Optical composite film of composite polylactic acid core layer and preparation method of optical composite film

Abstract

The invention relates to the technical field of films, in particular to an optical composite film of a composite polylactic acid core layer and a preparation method of the optical composite film. The optical composite film is of a three-layer structure and comprises a composite polylactic acid core layer made of specific raw materials and amorphous polylactic acid skin layers on the two sides. The core layer is prepared from the following raw materials: poly (L-lactic acid) resin, poly (D-lactic acid) macromolecules, carboxyl-terminated poly (D-lactic acid) oligomers, a multifunctional epoxy chain amplification agent and aminated nano silicon dioxide. During preparation, key processes such as step-by-step addition of the chain extender, the D-lactic acid component and the aminated nano silicon dioxide and three-layer co-extrusion stretching are carried out, so that the molecular structure and the dispersion state of the material are synergistically optimized. The method effectively solves the technical contradiction that a traditional polylactic acid optical film is difficult to consider high light transmittance, low birefringence and low thermal shrinkage at the same time.

Description

Technical Field

[0001] This invention relates to the field of membrane technology, and in particular to an optical composite film with a composite polylactic acid core and its preparation method. Background Technology

[0002] Polylactic acid (PLA), as a bio-based biodegradable polymer, has shown potential to replace petroleum-based materials in the field of optical films, particularly in applications such as polarizer protective films, temporary protective layers before optical adhesive (OCA) lamination, and high-end biodegradable transparent packaging. However, existing PLA transparent films still face a series of technical bottlenecks that urgently need to be addressed when pushing them towards these high-performance applications.

[0003] Firstly, in terms of optical performance, the high birefringence caused by the crystallization characteristics of polylactic acid (PLA) material itself and the subsequent stretching process can cause the film to produce obvious rainbow patterns or interference fringes under polarized light or specific lighting conditions, severely affecting the visual effect and imaging quality of the display module. Simultaneously, the haze level of the film often fails to meet the requirements of high-end optical applications for extremely high transmittance and clarity. The persistently high haze value of conventional PLA films has become a key obstacle limiting their application in the field of high-definition displays.

[0004] Secondly, regarding dimensional stability and thermal properties, polylactic acid (PLA) films are prone to uncontrollable thermal shrinkage when heated during subsequent processing (such as lamination) or in the usage environment due to the release of residual orientation stress. This thermal shrinkage directly leads to film warping, adhesion failure to the substrate, or pattern misalignment, severely affecting product yield and long-term reliability. Although increasing crystallinity can improve heat resistance to some extent, it usually comes with an increase in the crystalline region and enhanced light scattering, which in turn leads to increased haze and decreased light transmittance, resulting in a dilemma where performance cannot be balanced.

[0005] To overcome these shortcomings, existing technologies typically employ single modification methods. For example, adding multifunctional chain extenders can increase the molecular weight and melt strength of polylactic acid (PLA) to reduce internal stress; or direct blending of small amounts of poly(D-lactic acid) can induce the formation of stereocomplex crystals, thereby improving the material's heat resistance. However, these methods often introduce new problems. Adding chain extenders all at once may lead to the rapid consumption of reaction sites in the early stages of processing, leaving insufficient reactive sites for the subsequent introduction of functional components, and the control over melt flow and stress relaxation is too crude. Simply blending poly(D-lactic acid) can easily result in excessively large stereocomplex crystal regions due to uneven dispersion or improper control of crystallization kinetics, becoming new light scattering centers and degrading the film's transparency.

[0006] In addition, introducing nanoparticles (such as silica) as reinforcing or nucleating components is another common approach. However, untreated nanoparticles are prone to agglomeration in polylactic acid (PLA) matrices, which not only fails to effectively reduce birefringence and thermal shrinkage but also significantly increases haze due to severe interfacial light scattering. Even with surface modification of nanoparticles, if their addition timing and location are inappropriate, such as blending with the resin matrix in the early stages of processing, nanoparticles may still migrate and agglomerate during the long melt processing, making it difficult to achieve uniform and stable dispersion.

