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Preparation of High-Tack Value Adhesive for Offset Printing Inline Cold Foil Stamping and Its Performance Characterization.

INTRODUCTION

Gold foil stamping, which is also known as hot stamping, is a method of transferring metallic foil or holographic laser anticounterfeiting metallic foil onto paper through the use of pressure and high temperature [1]. This method is a decoration technology widely used for printing packaging products. Offset printing products with high printing accuracy and colorful expression occupy over one-third of the market shares of printed packaging materials [2, 3], Cold foil stamping, also known as cold foil printing, is a modern method of printing metallic foil on a substrate to enhance the esthetic of the final product. Cold foil printing can be done two ways: the dry lamination process common in the offset printing industry, or the wet lamination process, which is dominant in the flexo label industry. Cold foil takes the idea of hot foil stamping and makes it more convenient and cost-effective. The cold foil functions like an additional ink, very fast lamination adhesive and can be bonded inline in a single run using a printing plate for either flexographic web printing or offset sheet-fed printing. It can be applied precisely with high resolution, even for fine structures such as raster gradients and thin lines. Typeface is legible from about 5 points upwards and has excellent edge definition [4], The first case of cold foil stamping was was produced on a Manroland 706 printing press by a collaboration between German stamping foil manufacturer Kurz and ink producer Zeller + Gmelin [5].Offset printing inline cold foil stamping is done by reconstructing two printing units of offset press. On the first printing unit, adhesive is pressed on paper surface using an offset forme. In regions pressed with adhesive, that is, intend-stamped dots, lines, planes, characters, and images, metallic foil will be transfered to the regions on the surface of paper. On the second printing unit, metallic foil will pass through the space between the dedicated rubber blanket and the impression cylinder. The regions with adhesive will adhere to the metal layer of coldstamping foils onto the paper, whereas the base substrate of the cold-stamping foil and "resting" metal foils in the free region will be collected by the rewinding unit at the top of the third printing unit. The traditional multi-color printing of papers started from the third printing unit. Inline cold foil stamping can copy dots, lines, planes, minor details of images, and screen tone dots. The resolution can reach 150 dpi, and the reproducible excellent percent dot is 15% to 90%. ROLAND 500 and ROLAND 700 offset press are equipped with inline cold foil stamping, which is also called as Inline Foiler [6], Offset printing inline cold foil stamping has been extensively applied in printed packages. This method integrates printing and cold foil stamping into one step, and significantly increases production efficiency. It is characterized by low consumptions of stamping film, energy sources, and labor forces, free costly hot dies, as well as the need for low volatile organic compounds. Offset printing inline cold foil stamping is a one of the significant green printing technology.

Adhesive is an essential raw material in inline cold foil stamping. However, the cold foil stamping film-adhesive interface in China always has an aluminum layer or ZnS layer on transfer paper, which belong to non-absorptive printing media. Most existing adhesives are only applicable to flexographic printing inline foil stamping and other offline foil stamping methods [7], Existing adhesives in offset printing inline stamping have unsatisfying color expression, fuzzy color tones, and poor detail expression of graphic printing dots, because of their small tack value. Moreover, many stamping microcracks are formed because of high viscosity and low ink flow. Thus, the current study investigated the performances of two adhesives with different tack values.

EXPERIMENT

Materials

The materials used to prepare the adhesive include rosin-modified phenolic resin, terpene resin, Exxon Mobil S200 solvent oil, industrial soybean oil, defoaming agent, leveling agent, drying agent, cold-stamping foil, and transfer papers.

Preparation of Adhesives

According to the typical formula of adhesives, 33 wt% rosin-modified phenolic resin, 6 wt% terpene resin, 33 wt% industrial soybean oil, and 28 wt% S200 solvent oil were immersed in reaction vessel for 12 h. The ingredients were heated to 200[degrees]C for 30 min to ensure that the resins in the reaction vessel are dissolved completely. Temperature was then reduced to 150[degrees]C. The mixture was continuously stirred for 3 h. Then, temperature was lowered to 100[degrees]C. The defoaming agent, drying agent, and leveling agent were added at this time, and the mixture was stirred for 1 h. Finally, the mixture, that is adhesive sample, was cooled to room temperature. The adhesive sample with high tack value was named SI. The whole preparation process is just a dissolving process with the protection of pure [N.sub.2]. The terpene resin content in the low-tack-value adhesive sample, named as S2, was 2 wt% lower than that of the high-tack-value adhesive sample, S1.

Characterization of Adhesive Samples

Tack Value. The tack value of adhesive samples was tested using 106 THWING-ALBERT electronic ink (USA). Specifically, 2 mL of adhesive samples were collected using an ink injector and coated evenly using a rider roller. Water bath temperature and rotation speed of the tester were set at 32[degrees]C and 400 rpm, respectively. Data were recorded every 60 s until 15 min.

