A method for preparing a low-density PLA foamed material
By blending anti-hydrolysis agents and nucleating agents and controlling multiple foaming steps, the problems of gas escape and hydrolysis in PLA foam materials were solved, and PLA foam materials with high foaming ratio and uniform cell structure were prepared.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- EAST CHINA UNIV OF SCI & TECH
- Filing Date
- 2024-01-18
- Publication Date
- 2026-07-24
AI Technical Summary
PLA foam materials are prone to gas escape and hydrolysis during the foaming process, resulting in unsatisfactory cell structure and difficulty in increasing the foaming ratio.
A method is adopted by blending anti-hydrolysis agent and nucleating agent, and foaming through multiple saturated adsorption and depressurization steps, including first, second and third foaming steps, controlling temperature, pressure and time, and utilizing the dissolution and branching reaction of gaseous foaming agent to support cell nucleation and growth.
It significantly improved the foaming ratio of PLA foam material, achieving a high-density and uniform cell structure, meeting the demand for lightweight high-performance materials.
Smart Images
Figure CN117903486B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of polymer foaming, and more particularly to a method for preparing a low-density PLA foam material. Background Technology
[0002] To address the issue of plastic products being difficult to degrade in the natural environment, the use of non-degradable plastics should be restricted, and biodegradable plastics should be actively promoted.
[0003] Biodegradable polymer foam materials are far superior to traditional solid polymer materials due to their lightweight, thermal insulation, electrical insulation, sound insulation, and shock absorption properties. They are widely used in aerospace, medical devices, automotive parts, packaging and sports equipment, microelectronics and other fields, and are one of the most popular lightweight high-performance materials.
[0004] In the chemical foaming process, organic chemical foaming agents can produce toxic and harmful substances such as azo compounds. Physical foaming agents, on the other hand, have advantages such as wide availability, non-toxicity, and non-flammability. They have become a new type of foaming agent to replace traditional chemical foaming agents, especially carbon dioxide (CO2) and nitrogen (N2) as foaming agents, which have broad application prospects.
[0005] Polylactic acid (PLA) is a biodegradable green polymer material produced from renewable resources such as corn stalks and wheat. PLA has good transparency, biocompatibility, and biodegradability. In the natural environment, it is easily decomposed by microorganisms and enzymes in plants and animals, producing water and CO2 without causing any pollution.
[0006] However, PLA has low melt strength and a slow crystallization rate, making it prone to foaming agent gas escape during foaming, leading to bubble coalescence and even rupture, and making it difficult to form a satisfactory cell structure. Especially in autoclaving foaming with water as the dispersant, large amounts of water can cause hydrolysis of PLA, further affecting its foaming behavior. Therefore, improving the foaming ratio of polylactic acid is a key challenge in the preparation of PLA foam materials. Summary of the Invention
[0007] The purpose of this invention is to provide a method for preparing low-density PLA foam material, which produces PLA foam material with a high foaming ratio.
[0008] To achieve the above-mentioned objectives, the present invention provides the following technical solution:
[0009] This invention provides a method for preparing a low-density PLA foam material, comprising the following steps:
[0010] PLA, anti-hydrolysis agent and nucleating agent are mixed to undergo a branching reaction. The resulting mixture is then granulated to obtain the foam to be expanded.
[0011] The anti-hydrolysis agent is one or more of carbodiimide, isocyanate, oxazoline, and epoxy compound;
[0012] The material to be foamed is mixed with water, and then subjected to first saturation adsorption in an atmosphere containing a first gaseous foaming agent, followed by first pressure relief foaming to obtain a primary foamed body; the temperature of the first saturation adsorption is (T). m -70℃~(T m -10℃), the T m The melting temperature of the PLA is 4–20 MPa; the pressure is 4–20 MPa; the time is d1 × (60–180) min, where d1 is the radius of the foam to be expanded in mm.
[0013] The primary foam is subjected to a second saturation adsorption in an atmosphere containing a second gaseous foaming agent, followed by a second pressure relief foaming to obtain a secondary foam; the temperature of the second saturation adsorption is (T). m -130℃~(T m -70℃);
[0014] After mixing the secondary foam with the second water, a third saturation adsorption process is performed, followed by a third depressurization foaming process to obtain the low-density PLA foam material.
[0015] Preferably, the mass of the anti-hydrolysis agent is 0.1% to 3% of the mass of PLA.
