A test liquid for detecting barrier uniformity of a high-barrier coated paper and a method of using the same

Abstract

The application discloses a kind of high-barrier coating paper barrier uniformity detection test liquid, its preparation method and application method.To solve the technical problems that existing detection is time-consuming, cannot full-width screening, cannot directly show coating local thin coating / leak coating / pinhole defect distribution.The test liquid comprises water, anionic surfactant and edible pigment;The anionic surfactant is a complex system of sodium dodecyl benzene sulfonate and sodium dioctyl sulfosuccinate;The edible pigment is water-soluble red edible pigment;The surface tension of the test liquid is 22mN / m-32mN / m.The application can complete detection in tens of seconds without large instrument, and the position and distribution of pinhole, thin coating and other defects can be directly determined by naked eye after full-width coating, which breaks through the limitations of traditional single-point sampling detection.

Description

Technical Field

[0001] This invention relates to the field of testing technology for food packaging paper, and more specifically, to a test solution for testing the barrier uniformity of high-barrier coated paper, a method for preparing the test solution for testing the barrier uniformity of high-barrier coated paper, and a method for applying the test solution for testing the barrier uniformity of high-barrier coated paper. Background Technology

[0002] High-barrier, oil-resistant coated paper cups typically use acrylic or EAA (ethylene-acrylic acid copolymer) coatings, with a coating weight of 8–10 g / m². Due to their excellent oil-resistant and heat-sealing properties, they are widely used in the production of oil-resistant paper cups for fast food. The quality of the coating's barrier properties directly determines the paper cup's oil-proof performance and safety; therefore, testing the coating's barrier properties is a crucial step in the quality control of paper cup base paper production.

[0003] Currently, the standard testing methods used in the industry to evaluate the barrier properties of coatings mainly include: Cobb water absorption test (assessing the paper's resistance to liquid water absorption), WVTR water vapor transmission rate test (assessing the barrier performance against water vapor molecules), OTR oxygen transmission rate test (assessing the barrier performance against oxygen molecules), and KIT oil resistance rating test (assessing the ability to prevent the penetration of oily substances). However, the above-mentioned existing testing methods have the following obvious shortcomings in practical applications: First, the testing cycle is long and relies on large, precision instruments. WVTR and OTR tests require specialized gas permeability analyzers, and samples must be sent to specialized laboratories, with each test taking several hours. This not only results in low testing efficiency, but more importantly, the test results lag significantly behind the production process. When barrier performance is found to be substandard, a large number of semi-finished or finished products are often in a state of pending judgment, failing to meet the needs of continuous production line for real-time quality control and making it impossible to effectively intercept production abnormalities at the first moment.

[0004] Secondly, all methods involve single-point sampling and cannot achieve full-width screening. Whether it's the WVTR, OTR, Cobb, or KIT method, each involves cutting a small sample from a specific area of ​​the paper being tested. The results reflect the barrier properties of that sampling point and cannot reflect the uniformity of the coating across the entire paper surface. However, in continuous coating production, coating defects, such as localized thin coatings, missed coatings, and pinholes, are often randomly distributed. Single-point sampling easily misses defective areas, leading to a large amount of paper with weak localized barrier properties flowing into subsequent paper cup processing steps, ultimately causing localized oil leakage quality incidents in actual use.

[0005] Third, the test results cannot visually present the location and distribution of defects. While the Cobb and KIT methods can assess the overall barrier grade of paper, they cannot show the specific location, quantity, and distribution pattern of defects, thus failing to provide a direct basis for targeted adjustments to the coating process. Operators find it difficult to determine which area or parameter of the coating equipment is abnormal based on the test results, limiting the guiding role of testing in process optimization.

[0006] Therefore, the industry urgently needs a dedicated test solution that is low-cost, can quickly develop colors on-site, and allows for full-width visualization to determine the uniformity of coatings. Summary of the Invention

[0007] To address the technical problems of existing methods for testing the barrier performance of high-barrier coated paper, such as long testing time, reliance on large instruments, single-point sampling only (inability to screen the entire surface), and inability to visually display the distribution of local thin coating / missing coating / pinhole defects, this invention provides a test solution for testing the barrier uniformity of high-barrier coated paper, a method for preparing the test solution, and a method for applying the test solution. Through the synergistic effect of anionic surfactant compound system and red food coloring, the surface tension of the test solution is precisely controlled within a narrow window of 22–32 mN / m, achieving selective penetration and color development of coating defects. This test solution has a simple formulation, low cost, and can complete the test within tens of seconds without the need for large instruments. Furthermore, after full-width coating, the location and distribution of defects such as pinholes and thin coatings can be visually determined, overcoming the limitations of traditional single-point sampling testing.

[0008] In a first aspect, the present invention provides a test solution for detecting the barrier uniformity of high-barrier coated paper, the test solution comprising water, anionic surfactant and food coloring; The anionic surfactant is a compound system of sodium dodecylbenzenesulfonate and sodium dioctyl sulfosuccinate; The food coloring is a water-soluble red food coloring; The surface tension of the test liquid is 22 mN / m to 32 mN / m.

[0009] Furthermore, the weight ratio of sodium dodecylbenzenesulfonate to sodium dioctyl sulfosuccinate is 1:2 to 2:1.

[0010] Furthermore, the total weight percentage of the anionic surfactant in the test solution is 0.5% to 2.0%, and the weight percentage of the food coloring in the test solution is 0.3% to 1.5%.

[0011] Furthermore, the test solution is composed of the following components in parts by weight: Water: 97-99 parts Sodium dodecylbenzenesulfonate: 0.15–0.7 parts Sodium dioctyl sulfosuccinate: 0.3–1.4 parts Water-soluble red food coloring: 0.3 to 1.5 parts.

[0012] Furthermore, the test solution is composed of the following components in parts by weight: Water: 97.5–98.5 parts Sodium dodecylbenzenesulfonate: 0.3–0.5 parts Sodium dioctyl sulfosuccinate: 0.6–1.0 parts Water-soluble red food coloring: 0.5 to 1.0 parts.

