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A thermo-sensitive imaging coating derived from polymer nanoparticles containing infrared absorbing dye
European Polymer Journal 52 (2014) 166–171
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
A thermo-sensitive imaging coating derived from polymer nanoparticles containing infrared absorbing dye Li An, Zhejing Cai, Weiwei Wang, Jialing Pu, Zhongxiao Li ⇑ Laboratory of Printing & Packaging Material and Technology, Beijing Institute of Graphic Communication, Xinghua Avenue (Band Two), Daxing, Beijing 102600, PR China
MACROMOLECULAR NANOTECHNOLOGY
a r t i c l e
i n f o
Article history: Received 24 May 2013 Received in revised form 19 December 2013 Accepted 5 January 2014 Available online 13 January 2014 Keywords: Polymer nanoparticles Infrared dye Laser-imageable Emulsion polymerization
a b s t r a c t The present study relates to a method for incorporating a near infrared dye (the absorption maximum at approximately 830 nm, IR-830) into polymer particles via miniemulsion polymerization and the preparation of an infrared laser-imageable coating based on the polymer particles as well as the potentiality in developing environment-friendly computer-to-plate (CTP) precursor. Polymer particles containing the IR-830 were prepared through miniemulsion polymerization technology in the presence of sodium dodecyl sulfate and hexadecanol as the emulsifier blend. Polyvinyl alcohol was used as a water soluble polymer binder resin for the production of latex coatings. Using the above prepared materials as the main components, an IR laser-imageable coating was prepared. Upon computer-controlled laser exposure, the IR dye-containing polymer particles absorb IR laser energy and produce high temperature, causing great changes of the imaged areas of the latex coating. As a result, the imaged areas could not be removed with water cleaning, whereas the polymer particles of the non-imaged areas remain unchanged, and still could be easily removed by water cleaning. When developing with water, negative graphics were obtained. The results of the research can be used in developing chemicalfree CTP plates required by green printing industry. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor. Computer to plate (CTP) is an imaging technology used in modern printing processes. In this technology, an image created in a desktop publishing (DTP) application is output directly to a printing plate, which can form a latent image by using the heat or the light of a laser [1]. There are several kinds of CTP plates such as photopolymer plates, silver halogen plates and thermal plates [2]. As thermal materials offer the advantage of daylight stability, thermal sensitive ⇑ Corresponding author. Tel.: +86 010 60261107. E-mail address: [email protected] (Z. Li). 0014-3057/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2014.01.002
printing plate precursors which form latent images by 830 nm infrared laser have become very popular at present. The latent images which are produced by 830 nm infrared laser exposure are not yet ready for printing and must be transformed into a durable printing image such as chemical development (Scheme 1). However, traditional development processes will generate a large number of pollution by using chemical reagents, therefore, chemical-free plates are extremely desirable for environment reasons. Emulsion polymerization is a unique technique which could produce polymers under the environmentallyfriendly process [3]. This technology using water as a reaction medium and easy handing of the final latex, thus, it is used to form continuous films in adhesives, paper coating, paints, etc. [4,5]. Miniemulsion polymerization offers the efficient method for the encapsulation of different materials. Thus it is popularly used to form complex structured
L. An et al. / European Polymer Journal 52 (2014) 166–171
167
polymeric nanoparticles [6–9]. Many different materials, ranging from organic and inorganic pigments, magnetite, or other solid nanoparticles, to hydrophobic and hydrophilic liquids, such as fragrances, drugs, or photoinitators, can be encapsulated to prepare functionalized nanoparticles [10–13]. Oil-soluble dyes dispersed in water to form colored polymer latex particles with encapsulation strategies have been gathering great attention in recent years. Many researches encapsulated oil-soluble dyes into polymer latex particles by miniemulsion polymerization method. For example, Takasu et al. [14–16] prepared polymer particles containing copper phthalocyanine dyes and styryl dyes as oil-soluble dye via miniemulsion polymerization using azo-bisisobutyronitrile as initiator. Zhao et al. [17] prepared polystyrene/Sudan black B latex particles also by a miniemulsion polymerization technique. All the above cases can obtain stable and durable colored latex. Thermal sensitive printing plate precursors usually employ infrared radiation (IR) dye and polymer resin. When these materials are exposed to infrared laser, the IR dye absorbs the laser energy to generated heat triggering physico-chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer, heat-induced solubilization or particle coagulation of a thermoplastic polymer latex. With the above mentioned process, the printing plate precursors can finally form a latent image. Thermal sensitive printing plate precursors with particle coagulation of a thermoplastic polymer imaging mechanism can easily realize chemical-free plate making, because unexposed polymer particles is aqueous coating layer made by thermoplastic polymer latex. Thus, the latent images can be transformed into a durable printing image by water. In this case, encapsulation of oil-soluble IR dyes into thermoplastic polymer particles via miniemulsion polymerization can make the water development printing plate come true. The mechanism of this case is
shown in Scheme 2. Although many researchers concerned encapsulation oil-soluble dyes into polymer particles by miniemulsion polymerization, to the best of our knowledge, few articles concerning IR dye encapsulation in polymer particles via miniemulsion polymerization method and its application in thermal sensitive imaging system have been published as so far. In this paper, we present the preparation of the IR dyecontaining polymer nanoparticles and study the use in lithographic printing plate, which can be imaged using digitally controlled laser output and developed with neutral water. 2. Experimental 2.1. Materials Styrene, acrylonitrile, methyl methacrylate (MMA) and butyl methacrylate (BMA) which were purchased from Beijing Chemicals Co. were distilled under reduced pressure and stored at 15 °C. Sodium dodecyl sulfate (SDS) and hexadecanol as the emulsifier blend, L-ascorbic acid (LAA) and tert-butyl hydroperoxide (TBH) as redox initiator, polyvinyl alcohol (PVA) as binder polymer and the IR dye (Fig. 1) with the maximum absorption near 830 nm (IR-830) were used as received. They were all commercial products from Acros Organics. Distilled deionized water (DDW) was used as the polymerization medium. 2.2. Preparation of the polymer nanoparticles containing IR830 by miniemulsion polymerization The aqueous phase was formed by a mixture of SDS (0.4 g), LAA (0.15 g) and DDW (50 ml) with magnetic stirrer for several minutes. Styrene (9 g) and acrylonitrile (6 g) solution of hexadecanol (0.1 g) and various amount
Scheme 2. Schematic diagram of the laser imaging process by water development.
