Characterisation of expandable graphite and its flame retardant abilities in flame retardant systems for polyethylene

Abstract

In the pursuit of lower cost intumescent flame retardant (IFR) systems, the compound expandable graphite (EG) was identified. This compound delivers high flame retardant performance but provides non-uniform thermal shielding when exposed to open flame from below due to negative gravitational effects. It was theorised that this may be remedied either through ion exchange of the interstratified ions with low glass transition ions or through use in binary systems with other compounds. Two classes of commercial EG were identified, namely a low and a high expansion onset temperature EG compound. Extensive characterisation of each EG compound was undertaken to assess its composition, expansion mechanisms and onset temperatures in order to identify compatible compounds for binary use. The susceptibility of each compound to ion exchange was also assessed. An industrial IFR ethylenediamine phosphate (EDAP) and a novel flame retardant were synthesised for assessment in binary use with EG. Coupled with the above study, this project developed two novel fire testing techniques as low cost alternatives to well-established fire testing methods such as cone calorimetry. The first technique involved an open flame fire testing method which allowed vertical or horizontal testing. Digital and infrared (IR) video recording during operation facilitated comparison of multiple performance indicators further strengthening this method. The second technique allowed assessment of the mass loss resistance of each compound during laser pyrolysis. Characterisation of the EG compounds allowed development of structural models to describe each compound and explain the mechanisms of their expansion and gaseous release. Exhaustive ion exchange testing did not deliver favourable results, necessitating the pursuit of compounds for binary use with EG. A novel IFR was synthesised by neutralising 3,5-diaminobenzoic acid hydrochloride salt with ammonium dihydrogen phosphate. This compound, which melts at 257 °C, decomposes concurrently to release carbon dioxide gas which promotes intumescent charring. The flame retardant performance of this compound and EDAP as primary flame retardants and in combination with expandable graphite was evaluated. As a proof of concept, the novel compound was tested as a primary flame retardant using cone calorimetry after which its utility in binary systems with low temperature expandable graphite was tested. Substantial decreases in peak heat release rate (pHRR) and flame out time were achieved for all binary systems. This success led to testing of a number of combinations of low and high expansion onset EG and the other IFRs to identify the highest performing combination, which proved to be the 10-10 EDAP-EG system. Combinations of EG and the novel compound also showed excellent results. The novel fire testing techniques proved effective in identifying high performance combinations and showed comparable trends to those measured in cone calorimetry, at a greatly reduced cost and material requirement. IR analysis of open flame fire testing indicated increases in the temperatures required for ignition and burn through of the substrate. Observations, corroborated by optical video, showed that cohesive and uniform thermal shielding was achieved in all binary systems tested. This study illustrates that systems of 10% EG combined with either 10% DABAP or 10% EDAP are both the most economical binary systems tested but are extremely high performance systems as well. Both of these systems delivered excellent results while being more economic than the widely used industrial system with a 25-30% EDAP loading. It is recommended that these compounds be considered for industrial use. Furthermore, the effective fire testing techniques developed in this study may be utilised in future fire testing to identify high performance compounds at a lower cost prior to further assessment through methods such as cone calorimetry.