[0013] Furthermore, the food coloring is carmine or allura red.

[0014] In a second aspect, the present invention provides a method for preparing a test solution as described in any one of the first aspects, comprising the following steps: Heat the water to 70℃~95℃; Add the prescribed amount of the anionic surfactant compound system to the heated water and stir until dissolved; Add the recommended amount of food coloring and stir until fully mixed. Cool to room temperature to obtain the test solution.

[0015] Thirdly, the present invention provides a method for applying a test solution to test the barrier uniformity of high-barrier coated paper, wherein the barrier uniformity of high-barrier coated paper is visually evaluated using the test solution described in any one of the first aspects, comprising the following steps: S1. Apply the test liquid to the coating surface of the high-barrier coated paper to be tested; S2, let stand for 60s to 300s; S3. Gently wipe the paper surface with a damp cloth. If there are no colored spots on the paper surface, the coating in the test area is intact; if there are colored spots on the paper surface that cannot be wiped off, there is a coating defect at the colored spots.

[0016] Furthermore, the application method in step S1 is either the droplet method or the coating bar method; The droplet method includes: adding liquid droplets to the coating surface using a dropper, with each droplet having a volume of 0.03 mL to 0.08 mL and a droplet spacing of 20 mm to 30 mm; The coating bar method includes: applying the test liquid to one end of the coating surface, and scraping the coating surface uniformly with a coating bar at a speed of 50 mm / s to 150 mm / s, wherein the gap between the coating bar and the coating surface is 0 to 0.15 mm.

[0017] Furthermore, the ambient temperature of the application method is 15℃~30℃, and the relative humidity is 40%~60%.

[0018] Beneficial effects

[0019] The test solution of this invention, through a compound system of sodium dodecylbenzenesulfonate and sodium dioctyl sulfosuccinate, synergistically achieves accurate identification of coating defects. Sodium dodecylbenzenesulfonate provides stable wetting, emulsifying, and dispersing properties, ensuring uniform and stable dispersion of the pigment in the solution without aggregation or sedimentation. Sodium dioctyl sulfosuccinate, with its excellent penetrating properties, drives the liquid to rapidly enter the micropores of the coating. The synergistic effect of the two surfactants, combined with the excellent solubility and coloring power of edible red pigment, allows the test solution to selectively penetrate into the defective areas of the coating through capillary action and remain there for color development, while only adhering to the surface of intact coatings, which can be wiped away. This completely solves the problem of false positives and false negatives in traditional methods. The test solution uses water as a solvent and food-grade ingredients as raw materials, offering the advantages of safety, environmental friendliness, and no damage to paper; it exhibits excellent stability at room temperature, requires no special storage conditions, and is convenient to use. Moreover, this test solution breaks through the technical limitations of traditional testing, which can only sample at a single point and rely on instrument readings. It enables real-time screening and intuitive interpretation of the entire paper surface in an open production line environment. The test results are not significantly affected by fluctuations in ambient temperature and humidity, and have strong industrial applicability and on-site adaptability.

[0020] The application method of this invention adopts a simplified three-step operation process of "coating-standing-wiping verification," compressing the detection time to tens of seconds to several minutes. No precision instruments or professional training are required, and production line workers can complete the process independently. The full-width coating method achieves complete coverage screening of the entire paper surface, accurately locating the position, size, and distribution density of each pinhole, thin coating, and missed coating defect, providing a direct basis for targeted adjustments to the coating process. Utilizing the physical difference that intact coatings only adhere to the surface while defective coatings retain pigment, the judgment results are objective and reliable, unaffected by the operator's subjective judgment. The detection process does not damage the paper structure. Furthermore, this method has strong tolerance to changes in temperature and humidity in the production line environment, eliminating the need for strict control of detection conditions. This enables industrial application from the laboratory to the production site, providing enterprises with an efficient, economical, and reliable on-site detection solution for quality control. Attached Figure Description

[0021] 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.

[0022] Figure 1A schematic flowchart illustrating a method for preparing a test solution for detecting the barrier uniformity of high-barrier coated paper, provided for an embodiment of the present invention. Figure 2 A flowchart illustrating the application method of a test solution for detecting the barrier uniformity of high-barrier coated paper, provided for an embodiment of the present invention. Figure 3 This is a schematic diagram showing the state of pure water on the surface of high-barrier coated paper. Figure 4 A schematic diagram showing the state of the test liquid applied to the surface of high-barrier coated paper according to an embodiment of the present invention; Figure 5 The results of testing the coating using the test solution of the present invention after drying under different drying conditions are shown in the figure. Figure 6 This is a diagram showing the test results of a coating sample using the test solution of the present invention in an embodiment of the present invention; Figure 7 This is a graph showing the test results of another coating sample using the test solution of the present invention in an embodiment of the present invention; Figure 8 This is a graph showing the test results of another coating sample using the test solution of the present invention in an embodiment of the present invention; Figure 9 The results of testing known defective coatings using the test solution of this invention in Examples 1-3 of this invention are shown in the figure. Figure 10 The image shows the results of testing a coating with known defects using the test solution of this invention, as shown in Comparative Example 1 of this invention. Detailed Implementation

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] This invention provides a test solution for detecting the barrier uniformity of high-barrier coated paper, the test solution comprising water, anionic surfactant and food coloring; The anionic surfactant is a compound system of sodium dodecylbenzenesulfonate and sodium dioctyl sulfosuccinate; The food coloring is a water-soluble red food coloring; The surface tension of the test liquid is 22 mN / m to 32 mN / m.

[0029] Unless otherwise specified, all raw materials used in this invention are commercially available products commonly used in the field. Specifically, sodium dodecylbenzenesulfonate has a purity of not less than 90%, sodium dioctyl sulfosuccinate has a purity of not less than 99%, the food coloring is food-grade carmine or Allura Red with a purity of not less than 85%, and the water is distilled water.

