Method for preparing polylactic acid based on lactic acid fermentation of salix psammophila

Abstract

The invention relates to the technical field of environmental protection, and particularly discloses a method for preparing polylactic acid based on lactic acid fermentation of salix psammophila. B, crushing treatment; c, steam explosion; d, performing bacterium and enzymolysis; e, fermenting; f, purifying; g, preparing an intermediate product; h, ring opening polymerization; the prepared polylactic acid material has good mechanical performance, thermal stability and biodegradability, the tensile strength and the elongation at break of salix mongolica-polylactic acid are equivalent to those of traditional petroleum-based polylactic acid, the requirements of common packaging materials can be met, meanwhile, the salix mongolica-polylactic acid further has good biodegradability, and therefore the packaging material can be used for packaging materials. And the material can be quickly degraded under the simulated natural environment condition, so that the pollution of plastic wastes to the environment is greatly reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of environmental protection, and particularly relates to a method for preparing polylactic acid based on lactic acid fermentation of Salix psammophila. Background Art

[0002] Polylactic acid (PLA), as a bio-based, degradable polymer material, is widely used in packaging materials, agricultural mulch films, biomedical materials and other fields due to its excellent biocompatibility, degradability and processing properties. However, the traditional preparation method of PLA mainly relies on fossil resources such as oil and natural gas, which not only increases production costs, but also aggravates global climate change and environmental pollution problems.

[0003] Salix psammophila is a superior plant resource widely distributed in my country's desert areas. It has the characteristics of drought resistance, salt-alkali resistance, and rapid growth. It is an ideal choice for the development and utilization of biomass resources. However, the current resource utilization methods of Salix psammophila are still relatively single, mainly concentrated in the fields of fuel ethanol and biodiesel. Therefore, the present invention aims to provide a method for preparing polylactic acid based on lactic acid fermentation of Salix psammophila. Summary of the invention

[0004] The object of the present invention is to provide a method for preparing polylactic acid based on lactic acid fermentation of Salix psammophila, so as to solve the problems raised in the above background technology.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A method for preparing polylactic acid based on lactic acid fermentation of Salix psammophila, comprising the following steps:

[0007] a. Material collection and pruning: Select Salix psammophila, a superior plant in the Maowusu Sandy Land of Ordos, and carry out pruning treatment;

[0008] b. Crushing: The stubble was crushed to obtain 20-40 mesh safflower powder;

[0009] c. Steam explosion: steam explosion treatment is performed on Salix psammophila powder to destroy its cell structure;

[0010] d. Bacterial and enzymatic hydrolysis: The Salix psammophila powder after steam explosion is subjected to bacterial and / or enzymatic hydrolysis to convert the polysaccharides therein into mixed sugars;

[0011] e. Fermentation: Using mixed sugar as raw material, lactic acid bacteria are used for fermentation to obtain lactic acid;

[0012] f. Purification: Purifying the lactic acid to obtain high-purity lactic acid monomer;

[0013] g. Preparation of intermediate products: Lactic acid monomer is converted into the intermediate product - lactide through a special concentration process;

[0014] h. Ring-opening polymerization: In the presence of a catalyst, lactide is ring-opened to obtain polylactic acid.

[0015] Preferably, in step a, the stubble height is controlled to be 10-20 cm above the ground.

[0016] Preferably, in the step b, the trimmed Salix psammophila stems are fed into a mechanical crusher for crushing, and the crushing speed is 1500-2000 rpm.

[0017] Preferably, in step c, the Salix psammophila powder is loaded into a steam explosion device, and high-temperature and high-pressure steam is introduced, the steam pressure is 1.5-2.0 MPa, the temperature is 180-200° C., and the processing time is 10-15 minutes.

[0018] Preferably, in step d, the inoculation amount of lactic acid bacteria is controlled at 10% by mass, cellulase is added at the same time, the enzymatic hydrolysis temperature is controlled at 45-50°C, the pH value is controlled at 5.0, and the enzymatic hydrolysis time is 24-48 hours. After enzymatic hydrolysis, the polysaccharides in Salix psammophila are converted into fermentable mixed sugars.

[0019] Preferably, in step e, the fermentation temperature is controlled at about 37° C., the pH value is adjusted to 5.0-5.5 by adding ammonia water or hydrochloric acid, the fermentation time is controlled at 48-72 hours, and after the fermentation is completed, lactic acid is extracted from the fermentation liquid by centrifugation and filtration.

[0020] Preferably, in the step f, a reduced pressure distillation method is adopted, the distillation temperature is controlled at 120-130° C., and the vacuum degree is controlled at -0.09 to -0.1 MPa. After distillation, a lactic acid monomer with a purity higher than 95% is obtained.

