阅读说明：本技术 一种基于成核剂的制备高分子量聚乳酸的方法 (Method for preparing high-molecular-weight polylactic acid based on nucleating agent ) 是由 余承涛 郑映 潘鹏举 周健 于 2021-08-20 设计创作，主要内容包括：本发明涉及聚乳酸合成领域,旨在提供一种基于成核剂的制备高分子量聚乳酸的方法。该方法是在乳酸直接脱水缩聚得到聚乳酸的一步法工艺过程中添加成核剂,最终得到高分子量聚乳酸；成核剂添加方式有两种：在开始缩聚反应前,先将成核剂与催化剂、乳酸溶液混合；或者,将缩聚所得聚乳酸预聚物粉碎后与成核剂混匀,然后再进行预结晶和缩聚反应。本发明通过成核剂的运用,能够提高乳酸直接缩聚制备聚乳酸的分子量。采用的成核剂与乳酸溶液和聚乳酸预聚物具有较好的相容性,可在乳酸溶液中和聚乳酸预聚物中均匀分散。本发明工艺简单、技术成本低、无污染,可实现大规模工业化生产。(The invention relates to the field of polylactic acid synthesis, and aims to provide a method for preparing high-molecular-weight polylactic acid based on a nucleating agent. The method is characterized in that a nucleating agent is added in the one-step process of directly dehydrating and polycondensing lactic acid to obtain polylactic acid, and finally high molecular weight polylactic acid is obtained; the nucleating agent is added in two ways: before the polycondensation reaction, firstly, mixing a nucleating agent, a catalyst and a lactic acid solution; or crushing the polylactic acid prepolymer obtained by polycondensation, mixing the crushed polylactic acid prepolymer with the nucleating agent uniformly, and then performing pre-crystallization and polycondensation reaction. The invention can improve the molecular weight of polylactic acid prepared by direct polycondensation of lactic acid by using the nucleating agent. The nucleating agent has good compatibility with the lactic acid solution and the polylactic acid prepolymer, and can be uniformly dispersed in the lactic acid solution and the polylactic acid prepolymer. The invention has simple process, low technical cost and no pollution, and can realize large-scale industrial production.)

1. A method for preparing high molecular weight polylactic acid based on nucleating agent is characterized in that the nucleating agent is added in the one-step process of directly dehydrating and polycondensing lactic acid to obtain polylactic acid, and finally the high molecular weight polylactic acid is obtained;

the nucleating agent is added in two ways: (1) before the polycondensation reaction, firstly, mixing a nucleating agent, a catalyst and a lactic acid solution; or (2) crushing the polylactic acid prepolymer obtained by polycondensation to obtain prepolymer powder, mixing the prepolymer powder with a nucleating agent uniformly, and then performing pre-crystallization and polycondensation reaction;

the high molecular weight polylactic acid refers to,number average molecular weight M of polylactic acid_n57-107kg/mol, weight average molecular weight M_wIs 87-182 kg/mol.

2. The method according to claim 1, characterized in that it comprises in particular the steps of:

(1) adding a nucleating agent into a lactic acid solution containing a catalyst, fully mixing, heating to 100-140 ℃, and carrying out polycondensation for 2-6 h under normal pressure; then gradually raising the temperature to 160-180 ℃, and carrying out polycondensation for 4-10 h under 50-200 Pa to obtain a polylactic acid prepolymer;

(2) quenching the polylactic acid prepolymer to room temperature, crushing the polylactic acid prepolymer, and pre-crystallizing the polylactic acid prepolymer for 0.5 to 3 hours at the temperature of between 90 and 120 ℃; and then continuously polycondensing for 6-15 h at 130-150 ℃ under 50-200 Pa to obtain the high molecular weight polylactic acid.

3. The method according to claim 1, characterized in that it comprises in particular the steps of:

(1) heating the lactic acid solution containing the catalyst to 100-140 ℃, and carrying out polycondensation for 2-6 h under normal pressure; then gradually raising the temperature to 160-180 ℃, and carrying out polycondensation for 4-10 h under 50-200 Pa to obtain a polylactic acid prepolymer;

(2) quenching the polylactic acid prepolymer to room temperature, crushing the polylactic acid prepolymer, fully mixing the polylactic acid prepolymer with a nucleating agent, and pre-crystallizing the polylactic acid prepolymer for 0.5 to 3 hours at the temperature of between 90 and 120 ℃; and then continuously polycondensing for 6-15 h at 130-150 ℃ under 50-200 Pa to obtain the high molecular weight polylactic acid.

4. The method according to claim 2 or 3, wherein the lactic acid solution is an aqueous lactic acid solution, and the mass fraction of lactic acid in the lactic acid solution is 80-98%.

