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MATERIALS AND INTERFACES Pigmentation of Boc-indanthrone in Organic Solvent/Photopolymer through a Thermal Process Tang-Hao Liu,† Wen-Tung Cheng,*,† and Kuo-Tung Huang‡ Department of Chemical Engineering, National Chung Hsing UniVersity, Taichung 40227, Taiwan, and Material and Chemical Research Laboratory, Industrial Technology Research Institute, Hsinchu 310, Taiwan
This work explores a pigment converted from a latent pigment in organic solvent and photopolymer through a thermal process. Measurements by thermogravimetric analysis (TGA), ultraviolet-visible (UV-vis) spectroscopy, and Fourier transform infrared (FTIR) spectroscopy showed that regeneration of the pigment from boc-indanthrone in the solvent or photopolymer could be achieved by heating. X-ray diffraction (XRD) was employed to characterize the regenerated pigments. As shown in the results, the recrystallized structure is similar to the structure of the parent pigment. Analysis using field-emission scanning electron microscopy (FESEM) indicated that the particles of the re-formed pigment in the solvent were initially cubic, then aggregated into loose bars, and finally became firmly slated and flat for hydrogen-bonding and π-π stacking interactions. In addition, the morphology of the reproduced pigment was found to be cubic, as in the photopolymer. 1. Introduction Pigment dispersion has played an essential role in significantly influencing the quality and properties of the products in binding materials over the past decade. Conventionally, treating a pigment with grinding, milling, or use of a solvent is necessary to improve the dispersion and decrease its particle size to achieve a good dispersion in the application substrate.1 However, this method consumes a large amount of time and energy. In addition, the pigment needs to be dispersed and embedded with auxiliaries such as dispersing agents, surfactants, and so on, to avoid instability in the vehicle for application.2 Pigments are conventionally applied through dispersion in various types of vehicles, polymers, and binders.3-6 Latent pigment, a precursor of pigment, is synthesized by substituting the hydrogen in the amine or imino group of the parent pigment with protective groups such as t-butoxycarbonyl (t-boc) radical.7-9 The resulting latent pigment is soluble and easily disperses in the application medium without any timeand energy-consuming treatment. The protective groups in the latent pigment can be removed from the application medium, and the latent pigment can be converted into parent pigment in situ through thermal or acid treatment.10,11 Many researchers use the same technique to produce latent pigments and investigate their applications in several fields.12-14 Latent pigment technology involves UV photoirradiation or thermal treatment to remove the boc protecting group from the pigment precursors.2,15,16 Although research on latent pigments has been published, to the best of our knowledge, few studies on the effects of the pigmenting conditions, such as environment and processing temperature, on the morphology, particle size, and crystal phase of the converted pigment obtained from the latent pigment have * To whom correspondence should be addressed. Tel.: 886-422840510 #709. Fax: 886-4-22871787. E-mail: wtcheng@ dragon.nchu.edu.tw. † National Chung Hsing University. ‡ Industrial Technology Research Institute.
been reported in the literature. To address this shortfall, this work focuses on the appearance and particle size of the re-formed pigment obtained from latent pigment by thermal treatment in either a solvent or a photopolymer. Indanthrone, a high-performance pigment, was used as the model compound in this investigation. The latent pigment of boc-indanthrone was synthesized by replacing the H atom in the NH group of indanthrone pigment with a compound containing the t-butyloxycarbonyl (t-boc) group. Ultraviolet-visible (UV-vis) and Fourier transform infrared (FTIR) spectroscopies were used to monitor the pigmentation of the latent pigments in the solvent or photopolymer through the heating process. X-ray diffraction (XRD) was employed to explore the variation of crystal phases during pigmentation. Furthermore, field-emission scanning electron microscopy (FESEM) was used to examine the morphology of the refabricated pigment in organic solvent and polymeric matrix. 2. Experimental Section 2.1. Materials. Indanthrone was purchased from TCI; ditert-butyldicarbonate, 4-dimethylaminopyridine, and dichloromethane were donated by Acros Chemical Co. N-Methyl-2pyrrolidinone (NMP) was obtained from Tedia Chemical Co. The reactive monomer trimethylolpropane triacrylate (TMPTA, Mw ) 296.32) and the photoinitiator 2-benzyl-2-N,N-dimethylamino-1-(4-morpholinophenyl)-1-butanone (Irgacure369, λmax. ) 327 nm) were purchased from UCB Chemicals and Ciba Specialty Chemicals Inc., respectively. All materials were used as received. 2.2. Physical Measurements. Thermal properties were determined by thermogravimetric analysis (TGA), which was performed on a Perkin-Elmer thermogravimetric analyzer (Pyris 1 TGA) at a heating rate of 10 °C/ min under nitrogen atmosphere. 1H NMR spectra were obtained with a Varian Oxford FT-NMR spectrometer. FTIR spectra were recorded with a Perkin-Elmer One B model spectrometer. UV-vis spectra
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Figure 1. Synthesis and thermal pigmentation of boc-indanthrone.
