Preparation Of Flame Retardant Ramie Fabric Via Layer-by-layer Assembly And Its Application In Ramie Fabric/polybenzoxazine Laminates

As a kind of interesting natural cellulosic fibers, bast fiber shows promising applications as reinforcement of composites in many advanced fields (such as automotive, aerospace, military and construction industries, etc.) besides home textile, because of its biodegradability, low price, low density and excellent mechanical properties. However, the highly flammable nature limits its practical applications. In view of the special chemical composition and structure of ramie fabric, we have constructed intumescent flame retardant coating on ramie fabric, and then applied the flame retardant ramie fabric to prepare ramie fabric/polybenzoxazine laminates.Firstly, three kinds of novel aromatic diamine-based benzoxazines containing naphthalene, propane-2,2-diyldibenzene and neopentyl group in the backbone, respectively (designated as BAPNCP, BAPBACP and BAPNPGCP, respectively), have been synthesized and characterized. In addition, the effect of backbone structures on curing behaviors of the monomers and thermal and flammability properties of the resulting polymers were studied. The results indicated that BAPNPGCP displayed the highest enthalpy of the curing reaction associated with the ring-opening of benzoxazine, which was due to the effect of benzoxazine ring content per unit mass. Interestingly, the5wt%weight loss temperature and char residue at900℃for poly(BAPNPGCP) were8℃and7wt%higher than those of poly(BAPBACP). Meanwhile, the total heat release of poly(BAPNPGCP) was less than half that for poly(BAPBACP), indicating the substantial effect of benzoxazine ring content on flammability and char formation. Furthermore, it was found that poly(BAPNCP) gave the best thermal stability and flame retardancy, which was due to synergistic effect between naphthalene group and benzoxazine ring content.Secondly, a new flame retardant nanocoating has been constructed by the alternate adsorption of polyelectrolyte amino-functionalized multiwall carbon nanotube (MWNT-NH2) and ammonium polyphosphate (APP) onto flexible and porous ramie fabric, and the film growth was monitored by UV-visible spectrometry (UV-vis), scanning electron microscopy (SEM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and energy-dispersive X-ray analysis (EDX). SEM indicated that the adsorbed carbon nanotube coating was a randomly oriented and overlapped network structure, which is a promising candidate for flame retardancy applications. ATR-FTIR and EDX confirmed that the APP was successfully incorporated into the multilayers sequentially. Assessment of the thermal and flammability properties for the pristine and nanocoated ramie fabrics showed that the thermal stability, flame retardancy and residual char were enhanced as the concentration of MWNT-NH2suspension and number of deposition cycles increases. The enhancements are mostly attributed to the barrier effect of intumescent network structure, which is composed of MWNT-NH2and the absorbed APP.Thirdly, self-extinguishing multilayer coatings consisting of polyelectrolyte polyethylenimine (PEI) and ammonium polyphosphate (APP) have been constructed by layer-by-layer assembly technique onto flexible and porous ramie fabric, and the film growth was monitored by ATR-FTIR, SEM, EDX and Atomic force microscopy (AFM). ATR-FTIR and EDX directly confirmed that PEI and APP were successfully incorporated onto the surface of ramie fabric sequentially. Assessment of the thermal and flammability properties for the coated ramie fabrics showed that the char residue at temperature ranging from400to600℃during thermogravimetric analysis (TGA) and the self-extinguishing ability during vertical flame test were significantly enhanced as compared with the pristine sample, which showed strong dependency on the number of deposited layers, especially on the concentration collocation of both polyelectrolytes.Finally, we prepared polybenzoxazine matrix laminates reinforced with pristine and treated ramie fabrics with20bilayers of MWNT/APP or PEI/APP (designated as BZ/ramie, BZ/ramie/MWNT/APP and BZ/ramie/PEI/APP, respectively), and studied the influence of treated ramie fabric on mechanical and flame retardant properties for laminates. Compared to BZ/ramie, the total heat release and maximum peak values of BZ/ramie/MWNT/APP and BZ/ramie/PEI/APP decreased substantially, and the limiting oxygen index values were increased significantly. The burning ratings of BZ/ramie/MWNT/APP and BZ/ramie/PEI/APP during UL-94vertical flame test were V-0and V-1, respectively. Flexural and tensile tests indicated that multilayers of flame retardant coating can enhance interfacial adhesion between the matrix and reinforcement, so as to improve flexural strength, tensile strength and elongation at break.