INTRODUCTION:

Cotton is important textile fibre widely used in many applications such as home furnishing, sleepwear to work wear, military tents, interior fabrics in aircraft and automobile but cotton fabric burns easily when heated at high temperature and ignited by flame, and thus causes the fire hazards. It is observed that the complete protection against fire is impossible. But flame retardants can inhibit burning tendency of materials, thus allowing sufficient time for safety measures to be taken. Therefore, researchers are endeavouring to develop flame retarded materials to be used in isolated places for public safety.

Phosphorous based flame retardants are considered environmental friendly and are used as additive and reactive flame retardants to protect buildings, electronic equipment and textiles from fire hazards¹. Flame retardant efficiency depends on the phosphorous content present in the modified material². The surfactants play the key role in case of coating of flame retardant on the substrate polymer depending upon their nature as well as of substrate.

In this paper, two types of surfactants (cationic and anionic) were used for coating a polymeric film on cotton fabric by carrying out admicellar polymerization of phosphorous based monomer. The type of surfactant can also influence the extent of coating and polymer formation, and hence the amount of phosphorous content on coated fabric. Admicellar polymerization^3,4,5 is a novel technique that can be used to form thin polymeric film on cotton fabric surface with the assistance of surfactant. It consists of four steps: (i) admicelle formation (ii) monomer partitioning (iii) in-situ polymerization and (iv) washing. In this study, phosphoric acid 2 hydroxy ethyl methacrylate ester (PHEM) monomer has been polymerized using azobisisobutyronitrile initiator on the surface of cotton fabric. The thermal and burning behaviour of cotton fabric samples were studied using thermogravimetry (TG) and flammability tests, respectively.

EXPERIMENTAL:

Materials:

Phosphoric acid 2 hydroxy ethyl methacrylate ester (PHEM), dodecylbenzenesulphonic acid sodium salt (DBSA), cetylpyridinium chloride (CPC) and azobisisobutyronitrile (AIBN) were purchased from Sigma Aldrich (India). All these materials were used as received without further purification. Simple woven cotton fabric (110 g/m²) was purchased from Amartex Industries Limited (Panchkula, Haryana, India). Its specifications are warp count-40/1Ne, weft count-40/1Ne, GSM-110, ends per cm-34.64 and picks per cm-29.92.

Determination of adsorption isotherm:

The adsorption isotherm of (i) DBSA surfactant at pH 4 and (ii) CPC surfactant was obtained by exposing a 20 cm × 5 cm piece of cotton fabric to 30 ml of the surfactant solution of known initial concentration. The adsorption was allowed to take place at 75 ⁰C for 8 h in 36 ml vial. The amount of surfactant in supernatant was measured by a UV-VIS spectrophotometer (Varian cary 5000). The surfactant adsorption isotherm was obtained by plotting of the amount of adsorbed surfactant versus supernatant concentration.

Admicellar polymerization process:

Phosphoric acid 2 hydroxy ethyl methacrylate ester (PHEM) monomer was polymerized on cotton fabric using AIBN initiator for each of the two surfactants. A piece of cotton fabric (20cm × 5 cm) was placed in a vial containing the 1.5 ml surfactant. The surfactants concentration used were 0.55 mM and 0.60 mM for DBSA and CPC, respectively. The vial was then wrapped with aluminium foil and then lid was screwed on and sealed with paraffin film. Both the vials were placed into a water shaker thermostat for 8 h at 75 ⁰C to allow adsorption of surfactants. Then to initiate the polymerization reaction, PHEM monomer (100 mM) and 3 ml of 0.01M AIBN as initiator dissolved in isopropyl alcohol was added in both the above vials and shaking was continued for 8 h at 75 ⁰C. After polymerization, the vials were removed and cooled down to room temperature. Then fabric samples were removed from the vial and dried first in air and then in an oven at 60 ⁰C overnight.

