SYNTHESIS AND CHARACTERIZATION OF HYBRID PHOSPHORUS-SILICA FILMS FOR HALOGEN-FREE FLAME RETARDANT COTTON FABRICS

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1 SYNTHESIS AND CHARACTERIZATION OF HYBRID PHOSPHORUS-SILICA FILMS FOR HALOGEN-FREE FLAME RETARDANT COTTON FABRICS G. Rosace 1, J. Alongi 2, C. Colleoni 1, E. Guido 1, G. Malucelli 2 1 Dipartimento di Ingegneria Industriale, Università di Bergamo, Viale Marconi 5, Dalmine (Bg), Italy 2 Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, sede di Alessandria, Viale Michel 5, Alessandria, Italy giuseppe.rosace@unibg.it Abstract Diethylphosphatoethyltriethoxysilane has been used as a functional alkoxysilane in consecutive sol-gel depositions on cotton fabrics, for obtaining architectures with a different number of layers (namely, 1, 3 or 6 layers). The role of such architectures has been deeply investigated and correlated with the final properties of the treated fabrics. Afterwards, 3- aminopropyltriethoxysilane and N,N,N',N',N'',N''-hexakis-methoxymethyl-[1,3,5]triazine- 2,4,6-triamine have been reacted with diethylphosphatoethyltriethoxysilane to investigate the effect of the concurrent presence of Si, P and N on the flame retardancy properties of the cellulosic fabrics. Finally, the synergistic or additive effects due to the concurrent presence of phosphorus and nitrogen-based compounds have been thoroughly investigated. Flammability and cone calorimetry tests have shown that only phosphorus (i.e. bisphosphonate) is able to promote a certain synergism with the sol-gel derived oxide phases in terms of residue, heat release rate and total burning time. Introduction The massive use of polymeric materials in the textile field is driven by their remarkable combination of properties, low weight and ease of processing. Apart from comfort and aesthetics, the consumers seek for textiles having additional high performance properties such as anti-static, water and oil repellency and fire retardancy. Due to their chemical structure, made up mainly of carbon and hydrogen, cellulose-based textile materials are highly combustible, so the improvement of their thermal stability is a challenging topic. There is a large number of commercial flame retardant treatments available for cotton that can be classified into three categories: flame retardants based on phosphorus (condensed phase mechanism) and halogens (gas phase mechanism), synergistic systems containing flame retardant enhancers (nitrogen-phosphorus and antimony-halogens) and flame retardants with physical effects (alumina, borate) [1]. Some of these treatments show serious drawbacks: for instance, the use of halogens as flame retardants is restricted because of the toxicity of their combustion gases, whereas inorganic salts have very poor laundering durability, although they can provide excellent flame retardant properties for cellulose. In addition, these treatments often show some drawbacks or have a negative impact on the desirable properties of the fibres. Our research group has demonstrated that sol gel process can be successfully exploited as flame retardant treatments with a low environmental impact for textile fabrics [2,3]. Indeed, the treatment of cellulosic materials with hydrolyzed metal alkoxide solutions is an excellent tool to convey new properties to polymer surfaces, particularly if organic components are incorporated into the formulation. In this composition, the hybrid organicinorganic materials (HOIMs) [4] are able to conjugate the properties of both the phases. Therefore, the thermal stability at high temperatures proper of inorganic constituents can be combined with the flame retardancy of organic intumescent phosphorus-based compounds. 1

2 To this aim, in the present work, diethylphosphatoethyltriethoxysilane (DPTS) has been exploited in a novel multistep process consisting of 1-6 consecutive depositions in order to form architectures differing for the number of layers. Afterwards, the same silica based precursor has been studied in presence of co-reactants containing P and N elements, aiming to assess their effect on the thermal stability and flammability of cotton fabrics. Finally, the sol has been doped with a bisphosphonate (1-hydroxyethane 1,1-diphosphonic acid), melamine or urea, in order to investigate the eventual synergisms or additive effects that can derive from the combination of the thermal insulating effect of silica coatings with such wellknown flame retardant species. In a first 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 films. In a second step, FT-IR, SEM- EDX and TGA analyses were carried out on the treated fabrics for their chemical, morphological and thermal characterizations. The combustion behaviour was then investigated using cone calorimetry. Finally, flammability tests were carried out on untreated and treated fabrics according to ASTM D standard. Experimental part Scoured and bleached 100% plain-weave cotton fabrics (240 g/m 2 ) were used. The fabrics were washed in 