Oxidized-crosslinked modified starch with anti-retrogradation properties, preparation method and application in PBAT

Abstract

The present invention discloses an oxidized-crosslinked modified starch with anti-retrogradation, a preparation method and an application in PBAT. The preparation method of the oxidized-crosslinked modified starch has the following steps: S1: dissolving starch in water to prepare starch milk, adding Dess-Martin oxidant to the starch milk, washing and drying the product to obtain oxidized starch; S2: dissolving tetraethylenepentamine and glycerol triglycidyl ether in dimethyl sulfoxide and heating at a constant temperature to synthesize a crosslinking agent; S3: mixing the oxidized starch, the crosslinking agent and the plasticizer in a high-speed mixer and then extruding the oxidized-crosslinked modified starch through a twin-screw extruder. The present invention synthesizes a crosslinking agent with both crosslinking and plasticizing functions, which can improve the crosslinking efficiency and form a stable hydrogen bond, thereby inhibiting the retrogradation of starch. In addition, the oxidized starch and the crosslinking agent react with a Schiff base to form a dynamic crosslinking network inside the starch molecules, which can improve the thermal stability of the starch.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer material modification, and in particular to an oxidized-crosslinked modified starch with anti-retrogradation properties, a preparation method thereof, and an application in PBAT. Background Art

[0002] Polybutylene terephthalate-adipate (PBAT) is a commercially available fully biodegradable material widely used in disposable packaging, agricultural mulch and plastic packaging. At present, PBAT has been commercialized, but its promotion and use are limited due to its high cost. Therefore, reducing its material cost has become a hot topic in current research. In addition, starch is a natural polymer with the advantages of low cost and renewability, so using starch to fill PBAT is an effective strategy to reduce costs. Due to the presence of a large number of hydroxyl groups in starch molecules, its viscosity flow temperature is greater than the decomposition temperature, it is not thermoplastic, and it is difficult to melt process. At the same time, starch has the problem of poor mechanical properties, which further limits its application in packaging and daily fields; secondly, starch is prone to retrogradation during use or storage, which causes changes in the hardness, viscosity, mechanical properties, etc. of starch, affecting its use. Therefore, it is particularly important to modify starch as necessary.

[0003] Related studies have shown that by modifying starch, the retrogradation of starch can be inhibited and its application value can be improved. At present, common starch modification methods include physical modification, chemical modification, biological modification and other methods. At present, chemical methods are often used to modify starch. Chemical modification is a modification method that changes its molecular structure and properties by introducing chemical reactions. Different chemical modifications can meet different needs of starch. For example, esterified starch is generated by esterification reaction between hydroxyl groups in starch and introduced carboxyl groups. This modification can improve the hydrophobicity of starch and reduce the hydrogen bonding effect within starch molecules; for example, using hydrogen peroxide to oxidatively modify starch to obtain dialdehyde starch. This modification method reduces the molecular weight of starch and weakens the hydrogen bonding effect between starch molecules, which can improve the fluidity and thermal stability of starch molecules. The above common modification methods are all single chemical modifications, and the hydrogen bonding effect generated within the molecule is not obvious. At the same time, there are problems of poor modification effect and low modification efficiency, which still restricts its application in PBAT. Summary of the invention

[0004] Based on the technical problems existing in the background technology, the present invention proposes an oxidized-cross-linked modified starch with anti-retrogradation, a preparation method and an application in PBAT. The oxidized-cross-linked modified starch has high strength, high toughness and excellent anti-retrogradation performance, and significantly improves the processing performance of the composite material after blending with PBAT.

[0005] The method for preparing the oxidized-cross-linked modified starch with anti-retrogradation property proposed by the present invention comprises the following steps:

[0006] S1: dissolving starch in water, adding a Dess-Martin oxidant to react, and washing and drying the product to obtain oxidized starch;

[0007] S2: dissolving tetraethylenepentamine and propylene glycol triglycidyl ether in dimethyl sulfoxide, and heating the mixture to react to obtain a cross-linking agent;

[0008] S3: The oxidized starch, the cross-linking agent and the plasticizer are mixed at high speed and then melt-extruded to obtain an oxidized-cross-linked modified starch having anti-retrogradation properties.

[0009] Preferably, the mass ratio of starch to Dess-Martin oxide in S1 is 100:1-1.5.

[0010] Preferably, the reaction temperature in S1 is 40-60°C and the reaction time is 2-4h.

[0011] Preferably, the mass ratio of tetraethylenepentamine to propylene glycol triglycidyl ether in S2 is 1-1.5:2-3.

