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. 2023 May 31;8(23):21000–21007. doi: 10.1021/acsomega.3c01816

Improving the Optical Properties and Filler Content of White Top Testliners by Using a Size Press

Mustafa Çiçekler †,*, Tamer Sözbir ‡, Ahmet Tutuş †
PMCID: PMC10268623  PMID: 37332820

Abstract

graphic file with name ao3c01816_0005.jpg

This study aims to investigate the impact of coating with ground calcium carbonate (GCC) on the optical properties and filler content of white top testliner (WTT) papers. The paper properties investigated include brightness, whiteness, opacity, color coordinates, and yellowness. The results showed that the amount of filler mineral used in the coating process significantly affected the optical properties of the paper. The use of 15% total solids of GCC in the coating suspension resulted in the highest level of whiteness and improved the brightness value by 6.8%. The use of 7% total solids of starch and 15% total solids of GCC reduced the yellowness index by 85%. However, the use of only 7 and 10% total solids of starch had an adverse effect on the yellowness values. The surface treatment led to a significant increase in the filler content of the papers, with a maximum of 23.8% achieved using a coating suspension with 10% total solids of starch solution, 15% total solids of GCC suspension, and 1% dispersant. The starch and GCC in the coating suspension were found to have a direct impact on the filler content of the WTT papers. The addition of a dispersant improved the uniform distribution of the filler minerals and increased the filler content of the WTT. The water resistance of WTT papers increases due to the use of GCC, while their surface strength remains at an acceptable level. The study demonstrates the potential benefits of the surface treatment in terms of cost savings and provides valuable information on the impact of the treatment on the properties of WTT papers.

Introduction

Paper production and consumption are significant global issues due to their widespread use and impact on other countries. The increasing demand for paper has led to a shortage of fiber sources and a need for investment in cellulose production. However, producing cellulose is a capital-intensive process, which presents a challenge for companies and countries looking to increase their cellulose production. Sources such as pine forests are needed to meet the demand for cellulose, but the large amount of capital required to produce it is a limiting factor in its production. The need for alternative sources of fiber and more sustainable production methods is becoming increasingly pressing in light of the global fiber shortage.1 This has been noted by various studies.2−4

The use of fillers in paper production is becoming increasingly popular as papermakers look for alternative and cost-effective ways to reduce the cost of cellulose and fiber. Fillers, which are nonfibrous inorganic materials, are much cheaper than fibrous materials and offer a faster drying time compared to fibrous raw materials. This makes them a highly attractive option for papermakers, especially for the production of graphic paper or packaging paper.5−7 As a result, fillers play an important role in reducing production costs and increasing production efficiency. The cost-effectiveness and faster drying time of fillers have been confirmed by various studies.8−11

In addition to conventional paper production, the production of multifunctional paper types is gaining importance day by day. In this way, it is aimed to produce multifunctional papers with high added value and innovative and attractive features and to increase the market share compared to traditional papers.12−14 White top testliner (WTT) is one of the most important types of paper and consists of two layers. The top layer is made from either bleached virgin fibers or high-quality deinking recycled fibers. The underlay is made entirely of recycled fibers. One of the most important features of WTT is its excellent printability. The quality of WTT made from bleached virgin pulp is better than those made from recycled pulp. However, the cost of bleached virgin pulp is quite high.

Paper producers are seeking alternative materials to lower cellulose-related paper and packaging costs. As a result, the fillers used to improve the characteristics of paper are critical.1,4,15 Nonfiber inorganic materials are preferred over fibrous materials because they lower manufacturing costs and allow for faster drying than fibrous raw materials, hence boosting output.16,17 Many industrial raw materials are evaluated and employed in the papermaking process for two purposes: filling and coating. Some pigments are only used for filling or coating, whereas others may do both.18

The coating process of paper aims to make the surface even and smooth. In this process, a coating suspension, made up of pigments and adhesives, is applied to the surface of the paper. This method, similar to that used by wall plasterers, helps in reducing the roughness of the paper surface. The term “coated paper” refers to the process of applying the coating to the surface of the paper.19−21 Traditional coating formulas commonly include an inorganic pigment such as kaolin, calcium carbonate (CaCO3), or titanium dioxide (TiO2); a binder like starch, polyvinyl alcohol, acrylic acid, carboxymethyl cellulose, or styrene butadiene latex; and other auxiliary components.22 The inorganic pigment serves as a colorant and filler, while the binder helps in holding the pigments together and adhering the coating to the paper surface. The auxiliary components are added to enhance specific properties, such as brightness or durability.

