Effects of Zirconium-Based Crosslinkers with Different Zirconium Contents on Pigment Coating in Paper

Abstract

This study investigates the impact of zirconium-based KZC series crosslinkers with varying zirconium contents and the polyamine-based crosslinker (PBC) on the properties of coated paper, focusing on key performance metrics such as viscosity, wet rub and pick resistance, dry pick resistance, gloss, brightness, ink set-off, and print mottle. The findings reveal that crosslinkers’ type and concentration significantly influence the coating colors’ viscosity, with PBC demonstrating lower low shear viscosity at lower concentrations. The KZC series showed stable viscosity across a zirconium content range of 7% to 20%, and both crosslinker types enhanced wet rub resistance at higher concentrations. Notably, the KZC series, particularly KZC7, exhibited superior wet pick resistance at increased concentrations, highlighting its effectiveness in enhancing the durability of the coating layer. All crosslinkers maintained excellent dry pick resistance, ensuring robust coating performance. While gloss and brightness were generally unaffected, KZC20, which had the highest zirconium content, slightly reduced these optical properties. Ink set-off tests indicated that the KZC series performed better at lower concentrations, whereas higher concentrations led to increased ink set-off, potentially due to over-crosslinking. Print mottle remained consistent across all crosslinkers, indicating uniform coating quality. Overall, the zirconium-based KZC series significantly enhances wet resistance and maintains high performance across various properties, though it may slightly reduce gloss and brightness and increase ink set-off at higher concentrations. PBC offers a balanced performance profile, emphasizing the need for careful crosslinker type and concentration optimization to achieve the desired coating properties for specific applications. This comprehensive evaluation provides valuable insights for developing and optimizing high-performance coated papers.

1. Introduction

The utilization of crosslinkers in polymer chemistry has profoundly impacted the development and performance of various materials. Among these, zirconium-based crosslinkers have garnered significant attention due to their unique chemical properties and their ability to enhance the structural integrity and functionality of polymers [1,2,3]. Crosslinking, a process that involves linking polymer chains through chemical bonds, can dramatically alter the physical properties of materials, making them more suitable for a wide range of applications, from coatings and adhesives to biomedical devices [4,5] and enhanced oil recovery [6,7].

Crosslinkers are essential in the formulation of paper pigment coatings, where they play a critical role in improving the durability, gloss, water resistance, and printability of coated papers [8,9,10,11]. Various types of crosslinkers have been employed in this domain, each offering distinct advantages and limitations. Formaldehyde-based crosslinkers such as urea-formaldehyde (UF) and melamine-formaldehyde (MF) resins were widely used until the advent of methylated MF and UF in the 1970s in paper pigment coating. These resins contain functional methoxymethyl (=NCH₂OCH₃) groups that react with hydroxyl groups of starch and polyvinyl alcohol (PVA) and carboxyl groups of latex, forming methylene and dimethylene ether linkages. These resins are sensitive to temperature and pH because they undergo acid-catalyzed condensation reactions [12,13]. However, due to environmental and health concerns arising from formaldehyde emissions, regulatory restrictions and consumer demand for safer alternatives have significantly reduced their usage [14,15,16].

Conventional crosslinkers like glyoxal face significant challenges, including limited efficacy for synthetic water-soluble binders and ineffectiveness at higher pH levels, specifically above pH 8.5 [12,17]. These limitations restrict their versatility and applicability across different binder systems. Additionally, glyoxal and its derivatives tend to increase the viscosity of wet formulations due to their reaction with hydroxyl groups in the binder, complicating the coating process and affecting the final product quality [18,19,20]. Environmental and health concerns are also prominent, as glyoxal is a known irritant and poses risks during handling and application [21,22,23]. Moreover, due to their specific chemical interactions with the binder, conventional crosslinkers can negatively impact the physical properties of coated paper, such as flexibility, printability, and adhesion [24]. These challenges underscore the need for more effective and environmentally friendly alternatives in paper coating applications.