5. The method of claim 4, wherein the lactic acid is a high gloss pure L-form or D-form lactic acid.

6. The method of claim 2 or 3, wherein the catalyst is stannous octoate (Sn (Oct))₂) N-butyl titanate (TBT), stannous chloride dihydrate (SnCl)₂·2H₂O), p-toluenesulfonic acid.

7. The method according to claim 2 or 3, characterized in that the nucleating agent is at least one of zinc phenylphosphonate (PPZn), orotic acid, diphenyldihydrazide Sebacate (SPH), diphenyldihydrazide Adipate (APH), N', N "-tricyclohexylphosphine-1, 3, 5-Benzenetricarboxamide (BTCA).

8. The method according to claim 2 or 3, wherein the mass ratio of the catalyst to the mass of the lactic acid solution in the reaction system of the whole process is 0.4%, and the mass ratio of the nucleating agent to the mass of the lactic acid solution is 0.1-1.0%.

9. The method according to claim 2 or 3, wherein the particle size of the crushed polylactic acid prepolymer is between 0.1 and 100 microns.

10. The method according to claim 2 or 3, wherein the nucleating agent is mixed with the lactic acid solution or with the pulverized polylactic acid prepolymer by a mechanical stirring method.

Technical Field

The invention relates to the field of polylactic acid synthesis, in particular to a method for preparing high molecular weight polylactic acid based on a nucleating agent.

Background

At present, the annual output of plastics in China exceeds 8000 ten thousand tons, wherein nearly 30 hundred million non-degradable plastic bags such as polyethylene, polyvinyl chloride and the like are used every day, and the annual usage amount is nearly 500 ten thousand tons. The huge usage amount inevitably causes a serious white pollution problem, researches find that the floating garbage in the surface water body of the ocean in China reaches 4027/square kilometer, and the ocean ecology suffers from great challenges. Therefore, the biodegradable plastic is used for replacing part of non-degradable plastic, has important significance particularly in the field of disposable products, and is beneficial to the sustainable development of human and nature.

The polylactic acid is a common biodegradable plastic, has good degradability, biocompatibility and mechanical property, and can replace part of common plastics to be used in the fields of agriculture, packaging materials, clothing and the like. At present, the preparation of high molecular weight polylactic acid mainly applies a two-step method, namely, lactic acid is firstly dehydrated and condensed to prepare polylactic acid oligomer, then depolymerized and cyclized to prepare lactide, and the lactide can be cyclized and polymerized to prepare the high molecular weight polylactic acid after being purified. The early investment of the process route is large, and the production and purification processes of the intermediate product lactide are long, so that the cost of the polylactic acid is high, and the large-scale application of the polylactic acid is limited. If the polylactic acid is obtained by adopting a one-step process, namely direct dehydration and polycondensation of lactic acid, the equilibrium constant of lactic acid polycondensation is very small, and the high molecular weight polylactic acid is difficult to generate from the thermodynamic viewpoint, although the monomer conversion rate is high, the process is simple and the cost is low.

In order to solve the problem of low molecular weight of polylactic acid in the "one-step" process, studies have indicated that low molecular weight polylactic acid obtained by melt polycondensation can be subjected to solid phase polycondensation between the crystallization temperature and the melting point temperature of the polymer after pre-crystallization treatment, thereby further increasing the molecular weight of polylactic acid (Maharana et al, prog.polym.sci.2009,34, 99-124). The method can prepare a large amount of polylactic acid with the weight-average molecular weight of about 6 ten thousand, but has a large difference with the polylactic acid (with the weight-average molecular weight of more than 13 ten thousand) prepared by the two-step process.

As mentioned above, the preparation of high molecular weight polylactic acid by direct polycondensation of lactic acid still has great challenges.

Disclosure of Invention

The invention aims to solve the technical problem of overcoming the defects in the prior art and provides a method for preparing high molecular weight polylactic acid based on a nucleating agent.

In order to solve the technical problem, the solution of the invention is as follows:

the method for preparing the high molecular weight polylactic acid based on the nucleating agent is characterized in that the nucleating agent is added in the one-step process of directly dehydrating and polycondensing the lactic acid to obtain the polylactic acid, and finally the high molecular weight polylactic acid is obtained;

the nucleating agent is added in two ways: (1) before the polycondensation reaction, firstly, mixing a nucleating agent, a catalyst and a lactic acid solution; or (2) crushing the polylactic acid prepolymer obtained by polycondensation, mixing the crushed polylactic acid prepolymer with a nucleating agent uniformly, and then performing pre-crystallization and polycondensation reaction;

the high molecular weight polylactic acid refers to the number average molecular weight M of the polylactic acid_n57-107kg/mol, weight average molecular weight M_wIs 87-182 kg/mol.

