An environment degradable kraft paper lunch box, composition and preparation process

Abstract

The present application relates to the technical field of green packaging materials, and particularly relates to an environment-degradable kraft paper lunch box, a composition and a preparation process, the environment-degradable kraft paper lunch box is prepared from food-grade unbleached kraft paper as a base material layer of the kraft paper lunch box, a dense anti-leakage inner layer of the kraft paper lunch box is formed on one side of the base material layer by PLA film coating, an environment-degradable composition coating composed of nanocellulose, modified starch and sodium bicarbonate is formed on the other side of the base material layer by in-situ foaming to form a microporous fluffy structure layer of the kraft paper lunch box, and an environment-degradable closed layer formed by low-temperature coating of water-based bio-based coating is arranged on the outer surface of the microporous fluffy structure layer; the present application constructs an efficient heat insulation structure with precisely matched thickness and performance requirements, and provides a kraft paper lunch box with excellent heat insulation performance, full environmental protection, degradability and a gradient structure that can be uniformly and controllably realized by a precise process, and an efficient preparation method thereof.

Description

Technical Field

[0001] This invention relates to the field of green packaging materials technology, specifically to an environmentally degradable kraft paper lunch box, its composition, and its preparation process. Background Technology

[0002] Environmentally friendly and biodegradable kraft paper lunch boxes are disposable food packaging containers made primarily of natural kraft paper. Due to their excellent environmental performance and biodegradability, they have been widely used in recent years in areas such as food delivery, fast food packaging, and fresh food distribution. Kraft paper is made from natural wood pulp (usually unbleached sulfate wood pulp), containing no plastic or only a small amount of food-grade waterproof coating (such as PLA polylactic acid). It can be decomposed by microorganisms in the natural environment, ultimately transforming into water, carbon dioxide, and organic matter.

[0003] Environmentally friendly and biodegradable kraft paper lunch boxes face a common pain point in use: their mainstream structure (kraft paper substrate and PLA leak-proof film layer) is dense, resulting in insufficient heat insulation performance. When containing hot food, the outer wall is easy to get hot to the touch, affecting the safety and experience of use.

[0004] To improve thermal insulation, existing technologies mainly employ two approaches: one is to apply a high-efficiency thermal insulation coating to the outer surface, but this type of coating typically cannot be environmentally degraded, which is detrimental to environmental protection; the other is to use a double-layered structure, utilizing an intermediate air layer for insulation. However, while the latter approach theoretically improves thermal insulation, it is difficult to become an ideal solution due to inherent flaws in its design logic. First, there is a significant "performance redundancy" and cost imbalance: for the short-term heat insulation requirements of disposable lunch boxes, the heat insulation capacity provided by the double-layer structure is unnecessarily redundant. It solves the problem by simply physically stacking ("adding") rather than through optimized design of materials and structure. This directly leads to a doubling of the amount of raw materials such as paper and PLA coating, resulting in a significant increase in cost, which runs counter to the reduction principle that environmentally friendly products should follow.

[0005] Secondly, it sacrifices production efficiency and product usability: the solution requires manufacturing two separate boxes and fitting them together, resulting in a complex and interrupted process, leading to low production efficiency and increased energy consumption. Furthermore, the double-layer structure inherently suffers from unstable connections and increased storage and transportation volume, thereby compromising product reliability and commercial distribution efficiency.

[0006] Fundamentally, the two-tier solution fails to adhere to the principle of "good enough" in design. In attempting to address a localized performance bottleneck, it introduces systemic resource waste and complexity, making it neither economical nor sophisticated.

[0007] Therefore, the industry urgently needs a new solution: based on a single-layer substrate, it should focus on constructing a highly efficient insulation structure with a precise match between thickness and performance requirements along the critical heat conduction paths of the food container through material composites and precision processes. The ideal solution should achieve a "just right" insulation effect with minimal material increments and highly integrated continuous processes, while ensuring the overall mechanical strength, reliability, and complete environmental biodegradability of the food container. Summary of the Invention

[0008] To address the aforementioned problems, this invention proposes an environmentally degradable, heat-insulating, and heat-resistant kraft paper lunchbox and its manufacturing process. The aim is to construct a highly efficient heat-insulating structure with a precise match between thickness and performance requirements, and to provide a kraft paper lunchbox with excellent heat insulation performance, complete environmental friendliness and biodegradability, and a uniform and controllable gradient structure achievable through precise processes, along with its efficient manufacturing method. The specific technical solution is as follows: An environmentally degradable kraft paper lunch box comprises a food-grade unbleached kraft paper as the base material layer, a dense, leak-proof inner layer formed by PLA coating on one side of the base material layer, an environmentally degradable coating composed of nanocellulose, modified starch, and sodium bicarbonate forming an outer microporous and fluffy structure layer on the other side of the base material layer through in-situ foaming, and an environmentally degradable sealing layer formed by low-temperature application of a water-based bio-based coating on the outer surface of the microporous and fluffy structure layer.

[0009] Preferably, the average porosity of the microporous loose structure layer is ≥65%, and the thickness after shaping is 200–250 μm; the sealing layer is a non-thermal-sealing water-based bio-based coating sealing layer, or a thermal-sealing water-based bio-based coating sealing layer, with a construction drying temperature ≤60℃ and a thickness after shaping of 5–10 μm.

[0010] In this invention, the microporous fluffy structure layer is used for heat insulation and anti-scalding, and the sealing layer is used to protect the fragile porous structure and provide a printable or grease-resistant surface; optional non-heat-sealing water-based bio-based coating sealing layers and heat-sealing water-based bio-based coating sealing layers are used for different lunch box needs: when using a non-heat-sealing water-based bio-based coating sealing layer, the lunch box overlap sealing relies on the heat sealing of the PLA coating layer; when using a heat-sealing water-based bio-based coating sealing layer, the lunch box overlap sealing can be achieved by simultaneously utilizing the heat sealing of the PLA coating layer and the sealing layer.

[0011] In this invention, the environmentally degradable composition coating is composed of the following components in parts by weight: 2.8–3.8 parts of nanocellulose, 13–16 parts of octenyl succinic acid starch, 7–9 parts of soy protein isolate, 2.8–3.5 parts of micron-sized sodium bicarbonate ultrafine powder, 1.5–2.5 parts of glycerol, 0.2–0.4 parts of polyether-modified siloxane, and the balance being water, with a solid content of 28–32%; the particle size of the micron-sized sodium bicarbonate ultrafine powder is 10–50 μm.

[0012] The non-thermal-sealing waterborne bio-based coating is composed of the following components in parts by weight: 6–10 parts modified starch emulsion, 1.5–3.0 parts chitosan, 0.8–1.5 parts nanocellulose, 0.3–0.8 parts glycerol, 0.1–0.3 parts polyglycerol fatty acid ester, with the balance being water, and a solid content of 15–18%.

[0013] The water-based bio-based coating with heat-sealing function is composed of the following components in parts by weight: 8-12 parts thermoplastic starch, 5-8 parts polyhydroxyalkanoate emulsion, 3-5 parts oxidized starch, 0.5-1.0 parts nanocellulose, 0.8-1.5 parts triethyl citrate, with the balance being water, and a solid content of 15-18%; the polyhydroxyalkanoate emulsion has a particle size ≤200 nm and a melting point of 90-95℃.

[0014] A process for preparing an environmentally degradable kraft paper lunch box and a composition coating formulation includes sequentially passing through a first-stage kraft paper roll pretreatment process and a second-stage kraft paper roll shearing and hot pressing forming process to obtain the environmentally degradable kraft paper lunch box. The first stage of the kraft paper roll pretreatment process includes using a roll of single-sided PLA-coated kraft paper, which is unwound by an unwinding device and wound up by a winding device, with the non-PLA coated side facing upwards after unwinding. Between the unwinding and winding devices, a paper storage buffer station, a corona treatment station, a coating station, an ultrasonic pre-regulation gradient distribution station, a low-temperature hot air drying station, and an infrared heating and ultrasonic synergistic refining foaming station are sequentially arranged to pretreat the kraft paper roll. The specific process steps include the following: S1. Corona treatment: After the kraft paper roll is unwound, it enters the paper storage buffer station, and then comes out from the paper storage buffer station to the corona treatment station, where the non-PLA coated side of the kraft paper is corona treated to enhance the adhesion of subsequent coatings. S2. Coating with foam coating: After corona treatment, the kraft paper enters the coating station, and the environmentally degradable composition coating formula is applied to the non-PLA coated side of the kraft paper by a doctor blade coating method. S3. Ultrasonic pre-controlled gradient distribution: The coated kraft paper enters the ultrasonic pre-controlled gradient distribution station and is subjected to ultrasonic vibration at room temperature. The ultrasonic mechanical vibration is used to destroy the elastic resistance of the nanocellulose network and accelerate the sedimentation of sodium bicarbonate particles inside the coating in the gravitational field, forming a preset gradient distribution with a high concentration of sodium bicarbonate particles at the bottom and a low concentration of sodium bicarbonate particles at the top; the ultrasonic vibration direction is perpendicular to the coating surface. S4. Low-temperature hot air drying: After ultrasonic treatment, the coated kraft paper enters the low-temperature hot air drying station and adopts a segmented drying process. The first stage is carried out at 40–50°C; the second stage is carried out at ≤60°C to ensure that it dries without significant foaming. S5. Segmented Infrared Heating and Dynamic Ultrasonic Co-foaming: After low-temperature hot air drying, the kraft paper enters the infrared heating and ultrasonic co-foaming station. Three-stage infrared radiation heating and dynamically matched ultrasonic vibration are used to achieve infrared heating and dynamic ultrasonic co-foaming. The first stage of infrared heating raises the coating temperature to 60–75°C, while simultaneously applying low-frequency, low-amplitude ultrasonic vibration. The second stage of infrared heating raises the coating temperature to 75–90°C, while simultaneously applying medium-frequency, medium-amplitude ultrasonic vibration. The third stage of infrared heating raises the coating temperature to 90–100°C, while simultaneously applying high-frequency, high-amplitude ultrasonic vibration. S6. Cooling and Shaping of Microporous Loose Structure Layer: After foaming, the kraft paper enters the cooling zone and is cooled by cold air to reduce the coating temperature to room temperature, thereby solidifying and shaping the microporous structure and obtaining the pretreated kraft paper roll.

