CN115926401A - In situ core-shell starch-reinforced and toughened polyesters formed during reactive extrusion to urea-formaldehyde - Google Patents

Abstract

The invention relates to the technical field of biodegradable polymers, in particular to a polyester reinforced and toughened by forming core-shell starch in situ during reactive extrusion to generate urea-formaldehyde. Modified starch with a very low amount of urea-formaldehyde reaction precursor hydroxymethyl urea solution to obtain plasticized starch, and then extruded its mixture with biodegradable polyester through a screw extruder. During the extrusion process, high temperature and high Under the action of shearing, methylol urea preferentially generates urea-formaldehyde at the interface between the two phases through polycondensation reaction, and interacts with the active functional groups on the surface of starch and polyester through the hydrophilic groups on its macromolecules. Partitioned interfacial tension spontaneously forms starch granules in the polyester matrix during reactive extrusion - core-shell structured particles of polyester/urea-formaldehyde/thermoplastic starch, which can be obtained by conventional industrial extrusion processes while having excellent processing Low-cost biodegradable polyester/thermoplastic starch blends with properties, mechanical properties and biodegradability.

Description

In situ core-shell starch-reinforced and toughened polyesters formed during reactive extrusion to urea-formaldehyde

technical field

The invention relates to the field of biodegradable polymers, in particular to a polyester reinforced and toughened by in-situ formation of core-shell starch during reactive extrusion to generate urea-formaldehyde.

Background technique

A large amount of non-degradable plastic waste has caused serious environmental pollution all over the world. In order to effectively control plastic pollution, nearly 90 countries and regions around the world have introduced policies to control or prohibit the use of disposable non-degradable plastic products. The European Union will ban or restrict the use of ten types of single-use plastic products starting in 2021. In 2020, China promulgated the "Opinions on Further Strengthening Plastic Pollution Control", proposing to ban or restrict the use of non-degradable plastic products such as disposable plastic products and express plastic packaging in steps and fields, and encourage the use of degradable plastic products.

Biodegradable plastics refer to polymer materials that can be degraded by microorganisms existing in nature and finally completely converted into CO ₂ , water, mineralized inorganic salts of contained elements, and new substances. At present, the chemically synthesized biodegradable polymer materials widely studied and used at home and abroad are mainly polyesters containing ester groups that can be decomposed by microorganisms or enzymes in the molecular chain, including aliphatic polyesters and aliphatic-aromatic copolyesters. The representative ones are polylactic acid, polybutylene succinate, carbon dioxide-propylene oxide copolymer, polycaprolactone, polydioxanone, polyfurandicarboxylic acid, polyadipate terephthalate Butylene glycol formate. However, in order to ensure biodegradability, biodegradable polyester requires a high proportion of amorphous structure, so the crystallinity is low, the mechanical properties are poor, and its processing performance is also affected, which restricts the application of biodegradable polyester scope. Therefore, at present, how to improve the mechanical properties of biodegradable polyester is one of the key points to expand and promote its application. In addition, the high price of biodegradable plastics further restricts the widespread application of biodegradable polymers.

Starch is currently a useful material in the field of thermoplastics due to its biodegradability, non-toxicity, high purity, and low cost. However, since the starch molecular chain contains a large number of hydroxyl groups, there are strong hydrogen bond interactions between the macromolecules, so it has good hydrophilicity, but this property also makes it sensitive to water and difficult to process. . In addition, starch itself has poor mechanical properties and cannot replace commercial non-degradable plastics. In order to improve the processing performance and mechanical properties of starch, various thermoplastic starches or blends with other polymers can be used to expand the application range of starch after thermoplasticizing starch with appropriate technology.

A promising solution for making biodegradable polyesters cost-effective is to blend them with thermoplastic starch in proportions of 20-30 wt%. In many studies so far, starch has been added to biodegradable polyester matrices to reduce costs and improve composite properties. However, the poor compatibility between the hydrophobic biodegradable polyester and the hydrophilic starch leads to a significant decrease in the performance of the composite. The addition of reactive compatibilizers is the first choice to alleviate insufficient interfacial adhesion between the biodegradable polyester and thermoplastic starch phases while maintaining an acceptable balance between strength, stiffness, and elongation at break. Reactive melt blending is carried out in the presence of a compatibilizer, the compatibilizer will link the components through covalent bonds and significantly improve the interfacial adhesion, which contributes to the effective stress transfer between the component phases, thereby Improving the mechanical properties of biodegradable polyester/thermoplastic starch composites. A variety of reactive compatibilizers for biodegradable polyester/thermoplastic starch blends have been reported, the representative ones mainly include organic acids, maleic anhydride (MAH) and glycidyl methacrylate class (GMA). The carboxyl groups of organic acids, the anhydride groups of MAH, and the epoxy groups of GMA can improve the mechanical properties of biodegradable polyester/thermoplastic starch blends by enhancing the interfacial adhesion between the two phases. However, low-molecular-weight organic acids are difficult to sustainably interact with the polyester matrix, resulting in limited compatibility. The grafting efficiency of MAH on biodegradable polyesters is relatively low. The thermal polymerization products of GMA are complicated. With the increase of GMA, a large number of by-products will be formed during the mixing process to act as "plasticizers", which will reduce the strength of the material. Therefore, to enhance the interaction between lipophilic biodegradable polyesters and hydrophilic thermoplastic starches, a more effective compatibilizer or compatibilization strategy is highly necessary.

