A non-plastic coating defect gradient color development detection solution and a preparation and application method thereof

Abstract

This invention discloses a gradient colorimetric detection solution for defects in plastic-free coatings, its preparation method, and its application method. This addresses the problems of incompatibility between the detection solution and plastic-free coatings, insufficient detection accuracy, and safety and operability deficiencies. The detection solution comprises the following components by mass percentage: 0.08%–0.4% composite colorimetric indicator; 0.15%–1.8% food-grade wetting and penetrating agent; 0.1%–0.9% pH buffer adjuster; 0.06%–0.35% thickening and stabilizing agent; and the balance being deionized water. The composite colorimetric indicator is a mixture of bromocresol purple and neutral red, with a mass ratio of bromocresol purple to neutral red of 1:0.8–1.2. This invention enables rapid, non-destructive, gradient-quantitative, food-grade safe, and diffusion-resistant detection of plastic-free coatings.

Description

Technical Field

[0001] This invention relates to the field of nondestructive testing technology for coatings, and more specifically, to a gradient colorimetric detection solution for defects in plastic-free coatings, a method for preparing the gradient colorimetric detection solution for defects in plastic-free coatings, and a method for applying the gradient colorimetric detection solution for defects in plastic-free coatings. Background Technology

[0002] With the global implementation of plastic restriction policies, plastic-free water-based coatings such as starch-based, polyvinyl alcohol (PVA)-based, and plant-based adhesives have been widely used in food packaging paper cups, paper bowls, and composite paper products to replace traditional PE and PLA coated plastics. Plastic-free coatings achieve waterproof, oil-proof, and gas-barrier properties through their coating thickness and film density. However, in actual production, the coating process is prone to uneven coating, localized thin coatings, micron-level micropore defects, and incomplete curing, resulting in weak points in the coating. These weak areas significantly reduce the barrier properties, directly leading to leakage or water resistance failure in the final product, becoming a core pain point in the quality control of this type of material production.

[0003] Currently, colorimetric detection technologies for coatings are mainly divided into two categories, but both have insurmountable technical defects: First, existing testing solutions are incompatible with plastic-free coatings. Most commercially available testing solutions are designed for plastic film coatings (such as PE and PP) or metal anti-corrosion coatings. To achieve rapid penetration, these solutions typically contain high concentrations of organic solvents (such as acetone, ethanol, and n-hexane) or potent anionic surfactants (such as sodium dodecylbenzene sulfonate). Directly applying such solutions to hydrophilic starch-based or PVA-based plastic-free coatings would cause irreversible chemical swelling or dissolution of the coating structure, resulting in secondary damage and making non-destructive testing impossible.

[0004] Secondly, the detection accuracy is insufficient, only able to make qualitative judgments and unable to quantify the gradient. Conventional physical staining penetration methods (such as red pigment staining) only make binary judgments based on "spots / no spots," which can detect penetrating missed coatings or large pinholes, but cannot identify the "weak transition zone" (i.e., the sub-healthy area with a thickness of 3~7μm) where the coating thickness has become thinner but not yet fully exposed. This leads to a large number of thin-coated products with potential quality problems being misjudged as qualified products.

[0005] Thirdly, there are safety and operational deficiencies. Most testing solutions contain toxic and harmful substances, failing to meet the safety and hygiene requirements for testing food contact materials. Furthermore, due to a lack of rheological control methods, existing testing solutions easily diffuse and bleed on coating surfaces, resulting in blurred color boundaries for micron-level defects, making precise on-site marking and rapid screening impossible on the production line.

[0006] Therefore, there is an urgent need for a rapid detection method that can achieve non-destructive, gradient quantization, food-grade safety, and non-proliferation localization. Summary of the Invention

[0007] To address the aforementioned technical problems in existing technologies, this invention provides a gradient colorimetric detection solution for defects in plastic-free coatings, a method for preparing the solution, and a method for applying it. The solution achieves non-destructive testing of plastic-free coatings through a water-based non-ionic system; it utilizes a mixture of bromocresol purple and neutral red to convert differences in coating density into a three-color gradient signal, accurately distinguishing intact, weak, and uncoated areas; it uses food-grade raw materials to ensure safety; and it employs xanthan gum to regulate rheological properties and inhibit diffusion, achieving precise localization of micron-level defects.

[0008] In a first aspect, the present invention provides a gradient colorimetric detection solution for defects in plastic-free coatings, comprising the following components by mass percentage: Composite colorimetric indicator: 0.08%–0.4%; Food-grade wetting and penetrating agent: 0.15%–1.8%; pH buffer 0.1%–0.9%; Thickening stabilizer: 0.06%–0.35%; The remainder is deionized water; The composite colorimetric indicator is a mixture of bromocresol purple and neutral red, with a mass ratio of bromocresol purple to neutral red of 1:0.8 to 1.2.

[0009] Furthermore, the food-grade wetting and penetrating agent is polysorbate-80.

[0010] Furthermore, the pH buffer adjuster is a phosphate buffer system composed of sodium dihydrogen phosphate and disodium hydrogen phosphate.

[0011] Furthermore, the thickening stabilizer is food-grade xanthan gum.

[0012] Furthermore, the pH value of the colorimetric detection solution is stable at 6.8–7.2, the surface tension is 32–38 mN / m, and the dynamic viscosity is 15–30 mPa·s.

[0013] In a second aspect, the present invention provides a method for preparing a colorimetric detection solution as described in any of the first aspects, wherein the preparation environment temperature is controlled at 15–35°C and the stirring speed is controlled at 200–400 rpm throughout the process, and the preparation method includes the following steps: Step 1: Add the measured amount of deionized water to the stirred tank, turn on the stirrer, and add the pH buffer adjuster and thickening stabilizer in full proportion in sequence. Continue stirring for 40 to 60 minutes until the solution is uniform, transparent and free of suspended lumps to obtain the basic buffer solution. Step 2: Take 2 to 5 times the total mass of the composite colorimetric indicator as a pre-dissolving solvent, add the composite colorimetric indicator to the anhydrous ethanol, and stir at room temperature until it is completely transparent and dissolved to obtain the indicator pre-solution; slowly and uniformly add the indicator pre-solution to the basic buffer solution, and continue stirring until the system has a uniform color without local color patches after the addition is complete; Step 3: Add the full proportion of food-grade wetting and penetrating agent to the mixture obtained in Step 2, stir at room temperature for 15-25 minutes, then let it stand to defoam, filter to remove insoluble impurities, and the finished colorimetric detection solution is obtained.

[0014] Furthermore, the settling and defoaming time in step three is 25 to 35 minutes; the filtration uses a microporous filter membrane with a pore size of 0.45 to 1.0 μm.

