Phosphorus- and nitrogen-doped silica coatings for enhancing the flame retardancy of cotton: Synergisms or additive effects?

Polymer Degradation and Stability 98 (2013) 579e589

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Phosphorus- and nitrogen-doped silica coatings for enhancing the flame retardancy of cotton: Synergisms or additive effects? Jenny Alongi a, *, Claudio Colleoni b, Giuseppe Rosace b, Giulio Malucelli a a b

Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, Sede di Alessandria and Local INSTM Unit, Viale Teresa Michel 5, 15121 Alessandria, Italy Dipartimento di Ingegneria Industriale, Università di Bergamo, Viale Marconi 5, 24044 Dalmine, Bergamo, Italy

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 October 2012 Received in revised form 23 November 2012 Accepted 26 November 2012 Available online 1 December 2012

In the present work, the synergistic or additive effects due to the concurrent presence of phosphorusand/or nitrogen-based compounds on the flame retardancy of cotton fabrics previously treated by sol egel processes have been thoroughly investigated. Indeed, although additives containing P and/or N structures have been exploited for imparting fire resistance to cellulosic substrates, the quantification of their synergistic level has not been fully considered. More specifically, the concept of “synergistic effectiveness” has been applied to cotton fabrics treated with a phosphorus-doped silica coating, further doped with a bisphosphonate, melamine or urea. Flammability and cone calorimetry tests have shown that only phosphorus (i.e. bisphosphonate) is able to promote a certain synergism with the solegel derived oxide phases in terms of residue, heat release rate and total burning time. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Cotton Solegel processes Flame retardancy Synergisms

1. Introduction Cotton is one of the most important natural fibres employed in the textile field. The growing of the request market to find innovatory solutions for conferring enhanced and durable flame retardancy properties to cotton has always encouraged and motivated both the academic and the industrial research. Different approaches have been proposed to solve this problem and achieve the performances of the major industrial target compounds, i.e. ProbanÒ and PyrovatexÒ. The most comprehensive reviews describing the flame retardant additives and their applications currently attempted or exploited have been reported by Horrocks [1] and Weil et al. [2]. In order to design a flame retardant formulation, it is necessary to clarify its action both during the thermal degradation (in air) and during the combustion of cotton: indeed, the effect of heat on a fibre can produce physical as well as chemical effects. As thoroughly described by Horrocks [3], thermoplastic fibres show physical changes, since they initially soften (above their glass transition temperature) and subsequently melt (at the melting temperature). On the other hand, chemical changes start to occur at pyrolysis temperature, then proceed during the oxidation at high temperatures and are completed during the combustion. As already

* Corresponding author. Tel.: þ39 (0)13122937; fax: þ39 (0)131229399. E-mail address: [email protected] (J. Alongi). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2012.11.017

widely demonstrated, the chemical action of heat on cotton fibres plays the key role as far as their combustion is considered [3]. Indeed, upon heating, cellulose degrades by two competitive routes, i.e. depolymerisation and dehydration: the former induces the production of levoglucosan that subsequently pyrolyses, giving rise mainly to low molecular weight, highly flammable species. In the presence of oxygen, these pyrolysis products are oxidised and the highly exothermic character of the combustion forces more cellulose to pyrolyse. On the other hand, dehydration gives a carbonaceous residue, known as char. The equilibrium between these processes depends on the heating rate, as recently demonstrated [4]. In the current opinion, any flame retardant formulation for cellulosics should: i) remove the heat, ii) reduce the gas generation and combustible and promote the char formation, iii) prevent oxygen access to the flame and/or iv) interfere with the oxidation of flammable species. Therefore, the ideal flame retardant candidates for cotton are all the species able to acid-catalyse the dehydration and hence to act as char former in the condensed phase; in particular, such compounds as sulphuric and phosphoric acid (and their organic compounds), zinc chloride, aluminium sulphate and Lewis acids have been proven efficient flame retardants. Furthermore, organophosphorus flame retardants that contain synergistically active nitrogen could show higher effectiveness as compared with pure phosphorus counterparts. The consensus of current opinion [5e7] suggests that nitrogen in PeN synergistic retardants acts by a nucleophilic attack on the phosphate, creating polymeric