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Scheme 1. The imaging process of conventional CTP technology.
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H3C
CH3
CH3
Cl
Cl
N
N
CH3
CH3
H3C
SO3
Fig. 1. Molecular structure of IR-830 dye.
MACROMOLECULAR NANOTECHNOLOGY
Table 1 Recipes and global results.
a b c
Run
[IR-830]/[monomers]a
Dye contentb
Conversion (%)
dp (nm)c
PDI
1 2 3 4
0 2.5% 3.5% 4.4%b
0 2.7% 4.1% 5.2%
92 92 88 85
44.7 49.1 61.8 80.9
0.01 0.03 0.05 0.09
f potential (mV) 49.3 46.0 42.7 38.3
Concentration ratio is weight ratio. This concentration is saturation state for IR-830 in syrene/acrylonitrile (3:2) mixtures. Number-average diameter (dp), its polydispersity index (PDI) and f potential were measured by using a Malvern ZETASIZER Nano at 25 °C.
of the IR-830 were added to the aqueous phase and stirred at room temperature for 10 min. Table 1 presents the different weight ratios between IR-830 and monomer that used in the emulsion polymerizations. Subsequently, the mixture was treated with a high pressure homogenizer for 5 min. The miniemulsion obtained after high pressure homogenization was placed in a four-necked flask and mechanically stirred for 0.5 h at 40 °C. Aqueous solution of TBH (0.12 g) in water (10 ml) was added into the mixture by peristaltic pump to start the polymerization. The feeding rate was set at 3 ml/min. The polymerization was carried out under nitrogen atmosphere at 40 °C for 1 h. Finally, the IR dye-containing poly(styrene-co-acrylonitrile) (PSA) emulsion was filtered with a Buchner funnel. The filtrate was collected and kept for later use.
2.3. Preparation of the thin coating derived from the polymer nanoparticle miniemulsion and evaluation of the imaging performance In a typical experiment, PVA water solution (3.0 g, 10 wt.%) were successively added to the prepared polymer emulsion (20 g, 20.8 wt.%) under magnetic stirring. The mixture was spin-coated on a clean anodized aluminum plate (aluminum plate for offset printing with a specially prepared surface), and this was followed by drying at 60 °C in an oven for 10 min. Then the sample plate with a thin coating was mounted on the exposure device for laser scanning. Finally, the exposed plate was developed with water at 25 °C and the surface topography was recorded with optical microscope and scanning electron microscope (SEM).
2.4. Characterization The solubility of IR-830 dye in solvent and monomer conversion was obtained by gravimetry. The encapsulated IR-830 ratio was measured with an UV-2501PC spectrophotometer at room temperature. IR-830 or dried polymer particles were dissolved in THF/ethanol (1:1 by volume) and the concentration of the solution was adjusted to from 0.001 mg/ml to 0.05 mg/ml. If the IR-830 dye concentration is low enough, the absorbance of solution obeys Beer’s law. Then, the IR-830 dye content in the colored latex was determined from the absorbance at kmax (826 nm in THF/ ethanol) using the calibration curve of the absorbance at 826 nm versus IR-830 concentration (Fig. 4). Absorption spectra of IR-830 dyes are shown in Fig. 3. The numberaverage particle size (dp) and f potential measurements were done by a Malvern ZETASIZER Nano at 25 °C. All latex dispersions were diluted to 500 times with DDW and filtrated through 0.45 lm filters before the measurement. For each measurement, the obtained number-average particle size data was averaged over three individual run. The morphology of the polymer particles in emulsion was measured by transmission electron microscopy (TEM) with an H-7000 apparatus (Hitachi, Japan). Differential scanning calorimetry (DSC) was recorded on a Netzsch DSC200PC analysis apparatus at a heating rate of 10 °C/min. Laser imaging was conducted on a TP-46XX thermal plate-setter (the laser wavelength is 830 nm and its pulse width is 10 ns, Hangzhou CRON Machinery & Electronics Co., Ltd.). Surface water contact angles were measured by Kruss Optical contact angle measuring instrument. Hitachi SEM SU8020 was used to characterize the morphology of plate surface.
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3. Results and discussion 3.1. Preparation of the IR dye-containing polymer particles
Fig. 3. Absorption spectra of THF/ethanol solutions of IR-830(0.004 mg/ ml) and the dried polymer particles of Run 2 (0.04 mg/ml).
each sample was dried in vacuum oven at 40 °C, and the dried polymer particles were dissolved in THF/ethanol (1:1 by volume) on a quartz plate. The prepared polymer
1.5
IR-830 IR-830 in Poly(S-co-A) nanoparticles(Run4)
1.0
0.5
0.0 0.000
0.001
0.002
0.003
0.004
Conc.[mg/ml] Fig. 4. Calibration curve of the absorbance at 820 nm versus dye concentration data (in THF/ethanol).
