Environment-friendly packaging plastic film and preparation method thereof

Abstract

The invention relates to the technical field of high polymer material processing, and discloses an environment-friendly packaging plastic film and a preparation method thereof, and the film is prepared from PBAT, polylactic acid, epoxidized soybean oil, anhydrous citric acid micro powder, pentaerythritol stearate, zinc stearate and polymeric carbodiimide. And adding anhydrous citric acid later, and controlling the in-situ branching reaction to be carried out in a controlled rheological environment by matching with reverse temperature gradient setting at the middle-rear section of the extruder, so as to construct a uniformly distributed cross-linked network. Pentaerythritol stearate provides steric hindrance by using a star-shaped long-chain structure and inhibits regular folding of polylactic acid in a cooling section. Polymeric carbodiimide is independently injected in the later stage, unreacted free carboxyl in the system is quenched in situ, and the catalytic degradation effect of an acidic terminal group on a polyester main chain is eliminated. The plasticizer molecules are effectively locked, the migration precipitation rate is reduced, meanwhile, gel defects on the surface of the film are reduced, and the processing stability and the longitudinal tearing strength of the film are improved.

Description

Technical Field

[0001] This invention relates to the field of polymer material processing technology, specifically to an environmentally friendly packaging plastic film and its preparation method. Background Technology

[0002] Currently, biodegradable matrix resins, such as polybutylene terephthalate (PET) and polylactic acid (PLA), are widely used in the field of environmentally friendly packaging plastic films. To improve the flexibility of the film, plasticizers are usually added to the resin matrix. Since the relationship between plasticizers and polyester macromolecules is mostly a simple physical mixture, small-molecule plasticizers tend to migrate and precipitate to the surface during film storage and use, affecting the overall performance and service life of the film.

[0003] To limit plasticizer migration, some existing technologies introduce crosslinking agents to modify the polyester system. However, in conventional continuous melt extrusion processes, the rate of crosslinking reactions is often difficult to control effectively. Excessive local crosslinking can lead to the formation of large molecular gel clusters in the polyester matrix. These clusters manifest as gel defects on the film surface after film formation, not only reducing the appearance quality of the packaging film, but also becoming stress concentration sources in the structure, resulting in a decrease in the longitudinal right-angle tear strength of the film.

[0004] Furthermore, due to processing time constraints, the introduced crosslinking system often contains residual acidic substances such as unreacted free carboxyl groups. When the modified masterbatch enters a single-screw blown film mill for secondary high-temperature processing, these residual acidic substances catalyze the hydrolysis and thermal degradation of the polyester backbone, leading to chain breakage. The acid degradation of the matrix resin causes abnormal fluctuations in the melt flow rate, resulting in poor processing stability during film formation and increasing the defect rate. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides an environmentally friendly packaging plastic film and its preparation method, solving the problems of easy migration and precipitation of plasticizers, high density of gel defects on the film surface, and easy acid degradation of the matrix resin during secondary processing of biodegradable packaging films.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] In a first aspect, the present invention provides an environmentally friendly packaging plastic film, which adopts the following technical solution: it is made from raw materials comprising the following parts by weight: 60.0-80.0 parts of polybutylene terephthalate-adipate; 10.0-30.0 parts of polylactic acid; 5.0-12.0 parts of epoxidized soybean oil; 1.0-3.0 parts of anhydrous citric acid micro powder; 0.5-1.5 parts of pentaerythritol stearate; 0.1-0.5 parts of zinc stearate; and 0.3-1.0 parts of polymeric carbodiimide.

[0008] By adopting the above technical solution, the flexibility of the film can be improved by using polybutylene terephthalate (PET) and polylactic acid (PLA) as biodegradable matrices and epoxidized soybean oil as a plasticizer. In this system, anhydrous citric acid micropowder acts as a multifunctional crosslinking agent, reacting in situ with epoxidized soybean oil containing polyepoxy groups and the active groups at the ends of polyester macromolecular chains during melt extrusion.

[0009] The specific reaction mechanism is as follows: the carboxyl groups in anhydrous citric acid undergo a ring-opening addition reaction with the epoxy groups in epoxidized soybean oil to generate esterified products containing hydroxyl groups; these products further combine with the hydroxyl or carboxyl groups at the chain ends of polybutylene terephthalate (PET) or polylactic acid (PLA) to construct a branched network structure with a certain degree of crosslinking within the matrix. This branched network, through physical entanglement and chemical anchoring, confines free epoxidized soybean oil molecules within the intermoleculeal spaces of the polyester, increasing the resistance to plasticizer migration to the film surface and thus reducing the plasticizer migration rate.

[0010] Meanwhile, pentaerythritol stearate contains four star-shaped stearate chains, which can provide steric hindrance at the blend phase interface. This steric hindrance can interfere with the regular folding of polylactic acid segments during the extrusion cooling process, delay the crystallization kinetics, prevent the material from undergoing supercooled crystallization in the low-temperature zone of the extruder, and maintain the melt rheological properties required for the in-situ network construction stage.

