WO2024094672A1

WO2024094672A1 - Pla aerogel based phase-change composites for thermal energy storage and heat management

Info

Publication number: WO2024094672A1
Application number: PCT/EP2023/080316
Authority: WO
Inventors: Guang-zhong YIN; De-Yi Wang
Original assignee: Fundación Imdea Materiales; Fundación Universidad Francisco de Vitoria
Priority date: 2022-11-02
Filing date: 2023-10-31
Publication date: 2024-05-10

Abstract

The invention relates to a composite phase-change material (PCM) comprising a highly porous aerogel and a PCM, wherein the PCM occupies an internal volume of the aerogel. This material takes full advantage of the highly porous aerogel which is capable of entrapment of high amounts of PCM, while avoiding seepage of the PCM. A method for the preparation of these materials is also within the scope of the invention as well as its use and applications. The invention further relates to a thermal energy storage device comprising the composite PCM.

Description

PLA AEROGEL BASED PHASE-CHANGE COMPOSITES FOR THERMAL ENERGY STORAGE AND HEAT MANAGEMENT

FIELD OF THE INVENTION

This invention relates to the field of thermal energy storage and heat management, more precisely to the field of latent heat energy storage and heat management based on phase-change composite materials. The materials of the invention comprise a highly- porous Polylactic acid (PLA) aerogel and a PCM work substance.

BACKGROUND

The increasingly prominent energy and environmental problems are pushing the requirements of our society for improved energy conservation and environmental protection. The requirements for efficient energy use are also increasingly higher. Thermal energy storage (TES) technologies are valuable components in many energy systems and could be an important tool in achieving a low-carbon future. According to the storage principle, TES technologies can be divided into three categories: sensible heat storage, latent heat storage and thermochemical heat storage. Sensible heat storages are the most commonly deployed type of TES. However, latent heat storage technologies based on Phase-change Materials (PCMs) are particularly attractive for applications where thermal energy has to be stored or delivered over a narrow temperature range or when compactness is a requirement. Indeed, PCMs are capable of absorbing or releasing great amounts of energy in the form of latent heat during phase transitions at nearly constant temperature. They enable compact TES systems with volumetric storage capacity five to ten times greater than that of sensible heat storage systems.

During the development of PCMs, many kinds of materials have been deeply studied, including inorganic compounds (salts and hydrated salts) and organic compounds, such as, paraffins, fatty acids, and polyethylene glycols (PEGs). Studying the relationship between the basic structure and energy storage properties of PCMs is helpful to determine their ultimate energy storage/release mechanism. Generally, the ideal PCMs should satisfy the required thermophysical and chemical properties, such as suitable phase transition temperature, high energy storage density, good thermal conductivity, anti-leakage, shape-stability and both good chemical and cycling stability.

Most widely used PCMs undergo solid-liquid transitions (solid-liquid PCM). However, solid-liquid PCMs require encapsulation in order to avoid leakage of the liquid phase at temperatures above the melting point which may limit the practical use of PCMs. At present, there are various methods that try to solve the leakage problem of PCMs, which generally consist of porous solid supports capable of PCMs entrapment. Methods for obtaining leakage-free materials include the adsorption method, microencapsulation method, sol-gel method, membrane method or the electrospinning method. Common porous materials for adsorption method include clay minerals, porous carbon materials, and metal foams.

In this way, composite PCM refers to a class of materials composed of a supporting material which encloses the PCM work substance, examples being micro-encapsulated PCMs and the so-called shape-stabilized PCMs.

Micro-encapsulation is the process of coating individual particles or droplets with a film to produce capsules at micrometer to millimeter size (<1000 pm), known as a microcapsule, microparticles or microspheres. Capsules at nanometer in size (<100 nm) are known as nanocapsules, nanoparticles or nano-sphere. These capsules are composed of two main parts: the core comprising a solid-liquid PCM and a shell comprising a polymer, inorganic or hybrid material. The shell will act as a container to protect the inner core PCMs from the external environment and to avoid leakage of the PCM when in liquid phase. Micro- or nano-capsules exist in several shapes such as spherical, tubular, oval or irregular shape (Khadiran, T., Hussein, M.Z., Zainal, Z. and Rusli, R., Encapsulation techniques for organic phase-change materials as thermal energy storage medium: A review. Solar Energy Materials and Solar Cells, 2015, 143, 78-98; Sarier, N. and Onder, E., Organic phase-change materials and their textile applications: An overview. Thermochimica Acta, 2012, 540, 7-60). Micro-encapsulated PCMs yield encapsulation efficiency values from 60 to 90 % and PCM-shell mass proportion of up to 80%. However, microcapsules are expensive, can break, often require additional resinous binders for adhesion, and can cause poor fabric flexibility and properties.

Shape-stabilized phase-change materials (SPCMs), also referred to as form-stable PCMs, can be defined as a class of solid-liquid PCM materials which maintain their original shape even when the temperature of the PCM is above the melting point (liquid state). SPCMs can be prepared as polymer-based organic materials, which are obtained by blending and characterized by a continuous polymeric matrix that encloses the PCM or as inorganic porous materials that are produced by incorporating a liquid PCM into the porous structure of the supporting material by vacuum assisted infiltration method or by the sol-gel method. PCMs encapsulated into porous structures do not suffer leakage due to capillary forces and hydrogen bonding and yield PCM-supporting material mass proportion values ranging from 35 to 95%.

Conventional PCM composite materials as the ones described above have the advantage of containment of the liquid phase. However, they lack good encapsulation efficiencies, have low enthalpy values and their preparation methods often require consumption of large quantity organic solvents and require high temperature processes. The confinement of phase change material (PCM) within 3D porous structure has been identified as the most attractive and efficient strategy to fabricate high performance shape stable PCMs. Encapsulation and/or impregnation methodologies are applied in the design of shape-stable PCMs by incorporating pristine PCMs into supporting materials.

The work of Pavlos K. Pandis, et al. (“Differential scanning calorimetry based evaluation of 3D printed PLA for phase change materials encapsulation or as container material of heat storage tanks", Energy Procedia, Volume 161, Pages 429-437, 2019) discloses the use of PLA (polylactic acid) as a 3D-printed material for use in the encapsulation of PCMs, or as a material in the manufacture of storage tanks and custom heat storage vessels. The PLA support material was printed using a 3D printer and PLA filament. According to this disclosure, the mass uptake of the PLA structure after impregnation with liquid PCMs for 40 days was very low, at best only 0.05%/cm² increase.