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Fig. 2. TEM photographs of IR-830-containing polymer particles.
Absorbance
Successful encapsulation of oil-soluble dye into polymer latex particles by miniemulsion polymerization method requires the oil-soluble dye having good solubility in monomer. Therefore, the solubility of IR-830 in various monomers was firstly investigated to determine the monomers candidates in this miniemulsion polymerization system. The solubility of IR-830 in acrylonitrile is about 10 wt.%, but very poor in styrene, MMA and BMA. Additionally, in considering the processing temperature required by the practical application, the glass transition temperature of the polymer nanoparticles should withstand the processing temperature (70–80 °C). Thus, styrene and acrylonitrile were chosen as the comonomers in this study. PSA nanoparticles containing IR-830 were prepared through miniemulsion polymerization of the styrene/acrylonitrile solution of the dye. Miniemulsion polymerization was initiated effectively with the addition of LAA/TBH aqueous solution. The IR-830 is quite sensitive to peroxides, and color fading will take place if TBH is added to the IR-830-containing pre-emulsion in one portion. However, IR-830 is quite stable in the presence of LAA. Thus, LAA was firstly mixed with pre-emulsion and TBH was added dropwise to avoid excessive oxidant in the reaction mixture. The active free radicals generated in the aqueous phase diffused into the monomer droplets and initiated polymerization. With the progress of emulsion polymerization, the color of the reaction mixture gradually deepened, and finally into a dark green. The conversion of monomer coexisting with IR-830 reached as high as above 85% for 1 h. The emulsion was quite stable and no precipitation was found. This can be supported by the high f potential value of the each obtained aqueous dispersion. In miniemulsion polymerization process, SDS and hexadecanol were used as emulsifier and co-emulsifier, respectively. The co-emulsifier makes it easier to reduce the particle size. It should be noted that high pressure homogenization is an indispensible process for producing a stable pre-emulsion with smaller droplet size. Dynamic light scattering (DLS) technique is used for measuring the final particle size and distribution of the emulsion. Table 1 shows that number-average diameters of the latex particles increased with increasing the concentration of IR-830 in the oil phase. It directly indicates that most of the IR-830 can be encapsulated into the latex particles. This phenomenon can be found in previous work [14]. The morphology of the polymer particles in emulsion was detected by TEM (Fig. 2), it manifested the PSA particles are spherical and monodisperse with a diameter of around 50 nm (Run 2), which was close to the number average particle diameter detected by DLS technique (Table 1). Fig. 3 depicts the UV–Vis spectra of IR-830 and the prepared polymer nanoparticles containing IR-830 (Run 2). The miniemulsion products were filtered through 450 nm microfilters to remove the dye molecule clusters. Then,
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nanoparticles containing IR-830 should have almost the same spectrum as the IR-830 in the mixed solution. As seen, the UV–Vis spectral wave length of IR-830 in mixed solution (the solid line) was almost the same as that of the prepared polymer nanoparticles containing IR-830 (the dash line), displaying a maximum absorption wavelength around 820 nm. That is to say, the IR-830 molecular structure did not change in the polymerization, and was distributed well in the polymer particles. The amount of IR-830 incorporated into the polymer particles was determined by using the calibration curve
3.2. Thermal properties of the IR dye-containing polymer nanoparticle
exo 0.4
DSC/mW/mg
0.2
MACROMOLECULAR NANOTECHNOLOGY
as Fig. 4. The absorbance at 820 nm of the mixed solution was measured (Fig. 3) and the IR-830 content of the polymer particles was calculated from the calibration curve (Fig. 4). The encapsulation IR-830 ratio of each run final polymer particles is a little higher than the initial ratio between IR-830 and monomers, because monomer conversions have not reached 100%. Thus, the latex particles contained IR-830 of each ratio in accordance with the theoretical value in consideration of monomer conversion. This is a reasonable result, reflecting successful full encapsulation of IR-830 by miniemulsion polymerization method.
0.0
-0.2
-0.4 0
100
200
300
400
Temperature (°C) Fig. 5. DSC analysis of the IR dye-containing polymer nanoparticle.
Fig. 6. SEM photographs of fusion of the latex polymer particles.
Thermal properties of the latex particle were measured by DSC (Fig. 5). The DSC curve shows a small exothermal peak around 104 °C, which is the glass transition temperature of PSA. As the result, the particles should withstand the processing temperature (70–80 °C) required for the coating preparation with the emulsion. That is to say, the polymer particles can retain the shape and structure during the coating preparation. Thus, the properties of the polymer particles should remain unchanged. This is important for producing an effective laser-imageable thin coating. The endothermic peak starting at about 350 °C is ascribed to the fast decomposition of PSA. 3.3. Laser imaging of the coating derived from IR dyecontaining polymer particles The aim of this study is to develop a water developable IR laser-imaginable coating based on the IR dye-containing polymer nanoparticles. As mentioned above, the coating on the aluminum substrate consists of the following components: the prepared polymer emulsion, PVA used as binder resin and small amount addition agents. The original coating is hydrophilic and can be easily removed with water cleaning. Scheme 2 depicts the laser imaging process. Laser imaging was carried out according to the procedure described in the experimental section. Upon imagewise laser exposure, the areas of the coating that received laser radiation cure to a durable oleophilic state by the action of heat. The IR dye-containing polymer particles fused and the original structure was broken down (Fig. 6). Because the hydrophobic polymer together with the IR dye comprised the majority (about 85%) of the coating, the areas which received laser radiation became resistant to water
Fig. 7. Drop of water on the anodized aluminum substrate (a) and the imaged area (b).
