Aerogels from post-consumer packaging waste

Abstract

A simple method, at room temperature, for the preparation of poly(ethylene terephthalate) (PET)-based aerogels, using virgin PET or recycled packaging or recycled PET (rPET) industrial wastes as raw material. It consists in the dissolution of PET followed by precipitation and subsequent gelation, and then the aerogel is obtained by freeze-drying. Unreinforced and organic or inorganic fiber-reinforced PET aerogels were prepared using this method. The resulting aerogels exhibit good mechanical properties and do not break during handling. They present thermal insulation capacity at the level of commercial insulators. These aerogels have a porosity between 80 and 86%, a bulk density between 150 and 210 kg / m3, a thermal conductivity between 35 and 50 mW / m.K, and a Young's modulus between 1.5 and 3.5 MPa, with reinforced aerogels being more mechanically resistant, but more flexible (lower modulus).

Description

AEROGELS FROM POST-CONSUMER PACKAGING WASTE

[0001] This invention relates to polymeric aerogels based on poly(ethylene terephthalate) (PET) from post-consumer packaging waste, unreinforced and reinforced with fibers, and the respective preparation method.

[0002] Poly(ethylene terephthalate) (PET) is an aliphatic-aromatic polyester widely used in the packaging industry, mainly in beverage bottles. It is estimated that PET represents 16% of total plastic consumption in Europe [1] and it is expected that the market for this polyester will reach 114.7 million tons by 2028, with a compound annual growth rate (CAGR) of 5.2% [2].

[0003] PET, like many other polymers used in the packaging industry, is a non-biodegradable polymer, and its disposal in the environment presents serious risks for the future of the planet. Regarding this, several efforts have been made to recycle post-consumer PET waste in a more effective way. In 2020, around 2.4 million tons of PET were mechanically recycled in the EU27+3. The recycled PET (rPET) obtained was then used in the production of trays and sheets (32%), and bottles for food contact (29%) [3]. However, it should be noted that, in this type of bottles, rPET is used blended with virgin PET, up to 17% [3]. Despite the growing trend towards PET recycling, it should be noted that the possibilities for mechanical recycling are not unlimited. Mechanical recycling can lead to irreversible changes in the PET structure due to undesirable secondary reactions promoted by both thermal and mechanical action [4]. These structural changes generally deteriorate the mechanical and rheological properties of PET, making rPET difficult to be used in the same applications as virgin material. Furthermore, repeated mechanical recycling of PET can lead to the formation of acetaldehydes in significant quantities, which may hinder the use of rPET in food contact applications [1]. Given these facts, a question arises: how to transform rPET, which has gone through several recycling cycles, into a useful and value-added material, thus avoiding its disposal in the environment? A possible answer to this question may involve the use of rPET in the production of advanced products, namely aerogels.

[0004] Aerogels are known for their exceptional properties, such as capacity for thermal and acoustic insulation, adsorption of pollutants, storage of active compounds, among other applications. They are synthetic, porous and ultra-lightweight materials, derived from a gel in which the liquid component has been replaced by a gas, usually air. The combination of very small pores (mesopores) with high porosity (>90%) leads to the unique properties of aerogels: solids with very low thermal conductivity and density [5-9]. For about 70 years, all aerogels produced were inorganic, with the first polymeric aerogel being developed in the late 1980s. Since then, several polymer-based aerogels have been developed. Compared to silica-based aerogels, polymeric aerogels are cheaper, less brittle and do not easily disintegrate during handling

[0010] .

[0005] Koh et al.

[0012] developed PET aerogels with fibers obtained from plastic bottles and poly(vinyl alcohol) (PVA), using glutaraldehyde (GA) as crosslinker. In addition to the covalent bonds between PET and PVA mediated by GA, hydrogen bonds also contributed to the gel formation. This gel was then freeze-dried to obtain hydrophobic aerogels, with very low density (0.007–0.026 g / cm3), highly porous network structure (98.3–99.5%) and low elastic modulus (1.16–2.87 kPa). They also exhibited low thermal conductivities (0.035–0.038 W / (m.K)). In another approach, Roy et al.

[0013] developed a PET aerogel through the electrospinning (ES) process, using waste PET bottles as raw material. PET bottles were crushed and dissolved in a mixture of trifluoroacetic acid (TFA) and dichloromethane (DCM) (1:3 ratio), and the solution was subjected to the ES process. The fibers obtained were then collected from a solvent bath container with an alcohol. After removing the fibers from the solvent bath, they were heated to 120 °C in a hydrothermal treatment in an autoclave, containing 1% (percentage by mass) of an aqueous solution of 1,3-benzene disulfonyl azide. The gel-like sample was washed with deionized water and ethanol. Finally, the sample was freeze-dried, resulting in a fibrous PET aerogel with a three-dimensional structure. The final product was described as extremely flexible, soft, yet structurally durable. It has also shown an exceptionally high ability to remove several heavy metals from polluted water. Puguan et al.

