On the flameproof treatment of ramie fabrics using a spray-assisted layer-by-layer technique
Introduction
The fiber of the ramie plant has long history of being woven into fabric. With properties of renewability, biodegradability, low cost, low density and excellent mechanical strength, it has great potential for applications in automotive, aerospace, construction, and military areas as reinforcement in composites, besides its traditional use in home textile and furnishing industries [1], [2]. However, the intrinsic drawbacks of natural cellulosic fabrics, such as low limiting oxygen index (LOI) and combustion temperature, make it highly flammable and impose great restrictions on their fields of application. Thus, flame retardant modification for natural fiber fabrics has attracted increasing attentions from both academe and industry.
Until now, a number of treatments, such as dyeing and finishing [3], [4], surface coating [5], [6], graft copolymerization [7], [8], [9], [10], [11], sol–gel [12], [13], [14] and LBL assembly, have been developed to enhance the thermal stability and flame retardancy of fabrics. Among them, LBL assembly represents an innovative approach suitable for various substrates (fabrics, ceramics, plastics, metals, etc.). It generally consists of alternately immersing the substrates into oppositely charged polyelectrolyte solutions or suspensions, which results in the construction of multiple positively and negatively charged layers on the surface of the substrates. The initial attempt to introduce this method to the textile flame retardant field was carried out by Grunlan and co-workers [15]. Multilayer flame-retardant architectures consisting of polyethylenimine (PEI) coupled with laponite nano-platelets were produced on cotton fabrics. However, the FR improvement of treated fabrics was less than satisfactory because the only physical barrier effect produced by limited laponite nano-platelets. Alternatively to laponite, different negative-charged inorganic materials such as SiO2 [16], clay [17], carbon nanofiber [18], POSS [19] and MWCNT-NH2 [20] paired with PEI have been developed and successfully applied to fiber fabrics and polyurethane foam. In order to further improve the FR performance, the LBL architectures contains completely organic coatings, namely, intumescent LBL coatings (such as PAA/PSP, chitosan/PSP, chitosan/APP, PAA/APP, etc.) [19], [21], [22], [23], [24] have been developed on plant fabrics. More specifically, the chitosan/APP could act as a typical intumescent system, since chitosan represents a carbon source and foaming agent as well, whereas APP in situ generates phosphoric acid at high temperatures, favoring the char formation of both chitosan and cellulose. Additionally, intumescent multilayer nano-coatings made only with renewable biomass polyelectrolytes have been applied to cotton quite recently with remarkable decrease of both peak heat release rate (PHRR) and total heat release (THR) [25], [26]. When referring to natural fabrics, the intumescent systems were found to be the most promising flame retardant strategy.
All the above-mentioned LBL coatings were implemented by means of the dipping method. In contrast, the spraying method could represent a more promising approach for its high efficiency and easy implementation in large industrial scale, as comprehensively reviewed by Schaaf and co-workers [27]. The results have shown that the spraying method could obtain the same homogeneity and consistency level of the depositing coatings, resulting in the desirable properties on substrate materials. Until very recently, horizontal spraying has been developed and successfully applied to impart flame retardant properties to fabrics. This was found to be relatively adaptive for both cotton and polyester textiles, with satisfactory results from flammability and combustion tests [28], [29].
In this work, a simple coating system of polyethyleneimine (PEI) paired with commercially available ammonium polyphosphate (APP) was employed, using both dipping and spraying methods. The fire performance as well as the depositing process, morphology and thermal stability of the both spraying-coated and dipping-coated ramie fabrics were evaluated and compared. The results demonstrated the superiority of the spraying technique.
Section snippets
Chemicals and substrates
Plain ramie fabrics, purchased from Jiangxi Jingzhu Ramie Textile Co., Ltd. (China), were cut into pieces and washed with detergent in deionized water several times, followed by drying under vacuum at 60 °C for 2 h.
Polyethylenimine (PEI 50 wt% aqueous solution, Mw = 70,000) was purchased from Aladdin. Ammonium polyphosphate (APP, JLS-APP 104MF, P% = 28.0–30.0 wt%) was obtained from Hangzhou JLS Flame Retardants Chemical Co., Ltd. (China). Sodium hydroxide (NaOH, ≥96.0%) was purchased from
Characterization of the (PEI/APP)n coating
The ATR-FTIR spectra of the uncoated and coated fabrics are shown in Fig. 2, the surface chemical structure of fabrics was determined to monitor the LBL growth process. For the both spraying and dipping methods, the coated fabrics showed strong new absorption bands at near 1252 and 862 cm−1, which could be ascribed to PO and P–O–P vibration peaks in APP, respectively [22]. With the increase of the bilayer number, the intensity of the above two characteristic peaks increased accordingly.
Conclusions
In this work, intumescent (PEI/APP)n coatings have been deposited on the external surface of ramie fabrics, using both LBL-spraying and LBL-dipping methods. It can be concluded that the spraying method is more effective and practical in comparison with the traditional LBL-dipping technique. TGA, MCC as well as cone results all revealed that the spraying method could achieve the similar thermal stabilization and flame-retardant effect in fewer coating compared with the LBL dipping method.
Acknowledgments
This work was financially supported by the National Basic Research Program of China (No. 2010CB631105) and Ningbo Natural Science Foundation of China (No. 2015A610028). We also want to thank Prof. John Billingsley for his generous help in the modification of the manuscript.
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