Phosphorus-Silica Sol-Gel Hybrid Coatings for Flame Retardant Cotton

This work investigates the use of organic-inorganic sol-gel coatings based on silica and phosphorous compounds for providing cotton fabrics with fl ame retardant features. To this aim, diethylphosphatoethyltriethoxysilane precursor was employed for the synthesis of several sols in combination with diff erent chemical additives. Sols were reacted with azo-based compounds and repeatedly applied onto the cellulosic substrate in a multilayer assembly, aiming at assessing the eff ect of the concurrent presence of Si, P and N on the overall fi re behaviour of the fabric. In order to evaluate the fl ame retardancy of treated cellulosic fabrics, fl ammability tests were carried out. The obtained results showed that the phosphorus-silica coating is able to promote the formation of a stable char that acts as insulator barrier. Finally, an additive P-N eff ect of the ceramic oxide coating in terms of increased residue and decreased heat release rate and total burning time was observed in cone calorimetry tests.


Introduction
In the last years, there has been a considerable interest in the development of fl ame retardants for cellulosic materials.Generally speaking, fabrics made from natural products, such as cotton and linen easily and vigorously burn with a high combustion rate.In addition to fi bres, the fl ame-spread rate of fabrics is also dependent on the fabric weight (g/m 2 ) and texture; lightweight and loose fabrics are more prone to catching fi re quickly.Among all textile fibres, cotton is the one most commonly used in Izvleček V prispevku je raziskana uporaba organsko-anorganskih premazov sol-gel na podlagi silicijevih in fosforjevih spojin, s katerimi bi bombažnim tkaninam zagotovili odpornost proti gorenju.Za ta namen je bil uporabljen prekurzor dietil-fosfatoetil-trietoksisilan za sintezo različnih solov v kombinaciji z različnimi kemičnimi aditivi.Soli, ki so reagirali z azo-spojinami, so bili v več plasteh nanešeni na celulozni substrat z namenom ugotoviti, kako sočasna prisotnost Si, P in N vpliva na obnašanje vlaken v primeru gorenja.Odpornost obdelanih celuloznih vlaken proti gorenju je bila določena s testi gorljivosti.Rezultati testov so pokazali, da je fosfor-silicijev premaz pospešil nastanek stabilnega pooglenelega ostanka, ki deluje kot izolacijska pregrada.Iz rezultatov kalorimetričnih meritev je bil razviden aditivni učinek P-N silicijevega oksidnega premaza v smislu povečanega pooglenelega ostanka, zmanjšane hitrosti sproščanja toplote in skrajšanega celotnega časa gorenja.Ključne besede: toplotna stabilnost, celulozna tkanina, sol-gel, dietil-fosfatoetil-trietoksisilan, hibridna snov domestic applications (clothes, beddings, furniture, wall-hangings, etc.).Th us, for public safety it is of primary importance to fi nd ways to make this material less fl ammable, in the most economically and environmentally-friendly way.Various forms of organophosphorus-based fl ame retardants such as phosphates, phosphoramide, phosphonate and phosphonium salts have been developed and used in fl ame retardant fi nishing of cellulosic fabrics for a long time.However, only a few of these products were commercially successful.Current government regulations have raised safety and environmental protection standards on fl ame retardants, which led to a renewed interest on the development of environmentally-friendly structures.Flame retardant for cottons are usually produced by exploiting a chemical treatment performed during the textile fi nishing process, which, depending on their chemical process, generates fl ame retardant properties having varying degrees of durability to various laundering processes.Flame retardants [1] may be (i) simple salts (e.g.ammonium phosphates, polyphosphate and bromide borate-boric acid mixtures) providing non-durable fi nishes due to their water-solubility; (ii) functional fi nishes (e.g.organophosphorus and nitrogen-containing monomers such as alkylphosphonamide derivatives, or polycondensates such as tetrakis(hydroxylmethyl)phosphonium salt) which contain reactive groups which may chemically react with surface functionalities to confer more-durable fl ame retardancy, or (iii) back-coatings, which usually comprise a resin-bonded antimony-bromine fl ame retardant system.A sol-gel represents a versatile synthetic route based on a two-step reaction (hydrolysis and condensation), starting from (semi) metal alkoxides (e.g.tetraethoxysilane, tetramethoxysilane, titanium tetraisopropoxide), which leads to the formation of completely hybrid inorganic or organic-inorganic coatings at room temperature [2].Th ese coatings are capable to protect the polymer surface by creating a physical barrier, thus improving some specifi c properties, such as for example water or oil repellence and wear resistance [2].As far as textile applications are considered, usually the sol-gel technique has been proposed for conferring new functional properties to fabrics, such as antimicrobial or UV radiation protection [3], dye fastness [4], anti-wrinkle fi nish [5] and hydrophobicity [6].With the goal of improving fl ame resistance of fabrics, in the last years, textile fi nishing by sol-gel method has been systematically studied [7][8][9][10][11][12][13]. Th e treatment of textile materials with hydrolysed metal alkoxide solutions is an excellent tool to convey new properties to polymer surfaces, particularly if organic components, like phosphorous-based chemicals, are incorporated into the formulation [9].Consequently, developed studies were focused on the investigation of the synergistic eff ects among silica, nitrogen and phosphorus on the thermal behaviour of treated fabrics [14].Previous results demonstrated that sol-gel processes could be exploited as fi reproof treatments with lower environmental impact for textile fabrics as compared with common fl ame retardant processes [9].In this composition, the hybrid organic-inorganic materials (HOIMs) [8,9,14] show the properties of both the phases.Th erefore, the thermal stability at high temperatures, proper of inorganic constituents, can be combined with the fl ame retardancy provided by organic phosphorusbased compounds.To this aim, in the present work, diethylphosphatoethyltriethoxysilane (DPTS) has been used in a novel multistep process consisting of 1-6 consecutive depositions in order to form architectures diff ering for the number of layers.In the fi rst step, a FT-IR study of xerogels applied on glass slides was carried out in order to investigate the chemical structure of the produced thin fi lms.Th en, Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and thermogravimetric analyses (TGA) in inert and oxidative atmospheres were carried out in order to assess the chemical, morphological and thermal properties of the untreated and sol-gel treated fabrics.Finally, the fl ammability of untreated and treated fabrics was assessed according to the ASTM D1230 standard.Th e results showed a substantial enhancement of the char-forming properties and fl ame retardancy for the fabrics modifi ed with the oxidic thin fi lms.

