CN115926401B

CN115926401B - In-situ forming core-shell starch reinforced and toughened polyester when urea formaldehyde is generated by reactive extrusion

Info

Publication number: CN115926401B
Application number: CN202310017022.8A
Authority: CN
Inventors: 刘亚青; 刘泽宇; 武朝阳; 赵贵哲
Original assignee: North University of China; Shanxi Zhongbei New Material Technology Co Ltd
Current assignee: North University of China; Shanxi Zhongbei New Material Technology Co Ltd
Priority date: 2023-01-06
Filing date: 2023-01-06
Publication date: 2023-06-30
Anticipated expiration: 2043-01-06
Also published as: CN115926401A

Abstract

The invention relates to the technical field of biodegradable polymers, in particular to polyester which is reinforced and toughened by in-situ formed core-shell starch when urea formaldehyde is generated by reactive extrusion. The starch is modified by a reaction precursor methylol urea solution of very low amount of urea formaldehyde to obtain plasticized starch, then the plasticized starch and a biodegradable polyester mixture are extruded by a screw extruder, and under the high temperature and high shear action in the extrusion process, methylol urea preferentially generates urea formaldehyde in situ between two phase interfaces through polycondensation reaction and respectively interacts with active functional groups on the surfaces of starch and polyester through hydrophilic groups on macromolecules of the methylol urea, and core-shell structure particles of starch particles-polyester/urea formaldehyde/thermoplastic starch are spontaneously formed in a polyester matrix in the reaction extrusion process due to interfacial tension among the components, so that the low-cost biodegradable polyester/thermoplastic starch mixture with excellent processability, mechanical property and biodegradability can be obtained through a conventional industrialized extrusion process.

Description

In-situ forming core-shell starch reinforced and toughened polyester when urea formaldehyde is generated by reactive extrusion

Technical Field

The invention relates to the field of biodegradable polymers, in particular to polyester which is reinforced and toughened by in-situ forming core-shell starch when urea formaldehyde is generated by reactive extrusion.

Background

A large amount of non-degradable plastic waste has caused serious environmental pollution worldwide. In order to effectively treat plastic pollution, the policy of controlling or prohibiting the use of disposable non-degradable plastic products in the platform of nearly 90 countries and regions exists worldwide. The european union began in 2021 to prohibit or limit the use of ten disposable plastic articles. In 2020, china issued opinion about further enhanced pollution control of plastics, proposed to prohibit or limit the use of non-degradable plastic products such as disposable plastic products and express plastic packages in steps and fields, and encouraged the use of degradable plastic products.

Biodegradable plastics mean plastics which are capable of being degraded by microorganisms present in nature and are ultimately converted completely into CO ₂ Water, mineralized inorganic salt of the contained element and a polymer material of new substances. At present, chemical synthesis type biodegradable high molecular materials widely studied and used at home and abroad are mainly polyesters containing ester groups which can be decomposed by microorganisms or enzymes in molecular chains, including aliphatic polyesters and aliphatic-aromatic copolyesters, and are represented mainly by polylactic acid, polybutylene succinate, carbon dioxide-propylene oxide copolymer, polycaprolactone, polydioxanone, polyfurandicarboxylic acid and polybutylene adipate terephthalate. However, in order to secure biodegradability, the biodegradable polyester requires a higher proportion of amorphous structure, and thus has a lower crystallinity and poorer mechanical properties, and also affects its processability, which restricts the range of application of the biodegradable polyester. So at present, how to improve the mechanical properties of the biodegradable polyester is one of the important points for expanding and promoting the application of the biodegradable polyester. In addition, biodegradable plastics are expensive, which further restricts the wide application of biodegradable polymers.

Starch has become a useful material in the field of thermoplastics at present because of its advantages of biodegradability, non-toxicity, high purity, low cost, etc. However, since starch contains a large number of hydroxyl groups in the molecular chain, strong hydrogen bond interaction exists between macromolecules, so that the starch has good hydrophilicity, but the starch also has the problems of sensitivity to water, difficulty in processing and the like. In addition, the starch itself has poor mechanical properties and cannot replace commercial non-degradable plastics. In order to improve the processability and mechanical properties of starch, various thermoplastic starches or blends thereof with other polymers can be produced by thermoplasticizing starch by a suitable process, which can expand the application range of starch.

