A high melt strength polylactic acid and its preparation method

Abstract

The present invention belongs to the technical field of polymer materials, and particularly relates to a high melt strength polylactic acid and a preparation method thereof. The present invention uses an organic cyclic peroxide as a viscosity-increasing agent, and polylactic acid and a branching aid as blending raw materials, and uses an improved twin-screw extruder to prepare a high melt strength polylactic acid, wherein the mass ratio of the raw materials is 96-98.9% of polylactic acid, 0.1-2% of the viscosity-increasing agent, and 1-2% of the branching aid. The high melt strength polylactic acid obtained by the present invention has excellent melt strength and good processing fluidity. The process of the present invention is simple, the equipment is ordinary, and it is easy to realize industrial production, and the raw material polylactic acid used therein has a wide range of sources and is renewable.

Description

Technical Field

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

[0002] Polylactic acid (PLA) is an aliphatic polyester obtained by extracting and fermenting lactic acid monomers from crop fruits or crop straws and then chemically synthesizing them. It has good biocompatibility, can be completely degraded in nature, and the final degradation products are water and carbon dioxide, without causing pollution to the environment. At the same time, it has a relatively high tensile strength (>60 MPa), is easy to process and form, and has a relatively appropriate price. The melt strength of pure polylactic acid is very low. It can be processed by injection molding and casting molding, etc. However, when preparing foamed materials, due to the too low melt strength, the gas escape speed is too fast, so it is not easy to prepare foam plastics. This low melt strength characteristic limits the application of polylactic acid in the fields of foam plastics, etc.

[0003] In the current related fields of polymer materials, some blending modification technologies have been found to improve the melt strength of polylactic acid. Chinese Patent (CN113265129A) discloses a method for modifying polylactic acid by using organic peroxides and regulators, etc. The content of the organic peroxide is 0.1-2%, the content of the regulator and the lubricant is 0.1-3%, and the content of the nucleating agent is 0.1-5%, and the rest is polylactic acid. Through the modification of substances such as organic peroxides and regulators, the melt strength of the modified sample is enhanced to a certain extent, but the operation steps are relatively cumbersome, the modified components are relatively numerous, and the fluidity of the modified sample decreases to a certain extent. And in Chinese Patent (CN113929831A), a polylactic acid mixture is disclosed. Using benzoyl peroxide as an initiator, it is blended and modified with trifunctional acrylate to improve the melt strength of polylactic acid. The prepared polylactic acid has a high degree of branching entanglement, higher melt strength, and better cell properties. Chinese Patent (CN109575196B) blends polylactic acid and chain extender glycidyl methacrylate in a twin-screw extruder, and the increase in the maximum torque of the twin-screw extruder reflects the increase in melt strength. Chinese Patent (CN113265029A) discloses a long-chain branched polylactic acid with both high melt strength and excellent processing fluidity. By reacting and extruding polylactic acid, organic peroxides and branching regulators, a long-chain branched structure is introduced into the polylactic acid molecular chain, making it have both excellent melt strength and processing fluidity at the same time. These several methods of blending have modified polylactic acid, but the compatibility of the materials is relatively low, and the melt strength still cannot reach the ideal level.

[0004] While improving the melt strength of polylactic acid, its processing ability will also be reduced to a certain extent. How to ensure that its fluidity can meet the normal processing requirements while significantly improving the melt strength is the most critical issue in the field of polylactic acid melt strength modification technology. Summary of the Invention

[0005] To solve the above problems, the present invention provides a high melt strength polylactic acid and a preparation method thereof.

[0006] The technical solution of the present invention is as follows:

[0007] A high melt strength polylactic acid, by weight, the raw material ratio is: 96 - 98.9% of polylactic acid, 0.1 - 2% of organic cyclic peroxide tackifier, 1 - 2% of branching aid; wherein, the organic cyclic peroxide is one or a combination of two of cyclohexanone peroxide and 1,2,4-trioxolane peroxide.

[0008] Preferably, the structural formula of cyclohexanone peroxide is as shown in formula (1) or (2):

[0009]

[0010] Wherein R1-R4 is any one of methyl, ethyl or isopropyl.

