WO2024009250A1 - Functionalized silica particles, obtention and uses thereof - Google Patents

Functionalized silica particles, obtention and uses thereof Download PDF

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WO2024009250A1
WO2024009250A1 PCT/IB2023/056985 IB2023056985W WO2024009250A1 WO 2024009250 A1 WO2024009250 A1 WO 2024009250A1 IB 2023056985 W IB2023056985 W IB 2023056985W WO 2024009250 A1 WO2024009250 A1 WO 2024009250A1
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silica
previous
acid
qas
particles
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PCT/IB2023/056985
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Mariana Ivone ALVES DE ORNELAS CARDOSO
Ana Rita DE OLIVEIRA BARROS
Regina Andreia TORRES MALGUEIRO
Verónica DE CARVALHO ROQUE BOUÇA
Filipa OLIVEIRA GOMES
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Centitvc - Centro De Nanotecnologia E Materiais Técnicos Funcionais E Inteligentes
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Publication of WO2024009250A1 publication Critical patent/WO2024009250A1/en

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    • C09C1/00—Treatment of specific inorganic materials other than fibrous fillers; Preparation of carbon black
    • C09C1/28—Compounds of silicon
    • C09C1/30—Silicic acid
    • C09C1/3063—Treatment with low-molecular organic compounds
    • A—HUMAN NECESSITIES
    • A01—AGRICULTURE; FORESTRY; ANIMAL HUSBANDRY; HUNTING; TRAPPING; FISHING
    • A01N—PRESERVATION OF BODIES OF HUMANS OR ANIMALS OR PLANTS OR PARTS THEREOF; BIOCIDES, e.g. AS DISINFECTANTS, AS PESTICIDES OR AS HERBICIDES; PEST REPELLANTS OR ATTRACTANTS; PLANT GROWTH REGULATORS
    • A01N33/00—Biocides, pest repellants or attractants, or plant growth regulators containing organic nitrogen compounds
    • A01N33/02—Amines; Quaternary ammonium compounds
    • A01N33/12—Quaternary ammonium compounds
    • A—HUMAN NECESSITIES
    • A01—AGRICULTURE; FORESTRY; ANIMAL HUSBANDRY; HUNTING; TRAPPING; FISHING
    • A01N—PRESERVATION OF BODIES OF HUMANS OR ANIMALS OR PLANTS OR PARTS THEREOF; BIOCIDES, e.g. AS DISINFECTANTS, AS PESTICIDES OR AS HERBICIDES; PEST REPELLANTS OR ATTRACTANTS; PLANT GROWTH REGULATORS
    • A01N55/00—Biocides, pest repellants or attractants, or plant growth regulators, containing organic compounds containing elements other than carbon, hydrogen, halogen, oxygen, nitrogen and sulfur
    • A—HUMAN NECESSITIES
    • A01—AGRICULTURE; FORESTRY; ANIMAL HUSBANDRY; HUNTING; TRAPPING; FISHING
    • A01P—BIOCIDAL, PEST REPELLANT, PEST ATTRACTANT OR PLANT GROWTH REGULATORY ACTIVITY OF CHEMICAL COMPOUNDS OR PREPARATIONS
    • A01P1/00—Disinfectants; Antimicrobial compounds or mixtures thereof
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    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/113—Silicon oxides; Hydrates thereof
    • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
    • C01B33/18—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09C—TREATMENT OF INORGANIC MATERIALS, OTHER THAN FIBROUS FILLERS, TO ENHANCE THEIR PIGMENTING OR FILLING PROPERTIES ; PREPARATION OF CARBON BLACK  ; PREPARATION OF INORGANIC MATERIALS WHICH ARE NO SINGLE CHEMICAL COMPOUNDS AND WHICH ARE MAINLY USED AS PIGMENTS OR FILLERS
    • C09C1/00—Treatment of specific inorganic materials other than fibrous fillers; Preparation of carbon black
    • C09C1/28—Compounds of silicon
    • C09C1/30—Silicic acid
    • C09C1/3081—Treatment with organo-silicon compounds
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09C—TREATMENT OF INORGANIC MATERIALS, OTHER THAN FIBROUS FILLERS, TO ENHANCE THEIR PIGMENTING OR FILLING PROPERTIES ; PREPARATION OF CARBON BLACK  ; PREPARATION OF INORGANIC MATERIALS WHICH ARE NO SINGLE CHEMICAL COMPOUNDS AND WHICH ARE MAINLY USED AS PIGMENTS OR FILLERS
    • C09C1/00—Treatment of specific inorganic materials other than fibrous fillers; Preparation of carbon black
    • C09C1/28—Compounds of silicon
    • C09C1/30—Silicic acid
    • C09C1/309—Combinations of treatments provided for in groups C09C1/3009 - C09C1/3081
    • D—TEXTILES; PAPER
    • D06—TREATMENT OF TEXTILES OR THE LIKE; LAUNDERING; FLEXIBLE MATERIALS NOT OTHERWISE PROVIDED FOR
    • D06M—TREATMENT, NOT PROVIDED FOR ELSEWHERE IN CLASS D06, OF FIBRES, THREADS, YARNS, FABRICS, FEATHERS OR FIBROUS GOODS MADE FROM SUCH MATERIALS
    • D06M13/00—Treating fibres, threads, yarns, fabrics or fibrous goods made from such materials, with non-macromolecular organic compounds; Such treatment combined with mechanical treatment
    • D06M13/244—Treating fibres, threads, yarns, fabrics or fibrous goods made from such materials, with non-macromolecular organic compounds; Such treatment combined with mechanical treatment with compounds containing sulfur or phosphorus
    • D06M13/282—Treating fibres, threads, yarns, fabrics or fibrous goods made from such materials, with non-macromolecular organic compounds; Such treatment combined with mechanical treatment with compounds containing sulfur or phosphorus with compounds containing phosphorus
    • D06M13/288—Phosphonic or phosphonous acids or derivatives thereof
    • D—TEXTILES; PAPER
    • D06—TREATMENT OF TEXTILES OR THE LIKE; LAUNDERING; FLEXIBLE MATERIALS NOT OTHERWISE PROVIDED FOR
    • D06M—TREATMENT, NOT PROVIDED FOR ELSEWHERE IN CLASS D06, OF FIBRES, THREADS, YARNS, FABRICS, FEATHERS OR FIBROUS GOODS MADE FROM SUCH MATERIALS
    • D06M13/00—Treating fibres, threads, yarns, fabrics or fibrous goods made from such materials, with non-macromolecular organic compounds; Such treatment combined with mechanical treatment
    • D06M13/50—Treating fibres, threads, yarns, fabrics or fibrous goods made from such materials, with non-macromolecular organic compounds; Such treatment combined with mechanical treatment with organometallic compounds; with organic compounds containing boron, silicon, selenium or tellurium atoms
    • D06M13/51—Compounds with at least one carbon-metal or carbon-boron, carbon-silicon, carbon-selenium, or carbon-tellurium bond
    • D06M13/513—Compounds with at least one carbon-metal or carbon-boron, carbon-silicon, carbon-selenium, or carbon-tellurium bond with at least one carbon-silicon bond
    • D—TEXTILES; PAPER
    • D06—TREATMENT OF TEXTILES OR THE LIKE; LAUNDERING; FLEXIBLE MATERIALS NOT OTHERWISE PROVIDED FOR
    • D06M—TREATMENT, NOT PROVIDED FOR ELSEWHERE IN CLASS D06, OF FIBRES, THREADS, YARNS, FABRICS, FEATHERS OR FIBROUS GOODS MADE FROM SUCH MATERIALS
    • D06M16/00—Biochemical treatment of fibres, threads, yarns, fabrics, or fibrous goods made from such materials, e.g. enzymatic
    • D—TEXTILES; PAPER
    • D06—TREATMENT OF TEXTILES OR THE LIKE; LAUNDERING; FLEXIBLE MATERIALS NOT OTHERWISE PROVIDED FOR
    • D06M—TREATMENT, NOT PROVIDED FOR ELSEWHERE IN CLASS D06, OF FIBRES, THREADS, YARNS, FABRICS, FEATHERS OR FIBROUS GOODS MADE FROM SUCH MATERIALS
    • D06M23/00—Treatment of fibres, threads, yarns, fabrics or fibrous goods made from such materials, characterised by the process
    • D06M23/08—Processes in which the treating agent is applied in powder or granular form
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/113—Silicon oxides; Hydrates thereof
    • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
    • C01B33/14—Colloidal silica, e.g. dispersions, gels, sols
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2002/00—Crystal-structural characteristics
    • C01P2002/80—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70
    • C01P2002/82—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70 by IR- or Raman-data
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    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
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    • C01P2002/80—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70
    • C01P2002/88—Crystal-structural characteristics defined by measured data other than those specified in group C01P2002/70 by thermal analysis data, e.g. TGA, DTA, DSC
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/01—Particle morphology depicted by an image
    • C01P2004/03—Particle morphology depicted by an image obtained by SEM
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/60—Particles characterised by their size
    • C01P2004/61—Micrometer sized, i.e. from 1-100 micrometer
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/60—Particles characterised by their size
    • C01P2004/62—Submicrometer sized, i.e. from 0.1-1 micrometer
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
    • C08K2201/00—Specific properties of additives
    • C08K2201/002—Physical properties
    • C08K2201/005—Additives being defined by their particle size in general
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
    • C08K3/00—Use of inorganic substances as compounding ingredients
    • C08K3/34—Silicon-containing compounds
    • C08K3/36—Silica
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
    • C08K9/00—Use of pretreated ingredients
    • C08K9/04—Ingredients treated with organic substances
    • C08K9/06—Ingredients treated with organic substances with silicon-containing compounds

Definitions

  • the present disclosure relates to functionalized silica particles, specifically, functionalized hydrophobic silica particles comprising a flame retardant and an antimicrobial agent, wherein the antimicrobial agent is the linker between the silica particle and the flame retardant agent.
  • the present disclosure also relates to methods to obtain such functionalized hydrophobic silica particles, as well as the obtention of silica from natural sources through extraction processes.
  • the present invention relates to the use such functionalized hydrophobic silica particles, preferably in textiles, construction materials and interior of motor vehicle.
  • waste management comes as a global challenge, being the resource efficiency (reduction of oil and gas imports), the most assertive solution to help on a greener and more sustainable world [1-3],
  • silica could be performed by low temperature synthesis (micro-wave reduction in the presence of sodium hydroxide [14] or even by a sol-gel route) [15],
  • the use of alkali extraction followed by its precipitation using an acidic solution, such as hydrochloric or sulphuric acid is reported [16,17].
  • acidic solution such as hydrochloric or sulphuric acid
  • Multifunctionality is a highly desirable feature for silica particles, enhancing their versatility and the added value of the final materials.
  • Environmentally friendly pathways for the functionalization of biosilica can be accomplished with the use of bio-based compounds such as phytic acid (PA) or quaternary ammonium salts with silane groups, to impart multifunctional properties such as flame retardancy, antimicrobial, and hydrophobicity, that may be of interest for the final application of the particles.
  • bio-based compounds such as phytic acid (PA) or quaternary ammonium salts with silane groups, to impart multifunctional properties such as flame retardancy, antimicrobial, and hydrophobicity, that may be of interest for the final application of the particles.
  • Phytic acid is a bio-based flame retardant and environmentally friendly compound that has been shown to improve the flame retardancy of different polymeric matrices such as Polyethylene terephthalate (PET), Polylactic acid (PLA), Polypropylene (PP), Polyamide (PA66), cotton, among others [21], PA is a phosphorus-rich organic acid compound (mass fraction of about 28%) with 6 phosphate groups and 12 hydroxyl groups, and it can be extracted from plant seeds, such as rice bran, beans, seeds, among others [22], Several studies reported developments of organic-inorganic compounds by using silane agents, as tetraethoxysilane (TEOS, precursor of silica) and PA envisioning flame retardant properties.
  • TEOS tetraethoxysilane
  • Antimicrobial properties are also attractive for some applications of silica particles, such as those involving the need for anti-rotting features as in the case of construction applications.
  • fungal infections have been increasing and invasive forms can cause mortality, especially in immunocompromised or immunosuppressed patients [27], This is aggravated by the emergence of drug-resistant fungus and exploration on new antifungal agents must be made [28] including metal oxides (Ag, TiCh, ZnO nanoparticles) [29,30], amphotericin B [31], antifungal peptides [32], and quaternary ammonium compounds.
  • the antimicrobial activity of quaternary ammonium compounds has been attributed to the length of the N-alkyl chain that can affect its performance [33], It has been reported that the optimum chain length for activity towards gram-positive bacteria is 14 carbons, 16 carbons for gram-negative bacteria, and 12 carbons for yeast and fungi [34], Botequim et al. (2012), for example, reported antimicrobial silica nanoparticles modified with a didodecyldimethylammonium bromide (DDAB) with a large antimicrobial activity against fungi (Candida albicans), bacteria (Staphylococcus aureus, Escherichia coli) and virus (influenza A) [31],
  • DDAB didodecyldimethylammonium bromide
  • Silanes groups introduced in quaternary ammonium compounds are interesting compounds for the functionalization of silica particles, through sol-gel technology.
  • the authors compared the silica nanoparticles modified with quaternary ammonium silane with silica nanoparticles functionalized with octadecyltrimethoxy silane.
  • US2006178443A1 describes a nucleating agent to produce polyurethane (PU) foam comprising nanoparticles, a polyurethane foam comprising nanoparticles and the use of the nucleating agent for producing the polyurethane foam.
  • the nanoparticle dispersion is added to a flame retardant.
  • the nanoparticles comprise metal oxide selected from the group consisting of SiO2, ZnO2, AI2O3, ZrO2 and TiO2. No information is provided regarding the obtention of multifunctional SiO2 particles with hydrophobic, flame retardant and antimicrobial properties and the use of a quaternary amine as crosslinker for modified-silica particles.
