"FIBRE-REINFORCED AEROGEL COMPOSITES FROM MIXED SILICA AND RUBBER SOLS AND A METHOD TO PRODUCE THE RUBBER-SILICA

AEROGEL COMPOSITES"

Technical field

This application relates to rubber-silica aerogel composites reinforced with fibres and a method to produce said aerogel composites from silica and rubber sols.

Background art

One of the most critical issues in modern society is the constant increase of waste [1]. Currently, over 1.6 billion new tires and around 1 billion of waste tires are generated worldwide every year [2]. In the last decade, there has been a growth in the number of tires being discarded as end-of- life tires (ELTs) [1], which leads to serious environmental problems. In order to improve the waste management practices, the European legislation has established a priority order for dealing with wastes, from the most preferred option of reduction, followed by reuse, recycling, energy production to least preferred option of disposal [3]-[5]. However, significant difficulties are associated with the recovery and recycling of used tires, due to their composition and complex structure [6]. The main component of the tires (70— 80% of the total mass) is vulcanized rubber, and their disposal is an issue, as they are non-biodegradable and cannot be reprocessed in a simple process like the thermoplastics, remaining on the landfills [1], [6], [7].

To engage in more environmentally friendly solutions, and comply with more restrict legislations, new methods for the treatment or reuse/recycling of the basic components of ELTs are being developed. Dwivedi and co-authors [8] were able to use the carbon-rich solid (RCB) fraction, from the pyrolysis of waste tires, as a partial substitute to conventional carbon black (CB) in natural rubber-based conveyor belts cover composites. The mechanical properties of the RCB reinforced composites were lower than the material obtained with conventional CB. However, when a combination of RCB and commercial CB were used, these limitations could be overcome, and the impact in the mechanical properties is acceptable. This partial replacement reduces the cost of rubber compounds and provides a sustainable recycling option of waste tires.

Another approach was developed by Passaponti et al. [9], in which the authors were able to recover and valorize the chars, the least valuable by-product of waste tire pyrolysis. They were able to convert this material into highly efficient catalysts for the Oxygen Reduction Reaction (ORR), with a simple heat treatment. The sample prepared at 450°C displayed the maximum catalytic activity, with a conversion efficiency of O2 to H2O above 85%. The developed material has potential to be applied in alkaline fuel cells and metal air batteries.

Wang et al. [10] were able to directly convert waste tires into 3D graphene, by raising pyrolytic temperature to 1000°C and using an active K vapor to induce carbon atom rearrangement. The final material displays a well-defined porous gradient and high electrical conductivity (18.2 S.cm ¹ ). When applied as supercapacitor electrode, the graphene exhibits an excellent capacitive behaviour, with almost no degradation after 10000 cycles. This work provides a new consideration for the development of more high-value products from waste tires. Another possibility to obtain high value products is the synthesis of aerogels from waste tires, as presented by Thai et al. [7]. The authors were able to obtain rubber aerogels from recycled car tires fibres through the freeze-drying method, using polyvinyl alcohol and glutaraldehyde as crosslinkers. The rubber aerogel has densities as low as 35 kg.m ^-3 , porosities up to 96 %, thermal and sound insulation properties with thermal conductivities between 35 and 47 mW.m ^_1 .K ^_1 and a noise reduction coefficient of 0.41. Besides that, the aerogel also has an oil absorption capacity of 19.3 g.g ^-1 , showing the versatility of the developed material .

Some works have already treated rubber with peracetic acid for producing epoxidized rubber; however, the final product of these works was always a solid rubber [11-16]. In the present invention, the final product is a stable colloidal rubber solution in alcohol, that can be easily mixed with a silane sol in order to form a rubber-silica solution.

Evans et al. [17] developed a sulphur-containing organic- inorganic hybrid gel composition and aerogels. In this document, the authors were also able to obtain a composite from a silica precursor and rubber, however, unlike the present invention, they used a sulphur based cross linking agent to bridge the organic-inorganic components. They also performed a modification of the poly(styrene-butadiene) latex, for example, with a sulfidosilane coupling agent, to increase the organic loading in the final material.

A few other documents also developed composites with silica aerogel and rubber [18-21], but with very different procedures when compared to the one disclosed in this application. In the first [18], the authors obtained aerogel- blown rubber composite particles for sewage disposal. For the preparation of this composite, first the authors prepared a silica gel, and, in the sequence a demulsifier was added into the gel. The next step was to add blown rubber particles in the previous mixture, to be then dried using supercritical carbon dioxide.

For the second document [19], the developed material is an aerogel-containing high-weather-resistance foaming rubber. The synthesis of this material starts with plasticating the ethylene propylene diene monomer rubber and the styrene butadiene rubber, followed by the mixture of these materials to form a sizing material. In the next step, the rubber material is added into a mixer with lubricant, accelerator and an auxiliary agent. After this, the mixture is placed in an open mill; cross-linking, foaming and reinforcing agents were added together with the commercial silica aerogel to obtain the rubber compound. Then, vulcanization, foaming and molding were performed to obtain the foamed sample.

The third document [20] discloses a thermal insulation sheet comprised of an aerogel layer and a coating layer disposed on both sides of the aerogel layer, where the aerogel layer is formed by binding the aerogel particles with a rubber- based binder.