[0015] In step S5 above, segmented infrared heating and dynamic ultrasonic synergistic foaming, the ultrasonic frequency in the first stage is selected as 20-25 kHz, because the cavitation effect in this frequency range is moderate, mainly promoting the initial formation of micro bubble nuclei; if the frequency is too high (>35 kHz), the cavitation energy will be insufficient, and if it is too low (<18 kHz), it will easily lead to premature bubble merging.

[0016] In this invention, the gradient distribution of porosity within the microporous and fluffy structural layer is achieved through the synergistic interaction of steps S3 and S5. The working mechanism is further explained below: Step S3 utilizes the negative sedimentation effect of high-density sodium bicarbonate in reverse, combined with ultrasonic vibration, to achieve gradient enrichment of particles near the paper substrate: after coating is completed in step S2 and proceeding to step S3, the coating is in a wet liquid state; the sodium bicarbonate powder density is approximately 2.2 g / cm³. 3 The density is significantly higher than that of the water-based coating system (density approximately 1.0-1.2 g / cm³). 3Therefore, under the influence of gravity, there is a natural tendency for the material to settle, which is a "negative settlement" that is usually avoided because it can lead to uneven coating composition. This invention reverses this physical phenomenon and introduces low-frequency ultrasonic vibration (step S3) for active control.

[0017] Specifically, the applied ultrasonic vibration (26–30 kHz, 15–25 μm amplitude) generates a strong acoustic flow effect and tiny cavitation bubbles in the liquid coating. This acoustic flow produces directional microscale flow, accelerating particle transport. More importantly, the microjets generated by moderate cavitation can temporarily and locally disrupt the binding forces of the three-dimensional network structure formed by nanocellulose and starch, reducing the apparent viscosity of the system. In this "temporarily unbound" state, the gravitational sedimentation effect is significantly enhanced and accelerated. By controlling the ultrasonic treatment time (10–20 seconds), most sodium bicarbonate particles can be precisely allowed to settle to the surface area near the kraft paper substrate, while lighter components such as nanocellulose float relatively. This pre-constructs a chemical potential gradient distribution with a high sodium bicarbonate concentration at the bottom and a low concentration at the top in the vertical direction of the coating before the foaming reaction occurs. This creates a gradient foaming base, allowing the high concentration of sodium bicarbonate at the bottom to decompose and generate more gas during foaming. This results in a more porous and bubble-rich reinforced insulation layer that adheres closely to the substrate side where heat sources need to be blocked, maximizing thermal resistance. Meanwhile, the top area has less foaming agent, resulting in smaller and more compact cells. This provides a more robust and flat base for the subsequent coating of the sealing layer, ultimately giving the outer layer of the lunchbox better resistance to pressure and friction.

[0018] Step S5 improves the porosity structure by employing segmented infrared heating and dynamic ultrasonic synergistic foaming: This step is divided into three segments, each involving the synergy of infrared heating and ultrasonic treatment with specific parameters; the infrared heating uses infrared radiation with a wavelength of 3.0-3.5 μm, which can be efficiently absorbed by water and organic matter in the coating, achieving rapid and uniform volumetric heating from within the coating. The segmented heating (60-75°C → 75-90°C → 90-100°C) precisely corresponds to the decomposition kinetics of sodium bicarbonate and the evaporation process of water, avoiding surface overheating and crusting or insufficient internal foaming.

[0019] Specifically, the first stage (low-temperature start-up period): low-frequency, low-amplitude ultrasound (20-25 kHz) is applied. At this stage, sodium bicarbonate has just begun to decompose, and bubbles begin to nucleate. Low-frequency ultrasound helps promote more and more uniform bubble nucleation points. The second stage (main foaming period): the temperature rises to the range where sodium bicarbonate decomposes rapidly. Medium-frequency, medium-amplitude ultrasound (25-28 kHz) is applied. At this stage, bubbles grow rapidly and easily merge into large, unfavorable bubbles. The local high-pressure shock waves generated by the cavitation effect of ultrasound can effectively break (refine) the growing bubbles, while the acoustic flow effect can stir and disperse the bubbles, preventing them from agglomerating and merging, thus ensuring that the bubbles are small in size and uniformly distributed. The third stage (early shaping stage): the temperature is the highest, and the bubbles are basically formed but the structure is still unstable. High-frequency, high-amplitude ultrasound (28-32 kHz) is applied. The denser but relatively concentrated cavitation events generated by high-frequency ultrasound can "refine" the formed foam structure, further homogenize the cell structure, and use acoustic flow to promote the discharge of liquid between the bubble walls, making the cell structure tend to stabilize.

[0020] Through the three-stage infrared ultrasonic synergistic foaming process described above, a microporous structure with smaller average pore size and narrower distribution can be obtained, which greatly improves the uniformity and efficiency of the insulation layer and achieves finer and more uniform foam pores. At the same time, based on the preset sodium bicarbonate concentration gradient, this controlled fine foaming process ensures a gradual transition in pore size and density from bottom to top, perfectly transforming the "chemical gradient" into a "structural gradient" and achieving structural gradient enhancement. In addition, through rapid and uniform infrared heating and short-time ultrasonic treatment, the two work together to shorten the total time required for foaming and shaping, which is conducive to improving the consistency of product performance.

[0021] Preferably, the first-stage kraft paper roll pretreatment process further includes a foaming layer sealing station located after the infrared heating and ultrasonic synergistic refining foaming station, where the following process steps are performed: S7. Coating sealing layer: The non-heat-sealing water-based bio-based coating or the heat-sealing water-based bio-based coating is applied to the surface of the foamed layer by scraping, rolling or spraying to form a sealing layer. S8. Drying treatment of the sealing layer: The sealing layer after coating is dried using a segmented drying process. The first stage uses low-temperature hot air drying, and the second stage uses low-temperature far-infrared drying to avoid damaging the microporous structure of the foaming layer. The moisture content of the dried sealing layer decreases to 5-8%.

[0022] In this invention, the second-stage kraft paper roll shearing and hot-pressing forming process includes the following steps: A1. Unwinding and Die-cutting: Unwind the pre-treated kraft paper roll, cut it into lunch box unfolded pieces through the die-cutting station, and press crease lines on the lunch box unfolded pieces; A2. Folding and heat sealing: Fold the unfolded sheet along the fold lines to form a box shape, and use a heat sealing mold to heat seal the overlapping area to obtain an environmentally degradable kraft paper lunch box.

[0023] In this invention, ultrasonic vibration devices are respectively installed on the ultrasonic pre-regulation gradient distribution station and the infrared heating ultrasonic synergistic refining foaming station. The ultrasonic vibration device includes an ultrasonic vibration table, which is used to support and drive the kraft paper and its coating to vibrate. To address the issue of poor contact between the kraft paper and the vibration table surface caused by tension fluctuations, uneven surfaces, or air gaps during the forward movement of the kraft paper, and to ensure efficient and uniform transmission of ultrasonic vibration energy, the ultrasonic vibration table is equipped with a negative pressure cavity inside. The table surface of the ultrasonic vibration table is densely covered with negative pressure holes that communicate with the negative pressure cavity; the negative pressure cavity is connected to a vacuum system.

[0024] To further optimize and improve the uniformity of negative pressure adsorption and the efficiency of ultrasonic vibration energy transfer, and to avoid localized adhesion dead zones, the ultrasonic vibration table is also machined with crisscrossing microgrooves. These microgrooves are interconnected, forming a microgroove network covering the entire processing area. Preferably, the densely distributed negative pressure holes are located at the nodes where the microgroove network intersects. When the vacuum system is activated, air is rapidly drawn away through the microgroove network to the negative pressure holes at the nodes, thereby quickly establishing a uniform and stable negative pressure on the entire back of the kraft paper. This ensures that the kraft paper remains tightly and flatly adsorbed onto the table surface as it moves forward.