  • US2007231295A1 describes SiO z flakes, especially porous SiO z flakes, wherein 0.70 ⁇ z ⁇ 2.0, especially 0.95 ⁇ z ⁇ 2.0, especially porous SiO2 flakes, comprising an organic, or inorganic antimicrobial compound, or composition, which provide enhanced (long term) antimicrobial efficacy.
  • This document only provides examples regarding silver/copper/palladium/nickel coted silicon oxide flakes.
  • the present disclosure relates to functionalized silica particles, specifically, functionalized hydrophobic silica particles comprising a flame retardant and an antimicrobial agent, wherein the antimicrobial agent is the linker between the silica particle and the flame retardant agent.
  • the present disclosure also relates to methods to obtain such functionalized hydrophobic silica particles, as well as the obtention of silica from natural sources through extraction processes.
  • the present invention relates to the use such functionalized hydrophobic silica particles, preferably in textiles, construction materials and interior of motor vehicle.
  • the present disclosure relates to an innovative method to produce multifunctional silica particles extracted from RH and functionalized simultaneously with PA and QAS, promoting circular economy, while developing value-added silica particles (combination of flame retardant, antimicrobial and hydrophobic properties) that can be used in different industrial sectors, such as the automotive and the construction and building sectors, as nanofillers or in textile coatings.
  • SiO2@QAS+PA and “SiO2@PA+QAS” have the same meaning and refer exactly to the same particles, i.e., silica particles functionalized with QAS and PA.
  • QAS 3-(trimethoxysilyl)-propyldimethyloctadecyl ammonium chloride
  • CAS Chemical Abstracts Service
  • flame retardant agents can be employed. These agents are typically phosphorous-based compounds that could be phytic acid and its derivatives or; other compounds such as Ammonium polyphosphate (APP), melamine polyphosphate (MPP), pentaerythritol phosphate alcohol, phosphazenes, diethyl phosphonic metal salts and 9,10-dihydro-9-oxa-10- phosphaphenanthrene-10-oxide (DOPO), or mixtures thereof; among others preferably, phytic acid is used.
  • APP Ammonium polyphosphate
  • MPP melamine polyphosphate
  • DOPO 9,10-dihydro-9-oxa-10- phosphaphenanthrene-10-oxide
  • some fillers may also enhance the flame retardant properties, such as aluminum trihydroxide, magnesium carbonate or magnesium hydroxide and titanium dioxide, among others.
  • antimicrobial agents can be employed: N-allyl-N-decyl- N-methyl-N-trimethoxysilylpropylammonium iodide, Perfluorooctyl-containing quaternary ammonium salt, Triethoxysilylpropyl Succinic Anhydride Silane, proving one more time that quaternary amines are the group with the highest interest in this functionality.
  • 3-(trimethoxysilyl)- propyldimethyloctadecyl ammonium chloride (QAS) is used.
  • An aspect of the present disclosure relates to a functionalized hydrophobic silica particle comprising a flame retardant and an antimicrobial agent, wherein the antimicrobial agent is a linker between the silica particle and the flame retardant agent; wherein the flame retardant agent is selected from a list consisting of: phytic acid, ammonium polyphosphate, pentaerythritol phosphate alcohol, melamine polyphosphate, phosphazenes; diethyl phosphonic metal salts; 9.10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide; wherein the antimicrobial agent is selected from a list consisting of: 3-(trimethoxysilyl)- propyldimethyloctadecyl ammonium chloride, quaternary ammonium silane, quaternary amine, ethyl lauroyl arginate (ELA), quaternized carboxymethylchitosan (qCMChi
  • the flame retardant agent is phytic acid.
  • the antimicrobial agent is a quaternary ammonium silane.
  • the functionalized hydrophobic silica particle comprises phytic acid and a quaternary ammonium silane, wherein the quaternary ammonium silane is the linker between the silica particle and the phytic acid.
  • the quaternary ammonium silane is 3-(trimethoxysilyl)- propyldimethyloctadecyl ammonium chloride.
  • the functionalized hydrophobic silica particle comprises a particle size less than 2 pm.
  • the particle size is measured through dynamic light scattering (DLS).
  • DLS dynamic light scattering
  • Zeta potential measurements in DLS analysis were performed with a Zetasizer Nano ZS90 (Malvern) and a disposable capillary cell (DTS1070) at 25 °C.
  • bio-silica was dispersed in two different solvents, ethanol or deionized water, at a concentration of 1 g/L and subjected to ultrasonic agitation before the analysis.
  • Particle size was also evaluated by scanning electron microscopy (SEM) connected to an energy dispersive X-ray spectroscopy (EDS) using a NanoSEM - FEI Nova 200 (FEG/SEM) and EDAX - Pegasus X4M (EDS/EBSD) with high vacuum resolution 1.8 nm at 1 kV (SE) 1.0 nm at 15 kV (SE) or low vacuum resolution 1.8 nm at 3 kV (Helix detector) 1.5 nm at 10 kV (Helix detector).
  • SEM scanning electron microscopy
  • EDS energy dispersive X-ray spectroscopy
  • SE NanoSEM - FEI Nova 200
  • compositions comprising such functionalized hydrophobic silica particle.
  • the composition further comprises a filler, preferably such filler is selected from the list consisting of: aluminum trihydroxide, magnesium carbonate, magnesium hydroxide or titanium dioxide, or mixtures thereof.
  • Another aspect of the present disclosure relates to an article comprising the functionalized hydrophobic silica particle or the composition.
  • the article comprises a textile material, a construction material or a material of the interior of a motor vehicle.
  • Another aspect of the present disclosure relates to the use of the functionalized hydrophobic silica or the composition as a hydrophobic agent, as an antimicrobial agent and as a flame retardant agent.
  • Another aspect of the present disclosure relates to the use of the functionalized hydrophobic silica particle or the composition as a dirt-resistant agent.
  • Another aspect of the present disclosure relates to the use of the functionalized hydrophobic silica particle or the composition as a stain repellent agent.
  • the use of these particles is in textiles, and/or in construction materials, and/or in the interior of motor vehicles.
  • Another aspect of the present disclosure relates to a method to produce functionalized hydrophobic silica particle of the present disclosure, comprising the following steps: obtaining a silica particle; dispersing the silica particles in a solution of EtOH:H2O, preferably with a concentration ratio of 1:1 (v/v); adding a flame retardant agent and an antimicrobial agent to the obtained dispersion, preferably with sonication; wherein the flame retardant agent is selected from a list consisting of: phytic acid, ammonium polyphosphate, melamine polyphosphate, phosphazenes; diethyl phosphonic metal salts; 9.10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide; and wherein the antimicrobial agent is selected from a list consisting of: 3-(trimethoxysilyl)- propyldimethyloctadecyl ammonium chloride, quaternary ammonium silane, quaternary amine
  • the silica particles are obtained through an extraction process from an organic or inorganic source; preferably an organic source.
  • the silica particles are obtained through an extraction process from rice husk.
  • the extraction process comprises an acid extraction method or a sol-gel method.
  • the acid extraction method comprises the following steps: obtaining crude material comprising silica; preferably with particle size equal or below 1.5 mm; washing such crude material comprising silica; preferably with distilled water; preferably for 2h at room temperature; digesting with acid the washed crude material; preferably with an acid solution selected from a list consisting of: hydrochloric acid, nitric acid or citric acid, or mixtures thereof; preferably at 60°C for 2 h; filtering the digested crude material; preferably with a wire mesh of 130 pm; neutralizing the filtered digested crude material; preferably with deionized water;
  • Drying the neutralized material preferably at 110 °C for 12h; calcinating the dried material to obtain purified silica; preferably at 700 °C for 3h30 min after an initial heating ramp from 25 °C to 700 °C for 2h30 min.
  • the sol-gel method comprises the following steps: obtaining crude material comprising silica; preferably with particle size equal or below 1.5 mm; calcinating the crude material comprising silica; preferably at 700 °C for 3h30 min after an initial heating ramp from 25 °C to 700 °C for 2h30 min; extracting the silica from the calcinated product with an alkaline solution under reflux to obtain crude sodium silicate; preferably the alkaline solution is NaOH 10%; preferably at 90°C for 3h; filtrating the crude sodium silicate to eliminate char impurities; treating the filtrated crude sodium silicate with acid solution to obtain crude silica; preferably with hydrochloric acid, nitric acid, citric acid, phosphoric acid or sulfuric acid, or mixtures thereof washing the crude silica; preferably with water;
  • Figure 1 FTIR analysis of silica obtained with different conditions: without calcination, 600 °C, 700 °C and 800 °C.
  • Figure 2 Scanning electron microscopy images of the (a) rice husk, (b) silica without acid digestion, (c) silica obtained by acid digestion before calcination, (d) silica obtained by acid digestion after calcination, (e) Silica obtained by Stober method, at different scales such as (A) 5 pm and (B) 500 nm.
  • Figure 3 FTIR analysis of (a) silica obtained with RH acid digestion and (b) with Stober method.
  • Figure 4 FTIR analysis of biosilica obtained by the sol-gel method using phosphoric acid, sulfuric acid, citric acid, nitric acid and hydrochloric acid at the neutralization process.
  • Figure 5 FTIR spectra for the samples: RH, SiCh, SiC>2@QAS, SiO2@QAS+PA.
  • Figure 7 FTIR analysis of SiC>2@QAS+PA, PA, SiCh, SiC>2@PA unwashed and SiC>2@PA washed.
  • Figure 8 Scanning electron microscopy images and EDS spectra of the (a) SiCh, (b) SiO2@QAS, and (c) SiO2@QAS+PA.
  • Figure 9 ISO 6941:2003 vertical burning tests.
  • Teste scheme (a) test for cotton with SiO2 in chitosan matrix (b), test for cotton with SiO2@QAS+PA in chitosan matrix (c), test for cotton with SiO2@QAS in chitosan matrix (d) and test for cotton with 3% chitosan (3% w/v) (e).
  • the images show the caption of test specimens at 30 s. of the test.
  • Figure 10 1 Results obtained in the vertical burning tests according to the ISO 6941:2003: A) Cotton; B) Cotton with chitosan; C) Cotton with chitosan and SiO2 particles; D) Cotton with chitosan and SiO2@QAS; E) Cotton with chitosan and QAS+PA; F) Cotton with chitosan and SiO2@PA+QAS.
  • Figure 11 Antimicrobial illustrations for Staphyloccoccus aureus, Eschericia coli, Methicillin- Resistant Staph (MRSA), S. Cerevisiae and Candida albicans for non-functionalized and functionalized biosilica.
  • Figure 12 Water contact angle for the samples: (a) SiCh (CA: not detected), (b) SiO2@QAS (CA: 150.7 ⁇ 4.3°) and (c) SiO 2 @QAS+PA (CA: CA: 153.3 ⁇ 3.2°).
  • Figure 13 FTIR spectra for carboxymethyl chitosan.
  • Figure 14 FTIR results for (a) SiO 2 @ELA+PA and (a) SiO 2 @qCMChi+PA.
  • Figure 15 Flame retardancy (with three test pieces for each particle) for (a) SiO 2 @ELA+PA and (b) SiO 2 @qCMChi+PA.
  • the present disclosure relates to functionalized silica particles, specifically, functionalized hydrophobic silica particles comprising a flame retardant and an antimicrobial agent, wherein the antimicrobial agent is the linker between the silica particle and the flame retardant agent.
  • the present disclosure also relates to methods to obtain such functionalized hydrophobic silica particles, as well as the obtention of silica from natural sources through extraction processes.
  • the present invention relates to the use such functionalized hydrophobic silica particles, preferably in textiles, construction materials and interior of motor vehicle.
  • silica particles functionalized with a flame retardant agent it was surprisingly found that such functionalization is possible when an antimicrobial agent is used as a linker between the silica particle and the flame retardant agent. It was possible to obtain functionalized SiO 2 particles, comprising a flame retardant and an antimicrobial agent, wherein the antimicrobial agent is the linker between the silica particle and the flame retardant agent. The method proved to be efficient, and the functionalized particles obtained showed antimicrobial, flame retardant and hydrophobic properties.
  • the SiO 2 particles used in the present disclosure can be obtained commercially, or by synthesis using traditional procedures like Stober method or through extraction from natural sources, namely mineral, organic and/or vegetal sources.
  • the SiO 2 particles of the present disclosure are obtained from rice husk.
  • rice husk (RH) was purchased from Alvaro Alves Borges, Lda (Figueira da Foz, Portugal). Hydrochloric acid (HCI) 37 wt.%, sulfuric acid (>95%), and sodium chloride (NaCI) were purchased from Fisher Chemicals (Porto Salvo, Portugal). Ammonium molybdate tetrahydrate was acquired from Alfa Aesar (Kandel, Germany).
  • Sodium sulphite anhydrous, p-methylaminophenol sulfate, oxalic acid dehydrate and sodium metasilicate, phytic acid sodium salt hydrate, iron (III) chloride hexahydrate, dimethyloctadecyl(3-(trimethylsilyl)propyl ammonium chloride solution (QAS) 42 wt.% in methanol were purchased from Sigma-Aldrich (Schnelldorf, Germany). Inositol hexaphosphoric acid (phytic acid, PA; 50 wt.% aqueous solution) was obtained from Acros Organics (Geel, Belgium).
  • 5- sulphosalicylic acid dihydrate and glacial acetic acid (99.9%) were supplied from VWR International (Alfragide, Portugal). Chitosan of medium molecular weight and with >95% of deacetylation was purchased from Primex (ChitoClear, Iceland). Absolute ethanol (EtOH) was purchased from Aga (Prior Velho, Portugal). Ultra-pure water of Mil-li-Q. quality (Millipore, Italy) was used. All other reagents used were of analytical grade.
  • the yield is similar and hydrochloric acid, nitric acid and citric acid are good choices.