At present, a large number of studies have confirmed that the composite form of "core-shell" can exert the synergistic effect of the two materials, and finally realize the rigidity and toughness of the modified polymer. Core-shell structure impact modifiers have a relatively hard core or shell component, so they can effectively toughen materials while maintaining high strength and modulus, and have been widely used in many polymer systems. On this basis, the toughening method of in situ formation of core-shell structure particles by melt blending has been gradually developed. Using the interfacial tension between polymers as the driving force to spontaneously form core-shell particles in the blend system can ensure better stress transfer between the dispersed phase and the matrix, so that the material can be toughened and strengthened at the same time.

Contents of the invention

In order to enhance the interfacial interaction between lipophilic biodegradable polyester and hydrophilic thermoplastic starch, the invention provides a process for in-situ formation of core-shell starch particles in the process of reactive extrusion to form urea-formaldehyde to reinforce and toughen polyester.

The present invention is achieved through the following technical solutions: a process for in-situ formation of core-shell starch particles in the process of generating urea-formaldehyde by reactive extrusion, comprising the following steps:

(1) Mix the hydroxymethyl urea solution with the dry starch powder until there are no solid particles, then add the mixture to a kneader and knead it into a uniform powder without crystal points; keep the material sealed to form a homogeneous stable system, and obtain a plasticized starch;

(2) Add the dried biodegradable polyester and the plasticized starch prepared in step (1) into the high-speed mixer, and at the same time add a certain amount of compatibilizer maleic anhydride, mix and add to the twin-screw extruder During extrusion at a certain speed and temperature, the small molecule precursor methylol urea in the plasticized starch generates urea-formaldehyde polymer in situ during the extrusion process, and at the same time due to the interaction between biodegradable polyester, thermoplastic starch and urea-formaldehyde The interfacial tension of the biodegradable polyester matrix forms a starch granule-polyester matrix in which polyester macromolecular chains, urea-formaldehyde macromolecular chains and thermoplastic starch macromolecular chains interpenetrate and penetrate each other as the shell and starch granules as the core. Core-shell structure particles of ester/urea-formaldehyde/thermoplastic starch; extruded strips are cooled and pelletized by a pelletizer to obtain polyester blends.

In the step (1) of the present invention, the reaction precursor hydroxymethyl urea of the urea-formaldehyde polymer, which is a small molecule that is easy to diffuse and infiltrate, added in the form of a solution, due to its own viscosity and the strong hydrogen bond interaction between it and the starch macromolecule , so only the outer layer of starch granules is infiltrated, and the intermolecular and intramolecular hydrogen bonds in the starch infiltrated layer are broken through hydrogen bond interaction, thereby significantly improving the processing performance of starch; but at the same time, the strong hydrogen between the two The bond interaction also makes it possible that under the temperature and shear of the mixing process, hydroxymethyl urea does not penetrate further into the interior of the starch like other currently used plasticizers and replaces the intramolecular and The intermolecular hydrogen bond thus destroys the crystal structure of starch, but prevents the original rigid structure of starch granules from being destroyed, and finally makes the polyester blend both rigid and tough.

In the present invention, under the action of high temperature and high shear in the extrusion process, the hydrogen bonds between the starch macromolecules and intramolecules in the plasticized starch are destroyed, and the crystal structure in the starch granules is disintegrated by melting and shearing, forming starch high A disordered continuous phase of molecules to obtain thermoplastic starch.

In the step (2) of the present invention, the small molecule precursor hydroxymethyl urea in the plasticized starch is preferentially formed in the two phases of biodegradable polyester and starch through polycondensation reaction under the action of high temperature and high shear in the extrusion process. The urea-formaldehyde polymer is generated in situ at the interface, and then due to the interfacial tension between the biodegradable polyester, thermoplastic starch and urea-formaldehyde polymer, polyester macromolecular chains, urea-formaldehyde macromolecular chains and thermoplasticity are formed in the biodegradable polyester matrix. Starch granule-polyester/urea-formaldehyde/thermoplastic starch core-shell structure particle with interpenetrating and interpenetrating network structure of starch macromolecular chains as shell and starch granules as core. In the extrusion process, the macromolecular urea-formaldehyde polymer produced in situ by polycondensation reaction between the two-phase interface between the two phases of methylolurea, not only acts as a plasticizing compatibilizer, but also has excellent biodegradability. At the same time, the nutrient element nitrogen can be released to increase the activity of microorganisms. Therefore, the polyester blend prepared by the process of the present invention not only does not contain non-biodegradable polymers, but also has further optimized biodegradable properties.

As a further improvement of the technical solution of the present invention, in step (1), the mass ratio of hydroxymethylurea to starch powder is 1:9-2:8.

As a further improvement of the technical solution of the present invention, in step (2), the mass ratio of the biodegradable polyester to the starch powder in step (1) is 9:1-7:3.

As a further improvement of the technical solution of the present invention, in step (2), the amount of maleic anhydride added is less than or equal to 6% of the total mass of the biodegradable polyester and the starch powder in step (1).

As a further improvement of the technical solution of the present invention, in step (2), the rotational speed of the twin-screw extruder is 20-400 RPM.

As a further improvement of the technical solution of the present invention, in step (2), the temperature of the twin-screw extruder from the feeding zone to the machine head is set at 100-200°C.

As a further improvement of the technical solution of the present invention, the starch powder is one or a mixture of any two or more of potato starch, bean starch, cereal starch and vegetable starch.

As a further improvement of the technical solution of the present invention, the biodegradable polyester includes aliphatic polyester and aliphatic-aromatic copolyester.

As a further improvement of the technical solution of the present invention, polylactic acid, polybutylene succinate, carbon dioxide-propylene oxide copolymer, polycaprolactone, polydioxanone, polyfurandicarboxylic acid, polyadipic acid One or any mixture of two or more of butanediol terephthalates.

