Modified polylactic acid as well as preparation method and production device thereof

Abstract

The invention provides modified polylactic acid as well as a preparation method and a production device thereof. The modified polylactic acid is obtained by modifying polylactic acid with a segmented copolymer of polylactic acid and anhydride, has high elongation at break, low tensile strength and low elastic modulus, further has better toughness, has high thermal decomposition temperature and low degradation degree, and further shows higher stability.

Description

Technical Field

[0001] The invention relates to polymer material synthesis technology, and in particular to a modified polylactic acid and a preparation method and a production device thereof. Background Art

[0002] Polylactic acid, also known as polylactide, is a completely biodegradable polyester polymer material made by polycondensation of lactic acid. Currently, starch-containing biomass such as corn or straw cellulose is used as raw materials to ferment lactic acid, and then lactic acid is prepared into cyclic dimer lactide, which is then ring-opened and polymerized to generate polylactic acid. Waste from polylactic acid products can be decomposed into carbon dioxide and water by microorganisms in nature. Carbon dioxide and water form biomass through plant photosynthesis and continue to become raw materials for polylactic acid fermentation. Therefore, polylactic acid is currently recognized as an environmentally friendly material in the world.

[0003] Polylactic acid has been widely used in medical implants, surgical sutures, drug controlled release systems, fracture fixation and tissue regeneration due to its good biodegradability, biocompatibility and bioabsorbability, especially as a tissue engineering scaffold material. However, the carbonyl group in the traditional polylactic acid molecular chain is coplanar with the adjacent oxygen atom and is close to the adjacent carbon atom, which is not easy to rotate, resulting in poor flexibility of the molecular chain, making the polylactic acid material hard and brittle, with poor toughness and difficult to bend. In addition, polylactic acid materials also have problems such as insufficient tensile length, poor thermal stability and excessive degradation. These shortcomings limit the practical application of polylactic acid and are currently problems that need to be solved urgently. Summary of the invention

[0004] The invention provides a modified polylactic acid, which has high toughness, high elongation at break, low tensile strength, high thermal stability and low degradation.

[0005] The present invention provides a method for preparing the modified polylactic acid. The method can obtain a large amount of stable modified polylactic acid with excellent performance in a short time, which is convenient for subsequent large-scale production.

[0006] The present invention also provides a production device for modified polylactic acid. The production device can be used to produce modified polylactic acid, so a large amount of modified polylactic acid can be obtained quickly and simply, which is convenient for large-scale production.

[0007] The invention provides a modified polylactic acid, which is obtained by modifying polylactic acid with a block copolymer of polylactic acid and acid anhydride.

[0008] The modified polylactic acid as described above, wherein the mass ratio of the block copolymer of polylactic acid and acid anhydride to the polylactic acid is 1:(1-5).

[0009] In the modified polylactic acid as described above, the acid anhydride includes at least one of succinic anhydride, adipic anhydride, maleic anhydride, methylmaleic anhydride and dimethylmaleic anhydride.

[0010] The modified polylactic acid as mentioned above, wherein the grafting rate of the modified polylactic acid is 1.5%-3.5%.

[0011] The modified polylactic acid as described above, wherein the molecular weight of the modified polylactic acid is 2.5×10 5 -4.5×10 5 .

[0012] The present invention provides a method for preparing the modified polylactic acid, comprising the following steps:

[0013] Step 1, subjecting lactide, a first catalyst, a first initiator and anhydride to a first ring-opening polymerization reaction to obtain a block copolymer of polylactic acid and anhydride;

[0014] Step 2, subjecting lactide, a second catalyst and a second initiator to a second ring-opening polymerization reaction to obtain polylactic acid;

[0015] Step 3: melt-blending the block copolymer of polylactic acid and acid anhydride with polylactic acid to obtain modified polylactic acid.

[0016] The preparation method as described above, wherein the mass ratio of the total mass of lactide in steps 1 and 2 to the mass ratio of the acid anhydride in step 1 is (30-70):1;

[0017] And / or, in step 1, the mass ratio of lactide, the first catalyst, the first initiator and the acid anhydride is (5000-6000):1:(25-35):(500-600);

[0018] And / or, the mass ratio of the block copolymer of polylactic acid and acid anhydride to polylactic acid is 1:(1-5).

[0019] The preparation method as described above, wherein the temperature of the first ring-opening polymerization reaction is 140-180° C., the pressure is 0.01-0.5 MPa, and the time is 1-3 h;

[0020] and / or, the second ring-opening polymerization reaction is carried out at a temperature of 140-180° C., a pressure of 0.01-0.5 MPa, and a time of 2-4 h;

[0021] And / or, the temperature of the melt blending reaction is 170-190° C., the pressure is 12-20 MPa, and the time is 2-6 h.

[0022] The preparation method as described above, wherein the lactide comprises at least one of L-lactide, D-lactide, and meso-lactide;

[0023] and / or, the first catalyst and the second catalyst are each independently selected from at least one of tin pentanoate, stannous pentanoate, tin hexanoate, stannous hexanoate, tin heptanoate, stannous heptanoate, tin octoate, and stannous octoate;

[0024] and / or, the first initiator and the second initiator are each independently selected from at least one of butanediol, propylene glycol, and pentanediol;

[0025] And / or, the acid anhydride includes at least one of succinic anhydride, adipic anhydride, maleic anhydride, methylmaleic anhydride and dimethylmaleic anhydride.