[0012] Preferably, the temperature of the heating reaction in S2 is 120-140° C., and the reaction time is 2-4 h.

[0013] Preferably, the plasticizer in S3 is one or more of glycerol, sorbitol, urea, epoxy soybean oil and acetylated tributyl citrate.

[0014] Preferably, the mass ratio of oxidized starch, cross-linking agent and plasticizer is 100:1-1.5:20-30.

[0015] Preferably, the conditions for melt extrusion in S3 are: the screw speed is set to 100-200r / min, the feeding rate is set to 3-5r / min, and the extrusion temperature zone is set to 120, 125, 130, 135, 140, 145, 150, 150, 150, 150, 140, 145°C.

[0016] The oxidized-cross-linked modified starch with anti-retrogradation property provided by the present invention is prepared by adopting the above-mentioned preparation method.

[0017] The present invention proposes the use of the above-mentioned oxidation-cross-linked modified starch with anti-retrogradation in PBAT.

[0018] The preparation method of the modified PBAT proposed in the present invention comprises the following steps: melt-extruding polybutylene terephthalate-adipate and the above-mentioned oxidation-cross-linked modified starch with anti-retrogradation properties to obtain modified polybutylene terephthalate-adipate.

[0019] Preferably, the mass ratio of polybutylene terephthalate-adipate to oxidized-cross-linked modified starch is 100:10-30.

[0020] Preferably, the conditions for melt extrusion are: the screw speed is set to 200-300r / min, the feeding rate is set to 5-8r / min, and the extrusion temperature zone is set to 130, 135, 140, 145, 150, 155, 160, 160, 160, 160, 150, 155°C.

[0021] Beneficial technical effects of the present invention:

[0022] The invention proposes a cross-linking agent with both cross-linking and plasticizing functions, which is synthesized based on the amino-epoxy ring-opening reaction of tetraethylenepentamine and glycerol triglycidyl ether. The cross-linking agent can improve the cross-linking efficiency and generate a stable hydrogen bond, thereby realizing the anti-retrogradation of starch.

[0023] The present invention establishes an oxidation-crosslinking dual modification process, which forms a complex dynamic crosslinking network within the starch molecules, effectively improving the hydrophobicity and thermal stability of the starch, thereby improving the thermal processing performance between the starch and PBAT. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Infrared spectra of St, D-St, TT and DT-St proposed in the present invention;

[0025] Figure 2 The crystallization / melting curve comparison diagram of St, D-St, PBAT and PBAT / DT-St proposed in the present invention; wherein (a) is the temperature rise melting process of St and DT-St, (b) is the temperature rise melting process of PBAT and PBAT / DT-St, and (c) is the temperature drop crystallization process of PBAT and PBAT / DT-St;

[0026] Figure 3 The starch oxidation mechanism proposed by the present invention;

[0027] Figure 4 The synthesis mechanism of the cross-linking agent TT proposed in the present invention;

[0028] Figure 5 This is the preparation mechanism of the oxidative-cross-linked modified starch proposed by the present invention. DETAILED DESCRIPTION

[0029] The present invention will be further explained below in conjunction with specific embodiments.

[0030] The starch (St) of the present invention can be corn starch, cassava starch, potato starch, mung bean starch, etc.; the starch used in the specific implementation manner is corn starch, with a mesh size of 100 mesh, a moisture content of 9.69%, and a whiteness of 89.9%; the PBAT selected in the specific implementation manner of the present invention has a melt index (190°C, 2160g) of 3.42g / 10min, a melting point of 120.49°C, and a carboxyl content of 13.27mol / t.

[0031] Example 1

[0032] Weigh 200g of dry starch, use distilled water to configure the starch into a starch milk with a mass ratio of 30%, add 3.0g of Dess-Martin oxidant (DMP oxidant) to the starch emulsion, and use 6% sodium hydroxide solution to adjust the pH value of the reaction environment. The temperature of the constant temperature water bath is adjusted to 40°C, and the starch milk is placed in a three-necked flask (the constant temperature water bath is heated and the starch milk is configured at the same time), and the reaction is carried out for 3h. After the reaction is completed, a saturated sodium bicarbonate solution is added dropwise to terminate the reaction. The reactant is repeatedly centrifuged and washed with a large amount of distilled water until the pH value of the starch milk is 7. The washed product is placed in a glass dish and placed in an oven. The oven temperature is set to 50°C and dried for 6h to obtain oxidized starch, which is recorded as D-St-1.