One promising approach to provide cost-effective advantages to biodegradable polyesters is to mix thermoplastic starch with it in proportions of 20-30 wt.%. To date, in many studies, starch has been added to biodegradable polyester matrices to reduce costs and improve the properties of the composite. But the poor compatibility between the hydrophobic biodegradable polyester and the hydrophilic starch results in a significant decrease in the properties of the composite. The addition of a reactive compatibilizer is the first choice to alleviate the insufficient interfacial adhesion between the biodegradable polyester and thermoplastic starch while maintaining an acceptable balance of strength, hardness and elongation at break. When the reactive melt blending is performed in the presence of the compatilizer, the compatilizer can connect the components through covalent bonds and remarkably improve the interfacial adhesion, which is helpful for effective stress transmission among the components, thereby improving the mechanical properties of the biodegradable polyester/thermoplastic starch composite material. A variety of reactive compatibilizers for biodegradable polyester/thermoplastic starch blends have been reported, with representative examples mainly including organic acids, maleic Anhydride (MAH) and Glycidyl Methacrylate (GMA). The carboxyl groups of the organic acid, the anhydride groups of the MAH, the epoxy groups of the GMA can improve the mechanical properties of the biodegradable polyester/thermoplastic starch blend by enhancing the interfacial adhesion between the two phases. However, low molecular weight organic acids are difficult to interact with polyester matrices continuously, resulting in limited compatibility. The grafting efficiency of MAH onto biodegradable polyesters is relatively low. The hot polymerization products of GMA are complex, and as GMA increases, a large amount of by-products are formed during the mixing process to act as "plasticizers" which can reduce the strength of the material. Therefore, in order to enhance the interaction between the lipophilic biodegradable polyester and the hydrophilic thermoplastic starch, a more efficient compatibilizer or compatibilizer strategy is highly necessary.

At present, a large number of researches prove that the composite form of the core-shell can exert the synergistic effect of two materials, and finally the rigidity and toughness of the modified polymer are realized. Core-shell impact modifiers, which have a relatively stiff core or shell component, are widely used in a variety of polymer systems today, and thus provide effective toughening while maintaining relatively high strength and modulus. On the basis, the toughening method for forming the core-shell structure particles in situ by melt blending is gradually developed. The interfacial tension between polymers is used as driving force to spontaneously form core-shell structure particles in a blending system, so that better stress transmission between a disperse phase and a matrix can be ensured, and the material is toughened and reinforced simultaneously.

Disclosure of Invention

In order to enhance the interfacial interaction between the lipophilic biodegradable polyester and the hydrophilic thermoplastic starch, the invention provides a process for forming the core-shell starch particle reinforced and toughened polyester in situ in the process of forming urea formaldehyde by reactive extrusion.

The invention is realized by the following technical scheme: a process for in-situ forming core-shell starch particle reinforced and toughened polyester in the process of producing urea formaldehyde by reactive extrusion comprises the following steps:

(1) Mixing the methylol urea solution with the dried starch powder until no solid particles are present, and then adding the mixture into a kneader to be kneaded until uniform amorphous powder is obtained; sealing and preserving the material to form a homogeneous stable system to obtain plasticized starch;

(2) Adding the dried biodegradable polyester and the plasticized starch prepared in the step (1) into a high-speed mixer, adding a certain amount of compatibilizer maleic anhydride, mixing, adding into a double-screw extruder, extruding at a certain rotating speed and temperature, and forming urea formaldehyde polymer in situ by small molecular precursor methylol urea in the plasticized starch in the extruding process, wherein the core-shell structure particles of the starch particle-polyester/urea formaldehyde/thermoplastic starch which take polyester macromolecular chains, urea formaldehyde macromolecular chains and thermoplastic starch macromolecular chains as shells and starch particles as cores are formed in a biodegradable polyester matrix due to interfacial tension among the biodegradable polyester, thermoplastic starch and urea formaldehyde polymer; the extruded strands were cooled and pelletized by a pelletizer to obtain a polyester blend.

In the step (1), the reaction precursor methylol urea of the urea formaldehyde polymer which is added in the form of solution and is easy to diffuse and infiltrate only infiltrates the outer layer of the starch granule due to the self viscosity and the strong hydrogen bond interaction with the starch macromolecules, and damages the intermolecular and intramolecular hydrogen bonds in the starch infiltrate layer through the hydrogen bond interaction, thereby obviously improving the processing property of the starch; however, at the same time, the strong hydrogen bond interaction between the two also enables the methylol urea not to go deep into the starch and replace the intramolecular and intermolecular hydrogen bonds of the starch by the hydrogen bonds between the methylol urea and the starch so as to break the crystal structure of the starch, but enables the original rigid structure of the starch particles not to be broken, and finally enables the polyester blend to have rigidity and toughness under the action of the temperature and shearing action of the mixing process.

In the invention, the intermolecular and intramolecular hydrogen bonds of starch macromolecules in the plasticized starch are destroyed under the high temperature and high shear action in the extrusion process, so that the crystalline structure in the starch particles is disintegrated by melting and shearing action to form a disordered continuous phase of the starch macromolecules, thereby obtaining the thermoplastic starch.

In the step (2), the small molecular precursor methylol urea in the plasticized starch is preferentially subjected to polycondensation reaction under the high temperature and high shear action in the extrusion process to generate urea formaldehyde polymer in situ between the biodegradable polyester and the two-phase interface of the starch, and then the starch particle-polyester/urea formaldehyde/thermoplastic starch core-shell structure particles taking polyester macromolecular chains, urea formaldehyde macromolecular chains and thermoplastic starch macromolecular chains as shells and starch particles as cores are formed in the biodegradable polyester matrix due to interfacial tension among the biodegradable polyester, the thermoplastic starch and the urea formaldehyde polymer. The macromolecular urea formaldehyde polymer generated in situ by the polycondensation reaction of the methylol urea between two phase interfaces in the extrusion process has excellent biodegradability as well as is used as a plasticizing compatibilizer, and can release nutrient element nitrogen while being degraded by microorganisms, so that the microbial activity is improved, and therefore, the polyester blend prepared by the process of the invention does not contain non-biodegradable polymers, and has further optimized biodegradability.

