A daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material and its preparation method and application

Abstract

This invention proposes a daytime radiant cooling polylactic acid inorganically doped microfiber material, its preparation method, and its application. This invention belongs to the field of nonwovens technology and addresses the technical problem of poor radiant cooling performance of microfiber materials used as flexible packaging materials. The method comprises the following steps: adding SiO2 to a PEG melt and mixing uniformly to prepare PEG@SiO2 chips; mixing the PEG@SiO2 chips with PLA chips to obtain a blended masterbatch; and meltblowing the masterbatch to produce the daytime radiant cooling polylactic acid inorganically doped microfiber material. The microfiber material prepared in this invention exhibits excellent daytime radiant cooling performance, as well as excellent liquid shielding, printability, and customizable cutting properties. It is expected to provide a new flexible, waterproof, breathable material for radiant cooling in applications such as flexible packaging and outdoor sports.

Description

Technical Field

[0001] The present invention belongs to the technical field of nonwovens, and in particular relates to a daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material and a preparation method and application thereof. Background Art

[0002] Flexible packaging materials serve as the final barrier between supplies and the external environment, maintaining a constant temperature and shielding against external aggressors. They are crucial for ensuring the safety and long-term storage of supplies. Existing flexible packaging materials primarily consist of paper, film, and microfiber. Microfiber, a flexible, porous medium composed of 1-5μm fibers, combines the printability and processability of paper with the barrier and conformability of film, making it one of the most important development areas for flexible packaging. Traditional microfiber materials used in packaging are primarily petroleum-based polymers such as polypropylene (PP), polyethylene (PE), and polyester. While these offer advantages such as low cost and a fast molding process, they are unable to withstand temperature fluctuations during transportation and storage. Especially during the hot summer months, prolonged direct sunlight exposure can easily cause bag bulging, contamination, and shortened shelf life, posing a significant risk to waste paper and material safety. Furthermore, most packaging materials are disposable, and the extensive use of petroleum-based polymers can contribute to environmental problems such as white pollution and the greenhouse effect. Therefore, the development of green and environmentally friendly microfiber materials with passive cooling capabilities has become a common research hotspot in the fields of flexible packaging materials and safety protection textiles.

[0003] Radiative cooling materials are thermal management materials based on infrared radiation regulation. They can achieve long-term temperature regulation through radiative heat dissipation without external energy access, providing a new way for passive cooling and attracting widespread attention from scholars in recent years. For example, Li et al. (LiD, LiuX, LiW, et al. Scalable and hierarchically designed polymer film as a selective thermal emitter for high-performance all-day radiative cooling [J]. Nature Nanotechnology, 2021, 16(2): 153-158) prepared a polyethylene oxide (PEO) film by electrospinning, which achieved an effective cooling effect by virtue of the selective radiation properties of PEO. Song et al. (Song Y-N, Li Y, Yan D-X, et al. Novel passive cooling composite textile for both outdoor and indoor personal thermal management [J]. Composites Part A: Applied Science and Manufacturing, 2020, 130: 105738) prepared a nylon (PA)-polyvinylidene fluoride (PVDF)-nanoPE laminated composite material with infrared selective radiation, which can achieve a cooling effect of up to 6.5 ° C. In summary, existing studies have shown that the chemical bond characteristic peaks of polymers such as PVDF and PEO are in the atmospheric transparency window (8-14 μm), such as COC (1260-1100 cm -1 )、C-OH(1239-1030cm -1 )、CF(1148cm -1 ), which can effectively dissipate heat to the outside environment and reduce the temperature of objects. However, on a clear day, the constant sunlight will cause the heat absorbed by the object to far exceed the heat dissipated by radiation, making it difficult to achieve a cooling effect. Summary of the Invention

[0004] In response to the technical problem that ultrafine fiber materials have poor radiation cooling performance when used as flexible packaging materials, the present invention proposes a daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material and its preparation method and application. The prepared ultrafine fiber material has excellent daytime radiation cooling performance.

[0005] In order to achieve the above object, the technical solution of the present invention is achieved as follows:

[0006] A method for preparing a daytime radiation cooling polylactic acid inorganically doped ultrafine fiber material comprises the following steps:

[0007] (1) Adding SiO2 to the PEG melt and mixing evenly to prepare PEG@SiO2 slices;

[0008] (2) Mixing PEG@SiO2 chips and PLA chips to obtain a blended masterbatch;

[0009] (3) The blended masterbatch was prepared by melt-blowing method to obtain polylactic acid inorganic doped microfiber material for daytime radiation cooling.

[0010] The mass ratio of the PEG, SiO2 and PLA slices is 3:(1-3):(94-97).

[0011] In the step (1), PEG is heated to 60-80° C. to obtain a PEG melt.

[0012] In the step (3), the meltblowing process is as follows: main engine speed 3-10r / s, extrusion zone 1 temperature 170-190°C, extrusion zone 2 temperature 190-210°C, extrusion zone 3 temperature 210-230°C, metering pump temperature 210-230°C, metering pump speed 2-5r / min, die head temperature 210-230°C, hot air temperature 230-250°C, and hot air pressure 10-50KPa.

