A high-strength, high-toughness, and water-resistant starch / PVA composite film and its preparation method

Abstract

This invention discloses a high-strength, high-toughness, and water-resistant starch / PVA composite film and its preparation method, belonging to the technical field of modified starch-based composite materials. The invention involves mixing starch, polyvinyl alcohol, and a metal salt, mechanically activating the resulting mixture, adding a plasticizer, and then mixing again. The mixture is then subjected to extrusion granulation and extrusion blown film processing to obtain a high-strength, high-toughness, and water-resistant starch / PVA composite film. This starch / PVA composite film exhibits excellent mechanical properties, water resistance, and water barrier properties, with a tensile strength of 22–40 MPa, an elongation at break of 240–350%, a water contact angle of 52–98°, and a water vapor transmission rate of 0.7 × 10⁻⁶. ‑12 ~7.5×10 ‑ 12 g·cm / (cm 2 The film has a strength of ·s·Pa, which meets the performance requirements of petroleum-based materials. Moreover, the film is green and environmentally friendly, and its use as a new type of packaging material has good economic, social and environmental benefits.

Description

Technical Field

[0001] This invention belongs to the technical field of modified starch-based composite materials, specifically relating to a high-strength, high-toughness, and water-resistant starch / PVA composite film and its preparation method. Background Technology

[0002] The depletion of natural resources has led to growing concerns about petroleum-based materials, coupled with increasingly severe global environmental problems, bringing biopolymer-based materials for food packaging into focus. Starch, a sustainable, environmentally friendly, and inexpensive biopolymer, represents a promising approach for producing biocomposite materials. Starch-based films, however, exhibit low mechanical strength and toughness as well as high hydrophilicity, necessitating modification. A common industrial method is chemical modification to improve the performance of starch materials, but this is prone to pollution and is costly, significantly limiting the application of starch in the packaging industry.

[0003] The intermolecular forces and hydrogen bonds in starch make it difficult to process into thermoplastic materials. To overcome this problem, converting starch into thermoplastic starch (TPS) by adding small-molecule plasticizers is a common method, and TPS is considered a suitable candidate to replace synthetic polymers used in packaging. However, TPS has drawbacks such as hygroscopicity, low air permeability, and unsatisfactory water-blocking properties, as well as inherent problems such as poor mechanical properties and hygroscopicity, making it unsuitable for food packaging materials and medical materials. To prepare packaging materials that meet the requirements of practical applications, TPS must be blended with synthetic or natural polymers to enhance mechanical properties. TPS properties can be improved through starch modification, the addition of reinforcing agents, and blending with other polymers. Polyvinyl alcohol (PVA) is a biodegradable and biocompatible material with high mechanical properties, excellent adhesive properties, and chemical resistance. Due to its polyhydroxy structure, it is suitable for blending and modification with starch.

[0004] The preparation process of starch / PVA films can be divided into wet and dry processes. Although the wet process is most commonly used in the laboratory, it is too time-consuming and unsuitable for large-scale production. On the other hand, the dry film formation process has the advantages of high production efficiency, low equipment investment, and low material loss, making it more suitable for large-scale production. In the dry film formation process, extrusion blown film technology is widely used in plastic films due to its advantages such as simple production equipment, low investment, continuous production, and adjustable film size. However, the intermolecular and intramolecular hydrogen bonds in starch and PVA lead to difficulties in the thermal processing of the two materials. Their melting points and decomposition temperatures are very close. To improve the processing performance of starch / PVA blends, the most common method is to add plasticizers, which can lower the melting temperature of starch / PVA blends and improve their flexibility and melt processability. However, the addition of plasticizers will reduce the melt strength of the mixture. When extruding (or processing) thermoplastic starch (i.e., starch mixed with plasticizer and water) at high temperatures, poor melt toughness has been identified as one of the potential limitations. Melt toughness is defined here as the ability of a melt to deform without breaking. High melt strength makes the film bubble more stable during blow molding, enabling the production of thinner films and resulting in higher productivity and performance. Conversely, reduced melt strength affects extrusion blown film and the performance of extruded foamed products. Starch and PVA have poor compatibility, and with increasing starch content, the mechanical properties of the blend film decrease significantly, while hydrophilicity increases, greatly limiting the commercial application of starch / PVA-based materials.

[0005] Mechanical activation (MA) is an intense ball milling process that uses the impact, shearing, and friction forces generated by the high-speed movement of the milling media to disrupt the dense crystalline structure of solid materials and create close interactions between the components. Mechanical activation alters the apparent structure, physicochemical properties, and reactivity of solid materials, thus positively impacting subsequent processes and product characteristics. It is considered a simple and environmentally friendly pretreatment method for preparing functional composite materials.

