Citation

Xian-Sheng Zhang, Yan-Zhi Xia, Mei-Wu Shi, Xiong Yan. The flame retardancy of alginate/flame retardant viscose fibers investigated by vertical burning test and cone calorimeter[J]. Chin. Chem. Lett., 2018, 29(3): 489-492  

Xian-Sheng Zhang^a, Yan-Zhi Xia^b, Mei-Wu Shi^a,c,1, Xiong Yan^a    

Characterized with high efficiency and cost-effective, simple blending is widely used in manufacturing textiles, which can combine advantages of different categories of fibers, for example, balance of the mechanical performance, comfortability, dyeability and the cost. In addition, blending fibers can achieve excellent flame retardancy due to mutual influence between constituent fibers. As reported, better flammability can be achieved at certain ratio in viscose/modacrylic [1], wool/para-amide [2] and Nomex/viscose FR fiber [3].

Alginate is a kind of biopolymer extracted from brown seaweeds, which is a linear copolymer with the repeating monomeric units of β-1, 4-D-mannuronate (M) and α-1, 4-L-guluronate (G) [4]. It possesses better flame retardancy and biocompatibility, which can be applied in food industry, wound dressing and textile industry [5-7]. It was confirmed that alginate fibers with divalent or trivalent metal ion possess excellent flame retardancy [8, 9]. However, it is found that smoldering occurs in alginate fibers, which may have serious damage due to the incandescent residue can ignite the flammable materials [10, 11]. The smoldering weakness of alginate fibers may limit its application in the fire retardant protective textiles. To the best of our knowledge, rare method is proposed in the suppression of smoldering, which is necessary in expanding application of alginate fibers.

Viscose fiber is widely used in clothing due to its comfort and good dying properties, but the flammability limits its application. The flame retardant viscose (FRV) fiber can be obtained by mixing flame retardants during processing, among them, dithiopyrophosphate (DDPS) with the trade name of Sandoflame 5060 is the most widely used in the industry [12]. The DDPS can decompose to phosphoric acid when exposed to heat, which can induce the dehydration of the fiber and generate char [13]. Since the char can isolate the inner cellulose from air and prevent the heat transmission, the flame retardancy of FRV can be achieved.

However, how is the flame retardancy of alginate/FRV blending fibers? Whether can the FRV solve the smoldering weakness of alginate? With this doubt, the flame retardancy of alginate, FRV and alginate/FRV (50/50) were investigated by vertical burning and cone calorimeter tests. Here, in order to quickly and directly evaluate the flame retardancy of fibers, a simple and efficient method based on the vertical burning test was undertaken using twin wire as the sample. As vertical burning test turned out, FRV was confirmed to be effective in suppression of smoldering in alginate fibers. The corresponding mechanism was revealed by exploring the morphology, the chemical structure and thermal degradation behaviors. Moreover, cone calorimeter test showed that alginate can further enhance the flame retardancy of FRV fibers.

The alginate fibers (white, 1.62 dtex, 38 mm) were provided by Qingdao Kangtong Ocean Fibers Co., Ltd. FRV fibers (black, 1.67 dtex, 38 mm), with the fire retardant of Sandoflame 5060, were produced by Jilin Chemical Fiber Group Co., Ltd. Here, in order to quickly and directly evaluate the flame retardancy of fibers, a simple and efficient method based on the vertical burning test was undertaken using twin wire as the sample. For preparation of the samples for vertical burning test, firstly, the fibers of alginate, FRV and alginate/FRV (50/50) were carded and well mixed to fibers mat by small-scale carding machine. Secondly, the fibers mat were bunched and rolled to a roving yarn strip, then folded and twisted to twin wire (0.6 g (weight), 80–90 mm (length), 3 mm (diameter)). Finally, the twin wire was twined by thin steel wire to keep its shape during burning. The vertical burning test was performed by an alcohol burner. The flame height of the alcohol burner was about 3 cm. The sample tail was clamped with tweezers and kept upright. The sample head was put in the outer flame. The length of the sample in the outer flame was about 1 cm and the duration time was 24 s. Then the afterflame time, afterglow time and damaged length were recorded. Each sample was tested for three times and the average results were obtained.