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Acknowledgements This research was supported by Beijing Municipal Education Commission (KZ201110015018), BIGC Project (E-b-2014-15) and The Project of Construction of Innovative Teams and Teacher Career Development for Universities and Colleges under Beijing Municipality.
Fig. 8. Micrograph of the IR laser-exposed coating after water cleaning (25 °C).
and remained on the substrate after the coating was subjected to water cleaning (i.e. the developing process). However, the unexposed areas of the latex film could be easily washed away by water because the particles remain intact and still possessed a hydrophilic surface. Fig. 7 shows the water drops on the anodized aluminum substrate and the imaged area. The contact angle of water drop on the anodized aluminum plate was 53°, and then it increased to 67°, when the coating surfaces were exposed by the IR laser. This indicated that the microstructure of the coating layer has been changed after laser exposure. As seen in Fig. 8, the dark regions are the exposed areas or the image areas of the coating, and the background regions of the aluminum substrates are the un-exposed areas or non-image areas. The IR laser-imagineable coating is negative working. The exposure dose was about 250 mJ/cm2. The micrograph of the image shows that the unexposed areas of the original coating were completely removed from the substrate. However, those of the exposed areas survived the developing process and remained on the surface of the substrate, producing a negative image with sharp dot edge. 4. Conclusions In this study, narrowly distributed IR dye-containing polymer nanoparticles were synthesized through miniemulsion polymerization. The polymer nanoparticles emulsion has a maximum absorption near 830 nm. A latex coating was prepared with the polymer nanoparticles emulsion and a water soluble binder polymer (PVA). Negative image was obtained through imagewise exposure by IR laser radiation followed with water developing. This kind of coating derived from the IR dye-containing polymer particles can be used as the precursor for developing chemical-free thermal CTP plate.
[1] Tamaki E, Hashimoto Y, Leung OS. Computer-to-plate printing using the Grating Light Valve device. In: Proc. SPIE 5348, MOEMS display and imaging systems II, 2004, vol. 89. http://dx.doi.org/10.1117/ 12.525895. [2] Urano T, Nagao T, Takada A, Itoh H. Photosensitization mechanisms in photopolymer coating film containing photoinitiators sensitized by aminochalcone-type dye for computer-to-photopolymer plate. Polym Advan Technol 1999;10(4):244–50. [3] Monteiro MJ. Nanoreactors for polymerizations and organic reactions. Macromolecules 2010;43(3):1159–68. [4] Moad G. Solomon DH The chemistry of free radical polymerization. Elmsford, New York: Pergamon Press; 1995. [5] Nomura M. J Ind Eng Chem 2004;10:1182. [6] Yuan B, Wicks DA. Thermotropic color changing nanoparticles prepared by encapsulating blue polystyrene particles with a polyN-isopropylacrylamide gel. J Appl Polym Sci 2007;105(2):446–52. [7] Taniguchi T, Takeuchi N, Kobaru S, Nakahira T. Preparation of highly monodisperse fluorescent polymer particles by miniemulsion polymerization of styrene with a polymerizable surfactant. J Colloid Interf Sci 2008;327(1):58–62. [8] Tamai T, Watanabe M, Maeda H, Mizuno K. Fluorescent polymer particles incorporating pyrene derivatives. J Polym Sci Part A: Polym Chem 2008;46(4):1470–5. [9] Yuan B, States J, Sahagun C, Wicks D. Effects of three crosslinkers on colored pH-responsive core–shell latexes. J Appl Polym Sci 2008;107(6):4093–9. [10] Caruso F, Landfester K, Weiss CK. In: Caruso F, editor. Encapsulation by miniemulsion polymerization. Berlin Heidelberg: Springer; 2010. p. 1–49. [11] Skirtach AG, Antipov AA, Shchukin DG, Sukhorukov GB. Remote activation of capsules containing Ag nanoparticles and IR dye by laser light. Langmuir 2004;20(17):6988–92. [12] Chavez JL, Wong JL, Duran RS. Core shell nanoparticles: characterization and study of their use for the encapsulation of hydrophobic fluorescent dyes. Langmuir 2008;24(5):2064–71. [13] Tiarks F, Landfester K, Antonietti M. Encapsulation of carbon black by miniemulsion polymerization. Macromol Chem Phys 2001;202(1):51–60. [14] Takasu M, Shiroya T, Takeshita K, Sakamoto M, Kawaguchi H. Preparation of colored latex containing oil-soluble dyes with high dye content by mini-emulsion polymerization. Colloid Polym Sci 2003;282(2):119. [15] Takasu M, Shiroya T, Takeshita K, Sakamoto M, Kawaguchi H. Improvement of the storage stability and photostability of colored latex prepared by miniemulsion polymerization. Colloid Polym Sci 2004;282(7):740. [16] Takasu M, Kawaguchi H. Preparation of colored latex with polyurea shell by miniemulsion polymerization. Colloid Polym Sci 2005;283(7):805–11. [17] Zhao X, Zhou S, Chen M, Wu L. Effective encapsulation of Sudan black B with polystyreneusing miniemulsion polymerization. Colloid Polym Sci 2009;287:969–77.