[0014] developed a flexible nanofibrous aerogel with laser-cut perforations for sound absorption and insulation. First, PET waste was subjected to ES to obtain uniform nanofibers. These nanofibers were treated with an alkaline solution and then mixed with a PVA solution. Subsequently, the mixture was placed in a mold and freeze-dried to produce the 3D fibrous aerogel. The aerogel was subjected to a crosslinking step, mediated by GA / HCl vapors, at 25 ºC. The resulting aerogel was characterized as flexible and ultralightweight, with a Young's modulus of 2.28×10−2MPa and a noise reduction coefficient of 0.37 (at a sand density of 465 g / m2). In laser-drilled aerogels, a noise reduction coefficient of 0.54 was obtained (at a sand density of 930 g / m2). In turn, Gao et al.

[0015] used PET bottles to produce an aerogel with high thermomechanical performance, fire resistance and high compression resistance. PET bottles were crushed until they turned into a powder, and then dissolved in a solution of phenol and 1,1,2,2–perchloroethylene (1:1) and heated to 68 ºC. An aqueous solution of aluminum dihydrogen phosphate was added to this solution, as well as aluminum isopropoxide, forming an aluminum phosphate network. This solution was then continuously heated to 95 ºC-97 ºC while stirring. The reaction was considered complete when the mixture turned white. Finally, the resulting solution was poured into a centrifuge tube and freeze-dried to obtain the aerogel. The aerogel was able to withstand temperatures of up to 1300 ºC without degrading or releasing smoke. The maximum Young's modulus and compressive strength reported were 40.3 MPa and 29.1 MPa, respectively.

[0006] Common to all the cited works is the complexity of the used methods to prepare the aerogels, which include, for example, the application of temperature or crosslinking steps. Thus, in the present invention we present a simple procedure to prepare rPET aerogels by dissolution / gelation, carried out at room temperature, and finished by a freeze-drying step. The method also allows the incorporation of fibers of different natures as aerogel reinforcement, which opens the possibility to easily adjust its mechanical properties.General description

[0007] The present invention is related to polymeric PET aerogels and their reinforcement with organic and inorganic fibers. The percentage of fibers in the aerogel is between 8 and 12% w / w, depending on the type of fiber used. The bulk density of the aerogels is between 150 and 210 kg / m3, and their thermal conductivity is between 35 and 50 mW / m.K.

[0008] The present invention also relates to a method of preparing aerogels unreinforced and reinforced with fibers, which comprises the following steps:

[0009] – dissolving in an organic solvent, at room temperature and under stirring, the PET from recycled bottles or packaging wastes, or rPET from industrial wastes or virgin PET packaging;

[0010] – adding ethanol (non-solvent) to the previous solution, under magnetic stirring, until the formation of a precipitate;

[0011] – letting the precipitate dissolve and, when an increase on the solution turbidity and viscosity is observed, remove the magnet and let the solution stand to rest in a mold (container);

[0012] – in the case of reinforced aerogels, adding the fibers to the mold after the previous redissolution step;

[0013] – removing the gel from the mold and wash it with ethanol, followed by washing with water;

[0014] – drying the gel by freeze-drying to obtain the aerogel.

[0015] In one embodiment, the solvent is trifluoroacetic acid (TFA). In another embodiment, the solvent is a mixture of TFA with dichloromethane (DCM), with variable composition.

[0016] In one embodiment, the entire volume of ethanol is added at once.

[0017] In another embodiment, the volume of ethanol is added in steps.

[0018] In one embodiment, the amount of fibers added varies between 8 and 12% w / w.

[0019] In one embodiment, the fibers are added in dispersed form.

[0020] In another embodiment, the fibers are added in the form of a felt.

[0021] In one embodiment, the gel is immersed in liquid nitrogen, to freeze, before being subjected to freeze-drying process to obtain the dry material.

[0022] In another embodiment, the gel is placed in the refrigerated chamber of the freeze-dryer, to freeze, before being subjected to the drying step.

[0023] The permanent need to reduce the disposal of plastic in the environment means that it is necessary to think about strategies that allow transforming plastic that can no longer be recycled, due to the marked loss of properties, into materials with greater added value. Thus, the objective of the present invention is focused on the development of PET-based aerogels, through a simple method that is carried out at room temperature. The developed aerogels represent a unique opportunity to value plastic waste through the production of an advanced material, following the principles of circular economy.