Code
Name Chemical formula

Functional fi nishing of cotton fabric
Each sol was stirred for 4 h at room temperature prior to the application.0.0125 mol of APTES and 0.0125 mol of DPTS were hydrolysed, separately, with 0.0002 mol of HCl (0.1 N) in 45.07 mL of deionized water under vigorous stirring for 10 h at room temperature to obtain the silica sols.Th ree diff erent solutions containing the MF were prepared by adding 0.002 mol, 0.004 mol and 0.006 mol of MF (MF30, MF60, MF90 respectively) in deionized water up to 50 mL of total volume.For the DPTS-APTES05, DPTS-APTES1 and DPTS-APTES2 sols, the molar ratios of APTES (0.06, 0.12 and 0.25 M) and DPTS (0.25 M) were 1 : 4, 1 : 2 and 2 : 2, respectively.DPTS sols (0.25 M) containing MF were prepared by adding 0.0125 mol of DPTS in three diff erent solutions with, respectively, 0.002 mol, 0.004 mol and 0.006 mol of resin (DPTS-MF1, DPTS-MF2, DPTS-MF3 respectively).Th e diff erent sols were applied to the cotton samples by a pad-cure-method, using a Werner Mathis padder.Aft er having been dried at 80°C for 2 h, the fabrics were cured at 150°C for 2 min in a laboratory oven.

Characterization
Th ermogravimetric analyses were carried out on a TA Instruments Q500 thermobalance, operating under nitrogen and air atmosphere using an alumina pan containing samples of approximately 10 mg (± 0.5).Th e runs were performed over a temperature range between 50 and 800°C at 10°C/min heating rate and 60 mL/min fl ow.Th e burning behaviour of the samples was studied using the Flammability Tester Model 7633E by United States Testing Company, Inc., according to the ASTM D1230 standard.In these tests the sample is mounted in a frame and horizontally held in a cabinet, with a tilting angle of 45°.A standardized fl ame is applied to the surface from the bottom; the ignition time was set at 5 s.Five samples for each textile fabric were processed in order to get reproducible data; burning time, burning rate and the fi nal residue were measured.