2% non-ionic detergent at 40 C for 20 min, and then rinsed several times with deionized water, dried and put into drier for storage. The cleaned samples were conditioned under standard atmospheric pressure at 65 ± 4% relative humidity and 20 ± 2 C for at least 24 h before all the experiments. The sol-gel precursor (diethylphosphatoethyltriethoxysilane, DPTS, purity grade 95%) was purchased from Gelest and used as received; 3-aminopropyltriethoxysilane (APTES, purity grade 95%), 1- hydroxyethane 1,1-diphosphonic acid (P; 60% aqueous solution), melamine (M, 99%), urea (U, 99%), dibutyl tindiacetate (DBTA, condensation catalyst), hydrochloric acid, sodium hydroxide and ethanol were purchased from Sigma-Aldrich. N,N,N',N',N'',N''-hexakismethoxymethyl-[1,3,5] triazine-2,4,6-triamine (MF) in aqueous solution (50 w/v%) was a commercial product supplied by Europizzi S.p.A. (Italy). All the chemicals were used without any further purification. The chemical structures are schematized in Table 1. Table 1. Name, code and chemical structures of sol-gel precursors and synergists Name Code Chemical structure Diethylphosphatoethyltriethoxysilane D 3-Aminopropyltriethoxysilane A N,N,N',N',N'',N''-Hexakismethoxymethyl- [1,3,5]triazine-2,4,6-triamine MF 2

3 1-hydroxyethane 1,1-diphosphonic acid P Melamine M Urea U Sol-gel treatment on textile fabrics ml (0.03mol) of silane precursor (D) were hydrolysed with 8 ml (0.1mol) of HCl (37.5%), in presence of 5 ml of ethanol for 24 h under mechanical stirring at reflux. Subsequently, the obtained sol was added with water, reaching a final volume of 100 ml, and the ph was set at 5 by using sodium hydroxide. This sol was doped with 1-hydroxyethane 1,1-diphosphonic acid, melamine or urea, keeping silane precursor:p, silane precursor:m, silane precursor:u molar ratios equal to 2:1, 12:1 and 4:1, respectively. According to these stoichiometries, the resultant P:N atomic ratio was set at 2:1 for silane precursor-melamine and silane precursor-urea sols ml of APTES was hydrolyzed, separately, with 2 ml ( mol) of HCl (0.1 N) in ml of deionized water under vigorous stirring for 12 h at room temperature to obtain the silica sols. Three different solutions containing MF (crosslinker) were prepared by adding 1.5 g (0.002 mol), 3 g (0.004 mol) and 4.5 g (0.006 mol) of MF and dimethylbenzene sulphonic acid (20% on crosslinker mass) in deionized water up to 50 ml of total volume. For the APTES-DPTS05, APTES-DPTS1 and APTES-DPTS2 sols (50 ml of total volume), 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 3.98 ml ( mol) of DPTS in three different solutions with, respectively, 1.5 g (0.002 mol), 3 g (0.004 mol) and 4.5 g (0.006 mol) of resin and dimethylbenzene sulphonic acid (20% on crosslinker mass); subsequently, distilled water was added for achieving a final volume of 50 ml. Referring to the sol containing the silane precursor and 1-hydroxyethane 1,1-diphosphonic acid, its phosphorus atomic content was doubled with respect to that already present in silane precursor sol. Each sol was stirred for 4 h at room temperature prior to the application. The different sols were applied to the cotton fabrics by a pad-cure-method. The coating was carried out by a padding process using a two-roll laboratory padder (Werner Mathis, Zurich, Switzerland) with 70% of wet pick-up. After drying (80 C for 2 h) the fabrics were cured (at 150 C for 2 min) in a laboratory oven. Characterization techniques The formation of bonds between cotton samples and the different formulations applied onto the fabric samples was investigated by infrared spectroscopy. FT-IR spectra were recorded at room temperature in the range cm -1 (64 scans and 4 cm -1 resolution), using a Thermo Avatar 370 spectrophotometer, equipped with an attenuated total reflection accessory and ZnSe crystal. The thermal stability of the fabrics was evaluated by thermogravimetric (TG) analyses from 50 to 800 C with a heating rate of 10 C/min, both in nitrogen and in air (60 ml/min for both the atmospheres). To this aim, a TAQ500 thermogravimetric balance was used, placing the samples in open alumina pans (ca. 10 mg). The experimental error was 0.5% on weight and 1 C on temperature. The flammability of the prepared samples was measured using a vertical test, applying a methane flame for 5 s at the bottom of a fabric specimen (50 x 150 mm 2 ) and repeating the test 3 times for each formulation in order to 3

