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Eco-friendly flame retardant coating deposited on cotton fabrics from bio-based chitosan, phytic acid and divalent metal ions
International Journal of Biological Macromolecules 140 (2019) 303–310
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International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Eco-friendly flame retardant coating deposited on cotton fabrics from bio-based chitosan, phytic acid and divalent metal ions Zhihao Zhang, Zhongying Ma, Qian Leng, Yuhua Wang ⁎ Department of Material Science, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, China
a r t i c l e
i n f o
Article history: Received 2 May 2019 Received in revised form 19 July 2019 Accepted 6 August 2019 Available online 12 August 2019 Keywords: Bio-based flame retardant Coating Cotton fabrics Chitosan Phytic acid Metal ion
a b s t r a c t With the increasing awareness of environmental protection, the greening of flame retardants has become an inevitable choice for flame retardant technology. Bio-based materials have the advantages of wide source, recyclability and green environmental protection, and have received extensive attention. In this work, chitosan and phytic acid as intumescent flame retardant system and metal ion as a synergist were built on cotton fabrics to achieve efficient flame retardancy by facile dip-pad-dry process. Microscale combustion calorimetry results manifested that the heat release rate values of coated samples were lower than that of uncoated samples. The peak heat release rate and total heat release of uncoated samples were 333.1 W/g and 13.1 kJ/g, while these values of samples coated CH/PA/Ba/PA were 129.1 W/g and 5.4 kJ/g respectively and char residues also were increased sharply owing to the phosphate groups promote the formation of char layer covering the surface of the cotton fabrics, which can prevent heat transfer. Scanning electron microscopy confirmed that there were much more bubbles on the surface of fibers for the coated CH/PA/Ba/PA after horizontal flame tests. The results indicated that the synergy of the intumescent flame retardant system was achieved by the addition of metal ions which could accelerate the produce of a large number of nonflammable gases. Horizontal flame test and limit oxygen index (LOI) test showed that the burning rate and LOI values of samples coated CH/PA/Ba/PA, CH/PA /CH/PA, PA/Ba/PA/Ba were 1.5 mm/s and 22.0, 2.0 mm/s and 20.2 and 2.5 mm/s and 18.0 respectively, compared to 3.3 mm/s and 16.2 of uncoated samples. The samples with intumescent flame retardant system and metal ions (CH/PA/Ba/PA) had best flame retardant performance. Moreover its weight gain (5.2%) was less than that (10.7%) of the sample only with intumescent flame retardant system (CH/PA /CH/PA). The less weight gain had less effect on the softness of cotton fabrics. As a result, the coating is expected to be applied in practice. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Cotton fabrics are used in every aspect of people's daily life [1], however, they are extremely flammable and can complete combustion quickly in air [2]. All the time, fire problems caused by cotton fabrics pose a serious threat to people's lives and property. Therefore, it is of great practical significance to improve the flame retardant properties of cotton fabrics [3]. Moreover, with the development of science and technology and the advancement of society, the greening of flame retardants has become an inevitable choice for flame retardant technology. The use of bio-based materials in nature as a flame retardant meets the requirements of a green strategy, which not only mitigates the energy crisis, but also does not cause environmental pollution [4]. Furthermore, some halogenated flame retardants in traditional flame retardants are toxic, carcinogenic and bio-accumulative and so on for ⁎ Corresponding author at: Department of Material Science, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China. E-mail address: [email protected] (Y. Wang).
https://doi.org/10.1016/j.ijbiomac.2019.08.049 0141-8130/© 2019 Elsevier B.V. All rights reserved.
humans and animals [5,6]. Accordingly, renewable and environmentally friendly bio-based materials are potential alternatives to traditional flame retardants in the aspect of energy protection and environmental safety. Recently, chitosan as a polysaccharide consists of N-acetyl glucosamine and glucosamine residues which are connected by a-b (1–4) linkage and is the deacetylated derivatives of chitin which is an ecoenvironmental and carbon-rich natural polymer [7]. In many applications of chitosan, chitosan has been found to be used in the flame retardant treatment of cotton fabrics and is one of the components of an effective intumescent flame retardant [8–11]. Usually, not only chitosan is used to act as a carbon source due to the polyhydroxy structure, but also can be as a gas source because of the existence of nitrogen element, which results in the release of ammonia when chitosan degrades [12]. Phytic acid is a natural phosphorus-containing compound that is mainly stored in plant tissues (such as soybean seeds, chaff or oil grains) [13]. As a biocompatible and environmentally friendly organic phosphate compound, phytic acid has been widely used in various fields, especially in the flame retardant field [14–17]. One molecule of the phytic acid
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includes six molecules of phosphoric acid, and the phosphorus content is as high as 28%, which can potentially be used as a phosphoruscontaining flame retardant [14,18]. The halogen-free, non-toxic phytic acid which acts as acid source of intumescent flame retardant will significantly improve the flame retardant performance of the cotton fabrics. In previous research, the flame retardant coating, composed of cationic chitosan and anionic phytic acid and deposited on cotton fabrics by layer-by-layer assembly, has been proven to reduce flammability of cotton fabrics [19]. However, the work shown that only when the number of deposited layers reached 30 bilayers, it presented good flame retardant effect. This implies that the preparation process of flame retardant coatings is tedious and time-consuming. In addition, the weight gain of cotton fabrics was up to 18%, supposedly, which seriously affected the hand-feeling and softness of cotton fabrics. In our work, alkalinization pretreatment of cotton fabrics was carried out, which could improve the absorption capacity of flame retardant, increase the reactivity of fibers, and increase the pore diameter of micropores inside fibers. Then a dip-roll-dry experimental process was carried out. Rolling process could not only distribute the flame retardant more evenly on cotton fabrics, but also press the flame retardant into the gap of cotton fabrics. Drying process could allow the flame retardant to penetrate into the fiber sufficiently. Above experimental process could reduce the number of deposition, thereby reducing the experimental time. Finally, the flame retardant coating deposited on cotton fabrics, composed of chitosan, phytic acid and barium ion, was built by a dip-roll-dry process successfully. The results showed that a good flame retardant effect was achieved only by four depositions. In addition, the weight gain was about 5%, which has less effect on the softness of cotton fabrics. It has been reported that some metal ions (Zn2+, K+, Ca2+, Ba2+, Ni2 + , Co2+, Cu2+, Mn2+…) have been used to promote the reaction of intumescent flame retardants to improve their flame retardant efficiency [20–24]. These metal ions could be used not only as a catalyst for crosslinking of the resin to improve the thermal stability of composite material, but also could be used as a catalyst for the dehydrogenation reaction to increase the amount of char formation. Moreover, the direct use of metal ions as a synergist is simpler and easier than synthesis of new flame retardants. In our work, it was found that barium ions could have better catalytic effect in preparation experiment. Therefore, we directly use barium ions as synergists to increase the efficiency of intumescent flame retardants and reduce weight gain of cotton fabrics, which not only reduces the impact on the softness of cotton fabrics, but also reduces costs to a certain extent.
