A polylactic acid block copolymer, a method for preparing the same, and use thereof

Abstract

The application discloses a kind of polylactic acid block copolymer with excellent mechanical properties and degradability, the structure of the block copolymer contains polylactic acid segment and polyurethane segment, and the raw material of polyurethane segment is L-lysine diisocyanate and glycerol monooleate / glycerol monolinoleate, all segments in the block copolymer have excellent degradability, and the block copolymer has higher molecular weight and crystallinity, and the long-chain alkyl of oleic acid / linoleic acid can also be combined with the brittleness of polylactic acid segment, to give the block copolymer excellent mechanical properties.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of high molecular copolymer, and particularly relates to a polylactic acid block copolymer with excellent mechanical properties and degradability, a preparation method thereof and application thereof. BACKGROUND

[0002] As a biobased degradable polymer material, polylactic acid (PLA) has been widely used in packaging, fiber and disposable products. Its raw material is derived from renewable plant resources, and the degradation product is harmless to the environment, which meets the requirements of green circular economy development. However, the traditional polylactic acid material has obvious shortcomings: high brittleness, poor toughness, low impact resistance and elongation at break, which seriously limits its in-depth application in fields requiring high mechanical strength and flexibility, such as durable packaging, engineering plastics and agricultural mulching film.

[0003] At present, in order to improve the mechanical properties of polylactic acid, the method of blending modification is usually used, such as physical blending with plasticizer or other polymers (such as polybutylene adipate terephthalate, PBAT). However, this method has obvious disadvantages: the compatibility of the blending system is poor, and phase separation easily occurs, resulting in insufficient long-term use stability of the material and large fluctuation of mechanical properties; at the same time, the added non-degradable components may delay the overall degradation rate, or produce microplastic residues during degradation, affecting its application under strict environmental requirements.

[0004] In particular, in the field of agricultural mulching film, polylactic acid mulching film is concerned due to its degradable characteristics. However, the existing polylactic acid mulching film generally has the problems of insufficient flexibility, poor weather resistance and the like, and it is difficult to meet the needs of mechanical support for film laying and controllability of degradation rate, which has an adverse effect on the ecological environment of farmland and crop yield.

[0005] Therefore, it is a technical problem to be solved in the field to develop a new type of copolymerized polylactic acid with excellent mechanical properties and controllable degradation characteristics. SUMMARY

[0006] In view of the above-mentioned deficiencies in the prior art, a polylactic acid block copolymer with excellent mechanical properties and degradability is provided. The block copolymer contains polylactic acid segments and polyurethane segments in its structure, all of which have excellent degradability, and the block copolymer has high molecular weight and crystallinity. The long-chain alkyl of oleic acid / linoleic acid can also integrate the brittleness of the polylactic acid segment, and can give the block copolymer excellent mechanical properties.

[0007] The first object of the present application is to provide a polylactic acid block copolymer, which comprises a product obtained by reacting a polymer represented by formula (I) with an oligomer represented by formula (II);

[0008] Formula (I) is shown in the following formula (I);

[0009] Formula (II) is shown in the following formula (II);

[0010] The weight average molecular weight of the polymer shown in formula (I) is 3kDa~100kDa;

[0011] In formula (II), R represents the residue after removing one H atom from the carboxyl group in the oleic acid / linoleic acid molecule, and the number average molecular weight of the oligomer shown in formula (II) is 500~2000Da.

[0012] For the above-mentioned polylactic acid block copolymer, firstly, it contains polylactic acid segment, L-lysine diisocyanate and glycerol monooleate / glycerol monolinoleate generated polyurethane segment in structure, wherein the polylactic acid segment and the polyurethane segment both have degradability, thus the polylactic acid block copolymer has excellent degradability; secondly, the isocyanate groups at the ends of the oligomer shown in formula (II) can react with the hydroxyl groups and carboxyl groups at the ends of the polylactic acid shown in formula (I), the oligomer shown in formula (II) actually plays the role of a large chain extender, which can increase the molecular weight of the system without destroying the crystalline structure of the polylactic acid, so that the polylactic acid block copolymer has good mechanical strength, at the same time, the long-chain alkyl groups in the polyurethane segment can also integrate the brittleness of the polylactic acid segment, so that the polylactic acid block copolymer is both strong and tough, and has excellent mechanical properties.

