A method for regulating the strength of a wrapping paper

Abstract

This invention provides a method for controlling the strength of packaging paper, belonging to the field of papermaking technology. Addressing the problem that existing methods for uniformly adding cellulose nanofibers (CNFs) cannot simultaneously achieve surface density and core rigidity, this invention regulates the distribution morphology of cationic CNFs by constructing a charge density gradient in the pulp: the Zeta potential of the surface pulp is controlled between -28 mV and -40 mV, inducing uniform CNF dispersion to form a dense network and improve surface strength; the Zeta potential of the core pulp is controlled between -8 mV and -15 mV, inducing CNF micro-flocculation to form a skeletal structure and improve ring crush strength; this is further enhanced by online monitoring and feedback adjustment of the Zeta potential. This method achieves a synergistic improvement in the overall mechanical properties of packaging paper without increasing the total amount of chemicals used, and is suitable for the preparation of high-strength packaging paper.

Description

Technical Field

[0001] This invention belongs to the field of papermaking technology, specifically relating to a method for improving the overall strength of packaging paper by regulating the distribution morphology of cationic cellulose nanofibers using charge density gradient. Background Technology

[0002] Packaging paper is a key surface layer material for transport packaging such as corrugated boxes. In logistics, warehousing, and e-commerce express delivery, it plays a vital role in withstanding stacking pressure, protecting contents, preventing damage, and adapting to complex transportation environments. Its comprehensive mechanical properties, such as ring crush strength, bursting strength, internal bond strength, and flexural stiffness, directly affect the box's resistance to collapse, impact, and puncture during loading, handling, and long-term stacking. Therefore, obtaining packaging paper with controllable strength and stable performance while maintaining or reducing basis weight is one of the core issues that the packaging paper industry continues to focus on.

[0003] Cellulose nanofibers (CNFs) are considered highly promising paper strengthening agents due to their high aspect ratio and high strength. However, current CNF application technologies mostly focus on "homogeneous addition," that is, indiscriminately mixing CNFs into all pulp layers, or simply interlayer spraying. This approach ignores the morphological differences of CNFs under different microenvironments. In fact, cationic CNFs tend to spread and adsorb in a strong anionic environment, forming a dense film; while in a weak anionic environment, they tend to agglomerate or form bridging flocs. Existing technologies fail to utilize this characteristic, resulting in excessive CNF flocculation affecting uniformity. On the other hand, excessive dispersion leads to low retention and the inability to form an effective compressive strength skeleton, making it difficult to achieve a perfect balance between the surface density of packaging paper and the rigid support of the core layer.

[0004] Against the backdrop of "dual carbon" (carbon dioxide, carbon dioxide, and carbon emissions) and the increasing demand for lightweight packaging, the shortcomings of existing technologies in terms of cost control, performance designability, and environmental friendliness are becoming increasingly apparent. On the one hand, simply relying on increasing basis weight or strengthening pulping is no longer sufficient to achieve both high strength and lightweight. On the other hand, over-reliance on traditional chemical reinforcing agents is detrimental to green production processes and the stability of waste paper recycling systems. Therefore, there is an urgent need for an innovative method for controlling the strength of packaging paper that not only focuses on the amount of cationic CNF itself added, but also on the coupling relationship between the charge environment of different layers of the paper web and the CNF structure, to achieve a refined design of "strength balance." Summary of the Invention

[0005] The purpose of this invention is to provide a method for controlling the strength of packaging paper, aiming to solve the problems of low CNF strengthening efficiency and the inability to simultaneously achieve surface density and high core stiffness in existing technologies. A smart flocculation and distribution control strategy based on "charge density gradient" is proposed. Its core idea is not simply to control the amount of CNF added, but to actively regulate the anionic charge density environment of different pulp layers in the paper sheet, inducing cationic CNF to produce differentiated strengthening mechanisms.

[0006] The technical problem to be solved by this invention is achieved through the following technical solution:

[0007] A method for controlling the strength of packaging paper includes the following steps:

[0008] (1) Adjust the anionic charge density of the surface slurry and the core slurry respectively: Adjust the pulping process of the surface slurry and / or add anionic additives to adjust the surface slurry to have a higher anionic charge density. Adjust the pulping process of the core slurry and / or screen and clean to remove some soluble anionic substances, or add cationic fixatives, or reduce the degree of bleaching to retain lignin, so as to control the core slurry to have a lower anionic charge density, so that the absolute value of the Zeta potential of the surface slurry is higher than that of the core slurry.

[0009] (2) Add cationic cellulose nanofiber dispersion to the surface slurry and core slurry after step (1) respectively; by utilizing the difference in charge density between the surface slurry and the core slurry, the cationic cellulose nanofibers form a uniformly dispersed dense nano network in the surface slurry and a micro-flocculated skeleton structure in the core slurry.

[0010] (3) Monitor the Zeta potential of each layer of pulp after mixing in the papermaking flow system online, and adjust the amount of cationic cellulose nanofibers or charge regulators added by feedback to maintain the preset charge density gradient.

[0011] (4) High-strength packaging paper is produced by forming, pressing, drying and calendering through a multi-layer flow box.

