Process method for improving surface strength of uncoated paper cup paper

Abstract

The invention belongs to the technical field of papermaking, and particularly relates to a process method for improving the surface strength of uncoated cup paper. The process method for improving the surface strength of the uncoated paper cup paper comprises the following steps: respectively preparing surface layer slurry, core layer slurry and bottom layer slurry; wherein the surface layer pulp comprises bleached softwood pulp and bleached hardwood pulp, the core layer pulp comprises bleached softwood pulp, bleached hardwood pulp, bleached chemical thermo-mechanical pulp and broke pulp, and the bottom layer pulp comprises bleached softwood pulp and bleached hardwood pulp; respectively conveying the surface layer pulp, the core layer pulp and the bottom layer pulp to a surface net, a core net and a bottom net of a paper machine to carry out three-layer papermaking molding, and carrying out squeezing and pre-drying; and carrying out surface sizing on the paper sheet, and carrying out post-drying and calendaring treatment on the sized paper sheet to obtain the uncoated cup paper. According to the process method for improving the surface strength of the non-coated paper cup paper, the prepared non-coated paper cup paper is high in surface strength, and the process requirements of follow-up film spraying, printing and the like can be met.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of papermaking, and particularly relates to a process for improving the surface strength of uncoated paper cup paper. BACKGROUND

[0002] Paper cups, as widely used disposable food contact containers, the performance of the paper cup base paper directly determines the quality, safety and production cost of the final product. An ideal paper cup base paper must have excellent surface strength to withstand the subsequent key "film coating-printing" processing chain test, while having good stiffness, safety and cost-effectiveness. Film coating (usually PE film or degradable PLA corn starch film) and color printing are the core processes of paper cup production, both of which require high surface integrity of the base paper. Base paper with insufficient surface strength (usually represented by a wax rod value <14A) will cause uneven film adhesion and easy blistering during film coating; during high-speed printing, it will cause serious fluffing and powdering, resulting in blurred printing patterns and uneven ink, which seriously affects product qualification rate and production efficiency.

[0003] To address this industry-wide challenge, existing technologies mainly improve paper strength from two dimensions: pulp addition and surface treatment. However, these solutions have significant limitations when applied to cost-effective uncoated paper cup paper.

[0004] Adding reinforcing agents to the pulp is a common method to improve the overall strength of paper. For example, some technologies use a specific ratio of bleached coniferous wood and broadleaf wood pulp, and add cationic starch, wet strength agent, etc., aiming to improve physical indicators by optimizing fiber bonding. Some other solutions introduce special fillers, such as using polymer modifiers (such as KH550 silane coupling agent, polyamide epoxy chloropropane resin) to coat the surface of silicate glass powder, and then adding it as a functional filler to the pulp to improve the mechanical properties of the paper. However, the use of large amounts of chemical wood pulp or special fillers will significantly increase the cost of raw materials, and the complex pretreatment (such as high-temperature melting, fluidized bed modification) process is complicated, increasing production energy consumption and complexity. In addition, it is difficult to uniformly distribute the reinforcing agent in the pulp, and the targeted improvement of surface strength is limited. To achieve the target, excessive addition is often required, resulting in waste.

[0005] Targeted improvement of paper surface properties through surface sizing or coating is a more direct technical approach. Existing research discloses various surface sizing formulations, such as composite coatings containing cationic polymers and starch, utilizing the electrostatic adsorption between cationic polymers and negatively charged fibers to form a reinforcing layer on the paper surface. Other methods optimize the performance of baked base paper by precisely controlling the temperature and pressure parameters of multi-stage drying and calendering, combined with impregnation-based surface sizing. However, these surface treatment technologies primarily focus on improving the paper's dry strength, water resistance, or smoothness, lacking targeted design for the specific surface properties required for subsequent lamination processes of uncoated cup paper (such as high adhesion strength to PE or PLA films). More importantly, to achieve specific functionalities (such as high oil resistance), some technologies still require secondary coating or lamination treatment on the surface, essentially turning the paper into "coated paper," increasing processes and costs. Furthermore, the application of new environmentally friendly lamination materials (such as PLA corn starch films) places different compatibility and adhesion requirements on the base paper surface compared to traditional PE lamination. However, the high cost of relying on imported core raw materials such as PLA particles has forced the industry to seek more cost-effective solutions. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the above-mentioned defects of the prior art and provide a process method to improve the surface strength of uncoated paper cups. The uncoated paper cups prepared have high surface strength and can meet the requirements of subsequent coating, printing and other processes.

