Decorative base paper is made in a paper machine using high-quality wood pulp, titanium dioxide, functional fillers and additive as raw material. After been treated by some processes, such as printing, resin dipping, lamination and other methods, decorative base paper can be used in the surface decoration of wood-based panels including fiberboard and particleboard. It can effectively improve the appearance and resistance of wood-based panels. However, decorative base paper with low tensile strength, bad color reproduction ability, and poor image definition limits its use for the production of decorative laminates.
In order to improve the quality of decorative base paper, some studies in recent years focused on the materials and process of papermaking and impregnated technology. The effects of the pulp and fillers on the strength and absorbency of decorative base paper were studied by Drees et al. (2004). The combination of silicate and titanium dioxide could reduce the dosage of titanium dioxide, increase the retention rate of the pulp, and reduce the production cost (Dutt et al. 2011). Modified titanium dioxide which contains silicon and aluminum was used as fillers to improve the performance of decorative base paper (Schulz 2004, 2005). Schulz reported that the opacity of decorative could be improved by using pigments consisting of titanium dioxide, silica and alumina (Schulz 2004). Gokay Nemli (Nemli 2008) pointed out that the performance of wood-based panels depended on the impregnation process of decorative paper. The above mentioned works were dedicated to the improvement of the physical properties and application performance of decorative paper through optimizing pulp, fillers and impregnation process rather than to the surface treatment of such papers. Therefore, under the premise of retaining the production process of decorative base paper, it should be studied how to improve their quality by surface treatment such as sizing and pigment coating. Internal sizing and surface sizing were applied to reduce the resin absorption of decorative base paper during impregnation process and improve its opacity, gloss and surface durability (Mehta and Hill 1997). In order to increase the strength and reduce the release of formaldehyde, a modified starch was used for surface sizing of decorative base paper by Yang (2012). A modified starch with a special molecular weight and structure was applied to decorative paper by surface sizing to increase both the strength and absorption (Wicher 2012, Zhang et al. 2016a, 2016b). This research showed that surface treatment can effectively improve the physical properties of decorative base paper and control the release of impregnating resin and formaldehyde. However, there was less research on the effect of surface treatment on the printability of decorative base paper. Therefore, in order to systematically study the role of surface treatment in improving the printability of decorative base paper, in this paper two surface treatment methods, surface sizing and pigment surface sizing, were used to systematically investigate the effect and influence mechanism of the pigment surface sizing technology.
Surface sizing is a surface treatment method that refers to the application of modified starch, styrene-acrylate latex, polyvinyl alcohol, and other natural or synthetic sizing solution for paper surface treatment in order to improve paper strength and surface properties (Wang and Fang 2015). The surface strength, smoothness and ink jet printing performance of the paper could be improved by using cationic starch sizing (Lipponen et al. 2004, Saraiva et al. 2010, Zhang et al. 2016a, 2016b). Heat resistance, weather resistance, corrosion resistance, stain resistance, gloss and color retention of paper could be increased by surface sizing using styrene butadiene or styrene-acrylate latex (Bardak et al. 2011).
Pigment sizing is a surface treatment technology between surface sizing and pigment coating. In order to fill the surface pore, improve the uniformity, opacity, ink receptivity and fixing properties of paper, the pigment surface sizing liquid, prepared by adding a certain proportion of pigment into the sizing agent was coated on the surface of the paper. The effects of pigment proportion of sizing liquid, particle size, sizing amount on the transparency of bond paper sizing layer were systematically studied by Wang et al. (2016) under the condition of solid content of 5 % and 8 %. Zhang and Chen (2008) applied surface sizing technology to research its benefits on inkjet paper physical properties, optical properties and print performance.
Silica is a common pigment which has porous structure and excellent ink receptivity, where the silica particle size, in the range of 2–5 µm, is of much lower magnitude than the un-sized paper pore size, which is in the range of 5–7 µm. It can effectively enhance smoothness, gloss and opacity of the coating layer. Moreover, silica coating can promote water resistance and wet rub resistance of the coating layer. The application of industrial colloid silica in surface sizing and coating process was studied by Chen et al. (2006). The surface strength, ink absorbency and water resistance of the paper changed with the increase of colloid silica dosage. The effects of different weight ratios of PVA and silica on the physical properties and laser printing performance of paper were investigated by Zhang et al. (2013). Significant improvements of the brightness, roughness, opacity, color density and tone reproducibility of paper were obtained. Previous studies have found that small silica particles are advantageous for the formation of a denser coating layer with smaller pore size and pore area.
