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

ABSTRACT This work presents the synthesis of crosslinked hexamethylene diisocyanate β‐cyclodextrins (HDI‐CDs) by reacting β‐cyclodextrin (β‐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), 13C nuclear magnetic resonance spectroscopy, and nitrogen adsorption–desorption test. The thermogravimetric analysis (TGA) results indicate that HDI‐CDs have better char‐forming performance than β‐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 β‐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 β‐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 %. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 48320.