[0007] In summary, the modification problems of existing polylactic acid optical films are not isolated. Their high haze, high birefringence, and high thermal shrinkage are interconnected. Therefore, developing a new solution that can systematically solve this contradiction has become an urgent need for technological development in this field. Summary of the Invention

[0008] In view of this, the purpose of this invention is to propose an optical composite film with a composite polylactic acid core and its preparation method, so as to solve the technical contradiction that existing polylactic acid optical films are difficult to achieve high light transmittance, low birefringence and low thermal shrinkage in a coordinated manner.

[0009] To achieve the above objectives, the present invention provides an optical composite film with a composite polylactic acid core layer, comprising a composite polylactic acid core layer and a first amorphous polylactic acid skin layer and a second amorphous polylactic acid skin layer respectively disposed on both sides of the composite polylactic acid core layer. The composite polylactic acid core layer is prepared from the following raw materials by mass: 10,000 parts poly(L-lactic acid) resin, 40-70 parts poly(D-lactic acid) polymer, 20-40 parts carboxyl-terminated poly(D-lactic acid) oligomer, 50-70 parts multifunctional epoxy chain expander, and 12-20 parts aminated nano-silica; and the total amount of the poly(D-lactic acid) polymer and the carboxyl-terminated poly(D-lactic acid) oligomer is 80-90 parts.

[0010] Preferably, the thickness of the optical composite film is 38-47 μm.

[0011] Preferably, by mass parts, both the first amorphous polylactic acid skin layer and the second amorphous polylactic acid skin layer are prepared from 3000 parts of amorphous polylactic acid resin, 10 parts of antioxidant 1010 and 10 parts of antioxidant 168.

[0012] Preferably, the composite polylactic acid core layer further includes 20 parts of antioxidant 1010 and 20 parts of antioxidant 168 by weight.

[0013] Preferably, the aminated nano-silica is obtained by surface modification of nano-silica with 3-aminopropyltriethoxysilane.

[0014] Preferably, the carboxyl-terminated poly(D-lactic acid) oligomer is obtained by ring-opening polymerization of D-lactide under stannous octoate catalysis with D-lactic acid as an initiator, followed by devolatilization to remove unreacted monomers.

[0015] Preferably, the number average molecular weight of the carboxyl-terminated poly(D-lactic acid) oligomer is 4000-6000.

[0016] Preferably, the number-average molecular weight of the poly(D-lactic acid) polymer is 80,000-100,000.

[0017] Preferably, the multifunctional epoxy chain amplifying agent is Joncryl ADR-4468.

[0018] Preferably, at least one of the outer surfaces of the first amorphous polylactic acid skin layer and the second amorphous polylactic acid skin layer is corona treated.

[0019] Preferably, the poly(L-lactic acid) resin is of type Ingeo 4032D; and the amorphous polylactic acid resin used is of type Ingeo 4060D.

[0020] Furthermore, the present invention also provides a method for preparing an optical composite film with a composite polylactic acid core layer, comprising the following steps: (1) Poly(L-lactic acid) resin, carboxyl-terminated poly(D-lactic acid) oligomer and a portion of multifunctional epoxy chain expander are melt-extruded and vacuum degassing is performed during the extrusion process; aminated nano-silica is added in the middle section of the extrusion and the remaining portion of multifunctional epoxy chain expander is added; poly(D-lactic acid) polymer is added in the later section of the extrusion and extrusion granulation is performed to obtain composite polylactic acid core layer granules; (2) Amorphous polylactic acid resin is melt-extruded and granulated to obtain amorphous polylactic acid skin granules; (3) The amorphous polylactic acid skin granules and the composite polylactic acid core granules are co-extruded into a symmetrical structure of skin / composite polylactic acid core / skin by three-layer co-extrusion. (4) The co-extruded casting is machine stretched, transverse stretched and heat-set to obtain an optical composite film with a composite polylactic acid core layer.

[0021] Preferably, in step (1), a twin-screw extruder is used for melt extrusion. The temperatures of each zone of the twin-screw extruder are as follows: Zone 1 170±5℃, Zone 2 175±5℃, Zone 3 180±5℃, Zone 4 185±5℃, Zone 5 190±5℃, Zone 6 190±5℃, Zone 7 190±5℃, Zone 8 185±5℃, and the die head 185±5℃. The screw speed is 200 rpm, the main feed rate is 5 kg / h, and a vacuum exhaust of -0.08±0.01 MPa is set in Zone 5.