2.2. Fabrication of flame retardant coating Before deposition, the cotton fabrics were immersed in 10 wt% NaOH at room temperature for 10 min (mercerization process) and then dried in an oven. For the samples containing CH/PA/Ba/PA, they were firstly soaked in chitosan solution for 5 min, and then padded with roller. The Padded cotton fabrics were dried in an oven at 80 °C for 3 min. And then they were dipped PA, Ba, PA solutions in order using the same experimental steps. Finally, the coated samples were dried in an oven at 60 °C for 3 h. Fig. 1 shows the structures of chitosan, phytic acid and barium phytate and the coating process based on chitosan, phytic acid and barium ion by dip-pad-dry method. For the samples containing CH/PA/CH/PA and containing PA/Ba/PA/Ba, they were dipped, rolled and dried according to the above method, until the completion of four depositions. 2.3. Characterization and measurements The weight of the coating on the cotton fabrics was calculated on the basis of the equation: Adding-on = (W1\\W)/W × 100%, where W is the weight of the uncoated fabrics and W1 is the weight of coated fabrics. The micro morphology of all cotton fabrics and the char residues after burning was viewed by a scanning electron microscopy (SEM, S3400, Hitachi, Japan). Thermo gravimetric analysis (TGA) was performed on a TGA apparatus(STA PT 1600, Linseis, Germany) from room temperature to 600 °C at a heating rate of 15 °C/min under nitrogen and air atmosphere. The Microscale Combustion Calorimeter tests (MCC, developed by the Federal Aviation Administration) were carried out on a micro calorimeter (PCFC) from Fire Test Technology Limited of UK. The sample weights were 4.5–5 mg and were heated to 600 °C at a heating rate of 1 °C/s. For the purpose of qualitative analysis, the horizontal flame tests were performed by methane flame igniting the specimens for 3 s, whose sizes are 80 × 160 mm2. The limiting oxygen index (LOI) was measured with oxygen index meter (5801A) produced in VOUCH of China. The samples of 120 mm × 50 mm were combust in the oxygen and nitrogen mixture according to GB/T 5454 standard. To characterize the water durability of flame retardant cotton fabrics, the treated samples were washed with mechanical stirring at 300 rpm in deionized water for 6 h, and then dried at 60 °C for 6 h. 3. Results and discussion
2. Experimental section
3.1. Characterization of the coating
2.1. Materials
From Fig. 2, it is obvious that the weight gains of all treated fabrics are increased. The order of the weight gain of coated samples is containing CH/PA/CH/PA N containing CH/PA/Ba/PA N containing PA/Ba/PA/Ba. In general, the more weight gains, the better the flame retardant effect and the worse the softness, thus, the weight gain of the coating containing both intumescent flame retardant system and metal ions (CH/PA/ Ba/PA) is appropriate, which can not only improve the flame retardancy but also reduce the impact on softness of cotton fabrics. According to weight gain, it could be preliminarily guessed that the coatings were deposited on cotton fabrics successfully. SEM was used to investigate the surface micromorphology of cotton fabrics. In Fig. 3, the SEM pictures with different magnification of all samples were obtained. As presented in Fig. 3a–c, uncoated cotton fabrics have a smooth and neat surface. The adjacent fibers are not connected, which implies that the softness of cotton fabrics is very good. It can be seen from Fig. 3d–f that small particles adhere on the surface of the cotton fabrics treated with PA/Ba/PA/Ba, which makes the surface rough, and the adjacent fibers are not connected, which has little effect on the softness of the cotton fabrics. From Fig. 3g–i, the surface of the cotton fabrics treated by CH/PA/CH/PA is smooth and the adjacent fibers
Cotton fabrics (121 g/m2) were gained from Hongda Fabric Industry in Hebei of China. The cotton fabrics were washed with common detergent, ethyl alcohol and deionized water, and then dried for 6 h at 60 °C. Chitosan (powder, MW~ 50–190 kDa, 80–95% deacetylated, BR) was gained from Sinopharm Chemical Reagent Co., Ltd. Then, its deposition solutions were prepared by adding 1 wt% CH to deionized water whose pH was adjusted to 2 with hydrochloric acid. The solutions were magnetically stirred more than 24 h until the chitosan was dissolved. Phytic acid (70 wt% aqueous solutions, MW~660.04) was gained from Aladdin Chemistry Co., Ltd. Its deposition solutions were prepared by adding 3 wt% PA to deionized water and then stirred for 24 h. The pH of CH solutions and PA solutions were adjusted to 4 by HCl and NaOH respectively prior to deposition. Barium chloride was purchased from Kermel Company of China. Its aqueous solution concentration was 4 g/L. Deionized water with a resistance of 18.2 M was used in all experimental process. All above chemicals were analytical grade without further purification and were used as received without further purification.
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Fig. 1. The structures of chitosan, phytic acid and barium phytate and the coating process based on chitosan, phytic acid and barium ion by dip-pad-dry method.
are completely connected, which greatly affects the softness of the cotton fabrics and makes the cotton fabrics hard. In Fig. 3j–l, after treatment with CH/PA/Ba/PA, although there are convex parts on the surface of cotton fabrics, it still looks smooth, which means that the small particles produced by PA/Ba are wrapped in the film produced by CH. Moreover, almost no fibers are connected, which suggests that the coating has less effect on the softness of the cotton fabrics. The above results indirectly suggest the degree of softness of cotton fabrics and show that the coating was successfully deposited on cotton. 3.2. Thermal stability of all cotton fabrics TGA was carried out to obtain the data about thermal degradation process by measuring weight loss of materials as a function of temperature. Fig. 4a shows the TGA curves of all samples under a nitrogen
atmosphere. The relevant data including the initial degradation temperature (T-10%), the temperature of the maximum degradation rate (Tmax) and the amount of char residues from TGA is collected in Table 1. It is obvious that there is a one-step main decomposition occurring at 280–400 °C. Compared with the control, the coated cotton fabrics display the lower T-10%, which is mostly owing to the coating degradation at early stage. As well as, in all coated samples, both intumescent flame retardant system and metal ions work together to improve the thermal stability of the coating due to highest T-10% for CH/PA/Ba/PA coating. And metal ions could play a more important role, because the metal phytate complex formed during the coating process has higher thermal stability so that the T-10% of coated cotton fabrics containing metal ions are higher [25]. In addition, Tmax of coated samples shifts to lower temperature compared to the control. The main reason could be the phosphate groups promote the formation of char layer covering the surface of the
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Fig. 2. Relationship of weight gains and the coating of the different formulation. (Standard deviations of five measurements are given in brackets.)
cotton fabrics, which can prevent heat transfer. So their main decomposition temperature ranges get wider. Moreover, all coated cotton fabrics display sharp increase in char residues at 600 °C. The char residues are
increased to 17.0%, 29.2% and 29.4% respectively compared to 0% char residues of the control. It is noted that the char residues of cotton fabrics coated CH/PA/Ba/PA have highest char residue, which indicates metal ions could improve the ability of char forming of intumescent flame retardant system. Fig. 4b displays the TGA curves of all samples under air atmosphere. The relevant data is listed in Table 1. Unlike in nitrogen atmosphere, two steps are seen as previously reported [22] and the main stage of degradation occurs at 260–400 °C. Similarly, T-10% of coated samples are shifted down to lower temperature owing to the coating degradation at an early stage. The cotton fabrics coated with CH/PA/Ba/PA have better thermal stability due to highest T-10% among coated samples. This phenomenon could be because the metal phytate complex formed during the coating process has higher thermal stability and the intumescent flame retardant system and metal ions work together to improve the thermal stability of coated samples. Compared with the uncoated cotton fabrics, the Tmax of all coated cotton fabrics is lower, so the main decomposition temperature ranges get wider. This could be because that formed char from coatings cover the surface of cotton fabrics to hinder the release of degradation products and slow down the rate of degradation. The char residues at 600 °C are 0.0%, 2.4%, 10.3% and 18.9% for untreated cotton fabrics and treated cotton fabrics coated with PA/Ba/PA/
Fig. 3. SEM images of cotton fabrics uncoated (a–c), coated PA/Ba/PA/Ba (d–f), coated CH/PA/CH/PA (g–i), coated CH/PA/Ba/PA (j–l).
