry, Davutpasa Campus, Yildiz Technical University, Istanbul 34210, Turkey
a r t i c l e
i n f o
Article history: Received 19 November 2007 Accepted 7 July 2008 Keywords: Thioxanthone acetic acid derivatives Photoinitiators RT-FTIR
a b s t r a c t Photopolymerization of multifunctional acrylates with 2-thioxanthone-thioacetic acid (TXSCH2 COOH) and 2-(carboxymethoxy) thioxanthone (TXOCH2 COOH) as the one-component photoinitiator has been investigated by real-time Fourier transform infrared (RT-FTIR) spectroscopy. The photobleaching of these one-component nature initiators was performed in air. The irradiation time for total bleaching was 240 s for TXSCH2 COOH and 540 s for TXOCH2 COOH. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The UV-curing technique represents a major advance in the development of the coating, adhesive and ink industries [1–6]. Under intense illumination, reactive species such as free radicals or cations can be generated in high concentrations by photolysis of an initiator, and thus promote the polymerization of monomers and oligomers. UV-curing formulations provide some benefits such as fast cure response, excellent chemical resistance, good weathering characteristics, and broad formulating latitude. Low VOC is certainly the most talked about aspect of UV-curing but product performance and cost effectiveness are equally important features that lead to the decision to use UV [7]. Because of their vital role in photopolymerization, photoinitiators are the subject of particularly extensive research. Most of this research has focused on Type I photoinitiators, which upon irradiation undergo an ␣-cleavage process to form two radical species [1–5]. Type II photoinitiators are a second class of photoinitiators and are based on compounds whose triplet excited states are reacted with hydrogen donors thereby producing an initiating radical. Because the initiation is based on bimolecular reaction, they are generally slower than Type I photoinitiators which are based on unimolecular formation of radicals [1–5]. Recently we reported [8,9] photoinitiator acetic acid derivatives of TX, which also exhibit a one-component nature (Scheme 1). The light absorbing and hydrogen donating sites are contained in the photoinitiator molecules.
∗ Corresponding author. Tel.: +90 2123834186; fax: +90 2123834134. E-mail addresses: [email protected], [email protected] (N. Arsu). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.07.004
Notably, these one-component photoinitiators initiate the polymerization much more efficiently than two-component systems in which the related functions are composed in independent molecules. In this study, we performed photopolymerization of multifunctional acrylates with 2-thioxanthone-thioacetic acid (TXSCH2 COOH) and 2-(carboxymethoxy) thioxanthone (TXOCH2 COOH) in an air atmosphere. The polymerization reaction was followed by RT-FITR method and photobleaching of the initiators in N,N dimethylformamide (DMF) was also performed by a medium pressure mercury light.
2. Experimental 2.1. Materials TXSCH2 COOH and TXOCH2 COOH were synthesized as described previously [8]. Polyethylene glycol monoacrylate (PEG-6), tripropylene glycol diacrylate (TPGDA), 75% epoxyacrylate oligomer + 25% tripropylene glycol diacrylate (P-3038), trimethylol propane triacrylate (TMPTA), polyester resin + 40% TPGDA (E-525), and propoxylated neopentyl-glycol diacrylate (P-4127) were obtained from Cognis France. DMF was supplied by Aldrich. 2.2. Instruments IR spectra were recorded on an ATI Unicam Mattson 1000 FT/IR spectrophotometer on a KBr disc. A Flexi-cure system was employed which consists of a medium pressure lamp and a light guide.
2
F. Karasu et al. / Progress in Organic Coatings 64 (2009) 1–4
Scheme 1. Structure of the thioxanthone acetic acid derivatives.
2.3. Real-time infrared spectroscopy photopolymerization studies (RT-FTIR) The free radical polymerization of multifunctional acrylates was followed in real-time infrared spectroscopy by monitoring the decrease upon UV-exposure of the peak at 810 cm−1 of the acrylate double bond (C C stretch). In one set of experiments the formulations were coated onto potassium bromide plates in the usual way. The infrared spectrum of the non-irradiated material was recorded and then the wet film subjected to UV-exposure. The IR spectra of the films after UV-curing were taken and percentage conversions were calculated. Thus, conversion versus time curves were plotted for polymerization reactions occurring within seconds. The degree of conversion expressed as a percentage can be expressed by the following equation: Conv.% :
A0 − At A0
Fig. 1. Kinetic profiles demonstrating the photopolymerization of the PEG-6 + P3038 mixture (w/w) containing TXSCH2 COOH (—) and TXOCH2 COOH (- - -) with polychromatic light in an air atmosphere (for 180 and 3 s of irradiation). Formulations: (0.5%) PI + (49.75%) PEG-6 + (49.75%) P-3038. The inset shows the polymerization kinetics in the initial phase.