[0016] Preferably, the nucleating agent includes one or more of talc, silicon dioxide, magnesium oxide, and mica.
[0017] Preferably, the mass of the nucleating agent is 0.1% to 3% of the mass of PLA.
[0018] Preferably, the first gaseous blowing agent and the second gaseous blowing agent independently comprise one or more of CO2, N2, methanol and butane.
[0019] Preferably, the depressurization rate of the first depressurization foam is 0.1 to 300 MPa / s.
[0020] Preferably, the second saturation adsorption time is d2×(200~1000)min, where d2 is the radius of the primary foam body in mm.
[0021] Preferably, the depressurization rate of the second depressurization foam is 1 to 200 MPa / s.
[0022] Preferably, the temperature of the third saturated adsorption is (T) m -45℃~(T m-5℃), pressure 0.1~10MPa, time d3×(1~30)min, where d3 is the radius of the secondary foam body in mm.
[0023] Preferably, the pressure relief rate of the third pressure relief foam is 0.1 to 200 MPa / s.
[0024] This invention provides a method for preparing PLA foam material, comprising the following steps:
[0025] PLA, anti-hydrolysis agent and nucleating agent are mixed to undergo a branching reaction. The resulting mixture is then granulated to obtain the foam to be expanded.
[0026] The anti-hydrolysis agent is one or more of carbodiimide, isocyanate, oxazoline, and epoxy compound;
[0027] The material to be foamed is mixed with water, and then subjected to first saturation adsorption in an atmosphere containing a first gaseous foaming agent, followed by first pressure relief foaming to obtain a primary foamed body; the temperature of the first saturation adsorption is (T). m -70℃~(T m -10℃), the T m The melting temperature of the PLA is 4–20 MPa; the pressure is 4–20 MPa; the time is d1 × (60–180) min, where d1 is the radius of the foam to be expanded in mm.
[0028] The primary foam is subjected to a second saturation adsorption in an atmosphere containing a second gaseous foaming agent, followed by a second pressure relief foaming to obtain a secondary foam; the temperature of the second saturation adsorption is (T). m -130℃~(T m -70℃);
[0029] After mixing the secondary foam with the second water, a third saturation adsorption process is performed, followed by a third depressurization foaming process to obtain the low-density PLA foam material.
[0030] This invention inhibits PLA hydrolysis by adding a suitable anti-hydrolysis agent, which can also undergo a branching reaction with PLA, giving PLA a long-chain branched structure. This results in higher melt strength in PLA, supporting cell growth and thus increasing the foaming ratio. The addition of a nucleating agent increases the cell nucleation density, further improving the foaming ratio. Adjusting the first saturation adsorption time alleviates the problem of insufficient melt strength caused by PLA hydrolysis. Furthermore, within the scope of this invention, the first saturation adsorption time allows the gaseous blowing agent to fully dissolve in the polymer, ensuring sufficient gaseous blowing agent to support cell nucleation and growth during foaming, thereby increasing the foaming ratio. Controlling the temperature of the first saturation adsorption ensures that the PLA melt strength can support cell growth, further increasing the foaming ratio. Adjusting the pressure of the first saturation adsorption ensures that the adsorbed gas is sufficient to support cell nucleation and growth during depressurization foaming, also increasing the foaming ratio. The use of three-stage depressurization foaming further improves the foaming ratio of the PLA foam material. The results of the examples show that the PLA foam material prepared by this invention has a foaming ratio of 27.8–35.7. Attached Figure Description
[0031] Figure 1 Rheological properties of polylactic acid used in the examples, PLA foam material of Example 1, PLA foam material of Comparative Example 5, PLA foam material of Comparative Example 6, PLA foam material of Comparative Example 7 and PLA foam material of Comparative Example 12.
[0032] Figure 2 The image shows a cross-sectional electron microscope image of the polylactic acid foam product obtained in Example 1.
[0033] Figure 3 The image shows a cross-sectional electron microscope image of the polylactic acid foam product obtained in Comparative Example 1. Detailed Implementation
[0034] This invention provides a method for preparing a low-density PLA foam material, comprising the following steps:
[0035] PLA, anti-hydrolysis agent and nucleating agent are mixed to undergo a branching reaction. The resulting mixture is then granulated to obtain the foam to be expanded.
