CN112662152A - Polylactic acid-based degradable composite material, preparation method and application of polylactic acid-based degradable composite material as mulching film - Google Patents

Polylactic acid-based degradable composite material, preparation method and application of polylactic acid-based degradable composite material as mulching film Download PDF

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CN112662152A
CN112662152A CN202011537397.XA CN202011537397A CN112662152A CN 112662152 A CN112662152 A CN 112662152A CN 202011537397 A CN202011537397 A CN 202011537397A CN 112662152 A CN112662152 A CN 112662152A
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polylactic acid
composite material
hydroxyapatite
degradable composite
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CN112662152B (en
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刘畅
郝宏斌
刘思啸
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Shanxi Research Institute Of Biomass New Materials Industry Co ltd
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Abstract

The invention discloses a polylactic acid-based degradable composite material, a preparation method and application of the polylactic acid-based degradable composite material as a mulching film. The polylactic acid degradable composite material provided by the inventionThe organic fertilizer has good mechanical strength and high degradation rate, the degradation product does not influence the original acidity and alkalinity of soil, and the degradation product degrades and releases N, P and other nutrient elements, thereby promoting plant growth and improving the physical and chemical properties of soil.

Description

Polylactic acid-based degradable composite material, preparation method and application of polylactic acid-based degradable composite material as mulching film
Technical Field
The invention belongs to the technical field of agricultural film materials, and particularly relates to a polylactic acid degradable composite material, a preparation method thereof and application thereof as a mulching film.
Background
Polylactic acid (PLA), also known as polylactide, is a polyester obtained by polymerizing lactic acid as a raw material. The polylactic acid has excellent biodegradability, compatibility and absorbability. Polylactic acid is a non-toxic and non-irritant synthetic polymer material, and is prepared from lactic acid mainly from starch (such as corn and rice) by fermentation. PLA raw material has wide source, and products prepared from the PLA raw material can be directly composted or incinerated after being used and can be finally completely degraded into CO2And H2And O, meeting the requirement of sustainable development. PLA is inferior in mechanical strength, high in brittleness, insufficient in strength, poor in toughness, and easy to break, and therefore it is necessary to modify PLA by toughening.
Hydroxyapatite (HA) is a natural mineral of calcium apatite, HAs a composition and structure similar to those of human bone tissues, HAs good biocompatibility, and is a recognized bone repair material. Since the 80 s of the 20 th century, the polylactic acid and hydroxyapatite composite material is considered to be an ideal material for bone repair and bone replacement, and is currently applied to bone tissue repair materials.
As known to those skilled in the art, the mechanical properties of the inorganic particles and the polymer composite system are mainly determined by the interfacial bonding force between the inorganic particles and the polymer composite system, and the dispersibility of the inorganic particles and the bonding condition between the two phases can significantly affect various properties of the composite material. The hydroxyapatite has high surface energy and can generate serious agglomeration in a polylactic acid matrix. Secondly, the difference between the thermal expansion coefficients of the hydroxyapatite and the PLA is large, the bonding force with the PLA interface is small, and the mechanical property of the composite material is seriously reduced. Therefore, the key to improve the agglomeration of the hydroxyapatite and the compatibility with the polylactic acid is to improve the mechanical property of the composite material.
Patent document 201410254491.2 discloses a polylactic acid fiber-reinforced polylactic acid/hydroxyapatite composite material, which includes a plurality of polylactic acid-hydroxyapatite film layers arranged from top to bottom, a polylactic acid fiber layer is provided between two adjacent polylactic acid-hydroxyapatite film layers, and each polylactic acid-hydroxyapatite film layer and each polylactic acid fiber layer are thermally pressed together. Although in technical effect, the skilled person claims that the polylactic acid fibers and the polylactic acid matrix have the same chemical structure, they are completely compatible with each other, and there is no interface problem. However, those skilled in the art know that such layered composite materials prepared by hot pressing are not only complex in process and high in preparation condition requirements, but also not as strong as integrally crosslinked materials in use strength.
