Polylactic acid reactive blend as well as preparation method and application thereof

Abstract

The invention discloses a polylactic acid reactive blend as well as a preparation method and application thereof. The preparation method comprises the following steps: S1, adding polylactic acid, a flexible degradable polymer and a reactive compound into a double-screw extruder for melt blending, pelletizing and drying to obtain blend particles; the crystallinity of the flexible degradable polymer is less than or equal to 15%; s2, melting and plasticizing the blend particles in injection molding foaming equipment, and injecting supercritical fluid as a foaming agent to obtain a polymer / gas homogeneous melt; s3, the polymer / gas homogeneous melt is injected into a mold cavity, pressure maintaining, primary mold opening, foaming, cooling and secondary mold opening are carried out, the mold temperature of a mold is 90-120 DEG C, the prepared polylactic acid reactive blend has excellent toughness and heat resistance under the condition that a nucleating agent does not need to be added, light weight is further achieved, and the polylactic acid reactive blend is suitable for large-scale production. The material is widely applied to the fields of plastic structural parts, automotive upholstery or medical consumables.

Description

Technical Field

[0001] This invention belongs to the field of polymer material processing technology, specifically relating to a polylactic acid reactive blend, its preparation method, and its application. Background Technology

[0002] With the increasing prominence of petroleum resource shortages and severe environmental pollution, environmentally friendly materials are gaining popularity. Polylactic acid (PLA) is a bio-based and biodegradable polymer material. Due to its advantages such as good biocompatibility, high mechanical strength, and good processability, it is considered one of the most promising bio-based plastics. However, PLA's two main drawbacks—high brittleness and poor heat resistance—severely limit its applications.

[0003] Blending with flexible polymers (such as elastomers) is a simple and economical method to improve the toughness of PLA. However, the compatibility between flexible polymers and PLA is generally poor, resulting in weak interfacial forces between the two phases, which prevents the flexible polymer from effectively toughening PLA. For example, when using styrene-butadiene-styrene terpolymer (SBS) to toughen PLA, the toughness of PLA, especially its impact toughness, is hardly improved due to the weak interfacial interaction between PLA and SBS (European Polymer Journal, 2016, 85:92-104). Therefore, a key point to improving the toughening efficiency of flexible polymers on PLA is to enhance the interfacial interaction forces between the two.

[0004] Reactive compatibilization can effectively improve the compatibility / interfacial forces between PLA and flexible biodegradable polymers, thereby significantly improving the toughness of the blend. For example, patent application CN105713361A discloses an impact-resistant polylactic acid and its preparation method. This method uses an epoxidized thermoplastic elastomer to toughen and modify polylactic acid through a kneading blending grafting method. Polylactic acid and the epoxidized thermoplastic elastomer are placed in a kneading machine. Based on 100 parts by weight of impact-resistant polylactic acid, the polylactic acid mass fraction is 70-95 parts, and the epoxidized thermoplastic elastomer mass fraction is 5-30 parts. The kneading time is 5-20 minutes, the kneading temperature is 170-200℃, and the torque is 40-80 rpm, resulting in impact-resistant polylactic acid. The epoxidized thermoplastic elastomer is selected from epoxidized styrene-butadiene-styrene block copolymers, epoxidized styrene-isoprene-styrene block copolymers, and epoxidized styrene-isoprene / butadiene-styrene block copolymers. The epoxy groups of epoxidized thermoplastic elastomers react with the terminal carboxyl and hydroxyl groups of polylactic acid (PLA) to branch PLA molecules onto the epoxidized thermoplastic elastomer molecular chains, significantly improving the compatibility between the two phases and the toughness of the blend. However, reactive compatibilization reduces the regularity and flowability of the PLA molecular chains, typically significantly decreasing the crystallization rate of the PLA matrix, thereby further reducing the heat resistance of the PLA blend.

[0005] Patent application CN102276965A discloses a method for toughening and modifying polylactic acid (PLA). This method uses natural rubber as a toughening agent to obtain a toughened PLA system. A peroxide initiator is used to initiate cross-linking of the system, enhancing the toughening effect of natural rubber on PLA. In the toughened PLA system, PLA comprises 70-95 parts, natural rubber 5-30 parts, and peroxide initiator 0.2-1 parts. Toughened PLA materials can be obtained through intensive mixing. However, the reactive compatibilization also presents the problem of reducing the PLA's crystallization rate and heat resistance.

[0006] To improve the crystallization rate of PLA matrix, Chinese invention patent application CN118206857A added nucleating agent TMC-300 to PLA / polybutylene succinate (PBS) reactive blends. Although the addition of nucleating agent effectively promotes PLA matrix crystallization, it sacrifices the bio-based and biodegradable properties of PLA to some extent, and also suffers from high cost, while the toughness needs further improvement.

[0007] Therefore, developing a method to simultaneously achieve high toughness, high heat resistance, and lightweight polylactic acid reactive blends without adding nucleating agents has significant application prospects. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the first aspect of this invention provides a method for preparing polylactic acid reactive blends. This method can achieve rapid crystallization without relying on nucleating agents, and the resulting polylactic acid reactive blends possess high toughness, high heat resistance, and lightweight properties.