[0036] The anti-hydrolysis agent is one or more of carbodiimide, isocyanate, oxazoline, and epoxy compound;
[0037] The material to be foamed is mixed with water, and then subjected to first saturation adsorption in an atmosphere containing a first gaseous foaming agent, followed by first pressure relief foaming to obtain a primary foamed body; the temperature of the first saturation adsorption is (T). m -70℃~(T m -10℃), the T mThe melting temperature of the PLA is 4–20 MPa; the pressure is 4–20 MPa; the time is d1 × (60–180) min, where d1 is the radius of the foam to be expanded in mm.
[0038] The primary foam is subjected to a second saturation adsorption in an atmosphere containing a second gaseous foaming agent, followed by a second pressure relief foaming to obtain a secondary foam; the temperature of the second saturation adsorption is (T). m -130℃~(T m -70℃);
[0039] After mixing the secondary foam with the second water, a third saturation adsorption process is performed, followed by a third depressurization foaming process to obtain the low-density PLA foam material.
[0040] The present invention involves blending PLA, an anti-hydrolysis agent, and a nucleating agent, and then granulating the resulting mixture to obtain a foamed body.
[0041] In this invention, the PLA is preferably dried before blending. The drying process is not particularly limited; simply removing moisture from the PLA is sufficient. Specifically, in an embodiment of this invention, the PLA is dried in a vacuum oven at 60°C for 8 hours.
[0042] In this invention, the weight-average molecular weight of the PLA is preferably 180,000 to 200,000.
[0043] In this invention, the mass of the anti-hydrolysis agent is preferably 0.1-3% of the mass of PLA, more preferably 0.5-2%; the anti-hydrolysis agent is one or more selected from carbodiimide, isocyanate, oxazoline and epoxy compound;
[0044] In this invention, the mass of the nucleating agent is preferably 0.1-3% of the mass of PLA, more preferably 0.5-2%; the nucleating agent preferably includes one or more of talc, silicon dioxide, magnesium oxide and mica.
[0045] In this invention, the blending is preferably carried out in a torque rheometer or a twin-screw extruder. When the blending is carried out in a torque rheometer, the torque speed of the torque rheometer is preferably 50 to 100 rpm; when the blending is carried out in a twin-screw extruder, the torque speed of the twin-screw extruder is 100 to 500 rpm.
[0046] After obtaining the foam to be foamed, the present invention mixes the foam to be foamed with a first water, performs a first saturation adsorption in an atmosphere containing a first gaseous foaming agent, and then performs a first depressurization foaming to obtain a primary foam.
[0047] In this invention, after the material to be foamed is mixed with the first water, the material to be foamed and the water are preferably placed in a high-pressure foaming device at the first saturation adsorption temperature before the first saturation adsorption is performed in an atmosphere containing the first gaseous foaming agent. Then, inert gas is introduced into the high-pressure foaming device to purge and replace the air therein. Afterward, the first gaseous foaming agent is added to the high-pressure foaming device through a pressurizing device to the first saturation adsorption pressure.
[0048] In this invention, the first gaseous foaming agent preferably includes one or more of CO2, N2, methanol and butane.
[0049] In this invention, the preferred mass ratio of the foam to the first water is 1:3. Water can be used as a dispersant, heating medium, and co-foaming agent.
[0050] In this invention, the temperature of the first saturated adsorption is (T) m -70℃~(T m -10℃), preferably (T m -60℃~(T m -40℃), the T m The melting temperature of the PLA;
[0051] The pressure of the first saturated adsorption is 4 to 20 MPa, preferably 5 to 10 MPa.
[0052] The first saturation adsorption time is d1×(60~180)min, preferably d1×(100~140)min, where d1 is the radius of the foam body to be expanded, in mm. Excessive saturation adsorption time will cause severe hydrolysis of PLA, while insufficient saturation adsorption time will result in the gaseous blowing agent not completely dissolving in the PLA, which is insufficient to support cell nucleation and growth during the foaming process. The blowing agent inside the foam body in a gas-saturated state is used for cell nucleation and growth.
[0053] In the first saturated adsorption process, water and gaseous foaming agent are adsorbed.
[0054] In this invention, the depressurization rate of the first depressurization foaming process is 0.1–300 MPa / s, preferably 100–250 MPa / s. Depressurization rates within this range are beneficial for cell nucleation.
[0055] After the first depressurization foaming is completed, the present invention preferably places the obtained foamed material in ice water to cool it until the foam cells are fixed.