Silica aerogel was also used in document [21]. In this work, the incorporation of the silica aerogel is performed by milling this material with unvulcanized crepe rubber to form a product suitable for conventional purposes such as soling. The incorporation of silica aerogel nanoparticles into a natural rubber latex (NRL) matrix was also studied in an article by On et al. [22]. The authors developed a NRL-silica aerogel film by latex compounding and dry coagulant dipping to form a thin film where silica aerogel acts as filler. The addition of silica aerogel enhances the mechanical properties of the NRL-silica aerogel film, with 4 phr (parts per hundred rubber) of silica aerogel giving the optimum tensile, tear strength, and elongation at break.

Recycled car tire fibres were used to obtain a rubber aerogel by using polyvinyl alcohol (PVA) and glutaraldehyde (GA) as crosslinkers through a cost-effective freeze-drying method, in the works developed by Thai et al. [23-25]. The combination of properties of the aerogels, such as ultra-low density (0.020-0.091 g.cm ³ ), super-hydrophobic properties (water contact angle up to 153°), high sound absorption efficiency (noise reduction coefficient of 0.56) and low thermal conductivity (35 - 49 mW.nr ¹ .K ^_1 ), makes them useful to be applied in different areas, for example thermal insulation, sound absorption and oil spill-cleaning applications. It should be noted that this material does not contain silica, it is most likely a crosslinked fibre mat.

Even though the composite materials here cited have similar denominations as the present invention, the synthesis procedures are completely different than the one of the present application.

Summary

The present application relates to a fibre-reinforced rubber-silica aerogel composite comprising silica and tire rubber and being reinforced with fibres, wherein the composite comprises from 5 to 25% w/w of rubber, has a bulk density between 100 and 200 kg m ³ , contact angle with water between 120 and 160° and thermal conductivity between 14 and 30 mW.m ^_1 -K ^_1 .

The present application also relates to a method to produce the fibre-reinforced rubber-silica aerogel composite comprising the following steps: dissolving tire rubber in a solution comprising an oxidising agent and stirring to obtain a rubber colloidal solution, wherein the tire rubber is present in an amount from 1 to 10% w/v in the rubber colloidal solution;

- mixing the previous rubber colloidal solution with a solution comprising a silane, an alcohol and water;

- adding an aqueous basic catalyst solution to the previous mixture;

- pouring the previous mixture into a mold comprising fibres to form the aerogel composite;

- unmolding the aerogel composite and washing it first with an alcohol solution and afterwards with a non-polar solvent;

- modifying the surface of the aerogel composite by immersing the aerogel composite in a silylating solution comprising a dilution fluid and a first modifying agent, then adding a second modifying agent to the solution, and incubating the aerogel composite in the solution;

- drying the composite aerogel.

In one embodiment the tire rubber is used with a particle diameter lower than 1 mm.

In another embodiment an alcohol in further added to the rubber and oxidising agent solution and is selected from methanol, ethanol, n-propanol, isopropanol, or mixtures thereof.

In yet another embodiment the oxidizing agent is selected from peracetic acid, hydrogen peroxide, sulfuric acid, and nitric acid, a mixture of acetic acid or acetic anhydride and hydrogen peroxide, and is present in a concentration varying from 2% to 40% v/v of the rubber colloidal solution.

In one embodiment the molar ratio between silane and alcohol in the solution varies from 1:5 to 1:35.

In another embodiment silane is selected from tetramethylorthosilicate (TMOS), tetraethylorthosilicate (TEOS), methyltrimethoxysilane (MTMS), 3- aminopropyltrimethoxysilane (APTMS), aminopropyltriethoxysilane (APTES), vinyltrimethoxysilane (VTMS), or mixtures thereof.

In yet another embodiment the aqueous basic catalyst is selected from ammonium hydroxide, sodium hydroxide, potassium hydroxide, ammonium carbonate, and the concentration in the aqueous basic catalyst solution varies from 1 mol.L ^-1 to 15 mol.L ^-1 .

In one embodiment the fibres are used in their loose form or in the form of a blanket/mat/felt.

In another embodiment the solvent of the silylating solution is selected from hexane, heptane, octane, ethanol or isopropanol . In one embodiment the first modifying agent is selected from, hexamethyldisiloxane (HMDSO), hexamethyldisilazane (HMDZ), methyltrimethoxysilane (MTMS), dimethoxydimethylsilane (DMDMS), among other organosilanes with non-hydrolysable methyl groups, and is present in the silylating solution in a concentration between 15% and 35% v/v.

In one embodiment the second modifying agent is selected from trimethylchlorosilane , dimethyldichlorosilane or methyltrichlorosilane, and is present in the silylating solution in a concentration from 5% to 25% v/v.

In one embodiment the fibres are selected from recycled tire fibres, glass fibres, glass wool, silica fibres, polyester fibres, or other type of organic or inorganic fibres, in the form of loose fibres or a blanket/mat/felt.

In one embodiment the fibres are pre-treated with an alcoholic solution comprising an oxidising agent, stirred from 1 to 24 hours, filtered, washed and dried.

In one embodiment the fibre-reinforced rubber-silica aerogel composite is used as a thermal insulator, an acoustic insulator, or as an adsorbent material.

General description

Motivated by the environmental problems caused by ELTs, and the current potential markets for products obtained with recycled/reused materials, the goal of the present application is to disclose fibre-reinforced silica/rubber aerogel composites. The fibres imbedded in the aerogel matrix ensure its integrity while handling and improves the flexibility of the composite, as silica aerogels are inherently brittle and cannot be bent.