[0030] The test solution of this invention is prepared by dissolving anionic surfactant compound system and water-soluble red food coloring in water. The following provides a detailed description of the preferred formulation and preparation process parameters.

[0031] The core components of the test solution of this invention include water, an anionic surfactant compound system, and food coloring. Specifically: Water acts as a solvent, dispersing and dissolving the carrier. The anionic surfactant is a compound system of sodium dodecylbenzenesulfonate and sodium dioctyl sulfosuccinate. Sodium dodecylbenzenesulfonate (LAS) is a widely used anionic surfactant that significantly reduces the surface tension of water, provides excellent wetting and penetration, and simultaneously emulsifies and disperses, ensuring uniform and stable pigmentation. Furthermore, it is easily compounded with various additives and is chemically stable to alkalis, dilute acids, and hard water, forming the basis for constructing stable formulations. The core advantage of sodium dioctyl sulfosuccinate (also known as rapid penetrant T) lies in its exceptional penetrating power, strongly driving liquid penetration into the smallest pores and cracks of the coating, which is key to achieving high-sensitivity detection and accurate discovery of hidden defects. The combination of the two surfactants produces a synergistic effect, giving the test solution both excellent wetting and penetration properties.

[0032] The food coloring is a water-soluble red food coloring, also known as edible red pigment, preferably carmine or allura red. The choice of red as the indicator color for the test solution is based on a comprehensive consideration of visual contrast, chemical compatibility, and material properties: red forms the strongest color contrast with white or light-colored high-barrier coated paper substrates, and as one of the colors with the highest visual sensitivity in the long-wavelength spectrum, red is most likely to attract the observer's attention; commonly used red food colorings, such as allura red and carmine, mostly carry a negative charge, which keeps them stably dispersed with the similarly negatively charged anionic surfactants in the formula due to "like charges repel," effectively preventing pigment aggregation; in addition, red food coloring has excellent properties such as good solubility, light and heat resistance, and strong tinting strength, producing a noticeable color development effect even in small amounts.

[0033] The dosage of each component can be selected within the following range: Water: 97-99 parts, sodium dodecylbenzenesulfonate: 0.15-0.7 parts, sodium dioctyl sulfosuccinate: 0.3-1.4 parts, water-soluble red food coloring: 0.3-1.5 parts.

[0034] More preferably, the following components are used: Water: 97.5–98.5 parts, sodium dodecylbenzenesulfonate: 0.3–0.5 parts, sodium dioctyl sulfosuccinate: 0.6–1.0 parts, water-soluble red food coloring: 0.5–1.0 parts.

[0035] In the above weight ratio, the sum of the weight parts of each component is 100 parts. The test solution prepared in this ratio range has an anionic surfactant compound system in the test solution with a total weight percentage of 0.5% to 2.0% (preferably 1.0% to 1.5%), and food coloring in the test solution with a weight percentage of 0.3% to 1.5% (preferably 0.5% to 1.0%).

[0036] It should be noted that the dosage ranges of each component mentioned above are independent selectable ranges. In actual preparation, the actual dosage of each component can be selected independently within its respective range, and the sum of the weight parts of each component in the same embodiment is 100 parts. In other words, the above-mentioned weight part values ​​are any values ​​between the endpoints. Those skilled in the art can select any specific dosage within this range according to actual needs, as long as the sum of the selected specific dosages of each component is 100 parts. For example, 98.2 parts of water, 0.4 parts of sodium dodecylbenzenesulfonate, 0.8 parts of sodium dioctyl sulfosuccinate, and 0.6 parts of edible red food coloring can be selected, totaling 100 parts, but this is not the only option.

[0037] The present invention also provides a method for preparing the above-mentioned test solution, such as... Figure 1 As shown, it includes the following steps: Step S10: Heat the water to 70℃~95℃; Step S20: Add the formulated amount of anionic surfactant compound system to the heated water and stir until dissolved; Step S30: Add the amount of food coloring specified in the recipe and stir until completely mixed. Step S40: Cool to room temperature to obtain the test solution.

[0038] Specifically, water is poured into a clean stainless steel or glass container and heated to 70°C to 95°C, preferably 80°C; Add the formulated amount of anionic surfactant compound system, namely sodium dodecylbenzenesulfonate and sodium dioctyl sulfosuccinate, to warm water and stir at 200 rpm to 400 rpm until completely dissolved, which takes about 1 min to 5 min, preferably 2 min; Add the amount of water-soluble red food coloring powder specified in the formula, and continue stirring at 300 rpm to 500 rpm until the materials are completely mixed and homogeneous, which takes about 5 to 10 minutes. Allow it to cool naturally to room temperature for about 20 to 30 minutes, then let it stand to defoam, and you will get the finished test solution.

[0039] The resulting test solution is a red, transparent, and homogeneous liquid with a pH value of 6.5–7.5. This pH value is the natural result of the raw materials after dissolution and mixing. Under slightly neutral conditions, the optimal activity state of the anionic surfactant LAS and sodium dioctyl sulfosuccinate is ensured. The surface tension is 22 mN / m–32 mN / m at 25°C.

[0040] The test solution of this invention has a shelf life of 12 months when stored at room temperature in a sealed, light-protected environment. All three components are water-soluble and uniformly dispersed in water in molecular or ionic form, without insoluble particles or an oil phase, thus making them less prone to stratification or precipitation. The stability at different storage temperatures is shown in Table 1 below.

[0041] Table 1: Stability of the test solution of this invention at different storage temperatures

[0042] The test solution of this invention is used for the visual evaluation of the barrier uniformity of high-barrier coated paper, and its application method is as follows: Figure 2 As shown, it includes the following steps: Step S1: Apply the test liquid to the coating surface of the high-barrier coated paper to be tested; Step S2: Let stand for 60s to 300s; Step S3: Gently wipe the paper surface with a damp cloth. If there are no colored spots on the paper surface, the coating in the test area is intact; if there are colored spots on the paper surface that cannot be wiped off, there is a coating defect at the colored spots.