There are several techniques available for the preparation of PLA-based aerogels (or PLA based highly porous materials), such as porogen leaching/freeze-drying, phase separation/salt particle-leaching method, and electrospinning. These techniques have their outstanding advantages, and are widely used in different fields. However, there are also various problems in these methods, including the need to improve the foaming rate, the porosity and the inter pore connectivity. It is also difficult to prepare porous materials with ideal porous structure and morphology. Among these methods, the phase separation technique conventionally divides into thermal induced phase separation (TIPS) and non-solvent induced phase separation (NIPS). Both techniques have been used widely to prepare porous materials for tissue scaffolds. For example, the work of Bueno, A., et al. (“Production of polylactic acid aerogels via phase separation and supercritical CO2 drying: thermodynamic analysis of the gelation and drying process", J Mater Sci, Volume 56, pages 18926-18945, 2021) concerns the improved production of aerogels by via thermal-induced phase separation (TIPS) method, using two solvents for phase separation (solvent and anti-solvent). This scientific paper teaches the requirement of supercritical CO2 drying to preserve the mesoporous gel structured during the gelation of the aerogel. They describe a PLA aerogel, and identify its potential use as drug carrier for pharmaceutical applications.

Wei-Chi Lai et al. (“Novel green and sustainable shape-stabilized phase change materials for thermal energy storage", Journal of the Taiwan Institute of Chemical Engineers, Volume 117, Pages 257-264, 2020) report on polyethylene glycol PCMs blended with poly(L-lactic acid) and discuss how to improve the miscibility and morphology of the blends. The reported material is characterized by a latent heat during melting of 130.26 J/g. They mention that poly (L-lactic acid) (PLLA) is a sustainable biodegradable polyester with environmentally friendly characteristics and potential applications for biomedical uses.

In the report by Xiang Lu, et al. (“Bio-based poly (lactic acid)/high-density polyethylene blends as shape-stabilized phase change material for thermal energy storage applications", Solar Energy Materials and Solar Cells, Volume 192, Pages 170-178, 2019), shape-stabilized phase change materials are prepared via melt blending by employing PLA as the supporting matrix and high-density polyethylene (HDPE) as PCM for TES applications. The PLA supporting matrix is not an aerogel, and it reports that PLA/PCM ratios are only stable up to a 50/50 ratio, and that a ratio of 40/60 is not stable. Their material is characterized by a latent heat during melting of 100.1 J/g and a latent heat during solidification of 97.6 J/g.

Chinese patent application CN 113 136 172 A discloses a shape-stabilized composite phase-change material comprising polyvinyl alcohol aerogel as a supporting matrix and a phase-change material. However, the supporting matrix requires the presence of a crosslinker.

Chinese patent application CN 110 982 111 A discloses a complex 3D-printing method under cryogenic conditions to produce an aerogel of aramid fiber. The method requires printing the aramid fiber, followed by surface modification to obtain a hydrophobic material.

However, as will become apparent from the invention description below, a need still exists to improve existing solutions and provide more efficient latent heat storage technologies based on phase-change materials.

BRIEF DESCRIPTION OF THE INVENTION

The authors of the present invention have developed an improved composite phase change material (PCM) based on polylactic acid (PLA) aerogels, prepared by a phaseseparation method comprising both thermal induced phase separation (TIPS) and non- solvent induced phase separation (NIPS) procedures, characterized by a phase change enthalpy of 100-250 J/g. The materials are characterized by uniform, and reproducible structures.

To the best knowledge of the inventors, no such composite PCM material had ever been disclosed before.

Therefore, the invention disclosed herein and defined in the claims relates to composite PCMs in which a highly porous polylactic acid aerogel encapsulates/supports a PCM. It differs from previous works in the fact that the outer encapsulating/supporting material is a PLA aerogel, leading to major new features and advantages as described in the present disclosure.

In a first inventive aspect the invention provides a shape-stabilized composite phasechange material comprising: a) a polylactic acid aerogel; and b) a phase-change material, wherein the phase-change material occupies an internal volume of the aerogel.

A second aspect is directed to a method for the preparation of the composite phasechange material of the invention, wherein a polylactic acid aerogel is put in contact with a molten phase-change material, and wherein said polylactic acid aerogel is prepared by a method comprising the steps of: a) dissolving a polylactic acid in a first solvent; b) cooling the solution of step (a) for phase separation, thus obtaining a first polylactic acid gel; c) immersing the first polylactic acid gel formed in step (b) in a second solvent for solvent exchange, thus obtaining a second polylactic acid gel; d) optionally washing the second polylactic acid gel of step (c) with water, ethanol or mixtures thereof; and e) freeze drying the second polylactic acid gel obtained in step (c), or the washed second polylactic acid gel obtained in step (d), to obtain the aerogel.

In a third aspect, the invention is directed to a composite phase-change material obtainable according to the method of the second aspect of the invention.

In a fourth aspect, the invention is directed to the use of the composite phase-change material of the invention, in thermal energy storage or thermal management including peak energy reduction, preferably in electronics, power electronics, solar energy, batteries, buildings, waste heat recovery, air-conditioning, temperature-adaptable greenhouses and textiles. In a fifth aspect, the invention is directed at a thermal energy storage system comprising a composite phase-change material.

In a sixth aspect, the invention is directed to a product of manufacture, an electronic device, a solar energy system device, an energy storage device, an electronic device, a computer, a medical device, a storage unit, a building or building material, a container, an insulation or construction material, an automotive material, a vehicle, a boat, an airplane, a weapon or weapon system, industrial machinery, a pharmaceutical or a drug or a food package or storage device or container, a textile, a clothing or an apparel, footwear, a bedding or bedding system, a flame retardant material, comprising a composite phase-change material according to the invention, or comprising the thermal energy storage system of the fifth aspect.

These and other characteristics and advantages of the invention will become clearly understood in view of the detailed description of the invention which becomes apparent from a preferred embodiment of the invention, given just as an example and not being limited thereto, with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a picture of a typical aerogel sample of the invention (aerogel sample 3), showing its lightweight nature.

Figure 2 shows SEM images of the aerogel sample 1 (Fig 2a), aerogel sample 2 (Fig 2b), and of sample 3 (Fig 2c).