[0006] Therefore, this invention employs a mechanical activation and enhanced metal salt coordination method to treat starch / PVA and uses an extrusion blown film method to prepare a water-resistant, high-strength, and high-toughness fully biodegradable starch / PVA film as a novel packaging material, which can significantly improve mechanical properties and water resistance, enabling commercial application. Summary of the Invention

[0007] To address the above problems, this invention provides a method for preparing a high-strength, high-toughness, and water-resistant starch / PVA composite film. The method involves mixing starch, polyvinyl alcohol, and metal salt, followed by mechanical activation treatment, adding a plasticizer, and then using an extrusion blown film method to prepare the high-strength, high-toughness, and water-resistant starch / PVA composite film.

[0008] This invention is achieved through the following technical solution:

[0009] A high-strength, high-toughness, and water-resistant starch / PVA composite film is obtained by mixing starch, polyvinyl alcohol, and metal salt, mechanically activating the resulting mixture, adding plasticizer, mixing, and then extruding, granulating, and blowing the film to obtain the high-strength, high-toughness, and water-resistant starch / PVA composite film.

[0010] Further, the mass ratio of starch, polyvinyl alcohol and metal salt is 1:0.5-1.5:0.04-0.3; the mass ratio of plasticizer to mixture is 0.2-0.5:1.

[0011] Furthermore, the metal salt is any one of calcium acetate, zinc acetate, and calcium gluconate.

[0012] Furthermore, the starch is any one of corn starch, tapioca starch, or potato starch.

[0013] Further, the plasticizer is any two or three combinations of glycerol, water, urea, and sorbitol; the mass ratio of water, glycerol, and urea is 0.5–1:3–3.5:1; the mass ratio of water, urea, and sorbitol is 0.5–1:3–3.5:1; the mass ratio of water and urea is 1:4–4.5; and the mass ratio of water, glycerol, and sorbitol is 0.5–1.5:3–3.5:1.

[0014] Furthermore, the high-strength, high-toughness, and water-resistant starch / PVA composite film exhibits a tensile strength of 22.7–34.8 MPa, an elongation at break of 240–350%, a water contact angle of 52–98°, and a water vapor permeability of 0.71 × 10⁻⁶. -12 ~7.212×10 -12 g·cm / (cm 2 ·s·Pa).

[0015] A method for preparing a high-strength, high-toughness, and water-resistant starch / PVA composite film as described above includes the following steps:

[0016] (1) Mechanical activation: Starch and polyvinyl alcohol are pulverized, and the pulverized starch and polyvinyl alcohol are mixed with metal salt. The resulting mixture is placed in a ball mill jar for mechanical activation treatment to obtain an activated mixture.

[0017] (2) Extrusion granulation: Plasticizer is added to the obtained activated mixture and mixed, and then extruded and granulated through a twin-screw extruder to obtain masterbatch;

[0018] (3) Extrusion blown film: The obtained masterbatch is put into a single screw extruder for extrusion blown film to obtain a high-strength, high-toughness, and water-resistant starch / PVA composite film.

[0019] Further, in step (1), the mechanical activation treatment is as follows: the mixture and the ball milling media are added to the ball milling jar at a ratio of 0.5 kg: 500-800 mL, and the mixture is ball milled for 0.5-1.5 h under constant temperature conditions of 300-600 r / min and 30-70 °C. After the ball milling is completed, the product and the ball milling media are separated to obtain the activated mixture; the ball milling media is zirconium balls.

[0020] Furthermore, in step (2), the twin-screw extruder has a rotational speed of 100-150 r / min, a temperature of 120-200℃, and a compression ratio of 2-5.

[0021] Furthermore, in step (3), the single screw extruder has a rotational speed of 110-150 r / min, a temperature of 130-200℃, a blow-up ratio of 2-6, and a stretching ratio of 2-5.

[0022] The preparation principle of the high-strength, high-toughness, and water-resistant starch / PVA composite film of the present invention:

[0023] This invention effectively reduces the crystallinity of starch and PVA through mechanical activation and enhanced metal salt coordination treatment. Most crystalline regions are transformed into amorphous regions, lowering the melt processing temperature of the starch / PVA mixture and improving plasticization. Simultaneously, mechanical activation thoroughly refines the starch / PVA mixture within the metal salt, inducing cracking, plastic deformation, and various types of defects. This converts some mechanical energy into internal energy, resulting in thorough mixing and an interlocking doped state among the cracked and defective components. This improves the compatibility of the mixture, leading to tight bonding and structural stability, thus enhancing the efficiency of thermoplastic processing. The strong mechanical force also facilitates the entry of metal salts into the starch / PVA molecular chains. During extrusion granulation, the -OH groups in the starch / PVA can interact with metal ions through thermomechanical shear force to construct metal-organic coordination structures. The formation of these coordination structures significantly enhances melt strength, which is beneficial for subsequent extrusion blown film production, resulting in increased mechanical strength and toughness of the product.