For the first time, recycled rubber sols were produced and incorporated in the silica sols. Since the ELTs rubber is vulcanized, it is very difficult to link its granules to other compounds in composites. Even when strategies are implemented to cross-link the rubber granules with other organic matrices or organically-modified inorganic matrices, these linkages are normally not strong enough and the granules easily separate from the composite. Therefore, a strategy that transforms rubber granules into a rubber sol, by the use of an acidic strong oxidizer, will allow the intimate mixing of this sol with the silica sol (in the case of the aerogel), avoiding the segregation of the two phases. In addition, the acidic nature of the oxidizer will promote hydrolysis of the silica precursors without the need of using other acid catalysts.

The final aerogel composite materials herein disclosed were characterized regarding their chemical, physical, structural, and thermal/acoustic properties. Due to their very low thermal conductivity and bulk density, high vibration dissipation, hydrophobicity and good mechanical flexibility, these composite aerogels not only promote the recycling of waste tire, but also have a high potential for thermal and acoustic insulation applications (buildings, roads, automotive, aeronautics/aerospace, among others), as well as an adsorbent material for cleaning wastewaters with apolar pollutants (oils, organic solvents, pharmaceuticals, dyes, among others) by their sorption. 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 1 shows macro-photographs of a) the aerogel composite material with recycled tire fibres, b) the aerogel composite material with polyester fibre blanket, c) the aerogel composite material with silica fibre blanket and d) the aerogel composite material with glass wool.

Figure 2 shows a) FTIR spectrum and b) TGA of recycled tire fibre.

Figure 3 shows a) FTIR spectrum and b) TGA of polyester fibre.

Figure 4 shows a) FTIR spectrum and b) TGA of silica fibre.

Figure 5 shows a) FTIR spectrum and b) TGA of glass wool.

Figure 6 shows the reactions of hexamethyldisiloxane (HMDSO) and trimethylchlorosilane (TMCS) with silica surface (1) and reaction of HMDSO with HC1 forming TMCS (2).

Figure 7 shows thermogravimetric analysis (TGA) of a) aerogel composite material with recycled tire fibres, b) aerogel composite material with polyester fibre blanket, c) aerogel composite material with silica fibre blanket and d) aerogel composite material with glass wool. Figure 8 shows SEM images of a,b) aerogel composite with recycled tire fibres, and c,d) aerogel composite with polyester fibre blanket, e,f) aerogel composite with silica fibre blanket, g,h) aerogel composite with glass wool. a,c,e,g) Interaction of the silica phase with the fibres; b,d,f,h) rubber and silica matrix..

Figure 9 shows mechanical tests of the aerogel composite material with different fibres, a) Reversible compressive stress-strain curves of the composites until 10% strain; b) ten cycles of reversible compressive stress-strain curves of the composites until 10% strain with a load cell of 50 N; c) reversible compressive stress-strain curves of the composites until 25% strain; and d) uniaxial compression with a load cell of 3 kN.

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 application relates to fibre-reinforced rubber- silica aerogel composites and a method to produce said aerogel composites.

The aerogel composites of the present application have a silica/rubber composition and are reinforced with fibres as shown in Figure 1.

The aerogel composites were produced from rubber sols incorporated in silica sols, and the addition of fibres. The method to produce the fibre-reinforced rubber-silica aerogel composite of the present application comprises the following steps:

- Dissolving tire rubber in a solution comprising an oxidising agent, and stirring to obtain a rubber colloidal solution;

In one embodiment, the tire rubber is used with a particle diameter of less than 1 mm.

In one embodiment the rubber is present in an amount from 1 to 10% w/v in the rubber colloidal solution and 5 to 25% w/w of the aerogel composite. In a preferred embodiment, the rubber is 5% w/v of the rubber colloidal solution and 10% w/w of the composite.

In one embodiment alcohol is added to the solution comprising tire rubber and the oxidising agent.

In one embodiment, the alcohol is selected from, but not limited to, methanol, ethanol, n-propanol, isopropanol, or mixtures thereof.

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

The oxidizing agent concentration varies from 2% to 40% v/v in the rubber colloidal solution. In a preferred embodiment, the concentration of peracetic acid is 5% v/v.

In one embodiment, the stirring occurs from 1 to 30 hours. In a preferred embodiment, for a concentration of 5% v/v of oxidising agent in the rubber colloidal solution, the stirring time is 24 hours.

- Mixing the previous rubber colloidal solution with a solution comprising a silane, an alcohol and water; with the rubber colloidal solution being 15 to 30 vol.% of the resultant solution.

In one embodiment the colloidal solution is 23.5% v/v of the resultant solution.

In one embodiment, the molar ratio between silane and alcohol in the resultant solution varies from 1:5 to 1:35. In a preferred embodiment, the silane:alcohol molar ratio is

1:10.

In one embodiment, the molar ratio between silane and water in the resultant solution varies from 1:2 to 1:10. In a preferred embodiment, the silane:water molar ratio is 1:4.

In one embodiment, the silane is selected from, but not limited to, tetramethylorthosilicate (TMOS), tetraethylorthosilicate (TEOS), methyltrimethoxysilane (MTMS), 3-aminopropyltrimethoxysilane (APTMS), aminopropyltriethoxysilane (APTES), vinyltrimethoxysilane (VTMS), or mixtures thereof.

In one embodiment, the alcohol is selected from, but not limited to, methanol, ethanol, n-propanol, isopropanol, or mixtures thereof. In one embodiment, the mixture is stirred from 5 minutes to 24 hours.