Figure 3 shows photographic images of the composite materials PLA-PEG400 (top), PLA-PEG600 (middle) and PLA-PEG1000 (bottom) after 90 minutes of heating, and the graphs show the evolution of the weight of the samples with time.

Figure 4 shows the evolution of the weight of the samples with time. Sample A is a PLA- Paraffin composite according to the invention, and sample B is a comparative Chitosan- Paraffin composite.

DETAILED DESCRIPTION OF THE INVENTION

In a first inventive aspect the invention provides a shape-stabilized composite phasechange material comprising: a) a polylactic acid aerogel; and b) a phase-change material; wherein the phase-change material occupies an internal volume of the aerogel. The material of the invention is a composite material (also called a composition material or shortened to composite, which is the common name) which is produced from two or more constituent materials. In the present case, the composite material of the invention comprises a scaffold support made of polylactic acid (PLA) which is in the form of an aerogel. The highly porous PLA aerogel acts as a scaffold for a solid-liquid phase-change material (PCM), which is comprised within the 3D highly porous aerogel structure.

Aerogel

According to Bueno, A., et al., (“Production of polylactic acid aerogels via phase separation and supercritical CO2 drying: thermodynamic analysis of the gelation and drying process", J Mater Sci, Volume 56, pages 18926-18945, 2021), polylactic acid (PLA) is a biocompatible, biodegradable and immunologically inert synthetic polymer. The chiral nature of lactic acid allows for preparing three types of PLA: PLLA (poly-L- lactic acid) and PDLA (poly-D-lactic acid) as well as PDLLA (poly-DL-lactic acid). The latter is a copolymer composed of both stereoisomers. PLLA is semicrystalline, while PDLA and PDLLA are both amorphous. Unless explicitly mentioned otherwise, in the present disclosure the term PLA is used for any type of PLA. Therefore, in the present invention, the aerogel is made of PLA, wherein the PLA is selected from poly-L-lactic acid, poly-D-lactic acid, poly-D, L-lactic acid or mixtures thereof.

In the composite material of the invention, the polylactic acid aerogel is characterized by an overall porosity in the range of from 80% to 96%. In a particular embodiment, the overall porosity of the PLA aerogel is equal to or higher than 80%, preferably equal to or higher than 82%, or 84%, more preferably equal to or higher than 86%, and even more preferably equal to or higher than 88%.

The porosity of the aerogel may be calculated, for the purposes of the present disclosure, by means of equation (1) as shown below, by previously knowing the densities of the precursor PLA and of the PLA aerogel. While the density of the PLA used is known from the manufacturer, the density of the PLA aerogel can be determined directly from the ratio of mass to volume.

Porosity

In the present invention, the PLA aerogel is characterized by an internal nano-sheet or nano-fibrous structure. In this context, the term “nano-sheet” or “nano-fibrous” refers to the morphology of the aerogel of the invention, when observed under a Scanning electron microscope (SEM), which is characterized by structures that have at least one of its dimensions is not greater than 100 nm. For example, the aerogel of the invention is nano-fibrous because it contains needle-like structures, wherein at least one of its dimensions is not greater than 100 nm. Similarly, the aerogel of the invention exhibits a nano-sheet morphology because it contains sheet-like structures, wherein at least one of its dimensions is not greater than 100 nm.

The PLA aerogel scaffold support, in other words, the polylactic acid aerogel of the invention, may comprise at least one functional additive. In a particular embodiment, the aerogel comprises at least one functional additive selected from the group consisting of poly(caprolactone), poly(butylene succinate), polyhydroxyalkanoates, polybutylene adipate terephthalate, cellulose, alginate, chitosan and mixtures thereof. Preferably, the functional additive is selected from the group consisting of functionalized metal foams, functionalized ceramic porous materials, thermal conductive fillers, flame retardants, pigments and mixtures thereof.

Suitable thermal conductive fillers may be selected from the group consisting of Boron Nitride, graphene, MXene, biochar, carbon fiber and thermally conductive ceramic powders.

Suitable thermally conductive ceramic powders may be selected from AIN, BeO, SiaN4, SiC, AI2O3 and mixtures thereof.

Suitable flame retardants are selected from the group consisting of ammonium polyphosphate, phytic acid, chitosan, expandable graphite, lignin derivatives, polyhedral oligomeric silsesquioxane, metal-organic frameworks and layered double hydroxides.

In a particular embodiment, the PLA aerogel scaffold support does not comprise a crosslinker.

PCM

In the context of the present invention, a phase-change material (PCM) is a material which undergoes a first-order phase transition of its state or of its microcrystalline structure. For example, a phase transition is that of changing from the solid state to the liquid state or vice-versa (solidification-melting). In the application frame of this invention, the phase transition of the PCM is thermally activated. When the PCM reaches the phase transition temperature, its temperature remains constant during the phase transition since the external heat is no longer used to change the temperature of the PCM but to change the state of the material itself. As a consequence, the energy involved in the phase transition of the PCM, i.e., the specific enthalpy difference between both states of the PCM (at constant temperature) also known as latent heat, is absorbed or released on the melting-solidification process of the PCM.

Therefore, the phase-change material is a solid-liquid phase-change material.

In principle, any solid-liquid PCM can be selected for the composite PCM of the invention, as long as the solid-liquid PCM is characterized by a phase transition at a temperature range that is not as hot as being detrimental to the PLA aerogel.

Other desirable properties of the solid-liquid PCM are high values of heat capacity; high density; resistance to oxidation; non-toxicity; non-flammability; or low volumetric change during phase transition.

In a preferred embodiment, the latent heat of melting of the PCM is above 60 J/g, preferably above 100 J/g, more preferably above 150 J/g, and can be as high as 300 J/g. Preferably, it is comprised between 100 and 300 J/g, more preferably between 150 and 250 J/g.

The temperature at which the phase-change occurs and its total latent heat can be measured by means of Differential Scanning Calorimetry (DSC).

In a preferred embodiment, the PCMs are selected so that the phase-change material has a phase-change temperature comprised between -30 and 200 °C, preferably between 30 and 200 °C, preferably between 30 and 160 °C, even more preferably between 30 and 100 °C. The phase-change can be a melting or solidification.