In this paper, investigation of the influence of pigment sizing including silica, cationic starch and SAE on the printability and coating structure of decorative base paper are reported. The printability of decorative base paper was characterized in terms of physical performance and printing properties such as tensile strength, roughness, water absorption height, ink absorption, color density and printing dots. The analysis of coating structure was carried out by dynamic penetration testing, SEM and laser microscope.
Materials and methods
Decorative base paper used in the experiments was supplied by Shandong Zhengda Paper Company, with basis weight 85 g/m2, dry tensile of 1.63 kN/m, wet tensile of 0.332 kN/m, water absorption height of 23 mm, and air permeability of 3.19 s. Cationic starch was supplied by Guangxi Mingyang Biological Technology Co., Ltd. Silica was purchased from Shanxi Tianyi Co., Ltd. The silica has a particle size of about 5 μm, pH value of 5–7, whiteness of about 85 %, specific surface area of 236 m2/g, and pore volume of 1.7–1.8 ml/g. SAE with solid content of 25 %, pH value of 4.0–5.0, viscosity of 30–50 mPa·s, was synthesized in the laboratory. Water-based gravure ink with viscosity of 5.25 mPa·s and K&N ink S89-2 were purchased from Tianjin Toyo Ink Co., Ltd. IGT Helio test ink of red color was used to detect the loss of dots of gravure printing. Some Chemical reagents were obtained from Tianjin Yuan Li Chemical Co., Ltd.
SAE synthesis and preparation of silica/cationic starch/SAE sizing solution: The SAE synthesis was carried out using a four-necked flask following the steps below. First, certain amounts of monomers, emulsifier, initiator and water were poured into a four-port flask immersed in a thermostated water bath equipped with reflux condenser, mechanical stirrer, dropping funnels and inlet for nitrogen gas. The mixture was stirred for 0.5 h under nitrogen atmosphere. The temperature was raised from 40 °C to 80 °C, and the rest monomers, emulsifier and initiator were added dropwise. At the same time, APS (Ammonium Persulfate) aqueous solution (0.2 g of APS dissolved in 20 ml water) was also poured into the flask to initiate the polymerization at a constant temperature of 80 °C. When the addition of monomers mixture and APS solution was completed, the pH value of the suspension solution was adjusted to 7.0–8.0 by adding ammonia. The final mixed solution was stirred for 4 h at a temperature of 85 °C.
Aqueous starch solution (8 wt %) was prepared in a water bath held at >95 °C under continuous stirring for 60 min. The cationic starch and SAE solutions were mixed with oven dried cationic starch:SAE emulsion equal to 20:1. The silica suspension (60 wt %) was prepared using a high-speed disperser under continuous stirring for 60 min.
The pigment sizing solution was prepared by mixing the silica suspension and cationic starch/SAE solutions with different amounts of pigment as shown in Table 1. The ratio of cationic starch of oven dry and SAE solution: oven dried silica (denoted binder-to-pigment) was 90:10, 80:20, 70:30, 60:40, 50:50.
Pigment sizing and calendering: The pigment sizing solution was applied on decorative base paper by using a coating unit (CU5, Sumet-Messtechnik, Germany) under the conditions of No. 30 metering bar with pressure of 5 N, rubber roller pressure 150 N, speed of roller 9 m/min, 100 % infrared light energy, hot air temperature 100 °C, drying time 90 s. The calender treatment of pigment sized paper was carried out with a multifunctional calender (CA5) unit manufactured by SUMET Company (Germany) under the conditions pressure 30 N/mm, temperature 40 °C, speed 15 m/min.
Gravure proofing: IGT printability tester G1 (Netherlands IGT Company) was employed to gravure proofing using red water-based gravure ink with printing pressure of 800 N and two gear speed.
Physical property test: The tensile strength, print-surf roughness, water absorption height and ink absorption of the decorative paper were measured according to GB/T 12914-2008, GB/T 22363-2008, GB/T 461.1-2002 and GB/T 12911-1991 respectively.