[0012] Furthermore, the zeta potential of the surface slurry is controlled between -28 mV and -40 mV, and the zeta potential of the core slurry is controlled between -8 mV and -15 mV.

[0013] Furthermore, the surface slurry undergoes a higher degree of beating to expose more fiber surface hydroxyl and carboxyl groups, with the beating degree of the surface slurry controlled at 38°SR-50°SR; the core slurry undergoes light beating, with the beating degree of the core slurry controlled at 20°SR-25°SR.

[0014] Furthermore, the anionic additive is anionic polyacrylamide or anionic starch.

[0015] Furthermore, the cationic cellulose nanofibers have a charge density of 3.0-4.0 mmol / g, and are added to the surface slurry at a rate of 0.8%-1.5% of the oven-dry slurry mass, and to the core slurry at a rate of 0.5%-1.5% of the oven-dry slurry mass.

[0016] Furthermore, the feedback adjustment mechanism is set as follows: when the absolute value of the Zeta potential of the surface slurry decreases by more than 10% of the preset threshold, the addition rate of cationic cellulose nanofibers is reduced; when the absolute value of the Zeta potential of the core slurry increases by more than 10% of the preset threshold, the addition rate of cationic cellulose nanofibers is increased.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] This invention constructs a charge density gradient in the pulp, inducing cationic cellulose nanofibers to uniformly disperse and form a dense network on the surface and micro-flocculate and form a skeletal structure in the core layer, thereby achieving a synergistic improvement and on-demand distribution of the comprehensive mechanical properties of packaging paper.

[0019] (1) The surface properties are significantly improved. The highly dispersed CNF network on the surface significantly improves the smoothness, surface strength and burst strength of the paper, and reduces the phenomenon of printing lint and dust.

[0020] (2) The stiffness and compressive strength are enhanced at the same time. The micro-floc CNF skeleton of the core layer is compacted during the pressing and drying process but retains a moderate microporous structure. This structure is similar to the aggregate in concrete, which significantly improves the stiffness and ring crush strength of the paper.

[0021] (3) The overall performance is optimized. With the total amount of CNF added remaining unchanged, compared with uniform addition, the charge gradient control of the present invention significantly improves the strength balance of the paper and realizes the on-demand allocation of material properties. Detailed Implementation

[0022] The present invention will be further described below with reference to the embodiments.

[0023] Example 1: Standard Gradient Reinforced Packaging Paper

[0024] This embodiment prepares a standard gradient reinforced packaging paper that balances surface strength and ring crush strength.

[0025] 1. Slurry preparation and charge gradient construction

[0026] The surface pulp is made of bleached softwood pulp, and its freeness is controlled at 45°SR through a pulping process to fully expose the hydroxyl and carboxyl groups on the fiber surface. Anionic polyacrylamide (0.03% by weight of oven-dried pulp) is added to the pulp after pulping to stabilize the zeta potential of the surface pulp at a high negative charge level of -32 mV.

[0027] The core layer pulp uses AOCC (American waste corrugated cardboard box) pulp, with the beating degree controlled at 22°SR. After screening and appropriate washing to remove some dissolved anionic substances, its zeta potential is maintained at a low negative charge state of -12 mV.

[0028] 2. Preparation and Differentiated Addition of Cationic CNFs

[0029] A cationic CNF dispersion with a charge density of 3.5 mmol / g was prepared, and a differentiated addition strategy was implemented: 1.0% cationic CNF by weight of oven-dried slurry was added to the surface slurry, and 1.2% cationic CNF by weight of oven-dried slurry was added to the core slurry.

[0030] 3. Online monitoring and conveying forming

[0031] Online zeta potential monitoring systems (BTG zeta potential meters) were installed in the surface slurry and core slurry delivery pipelines, respectively, and data was collected every 30 seconds. When the absolute value of the surface slurry zeta potential dropped to 28 mV (a decrease of 12.5%, exceeding the 10% threshold), the system automatically reduced the speed of the cation CNF addition pump by 10%; when the absolute value of the core slurry zeta potential rose to 13 mV (an increase of 8.3%, not exceeding the threshold), the current addition rate was maintained.

[0032] 4. Three-layer wire forming device: The pulp is formed, pressed, and dried in three layers to obtain the finished product. During this process, due to the high-intensity anionic environment of the surface pulp, cationic CNFs are rapidly adsorbed and evenly spread in the form of single fibers or extremely fine bundles, forming a dense surface film structure. Meanwhile, the weaker anionic environment of the core layer promotes the formation of obvious micro-flocs between the cationic CNFs, constructing a "bridging" framework. Test results show that compared to the conventional process without gradient charge control, the burst strength index of the resulting packaging paper is increased by 25%, and the ring crush index is increased by 18%, achieving dual optimization of surface density and core layer support.

[0033] Example 2: High-rigidity heavy-duty packaging paper

[0034] This embodiment aims to prepare a high-rigidity packaging paper suitable for heavy-duty packaging.

[0035] The surface layer material is unbleached wood pulp, and the core layer material is OCC (waste corrugated cardboard) pulp. During the charge gradient construction stage, the surface pulp is pulped to 38°SR and the Zeta potential is adjusted to -28 mV to ensure the interfiber bonding force on the surface.