The flammability of the samples (alginate, FRV and alginate/FRV (50/50)) based on the simulated vertical burning test is shown in Table 1. Apparently, alginate fibers cannot be ignited and the afterflame time is 0 s, while extremely long afterglow time (605 s) and serious damaged length (85 mm) are occurred. Almost the whole sample is destroyed as shown in Fig. 1b. Since that the smoldering occurs from the head to tail of the sample, the corresponding temperature of the fibers are gradually decreased and lead to different degrees of damage. Thus, the head is destroyed seriously with gray char (b₁ in Fig. 1b), while the tail is destroyed slightly with black char (b₂ in Fig. 1b). In addition, due to the heat dissipation to the outside, the inner temperature of the sample is higher than that of external, resulting in the most serious destroyed char (white, b₀ in Fig. 1b). For the FRV fibers, the afterflame time is 2 s, but no afterglow time (0 s) and short damaged length (10 mm) are shared. Interestingly, when the alginate fibers are blended with FRV fibers with the weight ratio of 50/50, the smoldering of alginate fibers can be effectively improved, indicated by the much shorter afterglow time (85 s) and shorter damaged length (35 mm) than pure alginate fibers.

Table 1

Table 1 The vertical burning test results of alginate, FRV and alginate/FRV (50/50).

Table 1 The vertical burning test results of alginate, FRV and alginate/FRV (50/50).

Figure 1

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Fig. 1. Images of the alginate: (a) before burning, (b) after burning, (b1) and (b2) magnified char; FRV: (c) before burning, (d) after burning, (d1) and (d2) magnified char; alginate/FRV (50/50): (e) before burning, (f) after burning, (f1) and (f2) magnified char.

Then the morphology of the char was obtained by the scanning electron microscopy (SEM). Fig. 1b₁ demonstrates that the alginate fibers in head part are seriously destroyed and broken into small parts. Fig. 1b₂ shows that the fibers in tail part are also destroyed during smoldering. The difference of the char morphology is ascribed to the temperature distribution from head to tail of the fibers as described in the previous section. For the FRV fibers, in Fig. 1d₁, the fibers under flame are just slightly destroyed compared with the raw fibers (Fig. 1d₂). The FRV fibers can form char during heating, which is induced by the fire retardant of Sandoflame 5060. The char can act as the physical barrier in the heat transmission, which interprets inexistence of smoldering phenomenon in FRV. For the alginate/FRV (50/50) fibers, they are destroyed to some extent under flame (Fig. 1f₁), but the char formed by FRV fibers can prevent the heat transmission and protect the tail parts (Fig. 1f₂) from being destroyed. In this case, the char formed by FRV fibers can prevent the heat transmission and suppress the smoldering process of alginate fibers.

In order to investigate the chemical structure variation of the alginate fibers during the smoldering process, the Fourier transform infrared spectroscopy (FTIR) spectra of the alginate fibers and the char in different parts shown in Fig. 1b are indicated in Fig. 2a. The bands at 3300, 1620, 1440 and 1065 cm^-1 of pure alginate (b₃) are assigned to -OH stretching vibration, COO-asymmetric stretching, COO-symmetric stretching and the stretching vibration of the C-O-C groups, respectively [14, 15]. The spectrum of black char (b₂) displays the single peak of 1440 cm^-1 (COO-symmetric stretching), which demonstrates that the chain of the alginate has been destroyed seriously at the last stage of the smoldering process. That is, the lower temperature can trigger the degradation of alginate, which explains its easy smoldering behavior. The spectra of gray and white char (b₁ and b₀) present a sharp and new peak at 910 cm^-1, which is due to the generation of the carbonate during further degradation [16].

Figure

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Fig. .