[0022] Preferably, in step (1), the weight of the multifunctional epoxy chain amplifying agent added in the middle of the extrusion section is 20%-40% of the total weight of the multifunctional epoxy chain amplifying agent.

[0023] Preferably, in step (3), a symmetrical structure of skin / composite polylactic acid core / skin is formed by a three-layer co-extrusion distributor, the volume flow ratio is set to skin:core:skin = 1:8:1, the die head temperature is 190±5℃, the T-die head die gap is 0.7-0.9mm, the casting cooling roller temperature is 20-30℃, and the traction line speed is 5-7m / min.

[0024] Preferably, in step (4), the co-extruded sheet is preheated to 65°C and held for 60 seconds before being stretched in the machine direction. The machine direction stretching temperature is 65-68°C and the stretching ratio is 3.1-3.5 times. Then, it is stretched in the transverse direction. The transverse stretching temperature is 75-78°C and the transverse stretching ratio is 4.2-4.7 times. After stretching, it is heat-set at 108-115°C for 8-10 seconds.

[0025] Preferably, in step (4), after heat setting, the outer surface is subjected to corona treatment with a power of 0.8-1.2kW and a processing speed of 5-7m / min.

[0026] The beneficial effects of this invention are: First, by employing a two-step method—compensating for the addition of a multifunctional epoxy chain extender in the initial stage and a trace addition in the middle stage—and coordinating this with a specific vacuum exhaust zone, precise pre-reservation-compensation control of the polylactic acid melt's long-chain / branching reaction process was achieved. This method not only effectively improved melt strength and promoted the relaxation of orientation stress, but more importantly, it preserved sufficient and active epoxy reaction sites for the covalent bond fixation of the aminated nano-silica introduced in the middle stage. This created conditions for the stable anchoring of the inorganic nanophase at the molecular scale, fundamentally reducing birefringence caused by uneven internal stress and light scattering caused by nanophase aggregation.

[0027] Secondly, by designing a stepwise introduction strategy—introducing carboxyl-terminated poly(D-lactic acid) oligomers first and anchoring them to the epoxy sites, followed by the introduction of poly(D-lactic acid) polymers in subsequent processes—high-density, fine-grained stereocomposite crystal nuclei were ingeniously constructed. This model of first anchoring nucleation and then restricting growth makes the stereocomposite crystallization process more controllable, enabling the formation of fine and uniform crystal structures with extremely low total dosage. This significantly improves the material's heat resistance, effectively suppresses thermal shrinkage, and avoids the adverse effects of crystal coarsening on transparency, achieving a balance between heat resistance and high light transmittance.

[0028] Finally, by selectively introducing aminated nano-silica at specific positions in the chain extension reaction, its primary amine groups can undergo in-situ ring-opening reactions with reserved and added epoxy groups in the system, achieving covalent bonding between the nano-silica and the polylactic acid matrix. This covalent fixation strategy fundamentally changes the traditional dispersed state of nanoparticles as physical fillers, making them confined nodes in the polymer network. Covalently bonded nanoparticles not only effectively transfer and homogenize stress, suppressing the formation of large-size crystalline regions and thus simultaneously reducing birefringence and thermal shrinkage, but also greatly enhance interfacial bonding, avoiding performance degradation caused by interfacial debonding. This allows the film to maintain excellent optical properties while possessing more reliable dimensional stability and mechanical integrity. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0030] Example 1: The poly(L-lactic acid) resin used in this embodiment is Ingeo 4032D manufactured by NatureWorks LLC, USA; the amorphous polylactic acid resin used is Ingeo 4060D manufactured by NatureWorks LLC, USA; the multifunctional epoxy chain extender used is Joncryl ADR-4468 from BASF; and the nano-silica used is AEROSIL 200 hydrophilic fumed silica from Evonik Operations GmbH, Germany, with a nitrogen adsorption specific surface area of ​​215 m². 2 / g; The poly(D-lactic acid) polymer used is Poly(D-lactide) under the Sigma-Aldrich brand of Merck KGaA, Germany, product number 67122, with a number average molecular weight of approximately 90,000 and a weight average molecular weight of approximately 124,000.