[0007] The process for improving the surface strength of uncoated paper cups includes the following steps: Pulp preparation: Prepare surface pulp, core pulp and bottom pulp separately; wherein, surface pulp includes bleached softwood pulp and bleached hardwood pulp, core pulp includes bleached softwood pulp, bleached hardwood pulp, bleached chemithermomechanical pulp and waste pulp, and bottom pulp includes bleached softwood pulp and bleached hardwood pulp. Pulp formulation and forming: The surface slurry, core slurry and bottom slurry are respectively fed to the face wire, core wire and bottom wire of the paper machine for three-layer forming, and then subjected to pressing and pre-drying treatment; Surface sizing: The pre-dried paper is sizing. The starch solution used for sizing has a solid content of 26-30 wt% and a viscosity of 190-230 cps. The sizing amount on both sides is 1-2 g / m². 2 ; Post-processing: The sized paper sheets are then dried and calendered to obtain the uncoated cup paper.

[0008] The freeness of the bleached softwood pulp (NBKP), bleached hardwood pulp (LBKP), and bleached chemithermomechanical pulp (BCTMP) is 350~410mL; the freeness of the broke pulp is 180~220mL.

[0009] The surface layer pulp composition is 30-45% bleached softwood pulp (NBKP) and 55-70% bleached hardwood pulp (LBKP); the core layer pulp composition is 10-20% bleached softwood pulp, 10-20% bleached hardwood pulp, 45-55% bleached chemithermomechanical pulp (BCTMP), and 10-35% broke pulp; the bottom layer pulp composition is 55-70% bleached softwood pulp and 30-45% bleached hardwood pulp. Preferably, the surface layer pulp composition is 40% bleached softwood pulp and 60% bleached hardwood pulp; the core layer pulp composition is 15% bleached softwood pulp, 15% bleached hardwood pulp, 50% bleached chemithermomechanical pulp, and 20% broke pulp; and the bottom layer pulp composition is 65% bleached softwood pulp and 35% bleached hardwood pulp.

[0010] The pressing process employs a three-stage pressing method, with the arrangement being large roller pressing, shoe pressing, and smooth pressing. The dryness of the pressing section is 44~46wt%, preferably 45wt%.

[0011] The surface layer slurry, core layer slurry, and bottom layer slurry are diluted, purified, and screened before being fed into the grid.

[0012] Retention aids, bactericides, and other auxiliary agents are added to the surface slurry, core slurry, and bottom slurry.

[0013] The dryness after pre-drying treatment is 89~91wt%, preferably 90wt%.

[0014] The surface sizing solution used in surface sizing is prepared as follows: First, a starch solution is prepared with a solid content of 26-30 wt% and a viscosity of 190-230 cps. After preparation, the starch solution is stored in a starch storage tank and then transported to the tank before the sizing machine. Water is added and diluted according to the sizing process, along with bactericides and defoamers, to prepare the surface sizing solution for paper sizing. The solid content of the surface sizing solution is 13-17 wt%, and the application rate is 1-2 g / m³. 2 Viscosity 25~35cps.

[0015] First, the preparation of the starch solution for applying the sizing agent: The dryness of the paper sample after the post-drying treatment is 96~98wt%, preferably 97wt%.

[0016] Calendering is performed using a hard calender, with a pressing pressure of 20 kPa. Hard calendering occurs after the drying section and its purpose is to initially control the smoothness and thickness uniformity of the paper. The calender consists of two rollers: one is a hard, heated roller, and the other is also a hard roller. The paper thickness is controlled by adjusting the pressing pressure.

[0017] Specifically, the process for improving the surface strength of uncoated paper cups includes the following steps: 1. Pulp preparation: Four types of pulp, namely BCTMP, LBKP, NBKP and BROKE, are prepared with a freeness of 380±30mL. The high freeness of the pulp ensures the effective length of the fibers and helps to improve the surface strength of the paper.

[0018] Paper cups are manufactured using a three-layer papermaking process, therefore the pulp ratio is determined based on the properties of the surface, core, and bottom layers. The pulp ratio is as follows: Surface layer pulp: 40% NBKP, 60% LBKP, utilizing the high strength of softwood pulp and the fineness of hardwood pulp to balance surface strength and printability; Core layer pulp: 15% NBKP, 15% LBKP, 50% BCTMP, 20% BROKE, ensuring structural strength while reducing costs; Bottom layer pulp: 65% NBKP, 35% LBKP, ensuring mechanical strength and supporting the overall structure. This optimized pulp ratio imparts higher surface strength to the paper during the mixing process in the mixing tank.