[0013] In some embodiments of the present application, the polymer shown in formula (I) includes at least one of PLLA and PDLA.

[0014] In some embodiments of the present application, the preparation raw material of the oligomer shown in formula (II) is L-lysine diisocyanate and glyceride in a molar ratio of 1.5~2.5:1, and the glyceride is at least one of glycerol monooleate and glycerol monolinoleate.

[0015] In some embodiments of the present application, the preparation method of the oligomer shown in formula (II) includes the following steps: adding L-lysine diisocyanate into glyceride, and reacting at 55~85℃ for 1~5h until the NCO value is almost unchanged, to obtain the oligomer shown in formula (II).

[0016] In some embodiments of the present application, the preparation method of the oligomer shown in formula (II) further includes the step of adding a catalyst before the reaction.

[0017] In some embodiments of the present application, the preparation method of the oligomer shown in formula (II), the catalyst is an organic tin catalyst or an organic titanium catalyst.

[0018] In some embodiments of the present application, the amount of the catalyst used in the preparation method of the oligomer of formula (II) is 0.05-1 wt% of L-lysine diisocyanate.

[0019] In some embodiments of the present application, the mass ratio of the polymer of formula (I) to the oligomer of formula (II) is 100:0.1-20. By controlling the mass ratio of the two within the above range, the crystallinity of the polylactic acid block copolymer is maintained, thereby facilitating the obtaining of better mechanical properties.

[0020] The second object of the present application is to provide a preparation method of the polylactic acid block copolymer as described above, comprising the following steps: mixing the polymer of formula (I) with the oligomer of formula (II), reacting at 50-85°C for 2-24h, and then performing a purification step to obtain the polylactic acid block copolymer.

[0021] In some embodiments of the present application, the purification step comprises at least one of filtration, washing, dissolution, and precipitation.

[0022] In some embodiments of the present application, the preparation method of the polylactic acid block copolymer further comprises a step of adding a catalyst before the reaction.

[0023] In some embodiments of the present application, the catalyst used in the preparation method of the polylactic acid block copolymer is an organic tin or organic titanium catalyst.

[0024] In some embodiments of the present application, the amount of the catalyst used in the preparation method of the polylactic acid block copolymer is 0.05-2 wt% of the polymer of formula (I).

[0025] The third object of the present application is to provide a degradable composition comprising the polylactic acid block copolymer as described above and a biological additive; the biological additive comprises citrus fruit peel powder. By adding such a biological additive as citrus fruit peel powder to the degradable composition, the biocompatibility and degradability of the degradable composition are improved while ensuring environmental protection, and at the same time, the main components of citrus fruit peel powder are pectin, cellulose, lignin, and limonin, which can regulate the crystallinity of the polylactic acid segment, thereby also improving the mechanical properties of the degradable composition.

[0026] In some embodiments of the present application, the mass ratio of the polylactic acid block copolymer to the biological additive is 100:5-80. By controlling the mass ratio of the two within the above range, both excellent mechanical properties and degradability are facilitated.

[0027] In some embodiments of the present application, the citrus fruit peel in the citrus fruit peel powder is at least one selected from the group consisting of orange peel, lemon peel, grapefruit peel, and tangerine peel.

[0028] In some embodiments of the present application, the preparation method of the citrus fruit peel powder comprises the following steps: washing, chopping, freeze-drying, grinding, and sieving the citrus fruit peel.

[0029] In some embodiments of the present application, the bio-source additive further comprises chitosan. Chitosan is a natural high-molecular amino polysaccharide, which can interact with the citrus fruit peel powder to jointly regulate the crystallinity of the polylactic acid segment, thereby further improving the mechanical properties of the degradable composition.

[0030] In some embodiments of the present application, the mass ratio of the citrus fruit peel powder to the chitosan is 1:0.1-1. By controlling the mass ratio of the two in the reaction, the degradable composition can obtain excellent mechanical properties.

[0031] In some embodiments of the present application, the bio-source additive further comprises at least one of microcrystalline cellulose, nanocellulose, and starch, and the mass ratio of the bio-source additive to the citrus fruit peel powder is 0.1-1:1.

[0032] The fourth object of the present application is to provide the polylactic acid block copolymer or the degradable composition described above for use in preparing a degradable mulch film.