In one embodiment, the method can further comprise a storing step of the previous mixture, but it is not mandatory. The mixture can be stored at a temperature between 20°C to 50°C for a period from 1 to 24 hours.

- Adding an aqueous basic catalyst solution to the previous mixture;

In one embodiment the molar ratio between silane and water with the basic catalyst solution is from 1:2 to 1:8, but in a preferred embodiment this ratio is 1:4.

In one embodiment the mixture is under stirring from 1 to 10 minutes.

In one embodiment the concentration of the base in the aqueous basic catalyst varies from 1 mol.L ^-1 to 15 mol.L ^-1 .

In one embodiment the aqueous basic catalyst is selected from, but not limited to, ammonium hydroxide, sodium hydroxide, potassium hydroxide, ammonium carbonate, among other basic catalysts.

- Pouring the previous mixture into a mold comprising fibres to form the aerogel composite;

In one embodiment, the volume of the mold is filled with the fibres in their loose form or in the form of a blanket/mat/felt . - Unmolding the aerogel composite and wash it first with an alcohol solution and afterwards with a non-polar solvent;

In one embodiment, the washing step is performed at a temperature between 15°C and 60°C.

In one embodiment, the alcohol solution is selected from, but not limited to, methanol, ethanol, n-propanol, isopropanol, or their mixtures, and used in an amount of 50 to 95% v/v.

In one embodiment, the non-polar solvent is selected from, but not limited to, pentane, hexane, heptane, cyclohexane, or their mixtures, and used in an amount of 50 to 95% v/v.

- Modifying the surface of the aerogel composite by immersing the aerogel composite in a silylating solution comprising a dilution fluid and a first modifying agent, then adding a second modifying agent to the solution;

In one embodiment the dilution fluid is selected from hexane, heptane, octane, ethanol or isopropanol, in a concentration between 65% and 85% v/v in the silylating solution.

In one embodiment, the first modifying agent is present in the silylating solution in a concentration between 15% and 35% v/v.

In one embodiment the first modifying agent is selected from, but not limited to, hexamethyldisiloxane (HMDSO), hexamethyldisilazane (HMDZ), methyltrimethoxysilane (MTMS), dimethoxydimethylsilane (DMDMS), among other organosilanes with non-hydrolysable methyl groups.

In one embodiment the second modifying agent is selected from TMCS, dimethyldichlorosilane or methyltrichlorosilane, but preferentially TMCS, and is present in the silylating solution in a concentration from 5% to 25% v/v.

The silylating solution is stirred for a time between 5 and 60 minutes.

- Incubating the composite aerogel in the silylating solution;

In one embodiment, the composite aerogel is incubated in the silylating solution at a temperature between 15 °C and 60 °C, for a time between 24 h and 96 hours.

- Drying the composite aerogel.

In one embodiment, the composite aerogel is dried temperatures between 50 °C and 200 °C, depending on the thermal stability of the reinforcing fibres.

In one embodiment, the fibres for composite reinforcement are selected from recycled tire fibres, glass fibres, glass wool, silica fibres, polyester fibres, or other type of organic or inorganic fibres.

The fibres are used either in the form of a blanket/ /mat/felt or used as loose fibres dispersed layer-by-layer in the mold. In the context of this application, a fibre blanket is understood as a non-woven fibre network (for example like a felt), with the fibres randomly distributed or aligned in horizontal/longitudinal direction. In addition, when using recycled tire fibres, these can be assembled in a mat by needle-punching them before insertion in the mold.

Very good results were obtained when the silica fibres and polyester fibres (both in the form of felts), were used as reinforcement of the aerogel composites, these materials presenting superinsulation properties.

In the embodiment where recycled tire fibres are used for reinforcement, they can be used in their pristine state or after being modified by an acid solution pre-treatment.

The fibre pre-treatment step is achieved by mixing the tire fibres with an alcoholic solution comprising an oxidising agent. This mixture is stirred from 1 to 24 hours and then filtered. The fibres are then washed with alcohol, for example using the centrifuge, and then dried at a temperature between 40 °C and 250 °C.

In one embodiment the alcohol of this step is selected from, but not limited to, ethanol, methanol, isopropanol or butanol.

In one embodiment, the oxidizing agent of this step is selected from, but not limited to, peracetic acid, hydrogen peroxide, a mixture of acetic acid or acetic anhydride and hydrogen peroxide, sulfuric acid, and nitric acid, and is present in the pre-treatment alcoholic solution in a concentration 2 to 40% v/v. In one embodiment, the alcohol to wash the fibres is selected from, but not limited to, methanol, ethanol, or isopropanol or mixtures thereof, used in a concentration varying from 50 to 95% v/v.

The fibre-reinforced rubber-silica aerogel composite disclosed in the present application is made from the mixing of silica and rubber sols, comprising from 5 to 25% w/w of rubber in the composite. It has negligible shrinkage, bulk density between 100 and 200 kg m ³ , contact angle with water between 120 and 160°, thermal conductivity between 14 and 30 mW.m ^_1 -K ^_1 , and with negligible weight loss up to 400 °C in the case of the more thermally stable fibres and inert atmosphere .

