A surface sizing agent, its preparation method, and corrugated cardboard

Abstract

This invention discloses a surface sizing agent, its preparation method, and corrugated cardboard, belonging to the field of papermaking additives technology. To solve the problems of poor folding endurance and easy bursting of corrugated cardboard, this invention provides a surface sizing agent comprising a first modified component obtained by reacting polyvinyl alcohol with a first silane coupling agent containing epoxy groups, and a second modified component obtained by reacting nanocellulose with a second silane coupling agent containing amino groups. The preparation method includes modifying the two raw materials separately, wherein the nanocellulose is dispersed by ultrasonic treatment. Applying the sizing agent to the fold lines of the corrugated cardboard forms a double cross-linked network with both hydrogen bonds and covalent bonds on its surface, thereby significantly improving the folding endurance of the corrugated cardboard and effectively preventing bursting.

Description

Technical Field

[0001] This invention relates to the field of papermaking additives, and more specifically, to a surface sizing agent for improving the physical properties of corrugated board. The invention also relates to a method for preparing the surface sizing agent, and corrugated board with improved folding endurance obtained by applying the surface sizing agent. Background Technology

[0002] Corrugated cardboard plays an indispensable role in the outer packaging and transport packaging of goods due to its excellent cushioning and shock absorption properties and high structural strength. With increasing global attention to environmental protection issues and the rapid development of e-commerce, corrugated cardboard packaging made from renewable plant fibers is gradually becoming an important alternative to traditional plastic packaging, and its application scope has expanded from traditional packaging boxes to emerging fields such as exhibition displays and paper furniture.

[0003] The main raw material for corrugated cardboard is recycled waste corrugated boxes. The fibers in waste paper typically undergo multiple recycling processes, each cycle causing some degree of damage to the physical properties of the fibers. For example, the fiber length decreases, the content of fine particles increases, and the fibers themselves become more brittle. These factors collectively lead to a decline in the macroscopic physical properties of the cardboard produced from it, with a particularly significant reduction in folding endurance.

[0004] In practical applications of corrugated cardboard, especially as packaging for express delivery, insufficient folding endurance directly leads to a serious technical problem: "bursting." When ambient humidity is low, particularly in cold, dry winters or arid climates, the surface paper layer of the cardboard is prone to fiber breakage when folded along the crease line, forming visible cracks. Bursting not only severely affects the appearance of the carton, but more importantly, it damages the structural integrity of the cardboard, leading to a decrease in key mechanical properties such as compressive strength. This increases the risk of damage to goods during distribution and raises the scrap rate for related manufacturers.

[0005] To address the issue of crease lines bursting in corrugated cardboard, existing production processes typically employ physical aids, such as spraying moisture onto the creases before folding or adjusting the geometry of the die-cutting head. However, these methods do not improve the cardboard's toughness at its core, resulting in limited effectiveness and difficulty in controlling process stability. Another approach proposes applying chemical additives, i.e., surface sizing agents, to the paper surface to improve its mechanical properties. However, cardboard differs significantly from ordinary paper in structure and performance; cardboard has a much higher out-of-plane stiffness than paper, leading to more complex and concentrated stresses at the creases during folding. Therefore, existing surface sizing technologies developed for ordinary paper cannot directly and effectively solve the folding endurance problem of corrugated cardboard. Consequently, there is an urgent need in this field for a novel surface sizing agent that can fundamentally improve the fiber toughness of corrugated cardboard and effectively prevent crease lines from bursting during folding. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to address the problem that corrugated cardboard has insufficient folding endurance and is prone to bursting during folding due to the decline in fiber properties in the prior art. The present invention provides a surface sizing agent that can significantly improve the folding endurance of corrugated cardboard, its preparation method, and corrugated cardboard treated with the sizing agent.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] In a first aspect, the present invention provides a surface sizing agent comprising a first modified component obtained by reacting polyvinyl alcohol with a first silane coupling agent, and a second modified component obtained by reacting nanocellulose with a second silane coupling agent. The first silane coupling agent is an epoxy-containing silane coupling agent, and the second silane coupling agent is an amino-containing silane coupling agent.

[0009] As a preferred technical solution, the first silane coupling agent is γ-glycidoxypropyltrimethoxysilane, and the second silane coupling agent is 3-aminopropyltriethoxysilane.