In the pulp and paper industry, a suitable filler has the following characteristics: particle dispersion, good paper retention, low density, little obvious chemical reaction, low abrasion, and a reasonable cost. Following kaolin, calcium carbonate is a significant pigment in the paper industry.23 Calcium carbonate is a versatile material in the papermaking industry, serving as both a filler and a coating for the paper’s surface. The particle size of calcium carbonate can vary and can range from 2–3 μm up to 7–8 μm, depending on the type of carbonate used. In terms of its color, calcium carbonate has a lightness range of 93–98 ISO%, meaning that they are relatively light in color. Additionally, calcium carbonates are highly soluble in water, making them easy to work with during the papermaking process. These characteristics of calcium carbonate make it an attractive option for papermakers looking to improve the properties of their paper.3 Ground calcium carbonate (GCC) is a filler material widely used in the paper industry produced through a process of grinding natural limestone and mixing it with water. This mixture is then subjected to either air or water separation, resulting in a product with the desired particle size. GCC is the most commonly used filler and coating material in the paper industry, compared to precipitated calcium carbonate (PCC), which is produced through chemical processes. GCC is preferred for its natural origin, ease of production, and cost-effectiveness compared to PCC, which is more expensive to produce.24

In the production of WTT paper, the use of recycled fibers from mixed office wastes (MOW) and old magazine papers (OMP) is a cost-effective way to reduce cellulose costs. Even though recycled fibers are de-inked and bleached, their optical properties are not as good as those of bleached virgin pulp. However, the cost of recycled fibers is lower than that of virgin pulp. To produce WTT with high optical properties and high filler content, the aim of this study is to recycle MOW and OMP and use GCC pigments as a coating in the production of the top layer using a size press. The use of pigment/mineral in WTT production can significantly reduce cellulose costs while maintaining good optical properties.

Materials and Methods

Materials

The coating pigment used in the study was GCC, which was obtained from the mining company OMYA. The technical specifications of GCC, such as its particle size distribution and purity, are provided in Table 1 of the study. The cationic starch used in the study was procured from Amylum Starch Inc. and served as an ingredient to improve the binding properties of the coating. Additionally, a 40% aqueous solution of Coatex *Ecodis N dispersant was utilized as the binder in the coating formulation. This binder helped in distributing the GCC evenly and maintaining its suspension in the coating solution, ensuring consistent and uniform application of the coating on the substrate. The use of specific raw materials and additives in the coating formulation helped in achieving the desired properties and performance characteristics of the final product.

Table 1. Technical Specifications of GCC.

GCC properties values
45 mesh screen (ISO 787-7), % 0.01
top cut (D98, Malvern Mastersizer 2000), μm 13.0
<2 μm (Malvern Mastersizer 2000), % 37.1
average (D50, Malvern Mastersizer 2000), μm 2.8
brightness CIELAB (ISO 11664-4), L*, a*, b* 98.5, −0.03, 0.8
brightness RY (C/2°, DIN 53163), Ry 96.2
moisture content, % 0.1
CaCO3, % 98
MgCO3, % 1.7
Fe2O3, % 0.05
HCl insoluble content, % 0.2
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The raw material for producing WTT is obtained through recycling of waste papers. The bottom layer of the product is made from old corrugated cardboards (OCC), which were collected and processed for reuse. The top layer, on the other hand, is composed of a mixture of recycled MOW and OMP. These waste materials are collected and sorted to extract usable fibers, which are then blended and processed to form the top and bottom layers of the WTT papers.

WTT Production

The process of paper production was conducted using a semi-automatic Rapid Köthen paper machine, with the conditions specified by the ISO 5269-2 standard. The defibrated pulp was adjusted to a concentration of 0.3% in 10 L of water and then placed into the disintegrator. The WTT was produced by utilizing a bottom layer composed of OCC pulp with a weight of 80 g/m2 and a top layer composed of de-inked pulp obtained from a mixture of recycled MOW and OMP with a weight of 40 g/m2. Following the production process, the WTT paper was conditioned for 24 h in a climate chamber with a temperature of 23 ± 1 °C and 50 ± 2% relative atmospheric humidity, as specified in the ISO 187 standard. This conditioning process allows for the stabilization of the WTT’s physical properties, such as its moisture content, before further analysis or use.