  • Table 3 resumes the results obtained using different acids in the digestion process.
  • the particle morphology of silica extracted from rice husk is completely different of silica obtained by chemical processes (Stober).
  • Silica from rice husk shows agglomerates while silica Stober has circular morphology.
  • the presence of agglomerates in silica from RH could also be evidenced by DLS analysis (a high value of PDI).
  • FTIR is also different. Silica from RH did not present silanol groups and silica from Stober presents (it is possible to see that these groups influence the particle charge, so, zeta potential is much more negative in Stober particles).
  • Table 4 depicts the FTIR analysis of silica acid digestion and Silica Stober.
  • Table 4 Particle charge and zeta potential of Stober particles
  • the particle size is higher than 1pm and the polidespersion index obtained for particles dispersed in water has lower values.
  • This particle size results demonstrate that particles obtained from agroindustrial waste such as rice husk are not similar with particles synthetize chemically such as Stober particles.
  • Table 5 Particle charge and zeta potential of silica particles from rice husk with two different dispersants such as water and ethanol.
  • the RH was filtered using a wire mesh of 130 pm, followed by neutralization with deionized water and a drying process at 110 °C for 12 h. Finally, the SiCh particles were obtained through a calcination process of the digested RH previously milled to a mesh of 0.5mm (22 °C to 700 °C in 2h30 and then 700 °C for 3h30).
  • sol-gel method is applied as extraction process.
  • the silica in rice husk is dissolved quickly in alkaline conditions followed by precipitation in acidic conditions.
  • the rice husk ground or not
  • the rice husk is calcinated in a furnace at 700 °C for 3h30 (after an initial heating ramp from 25 °C to 700 °C for 2h30) to eliminate organic matter.
  • the silica is then extracted from the resulting rice husk ash by an alkali extraction using NaOH 10%.
  • the mixture is stirred under reflux for 3h at 90 °C to obtain the silicate precursors (sodium silicate).
  • the mixture is filtered to remove the remaining char and the silica in the filtrate is precipitated with HCI 10% or other acids such as nitric, citric, phosphoric and sulfuric ( Figure 4).
  • HCI 10% or other acids such as nitric, citric, phosphoric and sulfuric ( Figure 4).
  • the resulting precipitate is then filtered, washed with water and dried at 150 °C for 2h (or dried under vacuum using a freeze dryer) to obtain silica particles.
  • Table 7 depict the impact of the acid used in the neutralization in the particle size, Pdl and potential zeta of the silica particles obtained.
  • Table 7 impact of the acid used in the neutralization in the particle size, Pdl and potential zeta of the silica particles obtained.
  • Table 8 depict the impact of the acid used in the neutralization in the yield of and the yield and purity levels obtained. It is possible to observed that, high yields did not represent a good solution if accompanied by impurities (detected by EDS analysis). The best result was obtained using HCI, CgHsO? or HNO 3 .
  • Table 8 Impact of the acid used in the neutralization in the yield of and the yield and purity levels obtained.
  • the activated silica was functionalized with specific compounds that conferred antimicrobial and flame retardant properties namely QAS and PA, respectively. Briefly, 6 g of SiOa was dispersed in 360 mL EtOI-kHzO (1:1), and 240 mL of PA was added to this mixture being sonicated for 30 min. Then, 72 mL of QAS was added dropwise to the previous mixture and the reaction was left for 24 h at room temperature. Finally, the particles were centrifuged (9000 rpm for 10 min) and washed with deionized water. This procedure was performed 3 times and the obtained particles were dried at 40 °C, followed by a grinding process to obtain the particles in the powder form with less than 2 pm.
  • QAS antimicrobial and flame retardant properties
  • SiO2 modified with QAS was also performed following the same procedure, but without the addition of PA.
  • SIO2 modified with PA, without QAS was also performed. The same procedure was applied using different compounds with antifungal and flame retardant properties, substituting QAS and PA for the respective compound. All the other conditions of the procedure were not changed.
  • FTIR-ATR Fourier transform infrared spectroscopy
  • FTIR-ATR was used to evaluate the functional groups of the synthetized particles using a Perkin Elmer Spectrum 100 Series spectrophotometer with a spectral range and resolution factor of 4000 to 650 cm-1 and 8 cm-1, respectively.
  • DLS Dynamic light scattering
  • Zeta potential measurements in DLS analysis were performed with a Zetasizer Nano ZS90 (Malvern) and a disposable capillary cell (DTS1070) at 25 °C.
  • DTS1070 disposable capillary cell
  • bio-silica was dispersed in two different solvents, ethanol or deionized water, at a concentration of 1 g/L and subjected to ultrasonic agitation before the analysis.
  • TGA Thermogravimetric analysis
  • the morphology and chemical composition of the particles were characterized by scanning electron microscopy (SEM) connected to an energy dispersive X-ray spectroscopy (EDS) using a NanoSEM - FEI Nova 200 (FEG/SEM) and EDAX - Pegasus X4M (EDS/EBSD) with high vacuum resolution 1.8 nm at 1 kV (SE) 1.0 nm at 15 kV (SE) or low vacuum resolution 1.8 nm at 3 kV (Helix detector) 1.5 nm at 10 kV (Helix detector).
  • SEM scanning electron microscopy
  • EDS energy dispersive X-ray spectroscopy
  • SE NanoSEM - FEI Nova 200
  • Tensiometry contact angle analysis: In an embodiment, the water contact angle measurements of the particles were determined with a tensiometer (Attension Theta, Biolin Scientific) through the sessile drop method, with a water droplet volume of 3 pL, using the OneAttension software. The contact angle measurements were performed on circular pellets (1.3 cm in diameter), prepared with 180 mg of particles and using a Atlas Manual Hydraulic Press.
  • the gross heat of combustion (calorific value) of the SiO2@QAS+PA was determined in agreement with EN ISO 1716:2018. Prior to testing, the specimens were conditioned for a minimum period of 100 h at 23 ⁇ 2 9 C and 50 ⁇ 5 % relative humidity, having met the constant mass criterion. The samples were tested according to the crucible method using benzoic acid as a combustion aid.
  • the fire behavior of cotton (CO) textile fabrics samples was evaluated by using the cone calorimeter method at 25 kW/m2, according to ISO 5660-1:2015.
  • the modified silica particles, SiO2@QAS+PA, SiO2@QAS (15% (w/v), or non-modified bio-silica particles for control trials were mixed with 3% (w/v) of chitosan (previously dissolved in 1% (v/v) acetic acid) with the help of a dispersing machine (Ultra-Turrax) for 30 min.
  • a film of 400 pm was deposited on a cotton fabric through a film applicator.
  • the cotton fabrics were then dried in an oven at 120 °C for 30 s (several cycles until the textile was dried).
  • the specimens 100 mm x 100 mm
  • the initial weight of the CO, CO-SiO2, CO-SiO2@QAS, and CO-SiO2@QAS+PA fabrics were approximately 2 g.
  • the measured parameters were: following time to ignition (s); Average rate of heat release for test duration (kW/m2); maximum average heat release (MAHRE) (kW/m2); Time to MAH-RE (s); sample mass loss for test duration (%).
  • Flammability tests In one embodiment, the flammability behavior of the functionalized CO textile fabrics samples (SiO2@QAS+PA, SiC>2@QAS, and SiO2), prepared under the same conditions as reported in the Calorimetry tests, was determined by measuring the flame spread properties of a vertically oriented cotton fabric (560x170 mm) according to ISO 6941:2013. The source ignition was applied on the surface of the cotton fabric (at 90°) and exposed to the flame for 10 s. After the flame source was removed, the propagation times of the flame in the cotton textile were recorded.
  • Antifungal tests In an embodiment, the inhibition tests (antifungal assays) were performed with Candida albicans (C. albicans) (106 CFU/mL) suspension according with CLSI Antimicrobial Susceptibility Testing (AST) Standards by the method Broth Dilution Antifungal Susceptibility Testing of Yeasts. For each sample, a solution of silica particles was prepared with a concentration of 20 g/L. 100 pL of the sample solution (5 replicates per sample) were inserted in a 96-well plate and subsequently, 100 pL of the fungal suspension was added to this solution.
  • AST Antimicrobial Susceptibility Testing
  • the plates were incubated at 37 °C (optimal temperature for microorganism growth) for 24 h, and finally, the percentage of antifungal activity (fungus growth) was estimated. Additionally, other antimicrobial tests (for Staphylococcus aureus, Escherichia coli, MRSA and S. cerevisieae) were done by two methods. The first one was the disc diffusion method, in which discs of functionalized cotton were placed on a plate with the microorganism's inoculum, so that the growth inhibition zone could be measured. In the second method, 10 pL drops of microorganism inoculum were placed to grow on a suitable medium with functionalized sample dispersed on that medium (in a certain concentration), to obtain the MIC (minimum inhibitory concentration).
  • rice husk was selected as the raw material for the extraction of silica particles.
  • QAS and PA were chosen due to their potential for antifungal and flame retardant properties, respectively.
  • PA is a bio compound that could be extracted from RH
  • the PA content in the RH (percentage in dry weight) was found to be less than 1%, revealing that extracting this bio-compound from RH for the functionalization of the silica is a resource and time-consuming process.
  • the amount of PA needed for the functionalization process would imply that very high amounts of RH would have to be processed. For these reasons commercially available PA was used for further functionalization processes.
  • Figure 5 shows the FTIR spectra of functionalized silica particles, such as SiO2@QAS+PA and SiO 2 @QAS, as well as SiO 2 , SiO 2 -OH, and rice husk.
  • functionalized silica particles such as SiO2@QAS+PA and SiO 2 @QAS
  • SiO 2 , SiO 2 -OH, and rice husk The presence of characteristic peaks of C-H stretching and deformation vibration bands at 2917, 2852, and 1467 cm-1 from QAS [42] differentiate the SiO 2 @QAS+PA and SiO 2 @QAS spectra from SiO 2 , SiO 2 , and RH.
  • the spectra for all SiO2 particles (with and without functionalization) exhibited bands at 802 and 1059 cm 1 , which could be assigned to the symmetric and asymmetric stretching vibration of Si-O-Si bonds, respectively [44,45], These bands also appeared in the RH, which could indicate the presence of SiO2 content.
  • a band at 1647 cm' 1 can be observed, which appears in all spectra except for SiO2 particles, and that is being attributed to absorbed H2O [44],
  • Table 9 Zeta potential (in mV) for the extracted and functionalized silica particles in ethanol and deionized water.
  • Figure 6 (a) shows the thermogravimetric analysis (TGA) of extracted, activated, and functionalized silica, as well as RH.
  • TGA thermogravimetric analysis
  • the residual mass observed in the thermogram of silica particles was higher than 95 %, proving its inorganic nature and confirming the value obtained from the silica quantification method (around 93 %). Comparing the SiO 2 @QAS thermogram with QAS, a similar profile is observed.
  • the second loss in the temperature range of around 200-300°C is characteristic for the quaternary amine according to Galimberti et al. (2009) [47], Despite being an organic compound, the residual mass of QAS shows a quite high percentage (18.2 %) when compared to the PA (4.4 %) due to the presence of a silane group in QAS composition.
  • PA decomposes in many degradation temperatures that have been reported elsewhere corresponding to dehydration, carbonization, and char degradation processes [48], After 565 °C, thermal decomposition of the char takes place leading to a final char with a weight loss of 35.5 %.
  • the integration of PA into SiO2@QAS was one more time successfully proved with thermographic analysis.
  • the degradation temperature at 229-370 °C observed in Figure 6 (b) for SiO2@QAS+PA accounts for the highest weight loss (approximately 38 %). Comparing this value with the sum of the second lost for PA and QAS, the same weight loss is obtained (Table 10).
  • the functionalized SiCh particles were evaluated in terms of flame retardant properties.
  • one of the most relevant assays to evaluate this property is the determination of the gross heat of combustion (calorific value) using the EN ISO 1716:2018 namely in samples in the powder form.
  • the introduction of organic groups, from the PA and QAS composition, helps on the difference on heat of combustion obtained for SiO2 particles without functionalization of - 0.2 MJ/Kg and SiO2@QAS+PA of 19.7 MJ/Kg.
  • This assay was used as a preliminary test, but it yielded inconclusive results. Therefore, a different approach to demonstrate the flame retardancy of the modified particles was pursued.
  • the textile coating was chosen as this is one of the many possible applications of the incorporation of the modified silica particles of the present disclosure to enhance the intrinsic properties of the final product.
  • the particles were incorporated in a chitosan matrix (3% w/v) and applied in a textile substrate to assess their flame retardant properties.
  • the QAS+PA and QAS modified bio-silica particles were successfully deposited in the cotton (CO) textile substrate as described in section "Hydrophobic, antifungal, and flame retardant evaluation techniques" (non-modified silica particles were used for the preparation of control samples).
  • CO-SiO2@QAS+PA shows a heat release rate lower than CO-SiO2@QAS+PA but a value close to CO-SiCh demonstrating that, QAS did not influence the levels of heat release rate and that PA is the most important compound in such analysis.
  • Table 13 describes the results obtained with silica from different rice origins. No significative differences were observed between origins and acids used for silica extraction from rice husk. This assay was performed considering concentrations in v/v and not w/v due to the different densities of the material and for this reason it allows a right comparison.
  • Table 14 The concentration of the fungus Candida albicans after 24 h of incubation with biosilica particles (5 replicates per sample).
  • the hydrophobic behavior of the modified particles was also assessed by water contact angle (WCA) measurements.
  • WCA water contact angle
  • the results obtained for the water contact angle for SiCh, SiO2@QAS and SiO2@QAS+PA particles are shown in Figure 12.
  • the WCA of both functionalized SiCh particles (QAS and QAS+PA) increased when compared with non-functionalized SiCh due to the presence of long alkyl groups from QAS.
  • the functionalized particles exhibited similar WCA values and hydrophobic properties (0>9O°).