In the context of the present invention, alkanes, paraffin waxes, beeswax, metal salts, polyethylene glycols, carboxylic acids, fatty acids, fatty acid esters, fatty alcohols, sugar, sugar alcohols and mixtures thereof, are preferred as PCMs. Preferably, the phasechange material is selected from polyethylene glycol, paraffin, fatty acids, fatty alcohols, beeswax, n-eicosane, n-octadecane, n-hexadecane, nonadecane, heptadecane or mixtures thereof. Even more preferably, the phase-change material is selected from polyethylene glycol, paraffin, stearic acid, stearyl alcohol or a mixture thereof.

Exemplary PCMs, suitable for the purposes of the present invention, include linear n- alkanes (C_nH2n+2). The intermolecular forces holding n-alkane molecules together are van-der-Waals forces. Therefore, both the melting point and the latent heat of fusion increase progressively with the carbon chain length. Besides, many binary alkane systems make solid-state solutions with very narrow temperature windows (2-5°C) between solidus and liquidus lines (Ventola, L., et al., Molecular alloys as phase change materials for energy storage and thermal protection at temperatures from 70 to 85 °C, J. Phys. Chem. Solids, 2005, 66, 1668-1674). This property allows a blend to be tailored to a particular melting range. Alkanes are preferably straight n-chain alkanes of formula C_nH2n+2, preferably n being from 10 to 40, more preferably from 14 to 30, such as for example tetradecane, hexadecane, heptadecane, octadecane, nonadecane, eicosane, docosane, tetracosane, hexacosane, octacosane, or the like, more preferably n- eicosane, n-octadecane, n-hexadecane, nonadecane, heptadecane or mixtures thereof. Other exemplary PCMs, suitable for the purposes of the present invention, include polyethylene glycol (HO-(CH2-CH2-O)_n-CH2-CH2-OH). The melting temperatures and the latent heat values of these molecules increase gradually as the average molar weight (MW) of the PEG chains increase (Sundararajan, S., et al., Versatility of polyethylene glycol (PEG) in designing solid-solid phase change materials for thermal management and their application to innovative technology, Journal of Materials Chemistry A, 2017, 5, 18379-18396). Therefore, the melting point and heat capacity of a PEG system can be customized by selecting or mixing PEGs with different MWs. In a preferred embodiment, the phase-change material is a polyethylene glycol of an average molecular weight comprised between 500 g/mol and 20000 g/mol, preferably between 600 g/mol and 20000 g/mol, more preferably between 600 g/mol and 10000 g/mol, even more preferably between 2000 g/mol and 10000 g/mol.

Suitable fatty acids include, for example, capric acid, caprylic acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, lignoceric acid, cerotic acid, or mixtures thereof. Preferably, the fatty acids are selected from the group of consisting of capric acid, caprylic acid, lauric acid, myristic acid, palmitic acid, stearic acid, or mixtures thereof.

Fatty acid esters include alkyl (e.g. C1-C12 alkyl) esters of the above-mentioned fatty acids.

Suitable fatty alcohols include 1 -heptanol, 1 -octanol, pelargonic alcohol, 1 -decanol, undecyl alcohol, lauryl alcohol, tridecyl alcohol, myristyl alcohol, pentadecyl alcohol, cetyl alcohol, palmitoleyl alcohol, heptadecyl alcohol, stearyl alcohol, oleyl alcohol, nonadecyl alcohol, arachidyl alcohol, heneicosyl alcohol, behenyl alcohol, erucyl alcohol, lignoceryl alcohol, ceryl alcohol, montanyl alcohol, 1-nonacosanol, myricyl alcohol, 1- dotriacontanol, or mixtures thereof. Preferably, the fatty alcohols are selected from the group of consisting of capric alcohol, lauryl alcohol, myristic alcohol, cetyl alcohol, stearyl alcohol, or mixtures thereof.

As shown in table 1 , these PCMs have appropriate melting points. They also have high latent heat. In addition, some of them allows easy tuning of their melting temperature.

In a particular embodiment, the PCM is selected from alkanes, paraffin, fatty acids, fatty alcohols, PEG, or mixtures thereof. Preferably, the PCM is selected from linear alkanes, paraffin, stearyl alcohol, stearic acid, PEG, or mixtures thereof; more preferably the PCM is selected from paraffin, stearyl alcohol, stearic acid, PEG, or mixtures thereof.

Table 1. Melting temperature ranges and corresponding phase change latent heat per unit mass and per unit volume for exemplary PCMs.

Composite PCM of the invention

The composite PCM of the invention is shape-stabilized. This is because the PLA aerogel functions as support/encapsulating/enclosing material of the PCM and does not require any further compounds to stabilize the PCM. In this context, stabilization refers to the fact that the composite PCM of the invention does not leak PCM when this is in the liquid state, and the shape of the structure is maintained.

In the context of the present invention, reference to the PLA aerogel encapsulating or enclosing the solid-liquid PCM includes any configuration where the PLA aerogel is acting as supporting material for the solid-liquid PCM, i.e. where the solid-liquid PCM occupies an internal volume of the PLA aerogel, so that no seepage occurs even when the temperature of the system is such that the solid-liquid PCM is in liquid state.

The configuration of the composite PCM does not significantly affect the way the invention works, as long as the PLA aerogel functions as shape-stabilizer or support of the solid-liquid PCM.

The aerogel PLA and PCMs are chemically compatible with each other. In the context of the present invention, “chemically compatible” means the absence of irreversible chemical reactions between the materials.

The composite PCM of the invention can be tailored to specific needs, because the temperature at which the phase change occurs can be tuned by careful selection of the PCM. In fact, more than one PCM can be also used. In this case, the phase change temperature can be tuned by controlling the relative amounts of each solid-liquid PCM. In a preferred embodiment, the composite material of the invention comprises at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, or at least 90% of the PCM, by weight percent with respect to the total weight of the composite material of the invention, i.e., the total weight of the aerogel and PCM.

In a more preferred embodiment, the composite material of the invention comprises at least 30% of the PCM, preferably at least 50%, more preferably at least 70%, and even more preferably at least 80%, by weight percent with respect to the total weight of the composite material of the invention.

In another preferred embodiment, the composite material of the invention comprises less than 99%, less than 98%, less than 97%, less than 96% or less than 95% of a PCM, by weight percent with respect to the total weight of the composite material of the invention. In a most preferred embodiment, the composite material of the invention comprises from 75% to 98%, from 75% to 96%, or from 85% to 95% of PCM, by weight percent with regards to the total weight of the composite material of the invention.