[0010] As another preferred technical solution, the mass ratio of the first silane coupling agent to the polyvinyl alcohol used to prepare the first modified component is 1:6. The mass ratio of the second silane coupling agent to the nanocellulose is 1:5.

[0011] A second aspect of the present invention provides a method for preparing the surface sizing agent as described above, comprising the following steps:

[0012] Step S1: Prepare the first modified component by dissolving polyvinyl alcohol in deionized water, adding the first silane coupling agent dropwise, and reacting under alkaline conditions.

[0013] Step S2: Prepare the second modified component by dispersing nanocellulose in water, dissolving the second silane coupling agent in an ethanol-water mixed solvent, reacting the two under acidic conditions, obtaining the product, precipitating it by adding an organic solvent, centrifuging and drying to obtain the solid second modified component.

[0014] Step S3: Disperse the solid second modified component obtained in step S2 in water, mix it with the reaction product containing the first modified component obtained in step S1, and stir it under alkaline conditions.

[0015] Step S4: Mix the intermediate product obtained in step S3, the polyvinyl alcohol aqueous solution, and glycerol to obtain the surface sizing agent.

[0016] As a further limitation of the above preparation method, in step S1, the polyvinyl alcohol is dissolved at 90°C for 2 hours and then cooled to 60°C; the reaction is carried out by adding 10% sodium hydroxide solution to adjust the pH value to 7-8 and stirring for 2 hours.

[0017] As a further limitation of the above preparation method, in step S2, the nanocellulose is dispersed in water by ultrasonic treatment to form a 1wt% nanocellulose solution; the reaction is carried out at 70°C for 3 hours, during which glacial acetic acid is added to stabilize the pH of the reaction system at 4-5.

[0018] As a further limitation of the above preparation method, after step S2, the product is precipitated in acetone, wherein the volume ratio of acetone to product is 5:1, and the precipitate is subjected to three redispersion-centrifugation cycles and then vacuum dried at 60°C for 6 hours.

[0019] As a further limitation of the above preparation method, in step S3, the solid second modified component is prepared as a 3wt% aqueous solution, and after being mixed with the product of step S1, the pH value is adjusted to 7-8 by adding 10% sodium hydroxide solution, and mechanically stirred for 2 hours.

[0020] In a third aspect, the present invention provides a corrugated cardboard, wherein the folds of the corrugated cardboard are coated with any of the surface sizing agents described in the first aspect of the present invention.

[0021] Based on a deep understanding of the failure mechanism of corrugated cardboard, this invention proposes a technical solution with the synergistic effect of dual modification and dual crosslinking, which fundamentally improves the bonding strength between the sizing agent coating and the cardboard fiber and the toughness of the coating itself from the two levels of chemical bonding and physical network.

[0022] First, this invention recognizes that sizing agents based solely on polyvinyl alcohol (PVA) rely primarily on low-energy hydrogen bonds for bonding with paper fibers, a bond easily broken under folding stress. To address this issue, this invention chemically modifies PVA. A first silane coupling agent containing asymmetric epoxy groups (such as KH560) is selected. Under alkaline conditions, these epoxy groups can undergo a highly efficient ring-opening reaction with the abundant hydroxyl groups on the PVA molecular chain. Essentially, this reaction successfully attaches more reactive silanol functional groups to the PVA molecule. These introduced silanol groups lay the material foundation for the subsequent formation of a strong covalent-based chemical crosslinking network within the sizing agent coating and between the coating and fiber interface.