Emulsification Rate. A mixture of 30 g adhesive and 30 g water was stirred. The stirring rate of the emulsification tester (RH-100-III, Tianjin Chemical Machinery) was set to 100 rpm. The mixture was stirred for 2,000 revolutions and then placed static for 15 min. Unabsorbed water was then separated and weighted. The emulsification rate of adhesive is calculated by dividing the amount of absorbed water by the sum of the amount of adhesive and the amount of absorbed water.

Shear Viscosity, Viscoelasticity, and Thixotropy. Shear viscosity, viscoelasticity, and thixotropy were tested by the AR2000 rheometer at 25[degrees]C and using a 40-mm steel plate as rotor. Shearing rate ranged from 0.1 1/s to 100 1/s. The time frequency of viscoelasticity ranged from 1 Hz to 10 Hz. The shearing rate of thixotropy ranged from 0 1/s to 50 1/s in the linear growth mode and from 50 1/s to 0 1/s in the linear reduction mode.

Thread Length and Ink Flow of Ink. Ink thread length refers to the thread length of the adhesive in the current research. The adhesive thread length and ink flow were tested at a temperature of 25[degrees]C and 60% relative humidity. The panel viscosimeter was placed on a horizontal desk and filled with stirred adhesive samples. The spreading diameters (mm) of adhesive samples were recorded at 10, 60, and 100 s under the pressing of the upper parallel plate.

Thermogravimetric Analysis (TGA). Approximately 20 mg samples were placed on the crucible for TGA, which requires the use of the thermogravimetry analyzer (TG-209, Germany Netzsch Company). [N.sub.2] flow rate, heating temperature range, and heating rate were set at 20 mL/min, 30[degrees]C to 550[degrees]C, and 20[degrees]C/min, respectively.

Peel Strength Test. Peel strength was tested using INSTRON high- and low-temperature environmental electronic tensile machine according to GBIT 2790-1995 180[degrees] Peel Strength Test Method of Adhesive [8] under (23 [+ or -] 2)[degrees]C. The sample size, scale distance, and test rate were 15 cm X 2 cm, 100 mm, and 100 mm/min. Adhesive samples were coated on coated paper strips by a piece of 12 [micro]m wire rod. The length of the coated adhesive was 7.5 cm. Each group has at least five samples. The mean results were used.

Application of Adhesive Samples in Inline Cold Stamping

Two kinds of adhesive samples were used in the ROLAND 700 offset press for inline foil stamping. Printing rate was 10,000 pieces/h. Cold foil stamping samples were observed using a laser scanning confocal microscope (VK-X200K, Keyence) at magnification of 200 X.

RESULTS AND DISCUSSION

Performance Analysis of Adhesives

Relationship between Tack Value and Cold Stamping. In the offset printing inline cold foil stamping process, the adhesive is transfered from ink fountain, to ink roller and printing plate, and then to the surface of the paper. In this dynamic transfer process, the adhesive is splitted first, then transfered to the surface of the paper substrate. The tack value is very important for the adhesive transfer process and stamping quality. The adhesive was printed on the surface of the transfer paper using the printing form of the first printing unit of the offset press. First of all, the tack value, viscosity, ink thread length, ink flow, viscoelasticity, thixotropy, and emulsification rate of adhesives should meet the requirements of the offset press as the requirements to the offset inks. The tack value of the offset printing ink ranges from 4 to 10 [9]. An excessively low tack value is not suitable for ink transfer and will result in powerless ink thread resilience, which will easily cause printing dot extension, fuzzy edges, and obscure printed patterns. A high tack value will result in smooth and strong printing dot edges and clear pattern. However, an excessively high tack value will cause paper peeling or peeling of the previous ink film, also, detrimental to ink transfer [10]. The papers commonly used are white cardboard or aluminum-coated or ZnS-coated transfer papers. Offset printing inline cold stamping involves stamping followed by printing. There is no previous printed wet ink layer for adhesive to peel. The adhesive connects the paper surface and the aluminum layer of the cold-stamping foil. The anti-peeling resistance of the paper surface is significantly higher than that of wet ink layer in conventional offset printing. Therefore, the tack value of adhesive for inline cold foil stamping can be kept high to achieve a stamping pattern with clear tonal gradient. Two adhesives with different tack values were prepared for convenience of comparison. Test results on the tack performance of adhesive samples are shown in Fig. 1. According to microscopic features of cold stamping, the stamping color achieved using the adhesive with low tack value is a little light than that of with high tack value. Moreover, a high tack value will result in high stamping accuracy and improved stamping quality [11], Therefore, the tack value of adhesive must be kept high and stable. Based on numerous experiments, the adhesive with tack value that range from 20 to 30 has good stamping quality. In the current research, the tack value of the prepared adhesive was adjusted by terpene resin content. The higher the terpene resin content is, the higher the tack value will be.

