A study on a functional emulsion coating and infrared laser-induced

imaging performance

Junjie Gong1,a, Zhongxiao Li1,b ,Jialing Pu2,c

1Lab. of Printing & Packaging Material and Technology,

Beijing Institute of Graphical Communication; Beijing, P. R. China.

2State Key Laboratory of Pulp and Paper Engineering,

South China University of Technology, Guangzhou, China

aemail：gongjunjie891111@126.com

bemail：lizhongxiao@bigc.edu.cn

cemail：pujialing@bigc.edu.cn

Keywords: Polymer nanoparticle; Infrared dye; Laser-induced imaging

Abstract. In isopropanol-water mixture, a polymer emulsion was prepared through the radical

copolymerization[1,2]of styrene, acrylonitrile and 4-vinyl pyridine in the presence of

azodiisobutyronitrile (AIBN) and a reactive emulsifier which was synthesized through the addition

reaction of 2-isocyanatoethyl methacrylate and methoxypolyethylene glycols (Mn ≈ 2000). The

polymer particles were narrowly distributed and had an average size of about 110 nm in diameter.

Hydroxypropyl cellulose was used as the binder resin for the emulsion film. The emulsion coating

was prepared by coating the mixture of the polymer emulsion and the water solution of the binder

resin on aluminum substrate and dried at about 80˚C. The resulted film can be easily removed from

the substrate with water rinsing. However, once the coating is heated at a temperature which is

much higher than the glass transition temperature (Tg) of the particle polymer (e.g., 150˚C) for a

short period of time, it could no longer be removed from the substrate by water. On the basis of the

above work, a water soluble infrared-absorbing dye (IR dye) was incorporated into the emulsion

coating and exposed to computer-controlled IR laser (830nm) scanning. The exposed areas could

not be removed with water cleaning, whereas the non-exposed areas could still be easily removed

with water. Negative images were obtained. Hence, the emulsion coating can be used in developing

chemical-free CTP plates required by green printing industry.

Introduction

Water-developable information recording materials [3,4,5] have attracted much attention because

they should be environment-friendly, easy to operate and economic, etc. Most water-developable

materials involved chemical reactions, which were either induced by light or acid or other actions

during the application processes[6]. These materials have found wide applications in developing

computer-to-plate (CTP) plates. For example, a CTP plate based on thermo-fusable plastic particles

has been gotten to market, and the image formation is a simply physical process and can be

developed simply by neutral water .

Advanced Materials Research Vols. 1004-1005 (2014) pp 89-93 Submitted: 28.05.2014Online available since 2014/Aug/13 at www.scientific.net Accepted: 07.06.2014© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.1004-1005.89

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 130.207.50.37, Georgia Tech Library, Atlanta, USA-12/11/14,06:00:45)

In this paper, polymer particles with a hydrophilic shell were prepared by means of a simple

one-step method through precipitation polymerization. Preliminary works were also done to prepare

a thermo-sensitive[7] water-developable emulsion coating and investigate the imaging performance.

Experimental

Materials and methods. Styrene, acrylonitrile, 2-isocyanatoethyl methacrylate,

methoxypolyethylene glycols (MPEG-2000, Mn = 2000), azodiisobutyronitrile (AIBN), 4-vinyl

pyridine, isopropanol, dibutyltin dilaurate and hydroxypropyl cellulose were commercial products

from Beijing Chemicals Co. Styrene and acrylonitrile were purified by vacuum distillation before

use. Water soluble IR dye with the maximal absorption at 830nm (IR830 Fig. 1) was synthesized in

our laboratory.

Fig. 1. Chemical structure of IR-830

FTIR spectra were obtained with a Shimadzu FTIR-8400 infrared spectrophotometer. The

average particle size and distribution were measured by scanning electron microscope (SEM)

(S-4800, Hitachi). Differential scanning calorimetry (DSC) was recorded on a Netzsch DSC200PC

analysis apparatus. The thickness of the latex films was measured with a FTS-S3c ultra surface

analysis instrument (Taylor Hbson Ltd., England). Laser exposure was carried out on a TP-46XX

thermal plate-setter (the laser wavelength is 830nm and its pulse width is 10ns, Hangzhou CRON

Machinery & Electronics Co., Ltd).

Preparation of the polymer emulsion. In a 500ml, four-necked flask equipped with a mechanical

stirrer and a reflux condenser was placed 30g of MPEG-2000 and 2.33g of 2-isocyanatoethyl

methacrylate. The mixture was heated to 60˚C to give a clear solution. Then 0.1g of dibutyltin

dilaurate was added and stirred for 3h. A solution of isopropanol (600g) and water (180g) was

added to the reaction mixture and the temperature was raised to 80˚C. With good stirring under

nitrogen, a solution of AIBN (1.0g) in styrene (100g), 4-vinyl pyridine (40g), acrylonitrile (50g)

was added dropwise over 6 h. After the addition, the mixture was kept at 80˚C for 8h, and then 0.5g

of AIBN was added and stirred for another 8h. The mixture was allowed to cool to room

temperature with constant stirring. Finally, the poly(styrene-4-vinyl pyridine-acrylonitrile) (PSVA))

emulsion was filtered with a sintered filter funnel and the filtrate was collected and kept for later

use.