[0033] 0.50 g of tetraethylenepentamine (TEPA) and 1.50 g of propylene glycol triglycidyl ether (TPEG) were weighed and dissolved in a dimethyl sulfoxide solution and heated at 140° C. in a three-necked flask to obtain a cross-linking agent, which was recorded as TT-1.

[0034] Weigh 200g D-St-1, 2g cross-linking agent TT-1 and 40g glycerol, mix in a high-speed mixer at 500r / min at room temperature for 10min, and then melt extrude through a twin-screw extruder, with the screw speed set to 100r / min, the feeding rate set to 3r / min, and the extrusion temperature zone set to 120, 125, 130, 135, 140, 145, 150, 150, 150, 150, 140, 145°C to obtain thermoplastic oxidation-cross-linked modified starch, recorded as DT-St-1.

[0035] Weigh 200g DT-St-1 and 2kg polybutylene terephthalate-adipate (PBAT) and place them in a high-speed mixer, mix them at 500r / min for 10min at room temperature, and then melt-extrude the evenly mixed raw materials through a twin-screw extruder. The screw speed is set to 300r / min, the feeding rate is set to 5r / min, and the extrusion temperature zone is set to 130, 135, 140, 145, 150, 155, 160, 160, 160, 160, 150, 155°C to obtain a modified PBAT composite material, recorded as PBAT / DT-St-1.

[0036] Example 2

[0037] 0.625 g of TEPA and 1.875 g of TPEG were weighed and dissolved in a dimethyl sulfoxide solution and heated at 140° C. in a three-necked flask to obtain a cross-linking agent, which was recorded as TT-2.

[0038] Weigh 200g D-St-1, 2.5g cross-linking agent TT-2 and 40g glycerol in a high-speed mixer at room temperature and 500r / min for 10min, then melt-extrude through a twin-screw extruder, with the screw speed set to 150r / min, the feeding rate set to 4r / min, and the extrusion temperature zone set to 120, 125, 130, 135, 140, 145, 150, 150, 150, 150, 140, 145°C to obtain thermoplastic oxidation-cross-linked modified starch, recorded as DT-St-2.

[0039] Weigh 200g D-T-St-2 and 1kg PBAT into a high-speed mixer, mix at a speed of 500r / min for 10min at room temperature, and then melt-extrude the uniformly mixed raw materials through a twin-screw extruder. The screw speed is set to 300r / min, the feeding rate is set to 5r / min, and the extrusion temperature zone is set to 130, 135, 140, 145, 150, 155, 160, 160, 160, 160, 150, 155°C to obtain a modified PBAT composite material, recorded as PBAT / DT-St-2.

[0040] Example 3

[0041] 0.75 g of TEPA and 2.25 g of TPEG were weighed and dissolved in a dimethyl sulfoxide solution and heated at 140° C. in a three-necked flask to obtain a cross-linking agent, which was recorded as TT-3.

[0042] Weigh 200g D-St-1, 3.0g cross-linking agent TT-3 and 40g glycerol in a high-speed mixer at room temperature and 500r / min for 10min, and then melt-extrude through a twin-screw extruder, with the screw speed set to 200r / min, the feeding rate set to 5r / min, and the extrusion temperature zone set to 120, 125, 130, 135, 140, 145, 150, 150, 150, 150, 140, 145°C to obtain thermoplastic oxidation-cross-linked modified starch, recorded as DT-St-3.

[0043] Weigh 200g DT-St-3 and 1kg PBAT into a high-speed mixer, mix at a speed of 500r / min for 10min at room temperature, and then melt-extrude the uniformly mixed raw materials through a twin-screw extruder. The screw speed is 300r / min, the feeding speed is set to 5r / min, and the extrusion temperature zone is set to 130, 135, 140, 145, 150, 155, 160, 160, 160, 160, 150, 155°C to obtain a modified PBAT composite material, recorded as PBAT / DT-St-3.

[0044] Comparative Example 1

[0045] Weigh 200g of dry starch and 1kg of PBAT into a high-speed mixer, mix at 500r / min for 10min at room temperature, and then melt-extrude the uniformly mixed raw materials through a twin-screw extruder to prepare a PBAT / St masterbatch, wherein the screw speed is set to 300r / min, the feeding rate is set to 5r / min, and the extrusion temperature zone is set to 130, 135, 140, 145, 150, 155, 160, 160, 160, 160, 150, 155°C to obtain a modified PBAT composite material, recorded as PBAT / St.