Patent document 201510146352.2 discloses a method for preparing a novel hydroxyapatite-grafted polylactic acid, which comprises reacting-NH with 3-Aminopropyltriethoxysilane (APTES)2Modifying to the HA surface; activating carboxyl end groups on a polylactic acid molecular chain by using N, N' -Carbonyldiimidazole (CDI) to form a reaction intermediate, and then reacting the modified HA with polylactic acid to obtain hydroxyapatite graft polylactic acid (HA-PLLA). Although the method is feasible in principle, in the actual operation process, the reaction of APTES and HA will react to form-NH2Modification to the HA surface is very difficult. Since HA HAs a high surface energy, it is prone to agglomerate in inorganic or organic solvents, and cannot be chemically modified theoretically on the premise of poor dispersibility.
In order to prevent coagulation of HA during the reaction, it is a relatively conventional practice to surface-modify HA particles with a modifying agent such as a silane coupling agent, polyethylene glycol, dodecanol, or the like. Patent document 201610944507.1 discloses a modified nano-hydroxyapatite/polyethylene glycol composite hydrogel, which is prepared by grafting a silane coupling agent onto the surface of nano-hydroxyapatite to make the surface of the hydroxyapatite aminated; grafting polyethylene glycol containing terminal carboxyl with hydroxyapatite; and mixing the HA grafted with the polyethylene glycol with the multi-arm end amino PEG solution, and adding a cross-linking agent for incubation to obtain the modified nano hydroxyapatite/PEG composite hydrogel. According to the preparation method, polyethylene glycol is grafted on the surface of HA in order to improve the problem of HA coagulation, but the polyethylene glycol is a water-soluble long-chain compound, and the effect of preventing HA particles from coagulation by only depending on the chain-shaped high polymer material is not ideal in practice.
In order to overcome the defects in the aspects of the prior art and the application field, the technical personnel of the invention assume that the excellent characteristics of polylactic acid and hydroxyapatite are combined simultaneously, and develop and design a composite material with high mechanical strength and good toughness, wherein the composite material is quickly biodegraded, can release phosphorus elements after being degraded, promotes the growth of plants, and is very suitable for being used as an agricultural mulching film.
Disclosure of Invention
The invention aims to provide a polylactic acid-based degradable composite material, a preparation method thereof and application of the polylactic acid-based degradable composite material in the field of mulching films.
In a first aspect, the invention provides a polylactic acid-based degradable composite material, which is characterized in that the polylactic acid-based degradable composite material is obtained by compounding modified hydroxyapatite and polylactic acid, wherein the modified hydroxyapatite is prepared by performing surface modification on the hydroxyapatite by using polyethylene glycol and/or linear polyphosphazene as a modifier.
The linear polyphosphazene has a structure shown in a formula I:
Figure RE-GDA0002941908410000031
wherein n is an integer between 450 and 2000, and m is an integer between 0 and 16.
The synthetic route diagram of the linear polyphosphazene preparation method is shown in figure 1, and comprises the following steps:
(1) reacting polydichlorophosphazene with methoxy polyethylene glycol alkali metal salt in tetrahydrofuran or dioxane solvent at 40-100 deg.C for 12-48 hr to obtain methoxy polyethylene glycol ether substituted polyphosphazene;
(2) adding trimethyl iodosilane into polyphosphazene substituted by methoxy polyglycol ether for end group displacement, reacting at 30-35 deg.C for 1-4 days, and hydrolyzing to obtain linear polyphosphazene.
Preferably, the terminal group replacement reaction solvent is one or a combination of two or more of chloroform and dichloromethane, and the hydrolysis reaction solvent is one of tetrahydrofuran aqueous solution or dioxane aqueous solution.
Preferably, the polylactic acid-based degradable composite material comprises the following preparation raw materials in parts by weight: 90-100 parts of polylactic acid and 5-15 parts of modified hydroxyapatite. The polylactic acid is one or the combination of more than two of poly L-lactic acid (PLLA), poly D, L-lactic acid (PDLLA) or poly L-lactic acid-glycolic acid copolymer (PLGA), and the number average molecular weight is between 10 and 40 ten thousand.