[0023] Secondly, this invention introduces nanocellulose (CNF) as a reinforcing phase. CNF itself possesses excellent mechanical properties and a high aspect ratio, enabling it to act as a reinforcing filler, forming a dense nanofiber network in the coating and effectively filling the pores between fibers on the paperboard surface. However, the inventors recognized a key technical obstacle: the CNF surface is rich in strongly polar hydroxyl groups, which, in a relatively hydrophobic reinforcing system like modified PVA, are prone to agglomeration due to thermodynamic incompatibility, leading to uneven dispersion and failing to exert its reinforcing effect. Therefore, this invention specifically modifies the surface of CNF to improve its compatibility and dispersibility in the polymer matrix. A disilane coupling agent containing amino groups (such as KH550) is selected and pre-hydrolyzed under acidic conditions to hydrolyze its ethoxy groups into reactive silanol groups. Simultaneously, the acidic environment effectively inhibits premature self-condensation of the silanol groups. Subsequently, these silanol groups undergo a grafting reaction with the hydroxyl groups on the CNF surface, successfully attaching hydrophobic alkyl chains to the CNF surface. This step significantly reduces the surface energy of CNF, fundamentally solving its dispersion problem and ensuring that it can be uniformly dispersed at the nanoscale throughout the sizing agent system, thus guaranteeing the formation of a uniform, defect-free reinforced coating.

[0024] Finally, when the composite sizing agent containing two modified components prepared according to this invention is coated on the surface of corrugated cardboard, a three-dimensional network with both crosslinking mechanisms is formed. First, the abundant hydroxyl groups of all components in the system (modified PVA, modified CNF, and cardboard fibers) crosslink through hydrogen bonds to form a dense basic network, providing excellent film-forming properties and toughness. Second, and this is the core innovation of this invention, during the moisture evaporation and curing process, the active silanol groups grafted onto PVA and CNF undergo further dehydration condensation reactions to form chemically stable silicon-oxygen-silicon (Si-O-Si) covalent bonds. These covalent bonds not only crosslink within the sizing agent but also react with the hydroxyl groups on the surface of the paper fibers, forming a strong chemical anchor.

[0025] Therefore, the final coating formed by this invention is an interpenetrating network structure composed of a flexible hydrogen bond network and a rigid covalent bond skeleton. When corrugated cardboard is folded, external stress can be rapidly dispersed through this efficient stress transfer network, avoiding stress concentration at microscopic defects. Replacing fragile hydrogen bonds with covalent bonds as the main interfacial bonding force results in an order-of-magnitude increase in interfacial strength. This unique structure ultimately endows corrugated cardboard with unprecedented folding resistance, fundamentally solving the problem of bursting caused by fiber performance degradation. Attached Figure Description

[0026] Figure 1 This is a simplified schematic diagram illustrating the principle of the instrument for measuring the number of folds withstandd according to the present invention.

[0027] Figure 2A This is an external view of the corrugated cardboard face paper of Embodiment 1 of the present invention without being folded.

[0028] Figure 2B This is an external view of the lining paper of the corrugated cardboard in Embodiment 1 of the present invention without being folded.

[0029] Figure 3A This is a diagram of the appearance of the face paper of the corrugated cardboard after being folded 14,300 times according to Embodiment 1 of the present invention.

[0030] Figure 3B This is an image of the inner paper of the corrugated cardboard after being folded 14,300 times according to Embodiment 1 of the present invention.

[0031] Figure 4 This is a crack morphology diagram of the corrugated cardboard after being folded 14,300 times according to Embodiment 1 of the present invention. Detailed Implementation

[0032] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0033] Example 1.

[0034] This embodiment provides a method for preparing a surface sizing agent. First, 30 grams of polyvinyl alcohol (Shandong Keyuan Biochemical Co., Ltd., model 1799, hereinafter referred to as PVA1799) are weighed and added to 270 ml of deionized water. The solution is stirred and dissolved at 90°C for 2 hours, and then the solution is cooled to 60°C for later use.

[0035] Next, 5 grams of γ-glycidoxypropyltrimethoxysilane (Shanghai Maclean Biochemical Technology Co., Ltd., purity 97%, molecular weight 236.34, hereinafter referred to as KH560) were added dropwise to the above PVA1799 solution. Then, the pH of the mixed solution was adjusted to the range of 7-8 using a 10% sodium hydroxide aqueous solution, and the mixture was stirred continuously for 2 hours under these conditions to obtain the reaction product containing the first modified component, denoted as solution A.

[0036] Take another 2 grams of nanocellulose (Shandong Keyuan Biochemical Co., Ltd., diameter 4-10 nanometers, length 200 nanometers, hereinafter referred to as CNF) and add it to 198 ml of deionized water. Sonicate for 30 minutes to disperse it evenly, to obtain a 1 wt% CNF solution. Then add glacial acetic acid to adjust the pH of the solution to 4-5.