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Fig. 4. TGA curves of all cotton fabrics under nitrogen atmosphere (a) and air atmosphere (b).
Table 1 TGA data of all of cotton fabrics under nitrogen atmosphere and air atmosphere. (Standard deviations of five measurements are given in brackets.) Sample
Add-on (wt%)
Control PA/Ba/PA/Ba CH/PA/CH/PA CH/PA/Ba/PA
0.6 [0.3] 10.7 [0.5] 5.2 [0.5]
T -10%(°C)
T max(°C)
600 °C char residues (wt%)
N2
Air
N2
Air
N2
314.2 278.6 278.5 284.1
331.8 268.4 262.0 291.9
360.9 332.4 323.0 319.1
355.7 324.6 316.2 318.6
0 17.0 29.2 29.4
Air 0 2.4 10.3 18.9
Ba, CH/PA/CH/PA and CH/PA/Ba/PA, respectively. These results explain that the addition of metal ions improved the thermal stability of intumescent flame retardant coating and enhanced the ability of stable char residues to decrease the decomposition rate of cotton fabrics. 3.3. Microscale combustion calorimetry Previous research work has verified that the Microscale Combustion Calorimetry(MCC) measurement is convenient and well related to traditional flame tests [26]. Using this test method, some critical flame parameters can be acquired, including the peak heat release rate (pHHR; W/g), the temperature at the peak heat release rate (Tp; °C), the heat release capacity (HRC; J/(g·K)), the total heat release (THR; kJ/g) and the char residue yield. So it is applied for evaluating the flame retardant properties of all samples. Fig. 5 exhibits that the heat release rate
(HRR) values of all samples change with temperature. And the primary flaming parameters of the untreated and treated cotton fabrics are displayed in Table 2. According to Fig. 5 and Table 2, compared to untreated cotton fabrics, the treated cotton fabrics have lower pHRR, Tp, HRC and THR values and higher char residues yield. For the contrast of the control and three coated cotton fabrics (PA/Ba/PA/Ba, CH/PA/CH/PA and CH/PA/Ba/PA), the reduction values of pHRR are 45.0%, 56.6% and 61.2% respectively. It is obvious that when the intumescent flame retardant system and metal ions are present at the same time, the peak heat release rate reaches the most reduction values. Meanwhile, the values of HRC and THR also decrease from 331.7 J/(g·K) to 126.3 J/(g·K) and from 13.1 kJ/g to 5.4 kJ/g respectively. What is noteworthy is that the Tp values for the treated cotton fabrics shift in the direction of lower temperature in comparison with that of the control, from 375.7°Cto368.4 °C、352.6 °C and 341.6 °C respectively. The sample coated CH/PA/Ba/ PA reaches the lowest Tp values. Moreover, the char residues increase noticeably from 3.65% for the control to 18.5, 26.8 and 29.7% for the cotton fabrics coated PA/Ba/PA/Ba, CH/PA/CH/PA and CH/PA/Ba/PA, respectively. As mentioned above, both intumescent flame retardant system and metal ions are flame retardant contributors and they play a synergistic effect to reach the better flame retardant performance. These phenomena could be due to the formation of phytic acid metal salts during the coating process, which contributes to the improvement of the crosslinking degree and thermal stability of the intumescent flame retardant at high temperature and the improvement of the char formation ability. Moreover, phytic acid metal salts can improve the quality of the char layer, which could block gas and heat exchange more effectively. Thus CH/PA/Ba/PA could be an efficient coating for improving flame retardant property [9]. 3.4. Horizontal flame test The horizontal flame configuration was used to assess the flame retardant properties of all samples further. Fig. 6 shows the photos of the char residues after the horizontal flame test. It is seen that the uncoated cotton fabrics consume completely and there are no char residues. On
Table 2 MCC data of all cotton fabrics. (Standard deviations of three measurements are given in brackets.) Sample
Fig. 5. Heat release rate (HRR) curves of all cotton fabrics.
Control PA/Ba/PA/Ba CH/PA/CH/PA CH/PA/Ba/PA
pHRR (W/g) 333.1 [5.5] 183.1 [6.1] 144.4 [5.4] 129.1 [5.9]
Tp (°C)
HRC (J/g·K)
THR (kJ/g)
Residue (%)
374.1 [1.1] 368.2 [0.2] 350.2 [2.4] 343.6 [2.7]
331.7 [3.8] 180.7 [6.1] 141.0 [6.2] 126.3 [4.9]
13.1 [0.4] 8.5 [0.3] 6.1 [0.2] 5.4 [0.2]
3.65 [1.4] 18.5 [1.4] 26.8 [1.0] 29.7 [0.8]
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Fig. 6. Photos of all cotton fabrics after the horizontal flame test.