As a preferable scheme of the invention, the method specifically comprises the following steps:

(1) adding a nucleating agent into a lactic acid solution containing a catalyst, fully mixing, heating to 100-140 ℃, and carrying out polycondensation for 2-6 h under normal pressure; then, heating to 160-180 ℃, and carrying out polycondensation for 4-10 h under 50-200 Pa to obtain a polylactic acid prepolymer;

(2) quenching the polylactic acid prepolymer to room temperature, crushing the polylactic acid prepolymer, and pre-crystallizing the polylactic acid prepolymer for 0.5 to 3 hours at the temperature of between 90 and 120 ℃; and then continuously polycondensing for 6-15 h at 130-150 ℃ under 50-200 Pa to obtain the high molecular weight polylactic acid.

As a preferable scheme of the invention, the method specifically comprises the following steps:

(1) heating the lactic acid solution containing the catalyst to 100-140 ℃, and carrying out polycondensation for 2-6 h under normal pressure; then, heating to 160-180 ℃, and carrying out polycondensation for 4-10 h under 50-200 Pa to obtain a polylactic acid prepolymer;

(2) quenching the polylactic acid prepolymer to room temperature, crushing the polylactic acid prepolymer, fully mixing the polylactic acid prepolymer with a nucleating agent, and pre-crystallizing the polylactic acid prepolymer for 0.5 to 3 hours at the temperature of between 90 and 120 ℃; and then continuously polycondensing for 6-15 h at 130-150 ℃ under 50-200 Pa to obtain the high molecular weight polylactic acid.

In a preferred embodiment of the present invention, the lactic acid solution is an aqueous lactic acid solution, and the mass fraction of lactic acid in the lactic acid solution is 80-98%.

In a preferred embodiment of the present invention, the lactic acid is a high-gloss pure L-type or D-type lactic acid.

In a preferred embodiment of the present invention, the catalyst is stannous octoate (Sn (Oct)₂) N-butyl titanate (TBT), stannous chloride dihydrate (SnCl)₂·2H₂O), p-toluenesulfonic acid.

In a preferred embodiment of the present invention, the nucleating agent is at least one of zinc phenylphosphonate (PPZn), orotic acid, diphenyldihydrazide Sebacate (SPH), diphenyldihydrazide Adipate (APH), N', N "-tricyclohexylphosphine-1, 3, 5-Benzenetricarboxamide (BTCA).

In a preferable embodiment of the present invention, in the reaction system of the whole process, the mass ratio of the catalyst is 0.4% of the mass of the lactic acid solution, and the mass ratio of the nucleating agent is 0.1-1.0% of the mass of the lactic acid solution.

In a preferred embodiment of the present invention, the particle size of the pulverized polylactic acid prepolymer is 0.1 to 100 μm.

In a preferred embodiment of the present invention, the mixing method of the nucleating agent and the lactic acid solution or the pulverized polylactic acid prepolymer is a mechanical stirring method.

Description of the inventive principles:

in the invention, the nucleating agent has good compatibility with the lactic acid solution and the polylactic acid prepolymer powder, and can be uniformly dispersed in the lactic acid solution and the polylactic acid prepolymer powder matrix. The preparation of the high molecular weight polylactic acid by the one-step method mainly comprises the following steps: three steps of (melt) polycondensation, pre-crystallization and (solid phase) polycondensation. The addition of the nucleating agent mainly affects the pre-crystallization and solid phase polycondensation processes, so the action mechanism of the nucleating agent is consistent when the nucleating agent is added before the pre-crystallization. The action mechanism of the nucleating agent is as follows: the nucleating agent can promote the crystallization of the polylactic acid prepolymer and improve the crystallinity of the polylactic acid prepolymer in the pre-crystallization process, so that small molecular monomers, catalysts and end functional groups of polylactic acid can be better discharged to an amorphous region in the forming process of a polylactic acid crystal region, the collision probability between carboxylate functional groups and hydroxyl functional groups among polylactic acid molecules is increased, the continuous polycondensation reaction is promoted, water generated in a reaction system is taken away by means of vacuum, the reaction balance is moved to the positive and negative reaction direction, and the molecular weight of the polylactic acid is further improved. In the similar prior art, those skilled in the art often use nucleating agents to improve the crystallinity of polylactic acid, and further regulate the optical properties and mechanical properties of polylactic acid. The invention breaks through the thought of inertia and realizes the improvement of the molecular weight of the polylactic acid by reasonably using the nucleating agent.

