Flammability studies on biopolymers and their blends

Abstract

The effect of commercial expandable graphite (EG) on the flammability and thermal decomposition characteristics of two systems based on poly(lactic acid) (PLA) and poly(lactic acid)/poly(ε-caprolactone) (PLA/PCL) blend was investigated. Furthermore, the morphology, structure, melting and crystallization behaviour as well as the dynamic mechanical properties of flame retardant PLA/EG and PLA/PCL/EG composites were also studied. The flame retardant PLA/EG and PLA/PCL/EG composites were prepared by melt-mixing using the Brabender-Plastograph and were melt-pressed using the electrical hydraulichot melt press. The samples were characterized for their flammability performance and thermal stability via cone calorimeter and thermogravimetric analyser (TGA), respectively. They were also characterized for their volatile pyrolysis products during thermal degradation using simultaneous TGA-Fourier transform infrared spectroscopy (TGA-FTIR). The char residues obtained after combustion by cone calorimeter were further analysed with environmental scanning electron microscopy (ESEM). Furthermore, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to elucidate the structure and morphology of the flame retardant PLA/EG and PLA/PCL/EG composite systems. Their thermal behaviour (i.e. melting and crystallization) as well as their thermo-mechanical properties were respectively analysed by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) techniques. For the PLA/EG composite system, the thermal decomposition stability of the composites was improved in the presence of EG. However, the char content was less than expected as per the sum of the wt.% EG added into PLA and % residue of PLA after thermal decomposition. The flammability performance of the PLA/EG composites was improved, especially at 15 wt.% EG content, due to a thick and strong worm-like char structure. The peak heat release rate (PHRR) was improved by 74%, the total smoke production (TSP) was improved by 40% and the specific extinction area (SEA) by 55%. These improvements were due to the ability of EG to exfoliate at increased temperatures during which three effects occurred: i) cooling effect due to an endothermic exfoliation process; ii) dilution effect due to the release of H2O, SO2 and CO2 gases and iii) formation of a protective intumescent char layer. However, both the CO and CO2 yields were found to be unfavourably high due to the presence of EG. The graphite layers still existed in an aggregate structure with poor filler dispersion and lack of interfacial adhesion between EG and the PLA matrix. The presence of EG micro-particles: i) did not favour the crystallization of PLA, ii) increased the glass transition temperature and iii) showed a reduction in the crystallinity of the composites. The composites showed enhanced storage and loss moduli, especially at high EG contents (i.e. 10 and 15 wt.%). The glass transition from the loss modulus and damping factor varied inconsistently with the EG content. The use of commercial expandable graphite as filler in PLA could preserve the thermal properties of injection molding grade Cereplast PLA, while improving the fire resistance of PLA/EG flame retardant composites. In the case of PLA/PCL/EG flame retardant composite system, the thermal degradation stability of the composites was improved and the char content was found to have increased. Although the char content of the composites increased generally with EG loadings, the combined % residue from both the blend and wt.% EG initially added into the blend was higher than the observed % residue because of the thermal degradation mechanism that favoured the formation of CO and CO2 volatile gases rather than carbon. The flammability performance results indicated that the PLA/PCL blend was successfully modified with the EG micro-filler that resulted in fire resistant composites, especially at high filler loadings, due to the formation of intumescent carbonaceous char. This was confirmed by reductions of up to 64% in both the peak heat release rate (PHRR) and the total smoke release (TSR) and 54% in the specific extinction area (SEA). This was due to the EG acting mainly through a physical mode by cooling and fuel dilution and through the formation of an intumescent char layer. However, the effective heat of combustion (EHC) and carbon monoxide (CO) yields did not favourably improve. It was found that the melt mixing process could not separate the graphite layers, which existed as aggregate structures (i.e. EG layered stacks and/or lumps). In the composites, PCL was favoured to crystallize mainly on the surface of the microspheres and EG. The PLA/PCL blend showed an immiscibility feature, even in the presence of EG filler. Incorporation of EG in PLA/PCL blend influenced the melting and crystallization behaviour of PCL than that of the PLA component. Both the PCL and EG hindered the crystallization of the PLA component. From DSC and DMA, the glass transition of the composites occurred at high temperatures, suggesting that PLA polymer chains were immobilized in the presence of EG micro filler. The storage and loss moduli values were low for the composites when compared to PLA/PCL. The results suggest that the PLA/PCL/EG flame retardant composites have low thermal and thermo-mechanical properties due to the aggregation of EG and lack of interfacial adhesion between EG lumps and the polymer blend matrix.