Environment-friendly degradable packaging bag and preparation method thereof

Abstract

The application discloses an environment-friendly degradable packaging bag and a preparation method thereof, and belongs to the technical field of packaging products. The application adopts a double compatibilization mechanism of starch surface micro-crosslinking pre-activation and online in-situ grafting compatibilization to overcome the problem that corn starch and polyvinyl alcohol molecular chains are both strong polar polyhydroxy structures, and the interface compatibility is poor, which leads to serious phase separation. The application adopts electromagnetic dynamic plasticizing blow molding technology to replace traditional resistance heating, so as to overcome the problem that it is difficult to continuously and stably produce by using the melt extrusion method due to the poor thermal stability and easy pasting of starch. The in-situ grafting of EMMA forms a polymer brush structure at the interface of starch and PVA, and the micro-crosslinked starch constructs a nano cage rigid skeleton, which blocks the penetration of water molecules and overcomes the shortcomings that starch and PVA materials are easy to absorb moisture and soften due to the rich hydroxyl groups. Four-stage gradient temperature control pulse heat sealing is adopted to optimize the interface healing through the controllable diffusion-freezing mechanism of molecular chains.

Description

TECHNICAL FIELD

[0001] The application discloses a packaging product technology, and particularly relates to an environment-friendly degradable packaging bag and a preparation method thereof. BACKGROUND

[0002] Traditional petroleum-based plastic packaging bags such as PP and PE are difficult to degrade in a natural environment, causing serious white pollution.

[0003] In the prior art, although biodegradable materials such as polylactic acid (PLA) and polybutylene adipate-co-terephthalate (PBAT) have good mechanical properties, the high cost limits large-scale application. Corn starch is widely available and low in price, and has excellent biodegradability, but when blended with polyvinyl alcohol (PVA), there are problems such as poor compatibility, high water sensitivity and poor water resistance. Existing researches mostly use a laboratory-scale casting method to prepare a film, and the production efficiency is low, so it is difficult to realize continuous industrial production. SUMMARY

[0004] The application aims to solve the above problems and provides an environment-friendly degradable packaging bag and a preparation method thereof.

[0005] To achieve the above object, the application provides the following technical scheme: a preparation method of an environment-friendly degradable packaging bag, comprising the following steps:

[0006] S1, material pretreatment, corn starch is vacuum dried to a water content of less than 1%, and then subjected to plasma treatment for 3 minutes by using a dielectric barrier discharge plasma device, and then subjected to reaction for 5 minutes at 85 DEG C under microwave conditions by spraying a 2% solid-liquid mass ratio 10:1 citric acid solution, so as to form micro-crosslinked starch with a crosslinking degree of 0.5-1.5%, and then the plasma is frozen by using liquid nitrogen, and then the micro-crosslinked starch is broken in a high-speed pulverizer to an average particle size of 45 microns;

[0007] S2, reaction extrusion compatibilization grafting, the micro-crosslinked starch, polyvinyl alcohol, sorbitol and ethylene-methyl methacrylate copolymer (EMMA) are injected into a double-screw extruder through a lateral forced feeding system, the melt viscosity of the EMMA is controlled to be 800-1000 Pa·s, 0.03-0.08% peroxide initiator DCP is added in the reaction section at 195-200 DEG C through a precision metering pump, the ethylene-methyl methacrylate copolymer (EMMA) is in-situ grafted with the starch and the polyvinyl alcohol, and a grafted material is obtained, and the grafting rate is 1.0-2.0%;

[0008] S3, electromagnetic dynamic plasticization blow molding, the grafted material is sent into a screw extruder, heated to 180-185 DEG C through 20 kHz magnetic induction, plasticized under the action of a 0.5-1.0 T pulse electromagnetic field, and blown into a film through a rotating die, and the blow-up ratio is 4:1.

[0009] S4, gradient temperature control pulse heat sealing, after the film is positioned by vacuum adsorption, sequentially passing through a preheating section, a temperature rising section, a temperature maintaining section and a pulse cooling section, heat sealing is completed to obtain a finished packaging bag, and the heat sealing cycle is 3 seconds.

