Flame retardant sisal rope: combustion properties and characterization

A new mechano-chemical formulation is developed for making flame resistant sisal fibre rope with low chemical loading. Sisal yarn is treated with a different formulation of ammonium sulfamate (AS) (50 g/L and 100 g/L) by following dip-dry approach at room temperature. Limiting oxygen index (LOI) measures the minimum amount of oxygen required in the mixture of nitrogen and oxygen for burning of the sample. Treated sisal showed LOI values of 28–32 and self-extinguishment in vertical flammability test whereas control sisal yarn showed LOI value of 21 and burnt easily within a minute. Forced combustion results revealed that AS treated sisal yarn exhibited 50% lower peak heat release rate (PHRR) than the control sisal yarn. Sisal yarn based rope was prepared by following braiding technique with three single yarns, using different combinations of untreated and treated sisal yarn. Fire retardant sheath yarn is used to cover the untreated sisal yarn present in the core by twisting or braiding. Rope made from sisal yarn has shown LOI value 30–40. Besides, a new method of simultaneous dyeing and flame retardant finishing of sisal rope is also proposed. The physical properties of the ropes were measured and it was found that the extent of strength loss is statistically insignificant at 95% confidence level. The thermal stability of the AS-treated sisal yarn is measured by thermo-gravimetric analysis. Charring behaviour of the control and AS treated sisal fibre was examined using microscopic images and scanning electron microscopy. Besides, in detail mechanism behind flame retardancy is revealed in the context with the help of XRD and FTIR analysis techniques.

Abstract A new mechano-chemical formulation is developed for making flame resistant sisal fibre rope with low chemical loading. Sisal yarn is treated with a different formulation of ammonium sulfamate (AS) (50 g/L and 100 g/L) by following dip-dry approach at room temperature. Limiting oxygen index (LOI) measures the minimum amount of oxygen required in the mixture of nitrogen and oxygen for burning of the sample. Treated sisal showed LOI values of 28-32 and self-extinguishment in vertical flammability test whereas control sisal yarn showed LOI value of 21 and burnt easily within a minute. Forced combustion results revealed that AS treated sisal yarn exhibited 50% lower peak heat release rate (PHRR) than the control sisal yarn. Sisal yarn based rope was prepared by following braiding technique with three single yarns, using different combinations of untreated and treated sisal yarn. Fire retardant sheath yarn is used to cover the untreated sisal yarn present in the core by twisting or braiding. Rope made from sisal yarn has shown LOI value 30-40. Besides, a new method of simultaneous dyeing and flame retardant finishing of sisal rope is also proposed. The physical properties of the ropes were measured and it was found that the extent of strength loss is statistically insignificant at 95% confidence level. The thermal stability of the AS-treated sisal yarn is measured by thermo-gravimetric analysis. Charring behaviour of the control and AS treated sisal fibre was examined using microscopic images and scanning electron microscopy. Besides, in detail mechanism behind flame retardancy is revealed in the context with the help of XRD and FTIR analysis techniques.

Introduction
Sisal fibre is a wood-like composite structure made of cellulose, hemicellulose and lignin polymers (Shukla et al. 2016;Mukherjee and Satyanarayana 1984). Ropes made from sisal fibre are very common in heavy-weight industries, packaging, military activities, transport purposes etc., because of their high strength and toughness (Kambli et al. 2018;Teli et al. 2017;Gazi et al. 2021). Synthetic ropes made from nylon, aramid, and polyester are most popular in the market, however, they are not bio-degradable (Catling and Grayson 1982). Nylon fibre ropes loose strength on water absorption and degrade on exposure to a higher temperature for a long duration. Polyester ropes are stronger but very stiff to handle (Khan et al. 2011). Polypropylene ropes though have high thermal expansion behaviour and light weight but have poor UV resistance. Because of these inherent shortcomings of synthetic fibre ropes and due to increasing sustainability awareness in recent days, natural fibre ropes are preferred in some application areas. Among all the natural fibres, sisal is predominant for making rope due to its high strength, salt water resistance and comparatively more toughness than other natural fibres. However, sisal fibre ropes are not thermally stable (due to cellulose in its structure) and during fire accidents different toxic flammable gases and smoke get released (Horrocks 2004;Shukla et al. 2016). The tensile properties of most of the natural fibres treated with commercial flame retardant chemicals reduce due to process conditions, pH level of treatment etc. Therefore, for successful applications of sisal ropes, thermal stability