[0025] Because the kraft paper and the vibration table are perfectly bonded, the ultrasonic vibration energy can be transmitted to the inside of the coating with low loss and uniformity. Therefore, in step S3, ultrasonic pre-controlled gradient distribution, the elastic retardation of the nanocellulose network can be efficiently and consistently destroyed, accelerating the sedimentation of sodium bicarbonate particles in the gravitational field and forming a preset concentration gradient with a high bottom and a low top. In step S5, segmented infrared heating and dynamic ultrasonic synergistic foaming, the efficient segmented and graded control of the pores inside the coating can be further realized, further refining and stabilizing the pore structure.

[0026] Furthermore, considering that during the continuous movement of the kraft paper, the microgrooves and negative pressure holes located at the edges and corners (including the outer four-sided area and the corner areas adjacent to the outer four-sided area) where the kraft paper contacts the ultrasonic vibration table are more susceptible to external air disturbances, which can easily lead to a greater decrease and fluctuation in the vacuum level of these areas compared to the central area, the periphery of the vibration table is divided into four outer edge areas and four corner areas. The diameter of the negative pressure holes in each area is increased relative to that in the central area. Simultaneously, the negative pressure holes in each area are also increased in size. The pressure hole is connected to an adjustable automatic valve through a manifold. The airflow area of ​​the automatic valve is greater than the sum of the airflow areas of the enlarged negative pressure holes in that area. Each area is equipped with a negative pressure sensor connected to the microgroove. The valve opening of the automatic valve is adjusted according to the data measured by the negative pressure sensor, so that the negative pressure in the entire area where the kraft paper is in contact with the vibrating table is uniform and consistent. This ensures that the ultrasonic vibration energy is evenly transmitted to the kraft paper in the entire area, thereby ultimately achieving a uniform gradient distribution of sodium bicarbonate particles in the entire kraft paper coating.

[0027] In this invention, the manifold is led out from the internal negative pressure chamber of the ultrasonic vibration table to the outside and then back to the internal negative pressure chamber, and the automatic valve is set on the manifold at the external end.

[0028] Preferably, the ultrasonic vibration table comprises, from top to bottom: an upper plate, a lower plate, and a ring of side plates connecting the edges of the two. The three are sealed together to form a closed negative pressure cavity. The negative pressure hole and microgroove are provided on the upper plate. The ultrasonic vibration device includes a rod array composed of a number of rods that are erected inside the negative pressure cavity and rigidly connected to the lower end face of the upper plate. An ultrasonic transducer (piezoelectric ceramic) is installed at the lower end of each rod, thereby forming an ultrasonic transducer array inside the negative pressure cavity. Its vibration is transmitted to the upper plate of the ultrasonic vibration table through the rods.

[0029] Preferably, a hollow water-cooled plate is also provided inside the negative pressure cavity, and a cooling water jacket array is provided on the hollow water-cooled plate. The cooling water jackets in the cooling water jacket array are correspondingly sleeved on the outer periphery of the ultrasonic transducer, and a radial gap is provided between them and the ultrasonic transducer. A heat-absorbing coating is provided on the inner wall of the cooling water jacket, which can absorb the radiant heat emitted by the ultrasonic transducer in a timely manner and prevent the ultrasonic transducer from overheating.

[0030] Preferably, the cooling water jacket and the hollow water-cooled plate are connected by a welded sealing structure to prevent leakage of cooling water.

[0031] Preferably, the upper end of the hollow water-cooled plate is connected to a half-clamp. The half-clamp is located on the periphery of the rod and has a radial gap with the rod. A heat radiation absorbing coating is provided on the inner wall of the half-clamp to absorb the radiant heat of the rod, prevent the rod from undergoing micro-deformation in the length direction due to thermal expansion and contraction, and reduce the efficiency loss of ultrasonic vibration.

[0032] Preferably, the radial gap between the cooling water jacket and the ultrasonic transducer is 0.5-2 mm, and the radial gap between the half-clamp and the rod body is 0.5-2 mm.

[0033] Preferably, the heat-absorbing coating on the inner wall of the cooling water jacket and the half jacket is a black ceramic coating (such as a silicon carbide-based coating) or a diamond-like carbon film with high infrared absorption rate.

[0034] Preferably, the hollow water-cooled plate is installed on the lower platform of the ultrasonic vibration table via a column. The column is a hollow water-cooled column, which is connected to the hollow water-cooled plate to form a water-cooled circulation channel for the inlet and outlet of external cooling water.

[0035] The rod body is positioned to avoid the negative pressure hole.

[0036] Preferably, multiple miniature accelerometers (such as small-sized ultra-miniature triaxial piezoelectric accelerometers) are glued and fixed to the lower end face of the upper platform using high-strength thermally conductive epoxy resin adhesive. These sensors are positioned near the excitation points of each ultrasonic drive unit (rod and ultrasonic transducer), forming a miniature accelerometer array. The Z-axis of each miniature accelerometer is perpendicular to the plane of the upper platform, allowing direct measurement of the axial vibration acceleration at corresponding positions on the upper platform. Signals from each sensor are collected via thin-diameter shielded cables and led out to an external signal processing system through a vacuum-sealed multi-channel electrical feedthrough interface located on the side panel.

[0037] In this invention, the signal processing system incorporates a vibration field reconstruction algorithm. Based on the finite element dynamic model of the upper platform, this algorithm uses real-time measurement data from the aforementioned sensor array as input to calculate a continuous distribution cloud map of vibration acceleration, velocity, and displacement (amplitude) across the entire working area (two-dimensional plane) of the upper platform. These cloud maps visually reflect the spatial uniformity of the vibration field energy distribution, serving as feedback for closed-loop control. For example, when the signal processing system is operating, it first acquires the vibration signals from the sensor array located on the lower surface of the upper platform. Then, based on these vibration signals, it obtains the overall vibration energy distribution across the working surface of the upper platform and identifies locations or regions where the vibration energy distribution is too high or too low. Based on this, it independently adjusts the driving parameters of each ultrasonic transducer to ensure that the actual energy distribution at each location or region of the ultrasonic vibration table tends to be uniform.

[0038] Preferably, in order to reduce friction between the kraft paper and the vibrating table during movement, the upper surface of the ultrasonic vibrating table is provided with a friction-reducing coating (such as a polytetrafluoroethylene coating).

[0039] At each segment of the infrared heating and ultrasonic co-processing foaming station, the microgroove network on the ultrasonic vibration table not only provides uniform adsorption and adhesion for the moving kraft paper, but also protects the PLA coating on the back of the kraft paper from overheating because the external air entering the microgroove network can quickly remove heat.

[0040] Preferably, to further enhance the overheat protection of the PLA coating on the back of the kraft paper, a tabletop water-cooling device is provided in the ultrasonic vibration device of each segment of the infrared heating and ultrasonic co-processing foaming station. The tabletop water-cooling device includes a hollow upper section of the upper table and an array of negative pressure sleeves within it. Each negative pressure sleeve in the array is welded to the upper table using a sealed structure, and the inner hole of the negative pressure sleeve serves as a negative pressure hole. The hollow inner cavity of the upper table is connected to an external water-cooling circulation channel. By using the negative pressure sleeves, gas-liquid isolation is achieved, preventing leakage of cooling water. The tabletop water-cooling device works in conjunction with the negative pressure airflow of the microgroove network to cool and protect the moving PLA coating layer on the back of the kraft paper.

[0041] It should be further noted that the ultrasonic vibration device innovatively designed in this invention can also be applied independently and extensibly to other applications that require the balanced transfer of ultrasonic energy to materials on a stationary or moving flexible belt (such as a conveyor belt) to improve the quality of the material products.

[0042] The beneficial effects of this invention are: First, the present invention provides an environmentally degradable kraft paper lunchbox, composition, and preparation process. This involves constructing a microporous, fluffy structural layer with an average porosity ≥65% on the outer side of the substrate layer. This layer is filled with pores, utilizing the extremely low thermal conductivity of air to form a highly efficient thermal barrier. Testing shows that the lunchbox prepared using this invention, when filled with 95°C hot water, has an outer surface contact temperature that is 15-20°C lower than that of traditional single-layer PLA coated lunchboxes, significantly improving handling and usage safety.

[0043] Secondly, the present invention provides an environmentally degradable kraft paper lunch box, composition, and preparation process. Through "ultrasonic pre-controlled gradient distribution" and "segmented infrared heating and dynamic ultrasonic synergistic foaming" processes, a gradient microporous structure with "high porosity at the bottom (near the substrate) and low porosity at the top" is formed in the coating thickness direction. This maximizes the thermal insulation performance of the high-porosity area at the bottom. The relatively dense microporous structure at the top, combined with the outer sealing layer, provides sufficient surface hardness, wear resistance, and scratch resistance, solving the common problem of weak surface strength in porous materials. This "sparse inside, dense outside" porosity gradient design achieves an optimal balance between thermal insulation and mechanical strength.