Coating Suspension Preparation and Surface Application

In this study, the preparation of two distinct starch solutions was conducted by mixing starch with water to achieve dry matter contents of 7 and 10%. The starch solutions were then heated to a temperature of 93 ± 1 °C to enable gelatinization of the starch. Following cooking, the starch solutions were cooled down to a temperature of 72 ± 1 °C and were maintained at this temperature for a period of 30 min. Then, GCC suspensions were prepared by mixing GCC with water to attain total solid (TS) contents of 12 and 15%. The dispersant, which was used in certain trials at a rate of 1%, was added to the GCC suspensions to improve their dispersibility and stability.

The resulting coating suspensions, which consisted of the starch solutions, GCC suspensions, and dispersant, were applied to WTT papers. The different components of the coating suspension and their respective concentrations are detailed in Table 2 of the study. The table also lists the codes assigned to the 13 different coating suspension formulations that were applied to the WTT papers in the study. These formulations varied in terms of their dry matter content, which was controlled by adjusting the proportions of starch and GCC in the suspensions. The experiments aimed to evaluate the effect of different coating suspension formulations on the properties of the coated WTT papers.

Table 2. Components of Coating Suspensions.

code starch (7% TSa) starch (10% TS) GCC (12% TS) GCC (15% TS) dispersant (1%)
C          
7S X        
10S   X      
7S1D X       X
10S1D   X     X
7S12G X   X    
10S12G   X X    
7S15G X     X  
10S15G   X   X  
7S12G1D X   X   X
10S12G1D   X X   X
7S15G1D X     X X
10S15G1D   X   X X
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a

TS refers to total solid content (dry matter)

The coating process began by blending the prepared starch solution and GCC suspension in equal amounts by weight. The resulting coating suspension was then introduced into a laboratory-scale horizontal size press device, as illustrated in Figure 1 of the study. The WTT papers were passed through the press rollers, which were rotating at a speed of 3.2 m/min, and the coating suspensions, as specified in Table 2, were discharged between the rollers. The pressure applied to the WTT papers was 5 bar, which ensured a uniform and consistent coating application. The use of a laboratory-scale horizontal size press device and the specified operating conditions in this study are critical in achieving precise and reproducible coating results. The equal mixing of starch solution and GCC suspension by weight and the uniform application of pressure during the coating process helped in maintaining the integrity and quality of the coated WTT papers. The methodology employed in this study can be used as a basis for the coating process of WTT papers in various applications.

Figure 1.

Figure 1

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Laboratory-type horizontal size press.

During the sizing process, the WTT papers absorbed a proportion of the coating suspension into their inner structure due to the applied pressure. To assess the effectiveness of the coating process, 10 WTT sheets were run through each of the different coating suspensions in the laboratory-scale horizontal size press. The resulting coated WTT papers were then dried and conditioned following the procedures and conditions outlined in the ISO 187 standard. The ISO 187 standard specifies the necessary steps and conditions for conditioning paper and board samples before testing to ensure that they have reached a stable moisture content. The drying and conditioning process helps in eliminating any residual moisture from the coated WTT papers, thereby ensuring that their properties and characteristics are accurately measured and reported.

WTT Testing

The ash content of paper is an important parameter that provides information about the composition of the paper and its resistance to combustion. When paper is subjected to high temperatures that exceed the combustion temperature of its organic components, the inorganic materials present in the paper that do not burn at those temperatures form a residue known as ash.25 This ash content can be used to determine the retained filler content in the paper, which provides insight into the paper’s properties and performance.

In this study, the ash contents of the papers used before and after the surface application were determined according to the ISO 1762 standard in an Elektro-mag crematorium. The ISO 1762 standard provides a method for measuring the ash content of paper and board products, and the Elektro-mag crematorium is a specialized device used to perform this measurement. The ash content was calculated using the equation described in eq 1 below, which provides a way to determine the fraction of the original sample that remains after burning. This information was used to assess the effect of the surface application on the composition and properties of the papers and to determine the retained filler content in the coated papers

graphic file with name ao3c01816_m001.jpg 1

where A is the ash content (g) of WTT paper and W corresponds to the dry weight (g) of the WTT paper.