[0011] Furthermore, polymeric carbodiimide is used to treat unreacted residual anhydrous citric acid in the system. The reaction process involves the carbodiimide group (-N=C=N-) in the polymeric carbodiimide undergoing an electrophilic addition reaction with the free carboxyl groups in the system to generate a stable N-acylurea structure. This process consumes the acidic end groups that catalyze polyester hydrolysis, blocking the polyester backbone breakage pathway initiated by residual acid during secondary blown film processing, and maintaining the stability of the melt flow rate.

[0012] Preferably, the melt index of polybutylene terephthalate (PET) is 2.0-5.0 g / 10 min, and the melt index of polylactic acid (PLA) is 4.0-12.0 g / 10 min.

[0013] By adopting the above technical solution and selecting a matrix resin with a specific melt flow rate range, it is possible to ensure that the two polyesters have similar viscosities in the initial mixing stage, which is beneficial to forming a uniform phase distribution.

[0014] Preferably, the epoxy value of the epoxidized soybean oil is 6.0%-6.4%, the anhydrous citric acid powder passes through a 100-200 mesh sieve, and the effective carbodiimide group mass fraction of the polymeric carbodiimide is 8.0%-12.0%.

[0015] By employing the above technical solutions, the high epoxy value plasticizer provides more crosslinking sites, while the high-mesh anhydrous citric acid micropowder increases the reaction contact area, avoiding gel defects caused by excessive local crosslinking. The limited content of active groups in the polymeric carbodiimide ensures the amount of residual acid consumed in the reaction, reducing inherent defects in the film.

[0016] Secondly, the present invention provides a method for preparing an environmentally friendly packaging plastic film, employing the following technical solution: The method applies to the aforementioned environmentally friendly packaging plastic film and includes the following steps:

[0017] Polybutylene terephthalate-adipate and polylactic acid were mixed and then continuously dried in a drying device. Anhydrous citric acid micro powder and zinc stearate were placed in a mixing device and premixed at room temperature to obtain a premixed powder.

[0018] The dried polybutylene terephthalate, polylactic acid, and pentaerythritol stearate are fed into the main feed port of the twin-screw extruder through the main feed device. At the same time, liquid epoxidized soybean oil is injected into the second zone of the twin-screw extruder barrel through the liquid injection device, forming a mixture in the first to third zones of the twin-screw extruder barrel.

[0019] The premixed powder is pressed into the fourth zone of the barrel of a twin-screw extruder through a side-feeding device and melt-blended with the mixture.

[0020] Start the barrel cooling system of the twin-screw extruder and control the temperature of the 5th, 6th and 7th barrel zones of the twin-screw extruder to decrease sequentially, so that the mixture stays in the 5th to 7th barrel zones of the twin-screw extruder.

[0021] When the mixture enters zone 8 of the barrel of the twin-screw extruder, the heated and molten polymeric carbodiimide is injected into zone 8 of the barrel of the twin-screw extruder through a melt pumping device;

[0022] The vacuum exhaust system is activated in zone 9 of the twin-screw extruder barrel. The mixture passes through zone 10 of the twin-screw extruder barrel and is extruded as a melt strip by the extrusion die of the twin-screw extruder. The melt strip is then pelletized and centrifuged to obtain modified biodegradable masterbatch.

[0023] Modified biodegradable masterbatch is added to a blown film equipment for blow molding and then wound up to obtain an environmentally friendly packaging plastic film.

[0024] By employing the above technical solution, this invention separates chemical crosslinking and reaction quenching in a spatial segment by adjusting the timing of material addition and temperature settings. The specific mechanism is as follows:

[0025] In the front section of the twin-screw extruder (zones 1 to 3), only polyester matrix and liquid epoxidized soybean oil are added to establish a physical dispersion system to avoid uneven dispersion caused by premature intervention of crosslinking agents.

[0026] Subsequently, anhydrous citric acid micropowder was added later, in zone 4 of the barrel, and a decreasing reverse temperature gradient was applied from zones 5 to 7. This temperature gradient slowed the crosslinking reaction rate by lowering the melt temperature, inhibiting the formation of large molecular clusters and ensuring a uniform distribution of the crosslinked network in the matrix. This reduced the number of hard gel particles on the film surface, preventing stress concentration caused by gel particles and improving the longitudinal right-angle tear strength of the film. During this cooling process, the steric hindrance provided by pentaerythritol stearate prevented low-temperature crystallization of polylactic acid, avoiding a significant increase in melt viscosity and ensuring the stability of the continuous extrusion process.

[0027] Since cooling operations prevent anhydrous citric acid from reacting completely, this solution involves independently injecting polymeric carbodiimide into zone 8 of the barrel to prevent free carboxyl groups from causing degradation in subsequent processing. The polymeric carbodiimide binds to free carboxyl groups in situ via electrophilic addition, achieving online quenching of acidic end groups, blocking the catalytic degradation pathway of residual acid on the polyester backbone, and ensuring the processing stability of the masterbatch during secondary blown film forming.

[0028] Preferably, the drying equipment is a dehumidifying dryer, and the mixing equipment is a high-speed mixer; in the step of continuously drying the mixture of polybutylene terephthalate and polylactic acid in the dehumidifying dryer, the continuous drying temperature is controlled at 70-85℃, and the continuous drying time is 3.0-6.0 hours.

[0029] By adopting the above technical solution, the dehumidification and drying equipment can effectively remove the moisture absorbed by the polyester matrix and reduce the probability of hydrolysis reaction of the matrix during high-temperature melt extrusion.