with retained tensile properties is the foremost requirement (Basak et al. 2015. Most of the commercial fire retardant chemicals are found to be effective at acidic pH when applied on cellulosic and ligno-cellulosic fibres. Moreover a higher quantity of chemicals is used to achieve the self-extinguishing effect in the ligno-cellulosic fibres. Therefore, chemical add-on on the fibre surface and the tensile properties of treated fibre are both correlated with each other. Most of the chemicals used for making self-extinguish cellulosic materials contain phosphorous and nitrogen groups. Usually these chemicals restrict the formation of flammable gases during burning and also assist char formation. Shukla et al. (2016) have registered the efficacy of pyrovatex (N,N Dimethylol phosphopropionamide) as a flame resistant component for sisal yarn. They have used melamine resin, phosphoric acid, and catalysts into the formulation for crosslinking and flame-retardant action. According to their report, the treated yarn has shown an LOI value of around 30 and strength loss was reported around 20%. Moreover, the add-on% required for self-extinguishing action was more than 30% (Shukla et al. 2016). Pyrovatex treated sisal rope strands lost their toughness and strength by more than 25% and 30% respectively, after treatment. Therefore, maintaining the tensile strength property and toughness of the sisal fibre after the fire-retardant treatment is an important challenge for the researchers. Related to this line of work, another research group has reported a new process of making flame resistant sisal fibre by using nano zinc oxide. This treatment was suitable for chemical loading and flame-resistant efficacy (Seshama et al. 2017). Nano zinc oxide is an effective flame retardant, and treated sisal fibre has shown LOI value of 27 with only 1-2% chemical loading on its surface. Nano ZnO was used by the researchers for making fire retardant forest wood. As per the reported literature nano ZnO blocks the primary hydroxyl groups of cellulose present in the wood polymer (Habibzade et al. 2016;Sun et al. 2015) and hinders the release of the flammable gases. One research report has been registered on the flame retardant efficacy of the nano ZnO on the ligno-cellulosic jute polymer (Hady 2013;Samanta et al. 2020). However, researchers did not register any report on the tensile properties and durability of the flame retardant finish. In the present study Ammonium sulfamate (AS) has been used for the treatment as it could be loaded on the sisal surface at a neutral pH condition. Moreover, AS treatment did not affect the physical properties of the textile substrate.
As far as the flame retardant cellulosic materials are concerned, high add-on and usage of a larger quantity of chemical are the biggest challenges. To this end, our research group has developed a new mechano-chemical design for making fire retardant sisal rope. Developed process makes sisal fibre made rope, having flame retardancy effect at low add-on%. At the same time, it also maintains the strength parameters. As per the report, till date no mechanical approach has been reported by the researcher for making flame retardant cellulosic material as best of our knowledge. Therefore, for filling that research gap, a new mechano-chemical formulation has been developed and reported in the current context. Besides, a new method of simultaneous dyeing and flame-retardant finishing of sisal rope has also been proposed.

Material and methods
Mechano-chemical process for making sisal rope Sisal yarn of 2000 g/1000 m (2000 tex) was prepared in the mechanical processing Division of ICAR-NIN-FET following the process line mentioned in Fig. 1. Ammonium sulfamate (AS) based formulation was used to treat sisal yarn strand. The pH of the solution was around 8 and all the dipping treatments were carried out at room temperature. The treated samples were dried at 120 °C for 4-5 min. Thereafter different combination of AS treated sisal yarn (Formulation: ammonium sulfamate and 4% BTCA with 1% sodium hypophosphite) and untreated yarn were braided into a rope like structure as shown in Fig. 2. Three sisal yarns (a combination of treated and untreated) were used as braiding unit for making a sisal rope. Ropes developed are marked as 3UT, 2UT + 1 T, 2 T + 1UT, and 3 T where (UT-untreated, T-Treated).
Chemical loading on the rope Ammonium sulfamate (AS) loading on the rope surface was estimated by measuring the initial and final oven-dry weight.
represented are the oven-dried weights of the initial (untreated) and final (treated) yarn sample, respectively.
Flammability testing LOI (Limiting oxygen index) and vertical flammability of the ropes were measured by following IS13501 and as per 1871 method A, respectively ).
Thermo-gravimetry analysis (TGA) TGA of the sisal yarn (untreated and treated) was performed to understand the pyrolysis behaviour of the sisal fibre after AS treatment in a nitrogen atmosphere. TG test was carried out in Perkin Elmer Thermal Analyzer at a heating rate of 20 °C/min ).