[0013] The average particle size of the SiO2 is 1-5 μm.

[0014] The melt flow index of the PLA slice at 210° C. is 20.0-25.0 g / 10 min.

[0015] The molecular weight of the polyethylene glycol is 2000-6000.

[0016] Application of daytime radiation cooling polylactic acid inorganic doped microfiber materials in flexible packaging materials.

[0017] Beneficial effects of the present invention:

[0018] (1) The addition of SiO2 can effectively improve the crystallization ability and thermal stability of PLA polymer, and promote the melt-blown molding of PLA. At the same time, SiO2 particles can be more evenly loaded onto the surface of PLA fibers. As the proportion of SiO2 increases, the average fiber diameter of the fiber material gradually increases from 1-4μm to 3-8μm, and the fiber surface roughness increases from 7.832μm to 12.7μm.

[0019] (2) With the increase of SiO2 ratio, the mechanical properties of the microfiber material gradually increase. When the SiO2 ratio is 2.5%, the longitudinal breaking strength of the sample is 25.21N, the transverse breaking strength is 4.48N, and the bursting strength is 4.64N. At the same time, the microfiber material shows a significant daytime radiation cooling effect. When the SiO2 ratio is 2.5%, the emissivity and reflectivity of the material are the highest, which are 87.5% and 98.1% respectively. At this time, the fiber material has the strongest daytime radiation cooling performance, and the temperature difference can reach up to 8.9℃.

[0020] (3) PLA / PEG@SiO2 fiber material has excellent liquid barrier properties, and the liquid barrier performance improves with the increase of SiO2 ratio. When the SiO2 ratio is 2.5%, the contact angle can reach up to 134.4°, and the hydrostatic pressure resistance can reach up to 1064.2Pa. In addition, PLA / PEG@SiO2 fiber material also shows good writability, printability and foldability. It is hoped that this will provide new ideas for the preparation of waterproof and breathable radiation cooling materials and their functional applications in flexible packaging and outdoor sports.

[0021] (4) Meltblown technology is one of the main commercial preparation methods for ultrafine fiber materials. It is a non-woven forming technology that uses high-speed hot air flow to draw polymer melt into fibers in one step. It can not only realize the composite spinning of multiple raw materials, but also has the advantages of easy process adjustment, high production efficiency and non-toxic and pollution-free production. It can be well used for large-scale production of daytime radiation cooling PLA ultrafine fiber materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0023] Figure 1 Thermodynamic properties of PLA / PEG@SiO2 blend polymer samples: (a) secondary heating curve; (b) cooling curve; (c) TG curve; (d) rheological curve.

[0024] Figure 2 Surface morphology of polylactic acid inorganic doped microfiber material samples for daytime radiation cooling: (ae) surface electron microscope images; (f) average fiber diameter and roughness.

[0025] Figure 3Mechanical properties of PLA / PEG@SiO2 fiber materials: (a) longitudinal breaking strength; (b) transverse breaking strength; (c) bursting strength; (d) XRD pattern.

[0026] Figure 4 Schematic diagram of the radiation cooling mechanism of polylactic acid inorganic doped microfiber material for daytime radiation cooling: (a) Schematic diagram of the infrared spectrum curve of polylactic acid inorganic doped microfiber material for daytime radiation cooling; (b) Schematic diagram of the lattice vibration principle of SiO2 crystal.

[0027] Figure 5 Optical properties of PLA / PEG@SiO2 fiber materials with different SiO2 ratios: (a) reflectivity; (b) emissivity.

[0028] Figure 6 Radiative cooling capacity of polylactic acid inorganic doped microfiber material samples for daytime radiative cooling: (a) test device; (b) test temperature-time curve; (c) solar irradiance-time curve.

[0029] Figure 7 Liquid shielding properties of polylactic acid inorganic-doped microfiber material samples for daytime radiation cooling: (a) water contact angle; (b) hydrostatic pressure resistance; (c) droplet adhesion diagram.

[0030] Figure 8 Actual pictures of the writing, printing and folding plasticity tests of PLA / PEG@SiO2 fiber materials: (a) writing; (b) printing; (c) flexible folding pictures. DETAILED DESCRIPTION

[0031] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without creative work are within the scope of protection of the present invention.

[0032] The raw materials used in the following examples are as follows: PLA chips (6252D), melt flow index of 21.3 g / 10 min, density of 1.24 g / cm 3 , Nature Works, USA; polyethylene glycol (PEG 4000), molecular weight 4000, density 1.125 g / cm 3 Haian Petrochemical Plant, Jiangsu Province; SiO2 particles, average particle size 1 μm, density 2.2 g / cm 3 , Jinan Zhiding Co., Ltd.