[0043] Specifically, the operation process for each step is as follows: S1. Apply the test solution to the coating surface of the high-barrier coated paper to be tested. Application can be done using either the droplet method or the coating rod method. Droplet method: Using a pipette or microdropper, draw up the test solution and evenly drop it onto the surface of the high-barrier coated paper along its width. Each drop should be 0.03 mL to 0.08 mL, preferably 0.05 mL, with a droplet spacing of 20 mm to 30 mm. When adding the drop, keep the dropper tip approximately 5 mm to 10 mm away from the coating surface, allowing the droplet to fall naturally and avoiding impact on the coating. This method is simple to operate, requires no equipment, and can be used to test specific areas with low reagent consumption (approximately 0.75 mL to 1.6 mL per 100 cm²). However, it is less efficient for large-area testing, and the uniformity of droplet distribution depends on the operator's skill.

[0044] Coating rod method: Lay the high-barrier coated paper sample flat on a smooth glass stage and secure the paper with clips. Take an appropriate amount of test solution, about 2-3 mL, and evenly coat one end of the coating surface. Use a stainless steel coating rod with a diameter of 10 mm and scrape it across the coating surface at a constant speed of 50 mm / s to 150 mm / s to ensure the test solution evenly covers the entire detection area. The coating amount is controlled by the gap between the coating rod and the coating surface, which should be 0 mm to 0.15 mm. The coating rod surface must be cleaned before each coating to avoid cross-contamination. This method is suitable for rapid screening of large areas, and can cover an A4 size or larger area in a single application. It has good coating uniformity and strong result consistency, making it suitable for batch testing on production lines. However, the reagent consumption is relatively high for large areas with intact coatings; approximately 2-3 mL is needed for an A4 area.

[0045] S2. Static color development: After applying the test solution, allow it to stand at room temperature for 60s to 300s, preferably 300s. During the standing process, the test solution penetrates and develops color at the coating defects through capillary action.

[0046] S3. Judgment Result: After the standing period, gently wipe the paper surface with a damp cloth, damp towel, or damp paper towel. If no colored spots appear on the paper after the paper dye is wiped off, the coating in the test area is intact—indicating that the test liquid only stays on the surface of the coating and does not penetrate into the interior of the coating, the coating is dense and complete, and the barrier performance meets the standard. If there are indelible colored spots on the paper surface, there is a coating defect at the colored spot location—indicating that the test liquid has penetrated and remained in the paper fibers along the defect, such as pinholes, microcracks, thin coating, or missed coating, and the pigment cannot be carried out by the wet cloth.

[0047] The optimal ambient temperature for the detection method is 15℃~30℃, and the optimal relative humidity is 40%~60%. Under these conditions, the colorimetric reaction of the test solution is sufficient and the interpretation is reliable.

[0048] The working principle of the test solution of this invention is based on the capillary selective permeation mechanism regulated by surface tension.

[0049] Pure water has a high surface tension, approximately 72 mN / m, and a large contact angle, ranging from 90° to 120°. On the surface of a high-barrier coating, it shrinks into a bead-like shape, unable to spread or penetrate the coating's micropores. The behavior of pure water on the surface of a high-barrier coated paper is as follows: Figure 3 As shown, the droplets exhibit a distinctly bead-like or hemispherical shape, with a small contact area between the bottom and the coating surface, and an overall outline approaching a sphere. This morphology is typical of high surface tension liquids on low surface energy solid surfaces—the liquid tends to contract itself to reduce the gas-liquid interface area, resulting in a large contact angle. The liquid exhibits poor wettability on the coating, making it difficult for the liquid to spread across the coating surface; even with small defects in the coating, penetration is challenging.

[0050] This invention reduces the surface tension of the test solution from approximately 72 mN / m to 22 mN / m–32 mN / m by adding an anionic surfactant compound system, namely sodium dodecylbenzenesulfonate and sodium dioctyl sulfosuccinate. When the test solution is applied to the surface of high-barrier coated paper, its state is as follows: Figure 4 As shown, unlike the bead-like shrinkage morphology of pure water on the surface of a high-barrier coating, the test liquid droplets exhibit a significantly increased contact area between the bottom of the droplets and the coating surface due to the low surface tension and significantly reduced contact angle. The test droplets spread out on the coating surface, demonstrating a significant improvement in the wettability of the test liquid on the coating surface.

[0051] Intact coating area: The coating surface is dense and has low surface energy. The wetting power of the test liquid is insufficient to overcome the repulsive force of the coating surface. The liquid only stays on the coating surface, showing a uniform light red wetting state, and does not penetrate into the interior. After wiping with a damp cloth, the red color on the surface can be completely wiped away.

[0052] Defective areas: The coating contains penetrating defects such as pinholes, microcracks, or insufficient thickness. The exposed paper fibers have a porous structure and strong hydrophilicity, forming capillary channels. Under the action of capillary force, the test solution quickly penetrates into the defects, and the edible pigments it carries remain in the paper fibers, forming clear and persistent red spots. Even after repeated wiping with a damp cloth, the spots remain clearly visible and cannot be removed.

[0053] The threshold effect of surface tension is a key factor in this invention. When the amount of surfactant added is too low, the surface tension of the test solution is higher than 32 mN / m, resulting in insufficient wetting and penetration, and micro-defects cannot be detected. When the amount of surfactant added is too high, the surface tension of the test solution is lower than 22 mN / m, resulting in excessive penetration, which can penetrate intact coatings and cause false spots. Only when the surface tension is controlled within a narrow window of 22 mN / m to 32 mN / m can the test solution achieve accurate differentiation between "intact coatings not penetrating and defective coatings selectively penetrating." The relationship between the surface tension of the test solution and the detection performance is shown in Table 2 below.

