Flammability test
Sisal yarn was treated with different concentration of AS and the combinations of treated and untreated yarn has 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 as well as rope were examined by following the standard test method. Test results are reported in the below mentioned Table 1 and 2.
Sisal yarn and its thermal properties
Sisal yarn was treated with different concentration of ammonium sulfamate (AS) and the detail flammability properties of the treated yarns are listed below. It was observed from the 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 compound and lignin is resistance to thermal propagation due to its polyphenolic nature and branched chain structure and because of the presence of –OH groups in its structure. However, smoke and secondary burning of the sisal fibre made yarn is a serious problem and it can cause of 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., are harmful for the common people as it lowers the concentration of oxygen in the surrounding atmosphere. However, AS treated sisal yarns have shown higher LOI value (28 and 32) and specific char length. Afterglow and smoke generation was completely extinguished as more amount of oxygen gas is required for combustion. Moreover, both the treated yarns have shown lower add-on value (add-on 9-10%) as getting self-extinguishing effect at lower chemical loading is really a challenge. In practical observation during burning test, it was found that a lot of carbon mass generation are there for AS treated yarns, may be attributed with the condensation effect of the sulphur based ammonium sulphamate (AS). From the flammability results, 100g/L concentration of AS was optimized (depending on add-on and burning properties) for application purpose 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.
Table 1
Flammability properties of the untreated and AS treated sisal yarn
Chemical Parameters
|
Formulation and process condition for treatment
|
Without treatment
|
|
Formulation A treated sisal yarn
|
Formulation B treated sisal yarn
|
Untreated sisal yarn
|
Ammonium sulfamate (AS)
|
50 g/L
|
100g/L
|
-
|
Drying
|
1 min, 120°C
|
1 min, 120°C
|
-
|
Flammability parameters
|
Chemical loading (%)
|
8.5
|
9.26
|
-
|
LOI
|
28
|
32
|
21
|
Flammability test in vertical direction
|
Observation of flashing
|
No
|
No
|
No
|
Char length (mm)
|
90mm
|
64mm
|
Nil as total sample was burnt with afterglow and smoke
|
After flame (s)
|
No after flame
|
No after flame
|
3-4s
|
After glow (s)
|
70s with smoke and then self-extinguished
|
No afterglow
|
Sample burnt with continuous afterglow
|
Total burning time (s)
|
70s
|
-
|
600
|
Observed burning rate (mm/s)
|
1.2
|
-
|
2.8
|
Status after combustion cycle
|
Brittle black ash
|
More black ash
|
Greyish brittle char observed
|
Tensile strength loss after treatment
|
No strength loss observed as the treatment was done in almost neutral condition
|
No strength loss observed as the treatment was done in almost neutral condition
|
-
|
Sisal rope and its thermal properties
Table 2
Flammability testing of rope specimens
Parameter of sisal yarns in rope
|
3 UT
|
2 UT+ 1 T
|
1 UT+ 2 T
|
3 T
|
LOI
|
22
|
30
|
38
|
40
|
Vertical Flammability Test (350mm long rope)
|
After flame (s)
|
4s
|
Nil
|
Nil
|
Nil
|
Char length (mm)
|
No specific char length, sample burnt with severe afterglow and smoke
|
No self-extinguishment but very low burning rate and smoke production rate
|
50
|
25
|
Burning rate (mm/sec)
|
0.58
|
0.2
|
NA
|
NA
|
Afterglow (sec)
|
125
|
1750
|
Nil
|
Nil
|
After burning residue
|
Feathery grey color ash
|
Rigid char surrounding incompletely burnt core
|
Rigid char
|
Rigid char
|
Practical observation
|
Severe afterglow with full diameter of rope
|
Afterglow present only in the part of the rope and propagate slowly
|
No afterglow present
|
No afterglow present
|
Like sisal yarns, flammability properties of the 4 different types of ropes (3UT, 2UT+ 1T, 2T+ 1UT and 3T) 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 value of the rope developed by braiding of three untreated sisal yarns is 22. In addition, burning rate observed in vertical flammability test was also lower as compared to the control sisal yarn. It means requirement of oxygen is slightly more for combustion of rope as compared to the sisal yarn. It may be due to the more mass per unit length of rope structure and also may be due to the presence of braided structure, hinders the presence of 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 100g/L AS) has 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 with the presence of ammonium sulfamate (AS) on the sisal yarn surface and for the covered structure of the rope, restrict propagation of temperature throughout the untreated yarn. As a result rate of afterglow propagation has been decreased from 0.58mm/sec to 0.2 mm/sec. Flame resistance behaviour has been increased further with increasing the presence of treated yarn inside the rope structure. Thus, rope made with 2 AS treated yarn has shown almost same char length as compared to the rope 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 has been represented in Fig 4. Burning nature of the different ropes used for the experiment was represented in Fig 5 and Fig 6. It shows that red colour afterglow has covered the total circumference of the rope made with three untreated sisal yarn. However, presence of the afterglow has been limited in specific place of the surface when at least one treated yarn was present inside the rope surface.