Results and discussion
Sol-gel processes have been exploited for assessing the eff ects derived from the concurrent presence of silica, phosphorus and nitrogen on cotton thermal and fi re stability.To this aim, a specifi c combination of diethylphosphatoethyltriethoxysilane, with P and N donors (1,1-diphosphonic acid, N,N,N',N',N'',N''hexakis-methoxymethyl-[1,3,5]triazine-2,4,6-triamine, respectively and urea) was chosen and exploited for preparing hybrid phosphorus doped silica fi lms.Since infrared absorption bands characteristics of the hybrid thin fi lm applied onto the fabric surface are covered by the strong vibrational peaks of the cellulosic substrate, xerogels have been applied and annealed on glass slides and textile samples in order to perform FT-IR characterizations.Th e frequencies of major absorption bands are shown in  Th e Si-O-Si asymmetrical stretching absorption band at 1020-1027 cm -1 confi rms the presence of the SiOx phase on the treated cotton fabric.However, the hybrid xerogel applied onto textile fabric shows few changes in the absorption bands when compared with the treated glass substrate due to the interactions between silica and cellulose.Th e ν (P=O) absorption band, located at 1235 cm -1 in the FT-IR spectrum of the xerogel, is shift ed towards low frequencies in the spectra of the treated fabrics by about 10-25 cm -1 .Th is fi nding could indicate the involvement of the phosphoryl group in the formation of hydrogen bonds.SEM observations have been performed in order to establish the morphology of the hybrid fi nishing deposited on cotton fi bres through sol-gel processes.
As expected, the surface of raw cotton fi bres shows a certain level of heterogeneity and irregularity, which disappear when the fi bres are sol-gel treated.Indeed, their surface becomes smooth and fl awless.All the treated fi bres appear homogeneously covered by the coating, irrespective of the kind of treatment [18].Th e composition of the fi lms was determined by EDX analysis.For the treated cotton fabric surfaces, depending on the nature of coating, silicon, nitrogen and phosphorus were the only detected elements in addition to fabric reference, confi rming the occurred deposition of the hybrid coatings under investigation.Th e thermal and thermo-oxidative stability of the sol-gel treated samples have been evaluated by thermogravimetric analysis and compared with that of the untreated cotton fabrics.As reported in the literature, cotton shows a peculiar thermal degradation in nitrogen and air atmospheres [15,18].Usually, cotton pyrolyses in nitrogen according to two alternative pathways: (i) the decomposition of glycosyl units to char at lower temperature (namely, dehydration) and (ii) the depolymerisation of such units to volatile products containing levoglucosan at higher temperature.If cotton is treated with only APTES or DPTS (Figure 1A), its thermal stability increases, as indicated by the shift of Tmax values toward higher temperatures and by the fi nal residue at 750°C.By combining APTES and DPTS in diff erent molar ratios (Figure 1C), it is possible to observe a joint eff ect involving the sol-gel derived silica, P and N elements present in the hybrid fi lm (Figure 1).Th e concurrent presence of Si, P and N turns out to further hinder the formation of volatile products during the fi rst step of cellulose degradation and, at the same time, to favour the formation of the char.
In order to further investigate the behaviour of the hybrid coatings containing Si, P and N species, MF was used at diff erent concentrations and combined with DPTS as an alternative to APTES.When cotton was treated with MF only (Figure 1B), the mechanism and kinetics, through which the fabric degraded, did not change in a remarkable way, with the exception of the fi nal residue at 750°C.Similar results were found when a combination of MF and DPTS was employed (Figure 1D).In spite of a signifi cant anticipation of the degradation as compared with the untreated cotton, these hybrid fi lms are able to protect the cellulose from degradation, as already observed with DPTS-APTES-based formulations.At 750°C they show very high residues (41%) with respect to cotton (8%).For the investigated systems, two decomposition peaks of cotton in air are observable in between 340 and 470°C.With the exception of a slight anticipation in the Tonset for the APTES-based formulation, the fi rst degradation peak (Tmax1) turns out to be aff ected by the presence of each single species.On the contrary, in presence of both APTES and DPTS, regardless of the used molar ratio, the degradation profi le of cotton strongly changes: indeed, the formation of a thermally stable char is favoured, thus the second step disappears.Th is