Compared with the prior art, the invention has the beneficial effects that:

1. the invention can improve the molecular weight of polylactic acid prepared by direct polycondensation of lactic acid by using the nucleating agent.

2. The nucleating agent adopted by the invention has better compatibility with the lactic acid solution and the polylactic acid prepolymer, and can be uniformly dispersed in the lactic acid solution and the polylactic acid prepolymer.

3. The invention adopts the commercialized nucleating agent as the accelerant for improving the molecular weight of the polylactic acid, can adopt a mechanical stirring method for mixing, has simple process, low technical cost and no pollution, and can realize large-scale industrial production.

Detailed Description

The present invention is described in further detail below with reference to specific embodiments, but the scope of the invention as claimed is not limited to the scope expressed by the examples.

The raw materials used in the present invention are illustrated below: the L-lactic acid solution was purchased from Henan gold Dan lactic acid science and technology Co. Type D lactic acid solution, nucleating agents PPZn, SPH and APH were purchased from Sigma orniths (Sigma-Aldrich). The nucleating agent BTCA is purchased from Hangzhou xi Xin Tech Co. Catalyst SnCl₂·2H₂O was purchased from tokyo chemical industry co. Catalyst Sn (Oct)₂TBT and p-toluenesulfonic acid were purchased from sigma-orniths.

The method for preparing the high molecular weight polylactic acid based on the nucleating agent is to prepare the polylactic acidAdding a nucleating agent in the one-step technological process of directly dehydrating and polycondensing to obtain polylactic acid, and finally obtaining high-molecular-weight polylactic acid; the nucleating agent is added in two ways: (1) before the polycondensation reaction, firstly, mixing a nucleating agent, a catalyst and a lactic acid solution; or (2) crushing the polylactic acid prepolymer obtained by polycondensation to obtain prepolymer powder, mixing the prepolymer powder with a nucleating agent uniformly, and then performing pre-crystallization and polycondensation reaction; the high molecular weight polylactic acid refers to the number average molecular weight M of the polylactic acid_n57-107kg/mol, weight average molecular weight M_wIs 87-182 kg/mol.

The temperature, time and pressure in the polycondensation process of the present invention are not particularly limited, and can be carried out by a known method. The kind and amount of the catalyst are not particularly limited, and the reaction can be carried out by a known method. The lactic acid solution is a lactic acid aqueous solution, and high-light pure L-type or D-type lactic acid can be selected. The method for producing lactic acid itself in the present invention is not particularly limited, and it can be carried out by a known method, for example, fermentation process using corn, straw or the like. In the preparation of the high molecular weight polylactic acid, known plastic auxiliaries and additives such as heat stabilizers, antioxidants, fillers and the like may be used. The method of mixing the nucleating agent with the lactic acid solution or the polylactic acid prepolymer powder is a physical mixing method, and the physical mixing method may be performed by a known method, for example, mechanical stirring.

An exemplary method for preparing high molecular weight polylactic acid includes the following two schemes:

the first scheme is as follows: directly adding a nucleating agent into a lactic acid solution containing a catalyst, fully mixing, heating to 100-140 ℃, carrying out polycondensation for 2-6 h under normal pressure, gradually heating to 160-180 ℃, and carrying out polycondensation for 4-10 h under 50-200 Pa; after the reaction is finished, quenching the polylactic acid prepolymer to room temperature, crushing to obtain polylactic acid prepolymer powder, pre-crystallizing at 90-120 ℃ for 0.5-3 h, and continuously polycondensing at 130-150 ℃ and 50-200 Pa for 6-15 h to obtain the polylactic acid with high molecular weight. In the raw materials, the mass fraction of lactic acid in the lactic acid solution is 80-98%, the mass proportion of the catalyst is 0.4% of the mass of the lactic acid solution, and the mass proportion of the nucleating agent is 0.1-1.0% of the mass of the lactic acid solution.

Scheme II: heating the temperature of the lactic acid solution containing the catalyst to 100-140 ℃, performing polycondensation for 2-6 h under normal pressure, then heating the temperature to 160-180 ℃, and performing polycondensation for 4-10 h under 50-200 Pa; after the reaction is completed, the polylactic acid prepolymer is quenched to room temperature and then crushed. Fully mixing the prepolymer powder with a nucleating agent, pre-crystallizing at 90-120 ℃ for 0.5-3 h, and continuously polycondensing at 130-150 ℃ under 50-200 Pa for 6-15 h to obtain the polylactic acid with high molecular weight. In the raw materials, the mass fraction of lactic acid in the lactic acid solution is 80-98%, the mass proportion of the catalyst is 0.4% of the mass of the lactic acid solution, and the mass proportion of the nucleating agent is 0.1-1.0% of the mass of the lactic acid solution.

