CN115926401A - In-situ formation of core-shell starch reinforced and toughened polyester during urea formaldehyde reaction extrusion - Google Patents
In-situ formation of core-shell starch reinforced and toughened polyester during urea formaldehyde reaction extrusion Download PDFInfo
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Abstract
The invention relates to the technical field of biodegradable polymers, in particular to a core-shell starch reinforced and toughened polyester formed in situ during urea formaldehyde reaction and extrusion. The method comprises the steps of modifying starch with a reaction precursor hydroxymethyl urea solution of urea formaldehyde with an extremely low amount to obtain plasticized starch, extruding the plasticized starch and a mixture of biodegradable polyester through a screw extruder, wherein under the action of high temperature and high shear in the extrusion process, the hydroxymethyl urea preferentially generates urea formaldehyde in situ between two phase interfaces through polycondensation reaction, and the urea formaldehyde interacts with active functional groups on the surfaces of the starch and the polyester through hydrophilic groups on macromolecules of the urea formaldehyde.
Description
Technical Field
The invention relates to the field of biodegradable polymers, in particular to a core-shell starch reinforced and toughened polyester formed in situ during urea formaldehyde reaction and extrusion.
Background
A large amount of non-degradable plastic waste has caused serious environmental pollution worldwide. In order to effectively treat plastic pollution, nearly 90 countries and regions have policies for controlling or forbidding the use of disposable non-degradable plastic products in the world. The european union banned or restricted the use of ten disposable plastic articles since 2021. In 2020, china issued an opinion on further strengthening plastic pollution treatment, and proposed non-degradable plastic products such as disposable plastic products, express plastic packages and the like which are forbidden or limited to be used in different steps and different fields, so as to encourage the use of the degradable plastic products.
Biodegradable plastics are understood to mean plastics which can be degraded by microorganisms present in nature and are finally converted completely into CO 2 Water, mineralized inorganic salts containing elements and high molecular materials of new biomass. At present, the widely studied and used chemical synthesis biodegradable polymer materials 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 the representative ones mainly include polylactic acid, polybutylene succinate, carbon dioxide-propylene oxide copolymer, polycaprolactone, polydioxanone, polyfurandicarboxylic acid and polybutylene adipate-terephthalate. However, in order to ensure biodegradability, biodegradable polyesters require a high proportion of amorphous structure and therefore have a low degree of crystallinity and poor mechanical properties, while also affecting their processability, which limits the range of applications of biodegradable polyesters. Therefore, at present, how to improve the mechanical properties of biodegradable polyesters is one of the key points for expanding and promoting the applications of biodegradable polyesters. Furthermore, biodegradable plasticsThe price of (b) is high, which further restricts the wide application of biodegradable polymers.
Starch is a useful material in the field of thermoplastics because of its biodegradability, nontoxicity, high purity, low cost, etc. However, starch has good hydrophilicity due to a large number of hydroxyl groups contained in the molecular chain and strong hydrogen bonding interaction between macromolecules, but the property also causes problems in the aspects of sensitivity to water, difficulty in processing and the like. In addition, the mechanical properties of starch are poor, and the starch cannot replace commercial non-degradable plastics. In order to improve the processing property and mechanical property of the starch, the starch is thermoplasticized by adopting a proper process to prepare various thermoplastic starches or blends of the thermoplastic starches and other polymers, so that the application range of the starch can be expanded.
One promising solution to make biodegradable polyesters cost-effective is to mix thermoplastic starch in a proportion of 20-30 wt%. To date, in many studies, starch has been added to biodegradable polyester matrices to reduce costs and improve the performance of composite materials. However, the compatibility between the hydrophobic biodegradable polyester and the hydrophilic starch is poor, resulting in a significant decrease in the performance of the composite. The addition of a reactive compatibilizer is the first choice to alleviate the lack of interfacial adhesion between the biodegradable polyester and the thermoplastic starch while maintaining an acceptable balance between strength, hardness and elongation at break. When the reactive melt blending is carried out in the presence of the compatilizer, the compatilizer can connect the components through covalent bonds and remarkably improve the interface adhesive force, which is beneficial to the effective stress transfer among the components, thereby improving the mechanical property of the biodegradable polyester/thermoplastic starch composite material. Various reactive compatibilizers for biodegradable polyester/thermoplastic starch blends have been reported, representative of which include mainly organic acids, maleic Anhydrides (MAH), and Glycidyl Methacrylates (GMA). The carboxyl groups of the organic acids, the anhydride groups of the MAH, and the epoxy groups of the GMA improve the mechanical properties of the biodegradable polyester/thermoplastic starch blends by enhancing the interfacial adhesion between the two phases. However, low molecular weight organic acids have difficulty in sustained interaction with the polyester matrix, resulting in limited compatibility. The grafting efficiency of MAH on biodegradable polyesters is relatively low. The thermal polymerization product of GMA is complicated, and as GMA increases, a large amount of by-products are formed during mixing 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 effective compatibilizer or compatibilization strategy is highly necessary.
