A green and environment-friendly starch-based meal box formula and a forming method thereof

Abstract

The application belongs to the technical field of green and environmental protection materials, in particular to a green and environmental protection starch-based meal box formula and a forming method thereof, the formula comprising modified starch, main plasticizer, functional additive and biodegradable polyester, the modified starch is provided with rigid support by introducing hydrophobic long chains on the molecular chain of the modified starch to form physical entanglement points; the functional additive contains hydrophilic functional groups and hydrophobic long chains, forms hydrogen bonds with hydroxyl groups of the starch and constructs an interfacial compatible layer between the two phases; the main plasticizer reduces the glass transition temperature of the starch to realize thermoplastic processing; the biodegradable polyester is used as a dispersed phase to improve the overall hydrophobicity; through the synergistic effect of the above technical solutions, the meal box prepared by the application realizes effective combination of the starch and the polyester two phases while maintaining the integrity of the starch molecular chain, the bending strength and the elastic modulus are guaranteed, the water absorption rate is controlled, and the degradation period meets the ninety-day composting requirement.

Description

Technical Field

[0001] This invention belongs to the field of green and environmentally friendly materials technology, specifically a green and environmentally friendly starch-based lunch box formula and its molding method. Background Technology

[0002] Starch-based biodegradable lunch boxes are a core environmentally friendly product that replaces traditional plastic tableware and solves white pollution. Starch molecules are rich in hydroxyl groups, forming a dense three-dimensional network of hydrogen bonds, which endows the material with biodegradable properties and basic mechanical properties. However, it also has inherent material defects such as excessive rigidity that prevents direct thermoplastic processing and strong hydrophilicity and poor water resistance. Achieving a synergistic fit between mechanical properties, water resistance, processability and degradability is the core R&D goal for the industrialization of high-performance starch-based lunch boxes.

[0003] Currently, the industry generally adopts single or combined modification processes for modifying starch-based lunch boxes, such as small molecule polyol plasticizing modification to optimize processing performance, chemical hydrophobic / crosslinking modification to improve water resistance, and biodegradable polyester blending to improve mechanical properties. These processes aim to make up for the application shortcomings of pure starch materials and prepare biodegradable tableware products.

[0004] Existing modification processes suffer from a fundamental technical contradiction rooted in the shared hydrogen bond mechanism of starch. All modification methods are merely peripheral structural modifications and cannot optimize the inherent properties of starch molecules at their source. Plasticizer doping has problems such as unstable physical binding and easy migration and precipitation, which can cause products to become brittle and crack after long-term storage and pose food safety risks. Hydrophobic crosslinking modification blocks enzymatic hydrolysis sites and restricts molecular chain movement, significantly prolonging the degradation cycle. Polyester blending suffers from severe phase separation and weak interfacial bonding due to the difference in polarity between the two phases, and additional additives are required to improve compatibility. At the same time, the batch composition difference of starch raw materials will further amplify the performance fluctuations of various modification processes. Ultimately, the existing technology is always trapped in a performance balance dilemma of strength, toughness, water resistance, processability, and degradability, making it impossible to achieve simultaneous optimal adaptation of the comprehensive performance of starch-based food containers.

[0005] Therefore, this invention provides a green and environmentally friendly starch-based lunchbox formula and its molding method. Summary of the Invention

[0006] In order to overcome the shortcomings of the prior art, at least one technical problem raised in the background art is solved.

[0007] The technical solution adopted by the present invention to solve its technical problem is: a green and environmentally friendly starch-based lunch box formula of the present invention, which includes the following components: modified starch, main plasticizer, functional additives and biodegradable polyester.

[0008] Modified starch is used as the main base material. Specific modifying groups are introduced into its molecular structure. These modifying groups are introduced by grafting hydrophobic long chains with specific structures onto the hydroxyl positions of the starch molecular chains.

[0009] Functional additives play multiple synergistic roles in formulations: firstly, they participate in the construction of a hydrogen bond competition system between starch molecular chains, partially replacing the original hydrogen bond network between starch molecular chains through the intermolecular hydrogen bonds formed between the functional additive molecules and starch hydroxyl groups; secondly, they provide the structural basis for interfacial bonding with biodegradable polyesters, enabling functional additives to build an effective compatibility bridge between the starch phase and the polyester phase.

[0010] In this invention, the combined use of modified starch and functional additives constitutes the core design. The hydrophobic long chains on the modified starch molecular chains form physical entanglement points between the starch molecular chains. These physical entanglement points generate elastic resistance when the starch molecular chains undergo relative displacement, providing a certain degree of rigid support to the system.

[0011] The functional additive molecule contains both hydrophilic functional groups and hydrophobic long chain structures. The hydrophilic functional groups can form hydrogen bonds with starch hydroxyl groups, while the hydrophobic long chains can generate van der Waals forces with biodegradable polyester segments.

[0012] This dual function enables the functional additives to form an interfacial compatibility layer between the starch phase and the polyester phase, through which the starch phase and the polyester phase achieve effective bonding.

[0013] When external forces are applied to the composite material, the stress can be effectively transferred between the starch phase and the polyester phase through the interfacial compatibility layer, thereby improving the mechanical properties of the composite material.

[0014] The amount of the primary plasticizer added to the formulation is within a specific range of proportions by weight of starch, which is sufficient to reduce the glass transition temperature of the starch to a range that is operable in the molding process.

[0015] The main plasticizer molecule inserts between starch molecular chains and weakens the hydrogen bond network between starch molecular chains by forming hydrogen bonds with starch hydroxyl groups, thereby reducing the activation energy required for the movement of starch molecular chain segments.

[0016] This effect allows starch to soften at temperatures below its original glass transition temperature, thus meeting the basic requirements for thermoplastic processing.

[0017] During the subsequent molding process, the main plasticizer forms a uniform distribution in the starch phase, and its molecular motion maintains a dynamic equilibrium within a certain temperature range, thus maintaining the processing fluidity of the starch phase.

[0018] This invention also provides a green and environmentally friendly starch-based food container molding method, which includes the following process steps: raw material pretreatment, mixing and plasticizing, molding and processing, and post-treatment.

[0019] In the raw material pretreatment step, the modified starch is first dried to control its moisture content below a preset threshold in order to avoid moisture vaporization during subsequent high-temperature processing, which would cause product defects. At the same time, the biodegradable polyester is preheated to reach the softening temperature and become molten. Functional additives and main plasticizers are premixed evenly according to the formula ratio to form an additive mixture for later use.

[0020] In the mixing and plasticizing step, the pretreated modified starch, preheated biodegradable polyester and additive mixture are sequentially added to the mixing and plasticizing equipment. The mixing and plasticizing equipment is equipped with a screw structure that works together with each other. The screw structure rotates at a set speed under the action of the drive device.

[0021] The material is gradually melted and mixed under the conveying and shearing action of the screw. During the mixing and plasticizing process, the temperature of each section of the mixing and plasticizing equipment is distributed according to the preset temperature gradient, gradually increasing from the feeding section to the plasticizing section.

[0022] The shearing action of the screw fully disperses the starch particles in the molten polyester, and the functional additives are oriented at the interface between the two phases, with their hydrophilic functional groups facing the starch phase and their hydrophobic long chains facing the polyester phase. As the mixing and plasticizing proceeds, the functional additives gradually form a stable interfacial compatibility layer at the interface between the two phases, and the phase interface between the starch phase and the polyester phase becomes blurred.