As a further improvement of the technical scheme, in the step (1), the mass ratio of the methylol urea to the starch powder is 1:9-2:8.

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

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

As a further improvement of the technical scheme of the invention, in the step (2), the rotating speed of the double-screw extruder is 20-400RPM.

As a further improvement of the technical scheme of the invention, in the step (2), the temperature from the feeding area to the machine head of the twin-screw extruder is set at 100-200 ℃.

As a further improvement of the technical scheme of the 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 scheme of the invention, the biodegradable polyester comprises aliphatic polyester and aliphatic-aromatic copolyester.

The technical scheme of the invention is further improved to be one or a mixture of any two or more of polylactic acid, polybutylene succinate, carbon dioxide-propylene oxide copolymer, polycaprolactone, polydioxanone, polyfurandicarboxylic acid and polybutylene adipate-terephthalate.

Compared with the prior art, the process for in-situ forming the core-shell starch particle reinforced and toughened polyester in the process of producing urea formaldehyde by reactive extrusion has the following advantages:

(1) The reaction precursor methylol urea of the urea formaldehyde polymer which is added in the form of solution and is easy to diffuse and infiltrate only infiltrates the outer layer of the starch granule due to the self viscosity and the strong hydrogen bond interaction with the starch macromolecules, and the methylol urea does not penetrate into the starch and replace the intramolecular and intermolecular hydrogen bonds of the starch by the hydrogen bonds with the starch like other currently used plasticizers under the temperature and shearing action of the mixing process, so that the crystal structure of the starch is destroyed, the original rigid structure of the starch granule is not destroyed, and finally the polyester blend can be rigid and tough.

(2) According to the process, the small molecular precursor methylol urea adsorbed on the outer layer of the plasticized starch granule is preferentially subjected to polycondensation reaction under the high temperature and high shear action in the extrusion process to generate urea formaldehyde polymer in situ between biodegradable polyester and starch two-phase interface, and then the polyester macromolecular chain, the urea formaldehyde macromolecular chain and the thermoplastic starch macromolecular chain are mutually penetrated and mutually penetrated to form a network structure which takes the polyester macromolecular chain, the urea formaldehyde macromolecular chain and the thermoplastic starch macromolecular chain as a shell in a biodegradable polyester matrix due to interfacial tension among the biodegradable polyester, the thermoplastic starch and the urea formaldehyde polymer, and the starch granule is starch granule-polyester/urea formaldehyde/thermoplastic starch core-shell structure granule with the starch granule as a core. Thus, a low cost polyester blend having excellent processability, mechanical properties and biodegradability at the same time is easily obtained by a conventional industrial extrusion process.

(3) The macromolecular urea formaldehyde polymer generated in situ by the hydroxy methyl urea through polycondensation reaction between two phase interfaces in the extrusion process can be used as a plasticizing compatibilizer. And urea formaldehyde is the first agricultural nitrogen slow release product which is researched and applied, and has excellent biodegradability. The urea formaldehyde with good biocompatibility can release nutrient element nitrogen while being decomposed by microorganisms, so that the microbial activity is improved, and the biodegradability of the polyester blend is further optimized.

(4) The process for in-situ forming the core-shell starch particle reinforced and toughened polyester in the process of producing urea formaldehyde by reactive extrusion can obtain the polyester blend with extremely low content of urea formaldehyde plasticizing and compatibilizing. In addition, the byproduct water of the polycondensation reaction of the urea formaldehyde generated by in-situ extrusion of the methylol urea is also an excellent plasticizer of starch, so that the processability of a mixed system can be further improved, and the method has important significance for reducing the cost of the biodegradable polyester and improving the mechanical property of the biodegradable polyester, thereby further promoting the application of the biodegradable polyester and the starch.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are needed in the description of the embodiments or the prior art will be briefly described, and it is obvious that the drawings in the description below are some embodiments of the present invention, and other drawings can be obtained according to the drawings without inventive effort for a person skilled in the art.

FIG. 1 is a torque-residence time curve during mixing of 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.

FIG. 2 is a Scanning Electron Microscope (SEM) photograph of the surface of the PBAT/tapioca starch (PBAT/CS) blend prepared in comparative example 1.

FIG. 3 is a Scanning Electron Microscope (SEM) photograph of a quenched section of the PBAT/tapioca starch (PBAT/CS) blend prepared in comparative example 1.

FIG. 4 is a Scanning Electron Microscope (SEM) photograph of a stretched section of a PBAT/tapioca starch (PBAT/CS) blend prepared in comparative example 1.

