Green Radiative Cooling and Antimicrobial Materials Based on Polylactic Acid Oligomer Hexagonal Microspheres

1. Introduction

Excessive energy consumption and carbon emission lead to global climate warming [1,2], thus increasing diseases associated with heat stress and impeding the sustainable development of human society [3,4]. Passive radiative cooling methodology, which utilizes the enhanced reflection in the UV-Vis-NIR wave range (0.3-2.5 µm) [5,6,7]and emission in the MIR range (atmospheric window, 8-13 µm) [8,9,10], has become a promising direction with nearly zero energy cost and carbon emission to replace traditional energy-thirsty air-conditioning. The radiative cooling technology can be used in solar cells and fluid-cooling systems [11,12,13], it can also be applied in the textile industry for thermal management [7,9,14,15,16,17]. However, most reported radiative cooling materials were made from non-degradable polymers or nanoparticles containing heavy metals [7,16,17], which have potential hazard to the environment. We previously reported the successful developments of the manufacturing processes of degradable PLA/PHBV textiles [18,19], and the first discovery of their antimicrobial properties and the active ingredient being Poly(3-hydroxybutyrate) (PHB) oligomer [20,21]. In this work, we further investigated one of the homologues, polylactic acid oligomer (PLAO), illustrates that its radiative cooling effect when crystalized into microsphere with nanotexture, while maintaining effective antimicrobial properties similar to PHB oligomers. PLAOs with various degrees of polymerization (DP) and states are synthesized as solid and liquid PLAO coating materials for both radiative cooling and antimicrobial applications. This work offers a green and one-stop solution to prepare cooling and antimicrobial coating layer, thus providing a simple and effective technology for healthcare in multiaspect.

2. Materials and Methods

2.1. Synthesis of Polylactic Acid Oligomer (PLAO).

L-Lactic acid (≥85%, Dieckmann, Hong Kong) was added to the reactor, under nitrogen protection, stirred and heated up until 100 ~ 220 ℃, then maintained for 1 ~ 24 hours (< 4 h for liquid PLAO and > 6 h for solid PLAO). The pressure in the reactor is maintained at 1000Pa ~ 2MPa. After the reaction was finished, the temperature was reduced to normal temperature, and all the reaction products were collected.

2.2. Antimicrobial Finishing of PP Nonwoven Fabric

Liquid PLAO was mixed with solvent (PLAO: H2O = 1:10) to achieve homogenous solution, prior to be applied on the PP nonwoven fabric through double-roll padding process. After evaporation of water by drying, the liquid PLAO finished PP nonwoven fabric was cooled down and collected in ambient condition for further antimicrobial test.

2.3. Preparation of Hierarchically Porous PLA Oligomer (HPPLAO) Layer

Solid PLAO was heated to 160 for achieving liquid state, followed by a coating process on horizonal glass or fabric (PP, PET or cotton fabric) with doctor blading apparatus. Thickness of solid PLAO can be adjusted by controlling the height of blade. Solvent (DMSO, ethanol, acetone, ethyl acetate or hexane) was then dropped on the surface of solid PLAO. After evaporation of solvent and crystallization of PLAO for 12 h ~ 48 h in the ambient condition, the HPPLAO forms with white appearance.

2.4. Characterization

UV-Vis-NIR spectrometer (Perkin Elmer Lambda 1050) and FT-IR Spectrometer (Spectrum 100, Perkin Elmer) with Integrating Sphere (Mid-IR IntegratIR, PIKE) were used to test the reflectivity and transmissivity of radiative cooling device/fabrics in the UV-Vis-NIR range (0.25 ~ 2.5 μm) and MIR range (4 ~ 15 μm), respectively. The radiative cooling tests were carried out under a solar simulator (Oriel® Sol1A™ Class ABB systems, NEWPORT) with power of 1600 W[29] (Figure S3). Thermal Imaging Camera (E33, FLIR) and thermocouple (Multi-Channel Temperature Meter, AT4516, Anbai) was deployed for measuring the temperature of sample and surrounding environment. SEM (T3000, Hitachi) was applied for characterizing the morphology of HPPLAO.The roughness of transparent and opaque sample were measured by Optical Profiling System (New Zygo NexView) and 3D Optical Measuring System (Alicona IFM G4), respectively.

2.5. Antimicrobial Test

The antifungal tests of liquid PLAO were carried out according to ASTM E2149-20.