[0024] The aerogels described here were characterized in terms of their chemical structure, morphology, thermal properties, and mechanical properties. The aerogels showed good thermal stability and thermal conductivity similar to commercial thermal insulators. They also presented low density, due to their highly porous structure. The aerogels, both unreinforced and reinforced with fibers, did not break during handling and exhibited good mechanical properties.

[0025] For an easier understanding of the present application, figures representing the preferred forms of implementation are attached, which, however, are not intended to limit the technique described in this document.Fig.1

[0026] shows the visual appearance of a) unreinforced rPET aerogel, b) unreinforced rPET aerogel before drying.Fig.2

[0027] shows the proton nuclear magnetic resonance (1H NMR) spectrum of the rPET aerogel.Fig.3

[0028] shows the pore size distribution of the aerogel.Fig.4

[0029] shows SEM micrographs of the internal morphology of rPET aerogels: (A) rPET aerogel (rPETa) with 5 kx magnification; (B) rPETa with a magnification of 10 kx; (C) rPET aerogel reinforced with quartz fibers (rPETa_qz), with a magnification of 10 kx; (D) rPETa_qz with a magnification of 1 kx; (E) rPET aerogel reinforced with polyester fibers (rPETa_pe), with a magnification of 10 kx; (F) rPETa_pe magnified 500x.Fig.5

[0030] [Fig.5] shows the stress-strain curves obtained in the mechanical compression tests for unreinforced and reinforced rPET aerogels. The 'insets' in [Fig.5] show the macroscopic appearance of the aerogels after the compression test.

[0031] Now, the preferred ways to implement this application will be described in detail with reference to the attached drawings. However, they are not intended to limit the scope of this application.

[0032] The present invention relates to polymeric aerogels based on PET and the method of preparation thereof.

[0033] In one embodiment, the PET is selected from rPET or virgin PET.

[0034] In one embodiment, the aerogels of the present invention can be based on rPET () and can be reinforced with inorganic fibers, for example quartz fibers, or organic fibers, such as polyester fibers.

[0035] In the context of the present invention, the room temperature is defined in the range between 20 and 25ºC.

[0036] The method used to prepare the aerogels comprises the following steps:

[0037] – In a mold container, dissolving PET in an organic solvent, under magnetic stirring and at a temperature between 20 and 25ºC;

[0038] – Proceeding with the precipitation and gelation of the polymer by adding ethanol to obtain a gel;

[0039] – Allowing the formed gel to rest for up to 12 hours;

[0040] – Washing the gel after removing it from the container;

[0041] – Drying the gel by freeze-drying, to obtain the aerogel;

[0042] In one embodiment, PET may come from rPET from recycled water bottles, food packaging wastes, fabric fibers, or other types of recycled PET.

[0043] In one embodiment, the packaging can be composed of virgin PET or from industrial PET wastes.

[0044] In one embodiment, the organic solvent corresponds only to TFA.

[0045] In another embodiment, the organic solvent corresponds to a mixture of DCM with TFA. The TFA / DCM volumetric ratio is between 0.4 to 10.

[0046] In another embodiment, the TFA / DCM volumetric ratio is 0.4.

[0047] In yet another embodiment, the concentration of PET in the solution is between 10 and 20% (m / v), preferably 15% (m / v).

[0048] In one embodiment, the volume of ethanol to be added ranges between 10 and 25% (v / v), preferably 14% (v / v).

[0049] In one embodiment, the addition of ethanol can be done at once.

[0050] In another embodiment, the addition of ethanol could be in steps, and the number of additions can vary between 2 and 3, with varying volumes.

[0051] In yet another embodiment, the gel is considered to be formed when the solution acquires turbidity and increases viscosity.

[0052] In one embodiment, precipitation and gelation of the polymer, was made by adding ethanol, in the presence of fibers.

[0053] Washing and drying of the gel allows to remove the organic solvents, ethanol and water.

[0054] In one embodiment, the aerogel comprises 100% w / w of PET.

[0055] In one embodiment, the amount of fibers added is between 8 and 12% w / w of the total weight of the aerogel.

[0056] In one embodiment, a fiber felt is inserted in the solution when it begins to gain turbidity;

[0057] In another embodiment, the fibers are dispersed in the solution when it begins to gain turbidity.

[0058] In one embodiment, the gel is washed with ethanol up to 5 times, with the washes lasting up to 24 h.

[0059] In another embodiment, the gel is washed in distilled water, up to 5 times, with each wash lasting up to 24 hours.

[0060] In one embodiment, the gel washed with ethanol and water is pre-frozen by immersion in liquid nitrogen before being subjected to freeze-drying process to obtain the aerogel (dry material).