the contrary, for the coated cotton fabrics, a lot of char residues are gained after the horizontal flame test. The samples coated PA/Ba/PA/ Ba shows fragmentary and fractured char residues, while the samples coated with CH/PA/CH/PA and CH/PA/Ba/PA show intact and continuous char residues, which could be owing to lack of carbon source in the samples coated with PA/Ba/PA/Ba to reduce char residues formation ability. In addition, the data from horizontal flame test are listed in Table 3. It is observed that the burning rates of the samples coated PA/ Ba/PA/Ba, CH/PA/CH/PA and CH/PA/Ba/PA are lower up to 2.5 mm/s,2.0 mm/s and 1.5 mm/s respectively compared to that of the control up to 3.3 mm/s. Among these, the samples coated CH/PA/Ba/ PA have the lowest burning rate. This could be because the addition of metal ions can promote char formation ability which can hinder flammable gases and heat transfer so that the flame spread rate on the treated cotton fabrics is decreased [20,27]. Therefore, the synergistic effect of intumescent flame retardant system and metal ions is achieved. Furthermore, Fig. 7 shows that the SEM pictures of the char residues of the coated cotton fabrics after horizontal burning. It is clear that the weave structure is maintained very well for the coated cotton fabrics and the gaps among the yarns slightly increase a bit after burning. But the gaps among the yarns of the char residues of the samples treated with PA/Ba/PA/Ba is larger and some of fibers are also burn off, which explains that its flame retardant effect is not as well as the samples treated with CH/PA/CH/PA and CH/PA/Ba/PA. This could be lack of char forming agent in the samples treated with PA/Ba/PA/Ba. Comparing the cotton fabrics coated CH/PA/CH/PA with the cotton fabrics coated CH/PA/Ba/PA, the only difference between the both is that there are much more bubbles on the fiber treated with CH/PA/Ba/PA. All the fibers are remained under the bubbles which act as a barrier to hinder heat and flammable volatile substance transfer. Therefore, the coatings treated with CH/PA/Ba/PA have the better role to protect the underlying cotton fabrics during combustion. Table 3 Flammability data of the untreated and coated cotton fabrics. Sample Control PA/Ba/PA/Ba CH/PA/CH/PA CH/PA/Ba/PA Washed PA/Ba/PA/Ba Washed CH/PA/CH/PA Washed CH/PA/Ba/PA
Char length (mm)
Burning rate (mm/s)
16 14 14 14 14 14 14
3.3 ± 0.1 2.5 ± 0.1 2.0 ± 0.1 1.5 ± 0.1 2.8 ± 0.2 2.3 ± 0.1 1.8 ± 0.1
3.5. Limit oxygen index test The limiting oxygen index (LOI) refers to the minimal volume fraction concentration of oxygen in the oxygen and nitrogen mixed gas when it can support combustion of a particular polymer. LOI is an important parameter to estimate flame retardant property of materials. Table 4 demonstrates that the LOI values of all samples. It is obvious that the LOI values of all coated cotton fabrics are higher than that of uncoated cotton fabrics, which mean that all coated cotton fabrics are more difficult to combust compared to the control. Moreover, the LOI value of CH/PA/Ba/PA is higher than that of CH/PA/CH/PA and the value of CH/PA/CH/PA is higher than that of PA/Ba/PA/Ba, which indicates that the order of flame retardancy is CH/PA/Ba/PA N CH/PA/CH/ PA N PA/Ba/PA/BaNthe control. The Above results show that only when the intumescent flame retardant system and metal ions exist at the same time, can the better flame retardant effect be achieved. In order to prove synergy in a quantitative way, Lewin's Synergistic Effectiveness parameter [28], Es may be defined as: ES ¼ LOI½frþs −LOIc =f﹙LOIfr −LOIc ﹚ þ ﹙LOIs −LOIc ﹚g where LOI is the effective parameter of flame retardancy and LOIc, LOIfr, LOIs, LOI[fr+s],are the values for cotton fabrics, cotton fabrics plus flame retardant, cotton fabrics plus synergist, cotton fabrics plus the combined synergist and flame retardant, respectively. LOIc, LOIfr, LOIs and LOI[fr+s] obtained by the oxygen index test are 16.2, 20.2, 16.4, and 22.0,respectively. Thus, use of Lewin's Synergistic Effectiveness parameter (ES = 1.4) showed that the synergistic effectiveness of metal ions increased the flame retardant effect of the intumescent flame retardant system by nearly 1.4 times with respect to limiting oxygen index (LOI) as the measure of flame retardancy. However, the data could not be very accurate, because the SE values (Lewin's Synergistic Effectiveness parameter) are based in most cases on results obtained for additive/synergist ratios yielding the highest effect [29]. Compared to the Es calculated by the additive flame retardant synergist, the Es calculated from the coating containing the synergist could have error duo to uncertain additive/synergist ratios, but the calculation results do indicate this trend that the metal ion and the intumescent flame retardant system play a synergistic effect. In summary, intumescent flame retardant system and metal ions exhibit synergy effect to improve the fire safety of cotton fabrics. A schematic diagram of a possible flame retardant mechanism is exemplified in Fig. 8. Firstly, metal ions act as a catalyst for dehydrogenation to
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Fig. 7. SEM images of coated cotton fabrics after the horizontal flame test.
Table 4 The LOI values of all cotton fabrics. Sample
Untreat
PA/Ba/PA/Ba
CH/PA/CH/PA
CH/PA/Ba/PA
LOI(%)
16.2
18.0
20.2
22.0
catalyze char formation which could prevent heat and oxygen transmission. During this period, the release of water can dilute the concentration of flammable gases and remove part of the heat. Secondly, metal
ions act as a catalyst crosslinking of polymers to form higher quality char layer, which could play a more effective barrier compared with formed char layer when there is no metal ion. Thirdly, metal ions could facilitate to releases a large number of non-combustion gases such as H2O and etc. The non-combustion gases are helpful for both diluting the concentration of the flammable gases and absorbing heat energy. Therefore, the flame retardant operates in both condensed phase and gaseous phases during the burning process for cotton fabrics, suggesting that the intumescent flame retardant system and metal ions exhibit a synergistic effect.
Fig. 8. Schematic illustration of a possible flame retardant mechanism for cotton fabrics treated with CH/PA/Ba/PA during burning.
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References
Fig. 9. Photos of all washed cotton fabrics after the horizontal flame test.
3.6. Water durability of coated cotton fabrics In order to evaluate the washing durability of the coatings, the coated cotton fabrics were washed with mechanical stirring at 300 rpm in deionized water for 6 h. After washing, the horizontal flame test was carried out. Table 3 lists the burning rates of washed samples coated with PA/Ba/PA/Ba, CH/PA/CH/PA and CH/PA/Ba/PA. The burning rates of washed samples were only increased a little compared with those before washing, but their burning rates are lower than that of the untreated cotton fabrics, which means that the washed samples still have flame retardancy. Meanwhile, as shown in Fig. 9, the char residues of washed cotton fabrics were preserved well after the horizontal flame test. In these char residues, the char residue of the sample coated with CH/PA/Ba/PA was the more complete and continuous. This could be because that metal ions act as a catalyst of crosslinking of polymers to form higher quality char layer. The washing durability test in the work is not as strict as laundering process, but these results explain that the water durability is obtained after coating treatment. 4. Conclusion In conclusion, the eco-friendly flame retardant coating, composed of bio-based intumescent flame retardant system (chitosan and phytic acid) and metal ions, was successfully deposited on cotton fabrics and effectively improved the thermal stability and flame retardant performance of cotton fabrics by using dip-roll-dry method. Several characterization and measurements including scanning electron microscopy (SEM), thermo gravimetric analysis (TGA), microscale combustion calorimeter (MCC), horizontal flame tests (HFT) and limit oxygen index (LOI) have been used to examine the flame retardant property of all samples. Among these data, char residues and LOI values are increased from 3.65 to 29.7% and from 16.2 to 22.0, respectively. The burning rate and the peak heat release rate (pHHR) are decreased from 3.3 to 1.5 mm/s and from 331.1 to 129.1 W/g. All data and results confirmed that the sample coated CH/PA/Ba/PA had the better flame retardancy, which was attributed to the synergistic effect of the intumescent flame retardant system and metal ions. In addition, the weight gain was less so that this had less effect on the softness of cotton fabrics. Moreover, the experimental process was simple and not time consuming compared to layer-by-layer assembly method. Therefore, the biobased intumescent flame retardant system with metal ions and the experimental method had the choice to be used in practice.
