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Development of CO2 utilized flame retardant finishing: Solubility measurements of flame retardants and application of the process to cotton
Journal of CO₂ Utilization 37 (2020) 222–229
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Development of CO2 utilized flame retardant finishing: Solubility measurements of flame retardants and application of the process to cotton
T
Guohua Liua,b, Yitong Hana,b, Yuping Zhaoa,b, Huanda Zhenga,b,*, Laijiu Zhenga,b,* a b
National Supercritical Fluid Dyeing Technology Research Center, Dalian Polytechnic University, Dalian, Liaoning, 116034, China Liaoning Provincial Key Lab of Supercritical CO2 Waterless Dyeing, Dalian Polytechnic University, Dalian, Liaoning, 116034, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Supercritical CO2 Solubility Flame retardant Solution model Application
2,2′-oxybis-(5,5-dimethyl-1,3,2,-dioxaphosphorinane-2,2′-disulfide) (5060) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) are widely used in the flame-retardant finishing of polymers due to their excellent fire protection properties and environmentally friendly characteristics. In this work, solubility of 5060 and DOPO were measured at pressures ranging from 16 to 24 MPa with different temperatures in supercritical CO2. With the rising of system pressure and temperature, the solubility of 5060 and DOPO increase, and DOPO exhibits a higher solubility than 5060 in the same supercritical CO2 fluid conditions. Moreover, five semi-empirical models, namely Chrastil equation, Mendez-Santiago-Teja equation, Kumar-Johnston equation, GarlapatiMadras equation as well as Sung-Shim equation, were used to correlate the experimental solubility of the flame retardants. The results showed that Sung-Shim model presents better fitting effect with the AARD values of 21.5% for 5060 and 12.2% for DOPO. In addition, experiments were also conducted with 5060 and DOPO to determine the feasibility of flame-retardant finishing in supercritical CO2.
1. Introduction Supercritical CO2 is widely used as dyeing medium instead of water in textile dyeing due to its high dissolution capacity for low polarity dyes and the environmentally friendly characteristics [1–4]. Extensive studies have been carried out to confirm the application feasibility for polyester dyeing in supercritical CO2 since numerous disperse dyes can dissolve in it [5–7]. Lee measured the solubility of Disperse blue 3 and Disperse blue 79 with temperatures and pressures ranging from 323.7 to 413.7 K and 10.0 to 30.0 MPa in supercritical CO2 [8]. Yamini investigated the solubility of Disperse Yellow 184 and 232 from 308 to 348 K in supercritical CO2, and determined the dye-CO2 solvation enthalpies were 12.5 to 19.4 kJ/mol in the range of 12.1 and 35.5 MPa [9]. Tamura reported the maximum solubility of Disperse Violet 1 was 26.1 × 10−7 mol/mol from 323.15 to 383.15 K and 12.5 to 25.0 MPa [10]. Supported by a large number of dye solubility data, dyeing of polyester in supercritical CO2 has shifted from laboratory research to pilot production. Long reported the supercritical CO2 rope dyeing of polyester, and obtained the dyed fabrics with color fastness to washing and rubbing rated at 4–5 [11]. Zheng developed an industrial scale supercritical CO2 dyeing apparatus with two dyeing vessels (total volume: 1000 L), and commercially acceptable dyed polyester bobbins were produced [12]. ⁎
Apart from being used as a dyeing solvent, supercritical CO2 has also been selected as a green medium replacement of ethyl alcohol, decamethylcyclopentasiloxane and other organic solvents for other textile processing due to its features of reuse and savings in chemicals [13]. Mohamed demonstrated that supercritical CO2 could provide excellent coating of cotton with modified dimethylsiloxane polymers terminated with silanol groups in a layer between 1 and 2 um [14]. Long pretreated grey cotton fabric with enzymes in supercritical CO2, and validated the feasibility of desizing and scouring pretreatment with bis(2-ethylhexyl)sodium sulfosuccinate (AOT) [15]. Abate investigated the antimicrobial functionalization of polyester in supercritical CO2 with chitosan / derivative antimicrobial agents, and found 75–93% of Escherichia coli (ATCC 25922) bacteria was reduced within 1 h [16]. Moreover, there are some researches on preparation of functional polymers in supercritical CO2. Yang obtained the polyethylene terephthalate with electromagnetic shielding function in supercritical CO2 at temperatures ranging from 20 to 70 °C and pressures ranging from 5 to 30 MPa by using copper complex [17]. Dai found that ultraviolet protection factor of polyester fabric could be reached above 60 under supercritical CO2 conditions of 120 °C, 20 MPa and 90 min [18]. However, up to now, few studies have attempted to employ supercritical CO2 as an environmentally benign media to inject additives to textiles for achieving flame retardant application.
Corresponding authors at: National Supercritical Fluid Dyeing Technology Research Center, Dalian Polytechnic University, Dalian, Liaoning, 116034, China. E-mail addresses: [email protected] (H. Zheng), [email protected] (L. Zheng).
https://doi.org/10.1016/j.jcou.2019.12.015 Received 22 August 2019; Received in revised form 20 December 2019; Accepted 20 December 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
