Polyisocyanurate/stabilized phosphate microcapsules as effective flame retardants to minimize phosphate migration and vulcanization interference in self-extinguishing rubber blend composites

Abstract

Utilization of ammonium phosphates, the human-safe and effective flame retardants, are still limited in rubber composite manufacturing. Phosphate migration and vulcanization efficiency problems remain unsolved. Therefore, stabilized phosphate microcapsule (PIR@MADHP) was first introduced. The stabilized phosphate (MADHP) was synthesized from the reaction of melamine and ammonium dihydrogen phosphate (ADHP) (1:1 mole ratio) and then microencapsulated in polyisocyanurate (PIR) through isocyanate cyclotrimerization, yielding the PIR@MADHP. The characterized PIR@MADHP possessed double-shell-like layers of PIR and melamine and exhibited hydrophilic-to-hydrophobic transition (water contact angle = 110.6°). It contained substantial flame-retardant elements (46.1 % C, 30.9 % N and 7.7 % P). PIR enhancing filler compatibility and simultaneously improving fire suppression in elastomer composites is first introduced. When using this microcapsule with composite model of natural/synthetic rubber blend, epoxidized natural rubber/chloroprene rubber blend (RB), the RB/PIR@MADHP composite performed anti-dripping and self-extinguishing behaviors with elevated limiting oxygen index (LOI) to 23.8 %. Its peak of heat release rate, mass loss and maximum smoke production rate diminished from those of unfilled RB by 77.9, 67.8 and 75.0 %, respectively. Fire performance index of RB/PIR@MADHP was two-fold higher than RB/commercial ammonium polyphosphate (APP) flame retardant. Microencapsulation of phosphate enabled to eliminate acid-deactivating effects during composite vulcanization, leading RB/PIR@MADHP to have maximum crosslink density. This enhanced network formability and the improved compatibility helped RB/PIR@MADHP composite to prevent phosphate migration by minimizing the LOI reduction to 0.4 % after water erosion test. RB/PIR@MADHP also provided the highest abrasion resistance and tensile strength among the composites with crude phosphate, non-microencapsulated flame retardant and commercial APP.