[0023] When the mixing and plasticizing process reaches the set time, the screw speed and shear rate are dynamically adjusted according to the material state at that time. When the degree of melting of the material is detected to reach the preset standard, the system automatically reduces the screw speed and the shear rate to avoid excessive shearing and excessive degradation of starch molecular chains. When the uniformity of mixing of the material is detected to reach the preset requirements, the mixing and plasticizing step is completed and the material enters the next process.

[0024] In the molding process, the mixed and plasticized material enters the molding die under the pushing action of the screw. The temperature of the molding die is controlled within the temperature range where the main plasticizer remains active, so that the starch phase is kept in a softened state.

[0025] Under the action of molding pressure, the material fills into all parts of the mold cavity, fully contacts the mold wall and cools and solidifies rapidly. The setting of molding pressure takes into account the viscosity difference between the starch phase and the polyester phase and the interfacial bonding state, ensuring that the material flows smoothly in the mold cavity and the interfacial bonding is complete.

[0026] In the post-processing step, the molded lunch box product is removed from the mold and subjected to a shaping process. The shaping process is carried out under set temperature and humidity conditions to release the residual stress inside the product and stabilize the distribution of functional additives at the two-phase interface. After the shaping is completed, the product is subjected to quality inspection, including appearance inspection, dimensional measurement and performance sampling.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1. Through the synergistic effect of modified starch and functional additives, a dual-action mechanism between starch molecular chains is constructed. The hydrophobic long chains on the modified starch molecular chains form physical entanglement points, while the functional additives partially replace the original hydrogen bond network between starch molecules through hydrogen bond competition. The superposition of these two effects allows the starch phase to maintain a certain rigidity while achieving processing fluidity in the presence of the main plasticizer, thus ensuring the flexural strength and elastic modulus of the composite material. Since the physical entanglement points are a reversible physical effect rather than a chemical crosslinking, the glycosidic bonds on the starch molecular chains remain intact. The amylase secreted by microorganisms can recognize and bind to specific sites on the starch chain segments, and the efficiency of the enzymatic hydrolysis reaction is not significantly affected. The degradation cycle of the product in the composting environment can meet the 90-day composting treatment requirements.

[0029] 2. The amphiphilic structure of the functional additives enables them to form an interfacial compatibility layer between the starch and polyester phases. The hydrophilic functional groups of the functional additives form hydrogen bonds with the starch hydroxyl groups, while the hydrophobic long chains generate van der Waals forces with the polyester segments. These two effects coexist and synergistically interact at the interface. The formation of the interfacial compatibility layer enhances the bonding strength between the starch and polyester phases. When external forces are applied to the composite material, the stress can be effectively transferred between the two phases through this interfacial compatibility layer. This mechanism improves the compatibility between starch and polyester, achieving effective bonding between the two phases without the need for additional coupling agents or compatibilizers, thus simplifying the complexity of the formulation system.

[0030] 3. Biodegradable polyester is distributed as a dispersed phase in the continuous phase formed by modified starch. The hydrophobic structure of the polyester chain segments forms a physical barrier to the penetration of water molecules. At the same time, the hydrophobic long chain portion of the functional additive covers the starch hydroxyl groups, reducing the direct contact between water molecules and starch hydroxyl groups. The above dual effects control the water absorption rate of the composite material. When the product comes into contact with water or high humidity environment, the decrease in mechanical properties is reduced and the water resistance is improved.

[0031] 4. The physical entanglement formed between the hydrophobic long chains on the modified starch molecular chain and the functional additive molecules is dynamically reversible. During the storage and use of the product, the migration of functional additive molecules in the starch phase is hindered by the physical entanglement points, and their migration rate to the product surface is reduced. At the same time, the hydrogen bonding between the functional additive and the starch hydroxyl groups has reversible equilibrium characteristics. When some functional additives migrate to the surface, some hydrogen bonds will be reformed between the starch hydroxyl groups, but the physical entanglement points can still maintain a certain rigid support. This mechanism suppresses the change of the mechanical properties of the product during storage, and the performance stability is better than that of traditional formulation systems that rely solely on small molecule plasticizers. Attached Figure Description

[0032] The invention will now be further described with reference to the accompanying drawings.

[0033] Figure 1 This is a structural block diagram of a green and environmentally friendly starch-based lunchbox formula according to the present invention;

[0034] Figure 2 This is a flowchart of a green and environmentally friendly starch-based lunchbox molding method according to the present invention. Detailed Implementation

[0035] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0036] like Figure 1 As shown, the green and environmentally friendly starch-based lunchbox formula of the present invention comprises four basic components: modified starch, main plasticizer, functional additives, and biodegradable polyester.

[0037] Modified starch plays a dominant role in the formulation as a continuous phase matrix. The preparation method of modified starch is as follows: 100 parts of corn starch are added to a three-necked flask, and anhydrous ethanol is added to prepare a 30% starch emulsion. Under nitrogen protection, the temperature is raised to 55°C. 0.8 parts of cerium ammonium nitrate initiator are added, and after stirring and activation for 10 min, 12 parts of hexadecene grafted monomer are slowly added dropwise. The reaction is maintained at this temperature for 3 h. After the reaction, the product is precipitated with acetone, and the homopolymer is removed by Soxhlet extraction for 24 h. The product is then vacuum dried at 60°C to constant weight to obtain grafted modified starch. It is determined that 8 hexadecyl alkyl long chains are grafted onto every 100 glucose units, with a grafting rate of 9.2%. After specific modification treatment, its molecular structure forms a novel starch derivative with hydrophobic long chain side groups. These hydrophobic long chain side groups are fixed at the hydroxyl positions of the starch molecular chain by chemical grafting, forming the main chain side group structure of modified starch.

[0038] The main plasticizer is uniformly distributed in the continuous phase matrix formed by modified starch in a molecular-level dispersion. Its molecules can insert between the molecular chains of modified starch and form hydrogen bonds with the hydroxyl groups on the modified starch chains.

[0039] Functional additives play multiple synergistic roles in the formulation. Their molecular structure contains both hydrophilic functional groups and hydrophobic long chain structures. The hydrophilic functional groups can compete with the hydroxyl groups on the modified starch chain to form hydrogen bonds, while the hydrophobic long chains can interact with the segments of the biodegradable polyester chain through van der Waals forces.

[0040] Biodegradable polyester is distributed in the form of a dispersed phase within a continuous phase matrix formed by modified starch, and its chain segment structure interacts physically with the hydrophobic long chains of functional additives.

[0041] In the internal structure of the formulation system, the modified starch molecular chains form a spatial network structure through physical entanglement points. The formation of these physical entanglement points originates from the mutual entanglement between the hydrophobic long chains on the modified starch molecular chains. When the modified starch molecular chains undergo relative displacement under the action of external force, these physical entanglement points can generate elastic resistance, thereby constraining the slippage of the molecular chains.

[0042] The functional additive molecules interact with both the modified starch chain and the biodegradable polyester chain. Their hydrophilic functional groups form hydrogen bonds with the hydroxyl groups on the modified starch chain, while their hydrophobic long chains interact with the biodegradable polyester chain segments through van der Waals forces. This dual interaction enables the functional additive to form a stable interfacial compatibility layer at the interface between the modified starch phase and the biodegradable polyester phase.

[0043] The modified starch continuous phase and the biodegradable polyester dispersed phase are effectively bonded together through an interfacial compatibility layer. When external force is applied to the composite material, the stress can be effectively transferred between the two phases through this interfacial compatibility layer.

[0044] The modified starch described in this invention is a product obtained by introducing specific modifying groups into the molecular structure of natural starch.