(1)
3. Results and discussion Real-time infrared spectroscopy, with millisecond time resolution, is more useful in determining photopolymerization kinetics. In real-time spectra, reactions are followed by monitoring the disappearances of a reactive bond [10,11]. The relationship between monomer structure and reactivity was investigated extensively by Decker [12]. In this work, several formulations were prepared which consisted of novel one-component type initiators and various acrylates. Photocuring of formulations was followed by RT-FTIR. These initiators, namely TXSCH2 COOH and TXOCH2 COOH, and their mechanism involved intramolecular electron transfer followed by hydrogen abstraction and decarboxylation [9]. At the concentrations above 5 × 10−3 M, however, the respective intermolecular reactions may be operative. Formulations (see Table 1) were prepared and comprised a mixture of photoinitiator and monomer, which were coated onto a KBr tablet. The sample was placed in the compartment of a FTIR spectrophotometer where it was exposed for a few seconds to the UV radiation of a medium pressure mercury lamp via a fiber optic light pipe. The disappearance of monomer in a mixture was monitored continuously at the wavelength where the double bond, characteristic to the individual monomer, is infrared active: 1640 or 810 cm−1 for the acrylate. Conversion percentages of polymerization were
Fig. 2. Kinetic profiles demonstrating the photopolymerization of the PEG-6 + P3038 + TPGDA mixture (w/w) containing TXSCH2 COOH (—) and TXOCH2 COOH (- -) with polychromatic light in an air atmosphere (for 180 and 3 s of irradiation). Formulations: (0.5%) PI + (20%) PEG-6 + (39.75%) P-3038 + (39.75%) TPGDA. The inset shows the polymerization kinetics in the initial phase.
determined according to Eq. (1) and the results obtained from RTFTIR are given in Figs. 1–4. It is known that the rate of polymerization also depends on the reactivity of the functional group, its concentration and the viscosity of the resin. The chemical structure and functionality
Table 1 Prepared formulations for RT-FTIR studies No.
TXSCH2 COOH (w/w, %)
TXOCH2 COOH (w/w, %)
PEG-6 (w/w, %)
P-3038 (w/w, %)
TPGDA (w/w, %)
TMPTA (w/w, %)
E-525 (w/w, %)
P-4127 (w/w, %)
1 2 3 4 5 6 7 8
0.5 – 0.5 – 0.5 – 0.5 –
– 0.5 – 0.5 – 0.5 – 0.5
49.75 49.75 20 20 20 20 – –
49.75 49.75 39.75 39.75 – – – –
– – 39.75 39.75 39.75 39.75 – –
– – – – 39.75 39.75 – –
– – – – – – 49.75 49.75
– – – – – – 49.75 49.75
F. Karasu et al. / Progress in Organic Coatings 64 (2009) 1–4
Fig. 3. Kinetic profiles demonstrating the photopolymerization of the PEG6 + TMPTA + TPGDA mixture (w/w) containing TXSCH2 COOH (—)and TXOCH2 COOH (- - -) with polychromatic light in an air atmosphere (for 180 and 3 s of irradiation). Formulations: (0.5%) PI + (20%) PEG-6 + (39.75%) TMPTA + (39.75%) TPGDA. The inset shows the polymerization kinetics in the initial phase.
