CN114851427A

CN114851427A - High-toughness high-transparency heat-resistant polylactic acid material and preparation method thereof

Info

Publication number: CN114851427A
Application number: CN202210456068.5A
Authority: CN
Inventors: 况太荣; 张精兵; 刘通; 周楚仪; 陈枫; 杨晋涛; 费炎培
Original assignee: New Materials Research Institute Of Zhejiang University Of Technology Pinghu City; Zhejiang University of Technology ZJUT
Current assignee: New Materials Research Institute Of Zhejiang University Of Technology Pinghu City; Zhejiang University of Technology ZJUT
Priority date: 2022-04-27
Filing date: 2022-04-27
Publication date: 2022-08-05
Anticipated expiration: 2042-04-27
Also published as: CN114851427B

Abstract

The invention discloses a preparation method of a high-toughness high-transparency heat-resistant polylactic acid material, which comprises the steps of carrying out compression molding on polylactic acid powder obtained by ball milling to obtain a blocky sample; and (3) placing the block sample in a self-made limited processing die and a flat vulcanizing machine, and carrying out uniaxial forced flow orientation in a solid state in the limited die so as to promote the change of the microscopic crystal structure in the material. The prepared polylactic acid product shows greatly improved tensile strength, elongation at break, impact strength and heat resistance, maintains high transparency, and effectively overcomes the defects of brittleness and poor heat resistance of polylactic acid, and reduced transparency while improving toughness. The preparation method has the advantages of simple required equipment, easy operation of process, low energy consumption, no need of adding any second phase, environmental protection and low cost.

Description

High-toughness high-transparency heat-resistant polylactic acid material and preparation method thereof

Technical Field

The invention relates to the field of high polymer material molding processing, in particular to a method for preparing a high-toughness high-transparency heat-resistant polylactic acid material.

Background

With the rapid development of polymer materials and engineering subjects, compared with traditional metal materials, polymer materials play a very important role in daily production and life of people with low price, excellent performance and diversified functions, but a great deal of waste plastic products which are difficult to degrade also cause huge burden on the environment, so how to safely and thoroughly treat the plastic wastes becomes the key point of research in academic circles and industrial circles of various countries in the world. In recent decades, biodegradable polymer materials have attracted more and more attention, and polylactic acid (PLA) is considered to have the most potential to replace traditional petroleum-based plastics due to its characteristics of non-toxicity, no irritation, high strength, good transparency, good biocompatibility, biodegradability and the like. However, the inherent defects of the PLA material, such as poor toughness, poor heat resistance, slow crystallization rate, etc., limit its application in high-value and high-performance engineering fields. Therefore, with the increasing demand of biodegradable polymer materials, people have higher and higher performance requirements, and the high performance of PLA is also a research hotspot in the field of polymer material molding processing.

In order to improve the performance of the PLA material and realize the strengthening and toughening of the PLA material, the main approaches can be divided into two main categories of chemical modification and physical modification. Compared with chemical modification, the physical modification strategy is simpler and cheaper, and is more suitable for industrial production, and the method mainly comprises the modes of plasticizing modification, nucleating agent modification, blending modification, molding process regulation and control and the like. However, the traditional physical modification method is still not easy to realize the reinforcement and toughening of the PLA material, and the toughness is greatly improved and the strength is reduced; or the toughness is reduced or improved only to a limited extent while the strength is increased. Furthermore, the addition of a second phase is often required, which also destroys the high transparency of PLA itself, and limits its application in the field of packaging and the like. Therefore, how to overcome the strength-toughness contradiction of the PLA material and obtain a PLA material with high strength, high toughness, high heat resistance and high transparency becomes the focus of research on high performance research.

Disclosure of Invention

The invention aims to overcome the defects of high toughness, poor heat resistance, improved toughness and reduced transparency of a PLA material and the like, and provides a preparation method capable of realizing high toughness, high heat resistance and high transparency of the PLA material. The method has the advantages of simple operation, low energy consumption, environmental protection, no need of any additive and the like, and can effectively avoid the problems of polymer degradation, filler introduction, biocompatibility influence, transparency and the like caused by processing at high temperature by only performing microstructure regulation and control of the material at relatively low temperature and pressure by a processing means.

In order to achieve the purpose, the invention adopts the following technical scheme:

the invention provides a high-toughness high-transparent heat-resistant polylactic acid material, which is prepared by the following method:

(1) carrying out primary drying, ball milling and screening on polylactic acid particles to obtain powder with the particle size of less than 500 mu m; the powder is dried for the second time, and the dried powder is subjected to compression molding forming to obtain a polylactic acid sheet; the mass of the dried powder is calculated by a method conventional in the art, i.e. slightly greater than the mass of a theoretical formed sheet, which is the volume of the cavity of the mold x the density of the polylactic acid.