[0045] Natural starch molecules are composed of two parts: amylose and amylopectin. Amylose is a linear polymer composed of α-D-glucopyranose units linked by α-1,4 glycosidic bonds. In addition to linear segments linked by α-1,4 glycosidic bonds, amylopectin also contains branched structures linked by α-1,6 glycosidic bonds.

[0046] Natural starch molecules have a large number of hydroxyl groups distributed on them. These hydroxyl groups can form intramolecular hydrogen bonds and intermolecular hydrogen bonds, forming a dense hydrogen bond network structure. This hydrogen bond network structure is the fundamental reason why natural starch has high rigidity and low thermoplastic processability.

[0047] The modified starch of the present invention is prepared by chemical grafting modification, in which hydrophobic long chains with specific structures are grafted onto the hydroxyl positions of the starch molecular chains.

[0048] This chemical grafting reaction is usually carried out in a reaction system containing a dispersion medium, which can be an organic solvent or a mixture of an organic solvent and water.

[0049] The grafting monomer is selected from olefinic compounds with a long-chain alkyl structure, with the number of carbon atoms between twelve and twenty-two, preferably with an alkyl structure of sixteen to eighteen carbon atoms.

[0050] The initiation system uses chemical initiators or radiation initiation methods that can generate free radicals. Under the action of the initiator, olefin monomers undergo graft copolymerization with the hydroxyl positions on the starch molecular chain to form a novel polymer structure with starch as the main chain and long-chain alkyl groups as side groups.

[0051] After the above chemical grafting modification treatment, some of the hydroxyl groups on the starch molecular chain are converted into hydrophobic long-chain side groups connected by ester bonds or ether bonds. The length of the hydrophobic long-chain side groups should be sufficient to form physical entanglement between adjacent modified starch molecular chains. When the number of carbon atoms in the hydrophobic long chain is too small, its entanglement ability with adjacent molecular chains is insufficient, and it cannot form effective physical cross-linking points. When the number of carbon atoms in the hydrophobic long chain is too large, it may excessively reduce the hydrophilicity of the modified starch, affecting its compatibility with other components in subsequent processing. Experimental studies have shown that alkyl long chains with sixteen to eighteen carbon atoms can achieve the best balance between physical entanglement ability and hydrophilicity retention.

[0052] The hydrophobic long chains on the modified starch molecular chain form physical entanglement points between starch molecular chains, and these physical entanglement points have reversible dynamic characteristics.

[0053] When modified starch is in a molten state, the thermal motion of the molecular chains intensifies, and the physical entanglement points are in a dynamic equilibrium of continuous dissociation and reconstruction. When the modified starch is cooled to room temperature, the physical entanglement points reform, providing rigid support for the composite material.

[0054] Unlike chemical crosslinking, physical entanglements are formed through van der Waals forces and hydrophobic interactions between molecular chains. Their formation and destruction do not involve the breaking and reconstruction of covalent bonds, thus physical entanglements are reversible. This characteristic is of great significance for subsequent degradation and processing performance.

[0055] The main plasticizer described in this invention plays a key role in the formulation system by reducing the glass transition temperature of modified starch and enabling the modified starch to acquire thermoplastic processing properties.

[0056] Modified starch, as a polymer material, has its molecular chain segment movement restricted in a dense network structure formed by intermolecular hydrogen bonds. It exhibits a glassy state at room temperature and has high rigidity and brittleness.

[0057] When the temperature rises above the glass transition temperature, the molecular chain segments of modified starch can obtain sufficient degrees of freedom of movement, exhibiting a highly elastic or viscous flow state, thus possessing the basic conditions for thermoplastic processing.

[0058] However, the glass transition temperature of natural starch or simply modified starch is usually above 200 degrees Celsius. Processing under such high temperature conditions will cause the starch molecular chains to undergo a violent thermal degradation reaction, resulting in a serious decline in product performance.

[0059] The addition of the main plasticizer can effectively solve the above-mentioned technical problems. The main plasticizer selected in this invention is a polyol compound, preferably a small molecule polyol such as glycerol, sorbitol or xylitol. The molecular structure of this type of compound contains multiple hydroxyl groups, which can form hydrogen bonds with the hydroxyl groups on the modified starch chain.

[0060] When the main plasticizer molecule inserts between the modified starch molecular chains, its hydroxyl groups form new hydrogen bonds with the hydroxyl groups on the modified starch chains, partially replacing the original hydrogen bond network between the starch molecular chains. This substitution effect leads to a decrease in the hydrogen bond density between the modified starch molecular chains, and the activation energy required for the movement of molecular chain segments also decreases accordingly. Macroscopically, this manifests as a shift in the glass transition temperature of the modified starch towards lower temperatures.

[0061] The amount of main plasticizer added is a key parameter affecting the thermoplastic processing performance of modified starch. When the amount added is too small, the main plasticizer molecules are not enough to fully insert between the modified starch molecular chains, and the glass transition temperature of the modified starch is still too high, making it impossible to thermoplastic mold within a reasonable processing temperature range.

[0062] When the amount added is too large, although the glass transition temperature of modified starch can be significantly reduced, too much main plasticizer will cause excessive loss of rigidity of the continuous phase of modified starch. The flexural strength and elastic modulus of the composite material cannot meet the requirements for use in lunch box products. In addition, too much main plasticizer also poses a risk of migration and precipitation, affecting the long-term performance of the product.

[0063] The present invention optimizes the amount of the main plasticizer, controlling it within the range of 25% to 40% of the modified starch mass.

[0064] Within this dosage range, the main plasticizer molecules can fully insert into the modified starch molecular chains, reducing the glass transition temperature of the modified starch to the range of 120°C to 150°C, so that the modified starch exhibits a good softening and flow state under processing temperature conditions of 160°C to 180°C.

[0065] At the same time, under this dosage condition, the hydroxyl positions on the modified starch molecular chains that are not occupied by the main plasticizer still retain a certain number of intermolecular hydrogen bonds. In addition, the rigid support provided by the physical entanglement points between the modified starch molecular chains allows the flexural strength and elastic modulus of the composite material to be maintained above the requirements for use in lunch box products.

[0066] The main plasticizer plays an important role in maintaining processing fluidity during the mixing and plasticizing process. Under the high temperature and high shear environment of the mixing and plasticizing equipment, the hydrogen bonding between the main plasticizer molecules and the modified starch molecular chains is in a dynamic equilibrium state. The main plasticizer molecules continuously undergo dynamic processes of adsorption and desorption in the modified starch phase.

[0067] This dynamic balance allows the modified starch phase to maintain stable processing fluidity within a certain temperature range, preventing the material from becoming sticky due to excessive activity of the main plasticizer, and also preventing the material from becoming insufficiently fluid due to excessive stability of the main plasticizer.

[0068] During subsequent molding and post-processing, the main plasticizer molecules gradually achieve uniform distribution in the modified starch phase, and their molecular motion tends to stabilize under room temperature conditions, thus ensuring the dimensional stability and performance of the product.

[0069] The functional additives described in this invention are key components for achieving interfacial bonding between modified starch and biodegradable polyester. The functional additive molecules contain both hydrophilic functional groups and hydrophobic long-chain structures in their structure. This unique amphiphilic structure enables them to play a bridging role at the interface between the modified starch phase and the biodegradable polyester phase.

[0070] The hydrophilic functional groups of functional additives are usually polar functional groups such as hydroxyl, carboxyl or amino groups, which can form hydrogen bonds with the hydroxyl groups on the modified starch chain; the hydrophobic long chains of functional additives are usually alkyl chains or ester long chains with twelve to twenty carbon atoms, which can generate van der Waals forces with the segments of biodegradable polyester.

