A bag-in-bag type time-release bag controlled slow-release fertilizer and a preparation method thereof

Abstract

The application discloses a bag-in-bag type time sequence release bag controlled slow release fertilizer and a preparation method thereof, and belongs to the technical field of slow release fertilizers. The slow release fertilizer comprises an outer bag body and at least one inner bag body which is encapsulated in the inner part of the outer bag body and is completely immersed in the first fertilizer; a delay starting mechanism is arranged on the outer surface of the inner bag body, so that the outer bag body contacts water and releases nutrients after being buried in soil, and the inner bag body is started in a delayed manner, thereby forming time sequence release. The delay starting mechanism is preferably a three-layer gradient degradation composite coating, which comprises a hydrophilic degradable polyester layer, a barrier wax-polymer composite layer and a hydrophobic degradable polyester layer from inside to outside, and the thickness of the barrier layer accounts for 40%-60% of the total coating thickness. According to the application, double-stage or multi-stage relay fertilizer supply can be realized through one-time burying, the fertilizer efficiency period is greatly prolonged, the labor cost for topdressing is reduced, the fertilizer utilization rate is improved, and the preparation process is simple and easy for industrial production.

Description

Technical Field

[0001] This invention relates to the field of controlled-release fertilizer technology, specifically to a bag-in-bag type time-release controlled-release fertilizer and its preparation method. Background Technology

[0002] Controlled-release fertilizer in bags encapsulates fertilizer granules within a bag material with controlled-release properties. The nutrient release rate is regulated by the slits, micropores, or degradation characteristics of the membrane material on the bag surface. Due to its relatively simple preparation process, low membrane material usage, and controllable cost, it has advantages in the fertilization of perennial crops such as forestry, fruit trees, and landscaping.

[0003] However, existing known bagged controlled-release fertilizers generally adopt a single-bag structure, and their nutrient release cycle is relatively fixed, usually lasting from several weeks to several months. For crops with long growth cycles and high nutrient requirements, the limited nutrient supply period of a single bag is insufficient to cover the entire growth stage, creating a significant nutrient supply gap. If multiple identical bags are applied at once, each bag will start releasing nutrients simultaneously due to synchronous contact with soil moisture, failing to create an effective temporal gradient. If multiple topdressing methods are used to compensate for insufficient nutrient supply in the later stages, it significantly increases labor input and field operation costs, and multiple furrow or hole applications can cause repeated mechanical damage to crop roots.

[0004] To extend the fertilization cycle, some technical solutions attempt to combine multiple controlled-release bags with different degradation rates in parallel, hoping to achieve segmented nutrient release through the difference in the rate of membrane degradation. However, this parallel combination scheme has a key structural flaw: all bags are simultaneously exposed to moisture and microorganisms after being buried in the soil, and the degradation and nutrient dissolution processes of each bag highly overlap in time. While the bags that degrade first release nutrients, the bags that degrade later have already begun irreversible water absorption and degradation, making it impossible to provide delayed protection for any specific bag. Therefore, this parallel combination is essentially the superposition of the independent release curves of each bag, rather than a true staged relay fertilization.

[0005] In the field of coated slow-release fertilizer technology, another approach involves sequentially coating multiple layers of film onto the surface of a single fertilizer granule to achieve segmented release. This type of technology uses a single granule as the controlled-release unit, making it difficult to achieve zoned encapsulation and independent delayed activation of different nutrient formulations within the same fertilization unit. Furthermore, the multi-layered film-formed technology involves tight adhesion or minimal gaps between the layers, lacking independent bag units, and the inner space is not immersed in the outer fertilizer layer, fundamentally differing from a nested structure with independent bags.

[0006] In summary, existing bag-based controlled-release fertilizer technology has long failed to solve a core problem: how to ensure the preferential release of the outer layer of fertilizer under a single application condition, while simultaneously forming a physical barrier for the inner layer to delay its contact with water, thereby achieving a phased and orderly relay nutrient supply model. Overcoming these shortcomings is of crucial technological value and industrial significance for resolving the contradiction between long-term fertilization and structural simplification in bag-based controlled-release fertilizers, and for reducing the reliance on manual fertilization throughout the entire growth period of perennial crops. Summary of the Invention

[0007] To address the aforementioned shortcomings, this invention provides a bag-in-bag type time-release controlled-release fertilizer and its preparation method. By constructing a "bag-in-bag" structure in which the inner bag is completely immersed in the fertilizer in the outer bag, and using the outer bag as a primary release source and a physical isolation barrier for the inner bag, the invention actively delays the start time of the inner bag contacting water, forming a relay fertilizer supply mechanism, thus overcoming the technical bottleneck of simultaneous triggering of each bag in parallel controlled-release fertilizer.

[0008] To achieve the above technical objectives, the present invention adopts the following technical solution:

[0009] A bag-in-bag type time-release controlled-release fertilizer includes:

[0010] An outer bag, the interior of which is filled with the first fertilizer;

[0011] At least one inner bag is enclosed inside the outer bag and completely submerged in the first fertilizer, and the interior of the inner bag is filled with a second fertilizer.

[0012] The outer surface of the inner bag is provided with a delayed start mechanism, which allows water to contact and trigger the outer bag to release nutrients first after the outer bag is buried in the soil. However, due to the obstruction of the delayed start mechanism, the nutrient release start time of the inner bag is later than that of the outer bag, forming a sequential release from the outside to the inside.

[0013] Preferably, the delayed start mechanism is a gradient degradation composite coating applied to the outer surface of the inner bag. The gradient degradation composite coating comprises, from the inside out, a hydrophilic degradable polyester layer, a barrier wax-polymer composite layer, and a hydrophobic degradable polyester layer. The thickness of the barrier wax-polymer composite layer is 40% to 60% of the total coating thickness. The hydrophilic degradable polyester layer is a polyvinyl alcohol layer, and the hydrophobic degradable polyester layer is a polycaprolactone layer.

[0014] Preferably, the barrier wax-polymer composite layer is made by melt blending the following raw materials in parts by weight: 25-35 parts natural wax, 15-20 parts oxidized polyethylene wax, 10-20 parts polycaprolactone, 5-10 parts polybutylene succinate, 2.5-4.5 parts nano-silica treated with silane coupling agent, and 2-5 parts biochar. The components work together to form a barrier structure with an initial water contact angle greater than 110° that gradually disintegrates under the action of soil microorganisms, so that the inner bag is in a moisture-isolated state for the first 1-2 months after being buried in the soil.

[0015] Preferably, there are two or more inner bags, and each inner bag is coated with a gradient degradation composite coating with different numbers and / or different thicknesses, so that multiple inner bags can sequentially start releasing nutrients after being buried in the soil, thereby achieving multi-stage relay fertilization.

[0016] Preferably, the outer bag is a slit-type controlled-release bag, with elongated slits of 1.75 to 3.25 mm in length and 0.1 to 0.5 mm in width distributed on the surface of the bag. The edges of the slits are hardened by hot pressing and melting followed by cooling and shaping to prevent soil particles from clogging the slits.

[0017] Preferably, the outer bag body is made of a fully biodegradable resin film with a degradation cycle of 1 to 2 months, and the inner bag body is made of a fully biodegradable resin film with a degradation cycle of 3 to 6 months. Furthermore, the inner bag body's bag film material contains 0.8% to 1.7% of a degradation inhibitor by weight of the total film material. The degradation inhibitor is at least one of carbodiimide compounds, epoxidized soybean oil, and organophosphonate compounds.

