Santanu Basak1, Ankur Shukla2
1ICAR-National Institute of Natural Fibre Engineering and Technology, 12 Regent Park, Kolkata: 700040
2Department of Textile and Fibre Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi: 110016
Corresponding author email: [email protected]
Among the various functional finishing of textile substrates, the flame retardant finishing is important as it is directly related to human health and hazards. Cellulosic, ligno-cellulosic fibres such as cotton, flax, ramie, jute are mostly used in apparels and home-furnishings. Besides their apparel and home-furnishing applications, natural fibres are also used in hotels, hospitals, automobiles, railways and airways as tapestries and upholstery. For apparel and home textiles, mostly cotton is preferred due to its advantages of soft feel, good moisture regain and adequate thermal insulation. But being cellulosic in nature with low limiting oxygen index (LOI) of 18, cotton catches flame readily and burns vigorously in an open atmosphere which is difficult to extinguish, and also sometimes causes accidental death. Though cotton and jute cellulose have pyrolysis temperatures of 350 ºC, during real time burning the temperature can go as high as 400-450 °C and 400°C respectively, producing hardly any residual mass. It may be noted that textiles with an LOI of ≤21 catches flame readily and burns in an open atmosphere rapidly. The samples with an LOI of ≥ 21 to ≤26 also catch flame, however, burn slowly in an open atmosphere. On the other hand, the sample with an LOI of ≥26 is generally considered to be a flame retardant textile. The situation is slightly better with the ligno-cellulosic textiles, such as jute with the LOI of 21 or more, making them suitable for packaging of agricultural crops and food products, and upholstery and home furnishing applications.
The flame retardant finishing of textiles can be categorized as non-durable, semi-durable, and durable based on their efficacy in performance after successive washing cycles. The simple and well-known non-durable flame retardant chemicals available in the market are inorganic salts, borax and boric acid mixture, di-ammonium phosphate and urea [1-2]. There have been some works where these properties have been utilised for making flame retardant textile structures also. The flame retardant property of some synthetic fibres and chemicals can be combined together to get very effective results.
Mechanism of imparting flame retardant finishing to textiles
When a cellulosic material or fabric is heated, its temperature has been raised and at a certain point it reached pyrolysis temperature i.e., 350°C. At this particular temperature lots of volatile flammable gases like levoglucosan, pyroglucosan etc. are released. These volatile flammable gases in presence of heat and oxygen help to burn the material continuously.Hence, for formulating an effective flame retardant chemical, the points to be taken care off are either to decrease the pyrolysis temperature of the fibre and formation of char and non-flammable gases and/or to decrease the formation of flammable gases and masking the availability of oxygen. A few approaches followed for the purpose are described below.
- Apply a suitable chemical that will form a glassy / foamy insulating layer on the textile substrates during combustion to restrict the flow of heat and/or oxygen
- Use those chemicals that thermally decompose through a strong endothermic reaction to ensure less heat production.
- Altering the pyrolysis temperature of the polymer to ensure formation of more char and less flammable volatile gases.
- Dilute or replace the surrounding oxygen in the burning microclimate with inert or non-flammable gases to reduce or stop combustion.
Apply a suitable chemical that will form a glassy / foamy insulating layer on the textile substrates during combustion to restrict the flow of heat and/or oxygen Use those chemicals that thermally decompose through a strong endothermic reaction to ensure less heat production. Altering the pyrolysis temperature of the polymer to ensure formation of more char and less flammable volatile gases. Dilute or replace the surrounding oxygen in the burning microclimate with inert or non-flammable gases to reduce or stop combustion.
Flame retardancy to cellulosic textiles
Nitrogen and phosphorous based chemicals
Di-ammonium phosphate along with urea and ammonium salts of phosphoric acid also has been used as a common fire retardant for cellulosic substrates. They act in the condensed phase mechanism. When a nitrogen and phosphorous-based chemical is applied for improving the thermal stability of the cellulosic substrate, it reduces the pyrolysis temperature and promotes production of more residual char and less-flammable volatiles. This is happened due to the fact that phosphorous containing flame-retardants produce phosphoric acid at an elevated temperature, which then crosslinks with the hydroxyl group of the cellulose, thus causing an early dehydration. In such type of formulations, nitrogen acts in synergism with phosphorous and depict a better efficiency. The interaction between phosphorous and nitrogen also alters the thermal decomposition pathway of cellulose. Possibly, it delays the thermal decomposition of cellulose by de-polymerization. As a result, the limiting oxygen index (LOI) of the cellulosic fabric is enhanced through the accelerated dehydration on heating. In this regard, the synergistic effect of trimethyl melamine (TMM) and dimethyl dihydroxy ethylene urea (DMDHEU) as a nitrogen provider with organo-phosphorous compound has been reported in the literature [3-4]. The TMM serves as a better nitrogen donor compared to DMDHEU, depending on the measurement of improvement in the LOI after the application. A group of researchers reported that probably the organic nitrogen is helping in controlling the pH during the crosslinking reaction of phosphoric acid. Here, nitrogen gets protonated, thus reducing the amount of available acid required for crosslinking. It also might have been converted into phosphorous acid amide that can catalyse the dehydration and carbonisation of cellulose. The water insoluble ammonium polyphosphate is also known as an effective flame retardant, when it is applied to binder systems in the coating. However, the majority of the above-mentioned formulations are not durable to washing.