[0021] Preferably, in step g, the purified lactic acid monomer is fed into a condensation reactor, a condensing agent is added, the reaction temperature is controlled at 150-180° C., the reaction time is 6-8 hours, and after the reaction is completed, lactide is obtained by cooling, crystallization and filtering.

[0022] Preferably, in the step h, lactide is fed into a polymerization reactor, stannous octoate is added, the polymerization temperature is controlled at 180-200° C., the reaction time is 4-6 hours, and after the polymerization is completed, polylactic acid particles are obtained by cooling and pelletizing.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) The polylactic acid material prepared by the method of the present invention has good mechanical properties, thermal stability and biodegradability. The tensile strength and elongation at break of the salix salix-polylactic acid are comparable to those of traditional petroleum-based polylactic acid, which can meet the requirements of general packaging materials. At the same time, the salix salix-polylactic acid also has good biodegradability and can be rapidly degraded under simulated natural environmental conditions, greatly reducing the pollution of plastic waste to the environment.

[0025] (2) By selecting Salix psammophila as raw material, it not only provides a new way for the development and utilization of Salix psammophila as a biomass resource, but also helps to alleviate the environmental problems caused by over-reliance on fossil resources. Salix psammophila, as an easily accessible and low-cost biomass resource, greatly reduces the production cost of polylactic acid. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a flow chart of the present invention. DETAILED DESCRIPTION

[0027] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] Embodiment 1:

[0029] See also Figure 1 As shown, a method for preparing polylactic acid based on lactic acid fermentation of Salix psammophila comprises the following steps:

[0030] a. Material collection and pruning: Select Salix psammophila, a superior plant in the Maowusu Sandy Land of Ordos, and carry out pruning treatment;

[0031] b. Crushing: The stubble was crushed to obtain 20-40 mesh safflower powder;

[0032] c. Steam explosion: steam explosion treatment is performed on Salix psammophila powder to destroy its cell structure;

[0033] d. Bacterial and enzymatic hydrolysis: The Salix psammophila powder after steam explosion is subjected to bacterial and / or enzymatic hydrolysis to convert the polysaccharides therein into mixed sugars;

[0034] e. Fermentation: Using mixed sugar as raw material, lactic acid bacteria are used for fermentation to obtain lactic acid;

[0035] f. Purification: Purifying the lactic acid to obtain high-purity lactic acid monomer;

[0036] g. Preparation of intermediate products: Lactic acid monomer is converted into the intermediate product - lactide through a special concentration process;

[0037] h. Ring-opening polymerization: In the presence of a catalyst, lactide is ring-opened to obtain polylactic acid.

[0038] In one embodiment of the present invention, in step a, the stubble height is controlled to be 10-20 cm above the ground.

[0039] In one embodiment of the present invention, in step b, the trimmed Salix psammophila stems are fed into a mechanical crusher for crushing, and the crushing speed is 1500-2000 rpm.

[0040] In one embodiment of the present invention, in step c, the Salix psammophila powder is loaded into a steam explosion device, and high-temperature and high-pressure steam is introduced, the steam pressure is 1.5-2.0 MPa, the temperature is 180-200°C, and the processing time is 10-15 minutes.

[0041] In one embodiment of the present invention, in step d, the inoculation amount of lactic acid bacteria is controlled at 10% by mass, cellulase is added at the same time, the enzymatic hydrolysis temperature is controlled at 45-50°C, the pH value is controlled at 5.0, and the enzymatic hydrolysis time is 24-48 hours. After enzymatic hydrolysis, the polysaccharides in Salix psammophila are converted into fermentable mixed sugars.

[0042] In one embodiment of the present invention, in step e, the fermentation temperature is controlled at about 37°C, the pH value is adjusted to 5.0-5.5 by adding ammonia water or hydrochloric acid, the fermentation time is controlled at 48-72 hours, and after the fermentation is completed, lactic acid is extracted from the fermentation broth by centrifugation and filtration.

[0043] In one embodiment of the present invention, in step f, a reduced pressure distillation method is adopted, the distillation temperature is controlled at 120-130° C., and the vacuum degree is controlled at -0.09 to -0.1 MPa. After distillation, lactic acid monomer with a purity higher than 95% is obtained.

[0044] In one embodiment of the present invention, in step g, the purified lactic acid monomer is fed into a condensation reactor, a condensing agent is added, the reaction temperature is controlled at 150-180° C., the reaction time is 6-8 hours, and after the reaction is completed, lactide is obtained by cooling, crystallization and filtering.