[0018] Preferably, the first fertilizer is a fast-acting chemical fertilizer granule, and the second fertilizer is a slow-release fertilizer granule treated with a double coating of urease inhibitor and nitrification inhibitor; the total nutrient content of the first fertilizer and the second fertilizer may be the same or different.

[0019] Preferably, a water-soluble isolation film is attached to the inner wall of the outer bag. After the outer bag is sealed, the water-soluble isolation film protects the outer surface coating of the inner bag from erosion by free water in the first fertilizer during storage and dissolves rapidly under the action of soil moisture.

[0020] A method for preparing a bag-in-bag type time-release controlled-release fertilizer includes the following steps:

[0021] S1. Prepare the outer bag body and inner bag body membrane materials;

[0022] S2. The inner bag material is made into an inner bag, the second fertilizer is filled into the inner bag and then sealed.

[0023] S3. A gradient degradation composite coating is formed on the outer surface of the sealed inner bag using a layer-by-layer impregnation-drying process: First, the inner bag is immersed in a polyvinyl alcohol solution and pulled dry to form a hydrophilic biodegradable polyester layer; then, it is immersed in a molten blend containing natural wax, oxidized polyethylene wax, polycaprolactone, polybutylene succinate, nano-silica surface-modified with silane coupling agent, and biochar and pulled cool to form a barrier wax-polymer composite layer; finally, it is immersed in a polycaprolactone solution and pulled dry to form a hydrophobic biodegradable polyester layer. The thickness of each layer is controlled by adjusting the solution concentration and the number of pulls to obtain a slow-release start-up delayed inner bag with a coating.

[0024] S4. Fill the outer bag with the first fertilizer to the predetermined filling amount, and then place the coated slow-release start-up delay inner bag into the fertilizer inside the outer bag, so that the inner bag is located at the geometric center or off-center of the outer bag.

[0025] S5. Seal the outer bag so that the inner bag is completely sealed inside the outer bag and immersed in the first fertilizer.

[0026] Preferably, before the first fertilizer is filled into the outer bag, a polyvinyl alcohol aqueous solution is sprayed onto the inner wall of the outer bag and dried to form the water-soluble isolation film; and during the preparation of the outer bag, a hot-press cutter is used to form the slit on the surface of the bag, while the hot-press process is used to melt the edge of the slit, which naturally forms the hardened edge after cooling.

[0027] Compared with the prior art, the present invention has the following advantages and technical effects:

[0028] First, it pioneered a controlled-release structure that integrates physical isolation and time-sequential release, fundamentally resolving the contradiction between single-application fertilization and long-term fertilization.

[0029] Existing controlled-release fertilizers typically use parallel bags or single controlled-release bags. All fertilizer units come into contact with water and begin nutrient dissolution simultaneously after being buried in the soil, failing to create a sequential, relay-like fertilization process. Extending the fertilization cycle requires multiple topdressings, increasing labor input. This invention utilizes a nested structure where an outer bag encloses an inner bag, with the inner bag submerged in the fertilizer within the outer bag. This gives the outer bag a dual function: it preferentially dissolves and releases the first fertilizer, meeting the crop's early nutrient needs; simultaneously, it forms a physical barrier, systematically delaying water penetration into the inner bag, ensuring the inner bag's nutrient release reliably starts later than the outer bag. This physical barrier mechanism is simple in structure, does not rely on complex chemical control methods, and its delay effect is clear and controllable. For the first time, it achieves seamless transition between early and late-stage fertilization with a single application, significantly extending the fertilizer's effective period and reducing application frequency and labor costs.

[0030] II. The gradient degradation composite coating serves as a delayed start mechanism, enabling precise control of the inner bag release start time.

[0031] Unlike methods that rely solely on differences in membrane degradation rates or single-layer coatings to control the initiation timing, the gradient degradation composite coating of this invention sequentially comprises a hydrophilic biodegradable polyester layer, a barrier wax polymer composite layer, and a hydrophobic biodegradable polyester layer from the inside out, forming a humidity gradient barrier from the inside out. The barrier layer is composed of natural wax, oxidized polyethylene wax, polycaprolactone, polybutylene succinate, surface-modified nano-silica with a silane coupling agent, and biochar, all synergistically melt-blended. The natural wax provides the basic hydrophobic framework, while the oxidized polyethylene wax bridges and toughens the wax and polyester components through polar groups, forming a dense, crack-resistant composite matrix. Polycaprolactone and polybutylene succinate constitute a controllable degradation network; microorganisms preferentially erode the faster-degrading polybutylene succinate, causing the dense structure of the barrier layer to gradually loosen and collapse, thus periodically releasing the moisture barrier. The surface-modified nano-silica is uniformly dispersed in the matrix, utilizing the labyrinth effect of nanoparticles to greatly extend the water molecule penetration path, giving the coating excellent initial water-blocking performance. Porous biochar adsorbs and enriches soil microorganisms and enzymes, serving as continuous degradation trigger sites to anchor and accelerate the hydrolysis and disintegration process of the barrier layer. The barrier layer, synergistically formed by these components, effectively isolates external moisture during the first one to two months after burial in the soil, protecting the inner bag from initiating nutrient release. Subsequently, it gradually disintegrates under the continuous action of soil microorganisms, allowing moisture to permeate into the inner bag and triggering the second stage of nutrient release. This delayed mechanism driven by exogenous microorganisms is less affected by fluctuations in soil temperature and pH compared to simple hydrolysis or swelling-type delayed mechanisms, and exhibits greater controllability and reproducibility in initiation time.

[0032] Third, the differentiated design of the inner bag coating allows for flexible multi-stage relay fertilization, accurately matching the fertilization needs of crops throughout their entire growth period.

[0033] This invention allows multiple inner bags to be encapsulated within the same outer bag, with each inner bag coated with a gradient degradation composite coating of varying numbers or thicknesses, resulting in different nutrient release initiation times. A single fertilizer product can sequentially provide nutrient release at multiple stages—early, middle, and late—after crop transplanting, constructing a nutrient supply spectrum highly consistent with the crop's growth curve. For example, the outer bag provides fast-acting nutrients for root development and growth after transplanting, the first inner bag initiates release during flowering, and the next inner bag initiates release during fruit enlargement. This highly integrated multi-stage controlled-release scheme provides a convenient way to achieve precise nutrient management for fruit trees, forest trees, and perennial economic crops, avoiding the problems of inaccurate dosage control and timing of fertilization in traditional multi-stage fertilization.

[0034] IV. The slit-controlled release structure and hardened edge design of the outer bag body ensure the stability and anti-clogging properties of the initial nutrient release.

[0035] The outer bag surface features elongated, narrow slits formed by hot-pressing and melting, serving as the main channels for nutrient leaching. The size of these slits directly controls the initial nutrient release rate of the outer bag. The hot-pressing and melting process creates a hardened edge at the slit's edge, providing high mechanical strength and a smooth surface. This effectively prevents fine soil particles from becoming embedded in the slits and clogging the channels due to bag expansion or external forces, ensuring the continuity and uniformity of nutrient release from the outer bag and maintaining the predetermined time window for nutrient release from both the inner and outer bags.

[0036] Fifth, the coordinated configuration of the degradation cycles of inner and outer bag film materials and the regulation and use of degradation inhibitors enable the environmental fate of packaging materials to be highly synchronized with the nutrient release cycle.

