A polylactic acid composite film with high conductivity and balanced rigidity and toughness and a preparation method thereof

Abstract

The application discloses a kind of high conductivity and rigid tough balance polylactic acid composite film and preparation method, comprising the following steps: (1) raw material drying;(2) polylactic acid / carbon black blend granules are prepared by melt blending;(3) polylactic acid / carbon black blend casting film is prepared by extrusion;(4) polylactic acid / carbon black blend stretched film is prepared by annealing and stretching;(5) polylactic acid / carbon black composite film is prepared by heat treatment.The polylactic acid / carbon black composite film prepared by the application has excellent conductivity, which is about 10 orders of magnitude higher than that of pure polylactic acid film.Meanwhile, it also maintains excellent mechanical properties, with an elongation at break of 44.3% and a tensile strength of 97.8 MPa, which are 352.0% and 41.1% higher than those of pure polylactic acid respectively.In addition, the film has excellent heat resistance.The polylactic acid used in the method is derived from renewable resources and is completely biodegradable.The preparation process of the composite film is simple and stable, with a short preparation period and easy industrialization.

Description

Technical Field

[0001] This invention relates to the field of biodegradable films and their preparation, and particularly to a polylactic acid composite film with both high conductivity and a balance of rigidity and toughness, and its preparation method. Background Technology

[0002] Using antistatic packaging for electronic devices can effectively eliminate static electricity generated during transportation and storage, preventing irreversible damage caused by charge accumulation. Currently, antistatic films on the market are mainly based on non-degradable traditional petroleum-based polymers such as polyethylene and polypropylene, and most packaging is discarded after "single use," which inevitably exacerbates the problem of "white pollution" and the energy crisis (Composites Part B: Engineering, 2020, 200: 108-254.).

[0003] Polylactic acid (PLA), a fully biodegradable polymer, is derived from biomass and degrades into carbon dioxide and water after use. It possesses high mechanical strength and good molding and processing properties. Furthermore, due to its declining production costs year by year, it is currently one of the research hotspots in the field of renewable resources and is expected to replace traditional petroleum-based polymers (Advanced Materials, 2000, 12(23): 1841-1846; Polymer Degradation and Stability, 2010, 95(11): 2200-2206.). In the existing technology, there are related technologies to improve the conductivity of PLA by adding conductive fillers, such as invention patents with publication numbers CN103232691A and CN111269541A, and "Research Progress of Polylactic Acid Conductive Polymer Composites" (Materials Reports, 2013, 27(21): 66-72.). However, similar to other polymer-based conductive composites, a high content of conductive fillers is usually required in the polymer matrix to construct a conductive network, thereby improving conductivity, but this is detrimental to the improvement of mechanical properties, especially ductility (Polymers, 2019, 11(10); Journal of Applied Polymer Science, 2019, 136(13): 47273.). In addition, given the inherent brittleness of polylactic acid, balancing the conductivity and mechanical properties (strength and ductility) of polylactic acid-based composites remains a significant challenge.

[0004] To improve its shortcomings, polylactic acid (PLA) is often toughened by introducing elastomers or rigid toughening fillers (Polymers, 2021, 13(12); Materials Today Communications, 2019, 19: 374-382.), but this usually results in a decrease in strength or insufficient toughening effect. In contrast, annealing and stretching the PLA substrate to control the multi-level structure of the material (such as molecular chain stacking, crystal structure, etc.) can also improve the comprehensive mechanical properties of PLA films (Biomacromolecules, 2014, 15(11), 4054-4064; Macromolecules, 2019, 52(14): 5278-5288.). However, under normal circumstances, the introduction of a tensile external field will lead to the deformation and destruction of the conductive network (European Polymer Journal, 2009, 45(10): 2741-2748.), thereby reducing the conductivity of PLA-based conductive composite materials and affecting their conductivity. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by using a "melt extrusion-annealing and stretching-heat treatment" process to prepare a polylactic acid (PLA) composite film material that maintains high conductivity while possessing a balance of rigidity and toughness. The mechanism involves melt extruding a PLA / carbon black blend to obtain a PLA-based cast film, followed by annealing and stretching within a specific temperature range (60–150°C). By changing the stretching roller speed and adjusting the stretching ratio, the crystalline and amorphous regions of the PLA are oriented along the stretching direction. The orientation structure is then fixed by quenching, significantly improving the strength and toughness of the cast film. Finally, the stretched film is heat-treated to obtain the PLA / carbon black composite film. Specifically, controlling the orientation and crystallization induced by the stretching external field can enhance the PLA / carbon black composite film's strength and toughness; subsequent heat treatment effectively eliminates the negative impact of stretching on the conductive network while maintaining the high strength and high toughness of the composite film.