At present, a great deal of research has proved that the composite form of the core-shell can exert the synergistic effect of the two materials, and finally the rigidity and toughness of the modified polymer are considered. The core-shell structure impact modifier has a harder core or shell component, so that the impact modifier can effectively toughen materials and simultaneously keep higher strength and modulus, and is widely applied to various polymer systems at present. On the basis, a toughening method for forming core-shell structure particles in situ by melt blending is gradually developed. The interfacial tension between the polymers is used as a driving force to spontaneously form core-shell structure particles in a blending system, and better stress transfer between a dispersed phase and a matrix can be ensured, so that the material is toughened and reinforced at the same time.
Disclosure of Invention
In order to enhance the interface interaction between the lipophilic biodegradable polyester and the hydrophilic thermoplastic starch, the invention provides a process for in-situ forming core-shell starch particle reinforced and toughened polyester in the process of generating 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 generating urea formaldehyde through reactive extrusion comprises the following steps:
(1) Mixing the hydroxymethyl urea solution with the dried starch powder until no solid particles exist, and then adding the mixture into a kneader to be kneaded until uniform amorphous point powder exists; sealing and storing 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 a compatibilizer maleic anhydride, mixing, adding into a double-screw extruder, extruding at a certain rotating speed and temperature, in-situ generating a urea-formaldehyde polymer from a micromolecule precursor hydroxymethyl urea in the plasticized starch in the extrusion process, and forming a core-shell structure particle of starch granules, polyester/urea-formaldehyde/thermoplastic starch, wherein the core-shell structure particle takes a polyester macromolecular chain, a urea-formaldehyde macromolecular chain and a thermoplastic starch macromolecular chain which are mutually penetrated and mutually penetrated as a shell and takes the starch granules as a core in a biodegradable polyester matrix due to the interfacial tension among the biodegradable polyester, the thermoplastic starch and the urea-formaldehyde polymer; and cooling the extruded strip-shaped object, and then cutting the extruded strip-shaped object into particles by a granulator to obtain the polyester blend.
In the step (1), the small molecular reaction precursor hydroxymethyl urea of the urea-formaldehyde polymer which is easy to diffuse and infiltrate and is added in the form of solution only infiltrates the outer layer of the starch granules due to the self viscosity and the strong hydrogen bond interaction between the small molecular reaction precursor hydroxymethyl urea and the starch macromolecules, and the intermolecular and intramolecular hydrogen bonds in the starch infiltration layer are destroyed through the hydrogen bond interaction, so the processing performance of the starch is obviously improved; at the same time, however, the strong hydrogen bond interaction between the two also enables the hydroxymethyl urea not to go deep into the starch further like other currently used plasticizers under the temperature and shearing action of the mixing process and to replace the intramolecular and intermolecular hydrogen bonds of the starch through the hydrogen bonds between the hydroxymethyl urea and the starch so as to destroy the crystal structure of the starch, but enables the original rigid structure of the starch granule not to be destroyed, and finally enables the polyester blend to have both rigidity and toughness.
In the invention, hydrogen bonds among starch macromolecules and intramolecular hydrogen bonds in plasticized starch are destroyed under the action of high temperature and high shear in the extrusion process, so that a crystalline structure in starch granules is disintegrated through melting and shearing to form a disordered continuous phase of the starch macromolecules, thereby obtaining the thermoplastic starch.
In the step (2), a micromolecule precursor methylol urea in the plasticized starch preferentially generates a urea formaldehyde polymer in situ between two phase interfaces of biodegradable polyester and starch through a polycondensation reaction under the action of high temperature and high shear in the extrusion process, and then a starch granule-polyester/urea formaldehyde/thermoplastic starch core-shell structure granule which takes a polyester macromolecular chain, a urea formaldehyde macromolecular chain and a thermoplastic starch macromolecular chain as a shell and takes a starch granule as a core is formed in a biodegradable polyester matrix due to the interfacial tension among the biodegradable polyester, the thermoplastic starch and the urea formaldehyde polymer. In the extrusion process, the macromolecular urea-formaldehyde polymer generated in situ by the polycondensation reaction between two phase interfaces of hydroxymethyl urea has excellent biodegradability while being used as a plasticizing compatibilizer, and can release nutrient element nitrogen while being degraded by microorganisms so as to improve the activity of the microorganisms, so that the polyester blend prepared by the process disclosed by the invention does not contain non-biodegradable polymers and has further optimized biodegradability.