The preparation raw materials of the modified hydroxyapatite comprise: the modifier, diisocyanate and hydroxyapatite, wherein the quantity ratio of hydroxyl functional groups in the modifier to substances of diisocyanate and hydroxyapatite is (5-10): 10: (3-10), wherein the modifier is polyethylene glycol and/or linear polyphosphazene. Preferably, the modifier is polyethylene glycol and linear polyphosphazene, and the amount ratio of the substances of the hydroxyl functional groups on the polyethylene glycol and the linear polyphosphazene is (1-10): 1,. Most preferably, the modifier is polyethylene glycol and linear polyphosphazene in a mass ratio of (1-3): 1.
in a second aspect, the present invention provides a method for preparing a polylactic acid-based degradable composite material, comprising the following steps:
(1) dissolving diisocyanate and a catalyst in a solvent, adding a modifier for reaction, adding hydroxyapatite, and stirring for reaction to obtain modified hydroxyapatite;
(2) drying the polylactic acid particles at 50-60 ℃ for 4-5 hours, and mixing the dried polylactic acid particles with the modified hydroxyapatite according to the mass ratio of 5-20:1 to obtain a mixed material;
(3) adding the mixed material into a double-screw extruder, blending, extruding and granulating to obtain polylactic acid granules;
(4) adding the polylactic acid granules into film forming equipment for blow molding to form a film, and preparing the polylactic acid-based degradable composite material with the thickness of 0.005-0.01 mm.
Wherein, the modified hydroxyapatite in the step (1) is prepared according to the following method: dissolving diisocyanate and a catalyst dibutyltin dilaurate in an organic solvent at 35-40 ℃ in a nitrogen atmosphere, adding polyethylene glycol and/or linear polyphosphazene to react in a reaction system for 12-24 hours, adding hydroxyapatite, stirring and reacting for 12-24 hours, centrifuging, washing, and drying to remove residual solvent to obtain the modified hydroxyapatite.
The linear polyphosphazene is prepared by the method provided by the invention.
Preferably, the mass ratio of the polyethylene glycol and/or linear polyphosphazene to the diisocyanate is 1: 1-2. The adding amount of the hydroxyapatite is 50-100% of the amount of the diisocyanate substance.
Preferably, the organic solvent is one or a combination of more than two of chloroform, acetone and ethyl acetate.
The diisocyanate used in the present invention includes one or a combination of two or more of isophorone diisocyanate, xylylene diisocyanate, 1, 6-hexamethylene diisocyanate, and 4,4' -dicyclohexylmethane diisocyanate (HMDI).
In a most preferred embodiment of the invention, the diisocyanate is 4,4' -dicyclohexylmethane diisocyanate (HMDI).
In a third aspect, the invention provides application of a polylactic acid-based degradable composite material in the field of agricultural mulching films.
The polylactic acid degradable composite material provided by the invention has the following technical advantages: 1, polyethylene glycol and/or linear polyphosphazene are used as a modifier to overcome the defects of uneven dispersion and easy coagulation of hydroxyapatite, so that the hydroxyapatite provides stress concentration points for a polylactic acid film, and the defects of poor mechanical strength, insufficient toughness and easy breakage of the polylactic acid film are effectively overcome; 2, the degradation rate is high, and the degradation product does not influence the original acidity and alkalinity of the soil; 3, degrading to release N, P and other nutrient elements, promoting plant growth and improving soil physical and chemical properties.
The linear polyphosphazene is a polymer with an inorganic main chain, wherein the main chain is formed by nitrogen atoms and phosphorus atoms which are alternately connected by single bonds and double bonds, each phosphorus atom is connected with two side chains, and the side chains are polyethylene glycol chains containing hydroxyl groups. The linear polyphosphazene is a degradable biological material, degradation is initiated by a polyethylene glycol side chain, the side chain is hydrolyzed and shed from a main chain, the phosphorus-nitrogen main chain is hydrolyzed, acidic products such as ethanol and glycolic acid generated by side chain hydrolysis can promote degradation of the polyphosphazene, basic substances such as alkaline substance phosphate and ammonia generated by polyphosphazene degradation can effectively neutralize the acidic degradation products, and the neutrality of the system is kept.