FIG. 5 is a Scanning Electron Microscope (SEM) photograph of the surface of the polyester blend (PBAT/TPCS) prepared in example 1.

FIG. 6 is a Scanning Electron Microscope (SEM) photograph of a quenched section of the polyester blend (PBAT/TPCS) prepared in example 1.

FIG. 7 is a Scanning Electron Microscope (SEM) photograph of a stretched section of the polyester blend (PBAT/TPCS) prepared in example 1.

FIG. 8 is an infrared spectrum of polybutylene adipate terephthalate (PBAT), PBAT/tapioca starch (PBAT/CS) blends prepared in comparative example 1, and polyester blends (PBAT/TPCS) prepared in example 1.

Fig. 9 is a partial enlarged view of fig. 8.

Fig. 10 is another enlarged partial view of fig. 8.

FIG. 11 is a graph comparing Differential Scanning Calorimeter (DSC) curves of cooling processes of polybutylene adipate terephthalate (PBAT), PBAT/tapioca starch (PBAT/CS) blend prepared in comparative example 1, and polyester blend (PBAT/TPCS) prepared in example 1.

FIG. 12 is a graph comparing Differential Scanning Calorimeter (DSC) curves of polybutylene adipate terephthalate (PBAT), PBAT/tapioca starch (PBAT/CS) blends prepared in comparative example 1, and polyester blends (PBAT/TPCS) prepared in example 1 during heating.

FIG. 13 is a graph comparing thermogravimetric curves of polybutylene adipate terephthalate (PBAT), PBAT/tapioca starch (PBAT/CS) blend prepared in comparative example 1, and polyester blend (PBAT/TPCS) prepared in example 1.

FIG. 14 is a graph comparing the differential thermogravimetric curves of 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.

FIG. 15 is a graph comparing X-ray diffraction (XRD) curves of 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.

FIG. 16 is a graph showing the biodegradation rate of 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 over time.

Detailed Description

The following description of the present invention will be made clearly and fully, and it is apparent that the embodiments described are some, but not all, of the embodiments of the present invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

The invention provides a specific embodiment of a process for forming core-shell starch particle reinforced and toughened polyester in situ in the process of producing urea formaldehyde by reactive extrusion, which comprises the following steps:

In the present invention, the mixing of the methylol urea solution with the dried starch powder in step (1) is performed in a high speed mixer. In the embodiment of the invention, no special regulations are provided for the specific model and the rotating speed of the high-speed mixer, so long as the methylol urea solution and the dried starch powder can be mixed until no solid particles exist.

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

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

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

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

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

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 comprises aliphatic polyester and aliphatic-aromatic copolyester, and is one or a mixture of any two or more of polylactic acid, polybutylene succinate, carbon dioxide-propylene oxide copolymer, polycaprolactone, polydioxanone, polyfurandicarboxylic acid and polybutylene adipate terephthalate.

The specific test method adopted by the invention is as follows:

rheological properties: the materials were thoroughly mixed in proportions, the torque rheometer was opened and the temperature was set to the melting temperature of the biodegradable polyester, and then 50g of the weighed material was added to the rheometer until the torque was constant.

Mechanical properties of the material: according to GB/T1040.1-2010 plastics tensile properties test.

Biodegradability: according to the test method of GB/T19277, the final aerobic biodegradability of a mulch film is determined by measuring the amount of carbon dioxide released under controlled composting conditions. The method comprises the following steps: adding 10g of material sample to be tested, 60g of compost and 320g of sea sand which are uniformly mixed into a culture flask, keeping the humidity at 40%, culturing at 58 ℃ in a sealed constant temperature incubator, and putting the same amount of compost and sea sand into a blank (namely CK treatment). Continuous CO-free introduction into a culture flask ₂ Introducing the gas generated in the culture flask into NaOH solution for collectionSampling every 5 days, and measuring the carbon content of the obtained material sample by an organic carbon analyzer. Calculating the theoretical release amount of carbon dioxide generated by the material to be tested in each culture flask according to the following methodm(ThCO ₂ ) Expressed in grams (g):

wherein:

m: the mass of the material to be measured in the culture flask is given in grams (g);

w _c : the carbon content of the material to be measured is obtained by a chemical formula or by testing with an elemental analysis tester and is expressed by mass fraction;

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

Each test node calculates the biodegradation percentage D of the material to be tested in each culture flask according to the accumulated released carbon dioxide by the following formula _t （%）：

Wherein:

: the carbon dioxide quantity released by the culture flask containing the material to be tested is accumulated until the time t of the test, wherein the unit is gram (g);

: average value of accumulated released carbon dioxide amount of culture flask from start of test to time t blank test (average value of two groups of blank tests), wherein the unit is gram (g);

: the theoretical release of carbon dioxide in grams (g) is produced by the material to be tested in each flask.

The technical scheme of the invention is described in detail through specific embodiments.

