WO2023227931A1 - Aramid-reinforced silica aerogel composites comprising aluminosilicates - Google Patents

Abstract

The present application relates to composites based on aramid fiber-reinforced silica aerogels incorporating natural aluminosilicates. The incorporation of aluminosilicates, such as kaolin or palygorskite, replacing a part of synthetic silica, results in homogeneous monolithic s amp les. The influence of di f ferent parameters that have a strong influence in the materials key properties were investigated, namely the type and amount of aramid fiber, and the type and amount of mineral used. The composites obtained have low shrinkage and high porosity, and low bulk densities. For composites obtained with both minerals, very low thermal conductivity values were obtained and also showed an excellent thermal stability. The addition of short length fibers produced sti f fer composites, while the longer fibers led to more flexible composites.

Description

ARAMID-REINFORCED SILICA AEROGEL COMPOSITES COMPRISING

ALUMINOSILICATES

Technical field

This application relates to composites of silica-based aerogels comprising aluminosilicates and reinforced with aramid fibers.

Background art

Silica aerogels are lightweight 3D porous materials, typically displaying a very high surface area (500 - 100 m² g^-1) and pore volume (>90%) [1]. Consequently, these materials present very low thermal conductivity (k) (15 - 25 mW.m^-1.K^-1) [1], [2]. These remarkable properties, associated with their non-flammable character, make them very attractive materials for thermal insulation, in different environments from mild to high temperatures [1], [3]. Until now, several materials based in silica aerogels have been developed for thermal insulation in mild conditions [4]. Haq et al. fabricated MTMS-based aerogels reinforced with inorganic fibers presenting extraordinary k values, as low as 12 mW.m^-1.K^-1 [5].

However, silica-based aerogels, due to their brittle nature, pose some restrictions on their use in monolithic form without disintegration, compromising their performance [1,

6, 7]. To overcome this issue, different reinforcement strategies have been employed. The use of polymers as a reinforcement material has been proven to be an efficient strategy to reinforce the aerogels' structure [6]. However, it usually increases the aerogel density and thermal conductivity, dduuee ttoo the polymers' properties, and the majority of polymers degrade at temperatures lower than 400 °C [6]. Such conditions limit the use of polymer reinforced aerogels. To increase the thermal resistance, the uussee of polymers such as aramids is suitable due to their excellent mechanical strength and thermal stability [8]. The non- flammable and self-extinguishing behavior of aramids, presenting limiting oxygen index values around 30 [8], makes these polymers excellent reinforcing materials. In the form of fibers, they can be incorporated into aerogels, acting as network support. The aerogel matrix fills the spaces between the fibers and also coats tthheemm,, protecting against their thermal degradation.

Silica aerogels reinforced with aramid fibers with very low thermal conductivities (k) (21-27 mW.m^-1.K^-1) [9]-[ll] were developed by Li and co-workers . However, despite the remarkable k values, thermal stability is in the range of common polymers (up to 280 o C), which causes restrictions if the material is subjected to high temperature conditions.

One possible solution to increase the thermal stability of pure silica aerogels is the introduction of a refractory phase ((aalluummiinnaa)) wwiitthh aa higher thermal and dimensional stability. Previous works demonstrated that the introduction of small amounts of alumina results in the maintenance of pore stability of silica aerogel at elevated temperatures, and therefore, the maintenance of the insulation properties

[12]-[14].

Alumina-silica aerogels can be obtained from natural sources such as clays or minerals that have in their composition high percentages of aluminum and silicon [15], [16]. The resulting aerogels exhibited properties close to those obtained using synthetic precursors [17], indicating the potential to produce aerogels using these minerals, and, at the same time, the circular economy is promoted, ensuring the sustainability. Until now, aerogel materials have been prepared from a few aluminosilicate sources, either natural or in industrial wastes, which include kaolin [15], [16], palygorskite fibers [18], coal gangue [19] and fly ash [20].

These materials exhibited different properties depending on the alumina and silica amounts [17]. Regarding thermal properties, these aerogels present excellent thermal stability [17] and, in general, low values of thermal conductivity (30-39 mW.m^-1.K^-1) were obtained [16]. The materials can be calcinated at 400 °C, resulting in a slight decrease of thermal conductivities to 25-28 mW.m^-1.K^-1.

However, the values increase for temperatures up to 800 °C

(57-74 mW.m^-1.K^-1), due to densification of the material [16],

[17].

A thermal insulation material and preparation method, consisting of a silica composite (TEOS-based) incorporating an opacifier (titanium dioxide) and minerals (Kaolin) as fillers, reinforced with inorganic fibers were reported in

CN101628804A. The material has shown densities (ρ) around

300 kg.m^-3 and thermal conductivities (k) of ~50 mW.m^-1.K^-1 at room temperature and Young's modulus of 2 MPa. In comparison, the presently disclosed material reaches half these values with ρ of 150 kg.m^-3 and k values about ~25 mW.m^-1.K^-1

Furthermore, despite the incorporation of the minerals was in acid conditions, it was carried out without a pre- treatment step, limiting the dissolution of the minerals in the silica sol and their availability to interact with the silica matrix. Summary

The present invention relates to a silica aerogel composite comprising aluminosilicates selected ffrroomm kaolin, palygorskite, or mixtures thereof, in an amount between 5 and 35 wt%, and being reinforced with aramid fibers in an amount between 3 to 15% wt% of the final aerogel weight; wherein the aerogel composite has a bulk density between 100 and 170 kg.m^-3 and thermal conductivity between 22 and 25 mW K^-1 m^-1.

In one embodiment the kaolin, palygorskite, or mixtures thereof, are present in an amount between 10 and 20 wt%.

In one embodiment the aramid fibers are selected from para- aramids, meta-aramids or a mixture thereof.

In one embodiment the aramid fibers have lengths between 0.3 mm to 1 mm or between 3 to 5 cm.

The invention also relates to a method to produce the silica aerogel composite comprising aluminosilicates and reinforced with aramid fibers comprising the following steps:

- Mixing at least one aluminosilicate selected from kaolin or palygorskite, or mixtures tthheerreeooff,, in a concentration between 5 wt % and 35 wt % of the final aerogel concentration with a solution comprising an acid in a concentration between 0.1 and 1 M in a solution that is 50 to 95% v/v of ethanol;

- Stirring the resulting solution for a time between 1 and

5 days at a temperature between 10 and 40 °C;

- Adding to the previous solution a silica precursor and ethanol in a molar ratio from 1:5 to 1:35, and stirring for a time between 5 and 30 minutes; The aramid fibers in an amount between 3 to 15% wt% are added to the solution together with the silica precursor or arranged in a mould aanndd the silica solution prepared according to the method is poured into the mould;

- Storing the solution at a temperature between 20 and 50°C for a time between 1 and 24 hours to promote hydrolysis;

- Adding to the solution NH₄F or NH₄OH with a concentration between 0.1 and 5 M and stir for a time between 20 and 60 seconds;

- Gelation and aging of the solution by keeping the previous solution comprising aramid fibers at a temperature between 20 and 50°C for a time between 2 and 6 days to obtain the aerogel;

- After gelation and aging, the surface of the aerogel is modified with a silylating solution of a silylating agent in heptane with a concentration between 10 and 40 (v/v), at a temperature between 15 and 60 °C for a time between 12 and 96 hours;

- Drying the aerogel and submit it to a temperature between

20 and 80 °C between 1 and 6 hours followed by a temperature between 100 and 200 °C at a time between 1 and 3 hours.

In one embodiment the acid is selected from nitric acid, sulfuric acid, peracetic acid, a mixture of acetic acid and hydrogen peroxide.

In one embodiment the silica precursor is selected from tetramethylorthosilicate, tetraethylorthosilicate, vinyltrimethoxysilane, 3-aminopropyltrimethoxysilane, aminopropyltriethoxysilane, methyltrimethoxysilane and methyltriethoxysilane, or mixtures thereof. In one embodiment the silica precursor and ethanol are added in a molar ratio of 1:10.

In one embodiment the silylating agent is selected from

Hexamethyldisilazane, Trimethylchlorosilane,

Methyltrimethoxysilane, Methyl triethoxysilane,

Dimethyldimethoxysilane, Hexamethyldisiloxane or mixtures thereof.

In one embodiment after gelation and aging the aerogels are washed with ethanol and heptane or methanol or hexane, at a temperature between 20 and 60 °C.

The invention also relates to a silica aerogel composite for use as thermal insulators for space devices, equipment, buildings or piping.

General description

Herein it is disclosed the preparation and characterization on silica aerogel composites with different amounts of two minerals, kaolin and palygorskite, incorporated in the matrix and reinforced with aramid fibers. An extensive characterization of their physico-chemical and, in special, the thermomechanical properties was made. The results show the potentiality of use the developed materials as thermal insulators for space devices, equipment, buildings, piping, among other.

Monolithic samples of aramid reinforced silica aerogel composites, incorporating different amounts of natural minerals were obtained. The minerals are well dispersed in the matrices, indicating aa good interaction between the different phases during the formation of the gel network.

Palygorskite-containing composite samples are more flexible (with lower Young's modulus) when compared with kaolinite- based ones. Composites with elongated fibers are more flexible when compared with those reinforced with short ones. This fact is correlated with the slightly higher density of the latter.