of both monomer and oligomer are also important, for they will determine the final degree of polymerization as well as the physical and chemical characteristics of the UV-cured polymer. The influence of the resin constituents on the kinetic parameters can be readily evaluated by RTIR spectroscopy which seems to be an ideal tool for assessing the new initiators. Therefore, we employed several types of monomers and changed their stochiometries as well. Photoinitiator concentration (w/w) was constant in all formulations. Formulation 1 consisted of PEG monoacrylate and P-3038 which is a mixture of EA and 25% TPGDA. PEG monoacrylate is known to lead to increased flexibility of coating and epoxydiacrylate increases the cure speed. Indeed, the conversion percentage of mixture of acrylates reached 59 for the first 0.1 s of irradiation for TXSCH2 COOH and 30% conversion was obtained when the photoinitiator was changed to TXOCH2 COOH. When the irradiation was prolonged to 3 s, conversion percentage values were slightly increased. When TXSCH2 COOH and TXOCH2 COOH were used as initiators for formulation 1, 82 and 78 conversion per-
Fig. 4. Kinetic profiles demonstrating the photopolymerization of the PEG6 + TMPTA + TPGDA mixture (w/w) containing TXSCH2 COOH (—) and TXOCH2 COOH (- - -) with polychromatic light in an air atmosphere (for 180 and 3 s of irradiation). Formulations: (0.5%) PI + (49.75%) E-525 + (49.75%) P-4127. The inset shows the polymerization kinetics in the initial phase.
3
Fig. 5. Absorption spectra of TXOCH2 COOH (3.5 × 10−4 M) in air saturated dimethylformamide (DMF) solution after irradiation at 365 nm for 0, 10, 20, 50, 80 and 540 s.
centage values, respectively, were obtained at 180 s of irradiation time. As can be seen clearly from Fig. 1, TXOCH2 COOH is as efficient as TXSCH2 COOH for formulation 1 in prolonged irradiation times. For formulation 2, the amount of PEG monoacrylate was reduced and more TPGDA was added, as it is known that this diacrylate has high reactivity and is a good solvent. But the results were not satisfying for either initiator conversion percentages of acrylates were limited to 55 and 58 at 180 s of irradiation time for TXSCH2 COOH and TXOCH2 COOH. TMPTA is known to bring high reactivity to the formulations and also has good miscibility with other prepolymers. Instead of P-3038, as in formulation 2, TMPTA was used for formulation 3 and the results are given in Fig. 3. As indicated in Fig. 3, a 58% conversion was achieved with TXSCH2 COOH for the PEG 6 + TMPTA + TPGDA formulation and 23% conversion was obtained with TXOCH2 COOH for the same formulation in 0.1 s of irradiation time. The experiments demonstrated
Fig. 6. Absorption spectra of TXSCH2 COOH (4.5 × 10−4 M) in air saturated dimethylformamide (DMF) solution after irradiation at 365 nm for 0, 15, 30, 40, 60, 120 and 240 s.
4
F. Karasu et al. / Progress in Organic Coatings 64 (2009) 1–4
that the formulation consisting of TXSCH2 COOH reached a 63% conversion while the conversion percentage value was 42% with TXOCH2 COOH at an early irradiation time (3 s) and when the irradiation was prolonged to 180 s, 75 and 60% conversions were achieved for formulation 3, with TXSCH2 COOH and TXOCH2 COOH, respectively. TXSCH2 COOH was the most efficient initiator for the polymerization of multifunctional acrylate formulations. Especially at early irradiation times it was more efficient for formulations 1 and 3 (see Figs. 2 and 4). For the last one, formulation 4, the polyester oligomer and propoxylated neopentylglycoldiacrylate were used. In this formulation TXOCH2 COOH, as initiator, helped to increase the conversion percentage values of acrylates. In the first 3 s of irradiation time, it reached 58% conversion value, while the other formulation containing TXSCH2 COOH gave 70% conversion to the polymer (see Fig. 4). When formulation 4, which consisted of TXSH2 COOH, was subjected to further irradiation (180 s), a 78% conversion was achieved. A very important constituent in a photopolymerizable formulation is certainly the photoinitiator, since even the most reactive acrylate hardly polymerizes when exposed in pure form to UV light. It plays a key role in UV-curable systems by controlling both the initiation rate and the cure depth. Therefore, it is essential to select the right photoinitiator showing the highest initiation efficiency and undergoing fast bleaching upon UV-exposure in order to achieve a deep through cure [13,14]. Photobleaching of TXSCH2 COOH and TXOCH2 COOH was performed in an air atmosphere. Representative results for the decomposition of initiators in an air atmosphere are shown in Figs. 5 and 6. UV spectra were recorded after the solution had been exposed to the light of the UV lamp for certain times. These compounds were used as initiators for polymerization of multifunctional acrylates. The irradiation time for bleaching of TXSCH2 COOH was found to be 240 s while it took 540 s for total bleaching of TXOCH2 COOH in DMF.