Charring behaviour
Surface morphology of the sisal fibre used for the experiment was represented in Fig 7. It showed rough surface with the presence of craters and longitudinal striations through out the surface. Cross-section of the sisal fibre shows countless small holes of elliptical shape inside the fibre structure. Fig 5C shows the presence of AS coating on the surface of the fibre and it has been speeded uniformly spread through-out the structure. Surface morphology of the yarn after burning has been represented in Fig 7. It shows that untreated sisal yarn was fragmented and fragile in nature after completion of burning process whereas AS treated yarn shows more amount of mass left after burning. For more clear understanding, microscopical images are represented in Fig 8. This Fig enlightens us with the clear scientific difference of the char of treated and untreated sisal rope. Sisal rope made with untreated yarns has shown greyish, fragile, ash like char structure (Fig 6C, D) whereas rope made with 3 treated yarns has shown black colour rigid char. Indeed, structure of the surface of the sisal fibre still remain intact after the completion of the burning process, as shown from the image Fig 8 G, H. One another interesting observation is that the charred fibre of the 3T 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 it has been performed in 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 with 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 is represented third peak of the degradation and degradation has been occurred depending on the molecular weight and thermal behaviour (Basak et al. 2016, Basak et al. 2019) of different parts of lignin and hemi-cellulose. On the contrary, it was found from the curve that the AS treatment did not have much effect on the T5 and T10 % of the treated sisal. Moreover, extent of char mass remained at higher temperature is almost same as compared to the control sisal yarn. However, major difference between the thermal degradation of the control and treated one is attributed with the location of major mass loss peak. It was observed that the AS treatment has catalysed the pyrolysis process by around 100°C (from 320°C 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 from the EDX analysis of AS treated yarn and its char. Char of the treated yarn has shown less quantity of nitrogen as compared to the treated one, may be due to the release of nitrogen as non-flammable gas during burning. Rate of weight loss of the treated yarn is less as compared to the control sisal yarn and for treated yarn char formation has been started earlier, before depolymerisation of cellulosic and lignin chain. Another interesting observation is that the AS treated sisal fibre has shown very steep pyrolysis zone as compared to the control one, may be related with the reaction of ammonium sulfamate with the cellulose backbone. Sulphur present on the AS treated yarn also assists in carbon generation during 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 longer time period of 20 min. It proves the dehydration or aromatisation of the cellulosic part of sisal with the ammonium sulfamate at higher temperature.
Table 3
TG analysis of the control and treated sisal yarn
Parameters
|
Control sisal yarn (3UT)
|
Treated sisal yarn (3T)
|
Treated sisal yarn in isothermal condition (hold at 300°C for 20 min)
|
T5 (°C)
|
100
|
75
|
75-80
|
T10(°C)
|
200
|
200
|
200
|
Char remained at 500°C (%)
|
30-35
|
30-35
|
Not applicable
|
Char remained at 600°C (%)
|
30
|
30
|
Not applicable
|
Major mass loss peak (°C)
|
320
|
220
|
At around 210°C-220°C
|
Total number of peaks as observed from derivative curve
|
Three stages at 150, 300 and 450°C
|
One stage at 200°C
|
Only one peak with 50% char mass remained at the end of the TG programme
|
Mass remained at each stages
|
1st stage (85%)
2nd stage (50%)
3rd stage (35%)
|
1st stage (60%)
|
50% char mass remained
|
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 the part of the fibre structure and also may come from the chemical based formulation used for coating purposes. From the results, it has been observed that the control sisal fibre showed the presence of carbon (59.11%) and oxygen (39.07%) predominantly. However, some part 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 amount 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 and it reveals the presence of carbonaceous element on the char mass. Carbon content of the char mass of AS treated sisal fibre is almost 20% higher as 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 char mass of treated sisal yarn also has been fortified from the microscopical morphology of the char mass, showed blackish colour mass left. Atomic weight percentage of nitrogen present in the char mass was almost half as compared to the AS treated sisal fibre. It may be due to the formation of nitrogen containing non-flammable ammonia gas during burning process of the fibre. This phenomenon also has been concluded from the TG analysis of the AS treated sisal fibre. EDX curves are represented in Fig 11. More amount of carbon present in the treated char again fortify 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 has shown no significant differences. Control sisal yarn has shown the major characteristic peaks representing the C-O-C vibration, CH2 bending, -OH stretching vibration etc. These groups are the major 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, AS treated sisal yarn shows two distinct peaks at around 1600cm-1 and 1400cm-1, may be attributed with the sulphur group present on the surface of sisal yarn [Mostashari et al., 2008]. Apart from it no other significant difference has been observed from this analysis. Presence of sulphur on the surface of the treated sisal fibre also has been proved from the EDX analysis.