Preferably, the modifier for modifying the hydroxyapatite is a combination of polyethylene glycol and linear polyphosphazene, the molecular structure of the linear polyphosphazene also contains a polyethylene glycol chain segment, and the polyethylene glycol is used as a flexible chain segment, so that the hydroxyapatite can be uniformly dispersed and is not easy to coagulate, the interfacial bonding force between the hydroxyapatite and polylactic acid can be effectively improved, the polylactic acid can be plasticized, and the mechanical property of the polylactic acid film can be improved.
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FIG. 1 is a scheme for the preparation of linear polyphosphazenes.
FIG. 2 is a scheme for the preparation of linear polyphosphazenes according to the examples.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below, and it is obvious that the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Preparation of linear polyphosphazenes
The preparation scheme of the linear polyphosphazene is shown in figure 2.
S1: dissolving 1.16g (0.01mol, the polymerization degree is about 450, and the molecular weight is about 5 ten thousand) of linear polydichlorophosphazene cyclotrimer into 10ml of dry tetrahydrofuran, slowly dropwise adding 50ml of tetrahydrofuran solution of 7.0g (0.02mol, the polyether molecular weight is about 350) of methoxypolyethylene glycol sodium, refluxing for 24 hours at 60 ℃, filtering to remove sodium chloride, and precipitating by using normal hexane to obtain methoxypolyethylene glycol substituted polyphosphazene;
s2: 7.45g of methoxypolyethylene glycol-substituted polyphosphazene prepared above was dissolved in 50ml of chloroform, 2.0g (0.01mol) of trimethylsilyliodiane was added, the reaction was carried out at 35 ℃ for 3 days, the solvent was distilled off under reduced pressure, the product was dissolved in tetrahydrofuran, an aqueous tetrahydrofuran solution (tetrahydrofuran/water, v/v ═ 5:1) was added thereto, hydrolysis was carried out at room temperature for 1 hour, and the solvent and water were removed to give a linear polyphosphazene.
Preparation of modified hydroxyapatite
Preparation example 1
S1: under the conditions of 35 ℃, vigorous stirring and nitrogen protection, 2.62g (0.01mol) of diisocyanate HMDI and 5 drops of dibutyltin dilaurate are dissolved in 250ml of trichloromethane reaction system, 3.5g (0.01mol, polyether molecular weight is about 350) of polyethylene glycol is dissolved in 80ml of trichloromethane, and the mixture is slowly dropped into the reaction system and stirred for reaction overnight.
S2: mixing hydroxyapatite 5g (Ca)10(PO4)6(OH)20.005mol) and 5 drops of dibutyltin dilaurate are dissolved in 100ml of trichloromethane, and are slowly dropped into the reaction system, the temperature of the reaction system is raised to 40 ℃, and the reaction is continuously stirred overnight. And after the reaction is finished, centrifuging the reaction solution for 15 minutes at the speed of 5000 r/min, removing the supernatant, washing with trichloromethane, centrifuging and drying to obtain the modified hydroxyapatite taking polyethylene glycol as a modifier.
Preparation example 2
S1: under the conditions of 35 ℃, vigorous stirring and nitrogen protection, 2.62g (0.01mol) of diisocyanate HMDI and 5 drops of dibutyltin dilaurate are dissolved in 250ml of trichloromethane reaction system, 3.9g (wherein, the hydroxyl functional group is 0.01mol, the polymerization degree is 450) of linear polyphosphazene prepared by the invention is dissolved in 80ml of trichloromethane, and the linear polyphosphazene is slowly dropped into the reaction system and stirred for reaction overnight. S2 the procedure is exactly the same as in preparation example 1, and modified hydroxyapatite using linear polyphosphazene as a modifier is prepared.