[0031] S1: Add 300g anhydrous ethanol, 15g deionized water and 2g glacial acetic acid to a three-necked flask, stir at 300rpm and heat to 25℃, then add 3g 3-aminopropyltriethoxysilane and continue stirring for 20min; then add 15g nano silica and simultaneously perform ultrasonic dispersion for 30min (ultrasonic power 300W), then heat to 80℃ and reflux and stir for 2h, filter, wash the filter cake twice with anhydrous ethanol, and then place the filter cake in a vacuum dryer at 120℃ and -0.09MPa for 12h to obtain aminated nano silica powder; S2: 100g of D-lactide was dried under vacuum at 40℃ and -0.09MPa for 12h to remove trace amounts of moisture; 2g of D-lactic acid was dehydrated under vacuum at 60℃ and -0.09MPa for 2h to reduce free water content; 100g of dried D-lactide and 2g of dehydrated D-lactic acid were added to a three-necked flask, and the mixture was heated to 130℃ under nitrogen protection to melt the monomers and stirred at 200rpm. After the system was homogeneous, 100mg of stannous octoate was added and the temperature was raised to 160℃ and maintained for 3h. Then the temperature was lowered to 80℃ and the mixture was subjected to devolatilization at 120℃ and -0.09MPa for 4h to remove unreacted monomers, yielding carboxyl-terminated poly(D-lactic acid) oligomers (number average molecular weight 5252, determined by gel permeation chromatography). S3: 10,000g of poly(L-lactic acid) resin, 3,000g of amorphous polylactic acid resin, 50g of poly(D-lactic acid) polymer, 30g of carboxyl-terminated poly(D-lactic acid) oligomer, 60g of multifunctional epoxy chain expander, 20g of antioxidant 1010, 20g of antioxidant 168, and 16g of aminated nano-silica were placed in a dehumidifying drying oven for drying. The poly(L-lactic acid) resin was dried at 80℃ for 4 hours, the 3,000g of amorphous polylactic acid resin was dried at 45℃ for 4 hours, and the remaining solid additives were dried at 60℃ for 2 hours. S4: 10,000g of dried poly(L-lactic acid) resin, 20g of antioxidant 1010, 20g of antioxidant 168, 30g of carboxyl-terminated poly(D-lactic acid) oligomer, and 40g of multifunctional epoxy chain expander are premixed for 5 minutes in a closed high-speed mixer and then added as the main feed to the main feed port of a twin-screw extruder. The twin-screw extruder temperatures are set as follows: Zone 1: 170℃, Zone 2: 175℃, Zone 3: 180℃, Zone 4: 185℃, Zone 5: 190℃, Zone 6: 190℃, Zone 7: 190℃. The temperature is set at 185°C in Zone 8 and at the die head, with a screw speed of 200 rpm and a main feed rate of 5 kg / h. A vacuum exhaust of -0.08 MPa is set in Zone 5 to remove trace amounts of volatiles. During continuous and stable extrusion into Zone 6, 16 g of aminated nano-silica is added through the side feed port, and 20 g of multifunctional epoxy chain expander is added through the micro-feed port. In Zone 7, 50 g of poly(D-lactic acid) polymer is added through the side feed port. The extrudate is water-cooled, stretched, and pelletized to obtain composite polylactic acid core layer granules. S5: After drying, 3000g of amorphous polylactic acid resin, 10g of antioxidant 1010 and 10g of antioxidant 168 are mixed and added to the main feed port of a single screw extruder for conventional melt extrusion granulation. The temperature of the single screw extruder is set to 170℃ in zone 1, 180℃ in zone 2, 190℃ in zone 3 and 190℃ at the die head, and the screw speed is 80rpm. The extruder is then stretched into strips, water-cooled and pelletized to obtain amorphous polylactic acid skin granules. S6: Add 10000g of composite polylactic acid (PLA) core layer granules, 1510g of amorphous PLA skin layer granules, and 1510g of amorphous PLA skin layer granules to the hoppers of three extruders respectively. Through a three-layer co-extrusion distributor, a symmetrical structure of skin / composite PLA core layer / skin layer is formed. The volumetric flow rate ratio is set to skin:core:skin = 1:8:1, the die head temperature is 190℃, the T-die clearance is 0.8mm, the casting cooling roller temperature is 25℃, and the traction line speed is 6m / min, resulting in a three-layer co-extruded casting sheet; then, at the dew point... After preheating to 65℃ and holding at that temperature for 60 seconds in a dry air environment at 35℃, the material is stretched in the machine direction at a temperature of 66℃ and a stretching ratio of 3.3 times. The pre-stretched sheet is then placed in the transverse stretching zone at a temperature of 76℃ and a stretching ratio of 4.5 times. After stretching, the sheet is heat-set at 110℃ for 8 seconds and then corona-treated on the outer surface at a power of 1kW and a processing speed of 6m / min. The sheet is then slit and wound up to obtain an optical composite film with a composite polylactic acid core layer and a thickness of 41.5μm.