Composites with palygorskite exhibit smaller pore diameters than their kaolinite counterparts. It was observed that using different types of aramid fibers influences in the porous structure formation. TThhee ssaammpplleess synthesized with short fibers showed a closed structure, with slightly smaller pores, wwhheenn compared with the composites with elongated fibers composites.

Considering all the properties in an integrated form, the use of 15% of minerals seems to produce the best composite samples in tteerrmmss of thermal insulation, density and mechanical integrity. Despite the use of elongated fibers as reinforcement material typically led ttoo improved thermomechanical properties, for large samples production, short fibers aarree the best option, since the large fiber length will provide composites with less shrinkage during drying and increased structure integrity.

Brief description of drawings

For easier understanding of this application, figures are attached in the annex that represent the preferred forms of implementation which nevertheless are not intended to limit the technique disclosed herein. Figure la shows the infrared spectra of palygorskite (P). Figure lb shows the infrared spectra of kaolinite (K).

Figure 2a shows XRD patterns of palygorskite. Figure 2b shows XRD patterns of kaolinite.

Figure 3 shows SEM micrographs for a - palygorskite and b - kaolinite minerals.

Figure 4 shows thermogravimetric curves for palygorskite and kaolinite samples.

Figure 5 shows the infrared spectra of Kevlar Pulp (KP) and Teijinconex fibers (TJ).

Figure 6 shows SEM images of different aramid fibers: KP

(a,b) and TJ (c,d).

Figure 7 shows thermogravimetric curves for KP and TJ fibers.

Figure 8a shows FTIR spectra for the mineral-silica aerogel composites with 5 wt% of KP. Figure 8b shows FTIR spectra for the mineral-silica aerogel composites with 5 wt% of TJ as reinforcement phase.

Figure 9 shows powder X-ray diffraction patterns of mineral- silica aerogel composites.

Figure 10 shows SEM micrographs of mineral-silica aerogel reinforced with aramid fibers, a) 15 wt% of P and 5wt% of

KP; b) 20 wt% of P and 5 wt% of KP; c) 15 wt% of K and 5 wt% of KP; d) 20 wt% of K and 5 wt% of KP; e) 15 wt% of P and 5wt% of TJ; f) 20 wt% of P and 5 wt% of TJ; g) 15 wt% of K and 5 wt% of TJ; h) 20 wt% of K and 5 wt% of TJ.

Figure 11a shows the thermogravimetric curves the mineral- silica aerogel composites with 5 wt% of KP. Figure lib shows the thermogravimetric ccuurrvveess tthhee mineral-silica aerogel composites with 5 wt% of TJ as reinforcement phase.

Figure 12a shows mechanical tests of the composite materials with different minerals and fibers - Reversible compressive stress-strain curves of the composites until 15% strain with

KP. Figure 12b shows mechanical tests ooff the composite materials with different minerals and fibers Reversible compressive stress-strain curves of the composites until 15% strain with TJ. Figure 12c shows mechanical tests of the composite materials with different minerals and fibers - ten cycles of reversible compressive stress-strain curves of the composites until 25% strain with KKPP.. Figure 12d shows mechanical tests of the composite materials with different minerals and fibers - ten cycles of reversible compressive stress-strain curves of the composites until 25% strain with

TJ.

Detailed description of embodiments

Now, preferred embodiments of the present application will be described in detail with reference to the annexed drawings. However, they are not intended to limit the scope of this application. The present invention relates to a silica aerogel composite comprising aluminosilicates and reinforced with aramid fibers.

The aramid fibers can be short or long fibers. By short fibers it is understood as shredded fibers between 0.3 mm to

1 mm of length, and long fibers refer to fibers in the range from 3 to 5 cm.

In one embodiment the aluminosilicates are selected from kaolin or palygorskite, or mixtures thereof.

In one embodiment, the silica aerogel composite comprises kaolin in an amount between 5 and 35 wt%. In another embodiment, the silica aerogel composite comprises kaolin in an amount between 10 and 20 wt%.

In one embodiment, the silica aerogel composite comprises palygorskite in an amount between 5 and 35 wt%. In another embodiment, the silica aerogel composite comprises palygorskite in an amount between 10 and 20 wt%.

In one embodiment, the aramid fibers are present in an amount between 3 to 15% wt% of the final aerogel weight.

In one embodiment, the silica aerogel composite comprises fibers of types selected from aramids, either para- or meta- aramids or a mixture thereof.

Some of the characteristics of the composites of the present application are as follows:

Shrinkage between 0 and 5 % volume;

Bulk density between 100 and 170 kg.m^-3; Surface area between 450 and 600 m².g^-1;

Thermal conductivity between 22 and 25 mW K^-1 m^-1;

Young's modulus between 200 and 1000 kPa.

The present invention also relates to a method to produce the silica aerogel composite comprising aluminosilicates and reinforced with aramid fibers, comprising the following steps:

- Mixing at least one aluminosilicate in a concentration between 5 wt % and 35 wt % of the final aerogel concentration with a solution comprising an acid in a concentration between 0.1 and 1 M in a solution that is 50 to 95% (v/v) of ethanol;

- Stirring the resulting solution for a time between 1 and

5 days at a temperature between 10 and 40 °C;

- Adding to the previous solution a silica precursor and ethanol in a molar ratio from 1:5 to 1:35, and stirring for a time between 5 and 30 minutes;

The aramid fibers in an amount between 3 to 15% wt% are added to the solution together with the silica precursor or arranged in aa mould and the silica solution prepared according to the method is poured into the mould;

- Storing the solution at a temperature between 20 and 50°C for a time between 1 and 24 hours to promote hydrolysis;

- Adding to the solution NH₄F or NH₄OH with a concentration between 0.1 and 5 M and stir for a time between 20 and 60 seconds;

- Gelation and aging of the previous solution by keeping the solution comprising aramid fibers at a temperature between 20 and 50°C for a time between 2 and 6 days to obtain the aerogel;

- After gelation and aging, the surface of the aerogel is modified with a silylating solution of a silylating agent in heptane with a concentration between 10 and 40 (v/v), at a temperature between 15 and 60 C for a time between 12 and

96 hours;

- Drying the aerogel and submit it to a temperature between

20 and 80 ° C between 1 and 6 hours followed by a temperature between 100 and 200 °C at a time between 1 and 3 hours.

In one embodiment the aluminosilicates are preferably selected from kaolin or palygorskite, or mixtures thereof.

In one embodiment, the acid is selected from, but not limited to, nitric acid, sulfuric acid, peracetic acid, a mixture of acetic acid and hydrogen peroxide.

In one embodiment, the silica precursor is selected from, but not limited to, tetramethylorthosilicate (TMOS), tetraethylorthosilicate (TEOS), vinyl trimethoxysilane

(VTMS), 3-aminopropyltrimethoxysilane (APTMS), aminopropyltriethoxysilane (APTES), methyltrimethoxysilane

(MTMS) and methyltriethoxysilane (MTES), or mixtures thereof.

In one embodiment, the silica precursor and ethanol are added in a molar ratio of 1:10.

In one embodiment, the silylating agent is selected from, but not limited to, Hexamethyldisilazane (HMDZ),

Trimethylchlorosilane (TMCS), Methyl trimethoxysilane

(MTMS), Methyltriethoxysilane (MTES),

Dimethyldimethoxysilane (DMDMS), Hexamethyldisiloxane

(HMDSO) or mixtures thereof.

In one embodiment, the aramid fibers are added to the solution together with the silica precursor. In another embodiment, the aramid fibers are arranged in a mould and the silica solution prepared according to the method is poured into the mould prior to the gelation step.

In one embodiment after gelation and aging the aerogels are washed with ethanol and heptane at a temperature between 20 and 60 °C. Other solvents can be used, such as methanol or hexane, although being more hazardous.

Non-limitative examples and experimental data:

1. Materials

Ethanol (C₂H₅OH, purity ≥99.8%, Thermo Scientific), tetraethylorthosilicate (TEOS, Si(OC₂H₅)₄, purity £99%,

Aldrich), nitric acid (HNOa, 65%, Aldrich), ammonium fluoride

(NH₄F, 40% in aqueous solution, Fluka Analytical), n-heptane

(C₇H₁₆, purity >99.5%, Thermo Scientific) and hexamethyldisilazane (HMDZ, C₆H₁₉NSi₂,purity of 98.5%, Thermo

Scientific)were used as received. Two clay minerals, palygorskite (P) and kaolinite (K), which were gently offered by Omya SA, were incorporated in the composites. And two different types of aramid materials were used as reinforcement for the composites: a pulp of para-aramid,

Kevlar® pulp (KP) fabricated by DuPont (USA), and a fiber of meta-aramid, Teijinconex® (TJ; 100% meta-aramid fiber) which was kindly offered by Teij in Aramid GmbH (Wuppertal,

Germany) . High purity water was used to prepare the aqueous solutions of nitric acid and ammonium fluoride.

2. Composite synthesis

The clay mineral-silica gels were synthesized by the sol-gel process . To reduce the size of the mineral particles (achieving colloidal size) and improve the interaction of the minerals with the silica network, the minerals were submitted to a pre-treatment step, before adding the silica precursor. Nitric acid, with a concentration of 0.5 M, was used as pre-treatment agent for both minerals. In this first stage, an amount of mineral (10 wt% to 35 wt% of the final aerogel concentration) was mixed in a solution containing ethanol (ca. 2.5 mL) and nitric acid (~ 1 mL). The solution was stirred for three days at a temperature between 15 and

30°C.