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References
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
A thermo-sensitive imaging coating derived from polymer nanoparticles containing infrared absorbing dye Li An, Zhejing Cai, Weiwei Wang, Jialing Pu, Zhongxiao Li ⇑ Laboratory of Printing & Packaging Material and Technology, Beijing Institute of Graphic Communication, Xinghua Avenue (Band Two), Daxing, Beijing 102600, PR China
MACROMOLECULAR NANOTECHNOLOGY
a r t i c l e
i n f o
Article history: Received 24 May 2013 Received in revised form 19 December 2013 Accepted 5 January 2014 Available online 13 January 2014 Keywords: Polymer nanoparticles Infrared dye Laser-imageable Emulsion polymerization
a b s t r a c t The present study relates to a method for incorporating a near infrared dye (the absorption maximum at approximately 830 nm, IR-830) into polymer particles via miniemulsion polymerization and the preparation of an infrared laser-imageable coating based on the polymer particles as well as the potentiality in developing environment-friendly computer-to-plate (CTP) precursor. Polymer particles containing the IR-830 were prepared through miniemulsion polymerization technology in the presence of sodium dodecyl sulfate and hexadecanol as the emulsifier blend. Polyvinyl alcohol was used as a water soluble polymer binder resin for the production of latex coatings. Using the above prepared materials as the main components, an IR laser-imageable coating was prepared. Upon computer-controlled laser exposure, the IR dye-containing polymer particles absorb IR laser energy and produce high temperature, causing great changes of the imaged areas of the latex coating. As a result, the imaged areas could not be removed with water cleaning, whereas the polymer particles of the non-imaged areas remain unchanged, and still could be easily removed by water cleaning. When developing with water, negative graphics were obtained. The results of the research can be used in developing chemicalfree CTP plates required by green printing industry. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor. Computer to plate (CTP) is an imaging technology used in modern printing processes. In this technology, an image created in a desktop publishing (DTP) application is output directly to a printing plate, which can form a latent image by using the heat or the light of a laser [1]. There are several kinds of CTP plates such as photopolymer plates, silver halogen plates and thermal plates [2]. As thermal materials offer the advantage of daylight stability, thermal sensitive ⇑ Corresponding author. Tel.: +86 010 60261107. E-mail address: [email protected] (Z. Li). 0014-3057/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2014.01.002
printing plate precursors which form latent images by 830 nm infrared laser have become very popular at present. The latent images which are produced by 830 nm infrared laser exposure are not yet ready for printing and must be transformed into a durable printing image such as chemical development (Scheme 1). However, traditional development processes will generate a large number of pollution by using chemical reagents, therefore, chemical-free plates are extremely desirable for environment reasons. Emulsion polymerization is a unique technique which could produce polymers under the environmentallyfriendly process [3]. This technology using water as a reaction medium and easy handing of the final latex, thus, it is used to form continuous films in adhesives, paper coating, paints, etc. [4,5]. Miniemulsion polymerization offers the efficient method for the encapsulation of different materials. Thus it is popularly used to form complex structured
L. An et al. / European Polymer Journal 52 (2014) 166–171
167
polymeric nanoparticles [6–9]. Many different materials, ranging from organic and inorganic pigments, magnetite, or other solid nanoparticles, to hydrophobic and hydrophilic liquids, such as fragrances, drugs, or photoinitators, can be encapsulated to prepare functionalized nanoparticles [10–13]. Oil-soluble dyes dispersed in water to form colored polymer latex particles with encapsulation strategies have been gathering great attention in recent years. Many researches encapsulated oil-soluble dyes into polymer latex particles by miniemulsion polymerization method. For example, Takasu et al. [14–16] prepared polymer particles containing copper phthalocyanine dyes and styryl dyes as oil-soluble dye via miniemulsion polymerization using azo-bisisobutyronitrile as initiator. Zhao et al. [17] prepared polystyrene/Sudan black B latex particles also by a miniemulsion polymerization technique. All the above cases can obtain stable and durable colored latex. Thermal sensitive printing plate precursors usually employ infrared radiation (IR) dye and polymer resin. When these materials are exposed to infrared laser, the IR dye absorbs the laser energy to generated heat triggering physico-chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer, heat-induced solubilization or particle coagulation of a thermoplastic polymer latex. With the above mentioned process, the printing plate precursors can finally form a latent image. Thermal sensitive printing plate precursors with particle coagulation of a thermoplastic polymer imaging mechanism can easily realize chemical-free plate making, because unexposed polymer particles is aqueous coating layer made by thermoplastic polymer latex. Thus, the latent images can be transformed into a durable printing image by water. In this case, encapsulation of oil-soluble IR dyes into thermoplastic polymer particles via miniemulsion polymerization can make the water development printing plate come true. The mechanism of this case is
shown in Scheme 2. Although many researchers concerned encapsulation oil-soluble dyes into polymer particles by miniemulsion polymerization, to the best of our knowledge, few articles concerning IR dye encapsulation in polymer particles via miniemulsion polymerization method and its application in thermal sensitive imaging system have been published as so far. In this paper, we present the preparation of the IR dyecontaining polymer nanoparticles and study the use in lithographic printing plate, which can be imaged using digitally controlled laser output and developed with neutral water. 2. Experimental 2.1. Materials Styrene, acrylonitrile, methyl methacrylate (MMA) and butyl methacrylate (BMA) which were purchased from Beijing Chemicals Co. were distilled under reduced pressure and stored at 15 °C. Sodium dodecyl sulfate (SDS) and hexadecanol as the emulsifier blend, L-ascorbic acid (LAA) and tert-butyl hydroperoxide (TBH) as redox initiator, polyvinyl alcohol (PVA) as binder polymer and the IR dye (Fig. 1) with the maximum absorption near 830 nm (IR-830) were used as received. They were all commercial products from Acros Organics. Distilled deionized water (DDW) was used as the polymerization medium. 2.2. Preparation of the polymer nanoparticles containing IR830 by miniemulsion polymerization The aqueous phase was formed by a mixture of SDS (0.4 g), LAA (0.15 g) and DDW (50 ml) with magnetic stirrer for several minutes. Styrene (9 g) and acrylonitrile (6 g) solution of hexadecanol (0.1 g) and various amount
Scheme 2. Schematic diagram of the laser imaging process by water development.
