In addition to inorganic fire retardants, certain plants have developed a defense mechanism against fire invasions due to their specific molecular structure. Bio based compounds in these plants owe their intrinsic fire retardancy to the formation of a thermally stable char layer when exposed to fire. A review of bio based flame retardants has been previously published by Costes et al.13 Briefly, char formation mechanism is initiated as the stored water in wood starts to be liberated during thermal decomposition of wood components (lignin, cellulose, etc.). This barrier char layer insulates the underlying wood from further burning by hindering the heat exposure.
Biomass is the largest resource for bio-based materials and a massive range of chemicals and biofuels are produced based on biomass derivatives, thanks to their abundant resources and reasonable pricing. The inherent char formation ability of biomass and the resulting fire retardancy makes it desirable for FR applications.61 Up to 75% of biomass is composed of saccharide-based products (e.g. cellulose, hemicellulose, and lignin), while the rest is mostly energy storage components (e.g. starch), proteins, and vegetable oils.
Lignin is one of the most abundant bio-based products with a high molecular weight. Thermal degradation of lignin is accompanied by a stable char formation; it first starts around 200°C with water release and condensation reaction. Lignin experiences a mass loss over a broad range of temperatures up to around 800°C, which is resulted from its structural decomposition and carbonized layer formation.62 Chirico et al reported an improvement in fire behavior of polypropylene in the presence of 15 to 20 wt% of lignin. The pHRR and mass loss rate of the composite was generally lower compared with the virgin polymer due to the char formation mechanism of the lignin.
Starch is another bio-based compound with an interesting fire retardancy mechanism. It is a polysaccharide composed of amylose and amylopectin polymers. During starch decomposition, a physical dehydration happens by releasing the stored moisture, followed by thermal condensation of hydroxyl groups in higher temperatures. Consequently, aromatic rings (e.g. furan) are formed in temperatures around 400°C as a result of abovementioned water liberation and ether segment formations. These aromatic rings later participate in carbonization and char formation.64-66 It is worth to mention that thermal stability of starch is highly dependent on its composition, structure, chemical modification, and extraction methods. Starch's high decomposition temperature makes it an interesting option for polymer composites with high processing temperatures.
Starch is a rich carbon source and when combined with IFRs can result in flame retardancy properties for commercial polymers, such as, PLA.13, 70 Wang et al prepared a PLA composite using 10 wt% starch and 20 wt% IFR. Here, microencapsulated Ammonium Polyphosphate served as acid source, melamine a blowing agent, while starch as the carbonization agent. The resulting composite showed 41% LOI value along with UL 94 V-0 classification and modified dripping behaviors compared with the neat polymer.68 It is important to note that addition of starch and IFR separately to PLA did not yield successful fire retardancy behavior compared with the neat polymer, suggesting the potential synergetic effect of starch/IFR system.
Proteins also have attracted research interests in fire retardancy applications. Casein, milk's major protein, is a phosphorous containing molecule, which is used as a coating for cotton or polyester fabrics. The fire behavior of casein-coated fabrics is reported to be improved in terms of significant reduction in total burning rate, thanks to the char formation mechanism of proteins. During the thermal decompostion, proteins like casein favor the char formation mechanism and volatile supression by releasing acidic compounds, such as, phosporic acid. Carosie et al reported a remarkable decrease of the burning rate (−70%) and an increase of limiting oxygen index (from 21 to 26%) of polyester by adding a suspension of casein with total of 20 wt% dry solid add-on.
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