The next step consisted in adding ethanol (5.25 mL) and TEOS

(1.45 mL) into the mineral solution, while maintaining the solution under stirring for more 30 min. The mineral-silica solution was then stored in an oven at 27 °C for 24 h for the hydrolysis step. A solution of NH₄F (0.75 M; 0.96 mL) was added to the former mixture and kept under strong agitation for 1 minute. After this step the gelation of the samples takes place. In the case of KP, the reinforcement phase was added together with the silica precursor, while when Teijinconex was used, the final solution was poured into a mold with the fibers. The samples were kept in the oven at 27 ° C during 6 days for aging.

The composite gels were then unmolded and washed two times with ethanol and four times with heptane at 50 °C, in order to remove any trace of unreacted precursors or reagents. The next step consisted in the surface modification; the samples were placed in a flask containing a silylating solution (20% v/v of HMDZ in heptane) for 24 h at 50 °C. The samples were dried in ambient pressure, being submitted to temperatures of 60 °C for 4 h and 150 ° C for 2 h. 3. Characterizations

3.1 Chemical characterization

For the fibers and composite materials, the chemical structure was assessed by attenuated total reflection (ATR)

Fourier-transform infrared spectroscopy (FTIR) (FT/TR 4200,

Jasco). The spectra were collected between a wavenumber of 4000 and 500 cm^-1, with a resolution of 4 cm^-1 and 128 scans.

In the case of the minerals, the spectra were obtained in the potassium bromide (KBr) pellet method, using KBr pellets composed by 78 mg of KBr and 0.2-0.3 mg of mineral, in a wavenumber range of 4000-400 cm^-1.

The contact angles were measured by dropping high purity water droplets on the surface of the samples, in order to obtain the hydrophobicity/hydrophilicity level of each composite. The analyses were performed at room temperature with an OCA 20 system (Dataphysics) by the sessile drop method.

3.2 Structural characterization

The crystalline structures of the minerals were evaluated using powder X-ray diffraction (XRD). The XRD patterns were obtained by a Bruker 8D Advance diffractometer, operating in

Bragg-Brentano geometry (θ-2θ), using Cu Kα radiation (λ =

0.154184 nm), an applied voltage of 40 kV and current of 40 mA. The information was collected in the range of 5°- 80°

(2θ), with step size of 0.03°, at room temperature, and a recording time of 7 s per step. The phases’ identification was performed using the database ICDD-JCPDS installed on software DIFFRAC.SUITE EVA.

Linear and volumetric shrinkages were determined by using the diameter or volume of the samples before (d₀ and V₀) and after drying (d and V), respectively, as described by Equations 1 and 2 [21].

Linear shrinkage = (1- d/d₀) × 100 (1)

Volumetric shrinkage = (1- V/V₀) × 100 (2)

Bulk density (ρ_b) of the monoliths of mineral-silica aerogel composites was assessed from the weight, which was determined with a microbalance of 10^-5 g precision, aanndd volume, with their dimensions being measured with a caliper of 0.01 mm resolution. TThhee skeletal density (ρ_s) wwaass determined by Helium pycnometry (Accupyc 1330, Micromeritics) after finely grinding the composites. The specific surface area (S_BET) was estimated by nitrogen gas adsorption at 77 K (.Gemini V2.00, Micromeritics Instrument Corp.), and the Brunauer-Emmett-

Teller (BET) theory was applied in the relative pressure interval 0.05-0.25 of the adsorption isotherm.

The porosity (Equation 3) and pore volume (V_P, Equation 4) were estimated by using the values of bulk and skeletal densities. The average pore size (Equation 5) was computed with the values of pore volume and the specific surface:

Porosity (%) = (1 - ρ_b/ ρ_s) × 100 (3)

V_P = 1/ρ_b - 1/ρ_s (4)

Average pore diameter = 4( V_P)/S_BET (5)

The microstructure and identification of chemical elements of the mineral-silica aerogel composites were investigated with aa TESCAN Vega3 SBH scanning electron microscope, operated at a voltage of 5 kV, equipped with an energy- dispersive X-ray spectroscopy (EDS) detector Bruker XFlagh 410 M, operating at a voltage of 20 kV. All images were obtained after coating the samples with a thin layer of Au/Pd, using Physical Vapor Deposition during 90 s.

3.3 Thermo-mechanical characterization

The thermal stability of the materials was assessed by using a DSC/TGA equipment (SDT Q500, TA Instruments), from room temperature to 1200 °C,, at a 10 °C.min^-1 heating rate under air ffllooww.. Thermal conductivity, k, wwaass evaluated with a Thermal Constants Analyzer TPS 2500 S (Hot Disk), using the transient plane source method with two replicates of the same sample, or, in the case of the minerals and fibers, the samples were placed in the sample holder for powders.

The composites mechanical properties were investigated in an Inspekt mini-series equipment (Hegewald & Peschke), by performing uniaxial compression-decompression static tests with a strain rate of 1 mm.min^-1. The Young's modulus was obtained from the initial linear region of the loading curve of compression tests, with a load cell of 50 N using a strain from 0 to 15%. Destructive tests (compression to the limit) were performed with a cell of 3 kN. Additionally, to evaluate the materials capacity to withstand repeated loads, axial cyclic compression tests (10 cycles), were performed for selected composites until a strain of 25%, also with the 3 kN load cell.

4. Results

4.1 Minerals' characterization

The first step for the development of the clay mineral-based aerogels was the characterization of the minerals. In the

FTIR spectra (Figure la and lb), the characteristic bands of palygorskite and kaolinite wweerree identified. IInn the palygorskite spectrum, the adsorption bands at approximately

3620, 3580 and 3544 cm^-1 can be attributed to the vibrations of AI₂-OH, Al,Fe³⁺-OH and Mg,Fe³⁺-OH, respectively [22]—[24]. An asymmetric band centered at around 1640 cm^-1 is assigned to the bending modes from hygroscopic, adsorbed water or from structural OH [23]. Between 1200 and 1000 cm^-1 are observed the Si-O vibration bands of palygorskite, corresponding to Si-O-Si and SiO₄ tetrahedral vibrations

[22], [24]-[26]. The band detected at 987 cm^-1 is ascribed to Si-non bridging O-Mg asymmetric stretching vibration

[24], [26]. The bands at 909 and 880 cm^-1 can be attributed to the bending vibration of Al-OH-Al and Al-OH-Fe, respectively [22], [24], [25], while the one at 648 cm^-1 is assigned to the symmetric stretching vibration of H₂O-Mg-H₂O

[24]. The last three bands in the palygorskite spectrum,

510, 480 and 440 cm^-1, correspond to the bending vibration of Si-O-Si and O-Si-O and the symmetric stretching vibration of MgO₆ octahedron, respectively [23], [24], [26].

The kaolinite spectrum has typical absorption bands, between

3700 and 3500 cm^-1, which corr espond to stretching frequencies of OH groups [27], [28]. When four bands (3695,

3670, 3650 and 3620 cm^-1) are present and well defined, it is an indication that the sample has a high structural ordering. However, when the middle two bands are replaced by a single broader band at 3654 cm^-1, as observed for the kaolinite in Figure 1b, it indicates that the samples have only a partially ordered structure [27], [29], [30]. The spectrum of this clay mineral shows absorptions attributed to Si-O, Al-O and -OH groups in the 1300-400 cm^-1 range [27]—

[30]. The longitudinal mode Si-O stretching band vibration was observed at 1106 cm^-1, while in-plane Si-O stretching bands were detected at 1032 and 1009 cm^-1. The band attributed to the OH deformation of inner hydroxyl groups, from Al-OH, appeared around 915 cm^-1. Three bands assigned to Si-O were observed at 790, 761 and 697 cm^-1.At 539 cm^-1, it was detected the band assigned to the Al-O-Si deformation, while the bands at 470 and 429 cm^-1 are ascribed to the Si-O-Si deformation and Si-O deformation, respectively.

The next step was the characterization of the minerals by

XRD to confirm that palygorskite and kaolin were the main phases present in the used minerals. The results are shown in Figure 2a and b.

The characteristic diffraction peaks of palygorskite at 2θ values of 8.5° (110), 13.9° (20 0), 16.4° (13 0), 19.8°

(040), 21.5° (310), 27.9° (40 0) and 42.5° (600) were detected in the XRD pattern [24], [31]-[33]. The peak at

8.5° is attributed to the basal space of the palygorskite framework, and the peaks at 13.9°, 16.4° and 19.8° are assigned to the Si-O-Si crystalline layers of the clay [33].

From the XRD pattern of the raw material it was possible to detect several characteristic peaks from kaolinite for 2θ values of 12.3° (001), 19.8° (020), 20.3° (-11 0), 23.1°

(0 -2 1), 24.8° (0 0 2), 34.9° (1 3 0), 35.4° (-1 -3 0),

35.9° (20 0), 36.5° (003), 37.7° (1 -31) and 38.4° (13

1) [27], [34]—[36]. The kaolinite structure consists of two

(001) basal planes: one consisting in a silica tetrahedral layer, with 0 atoms bonded to Si atoms so-called "siloxane surface", and the other described as aluminum hydroxide octahedral layer, with OH groups bonded to Al known as

"aluminol surface" [37], [38]. Besides these minerals, the presence of muscovite, another phyllosilicate mineral, was verified with the peaks at ca. 9° ((000022)), 18° (004), 2977° (006) and 45.5° (0010) [39]. Quartz impurities was also present in the samples, as demonstrated by the presence of the peaks at 20.8° and 27.5° [32], were also detected in tthhee diffractograms. However, the XRD patterns iinnddiiccaatteedd tthhaatt palygorskite (Figure 2a) and kaolinite (Figure 2b) have the largest peaks, showing that they are the main minerals, as desired.

