Preparation method of polylactic acid material with high impact strength

Abstract

The present invention relates to the technical field of polymer materials, and particularly relates to a preparation method of a polylactic acid material with high impact strength. The polylactic acid preform is subjected to rolling treatment in a temperature environment of 90 °C, and rolled until the deformation amount is higher than 60%, and then a polylactic acid material with high impact strength can be obtained. In the rolling process of the present invention, the molecular chains are oriented and arranged along the stretching direction, and a densely arranged oriented structure is formed inside the system, so that the polylactic acid material can be toughened greatly without adding any additives. This method is easy to operate and easy to implement, so it has good application prospects.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer materials, and particularly relates to a preparation method of a polylactic acid material with high impact strength. Background Art

[0002] Degradable materials have become the main research direction in the development of polymer materials, and the application fields of degradable materials are continuously expanding. The development of new biodegradable materials is rapid and the output is extremely high. Especially for polylactic acid, Natureworks Company in the United States has developed different types of polylactic acid, and domestic companies such as Hisun and Fengyuan also produce polylactic acid. The output of polylactic acid has developed rapidly and has become one of the products with the fastest growth rate and the highest output among degradable plastics. Polylactic acid has good comprehensive properties, such as easy availability of raw materials, good biocompatibility, good biodegradability; high transparency, high strength, good processing performance; safe decomposition, good air permeability, excellent antibacterial and anti-enzyme properties. Due to environmental protection issues, polylactic acid has been widely used as non-woven fabric. In addition, it also has unique advantages in fields such as heart stents, bone nails and bone plates, and disposable tableware. However, polylactic acid products also have the defects of high brittleness, low notched impact strength and low elongation at break, which limits the wider application of polylactic acid products. Therefore, how to obtain a polylactic acid material with high toughness has always been one of the main directions in the development of high-performance polylactic acid.

[0003] At present, there are mainly two types of methods for toughening and modifying polylactic acid at home and abroad:

[0004] The first type is to add a second component or a third component. A flexible second component, such as polyethylene oxide, polycaprolactone, etc., is added to polylactic acid. By adding a compatible second component such as EGMA or PEO, the toughness of polylactic acid can be greatly improved, but the strength of polylactic acid will be reduced. In addition, a third component such as nylon 66 can also be added, which can avoid the strength reduction problem caused by adding the second component. There is a literature report that by adding 40% content of EGMA as the second component and 20% content of PA11 as the third component, the notched impact strength at room temperature can reach up to 88.3 kJ / m 2 , and the tensile yield strength and elongation at break are 75.7 MPa and 121% respectively. However, this requires the processing speed to reach 1000 rpm, which has relatively strict requirements for processing conditions, and not all companies have instruments with such high speeds. Chinese Patent No. CN118063947B discloses adding a lead alloy and modified inorganic nano-silica to acyl chloride-treated polylactic acid, and the silica is bonded to the polylactic acid molecular chain to obtain a wear-resistant material with good impact resistance. However, this method is complex and the toughness is not significantly improved.

[0005] The second category is modified by post-treatment methods. By methods such as melt stretching and thermal stretching, the impact resistance of polylactic acid can be effectively improved.

[0006] The Chinese patent with publication number CN114773644B discloses that polylactic acid is prepared by an extrusion casting and stretching process to obtain polylactic acid with a fracture strength of more than 164.7 MPa, a Young's modulus of more than 3.2 GPa, and an elongation at break of more than 12.4%. However, this patent does not mention the notched impact strength, and the elongation at break has not been significantly improved.

[0007] There are also literature reports on preparing polylactic acid with a special shish-kebab crystal structure by a thermal stretching process. The polylactic acid prepared by this method has an elongation at break of 294.9% and a yield strength of 135.5 MPa. However, the polylactic acid obtained by this method has a high crystallinity, and a high crystallinity will greatly affect the transparency of the material, and this literature also does not have impact performance tests. (Gao X R, Li Y, Huang H D, et al. Extensional stress-induced orientation and crystallization can regulate the balance of toughness and stiffness of polylactide films: interplay of oriented amorphous chains and crystallites. Macromolecules, 2019, 52(14): 5278-5288.).