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species having PeN bonds. These latter are more polar than the already present PeO bonds, and the enhanced electrophilicity of the phosphorus atom increases its ability to phosphorylate the C(6) primary hydroxyl group of cellulose [3]. By this way, the intramolecular C(6)-C(1) rearrangement reaction forming levoglucosan is blocked. Meanwhile, the auto-crosslinking of cellulose promotes and consolidates the char formation derived by the action of the same flame retardants. Although numerous studies referring to the PeN synergism for cotton have been published so far, to the best of our knowledge, only a qualitative observation of this phenomenon has been often argued. Lewin [8] and Horrocks [9] have demonstrated that it is possible to identify a real synergism between two species (namely, phosphorus and nitrogen) only through the calculation of a synergism effectiveness parameter. Indeed, in some cases, the effect of the two species can be merely additive or even antagonist. In this scenario, novel smart coatings deposited on fibres by different techniques, such as nanoparticle adsorption, Layer by Layer assembly, plasma treatments and solegel processes may represent effective and alternative routes for enhancing the flame retardancy of natural and synthetic fibres [10]. In particular, we have demonstrated that solegel processes represent a simple a versatile route by which it is possible to prepare completely inorganic or hybrid organiceinorganic coatings able to impart flame retardancy properties to cotton [11]. More specifically, these architectures are able to protect the polymer substrate, by: i) absorbing the heat from the surrounding area, ii) creating a physical barrier to oxygen and heat transfer (thermal insulating effect), iii) hindering the formation of volatile species that fuel further degradation and, at the same time, iv) favouring the formation of char. Since these coatings can only operate in the condensed phase during the combustion of a polymeric material, solegel derived architectures cannot be fully considered effective flame retardant systems [1,11]. To overcome this limitation, synergistic or additive effects achieved by combining the solegel oxidic phases with active species (namely, phosphorus and/or nitrogen) can be exploited [12e15]. Alternatively, in order to achieve better performances, the silica precursor can also bear a phosphate group: by this way, hybrid phosphorus-doped silica architectures can be prepared [16]. In the present work, cotton fabrics treated with phosphorusdoped silica coatings have been further doped with phosphorus or nitrogen agents, aiming to assess the eventual synergisms or additive effects that can be derived from the combination of thermal insulating effect of the silica coatings with the well-known flame retardant character of species such as a bisphosphonate (1hydroxyethane 1,1-diphosphonic acid), melamine or urea. To this aim, the synergistic effectiveness (SE) described by Lewin [8] has been applied to the flame retardant parameters collected by flammability and/or combustion tests. SE can be defined according to the following equation:

ðFpÞfrþs  ðFpÞp    SE ¼  ðFpÞfr  ðFpÞp þ ðFpÞs  ðFpÞp

By using Equation (1), we have thoroughly assessed the possible synergisms or additive effects between solegel phosphorus-doped silica architectures and three potential doping agents (bisphosphonate, melamine and urea), on the flame retardant properties of cotton. The morphology of the differently doped systems has been studied by Scanning Electron Microscopy and elemental analyses; furthermore, the thermal and thermo-oxidative stability as well as the flammability and combustion behaviour have been assessed by thermogravimetry in nitrogen and air, flammability and cone calorimetry tests, respectively. 2. Experimental part 2.1. Materials Scoured and bleached 100% plain-weave cotton fabrics (240 g/ m2) were supplied by Mascioni Spa, Varese, Italy. The fabrics were washed in 2% non-ionic detergent for 20 min at 40  C, 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 prior to all the tests. The silica precursor (diethylphosphatoethyltriethoxysilane, D, purity grade 95%) was purchased from Gelest and used as received; hydrochloric acid, ethanol, 1-hydroxyethane 1,1-diphosphonic acid (P; 60% aqueous solution), melamine (M, 99%) and urea (U, 99%) were supplied by SigmaeAldrich and used without any further purification. The chemical structure of D, P, M and U are reported in Table 1. 2.2. Solegel treatments on cotton fabrics 10.07 mL (0.03mol) of silane precursor (D) were hydrolysed with 8 mL (0.1mol) of HCl (37.5%), in the 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 precursorurea sols. Referring to the sol containing the silane precursor and 1-

Table 1 Name, code and chemical structures of solegel precursor and synergist agents. Name

Code

Diethylphosphatoethyltriethoxysilane

D

1-hydroxyethane 1,1-diphosphonic acid

P

(1)

where (Fp) is a given flammability parameter (from flammability or combustion tests), (Fp)p is the flame retardant property of the polymer alone, (Fp)fr is that of the polymer and flame retardant, (Fp)s is that of the polymer treated with the synergist, and (Fp)frþs is that of the full formulation comprising flame retardant and synergist. This parameter allows the direct quantitative comparison of the synergistic properties between different flame retardants. SE > 1 means that synergy is occurring; 0 < SE  1 points out a simply additive or cumulative effect, SE < 0 implies antagonism [9].