[0037] The outer bag uses a fully biodegradable resin membrane with a short degradation cycle, which is compatible with its short-term fertilization function. After the nutrients are released, the bag can degrade rapidly and will not remain in the soil for a long time. The inner bag uses a fully biodegradable resin membrane with a longer degradation cycle, and selectively adds carbodiimide compounds, epoxidized soybean oil, or organophosphonate degradation inhibitors to precisely control the degradation rate, so that the degradation cycle matches the mid-to-late stage of nutrient slow release.

[0038] VI. The water-soluble isolation membrane on the inner wall of the outer bag improves the product's storage stability and reliability.

[0039] During storage and transportation, free moisture in the first fertilizer may slowly erode the gradient degradation composite coating on the outer surface of the inner bag, leading to a decrease in its barrier performance or even premature failure. This invention pre-forms a polyvinyl alcohol water-soluble barrier film on the inner wall of the outer bag, physically separating the inner bag coating from the fertilizer particles in the outer bag during the shelf life, effectively protecting the coating from erosion. After the product is buried in the soil, this barrier film quickly dissolves and disappears under the influence of soil moisture, without hindering the release of nutrients from the outer bag or the delayed activation of the inner bag.

[0040] VII. Functional combination of internal and external fertilizer components improves nutrient utilization efficiency and reduces the risk of environmental loss.

[0041] The outer bag contains a first layer of fast-acting chemical fertilizer granules, quickly providing the nutrients needed for early crop growth. The inner bag contains a second layer of slow-release fertilizer granules, double-coated with urease inhibitors and nitrification inhibitors. These two inhibitors delay urea hydrolysis and the conversion of ammonium nitrogen to nitrate nitrogen, respectively, reducing nitrogen loss through ammonia volatilization, denitrification, and leaching. Encapsulating this type of slow-release fertilizer in a delayed-start inner bag ensures a sustained and stable nitrogen supply for crops during the peak nutrient demand period in the mid-to-late stages, improving fertilizer utilization efficiency and reducing the excessive nitrogen emissions into water bodies and the atmosphere. Detailed Implementation

[0042] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0043] A bag-in-bag type time-release controlled-release fertilizer includes:

[0044] An outer bag, the interior of which is filled with the first fertilizer;

[0045] At least one inner bag is enclosed inside the outer bag and completely submerged in the first fertilizer, and the interior of the inner bag is filled with a second fertilizer.

[0046] The outer surface of the inner bag is provided with a delayed start mechanism, which allows water to contact and trigger the outer bag to release nutrients first after the outer bag is buried in the soil. However, due to the obstruction of the delayed start mechanism, the nutrient release start time of the inner bag is later than that of the outer bag, forming a sequential release from the outside to the inside.

[0047] The delayed start mechanism is a gradient degradation composite coating applied to the outer surface of the inner bag. The gradient degradation composite coating includes, from the inside out, a hydrophilic degradable polyester layer, a barrier wax-polymer composite layer, and a hydrophobic degradable polyester layer. The thickness of the barrier wax-polymer composite layer is 40% to 60% of the total coating thickness. The hydrophilic degradable polyester layer is a polyvinyl alcohol layer, and the hydrophobic degradable polyester layer is a polycaprolactone layer.

[0048] The barrier wax-polymer composite layer is made by melt blending the following raw materials in parts by weight: 25-35 parts natural wax, 15-20 parts oxidized polyethylene wax, 10-20 parts polycaprolactone, 5-10 parts polybutylene succinate, 2.5-4.5 parts nano-silica with surface modification by silane coupling agent, and 2-5 parts biochar. The components work together to form a barrier structure with an initial water contact angle greater than 110° that gradually disintegrates under the action of soil microorganisms, so that the inner bag is in a state of moisture isolation for the first 1-2 months after being buried in the soil.

[0049] The number of inner bags is two or more, and each inner bag is coated with a gradient degradation composite coating with different numbers of layers and / or different thicknesses, so that multiple inner bags can sequentially start releasing nutrients after being buried in the soil, realizing multi-stage relay fertilization.

[0050] The outer bag is a slit-type controlled-release bag, with long strip-shaped slits of 1.75 to 3.25 mm in length and 0.1 to 0.5 mm in width distributed on its surface. The edges of the slits are hardened by hot-pressing and melting followed by cooling and shaping to prevent soil particles from clogging the slits.

[0051] The outer bag is made of a fully biodegradable resin film with a degradation cycle of 1 to 2 months, and the inner bag is made of a fully biodegradable resin film with a degradation cycle of 3 to 6 months. Furthermore, the inner bag contains 0.8% to 1.7% degradation inhibitors by weight of the film material, which are at least one of carbodiimide compounds, epoxidized soybean oil, and organophosphonate compounds.

[0052] The first fertilizer is a fast-acting chemical fertilizer granule, and the second fertilizer is a slow-release fertilizer granule treated with both urease inhibitor and nitrification inhibitor; the total nutrient content of the first fertilizer and the second fertilizer may be the same or different.

[0053] A water-soluble isolation film is attached to the inner wall of the outer bag. After the outer bag is sealed, the water-soluble isolation film protects the outer surface coating of the inner bag from erosion by free water in the first fertilizer during storage and dissolves rapidly under the action of soil moisture.

[0054] A method for preparing a bag-in-bag type time-release controlled-release fertilizer includes the following steps:

[0055] S1. Prepare the outer bag body and inner bag body membrane materials;

[0056] S2. The inner bag material is made into an inner bag, the second fertilizer is filled into the inner bag and then sealed.

[0057] S3. A gradient degradation composite coating is formed on the outer surface of the sealed inner bag using a layer-by-layer impregnation-drying process: First, the inner bag is immersed in a polyvinyl alcohol solution and pulled dry to form a hydrophilic biodegradable polyester layer; then, it is immersed in a molten blend containing natural wax, oxidized polyethylene wax, polycaprolactone, polybutylene succinate, nano-silica surface-modified with silane coupling agent, and biochar and pulled cool to form a barrier wax-polymer composite layer; finally, it is immersed in a polycaprolactone solution and pulled dry to form a hydrophobic biodegradable polyester layer. The thickness of each layer is controlled by adjusting the solution concentration and the number of pulls to obtain a slow-release start-up delayed inner bag with a coating.

[0058] S4. Fill the outer bag with the first fertilizer to the predetermined filling amount, and then place the coated slow-release start-up delay inner bag into the fertilizer inside the outer bag, so that the inner bag is located at the geometric center or off-center of the outer bag.

[0059] S5. Seal the outer bag so that the inner bag is completely sealed inside the outer bag and immersed in the first fertilizer.

[0060] Before the first fertilizer is loaded into the outer bag, a polyvinyl alcohol aqueous solution is sprayed onto the inner wall of the outer bag and dried to form the water-soluble isolation film. During the preparation of the outer bag, a hot-press cutter is used to form the slit on the surface of the bag, and the edge of the slit is melted by the hot-pressing process. After cooling, the hardened edge is formed naturally.

[0061] The working mechanism and synergistic principle of each step in this invention:

[0062] I. Overall Technical Concept and Solution Approach

[0063] This invention addresses the industry challenge of achieving continuous fertilization through single-application fertilization by innovatively combining a "bag-within-a-bag" physical topology with a gradient degradation coating on the inner bag's surface. Its core innovation lies in constructing a time-triggered microenvironment from the outside in: the outer bag serves as both the first-stage nutrient supply unit and the physical protective barrier for the inner bag; the gradient coating on the inner bag's surface acts as a delayed-start passive actuator. This design, which synergistically integrates macroscopic spatial architecture with material surface engineering, is not found in existing controlled-release fertilizer technologies and represents a novel approach to solving the problem of long-term fertilization.

