Acid-neutralizing nanopha paperboard enhancer and preparation and application thereof

Abstract

The application discloses an acid-neutral nano PHA paperboard reinforcing agent and preparation and application thereof, and belongs to the technical field of papermaking chemicals. The reinforcing agent is a bio-based nano water-based emulsion, and the core is that a stable high positive charge nanoparticle interface is constructed under an acid-neutral window through synergistic effect of polyphenols and amine polymers. The application solves the environmental safety hidden danger of traditional wet strength agents, and does not contain free formaldehyde and epichlorohydrin. The outstanding advantages are as follows: completely compatible with modern acid-neutral papermaking process, overcoming the defect that similar bio-based technologies need to react under alkaline; flexible application, can be efficiently used for wet part addition, or stably compounded with oxidized starch for surface sizing; high efficiency and energy saving, under the condition of lower addition amount and 105 DEG C conventional industrial drying, excellent wet strength retention rate and dry strength improvement can be realized, and no additional high temperature curing is needed.

Description

Technical Field

[0001] This invention belongs to the field of papermaking chemicals technology, specifically relating to acid-neutral nano-PHA paperboard reinforcing agent and its preparation and application. Background Technology

[0002] Packaging paperboard, especially linerboard and corrugated base paper used to manufacture corrugated boxes, plays a crucial role in modern logistics and warehousing. This type of paperboard not only needs sufficient dry strength but also faces stringent requirements regarding its ability to maintain strength in humid environments. This directly affects the stacking life and compression performance of the boxes, such as edge crush strength (ECT) and box crush strength (BCT). However, the field has long faced a core contradiction: the difficulty in simultaneously achieving high-efficiency wet strength performance and ensuring environmental safety and process compatibility.

[0003] Currently, the technical approaches in this field can be mainly categorized as follows:

[0004] The first category is the "traditional chemical crosslinking wet strength agent system." This type of technology relies on synthetic polymers forming a chemical crosslinking network between fibers and is currently the mainstream in industrial applications. Examples include polyamide-epoxychloropropane (PAE) and glyoxal-modified polyacrylamide (GPAM). A common drawback of this type of technology is its serious environmental and safety hazards: PAE inevitably introduces adsorbable organic halides (AOX) and chloropropanol byproducts (such as 1,3-dichloro-2-propanol, 1,3-DCP; 3-chloro-1,2-propanediol, 3-MCPD) during production and application, substances which are subject to strict global regulation; while the GPAM system carries the risk of releasing free formaldehyde. Furthermore, even with the use of these traditional wet strength agents, the compressive strength of corrugated cardboard still significantly decreases under high humidity conditions.

[0005] The second category is "melt blending modification of bio-based materials." This type of technology attempts to improve the overall mechanical properties of bio-based polymers, such as polyhydroxyalkanoates (PHAs), by blending them with other materials (e.g., polybutylene adipate terephthalate (PBAT) and polyvinyl alcohol (PVOH) disclosed in patent CN119912795A) through high-temperature melt processes such as twin-screw extrusion. The limitation of this type of technology is that it was developed for the plastic film or injection molding fields, and its high-temperature, anhydrous processing method is completely incompatible with the wet-end chemistry or water-based surface sizing processes in the paper industry, making it unsuitable for use as a water-based additive in the papermaking process.

[0006] The third category is "μm-level aqueous dispersions of bio-based materials." This type of technology prepares bio-based polymers such as PHA into aqueous dispersions, primarily for use as coatings on paper surfaces. For example, patent CN117769583A discloses an aqueous dispersion of PHA with a particle size of 0.5–5 μm (i.e., 500–5000 nm) for use as a barrier and water-resistant coating on paper. The drawback of this technology is its excessively large particle size, typically at the μm level. Its design purpose is to form a physical barrier film on the paper surface, rather than as a highly efficient reinforcing agent. The excessively large particle size and uncertain surface charge characteristics result in extremely low fiber retention when added in the wet end, or poor colloidal stability when compounded with oxidized starch for surface sizing, making it difficult to meet the high-speed operation requirements of modern paper machines.

[0007] The fourth category is "polyphenol-amine based interfacial adhesion systems." This type of technology utilizes the reaction between polyphenols (such as tannic acid and dopamine) and amine polymers (such as polyethyleneimine, PEI) to construct functional interfacial layers. For example, patent EP2239370A1 discloses a scheme using cationic tannic acid to improve paper strength. However, a key drawback of this type of technology lies in its reaction conditions. Most existing disclosures (such as the self-polymerization of dopamine) rely on alkaline conditions (e.g., pH 8.5) to achieve the oxidative polymerization and co-deposition of polyphenols. This alkaline process contradicts the acid-neutral papermaking system (pH 6.5–7.5) commonly used in modern papermaking. Under the acid-neutral window, this type of reaction cannot proceed effectively, leading to adhesion failure.

[0008] In summary, existing technologies fail to provide a high-performance bio-based enhancement solution that can completely eliminate regulated substances such as AOX, chloropropanol, and formaldehyde, while also being fully compatible with acid-neutral papermaking processes (pH 6.5–7.5). A long-standing technological gap in this field lies in how to design a bio-based PHA nano-aqueous emulsion that, within a specific acid-neutral window, can achieve a nanoparticle size (D) through precise polyphenol-amine synergistic anchoring interface engineering. 50 The unique combination of parameters—100–140 nm, high positive potential (ζ potential +16 to +21 mV), and low dispersion index (PDI ≤ 0.15)—is key to ensuring efficient retention and stable compounding of the reinforcing agent, whether added in the wet end or applied to the surface. Ultimately, it enables high wet strength retention (WSR) and excellent white water properties (low charge demand, low turbidity, and low chemical oxygen demand (COD)) under conventional industrial drying conditions (105°C). This constitutes the unique technical challenge that this invention aims to address. Summary of the Invention

[0009] The purpose of this invention is to overcome the shortcomings of the prior art and provide an acid-neutral nano-PHA paperboard reinforcing agent and its preparation and application, aiming to solve the problems of environmental risks, high energy consumption, and insufficient application of bio-based materials in the field of paperboard reinforcement of existing wet strength agents.

[0010] To achieve the above objectives, the present invention provides the following technical solution:

[0011] An acid-neutral nano-PHA paperboard reinforcing agent, wherein the reinforcing agent is an aqueous emulsion, and is composed of the following components by weight based on emulsion solids:

[0012] PHA: 90–96.5 parts by weight;

[0013] Polyphenols: 0.8–1.2 parts by weight;

[0014] Amine polymers: 1.2–2.4 parts by weight;

[0015] Nonionic surfactant: 0.28–0.50 parts by weight;

[0016] Plasticizer: 0.35–0.80 parts by weight;

[0017] For example, the mass fraction of the PHA can be: 90 parts by mass, 91 parts by mass, 92 parts by mass, 93 parts by mass, 94 parts by mass, 95 parts by mass, 95.5 parts by mass, 95.8 parts by mass, 96 parts by mass, 96.2 parts by mass, or 96.5 parts by mass. The mass fraction of the polyphenol can be: 0.8 parts by mass, 0.85 parts by mass, 0.9 parts by mass, 0.95 parts by mass, 1.0 parts by mass, 1.05 parts by mass, 1.1 parts by mass, 1.15 parts by mass, or 1.2 parts by mass. The mass fraction of the amine polymer can be: 1.2 parts by mass, 1.3 parts by mass, 1.4 parts by mass, 1.5 parts by mass, 1.6 parts by mass, 1.7 parts by mass, 1.8 parts by mass, 1.9 parts by mass, 2.0 parts by mass, 2.1 parts by mass, 2.2 parts by mass, 2.3 parts by mass, or 2.4 parts by mass. The nonionic surfactant can be present in the following proportions by weight: 0.28 parts by weight, 0.30 parts by weight, 0.32 parts by weight, 0.35 parts by weight, 0.38 parts by weight, 0.40 parts by weight, 0.42 parts by weight, 0.45 parts by weight, 0.48 parts by weight, or 0.50 parts by weight. The plasticizer can be present in the following proportions by weight: 0.35 parts by weight, 0.40 parts by weight, 0.45 parts by weight, 0.50 parts by weight, 0.55 parts by weight, 0.60 parts by weight, 0.65 parts by weight, 0.70 parts by weight, 0.75 parts by weight, or 0.80 parts by weight.

