Advanced Materials Research Online: 2014-05-07 ISSN: 1662-8985, Vol. 937, pp 508-514 doi:10.4028/www.scientific.net/amr.937.508 2014 Trans Tech Publications, Switzerland Research on Environmentally Friendly Water-based Covering Ink and Its Application in Scratch-type Invoice Printing Ying GONG 1,a, Bei-qing HUANG 1,b, and Xian-fu WEI 1,c* School of Printing and Packaging Engineering, Beijing Institute of Graphic Communication, Beijing 102600, China a gyer0716@163.com, b huangpeiqing@bigc.edu.cn, c weixianfu@bigc.edu.cn Keywords: Environmentally friendly water-based covering ink, Ink performance, scratch-type invoice printing, Environment protection. Abstract. With the development of printing technology, greater emphasis has been laid on environmentally friendly materials. Due to its printability and environmental friendliness, Water-based flexo ink is gradually gaining popularity at home and abroad, and has been widely applied in the field of printing. Different from other kinds of bills, scratch-type invoices have an additional scratch area on the surface, which highly covering performance is required for the ink applied. This paper emphatically analyzes and discusses: (a) flexo water-based covering ink and its application in scratch-type invoice covering region printing; (b) how to explore and evaluate new developed green inks. Compared with traditional inks, the water-based flexo ink is free of emissions of volatile organic compounds (VOC), and its popularization might have significant meanings for the environment protection and sustainable development. Introduction With the development of society, printing becomes indispensible and plays a greater role in the industry. But unfortunately, printing industry produces lots of pollution, of which solvent-based ink is to blame [1-3]. Solvent-based ink uses organic solvent and is criticized for the emission of volatile organic compounds (VOC), which does harm to human body as well as the environment [4]. As people's awareness of environment protection increases, it becomes demanding for printing field to explore new environmentally friendly materials [5]. Under the call for sustainable development, the green inks, which mean environmentally friendly, are being explored [6-9]. The covering ink is an opaque ink that can protect the information subjacent to it from being perceived before scratched. It has already been used in the printing of scratch-type invoice [10]. The scratch-type invoice (e.g. Fig.1) is a kind of business voucher of economic activity, and the anti-counterfeiting methods have strict requirements on the covering ink s covering property (CP). Within the past 10 years, due to the development in new inks and advancement in the printing technology, flexographic printing, favored by its environment protection performance, has become a very influential printing method, hankering for new environmentally friendly inks to enhance its development [11-13]. This also can be inferred from the huge and increasing demand of flexo covering inks every year. Invoices have different subspecies. They differentiate in social functions but have the same demand for anti-counterfeiting methods. On the surface of the invoice, the covering ink can be scraped by a coin or fingernail, and then the subjacent information can be perceived. So the covering region (Fig.1), for security reasons, have special requirement on the CP of the covering ink applied. The water-based covering ink being discussed in this paper is one of the green inks and experiment has been carried out to test their printability in scratch-type invoice covering region printing. 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 Trans Tech Publications, www.ttp.net. (ID: 130.203.136.75, Pennsylvania State University, University Park, USA-11/05/16,10:17:16)
Advanced Materials Research Vol. 937 509 Environmentally Friendly Water-based Ink Literally, water-based ink uses water as solvent that contains aqueous emulsion polymer resin, organic pigments and related additives made by a mixture of physical and chemical processes. Water-based ink does not contain volatile organic solvents, so that it can greatly reduce the emissions of VOC, thus causing less air pollution, bettering printing operating environment. Water-based ink is free of toxic and hazardous substances which are harmful to the human, the packaged goods and the environment, thus making it environmentally friendly. It is particularly suitable for the packaging of tobacco, alcohol, food, beverages, medicines, children's toys and other products which have strict requirements on health, packaging and printing conditions. Furthermore, it not only can reduce the risk of fire caused by static electricity, flammable solvents and pitfalls, but also can reduce the residual toxicity on the printing surface, and the printing equipment is easy to clean. Water-based ink is also called a new type of environmentally friendly printing materials. Scratch-type Invoice Printing The Composition of Scratch-type Invoice Fig.1 is a sketch map of the surface composition of an invoice, on the surface, the words, numbers and images are commonly printed by hectograph, while the covering region is mainly printed by flexograph. Excellent transparency outer coating (light oil in Fig.1) must be implemented after the offset printing process. The coated layer stabilizes the printed graphic as well as enhances the covering ink s scratch resistance [14]. After that, the flexo covering ink can be printed on the coated layer, to prevent certain information to be perceived. Fig. 1. Sketch map of scratch-type invoice surface composition Requirements of Water-based Ink in Scratch-type Invoice Printing (A) The water-based ink applied in scratch-type invoice covering region printing should be stable and should be easily stripped by fingernails or coins. Invoices commonly have a circulation of one year, which require that the covering ink should keep in good condition during the circulation. (B) After the covering ink is being removed, the revealed subjacent information should be clear, without the contamination of the water-based ink. (C) The CP of the water-based ink should be high enough to make the covering region opaque. (D) High toughness of the ink film of the water-based ink is also required. The ink film should not be mangled by operations, such as moving, drying or cutting. It also should bear the ability to resistant other forms of external forces during the circulation. It can be safely drawn that (C) and (D) is almost decided by the properties of covering ink, while (A) and (B) is determined by the subjacent light oil and degree of ink adhesion. It can be inferred that the water-based ink plays a primordial role in the invoices covering region printing.