[0044] Third, the present invention provides an environmentally degradable kraft paper lunch box, composition, and preparation process. By setting up an ultrasonic pre-controlled gradient distribution and a segmented infrared heating-dynamic ultrasonic synergistic foaming process, the gradient pre-distribution of the foaming area and the process control of precision foaming are achieved. First, by using ultrasonic vibration to assist gravity sedimentation, a concentration gradient of sodium bicarbonate is pre-established in the liquid coating, so as to pre-enrich the foaming agent (sodium bicarbonate) at the bottom of the coating, laying the structural foundation for the subsequent formation of a gradient pore structure. Subsequently, in the foaming stage, infrared heating provides a uniform heat source to trigger decomposition, while using ultrasound of different frequencies and amplitudes to dynamically control the nucleation, growth, and merging of bubbles, thereby achieving the refinement and gradient distribution of bubbles, and finally obtaining a gradient pore structure with unique high bottom porosity (efficient heat insulation) and low top porosity (maintaining surface strength).

[0045] Fourth, the present invention provides an environmentally degradable kraft paper lunchbox, composition, and preparation process, employing a three-stage infrared heating and dynamically matched ultrasonic vibration synergistic effect. Infrared radiation provides uniform and efficient bulk heating, ensuring synchronous decomposition of the foaming agent; dynamically changing ultrasound (low to high frequency) intervenes at each stage of bubble nucleation, growth, and merging through cavitation and acoustic flow effects, effectively breaking up large bubbles, guiding bubble refinement and uniform distribution, and ultimately obtaining a gradient microporous structure with small and uniform pore size. This synergistic process overcomes the difficulty of controlling bubble size and distribution in traditional heating foaming methods.

[0046] Fifth, the present invention provides an environmentally degradable kraft paper lunchbox, its composition, and its preparation process. From the base material kraft paper and PLA coating layer, to the microporous layer composition (nanocellulose, modified starch, sodium bicarbonate), and then to the outer sealing layer coating (modified starch, chitosan, polyhydroxyalkanoates, etc.), all major components are derived from renewable biomass or substances that can be completely degraded in natural / composting environments. The lunchbox waste is expected to completely degrade within 90-180 days under industrial composting conditions, with no risk of microplastic residue, truly achieving full life-cycle environmental protection from production to disposal.

[0047] Sixth, the present invention provides an environmentally degradable kraft paper lunch box, composition, and preparation process. By employing an ultrasonic vibration table with vacuum adsorption and microgroove network surface, the problem of poor contact between kraft paper and the vibration table surface caused by tension fluctuations, uneven surface, or air gaps during high-speed movement is completely eliminated. On the one hand, this design achieves high efficiency and uniformity in ultrasonic energy transfer, enabling near-ideal mechanical coupling between the kraft paper and the vibration source (table), greatly reducing ultrasonic energy reflection and loss at the interface, and ensuring highly consistent ultrasonic energy density received by each coating layer. On the other hand, this design achieves controllability and repeatability of gradient distribution. Its uniform ultrasonic field ensures that the nanocellulose network in each region of the coating is "unbound" to the same degree, making the sedimentation behavior of sodium bicarbonate particles highly controllable. This allows for the repeated acquisition of a completely consistent concentration gradient preset in each production run, laying a solid foundation for the subsequent formation of a stable and excellent gradient foaming structure. Furthermore, this design achieves process stability and improved production yield, avoiding processing dead zones or uneven effects caused by poor local contact, reducing product performance fluctuations, and improving the stability of the entire production process and the yield of the final product.

[0048] Seventh, the present invention provides an environmentally degradable kraft paper lunch box, composition, and preparation process. The negative pressure chamber of the vibration table is equipped with a miniature accelerometer array for monitoring the distribution of ultrasonic vibration energy on the upper surface. The signal processing system takes the microstructure of "gradient void" as the final control target and achieves a balanced and consistent distribution of vibration energy on the upper surface through intelligent closed-loop control of the vibration field. This makes the porosity gradient distribution inside the outer coating of the kraft paper uniform and consistent throughout the entire area, which is beneficial to improving the consistency of product quality.

[0049] Eighth, the present invention provides an environmentally degradable kraft paper lunch box, composition, and preparation process. The ultrasonic vibration device incorporates a zoned intelligent negative pressure balancing system, aiming to fundamentally solve the problem of uneven adsorption force caused by the "edge effect" in large-area vacuum adsorption stages, thereby laying a stable physical foundation for uniform ultrasonic energy transfer upstream. By setting eight independent control zones located on the four outer sides and four corners, the system can monitor and compensate for vacuum attenuation caused by positional differences in real time, ensuring that the kraft paper substrate maintains a high degree of consistency in its adhesion to each area of ​​the upper plate throughout the ultrasonic treatment process. This eliminates ultrasonic energy transfer loss or distortion caused by poor local adhesion, which is a physical prerequisite for subsequent vibration field balancing. Simultaneously, the zoned intelligent negative pressure balancing system can adaptively adjust and maintain the set adsorption balance state according to changes in the smoothness of different batches of kraft paper, production line speed fluctuations, and tension variations. This significantly reduces product quality risks caused by adsorption force fluctuations, improves the robustness of the entire intelligent processing process and the stability of continuous production, and is beneficial for improving the uniformity of the sodium bicarbonate gradient distribution throughout the coating.

[0050] Ninth, the present invention provides an environmentally degradable kraft paper lunch box, composition, and preparation process. The ultrasonic vibration device adopts a negative pressure cavity with an internal ultrasonic transducer, which utilizes the sound shielding effect of vacuum to greatly reduce environmental noise. The heat of the ultrasonic transducer and the rod is absorbed by the cooling water jacket, which improves the working reliability and efficiency of the ultrasonic transducer.

[0051] Tenth, the present invention provides an environmentally degradable kraft paper lunch box, composition, and preparation process. The entire kraft paper roll pretreatment process can be completed continuously on a roll-to-roll production line. The process is continuous and efficient, and can be seamlessly connected with subsequent die-cutting and hot-pressing processes. It is suitable for large-scale industrial production, with high efficiency and controllable cost. Attached Figure Description

[0052] Figure 1 This is a schematic diagram of the process flow for an environmentally degradable kraft paper lunch box, composition, and preparation process according to the present invention. Figure 2 This is a schematic diagram of the ultrasonic vibration device used in the manufacturing process; Figure 3 Is Figure 2 A schematic diagram of a microgroove network on the upper surface of an ultrasonic vibration table; Figure 4 Is Figure 2 The ultrasonic vibration device has been increased.

[0053] In the diagram: 1. Ultrasonic vibration device, 2. Ultrasonic vibration table, 3. Negative pressure chamber, 4. Negative pressure hole, 5. Vacuum system, 6. Peripheral edge area, 7. Corner area, 8. Manifold, 9. Automatic valve, 10. Negative pressure sensor, 11. Upper platform, 12. Lower platform, 13. Side panel, 14. Rod array, 15. Rod, 16. Ultrasonic transducer, 17. Ultrasonic transducer array, 18. Hollow water-cooled plate, 19. Cooling water jacket array, 20. Cooling water jacket, 21. Half jacket, 22. Hollow water-cooled column, 23. Water-cooled circulation channel, 24. Miniature accelerometer, 25. Miniature accelerometer array, 26. Negative pressure sleeve, 27. Hollow inner cavity, 28. Microgroove, 29. Microgroove network, 30. Kraft paper (roll). Detailed Implementation

[0054] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and examples. The following examples are only used to more clearly illustrate the technical solutions of the present invention and should not be construed as limiting the scope of protection of the present invention.

[0055] Example 1: like Figures 1 to 4 The illustration shows an embodiment of an environmentally degradable kraft paper lunch box, composition, and preparation process of the present invention. Food-grade unbleached kraft paper 30 is used as the substrate layer of the kraft paper lunch box. A dense, leak-proof inner layer of the kraft paper lunch box is formed on one side of the substrate layer by PLA lamination. An environmentally degradable composition coating composed of nanocellulose, modified starch, and sodium bicarbonate is foamed in situ on the other side of the substrate layer to form an outer microporous, fluffy structural layer of the kraft paper lunch box. An environmentally degradable sealing layer formed by low-temperature coating of a water-based bio-based coating is provided on the outer surface of the microporous, fluffy structural layer.

[0056] Preferably, the average porosity of the microporous loose structure layer is ≥65%, and the thickness after shaping is 200–250 μm; the sealing layer is a non-thermal-sealing water-based bio-based coating sealing layer, or a thermal-sealing water-based bio-based coating sealing layer, with a construction drying temperature ≤60℃ and a thickness after shaping of 5–10 μm.

[0057] In this embodiment, the microporous fluffy structure layer is used for heat insulation and anti-scalding, and the sealing layer is used to protect the fragile porous structure and provide a printable or grease-resistant surface; optional non-heat-sealing water-based bio-based coating sealing layers and heat-sealing water-based bio-based coating sealing layers are applied to different lunch box needs: when using a non-heat-sealing water-based bio-based coating sealing layer, the lunch box overlap sealing relies on the heat sealing of the PLA coating layer; when using a heat-sealing water-based bio-based coating sealing layer, the lunch box overlap sealing can be achieved by simultaneously utilizing the heat sealing of the PLA coating layer and the sealing layer.

[0058] Example 2: The environmentally degradable composition coating described in Example 1 is composed of the following components in parts by weight: 2.8–3.8 parts nanocellulose, 13–16 parts octenyl succinic acid starch, 7–9 parts soy protein isolate, 2.8–3.5 parts micron-sized sodium bicarbonate ultrafine powder, 1.5–2.5 parts glycerol, 0.2–0.4 parts polyether-modified siloxane, with the balance being water, and a solid content of 28–32%; the particle size of the micron-sized sodium bicarbonate ultrafine powder is 10–50 μm.