4 obtain reproducible data; burning time, burning rate and final residue were measured. This test aims to mimic the procedure described in ISO standard, commonly employed for protective garments, although the specimen size is different (200 x 16 mm 2 in ISO 15025). The combustion behaviour of square fabric samples (50 x 50 mm 2 ) was investigated using cone calorimetry (Fire Testing Technology, FTT). The measurements were carried out under a 35kW/m 2 irradiative heat flow in horizontal configuration. Such parameters as Time To Ignition (TTI, s), Flame Out time (FO, s), Total Heat Release (THR, kw/m 2 ), and peak of Heat Release Rate (pkhrr, kw/m 2 ) were measured. Total Smoke Release (TSR, m 2 /m 2 ) and peak of Smoke Release Rate (pksrr, 1/s) were evaluated, as well. The experiments were repeated four times for each investigated material to ensure reproducible and significant data; the experimental error was within 5%. Prior to flammability and combustion tests, all the specimens were conditioned at 23 ±1 C, for 48 h at 50% R.H. in a climatic chamber. Results and discussion FT-IR characterization FT-IR spectra were collected to characterize the absorption bands related to the vibrational modes of the functionalities present on the cotton fabrics treated with the hydrolyzed pure precursor sol (DPTS or APTES) and MF. In addition, the FT-IR spectra of the hybrid thin films, obtained mixing DPTS with MF or APTES, applied and annealed on the textile samples were collected. The frequencies of the main absorption bands are shown in Table 2. In all the treated samples, a Si O-Si basic skeleton can be foreseen by the presence of signals at 1090 cm -1 (Si O stretching) and 800 cm -1 (Si O bending). As far as the DPTS spectrum is concerned, the strongest intensity of the absorption band (Si O bending) can be attributed to the overlapping with P O vibration. In the same spectrum, the P=O absorption band (located at 1241 cm -1 for pure DPTS) appears at 1210 cm -1 in xerogel spectra, e.g. shifted towards low frequency range. Such a shift is an indication of the involvement of phosphonyl groups in the formation of hydrogen bonds. An absorption band at 784 cm -1 is also observable, the intensity of which can be partially ascribed to the P O vibration. The characteristic band of MF at 1556 cm -1 is ascribed to the stretching of triazine ring, whereas that at 814 cm -1 is due to triazine ring out-of-plane bending. Table 2 main vibration modes of DPTES, APTES and MF thin films on cotton samples DPTS APTES MF Peak characteristic 1020 ν (Si-O-Si) stretching δ (Si-O-Si) δ (Si-C) 1577 δ (N-H) ν (C-H) ν (P-O) 1230 ν (P=O) ν (C=N) triazine δ (C=N) triazine Thermal stability The thermal and thermo-oxidative stability of the sol-gel treated samples has been assessed by thermogravimetric analysis and compared with that of pure cotton fabrics. The collected 4

5 results [2] prove that the films deposited on the cotton fibres play a protective role against the thermal decomposition of cotton, favouring the char formation both in nitrogen and air. The 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 significantly higher with respect to that of pure cotton. Furthermore, the residue increases by increasing the number of applied layers. By combining DPTS and APTES in different molar ratios [5], it is possible to observe a joint effect amongst the sol gel derived silica and P and N elements present in the hybrid film. As it is well described in the literature, silica is able to act as a physical barrier to the thermal and thermo-oxidation of cotton, blocking the heat and oxygen transfer towards the fabric. The concurrent presence of Si, P and N in the hybrid coating turns out to further hinder the formation of volatile products during the first step of cellulose degradation and, at the same time, favours the formation of the char. Similar results were found when a combination of MF and DPTS was employed. In spite of a significant anticipation of the degradation that starts at lower temperatures with respect to untreated cotton, these films are able to protect the cellulose from its degradation, as already observed with APTES DPTS-based formulations. This conclusion does not indicate the occurrence of a synergistic effect due to the presence of the different elements within the hybrid coating, even though it seems quite evident that the combination of these species is capable to further improve the thermooxidative stability of the fabric. To understand the influence of dopants or synergic molecules on the thermal stability of the treated textile fabric, 1-hydroxyethane 1,1-diphosphonic acid (DP), melamine (DM) and urea (DU) were considered in combination with DPTS sols [6]. The sol added with bisphosphonate (code: DP) showed interesting results due to phosphoric acid release, during its degradation at ca. 260 C, that is able to acid-catalyse the dehydration, phosphorylating the C(6) primary hydroxyl groups of cellulose and inhibiting the C(6)-C(1) intramolecular rearrangement that produces levoglucosan and thus promoting the char formation. On the contrary, both melamine and urea do not significantly affect the thermal degradation of cotton, with the exception of the final residue. The other two doping agents (i.e. melamine and urea) do not significantly change the thermal degradation of cellulose (only melamine slightly decreases the char formation). Flame retardancy The flame retardant properties of the treated fabrics were evaluated in terms of flammability: the prepared samples were ignited directly by a flame and the response of the material