[0006] The technical solution of the present invention for achieving the above objectives is as follows:

[0007] A polylactic acid composite film that combines high conductivity with a balance of rigidity and toughness is characterized by comprising the following steps:

[0008] (1) Raw material drying: Polylactic acid is placed in a dehumidifying dryer to dry thoroughly, while carbon black does not need to be dried and can be used directly;

[0009] (2) Preparation of polylactic acid / carbon black blend granules: polylactic acid and carbon black were melt-blended and extruded using a twin-screw extruder and then granulated by a granulator to obtain polylactic acid / carbon black blend granules;

[0010] (3) Preparation of polylactic acid / carbon black blended film: The blended granules in step (2) are dried again and sent to a single screw extruder. After melting, they are extruded through a slit die and naturally cooled in air to form a blended film.

[0011] (4) Preparation of polylactic acid / carbon black blend stretched film: The blend film obtained in step (3) is sent to a stretching device for stretching treatment. The stretching ratio is controlled by changing the speed difference of the stretching rollers of the stretching device. The polylactic acid / carbon black blend stretched film is formed by quenching the annealing roller.

[0012] (5) Heat treatment of polylactic acid / carbon black composite film: The blend obtained in step (4) is stretched into a film and heat-treated, and then quenched to form the final polylactic acid / carbon black composite film.

[0013] Preferably, in step (1), the content of dextro-lactic acid in polylactic acid is 2-12%, more preferably 2-4%, and most preferably 2%.

[0014] Preferably, in step (2), the mass ratio of carbon black to polylactic acid is (0.05-0.4):1, more preferably (0.15-0.25):1, and most preferably 0.25:1.

[0015] Preferably, in step (2), the temperature range of the twin-screw extruder from the feed port to the die is 150-210℃, more preferably 160-200℃, and most preferably 160-190℃; the screw speed is 60-300r / min, more preferably 110-200r / min, and most preferably 160r / min.

[0016] Preferably, in step (3), the temperature range of the single screw extruder from the feed port to the die is 170-220°C, more preferably 180-210°C, and most preferably 180-200°C; the screw speed is 50-150 r / min, more preferably 50-100 r / min, and most preferably 80 r / min.

[0017] Preferably, in step (4), the stretching ratio is controlled to be 1.0-7.0, more preferably 1.0-4.0, and most preferably 3.0.

[0018] Preferably, in step (4), the temperature of the stretching roller is 60-150°C, more preferably 80-110°C, and most preferably 90°C; the speed of the stretching roller is 0.5-100.0 m / min, more preferably 0.8-2.0 m / min, and most preferably 1.5 m / min.

[0019] Preferably, in step (5), the heat treatment temperature is 110-190°C, more preferably 130-170°C, and most preferably 150°C.

[0020] Beneficial effects

[0021] The polylactic acid composite film and its preparation method provided by this invention, which combine high conductivity and a balance of rigidity and toughness, have advantages over existing methods, such as a balance of rigidity and toughness, high conductivity, simple preparation process, and suitability for large-scale industrial production.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] (1) This invention improves the conductivity of polylactic acid by adding carbon black to the polylactic acid matrix to form a conductive network, while coupling the tensile flow field and the temperature field, which is the first of its kind in the technical field.

[0024] (2) While taking into account the strength and toughness of polylactic acid, this invention also makes polylactic acid have excellent conductivity, which solves the shortcomings of poor mechanical properties of traditional conductive polymer materials.

[0025] (3) The polylactic acid / carbon black composite film prepared by the present invention can maintain its own resistance value at 150°C, which has excellent heat resistance and shows potential for application in high temperature environment, thus broadening its application range.