4. Conclusion Photoinitiated polymerization of multifunctional acrylates with one-component initiators was achieved and such ultrafast polymerizations were followed by RT-FTIR spectroscopy, a technique that allows the formation of the polymer to be monitored in situ on a millisecond time scale. As a result of this study, the influence of initiators for the formulations containing various acrylate systems was determined. TXSCH2 COOH was found to be a very efficient one-component initiator. Acknowledgement The authors wish to thank TUBITAK, Yildiz Technical University Research Fund and DPT for their financial Support. References [1] S.P. Pappas, in: N.S. Allen (Ed.), Photopolymerization and Photoimaging Science and Technology, Elsevier, London, 1991. [2] H.S. Hageman, in: N.S. Allen (Ed.), Photopolymerization and Photoimaging Science and Technology, Elsevier, London, 1989. [3] J.P. Fouassier, Photoinitiation, Photopolymerization and Photocuring, Hanser Verlag, Munich, 1995. [4] K. Dietliker, Chemistry & Technology of UV & EB Formulation for Coatings, Inks & Paints, vol III, SITA Technology Ltd., London, 1991. [5] R.S. Davidson, Exploring the Science, Technology and Applications of UV and EB Curing, SITA Technology Ltd., London, 1999. [6] C. Decker, Prog. Polym. Sci. 21 (1996) 593. [7] N. Kayaman Apohan, A. Amanoel, N. Arsu, A. Güngör, Prog. Org. Coat. 49 (2004) 23. [8] M. Aydin, N. Arsu, Y. Yagci, Macromol. Rapid Commun. 24 (2003) 718. [9] M. Aydin, N. Arsu, Y. Yagci, S. Jockusch, N.J. Turro, Macromolecules 38 (2005) 4133. [10] C. Decker, K. Moussa, Macromol. Chem. 189 (1988) 2381. [11] C. Decker, K. Moussa, Macromolecules 22 (1989) 4455. [12] C. Decker, Macromol. Rapid. Commun. 23 (2002) 1067. [13] C. Decker, Polym. Int. 45 (1998) 133. [14] V. Ivanov, C. Decker, Polym. Int. 50 (2001) 113.
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Determination of photoinitiated polymerization of multifunctional acrylates with acetic acid derivatives of thioxanthone by RT-FTIR Feyza Karasu, Meral Aydin, M. Arif Kaya, Demet Karaca Balta, Nergis Arsu ∗ Department of Chemistry, Davutpasa Campus, Yildiz Technical University, Istanbul 34210, Turkey
a r t i c l e
i n f o
Article history: Received 19 November 2007 Accepted 7 July 2008 Keywords: Thioxanthone acetic acid derivatives Photoinitiators RT-FTIR
a b s t r a c t Photopolymerization of multifunctional acrylates with 2-thioxanthone-thioacetic acid (TXSCH2 COOH) and 2-(carboxymethoxy) thioxanthone (TXOCH2 COOH) as the one-component photoinitiator has been investigated by real-time Fourier transform infrared (RT-FTIR) spectroscopy. The photobleaching of these one-component nature initiators was performed in air. The irradiation time for total bleaching was 240 s for TXSCH2 COOH and 540 s for TXOCH2 COOH. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The UV-curing technique represents a major advance in the development of the coating, adhesive and ink industries [1–6]. Under intense illumination, reactive species such as free radicals or cations can be generated in high concentrations by photolysis of an initiator, and thus promote the polymerization of monomers and oligomers. UV-curing formulations provide some benefits such as fast cure response, excellent chemical resistance, good weathering characteristics, and broad formulating latitude. Low VOC is certainly the most talked about aspect of UV-curing but product performance and cost effectiveness are equally important features that lead to the decision to use UV [7]. Because of their vital role in photopolymerization, photoinitiators are the subject of particularly extensive research. Most of this research has focused on Type I photoinitiators, which upon irradiation undergo an ␣-cleavage process to form two radical species [1–5]. Type II photoinitiators are a second class of photoinitiators and are based on compounds whose triplet excited states are reacted with hydrogen donors thereby producing an initiating radical. Because the initiation is based on bimolecular reaction, they are generally slower than Type I photoinitiators which are based on unimolecular formation of radicals [1–5]. Recently we reported [8,9] photoinitiator acetic acid derivatives of TX, which also exhibit a one-component nature (Scheme 1). The light absorbing and hydrogen donating sites are contained in the photoinitiator molecules.