Compared with the prior art, the process of forming core-shell starch particle reinforced toughened polyester in situ in the process of reactive extrusion to generate urea-formaldehyde according to the present invention has the following advantages:

(1) The reaction precursor hydroxymethyl urea of the urea-formaldehyde polymer that is easy to diffuse and infiltrate the small molecule that the present invention adds in solution form, because self viscosity and its strong hydrogen bond interaction with starch macromolecule, therefore only make The outer layer of the starch granules is infiltrated, and under the temperature and shear of the mixing process, hydroxymethylurea does not penetrate further into the interior of the starch and replace the starch through hydrogen bonds with the starch like other currently used plasticizers The intramolecular and intermolecular hydrogen bonds can destroy the crystal structure of starch, but can prevent the original rigid structure of starch granules from being destroyed, and finally make the polyester blend both rigid and tough.

(2) In the process of the present invention, the small molecule precursor hydroxymethyl urea adsorbed on the outer layer of the plasticized starch granules is preferentially formed in biodegradable polyester and starch through polycondensation reaction under the action of high temperature and high shear in the extrusion process. The urea-formaldehyde polymer is generated in situ at the interface between the two phases, and then due to the interfacial tension between the biodegradable polyester, thermoplastic starch and urea-formaldehyde polymer, polyester macromolecular chains and urea-formaldehyde macromolecular chains are formed in the biodegradable polyester matrix. And thermoplastic starch macromolecular chains interpenetrate and interpenetrate the network structure as the shell, starch granules as the core starch granule-polyester/urea-formaldehyde/thermoplastic starch core-shell structure particles. Therefore, it is easy to obtain a low-cost polyester blend with excellent processing properties, mechanical properties and biodegradable properties through a conventional industrial extrusion process.

(3) The macromolecular urea-formaldehyde polymer produced in situ by polycondensation reaction between the two-phase interface of methylolurea during the extrusion process of the present invention can be used as a plasticizing compatibilizer. Urea-formaldehyde is the first agricultural nitrogen slow-release product that has been researched and applied, and has excellent biodegradability. The urea-formaldehyde with good biocompatibility can also release the nutrient element nitrogen when it is decomposed by microorganisms, which improves the activity of microorganisms, thereby further optimizing the biodegradation performance of polyester blends.

(4) The process of forming core-shell starch particle reinforced and toughened polyester in situ in the process of reactive extrusion to form urea-formaldehyde of the present invention can obtain polyester blends plasticized and compatibilized with very low content of urea-formaldehyde. In addition, water, a by-product of the polycondensation reaction of methylol urea produced by in-situ extrusion of urea-formaldehyde, is also an excellent plasticizer for starch, so it can further improve the processing performance of the hybrid system, which is helpful for reducing the cost of biodegradable polyester and improving the productivity. It is of great significance to further promote the application of biodegradable polyester and starch by studying the mechanical properties of biodegradable polyester.

Description of drawings

In order to more clearly illustrate the specific implementation of the present invention or the technical solutions in the prior art, the following will briefly introduce the accompanying drawings that need to be used in the specific implementation or description of the prior art. Obviously, the accompanying drawings in the following description The drawings show some implementations of the present invention, and those skilled in the art can obtain other drawings based on these drawings without any creative work.

Fig. 1 is polybutylene adipate terephthalate (PBAT), the PBAT/tapioca starch (PBAT/CS) blend prepared in Comparative Example 1 and the polyester blend prepared in Example 1 (PBAT/CS) TPCS) torque-residence time curves during mixing.

2 is a scanning electron microscope (SEM) photo of the surface of the PBAT/tapioca starch (PBAT/CS) blend prepared in Comparative Example 1.

3 is a scanning electron microscope (SEM) photo of the quenched section of the PBAT/tapioca starch (PBAT/CS) blend prepared in Comparative Example 1.

Figure 4 is a scanning electron microscope (SEM) photo of the tensile section of the PBAT/tapioca starch (PBAT/CS) blend prepared in Comparative Example 1.

FIG. 5 is a scanning electron microscope (SEM) photo of the surface of the polyester blend (PBAT/TPCS) prepared in Example 1.

Fig. 6 is a scanning electron microscope (SEM) photo of the quenched section of the polyester blend (PBAT/TPCS) prepared in Example 1.

FIG. 7 is a scanning electron microscope (SEM) photo of the tensile section of the polyester blend (PBAT/TPCS) prepared in Example 1.

Figure 8 shows polybutylene adipate terephthalate (PBAT), the PBAT/tapioca starch (PBAT/CS) blend prepared in Comparative Example 1 and the polyester blend prepared in Example 1 (PBAT/CS) TPCS) infrared spectrum.

FIG. 9 is a partially enlarged view of FIG. 8 .

FIG. 10 is another partial enlarged view of FIG. 8 .

Figure 11 shows polybutylene adipate terephthalate (PBAT), the PBAT/tapioca starch (PBAT/CS) blend prepared in Comparative Example 1 and the polyester blend prepared in Example 1 (PBAT/CS) Comparison of differential scanning calorimetry (DSC) curves of the cooling process of TPCS).

Figure 12 shows polybutylene adipate terephthalate (PBAT), the PBAT/tapioca starch (PBAT/CS) blend prepared in Comparative Example 1 and the polyester blend prepared in Example 1 (PBAT/CS) Comparison of differential scanning calorimetry (DSC) curves during the heating process of TPCS).