[0026] (4) The production process of this invention is simple, requiring only simple blending, stretching and heat treatment. It has high production efficiency, stable and easy-to-achieve production conditions, and is suitable for mass production in industry. Attached Figure Description

[0027] Figure 1 The energy storage modulus variation curves are for Comparative Examples 8, 11, 23(a) and Examples 17, 45, 57(b).

[0028] Figure 2 The temperature-resistance curves are for Examples 17, 45 and Comparative Examples 11, 23.

[0029] Figure 3 Two-dimensional wide-angle X-ray diffraction patterns for Examples 17(c), 45(f), 57(e) and Comparative Examples 8(a), 11(b), 23(d).

[0030] Figure 4 Line graphs showing the crystallinity, active amorphous phase (MAF), and rigid amorphous phase (RAF) content of Examples 17, 45, 57, 59, and Comparative Example 11. Detailed Implementation

[0031] The present invention will be further illustrated below with reference to specific embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Non-essential improvements and adjustments made to the present invention by those skilled in the art based on the above-described invention are still within the scope of protection of the present invention.

[0032] Examples 1-24 are shown in Table 1 below:

[0033] (1) Raw material drying: Polylactic acid is placed in a dehumidifying dryer for thorough drying. Carbon black does not need to be dried and can be used directly. The mass ratio of polylactic acid to carbon black is shown in Table 1.

[0034] (2) Preparation of polylactic acid / carbon black blend granules: The fully dried polylactic acid and carbon black in step (1) were melt-blended and extruded using a twin-screw extruder. The temperature range from the feed port to the die of the twin-screw extruder was 150 to 210°C, and the screw speed was 60 to 300 r / min. The polylactic acid / carbon black blend granules were obtained by granulation.

[0035] (3) Preparation of polylactic acid / carbon black blend cast film: The blend granules from step (2) are dried again and fed into a single-screw extruder. The temperature range from the feed port to the die of the single-screw extruder is 170-220℃, and the screw speed is 50-150 r / min. Then, the film is extruded through a slit die and naturally cooled in air to form a blend cast film.

[0036] (4) Preparation of polylactic acid / carbon black blend stretched film: The cast film obtained in step (3) is sent to a stretching device for stretching treatment. The stretching ratio is controlled by changing the speed difference of the stretching rollers in the stretching device. Annealing and stretching are performed at 60-150℃, and the polylactic acid / carbon black blend stretched film is formed by quenching the annealing rollers. The speed of the stretching rollers is 0.5-100.0 m / min, and the stretching ratio (abbreviated as DR) is controlled at 1.0-7.0.

[0037] Comparative Examples 1-7 are shown in Table 1 below:

[0038] (1) Raw material drying: Place polylactic acid in a dehumidifying dryer to dry it thoroughly.

[0039] (2) Preparation of polylactic acid (PLA) cast film: PLA granules are fed into a single-screw extruder. The temperature range from the feed port to the die of the single-screw extruder is 170-220℃, and the screw speed is 50-150 r / min. Then, the film is extruded through a slit die and naturally cooled in air to form a PLA cast film. The carbon black / PLA mass ratio is set to 0, i.e., it contains no carbon black.

[0040] (3) Preparation of polylactic acid stretched film: The cast film obtained in step (2) is sent to a stretching device for stretching treatment. The stretching ratio is controlled by changing the speed difference of the stretching rollers of the stretching device. Annealing stretching is carried out at 60-150℃ and then quenched by the annealing rollers to form the film.

[0041] Comparative Example 8 is shown in Table 1 below:

[0042] (1) Raw material drying: Place polylactic acid in a dehumidifying dryer to dry it thoroughly.

[0043] (2) Preparation of polylactic acid cast film: Polylactic acid granules are fed into a single-screw extruder. The temperature range from the feed port to the die of the single-screw extruder is 170-220℃, and the screw speed is 50-150 r / min. Then, the film is extruded through a slit die and naturally cooled in air to form a polylactic acid cast film. The stretch ratio of the obtained cast film is set to 0.

[0044] Comparative Examples 9-12 are shown in Table 1 below:

[0045] (1) Raw material drying: Polylactic acid is placed in a dehumidifying dryer for thorough drying. Carbon black does not need to be dried and can be used directly. The mass ratio of polylactic acid to carbon black is shown in Table 1.

