Preparation method of modified starch composite material with both thermoplasticity and cross-linking properties

Abstract

The present invention discloses a method for preparing a modified starch composite material having both thermoplasticity and cross-linking properties, and belongs to the technical field of polymer material modification. First, triphenylmethane triisocyanate is mixed with corn starch to react and prepare cross-linked starch; secondly, N, N-dimethylformamide is mixed with sorbitol and then a composite plasticizer is prepared, and after mixing with the cross-linked starch, melt extrusion is performed to obtain thermoplastic cross-linked starch; finally, thermoplastic cross-linked starch and polybutylene terephthalate adipate are melt-extruded to obtain a modified starch composite material having both thermoplasticity and cross-linking properties. The modified starch composite material having both thermoplasticity and cross-linking properties prepared by the present invention has high strength and high toughness, with tensile strength increased from 8.23Mpa to 22.53Mpa, flexural strength increased from 2.14Mpa to 4.26Mpa, and elongation at break increased from 382% to 650%.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer material modification, and in particular to a method for preparing thermoplastic-crosslinked starch and application of the same in polybutylene terephthalate-adipate (PBAT). Background Art

[0002] The extensive use of traditional non-degradable plastics has caused serious environmental pollution. With increasing environmental awareness, biodegradable plastics have received widespread attention. Polybutylene terephthalate (PBAT) is one of the most commercially available fully biodegradable materials. It exhibits excellent thermoplasticity and thermal stability, along with high elongation at break, elastic modulus, and tensile strength. It is widely used in applications such as disposable packaging and agricultural mulch films. Although PBAT has been commercialized, its high cost has limited the promotion and use of PBAT films. Reducing the cost of fully biodegradable materials has become a research hotspot. Starch is a natural polymer material with the advantages of low cost, renewable nature, and non-toxicity. Filling PBAT with low-cost starch is an effective method to reduce the cost of PBAT. However, starch molecules contain a large number of hydroxyl groups and have high polarity. Hydrogen bonds are easily formed between hydroxyl groups, and the number of hydrogen bonds formed is large and the strength is strong. The strong intermolecular hydrogen bonding force makes the viscosity flow temperature of starch higher than its decomposition temperature, making it not thermoplastic and difficult to melt process. In addition, the strength and toughness of thermoplastic modified starch are low, which limits its application in packaging and daily fields. Therefore, achieving thermal processing and high performance of starch through modification is an important means to expand the application range of starch materials. Moreover, blending the modified starch with polybutylene terephthalate-adipate (PBAT) has low cost and high performance, which has great research value.

[0003] Currently, physical and chemical methods are commonly used to modify starch. In the process of physical modification of starch, glycerol is often used as a plasticizer to prepare thermoplastic starch. In a method for preparing a polylactic acid-starch-based bioplastic composite material, glycerol is used as a plasticizer to prepare thermoplastic starch by high-speed stirring, and a composite material is prepared with polylactic acid (PLA). However, the hydrogen bond formed by glycerol as a plasticizer with starch is weak, and when left for a long time and stored for a long time, the plasticizer is easy to migrate, causing the starch to recrystallize, resulting in poor plasticization effect. Chemical modification mainly uses chemical reactions, in which the modifier reacts with the hydroxyl groups in the starch to improve the starch properties. In a high-α-degree pregelatinized starch and its preparation method, citric acid is used as a crosslinking agent to crosslink the starch through an esterification reaction to promote starch gelatinization. However, the reaction process conditions of citric acid as a crosslinking agent with starch are complex, and the degree of crosslinking is low. Summary of the Invention

[0004] In order to solve the problems of low strength, poor toughness and difficulty in thermoplastic processing of starch, the present invention proposes a method for preparing a modified starch composite material with both thermoplastic and cross-linking properties.