[0059] The non-thermal-sealing waterborne bio-based coating described in Example 1 is composed of the following components in parts by weight: 6–10 parts modified starch emulsion, 1.5–3.0 parts chitosan, 0.8–1.5 parts nanocellulose, 0.3–0.8 parts glycerol, 0.1–0.3 parts polyglycerol fatty acid ester, with the balance being water, and a solid content of 15–18%.

[0060] The water-based bio-based coating with heat-sealing function described in Example 1 is composed of the following components in parts by weight: 8-12 parts thermoplastic starch, 5-8 parts polyhydroxyalkanoate emulsion, 3-5 parts oxidized starch, 0.5-1.0 parts nanocellulose, 0.8-1.5 parts triethyl citrate, with the balance being water, and a solid content of 15-18%; the polyhydroxyalkanoate emulsion has a particle size ≤200 nm and a melting point of 90-95℃.

[0061] Example 3: A preparation process for an environmentally degradable kraft paper lunch box and a composition coating formulation based on Examples 1 and 2 includes sequentially passing through a first-stage kraft paper roll pretreatment process and a second-stage kraft paper roll shearing and hot pressing forming process to obtain the environmentally degradable kraft paper lunch box. The first stage of the kraft paper roll pretreatment process includes using a roll of single-sided PLA-coated kraft paper 30, which is unwound by an unwinding device and wound up by a winding device, with the non-PLA coated side facing upwards after unwinding. Between the unwinding and winding devices, a paper storage buffer station, a corona treatment station, a coating station, an ultrasonic pre-regulation gradient distribution station, a low-temperature hot air drying station, and an infrared heating and ultrasonic synergistic refining foaming station are sequentially arranged to pretreat the kraft paper roll 30. The specific process steps include the following: S1. Corona Treatment: After the kraft paper roll 30 is unwound, it enters the paper storage buffer station, and then exits from the paper storage buffer station to the corona treatment station. The non-PLA coated side of the kraft paper 30 is subjected to corona treatment to enhance the adhesion of subsequent coatings. The power density of the corona treatment is 1.4–1.8 kW•min / m. 2 The linear velocity is 100–130 m / min, resulting in a surface energy ≥46 mN / m; S2. Coating with foam coating: After corona treatment, the kraft paper 30 enters the coating station, and the environmentally degradable composition coating formula is applied to the non-PLA coated side of the kraft paper 30 by a scraper coating method. S3. Ultrasonic Pre-Controlled Gradient Distribution: The coated kraft paper 30 enters the ultrasonic pre-controlled gradient distribution station and is subjected to ultrasonic vibration at room temperature. The ultrasonic vibration frequency is 26–30 kHz and the amplitude is 15–25 μm for 10–20 s. The ultrasonic mechanical vibration is used to destroy the elastic hindrance of the nanocellulose network and accelerate the sedimentation of sodium bicarbonate particles inside the coating in the gravitational field, forming a preset gradient distribution with a high concentration of sodium bicarbonate particles at the bottom and a low concentration of sodium bicarbonate particles at the top. The direction of the ultrasonic vibration is perpendicular to the coating surface. S4. Low-temperature hot air drying: After ultrasonic treatment, the coated kraft paper 30 enters the low-temperature hot air drying station and adopts a segmented drying process. The first stage is carried out at 40–50°C for 10–15 s; the second stage is carried out at ≤60°C for 15–20 s, ensuring that it is dried without significant foaming (the decomposition of sodium bicarbonate is mainly triggered by the directional infrared radiation heating in the subsequent S5 stage). S5. Segmented Infrared Heating and Dynamic Ultrasonic Co-foaming: After low-temperature hot air drying, the kraft paper 30 enters the infrared heating and ultrasonic co-foaming station. Three-stage infrared radiation heating and dynamically matched ultrasonic vibration are used to achieve infrared heating and dynamic ultrasonic co-foaming. In the first stage, infrared heating raises the coating temperature to 60–75°C and maintains it for 5–10 seconds, while simultaneously applying low-frequency, low-amplitude ultrasonic vibration at a frequency of 20–25 kHz (this frequency range provides a moderate cavitation effect, primarily promoting the initial formation of microbubble nuclei; if the frequency is too high (e.g., >35 kHz), the cavitation energy is insufficient; if it is too low (e.g., <18 kHz), premature bubble coalescence is likely), with an amplitude of 10–15 μm. In the second stage, infrared heating raises the coating temperature to 75–90°C and maintains it for 10–15 seconds, while simultaneously applying medium-frequency, medium-amplitude ultrasonic vibration at a frequency of 25–28 kHz and an amplitude of 15–20 μm. In the third stage, infrared heating raises the coating temperature to 90–100°C. (Preferably 92–98°C), maintained for 8–12 seconds, simultaneously subjected to high-frequency, high-amplitude ultrasonic vibration with a frequency of 28–32 kHz and an amplitude of 20–25 μm; the total duration of infrared heating is 23–37 seconds, with a wavelength of 3.0–3.5 μm, and the power density of each stage is independently adjustable (8–12 kW / m² in the first stage). 2 Phase II 14–18 kW / m 2 Phase III 18–22 kW / m 2The ultrasonic vibration breaks up the growing bubbles through cavitation and acoustic flow effects, refining the bubble size and forming a refined micropore gradient distribution structure with high porosity at the bottom and low porosity at the top. S6. Cooling and shaping of the microporous loose structure layer: The foamed kraft paper 30 enters the cooling zone and is cooled by cold air at a temperature of 10–15°C for 10–15 seconds, so that the coating temperature drops to room temperature and the microporous structure is cured and shaped, thus obtaining the pretreated kraft paper roll 30.

[0062] In step S5 above, segmented infrared heating and dynamic ultrasonic synergistic foaming, the ultrasonic frequency in the first stage is selected as 20-25 kHz, because the cavitation effect in this frequency range is moderate, mainly promoting the initial formation of micro bubble nuclei; if the frequency is too high (>35 kHz), the cavitation energy will be insufficient, and if it is too low (<18 kHz), it will easily lead to premature bubble merging.

[0063] In this embodiment, the gradient distribution of porosity inside the microporous and fluffy structure layer is achieved through the synergy of steps S3 and S5. The working mechanism is further explained as follows: Step S3 utilizes the negative sedimentation effect of high-density sodium bicarbonate in reverse, combined with ultrasonic vibration, to achieve gradient enrichment of particles near the paper substrate: after coating is completed in step S2 and proceeding to step S3, the coating is in a wet liquid state; the sodium bicarbonate powder density is approximately 2.2 g / cm³. 3 The density is significantly higher than that of the water-based coating system (density approximately 1.0-1.2 g / cm³). 3 Therefore, under the influence of gravity, there is a natural tendency for the material to settle, which is a "negative settlement" that is usually avoided because it can lead to uneven coating composition. This embodiment reverses this physical phenomenon and introduces low-frequency ultrasonic vibration (step S3) for active control.

[0064] Specifically, the applied ultrasonic vibration (26–30 kHz, 15–25 μm amplitude) generates a strong acoustic flow effect and tiny cavitation bubbles in the liquid coating. This acoustic flow produces directional microscale flow, accelerating particle transport. More importantly, the microjets generated by moderate cavitation can temporarily and locally disrupt the binding forces of the three-dimensional network structure formed by nanocellulose and starch, reducing the apparent viscosity of the system. In this "temporarily unbound" state, the gravitational sedimentation effect is significantly enhanced and accelerated. By controlling the ultrasonic treatment time (10–20 seconds), most sodium bicarbonate particles can be precisely allowed to settle to the surface area near the kraft paper substrate, while lighter components such as nanocellulose float relatively. This pre-constructs a chemical potential gradient distribution with a high sodium bicarbonate concentration at the bottom and a low concentration at the top in the vertical direction of the coating before the foaming reaction occurs. This creates a gradient foaming base, allowing the high concentration of sodium bicarbonate at the bottom to decompose and generate more gas during foaming. This results in a more porous and bubble-rich reinforced insulation layer that adheres closely to the substrate side where heat sources need to be blocked, maximizing thermal resistance. Meanwhile, the top area has less foaming agent, resulting in smaller and more compact cells. This provides a more robust and flat base for the subsequent coating of the sealing layer, ultimately giving the outer layer of the lunchbox better resistance to pressure and friction.

[0065] Step S5 improves the porosity structure by employing segmented infrared heating and dynamic ultrasonic synergistic foaming: This step is divided into three segments, each involving the synergy of infrared heating and ultrasonic treatment with specific parameters; the infrared heating uses infrared radiation with a wavelength of 3.0-3.5 μm, which can be efficiently absorbed by water and organic matter in the coating, achieving rapid and uniform volumetric heating from within the coating. The segmented heating (60-75°C → 75-90°C → 90-100°C) precisely corresponds to the decomposition kinetics of sodium bicarbonate and the evaporation process of water, avoiding surface overheating and crusting or insufficient internal foaming.