[0019] 2. Flow System: Retention aids, bactericides, and other additives are added to each of the three pulp layers. The prepared pulp for each layer is diluted, purified, and screened to remove impurities, ensuring a clean and consistent pulp on the wire. The flow system delivers the uniformly dispersed fiber pulp into the headbox and sprays it onto the wire for forming. After verifying that all pulp parameters meet process requirements, the flow system feeds the pulp into the wire section headbox for paper sheet formation. The process requirements for various parameters of the slurry (concentration, retention rate, pH): Surface layer mixed slurry: concentration 0.3~0.5wt%, retention rate 85~95wt%, pH 4.5~5.5; Core layer mixed slurry: concentration 1.0~1.6wt%, retention rate 85~95wt%, pH 4.8~5.8; Bottom layer mixed slurry: concentration 0.2~0.4wt%, retention rate 85~95wt%, pH 4.5~5.5.

[0020] 3. Net Section: The slurry is formed on the net. The core layer is equipped with a top net forming device, which accelerates the dehydration of the core layer and shapes it. An air-cushion headbox (the core layer is a headbox with diluted water) is used to ensure stable and consistent slurry on the net. After the three layers of slurry are formed, a layer is synthesized by spraying starch (the spray liquid is 0.1wt% ultra-diluted starch liquid). Then, according to the process standards, the machine speed, pressure and other indicators are controlled for dehydration and pressing (net speed 600m / min, slurry-to-net speed ratio 1.0, opening of the three lip plates of the face, core and bottom: 12%, 20%, 11%) to remove moisture.

[0021] 4. Press Section: The wet paper undergoes pressing, employing a three-stage pressing process. The arrangement includes large-roll press, shoe press, and calendering. The advanced shoe press, with its wider pressing zone (0.3m) and higher linear pressure (52kPa for the first press, 800kPa for the second, and 30kPa for the third), removes more moisture from the paper, reducing the loss of bulk. Calendering primarily finishes the surface of the paper without dehydration. The dryness of the paper exiting the press section is 43-45 wt%.

[0022] 5. Pre-drying: After the press section, the moisture of the wet paper continues to be removed. Steam is introduced into the drying cylinder. The steam pressure of the pre-drying is 67 kPa and the steam pressure of the post-drying is 70 kPa. The steam ratio of the pre-drying is 0 and the steam ratio of the post-drying is 10. The wet paper is heated by the drying cylinder wall to dry it. The paper undergoes a large amount of dehydration in the drying cylinder under the action of heat radiation and pressure. The dryness of the paper exiting the drying section is 89~91%.

[0023] 6. Sizing: At the end of the front drying section of the paper machine, a layer of sizing liquid is sprayed through the sizing roller (sizing amount set: 1.5±0.5g / m). 2 The amount of adhesive applied is controlled by adjusting the adhesive metering rod.

[0024] (1) Preparation of sizing solution: The surface sizing solution is prepared from cooked starch solution. When preparing the cooked starch solution, the solid content of the starch solution is reduced from 30 wt% to 28 wt%. At the same time, the amount of amylase is controlled, reduced from 3 wt% to 1 wt%, and the viscosity of the starch solution is controlled within the standard range, increasing from 110 cps to 210 cps. Then the starch solution is diluted according to the solid content of the surface sizing solution, with a surface sizing solid content of 15 wt%, and the sizing viscosity of the sizing starch is increased from 20 cps to 30 cps. This "low solids, high viscosity" strategy optimizes the penetration and film formation balance of starch molecules on the fiber surface, enhances the binding force between starch and fiber, reduces porosity, and thus improves the surface strength of the paper.

[0025] After the starch solution is prepared according to the new process specifications, a sample is taken to test whether its properties meet the requirements. If there are no errors, it is quantitatively diluted according to the paper weight of the paper cup and the process requirements. At the same time, bactericides, defoamers and other auxiliary agents are added to prepare a surface sizing solution for sizing.

[0026] 7. Post-drying: After sizing, further dewatering is performed, and the paper sample dryness is 96~98wt%.

[0027] 8. Calendering: Hard calendering is performed after the drying section. The calender consists of two rollers, one is a hard heated roller, and the other is also a hard roller. The paper thickness is controlled by adjusting the pressure of the pressing zone. The calendering process parameter is 20 kPa.

[0028] 9. Rewinding: A paper roll is used to change rolls according to the required length; the function of the rewinder is to cut and process the original paper rolls according to the specifications required by the customer (such as diameter, length, and width).

[0029] Compared with the prior art, the beneficial effects of the present invention are: (1) This invention significantly improves the surface strength and overall performance of uncoated cup paper by synergistically optimizing surface sizing, pulp ratio, and pulp preparation processes. Specifically, a surface sizing process with "low solids content and high viscosity" is adopted to optimize the film-forming and permeability of starch molecules on the fiber surface, thereby enhancing the bonding force. Differentiated pulp ratios are designed for the surface layer, core layer, and bottom layer. High-strength pulp is used on the surface to ensure performance, while chemimechanical pulp and waste pulp are scientifically incorporated into the core layer to maintain the skeleton. At the same time, the freeness of each pulp is uniformly increased and controlled within a high range, effectively preserving the fiber length. These measures work synergistically to make the surface structure of the paper dense and the fibers firmly bonded, fundamentally solving the problems of lint and powder shedding, and ensuring that the subsequent coating adhesion is firm and the printed pattern is clear and uniform.