[0026] The present invention also provides a production device for the modified polylactic acid, comprising a first reaction kettle, a second reaction kettle and a tower reactor;

[0027] The first reactor and the second reactor are arranged in parallel, and the discharge ports of the first reactor and the second reactor are respectively connected to the feed port of the tower reactor;

[0028] The first reaction kettle and the second reaction kettle are used for performing a first ring-opening polymerization reaction and a second ring-opening polymerization reaction, respectively;

[0029] Tower reactors are used to carry out melt blending reactions.

[0030] The invention provides a modified polylactic acid. The modified polylactic acid is obtained by modifying polylactic acid with a block copolymer of polylactic acid and anhydride. Compared with the modified polylactic acid obtained by directly modifying polylactic acid with anhydride, the modified polylactic acid has higher elongation at break, lower tensile strength and lower elastic modulus, and thus has better toughness; it has higher thermal decomposition temperature and lower degradation degree, and thus shows higher stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a modified polylactic acid production device according to one embodiment of the present invention.

[0032] Description of reference numerals:

[0033] 1: the first reactor;

[0034] 2: The second reactor;

[0035] 3: Tower reactor;

[0036] 31: first reaction unit;

[0037] 32: second reaction unit;

[0038] 33: the third reaction unit;

[0039] 34: The fourth reaction unit. DETAILED DESCRIPTION

[0040] In order to enable those skilled in the art to better understand the scheme of the present invention, the present invention is further described in detail below. The specific implementation methods listed below are only for describing the principles and features of the present invention. The examples are only used to explain the present invention and are not intended to limit the scope of the present invention. Based on the embodiments of the present invention, all other implementation methods obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention.

[0041] At present, in order to improve the toughness of polylactic acid, polylactic acid is usually modified by grafting anhydride to polylactic acid. However, this graft copolymerization may cause the anhydride to be unable to be uniformly grafted to the molecular chain of polylactic acid, and it is also difficult to control the position and amount of grafting, resulting in fluctuations in the performance of the obtained polylactic acid. In addition, graft copolymerization requires more stringent reaction conditions, such as high temperature, which may also cause damage to the main chain of polylactic acid. Due to the poor compatibility between anhydride and polylactic acid, directly grafting anhydride to polylactic acid will result in a low embedding rate of anhydride, which in turn affects the improvement of the performance of polylactic acid by anhydride. It is also easy to cause the main chain to break due to the attack of free radicals on the main chain of polylactic acid during the graft copolymerization process, thereby reducing the molecular weight of the modified polylactic acid; and the lower molecular weight will make the modified polylactic acid show lower toughness, which is more likely to break or break during use, and will also affect the glass transition temperature and melting point of the modified polylactic acid, further affecting its stability under high temperature conditions, and will also affect the degradation performance of the modified polylactic acid, causing it to degrade. In addition, the inventors found in the actual production process that if acid anhydride is directly copolymerized with polylactic acid, a complex may sometimes be generated, thereby causing the stability of the modified polylactic acid to decrease.

[0042] Therefore, the present invention provides a modified polylactic acid, which is obtained by modifying polylactic acid through a block copolymer of polylactic acid and anhydride. The present invention uses a block copolymer to more accurately control the ratio and distribution of polylactic acid and anhydride, can provide a more uniform molecular structure, improve the performance consistency of modified polylactic acid, and the synthesis of the block copolymer can also be carried out under relatively mild conditions to reduce the damage to the main chain of polylactic acid. The structural orderliness of the block copolymer of polylactic acid and anhydride prompts it to show better compatibility, so the block copolymer can be used to obtain a modified polylactic acid with a higher anhydride embedding rate, and the synthesis of the block copolymer is usually carried out by a non-radical reaction, which can effectively avoid the degradation of the main chain of polylactic acid, so as to better improve the molecular weight of the modified polylactic acid. At the same time, the block copolymer synthesized by polylactic acid and anhydride is a linear copolymer formed by alternating polymerization of segments with different chemical structures of polylactic acid and anhydride, with stable chemical bonds, and thus compared with the copolymerization of anhydride directly with polylactic acid, it has higher stability.

[0043] Specifically, the modified polylactic acid provided by the present invention has higher elongation at break, lower tensile strength and lower elastic modulus, and thus has better toughness, and has a higher thermal decomposition temperature and lower degradation degree, and thus exhibits higher stability, compared with the modified polylactic acid obtained by directly modifying polylactic acid using anhydride.

[0044] Furthermore, in order to promote a good matching effect between the block copolymer of polylactic acid and anhydride and polylactic acid to improve the toughness of the modified polylactic acid and at the same time improve the stability of the modified polylactic acid, the mass ratio of the block copolymer of polylactic acid and anhydride to polylactic acid can be regulated in the present invention. Specifically, the mass ratio is 1:(1-5).