A commonly used form of zirconium crosslinker is ammonium zirconium carbonate (AZC), a soluble alkaline salt of zirconium available as an aqueous solution known for its insolubilization properties since the early 1960s [25,26]. Among zirconium-based crosslinkers, AZC and potassium zirconium carbonate (KZC) are notable for their capability to insolubilize coatings. KZC was chosen for this study due to its broader operational pH range (3 to 8) and its effective crosslinking mechanism, which does not involve the evaporation of ammonia like AZC [27,28]. The ammonia in AZC can react with clay dispersion, causing an undesirable increase in viscosity during storage times. As KZC is new on the market compared to AZC, it offers more excellent stability and ease of handling, making it more versatile and efficient in a wider range of coating formulations.

Despite the growing interest in zirconium-based crosslinkers, there remains a gap in the comprehensive understanding of how varying the zirconium content affects the final properties of crosslinked materials. Zirconium content is a critical parameter in the formulation of these crosslinkers, influencing the viscosity, reactivity, and overall performance of the crosslinked network [29,30,31]. However, systematic studies exploring the correlation between zirconium content and material properties are limited.

This study aims to fill this gap by investigating the effect of zirconium-based crosslinkers with different zirconium contents on the properties of crosslinked polymers used for paper pigment coatings. By systematically varying the zirconium contents, we seek to establish a clear relationship between zirconium content and coating performance in coating color. Such insights are crucial for optimizing the formulation of zirconium-based crosslinkers for paper pigment coating. The findings from this study will provide valuable guidelines for the formulation of advanced crosslinkers and contribute to the broader understanding of paper pigment coatings.

2. Materials and Methods

2.1. Preparing Coating Color

The formulation of the coating color for paper pigment coating is summarized in

Table 1. Formulation of coating color for paper pigment coating.

Types	pph	Solid Content (%)	Products
GCC	100	77.74	GM-95HS, GMC Co., Ltd., Incheon, the Republic of Korea
SB latex	12	62.085	DTO6115, Trinseo Korea Ltd., Seoul, the Republic of Korea
Rheology modifier	0.15	58.52	PM-7700A, Jeong Won Chemical Co., Ltd., Busan, the Republic of Korea
Lubricant	0.4	59.02	Nopcote C155, GEO Specialty Chemicals Inc., Ambler, PA, USA
Crosslinkers	0.5, 0.9	53.02	PBC, WooJin Industry Co., Ltd., Ansan, the Republic of Korea
NaOH	0.12	26, 32, 53 # 90	KZC7, KZC10, KZC20 *, HanKyung TEC, Jinju, the Republic of Korea SamChun Chemicals Co., Ltd., Seoul, the Republic of Korea
Dispersant	0.2	64.94	WT-117, Jeong Won Chemical Co., Ltd., Busan, the Republic of Korea

* The number following KZC indicates the concentration of ZrO₂. ^# The KZC series was added assuming 100% solution, ignoring the actual solid contents.

. After adjusting the pH of the water by adding NaOH, the dispersant and pigment were added and stirred into a slurry for approximately 20 min. Subsequently, each additive was added in the following order: rheology modifier, binder, lubricant, and crosslinker, to complete the preparation of the coating formulation. For reference, among the crosslinkers used, the KZC series was an KZC-based solution with zirconium contents of 7%, 10%, and 20%.

Unlike the polyamine series PBC, 0.5 and 0.9 parts of each KZC7, KZC10, and KZC20 were added when preparing the coating color, assuming a 100% solid content solution. Figure 1 shows the basic chemical structure of KZC. The final zirconium content of the prepared coating colors was 65%.

The rationale behind selecting the specific zirconium content percentages of 7%, 10%, and 20% was to systematically investigate the effects of varying zirconium concentrations on the coating properties. These concentrations were chosen to provide a comprehensive understanding of the performance range of zirconium-based crosslinkers, from lower to higher content, and to identify the optimal concentration that balances performance improvements with material efficiency. This range allows for the assessment of both incremental and more significant changes in the zirconium content, thereby ensuring a thorough evaluation of its impact on the mechanical strength, thermal stability, and water resistance of the coated paper.