Example 1:

the process of preparing PBAT/TPCS with in-situ core-shell starch particle reinforced and toughened polyester in the process of producing urea formaldehyde through reaction extrusion includes the following steps:

MU aqueous solution with a concentration of 66.7% was dried with CS powder (tapioca powder) at 40 ℃ for 8 hours according to a mass ratio of 1:9 mixing by a high speed mixer (2500Y, yongkang platinum European hardware Co., ltd.) at 2000r/min for 15min at room temperature until no solid particles. The mixture was fed into a kneader (Hongxin mechanical Co., ltd., lycra) and kneaded at 30℃for 1 hour at 30r/min to a uniform amorphous powder. The material was stored in a sealed condition at 25℃for 24 hours to form a homogeneous stable system, yielding a plasticized starch (TPCS).

The PBAT pellets were dried in a vacuum oven at 80 ℃ for 8 hours and the compatibilizer maleic anhydride at 40 ℃ for 8 hours. According to the mass ratio of PBAT to CS powder of 8:2 adding PBAT and the plasticized starch prepared above into a high-speed mixer, simultaneously adding maleic anhydride accounting for 4% of the total mass of PBAT granules and CS powder, mixing for 10 minutes at 2000r/min, and then adding into a co-rotating twin-screw extruder (TE-20, ke-time Long Keya (Nanj) mechanical Co., ltd.), wherein the screw diameter is 21mm and the length-diameter ratio is 36:1. The screw speed was 35RPM and the temperature from the feed zone to the head was 135-135-145-145-140 ℃. The extrudate was cooled and pelletized by a pelletizer (180, sail mechanical Co., ltd.) to obtain PBAT/TPCS polyester blend pellets.

Examples 2 and 3: the method for preparing PLA/TPCS by the process of forming core-shell starch particle reinforced and toughened polyester in situ in the process of producing urea formaldehyde by reactive extrusion comprises the following steps:

(1) An aqueous solution with a MU concentration of 66.7% was mixed with CS powder (tapioca starch powder) dried at 40 ℃ for 8 hours at a mass ratio at room temperature by a high speed mixer (2500Y, platinum euro hardware ltd. In Yongkang city) for 15min at 2000r/min until no solid particles were present. The mixture was fed into a kneader (Hongxin mechanical Co., ltd., lycra) and kneaded at 30℃for 1 hour at 30r/min to a uniform amorphous powder. The material was stored in a sealed condition at 25℃for 24 hours to form a homogeneous stable system, yielding a plasticized starch (TPCS).

(2) The PLA pellets were dried in a vacuum oven at 80℃for 8 hours and the compatibilizer maleic anhydride at 40℃for 8 hours. PLA and the above-prepared plasticized starch (TPCS) were added to a high-speed mixer at a certain mass ratio, and maleic anhydride accounting for 4% of the total mass of PLA particles and starch was added at the same time, mixed at 2000r/min for 10 minutes, and then added to a co-rotating twin-screw extruder (TE-20, ke-time Long Keya (Nanj) mechanical Co., ltd.), screw diameter of 21mm and aspect ratio of 36:1. The screw speed was 35RPM and the temperature from the feed zone to the head was 160-160-170-170 ℃. The extrudate was cooled and pelletized by a pelletizer (180, sail mechanical Co., ltd.) to give PLA/TPCS pellets.

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

Comparative example 1

A method of making a PBAT/CS polyester blend comprising the steps of:

the PBAT pellets were dried in a vacuum oven at 80 ℃ for 8 hours and the compatibilizer maleic anhydride at 40 ℃ for 8 hours. According to the mass ratio of PBAT granules to CS powder of 8:2 PBAT and dried CS were added to a high speed mixer while adding maleic anhydride in an amount of 4% by mass of the total of PBAT particles and starch, mixed at 2000r/min for 10 minutes, and then added to a co-rotating twin screw extruder (TE-20, family power Long Keya (south kyo mechanical limited), screw diameter 21mm, length to diameter ratio 36:1. The screw speed was 35RPM and the temperature from the feed zone to the head was 135-135-145-145-140 ℃. The extrudate was cooled and pelletized by a pelletizer (180, sail mechanical Co., ltd.) to obtain PBAT/CS pellets.

Comparative example 2:

polylactic acid/starch (PLA/CS) blends were prepared as a comparison by the formulation and process of example 2 except that no methylol urea was added, i.e., comparative example 2 differed from example 2 only in that no methylol urea was added.

Table 1 comparison of properties of polyester blends prepared in examples and comparative examples

In table 1, the tensile strength and elongation at break of the PBAT/TPCS polyester blend prepared in example 1 are significantly improved compared with those of the PBAT/CS polyester blend prepared in comparative example 1 and the pure PBAT resin, and the remarkable reinforcing and toughening effects of the core-shell starch particles in the system are shown. In addition, as can be seen from the comparison of examples 2 and 3 with comparative example 2, as the urea formaldehyde content of the plasticizing compatibilizer generated by in-situ polycondensation of reactive extrusion increases, the tensile strength and the elongation at break of the prepared PLA/starch blend are both improved, and the material shows better comprehensive mechanical properties. The process can obviously improve the interfacial binding force between the components of the PLA/starch blend, so that the strength and toughness of the blend material are obviously improved compared with those of the polylactic acid/unmodified starch blend.