Table1: Description of samples

Sample name	Description of sample
CF	Pure cotton fabric
CCF-A	Coated cotton fabric with 100 mM monomer, DBSA surfactant and initiator
CCF-C	Coated cotton fabric with 100 mM monomer, CPC surfactant and initiator
CCF-AL	After first home laundering of CCF-A
CCF-CL	After first home laundering of CCF-C

Determination of phosphorous content in coated cotton fabric:

Coated cotton fabric (0.1 g) was digested with 2 ml of concentrated sulphuric acid and then 10 ml of 30 % H₂O₂ was added dropwise to the mixture and stirred continuously on hot plate for 1h. Then the mixture was transferred to a 50 ml volumetric flask after complete digestion and diluted with distilled water. The prepared samples were analysed by UV-VIS spectrophotometer (Varian cary 5000) by stannous chloride method for phosphate determination⁶. The phosphorous content was calculated from phosphate concentration.

Techniques used:

FTIR spectra of samples were recorded using Shimadzu IR affinity-I 8000 FTIR spectrometer in the range 4000-400 cm^-1for 15 scans with a resolution of 4 cm^-1 to characterize the surface of pure cotton fabric (CF), coated cotton fabric using DBSA (CCF-A) and coated cotton fabric using CPC (CCF-C).

TG thermograms of samples were obtained using TA Instruments DSCQ10 (Differential Calorimeter Thermal Analyzer) by taking about 10 mg of samples in alumina crucibles under nitrogen flow of 100 ml min^-1from ambient temperature to 700 ⁰C at a heating rate of 10 ⁰C min^-1.

The burning behaviour of cotton fabric samples were studied using the Paramount 45⁰ Automatic Flammability Tester⁷ (An ISO 9001:2008 group) according to ASTM D1230 (Standard Test Method for Flammability of Apparel Textiles). In this study, the ignition time was set at 1 second. This test measures the time required for flame to propagate through the length of the fabric and ease of ignition. The specimen (15 cm × 5 cm) was then exposed to flame and then burning time and burning characteristics are recorded. The burning behaviour of fabric was recorded by a digital camera.

Limiting oxygen index (LOI) analysis of samples was performed using Limiting Oxygen Indexer (ISO 4859-2 as per ASTM D 2863). The cotton specimen (15 cm × 5 cm) was ignited at the top. The oxygen concentration was adjusted until the specimen just support combustion. The oxygen concentration is reported as volume percent.

RESULTS AND DISCUSSION:

Adsorption isotherms of DBSA and CPC surfactants on cotton fabric:

The adsorption isotherms of (i) DBSA surfactant⁵ and (ii) CPC surfactant⁸ are shown in Fig.1. The amount of surfactant adsorption reached a constant value at 0.55 mM of equilibrium surfactant concentration (considered as critical micelle concentration-CMC) in case of DBSA surfactant and 0.60 mM in case of CPC. In the admicellar polymerization, the concentration of surfactant should be just below or near its CMC to ensure significant coverage and to avoid the emulsion polymerization⁹.

Therefore, from the adsorption isotherm, a concentration of 0.55 mM DBSA and 0.60 mM CPC which is near the CMC was chosen for the admicellar polymerization reaction. Figure 1 shows that high adsorption takes place in case of cationic surfactant (CPC) as compared to anionic surfactant (DBSA) in same conditions. This is because of negatively charged cotton surfaces in aqueous medium.

Figure 1: Adsorption isotherm on cotton fabric of CPC and DBSA

Identification of polymeric film on the coated cotton:

FTIR spectra of pure and coated cotton fabrics are shown in Figure 2. The additional peaks at 1720 cm^-1(C=O str.), 1260 cm^-1(P=O str.) and 1076 cm^-1(P ̶̶ O ̶̶ C str.) are seen in coated cotton fabric CCF-A and CCF-C indicating that a layer of phosphorus based polymer is formed on both of the samples.

Figure 2: FTIR spectra of untreated CF, CCF-A and CCF-C cotton fabric samples

Effect of surfactants:

In admicellar polymerization process, the type of surfactant influences the extent of polymer formation on the substrate. Cotton generally acquires a negative surface charge when immersed in water. As result of this, the cationic surfactant adsorbed preferably more on the cotton surface in comparison to the use of anionic surfactant^10,11. More the adsorption of the surfactant, higher is the amount of polymer formed on cotton surface and higher is the amount of phosphorous content on the coated fabric. The higher is the amount of phosphorous content on coated cotton fabric, higher is its flame retardant efficiency.