[0033] Example 1

[0034] PLA slices were placed in a vacuum drying oven (DHG-9076A, Shanghai Jinghong Laboratory Equipment Co., Ltd.) and dried at 80°C for 12 hours to obtain dry PLA slices. PEG (3 wt%) was dissolved in an 80°C beaker and melted to form a PEG melt. SiO2 particles (1.0 wt%) with an average particle size of 1 μm were then added to the PEG melt, and solid PEG@SiO2 slices were formed after cooling to room temperature. The PEG@SiO2 slices and dried PLA slices were placed in a high-speed blender and stirred for 30 minutes to obtain a fully mixed meltblown masterbatch (PLA / PEG / SiO2 = 96.0 / 3.0 / 1.0). The obtained blended masterbatch was then poured into the screw extruder of a meltblown test system (MB-300, Suzhou Duoyu, China), and the screw extruder parameters were set as follows: main engine speed 5 r / s, extrusion zone 1 temperature 180°C, extrusion zone 2 temperature 200°C, extrusion zone 3 temperature 220°C, metering pump temperature 220°C, metering pump speed 3 r / min, die head temperature 220°C, hot air temperature 240°C, and hot air pressure 30 kPa. The blended masterbatch was softened and melted into a melt. Thereafter, the blended melt formed PLA / PEG@SiO2 ultrafine fibers under the drawing action of high-speed hot air, and formed a daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material in a self-adhesive form.

[0035] Example 2

[0036] PLA slices were placed in a vacuum drying oven (DHG-9076A, Shanghai Jinghong Laboratory Equipment Co., Ltd.) and dried at 80°C for 12 hours to obtain dry PLA slices. PEG (3 wt%) was dissolved in an 80°C beaker and melted to form a PEG melt. SiO2 particles (1.5 wt%) with an average particle size of 1 μm were then added to the PEG melt, and solid PEG@SiO2 slices were formed when the melt cooled to room temperature. The PEG@SiO2 slices and the dried PLA slices were placed in a high-speed blender and stirred for 30 minutes to obtain a fully mixed meltblown masterbatch (PLA / PEG / SiO2 = 95.5.0 / 3.0 / 1.5). The obtained blended masterbatch was then poured into the screw extruder of a meltblown test system (MB-300, Suzhou Duoyu, China), and the screw extruder parameters were set as follows: main engine speed 5 r / s, extrusion zone 1 temperature 180°C, extrusion zone 2 temperature 200°C, extrusion zone 3 temperature 220°C, metering pump temperature 220°C, metering pump speed 3 r / min, die head temperature 220°C, hot air temperature 240°C, and hot air pressure 30 kPa. The blended masterbatch was softened and melted into a melt. Thereafter, the blended melt formed PLA / PEG@SiO2 ultrafine fibers under the drawing action of high-speed hot air, and formed a daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material in a self-adhesive form.

[0037] Example 3

[0038] PLA slices were placed in a vacuum drying oven (DHG-9076A, Shanghai Jinghong Laboratory Equipment Co., Ltd.) and dried at 80°C for 12 hours to obtain dry PLA slices. PEG (3 wt%) was dissolved in an 80°C beaker and melted to form a PEG melt. SiO2 particles (2.0 wt%) with an average particle size of 1 μm were then added to the PEG melt, which was cooled to room temperature to form solid PEG@SiO2 slices. The PEG@SiO2 slices and dried PLA slices were placed in a high-speed blender and stirred for 30 minutes to obtain a fully mixed meltblown masterbatch (PLA / PEG / SiO2 = 95.0 / 3.0 / 2.0). The obtained blended masterbatch was then poured into the screw extruder of a meltblown test system (MB-300, Suzhou Duoyu, China), and the screw extruder parameters were set as follows: main engine speed 5 r / s, extrusion zone 1 temperature 180°C, extrusion zone 2 temperature 200°C, extrusion zone 3 temperature 220°C, metering pump temperature 220°C, metering pump speed 3 r / min, die head temperature 220°C, hot air temperature 240°C, and hot air pressure 30 kPa. The blended masterbatch was softened and melted into a melt. Thereafter, the blended melt formed PLA / PEG@SiO2 ultrafine fibers under the drawing action of high-speed hot air, and formed a daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material in a self-adhesive form.

[0039] Example 4

[0040] PLA slices were placed in a vacuum drying oven (DHG-9076A, Shanghai Jinghong Laboratory Equipment Co., Ltd.) and dried at 80°C for 12 hours to obtain dry PLA slices. PEG (3 wt%) was dissolved in an 80°C beaker and melted to form a PEG melt. SiO2 particles (2.5 wt%) with an average particle size of 1 μm were then added to the PEG melt, and the mixture was allowed to cool to room temperature to form solid PEG@SiO2 slices. The PEG@SiO2 slices and dried PLA slices were placed in a high-speed blender and stirred for 30 minutes to obtain a fully mixed meltblown masterbatch (PLA / PEG / SiO2 = 94.5 / 3.0 / 2.5). The obtained blended masterbatch was then poured into the screw extruder of a meltblown test system (MB-300, Suzhou Duoyu, China), and the screw extruder parameters were set as follows: main engine speed 5 r / s, extrusion zone 1 temperature 180°C, extrusion zone 2 temperature 200°C, extrusion zone 3 temperature 220°C, metering pump temperature 220°C, metering pump speed 3 r / min, die head temperature 220°C, hot air temperature 240°C, and hot air pressure 30 kPa. The blended masterbatch was softened and melted into a melt. Thereafter, the blended melt formed PLA / PEG@SiO2 ultrafine fibers under the drawing action of high-speed hot air, and formed a daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material in a self-adhesive form.