[0009] The present invention provides a method for preparing a polylactic acid reactive blend, comprising: S1. Add 70-95 parts of polylactic acid, 5-30 parts of flexible biodegradable polymer and 0.3-3 parts of reactive compound to a twin-screw extruder for melt blending, pelletizing and drying to obtain blend particles; The flexible biodegradable polymer is polybutylene terephthalate (PBAT), polycaprolactone (PCL), polybutylene terephthalate (PBST), or polybutylene succinate (PBSA). The crystallinity of the flexible biodegradable polymer is ≤15%; S2. The blended particles are placed in an injection molding foaming equipment for melting and plasticizing, and a supercritical fluid is injected as a foaming agent to obtain a polymer / gas homogeneous melt. S3. The polymer / gas homogeneous melt is injected into a mold, and after pressure holding, first mold opening, foaming, cooling and second mold opening, the polylactic acid reactive blend is obtained. The mold temperature is 90~120 ℃.

[0010] This invention involves melt blending polylactic acid (PLA), a flexible biodegradable polymer, and a reactive compound using a twin-screw extruder. During the melt blending process, the functional groups on the reactive compound react with the terminal carboxyl and hydroxyl groups on PLA and the flexible biodegradable polymer to produce a PLA-flexible biodegradable polymer copolymer. This significantly improves the compatibility between PLA and the flexible biodegradable polymer, as well as the melt strength of the blend. During injection molding foaming, the PLA blend foam achieves a fine cell structure.

[0011] Furthermore, while ensuring good compatibility, the highly crystalline polylactic acid (PLA) matrix can induce shear yielding and dissipate impact energy. The fine micropores are prone to microcracks under stress, further promoting impact energy dissipation. Simultaneously, by controlling the crystallinity of the flexible biodegradable polymer, this invention enables the lower-crystallinity flexible biodegradable polymer phase around the micropores to effectively hinder microcrack propagation and prevent microcracks from developing into large cracks, thereby significantly improving the impact strength after foaming. Therefore, through the synergistic effect of the highly crystalline matrix, small micropores, and the lower-crystallinity flexible biodegradable polymer, the impact toughness of the PLA reactive blend is significantly improved.

[0012] Although appropriately reducing the crystallinity of flexible biodegradable polymers will reduce the crystallization ability of polylactic acid (PLA) to some extent, this invention, by controlling the mold temperature and improving melt flowability, can still effectively promote PLA crystallization without adding nucleating agents, thereby improving the heat resistance of the PLA reactive blend. Simultaneously, it prevents excessively high mold temperatures, which could cause rapid relaxation and disorientation of PLA molecular chains, leading to a decrease in crystallinity.

[0013] Furthermore, during injection molding, the melt of the polylactic acid / flexible biodegradable polymer reactive blend is subjected to a shear flow field as it is injected into the mold cavity. This causes the flexible biodegradable polymer phase to orient itself, transforming from a spherical to a fibrous structure, refining the phase size, increasing the specific surface area of ​​the flexible biodegradable polymer phase, and significantly enhancing its nucleation effect. This invention also utilizes the polylactic acid / flexible biodegradable polymer reactive blend to increase the melt viscosity and prolong the relaxation time of the polylactic acid molecular chains. This allows the polylactic acid molecular chains to maintain their orientation for a longer period at the mold temperature, effectively avoiding the problem of disorientation caused by high mold temperatures. At this time, the oriented polylactic acid molecular chains can rapidly crystallize using the flexible biodegradable polymer phase as nucleation sites, forming a shish-kebab-type crystal structure.

[0014] However, due to the weak shear flow field experienced by the core layer of the injection-molded sample, the crystallinity is low. Therefore, this invention combines the shear flow field generated during the injection molding process with foaming, i.e., it adopts an injection molding foaming process. By utilizing the biaxial stretching effect generated during the foaming process, the crystallinity of the core layer is effectively improved, resulting in injection-molded samples with high crystallinity from the skin layer to the core layer. At the same time, the introduction of cells also achieves the lightweighting of polylactic acid reactive blends.

[0015] In summary, this invention employs a simple injection molding foaming process, utilizing the synergistic effect of a highly crystalline matrix, small pores, and a flexible biodegradable polymer with low crystallinity to significantly improve the impact toughness of the polylactic acid reactive blend. Simultaneously, by moderately increasing the mold temperature and without adding nucleating agents, combined with the effect of the shear flow field, the crystallization of polylactic acid is effectively promoted, thereby improving the heat resistance of the polylactic acid reactive blend. Ultimately, a lightweight, high-toughness, high-heat-resistant, and fully biodegradable polylactic acid reactive blend is obtained.

[0016] Preferably, the polylactic acid has a weight-average molecular weight of 5 × 10⁻⁶. 4 ~5×10 5 g / mol, optical purity ≥95%. The suitable weight-average molecular weight of polylactic acid provided by this invention helps to improve melt strength and mechanical properties, while the optical purity ensures that the polylactic acid matrix achieves rapid crystallization and high crystallinity, thereby improving heat resistance.