Journal of CO₂ Utilization 37 (2020) 222–229
G. Liu, et al.
Use of flame-retardant textiles can decrease the combustibility and delay fire spread, which provides one of the most effective fire protection measures. Generally, in order to obtain the flame retardant property, coating, dipping and other post-finishing methods are adopted extensively in fabrics [19–21]. Nevertheless, serious water pollution is caused by the above aqueous flame-retardant process. Furthermore, large quantities of harmful gases such as formaldehyde and ammonia are produced, exacerbating the problem of environmental pollution [22,23]. Adapting supercritical CO2 to flame-retardant finishing will offer outstanding environmental benefits because of no water and recycling characteristics of gases and auxiliaries. Simultaneously, technological process can be significantly shorted in supercritical CO2 which shows the potential in broadening the application scope in textile industries. Marosi reported the flammability improvement of poly(lactic acid) foams by supercritical CO2 assisted extrusion [24]. Nevertheless, to the best of our knowledge, the data on the flame retardant solubility in supercritical CO2 are scarce although the solubility behavior determines whether this technique is feasible or not. As new organophosphorus flame retardants with high efficiency, 2,2′-oxybis-(5,5-dimethyl-1,3,2,-dioxaphosphorinane-2,2′-disulfide) (5060) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) have been receiving great interest due to their excellent fire protection property, halogen-free, non-toxic and environmentallyfriendly characteristics [25]. Among these, 5060 is widely used for the flame-retardant finishing of viscose fiber while DOPO is extensively used in the flame-retardant finishing of polyester, polypropylene and other polymers. In this work, the solubilities of 5060 and DOPO were measured for the first time in supercritical CO2. The data were then correlated with different semi-empirical models, namely Chrastil equation, Mendez-Santiago-Teja equation, Kumar-Johnston equation, Garlapati-Madras equation, and Sung-Shim equation, respectively. In addition, flame-retardant finishing of cotton with 5060 and DOPO was also conducted to determine the feasibility in supercritical CO2.
constant weight. 1 g DOPO or 5060 with a particle size of 0.1 mm was firstly wrapped in stainless steel mesh with a mesh number of 1000 to avoid the leakage of solid, and placed into an equilibrium cell (20 ml). Liquid CO2 in a gas cylinder (1) was injected into the equilibrium cell until the pressure was balanced, and was then heated and pressurized to tune into supercritical state by using a liquid pump and an electric heater (6). Supercritical CO2 was then stirred adequately by an electromagnetic agitator (7) when the experimental temperatures and pressure were reached. Preliminary test results showed that the dissolving equilibrium of the flame retardants was reached in 120 min. Afterwards, the saturated supercritical CO2 was rapidly discharged by a relief valve in 1–2 second. Residual DOPO and 5060 were dried at 60 °C for 12 h. During supercritical CO2 dissolution, pressure was controlled within ± 0.1 MPa, and temperature was within ± 0.1 K in the equilibrium cell. The data shown for each point are averages of 3 single measurements. The average uncertainty of the solubility was less than ± 5.0%. 2.3. Flame-retardant finishing procedure Finishing experiments were conducted in a self-built apparatus according to our previous report [26]. Scoured cotton fabrics was firstly wound around a porous dyeing beam and was placed into a dyeing vessel. The flame retardants with a ratio of 10% o.m.f. (on the mass of fabric), was placed into a dye vessel. Liquid CO2 was injected into supercritical fluid apparatus, and then was pressurized to above 7.38 M P using a high-pressure pump and was heated to above 304.25 K using a heat exchanger. Cotton samples were treated at 22 MPa for 120 min with different temperatures according to the obtained solubility results in supercritical CO2. Furthermore, allowing for the possible swelling of the cotton fibers, a higher temperature of 403. 15 K was adopted. After the separation recycling of CO2 and the dissolved flame retardants was finished, the cotton fabric was removed and used for combustion measurement.
2. Experimental 2.4. Quantitative measurement of solubility 2.1. Materials To quantitatively measure the solubility data, a simple gravimetric method was employed since the dissolved mass of the DOPO and 5060 are higher than 10−3 g under test conditions [27]. Thus, the weight of the two flame retardants was measured before and after supercritical CO2 dissolving by a MS105/A METTLER TOLEDO microbalance with a sensitivity of 0.01 mg. The solubilities of the flame retardants under various supercritical CO2 conditions were then determined in terms of the mole fraction (y) according to Eqs. (1) and (2):
CO2 with a certified purity of 99.99% was purchased from China Haohua (Dalian) Research & Design Institute of Chemical Industry Co., Ltd. DOPO (> 99%, CAS No.35948-25-5) and 5060 (> 99%, CAS No.4090-51-1) were supplied by Dalian Xinyuan Chemistry Co., Ltd (China), and used without further purification. Scoured cotton fabrics (289 g/m2) was supplied by Liaoning Hongfeng Dyeing and Finishing Co., Ltd (China). The detailed information for all the chemical samples is shown in Table 1.
y= 2.2. Solubility measurement
n2 = n1 + n2
m2 M2 m1 M1
+
m2 M2
(1)
m = ρV
Solubilities of DOPO and 5060 were measured by employing a SPM 20 Phase Equilibrium System (Waters, US), and the schematic diagram of the apparatus is displayed in Fig. 1. Flame retardants were dried at 60 °C for 12 h in a vacuum drying chamber (Jinghong, China) to
(2)
where m1, n1, M1 and m2, n2, M2 denote the weight, the mole number and the mole mass of CO2 and solute, respectively; ρ and V are density and volume of CO2 and solute.
Table 1 Description of the chemical samples. Chemical name
CAS
Chemical structure
Molecular weight (g/mol)
Source
Carbon dioxide DOPO
124-38-9 35948-25-5
CO2
44.01 216
Haohua (Dalian) Xinyuan (Dalian)
5060
4090-51-1
346.34
Xinyuan (Dalian)
223
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Fig. 1. Phase equilibrium apparatus: (1) Gas cylinder, (2) Purifier, (3) Refrigerator, (4) Liquid pump, (5) Equilibrium cell, (6) Electric heater, (7) Electromagnetic agitator, (8) Control terminal.
When CO2 fluid was injected into the equilibrium cell, supercritical state was achieved with increasing system temperature and pressure, which resulted in the continuous dissolving of 5060 and DOPO in Fig. 2(b) and (e). Homogeneous supercritical CO2 phase appeared in Fig. 2(c) and (f) with the extension of time due to the dissolving of 5060 and DOPO. Simultaneously, it can be observed from Fig. 2(c) and (f) that the residual 5060 and DOPO decreased significantly in comparison with Fig. 2(a) and (d). This proves the solubility feasibility of 5060 and DOPO in supercritical CO2. On this basis, the solubilities of 5060 and DOPO in supercritical CO2 were measured from 16 to 24 MPa with different temperatures, and the resulting solubilities of 5060 and DOPO have been listed in Tables S1 and S2, respectively.