Shear Viscosity. Adhesive in offset printing inline cold foil stamping is supplied by the printing unit of the offset press. The viscosity of the adhesive must meet the requirements on ink transfer for offset printing. The shear viscosity curves of adhesives are shown in Fig. 2. The stability of the shear viscosity of adhesive is the premise of the consistent color of the stamping images. The viscosity of adhesive should remain stable (approximately 80Pa-s) within the shearing rate range of 0.1 to 90 (1/s), which not only ensures uniform adhesive transfers, but also achieves good stamping quality. If viscosity is low, the leveling property will be excessive, thereby easily expanding printing dots. Low viscosity will result in inaccurate stamping of small dots, fine lines, and characters.

Viscoelasticity. The results of the viscoelasticity test on adhesive samples are shown in Fig. 3, where G' is the storage modulus that represents the elasticity (solid) of the adhesive, and G" is the loss modulus that represents the viscosity (liquid) of the adhesive [12]. The loss modulus during changes of angular frequency is always higher than that of the storage modulus, which indicates that liquid features are dominant characteristics of adhesives, which is in favor of uniform ink transfer. Generally, adhesive thread with high elasticity withdraws quickly at breakage. The storage modulus of high-tack-value adhesive is higher than that of low-tack-value adhesive, but loss modulus is similar. Therefore, high-tack-value adhesive has better elasticity. When adhesive is drawn into threads during stamping, the adhesive thread head withdraws rapidly after the breakage, which can prevent ink misting and improve reproduction of small stamping dots.

Thixotropy. Thixotropy is usually characterized by the area size of rheological curve hysteresis loop (thixotropy-loop). Figure 4 presents the thixotropy of adhesives. At low shearing rate, the stress and strain of adhesives are synchronous, the structure recovers quickly, and the viscosity remains stable, which will increase the foil stamping consistency. Thixotropy occurs only at high shearing rate, which shows a certain level of thixotropy. An appropriate thixotropic level is conducive to the transfer of adhesives and printing dot regularity [13]. Stamping dots will turn out unclear if thixotropy is low. Nevertheless, excessive thixotropy will lead to inconsistent stamping color.

Ink Thread Length and Ink Flow. The length of ink thread is measured indirectly using a panel viscometer. The spreading diameter of adhesive (d) on the panel viscometer and logarithm of spreading time (lg t) have an approximately linear relationship [14],

d=S lg t+s (1)

This expression is generally accepted as the characteristic Equation of ink (adhesive). In Eq. 1, S is the slope of the ink characteristic line and I is the intercept. A high I indicates a soft structure of the adhesive.

S can be calculated using the following formula:

S=d100 - d10 (2)

where S is the slope of the ink characteristic line (mm), dlO is the spreading diameter of ink at 10 s (mm), and d 100 is the spreading diameter of ink at 100 s (mm).

The calculation formula of I is:

I=d10 - S (3)

The measurement data of the two kinds of prepared adhesive samples using the panel viscometer are listed in Table 1. The S of the two kinds of adhesive samples could be calculated from Eqs. (1-3). The result indicates that the length of the ink thread is measured by the length of the adhesive at breakage. For offset printing ink, S shall be controlled between 5 and 6 and should not be lower than 4 [15]. A large S brings high viscosity, long ink thread, high elasticity, and quick withdrawal, which will improve stamping quality. A low S results in a short thread, thick ink film, and uniform transfer. However, an excessively small S will result in poor ink flow and weak influence ink transfer [16]. The length of ink thread depends on the composition of the adhesive. By adding more leveling agents, the ink flow of high-tack-value adhesive is adjusted better than that of low-tack-value adhesive. Good ink flow will result in better large-scale plane stamping quality.

Relationship between Emulsification and Cold Foil Stamping.

* Ink emulsification pertains to the phenomenon wherein ink absorbs a fountain solution during printing [17]. The emulsification rate of conventional offset printing ink ranges from 15% to 26% [18]. Water-in-oil (W/O) emulsion is formed, which is suitable to the ink on the form at the lowest water volume, that is, water-ink balance [19]. The lack of appropriate emulsification makes the transfer of ink for offset printing impossible to satisfy. However, excessive ink emulsification will result in an unglazed pattern, serious deformation of printing dots, slow drying, and dirty adhesion at the back of printed matter [20]. Offset printing inline cold foil stamping will control the amount of adhesive by printing dot density on the form. Thus, tonal gradation is controlled by the amount of aluminum that has been obtained from aluminum layers in the adhesive stamping film. A tonal gradation sample is illustrated in Fig. 5. Compared with conventional offset printing ink, a slightly higher emulsification rate does not result in light stamping color given that the adhesive has high tack value. If the emulsification rate is low, the high-tack-value adhesive will cause poor ink transfer. However, an extremely high emulsification rate will decrease the tack value of the adhesive. The adhesive will swell after water absorption, and printing dots will expand after stamping, thereby causing stamping defects. Based on the results of offset printing inline cold foil stamping, the emulsification rate of adhesive should be high and kept within the range of 30% to 40%.