510 Material Science and Environmental Engineering Water-based Covering Ink and Its Application in Scratch-type Invoice Printing The ink s CP is a key factor in manufacturing scratch-type invoice. The water-based covering should have good printability as well as good CP. The CP should be high enough to cover the words, numbers and graphs to protect the information, the ink film should be liable to stripe off, resistant to high temperature and severe cold. It also should have a storage life of at least one year. The authors have carried out a series of experiments, evaluating different compositions of water-based covering ink and their printability, and have obtained feasible evaluation methods and system to test the overall performance of the water-based covering ink applied in scratch-type invoice printing. CP Ink film thickness has the most influence on CP. If the film thickness is too thin, the subjacent information is perceivable, thus making the invoice useless; if the thickness is too thick, it takes a long time to dry, thus reducing the production efficiency. So, in the printing process, ink film thickness should be strictly controlled. In the printing practice, the film thickness varies and is experiential dependent. The film thickness subjects to the principle that as long as it can properly cover the subjacent layer, the thinner, the better. Apart from this, the ink dispersion is another factor affecting the CP, the greater the dispersion is, the higher the CP of the ink is. Compositions can be altered to improve the ink dispersion, thus enhancing the CP. The Ref.310 CP Tester is applied for the test of ink s CP value. Generally, white CP paperboard is used for the test of black and color ink, while black CP paperboard is prepared for the test of white ink. In this experiment, ink sample is applied to the white CP paperboard; RB is the testing value of the black grids on the paperboard, while RW is the testing value of the white grids. The CP value can be calculated by the equation: CP=RB/RW. The greater the CP value is, the better CP performance can be achieved, vice versa. Inks with different pigment content achieve different CP; the testing results are shown in Table 1. Pigment content[%] Table 1. Different pigment contents and their CP 20 23 25 28 30 33 35 38 RB 61.1 62.7 64.1 64.4 65.5 65.9 66.0 66.2 RW 71.3 72.7 74.2 74.5 75.7 76.1 76.2 76.4 CP 0.8569 0.8624 0.8639 0.8644 0.8653 0.8660 0.8661 0.8665 As it can be seen from Table1 and Fig.2, when the pigment content is within 20%~38%, with the pigment content increases, the CP value gets better gradually, indicating that the ink s CP performance is being enhanced. When the pigment content comes to a certain extent, the content of pigment increase has little influence on CP value. For the experimented water-based samples, 38% pigment content is highly recommended, for it achieves the best CP with acceptable pigment content. However, it must be pointed out that the optimal pigment content varies with different ink formula.
Advanced Materials Research Vol. 937 511 Fig. 2. Pigment content and its CP Scratch Resistance (SR) Covering ink is printed on the surface of the light oil (Fig.1), certain SR is required. For the circulation of the invoices, good SR is required to remain at least one year. And after the covering ink is being removed, the revealed subjacent information should be clear and intact. The Scratch Tester is applied for the test of ink s SR value. During the process, the testing samples are fixed on the texting board. A fixed stylus which is 45 o (from vertical) scratches the surface of the testing sample. By pushing the testing board, adding the pressure on it until the stylus stripped the ink layer off. The pressure on the testing board can be converted into the SR value. In scratch-type invoice covering region printing, the testing sample s normal temperature resistance(nt) are required at 75~100g, after heated in the condition of temperature(t) =80, humidity(h) =80% for one hour(h80t80-1), 120~150g, after heated in T=80, H=80% for two hours(h80t80-2), 130~175g. After heated in T=80, H=80% for twenty four hours(h80t80-24), the acceptable scratch resistance is under 450g. The testing samples SRs are shown in Table 2. Table 2. SR value after different conditions (UTSO: unable to scratch off) Ink Samples 1# 2# 3# 4# 5# 6# NT 100 75 100 150 125 200 SR[g] H80T80-1 150 100 150 250 150 300 H80T80-2 250 150 300 400 300 450 H80T80-24 500 400 500 UTSO UTSO UTSO It can be seen clearly from Table 2 that some of the inks explored do not fit for invoice covering region printing. Sample 4#, 5#and 6#fail all the test; sample 1#, 3# have acceptable SR in NT and H80T80-1 test, but fail in the H80T80-2 and H80T80-24 test. Sample 2# meets all the requirements in the SR test and is hopeful for the practice application. Further improvement of Sample 2# is underway. Stability Covering ink should have excellent stability. Invoices commonly have a circulation of one year, which requires that the covering ink keep in stable condition during the circulation. Any crack and contamination are undesirable. High temperature and humidity resistance is also required. If the water-based covering ink is of good resistance, when it is exposed to T=80, H=80% conditions, it can still holds acceptable CP and SR performance. To grantee the subjacent information from being perceived or detected by other apparatus, enough film thickness (usually 20~40um) is necessary.