[0030] Preferably, the main feeding device is a loss-in-weight feeder, the twin-screw extruder is a co-rotating twin-screw extruder, and the liquid injection device is a liquid metering pump; the length-to-diameter ratio of the co-rotating twin-screw extruder is 40-48, the temperature of the barrel zones 1 to 3 of the co-rotating twin-screw extruder is set at 160-180℃, and the screw speed of the co-rotating twin-screw extruder is 200-300rpm; the side feeding device is a side forced feeder, and the temperature of the barrel zone 4 of the twin-screw extruder is set at 160-170℃.

[0031] By adopting the above technical solutions, the co-rotating twin-screw extruder with a large length-to-diameter ratio provides the required reaction stroke for multi-segment temperature control and reaction separation, and the side-forced feeder ensures the forced insertion and rapid dispersion of low-density powder materials in the high-temperature melt.

[0032] Preferably, the temperature of zone 5 of the twin-screw extruder barrel is set to 150-160°C, the temperature of zone 6 of the twin-screw extruder barrel is set to 145-155°C, and the temperature of zone 7 of the twin-screw extruder barrel is set to 140-150°C; the average residence time of the mixture in zones 5 to 7 of the twin-screw extruder barrel is controlled to be 30-75 seconds.

[0033] By adopting the above technical solution, the reverse temperature gradient and material residence time are precisely controlled, so that the crosslinking reaction dominated by anhydrous citric acid is in a controlled rheological environment, avoiding local crosslinking runaway caused by excessively high thermal field temperature.

[0034] Preferably, the melt pumping equipment is a high-pressure melt gear pump, and the temperature of the 8th zone of the barrel of the twin-screw extruder is set to 150-160℃.

[0035] By adopting the above technical solution, the high-pressure melt gear pump can overcome the melt back pressure inside the extruder barrel and quantitatively inject polymeric carbodiimide into the reaction system, ensuring the metering accuracy of the end-group quenching reaction.

[0036] Preferably, the temperatures of the 9th zone of the barrel, the 10th zone of the barrel, and the extrusion die of the twin-screw extruder are set to 155-165°C, and the vacuum degree of the vacuum exhaust system is controlled to be -0.06 to -0.09 MPa.

[0037] By adopting the above technical solution, opening the exhaust system can extract small molecule volatiles and encapsulated air generated during the crosslinking and quenching reactions, preventing bubble defects in the extruded strips and the final film products.

[0038] Preferably, the blown film equipment is a single-screw blown film machine, with the temperature of each zone and the die opening of the single-screw blown film machine set at 140-165℃, the blow-up ratio of the single-screw blown film machine set at 2.0-4.0, and the traction ratio of the single-screw blown film machine set at 3.0-6.0.

[0039] By adopting the above technical solution, the set molding temperature and stretching ratio are adapted to the melt strength characteristics of the modified masterbatch, so that the film with a specific cross-linked network structure can form a regular crystalline orientation structure under stretching, which meets the physical and mechanical properties required for actual packaging applications.

[0040] This invention provides an environmentally friendly packaging plastic film and its preparation method. It has the following beneficial effects:

[0041] 1. This invention constructs a molecular branched network by in-situ crosslinking reaction between anhydrous citric acid, epoxidized soybean oil, and polyester matrix during melt extrusion. The plasticizer is locked inside the matrix by chemical anchoring and physical entanglement, which effectively reduces the migration and precipitation rate of plasticizer during film use.

[0042] 2. By adding anhydrous citric acid later and using a reverse temperature gradient process in the later stage of the extruder, this invention controls the rate and uniformity of the crosslinking reaction, inhibits the formation of macromolecular gel clusters, reduces gel defects on the film surface, and thus improves the appearance quality and longitudinal tear strength of the packaging film.

[0043] 3. This invention eliminates unreacted free carboxyl groups in the system by injecting polymeric carbodiimide in the later stage of extrusion, thereby quenching acidic substances that can catalyze the hydrolysis of polyester online. This ensures the stability of the melt flow rate of the modified masterbatch during the secondary blown film processing and reduces the risk of thermal degradation and chain breakage during film formation. Attached Figure Description

[0044] Figure 1 The figures show a comparison of processing stability and retention rate tests in embodiments and comparative examples of the present invention. Figure 1 (a) is a graph showing the variation of the average torque load rate of the extruder according to the present invention. Figure 1 (b) is a graph showing the retention rate of melt flow rate before and after film forming from the masterbatch of the present invention;

[0045] Figure 2 These are test graphs showing the migration rate of plasticizers in the films of the embodiments and comparative examples of the present invention;

[0046] Figure 3 This is a statistical graph showing the density of gel defects in the thin film of the embodiments and comparative examples of the present invention;

[0047] Figure 4 This is a comparison diagram of the longitudinal right-angle tear strength of the films in the embodiments and comparative examples of the present invention after being placed for 30 days. Detailed Implementation

[0048] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings, examples, comparative examples, and test examples. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0049] Examples 1-3:

[0050] Example 1:

[0051] This embodiment provides an environmentally friendly packaging plastic film and its preparation method, including the following steps:

[0052] By weight, accurately weigh 60.0 parts of polybutylene terephthalate (PBAT, melt index 2.0 g / 10 min), 10.0 parts of polylactic acid (melt index 4.0 g / 10 min), 5.0 parts of epoxidized soybean oil (epoxidation value 6.0%), 1.0 part of anhydrous citric acid micro powder (passed through a 100-mesh sieve), 0.5 parts of pentaerythritol stearate, 0.1 parts of zinc stearate, and 0.3 parts of polymeric carbodiimide (effective carbodiimide group mass fraction 8.0%).