[0032] Example 2: Based on Example 1, in step S3, the amount of poly(D-lactic acid) polymer was 70g, the amount of carboxyl-terminated poly(D-lactic acid) oligomer was 20g, the amount of aminated nano-silica was 12g, and the total amount of multifunctional epoxy chain amplifying agent was 60g; in step S4, 45g of multifunctional epoxy chain amplifying agent was premixed and 15g of multifunctional epoxy chain amplifying agent was added through a micro-feeding port, and the vacuum exhaust in zone 5 was set to -0.07MPa; in step S6, the machine direction stretching temperature was set to 67℃ and the stretching ratio to 3.4 times, the transverse stretching temperature was set to 77℃ and the transverse stretching ratio to 4.7 times, and the heat setting temperature was set to 112℃ and the time to 8s, resulting in an optical composite film with a composite polylactic acid core layer with a thickness of 38.5μm; the remaining conditions were the same as in Example 1.

[0033] Example 3: Based on Example 1, in step S3, the amount of poly(D-lactic acid) polymer was 40g, the amount of carboxyl-terminated poly(D-lactic acid) oligomer was 40g, the amount of aminated nano-silica was 20g, and the total amount of multifunctional epoxy chain amplifying agent was 70g; in step S4, 50g of multifunctional epoxy chain amplifying agent was premixed and 20g of multifunctional epoxy chain amplifying agent was added through a micro-feeding port; in step S6, the machine direction stretching temperature was set to 65℃ and the stretching ratio to 3.1 times, the transverse stretching temperature was set to 75℃ and the transverse stretching ratio to 4.2 times, and the heat setting temperature was set to 115℃ and the time to 10s, resulting in an optical composite film with a composite polylactic acid core layer thickness of 47.0μm; the remaining conditions were the same as in Example 1.

[0034] Example 4: Based on Example 1, in step S3, the amount of poly(D-lactic acid) polymer was 55g, the amount of carboxyl-terminated poly(D-lactic acid) oligomer was 25g, the amount of aminated nano-silica was 16g, and the total amount of multifunctional epoxy chain amplifying agent was 55g; in step S4, 35g of multifunctional epoxy chain amplifying agent was premixed and 20g of multifunctional epoxy chain amplifying agent was added through a micro-feeding port, and the vacuum exhaust in zone 5 was set to -0.09MPa; in step S6, the machine direction stretching temperature was set to 68℃ and the stretching ratio to 3.5 times, the transverse stretching temperature was set to 78℃ and the transverse stretching ratio to 4.6 times, and the heat setting temperature was set to 108℃ and the time to 8s, resulting in an optical composite film with a composite polylactic acid core layer with a thickness of 38.0μm; the remaining conditions were the same as in Example 1.

[0035] Example 5: Based on Example 1, in step S3, the amount of poly(D-lactic acid) polymer was 60g, the amount of carboxyl-terminated poly(D-lactic acid) oligomer was 30g, the amount of aminated nano-silica was 18g, and the total amount of multifunctional epoxy chain amplifying agent was 65g; in step S4, 45g of multifunctional epoxy chain amplifying agent was premixed and 20g of multifunctional epoxy chain amplifying agent was added through a micro-feeding port; in step S6, the machine direction stretching temperature was set to 66℃ and the stretching ratio to 3.2 times, the transverse stretching temperature was set to 76℃ and the transverse stretching ratio to 4.4 times, and the heat setting temperature was set to 110℃ and the time to 8s, resulting in an optical composite film with a composite polylactic acid core layer thickness of 43.5μm; the remaining conditions were the same as in Example 1.