[0033] The fifth object of the present application is to provide a degradable mulch film comprising the polylactic acid block copolymer or the degradable composition described above.

[0034] Compared with the prior art, the present application has the following beneficial effects:

[0035] (1) The polylactic acid block copolymer of the present application has a relatively large molecular weight, and contains a polylactic acid segment and a polyurethane segment with degradability in structure. Moreover, the embedded polyurethane segment does not destroy the crystalline structure of the polylactic acid, and also integrates the brittleness of the polylactic acid segment, so that a polylactic acid block copolymer with excellent mechanical properties and degradability can be obtained.

[0036] (2) The preparation method of the polylactic acid block copolymer in the present application is simple and controllable, which is conducive to ensuring the performance stability of products of different batches.

[0037] (3) The preparation raw materials of the polylactic acid block copolymer, the degradable composition, and the degradable mulch film in the present application are green in origin, which conforms to the concept of green environmental protection. DETAILED DESCRIPTION

[0038] In the following, the technical solutions of the present application will be described clearly and completely in connection with the embodiments of the present application, so that those skilled in the art can better understand the technical solutions in the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor should fall within the scope of protection of the present application.

[0039] All raw materials in the present application are from commercially available products, and are calculated by weight parts, unless otherwise specified.

[0040] The following preparation examples are used to prepare the oligomer shown in formula (II).

[0041] Preparation Example 1

[0042] The present preparation example provides a preparation method of the oligomer O1 shown in formula (II), which comprises the following steps:

[0043] In 35.65 parts of glycerol monooleate treated by drying, 45.25 parts of L-lysine diisocyanate and 0.02 parts of dibutyltin dilaurate are added, and the reaction is carried out at 70℃ for 2h until the NCO value is constant, to obtain the oligomer O1 shown in formula (II). The NCO amount at the end of the oligomer O1 is determined by titration method, and the number average molecular weight is determined to be 912 Da.

[0044] Preparation Example 2

[0045] The present preparation example provides a preparation method of the oligomer O2 shown in formula (II), which comprises the following steps:

[0046] In 35.45 parts of glycerol monolinoleate treated by drying, 45.25 parts of L-lysine diisocyanate and 0.02 parts of dibutyltin dilaurate are added, and the reaction is carried out at 70℃ for 2h until the NCO value is constant, to obtain the oligomer O2 shown in formula (II). The NCO amount at the end of the oligomer O2 is determined by titration method, and the number average molecular weight is determined to be 850 Da.

[0047] Preparation Example 3

[0048] The present preparation example provides a preparation method of the oligomer O3 shown in formula (II), which comprises the following steps:

[0049] In 17.83 parts of dried glycerol monooleate, 17.73 parts of glycerol monolinoleate, 45.25 parts of L-lysine diisocyanate are added, 0.02 parts of dibutyltin dilaurate is added, and the reaction is carried out at 70°C for 2h until the NCO value is constant, to obtain the oligomer O3 shown in formula (II). The NCO amount at the end of the oligomer O3 is determined by titration method, and the number average molecular weight is determined to be 958 Da.

[0050] The following examples are used to prepare polylactic acid block copolymer.

[0051] Example 1

[0052] The present example provides a preparation method of polylactic acid block copolymer CP1, comprising the following steps: 100 parts of dried PLLA (model number DG-LOH030 from Jinan Daigang Biology) are mixed with 0.5 parts of the oligomer O1 prepared in the above preparation example 1, 0.5 parts of dibutyltin dilaurate is added, and the reaction is carried out at 60°C for 5h, the obtained product is dissolved with dichloromethane, precipitated and separated out with methanol, and then filtered and dried to obtain the polylactic acid block copolymer CP1.

[0053] Example 2

[0054] The present example provides a preparation method of polylactic acid block copolymer CP2, comprising the following steps: 100 parts of dried PLLA (model number DG-LOH030) are mixed with 5 parts of the oligomer O2 prepared in the above preparation example 2, 0.5 parts of dibutyltin dilaurate is added, and the reaction is carried out at 60°C for 5h, the obtained product is dissolved with dichloromethane, precipitated and separated out with methanol, and then filtered and dried to obtain the polylactic acid block copolymer CP2.