[0046] Comparative Example 2

[0047] Weigh 200g D-St-1, 20g glycerol and 1kg PBAT in a high-speed mixer, mix at 500r / min for 10min at room temperature, and then melt-extrude the uniformly mixed raw materials through a twin-screw extruder to prepare a PBAT / D-St masterbatch, wherein the screw speed is set to 300r / min, the feeding rate is set to 5r / min, and the extrusion temperature zone is set to 130, 135, 140, 145, 150, 155, 160, 160, 160, 160, 150, 155°C to obtain a modified PBAT composite material, recorded as PBAT / D-St.

[0048] The present invention conducts mechanical property tests on the samples prepared in Examples 1-3 and Comparative Examples 1-2 and the pure PBAT samples, and the results are shown in Table 1. Among them, the standard for the tensile strength and elongation at break test is GB / T1040.1-2018 "Plastics - Determination of tensile properties".

[0049] Table 1 Mechanical properties of samples

[0050]

[0051] As shown in Table 1, the sample prepared in Example 1 has the best mechanical properties, with a tensile strength of 28.95 MPa and an elongation at break of 810%. Overall, the tensile strength of the PBAT / DT-St-1 sample prepared in Example 1 is reduced from 32.11 MPa to 28.95 MPa, a decrease of 9%, and the elongation at break is increased from 760% to 810%, an increase of 6.6%, relative to the pure PBAT sample. In Example 1, 20% modified starch is filled. Since the original starch and PBAT are incompatible systems, the reduction of mechanical properties is an inevitable trend; however, filling with 20% modified starch effectively reduces the material cost, and the performance far exceeds the daily use requirements.

[0052] Among them, the mechanical properties of the samples in Examples 2 and 3 are lower than those in Example 1, which is mainly due to excessive crosslinking. From the comparative example, it can be seen that the mechanical properties of the unmodified and oxidatively modified samples are poor, because the unmodified starch and the single modified starch have poor modification effects and poor hydrophobic properties, resulting in poor compatibility with PBAT.

[0053] The present invention has carried out infrared detection and analysis on St, D-St-1, TT-1 and DT-St-1, and the results are as follows Figure 1 As shown. Figure 1 It can be seen that 1000cm -1 The stretching vibration peak of the glycosidic bond COC of St, D-St-1 and DT-St-1 samples. Compared with pure starch, oxidized starch has a higher peak at 3430 cm -1 The broad peak at 1751cm is somewhat weakened, which is related to the stretching vibration of OH. After starch is oxidized, the hydroxyl groups in the molecule are converted into carbonyl groups, which will cause the peak intensity of OH to weaken. In addition, the peak at 1751cm of D-St-1 sample is -1 A strong carbonyl stretching vibration absorption peak appeared, indicating that the starch was successfully oxidized.

[0054] From the infrared spectrum of the crosslinking agent TT-1 sample, we can see that 1664cm -1 It is the characteristic peak formed by the in-plane deformation vibration of -NH- on the amide group, 3380cm -1 Corresponding to the stretching vibration peak of -NH-. And at 910cm -1 No obvious epoxy characteristic absorption peak was observed at 3400 cm -1 Obvious hydroxyl stretching vibration peaks were observed, which proved the presence of crosslinker TT-1. From the infrared spectrum of DT-St-1 sample, it can be seen that compared with St and DT-St-1 samples, the peak at 3430 cm -1 The peak area at 1060cm-1 decreased, indicating that the absorption peak intensity of the hydroxyl group decreased, which further proved that the starch was successfully modified.-1 The CN stretching vibration appears at 1558cm -1 The bending vibration peak of NH appears at 1751 cm -1 Blue shift to 1780cm -1 , which indicates that a Schiff base reaction occurred between D-St-1 and TT-1.

[0055] The present invention performs differential scanning calorimetry analysis on St, D-St-1, PBAT and PBAT / DT-St-1 samples. The results are as follows: Figure 2 As shown. Part (a) shows the melting process of St and DT-St-1 during heating, part (b) shows the melting process of PBAT and PBAT / DT-St-1 during heating, and part (c) shows the crystallization process of PBAT and PBAT / DT-St-1 during cooling. Figure 2 In part (a), St has a strong melting peak between 100-130°C, however, DT-St-1 has a strong melting peak between 165-180°C, which indicates that DT-St-1 has stronger thermal stability; among them, the melting temperature of St is 120.06°C, and the melting temperature of DT-St-1 is 174.85°C, which further indicates that the thermal stability of starch is greatly improved after oxidation-cross-linking modification.