2.2. Properties of Coating Color

The viscosity of the prepared coating color was measured using a low shear viscometer (Brookfield, Middleboro, MA, USA) equipped with a No.4 spindle and 25 mL of coating color in a falcon tube at 60 rpm. The high shear viscosity was measured using a high shear viscometer (ACAV AX150, Polvijärvi, AMT-Systems Oy, Finland) by connecting a 0.5 × 50 mm capillary tube to a cylinder containing 180 mL of coating color and applying a shear rate of 1,000,000 s⁻¹. The pH was measured using a pH meter (pH/Temp Meter p25, ISTEK INC., Seoul, the Republic of Korea). The water retention of the coating color was evaluated using a gravimetric water retention meter (AA-GWR, Kaltec Scientific Inc., Livonia, MI, USA) by measuring the amount of water drained over 30 s.

2.3. Preparing Coated Paper

The base paper for coating had a basis weight of approximately 104 g/m² and was provided by Moorim Paper Co., Ltd., located in Jinju, the Republic of Korea. The coated paper was prepared using a laboratory semi-automatic coater (Auto bar coater, HanTech Co., Ltd., Ulsan, the Republic of Korea) at a speed of 70 mm/s. The coating weight was set at 50 ± 2 g/m² per side. After coating, the paper was dried for 30 s in a hot air dryer (OF-21E, JEIO TECH, Daejeon, the Republic of Korea) at 105 °C. Subsequently, the coated paper was passed once through a supercalender (M2-250A, DUCO Co., Ltd., Daejeon, the Republic of Korea) with the temperature set at 70 °C and a nip pressure of 50 ± 10 kg, ensuring the coated side faced the steel roll at 0.05 m/s.

2.4. Properties of Coated Paper

The gloss of the coated paper was measured using a gloss meter (T480A, Technidyne Corporation, New Albany, IN, USA) according to ISO 8254-1, and the brightness and opacity were measured using an Elrepho (LAMBDA-900, Lorentzen & Wettre, Stockholm, Sweden) according to ISO 2470-1 and ISO 2471 [32,33,34]. The printability of the coated paper, including ink set-off, print mottle, dry pick, and wet pick strength, was evaluated using an RI printability tester (RI-3, AkiraSeisakusho, Tokyo, Japan) according to ISO 8789, ISO 13660, and ISO 3783 [35,36,37]. For the wet rubbing test of the coated paper, 3 × 20 cm specimens were secured using the built-in fixture of the rubbing tester (COAD.105, OCEAN SCIENCE, Uiwang, the Republic of Korea), subjected to 10 back-and-forth strokes with a lint-free cloth, then the cloth with the abraded coating color was immersed in 15 mL of distilled water for the turbidity test (DR/890 Colorimeter, Hach Company, Loveland, CO, USA), with the average turbidity value calculated after six repetitions. A field-emission scanning electron microscope (FE-SEM, JSM-7610F, JEOL, Tokyo, Japan) and cross-section polisher (IB-09020CP, JEOL, Tokyo, Japan) were employed to observe the surface structure and cross-section of the coated paper. The FE-SEM was operated under optimized conditions to achieve high-resolution imaging, allowing for detailed visualization of the coating morphology. The images illustrated the coating characteristics of different crosslinkers on the base paper.

3. Results and Discussion

3.1. Properties of Coating Color

As shown in Figure 2, the pH of the coating color remained largely unchanged across different zirconium contents despite the addition of crosslinkers ranging from 0.5 to 0.9 parts. The pH of all coating colors was consistently maintained at approximately 9. This indicates that crosslinkers from the polyamine and KZC series exhibit low sensitivity to pH variations [38].

Figure 3 compares the water retention values of the coating colors at different zirconium contents of the crosslinkers. Among the KZC series, the crosslinker KZC7, with a 7% zirconium content, exhibited a water-holding capacity comparable to that of PBC from the polyamine series. As the zirconium content increased, the crosslinkers KZC10 and KZC20 demonstrated slight improvements in water retention compared to PBC and KZC7. The KZC series crosslinkers were added at 0.5 part and 0.9 part, assuming a 100% solution regardless of the actual solid contents of 7%, 10%, and 20%, meaning that much less of the KZC crosslinkers were used in the coating color formulation compared to the polyamine-based PBC. Considering this, the KZC series crosslinkers can be regarded as having better water holding ability than the polyamine series crosslinker. Specifically, the water retention ability of the KZC series showed noticeable enhancements as more crosslinkers were added. This suggests that higher zirconium contents in the KZC series contribute to improved water retention in the coating colors, likely due to zirconium’s covalent bonds with carboxyl groups and the weaker hydrogen bonds with hydroxyl groups of binders (refer to Figure 4). Thus, the water sensitivity can be decreased by crosslinking soluble binders with the crosslinkers or by forming an insoluble net around the binders [10,11]. This observation underscores the importance of optimizing the zirconium content in KZC-based crosslinkers to achieve the desired performance characteristics in pigment coatings.