[0024] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0025] 1. This invention involves mixing starch, polyvinyl alcohol, and a metal salt, mechanically activating the resulting mixture, adding a plasticizer, mixing again, and then extruding, granulating, and blowing the film to obtain a high-strength, high-toughness, and water-resistant starch / PVA composite film. This starch / PVA composite film exhibits excellent mechanical properties, water resistance, and water-blocking properties, with a tensile strength of 22–40 MPa, an elongation at break of 240–350%, a water contact angle of 52–98°, and a water vapor transmission rate of 0.7 × 10⁻⁶. -12 ~7.5×10-12 g·cm / (cm 2 The film has a strength of ·s·Pa, which meets the performance requirements of petroleum-based materials. Moreover, the film is green and environmentally friendly, and its use as a new type of packaging material has good economic, social and environmental benefits.

[0026] 2. The starch / PVA of the present invention undergoes mechanical activation and synergistic metal salt treatment, which more effectively destroys the crystalline region, allowing the plasticizer to quickly and uniformly penetrate into both phases of starch / PVA, resulting in good plasticizing performance. Furthermore, the metal salt is more uniformly anchored on the starch / PVA molecular particles, improving the reaction efficiency of the organic-coordination structure constructed by twin-screw extrusion. This results in a stronger organic coordination structure, improved product performance, and higher melt strength in the obtained masterbatch during single-screw extrusion. This also improves the production efficiency of extrusion blown film and the mechanical properties of the finished film.

[0027] 3. The starch / PVA composite film of the present invention has excellent mechanical properties and water resistance. Mechanical activation and synergistic metal salt treatment improve the compatibility of the starch / PVA two phases, significantly improving the mechanical properties of the material. The organic-coordination interaction formed between metal ions and starch / PVA can act as a bridge dispersed between the two phases, making the interaction between the materials stronger, improving the compactness of the material, thereby increasing the water contact angle of the material and reducing the water vapor permeability, greatly enhancing the water resistance of the material.

[0028] 4. This invention uses starch, PVA, plasticizer, and edible organometallic salts as raw materials, and continuously produces the film using a screw extruder with shear plasticizing properties. During this process, the film exhibits excellent thermoplastic processing performance, high melt strength, stable extrusion blow molding process, and is easy to control, resulting in low production costs and enabling economical large-scale production. The starch / PVA composite film obtained by this invention possesses excellent mechanical properties, water resistance, and water-blocking properties. Furthermore, it is inexpensive, non-toxic, non-polluting, and environmentally friendly, making it a potential new packaging material. It also holds promise as a replacement for petroleum-based plastics, helping to completely solve the pollution problems associated with traditional petroleum-based plastics. Attached Figure Description

[0029] Figure 1 The images are SEM images of ST / PVA obtained in Comparative Example 1 at different magnifications.

[0030] Figure 2 The image shows SEM images of MA-ST / PVA obtained in Comparative Example 2 at different magnifications.

[0031] Figure 3 The image shows SEM images of ST / PVA / CA obtained in Comparative Example 3 at different magnifications.

[0032] Figure 4The images show SEM images of MA-ST / PVA / CA obtained in Example 4 at different magnifications.

[0033] Figure 5 The images show the XRD patterns of ST / PVA, MA-ST / PVA, ST / PVA / CA, and MA-ST / PVA / CA prepared in Comparative Examples 1-3 and Example 4, respectively.

[0034] Figure 6 The rheological test diagrams are for ST / PVA, MA-ST / PVA, ST / PVA / CA, and MA-ST / PVA / CA prepared in Comparative Examples 1-3 and Example 4, respectively.

[0035] Figure 7 The tensile strength and elongation at break of ST / PVA, MA-ST / PVA, ST / PVA / CA, and MA-ST / PVA / CA prepared in Comparative Examples 1-3 and Example 4 are shown in the diagram.

[0036] Figure 8 Young's modulus diagrams of ST / PVA, MA-ST / PVA, ST / PVA / CA, and MA-ST / PVA / CA prepared in Comparative Examples 1-3 and Example 4, respectively.

[0037] Figure 9 The TG curves are for ST / PVA, MA-ST / PVA, ST / PVA / CA, MA-ST / PVA / CA and CA prepared in Comparative Examples 1-3 and Example 4, respectively.