[0018] And the physical properties of the emulsion satisfy:

[0019] Solid content is 15%–45%;

[0020] For example, the solid content is 15%, 18%, 20%, 22%, 25%, 28%, 30%, 33%, 35%, 38%, 40%, 42%, or 45%.

[0021] Viscosity at 25℃ ≤2000mPa·s;

[0022] For example, the viscosity at 25°C can be 500 mPa·s, 600 mPa·s, 700 mPa·s, 800 mPa·s, 900 mPa·s, 1000 mPa·s, 1100 mPa·s, 1200 mPa·s, 1300 mPa·s, 1400 mPa·s, 1500 mPa·s, 1600 mPa·s, 1700 mPa·s, 1800 mPa·s, 1900 mPa·s, or 2000 mPa·s.

[0023] Median particle size D 50 The wavelength is 100–140 nm.

[0024] For example, the median particle size D 50 The wavelengths are 100nm, 105nm, 110nm, 115nm, 120nm, 125nm, 130nm, 132nm, 135nm, 138nm, or 140nm.

[0025] The dispersion index (PDI) is ≤0.15.

[0026] For example, the dispersion index (PDI) is 0.05, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, or 0.15.

[0027] In 10 mmol / L phosphate buffer, at pH 6.65–6.95, the zeta potential was measured to be +16–+21 mV, and the zeta potential drift was ≤1.5 mV within 60 min.

[0028] For example, the zeta potential is +16mV, +16.5mV, +17mV, +17.5mV, +18mV, +18.5mV, +19mV, +19.5mV, +20mV, +20.5mV, or +21mV. The zeta potential drift within 60 minutes is, for example, 0.3mV, 0.5mV, 0.7mV, 0.8mV, 0.9mV, 1.0mV, 1.1mV, 1.2mV, 1.3mV, 1.4mV, or 1.5mV.

[0029] Based on emulsion solids, the content of acrylic polymers, methacrylic polymers and their copolymers is <0.5%, and the content of PBAT, PVOH and polylactic acid is <1.0% each;

[0030] The total organic fluorine was not detected by EN 14582:2016 oxygen bomb combustion-ion chromatography method; the system did not contain free formaldehyde or epichlorohydrin.

[0031] This combination of component limitations and physical property parameters ensures efficient retention and stability of the reinforcing agent within the acid-neutral papermaking window, as well as the environmental friendliness of the final product, in stark contrast to traditional wet strength agents that rely on AOX or formaldehyde crosslinking.

[0032] The PHA is selected from one or more of short-chain PHA, medium- and long-chain PHA, or PHA formed by copolymerization of short-chain and medium- and long-chain monomers.

[0033] The short-chain PHA is selected from one or more of poly-3-hydroxybutyrate, poly-3-hydroxybutyrate-co-3-hydroxyhexanoate, poly-3-hydroxybutyrate-co-4-hydroxybutyrate, and poly-3-hydroxybutyrate-co-3-hydroxyvalerate.

[0034] The medium- to long-chain monomers have 6 to 14 carbon atoms.

[0035] The polyphenols are selected from one or more of hydrolyzed tannins, condensed tannins, ortho- and oxaloacetic acid or catechins, polyhydroxy flavonoids, or tea polyphenols; the hydrolyzed tannins are one or more of tannic acid, gallic acid, ellagic acid, and their salts; the condensed tannins are proanthocyanidins; the ortho- and oxaloacetic acid or catechins are one or more of catechins, dopamine, and their salts; the polyhydroxy flavonoids are one or more of quercetin and myricetin; and the tea polyphenols are one or more of epigallocatechin gallate and epicatechin gallate.

[0036] The amine polymer is selected from one or more of quaternized chitosan, polyethyleneimine, or polydimethyldiallylammonium chloride, wherein the number average molecular weight of polyethyleneimine is 600–1800 g / mol, for example, the number average molecular weight of polyethyleneimine can be 600 g / mol, 700 g / mol, 800 g / mol, 900 g / mol, 1000 g / mol, 1100 g / mol, 1200 g / mol, 1300 g / mol, 1400 g / mol, 1500 g / mol, 1600 g / mol, 1700 g / mol, or 1800 g / mol.

[0037] The molar ratio of phenolic hydroxyl groups in polyphenols to protonable nitrogen in amine polymers is 1.1 to 1.9. For example, this molar ratio can be 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, 1.55, 1.6, 1.65, 1.7, 1.75, 1.8, 1.85, or 1.9.

[0038] The plasticizer is triethyl citrate, tributyl citrate, or polyethylene glycol with a number average molecular weight of 200-600, such as polyethylene glycol with a number average molecular weight of 200 g / mol, 250 g / mol, 300 g / mol, 350 g / mol, 400 g / mol, 450 g / mol, 500 g / mol, 550 g / mol, or 600 g / mol.

[0039] The nonionic surfactant has an HLB value of 11.5 to 12.5 and is selected from one or more of fatty alcohol polyoxyethylene ethers, alkyl glucosides, polyoxyethylene-polyoxypropylene block copolymers, polyoxyethylene castor oil, or polyethylene glycol fatty acid esters, for example, an HLB value of 11.5, 11.6, 11.7, 11.8, 11.9, 12.0, 12.1, 12.2, 12.3, 12.4, or 12.5.

[0040] The emulsion has a volatile organic compound (VOC) content of ≤0.3% and a 0.45µm membrane residue content of ≤0.3%, for example, VOCs of 0.05%, 0.1%, 0.15%, 0.18%, 0.2%, 0.22%, 0.25%, 0.28%, or 0.3%; and a 0.45µm membrane residue content of 0.05%, 0.08%, 0.1%, 0.12%, 0.15%, 0.18%, 0.2%, 0.23%, 0.25%, 0.27%, or 0.3%.

[0041] The method for preparing the acid-neutral window polyphenol-amine synergistic anchored nano-PHA paperboard reinforcing agent includes the following steps:

[0042] Step 1. Dissolve PHA in a water-nonionic surfactant system at 65℃~75℃, for example, 65℃, 66℃, 67℃, 68℃, 69℃, 70℃, 71℃, 72℃, 73℃, 74℃ or 75℃, for ≥2000s -1 High shear rates, such as 2000s -1 2200s -1 2500s -1 2800s -1 or 3000s -1 Pre-emulsify for 30 minutes to obtain a pre-dispersion;

[0043] Step 2. Cool the pre-dispersion to 30℃~40℃, for example 30℃, 31℃, 32℃, 33℃, 34℃, 35℃, 36℃, 37℃, 38℃, 39℃ or 40℃, and add the polyphenol diluent at a uniform rate over 10~30min, for example 10min, 12min, 15min, 18min, 20min, 22min, 25min, 28min or 30min, to obtain the polyphenol modified dispersion;

[0044] Step 3. Maintaining the pH at 6.65–6.95 in 10 mmol / L phosphate buffer, add the amine polymer solution dropwise at a constant rate over 30–120 min, for example, 30 min, 40 min, 45 min, 50 min, 60 min, 70 min, 80 min, 90 min, 100 min, 110 min, or 120 min, with a stirring shear rate ≥2000 s. -1 The reaction mixture is obtained;

[0045] Step 4. After the reaction mixture is allowed to stand for 10 minutes, it is then subjected to a 100µm pre-filtration and a 0.45µm pressure filtration to obtain the emulsion.