[0005] The preparation steps of a modified starch composite material having both thermoplastic and cross-linking properties are as follows:

[0006] (1) Preparation of cross-linked starch (XSt)

[0007] Triphenylmethane triisocyanate (TTI) and corn starch (St) are mixed at a mass ratio of 1-1.5:100 in a high-speed mixer at a temperature of 90-110° C. to react to obtain cross-linked starch (XSt);

[0008] (2) Preparation of thermoplastic-crosslinked starch (TP-XSt)

[0009] (2.1) N,N-dimethylformamide (DMF) and sorbitol (SO) are uniformly mixed in a mass ratio of 3-5:1 to obtain a composite plasticizer;

[0010] (2.2) placing the composite plasticizer and the cross-linked starch (XSt) in a high-speed mixer at a mass ratio of 20-30:100 and mixing at room temperature;

[0011] (2.3) then melt-extruded through a twin-screw extruder to obtain thermoplastic-crosslinked starch (TP-XSt);

[0012] (3) Preparation of modified starch composite materials with both thermoplastic and cross-linking properties

[0013] Thermoplastic cross-linked starch (TP-XSt) and polybutylene terephthalate adipate (PBAT) are mixed at a mass ratio of 10-30:100 at room temperature and high speed;

[0014] The resulting mixture is then melt-extruded through a twin-screw extruder to obtain thermoplastic-crosslinked starch-modified polybutylene terephthalate-adipate (PBAT / TP-XSt), a modified starch composite material with both thermoplastic and cross-linking properties.

[0015] The modified starch composite material with both thermoplastic and cross-linking properties has a tensile strength of 20.05-22.53 MPa, a flexural strength of 4.03-4.26 MPa, an elongation at break of 600-650%, a melt index of 4.96-6.26 g / 10 min, and a crystallinity of 10.07-11.27%.

[0016] The technical solutions are further defined as follows:

[0017] In step (1), the high-speed mixing speed is 1000-1500 r / min and the time is 20-30 min.

[0018] In step (2.2), the high-speed mixing speed is 1000-1500 r / min and the time is 10-20 min.

[0019] In step (2.3), the screw speed of the twin-screw extruder is 100-200 r / min, the feeding rate is set to 3-5 r / min, and the extrusion temperature zone is set to 120, 125, 130, 135, 140, 145, 150, 150, 150, 150, 140, 145°C.

[0020] In step (3), the high-speed mixing condition at room temperature is: mixing at a rotation speed of 1000 r / min for 20 minutes.

[0021] In step (3), under the conditions of temperature 190° C. and mass 2160 g, the polybutylene terephthalate-adipate (PBAT) has a melt index of 3.42 g / 10 min, a melting point of 118.49° C., and a carboxyl content of 13.27 mol / t.

[0022] In step (3), the screw speed of the twin-screw extruder is 300r / min, the feeding rate is 5r / min, and the extrusion temperature zones are set to 130, 135, 140, 145, 150, 155, 160, 160, 160, 160, 150, and 155°C.

[0023] The beneficial technical effects of the present invention are embodied in the following aspects:

[0024] (1) The modified starch composite material with both thermoplastic and cross-linking properties prepared by the present invention has high strength and high toughness. This is because: on the one hand, due to the cross-linking effect, the interaction between the thermoplastic-cross-linked starch (TP-XSt) molecules and the polybutylene terephthalate-adipate (PBAT) molecular chains is stronger, which improves the mechanical properties of the composite material. Compared with the untreated starch-modified polybutylene terephthalate-adipate composite material (PBAT / St), the tensile strength of the composite material (PBAT / TP-XSt) prepared by the present invention increases from 8.23 MPa to 22.53 MPa, and the flexural strength increases from 2.14 MPa to 4.26 MPa. The elongation at break increased from 382% to 650%. On the other hand, due to the enhanced fluidity of the molecular chains of thermoplastic-cross-linked starch (TP-XSt), the regular arrangement of the molecular chains is promoted, so that the composite material has higher processing performance, heat resistance and crystallization performance. Compared with the untreated starch-modified polybutylene terephthalate-adipate composite material (PBAT / St), the melt index of the composite material (PBAT / TP-XSt) prepared by the present invention is reduced from 10.25 g / 10 min to 4.96 g / 10 min, the melting temperature is increased from 118.49°C to 123.30°C, and the crystallinity is increased from 8.95% to 11.27%.