[0008] Although the above methods can all improve the impact resistance of polylactic acid to a certain extent, relatively speaking, adding the second component and the third component will reduce the degradability or strength of the material; and the post-treatment processing method has high equipment requirements and is not suitable for industrial production. Summary of the Invention

[0009] The object of the present invention is to provide a preparation method of a polylactic acid material with high impact strength. The polylactic acid preform is roll-pressed at an ambient temperature of 90 °C. During the roll-pressing process, the molecular chains are oriented and arranged along the stretching direction, and a highly oriented structure is formed inside the system, enabling the polylactic acid material to be significantly toughened without adding any additives. This method is easy to operate and easy to implement, so it has good application prospects.

[0010] The object of the present invention can be achieved by the following technical solutions:

[0011] A preparation method of a polylactic acid material with high impact strength, comprising the following steps:

[0012] Mold the polylactic acid and cool and solidify it to obtain a preform; then place the preform at 90 °C for 30 min to ensure complete crystallization; place it at room temperature to ensure that there are no obvious changes in the internal structure, and obtain a polylactic acid preform;

[0013] Perform rolling treatment on the polylactic acid preform in an environment at a temperature of 90 °C. During the rolling process, the molecular chains are oriented and arranged along the stretching direction, and densely arranged oriented molecular chains are formed inside the system. The oriented molecular chains and the crystalline regions act together to resist high-temperature deformation. At the same time, the oriented molecular chains are closely arranged to reduce the molecular gap, and a polylactic acid material with high impact strength is obtained;

[0014] Among them, the impact strength is 60.55 kJ / m 2 ~69 kJ / m 2， The fracture strength of polylactic acid is 93 - 103 MPa, the Young's modulus is 2100 - 2400 MPa, the heat distortion temperature is 168 - 181 °C, and the water vapor barrier rate is 0.03 - 0.08 m 2 ·day·atm.

[0015] As a further scheme of the present invention: the preparation process of the polylactic acid preform is:

[0016] Mold the polylactic acid raw material dried at 60 °C for 12 h into a preform in a molding machine.

[0017] It should be explained that: the polylactic acid is molded after drying. The drying temperature is 50 °C to 90 °C; the drying time is 2 h to 72 h; select an appropriate drying time according to different drying temperatures until the water content of the pellets is less than 0.01%. Drying the polylactic acid can prevent thermal degradation of the polylactic acid during subsequent injection molding or compression molding due to increased moisture;

[0018] As a further scheme of the present invention: during the preparation of the polylactic acid material with high impact strength, no additives are added except for the polylactic acid raw material.

[0019] As a further scheme of the present invention: the compression molding temperature is 200 °C.

[0020] As a further scheme of the present invention: the compression molding pressure is 10 MPa.

[0021] As a further scheme of the present invention: the compression molding time is 10 min.

[0022] As a further scheme of the present invention: before the rolling process, the preform is cooled to room temperature by quenching with ice water; it is kept warm in a vacuum oven at 90 °C for 30 min, and then taken out and cooled to room temperature at 23 °C.

[0023] It should be noted that: the forming method is injection molding or compression molding. The temperature of the compression molding is 180°C to 250°C; the pressure of the compression molding is 2 MPa to 12 MPa; the time of the compression molding is 1 min to 120 min; after the compression molding, it is quenched and cooled with ice water to obtain a preform; the obtained preform is placed at room temperature for 24 h - 72 h to reach a stable state. In the present application, the size of the "preform" is not particularly limited, and usually, "preforms" of different sizes are pressed by the mold in the press machine. For example, it can be 2 mm to 4 mm thick, 10 mm to 80 mm wide, and 80 mm to 100 mm long. According to the different sizes of the preform, the materials formed by roll pressing can include film materials or sheet materials.

[0024] As a further aspect of the present invention: the roll pressing rate is 24 rpm, and the radius of the roller is 48 mm.

[0025] As a further aspect of the present invention: the amount of deformation of the roll pressing is 30 - 65%.