[0030] (2) This invention effectively controls production costs and enhances the market competitiveness of the product while improving performance. Through innovative design of pulp ratio, while ensuring the key performance of the surface and bottom layers, the proportion of lower-cost chemimechanical pulp and recycled waste pulp in the core layer is significantly increased, significantly reducing the amount of expensive bleached chemical wood pulp and optimizing raw material costs. The produced uncoated paper cups meet the requirements of high-end lamination and printing processing while maintaining a cost advantage.

[0031] (3) The present invention adopts a high pulp freeness process, which reduces excessive cutting and wear of fibers, thereby reducing energy consumption in the pulping process and wear on equipment. At the same time, the entire process does not rely on complex chemical additives or additional coating steps, and achieves the goal by optimizing the inherent parameters of the existing papermaking process, resulting in a simple process. Detailed Implementation

[0032] The present invention will be further described below with reference to specific embodiments.

[0033] The bleached softwood pulp (NBKP), bleached hardwood pulp (LBKP), and bleached chemithermomechanical pulp (BCTMP) of this invention are all purchased pulp boards, and their freeness after refining is 360~400mL; the broke pulp is obtained by refining recycled paper, and its freeness is 180~220mL.

[0034] Example 1 The process for improving the surface strength of uncoated paper cups includes the following steps: 1. Preparation of pulp: Four types of pulp, namely BCTMP, LBKP, NBKP and BROKE, were prepared with freeness of 371 mL, 384 mL, 379 mL and 213 mL, respectively. Paper cups are made using a three-layer papermaking process, therefore the pulp ratio is determined based on the properties of the surface, core, and bottom layers. Pulp ratios are as follows: Surface layer pulp: 45% NBKP, 55% LBKP; Core layer pulp: 20% NBKP, 10% LBKP, 55% BCTMP, 15% BROKE; Bottom layer pulp: 70% NBKP, 30% LBKP.

[0035] 2. Heading System: Each of the three slurry layers contains retention aids PAM 0.6wt%, PAE 0.45wt%, and cationic starch 0.12wt% (% are calculated relative to the total mass of the slurry). The prepared slurries for each layer are diluted, purified, and screened to remove impurities, ensuring a clean and consistent slurry on the wire. The heading system delivers the uniformly dispersed fiber slurry into the headbox and sprays it onto the wire for forming.

[0036] The process requirements for various parameters of the slurry (concentration, retention rate, pH): Surface layer mixed slurry: concentration 0.5wt%, retention rate 90wt%, pH 5.0; Core layer mixed slurry: concentration 1.2wt%, retention rate 88wt%, pH 5.5; Bottom layer mixed slurry: concentration 0.3wt%, retention rate 90wt%, pH 5.0.

[0037] 3. Net Section: The slurry is formed on the net. The core layer is equipped with a top net forming device, and an air-cushion headbox (the core layer is a headbox for diluted water) is used to ensure stable and consistent slurry flow on the net. After the three layers of slurry are formed, they are combined into one layer by spraying starch (the spray liquid is 0.12wt% ultra-diluted starch liquid). The net speed is controlled at 600m / min, the slurry-to-net speed ratio is 1.0, and the opening of the three lip plates (face, core, and bottom) is 12%, 20%, and 11%, respectively. After dehydration and pressing, 22% of the moisture is removed before leaving the net section.

[0038] 4. Press Section: The wet paper is pressed using a three-stage pressing process, arranged as a large roller press, a shoe press, and a light press. The advanced shoe press, with its wider pressing zone (0.3m) and higher linear pressure (52kPa for the first press, 800kPa for the second press, and 30kPa for the third press), removes more moisture from the paper and reduces the loss of bulk. The light press mainly finishes the surface of the paper without dewatering. The dryness of the paper exiting the press section is 45wt%.

[0039] 5. Pre-drying: After the press section, the moisture of the wet paper continues to be removed. Steam is introduced into the drying cylinder. The steam pressure of the pre-drying is 67 kPa and the steam pressure of the post-drying is 70 kPa. The steam ratio of the pre-drying is 0 and the steam ratio of the post-drying is 10. The wet paper is heated by the drying cylinder wall to dry it. The paper undergoes a large amount of dehydration in the drying cylinder under the action of heat radiation and pressure. The dryness of the paper exiting the drying section is 90%.

[0040] 6. Sizing: At the end of the front drying section of the paper machine, a layer of sizing liquid is sprayed through the sizing roller (sizing amount set: 1.5±0.5g / m). 2 The amount of adhesive applied is controlled by adjusting the adhesive metering rod.

