Crosslinking of β-cyclodextrin and combining with ammonium polyphosphate for flame-retardant polypropylene

This work presents the synthesis of crosslinked hexamethylene diisocyanate beta-cyclodextrins (HDI-CDs) by reacting beta-cyclodextrin (beta-CD) with HDI as a crosslinking agent at different feed ratios. As a novel char-forming agent, the HDI-CDs are combined with ammonium polyphosphate (APP) and applied in polypropylene (PP) to form intumescent flame-retardant composites. The structure of HDI-CDs is characterized by Fourier transform infrared spectra (FTIR), C-13 nuclear magnetic resonance spectroscopy, and nitrogen adsorption-desorption test. The thermogravimetric analysis (TGA) results indicate that HDI-CDs have better char-forming performance than beta-CD. FTIR spectra, X-ray diffraction, and Raman spectra characterization demonstrate that the reaction between HDI-CDs and APP contributes to the formation of a more stable char layer than beta-CD and APP. According to the results of TGA, scanning electron microscopy, limiting oxygen index (LOI), UL-94, and X-ray photoelectron spectroscopy test, when the crosslinking degree of HDI-CDs is high enough (not less than beta-CD:HDI = 1:3.6), the PP/APP/HDI-CDs composites can form a compact and dense char layer during combustion. Among all composites, PP/APP/HDI-CD(4) shows the best flame-retardant performance, which can pass the UL 94 V-0 rating with an LOI value of 32.8% when the loading of flame retardants is 28 wt %. (c) 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 48320.