Thermal properties and fire behavior of PLA nanocomposite films

Background and objectives: Improvement of polymeric materials properties, in terms of flammability resistance and thermal stability has always been interested in the scientific and industrial sectors. Polylactic acid (PLA) is also flammable like other polymers to its own intrinsic chemical composition and molecular structure. The use of nanoscale fillers can play a useful role in improving the thermal stability and flame retardancy of PLA. This research was designed to investigate the effect of cellulose nanocrystal and nanoclay on thermal properties and fire behavior of PLA films.
Materials and methods: PLA and PLA-based nanocomposite films were fabricated using a solvent casting method. In order to improve the compatibility and miscibility of the whole system with respect to PLA matrix, cellulose nanocrystal was treated with oleic acid. Firstly, the PLA was dissolved in 100 mL of chloroform while mixing vigorously at room temperature. Then, the cellulose nanocrystal and nanoclay with different loads (0 and 3 wt %) was dispersed in chloroform solvent by sonication for 30 min. The nanoparticles suspension was mixed with the PLA solution by sonicator for achievement of dispersion homogeneity. Finally, the dissolved solution was poured onto a leveled Teflon film and then allowed to evaporate solvent. The resultant film was peeled from the casting surface, and then the samples were dried in an oven. For evaluating the thermal and fire properties of films, melting temperature, crystallization temperature, degree of crystallinity, heat release rate, time to ignition, mass loss rate and limiting oxygen index were measured. To meet this objective, the differential scanning calorimetry (DSC) and cone calorimetry were used.
Results: DSC results found that with incorporation of nanoparticles, the melting temperature, crystallization temperature and degree of crystallinity increased. Cone calorimetry test revealed that, the presence of cellulose nanocrystal in the polymer matrix decreased the mass loss rate and heat release rate compared to those of pure PLA. However, the time to ignition increased with increase of cellulose nanocrystal loading. Moreover, the formation of carbonaceous chars from nanoclay on the surface of the films which reduces the burning process and delay the thermal degradation. Furthermore, the reduction of limiting oxygen index with the presence of both nanofillers is attributed to protect the underlying composites by formation of char shield on the surface and the limitation of polymer chains mobility.
Conclusion: The findings showed that the usage of each cellulose nanocrystal and nanoclay individually, has an effective role in improving the thermal and fire properties of PLA films. Furthermore, it was found that the highest thermal stability was attained by incorporating 3% cellulose nanocrystal and 3% nanoclay in composite formulation. This knowledge has an important role to design the new generation of environment-friendly flame retardant and biodegradable polymer nanocomposites.