were obtained with Shimadzu UV-1601 spectrometer. The morphology was examined with a JEOL JSM-6700F FESEM instrument. Vilber Lourmat provided the polymeric film and the light source (BLX-365, 80 W). XRD was performed using Mac Science equipment (MXP-III diffractometer, Cu KR radiation, λ ) 1.5406 Å; generator voltage ) 40 kV, current ) 30 µA). Powder samples were scanned in 2θ ranges from 10 to 40° at a rate of 1°/min. Measurements were recorded at every 0.01°. 2.3. Synthesis of the Boc-indanthrone. Boc-indanthrone was prepared from indanthrone by grafting it with di-tert-butyldicarbonate.4,8,10 The imino groups in indanthrone were substituted by t-boc groups to form boc-indanthrone in the presence of 4-dimethylaminopyridine. The pigmentation from boc-indanthrone was used to regenerate indanthrone through thermal treatment in the solvent and photopolymer, as shown in Figure 1. 2.4. Pigmentation of Boc-indanthrone in Organic Solvent. A solution of 0.01 gof boc-indanthrone was dissolved in 10 g of NMP and heated at 200 °C in an oven. The temporal change in the pigmentation process was detected and monitored by UV-vis and FTIR spectra. The morphology of the pigment converted from boc-indanthrone was analyzed by FESEM, for which sample preparation was as follows: The latent pigment was dissolved in the solvent (0.01 g, 1.56 × 10-2 mmol of bocindanthrone in 10 g of NMP) and heated at 200 °C for 30, 60, or 120 min in an oven. After thermal treatment, the samples were coated onto a glass substrate and dried at 90 °C for 10 min. 2.5. Pigmentation of Boc-indanthrone in Photopolymer. A solution composed of boc-indanthrone (0.04 g, 6.23 × 10-2 mmol), Irgacure369 (0.02 g, 5.46 × 10-2 mmol), and TMPTA (0.4 g, 1.35 mmol) dissolved in NMP (4 g) was prepared and spun-cast onto a glass substrate with two stages of 1000 rpm for 10 s and a stage of 2000 rpm for 10 s to form a photopolymer film. The film was prebaked at 70 °C for 10 min, irradiated (standard UV lamp, λ ) 365 nm) for 20 min, and postbaked at 90 °C for 30 min to remove residual solvent from the solidified film. These films were thermally treated in an oven at 200 °C for a certain time. All processes of pigmentation were monitored using by UV-vis and FTIR spectroscopies. The same samples were also analyzed by FESEM. 3. Results and Discussion 3.1. Characterization of Boc-indanthrone. The chemical structure of boc-indanthrone is identified by 1H NMR spectrum, as shown in Figure 2 [1H NMR (200 MHz, CDCl3) δ (ppm): 1.35 (s, 18H), 7.59 (d, 2H), 7.73 (d, 2H), 7.98 (d, 4H), 8.26 (t, 4H)]. The chemical shift and integration values of the resonance peaks are in agreement with the proposed chemical structure of boc-indanthrone. Figure 3 illustrates the thermal properties of indanthrone and boc-indanthrone, as determined by TGA under nitrogen atmosphere. As can be seen in the figure, the decomposition temperature (5% weight loss) of indanthrone is
Figure 2. 1H NMR spectrum of as-synthesized boc-indanthrone in this study.
Figure 3. TGA curves of indanthrone and boc-indanthrone.
about 460 °C, and decomposition is complete at about 600 °C. In contrast, boc-indanthrone exhibits a two-step decomposition at about 210 and 460 °C because of the deprotection of the t-boc groups on indanthrone; the weight loss is about 31%, close to the theoretical value. The secondary decomposition begins at 460 °C after the deprotection of the t-boc groups on bocindanthrone completely and finishes at about 600 °C, which can attribute to the decomposition of indanthrone. Additionally, the difference between the parent and latent pigments was discriminated using FTIR spectroscopy. The characteristic absorptions of sNH and anthraquinone in indanthrone at 3357 and 1658 cm-1 are clearly viewed for the disappearance of hydrogen in the imino group after substitution by the t-boc groups. The characteristic absorption of the ester CdO in boc-indanthrone at 1717 cm-1 is sufficient to identify the chemical structure of the latent pigment. Figure 4 shows FESEM images of indanthrone and boc-indanthrone, respectively. As shown in the images, the morphology of indanthrone is in the form of elongated laths, with the aggregated particles in flat layers because of the hydrogen bonding between sNH and )O. After
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Figure 6. Powder X-ray diffraction patterns of (a) indanthrone, (b) bocindanthrone, and (c) the regenerated pigment with heating at 200 °C for 60 min.