[0055] Example 3

[0056] The present example provides a preparation method of polylactic acid block copolymer CP3, comprising the following steps: 100 parts of dried PLLA (model number DG-LOH030) are mixed with 15 parts of the oligomer O3 prepared in the above preparation example 3, 0.5 parts of dibutyltin dilaurate is added, and the reaction is carried out at 60°C for 5h, the obtained product is dissolved with dichloromethane, precipitated and separated out with methanol, and then filtered and dried to obtain the polylactic acid block copolymer CP3.

[0057] Example 4

[0058] The embodiment provides a preparation method of a polylactic acid block copolymer CP4, which comprises the following steps: 100 parts of PLLA (type number: DG-LOH070) subjected to drying treatment are mixed with 5 parts of the oligomer O1 prepared in the preparation example 1, 0.5 parts of dibutyltin dilaurate is added, and reaction is carried out at 60 DEG C for 5 h; the obtained product is dissolved in dichloromethane, precipitated and separated out by using methanol, and then filtered and dried to obtain the polylactic acid block copolymer CP4.

[0059] Example 5

[0060] The embodiment provides a preparation method of a polylactic acid block copolymer CP5, which comprises the following steps: 100 parts of PLLA (type number: DG-LOH100) subjected to drying treatment are mixed with 5 parts of the oligomer O1 prepared in the preparation example 1, 0.5 parts of dibutyltin dilaurate is added, and reaction is carried out at 60 DEG C for 5 h; the obtained product is dissolved in dichloromethane, precipitated and separated out by using methanol, and then filtered and dried to obtain the polylactic acid block copolymer CP5.

[0061] Example 6

[0062] The embodiment provides a preparation method of a polylactic acid block copolymer CP6, which comprises the following steps: 100 parts of PDLA (type number: DG-DOH070) subjected to drying treatment are mixed with 5 parts of the oligomer O1 prepared in the preparation example 1, 0.5 parts of dibutyltin dilaurate is added, and reaction is carried out at 60 DEG C for 5 h; the obtained product is dissolved in dichloromethane, precipitated and separated out by using methanol, and then filtered and dried to obtain the polylactic acid block copolymer CP6.

[0063] Example 7

[0064] The embodiment provides a preparation method of a polylactic acid block copolymer CP7, which comprises the following steps: 50 parts of PLLA (type number: DG-LOH070) subjected to drying treatment and 50 parts of PDLA (type number: DG-DOH070) subjected to drying treatment are mixed with 5 parts of the oligomer O1 prepared in the preparation example 1, 0.5 parts of dibutyltin dilaurate is added, and reaction is carried out at 60 DEG C for 5 h; the obtained product is dissolved in dichloromethane, precipitated and separated out by using methanol, and then filtered and dried to obtain the polylactic acid block copolymer CP7.

[0065] The following application examples and comparative application examples are used for preparing degradable compositions.

[0066] Application Example 1

[0067] The application example provides a degradable composition C1, which is prepared from 100 parts of the polylactic acid block copolymer CP4 obtained in the example 4 and 10 parts of pomelo peel powder, and the preparation method is as follows:

[0068] (1) The pomelo peel is cut into pieces, freeze-dried, ground, and passed through a 100-mesh sieve to obtain pomelo peel powder;

[0069] (2) The polylactic acid block copolymer CP4 obtained in Example 4 after drying treatment is mixed with the pomelo peel powder according to the weight parts to obtain a degradable composition C1.

[0070] Application Example 2

[0071] This application example provides a degradable composition C2, which is prepared from 100 parts of the polylactic acid block copolymer CP4 obtained in Example 4 and 30 parts of pomelo peel powder, and the preparation method is consistent with Application Example 1.

[0072] Application Example 3

[0073] This application example provides a degradable composition C3, which is prepared from 100 parts of the polylactic acid block copolymer CP4 obtained in Example 4 and 50 parts of pomelo peel powder, and the preparation method is consistent with Application Example 1.

[0074] Application Example 4

[0075] This application example provides a degradable composition C4, which is prepared from 100 parts of the polylactic acid block copolymer CP4 obtained in Example 4 and 30 parts of orange peel powder, and the preparation method is:

[0076] (1) The orange peel is cut into pieces, freeze-dried, ground, and passed through a 100-mesh sieve to obtain orange peel powder;

[0077] (2) The polylactic acid block copolymer CP4 obtained in Example 4 after drying treatment is mixed with the orange peel powder according to the weight parts to obtain a degradable composition C4.