The optical properties of the WTT papers were quantitatively measured using a Datacolor Elrepho spectrophotometer. The spectrophotometer is a specialized device used for making optical measurements of materials, and it provides precise and accurate data on the color and optical properties of the papers. The properties of whiteness, brightness, opacity, color coordinates, and yellowness were determined according to various international standards. The whiteness of the papers was measured according to the ISO 11475 standard, which provides a method for determining the degree of whiteness of a paper sample. The brightness of the papers was measured according to the ISO 2470-1 standard, which provides a method for determining the reflectance of a paper sample under standardized conditions. The opacity of the papers was measured according to the ISO 2471 standard, which provides a method for determining the degree to which a paper sample is able to obstruct the transmission of light. The color coordinates of the papers were measured according to the ISO 11664-4 standard, which provides a method for determining the colorimetric properties of a paper sample in the CIE Lab* color space. Finally, the yellowness of the papers was measured as an index according to the ASTM E313 standard, which provides a method for determining the degree of yellowness in a paper sample. By determining these properties according to well-established standards, the study ensured that the results obtained for the WTT papers were comparable and reliable and that the effects of the surface application on the color and optical properties of the papers could be accurately assessed.

The Dennison Wax Pick test was used to analyze the surface strength of the WTT papers or how well the coating materials adhered to the surface. It is performed according to TAPPI test method T459. In this test, heated waxes of varying hardness were applied to the surface to be tested. They were drawn away from the surface once they have cooled. Higher numbers are assigned to harder waxes when numbering waxes, i.e., the stronger the surface strength of the paper, the higher the wax pick number. The paper’s numerical rating was determined by the highest wax pick number that does not disturb the surface of the paper.

The Cobb test is a standard method (TAPPI T441) used to measure the water absorbency of paper products. The test consists of subjecting a sample of paper to a specific load for a specific period of time (usually 60 s) and then measuring the amount of water absorbed by the paper. The Cobb test was used to determine the water absorption properties of WTT papers and to evaluate the printability and water resistance of the papers.

Results and Discussion

The term “control papers (C)” in the text refers to uncoated WTT papers, which have not undergone any additional surface treatment beyond their initial production. The control papers serve as a baseline for comparison with the surface treated papers, which have had a coating mixture of starch solution and GCC applied to their surfaces. The filler content of the surface-treated papers was determined and is provided in Table 3. The filler content refers to the amount of GCC that was retained on the surface of the WTT papers after the coating process. The control papers, on the other hand, have a filler content of 9.3%, which is due to the fact that they were produced by recycling waste papers.

Table 3. Filler Contents of WTT Papersl.

code starch (7% TS) starch (10% TS) GCC (12% TS) GCC (15% TS) dispersant (1%) filler content (%)
C           9.3jk
7S X         8.7k
10S   X       10.2j
7S1D X       X 9.6i
10S1D   X     X 11.4h
7S12G X   X     15.4g
10S12G   X X     16.7f
7S15G X     X   18.1e
10S15G   X   X   19.9d
7S12G1D X   X   X 20.1d
10S12G1D   X X   X 21.3c
7S15G1D X     X X 22.6b
10S15G1D   X   X X 23.8a
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l

Mean values with the same lowercase letters are not significantly different according to Duncan’s mean separation test

By determining the filler content of the surface-treated papers, the study was able to assess the effectiveness of the surface treatment in increasing the filler content of the WTT papers and compare the filler content of the control papers with that of the surface treated papers. The results of this comparison provide valuable information on the impact of the surface treatment on the properties of the WTT papers and on the potential benefits that can be achieved by using the surface treatment in the production of these papers.

The study found that the highest filler retention of 23.8% was achieved with a coating suspension that was prepared using 10% TS starch solution, 15% TS GCC suspension, and 1% dispersant. Starch was used as a binder in the coating process because of its properties that make it ideal for this purpose. Starch has a high molecular weight and forms a gel when mixed with water, which helps in binding the coating pigments together and anchor them to the cellulosic fibers in the WTT paper.1,26 This enhances the adhesion of the coating to the substrate and contributes to the overall performance of the coated WTT papers.

Figure 2 compares the filler contents of the control papers with those of the coated WTT papers. The results show that the surface treatment process led to a significant increase in the filler content of the WTT papers as compared to the control papers. The results also demonstrate the potential benefits of using the coating suspension in the production of WTT papers as it leads to higher filler retention and improved properties of the final product.

Figure 2.

Figure 2

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Filler content of white top testliner papers.