(2) Cutting the polylactic acid sheet obtained in the step (1) into a square material matched with a limited mold with a cuboid cavity, wherein a cavity is reserved in the axial direction of the limited mold (preferably, cavities are reserved at two ends) when the square material is placed in the limited mold, and the square material is subjected to uniaxial forced flow orientation in the limited mold for 5-15 min under the conditions of 100-120 ℃ and 150-450 MPa to obtain the high-toughness high-transparency heat-resistant polylactic acid material.

Cutting: and (2) cutting the polylactic acid sheet obtained in the step (1) into a square material which has the same width as the cuboid, the length of the square material is less than that of the cuboid, and the thickness of the square material is less than that of the cuboid.

Firstly, carrying out compression molding on PLA powder to obtain a block sample; then, the block sample is placed in a self-made limited processing mold (the schematic diagram of the mold is shown in fig. 1) and is placed into a flat vulcanizing machine, the processing temperature is higher than the glass transition temperature and lower than the melting temperature of the PLA material, and the block sample generates solid uniaxial forced flow orientation in the limited mold under proper processing pressure, so that the microstructure in the block sample is promoted to generate orientation deformation. The unique microscopic morphological structure enables the PLA material to have excellent strength, toughness and heat resistance, maintains high transparency, and effectively overcomes the defects that the transparency is reduced while the brittleness and the heat resistance of the PLA are poor, the toughness is improved.

The preparation method has the advantages of simple equipment, easy operation of process, low energy consumption, no need of adding any second phase, environmental protection and low cost, and is a molding processing method with wide application prospect. The processing method not only enables the PLA material to be processed at a lower temperature and avoids the degradation of the PLA caused by high temperature, but also enables the obtained PLA product to have excellent performances of high toughness, high heat resistance, high transparency and the like.

Preferably, the polylactic acid particles in step (1) are NatureWorks 4032D, USA, and have a melting point of 160 ℃ and a density of 1.24g/cm ³ The grain diameter is 2000-2500 μm.

Preferably, the first drying in the step (1) is drying at 60-80 ℃ for 24 h.

Specifically, the molding process in step (1) may be performed according to a conventional process well known in the art, and the preparation process may be performed on a conventional molding apparatus such as a press vulcanizer. Preferably, the press molding process parameters on the press vulcanizer are as follows: the temperature is 190 ℃, the pressure is 10MPa, and the processing time is 20 min.

Preferably, the second drying in the step (1) is drying at 60-80 ℃ for 24 h.

Preferably, the uniaxial forced flow orientation in step (2) may also be performed on a press, i.e., the constrained mold is placed on a press for molding. The technological parameters are as follows: the pressure is 150-450 MPa, the processing time is 5-15 min, a PLA sample is placed in the middle of a die cavity in the processing process, and the die is pressurized through equipment such as a flat vulcanizing machine, so that the PLA sample in the die cavity is subjected to uniaxial orientation deformation under the action of the pressure, and PLA products with different performances can be finally obtained by adjusting the pressure.

Preferably, the shape of the cavity of the restricted mold in step (2) is a cuboid, the length of the cuboid is far longer than that of the cut square material, for example, the length of the square material is less than one third of the length of the cuboid, so that the one-way flow restriction of the square material is avoided.

In summary, compared with the prior art, the invention has the beneficial effects that:

(1) according to the invention, PLA materials (including granules and powder) with different particle sizes are selected to be subjected to compression molding, and then the microstructure of the PLA material is regulated, so that the PLA material is reinforced and toughened at the same time, the heat resistance of the PLA material is improved, and the high transparency of the PLA material is maintained. Wherein, the reinforcing and toughening effects of the PLA powder are far better than those of the PLA granules.

(2) The processing method provided by the invention is simple to operate, low in energy consumption, green and environment-friendly, does not add any additive and the like, only performs uniaxial forced orientation processing on the block sample prepared from the PLA powder at relatively low temperature and proper pressure by a processing means, avoids the problems of polymer degradation caused by processing at high temperature, influence of filler on biocompatibility and transparency and the like, and is a green, environment-friendly and cheap preparation method.

Drawings

Fig. 1 is a schematic view of a constrained mold according to the present invention and a schematic view of the fabrication thereof.

Fig. 2 is a pictorial view of a constrained mold according to the present invention.