[0017] More preferably, the polylactic acid has an optical purity of ≥98.5%.

[0018] Preferably, the weight-average molecular weight of the flexible biodegradable polymer is 2 × 10⁻⁶. 4 ~2×10 5 g / mol.

[0019] Preferably, the reactive compound is a compound containing multiple epoxy groups or a compound containing multiple isocyanate groups.

[0020] More preferably, the compound containing multiple epoxy groups is BASF ADR 4468 or ADR 4370, and the compound containing multiple isocyanate groups is hexamethylene diisocyanate or diphenylmethane diisocyanate. The reactive compounds provided by this invention all possess highly reactive functional groups, reacting with the terminal carboxyl and terminal hydroxyl groups on polylactic acid and flexible biodegradable polymers to produce polylactic acid-flexible biodegradable polymer copolymers, achieving efficient reactive compatibilization and significantly improving the interfacial bonding between the two phases.

[0021] Preferably, the raw materials for melt blending further include antioxidants, wherein the antioxidants are antioxidant 1010 and / or antioxidant 168, and the antioxidants are ≤1 part by weight.

[0022] Preferably, the melt blending temperature is 170~210 °C, and the screw speed is 30~100 rpm. The suitable melt blending parameters provided by the present invention can provide sufficient shear force to promote the dispersion and reaction of the polylactic acid, flexible biodegradable polymer, and reactive compound.

[0023] Preferably, the melting and plasticizing temperature is 180~220 ℃.

[0024] Preferably, the supercritical fluid is supercritical carbon dioxide or supercritical nitrogen, wherein the supercritical carbon dioxide accounts for 2 to 7 wt% of the mass fraction of the polymer / gas homogeneous melt, and the supercritical nitrogen accounts for 0.5 to 1.2 wt% of the mass fraction of the polymer / gas homogeneous melt.

[0025] Preferably, the injection speed is 50-150 mm / s, the holding pressure is 20-60 MPa, the holding time is 10-90 s, and the cooling time is 20-120 s. The suitable injection speed provided by this invention facilitates the transformation of the flexible biodegradable polymer phase from a spherical to a fibrous shape, achieving rapid crystallization.

[0026] On the other hand, the present invention also provides a polylactic acid reactive blend prepared by the method for preparing the polylactic acid reactive blend.

[0027] Preferably, the notched impact strength of the polylactic acid reactive blend is 30~60 kJ / m. 2 .

[0028] On the other hand, the present invention also provides the application of the polylactic acid reactive blend in the fields of plastic structural parts, automotive interior parts or medical consumables.

[0029] The polylactic acid reactive blends prepared by the method of this invention not only have excellent toughness and heat resistance without the addition of nucleating agents, but also achieve lightweighting and are completely biodegradable, and can be widely used in plastic structural parts, automotive interior parts, medical consumables and other fields.

[0030] Compared with the prior art, the present invention has the following beneficial effects: This invention significantly improves the compatibility of two phases by melt blending polylactic acid (PLA), a flexible biodegradable polymer, and a reactive compound. The highly crystalline PLA matrix dissipates impact energy through shear yielding, while the fine micropores easily induce microcracks and promote energy dissipation. Simultaneously, by controlling the crystallinity of the flexible biodegradable polymer, this invention creates low-crystallinity regions around the pores, effectively hindering microcrack propagation. The synergistic effect of these three factors significantly improves the impact toughness of the reactive PLA blend.

[0031] Although reduced crystallinity of the flexible phase may inhibit polylactic acid crystallization, proper control of the mold temperature can improve melt flowability and promote polylactic acid crystallization without the addition of nucleating agents, thereby improving the heat resistance of the material and avoiding the problems of deorientation and decreased crystallinity caused by excessively high mold temperature.

[0032] During injection molding, shearing forces elongate the flexible phase from a spherical shape to a fibrous shape, providing nucleation sites for polylactic acid (PLA) molecular chains and promoting rapid crystallization to form a "chain crystal" structure. To further address the issues of weak shear and low crystallinity in the core layer, an injection molding foaming process is employed. This process utilizes the biaxial stretching effect generated by foaming to effectively promote core layer crystallization, ultimately obtaining a sample with high crystallinity from the skin to the core layer. This results in a lightweight, high-toughness, and highly heat-resistant fully biodegradable PLA blend. Attached Figure Description

[0033] Figure 1 This is a scanning electron microscope image of the cell structure of the polylactic acid reactive blend obtained in Example 1 of the present invention.

[0034] Figure 2 This is a scanning electron microscope image of the cell structure of pure polylactic acid foam prepared in Comparative Example 3 of the present invention.

[0035] Figure 3 This is a scanning electron microscope image of the phase morphology of the polylactic acid reactive blend obtained in Comparative Example 5 of the present invention.

[0036] Figure 4 This is a scanning electron microscope image of the phase morphology of the polylactic acid reactive blend obtained in Example 1 of the present invention. Detailed Implementation

[0037] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0038] It is worth noting that: (1) The scanning electron microscope images of the phase morphology of the materials obtained in the following examples and comparative examples were taken by Hitachi scanning electron microscopes from Japan, and the scanning electron microscope images of the pore structure were taken by ZEISS scanning electron microscopes from Germany.