[0064] II. Mechanism of Action of the Rapid-Acting Stage of the Outer Bag

[0065] The outer bag is designed as a slit-type controlled-release bag, with slits distributed on its surface through hot-pressing and melting. The molten and hardened edges are crucial for ensuring release stability: they eliminate the dimensional changes caused by the softening of the membrane material due to moisture absorption in traditional mechanical cuts, ensuring the slit opening remains constant throughout the nutrient release cycle, thus controlling the dissolution rate of the first fertilizer through physical flow restriction. The outer bag body is made of a fully biodegradable resin with a short degradation cycle. Its design intention is that after the first fertilizer has been largely released within the predetermined cycle, the bag itself disintegrates due to the action of soil microorganisms and moisture, creating the necessary conditions for soil moisture to contact the inner bag. This design, sacrificing functionality, achieves a seamless transition from rapid-acting to long-acting stages.

[0066] III. Construction of Gradient Degradation Composite Coating and Synergistic Principle of Each Layer

[0067] The delayed start mechanism of the inner bag is its core, which is achieved by a three-layer gradient composite coating.

[0068] The first layer is a polyvinyl alcohol layer, which serves as an interface buffer and adhesion enhancer. Its function is to provide a flexible base for the subsequent coating of the more brittle barrier layer by virtue of its good wettability to the base film, preventing the coating from cracking due to deformation of the inner bag film material or thermal expansion and contraction.

[0069] The second layer is a barrier-resistant wax polymer composite layer, made by melt blending five components. Each component has a clear function, working synergistically to achieve highly efficient initial water barrier and controlled disintegration later. Natural wax provides the basic hydrophobic framework. Oxidized polyethylene wax acts as a solvent and toughening agent, firmly bridging the wax and polyester through its polar groups to form a dense and crack-free composite matrix. Polycaprolactone and polybutylene succinate form a degradation regulation network. Microorganisms preferentially erode and degrade the faster-growing polybutylene succinate, causing the dense structure of the barrier layer to loosen and collapse, thus releasing the water barrier at regular intervals. Nano-silica, surface-treated with a silane coupling agent, is uniformly dispersed in the matrix. Utilizing the labyrinth effect of nanoparticles, it greatly extends the water molecule penetration path, which is key to the coating's excellent initial barrier properties. Porous biochar adsorbs and enriches soil microorganisms and enzymes, serving as continuous degradation trigger sites, anchoring and accelerating the hydrolysis and disintegration process of the composite layer. The composite layer composed of the above components achieves a precise time-sequential function of highly efficient initial water barrier and subsequent microbial-triggered disintegration.

[0070] The third layer is a polycaprolactone layer, which serves as a protective and auxiliary barrier. It comes into direct contact with the fertilizer in the outer bag, preventing fertilizer particles from causing physical scratches or chemical erosion to the barrier layer during storage and transportation. At the same time, it forms an inward hydrophobic gradient with the inner layer in the soil environment.

[0071] IV. Design for Combined Use of Internal and External Fertilizers and Storage Stability

[0072] The inner and outer fertilizers are designed with differentiated formulas. The outer bag contains fast-acting chemical fertilizer to quickly respond to the early needs of crops. The inner bag contains slow-release fertilizer that has been treated with both urease inhibitors and nitrification inhibitors. This achieves chemical inhibition at the nitrogen form conversion level, complementing the physical controlled release, and making the nutrient supply curve more closely match the crop's fertilizer requirements.

[0073] The water-soluble temporary isolation film formed by spraying polyvinyl alcohol aqueous solution onto the inner wall of the outer bag cleverly utilizes the material's properties of being water-resistant in the dry state and rapidly dissolving in the wet state. This protects the inner bag coating from the erosion of free moisture in the first fertilizer during its shelf life, while dissolving instantly upon application to the soil, thus removing the protection and providing engineering assurance for the reliability of field applications.

[0074] To make the present invention more fully disclosed, more specific embodiments are described below.

[0075] I. Implementation Examples

[0076] Example 1

[0077] S1. Preparation of outer and inner bag membrane materials: The outer bag uses a fully biodegradable resin membrane (PBAT / PLA blend, mass ratio 60:40, thickness 60μm) with a degradation cycle of 1.5 months, and the inner bag uses a fully biodegradable resin membrane (PBAT / PLA blend, mass ratio 70:30, thickness 50μm) with a degradation cycle of 4.5 months. A carbodiimide degradation inhibitor (N,N-dicyclohexylcarbodiimide) accounting for 1.25% of the total mass of the membrane material is added to the inner bag membrane material. The membrane is prepared by twin-screw granulation and blown film extrusion.

[0078] S2. The inner bag membrane material is hot-pressed into an inner bag with a volume of 50mL. 35g of the second fertilizer is filled into the inner bag and then heat-sealed. The second fertilizer is a urea-diammonium phosphate-potassium chloride compound fertilizer (total N-P2O5-K2O content 45%, formula 15-15-15) that has been double-coated with urease inhibitor (NBPT, added at 0.5% of pure nitrogen) and nitrification inhibitor (DMPP, added at 0.8% of pure nitrogen).

[0079] S3. A gradient degradation composite coating is formed on the outer surface of the sealed inner bag using a layer-by-layer impregnation-drying process:

[0080] (a) Immerse the inner bag in an 8% (w / w) aqueous solution of polyvinyl alcohol (PVA1788), pull it up at a speed of 0.5 m / min, and dry it at 60°C for 30 min to form a hydrophilic biodegradable polyester layer with a thickness of about 20 μm;

[0081] (b) Preparation of the melt blend liquid for the barrier wax-polymer composite layer: Weigh 30 parts of natural beeswax, 18 parts of oxidized polyethylene wax, 15 parts of polycaprolactone (number average molecular weight 80,000), 7.5 parts of polybutylene succinate (number average molecular weight 100,000), 3.5 parts of nano-silica (particle size 20nm) surface-modified with silane coupling agent KH-550, and 3.5 parts of biochar (particle size 200 mesh). Melt blend at 120°C and stir continuously for 40 minutes until uniform to obtain the melt blend liquid. Immerse the inner bag body obtained in step (a) into the melt blend liquid, pull it up at a speed of 0.3m / min, and cool and shape it in a cold airflow at 5°C to form a barrier wax-polymer composite layer with a thickness of about 55μm.

[0082] (c) Immerse the inner bag obtained in step (b) in a 10% dichloromethane solution of polycaprolactone (number average molecular weight 60,000), pull it up at a speed of 0.5 m / min, and dry it at 40°C for 20 min to form a hydrophobic and biodegradable polyester layer with a thickness of about 22 μm.

[0083] The total thickness of the resulting gradient degradation composite coating is approximately 97 μm, of which the barrier layer accounts for approximately 56.7%.

[0084] S4. Preparation of the outer bag: A long, narrow slit, 2.5 mm in length and 0.3 mm in width, is formed on a 60 μm thick fully biodegradable resin film using a hot-press cutter. The hot-press temperature is 120℃, the pressure is 0.4 MPa, and the hot-press time is 2 seconds, allowing the edges of the slit to melt and naturally cool to form a hardened edge. Then, a 5% (w / w) polyvinyl alcohol aqueous solution is sprayed onto the inner wall of the outer bag (volume 200 mL), and dried at 50℃ for 15 min to form a water-soluble release film with a thickness of approximately 5 μm.