  • the WCA of the non-functionalized SiO2 was not determined due to the high intrinsic hydrophilic behavior of these particles.
  • rice husk a residue that is widely available in agroindustry, was used to make the extraction of bio-silica (obtained using acidic conditions) in a circular economy approach with yields of around 10% and purity higher than 90%.
  • bio-SiO2 with QAS compound (anti-fungal and hydrophobic agent) and PA (flame retardant agent) was successfully achieved, and the particles were well characterized by structural and thermal techniques demonstrating functional groups and degradation temperatures that are characteristics of the compounds used for functionalization. Complementary analyses were performed to verify the morphology and the particle size around less than 2 pm, obtained with the acidic process.
  • the particles also demonstrated to be able to improve the flame retardancy performance when compared to SiO2@QAS proving that, the introduction of PA markedly makes the particles more promising to be used as flame retardants. Furthermore, the SiO2@QAS+PA were able to reduce the growth of Candida albicans, while SiO2 and SiO2@QAS particles continued to promote the growth of the fungus.
  • the SiO2@QAS+PA particles were applied to a cork-based substrate and the flame retardancy properties of the modified surface were assessed.
  • the flame retardancy of the samples was evaluated and promising results were obtained: after the ignition time (10 s.), the samples coated with a mixture of chitosan and SiO2@QAS+PA exhibited a promising flame retardant behaviour, resulting in an average flame progression of 8.6 cm against 13 cm obtained for the control sample (without modified particles).
  • SiO2 + PA were functionalized with different antimicrobial compounds: ELA (Ethyl Lauroyl Arginate) and Quaternized Carboxymethylchitosan (qCMChi).
  • ELA was used as commercial agent and qCMChi was synthetized.
  • the synthesis of qCMChi was adapted from Abgovi et al. (2016) [53] and Nhung et al. (2020) [54], Firstly, carboxymethyl chitosan was synthetized using 10 g of chitosan mixed with NaOH prepared in Isopropanol (13.5 % (m/v)) and placed at 50 °C during lh.
  • the FTIR spectrum for carboxymethyl chitosan is similar to the data described by Agbovi et al. (2016).
  • the IR spectrum of chitosan has stretching bands at 3435 cm' 1 characteristic of O-H and 2867 cm' 1 for C-H ( Figure 13). At lower wavenumber it is possible to see the bands at 1142 cm' 1 for C-O-C and 1109 cm-1 for C-O, and 1591 cm' 1 for N-H bending. When it is made the carboxymethylation, it is observing the appearance of different bands at 1570 cm 1 for -COO- group and 1410 cm' 1 for -CH3 groups.
  • the other bands are similar to the chitosan (O-H and C-H broad spectral bands between 3440 and 2980 cm' 1 ).
  • the quaternization demonstrates the introduction of a quaternary ammonium salt group on the carboxymethyl chitosan structure and the peak at 1480 cm -1 is characteristic of C-H asymmetric bending vibration of the trimethylammonium group, confirming the existence of this quaternary amine.
  • the N-H bending (1596 cm -1 ) of the primary amine diminished because of the transformation of the primary amine into the secondary amine (aliphatic).
  • the spectrum exhibits a broad band at approximately 3450 cm -1 , which became higher than that of chitosan and carboxymethyl chitosan due to the increase number of hydroxyl groups.
  • ranges are included. Furthermore, it should be understood that unless otherwise indicated or otherwise evident from the context and/or understanding of a technical expert, the values which are expressed as ranges may assume any specific value within the ranges indicated in different achievements of the invention, at one tenth of the lower limit of the interval, unless the context clearly indicates the contrary. It should also be understood that, unless otherwise indicated or otherwise evident from the context and/or understanding of a technical expert, values expressed as range may assume any sub-range within the given range, where the limits of the sub-range are expressed with the same degree of precision as the tenth of the unit of the lower limit of the range.

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Abstract

The present disclosure relates to functionalized silica particles, specifically, functionalized hydrophobic silica particles comprising a flame retardant and an antimicrobial agent, wherein the antimicrobial agent is the linker between the silica particle and the flame retardant agent. The present disclosure also relates to methods to obtain such functionalized hydrophobic silica particles, as well as the obtention of silica from natural sources through extraction processes. Furthermore, the present invention relates to the use such functionalized hydrophobic silica particles, preferably in textiles, construction materials and interior of motor vehicle.

Description

D E S C R I P T I O N
FUNCTIONALIZED SI LICA PARTICLES, OBTENTION AND USES TH EREOF
TECHNICAL FIELD
[0001] The present disclosure relates to functionalized silica particles, specifically, functionalized hydrophobic silica particles comprising a flame retardant and an antimicrobial agent, wherein the antimicrobial agent is the linker between the silica particle and the flame retardant agent. The present disclosure also relates to methods to obtain such functionalized hydrophobic silica particles, as well as the obtention of silica from natural sources through extraction processes. Furthermore, the present invention relates to the use such functionalized hydrophobic silica particles, preferably in textiles, construction materials and interior of motor vehicle.
BACKGROUND
[0002] Nowadays, waste management comes as a global challenge, being the resource efficiency (reduction of oil and gas imports), the most assertive solution to help on a greener and more sustainable world [1-3],
[0003] In this context, the use of waste or by-products from the agricultural sector has received greater attention from the scientific and ecological communities. Specifically, rice husk (RH) is an abundantly available waste material in all rice producing countries, being China and India the countries with the highest production in 2017 of around 42.54 and 33.70 million tons, respectively, in a world total of 153.95 million tons [4], Consequently, RH is one of the most abundant residues of the agro-industry and produces, when incinerated, the highest percentage of ash (23 %), compared to other waste from cereals, such as sugarcane bagasse (15%), corn leaves, and wheat (12 % and 10 %, respectively) [5], Due to its high availability and the high percentage of silica [6] (typically around 90 %, but it can be as low as 70 %), this resource has latent potential for being reused as a secondary raw material to produce silica- based materials for different applications, such as drug delivery [7], biomedical [8], optical devices [9], catalysis [10], agriculture [11] or in the production of value-added ceramics [12], Indeed, the work by Aguilar et al. (2020) [13] showed that this residue yields a higher amount of silica (10 %) and the highest purity of the obtained silica particles (around 93 %).
[0004] The production of silica could be performed by low temperature synthesis (micro-wave reduction in the presence of sodium hydroxide [14] or even by a sol-gel route) [15], The use of alkali extraction followed by its precipitation using an acidic solution, such as hydrochloric or sulphuric acid is reported [16,17]. Different works have shown that it is also possible to extract high-purity silica from RH just using acidic leaching, followed by thermal treatment of the digested husk [18-20],
[0005] Multifunctionality is a highly desirable feature for silica particles, enhancing their versatility and the added value of the final materials.
[0006] Environmentally friendly pathways for the functionalization of biosilica can be accomplished with the use of bio-based compounds such as phytic acid (PA) or quaternary ammonium salts with silane groups, to impart multifunctional properties such as flame retardancy, antimicrobial, and hydrophobicity, that may be of interest for the final application of the particles.
[0007] Phytic acid (PA) is a bio-based flame retardant and environmentally friendly compound that has been shown to improve the flame retardancy of different polymeric matrices such as Polyethylene terephthalate (PET), Polylactic acid (PLA), Polypropylene (PP), Polyamide (PA66), cotton, among others [21], PA is a phosphorus-rich organic acid compound (mass fraction of about 28%) with 6 phosphate groups and 12 hydroxyl groups, and it can be extracted from plant seeds, such as rice bran, beans, seeds, among others [22], Several studies reported developments of organic-inorganic compounds by using silane agents, as tetraethoxysilane (TEOS, precursor of silica) and PA envisioning flame retardant properties. Cheng et al. (2020) developed a PA/silica organic-inorganic hybrid sol by using TEOS as a silica precursor [23], PA was used to enhance the flame retardancy properties of cotton fabrics, where sodium alginate was applied as an anti-agglomeration agent and stabilizer for the hybrid silica sol and citric acid as a cross-linking agent that promoted the interaction between alginate and cotton fiber. Also, Barbalini et al. (2019) developed a sol-gel hybrid organic-inorganic coating based on PA and TEOS with different ratios to enhance the flame retardant properties in cotton fabrics [24], Sui et al. (2020) developed a flame-retardant system for epoxy resin composed by 3D organic-inorganic core-shell nanoparticles, that are aminated silica nanoparticles post functionalized with PA and nickel (nickel phytate complex) [25], The incorporation of the modified particles has improved the thermal stability of the resin. Fu et al. (2019) also re-ported a coating with hydrophobic and flame retardant properties for cotton fabric composed by bilayer of chitosan/PA and subsequently a layer of hydrophobic silica nanoparticles [26],
[0008] Antimicrobial properties are also attractive for some applications of silica particles, such as those involving the need for anti-rotting features as in the case of construction applications. In the last two decades, fungal infections have been increasing and invasive forms can cause mortality, especially in immunocompromised or immunosuppressed patients [27], This is aggravated by the emergence of drug-resistant fungus and exploration on new antifungal agents must be made [28] including metal oxides (Ag, TiCh, ZnO nanoparticles) [29,30], amphotericin B [31], antifungal peptides [32], and quaternary ammonium compounds. [0009] The antimicrobial activity of quaternary ammonium compounds has been attributed to the length of the N-alkyl chain that can affect its performance [33], It has been reported that the optimum chain length for activity towards gram-positive bacteria is 14 carbons, 16 carbons for gram-negative bacteria, and 12 carbons for yeast and fungi [34], Botequim et al. (2012), for example, reported antimicrobial silica nanoparticles modified with a didodecyldimethylammonium bromide (DDAB) with a large antimicrobial activity against fungi (Candida albicans), bacteria (Staphylococcus aureus, Escherichia coli) and virus (influenza A) [31],
[0010] Silanes groups introduced in quaternary ammonium compounds are interesting compounds for the functionalization of silica particles, through sol-gel technology. Song et al. (2011), for example, functionalized silica particles with an ammonium chloride silane called 3-(trimethoxysilyl)- propyldimethyloctadecyl ammonium chloride (QAS), but only evaluated its antibacterial activity. The authors compared the silica nanoparticles modified with quaternary ammonium silane with silica nanoparticles functionalized with octadecyltrimethoxy silane. Both modified particles exhibited similar hydrophobicity, but QAS-modified silica showed better results in reducing the growth of bacteria due to the presence of the quaternary ammonium groups in its structure [35], Gong et al. (2014) have also reported one-pot modification of silica particles with QAS and 3-methacryloxypropyltrimethoxysilane; these particles, when incorporated into bisphenol A-glycidyl methacrylate/ Triethylene glycol dimethacrylate (bis-GMA/TEGDMA) resin, exhibited antimicrobial activities against Streptococcus mutans, Actinomyces naeslundii, and Candida albicans [36], Wang and co-workers also prepared a layer- structured montmorillonite nanocomposite containing QAS-grafted silica nanoparticles that exhibited an excellent antibacterial capability against Gram-negative and Gram-positive bacteria [37],
[0011] US2006178443A1 describes a nucleating agent to produce polyurethane (PU) foam comprising nanoparticles, a polyurethane foam comprising nanoparticles and the use of the nucleating agent for producing the polyurethane foam. In an embodiment, the nanoparticle dispersion is added to a flame retardant. In particular, the nanoparticles comprise metal oxide selected from the group consisting of SiO2, ZnO2, AI2O3, ZrO2 and TiO2. No information is provided regarding the obtention of multifunctional SiO2 particles with hydrophobic, flame retardant and antimicrobial properties and the use of a quaternary amine as crosslinker for modified-silica particles.
[0012] US2007231295A1 describes SiOz flakes, especially porous SiOz flakes, wherein 0.70<z<2.0, especially 0.95<z<2.0, especially porous SiO2 flakes, comprising an organic, or inorganic antimicrobial compound, or composition, which provide enhanced (long term) antimicrobial efficacy. This document only provides examples regarding silver/copper/palladium/nickel coted silicon oxide flakes. These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure.
[0013] These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure. GENERAL DESCRIPTION
[0014] The present disclosure relates to functionalized silica particles, specifically, functionalized hydrophobic silica particles comprising a flame retardant and an antimicrobial agent, wherein the antimicrobial agent is the linker between the silica particle and the flame retardant agent. The present disclosure also relates to methods to obtain such functionalized hydrophobic silica particles, as well as the obtention of silica from natural sources through extraction processes. Furthermore, the present invention relates to the use such functionalized hydrophobic silica particles, preferably in textiles, construction materials and interior of motor vehicle.
[0015] In an embodiment, the present disclosure relates to an innovative method to produce multifunctional silica particles extracted from RH and functionalized simultaneously with PA and QAS, promoting circular economy, while developing value-added silica particles (combination of flame retardant, antimicrobial and hydrophobic properties) that can be used in different industrial sectors, such as the automotive and the construction and building sectors, as nanofillers or in textile coatings.
[0016] Within the present disclosure, the terms "SiO2@QAS+PA" and "SiO2@PA+QAS" have the same meaning and refer exactly to the same particles, i.e., silica particles functionalized with QAS and PA.
[0017] 3-(trimethoxysilyl)-propyldimethyloctadecyl ammonium chloride (QAS), also known as Dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride, is identified by the CAS (Chemical Abstracts Service) Registry Number: 27668-52-6.
[0018] Within the present disclosure, different flame retardant agents can be employed. These agents are typically phosphorous-based compounds that could be phytic acid and its derivatives or; other compounds such as Ammonium polyphosphate (APP), melamine polyphosphate (MPP), pentaerythritol phosphate alcohol, phosphazenes, diethyl phosphonic metal salts and 9,10-dihydro-9-oxa-10- phosphaphenanthrene-10-oxide (DOPO), or mixtures thereof; among others preferably, phytic acid is used.