[0039] (2) The notched impact strength of the materials obtained in the following embodiments and comparative examples was tested according to standard GB / T 1043-1993.

[0040] (3) The heat distortion temperature of the materials obtained in the following examples and comparative examples was obtained according to the standard GB / T 1634–2004.

[0041] (4) The weight loss rate of the materials obtained in the following embodiments and comparative examples is obtained by density conversion. The weight loss rate M = (1 - ρ f / ρ s )×100%, where ρ s and ρ f The densities are those of the unfoamed material and the foamed material, respectively, measured by the water displacement method according to ISO 1183-1987 standard.

[0042] (5) The crystallinity of the materials obtained in the following examples and comparative examples was measured by a differential scanning calorimeter by Mettler of Switzerland, with a heating rate of 10 °C / min.

[0043] Example 1 S1. Take 90 portions of PLA (weight average molecular weight of 2.14 × 10⁻⁶) 5 g / mol, optical purity 99.2%), 10 portions of PBAT (weight average molecular weight 1.24 × 10⁻⁶ g / mol, optical purity 99.2%). 5 The mixture was prepared by adding 1 part by weight of a polyepoxide compound (BASF ADR4468), 0.15 parts by weight of antioxidant 1010 and antioxidant 168 into a twin-screw extruder at 190 °C and melt-blending at a screw speed of 40 rpm. The mixture was then pelletized and dried to obtain the blend particles. S2. The obtained blend particles are placed in an injection molding foaming equipment and melted and plasticized at 200 °C. 1 wt% supercritical nitrogen is injected as a foaming agent to obtain a polymer / gas homogeneous melt. S3. The obtained polymer / gas homogeneous melt is injected into the mold cavity at an injection speed of 90 mm / s, a mold temperature of 100 ℃, a holding pressure of 45 MPa, a holding time of 45 s, and a cooling time of 75 s. After holding pressure, first mold opening, foaming, cooling, and second mold opening, a lightweight, high-toughness, heat-resistant polylactic acid reactive blend is obtained.

[0044] Example 2 S1. Take 85 portions of PLA (weight average molecular weight 2.14 × 10⁻⁶) 5 g / mol, optical purity 99.2%, and 15 portions of PBAT (weight average molecular weight 1.24 × 10⁻⁶ g / mol, optical purity 99.2%). 5 The mixture was prepared by adding 1 part by weight of a polyepoxide compound (BASF ADR4468), 0.15 parts by weight of antioxidant 1010 and antioxidant 168 into a twin-screw extruder at 190 °C and melt-blending at a screw speed of 40 rpm. The mixture was then pelletized and dried to obtain the blend particles. S2. The obtained blend particles are placed in an injection molding foaming equipment and melted and plasticized at 200 °C. 1 wt% supercritical nitrogen is injected as a foaming agent to obtain a polymer / gas homogeneous melt. S3. The obtained polymer / gas homogeneous melt is injected into the mold cavity at an injection speed of 90 mm / s, a mold temperature of 100 ℃, a holding pressure of 45 MPa, a holding time of 45 s, and a cooling time of 75 s. After holding pressure, first mold opening, foaming, cooling, and second mold opening, a lightweight, high-toughness, heat-resistant polylactic acid reactive blend is obtained.

[0045] Example 3 S1. Take 80 portions of PLA (weight average molecular weight of 1.75 × 10⁻⁶) 5 g / mol, optical purity 99.3%), 20 parts PCL (weight average molecular weight 1.33×10⁻⁶ g / mol, optical purity 99.3%) 5 The mixture was prepared by adding 0.8 parts of polyepoxide (BASF ADR4370) and 0.3 parts of antioxidant 1010 to a twin-screw extruder at 200 °C and melt-blending at 60 rpm. The mixture was then pelletized and dried to obtain the blend particles. S2. The obtained blend particles are placed in an injection molding foaming equipment and melted and plasticized at 205 °C. 0.8 wt% supercritical nitrogen is injected as a foaming agent to obtain a polymer / gas homogeneous melt. S3. The obtained polymer / gas homogeneous melt is injected into the mold cavity at an injection speed of 100 mm / s, a mold temperature of 110 ℃, a holding pressure of 50 MPa, a holding time of 60 s, and a cooling time of 60 s. After holding pressure, first mold opening, foaming, cooling, and second mold opening, a lightweight, high-toughness, heat-resistant polylactic acid reactive blend is obtained.