The skilled person readily understands that, when the composite PCM of the invention includes a definition of the amounts of PCM it comprises, expressed in terms of weight percentage values, these values can never sum up to a value which is greater than 100%.

The present invention further contemplates a surface coating or layer that protects the composite PCM of the invention. In a particular embodiment, said coating or layer serves as additional protection against external environment conditions that could reduce the efficacy of the composite PCM of the invention.

Method for the preparation of the composite PCM of the invention

The present invention is a composite material comprising an aerogel of PLA and a PCM, wherein the aerogel serves as a scaffold support for the PCM. The aerogel is prepared following the steps (a) to (e), identified above. The method of the invention comprises both thermal-induced phase separation (TIPS) and nonsolvent induced phase separation (NIPS).

In general, the aerogel of the invention can be prepared by first dissolving an appropriate amount of polylactic acid (PLA, PDLA, PLLA or mixtures) in a suitable solvent selected from, for example, acetone, ethyl acetate (EtOAc), or mixtures thereof.

In a particular embodiment of the method of the invention, the first solvent in step (a) is an organic solvent. Preferably, the first solvent in step (a) is selected from the group consisting of acetone, tetra hydrofuran, ethyl acetate, dichloromethane, N,N- dimethylformamide and mixtures thereof. More preferably, the first solvent in step (a) is acetone.

In a particular embodiment of the method of the invention, step (a) comprises dissolving a polylactic acid in a first solvent at a temperature comprised between 20 and 100 °C, preferably between 20 and 80 °C, more preferably between 25 and 80 °C.

In another particular embodiment of the method of the invention, step (a) comprises dissolving a polylactic acid in a first solvent such that the resulting concentration of polylactic acid is comprised between 0.01 and 0.2 g/mL, preferably between 0.02 and 0.2 g/mL, more preferably between 0.03 and 0.15 g/mL and even more preferably between 0.03 and 0.1 g/mL.

In a preferred embodiment of the method of the invention, step (a) comprises completely dissolving a polylactic acid in a first solvent.

At this point a further optional functional additive may be added. Said optional functional additive ideally imparts improved properties to the aerogel. Exemplary additives include flame retardants, thermal conductive fillers and mechanically enhancement additives. Therefore, in a particular embodiment of the method of the invention, step (a) comprises dissolving a polylactic acid, and a functional additive, in a first solvent. In a particular embodiment of the method of the invention, step (a) does not comprise the addition of a crosslinker. The next step of the method of the invention comprises lowering the temperature of the PLA solution, at a temperature low enough so that phase separation occurs. This step leads to the formation of a PLA gel.

In this way, step (b) of the method of the invention comprises lowering the temperature to achieve phase separation, thus obtaining a first polylactic acid gel. In a particular embodiment of the method of the invention, step (b) comprises cooling the solution of step (a) to a temperature lower than 10 °C, preferably lower than -5 °C, more preferably lower than -10 °C, more preferably lower than -15 °C.

In a particular embodiment of steps (a) and (b), a single solvent is used. In other words, in a particular embodiment of the method of the invention, the thermal induced phase separation comprises the use of a single solvent.

The next step comprises putting the first PLA gel of step (b) in a different solvent, such as methanol, ethanol, water, or mixtures thereof, thus obtaining a second polylactic acid gel. This step may be conducted at any temperature, and for any appropriate duration, as long as solvent exchange occurs. An example would be conducting the solvent exchange at room temperature, and/or for a period of time between 1 hour and 24 hours. Step (c) of the method of the invention thus comprises the addition of a non-solvent to replace the solvent with one that does not solubilize the PLA. In a particular embodiment of the method of the invention, the second solvent in step (c) is a polar solvent, preferably selected from the group consisting of methanol, ethanol, water and mixtures thereof. This step allows obtaining uniform microstructures.

In a particular embodiment, step (c) of the method of the invention comprises immersing the polylactic acid gel in said second solvent at a temperature greater or equal to 0 °C, preferably greater or equal to 10 °C, more preferably greater or equal to 20 °C.

In a particular embodiment, step (c) of the method of the invention comprises immersing the polylactic acid gel in said second solvent for a duration of time of at least 1 h, at least 2 h, at least 3 h, at least 6 h. Preferably, step (c) is conducted for a duration of time of no more than 48 h, preferably no more than 24 h. Therefore, in a particularly preferred embodiment, step (c) is conducted for a duration of time of 1 to 24 h, preferably 1 to 6 h. The solvent exchange step (c) can optionally be improved by subsequently putting the PLA gel in pure water for an additional duration of time, for example 24 or 48 hours, while changing the water with fresh water for several times, for example 2-5 times a day. This optional step ensures that the solvent was completely exchanged. Thus, a particular embodiment of the method of the invention comprises step (d), wherein the aerogel of step (c) is further washed with water, ethanol or mixtures thereof. In a particular embodiment, step (d) is carried out and the gel is washed with fresh water at least two times a day for a duration of at least one, preferably at least two days.

Finally, the PLA aerogels of the invention are obtained by freeze drying the solvent- exchanged PLA gels during any suitable period of time such as between 24 hours and 72 hours. In a particular embodiment, step (e) of the method of the invention comprises freeze-drying the material for at least 24 h.

The inventors have surprisingly found that this method allows obtaining an aerogel with a continuous and homogeneous structure, with nano-fibrous structure, as shown in Fig. 1.

In the method for the preparation of the composite phase-change material of the invention, the polylactic acid aerogel prepared according to the above disclosure is put in contact with a molten phase-change material. This step is conducted in such a way that it leads to the entrapment of the PCM in the pores of the aerogel.

In a particular embodiment, said polylactic acid aerogel is put in contact with said molten phase-change material for at least 1 h, preferably under vacuum. Preferably, this step of putting the aerogel in contact with the PCM can be performed by known techniques such as immersion of the aerogel in a liquid sample of the molten PCM, or vacuum impregnation, to name a few. Preferably, the aerogel is immersed in the molten PCM.

The method of the invention is preferably performed under ambient air conditions, without any inert gas.