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 forced combustion process, results and curves are depicted in Fig 12. 35kW/m2 heat flux level assigned with the mild fire exposure and it is especially used for light weight upholsters furniture, curtain, mattress component, etc. Therefore, sisal yarns have been exposed to the aforementioned heat flux level (35 kW/m2) and the detail cone calorimetric results have been represented in Table 4 and Table 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 and the time to ignition is noted as 10s (time has been noted from the heat exposure by cone heater to ignition initiated in the sample by spark igniter source) whereas, the 300g/L AS formulation treated sisal yarns did not show any flame catch up (no flame observed even after repeated ignition by spark igniter) but the sample has been combusted with afterglow. Another observation from the experiment is that the control sisal yarns have been completely burnt with flame within 300s. On the contrary, treated yarns did not catch flame but completely combusted throughout 600s which is more than the control yarns. This may be because of the presence of 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 time to ignition value. At the end of the test, peak heat release rate (PHRR) of the control yarns was found 103.2 kW/m2 whereas the AS formulation treated yarns have registered peak heat release rate as 48.3kW/m2 (almost less than half of the control sisal yarns). HRR curve of the treated yarns have shown 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 with the dilution of the flammable volatiles or with the insulated char mass generation. Pertaining to this concept, at the end of the combustion process, treated sisal yarns showed intense black colour char mass 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 come from the gradient of the tangent of peak heat release rate) is more in the control 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, formed 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/m2 whereas the heat release value for the treated yarn is around 1.85 MJ/m2. 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 concern parameter is a good measure of the propensity of the fire development under real scale condition. 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 out from the curve and it is around 0.03m2/s for the control sisal whereas the treated sisal showed smoke production rate of 0.06m2/s. Extent of carbon monoxide generation is almost similar for the control and the treated sisal . However, the amount of carbon di-oxide liberated during the combustion of the treated sisal is around 0.06% which is nearly five times lower compared to the control sisal .
Table 4
Summary of the cone calorimeter data of the control and the treated sisal yarns
Test series No.
|
Material
|
PHRRa
(kW/m2)
|
Time to sustained ignitiona(sec)
|
Time to PHRRa
(sec)
|
Total HRa
(MJ/m2)
|
MARHEa
(kW/m2)
|
CO
(%)
|
CO2
(%)
|
1
|
Control
|
103.2 (3.2%)
|
10 (2%)
|
110(2.2%)
|
4.75(2.2%)
|
13(3.2%)
|
0.001
|
0.32
|
2
|
Treated
|
48.3(4.4%)
|
No ignition
|
385(4%)
|
1.85(3.2%)
|
8.2(4.5%)
|
0.001
|
0.06
|
Note: H.R.R: Heat release rate, T.H.R: Total heat release, MARHE: Maximum average rate of heat emission. Parentheses represent the CV% of data.
Table 5
Summary of the mass loss and smoke related properties of the control and the treated sisal yarns
Test series No.
|
Material
|
Initial mass (gm)
|
Mass loss rate (g/m2s-1)
|
Smoke production rate (kW/m2)
|
Total smoke (m2/m2)
|
Sample thickness (mm)
|
1
|
Control
|
3.0
|
5.7 (2%)
|
0.04(2.2%)
|
16(2.2%)
|
1
|
2
|
Treated
|
3.5
|
3 (3.2%)
|
0.01 (4%)
|
10 (3.2%)
|
1.2
|
Mechanism of flame retardancy
Sisal fibre contains cellulose, hemi-cellulose, lignin etc., as main structural unit of the polymer. Sisal fibre is very much flammable in open atmosphere as cellulose, lignin, hemi-cellulose present in it have been depolymerised and different flammable gases like levoglucosan, furan, pyroglucosan, carbon monoxide etc., get released [Kandola et al., 1996]. Moreover, secondary burning and smoke production both are other serious disadvantages in case of the burning course of sisal polymer. Therefore, for minimizing these dangerous outcomes, one mechano-chemical approach has been taken by the researchers. It was found from the earlier flammability results, that combination of three untreated sisal yarns have restricted the flow of oxygen inside the rope structure and as a result burning rate is going down from 2.58 mm/sec to 0.5mm/sec. In case of the combination of 1untreated yarns and 2 treated yarn, untreated yarn remains in the core and it was surrounded by 2 AS treated yarn. In addition of oxygen hindrance, AS treated yarn also covers the every part of the untreated yarn and assist to restrict the heat flow inside the rope structure. In addition, main load bearing part of the yarn also remains intact (as shown in Fig 1) as it has not been treated with any kind of chemical. Flame resistance property of the rope made with 3 untreated sisal yarns was also similar like earlier one. It means 8-9% AS chemical add-on can be avoided by using such kind of mechanical design. Moreover, structure will maintain more tensile strength as it is one of the important parameter for any kind of sisal based product. Chemical