[0045] In one embodiment of the present invention, in step h, lactide is fed into a polymerization reactor, stannous octoate is added, the polymerization temperature is controlled at 180-200° C., the reaction time is 4-6 hours, and after the polymerization is completed, polylactic acid particles are obtained by cooling and pelletizing.

[0046] Experimental example:

[0047] Select an appropriate amount of Salix psammophila, and perform pruning treatment in autumn, with the pruning height controlled at 15 cm above the ground. Send the pruning Salix psammophila stems into a grinder and crush them to a particle size of about 0.6 mm (about 30 meshes). After crushing, about 200 g of Salix psammophila powder is obtained. The Salix psammophila powder is loaded into a steam explosion device, and high-temperature and high-pressure steam is introduced. The steam pressure is controlled at 1.8 MPa, the temperature is controlled at 190°C, and the treatment time is 12 minutes. After the explosion, the color of the Salix psammophila powder deepens and the volume expands slightly. The Salix psammophila powder after the steam explosion treatment is added to a bacteria and enzymolysis reactor, cellulase and hemicellulase are added, and the total amount of the enzyme preparation added accounts for 2% (mass ratio) of the mass of the Salix psammophila powder. Lactobacillus strains are inoculated, and the inoculation amount accounts for 10% (mass ratio) of the mass of the Salix psammophila powder. The pH value is adjusted to 5.0, the enzymolysis temperature is controlled at 48°C, and the enzymolysis time is 36 hours. After the enzymolysis is completed, the solid residue and the enzymolysis liquid are separated by a centrifuge;

[0048] The enzymatic hydrolysate contains fermentable mixed sugars, and the total sugar concentration is measured to be 10% (mass ratio). The enzymatic hydrolysate is sent to a fermentation tank, and an appropriate amount of lactic acid bacteria is added. The fermentation temperature is controlled at 37° C., and the pH value is adjusted to 5.2 by adding ammonia water. The fermentation time is controlled at 60 hours, and the lactic acid concentration is regularly tested during the fermentation. After the fermentation is completed, the lactic acid concentration is measured to be 12% (mass ratio). The lactic acid fermentation liquid and the bacteria are separated by a centrifuge to obtain a lactic acid solution.

[0049] The lactic acid solution is fed into a vacuum distillation apparatus, the distillation temperature is controlled at 125°C, the vacuum degree is controlled at -0.095 MPa, and about 18 grams of lactic acid monomer with a purity higher than 95% is obtained after distillation. The lactic acid monomer is fed into a condensation reactor, and an appropriate amount of acetic acid is added as a condensation agent. The molar ratio of acetic acid to lactic acid monomer is 1:2. The reaction temperature is controlled at 165°C, and the reaction time is 7 hours. After the reaction is completed, about 15 grams of lactide is obtained by cooling, crystallization, filtering, etc.;

[0050] Feed lactide into a polymerization reactor, add an appropriate amount of stannous octoate as a catalyst, the amount of catalyst accounts for 0.1% (mass ratio) of the mass of lactide, the polymerization temperature is controlled at 190°C, the reaction time is 5 hours, and after the polymerization is completed, about 13 grams of polylactic acid particles are obtained by cooling, pelletizing and other methods.

[0051] Application experiment example:

[0052] Polylactic acid (hereinafter referred to as "Salix salix-PLA") prepared by lactic acid fermentation of Salix salix was applied to the field of packaging materials to evaluate its feasibility and performance as a packaging material. Salix salix-PLA and traditional petroleum-based PLA were processed into packaging samples of the same size using an injection molding machine.

[0053] Use a tensile testing machine to test the tensile strength, elongation at break and other mechanical properties of the packaging samples. The specific experimental data are as follows:

[0054] Sample type Tensile strength(MPa) Elongation at break (%) Salix psammophila-PLA 45.6±2.3 12.5±1.1 Traditional petroleum-based PLA 47.2±1.9 11.8±0.9

[0055] The thermal stability of the packaging samples was tested using a thermogravimetric analyzer, including the initial decomposition temperature, the maximum weight loss rate temperature, etc. The specific experimental data are shown in the following table:

[0056] Sample type Initial decomposition temperature (℃) Maximum weight loss rate temperature (℃) Salix psammophila-PLA 345.7±3.1 382.5±2.8 Traditional petroleum-based PLA 348.2±2.6 385.1±2.4

[0057] The packaging samples were placed in a biodegradability test device to simulate the degradation process in a natural environment. The degradation of the samples was regularly observed and recorded, such as shape change, mass loss, etc. The specific experimental data are shown in the following table:

[0058] Sample type Degradation time (days) Mass loss rate (%) Salix psammophila-PLA 60 68.3±4.2 Traditional petroleum-based PLA 60 12.5±1.5

[0059] From the above, we can see that the tensile strength of Salix psammophila-PLA is slightly lower than that of traditional petroleum-based PLA, but the elongation at break is slightly higher. The overall mechanical properties of the two are comparable and can meet the requirements of general packaging materials.