2.5. Combustion performance measurement Combustion performance of the cotton fabrics was evaluated according to GB/T 5455-2014 and GB/T 5454-1997 by using the afterflame time, the afterglow time as well as limiting oxygen index (LOI). Theoretically, the shorter the after-flame time and the afterglow time, the better the flame retardant effect. The larger the LOI value, the better the flame retardant effect. 3. Results and discussion 3.1. The solubility data of 5060 and DOPO To explore whether 5060 and DOPO can be dissolved in supercritical CO2, their dissolution process was firstly observed by employing the visible equilibrium cell in the SPM Phase Equilibrium System. As shown in Fig. 2(a) and (d), white solid flame retardants particles presented in the equilibrium cell under atmospheric pressure without CO2.
3.2. Effect of experimental parameters on the solubilities of 5060 and DOPO Fig. 3 shows the experimental solubilities of 5060 and DOPO in
Fig. 2. Dissolution process of 5060 (a, b, and c) and DOPO (d, e, and f) in supercritical CO2. 224
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Fig. 3. Relationship between the solubilities of flame retardants and the pressures: (a) 5060 and (b) DOPO.
5060 in supercritical CO2 because of the different molecular structure and molecular interactions between CO2 and flame retardants [29]. On the one hand, smaller molecular structure of DOPO favors its dissolution in supercritical CO2. On the other hand, a weak dispersion-dominated interaction of the CO2 quadrupole with the delocalized π electrons of the aromatic ring increases the solubility of DOPO [32]. In addition, solid DOPO starts to melt as the temperature rises up to over 353.15 K, and thus no further solubility measurements were made.
Table 2 The equation of the empirical models. Model
Equation
Chrastil
ln y = A +
Mendez-Santiago-Teja Kumar-Johnston
T ln(yp) = A + Bρ + CT
B T
B T B ln y = A + T B ln y = A + T
ln y = A +
Garlapati-Madras Sung-Shim
+ C ln ρ
+ Cρ
+ C ln(ρT ) D T
+ (C+ ) ln ρ
3.3. The experimental solubility correlation
supercritical CO2. The solubility of flame retardants depends mainly on the CO2 density, system pressure as well as temperature [29]. With the rising of system pressure and temperature, the solubilities of 5060 and DOPO continue to increase. Smaller solubilities increase is presented at pressures lower than 20 MPa as the density of supercritical CO2 becomes less dense at lower pressure regions under specified temperature [30]. Moreover, it is clear from Fig. 3(a) that the solubility of 5060 at 353.15 K displays a crossover between 16 and 20 MPa for the isotherms at 363.15 K where the solubility is enhanced with the temperature increase above the crossover pressure, but an opposite change trend exhibits below the crossover pressure. The crossover appeared can be mainly attributed to the existence of retrograde vaporization in binary supercritical CO2 - solid systems [31]. At crossover pressure, the competitions between solid vapor pressure and CO2 density on solid solubility balance each other. At pressures between 16 and 20 MPa, the solubility of 5060 reduces with the temperature increasing as a consequence of the rapid decrease in CO2 density. It is also noted that the DOPO solubility is apparently higher than
Empirical models are widely used to correlate solute solubility in supercritical CO2 because of their excellent accuracy and applied range. For understanding the capability in representing the solubility, the experimental data of 5060 and DOPO were further correlated with different semi-empirical density-based models, including Chrastil equation [33], Mendez-Santiago-Teja equation [34], Kumar-Johnston equation [35], Garlapati-Madras equation [36] as well as Sung-Shim equation [37], which denotes the relationship between solubility and solvent density (ρ), temperature (T) and pressure (P). Table 2 lists the equations of the models employed for correlation. The parameters in the above empirical equations, A, B, C and D were determined by using least-squares method according to the experimental solubility. Furthermore, the average absolute relative deviation (AARD) between the calculated solubility (ycal) and the experimental solubility (yexp) was obtained by using Eq. (3) [38].
AARD (%) =
100 N
∑ n
|y cal − y exp | y exp
(3)
Table 3 Parameters for 5060 and DOPO and AARD between the experimental and calculated values. Model
5060 Chrastil Mendez-Santiago-Teja Kumar-Johnston Garlapati-Madras Sung-Shim DOPO Chrastil Mendez-Santiago-Teja Kumar-Johnston Garlapati-Madras Sung-Shim
Parameters
AARD (%)
A
B
C
−3.48 −14458.13 25.68 −38.89 −63.13
−9450.49 4.43 −10286.00 −7577.66 12722.54
5.15 36.65 0.01 5.14 14.42
2.61 −11219.26 21.59 −20.02 176.03
−7016.56 2.74 −7414.78 −5904.42 −68477.87
3.30 31.68 0.005 3.31 −23.13
225
D
−3445.43
22.9 26.5 21.9 22.7 21.5
9245.53
16.2 15.5 26.4 16.2 12.2
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Fig. 4. Correlated results of the solubilities of 5060 from (a) Chrastil, (b) Mendez-Santiago-Teja, (c) Kumar-Johnston, (d) Garlapati-Madras and (e) Sung-Shim models: ◇-333.15 K, ▽-343.15 K, △-353.15 K, ◃-363.15, ○-373.15 K, □-383.15 K, ―Calculated.
in Figs. 4 and 5 may be attributed to the CO2 density changes at lower or higher temperature [40].