[0056] Table 2 Crystallization parameters of samples

[0057]

[0058] The details of the crystallization parameters of PBAT and PBAT / DT-St-1 samples are shown in Table 2. As can be seen from Table 2, the PBAT / DT-St-1 sample has a higher melting enthalpy than pure PBAT, and the crystallinity of the PBAT / DT-St-1 sample is greatly improved compared with pure PBAT, which is mainly due to the heterogeneous nucleation of starch. Therefore, the PBAT / DT-St-1 sample has better thermal stability and crystallization performance.

[0059] Figure 3 This is the oxidation mechanism proposed by the present invention. By reducing the number of hydroxyl groups in the starch molecules after oxidation modification, its hydrophobicity can be improved. At the same time, a large number of hydroxyl groups in the starch molecules are oxidized into aldehyde groups, which can improve the thermal stability of the starch molecules; in addition, since the molecular weight of the oxidatively modified starch is reduced, the fluidity of the starch molecules can be improved. Figure 4 This is the synthesis mechanism of the cross-linking agent proposed by the present invention. Figure 5This is the oxidation-crosslinking modification mechanism proposed by the present invention. During the crosslinking modification of oxidized starch, the amino groups in the crosslinking agent react with the aldehyde groups in the oxidized starch to form a dynamic crosslinking network. In addition, the crosslinking agent also acts as a plasticizer, which penetrates into the starch molecules through penetration, increases the distance between the starch molecules, and improves the fluidity of the starch molecules; at the same time, the crosslinking agent can form more stable hydrogen bonds between the starch molecules, thereby playing an anti-retrogradation role.

Claims

1. A method for preparing an oxidized-crosslinked modified starch having anti-retrogradation properties, characterized in that: The steps are as follows: S1: dissolving starch in water, adding a Dess-Martin oxidant to react, and washing and drying the product to obtain oxidized starch; S2: dissolving tetraethylenepentamine and propylene glycol triglycidyl ether in dimethyl sulfoxide, and heating the mixture to react to obtain a cross-linking agent; S3: mixing the oxidized starch, the cross-linking agent and the plasticizer at high speed and then melt-extruding to obtain an oxidized-cross-linked modified starch with anti-retrogradation properties; The mass ratio of tetraethylenepentamine to glycerol triglycidyl ether in S2 is 1-1.5:2-3; The mass ratio of oxidized starch, cross-linking agent and plasticizer in S3 is 100:1-1.5:20-30.

2. The method for preparing the oxidized-cross-linked modified starch with anti-retrogradation properties according to claim 1, characterized in that: The mass ratio of starch to Dess-Martin oxidant in S1 is 100:1-1.

5.

3. The method for preparing the oxidized-crosslinked modified starch with anti-retrogradation properties according to claim 1, characterized in that: The reaction temperature in S1 is 40-60°C and the reaction time is 2-4h.

4. The method for preparing the oxidized-cross-linked modified starch with anti-retrogradation properties according to claim 1, characterized in that: The temperature of the heating reaction in S2 is 120-140°C, and the reaction time is 2-4h.

5. The method for preparing the oxidized-cross-linked modified starch with anti-retrogradation properties according to claim 1, characterized in that: The plasticizer in S3 is one or more of glycerol, sorbitol, urea, epoxy soybean oil and acetylated tributyl citrate.

6. The method for preparing the oxidized-crosslinked modified starch with anti-retrogradation properties according to claim 1, characterized in that: The conditions for melt extrusion in S3 are: the screw speed is set to 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, 145°C.

7. An oxidized-crosslinked modified starch having anti-retrogradation properties, characterized in that: The method is prepared by the preparation method according to any one of claims 1 to 6.

8. Use of the oxidized-crosslinked modified starch with anti-retrogradation properties as claimed in claim 7 in PBAT.

9. A method for preparing modified PBAT, characterized in that: The method comprises the following steps: melt-extruding polybutylene terephthalate adipate and the oxidation-cross-linked modified starch with anti-retrogradation properties as claimed in claim 7 to obtain modified polybutylene terephthalate adipate; The mass ratio of polybutylene terephthalate adipate to oxidized-crosslinked modified starch is 100:10-30; The conditions for melt extrusion are: the screw speed is set to 200-300 r / min, the feeding rate is set to 5-8 r / min, and the extrusion temperature zones are set to 130, 135, 140, 145, 150, 155, 160, 160, 160, 160, 150, 155°C.

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