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International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Eco-friendly flame retardant coating deposited on cotton fabrics from bio-based chitosan, phytic acid and divalent metal ions Zhihao Zhang, Zhongying Ma, Qian Leng, Yuhua Wang ⁎ Department of Material Science, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, China
a r t i c l e
i n f o
Article history: Received 2 May 2019 Received in revised form 19 July 2019 Accepted 6 August 2019 Available online 12 August 2019 Keywords: Bio-based flame retardant Coating Cotton fabrics Chitosan Phytic acid Metal ion
a b s t r a c t With the increasing awareness of environmental protection, the greening of flame retardants has become an inevitable choice for flame retardant technology. Bio-based materials have the advantages of wide source, recyclability and green environmental protection, and have received extensive attention. In this work, chitosan and phytic acid as intumescent flame retardant system and metal ion as a synergist were built on cotton fabrics to achieve efficient flame retardancy by facile dip-pad-dry process. Microscale combustion calorimetry results manifested that the heat release rate values of coated samples were lower than that of uncoated samples. The peak heat release rate and total heat release of uncoated samples were 333.1 W/g and 13.1 kJ/g, while these values of samples coated CH/PA/Ba/PA were 129.1 W/g and 5.4 kJ/g respectively and char residues also were increased sharply owing to the phosphate groups promote the formation of char layer covering the surface of the cotton fabrics, which can prevent heat transfer. Scanning electron microscopy confirmed that there were much more bubbles on the surface of fibers for the coated CH/PA/Ba/PA after horizontal flame tests. The results indicated that the synergy of the intumescent flame retardant system was achieved by the addition of metal ions which could accelerate the produce of a large number of nonflammable gases. Horizontal flame test and limit oxygen index (LOI) test showed that the burning rate and LOI values of samples coated CH/PA/Ba/PA, CH/PA /CH/PA, PA/Ba/PA/Ba were 1.5 mm/s and 22.0, 2.0 mm/s and 20.2 and 2.5 mm/s and 18.0 respectively, compared to 3.3 mm/s and 16.2 of uncoated samples. The samples with intumescent flame retardant system and metal ions (CH/PA/Ba/PA) had best flame retardant performance. Moreover its weight gain (5.2%) was less than that (10.7%) of the sample only with intumescent flame retardant system (CH/PA /CH/PA). The less weight gain had less effect on the softness of cotton fabrics. As a result, the coating is expected to be applied in practice. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Cotton fabrics are used in every aspect of people's daily life [1], however, they are extremely flammable and can complete combustion quickly in air [2]. All the time, fire problems caused by cotton fabrics pose a serious threat to people's lives and property. Therefore, it is of great practical significance to improve the flame retardant properties of cotton fabrics [3]. Moreover, with the development of science and technology and the advancement of society, the greening of flame retardants has become an inevitable choice for flame retardant technology. The use of bio-based materials in nature as a flame retardant meets the requirements of a green strategy, which not only mitigates the energy crisis, but also does not cause environmental pollution [4]. Furthermore, some halogenated flame retardants in traditional flame retardants are toxic, carcinogenic and bio-accumulative and so on for ⁎ Corresponding author at: Department of Material Science, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China. E-mail address: [email protected] (Y. Wang).
https://doi.org/10.1016/j.ijbiomac.2019.08.049 0141-8130/© 2019 Elsevier B.V. All rights reserved.
humans and animals [5,6]. Accordingly, renewable and environmentally friendly bio-based materials are potential alternatives to traditional flame retardants in the aspect of energy protection and environmental safety. Recently, chitosan as a polysaccharide consists of N-acetyl glucosamine and glucosamine residues which are connected by a-b (1–4) linkage and is the deacetylated derivatives of chitin which is an ecoenvironmental and carbon-rich natural polymer [7]. In many applications of chitosan, chitosan has been found to be used in the flame retardant treatment of cotton fabrics and is one of the components of an effective intumescent flame retardant [8–11]. Usually, not only chitosan is used to act as a carbon source due to the polyhydroxy structure, but also can be as a gas source because of the existence of nitrogen element, which results in the release of ammonia when chitosan degrades [12]. Phytic acid is a natural phosphorus-containing compound that is mainly stored in plant tissues (such as soybean seeds, chaff or oil grains) [13]. As a biocompatible and environmentally friendly organic phosphate compound, phytic acid has been widely used in various fields, especially in the flame retardant field [14–17]. One molecule of the phytic acid
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includes six molecules of phosphoric acid, and the phosphorus content is as high as 28%, which can potentially be used as a phosphoruscontaining flame retardant [14,18]. The halogen-free, non-toxic phytic acid which acts as acid source of intumescent flame retardant will significantly improve the flame retardant performance of the cotton fabrics. In previous research, the flame retardant coating, composed of cationic chitosan and anionic phytic acid and deposited on cotton fabrics by layer-by-layer assembly, has been proven to reduce flammability of cotton fabrics [19]. However, the work shown that only when the number of deposited layers reached 30 bilayers, it presented good flame retardant effect. This implies that the preparation process of flame retardant coatings is tedious and time-consuming. In addition, the weight gain of cotton fabrics was up to 18%, supposedly, which seriously affected the hand-feeling and softness of cotton fabrics. In our work, alkalinization pretreatment of cotton fabrics was carried out, which could improve the absorption capacity of flame retardant, increase the reactivity of fibers, and increase the pore diameter of micropores inside fibers. Then a dip-roll-dry experimental process was carried out. Rolling process could not only distribute the flame retardant more evenly on cotton fabrics, but also press the flame retardant into the gap of cotton fabrics. Drying process could allow the flame retardant to penetrate into the fiber sufficiently. Above experimental process could reduce the number of deposition, thereby reducing the experimental time. Finally, the flame retardant coating deposited on cotton fabrics, composed of chitosan, phytic acid and barium ion, was built by a dip-roll-dry process successfully. The results showed that a good flame retardant effect was achieved only by four depositions. In addition, the weight gain was about 5%, which has less effect on the softness of cotton fabrics. It has been reported that some metal ions (Zn2+, K+, Ca2+, Ba2+, Ni2 + , Co2+, Cu2+, Mn2+…) have been used to promote the reaction of intumescent flame retardants to improve their flame retardant efficiency [20–24]. These metal ions could be used not only as a catalyst for crosslinking of the resin to improve the thermal stability of composite material, but also could be used as a catalyst for the dehydrogenation reaction to increase the amount of char formation. Moreover, the direct use of metal ions as a synergist is simpler and easier than synthesis of new flame retardants. In our work, it was found that barium ions could have better catalytic effect in preparation experiment. Therefore, we directly use barium ions as synergists to increase the efficiency of intumescent flame retardants and reduce weight gain of cotton fabrics, which not only reduces the impact on the softness of cotton fabrics, but also reduces costs to a certain extent.