[0019] In an embodiment, some fillers may also enhance the flame retardant properties, such as aluminum trihydroxide, magnesium carbonate or magnesium hydroxide and titanium dioxide, among others.
[0020] Within the present disclosure, different antimicrobial agents can be employed: N-allyl-N-decyl- N-methyl-N-trimethoxysilylpropylammonium iodide, Perfluorooctyl-containing quaternary ammonium salt, Triethoxysilylpropyl Succinic Anhydride Silane, proving one more time that quaternary amines are the group with the highest interest in this functionality. Preferably, 3-(trimethoxysilyl)- propyldimethyloctadecyl ammonium chloride (QAS) is used. [0021] An aspect of the present disclosure relates to a functionalized hydrophobic silica particle comprising a flame retardant and an antimicrobial agent, wherein the antimicrobial agent is a linker between the silica particle and the flame retardant agent; wherein the flame retardant agent is selected from a list consisting of: phytic acid, ammonium polyphosphate, pentaerythritol phosphate alcohol, melamine polyphosphate, phosphazenes; diethyl phosphonic metal salts; 9.10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide; wherein the antimicrobial agent is selected from a list consisting of: 3-(trimethoxysilyl)- propyldimethyloctadecyl ammonium chloride, quaternary ammonium silane, quaternary amine, ethyl lauroyl arginate (ELA), quaternized carboxymethylchitosan (qCMChi), N-allyl-N-decyl-N-methyl-N- trimethoxysilylpropylammonium iodide, Perfluorooctyl-containing quaternary ammonium salt, Triethoxysilylpropyl Succinic Anhydride Silane.
[0022] In a preferred embodiment, the flame retardant agent is phytic acid.
[0023] In a preferred embodiment, the antimicrobial agent is a quaternary ammonium silane.
[0024] In an embodiment, the functionalized hydrophobic silica particle comprises phytic acid and a quaternary ammonium silane, wherein the quaternary ammonium silane is the linker between the silica particle and the phytic acid.
[0025] In an embodiment, the quaternary ammonium silane is 3-(trimethoxysilyl)- propyldimethyloctadecyl ammonium chloride.
[0026] In an embodiment, the functionalized hydrophobic silica particle comprises a particle size less than 2 pm.
[0027] In an embodiment, the particle size is measured through dynamic light scattering (DLS). Zeta potential measurements in DLS analysis were performed with a Zetasizer Nano ZS90 (Malvern) and a disposable capillary cell (DTS1070) at 25 °C. For the measurements, bio-silica was dispersed in two different solvents, ethanol or deionized water, at a concentration of 1 g/L and subjected to ultrasonic agitation before the analysis. Particle size was also evaluated by scanning electron microscopy (SEM) connected to an energy dispersive X-ray spectroscopy (EDS) using a NanoSEM - FEI Nova 200 (FEG/SEM) and EDAX - Pegasus X4M (EDS/EBSD) with high vacuum resolution 1.8 nm at 1 kV (SE) 1.0 nm at 15 kV (SE) or low vacuum resolution 1.8 nm at 3 kV (Helix detector) 1.5 nm at 10 kV (Helix detector). The electron beam resolution 0.8 nm at 30 kV (STEM), beam landing energy: 200V to 30 kV, high stability Schottky field emission gun with automatic operation, probe current: 0.3 pA to 22 nA, chamber vacuum (high vacuum): <10-4 mBar and chamber vacuum (low vacuum): <2 mBar was selected as the optimal operation conditions.
[0028] Another aspect of the present disclosure relates to a composition comprising such functionalized hydrophobic silica particle. [0029] In an embodiment, the composition further comprises a filler, preferably such filler is selected from the list consisting of: aluminum trihydroxide, magnesium carbonate, magnesium hydroxide or titanium dioxide, or mixtures thereof.
[0030] Another aspect of the present disclosure relates to an article comprising the functionalized hydrophobic silica particle or the composition.
[0031] In an embodiment, the article comprises a textile material, a construction material or a material of the interior of a motor vehicle.
[0032] Another aspect of the present disclosure relates to the use of the functionalized hydrophobic silica or the composition as a hydrophobic agent, as an antimicrobial agent and as a flame retardant agent.
[0033] Another aspect of the present disclosure relates to the use of the functionalized hydrophobic silica particle or the composition as a dirt-resistant agent.
[0034] Another aspect of the present disclosure relates to the use of the functionalized hydrophobic silica particle or the composition as a stain repellent agent.
[0035] In an embodiment, the use of these particles is in textiles, and/or in construction materials, and/or in the interior of motor vehicles.
[0036] Another aspect of the present disclosure relates to a method to produce functionalized hydrophobic silica particle of the present disclosure, comprising the following steps: obtaining a silica particle; dispersing the silica particles in a solution of EtOH:H2O, preferably with a concentration ratio of 1:1 (v/v); adding a flame retardant agent and an antimicrobial agent to the obtained dispersion, preferably with sonication; wherein the flame retardant agent is selected from a list consisting of: phytic acid, ammonium polyphosphate, melamine polyphosphate, phosphazenes; diethyl phosphonic metal salts; 9.10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide; and wherein the antimicrobial agent is selected from a list consisting of: 3-(trimethoxysilyl)- propyldimethyloctadecyl ammonium chloride, quaternary ammonium silane, quaternary amine, ethyl lauroyl arginate (ELA), quaternized carboxymethylchitosan (qCMChi), N-allyl-N-decyl-N- methyl-N-trimethoxysilylpropylammonium iodide, Perfluorooctyl-containing quaternary ammonium salt, Triethoxysilyl propyl Succinic Anhydride Silane; centrifuging the obtained dispersion; preferably at 9000 rpm for 10 min; washing the precipitated; preferably with deionized water; repeating the two previous steps; preferably 3 times; drying the washed precipitated until obtained a dried powder; preferably during 48h at 40 °C; milling the obtained powered until obtained a particle size less than 2pm in order to obtain the functionalized silica particle.
[0037] In an embodiment, the silica particles are obtained through an extraction process from an organic or inorganic source; preferably an organic source.
[0038] In an embodiment, the silica particles are obtained through an extraction process from rice husk.
[0039] In an embodiment, the extraction process comprises an acid extraction method or a sol-gel method.
[0040] In an embodiment, the acid extraction method comprises the following steps: obtaining crude material comprising silica; preferably with particle size equal or below 1.5 mm; washing such crude material comprising silica; preferably with distilled water; preferably for 2h at room temperature; digesting with acid the washed crude material; preferably with an acid solution selected from a list consisting of: hydrochloric acid, nitric acid or citric acid, or mixtures thereof; preferably at 60°C for 2 h; filtering the digested crude material; preferably with a wire mesh of 130 pm; neutralizing the filtered digested crude material; preferably with deionized water;
Drying the neutralized material; preferably at 110 °C for 12h; calcinating the dried material to obtain purified silica; preferably at 700 °C for 3h30 min after an initial heating ramp from 25 °C to 700 °C for 2h30 min.
[0041] In an embodiment, the sol-gel method comprises the following steps: obtaining crude material comprising silica; preferably with particle size equal or below 1.5 mm; calcinating the crude material comprising silica; preferably at 700 °C for 3h30 min after an initial heating ramp from 25 °C to 700 °C for 2h30 min; extracting the silica from the calcinated product with an alkaline solution under reflux to obtain crude sodium silicate; preferably the alkaline solution is NaOH 10%; preferably at 90°C for 3h; filtrating the crude sodium silicate to eliminate char impurities; treating the filtrated crude sodium silicate with acid solution to obtain crude silica; preferably with hydrochloric acid, nitric acid, citric acid, phosphoric acid or sulfuric acid, or mixtures thereof washing the crude silica; preferably with water;
Drying the washed crude silica to obtain dried silica particles; preferably at 150°C for 2 h. BRIEF DESCRIPTION OF THE DRAWINGS
[0042] The following figures provide preferred embodiments for illustrating the disclosure and should not be seen as limiting the scope of invention.
[0043] Figure 1: FTIR analysis of silica obtained with different conditions: without calcination, 600 °C, 700 °C and 800 °C.
[0044] Figure 2: Scanning electron microscopy images of the (a) rice husk, (b) silica without acid digestion, (c) silica obtained by acid digestion before calcination, (d) silica obtained by acid digestion after calcination, (e) Silica obtained by Stober method, at different scales such as (A) 5 pm and (B) 500 nm.
[0045] Figure 3: FTIR analysis of (a) silica obtained with RH acid digestion and (b) with Stober method.
[0046] Figure 4: FTIR analysis of biosilica obtained by the sol-gel method using phosphoric acid, sulfuric acid, citric acid, nitric acid and hydrochloric acid at the neutralization process.
[0047] Figure 5: FTIR spectra for the samples: RH, SiCh, SiC>2@QAS, SiO2@QAS+PA.
[0048] Figure 6: TG (a) and DTG (b) curves were obtained at 20°C/min under a dynamic synthetic air atmosphere for the samples: RH, SiCh, PA, QAS, SiC>2@QAS, and SiO2@QAS+PA.
[0049] Figure 7. FTIR analysis of SiC>2@QAS+PA, PA, SiCh, SiC>2@PA unwashed and SiC>2@PA washed.
[0050] Figure 8: Scanning electron microscopy images and EDS spectra of the (a) SiCh, (b) SiO2@QAS, and (c) SiO2@QAS+PA.
[0051] Figure 9: ISO 6941:2003 vertical burning tests. Teste scheme (a), test for cotton with SiO2 in chitosan matrix (b), test for cotton with SiO2@QAS+PA in chitosan matrix (c), test for cotton with SiO2@QAS in chitosan matrix (d) and test for cotton with 3% chitosan (3% w/v) (e). The images show the caption of test specimens at 30 s. of the test.
[0052] Figure 10: 1 Results obtained in the vertical burning tests according to the ISO 6941:2003: A) Cotton; B) Cotton with chitosan; C) Cotton with chitosan and SiO2 particles; D) Cotton with chitosan and SiO2@QAS; E) Cotton with chitosan and QAS+PA; F) Cotton with chitosan and SiO2@PA+QAS.
[0053] Figure 11: Antimicrobial illustrations for Staphyloccoccus aureus, Eschericia coli, Methicillin- Resistant Staph (MRSA), S. Cerevisiae and Candida albicans for non-functionalized and functionalized biosilica.
[0054] Figure 12: Water contact angle for the samples: (a) SiCh (CA: not detected), (b) SiO2@QAS (CA: 150.7 ± 4.3°) and (c) SiO2@QAS+PA (CA: CA: 153.3 ± 3.2°).
[0055] Figure 13: FTIR spectra for carboxymethyl chitosan. [0056] Figure 14: FTIR results for (a) SiO2@ELA+PA and (a) SiO2@qCMChi+PA.
[0057] Figure 15: Flame retardancy (with three test pieces for each particle) for (a) SiO2@ELA+PA and (b) SiO2@qCMChi+PA.
DETAILED DESCRIPTION
[0058] The present disclosure relates to functionalized silica particles, specifically, functionalized hydrophobic silica particles comprising a flame retardant and an antimicrobial agent, wherein the antimicrobial agent is the linker between the silica particle and the flame retardant agent. The present disclosure also relates to methods to obtain such functionalized hydrophobic silica particles, as well as the obtention of silica from natural sources through extraction processes. Furthermore, the present invention relates to the use such functionalized hydrophobic silica particles, preferably in textiles, construction materials and interior of motor vehicle.
[0059] Throughout an investigation work carried out to obtain silica particles functionalized with a flame retardant agent, it was surprisingly found that such functionalization is possible when an antimicrobial agent is used as a linker between the silica particle and the flame retardant agent. It was possible to obtain functionalized SiO2 particles, comprising a flame retardant and an antimicrobial agent, wherein the antimicrobial agent is the linker between the silica particle and the flame retardant agent. The method proved to be efficient, and the functionalized particles obtained showed antimicrobial, flame retardant and hydrophobic properties.
[0060] In an embodiment, the SiO2 particles used in the present disclosure can be obtained commercially, or by synthesis using traditional procedures like Stober method or through extraction from natural sources, namely mineral, organic and/or vegetal sources. Preferably, the SiO2 particles of the present disclosure are obtained from rice husk.
Materials and Methods / Materials
[0061] In an embodiment, rice husk (RH) was purchased from Alvaro Alves Borges, Lda (Figueira da Foz, Portugal). Hydrochloric acid (HCI) 37 wt.%, sulfuric acid (>95%), and sodium chloride (NaCI) were purchased from Fisher Chemicals (Porto Salvo, Portugal). Ammonium molybdate tetrahydrate was acquired from Alfa Aesar (Kandel, Germany). Sodium sulphite anhydrous, p-methylaminophenol sulfate, oxalic acid dehydrate and sodium metasilicate, phytic acid sodium salt hydrate, iron (III) chloride hexahydrate, dimethyloctadecyl(3-(trimethylsilyl)propyl ammonium chloride solution (QAS) 42 wt.% in methanol were purchased from Sigma-Aldrich (Schnelldorf, Germany). Inositol hexaphosphoric acid (phytic acid, PA; 50 wt.% aqueous solution) was obtained from Acros Organics (Geel, Belgium). 5- sulphosalicylic acid dihydrate and glacial acetic acid (99.9%) were supplied from VWR International (Alfragide, Portugal). Chitosan of medium molecular weight and with >95% of deacetylation was purchased from Primex (ChitoClear, Iceland). Absolute ethanol (EtOH) was purchased from Aga (Prior Velho, Portugal). Ultra-pure water of Mil-li-Q. quality (Millipore, Italy) was used. All other reagents used were of analytical grade.