[0045] The inventor proceeds from the selection of anhydride types, and finds that cyclic anhydride can be converted into linear copolymer by ring-opening polymerization in the process of forming block copolymer with polylactic acid. On the one hand, cyclic anhydride has higher reactivity than other anhydrides, and the tension in its cyclic structure makes it easier to open and react with other molecules. On the other hand, cyclic anhydride usually has a smaller molecular weight and higher diffusivity, which makes them easier to be evenly distributed in the polymer matrix, thereby improving the uniformity of the reaction and the embedding rate of anhydride. On this basis, because succinic anhydride, adipic anhydride, maleic anhydride, methyl maleic anhydride, dimethyl maleic anhydride not only have good compatibility with polylactic acid, can improve the anhydride embedding rate, improve the toughness of modified polylactic acid, and can also give modified polylactic acid good chemical stability. Therefore, in the present invention, anhydride can include at least one of succinic anhydride, adipic anhydride, maleic anhydride, methyl maleic anhydride, and dimethyl maleic anhydride.

[0046] Based on the above research, by adjusting the grafting rate of modified polylactic acid to 1.5%-3.5%, its main body is basically straight chain with only a few side chains, which can avoid the problems of poor consistency, low anhydride embedding rate and low molecular weight existing in graft copolymers. It has the advantages of good consistency, high anhydride embedding rate and high molecular weight.

[0047] In the present invention, the molecular weight of the modified polylactic acid can be adjusted to 2.5×10 5 -4.5×10 5 , which makes the toughness and stability of the modified polylactic acid approach the required range. If the molecular weight of the modified polylactic acid is too small, that is, the molecular weight is less than 2.5×10 5 When the molecular weight of the modified polylactic acid is too large, that is, the molecular weight is greater than 4.5×10 5 At the same time, it may also cause the modified polylactic acid to be too rigid, reduce its toughness, and affect its stability to a certain extent.

[0048] The present invention provides a method for preparing the modified polylactic acid, comprising the following steps:

[0049] Step 1, subjecting lactide, a first catalyst, a first initiator and anhydride to a first ring-opening polymerization reaction to obtain a block copolymer of polylactic acid and anhydride;

[0050] Step 2, subjecting lactide, a second catalyst and a second initiator to a second ring-opening polymerization reaction to obtain polylactic acid;

[0051] Step 3: melt-blending the block copolymer of polylactic acid and acid anhydride with polylactic acid to obtain modified polylactic acid.

[0052] Among them, ring-opening polymerization refers to the reaction in which a cyclic compound monomer is converted into a linear polymer through ring-opening addition. Melt blending refers to a method in which the polymer components required for blending are prepared into a uniform polymer co-melt by a mixing device above their viscosity flow temperature, and then cooled, crushed or granulated. It can be understood that in the present invention, step 1 and step 2 can be carried out sequentially or randomly.

[0053] The present invention uses a block copolymer of polylactic acid and anhydride to modify polylactic acid. Compared with directly modifying polylactic acid with anhydride, a modified polylactic acid with higher elongation at break, lower tensile strength and lower elastic modulus can be obtained, thereby having better toughness. At the same time, the modified polylactic acid also has a higher thermal decomposition temperature and a lower degradation degree, thereby showing higher stability. Therefore, the preparation method of the present invention can obtain a large amount of stable and excellent modified polylactic acid in a short time, which is convenient for subsequent large-scale production.

[0054] In the scheme of the present invention, the total mass of lactide in steps 1 and 2 and the mass ratio of anhydride in step 1 can be adjusted to (30-70): 1, so that the content of lactide and anhydride can form a good matching effect, and a matching block copolymer of polylactic acid and anhydride can be provided for subsequent reactions; the mass ratio of lactide, the first catalyst, the first initiator and the anhydride in step 1 can also be adjusted to (5000-6000): 1: (25-35): (500-600) to further improve the matching of the obtained block copolymer of polylactic acid and anhydride with subsequent reactions; the mass ratio of the block copolymer of polylactic acid and anhydride to polylactic acid can also be adjusted to 1: (1-5), so that the block copolymer of polylactic acid and anhydride and polylactic acid can form a good matching effect to improve the toughness of the modified polylactic acid, and at the same time improve the stability of the modified polylactic acid. It is understandable that those skilled in the art can perform combined regulation or single regulation in the above scheme according to actual conditions.

[0055] In the scheme of the present invention, the temperature, pressure and time of the first ring-opening polymerization reaction can be adjusted to promote the reaction to occur fully, so as to better obtain the block copolymer of polylactic acid and anhydride. Specifically, the temperature is 140-180°C, the pressure is 0.01-0.5Mpa, the time is 1-3h, and further, the time can be 1.5-2.5h; the temperature, pressure and time of the second ring-opening polymerization reaction can also be adjusted to promote the reaction to occur fully, so as to better obtain polylactic acid. Specifically, the temperature is 140-180°C, the pressure is 0.01-0.5Mpa, the time is 2-4h, and further, the time can be 2.5-3.5h; the temperature, pressure and time of the melt blending reaction can also be adjusted to promote the reaction to occur fully, so as to better obtain the modified polylactic acid. Specifically, the temperature is 170-190°C, the pressure is 12-20Mpa, the time is 2-6h, and further, the time can be 3-5h. For example, in the melt blending reaction, within a suitable temperature range and pressure range, the block copolymer of polylactic acid and anhydride and polylactic acid are fully mixed, and the modified polylactic acid with anhydride uniformly embedded can be obtained within an appropriate time, which will further increase the anhydride embedding rate and molecular weight of the modified polylactic acid, improve the toughness of the modified polylactic acid and improve the stability. It is understandable that those skilled in the art can also perform combined regulation or separate regulation in the above scheme according to actual conditions.

