Characterisation of Halogen Free Fire Retardant Polypropylene Composites Reinforced by Short Natural Fibres

Abstract

With the increasing concerns about the sustainability issues and the existing ethics/environment related regulations implemented during the last few decades have diverted the focus of the composites manufacturing industry towards replacing synthetic fibres with their natural counterparts. However, the limited thermal stability of natural fibres, which leads to degradation during processing mostly beyond 200⁰C, restricts the mass manufacturing methods, such as injection moulding. Halogen-based compounds containing bromine or chlorine are extremely active fire retardant elements in the gas phase of the combustion process, but with the worldwide stringent fire regulation policies, the governments have been pressured to ban or severely restrict the use of halogens with their serious health related and environmental hazards. Therefore, the phosphorus-based fire retardant fillers play a prominent part in the fire retardant development pursuit. Hence, this thesis has embarked on a systematic approach to developing a halogenfree but cost-effective fire retardant for short natural fibre composites. The major drawback of currently available filler type flame retardants is that, they require considerably large amount of filler (by weight) to achieve the standard flammability ratings and thereby promote adverse effects on the mechanical properties of the composites. Use of intumescent ammonium polyphosphate (APP) in composites has substantially addressed this issue due to its inherent intumescent properties and is an effective way to improve flame retardance of natural fibre composites. However, the widely available different commercial grades of these flame retardants has limited the identification of suitable flame retardant for the natural fibre composites. Therefore, in this work the first task was to enhance the thermal and mechanical performances of polypropylene/kenaf (PP/Ke) composites. Different grades of commercially available intumescent APPs were initially selected and tested for PP/kenaf composites. From the preliminary phase of this work, it was identified that from an overall flame retardant perspective (forced and sustained combustions) the APP particles would perform better over the surface energy modifications. Even with these unique findings relatively small flame retardant efficiency of the IFR system needed a large proportion of flame retardant additive, which could impede their commercial applications. The use of synergistic nano fillers (montmorillonite nano clays and halloysite nanotubes) augments the flame resistance properties of PP/kenaf/APP composites through the formation of cross-linked aluminosilicate phosphor-carbonaceous structure. The agglomeration of particles tends to reduce the mechanical strengths of these composites providing low compatibilisation and crack propagation. The decisive advantages obtained through filler particle synergism provided encouragement to implement the “synergistic natural filler hybridisation” technique in the next stage of development in this research. Natural wool has been added to PP/kenaf/APP composite as the synergistic additive to enhance the thermal properties of the composite by overcoming various deficiencies. It has been found that the secondary decomposition of intumescent APP activates the hydrolytic scissions of wool keratin and create cross-linked aromatic structure. This further reduces the degradation of the composite by increasing the mechanical strength and stability of the char and promotes competitive mechanical properties. Conducive to commercialisation, the design of experiments (DoE) methodology has been used to develop a favourable flame retardant combination for PP/Ke composites. On the basis of this parametric study, it has been found that APP and wool content are the two most significant influencing factors in the development of desired flame retardant combination. The developed synergistic hybridised favourable flame retardant (SHFFR) masterbatch has been proven to be effective for other PP natural fibre combinations. Further, the fire performnce validation has also been carried out with the help of Building Research Association of New Zealand (BRANZ).