Layer-by-Layer Nanocoatings with Flame Retardant and Oxygen Barrier Properties: Moving Toward Renewable Systems



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Numerous studies have focused on enhancing the flame retardant behavior of cotton and polyurethane foam. Some of the most commonly used treatments (e.g., brominated compounds) have raised concerns with regard to toxicity and environmental persistence. These concerns have led to significant research into the use of alternative approaches, including polymer nanocomposites prepared from more environmentally benign nanoparticles. These particles migrate to the surface from the bulk during fire exposure to form a barrier on the surface that protects the underlying polymer. This theory of fire suppression in bulk nanocomposites inspired the use of layer-by-layer (LbL) assembly to create nanocoatings in an effort to produce more effective and environmentally-benign flame retardant treatments.

Negatively charged silica nanoparticles of two different sizes were paired with either positively charged silica or cationic polyethylenimine (PEI) to create thin film assemblies. When applying these films to cotton fabric, all coated fabrics retained their weave structure after being exposed to a vertical flame test, while uncoated cotton was completely destroyed. Micro combustion calorimetry confirmed that coated fabrics exhibited a reduced peak heat release rate, by as much as 20% relative to the uncoated control. Even so, this treatment would not pass the standard UL94 vertical flame test, necessitating a more effective treatment.

Positively- charged chitosan (CH) was paired with montmorillonite (MMT) clay to create a renewable flame retardant nanocoating for polyurethane foam. This coating system completely stops the melting of a flexible polyurethane foam when exposed to direct flame from a butane torch, with just 10 bilayers (~ 30 nm thick). The same coated foam exhibited a reduced peak heat release rate, by as much as 52%, relative to the uncoated control. This same nanobrick wall coating is able to impart gas barrier to permeate plastic film.

Multilayered thin films were assembled with "green" food contact approved materials (i.e., chitosan, polyacrylic acid (PAA) and montmorillonite clay). Only ten CH-PAA-CH-MMT quadlayers (~90 nm thick) cause polylactic acid (PLA) film to behave like PET in terms of oxygen barrier. A thirty bilayer CH-MMT assembly (~100 nm thick) on PLA exhibits an oxygen transmission rate (OTR) below the detection limit of commercial instrumentation (<= 0.005 cm^3/(m^2dayatm)). This is the same recipe used to impart flame retardant behavior to foam, but it did not provide effective FR to cotton fabric, so a very different recipe was used.

Thin films of fully renewable electrolytes, chitosan and phytic acid (PA), were deposited on cotton fabric in an effort to reduce flammability through an intumescent effect. Altering the pH of aqueous deposition solutions modifies the composition of the final nanocoating. Fabrics coated with highest PA content multilayers completely extinguished the flame and reduced peak heat release (pkHRR) and total heat release of 60% and 76%, respectively. This superior performance is believed to be due to high phosphorus content that enhances the intumescent behavior of these nanocoatings.