[0041] Example 5

[0042] PLA slices were placed in a vacuum drying oven (DHG-9076A, Shanghai Jinghong Laboratory Equipment Co., Ltd.) and dried at 80°C for 12 hours to obtain dry PLA slices. PEG (3 wt%) was dissolved in a 60°C beaker and melted to form a PEG melt. SiO2 particles (3 wt%) with an average particle size of 3 μm were then added to the PEG melt, which was allowed to cool to room temperature to form solid PEG@SiO2 slices. The PEG@SiO2 slices and dried PLA slices were placed in a high-speed blender and stirred for 30 minutes to obtain a fully mixed meltblown masterbatch (PLA / PEG / SiO2 = 94 / 3.0 / 3.0). The obtained blended masterbatch was then poured into the screw extruder of a meltblown test system (MB-300, Suzhou Duoyu, China), and the screw extruder parameters were set as follows: main engine speed 8 r / s, extrusion zone 1 temperature 170°C, extrusion zone 2 temperature 190°C, extrusion zone 3 temperature 210°C, metering pump temperature 210°C, metering pump speed 3 r / min, die head temperature 210°C, hot air temperature 230°C, and hot air pressure 10 KPa. The blended masterbatch was softened and melted into a melt. Thereafter, the blended melt formed PLA / PEG@SiO2 ultrafine fibers under the drawing action of high-speed hot air, and formed a daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material in a self-adhesive form.

[0043] Example 6

[0044] PLA slices were placed in a vacuum drying oven (DHG-9076A, Shanghai Jinghong Laboratory Equipment Co., Ltd.) and dried at 80°C for 12 hours to obtain dry PLA slices. PEG (3 wt%) was dissolved in a 70°C beaker and melted to form a PEG melt. SiO2 particles (1.0 wt%) with an average particle size of 5 μm were then added to the PEG melt, which was cooled to room temperature to form solid PEG@SiO2 slices. The PEG@SiO2 slices and dried PLA slices were placed in a high-speed blender and stirred for 30 minutes to obtain a fully mixed meltblown masterbatch (PLA / PEG / SiO2 = 96.0 / 3.0 / 1.0). The obtained blended masterbatch was then poured into the screw extruder of a meltblown test system (MB-300, Suzhou Duoyu, China), and the screw extruder parameters were set as follows: main engine speed 8 r / s, extrusion zone 1 temperature 190°C, extrusion zone 2 temperature 210°C, extrusion zone 3 temperature 230°C, metering pump temperature 230°C, metering pump speed 3 r / min, die head temperature 230°C, hot air temperature 250°C, and hot air pressure 50 kPa. The blended masterbatch was softened and melted into a melt. Thereafter, the blended melt formed PLA / PEG@SiO2 ultrafine fibers under the drawing action of high-speed hot air, and formed a daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material in a self-adhesive form.

[0045] Comparative Example

[0046] PLA slices were placed in a vacuum drying oven (DHG-9076A, Shanghai Jinghong Laboratory Equipment Co., Ltd.) and dried at 80°C for 12 hours to obtain dry PLA slices. PEG (3 wt%) was dissolved in an 80°C beaker and melted to form a PEG melt. The melted PEG melt and the dried PLA slices were stirred in a high-speed blender for 30 minutes to obtain a fully mixed meltblown masterbatch (PLA / PEG / SiO2 = 97.0 / 3.0). The obtained blended masterbatch was then poured into the screw extruder of a meltblown test system (MB-300, Suzhou Duoyu, China), and the screw extruder parameters were set as follows: main engine speed 5 r / s, extrusion zone 1 temperature 180°C, extrusion zone 2 temperature 2000°C, extrusion zone 3 temperature 220°C, metering pump temperature 220°C, metering pump speed 3 r / min, die head temperature 220°C, hot air temperature 240°C, and hot air pressure 30 kPa. The blended masterbatch was softened and melted into a melt, and then the blended melt was formed into PLA / PEG ultrafine fibers under the drawing action of high-speed hot air, and the PLA / PEG ultrafine fiber material was formed in a self-adhesive form.

[0047] Performance Testing

[0048] Examples 1-4 and comparative examples were tested. The samples of Examples 1-4 were recorded as PLA / PEG / SiO2=96.0 / 3.0 / 1.0, PLA / PEG / SiO2=95.5.0 / 3.0 / 1.5, PLA / PEG / SiO2=95.0 / 3.0 / 2.0 and PLA / PEG / SiO2=94.5 / 3.0 / 2.5, and the comparative example sample was recorded as PLA / PEG / SiO2=97.0 / 3.0.