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Scheme 1. The imaging process of conventional CTP technology.
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L. An et al. / European Polymer Journal 52 (2014) 166–171
H3C
CH3
CH3
Cl
Cl
N
N
CH3
CH3
H3C
SO3
Fig. 1. Molecular structure of IR-830 dye.
MACROMOLECULAR NANOTECHNOLOGY
Table 1 Recipes and global results.
a b c
Run
[IR-830]/[monomers]a
Dye contentb
Conversion (%)
dp (nm)c
PDI
1 2 3 4
0 2.5% 3.5% 4.4%b
0 2.7% 4.1% 5.2%
92 92 88 85
44.7 49.1 61.8 80.9
0.01 0.03 0.05 0.09
f potential (mV) 49.3 46.0 42.7 38.3
Concentration ratio is weight ratio. This concentration is saturation state for IR-830 in syrene/acrylonitrile (3:2) mixtures. Number-average diameter (dp), its polydispersity index (PDI) and f potential were measured by using a Malvern ZETASIZER Nano at 25 °C.
of the IR-830 were added to the aqueous phase and stirred at room temperature for 10 min. Table 1 presents the different weight ratios between IR-830 and monomer that used in the emulsion polymerizations. Subsequently, the mixture was treated with a high pressure homogenizer for 5 min. The miniemulsion obtained after high pressure homogenization was placed in a four-necked flask and mechanically stirred for 0.5 h at 40 °C. Aqueous solution of TBH (0.12 g) in water (10 ml) was added into the mixture by peristaltic pump to start the polymerization. The feeding rate was set at 3 ml/min. The polymerization was carried out under nitrogen atmosphere at 40 °C for 1 h. Finally, the IR dye-containing poly(styrene-co-acrylonitrile) (PSA) emulsion was filtered with a Buchner funnel. The filtrate was collected and kept for later use.
2.3. Preparation of the thin coating derived from the polymer nanoparticle miniemulsion and evaluation of the imaging performance In a typical experiment, PVA water solution (3.0 g, 10 wt.%) were successively added to the prepared polymer emulsion (20 g, 20.8 wt.%) under magnetic stirring. The mixture was spin-coated on a clean anodized aluminum plate (aluminum plate for offset printing with a specially prepared surface), and this was followed by drying at 60 °C in an oven for 10 min. Then the sample plate with a thin coating was mounted on the exposure device for laser scanning. Finally, the exposed plate was developed with water at 25 °C and the surface topography was recorded with optical microscope and scanning electron microscope (SEM).
2.4. Characterization The solubility of IR-830 dye in solvent and monomer conversion was obtained by gravimetry. The encapsulated IR-830 ratio was measured with an UV-2501PC spectrophotometer at room temperature. IR-830 or dried polymer particles were dissolved in THF/ethanol (1:1 by volume) and the concentration of the solution was adjusted to from 0.001 mg/ml to 0.05 mg/ml. If the IR-830 dye concentration is low enough, the absorbance of solution obeys Beer’s law. Then, the IR-830 dye content in the colored latex was determined from the absorbance at kmax (826 nm in THF/ ethanol) using the calibration curve of the absorbance at 826 nm versus IR-830 concentration (Fig. 4). Absorption spectra of IR-830 dyes are shown in Fig. 3. The numberaverage particle size (dp) and f potential measurements were done by a Malvern ZETASIZER Nano at 25 °C. All latex dispersions were diluted to 500 times with DDW and filtrated through 0.45 lm filters before the measurement. For each measurement, the obtained number-average particle size data was averaged over three individual run. The morphology of the polymer particles in emulsion was measured by transmission electron microscopy (TEM) with an H-7000 apparatus (Hitachi, Japan). Differential scanning calorimetry (DSC) was recorded on a Netzsch DSC200PC analysis apparatus at a heating rate of 10 °C/min. Laser imaging was conducted on a TP-46XX thermal plate-setter (the laser wavelength is 830 nm and its pulse width is 10 ns, Hangzhou CRON Machinery & Electronics Co., Ltd.). Surface water contact angles were measured by Kruss Optical contact angle measuring instrument. Hitachi SEM SU8020 was used to characterize the morphology of plate surface.
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3. Results and discussion 3.1. Preparation of the IR dye-containing polymer particles
Fig. 3. Absorption spectra of THF/ethanol solutions of IR-830(0.004 mg/ ml) and the dried polymer particles of Run 2 (0.04 mg/ml).
each sample was dried in vacuum oven at 40 °C, and the dried polymer particles were dissolved in THF/ethanol (1:1 by volume) on a quartz plate. The prepared polymer
1.5
IR-830 IR-830 in Poly(S-co-A) nanoparticles(Run4)
1.0
0.5
0.0 0.000
0.001
0.002
0.003
0.004
Conc.[mg/ml] Fig. 4. Calibration curve of the absorbance at 820 nm versus dye concentration data (in THF/ethanol).