The results of the study showed that the amount of starch used in the coating process has a direct impact on the amount of filler minerals that adhere to the WTT papers. When the starch solution solid content was increased from 7 to 10% dry matter in the coating suspension, which also contained GCC and a dispersant, the filler content of the WTT papers increased by 1.3 units. This is because starch acts as an adhesive to bind the coating pigments and anchor them to the cellulosic fibers of the WTT paper. Similarly, the filler content of WTT papers increased with an increase in the GCC concentration in the coating suspension.

The use of a dispersant, also known as a dispersing agent, helps in improving particle separation in the coating suspension and preventing settling or clumping of the solid particles. This results in a more uniform distribution of filler minerals in the coating layer, leading to higher filler retention in the final WTT product.27 The addition of the dispersant to the coating suspension helped in improving the filler content of the WTT. This is due to the improved homogeneity of the coating suspension, which leads to an even distribution of the coating components during the coating process.28−30 The result is a higher retention of filler minerals in the WTT, which is beneficial as it reduces the cost of the produced paper. The use of a higher concentration of starch and GCC in the coating suspension also leads to an increase in the filler content of the WTT, which results in cost savings. Figure 2 demonstrates the effects of the dispersant, starch, and GCC on the filler content of the WTT papers. The results show that by increasing the dry matter content of the coating components, the filler content of the WTT can be increased, and thus the cost of the produced paper can be reduced.

In the study, the optical properties of the WTT papers were measured after the surface application of the GCC coating. Table 4 presents the results of the optical property measurements for the WTT papers. It was observed that the use of GCC as the coating pigment had a positive impact on the optical properties of the WTT papers. The values for whiteness, brightness, opacity, color coordinates, and yellowness improved after the application of the GCC coating. This result indicates that the use of GCC coating increases the overall appearance and visual quality of the WTT papers. Additionally, the improvement in the optical properties can lead to higher demand for the WTT papers as well as an increase in their market value.

Table 4. Optical Properties of the WTT Papersg.

code whiteness (ISO%) brightness (ISO%) yellowness (E313) L a b
C 62.3d 68.0dc –11.2d 83.1 0.84 –5.34
7S 62.2d 66.8d –10.1e 83.0 0.58 –4.31
10S 63.0cd 69.8dc –10.1e 83.5 1.26 –4.33
7S1D 62.8cd 70.0bc –14.1b 83.2 1.37 –6.79
10S1D 62.5d 69.7bc –14.1b 83.2 1.09 –6.68
7S12G 63.3c 70.5bc –14.0b 83.6 1.29 –6.72
10S12G 62.6d 67.4c –10.4e 83.2 0.53 –4.43
7S15G 62.3d 66.7c –10.8e 83.1 0.17 –4.24
10S15G 63.0c 67.6bc –10.0ef 83.5 0.50 –4.23
7S12G1D 63.9bc 72.2b –16.5a 83.9 1.11 –7.23
10S12G1D 63.9bc 72.5a –16.4a 83.9 1.60 –7.90
7S15G1D 63.4bc 72.2b –16.8a 83.7 2.01 –8.22
10S15G1D 65.8a 72.6a –12.7c 84.8 1.08 –5.74
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g

Mean values with the same lowercase letters are not significantly different according to Duncan’s mean separation test

The changes in the optical properties of the WTT papers as a result of coating with GCC are mainly attributed to the light scattering and light absorption behavior of the applied layer. The properties and amount of filler mineral used in the coating process play a crucial role in determining the final optical properties of the paper. As the filler mineral is added to the coating suspension, it creates a layer on the surface of the WTT that can either scatter or absorb light, affecting the final optical properties of the paper. The type of filler mineral and its concentration can thus have a significant impact on the brightness, whiteness, opacity, color coordinates, and yellowness of the paper,31 as shown in Table 4. The use of calcium carbonate (CaCO3) as a pigment in the coating of paper can improve the brightness and whiteness of the paper by reflecting light and creating a white, opaque layer that enhances the visual appearance of the paper. Whiteness is a color parameter that evaluates the ability of a substrate to reflect light across the entire visible light spectrum (400–800 nm), providing information about the color rendering capacity of the substrate. The bright white appearance of CaCO3 contributes to an improved visual appearance and enhances the overall quality of the paper product.32 The results indicated that the WTT paper coated with the suspension that contained 15% total solids of GCC showed the highest level of whiteness according to the ISO 11475 standard. This high level of whiteness was achieved as a result of several factors. First, the dispersant used in the coating process helped in creating a more homogeneous suspension,27 which contributed to an even distribution of the GCC coating pigment. Second, the addition of 10% total solids of starch as a binder helped in binding the coating pigments together and improved the retention of the filler mineral.22 The presence of both GCC and starch in the coating suspension created a favorable environment for high levels of light reflectance, which improved the whiteness of the WTT paper. The whiteness of a paper is a color parameter that provides information about the color rendering capacity of the substrate and measures its reflectance across all wavelengths in the visible light spectrum (400–800 nm).