[0060] Thermal stability: The initial decomposition temperature and maximum weight loss rate temperature of Salix psammophila-PLA are similar to those of traditional petroleum-based PLA, indicating that it has good thermal stability;

[0061] Biodegradability: Salix psammophila-PLA exhibits significant biodegradability under simulated natural environmental conditions, and its mass loss rate is much higher than that of traditional petroleum-based PLA, meeting the requirements of environmentally friendly packaging materials.

[0062] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing polylactic acid based on lactic acid fermentation of Salix psammophila, characterized in that: The following steps are involved: a. Material collection and pruning: Select Salix psammophila, a superior plant in the Maowusu Sandy Land of Ordos, and carry out pruning treatment; b. Crushing: The stubble was crushed to obtain 20-40 mesh safflower powder; c. Steam explosion: steam explosion treatment is performed on Salix psammophila powder to destroy its cell structure; d. Bacterial and enzymatic hydrolysis: The Salix psammophila powder after steam explosion is subjected to bacterial and / or enzymatic hydrolysis to convert the polysaccharides therein into mixed sugars; e. Fermentation: Using mixed sugar as raw material, lactic acid bacteria are used for fermentation to obtain lactic acid; f. Purification: Purifying the lactic acid to obtain high-purity lactic acid monomer; g. Preparation of intermediate products: Lactic acid monomer is converted into the intermediate product - lactide through a special concentration process; h. Ring-opening polymerization: In the presence of a catalyst, lactide is ring-opened to obtain polylactic acid.

2. The method for preparing polylactic acid based on lactic acid fermentation of Salix psammophila according to claim 1, characterized in that: In step a, the stubble height is controlled to be 10-20 cm above the ground.

3. The method for preparing polylactic acid based on lactic acid fermentation of Salix psammophila according to claim 1, characterized in that: In the step b, the trimmed Salix psammophila stems are fed into a mechanical crusher for crushing at a speed of 1500-2000 rpm.

4. The method for preparing polylactic acid based on lactic acid fermentation of Salix psammophila according to claim 1, characterized in that: In the step c, the Salix psammophila powder is loaded into a steam explosion device, and high-temperature and high-pressure steam is introduced, the steam pressure is 1.5-2.0 MPa, the temperature is 180-200° C., and the processing time is 10-15 minutes.

5. The method for preparing polylactic acid based on lactic acid fermentation of Salix psammophila according to claim 1, characterized in that: In the step d, the inoculation amount of lactic acid bacteria is controlled at 10% by mass, cellulase is added at the same time, the enzymatic hydrolysis temperature is controlled at 45-50°C, the pH value is controlled at 5.0, and the enzymatic hydrolysis time is 24-48 hours. After enzymatic hydrolysis, the polysaccharides in Salix psammophila are converted into fermentable mixed sugars.

6. The method for preparing polylactic acid based on lactic acid fermentation of Salix psammophila according to claim 1, characterized in that: In the step e, the fermentation temperature is controlled at about 37° C., the pH value is adjusted to 5.0-5.5 by adding ammonia water or hydrochloric acid, the fermentation time is controlled at 48-72 hours, and after the fermentation is completed, lactic acid is extracted from the fermentation liquid by centrifugation and filtration.

7. The method for preparing polylactic acid based on lactic acid fermentation of Salix psammophila according to claim 1, characterized in that: In the step f, a reduced pressure distillation method is adopted, the distillation temperature is controlled at 120-130° C., and the vacuum degree is controlled at -0.09 to -0.1 MPa. After distillation, lactic acid monomer with a purity higher than 95% is obtained.

8. The method for preparing polylactic acid based on lactic acid fermentation of Salix psammophila according to claim 1, characterized in that: In the step g, the purified lactic acid monomer is fed into a condensation reactor, a condensing agent is added, the reaction temperature is controlled at 150-180° C., the reaction time is 6-8 hours, and after the reaction is completed, lactide is obtained by cooling, crystallization and filtering.

9. The method for preparing polylactic acid based on lactic acid fermentation of Salix psammophila according to claim 1, characterized in that: In the step h, lactide is fed into a polymerization reactor, stannous octoate is added, the polymerization temperature is controlled at 180-200° C., the reaction time is 4-6 hours, and after the polymerization is completed, polylactic acid particles are obtained by cooling and pelletizing.

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