Solid Content in Adhesives. Solid content refers to the proportion of the solid mass of dried adhesive in adhesive dose, including resin and some residues of high-boiling-point solvent. Figure 6 shows that solid content in two kinds of adhesive samples is generally 82% to 86% at the temperature range of 180[degrees]C to 380[degrees]C. The solid content in low-tack-value adhesive is 82.6%. The shrinkage of the adhesive layer caused by solvent evaporation after stamping is small and uniform if the solid content in the adhesive is high. This condition can increase stamping accuracy and prevent the formation of microcracks on the stamped foil surface. Therefore, solid content shall be as high as possible while maintaining a satisfying tack value, printability, and drying rate.

Peel Strength of Adhesives. Adhesive substrate in offset printing inline cold stamping uses different types of paper. In this study, the peel strength of adhesives was tested using aluminum transfer paper according to GB/T 2790-1995 180[degrees] Peel Strength Test Method of Adhesive [8], The peel strengths of high-tack-value and low-tack-value adhesives are 2.84 N and 2.23 N, respectively. The adhesive force in cold stamping was tested using 3M adhesive tape. No peeling was observed.

Stamping Quality

Figure 7 presents the laser scanning confocal microscopic images of stamping samples using adhesives with high tack value (a) and low-tack value (b). Incomplete stamping and break lines are observed in the stamping sample when low-tack-value adhesive was used.

Stamping quality was evaluated based on gradual changes of printing dots, lines, small characters, and large-scale plane stamping. Figure 5 illustrates the gradual changes of the cold foil stamping color with different tones. Generally, stamping under different printing dot densities is uniform, consistent, and does not exhibit fading, stripes, shadows, and orange peel, indicating the high reduction rate of printing dots. The large-scale plain stamping samples show clear holographic effect, strong stereoscopic impression, full colors, uniform brightness, and high consistency.

CONCLUSIONS

A high-tack-value adhesive for offset printing inline cold foil stamping is prepared using rosin-modified phenolic resin, terpene resin, industrial soybean oil, and solvent oil as the main raw materials. The tack value of the adhesive is controlled by terpene resin content, and the ink flow is adjusted by the leveling agent. A representative formula is 33 wt% rosin-modified phenolic resin, 6 wt% terpene resin, 33 wt% industrial soybean oil, and 28 wt% Exxon Mobil S200 solvent oil. Small amounts of leveling agent, defoaming agent, and drying agent were added. The prepared adhesive has high and stable tack value (20-30), stable viscosity (80 Pa-s), emulsification rate of 30% to 40%, and appropriate levels of viscoelasticity, thixotropy and ink flow.

ACKNOWLEDGMENTS

This study is funded by the Key Science and Technology Project of Beijing Municipal Education Commission (KZ201610015015), and by the Research & Development Program Project of Beijing Institute of Graphic Communication (EC201804).

REFERENCES

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Xiao Liu [iD], (1), Shiyong Luo, (1) Xuemei Li, (1) Wenyu Zhang, (2) Nengjian Zhang, (2) Gang Deng, (2) Shaoguo Li (2)

(1) Beijing Key Laboratory of Printing and Packaging Materials and Technology, Beijing Institute of Graphic Communication, Beijing, 102600, China

(2) Shaoxing Hucais Laser Materials Technology Co., Ltd, China

Correspondence to: S. Luo; e-mail: 626852658@qq.com

DOI 10.1002/pen.24879

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Tack value test results of adhesives.

Caption: FIG. 2. Shear viscosity curves of adhesives.

Caption: FIG. 3. Viscoelasticity of adhesives.

Caption: FIG. 4. Thixotropy of adhesives.

Caption: FIG. 5. Tone steps of the cold foil stamp print. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. TGA results of adhesive samples.

Caption: FIG. 7. Laser scanning confocal microscopic images of stamping samples by using adhesives with high-tack value and low-tack value (b). [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Ink thread length and flowability of adhesives.

Sample No.   S(Slope)   I (Intercept)   Flowability (mm)

S1              8            22                38
S2              7            22                36
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Author:Liu, Xiao; Luo, Shiyong; Li, Xuemei; Zhang, Wenyu; Zhang, Nengjian; Deng, Gang; Li, Shaoguo
Publication:Polymer Engineering and Science
Article Type:Report
Date:Jan 1, 2019
Words:3665
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