The morphology of both minerals was assessed by SEM (Figure 3). Palygorskite (Figure 3a) has a distinctive fibrillar structure, as also observed by different authors [40]-[43].

The fiber diameters are around 50 nm and the lengths have a broad range, varying from hundreds of nanometers to a few micrometers. In the SEM images of kaolinite (Figure 3b), it was observed that the mineral consists of a variety of morphologies, including pseudo-hexagonal particles, booklet- like shape, or sheets stacks [37], [44], [45], with sizes generally lower than 2 μm.

EDS analyses were also performed in both mineral samples, and the results are in agreement with the ones obtained by XRD. Only minor amounts of potassium were detected, and this element can be attributed to the presence of muscovite in the samples, as it is the only phyllosilicate here detected that contains K in its molecular formula

(KAI₂ (Si₃Al)O₁₀ (OH,F)₂), which once again indicates the high purity of the clay samples.

The thermal behavior of the minerals was first assessed by TGA. The thermogravimetric curves are represented in Figure 4, and the corresponding temperatures and weight losses for the thermal phenomena were also measured (Table 1). In the case of palygorskite, four distinct steps were observed: (i) the weight loss between 25°C and 100°C corresponds to the removal of physisorbed water and part of the zeolitic water

(located in the channels); (ii) the second loss of weight, from 100 to 200 °C, is attributed to the loss of the remaining zeolitic water and some weakly bound structural water; (iii) the following weight loss, up to 550°C, is related to the removal of the residual structural water, with the formation of palygorskite anhydride; and (iv) in the final loss, from

550 °C to 1200 °C, dehydroxylation of Mg-OH groups and partial destruction of the palygorskite crystal structure [46]-[50].

Table 1 Identification of onset temperatures and weight losses observed in the thermograms of Figure 4.

The TGA curve of kaolinite shows two regions of weight loss.

A small one up to 200 ° C associated with the loss of adsorbed water, and a significative one, at temperatures higher than

400 °C, related to the dehydroxylation of the mineral and the removal of structural water [27], [51], [52].

The thermal conductivities of both minerals were also measured, with palygorskite presenting a value of 70.7 ± 0.1 mW.m^-1.K^-1, and kaolin a value of 209.9 ± 0.2 mW.m^-1.K^-1 It is important to note that due to the layer of surface hydroxyl groups of the octahedral sheet in kaolinite, which has a hydrophilic character, the measured value could have been negatively affect by the presence of moisture during the measurement. The values obtained for the minerals are quite high, especially that of kaolin, if compared with insulation materials, such as polyurethane foam (~20

29 mW.m^-1.K^-1), foam glass (~38 - 50 mW.m^-1.K^-1) and extruded polystyrene (XPS) (~29 48 mW.m^-1.K^-1) [ [11]],, ssoo it is important to control the amount of minerals added into the final composite, and determine the optimum value, so the composites' thermal conductivity can be controlled.

4.2 Fibers' characterization

The FTIR spectra of the fibers, KP and TJ, were collected and the typical spectra are shown in Figure 5. Both aramid fibers showed characteristic amide absorption bands at approximately 3300 cm^-1, 1640 cm^-1 (Amide I bands), 1540 cm^-1 (Amide II bands) and 1300 cm^-1 (Amide III bands) attributed to N-H stretching vibration, to the carbonyl (C=O) stretching vibrations, the N-H rocking vibrations and the asymmetrical

C-N stretching vibrations, respectively [53]-[55]. The bands around 3060 cm^-1, 2920 cm^-1 and 2850 cm^-1 are also present in both spectra, but more intense in TJ spectrum, and are assigned to the stretching of aromatic C-H, methylene and methyl groups, respectively [9], [56]. For TJ, a meta-aramid fiber, a new band appears near 1600 cm^-1 , near the carbonyl stretching vibration, which is attributed to C=C stretching vibration [54], [57], and one band at 1475 cm^-1, that can be assigned to in plane vibrations of CH and NH [58]. The bands between 1230 cm^-1 aanndd 662200 cm^-1 in both fibers are mainly related to out of plane and in plane vibrations of NH and CH groups [58].

The morphology and structure of the fibers were assessed by

SEM (Figure 6). Through the analyses of the images of KP

(Figure 6a and b) it was possible to observe fibers with diameters around 20 μm and a significant amount of nanofibers from the fiber stems, as also observed in other aramid pulps

[59], [60], which helps to create a multiscale three- dimensional network structure. The TJ micrographs (Figure 6c and d) show longer fibers than the ones observed in KP, and all presenting diameters around 15 μm. It is also possible to observe a well-defined cylindrical shape and some level of roughness on the TJ fibers' surfaces.

After the chemical and morphological characterizations, the thermal properties of the fibers were assessed. Figure 7 shows the thermograms for KP and TJ fibers, and the corresponding temperatures and weight losses for the thermal phenomena were also measured (Table 2).

Table 2 Identification of onset temperatures and weight losses observed in the thermograms of Figure 7.

Both fibers present a first mass loss of around 4% up to 100

°C, which is ascribed to the removal of adsorbed moisture

[54]. After that temperature, significant differences are observed in the spectra as KP decomposes in a single step process, TJ shows four weight losses, and between these fibers, KP exhibits the highest thermal stability. The substantial weight loss for KP has an onset temperature of

560 °C, and is attributed to the initial backbone collapse, this event occurring mainly due to the decomposition of amide groups by the cleavage of C=O and C-N bonds [61], [62].

In the case of TJ, after the loss of adsorbed water, a barely noticeable weight loss (less than 2%) takes place up to 350 ° C, which is related to the cleavage of hydrogen bonds among the fibers macrochains [63]. The major weight loss starts at around 420 °C, and, in the case of this meta-aramid fiber, the fiber decomposition is a two-step process. The first step takes place in the temperature range of 390 °C to 475 °C, and is usually attributed to the heterolytic breaking of the amide groups, which causes the release of carboxylic acid and primary amines [54], [61], [63], [64]. The second decomposition, which occurs in the temperatures between 475

°C and 630 °C, is ascribed to homolytic cleavages of amide bonds, yielding aryl nitriles, and the breaking of bonds between the amide groups and the benzene rings at meta- positions [54], [61], [63], [64]. A final, and smaller, weight loss is detected in temperatures higher than 630 °C; this variation is probably due to further condensation/polycondensation reactions to form large polyaromatic compounds and the final char products [63].

The weight loss was higher for the KP, which indicates that the difference in the substitutions' positions in the aromatic rings, para- or meta-, besides changing the decomposition temperatures as previously discussed, also affects the final char yield. The para-substitution in KP structure leads to a linear structure that does not favor the cross-linking and promotes the formation of more volatile compounds [54], [61].

The thermal conductivity of both fibers was also determined, with the KP having a value of 74.4 ± 0.1 mW.m^-1.K^-1 and TJ a value of 43.6 ± 0.2 mW.m^-1.K^-1. These results are lower than the ones obtained for the minerals, and, while the TJ's thermal conductivity is in the range of the insulation materials, KP still presents a higher value than common materials used as thermal insulators, as previously described.

4.3 Composites' characterization 4.3.1 Composite selection based on experimental observations

After the complete characterization of minerals and reinforcement phase, the syntheses of the composites were performed. The influence of the amount of mineral in the final composite properties was the first evaluated parameter. Some of the physical and thermal properties of these materials are reported in Table 3.

Table 3 Physical and thermal properties of the mineral- silica aerogel composites with 5 wt% of KP and different wt% of minerals.

With the exception of tthhee samples containing 35 wt% of minerals, the samples present very low shrinkages after the drying (Table 3). Most of the shrinkage values here obtained are much lower than others works that also developed silica aerogel composites with aramid pulp, in which they have linear and volumetric shrinkages higher than 20 % [10], [57], [65]. The low shrinkage is a good indicator that, up to 20 wt% of both clays, the mineral is interacting with the silica phase and with tthhee fibers, aanndd nnoott preventing, or interrupting, the formation of the three-dimensional network. The addition of the fibers, as they can act as a supporting skeleton of tthhee whole aerogel [21], and the hydrophobization of the gels through the modification step, which allows the "spring-back" effect of the matrix, also contribute to the low shrinkage of the composites.

Regarding the bulk densities, the higher values were obtained for the samples synthesized with 35 wt%, as expected, due to their higher shrinkages during the drying. The remaining samples all present values between 120 and 170 kg.m^-3 (Table

3), similar to other composites developed with aramid fibers [9]—[11]. As it is well-known for aerogels, their density has a significant influence in the materials' thermal conductivity [66], [67], and most of the superinsulating SiC>2 aerogels commercially available present densities between 80 and 200 kg m^-3 [68]. The thermal conductivity of the materials was also assessed (Table 3), and except for the composite containing 35 wt% of palygorskite, all the mineral-silica aerogel composites have values of thermal conductivity lower than 25 mW.m^-1.K^-1, which allow them to be classified as superinsulating materials [68].