∗ Corresponding author. Tel.: +90 2123834186; fax: +90 2123834134. E-mail addresses: [email protected], [email protected] (N. Arsu). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.07.004
Notably, these one-component photoinitiators initiate the polymerization much more efficiently than two-component systems in which the related functions are composed in independent molecules. In this study, we performed photopolymerization of multifunctional acrylates with 2-thioxanthone-thioacetic acid (TXSCH2 COOH) and 2-(carboxymethoxy) thioxanthone (TXOCH2 COOH) in an air atmosphere. The polymerization reaction was followed by RT-FITR method and photobleaching of the initiators in N,N dimethylformamide (DMF) was also performed by a medium pressure mercury light.
2. Experimental 2.1. Materials TXSCH2 COOH and TXOCH2 COOH were synthesized as described previously [8]. Polyethylene glycol monoacrylate (PEG-6), tripropylene glycol diacrylate (TPGDA), 75% epoxyacrylate oligomer + 25% tripropylene glycol diacrylate (P-3038), trimethylol propane triacrylate (TMPTA), polyester resin + 40% TPGDA (E-525), and propoxylated neopentyl-glycol diacrylate (P-4127) were obtained from Cognis France. DMF was supplied by Aldrich. 2.2. Instruments IR spectra were recorded on an ATI Unicam Mattson 1000 FT/IR spectrophotometer on a KBr disc. A Flexi-cure system was employed which consists of a medium pressure lamp and a light guide.
2
F. Karasu et al. / Progress in Organic Coatings 64 (2009) 1–4
Scheme 1. Structure of the thioxanthone acetic acid derivatives.
2.3. Real-time infrared spectroscopy photopolymerization studies (RT-FTIR) The free radical polymerization of multifunctional acrylates was followed in real-time infrared spectroscopy by monitoring the decrease upon UV-exposure of the peak at 810 cm−1 of the acrylate double bond (C C stretch). In one set of experiments the formulations were coated onto potassium bromide plates in the usual way. The infrared spectrum of the non-irradiated material was recorded and then the wet film subjected to UV-exposure. The IR spectra of the films after UV-curing were taken and percentage conversions were calculated. Thus, conversion versus time curves were plotted for polymerization reactions occurring within seconds. The degree of conversion expressed as a percentage can be expressed by the following equation: Conv.% :
A0 − At A0
Fig. 1. Kinetic profiles demonstrating the photopolymerization of the PEG-6 + P3038 mixture (w/w) containing TXSCH2 COOH (—) and TXOCH2 COOH (- - -) with polychromatic light in an air atmosphere (for 180 and 3 s of irradiation). Formulations: (0.5%) PI + (49.75%) PEG-6 + (49.75%) P-3038. The inset shows the polymerization kinetics in the initial phase.
(1)
3. Results and discussion Real-time infrared spectroscopy, with millisecond time resolution, is more useful in determining photopolymerization kinetics. In real-time spectra, reactions are followed by monitoring the disappearances of a reactive bond [10,11]. The relationship between monomer structure and reactivity was investigated extensively by Decker [12]. In this work, several formulations were prepared which consisted of novel one-component type initiators and various acrylates. Photocuring of formulations was followed by RT-FTIR. These initiators, namely TXSCH2 COOH and TXOCH2 COOH, and their mechanism involved intramolecular electron transfer followed by hydrogen abstraction and decarboxylation [9]. At the concentrations above 5 × 10−3 M, however, the respective intermolecular reactions may be operative. Formulations (see Table 1) were prepared and comprised a mixture of photoinitiator and monomer, which were coated onto a KBr tablet. The sample was placed in the compartment of a FTIR spectrophotometer where it was exposed for a few seconds to the UV radiation of a medium pressure mercury lamp via a fiber optic light pipe. The disappearance of monomer in a mixture was monitored continuously at the wavelength where the double bond, characteristic to the individual monomer, is infrared active: 1640 or 810 cm−1 for the acrylate. Conversion percentages of polymerization were
Fig. 2. Kinetic profiles demonstrating the photopolymerization of the PEG-6 + P3038 + TPGDA mixture (w/w) containing TXSCH2 COOH (—) and TXOCH2 COOH (- -) with polychromatic light in an air atmosphere (for 180 and 3 s of irradiation). Formulations: (0.5%) PI + (20%) PEG-6 + (39.75%) P-3038 + (39.75%) TPGDA. The inset shows the polymerization kinetics in the initial phase.
determined according to Eq. (1) and the results obtained from RTFTIR are given in Figs. 1–4. It is known that the rate of polymerization also depends on the reactivity of the functional group, its concentration and the viscosity of the resin. The chemical structure and functionality
Table 1 Prepared formulations for RT-FTIR studies No.