[0037] Take another 3 mL of 3-aminopropyltriethoxysilane (Shandong Keyuan Biochemical Co., Ltd., purity 99%, molecular weight 221.37, density approximately 0.945 g / mL, hereinafter referred to as KH550), add it to 97 mL of a mixed solvent consisting of ethanol and water in a volume ratio of 8:2, stir for 1 hour, and obtain a KH550 stock solution with a total volume of approximately 100 mL.

[0038] Accurately transfer 14 mL from the aforementioned KH550 stock solution and slowly add this 14 mL solution dropwise to the aforementioned CNF solution. The 14 mL of KH550 solution transferred in this step contains approximately 0.4 g of KH550, which, when added to the 2 g CNF, precisely constitutes a 1:5 mass ratio. The mixed system is then reacted in a 70°C water bath for 3 hours, with glacial acetic acid continuously added during the reaction to maintain a stable pH of 4-5.

[0039] After the reaction, the obtained product was mixed with acetone (Tianjin Fuyu Fine Chemical Co., Ltd., analytical grade) at a volume ratio of 1:5 to precipitate the modified CNF. The precipitate was collected by centrifugation and redispersed with acetone by centrifugation and washing, a process repeated three times. Finally, the collected paste was dried in a vacuum drying oven at 60°C for 6 hours to obtain the solid second modified component product.

[0040] Weigh 0.3 g of the above-mentioned solid second modified component product, disperse it in water to prepare a 1% suspension, and stir for 2 hours to obtain solution B. Add solution B to solution A, adjust the pH of the system to 7-8 again using a 10% sodium hydroxide aqueous solution, and mechanically stir for 2 hours to obtain the intermediate product.

[0041] Alternatively, 10 grams of PVA1799 were added to 90 ml of deionized water and dissolved at 90°C for 2 hours. The solution was then cooled to 60°C to obtain an aqueous solution of PVA1799.

[0042] Finally, take 10 ml of the above intermediate product, 40 ml of the above PVA1799 aqueous solution, and 10 ml of glycerol, and mix them thoroughly. Then add 30 ml of deionized water to the mixture to obtain the final surface sizing agent.

[0043] Example 2.

[0044] This embodiment provides a method for preparing a surface sizing agent using a different process route. The main difference between this method and Example 1 lies in the preparation step of the second modified component.

[0045] The steps for preparing the first modified component are exactly the same as in Example 1, namely, weighing 30 grams of PVA1799 and adding 5 grams of KH560, reacting under alkaline conditions to obtain solution A containing the first modified component.

[0046] The steps for preparing the second modified component are as follows: 6 mL of KH550 (approximately 5.67 g) was added to a mixed solvent consisting of 135 mL of ethanol and 15 mL of deionized water. The pH was adjusted to 4-5 using glacial acetic acid, and the mixture was stirred for 2 hours. Subsequently, 2 g of CNF was added to the aforementioned solution, and the mixture was reacted at 65°C for 4 hours, with continued stirring for another 4 hours. The KH550 to CNF ratio used in this step was approximately 2.84:1 by mass. This example aims to verify the effect of a different technical solution than the 1:5 ratio specified in Example 1.

[0047] After the reaction, the subsequent purification steps and the final preparation steps were exactly the same as those described in Example 1. That is, acetone was used for precipitation purification to obtain a solid second modified component, which was then prepared into a 2% suspension (0.6 g) and mixed with solution A. Finally, it was mixed with PVA1799 aqueous solution and glycerol to obtain the final surface sizing agent.

[0048] Comparative Example 1: Surface sizing agent containing only modified polyvinyl alcohol

[0049] The preparation method of this comparative example is basically the same as that of Example 1, except that its sizing agent does not contain the modified nanocellulose component. That is, only the preparation of solution A in Example 1 is carried out, and it is mixed with polyvinyl alcohol aqueous solution and glycerol in a certain proportion to obtain the surface sizing agent.

[0050] Comparative Example 2: Surface sizing agent with varying amounts of modified nanocellulose

[0051] The preparation method of this comparative example is basically the same as that of Example 1, except for the amount of the second modified component (modified nanocellulose) used in the preparation of the intermediate product. Specifically, in this comparative example, the 0.3 g of solid second modified component (prepared as a 1% suspension) used in Example 1 is replaced with 0.6 g (prepared as a 2% suspension).