[0010] Preferably, in step S2, the twin-screw extruder is provided with a reaction section in the fourth to sixth zones, equipped with a two-stage vacuum devolatilization system with vacuum degrees of -0.08 MPa and -0.095 MPa respectively; a side forced feeding system is located at the end of the second section of the extruder, and the EMMA melt is preheated to 185-195 DEG C in an auxiliary extruder.

[0011] Preferably, in step S3, the magnetic induction heating and the pulse electromagnetic field are respectively operated by winding a high-frequency induction coil on the outer wall of the screw extruder barrel and installing a pulse magnetic field generator at the front end of the screw.

[0012] Preferably, in step S4, a four-section heat sealing knife is used, each section is independently temperature controlled, and a semiconductor refrigeration sheet and a pressure sensor are built-in, and the process window is set as follows:

[0013] Preheating section: 120 DEG C, pressure 0.2 MPa, time 0.5 s;

[0014] Temperature rising section: 180 DEG C, pressure 0.5 MPa, time 1.5 s;

[0015] Temperature maintaining section: 160 DEG C, pressure 0.3 MPa, time 0.5 s;

[0016] Pulse cooling section: -5 DEG C, pressure 0.1 MPa, time 0.5 s.

[0017] An environmentally friendly degradable packaging bag is prepared by the above preparation method and comprises the following raw materials in parts by mass:

[0018] Surface micro-crosslinked starch: 28-32 parts;

[0019] Polyvinyl alcohol: 28-32 parts;

[0020] Glycerol: 14-16 parts;

[0021] Sorbitol: 4-6 parts;

[0022] EMMA: 2 parts;

[0023] Initiator: 0.05 parts;

[0024] Auxiliary agent: 1.5 parts.

[0025] Compared with the prior art, the present application has the following beneficial effects:

[0026] First, the starch surface micro-crosslinking pre-activation and in-situ grafting compatibilization dual compatibilization mechanism is adopted to overcome the problem of serious phase separation caused by the poor interface compatibility of corn starch and polyvinyl alcohol molecular chains which are both strong polar polyhydroxy structures;

[0027] Second, electromagnetic dynamic plasticizing blowing technology is adopted to replace traditional resistance heating to overcome the problem of difficult continuous and stable production caused by poor thermal stability and easy pasting of starch in melt extrusion method;

[0028] Third, the EMMA in-situ grafting forms polymer brush structure at the interface of starch and PVA, and the micro-crosslinked starch constructs a nano-cage rigid skeleton to block the penetration of water molecules, overcoming the shortcomings of easy moisture absorption and softening of starch and PVA materials due to the rich hydroxyl groups;

[0029] Fourth, four-stage gradient temperature control pulse heat sealing is adopted to optimize the interface healing through the controllable diffusion-freezing mechanism of molecular chains. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 The final data chart obtained according to the control experiment of the auxiliary system design;

[0031] Figure 2 The surface micro-morphology chart of the packaging bag without adding EMMA;

[0032] Figure 3 The surface micro-morphology chart of the packaging bag with adding EMMA. DETAILED DESCRIPTION

[0033] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. In the description of the present application, it should be understood that the terms "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only used to facilitate the description of the present application and simplify the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, therefore, it cannot be understood as a limitation on the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of the present application.

[0034] A preparation method of an environmentally friendly degradable packaging bag, comprising the following steps:

[0035] S1, material pretreatment micro-crosslinking pre-activation, corn starch was vacuum dried to a moisture content of <1%, then treated by plasma for 3 min through a dielectric barrier discharge plasma device, and then sprayed with a 2% solid-liquid mass ratio 10:1 citric acid solution under microwave conditions at 85°C for 5 min to form micro-crosslinked starch with a crosslinking degree of 1%. The plasma used argon gas with a flow rate of 20 L / min. Then the micro-crosslinked starch was broken by a high-speed pulverizer to an average particle size of 45 μm after quick freezing with liquid nitrogen;

[0036] The micro-crosslinking structure forms a nanocage on the surface of the starch granule, which prevents starch gelatinization during processing and retains enough hydroxyl groups for reaction with PVA. If the crosslinking degree is less than 0.5%, the thermal stability is insufficient, and if it is higher than 1.5%, the reactivity is reduced. High-energy electrons are bombarded at 10-20 eV, and high-energy electrons in argon plasma break the C1-C4 glycosidic bonds of starch molecules to produce free radicals at the C2 and C3 positions of glucose:

[0037] Starch-O-CH2-O-CH(OH)-CH(OH)→Starch-O·+·CH(OH)-CH(OH);

[0038] Surface hydroxyl activation, O2 plasma introduces peroxide radicals (-OO·), which oxidize some hydroxyl groups to carboxyl groups (-COOH); the surface of the original starch granule is smooth, and after plasma treatment, 10-20 nm shallow pits are produced, the roughness Ra is increased, and the specific surface area is increased, providing more active sites for subsequent crosslinking reactions;

[0039] Molecular behavior of microwave-assisted citric acid micro-crosslinking, crosslinking reaction path:

[0040] Citric acid dehydration: under the rapid heating of a microwave field at 2450 MHz, citric acid molecules are dehydrated to form cyclic anhydride intermediates;

[0041] Esterification reaction: SN2 nucleophilic substitution occurs between the acid anhydride and the hydroxyl groups of starch, forming single ester crosslinking bridges accounting for 70% and double ester crosslinking rings accounting for 30%;

[0042] Starch-OH+HOOC-CH2-C(OH)(COOH)-CH2-COOH→Starch-O-CO-CH2-C(OH)(COOH)-CH2-COOH;

[0043] By adjusting the citric acid concentration to 2% and the microwave power to 500 W, the crosslinking density is controlled at 1.0±0.2%, corresponding to 1-1.5 crosslinking points per 100 glucose units;

[0044] Nanocage structure formation: crosslinking point spacing about 50-80 nm, forming a three-dimensional network space, the crosslinking domain is limited, the glass transition temperature Tg from 65 ℃ to 78 ℃; Key effect: at a processing temperature of 180 ℃, the crosslinking region maintains the skeleton structure without collapse, while the free hydroxyl group can still react with PVA, realizing the dual functions of rigid support and flexible reaction;

[0045] Low temperature embrittlement mechanism:

[0046] Liquid nitrogen-196 ℃ makes the amorphous region of starch brittle, the glass transition temperature Tg decreases to-15 ℃, under high-speed impact, the crack preferentially expands along the amorphous-crystalline region interface, the wafer thickness is thinned from 15 nm to 8 nm, and the crystalline region ratio is reduced;

[0047] Criticality of particle size control:

[0048] When pulverized to 45 μm, the crystalline-amorphous structure in the particles is preserved but the size is reduced, too large >100 μm leads to uneven dispersion, and too small <20 μm reduces the strength by destroying the crystalline region;

[0049] S2, reactive extrusion compatibilization grafting, micro-crosslinked starch, polyvinyl alcohol, sorbitol and ethylene-methyl methacrylate copolymer EMMA are injected into a twin-screw extruder through a lateral forced feeding system, the lateral feeding temperature is controlled at 190 ℃, the EMMA melt viscosity is controlled at 800-1000 Pa·s, the main screw speed is 120 r / min, the residence time is 90-120 s, 0.05% peroxide initiator DCP is added at 195-200 ℃ in the reaction section through a precision metering pump, the grafting reaction activation energy is reduced to 85 kJ / mol, the ethylene-methyl methacrylate copolymer EMMA is in-situ grafted with starch and polyvinyl alcohol, and a grafted material is obtained, with a grafting rate of 1.5%;

[0050] In the molten state, DCP decomposes to produce free radicals, which take away the methylene hydrogen on the EMMA molecular chain to form EMMA macromolecular radicals. The radicals attack the hydroxyl groups of starch and PVA to realize in-situ grafting through hydrogen abstraction reaction, with a grafting efficiency of 1.5%, which is better than the traditional blending of 0.3%, and the grafting chains form a skeleton at the interface, and the interfacial shear strength is increased from 8.2 MPa to 18.5 MPa;

[0051] Melt stratified flow, the main melt starch / PVA / plasticizer flows at a speed of 1.2 m / s in the screw groove, and the EMMA melt injected laterally at 190 ℃ vertically cuts in, and the two phases form extensional flow in the kneading block area, the interfacial area is expanded, and nanoscale dispersion is realized;