Thermal study:

Figure 3 shows TG curves of CF, CCF-A and CCF-C. For CF, there is only one stage of thermal degradation in temperature range 100-600°C with 4% residue remaining at 600°C in nitrogen atmosphere. On heating cotton fabric, the weight loss occurs due to dehydration and decomposition process leading to the formation of major volatile product mainly levoglucosan. The coated cotton samples (CCF-A and CCF-C), show two stages of thermal degradation. The first stage of degradation occurred due to catalyzed dehydration, phosphorylation and dephosphorylation of cotton. In case of coated samples, the second stage is the major stage in which most of the sample degraded due to decomposition and pyrolysis leaving higher char yields. The DTG decomposition peak shifted to lower temperatures in case of CCF-A and CCF-C due to acid catalysed dehydration by phosphoric acid released from decomposition of phosphorous containing flame retardant. Table 2 shows that the char yield of CCF-C (26%) is more than CCF-A (20%) indicating the high efficacy of cationic surfactant in comparison to anionic surfactant.

Figure 3: TG curves of CF, CCF-A and CCF-C cotton fabric samples in nitrogen atmosphere

Flammability behavior:

Auto flammability test:

The auto flammability test of CF, CCF-A, and CCF-C samples was carried out at the same ignition time of 1 sec and their images are shown in Figure 4.For CF, after removing the ignition source, the flame spreads quickly and burned the entire fabric with no residue within 12 sec. In case of CCF-C, the flame spreads slowly leaving a spot of char of length of 2.8 cm. For CCF-A, the char length increased to 9.1 cm. This indicates that the use of cationic surfactant (CPC) gives improved flame retardant behaviour as compared to that of anionic surfactant (DBSA).Table 3 also shows that even after first home laundering the PHEM-coated cotton fabric with the use of cationic surfactant retains the flame retardant property.

(a) CCF-A (b) CCF-C (c) CCF-CL

Figure 4: Burning behaviour of (a) CCF-A, (b) CCF-C and (c) CCF-CL cotton fabric samples after auto flammability test

Limiting oxygen index (LOI):

LOI value for CF is 18.0 %, which is increased to 22 % for CCF-A and 25 % for CCF-C(Table-3). LOI test also indicates the use of cationic surfactant (CPC) gives improved flame retardant behaviour as compared to that of anionic surfactant (DBSA).

CONCLUSION:

In this research work, a cationic surfactant was found more effective for adsorption and admicellar polymerization process in comparison to anionic surfactant as cotton generally acquires a negative surface charge when immersed in water The polymeric film on cotton fabric using cationic surfactant (CPC) has higher content of phosphorous than that of using anionic surfactant(DBSA). The decomposition temperature of the coated cotton is decreased with the increase of phosphorus content, but increased the char yield at the expense of combustion products. The use of cationic surfactant gives improved flame retardant behaviour as compared to that of anionic surfactant (DBSA).

Table 2: TG data of pure and coated cotton fabrics

Samples	Stages	Temp.range (°C)	Weight loss (%)	DTG (°C)	T_10wt%	T_50wt%	Char at 600°C (%)
CF	1^st	100-600	87.0	355	267	350	4.0
CCF-A	1^st 2^nd	100-280 280-600	17.5 61.5	230 400	248	408	20.0
CCF-C	1^st 2^nd	100-275 275-600	16.8 55.2	225 360	238	380	26.0

Table 3: Flammability test and phosphorous content data for cotton fabric samples

Sample name	Flame spread time (sec)	Char length (cm)	Pass/Fail	LOI(%)	Phosphorous content (mg/g cotton)
CF	12	BL	Fail	18.0	-
CCF-A	NI	9.1	Pass	22	4.5
CCF-C	NI	2.8	Pass	25	6.8
CCF-AL	52	BL	Fail	20	3.7
CCF-CL	NI	12.2	Pass	24	5.6

NI = not ignite, BL = burn entire length

ACKNOWLEDGEMENT:

One of the authors (Shubha) is grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi for the award of Senior Research Fellowship.

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Received on 08.08.2017 Modified on 20.09.2017

Asian J. Research Chem. 2017;10(5): 680-684.

DOI: 10.5958/0974-4150.2017.00115.8