[0071] During the mixing and plasticizing process, functional additive molecules spontaneously migrate to the interface between the two phases and align themselves, with their hydrophilic functional groups facing the modified starch phase and their hydrophobic long chains facing the biodegradable polyester phase.

[0072] This directional arrangement enables functional additive molecules to form a stable adsorption layer at the interface between the two phases. The functional additive molecules form hydrogen bonds with the modified starch chain through their hydrophilic functional groups, and at the same time form van der Waals forces with the biodegradable polyester chain segments through their hydrophobic long chains.

[0073] When an external force is applied to the composite material, the stress is first transferred to the interfacial compatibility layer, and then transferred from the interfacial compatibility layer to the modified starch phase and the biodegradable polyester phase, respectively, thus achieving effective stress dispersion and transfer.

[0074] Functional additives also play an important role in the formulation system by constructing a hydrogen bond competition system between starch molecular chains.

[0075] The hydrogen bond network between modified starch molecular chains is a key factor affecting the rigidity and processing performance of composite materials. An overly strong hydrogen bond network makes modified starch difficult to process, while an overly weak hydrogen bond network results in insufficient rigidity of the composite material. Functional additive molecules form hydrogen bonds with the hydroxyl groups on the modified starch chains through their hydrophilic functional groups, partially replacing the original hydrogen bond network between starch molecular chains.

[0076] This hydrogen bond competition can effectively reduce the hydrogen bond density between modified starch molecular chains, making the modified starch more prone to chain segment movement under external force, thereby achieving processing fluidity at lower temperature conditions.

[0077] The hydrogen bond competition system formed by the functional additives and the physical entanglement points between the modified starch molecular chains create a synergistic effect.

[0078] Physical entanglement points provide rigid support at the molecular chain level, similar to a physical cross-linking network; the hydrogen bond competition formed by functional additives reduces the bonding strength between molecular chains, making them more prone to relative slippage under external forces.

[0079] The combined effect of these two actions allows the modified starch to maintain a certain rigidity while achieving processing fluidity. When the effect of the main plasticizer in lowering the glass transition temperature of the modified starch is combined with the above two effects, the thermoplastic processing window of the modified starch is significantly widened, and the composite material can maintain good mechanical properties while meeting the processing fluidity requirements.

[0080] The amount of functional additives added to the formula also needs to be precisely controlled.

[0081] When the amount added is too small, the functional additive molecules are insufficient to form a complete interfacial compatibility layer at the interface between the two phases, resulting in insufficient bonding strength between the modified starch phase and the biodegradable polyester phase. The composite material is prone to failure at the interface when subjected to stress.

[0082] When the amount of functional additives added is too high, the functional additive molecules may excessively weaken the hydrogen bond network between the modified starch molecular chains, resulting in an excessive reduction in the rigidity of the composite material. At the same time, too many functional additive molecules may self-aggregate to form independent phase regions, which may affect the uniformity of the composite material. Experimental studies have shown that when the amount of functional additives added is controlled within the range of 3% to 8% of the mass of modified starch, the best balance can be achieved between interfacial compatibility and composite material performance.

[0083] The biodegradable polyester described in this invention exists as a dispersed phase in the formulation system, and its main function is to improve the hydrophobic properties and mechanical strength of the composite material.

[0084] Biodegradable polyesters are a class of polyester polymers that can be decomposed by microorganisms under specific environmental conditions. Common examples include polylactic acid, polyhydroxyalkanoates, polybutylene succinate and their copolymers. These materials contain organic segments linked by ester bonds in their molecular structure. These ester bonds can be hydrolyzed and broken by enzymes secreted by microorganisms, and are eventually decomposed into carbon dioxide, water and biomass.

[0085] The amount of biodegradable polyester added to the formulation directly affects the performance of the composite material. When the amount added is too small, the hydrophobic barrier effect of the biodegradable polyester dispersed phase is not obvious, the water absorption rate of the composite material is still high, and the water resistance cannot be effectively improved.

[0086] When the amount added is too large, the volume fraction of the biodegradable polyester dispersed phase is too high, which may cause the dispersed phase to aggregate in the modified starch continuous phase, forming large dispersed phase particles, which in turn weakens the interfacial bonding strength of the composite material.

[0087] Experimental studies have shown that when the amount of biodegradable polyester added is controlled within the range of 15% to 30% of the modified starch mass, the best balance can be achieved between hydrophobic barrier effect and interfacial bonding strength.

[0088] During the mixing and plasticizing process, biodegradable polyester needs to be preheated to a molten state. Under the high temperature environment of the mixing and plasticizing equipment, the biodegradable polyester particles gradually soften and melt to form a continuous polymer melt.

[0089] The modified starch particles are gradually dispersed in the melt, and the functional additive molecules are oriented at the interface between the two phases to form a stable interfacial compatibility layer. As the mixing and plasticizing proceeds, the dispersion uniformity of the biodegradable polyester dispersed phase gradually increases, and the size of the dispersed phase particles gradually decreases and tends to be uniform.

[0090] When the mixing and plasticizing process reaches the set time, the biodegradable polyester dispersed phase forms a uniformly dispersed fine particle structure in the modified starch continuous phase. The size of the dispersed phase particles is usually controlled in the range of five to fifty micrometers.

[0091] The hydrophobic segments of the biodegradable polyester dispersion phase significantly improve the water absorption rate of the composite material.

[0092] When the composite material comes into contact with water or a high-humidity environment, water molecules need to first penetrate through the modified starch continuous phase before entering the interior of the material.

[0093] The presence of the biodegradable polyester dispersion phase increases the path length for water molecule penetration, requiring water molecules to bypass the dispersion phase particles before continuing to penetrate into the material.

[0094] Meanwhile, the hydrophobic long chain portion of the functional additive covers the hydroxyl groups on the modified starch chain, further reducing the direct contact between water molecules and modified starch. The superposition of these two effects significantly reduces the water absorption rate of the composite material, and reduces the decline in mechanical properties of the product when it comes into contact with water or high humidity environments.

[0095] Based on the above formula, the present invention also provides a green and environmentally friendly starch-based lunch box molding method, the molding method comprising the following process steps:

[0096] Step 1: The raw material pretreatment step is the foundation of the entire molding method. Its purpose is to provide the raw material state that meets the processing requirements for subsequent mixing and plasticizing. In the drying process of modified starch, the moisture content in the modified starch raw material needs to be strictly controlled to below one percent.

[0097] Excess moisture in modified starch will vaporize and escape during the subsequent high-temperature mixing and plasticizing process, forming bubble defects in the composite material, which seriously affects the appearance quality and mechanical properties of the product.

[0098] Drying is usually carried out in a vacuum drying oven or fluidized bed drying equipment, with the drying temperature controlled in the range of 80 to 100 degrees Celsius. The drying time is adjusted appropriately according to the initial moisture content of the modified starch and the batch of raw materials.

[0099] Preheating of biodegradable polyester is another important step in the raw material pretreatment process. Biodegradable polyester exists as solid granules at room temperature, and its softening temperature is typically in the range of 100 to 120 degrees Celsius.

[0100] By preheating, the biodegradable polyester is heated to above the softening temperature, making it softened and fluid. This facilitates thorough mixing with modified starch during the subsequent mixing and plasticizing process. The preheating temperature is usually controlled within the range of 130~150℃, and the preheating time is based on the complete softening of the biodegradable polyester particles, usually ten to thirty minutes.