[0025] (2) The present invention designs a "crosslinking-plasticizing" process, which adopts a process of first crosslinking and then plasticizing. First, see Figure 1 , using triphenylmethane triisocyanate (TTI) as a crosslinking agent, where TTI contains multiple isocyanate groups, which react with the hydroxyl (-OH) groups of starch to form carbamate, forming stronger chemical bonds between starch molecules, cross-linking to form a multi-dimensional network structure, increasing the mechanical strength of starch, and reducing the hydroxyl content of starch, reducing the hydrophilicity of starch, and improving the compatibility of starch with polybutylene adipate terephthalate (PBAT). Secondly, see Figure 2 N,N-dimethylformamide (DMF) and sorbitol (SO) are used as composite plasticizers. Among them, N,N-dimethylformamide (DMF) acts as a hydrogen bond acceptor, forming more stable hydrogen bonds with starch and inhibiting starch retrogradation; sorbitol (SO) mostly acts as a hydrogen bond donor, and has a similar molecular structure to glucose. It has a large molecular weight, which expands the steric hindrance of starch and has a stronger plasticizing effect. In addition, cross-linked starch first cross-linked with triphenylmethane triisocyanate (TTI) as a cross-linking agent may have unreacted isocyanate groups. Adding a plasticizer sorbitol (SO) with a hydroxyl functional group can further improve the degree of cross-linking. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a diagram of the cross-linking mechanism of starch cross-linked with triphenylmethane triisocyanate (TTI);

[0027] Figure 2This is a diagram of the hydrogen bonding mechanism of composite plasticizers;

[0028] Figure 3 This is a comparison of Fourier transform infrared (FT-IR) images of corn starch (St) and thermoplastic-crosslinked starch (TP-XSt);

[0029] Figure 4 This is a comparison of the crystallization / melting curves of pure poly (butylene terephthalate)-adipate (PBAT) raw material, the starch-modified poly (butylene terephthalate)-adipate composite material (PBAT / St) prepared in Comparative Example 1, and the thermoplastic-cross-linked starch-modified poly (butylene terephthalate)-adipate composite material (PBAT / TP-XSt) prepared in Example 1; (a) is the cooling crystallization process, and (b) is the heating melting process. DETAILED DESCRIPTION

[0030] The present invention is further described in detail below through examples.

[0031] Example 1

[0032] The preparation steps of a modified starch composite material having both thermoplastic and cross-linking properties are as follows:

[0033] (1) Preparation of cross-linked starch (XSt)

[0034] 15 g of triphenylmethane triisocyanate (TTI) and 1 kg of corn starch (St) were placed in a high-speed mixer and mixed at a temperature of 90° C. and a rotation speed of 1000 r / min for 30 min to obtain cross-linked starch (XSt).

[0035] (2) Preparation of thermoplastic-crosslinked starch (TP-XSt)

[0036] (2.1) Weigh 160 g of N,N-dimethylformamide (DMF) and 40 g of sorbitol (SO) in a beaker and mix uniformly to obtain a composite plasticizer.

[0037] (2.2) Place 200 g of composite plasticizer and 1 kg of cross-linked starch (XSt) in a high-speed mixer and mix at 1000 rpm for 20 min at room temperature;

[0038] (2.3) The product was melt-extruded through a twin-screw extruder at a screw speed of 200 r / min, a feed rate of 4 r / min, and an extrusion temperature zone of 120, 125, 130, 135, 140, 145, 150, 150, 150, 150, 140, and 145°C to obtain thermoplastic-cross-linked starch (TP-XSt).

[0039] (3) Preparation of modified starch composite materials with both thermoplastic and cross-linking properties

[0040] 200 g of thermoplastic cross-linked starch (TP-XSt) and 1 kg of polybutylene terephthalate adipate (PBAT) were placed in a high-speed mixer and mixed at room temperature and a speed of 1000 r / min for 20 min.

[0041] At a temperature of 190° C. and a mass of 2160 g, the polybutylene terephthalate-adipate has a melt index of 3.42 g / 10 min, a melting point of 118.49° C., and a carboxyl content of 13.27 mol / t.

[0042] The uniformly mixed material was then melt-extruded through a twin-screw extruder at a screw speed of 300 r / min, a feed rate of 5 r / min, and an extrusion temperature zone of 130, 135, 140, 145, 150, 155, 160, 160, 160, 160, 150, and 155° C. to obtain a thermoplastic-cross-linked starch modified polybutylene terephthalate-adipate composite material (PBAT / TP-XSt), i.e., a modified starch composite material with both thermoplastic and cross-linked properties;

[0043] The modified starch composite material with both thermoplastic and cross-linking properties prepared in Example 1 has a tensile strength of 22.53 MPa, a flexural strength of 4.26 MPa, an elongation at break of 650%, a melt index of 4.96 g / 10 min, and a crystallinity of 11.27%.