TXSCH2 COOH (w/w, %)
TXOCH2 COOH (w/w, %)
PEG-6 (w/w, %)
P-3038 (w/w, %)
TPGDA (w/w, %)
TMPTA (w/w, %)
E-525 (w/w, %)
P-4127 (w/w, %)
1 2 3 4 5 6 7 8
0.5 – 0.5 – 0.5 – 0.5 –
– 0.5 – 0.5 – 0.5 – 0.5
49.75 49.75 20 20 20 20 – –
49.75 49.75 39.75 39.75 – – – –
– – 39.75 39.75 39.75 39.75 – –
– – – – 39.75 39.75 – –
– – – – – – 49.75 49.75
– – – – – – 49.75 49.75
F. Karasu et al. / Progress in Organic Coatings 64 (2009) 1–4
Fig. 3. Kinetic profiles demonstrating the photopolymerization of the PEG6 + TMPTA + TPGDA mixture (w/w) containing TXSCH2 COOH (—)and TXOCH2 COOH (- - -) with polychromatic light in an air atmosphere (for 180 and 3 s of irradiation). Formulations: (0.5%) PI + (20%) PEG-6 + (39.75%) TMPTA + (39.75%) TPGDA. The inset shows the polymerization kinetics in the initial phase.
of both monomer and oligomer are also important, for they will determine the final degree of polymerization as well as the physical and chemical characteristics of the UV-cured polymer. The influence of the resin constituents on the kinetic parameters can be readily evaluated by RTIR spectroscopy which seems to be an ideal tool for assessing the new initiators. Therefore, we employed several types of monomers and changed their stochiometries as well. Photoinitiator concentration (w/w) was constant in all formulations. Formulation 1 consisted of PEG monoacrylate and P-3038 which is a mixture of EA and 25% TPGDA. PEG monoacrylate is known to lead to increased flexibility of coating and epoxydiacrylate increases the cure speed. Indeed, the conversion percentage of mixture of acrylates reached 59 for the first 0.1 s of irradiation for TXSCH2 COOH and 30% conversion was obtained when the photoinitiator was changed to TXOCH2 COOH. When the irradiation was prolonged to 3 s, conversion percentage values were slightly increased. When TXSCH2 COOH and TXOCH2 COOH were used as initiators for formulation 1, 82 and 78 conversion per-
Fig. 4. Kinetic profiles demonstrating the photopolymerization of the PEG6 + TMPTA + TPGDA mixture (w/w) containing TXSCH2 COOH (—) and TXOCH2 COOH (- - -) with polychromatic light in an air atmosphere (for 180 and 3 s of irradiation). Formulations: (0.5%) PI + (49.75%) E-525 + (49.75%) P-4127. The inset shows the polymerization kinetics in the initial phase.
3
Fig. 5. Absorption spectra of TXOCH2 COOH (3.5 × 10−4 M) in air saturated dimethylformamide (DMF) solution after irradiation at 365 nm for 0, 10, 20, 50, 80 and 540 s.
centage values, respectively, were obtained at 180 s of irradiation time. As can be seen clearly from Fig. 1, TXOCH2 COOH is as efficient as TXSCH2 COOH for formulation 1 in prolonged irradiation times. For formulation 2, the amount of PEG monoacrylate was reduced and more TPGDA was added, as it is known that this diacrylate has high reactivity and is a good solvent. But the results were not satisfying for either initiator conversion percentages of acrylates were limited to 55 and 58 at 180 s of irradiation time for TXSCH2 COOH and TXOCH2 COOH. TMPTA is known to bring high reactivity to the formulations and also has good miscibility with other prepolymers. Instead of P-3038, as in formulation 2, TMPTA was used for formulation 3 and the results are given in Fig. 3. As indicated in Fig. 3, a 58% conversion was achieved with TXSCH2 COOH for the PEG 6 + TMPTA + TPGDA formulation and 23% conversion was obtained with TXOCH2 COOH for the same formulation in 0.1 s of irradiation time. The experiments demonstrated
Fig. 6. Absorption spectra of TXSCH2 COOH (4.5 × 10−4 M) in air saturated dimethylformamide (DMF) solution after irradiation at 365 nm for 0, 15, 30, 40, 60, 120 and 240 s.