The difference of the two schemes is the adding time of the nucleating agent, and the scheme is that the nucleating agent is firstly mixed with a catalyst and a lactic acid solution before the polycondensation reaction is started; and in the second scheme, the polylactic acid prepolymer obtained by polycondensation is crushed to obtain powder, the powder is uniformly mixed with the nucleating agent, and then pre-crystallization and polycondensation reaction are carried out.

The following examples are presented to enable those skilled in the art to more fully understand the present invention and are not intended to limit the invention in any way.

Scheme one

Example 1: preparation of PLLA1 samples

According to the first scheme, PPZn nucleating agent which accounts for 0.1 percent of the mass of the L-type lactic acid solution is directly added into SnCl which accounts for 0.4 percent of the mass of the lactic acid solution₂·2H₂And fully mixing the lactic acid solution of the O by mechanical stirring, wherein the mass fraction of the lactic acid in the lactic acid solution is 95%. Heating to 120 ℃, carrying out polycondensation for 4h under normal pressure, then heating to 170 ℃, and carrying out polycondensation for 7h under 100 Pa; after the reaction is finished, the polylactic acid prepolymer is quenched to room temperature and then is crushed, and prepolymer powder with the particle size of about 0.1 micron is obtained. Pre-crystallizing at 105 deg.c for 1 hr, and polycondensing at 145 deg.c and 100Pa for 10 hr to obtain PLLA1 as polylactic acid sample.

Example 2: preparation of PLLA2 samples

According to the first scheme, PPZn nucleating agent accounting for 0.5 percent of the mass of the L-type lactic acid solution is directly added into SnCl which is a catalyst accounting for 0.4 percent of the mass of the lactic acid solution₂·2H₂And fully mixing the lactic acid solution of the O by mechanical stirring, wherein the mass fraction of the lactic acid in the lactic acid solution is 95%. Heating to 120 ℃, carrying out polycondensation for 4h under normal pressure, then heating to 170 ℃, and carrying out polycondensation for 7h under 100 Pa; after the reaction is finished, the polylactic acid prepolymer is quenched to room temperature and then is crushed, and prepolymer powder with the particle size of about 0.1 micron is obtained. Pre-crystallizing at 105 deg.c for 1 hr, and polycondensing at 145 deg.c and 100Pa for 10 hr to obtain PLLA2 as polylactic acid sample.

Example 3: preparation of PLLA3 samples

According to the first scheme, PPZn nucleating agent accounting for 1.0 percent of the mass of the L-type lactic acid solution is directly added into SnCl which is a catalyst accounting for 0.4 percent of the mass of the lactic acid solution₂·2H₂And fully mixing the lactic acid solution of the O by mechanical stirring, wherein the mass fraction of the lactic acid in the lactic acid solution is 95%. Heating to 120 ℃, carrying out polycondensation for 4h under normal pressure, then heating to 170 ℃, and carrying out polycondensation for 7h under 100 Pa; after the reaction is finished, the polylactic acid prepolymer is quenched to room temperature and then is crushed, and prepolymer powder with the particle size of about 0.1 micron is obtained. Pre-crystallizing at 105 deg.c for 1 hr, and polycondensing at 145 deg.c and 100Pa for 10 hr to obtain PLLA3 as polylactic acid sample.

Example 4: preparation of PLLA4 samples

According to the first scheme, PPZn nucleating agent which accounts for 0.1 percent of the mass of the L-type lactic acid solution is directly added into SnCl which accounts for 0.4 percent of the mass of the lactic acid solution₂·2H₂And (3) fully mixing the lactic acid solution of O by mechanical stirring, wherein the mass fraction of the lactic acid in the lactic acid solution is 80%. Heating to 100 ℃, carrying out polycondensation for 2h under normal pressure, then heating to 160 ℃, and carrying out polycondensation for 4h under 200 Pa; after the reaction is finished, the polylactic acid prepolymer is quenched to room temperature and then is crushed, and prepolymer powder with the particle size of about 100 microns is obtained. Pre-crystallizing at 90 deg.C for 0.5h, and continuously polycondensing at 130 deg.C under 200Pa for 6h to obtain polylactic acid sample PLLA 4.