[0015] Thirdly, the present invention provides a method for applying a gradient colorimetric detection solution for defects in plastic-free coatings, which uses any of the colorimetric detection solutions described in the first aspect to identify weak points in the plastic-free coating. The application method includes the following steps: S1. Lay the paper-based plastic-free coating sample to be tested flat on a horizontal platform, wipe away surface dust and oil, and ensure that the coating surface is dry and clean; S2. Apply the colorimetric test solution to the surface of the coating to be tested using a dropper or spray bottle. When using a dropper, the volume of a single drop should be 5–20 μL, and the drop density should be 0.5–2 drops / cm², so that the droplets are evenly distributed on the coating surface and adjacent droplets do not overlap. When using a spray bottle, the spraying pressure should be 0.05–0.1 MPa, the nozzle should be 10–15 cm away from the coating surface, and the solution should be sprayed evenly and thinly until there is no liquid flow or accumulation.

[0016] S3. After applying the solution, let it stand for 1-5 seconds and observe the color development of the coating surface: If it appears as a light blue-purple color, it is determined that the coating thickness meets the standard and the film layer is dense and intact; If it appears light pink, it is determined that the coating has thin coating, micropores, or incomplete curing. If it appears as a deep pink color, it is determined to be an area where the coating has been missed or damaged. S4. Mark the weak points according to the color development location. After the test is completed, wipe the coating surface with a cotton cloth moistened with water to remove the test solution residue.

[0017] Furthermore, the standard for judging the color development state in step S3 is to use a standard paper colorimetric card or a colorimeter for auxiliary quantitative determination: the light blue-purple corresponds to the CIE Lab color space range of L=62~68, a=-4~-1, b=-8~-4; the light pink corresponds to L=70~78, a=6~12, b=2~6; and the dark pink corresponds to L=80~88, a=15~22, b=5~10.

[0018] Furthermore, the plastic-free coating is an aqueous coating on the surface of food packaging paper, including starch-based coating, polyvinyl alcohol-based coating, or plant gum coating; the intact and dense thickness of the plastic-free coating is ≥8μm, the thickness of the weak and defective areas is 3-7μm, and the thickness of the uncoated areas is 0-2μm.

[0019] Beneficial effects

[0020] This invention's gradient colorimetric detection solution for defects in plastic-free coatings uses deionized water as the main solvent, completely eliminating organic solvents. It incorporates non-ionic Tween 80 as a wetting and penetrating agent, avoiding chemical swelling and dissolution of the hydrophilic plastic-free coating. This achieves zero damage to the coating during the testing process, allowing the product to be used normally or continue for other physicochemical tests after testing. Utilizing the complementary color effect of bromocresol purple and neutral red within a specific pH range, the difference in coating density is transformed into a visually perceptible three-color gradient—a light blue-purple for intact coatings, a light pink for thin areas, and a dark pink for uncoated areas. This represents a technological leap from a binary qualitative judgment of "presence or absence of uncoated areas" to a three-level quantitative assessment of "intact / weak / uncoated areas," accurately locating quality hazard areas that are thin but not yet uncoated. All components are made from raw materials that meet food contact material safety standards, using water as the sole solvent, without introducing any toxic or harmful organic solvents. After testing, wiping with water leaves no residue, meeting the hygiene and safety requirements of food packaging production lines. The addition of trace amounts of xanthan gum adjusts the viscosity of the detection solution to a suitable range, effectively suppressing the lateral diffusion of droplets on the coating surface, concentrating the colorimetric signal at the defect location with clear boundaries, and realizing the pinpoint marking of micron-level defects, making it suitable for precise real-time quality inspection on the production line.

[0021] The preparation method of this invention, through a standardized and fully quantitative three-step preparation process, including key control steps such as pre-dissolution, gradient addition, and defoaming filtration, ensures the uniformity and storage stability of the test solution between batches. The process conditions are mild, the operation is simple, and it does not rely on special equipment, making it suitable for industrial mass production.

[0022] The application method of this invention achieves rapid on-site testing without the need for large instruments through standardized liquid application operations and clear three-color interpretation standards. The matching standard colorimetric card or colorimeter can further eliminate human interpretation errors. After testing, there is no residue after wiping with water. It is convenient, repeatable and adaptable to production lines. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0024] Figure 1 A schematic flowchart illustrating a method for preparing a gradient colorimetric detection solution for defects in plastic-free coatings, provided for an embodiment of the present invention; Figure 2 A schematic flowchart illustrating the application method of a gradient colorimetric detection liquid for defects in plastic-free coatings provided for an embodiment of the present invention; Figure 3 A gradient colorimetric detection result diagram of different coating states of non-plastic coating provided for an embodiment of the present invention; Figure 4 This is a graph showing the pH change during room temperature storage for Embodiment 1 and Comparative Example 5 of the present invention. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] In this invention, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0027] It should be noted that if the embodiments of the present invention involve directional indicators (such as up, down, left, right, front, back, etc.), the directional indicators are only used to explain the relative positional relationship and movement of the components in a specific posture. If the specific posture changes, the directional indicators will also change accordingly.

[0028] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are feasible for those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0029] To better understand the above technical solutions, the present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0030] An embodiment of the present invention provides a gradient colorimetric detection solution for defects in plastic-free coatings, comprising the following components by mass percentage: Composite colorimetric indicator: 0.08%–0.4%; Food-grade wetting and penetrating agent: 0.15%–1.8%; pH buffer 0.1%–0.9%; Thickening stabilizer: 0.06%–0.35%; The remainder is deionized water; The composite colorimetric indicator is a mixture of bromocresol purple and neutral red, with a mass ratio of bromocresol purple to neutral red of 1:0.8 to 1.2.

[0031] The core concept of the gradient colorimetric detection solution for plastic-free coatings provided by this invention lies in: constructing a mild water-based detection system using deionized water as the sole solvent; using the nonionic surfactant Tween 80 to regulate surface tension and achieve rapid wetting and penetration; using food-grade xanthan gum to regulate rheological properties and inhibit lateral diffusion; using a phosphate buffer system to stabilize the background pH of the detection solution; and using a mixture of bromocresol purple and neutral red as a gradient colorimetric indicator. The synergistic effect of these five components enables the detection solution to exhibit differentiated colorimetric responses on coating surfaces with varying densities, thereby achieving a visual grading and determination of coating quality.

[0032] The composite colorimetric indicator of the present invention is composed of bromocresol purple and neutral red in a mass ratio of 1:0.8 to 1.2. The total mass percentage of the composite indicator in the detection solution is 0.08% to 0.4%, preferably 0.15% to 0.27%.