[0056] After obtaining the primary foam body, the present invention performs a second saturation adsorption on the primary foam body in an atmosphere containing a second gaseous foaming agent, followed by a second depressurization foaming to obtain a secondary foam body.
[0057] In this invention, before the primary foam body undergoes second saturation adsorption in an atmosphere containing a second gaseous foaming agent, the primary foam body is preferably placed in a high-pressure foaming device at a temperature equal to the second saturation adsorption temperature. Then, inert gas is introduced into the high-pressure foaming device to purge and replace the air therein. Subsequently, a second gaseous foaming agent is added to the high-pressure foaming device through a pressurizing device to the pressure of the second saturation adsorption.
[0058] In this invention, the second gaseous foaming agent preferably includes one or more of CO2, N2, methanol and butane.
[0059] In this invention, the temperature of the second saturated adsorption is (T) m -130℃~(T m -70℃), preferably (T m -110℃~(T m -90℃);
[0060] The pressure of the second saturated adsorption is 0.1–10 MPa, preferably 0.5–5 MPa;
[0061] The second saturation adsorption time is d2×(200~1000)min, preferably d2×(600~800)min, where d2 is the radius of the primary foam in mm. Excessive saturation adsorption time wastes time, while insufficient saturation adsorption time results in the physical foaming agent not completely dissolving in the polymer.
[0062] In this invention, the depressurization rate of the second depressurization foaming is preferably 1–200 MPa / s, more preferably 50–150 MPa / s. Depressurization rates within the above range are beneficial for cell nucleation and growth.
[0063] After the second depressurization foaming is completed, the present invention preferably places the obtained foamed material in ice water to cool it until the foam cells are fixed.
[0064] After obtaining the secondary foam, the present invention mixes the secondary foam with a second water, performs a third saturation adsorption, and then performs a third depressurization foam to obtain the low-density PLA foam material.
[0065] In this invention, after the foam to be expanded is mixed with the second water, the foam to be expanded and the water are placed in a high-pressure foaming device at a temperature of the third saturation adsorption temperature, and then an inert gas is introduced into the high-pressure foaming device to purge and replace the air therein.
[0066] In this invention, the preferred mass ratio of the secondary foam to the second water is 7:1. The mass of water carried by the foam cells due to cell shaping in the secondary foam product is negligible.
[0067] In this invention, the preferred temperature for the third saturated adsorption is (T). m -45℃~(T m -5℃), more preferably (T m -35℃~(T m -15℃); the pressure is preferably 0.1~10MPa, more preferably 0.2~4MPa; the time is preferably d3×(1~30)min, more preferably d3×(2~15)min, where d3 is the radius of the secondary foam in mm.
[0068] In this invention, the depressurization rate of the third depressurization foam is preferably 0.1 to 200 MPa / s, more preferably 10 to 150 MPa / s.
[0069] After the third depressurization foaming is completed, the present invention preferably places the obtained foamed material in ice water to cool it until the foam cells are fixed.
[0070] The following detailed description of the preparation method of PLA foam material provided by the present invention, with reference to the embodiments, should not be construed as limiting the scope of protection of the present invention.
[0071] Example 1
[0072] Polylactic acid, 1% carbodiimide (by mass of polylactic acid), and 1% talc (by mass of polylactic acid) were dried in a vacuum oven at 60°C for 8 hours. The mixture was then blended in a torque rheometer at 220°C and 50 rpm for 10 minutes. The resulting blend was then granulated to obtain a foam with a radius of 0.5 mm.
[0073] Three times the mass of the material to be foamed was injected into a high-pressure foaming device, and the material was placed inside. The air in the device was replaced by CO2 purging. CO2 was injected to a pressure of 5 MPa, and the polymer sample was saturated in a CO2 atmosphere at 110°C for 1 hour. Then, the pressure was rapidly released at a rate of 200 MPa / s, and the resulting primary foamed product was quickly placed in ice water to cool until the cells were set.
[0074] After the primary foaming product (1 mm radius) is removed, it is placed in a high-pressure foaming device for 5 minutes. In this device, the air is replaced by purging with N2. N2 is then injected to a pressure of 0.8 MPa. The primary foaming product is saturated in a CO2 atmosphere at 50°C for 10 hours, and then rapidly depressurized at a rate of 100 MPa / s. The resulting secondary foaming product is then quickly placed in ice water to cool until the cell structure is set.
