Effects of Wool Fibres on Mechanical and Flammability Characteristics of Wool-Polypropylene Composites

Abstract

Natural fibre reinforced polymer composites have been intensely studied and developed to create alternatives for synthetic fibre reinforced counterparts due to their unique advantages, such as CO2 neutrality, relatively high specific strength, and stiffness. One such natural fibre is wool, which is protein based and made up of the biodegradable keratin. The fibre’s complex physical structure and chemical compositions are responsible for its high thermal stability and mechanical performance. Also, wool is a naturally fire resistant fibre because of its high nitrogen and sulphur contents as well as char forming ability. In spite of the advantages of wool, studies regarding wool based polymeric composites have not been thoroughly performed. Therefore, this thesis has commenced a systematic investigation relating to the effects of wool on composites mechanical and fire retardant performance and development of a fire dynamics model based on computational fluid dynamics (CFD). Short wool fibre and polypropylene (PP) based composite sheets have been fabricated by a continuous extrusion process. Taguchi design of experiment method has been applied to comprehend the effects of wool in conjunction with other constituent’s conditions, such as maleic anhydride grafted polypropylene (MAPP) and polymer viscosity, on the mechanical properties of the extruded composites. The contribution of the selected factors towards the composites mechanical performance has been identified by the parametric study. Wool characteristics of the extruded composites, namely fibre length, orientation, and fracture behaviour, have also been investigated by an image analysis to determine their influences on the improvement of mechanical properties. The mechanical properties of the composites have been enhanced through the identification of suitable constituents’ conditions (using the Taguchi analysis) coupled with the improved orientation of wool fibres through the extrusion process. The effect of fire resistance of wool on composites flammability has been comprehensively explored in the second phase of this doctoral research. The enhancement of fire retardancy of the wool-PP composites through the addition of wool and ammonium polyphosphate (APP) has been confirmed by flammability tests. In particular, cone calorimeter and vertical burn tests have exhibited a significant decrease in peak heat release rate (PHRR) and a direct flame self-extinguishment of composites, respectively. Furthermore, the cone calorimeter experiments and char morphology have revealed that different polymer viscosities in the composites can influence the APP particle dispersion, leading to different PHRR results. The charring tendency of wool has contributed to an increase in the amount of residues with formation of compact micro-structures in a rigid char layer of the wool-PP-APP composites. Further investigation on the wool-PP composites flammability has been carried out to evaluate the effects of additives, such as APP types, MAPP, and talc, on fire retardant and mechanical properties. It has been revealed that the nitrogen content in APP is an important factor to create compact char structure and reduce the PHRR. Also, the positive effect of talc on char formation has been manifested in structure and chemical elements analyses of char. The influences of wool and the additives on mechanical properties have also been determined where the tensile moduli of the wool-PP-APP composites improved when compared to the neat PP. Finally, the CFD model has been developed using fire dynamics simulator (FDS) to simulate burning behaviour of wool-PP composite in the cone calorimeter test. The FDS model has been applied to predict the heat release rate of the composite and possible validation with the experimental result. A reasonable agreement of HRR between the simulation and the experiment has been observed.