[0056] In addition, in the present invention, in order to further improve the performance of the target product, the lactide may include at least one of L-lactide, D-lactide, and meso-lactide, and further, may be L-lactide; in order to further promote the reaction toward the target product, the first catalyst and the second catalyst may be independently selected from at least one of tin pentanoate, stannous pentanoate, tin hexanoate, stannous hexanoate, tin heptanoate, stannous heptanoate, tin octoate, and stannous octoate; in order to further promote the reaction and increase the reaction rate, the first initiator and the second initiator may be independently selected from at least one of butanediol, propylene glycol, and pentanediol; in order to further improve the performance of the modified polylactic acid, the anhydride may include at least one of succinic anhydride, adipic anhydride, maleic anhydride, methyl maleic anhydride, and dimethyl maleic anhydride. It is understood that those skilled in the art may also select in combination or individually from the above reagent range according to actual conditions.

[0057] The present invention also provides a production device for the modified polylactic acid, comprising a first reaction kettle 1, a second reaction kettle 2 and a tower reactor 3;

[0058] The first reactor 1 and the second reactor 2 are arranged in parallel, and the discharge ports of the first reactor 1 and the second reactor 2 are respectively connected to the feed port of the tower reactor 3;

[0059] The first reaction kettle 1 and the second reaction kettle 2 are used for performing a first ring-opening polymerization reaction and a second ring-opening polymerization reaction respectively;

[0060] The tower reactor 3 is used for performing melt blending reaction.

[0061] In the present invention, the tower reactor 3 is a multi-stage direct-connected tower reactor, and the number of stages is greater than 1, and further, the number of stages may be 4.

[0062] In one embodiment of the present invention, the production device of modified polylactic acid is as follows Figure 1 As shown, the tower reactor 3 includes a first reaction unit 31, a second reaction unit 32, a third reaction unit 33 and a fourth reaction unit 34, the feed port of the tower reactor 3 is arranged in the first reaction unit 31, and the first reaction unit 31, the second reaction unit 32, the third reaction unit 33 and the fourth reaction unit 34 are arranged in series through a pipeline, and a plurality of extraction ports are arranged in the tower reactor 3 to prepare modified polylactic acid of different molecular weights by adjusting the melt blending reaction time, and an extraction port is arranged between every two reaction units to extract the modified polylactic acid, that is, the first extraction port is arranged on the pipeline between the second reaction unit 32 and the third reaction unit 33, and the second extraction port is arranged on the outlet pipeline of the fourth reaction unit 34, and the melt blending product extracted through the extraction port can be transported to other devices for downstream production, for example, it can be transported to a devolatilization unit for devolatilization, and then water-cooled, pelletized and dried to obtain modified polylactic acid.

[0063] It can be understood that during the preparation process of modified polylactic acid, the first ring-opening polymerization reaction and the second ring-opening polymerization reaction can share a catalyst and an initiator. At this time, the same feed pipeline can be set to respectively enter the first reactor 1 and the second reactor 2, and the polylactic acid raw material can also share a feed pipeline together with the catalyst and the initiator.

[0064] In addition, the product of the first reactor 1 and the product of the second reactor 2 can be converged into the same discharge pipeline and then introduced into the feed port of the tower reactor 3 .

[0065] The use of the above-mentioned production device for the production of modified polylactic acid can quickly and simply obtain a large amount of modified polylactic acid with high toughness, high elongation at break, low tensile strength, low elastic modulus, high thermal stability and low degradation, which is convenient for large-scale production.

[0066] The technical solution of the present application will be further explained below in conjunction with specific embodiments.

[0067] In the following examples, L-lactide was purchased from Corbin Company and was industrial grade; maleic anhydride was purchased from Sinopharm Chemical Reagent Co., Ltd.; other raw materials were commercially available and of analytical purity; the remaining reagents used, unless otherwise specified, were commercially available or available from public channels; experimental methods without specific conditions were usually carried out under conventional conditions or under conditions recommended by the manufacturer.

[0068] Embodiment 1:

[0069] This embodiment provides a method for preparing modified polylactic acid, which adopts Figure 1 The production device shown in the figure specifically comprises the following steps:

[0070] (1) In a nitrogen environment, 5000 g of lactide, 1 g of stannous octoate catalyst, 25 g of initiator 1,4-butanediol and 500 g of succinic anhydride were continuously introduced into the first reactor 1 for ring-opening polymerization at a reaction temperature of 150° C., a reaction time of 2 h, a reaction pressure of 0.1 MPa, and a stirring speed of 200 rpm to obtain 5250 g of a block copolymer of polylactic acid and anhydride.

[0071] (2) In a nitrogen environment, 25000 g of lactide, 5 g of catalyst stannous octoate, and 125 g of initiator 1,4-butanediol were continuously introduced into the second reactor 2 for ring-opening polymerization. The reaction temperature was 150° C., the reaction time was 3 h, the reaction pressure was 0.1 MPa, and the stirring speed was 200 rpm to obtain 23750 g of polylactic acid.