After determining that a small variation in the amount of mineral, such as 2.5 wt%, do not have a significant impact in the final composite properties and that 35 wt% of clay leads to higher values of density and thermal conductivity, only three amounts of mineral (10 wt%, 15 wt% and 20 wt%) were selected for further studies. The next step was to study the influence of a higher amount of KP (10 wt%) in the system. The results are displayed in Table 4.

Table 4 Physical and thermal properties of the mineral- silica aerogel composites with 10 wt% of KP and different wt% of minerals.

As expected, much lower shrinkages (Table 4) were observed for the systems with 10 wt% of the reinforcement phase, as the fibers act as supporting skeleton to the composite, as previously mentioned, preventing the collapse of the network. This also has an impact in the materials' densities (Table 4), with all of them presenting lower values than the composites with 5 wt% of KP (Table 3). However, the addition of more fibers into the systems caused a negative impact in the thermal conductivities of the composites, with all the samples exhibiting values slightly above 25 mW.m^-1.K^-1.

After the first screening processes, as better insulation properties are a desirable feature, the samples synthesized with 5 wt% of reinforcement, due to their lower thermal conductivities, were selected for further characterizations.

Besides, as one of the main goals of the work was to add minerals into the silica aerogel composites, the samples containing 15 wt% and 20 wt% of minerals were selected for additional studies, as no significant difference was observed between their properties and the ones synthesized with 10 wt% of clay. So,, from now on,, only samples with 5 wt% of reinforcement phase (KP and TJ) and with higher amount of minerals (15 wt% and 20 wt%) will be presented.

4.3.2 Chemical characterization

The characteristic vibrations bands of the mineral-silica aerogel composites with two different reinforcement phases were assessed by ATR-FTIR (Figure 8a and b). All the spectra are very similar, in which is possible to observe the typical vibration bands of the skeletal SiO₂ network at around 1170 cm^-1 and 1060 cm^-1, related to the longitudinal and transversal-optical components of the asymmetric stretching vibration of Si-O-Si, respectively, and 755 cm^-1, ascribed to the symmetric stretching vibrations of the Si-O-Si bonds

[21], [57], [69], . The bands detected at 2900-3000 cm-

¹ , around 1250 cm^-1 and 845 cm^-1, indicate that the samples were successfully modified by the silylating agent (HMDZ), as these bands are attributed to the stretching and deformation vibrations of CH groups and the stretching of Si-CH₃ bonds, respectively [21], [57], [70].

Small bands were detected at 1650 cm^-1 and 1540 cm^-1, and they confirm the presence of the aramid fibers in the composites, as these bands are attributed to the amide I band (stretching of the carbonyl group) and to the amide II band (N-H rocking vibrations), respectively [53]—[55]. The low intensity of these bands is expected since they are probably coated by the silica matrix, not being fully exposed in the composite. The main differences between the samples were detected for the different minerals, while the composites synthesized with palygorskite presented a band around 985 cm^-1, assigned to Si-non bridging O-Mg asymmetric stretching vibration

[24], [26], the materials obtained with kaolinite display bands at 910 cm^-1 and 690 cm^-1, which are characteristic to the Al-OH bending and to Si-O group, respectively [27]-

[30]. By the analyses of the FTIR spectra it was possible to confirm the presence of the three phases (mineral, silica, and fibers) in all the final composites, indicating a good interaction between them.

The wetting properties of the composites were determined by the contact angle mmeeaassuurreemmeennttss.. The samples have a hydrophobic character, with contact angles between 145° and 150°, independently of the mineral or fiber used in the synthesis. These results are in agreement with the FTIR analysis, indicating aa successfully modification of the mineral-silica network by the HMDZ, and a high number of methyl groups attached to the composites surface.

4.3.3 Structural characterization

Figure 9 shows the powder XRD patterns obtained for two of the mineral-silica aerogel composites. For both samples, a broad diffraction band was observed around 22°° ((2266)),, which is typical of silica materials with low crystallinity [71]- [73]. This band is attributed to the spacing between silicon atoms and the angle of Si-O-Si group [71], [72].

Moreover, some of the characteristic crystalline peaks of palygorskite and kaolinite were observed in their respective composites. In the case of the palygorskite composite, the peaks attributed to the basal space of the framework, and the ones assigned to the Si-O-Si crystalline layers of the mineral [33] were detected. In the case of the kaolinite sample, the main peak, ascribed to the two basal planes, was identified [37], [38].

In table 3, the information regarding the microstructure of developed composites is presented. In general, all composites display a highly porosity (> 90%) with low pore size, which can explain the reduced thermal conductivity values obtained for these materials.

Despite the fact that no significant changes in the skeletal density values were observed, the values tend to be slightly higher in kaolin-based composites. In any case, the skeletal density values are in the range of those obtained previously for silica composites with aramids fibers as reinforcement material [9], [11], [57], [65]. The specific surface areas of the aerogels were lower than those for other aramid reinforced silica aerogels (-950-970 m² g^-1) [9-11], but similar to those obtained by Almeida el al. (-600 m² g^-1) [57] for TEOS/VTMS system, using the same fibers. The higher specific surface area showed by the KP- based composites can be justified by their smaller pore diameters (Table 5), probably due to higher affinity between the KP fibers and the minerals.

Table 5 - Microstructure properties of KP (5%) and TJ (5%)- reinforced silica aerogel composites with different minerals.

By using TJ, high pore sizes are obtained, as well as using kaolin instead palygorskite. In case of composites with KP as reinforcement material, an increase oonn mineral concentration leads to a decrease on the pore size. However, the same behavior is not observed when using TJ. In this case, the pore size increases with the mineral concentration. Once again, the possible explanation could be related to the strong interaction between the mineral components and KP fiber, leading to a well-organized structure and decreasing the pore size. Such fact was confirmed by SEM images in Figure 10.

The microstructure of the mineral-silica aerogel composites, with different clays and fibers, are shown in Figure 10.

It is possible ttoo oobbsseerrvvee tthhaatt aallll samples present an interconnected three-dimensional aerogel matrix with a fine microstructure. Even though the two minerals may interact differently with the silica precursor, it can be concluded that their presence did not prevent the formation of the network. It was also determined, by EDX technique, that the minerals are well dispersed throughout the matrices, which indicates a good interaction between these two phases during the formation of the network. Regardless of the similarities, the palygorskite composites exhibit smaller pore diameters than their kaolinite counterparts. From tthhee SEM images (Figure 10), it was possible to establish that the type of aramid fibers also has some influence in the porous structure formation. The samples synthesized with KP present a more closed structure, with slightly smaller pores, if compared to the TJ composites. These assumptions seem ttoo bbee in agreement with the average pore diameter and pore volume (Table 4), as the aerogels with TJ have higher values than the ones with KP.

4.3.4 Thermo-mechanical characterization

To evaluate the thermal stability of the materials, the composites were submitted to a thermogravimetric analysis from 20 °C to 1200 °C (Figure 11a and b). The information of mineral-silica aerogels without reinforcement fibers was also obtained for comparison. As expected, the composites synthesized with kaolinite present a better thermal stability, and the samples reinforced with TJ fiber show lower weight losses.

For the samples synthesized with palygorskite, for both amounts, and KP (Figure 11a), six weight losses were observed (Table 6). Up to 450 °C, the three weight losses that happen are attributed to the removal of water from the system [46]-

[50]. The fourth phenomenon, with an onset temperature of approximately 475 °C,, can be ascribed to the oxidation of surface organic groups from the silica aerogel [74]. The next weight loss (T_onset = ~ 520 °C) happens due to the partial destruction of the palygorskite crystalline structure, The final weight loss is related with the degradation of the

Kevlar pulp [61], [62]. For the composites with kaolinite and KP (Figure 11a), four thermal phenomena were detected.

The two first weight losses (T_onset = 413 ° C and ~435 °C) are related to the removal of water, residual solvents and the dehydroxylation of kaolinite. The two final events, with onset temperatures of around 500 °C and 555 ° C, also observed in the samples with palygorskite, were attributed to the oxidation of the organic groups and decomposition of the aramid fiber, respectively [61], [62]. Table 6- Identification of onset temperatures and weight losses observed in the thermograms of mineral-silica aerogel composites with 5 wt% of KP represented in Figure 12a.

Table 7- Identification of onset temperatures and weight losses observed in the thermograms of mineral-silica aerogel composites with 5 wt% of TJ represented in Figure 12b.

The thermograms for the composites with TJ fiber (Figure

11b) are very similar to the ones obtained for the samples with KP. The composites with palygorskite displayed six weight losses (Table 7), while the ones with kaolinite presented only four phenomena. The only differences were observed for the fiber degradations, with the TJ presenting one weight loss at an onset temperature of 415 ° C, attributed to the heterolytic breaking of amide groups, and one at ca.

540 °C, ascribed to the homolytic cleavages of the amide bonds and the breaking of the bonds between the amide group and the aromatic ring. In all cases, after the oxidation of the surface groups, it is expected that the composites lose their hydrophobic character [21], [75].