[0049] Thermal properties of polymer samples were tested using differential scanning calorimetry (DSC) using a differential scanning calorimeter (DSC25, TA, USA). The test conditions were: heating to 300°C twice under nitrogen at a rate of 10°C / min, stabilizing for 3 minutes, and then cooling to room temperature. Thermal stability of polymer samples was tested using thermogravimetric analysis (TG) using a thermogravimetric analyzer (209F1 Iris, NETZSCH, Germany). The test conditions were: heating to 800°C under nitrogen at a rate of 10°C / min. Rheological properties of polymer melts were tested using a rotational rheometer (HR20, TA, USA) using a shear rate of 0.1-400 rad / s and a temperature of 220°C. The fiber morphology of the samples was observed using a scanning electron microscope (SEM, JSM-IT200, JEOL, Japan), and the images were analyzed using SmileViewMap software (V9.3, JEOL, Japan) to determine the fiber diameter and surface roughness. Fourier transform infrared spectra and absorptivity were measured using a Nicolet 6700, Thermo Fisher, USA, equipped with an integrating sphere accessory. The crystal structure of the samples was determined using an X-ray diffractometer (D8, BRUKER, Germany). The longitudinal and transverse tensile strength of the samples was measured using an electronic fabric strength tester (HD026S, Nantong Hengda, China). The bursting strength of the samples was measured using a bursting strength tester (YG026MD, Wenzhou Fangyuan, China). The reflectivity of the samples was measured using a UV-visible spectrophotometer (Color i5, X-rite, USA). The radiative cooling capacity of the samples was tested using a custom-made radiative cooling test device. The device consists of a foam box wrapped in aluminum foil and a multi-channel temperature tester. The foam box has a 10 cm × 10 cm square test hole. The test sample is placed in the test hole and covered with a layer of PE film. The multi-channel temperature tester monitors the microenvironment temperature within the foam box, and the microenvironment temperature is used to indicate the radiative cooling capacity. A solar power meter (TES1333R, Taiwan TES, China) was used to measure the solar irradiation intensity. A contact angle tester (SDC350, Dongguan Shengding, China) was used to measure the liquid water contact angle of the samples. A fully automatic hydrostatic pressure tester (YG826G, Ningbo Textile, China) was used to test the hydrostatic pressure of the samples.

[0050] 1. Thermodynamic properties of polymers

[0051] The secondary heating and cooling curves of PLA / PEG@SiO2 blend melt are as follows: Figure 1 a and Figure 1 As shown in b. First, Figure 1It can be seen from a that the PLA / PEG@SiO2 polymer blend has a clear exothermic peak near 115.3°C, which is also the cold crystallization peak of the PLA / PEG@SiO2 polymer blend. The activity of the PLA macromolecular chain gradually increases with increasing temperature, which causes the PLA macromolecular chain to arrange regularly and rearrange the crystals. Secondly, as the SiO2 ratio increases from 0% to 2.5%, the peak value of the cold crystallization peak of the PLA / PEG@SiO2 polymer blend decreases from 119.2°C to 110.7°C. This is because SiO2 particles play a role of heterogeneous nucleation in the PLA crystallization process, promoting the formation of PLA crystals. In addition, from Figure 1 It can also be seen that a melting range consisting of two endothermic peaks of different sizes appeared near 150.2℃. This is because the PLA crystallization is imperfect, resulting in recrystallization during the second heating process, forming more complete crystals. Figure 1 It can also be seen that as the SiO2 ratio increases from 0% to 2.5%, the main melting peak temperature of the PLA / PEG@SiO2 polymer blend decreases from 151.9℃ to 142.7℃, and the secondary melting peak temperature decreases from 158.6℃ to 150.2℃, that is, SiO2 particles promote the crystallization behavior of PLA. Figure 1 b It can be seen that the PLA / PEG@SiO2 blend polymer has no obvious crystallization peak during the cooling process, which indicates that the crystallization ability of PLA is weak. Figure 1 c is the TG curve of PLA / PEG@SiO2 blend. It can be seen that the thermal decomposition process of PLA is a one-step reaction. The T 10wt％ 、T 50wt％ and T 95wt％ The T values ​​of PLA / PEG@SiO2 blends were 288.3℃, 341.7℃ and 366.4℃ respectively. This shows that PLA / PEG blends have good thermal stability. As the SiO2 ratio increases from 0% to 2.5%, the T values ​​of PLA / PEG@SiO2 blends increase. 10wt％ 、T 50wt％ and T 95wt％ The temperature increases to 314.2℃, 355.7℃ and 388.7℃ respectively. The reason may be that the addition of SiO2 increases the interaction between polymer molecular chains, restricts the movement of PLA macromolecular chains, and effectively improves the thermal stability of PLA macromolecular chains. Figure 1Figure d is the rheological curve of the PLA / PEG@SiO2 blend polymer melt. It can be seen that the melt viscosity of the PLA / PEG@SiO2 blend polymer decreases with increasing shear rate, which is a typical shear-thinning fluid. At the same time, compared with the PLA / PEG blend polymer without SiO2 addition, the PLA / PEG@SiO2 blend polymer melt exhibits a larger complex viscosity. This may be because SiO2 acts as an entanglement point in the blend polymer melt, adsorbing PLA macromolecular chains on the surface, and the entanglement points are difficult to break, making it difficult for the macromolecular chains to disentangle. Excessively high complex viscosity may affect the formation of ultrafine fibers, so the SiO2 addition ratio needs to be properly controlled during sample preparation.