Preparation example 3
S1: under the conditions of 35 ℃, vigorous stirring and nitrogen protection, 2.62g (0.01mol) of diisocyanate HMDI and 5 drops of dibutyltin dilaurate are dissolved in 250ml of trichloromethane reaction system, 1.75g (0.005mol, polyether molecular weight about 350) of polyethylene glycol and 1.95g (0.005mol, polymerization degree of 450) of linear polyphosphazene prepared by the invention are dissolved in 80ml of trichloromethane, and the mixture is slowly dropped into the reaction system for stirring and reacting overnight. S2 the procedure is exactly the same as in preparation example 1, and modified hydroxyapatite using polyethylene glycol and linear polyphosphazene (substance amount ratio 2:1) as a modifier is prepared.
Preparation example 4
S1: under the conditions of 35 ℃, vigorous stirring and nitrogen protection, 2.62g (0.01mol) of diisocyanate HMDI and 5 drops of dibutyltin dilaurate are dissolved in 250ml of trichloromethane reaction system, 2.1g (0.006mol, polyether molecular weight about 350) of polyethylene glycol and 1.6g (0.004 mol of hydroxyl functional group, polymerization degree of 450) of linear polyphosphazene prepared by the invention are dissolved in 80ml of trichloromethane, and the mixture is slowly dropped into the reaction system for stirring and reacting overnight. S2 the procedure is exactly the same as in preparation example 1, and modified hydroxyapatite using polyethylene glycol and linear polyphosphazene (substance amount ratio 3:1) as a modifier is prepared.
Preparation example 5
S1: under the conditions of 35 ℃, vigorous stirring and nitrogen protection, 2.62g (0.01mol) of diisocyanate HMDI and 5 drops of dibutyltin dilaurate are dissolved in 250ml of trichloromethane reaction system, 3.15g (0.009mol, polyether molecular weight of about 350) of polyethylene glycol and 0.4g (0.001 mol of hydroxyl functional group, polymerization degree of 450) of linear polyphosphazene prepared by the invention are dissolved in 80ml of trichloromethane, and the mixture is slowly dropped into the reaction system for stirring and reacting overnight. S2 the procedure is exactly the same as in preparation example 1, and modified hydroxyapatite using polyethylene glycol and linear polyphosphazene (substance amount ratio 18:1) as a modifier is prepared.
Preparation of polylactic acid-based degradable mulching film
Example 1
Drying 100 parts of polylactic acid particles (poly-L-lactic acid, having a number average molecular weight of about 18 ten thousand) at 60 ℃ for 4 hours, removing water therefrom, and mixing the dried polylactic acid particles with 10 parts of the modified hydroxyapatite obtained in preparation example 1 to obtain a mixed material; adding the mixed material into a double-screw extruder, blending, extruding and granulating to obtain polylactic acid granules; adding the polylactic acid granules into film forming equipment for blow molding to form a film, and preparing the polylactic acid-based degradable mulching film with the thickness of about 0.1 mm.
Example 2
The preparation method was the same as in example 1 except that 10 parts of the modified hydroxyapatite obtained in preparation example 2 was added to the dried polylactic acid particles to obtain a mixed material.
Example 3
The preparation method was the same as in example 1 except that 10 parts of the modified hydroxyapatite obtained in preparation example 3 was added to the dried polylactic acid particles to obtain a mixed material.
Example 4
The preparation method was the same as in example 1 except that 10 parts of the modified hydroxyapatite obtained in preparation example 4 was added to the dried polylactic acid particles to obtain a mixed material.
Example 5
The preparation method was the same as in example 1 except that 10 parts of the modified hydroxyapatite obtained in preparation example 4 was added to the dried polylactic acid particles to obtain a mixed material.
Examples of effects
Detection of mechanical property of polylactic acid-based degradable mulching film
The mulching films prepared in the embodiments 1 to 5 of the present invention were subjected to mechanical property detection. The tensile strength, the elongation at break and the impact strength were measured according to GB/T1040.3-2006 "test conditions for tensile Properties of plastics" part 3 (film and sheet). The results are shown in the following table.