where N refers to the data points number. Table 3 lists the parameters of the employed semi-empirical models. It is to be noted that acceptable agreement of the experimental solubility with the calculated results was displayed. Relatively poor predictions for 5060 may be due to the changes in the physicochemical properties of the solute at higher temperature in supercritical CO2 [39]. In all the models, the model which contains 4 adjustable parameters presents better fitting effect than those of 3 parameters with the AARD values of 21.5% for 5060 and 12.2% for DOPO. Moreover, as observed in Figs. 4 and 5 , good solubility prediction results of 5060 and DOPO can be achieved with Sung-Shim model. The emerged fitting deviation
3.4. Flame-retardant finishing of cotton in supercritical CO2 As listed in Table 4, with CO2 temperature increasing, the afterflame time and the afterglow time of cotton samples were reduced moderately. It is noticeable that the afterglow time of the cotton was 0 s after treated at 403.15 K in supercritical CO2, showing lower combustibility and better flame retardant property in comparison with the untreated cotton sample. Moreover, the LOI values was increased from 226
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Fig. 5. Correlated results of the solubilities of DOPO from (a) Chrastil, (b) Mendez-Santiago-Teja, (c) Kumar-Johnston, (d) Garlapati-Madras and (e) Sung-Shim models:◇-313.15 K, ▽-323.15 K, △-333.15 K, ○-343.15 K, □-353.15 K, ―Calculated.
20.3 to 22.6, which presents consistent flame retardant results. The cotton fabrics treated in supercritical CO2 and their carbon residue after combustion are also investigated by Scanning Electron Microscope (SEM) analysis. It can be seen from Fig. 6(a) that original fiber surface appears numerous winkles. After supercritical CO2 treatment, the winkles reduces significantly because of the flame retardant particles coating on the surface of the cotton fiber, as displayed in Fig. 6(c) and (e). Furthermore, fabric structure of the control cotton fabric is significantly damaged due to the collapse of the carbon structure, as shown in Fig. 6(b). However, the samples treated in
supercritical CO2 with 5060 and DOPO still maintain better fabric integrity with continuous dense carbon layer, as shown in Fig. 6(d) and (f). This may be because the char-formed layer on the fiber surface presents the functions of insulation of heat, fuel and oxygen transmission, resulting in the protection of cotton from further burning [23]. Moreover, it can be observed that more DOPO particles adheres to the surface of the cotton fiber, which is consistent with the solubility data. More flame retardants can promote carbon formation during burning, thus improving flame retardant performance. Therefore, it is feasible to conduct the flame-retardant finishing of cotton with 5060 and DOPO in 227
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supercritical CO2 although further investigation on the finishing procedure is still needed.
Table 4 Combustion performance of the cotton fabrics. Finishing conditions
The control sample 383.15 K × 22 MPa × 120 min 393.15 K × 22 MPa × 120 min 403.15 K × 22 MPa × 120 min
Flame retardants
– 5060 DOPO 5060 DOPO DOPO
After-flame time a (s)
After-glow time b (s)
34.1 31.6 25.9 29.1 24.4 20.6
64.3 31.8 16.1 34.8 8.7 0
LOI (%)
c
4. Conclusions 20.3 21.5 21.8 21.9 22.0 22.6
The solubility of two flame retardants, 5060 and DOPO, was measured in supercritical CO2 at pressures of 16, 18, 20, and 24 MPa and temperatures of 333.15, 343.15, 353.15, 363.15, 373.15, and 383.15 K and 313.15, 323.15, 333.15, 343.15, 353.15 K, respectively. Effect of system pressure and temperature on the solubilities of 5060 and DOPO was investigated in supercritical CO2. The results showed that the solubility of 5060 and DOPO increase with system pressure and temperature increase. The experimental solubility of 5060 and DOPO were correlated successfully by using Chrastil model, Mendez-Santiago-Teja model, Kumar-Johnston model, Garlapati-Madras as well as Sung-Shim model. Furthermore, flame-retardant finishing results of cotton shows that it is potentially feasible to achieve the eco-friendly flame-retardant finishing of textiles using supercritical CO2.
a After-flame time is the length of time for which a material continues to flame after the ignition source has been removed. b Afterglow time is the time afterglow continues after the removal of the ignition source and the cessation of flaming. c LOI is the minimum volume percent of oxygen in a mixture of oxygen and nitrogen that will just support flaming combustion of a material.
Fig. 6. SEM images of the cotton fabrics treated at 393.15 K in supercritical CO2: (a) the control sample before treatment and (b) its residues after burning; (c) with 5060 and (d) its residues after burning; (e) with DOPO and (f) its residues after burning. 228
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CRediT authorship contribution statement
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Guohua Liu: Data curation, Investigation. Yitong Han: Data curation, Investigation. Yuping Zhao: Investigation. Huanda Zheng: Supervision, Methodology, Writing - original draft. Laijiu Zheng: Supervision, Methodology, Writing - original draft. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to thank the financial support from the National Natural Science Foundation of China (21908015), Liaoning Natural Science Foundation for Guidance Project (2019-ZD-0285), and China Postdoctoral Science Foundation (2017M611420). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2019.12.015. References [1] X. Kong, T. Huang, H. Cui, D. Yang, J. Lin, Multicomponent system of trichromatic disperse dye solubility in supercritical carbon dioxide, J. CO2 Util. 33 (2019) 1–11. [2] T. Bai, K. Kobayashi, K. Tamura, Y. Jun, L. Zheng, Supercritical CO2 dyeing for nylon, acrylic, polyester, and casein buttons and their optimum dyeing conditions by design of experiments, J. CO2 Util. 33 (2019) 253–261. [3] Z. Liu, J. Chen, Z. Liu, J. Lu, New process for synthesizing fluorinated polymers in supercritical carbon dioxide, Macromolecules 41 (2008) 6987–6992. [4] X. Jing, Y. Han, L. Zheng, H. Zheng, Surface wettability of supercritical CO2 - ionic liquid processed aromatic polyamides, J. CO2 Util. 27 (2018) 289–296. [5] T.A. Elmaaty, E.A. Elaziz, Supercritical carbon dioxide as a green media in textile dyeing: a review, Text. Res. J. 88 (2017) 1184–1212. [6] J. Long, H. Xu, C. Cui, X. Wei, F. Chen, A. Cheng, A novel plant for fabric rope dyeing in supercritical carbon dioxide and its cleaner production, J. Clean. Prod. 65 (2014) 574–582. [7] T.A. Elmaaty, F. El-Taweel, H. Elsisi, S. Okubayashi, Water free dyeing of polypropylene fabric under supercritical carbon dioxide and comparison with its aqueous analogue, J. Supercrit. Fluid. 