[0066] Specifically, the first stage (low-temperature start-up period): low-frequency, low-amplitude ultrasound (20-25 kHz) is applied. At this stage, sodium bicarbonate has just begun to decompose, and bubbles begin to nucleate. Low-frequency ultrasound helps promote more and more uniform bubble nucleation points. The second stage (main foaming period): the temperature rises to the range where sodium bicarbonate decomposes rapidly. Medium-frequency, medium-amplitude ultrasound (25-28 kHz) is applied. At this stage, bubbles grow rapidly and easily merge into large, unfavorable bubbles. The local high-pressure shock waves generated by the cavitation effect of ultrasound can effectively break (refine) the growing bubbles, while the acoustic flow effect can stir and disperse the bubbles, preventing them from agglomerating and merging, thus ensuring that the bubbles are small in size and uniformly distributed. The third stage (early shaping stage): the temperature is the highest, and the bubbles are basically formed but the structure is still unstable. High-frequency, high-amplitude ultrasound (28-32 kHz) is applied. The denser but relatively concentrated cavitation events generated by high-frequency ultrasound can "refine" the formed foam structure, further homogenize the cell structure, and use acoustic flow to promote the discharge of liquid between the bubble walls, making the cell structure tend to stabilize.

[0067] Through the three-stage infrared ultrasonic synergistic foaming process described above, a microporous structure with smaller average pore size and narrower distribution can be obtained, which greatly improves the uniformity and efficiency of the insulation layer and achieves finer and more uniform foam pores. At the same time, based on the preset sodium bicarbonate concentration gradient, this controlled fine foaming process ensures a gradual transition in pore size and density from bottom to top, perfectly transforming the "chemical gradient" into a "structural gradient" and achieving structural gradient enhancement. In addition, through rapid and uniform infrared heating and short-time ultrasonic treatment, the two work together to shorten the total time required for foaming and shaping, which is conducive to improving the consistency of product performance.

[0068] Preferably, the first-stage kraft paper roll pretreatment process further includes a foaming layer sealing station located after the infrared heating and ultrasonic synergistic refining foaming station, where the following process steps are performed: S7. Sealing Layer: The non-heat-sealing water-based bio-based coating or the heat-sealing water-based bio-based coating is applied to the surface of the foamed layer by scraping, rolling, or spraying to form a sealing layer with a wet film thickness of 20-40 μm; the coating speed is 50-150 m / min, and the coating temperature is controlled at 15-30°C; the solid content of the non-heat-sealing water-based bio-based coating is 15-25%, and the solid content of the heat-sealing water-based bio-based coating is 20-30%; S8. Drying treatment of the sealing layer: The sealed layer after coating is dried using a segmented drying process. The first stage uses low-temperature hot air drying at 52±2°C for 15 seconds, and the second stage uses low-temperature far-infrared drying at 58±2°C for 10 seconds to avoid damaging the microporous structure of the foam layer. The moisture content of the dried sealed layer decreases to 5–8%. The surface of the dried sealed layer is smooth and the interior is dense, forming a firm bond with the foam layer.

[0069] In this embodiment, the second-stage kraft paper roll shearing and hot-pressing forming process includes the following steps: A1. Unwinding and Die-cutting: Unwind the pre-treated kraft paper roll 30, cut it into lunch box unfolded pieces through the die-cutting station, and press crease lines on the lunch box unfolded pieces; A2. Folding and Heat Sealing: Fold the unfolded sheet along the creases to form a box shape. Use a heat sealing mold to heat seal the overlapping areas. The heat sealing temperature is 140-160°C, the heat sealing pressure is 0.2-0.4 MPa, and the heat sealing time is 2-3 seconds. After heat sealing, an environmentally degradable kraft paper lunch box is obtained. The kraft paper lunch box has a substrate thickness of 0.22–0.28 mm and a basis weight of 90–140 g / m³. 2 The PLA coating layer has a thickness of 15–25 μm; in the pretreated kraft paper roll 30, the microporous outer layer, the kraft paper substrate layer and the PLA coating inner layer form a three-layer composite structure with a total thickness of 0.42–0.53 mm.

[0070] In this embodiment, ultrasonic vibration devices 1 are respectively installed on the ultrasonic pre-regulation gradient distribution station and the infrared heating ultrasonic co-processing foaming station. The ultrasonic vibration device 1 includes an ultrasonic vibration table 2, which is used to carry and drive the kraft paper 30 and its coating to vibrate. To address the issue of poor contact between the kraft paper 30 and the vibration table surface caused by tension fluctuations, uneven surfaces, or air gaps during the forward movement of the kraft paper 30, and to ensure efficient and uniform transmission of ultrasonic vibration energy, the ultrasonic vibration table 2 is equipped with a negative pressure cavity 3 inside. The table surface of the ultrasonic vibration table 2 is densely covered with negative pressure holes 4 that communicate with the negative pressure cavity 3. The negative pressure cavity 3 is connected to a vacuum system 5.

[0071] To further optimize and improve the uniformity of negative pressure adsorption and the efficiency of ultrasonic vibration energy transfer, and to avoid localized dead zones in adhesion, the ultrasonic vibration table 2 is also machined with crisscrossing microgrooves 28. These microgrooves 28 are interconnected, forming a microgroove network 29 covering the entire processing area. Preferably, the densely distributed negative pressure holes 4 are located at the nodes where the microgroove network 29 intersects. When the vacuum system 5 is activated, air is rapidly drawn away through the microgroove network 29 to the negative pressure holes 4 at the nodes, thereby quickly establishing a uniform and stable negative pressure on the entire back of the kraft paper 30, ensuring that the kraft paper 30 remains tightly and flatly adsorbed onto the table surface as it moves forward.

[0072] Preferably, the depth of the microgrooves 28 is 0.2-0.5 mm, the width is 1-2 mm, and the spacing between adjacent grooves is 20-50 mm. This design effectively guides airflow without leaving obvious indentations on the back of the kraft paper 30.

[0073] Because the kraft paper 30 and the vibration table are perfectly bonded, the ultrasonic vibration energy can be transmitted to the inside of the coating with low loss and uniformity. Therefore, in step S3, ultrasonic pre-controlled gradient distribution, the elastic retardation of the nanocellulose network can be efficiently and consistently destroyed, accelerating the sedimentation of sodium bicarbonate particles in the gravitational field and forming a preset concentration gradient with a high bottom and a low top. In step S5, segmented infrared heating and dynamic ultrasonic synergistic foaming, the efficient segmented and graded control of the pores inside the coating can be further realized, further refining and stabilizing the pore structure.

[0074] Furthermore, considering that during the continuous movement of the kraft paper 30, the microgrooves 28 and negative pressure holes 4 located at the edges and corners (including the outer four-sided area and the corner area adjacent to the outer four-sided area) of the kraft paper 30 in contact with the ultrasonic vibration table 2 are more susceptible to external air disturbances, which can easily lead to a greater decrease and fluctuation in the vacuum degree of this area compared to the central area, the periphery of the vibration table is divided into four outer edge areas 6 and four corner areas 7. The diameter of the negative pressure hole 4 in each area is increased relative to the diameter of the negative pressure hole 4 in the central area. At the same time, the negative pressure holes in each area are also increased. 4. A manifold 8 is connected to an adjustable automatic valve 9. The airflow area of ​​the automatic valve 9 is greater than the sum of the airflow areas of the enlarged negative pressure holes 4 in the area. Each area is equipped with a negative pressure sensor 10 connected to the microgroove 28. The valve opening of the automatic valve 9 is adjusted according to the data measured by the negative pressure sensor 10, so that the negative pressure in the entire area where the kraft paper 30 is in contact with the vibrating worktable is uniform and consistent. This ensures that the ultrasonic vibration energy is uniformly transmitted to the kraft paper 30 in the entire area, thereby ultimately achieving a uniform gradient distribution of sodium bicarbonate particles in the entire kraft paper coating.

[0075] In this embodiment, the manifold 8 is led out from the internal negative pressure chamber 3 of the ultrasonic vibration table 2 to the outside and then back to the internal negative pressure chamber 3, and the automatic valve 9 is set on the manifold 8 located on the outside.

[0076] Preferably, the ultrasonic vibration table 2 comprises, from top to bottom: an upper plate 11, a lower plate 12, and a ring of side plates 13 connecting the four edges of the two. The three are sealed together to form a closed negative pressure cavity 3. The negative pressure hole 4 and the microgroove 28 are provided on the upper plate 11. The ultrasonic vibration device 1 includes a rod array 14 composed of a number of rods 15, which are erected inside the negative pressure cavity 3 and rigidly connected to the lower end face of the upper plate 11. An ultrasonic transducer 16 is installed at the lower end of each rod 15, thereby forming an ultrasonic transducer array 17 inside the negative pressure cavity 3. Its vibration is transmitted to the upper plate 11 of the ultrasonic vibration table 2 through the rods 15.

