Article pubs.acs.org/IECR
Flame Retardancy of Polyester and Polyester−Cotton Blends Treated with Caseins Federico Carosio, Alessandro Di Blasio, Fabio Cuttica, Jenny Alongi,* and Giulio Malucelli Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino and Local INSTM Unit, Viale Teresa Michel 5, 15121 Alessandria, Italy ABSTRACT: For the first time, polyester and polyester−cotton fabrics have been treated with an aqueous suspension of caseins to increase their thermal stability and flame retardancy. The effectiveness of the fabric treatment as well as the morphology of the deposited coatings have been assessed by infrared spectroscopy and scanning electron microscopy, respectively. The thermal stability of the treated fabrics in nitrogen and air, as well as their resistance to a flame application and to an irradiative heat flux of 35 kW/m2, has proven to be strongly affected by caseins. Indeed, in the case of polyester, a remarkable decrease of the burning rate (−70%) and a significant increase of its limiting oxygen index (from 21 to 26%) has been reached. In the case of polyester− cotton blends, caseins turned out to slow down the fabric burning rate (about −40%) and its resistance to an irradiative flux as assessed by cone calorimetry.
1. INTRODUCTION The continuous search for novel green flame retardants in the textile field has recently encouraged academic research toward the use of alternative and more eco-friendly molecules.1,2 More specifically, current attention is being focused on the production of effective halogen-free formulations for coatings and back-coated textiles, and on the replacement of Proban (i.e., (hydroxylmethyl)phosphonium salts) because of the formaldehyde release, for cellulosic textiles.2 On the other hand, as far as synthetic fabrics like polyester are concerned, one of the main goals is still to develop new halogen-free flame retardants capable of stimulating char formation and stopping undesired dripping. Indeed, this latter result is one of the most important issues (as the formation of incandescent melt droplets can easily spread the fire to other ignitable materials) on which both industrial and academic efforts are currently focused.3 In this scenario, the efficiency of biomacromolecules like proteins4 and nucleic acids5−7 as novel green flame retardant systems for cellulosic substrates has been recently investigated and demonstrated. Surprising results have been found: indeed, it has been possible to achieve the selfextinguishment of cotton by impregnating the fabric with deoxyribose nucleic acid (DNA),5,6 even exploiting the building up of a layer by layer assembly of this biomacromolecule with chitosan.7 Very recently, some preliminary results have clearly indicated that caseins can be exploited as new green flame retardants for cotton fabrics because of their high phosphorus content.8 More specifically, caseins were shown to be capable of favoring the dehydration of cellulose toward the formation of char, as opposed to the depolymerization, which is responsible for the further production of combustible volatile species. Hence, a higher flame resistance of cotton has been achieved, as indicated by the significant decrease of total burning rate as well as by the formation of a coherent residue found at the end of the flammability tests. In addition, the presence of caseins has also modified the resistance to a heat flux of 35 kW/m2 with a significant reduction of the peak heat release rate (−30%). © 2014 American Chemical Society
As previously mentioned, the efficiency of caseins can be ascribed to their high phosphorus content: indeed, casein is made up of many components, among which αs1-casein, αs2casein, β-casein, and κ-casein are prevalent. In particular, αscaseins are the major casein proteins and contain 8−10 serylphosphate groups, while β-casein contains about 5 phosphoserine residues.9,10 When their behavior is compared with that of ammonium polyphosphate (APP), a common flame retardant currently employed in intumescent formulations for plastics and textiles,2,3 several similitudes have been found.8 Given the results of this research, in the present work, the thermal and flame retardant properties of caseins have been thoroughly investigated on polyester and polyester−cotton fabrics that have been treated with an aqueous suspension of these proteins. To this aim, thermogravimetric analyses, flammability tests (in horizontal configuration), limiting oxygen index measurements, and combustion experiments through cone calorimetry have been carried out. To evaluate the effect of cotton on the behavior of polyester−cotton blends, the same tests have been also performed on neat cotton fabrics treated with caseins.