[0046] (2) Preparation of polylactic acid / carbon black blend granules: The fully dried polylactic acid and carbon black in step (1) were melt-blended and extruded using a twin-screw extruder. The temperature range from the feed port to the die of the twin-screw extruder was 150 to 210°C, and the screw speed was 60 to 300 r / min. The polylactic acid / carbon black blend granules were obtained by granulation.

[0047] (3) Preparation of polylactic acid / carbon black blend cast film: The blend granules from step (2) are fed into a single-screw extruder, melted, and extruded through a slit die. After cooling by a low-temperature casting roller, a cast film is formed. The temperature range from the feed port to the die in the single-screw extruder is 150–225°C, and the screw speed is 50–200 r / min. The stretch ratio of the resulting cast film is set to 0.

[0048] Table 1. Mechanical properties and electrical conductivity of Examples 1-28 and Comparative Examples 1-11 under different filler contents and stretching conditions.

[0049]

[0050]

[0051] Examples 1-28 and Comparative Examples 1-12 in Table 1 were subjected to heat treatment under the same conditions to obtain Examples 29-56 and Comparative Examples 13-24.

[0052] Examples 29-56 are shown in Table 2 below:

[0053] Preparation of polylactic acid / carbon black composite film: The polylactic acid / carbon black blend is stretched into a film and heat-treated at 110-190℃, and then quenched to form the final polylactic acid / carbon black composite film.

[0054] Comparative Examples 13-19 are shown in Table 2 below:

[0055] Preparation of polylactic acid heat-treated film: Polylactic acid stretched film is heat-treated at 110-190℃ and then quenched to form the final polylactic acid heat-treated film.

[0056] Comparative Example 20 is shown in Table 2 below:

[0057] Preparation of polylactic acid heat-treated cast film: Polylactic acid blend cast film is heat-treated at 110-190℃ and then quenched to form the final polylactic acid heat-treated cast film.

[0058] Comparative Examples 21-24 are shown in Table 2 below:

[0059] Preparation of polylactic acid / carbon black blend heat-treated cast film: The polylactic acid / carbon black blend cast film is heat-treated at 110-190℃ and then quenched to form the final polylactic acid / carbon black blend heat-treated cast film.

[0060] Table 2 shows the mechanical properties and electrical conductivity of different filler contents and stretching conditions in Examples 29-56 and Comparative Examples 13-24.

[0061]

[0062]

[0063] Examples 19 in Table 1 were heat-treated under different conditions to obtain Examples 57-59, as shown in Table 3 below:

[0064] Polylactic acid / carbon black composite films annealed at different heat treatment temperatures: Polylactic acid / carbon black blends with a mass ratio of 0.25:1 and a stretch ratio of 3.0 were stretched into films and subjected to heat treatment at 130, 170 and 190°C to obtain polylactic acid / carbon black composite films annealed at different heat treatment temperatures.

[0065] Table 3 Mechanical properties and electrical conductivity of Example 19 after annealing at different heat treatment temperatures

[0066]

[0067] Material characterization experiments

[0068] 1) Mechanical properties:

[0069] Regarding mechanical properties, as shown in Table 1, the method provided by this invention can effectively improve the mechanical properties of polylactic acid (PLA) materials. Comparative Example 8 exhibits typical brittleness of PLA (elongation at break 9.8%). Taking Examples 1-7, 8-14, and 15-21 as examples, the stretched PLA / carbon black films exhibit excellent mechanical properties (up to 74.2% elongation at break and up to 171.6 MPa tensile strength), which is significantly improved compared to unstretched pure PLA films; while for cast films that have only undergone stretching without stretching (Examples 1, 8, and 15), both elongation at break and tensile strength are somewhat reduced. For stretched films, the stretching ratio has a significant impact on the mechanical properties of the film. Taking Examples 1-7 as examples, due to the formation of the orientation structure, the tensile strength gradually increases with the increase of the stretching ratio, while the elongation at break shows a pattern of first increasing and then decreasing. In addition, due to the addition of rigid filler carbon black, the strength improvement is more significant than that of Comparative Examples 1-7.