[0062] In an embodiment, commercial Ethyl Lauroyl Arginate Hydrochloride (95%) was purchased from abcr GmbH with the CAS number of 60372-77-2 and the reagents needed to quaternized carboxymethyl chitosan synthesis (qCMCHi) were: Chitosan (hbglO ChitoClear)( 95% CAS: 9012-76-4; Supplier : Primex, deacetylation degree 93.7%, M=2xl0-4), NaOH (pellets) - (CAS: 1310-73-2; SupplienEka), Isopropanol - (99.7% CAS: 67-63-0; Supplier : VWR Chemicals), 3-chloropropyltrimethoxysilane (CAS 2530-87-2, TCI/C0840), EtOH ( 99,8% CAS: 64-17-5; Supplier: Aga), acetic acid glacial (99,7% CAS: 64-19-7; Supplier : Fisher Chemical), Glycidyltrimethylammonium chloride - GTMAC (ca. 80% in water) )( CAS: 3033-77-0; Supplier : TCI / G0476); Acetone (pure ; CAS: 67-64-1; Supplier : Quimijuno); Methanol (CAS: 67-56-1 Supplier : Labkem / MTOL-OIA-2K5).
Preparation of RH and SiO2 extraction
[0063] In an embodiment, to optimize the silica extraction procedure, different conditions were assessed, such as granulometry (0.25 to >1.5 mm, without griding), calcination temperature (600, 700 or 800 °C), acid digestion after, before or without and the type of acid to digestion (hydrochloric, citric, acetic, or nitric) to understand the affectation of these parameters. Granulometry did not affect the silica purity and yield, as depicted in Table 1.
Table 1. Granulometry and yield.
Granulometry (mm) Yield (%)
> 1.50 (without griding) 11.2
1.50 11.6
1.00 11.1
0.50 11.2
0.25 11.7
[0064] On the other hand, the calcination temperature seems to be an important parameter. Through FTIR, TGA and EDS it was possible to concluded that 700 °C was the temperature that allows the best result.
[0065] In an embodiment, Silica without acid digestion demonstrate some impurities (trough FTIR, Figure 1) and EDS analysis with the presence of Mg, K and Ca) and acid digestion after calcination seems not to be the best option since a lot of silica is lost in the process with lower yields being an uncontrollable method, according with Table 2. For this reason, the selected conditions were 0.5mm of particle size, acid digestion before calcination, and calcination of temperature of 700°C (2h30-25°C to 700°C; 3h30 at 700°C). [0066] Tabela 2. Acid digestion before and after calcination at different temperatures
Yield (%)
Calcination temperature (°C) Add digestion before Acid digestion after
_ calcination _ calcination _ 600 12.5 15.5
700 11.2 7.11
800 12.3 10.1
[0067] In an embodiment, in terms of acids used in the acid digestion process, the yield is similar and hydrochloric acid, nitric acid and citric acid are good choices. Table 3 resumes the results obtained using different acids in the digestion process.
Table 3. Results obtained using different acids in the digestion process.
Acid ,
Si+O (through Temperature digestion . , w. , , . . . ,n,. Ratio
, Acid Yield (%) EDS analysis) Si (%)
(°C) after, before ’ (O/Si) or without
Before HCI 12.54 96 36 1.67
600 After HCI 15.50 96 36 1.67
Without - 14.75 91 35 1.60
HCI 12.28 91 36 1.53
HNO3 12.30 95 38 1.50
Before CH3COOH 13.00 90 34 1.65
700 C6H8O7
10.90 96 42 1.28
After HCI 7.11 91 34 1.68
Without - 14.05 95 37 1.57
Before HCI 11.16 87 40 1.12
800 After HCI 10.07 90 31 1.90
Without - 13.36 88 31 1.84
[0068] In an embodiment, the particle morphology of silica extracted from rice husk is completely different of silica obtained by chemical processes (Stober). Silica from rice husk shows agglomerates while silica Stober has circular morphology. The presence of agglomerates in silica from RH could also be evidenced by DLS analysis (a high value of PDI). FTIR is also different. Silica from RH did not present silanol groups and silica from Stober presents (it is possible to see that these groups influence the particle charge, so, zeta potential is much more negative in Stober particles). These results are depicted in Table 4 and Table 5 and in Figure 2. Figure 3 depicts the FTIR analysis of silica acid digestion and Silica Stober. [0069] Table 4: Particle charge and zeta potential of Stober particles
Method Particle size (nm) /PDI Zeta potential (mV)
Acid digestion 762±57 / 0.62 -18±1
Stober 524±5 / 0.11 -35
[0070] In general, the particle size is higher than 1pm and the polidespersion index obtained for particles dispersed in water has lower values. This particle size results demonstrate that particles obtained from agroindustrial waste such as rice husk are not similar with particles synthetize chemically such as Stober particles.
[0071] Table 5: Particle charge and zeta potential of silica particles from rice husk with two different dispersants such as water and ethanol.
Sample Dispersion in water Dispersion in EtOH
1410 ± 85 2586 ± 513
Calcination 700°C RSD = 0.060 RSD = 0.198
Pdl = 0.572 Pdl = 0.813
1696 ± 352 3926 ± 1359
Calcination 700°C + acid digestion RSD = 0.207 RSD = 0.346
Pdl = 0.574 Pdl = 0.518
804 ± 52 1548 ± 17
Acid digestion + Calcination 700°C RSD = 0.064 RSD = 0.011
Pdl = 0.270 Pdl = 0.492
[0072] In an embodiment, the effect of rice husk origin to obtain silica particles by acid digestion with citric or nitric acids, was evaluated and the yield and purity was determined, as depicted in Table 6. In general, there are no significant differences between origins in terms of purity > 95% (nitric acid shows some contaminants such as Mg, S, Na and Ca). However, the yield seems to be different in some samples.
Table 6. Effect of rice husk origin to obtain silica particles by acid digestion with citric or nitric acids
Yield (%) Si+O (through EDS analysis) (At %)
Figure imgf000013_0001
HNO C H O HNO C H O
0 Portugal Carolino 9.93 10.43 100.0 99.90
1 Portugal Agulha 11.26 11.14 95.02 96.00
4 Portugal Carolino 12.41 13.20 96.71 97.50
6 Greece Carolino 11.17 12.35 94.60 100.0
7 Italy Arboreo 9.60 11.82 98.06 100.0
9 Brasil Agulha 11.79 9.19 97.54 100.0
10 Spain Integral 7.94 8.31 98.40 100.0
[0073] In an embodiment the acidic extraction process of the silica from the silicate-rich was adapted from the procedure described by Aguilar et al. (2019) [41], Initially, the RH was milled to obtain a particle size of 1.5 mm with the help of a Universal Cutting Mill (Pulverisette 19-Fritsch, P=5kw). Then, the RH was washed with distilled water under stirring for 2 h at room temperature. Next, the washed RH underwent acidic digestion with HCI 10% wt. at 60 °C for 2 h and a complete acidic digestion was observed. Then, the RH was filtered using a wire mesh of 130 pm, followed by neutralization with deionized water and a drying process at 110 °C for 12 h. Finally, the SiCh particles were obtained through a calcination process of the digested RH previously milled to a mesh of 0.5mm (22 °C to 700 °C in 2h30 and then 700 °C for 3h30).
[0074] Alternatively to acid digestion (extraction), other extraction processes can also be employed to extract bio-silica from agro-industrial resources.
[0075] In an embodiment, sol-gel method is applied as extraction process. For this method, the silica in rice husk is dissolved quickly in alkaline conditions followed by precipitation in acidic conditions. For that, the rice husk (ground or not) is calcinated in a furnace at 700 °C for 3h30 (after an initial heating ramp from 25 °C to 700 °C for 2h30) to eliminate organic matter. The silica is then extracted from the resulting rice husk ash by an alkali extraction using NaOH 10%. The mixture is stirred under reflux for 3h at 90 °C to obtain the silicate precursors (sodium silicate). After cooling, the mixture is filtered to remove the remaining char and the silica in the filtrate is precipitated with HCI 10% or other acids such as nitric, citric, phosphoric and sulfuric (Figure 4). The resulting precipitate is then filtered, washed with water and dried at 150 °C for 2h (or dried under vacuum using a freeze dryer) to obtain silica particles.
[0076] Table 7 depict the impact of the acid used in the neutralization in the particle size, Pdl and potential zeta of the silica particles obtained.
[0077] Table 7: impact of the acid used in the neutralization in the particle size, Pdl and potential zeta of the silica particles obtained.
Acid Particle size (nm) PDI Zeta potential (mV)
HCI 316 ± 10 0.377 -34 ± 1
C bHo O/ 576 ± 25 0.745 -16 ± 1
HNO3 261 ± 18 0.505 -32 ± 1
H3PO4 258 ± 7 0.526 -32 ± 1
H2SO4 285 ± 14 0.431 -29 ± 1
[0078] Table 8 depict the impact of the acid used in the neutralization in the yield of and the yield and purity levels obtained. It is possible to observed that, high yields did not represent a good solution if accompanied by impurities (detected by EDS analysis). The best result was obtained using HCI, CgHsO? or HNO3.
Table 8: Impact of the acid used in the neutralization in the yield of and the yield and purity levels obtained.
Acid to Si+O (through EDS .... _ _
Neu *tra 1l-iza *t-ion Yield (%) ana 'lys .is) (At %) Si (%) Ratio (O/Si)
HCI 11.90 90.92 31.81 1.86
C6H8O7 13.10 89.22 34.82 1.56 HNO3 10.40 94.35 36.12 1.61
H3PO4 33.20 65.06 18.40 2.54
H2SO4 20.00 68.12 19.92 2.42
Functionalization of SiCh particles with active agents
[0079] In an embodiment, the activated silica was functionalized with specific compounds that conferred antimicrobial and flame retardant properties namely QAS and PA, respectively. Briefly, 6 g of SiOa was dispersed in 360 mL EtOI-kHzO (1:1), and 240 mL of PA was added to this mixture being sonicated for 30 min. Then, 72 mL of QAS was added dropwise to the previous mixture and the reaction was left for 24 h at room temperature. Finally, the particles were centrifuged (9000 rpm for 10 min) and washed with deionized water. This procedure was performed 3 times and the obtained particles were dried at 40 °C, followed by a grinding process to obtain the particles in the powder form with less than 2 pm. For control trials, SiO2 modified with QAS was also performed following the same procedure, but without the addition of PA. SIO2 modified with PA, without QAS, was also performed. The same procedure was applied using different compounds with antifungal and flame retardant properties, substituting QAS and PA for the respective compound. All the other conditions of the procedure were not changed.
Characterization techniques
Structural and thermal analysis
[0080] Attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy (FTIR-ATR): In an embodiment, FTIR-ATR was used to evaluate the functional groups of the synthetized particles using a Perkin Elmer Spectrum 100 Series spectrophotometer with a spectral range and resolution factor of 4000 to 650 cm-1 and 8 cm-1, respectively.
[0081] Dynamic light scattering (DLS): In an embodiment, Zeta potential measurements in DLS analysis were performed with a Zetasizer Nano ZS90 (Malvern) and a disposable capillary cell (DTS1070) at 25 °C. For the measurements, bio-silica was dispersed in two different solvents, ethanol or deionized water, at a concentration of 1 g/L and subjected to ultrasonic agitation before the analysis.
[0082] Thermogravimetric analysis (TGA): In an embodiment, TGA was performed on a TG 209 Fl Libra apparatus (Netzsch) with a Top-loading type of thermobalance. The test conditions were carried out by an internal method "Evaluation of the thermal stability of materials by thermogravimetry" based on the standard ISO 11358:1997(E). The samples were pre-conditioned at laboratory's ambient conditions: room registered temperature (= 23 °C) and relative humidity (= 51%). For the sample pre-treatment, each one was properly separated, in order to fit inside the crucible (volume of 85 pL) and to ensure maximum contact with its bottom. The tests were accomplished in a synthetic air atmosphere, following a temperature program from 30 to 700 °C, with a heating rate of 20 °C/min.
Morphological analysis
[0083] In an embodiment, the morphology and chemical composition of the particles were characterized by scanning electron microscopy (SEM) connected to an energy dispersive X-ray spectroscopy (EDS) using a NanoSEM - FEI Nova 200 (FEG/SEM) and EDAX - Pegasus X4M (EDS/EBSD) with high vacuum resolution 1.8 nm at 1 kV (SE) 1.0 nm at 15 kV (SE) or low vacuum resolution 1.8 nm at 3 kV (Helix detector) 1.5 nm at 10 kV (Helix detector). The electron beam resolution 0.8 nm at 30 kV (STEM), beam landing energy: 200V to 30 kV, high stability Schottky field emission gun with automatic operation, probe current: 0.3 pA to 22 nA, chamber vacuum (high vacuum): <10-4 mBar and chamber vacuum (low vacuum): <2 mBar was selected as the optimal operation conditions.
Hydrophobic, antifungal, and flame retardant evaluation techniques
[0084] Tensiometry (contact angle analysis): In an embodiment, the water contact angle measurements of the particles were determined with a tensiometer (Attension Theta, Biolin Scientific) through the sessile drop method, with a water droplet volume of 3 pL, using the OneAttension software. The contact angle measurements were performed on circular pellets (1.3 cm in diameter), prepared with 180 mg of particles and using a Atlas Manual Hydraulic Press.
Calorimetry tests
[0085] In an embodiment, the gross heat of combustion (calorific value) of the SiO2@QAS+PA was determined in agreement with EN ISO 1716:2018. Prior to testing, the specimens were conditioned for a minimum period of 100 h at 23 ± 2 9C and 50 ± 5 % relative humidity, having met the constant mass criterion. The samples were tested according to the crucible method using benzoic acid as a combustion aid.