[0026] Advantages of the present invention:

[0027] The present invention performs roll pressing treatment at an ambient temperature of 23°C until the required deformation is achieved, so that the polylactic acid preform obtains a highly oriented structure, significantly improving the impact strength of the material. The notched impact strength of the polylactic acid material after roll pressing treatment at room temperature increases from 2.6 kJ / m 2 to 69 kJ / m 2 , and the toughening effect is very significant.

[0028] Compared with other toughening methods, the toughening method of roll pressing treatment at room temperature does not require the addition of any additives, does not require heating, is easy to operate, saves energy, and can also improve the mechanical properties of the material, having good application prospects.

[0029] The impact strength of the high impact strength polylactic acid material obtained by this method is 60.55 kJ / m 2 -69 kJ / m 2 .

[0030] The ultra-high impact strength pure polylactic acid material obtained by the above preparation method of the ultra-high impact strength pure polylactic acid material can be used to manufacture packaging materials, medical materials, fibers, non-woven fabrics, etc., and can be used in the fields of clothing (underwear, outerwear), industry (construction, agriculture, forestry, papermaking, automotive manufacturing), and medical and health devices, etc.;

[0031] The present invention performs rolling treatment on pure polylactic acid materials at room temperature. During the rolling process, the molecular chains are oriented and arranged along the stretching direction, and a highly oriented structure is formed inside the system, enabling the polylactic acid material to be significantly toughened without adding any additives. This method is easy to operate and implement, so it has good application prospects.

[0032] The polylactic acid formed by the rolling method in the present invention forms highly oriented molecular chains. These oriented molecular chains can act together with the crystalline regions to resist high-temperature deformation during the high-temperature process. At the same time, the oriented molecular chains are closely arranged, reducing the molecular gap and capable of blocking gases. Brief Description of the Drawings

[0033] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0034] Figure 1 It is a graph showing the relationship between the cantilever beam notched impact strength and strain of the pure polylactic acid material samples obtained in Examples 1 to 4 and Comparative Example 1 of the present invention. Detailed Embodiments

[0035] In order to enable those skilled in the art to better understand the solution of the present invention, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0036] The reagents used in the following examples are all commercially available. The polylactic acid raw materials used in the examples were all purchased from Natureworks, with the grade of 4032D. The molecular weight of this sample is 86 kg / mol, and the molecular weight distribution is 1.35. The impact performance test in the embodiments of the present invention was carried out according to the standard GB / T1843-2008.

[0037] Example 1

[0038] The polylactic acid raw material dried at 60°C for 12 h was molded into a preform of 80 mm × 80 mm × 1.5 mm (for preparing impact splines) in a molding press. The molding temperature was 200°C, the pressure was 10 MPa, and the time was 10 min. The preform was cooled to room temperature by quenching with ice water. It was kept in a vacuum oven at 90°C for 30 min and then taken out and cooled to room temperature at 23°C. On a rolling equipment, the preform was rolled. The rolling chamber temperature was 90°C, the rolling rate was 24 rpm, the radius of the roller was 48 mm, and the rolling strain was 30%. After reaching the predetermined strain, a pure polylactic acid material sample with a rolling strain of 30% was obtained.

[0039] The rolled sample was prepared into a notched rectangular impact spline of 2 mm × 10 mm × 80 mm, and then the impact resistance performance test was carried out. The notched impact strength of the polylactic acid material sample was 12.42 kJ / m 2 。

[0040] Example 2

[0041] The polylactic acid raw material dried at 60°C for 12 h was molded into a preform of 80 mm × 80 mm × 1.5 mm (for preparing impact splines) in a molding press. The molding temperature was 200°C, the pressure was 10 MPa, and the time was 10 min. The preform was cooled to room temperature by quenching with ice water. It was kept in a vacuum oven at 90°C for 30 min and then taken out and cooled to room temperature at 23°C. On a rolling equipment, the preform was rolled. The rolling chamber temperature was 90°C, the rolling rate was 24 rpm, the radius of the roller was 48 mm, and the rolling thickness strain was 50%. After reaching the predetermined strain, a pure polylactic acid material sample with a rolling strain of 30% was obtained.

