Expandable graphite: A fire retardant additive for polyurethane coatings

Abstract

SUMMARYThe study investigates the mechanism of fire retardancy induced by expandable graphite in a polyurethane coating. A complementary investigation to the spectroscopic study performed previously confirms that little interaction occurs between the matrix and the additive. X-ray diffraction study demonstrates that the crystalline structure of the graphite is maintained during the intumescent process. The presence of free radicals in the protective shield is then demonstrated but at low concentration compared with classical intumescent systems, such as for example polyurethane/ammonium polyphosphate. Finally, the physical properties of the char are then investigated. Blowing measurement demonstrates the high degree of expansion of the layer which may partially explain the fire retardant performance of expandable graphite in polyurethane. Surface analyses by optical microscopy and optical profilometry also enable a better understanding of the efficiency of expandable graphite. In the absence of a fire retardant additive the coating displays cracks and holes, providing a pathway for oxygen diffusion as well as heat and mass transfer between the flame and the virgin polymer. Both phenomena are limited when expandable graphite is added to polyurethane. Consequently, fire retardancy of expandable graphite results more from a physical than from a chemical action.

Preparation of Boric Acid Modified Expandable Graphite and Its Influence on Polyethylene Combustion Characteristics

“…In the referenced EG, the S content is relative higher, which reveals the intercalation of H 2 SO 4 /HSO 4 -. 10 Although the S content in EG B is still higher than that of natural graphite, it's lower than that of EG, which dues to the assistant intercalation of H 3 BO 3 . Therefore, EG B will release less SO 2 gas than the referenced EG.…”

“…At the same time, the intercalated H 2 SO 4 reacts with graphite causing release of CO 2 , H 2 O and SO 2 , 7-9 which leads to the expansion of GICs and the formation of the "worm like" expanded graphite particles. 10 The EG B should show better flame retardancy than EG for its good dilatability.…”

“…Due to its outstanding anti-flame capability, EG has been used in the flame retardance of polymer materials such as polyurethane (PU) or polyurethane coatings, 10,11 polyolefin blends, 12,13 acrylonitrile-butadiene-styrene (ABS), 14,15 ethylene vinyl acetate (EVA). 16,17 However, there are still some shortcomings when EG is used as a FR.…”

“…Moreover, more SO 2 will emit when EG is prepared with only H 2 SO 4 as intercalator. 19,20 So it is very important to enhance efficiency in order to meet environmental protection standard.…”

“…It was reported the H 2 SO 4 /APP (APP, ammonium polyphosphate, a assistant intercalating agent) GIC, prepared through two-step method, exhibited a higher expansion volume (EV, 240 mL g -1 ) than that of the H 2 SO 4 intercalated EG (210 mL g -1 ). 19 An EG, intercalated by H 2 SO 4 and Na 2 SiO 3 through two-step intercalation reaction, 17 presented an EV of 517 mL g -1 and combustion limiting oxygen index (LOI) of 28.7% for EVA. However, the EV of referenced EG, which is intercalated by only H 2 SO 4, was merely 433 mL g -1 , and the LOI was just 24.4%.…”