Fig. 3 is a pictorial representation of the deformation of bulk PLA samples before and after processing.

Fig. 4 is a stress-strain comparison graph of PLA materials made in comparative example 4, examples 1-3.

Fig. 5 is a graph comparing stress-strain of PLA materials prepared in comparative examples 1-3 and examples 1-3.

Fig. 6 is a graph of stress-strain comparison of PLA materials made in comparative examples 5-8, examples 1-3.

Fig. 7 is an SEM image of the PLA material prepared in comparative example 4, example 1.

Fig. 8 is an SEM image of the PLA material prepared in comparative example 4 and example 1 after etching.

Fig. 9 is a graph of the thermal deformation of the PLA material prepared in comparative example 4 and example 1 at different temperatures.

Fig. 10 is a graph comparing light transmittance of PLA materials prepared in comparative example 4 and example 1.

Detailed Description

The technical solution of the present invention is further specifically described below by using specific embodiments and with reference to the accompanying drawings.

In the present invention, all the equipment and materials are commercially available or commonly used in the art, and the methods in the following examples are conventional in the art unless otherwise specified.

The press vulcanizer in the following example was purchased from precision instruments ltd, model No.: BP-8170-A.

Example 1

(1) Mixing PLA granules (NatureWorks 4032D, USA, melting point 160 deg.C, density 1.24g/cm ³ ) Drying in a 60 ℃ oven for 24 h;

(2) adding the dried polylactic acid particles into a planetary ball mill, and carrying out ball milling for 24h under the condition of 400rpm to obtain PLA powder with different particle sizes;

(3) screening the ball-milled PLA powder by using a 35-mesh (500 mu m) screen to obtain powder with the particle size of less than 500 mu m;

(4) placing the PLA powder with the particle size of less than 500 mu m in a 60 ℃ oven again for drying for 24 h;

(5) taking 60g of dried PLA powder, and carrying out compression molding by using a flat vulcanizing machine, wherein the set parameters of the flat vulcanizing machine are that the temperature is 190 ℃, the preheating is 8min, the pressure is 10MPa, and the pressure is maintained for 2min, so that a PLA sheet with the size of 100mm multiplied by 4mm is obtained;

(6) the obtained sheet was cut into a block sample of 30mm × 10mm × 4mm in size, and then placed in a self-made constrained mold (as shown in fig. 1 and 2, the mold cavity size of the mold is 120mm × 10mm × 12mm), the block sample was subjected to uniaxial forced orientation deformation by a flat vulcanizing machine under the process parameters of 110 ℃ temperature, 450MPa pressure and 15min processing time, and the physical diagram of the sample deformation before and after processing is shown in fig. 3, and finally a PLA article having excellent strength and toughness (fig. 4), good heat resistance (fig. 9) and high transparency (fig. 10) was obtained.

Example 2

Example 2 is different from example 1 in that in step (6), the press machines applied pressure during the processing was 150MPa respectively, and the other operation steps were the same.

Example 3

Example 3 is different from example 1 in that in step (6), the press machines applied pressure during the processing was 300MPa, and the other operation steps were the same.

Comparative example 1

Comparative example 1 differs from example 1 in that the raw material used in the comparative example was unground PLA particles (NatureWorks 4032D, U.S. Pat. No. 2D, melting Point 160 ℃, density 1.24g/cm ³ )，

(1) Mixing polylactic acid granules (American Na)TureWorks 4032D, melting point 160 ℃, density 1.24g/cm ³ ) Drying in a 60 ℃ oven for 24 h;

(2) taking 60g of dried PLA granules, and carrying out compression molding by using a flat vulcanizing machine, wherein the set parameters of the flat vulcanizing machine are temperature of 190 ℃, preheating for 8min, pressure of 10MPa and pressure maintaining for 2min, so as to obtain a PLA sheet with the size of 100mm multiplied by 4 mm;

(3) the obtained sheet was cut into a block sample having dimensions of 30mm × 10mm × 4mm, and then placed in a self-made constrained mold (mold cavity having dimensions of 120mm × 10mm × 12mm as shown in fig. 1 and 2), and the block sample was subjected to uniaxial forced orientation deformation under process parameters of a temperature of 110 ℃, a pressure of 450MPa, and a processing time of 15 min.

Comparative example 2

Comparative example 2 differs from example 2 in that the material raw material used was PLA particles which were not pulverized by a ball mill, and the remaining operation steps were the same.

Comparative example 3

Comparative example 3 differs from example 3 in that the material used as the starting material is PLA granules which have not been comminuted by means of a ball mill, and the remaining operating steps are identical.