Examples

Materials

Tire rubber (diameter < 0.8 mm), ethanol (absolute, Fluka; C2H5OH), peracetic acid (38-40%, Merck; CH3CO3H), tetraethylorthosilicate (TEOS; purity ³ 99%, Aldrich; Si (OC2H5)4), ammonium hydroxide (25% NH3 in H2O, Fluka Analytical; NH4OH), n-hexane (C6H14, purity > 95%, Fisher Chemical), hexamethyldisiloxane (HMDSO, (CH3)3SiOSi(CH3)3, purity > 98%, Acros Organics), trimethylchlorosilane (TMCS, (CH3)3SiCl, purity ³ 98%, Sigma Aldrich) and several types of fibres were used in the composites production. The fibres acted as reinforcement of composites. The tire fibres include the following three main components: i) natural rubber and synthetic rubber polymers, ii) steel wire and iii) textile fibres (Nylon 6 and 66, Polyester, Kevlar, Rayon and Glass). A polyester fibre blanket, a silica fibre felt, and glass wool were also applied as alternatives to tire fibres. 1 . Aerogel composite synthesis

In the first step for the composite synthesis, 0.5 g of tire rubber was dissolved in a 10 mL solution containing ethanol and peracetic acid, with acid concentration of 5.0 vol.%. The solution was stirred for 24 h. After this period, a black colloidal solution was obtained. In one embodiment, the concentration of peracetic acid solution was 5 vol.% with a dissolution time of 24 h.

In the next step, the rubber colloidal solution was mixed with the silica sol. So, ethanol, TEOS and double distilled water were added, with a molar ratio of 10:1:4, to the rubber colloidal solution, and the mixture was stirred for 30 min. This solution was then stored in an oven at 27 °C for 24 h for the hydrolysis step. A basic solution, NH ₄ OH 2.5 M, was added to the former solution and kept under strong agitation for 1 minute and then poured into the mold with the fibre blanket. The samples were kept in the oven at 27 °C during 5 days for aging.

Different types of fibres were used for the composites' reinforcement, from recycled tire fibres to glass/silica and polyester fibres. In the case of recycled tire fibres, they were used in their pristine state or after being modified by an acid solution treatment. For this treatment, 5 g of tire fibres were mixed with an alcoholic solution with 2.5 vol.% of peracetic acid. The mixture was stirred for 1-2 h and then filtered. The fibres were washed two times with ethanol using the centrifuge and then dried at 60 °C in an oven.

The composite aerogels were unmolded and washed with ethanol and hexane at 50 °C. The aerogel samples were then subjected to a surface modification. The silylating solution comprises hexane, HMDSO and TMCS (70:20:10 volumetric percentages). First, hexane and HMDSO were added to the aerogel samples, and the solution was stirred for 30 min, and then TMCS was added, and the solution is stirred for another 30 min. The aerogel samples were immersed in the silylating solution and then placed in an oven at 50 °C and kept at that temperature for 8 h. After that, the samples are kept in the silylating solution for another 48 hours at a temperature between 15 and 35 °C. To dry the samples, the solution was removed, and the samples are kept in a hood for 24 h and then subjected to 100 °C for 3 h and 150 °C for 3 h.

2 . Characterization procedures

The properties of the final composite aerogel materials were assessed by different characterization techniques. The bulk density (p _b ) was calculated from the weight and volume of regular pieces of the samples. The chemical structure was evaluated by attenuated total reflection (ATR) Fourier- transform infrared spectroscopy (FTIR) (FT/IR 4200, Jasco), collecting the spectra between a wavenumber of 4000 and 400 cm ^-1 , with 128 scans and 4 cm ^-1 of resolution. For the rubber, elemental analysis (EA) was also performed (EA 1108 CHNS-O, Fision Instruments), in terms of C, N, H and S elements. Scanning electron microscopy (SEM) images were obtained using a Compact/VPCompact FESEM (Zeiss Merlin) microscope, after coating the aerogel samples with a thin gold layer by Physical Vapor Deposition, during 20 s. Thermal properties were assessed by thermal gravimetric analysis and thermal conductivity. The thermal stability of different materials was obtained by using a DSC/TGA equipment (TGA-Q500, TA Instruments), from 20 °C to 800 °C, at a 10 °C.min ^_1 heating rate under nitrogen flow. Thermal conductivity, k, was measured with a Thermal Constants Analyzer TPS 2500 S (Hot Disk), using the transient plane source method with two samples maintained at 20 °C. For samples with dimensions of 21.5 x 21.5 cm ² , the thermal conductivity was also determined using Heat Flow Meter HFM 436/3/1 Lambda (EN 1946-1:1999), from NETZSCH, at 23 °C. The results obtained in this equipment are usually in close agreement with those obtained by the Guarded Hot Plate method [26].

For the composite with lower thermal conductivity, i.e. that reinforced with polyester fibres, the dynamic stiffness, s _t ', was measured following the test procedures defined in standard ISO 9052-1 and the test-samples have a thickness of 15 mm and an area of 20.0 x 20.0 cm ² .

Sorption tests were also performed with the composite reinforced with the polyester fibre blanket. The composite was placed floating on water with motor lubricant oil for 10 minutes and the percentage of oil removed from the water by the composite was recorded. The test samples have a thickness of 13 mm and an area 3.0 x 1.0 cm ² and they were turned 180° after 5 minutes of exposure.