TABLE 1 polylactic acid based degradable mulch mechanical property results
Impact Strength (KJ/m)2) Elongation at Break (%) Tensile Strength (MPa)
Example 1 19.2 313.3 46.1
Example 2 20.3 330.0 46.7
Example 3 23.0 337.2 49.3
Example 4 21.9 340.6 48.5
Example 5 20.7 321.4 46.5
The hydroxyapatite serving as rigid inorganic particles can provide stress concentration points for the polylactic acid mulching film and improve the mechanical property of the polylactic acid mulching film. However, if the interfacial bonding force between hydroxyapatite and polylactic acid is small, and coagulation and the like are generated during the mixing process, the improvement of the mechanical properties of polylactic acid by hydroxyapatite is affected. Comparing the results of example 1 and example 2, the effect of modifying hydroxyapatite with linear polyphosphazene is better than that of modifying hydroxyapatite with polyethylene glycol alone, probably because linear polyphosphazene is a polymer with two side chains, and the effect is better in preventing coagulation of hydroxyapatite and improving the interfacial bonding force between hydroxyapatite and polylactic acid. However, the above results show that when the polyethylene glycol and the linear polyphosphazene are used as the modifier to modify the hydroxyapatite, the finally prepared polylactic acid mulching film has better mechanical effect. The analysis reason is probably because the polyethylene glycol and the linear polyphosphazene can form a more compact reticular structure as the modifier simultaneously, the coagulation prevention effect on the hydroxyapatite is better, the dispersion uniformity of the hydroxyapatite in the polylactic acid is better, and the improvement effect on the mechanical property of the polylactic acid is better.
Polylactic acid-based degradable mulching film degradation and nitrogen and phosphorus release rate
(1) The mulching film prepared in the embodiment 1-5 of the invention is cut into 20cm by 20cm samples, the samples are buried in 10-15cm underground of a test field after being weighed, the samples are taken out after 90 days, and the samples are placed in clean water to be repeatedly cleaned to remove soil and attachments on the surfaces of the samples until the clean water is not turbid any more. And (3) placing the treated sample in a beaker, adding deionized water, ultrasonically cleaning for 5 minutes by using an ultrasonic cleaner, removing impurities on the surface which are not easy to remove, drying the sample in the shade under natural conditions, weighing again, and calculating the weight loss rate of the sample.
(2) Collecting soil samples before burying the samples in the test field, detecting the original nitrogen and phosphorus content in the soil, collecting the soil samples again after the experiment is finished for 90 days, detecting the nitrogen and phosphorus content in the soil, and calculating the accumulated release rate of the nitrogen and phosphorus generated by biodegradation. Wherein, the total nitrogen content is determined by adopting a Kjeldahl method, and the total phosphorus content is determined by adopting a molybdenum-antimony colorimetric resistance method. Cumulative release rate (1-residue/total) 100%.
The results of the experiments are shown in the following table.
TABLE 2 degradation of polylactic acid-based degradable mulching film and nitrogen and phosphorus release rate
Figure RE-GDA0002941908410000101
Figure RE-GDA0002941908410000111
According to the data and effects of the table above, the difference of the modified hydroxyapatite modifier can affect the degradation rate of the finally prepared polylactic acid mulching film. Wherein, the 90-day weight loss rate of the mulching film prepared from hydroxyapatite modified by polyethylene glycol is 9.8%, and the 90-day weight loss rate of the mulching film prepared from hydroxyapatite modified by linear polyphosphazene as a modifier is 10.3%. The inventors have unexpectedly found that the degradation rate of the mulching film obtained by using polyethylene glycol and linear polyphosphazene as a modifier to modify hydroxyapatite is further increased. The analytical reason may be that the linear polyphosphazene degradation can produce alkaline degradants, which accelerate the degradation of polylactic acid.
Secondly, the release of nitrogen and phosphorus elements generated by the degradation of the mulching film is in a positive correlation trend with degradation data, as shown in the data results in the table above, the modified hydroxyapatite modifiers adopted by the mulching films prepared in examples 3-5 are all the combination of polyethylene glycol and linear polyphosphazene, but the results in examples 3-4 are obviously superior to those in example 5, so when the amount of hydroxyl functional groups in the linear polyphosphazene is equivalent to that in the polyethylene glycol, and the preferred amount ratio of the two polymer hydroxyl functional group substances is 1:1-2, the linear polyphosphazene has the best promotion effect on the degradation rate and the release rate of the nitrogen and phosphorus elements.