[0085] S5. Fill the outer bag with 100g of the first fertilizer to the predetermined filling amount. The first fertilizer is a fast-acting urea-monoammonium phosphate-potassium sulfate compound fertilizer granules (total nutrient content of N-P2O5-K2O 45%, formula 20-10-15). Then place the coated slow-release start-up delay inner bag obtained in step S3 into the fertilizer inside the outer bag, so that it is located at the geometric center of the outer bag.

[0086] S6. Heat seal the outer bag to completely enclose the inner bag inside the outer bag and immerse it in the first fertilizer to obtain a bag-in-bag type time-release controlled-release fertilizer.

[0087] Single-factor experimental screening of key process parameters

[0088] To verify the influence of the various technical features of this invention on product performance, and to further prove the rationality and non-obviousness of the parameter selection of this invention, single-factor experiments were conducted on six key parameters—the amount of natural wax, the amount of polycaprolactone, the amount of nano-silica, the slit length, the amount of degradation inhibitor added, and the degradation cycle of the inner bag film—based on the process conditions of Example 1, with other conditions remaining unchanged.

[0089] (a) The effect of the amount of natural wax used

[0090] Based on the process conditions of Example 1, and with other conditions remaining unchanged, only the amount of natural wax was changed. The test results of the obtained samples are shown in Table 1.

[0091]

[0092] Table 1 shows that when the amount of natural wax is 30 parts, the delayed release time is 46 days, and the coating integrity retention rate is 98.4% after 60 days, exhibiting the best overall performance. When the amount is less than 30 parts, the overall hydrophobicity of the barrier layer is insufficient, the water penetration rate is accelerated, the inner bag activation time is advanced, the delayed release time is shortened, and the total effective fertilizer supply period is correspondingly reduced. When the amount is more than 30 parts, the excessive wax content increases the rigidity of the coating, reduces the interfacial compatibility with the inner and outer polyester layers, and makes it prone to microcracks under alternating wet and dry soil conditions. The coating integrity retention rate decreases from 98.4% to 96.7%. Although the delayed release time increases, the increased coating defects lead to poorer reproducibility of the delayed release time. This demonstrates that 30 parts is the optimal amount to balance the delayed release effect and the coating integrity.

[0093] (II) Effect of polycaprolactone dosage

[0094] Based on the process conditions of Example 1, with other conditions remaining unchanged, only the amount of polycaprolactone was changed, and the test results are shown in Table 2.

[0095]

[0096] Table 2 shows that 15 parts of polycaprolactone (PVC) yielded the best performance. As a semi-crystalline biodegradable polyester, PVC forms a dense interpenetrating network when blended with wax components. When the dosage is too low, the polyester network within the barrier layer is incomplete, resulting in insufficient moisture barrier capacity, premature inner bag activation, and compromised coating integrity. Conversely, when the dosage is too high, the barrier layer becomes excessively hydrophobic and the degradation rate is too slow, extending the delay time. However, under prolonged humid conditions, the coating is prone to microscopic peeling due to swelling stress, leading to decreased retention. A dosage of 15 parts provides a suitable delay time and optimal coating integrity.

[0097] (III) Effect of the dosage of nano-silica

[0098] Based on the process conditions of Example 1, with other conditions remaining unchanged, only the amount of nano-silica was changed, and the test results are shown in Table 3.

[0099]

[0100] Table 3 shows that a dosage of 3.5 parts of nano-silica resulted in the optimal delay time and coating integrity. Appropriate amounts of nano-silica, modified with silane coupling agents, can be uniformly dispersed in the wax-polymer matrix, extending the water molecule penetration path through the labyrinth effect and improving initial water-blocking performance. When the dosage is too low, the labyrinth effect weakens, resulting in insufficient water-blocking capacity and a shortened delay time. When the dosage is too high, nanoparticles are prone to soft agglomeration, forming stress concentration points in the coating. Under the influence of soil microorganisms and moisture, microscopic defects preferentially appear, reducing the coating integrity retention rate from 98.4% to 96.4%. Although the delay time increases, the coating reliability decreases.

[0101] (iv) The influence of slit length

[0102] Based on the process conditions of Example 1, with other conditions remaining unchanged, only the slit length was changed, and the test results are shown in Table 4.

[0103]

[0104] Table 4 shows that with a slit length of 2.5 mm, the outer bag release time is 32 days, and the blockage rate is only 1.2%, exhibiting the best overall performance. If the slit is too short, the nutrient dissolution channel area is insufficient, resulting in slow release from the outer bag and extending the release time to 48 days, leading to delayed nutrient supply. Although the delayed release time remains essentially unchanged, early nutrient supply is insufficient. If the slit is too long, the outer bag releases too quickly, shortening the release time to 24 days. Simultaneously, the excessively large slit opening is more easily squeezed into by fine soil particles, increasing the blockage rate to 5.6%, causing poor nutrient release in the later stages and disrupting the stability of the release sequence. 2.5 mm achieves the optimal balance between release rate and anti-blockage performance.

[0105] (v) Effect of the amount of degradation inhibitor added

[0106] Based on the process conditions of Example 1, and with other conditions remaining unchanged, only the amount of carbodiimide degradation inhibitor added was changed. The test results are shown in Table 5.

[0107]

[0108] Table 5 shows that when the degradation inhibitor dosage is 1.25%, the inner bag activation time is 78 days and the delay is 46 days, which is the optimal match with the coating degradation cycle. If the dosage is too low, the inner bag film degrades and ruptures prematurely, allowing moisture to enter early, significantly advancing the inner bag activation time and greatly reducing the delay effect. If the dosage is too high, the inner bag film degradation is excessively delayed, extending the activation time to 97 days, resulting in an excessively long total fertilization period, which may lead to nutrient oversupply in the later stages of crop growth and nutrient deficiency in the middle stages. 1.25% coordinates the degradation of the inner bag film with the coating failure time, achieving precise relay fertilization.

[0109] (vi) The impact of the degradation cycle of the inner bag film

[0110] Based on the process conditions of Example 1, and with other conditions remaining unchanged, only the PBAT / PLA blending ratio was adjusted to change the degradation cycle of the inner bag film. The test results are shown in Table 6.

[0111]

[0112] Table 6 shows that when the inner bag film has a degradation cycle of 4.5 months, the delayed release time is 46 days, which closely matches the degradation start-up window of the barrier coating. If the degradation cycle is too short, the inner bag film degrades before the coating is completely ineffective, allowing water to enter prematurely and shortening the delay time. If the degradation cycle is too long, even if the coating has disintegrated, the inner bag film itself still constitutes an additional water-blocking layer, resulting in an excessively delayed start-up time. Although the total nutrient supply period is long, the concentrated release of nutrients in the middle and late stages may exceed the crop's absorption capacity, leading to waste. The 4.5-month cycle, combined with the gradient coating, achieves synergistic complementarity in time, resulting in the best technical effect.

[0113] Example 2

[0114] Same as Example 1, except that in step S3(b), the composition of the barrier layer melt blend is 25 parts of natural beeswax, 20 parts of oxidized polyethylene wax, 20 parts of polycaprolactone, 5 parts of polybutylene succinate, 2.5 parts of nano-silica treated with silane coupling agent, and 5 parts of biochar; the barrier layer thickness is about 48 μm, the total coating thickness is about 90 μm, and the barrier layer accounts for about 53.3%.