[0041] (1) Preparation of sizing solution: The surface sizing solution used for surface sizing is as follows: First, dissolve starch in water. After the starch solution is prepared, put it into a starch storage tank for storage (the storage data in Table 2 is obtained from this) and transport it to the front tank of the sizing machine. Dilute it with water according to the sizing process, and add 1.2wt% Amazon bactericide 9058 and 0.8wt% defoamer (AmiDefoam5873 produced by Shanghai Emerson Chemical Co., Ltd.) to prepare a surface sizing solution for paper sizing. The front sizing parameters and sizing amount are as shown in the forward reflux parameters in Table 2, and the back sizing parameters and sizing amount are as shown in the reverse reflux parameters in Table 2.

[0042] 7. Post-drying: After sizing, further dehydration is carried out, and the paper sample has a dryness of 97 wt%.

[0043] 8. Calendering: Hard calendering is performed after the drying section. The calender consists of two rollers, one is a hard heated roller, and the other is also a hard roller. The paper thickness is controlled by adjusting the pressure of the pressing zone. The calendering process parameter is 20 kPa.

[0044] 9. Curl and re-curl.

[0045] Example 2 The process for improving the surface strength of uncoated paper cups includes the following steps: 1. Preparation of pulp: Four types of pulp, namely BCTMP, LBKP, NBKP and BROKE, were prepared with freeness of 377mL, 388mL, 382mL and 209mL, respectively. Paper cups are made using a three-layer papermaking process, therefore the pulp ratio is determined based on the properties of the surface, core, and bottom layers. Pulp ratios are as follows: Surface layer pulp: 35% NBKP, 65% LBKP; Core layer pulp: 10% NBKP, 10% LBKP, 45% BCTMP, 35% BROKE; Bottom layer pulp: 60% NBKP, 40% LBKP.

[0046] 2. Flow system: Each of the three slurry layers contains retention aids PAM 0.45wt%, PAE 0.35wt%, and cationic starch 0.08wt% (% are calculated relative to the total slurry mass). The prepared slurries for each layer are diluted, purified, and screened to remove impurities. The flow system then delivers the uniformly dispersed fiber slurry into the headbox and sprays it onto the mesh for forming. The process requirements for the various slurry parameters (concentration, retention rate, pH) are as follows: Surface layer mixed slurry: concentration 0.55wt%, retention rate 93wt%, pH 5.6; Core layer mixed slurry: concentration 1.8wt%, retention rate 92wt%, pH 5.8; Bottom layer mixed slurry: concentration 0.5wt%, retention rate 93wt%, pH 5.6.

[0047] 3. Net Section: The slurry is formed on the net. The core layer is equipped with a top net forming device, and an air-cushion headbox (the core layer is a headbox for diluted water) is used to ensure stable and consistent slurry flow on the net. After the three layers of slurry are formed, they are combined into one layer by spraying starch (the spray liquid is 0.08wt% ultra-diluted starch liquid). Then, the net speed is controlled at 600m / min, the slurry-to-net speed ratio is 1.0, and the opening of the three lip plates (face, core, and bottom) is 12%, 20%, and 11%, respectively, for dehydration and pressing. 22.5% of the moisture is removed before leaving the net section.

[0048] 4. Press Section: The wet paper is pressed using a three-stage pressing process, arranged as a large roller press, a shoe press, and a light press. The advanced shoe press, with its wider pressing zone (0.3m) and higher linear pressure (52kPa for the first press, 800kPa for the second press, and 30kPa for the third press), removes more moisture from the paper and reduces the loss of bulk. The light press mainly finishes the surface of the paper without dewatering. The dryness of the paper exiting the press section is 45wt%.

[0049] 5. Pre-drying: After the press section, the moisture of the wet paper continues to be removed. Steam is introduced into the drying cylinder. The steam pressure of the pre-drying is 67 kPa and the steam pressure of the post-drying is 70 kPa. The steam ratio of the pre-drying is 0 and the steam ratio of the post-drying is 10. The wet paper is heated by the drying cylinder wall to dry it. The paper undergoes a large amount of dehydration in the drying cylinder under the action of heat radiation and pressure. The dryness of the paper exiting the drying section is 89%.

[0050] 6. Sizing: At the end of the front drying section of the paper machine, a layer of sizing liquid is sprayed through the sizing roller (sizing amount set: 1.5±0.5g / m). 2 The amount of adhesive applied is controlled by adjusting the adhesive metering rod.