Figure 13 shows polybutylene adipate terephthalate (PBAT), the PBAT/tapioca starch (PBAT/CS) blend prepared in Comparative Example 1 and the polyester blend prepared in Example 1 (PBAT/CS) TPCS) thermogravimetric curve comparison chart.

Figure 14 shows polybutylene adipate terephthalate (PBAT), the PBAT/tapioca starch (PBAT/CS) blend prepared in Comparative Example 1 and the polyester blend prepared in Example 1 (PBAT/CS) TPCS) comparison chart of the derivative thermogravimetric curve.

Figure 15 shows polybutylene adipate terephthalate (PBAT), the PBAT/tapioca starch (PBAT/CS) blend prepared in Comparative Example 1 and the polyester blend prepared in Example 1 (PBAT/CS) TPCS) X-ray diffraction (XRD) curve comparison chart.

Figure 16 shows polybutylene adipate terephthalate (PBAT), the PBAT/tapioca starch (PBAT/CS) blend prepared in Comparative Example 1 and the polyester blend prepared in Example 1 (PBAT/CS) TPCS) biodegradation rate versus time curve.

Detailed ways

The technical solutions of the present invention are clearly and completely described below, and obviously, the described embodiments are part of the embodiments of the present invention, but not all of them. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

The invention provides a specific embodiment of the process of forming core-shell starch particle reinforced toughened polyester in situ in the process of reactive extrusion to generate urea-formaldehyde, comprising the following steps:

(1) Mix the hydroxymethyl urea solution with the dry starch powder until there are no solid particles, then add the mixture to a kneader and knead it into a uniform powder without crystal points; keep the material sealed to form a homogeneous stable system, and obtain a plasticized starch;

(2) Add the dried biodegradable polyester and the plasticized starch prepared in step (1) into the high-speed mixer, and at the same time add a certain amount of compatibilizer maleic anhydride, mix and add to the twin-screw extruder During extrusion at a certain speed and temperature, the small molecule precursor methylol urea in the plasticized starch generates urea-formaldehyde polymer in situ during the extrusion process, and at the same time due to the interaction between biodegradable polyester, thermoplastic starch and urea-formaldehyde The interfacial tension of the biodegradable polyester matrix is formed in the biodegradable polyester matrix. The polyester macromolecular chain, the urea-formaldehyde macromolecular chain and the thermoplastic starch macromolecular chain interpenetrate and interpenetrate the network structure as the shell, and the starch granule as the core. Core-shell structure particles of ester/urea-formaldehyde/thermoplastic starch; extruded strips are cooled and pelletized by a pelletizer to obtain polyester blends.

In the present invention, the mixing of the hydroxymethyl urea solution and the dry starch powder in step (1) is carried out in a high-speed mixer. In the embodiment of the present invention, there are no special regulations on the specific model and rotating speed of the high-speed mixer, as long as the methylol urea solution and the dry starch powder can be mixed until there are no solid particles.

Further, in step (1), the mass ratio of hydroxymethyl urea (the solute in the hydroxymethyl urea solution) to starch is 1:9-2:8. Preferably, the mass ratio of hydroxymethylurea to starch powder is 1:9-1.5:8.

Further, in step (2), the mass ratio of the biodegradable polyester to the starch powder in step (1) is 9:1-7:3. Preferably, the mass ratio of the biodegradable polyester to the starch powder in step (1) is 8:2-7:3.

Further, in step (2), the amount of maleic anhydride added is less than or equal to 6% of the total mass of the biodegradable polyester and the starch powder in step (1). Preferably, the addition of maleic anhydride is equal to 3-6% of the total mass of starch powder in the biodegradable polyester and step (1); more preferably, the addition of maleic anhydride is equal to the biodegradable polyester and step (1). (1) 4-5% of the total mass of starch powder.

Further, in step (2), the rotational speed of the twin-screw extruder is 20-400 RPM. Preferably, the rotational speed of the twin-screw extruder is 20-300 RPM; more preferably, the rotational speed of the twin-screw extruder is 25-200 RPM.

Further, in step (2), the temperature of the twin-screw extruder from the feeding zone to the machine head is set at 100-200°C. Preferably, the temperature of the twin-screw extruder from the feed zone to the head is set at 100-190°C; more preferably, the temperature of the twin-screw extruder from the feed zone to the head is set at 110-180°C.

Further, the starch powder is one or a mixture of any two or more of potato starch, bean starch, cereal starch and vegetable starch.

Further, the biodegradable polyester includes aliphatic polyester and aliphatic-aromatic copolyester, such as polylactic acid, polybutylene succinate, carbon dioxide-propylene oxide copolymer, polycaprolactone , polydioxanone, polyfurandicarboxylic acid, polybutylene adipate terephthalate, or a mixture of any two or more.

The concrete test method that the present invention adopts is as follows:

Rheological properties: Mix the materials thoroughly according to the ratio, turn on the torque rheometer and set the temperature to the melting temperature of the biodegradable polyester, then add 50g of the material weighed into the rheometer until the torque is constant.

Material mechanical properties: According to GB/T1040.1-2010 plastic tensile properties test.

Biodegradability: According to the test method of GB/T 19277, the final aerobic biodegradability of the film is determined by measuring the amount of carbon dioxide released under controlled composting conditions. Specifically: Add 10g of the material sample to be tested, 60g of compost and 320g of sea sand mixed evenly into the culture bottle, keep the humidity at 40%, culture it in a sealed constant temperature incubator at 58°C, put it into the blank treatment (ie CK treatment), etc. amount of compost and sea sand. Continuously inject _CO2 -free air into the culture bottle, and collect the gas generated in the culture bottle into the NaOH solution. Samples are taken every 5 days, and the carbon content of the collected material samples is determined by an organic carbon analyzer. Calculate the theoretical carbon dioxide release m (ThCO ₂ ) produced by the material to be tested in each culture bottle according to the following formula, expressed in grams (g):

In the formula:

m : the mass of the material to be tested in the culture bottle, in grams (g);

w _c : the carbon content of the material to be tested, which is obtained from a chemical formula or tested by an elemental analysis tester, expressed in mass fraction;

44 and 12: represent the molecular weight of carbon dioxide and the atomic weight of carbon, respectively.