[0044] Example 2

[0045] The preparation steps of a modified starch composite material having both thermoplastic and cross-linking properties are as follows:

[0046] (1) Preparation of cross-linked starch (XSt)

[0047] 12.5 g of triphenylmethane triisocyanate (TTI) and 1 kg of corn starch (St) were placed in a high-speed mixer and mixed at a temperature of 90° C. and a rotation speed of 1000 r / min for 30 min to obtain cross-linked starch (XSt).

[0048] (2) Preparation of thermoplastic-crosslinked starch (TP-XSt)

[0049] (2.1) Weigh 160 g of N,N-dimethylformamide (DMF) and 40 g of sorbitol (SO) in a beaker and mix uniformly to obtain a composite plasticizer.

[0050] (2.2) Place 200 g of composite plasticizer and 1 kg of cross-linked starch (XSt) in a high-speed mixer and mix at 1000 rpm for 20 min at room temperature;

[0051] (2.3) The product was melt-extruded through a twin-screw extruder at a screw speed of 200 r / min, a feed rate of 4 r / min, and an extrusion temperature zone of 120, 125, 130, 135, 140, 145, 150, 150, 150, 150, 140, and 145°C to obtain thermoplastic-cross-linked starch (TP-XSt).

[0052] (3) Preparation of modified starch composite materials with both thermoplastic and cross-linking properties

[0053] 200 g of thermoplastic cross-linked starch (TP-XSt) and 1 kg of polybutylene terephthalate adipate (PBAT) were placed in a high-speed mixer and mixed at room temperature and a speed of 1000 r / min for 20 min.

[0054] At a temperature of 190° C. and a mass of 2160 g, the polybutylene terephthalate-adipate has a melt index of 3.42 g / 10 min, a melting point of 118.49° C., and a carboxyl content of 13.27 mol / t.

[0055] The uniformly mixed material was then melt-extruded through a twin-screw extruder at a screw speed of 300 r / min, a feed rate of 5 r / min, and an extrusion temperature zone of 130, 135, 140, 145, 150, 155, 160, 160, 160, 160, 150, and 155° C. to obtain a thermoplastic-cross-linked starch modified polybutylene terephthalate-adipate composite material (PBAT / TP-XSt), i.e., a modified starch composite material with both thermoplastic and cross-linked properties;

[0056] The modified starch composite material with both thermoplastic and cross-linking properties prepared in Example 2 has a tensile strength of 21.25 MPa, a flexural strength of 4.15 MPa, an elongation at break of 630%, a melt index of 5.38 g / 10 min, and a crystallinity of 10.85%.

[0057] Example 3

[0058] The preparation steps of a modified starch composite material having both thermoplastic and cross-linking properties are as follows:

[0059] (1) Preparation of cross-linked starch (XSt)

[0060] 10 g of triphenylmethane triisocyanate (TTI) and 1 kg of corn starch (St) were placed in a high-speed mixer and mixed at a temperature of 90° C. and a rotation speed of 1000 r / min for 30 min to obtain cross-linked starch (XSt).

[0061] (2) Preparation of thermoplastic-crosslinked starch (TP-XSt)

[0062] (2.1) Weigh 160 g of N,N-dimethylformamide (DMF) and 40 g of sorbitol (SO) in a beaker and mix uniformly to obtain a composite plasticizer.

[0063] (2.2) Place 200 g of composite plasticizer and 1 kg of cross-linked starch (XSt) in a high-speed mixer and mix at 1000 rpm for 20 min at room temperature;

[0064] (2.3) The product was melt-extruded through a twin-screw extruder at a screw speed of 200 r / min, a feed rate of 4 r / min, and an extrusion temperature zone of 120, 125, 130, 135, 140, 145, 150, 150, 150, 150, 140, and 145°C to obtain thermoplastic-cross-linked starch (TP-XSt).

[0065] (3) Preparation of modified starch composite materials with both thermoplastic and cross-linking properties

[0066] 200 g of thermoplastic cross-linked starch (TP-XSt) and 1 kg of polybutylene terephthalate adipate (PBAT) were placed in a high-speed mixer and mixed at room temperature and a speed of 1000 r / min for 20 min.