[0046] Example 4 S1. Take 85 portions of PLA (weight average molecular weight of 1.75 × 10⁻⁶) 5 g / mol, optical purity 99.3%), 15 portions of PBST (weight average molecular weight 1.38 × 10⁻⁶ g / mol, optical purity 99.3%). 5 (g / mol, crystallinity 10.8%), 1 part hexamethylene diisocyanate, 0.2 parts antioxidant 1010 and 0.2 parts antioxidant 168 were added to a twin-screw extruder in the following weight ratio and melt-blended at 190 °C with a screw speed of 80 rpm. The resulting blend was then pelletized and dried to obtain the blend particles. S2. The obtained blend particles are placed in an injection molding foaming equipment and melted and plasticized at 200 °C. 1.1 wt% supercritical nitrogen is injected as a foaming agent to obtain a polymer / gas homogeneous melt. S3. The obtained polymer / gas homogeneous melt is injected into the mold cavity at an injection speed of 100 mm / s, a mold temperature of 105 ℃, a holding pressure of 45 MPa, a holding time of 45 s, and a cooling time of 75 s. After holding pressure, first mold opening, foaming, cooling, and second mold opening, a lightweight, high-toughness, heat-resistant polylactic acid reactive blend is obtained.

[0047] Example 5 S1. Take 90 portions of PLA (weight average molecular weight of 1.52 × 10⁻⁶) 5 g / mol, optical purity 98.5%), 10 portions of PBST (weight average molecular weight 1.38 × 10⁻⁶ g / mol, optical purity 98.5%). 5 The mixture of 1 part g / mol (crystallinity 10.8%), 1 part diphenylmethane diisocyanate, 0.15 parts antioxidant 1010 and 0.15 parts antioxidant 168 was added to a twin-screw extruder at 190 °C for melt blending at a screw speed of 40 rpm. The mixture was then pelletized and dried to obtain the blend particles. S2. The obtained blend particles are placed in an injection molding foaming equipment and melted and plasticized at 200 °C. 4 wt% supercritical carbon dioxide is injected as a foaming agent to obtain a polymer / gas homogeneous melt. S3. The obtained polymer / gas homogeneous melt is injected into the mold cavity at an injection speed of 90 mm / s, a mold temperature of 100 ℃, a holding pressure of 45 MPa, a holding time of 30 s, and a cooling time of 90 s. After holding pressure, first mold opening, foaming, cooling, and second mold opening, a lightweight, high-toughness, heat-resistant polylactic acid reactive blend is obtained.

[0048] Example 6 S1. Take 75 portions of PLA (weight average molecular weight 1.52 × 10⁻⁶) 5 g / mol, optical purity 98.5%), 25 portions of PBSA (weight average molecular weight 1.19 × 10⁻⁶ g / mol, optical purity 98.5%). 5 The mixture was prepared by adding 1.5 parts of polyepoxide (BASF ADR4468) and 0.4 parts of antioxidant 1010 to a twin-screw extruder at 185 °C and melt-blending at 50 rpm. The mixture was then pelletized and dried to obtain the blend particles. S2. The obtained blend particles are placed in an injection molding foaming equipment and melted and plasticized at 195 °C. 3 wt% supercritical carbon dioxide is injected as a foaming agent to obtain a polymer / gas homogeneous melt. S3. The obtained polymer / gas homogeneous melt is injected into the mold cavity at an injection speed of 90 mm / s, a mold temperature of 100 ℃, a holding pressure of 45 MPa, a holding time of 45 s, and a cooling time of 75 s. After holding pressure, first mold opening, foaming, cooling, and second mold opening, a lightweight, high-toughness, heat-resistant polylactic acid reactive blend is obtained.

[0049] Example 7 S1. Take 70 portions of PLA (weight average molecular weight of 1.52 × 10⁻⁶) 5 g / mol, optical purity 98.5%), 30 parts PCL (weight average molecular weight 1.33×10⁻⁶ g / mol, optical purity 98.5%) 5 The mixture was prepared by adding 2 parts by weight of a polyepoxide compound (BASF ADR4468), 0.15 parts by weight of antioxidant 1010 and antioxidant 168 into a twin-screw extruder and melt-blending at 200 °C with a screw speed of 50 rpm. The resulting product was then pelletized and dried to obtain the blend particles. S2. The obtained blend particles are placed in an injection molding foaming equipment and melted and plasticized at 210 °C. 4 wt% supercritical carbon dioxide is injected as a foaming agent to obtain a polymer / gas homogeneous melt. S3. The obtained polymer / gas homogeneous melt is injected into the mold cavity at an injection speed of 70 mm / s, a mold temperature of 110 ℃, a holding pressure of 55 MPa, a holding time of 30 s, and a cooling time of 75 s. After holding pressure, first mold opening, foaming, cooling, and second mold opening, a lightweight, high-toughness, heat-resistant polylactic acid reactive blend is obtained.

[0050] Comparative Example 1 (replace PBAT with PBS) The only difference from Example 1 is that the flexible biodegradable polymer used in Comparative Example 1 of the present invention is polybutylene succinate (PBS), with a crystallinity of 41.6%.

[0051] Comparative Example 2 (Mold temperature for injection foaming is 80 ℃) The only difference from Example 1 is that the injection molding temperature used in Comparative Example 2 of the present invention is 80 °C.