[0011] Preferably, the structural formula of 1,2,4-trioxolane peroxide is as shown in formula (3):

[0012]

[0013] Wherein R 1 -R 3 is any one of methyl, ethyl or isopropyl.

[0014] Preferably, the organic cyclic peroxide tackifier is one or a combination of more than one of 3,3,6,6-tetramethyl-1,2-dioxane, 3,3,6,6-tetramethyl-1,2,4,5-tetroxane, 7,7-dimethyl-2,3,5,6-tetraoxabicyclo[2.2.1]heptane (CA-1) or 3,3,5,7,7-pentamethyl-1,2,4-trioxepane (CA-2); the structural formulas of CA-1 and CA-2 are as shown in formulas (4)-(5):

[0015]

[0016] Preferably, the branching aid is one or a combination of butyl acrylate, 1,5-pentanediol diacrylate, pentaerythritol triacrylate, pentaerythritol acrylate, trimethylolpropane triacrylate, tris(2,3-epoxypropyl)isocyanurate, tris(4-hydroxyphenyl)methane triglycidyl ether, trimethylolpropane triglycidyl ether, 1,4-butanediol diglycidyl ether, phthalic anhydride, terephthalic anhydride, 4-hydroxybutyl glycidyl acrylate or glycidyl methacrylate.

[0017] A method for preparing high melt strength polylactic acid, comprising the following steps:

[0018] (1) By weight, the raw material ratio is: 96-98.9% of polylactic acid, 0.1-2% of organic cyclic peroxide tackifier, and 1-2% of branching aid;

[0019] (2) Place the polylactic acid in a vacuum drying oven and dry it at 70°C for 2 h, then cool it to room temperature;

[0020] (3) Feed the dried polylactic acid into the extruder from the feeding port of the modified twin-screw extruder, add the tackifier and the branching aid at the liquid feeding port, and extrude to obtain high melt strength polylactic acid; the temperature of the extruder screw is set at 140°C - 190°C, where the first section of the forward conveying element is 140 - 180°C, the mixing element and the reverse conveying element are both 180°C, and the temperature gradually rises from 180°C to 190°C from the last section of the forward conveying element to the die; the length-diameter ratio of the extruder screw is more than 64:1, and the screw speed is 50 - 400 rpm.

[0021] Preferably, the modified twin-screw extruder is provided with a set of mixing elements at a length-diameter ratio of 30 - 40 times the screw distance from the feeding port, and the stagger angle of the mixing elements is 60°; a set of reverse conveying elements is provided at a length-diameter ratio of 40 - 50 times the screw distance from the feeding port, and the helix angle of the elements is 30°.

[0022] It can be understood that making such improvements to the twin-screw extruder has the following advantages:

[0023] (1) A set of mixing elements with a stagger angle of 60° is provided at a length-diameter ratio of 30 - 40 times the screw distance from the feeding port, which is beneficial for evenly distributing the tackifier and the branching aid in the polylactic acid matrix before the polymerization reaction, avoiding waste of the aid and low branching efficiency caused by insufficient participation in the reaction due to uneven distribution.

[0024] (2) One set of reverse conveying elements with a helix angle of 30° is arranged at a position where the length-diameter ratio of the screw from the feeding port is 40 - 50 times. The advantage of the reverse conveying elements is to extend the residence time of polylactic acid, the viscosity-increasing agent, and the branching aid in the screw. Sufficient residence time can ensure that the polymerization branching reaction can proceed to the end, so that polylactic acid can react completely with the viscosity-increasing agent and the branching aid under high-temperature conditions, greatly improving the modification efficiency.