2. EXPERIMENTAL SECTION 2.1. Materials. Cotton (COT, 200 g/m2), polyester (PET, 175 g/m2), and a blend consisting of 65% PET and 35% COT (PET−COT, 245 g/m2) were purchased from Fratelli Ballesio S.r.l. (Torino, Italy). Casein powder (C, reagent grade) was purchased from Sigma-Aldrich (Italy); its approximate composition (in grams per liter, as stated in the product information sheet) is as follows: 12−15 αs1, 3−4 αs2, 9−11 β, and 2−4 κ. Received: Revised: Accepted: Published: 3917
December 3, 2013 February 10, 2014 February 18, 2014 February 18, 2014 dx.doi.org/10.1021/ie404089t | Ind. Eng. Chem. Res. 2014, 53, 3917−3923
Industrial & Engineering Chemistry Research
Article
3. RESULTS AND DISCUSSION 3.1. ATR Spectroscopy Measurements. ATR spectroscopy has been employed in order to verify the occurred deposition of caseins on COT, PET, and PET−COT fabrics. As shown in Figure 1, caseins ATR spectra are characterized by
Hereafter, the fabrics treated with caseins will be coded as COT_C, PET_C and PET−COT_C, respectively. 2.2. Coating of Cotton Fabrics with Caseins. The caseins powder (5 wt %) was dispersed in distilled water under mechanical stirring (300 rpm). Then the suspension was heated at 80 °C in a thermostatic bath, and the pH was adjusted to 10 using 1 M NaOH solution. Cotton fabrics were dipped in the casein suspension for 5 min, squeezed with a lab padder (Atlas Electric Devices Co., Chicago), and dried to constant weight in an oven at 80 °C. The total dry solids add-on on the fabrics (A, wt %) was determined by weighing each sample before (Wi) and after the impregnation with the suspension and the subsequent drying (Wf) using a Sartorius balance (±10−4g); the following equation was then applied: A=
Wf − Wi 100 Wi
(1)
The final total dry solid add-on on COT_C, PET_C, and PET−COT_C was 20 wt %. 2.3. Characterization Techniques. The chemical structure of the untreated and treated fabrics was assessed by attenuated total reflectance (ATR) spectroscopy. ATR spectra were recorded at room temperature in the 4000−600 cm−1 range (32 scans and 4 cm−1 resolution), using a Perkin-Elmer Frontier FT-IR/FIR spectrophotometer equipped with a diamond crystal (1.66 μm penetration depth as stated by the manufacturer). The surface morphology of the treated samples was studied using a LEO-1450VP scanning electron microscope (SEM; 5kV beam voltage); an X-ray probe (INCA Energy Oxford, Cu Kα X-ray source, k = 1.540562 Å) was used to perform elemental analysis (EDS). Fabric pieces (5 × 5 mm2) were cut and fixed to conductive adhesive tapes and gold-metallized. The thermal and thermo-oxidative stabilities were evaluated by thermogravimetric (TG) analyses in nitrogen and in air, respectively, from 50 to 800 °C, with a heating rate of 10 °C/ min. A TAQ500 analyzer was used, placing the samples (ca. 10 mg) in open alumina pans in inert or oxidative atmosphere (gas flow, 60 mL/min). The collected data were Tonset10% (temperature at 10% of weight loss), Tmax (temperature at maximum rate of weight loss), and residue at 600 °C. The resistance to a flame application of untreated and caseintreated fabrics was tested by flammability tests in horizontal configuration, applying a 25 mm methane flame for 3s on the short side of the specimen (50 × 100 mm2). These tests were repeated 3 times for each formulation. Total burning time and rate after the flame application as well as the final residue were measured. In addition, limiting oxygen index (LOI)tests were performed with a FIRE oxygen index apparatus according to the ASTM D2863 standard. The resistance to an irradiative heat flux of both untreated and casein-treated fabrics (100 × 100 mm2) was investigated by cone calorimetry (Fire Testing Technology, FTT). The measurements were carried out under a 35 kW/m2 irradiative heat flux in horizontal configuration, following the procedure described elsewhere.11 Time to ignition (TTI, in seconds) and peak of heat release rate normalized on the specimen weight (PHRR, in kilowatts per square metter per gram) were measured. The experiments were repeated four times for each sample to ensure reproducible data; the experimental error was within 5%. Prior to combustion tests, all the specimens were conditioned at 23 ± 1 °C for 48 h at 50% relative humidity (RH) in a climatic chamber.
Figure 1. ATR-FTIR spectra of untreated and protein-treated COT, PET, and PET−COT fabrics.
two peaks at 1635 and 1510 cm−1 attributable to amide I and amide II vibrational modes, typical of proteins.12 Regardless of the used type of fabric, caseins are able to form a coating on the substrate; indeed, their typical absorption bands (indicated by two asterisks in Figure 1) are still observable in the ATR spectra of the treated fabrics. 3.2. Morphological Characterization. SEM observations coupled with EDS analysis have been used for investigating the morphology of the treated fabrics, as compared with the untreated counterparts. Figure 2 shows some typical SEM micrographs of COT (2A), COT_C (2B), PET (2C), PET_C (2D), PET−COT (2E), and PET−COT_C (2F). Regardless of the substrate type, caseins form a continuous and homogeneous coating able to interconnect the adjacent fibers (Figure 2B,D,F). Furthermore, the presence of phosphorus can be found in the coating, as assessed by EDS analysis (Figure 3). In particular, Table 1 collects weight percentage of P element for both pristine caseins and treated fabrics. The P content found for pristine caseins is within the average composition reported in the literature (from 0.7 to 1.0 wt %).13 When the biomacromolecule coating is applied to fabrics, a decrease of P percentage occurs. This finding can be ascribed to 3918