[0036] Comparative Example 1: The difference from Example 1 is that in step S4, the 40g of multifunctional epoxy chain amplifying agent added to the main feed premix is ​​adjusted to 60g of multifunctional epoxy chain amplifying agent, and the multifunctional epoxy chain amplifying agent is no longer added through the micro-feeding port; the other conditions are the same as in Example 1.

[0037] Comparative Example 2: The difference from Example 1 is that in step S4, 30g of carboxyl-terminated poly(D-lactic acid) oligomer in the main feed is replaced with 30g of poly(L-lactic acid) resin; the other conditions are the same as in Example 1.

[0038] Comparative Example 3: The difference from Example 1 is that in step S4, the 50g of poly(D-lactic acid) polymer added to the 7th zone through the side feed port is replaced with 50g of poly(L-lactic acid) resin; the other conditions are the same as in Example 1.

[0039] Comparative Example 4: The difference from Example 1 is that in step S4, the 16g of aminated nano-silica added in zone 6 through the side feed port is moved to the main feed premixing addition, and aminated nano-silica is no longer added in zone 6 through the side feed port; the other conditions are the same as in Example 1.

[0040] Comparative Example 5: The difference from Example 1 is that in step S4, the 16g of aminated nano-silica added to the 6th zone through the side feed port is replaced with 16g of nano-silica; the other conditions are the same as in Example 1.

[0041] Performance testing: Thickness measurement: The film thickness was measured using a mechanical measurement method. Ten measurement points were taken at equal intervals along the width of each sample film. The thickness of each measurement point was recorded and the average value was calculated. The average thickness was used for subsequent calculations of birefringence, thermal shrinkage rate and tensile properties.

[0042] Transmittance and haze: Transmittance and haze were determined according to Method A (haze meter method) of GB / T 2410-2008. Five 50mm×50mm test pieces were cut from each sample and measured at 23℃ using a standard haze meter. The arithmetic mean of the five test pieces was taken as the transmittance and haze results of the sample.

[0043] Birefringence: Birefringence was determined according to GB / T 28609-2012; three 100mm×100mm specimens were cut from each sample, and the optical path difference was measured using a birefringence meter at a wavelength of 589nm. The birefringence value was calculated in conjunction with the thickness obtained from test item one; nine measuring points were selected for each specimen in a 3×3 grid, and the arithmetic mean of the 27 measuring points was taken as the birefringence result of the sample.

[0044] Heating dimensional change rate: The heating dimensional change rate was determined according to GB / T 12027-2004; for each sample, three 100mm×100mm specimens were cut along the machine direction and three along the transverse direction. The initial gauge length of 100mm was marked on the specimens. The specimens were heated in an 80℃ constant temperature blower for 30min, then removed and cooled in a 23℃ environment for 10min. The gauge length after heating was measured and the dimensional change rate in the machine direction and the transverse direction was calculated. The arithmetic mean of the three specimens was taken as the result of the heating dimensional change rate in the machine direction and the transverse direction of the sample.

[0045] Tensile properties: Tensile strength and elongation at break were determined according to GB / T 1040.1-2025. Five strip specimens with a width of 15 mm and a total length of 150 mm were cut along the machine direction and five along the transverse direction, with a clamping distance of 100 mm. The tensile speed was set to 200 mm / min, and tensile tests were conducted at 23℃. The tensile strength and elongation at break were recorded, and the arithmetic mean of the five specimens was taken. The test results are recorded in Table 1.