[0046] This preparation method, through specific temperature control, feeding sequence, and pH window, enables the construction of a stable polyphenol-amine synergistic anchoring interface layer under acidic and neutral conditions.

[0047] A wet-end method for enhancing the strength of paper or paperboard, the method comprising: adding 0.5% to 3.0% of the aforementioned reinforcing agent to the pulp based on oven-dry fiber content.

[0048] For example, the amount of reinforcing agent added can be 0.5%, 0.75%, 1.0%, 1.25%, 1.5%, 1.75%, 2.0%, 2.25%, 2.5%, 2.75%, or 3.0%.

[0049] And add 0.3% to 1.5% cationic starch and 0.02% to 0.10% retention aid and microparticles;

[0050] For example, the amount of cationic starch added can be 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, or 1.5%. The amount of retention aid and microparticles added can be 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, or 0.10%.

[0051] Then dry at 105℃±5℃ for 8~12min;

[0052] For example, the drying time is 8 min, 9 min, 10 min, 11 min or 12 min.

[0053] The resulting paper or paperboard has a WSR ≥ 72%.

[0054] For example, the WSR of the resulting paper or paperboard can be 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, or 80%.

[0055] Furthermore, the white water meets the following specifications: charge requirement ≤ 430 µeq / L, turbidity ≤ 155 nTU, and chemical oxygen demand ≤ 850 mg / L.

[0056] For example, charge requirements can be 340 µeq / L, 350 µeq / L, 360 µeq / L, 370 µeq / L, 380 µeq / L, 390 µeq / L, 400 µeq / L, 410 µeq / L, 420 µeq / L, or 430 µeq / L. Turbidity can be 135 nTU, 138 nTU, 140 nTU, 142 nTU, 145 nTU, 148 nTU, 150 nTU, 152 nTU, or 155 nTU. Chemical oxygen demand can be 790 mg / L, 800 mg / L, 810 mg / L, 820 mg / L, 830 mg / L, 840 mg / L, 845 mg / L, or 850 mg / L.

[0057] A surface sizing method, the method comprising: mixing the aforementioned acid-neutral window polyphenol-amine synergistically anchored nano-PHA paperboard reinforcing agent with oxidized starch at a solid mass ratio of 1:2 to 1:5.

[0058] For example, the solid mass ratio is, for example, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5 or 1:5.

[0059] Prepare a sizing solution with a total solids content of 10%; apply the sizing solution by pressing or coating, and control the dry adhesive content to be 2-6 g / m².

[0060] For example, the dry adhesive content is, for example, 2g / m², 2.5g / m², 3g / m², 3.5g / m², 4g / m², 4.5g / m², 5g / m², 5.5g / m², or 6g / m².

[0061] Dry at 105℃±5℃ for 8~12min;

[0062] For example, the drying time is 8 min, 9 min, 10 min, 11 min or 12 min.

[0063] The resulting paper or paperboard has a WSR ≥ 78%, Cobb 60 The value is ≤32g / m², and the short-distance compressive strength is improved.

[0064] For example, WSR can be 78%, 78.5%, 79%, 79.5%, 80%, 81%, 82%, 83%, or higher. (Cobb) 60The value can be 32.0 g / m², 31.9 g / m², 31.8 g / m², 31.7 g / m², 31.6 g / m², 31.5 g / m², 31.3 g / m², 31.0 g / m² or lower.

[0065] The retention aid and microparticles are one or more of bentonite, silica sol, colloidal silica, aluminum silicate, or magnesium silicate.

[0066] The emulsion exhibits the following storage stability: after 6 months of storage at 25°C protected from light, viscosity drift ≤ ±15%, for example, ±3%, ±5%, ±7%, ±8%, ±10%, ±12%, or ±15%; after 14 days of storage at 40°C, viscosity drift ≤ ±10%, for example, ±2%, ±3%, ±4%, ±5%, ±6%, ±7%, ±8%, ±9%, or ±10%; after one freeze-thaw cycle, the median particle size D... 50 The upward shift is <10nm, for example, 1nm, 2nm, 3nm, 4nm, 5nm, 6nm, 7nm, 8nm or 9nm; and the absolute value of the zeta potential change measured at pH 6.65 to 6.95 is ≤2.0mV, for example, 0.2mV, 0.5mV, 0.8mV, 1.0mV, 1.2mV, 1.5mV, 1.8mV or 2.0mV.

[0067] The minimum film-forming temperature is ≤60°C as determined by ASTM D2354, for example, 45°C, 47°C, 49°C, 50°C, 51°C, 52°C, 53°C, 54°C, 55°C, 58°C, or 60°C.

[0068] The method is applicable to corrugated base paper, linerboard, and kraft paper.

[0069] The edge crush strength improvement rate of the prepared single-wall B-flute cardboard is ≥15%;

[0070] For example, the edge compression strength improvement rate can be 15.0%, 15.2%, 15.5%, 15.6%, 15.8%, 16.0%, 16.2%, 16.3%, 16.5%, or 16.7%.

[0071] Furthermore, with a sample size of ≥8, the Pearson correlation coefficient between the longitudinal short-distance compressive strength and the edge crush strength of the paperboard is ≥0.90, for example, 0.90, 0.92, 0.925, 0.93, 0.935 or 0.94.

[0072] Compared with the prior art, the use of this invention can achieve the following significant beneficial effects:

[0073] High efficiency and energy saving: WSR≥70% is achieved under conventional industrial drying conditions of 105℃±5℃ for 8–12 min; in surface sizing conditions, the WSR value is 80%, while avoiding high-temperature curing.

[0074] High application flexibility: The precisely controlled nanoparticle size and positive potential window enable the product to be used efficiently for wet-end addition, and can also be stably compounded with oxidized starch for surface sizing, making it suitable for a wide range of applications.

[0075] High engineering robustness: By controlling indicators such as viscosity, accelerated aging stability and membrane residue, the reliability of the product in industrial environments for storage, transportation and online preparation is guaranteed.

[0076] Cost and process synergy: The target WSR window can be achieved at a typical dosage of 0.5% to 1.5%. Compared with the PAE / GPAM system, it can avoid halogen / aldehyde curing, reduce energy consumption and regulatory costs. It can reduce unit reinforcement cost and improve process robustness by synergizing with oxidized starch and retention-microparticle systems at both the wet end and sizing end.

[0077] Recycling friendly: The reinforced paperboard has good repulping and recyclability, which meets the requirements of the circular economy. Detailed Implementation

[0078] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention. Unless otherwise stated, the raw materials used in this embodiment are commercially available industrial products or can be prepared by conventional methods. Unless otherwise specified, performance testing methods are performed according to the standards described in the invention summary section.

[0079] Main reagents and raw materials:

[0080] Table 1. Main reagent and raw material names, brands / specifications, and manufacturers:

[0081]

[0082] Main analytical and testing instruments:

[0083] Table 2 mainly analyzes the names, models / specifications, and manufacturers of the testing instruments:

[0084]

[0085] Main testing standards:

[0086] Table 3 Main Test Standards and Methods:

[0087]

[0088] Terminology and Calculation Methods:

[0089] Wet stiffness: According to ISO 2493-2:2020; the sample is immersed in deionized water at 23°C for 60 min and tested within 5 min after removal.

[0090] WSR(%) = (wet stiffness / dry stiffness) × 100.

[0091] Foaming volume: Take 200mL of sizing solution into a 250mL graduated cylinder, mechanically stir at 25℃ and 500rpm for 3min, let stand for 1min and take the reading.

[0092] White water charge requirement: PCD determination; turbidity: in nTU; COD: according to ISO 15705:2002.