[0061] In another embodiment, the gel washed with ethanol and water is pre-frozen in the chilled chamber of the freeze-dryer, for a period up to 6 hours, before the drying process to obtain the aerogel (dry material).

[0062] The aerogels presented in this invention have a porosity between 80 and 86%, bulk density between 150 and 210 kg / m3, a thermal conductivity between 35 and 50 mW / m.K, and a Young's modulus between 1.5 and 3.5 MPa, with reinforced aerogels being more mechanically resistant but more flexible (lower modulus).

[0063] In one embodiment, aerogels are for use as thermal insulators, acoustic barriers, adsorption materials, delivery systems for active ingredients, or carriers of active ingredients.Examples

[0064] Materials:

[0065] Dichloromethane (DCM; 99.95%) and ethanol (EtOH; 96%) were purchased from José Manuel Gomes dos Santos, Lda (Odivelas, Portugal). Trifluoroacetic acid (TFA; >99.0%) was obtained from TCI Europe (Zwijndrecht, Belgium). Deuterated trifluoroacetic acid (TFA-d) and deuterated chloroform (CDCl3) were purchased from Eurisotop (St. Aubin, France). All chemicals were used without further purification. Distilled water was obtained on site via reverse osmosis using equipment from Diwer Technologies. Recycled poly(ethylene terephthalate) (rPET) was obtained from waste PET bottles that had already been recycled. The quartz and polyester fibers were obtained from Saint-Gobain Quartz and Tecidos de Coimbra, respectively.

[0066] Preparation of rPET aerogels:

[0067] First, the rPET bottles were properly washed to remove all impurities and randomly cut into small pieces. A rPET solution was prepared with a mixture of TFA:DCM solvents. The solution was magnetically stirred for about 10-15 minutes, until the polymer was completely dissolved. EtOH was then added and a precipitate was formed. Stirring continued until the precipitate dissolved and the solution began to increase in turbidity and viscosity. At this point, the magnet was removed and the solution was left to stand until a gel formed in the mold container (up to 12 h). Both dissolution and gelation steps occurred at room temperature. To remove residual solvent and other contaminants, the gel was washed up to 5 times with EtOH (up to 24 h / wash). Then, four more washes were carried out with distilled water (up to 24h / wash) to remove the EtOH, so that the material was ready for freeze-drying, to be dried. At this stage, the gel was quickly pre-frozen in liquid nitrogen or in the chilled chamber of the freeze-dryer, for a period up to 6 hours. Subsequently, the gel was freeze-dried for approximately 20 h. For fiber-reinforced rPET aerogels, the process was identical to unreinforced counterparts. The only difference was that at the point at which the precipitate dissolved and the turbidity and viscosity of the solution began to increase, the magnet was removed and the commercial fibers were immersed in the solution.

[0068] Table 1 presents an example of the conditions used in the preparation of unreinforced and reinforced rPET aerogels, in terms of polymer concentration and solvent volumes.

[0069] Table 1: Example of preferred conditions used in the preparation of unreinforced and reinforced rPET aerogels (volumes – V – and masses – m).

[0070] SamplePolymer concentration(% m / v)VTFA(mL)VDCM(mL)VEtOH(mL)mquartz fibers(g)mpolyester fibers(g)rPETa15251--rPETa_qz0,114-rPETa_pe-0,136

[0071] In Table 2 other examples of conditions used in the preparation of the aerogels that are the subject of this invention are presented. All preparations were carried out at room temperature.

[0072] Table 2: Example of other preparation conditions for unreinforced rPET aerogels.

[0073] SamplesPolymer concentration%(m / v)SolventsVsolvent(mL)VEtOH(mL)ObservationsrPETa_115TFA71.5(0.5+0.5+0.5)A cylindrical glass container with a Teflon lid was used. The addition of EtOH was in steps. On the third addition of ethanol, a precipitate was formed, which could not be dissolved. The solution began to gain viscosity and turbidity at room temperature and under magnetic stirring. The magnetic stirrer and precipitate were removed. A fragile gel was obtained.rPETa_215TFA / DCM2 / 51.2(0.5+0.5+0.2)After the third addition of EtOH, small precipitates were formed, with the concomitant formation of the gel. To remove the gel from the glass container, the lid was left open for 5-10 min and the gel ended up detaching from the walls of the container, resulting in an intact monolith.rPETa_315TFA71.3 (0.5+0.5+0.2+0.1)Small precipitates were formed, but gel formation occurred. The gel adhered to the glass, making more complex to remove it from the mold.rPETa_415TFA / DCM2 / 51.25(1+0.25)Small precipitates formed, but gel formation occurred, as previously.rPETa_515TFA / DCM2 / 50.9(0.7+0.1+0.1)The glass container was replaced by a plastic one, resistant to the solvents’ mixture. The solution was stirred, and the gel was formed after 1 h.rPETa_615TFA / DCM2 / 51(0.8+0.1+0.1)Plastic container. Gel formation occurred, which was easily removed from the container.rPETa_715TFA / DCM2 / 51Single addition of the entire volume of EtOH was tested. Gel formation was observed, before the precipitate being completely dissolved.rPETa_815TFA / DCM2 / 50.9Adding a small amount of EtOH at once, resulted in longer time for gel formation.No precipitate formation was observed.