Among them, polybutylene adipate terephthalate (PBAT), PBAT/tapioca starch (PBAT/CS) blend prepared in comparative example 1, and polyester blend (PBAT/TPCS) prepared in example 1 were studied as follows:

the torque rheometer can simulate the actual production and processing process and is widely used for representing the evolution of the raw material blending process in the processing and the processing flow property of the composite material. As can be seen from fig. 1, the torque of pure PBAT increases rapidly to a maximum value with the extension of the mixing time, which is attributable to the viscosity increase caused by the melting of the polymer. With a subsequent further increase in kneading time, the torque of the pure PBAT system was reduced and reached an equilibrium torque value of 5.8n.m. Comparative example 1 the overall trend of PBAT/CS with the addition of unplasticized starch was similar to that of pure PBAT, and the equilibrium torque value was also substantially 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 reduced by nearly 40% compared with PBAT and PBAT/CS, indicating excellent processability of the system.

FIG. 2 shows that the PBAT/CS prepared in comparative example 1 has an island-in-sea structure with a typical PBAT as a continuous phase and CS as a dispersed phase. But the size of the dispersed phase is not uniform and the interface between the two phases is clear. As can be seen from fig. 3 and 4, the surface of the pit of the starch phase pulled out by the PBAT/CS during the fracture process is smooth, which indicates that the compatibility between the PBAT and the CS is poor and the interface interaction is weak.

Fig. 5 shows that the PBAT/TPCS prepared in example 1 of the present invention still exhibits an islands-in-the-sea structure, but the spherical structure of the starch is intact, the particle size is significantly increased, and the dispersion in the resin matrix is also more uniform. In particular, it is evident that the core-shell structure of TPCS in PBAT/TPCS blends. The larger scale inset in the upper left corner of fig. 5 more clearly shows that the surface of the starch granules in the PBAT matrix has a shell layer of a certain thickness, which is also the main reason for the significant increase in particle size of TPCS. Figures 6 and 7 show that the pits of the starch phase of the PBAT/TPCS are significantly reduced during the fracture process, indicating that the interfacial interaction between the two phases is enhanced and the interfacial compatibility is significantly improved. In addition, the fracture surface showed significant matrix yielding deformation and the presence of elongated fibers. Research shows that the main toughening mechanism of the core-shell structure toughening agent is that microfibers are formed between the matrix and the particle interface of the core-shell structure toughening agent, and the microfibers can effectively transfer stress to cause the matrix to generate plastic deformation, so that the toughening matrix polymer is reinforced at the same time.

In FIG. 8, 1710cm ^-1 The nearby significant absorption peak was derived from the stretching vibration of c=o of the ester group of PBAT, and fig. 9 shows that the positions of this absorption peak of PBAT and PBAT/CS prepared in comparative example 1 are identical, indicating that no interaction between PBAT and CS occurs in PBAT/CS, and that the two components are simply physically mixed. While the peak in the PBAT/TPCS prepared in example 1 was red shifted. CS alone is at 3300cm due to the presence of a large number of hydroxyl groups (-OH) in its molecular chain ⁻¹ Broad peaks appear nearby. 3327cm in the pure MU (methylol urea) and UF (urea formaldehyde) curves ^-1 at-CONH-H-representing an amide bond-bending vibrations of N-H. As can be seen from fig. 10, in the spectrum of TPCS, the width ratio CS of the corresponding absorption peak is narrowed and shifted from both peaks. Furthermore, TPCS is at 1550cm compared to MU ^-1 Corresponds to-CONH ₂ -COO-stretching vibration peak in group and 2900cm ^-1 The absorption peaks at the positions are red shifted. The shift of the representative absorption peak shows that the reaction precursor MU which is added in the form of solution and is easy to diffuse and infiltrate can break the intermolecular and intramolecular hydrogen bonds in the starch infiltrate layer, thereby facilitating the movement of starch molecular chains. In addition, the PBAT in PBAT/TPCS forms hydrogen bond interactions with other components.

Fig. 11 and 12 show that CS has no significant melt and crystallization transitions. As can be seen from fig. 11, the crystallization transition of pure PBAT occurs around 42 ℃ (tc=42 ℃). As can be seen from fig. 12, pure PBAT shows a transition at 125 ℃, corresponding to melting of crystalline PBAT (tm=125 ℃). For the PBAT/CS prepared in comparative example 1, it can be seen from FIG. 11 that there are two peaks corresponding to 85℃and 110℃and that the strong peak at 85℃is attributed to the Tc of PBAT, and that the substantial increase from 42℃to 85℃of pure PBAT is due to the promotion of the movement and ordered arrangement of the molecular chains of PBAT by CS particles at higher temperatures. Another weak peak at 110℃is designated as 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 ℃. One possible reason for the simultaneous melting and crystallization transition in DSC is that high temperature and strong shear promote rearrangement of starch chains, thereby facilitating crystallization of starch. The presence of two distinct absorption peaks for PBAT/CS suggests that the 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 a distinct absorption peak of PBAT, and that there is almost no CS present. In addition, the peak temperature corresponding to the PBAT absorption peak of the PBAT/TPCS is higher than that of pure PBAT, which indicates that TPCS particles in the system can promote the movement and the regular arrangement of PBAT molecular chains at a higher temperature.