Chemical structure

O O Si O

O P HO HO

HO

O P O O

OH P OH O

NH2 Melamine

N

M

H2N Urea

U

H2N

N N O NH2

NH 2

J. Alongi et al. / Polymer Degradation and Stability 98 (2013) 579e589 Table 2 Composition, wet pick-up and total dry solids add-on of sols on cotton samples. Sample

Wet pick-up [wt.-%]

Add-on [wt.-%]

O_D O_P O_M O_U D_P D_M D_U

70 17 <2 <2 85 52 58

12.3 3.4 0.3 0.4 15.5 9.9 10.5

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 fabrics (20  30 cm2) were impregnated with the sols and afterward passed through a two-roll laboratory padder (Werner Mathis, Zurich, Switzerland), working with 3 bar nip pressure. The cotton fabrics treated with only diethylphosphatoethyltriethoxysilane (O_D sample) gained a wet pickup of 70wt.-%; subsequently they were cured at 170  C for 4 min, in a gravity convection oven, after having been dried at 80  C for 10 min. The fabrics treated with D and P, D and M or D and U were coded as D_P, D_M and D_U, respectively. In this case, a wet pick-up of ca. 60wt.-% was achieved (Table 2). The total dry solids add-on on cotton samples (A, wt.-%) was determined by weighting each sample before (Wi) and after the impregnation with the sols and subsequent thermal treatment

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(Wf), using a Sartorius balance (accuracy: 104 g). The precursor uptake (given in Table 2) was calculated according to the following equation:

A ¼

Wf  Wi 100 Wi

In order to calculate the “synergistic effectiveness”, the fabrics were also treated with only P, M or U (O_P, O_M or O_U samples, respectively) following the procedure described above. A schematic representation of the possible chemical structures formed on the fabrics treated by each single additive (i.e. D, P, M or U) is presented in Fig. 1: further interactions can be expected when a combination of these components is used at the same time. 2.3. Characterization techniques The surface morphology of the treated samples was studied using a LEO-1450VP Scanning Electron Microscope (beam voltage: 5 kV), equipped with an X-ray probe (INCA Energy Oxford, Cu-Ka Xray source, k ¼ 1.540562 A), to perform elemental analysis (EDS). Fabric pieces (0.5  0.5 cm2) were cut and fixed to conductive adhesive tapes and gold-metallized. 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

Fig. 1. Scheme of some possible chemical structures formed on the treated fabrics.

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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  150 mm2) and repeating the test 3 times for each formulation in order to obtain reproducible data; burning time, burning rate and final residue were measured. This test aims to mimic the procedure described in ISO 15025 standard, commonly employed for protective garments, although the specimen size is different (200  16 mm2 in ISO 15025). The combustion behaviour of square fabric samples (50  50 mm2) was investigated using cone calorimetry (Fire Testing Technology, FTT). The measurements were carried out under a 35kW/m2 irradiative heat flow in horizontal configuration, following the procedure described elsewhere [17]. Such parameters as Time To Ignition (TTI, s), Flame Out time (FO, s), Total Heat Release (THR, kW/m2), and peak of Heat Release Rate (pkHRR, kW/ m2) were measured. Total Smoke Release (TSR, m2/m2) and peak of Smoke Release Rate (pkRSR, 1/s) were evaluated, as well. The experiments were repeated four times for each material investigated 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.