[0025] The beneficial effects of the present invention are as follows:

[0026] 1. The present invention uses an organic cyclic peroxide as the viscosity-increasing agent. Due to the unique structural properties of the organic cyclic peroxide itself, more branched chains will be generated in the molecular chain segments of the modified polylactic acid, and the crosslinking degree will be reduced. The more branched chain structures will significantly enhance the melt strength of polylactic acid and maintain good processing fluidity. At the same time, after adding the branching aid, the organic cyclic peroxide can effectively regulate the free radical activity under the synergistic effect of the branching aid, thereby improving the utilization efficiency of the organic cyclic peroxide. In addition, it can also make the end groups of polylactic acid participate in the reaction, further reducing the crosslinking degree, increasing the branching degree and branching efficiency, and thus optimizing the topological structure of the polylactic acid molecular chain.

[0027] 2. The combination of polylactic acid / organic cyclic peroxide viscosity-increasing agent / branching aid selected in the present invention can still have a flow property that meets the processing requirements while significantly improving the melt strength.

[0028] 3. The present invention improves the structure of the twin-screw extruder, extends the residence time of polylactic acid in the screw, makes the polylactic acid matrix, the organic cyclic peroxide viscosity-increasing agent, and the branching aid mix evenly, and the reaction is more sufficient. At the same time, it avoids the problems of waste of additives and low reaction efficiency caused by some reactants not participating in the reaction sufficiently. Description of the Drawings

[0029] Figure 1 is the chemical reaction principle diagram of the organic cyclic peroxide viscosity-increasing agent and the branching aid with polylactic acid;

[0030] Figure 2 is the structural schematic diagram of the improved twin-screw extruder in the embodiment; wherein the reference numerals are: 1. feeding port, 2. screw, 2-1. forward conveying element, 2-2. mixing element, 2-3. reverse conveying element, 3. liquid feeding port;

[0031] Figure 3 is Figure 2 the schematic diagram of the forward conveying element of the screw in

[0032] Figure 4 is Figure 2 the schematic diagram of the mixing element of the screw in

[0033] Figure 5 For Figure 2 Schematic diagram of the reverse conveying element of the middle screw. Specific implementation mode

[0034] The twin-screw extruder used in the following examples can be seen in Figures 2 to 5 , which is provided with a screw, and a feeding port 1 and a liquid feeding port 3 arranged at intervals along the axial direction. The screw includes two sections of forward conveying elements 2-1, a mixing element 2-2 and a reverse conveying element 2-3 arranged between the two sections of forward conveying elements 2-1. One section of the forward conveying element 2-1 is located at the discharge port of the screw extruder, and the other section of the forward conveying element 2-1 extends from the feeding port 1 to the liquid feeding port 3. The mixing element 2-2 is located at a position where the ratio of the length to the diameter of the screw from the feeding port is 30-40 times, and the stagger angle of the mixing element is 60°. The reverse conveying element 2-3 is located at a position where the ratio of the length to the diameter of the screw from the feeding port is 40-50 times, and the helix angle of the reverse conveying element is 30°. The materials fed into the feeding port 1 and the liquid feeding port 3 are conveyed to the mixing element 2-2 through the other section of the forward conveying element 2-1, and after mixing, they enter the reverse conveying element 2-3.

[0035] Example 1

[0036] (1) By weight, the raw material ratio is: polylactic acid: 1.96 kg, tackifier (CA-1 accounts for 100%): 0.02 kg, branching aid (4-hydroxybutyl glycidyl acrylate): 0.02 kg.

[0037] (2) Place the polylactic acid in a vacuum drying oven and dry it at a temperature of 70°C for 2 h, and then cool it to room temperature.

[0038] (3) Add the dried polylactic acid into the improved twin-screw extruder through the feeding port, and then add the tackifier liquid and the branching aid to the extruder at the liquid feeding port for reaction and mixing, and extrude and pelletize to obtain high melt strength polylactic acid. The temperature of the screw of the improved screw extruder is set to 140-190°C, where the first section of the forward conveying element part is 140-180°C, the mixing element and the reverse conveying element parts are both 180°C, and the temperature gradually rises from 180°C to 190°C from the last section of the forward conveying element part to the die. The screw speed is set to 280 rpm, and the length-diameter ratio is set to 68:1.