Char and surface morphology analysis
Scanning Electron Microscope (SEM) was used to understand the char morphology of the control sisal fibre and the AS formulation treated sisal fibres. Surface morphology of the materials were studied with the help of a high resolution (up to 3 nm) scanning electron microscope (ZEISS EVO 50) using SE detector. For experimental purposes, a sputter coater was used for the required coating of the samples and were examined under the SEM. Voltage maintained for analysis was 12 kV. Types of elements present on the rope surface were examined by Energy dispersive X-ray (EDX) analysis (Basak et al. 2014). NIKON make optical microscope (Model No: SMZ18) was Add -on % = A 2− A 1 ∕A 1 Fig. 1 Sisal fibre from plant and yarn made from sisal fibre by mechanical process used for surface morphology analysis at different magnification.
FTIR analysis FTIR analysis of the control and the treated sisal yarns was carried out by using Perkin Elmer Spectrum system over the wavelength of 4000-500 cm − 1 . ATR transmittance model was used for the analysis Basak and Ali 2016).

Tensile strength testing
The Tensile strength of the sisal yarn based ropes was evaluated following ASTM D2256 method using Tinius Olsen machine (Model: H50K). All the samples have been tested at 20 mm/min speed with a sample length of 25 mm.

Crystallinity measurement
The crystallinity values of the AS treated and untreated rope samples were measured by using X-ray diffractometer, Rigaku, Model: Smart Lab, Japan. XRD testing of the samples was performed with X-ray generator at 40 kV, 75 mA having Cu emitter and Kβ filter. Scan range was 5-90° with step width 0.02°. For testing each sample, the original XRD diffraction was measured and a blank run having only the sample holder was carried out at same setting. To get corrected diffraction pattern the blank run diffraction patterns was deducted from the original XRD patterns.
The crystallinity of the sample was measured as per Segal et al. (1959). The crystallinity index was calculated as per equation where CrI expresses the relative degree of crystallinity, I 200 denotes the maximum intensity of 200 lattice Dyeing of sisal braided structure Direct dye was used for dyeing the blended yarn at alkaline conditions (pH-9) and in the presence of sodium sulphate salt (20 g/L) for better levelling properties. Temperature and time of the dyeing cycle was 90 °C for 40 min. Ammonium sulphamate (AS) 10% (w/v) was added to that solution for the self-extinguishing property of the rope. Structure of ammonium sulfamate and direct dyes used for the process are represented in Fig. 3.

Cone calorimeter analysis
A Cone calorimeter (manufactured by DnG Technologies Pvt. Ltd., India) was used to test '100 × 100' mm 2 control and AS formulation treated sisal yarns following the procedure of ISO 5660-1 standard. Specimens were tested at horizontal orientation with a heat flux of 35 kW/m 2 generated by the cone calorimeter and to the direct application of the propane flame. Before testing, all the samples were conditioned at 65% R.H and 27 °C. Samples were in horizontal condition during the combustion process. The parameters that have been measured are total heat release (MJ/m 2 ), heat release rate (kW/m 2 ), time to ignition, the maximum average rate of heat emission (MARHE), and the peak heat release rate (PHRR).
In each case, three replicates of the samples have been tested, and the CV% values have also been represented.

Flammability test
Sisal yarn was treated with different concentrations of AS, and the combinations of treated and untreated yarn have been used for making rope by following the braiding process. Length of one repeat unit of braiding is 1 cm, and the angle of each braid was maintained 55. Detail flammability parameters of the untreated and treated yarn and rope were examined by following the standard test method. Test results are reported in the below mentioned Tables 1 and 2.
Sisal yarn and its thermal properties Sisal yarn was treated with different concentrations of ammonium sulfamate (AS). The detail flammability properties of the treated yarns are listed below. It was observed from Table 1 that the control sisal yarn showed no self-extinguishing property as the total yarn was burnt slowly with afterglow and smoke generation. Sisal fibre contains polyphenolic lignin based compounds, and lignin is resistant to thermal propagation due to its polyphenolic nature, branched chain structure and presence of -OH groups in its structure. However, smoke and secondary burning of the sisal fibre made yarn is a severe problem, and it can cause many major fire related accidents in the packaging industries, marine sectors, sisal fibre made furnitures etc. Smoke contains toxic gases like carbon monoxide, furans, volatile organic liquids, formaldehyde, etc., which are harmful to ordinary people as it lowers oxygen concentration in the surrounding atmosphere. However, AS treated sisal yarns have shown higher LOI values (28 and 32) and specific char lengths. Afterglow and smoke generation were wholly extinguished as more oxygen gas was required for combustion. Moreover, Fig. 3 Ammonium sulfamate and direct dye used for the chemical processing both the treated yarns have shown lower add-on values (add-on 9-10%) as getting self-extinguishing effect at lower chemical loading is challenging. In practical observation during the burning test, it was found that a lot of carbon mass generation are there for AS treated yarns, which may be attributed to the condensation effect of the sulphur-based ammonium sulphamate (AS). From the flammability results, 100 g/L concentration of AS was optimized (depending on add-on and burning properties) for application purposes, and the said yarns were used further for the rope making.