[0046] Table 1 Performance Test Results Data Analysis: As can be seen from the data in Table 1, the optical composite film with a polylactic acid core layer prepared by this invention can maintain high transmittance and low haze under different formulation windows and stretching and heat setting windows. Furthermore, the birefringence and heating dimensional change rate are generally on the low order of magnitude, reflecting that the film still has low internal stress and a relatively uniform orientation distribution after orientation. Simultaneously, the tensile strength in the machine direction and transverse direction remains at a high level, coupled with an appropriate elongation at break, indicating that the film does not exhibit significant embrittlement due to improved dimensional stability. The possible reasons are as follows: the multifunctional epoxy chain expander is added at the beginning and supplemented at the middle, which enables the poly(L-lactic acid) backbone to form a more stable long-chain / branched structure and reduce orientation stress during the extrusion reaction; the carboxyl-terminated poly(D-lactic acid) oligomer is added first as a reaction anchor and induces the formation of fine and uniform stereocomposite crystal nuclei, and the poly(D-lactic acid) polymer added later is more likely to grow in a controlled environment, thus taking into account both heat resistance and transparency; in addition, the aminated nano-silica is inserted at the middle site and undergoes a covalent ring-opening reaction with the epoxy site to achieve covalent fixation of the nanophase, so that the nanophase participates in stress transfer and crystal growth regulation in the system as a confined node, synergistically reducing the risk of birefringence interference caused by orientation inhomogeneity and reducing the risk of warping caused by thermal shrinkage.

[0047] As can be seen from the data in Example 1 and Comparative Example 1 in Table 1, when the multifunctional epoxy chain extender is added only once without intermediate replenishment, the haze, birefringence, and heating dimensional change rate of the film all show a deteriorating trend, and the combined performance of tensile strength and elongation at break decreases. The main reason is that the one-time reaction easily leads to the rapid consumption of epoxy reaction sites in the early stage, making it difficult to provide sufficient sites for the covalent fixation of aminated nano-silica when entering the intermediate site insertion stage. The nano-phase is more likely to exist in a physical agglomeration form, introducing scattering and local stress concentration. At the same time, the lack of a window for reaction site compensation also makes it difficult to continuously control the degree of chain lengthening / branching and the stress relaxation process, making it more difficult to simultaneously achieve orientation inhomogeneity and thermal shrinkage. It can be seen that the dual-window control of the chain extender is not a conventional means of simply increasing the molecular weight, but a key condition for achieving low scattering and low internal stress in synergy with nano-phase fixation.

[0048] As can be seen from the data in Table 1 for Example 1 and Comparative Examples 2 and 3, when the introduction of poly(D-lactic acid) no longer follows the two-stage comparative path of adding carboxyl-terminated poly(D-lactic acid) oligomer first and then poly(D-lactic acid) polymer, the film exhibits different degrees of inverse changes in haze, birefringence, and heating dimensional change rate. The lack of carboxyl-terminated oligomer leads to a greater decrease in transparency, while the lack of the latter poly(D-lactic acid) polymer results in a more significant decrease in dimensional stability. This may be because the carboxyl-terminated poly(D-lactic acid) oligomer, acting as a reaction anchor, preferentially reacts with epoxy sites to form high-density, fine crystal nuclei. If poly(L-lactic acid) is used instead, the controlled occurrence of crystal nuclei is weakened, making subsequent crystal growth more prone to coarsening and increasing scattering. Furthermore, the lack of the latter poly(D-lactic acid) polymer limits the effective growth of the stereocomposite structure, making it difficult to provide sufficient confinement and support for chain segment retraction under thermal excitation, thus significantly reducing the thermal shrinkage suppression effect. This indicates that the introduction of two-stage poly(D-lactic acid) is coupled with the allocation of chain extension reaction sites, which can bring about the expected comprehensive performance improvement that is difficult to replace with a single component.

[0049] As can be seen from the data in Table 1 for Example 1 and Comparative Examples 4 and 5, when the aminated nano-silica is no longer inserted at the intermediate site but is moved forward to the premixing of the main feed, or when the aminated nano-silica is replaced with unmodified nano-silica, the haze of the film increases, the birefringence increases, and the rate of change of size upon heating increases, while the elongation at break decreases more significantly. The main reason is that the process arrangement of inserting at the intermediate site allows the nano-silica to enter the system under a more suitable viscosity and shear environment and undergo a covalent ring-opening reaction with the reserved / added epoxy sites, thereby achieving covalent fixation of the nano-phase and avoiding agglomeration. If the premixing is moved forward, the nano-phase is more likely to form agglomeration nuclei during the longer melting process, interfering with subsequent orientation and crystal growth, resulting in enhanced scattering and uneven stress field. If the aminated modification is lacking, the covalent fixation path is cut off, and the nano-silica tends to exist as a physical filler and form stress concentration points, making it difficult to simultaneously achieve transparency, orientation uniformity, and elongation toughness. It is evident that there is a significant synergistic effect between the amination, intermediate site insertion, and epoxy covalent fixation of nano-silica and the control of chain extension reaction sites and stereocomposite crystal nuclei, which can achieve a comprehensive performance effect greater than 2.