Printing performance evaluation: Spectrophotometer (X-rite 528, X-Rite Instrument Co., Ltd) was used to measure the color density of the color-block consisting of the dots with different gravure depths. Verity IA printing software (print target v3, Verity IA Company) was used for analyzing the sharpness and missing situation of dots.
Dynamic penetration test: Based on the principle of ultrasonic attenuation, a dynamic permeation analyzer (PDA.C 02, Emtec Company, Germany) was used to measure the dynamic permeation of the paper using water as test liquid.
The specific surface area and pore volume: The specific surface area and porosity was measured using an automatic specific surface area and pore analyzer (V-Sorb 2800P, Gold APP Instrument Corporation, China) according to GB/T 19587-2004.
Apparent morphology test: SEM micrographs of the paper were determined using a scanning electron microscope (SU1510, HITACHI Company, Japanese) with an accelerating voltage of 10 kV. The beam current (measured between the condenser lens and objective lens) was approximately 1 pA to 1 mA. Prior to the examination, a thin coating (∼10 nm) of gold was deposited on the sample using auto fine coater, to enhance the conductivity and secondary electron emission characteristics. The two-dimensional and three-dimensional structure of the paper surface was observed with Confocal laser microscope (VK-X100/X200, Keyence Co., Ltd).
Results and discussion
The effect of pigment sizing on the printability and dynamic permeability of decorative base paper
Physical properties analysis: Details of the variation of physical properties of different paper samples are listed in Table 2.
Table 2 shows that the tensile strength of decorative base paper of pigment sizing was improved by 19.63 %–33.13 % compared to the base paper. After been sized with cationic starch/SAE, the tensile strength of the paper was increased due to the more hydrogen bonds between the fiber and binder. However, with the increase of pigment content, the effect of hydrogen bonds was decreased by pigment. The more pigment content of the sizing solution, the less improvement of tensile strength was obtained.
The print-surf roughness of the paper could be reduced by surface sizing and pigment sizing. When the ratio of binder-to-pigment ratio was 70:30 and 60:40, the print-surf roughness was decreased by 16.81 % and 17.98 % respectively compared with the base paper. Due to the excellent film forming ability of SAE, it can improve the smoothness of paper surface after sizing with cationic starch/SAE. The positive charge of the cationic starch enable its favorable ability of being retained in the paper fiber with negative charge, which resulted in a better synergistic effect (Xu and Hu 2012, Ashori et al. 2013). In addition to the above synergistic effect, the appropriate amount of pigment added in the sizing solution could fill the pores of paper surface and decrease the print-surf roughness. However, for the pigment sizing solutions of low viscosity, if the pigment content is too high, it can lead to flocculation of pigments, resulting in an increase of the surface roughness.
The determination of the water absorption height of the paper can reflect the overall dipping liquid capacity of the decorative base paper. As shown in the Table 2, it can be concluded that water absorption height was reduced after sizing. Nevertheless, the pigment sizing could improve the water absorption height of the paper. The more pigment content of the pigment sizing solution, the larger water absorption height of the paper. When the ratio of binder-to-pigment was 70:30, the water absorption height of the paper was increased to 25.5 mm. It is because that the fine and uniform pore structure between the pigment particles can produce the more capillary suction inducing the higher water absorption height.
The absorption ability of ink on the paper surface was characterized by ink absorption. If the amount of ink adsorbed onto the paper is too large, it would lead to lower printing gloss, and cause print quality problem. On the contrary, if the ink absorption of the paper is very small, it will reduce the drying speed of ink, resulting in the back of dirt and the decrease of ink adhesion. As a result, ink absorption should be controlled within a reasonable range.
Table 2 shows that both surface sizing and pigment sizing reduced ink absorption of the paper surface slightly. However, with the increase of the amount of silica pigment, the ink absorption of the paper was increased. Therefore, the pigment sizing can be used to control the ink absorption and improve the printability of the paper by changing the pigment ratio.