2.1 Fibre characterization

2.1.1 Recycled tire fibre

FTIR analysis was also performed in the recycled tire fibre, and the spectrum is presented in Figure 2a. The spectrum shows different bands that can be attributed to: (1) hydroxyl groups; (2) stretching vibration of hydrogen bonds in amide II and/or -NH- stretching; (3) and (4) aliphatic C-H stretching; (5) stretching of C=0 of carboxylic acid group; (6) bending vibrations in hydrogen bonds of amide in mode I and/or -C=0 stretching; (7) bending vibrations in hydrogen bonds of amide in mode II and/or combined N-H deformation and C-N stretching vibration modes; (8) -CH2- scissors vibration; (9) -CH2- scissors vibration and/or para- substituted benzene ring; (10) -CH ₃ group deformation; (11) -CH ₂ wagging; (12) (C=0)-C stretching of ester group; (13) and (14) O-CH ₂ stretching; (15) Para-substituted benzene ring; (16) trans O-CH ₂ stretching; (17) Para-substituted benzene ring and/or out of plane vibration of aromatic C-H;

(18) trans CH2 rocking; (19) C=0 + CCO bending and/or vibrations of adjacent two aromatic H in p-substituted compounds; (20) interaction of polar ester groups and benzene rings and/or bending of aromatic C-C ; (21) and (22) are assigned to the torsion in hydrogen bonds of the mode I and mode II of amide.

After analysis the FTIR results, it was possible to confirm the presence of two main types of fibres: (1) a polyester fibre - poly(ethylene terephthalate) (PET) fibre, and (2) a polyamide fibre - nylon 6,6. The bands associated with the polyester fibres in the FTIR spectra are 1, 3, 5, 9, 11-20, while for the polyamide, the bands are 2, 4, 6-10; 21, 22.

These findings are in agreement with the literature, as PET and nylon 6,6 are the most commonly used polymers in tires. However, other fibres such as glass or rayon can also be present in the textile fibre used, but in lower amounts than the ones detected by FTIR analysis.

The TGA analysis (Figure 2b) of the recycled tire fibres also indicates a mixture of fibres, as detected in FTIR, as a two-step process was observed. However, for both fibres found in the chemical characterization (PET and Nylon 6,6), higher onset temperatures were expected (around 400 °C). The first degradation step consisted in a weight loss of 11.4% with an onset temperature of 324.3 °C, and the second step showed a weight loss of 66 % and onset temperature of 377 °C, with a maximum at 417 °C. The thermal degradation between 25-100 °C can be attributed to the loss of water. It is possible that the first degradation step is related to the other fibres present in the mixture, such as rayon fibre, that presents a thermal degradation temperature between 250 °C and 350 °C, while the second step is attributed to the degradation of both polyester and polyamide fibres.

The thermal conductivity of this fibre mat is 67.50 ± 0.14 mW.m ^_1 .K ^_1 (Hot Disk). This result is lower than the one obtained for the rubber (91.09 ± 0.13 mW.m ^_1 .K ^_1 ), but still higher than common materials used for thermal insulation, as previously described.

2.1.2 Polyester Fibre

Polyester fibre blanket was also submitted to FTIR analysis, and the spectrum is presented in Figure 3a. The spectrum shows different bands that can be attributed to: (1) hydroxyl groups; (2) aromatic C-H stretching; (3) and (4) aliphatic C-H stretching; (5) Stretching of C=0 of carboxylic group; (6) and (7) interplane skeletal vibrations of the aromatic ring; (8) CH2 bending; (9) Para-substituted benzene ring and/or in plane deformation of aromatic C-H and/or stretching of aromatic C-C; (10) CH2 wagging; (11) (C=0)-C stretching of ester group; (12) Para-substituted benzene ring and/or or in plane bending of aromatic C-H; (13) and (14) O-CH2 stretching; (15) Para-substituted benzene ring; (16) trans O-CH2 stretching; (17) Para-substituted benzene ring and/or out of plane vibration of aromatic C-H; (18) trans C¾ rocking; (19) C=0 + CCO bending and/or vibrations of adjacent two aromatic H in p-substituted compounds; (20) interaction of polar ester groups and benzene rings and/or bending of aromatic C-C. After analysis of the FTIR results, it was confirmed that the polyester fibre used in this work is a poly (ethylene terephthalate) (PET) fibre.

The thermal stability of the polyester fibres was investigated using TGA measurement, Figure 3b. The TGA data showed an onset temperature of 406 °C, and total weight loss of 77.5%, which is in agreement with the literature data for PET samples. The thermal conductivity of the PET was also assessed, with a value of 33.90 ± 0.05 mW.m ^_1 .K ^_1 (Hot Disk). This result is much lower than the ones obtained for the rubber and the recycled tire fibres, however, it is still higher than the very low thermal conductivities of silica aerogels, that are typically in the order of 15 mW.m ^_1 .K ^_1 , at ambient temperature, pressure and relative humidity.

2.1.3 Silica fibre

The silica fibre felt was also analysed by FTIR, and the spectrum is presented in Figure 4a. The spectrum of this inorganic fibre shows different bands that can be attributed to: (1) silanol groups; (2) stretching of aliphatic C-H groups; (3) asymmetric bending of C-H groups; (4) and (5) asymmetric stretching vibration of Si-O-Si groups; and (6) Si-0 symmetric stretching vibration. The C-H groups are due to a thin coating of varnish on the top of the fibre felt. The thermal degradation of the silica fibres was assessed through thermal gravimetric analysis. As observed in Figure 4b, this fibre only presented one small weight loss (around 3.9%), with an onset temperature of 270 °C, that is attributed to the degradation of the finishing organic coating. The silica fibres presented a thermal conductivity of 29.08 ± 0.20 mW.m ^_1 .K ^_1 (Hot Disk), the lowest between the fibres used in this work, however, as already mentioned, it is still higher than the values obtained for the silica aerogels.