As a further improvement of the technical scheme of the invention, in the step (1), the mass ratio of the methylol urea to the starch powder is 1 to 9-2.
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 to 7.
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 of the double-screw extruder from the feeding area to the head is set to be 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 proposal of the invention is further improved by one or a mixture of any two or more than two 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 generating the urea formaldehyde by reactive extrusion has the following advantages:
(1) According to the invention, the small molecule reaction precursor hydroxymethyl urea of the urea-formaldehyde polymer which is easy to diffuse and infiltrate and is added in the form of solution only infiltrates the outer layer of the starch granules due to the self viscosity and the strong hydrogen bond interaction between the small molecule reaction precursor hydroxymethyl urea and the starch macromolecules, and under the temperature and shearing action of the mixing process, the hydroxymethyl urea does not penetrate into the starch further like other currently used plasticizers and replaces the intramolecular and intermolecular hydrogen bonds of the starch through the hydrogen bonds with the starch so as to destroy the crystal structure of the starch, but the original rigid structure of the starch granules can not be destroyed, and finally the polyester blend can have both rigidity and toughness.
(2) According to the process, under the action of high temperature and high shear in the extrusion process, micromolecule precursor hydroxymethyl urea adsorbed on the outer layer of plasticized starch granules preferentially generates a urea formaldehyde polymer in situ between two phase interfaces of biodegradable polyester and starch through polycondensation reaction, and then a starch granule-polyester/urea formaldehyde/thermoplastic starch core-shell structure particle with a core of polyester macromolecular chains, urea formaldehyde macromolecular chains and thermoplastic starch macromolecular chains penetrating and mutually permeating is formed in a biodegradable polyester matrix due to the interfacial tension among the biodegradable polyester, the thermoplastic starch and the urea formaldehyde polymer. So that the low-cost polyester blend with excellent processability, mechanical properties and biodegradability can be easily obtained through the conventional industrialized extrusion process.
(3) In the extrusion process, macromolecular urea-formaldehyde polymer generated in situ by the hydroxymethyl urea between two phase interfaces through polycondensation can be used as a plasticizing compatibilizer. The urea formaldehyde is the first agricultural nitrogen slow release product researched and applied and has excellent biodegradability. The urea formaldehyde with good biocompatibility can release the nitrogen as a nutrient element while being decomposed by microorganisms, so that the activity of the microorganisms is improved, and the biodegradation performance 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 urea formaldehyde reaction extrusion process can obtain the polyester blend with extremely low urea formaldehyde content for plasticizing and compatibilization. In addition, the byproduct water of the condensation polymerization reaction of urea formaldehyde generated by in-situ extrusion of hydroxymethyl urea is also an excellent plasticizer of starch, so that the processing performance of a mixed system can be further improved, and the method has important significance for reducing the cost of biodegradable polyester and improving the mechanical property of the biodegradable polyester, thereby further promoting the application of the biodegradable polyester and starch.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and other drawings can be obtained by those skilled in the art without creative efforts.
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 a PBAT/tapioca starch (PBAT/CS) blend prepared in comparative example 1.
FIG. 3 is a Scanning Electron Microscope (SEM) photograph of a quench section of a PBAT/tapioca starch (PBAT/CS) blend prepared in comparative example 1.
FIG. 4 is a Scanning Electron Microscope (SEM) photograph of a tensile 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 face of the polyester blend (PBAT/TPCS) prepared in example 1.
FIG. 7 is a Scanning Electron Microscope (SEM) photograph of a tensile 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) blend prepared in comparative example 1, and polyester blend (PBAT/TPCS) prepared in example 1.
Fig. 9 is a partially enlarged view of fig. 8.
Fig. 10 is another partial enlarged view of fig. 8.
FIG. 11 is a Differential Scanning Calorimetry (DSC) curve comparison of the cooling process for 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 Differential Scanning Calorimetry (DSC) curve during heating comparing 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. 13 is a thermogravimetric plot comparison 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 quotient 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. 15 is a graph comparing the X-ray diffraction (XRD) profiles of polybutylene adipate terephthalate (PBAT), PBAT/tapioca starch (PBAT/CS) blends prepared in comparative example 1, and polyester blends prepared in example 1 (PBAT/TPCS).