Preparation of the emulsion coating derived from the polymer emulsion and evaluation of

infrared laser-induced imaging performance. 0.18g of IR-830, 8.0g aqueous solution of

hydroxypropyl cellulose (5 wt.%) and 1.86g of water were successively added to the prepared

polymer emulsion (20g, 22.2 wt.%) under magnetic stirring. The mixture was filtered with a

sintered filter funnel and the filtrate was spin-coated on a clean anodized aluminium plate

(aluminium plate for offset printing with a specially prepared surface), and this was followed by

drying at 100˚C for 5 min. Then the sample CTP plate was mounted on the exposure device for laser

90 Advanced Materials and Technologies

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).

Results and Discussion

Preparation and properties of the PSVA emulsion. PSVA emulsion was prepared through

precipitation co-polymerization of styrene, 4-vinyl pyridine and acrylonitrile in the presence of a

reactive surfactant, which was derived from MPEG-2000 and 2-isocyanatoethyl methacrylate. The

reactive macro surfactant formed a hydrophiphillic shell around the particle surface. The emulsion

was detected by SEM (Fig. 2). The average diameter of the polymer particles was around 120 nm.

Fig.2. TEM of PSVA polymer particles

Thermal properties of the latex particles were measured by DSC. The DSC curve (Fig. 3) shows a

small exothermal peak around 106˚C, which is the glass transition temperature of PSVA. Therefore

the particles should resist the high temperature (100˚C) during the drying process of the emulsion

coating. In other word, the shape and structure of the polymer particles will remain unchanged

during the coating preparation. This is verified that the dried emulsion coating could be easily

washed off by water from the substrate. The endothermic peak starting at about 350˚C is due to the

fast decomposition of PSVA.

0 100 200 300 400

-0.4

-0.2

0.0

0.2

0.4

DS

C/

mW

/mg

Temperature (oC)

exo

Fig. 3. DSC analysis of the polymer particles

Laser imaging of the coating derived from the polymer particles. In this study, we investigated

the possibility of a water-developable laser imaging material based on polymer emulsion, which can

be used to develop environmentally friendly CTP plate. As mentioned, the laser imaging coating

consists of the following main components: the polymer emulsion, water soluble IR-absorbing dye

Advanced Materials Research Vols. 1004-1005 91

and hydroxypropyl cellulose acting as the binder resin. The coating is originally very hydrophilic

and the contact angle with water at 25°is smaller than 30°. However, once the coating was heated at

a higher temperature (e.g., 150˚C) for short period of time, the contact angle with water of the

coating became larger than 65°. The affinity change of the coating was the result of thermo-induced

damage of the particle structures. Moreover, the heat-treatment process also made the coating

adhered to the substrate and difficult to remove. Fig. 4 depicts the laser imaging process. Laser

imaging was conducted with the laser energy density of 180 mJ/m2. Upon imagewise IR laser

scanning, the particles in the imaged areas of the coating fused and lost water dispersibility. This is

because that the hydrophobic polymer, PSVA, comprised the majority (about 80%) of the coating,

the areas which received laser energy became resistant to water cleansing (i.e. the developing

process). In addition, the 4-vinyl pyridine unity of PSVA should hydrogen-bonded complexatioin

with the sulfonic acid group of the water-soluble IR dye, which is beneficial to the enhancement of

the water resistance of the imaged coating. However, the non-imaged areas of the coating were

easily washed away by water because the particles remain intact and could still be dispersed in

water. As seen in Fig. 5, the dark areas are the imaged areas of the coating, and the light-colored

areas belong to the aluminium substrate where the coating was completely removed with water. The

exposure dose was set at about180 mJ/cm2. The coating thickness is about 1µm (1.7 g/m

2).

Fig. 4. Laser imaging process of the functional emulsion coating

Fig. 5. Micrograph of the IR laser-exposed coating after water developing (25 oC)

Conclusions

In this study, stable and narrowly distributed polymer emulsion was prepared through

precipitation polymerization in the presence of a reactive surfactant. Uniform emulsion coatings

were made with the emulsion and the water soluble binder polymer, which showed good

re-dispersing performance in water. When a water soluble IR-absorbing dye was incorporated into

92 Advanced Materials and Technologies

the emulsion coating, it showed sensitivity to IR laser and gave clear negative image after water

developing. This kind of thermo-sensitive coating based on polymer particles may be used in

developing chemical-free thermal CTP plate.

References

[1] De. Brouwer. H, M. A. J. Schellekens, B. Klumperman, M. J. Monteiro and A. L. German:

Polymer Chemistry. 38 , (2000), p. 3596

[2] S. V. Arehart and K. Matyjaszewski: Macromolecules. 32 , (1999), p. 2221

[3] S. L. Chu, E. Golda and A. L. Wilkes, U.S. Patent 4,104,072. (1978)

[4] K. Sakurai and S. Diego, U.S. Patent 4,540,649. (1985)

[5] K. Ishikawa, K. Konishi and H. Kusuda, U.S. Patent 5,175,076. (1992).

[6] Z. X. Li, Y. G. Feng, L. H. Li, J. L. Pu and H. J. Yang, European Polymer Journal. 44, (2008), p.

1899

[7] X. C.Xiao, L. Y. Chu, W. M. Chen, S. Wang and Y. Li, Advanced Functional Materials. 13,

(2003), p. 847

Advanced Materials Research Vols. 1004-1005 93

Advanced Materials and Technologies 10.4028/www.scientific.net/AMR.1004-1005 A Study on a Functional Emulsion Coating and Infrared Laser-Induced Imaging Performance 10.4028/www.scientific.net/AMR.1004-1005.89