Figure 4. FESEM images of 2 wt % (a) indanthrone and (b) boc-indanthrone dispersed and dissolved in NMP solution, cast on a glass substrate, and dried at 30 °C for 24 h.
modification, the hydrogen on sNH is replaced by t-boc groups. Indanthrone and boc-indanthrone are essentially different morphologies. Whereas the hydrogen bonding between parent pigments is destroyed by replacing t-boc groups at the sNH position in the parent pigment, the residual interaction between latent pigments is π-π stacking, leading to the morphology of the latent pigment as an isolated lath. As demonstrated in Figure 5, indanthrone undergoes aggregation caused by π-π stacking and hydrogen bonding, but after the introduction of t-boc groups to replace the hydrogen atoms on the imino groups, the interaction of boc-indanthrone molecules is dominated by only π-π stacking.
Figure 6 displays the XRD spectra of indanthrone, bocindanthrone, and the pigment regenerated from boc-indanthrone (treated by being heated at 200 °C for 60 min in the solvent). Figure 6a shows the diffraction pattern of R-form indanthrone, clearly presenting the peaks at 2θ ) 11.42°, 12.66°, 23.14°, 24.78°, 25.88°, 27.34°, 28.44°, and 39.44°.11 The diffraction of the boc-indanthrone, as shown in Figure 6b, is located at 2θ ) 11.74°, 12.52°, 13.04°, 14.24°, 16.08°, 16.72°, 17.50°, 18.42°, 22.06°, 23.56°, 24.88°, and 27.66° respectively. The diffraction of boc-indanthrone produces new peaks at 2θ ) 13.04°, 14.24°, 16.08°, 16.72°, 17.50°, 18.42°, and 22.06°, but the peak at 2θ ) 25.88° disappears compared to the pattern for indanthrone. Furthermore, Figure 6c shows the diffraction of the regenerated pigment, displaying the same peaks as the crystal phase of R-form indanthrone. 3.2. Pigmentation of Boc-indanthrone in Organic Solvent by Heating Process. This work investigates the thermal behavior of the precursors and performs spectroscopic analysis of the pigmentation in the organic solvent. Three temperatures (130, 180, and 200 °C) were set to study the effects of thermal treatment on the pigmentation of boc-indanthrone in NMP solution. The results indicate that boc-indanthrone remains stable even after prolonged heating at 130 °C, and hence, there is no change in the UV-vis spectrum (Figure 7a). Figure 7b shows that deprotection of boc-indanthrone commences upon heating
Figure 5. Schematic molecular structures of (a) indanthrone and (b) boc-indanthrone.
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Figure 8. FESEM images of boc-indanthrone treated thermally at 200 °C for (a) 30, (b) 60, and (c) 120 min in NMP solution, spun-cast as a film onto a glass substrate, and dried at 90 °C for 30 min.
Figure 7. UV-vis spectra of boc-indanthrone treated thermally at (a) 130, (b) 180, and (c) 200 °C and (d) FTIR spectra of boc-indanthrone treated thermally at 200 °C in NMP solution.