[0052] Shear-induced orientation, the shear rate is high in the kneading block area, and the EMMA molecular chains are oriented along the flow direction, and the oriented EMMA chains are more likely to contact with the hydroxyl groups of starch and PVA, and the reaction probability is improved;

[0053] In step S2, the twin-screw extruder is provided with a reaction section in the fourth to sixth zones, equipped with a two-stage vacuum devolatilization system with vacuum degrees of -0.08 MPa and -0.095 MPa respectively, the first stage vacuum -0.08 MPa removes unreacted DCP decomposition products acetophenone, methane, etc., and the second stage vacuum -0.095 MPa removes EMMA short chains with a molecular weight <1000.

[0054] In step S3, electromagnetic dynamic plasticization blowing, the grafted material is fed into the screw extruder, heated to 180℃ by 20 kHz magnetic induction, plasticized under the action of 0.8T pulse electromagnetic field, and blown into a film through a rotating die with a blow ratio of 4:1;

[0055] In step S3, the magnetic induction heating and the pulse electromagnetic field work through winding a high-frequency induction coil on the outer wall of the screw extruder barrel and installing a pulse magnetic field generator at the front end of the screw respectively, the high-frequency induction coil has a frequency of 20 kHz and a power density of 50 W / cm 2 , the pulse magnetic field generator has a magnetic field strength of 0.8T and a frequency of 50Hz, the barrel temperature is 180℃, which is lower than the traditional 210℃, the melt temperature uniformity is ±2℃, and the die rotating speed is 10rpm;

[0056] The magnetic induction heating realizes volumetric heating with a heating efficiency of 95%, avoiding the temperature gradient of traditional resistance heating, the pulse magnetic field causes the magnetic orientation of polar molecular chains, the melt elastic modulus is improved, the film bubble stability is enhanced, the electromagnetic ring cooling realizes rapid shaping of the film bubble through eddy current effect, the crystallization rate is increased by 50%, and the spherulite size is refined to 20-30nm;

[0057] The alternating magnetic field of 20 kHz generates eddy current on the surface of the screw, and for a stainless steel screw, heat is generated uniformly in the volume rather than surface conduction; the radial temperature difference ΔT of the traditional resistance heated barrel is 15-20℃, while the magnetic induction heating has ΔT<3℃, and the melt temperature fluctuation is ±2℃;

[0058] The starch hydroxyl group -OH and the PVA hydroxyl group have paramagnetism, the pulse magnetic field of 0.8T and 50Hz causes the hydroxyl group to be oriented along the magnetic field direction, and the orientation degree is determined by the dichroism S≈0.3; without magnetic field, the molecular chain segment relaxation time τ0=0.05s, and under the pulse magnetic field, τ0is shortened to 0.03s, the molecular chain activity is improved, and the apparent viscosity is reduced from 1200Pa·s to 850Pa·s;

[0059] Moreover, the viscosity reduction amplitude is increased from 65% to 85% at a shear rate of 100s⁻¹, the processing window is widened by 30℃, and high temperature degradation is avoided;

[0060] S4, gradient temperature control pulse heat sealing, after the film is positioned by vacuum adsorption, sequentially passes through a preheating section, a temperature rising section, a temperature maintaining section and a pulse cooling section to complete heat sealing to obtain a finished packaging bag, and a heat sealing cycle is 3 seconds;

[0061] In step S4, a four-section heat sealing knife is used, each section is independently temperature controlled, a semiconductor refrigeration sheet and a pressure sensor are arranged therein, and a process window is set as follows:

[0062] The preheating section is at 120 DEG C, the pressure is 0.2 MPa, and the time is 0.5 s;

[0063] The temperature rising section is at 180 DEG C, the pressure is 0.5 MPa, and the time is 1.5 s;

[0064] The temperature maintaining section is at 160 DEG C, the pressure is 0.3 MPa, and the time is 0.5 s;

[0065] The pulse cooling section is at -5 DEG C, the pressure is 0.1 MPa, and the time is 0.5 s.