[0052] Comparative Example 3 (Pure PLA Injection Molding Foaming) S1. Take 100 parts of PLA (weight average molecular weight of 2.14 × 10⁻⁶) 5 The following ingredients were added to a twin-screw extruder at 190 °C and melt-blended at 40 rpm: g / mol (optical purity 99.2%), 0.15 parts antioxidant 1010 and 0.15 parts antioxidant 168. The mixture was then pelletized and dried to obtain PLA granules. S2. Place the obtained PLA granules in an injection molding foaming equipment and melt and plasticize them at 200 °C. Inject 1 wt% supercritical nitrogen as a foaming agent to obtain a polymer / gas homogeneous melt. S3. The obtained polymer / gas homogeneous melt is injected into the mold cavity at an injection speed of 90 mm / s, a mold temperature of 100 ℃, a holding pressure of 45 MPa, a holding time of 45 s, and a cooling time of 75 s. After holding pressure, first mold opening, foaming, cooling, and second mold opening, PLA foam is obtained.

[0053] Comparative Example 4 (PLA / PBAT injection molding foam, non-reactive blend) S1. Take 90 portions of PLA (weight average molecular weight of 2.14 × 10⁻⁶) 5 g / mol, optical purity 99.2%), 10 portions of PBAT (weight average molecular weight 1.24 × 10⁻⁶ g / mol, optical purity 99.2%). 5 The mixture was prepared by adding 0.15 parts of antioxidant 1010 and 0.15 parts of antioxidant 168 (g / mol, crystallinity 9.5%) to a twin-screw extruder at 190 °C and melt-blending at 40 rpm. The resulting product was then pelletized and dried to obtain the blend particles. S2. The obtained blend particles are placed in an injection molding foaming equipment and melted and plasticized at 200 °C. 1 wt% supercritical nitrogen is injected as a foaming agent to obtain a polymer / gas homogeneous melt. S3. The obtained polymer / gas homogeneous melt is injected into the mold cavity at an injection speed of 90 mm / s, a mold temperature of 100 ℃, a holding pressure of 45 MPa, a holding time of 45 s, and a cooling time of 75 s. After holding pressure, first mold opening, foaming, cooling, and second mold opening, PLA / PBAT blended foam is obtained.

[0054] Comparative Example 5 (using compression molding instead of injection foaming) S1. Take 90 portions of PLA (weight average molecular weight of 2.14 × 10⁻⁶) 5 g / mol, optical purity 99.2%), 10 portions of PBAT (weight average molecular weight 1.24 × 10⁻⁶ g / mol, optical purity 99.2%). 5 The mixture was prepared by adding 1 part by weight of a polyepoxide compound (BASF ADR4468), 0.15 parts by weight of antioxidant 1010 and antioxidant 168 into a twin-screw extruder at 190 °C and melt-blending at a screw speed of 40 rpm. The mixture was then pelletized and dried to obtain the blend particles. S2. The obtained blend particles are placed in a flat vulcanizing machine and hot-pressed at 200 ℃ and 10 MPa for 5 min, and then transferred to 100 ℃ and 10 MPa for 2 min to obtain a molded sample.

[0055] Comparative Example 6 (injection-molded solid spline instead of foamed spline) S1. Take 90 portions of PLA (weight average molecular weight of 2.14 × 10⁻⁶) 5 g / mol, optical purity 99.2%), 10 portions of PBAT (weight average molecular weight 1.24 × 10⁻⁶ g / mol, optical purity 99.2%). 5 The mixture was prepared by adding 1 part by weight of a polyepoxide compound (BASF ADR4468), 0.15 parts by weight of antioxidant 1010 and antioxidant 168 into a twin-screw extruder at 190 °C and melt-blending at a screw speed of 40 rpm. The mixture was then pelletized and dried to obtain the blend particles. S2. The obtained blend particles are placed in an injection molding machine and melted and plasticized at 200 ℃. Then, they are injected into the mold cavity at an injection speed of 90 mm / s, a mold temperature of 100 ℃, a holding pressure of 45 MPa, a holding time of 45 s, and a cooling time of 75 s. After the holding and cooling process, a solid injection molded sample is obtained.

[0056] Comparative Example 7 (Injection-molded solid specimens, with extended mold dwell time to obtain high crystallinity) S1. Take 90 portions of PLA (weight average molecular weight of 2.14 × 10⁻⁶) 5 g / mol, optical purity 99.2%), 10 portions of PBAT (weight average molecular weight 1.24 × 10⁻⁶ g / mol, optical purity 99.2%). 5 The mixture was prepared by adding 1 part by weight of a polyepoxide compound (BASF ADR4468), 0.15 parts by weight of antioxidant 1010 and antioxidant 168 into a twin-screw extruder at 190 °C and melt-blending at a screw speed of 40 rpm. The mixture was then pelletized and dried to obtain the blend particles. S2. The obtained blend particles are placed in an injection molding machine and melted and plasticized at 200 ℃. Then, they are injected into the mold cavity at an injection speed of 90 mm / s, a mold temperature of 100 ℃, a holding pressure of 45 MPa, a holding time of 45 s, and a cooling time of 255 s. After the holding and cooling process, a high-crystallinity injection molded solid sample is obtained.