Figure 5 illustrates the viscosity of coating colors with varying zirconium contents of crosslinkers, highlighting its crucial role in the coating process by affecting application uniformity, coating weight control, flow and leveling, drying behavior, equipment compatibility, and ultimately, the quality of the finished product [2,39]. At a low concentration of 0.5 parts, the polyamine-based PBC demonstrated lower low shear viscosity compared to the zirconium-based KZC series, indicating that crosslinker type plays a pivotal role in viscosity at lower concentrations. Interestingly, within the KZC series, low shear viscosity remained relatively stable across a zirconium content range of 7% to 20%, highlighting an insensitivity to zirconium content variations within this range. However, at an increased concentration of 0.9 parts, a sharp rise in low shear viscosity was observed for all crosslinkers. KZC crosslinkers with zirconium contents above 10% exhibited slightly higher viscosities than PBC and KZC7. The zirconium-based KZC series is much more sensitive than the polyamine-based PBC due to the inherent chemical and physical properties of zirconium compounds, which tend to form stronger and more numerous crosslinks within the polymer matrix. This increased crosslink density results in a more pronounced increase in viscosity with the addition of zirconium content compared to the polyamine-based PBC. Moreover, the KZC series crosslinkers were added at 0.5 part and 0.9 part, assuming a 100% solution, regardless of the actual solid contents (7%, 10%, and 20%). This means that much less of the KZC crosslinkers were used in the coating color formulation compared to PBC, yet they still exhibited higher sensitivity in viscosity changes. This suggests that while crosslinker type is a critical factor at lower concentrations, both the concentration and specific crosslinker characteristics influence viscosity at higher levels, which in turn affects the overall quality and performance of the coating process and final product. This insight is essential for formulating coating colors to achieve desired processing and performance outcomes.

There was no remarkable difference among the types of crosslinkers in high shear viscosity. However, the viscosity of the KZC crosslinkers slightly increased as the zirconium content rose from 7% to 20% and the concentration increased from 0.5 part to 0.9 part. The KZC series was slightly more sensitive in high shear viscosity than the PBC. This indicates that higher zirconium content and greater concentration of KZC crosslinkers lead to increased resistance to flow under high shear conditions. Thus, understanding these relationships is crucial for selecting the appropriate crosslinker and its concentration to achieve the desired viscosity characteristics for specific applications, particularly in paper pigment coating processes where both low and high shear viscosities play pivotal roles.

3.2. Printing Properties of Coated Paper

Figure 6 illustrates the wet rub resistance of coated paper using coating colors with varying zirconium contents of crosslinkers and the polyamine-based crosslinker, highlighting the critical role of crosslinkers in reducing the water sensitivity of the coating layer. This water sensitivity is evaluated through wet rub and wet pick resistance tests. At a concentration of 0.5 parts, KZC7 demonstrated good wet rub resistance, while PBC, KZC10, and KZC20 exhibited comparable performance. Increasing the concentration to 0.9 parts resulted in enhanced wet rub resistance for all crosslinkers, indicating that higher zirconium contents contribute to improved water resistance in the coating layer, comparable to PBC. The data suggest that the zirconium-based crosslinkers, such as the KZC series, effectively enhance the coating layer’s wet rub resistance. While KZC7 showed good performance at lower concentrations, all crosslinkers, including PBC, KZC10, and KZC20, benefitted from increased concentrations, showcasing improved wet rub resistance. This trend underscores the importance of zirconium content in the formulation, as higher zirconium levels lead to a more robust and water-resistant coating layer.