[0093] Zeta potential drift (Δζ, mV): Take emulsion (30±1wt%) pH 6.8 (10mmol / L phosphate buffer), keep it at 25℃, sonicate for 2min to defoam, and let it stand. Record the absolute value of the difference between the average ζ values ​​at 0min and 60min; the qualified threshold Δζ≤1.5mV.

[0094] Surface contact angle criterion: 5µL deionized water droplet, 23℃; initial contact angle ≥98°, decay ≤4° within 10s.

[0095] TOF non-detection criterion: According to EN 14582:2016, the sample was determined by oxygen bomb combustion-ion chromatography; the result ≤5mg / kg was judged as "non-detection".

[0096] General Emulsion Preparation Process

[0097] Step 1. Dilute the polyphenol and amine solutions with deionized water to a solid content of ≤2%, let stand for 10 minutes, and obtain diluted solution a and diluted solution b.

[0098] Step 2. Disperse PHA powder in a water and nonionic surfactant system, and pre-emulsify under high shear at 65-75°C for 30 min to obtain a pre-dispersion.

[0099] Step 3. Cool the pre-dispersion obtained in Step 2 to 30–40°C, and add dilution solution a dropwise over 10–30 min while maintaining a shear rate ≥2000 s. -1 A polyphenol-modified dispersion was obtained.

[0100] Step 4. Slowly add dilution solution b to the polyphenol modified dispersion obtained in step 3 over a period of 30 to 120 minutes, maintaining the pH at 6.65 to 6.95 with phosphate buffer while continuously stirring to obtain the reaction solution.

[0101] Step 5. After the reaction solution obtained in step 4 is allowed to stand for 10 minutes, it is filtered through a 100µm filter screen, and then filtered with a filter membrane pressure of 0.45µm to detect the membrane residue, thus obtaining the target emulsion.

[0102] Step 6. Measure and record the solids content, viscosity at 25°C, and PDI of the target emulsion for subsequent applications.

[0103] Example:

[0104] Table 4 Formulations and Particle Size-Electrical Characterization of Examples:

[0105]

[0106] Note: PHA, polyphenols, amine polymers, nonionic surfactants, and plasticizers in the table are provided in parts by weight.

[0107] Example 1 uses TA as the polyphenol; quaternized chitosan as the amine polymer; and PEG-400 as the plasticizer.

[0108] Example 2 uses TA as the polyphenol; quaternized chitosan as the amine polymer; and PEG-400 as the plasticizer.

[0109] Example 3 uses TA as the polyphenol; PEI as the amine polymer; and TEC as the plasticizer.

[0110] Example 4 uses GA as the polyphenol; quaternized chitosan as the amine polymer; and PEG-400 as the plasticizer.

[0111] Example 5 uses TA as the polyphenol; PDADMAC as the amine polymer; and TEC as the plasticizer.

[0112] Example 6 uses TA as the polyphenol; quaternized chitosan as the amine polymer; and TEC as the plasticizer.

[0113] Example 7 uses DA·HCl as the polyphenol; quaternized chitosan as the amine polymer; and PEG-400 as the plasticizer.

[0114] Example 8 uses TA as the polyphenol; quaternized chitosan as the amine polymer; and PEG-400 as the plasticizer.

[0115] Comparative example:

[0116] Table 5 Comparative Example Formulation Explanation:

[0117]

[0118] Note: PHA, polyphenols, amine polymers, nonionic surfactants, and plasticizers in the table are provided in parts by weight.

[0119] Application example:

[0120] Application Example 1: Applying the product to the wet area.

[0121] Experimental Description: Bleached softwood pulp and waste paper pulp (30:70) were mixed and beaten to a Canadian Standard Freeness (CSF) of 300 mL to prepare 200 g / m² hand-made sheets. Reinforcing agent emulsions from Tables 4 and 5 were added to the pulp at three gradients based on oven-dry fiber content: 0.5%, 1.0%, and 2.5%. All wet paper sheets were dried at 105℃±5℃ for 10 min. The stiffness, dry burst strength, WSR, and water resistance (Cobb) of each hand-made sheet were tested and compared. 60 ).

[0122] Table 6. Application performance of wet-end additives:

[0123]

[0124] Results Analysis: Table 6 shows that the reinforcing agent of the present invention has a significant effect when added in the wet end. In all examples, at the minimum dosage of 0.5%, the WSR reached over 72%, and as the dosage increased to 2.5%, the WSR increased to 80% (Example 6). Meanwhile, Cobb... 60 The wet strength (WSR) dropped to 27.0 g / m², demonstrating excellent synergistic reinforcement and water resistance. In contrast, the WSR of Comparative Example 1 (pure PHA) was only in the single digits, while the WSRs of Comparative Examples 2–5 (components missing or inappropriate proportions) were all far below 70%. This indicates that the synergistic effect of polyphenols and amine polymers within a defined ratio is a key technological component for achieving high wet strength in acid-neutral systems.

[0125] Application Example 2: Surface application of adhesive.

[0126] Experimental Description: Unsized kraft paperboard with a surface area of ​​175 g / m² was used as the substrate. The reinforcing agent emulsions from Tables 4 and 5 were mixed with commercially available oxidized starch at solid mass ratios of 1:2, 1:4, and 1:5 to prepare sizing solutions with a total solids content of 10%. A laboratory coating machine was used for sizing, controlling the dry glue amount to 2 g / m², 4 g / m², and 6 g / m², respectively. The sized paperboard was dried in an oven at 105℃±5℃ for 10 min. The dry / wet stiffness, WSR, and Cobb stiffness of the treated paperboard were tested and compared. 60 value.

[0127] Table 7 Surface Adhesive Application Performance (Dry Adhesive Weight 2g / m²):

[0128]

[0129] Table 8 Surface Adhesive Application Performance (Dry Adhesive Weight 4g / m²):

[0130]

[0131] Table 9 Surface Adhesive Application Performance (Dry Adhesive Weight 6g / m²):

[0132]

[0133] Results Analysis: The data in Tables 7, 8 and 9 clearly demonstrate the combined effects of dry adhesive amount and compounding ratio on surface sizing effect.

[0134] First, the main effect of dry adhesive content was significant: as the dry adhesive content increased from 2 g / m² (Table 7) to 6 g / m² (Table 9), the dry stiffness and wet stiffness of all samples (including examples and comparative examples) steadily increased, while the water resistance (Cobb) also improved. 60 The value also improved (i.e., the value decreased). This is consistent with the stronger film-forming and covering effect resulting from the increased application amount.

[0135] Secondly, the effect of the compounding ratio is subtle and crucial: at the same dry glue amount, the 1:2 and 1:4 ratios have similar performance and both show excellent results, with both significantly better than the 1:5 ratio. For example, under the conditions of 4 g / m² (Table 8) and Example 6, the 1:2 ratio (WSR=83.2%, Cobb 60 =27.1g / m²) and a 1:4 ratio (WSR=82.2%, Cobb 60 The performance of WSR (79.4%) and Cobb (27.5 g / m²) is at a high level. Meanwhile, the 1:5 ratio of WSR (79.4%) and Cobb... 60 The concentration (28.5 g / m²) showed a significant decrease, indicating that when the reinforcing agent ratio was below 1:4 (i.e., the starch ratio was above 4), the performance began to reach an inflection point. This suggests that a 1:4 ratio is the optimal point for achieving a balance between high strength, high water resistance, and cost (less reinforcing agent usage).

[0136] In summary, the example group outperformed the comparative group under all test conditions, demonstrating the high efficiency and synergistic effect of the reinforcing agent of the present invention in compound applications with oxidized starch.

[0137] Application Example 3: Edge Crush Strength (ECT) Test of Corrugated Board.