The residues of flammability and combustion tests were characterised by Attenuated Total Reflectance (ATR) spectroscopy and by EDS (beam voltage: 20 kV). The ATR spectra were recorded at room temperature in the range 650e4000 cm1 (32 scans and 4 cm1 resolution), using a Frontier FT-IR/FIR spectrophotometer, equipped with a Ge crystal. 3. Results and discussion Cotton fabrics have been solegel treated in order to assess the possible synergistic or additive effects between phosphorus-doped silica architectures and bisphosphonate, melamine or urea. Table 2 collects the corresponding add-on values achieved for each formulation investigated. When cotton is covered by only phosphorusdoped silica coating (O_D), the add-on is 12.3wt.-%; by further doping the D sol with bisphosphonate, the add-on increases up to 15.5wt.-%, which substantially corresponds to the sum of O_D and O_P (3.4wt.%) add-ons. On the contrary, when the D-based architectures are doped with melamine or urea, the add-on decreases in a remarkable way (9.9 and 10.5 vs. 12.3wt.-% for D_M, D_U and O_D, respectively). This finding can be ascribed to the lower wet pick-up exhibited by these samples (Table 2), probably due to a change of the sol viscosity or of cotton wettability. In addition, as far as urea is concerned, the add-on decrease can be attributed to its thermal decomposition at the curing temperature [18].

Fig. 2. SEM magnifications of pure cotton (A), O_D (B) and D_P (C), D_M (D) and D_U (E) samples.

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Fig. 3. Elemental analysis of D_P (A), D_M (B) and D_U (C) samples.

3.1. Morphology SEM observations have been performed in order to assess the morphology of the fibres after the various treatments. As is wellknown, pure cotton fibres are characterized by a certain inhomogeneity level due to the natural growing, as depicted in Fig. 2A. When the fibres are solegel treated with only D, their surface becomes rougher (Fig. 2B), due to the presence of the coating that covers the fibres completely [11,16]. The doping of such architectures with bisphosphonate (Fig. 2C), melamine (Fig. 2D) or urea

Table 3 Elemental analysis of the treated fabrics and of the residues after flammability and cone calorimetry tests. C [%] Samples O_D 42 D_P 38 D_M 41 D_U 41 Residues after flammability tests D_P 49 D_M 58 D_U 55 Residues after combustion tests D_P 4.4 D_M 6.2 D_U 1.3

O [%]

Si [%]

P [%]

54 52 53 54

1.9 1.3 1.5 1.4

1.7 2.6 1.6 0.9

30 28 30

3.0 3.4 2.5

7.1 3.1 4.0

46 45 44

11.8 3.7 12.8

18 14 12

(Fig. 2E) does not change the observed morphologies in a remarkable way. The elemental analysis carried out by EDS shows a uniform and homogeneous distribution of Si and P elements on all the three coatings, regardless of the doping agent used (Fig. 3). Although the EDS investigation allows only a semi-quantitative analysis, by employing a high beam voltage (i.e. 20 kV) it is possible to exploit the high penetration of the incident beam to discriminate among the depositions obtained with the three different doping agents. The collected data are listed in Table 3 together with the values of residues after the flammability and combustion tests, which will be commented further on. When the cotton is treated with D (namely, O_D), the Si content is higher than that found when the sol is doped (1.9 vs. 1.3, 1.5 and 1.4wt.-% for D_P, D_M and D_U, respectively). A similar consideration can be argued for the P

Table 4 TGA data in nitrogen. Sample

Tonset10% [ C]

Tmaxa [ C]

Residue at Tmax [%]

Residue at 700  C [%]

COT O_D O_P O_M O_U D_P D_M D_U

311 253 268 295 307 251 245 262

344 335 327 350 349 328 335 334

50.6 59.2 58.0 44.9 48.4 68.6 57.4 59.0

6.7 29.3 25.6 8.4 8.1 34.6 27.2 29.1

a

From dTG curves.

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Table 5 TGA data in air. Sample

Tonset10% [ C]

Tmax1a [ C]

Tmax2a [ C]

Residue at Tmax1 [%]

Residue at Tmax2 [%]

Residue at 700  C [%]

COT O_D O_P O_M O_U D_P D_M D_U

316 285 297 315 318 289 292 289

336 319 313 333 336 318 320 320

451 481 482 466 456 486 482 483

42.3 64.0 63.2 48.4 48.3 67.9 68.2 64.9

3.1 17.7 13.9 4.6 3.9 22.8 20.4 18.4

0.4 7.3 4.4 0.6 0.3 9.2 7.1 7.5

a

From dTG curves.

amount with the exception of D_P, where the phosphorus content (2.6wt.-%) is obviously the highest as a consequence of the doping with bisphosphonate. 3.2. Thermal stability Fig. 5. TG and dTG curves of pure cotton and D_P, D_M and D_U samples in nitrogen.