As shown in FIGS. 13 and 14, the TG curve of UF (urea formaldehyde) is significantly different from MU (methylol urea) in that MU has a thermal decomposition zone at 110-214℃which is the thermal decomposition zone of unreacted urea during synthesis. Whereas UF has mainly two decomposition intervals of 180-220 ℃, 220-600 ℃. TPCS exhibits a very small pyrolysis peak at 110-214℃due to the low MU content. The PBAT/TPCS prepared in example 1 showed no thermal decomposition peak at 110-214℃, indicating that during extrusion, the polycondensation reaction of MU did occur to form UF. PBAT showed a single degradation step from 340℃to 460℃and the peak temperature value in the DTG curve was 428℃Tp _PBAT =428℃). CS undergoes a three-step degradation process. The first degradation step occurs at 50 ℃ to 110 ℃, corresponding to the elimination of water and other low molecular weight compounds. The second degradation step (shoulder temperature 305 ℃) and the apparent third degradation step (main peak temperature 340 ℃) correspond to the degradation of amylose and amylopectin, respectively. The degradation peaks corresponding to amylose and amylopectin in the curve of TPCS are respectively at 290 ℃ and 328 ℃, and the degradation peak is lower than that of pure CS, which indicates that the reaction precursor MU which is easy to diffuse and infiltrate of small molecules leads to plasticization of CS, and the hydrogen bonding acting force in the molecules is destroyed, so that the thermal stability is reduced. Degradation peaks corresponding to starch and PBAT appear in the curves for both PBAT/CS prepared in comparative example 1 and PBAT/TPCS prepared in example 1, with peak positions substantially consistent with tp=428 ℃ for pure PBAT, but with a larger difference from t=340 ℃ for CS. T of PBAT/CS _CS A significant decrease in comparison to pure CS, at 326 ℃, indicates that the hydrogen bonding forces and crystalline structure in the starch molecule are destroyed and thus their thermal stability is reduced. While T of PBAT/TPCS _CS Increased =345 ℃, compared to pure CS, indicates that UF generated in situ during reactive extrusion increases the number of hydrogen bonds formed between macromolecular chains of each component, resulting in increased degradation temperatures.

As can be seen from FIG. 15, the pure CS (starch) is at 2θ4 strong diffraction peaks appear at 15.10 degrees, 17.12 degrees, 17.84 degrees and 22.95 degrees, and one weak diffraction peak appears at 20.12 degrees, which indicates that the crystal is a double-helix structure type A crystal. The main characteristic peaks of plasticized starch (TPCS) are substantially consistent with those of CS, indicating that the readily diffusible and wettable reactive precursor MU of small molecules added in solution does not disrupt the crystalline structure of CS, but results in a significant decrease in crystallinity (23.58%) of TPCS over that of CS (41.55%). The reason for this phenomenon should beThe small molecular precursor MU adsorbed on the surface of the TPCS particles forms strong hydrogen bond interaction with CS macromolecules in the wetting layer, so that the crystallinity of the TPCS is obviously reduced compared with that of the CS. At the same time, however, the strong hydrogen bond interaction between the two also makes MU not go deep into the interior of starch and replace the intramolecular and intermolecular hydrogen bonds of starch by the hydrogen bonds between the MU and the starch to destroy the A-type crystal structure of the starch under the temperature and shearing action of the mixing process as other currently used plasticizers. While the retention of the rigid starch crystal structure should be more beneficial to the enhancement of the strength of the 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). Characteristic peaks of 2θ=22.31 °, 24.71 °, and 31.03 ° confirm the presence of a definite crystalline region in UF. The PBAT/CS blend prepared in comparative example 1 exhibited a crystallization peak attributed to PBAT and a crystallization peak of the type A crystals of starch. Whereas the PBAT/TPCS blend prepared in example 1 showed a small crystalline peak at 24.71 corresponding to UF in addition to the two crystalline peaks described above, indicating that MU did react to form UF during reactive extrusion. In addition, fig. 15 also shows that the crystallinity of PBAT was 22.06%, and that after 20wt.% CS with crystallinity of 41.55% was added, the crystallinity of the PBAT/CS blend prepared in comparative example 1 reached 23.55%, which is a small increase over pure PBAT; whereas the crystallinity of the PBAT/TPCS prepared in example 1 was 20.36%, it was lower than that of pure PBAT. The reason for this result should be that the small molecule precursor MU adsorbed on the CS particle surface preferentially generates UF (urea formaldehyde) polymer in situ between two phase interfaces through polycondensation reaction under the high temperature and high shear effect of extrusion process, whereas UF with lower molecular chain movement capability forms hydrogen bond interaction with hydroxyl groups on the CS surface and carboxyl or hydroxyl end groups on the PBAT surface through hydroxymethyl, amide groups and amino end groups on its macromolecules, respectively, thereby further promoting plasticization of CS and compatibilizing PBAT/TPCS blend through hydrogen bond bridging action while not affecting their crystal structure.