FIG. 16 is a graph of the biodegradation rate of 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/TPCS) as a function of time.
Detailed Description
The technical solutions of the present invention are described clearly and completely below, and it is obvious that the described embodiments are some, not all embodiments of the present invention. All other embodiments, which can be obtained by a person skilled in the art without making any creative effort based on the embodiments in the present invention, belong to the protection scope of the present invention.
The invention provides a specific embodiment of a process for in-situ forming core-shell starch particle reinforced and toughened polyester in the process of generating urea formaldehyde through reactive extrusion, which comprises the following steps:
(1) Mixing the hydroxymethyl urea solution with the dried starch powder until no solid particles exist, and then adding the mixture into a kneader to be kneaded into uniform amorphous powder; sealing and storing 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 a compatibilizer maleic anhydride, mixing, adding into a double-screw extruder, extruding at a certain rotating speed and temperature, in-situ generating a urea-formaldehyde polymer from a micromolecule precursor hydroxymethyl urea in the plasticized starch in the extrusion process, and forming a core-shell structure particle of starch granules, polyester/urea-formaldehyde/thermoplastic starch, wherein the core-shell structure particle takes a polyester macromolecular chain, a urea-formaldehyde macromolecular chain and a thermoplastic starch macromolecular chain which are mutually penetrated and mutually penetrated as a shell and takes the starch granules as a core in a biodegradable polyester matrix due to the interfacial tension among the biodegradable polyester, the thermoplastic starch and the urea-formaldehyde polymer; and cooling the extruded strip-shaped object, and then cutting the extruded strip-shaped object into particles by a granulator to obtain the polyester blend.
In the present invention, the mixing of the methylol urea solution with the dried starch powder in step (1) is carried out in a high speed mixer. In the embodiment of the present invention, there is no particular limitation on the specific type and rotation speed of the high-speed mixer, as long as the methylol urea solution and the dried starch powder can be mixed until there is no solid particle.
Further, in the step (1), the mass ratio of hydroxymethyl urea (solute in hydroxymethyl urea solution) to starch is 1 to 9-2. Preferably, the mass ratio of the hydroxymethyl urea to the starch powder is 1.
Further, in the step (2), the mass ratio of the biodegradable polyester to the starch powder in the step (1) is 9 to 7. Preferably, the mass ratio of the biodegradable polyester to the starch powder in the step (1) is (8) - (7).
Further, in the step (2), the addition amount of the maleic anhydride is less than or equal to 6% of the total mass of the biodegradable polyester and the starch powder in the step (1). Preferably, the addition amount of the maleic anhydride is equal to 3-6% of the total mass of the biodegradable polyester and the starch powder in the step (1); more preferably, the amount of maleic anhydride added is equal to 4-5% of the total mass of the biodegradable polyester and the starch powder in step (1).
Further, in the step (2), the rotating speed of the double-screw extruder is 20-400RPM. Preferably, the rotation speed of the double-screw extruder is 20-300RPM; more preferably, the twin screw extruder is operated at 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 to 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 to 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 are fully mixed according to the proportion, a torque rheometer is opened, the temperature is set to the melting temperature of the biodegradable polyester, and then 50g of the weighed materials are added into the rheometer until the torque is constant.
Mechanical properties of the material are as follows: according to the GB/T1040.1-2010 plastic tensile property test.
Biodegradability: according to the test method of GB/T19277, the final aerobic biodegradation capacity of the mulching film is determined by measuring the amount of released carbon dioxide under controlled composting conditions. The method specifically comprises the following steps: 10g of a material sample to be tested, 60g of compost and 320g of sea sand which are uniformly mixed are added into a culture flask, the humidity is kept at 40%, the mixture is cultured in a sealed constant-temperature incubator at 58 ℃, and equal amounts of compost and sea sand are added into blank treatment (namely CK treatment). Continuously introducing CO-free gas into the culture flask 2 The air generated in the culture bottle is introduced into NaOH solution for collection, the sampling is carried out once every 5 days, and the carbon content of the taken material sample is determined by an organic carbon analyzer. The theoretical release amount of carbon dioxide generated by the material to be measured in each culture flask was calculated according to the following formulam(ThCO 2 ) Expressed in grams (g):
in the formula:
m: the mass of the material to be measured in the culture bottle is gram (g);
w c : the carbon content of the material to be tested is obtained by a chemical formula or by an element analysis tester and is expressed by mass fraction;
44 and 12: respectively, the molecular weight of carbon dioxide and the atomic weight of carbon.