Further aspects

In a fourth aspect, the invention is directed to the use of the composite phase-change material of the invention, in thermal energy storage or thermal management including peak energy reduction, preferably in electronics, power electronics, solar energy, batteries, buildings, waste heat recovery, air-conditioning, temperature-adaptable greenhouses and textiles.

In a fifth aspect, the invention is directed at a thermal energy storage system comprising a composite phase-change material.

Particular embodiments

Embodiment 1. Shape-stabilized composite phase-change material comprising: a) a polylactic acid aerogel; and b) a phase-change material; wherein the phase-change material occupies an internal volume of the aerogel.

Embodiment 2. The composite according to embodiment 1 , wherein the phase-change material is a solid-liquid phase-change material.

Embodiment 3. The composite according to any one of embodiments 1 or 2, comprising at least 30%, preferably at least 50%, more preferably at least 70%, by weight percent of the phase-change material with respect to the total weight of the composite material.

Embodiment 4. The composite according to any one of embodiments 1 to 3, wherein the polylactic acid is selected from poly-L-lactic acid, poly-D-lactic acid, poly-D, L-lactic acid or mixtures thereof.

Embodiment 5. The composite according to any one of embodiments 1 to 4, wherein the polylactic acid aerogel further comprises at least one functional additive.

Embodiment 6. The composite according to embodiment 5, wherein the functional additive is selected from the group consisting of poly(caprolactone), poly(butylene succinate), polyhydroxyalkanoates, polybutylene adipate terephthalate, cellulose, alginate, chitosan and mixtures thereof.

Embodiment 7. The composite according to embodiment 5, wherein the functional additive is selected from the group consisting of functionalized metal foams, functionalized ceramic porous materials, thermal conductive fillers, flame retardants, pigments and mixtures thereof. Embodiment 8. The composite according to embodiment 7, wherein the thermal conductive fillers are selected from the group consisting of Boron Nitride, graphene, MXene, biochar, carbon fiber and thermally conductive ceramic powders.

Embodiment 9. The composite according to any one of embodiments 7 or 8, wherein the flame retardants are selected from the group consisting of ammonium polyphosphate, phytic acid, chitosan, expandable graphite, lignin derivatives, polyhedral oligomeric silsesquioxane, metal-organic frameworks and layered double hydroxides.

Embodiment 10. The composite according to any one of embodiments 1 to 9, wherein the phase-change material is selected from alkanes, paraffin waxes, beeswax, metal salts, polyethylene glycols, carboxylic acids, fatty acids, fatty acid esters, fatty alcohols, sugar, sugar alcohols.

Embodiment 11 . The composite according to embodiment 10, wherein the fatty acids are selected from caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, lignoceric acid, cerotic acid or mixtures thereof.

Embodiment 12. The composite according to any one of embodiments 10 or 11 , wherein the fatty alcohols are selected from 1-heptanol, 1-octanol, pelargonic alcohol, 1-decanol, undecyl alcohol, lauryl alcohol, tridecyl alcohol, myristyl alcohol, pentadecyl alcohol, cetyl alcohol, palmitoleyl alcohol, heptadecyl alcohol, stearyl alcohol, oleyl alcohol, nonadecyl alcohol, arachidyl alcohol, heneicosyl alcohol, behenyl alcohol, erucyl alcohol, lignoceryl alcohol, ceryl alcohol, montanyl alcohol, 1-nonacosanol, myricyl alcohol, 1- dotriacontanol, or mixtures thereof.

Embodiment 13. A method for the preparation of a composite phase-change material as defined in any one of embodiments 1 to 12, wherein a polylactic acid aerogel is put in contact with a molten phase-change material, and wherein said polylactic acid aerogel is prepared by a method comprising the steps of: a) dissolving a polylactic acid in a first solvent; b) cooling the solution of step (a) for phase separation, thus obtaining a first polylactic acid gel; c) immersing the first polylactic acid gel formed in step (b) in a second solvent for solvent exchange, thus obtaining a second polylactic acid gel; d) optionally washing the second polylactic acid gel of step (c) with water, ethanol or mixtures thereof; and e) freeze drying the second polylactic acid gel obtained in step (c), or the washed second polylactic acid gel obtained in step (d), to obtain the aerogel.

Embodiment 14. Use of a composite phase-change material according to any one of embodiments 1 to 12 in thermal energy storage or thermal management including peak energy reduction, preferably in electronics, power electronics, solar energy, batteries, buildings, waste heat recovery, air-conditioning, temperature-adaptable greenhouses and textiles.

Embodiment 15. A product of manufacture, an electronic device, a solar energy system device, an energy storage device, an electronic device, a computer, a medical device, a storage unit, a building or building material, a container, an insulation or construction material, an automotive material, a vehicle, a boat, an airplane, a weapon or weapon system, industrial machinery, a pharmaceutical or a drug or a food package or storage device or container, a textile, a clothing or an apparel, footwear, a bedding or bedding system, a flame retardant material, comprising a composite phase-change material according to any one of claims 1 to 12.

Advantages of the composite PCM of the invention

The present invention provides the following further advantages:

- There is no leakage of the PCM because the PCM occupies an internal volume of the PLA aerogel, and the aerogel is made of biocompatible PLA;

This provides easier handling and integration into devices. Other advantages include protection from external environment, local management of volume changes, and mitigation of phase separation and sedimentation problems.

- The choice of polylactic acid improves sustainability, allows compatibility with classical PCM work substances, and allows an eco-friendly fabrication process. Furthermore, the composite material of the invention does not require the presence of a crosslinker; - The method for obtaining the PLA aerogel (single solvent for TIPS followed by NIPS) is responsible for high-quality aerogels, i.e., high-porosity uniform aerogels exhibiting nano-fibrous or nano-sheet structures, which can be obtained with good reproducibility;

- The composite material has a very high load capacity of PCM since the entirety of the enclosing or supporting material is a highly porous aerogel, with porosities around 90% or more;

This provides materials with higher storage capacity than known composite PCMs because in the present invention usually more than 80% of the weight of the composite material is a PCM, which contributes to the latent heat storage. The novel composite PCMs can provide significantly higher storage capacity than standard composite PCMs, in which the carrier/encapsulating material is not capable of enclosing such high quantities of PCM. The present invention provides leak-resistant and shape-stable PCMs with melting enthalpies greater than 200 J/g; and

- The method for the preparation of the composite PCM only requires simple steps and, by simply tuning the relative amounts of the PCMs, the resulting composite material of the invention covers a wide range of temperatures, from 40 °C to 200 °C, preferably from 40 °C to 160 °C.