The thermal and thermo-oxidative stability of the solegel treated samples has been assessed by thermogravimetric analysis and compared with that of pure cotton. Tables 4 and 5 collect the data of relevant parameters in nitrogen and air, respectively; Figs. 4e7 plot the full TG and dTG curves of the samples for both the atmospheres. As previously demonstrated, the phosphorus-doped silica architectures (O_D) are responsible for a significant anticipation of cotton degradation (see Tonset10% in Table 4 and Fig. 4) due to their char-forming feature [11,16,19]. Indeed, the cotton covered by these architectures starts to degrade at lower temperatures with respect to the pure fabric and the coating, which protects the cellulose, favours the dehydration of the latter and thus the char formation instead of its depolymerisation, as shown by the residues found at Tmax and 700  C. The same behaviour is observed in the presence of bisphosphonate: indeed, this species degrades at ca. 260  C releasing phosphoric acid 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 [1]. On the contrary, both melamine and urea do not significantly affect the thermal degradation of cotton, with the exception of the final residue (Table 4 and Fig. 4). The D_P sample,

as reported in Table 4 and Fig. 5, shows the highest residues at Tmax and at 700  C (68.6 and 34.6 vs. 59.2 and 29.3, 58.0 and 25.6% for D_P, O_D and O_P, respectively). As a consequence, it is reasonable to expect the occurrence at least of an additive effect between D and P. 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). Notwithstanding, all the three formulations are capable of thermally protecting the cotton. In air, the degradation mechanism of cotton in the presence of the phosphorus-doped silica coating is similar to that discussed in nitrogen: the only difference is the oxidation of the char formed after the first step of degradation, which takes place at higher temperatures. Referring to pure cotton, the char formation is initiated by a rapid auto-crosslinking due to the formation of ether oxygen bridges derived from condensation reactions taking place between hydroxyl groups of adjacent chains. However, this spontaneous behaviour of cellulose is not completely efficient to inhibit its thermo-oxidation, since a partial aliphatic and not a fully aromatic char is probably formed [20]. When cotton is treated with D (O_D sample), polynuclear aromatic structures are formed, giving

Fig. 4. TG and dTG curves of pure cotton and O_D, O_P, O_M and O_U samples in nitrogen.

Fig. 6. TG and dTG curves of pure cotton and O_D, O_P, O_M and O_U samples in air.

J. Alongi et al. / Polymer Degradation and Stability 98 (2013) 579e589

Fig. 7. TG and dTG curves of pure cotton and D_P, D_M and D_U samples in air.

a coherent and thermally stable char after the first degradation step (Fig. 6). By this way, the maximum temperatures at which oxidation occurs shift to higher values (Tmax2, Table 5) and a carbonaceous residue is still found at 700  C (7.3 vs. 0.4% for O_D and pure cotton, respectively). Once again, bisphosphonate behaves similarly to O_D, while melamine and urea (O_M and O_U samples) do not interfere with the thermo-oxidation of cellulose. In conclusion, all the three agents act through an additive effect increasing the thermal stability of cotton in air (Fig. 7), as also clearly shown by the residues at Tmax1, Tmax2 and 700  C (Table 5).

stable residue at the end of the test (25 and 10% for O_D and O_P, respectively). In the D_P combination, the deposited coating is able to protect the cotton, 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 (i.e. D_M and D_U samples) are not able to ameliorate the performances of O_D. Indeed, no variations of flammability have been found in presence of urea, while melamine is only able to strongly reduce the total burning time, but also to significantly increase the burning rate. Once again, the PeN synergism does not occur, as it will be discussed later. As mentioned in the Introduction, it is possible to assess a synergism between two flame retardant species exploiting the concept of synergistic effectiveness (SE), as described by Lewin [8]. The calculated SE values from flammability data are collected in Table 6 for the three formulations under investigation. For D_P, the synergism between the two components occurs only when referring to the formation of the final residue, for which SE > 1: this means that their action in the condensed phase is not only merely additive, but synergistic. They may interact during the combustion as bisphosphonate starts to decompose at ca. 260  C and produce acid species that catalyse the cellulose dehydration; meanwhile, the silica coating, acting as a thermal insulator, further helps in the formation of an aromatic char resistant to the flame propagation. Such hypothesis has been confirmed by ATR spectroscopy measurements performed on the final residues: indeed, in the ATR spectra of O_D and O_P (Fig. 8) an intense band at ca. 1600 cm1 (and another one or a shoulder at 1240 cm1) can be ascribed to the