The effect of pigment sizing on printing properties of the decorative base paper: Figure 1 demonstrates the effect of the ratio of binder-to-pigment on the color density and tone reproduction of the paper. As can be seen from Figure 1, the color density of the paper was improved by pigment sizing. Compared to the sizing of cationic starch/SAE, the color density of paper using pigment sizing with silica had a considerable increase in dark tone. The larger the slope of curve, the better the performance of color reproduction of the paper. With the increase of silica dosage, the slope of curve was improved gradually resulting in better color reproduction. This is due to the good synergistic effects with respect to the positive charge of the starch, excellent ink absorption of silica and the perfect film forming ability of SAE, which was created by the pigment sizing solution made from silica/cationic starch/SAE. Taking the physical properties of the paper into account, the appropriate ratio of binder-to-pigment was 70:30. Thus, it can be concluded that by pigment sizing with a good control of the proportion of binder and pigment a porous structure can be created that avoids the reduction of porosity and the poor ink adsorption resulting from the use of pure sizing.
Dynamic penetration analysis: By measuring dynamic permeability of paper with dynamic permeability tester, using water as test liquid, the degree of openness of paper surface can be characterized, which could characterize the reception and absorption of ink on the paper surface. Figure 2 shows the variation of ultrasound energy within 0–0.15 s after the contact of the paper with water. The more the ultrasonic energy decreases, the stronger is the capillary action on the surface of the paper, which is beneficial to the receptivity and surface absorption of the water-based ink. The ultrasonic energy of the base paper decreased by about 50 %, while the ultrasonic energy of the paper sized with cationic starch/SAE was decreased by about 65 %. For the paper coated by pigment sizing in which the ratio of binder-to-pigment is of 70:30 ultrasonic energy was decreased by about 75 %. The following explanation for the observed effect can be suggested. Due to the high proportion of pigments on the surface of the decorative base paper and the strong surface absorption of water, the water does not permeate evenly at the instant of contact, and air above the surface is isolated to form bubbles. This induces an increase of ultrasonic wave scattering and a decrease of energy reception. Compared to the base paper, the hydrophilic groups of SAE emulsion and starch accelerate the penetration of water into the paper surface and the speed of bubble formation, resulting in a faster decline of ultrasonic energy. After adding a certain amount of silica pigment, the water resistance of the paper surface is increased and the degree of openness of paper surface is decreased, which results in a slower attenuation of ultrasonic energy. However, when the ratio of binder-to-pigment is 70:30, many small pores are formed on the surface of the paper. This enhances the capillary action and the formation of more small bubbles, which leads to faster and greater energy decline. Excessive addition of pigment results in flocculation of the pigment, reduction of the amount of fine pores in the paper surface and weakening of the capillary action. As a result, the water permeation rate, the production speed and quantity of the bubbles is reduced, and the decrease of the energy is slowed down. In summary, with the proper binder-to-pigment ratio and avoiding the flocculation of pigment, sizing can improve the water receptivity and absorption of decorative base paper.
T95 indicates the time required for the ultrasonic energy to drop from 100 % to 95 %, which having a good correlation with the depth of penetration of water, is able to indicate the porosity of paper. As shown in the Figure 3, the smallest T95 value was of base paper, and the T95 value of the samples coated with cationic starch/SAE was 8.20 % bigger than the base paper. All of pigment sizing samples’ T95 values was bigger than the samples only coated with sizing solution. As a surface treated technique, pigment sizing simply filled larger pores in the base paper and prevented excessive penetration of ink of subsequent printing so that color density and printing gloss could be improved.
Analysis of action mechanism of pigment sizing
To further explore the mechanism of the printability improvement of the decorative base paper treated by pigment sizing, paper samples sized with cationic starch/SAE and paper samples sized with silica/cationic starch/SAE (binder-to-pigment ratio was 70:30), loss and definition of dots, pore volume as well as two-dimensional and three-dimensional shape of the coating surface from different angles, coating microstructure and surface morphology were comparatively analyzed.
Dot analysis of printed products: IGT Helio is a method designed to evaluate the loss of dots after gravure printing. The IGT Helio module was applied, using special software verity IA Print Target v3 for analyzing Helio proofing test strips to obtain loss percentage of dots from samples (Table 3). The percentage of dots loss after pigment sizing was decreased by 21.33 % and 19.18 % compared to the base paper and sizing paper, respectively. This may be because pigment sizing has improved surface properties significantly such as smoothness, which is advantageous for the generation of even ink coating on the surface. Silica has improved the acceptance and adsorption of ink due to its porous structure and outstanding color reproduction ability.