[0039] Example 2

[0040] Change the raw material ratio in Example 1, polylactic acid: 1.96 kg, tackifier (CA-2 accounts for 100%): 0.02 kg, branching aid (4-hydroxybutyl glycidyl acrylate): 0.02 kg. Other steps are the same as those in Example 1.

[0041] Example 3

[0042] Change the raw material ratio in Example 1: polylactic acid: 1.978 kg, tackifier (50% each of CA-1 and CA-2): 0.002 kg, branching aid (4-hydroxybutyl glycidyl acrylate): 0.02 kg. Other steps are the same as in Example 1.

[0043] Example 4

[0044] Change the raw material ratio in Example 1: polylactic acid: 1.96 kg, tackifier (50% each of CA-1 and CA-2): 0.02 kg, branching aid (4-hydroxybutyl glycidyl acrylate): 0.02 kg. Other steps are the same as in Example 1.

[0045] Example 5

[0046] Change the raw material ratio in Example 1: polylactic acid: 1.95 kg, tackifier (50% each of CA-1 and CA-2): 0.02 kg, branching aid (4-hydroxybutyl glycidyl acrylate): 0.03 kg. Other steps are the same as in Example 1.

[0047] Example 6

[0048] Change the raw material ratio in Example 1: polylactic acid: 1.93 kg, tackifier (50% each of CA-1 and CA-2): 0.04 kg, branching aid (4-hydroxybutyl glycidyl acrylate): 0.03 kg. Other steps are the same as in Example 1.

[0049] Example 7

[0050] Change the raw material ratio in Example 1: polylactic acid: 1.92 kg, tackifier (50% each of CA-1 and CA-2): 0.04 kg, branching aid (4-hydroxybutyl glycidyl acrylate): 0.04 kg. Other steps are the same as in Example 1.

[0051] Example 8

[0052] Change the raw material ratio in Example 1: polylactic acid: 1.9 kg, tackifier (50% each of CA-1 and CA-2): 0.04 kg, branching aid (4-hydroxybutyl glycidyl acrylate): 0.06 kg. Other steps are the same as in Example 1.

[0053] Example 9

[0054] Change the raw material ratio in Example 1: polylactic acid: 1.88 kg, tackifier (50% each of CA-1 and CA-2): 0.06 kg, branching aid (4-hydroxybutyl glycidyl acrylate): 0.06 kg. Other steps are the same as in Example 1.

[0055] Comparative Example 1

[0056] Change the raw material ratio in Example 1, polylactic acid: 2 kg. Other steps are the same as those in Example 1.

[0057] Comparative Example 2

[0058] Change the raw material ratio in Example 1, polylactic acid: 1.96 kg, tackifier (CA-1 and CA-2 each account for 50%): 0.04 kg. Other steps are the same as those in Example 1.

[0059] Comparative Example 3

[0060] Change the raw material ratio in Example 1, polylactic acid: 1.96 kg, branching aid (4-hydroxybutyl glycidyl acrylate): 0.04 kg. Other steps are the same as those in Example 1.

[0061] Comparative Example 4

[0062] Add the dried polylactic acid, tackifier and branching aid into a conventional screw extruder together, extrude and pelletize to obtain high melt strength polylactic acid. Among them, the temperature of the screw extruder is set at 180 °C, the screw speed is set at 280 rpm, the length-diameter ratio is set at 68:1, and other steps are the same as those in Example 1.

[0063] Comparative Example 5

[0064] Change the raw material components and ratio in Example 1, polylactic acid: 1.92 kg, tackifier (3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxynonane): 0.04 kg, branching aid (4-hydroxybutyl glycidyl acrylate): 0.04 kg. Other steps are the same as those in Example 1.

[0065] Table 1 shows the test results of the maximum torque, zero-shear viscosity and melt flow rate index (MFR) at 210 °C and 2.16 kg obtained from Examples 1-9 and Comparative Examples 1-5.