[0053] PBAT and polylactic acid were mixed and placed in a dehumidifying dryer and dried continuously at 70°C for 3.0 hours; anhydrous citric acid and zinc stearate were placed in a high-speed mixer and premixed at room temperature to obtain a premixed powder for later use.

[0054] Next, the dried PBAT, polylactic acid, and weighed pentaerythritol stearate are added to the main feed port of a co-rotating twin-screw extruder with a length-to-diameter ratio of 40 through a main loss-in-weight feeder. At the same time, liquid epoxidized soybean oil is injected into the second zone of the barrel of the co-rotating twin-screw extruder through a liquid metering pump. The temperature of the barrel zones 1 to 3 of the co-rotating twin-screw extruder is set to 160°C, and the screw speed of the co-rotating twin-screw extruder is set to 200 rpm to establish a homogeneous physical dispersion system and form a mixture.

[0055] The premixed powder is then fed into the fourth zone of the barrel of a co-rotating twin-screw extruder via a side-forced feeder. The temperature of the fourth zone of the barrel is set to 160°C, and the powder is melt-blended with the mixture.

[0056] Then, the barrel cooling system of the co-rotating twin-screw extruder is started, and the temperature of the fifth zone of the barrel is set to 150°C, the temperature of the sixth zone of the barrel is set to 145°C, and the temperature of the seventh zone of the barrel is set to 140°C. The average residence time of the mixture in the fifth to seventh zones of the barrel is controlled to be 30 seconds, and in-situ network construction under the reverse temperature gradient is carried out.

[0057] When the mixture enters zone 8 of the barrel of the co-rotating twin-screw extruder, a heated molten polymeric carbodiimide is injected into zone 8 of the barrel by a high-pressure melt gear pump. The temperature of zone 8 of the barrel is set to 150°C to carry out reaction kinetic quenching and chemical anchoring.

[0058] The temperatures of the 9th and 10th zones of the barrel and the extrusion die of the co-rotating twin-screw extruder were further set to 155°C; the vacuum exhaust system was turned on in the 9th zone of the barrel to control the vacuum degree to -0.06MPa; the melt strip of the mixture was extruded from the extrusion die, and the melt strip was pelletized and centrifuged to obtain modified biodegradable masterbatch.

[0059] Finally, the obtained masterbatch is added to a single-screw blown film machine for blow molding. The temperature of each zone of the blown film machine and the die opening is set to 140℃, the blow ratio is 2.0, the traction ratio is 3.0, and the film is wound up to obtain an environmentally friendly packaging plastic film.

[0060] Example 2:

[0061] This embodiment provides an environmentally friendly packaging plastic film and its preparation method, including the following steps:

[0062] Accurately weigh 70.0 parts by weight of PBAT (melt index 3.5 g / 10 min), 20.0 parts of polylactic acid (melt index 8.0 g / 10 min), 8.5 parts of epoxidized soybean oil (epoxidation value 6.2%), 2.0 parts of anhydrous citric acid micronized powder (passed through a 150-mesh sieve), 1.0 part of pentaerythritol stearate, 0.3 parts of zinc stearate, and 0.65 parts of polymeric carbodiimide (effective carbodiimide group mass fraction 10.0%). Mix PBAT and polylactic acid and place in a dehumidifying dryer, drying continuously at 77°C for 4.5 hours. Premix anhydrous citric acid and zinc stearate in a high-speed mixer at room temperature to obtain a premixed powder for later use.

[0063] Next, the dried PBAT, polylactic acid, and weighed pentaerythritol stearate were added to the main feed port of a co-rotating twin-screw extruder with a length-to-diameter ratio of 44 through a main loss-in-weight feeder. At the same time, liquid epoxidized soybean oil was injected into the second zone of the barrel of the co-rotating twin-screw extruder through a liquid metering pump. The temperature of the barrel zones 1 to 3 of the co-rotating twin-screw extruder was set to 170°C, and the screw speed of the co-rotating twin-screw extruder was set to 250 rpm to establish a homogeneous physical dispersion system and form a mixture.

[0064] The premixed powder is then fed into the fourth zone of the barrel of a co-rotating twin-screw extruder via a side-forced feeder. The temperature of the fourth zone of the barrel is set to 165°C, and the powder is melt-blended with the mixture.

[0065] Then, the barrel cooling system of the co-rotating twin-screw extruder is started, and the temperature of the fifth zone of the barrel is set to 155°C, the temperature of the sixth zone of the barrel is set to 150°C, and the temperature of the seventh zone of the barrel is set to 145°C. The average residence time of the mixture in the fifth to seventh zones of the barrel is controlled to be 52 seconds, and in-situ network construction under the reverse temperature gradient is carried out.