[0033] Bromocresol purple is a sulfonyl phthalein acid-base indicator with a color change range of pH 5.2–6.8. It appears yellow under acidic conditions and purple-red under alkaline conditions. The phenolic hydroxyl group in its molecular structure loses a proton under alkaline conditions, resulting in a conjugated rearrangement that causes a red shift in the absorption spectrum, thus producing a color change visible to the naked eye.

[0034] Neutral red is a phenazine-based acid-base indicator with a color change range of pH 6.8–8.0. It appears red under acidic conditions and amber to yellow under alkaline conditions. In this invention, neutral red is not used as an independent pH indicator, but rather as a color contrast enhancer. Its function is as follows: when bromocresol purple appears purple in well-coated areas, the amber color of neutral red superimposes with the purple to form a clear light blue-purple; in weak areas, bromocresol purple partially turns yellow, superimposing with the red of neutral red to form a light pink; in uncoated areas, bromocresol purple completely turns yellow, superimposing with the red of neutral red to form a deep pink.

[0035] When used alone, bromocresol violet only exhibits a two-color change from yellow to purple in the pH range of 5.2–6.8, failing to form a three-color gradient. With the addition of neutral red, the color superposition effect of the two in different pH ranges achieves a three-stage transition from deep pink to light pink to light blue-purple within a pH range of approximately 5.0–7.2. Specifically: when pH < 5.5, bromocresol violet is yellow and neutral red is red, resulting in a deep pink; when pH 6.0–6.5, bromocresol violet is in a yellow-purple transition state while neutral red remains red, resulting in a light pink; when pH > 6.8, bromocresol violet is purple and neutral red turns amber, resulting in a light blue-purple.

[0036] The pKa values ​​of the two are approximately 6.3 (bromocresol purple) and 6.8 (neutral red), respectively, forming a gradient response within the pH range of 6.0–7.0. This precisely covers the pH variation range of the microenvironment for the plastic-free coating, from intact (pH≈7.0) to thin (pH≈6.0–6.5) to incomplete coating (pH≈5.0–5.5). Moreover, the deep pink, light pink, and light blue-purple colors exhibit extremely high contrast against a white paper background, allowing for visual interpretation without the need for instruments.

[0037] The deionized water of this invention is first-grade deionized water (conductivity ≤ 5 μS / cm), which serves as the sole base solvent. Its mass percentage is adjusted according to the amount of the other four components to achieve a total mass of 100%.

[0038] This invention uses the nonionic surfactant Tween 80 (polysorbate-80), with a mass percentage of 0.15%–1.8%, preferably 0.8%–1.2%. Tween 80 is polyoxyethylene sorbitan monooleate, a nonionic surfactant. Its molecular structure contains a long-chain fatty acid hydrophobic group and multiple polyoxyethylene hydrophilic chains, exhibiting excellent emulsifying, wetting, and dispersing properties. Its hydrophilic-lipophilic balance (HLB) value is approximately 15, classifying it as a hydrophilic surfactant. In water, it can form a transparent micelle solution, reducing the surface tension of deionized water from 72 mN / m to 32–38 mN / m, enabling the detection liquid to wet the walls of the micron-scale capillary channels in the coating, overcoming capillary resistance, and accelerating liquid penetration into the defect area. Moreover, Tween 80 is a nonionic surfactant that does not dissociate into charged ions in aqueous solution. It does not interact strongly with charged groups in the coating (such as the hydroxyl groups of PVA and the carboxyl groups of starch), thus it does not destroy the hydrogen bond network structure between the molecular chains of the coating, enabling non-destructive testing.

[0039] This invention selects a sodium dihydrogen phosphate-disodium hydrogen phosphate compound buffer salt system as a pH buffer regulator, with a mass percentage of 0.1%–0.9%, preferably 0.3%–0.5%. Sodium dihydrogen phosphate (NaH₂PO₄) and disodium hydrogen phosphate (Na₂HPO₄) form a weak acid-conjugate base buffer pair with a pKa₂≈7.20, exhibiting the strongest buffering capacity in the pH range of 6.8–7.2. When a small amount of acidic or basic substances are introduced into the detection solution, the buffer pair maintains pH stability through the following equilibrium reaction: H₂PO₄ - H + +HPO4² - When H appears in the system + At that time, HPO4² - It combines with to form H2PO4 - When OH appears in the system - At that time, H2PO4 - It reacts to produce HPO4² - Water. This buffering mechanism stabilizes the background pH of the detection solution at 6.8–7.2. It keeps bromocresol violet in its purple state and neutral red in the red / amber transition zone, providing a sensitive starting point for subsequent gradient color changes.

[0040] Fluctuations in ambient temperature and humidity can cause deionized water to absorb CO2 from the air and become slightly acidic, or cause it to evaporate and concentrate. The presence of buffer salts can effectively counteract these external interferences, ensuring consistency of test results across batches, seasons, and regions. Furthermore, because the buffer salts place the background pH of the test solution near the color change critical point, even a small pH drop (approximately 0.5–1.0 pH units) caused by weak areas of the coating can trigger a significant color change response in the indicator, thus improving detection sensitivity.

[0041] The present invention uses food-grade xanthan gum as a thickening and stabilizing agent, with a mass percentage of 0.06% to 0.35%, preferably 0.10% to 0.25%.

[0042] Xanthan gum is a high-molecular-weight extracellular heteropolysaccharide produced by the fermentation of *Xanthomonas campestris*. Its backbone is composed of β-D-glucose linked by β-1,4 glycosidic bonds, while its side chains contain mannose and glucuronic acid. In aqueous solution, xanthan gum molecules form a helical network structure through hydrogen bonding and van der Waals forces, endowing the solution with unique rheological properties—shear thinning and weak gel behavior. These properties allow it to effectively increase the viscosity of aqueous solutions at extremely low concentrations, with minimal viscosity change with temperature, exhibiting good thermal and acid-base stability. This achieves the effects of inhibiting lateral diffusion, ensuring vertical penetration, and extending shelf life.

[0043] The core mechanism by which the detection liquid of this invention achieves gradient color development can be summarized as a four-order causal response chain: "coating density → water penetration rate → microenvironment pH change → optical gradient color development", which is described in detail below: First stage: The physical structure of the coating determines the water penetration rate. The barrier performance of a plastic-free coating depends on its coating thickness and film density. In a well-formed coating (thickness ≥ 8 μm), the molecular chains are densely arranged with a low free volume fraction, making it almost impossible for water molecules to penetrate into the coating interior; they remain only on the surface. In a thin coating (thickness 3–7 μm), there are micron-sized pinholes or increased free volume between molecular chains, forming capillary channels, allowing water molecules to slowly penetrate into the coating interior under capillary action. In areas with poor coating (thickness 0–2 μm), the paper base fibers are exposed, exhibiting a porous structure, allowing water molecules to rapidly and extensively penetrate into the paper base without hindrance.

