Promotion of the flame retardancy of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide grafted natural rubber using expandable graphite

In this work, the problem of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) inhibiting vulcanization of natural rubber was solved. The flame retardancy of natural rubber was improved by introducing diglycydylepoxy natural rubber (DGENR) and expandable graphite (EG). DGENR was synthesized by grafting DOPO onto epoxidized natural rubber (ENR). The structure of DGENR was confirmed using Fourier transform infrared spectroscopy (FTIR) and vulcanization behavior testing. The flame retardancy of the resulting material was evaluated using the limited oxygen index (LOI), the UL-94, tensile testing, thermogravimetric analysis (TG) and cone calorimetry testing. Cone calorimetry testing showed that the peak heat release rate (pHRR) of NR decreased by 25.6% when 33 wt% DGENR and 7 wt% EG were added to pure NR. The NR containing 33 wt% DGENR and 7 wt% EG displayed LOI value of 25.4% and UL-94 V-0 rating. However, mechanical properties analysis showed that presence of EG reduced the mechanical properties of the matrix. These indicated that the addition of EG/DGENR was a good approach to prepare high effective flame-retardant NR, but had a negative effect on its mechanical properties.