[0052] Comparative Example 3: Surface sizing agent with further variations in the amount of modified nanocellulose.

[0053] The preparation method of this comparative example is basically the same as that of Example 1, except that the 0.3 g of solid second modified component (prepared as a 1% suspension) used in Example 1 is replaced with 0.9 g (prepared as a 3% suspension).

[0054] Comparative Example 4: Surface sizing agents based on different processes and varying amounts of modified nanocellulose

[0055] The preparation method of this comparative example is basically the same as that of Example 2, except that the 0.6 g of solid second modified component (prepared as a 2% suspension) used in Example 2 is replaced with 0.3 g (prepared as a 1% suspension).

[0056] Comparative Example 5: Surface sizing agents based on different processes and further variations in the amount of modified nanocellulose.

[0057] The preparation method of this comparative example is basically the same as that of Example 2, except that the 0.6 g of solid second modified component (prepared as a 2% suspension) used in Example 2 is replaced with 0.9 g (prepared as a 3% suspension).

[0058] Comparative Example 6: Surface sizing agents based on different processes and with varying amounts of modified nanocellulose

[0059] The preparation method of this comparative example is basically the same as that of Example 2, except that the 0.6 g of solid second modified component (prepared as a 2% suspension) used in Example 2 is replaced with 1.2 g (prepared as a 4% suspension).

[0060] Comparative Example 7: Blank Control

[0061] This comparative example is a sample of raw corrugated cardboard without any surface sizing treatment.

[0062] Cardboard folding endurance test.

[0063] To verify the beneficial effects of the surface sizing agent prepared in the embodiments of the present invention, the folding endurance of the corrugated cardboard prepared in the above embodiments and comparative examples was tested. First, all corrugated cardboard samples to be tested were pretreated for 48 hours in a constant temperature and humidity environment at a temperature of 23.0±1℃ and a relative humidity of 50.0±2.0% in accordance with the national standard GB / T10739.

[0064] The testing process was conducted in accordance with the national standard GB / T 457-2008. The structure of the testing instrument is as follows: Figure 1 As shown, the sample is placed flat in the slot 3 of the testing instrument and folded back and forth along the fold seam made during pretreatment. One complete backward fold of 135° and contact with the upper surface 1, followed by a forward fold of 135° and contact with the lower surface 2, constitutes one folding resistance cycle.

[0065] During the test, the number of folds and the length of the crack when the sample first cracked were continuously observed and recorded. When the total length of the cracks on the sample surface exceeded 10 mm, the sample was considered to have failed, the test was stopped, and the number of folds at this point was recorded as the number of folds affecting usability. Before the test, the folds of the surface-coated corrugated cardboard obtained in Example 1 were as follows: Figure 1 As shown, after 14,300 folds, the creases of the corrugated cardboard with surface sizing (coating) obtained in Example 1 are as follows: Figure 2A and Figure 2B As shown, the crack morphology on it is as follows Figure 3A and Figure 3B As shown.

[0066] The test results of the final embodiment and the comparative example are summarized in Table 1 below.

[0067] Table 1

[0068] Flexural strength test for the first appearance of a crack Affecting the number of folds used The length of the first crack Example 1 15400 times 20200 times 6 mm Example 2 5860 times 5860 times 58 mm Comparative Example 1 3033 times 6020 times 5.6 mm Comparative Example 2 4372 times 7690 times 13.3 mm Comparative Example 3 12366 times 13833 times 11.2 mm Comparative Example 4 2345 times 4703 times 13 mm Comparative Example 5 325 times 325 times 55 mm Comparative Example 6 96 times 96 times 66 mm Comparative Example 7 10 times 10 times 71 mm

[0069] As can be clearly seen from the data in Table 1, the technical solution proposed in this invention can significantly improve the folding endurance of corrugated cardboard compared to the untreated blank sample (Comparative Example 7) and the sizing agent using only modified polyvinyl alcohol (Comparative Example 1).