[0078] Application Example 5

[0079] This application example provides a degradable composition C5, which is prepared from 100 parts of the polylactic acid block copolymer CP4 obtained in Example 4 and 30 parts of chitosan, and the preparation method is: The polylactic acid block copolymer CP4 obtained in Example 4 after drying treatment is mixed with chitosan with a degree of deacetylation ≥75% (from Aladdin, product number C299272) according to the weight parts to obtain a degradable composition C5.

[0080] Application Example 6

[0081] The application example provides a degradable composition C6, which is prepared from 100 parts of the polylactic acid block copolymer CP4 obtained in the embodiment 4 and 30 parts of microcrystalline cellulose, and the preparation method is as follows: the polylactic acid block copolymer CP4 obtained in the embodiment 4 after drying treatment is fully mixed with the microcrystalline cellulose (from Qufu Tianli) according to the weight parts to obtain the degradable composition C6.

[0082] Application example 7

[0083] The application example provides a degradable composition C7, which is prepared from 100 parts of the polylactic acid block copolymer CP4 obtained in the embodiment 4 and 20 parts of pomelo peel powder and 10 parts of chitosan, and the preparation method is as follows:

[0084] (1) the pomelo peel is cut, freeze-dried, ground and passed through a 100 mesh sieve to obtain pomelo peel powder;

[0085] (2) the polylactic acid block copolymer CP4 obtained in the embodiment 4 after drying treatment is fully mixed with the pomelo peel powder and chitosan (from Aladdin, product number C299272) with a degree of deacetylation of ≥75% according to the weight parts to obtain the degradable composition C7.

[0086] Application example 8

[0087] The application example provides a degradable composition C8, which is prepared from 100 parts of the polylactic acid block copolymer CP4 obtained in the embodiment 4 and 20 parts of pomelo peel powder and 10 parts of microcrystalline cellulose, and the preparation method is as follows:

[0088] (1) the pomelo peel is cut, freeze-dried, ground and passed through a 100 mesh sieve to obtain pomelo peel powder;

[0089] (2) the polylactic acid block copolymer CP4 obtained in the embodiment 4 after drying treatment is fully mixed with the pomelo peel powder and microcrystalline cellulose (from Qufu Tianli) according to the weight parts to obtain the degradable composition C8.

[0090] Application example 9

[0091] The application example provides a degradable composition C9, which is prepared from 100 parts of the polylactic acid block copolymer CP4 obtained in the embodiment 4 and 10 parts of pomelo peel powder and 20 parts of chitosan, and the other parts are consistent with the application example 7.

[0092] Application example 10

[0093] The application example provides a degradable composition C10, which is prepared from 100 parts of the polylactic acid block copolymer CP4 obtained in the embodiment 4 and 15 parts of pomelo peel powder and 15 parts of chitosan, and the other parts are consistent with the application example 7.

[0094] Application Example 11

[0095] This application example provides a degradable composition C11, which is prepared from 100 parts of the polylactic acid block copolymer CP4 obtained in Example 4, 25 parts of pomelo peel powder, and 5 parts of chitosan, and other conditions are the same as in Application Example 7.

[0096] Comparative Application Example 1

[0097] This application example provides a degradable composition CC1, which is prepared from 100 parts of PLLA and 30 parts of pomelo peel powder, and the preparation method is as follows:

[0098] (1) The pomelo peel is cut, freeze-dried, ground, and passed through a 100-mesh sieve to obtain pomelo peel powder;

[0099] (2) The dried PLLA (model DG-LOH070) is mixed with the pomelo peel powder according to the weight parts to obtain the degradable composition CC1.

[0100] Performance test:

[0101] 1. Molecular weight

[0102] The weight average molecular weight of PLLA (model DG-LOH070) and the polylactic acid block copolymers CP1-CP7 obtained in Examples 1-7 is determined by using waters GPC, THF as solvent, and PS as standard sample. The results are shown in Table 1.

[0103] 2. Mechanical properties

[0104] The mechanical properties of the Type II dumbbell test pieces made of PLLA (model DG-LOH070), the polylactic acid block copolymers CP1-CP7 obtained in Examples 1-7, the degradable compositions C1-C11 obtained in Application Examples 1-11, and the degradable composition CC1 obtained in Comparative Application Example 1 are determined at room temperature according to GB13022-91, and the tensile speed is 5 mm / min. The results are shown in Table 1.