2.2. Fabrication of flame retardant coating Before deposition, the cotton fabrics were immersed in 10 wt% NaOH at room temperature for 10 min (mercerization process) and then dried in an oven. For the samples containing CH/PA/Ba/PA, they were firstly soaked in chitosan solution for 5 min, and then padded with roller. The Padded cotton fabrics were dried in an oven at 80 °C for 3 min. And then they were dipped PA, Ba, PA solutions in order using the same experimental steps. Finally, the coated samples were dried in an oven at 60 °C for 3 h. Fig. 1 shows the structures of chitosan, phytic acid and barium phytate and the coating process based on chitosan, phytic acid and barium ion by dip-pad-dry method. For the samples containing CH/PA/CH/PA and containing PA/Ba/PA/Ba, they were dipped, rolled and dried according to the above method, until the completion of four depositions. 2.3. Characterization and measurements The weight of the coating on the cotton fabrics was calculated on the basis of the equation: Adding-on = (W1\\W)/W × 100%, where W is the weight of the uncoated fabrics and W1 is the weight of coated fabrics. The micro morphology of all cotton fabrics and the char residues after burning was viewed by a scanning electron microscopy (SEM, S3400, Hitachi, Japan). Thermo gravimetric analysis (TGA) was performed on a TGA apparatus(STA PT 1600, Linseis, Germany) from room temperature to 600 °C at a heating rate of 15 °C/min under nitrogen and air atmosphere. The Microscale Combustion Calorimeter tests (MCC, developed by the Federal Aviation Administration) were carried out on a micro calorimeter (PCFC) from Fire Test Technology Limited of UK. The sample weights were 4.5–5 mg and were heated to 600 °C at a heating rate of 1 °C/s. For the purpose of qualitative analysis, the horizontal flame tests were performed by methane flame igniting the specimens for 3 s, whose sizes are 80 × 160 mm2. The limiting oxygen index (LOI) was measured with oxygen index meter (5801A) produced in VOUCH of China. The samples of 120 mm × 50 mm were combust in the oxygen and nitrogen mixture according to GB/T 5454 standard. To characterize the water durability of flame retardant cotton fabrics, the treated samples were washed with mechanical stirring at 300 rpm in deionized water for 6 h, and then dried at 60 °C for 6 h. 3. Results and discussion
2. Experimental section
3.1. Characterization of the coating
2.1. Materials
From Fig. 2, it is obvious that the weight gains of all treated fabrics are increased. The order of the weight gain of coated samples is containing CH/PA/CH/PA N containing CH/PA/Ba/PA N containing PA/Ba/PA/Ba. In general, the more weight gains, the better the flame retardant effect and the worse the softness, thus, the weight gain of the coating containing both intumescent flame retardant system and metal ions (CH/PA/ Ba/PA) is appropriate, which can not only improve the flame retardancy but also reduce the impact on softness of cotton fabrics. According to weight gain, it could be preliminarily guessed that the coatings were deposited on cotton fabrics successfully. SEM was used to investigate the surface micromorphology of cotton fabrics. In Fig. 3, the SEM pictures with different magnification of all samples were obtained. As presented in Fig. 3a–c, uncoated cotton fabrics have a smooth and neat surface. The adjacent fibers are not connected, which implies that the softness of cotton fabrics is very good. It can be seen from Fig. 3d–f that small particles adhere on the surface of the cotton fabrics treated with PA/Ba/PA/Ba, which makes the surface rough, and the adjacent fibers are not connected, which has little effect on the softness of the cotton fabrics. From Fig. 3g–i, the surface of the cotton fabrics treated by CH/PA/CH/PA is smooth and the adjacent fibers
Cotton fabrics (121 g/m2) were gained from Hongda Fabric Industry in Hebei of China. The cotton fabrics were washed with common detergent, ethyl alcohol and deionized water, and then dried for 6 h at 60 °C. Chitosan (powder, MW~ 50–190 kDa, 80–95% deacetylated, BR) was gained from Sinopharm Chemical Reagent Co., Ltd. Then, its deposition solutions were prepared by adding 1 wt% CH to deionized water whose pH was adjusted to 2 with hydrochloric acid. The solutions were magnetically stirred more than 24 h until the chitosan was dissolved. Phytic acid (70 wt% aqueous solutions, MW~660.04) was gained from Aladdin Chemistry Co., Ltd. Its deposition solutions were prepared by adding 3 wt% PA to deionized water and then stirred for 24 h. The pH of CH solutions and PA solutions were adjusted to 4 by HCl and NaOH respectively prior to deposition. Barium chloride was purchased from Kermel Company of China. Its aqueous solution concentration was 4 g/L. Deionized water with a resistance of 18.2 M was used in all experimental process. All above chemicals were analytical grade without further purification and were used as received without further purification.
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Fig. 1. The structures of chitosan, phytic acid and barium phytate and the coating process based on chitosan, phytic acid and barium ion by dip-pad-dry method.
are completely connected, which greatly affects the softness of the cotton fabrics and makes the cotton fabrics hard. In Fig. 3j–l, after treatment with CH/PA/Ba/PA, although there are convex parts on the surface of cotton fabrics, it still looks smooth, which means that the small particles produced by PA/Ba are wrapped in the film produced by CH. Moreover, almost no fibers are connected, which suggests that the coating has less effect on the softness of the cotton fabrics. The above results indirectly suggest the degree of softness of cotton fabrics and show that the coating was successfully deposited on cotton. 3.2. Thermal stability of all cotton fabrics TGA was carried out to obtain the data about thermal degradation process by measuring weight loss of materials as a function of temperature. Fig. 4a shows the TGA curves of all samples under a nitrogen
atmosphere. The relevant data including the initial degradation temperature (T-10%), the temperature of the maximum degradation rate (Tmax) and the amount of char residues from TGA is collected in Table 1. It is obvious that there is a one-step main decomposition occurring at 280–400 °C. Compared with the control, the coated cotton fabrics display the lower T-10%, which is mostly owing to the coating degradation at early stage. As well as, in all coated samples, both intumescent flame retardant system and metal ions work together to improve the thermal stability of the coating due to highest T-10% for CH/PA/Ba/PA coating. And metal ions could play a more important role, because the metal phytate complex formed during the coating process has higher thermal stability so that the T-10% of coated cotton fabrics containing metal ions are higher [25]. In addition, Tmax of coated samples shifts to lower temperature compared to the control. The main reason could be the phosphate groups promote the formation of char layer covering the surface of the
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Fig. 2. Relationship of weight gains and the coating of the different formulation. (Standard deviations of five measurements are given in brackets.)
cotton fabrics, which can prevent heat transfer. So their main decomposition temperature ranges get wider. Moreover, all coated cotton fabrics display sharp increase in char residues at 600 °C. The char residues are
increased to 17.0%, 29.2% and 29.4% respectively compared to 0% char residues of the control. It is noted that the char residues of cotton fabrics coated CH/PA/Ba/PA have highest char residue, which indicates metal ions could improve the ability of char forming of intumescent flame retardant system. Fig. 4b displays the TGA curves of all samples under air atmosphere. The relevant data is listed in Table 1. Unlike in nitrogen atmosphere, two steps are seen as previously reported [22] and the main stage of degradation occurs at 260–400 °C. Similarly, T-10% of coated samples are shifted down to lower temperature owing to the coating degradation at an early stage. The cotton fabrics coated with CH/PA/Ba/PA have better thermal stability due to highest T-10% among coated samples. This phenomenon could be because the metal phytate complex formed during the coating process has higher thermal stability and the intumescent flame retardant system and metal ions work together to improve the thermal stability of coated samples. Compared with the uncoated cotton fabrics, the Tmax of all coated cotton fabrics is lower, so the main decomposition temperature ranges get wider. This could be because that formed char from coatings cover the surface of cotton fabrics to hinder the release of degradation products and slow down the rate of degradation. The char residues at 600 °C are 0.0%, 2.4%, 10.3% and 18.9% for untreated cotton fabrics and treated cotton fabrics coated with PA/Ba/PA/
Fig. 3. SEM images of cotton fabrics uncoated (a–c), coated PA/Ba/PA/Ba (d–f), coated CH/PA/CH/PA (g–i), coated CH/PA/Ba/PA (j–l).