[0044] The second stage: Differences in water permeation rates lead to changes in the local microenvironment pH. Deionized water, as the mass transfer medium, ensures that the permeation process is not interfered with by external impurity ions. When water molecules permeate into the coating, they dissolve and carry away soluble substances from the coating material or paper base. PVA-based coatings may retain trace amounts of acetic acid (a byproduct of PVA alcoholysis) during production, starch-based coatings may introduce carboxyl groups during oxidative modification, and the paper base contains a large amount of weakly acidic substances (such as lignin degradation products, alum hydrolysis products, rosin sizing agents, etc.). In intact coatings, water molecules cannot penetrate, and the microenvironment pH remains at the background pH of the test solution (approximately 6.8–7.2). In weak coatings, a small amount of water permeates and dissolves weakly acidic substances, causing the local pH to drop to approximately 6.0–6.5. In areas with poor coating, a large amount of water permeates into the paper base, and a large amount of acidic substances dissolve rapidly, further lowering the local pH to approximately 5.0–5.5.

[0045] The third stage: pH differences in the microenvironment trigger the gradient color development of the indicator. Bromocresol purple (pKa≈6.3) and neutral red (pKa≈6.8) exhibit complementary color-changing responses within the pH range of 5.0–7.2. In a neutral environment with an intact coating, bromocresol purple appears purple and neutral red appears amber, resulting in a light blue-purple color. In a weakly acidic environment with a thin coating, bromocresol purple transitions to a yellow-purple transition state, while neutral red remains red, resulting in a light pink color. In a stronger acidic environment in areas with blemishes, bromocresol purple completely transforms into yellow, while neutral red remains red, resulting in a deep pink color. The color superposition effect of the two indicators allows for clear visual distinction between the three coating states.

[0046] Through the above-mentioned fourth-order causal chain, the present invention transforms the static information of the coating physical structure into a dynamic chemical signal output, realizing a technological leap from the binary qualitative judgment of "whether there is a missing coating" to the three-level quantitative evaluation of "intact / weak / missing coating".

[0047] The present invention also provides a method for preparing the colorimetric detection solution as described in any of the preceding claims, wherein the preparation environment temperature is controlled at 15–35°C, and the stirring speed is controlled at 200–400 rpm throughout the process. Figure 1 As shown, the preparation method includes the following steps: Step 1: Add the measured amount of deionized water to the stirred tank, turn on the stirrer, and add the pH buffer adjuster and thickening stabilizer in full proportion in sequence. Continue stirring for 40 to 60 minutes until the solution is uniform, transparent and free of suspended lumps to obtain the basic buffer solution. Step 2: Take 2 to 5 times the total mass of the composite colorimetric indicator as a pre-dissolving solvent, add the composite colorimetric indicator to the anhydrous ethanol, and stir at room temperature until it is completely transparent and dissolved to obtain the indicator pre-solution; slowly and uniformly add the indicator pre-solution to the basic buffer solution, and continue stirring until the system has a uniform color without local color patches after the addition is complete; Step 3: Add the full proportion of food-grade wetting and penetrating agent to the mixture obtained in Step 2, stir at room temperature for 15-25 minutes, then let it stand to defoam, filter to remove insoluble impurities, and the finished colorimetric detection solution is obtained.

[0048] The preparation of the detection solution described in this invention is carried out at room temperature (15–35°C) without the need for heating or cooling. The stirring equipment is a sealed stirring vessel with a stainless steel stirring paddle, and the power of the stirring motor is determined according to the production scale. The stirring speed is controlled at 200–400 rpm, preferably 250–350 rpm, to ensure that all components are fully dissolved without causing excessive shear.

[0049] In step one, the pH buffer stabilizer for preparing the basic buffer solution is sodium dihydrogen phosphate and disodium hydrogen phosphate, and the thickening stabilizer is xanthan gum. Xanthan gum should not be added simultaneously with the indicator, otherwise it is easy to form an insoluble complex. Furthermore, xanthan gum should be added slowly into the vortex at the liquid surface to avoid the formation of insoluble clumps from a large amount added at once.

[0050] In step two, during the pre-dissolution and dropwise addition of the composite indicator, the dropwise addition rate of the indicator ethanol pre-solution is controlled at 1 / 10 to 1 / 5 of the total amount added per minute. The stirring speed is maintained at 300 rpm throughout the dropwise addition process to ensure rapid and uniform dispersion of the pre-solution after it enters the reactor. After the dropwise addition is complete, stirring continues for 15 to 20 minutes until the system has a uniform color without any localized color patches or bands.

[0051] Step three involves the compounding, defoaming, and filtration of the wetting and penetrating agent. The full proportion of Tween 80 is added to the mixture obtained in Step two, and the mixture is stirred at 300 rpm at room temperature for 15–25 minutes, preferably 20 minutes. After stirring, the mixture is transferred to a settling tank and allowed to stand for 25–35 minutes to completely eliminate any micro-air bubbles introduced during stirring. After defoaming, the mixture is filtered using a microporous membrane or filter cartridge with a pore size of 0.45–1.0 μm to remove insoluble impurities and any trace amounts of insoluble matter, yielding the final test solution.

[0052] Each batch of finished test solution must pass the following quality inspections before it can leave the factory: Visual inspection: It should be a light purple, homogeneous, transparent liquid, without layering, sediment, suspended matter, or visible impurities.

[0053] pH value determination: Use a calibrated pH meter for measurement, and the value should be within the range of 6.8 to 7.2.

[0054] Surface tension measurement: The measurement should be performed at 25℃ using the platinum plate method or the pendant drop method, and should be within the range of 32 to 38 mN / m.

[0055] Dynamic viscosity was measured using a rotational viscometer at 25°C and a shear rate of 100 s. - ¹The measurement should be performed under the conditions of 15–30 mPa·s.

[0056] Color development performance verification: Take one standard starch-based coating test piece each of known intact, thin and missing coating. After applying the test solution, the test pieces should show light blue-purple, light pink and dark pink respectively within 3 seconds, and the color difference ΔE between the three colors should be greater than 12.

[0057] This invention also provides a method for applying a gradient colorimetric detection solution for defects in plastic-free coatings, using any of the colorimetric detection solutions described above to identify weak points in the plastic-free coating, such as... Figure 2 As shown, the application method includes the following steps: S1. Lay the paper-based plastic-free coating sample to be tested flat on a horizontal platform, wipe away surface dust and oil, and ensure that the coating surface is dry and clean; S2. Apply the colorimetric test solution to the surface of the coating to be tested using a dropper or spray bottle. When using a dropper, the volume of a single drop should be 5–20 μL, and the drop density should be 0.5–2 drops / cm², so that the droplets are evenly distributed on the coating surface and adjacent droplets do not overlap. When using a spray bottle, the spraying pressure should be 0.05–0.1 MPa, the nozzle should be 10–15 cm away from the coating surface, and the solution should be sprayed evenly and thinly until there is no liquid flow or accumulation.