[0105] 3. Degradability

[0106] Using a blown film machine, films with a thickness of 16 μm were made from PLLA (model DG-LOH070), polylactic acid block copolymer CP4 obtained in Example 4, biodegradable compositions C1-C11 obtained in Application Examples 1-11, and biodegradable composition CC1 obtained in Comparative Application Example 1. Five films with a size of 10 cm × 10 cm were taken, placed in a mesh nylon bag with a large mesh opening, and sealed. The bags were dried at 50 °C for 5 hours to obtain test samples, and the mass m0 at this time was recorded. The test samples were buried at a depth of 30 cm in the company's green lawn (the mass ratio of soil to farmyard manure was 5:1) where farmyard manure (chicken manure) had been applied. Each group of test samples had 4 samples, which were weighed on the 7th, 20th, 60th, and 100th days after burying the soil (the test samples were cleaned and dried at 50 °C before weighing), and the obtained masses were m7, m... 20 m 60 m 100 According to [m7 (or m) 20 m 60 m 100 The degradation rate was calculated by multiplying [m0] by 100% after different numbers of days of natural composting. The results are shown in Table 1. Furthermore, the appearance of the mesh nylon bag remained almost unchanged throughout the entire process.

[0107] Table 1

[0108]

[0109] As shown in Table 1, the polylactic acid block copolymer of the present invention has superior mechanical strength, elongation at break, and biodegradability compared to ordinary polylactic acid. The biodegradable composition prepared by the polylactic acid block copolymer of the present invention and a bio-derived additive including citrus fruit peel powder not only has higher mechanical strength but also has biodegradability comparable to that of the polylactic acid block copolymer. In particular, when the bio-derived additive includes a certain mass ratio of citrus fruit peel powder and chitosan, higher mechanical strength can be obtained.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that after reading this application specification, they can still modify or make equivalent substitutions to the specific implementation of the present invention, but these modifications or changes do not depart from the protection scope of the pending claims of the present invention.

Claims

1.A polylactic acid block copolymer, characterized in that, the polylactic acid block copolymer comprises a product obtained by reacting a polymer represented by formula (I) with an oligomer; Formula (I); wherein the polymer represented by formula (I) has a weight average molecular weight of 3 kDa to 100 kDa; the oligomer is prepared from L-lysine diisocyanate and a glyceride in a molar ratio of 1.5 to 2.5: 1, the glyceride being at least one of glyceryl monooleate and glyceryl monolinoleate; the preparation method of the oligomer comprises the following steps: adding L-lysine diisocyanate into the glyceride, and reacting at 55 to 85 ℃ for 1 to 5 h until the NCO value is unchanged to obtain the oligomer; the number average molecular weight of the oligomer is 500 to 2000 Da. 2.The polylactic acid block copolymer according to claim 1, characterized in that, the polymer represented by formula (I) comprises at least one of PLLA and PDLA. 3.The polylactic acid block copolymer according to claim 1, characterized in that, the mass ratio of the polymer represented by formula (I) to the oligomer is 100: 0.1 to 20. 4.A preparation method of the polylactic acid block copolymer according to any one of claims 1 to 3, characterized in that, the polymer represented by formula (I) is mixed with the oligomer, and reacted at 50 to 85 ℃ for 2 to 24 h, and then subjected to a purification step to obtain the polylactic acid block copolymer. 5.A degradable composition, characterized in that, comprising the polylactic acid block copolymer according to any one of claims 1 to 3, and a biological additive; the biological additive comprises citrus fruit peel powder. 6.The degradable composition according to claim 5, characterized in that, the mass ratio of the polylactic acid block copolymer to the biological additive is 100: 5 to 80. 7.The degradable composition according to claim 5, characterized in that, the biological additive further comprises chitosan. 8.Use of the polylactic acid block copolymer according to any one of claims 1 to 3 or the degradable composition according to any one of claims 5 to 7 in the preparation of a degradable mulch film. 9.A degradable mulch film, characterized in that, comprising the polylactic acid block copolymer according to any one of claims 1 to 3 or the degradable composition according to any one of claims 5 to 7.

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