[0101] The functional additives and main plasticizers are pre-mixed evenly according to the formulation ratio to form an additive mixture for later use. The functional additives and main plasticizers are added to the mixing equipment in proportion and stirred and mixed under normal temperature or slight heating conditions to form a homogeneous mixture of the two components.

[0102] The mixing equipment can be a high-speed mixer, ribbon mixer or conical mixer or other conventional mixing equipment. The mixing time is based on the complete and uniform mixing of the two components, usually five to fifteen minutes. The preparation of the additive mixture should be completed in the raw material pretreatment stage to avoid measurement errors caused by temporary preparation in the subsequent processing.

[0103] Step 2: The mixing and plasticizing step is the core process of melting and mixing the pretreated raw material components in the mixing and plasticizing equipment to form a uniform composite system.

[0104] This invention uses a screw extruder as a mixing and plasticizing device. The device has a screw structure that works together. The screw structure is driven by a drive device to rotate at a set speed. The material is gradually melted and mixed under the conveying and shearing action of the screw to form a uniform composite material melt. The screw structure usually adopts a split screw or pin screw design to enhance the dispersion and mixing effect during the mixing and plasticizing process.

[0105] The temperature of each section of the mixing and plasticizing equipment is distributed according to a preset temperature gradient, gradually increasing from the feeding section to the plasticizing section. The temperature of the feeding section is controlled within the range of 100 to 120 degrees Celsius, mainly used to preheat the material entering the equipment; the temperature of the compression section is controlled within the range of 140 to 160 degrees Celsius, where the material gradually softens and begins to melt; the temperature of the plasticizing section is controlled within the range of 160 to 180 degrees Celsius, where the material completes melting and mixing and achieves a uniform dispersion state.

[0106] The specific temperature settings for each section need to be optimized and adjusted based on the glass transition temperature of the modified starch, the volatility of the main plasticizer, and the melting characteristics of the biodegradable polyester.

[0107] The movement of materials during the mixing and plasticizing process follows specific rules. After the pretreated modified starch granules are added to the equipment from the feed hopper, they move forward under the pushing action of the screw and gradually enter the high temperature zone.

[0108] As the temperature rises and the shearing effect intensifies, the modified starch particles gradually soften and adsorb the main plasticizer molecules. The hydrogen bond network between the molecular chains is weakened under the combined action of the main plasticizer and functional additives. The particle morphology gradually disappears and transforms into a continuous melt phase. The biodegradable polyester completely melts in the high-temperature environment of the plasticizing section and forms a two-phase mixed system with the modified starch melt. During this process, the functional additive molecules migrate to the interface between the two phases and are oriented. Their hydrophilic functional groups face the modified starch phase, and their hydrophobic long chains face the polyester phase, gradually forming a stable interfacial compatibility layer.

[0109] During the set mixing and plasticizing process, the screw speed and shear rate are dynamically adjusted according to the material state. When the degree of melting of the material reaches the preset standard, the system automatically reduces the screw speed and shear rate to avoid excessive shearing that could cause excessive degradation of the modified starch molecular chains. When the uniformity of the mixture reaches the preset requirements, the mixing and plasticizing step is completed, and the material proceeds to the next process.

[0110] The degree of material melting can be tested by measuring the stability of extrusion pressure and the uniformity of melt temperature. When the melt viscosity fluctuation is less than 5% for 5 consecutive minutes, the degree of melting is considered to meet the standard. The degree of mixing uniformity can be tested by sampling and analyzing the size distribution of dispersed phase particles. When the average size of dispersed phase particles is in the range of 10~30μm and the particle size distribution index is less than 0.4, the degree of mixing uniformity is considered to meet the requirements.

[0111] The composite melt obtained after the mixing and plasticizing step should meet the following quality requirements: the modified starch continuous phase and the biodegradable polyester dispersed phase are evenly distributed, the dispersed phase particle size is in the range of five to fifty micrometers and is evenly distributed; the functional additives form a complete interfacial compatibility layer at the interface between the two phases, and the thickness of the interfacial compatibility layer is sufficient to achieve effective bonding between the two phases; the degree of polymerization of the modified starch molecular chain is maintained within a reasonable range, and the molecular chain has not undergone excessive degradation; the viscosity of the composite melt is within a suitable molding range, and the melt temperature is uniform and stable.

[0112] Step 3: The molding process is a key step in transforming the mixed and plasticized composite material melt into the shape of a lunchbox product.

[0113] After being mixed and plasticized, the material enters the molding die under the pushing action of the screw. The temperature of the molding die needs to be precisely controlled to ensure the quality of the product. If the mold temperature is too high, it will cause the product to be difficult to demold and stick to the surface. If the mold temperature is too low, the material will cool and solidify too early during the process of filling the mold cavity, resulting in incomplete filling or surface defects.

[0114] The present invention controls the mold temperature within the temperature range where the main plasticizer remains active, namely between 100 degrees Celsius and 120 degrees Celsius, so that the modified starch phase is kept in a softened state without excessive curing.

[0115] The setting of molding pressure needs to take into account the viscosity difference between the starch phase and the polyester phase, as well as the interfacial bonding state.

[0116] There is a certain difference in viscosity between the modified starch continuous phase and the biodegradable polyester dispersed phase. If the molding pressure is too low, the areas with higher viscosity may not be fully filled; if the molding pressure is too high, it may cause flash defects or interface debonding problems in the product.

[0117] The present invention sets the molding pressure in the range of eight to fifteen MPa. Under this pressure condition, the material can flow smoothly in the mold cavity and fully fill all parts, while the interface bonding remains intact.

[0118] After the material is filled into various parts of the mold cavity under the action of molding pressure, it fully contacts the mold wall and cools and solidifies rapidly to form a product with the shape of a lunch box.

[0119] Step 4: The post-processing steps include demolding, shaping, and quality inspection. After the molded lunch box product is removed from the mold, there may be residual stress inside. If this residual stress is not released in time, it will cause the product to warp and deform during subsequent storage and use.

[0120] The setting process is carried out under set temperature and humidity conditions, preferably in an environment with a temperature of 40 to 60 degrees Celsius and a relative humidity of 40% to 60% for two to four hours, so that the residual stress inside the product can be fully released and the distribution of functional additives at the two-phase interface can be stabilized.

[0121] After the shaping process is completed, the products undergo comprehensive quality inspection, including appearance inspection, dimensional measurement and performance sampling, to ensure that every product leaving the factory meets the quality requirements.

[0122] There is a close synergistic relationship between the formulation composition and the molding method of this invention, and each process parameter needs to be optimized and adjusted according to the formulation composition.

[0123] In terms of formulation composition, the hydrophobic long-link branching rate of modified starch directly affects the density of physical entanglement points, thereby affecting the rigidity and degradation performance of the composite material; the amount of main plasticizer added directly affects the glass transition temperature and processing fluidity of modified starch; the amount and molecular structure of functional additives added directly affect the formation effect of the interfacial compatibility layer and the bonding strength between the two phases; the amount and molecular weight of biodegradable polyester added directly affect the dispersion state of the dispersed phase and the comprehensive performance of the composite material.

[0124] Regarding molding methods, the selection of the mixing and plasticizing temperature needs to simultaneously meet the requirements of modified starch softening, biodegradable polyester melting, and interfacial activity of functional additives. Too low a temperature will result in insufficient softening of the modified starch and incomplete melting of the biodegradable polyester, leading to poor mixing and plasticizing effects; too high a temperature may cause thermal degradation of the modified starch and excessive volatilization of the main plasticizer, resulting in decreased product performance. Controlling the mixing and plasticizing time requires striking a balance between uniform mixing and protection of the modified starch molecular chains. Too short a time will lead to uneven dispersion and poor interfacial bonding, while too long a time may result in excessive shear degradation of the modified starch molecular chains.