[0074] Characterization techniquesof therPETaerogels:

[0075] 1H NMR analysis, at 25 °C, was performed on a Bruker Avance 400 MHz spectrometer, using a 5 mm broadband NMR probe. Deuterated trifluoroacetic acid (TFA-d) and deuterated chloroform (CDCl3) were used as solvents (33 / 67% v / v).

[0076] The bulk density (ρb) of the rPET aerogels was determined by weighing them with a 10-4g precision microbalance, and measuring their dimensions, using a caliper with a resolution of 0.01 mm, and then by using equation (1):

[0077] Pb = m / V (1)

[0078] Wheremis the mass of the aerogel andVis the volume of the aerogel, including the volume of all pores.

[0079] The skeletal density (ρs) of the unreinforced rPET aerogel matrix was measured by helium (He) pycnometry using an Accupyc 1330, from Micromeritics Instrument Corp, after the samples were milled, to open the closed pores. The value obtained was 1045 ± 38 kg / m3.

[0080] The porosity of the rPET aerogels was determined from the apparent and skeletal densities using equation (2):

[0081] Porosity (%) = 1 - (2)

[0082] The morphological analysis of rPET aerogels was carried out on a field emission scanning electron microscope (FESEM), ZEISS MERLIN Compact / VPCompact, Gemini II. Prior to SEM analysis, samples were coated with gold for 30 seconds by physical vapor deposition.

[0083] The specific surface area (SBET) of the rPET aerogels was measured by adsorption of N2gas at 77 K (ASAP 2000 from Micromeritics Instrument Corp.) and application of the Brunauer-Emmett-Teller (BET) theory in the relative pressure range 0.05–0.3 of the adsorption isotherm.

[0084] To evaluate the presence of macropores, equations (3) and (4) were used, for comparison with the porosimetry results obtained by the N2desorption isotherm with the Barrett, Joyner and Halenda (BJH) model (with pore size limitation from 1 to 300 nm):

[0085] Vp= (3)

[0086] dp= (4)

[0087] Here,Vpis the total pore volume,SBETis the specific surface area anddpis the average pore diameter (all pore ranges).

[0088] The thermal conductivity of the rPET aerogels was measured by a transient method, using a thermal constant analyzer TPS 2500 S (Hot Disk®), at 22 ºC, placing the 5501 sensor (diameter = 6.4 mm) between two replicates of the same sample.

[0089] The thermal stability of aerogels and rPET fibers was measured by thermogravimetric analysis (TGA) using the STA 449 F3 Jupiter instrument from TA Instruments. The analysis was carried out between 25 and 600 ºC, under an N2atmosphere, at a heating rate of 10 ºC / min.

[0090] The mechanical properties of the rPET aerogels were measured via uniaxial compression tests using an Inspekt mini-series testing machine (Hegewald & Peschke) equipped with a 3000 N load cell. Samples with 2.8 cm in diameter and with 1 cm in thickness were compressed until fracture / densification, at a strain rate of 1 mm / min.

[0091] Results ofaerogelscharacterization:

[0092] shows the visual appearance of unreinforced rPET aerogels.

[0093] The unreinforced rPET aerogel was characterized in terms of its chemical structure, by1H NMR, to check whether it had undergone any changes due to the preparation process. The1H NMR spectrum is shown in.

[0094] The rPET aerogel has peaks (aandb) at around 8.16 ppm, corresponding to protons of the aromatic ring, and at around 4.82 ppm, attributed to protons of the aliphatic chain (c)

[0016] . It is also possible to observe two peaks, at 4.67 ppm and 4.16 ppm, which may be associated with the presence of diethylene glycol (DEG) in the PET structure. DEG is generally formed as a by-product during polycondensation and incorporated into the polyester chain

[0017] . The fact that the1H NMR spectrum presents peaks at the expected chemical shifts and with the expected relationship between integrals, indicates that the preparation process did not lead to changes in the chemical structure of the rPET.

[0095] Table 3 presents the structural properties of rPET aerogels.

