Renewable Cardanol-Based Surfactant Modified Layered Double Hydroxide as a Flame Retardant for Epoxy Resin

A biobased modifier (cardanol-BS) was successfully synthesized from renewable resource cardanol via the ring-opening of 1, 4-butane sultone (BS). Cardanol-BS modified layered double hydroxide (m-LDH) was developed through a one-step coprecipitation method and subsequently incorporated into epoxy resins (EPs) with different loadings using a combined technique of three-roll mill and ultrasonication. As a comparison, a pristine LDH/EP composite was also prepared using the same procedure. The XRD result indicated that the interlayer spacing of m-LDH was about 5-fold enlarged compared with that of pristine LDH. As a result, the enlarged interlayer spacing of m-LDH facilitated the homogeneous dispersion of the nanoadditive in the epoxy matrix, as evidenced by TEM and XRD results. The flame retardant properties were improved with the increase of the m-LDH loading. With only 6 wt % m-LDH, the EP composite reached a limiting oxygen index (LOI) of 29.2% and UL-94 V0 rating. The peak heat release rate (PHRR), total heat release (THR), and total smoke production (TSP) values of EP/m-LDH-6% were decreased by 62%, 19%, and 45%, respectively, compared to those of pure EP. In contrast, pristine LDH did not show so high an efficiency as m-LDH in terms of the reduced PHRR, THR, and TSP, and also the EP/LDH-6% composite exhibited no rating in the UL-94 vertical burning test. These findings supported that the flame retardant behavior increased with improved dispersion of nanofiller in the polymer matrix. The well-dispersed m-LDH nanofillers were beneficial to improving the quality of char residue, which effectively inhibited flammable volatiles escaping from the interiors and served as an effective thermal insulation layer to shield the underlying matrix from the exterior heat irradiation.