3.3. Flame retardancy The flame retardant properties of the treated fabrics have been evaluated in terms of flammability and combustion behaviour by vertical flame tests and cone calorimetry, respectively. As far as flammability is concerned, pure cotton and the solegel treated samples have been ignited directly by a methane flame, measuring total burning time, total burning rate and the final residue. The collected data are summarised in Table 6. In the O_D sample, the coating is able to act as a flame retardant system for cotton, reducing the total burning time and increasing the final residue in a remarkable way, in spite of a significant increase of the burning rate. A similar trend has been observed when cotton is treated only with bisphosphonate (O_P), as already observed in TGA experiments. Both O_D and 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

Table 6 Flammability data.

COT O_D O_P O_M O_U D_P D_M D_U SEa (D_P) SEa (D_M) SEa (D_U) a

Total burning time [s]

Burning rate [mm/s]

Residue [%]

36 24 23 25 36 38 15 25 0.58 0.70 0.33

0.42 0.62 0.65 0.60 0.42 0.40 1.00 0.60 0.66 0.98 0.26

e 25 10 2.0 2.0 40 24 22 1.1 0.89 0.81

Synergistic effectiveness.

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Fig. 8. ATR spectra of O_D and O_P samples.

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Fig. 9. SEM micrographs of the residues left by D_P (A), D_M (B) and D_U (C) with the corresponding elemental analysis of the latter (D) after the flammability tests.

Table 7 Cone calorimetry data. Sample

TTI [s]

THR [MJ/m2]

pkHRR [kW/m2]

FO [s]

TSR [m2/m2]

pkRSR [1/s]

Residue [%]

COT O_D O_P O_M O_U D_P D_M D_U SEa (D_P) SEa (D_M) SEa (D_U)

36 20 18 27 18 14 12 12 0.65 0.96 0.71

3.8 1.8 3.3 3.4 3.2 3.0 2.4 2.5 0.32 0.58 0.50

128 75 106 133 125 50 95 65 1.04 0.69 1.13

100 100 130 140 86 140 140 140 2.29 1.33 1.07

38.6 9.4 4.0 10.3 17.3 5.7 0.8 3.0 0.52 0.66 0.70

2.8 0.9 0.9 0.8 1.2 0.4 0.4 0.7 0.63 0.62 0.60

<1 7 6 2 2 9 7 8 0.7 0.9 1.0

a

Synergistic effectiveness.

Fig. 10. HRR curves of pure cotton, O_D, O_P and D_P samples.

presence of polynuclear aromatic structures (skeletal vibration of an aromatic molecule e C]C stretching e and vibration of CH groups e out of plane bending e, respectively). Therefore, both the systems exploit the same flame retardant mechanism (in condensed phase), but in a synergistic way. However, this synergism is not able to reduce the total burning time and rate (SE < 1 for both these parameters). Referring to D_M and D_U samples, a simple additive effect occurs, since 0 < SE < 1. Therefore, in these systems, no PeN synergism has been observed. However, all the three doped systems leave a consistent and coherent residue, the morphology of which has been studied by scanning electron microscopy. Fig. 9 shows three typical SEM micrographs of the residues left by D_P (Fig. 9A), D_M (Fig. 9B) and D_U (Fig. 9C). It is noteworthy that the fibres are still compact in their structure and well covered by the coating. As an example, the

Fig. 11. HRR curves of pure cotton, O_D, O_M and D_M samples.

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Fig. 12. HRR curves of pure cotton, O_D, O_U and D_U samples.

presence of the coating constituents is confirmed by the elemental analysis of D_U depicted in Fig. 9D. By the semi-quantitative analysis carried out by EDS (Table 3, 2nd group of data), it is possible to correlate the best performances of D_P system and the synergism between the two components with the P amounts found in the residue; conversely, within the experimental error, in D_M and D_U residues, the Si and O amounts are practically unchanged with respect to D_P, while a very high C amount has been found.