Example 5: preparation of PLLA5 samples

According to the first scheme, PPZn nucleating agent accounting for 1.0 percent of the mass of the L-type lactic acid solution is directly added into SnCl which is a catalyst accounting for 0.4 percent of the mass of the lactic acid solution₂·2H₂Mixing with lactic acid solution of O, wherein the lactic acid is in milkThe mass fraction in the acid solution was 98%. Raising the temperature to 140 ℃, carrying out polycondensation for 6h under normal pressure, then raising the temperature to 180 ℃, and carrying out polycondensation for 10h under 50 Pa; after the reaction is finished, the polylactic acid prepolymer is quenched to room temperature and then is crushed, and prepolymer powder with the particle size of about 1.0 micron is obtained. Pre-crystallizing at 120 deg.c for 3 hr, and polycondensing at 150 deg.c and 50Pa for 15 hr to obtain PLLA5 as polylactic acid sample.

Example 6: preparation of PLLA6 samples

According to the first scheme, 0.5% of nucleating agent SPH in the mass of the L-shaped lactic acid solution is directly added into the lactic acid solution containing 0.4% of catalyst TBT in the mass of the lactic acid solution, and the lactic acid solution is fully mixed by mechanical stirring, wherein the mass fraction of the lactic acid in the lactic acid solution is 95%. Heating to 120 ℃, carrying out polycondensation for 2h under normal pressure, then heating to 170 ℃, and carrying out polycondensation for 8h under 100 Pa; after the reaction is finished, quenching the polylactic acid prepolymer to room temperature and crushing the polylactic acid prepolymer to obtain prepolymer powder with the particle size of about 0.1 micron; pre-crystallizing at 105 deg.c for 1 hr, and polycondensing at 145 deg.c and 100Pa for 10 hr to obtain PLLA6 as polylactic acid sample.

Example 7: preparation of PLLA7 samples

According to the first scheme, 0.5 percent of nucleating agent APH in the mass of the L-type lactic acid solution is directly added into 0.4 percent of catalyst Sn (Oct) in the mass of the lactic acid-containing solution₂The lactic acid solution of (2) is fully mixed by mechanical stirring, wherein the mass fraction of the lactic acid in the lactic acid solution is 95%. Heating to 120 ℃, carrying out polycondensation for 2h under normal pressure, then heating to 170 ℃, and carrying out polycondensation for 8h under 100 Pa; after the reaction is finished, quenching the polylactic acid prepolymer to room temperature and crushing the polylactic acid prepolymer to obtain prepolymer powder with the particle size of about 0.1 micron; pre-crystallizing at 105 deg.c for 1 hr, and polycondensing at 145 deg.c and 100Pa for 10 hr to obtain PLLA7 as polylactic acid sample.

Example 8: preparation of PLLA8 samples

According to the first scheme, 0.5% of nucleating agent BTCA in the mass of the L-type lactic acid solution is directly added into the lactic acid solution containing 0.4% of catalyst p-toluenesulfonic acid in the mass of the lactic acid solution, and the lactic acid solution is fully mixed by mechanical stirring, wherein the mass fraction of the lactic acid in the lactic acid solution is 95%. Heating to 120 ℃, carrying out polycondensation for 2h under normal pressure, then heating to 170 ℃, and carrying out polycondensation for 8h under 100 Pa; after the reaction is finished, quenching the polylactic acid prepolymer to room temperature and crushing the polylactic acid prepolymer to obtain prepolymer powder with the particle size of about 0.1 micron; pre-crystallizing at 105 deg.c for 1 hr, and polycondensing at 145 deg.c and 100Pa for 10 hr to obtain PLLA8 as polylactic acid sample.

Example 9: preparation of PLLA9 samples

According to the first scheme, nucleating agents of orotic acid and PPZn (the mass of orotic acid and PPZn and the like) accounting for 0.5 percent of the mass of the L-type lactic acid solution are directly added into a catalyst of SnCl accounting for 0.4 percent of the mass of the lactic acid-containing solution₂·2H₂O and p-toluenesulfonic acid (SnCl)₂·2H₂O, p-toluenesulfonic acid and the like), wherein the mass fraction of lactic acid in the lactic acid solution is 95%. Heating to 120 ℃, carrying out polycondensation for 2h under normal pressure, then heating to 170 ℃, and carrying out polycondensation for 8h under 100 Pa; after the reaction is finished, quenching the polylactic acid prepolymer to room temperature and crushing the polylactic acid prepolymer to obtain prepolymer powder with the particle size of about 0.1 micron; pre-crystallizing at 105 deg.c for 1 hr, and polycondensing at 145 deg.c and 100Pa for 10 hr to obtain PLLA9 as polylactic acid sample.