Agar diffusion plate tests of solid PLAO against K. pneumoniae and multi-drug resistant S. aureus were implemented according to ISO 20645-2004. Antiviral and antimicrobial properties of liquid PLAO finished PP nonwoven fabrics were evaluated according to ISO 18184: 2019 and ASTM E2149-20.

3. Results and Discussion

3.1. Structure, Morphology Measured Optical Spectra of PLAOs

Similar to the synthesis process of PHB oligomer[20], PLAO was synthesized by simple polymerization of lactic acid monomer. The degree of polymerization (DP) increases with prolongation of synthesizing time. The state of PLAO can be adjusted from liquid state to solid through different polymerizing time. Generally, the state of PLAO is liquid when the polymerizing time is less than two hours and the medium degree of polymerization (DP) is about 5 (Figure 5c), whereas the state can become solid when the polymerizing time is over 6 hours and the medium degree of polymerization is about 11 (Figure 5d). The liquid PLAO is transparent (Figure 1a) and can be readily coated on nonwoven polypropylene (PP) fabric through a simple soak-pad-dry process after diluted with water. An antimicrobial function can be imparted onto the nonwoven PP fabric without any change of appearance (Figure 1b). Solid PLAO is also transparent initially after polymerizing for over 8 hours, whereas the appearance may become semitransparent if absorbing moisture in the ambient condition (Figure 1c). After solvent treatment for over 12 hours, crystallization happens on the surface of solid PLAO, thus forming hierarchically porous PLAO (HPPLAO) (Figure 1d), which contains nano/micro porous layer (pore size: 300 nm ~ 2 μm, Figure 1e ~ 1f) in the surface layer and hexagonal microsphere (PLAOHM) (microsphere diameter: about 16 μm) with nanoscale texture (texture size: about 300 nm) (Figure 1g ~ 1h) in the sublayer. It can be seen from Figure 1i that the glass/HPPLAO device have high averaged reflectivity of about 80% in the main solar spectrum range of 0.3 ~ 1.4 μm (reduce the absorption of sunlight), while possess relative high emissivity of about 90% in the mid-infrared (MIR) range of 4 ~ 15 μm (increase the power radiated out of the device), thus obtaining potential property for cooling effect. PLAO itself is transparent and rarely absorb/reflect the UV-Vis light, whereas the HPPLAO with micro/nano-structure can scatter the UV-Vis light effectively through multiple scattering effects, thus achieving high reflectivity in solar spectrum range. Meanwhile, the HPPLAO layer is thick enough (0.8mm) to prevent the transmission of MIR light, resulting in high emissivity for the glass/HPPLAO device. Similarly, HPPLAO coating can impart the same property on polypropylene (PP) nonwoven fabric. The short dot line in Figure 1j shows the low reflectivity and emissivity of original PP nonwoven fabric, which is used for personal protective equipment (e. g. protective clothing for medical and sanitary use etc.). After coating a HPPLAO layer on the PP nonwoven fabric substrate, the reflectivity doubled from 30% to about 60% among the main solar spectrum range and the emissivity nearly quadruples from 20% to about 80% in the atmosphere window range of 7 ~ 14 μm. This remarkable promotion of reflectivity and emissivity on the PP fabric certifies a potential cooling effect of PP/HPPLAO fabric.

3.2. Factors Influencing Morphology and Optical Spectra

3.2.1. Solvent Treatments

As HPPLAO is prepared through soaking and evaporation of solvents, the effect of polarity on the formation of HPPLAO was investigated with five solvents from high polarity to low polarity, i. e. dimethyl sulfoxide (DMSO), ethanol, acetone, ethyl acetate (EA) and hexane. Figure 2a ~ 2e shows the appearance of HPPLAO after treatment of different solvent, wherein DMSO and hexane treated HPPLAO are more transparent than the samples treated by ethanol, acetone and EA. The high boiling temperature of DMSO (189 ℃) makes it difficult to vaporize thus opaque HPPLAO films are formed. Besides, the polarity of hexane (no polar group and low polarity) is different from that of PLAO (many polar ester groups and high polarity), leading to incompatibility and no interfacial role to form HPPLAO. On the contrast, ethanol, acetone and EA have approximately same polarity with PLAO, so these solvents are compatible with PLAO during the soaking process. After ambient evaporation of solvent and crystallization of PLAO, opaque HPPLAO forms with porous surface layer and PLAOHM sublayer. Excessive solvents were applied to soak PLAO for investigating the effect of quantity of solvents on formation of HPPLAO. Figure 2f ~ 2h shows that larger quantity of solvents can promote the formation of HPPLAO and higher whiteness.