587

This result can be ascribed to the presence of aliphatic hydrocarbon species that further fuel the cotton combustion and thus worsen the performances of such treatments, as compared to D_P. As far as the combustion behaviour is concerned, the resistance of these coatings to an irradiating heat flow has been assessed by cone calorimetry. The collected data are listed in Table 7. 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 D_P, D_M or D_U samples (SE < 1). On the contrary, unlike urea, bisphosphonate and melamine are able to postpone the FO (SE values > 1 for D_P and D_M). These results are expected as the systems under study are active flame retardants in the condensed and not in the gas phase. Referring to D_P sample, THR and pkHRR (Fig. 10) as well as TSR and RSR are much lower than those of pure cotton, O_D and O_P. The combustion kinetics turned out to be affected by the synergistic action of D with the bisphosphonate, as observable by the SE values collected in Table 7. On the contrary, the smoke parameters (TSR and RSR) do not show any synergistic effect (0 < SE < 1). As far as the doping with melamine (D_M, Fig. 11) or urea (D_U, Fig. 12) is concerned, a remarkable decrease of THR, pkHRR, TSR and RSR has been observed. However, on the basis of the calculated SE values, only a simply additive effect occurs between the two components. Once again, the combination of P and N does not give any dramatic amelioration of the flame retardancy level for the latter systems. At the end of the tests, the systems doped with P, M or U still leave a significant final residue, as depicted in Fig. 13 and reported in Table 3 (last set of data). SEM analysis shows that the residue

Fig. 13. Residues of pure cotton and of the solegel treated fabrics.

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Fig. 14. SEM micrographs of the residues left by D_P (A), D_M (B) and D_U (C) after cone calorimetry tests.

substantially consists of hollow fibres made of Si, O and P elements, as shown in Fig. 14 and reported in Table 3. The amount of C is almost negligible. As already observed for the flammability tests, the most coherent and consistent residue has been found for D_P, which exhibits the highest P content. On the basis of the obtained flammability results, the flame retardant effects exhibited by the combination of hybrid phosphorus-doped silica structures with bisphosphonate, melamine or urea seem to be lower with respect to conventional PeNcontaining flame retardants, currently applied to cotton at concentrations similar to those used in this work (Table 3) [1,3]. Indeed, the solegel process yields an external fibre coating with a very limited fibre penetration: this limits the diffusion of the flame retardants from the outside to the inside of the fibres. Therefore, it seems that the overall flammability of the solegel treated fibres depends on the competition between the rate at which the PeN-containing species in the coating diffuse into the fibres and the rates of char and volatilisation formation. Since diffusion is limited, the formation of char and volatile products seems to be favoured. In addition, since the presence of melamine or urea does not substantially contribute to the formation of char, as assessed by the residues collected in Table 6 (24, 22 vs. 25% for D_M, D_U and O_D, respectively), the levoglucosan formation is the preferred mechanism. 4. Conclusions In the present paper, we have investigated 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 solegel processes. 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 occurs when the hybrid phosphorus-

doped silica coatings are 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, meanwhile the hybridphosphorus silica coating, acting as a thermal insulator, further helps in the formation of an aromatic char resistant to the flame propagation. From an overall point of view, the solegel derived coatings doped with melamine or urea seem to show lower flammability performances with respect to conventional PeNcontaining flame retardants. This could be ascribed to the limited diffusion of P and N species from the outside to the inside of the fibres, because of the presence of the silica coating. As a consequence, the presence of melamine or urea does not substantially contribute to the formation of char, thus indicating that levoglucosan formation is favoured. Acknowledgements The authors would like to thank Mr. Fabio Cuttica and Mr. Alessandro Di Blasio for the technical support in the flammability and combustion tests. European COST Action MP1105 “Sustainable flame retardancy for textiles and related materials based on nanoparticles substituting conventional chemicals e FLARETEX e is also gratefully acknowledged. References [1] Horrocks AR. Flame retardant challenges for textiles and fibres: new chemistry versus innovatory solutions. Polym Degrad Stab 2011;96(3):377e92. [2] Weil ED, Levchik SV. Flame retardants in commercial use or development for textiles. J Fire Sci 2008;26(3):243e81. [3] Horrocks AR. An introduction to the burning behaviour of cellulosic fibres. J Soc Dyes Col 1983;99(7e8):191e7. [4] Alongi J, Camino G, Malucelli G. Heating rate effect on char yield from cotton, poly(ethylene terephthalate) and blend fabrics. Carbohydr Polym 2013;92: 1327e34.

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