[0075] After the secondary foaming product (radius of 1.4 mm) is removed, it is placed in a high-pressure foaming device for 5 minutes. Water with a mass of 7 times that of the secondary foaming product is injected into the high-pressure foaming device. The secondary foaming product is saturated at a temperature of 125°C for 10 minutes. Then, the pressure is rapidly released at a rate of 100 MPa / s. The resulting tertiary foaming product is then rapidly placed in ice water to cool until the foam cells are set.
[0076] Examples 2-10
[0077] The parameters that differ from those in Example 1 are shown in Table 1; all other parameters are the same as in Example 1.
[0078] Table 1. Parameters that differ from Example 1 in Examples 2-10.
[0079]
[0080]
[0081]
[0082] Comparative Example 1
[0083] Polylactic acid, 1% carbodiimide (by mass of polylactic acid), and 1% talc (by mass of polylactic acid) were dried in a vacuum oven at 60°C for 8 hours. The mixture was then blended in a torque rheometer at 220°C and 50 rpm for 10 minutes. The resulting blend was then granulated to obtain the foam to be expanded.
[0084] Water is injected into a high-pressure foaming device, and the material to be foamed is placed inside. The air in the device is replaced by CO2 purging. CO2 is injected to a pressure of 5 MPa, and the polymer sample is saturated in a CO2 atmosphere at 110°C for 1 hour. Then, the pressure is rapidly released at a rate of 200 MPa / s, and the resulting primary foamed product is quickly placed in ice water to cool until the cells are set.
[0085] Comparative Example 2
[0086] Polylactic acid, 1% carbodiimide (by mass of polylactic acid), and 1% talc (by mass of polylactic acid) were dried in a vacuum oven at 60°C for 8 hours. The mixture was then blended in a torque rheometer at 220°C and 50 rpm for 10 minutes. The resulting blend was then granulated to obtain the foam to be expanded.
[0087] Water is injected into a high-pressure foaming device, and the material to be foamed is placed inside. The air in the device is replaced by CO2 purging. CO2 is injected to a pressure of 5 MPa, and the polymer sample is saturated in a CO2 atmosphere at 110°C for 1 hour. Then, the pressure is rapidly released at a rate of 200 MPa / s, and the resulting primary foamed product is quickly placed in ice water to cool until the cells are set.
[0088] After the primary foaming product is removed, it is placed in a high-pressure foaming device within 5 minutes. In this device, the air is replaced by purging with N2. N2 is then injected to a pressure of 0.8 MPa. The primary foaming product is saturated in a CO2 atmosphere at 50°C for 10 hours, and then rapidly depressurized at a rate of 100 MPa / s. The resulting secondary foaming product is then quickly placed in ice water to cool until the cells are set.
[0089] Comparative Example 3
[0090] Polylactic acid, 1% carbodiimide (by mass of polylactic acid), and 1% talc (by mass of polylactic acid) were dried in a vacuum oven at 60°C for 8 hours. The mixture was then blended in a torque rheometer at 220°C and 50 rpm for 10 minutes. The resulting blend was then granulated to obtain the foam to be expanded.
[0091] Water is injected into a high-pressure foaming device, and the material to be foamed is placed inside. The air in the device is replaced by CO2 purging. CO2 is injected to a pressure of 5 MPa, and the polymer sample is saturated in a CO2 atmosphere at 110°C for 1 hour. Then, the pressure is rapidly released at a rate of 200 MPa / s, and the resulting primary foamed product is quickly placed in ice water to cool until the cells are set.
[0092] After the primary foaming product is removed, it is placed in a high-pressure foaming device for 5 minutes. Water is injected into the high-pressure foaming device, and the primary foaming product is saturated at a temperature of 125°C for 10 minutes. Then, the pressure is rapidly released at a rate of 100 MPa / s, and the resulting secondary foaming product is quickly placed in ice water to cool until the foam cells are set.
[0093] Comparative Example 4
[0094] Polylactic acid, 1% carbodiimide (by mass of polylactic acid), and 1% talc (by mass of polylactic acid) were dried in a vacuum oven at 60°C for 8 hours. The mixture was then blended in a torque rheometer at 220°C and 50 rpm for 10 minutes. The resulting blend was then granulated to obtain the foam to be expanded.