[0067] At a temperature of 190° C. and a mass of 2160 g, the polybutylene terephthalate-adipate has a melt index of 3.42 g / 10 min, a melting point of 118.49° C., and a carboxyl content of 13.27 mol / t.

[0068] The uniformly mixed material was then melt-extruded through a twin-screw extruder at a screw speed of 300 r / min, a feed rate of 5 r / min, and an extrusion temperature zone of 130, 135, 140, 145, 150, 155, 160, 160, 160, 160, 150, and 155° C. to obtain a thermoplastic-cross-linked starch modified polybutylene terephthalate-adipate composite material (PBAT / TP-XSt), i.e., a modified starch composite material with both thermoplastic and cross-linked properties;

[0069] The modified starch composite material with both thermoplastic and cross-linking properties prepared in Example 3 has a tensile strength of 20.05 MPa, a flexural strength of 4.03 MPa, an elongation at break of 600%, a melt index of 6.26 g / 10 min, and a crystallinity of 10.07%.

[0070] Comparative Example 1

[0071] 200 g of corn starch (St) and 1 kg of polybutylene terephthalate-adipate (PBAT) were weighed and placed in a high-speed mixer. The mixture was mixed at a speed of 1000 r / min for 20 min at room temperature. The uniformly mixed raw materials were melt-extruded through a twin-screw extruder at a screw speed of 300 r / min, a feeding rate of 5 r / min, and an extrusion temperature zone of 130, 135, 140, 145, 150, 155, 160, 160, 160, 160, 150, and 155 ° C to obtain a starch-modified polybutylene terephthalate-adipate composite material (PBAT / St).

[0072] Comparative Example 2

[0073] First, 15g of triphenylmethane triisocyanate (TTI) and 1kg of corn starch (St) were weighed and placed in a high-speed mixer, the high-speed mixer temperature was set to 900°C, the speed was set to 1000r / min, and high-speed mixing was performed for 30min to obtain XSt. Then, 200g of cross-linked starch (XSt) and 1kg of polybutylene terephthalate-adipate (PBAT) were weighed and placed in a high-speed mixer and mixed at a speed of 1000r / min for 20min at room temperature. Finally, the mixed raw materials were melt-extruded through a twin-screw extruder with a screw speed of 300r / min, a feed rate of 5r / min, and an extrusion temperature zone of 130, 135, 140, 145, 150, 155, 160, 160, 160, 160, 150, 155°C to obtain a cross-linked starch modified polybutylene terephthalate-adipate composite material (PBAT / XSt).

[0074] Comparative Example 3

[0075] First, 160 g of N,N-dimethylformamide (DMF) and 40 g of sorbitol (SO) were weighed and placed in a beaker and evenly mixed. The mixed composite plasticizer and 1 kg of corn starch (St) were placed in a high-speed mixer and mixed at 1000 r / min at room temperature for 20 min. The mixture was then melt-extruded through a twin-screw extruder with a screw speed of 200 r / min, a feeding rate of 4 r / min, and an extrusion temperature zone of 120, 125, 130, 135, 140, 145, 150, 150, 150, 150, 140, and 145°C to obtain thermoplastic starch (TPSt). Next, 200 g of thermoplastic starch (TPSt) and 1 kg of polybutylene terephthalate-adipate (PBAT) were weighed and placed in a high-speed mixer. The mixing speed was set to 5 r / min at room temperature at a speed of 1000 r / min, and the extrusion temperature zone was set to 130, 135, 140, 145, 150, 155, 160, 160, 160, 160, 150, and 155 ° C to obtain a thermoplastic starch modified polybutylene terephthalate-adipate composite material (PBAT / TPSt).

[0076] The present invention utilizes the reaction between isocyanate and hydroxyl groups to cross-link triphenylmethane triisocyanate (TTI) having multiple isocyanate groups with the hydroxyl groups on the starch molecules. The specific reaction formula is as follows: Figure 1 As shown in the figure, the hydroxyl content of starch decreases after cross-linking and a large number of carbamate bonds are generated, which increases the hydrophobicity and forms a multi-dimensional network structure. The intermolecular and intramolecular interactions are enhanced, and the tensile strength is improved. The molecular weight increases, and the chemical and physical properties are changed, effectively improving the various properties of starch.