Another advantage was that the pH range used for AS treatment was on the neutral side (7-8), and it did not affect the tensile properties of the cellulose based sisal yarn.  Sisal rope and its thermal properties Like sisal yarns, the flammability properties of the four different types of ropes (3UT, 2UT + 1 T, 2 T + 1UT, and 3 T) also have been measured, and the analysis reports are reported in Table 2. Vertical burning behaviour of the rope with time has been exhibited in Fig. 5. LOI is the minimum quantity of oxygen required in the mixture of oxygen and nitrogen of air for just candle like burning of the sample. LOI value of the rope developed by braiding three untreated sisal yarns is 22. In addition, the burning rate observed in the vertical flammability test was also lower than in the control sisal yarn. This phenomenon may be attributed to more oxygen consumption for braided sisal structures than the sisal yarn. It may be indirectly linked with the more mass per unit length of the rope structure and also may be due to the presence of a braided structure, hindering oxygen inside the structure. The result showed that the rope consisting of untreated yarns burns readily within 2-3 min with flame and afterglow whereas all the ropes made by at least one treated yarn (with 100 g/L AS) have shown resistance against flame and afterglow propagation. In addition, extent of smoke generation also has been arrested, notably in all the rope samples. It may be attributed to the presence of ammonium sulfamate (AS) on the sisal yarn surface and for the covered structure of the rope, restricting the propagation of temperature throughout the untreated yarn. As a result, the afterglow propagation rate decreased from 0.58 mm/s to 0.2 mm/s. Flame resistance behaviour has increased further with the increasing the presence of treated yarn inside the rope structure. Thus, rope made with 2 AS treated yarn has almost the same char length as that made with 3 AS treated yarn. It means two AS treated yarn has covered the surface of untreated yarn (as shown in Fig. 1) and rendered to stop the propagation of afterglow inside the rope structure. Braided structure and the surface of the sisal fibre of the rope have been represented in Fig. 4. Burning nature of the different ropes used for the experiment is represented in Figs. 5 and 6. It shows that the red colour afterglow has covered the total circumference of the rope made with three untreated sisal yarns. However, the presence of the afterglow has been limited in a specific place on the surface when at least one treated yarn was present inside the rope surface.

Charring behaviour
The surface morphology of the sisal fibre used for the experiment is represented in Fig. 7. It showed a rough surface with the presence of craters and longitudinal striations throughout the surface. Cross-section of the sisal fibre shows countless small holes of elliptical shape inside the fibre structure. Figure 5C shows the presence of AS coating on the surface of the fibre and it has been speeded uniformly spread throughout the structure. Surface morphology of the yarn after burning is represented in Fig. 7. It shows that untreated sisal yarn was fragmented and fragile after the burning process, whereas AS-treated yarn shows more mass left after burning. For more clear understanding, microscopical images are represented in Fig. 8. This  Fig enlightens us with the precise scientific difference between the char of treated and untreated sisal rope. Sisal rope made with untreated yarns has shown a greyish, fragile, ash-like char structure (Fig. 6C, D), whereas rope made with three treated yarns has shown black colour rigid char. Indeed, the structure of the surface of the sisal fibre remains intact after the completion of the burning process, as shown from the image Fig. 8G, H. One another interesting observation is that the charred fibre of the 3 T rope has maintained its circular diameter even after burning; however, sisal fibre of 3UT has lost its structural integrity, and it looks like an ashy skeleton.