For each test node, use the following formula to calculate the biodegradation percentage D _t (%) of the material to be tested in each culture bottle based on the cumulative amount of released carbon dioxide:

In the formula:

: the cumulative amount of carbon dioxide released from the culture bottle containing the material to be tested from the start of the test to time t, in grams (g);

: the mean value of the amount of carbon dioxide released by the culture bottle of the time t blank experiment from the beginning of the test (the average value of the two groups of blank experiments), in grams (g);

: The theoretical release amount of carbon dioxide produced by the material to be tested in each culture bottle, in grams (g).

The technical solution of the present invention will be described in detail below through specific examples.

Example 1:

The method for preparing PBAT/TPCS by in-situ formation of core-shell starch particles in the process of generating urea-formaldehyde by reactive extrusion to reinforce and toughen polyester comprises the following steps:

The MU aqueous solution with a concentration of 66.7% and the CS powder (tapioca starch powder) dried at 40°C for 8 hours were passed through a high-speed mixer (2500Y, Yongkang Boou Hardware Products Co., Ltd.) at a rate of 2000r/min at room temperature according to a mass ratio of 1:9. Mix for 15 minutes until there are no solid particles. Add the mixture into a kneader (Laizhou Hongxin Machinery Co., Ltd.), and knead at 30°C for 1h at 30r/min until it becomes a uniform powder without crystal points. The material was sealed and stored at 25°C for 24 hours to form a homogeneous and stable system to obtain plasticized starch (TPCS).

The PBAT particles were dried in a vacuum oven at 80°C for 8 hours, and the compatibilizer maleic anhydride was dried at 40°C for 8 hours. According to the mass ratio of PBAT and CS powder 8:2, PBAT and the plasticized starch prepared above were added to the high-speed mixer, and at the same time, 4% maleic anhydride was added to the total mass of PBAT particles and CS powder, and mixed at 2000r/min for 10 minutes , and then fed into a co-rotating twin-screw extruder (TE-20, Coperion Keya (Nanjing) Machinery Co., Ltd.), with a screw diameter of 21 mm and a ratio of length to diameter of 36:1. The screw speed is 35RPM, and the temperature from the feed zone to the machine head is 135-135-145-145-140°C in sequence. After the extruded material is cooled, it is pelletized by a pelletizer (Type 180, Yifan Machinery Co., Ltd.) to obtain PBAT/TPCS polyester blend pellets.

Embodiment 2 and 3: the method for preparing PLA/TPCS by the process of forming core-shell starch particles in situ in the process of generating urea-formaldehyde by reactive extrusion to strengthen and toughen polyester, comprising the following steps:

(1) Pass the aqueous solution with MU concentration of 66.7% and the CS powder (tapioca starch powder) dried at 40°C for 8 hours according to the mass ratio at room temperature through a high-speed mixer (2500Y, Yongkang Boou Hardware Products Co., Ltd.) at 2000r/min Mix for 15 minutes until there are no solid particles. Add the mixture into a kneader (Laizhou Hongxin Machinery Co., Ltd.), and knead at 30°C for 1h at 30r/min until it becomes a uniform powder without crystal points. The material was sealed and stored at 25°C for 24 hours to form a homogeneous and stable system to obtain plasticized starch (TPCS).

(2) Dry the PLA particles in a vacuum oven at 80°C for 8 hours, and dry the compatibilizer maleic anhydride at 40°C for 8 hours. Add PLA and the plasticized starch (TPCS) prepared above into a high-speed mixer according to a certain mass ratio, and at the same time add PLA granules and 4% maleic anhydride of the total mass of starch, mix at 2000r/min for 10 minutes, and then add to the same Into a twin-screw extruder (TE-20, Coperion Keya (Nanjing) Machinery Co., Ltd.), with a screw diameter of 21mm and a length-to-diameter ratio of 36:1. The screw speed is 35RPM, and the temperature from the feeding zone to the machine head is 160-160-170-170°C in sequence. After the extruded material is cooled, it is pelletized by a pelletizer (Type 180, Yifan Machinery Co., Ltd.) to obtain PLA/TPCS pellets.

The mass ratio of PLA and CS powder in Example 2 is 7:3, and the mass ratio of methylol urea to CS powder is 1.5:8.

The mass ratio of PLA and CS powder in Example 3 is 7:3, and the mass ratio of methylol urea to CS powder is 2:8.

Comparative example 1

The preparation method of PBAT/CS polyester blend comprises the following steps:

The PBAT particles were dried in a vacuum oven at 80°C for 8 hours, and the compatibilizer maleic anhydride was dried at 40°C for 8 hours. According to the mass ratio of PBAT granules and CS powder 8:2, PBAT and dried CS were added to the high-speed mixer, and at the same time, PBAT granules and maleic anhydride of 4% of the total mass of starch were added, mixed at 2000r/min for 10 minutes, and then Add it into a co-rotating twin-screw extruder (TE-20, Coperion Keya (Nanjing) Machinery Co., Ltd.), with a screw diameter of 21 mm and a ratio of length to diameter of 36:1. The screw speed is 35RPM, and the temperature from the feed zone to the machine head is 135-135-145-145-140°C in sequence. After the extruded material was cooled, it was pelletized by a pelletizer (Type 180, Yifan Machinery Co., Ltd.) to obtain PBAT/CS pellets.