[0086] In an embodiment, the fire behavior of cotton (CO) textile fabrics samples (SiO2@QAS+PA, SiO2@QAS, and SiO2) was evaluated by using the cone calorimeter method at 25 kW/m2, according to ISO 5660-1:2015. For this purpose, the modified silica particles, SiO2@QAS+PA, SiO2@QAS (15% (w/v), or non-modified bio-silica particles for control trials, were mixed with 3% (w/v) of chitosan (previously dissolved in 1% (v/v) acetic acid) with the help of a dispersing machine (Ultra-Turrax) for 30 min. Afterwards, a film of 400 pm was deposited on a cotton fabric through a film applicator. The cotton fabrics were then dried in an oven at 120 °C for 30 s (several cycles until the textile was dried). Prior to the calorimeter tests, the specimens (100 mm x 100 mm) were conditioned for 72 hours at 23 ± 2°C and 50%±5 relative humidity, having met the constant mass criterion. The initial weight of the CO, CO-SiO2, CO-SiO2@QAS, and CO-SiO2@QAS+PA fabrics were approximately 2 g. The measured parameters were: following time to ignition (s); Average rate of heat release for test duration (kW/m2); maximum average heat release (MAHRE) (kW/m2); Time to MAH-RE (s); sample mass loss for test duration (%).
[0087] Flammability tests: In one embodiment, the flammability behavior of the functionalized CO textile fabrics samples (SiO2@QAS+PA, SiC>2@QAS, and SiO2), prepared under the same conditions as reported in the Calorimetry tests, was determined by measuring the flame spread properties of a vertically oriented cotton fabric (560x170 mm) according to ISO 6941:2013. The source ignition was applied on the surface of the cotton fabric (at 90°) and exposed to the flame for 10 s. After the flame source was removed, the propagation times of the flame in the cotton textile were recorded.
[0088] Antifungal tests: In an embodiment, the inhibition tests (antifungal assays) were performed with Candida albicans (C. albicans) (106 CFU/mL) suspension according with CLSI Antimicrobial Susceptibility Testing (AST) Standards by the method Broth Dilution Antifungal Susceptibility Testing of Yeasts. For each sample, a solution of silica particles was prepared with a concentration of 20 g/L. 100 pL of the sample solution (5 replicates per sample) were inserted in a 96-well plate and subsequently, 100 pL of the fungal suspension was added to this solution. Then, the plates were incubated at 37 °C (optimal temperature for microorganism growth) for 24 h, and finally, the percentage of antifungal activity (fungus growth) was estimated. Additionally, other antimicrobial tests (for Staphylococcus aureus, Escherichia coli, MRSA and S. cerevisieae) were done by two methods. The first one was the disc diffusion method, in which discs of functionalized cotton were placed on a plate with the microorganism's inoculum, so that the growth inhibition zone could be measured. In the second method, 10 pL drops of microorganism inoculum were placed to grow on a suitable medium with functionalized sample dispersed on that medium (in a certain concentration), to obtain the MIC (minimum inhibitory concentration).
[0089] In an embodiment, rice husk was selected as the raw material for the extraction of silica particles.
[0090] In an embodiment, for the modification of the bio-silica particles, QAS and PA were chosen due to their potential for antifungal and flame retardant properties, respectively. As PA is a bio compound that could be extracted from RH, an attempt was made to extract this functional compound from the residues (described in the previous section), which would enhance the sustainability of the process (see the detailed procedure in the annex section). However, the PA content in the RH (percentage in dry weight) was found to be less than 1%, revealing that extracting this bio-compound from RH for the functionalization of the silica is a resource and time-consuming process. Moreover, the amount of PA needed for the functionalization process would imply that very high amounts of RH would have to be processed. For these reasons commercially available PA was used for further functionalization processes.
Structural and thermal characterization Spectroscopic analysis of RH and silica particles
[0091] Figure 5 shows the FTIR spectra of functionalized silica particles, such as SiO2@QAS+PA and SiO2@QAS, as well as SiO2, SiO2-OH, and rice husk. The presence of characteristic peaks of C-H stretching and deformation vibration bands at 2917, 2852, and 1467 cm-1 from QAS [42] differentiate the SiO2@QAS+PA and SiO2@QAS spectra from SiO2, SiO2, and RH. The broad band at 2114 - 2480 cm'1 corresponds to the OH — P=O groups from PA in SiO2@QAS+PA particles [43], As expected, the spectra for all SiO2 particles (with and without functionalization) exhibited bands at 802 and 1059 cm 1, which could be assigned to the symmetric and asymmetric stretching vibration of Si-O-Si bonds, respectively [44,45], These bands also appeared in the RH, which could indicate the presence of SiO2 content. Additionally, a band at 1647 cm'1 can be observed, which appears in all spectra except for SiO2 particles, and that is being attributed to absorbed H2O [44],
Thermal degradation of RH and silica particles
[0092] In Table 9 the values obtained for the charge of the particles evaluated by zeta potential are reported. Silica particles without functionalization showed a negative charge for both solvents (deionized water and EtOH). In aqueous medium, the SiO2 particles exhibited -24.6 ± 0.5 mV.. It should be noted that the zeta potential for SiO2 particles in ethanol and deionized water has not changed. As expected, the modification of SiO2 with QAS increased the zeta potential to a more positive value, in line with what was previously reported by Wang et al. (2017) [36], The addition of PA did not change the zeta potential of particles. Normally, the hydroxyl groups of PA do not appear protonated (this only happens in an extremely low pH (pH < 1.3) [46], leading to an electrostatic interaction between PA and QAS.
[0093] Table 9. Zeta potential (in mV) for the extracted and functionalized silica particles in ethanol and deionized water.
_ , Zeta potential / mV sample , ,
Ethanol Deionized water
SiO2 -24 ± 1 -25 ± 1
SiO2@QAS 44 ± 2 n. d.
SiO2@PA+QAS 41 ± 2 n. d. n.d. - non detected.
[0094] In an embodiment, Figure 6 (a) shows the thermogravimetric analysis (TGA) of extracted, activated, and functionalized silica, as well as RH. The residual mass obtained for RH of 11% (Table 10) is in accordance with the yield reported for SiO2 extraction from this raw material (10 %).
[0095] In an embodiment, the residual mass observed in the thermogram of silica particles was higher than 95 %, proving its inorganic nature and confirming the value obtained from the silica quantification method (around 93 %). Comparing the SiO2@QAS thermogram with QAS, a similar profile is observed. The second loss in the temperature range of around 200-300°C is characteristic for the quaternary amine according to Galimberti et al. (2009) [47], Despite being an organic compound, the residual mass of QAS shows a quite high percentage (18.2 %) when compared to the PA (4.4 %) due to the presence of a silane group in QAS composition.
[0096] In an embodiment, PA decomposes in many degradation temperatures that have been reported elsewhere corresponding to dehydration, carbonization, and char degradation processes [48], After 565 °C, thermal decomposition of the char takes place leading to a final char with a weight loss of 35.5 %. The integration of PA into SiO2@QAS was one more time successfully proved with thermographic analysis. The degradation temperature at 229-370 °C observed in Figure 6 (b) for SiO2@QAS+PA accounts for the highest weight loss (approximately 38 %). Comparing this value with the sum of the second lost for PA and QAS, the same weight loss is obtained (Table 10).
Table 10. Values of onset and end temperatures (°C), mass change (%), and residual mass (%) for the extracted and functionalized silica particles.
Figure imgf000019_0001
[0097] With FTIR analysis it was possible to observe that SiO2@PA washed is similar to SiO2 with nonefficient particles functionalization (Figure 7). When the particles are unwashed, some differences could be observed at =1600, 2300 and 3100-3600 cm 1 but with QAS, it is possible to obtain signal with higher transmittance in these wavenumbers, proving, one more time, the surprising role of QAS as linker. These results provide evidence that the functionalization of SiO2 directly with PA, without using QAS as linker, is not effective.
Morphology and elemental composition of particles
[0098] SEM images for 3 different samples (SiCh, SiO2@QAS, and SiO2@QAS+PA) are shown in Figure 8, as well as the corresponding chemical composition determined by EDS. The SiCh particles exhibited a spherical morphology in the range of 50-80 nm. However, with the incorporation of QAS and QAS+PA, its morphology changed to a more agglomerated status, probably formed by the self-condensation of QAS, as it was already suggested in the work by Gong et al. (2014)[35], The presence of Cl elements in SiO2@ AS and C and P elements in SiO2@QAS+PA, characteristics of QAS and PA composition, respectively, justify one more time, the efficiency of the silica functionalization process (Table 11).
Table 11. EDS analysis. EDS analysis (At % w/w))
Element SiO2 SiO2@QAS SiO2@QAS+PA
C 9 n.d. 67.11
O 55 55.40 22.09
Si 36 41.52 5.30
Cl n.d. 2.44 1.39
Ca n.d. 0.63 n.d.
P n.d. n.d. 4.11
Multifunctional properties of functionalized-SiCh
[0099] In an embodiment, the functionalized SiCh particles were evaluated in terms of flame retardant properties. According to the literature, one of the most relevant assays to evaluate this property is the determination of the gross heat of combustion (calorific value) using the EN ISO 1716:2018 namely in samples in the powder form. The introduction of organic groups, from the PA and QAS composition, helps on the difference on heat of combustion obtained for SiO2 particles without functionalization of - 0.2 MJ/Kg and SiO2@QAS+PA of 19.7 MJ/Kg. This assay was used as a preliminary test, but it yielded inconclusive results. Therefore, a different approach to demonstrate the flame retardancy of the modified particles was pursued. To the best of our knowledge, there is not a method that is suitable to directly assess the flame retardant properties of particles (powder form), thus a test that evaluates the flame retardancy of textiles was used. As described in the introduction section, the textile coating was chosen as this is one of the many possible applications of the incorporation of the modified silica particles of the present disclosure to enhance the intrinsic properties of the final product. To this purpose, the particles were incorporated in a chitosan matrix (3% w/v) and applied in a textile substrate to assess their flame retardant properties. The QAS+PA and QAS modified bio-silica particles were successfully deposited in the cotton (CO) textile substrate as described in section "Hydrophobic, antifungal, and flame retardant evaluation techniques" (non-modified silica particles were used for the preparation of control samples).
[00100] In an embodiment, initially, textile samples were tested according to ISO 5660-1:2015 with the determination of heat release rate (cone calorimeter method). The parameters presented in Table 12 such as heat release, time to ignition, and mass loss provide the most important information that could be related to each other. According to the literature, the combination between a high maximum average heat release and a low time of ignition may cause fast fire propagation [49,50], Concerning time to ignition, the CO-SIO2 and CO-SiO2@QAS ignited at 24.7 s and 30.3 s, respectively, whereas the samples with CO-SiO2@PAS+QAS decreased to 17.7 s. However, the time for the maximum average heat release of the CO-SiO2@QAS+PA was slightly lower than other samples without PA. Nevertheless, the values obtained for the average rate of heat release for test duration and the maximum average heat release are higher in the CO-SiO2@QAS+PA sample of approximately 35 MJ/m2, when compared to the other samples without flame retardant additive (PA). CO-SiO2@QAS shows a heat release rate lower than CO-SiO2@QAS+PA but a value close to CO-SiCh demonstrating that, QAS did not influence the levels of heat release rate and that PA is the most important compound in such analysis. In addition, comparing the samples' mass loss registered during the trial, it was possible to observe that the mass loss of the CO-SiO2 sample was higher (84.2%) than the CO-SiO2@QAS+PA (71.8%) and can be explained by the formation of a char residue in the CO-SiO2@QAS+PA. This char is visible at the end of this cone calorimeter test, and the burning vertical test will be discussed further. The formation of the char may due to the presence of the PA, which acts as a protective barrier for the material [49],
[00101] In an embodiment, through the cone calorimeter test is difficult to show the flame retardancy mechanism of SiO2@QAS+PA particles, these results can be explained by the heat flux (25 kW/m2) [50] applied to the tests, which could be not appropriate to evaluate the differences between the particles. Even though with the obtained data the flame retardancy was not completely proved, this technique could confirm, one more time, the efficiency in the bio-silica particles functionalization.
Table 12. Cone calorimetry parameters obtained for silica extracted from rice husk and functionalized with PA and/or QAS.
Sample Tl (s) HRTD (kW/m2) MAHR (kW/m2) TMAHR (s) MLTD (%)
CO n.d. 2.8 ± 0.2 3.3 ± 0.3 33 ± 36 81 ± 2
CO treated with SiO2 25 ± 1 7 ± 1 16 ± 2 36 ± 2 84 ± 2
CO treated with SiO2@QAS 30 ± 4 7 ± 2 9 ± 1 45 ± 7 71 ± 2
CO treated with „ „ „ „ „ „„ „
18 ± 4 9 ± 3 34 ± 17 31 ± 7 72 ± 1
SiO2@QAS+PA time to ignition (Tl); Average rate of heat release for test duration (HRTD); Maximum average heat release (MAHR); Time to maximum average heat release (TMAHR); Mass loss for test duration (MLTD))
[00102] In an embodiment, due to the difficulty to justify the flame retardancy of the developed bio- silica particles, another test was performed (according to the ISO 6941:2003). Figure 9 shows the scheme used in this analysis (vertical burning) as well as the photographs of the test performed for the SiO2 particles without and with QAS+PA or only modified with QAS. Due to the existence of an "external" element that was used to facilitate the impregnation of particles, an assay with the condition of cotton with chitosan was also accomplished. The three textile samples of chitosan, SiO2, and SiO2@QAS exhibited higher flammability in the vertical burning test, and whole samples were completely burned within the ignition time (10 s. of ignition - time pre-established in ISO 6941:2003), which results in a total char length about 51.0 cm. However, during the flame test, the fabric treated with SiO2@QAS+PA particles exhibited a good char formation, and the flame self-extinguished after the ignition time (with a char length <11 cm). Moreover, compared to the other particles (SiO2 and SiO2@ AS), PA proves to have a crucial role to promote the flame retardancy behavior on the textile substrate. Figure 10 also depict the results obtained in the vertical burning tests.
[00103] Table 13 describes the results obtained with silica from different rice origins. No significative differences were observed between origins and acids used for silica extraction from rice husk. This assay was performed considering concentrations in v/v and not w/v due to the different densities of the material and for this reason it allows a right comparison.