TG analysis TG analysis clears the pyrolysis behaviour of the control and the AS treated sisal fibre as performed in a nitrogen atmosphere. Results are represented in Table 3, and the curves are shown in Fig. 9. TG curve of control sisal has shown three peaks. First peak is small at 100 °C and may be corroborated by the evaporation of moisture. Second peak represents mainly the depolymerisation of cellulose (320 °C) and it is attributed with the liberation of flammable gases like furans, levoglucosan etc. Apart from the depolymerisation of cellulose, depolymerisation of different parts of lignin and hemi-cellulose are reflecting at around 400-500 °C. It represents the third degradation peak and degradation occurs depending on the molecular weight and thermal behaviour  Ali 2019) of different parts of lignin and hemicellulose. On the contrary, it was found from the curve that the AS treatment did not have much effect on the T 5 and T 10 % of the treated sisal. Moreover, the extent of char mass remaining at higher temperatures is almost the same as the control sisal yarn. However, the major difference between the thermal degradation of the control and treated one is attributed to the location of the significant mass loss peak. It was observed that the AS treatment had catalysed the pyrolysis process by around 100 °C (from 320 to 220 °C), and as a result, it hinders the formation of flammable gases like levoglucosan, etc., and at this temperature, AS may break down and ammonia like non-flammable gas released. This phenomenon has been fortified by the EDX analysis of AS-treated yarn and its char.  Char of the treated yarn has shown less quantity of nitrogen than the char of the control sisal yarn. It may be attributed to the leveraging of ammonia as nonflammable gas during burning. Rate of weight loss of the treated yarn is less than the control sisal yarn, and for treated yarn, char formation has been started earlier, before depolymerisation of the cellulosic and lignin chain. Another interesting observation is that the AS treated sisal fibre has shown a very steep pyrolysis zone as compared to the control one, which may be related to the reaction of ammonium sulfamate with the cellulose backbone. Sulphur present on the AS treated yarn also assists in carbon generation during the burning sequence. Isothermal TG analysis of the treated sisal also has proved the weight stability of the sisal polymer (50% remained) with pyrolysis temperature (300 °C) for a more extended time of 20 min. It proves the dehydration or aromatisation of  EDX and FTIR analysis EDX analysis of the control and the treated char showed the presence of different elements on the surface of the yarn structure. These elements are part of the fibre structure and may come from the chemicalbased formulation used for coating purposes. Results showed that the control sisal fibre predominantly showed the presence of carbon (59.11%) and oxygen (39.07%) predominantly. However, some parts of calcium, sodium, sulphur are also present on it in minor quantities. On the contrary, apart from carbon and oxygen, AS treated sisal yarn has shown the presence of nitrogen (11.01%), sodium (0.34%), calcium (0.52%), and sulphur (1.2%) in more amounts as compared to the control sisal fibre. Presence of nitrogen, sulphur may have come from the AS coating. EDX analysis of char samples also has been performed, revealing the presence of carbonaceous elements on the charred mass. Carbon content of the charred mass of AS-treated sisal fibre is almost 20% higher compared to the untreated one, and it also depicts the presence of nitrogen (6.7%), sulphur (0.5%) in the residual mass. More amount of carbon present in the charred mass of treated sisal yarn also has been fortified from the microscopical morphology of the charred mass, showing a blackish colour mass left. Atomic weight percentage of nitrogen present in the char mass was almost half compared to the AS treated sisal fibre. It may be due to the formation of nitrogencontaining non-flammable ammonia gas during the burning process of the fibre. This phenomenon also has been concluded from the TG analysis of the AStreated sisal fibre. EDX curves are represented in Fig. 11. More amount of carbon present in the treated char again fortifies the conclusion of dehydration of the cellulosic structure after treatment with AS. FTIR analysis of the control and the AS-treated sisal fibre was conducted to understand the presence of different active groups, depicted in Fig. 10. FTIR curves of the control and the AS-treated yarn have shown no significant differences. Control sisal yarn has shown the major characteristic peaks representing the C-O-C vibration, CH 2 bending, -OH stretching vibration etc. These groups are a significant part of the chemical groups present in the cellulose polymer. Apart from it, some other peaks are also present, representing the presence of lignin inside the structure. However, AStreated sisal yarn shows two distinct peaks at around 1600 cm −1 and 1400 cm −1 , which may be attributed to the sulphur group present on the surface of sisal yarn (Mostashari and Mostashari 2008). Apart from this analysis, no other significant difference has been observed. Presence of sulphur on the surface of the treated sisal fibre also has been proved from the EDX analysis represented in Fig. 11.