[0072] (3) The block copolymer of polylactic acid and anhydride obtained in the first reactor 1 and the polylactic acid obtained in the second reactor 2 are mixed and introduced into a tower reactor 3 for melt blending at a temperature of 180° C. for 5 h and a pressure of 15 MPa. After melt blending, the polylactic acid is taken out through the second outlet to a devolatilization unit for devolatilization, and then water-cooled, pelletized and dried to obtain modified polylactic acid.

[0073] Embodiment 2:

[0074] This embodiment provides a method for preparing modified polylactic acid. The specific steps can be referred to in Example 1, and the only difference is step (3). Step (3) of this embodiment is as follows:

[0075] The block copolymer of polylactic acid and anhydride obtained in the first reactor 1 and the polylactic acid obtained in the second reactor 2 are mixed and introduced into the tower reactor 3 for melt blending at a temperature of 180°C, a time of 3 hours, and a pressure of 15 MPa. After melt blending, the polylactic acid is taken out through the first outlet to the devolatilization unit for devolatilization, and then water-cooled, pelletized, and dried to obtain modified polylactic acid.

[0076] Embodiment 3:

[0077] This embodiment provides a method for preparing modified polylactic acid, which adopts Figure 1 The production device shown in the figure specifically comprises the following steps:

[0078] (1) In a nitrogen environment, 5000 g of lactide, 1 g of stannous octoate catalyst, 25 g of initiator 1,4-butanediol and 600 g of succinic anhydride were continuously introduced into the first reactor 1 for ring-opening polymerization at a reaction temperature of 150° C., a reaction time of 2 h, a reaction pressure of 0.1 MPa, and a stirring speed of 200 rpm to obtain 5350 g of a block copolymer of polylactic acid and anhydride.

[0079] (2) In a nitrogen environment, 20,000 g of lactide, 4 g of catalyst stannous octoate, and 100 g of initiator 1,4-butanediol were continuously introduced into the second reactor 2 for ring-opening polymerization. The reaction temperature was 150° C., the reaction time was 3 h, the reaction pressure was 0.1 MPa, and the stirring speed was 200 rpm to obtain 19,000 g of polylactic acid.

[0080] (3) The block copolymer of polylactic acid and anhydride obtained in the first reactor 1 and the polylactic acid obtained in the second reactor 2 are mixed and introduced into a tower reactor 3 for melt blending at a temperature of 180° C. for 5 h and a pressure of 15 MPa. After melt blending, the polylactic acid is taken out through the second outlet to a devolatilization unit for devolatilization, and then water-cooled, pelletized and dried to obtain modified polylactic acid.

[0081] Embodiment 4:

[0082] This embodiment provides a method for preparing modified polylactic acid. The specific steps can be referred to in Example 3, the only difference being step (3). Step (3) of this embodiment is as follows:

[0083] The block copolymer of polylactic acid and anhydride obtained in the first reactor 1 and the polylactic acid obtained in the second reactor 2 are mixed and introduced into the tower reactor 3 for melt blending at a temperature of 180°C, a time of 3 hours, and a pressure of 15 MPa. After melt blending, the polylactic acid is taken out through the first outlet to the devolatilization unit for devolatilization, and then water-cooled, pelletized, and dried to obtain modified polylactic acid.

[0084] Embodiment 5:

[0085] This embodiment provides a method for preparing modified polylactic acid, which adopts Figure 1 The production device shown in the figure specifically comprises the following steps:

[0086] (1) In a nitrogen environment, 5000 g of lactide, 1 g of stannous octoate catalyst, 25 g of initiator 1,4-butanediol and 500 g of succinic anhydride were continuously introduced into the first reactor 1 for ring-opening polymerization at a reaction temperature of 150° C., a reaction time of 2 h, a reaction pressure of 0.1 MPa, and a stirring speed of 200 rpm to obtain 5250 g of a block copolymer of polylactic acid and anhydride.

[0087] (2) In a nitrogen environment, 5500 g of lactide, 1.1 g of catalyst stannous octoate, and 27.5 g of initiator 1,4-butanediol were continuously introduced into the second reactor 2 for ring-opening polymerization. The reaction temperature was 150° C., the reaction time was 3 h, the reaction pressure was 0.1 MPa, and the stirring speed was 200 rpm to obtain 5225 g of polylactic acid.

[0088] (3) The block copolymer of polylactic acid and anhydride obtained in the first reactor 1 and the polylactic acid obtained in the second reactor 2 are mixed and introduced into a tower reactor 3 for melt blending at a temperature of 180° C. for 5 h and a pressure of 15 MPa. After melt blending, the polylactic acid is taken out through the second outlet to a devolatilization unit for devolatilization, and then water-cooled, pelletized and dried to obtain modified polylactic acid.

[0089] Embodiment 6:

[0090] This embodiment provides a method for preparing modified polylactic acid. The specific steps can be referred to in Example 5, the only difference being step (3). Step (3) of this embodiment is as follows:

[0091] The block copolymer of polylactic acid and anhydride obtained in the first reactor 1 and the polylactic acid obtained in the second reactor 2 are mixed and introduced into the tower reactor 3 for melt blending at a temperature of 180°C, a time of 3 hours, and a pressure of 15 MPa. After melt blending, the polylactic acid is taken out through the first outlet to the devolatilization unit for devolatilization, and then water-cooled, pelletized, and dried to obtain modified polylactic acid.