139 (2018) 114–121. [8] J.W. Lee, M.W. Park, H.K. Bae, Measurement and correlation of dye solubility in supercritical carbon dioxide, Fluid Phase Equilibr. 179 (2001) 387–394. [9] Y. Yamini, M. Moradi, M. Hojjati, F. Nourmohammadian, A. Saleh, Solubilities of some disperse yellow dyes in supercritical CO2, J. Chem. Eng. Data 55 (2010) 3896–3900. [10] R.S. Alwi, T. Tanaka, K. Tamura, Measurement and correlation of solubility of anthraquinone dyestuffs in supercritical carbon dioxide, J. Chem. Thermodyn. 74 (2014) 119–125. [11] M. Yang, J. Liu, Y. Zhang, C. Chen, K. Wang, C. Peng, J. Long, Rope dyeing of fabric in supercritical carbon dioxide for commerical purposes, Color. Technol. 130 (2014) 102–111. [12] H. Zheng, J. Zhang, J. Yan, L. Zheng, An industrial scale multiple supercritical carbon dioxide apparatus and its eco-friendly dyeing production, J. CO2 Util. 16 (2016) 272–281. [13] G. Lee, J. Chae, S. Lee, S. Kim, J. Lee, Supercritical CO2 dyeing and finishing technology – a review, Tex. Color. Finis. 31 (2019) 48–64. [14] A.L. Mohamed, M. Er-Rafik, M. Moller, Supercritical carbon dioxide assisted silicon based finishing of cellulosic fabric: a novel approach, Carbohyd. Polym. 98 (1)
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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Development of CO2 utilized flame retardant finishing: Solubility measurements of flame retardants and application of the process to cotton
T
Guohua Liua,b, Yitong Hana,b, Yuping Zhaoa,b, Huanda Zhenga,b,*, Laijiu Zhenga,b,* a b
National Supercritical Fluid Dyeing Technology Research Center, Dalian Polytechnic University, Dalian, Liaoning, 116034, China Liaoning Provincial Key Lab of Supercritical CO2 Waterless Dyeing, Dalian Polytechnic University, Dalian, Liaoning, 116034, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Supercritical CO2 Solubility Flame retardant Solution model Application
2,2′-oxybis-(5,5-dimethyl-1,3,2,-dioxaphosphorinane-2,2′-disulfide) (5060) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) are widely used in the flame-retardant finishing of polymers due to their excellent fire protection properties and environmentally friendly characteristics. In this work, solubility of 5060 and DOPO were measured at pressures ranging from 16 to 24 MPa with different temperatures in supercritical CO2. With the rising of system pressure and temperature, the solubility of 5060 and DOPO increase, and DOPO exhibits a higher solubility than 5060 in the same supercritical CO2 fluid conditions. Moreover, five semi-empirical models, namely Chrastil equation, Mendez-Santiago-Teja equation, Kumar-Johnston equation, GarlapatiMadras equation as well as Sung-Shim equation, were used to correlate the experimental solubility of the flame retardants. The results showed that Sung-Shim model presents better fitting effect with the AARD values of 21.5% for 5060 and 12.2% for DOPO. In addition, experiments were also conducted with 5060 and DOPO to determine the feasibility of flame-retardant finishing in supercritical CO2.
1. Introduction Supercritical CO2 is widely used as dyeing medium instead of water in textile dyeing due to its high dissolution capacity for low polarity dyes and the environmentally friendly characteristics [1–4]. Extensive studies have been carried out to confirm the application feasibility for polyester dyeing in supercritical CO2 since numerous disperse dyes can dissolve in it [5–7]. Lee measured the solubility of Disperse blue 3 and Disperse blue 79 with temperatures and pressures ranging from 323.7 to 413.7 K and 10.0 to 30.0 MPa in supercritical CO2 [8]. Yamini investigated the solubility of Disperse Yellow 184 and 232 from 308 to 348 K in supercritical CO2, and determined the dye-CO2 solvation enthalpies were 12.5 to 19.4 kJ/mol in the range of 12.1 and 35.5 MPa [9]. Tamura reported the maximum solubility of Disperse Violet 1 was 26.1 × 10−7 mol/mol from 323.15 to 383.15 K and 12.5 to 25.0 MPa [10]. Supported by a large number of dye solubility data, dyeing of polyester in supercritical CO2 has shifted from laboratory research to pilot production. Long reported the supercritical CO2 rope dyeing of polyester, and obtained the dyed fabrics with color fastness to washing and rubbing rated at 4–5 [11]. Zheng developed an industrial scale supercritical CO2 dyeing apparatus with two dyeing vessels (total volume: 1000 L), and commercially acceptable dyed polyester bobbins were produced [12]. ⁎
Apart from being used as a dyeing solvent, supercritical CO2 has also been selected as a green medium replacement of ethyl alcohol, decamethylcyclopentasiloxane and other organic solvents for other textile processing due to its features of reuse and savings in chemicals [13]. Mohamed demonstrated that supercritical CO2 could provide excellent coating of cotton with modified dimethylsiloxane polymers terminated with silanol groups in a layer between 1 and 2 um [14]. Long pretreated grey cotton fabric with enzymes in supercritical CO2, and validated the feasibility of desizing and scouring pretreatment with bis(2-ethylhexyl)sodium sulfosuccinate (AOT) [15]. Abate investigated the antimicrobial functionalization of polyester in supercritical CO2 with chitosan / derivative antimicrobial agents, and found 75–93% of Escherichia coli (ATCC 25922) bacteria was reduced within 1 h [16]. Moreover, there are some researches on preparation of functional polymers in supercritical CO2. Yang obtained the polyethylene terephthalate with electromagnetic shielding function in supercritical CO2 at temperatures ranging from 20 to 70 °C and pressures ranging from 5 to 30 MPa by using copper complex [17]. Dai found that ultraviolet protection factor of polyester fabric could be reached above 60 under supercritical CO2 conditions of 120 °C, 20 MPa and 90 min [18]. However, up to now, few studies have attempted to employ supercritical CO2 as an environmentally benign media to inject additives to textiles for achieving flame retardant application.