[0058] S3. After applying the solution, let it stand for 1-5 seconds and observe the color development of the coating surface: If it appears as a light blue-purple color, it is determined that the coating thickness meets the standard and the film layer is dense and intact; If it appears light pink, it is determined that the coating has thin coating, micropores, or incomplete curing. If it appears as a deep pink color, it is determined to be an area where the coating has been missed or damaged. S4. Mark the weak points according to the color development location. After the test is completed, wipe the coating surface with a cotton cloth moistened with water to remove the test solution residue.

[0059] When using the colorimetric detection solution of this invention, the ambient temperature is controlled at 15–40°C and the relative humidity is controlled at 30%–70%, covering the conventional environmental conditions of general factory workshops and laboratories.

[0060] This invention provides two application methods to adapt to different detection scenarios: Method A (Rapid Sampling Inspection on Production Line, Dropper Addition): Use a 10μL quantitative dropper or precision pipette, with a single drop volume of 5–20μL (preferably 10μL). The dropping density is 0.5–2 drops / cm² (preferably 1 drop / cm²), ensuring the droplets are evenly distributed on the coating surface, with adjacent droplets spreading naturally without overlapping or merging. This method is suitable for point sampling inspection, small-area samples, or situations requiring precise defect location.

[0061] Method B (Large-area roll inspection, spraying): Use a small, low-pressure sprayer, controlling the spraying pressure to 0.05–0.1 MPa. Keep the nozzle 10–15 cm away from the coating surface, and apply a thin, uniform spray in a single pass, aiming to form a thin, uniform liquid film on the coating surface without any dripping or accumulation. This method is suitable for rapid screening of large areas across the entire roll of paper, allowing for direct observation of the overall defect distribution.

[0062] Observe and interpret the results within 1 to 5 seconds after application, with the optimal observation window being 2 to 3 seconds. After 5 seconds, the water in the test solution gradually evaporates, and the increased indicator concentration may lead to color distortion, making it unsuitable as a basis for judgment.

[0063] The color development of the coating surface is observed with the naked eye and judged according to the following standards: Light blue-purple area: Corresponds to a coating thickness ≥ 8μm, a dense and complete film layer, free from thin coating, micropores, and curing defects, and meets the barrier performance standards. When measured with a colorimeter, its CIE Lab color space range is: L = 62~68, a = -4~-1, b =-8~-4.

[0064] Light pink area: Corresponding to a coating thickness of 3–7 μm, the film contains micro-pinholes, localized thin coating, or incomplete curing, and is judged as a weak defect area in the coating. The barrier performance has decreased, but the coating has not completely failed. Its Lab range is: L=70–78, a=6–12, b =2~6.

[0065] Dark pink area: Corresponds to a coating thickness of 0–2 μm, where the coating is completely missed or the film is damaged, exposing the paper base, and the barrier properties are completely lost. Its Lab range is: L=80–88, a=15–22, b =5~10.

[0066] The gradient colorimetric detection results of different coating states of the non-plastic coating in the embodiments of the present invention are as follows: Figure 3 As shown, from left to right, these are comparison photos of the color rendering effect under different coating thicknesses (normal coating vs. thin coating vs. missed coating).

[0067] For high-precision testing, a handheld colorimeter can be used to measure the Lab value for quantitative judgment, eliminating subjective errors from visual interpretation. For rapid on-site screening, standard paper colorimetric cards or plastic color charts can be provided as a reference standard for visual interpretation. Before use, the colorimetric card and the sample to be tested should be compared under the same light source.

[0068] After marking the coating, wipe the surface twice with a clean, lint-free cotton cloth or damp wipe moistened with pure water, and allow it to air dry at room temperature or be gently blown dry. After wiping, re-measure the tested area using a colorimeter and compare the color difference with the untested original coating area. When ΔE < 2, it can be determined that there is no residual color indicator or additive, the coating has not been chemically damaged, and it can proceed to the next production process or be used for other physicochemical property tests.

[0069] After the testing and marking are completed, wipe the coating surface twice with a clean cotton cloth soaked in pure water, and let it air dry at room temperature.

[0070] After wiping with the test liquid of the present invention, the coating surface was retested with a colorimeter. The color difference ΔE between the coating and the original untested coating was less than 2. There were no indicator or additive residues, and the waterproof and oil-proof properties of the coating were not changed.

[0071] The detection solution of this invention uses deionized water as the main solvent, completely eliminating organic solvents. Combined with non-ionic Tween 80 as a wetting and penetrating agent, it fundamentally eliminates the risk of chemical corrosion to hydrophilic non-plastic coatings (starch-based, PVA-based, plant-based adhesives). By combining bromocresol purple and neutral red, the difference in coating density is converted into a three-color gradient signal, simultaneously identifying three states: intact coatings, weak defects, and uncoated areas. Furthermore, this invention completes color development within 1-5 seconds after application, far exceeding the 60-300 seconds of existing technologies. This invention also uses a trace amount of xanthan gum to adjust the viscosity of the detection solution to 15-30 mPa·s, effectively suppressing the lateral spread of droplets on the coating surface. This "locks" the color signal to the defect area, significantly improving boundary clarity.

[0072] The detection solution of this invention exhibits excellent stability under sealed storage conditions at room temperature. The phosphate buffer system ensures that the pH value of the product does not drift with ambient temperature fluctuations during storage; the three-dimensional network structure of xanthan gum effectively prevents the sedimentation and aggregation of indicator particles through steric hindrance and the suspension effect of a weak gel network, and reduces water activity, thereby slowing down the oxidative degradation of the indicator. Accelerated testing has verified that the product shows no stratification or precipitation after 12 months of storage at room temperature, with pH fluctuation ≤0.1, and a color difference ΔE <4 compared to freshly prepared samples, meeting the requirements for commercial storage and distribution.

[0073] To further illustrate the influence of the key components and dosages of this embodiment on the performance of the colorimetric detection solution, the colorimetric detection solutions prepared in Examples 1 to 5 and Comparative Examples 1 to 6 are provided and tested.

[0074] Example 1

[0075] This embodiment provides a colorimetric detection solution, which is prepared in the following manner.

[0076] Formula: 0.15 parts bromocresol purple, 0.12 parts neutral red, total mass percentage of composite indicator is 0.27%, ratio of the two is 1:0.8. 0.8 parts Tween 80, 0.4 parts sodium dihydrogen phosphate-disodium hydrogen phosphate buffer, 0.15 parts food-grade xanthan gum, balance (98.38 parts) of primary deionized water, total mass 100 parts.