[0125] The dynamic adjustment of screw speed and shear rate is a key technical innovation of the molding method of this invention. In the initial stage of mixing and plasticizing, the viscosity of the material is high, requiring a high screw speed and shear rate to achieve effective dispersion and mixing; in the later stage of mixing and plasticizing, the material gradually becomes uniform and the viscosity decreases, and continuing to maintain high speed and high shear rate will cause unnecessary energy consumption and heat accumulation.

[0126] Therefore, the present invention adopts a dynamic adjustment strategy based on the material state. When the degree of melting of the material is detected to reach the preset standard, the screw speed is automatically reduced, and when the mixing uniformity is detected to reach the preset requirement, the mixing and plasticizing step is ended.

[0127] To enable those skilled in the art to more clearly understand the technical solution of the molding method of the present invention, a specific embodiment is described in detail below.

[0128] In the raw material pretreatment stage, the modified starch raw material is first placed in a vacuum drying oven for drying. The initial moisture content of the modified starch raw material is 8% to 12%. After vacuum drying at 80°C to 100°C for 8 to 12 hours, its moisture content is reduced to below 0.8%, meeting the moisture content requirements for subsequent processing. The biodegradable polyester raw material is polylactic acid particles, and its softening temperature is 120 to 130°C. It is placed in an oven at 140 to 150°C for preheating treatment for 20 to 30 minutes to make it completely softened and present a transitional state before melting. The functional additive is glyceryl monostearate, and the main plasticizer is glycerol. The two are added to a high-speed mixer in a mass ratio of 1:3 and mixed at room temperature for 10 to 15 minutes to form a uniform additive mixture.

[0129] During the mixing and plasticizing stage, the dried modified starch, preheated and softened biodegradable polyester, and additive mixture are sequentially added to the hopper of a twin-screw extruder. The twin-screw extruder has a screw diameter of 35 mm, a length-to-diameter ratio of 40:1, and a screw speed controlled within the range of 150 to 250 revolutions per minute.

[0130] The temperature settings for each section are as follows: the temperature of the feeding section is 110 degrees Celsius, the temperature of the compression section is 145 degrees Celsius, the temperature of the plasticizing section is 170 degrees Celsius, and the temperature of the die head is 165 degrees Celsius. The material gradually melts and mixes under the conveying and shearing action of the screw, and the functional additive molecules are oriented at the interface between the two phases to form an interfacial compatibility layer.

[0131] When the mixing and plasticizing process reaches 80 to 120 seconds, the change in melt viscosity of the material is detected by an online viscosity measuring device installed on the extruder. When the melt viscosity is detected to be stable, it indicates that the degree of melting of the material has reached the preset standard. The system automatically reduces the screw speed to 80 to 120 revolutions per minute to reduce the shear rate and avoid excessive shearing.

[0132] After mixing for another 30 to 60 seconds, samples are taken through the sampling port installed on the die head to analyze the size distribution of the dispersed phase particles. When the average size of the dispersed phase particles is within the range of 10 to 30 micrometers and is uniformly distributed, it indicates that the mixing uniformity has reached the preset requirements, and the mixing and plasticizing step is complete. The composite material melt obtained by extrusion is cooled in a water bath and then pelletized to obtain uniform composite material particles.

[0133] During the molding process, composite material particles are added to the injection molding machine to form the lunchbox product. The barrel temperature of the injection molding machine is controlled within the range of 160 to 175 degrees Celsius, the mold temperature is controlled within the range of 105 to 115 degrees Celsius, the molding pressure is set to 10 to 12 MPa, and the holding time is set to 8 to 12 seconds. Under the action of injection pressure, the material fills into all parts of the mold cavity, fully contacts the mold wall, and cools and solidifies rapidly to form a product with a standard lunchbox shape.

[0134] In the post-processing stage, after the molded lunch box products are removed from the mold, they are placed in an environment with a temperature of 50 degrees Celsius and a relative humidity of 50% for three to four hours for setting. After the setting process is completed, the products undergo comprehensive quality inspection, including appearance inspection, size measurement, water absorption test, bending strength test and degradation performance test.

[0135] The starch-based lunchbox formulation and molding method of the present invention produce lunchbox products with good biodegradability, which can meet the requirements of 90 days of composting. This degradation performance is achieved technically based on the following mechanism of action.

[0136] The glycosidic bonds on the modified starch molecular chain remain intact. The degradation of natural starch is achieved by microbial-secreted amylases recognizing and binding to specific sites on the starch chain segments, thereby catalyzing the hydrolysis and breakage of the glycosidic bonds. This invention provides rigid support through physical entanglement points rather than chemical cross-linking. The formation and destruction of these physical entanglement points do not involve the breaking of glycosidic bonds; therefore, the glycosidic bonds on the modified starch molecular chain remain intact, enabling effective recognition and catalytic hydrolysis by amylases.

[0137] The hydrogen bond competition system formed by functional additives accelerates the degradation process. The hydrogen bonds formed between the functional additives and the hydroxyl groups on the modified starch chains are in a reversible dynamic equilibrium. When amylase molecules approach the modified starch chain segments, some functional additive molecules desorb from the modified starch chains, providing binding sites for the amylase molecules. Simultaneously, the hydrogen bonds between functional additive molecules and modified starch chains are weaker than the hydrogen bonds between the modified starch chains themselves, making it easier for amylase molecules to disrupt the hydrogen bond connections between the functional additives and modified starch, thus initiating the degradation phase.

[0138] The addition of biodegradable polyester improves the controllability of degradation performance. Biodegradable polyester itself can be decomposed by microorganisms in a composting environment, and its decomposition products are carbon dioxide, water, and biomass. The presence of the biodegradable polyester dispersed phase gives the composite material a two-phase structure. The modified starch continuous phase provides a convenient permeation channel for amylase hydrolysis. The degradation process starts from the modified starch continuous phase and gradually extends to the biodegradable polyester dispersed phase, ultimately achieving complete degradation of the product.

[0139] To verify the technical effectiveness of the present invention, performance comparison tests are conducted below through specific embodiments and comparative examples.

[0140] Example 1 is a specific implementation of the technical solution of the present invention. The formulation used is as follows: 100 parts by weight of modified starch, 35 parts by weight of glycerol as the main plasticizer, 6 parts by weight of glyceryl monostearate as the functional additive, and 20 parts by weight of biodegradable polyester polylactic acid. The hydrophobic long-chain grafting rate of the modified starch is 8 hexadecyl alkyl long chains grafted onto every 100 glucose units. The molding method is carried out according to the process flow of the above-mentioned method example. The mixing and plasticizing temperature is 170°C, the screw speed is 200 rpm, and the molding pressure is 11 MPa.

[0141] Comparative Example 1 is a formulation system that uses natural starch to replace modified starch. Other components and molding methods are the same as in Example 1. Natural starch molecules do not have hydrophobic long chains and cannot form physical entanglement points. Its rigidity depends entirely on the intermolecular hydrogen bond network.

[0142] Comparative Example 2 is a formulation system without added functional additives. Other components and molding methods are the same as in Example 1. This formulation system lacks an interfacial compatibility layer, and there is no effective bond between the modified starch phase and the biodegradable polyester phase.

[0143] Comparative Example 3 is a formulation system in which the amount of the main plasticizer is reduced to 15 parts by weight. The other components and molding methods are the same as in Example 1. In this formulation system, the glass transition temperature of the modified starch is not reduced enough and the processing temperature window is too narrow.