As previously mentioned, the mineral-silica aerogel composites with only 5 wt% of Kevlar pulp as reinforcement phase presented thermal conductivities in the super- insulator range (< 25 mW.m^-1.K^-1). Very similar values were obtained for the composites reinforced with Teijinconex fibers, as shown in Table 8. The fibers have a positive influence in the thermal insulation properties of the composites, as the samples without fibers have higher thermal conductivity, with the palygorskite-silica aerogel having values around 28 mW.m^-1.K^-1 and the kaolinite-silica aerogel presenting values of approximately 27 mW.m^-1.K^-1

Table 8 Thermo-mechanical properties of mineral-silica aerogel composites.

Most of the values achieved here are lower than other silica aerogel composites with aramid fibers reported in the literature [10], [11], [57], [65], which indicates a positive impact of the addition of minerals into the composite. For example, for a composite system with TEOS and 5 wt% of KP, but with different solvent ratios, Ghica et al. [65] were able to achieve thermal conductivities around 55 mW.m^-1.K^-1

And, while Li et al. [10] were also able to achieve super insulation materials (23.2 mW.m^-1.K^-1), their silica aerogel/aramid pulp composite was modified with trimethylchlorosilane (TMCS), which is a major drawback, as the HCl production during the modification step can damage equipment and be harmful for the operators [21]. So, besides having great insulation properties, the synthesis procedure here developed is safer than others reported in the literature .

The mechanical behavior of the composites was assessed by compression-decompression tests, and the results are reported in Figure 12a, 12b, 12c and 12d. The Young's modulus was evaluated from the linear region of stress/strain loading curve of the compression test until 15% of strain (Figure

12a, Figure 12b and Table 8) and the values are all in the same order of magnitude. In general, the palygorskite samples are more flexible (lower Young's modulus) than ones with kaolinite . Regarding the fibers, the composites obtained with KP are more rigid than the ones with TJ, and these finds are in agreement with the literature [10], [57], in which smaller values of Young' s modulus were observed when elongated fibers were used. The difference in the values can be justified by the interaction between the mineral-silica aerogel and the fibers. The substantial number of nanofibers present in the aramid pulp (Figure 6a and 6b) allows an improved interface contact between KP and the aerogel matrix.

Even though no chemical bonds are formed, the three- dimensional network structure of the pulp favors interfacial adhesion between the two phases [9], [10], which allows KP and the aerogel to have a more tightly combination. As the

KP is well dispersed and does not have a preferential organization, the external stress can be transferred across the whole sample, preventing stress concentration, and, consequently, the composites can withstand higher compressive stresses [10], [65]. In the case of TJ, the fibers act as the composites' backbone, allowing the external stress to spread through the samples, preventing the structure failure [57]. However, as the fibers are mainly horizontally oriented, this led to different mechanical resistances if compared with samples with KP. The Young's modulus values here obtained are similar to the ones obtained for silica aerogels reinforced with other types of long fibers [57].

Axial cyclic compression-decompression tests (10 cycles) were also performed by submitting the samples to 25% strain

(Figure 12c and 12d). The composites' capacity to recover is an important feature for different purposes, such as in building and space applications [65]. The results indicate an excellent behavior, as the samples can withstand cyclic loads without disintegration or any noticeable fissures and show very high recoveries even after 10 compression- decompression cycles up to 25% strain (Table 8).

References

[1] M. A. Aegerter, NN.. Leventis, aanndd MM.. M. Koebel, Aerogels handbook. Springer Science & Business Media, 2011.

[2] J. L. Gurav, I.-K. Jung, H.-H. Park, EE.. SS.. Kang, and D. Y. Nadargi, "Silica aerogel: synthesis and applications," J. Nanomater., vol. 2010, 2010.

[3] F. Sabri, J. Marchetta, and K. M. Smith, "Thermal conductivity studies of a polyurea cross-linked silica aerogel-RTV 665555 compound for cryogenic propellant tank applications in space," Acta Astronaut., vol. 91, PP- 173-

179, 2013.

[4] A. Lamy-Mendes, A. D. R. Pontinha, P. Alves, P. Santos, and

L. Duraes, "Progress in silica aerogel-containing materials for buildings' thermal insulation," CCoonnssttrr.. Build. Mater., vol. 286, p. 122815, 2021.

[5] E. U. Haq, S. F. A. Zaidi, M. Zubair, M. RR.. AA.. Karim, S. K. Padmanabhan, and A. Licciulli, "Hydrophobic silica aerogel glass-fibre composite with higher strength and thermal insulation based on methyltrimethoxysilane (MTMS) precursor," Energy Build., vol. 151, pp. 494-500, 2017.

[6] H. Maleki, L. Duraes, and A. Portugal, "An overview on silica aerogels synthesis and different mechanical reinforcing strategies," JJ.. Non. Cryst. Solids, vvooll.. 385, pp. 55-74, 2014, doi: 10.1016/j.jnoncrysol.2013.10.017.

[7] P. C. Thapliyal and K. Singh, "Aerogels as promising thermal insulating materials: An overview," J. Mater., vol. 2014,

2014.

[8] M. O. Seydibeyoglu, A. KK.. Mohanty, and M. Misra, Fiber technology for fiber-reinforced composites. Woodhead

Publishing, 2017.

[9] Z. Li, X. Cheng, S. He, X. Shi, L. Gong, and H. Zhang, "Aramid fibers reinforced silica aerogel composites with low thermal conductivity and improved mechanical performance," Compos. Part A Appl. Sci. Manuf., vol. 84, pp. 316-325, 2016.

[10] Z. Li, L. Gong, C. Li, Y. Pan, Y. Huang, and X. Cheng, "Silica aerogel/aramid pulp composites with improved mechanical and thermal properties," J. Non. Cryst. Solids, vol. . PP- 1-

7, 2016, doi: https://doi.org/10.1016/j.jnoncrysol.2016.10.015.

[11] Z. Li, L. Gong, X. Cheng, S. He, C. Li, and H. Zhang, "Flexible silica aerogel composites strengthened with aramid fibers and their thermal behavior, " Mater. Des., vol. 99, pp. 349-355, 2016, doi: https://doi.org/10.1016/j.matdes.2016.03.063.

[12] P. R. Aravind, PP.. Mukundan, P. KK.. Pillai, and K. G. K.

Warrier, "Mesoporous silica-alumina aerogels wwiitthh high thermal pore stability through hybrid sol-gel route followed by subcritical drying," Microporous mesoporous Mater., vol.

96, no. 1-3, pp. 14-20, 2006.

[13] C. Rutiser, S. Komarneni, and R. Roy, "Composite aerogels of silica and minerals of different morphologies," Mater. Lett., vol. 19, no. 5-6, pp. 221-224, 1994.

[14] M. E. Ghica, C. M. R. Almeida, L. S. D. Rebelo, G. C. Cathoud-

Pinheiro, B. F. O. Costa, and L. Duraes, "Novel Kevlar® pulp- reinforced alumina-silica aerogel composites for thermal insulation at high temperature," J. Sol-Gel Sci. Technol., pp. 1-16, 2022.

[15] W. Hu, M. Li, W. Chen, N. Zhang, B. Li, M. Wang, and Z. Zhao, "Preparation of hydrophobic silica aerogel with kaolin dried at ambient pressure," Colloids Surfaces A Physicochem. Eng.

Asp., vol. 501, pp. 83-91, 2016.

[16] X. Ling, B. Li, M. Li, W. Hu, and W. Chen, "Thermal stability of Al-modified silica aerogels through epoxide-assisted sol- gel route followed by ambient pressure drying, " J. Sol-Gel Sci. Technol., vol. 87, no. 1, PP- 83-94, 2018.

[17] C. M. R. Almeida, M. E. Ghica, and L. Duraes, "An overview on alumina-silica-based aerogels," Adv. Colloid Interface Sci., vol. 282, p. 102189, 2020.

[18] J. Li, Y. Lei, D. Xu, F. Liu, J. Li, A. Sun, J. Guo, and G.

Xu, "Improved mechanical and thermal insulation properties of monolithic nanofiber/silica aerogel composites dried at ambient pressure, " J. Sol-Gel Sci.

Technol., vol. 82, no. 3, pp. 702-711, 2017, doi:

10.1007/S10971-017-4359-2.

[19] J. Zhu, S. Guo, and X. Li, "Facile preparation of a SiO2-

A12O3 aerogel using coal gangue aass a raw material via an ambient pressure drying method and its application in organic solvent adsorption, " RSC Adv., vol. 5, no. 125, pp. 103656-

103661, 2015.

[20] X. Wu, M. Fan, J. F. Mclaughlin, X. Shen, and G. Tan, "A novel low-cost method of silica aerogel fabrication using fly ash and trona ore with ambient pressure drying technique,"

Powder Technol., vol. 323, pp. 310-322, 2018.

[21] R. B. Torres, J. P. Vareda, A. Lamy-Mendes, and L. Duraes,

"Effect of different silylation agents on the properties of ambient pressure dried and supercritically dried vinyl- modified silica aerogels," JJ.. Supercrit. Fluids, vol. 147, pp. 81-89, 22001199,, doi: 10.1016/j.supflu.2019.02.010.

[22] M. Suarez and E. Garcia-Romero, "FTIR spectroscopic study of palygorskite: influence of the composition of the octahedral sheet," Appl. Clay Sci., vol. 31, no. 1-2, pp. 154-163, 2006.