[0077] After cross-linking, starch still has a certain amount of hydroxyl groups, and the hydrogen bonding between molecules and within molecules is strong. Moreover, the slippage between starch molecules is also restricted after cross-linking, resulting in poor plasticity of the material and difficulty in processing. Therefore, the present invention modifies starch by adding N, N-dimethylformamide (DMF) and sorbitol (SO) composite plasticizer, wherein the hydrogen bonding mechanism of the composite plasticizer is as follows: Figure 2 As shown. N,N-dimethylformamide (DMF), an amide plasticizer, penetrates the starch molecules through osmotic action. Its amide groups stabilize and disrupt the hydrogen bonds between native starch molecules while forming more stable hydrogen bonds with the starch hydroxyl groups. This increases the distance between starch molecules, thereby reducing intermolecular forces and achieving the purpose of plasticizing starch to produce thermoplastic starch while inhibiting its retrogradation. Sorbitol (SO) has a molecular structure more similar to that of glucose. Therefore, it interacts more strongly with starch molecular chains. Its larger molecular weight also increases starch steric hindrance. Furthermore, sorbitol forms a greater variety of hydrogen bonds with starch hydroxyl groups, providing a better plasticizing effect and further plasticizing the starch.

[0078] The present invention performs infrared detection and analysis on pure corn starch (St) samples and thermoplastic cross-linked starch (TP-XSt) samples. The results are as follows: Figure 3 As shown. Figure 3 It can be seen that at 1000cm -1 is the stretching vibration peak of the glycosidic bond COC of corn starch (St) sample and thermoplastic cross-linked starch (TP-XSt) sample; thermoplastic cross-linked starch (TP-XSt) at 1650 cm -1 The stretching vibration peak of carbamate appeared at 2260 cm-1, which proved that the isocyanate group in triphenylmethane triisocyanate (TTI) reacted with the hydroxyl group on starch to form a carbamate bond. -1 There is no obvious characteristic absorption peak of isocyanate group, indicating that the reaction of triphenylmethane triisocyanate (TTI) is complete; the center position of the hydroxyl peak of corn starch (St) is 3300 cm -1 The absorption peak is relatively broad, which is due to the high free hydroxyl groups in the starch molecular chain. After cross-linking modification, the peak area of the hydroxyl characteristic peak of thermoplastic cross-linked starch (TP-XSt) is reduced, indicating that the hydroxyl groups in the starch react with the active groups in the cross-linking agent, reducing the hydroxyl content in the starch. In addition, by adding N, N dimethylformamide (DMF) and sorbitol (SO) composite plasticizers, the hydrogen bonds between starch molecules are destroyed, and the hydroxyl peak vibration shifts to a higher wave number, from 3300 cm -1 Blue shift to 3326 cm -1 See also Figure 3 Infrared spectrum analysis showed that the isocyanate group of triphenylmethane triisocyanate (TTI) reacted with the hydroxyl group of starch, and the reaction between triphenylmethane triisocyanate (TTI) and starch produced a cross-linked structure; the composite plasticizer destroyed the intermolecular hydrogen bonds and caused the hydroxyl characteristic peak to blue-shift, with an obvious plasticizing effect.

[0079] The present invention conducts differential thermal scanning test on polybutylene terephthalate adipate (PBAT) sample, comparative example 1 sample (PBAT / St) and example 1 composite material sample (PBAT / TP-XSt). The results are as follows: Figure 4 As shown, Figure 4 (a) is the cooling crystallization process, Figure 4 (b) is the melting process; the crystallization parameters of the samples are shown in Table 1. Figure 4In the cooling crystallization curve shown in (a), pure polybutylene terephthalate-adipate (PBAT) shows a clear exothermic peak curve, and a strong crystallization peak is observed at 38.42°C. With the addition of corn starch (St) and thermoplastic-cross-linked starch (TP-XSt), the crystallization temperature of polybutylene terephthalate-adipate (PBAT) is significantly increased, indicating that the addition of corn starch (St) and thermoplastic-cross-linked starch (TP-XSt) promotes the crystallization of the material. Among them, the crystallization temperature of the composite material sample (PBAT / TP-XSt) of Example 1 reaches 84.64°C, and the crystallization effect is the best. This is because the amorphous phase of thermoplastic-cross-linked starch (TP-XSt) activates the chain mobility of polybutylene terephthalate-adipate (PBAT), which makes it easier for the chain movement to be arranged regularly, thereby improving the crystallization. Figure 4 In the heating melting curve shown in (b), PBAT has a glass transition at around -35.81°C, followed by a melting peak at 118.49°C. After adding corn starch (St) and thermoplastic-crosslinked starch (TP-XSt), the glass transition temperature (Tg) of PBAT moves toward the low temperature direction. The decrease in the glass transition temperature (Tg) indicates that thermoplastic-crosslinked starch (TP-XSt) reduces the movement of polybutylene terephthalate-adipate (PBAT) molecular chains. Polybutylene terephthalate-adipate (PBAT) and thermoplastic-crosslinked starch (TP-XSt) have high compatibility, and the glass transition temperature (Tg) reaches -38.69°C; the melting peak moves toward the high temperature direction, indicating that the interaction between thermoplastic-crosslinked starch (TP-XSt) molecules and polybutylene terephthalate-adipate (PBAT) molecular chains is stronger, the heat resistance is improved, and the melting temperature reaches 123.30°C.