[0096] Sampleρb(g / cm3)Porosity (%)SBET(m2 / g)Vpore(cm3 / g)dpore(nm)k(mW / (m.K))T5%(ºC)rPETa0.151 ± 0.00986176 ± 35.6712839.2393rPETa_qz0.163 ± 0.00484119 ± 25.1717445.3(-)rPETa_pe0.207 ± 0.0158096 ± 23.8716147.4(-)

[0097] Regarding the bulk density of the aerogels (Table 3), it is observed that the rPET aerogel reinforced with quartz fibers (rPETa_qz) presents aρbslightly higher than the unreinforced aerogel, while the one reinforced with polyester fibers (rPETa_pe) presents a significant increase in theρbvalue. In the case of rPETa_pe, the result may be a consequence of the greater interaction between the fibers and the aerogel matrix, due to their similar chemical nature. Furthermore, the possibility of some degree of dissolution of the fibers and consequent fusion with the rPET aerogel cannot be ruled out. This situation would also contribute to a denser aerogel matrix. For rPETa_qz, although gelation occurs in a condition favored by a larger surface available for nucleation (thin fibers), the lowρbvalue observed suggests a low affinity between the fibers and the aerogel matrix, which can lead to a significant number of voids inside the aerogel and enlargement of the pores, as observed in Table 3. Also, since the fibers are inorganic, they resist better to chemical attacks, maintaining the integrity of the felt and more efficiently hindering the shrinkage of the aerogel during drying.

[0098] The Pbvalues ​​are similar to those reported in the literature for unreinforced polymeric aerogels based on polypropylene

[0018] and polyamide

[0019] , and also for silica aerogels reinforced with Kevlar pulp

[0020] .

[0099] The determination of the specific surface area (SBET) of the aerogels was done by adsorption of N2 gas and applying the BET theory (Table 3). The adsorption isotherms for these aerogels present an IUPAC type IV classification with H2 hysteresis

[0021] , this profile being typical of mesoporous materials

[0022] . The BET surface area of ​​the rPET aerogel is larger than that of the reinforced aerogels, due to the lower density / higher porosity of the unreinforced samples. It is a fact that the addition of fibers tends to contribute to a greater extent of gelation due to the presence of the fiber surface as nucleation points. The surface area decreases with increasing density and decreasing porosity, as expected.

[0100] The pore size distribution in the aerogels can be observed in, relatively to the size range of mesopores and small macropores.

[0101] The pore size distribution ranges from 5 to 40 nm, indicating that the aerogels have an extensive network of mesopores (2-50 nm), although some micropores (diameter < 2 nm) were also detected. Furthermore, it is possible to observe that the curves tend to increase in the macropore range (> 50 nm), which cannot be measured by this technique. This is evidence of the existence of macropores in the samples.

[0102] It should be noted that the values ​​of the average pore sizes (dp) presented in Table 3 were determined considering equations (3) and (4), in order to include all pore size ranges (micropores, mesopores and macropores). The results obtained confirm that the samples present some macropores, however, not very large in size, given the proximity of the values ​​obtained with the range of sizes in.

[0103] The thermal conductivity of the aerogels (k) was determined to evaluate their potential for use as thermal insulator. The thermal conductivity of silica aerogels at room temperature ranges from 12 to 20 mW / m.K, which is much lower than that of the most common thermal insulators

[0023] . The thermal conductivity of the prepared unreinforced rPET aerogels is higher than that of the silica aerogels used for thermal insulation. This is a consequence of the lower porosity of the former and also of the larger pores present in the PET matrix when compared to pure silica aerogels (mainly mesoporous). These larger pores tend to contribute to higher thermal conductivities, since the Knudsen effect is not relevant in this context. Comparing the thermal conductivity values ​​of reinforced and unreinforced rPET aerogels, the values ​​increase with increasing bulk density (decreasing porosity) and larger pore sizes, as expected.

[0104] When comparing the thermal conductivities of the unreinforced rPET aerogel with reinforced silica aerogels, it was found that the former has similar values ​​to silica aerogel composites reinforced with glass wool or recycled tire textile fibers

[0024] , or with Twaron fibers

[0020] . Although the aerogels produced in this work cannot be considered superinsulating materials, the range of thermal conductivities obtained allows them to be considered insulating materials [7].

[0105] The thermal stability of the rPET aerogel was evaluated in N2atmosphere, in a temperature range of 25 ºC - 600 ºC. The temperature obtained for 5% degradation is excellent, considering the usual service temperature ranges of insulating materials. However, it restricts its use for insulating furnaces or other high-temperature equipment.

[0106] The internal morphology of the unreinforced and reinforced rPET aerogels was evaluated by FESEM, as shown in.