Except for the surficial appearance, the solvents can also influent the structure of prepared PLAOHM. Figure 2i ~ 2n illustrate the morphology of PLAOHM prepared by different solvents, wherein the edges of hexagonal microsphere are clearer for the PLAO treated by DMSO (though taking long time of over one week to form), acetone and EA, comparing with PLAO treated by ethanol. It is deducted that the higher moisture absorption of ethanol leads to partially hydrolysis of HPPLAO, thus losing the clear hexagonal edges.

3.2.2. Substrates

In this work, four kinds of substates are used for supporting HPPLAO, which is rigid independently, including glass, polyethylene terephthalate (PET) woven fabric, polypropylene (PP) nonwoven fabric and cotton woven fabric. The roughness of different substrates is shown in Figure 3a, while the roughness of HPPLAO based on different substrates is shown in Figure 3b. The roughness order of substates is glass < PET < cotton < PP, whereas the roughness of HPPLAO-based device/fabrics is glass < PP < PET < cotton, indicating that the roughness of PP remarkably decreased after coated by HPPLAO. Reflectivity of HPPLAOs on different substrate are demonstrated in Figure 3c, showing that the reflectivity of HPPLAO is much higher if the roughness of corresponding substrate is lower. It is deduced that smooth surface of substrate facilitates the formation of more PLAOHM, which can reflect the spectrum more effectively.

As shown in Figure 3d, PP fabric has lowest reflectivity in the UV-Vis-NIR range (0.25 μm ~ 1.1 μm) and MIR range (4 μm ~ 15 μm). PP/HPPLAO fabrics prepared by evaporation of different solvents possess higher reflectivity than the PP fabric, wherein effect of acetone and EA are better than that of ethanol. It is deduced that the polarity of ethanol is much higher than the PP fabric, thus PP substrate cannot retain ethanol for enough time to form HPPLAO. Oppositely, acetone has similar polarity with PP fabric and can stay longer on the substrate to form HPPLAO, which contributes to higher reflectivity.

3.2.3. Thickness

As shown in Figure 3e ~ 3f, reflectivity of cotton/HPPLAO fabric increases remarkably from 40% to 70% in the visible spectrum range when the thickness rises from 0.1 mm to 0.8 mm. Also, the thickness of HPPLAO shows slight promoting effect on the emissivity of cotton/HPPLAO fabric in the MIR range. These results illustrate that thicker HPPLAO layer can offer more effective unit to achieve higher reflectivity in the UV-Vis-NIR range and higher emissivity in the MIR range.

3.3. Radiative Cooling of Coating

For assessing the cooling effect of HPPLAO, glass/HPPLAO device and PP/HPPLAO fabric were tested under solar spectrum (AM1.5) though thermal image and thermocouples (Figure 4a~4d). Thermal image camera was used for testing the surface temperature of sample and paper, while thermocouples were used for testing temperature below/above the sample. The detailed temperature of each sample is shown in Table 1. Without HPPLAO coating, the temperature below the glass is 39.6 ℃ after irradiation of solar spectrum for 3 min, whereas the temperature below the glass/HPPLAO is only 32.8 ℃, realizing temperature reduction of 6.8 ℃. Similarly, comparing with PP nonwoven fabric, the PP/HPPLAO fabric achieves a temperature reduction of 4.3 ℃ at position below the sample with the cooling effect of HPPLAO.

The carbon emission, biodegradability, antimicrobial property and biological safety of glass/HPPLAO cooling devices and PP/HPPLAO cooling fabric are compared with other types of cooling devices/fabrics (Figure 4e~4f), including radiative cooling fabrics made by TiO2 doped PLA with PTFE layer[17], ZnO doped PE textile[14], nanoporous polyethylene textile[22], film based on polyethylene oxide (PEO) nanofiber[23] and hierarchically porous P(VdF-HFP) film[7].

Figure 4e shows that glass/HPPLAO cooling devices and PP/HPPLAO cooling fabric are eco-friendly functional device/fabric due to the degradability, as well as lowest emission of CO₂ of 0.5 kg CO₂ eq/kg, compared with other types of cooling devices/textiles[24,25,26,27,28]. Therefore, the radiative cooling and eco-friendly properties of glass/HPPLAO device and PP/HPPLAO fabric are comparative or better than the films/fabrics reported previously.

It can be seen from Figure 5f that antimicrobial reduction (against E. Coli) of glass/HPPLAO cooling devices and PP/HPPLAO cooling fabric are 99.99%, which are significantly higher than other types of cooling devices/textiles, showing excellent antimicrobial property. Meanwhile, their LD50 values (dose of a material causing 50% death of test animal) are 5000 mg/kg, exhibiting higher biological safety than the devices that contain ZnO and PEO.