Calculating the biodegradation percentage D of the material to be tested in each culture bottle according to the quantity of the carbon dioxide released cumulatively by using the following formula at each test node t （%）：
In the formula:
The technical solution of the present invention will be described in detail by specific examples.
Example 1:
the method for preparing PBAT/TPCS by the process of in-situ forming core-shell starch particle reinforced and toughened polyester in the process of generating urea formaldehyde by reactive extrusion comprises the following steps:
mixing an MU aqueous solution with the concentration of 66.7% and CS powder (cassava starch powder) dried at 40 ℃ for 8 hours according to the mass ratio of 1:9 at room temperature by means of a high-speed mixer (2500Y, platinum European hardware GmbH, yongkang) at 2000r/min for 15min until there are no solid particles. The mixture was charged into a kneader (Hongxin mechanical Co., ltd., ri., ltd.) and kneaded at 30r/min for 1 hour at 30 ℃ to obtain a homogeneous amorphous powder. The material was stored in a sealed condition at 25 ℃ for 24 hours to form a homogeneous stable system, and plasticized starch (TPCS) was obtained.
The PBAT pellets were dried in a vacuum oven at 80 ℃ for 8 hours and the compatibilizer maleic anhydride was dried at 40 ℃ for 8 hours. According to the mass ratio of the PBAT powder to the CS powder of 8:2 adding PBAT and the plasticized starch prepared above to a high-speed mixer, adding maleic anhydride in an amount of 4% by mass of the total of PBAT granules and CS powder at the same time, mixing at 2000r/min for 10 minutes, and then adding to a co-rotating twin-screw extruder (TE-20, machinery Co., ltd., ko-Longya (Nanjing)), screw diameter 21mm, aspect ratio 36. The screw speed was 35RPM and the temperature from the feed zone to the head was 135-135-145-145-140 ℃ in this order. The extrudate was cooled and pelletized by a pelletizer (model 180, one Sail mechanical Co., ltd.) to obtain PBAT/TPCS polyester blend pellets.
Examples 2 and 3: the method for preparing the PLA/TPCS by the process of in-situ forming the core-shell starch particle reinforced and toughened polyester in the urea formaldehyde reaction extrusion generation process comprises the following steps:
(1) An aqueous solution having an MU concentration of 66.7% was mixed with CS powder (tapioca powder) dried at 40 ℃ for 8 hours at a mass ratio at room temperature by a high speed mixer (2500Y, platinum European hardware Co., yongkang) at 2000r/min for 15min until no solid particles were present. The mixture was charged into a kneader (Hongxin mechanical Co., ltd., ri., ltd.) and kneaded at 30r/min for 1 hour at 30 ℃ to obtain a homogeneous amorphous powder. The material is hermetically stored for 24h at 25 ℃ to form a homogeneous stable system, and plasticized starch (TPCS) is obtained.
(2) The PLA granules were dried in a vacuum oven at 80 ℃ for 8 hours and the compatibilizer maleic anhydride was dried at 40 ℃ for 8 hours. PLA and the plasticized starch (TPCS) prepared above were added in a certain mass ratio to a high-speed mixer, and at the same time, maleic anhydride of 4% of the total mass of PLA pellets and starch was added, and mixed at 2000r/min for 10 minutes, and then added to a co-rotating twin-screw extruder (TE-20, machinery Co., ltd., ko-Longya (Nanjing) with a screw diameter of 21mm and a length-to-diameter ratio of 36. The screw speed was 35RPM and the temperature from the feed zone to the head was 160-160-170 ℃ in sequence. After cooling, the extrudate was cut into pellets by a pelletizer (model 180, one Sail machinery Co., ltd.) to obtain PLA/TPCS pellets.
The mass ratio of PLA to CS powder in example 2 was 7:3, the mass ratio of methylolurea to CS powder is 1.5.
The mass ratio of PLA to CS powder in example 3 was 7:3, the mass ratio of methylol urea to CS powder is 2.
Comparative example 1
A method for preparing 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 was dried at 40 ℃ for 8 hours. According to a mass ratio of 8 of PBAT particles to CS powder: 2 adding PBAT and the dried CS into a high-speed mixer, simultaneously adding maleic anhydride accounting for 4% of the total mass of PBAT particles and starch, mixing at 2000r/min for 10 minutes, and then adding into a co-rotating twin-screw extruder (TE-20, machinery Co., ltd., ko-Longkeya (Nanjing)), wherein the diameter of a screw is 21mm, and the length-diameter ratio is 36. The screw speed was 35RPM and the temperature from the feed zone to the head was 135-135-145-145-140 ℃ in this order. The extrudate was cooled and cut into PBAT/CS pellets by a cutter (type 180, one Sail machinery Co., ltd.).