[0070] As shown in Table 2, for the heat-treated composite films, all stretched films exhibit a critical stretch ratio. When the stretch ratio exceeds this value, the elongation at break of the heat-treated films increases to some extent. This is mainly because the improved initial orientation crystal structure during heat treatment inhibits the pull-out of molecular chains during the tensile test, resulting in better toughness. Conversely, samples with lower stretch ratios experience relaxation during heat treatment due to imperfect initial orientation structures, leading to deteriorated mechanical properties. Furthermore, all stretched samples show some strength loss after annealing. However, the overall strength remains higher than that of cast films, meaning it still provides a reinforcing effect compared to polylactic acid.

[0071] 2) Electrical conductivity:

[0072] Regarding conductivity, as shown in Table 1, at higher filler contents (Comparative Examples 11 and 12), the conductivity of the film improved to some extent after this method was applied. The conductivity of the polylactic acid / carbon black blend cast film reached 1.07 S / m, which is about 9 orders of magnitude higher than that of the polylactic acid cast film. However, the conductivity decreased due to the disruption of the conductive network by the stretching flow field during the stretching process (Examples 15-28). At lower filler contents (Examples 1-14), the conductivity of the polylactic acid / carbon black blend stretched film was similar to that of the polylactic acid stretched film (Comparative Examples 1-7) because a complete conductive path could not be formed, and both were in an insulating state.

[0073] As shown in Table 2, the conductivity of the heat-treated composite films was restored in all samples with high filler content (Examples 43-56 and Comparative Examples 23, 24). This is related to the relaxation of molecular chains during heat treatment, which restored the conductive network. Example 52 had the highest conductivity of 6.05 S / m, which was about 10 orders of magnitude higher than Comparative Example 8, exhibiting excellent conductivity. However, its mechanical properties were affected due to its higher filler content. Example 45 had a similar conductivity to Example 52, but its mechanical properties were significantly better, resulting in better overall performance. In addition, the conductivity of Examples 29-42 and Comparative Examples 20-22 was not significantly affected by heat treatment due to the imperfection of the initial conductive network. Their final conductivity was similar to that of Comparative Example 8, and they remained in an insulating state.

[0074] 3) Heat resistance:

[0075] Regarding heat resistance, such as Figure 1 As shown, the stretched film prepared by this invention exhibits minimal loss of storage modulus during heating and retains certain mechanical properties. Taking Comparative Examples 8 and 11 as examples, the loss of storage modulus is reduced due to the addition of rigid fillers and the increase in crystallinity during heat treatment. In Example 17, the film was heat-treated at 130°C and 150°C respectively. Although the storage modulus of the film decreased slightly compared to the stretched film, the loss was still much smaller than that of the polylactic acid cast film (Comparative Example 8), indicating that the sample has high heat resistance. On the other hand, as... Figure 2 As shown in the temperature rise resistance curves of Examples 17, 45 and Comparative Examples 11, 23, it can also be seen that the sample (Example 45) after "annealing and stretching-heat treatment" can maintain the stability of volume resistivity at a maximum of 150°C, which also indicates that the heat resistance of the composite film has been greatly improved.

[0076] 4) Oriented crystal structure:

[0077] To establish the relationship between crystal structure and the final properties of the thin film, a two-dimensional wide-angle X-ray diffraction characterization method was employed. Figure 3 In the middle, polylactic acid cast film (Comparative Example 8, Figure 3 (a) and cast polylactic acid / carbon black blend film (Comparative Example 11) Figure 3 (b) The diffuse ring signal indicates a completely amorphous phase. The blend was stretched into a cast film (Example 17). Figure 3 (c)) A distinct diffraction arc signal representing the (110 / 200) and (203) crystal planes of polylactic acid α crystals appeared in the meridian direction, proving the formation of stretch-induced oriented crystals, which is also the direct reason for the improvement of mechanical properties and heat resistance.

[0078] For the blended emulsion film subjected to direct heat treatment (Comparative Example 23), Figure 3 (d) shows a bright diffraction ring signal, indicating that the generated crystals are mainly isotropic spherulites, which do not contribute much to the mechanical properties but instead increase the brittleness of polylactic acid. However, after heat treatment of the stretched film of the blend (Examples 57, 45), Figure 3 As can be seen from (e) and (f), the initial diffraction arc becomes brighter during the heat treatment process, and new diffraction arcs also appear, indicating that the crystal structure of polylactic acid is further grown and improved.