[0144] The performance of the lunch box products prepared by the above four formulation systems was tested, and the test items and test methods were carried out in accordance with the relevant national standards.

[0145] The bending strength test was conducted in accordance with GB / T 9341 standard, with a test span of 50 mm and a loading rate of 2 mm per minute. The water absorption test involved immersing the product in water at 20 degrees Celsius for 24 hours and then measuring the rate of change in mass.

[0146] The degradation performance test involved placing the product in a composting environment at 50 degrees Celsius and 95% relative humidity, and taking samples every 15 days to measure the weight loss rate and molecular weight change.

[0147] Test Project Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Bending strength (MPa) 18.5 12.3 14.2 22.1 Flexural modulus (GPa) 2.8 1.9 2.3 3.5 Impact strength (kilojoules per square meter) 8.2 5.6 6.8 6.1 Water absorption rate (24-hour period) 8.3 15.7 12.1 9.2 15-day degradation weight loss (percentage) 32.5 28.3 30.1 15.8 30-day degradation weight loss (percentage) 58.7 45.2 51.6 32.5 Degradation weight loss rate (percentage) after 45 days 78.3 62.1 69.8 48.7 Weight loss rate after 60 days (percentage) 91.2 76.5 84.3 62.3 Degradation weight loss rate (percentage) over 90 days 98.6 88.2 94.1 81.5

[0148] The following conclusions can be drawn from the above comparative data.

[0149] The flexural strength of Example 1 was 18.5 MPa, which was significantly higher than that of Comparative Example 1 (12.3 MPa) and Comparative Example 2 (14.2 MPa).

[0150] In Comparative Example 1, natural starch was used to replace modified starch. Due to the lack of physical entanglement points formed by hydrophobic long chains, the rigidity of the composite material could not be effectively supported, and the bending strength decreased significantly.

[0151] In Comparative Example 2, without the addition of functional additives, there was a lack of effective interfacial bonding between the modified starch phase and the biodegradable polyester phase, which also affected the flexural strength.

[0152] In Comparative Example 3, the amount of main plasticizer was too small, and the glass transition temperature of the modified starch was not reduced sufficiently. Although the rigidity of the composite material was relatively high, the impact strength was only 6.1 kJ / m², which was too brittle and could not meet the requirements for use in lunch box products.

[0153] The water absorption rate of Example 1 was 8.3%, which was significantly lower than that of Comparative Example 1 (15.7%) and Comparative Example 2 (12.1%).

[0154] The presence of the biodegradable polyester dispersion phase increases the path length for water molecule penetration, and the hydrophobic long chains of the functional additives cover part of the modified starch hydroxyl groups. The combined effect of these two factors significantly improves the water resistance of Example 1.

[0155] Comparative Example 1 uses natural starch, whose hydrophilic hydroxyl groups are fully exposed, resulting in the highest water absorption rate; Comparative Example 2 does not contain any functional additives, and lacks the barrier effect of the hydrophobic long chains of functional additives, resulting in the second highest water absorption rate.

[0156] The degradation weight loss rate of Example 1 after 90 days was 98.6%, which fully meets the requirements for composting.

[0157] Comparative Example 1 uses natural starch, which has good degradation performance, but due to the lack of reversible physical entanglement points, the degradation rate is slightly lower than that of Example 1. The molecular chains still need to overcome some physical entanglement obstacles in the early stage of degradation.

[0158] In Comparative Example 2, without the addition of functional additives, the lack of an interfacial compatibility layer led to the obstruction of the degradation channels in the biodegradable polyester dispersion phase, resulting in a significant slowdown in the degradation rate after 45 days.

[0159] In Comparative Example 3, the amount of main plasticizer was too small, resulting in insufficient reduction of the glass transition temperature of the modified starch. As a result, the amylase molecules could not penetrate into the interior of the modified starch continuous phase, and the initial weight loss rate was only 15.8%, far lower than the 32.5% in Example 1. After ninety days, the weight loss rate was only 81.5%, which failed to meet the requirements for composting.

[0160] The above comparative data fully demonstrate that, through the synergistic effect of modified starch and functional additives, the optimization of the dosage of the main plasticizer, and the combination with the molding method, the present invention achieves optimized design of degradation performance while maintaining good mechanical properties of composite materials, and all performance indicators meet or exceed the preset technical goals.

[0161] To further verify the structure and function of the interfacial compatibility layer formed by the functional additives at the interface between the two phases, the present invention used scanning electron microscopy to characterize and analyze the interfacial microstructure of the composite material.

[0162] After the composite material sample prepared in Example 1 was subjected to liquid nitrogen brittle fracture treatment, the interface region of the brittle fracture surface was observed by scanning electron microscopy. The observation results showed that there was a transition layer with a thickness of about 0.5 to 2 micrometers in the interface region between the modified starch continuous phase and the biodegradable polyester dispersed phase. The morphology of the transition layer was different from that of the continuous phase and dispersed phase on both sides, indicating that its composition and structure were different from those of the two phase regions.

[0163] The elemental distribution of the transition layer was determined by energy dispersive spectroscopy (EDS). The results showed that the transition layer contained carbon and oxygen, characteristic elements of modified starch, as well as carbon and oxygen, characteristic elements of biodegradable polyester. The distribution of oxygen showed a gradient variation, confirming that the transition layer was an interfacial compatibility layer enriched with functional additives.

[0164] In Comparative Example 2, without the addition of functional additives, the brittle fracture interface region of the composite material exhibits obvious phase separation characteristics. The interface between the modified starch continuous phase and the biodegradable polyester dispersed phase is clearly distinguishable, and there are obvious gaps at the interface, indicating that the bonding strength between the two phases is weak.

[0165] The fracture morphology of the composite material samples of Example 1 and Comparative Example 2 after tensile testing was observed. The fracture surface of Example 1 showed ductile fracture characteristics, with obvious plastic deformation traces visible on the fracture surface, indicating that the presence of the interfacial compatibility layer enabled the stress to be effectively transferred between the two phases, and the material could undergo sufficient plastic deformation before fracture. The fracture surface of Comparative Example 2 showed brittle fracture characteristics, with a smooth fracture surface and obvious debonding traces visible at the interface, indicating that the interfacial bonding strength was insufficient and the stress could not be effectively transferred.

[0166] The above microstructure characterization results experimentally verify the effectiveness of the technical solution of forming an interfacial compatibility layer at the interface between two phases by the functional additives, providing a reliable scientific basis for the technical solution of the present invention.

[0167] The starch-based lunchbox product of the present invention can maintain good performance stability during storage and use. This characteristic is technically based on the following mechanism of action.

[0168] The hydrogen bonds formed between functional additives and the hydroxyl groups on the modified starch chain have reversible equilibrium characteristics. During the storage and use of the product, if some functional additive molecules migrate to the surface of the product, the hydroxyl positions on the modified starch chain that formed the hydrogen bonds with them will be temporarily vacated.

[0169] The hydroxyl groups on adjacent modified starch chains have the opportunity to reform hydrogen bonds, partially restoring the weakened hydrogen bond network. This reversible equilibrium property prevents the product from experiencing a continuous decline in mechanical properties during storage due to the gradual migration of functional additives.

[0170] The migration of functional additive molecules in the modified starch phase is also hindered by physical entanglement points. Functional additive molecules need to break free from the hydrogen bonds formed with the modified starch chains, and at the same time, they need to overcome the restriction of molecular chain movement by physical entanglement points in order to achieve migration.

[0171] This dual barrier significantly reduces the migration rate of functional additive molecules, ensuring the performance stability of the product under normal use conditions.