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Fig. 4. TGA curves of all cotton fabrics under nitrogen atmosphere (a) and air atmosphere (b).
Table 1 TGA data of all of cotton fabrics under nitrogen atmosphere and air atmosphere. (Standard deviations of five measurements are given in brackets.) Sample
Add-on (wt%)
Control PA/Ba/PA/Ba CH/PA/CH/PA CH/PA/Ba/PA
0.6 [0.3] 10.7 [0.5] 5.2 [0.5]
T -10%(°C)
T max(°C)
600 °C char residues (wt%)
N2
Air
N2
Air
N2
314.2 278.6 278.5 284.1
331.8 268.4 262.0 291.9
360.9 332.4 323.0 319.1
355.7 324.6 316.2 318.6
0 17.0 29.2 29.4
Air 0 2.4 10.3 18.9
Ba, CH/PA/CH/PA and CH/PA/Ba/PA, respectively. These results explain that the addition of metal ions improved the thermal stability of intumescent flame retardant coating and enhanced the ability of stable char residues to decrease the decomposition rate of cotton fabrics. 3.3. Microscale combustion calorimetry Previous research work has verified that the Microscale Combustion Calorimetry(MCC) measurement is convenient and well related to traditional flame tests [26]. Using this test method, some critical flame parameters can be acquired, including the peak heat release rate (pHHR; W/g), the temperature at the peak heat release rate (Tp; °C), the heat release capacity (HRC; J/(g·K)), the total heat release (THR; kJ/g) and the char residue yield. So it is applied for evaluating the flame retardant properties of all samples. Fig. 5 exhibits that the heat release rate
(HRR) values of all samples change with temperature. And the primary flaming parameters of the untreated and treated cotton fabrics are displayed in Table 2. According to Fig. 5 and Table 2, compared to untreated cotton fabrics, the treated cotton fabrics have lower pHRR, Tp, HRC and THR values and higher char residues yield. For the contrast of the control and three coated cotton fabrics (PA/Ba/PA/Ba, CH/PA/CH/PA and CH/PA/Ba/PA), the reduction values of pHRR are 45.0%, 56.6% and 61.2% respectively. It is obvious that when the intumescent flame retardant system and metal ions are present at the same time, the peak heat release rate reaches the most reduction values. Meanwhile, the values of HRC and THR also decrease from 331.7 J/(g·K) to 126.3 J/(g·K) and from 13.1 kJ/g to 5.4 kJ/g respectively. What is noteworthy is that the Tp values for the treated cotton fabrics shift in the direction of lower temperature in comparison with that of the control, from 375.7°Cto368.4 °C、352.6 °C and 341.6 °C respectively. The sample coated CH/PA/Ba/ PA reaches the lowest Tp values. Moreover, the char residues increase noticeably from 3.65% for the control to 18.5, 26.8 and 29.7% for the cotton fabrics coated PA/Ba/PA/Ba, CH/PA/CH/PA and CH/PA/Ba/PA, respectively. As mentioned above, both intumescent flame retardant system and metal ions are flame retardant contributors and they play a synergistic effect to reach the better flame retardant performance. These phenomena could be due to the formation of phytic acid metal salts during the coating process, which contributes to the improvement of the crosslinking degree and thermal stability of the intumescent flame retardant at high temperature and the improvement of the char formation ability. Moreover, phytic acid metal salts can improve the quality of the char layer, which could block gas and heat exchange more effectively. Thus CH/PA/Ba/PA could be an efficient coating for improving flame retardant property [9]. 3.4. Horizontal flame test The horizontal flame configuration was used to assess the flame retardant properties of all samples further. Fig. 6 shows the photos of the char residues after the horizontal flame test. It is seen that the uncoated cotton fabrics consume completely and there are no char residues. On
Table 2 MCC data of all cotton fabrics. (Standard deviations of three measurements are given in brackets.) Sample
Fig. 5. Heat release rate (HRR) curves of all cotton fabrics.
Control PA/Ba/PA/Ba CH/PA/CH/PA CH/PA/Ba/PA
pHRR (W/g) 333.1 [5.5] 183.1 [6.1] 144.4 [5.4] 129.1 [5.9]
Tp (°C)
HRC (J/g·K)
THR (kJ/g)
Residue (%)
374.1 [1.1] 368.2 [0.2] 350.2 [2.4] 343.6 [2.7]
331.7 [3.8] 180.7 [6.1] 141.0 [6.2] 126.3 [4.9]
13.1 [0.4] 8.5 [0.3] 6.1 [0.2] 5.4 [0.2]
3.65 [1.4] 18.5 [1.4] 26.8 [1.0] 29.7 [0.8]
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Fig. 6. Photos of all cotton fabrics after the horizontal flame test.