[0077] Preferably, a hollow water-cooled plate 18 is also provided inside the negative pressure cavity 3, and a cooling water jacket array 19 is provided on the hollow water-cooled plate 18. The cooling water jacket 20 in the cooling water jacket array 19 is correspondingly sleeved on the outer periphery of the ultrasonic transducer 16, and a radial gap is provided between it and the ultrasonic transducer 16. A heat-absorbing coating is provided on the inner wall of the cooling water jacket 20, which can absorb the radiant heat emitted by the ultrasonic transducer 16 in a timely manner and prevent the ultrasonic transducer 16 from overheating.

[0078] Preferably, the cooling water jacket 20 and the hollow water-cooled plate 18 are connected by a welded sealing structure to prevent leakage of cooling water.

[0079] Preferably, the upper end of the hollow water-cooled plate 18 is connected to a half-clamp 21. The half-clamp 21 is located around the rod 15 and has a radial gap with the rod 15. A heat radiation absorbing coating is provided on the inner wall of the half-clamp 21 to absorb the radiant heat of the rod 15, prevent the rod 15 from undergoing micro-deformation in the length direction due to thermal expansion and contraction, and reduce the efficiency loss of ultrasonic vibration.

[0080] Preferably, the radial gap between the cooling water jacket 20 and the ultrasonic transducer 16 is 0.5-2mm, and the radial gap between the half-clamp 21 and the rod 15 is 0.5-2mm.

[0081] Preferably, the heat-absorbing coating on the inner wall of the cooling water jacket 20 and the half jacket 21 is a black ceramic coating (such as a silicon carbide-based coating) or a diamond-like carbon film with high infrared absorption rate.

[0082] Preferably, the hollow water-cooled plate 18 is installed on the lower platform 12 of the ultrasonic vibration table 2 via a column. The column is a hollow water-cooled column 22, which is connected to the hollow water-cooled plate 18 to form a water-cooled circulation channel 23 for external cooling water to enter and exit.

[0083] The rod body 15 is positioned to avoid the negative pressure hole 4.

[0084] Preferably, multiple miniature accelerometers 24 (e.g., small-sized ultra-miniature triaxial piezoelectric accelerometers) are glued and fixed to the lower end face of the upper platform 11 using high-strength thermally conductive epoxy resin adhesive. Their positions correspond to the excitation points of each ultrasonic drive unit (rod 15 and ultrasonic transducer 16), forming a miniature accelerometer array 25. The Z-axis of each miniature accelerometer 24 is perpendicular to the plane of the upper platform 11, used to directly measure the axial vibration acceleration at corresponding positions on the upper platform 11. The signals from each sensor are collected via thin-diameter shielded cables and led out to an external signal processing system through a vacuum-sealed multi-channel electrical feedthrough interface located on the side panel 13.

[0085] In this embodiment, the signal processing system incorporates a vibration field reconstruction algorithm. Based on the finite element dynamic model of the upper platform 11, and using real-time measurement data from the sensor array 25 as input, the algorithm calculates a continuous distribution cloud map of vibration acceleration, velocity, and displacement (amplitude) across the entire working area (two-dimensional plane) of the upper platform 11. These cloud maps visually reflect the spatial uniformity of the vibration field energy distribution, serving as feedback for closed-loop control. For example, when the signal processing system is operating, it first acquires the vibration signal from the sensor array 25 located on the lower surface of the upper platform 11. Then, based on the vibration signal, it obtains the overall vibration energy distribution across the working surface of the upper platform 11 and identifies locations or areas where the vibration energy distribution is too high or too low. Based on this, it independently adjusts the driving parameters of each ultrasonic transducer 16 to ensure that the actual energy distribution at each position or region of the ultrasonic vibration table 2 tends to be uniform.

[0086] Preferably, in order to reduce friction between the kraft paper 30 and the vibrating table when it moves, the upper surface of the ultrasonic vibration table 2 is provided with a friction-reducing coating (such as a polytetrafluoroethylene coating).

[0087] At each segment of the infrared heating and ultrasonic co-processing foaming station, the microgroove network 29 on the ultrasonic vibration table 2 can not only uniformly adsorb and adhere the moving kraft paper 30, but also protect the PLA coating on the back of the kraft paper 30 from overheating because the external air entering the microgroove network 29 can quickly remove heat.

[0088] Preferably, to further enhance the overheat protection of the PLA coating on the back of the kraft paper 30, a tabletop water-cooling device is provided in the ultrasonic vibration device 1 of each segment of the infrared heating and ultrasonic co-processing foaming station. The tabletop water-cooling device includes a hollow upper section of the upper platform 11, with a negative pressure sleeve array within it. Each negative pressure sleeve 26 in the array is welded to the upper platform 11, and the inner hole of the negative pressure sleeve 26 serves as the negative pressure hole 4. The hollow inner cavity 27 of the upper platform 11 is connected to an external water-cooling circulation channel 23. By providing the negative pressure sleeves 26, gas-liquid isolation is achieved, preventing leakage of cooling water. The tabletop water-cooling device works in conjunction with the negative pressure airflow of the microgroove network 29 to cool and protect the PLA coating layer on the back of the moving kraft paper 30.

[0089] It should be noted that the ultrasonic vibration device 1 innovatively configured in this embodiment can also be applied independently and extensibly to other applications that require the balanced transmission of ultrasonic energy to materials on a stationary or moving flexible belt (such as a conveyor belt) to improve the quality of material products.

[0090] Example 4: Based on Examples 1-3, the technical parameters are further optimized as follows: Preparation of an environmentally degradable, heat-insulating, and heat-resistant kraft paper lunch box, the substrate being single-sided PLA-coated food-grade kraft paper 30 (substrate weight 120g / m²). 2 Thickness 0.25mm; PLA coating layer thickness 20μm).

[0091] Foaming coating formula: Weigh the following by weight: 3.5 parts nanocellulose, 15 parts octenyl succinic acid starch, 8 parts soy protein isolate, 3.2 parts sodium bicarbonate powder with a particle size of 30μm, 2.0 parts glycerin, 0.3 parts polyether modified siloxane, and the balance is water, with a solid content of 30%.

[0092] Sealing layer formulation: Thermo-sealing functional water-based bio-based coating is used: 10 parts thermoplastic starch, 7 parts polyhydroxyalkanoate emulsion (melting point 92°C), 4 parts oxidized starch, 0.8 parts nanocellulose, 1.2 parts triethyl citrate, and 16% dehydrated to solid content.

[0093] Preparation process: Pretreatment: Proceed according to steps S1-S8. Key parameters are taken as median values: Corona power 1.6 kW•min / m 2 S3 Ultrasonic frequency 28kHz, amplitude 20μm, time 15s; S5 Three-segment infrared-ultrasonic synergistic foaming (68°C / 25kHz→82°C / 26kHz→95°C / 30kHz); S7 Coating sealing layer wet film thickness 30μm.

[0094] Molding: Die-cut and hot-press molding are performed according to steps A1-A2 (heat sealing temperature 150°C).

[0095] Example 5: The preparation of an environmentally degradable, heat-insulating, and heat-resistant kraft paper lunch box differs from Example 4 in that the sealing layer formulation is different. Details are as follows: Sealing layer formulation: A non-thermal-sealing water-based bio-based coating is used: 8 parts modified starch emulsion, 2.0 parts chitosan, 1.2 parts nanocellulose, 0.5 parts glycerol, 0.2 parts polyglycerol fatty acid ester, and the balance is water, with a solid content of 17%.

[0096] Example 6: The preparation of an environmentally degradable, heat-insulating, and heat-resistant kraft paper lunch box differs from Example 4 only in that the gradient control in step S3 is omitted.

[0097] Example 7: The preparation of an environmentally degradable, heat-insulating, and heat-resistant kraft paper lunch box differs from Example 5 only in that the gradient control in step S3 is omitted.

[0098] Example 8: The environmentally degradable heat-insulating and heat-resistant kraft paper lunch boxes prepared in Examples 5-8 were tested for heat insulation and heat-resistant performance, and Comparative Example 1 and Comparative Example 2 were compared.

[0099] Comparative Example 1 is a single-layer PLA coated paper cup, and Comparative Example 2 is a double-layer hollow paper cup.

[0100] Test conditions: The lunch box of the present invention or the paper cup of the comparative example are filled with 95°C hot water, and the outer surface temperature is measured with a thermal imager after 30 seconds.

[0101] Measurement results: Example 4 (using gradient control process): 59°C; Example 5 (using gradient control process): 59°C; Example 6 (gradient control process omitted): 65°C; Example 7 (gradient control process omitted): 65°C; Comparative Example 1 (single-layer PLA coated paper cup): 80°C; Comparative Example 2 (double-walled hollow paper cup): 48°C; The experimental results show that: 1. The lunch box of the present invention has better heat insulation performance than a single-layer PLA paper cup, and is between that of a single-layer PLA paper cup and a double-layer hollow paper cup.

[0102] 2. The heat insulation performance of the lunch box with gradient control in this invention is better than that of the lunch box without gradient control.