[0038] Figure 10 The DTG curves are for ST / PVA, MA-ST / PVA, ST / PVA / CA, MA-ST / PVA / CA and CA obtained in Comparative Examples 1-3 and Example 4, respectively.

[0039] Figure 11 The diagram shows the water vapor transmission rate and water contact angle of ST / PVA, MA-ST / PVA, ST / PVA / CA, and MA-ST / PVA / CA prepared in Comparative Examples 1-3 and Example 4, respectively.

[0040] Figure 12 This is a diagram of the chelation structure of starch / PVA and calcium acetate during the mechanical activation process in Example 4. Detailed Implementation

[0041] The present invention will be further described in detail below through embodiments. These embodiments are only used to illustrate the present invention and do not limit the scope of protection of the present invention.

[0042] Example 1

[0043] Corn starch was pulverized to 500 mesh, and PVA to 100 mesh. 500g of the pulverized starch, PVA, and calcium acetate (mass ratio 50:50:2) were placed separately into a mechanically activated ball mill jar, along with 700mL of zirconium balls. The milling speed was adjusted to 400 rpm, the constant temperature water bath was set to 50℃, and the milling time was 1 hour to obtain an activated mixture. The sample was then removed, and 175g of plasticizer (water, glycerol, and urea in a mass ratio of 1:3.5:1) was added and mixed. The mixture was then extruded and granulated using a twin-screw extruder at an extrusion temperature of 130–150℃, a screw speed of 120 rpm, and a screw compression ratio of 5:2 to obtain masterbatch. The masterbatch was then fed into a single-screw extruder for extrusion blown film production. The extrusion temperature was 140–155°C, the screw speed was 130 r / min, the blow-up ratio was 4, and the stretch ratio was 3, resulting in a high-strength, high-toughness, and water-resistant starch / PVA composite film (denoted as MA-ST / PVA / CA).

[0044] The tensile strength of the starch / PVA composite film was measured to be 31.7 MPa, the elongation at break was 240%, the water contact angle was 52°, and the water vapor transmission rate was 7.212 × 10⁻⁶. -12 g·cm / (cm 2 ·s·Pa).

[0045] Example 2

[0046] Corn starch was pulverized to 500 mesh, and PVA to 100 mesh. 500g of the pulverized starch, PVA, and calcium acetate (mass ratio 50:50:5) were placed separately into a mechanically activated ball mill jar, along with 700mL of zirconium balls. The milling speed was adjusted to 400 rpm, the constant temperature water bath was set to 50℃, and the milling time was 1 hour to obtain an activated mixture. The sample was then removed, and 175g of plasticizer (water and urea mass ratio 1:4) was added and mixed. The mixture was then extruded and granulated using a twin-screw extruder at an extrusion temperature of 130–150℃, a screw speed of 120 rpm, and a screw compression ratio of 3:1 to obtain masterbatch. The masterbatch was then fed into a single-screw extruder for extrusion blown film production. The extrusion temperature was 140–155°C, the screw speed was 130 r / min, the blow-up ratio was 4, and the stretch ratio was 3, resulting in a high-strength, high-toughness, and water-resistant starch / PVA composite film (denoted as MA-ST / PVA / CA).

[0047] The tensile strength of the starch / PVA composite film was measured to be 32.5 MPa, the elongation at break was 275%, the water contact angle was 58°, and the water vapor transmission rate was 4.602 × 10⁻⁶. -12 g·cm / (cm 2 ·s·Pa).

[0048] Example 3

[0049] Corn starch was pulverized to 500 mesh, and PVA to 100 mesh. 500g of the pulverized starch, PVA, and calcium acetate (mass ratio 50:50:8) were placed separately into a mechanically activated ball mill jar, along with 700mL of zirconium balls. The milling speed was adjusted to 400r / min, the constant temperature water bath was set to 50℃, and the milling time was 1h to obtain an activated mixture. The sample was then removed, and 150g of plasticizer (water, urea, and sorbitol in a mass ratio of 1:3:1) was added and mixed. The mixture was then extruded and granulated using a twin-screw extruder at an extrusion temperature of 130–160℃, a screw speed of 120r / min, and a screw compression ratio of 3:1 to obtain masterbatch. The masterbatch was then fed into a single-screw extruder for extrusion blown film production. The extrusion temperature was 140–155°C, the screw speed was 130 r / min, the blow-up ratio was 4, and the stretch ratio was 3, resulting in a high-strength, high-toughness, and water-resistant starch / PVA composite film (denoted as MA-ST / PVA / CA).