behaviour can be attributed to the thermal insulation exerted by the formed silica, which promotes cotton dehydration towards the char formation, instead of the formation of volatile combustible products.An increased thermal stability of cotton in air was achieved for all the compositions under study.Th ese results are very important since cotton fl ame retardancy could increase as a consequence of its increased thermal stability in air (Table 3).Th e collected results prove that the fi lms deposited on cotton fi bres play a protective role against the thermal decomposition of cotton, favouring the char formation.Th e presence of the coating induces the anticipation of the cotton decomposition and of the maximum weight loss, but favours the formation of a thermally stable char that evolves at high temperatures and leaves a residue signifi cantly higher with respect to that of pure cotton.Furthermore, the residue increases as the number of applied layers increases.By combining DPTS and APTES in diff erent molar ratios, it is possible to observe a joint eff ect amongst the sol-gel derived silica and P and N elements present in the hybrid fi lm.Th e fl ammability data (Table 4) show that the treatments starting from DPTS sols are able to significantly reduce the total burning time of cotton (in spite of a slight increase of the burning rate) and to protect the treated fabric from the fl ame, favouring the char formation.Th e fl ammability data show that the formulations containing APTES or MF acting as reactive agents with DPTS are able to further reduce the total burning time and rate of cotton and protect the treated fabric from fl ame, as they favour the char formation instead of the production of volatile species that could promote a further combustion.Th e most important result is the achievement of high residues at the end of the test in comparison with the untreated cotton or the cotton separately treated with APTES, DPTS or MF.Finally, in the DP combination, the deposited coating is able to protect the fabric, leaving a very coherent and consistent final residue at the end of the test (40%), aligning the fl ammability parameters with cotton.Finally, the cotton fabrics treated with phosphorusdoped silica coatings have been further doped with phosphorus or nitrogen agents, aiming to assess the eventual synergisms or additive eff ects that can be derived from the combination of thermal insulating eff ect of the silica coatings with the well-known fl ame retardant character of species such as a bisphosphonate (1-hydroxyethane 1,1-diphosphonic acid, P), melamine (M) or urea (U).Both samples treated with only DPTS (O_D) and only bisphosphonate (O_P) are able to act in the condensed phase during the combustion of cellulose, favouring its dehydration and thus the formation of a coherent and thermally stable residue at the end of the test (25 and 10% for O_D and O_P, respectively) (Table 5).
In the DPTS-bisphosphonate combination (D_P), the deposited coating is able to protect the cotton, leaving a very coherent and consistent fi nal residue at the end of the test (40%), aligning the fl ammability parameters with cotton.On the contrary, the other two formulations (i.e.D_M and D_U samples) are not able to ameliorate the performances of O_D.Indeed, no variations of fl ammability have been found in presence of urea, while melamine is only able to strongly reduce the total burning time, but also to signifi cantly increase the burning rate.

Conclusion
Th is work has clearly shown that hybrid phosphorus-doped silica architectures obtained by sol-gel technique can be successfully applied on cotton fabrics to enhance their thermal stability and fl ame retardancy.Th e deposition of hybrid silica network promotes a signifi cant increase of the residues at high temperature and is responsible for a strong anticipation of cellulose decomposition.Th e presence of phosphorous additive in the silica matrix favours the char formation, as shown by thermogravimetry and fl ammability test.Indeed, the acidic species, released during the decomposition of the bisphosphonate (at ca.260°C), catalyse the cellulose dehydration, favouring the appearance of the carbonaceous residue.In addition, the hybrid phosphorus silica coating, acting as a thermal insulator, further helps in the formation of an aromatic char resistant to the fl ame propagation.

Figure 1 :
Figure 1: TG curves of pure and treated cotton fabrics in nitrogen: Weight/% vs Temperature/°C

Table 2 :
Main vibration modes ascribable to DPTS in xerogel thin fi lms on glass slides and on cotton fabrics

Table 3 :
TGA data of untreated and treated cotton fabrics in air

Table 4 :
Collected data of untreated and treated fabrics by fl ammability tests