2.1.4 Glass wool

FTIR analysis was also performed at Glass wool, and the spectrum is presented in Figure 5a. The spectrum of the glass wool shows similar bands as the ones obtained for the silica felt as expected, since both materials are mainly composed by S1O2, and these bands can be attributed to: (1) stretching of C=C; (2) deformation vibration of C-H; (3) symmetric deformation vibration of C-H; (4) asymmetric stretching vibration of Si-O-Si groups; (5) in-plane stretching vibration of Si-0 groups and (6) Si-0 symmetric stretching vibration.

Regarding the thermal stability of glass wool, the weight loss of around 35% observed in the TGA (Figure 5b) is probably due to the evaporation of adsorbed water and the degradation of organic compounds added/adsorbed in the sample during the manufacturing process. This also explains the C-containing bonds found in the FTIR spectrum. The thermal conductivity of the glass wool was also measured, and the sample had a value of 56.71 ± 0.03 mW.m ^_1 .K ^_1 (Hot Disk).

2.2 Composite aerogel characterization

Table 1 shows some composites' properties and Figure 1 the macro-photographs of the samples. The composites displayed negligible shrinkage during the drying step (Table 1), thus keeping intact the pore structure of the gel, which contributes to the excellent insulation performance and sorption capacity. Two main factors contribute to the absence of shrinkage. First, when fibres are added into the aerogel matrix, they are able to resist lateral capillary stresses developed during the drying procedure and thus act as supporting skeleton. The second factor is related to the modification of the silica matrix. After the modification step, the silica gel has a hydrophobic character (Table 1, see contact angle), which makes possible the "spring-back" effect of the matrix (reversible shrinkage). During ambient pressure drying, first there is a contraction of the gel due to capillary pressure, followed by a partial recover to its initial volume. This recovery is caused by the presence of non condensable moieties/non-polar groups grafted in the silica matrix surface. With the simultaneous use of HMDSO and TMCS in the modification step, almost all OH groups are converted to 0-Si-(CH3)3. The CH3 groups on the surface repel each other during the drying, leading to the referred "spring-back" effect.

The individual reactivities of HMDSO and TMCS are complementary; TMCS enhances the reactivity of HMDSO, since the HC1 needed for the cision of the HMDSO is formed during the reaction of TMCS with silica pendant hydroxyl groups. The occurring reactions are displayed in Figure 6. As these chain reactions occur, they enhance the surface modification and lead to the formation of an aerogel matrix with uniform structure and low density, as observed in the composites here developed.

Comparing the composites in Table 1, a lower value of bulk density was obtained for the composite synthesized with the polyester fibre blanket, silica fibre blanket and glass wool, with these being lower than the values obtained for the composites with recycled tire fibre, which contributes to better insulation properties. This difference is mainly due to the different densities of the fibres themselves, with the recycled tire fibre having 144.2 kg.m ^-3 while for example the polyester has a density lower than 10 kg.m ^-3 .

Even though a high value was obtained for the composite with recycled tire fibre, the materials have densities in the same range of other fibre-reinforced silica aerogel composites dried in ambient pressure conditions.

In aerogels, the density has a high influence in the thermal conductivity of the samples, with most of the relevant superinsulating S1O2 aerogels commercially available having densities between 80 and 200 kg m ^-3 .

As all materials in Table 1 have densities in this range, the thermal conductivities of the developed composite are expected to be very low and were assessed by two techniques.

This particular property is a crucial factor to establish the possibility of applying the developed composites as thermal insulators. The aerogel composite materials with recycled tire fibres and glass wool have higher values of thermal conductivity than the ones with polyester fibres and silica fibres. This was expected due to the different values for this property displayed by the fibres themselves, which dominates over the effect of density, because the fibres constitute an interconnected network that favours the heat transfer through the composite.

It is important to mention that the addition of the recycled rubber sol into the silica sol did not cause an increase in thermal conductivity of the final aerogel. The pure silica aerogel, TEOS-based matrix with the same modification procedure than the composites, has a thermal conductivity of 24.67 ± 0.14 mW.iti ^-1 .K ^_1 , while the rubber-silica aerogel (addition of rubber into the TEOS sol and modified with HMDSO/TMCS) exhibits a value of 24.82 ± 0.05 mW.m ^_1 .K ^_1 , when measuring by Hot Disk transient method. The similar values of both aerogels indicate a good interaction between the two phases to form the three-dimensional network, which was later confirmed by SEM images.

When measured with a steady-state thermal conductivity method, all composites disclosed in Table 1 display lower thermal conductivities than the typical building insulation materials used in walls such as fibreglass (33-40 mW.m ^_1 .K ¹ ), rockwool (37 mW.nr ¹ .K ^_1 ), polyethylene (41 mW.nr ¹ .K ^_1 ), expanded polystyrene (37-38 mW.nr ¹ .K ^_1 ), extruded polystyrene (30-32 mW.nr ¹ .K ^_1 ) and cellulose (46-54 mW.m ^_1 .K ^_1 ).

The lowest values were achieved by the polyester and silica fibres-silica/rubber aerogel composites, that have a thermal conductivity lower than 25 mW.m ^_1 .K ^_1 , being classified as a superinsulating materials.

Table 1 - Structural and thermal properties and hydrophobicity of the aerogel composites. a Panels with 21.5 x 21.5 cm ² of area.