[0066] When the mixture enters zone 8 of the barrel of the co-rotating twin-screw extruder, a heated molten polymeric carbodiimide is injected into zone 8 of the barrel by a high-pressure melt gear pump. The temperature of zone 8 of the barrel is set to 155°C to carry out reaction kinetic quenching and chemical anchoring.

[0067] The temperatures of the 9th and 10th zones of the barrel and the extrusion die of the co-rotating twin-screw extruder were further set to 160°C; the vacuum exhaust system was turned on in the 9th zone of the barrel to control the vacuum degree to -0.075MPa; the melt strip of the mixture was extruded from the extrusion die, and the melt strip was pelletized and centrifuged to obtain modified biodegradable masterbatch.

[0068] Finally, the prepared masterbatch is added to a single-screw blown film machine for blow molding. The temperature of each zone and the die opening of the blown film machine is set to 152℃, the blow-up ratio is 3.0, the traction ratio is 4.5, and the film is wound up to obtain an environmentally friendly packaging plastic film.

[0069] Example 3:

[0070] This embodiment provides an environmentally friendly packaging plastic film and its preparation method, including the following steps:

[0071] Accurately weigh 80.0 parts by weight of PBAT (melt index 5.0 g / 10 min), 30.0 parts of polylactic acid (melt index 12.0 g / 10 min), 12.0 parts of epoxidized soybean oil (epoxidation value 6.4%), 3.0 parts of anhydrous citric acid micronized powder (passed through a 200-mesh sieve), 1.5 parts of pentaerythritol stearate, 0.5 parts of zinc stearate, and 1.0 part of polymeric carbodiimide (effective carbodiimide group mass fraction 12.0%). Mix PBAT and polylactic acid and place in a dehumidifying dryer for continuous drying at 85°C for 6.0 hours. Premix anhydrous citric acid and zinc stearate in a high-speed mixer at room temperature to obtain a premixed powder for later use.

[0072] Next, the dried PBAT, polylactic acid, and weighed pentaerythritol stearate are added to the main feed port of a co-rotating twin-screw extruder with a length-to-diameter ratio of 48 through a main loss-in-weight feeder. At the same time, liquid epoxidized soybean oil is injected into the second zone of the barrel of the co-rotating twin-screw extruder through a liquid metering pump. The temperature of the barrel zones 1 to 3 of the co-rotating twin-screw extruder is set to 180°C, and the screw speed of the co-rotating twin-screw extruder is set to 300 rpm to establish a homogeneous physical dispersion system and form a mixture.

[0073] The premixed powder is then fed into the fourth zone of the barrel of a co-rotating twin-screw extruder via a side-forced feeder. The temperature of the fourth zone of the barrel is set to 170°C, and the powder is melt-blended with the mixture.

[0074] Then, the barrel cooling system of the co-rotating twin-screw extruder is started, and the temperature of the fifth zone of the barrel is set to 160°C, the temperature of the sixth zone of the barrel is set to 155°C, and the temperature of the seventh zone of the barrel is set to 150°C. The average residence time of the mixture in the fifth to seventh zones of the barrel is controlled to be 75 seconds, and in-situ network construction under the reverse temperature gradient is carried out.

[0075] When the mixture enters zone 8 of the barrel of the co-rotating twin-screw extruder, the heated molten polymeric carbodiimide is injected into zone 8 of the barrel by a high-pressure melt gear pump. The temperature of zone 8 of the barrel is set to 160°C to carry out reaction kinetic quenching and chemical anchoring.

[0076] The temperatures of the 9th and 10th zones of the barrel and the extrusion die of the co-rotating twin-screw extruder were further set to 165°C; the vacuum exhaust system was turned on in the 9th zone of the barrel to control the vacuum degree to -0.09MPa; the melt strip of the mixture was extruded from the extrusion die, and the melt strip was pelletized and centrifuged to obtain modified biodegradable masterbatch.

[0077] Finally, the obtained masterbatch is added to a single-screw blown film machine for blow molding. The temperature of each zone and the die opening of the blown film machine is set to 165℃, the blow ratio is 4.0, the traction ratio is 6.0, and the film is wound up to obtain an environmentally friendly packaging plastic film.

[0078] Comparative Examples 1-5:

[0079] Comparative Example 1:

[0080] Compared with Example 2, the difference is that: anhydrous citric acid micro powder and zinc stearate were not added, and the operation of placing anhydrous citric acid and zinc stearate in a high-speed mixer at room temperature for premixing was not performed. At the same time, the operation of pressing the material into the fourth zone of the barrel of the co-rotating twin-screw extruder by a side-forced feeder was not performed. All other aspects are the same.

[0081] Comparative Example 2:

[0082] Compared with Example 2, the difference is that pentaerythritol stearate was not added, and the material added to the main feed port of the co-rotating twin-screw extruder through the main loss-in-weight feeder does not contain pentaerythritol stearate; otherwise, they are the same.

[0083] Comparative Example 3:

[0084] Compared with Example 2, the difference is that no polymeric carbodiimide was added, and when the mixture enters Zone 8 of the barrel of the co-rotating twin-screw extruder, the operation of injecting heated molten polymeric carbodiimide into Zone 8 of the barrel via a high-pressure melt gear pump is not performed; all other operations are the same.