[0138] Experimental Description: To evaluate the contribution of the reinforcing agent to the performance of the final product, the surface-sized linerboard from Application Example 2 (represented by a 1:4 ratio and 4 g / m² dry adhesive content) was used and paired with a uniform batch of 150 g / m² corrugated core paper. Single-wall B-flute corrugated board was assembled using laboratory equipment. Subsequently, the edge crush strength (ECT) of the corrugated board was tested according to GB / T 6546-2021 standard, and its increase relative to the untreated base paper (Comparative Example 1) was calculated.

[0139] Table 10: Edge Crush Strength (ECT) Test Results for Corrugated Board

[0140]

[0141] Results Analysis: ECT (Effective Tissue Conversion) is a core indicator for measuring the compressive strength of corrugated boxes. Table 10 shows that the strengthening agent of this invention effectively transfers the reinforcement of the face paper to the overall structural strength of the corrugated board. All assembled corrugated boards from the various embodiments achieved a significant increase in ECT value exceeding 15%, with Example 6 showing an increase of 16.7%. In contrast, Comparative Example 5, which showed the highest increase among the comparative examples, only achieved 6.3%, with the others all below this value. This demonstrates that the performance gains brought about by this invention through improving the physical strength of the face paper (such as stiffness and compressive strength) are clear and efficient, improving the load-bearing capacity of the corrugated board without increasing its basis weight.

[0142] Application Example 4: Matching repulping properties with recyclability.

[0143] Experimental Description: To evaluate the impact of reinforcing agents on paperboard recycling, the paperboard prepared in Application Example 1 (with an addition of 1.0%) was re-pulped in the laboratory. The sieve residue content and macroscopic adhesion spot area in the re-pulped pulp were evaluated according to INGEDE method 12.

[0144] Acceptance criteria: Total area of ​​adhesive spots ≤ 2.0 mm² / m².

[0145] Table 11 Results of repulping and recyclability tests:

[0146]

[0147] Results Analysis: Resizing is a key indicator of paper product recyclability. Table 11 shows that after resizing, the adhesive spot area of ​​all paperboards treated in Examples 1–8 was significantly lower than the acceptable criterion of 2.0 mm² / m², with the lowest (Example 2) at only 0.62 mm² / m², indicating that the fibers can be recycled with high quality. Conversely, the adhesive spot areas of Comparative Examples 2–5 all exceeded the standard and were deemed unacceptable. This reveals that unbalanced formulations (such as those containing only polyphenols or amines in the comparative examples) can lead to the formation of difficult-to-disperse agglomerates during resizing, thus contaminating the recycled pulp. This invention, through precise synergistic formulation design, ensures compatibility between enhanced functionality and recyclability.

[0148] Application Example 5: Surface wettability and sizing compatibility.

[0149] Experimental Description: This application example evaluates the product from both process and performance perspectives. Regarding the process, the sizing solution prepared in Application Example 2 (1:4 ratio) was used to test its viscosity drift percentage over 30 minutes and foam volume after 3 minutes of mechanical stirring to assess its colloidal stability. Regarding performance, the initial static contact angle of a 5µL water droplet and the contact angle decay over 10 seconds were tested on the treated cardboard surface corresponding to Application Example 2 to characterize its surface hydrophobicity.

[0150] Table 12 Results of sizing compatibility and surface wettability tests:

[0151]

[0152] Results Analysis: Table 12 confirms the process applicability and product performance of the present invention. In terms of process, the viscosity drift of the sizing solution in all examples was controlled within +4.0%, and the foaming volume was low (≤18mL), demonstrating excellent compatibility with oxidized starch and colloidal stability, meeting industrial sizing requirements. In terms of performance, the initial contact angle of the paperboard in all examples was ≥98°, and the decay within 10 seconds was ≤4°, exhibiting stable hydrophobicity, which is consistent with the low Cobb properties in Application Example 2. 60 The values ​​are consistent. However, the comparative sample shows significant deficiencies in viscosity drift, foaming control, or surface hydrophobicity (low contact angle, rapid decay), failing to balance process and performance.

[0153] Application Example 6: Performance retention after damp heat cycling (WSR retention rate and ECT retention rate).

[0154] Experimental Description: To simulate the effect of temperature and humidity fluctuations on the strength of cardboard during warehousing and transportation, samples obtained from Application Example 1 (1.0% dosage) and Application Example 3 (1:4 ratio, 4 g / m²) were subjected to damp heat aging tests. The samples were alternately kept at 40℃ and 90% RH and 23℃ and 50% RH for 12 hours, for a total of 5 cycles. The WSR and ECT after each cycle were tested, and their retention rate relative to the initial values ​​before the cycles was calculated to evaluate the durability of the reinforcement effect.

[0155] Acceptance criteria: For the example, the WSR retention rate is ≥90% and the ECT retention rate is ≥90%; for the comparative example, there is no such requirement, only the data is recorded.

[0156] Table 13 Performance retention after damp heat cycling (after 5 cycles):

[0157]

[0158] Results Analysis: Durability is an important indicator for evaluating the practical application value of reinforcing agents. Table 13 shows that the reinforcing agent of this invention provides a long-lasting protective effect. After five harsh damp-heat cycles, all examples maintained a WSR and ECT retention rate of over 90%, with Example 6 achieving an even higher retention rate of 95% (WSR) and 96% (ECT). In contrast, the performance degradation of the comparative groups was significant, with the lowest WSR retention rate at only 68% (Comparative Example 1), and ECT retention rates all below 85%. This demonstrates that the polyphenol-amine synergistic anchoring interface layer of this invention has high stability and is not easily degraded or degraded during alternating damp-heat cycles.

[0159] Application Example 7: Wet end retention and white water environmental indicators (charge demand / turbidity / COD).

[0160] Experimental Description: To evaluate the retention efficiency of the reinforcing agent in the wet end and its impact on the papermaking water circulation system, an experimental papermaking apparatus was used under the 1.0% (oven-dry fiber) addition condition of Application Example 1. White water samples generated during the papermaking process were collected. Key environmental indicators of the white water, including charge demand (PCD method, µeq / L), turbidity (NTU), and chemical oxygen demand (COD, mg / L), were measured according to standard methods and compared with control examples.

[0161] Table 14. Charge requirements, turbidity, and COD of wet-end white water (1.0wt% dosage):

[0162]

[0163] Results Analysis: The white water index reflects the retention efficiency of chemicals and environmental impact. Table 14 shows that the charge requirement and turbidity of the white water in all embodiments were significantly lower than those in the comparative examples. For example, the charge requirement (340 µeq / L) and turbidity (135 nTU) of Example 6 were reduced by 47.7% and 38.6% respectively compared to Comparative Example 1 (650 µeq / L; 220 nTU). This is attributed to the precisely controlled positive zeta potential of the emulsion in this invention, which enables efficient retention in the fibers and reduces the loss of chemicals to the aqueous phase. Simultaneously, the COD values ​​of the embodiments were also controlled at a low level (≤850 mg / L). This indicates that the present invention improves performance while reducing the load on the white water system and increasing chemical efficiency.

[0164] Application Example 8: Correlation between short-range compressive strength (SCT) and ECT.

[0165] Experimental Description: To investigate the reinforcement mechanism, this application example tested the correlation between the compressive strength (SCT) of the linerboard itself and the final structural strength (ECT) of the paperboard. The linerboard with surface sizing from Application Example 2 (1:4 ratio, 4 g / m²) was used, and its longitudinal (MD) and transverse (CD) SCT values ​​were tested according to ISO 9895:2008. Pearson correlation analysis was performed on the SCT data and the corresponding ECT data from Application Example 3 (n=8 sample examples).

[0166] Table 15 Correlation between SCT and ECT of face paper (ratio 1:4, dry adhesive weight 4g / m²):

[0167]

[0168] Statistical results: Using the example sample of n=8, the Pearson correlation coefficient was used to calculate r(SCT-MD,ECT)=0.93 and r(SCT-CD,ECT)=0.90; the least squares method was used for linear regression.