[0066] Table 1: Maximum Torque, Zero-Shear Viscosity and MFR of Examples and Comparative Examples

[0067]

[0068]

[0069] In the present invention, as the addition amounts of the organic cyclic peroxide tackifier and the branching aid gradually increase, the maximum torque and zero-shear viscosity of the high melt strength polylactic acid continuously increase, and the MFR value slightly decreases. However, as can be seen from Example 8 and Example 9, if too much of the organic cyclic peroxide tackifier and the branching aid are added, the MFR value of the high melt strength polylactic acid will decrease significantly, which will seriously affect its processing fluidity.

[0070] As can be seen from Table 1, the maximum torque and zero-shear viscosity of the high melt strength polylactic acid prepared in Examples 1-7 are much higher than those of the pure polylactic acid in Comparative Example 1, indicating that the high melt strength polylactic acid prepared in Examples 1-7 has higher melt strength.

[0071] In Comparative Example 2, due to the lack of addition of the branching aid, the improvement effects of its maximum torque and zero-shear viscosity are not obvious.

[0072] In Comparative Example 3, due to the lack of addition of the organic cyclic peroxide tackifier, its test results are similar to those of Comparative Example 1. It can be seen that in the absence of the organic cyclic peroxide tackifier, the branching aid hardly participates in the reaction.

[0073] In Comparative Example 4, an ordinary twin-screw extruder was used. Since the modifier and the polylactic acid did not react sufficiently in the extruder, its measurement results were lower than those of Example 7 under the same raw material ratio, which indicates that the improved screw is beneficial to the full reaction of the raw materials.

[0074] In Comparative Example 5, a cyclic peroxide disclosed in the prior art was used as a tackifier. Compared with Example 7, under the same mass ratio, the degree of improvement of its melt strength is not as good as that of the organic cyclic peroxide selected in the present invention.

[0075] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and its concept of the present invention, makes equivalent substitutions or changes, and all should be covered within the protection scope of the present invention.

Claims

1. A high melt strength polylactic acid, characterized in that, By weight, the raw material ratio is as follows: 96 - 98.9% of polylactic acid, 0.1 - 2% of organic cyclic peroxide tackifier, and 1 - 2% of branching aid; wherein, the organic cyclic peroxide tackifier is composed of 7,7 - dimethyl - 2,3,5,6 - tetraoxabicyclo[2.2.1]heptane and 3,3,5,7,7 - pentamethyl - 1,2,4 - trioxepane; the branching aid is 4 - hydroxybutyl glycidyl acrylate or glycidyl methacrylate; the preparation method of the high melt strength polylactic acid: feeding the dried polylactic acid into the extruder from the feeding port of the modified twin - screw extruder, adding the organic cyclic peroxide tackifier and the branching aid at the liquid feeding port, and extruding and molding to obtain the high melt strength polylactic acid; the length - diameter ratio of the screw of the modified twin - screw extruder is above 64:1, and one set of mixing elements is arranged at a position where the length - diameter ratio of the screw from the feeding port is 30 - 40 times, and the stagger angle of the mixing elements is 60°; one set of reverse conveying elements is arranged at a position where the length - diameter ratio of the screw from the feeding port is 40 - 50 times, and the helix angle of the elements is 30°.

2. The preparation method of the high melt strength polylactic acid according to claim 1, characterized in that, It includes the following steps: (1) By weight, the raw material ratio is as follows: 96 - 98.9% of polylactic acid, 0.1 - 2% of organic cyclic peroxide tackifier, and 1 - 2% of branching aid; (2) Placing the polylactic acid in a vacuum drying oven at 70°C for 2 h, and then cooling it to room temperature; (3) Feeding the dried polylactic acid into the extruder from the feeding port of the modified twin - screw extruder, adding the organic cyclic peroxide tackifier and the branching aid at the liquid feeding port, and extruding and molding to obtain the high melt strength polylactic acid; the screw temperature of the extruder is set at 140°C - 190°C, wherein the first - stage forward conveying element part is 140 - 180°C, both the mixing element part and the reverse conveying element part are 180°C, and the temperature gradually rises from 180°C to 190°C from the last - stage forward conveying element part to the die; the length - diameter ratio of the screw of the extruder is above 64:1, and the screw speed is 50 - 400 rpm.

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