Figure 7 presents images of the coated paper after the wet pick test, revealing the performance of different crosslinkers and emphasizing the effects of zirconium content on wet pick resistance. At a concentration of 0.5 parts, all crosslinkers exhibited poor wet pick resistance, with the KZC series performing particularly poorly. However, at a higher concentration of 0.9 parts, the KZC series demonstrated superior wet pick resistance compared to the polyamine-based crosslinker PBC. Notably, KZC7, which has the lowest zirconium content among the KZC series, showed the best wet pick resistance at this higher concentration. This suggests that KZC7’s unique formulation might allow for more efficient crosslinking or better film formation, enhancing the coating layer’s durability and resistance to wet pick more effectively than KZC10, KZC20, and PBC. Conversely, increasing the zirconium content beyond 10% within the KZC series, even at a concentration of 0.9 parts, may lead to decreased wet pick resistance. This could be due to potential issues such as increased brittleness or suboptimal crosslinking density, which can negatively impact the coating’s flexibility and adhesion.

Thus, while higher concentrations of zirconium-based crosslinkers generally improve wet pick resistance, an optimal range of zirconium content maximizes this benefit. Understanding these relationships is crucial for selecting the appropriate crosslinker type and concentration to achieve the desired wet pick resistance for specific coated paper applications.

Figure 8 displays images of the dry pick resistance of coated paper treated with coating colors containing crosslinkers with different zirconium contents and a polyamine-based crosslinker, emphasizing the impact of zirconium content on this property. The results indicate that all crosslinkers exhibited excellent dry pick resistance, regardless of their type and zirconium content, suggesting that the crosslinkers effectively contribute to the dry pick resistance of the coating layer. Notably, there were no significant differences among the various crosslinkers, including the zirconium-based KZC series and the polyamine-based PBC. This indicates that the type and zirconium content of the crosslinker do not significantly affect the dry pick resistance of the coated paper.

This uniform performance underscores the robustness of the coating formulations in maintaining dry pick resistance, irrespective of the specific crosslinker used. It suggests that while zirconium content and crosslinker type are critical factors for other properties, such as wet rub and wet pick resistance, they have a negligible impact on dry pick resistance. This insight is valuable for formulating coating colors that require consistent dry pick resistance, offering flexibility in crosslinkers’ choice without compromising this property. This consistency in dry pick resistance across different crosslinkers and zirconium contents ensures that manufacturers can achieve reliable performance in their coated paper products.

Figure 9 presents images of the ink set-off test of coated paper treated with coating colors containing crosslinkers with different zirconium contents and a polyamine-based crosslinker, highlighting the effects of zirconium content on ink set-off properties. Ink set-off is a crucial parameter measured to ensure that excessive amounts of ink do not transfer from the printed packaging surface to the unprinted surfaces, which can affect the quality and appearance of printed materials [40,41]. At a concentration of 0.5 parts, all crosslinkers, including the potassium zirconium carbonate (KZC) series and the polyamine-based PBC, exhibited excellent ink set-off properties. This suggests that at lower concentrations, the crosslinkers effectively contribute to minimizing ink transfer, ensuring high-quality print results.

However, at an increased concentration of 0.9 parts, the KZC series demonstrated worse ink set-off properties compared to PBC. This indicates that the KZC series is more effective in lower concentrations and that increasing the zirconium content beyond a certain threshold may negatively impact ink set-off performance. The deterioration in ink set-off properties at higher concentrations for the KZC series could be due to over-crosslinking or changes in the coating layer’s surface characteristics, which may lead to increased ink transfer.

This analysis underscores the importance of optimizing crosslinker concentration in coating formulations. While the KZC series is effective at lower concentrations, careful consideration must be given to its concentration to avoid compromising ink set-off properties. Conversely, PBC appears to perform better at higher concentrations, offering more flexibility in formulation adjustments. Understanding these relationships is essential for selecting the appropriate crosslinker type and concentration to achieve desired ink set-off properties and ensure the quality of printed packaging materials.

Figure 10 presents images of the print mottle test of coated paper treated with coating colors containing crosslinkers with different zirconium contents and a polyamine-based crosslinker. Print mottle is tested for coated paper to evaluate the uniformity and quality of the printed surface [42,43]. The results indicate that the types of crosslinkers, whether zirconium-based or polyamine-based, and their concentration in the coating colors did not significantly affect the printing quality of the coated paper. All printed images were well-defined on the coated paper without noticeable mottle. This uniform performance across different crosslinkers suggests that both zirconium-based and polyamine-based crosslinkers are effective in producing a smooth and uniform coating layer that supports high-quality printing. The absence of print mottle indicates that the coating formulations, regardless of the crosslinker type, provide consistent ink absorption and distribution, leading to even color density and clear, sharp images.