[0115] Example 3

[0116] Same as Example 1, except that in step S3(b), the composition of the barrier layer melt blend liquid is 35 parts of natural beeswax, 15 parts of oxidized polyethylene wax, 10 parts of polycaprolactone, 10 parts of polybutylene succinate, 4.5 parts of nano-silica treated with silane coupling agent, and 2 parts of biochar; the barrier layer thickness is about 63 μm, the total coating thickness is about 105 μm, and the barrier layer accounts for about 60.0%.

[0117] Example 4

[0118] Same as Example 1, except that: there are two inner bags, each coated with a gradient degradation composite coating of different thicknesses. The total coating thickness of the first inner bag is 60 μm (including a 35 μm barrier layer), and the total coating thickness of the second inner bag is 130 μm (including an 80 μm barrier layer); the volume of the outer bag is correspondingly increased to 300 mL, and 150 g of the first fertilizer is filled in; the degradation cycle of the outer bag film, slit parameters, degradation inhibitors, and other conditions are the same as in Example 1.

[0119] Example 5

[0120] Same as Example 1, except that: in step S4, the outer bag slit size is 1.75 mm in length and 0.5 mm in width; the degradation cycle of the outer bag film is 2 months, the degradation cycle of the inner bag film is 3 months, and the degradation inhibitor in the inner bag film material is epoxidized soybean oil, which accounts for 1.7% of the total mass of the film material.

[0121] Comparative Example 1

[0122] The difference from Example 1 is that the inner bag is not coated with any gradient degradation composite coating, i.e. there is no delayed start mechanism, and the other conditions are the same as in Example 1.

[0123] Comparative Example 2

[0124] The difference from Example 1 is that the inner bag body is only coated with a single layer of wax coating (30 parts of natural beeswax and 18 parts of oxidized polyethylene wax are mixed and melt-coated), with a coating thickness of about 50 μm. The other conditions are the same as in Example 1.

[0125] Comparative Example 3

[0126] The difference from Example 1 is that the edge of the slit in the outer bag body is not heat-pressed and hardened, that is, the slit is processed by a cold cutting knife, the slit size is the same as in Example 1, and the other conditions are the same as in Example 1.

[0127] Comparative Example 4

[0128] The parallel combination controlled-release bag structure using existing technology is as follows: two independent controlled-release bags are prepared. One bag is filled with 100g of the first fertilizer as in Example 1 (the bag film material is a fully biodegradable resin film with a degradation cycle of 1.5 months, without coating). The other bag is filled with the second fertilizer inner bag body with a gradient degradation composite coating as in Example 1. The two independent controlled-release bags are placed side by side and buried in the soil at the same depth.

[0129] II. Summary of Experimental Results

[0130] (a) Performance testing methods

[0131] 1. Nutrient Release Cycle: The fertilizer sample to be tested was buried 15 cm deep in the field soil (pH 6.8, organic matter content 1.2%), maintaining soil moisture content at 60%–70% of field capacity. Samples were taken periodically at an ambient temperature of 25℃±3℃. The remaining nutrient content inside the bag was determined using a combination of drying and weighing method and Kjeldahl nitrogen determination, and the cumulative nutrient release rate was calculated. The time when the cumulative nutrient release rate of the outer bag reached 80% was recorded as the completion time of nutrient release from the outer bag; the time when the cumulative nutrient release rate of the inner bag reached 15% was recorded as the start time of nutrient release from the inner bag. The difference between the two is the delayed release time. The total effective fertilization period is from the date of burial until the cumulative nutrient release rate of both the inner and outer bags reaches 80%. Each treatment group had 3 replicates, and the results were averaged.

[0132] 2. Slit anti-clogging property: After the sample was buried in the soil for 90 days, it was taken out and carefully rinsed with deionized water to remove loose soil adhering to the surface of the bag. The percentage of the area blocked by soil particles in each slit was observed under a stereomicroscope (magnification 20×) to the original slit area. The average value of 20 slits was taken as the clogging rate.

[0133] 3. Coating Integrity Evaluation Method: After burying the inner bag to be tested in the soil for 60 days, remove it, carefully clean the surface adhering substances, and observe the coating surface under a stereomicroscope (magnification 30×). Count the unit area (per cm²). 2 The number of defects such as cracks, peeling, or pinholes appearing inside the coating was measured and compared with the observation results of the same location before embedding. The coating integrity retention rate (%) was calculated using the formula [1 - (number of defects after embedding / number of defects before embedding) × 100%]. Three inner bags were tested in each group, and the average value of the results was taken.

[0134] (II) Performance test results

[0135]

[0136] Note: Example 4 uses two inner bags. The data before and after " / " in the table correspond to the test results of the first and second inner bags, respectively. Comparative Example 4 uses two independent controlled-release bags. The data before and after " / " correspond to the data of the uncoated bag and the coated bag, respectively. The delayed release time is not applicable due to the different structures. Comparative Example 1 is uncoated. The coating integrity retention rate is not applicable.

[0137] (III) Data Comparison and Analysis

[0138] 1. Delayed Release Performance Analysis

[0139] The delayed release times of Examples 1-5 ranged from 30 to 99 days, with Example 1 reaching 68 days. Comparative Example 1 had a delay of only 2 days, Comparative Example 2 19 days, Comparative Example 3 36 days, and Comparative Example 4 failed to achieve an effective time delay. The delayed release time of Example 1 was improved by 3300.00%, 257.89%, and 88.89% compared to Comparative Examples 1, 2, and 3, respectively.

[0140] The delayed release time is essentially the time required for water molecules to penetrate the coating barrier and reach the surface of the inner bag membrane. This time parameter is jointly determined by the water-blocking capacity of the coating and the degradation and disintegration kinetics of the coating in the soil environment. The gradient degradation composite coating used in this invention constructs a three-layer functional system of "interface anchoring - main body barrier - external protection" from a structural design perspective. Each layer performs different functions and works synergistically with the others.

[0141] From an interfacial chemistry perspective, the innermost polyvinyl alcohol coating contains a large number of hydroxyl functional groups, which can form intermolecular hydrogen bonds with the ester groups and terminal hydroxyl groups on the surface of the PBAT / PLA blend of the inner bag membrane. Although the bond energy of hydrogen bonds is lower than that of covalent bonds, it is much stronger than ordinary van der Waals forces, which is sufficient to establish a stable physical connection at the coating-substrate interface and eliminate interfacial void defects. Without this anchoring layer, the coating and the bag membrane rely solely on mechanical adhesion, and interfacial peeling is very likely to occur under the influence of soil moisture swelling. Water molecules will bypass the barrier layer and directly contact the bag membrane surface along the peeling gap.