Example 10: preparation of PDLA1 sample

According to the first scheme, PPZn nucleating agent accounting for 0.5 percent of the mass of the D-type lactic acid solution is directly added into SnCl which is a catalyst accounting for 0.4 percent of the mass of the lactic acid solution₂·2H₂And fully mixing the lactic acid solution of the O by mechanical stirring, wherein the mass fraction of the lactic acid in the lactic acid solution is 95%. Heating to 120 ℃, carrying out polycondensation for 4h under normal pressure, then heating to 170 ℃, and carrying out polycondensation for 7h under 100 Pa; after the reaction is finished, the polylactic acid prepolymer is quenched to room temperature and then is crushed, and prepolymer powder with the particle size of about 0.1 micron is obtained. Pre-crystallizing at 105 deg.C for 1 hr, and continuously polycondensing at 145 deg.C under 100Pa for 10 hr to obtain PDLA1 sample.

Scheme two

Example 11: preparation of PLLA11 samples

According to the second scheme, a catalyst SnCl with the mass of 0.4 percent of that of the L-type lactic acid solution is added₂·2H₂The temperature of the lactic acid solution of O is raised to 120 ℃, wherein the mass fraction of lactic acid in the lactic acid solution is 95%. After 4 hours of normal pressure polycondensation, the temperature is raised to 170 ℃ and the polycondensation is carried out under 100PaPolymerizing for 8 hours; after the reaction is finished, quenching the polylactic acid prepolymer to room temperature and crushing to obtain prepolymer powder with the particle size of about 0.1 micron. And mechanically stirring and fully mixing the prepolymer and a PPZn nucleating agent accounting for 0.5 percent of the mass of the lactic acid solution, pre-crystallizing for 1h at 105 ℃, and continuously polycondensing for 10h at 145 ℃ and 100Pa to obtain a polylactic acid sample PLLA 11.

Example 12: preparation of PLLA12 samples

According to the second scheme, a catalyst SnCl with the mass of 0.4 percent of that of the L-type lactic acid solution is added₂·2H₂The temperature of the lactic acid solution of O is raised to 100 ℃, wherein the mass fraction of lactic acid in the lactic acid solution is 80%. After polycondensation is carried out for 2 hours under normal pressure, the temperature is raised to 160 ℃, and polycondensation is carried out for 4 hours under 200 Pa; after the reaction is finished, quenching the polylactic acid prepolymer to room temperature and crushing to obtain prepolymer powder with the particle size of about 100 microns. And mechanically stirring and fully mixing the prepolymer and a PPZn nucleating agent accounting for 0.1 percent of the mass of the lactic acid solution, pre-crystallizing for 0.5h at 90 ℃, and continuously polycondensing for 6h at 130 ℃ under 200Pa to obtain a polylactic acid sample PLLA 12.

Example 13: preparation of PLLA13 samples

According to the second scheme, a catalyst SnCl with the mass of 0.4 percent of that of the L-type lactic acid solution is added₂·2H₂The temperature of the lactic acid solution of O is raised to 140 ℃, wherein the mass fraction of lactic acid in the lactic acid solution is 98 percent. After polycondensation is carried out for 6 hours under normal pressure, the temperature is raised to 180 ℃, and polycondensation is carried out for 10 hours under 50 Pa; after the reaction is finished, quenching the polylactic acid prepolymer to room temperature and crushing to obtain prepolymer powder with the particle size of about 1.0 micron. And mechanically stirring and fully mixing the prepolymer and a PPZn nucleating agent accounting for 1.0 percent of the mass of the lactic acid solution, pre-crystallizing for 3.0 hours at 120 ℃, and continuously polycondensing for 15 hours at 150 ℃ under 50Pa to obtain a polylactic acid sample PLLA 13.

Example 14: preparation of PDLA2 sample

According to the second scheme, a catalyst SnCl with the mass of 0.4 percent of that of the D-type lactic acid solution is added₂·2H₂The temperature of the lactic acid solution of O is raised to 120 ℃, wherein the mass fraction of lactic acid in the lactic acid solution is 95%. After polycondensation is carried out for 4 hours under normal pressure, the temperature is raised to 170 ℃, and polycondensation is carried out for 8 hours under 100 Pa; after the reaction is completed, quenching the polylactic acid prepolymer toPulverizing at room temperature to obtain prepolymer powder with particle size of about 0.1 μm. And mechanically stirring and fully mixing the prepolymer and a PPZn nucleating agent accounting for 0.5 percent of the mass of the lactic acid solution, pre-crystallizing for 1h at 105 ℃, and continuously polycondensing for 10h at 145 ℃ and 100Pa to obtain a polylactic acid sample PDLA 2.