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Fig. 2. TEM photographs of IR-830-containing polymer particles.
Absorbance
Successful encapsulation of oil-soluble dye into polymer latex particles by miniemulsion polymerization method requires the oil-soluble dye having good solubility in monomer. Therefore, the solubility of IR-830 in various monomers was firstly investigated to determine the monomers candidates in this miniemulsion polymerization system. The solubility of IR-830 in acrylonitrile is about 10 wt.%, but very poor in styrene, MMA and BMA. Additionally, in considering the processing temperature required by the practical application, the glass transition temperature of the polymer nanoparticles should withstand the processing temperature (70–80 °C). Thus, styrene and acrylonitrile were chosen as the comonomers in this study. PSA nanoparticles containing IR-830 were prepared through miniemulsion polymerization of the styrene/acrylonitrile solution of the dye. Miniemulsion polymerization was initiated effectively with the addition of LAA/TBH aqueous solution. The IR-830 is quite sensitive to peroxides, and color fading will take place if TBH is added to the IR-830-containing pre-emulsion in one portion. However, IR-830 is quite stable in the presence of LAA. Thus, LAA was firstly mixed with pre-emulsion and TBH was added dropwise to avoid excessive oxidant in the reaction mixture. The active free radicals generated in the aqueous phase diffused into the monomer droplets and initiated polymerization. With the progress of emulsion polymerization, the color of the reaction mixture gradually deepened, and finally into a dark green. The conversion of monomer coexisting with IR-830 reached as high as above 85% for 1 h. The emulsion was quite stable and no precipitation was found. This can be supported by the high f potential value of the each obtained aqueous dispersion. In miniemulsion polymerization process, SDS and hexadecanol were used as emulsifier and co-emulsifier, respectively. The co-emulsifier makes it easier to reduce the particle size. It should be noted that high pressure homogenization is an indispensible process for producing a stable pre-emulsion with smaller droplet size. Dynamic light scattering (DLS) technique is used for measuring the final particle size and distribution of the emulsion. Table 1 shows that number-average diameters of the latex particles increased with increasing the concentration of IR-830 in the oil phase. It directly indicates that most of the IR-830 can be encapsulated into the latex particles. This phenomenon can be found in previous work [14]. The morphology of the polymer particles in emulsion was detected by TEM (Fig. 2), it manifested the PSA particles are spherical and monodisperse with a diameter of around 50 nm (Run 2), which was close to the number average particle diameter detected by DLS technique (Table 1). Fig. 3 depicts the UV–Vis spectra of IR-830 and the prepared polymer nanoparticles containing IR-830 (Run 2). The miniemulsion products were filtered through 450 nm microfilters to remove the dye molecule clusters. Then,
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nanoparticles containing IR-830 should have almost the same spectrum as the IR-830 in the mixed solution. As seen, the UV–Vis spectral wave length of IR-830 in mixed solution (the solid line) was almost the same as that of the prepared polymer nanoparticles containing IR-830 (the dash line), displaying a maximum absorption wavelength around 820 nm. That is to say, the IR-830 molecular structure did not change in the polymerization, and was distributed well in the polymer particles. The amount of IR-830 incorporated into the polymer particles was determined by using the calibration curve
3.2. Thermal properties of the IR dye-containing polymer nanoparticle
exo 0.4
DSC/mW/mg
0.2
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as Fig. 4. The absorbance at 820 nm of the mixed solution was measured (Fig. 3) and the IR-830 content of the polymer particles was calculated from the calibration curve (Fig. 4). The encapsulation IR-830 ratio of each run final polymer particles is a little higher than the initial ratio between IR-830 and monomers, because monomer conversions have not reached 100%. Thus, the latex particles contained IR-830 of each ratio in accordance with the theoretical value in consideration of monomer conversion. This is a reasonable result, reflecting successful full encapsulation of IR-830 by miniemulsion polymerization method.
0.0
-0.2
-0.4 0
100
200
300
400
Temperature (°C) Fig. 5. DSC analysis of the IR dye-containing polymer nanoparticle.
Fig. 6. SEM photographs of fusion of the latex polymer particles.
Thermal properties of the latex particle were measured by DSC (Fig. 5). The DSC curve shows a small exothermal peak around 104 °C, which is the glass transition temperature of PSA. As the result, the particles should withstand the processing temperature (70–80 °C) required for the coating preparation with the emulsion. That is to say, the polymer particles can retain the shape and structure during the coating preparation. Thus, the properties of the polymer particles should remain unchanged. This is important for producing an effective laser-imageable thin coating. The endothermic peak starting at about 350 °C is ascribed to the fast decomposition of PSA. 3.3. Laser imaging of the coating derived from IR dyecontaining polymer particles The aim of this study is to develop a water developable IR laser-imaginable coating based on the IR dye-containing polymer nanoparticles. As mentioned above, the coating on the aluminum substrate consists of the following components: the prepared polymer emulsion, PVA used as binder resin and small amount addition agents. The original coating is hydrophilic and can be easily removed with water cleaning. Scheme 2 depicts the laser imaging process. Laser imaging was carried out according to the procedure described in the experimental section. Upon imagewise laser exposure, the areas of the coating that received laser radiation cure to a durable oleophilic state by the action of heat. The IR dye-containing polymer particles fused and the original structure was broken down (Fig. 6). Because the hydrophobic polymer together with the IR dye comprised the majority (about 85%) of the coating, the areas which received laser radiation became resistant to water
Fig. 7. Drop of water on the anodized aluminum substrate (a) and the imaged area (b).
