Cellulose is the most abundant organic polymer, which is found in cell wall of plants as well as in fungi, bacteria, and algae. Cellulose has numerous glucose units with high degree of polymerization based on its extraction method. Figure 4 represents a cellulose fiber organization in a plant cell wall, which is consisted of many cellobiose repeating units.13, 73 Char formation mechanism is very complicated in cellulosic materials. During thermal decomposition, cellulose can produce an insulating char layer under certain specifications, depending on its extraction method and surface treatment. Degradation condition and existing species in the combustion environment govern the amount of produced char and its thermal stability.
In low temperatures, degradation of cellulose leads to the formation of anhydrocellulose. As temperature goes up, the remaining cellulose unzips into tar and further anhydrocellulose components and finally proceeds to char and gas formation. Many researches were carried out to improve fire retardancy of cellulose by chemical surface modifications or incorporation of other fire retardants (e.g. phosphorous FRs). These attempts proved to further assist the inherent char formation behavior of cellulose.73, 76
Cellulosic nanomaterials, such as, cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs), are generally used as nanofillers for various polymers in order to tailor their physical, mechanical, and thermal properties. They typically have a diameter between 5 and 500 nm and most of them are commercially available or can be extracted via well-known methods (e.g. acid hydrolysis).77, 78 Recently, flame retardancy potential of cellulose nanofillers has attracted a lot of interests. There are numerous reports on improved flame retardancy performance of commercial polymers in the presence of surface-modified cellulose nanocrystals or cellulose nanoparticles hybrids with other fire retardants.49, 79-81
A 10 wt% of CNCs extracted with phosphoric acid hydrolysis has proven to increase the thermal stability of polylactic acid (PLA).82 Phosphoric acid is well-known for its fire retardancy property.35 Moreover, CNCs have a highly crystalline structure with well-packed chains and hard-to-break inter-chain hydrogen bonding that protects them from melting in high temperatures. Therefore, the incorporation of phosphoric acid treated CNCs in PLA matrix would enhance the thermal stability of the polymer by formation of the char layer and hindering the heat energy.
CNFs-clay nanopaper composite has been reported as an effective flame retardant coating for wood.83 The hybrid system was prepared as a “brick and mortar” structure, where clay nanopapers represent the brick and CNFs represent the mortar. The thermal shielding provided by this system caused an 81% decrease in the heat flux of the wood as reported by cone calorimetry test.83 The char layer formed by this hybrid acted as an oxygen barrier insulating layer and increased the oxygen diffusion length. Additionally, clay char layer hindered the diffusion of CNFs volatiles, resulting in the formation of micro voids and further decline in thermal conductivity.
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