The thermogravimetric analysis (TGA) [17-19] was performed to study the thermal stability of alginate, FRV and alginate/FRV (50/ 50) fibers, which was used to investigate the origin of FRV fibers to suppress the smoldering for alginate fibers. TGA analysis was carried out under nitrogen atmosphere, with the heating rate of 20 ℃/min ranging from 50 ℃ to 700 ℃. The thermal degradation behaviors of the samples are displayed in Fig. 2b. It can be seen that a drop in remaining weight of alginate fiber occurs at about 100 ℃ (Region Ⅰ). At this point, the weight of alginate fibers is about 90.9%, indicating lower weight than that of FRV fiber (94.4%). This indicates better moisture adsorption performance of alginate fibers. In the temperature range 100–250 ℃ (Region Ⅱ), the remaining weight of alginate decreases from 90.9% to 79.1%, which is ascribed to the destruction of glycosidic bonds at relatively lower temperature. The lower initial thermal degradation temperature further explains the easy smoldering of alginate fibers. However, there is almost no weight loss for FRV in this temperature region. In the temperature range 250–300 ℃ (Region Ⅲ), both alginate and FRV fibers are significantly degraded. The obtained intermediates of alginate fibers were further destroyed. The phosphoruscontaining fire retardant induces the dehydration of the FRV fibers and generates the compact and stable char. In the temperature range 300–700 ℃ (Region Ⅳ), the loss of the weight for both fibers origins from the further degradation of the intermediate material and the oxidation of the char. For the sample of alginate/FRV (50/50) fibers, the main degradation temperature range is in 250–300 ℃ (Region Ⅲ), which is consist with alginate and FRV. From the discussion above, the significant thermal degradation temperature region is almost the same for both fibers, thus the char formed by FRV fibers during this region can effectively prevent the heat transmission and the resultant suppression of the smoldering in alginate fibers.

In addition, the cone calorimeter is widely used to measure the combustion behaviors of materials, which can obtain the heat release rate and total heat release based on the principle of oxygen consumption [20]. The samples for cone calorimeter were also carded and well mixed by small-scale carding machine, and the uniform fibrous web was obtained. The fibrous sample with the weight of 6 g and the dimension of 100 mm × 100 mm × 3 mm was put in the middle of an aluminum foil. A cross steel grid was applied to fix the fibrous sample. The detailed information of the preparation method for the samples was descried in our previous work [21]. The combustion behaviors of the samples are carried out by cone calorimeter at the heat flux of 35 kW/m². The data of time to ignition (TTI), heat release rate (HRR), peak heat release rate (PHRR) and total heat release (THR) for these samples are collected. The curves of HRR vs. time and THR vs. time are shown in Figs. 3a and b. For alginate, it cannot be ignited, which mainly attributed to less combustible gases emerged. In this case, the HRR of alginate is low and the THR is only 2.33 MJ/m² at 200 s. In comparison, FRV can be easily ignited with the TTI of 10 s and the PHRR of 99 kW/m². In addition, FRV can generate more heat with the THR of 7.75 MJ/m² at 200 s. For the blend of alginate/FRV (50/ 50), the sample can be ignited, but the TTI is delayed to 61 s, which is 51 s longer than that of FRV. Moreover, the PHRR and THR of alginate/FRV (50/50) are 72.6 kW/m² and 3.52 MJ/m², respectively. The significantly increase of TTI and decrease of PHRR and THR for alginate/FRV (50/50) is due to the flame retardant mechanism of metal ions in alginate. From the previous report, the metal ions (such as Na⁺ and Ca²⁺) can enhance the flame retardancy of cellulose fibers [22], which can explain the excellent flame retardancy of the blend of alginate/FRV (50/50). Combined the analysis of vertical burning and cone calorimeter tests, the addition of FRV can significantly suppress the smoldering of alginate. On the other hand, the alginate can increase the TTI and decrease the PHRR and THR of FRV. Therefore, the mixing fibers can reduce the weakness and combine the advantage of individual fibers.

Figure

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Fig. .

In summary, the flame retardancy of neat alginate, FRV and alginate/FRV (50/50) were investigated. The simulated vertical burning test using twin wire as the sample displayed that the afterglow time and the damaged length was respectively 605 s and 85 mm for alginate fibers, which were significantly decreased to 85 s and 35 mm after addition of FRV. The smoldering of alginate can be suppressed by simply blending with FRV. The smoldering of alginate was attributed to the fact that its structure can be destroyed at lower temperature. Since alginate and FRV fibers shared the concurrence of rapid degradation in the same temperature region of 250–300 ℃. In this way, the stable char formed by FRV can prevent the smoldering of alginate. Moreover, the cone calorimeter results showed that alginate can enhance the flame retardancy of FRV, and the blending fibers of alginate/FRV (50/50) possessed good flame retardancy with larger TTI and lower PHRR compared with FRV. In conclusion, this research proposes a method to suppress the smoldering of alginate fiber by blending with FRV fiber, which is useful for the application of alginate in the field of fireproof fabrics.

This work was generously supported by the Fundamental Research Funds for the Central Universities (No. CUSF-DH-D-2016012).