For zirconium-based crosslinkers, the ability to maintain low print mottle may be attributed to their strong crosslinking capabilities, which create a robust and uniform coating matrix. This uniformity helps in achieving even ink distribution during the printing process. Similarly, polyamine-based crosslinkers also effectively reduce print mottle, likely due to their ability to form flexible and evenly distributed crosslinks within the coating layer.

3.3. Optical Properties of Coated Paper

Figure 11 compares the gloss of coated papers treated with coating colors containing crosslinkers with different zirconium contents and a polyamine-based crosslinker, showing that the types and concentrations of crosslinkers in the coating colors do not significantly affect gloss. However, it should be noted that KZC20, with the highest zirconium content, showed a slight decrease in gloss. Higher zirconium content in KZC20 may lead to a higher crosslink density within the coating layer. While increased crosslinking can enhance certain properties, such as wet rub resistance [44,45], it can also create a more rigid and less smooth surface, which scatters light more diffusely and reduces gloss [28].

Gloss is primarily influenced by factors such as coating composition (types and proportions of pigments, binders, and additives), surface smoothness, drying and curing conditions, coating thickness, base paper quality, and the application method [46,47,48]. These elements collectively determine the smoothness and uniformity of the coating layer, which are crucial for achieving high gloss levels in coated paper products.

Figure 12 compares the brightness of coated papers treated with coating colors containing crosslinkers with different zirconium contents and a polyamine-based crosslinker. The results indicate that the brightness of the coated paper was hardly affected by the types and concentrations of the crosslinkers in the coating colors. However, similar to the gloss results observed in Figure 11, the use of KZC20 led to a slight decrease in brightness. This suggests that while most crosslinkers do not significantly impact brightness, the higher zirconium content in KZC20 may introduce surface irregularities or increase crosslink density, resulting in reduced light reflectance and a slight decrease in both gloss and brightness.

Despite these minor reductions in gloss and brightness with KZC20, it is important to note that these differences do not significantly affect the marketability or usability of the paper. The observed trade-offs are minimal and within acceptable ranges for most applications. Therefore, adjustments to the formula to mitigate these effects may not be necessary unless specific applications require exceptionally high gloss and brightness. Understanding these effects is crucial for optimizing coating formulations to achieve the desired optical properties in coated paper products, and further exploration of these parameters can help in fine-tuning the balance between optical properties and functional performance.

3.4. SEM Images of Coated Paper

Figure 13 shows SEM images of the surface and cross-section of coated paper treated with Figure 13a a polyamine-based crosslinker and Figure 13b a zirconium-based crosslinker. The SEM images revealed no significant differences in the morphology of the coating layer between the two crosslinkers. This suggests that both crosslinkers perform similarly in terms of bonding the coating to the base paper, indicating that either crosslinker can be effectively used for this purpose. It was determined that the analysis results discussed above would be more effective in verifying the effectiveness of the crosslinkers.

4. Conclusions

This study evaluated the effects of zirconium-based KZC series with different zirconium contents and polyamine-based PBC crosslinkers on coated paper properties, focusing on viscosity, wet rub and pick resistance, dry pick resistance, gloss, brightness, ink set-off, and print mottle. Results indicated that crosslinker type and concentration significantly impacted viscosity, with the KZC series showing consistent performance across a wide zirconium content range. Both crosslinker types improved wet rub resistance at higher concentrations, with the KZC series demonstrating superior wet pick resistance, particularly KZC7 at higher concentrations. All crosslinkers maintained excellent dry pick resistance, and gloss and brightness were mostly unaffected except for a slight reduction with high zirconium content in KZC20. Ink set-off was better at lower concentrations for the KZC series, and print mottle remained consistent across all crosslinkers. Overall, the zirconium-based KZC series significantly enhanced wet resistance and maintained high performance across various properties, though it might slightly reduce gloss and brightness and increase ink set-off at higher concentrations. Conversely, polyamine-based PBC offers balanced performance, underscoring the importance of optimizing crosslinker type and concentration to meet specific application requirements.