[0142] The intermediate barrier layer is the main component for delaying the function. Natural wax, as a continuous hydrophobic phase, exhibits intrinsic repulsion of water molecules due to its long-chain alkane structure. Oxidized polyethylene wax molecules contain oxygen-containing polar groups such as carboxyl groups, which can generate dipole-dipole interactions with the ester groups on the polycaprolactone and polybutylene succinate chains, improving the compatibility between the wax and polyester phases and resulting in a dense and uniform composite matrix rather than a phase-separated structure. Polycaprolactone and polybutylene succinate together form a biodegradable polyester network. The latter has a higher density of aliphatic ester bonds and better molecular chain flexibility, making it more sensitive to lipases and esterases secreted by microorganisms, preferentially undergoing enzymatic hydrolysis. This causes the barrier layer to gradually disintegrate through layer-by-layer erosion rather than randomly generating through-cracks. After surface modification with a silane coupling agent, the organic functional groups grafted onto the surface of nano-silica particles can form chemical bonds or strong physical entanglement with the polymer matrix, achieving uniform dispersion at the nanoscale. Uniformly dispersed nanoparticles create a tortuous effect along the diffusion path of water molecules, forcing water molecules to bypass the particles rather than penetrate in a straight line, effectively extending the mass transfer path. The hierarchical porous structure of biochar has an adsorption and enrichment effect on soil microorganisms, forming uniformly distributed microbial colonization sites within the coating, preventing degradation reactions from concentrating in individual areas and ensuring a uniform and controllable degradation process. The outermost polycaprolactone coating is hydrophobic and also serves as a mechanical protector of the barrier layer, preventing direct abrasion of the barrier layer by soil particles.

[0143] In Comparative Example 1, the inner bag had no coating, allowing unimpeded moisture release. Nutrient release began within two days of burial, demonstrating that a coating is essential for delayed initiation. Comparative Example 2 used only a single-layer wax coating, lacking inner interface anchoring and outer protection. Under the alternating swelling-shrinkage stress caused by soil wetting and drying, the inherent brittleness of the wax led to crack initiation and propagation, and without a degradation rate differential regulation mechanism, the delay was only 19 days. Although the coating system in Comparative Example 3 was the same as in Example 1, the outer bag's slit blockage resulted in slow nutrient release, causing the inner bag to initiate in 88 days. This 36-day delay was a passive waiting caused by the blockage, not a result of active coating regulation. Comparative Example 4 eliminated the physical isolation between the outer and inner bags, directly exposing the coated bag to the soil moisture field, significantly weakening the delay function. These comparisons, from both positive and negative perspectives, confirm that the functional division of the gradient coating layers and the bag-within-a-bag physical isolation structure are synergistically necessary conditions for achieving long-term controllable delay.

[0144] 2. Analysis of the total effective fertilizer supply period

[0145] The total effective fertilization period for Examples 1-5 ranged from 163 to 247 days, with Example 1 showing a maximum of 247 days. The total effective fertilization periods for Comparative Examples 1 to 4 were 94 days, 124 days, 163 days, and 116 days, respectively. Compared to Comparative Examples 1, 2, 3, and 4, Example 1 showed an increase in the total effective fertilization period of 162.77%, 99.19%, 51.53%, and 112.93%, respectively.

[0146] The total effective fertilization period consists of three time parameters: the completion time of nutrient release from the outer bag, the delayed start time of the inner bag, and the duration of nutrient release from the inner bag. The relationship between these three parameters determines the final fertilization window length. In Example 1, the outer bag completed the first fertilizer release in 31 days, and the inner bag started the second fertilizer release in 99 days. There was no time overlap between the two stages, and the nutrient supply curve showed a clear sequential connection. After the inner bag started, fertilization continued for approximately 148 days. This duration is closely related to the degradation cycle of the inner bag membrane material. The mechanism of action of carbodiimide degradation inhibitors lies in the presence of cumulative double bond functional groups in their molecular structure. These groups can undergo addition reactions with the carboxyl groups at the ends of the PBAT / PLA molecular chains, capping the active end groups and blocking the autocatalytic effect of carboxyl groups on the hydrolysis of ester bonds. This slows down the overall degradation rate of the membrane material, matching the degradation cycle with the time window of coating disintegration.

[0147] In Comparative Example 1, due to the lack of a delay mechanism, the inner and outer bags released nutrients synchronously, resulting in a complete overlap of nutrient supply times between the two stages, with a total fertilization period of only 94 days. In Comparative Example 2, the inner bag started releasing nutrients 51 days earlier, partially overlapping with the fertilization stage of the outer bag, failing to form an orderly relay. In Comparative Example 3, the outer bag's slit blockage caused slow nutrient release, delaying fertilization until 52 days. Although the total fertilization period was extended to 163 days due to the passive delay of the inner bag, a gap appeared in the early nutrient supply to the crop. In the parallel combination structure of Comparative Example 4, the two bags contacted the soil independently without any sequence, resulting in essentially synchronous nutrient release times and a total fertilization period of only 116 days, failing to achieve the synergistic effect required for relay fertilization. Example 1, through precise matching of three time parameters—controlled release through the slit of the outer bag, delayed inner bag coating, and regulation of inner bag membrane degradation—achieved a seamless, long-lasting fertilization supply from the early to the middle and late stages.

[0148] 3. Coating integrity analysis

[0149] The coating integrity retention rate of Examples 1-5 ranged from 97.2% to 99.3% after 60 days, with Example 1 achieving 99.3%, which was the optimal value. The single-layer wax coating of Comparative Example 2 had a retention rate of only 72.3%, while the coated bag in Comparative Example 4, directly exposed to soil, had a retention rate of 91.4%. The coating integrity retention rate of Example 1 was 37.34% and 8.64% higher than that of Comparative Examples 2 and 4, respectively.

[0150] The coating integrity retention rate is a key indicator for evaluating the reliability of delayed start mechanisms; a higher integrity rate results in better batch reproducibility of the delay time. The coating in Example 1 exhibited extremely low defect density 60 days after burial in the soil. From a materials mechanics perspective, the coating simultaneously withstands contact compressive stress from soil particles, alternating swelling-contraction stress from wet-dry cycles, and localized chemical stress caused by microbial degradation in the soil environment. The outer polycaprolactone coating, due to its flexible molecular chains and high elongation at break, acts as a flexible buffer layer, effectively absorbing contact stress from soil particles and preventing stress transmission to the brittle barrier layer. The intermolecular interactions between the polar groups of oxidized polyethylene wax and the polyester segments in the intermediate barrier layer toughen the wax matrix, improving its crack resistance under alternating stress. Nano-silica, when used in appropriate amounts, achieves uniform dispersion with particle spacing maintained within a reasonable range. This avoids both insufficient tortuosity when the dosage is too low and agglomeration and stress concentration points caused by van der Waals forces due to reduced particle spacing when the dosage is too high. Hydrogen bond anchoring at the interface between the inner polyvinyl alcohol layer and the bag film fundamentally eliminates the most serious failure mode of coating peeling off from the substrate surface.

[0151] The single-layer wax coating of Comparative Example 2, lacking polyester phase toughening, exhibited low intrinsic elongation at break. Under alternating wet and dry soil stress, cracks initiated and propagated, resulting in a significantly higher defect density at 60 days compared to Example 1. This directly led to a larger standard deviation in the delay time and insufficient reliability. Although the coating of Comparative Example 4 employed the same gradient structure as Example 1, the absence of an outer bag for cushioning meant the coating directly endured the sliding friction and compression of soil particles, resulting in a significantly lower integrity retention rate compared to Example 1. This difference quantifies the mechanical protection provided by the outer bag to the inner bag coating in the bag-in-bag structure.

[0152] 4. Slit Anti-clogging Analysis

[0153] The slit blockage rates in Examples 1-5 ranged from 1.0% to 3.8%, with Example 1 showing the lowest value of 1.0%. Comparative Example 3, due to the slit not undergoing hot-pressing hardening treatment, achieved a blockage rate of 13.7%. The slit blockage rate in Example 1 was 92.70% lower than that in Comparative Example 3.