[0051] The surface sizing solution used for surface sizing is prepared as follows: starch is first dissolved in water, and the starch solution is then stored in a starch storage tank (the storage data in Table 2 is obtained from this tank) and transported to the front tank of the sizing machine. Water is added and diluted according to the sizing process, and 1.0 wt% Amazon bactericide 9058 and 1.0 wt% defoamer (AmiDefoam5873 produced by Shanghai Emerson Chemical Co., Ltd.) are added at the same time to prepare the surface sizing solution for paper sizing. The front sizing parameters and sizing amount are shown in the forward reflux parameters in Table 2, and the back sizing parameters and sizing amount are shown in the reverse reflux parameters in Table 2.

[0052] 7. Post-drying: After sizing, further dehydration is carried out, and the paper sample has a dryness of 97 wt%.

[0053] 8. Calendering: Hard calendering is performed after the drying section. The calender consists of two rollers, one is a hard heated roller, and the other is also a hard roller. The paper thickness is controlled by adjusting the pressure of the pressing zone. The calendering process parameter is 20 kPa.

[0054] 9. Curl and re-curl.

[0055] Example 3 The process for improving the surface strength of uncoated paper cups includes the following steps: 1. Preparation of pulp: Four types of pulp, namely BCTMP, LBKP, NBKP and BROKE, were prepared with freeness of 384 mL, 397 mL, 399 mL and 211 mL, respectively. Paper cups are made using a three-layer papermaking process, therefore the pulp ratio is determined based on the properties of the surface, core, and bottom layers. Pulp ratios are as follows: Surface layer pulp: 30% NBKP, 70% LBKP; Core layer pulp: 20% NBKP, 20% LBKP, 50% BCTMP, 10% BROKE; Bottom layer pulp: 55% NBKP, 45% LBKP.

[0056] 2. Flow system: Each of the three slurry layers contains retention aids PAM 0.52wt%, PAE 0.42wt%, and cationic starch 0.15wt% (% are calculated relative to the total slurry mass). The prepared slurries for each layer are diluted, purified, and screened to remove impurities, ensuring a clean and consistent slurry on the wire. The flow system delivers the uniformly dispersed fiber slurry into the headbox and sprays it onto the wire for forming. The process requirements for the various slurry parameters (concentration, retention rate, pH) are as follows: Surface layer mixed slurry: concentration 0.25wt%, retention rate 89wt%, pH 4.6; Core layer mixed slurry: concentration 1.0 wt%, retention rate 89 wt%, pH 4.9; Bottom layer mixed slurry: concentration 0.2wt%, retention rate 89wt%, pH 4.6.

[0057] 3. Net Section: The slurry is formed on the net. The core layer is equipped with a top net forming device, and a hydraulic headbox is used to ensure more uniform fiber distribution on the net. After the three layers of slurry are formed, a layer is synthesized by spraying starch (the spray liquid is 0.15wt% ultra-diluted starch liquid). Then, according to the process standard, the net speed is controlled at 600m / min, the slurry-to-net speed ratio is 1.0, and the opening of the three lip plates of the face, core, and bottom is 12%, 20%, and 11%, respectively, to dewater and press out moisture. 21% of the moisture is removed before leaving the net section.

[0058] 4. Press Section: The wet paper is pressed using a three-stage pressing process, arranged as a large roller press, a shoe press, and a light press. The advanced shoe press, with its wider pressing zone (0.30m) and higher linear pressure (52kPa for the first press, 800kPa for the second press, and 30kPa for the third press), removes more moisture from the paper, thus reducing the loss of bulk. The light press mainly finishes the surface of the paper without dewatering. The dryness of the paper exiting the press section is 46wt%.

[0059] 5. Pre-drying: After the press section, the moisture of the wet paper continues to be removed. Steam is introduced into the drying cylinder. The steam pressure of the pre-drying is 67 kPa and the steam pressure of the post-drying is 70 kPa. The steam ratio of the pre-drying is 0 and the steam ratio of the post-drying is 10. The wet paper is heated by the drying cylinder wall to dry it. The paper undergoes a large amount of dehydration in the drying cylinder under the action of heat radiation and pressure. The dryness of the paper exiting the drying section is 90%.

[0060] 6. Sizing: At the end of the front drying section of the paper machine, a layer of sizing liquid is sprayed through the sizing roller (sizing amount set: 1.5±0.5g / m). 2 The amount of adhesive applied is controlled by adjusting the adhesive metering rod.

[0061] The surface sizing solution used for surface sizing is prepared as follows: starch is first dissolved in water, and the starch solution is then stored in a starch storage tank (the storage data in Table 2 is obtained from this tank) and transported to the front tank of the sizing machine. Water is added and diluted according to the sizing process, and 1.2wt% Amazon bactericide 9058 and 0.8wt% defoamer (AmiDefoam5873 produced by Shanghai Emerson Chemical Co., Ltd.) are added at the same time to prepare the surface sizing solution for paper sizing. The front sizing parameters and sizing amount are shown in the forward reflux parameters in Table 2, and the back sizing parameters and sizing amount are shown in the reverse reflux parameters in Table 2.