Comparative example 2:

Except that methylol urea was not added, the other polylactic acid/starch (PLA/CS) blends were prepared according to the formula and process of Example 2, that is, the difference between Comparative Example 2 and Example 2 was that no Hydroxymethylurea.

The performance contrast of the polyester blend prepared by table 1 embodiment and comparative example

In table 1, the tensile strength of the PBAT/TPCS polyester blend prepared in Example 1, the elongation at break are compared with the tensile strength, elongation at break of the PBAT/CS polyester blend prepared in Comparative Example 1 and pure PBAT resin The elongation was significantly improved, showing that the core-shell starch granules in this system have a significant strengthening and toughening effect. In addition, from the comparison of Examples 2 and 3 with Comparative Example 2, it can be seen that with the increase of the content of plasticizing compatibilizer urea-formaldehyde generated by reaction extrusion in-situ polycondensation, the tensile strength of the prepared PLA/starch blend The elongation at break and elongation at break are both improved, and the material shows better comprehensive mechanical properties. It can be seen that the process of the present invention can significantly improve the interfacial bonding force between the components of the PLA/starch blend, so that the strength and toughness of the blended material are significantly improved compared with the polylactic acid/unmodified starch blend.

Among them, polybutylene adipate terephthalate (PBAT), the PBAT/tapioca starch (PBAT/CS) blend prepared in Comparative Example 1 and the polyester blend (PBAT/TPCS) prepared in Example 1 ) related studies are as follows:

The torque rheometer can simulate the actual production and processing process, and is widely used to characterize the evolution of the blending process of raw materials during processing and the processing flow properties of composite materials. It can be seen from Fig. 1 that the torque of pure PBAT quickly rises to the maximum with the prolongation of mixing time, which can be attributed to the viscosity increase caused by the melting of the polymer. Then with the further increase of kneading time, the torque of pure PBAT system decreased and reached the equilibrium torque value of 5.8N•m. The overall change trend of PBAT/CS added with unplasticized starch in Comparative Example 1 is similar to that of pure PBAT, and the equilibrium torque value is basically the same as that of PBAT. The PBAT/TPCS prepared in Example 1 of the present invention not only greatly prolongs the time to reach the maximum torque value, but also has an equilibrium torque value of 3.9N m, which is nearly 40% lower than that of PBAT and PBAT/CS, indicating that the system Excellent processing properties.

Figure 2 shows that the PBAT/CS prepared in Comparative Example 1 has a typical sea-island structure in which PBAT is the continuous phase and CS is the dispersed phase. However, the size of the dispersed phase is not uniform, and the interface between the two phases is clear. It can be seen from Figures 3 and 4 that the surface of the pits where the starch phase was pulled out during the fracture process of PBAT/CS was smooth, indicating poor compatibility and weak interfacial interaction between PBAT and CS.

Figure 5 shows that the PBAT/TPCS prepared in Example 1 of the present invention still exhibits a sea-island structure, but the starch has a complete spherical structure, a significantly larger particle size and more uniform dispersion in the resin matrix. In particular, the core-shell structure of TPCS in PBAT/TPCS blends can be clearly seen. The illustration of a larger multiple in the upper left corner of Figure 5 clearly shows that the surface of the starch granules in the PBAT matrix has a certain thickness of the shell layer, which is also the main reason for the obvious increase in the particle size of TPCS. Figures 6 and 7 show that the pits pulled out by the starch phase of PBAT/TPCS were significantly reduced during the fracture process, indicating that the interfacial interaction between the two phases was enhanced and the interfacial compatibility was significantly improved. In addition, the fracture surface shows obvious yield deformation of the matrix, and there are elongated fibers. Studies have shown that the main toughening mechanism of the core-shell structure toughening agent is the formation of microfibers at the interface between the matrix and the toughening agent particles with a core-shell structure. These microfibers can effectively transfer stress and cause plastic deformation of the matrix, thereby simultaneously Reinforces toughened matrix polymer.

In Fig. 8, the significant absorption peak near 1710cm ^-1 originates from the stretching vibration of C=O of the ester group of PBAT, and Fig. 9 shows that the positions of the absorption peaks of PBAT and PBAT/CS prepared in Comparative Example 1 are exactly the same, which shows that There is no interaction between PBAT and CS in PBAT/CS, and the two components are simply mixed physically. However, the peak in the PBAT/TPCS prepared in Example 1 was red-shifted. Pure CS has a broad peak around 3300 cm ⁻¹ due to the presence of a large number of hydroxyl groups (−OH) in its molecular chain. In the curves of pure MU (methylol urea) and UF (urea-formaldehyde) at 3327 cm ⁻¹ is the bending vibration of HNH of -CONH- representing the amide bond. It can be seen from Figure 10 that in the spectrum of TPCS, the width of the corresponding absorption peak is narrower than that of CS and has shifted compared with the above two peaks. In addition, compared with MU, the absorption peak at 1550cm ^-1 corresponding to the –COO- stretching vibration in the –CONH ₂ group and the absorption peak at 2900cm ^-1 of TPCS are also red-shifted. The shifts of the above representative absorption peaks indicated that MU, a reaction precursor of small molecules that are easy to diffuse and infiltrate, added in the form of a solution, could break the intermolecular and intramolecular hydrogen bonds in the starch-infiltrated layer, thereby facilitating the movement of starch molecular chains. In addition, PBAT in PBAT/TPCS formed hydrogen bond interactions with other components.