[0052] 2. Morphology and structure

[0053] Figure 2 This is an electron microscope image of the surface structure of a polylactic acid inorganic doped ultrafine fiber material sample for daytime radiation cooling. Figure 2 a-2e (Comparative Example and Examples 1-4) show that the circular PLA / PEG@SiO2 ultrafine fibers are connected to each other to form a disordered three-dimensional network structure, showing a typical melt-blown nonwoven material structure. It can also be clearly seen from the electron microscope image that solid particles are evenly distributed on the surface of the circular fibers. And as the SiO2 ratio increases from 1% to 2.5%, the number of particles on the fiber surface gradually increases. Therefore, it can be inferred that the solid particles are SiO2, and it can also be known that the melt-blown method used in this article can disperse SiO2 particles well, thereby providing a structural basis for enhancing the radiation cooling characteristics. From Figure 2 It can also be seen from a-2e that the fiber diameter of the sample without SiO2 is mainly distributed between 1-4μm. As the proportion of SiO2 increases, the fiber diameter of the daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material sample also increases, and the fiber diameter is mainly distributed between 3-8μm. Figure 2 As can be seen from the figure, the average fiber diameters corresponding to samples with SiO2 mass ratios of 0% and 2.5% are 2.65μm and 7.11μm, respectively. This is mainly because the viscosity of the PLA / PEG@SiO2 melt increases with the increase in SiO2 mass ratio; the low-fluidity polymer melt is difficult to fully stretch, resulting in an increase in fiber diameter. It can also be seen that the surface roughness of the PLA / PEG fiber is 7.832μm. After the introduction of SiO2, the fiber surface roughness increases significantly, and then continues to increase with the increase in SiO2 content, reaching 12.7μm when the SiO2 ratio is 2.5%.

[0054] 3. Mechanical properties

[0055] Figure 3a-3b show the tensile displacement / fracture strength curves for a sample of inorganically doped PLA microfiber material subjected to daytime radiation cooling. As the SiO2 ratio increases, the longitudinal and transverse fracture strengths of the PLA / PEG@SiO2 fiber material gradually increase. The maximum strength is achieved when the SiO2 ratio is 2.5%, with a longitudinal fracture strength of 25.21N and a transverse fracture strength of 4.48N. This phenomenon may be due to the SiO2's enhanced crystallization. SEM analysis also reveals that SiO2 is relatively evenly dispersed within the fiber. At this point, SiO2, acting as an inorganic filler, has a certain reinforcing effect on the fabric structure, thereby enhancing the tensile fracture strength. Figure 3 Figure c shows the bursting strength curve for a sample of inorganically doped PLA microfibers subjected to daytime radiant cooling. Similar to the tensile displacement / fracture strength curve, the bursting strength of the PLA / PEG@SiO2 fiber material gradually increases with increasing SiO2 content, reaching a maximum bursting strength of 4.64 N at a SiO2 content of 2.5%. Figure 3 d is the XRD pattern of the polylactic acid inorganic doped ultrafine fiber material sample for daytime radiation cooling. Figure 3 As shown in Figure a, the XRD curves and peak shapes of all samples are essentially consistent, with a clear diffraction peak at 2θ = 27.1°. Furthermore, the position of the diffraction peak remains unchanged with changes in the SiO2 ratio, indicating that the PLA crystal structure remains unchanged. This indicates that the introduction of SiO2 does not alter the original PLA crystalline structure. Furthermore, the size and sharpness of the diffraction peak increase as the SiO2 ratio increases from 0% to 2.5%. This indicates that the polymer crystals grow larger with increasing SiO2 ratios, indicating that SiO2 promotes the formation of PLA crystals, which in turn enhances the mechanical properties of the fiber material samples.