512 Material Science and Environmental Engineering Drying Performance (DP) In general, water-based flexo ink can dry by evaporation, infiltration and curing reaction or combination of the three ways. Fast-drying is a notable feature of water-based flexo ink. In scratch-type invoice covering region printing, due to the thicker ink film, DP is of great concern. Practically, DP is associated with ph, from the experiment conducted; the ph and the corresponding DP are shown in Table 3. Table.3 Ink samples ph and corresponding DP ph Drying Performance <8.5 viscosity increases, enhance drying speed, scumming 8.5~9.5 prime printability and stability >9.5 ink viscosity, drying speed and water resistance are reduced Flexo printing ink is generally weak alkaline, normal ph range is 8.5 ~ 9.5, within which the water-based ink has prime printability and stability. When the ph value is higher than 9.5, the ink viscosity, drying speed and water resistance are reduced; when the ph value is lower than 8.5, water-based ink viscosity increases, enhance the drying speed, partially causing dried ink deposit on the plate and reticulate roller, and that leads to scumming. Water-based ink drying mechanism is shown in Table 4. Water and solvent volatilization. Table 4. Water-based ink drying mechanism Removal of amine Removal of water Viscosity Viscosity is the macro performance of ink cohesion, it is a physical attribute that represent the characterization of fluid molecules attraction which hinders the motion of the molecular. Viscosity directly affects the quality of printing products [15]. If the viscosity is too low, defects such as color dodge, dot gain, image blur may occur; if the viscosity is too high, problems such as poorly anilox roller ink delivery, uneven ink dispersion, filling in, scumming and poorly printability might occur. Flexographic printing ink viscosity is low, the range of viscosity of commonly used flexographic printing ink is 0.15~0.3Pa s (25 ). An optimal covering ink viscosity for scratch-type invoice covering region printing is not recommended here, but we strongly suggest that ink viscosity be taken into consideration when evaluate the overall performance of the test ink. Overall Evaluation and Rating System Due to environmentally friendliness, water-based ink has been explored and developed by numerous researchers and institutions. But not all the water-based inks developed are fit for the requirements. Pre-test of the prepared ink before put into use is fundamentally important. If an inappropriate ink come out of the laboratory and put into mass production, huge damage might occur both financially and environmentally.
Advanced Materials Research Vol. 937 513 Fig. 3. Core profile of overall evaluation and rating system During the experiment, in order to grantee all the inks developed meet all the requirements, an overall evaluation and rating system(fig.3) has been developed based on the standard acquired by the ink research and development laboratory, Beijing Institute of Graphic Communication. For each parameter, the higher the value is, the better performance can be achieved. The rating system proves to be feasible and can be applied in the development of other water-based inks. In Fig.3, the SC(standard curve) is the standard requirements for the testing ink, if the TC(testing curve) is within SC, it indicates that the testing sample has to be improved before put into use; if the TC is outside of the SC, it meets all the requirements and may put into practice. The TC in Fig.3 is one of the water-based covering ink developed, the TC shows that it meets all the requirements and has a overall rating score of 8 out of 10. Additional experiments have proved its practice use feasibility, which is in accord with Fig.3. Further Research Water-based ink is environmentally friendly printing material, and its application in practice has already made great contribution to the environment protection. However, the demand for new variety of water-based ink exceeds supply and existing water-based inks may have certain kinds of defects. So the research on water-based covering ink and its application in scratch-type invoice printing might be counted as the incipient work on how to explore and evaluate water-based ink. Further research on develop new types of water-based inks and improvement of their overall performance is due to carry out. Acknowledgement This research was financially supported by the Innovation Ability Enhancement Project of Beijing TJSHG201310015015. References [1] D.Z. Zhou, S.M. Song, C.Dong, Water-based ink development review (in Chinese), J. Chem.Technol. Market. 2007(12):6-10. [2] L.L. Lou, P.P. Jiang, J.L. Xia, Water-based ink research status (in Chinese), J. Chem. new Mater. 2013(1):9-11+17. [3] X.L. Xin, Y.N. Wei. Water-based ink review (in Chinese), J. Chin. Printing and Packaging Res. 2011(3):1-8. [4] Y.Chen, L.N. Li, C.Q. Yang, Countermeasures of priority control of toxic VOC air pollutants (in Chinese), J. Environ. Sci. 2011(12):3469-3475. [5] L.C. Wang, Green road for printing inks (in Chinese), J. Today's printing. 2011(3):37-38.
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Material Science and Environmental Engineering 10.4028/www.scientific.net/AMR.937 Research on Environmentally Friendly Water-Based Covering Ink and its Application in Scratch-Type Invoice Printing 10.4028/www.scientific.net/AMR.937.508