Development and characterisation of flame retardant nanoparticulate bio-based polymer composites

Abstract

Since the discovery of carbon nanotubes (CNTs) and nanoclays, there has been a great deal of research conducted for uses in applications such as: energy storage, molecular electronics, structural composites, biomedical to name but a few. Owing to their unique intrinsic properties and size means that they have an ever growing potential in the consumer and high technology sectors. In recent years the concept of using these as fillers in polymers has shown great potential. One such function is, as flame retardant additives. These possess much better environmental credentials than halogenated based additives as well as only needing to use a small loading content compared to traditional micron sized fillers. The combination of the above make these fillers ideal candidates for polymers and their composites. Especially with regards to natural fibre composites. Owing to environmental awareness and economical considerations, natural fibre reinforced polymer composites seem to present a viable alternative to synthetic fibre reinforced polymer composites such as glass fibres. However, merely substituting synthetic with natural fibres only solves part of the problem. Therefore selecting a suitable material for the matrix is key. Cellulose is both the most common biopolymer and the most common organic compound on Earth. About 33 % of all plant matter is cellulose; i.e. the cellulose content of cotton is 90 % and that of wood is 50 %. However just like their synthetic counterparts, the poor flame retardancy of bio-derived versions restricts its application and development in important fields such as construction and transportation. Abstract -vi- Traditional methods to improve the flame retardancy of polymeric material involve the use of the micron sized inorganic fillers like ammonium polyphosphate (APP) or aluminium trihydroxide (ATH). Imparting flame retardancy with these inorganic fillers is possible but only with relatively high loadings of more than 50 wt. %. This causes detrimental effects to the mechanical properties of the composite and embrittlement. Applying nanofillers can achieve similar if not better flame retarding performances to their micron sized counterparts but at much lower loading levels (<10 wt.%), thus preserving better the characteristics of the unfilled polymer such as good flow, toughness, surface finish and low density. This is the main focus of this study and it will be achieved by using various experimental techniques including the cone calorimeter and the newly developed microcalorimeter. After a comprehensive literature survey (Chapter 2), the experimental part of the thesis starts with a feasibility study of a flame retardant natural reinforced fibre sheet moulding compound (SMC) (Chapter 3). This work demonstrated that with a suitable flame retardant the peak heat release rate can be reduced. Chapter 4 deals with further improving the flame retardancy of the previously used unsaturated polyester resin. The aim is to study any synergistic behaviour by using aluminium trihydroxide in conjunction with ammonium polyphosphate whilst testing in the cone calorimeter. In Chapter 5, nanofillers are used to replace traditional micron sized fillers. In unsaturated polyester, multi-walled carbon nanotubes and sepiolite nanoclay are used together to create a ternary polymer nanocomposite. The microcalorimeter was employed for screening of the heat release rate. This work showed that the ternary nanocomposite showed synergistic behaviour with regards to significantly reducing the peak heat release rate. Abstract -vii- The same nanofillers were utilised in Chapters 6 and 7 but this time in combination with a thermoplastic (polypropylene) and bio-derived polymer (polylactic acid), respectively. In both systems an improved flame retardancy behavior was achieved whist meeting the recyclability objective. Chapter 8 attempts to show how the optimised natural fibre composite would behaviour in a large scale fire test. The ConeTools software package was used to simulate the single burning item test (SBI) and to classify the end product. This is a necessity with regards to commercialising the product for consumer usage. Finally, Chapter 9 is a summary of the work carried out in this research as well as possible future work that should be conducted.