[0169] Results Analysis: The data and statistical results in Table 15 reveal the transmission path of the reinforcing effect. The SCT values ​​of the face paper in all embodiments (e.g., 7.55 kN / m in the MD direction of Example 6) were significantly higher than those in all comparative examples (e.g., 6.40 kN / m in the MD direction of Comparative Example 1). Statistical analysis showed that the correlation coefficient r between SCT-MD and ECT was 0.93, and the correlation coefficient r between SCT-CD and ECT was 0.90. This extremely strong linear positive correlation indicates that the enhancement of SCT by the reinforcing agent of this invention can be directly and efficiently converted into an enhancement of ECT. This clear quantitative relationship provides a reliable basis for packaging engineering design, allowing for the accurate prediction and control of the final carton's ECT performance by controlling the face paper SCT index.

[0170] Application Example 9: Minimum Film Formation Temperature (MFFT) and Film Formation Adaptability.

[0171] Experimental Description: To evaluate the film-forming ability of the emulsion under industrial drying conditions, the minimum film-forming temperature of each sample emulsion was determined using an MFFT gradient plate according to ASTM D2354 standard. Simultaneously, actual film-forming tests were conducted on a non-porous glass plate at 105℃±5℃ for 10 min, and the continuity and integrity of the dry film were visually evaluated. The scoring criteria (0-5 levels) are defined as follows: Level 0: No film formation, powdering; Level 1: Severe cracking, large-area peeling; Level 2: Obvious cracks, discontinuous film (>50% defective area); Level 3: Slight cracks or pinholes (<50% defective area); Level 4: >Mostly continuous, with only a few pinholes (<5% defective area); Level 5: Smooth, continuous dry film with no visible defects. This index reflects the ability of the reinforcing agent to form an effective bond between the paperboard fibers.

[0172] Table 16 MFFT and film formation adaptability (n=3, average value):

[0173]

[0174] Results Analysis: MFFT is a key parameter determining whether an emulsion can form a continuous film at a given temperature. Table 16 shows that the MFFT of all examples was significantly reduced to the 49–54°C range, far lower than Comparative Example 1 (pure PHA, 68°C) and other comparative examples (64–67°C). This indicates that the polyphenol-amine synergistic anchoring of the interfacial layer effectively plasticizes and promotes film formation. The lower MFFT ensured sufficient coalescence capacity of the emulsion at a conventional industrial drying temperature of 105°C, forming a high-quality continuous dry film (integrity score ≥4.1), thus constructing an effective reinforcing network between fibers. In contrast, the comparative examples had excessively high MFFTs, resulting in poor film formation at the same temperature (score ≤2.4), failing to exert an effective reinforcing effect.

[0175] Application Example 10: Wet strength and water resistance under low-temperature drying (90℃±5℃).

[0176] Experimental Description: To investigate the applicability of the reinforcing agent under energy-saving drying (low-temperature) conditions, a drying process of 90℃±5℃ was simulated. Paperboards were prepared using the methods of Application Example 1 (wet-end addition, 1.0% dosage) and Application Example 2 (surface sizing, 1:4 ratio, 4 g / m²), but the drying temperature was reduced from 105℃ to 90℃, while the drying time remained unchanged (10 min). The WSR and Cobb strength of each paperboard sample were tested and compared under these low-temperature drying conditions. 60 And dryness and stiffness.

[0177] Table 17 Results of wet strength and water resistance tests under low-temperature drying (90℃±5℃):

[0178]

[0179] Results Analysis: Table 17 shows the good adaptability of the reinforcing agent of the present invention to low-temperature processes. Under drying conditions of 90°C, the WSR of all embodiments remained at a high level of 68%–74% (as in Example 6), Cobb 60 The value remained below 34 g / m², and the dryness and stiffness did not show a significant decrease. In contrast, all comparative examples showed a significant decrease in WSR to 61% or below at this low temperature, especially Comparative Example 1 (pure PHA), with a WSR of only 6%, and its water resistance (Cobb) was significantly lower. 60 The performance (46 g / m²) was also extremely poor. This is thanks to the low MFFT characteristic shown in Application Example 9, which enabled the emulsion in the example to form a film effectively at 90°C, while the comparative example could not form a continuous reinforcing layer due to the excessively low drying temperature, demonstrating the application potential of this invention in energy-saving and consumption-reducing processes.

[0180] Application Example 11: Total Organic Fluorines (TOF) Not Detected vs. Epichlorohydrin (ECH) Detection.

[0181] Experimental Description: To verify the environmental friendliness and safety compliance of the reinforcing agent of this invention, critical regulatory substances were tested on the emulsion samples of the examples. Total organic fluorine (TOF) was tested according to EN 14582:2016; free formaldehyde was tested according to ISO 11402:2004; and epichlorohydrin (ECH) was tested according to US EPAsW-846 8260D method. These tests aim to confirm that the product does not contain harmful substances such as AOX and formaldehyde associated with traditional wet-strength agents.

[0182] Table 18 Results of TOF, Free Formaldehyde and ECH Detection (Emulsion Samples, n=2):

[0183]

[0184] Results Analysis: The detection results in Table 18 were used to assess the content of specific regulated substances in the samples. The data shows that the total organic fluorine (TOF) content of all tested samples (including examples and comparative examples) was below the detection limit of 5 mg / kg, and was therefore determined to be "not detected." This indicates that no fluorinated surfactants or related additives were added to the tested system. Similarly, the free formaldehyde content of all samples was below the detection limit of 0.5 mg / kg (according to ISO 11402:2004), and the epichlorohydrin (ECH) content was below the detection limit of 5 µg / kg (according to US EPAsW-846 8260D), both of which were determined to be "not detected." This data indicates that the polyphenol-amine synergistic anchoring technology route adopted in this invention does not depend on epichlorohydrin and formaldehyde in its chemical composition, which differs from the chemical mechanism of traditional PAE or GPAM wet-strength agents.

[0185] Application Example 12: Validation of VOCs and 0.45µm permeabilization residue.

[0186] Experimental Description: This application example aims to determine the VOC content and 0.45µm membrane residue of an emulsion containing reinforcing agents. The VOC content in the finished emulsion was determined using gas chromatography (GC) according to ISO 11890-2:2020+Amd 1:2024. Simultaneously, following step 5 of the general emulsion preparation process, the emulsion sample was pressure filtered at 0.45µm after preparation. The residue on the filter membrane was collected and dried, and its mass percentage of the total solids content of the emulsion was calculated, i.e., the membrane residue rate.

[0187] Table 19 Test results of VOC and 0.45µm membrane residue (n=2):

[0188]

[0189] Results Analysis: The data in Table 19 were used to evaluate VOC content and 0.45µm membrane residue. According to the limitations of this invention, VOC content ≤0.3% and 0.45µm membrane residue ≤0.3% are the technical specifications of this invention.

[0190] The analysis results showed that the VOC content of all Examples 1–8 was within the range of 0.12%–0.23%, and the membrane residue was controlled within the range of 0.06%–0.22%, both of which were below the limit threshold of 0.3%, and were therefore deemed qualified. This data indicates that the preparation process and formulation system of the examples controlled the content of volatile components, and that the emulsion produced fewer agglomerates or coarse particles at the 0.45µm scale, which is consistent with the PDI ≤ 0.15 shown in Table 4.

[0191] In contrast, the VOC and residue parameters of Comparative Examples 1–3 were also deemed acceptable. However, the VOC of Comparative Example 4 (excessive polyphenols) and Comparative Example 5 (excessive amines) were 0.40% and 0.42%, respectively, and the membrane residue was 0.50% and 0.55%, respectively, both exceeding the limits of this invention. This indicates that the unbalanced formulation (as shown in Table 5, the viscosity of Comparative Examples 4 and 5 is >2000 mPa·s, and they have gelled) is related to the formation of more coarse particles or agglomerates in the system, making it unable to pass through 0.45 µm filtration.