at 180 °C for 30 min, and the protector is completely removed when the heating time is increased to 180 min. Figure 7c shows the UV-vis spectra of the pigmentation process of bocindanthrone in NMP solution at 200 °C. As can be seen in the figure, when the temperature is set at 200 °C for 10 min, the deprotection of boc-indanthrone occurs. As the heating time is increased, the characteristic peaks of boc-indanthrone at 270, 325, and 520 nm slowly disappear, but the peak at 618 nm
appears gradually. Finally, the deprotection is accelerated and is completed after heating for 120 min. Figure 7b presents FTIR spectra of the NMP solution with boc-indanthrone treated at 200 °C in an oven. The intensity of the characteristic peak of the ester CdO at 1717 cm-1 for boc-indanthrone decreases, but the intensity of NH at 3357 cm-1 for indanthrone pigment increases with increasing heated time. The decrease of the ester and the increase of NH indicate that the deprotection of the latent pigment and the re-formation of the parent pigment proceed gradually. The high temperature of thermal treatment accelerates the deprotection of boc-indanthrone. In Figure 7, the inset shows the UV-vis spectrum of the parent pigment in NMP solution, which is taken as the reference to verify the pigmentation of latent pigment in NMP solution. Figure 8 presents FESEM images of the progress of converting bocindanthrone into indanthrone in NMP solution by thermal treatment at 200 °C for 30, 60, and 120 min. After thermal treatment, spin-casting was used to form a film on the glass substrate, which was dried at 90 °C for 10 min. As shown in the images, after 30 min of heating, the morphology of the regenerated pigments was in the form of cubic particles
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Figure 9. Schematic mechanism for regenerating indanthrone pigment from boc-indanthrone molecules by thermal treatment in NMP solution.
distributed randomly because of the π-π stacking of the indanthrone molecules. After 60 min of heating, the cubic particles aggregated into loose bars. When the treatment time was increased to 120 min, these bars became firmly slated and formed laths as a result of hydrogen-bonding interactions. According to the above results, Figure 9 represents the schematic process for regenerating indanthrone in NMP solution. The bocindanthrone molecules are dissolved in NMP solution and converted to indanthrone through thermal treatment. As the thermal treatment time progresses, indanthrone molecules diffuse and aggregate to form indanthrone particles in loose bars, producing firmly slated laths as a result of π-π stacking and hydrogen-bonding interactions, which precipitate out from the NMP solution. 3.3. Pigmentation of Boc-indanthrone in Photopolymer by Heating Process. Figure 10a shows the change in the UV-vis spectra of the photopolymer film treated thermally at 200 °C. The results reveal that the characteristic absorptions at 275 and 518 nm decreased and that at 622 nm increased with time and that the deprotection was completed after approximately 120 min. The color of the photopolymer film changed from purple to blue after 10 min, and the hue deepened as the heating time increased, corresponding to the FTIR spectra. Figure 10b displays the spectral change of the photopolymer film before and after UV exposure and thermal treatment, showing transitions at 1664, 984, and 928 cm-1 that can be assigned to the dCH group of TMPTA before UV exposure. After UV radiation, the peaks of dCH disappear due to crosslinking. In the figure, the inset displays the UV-vis spectrum of the parent pigment used to verify the pigmentation of the latent pigment in the photopolymer. FESEM was used to determine the distribution and particle size of the regenerated indanthrone in the photopolymer film. Figure 11a shows FESEM images of the indanthrone pigment dispersed in the polymer, which exhibits serious agglomeration of the parent pigment in the polymeric matrix. Parts b-d of Figure 11 show FESEM images of photopolymer film containing latent pigment during the treatment process at 200 °C for 30, 60, and 120 min, respectively. As viewed in the figure, the regenerated pigment is well-dispersed in the polymeric matrix,
Figure 10. (a) UV-vis and (b) FTIR spectra of boc-indanthrone treated thermally at 200 °C, in a photopolymer matrix composed of TMPTA (0.4 g, 1.35 mmol), Irgacure369 (0.02 g, 5.46 × 10-2 mmol), and bocindanthrone (0.04 g, 6.23 × 10-2 mmol) in 4 g of NMP, coated on a glass substrate, prebaked at 70 °C for 10 min, exposed to 365-nm UV light for 20 min, and postbaked at 90 °C for 30 min. Inset: UV-vis spectrum of the parent pigment in the photopolymer matrix.
and the particle size of the pigment particles increases significantly with heating time; the minimum and maximum values
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Figure 11. FESEM images of (a) indanthrone and the pigment regenerated from boc-indanthrone in the polymeric matrix after heat treatment at 200 °C for (b) 30, (c) 60, and (d) 120 min, spin-coating on a glass substrate, prebaking at 70 °C for 10 min, radiating at 365 nm for 20 min, and postbaking at 90 °C for 30 min.