Comparative example 2:
a comparative polylactic acid/starch (PLA/CS) blend was prepared according to the formulation and process of example 2, except that the methylol urea was not added, i.e., comparative example 2 differed from example 2 only in that the methylol urea was not added.
TABLE 1 comparison of the Properties of the polyester blends prepared in the 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, showing the significant reinforcing and toughening effect of the core-shell starch particles in the system. In addition, the comparison between examples 2 and 3 and comparative example 2 shows that with the increase of the content of the plasticizing compatibilizer urea formaldehyde generated by the reaction extrusion in-situ polycondensation, the tensile strength and the elongation at break of the prepared PLA/starch blend are improved, and the material shows better comprehensive mechanical properties. Therefore, the process can obviously improve the interface bonding force among the components of the PLA/starch blend, and obviously improve the strength and toughness of the blend material compared with the polylactic acid/unmodified starch blend.
Among them, the relevant studies 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 were as follows:
the torque rheometer can simulate the actual production process and is widely used for representing the evolution of the raw material blending process in the process and the process flow property of the composite material. It can be seen from fig. 1 that the torque of pure PBAT rises rapidly to a maximum with increasing mixing time, which can be attributed to the viscosity increase caused by melting of the polymer. Subsequently, with further increase in kneading time, the torque of the pure PBAT system decreased and reached an equilibrium torque value of 5.8n.m. Comparative example 1 the overall trend of change in PBAT/CS with the addition of unplasticized starch was similar to that of pure PBAT, as was the equilibrium torque value. The PBAT/TPCS prepared in the embodiment 1 of the invention not only greatly prolongs the time for reaching the maximum torque value, but also greatly reduces the balance torque value to 3.9N.m, which is about 40 percent lower than that of the PBAT and the PBAT/CS, thereby showing the excellent processing performance of the system.
FIG. 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. But the size of the dispersed phase is not uniform and the interface between the two phases is clear. As can be seen from FIGS. 3 and 4, the surface of the pits from which the starch phase is extracted during the fracture of PBAT/CS is smooth, indicating that the compatibility between PBAT and CS is poor and the interfacial interaction is weak.
FIG. 5 shows that PBAT/TPCS prepared in example 1 of the present invention still exhibits sea-island structure, but the spherical structure of starch is intact, the particle size is significantly increased, and the dispersion in the resin matrix is more uniform. In particular, the core-shell structure of TPCS in PBAT/TPCS blends can be clearly seen. The larger-order inset in the upper left corner of fig. 5 shows more clearly that the surface of the starch particles in the PBAT matrix has a thick shell, which is also the main reason for the significant increase in the particle size of TPCS. FIGS. 6 and 7 show that the dishing of the starch phase during fracture of the PBAT/TPCS is significantly reduced, indicating that the interfacial interaction between the two phases is enhanced and that the interfacial compatibility is significantly improved. Furthermore the fractured surface showed a clear matrix yield deformation and elongated fibers were present. Researches show that the main toughening mechanism of the core-shell structure toughening agent is that microfibers are formed between a matrix and toughening agent particle interfaces with core-shell structures, and the microfibers can effectively transfer stress to cause the matrix to generate plastic deformation, so that the matrix polymer is reinforced and toughened at the same time.
In FIG. 8, 1710cm -1 A nearby significant absorption peak results from the C = O stretching vibration of the ester group of the PBAT, and fig. 9 shows that the position of this absorption peak is identical for PBAT and PBAT/CS prepared in comparative example 1, indicating that no interaction between PBAT and CS in PBAT/CS occurs, and that the two components are simply physically mixed. This peak was red-shifted in the PBAT/TPCS prepared in example 1. Simple CS is 3300cm at most due to the presence of a large number of hydroxyl groups (-OH) in its molecular chain −1 A broad peak appears nearby. 3327cm in the pure MU (hydroxymethyl Urea) and UF (Urea-Formaldehyde) curves -1 The point (B) is the bending vibration of H-N-H of-CONH-representing the amide bond. As is clear from fig. 10, in the spectrum of TPCS, the width of the corresponding absorption peak is narrower than that of CS and both peaks are shifted. Furthermore, TPCS was at 1550cm compared to MU -1 Corresponds to-CONH 2 -COO-stretching vibration peak in radical and 2900cm -1 The absorption peaks at (A) are all red-shifted. The displacement of the above representative absorption peaks indicates that the easily diffused and infiltrated reaction precursor MU of small molecules added in the form of solution can destroy intermolecular and intramolecular hydrogen bonds in the starch infiltration layer, thereby facilitating the movement of starch molecular chains. In addition, PBAT in the PBAT/TPCS forms hydrogen bond interactions with other components.
