Development of CO₂ utilized flame retardant finishing: Solubility measurements of flame retardants and application of the process to cotton

Highlights

• Solubilities of flame retardants have been measured in supercritical CO₂.

• Experimental data are correlated successfully by semi-empirical models.

• Sung-Shim equation works better in the representation of flame retardants solubility.

• Flame retardant properties of cotton were improved by CO₂ finishing process.

Abstract

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, Garlapati-Madras 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.

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], [2], [3], [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], [6], [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.

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], [20], [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 environmentally-friendly 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.