[0054] Table 2: Relationship between surface tension of test liquid and detection performance

[0055] In addition, the present invention uses red food coloring as an indicator, which has the following technical advantages: (1) High-barrier coated paper is usually white or light-colored substrate, and red forms the strongest color contrast with it, which is most likely to attract the attention of the observer; (2) Red food coloring (such as carmine and allura red) has a negative charge, and it maintains stable dispersion with anionic surfactants that also have a negative charge due to "like charges repel each other", which effectively prevents pigment aggregation and ensures the uniformity and storage stability of the test solution; (3) Red food coloring has good solubility, light and heat resistance, and strong coloring power, and a small amount can produce obvious color development effect; (4) Food coloring is safe, environmentally friendly, and has low toxicity, which meets the testing scenarios and relevant safety standards for food contact materials.

[0056] To facilitate the quantification and recording of test results, this invention provides the rating criteria shown in Table 3 below.

[0057] Table 3: Rating Criteria for Paper Surface Condition Test Results

[0058] In practical applications, corresponding qualification levels can be set according to production quality requirements, such as requiring a level of 3 or above to be considered qualified. This rating standard is simple and intuitive, requiring no professional training to operate, and is suitable for production line workers to quickly determine quality on-site.

[0059] The same material was dried at 120℃, 135℃, 150℃, 180℃, and 200℃ for the same drying time. Tests were then performed using the test solution of this invention. The results are as follows: Figure 5 As shown, Figure 5 The table below shows the test results for drying conditions at 120℃, 135℃, 150℃, 180℃, and 200℃, from top to bottom. It can be observed that no film formed at 120℃, and the test solution completely penetrated. At 135℃, a partial film formed, with localized seepage points on the red surfactant. A complete film formed at 150℃, and no seepage points were observed when the test solution was dried at 150℃ or higher. The test solution of this invention can effectively determine the integrity and defects of coatings in semi-finished or finished products.

[0060] This invention can also be used for on-site testing and auxiliary debugging during the production process of high-barrier coated paper, such as... Figures 6-8 The image shown is a test result diagram during the production debugging process. Figure 6 The red surface activity test results of the A2 sample from actual production were tested after the machine, and it was found that the sample coating was unqualified and the coating thickness did not meet the standard. Figure 7 For the test results of another sample, the B-side sample, the red surfactant test using the test solution of this invention revealed uneven coating; it also failed to meet the standard; parameter adjustment is required. Figure 8 The following are the test results of various samples during another production debugging process. In the first debugging (test 1 in the figure), there were large areas of red staining on the paper surface and many coating defects. After the second debugging (test 2 in the figure), some areas showed severe color development and uneven coating. After the third debugging (test 3 in the figure), the test effect of the single coating amount met the requirements. The barrier paper product requires a coating amount of 8-10g, which requires multiple coatings. This is the test result after a single coating with a coating amount of 4g. There are a few red spots on the surface. After one or more coatings, the final product requirements can be met.

[0061] To further illustrate the effect of the dosage of each key component in this embodiment on the performance of the pre-coated slurry, pre-coated slurries prepared in Examples 1 to 5 and Comparative Examples 1 to 3 are provided below and tested.

[0062] The preparation methods for the test solutions in all embodiments are the same, as detailed below: Pour 2L of distilled water into a clean stainless steel or glass container and heat it to 80℃. Add an appropriate amount of the anionic surfactant compound system, namely sodium dodecylbenzenesulfonate and sodium dioctyl sulfosuccinate, to the warm distilled water at a weight ratio of 1:2. Stir at 200-400 rpm for 2 minutes until completely dissolved. Then add an appropriate amount of dark red food coloring powder and continue stirring at 300-500 rpm until the materials are completely mixed and homogeneous, which takes about 5-10 minutes. Allow it to cool naturally to room temperature, which takes about 20-30 minutes, to obtain the finished test solution.

[0063] The resulting test solution is a red, transparent, and homogeneous liquid with a surface tension of 22~32 mN / m and a pH value of 6.5~7.5. It has a shelf life of twelve months when stored at room temperature in a sealed, light-protected container.

[0064] Example 1

[0065] The amounts of each component are as follows: 2L distilled water, 8g sodium dodecylbenzenesulfonate, 16g sodium dioctyl sulfosuccinate, and 16g carmine (food coloring). That is, the total amount of the anionic surfactant compound system is 24g / 2L, accounting for approximately 1.2% of the total weight; the weight ratio of sodium dodecylbenzenesulfonate to sodium dioctyl sulfosuccinate is 1:2; and the food coloring is 16g / 2L, accounting for approximately 0.8% of the total weight. Prepare according to the above standardized preparation method.

[0066] The testing method is the drip method: Using a dropper, draw up the test solution and evenly apply droplets along the width of the high-barrier coated paper, i.e., transversely. Each drop should be approximately 0.05 mL in volume, with a droplet spacing of approximately 20–30 mm. Place approximately 15–20 test points within a 100 cm² testing area. Observe after standing for 300 seconds and verify by gently wiping with a damp cloth.

[0067] Color rendering effect: On a coated paper surface with good barrier uniformity, the test solution forms a uniform, light red, circular wetting patch without any color deepening or penetration. After light wiping with damp paper or cloth, the red color can be completely wiped away without leaving a trace, indicating that the test solution has not penetrated into the coating. In areas with coating defects such as pinholes or microcracks, the test solution penetrates along the defect and forms a significantly darker red spot around it. Even after repeated wiping with damp paper or cloth, the spot remains clearly visible and cannot be removed, indicating that the test solution has penetrated into the defect, and the defect location can be clearly identified visually. During the test, areas with coating defects typically begin to show a color reaction within 10 seconds, and the color development is complete after 300 seconds, reaching maximum contrast with the intact coating area. Verification has shown that this test solution, combined with visual observation, can detect coating pinhole defects with a diameter ≥100μm.