[0066] Gradient heating makes the heat sealing interface form a controllable process of melting-diffusion-solidification, pulse cooling avoids disentanglement caused by slow cooling, the heat sealing interface forms an interpenetrating network structure IPN, and the peeling strength is improved; under the process window, the preheating section is used for eliminating internal stress of the film, the temperature rising section is used for quickly forming a melting layer, the temperature maintaining section is used for molecular chain diffusion and entanglement, and the cooling section is used for quickly freezing the structure;

[0067] Pulse electromagnetic field is used to realize non-contact rapid cooling, and the molecular chain orientation and crystallization behavior are regulated through the magnetic effect, so that the interface strength retention rate is increased; the crystallization process of the electromagnetic ring cooling is as follows:

[0068] Magnetic field opening period 0.1 s: molecular chain segment pre-orientation, and the nucleation energy barrier is reduced;

[0069] Magnetic field closing period 0.1 s: rapid cooling, and the nucleation density is increased;

[0070] Cyclic repetition: fine and uniform microcrystalline structure is formed;

[0071] An environmentally friendly degradable packaging bag is prepared by the above preparation method and comprises the following raw materials in parts by mass:

[0072] Surface micro-crosslinked starch: 30 parts;

[0073] Polyvinyl alcohol: 30 parts;

[0074] Glycerol: 15 parts;

[0075] Sorbitol: 5 parts;

[0076] EMMA: 2 parts;

[0077] Initiator: 0.05 parts;

[0078] Auxiliary agent: 1.2-1.6 parts.

[0079] In the case of keeping the rest of the raw materials unchanged, the auxiliary agent usually uses 0.5 parts of the antioxidant system and 0.5 parts of the lubricating and dispersing system as the necessary auxiliary agent, the main antioxidant uses antioxidant 1010, the internal lubricant calcium stearate 0.2 parts, the external lubricant EBS ethylene bis-stearamide 0.2 parts, the dispersing agent PE wax 0.1 parts, and according to the material performance, the rest of the auxiliary agent 0.2 parts can select the heat stabilizing system, the light stabilizing system and the antibacterial and mildew-proof system; Examples

[0080] In the heat stabilizing system, the rest of the auxiliary agent uses 0.1 parts of zinc stearate and 0.1 parts of epoxy soybean oil to capture the HCl generated in the process, prevent the PVA from degrading by de-HCl, and prevent the PVA from degrading by de-HCl; Examples

[0081] In the light stabilizing system, the rest of the auxiliary agent uses 0.2 parts of UV-326 to prevent the packaging bag from aging due to light during storage; Examples

[0082] In the antibacterial and mildew-proof system, the rest of the auxiliary agent uses 0.15 parts of ε-polylysine and 0.05 parts of natamycin, which meets the GB2760 food additive standard; Examples

[0083] Comprehensive selection of auxiliary agents in examples 1-3 system;

[0084] According to the auxiliary agent system design control experiment, the experimental grouping is as follows table 1:

[0085] Table 1, formula grouping details

[0086]

[0087] The processing performance index is as follows table 2:

[0088] Table 2, processing performance index

[0089]

[0090] The mechanical and practical performance index is as follows table 3:

[0091] Table 3, mechanical and practical performance index

[0092]

[0093] Thermal stability performance

[0094] Thermogravimetric analysis TGA: nitrogen atmosphere, 10℃ / min, record 5% weight loss temperature (T5%) and maximum degradation temperature T max ;

[0095] Oxidation induction period (OIT): 200℃, oxygen atmosphere, determination of oxidation initiation time;

[0096] Continuous processing stability: The rate of change of MFR was measured after 4 hours of continuous production;

[0097] photostable performance

[0098] UV aging test: GB / T16422.3, UVA-340 lamp, irradiance 0.76W / m 2 60℃, aged for 100 hours;

[0099] Performance retention rate after aging: tensile strength retention rate, elongation at break retention rate;

[0100] Yellowing index change Δb: the difference in b value before and after aging;

[0101] Antibacterial and antifungal properties

[0102] Antibacterial rate: GB / T31402, antibacterial rate against Staphylococcus aureus ATCC6538 and Escherichia coli ATCC8739;