Together with the simple preparing process (one-step solvent evaporation method) and low cost (cheap raw materials of lactic acid and ethanol), the cooling devices/fabrics based on HPPLAO are promising cooling solution in the applications with green, safe, hygienic and healthy requirements, e.g. curtain, tent, umbrella cloth etc.

Besides, HPPLAO prepared by surficial evaporation of excessive ethanol on solid PLAO shows lower temperature under illumination of solar spectrum (10 ~ 40 min) than other material or fabrics, e. g. glass, PP, PET, cotton etc. (Figure S1a~1c), illustrating the cooling effect generated by micro/nanostructure of HPPLAO with high reflectivity. For simulating sunshade device/fabric, the cooling devices/fabrics were set at height of 5 cm relative to flatwise paper. Compared with HPPLAO on fabric substrates, the glass/HPPLAO device can achieve largest temperature drop of 14.2 ℃ (upper surficial temperature of sample minus temperature of respective shadow on paper, Figure S1d~1e and Table S1), showing excellent radiative cooling effect if used as sunshade board.

In the remote radiative cooling test of glass/HPPLAO device and PP/HPPLAO fabric, the temperature differences between samples above (2 cm) and below (4 cm) were measured by thermocouple (Figure S2 and Table S2), showing obvious temperature drop of 3 ℃ and 2 ℃ under illumination of solar spectrum for 30 min. These temperature drop values are believed to be larger if the sizes of device/fabric are increased, as larger area of device/fabric can prevent the air convection and energy exchange more efficiently.

3.4. Wide-Spectrum Antimicrobial Properties of PLAO

Figure 5a shows the relationship among antifungal reduction against C. albicans, viscosity and polymerizing time of liquid PLAO. The viscosity of liquid PLAO increases slightly in the first hours, then rise sharply in the second hour, illustrating the generation of oligomer with higher DP. The antifungal reduction doesn’t change obviously with the increase of viscosity, as the main component and DP of PLAO haven’t changed much within polymerizing time of 2 hours. Figure 5b exhibits the antibacterial effect of solid PLAO against K. pneumoniae and multi-drug resistant S. aureus according to ISO 20645-2004. Obvious antibacterial rings form around the solid PLAOs, indicating the effective antibacterial performance against gram-positive, gram-negative and multi-drug resistant bacteria. Table 2 summarizes the wide-spectrum antimicrobial properties of liquid PLAO and its coated nonwoven PP fabrics against coronaviruses, gram-positive bacteria, gram-negative bacteria and fungi. The antiviral reductions against SARS-CoV-2 (COVID -19 virus) of 1% and 2% PLAO aqueous solution can reach 99.99% in 20 min and 1.5 h. The antiviral reductions of PLAO coated PP nonwoven fabric against H3N2 virus increases with longer contact time until reach high reduction of 99.99% with contact time of 2 hours. The liquid PLAO coated nonwoven PP fabrics also shows high antimicrobial reduction against other microbes, especially for gram-positive and gram-negative bacteria (99.99%). Considering the PLAO is homologue of PHB oligomer, its antimicrobial mechanism is probably same as that of PHB oligomer, i. e. the destruction on biofilm, cell membrane and protein of microbes[20].

4. Conclusions

In conclusion, we demonstrate a coating material based on HPPLAO that possess biodegradability, radiative cooling effect and wide-spectrum antimicrobial property simultaneously. The radiative cooling effect comes from the hexagonal microsphere geometry and nanotexture of PLAOHM crystal, which can enhance the reflectivity in the solar spectrum through multiple scattering effect and exhibit high emissivity in the mid-infrared atmospheric window due to the vibration modes in PLAO. Besides, the material shows wide-spectrum antimicrobial property and large antimicrobial reduction against coronaviruses (including COVID-19 virus), gram-positive bacteria, gram-negative bacteria, multi-drug resistant bacteria and fungi, owing to the destruction on cell membrane and protein of microbes. This work opens a door to develop green multifunctional, energy-saving and hygienic material with oligomer crystals. Along with this way, more similar multifunctional materials can be discovered through changing functional group, degree of polymerization, polarity, crystal size etc, thus satisfying diverse requirements for wide applications.

6. Patents

Publication No.: CN122037738A; Invention Name: Bio-based micro-nano material with radiation cooling and antibacterial functions, refrigeration device and application thereof