[0056] 4. Optical properties

[0057] Improving the reflectivity of the material in the solar energy band can effectively reduce the material's absorption of solar heat, thereby achieving daytime radiation cooling. The infrared spectrum of the polylactic acid inorganic doped microfiber material sample for daytime radiation cooling is as follows: Figure 4 As shown in a. It can be seen from the figure that at 757cm -1 、860cm -1 , 1080cm -1 、1180cm -1 There are clear absorption peaks near 757cm -1 、860cm -1 The infrared absorption peaks corresponding to the CH bond and CC bond are 1080cm -1 and 1180cm -1The infrared absorption peaks corresponding to the CO bond are all located in the infrared transparent range, indicating that the daytime radiation cooling polylactic acid inorganic doped microfiber material has high infrared emission characteristics. Figure 4 b is a schematic diagram of the lattice vibration of SiO2 crystals. Under strong sunlight, the symmetry of the SiO2 crystal's internal structure is disrupted, and the crystal's vibration activity is enhanced, resulting in phonon polarization resonance. This promotes the absorption of lattice vibrations and enhances infrared emissivity. Simultaneously, the high refractive index and surface flatness of SiO2 crystals result in high reflectivity, effectively reflecting sunlight. The high reflectivity of SiO2 crystals, lattice vibrations, and the high infrared emissivity of PLA work synergistically to effectively reduce the material's absorption of solar energy and increase heat dissipation. This provides the basis for the daytime radiative cooling capability of inorganically doped polylactic acid microfiber materials.

[0058] Figure 5 Figure a shows the reflectivity of a sample of inorganically doped polylactic acid microfibers used for daytime radiative cooling, along with the corresponding AM1.5 solar spectrum irradiance curve. The AM1.5 curve shows that sunlight energy in the 350-760nm band is high, representing a significant portion of solar energy. Improving the material's reflectivity in this wavelength band can effectively reflect sunlight and reduce its absorption. The reflectivity curves for PLA / PEG@SiO2 fibers with varying SiO2 mass ratios show that the reflectivity of the samples increases with increasing SiO2 content, reaching a maximum of 87.5% at 2.5%, a reflection of SiO2's high reflectivity. However, further increases in SiO2 mass ratio after reaching 2% do not significantly improve the reflectivity. This may be because excessive SiO2 is difficult to fully disperse within the fiber mass, preventing some of the SiO2 from effectively reflecting sunlight, thus limiting the effective reflectivity improvement. Figure 5 b is the emissivity of the polylactic acid inorganic doped microfiber material sample for daytime radiation cooling and the atmospheric transmittance curve of the corresponding wavelength. It can be seen that the atmosphere has a high transmittance to electromagnetic waves in the 8-14μm band. This band is called the atmospheric window. The infrared radiation of objects in the atmospheric window can efficiently pass through the atmosphere to reduce the temperature. Increasing the emissivity of the material in this band can significantly improve the thermal radiation capacity. It can be seen from the emissivity curves of PLA / PEG@SiO2 fiber materials with different SiO2 mass ratios that the emissivity of the sample gradually increases with the addition of SiO2, up to 98.1%. However, as the SiO2 mass ratio increases to 2.5%, the emissivity of the sample does not increase significantly. This trend is consistent with the Figure 6Similar to a, the possible reason is that excessive SiO2 will inhibit the vibration of the SiO2 crystal to a certain extent, thereby destroying the crystal's phonon polarization resonance and affecting the continued improvement of the sample's emission capability. The introduction of SiO2 brings about the increase in the emissivity and reflectivity of the ultrafine fiber material, which provides the basis for its daytime radiative cooling ability.

[0059] 5. Radiative cooling

[0060] Under direct sunlight, a self-made radiation cooling test device ( Figure 6 a) Monitor the microenvironment temperature in the enclosed space covered by the polylactic acid inorganic doped microfiber material sample for daytime radiation cooling. The results of solar irradiance and test temperature are shown in Figure 6 b-6c. From Figure 6 b) The time-temperature curve shows that compared with PLA / PEG fiber without SiO2, the PLA / PEG@SiO2 fiber sample with SiO2 can significantly reduce the temperature in the confined space. Solar irradiance increases from 10:20 to 10:58, reaching its maximum at 10:58. This period corresponds to the temperature increase from 10:20 to 11:19 in the temperature-time curve. This indicates that the PLA / PEG@SiO2 fiber has already achieved a certain cooling effect during this period, with a maximum temperature difference of 3.8°C. After a brief decrease in solar irradiance, it gradually increases again until reaching its maximum at 12:41. The corresponding temperature-time curve (11:08 to 12:41) also gradually rises, and the PLA / PEG@SiO2 fiber maintains its cooling effect, with a maximum temperature difference of 4.3°C. After that, the solar irradiance decreased again until it began to rise again at 13:01, reaching its maximum value at 13:50. The corresponding temperature-time curve (13:41-14:13) gradually rose. It can be seen that the PLA / PEG@SiO2 fiber material has the strongest cooling effect at this time, with a temperature difference of up to 8.9°C. The above test results show that the PLA / PEG@SiO2 fiber material has significant radiative cooling ability, and the cooling ability continues to increase as the SiO2 mass ratio increases from 0% to 2.5%, with a maximum cooling ability of 8.9°C. This also shows that it is effective to enhance the high emissivity of PLA by doping with SiO2 particles, while increasing the reflectivity of the fiber material, thereby improving the radiative cooling ability of the fiber material.