[0092] Embodiment 7:

[0093] This embodiment provides a method for preparing modified polylactic acid. The specific steps can be referred to in Example 1, and the only difference is step (2). Step (2) of this embodiment is as follows:

[0094] In a nitrogen environment, 33000g of lactide, 13.2g of catalyst stannous octoate, and 330g of initiator 1,4-butanediol were continuously introduced into the second reactor 2 for ring-opening polymerization. The reaction temperature was 150°C, the reaction time was 3h, the reaction pressure was 0.1Mpa, the stirring speed was 200rpm, and 31350g of polylactic acid was obtained.

[0095] Embodiment 8:

[0096] This embodiment provides a method for preparing modified polylactic acid. The specific steps can be referred to in Example 1, and the only difference is step (1). Step (1) of this embodiment is as follows:

[0097] (1) In a nitrogen environment, 5000 g of lactide, 1 g of stannous octoate catalyst, 25 g of initiator 1,4-butanediol and 500 g of itaconic anhydride were continuously introduced into the first reactor 1 for ring-opening polymerization at a reaction temperature of 150° C., a reaction time of 2 h, a reaction pressure of 0.1 MPa, and a stirring speed of 200 rpm to obtain 5250 g of a block copolymer of polylactic acid and anhydride.

[0098] Embodiment 9:

[0099] This embodiment provides a method for preparing modified polylactic acid. The specific steps can be referred to in Example 1, and the only difference is step (1) and step (2). Steps (1) and (2) of this embodiment are as follows:

[0100] (1) In a nitrogen environment, 5000 g of lactide, 1 g of stannous octoate catalyst, 25 g of initiator 1,4-butanediol and 200 g of succinic anhydride were continuously introduced into the first reactor 1 for ring-opening polymerization at a reaction temperature of 150° C., a reaction time of 2 h, a reaction pressure of 0.1 MPa, and a stirring speed of 200 rpm to obtain 4950 g of a block copolymer of polylactic acid and anhydride.

[0101] (2) In a nitrogen environment, 33000 g of lactide, 5 g of catalyst stannous octoate, and 125 g of initiator 1,4-butanediol were continuously introduced into the second reactor 2 for ring-opening polymerization. The reaction temperature was 150° C., the reaction time was 3 h, the reaction pressure was 0.1 MPa, and the stirring speed was 200 rpm to obtain 31350 g of polylactic acid.

[0102] Embodiment 10:

[0103] This embodiment provides a method for preparing modified polylactic acid, which adopts Figure 1 The production device shown in the figure specifically comprises the following steps:

[0104] (1) In a nitrogen environment, 5000 g of lactide, 1 g of stannous octoate catalyst, 25 g of initiator 1,4-butanediol and 500 g of succinic anhydride were continuously introduced into the first reactor 1 for ring-opening polymerization at a reaction temperature of 150° C., a reaction time of 0.5 h, a reaction pressure of 0.1 MPa, and a stirring speed of 200 rpm to obtain 5250 g of a block copolymer of polylactic acid and anhydride.

[0105] (2) In a nitrogen environment, 25000 g of lactide, 5 g of catalyst stannous octoate, and 125 g of initiator 1,4-butanediol were continuously introduced into the second reactor 2 for ring-opening polymerization. The reaction temperature was 150° C., the reaction time was 1 h, the reaction pressure was 0.1 MPa, and the stirring speed was 200 rpm to obtain 23750 g of polylactic acid.

[0106] (3) The block copolymer of polylactic acid and anhydride obtained in the first reactor 1 and the polylactic acid obtained in the second reactor 2 are mixed and introduced into a tower reactor 3 for melt blending at a temperature of 180° C. for 3 h and a pressure of 15 MPa. After melt blending, the polylactic acid is taken out through the first outlet to a devolatilization unit for devolatilization, and then water-cooled, pelletized and dried to obtain modified polylactic acid.

[0107] Comparative Example 1:

[0108] This comparative example provides a method for preparing polylactic acid, which adopts Figure 1 The production device shown in the figure specifically comprises the following steps:

[0109] (1) In a nitrogen environment, 20,000 g of lactide, 4 g of stannous octoate catalyst, and 100 g of 1,4-butanediol initiator were continuously introduced into the second reactor 2 for ring-opening polymerization at a reaction temperature of 150° C., a reaction time of 2 h, a reaction pressure of 0.1 MPa, and a stirring speed of 200 rpm to obtain 19,000 g of polylactic acid.

[0110] (2) The polylactic acid obtained from the second reaction kettle 2 is introduced into a tower reactor 3, and reacted at a temperature of 180° C., a time of 5 h, and a pressure of 15 MPa. The polylactic acid is taken out through a second production port to a devolatilization unit for devolatilization, and then water-cooled, pelletized, and dried to obtain the final polylactic acid.

[0111] Comparative Example 2:

[0112] This comparative example provides a method for preparing modified polylactic acid, which adopts Figure 1 The production device shown in the figure specifically comprises the following steps:

[0113] (1) In a nitrogen environment, 5000 g of lactide, 1 g of stannous octoate catalyst, 25 g of initiator 1,4-butanediol and 500 g of succinic anhydride were continuously introduced into the first reactor 1 for ring-opening polymerization at a reaction temperature of 150° C., a reaction time of 2 h, a reaction pressure of 0.1 MPa, and a stirring speed of 200 rpm to obtain 5250 g of a block copolymer of polylactic acid and anhydride.

