Environmentally-benign Flame Retardant Nanocoating for Foam and Fabric

Abstract

Halogen-additives are cost effective flame retardants (FRs) that scavenge H• and OH• radicals in the gas phase, but are under significant scrutiny due to the toxic smoke they release and their potential to leach out into the environment and possibly bio accumulate. One fire retarding solution is using layer-by-layer (LbL) assembly, which is a simple, bottom-up processing technique, to create functional nanocoatings through sequential adsorption of materials with complementary functional groups for the purpose of inhibiting or suppressing the combustion cycle. Inspiration for first applying polymer/clay thin films (i.e., nanobrick walls) as flame retardant (FR) coatings to polyurethane foam via LbL came from the final stage of a proposed flame suppression mechanism in a melt-mixture of polymer and clay, which depicts a physical barrier created from the build-up of impermeable flakes and carbonized char.

Intumescing nanobrick wall assemblies comprised of nitrogen and phosphorus-containing polymers (mortar) and clay platelets (bricks) were deposited on flexible polyurethane foam using layer-by-layer assembly. Four trilayers of the poly(allylamine hydrochloride) (PAH)/poly(phosphate sodium salt) (PSP)/montmorillonite (MMT) nanobrick wall assembly (< 3 wt% coating addition) are necessary to cut the pkHRR of polyether-based polyurethane by 54.8%, relative to control, uncoated foam.

The influence of clay aspect ratio and composition on fire behavior of coated polyurethane foam was studied as a function of polymer/clay (montmorillonite or vermiculite (VMT)) layers deposited and nanocoating weight addition. A single bilayer (BL) of polyethylenimine (PEI) and formulated-vermiculite clay, which adds only 3.2 wt% to the foam, successfully prevented formation of a melt pool of burning polymer and reduced peak heat release rate and total smoke release by 54% and 31%, respectively. MMT-nanobrick walls require 4 BL to match the fire performance of single BL VMT-nanobrick walls.

Aqueous coacervation was investigated as a single step process to deposit flame retardant nanocoatings on textiles quickly. Cotton soaked in environmentally-benign PSP/PEI complex for 1 min resulted in a 16.7% residue after vertical flame testing and a 52.7% reduction in total heat release in comparison to uncoated cotton in micro combustion calorimetry. Nanocoatings produced from a 10 min immersion result in fabric capable of self-extinguishing during vertical flame testing.