[0154] The slit blockage rate directly affects the rate and uniformity of nutrient release from the outer bag, thus impacting the accuracy of the time reference for the entire time-series release system. From a mass transfer perspective, the slit is the primary diffusion channel for nutrient dissolution, and the effective diffusion area is directly proportional to the nutrient release rate. Blockage reduces the effective diffusion area, decreases the release rate, and correspondingly prolongs the release completion time. The low slit blockage rate in Example 1 is attributed to both size design and processing technology. In terms of size, the slit opening size is larger than the particle size of typical soil aggregates, making it difficult for single particles to form direct blockages. Regarding the processing technology, hot-press melting and cutting causes the membrane material at the slit edge to undergo a phase transition process of melting-flowing-cooling and shaping, resulting in denser material in the edge area and a smoother, flatter surface, eliminating microcracks and burr-like protrusions common in cold-cutting processes. The smooth, dense edge surface reduces the probability of frictional adhesion and mechanical locking of fine soil particles at the slit edge. When the bag undergoes slight deformation under soil pressure, the edge is less prone to expanding the particle embedding area due to the propagation of microcracks.

[0155] Comparative Example 3, processed with a cold-cutting blade, exhibits a brittle fracture morphology at the slit edge, with a rough surface containing microcracks and burrs, providing abundant physical intercalation sites for fine particles. When the bag deforms under soil pressure, the microcracks expand, further enlarging the particle embedding area, significantly increasing the 90-day blockage rate, and extending the nutrient release completion time from 31 days to 52 days, disrupting the pre-set release window. This comparison, from the perspective of diffusion mass transfer area and interfacial friction, demonstrates the necessity of slit thermo-hardening edge treatment for maintaining unobstructed release channels and ensuring the accuracy of timed release.

[0156] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A bag-in-bag type time-release controlled-release fertilizer, characterized in that, include: An outer bag, the interior of which is filled with the first fertilizer; At least one inner bag is enclosed inside the outer bag and completely submerged in the first fertilizer, and the interior of the inner bag is filled with a second fertilizer. The outer surface of the inner bag is provided with a delayed start mechanism, which allows water to contact and trigger the outer bag to release nutrients first after the outer bag is buried in the soil. However, due to the obstruction of the delayed start mechanism, the nutrient release start time of the inner bag is later than that of the outer bag, forming a sequential release from the outside to the inside.

2. The bag-in-bag type time-release controlled-release fertilizer according to claim 1, characterized in that, The delayed start mechanism is a gradient degradation composite coating applied to the outer surface of the inner bag. The gradient degradation composite coating includes, from the inside out, a hydrophilic degradable polyester layer, a barrier wax-polymer composite layer, and a hydrophobic degradable polyester layer. The thickness of the barrier wax-polymer composite layer is 40% to 60% of the total coating thickness. The hydrophilic degradable polyester layer is a polyvinyl alcohol layer, and the hydrophobic degradable polyester layer is a polycaprolactone layer.

3. The bag-in-bag type time-release controlled-release fertilizer according to claim 2, characterized in that, The barrier wax-polymer composite layer is made by melt blending the following raw materials in parts by weight: 25-35 parts natural wax, 15-20 parts oxidized polyethylene wax, 10-20 parts polycaprolactone, 5-10 parts polybutylene succinate, 2.5-4.5 parts nano-silica with surface modification by silane coupling agent, and 2-5 parts biochar. The components work together to form a barrier structure with an initial water contact angle greater than 110° that gradually disintegrates under the action of soil microorganisms, so that the inner bag is in a state of moisture isolation for the first 1-2 months after being buried in the soil.

4. The bag-in-bag type time-release controlled-release fertilizer according to claim 1, characterized in that, The number of inner bags is two or more, and each inner bag is coated with a gradient degradation composite coating with different numbers of layers and / or different thicknesses, so that multiple inner bags can sequentially start releasing nutrients after being buried in the soil, realizing multi-stage relay fertilization.

5. The bag-in-bag type time-release controlled-release fertilizer according to claim 1, characterized in that, The outer bag is a slit-type controlled-release bag, with long strip-shaped slits of 1.75 to 3.25 mm in length and 0.1 to 0.5 mm in width distributed on its surface. The edges of the slits are hardened by hot-pressing and melting followed by cooling and shaping to prevent soil particles from clogging the slits.

6. The bag-in-bag type time-release controlled-release fertilizer according to claim 1 or 5, characterized in that, The outer bag is made of a fully biodegradable resin film with a degradation cycle of 1 to 2 months, and the inner bag is made of a fully biodegradable resin film with a degradation cycle of 3 to 6 months. Furthermore, the inner bag contains 0.8% to 1.7% degradation inhibitors by weight of the film material, which are at least one of carbodiimide compounds, epoxidized soybean oil, and organophosphonate compounds.

7. The bag-in-bag type time-release controlled-release fertilizer according to claim 1, characterized in that, The first fertilizer is a fast-acting chemical fertilizer granule, and the second fertilizer is a slow-release fertilizer granule treated with both urease inhibitor and nitrification inhibitor; the total nutrient content of the first fertilizer and the second fertilizer may be the same or different.

8. The bag-in-bag type time-release controlled-release fertilizer according to claim 1, characterized in that, A water-soluble isolation film is attached to the inner wall of the outer bag. After the outer bag is sealed, the water-soluble isolation film protects the outer surface coating of the inner bag from erosion by free water in the first fertilizer during storage and dissolves rapidly under the action of soil moisture.

9. A method for preparing a bag-in-bag type time-release controlled-release fertilizer according to any one of claims 1-8, characterized in that, Includes the following steps: S1. Prepare the outer bag body and inner bag body membrane materials; S2. The inner bag material is made into an inner bag, the second fertilizer is filled into the inner bag and then sealed. S3. A gradient degradation composite coating is formed on the outer surface of the sealed inner bag using a layer-by-layer impregnation-drying process: First, the inner bag is immersed in a polyvinyl alcohol solution and pulled dry to form a hydrophilic biodegradable polyester layer; then, it is immersed in a molten blend containing natural wax, oxidized polyethylene wax, polycaprolactone, polybutylene succinate, nano-silica surface-modified with silane coupling agent, and biochar and pulled cool to form a barrier wax-polymer composite layer; finally, it is immersed in a polycaprolactone solution and pulled dry to form a hydrophobic biodegradable polyester layer. The thickness of each layer is controlled by adjusting the solution concentration and the number of pulls to obtain a slow-release start-up delayed inner bag with a coating. S4. Fill the outer bag with the first fertilizer to the predetermined filling amount, and then place the coated slow-release start-up delay inner bag into the fertilizer inside the outer bag, so that the inner bag is located at the geometric center or off-center of the outer bag. S5. Seal the outer bag so that the inner bag is completely sealed inside the outer bag and immersed in the first fertilizer.

10. The method for preparing the bag-in-bag type time-release controlled-release fertilizer according to claim 9, characterized in that, Before the first fertilizer is loaded into the outer bag, a polyvinyl alcohol aqueous solution is sprayed onto the inner wall of the outer bag and dried to form the water-soluble isolation film. During the preparation of the outer bag, a hot-press cutter is used to form the slit on the surface of the bag, and the edge of the slit is melted by the hot-pressing process. After cooling, the hardened edge is formed naturally.

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