[0103] Anti-mildew grade: GB / T24128, Aspergillus niger and Penicillium cordiformis, cultured at 28℃ / 90%RH for 28 days, rated as 0-4;

[0104] Safety: Migration of additives (GB31604.1);

[0105] Degradation and Environmental Performance

[0106] Soil degradation rate: GB / T19277, natural soil, 25℃, burial depth 10cm, weight loss rate in 60 days;

[0107] Safety of degradation products: The impact of degradation solution on seed germination rate;

[0108] The final data obtained is as follows Figure 1 As shown in Groups A and B, the essential additives are indispensable, resulting in a 20.8% reduction in torque; a significant reduction in melt viscosity in the lubrication system, reducing equipment wear and increasing MFR by 71.4%; improved processing fluidity, enabling low-temperature molding and saving 15% in energy; a 31.3% increase in tensile strength; antioxidant 1010 preventing processing degradation and protecting the molecular chains; and a 48° increase in contact angle as the lubricant migrates to the surface to form a hydrophobic layer.

[0109] A 25°C increase in T5% significantly delays the oxidative degradation temperature and widens the processing window; essential additives are indispensable for ensuring process feasibility and basic performance.

[0110] From B group and C group, T5% again 15℃, zinc stearate as HCl capture agent, epoxy soybean oil auxiliary plasticizing and stabilizing system, aging retention rate increased by 12%, processing residual thermal stress decreased, product long-term stability enhanced, MFR increased by 8.3%: epoxy soybean oil weak plasticizing effect;

[0111] From B group and D group, aging retention rate increased by 16%, UV-326 molar extinction coefficient reached 15000L / (mol·cm) at 350nm, high efficiency shielding ultraviolet, mechanical properties almost unchanged: UV-326 dosage is only 0.2 parts, no dilution effect on the system, Δb value <3: small change in yellowing index, good appearance retention;

[0112] From B group and E group, it can be seen that the antibacterial rate reaches 99.0%, ε-polylysine adsorbs cell membrane through electrostatic force, destroys the integrity, and natamycin inhibits the synthesis of fungal ergosterol; weak influence on degradation, ε-polylysine slightly inhibits soil microorganisms, and the degradation rate decreases by 2%, which is still within the acceptable range; migration amount <0.05mg / dm 2 , far lower than the limit of 10mg / dm² in GB31604.1;

[0113] From B group and F group, it can be seen that the processing performance is optimal, the torque is the lowest 34N·m, the MFR is the highest 1.4, the functions of various additives are complementary, and the comprehensive performance is the best, with thermal stability, light stability and antibacterial; zinc stearate not only captures HCl, but also acts as a stabilizer for ε-polylysine to prevent its degradation at high temperature; UV-326 is enriched on the surface, and ε-polylysine is enriched in the hydrophilic phase, with clear functional zoning; epoxy soybean oil helps ε-polylysine disperse and prevents its agglomeration.

[0114] As shown in Figure 2 and Figure 3 , they are the surface microstructure diagrams of packaging bags without adding EMMA and adding EMMA respectively. The surface of the packaging bag without adding EMMA is uniform and smooth, with a small amount of impurity particles. When EMMA is added, the surface becomes smooth without obvious particles. This is because EMMA has a certain plasticizing effect. In a small amount, it can insert between the molecular chains of corn starch and PVA, weaken the interaction force between the molecular chains, increase the flexibility of the molecular chains, and make the molecular chains move and arrange more freely during film forming, so that a smooth surface is easily formed. At the same time, EMMA also has a compatibilizing effect. A small amount of addition can improve the compatibility between corn starch and PVA, making the film surface more uniform and smooth. Because EMMA can improve the compatibility between corn starch and PVA to some extent, the surface of the packaging bag becomes dense. EMMA molecules penetrate between the molecular chains of corn starch and PVA, forming an interwoven network structure and uniformly distributing in the film. As a compatibilizer, EMMA can reduce the interfacial tension between corn starch and PVA, making the film more uniform.