Forced Combustion analysis of the samples
Cone calorimeter analysis of the three replicates of the control and the treated sisal yarns have been performed to understand the heat release behaviour from the sample during the forced combustion process; results and curves are depicted in Fig. 12. 35 kW/ m 2 heat flux level assigned with mild fire exposure, and it is mainly used for light weight upholsters furniture, curtain, mattress component, etc. Therefore, sisal yarns have been exposed to the heat flux level (35 kW/m 2 ) as mentioned earlier and the detail cone calorimetric results have been represented in Tables 4 and 5. All the cone calorimetric parameters like time to ignition (TTI), heat release rate (HRR), total heat release (THR), peak heat release rate (PHRR) etc., normally varied with changing the heat flux level of combustion. It has been found from the cone calorimeter analysis that the control sisal yarns have been burnt out with flame. The time to ignition is noted as 10 s (time has been noted from the heat exposure by cone heater to ignition initiated in the sample by spark igniter) whereas, the 300 g/L AStreated sisal yarns did not show any flame catch up (no flame observed even after repeated ignition by spark igniter) B but the sample has been combusted with afterglow. Another observation from the experiment is that the control sisal yarns have been wholly burnt with a flame within 300 s. On the contrary, treated yarns did not catch flame but completely combusted throughout 600 s which is more than the control yarns. This may be because of the afterglow in the yarns, which propagated slowly during combustion. It also may be because of the high heat capacity of the coating material, which increases the time to ignition value. At the end of the test, the peak heat release rate (PHRR) of the control yarns was found to be 103.2 kW/m 2 , whereas the AS-formulation treated yarns registered a peak heat release rate of 48.3 kW/m 2 (almost less than half of the control sisal yarns). HRR curve of the treated yarns has shown a decreasing trend as compared to the heat release of the control sisal yarn, and as per literature decreasing trend of the curve may be assigned to the dilution of the flammable volatiles or with the insulated char mass generation. Pertaining to this concept, treated sisal yarns showed intense black colour char mass, at the end of the combustion process, whereas control yarns showed light grey colour fragile char mass. Heat release rate (HRR) curve of the control and the treated yarns has been represented in Fig. 12. From the nature of the heat release curve of the control and the treated sisal yarns, it also has been clear that the fire growth rate (concept of the fire growth rate comes from the gradient of the tangent of peak heat release rate) is more in the controlled yarn compared to the treated yarn. As per report, heat release rate has been calculated from the oxygen concentration in the fuel gases. The heat released from the sisal yarn is proportional to the oxygen consumed during combustion. Lower heat release rate of the treated yarn may be due to the fact that the ammonium sulfamate coating may act as barrier to fuel transport, forming a more insulated char mass with the dilution of the flammable gases and reradiate the flux from the cone calorimeter heater. Indeed, the concern treatment slowed down the release of flammable volatiles from the decomposed coating to the flame front. Total heat release (THR) for the control yarn is around 4.75 MJ/ m 2 whereas the heat release value for the treated yarn is around 1.85 MJ/m 2 . Actually, mass loss rate of the sample results from pyrolysis, controlled by heat flux, heat transfer, and thermal kinetics. Heat release rate of the sample is one function of the corresponding mass loss. Cone calorimeter analysis also determines the average rate of heat emission (ARHE), which signifies the cumulative heat emission divided by time and its peak value. Indeed, the concerned parameter is a good measure of the propensity of fire development under real scale conditions. Data value in Table 4 indicates that the MARHE value of the control sisal is around 40-42% higher compared to the treated sisal. Maximum smoke production rate has been found from the curve, and it is around 0.03 m 2 /s for the control sisal, whereas the treated sisal showed a smoke production rate of 0.06 m 2 /s. Extent of carbon monoxide generation is almost similar for the control and the treated sisal. However, the amount of carbon-dioxide liberated during the combustion of the treated sisal is around 0.06% which is nearly five times lower compared to the control sisal.