[0077] Preparation method: At an ambient temperature of 25℃, add a measured amount of primary deionized water to a stirred tank and start stirring at 250 rpm. Add sodium dihydrogen phosphate-disodium hydrogen phosphate buffer salt and food-grade xanthan gum sequentially, stirring continuously for 50 minutes until the solution is homogeneous, transparent, and free of suspended lumps, obtaining the basic buffer solution. Use anhydrous ethanol, three times the total mass of the composite colorimetric indicator, as a pre-dissolving solvent. Add the indicator to the anhydrous ethanol and stir at room temperature for 12 minutes until completely transparent and dissolved, obtaining the indicator pre-solution. Slowly and uniformly add the pre-solution dropwise to the basic buffer solution, maintaining a stirring speed of 300 rpm throughout the addition. After the addition is complete, continue stirring for 15 minutes until the system has a uniform color without localized color patches. Add Tween 80 to the mixture and stir at 300 rpm at room temperature for 20 minutes. Transfer to a settling tank and allow to stand for 30 minutes to defoam. Filter using a 0.65 μm microporous membrane to remove insoluble impurities, obtaining the final test solution.

[0078] The finished product testing solution has a pH value of 6.8–7.2, a surface tension of 35 mN / m, and a dynamic viscosity of 22 mPa·s. Coating weight change: The coating mass change after wiping and drying was <0.3%, with no film dissolution or loss.

[0079] Example 2

[0080] Other conditions are the same as in Example 1, except that the formula is adjusted as follows: 0.0444 parts of bromocresol purple, 0.0356 parts of neutral red (the total mass percentage of the composite indicator is 0.08%, and the ratio is 1:0.8), 0.15 parts of Tween 80, 0.1 parts of phosphate buffer, 0.06 parts of xanthan gum, and the balance of 99.61 parts of primary deionized water.

[0081] The viscosity of the test solution is low, at 16 mPa·s.

[0082] Example 3

[0083] Other conditions are the same as in Example 1, except that the formula is adjusted as follows: 0.218 parts of bromocresol purple, 0.182 parts of neutral red (the total mass percentage of the composite indicator is 0.4%, and the ratio is 1:0.8), 1.8 parts of Tween 80, 0.9 parts of phosphate buffer, 0.35 parts of xanthan gum, and the balance of 96.55 parts of primary deionized water.

[0084] With a viscosity of up to 28 mPa·s, it exhibits no smearing or diffusion, making it suitable for high-precision defect detection in laboratories.

[0085] Example 4

[0086] Other conditions are the same as in Example 1, except that the formula is adjusted to: 0.15 parts bromocresol purple, 0.12 parts neutral red (the total mass percentage of the composite indicator is 0.27%, and the ratio is 1:0.8), that is, the ratio of bromocresol purple to neutral red is taken as the lower limit of the present invention, Tween 80 is 0.8 parts, phosphate buffer is 0.4 parts, xanthan gum is 0.15 parts, and primary deionized water is the balance 98.38 parts.

[0087] Example 5

[0088] Other conditions are the same as in Example 1, except that the formula is adjusted to: 0.123 parts bromocresol purple, 0.147 parts neutral red (the total mass percentage of the composite indicator is 0.27%, and the ratio is 1:1.2), that is, the ratio of bromocresol purple to neutral red is taken as the upper limit of the present invention, Tween 80 is 0.8 parts, phosphate buffer is 0.4 parts, xanthan gum is 0.15 parts, and primary deionized water is the balance 98.38 parts.

[0089] Comparative Example 1

[0090] Other conditions are the same as in Example 1, except that the composite indicator is replaced with 0.27% bromocresol purple, neutral red is removed, and deionized water is added to bring the total to 100%.

[0091] Comparative Example 2

[0092] Other conditions are the same as in Example 1, except that the composite indicator is replaced with 0.27% neutral red, bromocresol purple is removed, and deionized water is added to bring the total to 100%.

[0093] Comparative Example 3

[0094] Other conditions are the same as in Example 1, except that Tween 80 is removed and the amount of deionized water is increased to maintain the total mass at 100%.

[0095] Comparative Example 4

[0096] Other conditions are the same as in Example 1, except that xanthan gum is removed and the amount of deionized water is increased to maintain a total mass of 100%.

[0097] Comparative Example 5

[0098] Other conditions are the same as in Example 1, except that the sodium dihydrogen phosphate-disodium hydrogen phosphate buffer salt is removed, and the amount of deionized water is increased accordingly to maintain the total mass at 100%.

[0099] Comparative Example 6

[0100] The other conditions were the same as in Example 1, except that Tween 80 was replaced with an equal mass of sodium dodecylbenzenesulfonate (an anionic surfactant).

[0101] The colorimetric detection solutions prepared in the above embodiments and comparative examples were subjected to the following performance tests: 1. Color development speed test Apply the test solution (10 μL / drop) to the surface of a standard starch-based coated specimen (one-third intact, one-third weak, and one-third uncoated). Record the time (in seconds) from when the droplet contacts the coating surface until the color stabilizes and no longer changes. Test each sample 10 times and take the average value.

[0102] 2. Color difference (ΔE) test

[0103] The CIELab colorimetric values ​​of the colored areas were measured 3 seconds after application using a handheld colorimeter (Konica Minolta CM-2600d). Using the Lab value of the intact coating area as a reference, the color difference ΔE between the weak and uncoated areas and the intact area was calculated. A larger ΔE value indicates better color differentiation. Formula: ΔE = [(ΔL)² + (Δa)² + (Δb)²] )²]^½.

[0104] 3. Minimum identifiable coating thickness test

[0105] A series of standard test pieces with starch-based coatings of varying thicknesses (2μm, 3μm...10μm) were prepared. After the test solution was applied, the color development results were observed. The minimum identifiable thickness was defined as the minimum thickness at which a color difference that could be clearly distinguished from that of an intact coating (≥8μm) was visible to the naked eye.

[0106] 4. Minimum identifiable micropore diameter test

[0107] A series of interconnected micropore arrays with different diameters (3μm, 4μm, 5μm, 8μm, 10μm, 15μm, 20μm, 50μm) were prepared on the surface of an intact starch-based coating. After the detection liquid was dropped onto the micropores, the appearance of colored spots was observed. The smallest micropore diameter that could clearly distinguish the colored spots was taken as the smallest identifiable micropore diameter.

[0108] 5. Diffusion Diameter Test

[0109] 10 μL of test solution was dropped onto the surface of a standard starch-based intact coating. After 30 seconds, the spreading diameter of the droplet on the coating surface was measured. The smaller the spreading diameter, the better the anti-diffusion performance.