In order to estimate the thermal stability of the composite materials, all aerogel samples were submitted to a thermogravimetric analysis from 20 °C to 600 °C, under N2 atmosphere (Figure 7).

For the composite with recycled tire fibres, a significant weight loss was detected, Figure 7a, with four phenomena being observed. The first weight loss starts right at room temperature (20-25°C) and is due to adsorbed water and residual solvents/by-products of the synthesis procedure (Tonset = 50.2 °C). The second phenomenon, onset temperature of 179 °C, can be attributed to the loss of structural hydroxyl groups of the silica matrix. The third and fourth weight losses (T _on set = 255 °C and 388 °C, respectively) are mainly due to the thermal degradation of the composites' fibres, first fibres such as rayon and later the polyester and polyamide fibres. However, the last phenomenon has also the contribution of the decomposition of methyl groups attached to the silica surface after the modification with

HMDSO and TMCS. For the composites made with polyester fibres, only a small weight loss was observed, around 8.5% (Figure 7b). The onset temperature was around 458 °C, much higher than the values obtained for the composite with recycled tire fibres. Very similar results were obtained for the samples obtained with silica fibres and glass wool (7c and 7d), with these composites presenting weight losses of 8.09% and 5.53%, and onset temperatures of 483 °C and 448 °C, respectively. The weight losses here observed are attributed to the thermal decomposition of the silica surface's methyl groups from silylation, as also verified in the other composite material, and overlapped with the onset of polyester degradation (when applicable) . After the degradation of -CH3 surface groups, it is expected that the materials lose the hydrophobic character of the modified aerogel.

A significant difference in the thermal degradation of the developed composites with organic fibres was observed in thermogravimetric analysis and is probably due to the different interaction of the rubber-silica aerogel with the fibres, as observed in the SEM images (Figure 8). In the case of polyester composites, the aerogel was able to completely cover the fibres (Figure 8c), increasing their thermal stability, while for the recycled tire fibres this was not observed (Figure 8a) and most of the fibres were still exposed. In the case of polyester fibres, it is easily observed that the aerogel grew around the fibre following the fibres' shape, while for the composites with the recycled tire fibres, the fibres do not interact significantly with the aerogel, with a clear separation between both phases. For the composites with silica fibres and glass wool (Figures 8e and 8g), some degree of interaction occurs between the two phases, with some of the fibres being covered by the aerogel while other remain exposed. For these two composites the interaction was not as good as in the case of polyester fibres (Figure 8c). However, in the case of silica fibres and glass wool the fibres themselves are quite stable up to the tested temperature in thermal analysis (600 °C), due to their predominant S1O2 composition.

Regarding the aerogel phase, all samples show similar microstructures (Figure 8) with an interconnected three- dimensional aerogel matrix. Thus, even though the interaction between both phases is different, it can be concluded that the type of fibres used does not affect the formation of the porous structure of the matrices. It was also verified that the presence of colloidal rubber did not prevent the formation of the network or interfered in the typical silica aerogel structure, as both samples present similar structures verified in other modified TEOS-based materials.

The mechanical behaviour of these materials was assessed by uniaxial compression tests, and the results are presented in Figure 9.

The material's capacity of recovery to its original shape is important for building applications, as it can regain its original shape after compression. This flexible behaviour also allows to adapt better to curved surfaces. Thus, recovery tests were performed submitting the samples to 10% and 25% strain (Figures 9a and 9c). The results indicate an excellent behaviour, as the samples are able to almost completely recover the original size, either after 10% strain or 25% strain (Figure 9 and Table 2), always above 94%, and in the case of polyester fibres and glass wool the observed recoveries exceeded 99%.

In order to further evaluate the capacity of the material to withstand dimensional load, axial cyclic compression tests (10 cycles) were performed until a strain of 10% (Figure 9c). After the test, the samples only display small reductions of their initial height (Table 2). These results indicate an excellent mechanical performance of the composites, in terms of flexibility, especially if compared with pristine silica aerogels. They are able to withstand cyclic loads without disintegration, which is an important feature for vibration dissipation and damping.

Table 2 - Mechanical properties of fibre-reinforced silica/rubber aerogels.

The samples were also submitted to a destructive test with the load cell of 3 kN, up to the maximum allowed force (Figure 9). Figure 9d presents the non-linear stress-strain curve, in which the compression progress of the sample contains three stages. At the first stage, with the strain ranging from 0% to around 30%, known as linear stage, the slope of the compression curve remains unchanged, and the open pores act as the main support of the composite. The second stage, the yielding stage (strain in the range of 30% to 60%), the stress increases at a fixed rate and the fibres become the main load-bearing part. In the final part, the densification stage (from 60% to ~95%), the collapse of the aerogel part and a significant increase in the curve slope are observed.

The samples did not recover their original height after the applied load was removed, as expected for this destructive test.

The measured dynamic stiffness of the new aerogel composite with polyester fibres was 11 MN.m ^-3 . In comparison with other materials (e.g., recycled tyre rubber: 61 MN.m ^-3 , and cork/rubber composite: 184 MN.m ^-3 ) the measured dynamic stiffness of the new aerogel composite is significantly lower.

Sorption tests for lubricant oil were carried out with the polyester-reinforced rubber-silica composites. The adsorption capacity of the composite was calculated in the first 5 min and it showed a value of 9.83 g.g ^-1 . The values obtained for removal are very high, 97%; this composite could be a very promising sorbent for oil spill cleaning.

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.

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