[0068] Example 2

[0069] Formula composition: 2L distilled water, 3g sodium dodecylbenzenesulfonate, 6g sodium dioctyl sulfosuccinate, 7g edible red food coloring (dark red powder).

[0070] The total amount of the anionic surfactant compound system is 9g / 2L, accounting for approximately 0.45% of the total weight; the edible red pigment is 7g / 2L, accounting for approximately 0.35% of the total weight; and the weight ratio of sodium dodecylbenzenesulfonate to sodium dioctyl sulfosuccinate is 1:2.

[0071] Prepared according to a unified preparation method.

[0072] Test method: Same as in Example 1, observe after standing for 300 seconds, and verify by wiping with a damp cloth.

[0073] Colorimetric Effect: Due to the low surfactant concentration (0.45%), the wetting and penetrating ability of the test solution decreased. On coating surfaces with good barrier uniformity, the test solution appeared as a lighter red, with a slightly smaller spreading area than in Example 1, and could be completely wiped away with a damp cloth. At areas with coating defects, although darker spots were observed, the spot density and contrast were not as pronounced as in Example 1, and the color was lighter, indicating insufficient penetration. During a 300-second settling period, pinhole defects with a diameter ≥150 μm were clearly identifiable, but the ability to identify smaller defects was insufficient. This indicates that when the surfactant concentration is too low, the penetration driving force of the test solution is insufficient, leading to decreased detection sensitivity.

[0074] Example 3

[0075] Formula composition: 2L distilled water, 14g sodium dodecylbenzenesulfonate, 28g sodium dioctyl sulfosuccinate, 28g edible red food coloring (dark red powder).

[0076] The total amount of the anionic surfactant compound system is 42g / 2L, accounting for about 2.1% of the total weight; the edible red pigment is 28g / 2L, accounting for about 1.4% of the total weight; and the weight ratio of sodium dodecylbenzenesulfonate to sodium dioctyl sulfosuccinate is 1:2.

[0077] Prepared according to a unified preparation method.

[0078] Test method: Same as in Example 1, observe after standing for 120 seconds, and verify by wiping with a damp cloth.

[0079] Color Development: Due to the high surfactant concentration of 2.1%, the wetting and penetrating ability of the test solution is significantly enhanced. On coating surfaces with good barrier uniformity, the test solution spreads rapidly and evenly, presenting a uniform red color. After standing for 120 seconds, it can be completely wiped away with a damp cloth. In areas with coating defects, the test solution penetrates rapidly, typically showing a color reaction within 10 seconds. At 120 seconds, the red spots are clear and have high contrast. However, it should be noted that if the surfactant concentration is too high, and the standing time exceeds 300 seconds, slight color penetration may occur in areas with intact coatings, leaving residue after wiping with a damp cloth, posing a risk of false positives. Therefore, it is recommended that the observation time for this formulation be controlled within 120-180 seconds.

[0080] Example 4: Formula composition: Same as Example 1.

[0081] Preparation method: Prepared according to the unified preparation method.

[0082] Test method: Take three samples of high-barrier coated paper from the same batch, all of which contain known defects. Apply the test solution using the drop method, and observe and record the color development effect at 60 seconds, 300 seconds and 600 seconds respectively. Wipe with a damp cloth to verify the penetration.

[0083] The color rendering effect is compared in Table 4 below. Table 4: Comparison of color development effects at different settling times in Example 4

[0084] Conclusion: This test solution exhibits a rapid onset of colorimetric reaction to coating defects, approximately 10 seconds. However, to ensure full color development of minute defects and achieve optimal contrast, a settling time of 300 seconds (5 minutes) is recommended. Within 600 seconds (10 minutes), the test solution showed no false penetration into intact coatings, demonstrating its excellent selectivity and time tolerance, making it suitable for the testing needs of different operating rhythms on production lines.

[0085] Example 5: Comparison of operational details of different coating methods

[0086] Formula composition: Same as Example 1.

[0087] Preparation method: Prepared according to the unified preparation method.

[0088] Test Method A – Droplet Method: Use a pipette or microdropper to draw up the test solution and evenly drop it onto the surface of the high-barrier coated paper along its width. Specific parameters are: 0.03–0.08 mL per drop, preferably 0.05 mL; droplet spacing of 20–30 mm; 15–20 test points within a 100 cm² detection area; and the dropper tip approximately 5–10 mm from the coating surface to allow the droplets to fall naturally, avoiding impact on the coating. After standing for 300 seconds, gently wipe with a damp cloth to verify and observe the color development results.

[0089] Advantages: Simple operation, no equipment required; can detect specific areas at fixed points; low reagent consumption, only about 0.75~1.6mL per 100cm². Disadvantages: Lower efficiency for large-area detection; droplet distribution uniformity depends on operator skill.

[0090] Test Method B – Coating Rod Method: Lay the high-barrier coated paper sample flat on a smooth glass stage and secure it with clips. Take an appropriate amount of test solution, approximately 2-3 mL, and evenly apply it to one end of the coating surface. Using a stainless steel coating rod with a diameter of 10 mm, scrape across the coating surface at a constant speed (50-150 mm / s recommended) to ensure the test solution evenly covers the entire testing area. Start timing immediately after coating. After standing for 300 seconds, gently wipe with a damp cloth to verify and observe the color development results. The coating amount is controlled by the gap between the coating rod and the coating surface, which should be 0-0.15 mm. Clean the coating rod surface before each coating to avoid cross-contamination.

[0091] Advantages: Suitable for rapid screening of large areas, capable of covering A4 size or larger in a single application; good coating uniformity and strong result consistency; suitable for batch testing on production lines. Disadvantages: Requires simple tools such as coating sticks; high reagent consumption for large areas with intact coatings, approximately 2-3 mL is needed for an A4 area.

[0092] The comparison of the two methods is summarized in Table 5 below.

[0093] Table 5: Comparison of Drop Method and Coating Rod Method

[0094] Comparative Example 1: The amount of surfactant added was too low.