[0114] (2) The block copolymer of polylactic acid and anhydride obtained in the first reaction kettle 1 is introduced into the tower reactor 3 for melt blending at a temperature of 180°C, a time of 5 hours, and a pressure of 15 MPa. After melt blending, it is taken out through the second outlet to the devolatilization unit for devolatilization, and then water-cooled, pelletized, and dried to obtain modified polylactic acid.

[0115] Test example:

[0116] (1) Grafting rate detection: The grafting rates of the modified polylactic acid obtained in the above examples and comparative examples were determined by chemical titration method. The results are shown in Table 1.

[0117] (2) Molecular weight detection: The molecular weight of the modified polylactic acid or polylactic acid obtained in the above examples and comparative examples was characterized by gel permeation chromatography. The results are shown in Table 1.

[0118] (3) Elongation at break and tensile strength test: The elongation at break and tensile strength of the modified polylactic acid or polylactic acid obtained in the above embodiments and comparative examples were tested using an Instron 5960 series mechanical testing machine in accordance with the method specified in GB / T1040.2-2006. Specifically, a 1A-shaped specimen was used and the test speed was 50 mm / min. The results are shown in Table 1.

[0119] (4) Elastic modulus test: The elastic modulus of the modified polylactic acid or polylactic acid obtained in the above embodiments and comparative examples was tested using a CTS-E500 microcomputer-controlled electronic universal testing machine. The results are shown in Table 1.

[0120] (5) Thermal stability test: The modified polylactic acid or polylactic acid obtained in the above examples and comparative examples was tested using a Perkins Elmer Pyris 1 differential scanning calorimeter. The sample was first heated at 20°C / min. When the instrument detected a change in the system temperature, the temperature was no longer increased and the thermal decomposition temperature of the sample was obtained at a constant temperature. The results are shown in Table 1.

[0121] (6) Degradation degree detection: The modified polylactic acid or polylactic acid obtained in the above examples and comparative examples was placed under composting conditions (the compost fertilizer consisted of straw and wood ash in a mass ratio of 3:1, the compost temperature was 30°C, the humidity was 60%, and the pH value was 6-8), and the degradation degree was calculated after 30 days. The results are shown in Table 1. The degradation degree was obtained by the following calculation method: (W1-W2) / W1*100%; wherein W1 is the original sample mass, g; W2 is the sample mass after 30 days of composting, g.

[0122] (7) Detection of succinic anhydride content: Nuclear magnetic resonance (1H-NMR) was used to characterize the chemical structure of the modified polylactic acid obtained in the above examples and comparative examples to obtain the anhydride content.

[0123] Table 1

[0124]

[0125] As shown in Table 1, compared with Example 2, Example 1 has a longer melt blending time, and its grafting rate, molecular weight, elongation at break, thermal decomposition temperature, and anhydride content are higher, while the tensile strength, degradation degree, and elastic modulus are lower, among which the slightly lower degradation degree may be caused by the higher grafting rate. The above results show that extending the melt blending time from 3 hours to 5 hours is beneficial to the performance of modified polylactic acid. Compared with Example 4, Example 3 has a longer melt blending time; compared with Example 6, Example 5 has a longer melt blending time, both of which show similar changes.

[0126] Compared with Example 1, Example 3 changes the amount of succinic anhydride added and the mass ratio of the block copolymer of polylactic acid and anhydride to polylactic acid. Its grafting rate, elongation at break, thermal decomposition temperature, and anhydride content are increased, while the molecular weight, tensile strength, degradation degree, and elastic modulus are decreased.

[0127] Compared with Example 1, Example 5 changes the mass ratio of the block copolymer of polylactic acid and anhydride to polylactic acid, significantly reduces the amount of polylactic acid added, and its elongation at break, thermal decomposition temperature, anhydride content and elastic modulus are increased, while the grafting rate, molecular weight, tensile strength, degradation degree and elastic modulus are decreased.

[0128] Example 7 Compared with Example 1, Example 7 changes the mass ratio of the block copolymer of polylactic acid and anhydride to polylactic acid, significantly increases the amount of polylactic acid added, and its tensile strength, degradation degree and elastic modulus increase significantly, while the grafting rate, molecular weight, elongation at break, thermal decomposition temperature and anhydride content decrease significantly. The above results show that significantly increasing the amount of polylactic acid added will be detrimental to the toughness and stability of the modified polylactic acid.

[0129] Compared with Example 1, Example 8 changes the type of anhydride, and its grafting rate, tensile strength, degradation degree, and elastic modulus are increased, while the molecular weight, elongation at break, thermal decomposition temperature, and anhydride content are decreased. The above results show that although the itaconic anhydride used is a cyclic anhydride, it has poor compatibility with polylactic acid, and its effect on promoting the toughness and stability of the modified polylactic acid is not as good as that of the anhydride defined in the present invention.