Figure 12. Schematic mechanism for regenerating indanthrone pigment from boc-indanthrone molecules by thermal treatment in the phoyopolymer matrix.
are about 100 and 1200 nm, respectively, resulting from π-π stacking interactions. The mechanism of regenerating indanthrone from the latent pigment in the photopolymer film is presented schematically in Figure 12. As proposed in the figure, the cross-linkable matrix is cured by UV irradiation in the presence of the photoinitiator Irgacure369 to form a solid thin film. The film is then treated thermally, and the boc-indanthrone is converted to indanthrone. Finally, indanthrone molecules diffuse and aggregate to form indanthrone particles in the presence of π-π stacking and hydrogen-bonding interactions. The morphology of the reproduced indanthrone particles is
cubic, which can be attributed to the resistance of the photopolymer matrix in this work. 4. Conclusions This study investigated the pigmentation of latent pigment by a thermal treatment in an organic solvent and a photopolymer matrix. UV-vis and FTIR spectroscopies and FESEM were used in situ to monitor pigmentation of boc-indanthrone in order to analyze the variations in both the morphology and the particle
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size and distribution of the converting pigment with heating time. The key results are summarized below. (1) The morphology of the pigment regenerated from bocindanthrone in NMP solvent consists of slates and laths because of π-π stacking and hydrogen-bonding interactions, and the crystallized structure is similar to that of the parent pigment, as confirmed by XRD. (2) The appearance of the pigment regenerated in the photopolymer matrix is cubic, with a particle size that increases uniformly with heating time. Acknowledgment The authors gratefully acknowledge the resource support from the Material and Chemical Research Laboratory, Industrial Technology Research Institute, Taiwan, R.O.C. Literature Cited (1) Cheng, W. T.; Yeh, W. T. Preparation and Characterization of Free Radical Photopolymer with Carbon Black Nano-particle. J. Photopolym. Sci. Technol. 2006, 19, 687. (2) Hunger K. J. Industrial Dyes: Chemistry, Properties, Applications; VCH Verlagsgesellschaft: Weinheim, Germany, 2003. (3) Zhang, T.; Fei, X.; Wang, S.; Zhou, C. Pigmentation of Vat Blue RS by Ball Milling in Solvents. Dyes Pigm. 2000, 45, 15. (4) Sabnis, R. W. Colour Filter Technology for Liquid Crystal Displays. Displays 1999, 20, 119. (5) Kudo, T.; Nanjo, Y.; Nozaki, Y.; Yamaguchi, H.; Kang, W. B.; Pawlowski, G. Polymer Optimization of Pigmented Photoresists for Colour Filter Production. Jpn. J. Appl. Phys. 1998, 37, 1010.
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(6) Sugiura, T. Development of Pigment-dispersed-type Color Filters for LCDs. J. Soc. Inf. Display 1993, 1/3, 341. (7) Zambounis, J. S.; Hao, Z.; Iqbal, A. Latent Pigment Activated by Heat. Nature 1997, 388, 132. (8) Zambounis, J. S.; Hao, Z.; Iqbal, A. Pyrrolo[3,4-c]pyrroles. U.S. Patent 5,484,943, 1996. (9) Schadeli, U.; Zambounis, J. S.; Iqbal, A.; Hao, Z. Compositions for Making Structured Color Images and Application Thereof. U.S. Patent 5,879,855, 1999. (10) Rawal, V. H.; Cava, M. Thermolytic Removal of t-Butyloxycarbonyl (BOC) Protecting Group on Indoles and Pyrroles. Tetrahedron Lett. 1985, 26, 6141. (11) Phanstiel, O., IV; Wang, Q. X.; Powell, D. H.; Ospina, M. P.; Leeson, B. A. Synthesis of Secondary Amines via N-(Benzoyloxy)amines and Organoboranes. J. Org. Chem. 1999, 64, 803. (12) Wang, S.; Liu, X.; Xiao, J.; Liu, B. A Study on the Application Behaviors of Latent Pigment Derived from C.I. Pigment Yellow 151. Dyes Pigm. 2004, 61, 99. (13) Okubo, K. Low Illumination Intensity-recordable Ink-jet Inks and the Printing Method Therewith. Japanese Patent JP 2007197634, 2007. (14) Ichimura, K.; Arimitsu, K.; Tahara, M. Photoacid-catalysed Pigmentation of Dyestuff Precursors Enhanced by Acid Amplifiers in Polymer Films. J. Mater. Chem. 2004, 14, 1164. (15) Radoslaw, W.; Andrzej, K.; Aleksandra, D.; Teofil, J. Effect of Surface Modification on Physicochemical Properties of Precipitated Sodiumaluminium Silicate, Used as a Pigment in Acrylic Dispersion Paints. Dyes Pigm. 2001, 50, 41. (16) Fei, X.; Zhang, T.; Zhou, C. Modification Study Involving a Naphthol as Red Pigment. Dyes Pigm. 2000, 44, 75.
ReceiVed for reView January 17, 2009 ReVised manuscript receiVed October 5, 2009 Accepted November 12, 2009 IE900078H