Fig. 11 and 12 show that CS has no significant melt transition and crystalline transition. As can be seen from fig. 11, the crystalline transition of pure PBAT occurred 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 ℃, the strong peak at 85 ℃ is attributed to the Tc of PBAT, and the large increase from 42 ℃ to 85 ℃ of pure PBAT is due to the CS particles promoting the movement and regular alignment of the PBAT molecular chains at higher temperatures. Another weak peak at 110 ℃ was assigned as the Tc of the CS phase in PBAT/CS. FIG. 12, the melt transition of the CS phase in the PBAT/CS prepared in comparative example 1 occurs at 130 ℃. One possible reason for the simultaneous presence of melting and crystallization transitions in DSC is that high temperature and strong shear promote rearrangement of the starch chains, thereby facilitating crystallization of the starch. The existence of two more distinct absorption peaks of PBAT/CS indicates that PBAT and CS are only physically mixed in the treatment. For the PBAT/TPCS prepared in example 1, it can be seen that there is only a distinct absorption peak for PBAT, with almost no presence of CS. In addition, the peak value 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 regular arrangement of PBAT molecular chains at higher temperature.
As shown in FIGS. 13 and 14, the TG curve of UF (urea formaldehyde) is clearly different from that of MU (methylol urea) which has a thermal decomposition zone at 110-214 deg.C, which is the thermal decomposition zone of unreacted urea during synthesis. UF mainly has two decomposition intervals of 180-220 deg.C and 220-600 deg.C. Due to the low MU content, TPCS showed a very small pyrolysis peak at 110-214 ℃. No thermal decomposition peak occurred at 110-214 ℃ in PBAT/TPCS prepared in example 1, indicating that during extrusion, the MU did undergo polycondensation to form UF. PBAT shows a single degradation step from 340 ℃ to 460 ℃ with a peak temperature value of 428 ℃ (Tp) in the DTG curve 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 third apparent degradation step (main peak temperature 340 ℃) correspond to the degradation of amylose and amylopectin, respectively. The peak values of degradation peaks corresponding to amylose and amylopectin in the TPCS curve are respectively at 290 ℃ and 328 ℃, and are lower than those of pure CS, which shows that small molecule reaction precursor MU which is easy to diffuse and infiltrate causes plasticization of CS, hydrogen bond acting force in the molecule is damaged, and therefore thermal stability is reduced. Degradation peaks corresponding to starch and PBAT appeared in both the PBAT/CS prepared in comparative example 1 and the PBAT/TPCS prepared in example 1, with peak positions substantially identical to Tp =428 ℃ for pure PBAT, but different from T =340 ℃ for CSIs relatively large. T of PBAT/CS CS =326 ℃, a significant reduction from pure CS, indicating that the hydrogen bonding forces and crystalline structure in the starch molecule are disrupted and thus its thermal stability is reduced. And T of PBAT/TPCS CS 345 c, higher than pure CS, indicating that UF generated in situ during reactive extrusion increases the number of hydrogen bonds formed between the macromolecular chains of each component, resulting in an increase in degradation temperature.