Comparative example 4

Comparative example 4 is different from example 1 in that dry PLA powder with a particle size of less than 500 μm was compression molded using a press vulcanizer with set parameters of 190 ℃, preheating for 8min, 10MPa, and holding pressure for 2min to obtain PLA sheets with a size of 100mm × 100mm × 4mm, and that no subsequent microstructure control, i.e., no uniaxial forced orientation deformation, was performed.

Comparative example 5

Comparative example 5 is different from example 1 in that the dried polylactic acid granules are added into a planetary ball mill, ball milling is carried out for 24h under the condition of 400rpm, PLA powder with different grain diameters is obtained, screening is carried out by using a 35-mesh (500 mu m) and 18-mesh (1000 mu m) sieve, PLA powder with the grain diameter larger than 500 mu m and smaller than 1000 mu m is obtained, the PLA powder with the grain diameter is used as a raw material, then the dried PLA powder with the same quality is subjected to compression molding by using a flat vulcanizing machine, the set parameters of the flat vulcanizing machine are 190 ℃, preheating is 8min, has the pressure of 10MPa, and is maintained for 2min, thus obtaining the PLA sheet with the size of 100mm multiplied by 4mm, and subsequent micro morphological structure regulation and control are not carried out, namely uniaxial forced orientation deformation is not carried out.

Comparative example 6

Comparative example 6 differs from example 1 in that in step (3) the material starting material used is a PLA powder with a particle size of more than 500 μm and less than 1000 μm, and the remaining operating steps are identical.

Comparative example 7

Comparative example 7 differs from example 2 in that the material used is a PLA powder with a particle size of more than 500 μm and less than 1000. mu.m, and the remaining operating steps are the same.

Comparative example 8

Comparative example 8 differs from example 3 in that the material used is a PLA powder with a particle size of more than 500 μm and less than 1000. mu.m, and the remaining operating steps are the same.

The mechanical property test of the PLA products prepared in examples 1-3 and comparative example 4 was carried out, and the test method and test setting conditions were as follows:

the dumbbell-shaped sample strip for the mechanical property test is manufactured into the dumbbell-shaped sample strip through a universal sampling machine, a tensile testing machine is used for tensile test, the tensile rate is 2mm/min, the tensile test standard refers to GB/T1040-.

As can be seen from FIG. 4, the tensile strength and elongation at break of the processed PLA powder are much better than those of the sample before processing. As can be seen from a comparison of example 1 and comparative example 4, the tensile strength of the processed PLA powder was 1.3 times that of the sample before the treatment, and the elongation at break was 11.6 times higher than that of the sample before the treatment.

The mechanical property test is carried out on the PLA products prepared in the examples 1-3 and the comparative examples 1-3, and the test method and the test setting conditions are as follows:

milling the dumbbell-shaped spline for mechanical property test into the dumbbell-shaped spline by a universal sampling machine, performing tensile test by using a tensile testing machine, wherein the tensile rate is 2mm/min, the tensile test standard refers to GB/T1040-.

As can be seen from FIG. 5, under the same pressure conditions, the tensile strength of the processed PLA pellets is slightly higher than that of the processed PLA powder, but the elongation at break of the processed PLA pellets is much lower than that of the processed PLA powder. It can be seen that the elongation at break of the PLA powder after the processing is 3 to 10 times that of the PLA pellets after the processing.

The mechanical property test is carried out on the PLA products prepared in the examples 1-3 and the comparative examples 5-8, and the test method and the test setting conditions are as follows:

As can be seen from FIG. 6, the tensile strength of the processed PLA powder with a particle size of less than 500 μm is significantly better than that of the processed PLA powder with a particle size of more than 500 μm under the same pressure conditions. In addition, the elongation at break of the PLA powder with the particle size of less than 500 μm after processing is far higher than that of the PLA powder with the particle size of more than 500 μm after processing. It can be seen that the elongation at break of the PLA powder having a particle size of less than 500 μm after the processing is 10 to 20 times that of the PLA powder having a particle size of more than 500 μm after the processing.

The mechanical properties of the PLA articles prepared in examples 1 to 3 and comparative examples 1 to 8 were measured, and the tensile strength and elongation at break obtained are shown in Table 1.

TABLE 1

The morphology of the articles of comparative example 1 and comparative example 4 is shown in FIG. 7. It is observed from fig. 7 that the cross-sectional morphology of the sample of comparative example 1 is relatively flat and smooth, while the cross-sectional morphology of example 1 can observe that the morphological structure of the sample becomes orderly and obviously oriented, and the formed highly oriented morphological structure is one of the main reasons for greatly improving the mechanical properties of the material.