Examples

Example 1. Preparation of PLA aerogel 1 .

An acetone PLA solution was put in a container at a concentration of 0.04 g/mL, and at a temperature of 80 °C until the PLA was completely dissolved. Then, from 5 mL to 10 mL of PLA solution was quickly poured into a pre-cooled cylinder container (20 mL with dimeter of 2 cm), which was immediately transferred to a refrigerator at -20 °C for phase separation. The newly formed PLA gel was then immersed into ethanol at room temperature for aging the gel. After 1 h, the PLA gels were transferred into excess water for an additional 24 h to make sure the solvent exchanged completely. Fresh water was changed 3 times a day. The aerogels were obtained after freeze drying for 24 h. The porosity, measured according to equation 1 , was 95.56 %. Example 2. Preparation of PLA aerogel 2.

An acetone PLA solution was put in a container at a concentration of 0.07 g/mL, and at a temperature of 80 °C until the PLA was completely dissolved. Then, from 5 mL to 10 mL of PLA solution was quickly poured into a pre-cooled cylinder container (20 mL with dimeter of 2 cm), which was immediately transferred to a refrigerator at -20 °C for phase separation. The newly formed PLA gel was then immersed into ethanol at room temperature for aging the gel. After 1 h, the PLA gels were transferred into excess water for an additional 24 h to make sure the solvent exchanged completely. Fresh water was changed 3 times a day. The aerogels were obtained after freeze drying for 24 h. The porosity, measured according to equation 1 , was 94.19 %.

Example 3. Preparation of PLA aerogel 3.

An acetone PLA solution was put in a container at a concentration of 0.10 g/mL, and at a temperature of 80 °C until the PLA was completely dissolved. Then, from 5 mL to 10 mL of PLA solution was quickly poured into a pre-cooled cylinder container (20 mL with dimeter of 2 cm), which was immediately transferred to a refrigerator at -20 °C for phase separation. The newly formed PLA gel was then immersed into ethanol at room temperature for aging the gel. After 1 h, the PLA gels were transferred into excess water for an additional 24 h to make sure the solvent exchanged completely. Fresh water was changed 3 times a day. The aerogels were obtained after freeze drying for 24 h. The porosity, measured according to equation 1 , was 92.42%.

Example 4. Preparation of PLA aerogel 4.

An acetone PLA solution was put in a container at a concentration of 0.12 g/mL, and at a temperature of 80 °C until the PLA was completely dissolved. Then, from 5 mL to 10 mL of PLA solution was quickly poured into a pre-cooled cylinder container (20 mL with dimeter of 2 cm), which was immediately transferred to a refrigerator at -20 °C for phase separation. The newly formed PLA gel was then immersed into ethanol at room temperature for aging the gel. After 1 h, the PLA gels were transferred into excess water for an additional 24 h to make sure the solvent exchanged completely. Fresh water was changed 3 times a day. The aerogels were obtained after freeze drying for 24 h. The porosity, measured according to equation 1 , was 90.46%. Example 5. Preparation of PLA aerogel 5.

An acetone PLA solution was put in a container at a concentration of 0.07 g/mL, and at a temperature of 80 °C until the PLA was completely dissolved. Then, 500 mL of PLA solution was quickly poured into a pre-cooled rectangular container (20 cm width x 40 cm length), which was immediately transferred to a refrigerator at -20 °C for phase separation. The newly formed PLA gel was then immersed into ethanol at room temperature for aging the gel. After 1 h, the PLA gels were transferred into excess water for an additional 24 h to make sure the solvent exchanged completely. Fresh water was changed 3 times a day. The aerogels sheets (of approximately 0.5 cm in thickness) were obtained after freeze drying for 24 h. The porosity, measured according to equation 1 , was 93.79 %.

Example 6. Preparation of PLA aerogel 6.

An acetone PLA solution with Boron Nitride (30 wt% of PLA) was put in a container at a concentration of 0.07 g/mL, and at a temperature of 80 °C until the PLA was completely dissolved. Then, from 5 mL to 10 mL of PLA solution was quickly poured into a precooled cylinder container (20 mL with dimeter of 2 cm), which was immediately transferred to a refrigerator at -20 °C for phase separation. The newly formed PLA gel was then immersed into ethanol at room temperature for aging the gel. After 1 h, the PLA gels were transferred into excess water for an additional 24 h to make sure the solvent exchanged completely. Fresh water was changed 3 times a day. The aerogels were obtained after freeze drying for 24 h. The porosity, measured according to equation 1 , was 88.12 %.

Example 7. Preparation of PLA aerogel 7.

An acetone PLA solution with graphene nanoplatelets (30 wt% of PLA) was put in a container at a concentration of 0.07 g/mL, and at a temperature of 80 °C until the PLA was completely dissolved. Then, from 5 mL to 10 mL of PLA solution was quickly poured into a pre-cooled cylinder container (20 mL with dimeter of 2 cm), which was immediately transferred to a refrigerator at -20 °C for phase separation. The newly formed PLA gel was then immersed into ethanol at room temperature for aging the gel. After 1 h, the PLA gels were transferred into excess water for an additional 24 h to make sure the solvent exchanged completely. Fresh water was changed 3 times a day. The aerogels were obtained after freeze drying for 24 h. The porosity, measured according to equation 1 , was 89.12 %. Exemplary method for the preparation of the composite phase-change material (PCM) of the invention.

The PLA aerogels are put in contact with a phase-change material (PCM) at a temperature above the melting point of the phase-change material. In principle any solidliquid phase-change material may be used, examples being paraffin, polyethylene glycol (with Mn=1000-20000 g/mol), stearic acid (and other fatty acids), stearyl alcohol (and other fatty alcohols), and mixtures thereof. Preferable PCMs are those that melt at a temperature not greater than 120 °C. The aerogels may be put in contact with the PCM for 1 h or longer durations, for example 24 h, in an appropriate oven at a temperature above the melting point of the PCM. After taking the PCM composite out of the oven, any PLA that remains on the surface may optionally be removed with any suitable means, such as filter paper, for example. The PCM absorption ratio can be calculated by their corresponding mass, and the key physical parameters can be characterized by DSC.