[0110] 6. Cobb value test (non-destructive verification)

[0111] Referring to GB / T 1540 "Determination of water absorption of paper and paperboard (Cobb method)", the 30-second Cobb water absorption value of the coated test pieces was tested before and after the test (after wiping and drying with clean water). The smaller the difference in Cobb value before and after the test, the less damage the test solution caused to the coating.

[0112] 7. Storage stability test

[0113] The test solution was sealed in a transparent glass bottle and stored at room temperature away from light. Samples were taken at 0, 1, 3, 6, and 12 months to observe the appearance (layering, precipitation), measure the pH value, and test the color development performance on a standard starch-based coated test piece. The color difference ΔE between the test solution and the fresh sample was calculated.

[0114] The test results of the samples prepared in each embodiment and each comparative example are shown in Table 1 and Table 2 below.

[0115] Table 1: Test results of each embodiment and each comparative example.

[0116]

[0117] Note: Comparative Example 5 suffers from inconsistent color development due to pH drift. Intact areas may appear pink while defective areas may appear purple. The color gradation is inconsistent in different areas of the same sample, making it impossible to obtain valid data.

[0118] Table 2: Storage stability test data (Comparison of Example 1 and Comparative Example 5)

[0119] Furthermore, in Example 2, slight discoloration occurred after 6 months of storage at room temperature, with a color difference ΔE = 2.5, but no stratification occurred. In Example 3, no stratification occurred after 12 months of storage at room temperature, with a color difference ΔE of 3.2 and pH fluctuation ≤ 0.1.

[0120] The pH change curves of Example 1 and Comparative Example 5 at room temperature storage are shown below. Figure 4 As shown, Example 1, containing a pH buffer system, maintained a stable pH within the effective colorimetric range during a 12-month storage period, while Comparative Example 5, without a buffer, failed within one month, with the pH dropping to 4.6. This comparison directly demonstrates the irreplaceable role of the sodium dihydrogen phosphate-disodium hydrogen phosphate buffer system in ensuring the stability of commercially available test solutions during storage.

[0121] Example 4 shows that the pink defect area has a soft color, which is suitable for the detection of light-colored transparent plastic-free coatings. It is not easy to cause visual fatigue when observed by the naked eye for a long time.

[0122] Example 5 shows a significant increase in the color difference between the light pink in the weak area and the dark pink in the uncoated area, which significantly enhances the identification of defects in the low-thickness ultra-thin coating, making it suitable for detection scenarios where the differentiation of pink gradients is critical.

[0123] Comparative Example 1 only displays blue-purple and cannot form a pink gradient. It is almost impossible to distinguish between intact coating and thin coating areas with the naked eye, and it cannot identify coating thickness gradients. This formula can only qualitatively determine whether the coating is covered, and cannot identify the difference between thin coating weak areas and missed coating areas, resulting in seriously insufficient detection accuracy.

[0124] Comparative Example 2 shows varying degrees of red or pink throughout, with no substantial color difference between different coating states. This formulation completely loses its defect identification capability and cannot distinguish between intact coatings and defective areas.

[0125] The surface tension of the test liquid in Comparative Example 3 was as high as 65 mN / m, making it unable to effectively wet and penetrate the micropores of the coating. The color development time was extended to more than 15 seconds, and tiny defects (diameter <50 μm) could not be detected at all. Due to the lack of wetting and penetrating agents, the test liquid could not reach the defect sites, thus losing its basic detection function.

[0126] The viscosity of the test solution in Comparative Example 4 was too low (approximately 1.5 mPa·s, comparable to pure water), causing it to spread rapidly laterally after being dropped, with droplet diameters reaching 10.5 mm. Due to severe smudging, the color boundaries of micropore defects were blurred, making it impossible to accurately locate micron-level weak points.

[0127] Comparative Example 5 showed inconsistent and unreliable color development results due to pH fluctuations (5.5–8.5) with different areas of the same sample. Intact areas may show pink while defective areas may show purple, making it impossible to obtain effective test data. The sample delaminated and became invalid after one month of storage.

[0128] Comparative Example 6: The anionic surfactant chemically eroded the PVA and starch-based hydrophilic coating film structure. After testing, the Cobb value of the coating increased by 42.5 g / m² (approximately 40%) compared to before testing, causing irreversible damage. Simultaneously, the anionic charge interfered with the ionic balance of the indicator, resulting in chaotic color development, irregular color spots, and an inability to form a stable three-color gradient. The detection sensitivity was significantly lower than that of Example 1.

[0129] Data Analysis

[0130] Examples 1-3 reveal the gradient effect of total component concentration. From Examples 2 to 3, as the total content of the four functional components (indicator, Tween 80, buffer salt, and xanthan gum) increased from 0.39% to 3.45%, the color development speed increased from 5 seconds to 1 second, the intact-weak color difference ΔE increased from 14.2 to 19.5, and the minimum identifiable thickness decreased from 6 μm to 2 μm. This indicates a significant positive synergistic effect among the components—the simultaneous increase in component concentration not only did not produce antagonism but also amplified their respective effects, simultaneously optimizing detection sensitivity, speed, and color discrimination.

[0131] Examples 4 and 5 demonstrate the value of precise control over the indicator ratio. When the ratio of bromocresol purple to neutral red is 1:0.8 (Example 4), the pink area shows a soft color, with an intact-to-weak ΔE of 15.3 and a weak-to-missing-paint ΔE of 13.0. When the ratio is increased to 1:1.2 (Example 5), the weak-to-missing-paint color difference ΔE increases from 13.0 to 15.5, and the deep pink in the missing-paint area becomes more prominent. This proves that appropriately increasing the proportion of neutral red helps to enhance the identification of defect areas. However, if the ratio is too high, the blue-purple hue of the intact area will be masked by the red; therefore, 1:0.8–1.2 is the optimal window verified by experiments.

[0132] Comparative Examples 1 and 2 verified the irreplaceable nature of the compound indicator. Bromocresol purple alone (Comparative Example 1) showed a ΔE of only 3.2 between the intact and weak areas, far lower than the 16.8 in Example 1, making it almost indistinguishable to the naked eye; Neutral Red alone (Comparative Example 2) showed a ΔE of only 0.5 between the intact and weak areas, completely losing its gradient recognition capability. This demonstrates that the color superposition effect of the two indicators is key to achieving a three-color gradient—bromocresol purple provides the primary color change signal for pH response, while neutral red provides an auxiliary signal for contrast enhancement; both are indispensable.