[0061] 6. Liquid barrier properties

[0062] Contact angle and hydrostatic pressure are often used to characterize the liquid barrier properties of packaging materials. Figure 7 a-7b is the curve of the water contact angle and hydrostatic pressure of the polylactic acid inorganic doped ultrafine fiber material sample under daytime radiation cooling as a function of SiO2 mass ratio. Figure 7As can be seen in a, the water contact angle of the sample without SiO2 is 130.8°. As the SiO2 mass ratio increases to 2.5%, the water contact angle increases to 134.4°. The reasons for this phenomenon are: 1) the SiO2 surface has a large number of hydroxyl groups and unsaturated bonds, which increase the surface energy of the fiber and thus improve its hydrophobicity; 2) based on the Cassie and Wenzel model (cosθ * =γ*cosθ,θ * = rough surface contact angle, γ = roughness, θ = smooth surface contact angle), the contact angle of a rough hydrophobic surface will be greater than the contact angle of a smooth hydrophobic surface of the same material, and the SiO2 particles on the fiber surface will increase the roughness of the fiber surface. Figure 7 b) also confirmed the above speculation, which is manifested in that as the SiO2 mass ratio increases from 0% to 2.5%, the hydrostatic pressure value increases from 918.4 Pa to 1064.2 Pa. In addition, the daytime radiation cooling polylactic acid inorganic doped microfiber material sample also has good resistance to common liquids in life such as water, tea, cola and milk. Figure 7 Figure c shows an optical photograph of the corresponding liquids attached to the surface of a 2.5% SiO2-doped polylactic acid microfiber sample for daytime radiant cooling 30 seconds after attachment. It can be clearly seen that each liquid appears as a round droplet on the sample surface, with no penetration or wetting observed, demonstrating that the prepared polylactic acid microfiber sample for daytime radiant cooling has strong shielding properties against liquids such as water.

[0063] In order to further characterize the feasibility of using the daytime radiation cooling polylactic acid inorganic doped microfiber material for flexible packaging, the daytime radiation cooling polylactic acid inorganic doped microfiber material with 2% SiO2 content in Example 3 was used as the substrate to conduct writing and flexible folding tests. The results are shown in Figure 2. Figure 8 a-8c. Figure 8 The characters in a from top to bottom are written by pencil, gel pen and ballpoint pen respectively. It can be seen that these three types of pens can not only write more delicate calligraphy and paintings fluently, but also have a strong sense of coloring and no "ink bleeding" phenomenon. This may be due to the unique ultrafine fiber network structure of the daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material. In addition, the ultrafine fiber network structure also shows good adsorption for toner. Figure 8 As shown in b, office lasers and printers can easily print various clear and visible texts and pictures on the surface of the daytime radiation cooling polylactic acid inorganic doped microfiber material sample. This will be beneficial to the printing of various types of information. In addition, the daytime radiation cooling polylactic acid inorganic doped microfiber material has good flexibility and foldability, such as Figure 8As shown in Figure c, the daytime radiation cooling polylactic acid inorganically doped microfiber material can be easily folded into a complex box-like structure, which is very useful for the bonding, protection, and transportation of various special-shaped parts. In summary, the PLA / PEG@SiO2 fiber material has excellent writability and printability, and can be adapted to various types of writing pens and printers. The relatively soft microfiber structure also gives the material excellent foldability, which has excellent application prospects in various fields such as various writing materials, flexible packaging materials, and outdoor sports.

[0064] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material, characterized in that: The following steps are involved: SiO2 was added to the PEG melt and mixed evenly to prepare PEG@SiO2 slices; Mix PEG@SiO2 chips and PLA chips to obtain a blended masterbatch; The blended masterbatch is made into daytime radiation cooling polylactic acid inorganic doped microfiber material through melt-blowing process; The mass ratio of the PEG, SiO2 and PLA slices is 3:(1-3):(94-97); The average particle size of the SiO2 is 1-5 μm; The molecular weight of the PEG is 2000-6000.

2. The method for preparing the daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material according to claim 1, characterized in that: In the step (1), PEG is heated to 60-80° C. to obtain a PEG melt.

3. The method for preparing the daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material according to claim 1, characterized in that: In the step (3), the meltblowing process is as follows: main engine speed 3-10 r / s, extrusion zone 1 temperature 170-190 ℃, extrusion zone 2 temperature 190-210 ℃, extrusion zone 3 temperature 210-230 ℃, metering pump temperature 210-230 ℃, metering pump speed 2-5r / min, die head temperature 210-230 ℃.

4. The method for preparing the daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material according to claim 3, characterized in that: In the melt-blowing process, the hot air temperature is 230-250° C., and the hot air pressure is 10-50 KPa.

5. The method for preparing the daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material according to claim 4, characterized in that: The PLA chips have a melt flow index of 20.0-25.0 g / 10 min at 210°C.

6. A daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material prepared by the method according to any one of claims 1 to 5.

7. Use of the daytime radiation cooling polylactic acid inorganic doped ultrafine fiber material according to claim 6 in flexible packaging materials.

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