Corresponding authors at: National Supercritical Fluid Dyeing Technology Research Center, Dalian Polytechnic University, Dalian, Liaoning, 116034, China. E-mail addresses: [email protected] (H. Zheng), [email protected] (L. Zheng).
https://doi.org/10.1016/j.jcou.2019.12.015 Received 22 August 2019; Received in revised form 20 December 2019; Accepted 20 December 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
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Use of flame-retardant textiles can decrease the combustibility and delay fire spread, which provides one of the most effective fire protection measures. Generally, in order to obtain the flame retardant property, coating, dipping and other post-finishing methods are adopted extensively in fabrics [19–21]. Nevertheless, serious water pollution is caused by the above aqueous flame-retardant process. Furthermore, large quantities of harmful gases such as formaldehyde and ammonia are produced, exacerbating the problem of environmental pollution [22,23]. Adapting supercritical CO2 to flame-retardant finishing will offer outstanding environmental benefits because of no water and recycling characteristics of gases and auxiliaries. Simultaneously, technological process can be significantly shorted in supercritical CO2 which shows the potential in broadening the application scope in textile industries. Marosi reported the flammability improvement of poly(lactic acid) foams by supercritical CO2 assisted extrusion [24]. Nevertheless, to the best of our knowledge, the data on the flame retardant solubility in supercritical CO2 are scarce although the solubility behavior determines whether this technique is feasible or not. As new organophosphorus flame retardants with high efficiency, 2,2′-oxybis-(5,5-dimethyl-1,3,2,-dioxaphosphorinane-2,2′-disulfide) (5060) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) have been receiving great interest due to their excellent fire protection property, halogen-free, non-toxic and environmentallyfriendly characteristics [25]. Among these, 5060 is widely used for the flame-retardant finishing of viscose fiber while DOPO is extensively used in the flame-retardant finishing of polyester, polypropylene and other polymers. In this work, the solubilities of 5060 and DOPO were measured for the first time in supercritical CO2. The data were then correlated with different semi-empirical models, namely Chrastil equation, Mendez-Santiago-Teja equation, Kumar-Johnston equation, Garlapati-Madras equation, and Sung-Shim equation, respectively. In addition, flame-retardant finishing of cotton with 5060 and DOPO was also conducted to determine the feasibility in supercritical CO2.
constant weight. 1 g DOPO or 5060 with a particle size of 0.1 mm was firstly wrapped in stainless steel mesh with a mesh number of 1000 to avoid the leakage of solid, and placed into an equilibrium cell (20 ml). Liquid CO2 in a gas cylinder (1) was injected into the equilibrium cell until the pressure was balanced, and was then heated and pressurized to tune into supercritical state by using a liquid pump and an electric heater (6). Supercritical CO2 was then stirred adequately by an electromagnetic agitator (7) when the experimental temperatures and pressure were reached. Preliminary test results showed that the dissolving equilibrium of the flame retardants was reached in 120 min. Afterwards, the saturated supercritical CO2 was rapidly discharged by a relief valve in 1–2 second. Residual DOPO and 5060 were dried at 60 °C for 12 h. During supercritical CO2 dissolution, pressure was controlled within ± 0.1 MPa, and temperature was within ± 0.1 K in the equilibrium cell. The data shown for each point are averages of 3 single measurements. The average uncertainty of the solubility was less than ± 5.0%. 2.3. Flame-retardant finishing procedure Finishing experiments were conducted in a self-built apparatus according to our previous report [26]. Scoured cotton fabrics was firstly wound around a porous dyeing beam and was placed into a dyeing vessel. The flame retardants with a ratio of 10% o.m.f. (on the mass of fabric), was placed into a dye vessel. Liquid CO2 was injected into supercritical fluid apparatus, and then was pressurized to above 7.38 M P using a high-pressure pump and was heated to above 304.25 K using a heat exchanger. Cotton samples were treated at 22 MPa for 120 min with different temperatures according to the obtained solubility results in supercritical CO2. Furthermore, allowing for the possible swelling of the cotton fibers, a higher temperature of 403. 15 K was adopted. After the separation recycling of CO2 and the dissolved flame retardants was finished, the cotton fabric was removed and used for combustion measurement.
2. Experimental 2.4. Quantitative measurement of solubility 2.1. Materials To quantitatively measure the solubility data, a simple gravimetric method was employed since the dissolved mass of the DOPO and 5060 are higher than 10−3 g under test conditions [27]. Thus, the weight of the two flame retardants was measured before and after supercritical CO2 dissolving by a MS105/A METTLER TOLEDO microbalance with a sensitivity of 0.01 mg. The solubilities of the flame retardants under various supercritical CO2 conditions were then determined in terms of the mole fraction (y) according to Eqs. (1) and (2):
CO2 with a certified purity of 99.99% was purchased from China Haohua (Dalian) Research & Design Institute of Chemical Industry Co., Ltd. DOPO (> 99%, CAS No.35948-25-5) and 5060 (> 99%, CAS No.4090-51-1) were supplied by Dalian Xinyuan Chemistry Co., Ltd (China), and used without further purification. Scoured cotton fabrics (289 g/m2) was supplied by Liaoning Hongfeng Dyeing and Finishing Co., Ltd (China). The detailed information for all the chemical samples is shown in Table 1.
y= 2.2. Solubility measurement
n2 = n1 + n2
m2 M2 m1 M1
+
m2 M2
(1)
m = ρV
Solubilities of DOPO and 5060 were measured by employing a SPM 20 Phase Equilibrium System (Waters, US), and the schematic diagram of the apparatus is displayed in Fig. 1. Flame retardants were dried at 60 °C for 12 h in a vacuum drying chamber (Jinghong, China) to
(2)
where m1, n1, M1 and m2, n2, M2 denote the weight, the mole number and the mole mass of CO2 and solute, respectively; ρ and V are density and volume of CO2 and solute.