[0130] Example 9 Compared with Example 1, Example 9 changes the mass ratio of the block copolymer of polylactic acid and anhydride to polylactic acid, significantly reduces the amount of succinic anhydride added, and increases the amount of polylactic acid added. Its tensile strength, degradation degree and elastic modulus increase significantly, while the grafting rate, molecular weight, elongation at break, thermal decomposition temperature and anhydride content decrease significantly. The above results show that changing the mass ratio of the block copolymer of polylactic acid and anhydride to polylactic acid will be detrimental to the toughness and stability of the modified polylactic acid.

[0131] Compared with Example 1, Example 10 shortens the reaction time of steps (1), (2), and (3), and its tensile strength, degradation degree, and elastic modulus increase significantly, while the grafting rate, molecular weight, elongation at break, thermal decomposition temperature, and anhydride content decrease significantly. The above results show that shortening the reaction time will be detrimental to the toughness and stability of the modified polylactic acid.

[0132] Comparative Example 1 Compared with Example 1, Comparative Example 1 deleted step (1) and changed the amount of lactide, catalyst and initiator added. Its grafting rate and anhydride content were 0, and the tensile strength, degradation degree and elastic modulus increased significantly, while the molecular weight, elongation at break and thermal decomposition temperature decreased significantly. The above results show that not using the block copolymer of polylactic acid and anhydride will be extremely unfavorable to the toughness and stability of the modified polylactic acid.

[0133] Compared with Example 1, Comparative Example 2 deleted step (2), and its grafting rate, tensile strength, degradation degree, acid glycoside content and elastic modulus increased significantly, while molecular weight, elongation at break and thermal decomposition temperature decreased significantly. The above results show that using only block copolymers of polylactic acid and anhydride to react in a tower reactor is also unfavorable to the toughness and stability of modified polylactic acid.

[0134] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than 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 the technical solutions described in the above embodiments can still be modified, or some or all of the technical features can be replaced by equivalents. However, these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A modified polylactic acid, characterized in that: The modified polylactic acid is obtained by modifying polylactic acid with a block copolymer of polylactic acid and acid anhydride.

2. The modified polylactic acid according to claim 1, characterized in that The mass ratio of the block copolymer of polylactic acid and acid anhydride to the polylactic acid is 1:(1-5).

3. The modified polylactic acid according to claim 1 or 2, characterized in that: The acid anhydride includes at least one of succinic anhydride, adipic anhydride, maleic anhydride, methylmaleic anhydride and dimethylmaleic anhydride.

4. The modified polylactic acid according to any one of claims 1 to 3, characterized in that: The grafting rate of the modified polylactic acid is 1.5%-3.5%.

5. The modified polylactic acid according to any one of claims 1 to 4, characterized in that: The molecular weight of the modified polylactic acid is 2.5×10 5 -4.5×10 5 .

6. A method for preparing the modified polylactic acid according to any one of claims 1 to 5, characterized in that: The steps include: Step 1, subjecting lactide, a first catalyst, a first initiator and anhydride to a first ring-opening polymerization reaction to obtain a block copolymer of polylactic acid and anhydride; Step 2, subjecting lactide, a second catalyst and a second initiator to a second ring-opening polymerization reaction to obtain polylactic acid; Step 3: melt-blending the block copolymer of polylactic acid and acid anhydride and the polylactic acid to obtain the modified polylactic acid.

7. The preparation method according to claim 6, characterized in that: The mass ratio of the total mass of lactide in steps 1 and 2 to the mass ratio of the anhydride in step 1 is (30-70):1; And / or, in step 1, the mass ratio of the lactide, the first catalyst, the first initiator and the acid anhydride is (5000-6000):1:(25-35):(500-600); And / or, the mass ratio of the block copolymer of polylactic acid and acid anhydride to the polylactic acid is 1:(1-5).

8. The preparation method according to claim 6 or 7, characterized in that: The first ring-opening polymerization reaction is carried out at a temperature of 140-180° C., a pressure of 0.01-0.5 MPa, and a time of 1-3 h; and / or, the temperature of the second ring-opening polymerization reaction is 140-180° C., the pressure is 0.01-0.5 MPa, and the time is 2-4 h; And / or, the temperature of the melt blending reaction is 170-190° C., the pressure is 12-20 MPa, and the time is 2-6 h.

9. The preparation method according to any one of claims 6 to 8, characterized in that: The lactide includes at least one of L-lactide, D-lactide, and meso-lactide; and / or, the first catalyst and the second catalyst are each independently selected from at least one of tin pentanoate, stannous pentanoate, tin hexanoate, stannous hexanoate, tin heptanoate, stannous heptanoate, tin octoate, and stannous octoate; And / or, the first initiator and the second initiator are each independently selected from at least one of butanediol, propylene glycol, and pentanediol; And / or, the acid anhydride includes at least one of succinic anhydride, adipic anhydride, maleic anhydride, methylmaleic anhydride and dimethylmaleic anhydride.

10. A production device for modified polylactic acid according to any one of claims 1 to 5, characterized in that: It includes a first reaction kettle, a second reaction kettle and a tower reactor; The first reactor and the second reactor are arranged in parallel, and the discharge ports of the first reactor and the second reactor are respectively connected to the feed port of the tower reactor; The first reactor and the second reactor are used for performing a first ring-opening polymerization reaction and a second ring-opening polymerization reaction, respectively; The tower reactor is used for performing melt blending reaction.

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