[0042] The rolled sample was prepared into a notched rectangular impact spline of 2 mm × 10 mm × 80 mm, and then the impact resistance performance test was carried out. The notched impact strength of the polylactic acid material sample was 20.75 kJ / m 2 。

[0043] Example 3

[0044] The polylactic acid raw material dried at 60°C for 12 h was molded into a preform of 80 mm × 80 mm × 1.5 mm (for preparing impact splines) in a molding press. The molding temperature was 200°C, the pressure was 10 MPa, and the time was from 10 min. The preform was cooled to room temperature by quenching with ice water. It was kept in a vacuum oven at 90°C for 30 min and then taken out and cooled to room temperature at 23°C. On a rolling equipment, the preform was rolled. The rolling chamber temperature was 90°C, the rolling rate was 24 rpm, the radius of the roller was 48 mm, and the rolling strain was 60%. After reaching the predetermined strain, a pure polylactic acid material sample with a rolling strain of 60% was obtained.

[0045] Prepare a notched rectangular impact spline of 2 mm×10 mm×80 mm from the roll-pressed sample, and then conduct an impact resistance performance test. The notched impact strength of the polylactic acid material sample is 20.75 kJ / m 2 .

[0046] Example 4

[0047] Mold the polylactic acid raw material dried at 60 °C for 12 h into a preform of 80 mm×80 mm×1.5 mm (for preparing impact splines) in a film press. The molding temperature is 200 °C, the pressure is 10 MPa, and the time is 10 min. Cool the preform to room temperature by quenching with ice water. Keep it in a vacuum oven at 90 °C for 30 min, and then take it out and cool it to room temperature at 23 °C. On a rolling equipment, conduct a rolling treatment on the preform. The rolling chamber temperature is 90 °C, the rolling rate is 24 rpm, the roller radius is 48 mm, and the rolling strain is 65%. After reaching the predetermined strain, obtain a pure polylactic acid material sample with a rolling strain of 60%.

[0048] Prepare a notched rectangular impact spline of 2 mm×10 mm×80 mm from the roll-pressed sample, and then conduct an impact resistance performance test. The notched impact strength of the polylactic acid material sample is 69 kJ / m 2 .

[0049] Comparative Example 1

[0050] Mold the polylactic acid dried at 60 °C for 12 h into a polylactic acid sheet in a film press. The molding temperature is 200 °C, the pressure is 10 MPa, and the time is 10 min. Cool the preform to room temperature by quenching with ice water. Keep it in a vacuum oven at 90 °C for 30 min, and then take it out and cool it to room temperature at 23 °C. Cut and process the sample to prepare a notched rectangular impact spline of 2 mm×10 mm×80 mm.

[0051] Performance test:

[0052] On a Sansi Feilong series PTM7000 plastic pendulum impact testing machine, analyze the pure polylactic acid material samples obtained in Examples 1-6 and Comparative Example 1 according to the standard GB / T1843-2008, and obtain the relationship diagram of the cantilever beam notched impact strength and strain, as Figure 1 shown:

[0053] When the pure polylactic acid is roll-pressed to 30% at room temperature (Example 1), the degree of orientation of the spline is low, and no dense structure is formed, and the impact strength does not change significantly.

[0054] When the pure polylactic acid is roll-pressed to 50% at room temperature (Example 2), the degree of orientation of the spline increases, but no dense oriented structure appears, and the impact strength does not increase significantly.

[0055] When the pure polylactic acid is rolled to 60% at room temperature (Example 3), the degree of orientation of the spline further increases, a dense oriented structure appears, and the impact strength is greatly improved, from 3.06 KJ / m 2 (Comparative Example 1) to 69 KJ / m 2 , which is nearly 23 times the impact strength of the pure polylactic acid before rolling.