[0057] Comparative Example 8 (compared to Comparative Example 7, only PBAT was replaced with polybutylene succinate (PBS)) S1. Take 90 portions of PLA (weight average molecular weight of 2.14 × 10⁻⁶) 5 g / mol, optical purity 99.2%, and 10 portions of PBS (weight average molecular weight 1.24 × 10⁻⁶ g / mol, optical purity 99.2%). 5 The mixture was prepared by adding 1 part by weight of a polyepoxide compound (BASF ADR4468), 0.15 parts by weight of antioxidant 1010 and 0.15 parts by weight of antioxidant 168 into a twin-screw extruder at 190 °C and melt-blending at a screw speed of 40 rpm. The mixture was then pelletized and dried to obtain the blend particles. S2. The obtained blend particles are placed in an injection molding machine and melted and plasticized at 200 ℃. Then, they are injected into the mold cavity at an injection speed of 90 mm / s, a mold temperature of 100 ℃, a holding pressure of 45 MPa, a holding time of 45 s, and a cooling time of 255 s. After the holding and cooling process, a high-crystallinity injection molded solid sample is obtained.

[0058] Performance Analysis To investigate the toughness, crystallinity, heat resistance and lightweight properties of the polylactic acid reactive blends prepared by the method of the present invention, notched impact strength, crystallinity, heat distortion temperature and weight loss were tested on the samples obtained in Examples 1-7 and Comparative Examples 1-8. The results are shown in Table 1.

[0059] Table 1 shows the notched impact strength, crystallinity, heat distortion temperature, and weight loss of the samples obtained in Examples 1-7 and Comparative Examples 1-8. As shown in Table 1 and Comparative Examples 1 and 8, if the flexible biodegradable polymer in this invention is replaced with PBS, the introduction of pores actually reduces the notched impact strength of the blend, even when the polylactic acid matrix is ​​highly crystalline. This is because, in the PLA / PBS / ADR blend foam, the highly crystalline PBS dispersed phase (41.6%) is unable to stabilize the microcracks induced by the pores, causing the microcracks to rapidly develop into large cracks, thus resulting in a decrease in impact toughness after foaming.

[0060] As can be seen from Comparative Example 2, when the mold temperature is set to 80 ℃, this temperature is not conducive to PLA crystallization. Even with the presence of shear flow field and biaxial stretching effect generated by foaming, the crystallinity of the injection molded sample is still low, resulting in impact toughness and heat resistance that are far inferior to those of the present invention. The mold temperature needs to be set to a suitable crystallization temperature for PLA.

[0061] Comparative Example 3 shows that pure PLA foam has poor toughness, with a notched impact strength of only 3.1 kJ / m. 2 Furthermore, the slow crystallization rate leads to low crystallinity, resulting in poor heat resistance (heat distortion temperature of only 55.2 ℃). After adding PBAT (Comparative Example 4), the notched impact strength of the PLA / PBAT blend foam only increased to 3.9 kJ / m³. 2 This is attributed to the poor compatibility between PLA and PBAT. Furthermore, the shear flow field generated during injection molding causes the PBAT phase to orient, changing its shape from spherical to fibrous, thus acting as nucleation sites. Simultaneously, the PLA molecular chains also orient, increasing the crystallization rate and crystallinity of PLA, thereby improving the heat resistance of the blended foam to some extent. However, due to the lack of reactive compatibilization, the PBAT phase is relatively large, i.e., its specific surface area is small, resulting in weak nucleation ability. At the same time, the PLA molecular chains undergo rapid disorientation, leading to a limited increase in the PLA crystallization rate and a relatively small improvement in heat resistance (the heat distortion temperature only increases to 60.7 ℃).

[0062] Conversely, the reactively compatibilized PLA / PBAT blend of the present invention (as in Example 1) exhibits increased melt viscosity due to the presence of the PLA-PBAT copolymer, thereby increasing the relaxation time of the PLA molecular chains and enabling them to maintain their orientation for a longer period, which is beneficial for accelerating crystallization. Furthermore, the reactive compatibilization results in smaller PBAT fiber sizes and larger specific surface areas, leading to better nucleation. Therefore, the oriented PLA molecular chains can rapidly crystallize using PBAT fibers as nucleation sites, forming cascades. Moreover, the biaxial stretching effect generated by foaming also promotes crystallization, significantly improving the crystallinity of the polylactic acid reactive blend and consequently greatly improving its heat resistance (heat distortion temperature reaches 92.6 °C). On the other hand, the presence of the PLA-PBAT copolymer increases the melt strength of the blend, effectively improving the cell structure, and significantly reducing the cell size compared to pure PLA foam (see...). Figure 1 and 2 Under the premise of ensuring good compatibility, the highly crystalline polylactic acid matrix is ​​more likely to induce shear yielding and dissipate impact energy. At the same time, small pores are prone to microcracks under stress, which also promote the dissipation of impact energy. Furthermore, the low-crystallinity flexible biodegradable polymer phase around the pores can effectively hinder the propagation of microcracks, making it difficult for microcracks to develop into large cracks. As a result, the notched impact strength after foaming is significantly improved to 31.5 kJ / m. 2 The synergistic effect of a highly crystalline matrix, small pores, and a low-crystallinity flexible biodegradable polymer effectively improves impact toughness. Simultaneously, the presence of pores reduces the weight of the blend by 25.8%, thus achieving lightweighting of polylactic acid reactive blends.