[0172] Biodegradable polyester dispersions also hinder the migration of functional additives. The hydrophobic long-chain portion of the functional additive molecules interacts with the biodegradable polyester segments through van der Waals forces. This interaction anchors the functional additive molecules at the interface between the two phases to some extent, further reducing the rate at which the functional additive molecules migrate to the surface of the product.

[0173] The synergistic effect of the aforementioned multiple functions enables the food container products of the present invention to maintain good performance stability during storage, with significantly lower changes in mechanical properties compared to traditional formulation systems that rely solely on small-molecule plasticizers. Experimental verification shows that after six months of storage under standard conditions of 23 degrees Celsius and 50% relative humidity, the flexural strength retention rate of the products of the present invention is 92.3%, and the impact strength retention rate is 88.7%, demonstrating significantly better performance stability than the comparative products.

[0174] The following technical points need to be noted during the industrial implementation of the technical solution of this invention to ensure the stability and consistency of product quality.

[0175] Quality control of modified starch raw materials is paramount. Different batches of modified starch may vary in hydrophobic long-link branching rate and molecular weight distribution, which directly affect the density of physical entanglements and the performance of composite materials. Therefore, it is necessary to establish incoming inspection standards for modified starch raw materials, testing key indicators such as branching rate, moisture content, and particle size distribution. Unqualified raw materials must not be used in production.

[0176] The uniformity of mixing of functional additives and primary plasticizers directly affects the formation of the interfacial compatibility layer. Inhomogeneous mixing leads to the accumulation of functional additives in some areas while they are scarce in others, resulting in an uneven distribution of the interfacial compatibility layer that negatively impacts the overall performance of the composite material. Therefore, it is necessary to ensure sufficient dispersion of functional additives and primary plasticizers during the mixing stage. If necessary, a two-step mixing method or high-shear mixing equipment can be used to improve mixing uniformity.

[0177] Parameter control during the mixing and plasticizing process is crucial to product quality. Fluctuations in parameters such as temperature, pressure, rotation speed, and time can affect the microstructure and macroscopic properties of composite materials. Therefore, real-time monitoring and automatic adjustment of key parameters are necessary during production to ensure that each batch of products is manufactured under identical process conditions. Furthermore, regular maintenance of the mixing and plasticizing equipment is required to ensure its precision and stability.

[0178] Mold design and process parameter optimization are equally important in the molding and processing stage. Lunchbox products typically have complex shapes with numerous corners and thin-walled areas, which are prone to defects such as incomplete filling or weld lines during molding. Therefore, it is necessary to optimize the mold gating system according to the specific shape of the product and finely adjust parameters such as molding pressure, holding time, and cooling time to ensure the molding quality of the product.

[0179] The post-processing conditions for setting the product need to be adjusted according to its thickness and shape. Thicker products require a longer setting time to release internal residual stress, while products with complex shapes require appropriate support and positioning methods to prevent warping and deformation. Simultaneously, the ambient temperature and humidity during the setting process must be strictly controlled to avoid variations in product performance due to fluctuations in environmental conditions.

[0180] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A green and environmentally friendly starch-based lunchbox molding method, characterized in that, Includes the following steps: Step 1: Raw material pretreatment step, the modified starch is dried to control its moisture content to below 1%; the biodegradable polyester is preheated to soften and melt; the functional additives and main plasticizers are premixed evenly in proportion to form an additive mixture for later use; Step Two: Mixing and Plasticizing Step. The pretreated modified starch, preheated biodegradable polyester, and additive mixture are sequentially added to a mixing and plasticizing device. This device contains a cooperating screw structure that rotates at a set speed under the action of a drive device. The temperature of each section of the mixing and plasticizing device follows a preset temperature gradient, gradually increasing from the feeding section to the plasticizing section. During mixing and plasticizing, the functional additives are oriented at the interface between the starch and polyester phases, with hydrophilic functional groups facing the starch phase and hydrophobic long chains facing the polyester phase, gradually forming an interfacial compatibility layer at the interface. When mixing and plasticizing reaches a set time, the screw speed and shear rate are dynamically adjusted according to the material state. When the degree of melting of the material reaches a preset standard, the screw speed is reduced to decrease the shear rate. When the mixing uniformity reaches the preset requirement, the mixing and plasticizing step is complete. Step 3: Molding process. The mixed and plasticized material enters the molding die under the pushing action of the screw. The temperature of the molding die is controlled within the temperature range where the main plasticizer remains active. Under the action of molding pressure, the material fills into all parts of the mold cavity, fully contacts the mold wall and cools and solidifies. Step 4: Post-processing step. The molded lunch box product is removed from the mold and subjected to shaping treatment. The shaping treatment releases the residual stress inside the product and at the same time stabilizes the distribution of functional additives at the interface.

2. The green and environmentally friendly starch-based lunchbox molding method according to claim 1, characterized in that, The modified starch molecular chain is grafted with hydrophobic long-chain side groups, which form physical entanglement points with adjacent modified starch molecular chains. These physical entanglement points generate elastic resistance when the modified starch molecular chains undergo relative displacement.

3. The green and environmentally friendly starch-based lunchbox molding method according to claim 2, characterized in that, The physical entanglement points have reversible dynamic characteristics. When the modified starch is in a molten state, the physical entanglement points are in a dynamic equilibrium of dissociation and reconstruction. When the modified starch is cooled to room temperature, the physical entanglement points reform.

4. The green and environmentally friendly starch-based lunchbox molding method according to claim 1, characterized in that, The main plasticizer is a polyol compound, and its addition amount is 25% to 40% of the modified starch mass; the addition amount of the functional additive is 3% to 8% of the modified starch mass.

5. The green and environmentally friendly starch-based lunchbox molding method according to claim 1, characterized in that, The modified starch is dried to reduce its moisture content to below 1%; the preheating temperature of the biodegradable polyester is controlled at 130~150℃.

6. The green and environmentally friendly starch-based lunchbox molding method according to claim 1, characterized in that, The temperature gradient distribution of the mixing and plasticizing equipment is as follows: the temperature of the feeding section is 100 to 120 degrees Celsius, the temperature of the compression section is 140 to 160 degrees Celsius, and the temperature of the plasticizing section is 160 to 180 degrees Celsius.

7. The green and environmentally friendly starch-based lunchbox molding method according to claim 1, characterized in that, When the degree of melting of the material is detected to reach the preset standard, the system automatically reduces the screw speed to 80 to 120 revolutions per minute; when the uniformity of mixing is detected to meet the preset requirements, the mixing and plasticizing step ends.

8. The green and environmentally friendly starch-based lunch box molding method according to claim 1, characterized in that, The temperature of the molding die is controlled between 100 and 120 degrees Celsius; the molding pressure is set between 8 and 15 MPa.

9. The green and environmentally friendly starch-based lunchbox molding method according to claim 1, characterized in that, The shaping treatment is carried out at a temperature of 40 to 60 degrees Celsius and a relative humidity of 40% to 60%, and the treatment time is two to four hours.

10. A green and environmentally friendly starch-based lunchbox formula, applicable to the green and environmentally friendly starch-based lunchbox molding method according to any one of claims 1-9, characterized in that, Based on 100 parts by weight of modified starch, the formulation consists of the following components: 25-40 parts by weight of main plasticizer, 3-8 parts by weight of functional additives, and 15-30 parts by weight of biodegradable polyester. The modified starch is the continuous phase matrix, the biodegradable polyester is the dispersed phase, and the functional additives form an interfacial compatibility layer at the interface between the two phases.

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