Mechanism of flame retardancy
Sisal fibre contains cellulose, hemicellulose, lignin, etc., as the main structural units (Catling and Grayson 1982). LOI value of sisal fibre is around 21, signifies it is very much flammable in an open atmosphere as a much lower quantity of oxygen is required for its combustion. Cellulose, lignin and hemicellulose present in it have been depolymerised during the course of combustion and different flammable gases like levoglucosan, furan, pyroglucosan, carbon monoxide etc., get released (Kandola et al. 1996). High extent of heat was also generated during combustion. Moreover, secondary burning (after glow) and smoke production are other severe disadvantages in the case of the burning course of sisal fibre. Therefore, to minimize these dangerous outcomes, one mechano-chemical approach has been adopted and described scientifically in this context. It was found from the earlier flammability results that the combination of three untreated sisal yarns restricted the flow of oxygen inside the rope structure. As a result burning rate goes down from 2.58 to 0.5 mm/s. Braiding design (length of the braid, braid angle, number of fibres present in the cross-section), available oxygen concentration and heat insulation by ammonium sulfamate are the major considering factors for the reduction of burning rate. In the case of the combination of one untreated yarn and two treated yarns, untreated yarn remained in the core, surrounded by two AS-treated yarn. In addition to oxygen hindrance, AS treated yarn also covers every part of the untreated yarn and assists in restricting the heat flow and oxygen availability inside the rope structure. Moreover, the main loadbearing part of the yarn also remains intact (as shown in Fig. 1) as it has not been treated with any chemical. Flame resistance property of the rope made with three treated sisal yarns was also similar to earlier. It means 8-9% AS chemical add-on can be avoided using such mechanical design. Moreover, the structure will maintain more tensile strength as it is one of the important parameters for any sisal rope applications. Chemical action of AS is also represented in the Figs. 13 and 14. It shows that at high temperatures, AS released non-flammable ammonia gas. Released ammonia gas may dilute the concentration of oxygen during burning. Presence of ammonia gas was proved by the loss of elemental nitrogen in the char mass of the AStreated sisal yarn. In addition, sulphuric acid has been released at higher temperatures and reacts with cellulose, hemicellulose, and lignin (major part of sisal polymer). Acid has attacked the glucosidic linkage of cellulose, aromatise the structure, and enhances char mass generation. Lignin part of sisal has also been dehydrated by acidic action by forming of lignin sulphonate groups. As a result of dehydration, the pyrolysis phenomenon of the treated sisal polymer was catalyzed (as observed from TG analysis) and assisted in minimizing the chances of the formation of flammable gases. Therefore, the total flame resistance action of ammonium sulfamate clears the fact that it follows the theory of condensed phase mechanism (catalytic pyrolysis action with more char formation) and gas phase mechanism (by releasing non-flammable ammonia gas and reduced the concentration of oxygen).
Analysis of physical properties of rope Figure 15 shows the stress-strain behaviour of the different braided yarns developed for testing. Untreated single sisal yarn has a tenacity of 10cN/tex (CV%-6.7) with elongation of 7%. Braided yarn developed with three untreated yarns showed maximum tenacity of 4.38 cN/tex (CV%-5) with an elongation of 18.2%. Energy required for breaking and work of rupture for the said yarn was around 27,900 mJ (CV%-3.4), and 0.00365 J/m/Tex respectively. Energy required for breaking was almost five times less than the control braided yarn as toughness of the structure has been increased after braiding. 2UT, 1 T braided yarn has shown tenacity 4.31 cN/tex (CV%-4.3) with elongation of 18.3%. (CV%-4) Energy required for breaking was around 23,900 mJ (7-10% lower than 3UT) with work of rupture 0.00360 J/m/Tex. However, 2 T, 1UT braided yarn has shown maximum tenacity of 3.92 cN/tex (CV%-6.2) with maximum strain of 20.6% (CV%-3.9). Energy required for breaking has been reduced further to 22,100 mJ with work of rupture 0.00380 J/m/Tex. Finally, 3 T braided yarn showed maximum tenacity of 4.59 cN/tex (CV%-3.6%) with a strain of 21.2% whereas energy required for breaking was around 24,200 mJ with work of rupture 0.00426 J/m/Tex. It means only 9% strength loss has been observed by using two AS-treated yarns in the braiding structure, while elongation has almost remained constant. This strength loss could be corroborated with the loss of some part of hemi-cellulose from the sisal structure due to light alkaline condition of the AS treatment. Ten samples in each case and representative curve (near about mean) are presented in the context. Sensitivity level of the machine was 75%. When one yarn in the braided structure was broken, the total load was shared by the rest two yarns and finally by one yarn. Therefore, two to three different points are there in the representative curve.
XRD analysis XRD analysis of the AS salt, control and AS treated sisal yarn also has been represented in Fig. 16. XRD diffraction pattern of ammonium sulfamate salt 16 (A) showed peaks at 2θ position 12°, 20°, 24°, 38°, 65°, as also reported by Kazachenko et al. (2021). XRD of control sisal fibre 16 (B) showed typical  (French 2014;and French and Santiago Cintrón 2013). Sharp peaks observed in the XRD patterns at 21.8° might be attributed to the presence of natural silicon oxide in the samples (Music et al. 2011;Nandanwar et al. 2015). Crystallinity has been calculated according to the procedure mentioned in material and method section. XRD curve shows that the control sisal fibre has a crystallinity of around 60.25% while after AS treatment, crystallinity has been reduced to 58.20%. From Fig. 16D it has been observed that treated and untreated sisal X-ray diffraction patterns completely overlap with each other, indicating no change in their crystal structure as well as crystallinity index. This phenomenon proved that the AS treatment has no adverse effect on the crystallinity of the sisal fibre, may be attributed with the application pH level in neutral condition. Crystallinity of fibre has direct relation with the tensile strength of the fibre. Unchanged crystallinity of the treated fibre also has been fortified by similar tensile properties of both treated and untreated sisal yarn (Fig. 15).