[0133] Comparative Examples 3 and 4 verified the functional complementarity of Tween 80 and xanthan gum. Removing Tween 80 alone (Comparative Example 3) resulted in a surface tension as high as 65 mN / m, preventing micropore penetration and making minute defects completely undetectable; removing xanthan gum alone (Comparative Example 4) resulted in a staining diameter of 10.5 mm, causing defect localization failure. Only when both are present can the ideal effect of "rapid vertical penetration and no lateral diffusion" be achieved.

[0134] Comparative Example 5 verified the role of the buffer salt in ensuring color development stability. Without the buffer salt, the pH of the test solution fluctuated with the environment (4.6–7.2), and the color development results were completely unpredictable, with different areas of the same sample exhibiting chaotic color gradations. This proves that although the buffer salt does not directly participate in the color development reaction, it provides a stable background pH environment for the indicator, making the color development results depend only on the density difference of the coating itself, rather than external environmental interference.

[0135] Comparative Example 6 verifies the irreplaceability of nonionic surfactants. While replacing Tween 80 with anionic surfactants can also reduce surface tension, their chemical erosion of the hydrophilic coating leads to a 42.5 g / m² increase in the Cobb value, completely contradicting the original intention of non-destructive testing. Furthermore, the anionic charge interferes with the ionic balance of the indicator, resulting in chaotic color development. This proves that the selection of Tween 80 in this invention is not only for "wetting effect" considerations, but also to achieve the core objective of "non-destructive penetration."

[0136] The above are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention specification and drawings under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

[0137] It should be noted that when numerical ranges are involved in this invention, it should be understood that the two endpoints of each numerical range and any value between the two endpoints can be selected. Since the steps and methods used are the same as those in the embodiments, preferred embodiments are described in order to avoid redundancy. Although preferred embodiments of this invention have been described, those skilled in the art can make other changes and modifications to these embodiments once they know the basic inventive concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of this invention.

[0138] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from the spirit and scope of this invention; if these modifications and variations of this invention fall within the scope of the claims of this invention and their equivalents, then this invention also intends to include these modifications and variations.

Claims

1. A gradient colorimetric detection solution for defects in plastic-free coatings, characterized in that, Includes the following components by mass percentage: Composite colorimetric indicator: 0.08%–0.4%; Food-grade wetting and penetrating agent: 0.15%–1.8%; pH buffer 0.1%–0.9%; Thickening stabilizer: 0.06%–0.35%; The remainder is deionized water; The composite colorimetric indicator is a mixture of bromocresol purple and neutral red, with a mass ratio of bromocresol purple to neutral red of 1:0.8 to 1.

2.

2. The colorimetric detection solution according to claim 1, characterized in that, The food-grade wetting and penetrating agent is polysorbate-80.

3. The colorimetric detection solution according to claim 1, characterized in that, The pH buffer adjuster is a phosphate buffer system composed of sodium dihydrogen phosphate and disodium hydrogen phosphate.

4. The colorimetric detection solution according to claim 1, characterized in that, The thickening stabilizer is food-grade xanthan gum.

5. The colorimetric detection solution according to claim 1, characterized in that, The pH value of the colorimetric detection solution is stable at 6.8–7.2, the surface tension is 32–38 mN / m, and the dynamic viscosity is 15–30 mPa·s.

6. A method for preparing the colorimetric detection solution as described in any one of claims 1 to 5, characterized in that, The preparation environment temperature is controlled at 15–35℃, and the stirring speed is controlled at 200–400 rpm throughout the process. The preparation method includes the following steps: Step 1: Add the measured amount of deionized water to the stirred tank, turn on the stirrer, and add the pH buffer adjuster and thickening stabilizer in full proportion in sequence. Continue stirring for 40 to 60 minutes until the solution is uniform, transparent and free of suspended lumps to obtain the basic buffer solution. Step 2: Take 2 to 5 times the total mass of the composite colorimetric indicator as a pre-dissolving solvent, add the composite colorimetric indicator to the anhydrous ethanol, and stir at room temperature until it is completely transparent and dissolved to obtain the indicator pre-solution; slowly and uniformly add the indicator pre-solution to the basic buffer solution, and continue stirring until the system has a uniform color without local color patches after the addition is complete; Step 3: Add the full proportion of food-grade wetting and penetrating agent to the mixture obtained in Step 2, stir at room temperature for 15-25 minutes, then let it stand to defoam, filter to remove insoluble impurities, and the finished colorimetric detection solution is obtained.

7. The preparation method according to claim 6, characterized in that, The settling and defoaming time in step three is 25 to 35 minutes; the filtration uses a microporous filter membrane with a pore size of 0.45 to 1.0 μm.

8. A method for applying a gradient colorimetric detection solution for defects in plastic-free coatings, characterized in that, The method of using the colorimetric detection solution described in any of claims 1-5 to identify weak points in the non-plastic coating includes the following steps: S1. Lay the paper-based plastic-free coating sample to be tested flat on a horizontal platform, wipe away surface dust and oil, and ensure that the coating surface is dry and clean; S2. Apply the colorimetric test solution to the surface of the coating to be tested using a dropper or spray bottle. When using a dropper, the volume of a single drop should be 5–20 μL, and the drop density should be 0.5–2 drops / cm², so that the droplets are evenly distributed on the coating surface and adjacent droplets do not overlap. When using a spray bottle, the spraying pressure should be 0.05–0.1 MPa, the nozzle should be 10–15 cm away from the coating surface, and the solution should be sprayed evenly and thinly until there is no liquid flow or accumulation. S3. After applying the solution, let it stand for 1-5 seconds and observe the color development of the coating surface: If it appears as a light blue-purple color, it is determined that the coating thickness meets the standard and the film layer is dense and intact; If it appears light pink, it is determined that the coating has thin coating, micropores, or incomplete curing. If it appears as a deep pink color, it is determined to be an area where the coating has been missed or damaged. S4. Mark the weak points according to the color development location. After the test is completed, wipe the coating surface with a cotton cloth moistened with water to remove the test solution residue.

9. The application method according to claim 8, characterized in that, The standard for judging the color development state in step S3 is to use a standard paper colorimetric card or a colorimeter for auxiliary quantitative determination: the light blue-purple corresponds to the CIELab color space range of L=62~68, a=-4~-1, b=-8~-4; the light pink corresponds to L=70~78, a=6~12, b=2~6; and the dark pink corresponds to L=80~88, a=15~22, b=5~10.

10. The application method according to claim 8, characterized in that, The plastic-free coating is a water-based coating on the surface of food packaging paper, including starch-based coatings, polyvinyl alcohol-based coatings, or plant-based gum coatings; The thickness of the intact and dense coating is ≥8μm, the thickness of the weak defect is 3~7μm, and the thickness of the uncoated area is 0~2μm.

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