[0062] 7. Post-drying: After sizing, further dehydration is carried out, and the paper sample has a dryness of 96 wt%.

[0063] 8. Calendering: Hard calendering is performed after the drying section. The calendering machine consists of two rollers, one of which is a hard heated roller and the other is also a hard roller. The calendering process parameter is 20 kPa.

[0064] 9. Curl and re-curl.

[0065] Example 4 The process for improving the surface strength of uncoated paper cups includes the following steps: 1. Preparation of pulp: Four types of pulp, namely BCTMP, LBKP, NBKP and BROKE, were prepared with freeness of 382 mL, 407 mL, 387 mL and 216 mL, respectively. Paper cups are made using a three-layer papermaking process, therefore the pulp ratio is determined based on the properties of the surface, core, and bottom layers. Pulp ratios are as follows: Surface layer pulp: 40% NBKP, 60% LBKP; Core layer pulp: 15% NBKP, 15% LBKP, 55% BCTMP, 15% BROKE; Bottom layer pulp: 60% NBKP, 40% LBKP.

[0066] 2. Flow system: Each of the three slurry layers contains retention aids PAM (0.48 wt%), PAE (0.38 wt%), and cationic starch (0.1 wt%), with an additional 0.05 wt% of a novel fiber reinforcing agent CMC (%) added (%). The prepared slurries for each layer are diluted, purified, and screened to remove impurities. The flow system then delivers the uniformly dispersed fiber slurry into the headbox and sprays it onto the wire mesh for forming.

[0067] The process requirements for various parameters of the slurry (concentration, retention rate, pH): Surface layer mixed slurry: concentration 0.35wt%, retention rate 87wt%, pH 5.2; Core layer mixed slurry: concentration 1.2wt%, retention rate 87wt%, pH 5.8; Bottom layer mixed slurry: concentration 0.25wt%, retention rate 87wt%, pH 5.2.

[0068] 3. Net Section: The slurry is formed on the net. The core layer is equipped with a top net forming device, and an air-cushion headbox (the core layer is a headbox for diluted water) is used to ensure stable and consistent slurry flow on the net. After the three layers of slurry are formed, they are combined into one layer by spraying starch (the spray liquid is 0.1wt% ultra-diluted starch liquid). Then, according to the process standard, the net speed is controlled at 600m / min, the slurry-to-net speed ratio is 1.0, and the opening of the three lip plates of the face, core, and bottom is 12%, 20%, and 11%, respectively. Dewatering and pressing are then carried out, and 22% of the moisture is removed before leaving the net section.

[0069] 4. Press Section: The wet paper is pressed using a three-stage pressing process, arranged as a large roller press, a shoe press, and a light press. The advanced shoe press, with its wider pressing zone (0.30m) and higher linear pressure (52kPa for the first press, 800kPa for the second press, and 30kPa for the third press), removes more moisture from the paper and reduces the loss of bulk. The light press mainly finishes the surface of the paper without dewatering. The dryness of the paper exiting the press section is 45wt%.

[0070] 5. Pre-drying: After the press section, the moisture of the wet paper continues to be removed. Steam is introduced into the drying cylinder. The steam pressure of the pre-drying is 67 kPa and the steam pressure of the post-drying is 70 kPa. The steam ratio of the pre-drying is 0 and the steam ratio of the post-drying is 10. The wet paper is heated by the drying cylinder wall to dry it. The paper undergoes a large amount of dehydration in the drying cylinder under the action of heat radiation and pressure. The dryness of the paper exiting the drying section is 90%.

[0071] 6. Sizing: At the end of the front drying section of the paper machine, a layer of sizing liquid is sprayed through the sizing roller (sizing amount set: 1.5±0.5g / m). 2 The amount of adhesive applied is controlled by adjusting the adhesive metering rod.

[0072] The surface sizing solution used for surface sizing is prepared as follows: starch is first dissolved in water, and the starch solution is then stored in a starch storage tank (the storage data in Table 2 is obtained from this tank) and transported to the front tank of the sizing machine. Water is added and diluted according to the sizing process, and 1.2wt% Amazon bactericide 9058 and 0.8wt% defoamer (AmiDefoam5873 produced by Shanghai Emerson Chemical Co., Ltd.) are added at the same time to prepare the surface sizing solution for paper sizing. The front sizing parameters and sizing amount are shown in the forward reflux parameters in Table 2, and the back sizing parameters and sizing amount are shown in the reverse reflux parameters in Table 2.

[0073] 7. Post-drying: After sizing, further dehydration is carried out, and the paper sample has a dryness of 97 wt%.