4
F. Karasu et al. / Progress in Organic Coatings 64 (2009) 1–4
that the formulation consisting of TXSCH2 COOH reached a 63% conversion while the conversion percentage value was 42% with TXOCH2 COOH at an early irradiation time (3 s) and when the irradiation was prolonged to 180 s, 75 and 60% conversions were achieved for formulation 3, with TXSCH2 COOH and TXOCH2 COOH, respectively. TXSCH2 COOH was the most efficient initiator for the polymerization of multifunctional acrylate formulations. Especially at early irradiation times it was more efficient for formulations 1 and 3 (see Figs. 2 and 4). For the last one, formulation 4, the polyester oligomer and propoxylated neopentylglycoldiacrylate were used. In this formulation TXOCH2 COOH, as initiator, helped to increase the conversion percentage values of acrylates. In the first 3 s of irradiation time, it reached 58% conversion value, while the other formulation containing TXSCH2 COOH gave 70% conversion to the polymer (see Fig. 4). When formulation 4, which consisted of TXSH2 COOH, was subjected to further irradiation (180 s), a 78% conversion was achieved. A very important constituent in a photopolymerizable formulation is certainly the photoinitiator, since even the most reactive acrylate hardly polymerizes when exposed in pure form to UV light. It plays a key role in UV-curable systems by controlling both the initiation rate and the cure depth. Therefore, it is essential to select the right photoinitiator showing the highest initiation efficiency and undergoing fast bleaching upon UV-exposure in order to achieve a deep through cure [13,14]. Photobleaching of TXSCH2 COOH and TXOCH2 COOH was performed in an air atmosphere. Representative results for the decomposition of initiators in an air atmosphere are shown in Figs. 5 and 6. UV spectra were recorded after the solution had been exposed to the light of the UV lamp for certain times. These compounds were used as initiators for polymerization of multifunctional acrylates. The irradiation time for bleaching of TXSCH2 COOH was found to be 240 s while it took 540 s for total bleaching of TXOCH2 COOH in DMF.
4. Conclusion Photoinitiated polymerization of multifunctional acrylates with one-component initiators was achieved and such ultrafast polymerizations were followed by RT-FTIR spectroscopy, a technique that allows the formation of the polymer to be monitored in situ on a millisecond time scale. As a result of this study, the influence of initiators for the formulations containing various acrylate systems was determined. TXSCH2 COOH was found to be a very efficient one-component initiator. Acknowledgement The authors wish to thank TUBITAK, Yildiz Technical University Research Fund and DPT for their financial Support. References [1] S.P. Pappas, in: N.S. Allen (Ed.), Photopolymerization and Photoimaging Science and Technology, Elsevier, London, 1991. [2] H.S. Hageman, in: N.S. Allen (Ed.), Photopolymerization and Photoimaging Science and Technology, Elsevier, London, 1989. [3] J.P. Fouassier, Photoinitiation, Photopolymerization and Photocuring, Hanser Verlag, Munich, 1995. [4] K. Dietliker, Chemistry & Technology of UV & EB Formulation for Coatings, Inks & Paints, vol III, SITA Technology Ltd., London, 1991. [5] R.S. Davidson, Exploring the Science, Technology and Applications of UV and EB Curing, SITA Technology Ltd., London, 1999. [6] C. Decker, Prog. Polym. Sci. 21 (1996) 593. [7] N. Kayaman Apohan, A. Amanoel, N. Arsu, A. Güngör, Prog. Org. Coat. 49 (2004) 23. [8] M. Aydin, N. Arsu, Y. Yagci, Macromol. Rapid Commun. 24 (2003) 718. [9] M. Aydin, N. Arsu, Y. Yagci, S. Jockusch, N.J. Turro, Macromolecules 38 (2005) 4133. [10] C. Decker, K. Moussa, Macromol. Chem. 189 (1988) 2381. [11] C. Decker, K. Moussa, Macromolecules 22 (1989) 4455. [12] C. Decker, Macromol. Rapid. Commun. 23 (2002) 1067. [13] C. Decker, Polym. Int. 45 (1998) 133. [14] V. Ivanov, C. Decker, Polym. Int. 50 (2001) 113.