[0080] Table 1 Crystallization parameters of samples

[0081]

[0082]

[0083] The crystallization parameters of the polybutylene terephthalate adipate (PBAT) sample, the comparative example 1 sample (PBAT / St) and the composite material sample of Example 1 (PBAT / TP-XSt) are shown in Table 1. As can be seen from Table 1, the composite material sample of Example 1 (PBAT / TP-XSt) has a higher melting enthalpy than the comparative example 1 sample (PBAT / St), and the composite material sample of Example 1 (PBAT / TP-XSt) has a higher crystallinity. This is because the molecular chain length of the thermoplastic-crosslinked starch (TP-XSt) after plasticization and cross-linking modification increases, and the interaction with polybutylene terephthalate adipate (PBAT) is enhanced; the hydrophilicity is reduced, and the compatibility with polybutylene terephthalate adipate (PBAT) is higher; the thermoplastic-crosslinked starch (TP-XSt) has strong plasticity, the molecular chain fluidity is enhanced, and the molecular chain movement and regular arrangement are promoted. Therefore, the composite material (PBAT / TP-XSt) obtained by blending thermoplastic-cross-linked starch (TP-XSt) with polybutylene terephthalate-adipate (PBAT) has high heat resistance and crystallization properties.

[0084] Table 2 Melt index of samples

[0085] sample Melt index (g / 10min) Blank example (pure raw material) 3.42 Comparative Example 1 10.25 Comparative Example 2 1.78 Comparative Example 3 8.24 Example 1 4.96

[0086] Melt index tests were conducted on polybutylene terephthalate-adipate (PBAT) samples, Comparative Example 1 sample (PBAT / St), Comparative Example 2 sample (PBAT / XSt), Comparative Example 3 sample (PBAT / TPSt), and the composite material sample (PBAT / TP-XSt) of Example 1. The results are shown in Table 2. As shown in Table 2, the melt index of the composite material sample (PBAT / TP-XSt) prepared in Example 1 was significantly lower than that of the samples (PBAT / St) and (PBAT / TPSt) of Comparative Example 3, but higher than that of the sample (PBAT / XSt) of Comparative Example 2. This is because polybutylene terephthalate-adipate has poor compatibility with starch, and the viscosity and melt strength of starch are reduced after simple modification with a plasticizer. The melt strength of the sample (PBAT / TPSt) of Comparative Example 3 is poor, but the melt index is higher. The melt index of the sample (PBAT / XSt) of Comparative Example 2, obtained by crosslinking starch, is significantly lower, but the flowability is poor, and the processing performance is reduced. Thermoplastic cross-linked starch (TP-XSt) not only increases the strength of corn starch (St) and improves its compatibility with polybutylene terephthalate-adipate (PBAT) by cross-linking it with triphenylmethane triisocyanate (TTI), but also enhances molecular chain mobility through plasticization with a composite plasticizer. As a result, the composite material (PBAT / TP-XSt) obtained by blending thermoplastic cross-linked starch (TP-XSt) with polybutylene terephthalate-adipate (PBAT) exhibits high melt strength and processing properties.