The crystal structures of example 1 and comparative example 4 were characterized by the following steps: preparing mixed solution by methanol and deionized water according to the volume ratio of 3:1, and then adding 0.025mol/L NaOH particles to obtain final etching solution; putting the sample into etching liquid and stirring for 24 hours at a low speed; finally, the sample is taken out, and the residual impurities on the surface of the sample are repeatedly cleaned by deionized water, and the result is shown in fig. 8. As can be seen from fig. 8, the crystal structure of comparative example 4 is a typical randomly arranged spherulite structure, while the spherulites in the crystal structure after etching in example 1 are obviously arranged in an orientation, and the formed crystal structure is also one of the main reasons for greatly improving the mechanical properties of the material.

The PLA articles of example 1 and comparative example 4 were characterized for heat resistance, respectively, and the results are shown in fig. 9. As can be seen from fig. 9, the samples before and after the control at 40 ℃ all maintain the original shape and are not deformed, the sample which is not processed by the control at 70 ℃ is obviously bent and deformed, the sample which is processed by the control does not deform, and the sample which is processed by the control at 110 ℃ still does not deform obviously, and these results show that the heat resistance of the PLA material which is processed by the control of the micro-morphological structure is improved significantly.

The clarity of the PLA materials of example 1 and comparative example 4 were characterized separately and the results are shown in fig. 10. As can be seen from FIG. 10, in the visible light band (380-780nm), the light transmittance of the PLA material prepared after the microstructure regulation is only slightly reduced compared with that before the regulation, and the light transmittance of the PLA product at 780nm is only reduced from 92.4% to 91.5% before processing, so that the transparency of the material is maintained to a higher degree, and the application of the high-performance PLA product in the fields of packaging and the like is further widened.

Claims

1. A high-toughness high-transparency heat-resistant polylactic acid material is characterized by being prepared by the following method:

(1) carrying out primary drying, ball milling and screening on polylactic acid particles to obtain powder with the particle size of less than 500 mu m; the powder is dried for the second time, and the dried powder is subjected to compression molding forming to obtain a polylactic acid sheet;

(2) cutting the polylactic acid sheet obtained in the step (1) into a square material matched with a limited mold with a cuboid cavity, wherein a cavity is reserved in the axial direction of the limited mold when the square material is placed in the limited mold, and carrying out uniaxial forced flow orientation on the square material in the limited mold for 5-15 min at the temperature of 100-120 ℃ and the pressure of 150-450 MPa to obtain the high-strength-toughness high-transparency heat-resistant polylactic acid material.

2. The high-toughness high-transparency heat-resistant polylactic acid material as claimed in claim 1, wherein: the polylactic acid particles in the step (1) are American NatureWorks 4032D, the melting point is 160 ℃, and the density is 1.24g/cm ³ The grain diameter is 2000-2500 μm.

3. The high-toughness high-transparency heat-resistant polylactic acid material as claimed in claim 1, wherein: the first drying in the step (1) is drying at 60-80 ℃.

4. The high-toughness high-transparency heat-resistant polylactic acid material as claimed in claim 1, wherein: the compression molding in the step (1) is carried out on a flat vulcanizing machine, and the process parameters are as follows: the temperature is 190 ℃, the pressure is 10MPa, and the processing time is 20 min.

5. The high-toughness high-transparency heat-resistant polylactic acid material as claimed in claim 1, wherein: in the step (1), the second drying is drying at 60-80 ℃.

6. The high-toughness high-transparency heat-resistant polylactic acid material as claimed in claim 1, wherein: and (3) when the square material is placed in the limited mould in the step (2), cavities are reserved at two axial ends of the limited mould.

7. The high-toughness high-transparency heat-resistant polylactic acid material as claimed in claim 1, wherein: and (3) carrying out uniaxial forced flow orientation on a flat vulcanizing machine in the step (2), namely placing the limited mould on the flat vulcanizing machine for mould pressing.

8. The high-toughness high-transparency heat-resistant polylactic acid material as claimed in claim 1, wherein: the temperature of the molding in the step (2) was 110 ℃.

9. The high-toughness high-transparency heat-resistant polylactic acid material as claimed in claim 1, wherein: the pressure of the die pressing in the step (2) is 450MPa, and the die pressing time is 15 min.

10. The high-toughness high-transparency heat-resistant polylactic acid material as claimed in claim 1, wherein: the length of the square material in the step (2) is less than one third of the length of the cuboid.