[0107] Regarding the rPET aerogel ((A) and (B)), it has an apparently cohesive internal structure with high porosity. Its “lace-like” appearance features a large number of visible macropores, consistent with the average pore size in Table 2, as well as mesopores. The internal morphology of the reinforced rPETa_qz aerogels ((C) and (D)) shows that the internal structure appears to be similar to that of unreinforced rPET, certainly due to the very close density values. On the other hand, for the rPETa_pe aerogels ((E) and (F)), the porous structure appears to be slightly more closed / denser than the unreinforced one. This result may be a consequence of the higher bulk density, but also of the cut performed on the sample before the SEM analysis. Unfortunately, all attempts to break the sample into liquid N2failed and it was decided to cut the sample at room temperature with a blade. This procedure led to plastic deformation of the polymer matrix, contributing to the closing of the pores on the surface with dragged polymer. Regarding the interaction of the polymeric matrix with the fibers, it can be observed that both types of fibers are lightly covered by the polymeric matrix, indicating a good interaction. However, this is much more evident in the rPETa_pe aerogel ((F)), as expected.

[0108] The stress-strain curves obtained from the destructive compression test of unreinforced and reinforced rPETa are shown in [Fig.5].

[0109] The curves of the rPET aerogels show the linear elastic stage and the densification stage, a common characteristic of aerogels [12, 25]. When the curve shows a shoulder, significant plastic deformation occurs due to several progressive fractures, as can be seen in the case of the unreinforced aerogel. In the case of rPETa_qz, an almost linear increase in compressive stress is observed before reaching the densification stage, which is not verified for rPETa and rPETa_pe. This indicates that the sample with quartz fibers presents greater flexibility, which can be explained by the maintenance of the felt shape and mechanical resistance. Regarding the start of the densification stage, it is observed that it starts later in the case of rPETa_qz, which is in line with the results previously presented. This aerogel also reaches the highest deformation values ​​when subjected to compression. Analyzed together, these results show that the rPETa_qz aerogel is mechanically more robust than rPETa and rPETa_pe due to its ability to better adapt to deformation under tension loads. The Young's modulus values increase in the following order, as expected: rPETa_qz (1873 kPa) < rPETa (2338 kPa) < rPETa_pe (3193 kPa). It is possible to see that the inclusion of polyester fibers makes the aerogels more rigid, and this is a consequence of the denser structure of the rPETa_pe aerogel relatively to the other aerogels studied.

[0110] The aerogels described in this work have a higher modulus of elasticity than silica aerogels reinforced with aramid fibers

[0020] or rPET aerogels formed by a crosslinking reaction with PVA in the presence of GA

[0012] .

[0111] This description is obviously not restricted in any way to the forms of implementation presented here, and anyone with an average knowledge of the field can offer many possibilities for modifying it without departing from the general idea defined by the claims. The preferred forms of implementation described above can obviously be combined with each other.References

[0112] 1. Nisticò R. Polymer Testing 2020;90:106707.

[0113] 2. https: / www.blueweaveconsulting.com / report / polyethylene-terephthalate-pet-resin-market#:~:text=The%20global%20polyethylene%20terephthalate%20(PET)%20resin%20market%20was%2080.9%20million,%2D2028%20(forecast%20period(access in July 2023).

[0114] 3. PET MARKET IN EUROPE STATE OF PLAY 2022 (https: / www.unesda.eu / pet-collection-rates / ) (access in July 2023).

[0115] 4. Nait-Ali LK, Colin X, and Bergeret A. Polymer Degradation and Stability 2011;96(2):236-246.

[0116] 5. Thapliyal PC and Singh K. Journal of Materials 2014;2014:127049.

[0117] 6. Maleki H, Durães L, and Portugal A. Journal of Non-Crystalline Solids 2014;385:55-74.

[0118] 7. Linhares T, Pessoa de Amorim MT, and Durães L. Journal of Materials Chemistry A 2019;7(40):22768-22802.

[0119] 8. Lamy-Mendes A, Pontinha ADR, Alves P, Santos P, and Durães L. Construction and Building Materials 2021;286:122815.

[0120] 9. Mekonnen BT, Ding W, Liu H, Guo S, Pang X, Ding Z, and Seid MH. Journal of Leather Science and Engineering 2021;3(1):25.

[0121] 10. Zuo L, Zhang Y, Zhang L, Miao Y-E, Fan W, and Liu T. Materials 2015;8(10):6806-6848.

[0122] 11. Nguyen PTT, Do NHN, Goh XY, Goh CJ, Ong RH, Le PK, Phan-Thien N, and Duong HM. Waste and Biomass Valorization 2022;13(4):1825-1847.

[0123] 12. Koh HW, Le DK, Ng GN, Zhang X, Phan-Thien N, Kureemun U, and Duong HM. Gels 2018;4(2):43.