Figures 11 and 12 show that CS has no obvious melting transition and crystallization transition. It can be seen from Figure 11 that the crystallization transition of pure PBAT occurs around 42°C (Tc=42°C). From Fig. 12, pure PBAT shows a transition at 125 °C, corresponding to the melting of crystalline PBAT (Tm = 125 °C). For the PBAT/CS prepared in Comparative Example 1, it can be seen from Figure 11 that there are two peaks corresponding to 85°C and 110°C, and the strong peak at 85°C is attributed to the Tc of PBAT, which is greatly increased from 42°C of pure PBAT to 85°C It is because CS particles promote the movement and regular arrangement of PBAT molecular chains at higher temperature. Another weak peak at 110 °C was assigned to the Tc of the CS phase in PBAT/CS. Fig. 12, in the PBAT/CS prepared in Comparative Example 1, the melting transition of the CS phase occurs at 130 °C. A possible reason for the simultaneous existence of melting and crystallization transitions in DSC is that high temperature and strong shear promote the rearrangement of starch chains, thereby favoring the crystallization of starch. There are two obvious absorption peaks in PBAT/CS, indicating that PBAT and CS are only physically mixed in this treatment. For the PBAT/TPCS prepared in Example 1, it can be seen that there is only an obvious absorption peak of PBAT, and that of CS hardly exists. In addition, the peak temperature corresponding to the PBAT absorption peak of PBAT/TPCS is higher than that of pure PBAT, indicating that the TPCS particles in this system can promote the movement and regular arrangement of PBAT molecular chains at higher temperatures.

As shown in Figures 13 and 14, the TG curve of UF (urea-formaldehyde) is obviously different from that of MU (hydroxymethylurea). The main difference is that MU has a thermal decomposition interval at 110-214 ° C, which is unreacted during synthesis. The thermal decomposition interval of urea. UF mainly has two decomposition intervals of 180-220°C and 220-600°C. Due to the low content of MU, TPCS exhibited a very small pyrolysis peak at 110–214 °C. The PBAT/TPCS prepared in Example 1 did not show a thermal decomposition peak at 110-214°C, indicating that during the extrusion process, the polycondensation reaction of MU did occur to form UF. PBAT exhibited a single degradation step from 340 °C to 460 °C, and the peak temperature value in the DTG curve was 428 °C (Tp _PBAT = 428 °C). CS undergoes a three-step degradation process. The first degradation step occurs at 50 °C to 110 °C, corresponding to the elimination of water and other low molecular weight compounds. The second degradation step (shoulder peak temperature 305 °C) and the obvious third degradation step (main peak temperature 340 °C) corresponded to the degradation of amylose and amylopectin, respectively. In the curve of TPCS, the degradation peaks corresponding to amylose and amylopectin are located at 290 °C and 328 °C, respectively, which are lower than those of pure CS, indicating that the small molecule MU, a reaction precursor that is easy to diffuse and infiltrate, leads to the plasticization of CS. The hydrogen bond force in its molecule is destroyed, thus the thermal stability decreases. Both the PBAT/CS prepared in Comparative Example 1 and the PBAT/TPCS prepared in Example 1 had degradation peaks corresponding to starch and PBAT, and the peak position was basically consistent with Tp=428°C of pure PBAT, but with Tp=428°C of CS. 340°C has a large difference. The T CS of PBAT/ _CS = 326°C, which was significantly lower than that of pure CS, indicating that the hydrogen bond force and crystal structure in starch molecules were destroyed, so its thermal stability decreased. However, the T _CS of PBAT/TPCS = 345 °C, which is higher than that of pure CS, indicating that the UF generated in situ during the reactive extrusion process increases the number of hydrogen bonds formed between the macromolecular chains of each component, resulting in an increase in the degradation temperature.

It can be seen from Figure 15 that pure CS (starch) has four strong diffraction peaks at 2 θ of 15.10º, 17.12º, 17.84º and 22.95º, and a weak diffraction peak at 20.12, indicating that it is Type A crystal with double helix structure. The main characteristic peaks of plasticized starch (TPCS) are basically consistent with those of CS, indicating that MU, a reaction precursor of small molecules that are easy to diffuse and infiltrate, added in the form of solution did not destroy the crystal structure of CS, but caused the crystallinity of TPCS (23.58%) is significantly lower than that of CS (41.55%). The reason for this phenomenon should be that the small molecule precursor MU adsorbed on the surface of TPCS particles forms a strong hydrogen bond interaction with the CS macromolecules in the wetting layer, which leads to a significant decrease in the crystallinity of TPCS compared with CS. But at the same time, the strong hydrogen bond interaction between the two also makes MU not penetrate further into the interior of the starch and pass between the starch and the starch under the temperature and shear action of the mixing process, unlike other currently used plasticizers. The hydrogen bonds of the starch replace the intramolecular and intermolecular hydrogen bonds to destroy the A-type crystal structure of the starch. The maintenance of the rigid starch crystal structure should be more conducive to improving the strength of PBAT. Pure PBAT has three strong diffraction peaks at 18.14° (010), 21.25° (101) and 23.93° (100), and two weak peaks at 15.84° (011) and 25.57° (111). The characteristic peaks at 2θ = 22.31°, 24.71°, and 31.03° confirm the presence of definite crystalline regions in UF. The PBAT/CS blend prepared in Comparative Example 1 showed a crystallization peak attributed to PBAT and a type A crystallization peak of starch. In the PBAT/TPCS blend prepared in Example 1, in addition to the above two types of crystallization peaks, as shown by the arrow, a small crystallization peak corresponding to UF also appeared at 24.71, indicating that MU did occur during the reactive extrusion process. The reaction produces UF. In addition, Figure 15 also shows that the crystallinity of PBAT is 22.06%. After adding 20wt.% of CS with a crystallinity of 41.55%, the crystallinity of the PBAT/CS blend prepared in Comparative Example 1 reaches 23.55%, which is higher than that of pure PBAT. Slightly improved; and the crystallinity of the PBAT/TPCS prepared in Example 1 is 20.36%, which is lower than pure PBAT. The reason for this result should be that the small molecule precursor MU adsorbed on the surface of CS particles preferentially generates UF (urea-formaldehyde) polymer in situ at the interface between the two phases through polycondensation reaction under the action of high temperature and high shear in the extrusion process. , while the UF with low molecular chain mobility forms hydrogen bond interactions with the hydroxyl groups on the CS surface and the carboxyl or terminal hydroxyl groups on the surface of PBAT through the hydroxymethyl group, amide group and terminal amino group on its macromolecule, respectively, so that it does not affect Their crystal structure also further promotes the plasticization of CS and compatibilizes PBAT/TPCS blends through hydrogen bond bridging.