[0085] Comparative Example 4:

[0086] Compared with Example 2, the difference is that: after anhydrous citric acid micro powder and zinc stearate are mixed with PBAT, polylactic acid and pentaerythritol stearate, they are added to the main feed port of a co-rotating twin-screw extruder with a length-to-diameter ratio of 44 in one go through a main loss-in-weight feeder. The operation of pressing the feeder into the fourth zone of the barrel of the co-rotating twin-screw extruder through a side forced feeder is not performed. All other operations are the same.

[0087] Comparative Example 5:

[0088] Compared with Example 2, the difference is that the barrel cooling system of the co-rotating twin-screw extruder is not started, and the temperature of the fifth zone of the barrel is set to 168°C, the temperature of the sixth zone of the barrel is set to 170°C, and the temperature of the seventh zone of the barrel is set to 175°C. All other settings are the same.

[0089] Test Example 1-2:

[0090] Test Example 1: Processing Stability and Reaction Mechanism Verification Test

[0091] Experimental steps:

[0092] The material combinations and processing conditions corresponding to Examples 1 to 3, as well as Comparative Examples 2, 3, and 5, were selected as experimental subjects.

[0093] During the stable operation phase of the twin-screw extruder, the percentage of motor torque load in zones 5 to 7 of the barrel is recorded. Specifically, the instantaneous torque value is recorded every 5 minutes, and the average value over 30 minutes of continuous operation is recorded as the average torque load rate. If an overload alarm occurs during operation, causing the machine to stop, the peak instantaneous torque value at the last moment before the stop is recorded, and the subsequent preparation process for that group is terminated.

[0094] Following the GB / T3682.1-2018 standard, masterbatch samples obtained after twin-screw extrusion pelletizing and film samples after single-screw blow molding were collected under conditions of 190℃ and 2.16kg load. The melt flow rate of the two samples was measured using a melt flow indexer and recorded as MFR before film formation and MFR after film formation, respectively. The MFR retention rate was obtained by calculating the ratio between the two.

[0095] Experimental data:

[0096] Table 1. Test results of torque and MFR retention rate during the machining process in the examples and comparative examples.

[0097] Group Average torque load rate (%) MFR (g / 10min) before film formation MFR (g / 10min) after film formation MFR retention rate (%) Example 1 67.8 3.45 3.61 95.57 Example 2 64.3 4.12 4.26 96.71 Example 3 61.5 4.88 4.96 98.39 Comparative Example 2 98.4 (Alarm shutdown) - - - Comparative Example 3 65.1 4.33 10.12 42.79 Comparative Example 5 72.6 3.21 3.48 92.24

[0098] Note: In Table 1, the "-" marked in the MFR before film formation, MFR after film formation, and MFR retention rate test items of Comparative Example 2 indicates that the test group failed to obtain masterbatch samples due to the surge in melt viscosity during the operation of the twin-screw extruder, which triggered the equipment overload alarm and forced shutdown. As a result, the subsequent single-screw blown film forming process and the corresponding melt flow rate measurement could not be carried out.

[0099] Conclusion Analysis:

[0100] According to Table 1 and Figure 1(a) shows the average torque load rate data, with the motor torque in the example group remaining within the range of 61.5% to 67.8% during the cooling range. In contrast, Comparative Example 2 triggered motor overload protection within a short period of actual testing. During routine processing observations, polylactic acid (PLA) exhibited a significant tendency to crystallize in environments below its melting point of 150°C. When the system was in a cooling range of 140°C to 150°C, without the addition of pentaerythritol stearate, PLA segments underwent regular folding and formed crystal nuclei, leading to a rapid increase in melt viscosity. This localized supercooling crystallization directly manifested as screw seizure in the extruder. Adding pentaerythritol stearate at the initial feeding stage allowed its tetrastearate star structure to provide steric hindrance at the phase interface, delaying the crystallization kinetics during the cooling phase and thus maintaining suitable fluidity of the material at low temperatures.

[0101] Under the premise of ensuring stable rheological state during processing, the structural stability of the prepared masterbatch during secondary melting processing is a key indicator for evaluating the quality of packaging film forming. Combined with... Figure 1 (b) The melt flow rate test results before and after film formation and the test records in Table 1 show that the MFR retention rate of Comparative Example 3 was only 42.79%, and the flow rate of its film product increased by more than double compared with the masterbatch stage. This abnormal increase in fluidity indicates that the matrix resin underwent acid hydrolysis and chain scission. Under the cooling blocking process, anhydrous citric acid failed to react completely in the pre-extrusion stage, and the residual free carboxyl groups accelerated the degradation of the polymer backbone under the high temperature and high shear conditions of the subsequent blown film process. Independently injecting polymeric carbodiimide in the rear section of the barrel can address this process problem in a timely manner. The injected carbodiimide combines with the free carboxyl groups through an electrophilic addition reaction to generate a stable N-acylurea structure, achieving end-group quenching at the reaction kinetics level. Examples 1 to 3, by separating the spatial nodes of the crosslinking reaction and the quenching reaction, completed network construction and maintained stable extrusion in the low-temperature stage, while controlling the degradation of residual acid by injecting carbodiimide in the rear section, thus solving the problem of difficulty in balancing conversion rate and processing stability in traditional reaction compatibilization processes.