FIG. 16 shows that the biodegradation rates of PBAT/CS prepared in comparative example 1 and PBAT/TPCS prepared in example 1 are significantly higher than those of pure PBAT. Microorganisms preferentially degrade readily degradable materials during degradation. CS, a naturally degradable polymer, is preferentially decomposed by microorganisms due to its good hydrophilicity. Therefore, the biodegradability of PBAT/CS is superior to that of PBAT. The UF with good biocompatibility can release nutrient element nitrogen while being decomposed by microorganisms, so that the activity of the microorganisms is improved, and the degradation rate of PBAT/TPCS is further optimized.

Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present invention, and not for limiting the same; although the invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some or all of the technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit of the invention.

Claims

1. The process of preparing PBAT/TPCS with in-situ forming core-shell starch particle reinforced toughened polyester in the process of producing urea formaldehyde through reaction extrusion includes the following steps:

(1) An aqueous solution of 66.7% methylol urea and tapioca starch powder dried at 40 ℃ for 8 hours were mixed according to a mass ratio of methylol urea to tapioca starch powder of 1:9, mixing for 15min at the room temperature by a high-speed mixer at 2000r/min until no solid particles exist; adding the mixture into a kneader, and kneading at 30 ℃ for 1h at 30r/min until the mixture is uniform and free of crystal point powder; sealing and preserving the material at 25 ℃ for 24 hours to form a homogeneous stable system, so as to obtain plasticized starch;

(2) The polybutylene adipate-terephthalate particles are dried in a vacuum oven at 80 ℃ for 8 hours, and the compatibilizer maleic anhydride is dried at 40 ℃ for 8 hours, according to the mass ratio of the polybutylene adipate-terephthalate to the tapioca starch powder of 8:2 adding polybutylene adipate terephthalate particles and the plasticized starch prepared in the step (1) into a high-speed mixer, simultaneously adding maleic anhydride accounting for 4% of the total mass of the polybutylene adipate terephthalate particles and the tapioca starch powder, mixing for 10 minutes at 2000r/min, then adding into a homodromous double-screw extruder, wherein the diameter of the screw is 21mm, the length-diameter ratio is 36:1, the rotating speed of the screw is 35RPM, the temperature from a charging area to a machine head is 135-135-145-145-140 ℃, and granulating the extruded material by a granulator after cooling to obtain PBAT/TPCS polyester blend particles.

2. The method for preparing PLA/TPCS by the process of forming core-shell starch particle reinforced and toughened polyester in situ in the process of producing urea formaldehyde by reactive extrusion is characterized by comprising the following steps:

(1) Mixing 66.7% concentration aqueous solution of methylol urea with 40 ℃ cassava starch powder for 8 hours according to the mass ratio of the methylol urea to the cassava starch powder of 1.5:8, at room temperature, through a high-speed mixer at 2000r/min for 15min until no solid particles exist, adding the mixture into a kneader, kneading at 30 ℃ for 1h at 30r/min until uniform crystal-free powder exists, and sealing and preserving the material at 25 ℃ for 24h to form a homogeneous stable system to obtain plasticized starch;

(2) Drying polylactic acid particles in a vacuum oven at 80 ℃ for 8 hours, and drying compatibilizer maleic anhydride at 40 ℃ for 8 hours according to the mass ratio of the polylactic acid particles to the tapioca starch powder of 7: and 3, adding polylactic acid particles and the plasticized starch prepared in the step 1 into a high-speed mixer, simultaneously adding maleic anhydride accounting for 4% of the total mass of the polylactic acid particles and the tapioca starch powder, mixing for 10 minutes at 2000r/min, then adding into a homodromous double-screw extruder, wherein the diameter of a screw is 21mm, the length-diameter ratio is 36:1, the rotating speed of the screw is 35RPM, the temperature from a feeding area to a machine head is 160-160-170-170 ℃, and granulating the extruded material by a granulator after cooling to obtain PLA/TPCS particles.

3. The method for preparing PLA/TPCS by the process of forming core-shell starch particle reinforced and toughened polyester in situ in the process of producing urea formaldehyde by reactive extrusion is characterized by comprising the following steps:

(1) Mixing 66.7% concentration aqueous solution of methylol urea with 40 ℃ cassava starch powder for 8 hours according to the mass ratio of the methylol urea to the cassava starch powder of 2:8, mixing at room temperature for 15min at 2000r/min by a high-speed mixer until no solid particles exist, adding the mixture into a kneader, kneading at 30 ℃ for 1h at 30r/min until uniform crystal-free powder exists, and sealing and preserving the material at 25 ℃ for 24h to form a homogeneous stable system to obtain plasticized starch;