The brightness of a sheet of paper is a measure of the amount of light that is reflected off of it. It is typically measured on a scale of 0–100% and the higher the value, the lighter the sheet of paper appears. This light reflection is mainly determined by the amount of blue light reflected, which corresponds to the brightness of the paper. The overall concept of brightness is an indication of how much light is reflected by the paper, providing information about its visual appearance and how well it will present the printed material.33−35 The brightness value of WTT papers can be improved through the use of high ratios of GCC and starch-TS in the production of suspensions for coating. The brightness value measures the amount of light reflection, which is determined by the reflection of blue light from a sheet of paper. The use of 10% TS starch, 15% TS GCC, and 1% dispersant in the preparation of coating suspension had a positive impact on the brightness values of WTT papers. This combination increased the brightness value by 6.8%. The dispersant played an important role in improving the brightness properties of the WTT papers by creating a more homogeneous suspension, which ensured even distribution in the coating process and resulted in a higher level of filler mineral retention. The high ratio of GCC and starch-TS also contributed to the improvement in brightness properties.

Paper yellowness is a common problem in the paper industry that can affect the visual appearance and legibility of paper products. It means that paper turns yellow or a different color over time, which can make it less bright and white and make it look old and unattractive.36 Many paper manufacturers and consumers struggle with the problem of paper yellowing. This can occur for a variety of reasons, including the use of recycled paper, the presence of yellowing agents such as printing inks and dyes, and exposure to light, heat, and moisture. The yellowing of paper can also be exacerbated by the aging process. When evaluating the effect of coating additives and pigments used in the coating process of WTT papers on the yellowness values, it was observed that the best results were achieved in papers coated with the 7S15G1D coating solution. This solution was made using 7% TS starch, 15% TS GCC (calcium carbonate), and 1% dispersant. The use of a dispersant in the coating suspension preparation is known to create a more homogeneous suspension and the use of a high ratio of GCC and starch can also help improve the yellowness values of the paper. The yellowness index was reduced from −11.2 to −16.8, an improvement of about 85%. However, the breakdown of starch in the coating process can lead to the production of acids,37 which can further contribute to the yellowing and deterioration of the paper over time. This highlights the need to consider the long-term effects of the coating process on the stability and preservation of the paper.

The use of only 7 and 10% TS starch in the coating process of WTT papers had an adverse effect on the yellowness values of the papers. This is because starch can break down and produce acids, which can contribute to the yellowing and degradation of the paper over time. To counter the negative impact of starch on paper yellowness, paper manufacturers may opt for alternative materials, such as CaCO3, that are less susceptible to yellowing and deterioration. The dispersant added to the coating suspension also plays a crucial role in reducing the yellowing caused by the starch. The dispersant increases the homogeneity of the suspension and allows for an even distribution of the coating materials. Furthermore, calcium carbonate is an alkaline material that can neutralize acids produced by the breakdown of other components in the paper, such as lignin and starch.

The Lab* color space is a numerical representation of the color of a piece of paper that is widely used in the printing industry. The color of a piece of paper can be described in terms of three values: L*, a*, and b*. The L* value is a measure of the lightness or darkness of the color, with a value of 0 being black and 100 being white. The a* value is a measure of the redness-greenness of the color, with positive values indicating redness and negative values indicating greenness. The b* value is a measure of the yellowness-blueness of the color, with positive values indicating yellowness and negative values indicating blueness. By using the Lab* color space, it is possible to accurately and objectively describe and quantify the color of a piece of paper, making it a useful tool for the papermaking industry.38−40 This statement suggests that the changes in the whiteness, brightness, and yellowness values of the WTT papers, as presented in Table 4, are consistent with the corresponding changes in the Lab* values. Thus, the agreement between the changes in the whiteness, brightness, and yellowness values with the Lab* values indicates that the changes in the color properties of the WTT papers are accurately reflected in the Lab* color space.