[0050] The tensile strength of the starch / PVA composite film was measured to be 34.5 MPa, the elongation at break was 307%, the water contact angle was 59°, and the water vapor transmission rate was 1.267 × 10⁻⁶. -12 g·cm / (cm 2 ·s·Pa).

[0051] Example 4

[0052] Corn starch was pulverized to 500 mesh, and PVA to 100 mesh. 500g of the pulverized corn starch, PVA, and calcium acetate (mass ratio 50:50:10) were placed separately into a mechanically activated ball mill jar, along with 700mL of zirconium balls. The milling speed was adjusted to 400 rpm, the constant temperature water bath was set to 50℃, and the milling time was 2 hours to obtain the activated mixture. The chelation structure of starch / PVA and calcium acetate during this process is shown in the diagram below. Figure 12 As shown. The above sample was taken out, and 150g of plasticizer (water, glycerol, and urea in a mass ratio of 1:3:1) was added and mixed. The mixture was then extruded and granulated using a twin-screw extruder at an extrusion temperature of 130–160℃, a screw speed of 120 r / min, and a screw compression ratio of 5:2 to obtain masterbatch. Subsequently, the masterbatch was fed into a single-screw extruder for blown film extrusion at an extrusion temperature of 140–180℃, a screw speed of 130 r / min, a blow-up ratio of 4, and a stretch ratio of 3 to obtain a high-strength, high-toughness, and water-resistant starch / PVA composite film (denoted as MA-ST / PVA / CA).

[0053] The tensile strength of the starch / PVA composite film was measured to be 34.3 MPa, the elongation at break was 273%, the water contact angle was 84°, and the water vapor transmission rate was 0.71 × 10⁻⁶. -12 g·cm / (cm 2·s·Pa).

[0054] Example 5

[0055] Cassava starch was pulverized to 500 mesh, and PVA to 100 mesh. 500g of the pulverized starch, PVA, and zinc acetate (mass ratio 50:50:5) were placed separately into a mechanically activated ball mill jar, along with 700mL of zirconium balls. The milling speed was adjusted to 500r / min, the constant temperature water bath was set to 50℃, and the milling time was 2h to obtain an activated mixture. The sample was then removed, and 175g of plasticizer (water, glycerol, and sorbitol in a mass ratio of 1:3:1) was added and mixed. The mixture was then extruded and granulated using a twin-screw extruder at an extrusion temperature of 140–165℃, a screw speed of 120r / min, and a screw compression ratio of 3:1 to obtain masterbatch. The masterbatch was then fed into a single-screw extruder for extrusion blown film production. The extrusion temperature was 150–190°C, the screw speed was 130 r / min, the blow-up ratio was 4, and the stretch ratio was 3, resulting in a high-strength, high-toughness, and water-resistant starch / PVA composite film (denoted as MA-ST / PVA / CA).

[0056] The tensile strength of the starch / PVA composite film was measured to be 24.7 MPa, the elongation at break was 350%, the water contact angle was 87°, and the water vapor transmission rate was 1.046 × 10⁻⁶. -12 g·cm / (cm 2 ·s·Pa).

[0057] Example 6

[0058] Cassava starch was pulverized to 500 mesh, and PVA to 100 mesh. 500g of the pulverized starch, PVA, and zinc acetate (mass ratio 50:50:10) were placed separately into a mechanically activated ball mill jar, along with 700mL of zirconium balls. The milling speed was adjusted to 400 rpm, the constant temperature water bath was set to 50℃, and the milling time was 2 hours. The activated mixture was obtained. The sample was then removed, and 150g of plasticizer (water, glycerol, and urea in a mass ratio of 0.5:3:1) was added and mixed. The mixture was then extruded and granulated using a twin-screw extruder at an extrusion temperature of 130–150℃, a screw speed of 120 rpm, and a screw compression ratio of 5:2 to obtain masterbatch. The masterbatch was then fed into a single-screw extruder for extrusion blown film production. The extrusion temperature was 140–155°C, the screw speed was 130 r / min, the blow-up ratio was 3, and the stretch ratio was 3, resulting in a high-strength, high-toughness, and water-resistant starch / PVA composite film (denoted as MA-ST / PVA / CA).

[0059] The tensile strength of the starch / PVA composite film was measured to be 22.7 MPa, the elongation at break was 270%, the water contact angle was 98°, and the water vapor permeability was 1.587 × 10⁻⁶. -12 g·cm / (cm2 ·s·Pa).

