Development and evaluation of fire retardent unsaturated polyester clay nanocomposites.
The research in this PhD thesis is an important part of a larger study aimed at developing the next generation of fire-resistant, lightweight, structural materials (Surefire). The Surefire project aimed to develop a new class of materials that exhibited properties such as strength, stiffness, hardness and fire performance to compete with conventional materials. The aim was to achieve this through the synergistic integration of both nano size (particulate) and micro size (fibrous) reinforcements within an unsaturated polyester matrix. A new fire retardant unsaturated polyester resin system capable of UL94 classification VO was developed. The novel approach used to achieve this innovative class of material did not include halogenated compounds, but instead combined state of the art nano-scale particles with existing but un-revealed know-how to achieve a new class of material.Initial studies concentrated on the characterisation of a range of alkylammonium and alkylphosphonium organoclays. This was followed by in-depth studies of the polymer layered silicate nanocomposite (PLSN) materials. A range of commercial nanoclays were sourced, and characterised. In addition a range of modified clays were developed to improve fire performance and compatibility. Intercalated nanoclays in polyester resin formed an integral, dense char and therefore provided good fire protection. Phosphonium-modified clays exhibited greater thermal stability and provided enhanced fire performance but were more expensive and batch quality was variable. The addition of nanoclays and the fire retardant dimethyl methyl phosphonate (DMMP) to the resin caused a synergistic improvement in fire performance in one particular type of resin, possibly due to the particular type of modification used, but not revealed. The addition of 5-10wt% clay can interfere with crosslink density of the composite resin so this issue was addressed by producing surfactants which offered a cross-linking group, vinyl and methacrylate groups in particular on the end of the alkyl tail. The best formulations incorporated tributylhexadecylphosphonium clay (Bul6-MMT) which achieved a UL94 VO classification making them competitive with the fully halogenated resins investigated in the benchmarking stage.A novel 'one-pot' synthesis method for the production of PLSN, by the in-situ polymerisation of UP in the presence of a tributylhexadecylphosphonium surfactant and Na+ Cloisite, was shown to be successful. Whereas, other novel PLSN formulations incorporating a range of triphenylphosphonium cations were encouraging but have, so far, only provided limited success.The Surefire resin system was estimated to be between 19 and 24% cheaper than brominated and chlorinated resins, while offering similar fire performance, together with the added benefit of eliminating toxic halogenated species. Surefire resins offer significant potential as non-halogenated, fire retardant resins, although additional work would be necessary to exploit these results commercially. The key areas of future focus should include (i) identifying why the most successful, non-halogenated resin exhibits better fire performance than the other resins, (ii) understanding the origins of the synergy between the resin and additives particularly the clay, (iii) developing a better appreciation of suitable applications and (iv) confirming suitability for purpose through further testing.
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Sheffield Hallam University (United Kingdom).