Table 5 gives the results of the Dennison wax bead test performed on the WTT papers. Three different locations were selected for each paper and the Dennison waxes were applied to these locations to assess surface strength. The test results demonstrate the ability of the WTT papers to withstand external forces without tearing or tearing. The data obtained from this test can provide valuable insight into the surface strength properties of the WTT papers, which is essential for evaluating their printability and durability.

Table 5. Dennison Wax Numbers of WTT Papers.

  wax pick number
code 1st 2nd 3rd mean
C 16A 14A 16A 15.3
7S 16A 16A 16A 16.0
10S 18A 16A 14A 16.0
7S1D 16A 16A 16A 16.0
10S1D 18A 18A 16A 17.3
7S12G 10A 12A 12A 11.3
10S12G 12A 12A 14A 12.7
7S15G 10A 8A 10A 9.33
10S15G 10A 10A 10A 10.0
7S12G1D 12A 10A 10A 10.7
10S12G1D 12A 12A 12A 12.0
7S15G1D 10A 8A 8A 8.67
10S15G1D 10A 8A 10A 9.33
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Analysis of the data in Table 5 shows that the surface strength of the WTT papers has decreased slightly as a result of the addition of GCC to the coating suspension. This is explained by the fact that the filling material weakens the binding of the fibers on the paper surface, which reduces the surface strength. Therefore, compared to control papers and papers containing only starch, WTT papers are more likely to experience surface defects such as picking and fuzzing GCC is not unexpected. The type and amount of filler used, the process used to make the paper, and the properties of the fiber can all affect the effects. In this study, the surface strength of the WTT papers was significantly reduced by using GCC as a filling material. This should be taken into account when assessing the suitability of WTT papers for printing and other applications. However, previous studies have shown that WTT papers with coated suspension, including GCC, have printability-acceptable surface strength wax numbers.41−43 Filler materials like GCC can reduce paper surface strength, but this may not always affect print quality. Studies suggest that the optimal surface strength wax number for printability depends on the printing method, ink type, and paper properties. Ink transfer and strike-through can improve with papers with lower surface strength wax numbers.44−46 Thus, while the presence of GCC lowers WTT papers’ surface strength wax numbers, other factors can affect printability.

Figure 3 refers to a graphical representation that visually displays the Cobb60 values of WTT papers that have been coated with different coating suspensions. The Cobb60 values represent the water absorbency of the paper samples under a standardized test condition of 60 s. The different coating suspensions that have been applied to the WTT papers represent various coating formulations or compositions that have been developed to enhance the surface properties of the paper.

Figure 3.

Figure 3

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Cobb60 values of the WTT papers.

It has been shown that surface treatments, depending on the figure, significantly reduce the Cobb values of WTT papers and thus improve their water resistance. The addition of minerals such as talc, GCC, and PCC to paper can significantly affect the paper’s water absorption ability. This is due to the fact that these minerals have a hydrophobic nature and do not mix well with water.47 When they are added to the paper, they penetrate between the hydrophilic fibers and reduce their water absorption ability. This improved water resistance is critical, especially in printing applications where the paper must maintain its integrity and prevent ink from smearing or bleeding. In fact, low Cobb values are often considered a requirement for high-quality printing papers as they ensure good ink acceptance and spreadability, resulting in sharp, clear images and text.47−50

Conclusions

The present study provides insights into the effects of surface treatment on WTT papers. The findings revealed that the application of the coating suspension with 10% total solids of starch solution, 15% total solids of GCC suspension, and 1% dispersant increased the filler content of the WTT papers. The addition of starch and GCC acted as an adhesive and filler mineral, respectively, resulting in improved homogeneity and retention of filler minerals. Moreover, the use of GCC coating enhanced the optical properties of WTT papers, making them visually appealing and increasing their market value. Although the application of GCC-containing coating suspension onto the surface of WTT papers reduces their surface strength properties, it is acceptable in terms of printability. The application of GCC and starch provides water resistance to WTT papers, improving their ink absorption and spreading compared to the control papers. Thus, the results indicate the potential benefits of using surface treatment to produce WTT papers with improved properties and cost savings. Further research could focus on optimizing the surface treatment parameters and evaluating its performance in different paper grades and applications.

The authors declare no competing financial interest.

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