[0102] Test Example 2: Comparative Test of Comprehensive Application Performance of Environmentally Friendly Films

[0103] Experimental steps:

[0104] Films prepared by single-screw blow molding in Examples 1 to 3, and Comparative Examples 1, 4, and 5 were selected as experimental subjects. Comparative Example 2 failed to produce a formed film due to overload shutdown during the twin-screw extrusion granulation stage, and Comparative Example 3 exhibited acid degradation issues; therefore, these two groups were not included in the film performance evaluation at this stage.

[0105] Each group of films was cut into 10cm × 10cm samples, and the initial mass was recorded using an analytical balance with an accuracy of 0.1mg. The samples were suspended in a constant temperature and humidity chamber set at 60℃ and 90% relative humidity for 30 days. After the specified time, the samples were removed, and any adhering substances on the film surface were wiped off with a lint-free cloth moistened with anhydrous ethanol. The samples were then dried in a vacuum drying oven at room temperature for 24 hours and weighed again. The plasticizer migration rate was determined by calculating the percentage difference in mass before and after the experiment.

[0106] One square meter of film sample was taken from each group of continuously produced films and laid flat on a transmission light box test bench equipped with a standard D65 light source. The number of gel particles with a diameter greater than 0.5 mm on the film surface was counted using a combination of manual marking and grid counting, and recorded as the gel defect density, expressed as particles / m². 2 .

[0107] Referring to GB / T16578.1-2008, the film samples that had been placed in the above environment for 30 days were cut into right-angle tear test strips. The test was conducted on a universal testing machine at a tensile speed of 200 mm / min. The fracture data of 5 valid strips in each group were recorded and the arithmetic mean was taken as the longitudinal right-angle tear strength.

[0108] Experimental data:

[0109] Table 2. Test results of comprehensive application performance of the thin films in the examples and comparative examples.

[0110] Group Plasticizer migration rate (%) <![CDATA[Gel defect density (number / m 2 )]]> Longitudinal right-angle tear strength (kN / m) Example 1 0.42 5 91.8 Example 2 0.23 3 95.4 Example 3 0.35 4 98.1 Comparative Example 1 8.46 2 54.3 Comparative Example 4 1.15 142 70.6 Comparative Example 5 0.58 217 45.9

[0111] Results analysis:

[0112] Based on the specific values ​​in Table 2 and Figure 2 The test curves for Examples 1 to 3 showed that after 30 days of exposure to high temperature and humidity, the plasticizer migration rate remained below 0.42%. During the actual experiment, it was observed that the surface of the Comparative Example 1 sample taken from the environmental chamber showed obvious oily deposits, and grease residue remained after wiping with a lint-free cloth. Its mass loss was as high as 8.46%, which is significant. Figure 2 The data shows a very prominent spike in the broken line. Typically, in polyester blending, small molecules tend to migrate to the surface due to compatibility differences. Since anhydrous citric acid was not added to the formulation of Comparative Example 1, a chemical network restricting the movement of small molecules could not be constructed within the system. The epoxidized soybean oil remained in a physically blended state within the polyester matrix, inevitably accumulating and precipitating to the outer surface over time. The example groups, relying on the branched structure generated by the in-situ reaction, locked the free plasticizer within the cross-linked network, blocking its outward migration path.

[0113] Controlling plasticizer precipitation mainly relies on the construction of the crosslinking system, but this often results in particle defects on the film due to uneven reaction. (Refer to Table 2 and...) Figure 3 The statistical data shows that Comparative Example 4 used a conventional one-time mixing and feeding method, resulting in a high concentration of anhydrous citric acid in the high-temperature zone at the front of the barrel. This led to an excessively rapid local cross-linking rate, and the resulting large molecular clusters could not be broken down by the screw shear force, ultimately leaving as many as 142 obvious hard fisheyes on a 1 square meter film. Although Comparative Example 5 used side feeding, it failed to coordinate with the reverse cooling in the middle and rear of the barrel. The high-temperature thermal field made the cross-linking reaction difficult to control, and the density of gel defects surged to 217 per m². 2 These dense gel particles become stress concentration points when subjected to force. During tear strength testing, it was found that the cracks in Comparative Examples 4 and 5 almost all originated from and penetrated through the edges of these fish-eye defects. Figure 4 The comparative trends clearly show that this macroscopic defect caused its longitudinal right-angle tear strength to decrease to 70.6 kN / m and 45.9 kN / m, respectively. In contrast, the example, by delaying the addition of the crosslinking agent to zone 4 and coordinating with a subsequent barrel temperature gradient reduction to 140°C, mitigated the intensity of the crosslinking reaction through process node adjustments, achieving a uniform distribution of the crosslinked network. From Table 2 and... Figure 3 , Figure 4 The combined test data shows that the embodiment controls the fisheye defects in the matrix to 5 per m. 2 Furthermore, the appropriate network structure, combined with hydrogen bonding, resulted in a relatively high longitudinal right-angle tear strength of 91.8 kN / m to 98.1 kN / m. These results demonstrate that adjusting the feeding process and setting a reasonable processing temperature have a substantial impact on the overall properties of the blend film.