the contrary, for the coated cotton fabrics, a lot of char residues are gained after the horizontal flame test. The samples coated PA/Ba/PA/ Ba shows fragmentary and fractured char residues, while the samples coated with CH/PA/CH/PA and CH/PA/Ba/PA show intact and continuous char residues, which could be owing to lack of carbon source in the samples coated with PA/Ba/PA/Ba to reduce char residues formation ability. In addition, the data from horizontal flame test are listed in Table 3. It is observed that the burning rates of the samples coated PA/ Ba/PA/Ba, CH/PA/CH/PA and CH/PA/Ba/PA are lower up to 2.5 mm/s,2.0 mm/s and 1.5 mm/s respectively compared to that of the control up to 3.3 mm/s. Among these, the samples coated CH/PA/Ba/ PA have the lowest burning rate. This could be because the addition of metal ions can promote char formation ability which can hinder flammable gases and heat transfer so that the flame spread rate on the treated cotton fabrics is decreased [20,27]. Therefore, the synergistic effect of intumescent flame retardant system and metal ions is achieved. Furthermore, Fig. 7 shows that the SEM pictures of the char residues of the coated cotton fabrics after horizontal burning. It is clear that the weave structure is maintained very well for the coated cotton fabrics and the gaps among the yarns slightly increase a bit after burning. But the gaps among the yarns of the char residues of the samples treated with PA/Ba/PA/Ba is larger and some of fibers are also burn off, which explains that its flame retardant effect is not as well as the samples treated with CH/PA/CH/PA and CH/PA/Ba/PA. This could be lack of char forming agent in the samples treated with PA/Ba/PA/Ba. Comparing the cotton fabrics coated CH/PA/CH/PA with the cotton fabrics coated CH/PA/Ba/PA, the only difference between the both is that there are much more bubbles on the fiber treated with CH/PA/Ba/PA. All the fibers are remained under the bubbles which act as a barrier to hinder heat and flammable volatile substance transfer. Therefore, the coatings treated with CH/PA/Ba/PA have the better role to protect the underlying cotton fabrics during combustion. Table 3 Flammability data of the untreated and coated cotton fabrics. Sample Control PA/Ba/PA/Ba CH/PA/CH/PA CH/PA/Ba/PA Washed PA/Ba/PA/Ba Washed CH/PA/CH/PA Washed CH/PA/Ba/PA
Char length (mm)
Burning rate (mm/s)
16 14 14 14 14 14 14
3.3 ± 0.1 2.5 ± 0.1 2.0 ± 0.1 1.5 ± 0.1 2.8 ± 0.2 2.3 ± 0.1 1.8 ± 0.1
3.5. Limit oxygen index test The limiting oxygen index (LOI) refers to the minimal volume fraction concentration of oxygen in the oxygen and nitrogen mixed gas when it can support combustion of a particular polymer. LOI is an important parameter to estimate flame retardant property of materials. Table 4 demonstrates that the LOI values of all samples. It is obvious that the LOI values of all coated cotton fabrics are higher than that of uncoated cotton fabrics, which mean that all coated cotton fabrics are more difficult to combust compared to the control. Moreover, the LOI value of CH/PA/Ba/PA is higher than that of CH/PA/CH/PA and the value of CH/PA/CH/PA is higher than that of PA/Ba/PA/Ba, which indicates that the order of flame retardancy is CH/PA/Ba/PA N CH/PA/CH/ PA N PA/Ba/PA/BaNthe control. The Above results show that only when the intumescent flame retardant system and metal ions exist at the same time, can the better flame retardant effect be achieved. In order to prove synergy in a quantitative way, Lewin's Synergistic Effectiveness parameter [28], Es may be defined as: ES ¼ LOI½frþs −LOIc =f﹙LOIfr −LOIc ﹚ þ ﹙LOIs −LOIc ﹚g where LOI is the effective parameter of flame retardancy and LOIc, LOIfr, LOIs, LOI[fr+s],are the values for cotton fabrics, cotton fabrics plus flame retardant, cotton fabrics plus synergist, cotton fabrics plus the combined synergist and flame retardant, respectively. LOIc, LOIfr, LOIs and LOI[fr+s] obtained by the oxygen index test are 16.2, 20.2, 16.4, and 22.0,respectively. Thus, use of Lewin's Synergistic Effectiveness parameter (ES = 1.4) showed that the synergistic effectiveness of metal ions increased the flame retardant effect of the intumescent flame retardant system by nearly 1.4 times with respect to limiting oxygen index (LOI) as the measure of flame retardancy. However, the data could not be very accurate, because the SE values (Lewin's Synergistic Effectiveness parameter) are based in most cases on results obtained for additive/synergist ratios yielding the highest effect [29]. Compared to the Es calculated by the additive flame retardant synergist, the Es calculated from the coating containing the synergist could have error duo to uncertain additive/synergist ratios, but the calculation results do indicate this trend that the metal ion and the intumescent flame retardant system play a synergistic effect. In summary, intumescent flame retardant system and metal ions exhibit synergy effect to improve the fire safety of cotton fabrics. A schematic diagram of a possible flame retardant mechanism is exemplified in Fig. 8. Firstly, metal ions act as a catalyst for dehydrogenation to
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Fig. 7. SEM images of coated cotton fabrics after the horizontal flame test.
Table 4 The LOI values of all cotton fabrics. Sample
Untreat
PA/Ba/PA/Ba
CH/PA/CH/PA
CH/PA/Ba/PA
LOI(%)
16.2
18.0
20.2
22.0
catalyze char formation which could prevent heat and oxygen transmission. During this period, the release of water can dilute the concentration of flammable gases and remove part of the heat. Secondly, metal
ions act as a catalyst crosslinking of polymers to form higher quality char layer, which could play a more effective barrier compared with formed char layer when there is no metal ion. Thirdly, metal ions could facilitate to releases a large number of non-combustion gases such as H2O and etc. The non-combustion gases are helpful for both diluting the concentration of the flammable gases and absorbing heat energy. Therefore, the flame retardant operates in both condensed phase and gaseous phases during the burning process for cotton fabrics, suggesting that the intumescent flame retardant system and metal ions exhibit a synergistic effect.
Fig. 8. Schematic illustration of a possible flame retardant mechanism for cotton fabrics treated with CH/PA/Ba/PA during burning.
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References
Fig. 9. Photos of all washed cotton fabrics after the horizontal flame test.
3.6. Water durability of coated cotton fabrics In order to evaluate the washing durability of the coatings, the coated cotton fabrics were washed with mechanical stirring at 300 rpm in deionized water for 6 h. After washing, the horizontal flame test was carried out. Table 3 lists the burning rates of washed samples coated with PA/Ba/PA/Ba, CH/PA/CH/PA and CH/PA/Ba/PA. The burning rates of washed samples were only increased a little compared with those before washing, but their burning rates are lower than that of the untreated cotton fabrics, which means that the washed samples still have flame retardancy. Meanwhile, as shown in Fig. 9, the char residues of washed cotton fabrics were preserved well after the horizontal flame test. In these char residues, the char residue of the sample coated with CH/PA/Ba/PA was the more complete and continuous. This could be because that metal ions act as a catalyst of crosslinking of polymers to form higher quality char layer. The washing durability test in the work is not as strict as laundering process, but these results explain that the water durability is obtained after coating treatment. 4. Conclusion In conclusion, the eco-friendly flame retardant coating, composed of bio-based intumescent flame retardant system (chitosan and phytic acid) and metal ions, was successfully deposited on cotton fabrics and effectively improved the thermal stability and flame retardant performance of cotton fabrics by using dip-roll-dry method. Several characterization and measurements including scanning electron microscopy (SEM), thermo gravimetric analysis (TGA), microscale combustion calorimeter (MCC), horizontal flame tests (HFT) and limit oxygen index (LOI) have been used to examine the flame retardant property of all samples. Among these data, char residues and LOI values are increased from 3.65 to 29.7% and from 16.2 to 22.0, respectively. The burning rate and the peak heat release rate (pHHR) are decreased from 3.3 to 1.5 mm/s and from 331.1 to 129.1 W/g. All data and results confirmed that the sample coated CH/PA/Ba/PA had the better flame retardancy, which was attributed to the synergistic effect of the intumescent flame retardant system and metal ions. In addition, the weight gain was less so that this had less effect on the softness of cotton fabrics. Moreover, the experimental process was simple and not time consuming compared to layer-by-layer assembly method. Therefore, the biobased intumescent flame retardant system with metal ions and the experimental method had the choice to be used in practice.
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