[0074] 8. Calendering: Hard calendering is performed after the drying section. The calendering machine consists of two rollers, one of which is a hard heated roller, and the other is also a hard roller. The calendering process parameter is 20 kPa.

[0075] 9. Curl and re-curl.

[0076] Comparative Example 1 The preparation of Comparative Example 1 is the same as that of Example 1, but the freeness of the slurry prepared in step 1 is BCTMP, LBKP, NBKP and BROKE, with freeness of 342mL, 336mL, 338mL and 218mL respectively. The viscosity of the starch solution prepared in step 6, as well as the sizing viscosity and other test results, are shown in Table 1. All other processes are the same as in Example 1.

[0077] Table 1 shows the test results for Comparative Example 1.

[0078] The paper prepared in the examples and comparative examples was subjected to performance testing according to standards, and the test results are shown in Table 3.

[0079] Table 2 Sizing Process Parameters

[0080] Table 3 Performance test results

[0081] As can be seen from the above, this invention, through innovative design of pulp ratio, ensures the key performance of the surface and bottom layers while significantly increasing the proportion of low-cost chemimechanical pulp and recycled waste pulp in the core layer, significantly reducing the amount of expensive bleached chemical wood pulp, thus optimizing raw material costs and maintaining the cost advantage of the produced uncoated paper cups.

Claims

1. A process for improving the surface strength of uncoated paper cups, characterized in that: Includes the following steps: Pulp preparation: Prepare surface pulp, core pulp and bottom pulp separately; wherein, surface pulp includes bleached softwood pulp and bleached hardwood pulp, core pulp includes bleached softwood pulp, bleached hardwood pulp, bleached chemithermomechanical pulp and waste pulp, and bottom pulp includes bleached softwood pulp and bleached hardwood pulp. Pulp formulation and forming: The surface slurry, core slurry and bottom slurry are respectively fed to the face wire, core wire and bottom wire of the paper machine for three-layer forming, and then subjected to pressing and pre-drying treatment; Surface sizing: The pre-dried paper is sizing. The starch solution used for sizing has a solid content of 26-30% and a viscosity of 190-230 cps. The sizing amount on both sides is 1-2 g / m². 2 ; Post-processing: The sized paper sheets are then dried and calendered to obtain the uncoated paper cup paper.

2. The process for improving the surface strength of uncoated cup paper according to claim 1, characterized in that: The freeness of the bleached softwood pulp, bleached hardwood pulp, and bleached chemithermomechanical pulp is 350-410 mL, while the freeness of the waste pulp is 180-220 mL.

3. The process for improving the surface strength of uncoated cup paper according to claim 2, characterized in that: The surface layer pulp composition is 30-45% bleached softwood pulp (NBKP) and 55-70% bleached hardwood pulp (LBKP); the core layer pulp composition is 10-20% bleached softwood pulp, 10-20% bleached hardwood pulp, 45-55% bleached chemithermomechanical pulp (BCTMP), and 10-35% broke pulp; the bottom layer pulp composition is 55-70% bleached softwood pulp and 30-45% bleached hardwood pulp.

4. The process for improving the surface strength of uncoated cup paper according to claim 1, characterized in that: The pressing process employs a three-stage pressing method, with the layout consisting of large roller pressing, shoe pressing, and smooth pressing. The dryness of the pressing section is 44-46%.

5. The process for improving the surface strength of uncoated cup paper according to claim 4, characterized in that: The surface layer slurry, core layer slurry, and bottom layer slurry are diluted, purified, and screened before being fed into the grid.

6. The process method for improving the surface strength of uncoated cup paper according to claim 5, characterized in that: Retention aids and bactericides are added to the surface slurry, core slurry, and bottom slurry.

7. The process method for improving the surface strength of uncoated cup paper according to claim 6, characterized in that: The dryness after pre-drying treatment is 89-91%.

8. The process method for improving the surface strength of uncoated cup paper according to claim 1, characterized in that: The surface sizing solution used in surface sizing is prepared as follows: First, a starch solution is prepared with a solid content of 26-30 wt% and a viscosity of 190-230 cps. After preparation, the starch solution is stored in a starch storage tank and then transported to the tank before the sizing machine. Water is added and diluted according to the sizing process, along with bactericides and defoamers, to prepare the surface sizing solution for paper sizing. The solid content of the surface sizing solution is 13-17%, and the application rate is 1-2 g / m³. 2 Viscosity 25~35cps.

9. The process method for improving the surface strength of uncoated cup paper according to claim 1, characterized in that: The dryness of the paper sample after the post-drying treatment was 96~98wt%.

10. The process method for improving the surface strength of uncoated cup paper according to claim 9, characterized in that: The calendering is completed using a hard calendering machine, with a pressing pressure of 20 kPa.

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