[0070] More notably, a deeper level of inventiveness of this invention can be revealed through in-depth comparative analysis of Examples 1 and 2, as well as related comparative examples. The inventors recognize that nanocellulose, due to its high surface energy, inherently presents a technical challenge of agglomeration within the system, which severely hinders its reinforcing effect. While the preparation process used in Example 2 also achieved good results, its mixing method was insufficient to completely overcome the agglomeration problem of nanocellulose, resulting in its synergistic effect with modified polyvinyl alcohol not reaching the ideal state.

[0071] Therefore, the optimized preparation process represented by Example 1 of this invention is proposed. This process, by introducing key steps such as ultrasonic dispersion, effectively breaks down the agglomerates of nanocellulose, ensuring that the modified nanocellulose can be uniformly and efficiently dispersed in the matrix at the nanoscale. It is under these optimized process conditions that the technical effect of the determined 1:5 mass ratio of the second silane coupling agent to nanocellulose is fully realized, allowing the two modified components to fully interact and form a structurally complete and defect-free cross-linked network, ultimately achieving a significant technological leap in flexural endurance from 5860 cycles (Example 2) to 20200 cycles (Example 1).

[0072] This fully demonstrates that the present invention not only provides an optimized component formulation, but also a preparation method that can fully realize the potential of this formulation. It is this synergistic matching of formulation and process that constitutes the core of the present invention, producing unexpected technical effects and thus perfectly solving the problem of insufficient folding endurance of corrugated cardboard in the prior art in a highly creative way.

Claims

1. A surface sizing agent, characterized in that, Comprising: a first modified component obtained by reacting polyvinyl alcohol with a first silane coupling agent; and a second modified component obtained by reacting nanocellulose with a second silane coupling agent; wherein the first silane coupling agent is gamma-glycidoxypropyltrimethoxysilane, and the second silane coupling agent is 3-aminopropyltriethoxysilane; the mass ratio of the second silane coupling agent to the nanocellulose is 1:5; the second modified component is obtained by dispersing the nanocellulose in water by ultrasonic treatment to form a 1wt% suspension, and then reacting with the second silane coupling agent.

2. The surface size according to claim 1, wherein the mass ratio of the first silane coupling agent to the polyvinyl alcohol used to prepare the first modified component is 1:

6.

3. A process for the preparation of a surface size as claimed in claim 1 or 2, characterized in that Comprising the following steps: S1: preparing a first modified component, dissolving polyvinyl alcohol in deionized water, adding the first silane coupling agent dropwise, and reacting under alkaline conditions; S2: preparing a second modified component, dispersing nanocellulose in water by ultrasonic treatment to form a 1wt% nanocellulose solution, dissolving the second silane coupling agent in an ethanol-water mixed solvent, reacting the two under acidic conditions, obtaining the product, and then precipitating by adding an organic solvent, centrifuging and drying to obtain the solid second modified component; S3: dispersing the solid second modified component obtained in step S2 in water, mixing with the reaction product containing the first modified component obtained in step S1, and stirring under alkaline conditions; S4: mixing the intermediate product obtained in step S3, a polyvinyl alcohol aqueous solution, and glycerol to obtain the surface sizing agent.

4. The preparation method according to claim 3, characterized in that, In step S1, the polyvinyl alcohol is dissolved at 90°C for 2 hours, then cooled to 60°C; the reaction is adjusted to a pH of 7-8 by adding a 10% sodium hydroxide solution, and stirred for 2 hours.

5. The preparation method according to claim 3, characterized in that, In step S2, the reaction is carried out at 70°C for 3 hours, during which the pH of the reaction system is stabilized at 4-5 by adding glacial acetic acid.

6. The production method according to claim 3 or 5, characterized by, The product is precipitated in acetone, with a volume ratio of acetone to product of 5:1, and the precipitate is subjected to 3 cycles of redispersion-centrifugation, and then vacuum dried at 60°C for 6 hours.

7. The preparation method according to claim 3, characterized in that, In step S3, the solid second modified component is configured as a 3wt% aqueous solution, mixed with the product of step S1, the pH is adjusted to 7-8 by adding a 10% sodium hydroxide solution, and mechanically stirred for 2 hours.

8. A corrugated paperboard, characterized by The corrugated paperboard is coated with the surface sizing agent of claim 1 or 2 at the crease.

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