[0124] 13. Roy S, Maji PK, and Goh K-L. Chemical Engineering Journal 2021;413:127409.

[0125] 14. Puguan JMC, Pornea AGM, Ruello JLA, and Kim H. ACS Applied Polymer Materials 2022;4(4):2626-2635.

[0126] 15. Gao B, Sun X, Yao C, and Mao L. Polymer 2022;254:125074.

[0127] 16. Han Z, Wang Y, Wang J, Wang S, Zhuang H, Liu J, Huang L, Wang Y, Wang W, Belfiore LA, and Tang J. Materials 2018;11(4):587.

[0128] 17. Hovenkamp SG and Munting JP. Journal of Polymer Science Part A-1: Polymer Chemistry 1970;8(3):679-682.

[0129] 18. Lang XH, Zhu TY, Zou L, Prakashan K, and Zhang ZX. Progress in Organic Coatings 2019;137:105370.

[0130] 19. Williams JC, Nguyen BN, McCorkle L, Scheiman D, Griffin JS, Steiner SA, III, and Meador MAB. ACS Applied Materials & Interfaces 2017;9(2):1801-1809.

[0131] 20. Almeida CMR, Ghica ME, Ramalho AL, and Durães L. Journal of Materials Science 2021;56(24):13604-13619.

[0132] 21. Sotomayor FJ, Cychosz KA, and Thommes M. Accounts of Materials & Surface Research 2018;3(2):34-50.

[0133] 22. Tompsett GA, Krogh L, Griffin DW, and Conner WC. Langmuir 2005;21(18):8214-8225.

[0134] 23. Fu Z, Corker J, Papathanasiou T, Wang Y, Zhou Y, Madyan OA, Liao F, and Fan M. Journal of Building Engineering 2022;57:104814.

[0135] 24. Lamy-Mendes A, Pontinha ADR, Santos P, and Duraes L. Materials 2022;15(22).

[0136] 25. Li Y, Ma B, Zhang R, and Luo X. Polymer 2022;253:125035.

Claims

Polymeric aerogels comprising poly(ethylene terephthalate) comprising a porosity between 80 and 86%, a bulk density between 150 and 210 kg / m3, a thermal conductivity between 35 and 50 mW / (m.K), and a Young's modulus between 1.5 and 3.5 MPa.Polymeric aerogels according to the previous claim, wherein the poly(ethylene terephthalate) is selected from recycled poly(ethylene terephthalate) or virgin poly(ethylene terephthalate).Polymeric aerogels according to any of the previous claims, wherein the poly(ethylene terephthalate) originates from recycled water bottles, food packaging wastes, fabric fibers or industrial wastes, and wherein the packaging can be also composed of virgin PET.Polymeric aerogels according to any of the previous claims, wherein the aerogels further comprise fibers in a weight ratio between 8 and 12% w / w the total weight of the aerogel.Polymeric aerogels according to the previous claim, wherein the fibers are selected from organic fibers or inorganic fibers.Method for preparing the aerogels described in any of the previous claims, comprising the following steps:- in a mold container, dissolving poly(ethylene terephthalate) in an organic solvent under magnetic stirring and at a temperature between 20 and 25ºC, making up a poly(ethylene terephthalate) concentration between 10 and 20% w / v in solution;- proceeding with the precipitation and gelation of the polymer by adding ethanol in a quantity between 10 and 25% v / v, until obtaining a gel;- allowing the formed gel to rest for up to 12 hours;- washing the gel after removing it from the container;- drying the gel by freeze-drying, to obtain the aerogel.Method according to the previous claim, characterized in that the organic solvent is trifluoroacetic acid.Method according to claim 6, wherein the organic solvent is a mixture of trifluoroacetic acid and dichloromethane in a volumetric ratio between 0.4 and 10.Method according to any one of the claims 6 to 8, wherein the fibers are added between 8 and 12% w / w.Method according to any one of claims 6 to 9, wherein the fibers are selected from organic fibers or inorganic fibers, added in the form of felt or dispersed in the polymeric solution.Method according to any one of claims 6 to 10, wherein washing of the gel is carried out with ethanol, up to 5 times, the washes lasting up to 24 hours, followed by washing with distilled water, up to 5 times, in which each washing lasts up to 24 hours.Method according to any one of claims 6 to 11, wherein the gel is pre-frozen by immersion in liquid nitrogen, or pre-frozen in the chilled chamber of the freeze-dryer, for up to 6 hours, before undergoing freeze-drying to form the aerogel.Use of the aerogel described in any one of claims 1 to 5 for thermal insulation, acoustic barrier, adsorption, delivery systems of active ingredients, or carriers of active ingredients.

Images