[0095] Water is injected into a high-pressure foaming device, and the material to be foamed is placed inside. The air in the device is replaced by purging with N2. N2 is injected to a pressure of 0.8 MPa. The foamed product is saturated in a CO2 atmosphere at 50°C for 10 hours, and then the pressure is rapidly released at a rate of 100 MPa / s. The resulting foamed product is then quickly placed in ice water to cool until the cells are set.
[0096] After the primary foaming product is removed, it is placed in a high-pressure foaming device for 5 minutes. Water is injected into the high-pressure foaming device, and the primary foaming product is saturated at a temperature of 125°C for 10 minutes. Then, the pressure is rapidly released at a rate of 100 MPa / s, and the resulting secondary foaming product is quickly placed in ice water to cool until the foam cells are set.
[0097] Comparative Examples 5-12
[0098] The parameters that differ from those in Example 1 are shown in Table 2; all other parameters are the same as in Example 1.
[0099] Table 2 shows the parameters that differ from Example 1 in Comparative Examples 5-12.
[0100]
[0101]
[0102] Effect Example
[0103] The rheological properties of polylactic acid used in the examples, PLA foam material of Example 1, PLA foam material of Comparative Example 5, PLA foam material of Comparative Example 6, PLA foam material of Comparative Example 7, and PLA foam material of Comparative Example 12 were obtained by small-amplitude shear oscillation rheological experiments. The results are as follows: Figure 1 As shown.
[0104] Depend on Figure 1 It can be seen that, compared with polylactic acid (PLA), the complex viscosity of PLA in Examples 1 and 7 with added anti-hydrolysis agents increased, while the complex viscosity of PLA in Examples 5 and 6 without added anti-hydrolysis agents decreased significantly. PLA in Example 12 with added anti-hydrolysis agent CUWR-AH01 did not have a long-chain branched structure, and its complex viscosity decreased significantly compared with PLA.
[0105] The weight-average molecular weights of the polylactic acid used in the examples, the PLA foam material of Example 1, the PLA foam material of Comparative Example 5, the PLA foam material of Comparative Example 6, the PLA foam material of Comparative Example 7, and the PLA foam material of Comparative Example 12 were analyzed using the GPC method: the weight-average molecular weight of polylactic acid was 192,760, the weight-average molecular weight of Example 1 was 214,350, the weight-average molecular weight of Comparative Example 5 was 174,650, the weight-average molecular weight of Comparative Example 6 was 174,800, the weight-average molecular weight of Comparative Example 7 was 214,010, and the weight-average molecular weight of Comparative Example 12 was 181,010.
[0106] Electron microscopy analysis was performed on the cross-sections of the polylactic acid foam products obtained in Example 1 and Comparative Example 1. The results are as follows: Figures 2-3 As shown.
[0107] Depend on Figures 2-3 It can be seen that the cell size of Example 1 is larger than that of Comparative Example 1, which indicates that three-stage foaming has a larger cell size than single-stage foaming, and thus has a larger foaming ratio.
[0108] The apparent density of the polymer foamed products obtained in Examples 1-10 and Comparative Examples 1-12 was determined using an analytical balance with an attached density component, and the expansion ratio was calculated. The foamed products were quenched using liquid nitrogen, and gold was sputtered onto the cross-section. The cell structure was then analyzed using a scanning electron microscope. The cell density, expansion ratio, and cell diameter of the foamed products obtained in Examples 1-10 and Comparative Examples 1-12 are shown in Table 3.
[0109] Table 3. Cell density, expansion ratio, and cell diameter of the foamed products obtained in Examples 1-10 and Comparative Examples 1-12
[0110]
[0111] The polymer foams manufactured using the methods of Examples 1-10 not only meet the requirements for high expansion ratios but also enable the easy preparation of various sizes of cells with uniform pores.
[0112] Comparative Example 1 lacked the second and third foaming steps, and the foam cells in the single-foaming sample did not grow further, resulting in a low foaming ratio in the prepared sample.
[0113] Comparative Example 2 lacked a third foaming step, and the foamed samples did not grow further, resulting in a lower foaming ratio.
[0114] Comparative Example 3 lacked a second foaming step, and the foamed samples did not grow further, resulting in a lower foaming ratio.
[0115] Comparative Example 4 lacked a first foaming step, and the content of physical foaming agent in the sample was too low, resulting in insufficient driving force for cell nucleation and cell growth, and the foaming ratio of the prepared sample was too low.