[0060] Example 7

[0061] Potato starch was pulverized to 500 mesh, and PVA to 100 mesh. 500g of the pulverized starch, PVA, and calcium gluconate (mass ratio 40:60:5) were placed separately into a mechanically activated ball mill jar, along with 700mL of zirconium balls. The milling speed was adjusted to 400 rpm, the constant temperature water bath was set to 50℃, and the milling time was 2 hours. The activated mixture was obtained. The sample was then removed, and 150g of plasticizer (water, glycerol, and urea in a mass ratio of 0.8:3.5:1) was added and mixed. The mixture was then extruded and granulated using a twin-screw extruder at an extrusion temperature of 130–150℃, a screw speed of 120 rpm, and a screw compression ratio of 5:2 to obtain masterbatch. The masterbatch was then fed into a single-screw extruder for extrusion blown film production. The extrusion temperature was 140–155°C, the screw speed was 130 r / min, the blow-up ratio was 3, and the stretch ratio was 3, resulting in a high-strength, high-toughness, and water-resistant starch / PVA composite film (denoted as MA-ST / PVA / CA).

[0062] The tensile strength of the starch / PVA composite film was measured to be 38.2 MPa, the elongation at break was 241%, the water contact angle was 77°, and the water vapor transmission rate was 1.150 × 10⁻⁶. -12 g·cm / (cm 2 ·s·Pa).

[0063] Example 8

[0064] Potato starch was pulverized to 500 mesh, and PVA to 100 mesh. 500g of the pulverized starch, PVA, and calcium gluconate (mass ratio 40:60:10) were placed separately into a mechanically activated ball mill jar, along with 700mL of zirconium balls. The milling speed was adjusted to 400 rpm, the constant temperature water bath was set to 50℃, and the milling time was 2 hours to obtain an activated mixture. The sample was then removed, and 150g of plasticizer (water, glycerol, and urea in a mass ratio of 1:3:1) was added and mixed. The mixture was then extruded and granulated using a twin-screw extruder at an extrusion temperature of 130–150℃, a screw speed of 120 rpm, and a screw compression ratio of 5:2 to obtain masterbatch. The masterbatch was then fed into a single-screw extruder for extrusion blown film production. The extrusion temperature was 140–155°C, the screw speed was 130 r / min, the blow-up ratio was 3, and the stretch ratio was 3, resulting in a high-strength, high-toughness, and water-resistant starch / PVA composite film (denoted as MA-ST / PVA / CA).

[0065] The composite film was measured to have a tensile strength of 30.5 MPa, an elongation at break of 349%, a water contact angle of 82°, and a water vapor permeability of 0.989 × 10⁻⁶. -12g·cm / (cm 2 ·s·Pa).

[0066] Comparative Example 1

[0067] The difference between Comparative Example 1 and Example 4 is that the raw materials in Comparative Example 1 do not contain calcium acetate and are not mechanically activated. The rest of the preparation process and conditions are the same as those in Example 4, and a starch / PVA film (denoted as ST / PVA) without metal salt doping and mechanical activation is obtained.

[0068] Comparative Example 2

[0069] The difference between Comparative Example 2 and Example 4 is that the raw materials in Comparative Example 2 do not contain calcium acetate, while the rest of the preparation process and conditions are the same as those in Example 4, resulting in a mechanically activated starch / PVA film (denoted as MA-ST / PVA).

[0070] Comparative Example 3

[0071] The difference between Comparative Example 3 and Example 4 is that no mechanical activation treatment was performed in Comparative Example 3, while the rest of the preparation process and conditions were the same as in Example 4, and a starch / PVA film doped with metal salt (denoted as ST / PVA / CA) was obtained.

[0072] Material characterization analysis

[0073] (I) Microscopic morphology analysis

[0074] The microstructure of ST / PVA, MA-ST / PVA, ST / PVA / CA, and MA-ST / PVA / CA prepared in Comparative Examples 1-3 and Example 4 were analyzed using scanning electron microscopy (SEM). The analysis results are as follows: Figure 1-4 As shown.

[0075] Depend on Figure 1-4 It is evident that the surface of the MA-ST / PVA sample is significantly smooth, indicating that mechanical force disrupts the crystalline structure of starch and PVA, thus improving plasticization. After mechanical activation to enhance metal salt coordination, the coordination between the metal salt and the hydroxyl groups of starch / PVA acts as a bridge between the two phases, resulting in a qualitative leap in the surface structure of the MA-ST / PVA / CA sample, exhibiting a dense, smooth, and uniform appearance. This result is crucial for improving the mechanical and barrier properties of starch / PVA mixtures. These significant changes in surface morphology may be attributed to improved interfacial adhesion, microdomain distribution, and thorough plasticization of starch and PVA.