The most successful durable flame retardant for cellulosic textiles is based on phosphorous and nitrogen containing chemicals that can directly react or crosslinks with the fibre. Tetrakis hydroxyl methyl phosphonium chloride (THPC) is one of those compounds, which in mixture with urea forms an insoluble structure on the cellulose. The process is commonly known as ‘Proban’ process. THPC is made of phosphine, formaldehyde, and hydrochloric acid. It can be applied to the cotton fabric with urea by pad-dry-cure method. Approximately, 25% of THPC along with 15% urea formulation was found to yield 4-5% phosphorous add-on in the cotton fabric. However, as discussed above, the treatment of cotton textile with such chemicals increases the stiffness of the sample with a simultaneous decrease in tensile and tear strengths. Further, it releases formaldehyde, a carcinogenic chemical during the processing. To overcome this drawback, THPC urea treated fabric was dried at 15% moisture content and then, exposed to the ammonia vapour in an enclosed chamber, followed by oxidation with hydrogen peroxide. The finish achieved by this method resulted in imparting a good fire retardant to the textiles with better retention of fabric physic-mechanical properties. However, the process is sensitive to dye molecules and the formulation affects different dye classes, such as Reactive, Direct, and Acid. Therefore, it is recommended that in the fire retardant finishing of cellulosic textile with THPC-urea, vat dyes should be used. Consequently, in the last five decades, flame-retardants based on the composition of phosphorous, nitrogen and halogen like Tetrakis phosphonium salt and N-alkyl phosphopropionamide derivatives have been widely used for commercial application [5-6]. In this regard, N-methylol dimethylphophopropionamide (Pyrovatex CP) in combination with trimethylol melamine and phosphoric acid is applied to the cotton fabric by pad-dry-cure process with an add-on of around 20-25%. After the treatment, a wash with alkali is given to remove the residual phosphoric acid from the treated fabric surface. Unlike in the earlier Proban process, the main advantages of this process are durability to 50-60 wash cycles and low-release of formaldehyde (below 40 ppm). It may be noted that melamine formaldehyde has the dual functions, viz. as a resin to crosslink Pyrovatex to cellulose and as an additional nitrogen donor to accelerate the dehydration of cellulose. The restriction on choice of dye classes is less in this process compared to the Proban process and the treated fabric remained comparatively softer. Presently, most of the textile industries use N-methylol dimethylphophopropionamide (Pyrovatex) with melamine resin for fire retardant finishing of cotton textile with the following recipe [7-8].
The fabric is padded with the above recipe, followed by drying at 100 °C for 5 min and curing at 150 °C for another 5 min, then washing in alkali water for the neutralisation. Though the process is quite advanced and popular in textile industry, but it suffers from higher cost of application and unpleasant odour of formaldehyde emitted during the processing. Besides, the chemicals cited above are hazardous, non-eco-friendly, and non-biodegradable. Due to the release of significant amounts of formaldehyde, the finishing process poses hazard to industrial workers, as it causes mucous and breathing problems. Considering the disadvantages, the Pyrovatex finished flame retardant textile is not recommended home furnishings application.
Flame retardant finishing using natural resource
Flame retardant finishing using natural resource is very utmost area of research for the modern researchers. In this regard, there is a need for development of sustainable green flame retardant chemicals preferably, produced from the renewable sources that will be easy to apply, cost-effective, environmental friendly, limited to no adverse effect on fabric mechanical properties, versatile to cellulosic, ligno-cellulosic, and protein textiles, and semi-durable to durable. In the recent years, a few researches have been reported on flame retardant finishing of cellulosic cotton textiles using bio-macromolecules, such as DNA from herring sperm, and salmon fish. It has been reported that the DNA consists of phosphate, carbonaceous deoxyribose units of polysaccharide dehydrate and some essential amino acids, which are responsible in the formation of more carbonaceous char and ammonia release to enhance the thermal stability of the cotton fabric [9-10]. Efforts were also directed to make cotton fabric flame retardant with whey proteins, casein, and hydrophobins, as these are rich in phosphate, disulphide and protein that can influence the pyrolysis by an early char formation .
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- Basak S, Patil PG, Shaikh AJ, Samanta AK (2016a) Green coconut shell extract and boric acid: new formulation for making thermally stable cellulosic paper. J Chem Technol Biotechnol 91: 2871-288
- Basak S, Ali S W (2016b) Sustainable fire retardancy of textiles using bio-macromolecules. Polym Degrad Stabil 133: 47-64.
- Basak S, Samanta KK, Chattopadhyay SK, Narkar R (2015) Thermally stable cellulosic paper made using banana pseudostem sap, a wasted by product. Cellulose 22: 2767-2776.