Figure 16 shows that the biodegradation rate of the PBAT/TPCS prepared in Comparative Example 1 and the PBAT/TPCS prepared in Example 1 is significantly higher than that of pure PBAT. During the degradation process, microorganisms will preferentially degrade easily degradable materials. CS, a natural degradable polymer, will be preferentially decomposed by microorganisms due to its good hydrophilicity. Therefore, the biodegradability of PBAT/CS is better than that of PBAT. UF with good biocompatibility can also release nutrient element nitrogen while being decomposed by microorganisms, which improves the activity of microorganisms, thereby further optimizing the degradation rate of PBAT/TPCS.

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, rather than limiting them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: It is still possible to modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the technical solutions of the various embodiments of the present invention. scope.

Claims (9)

1. a kind of reaction extrusion generates the technique that core-shell starch particle strengthens the toughening polyester in situ in generating urea-formaldehyde process, it is characterized in that, comprises the following steps: (1) Mix the hydroxymethyl urea solution with the dry starch powder until there are no solid particles, then add the mixture to a kneader and knead it into a uniform powder without crystal points; keep the material sealed to form a homogeneous stable system, and obtain a plasticized starch; (2) Add the dried biodegradable polyester and the plasticized starch prepared in step (1) into the high-speed mixer, and at the same time add a certain amount of compatibilizer maleic anhydride, mix and add to the twin-screw extruder During extrusion at a certain speed and temperature, during the extrusion process, the small molecule precursor methylol urea in the plasticized starch generates urea-formaldehyde polymer in situ, and at the same time due to biodegradable polyester, thermoplastic starch and urea-formaldehyde polymer In the biodegradable polyester matrix, the interpenetrating and interpenetrating network structure of polyester macromolecular chains, urea-formaldehyde macromolecular chains and thermoplastic starch macromolecular chains is formed as the shell, and the starch granules as the core. Starch granules- Core-shell structure particles of polyester/urea-formaldehyde/thermoplastic starch; extruded strips are cooled and pelletized by a pelletizer to obtain polyester blends. 2. The process of forming core-shell starch particle reinforced toughened polyester in situ in the process of reactive extrusion to generate urea-formaldehyde according to claim 1, characterized in that, in step (1), methylol urea and starch powder The mass ratio is 1:9~2:8. 3. The process of forming core-shell starch particle reinforced toughened polyester in situ in the process of reaction extrusion to generate urea-formaldehyde according to claim 1, characterized in that, in step (2), biodegradable polyester and step The mass ratio of starch powder in (1) is 9:1~7:3. 4. The process of forming core-shell starch particle reinforced toughened polyester in situ in the process of reactive extrusion to generate urea-formaldehyde according to claim 1, characterized in that, in step (2), the amount of maleic anhydride added is less than Equal to 6% of the total mass of biodegradable polyester and starch powder in step (1). 5. The process for in-situ formation of core-shell starch particle reinforced toughened polyester in the process of generating urea-formaldehyde by reactive extrusion according to claim 1, characterized in that, in step (2), the rotational speed of the twin-screw extruder is 20-400RPM. 6. The process of forming core-shell starch particle reinforced toughened polyester in situ in the process of reactive extrusion to generate urea-formaldehyde according to claim 1, characterized in that, in step (2), the twin-screw extruder starts from the feeding The temperature from zone to machine head is set at 100-200°C. 7. a kind of reaction extrusion according to claim 1 generates the technique of core-shell starch particle reinforced toughening polyester in situ in generating urea-formaldehyde process, it is characterized in that, described starch powder is potato starch, bean starch, One or a mixture of any two or more of cereal starch and vegetable starch. 8. a kind of reaction extrusion according to claim 1 generates the technology of in-situ core-shell starch particle reinforcement toughening polyester in generating urea-formaldehyde process, it is characterized in that, described biodegradable polyester comprises aliphatic polyester and Aliphatic-aromatic copolyesters. 9. a kind of reaction extrusion according to claim 1 generates the technology of in-situ core-shell starch particle reinforcement toughening polyester in generating urea-formaldehyde process, it is characterized in that, described biodegradable polyester is polylactic acid, polybutylene One or any of butylene diacid, carbon dioxide-propylene oxide copolymer, polycaprolactone, polydioxanone, polyfurandicarboxylic acid, polybutylene adipate terephthalate A mixture of two or more.

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