[23] S.-M. G.Antonio, G.-C. P. Ivan, and G.-R. J. Luis, "Influence of chemically treated palygorskite over the rheological behavior of polypropylene nanocomposites," Ing. Investig. y

Tecnol., vol. 16, no. 4, pp. 491-501, 2015.

[24] W. Yan, D. Liu, D. Tan, P. Yuan, and M. Chen, "FTIR spectroscopy study of the structure changes of palygorskite under heating, " Spectrochim. acta part A Mol. Biomol.

Spectrosc., vol. 97, pp. 1052-1057, 2012.

[25] A. Chahi, S. Petit, and A. Decarreau, "Infrared evidence of dioctahedral-trioctahedral site occupancy in palygorskite," Clays Clay Miner., vol. 50, no. 3, pp. 306-313, 2002.

[26] D. A. McKeown, J. E. Post, and E. S. Etz, "Vibrational analysis of palygorskite and sepiolite," Clays Clay Miner., vol. 50, no. 5, pp. 667-680, 2002.

[27] A. Tironi, M. A. Trezza, E. F. Irassar, and A. N. Scian,

"Thermal treatment of kaolin: effect on the pozzolanic activity," Procedia Mater. Sci., vol. 1, pp. 343-350, 2012.

[28] J. Madejova, "FTIR techniques in clay mineral studies," Vib.

Spectrosc., vol. 31, no. 1, PP- 1-10, 2003.

[29] L. Vaculikova, E. Plevova, S. Vallova, and I. Koutnik,

"Characterization and differentiation of kaolinites from selected Czech deposits using infrared spectroscopy and differential thermal analysis," 2011.

[30] N. N. Bukalo, G.-I. E. Ekosse, J. O. Odiyo, and J. S. Ogola,

"Fourier transform infrared spectroscopy of clay size fraction of cretaceous-tertiary kaolins in the Douala Sub- Basin, Cameroon," Open Geosci., vol. 9, no. 1, PP- 407-418,

2017.

[31] W. Dong, Y. Lu, W. Wang, M. Zhang, Y. Jing, and A. Wang, "A sustainable approach to fabricate new ID and 2D nanomaterials from natural abundant palygorskite clay for antibacterial and adsorption," Ghent. Eng. J., vol. 382, p. 122984, 2020.

[32] E. Mendelovici and D. Ci. Portillo, "Organic derivatives of attapulgite—I. Infrared spectroscopy and X-ray diffraction studies," Clays Clay Miner., vol. 24, no. 4, pp. 177-182,

1976.

[33] K. Wang, HH.. Wang, RR.. Wang, JJ.. Key, V. Linkov, and S. Ji, "Palygorskite hybridized carbon nanocomposite as a high- performance electrocatalyst support for formic acid oxidation," South African J. Ghent., vol. 66, no. 1, pp. 86-

91, 2013.

[34] V.A. Drits, A. Derkowski, B.A. Sakharov, and B. B. Zviagina, "Experimental evidence ooff the formation of intermediate phases during transition of kaolinite into metakaolinite," Am. Mineral., vol. 101, no. 10, pp. 2331-2346, 2016.

[35] Y. Cui, Y. Zheng, and W. Wang, "Synthesis of 4A Zeolite from Kaolinite-Type Pyrite Flotation Tailings (KPFT)," Minerals, vol. 8, no. 8, p. 338, 2018.

[36] I. Daou, G. L. Lecomte-Nana, N. Tessier-Doyen, C. Peyratout,

M. F. Gonon, and R. Guinebretiere, "Probing the

Dehydroxylation of Kaolinite and Halloysite by In Situ High Temperature X-ray Diffraction," Minerals, vol. 10, no. 5, p.

480, 2020.

[37] M. E. Awad, A. Lopez-Galindo, M. Setti, M. M. El-Rahmany, and C. V. Iborra, "Kaolinite in pharmaceutics and biomedicine, " Tnt. J. Pharm., vol. 533, no. 1, pp. 34-48,

2017.

[38] X. L. Hu and A. Michaelides, "The kaolinite (0 0 1) polar basal plane," Surf. Sci., vol. 604, no. 2, pp. 111-117, 2010. [39] L. Weng-Lip, N.M. Salleh, N. A. Rahman, N. S.A.A. Bakhtiar,

H. M. Akil, aanndd SS.. A. Zubir, "Enhanced intercalation of organo-muscovite prepared via hydrothermal reaction at low temperature," Bull. Mater. Sci., vol. 42, no. 5, PP- 1-11,

2019.

[40] L. Boudriche, R. Calvet, B. Hamdi, and H. Balard, "Effect of acid treatment on surface properties evolution of attapulgite clay: an application of inverse gas chromatography," Colloids Surfaces A Physicochem. Eng. Asp., vol. 392, no. 1, pp. 45-

54, 2011.

[41] E. Ferraz, L.Alves, P. Sanguine, J. Santaren, M. G. Rasteiro, and J. A. F. Gamelas, "Stabilization of palygorskite aqueous suspensions using bio-based and synthetic polyelectrolytes,"

Polymers (Basel)., vol. 13, no. 1, p. 129, 2021.

[42] M. Suarez, J. Garcia-Rivas, J. M. Sanchez-Migallon, and E.

Garcia-Romero, "Spanish palygorskites: geological setting, mineralogical, textural and crystal-chemical characterization," Eur. J. Mineral., vol. 30, no. 4, pp. 733-

746, 2018.

[43] D. Cisneros-Rosado and J. A. Uribe-Calderon, "Effect of surface modification of palygorskite on the properties of polypropylene/polypropylene-g-maleic anhydride/palygorskite nanocomposites," Tnt. J. Polym. Sci., vol. 2017, 2017.

[44] C. Choudhury and T. V. Bharat, "Wetting-induced collapse behavior of kaolinite: influence of fabric and inundation pressure," Can. Geotech. J., vol. 55, no. 7, pp. 956-967,

2018.

[45] A. Muhmed and D. Wanatowski, "Effect of lime stabilisation on the strength and microstructure of clay," IOSR J. Meeh. Civ. Eng., vol. 6, no. 3, pp. 87-94, 2013.

[46] R. Giustetto, F. XX.. Llabres i Xamena, G Ricchiardi, s.

Bordiga, A. Damin, R. Gobetto, and M. R. Chierotti, "Maya

Blue: a computational and spectroscopic study, " J. Phys.

Chem. B, vol. 109, no. 41, pp. 19360-19368, 2005.

[47] C. Dejoie, P. Martinetto, E. Dooryhee, R. Brown, S. Blanc, P. Bordat, P. Strobel, P. Odier, F. Porcher, and M. S. del

Rio, "Diffusion of indigo molecules inside the palygorskite clay channels," MRS Online Proc. Libr., vol. 1319, 2011.

[48] L. Boudriche, RR.. Calvet, BB.. Hamdi, and H. Balard, "Surface properties evolution of attapulgite by IGC analysis as a function of thermal treatment, " Colloids Surfaces A

Physicochem. Eng. Asp., vol. 399, pp. 1-10, 2012.

[49] J. Xu, W. Wang, and A. Wang, "Effects of solvent treatment and high-pressure homogenization process on dispersion properties of palygorskite," Powder Technol., vol. 235, pp.

652-660, 2013.

[50] A. Preisinger, "Sepiolite and related compounds: its stability and application," Clays Clay Miner., vol. 10, no.

1, pp. 365-371, 1961.

[51] A. K. Panda, B. G. Mishra, D. K. Mishra, and R. K. Singh,

"Effect of sulphuric acid treatment on the physico-chemical characteristics of kaolin clay, " Colloids Surfaces A

Physicochem. Eng. Asp., vol. 363, no. 1-3, pp. 98-104, 2010.

[52] N. M. Faqir, R. SShhaawwaabbkkeehh,, MM.. Al-Harthi, and H. A. Wahhab,

"Fabrication of geopolymers from untreated kaolin clay for construction purposes," Geotech. Geol. Eng., vol. 37, no. 1, pp. 129-137, 2019.

[53] C. Chen, X. Wang, F. Wang, and T. Peng, "Preparation and characterization of Para-Aramid fibers with the main chain containing heterocyclic units," J. Macromol. Sci. Part B, vol. 59, no. 2, pp. 90-99, 2020.

[54] S. Villar-Rodil, J. I. Paredes, A. Martinez-Alonso, and J.

M. D. Tascon, "Atomic force microscopy and infrared spectroscopy studies of the thermal degradation of Nomex aramid fibers," Chem. Mater., vol. 13, no. 11, pp. 4297-4304,

2001.

[55] K. Xu, Y. Ou, Y. Li, L. Su, M. Lin, Y. Li, J. Cui, and D.

Liu, "Preparation of robust aramid composite papers exhibiting water resistance by partial dissolution/regeneration welding," Mater. Des., vol. 187, p.

108404, 2020. [56] J. Zhao, "Effect of surface treatment on the structure and properties of para-aramid fibers by phosphoric acid," Fibers

Polym., vol. 14, no. 1, pp. 59-64, 2013. [57] C. M. R. Almeida, M. E. Ghica, A. L. Ramalho, and L. Duraes,

"Silica-based aerogel composites reinforced with different aramid fibres for thermal insulation in Space environments,"

J. Mater. Sci., pp. 1-16, 2021.

[58] S. M. Shebanov, I. K. Novikov, A. V Pavlikov, O. B. Anariin, and I. A. Gerasimov, "IR and raman spectra of modern aramid fibers," Fibre Chem., vol. 48, no. 2, pp. 158-164, 2016.