action of AS also has been represented in the Fig 13 and Fig 14. It shows that at high temperature, AS has released non-flammable ammonia gas. It may dilute concentration of oxygen during burning. Release of ammonia gas was proved indirectly by loss of elemental nitrogen in the char analysis of the AS treated sisal yarn. In addition, sulphuric acid also has been released at higher temperature and reacts with cellulose, hemi-cellulose and lignin (major part of sisal polymer). Acid has attacked the glucosidic linkage of cellulose, aromatise the structure and enhance char mass generation. Lignin part of sisal also has been dehydrated by acidic action by formation of lignin sulphonate groups. As a result of dehydration, pyrolysis phenomenon of the treated sisal polymer was catalyzed (as observed from TG analysis) and assisted to minimize the chances of the formation of flammable gases. Therefore, total flame resistance action of ammonium sulfamate clears the fact, that it acts by following both 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
Fig 15 has shown the stress strain behaviour of the different braided yarn developed for testing. Braided yarn developed with three untreated yarn showed maximum tenacity of 4.38 cN/tex (CV%-5) with elongation of 18.2%. Energy required for breaking was around 27900 mJ (CV%-3.4) with work of rupture was around 0.00365 J/m/Tex. Untreated single sisal yarn has shown tenacity of 10cN/tex (CV%-6.7) with elongation of 7%. Energy required for breaking was almost 5 times less as compared to the control braided yarn. It means toughness of the structure has been increased after braiding. On the contrary 2UT, 1T 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 23900 mJ (7-10% lower than 3UT) with work of rupture 0.00360 J/m/Tex. However, 2T, 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 22100 mJ with work of rupture 0.00380 J/m/Tex. Finally, 3T braided yarn has shown maximum tenacity of 4.59 cN/tex (CV%-3.6%) with strain of 21.2%. Energy required for breaking was around 24200 mJ with work of rupture 0.00426 J/m/Tex. It means only 9% strength loss has been observed by using two AS treated yarn in the braiding structure while elongation almost has been 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. XRD analysis of the control and AS treated sisal yarn also has been represented in Fig 16. It shows that the control sisal fibre has crystallinity of around 61.97% while after AS treatment, crystallinity has been reduced to 55.07%. 5-6% loss of crystallinity of the sisal polymer may be linked with the loss of hemi-cellulose or lignin part after AS treatment. These amorphous polymers act as binding agent between adjacent cellulose chains. This phenomenon may be attributed with the observed strength loss of the yarn (5-7%) after AS treatment. Strength loss is minimal as the AS treatment was carried out in neutral condition and some untreated sisal yarn was used in combination of AS treated yarn for making the sisal rope.
It was found from the newly developed process that chemical add-on percentage required for self-extinguishment of rope is lower as compared to the conventional flame resistant treatment. Analytically, 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 little yellowish after AS treatment, however, extent of colour change will depends 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 Material and Method section. Treated rope has shown uniform colour through out the surface of sisal braided structure. As ammonium sulphamate was added in the dye bath, it was deposited as coating chemical on the rope at alkaline treatment condition. Therefore, dyed rope has not only delivered uniform colour of the rope but also render to get a flame resistant rope structure. It was calculated that total add-on% of chemical on the rope was less than 20%. As far as the flame retardancy is concerned, this dyed rope also has shown self-extinguishing property. Limiting oxygen index of the concerned rope is 29 with no secondary burning has been observed during flammability test. Major advantage is that dyed rope did not loss its tensile strength after processing as the treatment was done in alkaline condition and no bleaching is required for uniform dyeing of sisal fibre. This kind of rope could be easily used in common household for drying cloths, fire fighting applications, trekking in hills, for packaging of goods etc.
Rubbing fastness of the rope
Rubbing fastness is the important properties for using sisal fibre made rope in outside applications, packaging, household applications etc. In all the cases a lot of handling is required. These dyed ropes are attractive and could be used in packaging purposes, marine sector etc. Rubbing fastness is very much important as rough handling is involved in all part of its applications. Rubbing of the rope was performed in machine for 20 cycles in dry and wet condition. After completion of rubbing cycles, color fastness and self-extinguishing property was measured in terms of LOI value. It was observed that colour fastness rating after both dry and wet rubbing is 4-5 whereas LOI value of the rubbed rope is 29 (average of five results). It means AP used for self-extinguishing effect of the rope was remained in the rope even after repeated rubbing in dry and wet condition.