[0050] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. An optical composite film with a composite polylactic acid core layer, characterized in that, It includes a composite polylactic acid core layer and a first amorphous polylactic acid skin layer and a second amorphous polylactic acid skin layer respectively disposed on both sides of the composite polylactic acid core layer; The composite polylactic acid core layer is prepared from the following raw materials by mass: 10,000 parts poly(L-lactic acid) resin, 40-70 parts poly(D-lactic acid) polymer, 20-40 parts carboxyl-terminated poly(D-lactic acid) oligomer, 50-70 parts multifunctional epoxy chain expander, and 12-20 parts aminated nano-silica; and the total amount of the poly(D-lactic acid) polymer and the carboxyl-terminated poly(D-lactic acid) oligomer is 80-90 parts.

2. The optical composite film with a composite polylactic acid core layer according to claim 1, characterized in that, The thickness of the optical composite film is 38-47 μm.

3. The optical composite film with a composite polylactic acid core layer according to claim 1, characterized in that, By mass fraction, both the first amorphous polylactic acid (PLA) skin layer and the second amorphous PLA skin layer are prepared from 3000 parts of amorphous PLA resin, 10 parts of antioxidant 1010, and 10 parts of antioxidant 168.

4. The optical composite film with a composite polylactic acid core layer according to claim 1, characterized in that, The composite polylactic acid core layer also includes 20 parts of antioxidant 1010 and 20 parts of antioxidant 168 by weight.

5. The optical composite film with a composite polylactic acid core layer according to claim 1, characterized in that, The aminated nano-silica is obtained by surface modification of nano-silica with 3-aminopropyltriethoxysilane.

6. The optical composite film with a composite polylactic acid core layer according to claim 1, characterized in that, The carboxyl-terminated poly(D-lactic acid) oligomer was obtained by ring-opening polymerization of D-lactide under stannous octoate catalysis with D-lactic acid as an initiator, followed by devolatilization to remove unreacted monomers.

7. The optical composite film with a composite polylactic acid core layer according to claim 1, characterized in that, The number average molecular weight of the carboxyl-terminated poly(D-lactic acid) oligomer is 4,000-6,000; the number average molecular weight of the poly(D-lactic acid) polymer is 80,000-100,000.

8. The optical composite film with a composite polylactic acid core layer according to claim 1, characterized in that, The multifunctional epoxy chain amplifying agent is Joncryl ADR-4468; the poly(L-lactic acid) resin is Ingeo 4032D; and the amorphous polylactic acid resin used is Ingeo 4060D.

9. A method for preparing an optical composite film with a composite polylactic acid core layer as described in any one of claims 1-8, characterized in that, Includes the following steps: (1) Poly(L-lactic acid) resin, carboxyl-terminated poly(D-lactic acid) oligomer and a portion of multifunctional epoxy chain expander are melt-extruded and vacuum degassing is performed during the extrusion process; aminated nano-silica is added in the middle section of the extrusion and the remaining portion of multifunctional epoxy chain expander is added; poly(D-lactic acid) polymer is added in the later section of the extrusion and extrusion granulation is performed to obtain composite polylactic acid core layer granules; (2) Amorphous polylactic acid resin is melt-extruded and granulated to obtain amorphous polylactic acid skin granules; (3) The amorphous polylactic acid skin granules and the composite polylactic acid core granules are co-extruded into a symmetrical structure of skin / composite polylactic acid core / skin by three-layer co-extrusion. (4) The co-extruded casting is machine stretched, transverse stretched and heat-set to obtain an optical composite film with a composite polylactic acid core layer.

10. The method for preparing the optical composite film with a composite polylactic acid core layer according to claim 9, characterized in that, In step (1), the weight of the multifunctional epoxy chain amplifying agent added in the middle of the extrusion section is 20%-40% of the total weight of the multifunctional epoxy chain amplifying agent.

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