[59] L. Wenbin, H. Jianfeng, FF.. JJiiee,, LL.. Zhenhai, C. Liyun, and Y. Chunyan, "Effect of aramid pulp on improving mechanical and wet tribological properties of carbon fabric/phenolic composites, " Tribal. Tnt., vol. 104, pp. 237-246, 2016.

[60] W. Yin-Kui, W. Xi-Wen, H. Jian, and L. Jin, "Wet-laid nonwoven preparation a separator for MH-Ni battery, " Int. J.

Electrochem. Sci, vol. 8, pp. 9287-9297, 2013.

[61] R. Ramani, T.M. Kotresh, R. I. Shekar, F. Sanai, U. K. Singh,

R. Renjith, and G. Amarendra, "Positronium probes free volume to identify para-and meta-aramid fibers and correlation with mechanical strength," Polymer (Guildf)., vol. 135, pp. 39-

49, 2018.

[62] M. E. G. Mosquera, M. Jamond, A. Martinez-Alonso, and J. M.

D. Tascon, "Thermal transformations of Kevlar aramid fibers during pyrolysis: infrared and thermal analysis studies, "

Chem. Mater., vol. 6, no. 11, PP- 1918-1924, 1994.

[63] C. Zhang, Y.-X. Jiang, J.-P. Sun, H. Xiao, M.-W. Shi, and

J.-J. Long, "Investigation of the influence of supercritical carbon dioxide treatment on meta-aramid fiber: Thermal decomposition behavior and kinetics," J. CO2 Util., vol. 37, pp. 85-96, 2020.

[64] G. Genq, B.Alp, D. Balkose, S. Ulku, and A. Cireli, "Moisture sorption and thermal characteristics of polyaramide blend fabrics," J. Appl. Polym. Sci., vol. 102, no. 1, pp. 29-38,

2006.

[65] M. E. Ghica, C. M. R. Almeida, M. Fonseca, A. Portugal, and L. Duraes, "Optimization of Polyamide Pulp-Reinforced Silica

Aerogel Composites for Thermal Protection Systems," Polymers

, vol. 12, no. 6. 2020, doi: 10.3390/polyml2061278.

[66] S. Groult and T. Budtova, "Thermal conductivity/structure correlations in thermal super-insulating pectin aerogels,"

Carbohydr. Polym., 2018, doi: 10.1016/j.carbpol.2018.05.026.

[67] A. Lamy-Mendes, W. J. Malfait, A. Sadeghpour, A. V Girao, R.

F. Silva, and L. Duraes, "Influence of ID and 2D carbon nanostructures in silica-based aerogels," Carbon N. ¥., vol.

180, pp. 146-162, 2021.

[68] M. Koebel, A. Rigacci, and P. Achard, "Aerogel-based thermal superinsulation: An overview," J. Sol-Gel Sci. Technol., vol.

63, no. 3, pp. 315-339, 2012, doi: 10.1007/S10971-012-2792-

9.

[69] A. Lamy-Mendes, A. V. V. Girao, R. F. F. Silva, and L. Duraes,

"Polysilsesquioxane-based silica aerogel monoliths with embedded CNTs," Microporous Mesoporous Mater., vol. 288,

2019, doi: 10.1016/j.micromeso.2019.109575.

[70] R. Al-Oweini and H. El-Rassy, "Synthesis and characterization by FTIR spectroscopy of silica aerogels prepared using several Si (OR) 4 and R Si (OR') 3 precursors," J. Mol.

Struct., vol. 919, no. 1-3, pp. 140-145, 2009.

[71] M. Ochoa, L. Duraes, A. M. Beja, and A. Portugal, "Study of the suitability of silica based xerogels synthesized using ethyltrimethoxysilane and/or methyltrimethoxysilane precursors for aerospace applications," J. sol-gel Sci.

Technol., vol. 61, no. 1, pp. 151-160, 2012.

[72] S. L. B. Lana and A. B. Seddon, "X-ray diffraction studies of sol-gel derived ORMOSILs based on combinations of tetramethoxysilane and trimethoxysilane," J. sol-gel Sci.

Technol., vol. 13, no. 1, pp. 461-466, 1998.

[73] B. Orel, R. Jese, A. Vilcnik, and U. L. Stangar, "Hydrolysis and solvolysis of methyltriethoxysilane catalyzed with HC1 or trifluoroacetic acid: IR spectroscopic and surface energy studies," J. Sol-Gel Sci. Technol., vol. 34, no. 3, pp. 251-

265, 2005. [74] P. M. Shewale, A. V. Rao, J. L. Gurav, and A. P. Rao,

"Synthesis and characterization of low density and hydrophobic silica aerogels dried at ambient pressure using sodium silicate precursor," J. Porous Mater., vol. 16, no.

1, pp. 101-108, 2009.

[75] L. Duraes, M. Ochoa, N. Rocha, R. Patricio, N. Duarte, V.

Redondo, and A. Portugal, "Effect of the drying conditions on the microstructure of silica based xerogels and aerogels,"

J. Nanosci. Nanotechnol., vol. 12, no. 8, pp. 6828-6834,

2012.

This description is of course not in any way restricted to the forms of implementation presented herein and any person with an average knowledge of the area can provide many possibilities for modification thereof without departing from the general idea as defined by the claims. The preferred forms of implementation described above can obviously be combined with each other. The following claims further define the preferred forms of implementation.

Claims

1. A silica aerogel composite characterized by comprising aluminosilicates selected from kaolin, palygorskite, or mixtures thereof, in an amount between 5 and 35 wt%, and being reinforced with aramid fibers in an amount between 3 to 15% wt% of the final aerogel weight; wherein the aerogel composite has a bulk density between 100 and 170 kg.im³ and thermal conductivity between 22 and 25 mW K^-1 m^-1.

2. Silica aerogel composite according to the previous claim, wherein the kaolin, palygorskite, or mixtures thereof, are present in an amount between 10 and 20 wt%.

3. Silica aerogel composite according to any of the previous claims, wherein the aramid fibers are selected from para-aramids, meta-aramids or a mixture thereof.

4. Silica aerogel composite according to any of the previous claims, wherein the aramid fibers have lengths between 0.3 mm to 1 mm or between 3 to 5 cm.

5. Method to produce the silica aerogel composite comprising aluminosilicates and reinforced with aramid fibers described in any of the previous claims, comprising the following steps:

- Mixing at least one aluminosilicate selected from kaolin or palygorskite, or mixtures thereof, in a concentration between 5 wt % and 35 wt % of the final aerogel weight with a solution comprising an acid in a concentration between 0.1 and 1 M in a solution that is 50 to 95% v/v of ethanol; - Stirring the resulting solution for a time between 1 and 5 days at a temperature between 10 and 40 °C;

- Adding to the previous solution a silica precursor and ethanol in a molar ratio from 1:5 to 1:35, and stirring for a time between 5 and 30 minutes;

- The aramid fibres in an amount between 3 to 15% wt% are added to the solution together with the silica precursor or arranged in a mould and the silica solution prepared according to the method is poured into the mould;

- Storing the solution at a temperature between 20 and 50°C for a time between 1 and 24 hours to promote hydrolysis;

- Adding to the solution NH₄F or NH₄OH with a concentration between 0.1 and 5 M and stir for a time between 20 and 60 seconds;

- Gelation and aging of the solution by keeping the previous solution comprising aramid fibres at a temperature between 20 and 50°C for a time between 2 and 6 days to obtain the aerogel;

- After gelation and aging, the surface of the aerogel is modified with a silylating solution of a silylating agent in heptane with a concentration between 10 and 40 (v/v), at a temperature between 15 and 60 °C for a time between 12 and 96 hours;

- Drying the aerogel and submit it to a temperature between 20 and 80 °C between 1 and 6 hours followed by a temperature between 100 and 200 °C at a time between 1 and 3 hours.

6. Method according to the previous claim, wherein the acid is selected from nitric acid, sulfuric acid, peracetic acid, a mixture of acetic acid and hydrogen peroxide.

7. Method according to any of the claims 5 to 6, wherein the silica precursor is selected from tetramethylorthosilicate, tetraethylorthosilicate, vinyltrimethoxysilane, 3-aminopropyltrimethoxysilane, aminopropyltriethoxysilane, methyltrimethoxysilane and methyltriethoxysilane, or mixtures thereof.

8. Method according to any of the claims 5 to 7, wherein the silica precursor and ethanol are added in a molar ratio of 1:10.

9. Method according to any of the claims 5 to 8, wherein the silylating agent is selected from Hexamethyldisilazane, Trimethylchlorosilane, Methyl trimethoxysilane, Methyltriethoxysilane, Dimethyldimethoxysilane, Hexamethyldisiloxane or mixtures thereof.

10. Method according to any of the claims 5 to 9, wherein after gelation and aging the aerogels are washed with ethanol and heptane or methanol or hexane, at a temperature between 20 and 60 °C.

11. A silica aerogel composite described in any of the claims 1 to 5 for use as thermal insulators for space devices, equipment, buildings or piping.