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Acknowledgements This research was supported by Beijing Municipal Education Commission (KZ201110015018), BIGC Project (E-b-2014-15) and The Project of Construction of Innovative Teams and Teacher Career Development for Universities and Colleges under Beijing Municipality.
Fig. 8. Micrograph of the IR laser-exposed coating after water cleaning (25 °C).
and remained on the substrate after the coating was subjected to water cleaning (i.e. the developing process). However, the unexposed areas of the latex film could be easily washed away by water because the particles remain intact and still possessed a hydrophilic surface. Fig. 7 shows the water drops on the anodized aluminum substrate and the imaged area. The contact angle of water drop on the anodized aluminum plate was 53°, and then it increased to 67°, when the coating surfaces were exposed by the IR laser. This indicated that the microstructure of the coating layer has been changed after laser exposure. As seen in Fig. 8, the dark regions are the exposed areas or the image areas of the coating, and the background regions of the aluminum substrates are the un-exposed areas or non-image areas. The IR laser-imagineable coating is negative working. The exposure dose was about 250 mJ/cm2. The micrograph of the image shows that the unexposed areas of the original coating were completely removed from the substrate. However, those of the exposed areas survived the developing process and remained on the surface of the substrate, producing a negative image with sharp dot edge. 4. Conclusions In this study, narrowly distributed IR dye-containing polymer nanoparticles were synthesized through miniemulsion polymerization. The polymer nanoparticles emulsion has a maximum absorption near 830 nm. A latex coating was prepared with the polymer nanoparticles emulsion and a water soluble binder polymer (PVA). Negative image was obtained through imagewise exposure by IR laser radiation followed with water developing. This kind of coating derived from the IR dye-containing polymer particles can be used as the precursor for developing chemical-free thermal CTP plate.
[1] Tamaki E, Hashimoto Y, Leung OS. Computer-to-plate printing using the Grating Light Valve device. In: Proc. SPIE 5348, MOEMS display and imaging systems II, 2004, vol. 89. http://dx.doi.org/10.1117/ 12.525895. [2] Urano T, Nagao T, Takada A, Itoh H. Photosensitization mechanisms in photopolymer coating film containing photoinitiators sensitized by aminochalcone-type dye for computer-to-photopolymer plate. Polym Advan Technol 1999;10(4):244–50. [3] Monteiro MJ. Nanoreactors for polymerizations and organic reactions. Macromolecules 2010;43(3):1159–68. [4] Moad G. Solomon DH The chemistry of free radical polymerization. Elmsford, New York: Pergamon Press; 1995. [5] Nomura M. J Ind Eng Chem 2004;10:1182. [6] Yuan B, Wicks DA. Thermotropic color changing nanoparticles prepared by encapsulating blue polystyrene particles with a polyN-isopropylacrylamide gel. J Appl Polym Sci 2007;105(2):446–52. [7] Taniguchi T, Takeuchi N, Kobaru S, Nakahira T. Preparation of highly monodisperse fluorescent polymer particles by miniemulsion polymerization of styrene with a polymerizable surfactant. J Colloid Interf Sci 2008;327(1):58–62. [8] Tamai T, Watanabe M, Maeda H, Mizuno K. Fluorescent polymer particles incorporating pyrene derivatives. J Polym Sci Part A: Polym Chem 2008;46(4):1470–5. [9] Yuan B, States J, Sahagun C, Wicks D. Effects of three crosslinkers on colored pH-responsive core–shell latexes. J Appl Polym Sci 2008;107(6):4093–9. [10] Caruso F, Landfester K, Weiss CK. In: Caruso F, editor. Encapsulation by miniemulsion polymerization. Berlin Heidelberg: Springer; 2010. p. 1–49. [11] Skirtach AG, Antipov AA, Shchukin DG, Sukhorukov GB. Remote activation of capsules containing Ag nanoparticles and IR dye by laser light. Langmuir 2004;20(17):6988–92. [12] Chavez JL, Wong JL, Duran RS. Core shell nanoparticles: characterization and study of their use for the encapsulation of hydrophobic fluorescent dyes. Langmuir 2008;24(5):2064–71. [13] Tiarks F, Landfester K, Antonietti M. Encapsulation of carbon black by miniemulsion polymerization. Macromol Chem Phys 2001;202(1):51–60. [14] Takasu M, Shiroya T, Takeshita K, Sakamoto M, Kawaguchi H. Preparation of colored latex containing oil-soluble dyes with high dye content by mini-emulsion polymerization. Colloid Polym Sci 2003;282(2):119. [15] Takasu M, Shiroya T, Takeshita K, Sakamoto M, Kawaguchi H. Improvement of the storage stability and photostability of colored latex prepared by miniemulsion polymerization. Colloid Polym Sci 2004;282(7):740. [16] Takasu M, Kawaguchi H. Preparation of colored latex with polyurea shell by miniemulsion polymerization. Colloid Polym Sci 2005;283(7):805–11. [17] Zhao X, Zhou S, Chen M, Wu L. Effective encapsulation of Sudan black B with polystyreneusing miniemulsion polymerization. Colloid Polym Sci 2009;287:969–77.
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References