[0116] In the saturation process of the first foaming in Comparative Example 5, the absence of nucleating agent in the sample resulted in low cell nucleation density and the formation of large cells. At the same time, the absence of anti-hydrolysis agent in the sample caused severe PLA hydrolysis and insufficient melt strength, leading to severe cell aggregation and rupture during the foaming process. As a result, the prepared sample had a low foaming ratio.
[0117] In Comparative Example 6, the lack of anti-hydrolysis agent in the sample during the saturation process of foaming caused severe hydrolysis of PLA and insufficient melt strength, resulting in severe cell coalescence and rupture during the foaming process, and the prepared sample had a low foaming ratio.
[0118] In the saturation process of the first foaming in Comparative Example 7, the lack of nucleating agent in the sample will result in low cell nucleation density and large cells. The excessively large cell size is not conducive to cell growth in the second and third foaming processes, and the foaming ratio of the prepared sample is low.
[0119] In Comparative Example 8, during the saturation process of a single foaming, an excessively short saturation time would result in the physical foaming agent not being completely dissolved in the polymer, which would be insufficient to support the nucleation and growth of cells during the foaming process, leading to an excessively low foaming ratio in the sample.
[0120] In Comparative Example 9, the saturation temperature of the first foaming was too high, and the PLA melt strength was too low to support cell growth, resulting in a low foaming ratio for the sample.
[0121] In Comparative Example 10, during the saturation process of a single foaming, the saturation pressure was too low to support the nucleation and growth of cells during the foaming process, resulting in a low foaming ratio of the sample.
[0122] In Comparative Example 11, during the secondary foaming saturation process, the foaming temperature was too high and the stiffness of PLA was too low, which could not support the cell structure. As a result, severe shrinkage occurred, leading to an excessively low foaming ratio in the sample.
[0123] The anti-hydrolysis agent CUWR-AH01 added in Comparative Example 12 does not undergo long-chain branching reaction with PLA. During the saturation process of foaming, the PLA molecular chain will undergo hydrolysis. The low melt strength of PLA is insufficient to support cell growth, resulting in a low foaming ratio of the sample.
[0124] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.
Claims
1. A method for preparing a low-density PLA foam material, characterized in that, Includes the following steps: PLA, anti-hydrolysis agent and nucleating agent are mixed to undergo a branching reaction. The resulting mixture is then granulated to obtain the foam to be expanded. The mass of the anti-hydrolysis agent is 0.1-3% of the mass of PLA; the mass of the nucleating agent is 0.1-3% of the mass of PLA. The anti-hydrolysis agent is one or more of carbodiimide, isocyanate, oxazoline, and epoxy compound; The material to be foamed is mixed with water, and then subjected to first saturation adsorption in an atmosphere containing a first gaseous foaming agent, followed by first pressure relief foaming to obtain a primary foamed body. The temperature of the first saturation adsorption is (Tm-70℃)~(Tm-10℃), where Tm is the melting temperature of the PLA; the pressure is 4~20MPa, and the time is d1×(60~180)min, where d1 is the radius of the material to be foamed in mm. The primary foam body is subjected to a second saturation adsorption in an atmosphere containing a second gaseous foaming agent, followed by a second pressure relief foaming to obtain a secondary foam body; the temperature of the second saturation adsorption is (Tm-130℃)~(Tm-70℃); The second saturation adsorption time is d2×(200~1000)min, where d2 is the radius of the primary foam body in mm; After mixing the secondary foam with the second water, a third saturation adsorption is performed, followed by a third depressurization foaming to obtain the low-density PLA foam material. The temperature for the third saturated adsorption is (Tm-45℃)~(Tm-5℃), the pressure is 0.1~10MPa, and the time is d3×(1~30)min, where d3 is the radius of the secondary foam in mm.
2. The preparation method according to claim 1, characterized in that, The nucleating agent includes one or more of talc, silicon dioxide, magnesium oxide, and mica.
3. The preparation method according to claim 1, characterized in that, The first gaseous blowing agent and the second gaseous blowing agent independently include one or both of CO2 and N2.
4. The preparation method according to claim 1 or 3, characterized in that, The depressurization rate of the first depressurization foam is 0.1~300MPa / s.
5. The preparation method according to claim 1 or 3, characterized in that, The pressure relief rate of the second pressure relief foam is 1~200MPa / s.
6. The preparation method according to claim 1, characterized in that, The depressurization rate of the third depressurization foam is 0.1~200MPa / s.