[0079] 5) Evaluation of amorphous structure:

[0080] To quantify the effects of stretching and heat treatment on the content of active amorphous phase, rigid amorphous phase, and crystalline phase in polylactic acid / carbon black blend films, modulated differential scanning calorimetry (MSC) was used to characterize the changes in reversible and irreversible heat flux during the film heating process, and the changes in the three-phase content were calculated to evaluate the influence of the amorphous structure on the mechanical properties of the film. Figure 4 As shown, the crystallinity of the blended cast film (Comparative Example 11) is almost 0, compared to... Figure 3 The diffuse diffraction ring signals were consistent, indicating that the sample interior was an amorphous phase. After annealing and stretching the film (Example 17), the content of the active amorphous phase decreased sharply from 95.5% to 51.6%, while the contents of both rigid amorphous phase and crystalline phase increased, with the crystallinity reaching 43.4%. This indicates that the stretching field simultaneously induced the consumption of the active amorphous phase and the formation of oriented rigid amorphous phase and crystalline phase. After heat treatment, the crystallinity of the film did not change significantly, but the content of the rigid amorphous phase increased, reaching a maximum of 18.7%. These oriented rigid amorphous phases also ensured that the film still possessed high toughness after heat treatment. When the heat treatment temperature exceeded the melting point of polylactic acid, the oriented structure melted, and the sample interior reverted to an amorphous phase (active amorphous phase content of 95.6%).

[0081] In summary, the polylactic acid / carbon black composite film prepared according to this invention possesses the characteristics of high conductivity and a balance of rigidity and toughness, demonstrating a significant improvement effect. The obtained polylactic acid-based film, while maintaining high conductivity and a balance of rigidity and toughness, also exhibits excellent heat resistance. This preparation method is stable and simple, requires minimal equipment, is suitable for large-scale industrial production, and holds promise for applications in antistatic packaging materials.

[0082] Although the present invention has been described above in conjunction with exemplary embodiments, it should be clear to those skilled in the art that various modifications and variations can be made to the above embodiments without departing from the spirit and scope of the claims.

Claims

1. A method for preparing a polylactic acid composite film with excellent conductivity and a balance of rigidity and toughness, comprising the following steps: (1) Raw material drying: Polylactic acid is placed in a dehumidifying dryer to dry thoroughly, while carbon black does not need to be dried and can be used directly; (2) Preparation of polylactic acid / carbon black blend granules: polylactic acid and carbon black were melt-blended and extruded at 150-210℃ using a twin-screw extruder and then granulated by a granulator to obtain polylactic acid / carbon black blend granules; (3) Preparation of polylactic acid / carbon black blended film: The blended granules in step (2) are dried again and sent to a single screw extruder. After melting at 170-220°C, they are extruded through a slit die and naturally cooled in air to form a blended film. (4) Preparation of polylactic acid / carbon black blend stretch film: The cast film obtained in step (3) is sent to a stretching device for stretching treatment. The stretching ratio is controlled in the range of 1.0-7.0 by changing the roller speed difference of the stretching device. Annealing stretching is carried out at 60-150℃. Polylactic acid / carbon black blend stretch film is prepared by cooling with annealing roller. (5) Heat treatment of polylactic acid / carbon black composite film: The stretched film obtained in step (4) is heat treated at 110-190℃ and then cooled to prepare the final polylactic acid / carbon black conductive composite film.

2. The preparation method according to claim 1, characterized in that: In step (1), the content of dextro-lactic acid in polylactic acid is 2-12%; the carbon black is conductive carbon black.

3. The preparation method according to claim 1, characterized in that: In step (2), the mass ratio of carbon black to polylactic acid is (0.25-0.4):

1.

4. The preparation method according to claim 1, characterized in that: In step (3), the cooling process of the extrudate from the single screw extruder uses low-temperature cast roller conduction cooling and / or air cooling and / or natural cooling.

5. The preparation method according to claim 1, characterized in that: In step (4), the speed of the stretching roller is 0.5 to 100 m / min.

6. A polylactic acid composite film with excellent conductivity and a balance of rigidity and toughness, prepared by the preparation method according to any one of claims 1-5.

Images