[0056] When the pure polylactic acid is rolled to 60% at room temperature (Example 3), the degree of orientation of the spline further increases, a dense oriented structure appears, and the impact strength is greatly improved, from 3.06 KJ / m 2 (Comparative Example 1) to 73 KJ / m 2 , which is nearly 24 times the impact strength of the pure polylactic acid before rolling;

[0057] The performance tests also include:

[0058] The polylactic acids obtained in Examples 1-4 and Comparative Example 1 were respectively subjected to tensile strength, Young's modulus, heat distortion temperature and gas barrier tests. Among them, the tensile strength test standard is GBT3923.1-2013, the Young's modulus test standard is GB / T22315-2008, the heat distortion temperature test standard is GB / T1634-1979, and the gas barrier test standard is GB / T1038.1-2022. The test results are shown in the following table:

[0059]

[0060] As can be seen from the above table, the polylactic acid prepared in Comparative Example 1 generally has a tensile strength of 90 MPa, a Young's modulus of 2000 MPa, a heat distortion temperature of 167 °C, and a water vapor barrier rate of 0.1 m 2 ·day·atm; while the polylactic acids prepared in Examples 1-4 of the present invention have a tensile strength of 93-103 MPa, a Young's modulus of 2100-2400 MPa, a heat distortion temperature of 168-181 °C, and a water vapor barrier rate of 0.03-0.08 m 2 ·day·atm; it can be known that the polylactic acid formed in the present invention is crystalline polylactic acid and has a very high crystallinity, so the obtained rolled crystalline polylactic acid samples have extremely high tensile strength, Young's modulus, heat distortion temperature and gas barrier.

[0061] The above has described in detail one embodiment of the present invention, but the content described is only a preferred embodiment of the present invention and cannot be considered as limiting the scope of implementation of the present invention. All equivalent changes and improvements made according to the scope of the present invention application should still fall within the scope covered by the patent of the present invention.

Claims

1. A preparation method of a polylactic acid material with high impact strength, characterized in that, It includes the following steps: Mold polylactic acid and cool and solidify it to obtain a preform; then place the preform at 90 °C for 30 min to ensure complete crystallization; place it at room temperature to ensure that there are no obvious changes in the internal structure, and obtain a polylactic acid preform; Perform rolling treatment on the polylactic acid preform in an environment at a temperature of 90 °C. During the rolling process, the molecular chains are oriented and arranged along the stretching direction, and densely arranged oriented molecular chains are formed inside the system. The oriented molecular chains and the crystalline regions work together to resist high-temperature deformation. At the same time, the oriented molecular chains are closely arranged, reducing the molecular gap, and a polylactic acid material with high impact strength is obtained; Among them, the impact strength is 60.55 kJ / m 2 ~69 kJ / m 2， The breaking strength of polylactic acid is 93 - 103 MPa, the Young's modulus is 2100 - 2400 MPa, the heat distortion temperature is 168 - 181 °C, and the water vapor barrier rate is 0.03 - 0.08 m 2 ·day·atm; The polylactic acid formed by processing through the rolling method forms highly oriented molecular chains. These oriented molecular chains can work together with the crystalline regions to resist high-temperature deformation during the high-temperature process. At the same time, the oriented molecular chains are closely arranged, reducing the molecular gap, and can play a role in blocking gases; The rolling rate is 24 rpm, and the radius of the roller is 48 mm; the amount of deformation during rolling is 30 - 65%.

2. The preparation method of a polylactic acid material with high impact strength according to claim 1, characterized in that The preparation process of the polylactic acid preform is as follows: Mold the polylactic acid raw material dried at 60 °C for 12 h into a preform in a molding machine.

3. The preparation method of a polylactic acid material with high impact strength according to claim 1, characterized in that, During the preparation of the polylactic acid material with high impact strength, no additives are added except for the polylactic acid raw material.

4. The preparation method of a polylactic acid material with high impact strength according to claim 2, characterized in that The molding temperature is 200 °C.

5. The preparation method of a polylactic acid material with high impact strength according to claim 3, characterized in that, The molding pressure is 10 MPa.

6. The preparation method of a polylactic acid material with high impact strength according to claim 3, characterized in that, The molding time is 10 min.

7. The preparation method of a polylactic acid material with high impact strength according to claim 1, characterized in that, Before the rolling process, cool the preform to room temperature by quenching with ice water; keep it in a vacuum oven at 90 °C for 30 min, and then take it out and cool it to room temperature at 23 °C.

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