Claims

1. An environmentally friendly packaging plastic film, characterized in that, The environmentally friendly packaging plastic film is made from raw materials comprising the following parts by weight: Polybutylene terephthalate (PET) 60.0-80.0 parts; Polylactic acid 10.0-30.0 parts; 5.0-12.0 parts of epoxidized soybean oil; 1.0-3.0 parts of anhydrous citric acid micro powder; Pentaerythritol stearate 0.5-1.5 parts; 0.1-0.5 parts of zinc stearate; 0.3-1.0 parts of polymeric carbodiimide.

2. The environmentally friendly packaging plastic film according to claim 1, characterized in that, The melt index of the polybutylene terephthalate-adipate is 2.0-5.0 g / 10 min, and the melt index of the polylactic acid is 4.0-12.0 g / 10 min.

3. The environmentally friendly packaging plastic film according to claim 1, characterized in that, The epoxy value of the epoxidized soybean oil is 6.0%-6.4%, the anhydrous citric acid powder passes through a 100-200 mesh sieve, and the effective carbodiimide group mass fraction of the polymeric carbodiimide is 8.0%-12.0%.

4. A method for preparing an environmentally friendly packaging plastic film, characterized in that, The application of an environmentally friendly packaging plastic film as described in any one of claims 1-3 includes the following steps: Polybutylene terephthalate-adipate and polylactic acid were mixed and then continuously dried in a drying device. Anhydrous citric acid micro powder and zinc stearate were placed in a mixing device and premixed at room temperature to obtain a premixed powder. The dried polybutylene terephthalate, polylactic acid, and pentaerythritol stearate are fed into the main feed port of the twin-screw extruder through the main feed device. At the same time, liquid epoxidized soybean oil is injected into the second zone of the barrel of the twin-screw extruder through the liquid injection device, forming a mixture in the first to third zones of the barrel of the twin-screw extruder. The premixed powder is pressed into the fourth zone of the barrel of the twin-screw extruder through a side-feeding device and melt-blended with the mixture. Start the barrel cooling system of the twin-screw extruder and control the temperature of the 5th, 6th and 7th barrel zones of the twin-screw extruder to decrease sequentially, so that the mixture stays in the 5th to 7th barrel zones of the twin-screw extruder; When the mixture enters the 8th zone of the barrel of the twin-screw extruder, the heated molten polymeric carbodiimide is injected into the 8th zone of the barrel of the twin-screw extruder by a melt pumping device; The vacuum exhaust system is activated in zone 9 of the barrel of the twin-screw extruder. The mixture passes through zone 10 of the barrel of the twin-screw extruder and is extruded as a melt strip by the extrusion die of the twin-screw extruder. The melt strip is then pelletized and centrifuged to obtain modified biodegradable masterbatch. The modified biodegradable masterbatch is added to a blown film equipment for blow molding and then wound up to obtain an environmentally friendly packaging plastic film.

5. The method for preparing an environmentally friendly packaging plastic film according to claim 4, characterized in that, The drying equipment is a dehumidifying dryer, and the mixing equipment is a high-speed mixer; in the step of continuously drying the polybutylene terephthalate-adipate and polylactic acid after mixing them in the dehumidifying dryer, the temperature of the continuous drying is controlled at 70-85℃, and the continuous drying time is 3.0-6.0 hours.

6. The method for preparing an environmentally friendly packaging plastic film according to claim 4, characterized in that, The main feeding device is a main loss-in-weight feed scale, the twin-screw extruder is a co-rotating twin-screw extruder, and the liquid injection device is a liquid metering pump; The length-to-diameter ratio of the co-rotating twin-screw extruder is 40-48, the temperature of the barrel zone 1 to the barrel zone 3 of the co-rotating twin-screw extruder is set to 160-180℃, and the screw speed of the co-rotating twin-screw extruder is 200-300rpm. The side-feeding device is a side-forced feeder, and the temperature of the fourth zone of the barrel of the twin-screw extruder is set to 160-170℃.

7. The method for preparing an environmentally friendly packaging plastic film according to claim 4, characterized in that, The temperature of the fifth zone of the barrel of the twin-screw extruder is set to 150-160℃, the temperature of the sixth zone of the barrel of the twin-screw extruder is set to 145-155℃, and the temperature of the seventh zone of the barrel of the twin-screw extruder is set to 140-150℃. The average residence time of the mixture in the barrel of the twin-screw extruder is controlled to be 30-75 seconds in zones 5 to 7.

8. The method for preparing an environmentally friendly packaging plastic film according to claim 4, characterized in that, The melt pumping equipment is a high-pressure melt gear pump, and the temperature of the 8th zone of the barrel of the twin-screw extruder is set to 150-160℃.

9. The method for preparing an environmentally friendly packaging plastic film according to claim 4, characterized in that, The temperatures of the 9th zone of the barrel, the 10th zone of the barrel, and the extrusion die of the twin-screw extruder are set to 155-165℃, and the vacuum degree of the vacuum exhaust system is controlled to be -0.06 to -0.09MPa.

10. The method for preparing an environmentally friendly packaging plastic film according to claim 4, characterized in that, The blown film equipment is a single-screw blown film machine. The temperature of each zone of the single-screw blown film machine and the die opening temperature of the single-screw blown film machine are set to 140-165℃. The blow-up ratio of the single-screw blown film machine is set to 2.0-4.

0. The traction ratio of the single-screw blown film machine is set to 3.0-6.0.

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