Compared to base paper and sizing paper, the roundness of dots for the pigment sizing paper was increased by 43.33 % and 26.6 %, respectively. Silica/cationic starch/SAE sizing solution was beneficial to form a smooth surface of paper inducing the larger fixation effect of ink. Good roundness of dots represents better printing quality, the increase of roundness may be affected by the increased smoothness of the paper after pigment sizing. As for brightness, that of pigment sized papers decreased by 2.63 % compared to base paper, while it was decreased by 1.01 % compared with that of sized paper. After pigment sizing, the brightness was reduced, which implies the improvement of color density. This is consistent with the conclusions in the analysis of effect of pigment sizing on printing properties.
Analysis of surface morphology and microstructure of the coating structure: Figure 4 shows the changing trend of pore volume of three paper samples. The pore volume of samples treated with cationic starch/SAE was reduced compared to base paper because when sizing solution penetrated into the body of the samples, the void between fibers was filled with sizing solution. Furthermore, the addition of silica filled the smaller pores so that the pore volume decreased markedly when compared to base paper.
Images of surface morphology were obtained by SEM to study the effect of addition of silica into sizing solution. Figure 5a shows the surface morphology of untreated decorative base paper. It can be seen that a lot of fibers interlace together, resulting in the formation of numerous complex pores. Fillers are dispersed around the fibers. Figure 5b and c show the surface morphology of paper samples coated with cationic starch/SAE and silica cationic starch/SAE, respectively. When the surface of the paper is covered with sizing solution, the pore structure of the paper surface becomes smaller. An explanation for this phenomenon is that sizing solution penetrates deeply into the internal structure of the paper and then combines with fibers and fillers. After the addition of silica, the reduction in paper porosity is sufficient to enhance greatly the visual appearance of printed sheets (contrast, image definition, etc.). The micro porous structure on paper surface is beneficial for the performance of instant ink receptivity.
Figure 6 shows the surface structure of paper samples under different sizing methods (in both two and three-dimensions). In the display, the salient area of the sheet is represented by red color; the depression area of the sheet is indicated by the blue color. As the color approaches red, the amount of salient features on the paper gets higher. On the other hand, when it gets closer to blue, the amount of depressions gets higher. There are significant differences in the color of the base paper, indicating that the surface of paper was roughened. It can be seen from the three-dimensional image that plenty of large pores were distributed in the body of the paper. Uneven arrangement of fibers, generated from papermaking, has formed fiber-crisscross pores. After sizing solution was added onto the surface of the paper samples, most of the large pores were filled. Further, a smooth coating structure was formed to reduce the roughness of the paper. The addition of pigment in sizing solution formed more small pores, which was beneficial for controlling the penetrated depth of ink into the body of the paper.
In order to solve the existing problems of decorative base paper, pigment sizing was employed to improve the printability of decorative base paper. The effect of silica/cationic starch/SAE sizing solution on decorative base paper was investigated from the perspective of coating structure. It not only provided a feasible method to modify decorative base paper, but enriched theories of the effect of pigment sizing on coating structure on paper.
This research shows that pigment sizing improved the tensile strength of decorative base paper. Both surface sizing and pigment sizing had a more obvious effect on the reduction of roughness. The water absorption height was decreased by surface sizing, but increased with higher proportion of silica due to the reduction of sealing of sizing. Both pigment sizing and surface sizing improve color density, where pigment sizing was more suitable for image reproduction in dark tone and surface sizing was suited to that in light tone. Pigment sizing has made the dynamic permeability of base paper complicated, and taken a more positive effect on dynamic permeability. Sizing agent also decreases the open degree of paper surface. Considering the increasing efficiency for physical properties and printability, the best ratio of binder-to-pigment was identified as 70:30.
With the addition of silica the percentage of dots loss decreases, the roundness of dots increases and thus better printing effect is achieved. SEM and three-dimensional image reflected directly that silica/cationic starch/SAE has filled pores of paper and forms uniform coating, which is consistent with the results of porosity measurement.
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About the article
Published Online: 2018-05-23
Published in Print: 2018-05-23
Funding Source: Tianjin University of Science and Technology
Award identifier / Grant number: 2014CXLG26
This project was financially supported by the Tianjin Research Program of Application Foundation and Advanced Technology (Grant Nos. 15JCQNJC42300, 13JCZDJC30900) and Young Innovation Foundation of Tianjin University of Science & Technology (Grant No. 2014CXLG26).
Conflict of interest: The authors do not have any conflicts of interest to declare.