[0095] Formula composition: 2L distilled water, 0.8g sodium dodecylbenzenesulfonate, 1.6g sodium dioctyl sulfosuccinate, 16g edible red food coloring (dark red powder).

[0096] That is, the total amount of the anionic surfactant compound system is 2.4g / 2L, accounting for about 0.12% of the total weight, which is less than 1 / 10 of Example 1; the edible red pigment is 16g / 2L, accounting for about 0.8% of the total weight; and the weight ratio of sodium dodecylbenzenesulfonate to sodium dioctyl sulfosuccinate is 1:2.

[0097] Prepared according to a unified preparation method.

[0098] Test method: Same as in Example 1, after standing for 300 seconds, wipe with a damp cloth for verification.

[0099] Results: At a surfactant concentration of 0.12%, the surface tension of the test solution only decreased from 72 mN / m in pure water to approximately 55-60 mN / m, indicating severely insufficient wetting and penetration capabilities. The test solution formed droplets on the coating surface, failing to spread effectively. In areas with pinhole defects, the test solution lacked sufficient penetration driving force and could not penetrate the tiny defects. After standing for 300 seconds, the defective areas showed almost no color change and no significant difference from the intact coating areas; after wiping with a damp cloth, almost all the red color on the surface was removed, making visual identification of defects impossible. These results indicate that when the surfactant concentration is below 0.2%, the test solution loses its basic detection function.

[0100] Comparative Example 2: Excessive surfactant addition

[0101] Formula composition: Distilled water: 2L, sodium dodecylbenzenesulfonate: 30g, sodium dioctyl sulfosuccinate: 60g, edible red food coloring (dark red powder): 16g.

[0102] That is, the total amount of the anionic surfactant compound system is 90g / 2L, accounting for about 4.5% of the total weight, which is about 3.75 times that of Example 1; the edible red pigment is 16g / 2L, accounting for about 0.8% of the total weight; and the weight ratio of sodium dodecylbenzenesulfonate to sodium dioctyl sulfosuccinate is 1:2.

[0103] Prepared according to a unified preparation method.

[0104] Test method: Same as in Example 1, observe after standing for 300 seconds, and verify by wiping with a damp cloth.

[0105] result: The surfactant concentration was too high, at 4.5%, significantly reducing the surface tension of the test solution and resulting in excessive penetration. After standing for 300 seconds, a large area of ​​uniform red background appeared in the intact coating area. Red residue remained after wiping with a damp cloth, indicating that the test solution had partially penetrated into the intact coating. However, the red spots in the defective areas were still obvious and could be distinguished from the stained background. Furthermore, the defective spots were magnified due to excessive penetration, and their actual displayed size was larger than the true size of the defect.

[0106] Although the high-concentration formula can still make the defects visible, the following problems exist: (1) The background is dyed red over a large area, the contrast between the defect and the background decreases, and the difficulty of judgment increases; (2) The defect area is over-magnified, and the true size and severity of the defect cannot be reflected; (3) After wiping with a damp cloth, there is still red residue in the intact area, and it is not possible to effectively distinguish between the real defect and the surface adhesion through wiping verification as in Example 1; (4) If the same wet cloth wiping verification is performed, there is residue in both the defect area and the background of the intact area, which causes the wiping verification to fail.

[0107] The results indicate that when the surfactant concentration is too high, i.e. greater than 3.0%, the test solution can still identify the defect location, but the background of the intact coating is stained over a large area, resulting in significant background interference, reduced reliability of the judgment, and the inability to use the wiping verification method for auxiliary judgment. Therefore, it is not the preferred formulation range.

[0108] Comparative Example 3: No surfactants were used, only pigment aqueous solution.

[0109] Formula composition: 2L distilled water, 16g of edible red food coloring (dark red powder). Contains no surfactants.

[0110] Preparation method: Pour 2L of distilled water into a clean container and heat to 80℃; add 16g of dark red food coloring powder and stir at 300~500rpm until completely dissolved, which takes about 5~10 minutes; let it cool naturally to room temperature and stand for 15 minutes to remove bubbles, and you will get a red pigment aqueous solution.

[0111] Test method: The droplet method was used, and observations were made at 30 seconds and 300 seconds respectively. At 300 seconds, a horizontal comparison was made with the same time point as Example 1 to eliminate the interference of time factors.

[0112] Results: The surface tension of the surfactant-free pigment aqueous solution was approximately 65-70 mN / m, exhibiting almost no wetting or penetration ability towards the high-barrier coating. After standing for 30 seconds, the pigment aqueous solution formed distinct water droplets on the coating surface and could not spread. After standing for 300 seconds, consistent with the detection time in Example 1, the droplets remained in a contracted state, and the coating surface showed no color change. After wiping with a damp cloth, all pigment could be completely removed, and the coating surface returned to its original state, making defect identification impossible.

[0113] The test solutions obtained from each embodiment and comparative example were subjected to the following tests: Surface tension measurement: The surface tension of the test solutions for each example and comparative example was measured using the platinum plate method under constant temperature conditions of 25℃±0.5℃. An automatic surface tension meter was used. Before measurement, the platinum plate was cleaned with distilled water and ethanol, and the instrument was calibrated. Each sample was measured in triplicate, and the arithmetic mean was taken.

[0114] Defect detection capability assessment: Take high-barrier coated paper samples from the same batch containing known defects, including pinholes and microcracks with a preset diameter of about 100 μm. Use the test solutions of each embodiment and comparative example to detect defects using the coating bar method. After standing for 300 seconds, wipe with a damp cloth to verify and record the clarity of defect color development and the smallest identifiable defect diameter.

[0115] The measurement results of each embodiment and comparative example are summarized in Table 6 below.