[0192] Application Example 13: Storage stability and freeze-thaw recovery.

[0193] Experimental Description: This application example aims to evaluate the storage stability of the reinforcing agent emulsion under different conditions. In accordance with the acceptance criteria of this invention, the emulsion samples of Examples 1–8 and Comparative Examples 1–5 were subjected to the following tests:

[0194] 1) Store at 25℃ in the dark for 6 months and test the viscosity drift (criterion: ≤±15%);

[0195] 2) Place at 40℃ for 14 days (accelerated aging) and test viscosity drift (criterion: ≤±10%);

[0196] 3) After undergoing one freeze-thaw cycle (freezing at -10°C for 24 hours, then thawing at 25°C), the median particle size D was measured. 50 The upward shift value (criterion: <10nm);

[0197] 4) In a buffer system with pH 6.65–6.95, measure the absolute value of the change in zeta potential of the emulsion after undergoing the above test (criteria: ≤2.0mV).

[0198] Table 20. Storage stability and freeze-thaw recovery test results:

[0199]

[0200] Results Analysis: The data in Table 20 were used to evaluate the storage stability of the emulsion. Based on the criteria established according to this invention, the samples were subjected to storage tests at 25°C for 6 months, at 40°C for 14 days, and freeze-thaw cycles.

[0201] Test results show that all Examples 1-8 meet the criteria of the present invention. Under storage conditions of 25°C for 6 months, the viscosity drift of the examples was between -10% and +12%, within the ±15% criterion range. After accelerated aging at 40°C for 14 days, the viscosity drift was between -7% and +9%, within the ±10% criterion range. After undergoing freeze-thaw cycles at -10°C, the particle size D of all examples... 50 The upward shift values ​​were all <10 nm (e.g., +9 nm in Example 4 and +4 nm in Example 7), and the absolute values ​​of the zeta potential changes were all ≤2.0 mV (e.g., +1.8 mV in Example 4 and -0.5 mV in Example 7). These data indicate that the emulsions in the examples maintained physical stability under the tested storage and freeze-thaw conditions.

[0202] In contrast, all Comparative Examples 1–5 failed all stability tests. Comparative Examples 1–3 exhibited viscosity shifts exceeding the criterion (+14%–+20%) during long-term storage and accelerated aging, and their post-freeze-thaw particle size increase (+20–+35 nm) and potential shift (-3.0–+4.0 mV) also exceeded the specified ranges. Comparative Examples 4 and 5 (imbalanced formulation) had high viscosity after preparation and gelled or aggregated during storage and freeze-thaw tests.

[0203] Analysis of experimental results:

[0204] Based on the experimental data from Examples 1-13, the following conclusions can be drawn to demonstrate the technical characteristics, application feasibility, and environmental impact of this reinforcing agent:

[0205] The effect of polyphenol-amine synergistic effect on performance:

[0206] Data from Application Example 1 (wet-end addition) and Application Example 2 (surface sizing) show that the reinforcing effects of Examples 1-8 are all superior to those of the comparative examples. When added in the wet end (1.0% dosage), the WSR range of the examples is 72%–79% (Table 6); when sizing on the surface (1:4 ratio, 4 g / m² dry adhesive amount), the WSR range is 81.9%–83% (Table 8).

[0207] In contrast, the performance data of Comparative Examples 1–5 were lower under the same conditions. The WSR of Comparative Example 1 (pure PHA) was 6%. The WSRs of Comparative Examples 2 and 4 (amine-deficient polymers) and Comparative Examples 3 and 5 (polyphenol-deficient polymers) did not exceed 66%. The data indicate that the combination and ratio of PHA substrate, polyphenols, and amine polymers are key factors affecting the achievement of corresponding enhancement effects within the acid-neutral window (pH 6.65–6.95).

[0208] Correlation between physicochemical parameters (particle size, potential, PDI) and performance:

[0209] The physical property data in Tables 4 and 5 show that the physicochemical parameters of the emulsions in the examples all fall within the defined window: median particle size D 50 The wavelength range is 100–140 nm, the dispersion index (PDI) is ≤0.15, and the zeta potential at pH 6.8 is +16–+21 mV.

[0210] The materialization parameters of the comparative scale deviate from this window (e.g., D). 50 >175nm, PDI>0.22, potential is negative or significantly deviates from the range of +16 to +21mV). This difference in physicochemical parameters is related to the performance in application examples (1, 2, 5, 7), such as retention rate, stability when compounded with oxidized starch (as shown in the foaming volume of application example 5), etc.

[0211] Film formation characteristics and process temperature adaptability:

[0212] Data from Application Example 9 (MFFT test) shows that the minimum film-forming temperature (MFFT) for all examples is in the range of 49–54 °C (Table 16), lower than that of the comparative example (>64 °C) and pure PHA (68 °C). This indicates that the emulsions of the examples have film-forming conditions at an industrial drying temperature of 105 °C, forming a dry film with good continuity (integrity score ≥4.1).

[0213] Data from Application Example 10 (low-temperature drying) further show that, under drying conditions at 90°C, the WSR of the example remained at 68% to 74% (Table 17), while the WSR of the comparative examples was all below or equal to 61%.

[0214] The transfer of paper reinforcement effects to corrugated cardboard:

[0215] Data from Application Example 3 (ECT test) shows that corrugated board assembled using the face paper treated with the reinforcing agent of the example has an edge crush strength (ECT) increase of 15.0% to 16.7% (Table 10), while the comparative example has the highest ECT increase of 6.3%.

[0216] Data analysis from Application Example 8 (SCT-ECT Correlation) shows a high linear positive correlation between the short-pitch compressive strength (SCT) of the face paper and the ECT of the corrugated board (r≥0.90, Table 15). This indicates that an increase in SCT can be converted into a corresponding increase in ECT.

[0217] Durability and process stability assessment:

[0218] Data from Application Example 6 (damp heat cycling) shows that after 5 damp heat cycles, the WSR and ECT retention rates of the embodiment are both above 90% (Table 13), while the performance retention rate of the comparative example is lower (WSR is the lowest at 68%).

[0219] Meanwhile, data from Application Example 5 (sizing compatibility) showed that when the emulsion of the example was compounded with oxidized starch, the viscosity drift (≤+4.0%) and foaming volume (≤18mL) (Table 12) were both controlled at low levels, indicating that it has process compatibility.

[0220] Environmental-related indicator assessment:

[0221] Wet-side white water index: Application Example 7 (white water index) shows that the white water charge requirement (340 µeq / L) and turbidity (135 nTU) of the examples (such as Example 6) are lower than those of Comparative Example 1 (650 µeq / L; 220 nTU) (Table 14), indicating that the examples have a higher retention rate in the wet end.

[0222] Recyclability: Data from Application Example 4 (Re-pulping) shows that the paperboard treated in the example had an adhesion spot area below the criterion of 2.0 mm² / m² after re-pulping (Table 11), while some comparative examples failed due to excessive adhesion spots.

[0223] Compliance testing: Data from Application Example 11 (compliance testing) showed that no total organic fluorine (TOF), free formaldehyde, or epichlorohydrin (ECH) were detected in the emulsions of all examples (Table 18), thus circumventing the relevant regulatory substances contained in traditional wet strength agents (PAE, GPAM).

[0224] In summary, this invention achieves the desired effect by allowing nano-PHA particles (D) to be produced within an acid-neutral window (pH 6.5–7.5). 50A polyphenol-amine synergistic anchoring interface layer with a specific ratio was constructed on a surface (100–140 nm), and its physicochemical parameters (ζ-potential +16–+21 mV; MFFT ≤ 60 °C) were defined to prepare a paperboard reinforcing agent. Experimental data show that this scheme exhibits reinforcing effects in both wet-end addition and surface sizing (WSR ≥ 70%; ECT improvement > 15%), and demonstrates corresponding engineering application characteristics in terms of durability, recyclability, white water cleanliness, and low-temperature process adaptability. This provides a technical route for paperboard reinforcement that does not rely on formaldehyde or epichlorohydrin.