[0115] It will be apparent to those skilled in the art that the application is not limited to the details of the above-exemplified embodiments and that the present application can be implemented in other particular forms without departing from the spirit or essential characteristics of the present application. The presently disclosed embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the application being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. No reference to an item in the claims to be construed as a disavowal of the item, even if the item is not recited in each claim.

[0116] Furthermore, it should be understood that although the description is made on embodiments, not every embodiment contains only one independent technical solution, and the description manner of the specification is only for the sake of clarity, and the skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be combined appropriately to form other embodiments which can be understood by the skilled in the art.

Claims

1. A method for preparing an environmentally degradable packaging bag, characterized by: The method comprises the following steps: S1, material pretreatment and micro-crosslinking pre-activation: corn starch is vacuum dried to a moisture content of <1%, then subjected to plasma treatment for 3 min by a dielectric barrier discharge plasma device, and then reacted with a 2% citric acid solution under microwave conditions at 85℃ for 5 min, wherein the solid-liquid mass ratio is 10:1, and the crosslinking degree of the micro-crosslinked starch is 0.5-1.5%; argon gas with a flow rate of 20 L / min is used for plasma; and then the micro-crosslinked starch is frozen in liquid nitrogen and broken in a high-speed pulverizer to an average particle size of 45 μm; S2, reaction extrusion compatibilization and grafting: the micro-crosslinked starch, polyvinyl alcohol, sorbitol, and ethylene-methyl methacrylate copolymer EMMA are injected into a twin-screw extruder through a side forced feeding system, the melt viscosity of the EMMA is controlled at 800-1000 Pa·s, and 0.03-0.08% of a peroxide initiator DCP is added in the reaction section at 195-200℃ through a precision metering pump, so that the ethylene-methyl methacrylate copolymer EMMA is in-situ grafted with the starch and polyvinyl alcohol to obtain a grafted material, and the grafting rate is 1.0-2.0%; S3, electromagnetic dynamic plasticization and blowing: the grafted material is sent into a screw extruder, heated to 180-185℃ through 20 kHz magnetic induction, plasticized under the action of a 0.5-1.0 T pulse electromagnetic field, and blown into a film through a rotating die at a blowing ratio of 4:1; S4, gradient temperature control pulse heat sealing: after the film is positioned through vacuum adsorption, it is sequentially subjected to a preheating section, a temperature rising section, a temperature maintaining section, and a pulse cooling section to complete heat sealing and obtain a finished packaging bag, and the heat sealing cycle is 3 seconds.

2. The method for preparing an environmentally friendly degradable packaging bag according to claim 1, characterized in that: In step S2, the twin-screw extruder is provided with a reaction section in the fourth to sixth zones, equipped with a two-stage vacuum devolatilization system with vacuum degrees of -0.08 MPa and -0.095 MPa, respectively; the side forced feeding system is located at the end of the second section of the extruder, and the EMMA melt is preheated to 185-195℃ in an auxiliary extruder.

3. The method for preparing an environmentally friendly biodegradable packaging bag according to claim 2, characterized in that: In step S3, the magnetic induction heating and the pulse electromagnetic field are respectively realized by winding a high-frequency induction coil on the outer wall of the screw extruder barrel and installing a pulse magnetic field generator at the front end of the screw.

4. The preparation method of the environment-friendly degradable packaging bag according to claim 3, characterized in that: In step S4, a four-section heat sealing knife is used, each section is independently temperature-controlled, and a semiconductor refrigeration sheet and a pressure sensor are built-in, and the process window is set as follows: Preheating section: 120℃, pressure 0.2 MPa, time 0.5 s; Temperature rising section: 180℃, pressure 0.5 MPa, time 1.5 s; Temperature maintaining section: 160℃, pressure 0.3 MPa, time 0.5 s; Pulse cooling section: -5℃, pressure 0.1 MPa, time 0.5 s.

5. An environmentally degradable packaging bag, characterized by: The method is prepared by the preparation method in any one of claims 1-4, and comprises the following mass fractions of raw materials: Surface micro-crosslinked starch: 28-32 parts; Polyvinyl alcohol: 28-32 parts; Glycerol: 14-16 parts; Sorbitol: 4-6 parts; EMMA: 2 parts; Initiator: 0.05 parts; Auxiliary agent: 1.5 parts.

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