Table 1 Description of the chemical samples. Chemical name
CAS
Chemical structure
Molecular weight (g/mol)
Source
Carbon dioxide DOPO
124-38-9 35948-25-5
CO2
44.01 216
Haohua (Dalian) Xinyuan (Dalian)
5060
4090-51-1
346.34
Xinyuan (Dalian)
223
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Fig. 1. Phase equilibrium apparatus: (1) Gas cylinder, (2) Purifier, (3) Refrigerator, (4) Liquid pump, (5) Equilibrium cell, (6) Electric heater, (7) Electromagnetic agitator, (8) Control terminal.
When CO2 fluid was injected into the equilibrium cell, supercritical state was achieved with increasing system temperature and pressure, which resulted in the continuous dissolving of 5060 and DOPO in Fig. 2(b) and (e). Homogeneous supercritical CO2 phase appeared in Fig. 2(c) and (f) with the extension of time due to the dissolving of 5060 and DOPO. Simultaneously, it can be observed from Fig. 2(c) and (f) that the residual 5060 and DOPO decreased significantly in comparison with Fig. 2(a) and (d). This proves the solubility feasibility of 5060 and DOPO in supercritical CO2. On this basis, the solubilities of 5060 and DOPO in supercritical CO2 were measured from 16 to 24 MPa with different temperatures, and the resulting solubilities of 5060 and DOPO have been listed in Tables S1 and S2, respectively.
2.5. Combustion performance measurement Combustion performance of the cotton fabrics was evaluated according to GB/T 5455-2014 and GB/T 5454-1997 by using the afterflame time, the afterglow time as well as limiting oxygen index (LOI). Theoretically, the shorter the after-flame time and the afterglow time, the better the flame retardant effect. The larger the LOI value, the better the flame retardant effect. 3. Results and discussion 3.1. The solubility data of 5060 and DOPO To explore whether 5060 and DOPO can be dissolved in supercritical CO2, their dissolution process was firstly observed by employing the visible equilibrium cell in the SPM Phase Equilibrium System. As shown in Fig. 2(a) and (d), white solid flame retardants particles presented in the equilibrium cell under atmospheric pressure without CO2.
3.2. Effect of experimental parameters on the solubilities of 5060 and DOPO Fig. 3 shows the experimental solubilities of 5060 and DOPO in
Fig. 2. Dissolution process of 5060 (a, b, and c) and DOPO (d, e, and f) in supercritical CO2. 224
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Fig. 3. Relationship between the solubilities of flame retardants and the pressures: (a) 5060 and (b) DOPO.
5060 in supercritical CO2 because of the different molecular structure and molecular interactions between CO2 and flame retardants [29]. On the one hand, smaller molecular structure of DOPO favors its dissolution in supercritical CO2. On the other hand, a weak dispersion-dominated interaction of the CO2 quadrupole with the delocalized π electrons of the aromatic ring increases the solubility of DOPO [32]. In addition, solid DOPO starts to melt as the temperature rises up to over 353.15 K, and thus no further solubility measurements were made.
Table 2 The equation of the empirical models. Model
Equation
Chrastil
ln y = A +
Mendez-Santiago-Teja Kumar-Johnston
T ln(yp) = A + Bρ + CT
B T
B T B ln y = A + T B ln y = A + T
ln y = A +
Garlapati-Madras Sung-Shim
+ C ln ρ
+ Cρ
+ C ln(ρT ) D T
+ (C+ ) ln ρ
3.3. The experimental solubility correlation
supercritical CO2. The solubility of flame retardants depends mainly on the CO2 density, system pressure as well as temperature [29]. With the rising of system pressure and temperature, the solubilities of 5060 and DOPO continue to increase. Smaller solubilities increase is presented at pressures lower than 20 MPa as the density of supercritical CO2 becomes less dense at lower pressure regions under specified temperature [30]. Moreover, it is clear from Fig. 3(a) that the solubility of 5060 at 353.15 K displays a crossover between 16 and 20 MPa for the isotherms at 363.15 K where the solubility is enhanced with the temperature increase above the crossover pressure, but an opposite change trend exhibits below the crossover pressure. The crossover appeared can be mainly attributed to the existence of retrograde vaporization in binary supercritical CO2 - solid systems [31]. At crossover pressure, the competitions between solid vapor pressure and CO2 density on solid solubility balance each other. At pressures between 16 and 20 MPa, the solubility of 5060 reduces with the temperature increasing as a consequence of the rapid decrease in CO2 density. It is also noted that the DOPO solubility is apparently higher than
Empirical models are widely used to correlate solute solubility in supercritical CO2 because of their excellent accuracy and applied range. For understanding the capability in representing the solubility, the experimental data of 5060 and DOPO were further correlated with different semi-empirical density-based models, including Chrastil equation [33], Mendez-Santiago-Teja equation [34], Kumar-Johnston equation [35], Garlapati-Madras equation [36] as well as Sung-Shim equation [37], which denotes the relationship between solubility and solvent density (ρ), temperature (T) and pressure (P). Table 2 lists the equations of the models employed for correlation. The parameters in the above empirical equations, A, B, C and D were determined by using least-squares method according to the experimental solubility. Furthermore, the average absolute relative deviation (AARD) between the calculated solubility (ycal) and the experimental solubility (yexp) was obtained by using Eq. (3) [38].