As can be seen in FIG. 15, 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 a weak diffraction peak appears at 20.12 degrees, which indicates that the crystal is an A-type crystal with a double helix structure. The main characteristic peak of the plasticized starch (TPCS) is substantially consistent with that of CS, indicating that the easily diffusible and wettable reaction precursor MU of small molecules added in solution form does not destroy the crystal structure of CS, but results in a significant decrease in crystallinity of TPCS (23.58%) over CS (41.55%). The reason for this should be that the small molecule precursor MU adsorbed on the surface of the TPCS particle forms strong hydrogen bond interaction with the CS macromolecule in the wetting layer, thus resulting in a significant decrease in crystallinity of the TPCS compared to the CS. But at the same time, the strong hydrogen bond interaction between the two also causes MU not to go further into the starch and replace the intra-and intermolecular hydrogen bonds of the starch by the hydrogen bonds with the starch to destroy the A-type crystal structure of the starch under the temperature and shearing action of the mixing process, unlike other currently used plasticizers. And the maintenance of the crystalline structure of the rigid starch should be more favorable for improving 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). The characteristic peaks of 2 θ =22.31 °, 24.71 ° and 31.03 ° confirm the presence of well-defined crystalline regions in UF. The PBAT/CS blend prepared in comparative example 1 exhibited a crystallization peak attributed to PBAT and a crystallization peak of type a crystals of starch. In addition to the two types of crystallization peaks described above, the PBAT/TPCS blend prepared in example 1 also exhibited a small crystallization peak corresponding to UF at 24.71 as indicated by the arrow, indicating that the MU did react to UF during the reactive extrusion. In addition, FIG. 15 also shows that the crystallinity of PBAT is 22.06%, 20wt being added% of the CS with a crystallinity of 41.55%, the crystallinity of the PBAT/CS blend prepared in comparative example 1 reached 23.55%, which was improved by a small amount compared to pure PBAT; the crystallinity of PBAT/TPCS prepared in example 1 was 20.36% lower than that of pure PBAT. The reason for this result should be that, under the action of high temperature and high shear during the extrusion process, the small molecule precursor MU adsorbed on the surface of CS particles preferentially generates the UF (urea formaldehyde) polymer in situ between two phase interfaces through a polycondensation reaction, and the UF with lower molecular chain mobility forms hydrogen bond interactions with hydroxyl on the surface of CS and terminal carboxyl or hydroxyl on the surface of PBAT through hydroxymethyl, amide and terminal amino on its macromolecule, respectively, thereby further promoting plasticization of CS and compatibilization of PBAT/TPCS blend through hydrogen bond bridging action without affecting their crystal structures.
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 for pure PBAT. During the degradation process, microorganisms will preferentially degrade materials that are easily degraded. CS, a natural degradable polymer, is preferentially decomposed by microorganisms due to its good hydrophilicity. Thus, biodegradability of PBAT/CS is superior to that of PBAT. Meanwhile, the UF with good biocompatibility can be decomposed by microorganisms and release nutrient element nitrogen, 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 used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and these modifications or substitutions do not depart from the spirit of the corresponding technical solutions of the embodiments of the present invention.
Claims (9)
1. A process for in-situ forming core-shell starch particle reinforced and toughened polyester in a urea formaldehyde reaction extrusion process is characterized by comprising the following steps:
(1) Mixing the hydroxymethyl urea solution with the dried starch powder until no solid particles exist, and then adding the mixture into a kneader to be kneaded until uniform amorphous point powder exists; sealing and storing 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 maleic anhydride as a compatibilizer, mixing, adding into a double-screw extruder, extruding at a certain rotating speed and temperature, wherein in the extruding process, a micromolecule precursor hydroxymethyl urea in the plasticized starch generates a urea-formaldehyde polymer in situ, and meanwhile, due to the interfacial tension among the biodegradable polyester, the thermoplastic starch and the urea-formaldehyde polymer, a core-shell structure particle of starch granules, polyester/urea-formaldehyde/thermoplastic starch, wherein the core-shell structure particle takes a polyester macromolecular chain, a urea-formaldehyde macromolecular chain and a thermoplastic starch macromolecular chain as mutually penetrated and mutually penetrated network structures as shells and takes the starch granules as cores; and cooling the extruded strip-shaped object, and then cutting the extruded strip-shaped object into particles by a granulator to obtain the polyester blend.
2. The process for in-situ formation of the core-shell starch particle reinforced and toughened polyester in the urea formaldehyde reaction extrusion forming process according to claim 1, wherein in the step (1), the mass ratio of the methylol urea to the starch powder is 1 to 9-2.
3. The process for in-situ formation of the core-shell starch particle reinforced and toughened polyester in the urea formaldehyde reaction extrusion forming process according to claim 1, wherein in the step (2), the mass ratio of the biodegradable polyester to the starch powder in the step (1) is 9 to 7.
4. The process of claim 1, wherein maleic anhydride is added in an amount of 6% or less of the total mass of the biodegradable polyester and the starch powder in step (1) in step (2).
5. The process of claim 1, wherein in step (2), the twin screw extruder is operated at 20-400RPM.
6. The process of claim 1, wherein the twin screw extruder is set at a temperature of 100-200 ℃ from the feed zone to the head in step (2).
7. The process of claim 1, wherein the starch powder is one or a mixture of any two or more of potato starch, bean starch, cereal starch and vegetable starch.
8. The process of claim 1, wherein the biodegradable polyester comprises aliphatic polyester and aliphatic-aromatic copolyester.
9. The process of claim 1, wherein the biodegradable polyester is one or a mixture of two or more of polylactic acid, polybutylene succinate, carbon dioxide-propylene oxide copolymer, polycaprolactone, polydioxanone, polyfuranic acid, and polybutylene adipate-terephthalate.
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