It was found from the newly developed process that the chemical add-on percentage required for self-extinguishment of rope is lower as compared to the conventional flame-resistant treatment. Analytically, a 27% ammonium sulfamate (AS) add-on is required for making self-extinguished rope by following conventional process whereas only 18% AS is required for making self-extinguished rope by following the above mentioned mechano-chemical process. It means, 9% chemicals have saved by following this process and it is a major step towards sustainable flame retardancy. In addition, not much colour difference also has been observed in the AS-treated sisal rope. As observed from Fig. 17, treated sisal rope has turned a little yellowish after AS treatment; however, extent of colour change will depend on the concentration of AS used for the treatment and on the number of AS treated yarns used for making rope.
Dyed sisal rope for commercial use Sisal rope was dyed with direct dye, as shown in Fig. 18, by following the suitable process condition as mentioned in the Material and Method section. Fig. 15 Tensile properties of the sisal braided rope structure made by three untreated (3UT); two untreated, 1 AS treated (2UT, 1 T); one untreated, 2 AS treated (1 UT, 2 T); three AS treated (3 T) yarn combinations Treated rope has shown uniform colour throughout the surface of the braided sisal structure. As ammonium sulphamate was added to the dye bath, it was deposited as a coating chemical on the rope at an alkaline treatment. Therefore, the dyed rope has not only delivered uniform colour of the rope but also rendered a flame-resistant rope structure. It was calculated that the total add-on% of chemicals on the rope was less than 20%. As far as flame retardancy is concerned, this dyed rope has shown selfextinguishing properties. Limiting oxygen index of the concerned rope is 29, with no secondary burning observed during the flammability test. Major advantage is that dyed rope did not lose its tensile strength after processing as the treatment was done in alkaline conditions, and no bleaching is required for uniform dyeing of sisal fibre. This kind of rope could be easily used in a common household for drying clothes, fire fighting applications, trekking in hills, for packaging goods etc.

Rubbing fastness of the rope
Rubbing fastness is an important property for using sisal fibre-made rope in outdoor applications, packaging, household applications, etc. In all cases, a lot of handling is required. These dyed ropes are attractive and could be used in packaging purposes, the marine sector, etc. Rubbing fastness is very much crucial as rough handling is involved in all parts of its applications. Rubbing of the rope was performed in the machine for 20 cycles in dry and wet condition. After the completion of rubbing cycles, color fastness and self-extinguishing properties were measured in terms of LOI value. It was observed that colour fastness rating after both dry and wet rubbing is 4-5 whereas the LOI value of the rubbed rope is 29 (average of five results). It means AP used for the self-extinguishing effect of the rope has remained in the rope even after repeated rubbing in dry and wet conditions. As the formulation contains 2% resin, it can withstand rain and other detrimental weather condition, although conventional laundering can mitigate the efficacy of the flame retardancy action (LOI of the product degraded from 40 to 28). However, this material could be a perfect substitute for packaging purposes and hanging cloths inside the shade or in the household, military training purposes, and as a lifesaving rope product for fire-extinguishing purposes.

Conclusion
The present study has shown the processing details of making fire retardant sisal rope using a combination of ammonium sulfamate (AS) treated and untreated sisal yarns, following the braiding process. Only 100 g/L AS-treated sisal yarn has shown LOI value of 32; however, rope made with two AS-treated yarns and one untreated yarn has shown an LOI value of 38. It is almost equal to the rope made from three AS treated sisal yarns. High LOI and thermal resistance with low add-on% were achieved by the combination of mechanical design and AS treatment. It concluded the fact that the same flame resistance property has been achieved by using 9-10% less chemical loading on sisal rope. As mentioned in the context, AS acts as a flame retardant by following the gas phase mechanism (by releasing non-flammable ammonia gas during the burning course) and following the path of condensed phase mechanism (by dehydration and catalytic pyrolysis action). Catalytic pyrolysis action of the AS treated sisal yarn has been observed from the TG analysis, and more carbon mass generation has been fortified by the elemental analysis of char. Char morphology of the treated yarn has shown a stronger and more compact structure than the charred mass of untreated sisal yarn. Most important finding is that the concerned treatment did not affect or have less effect on the tensile properties of the rope as chemical loading is less than 10%, and the prescribed design hinders the presence of oxygen inside the burning sample. This novel process can impart flame retardancy to the ropes, mats and sheets ropes used for packaging and covering necessary materials where many sisal yarns are used.