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 the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.

Claims (10)

1. The polylactic acid-based degradable composite material is characterized by being prepared by compounding modified hydroxyapatite and polylactic acid, wherein the modified hydroxyapatite is prepared by performing surface modification on the hydroxyapatite by using polyethylene glycol and/or linear polyphosphazene as a modifier.
2. The polylactic acid-based degradable composite material according to claim 1, wherein the linear polyphosphazene has a structure represented by formula I:
Figure FDA0002853529660000011
wherein n is an integer between 450 and 2000, and m is an integer between 0 and 16.
3. The polylactic acid-based degradable composite material according to claim 1 or 2, wherein the mass ratio of polylactic acid to modified hydroxyapatite is 90-100:5-15, the polylactic acid is one or a combination of more than two of poly-L-lactic acid, poly-D, L-lactic acid or poly-L-lactic acid-glycolic acid copolymer, and the number average molecular weight is 10-40 ten thousand.
4. The polylactic acid based degradable composite material as claimed in claim 2, wherein the preparation method of linear polyphosphazene comprises the following steps:
(1) reacting polydichlorophosphazene with methoxy polyethylene glycol alkali metal salt to obtain methoxy polyethylene glycol ether substituted polyphosphazene;
(2) and (3) adding trimethyl iodosilane into the methoxypolyethylene glycol ether substituted polyphosphazene for end group replacement, and hydrolyzing to obtain the linear polyphosphazene.
5. The polylactic acid based degradable composite material according to claim 4, wherein the reaction conditions of step (1) are a reaction temperature of 40-100 ℃ and a reaction time of 12-48 hours; and/or the reaction condition of the step (2) is that the reaction temperature is 30-35 ℃ and the reaction time is 1-4 days.
6. The polylactic acid-based degradable composite material according to claim 3, wherein the polylactic acid-based degradable composite material comprises the following preparation raw materials in parts by mass: 90-100 parts of polylactic acid and 5-15 parts of modified hydroxyapatite; the preparation raw materials of the modified hydroxyapatite comprise: the modifier, diisocyanate and hydroxyapatite, wherein the quantity ratio of hydroxyl functional groups in the modifier to substances of diisocyanate and hydroxyapatite is (5-10): 10: (3-10), wherein the modifier is polyethylene glycol and/or linear polyphosphazene.
7. The polylactic acid-based degradable composite material according to claim 6, wherein the modifier is polyethylene glycol and linear polyphosphazene, and the ratio of the amount of the substances with hydroxyl functional groups on the polyethylene glycol and the linear polyphosphazene is (1-10): 1, preferably (1-3): 1.
8. the method for preparing polylactic acid based degradable composite material according to any one of claims 1 to 7, comprising the steps of:
(1) dissolving diisocyanate and a catalyst in a solvent, adding a modifier for reaction, adding hydroxyapatite, and stirring for reaction to obtain modified hydroxyapatite;
(2) drying the polylactic acid particles at 50-60 ℃ for 4-5 hours, and mixing the dried polylactic acid particles with the modified hydroxyapatite according to the mass ratio of 90-100:5-10 to obtain a mixed material;
(3) adding the mixed material into a double-screw extruder, blending, extruding and granulating to obtain polylactic acid granules;
(4) adding the polylactic acid granules into film forming equipment for blow molding to form a film, and preparing the polylactic acid-based degradable composite material with the thickness of 0.005-0.01 mm.
9. The method according to claim 8, wherein the modified hydroxyapatite in the step (1) is prepared by the following method: dissolving diisocyanate and a catalyst dibutyltin dilaurate in an organic solvent at 35-40 ℃ in a nitrogen atmosphere, adding polyethylene glycol and/or linear polyphosphazene to react in a reaction system for 12-24 hours, adding hydroxyapatite, stirring and reacting for 12-24 hours, centrifuging, washing, and drying to remove residual solvent to obtain the modified hydroxyapatite.
10. Use of the polylactic acid-based degradable composite material according to any one of claims 1 to 7 in the field of agricultural mulching films.
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