[00104] Table 13. Results obtained with silica from different rice origins and acids used for silica extraction from rice husk.
Figure imgf000022_0001
N.A. - not analyzed
[00105] In an embodiment, to evaluate the antifungal activity of the modified particles and assess their multifunctionality, the attachment and growth of bacteria and Yeast as well as Candida albicans on the presence of SiO2@QAS+PA particles were analyzed. Notoriously, the functionalized silica with Q.AS+PA reduced the growth of this specific fungus on approximately 83.2%, as shown in Table 14. Despite its reduction, the fungus remains metabolically active, with the ability to multiply and contaminate the surrounding environment, but at a slower rate of growth. Although other works refer to QAS as an antifungal agent [32], a clear fungicide behavior was not observed in this work. This may be due to the low concentration of SiO2@QAS and SiO2@QAS+PA used during the evaluation. Nevertheless, the significant growth reduction observed for the SiO2@QAS+PA particles when compared to the slight growth of the fungus when in the presence of SiC>2@QAS particles indicates that the use of PA in the process has increased the amount of QAS linked to the silica particles.
[00106] Table 14. The concentration of the fungus Candida albicans after 24 h of incubation with biosilica particles (5 replicates per sample).
„ . Concentration of , . . . > . i „ .
Sample , , , Standard deviation / CFUs.mL'1 Reduction o t f growth / %
_ the fungus / CFUs.mL 1 _
SiO2 _ 2E+07 _ 3E+06 _ -18 _
SiO2@QAS _ 2E+07 _ 7E+06 _ -5 _
SiO2@QAS+PA _ 3E+06 _ 2E+06 _ 83 _
Positive control 2E+07 8E+06 0.0 [00107] However, concerning the other microorganisms selected for these particles such as Staphylococcus aureus, Escherichia coli, MRSA and S. cerevisieae different results are obtained. A total inhibition of its growth is observed in the presence of functionalized particles when compared to silica that was not functionalized, as depicted in Figure 11. This result demonstrates the multifunctionality presented by SiO2@QAS+PA particles concerning hydrophobicity, flame retardancy and antimicrobial behavior.
[00108] In an embodiment, the hydrophobic behavior of the modified particles was also assessed by water contact angle (WCA) measurements. The results obtained for the water contact angle for SiCh, SiO2@QAS and SiO2@QAS+PA particles are shown in Figure 12. The WCA of both functionalized SiCh particles (QAS and QAS+PA) increased when compared with non-functionalized SiCh due to the presence of long alkyl groups from QAS. The functionalized particles exhibited similar WCA values and hydrophobic properties (0>9O°). The WCA of the non-functionalized SiO2 was not determined due to the high intrinsic hydrophilic behavior of these particles.
[00109] In an embodiment, rice husk, a residue that is widely available in agroindustry, was used to make the extraction of bio-silica (obtained using acidic conditions) in a circular economy approach with yields of around 10% and purity higher than 90%.
[00110] In an embodiment, through the present disclosure, the modification of bio-SiO2 with QAS compound (anti-fungal and hydrophobic agent) and PA (flame retardant agent) was successfully achieved, and the particles were well characterized by structural and thermal techniques demonstrating functional groups and degradation temperatures that are characteristics of the compounds used for functionalization. Complementary analyses were performed to verify the morphology and the particle size around less than 2 pm, obtained with the acidic process.
[00111] The produced SiO2@QAS+PA particles according with the present disclosure demonstrated contact angles of 110°.
[00112] In an embodiment, the particles also demonstrated to be able to improve the flame retardancy performance when compared to SiO2@QAS proving that, the introduction of PA markedly makes the particles more promising to be used as flame retardants. Furthermore, the SiO2@QAS+PA were able to reduce the growth of Candida albicans, while SiO2 and SiO2@QAS particles continued to promote the growth of the fungus.
[00113] The combination of these characteristics in one particle can be of great interest for different applications, such as those intended in the textiles, automotive, and construction and building industries (as nanofillers) being a clear example of an added-value product obtained from agro-waste.
[00114] In an embodiment, the SiO2@QAS+PA particles were applied to a cork-based substrate and the flame retardancy properties of the modified surface were assessed. By using an adaptation of the method described in ISO 6941:2003, the flame retardancy of the samples was evaluated and promising results were obtained: after the ignition time (10 s.), the samples coated with a mixture of chitosan and SiO2@QAS+PA exhibited a promising flame retardant behaviour, resulting in an average flame progression of 8.6 cm against 13 cm obtained for the control sample (without modified particles).
[00115] In another examples, SiO2 + PA were functionalized with different antimicrobial compounds: ELA (Ethyl Lauroyl Arginate) and Quaternized Carboxymethylchitosan (qCMChi). ELA was used as commercial agent and qCMChi was synthetized. The synthesis of qCMChi was adapted from Abgovi et al. (2018) [53] and Nhung et al. (2020) [54], Firstly, carboxymethyl chitosan was synthetized using 10 g of chitosan mixed with NaOH prepared in Isopropanol (13.5 % (m/v)) and placed at 50 °C during lh. Then, 14.4 g of monochloroacetic acid dissolved in lOOmL isopropanol was added dropwise to the previous solution and leave at 50°C during 4 h. The next step was the addition of 200 mL 70% EtOH and then was adjusted to pH value to 7 with glacial acetic acid. Finally, the formed solid was filtered and washed with EtOH and dried at 40°C. To perform the quaternization, the 6 g of the previous solid was mixed with 60mL water and placed at 85°C. Then, 21.3mL of GTMAC was dropwise to this solution and leave at 85°C for 5h. 200mL of cold acetone was added and the mixture was left to rest overnight at 4°C. The final step was the addition of 100mL MeOH and immediately with 250 mL EtOH: acetone (4:1) to obtain a the qCMChi that was filtered, washed with EtOH and dried at 40°C.
[00116] The FTIR spectrum for carboxymethyl chitosan is similar to the data described by Agbovi et al. (2018). The IR spectrum of chitosan has stretching bands at 3435 cm'1 characteristic of O-H and 2867 cm' 1 for C-H (Figure 13). At lower wavenumber it is possible to see the bands at 1142 cm'1 for C-O-C and 1109 cm-1 for C-O, and 1591 cm'1 for N-H bending. When it is made the carboxymethylation, it is observing the appearance of different bands at 1570 cm 1 for -COO- group and 1410 cm'1 for -CH3 groups. The other bands are similar to the chitosan (O-H and C-H broad spectral bands between 3440 and 2980 cm'1). The quaternization demonstrates the introduction of a quaternary ammonium salt group on the carboxymethyl chitosan structure and the peak at 1480 cm-1 is characteristic of C-H asymmetric bending vibration of the trimethylammonium group, confirming the existence of this quaternary amine. It should also be noted that the N-H bending (1596 cm-1) of the primary amine diminished because of the transformation of the primary amine into the secondary amine (aliphatic). In addition, the spectrum exhibits a broad band at approximately 3450 cm-1, which became higher than that of chitosan and carboxymethyl chitosan due to the increase number of hydroxyl groups.
[00117] Similar procedure was used for SiO2@ELA+PA and SiO2@qCMChi+PA namely 1.2 g SiCh, 72mL EtOH:H2O (1:1), 48mL PA and 15mL qCMChi (4.43g in 15mL water) or 45mL ELA (42%) in EtOH (dropwise), with the obtained FTIR results presented at Figure 14. [00118] The term "comprising" whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
[00119] The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof. The above described embodiments are combinable.
[00120] Where ranges are provided, the range limits are included. Furthermore, it should be understood that unless otherwise indicated or otherwise evident from the context and/or understanding of a technical expert, the values which are expressed as ranges may assume any specific value within the ranges indicated in different achievements of the invention, at one tenth of the lower limit of the interval, unless the context clearly indicates the contrary. It should also be understood that, unless otherwise indicated or otherwise evident from the context and/or understanding of a technical expert, values expressed as range may assume any sub-range within the given range, where the limits of the sub-range are expressed with the same degree of precision as the tenth of the unit of the lower limit of the range.
[00121] The following dependent claims further set out particular embodiments of the disclosure.
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Claims

C L A I M S Functionalized hydrophobic silica particle comprising a flame retardant and an antimicrobial agent, wherein the antimicrobial agent is a linker between the silica particle and the flame retardant agent; wherein the flame retardant agent is selected from a list consisting of: phytic acid, pentaerythritol phosphate alcohol, melamine polyphosphate, phosphazenes; diethyl phosphonic metal salts; 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide; wherein the antimicrobial agent is selected from a list consisting of: quaternary ammonium silane, quaternary amine, ethyl lauroyl arginate, quaternized carboxymethylchitosan, N-allyl-N- decyl-N-methyl-N-trimethoxysilylpropylammonium iodide, Perfluorooctyl-containing quaternary ammonium salt, Triethoxysilylpropyl Succinic Anhydride Silane. Functionalized hydrophobic silica particle according to the previous claim wherein the flame retardant agent is phytic acid. Functionalized hydrophobic silica particle according to the previous claim 1 wherein the antimicrobial agent is a quaternary ammonium silane. Functionalized hydrophobic silica particle according to any of the previous claims comprising phytic acid and a quaternary ammonium silane, wherein the quaternary ammonium silane is the linker between the silica particle and the phytic acid. Functionalized hydrophobic silica particle according to any of the previous claims, wherein the quaternary ammonium silane is 3-(trimethoxysilyl)-propyldimethyloctadecyl ammonium chloride. Functionalized hydrophobic silica particle according to any of the previous claims comprising a particle size less than 2 pm. Composition comprising the functionalized hydrophobic silica particle according to any of the previous claims. Composition according to the previous claim further comprising a filler, preferably wherein such filler is selected from the list consisting of: aluminum trihydroxide, magnesium carbonate, magnesium hydroxide or titanium dioxide, or mixtures thereof. Article comprising the functionalized hydrophobic silica particle according to any of the previous claims 1-6 and/or the composition according to the previous claim. Article according to the previous claim further comprising a textile material, a construction material or a material of the interior of a motor vehicle. Article according to any of the previous claims 9-10 comprising a cork-based substract. Use of the functionalized hydrophobic silica particle according to any of the previous claims 1-6, or the composition according to the previous claim 8 as a hydrophobic agent, as an antimicrobial agent and as a flame retardant agent. Use of the functionalized hydrophobic silica particle according to any of the previous claims 1-6, or the composition according to the previous claim 8 as a dirt-resistant agent. Use of the functionalized hydrophobic silica particle according to any of the previous claims 1-6, or the composition according to the previous claim 8 as a stain repellent agent. Use according to any of the previous claims 12-14 in textiles, and/or in construction materials, and/or in the interior of motor vehicles. Method for the production of the functionalized hydrophobic silica particle according to any of the previous claims 1-6, comprising the following steps: obtaining a silica particle; dispersing the silica particles in a solution of EtOI-kl- O, preferably with a concentration ratio of 1:1 (v/v); adding a flame retardant agent and an antimicrobial agent to the obtained dispersion, preferably with sonication; wherein the flame retardant agent is selected from a list consisting of: phytic acid, , pentaerythritol phosphate alcohol melamine polyphosphate, phosphazenes; diethyl phosphonic metal salts; 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide; and wherein the antimicrobial agent is selected from a list consisting of: quaternary ammonium silane, quaternary amine, ethyl lauroyl arginate (ELA), quaternized carboxymethylchitosan (qCMChi), N-allyl-N-decyl-N-methyl-N-trimethoxysilylpropylammonium iodide, Perfluorooctyl- containing quaternary ammonium salt, Triethoxysilylpropyl Succinic Anhydride Silane; centrifuging the obtained dispersion; preferably at 9000 rpm for 10 min; washing the precipitated; preferably with deionized water; repeating the two previous steps; preferably 3 times; drying the washed precipitated until obtained a dried powder; preferably during 48h min at 40 °C; milling the obtained powered until obtained a particle size less than 2 pm in order to obtain the functionalized silica particle. Method according to the previous claim wherein the silica particles are obtained through an extraction process from an organic or inorganic source or residual waste; preferably an organic source. Method according to the previous claim wherein the silica particles are obtained through an extraction process from rice husk. Method according to any of the previous claims 16-18 wherein the extraction process comprises an acid extraction method or a solgel method. Method according to the previous claim wherein the acid extraction method comprises the following steps: obtaining crude material comprising silica; preferably with particle size equal or below 1.5 mm; washing such crude material comprising silica; preferably with distilled water; preferably for 2h at room temperature; digesting with acid the washed crude material; preferably with an acid solution selected from a list consisting of: hydrochloric acid, nitric acid or citric acid, or mixtures thereof; preferably at 60°C for 2 h; filtering the digested crude material; preferably with a wire mesh of 130 pm; neutralizing the filtered digested crude material; preferably with deionized water; drying the neutralized material; preferably at 110 °C for 12h; calcinating the dried material to obtain purified silica; preferably at 700 °C for 3h30 min after an initial heating ramp from 25 °C to 700 °C for 2h30 min. Method according to the previous claim 17 wherein the solgel method comprises the following steps: obtaining crude material comprising silica; preferably with particle size equal or below 1.5 mm; calcinating the crude material comprising silica; preferably at 700 °C for 3h30 min after an initial heating ramp from 25 °C to 700 °C for 2h30 min; extracting the silica from the calcinated product with an alkaline solution under reflux to obtain crude sodium silicate; preferably the alkaline solution is NaOH 10%; preferably at 90°C for 3h; filtrating the crude sodium silicate to eliminate char impurities; treating the filtrated crude sodium silicate with acid solution to obtain crude silica; preferably with hydrochloric acid, nitric acid, citric acid, phosphoric acid or sulfuric acid, or mixtures thereof; washing the crude silica; preferably with water; drying the washed crude silica to obtain dried silica particles; preferably at 150°C for 2 h.
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