Example 8. Preparation of exemplary composite PCM materials of the invention.

The aerogels identified in the table below (which were prepared according to the examples above) were placed in contact with molten PCM identified in the table below at 80 °C for 12 hours in a vacuum oven, for encapsulation of the PCM inside the PLA aerogel. Once the PCM composite was taken out, any free PCM remaining on the PLA aerogel surface was removed with filter paper. The thus obtained composites were characterized by their PCM loading ratios and DSC properties, as indicated in the table below.

Table 2. Composite PCM materials of the invention 1-10, prepared according to the method described above, and their key characteristics.

In table 2, the latent heat and the solidification enthalpy values are expressed as J/g. In table 2, the melting and solidification temperature values are expressed as °C, and they correspond to the peak temperatures of the DSC plots (not shown). In table 2, the thermal conductivity values are expressed as W/(m K).

Example 9. Leak resistance and low phase change temperature of exemplary composite PCM materials of the invention.

This example describes the anti-leaking properties and low phase change temperature of exemplary PLA-aerogel composite materials of the invention. The chosen PCM for this particular example was PEG 400, PEG 600 and PEG 1000, and the samples were prepared according to the same methodology described in the examples above. The relevant phase transition temperatures and other phase transition parameters are listed in table 2. The sample was weighted at time 0 and upon heating at 50 °C in a hotplate with one layer of filter paper for 20, 40, 60 and 90 minutes. Figure 3 shows the composite materials after 90 minutes of heating, and the evolution of the weight of the samples with time.

Example 10. Leak resistance comparison between an exemplary composite PCM material of the invention and a comparative composite PCM material.

This example shows the superior anti-leaking properties of an exemplary PLA-aerogel composite material of the invention, by direct comparison with a chitosan-aerogel. The chosen PCM for this particular example was paraffin.

The samples were prepared according to the same methodology described in the examples above. The sample was weighted at time 0 and upon heating on a filter paper at 80 °C in a hotplate for 15, 30, 45 and 60 minutes.

Figure 4 shows the evolution of the weight of the samples with time.

Claims

1. Shape-stabilized composite phase-change material comprising: a) a polylactic acid aerogel; and b) a phase-change material; wherein the phase-change material occupies an internal volume of the aerogel.

2. The composite according to claim 1 , wherein the phase-change material is a solidliquid phase-change material.

3. The composite according to any one of claims 1 or 2, comprising at least 30%, preferably at least 50%, more preferably at least 70%, by weight percent of the phase-change material with respect to the total weight of the composite material.

4. The composite according to any one of claims 1 to 3, wherein the polylactic acid is selected from poly-L-lactic acid, poly-D-lactic acid, poly-D, L-lactic acid or mixtures thereof.

5. The composite according to any one of claims 1 to 4, wherein the polylactic acid aerogel further comprises at least one functional additive.

6. The composite according to claim 5, wherein the functional additive is selected from the group consisting of poly(caprolactone), poly(butylene succinate), polyhydroxyalkanoates, polybutylene adipate terephthalate, cellulose, alginate, chitosan and mixtures thereof.

7. The composite according to claim 5, wherein the functional additive is selected from the group consisting of functionalized metal foams, functionalized ceramic porous materials, thermal conductive fillers, flame retardants, pigments and mixtures thereof.

8. The composite according to claim 7, wherein the thermal conductive fillers are selected from the group consisting of Boron Nitride, graphene, MXene, biochar, carbon fiber and thermally conductive ceramic powders.

9. The composite according to any one of claims 7 or 8, wherein the flame retardants are selected from the group consisting of ammonium polyphosphate, phytic acid, chitosan, expandable graphite, lignin derivatives, polyhedral oligomeric silsesquioxane, metal-organic frameworks and layered double hydroxides.

10. The composite according to any one of claims 1 to 9, wherein the phase-change material is selected from alkanes, paraffin waxes, beeswax, metal salts, polyethylene glycols, carboxylic acids, fatty acids, fatty acid esters, fatty alcohols, sugar, sugar alcohols.

11. The composite according to claim 10, wherein the fatty acids are selected from caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, lignoceric acid, cerotic acid or mixtures thereof.

12. The composite according to any one of claims 10 or 11 , wherein the fatty alcohols are selected from 1 -heptanol, 1 -octanol, pelargonic alcohol, 1 -decanol, undecyl alcohol, lauryl alcohol, tridecyl alcohol, myristyl alcohol, pentadecyl alcohol, cetyl alcohol, palmitoleyl alcohol, heptadecyl alcohol, stearyl alcohol, oleyl alcohol, nonadecyl alcohol, arachidyl alcohol, heneicosyl alcohol, behenyl alcohol, erucyl alcohol, lignoceryl alcohol, ceryl alcohol, montanyl alcohol, 1-nonacosanol, myricyl alcohol, 1-dotriacontanol, or mixtures thereof.

13. A method for the preparation of a composite phase-change material as defined in any one of claims 1 to 12, wherein a polylactic acid aerogel is put in contact with a molten phase-change material, and wherein said polylactic acid aerogel is prepared by a method comprising the steps of: a) dissolving a polylactic acid in a first solvent; b) cooling the solution of step (a) for phase separation, thus obtaining a first polylactic acid gel; c) immersing the first polylactic acid gel formed in step (b) in a second solvent for solvent exchange, thus obtaining a second polylactic acid gel; d) optionally washing the second polylactic acid gel of step (c) with water, ethanol or mixtures thereof; and e) freeze drying the second polylactic acid gel obtained in step (c), or the washed second polylactic acid gel obtained in step (d), to obtain the aerogel. Use of a composite phase-change material according to any one of claims 1 to 12 in thermal energy storage or thermal management including peak energy reduction, preferably in electronics, power electronics, solar energy, batteries, buildings, waste heat recovery, air-conditioning, temperature-adaptable greenhouses and textiles. A product of manufacture, an electronic device, a solar energy system device, an energy storage device, an electronic device, a computer, a medical device, a storage unit, a building or building material, a container, an insulation or construction material, an automotive material, a vehicle, a boat, an airplane, a weapon or weapon system, industrial machinery, a pharmaceutical or a drug or a food package or storage device or container, a textile, a clothing or an apparel, footwear, a bedding or bedding system, a flame retardant material, comprising a composite phase-change material according to any one of claims 1 to 12.