[0087] Table 3 Mechanical properties of samples

[0088] sample Tensile strength / Mpa Elongation at break / % Bending strength / Mpa Blank example (pure raw material) 18.22 600 3.13 Comparative Example 1 8.23 382 2.14 Comparative Example 2 15.28 498 3.86 Comparative Example 3 10.22 456 3.32 Example 1 22.53 650 4.26

[0089] The present invention conducted mechanical tests on polybutylene terephthalate-adipate (PBAT) samples, comparative example 1 sample (PBAT / St), comparative example 2 sample (PBAT / XSt), comparative example 3 sample (PBAT / TPSt) and the composite material sample (PBAT / TP-XSt) of Example 1, and the results are shown in Table 3. As can be seen from Table 3, the mechanical properties of the composite material sample (PBAT / TP-XSt) prepared in Example 1 are greatly improved compared to the polybutylene terephthalate-adipate (PBAT) samples, comparative example 1 sample (PBAT / St), comparative example 2 sample (PBAT / XSt) and comparative example 3 sample (PBAT / TPSt). This is because corn starch (St) is modified with a composite plasticizer after cross-linking, and a multidimensional network structure is first formed between the corn starch (St) molecules, which increases the mechanical strength of the starch. The composite plasticizer then destroys the hydrogen bonds between the cross-linked starch molecules and forms more stable hydrogen bonds, thereby expanding the steric hindrance between the molecules and enhancing the mobility of the molecular chains, thereby enhancing its thermoplasticity. Therefore, the composite material (PBAT / TP-XSt) obtained by blending thermoplastic-cross-linked starch (TP-XSt) with poly(butylene terephthalate-adipate) (PBAT) has higher tensile strength, elongation at break and flexural strength.

[0090] It is easy for those skilled in the art to understand that the above embodiments 1-3 are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a modified starch composite material having both thermoplasticity and cross-linking properties, characterized in that: The steps are as follows: (1) Preparation of cross-linked starch Triphenylmethane triisocyanate and corn starch are mixed at a mass ratio of 1-1.5:100 in a high-speed mixer at a temperature of 90-110° C. to react to obtain cross-linked starch; (2) Preparation of thermoplastic-cross-linked starch (2.1) Evenly mixing N,N-dimethylformamide and sorbitol in a mass ratio of 3-5:1 to obtain a composite plasticizer; (2.2) placing the composite plasticizer and the cross-linked starch in a high-speed mixer at a mass ratio of 20-30:100 and mixing at room temperature; (2.3) Melting and extruding the resulting starch through a twin-screw extruder to obtain thermoplastic cross-linked starch; (3) Preparation of modified starch composite materials with both thermoplastic and cross-linking properties Thermoplastic cross-linked starch and polybutylene terephthalate adipate are mixed at a mass ratio of 10-30:100 at room temperature and high speed; The mixture is then melt-extruded through a twin-screw extruder to obtain a thermoplastic-cross-linked starch modified polybutylene terephthalate-adipate composite material, that is, a modified starch composite material having both thermoplastic and cross-linking properties; At a temperature of 190° C. and a mass of 2160 g, the polybutylene terephthalate-adipate has a melt index of 3.42 g / 10 min, a melting point of 118.49° C., and a carboxyl content of 13.27 mol / t; The modified starch composite material with both thermoplastic and cross-linking properties has a tensile strength of 20.05-22.53 MPa, a flexural strength of 4.03-4.26 MPa, an elongation at break of 600-650%, a melt index of 4.96-6.26 g / 10 min, and a crystallinity of 10.07-11.27%.

2. The preparation method according to claim 1, wherein: In step (1), the high-speed mixing speed is 1000-1500 r / min and the time is 20-30 min.

3. The preparation method according to claim 1, wherein: In step (2.2), the high-speed mixing speed is 1000-1500 r / min and the time is 10-20 min.

4. The preparation method according to claim 1, wherein: In step (2.3), the screw speed of the twin-screw extruder is 100-200 r / min, the feeding rate is set to 3-5 r / min, and the extrusion temperature zones are set to 120, 125, 130, 135, 140, 145, 150, 150, 150, 150, 140, and 145 °C.

5. The preparation method according to claim 1, wherein: In step (3), the high-speed mixing condition at room temperature is: mixing at a rotation speed of 1000 r / min for 20 min.

6. The preparation method according to claim 1, wherein: In step (3), the screw speed of the twin-screw extruder is 300 r / min, the feeding rate is 5 r / min, and the extrusion temperature zones are set to 130, 135, 140, 145, 150, 155, 160, 160, 160, 160, 150, and 155 °C.

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