[0103] Example 9: Gradient-controlled coating density test: The outer coating sample prepared in Example 5 was completely peeled off from the kraft paper substrate and precisely cut into upper and lower layers along the thickness direction; the upper layer is the top coating far from the substrate, and the lower layer is the bottom coating near the substrate; the coating volume was automatically calculated by 3D scanning modeling software, the coating weight was obtained by precision electronic scale, and then the coating density was measured. The structure is as follows: The apparent density of the top coating is 0.62 g / cm³. 3 The apparent density of the bottom coating is 0.31 g / cm³. 3 ; Based on the formulation components, the density after dehydration was estimated, and the theoretical true density of the material was approximately 1.50 g / cm³. 3 ; Using the formula porosity = (1 - apparent density of material / theoretical true density of material) × 100%, the porosity results are as follows: The porosity of the top coating is 58.7%; the porosity of the bottom coating is 79.3%; and the average porosity is 69%.

[0104] Test results show that the top coating of the gradient design is denser and the bottom coating is more porous, forming a gradient distribution structure with high porosity at the bottom and low porosity at the top.

[0105] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. An environmentally degradable kraft paper lunch box, characterized in that, The kraft paper lunch box uses food-grade unbleached kraft paper as its base material. A dense, leak-proof inner layer of the kraft paper lunch box is formed on one side of the base material by PLA coating. An environmentally degradable coating composed of nanocellulose, modified starch, and sodium bicarbonate is formed on the other side of the base material by in-situ foaming to create a microporous, fluffy outer layer of the kraft paper lunch box. An environmentally degradable sealing layer formed by low-temperature application of a water-based bio-based coating is provided on the outer surface of the microporous, fluffy outer layer.

2. The environmentally degradable kraft paper lunch box according to claim 1, characterized in that, The average porosity of the microporous and fluffy structure layer is ≥65%, and the thickness after shaping is 200–250 μm; the sealing layer is a non-thermal-sealing water-based bio-based coating sealing layer, or a thermal-sealing water-based bio-based coating sealing layer, with a construction drying temperature ≤60℃ and a thickness after shaping of 5–10 μm.

3. A compositional coating formulation for an environmentally degradable kraft paper lunchbox according to any one of claims 1 to 2, characterized in that, The environmentally degradable coating composition comprises the following components in parts by weight: 2.8–3.8 parts nanocellulose, 13–16 parts octenyl succinic acid starch, 7–9 parts soy protein isolate, 2.8–3.5 parts micron-sized sodium bicarbonate ultrafine powder, 1.5–2.5 parts glycerol, 0.2–0.4 parts polyether-modified siloxane, with the balance being water, and a solid content of 28–32%; the micron-sized sodium bicarbonate ultrafine powder has a particle size of 10–50 μm.

4. The compositional coating formulation for an environmentally degradable kraft paper lunch box according to claim 3, characterized in that, The non-thermal-sealing waterborne bio-based coating is composed of the following components in parts by weight: 6–10 parts modified starch emulsion, 1.5–3.0 parts chitosan, 0.8–1.5 parts nanocellulose, 0.3–0.8 parts glycerol, 0.1–0.3 parts polyglycerol fatty acid ester, with the balance being water, and a solid content of 15–18%.

5. The compositional coating formulation for an environmentally degradable kraft paper lunch box according to claim 3, characterized in that, The water-based bio-based coating with heat-sealing function is composed of the following components in parts by weight: 8–12 parts thermoplastic starch, 5–8 parts polyhydroxyalkanoate emulsion, 3–5 parts oxidized starch, 0.5–1.0 parts nanocellulose, 0.8–1.5 parts triethyl citrate, with the balance being water, and a solid content of 15–18%; the polyhydroxyalkanoate emulsion has a particle size ≤200 nm and a melting point of 90–95℃.

6. A preparation process for an environmentally degradable kraft paper lunch box and a composition coating formulation according to any one of claims 1 to 5, characterized in that, The environmentally degradable kraft paper lunch box is produced by sequentially passing through a first-stage kraft paper roll pretreatment process and a second-stage kraft paper roll shearing and hot pressing forming process. The first stage of the kraft paper roll pretreatment process includes using a roll of single-sided PLA-coated kraft paper, which is unwound by an unwinding device and wound up by a winding device, with the non-PLA coated side facing upwards after unwinding. Between the unwinding and winding devices, a paper storage buffer station, a corona treatment station, a coating station, an ultrasonic pre-regulation gradient distribution station, a low-temperature hot air drying station, and an infrared heating and ultrasonic synergistic refining foaming station are sequentially arranged to pretreat the kraft paper roll. The specific process steps include the following: S1. Corona treatment: After the kraft paper roll is unwound, it enters the paper storage buffer station, and then comes out from the paper storage buffer station to the corona treatment station, where the non-PLA coated side of the kraft paper is corona treated to enhance the adhesion of subsequent coatings. S2. Coating with foam coating: After corona treatment, the kraft paper enters the coating station, and the environmentally degradable composition coating formula is applied to the non-PLA coated side of the kraft paper by a doctor blade coating method. S3. Ultrasonic pre-controlled gradient distribution: The coated kraft paper enters the ultrasonic pre-controlled gradient distribution station and is subjected to ultrasonic vibration at room temperature. The ultrasonic mechanical vibration is used to destroy the elastic resistance of the nanocellulose network and accelerate the sedimentation of sodium bicarbonate particles inside the coating in the gravitational field, forming a preset gradient distribution with a high concentration of sodium bicarbonate particles at the bottom and a low concentration of sodium bicarbonate particles at the top; the ultrasonic vibration direction is perpendicular to the coating surface. S4. Low-temperature hot air drying: After ultrasonic treatment, the coated kraft paper enters the low-temperature hot air drying station and adopts a segmented drying process. The first stage is carried out at 40–50°C and the second stage is carried out at ≤60°C to ensure that it is dried without significant foaming. S5. Segmented Infrared Heating and Dynamic Ultrasonic Co-foaming: After low-temperature hot air drying, the kraft paper enters the infrared heating and ultrasonic co-foaming station. Three-stage infrared radiation heating and dynamically matched ultrasonic vibration are used to achieve infrared heating and dynamic ultrasonic co-foaming. The first stage of infrared heating raises the coating temperature to 60–75°C, while simultaneously applying low-frequency, low-amplitude ultrasonic vibration. The second stage of infrared heating raises the coating temperature to 75–90°C, while simultaneously applying medium-frequency, medium-amplitude ultrasonic vibration. The third stage of infrared heating raises the coating temperature to 90–100°C, while simultaneously applying high-frequency, high-amplitude ultrasonic vibration. S6. Cooling and Shaping of Microporous Loose Structure Layer: After foaming, the kraft paper enters the cooling zone and is cooled by cold air to reduce the coating temperature to room temperature, thereby solidifying and shaping the microporous structure and obtaining the pretreated kraft paper roll.

7. The preparation process of the environmentally degradable kraft paper lunch box and the composition coating formulation according to claim 6, characterized in that, The first-stage kraft paper roll pretreatment process also includes a foaming layer sealing station located after the infrared heating and ultrasonic synergistic refining foaming station, where the following process steps are adopted: S7. Coating sealing layer: The non-heat-sealing water-based bio-based coating or the heat-sealing water-based bio-based coating is applied to the surface of the foamed layer by scraping, rolling or spraying to form a sealing layer. S8. Drying treatment of the sealing layer: The sealing layer after coating is dried using a segmented drying process. The first stage uses low-temperature hot air drying, and the second stage uses low-temperature far-infrared drying to avoid damaging the microporous structure of the foaming layer. The moisture content of the dried sealing layer decreases to 5-8%.

8. The preparation process of the environmentally degradable kraft paper lunch box and the composition coating formulation according to claim 6, characterized in that, The second stage of the kraft paper roll shearing and hot pressing process includes the following steps: A1. Unwinding and Die-cutting: Unwind the pre-treated kraft paper roll, cut it into lunch box unfolded pieces through the die-cutting station, and press crease lines on the lunch box unfolded pieces; A2. Folding and heat sealing: Fold the unfolded sheet along the fold lines to form a box shape, and use a heat sealing mold to heat seal the overlapping area to obtain an environmentally degradable kraft paper lunch box.

9. The preparation process of the environmentally degradable kraft paper lunch box and the composition coating formulation according to claim 7, characterized in that, Ultrasonic vibration devices are respectively installed on the ultrasonic pre-regulation gradient distribution station and the infrared heating ultrasonic co-refining foaming station. The ultrasonic vibration device includes an ultrasonic vibration table, which is used to support and drive the kraft paper and its coating to vibrate. The ultrasonic vibration table has a negative pressure cavity inside, and the table surface of the ultrasonic vibration table is densely covered with negative pressure holes that communicate with the negative pressure cavity. The negative pressure cavity is connected to a vacuum system.

10. The preparation process of the environmentally degradable kraft paper lunch box and the composition coating formulation according to claim 8, characterized in that, The ultrasonic vibration table comprises, from top to bottom, an upper plate, a lower plate, and a ring of side panels connecting the edges of the two. The three are sealed together to form a closed negative pressure cavity. The negative pressure holes and microgrooves are provided on the upper plate. The ultrasonic vibration device includes a rod array consisting of a number of rods that are erected inside the negative pressure cavity and rigidly connected to the lower end face of the upper plate. An ultrasonic transducer is installed at the lower end of each rod, thereby forming an ultrasonic transducer array inside the negative pressure cavity. Its vibration is transmitted to the upper plate of the ultrasonic vibration table through the rods.

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