dx.doi.org/10.1021/ie404089t | Ind. Eng. Chem. Res. 2014, 53, 3917−3923
Industrial & Engineering Chemistry Research
Article
PET, and PET−COT have been assessed by thermogravimetric analysis. Table 2 lists the collected data in nitrogen and air; Figures 4 and 5 plot the TG and derivative thermogravimetry (dTG) curves. As already demonstrated,14 the thermal degradation of cotton and polyester in nitrogen proceeds by only one step, during which the maximum weight loss is registered (354 and 426 °C for COT and PET, respectively; Figure 4 and Table 2). More specifically, cellulose pyrolyzes according to two alternative pathways, which involve the decomposition of the glycosyl units to char and the depolymerization of such units to volatile species at higher temperature (i.e., levoglucosan, furan, and furan derivatives). On the other hand, polyester thermal degradation in nitrogen occurs through two competitive volatilization−charring processes (Figure 4). Although different degradation mechanisms have been proposed, the interpretation of the first degradation step (within 400−500 °C) is still open.15−19 Indeed, it is not completely clear whether the primary chain scission can be ascribed to a heterolytic cleavage or a homolytic scission of ester bonds.15,16 However, most researchers agree on the chain scission through β CH-transfer reactions. In doing so, vinyl- and carboxyl-terminated oligomers that evolve to volatile species, like carbon mono- and dioxide, methane, ethylene, benzene, benzaldehyde, formaldehyde, and acetaldehyde, are formed. These reactions occur simultaneously to the chain depolymerization because of an intramolecular backbiting.17−19 The polyester−cotton blend exhibits a thermal degradation typical of a polymer blend, consisting of two independent steps: the former is attributable to cotton (351 °C, Table 2 and Figure 4) and the latter to polyester (423 °C).20 Casein-based coatings are responsible for a strong sensitization of both cellulose and polyester decomposition, as revealed by the reduction of Tonset10% values listed in Table 2 (−47 and −85 °C for COT_C and PET_C, respectively), and clearly depicted in Figure 4; a similar trend has been also observed for PET−COT_C and attributed to the phosphate groups present on the shell of casein micelles.9,10 Indeed, at high temperature, these bonds release phosphoric acid that catalyzes cellulose or polyester degradation, thus favoring the char production. In doing so, the degradation is anticipated; however, at the same time, the formation of a thermally stable char is promoted. Indeed, caseins behave similarly to APP salts, both starting to lose weight at about 250 °C and releasing phosphoric acid able to favor the formation of a residue stable up to 570 °C. This phenomenon has been assessed for both polyester and polyester−cotton blends, although in a less predominant way, as evidenced by comparing the residues left at Tmax1 and Tmax2 in Table 2 (49.0 vs 41.0, 53.0 vs 51.0, and 75.0 vs 73% for COT_C, PET_C, and PET−COT_C, respectively). These latter are partially thermally stable up to 600 °C and give rise to a residue increase of 19% (COT_C), 8% (PET_C), and 7% (PET−COT_C) as compared to the untreated fabrics. As far as the thermo-oxidation is concerned, cotton degradation usually occurs by three steps21−23 (Figure 5). The first (at 300−400 °C) involves two competitive pathways, which yield aliphatic char and volatile products; during the second step (at 400−800 °C), some aliphatic char converts to an aromatic form, yielding carbon mono- and dioxide as a consequence of the simultaneous carbonization and char oxidation. Within the last step (at ca. 800 °C), the char is further oxidized mainly to CO and CO2. In the present work, two decomposition peaks can be observed at 339 and 478 °C
Figure 2. SEM magnifications of COT (A), COT_C (B), PET (C), PET_C (D), PET−COT (E), and PET−COT_C (F).
Figure 3. SEM magnifications and phosphorus mapping of COT_C, PET_C, and PET−COT_C.
Table 1. Phosphorus Content (Weight Percent) As Assessed by EDS Analysis for Pristine Caseins and Treated Fabrics phosphorus content (wt %) pristine caseins COT_C PET_C PET−COT_C
0.73 0.39 0.38 0.62
± ± ± ±
0.08 0.14 0.10 0.12
the X-ray beam penetration into the fabric thickness, which overestimates C and O elements, thus reducing the P content. 3.3. Thermal Properties. The thermal and thermooxidative stabilities of untreated and casein-treated COT, 3919
dx.doi.org/10.1021/ie404089t | Ind. Eng. Chem. Res. 2014, 53, 3917−3923
Industrial & Engineering Chemistry Research
Article
Table 2. Thermogravimetric Data for Untreated and Protein-Treated Fabrics in Nitrogen and Air sample
Tonset10% (°C)
Tmax1* (°C)
Tmax2* (°C)
Tmax3* (°C)
COT COT_C PET PET_C PET−COT PET−COT_C
319 272 400 315 332 304
354 337 426 397 351 334
− − − − 423 405
− − − − − −
COT COT_C PET PET_C PET−COT PET−COT_C
318 242 392 310 323 311
339 327 422 404 339 335
478 482 547 538 419 416
− − − − 508 525
residue at Tmax1* (%) nitrogen 41.0 49.0 51.0 53.0 73.0 75.0 air 48.0 51.0 47.5 50.5 79.0 82.0
Residue at Tmax2* (%)
RESIDUE at Tmax3* (%)
residue at 600 °C (%)
− − − − 37.0 42.0
− − − − − −
2.0 21.0 14.0 22.0 15.0 22.0
4.0 10.0 1.5 13.0 37.0 43.0
− − − − 7.0 9.5