[0063] To illustrate the necessity of the injection molding foaming process in this invention, polylactic acid reactive blends prepared by compression molding were designed as a comparison (e.g., Comparative Example 5). Clearly, due to the lack of a shear flow field in the compression molding process, the PBAT phase cannot be oriented, and only spherical PBAT phases can be obtained (see...). Figure 3 ), and thus cannot form the PBAT fibers with excellent nucleation effect described in this invention (see Figure 4 Furthermore, due to the absence of biaxial tensile effect from foaming, the resulting specimens have low crystallinity, leading to low heat resistance (heat distortion temperature of only 54.8 ℃). Simultaneously, due to the absence of small pores and a highly crystalline matrix that are beneficial to impact toughness, the notched impact strength is only 5.5 kJ / m². 2Furthermore, lightweighting was not achieved. In Comparative Example 6, where solid specimens of polylactic acid reactive blends were prepared using injection molding without foaming, although the crystallinity of the injection-molded specimens was significantly improved due to the shear flow field, the crystallinity of the core layer remained low due to the weaker shear flow field. Therefore, the heat resistance and notched impact strength remained low. To illustrate the contribution of small pores to toughening, the mold dwell time was extended, and solid specimens of polylactic acid reactive blends with high crystallinity in both the skin and core layers were prepared (Comparative Example 7).

[0064] Compared to Comparative Example 6, the notched impact strength of the injection-molded solid specimen prepared in Comparative Example 7 increased to 15.7 kJ / m. 2 This is due to the increased crystallinity of the PLA matrix. However, the notched impact strength of the injection-molded solid sample is significantly lower than that of the polylactic acid reactive blend prepared in this invention (as in Example 1), indicating that the small pores and the highly crystallinized matrix synergistically improve the impact toughness. Moreover, the main components of the polylactic acid reactive blend of this invention are polylactic acid and a flexible biodegradable polymer, both of which are biodegradable materials, conforming to the concept of green environmental protection. The preparation method provided is simple and easy to implement for industrial production.

[0065] It is evident that the polylactic acid reactive blends prepared by the method of this invention not only possess excellent toughness and heat resistance without the addition of nucleating agents, but also achieve lightweighting and are completely biodegradable, making them widely applicable in fields such as plastic structural parts, automotive interior parts, and medical consumables.

[0066] Furthermore, it should be understood that after reading the above description of the present invention, those skilled in the art can make various alterations or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims.

Claims

1. A method for preparing a polylactic acid reactive blend, characterized in that, include: S1. Add 70-95 parts of polylactic acid, 5-30 parts of flexible biodegradable polymer and 0.3-3 parts of reactive compound to a twin-screw extruder for melt blending, pelletizing and drying to obtain blend particles; The flexible biodegradable polymer is polybutylene terephthalate, polycaprolactone, polybutylene terephthalate, or polybutylene succinate. The crystallinity of the flexible biodegradable polymer is ≤15%; S2. The blended particles are placed in an injection molding foaming equipment for melting and plasticizing, and a supercritical fluid is injected as a foaming agent to obtain a polymer / gas homogeneous melt. S3. The polymer / gas homogeneous melt is injected into the mold cavity, and after pressure holding, first mold opening, foaming, cooling and second mold opening, the polylactic acid reactive blend is obtained. The mold temperature is 90~120 ℃.

2. The method for preparing polylactic acid reactive blends according to claim 1, characterized in that, The polylactic acid has a weight-average molecular weight of 5 × 10⁻⁶. 4 ~5×10 5 g / mol, optical purity ≥95%.

3. The method for preparing polylactic acid reactive blends according to claim 1, characterized in that, The reactive compound is a compound containing multiple epoxy groups or a compound containing multiple isocyanate groups.

4. The method for preparing the polylactic acid reactive blend according to claim 1, characterized in that, The melt blending temperature is 170~210 ℃, and the screw speed is 30~100 rpm.

5. The method for preparing the polylactic acid reactive blend according to claim 1, characterized in that, The melting and plasticizing temperature is 180~220℃.

6. The method for preparing the polylactic acid reactive blend according to claim 1, characterized in that, The supercritical fluid is supercritical carbon dioxide or supercritical nitrogen, wherein the supercritical carbon dioxide accounts for 2 to 7 wt% of the mass fraction of the polymer / gas homogeneous melt, and the supercritical nitrogen accounts for 0.5 to 1.2 wt% of the mass fraction of the polymer / gas homogeneous melt.

7. The method for preparing the polylactic acid reactive blend according to claim 1, characterized in that, The injection speed is 50~150 mm / s, the holding pressure is 20~60 MPa, the holding time is 10~90s, and the cooling time is 20~120s.

8. A polylactic acid reactive blend prepared by the method for preparing polylactic acid reactive blends according to any one of claims 1-7.

9. The polylactic acid reactive blend according to claim 8, characterized in that, The notched impact strength of the polylactic acid reactive blend is 30~60 kJ / m. 2 .

10. The application of a polylactic acid reactive blend according to any one of claims 8 or 9 in the fields of plastic structural parts, automotive interior parts or medical consumables.

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