[0076] (II) Crystallinity Analysis

[0077] The crystal structures of ST / PVA, MA-ST / PVA, ST / PVA / CA, and MA-ST / PVA / CA prepared in Comparative Examples 1-3 and Example 4 were analyzed using X-ray diffraction (XRD). The analysis results are as follows: Figure 5 As shown.

[0078] Depend on Figure 5 It is evident that in the MA-ST / PVA / CA sample, mechanical activation combined with metal salt treatment disrupted the crystalline regions of starch, PVA, and metal salt, forming an amorphous structure. This improved the melt processability of the mixture and facilitated the dispersion of metal ions between the two phases, thereby enhancing the compatibility of the mixture. In the ST / PVA / CA sample, the presence of calcium acetate crystals was clearly observable, but the characteristic diffraction peaks of calcium acetate disappeared after mechanical activation. This confirms the existence of the calcium acetate crystal structure and verifies that mechanical activation disrupted the crystalline structures of starch, PVA, and calcium acetate, making it easier for metal ions to enter between the starch and PVA phases. This is beneficial for the subsequent hot extrusion reinforcement of the metal coordination structure between the metal salt and the starch / PVA mixture.

[0079] (III) Rheological Analysis

[0080] Rheological tests and analyses were performed on ST / PVA, MA-ST / PVA, ST / PVA / CA, and MA-ST / PVA / CA prepared in Comparative Examples 1-3 and Example 4, respectively. The results are as follows: Figure 6 As shown.

[0081] Depend on Figure 6 It was found that the apparent viscosity of all samples decreased linearly with increasing shear rate, and all curves exhibited shear-thinning behavior, clarifying the characteristics of non-Newtonian fluids. Mechanical ball milling disrupted the crystalline structure and intramolecular hydrogen bonds of starch and PVA, resulting in a significant reduction in the initial apparent viscosity of the starch / PVA mixture during melt processing. However, at high shear rates, the apparent viscosity of the MA-ST / PVA / CA sample was much higher than that of the ST / PVA / CA sample. Especially under high shear conditions, although the coordination effect was weakened, it was not destroyed by the high shear force. The higher melt viscosity at high shear rates indicates that the strong coordination effect ensures that the melt viscosity of the MA-ST / PVA / CA sample is higher than that of other samples under high shear conditions. The improvement of melt strength is crucial for the extrusion blown film performance of starch / PVA.

[0082] (iv) Mechanical performance analysis

[0083] The mechanical properties of ST / PVA, MA-ST / PVA, ST / PVA / CA, and MA-ST / PVA / CA prepared in Comparative Examples 1-3 and Example 4, respectively, were analyzed. The analysis results are as follows: Figure 7 and Figure 8 As shown.

[0084] Depend on Figure 7 and Figure 8 It was found that after adding calcium acetate, the tensile strength of the ST / PVA sample increased from 13.1 MPa to 16.2 MPa of the ST / PVA / CA sample, while the elongation at break decreased from 242% to 198%. The tensile strength of the MA-ST / PVA sample after mechanical activation treatment was also improved, exhibiting the highest elongation at break. The elastic modulus and tensile strength of the MA-ST / PVA / CA sample were 60 MPa and 34.3 MPa, respectively, with an elongation at break of 273%, all greater than other samples. These results indicate that mechanical activation significantly improves the compatibility between starch and PVA, and that mechanical activation can promote interfacial interactions between metal ions and the starch / PVA mixture. The elastic modulus, tensile strength, and elongation at break of the starch / PVA mixture depend on the interfacial interaction forces between the two phases. Hydrogen bonds play a dominant role in the mechanical action of the starch / PVA mixture, while the addition of metal ions can act as a bridge, building stronger interactions between the two phases.

[0085] (V) Thermal Stability Analysis

[0086] Thermal stability analysis was performed on ST / PVA, MA-ST / PVA, ST / PVA / CA, MA-ST / PVA / CA, and calcium acetate (denoted as CA) prepared in Comparative Examples 1-3 and Example 4, respectively. The results are as follows: Figure 9 and Figure 10 As shown.

[0087] Depend on Figure 9 and Figure 10 It can be seen that the thermogravimetric curve of the sample after mechanical activation and metal salt pretreatment is sharper, and the thermal decomposition peak of PVA at 410℃ is weaker, indicating that the compatibility between starch and PVA is significantly improved. Compared with the ST / PVA / CA sample without mechanical activation treatment, the thermal decomposition temperature of the MA-ST / PVA / CA sample is significantly increased, indicating that a stronger organic-coordination structure is formed, which enhances the thermal stability of the material.