was measured in terms of total burning time and final residue. The flammability data show that the treatments realized by 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 flame, favouring the char formation. The flammability data show that the formulations containing APTES or MF like reactive agents with DPTS are able to further reduce the total burning time and rate of cotton and protect the treated fabric from flame due to the protective role of the films deposited on the cotton surface that favour the char formation instead of the production of volatile species that could promote a further combustion. The most important result is the formation of high residues at the end of the test in comparison with pure cotton or cotton separately treated with APTES, DEPTES 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 flammability parameters with cotton. On the contrary, the other two formulations containing melamine or urea (DM and DU samples, respectively) are not able to ameliorate the performances of DPTS. Indeed, no variations of flammability have been found in presence of urea, while melamine was only able to strongly reduce the total burning time, but also to significantly increase the burning rate. 5

6 As far as the combustion behaviour is concerned, the resistance of these coatings to an irradiating heat flow has been assessed by cone calorimetry. From an overall consideration, it is important to underline that TTI is strongly reduced for all the formulations under study, regardless of the type of architecture or doping agent: hence, there is no synergy for DP, DM or DU samples. On the contrary, unlike urea, bisphosphonate and melamine are able to postpone the Flame Out time. These results are expected as the systems under study are active flame retardants in the condensed and not in the gas phase. Conclusion The sol-gel process has been successfully applied for obtaining hybrid phosphorus-doped silica architectures, derived from diethylphosphatoethyltriethoxysilane, which have shown to enhance the thermal stability and flame retardancy properties of cotton fabrics. The presence of the coating turned out to be responsible of a strong anticipation of the cellulose decomposition but, at the same time, promoted a significant increase of the residues at high temperatures. Two specific combination of DPTS with N donor agents like a silica precursor (3-aminopropyltriethoxysilane), and N,N,N',N',N'',N''-hexakis-methoxymethyl- [1,3,5]triazine-2,4,6-triamine were chosen and exploited for preparing hybrid phosphorus doped silica films on the surface of the fabric. Such sol gel treatments showed significant enhancements as far as both the thermal and thermo-oxidative stability and flammability resistance of cotton are considered, thanks to the protective role of the hybrid films that were found to promote the char formation and, at the same time, to limit the production of volatile species. The effect of three potentially synergistic agents (a bisphosphonate, melamine and urea) on the flame retardancy of cotton fabrics treated with phosphorus-doped silica coatings derived from sol-gel processes was assessed. The synergistic effectiveness concept has been exploited for demonstrating how merely an additive effect (observed in our systems) can be defined as synergism as a consequence of an incorrect interpretation of the flame retardancy data. In this work, only hybrid phosphorus-doped silica coatings turned out to synergistically act with 1-hydroxyethane 1,1-diphosphonic acid, while a simply additive effect was found to occur when the hybrid phosphorus-doped silica coatings were further doped with N- containing molecules such as melamine or urea. Indeed, silica and bisphosphonate are able to cooperate in the char formation, as shown by thermogravimetry, flammability and cone calorimetry tests. This finding can be probably ascribed to the decomposition of the bisphosphonate (at ca. 260 C), which gives rise to acidic species that catalyse the cellulose dehydration; in the meanwhile, the hybrid phosphorus silica coating, acting as a thermal insulator, further helps in the formation of an aromatic char resistant to the flame propagation. Acknowledgements The authors would like to thank European COST Action MP1105 Sustainable flame retardancy for textiles and related materials based on nanoparticles substituting conventional chemicals" (FLARETEX). References: [1] W.D. Schindler, P.J. Hauser, Chemical Finishing of Textiles; Eds.; Woodhead Pub.: Cambridge (2004). [2] J. Alongi, C. Colleoni, G. Malucelli, G. Rosace, Polymer Degradation and Stability, 97, 1334 (2012). [3] G. Brancatelli, C. Colleoni, M.R. Massafra, G. Rosace, Polymer Degradation and Stability, 96, 483 (2011). [4] J. Alongi, G. Malucelli, J. Mater. Chem., J. Mater. Chem. 22, (2012). [5] J. Alongi, C. Colleoni, G. Rosace, G. Malucelli, J Therm Anal Calorim 110, 1207 (2012). [6] J. Alongi, C. Colleoni, G. Rosace, G. Malucelli, Polymer Degradation and Stability 98, 579 (2013). 6