AARD (%) =
100 N
∑ n
|y cal − y exp | y exp
(3)
Table 3 Parameters for 5060 and DOPO and AARD between the experimental and calculated values. Model
5060 Chrastil Mendez-Santiago-Teja Kumar-Johnston Garlapati-Madras Sung-Shim DOPO Chrastil Mendez-Santiago-Teja Kumar-Johnston Garlapati-Madras Sung-Shim
Parameters
AARD (%)
A
B
C
−3.48 −14458.13 25.68 −38.89 −63.13
−9450.49 4.43 −10286.00 −7577.66 12722.54
5.15 36.65 0.01 5.14 14.42
2.61 −11219.26 21.59 −20.02 176.03
−7016.56 2.74 −7414.78 −5904.42 −68477.87
3.30 31.68 0.005 3.31 −23.13
225
D
−3445.43
22.9 26.5 21.9 22.7 21.5
9245.53
16.2 15.5 26.4 16.2 12.2
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Fig. 4. Correlated results of the solubilities of 5060 from (a) Chrastil, (b) Mendez-Santiago-Teja, (c) Kumar-Johnston, (d) Garlapati-Madras and (e) Sung-Shim models: ◇-333.15 K, ▽-343.15 K, △-353.15 K, ◃-363.15, ○-373.15 K, □-383.15 K, ―Calculated.
in Figs. 4 and 5 may be attributed to the CO2 density changes at lower or higher temperature [40].
where N refers to the data points number. Table 3 lists the parameters of the employed semi-empirical models. It is to be noted that acceptable agreement of the experimental solubility with the calculated results was displayed. Relatively poor predictions for 5060 may be due to the changes in the physicochemical properties of the solute at higher temperature in supercritical CO2 [39]. In all the models, the model which contains 4 adjustable parameters presents better fitting effect than those of 3 parameters with the AARD values of 21.5% for 5060 and 12.2% for DOPO. Moreover, as observed in Figs. 4 and 5 , good solubility prediction results of 5060 and DOPO can be achieved with Sung-Shim model. The emerged fitting deviation
3.4. Flame-retardant finishing of cotton in supercritical CO2 As listed in Table 4, with CO2 temperature increasing, the afterflame time and the afterglow time of cotton samples were reduced moderately. It is noticeable that the afterglow time of the cotton was 0 s after treated at 403.15 K in supercritical CO2, showing lower combustibility and better flame retardant property in comparison with the untreated cotton sample. Moreover, the LOI values was increased from 226
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Fig. 5. Correlated results of the solubilities of DOPO from (a) Chrastil, (b) Mendez-Santiago-Teja, (c) Kumar-Johnston, (d) Garlapati-Madras and (e) Sung-Shim models:◇-313.15 K, ▽-323.15 K, △-333.15 K, ○-343.15 K, □-353.15 K, ―Calculated.
20.3 to 22.6, which presents consistent flame retardant results. The cotton fabrics treated in supercritical CO2 and their carbon residue after combustion are also investigated by Scanning Electron Microscope (SEM) analysis. It can be seen from Fig. 6(a) that original fiber surface appears numerous winkles. After supercritical CO2 treatment, the winkles reduces significantly because of the flame retardant particles coating on the surface of the cotton fiber, as displayed in Fig. 6(c) and (e). Furthermore, fabric structure of the control cotton fabric is significantly damaged due to the collapse of the carbon structure, as shown in Fig. 6(b). However, the samples treated in
supercritical CO2 with 5060 and DOPO still maintain better fabric integrity with continuous dense carbon layer, as shown in Fig. 6(d) and (f). This may be because the char-formed layer on the fiber surface presents the functions of insulation of heat, fuel and oxygen transmission, resulting in the protection of cotton from further burning [23]. Moreover, it can be observed that more DOPO particles adheres to the surface of the cotton fiber, which is consistent with the solubility data. More flame retardants can promote carbon formation during burning, thus improving flame retardant performance. Therefore, it is feasible to conduct the flame-retardant finishing of cotton with 5060 and DOPO in 227
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supercritical CO2 although further investigation on the finishing procedure is still needed.
Table 4 Combustion performance of the cotton fabrics. Finishing conditions
The control sample 383.15 K × 22 MPa × 120 min 393.15 K × 22 MPa × 120 min 403.15 K × 22 MPa × 120 min
Flame retardants
– 5060 DOPO 5060 DOPO DOPO
After-flame time a (s)
After-glow time b (s)
34.1 31.6 25.9 29.1 24.4 20.6
64.3 31.8 16.1 34.8 8.7 0
LOI (%)
c
4. Conclusions 20.3 21.5 21.8 21.9 22.0 22.6
The solubility of two flame retardants, 5060 and DOPO, was measured in supercritical CO2 at pressures of 16, 18, 20, and 24 MPa and temperatures of 333.15, 343.15, 353.15, 363.15, 373.15, and 383.15 K and 313.15, 323.15, 333.15, 343.15, 353.15 K, respectively. Effect of system pressure and temperature on the solubilities of 5060 and DOPO was investigated in supercritical CO2. The results showed that the solubility of 5060 and DOPO increase with system pressure and temperature increase. The experimental solubility of 5060 and DOPO were correlated successfully by using Chrastil model, Mendez-Santiago-Teja model, Kumar-Johnston model, Garlapati-Madras as well as Sung-Shim model. Furthermore, flame-retardant finishing results of cotton shows that it is potentially feasible to achieve the eco-friendly flame-retardant finishing of textiles using supercritical CO2.
a After-flame time is the length of time for which a material continues to flame after the ignition source has been removed. b Afterglow time is the time afterglow continues after the removal of the ignition source and the cessation of flaming. c LOI is the minimum volume percent of oxygen in a mixture of oxygen and nitrogen that will just support flaming combustion of a material.
Fig. 6. SEM images of the cotton fabrics treated at 393.15 K in supercritical CO2: (a) the control sample before treatment and (b) its residues after burning; (c) with 5060 and (d) its residues after burning; (e) with DOPO and (f) its residues after burning. 228
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CRediT authorship contribution statement
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