| 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. 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. On the basis of the combustion mechanism of natural fiber fabric as well as the research status and development of flame retardants at home and abroad, we have constructed different assembly types of LBL coatings on the ramie fabric, and comparatively analyzed the functional mechanisms.Firstly, flame-retardant coatings consisted of opposite-charged polyelectrolyte polyethyleneimine (PEI) and ammonium polyphosphate (APP) were applied to ramie fabric using both a spray-assisted layer-by-layer (LBL) technique and a conventional dipping LBL method. Thermogravimetric analysis showed that all the coated fabrics left two to three times as much residual char as did uncoated ones. Use of the spraying method was able to achieve the similar thermal stabilization at fewer bilayer number in coating that compared with the dipping method. Additionally, both methods exhibited an obvious reduction in total heat release and peak heat release rate in the microscale combustion calorimeter test and cone calorimetry. These results demonstrate that the spray-assisted LBL technique represents a relatively efficient method with low layers, showing more industrial application prospect. Attributed to the limitation of manual operation process in this paper, the coatings was not as uniform as the conventional dipping LBL technique with high layers, making the spraying method inferior to the dipping one for imparting flame-retardant behaviour to ramie fabric.Secondly, negative-charged polyacrylic acid (PAA) was introduced into the bi-component PEI/APP multilayer flame-retardant coatings. The effect of PAA as a supplementary acid source on the flame retardancy of treated ramie fabrics has been studied. It is found that the char residues of the coated fabrics at high temperature during thermogravimetric analysis were significantly enhanced as compared with the untreated sample. Additionally, the microscale combustion calorimetric analysis revealed a deduction of both peak heat release rate (PHRR) and total heat release (THR). Compared with the bi-component (PEI/APP)n sample, the introduction of the third component PAA seemed to work at low assembly cycles by increasing the charge density of PEI, thus to improve the add-on of the FR coating on the fabric. While the same charges may repel each other from the confusion of APP and PAA with high layers, causing the FR effect not that favorable.Finally, α-zirconium phosphate, as an insulating inorganic flame retardant was introduced to construct LBL nanocoatings with different architectures on the ramie fabric:bi-component insulating FR coatings (PEI/ZrP), inside-intumescence-outside-barrier complex coatings (PEI/APP+PEI/ZrP) and inside-barrier-outside-intumescence coatings (PEI/ZrP+PEI/APP). It is found that the last sample possessed the most uniform and consistent coating surface morphology, as well as the highest content of N, P element, thus to show excellence in improving the flame retardancy of coated ramie fabrics. When the inside-barrier-outside-intumescence coated fabric was heated, the inner barrier layer effectively prevent oxygen and heat from penetrating into the substrate, the outer intumescent layer exposed to air also achieve good expansion at the same time, the synergistic effect formed during the combustion process could impart the fabrics with high flame retardancy. |
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