[0116] Table 6: Summary of measurement results for each example and comparative example

[0117] As shown in Table 6, there is a significant threshold effect between the detection performance of the test liquid and its surface tension: By preparing coatings with known defects, defect detection tests were conducted using the test solutions from Examples 1-3 and Comparative Example 1. The results are as follows: Figure 9 and Figure 10 As shown, Figure 9 The test results are shown in Examples 1-3, from top to bottom: Example 1, Example 2, and Example 3. Figure 10The results are for Comparative Example 1. It can be seen that the test effect of Example 1 is that the spots are clearly clear, the contrast is high, the test liquid can be completely wiped away without residue, Example 2 detects fewer defects, the penetration is shallower, and the contrast is insufficient, Example 3 has strong penetration ability and can detect defects, but there is slight background staining, and the specific condition of the defects cannot be clearly identified, while Comparative Example 1 cannot detect coating defects at all.

[0118] When the surface tension is >35 mN / m, for example, 55~60 mN / m in Comparative Example 1 and 65~70 mN / m in Comparative Example 3, the test solution cannot effectively wet the surface of the high-barrier coating, and the defective areas cannot be colored, resulting in a loss of detection function. When the surface tension is in the range of 22~32 mN / m, the test solution can selectively penetrate—intact coatings are impermeable and can be wiped away, while defective coatings penetrate and are colored due to capillary action, resulting in clear and high-contrast coloring of defects. When the surface tension is <22 mN / m, for example, 18.6 mN / m in Comparative Example 2, the test solution has excessive penetrating power, which can penetrate intact coatings and cause background staining. After wiping with a damp cloth, red residue remains, leading to false positives.

[0119] When the surfactant concentration is below 0.2%, the surface tension of the test solution is much higher than 32 mN / m, completely losing its detection function. At a surfactant concentration of 0.45% (Example 2), the surface tension is 35-40 mN / m, which can detect larger defects ≥150 μm, but its sensitivity is insufficient for small defects <150 μm. When the surfactant concentration is 1.0%–1.5%, the surface tension falls within the optimal window of 22–32 mN / m, resulting in the best detection performance. When the surfactant concentration is above 3.0%, the surface tension is below 22 mN / m, leading to false positives.

[0120] Moreover, this test solution exhibits rapid colorimetric reaction to coating defects and maintains detection selectivity over a wide time range, making it suitable for testing needs at different operating rhythms on the production line.

[0121] The coating bar method is suitable for large-area rapid screening and is ideal for batch testing on production lines. Both methods can be flexibly selected based on the actual testing scenario.

[0122] In summary, the test data from the embodiments and comparative examples fully verify the effectiveness and superiority of the technical solution of the present invention. By precisely controlling the surface tension within a narrow window of 22 to 32 mN / m and combining it with a simple operation process of "coating-standing-wiping verification", the present invention achieves rapid, visual, and full-width screening of the barrier uniformity of high-barrier coated paper, effectively solving the technical problems of existing detection methods that are time-consuming, can only be sampled at single points, and cannot intuitively view local defects in the coating.

[0123] 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.

[0124] 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.

[0125] 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 test solution for detecting the barrier uniformity of high-barrier coated paper, characterized in that, The test solution contains water, anionic surfactant, and food coloring. The anionic surfactant is a compound system of sodium dodecylbenzenesulfonate and sodium dioctyl sulfosuccinate; The food coloring is a water-soluble red food coloring; The surface tension of the test liquid is 22 mN / m to 32 mN / m.

2. The test solution according to claim 1, characterized in that, The weight ratio of sodium dodecylbenzenesulfonate to sodium dioctyl sulfosuccinate is 1:2 to 2:

1.

3. The test solution according to claim 1, characterized in that, The total weight percentage of the anionic surfactant in the test solution is 0.5% to 2.0%, and the weight percentage of the food coloring in the test solution is 0.3% to 1.5%.

4. The test solution according to claim 1, characterized in that, The test solution consists of the following components in parts by weight. composition: Water: 97-99 parts Sodium dodecylbenzenesulfonate: 0.15–0.7 parts Sodium dioctyl sulfosuccinate: 0.3–1.4 parts Water-soluble red food coloring: 0.3 to 1.5 parts.

5. The test solution according to claim 1, characterized in that, The test solution consists of the following components in parts by weight: Water: 97.5–98.5 parts Sodium dodecylbenzenesulfonate: 0.3–0.5 parts Sodium dioctyl sulfosuccinate: 0.6–1.0 parts Water-soluble red food coloring: 0.5 to 1.0 parts.

6. The test solution according to claim 1, characterized in that, The food coloring is carmine or allura red.

7. A method for preparing the test solution as described in any one of claims 1 to 6, characterized in that, Includes the following steps: Heat the water to 70℃~95℃; Add the prescribed amount of the anionic surfactant compound system to the heated water and stir until dissolved; Add the recommended amount of food coloring and stir until fully mixed. Cool to room temperature to obtain the test solution.

8. A method for applying a test solution to detect the barrier uniformity of high-barrier coated paper, characterized in that, Visually evaluating the barrier uniformity of high-barrier coated paper using the test solution described in any one of claims 1 to 6 includes the following steps: S1. Apply the test liquid to the coating surface of the high-barrier coated paper to be tested; S2, let stand for 60s to 300s; S3. Gently wipe the paper surface with a damp cloth. If there are no colored spots on the paper surface, the coating in the test area is intact; if there are colored spots on the paper surface that cannot be wiped off, there is a coating defect at the colored spots.

9. The application method according to claim 8, characterized in that, The application method in step S1 is either droplet application or coating bar application; The droplet method includes: adding liquid droplets to the coating surface using a dropper, with each droplet having a volume of 0.03 mL to 0.08 mL and a droplet spacing of 20 mm to 30 mm; The coating bar method includes: applying the test liquid to one end of the coating surface, and scraping the coating surface uniformly with a coating bar at a speed of 50 mm / s to 150 mm / s, wherein the gap between the coating bar and the coating surface is 0 to 0.15 mm.

10. The application method according to claim 8, characterized in that, The ambient temperature for the application method is 15℃~30℃, and the relative humidity is 40%~60%.

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