[0036] For the core layer slurry, the freeness is controlled at 20°SR, and 0.02% of polydimethyldiallylammonium chloride by weight of oven-dry slurry is added as a cationic stabilizer to neutralize some of the dissolved anionic charge, thereby precisely reducing the core layer zeta potential to -8 mV (extremely low negative charge state).

[0037] High-charge cationic CNF with a charge density of 4.0 mmol / g was selected for reinforcement (the preparation process was the same as in Example 1, but the degree of quaternary ammonium salt modification was increased). The addition amount in the surface slurry was set to 0.8%, while the addition amount in the core slurry was increased to 1.5%. The core mechanism of this example is that the extremely low core charge environment weakens the rapid adsorption capacity of the fibers to CNF, inducing the cationic CNF to form a large-sized flocculent skeleton in the interfiber spaces of the core layer. This structure is similar to coarse aggregate in concrete during the papermaking process, greatly enhancing the rigid support in the thickness direction of the paper. The final paper test showed that, under the same basis weight, the paper bulk increased by 5%, and the ring crush strength increased significantly by 30%, exhibiting excellent compression and stacking performance, making it suitable as the face paper material for heavy-duty packaging cartons.

[0038] Example 3: High surface strength printing grade packaging paper

[0039] This embodiment focuses on the preparation of printing-grade packaging paper with high surface strength.

[0040] All slurry layers use bleached sulfate slurry, but undergo strict layering and differentiation treatment. The surface slurry is intensified and beaten to 50°SR, and anionic starch is added as a charge enhancer to create an ultra-high negative charge environment with a zeta potential as high as -40 mV. The core slurry is beaten to 25°SR, and the zeta potential is controlled at -15 mV.

[0041] In terms of enhancement strategy, the focus is on strengthening the surface layer, with 1.5% cationic CNF (charge density 3.0 mmol / g) added to the surface layer and only 0.5% added to the core layer. In the highly negatively charged surface environment, the high concentration of cationic CNF is strongly attracted by electrostatics and rapidly adsorbs onto the fiber surface and between fine fibers in a nanoscale dispersion state, filling microscopic pores and forming an ultra-smooth nanocomposite surface layer; the small amount of CNF in the core layer mainly maintains basic interlayer bonding. The final product test results show that the packaging paper has excellent surface smoothness, and the printing surface strength (linting speed) is improved by 40% compared with conventional processes, effectively solving the problem of linting and powdering during high-speed printing, while maintaining good overall stiffness, meeting the needs of high-end fine printing packaging.

Claims

1. A method for controlling the strength of packaging paper, characterized in that, Includes the following steps: (1) Adjust the anionic charge density of the surface slurry and the core slurry respectively: Adjust the pulping process of the surface slurry and / or add anionic additives to adjust the surface slurry to have a higher anionic charge density. Adjust the pulping process of the core slurry and / or screen and clean to remove some soluble anionic substances, or add cationic fixatives, or reduce the degree of bleaching to retain lignin, so as to control the core slurry to have a lower anionic charge density, so that the absolute value of the Zeta potential of the surface slurry is higher than that of the core slurry. (2) Add cationic cellulose nanofiber dispersion to the surface slurry and core slurry after step (1); By utilizing the difference in charge density between the surface slurry and the core slurry, cationic cellulose nanofibers form a uniformly dispersed dense nanonetwork in the surface slurry and a micro-flocculent skeletal structure in the core slurry. (3) Monitor the Zeta potential of each layer of pulp after mixing in the papermaking flow system online, and adjust the amount of cationic cellulose nanofibers or charge regulators added by feedback to maintain the preset charge density gradient. (4) High-strength packaging paper is produced by forming, pressing, drying and calendering through a multi-layer flow box.

2. The method according to claim 1, characterized in that, The zeta potential of the surface slurry is controlled between -28 mV and -40 mV, and the zeta potential of the core slurry is controlled between -8 mV and -15 mV.

3. The method according to claim 1, characterized in that, The surface slurry is subjected to a high degree of beating to expose more fiber surface hydroxyl and carboxyl groups, with the beating degree of the surface slurry controlled at 38°SR-50°SR; the core slurry is lightly beating, with the beating degree of the core slurry controlled at 20°SR-25°SR.

4. The method according to claim 1, characterized in that, The anionic additive is anionic polyacrylamide or anionic starch.

5. The method according to claim 1, characterized in that, The cationic cellulose nanofibers have a charge density of 3.0-4.0 mmol / g, and are added to the surface slurry at a rate of 0.8%-1.5% of the oven-dry slurry mass, and to the core slurry at a rate of 0.5%-1.5% of the oven-dry slurry mass.

6. The method according to claim 1, characterized in that, The feedback adjustment mechanism is set as follows: when the absolute value of the Zeta potential of the surface slurry decreases by more than 10% of the preset threshold, the addition rate of cationic cellulose nanofibers is reduced; when the absolute value of the Zeta potential of the core slurry increases by more than 10% of the preset threshold, the addition rate of cationic cellulose nanofibers is increased.