[0088] (vi) Water resistance analysis

[0089] The water resistance properties of ST / PVA, MA-ST / PVA, ST / PVA / CA, and MA-ST / PVA / CA prepared in Comparative Examples 1-3 and Example 4 were analyzed using water contact angle and water vapor transmission rate tests. The analysis results are as follows: Figure 11 As shown.

[0090] Depend on Figure 11It can be seen that in the MA-ST / PVA / CA sample, mechanical activation enhanced the coordination of the metal salt, improved the reaction efficiency between the starch / PVA mixture and metal ions, improved the material compatibility, increased the water contact angle to 84°, and decreased the water vapor transmission rate to 0.71×10⁻⁶. -12 g·cm / (cm 2 ·s·Pa).

[0091] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-strength, high-toughness, water-resistant starch / PVA composite film, characterized by, The starch, polyvinyl alcohol and metal salt are mixed, the obtained mixture is subjected to mechanical activation treatment, then a plasticizer is added and mixed, and the high-strength high-toughness water-resistant starch / PVA composite film is obtained through extrusion granulation and extrusion film blowing. The mass ratio of the starch, polyvinyl alcohol and metal salt is 1:0.5-1.5:0.04-0.3; the mass ratio of the plasticizer to the mixture is 0.2-0.5:1; and the metal salt is any one of calcium acetate, zinc acetate and calcium gluconate. The preparation method of the high-strength high-toughness water-resistant starch / PVA composite film comprises the following steps: (1) mechanical activation: the starch and polyvinyl alcohol are pulverized, the pulverized starch and polyvinyl alcohol are mixed with a metal salt, and the obtained mixture is subjected to mechanical activation treatment in a ball mill tank to obtain an activated mixture; (2) extrusion granulation: a plasticizer is added to the obtained activated mixture and mixed, and then the mixture is subjected to extrusion granulation through a double-screw extruder to obtain a master batch; the rotation speed of the double-screw extruder is 100-150 r / min, the temperature is 120-200 DEG C, and the compression ratio is 2-5; (3) extrusion film blowing: the obtained master batch is put into a single-screw extruder for extrusion film blowing to obtain a high-strength high-toughness water-resistant starch / PVA composite film.

2. The high strength, high toughness, water resistant starch / PVA composite film according to claim 1, wherein, The starch is any one of corn starch, cassava starch and potato starch.

3. The high strength, high toughness, water resistant starch / PVA composite film according to claim 1, wherein, The plasticizer is any two or more than three kinds of combination of glycerol, water, urea and sorbitol.

4. The high-strength, high-toughness, water-resistant starch / PVA composite film according to any one of claims 1 to 3, characterized in that, The high-strength high-toughness water-resistant starch / PVA composite film has tensile strength of 22-40 Mpa, elongation at break of 240-350%, water contact angle of 52-98°, and water vapor permeability of 0.7*10 -12 -7.5*10 -12 g*cm / (cm 2 *s*Pa).

5. A method for preparing a high-strength, high-toughness, water-resistant starch / PVA composite film according to any one of claims 1 to 3, characterized by, The preparation method of the high-strength high-toughness water-resistant starch / PVA composite film comprises the following steps: (1) mechanical activation: the starch and polyvinyl alcohol are pulverized, the pulverized starch and polyvinyl alcohol are mixed with a metal salt, and the obtained mixture is subjected to mechanical activation treatment in a ball mill tank to obtain an activated mixture; (2) extrusion granulation: a plasticizer is added to the obtained activated mixture and mixed, and then the mixture is subjected to extrusion granulation through a double-screw extruder to obtain a master batch; the rotation speed of the double-screw extruder is 100-150 r / min, the temperature is 120-200 DEG C, and the compression ratio is 2-5; (3) extrusion film blowing: the obtained master batch is put into a single-screw extruder for extrusion film blowing to obtain a high-strength high-toughness water-resistant starch / PVA composite film.

6. The method for preparing the high-strength, high-toughness, water-resistant starch / PVA composite film according to claim 5, characterized in that, In step (1), the mechanical activation treatment is that the mixture and the ball milling medium are added into the ball mill tank at a ratio of 0.5 kg: 500-800 mL, and the ball milling is carried out at a rotation speed of 300-600 r / min and a constant temperature of 30-70 DEG C for 0.5-1.5 h; after the ball milling, the product and the ball milling medium are separated to obtain the activated mixture.

7. The method for preparing the high-strength, high-toughness, water-resistant starch / PVA composite film according to claim 5, characterized in that, In step (3), the rotation speed of the single-screw extruder is 110-150 r / min, the temperature is 130-200 DEG C, the blowing ratio is 2-6, and the stretching ratio is 2-5.

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