[0225] Those skilled in the art should understand that the above embodiments are merely exemplary and are not intended to limit the scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., made to the technical solutions of the present invention within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An acid-neutral nano-PHA paperboard reinforcing agent, characterized in that, The reinforcing agent is an aqueous emulsion, and is composed of the following components in parts by weight based on emulsion solids: Polyhydroxy fatty acid esters: 90–96.5 parts by weight; Polyphenols: 0.8–1.2 parts by weight; Amine polymers: 1.2–2.4 parts by weight; Nonionic surfactant: 0.28–0.50 parts by weight; Plasticizer: 0.35–0.80 parts by weight; And the physical properties of the emulsion satisfy: Solid content is 15%–45%; Viscosity at 25℃ ≤2000mPa·s; Median particle size D 50 The wavelength is 100–140 nm. The dispersion index (PDI) is ≤0.

15. In 10 mmol / L phosphate buffer, at pH 6.65–6.95, the zeta potential was measured to be +16–+21 mV, and the zeta potential drift was ≤1.5 mV within 60 min. Based on emulsion solids, the content of acrylic polymers, methacrylic polymers and their copolymers is <0.5%, and the content of polybutylene adipate terephthalate, polyvinyl alcohol and polylactic acid is <1.0% each; The reinforcing agent was determined according to EN 14582:2016 oxygen bomb combustion-ion chromatography, and no total organic fluorine was detected; the system did not contain free formaldehyde or epichlorohydrin. The minimum film-forming temperature of the reinforcing agent, as determined by ASTM D2354, is ≤60°C. The amine polymer is selected from one or more of quaternized chitosan, polyethyleneimine, or polydimethyldiallylammonium chloride, wherein the number average molecular weight of polyethyleneimine is 600 to 1800.

2. The reinforcing agent according to claim 1, characterized in that, The polyhydroxy fatty acid ester is selected from one or more of short-chain polyhydroxy fatty acid esters, medium- and long-chain polyhydroxy fatty acid esters, or polyhydroxy fatty acid esters formed by copolymerization of short-chain and medium- and long-chain monomers. The short-chain polyhydroxy fatty acid ester is selected from one or more of poly-3-hydroxybutyrate, poly-3-hydroxybutyrate-co-3-hydroxyhexanoate, poly-3-hydroxybutyrate-co-4-hydroxybutyrate, and poly-3-hydroxybutyrate-co-3-hydroxyvalerate. The medium- to long-chain monomers have 6 to 14 carbon atoms.

3. The reinforcing agent according to claim 1, characterized in that, The polyphenols are selected from one or more of hydrolyzed tannins, condensed tannins, ortho- and oxaloacetic acid or catechins, polyhydroxy flavonoids, or tea polyphenols; the hydrolyzed tannins are one or more of tannic acid, gallic acid, ellagic acid, and their salts; the condensed tannins are proanthocyanidins; the ortho- and oxaloacetic acid or catechins are one or more of catechins, dopamine, and their salts; the polyhydroxy flavonoids are one or more of quercetin and myricetin; and the tea polyphenols are one or more of epigallocatechin gallate and epicatechin gallate.

4. The reinforcing agent according to claim 1, characterized in that, The molar ratio of phenolic hydroxyl groups in polyphenols to protonated nitrogen in amine polymers is 1.1–1.

9.

5. The reinforcing agent according to claim 1, characterized in that, The plasticizer is triethyl citrate, tributyl citrate, or polyethylene glycol with a number average molecular weight of 200-600.

6. The reinforcing agent according to claim 1, characterized in that, The nonionic surfactant has an HLB value of 11.5 to 12.5 and is selected from one or more of fatty alcohol polyoxyethylene ethers, alkyl glucosides, polyoxyethylene-polyoxypropylene block copolymers, polyoxyethylene castor oil, or polyethylene glycol fatty acid esters.

7. The reinforcing agent according to claim 1, characterized in that, The emulsion has volatile organic compounds ≤0.3% and 0.45µm permeabilization residue ≤0.3%.

8. The reinforcing agent according to claim 1, characterized in that, The emulsion exhibits the following storage stability: viscosity drift ≤ ±15% after 6 months of storage at 25°C protected from light; viscosity drift ≤ ±10% after 14 days of storage at 40°C; and after one freeze-thaw cycle, the median particle size D... 50 The upward shift is <10 nm; and the absolute value of the zeta potential change measured at pH 6.65–6.95 is ≤2.0 mV.

9. A method for preparing the reinforcing agent according to claim 1, characterized in that, The method includes the following steps: Step 1. Dissolve polyhydroxyalkanoates in a water-nonionic surfactant system at 65℃~75℃, with a reaction time of ≥2000s. -1 Pre-emulsifying at a high shear rate for 30 min yields a pre-dispersion. Step 2. Cool the pre-dispersion to 30℃~40℃, and add the polyphenol diluent at a uniform rate over 10~30min to obtain the polyphenol modified dispersion; Step 3. Maintaining the pH at 6.65–6.95 in 10 mmol / L phosphate buffer, add the amine polymer solution dropwise at a constant rate over 30–120 min to the polyphenol-modified dispersion, with a stirring shear rate ≥2000 s. -1 The reaction mixture is obtained; Step 4. After the reaction mixture is allowed to stand for 10 minutes, it is then subjected to a 100µm pre-filtration and a 0.45µm pressure filtration to obtain the emulsion.

10. A wet-end method for enhancing the strength of paper or paperboard, characterized in that, The method comprises: adding 0.5% to 3.0% of the reinforcing agent described in claim 1 to the pulp based on oven-dry fiber content, and adding 0.3% to 1.5% of cationic starch and 0.02% to 0.10% of retention aid and microparticles; subsequently drying at 105℃±5℃ for 8 to 12 minutes; the resulting paper or paperboard has a wet strength retention rate ≥72%, and the white water index meets the following requirements: charge requirement ≤430µeq / L, turbidity ≤155nTU, and chemical oxygen demand ≤850mg / L.

11. The wet-end method for enhancing the strength of paper or paperboard according to claim 10, characterized in that, The retention aid and microparticles are one or more of bentonite, colloidal silica, aluminum silicate, or magnesium silicate. And / or the method is applicable to corrugated base paper, linerboard and kraft paper; And / or the edge crush strength improvement rate of the single-wall B-flute paperboard prepared by the method is ≥15%; and calculated with a sample size of ≥8, the Pearson correlation coefficient between the longitudinal short-distance compression strength and the edge crush strength of the paperboard is ≥0.

90.

12. A method for applying adhesive to a surface, characterized in that, The method includes: mixing the reinforcing agent described in claim 1 with oxidized starch at a solid mass ratio of 1:2 to 1:5 to prepare a sizing solution with a total solid content of 10%; applying the sizing solution by pressing or coating, controlling the dry glue content to be 2-6 g / m²; drying at 105℃±5℃ for 8-12 min; and achieving a wet strength retention rate of ≥78% for the resulting paper or paperboard. 60 The value is ≤32g / m², and the short-distance compressive strength is improved.

13. The surface sizing method according to claim 12, characterized in that, The method is applicable to corrugated base paper, linerboard and kraft paper; And / or the edge crush strength improvement rate of the single-wall B-flute paperboard prepared by the method is ≥15%; and calculated with a sample size of ≥8, the Pearson correlation coefficient between the longitudinal short-distance compression strength and the edge crush strength of the paperboard is ≥0.90.

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