Comparative example 1: preparation of PLLA01 samples

SnCl catalyst which does not contain nucleating agent and contains 0.4 percent of lactic acid solution by mass₂·2H₂The temperature of the lactic acid solution of O is raised to 120 ℃, wherein the mass fraction of lactic acid in the lactic acid solution is 95%. After polycondensation is carried out for 2 hours under normal pressure, the temperature is raised to 170 ℃, and polycondensation is carried out for 8 hours under 100 Pa; after the reaction is finished, quenching the polylactic acid prepolymer to room temperature and crushing to obtain prepolymer powder with the particle size of 0.1 micron; pre-crystallizing at 105 deg.c for 1 hr, and polycondensing at 145 deg.c and 100Pa for 10 hr to obtain PLLA01 as polylactic acid sample.

Testing of molecular weight: the Gel Permeation Chromatography (GPC) test was carried out at 30 ℃ with tetrahydrofuran as the mobile phase and at a flow rate of 1.0mL/min, using polystyrene as the calibration standard. M_nIs the number average molecular weight, M_wIs the weight average molecular weight.

Melting point test: using DSC to test, the test temperature is-50-200 ℃, the heating rate is 10 ℃/min, and the peak value of the polylactic acid melting peak is taken as the melting point T_m。

Testing of heat resistance: and (3) using a TGA test, wherein the test temperature is 50-600 ℃, and the heating rate is 10 ℃/min. T is_dIndicating the thermal degradation temperature.

Table 1 shows the statistics of the molecular weights (M) of the polylactic acids prepared in comparative example 1 and examples 1 to 14_nAnd M_w) Melting Point (T)_m) And thermal degradation temperature (T)_d). Analysis of comparative example 1 and examples 1 to 3 and 11 shows that the addition of the nucleating agent can significantly increase the molecular weight of the polylactic acid and the melting point and thermal degradation temperature of the polylactic acid are increased during the polycondensation of the lactic acid to prepare the polylactic acid. This is because the nucleating agent can promote the crystallization of the polylactic acid prepolymer and increase the degree of crystallinity during the pre-crystallization process, thereby better expelling the small-molecule monomer, the catalyst and the terminal functional group into the amorphous region and promoting the shrinkageThe polymerization reaction is continued, and then water in the reaction system is taken away by means of vacuum, so that the reaction balance moves towards the positive reaction direction, and the molecular weight of the polylactic acid is further improved. From example 2, it can be seen that when 0.5% of nucleating agent is added, the weight average molecular weight of polylactic acid can reach 18 ten thousand, which is more than 2 times higher than that of polylactic acid without nucleating agent, the melting point is increased by about 12 ℃, and the thermal degradation temperature is increased by about 50 ℃. As can be seen from the analysis of the molecular weights of the polylactic acids obtained in examples 1 to 3, when the nucleating agent content is less than 0.5%, the molecular weight of the polylactic acid is significantly increased with the increase of the nucleating agent, from 8.5 ten thousand to 18 ten thousand; when the nucleating agent accounts for 0.5% or more, it is found that the nucleating agent has a negligible effect on the molecular weight of the polylactic acid and the molecular weight of the polylactic acid is substantially stabilized by comparing examples 2 and 3.

In addition, it can be seen from comparative examples 2 and 11 that the solution has a better effect of increasing the molecular weight of polylactic acid than the solution two because the nucleating agent is more uniformly mixed in the lactic acid solution than in the polylactic acid prepolymer powder. Comparing examples 2 and 6-9, it can be found that the types of the catalyst and the nucleating agent also have certain influence on the molecular weight of the final polylactic acid; in the present invention, PPZn nucleating agent and SnCl₂·2H₂The O catalyst has the best effect.

TABLE 1

Molecular weight (M) of polylactic acid prepared in comparative example 1 and examples 1 to 14_nAnd M_w) Melting Point (T)_m) And thermal degradation temperature (T)_d)。

	Mn(kg/mol)	Mw(kg/mol)	Tm(℃)	Td(℃)
					Comparative example 1	55	85	168	198
Example 1	68	129	170	210
					Example 2	107	182	180	248
Example 3	103	176	178	240
					Example 4	63	92	163	208
Example 5	98	164	176	234
					Example 6	100	168	177	236
Example 7	101	173	178	235
					Example 8	97	158	172	223
Example 9	95	164	174	227
					Example 10	105	178	178	246
Example 11	90	158	173	228
					Example 12	57	87	159	203
Example 13	73	132	170	211
					Example 14	89	154	172	226

Finally, it should be noted that the above-mentioned list is only a specific embodiment of the present invention. It is obvious that the present invention is not limited to the above embodiments, but many variations are possible. All modifications which can be derived or suggested by a person skilled in the art from the disclosure of the present invention are to be considered within the scope of the invention.