COMPARISON OF BROMINATED AND HALOGEN FREE FLAME RETARDANT BEHAVIOURS IN GLASS-FIBRE- REINFORCED POLY(BUTYLENE TEREPHTHALATE)

In this work, 30 wt. % glass fibre reinforced poly( butylene terephthalate), PBT, was modified with brominated polystyrene combined with antimony trioxide or aluminium diethylphosphinate (Alpi) with/without organically modified Montmorillonite clay (NanoMMT). The efficiency of these fire retardants (FRs) was investigated in the cone calorimeter in both horizontal and vertical orientations by mea suring, at six different heat fluxes, key flammability parameters such as time to ignition, h eat release rate (HRR), smoke carbon monoxide and carbon dioxide yields and efficiency o f c mbustion. All formulations exhibit charring burning behaviour having two peaks in the mass loss rate (MLR) and HRR because they leave a residue while pyrolysing consisting of lass fibres and char and/or nanoparticles. Compared to the base formulation, (PBT-GF), bromina ted polystyrene does not reduce the MLR but reduces significantly the HRR because bromi ne released in the pyrolysis gases inhibits combustion producing excessive carbon mono xide and smoke. Alpi alone has limited effect on the reduction of both MLR and HRR but, wh en combined with Nano-MMT, a significant reduction occurs of MLR and HRR without an increase in CO and smoke. The decrease in mass loss rate is due to the improved c onsistency of the residue (glass fibre, nanoparticles and char) produced by Alpi in the pre sence of nano-MMT whereas the reduction in the HRR is due directly to the reducti on n the MLR because the measured heat of combustion is close to the heat of combustion of the basic formulation. Burning behaviour in horizontal or vertical orientations is similar e xc pt that lower HRRs are measured in vertical orientation owing to reduced flame heat fl uxes compared to the burning in horizontal orientation. No dripping was observed in vertical o rientation for all formulations. In addition, LOI and UL94 results for these materials are compar ed with the key flammability parameters in the cone calorimeter. Introduction In the past few years, the environmental and toxico logical hazards of brominated flame retardants (BFRs) have been highlighted by several studies. It was demonstrated in [1-5] that for some of the BFRs results in a strong bioaccumul ation in aquatic and terrestrial food chains, and a growing number of BFRs is found in in creasing concentrations in the human food chain, human tissues and breast milk. Furtherm ore, BFRs have also been found in indoor environment such as in dust at homes [6]. BFRs are very persistent and show serious toxicological effects such as endocrine disruption. Brain and nervous system were identified as one of the most vulnerable targets for the toxic actions PBDE’s [7]. It is therefore not surprising that some of the BFRs are or will be soo n phased out. Consequently it is essential, for ecological and economical reasons, to investiga te vailable environmental friendly alternatives. However, banning specific BFRs may im ply a serious risk if the introduction of non-brominated alternatives is not properly assesse d regarding environment and human health. It is also important that substitution opti ns do not affect the functionality and reliability of the end products and the fire behavi our is one of these aspects. This paper presents an evaluation of alternatives of brominate d polystyrene in glass-fibre-reinforced poly(butylene terephthalate) (PBT-GF) for its fire retardancy. Other environmental aspects are being studied within the ENFIRO Project. Materials Four formulations listed in Table 1 are investigate d in this study. The base formulation, PG1, consists of PBT reinforced with 30 wt. % of glass f ibres. A halogenated formulation, PG2, contains 10 wt. % brominated polystyrene, with 5 wt . % antimony trioxide to seek a synergism effect. Two halogen free flame retardants (HFFRs) are studied: PG4A contains 15 wt. % aluminium diethylphosphinate (Alpi) and PG3B contains 15.5 wt. % Alpi combined with 2.5 wt. % nano-montmorillonite (Nano-MMT). The virgin polymer and fire retardants were dried p rior to extruding at 120°C under vacuum in an oven for 6 hours. A Prism Twin Screw E xtruder (TSE) 16 TC was used to process the samples. The temperatures corresponding to the front zone, centre zone and rear zone were 255°C, 245°C and 240°C, respectively and the screw speed was set at 60 rpm. The extruded polymer was then pelletised and dried, as above. A BOY 22M was utilised to injection mould the samples. The temperatures corre sponding to the rear zone, centre zone, front zone and nozzle zone were 240°C, 250°C, 250°C , and 260°C, respectively. These temperatures were varied by ± 5°C depending on the level for fire retardant present. The melt temperature was 255°C and the water-cooled mould wa s set at 80°C. The thickness of the samples is 2.9mm with a surface area of 0.1 x 0.1 m 2 Table 1. Examined PBT-GF formulations. Compounds % weight of formulations Commercial product name and suppliers PG1 PG2 PG3B PG4A PBT-GF30 100 85 82 85 Arnite TV4 261 from DSM Brominated polystyrene 0 10 0 0 Milebrome 7010 from MPI Chemie Antimony trioxide 0 5 0 0 Timonox Red Star from Che mtura Alpi 0 0 15.5 15 Exolit OP 1230 from Clariant Nano-MMT 0 0 2.5 0 Cloisite 30B from Southern Clay Products UL94 and LOI Tests Prior to the cone calorimeter tests, LOI measuremen ts a d UL94 vertical tests were conducted using 3.2mm thick samples, according to BS EN 60695 -11-10 and BS EN ISO 4589-2 respectively. The results are summarised in Table 2 . The three FR-containing formulations are V0 rated while PG1 formulation does not pass UL94 t est in vertical position. From the UL94 results alone, it is not possible to differentiate behaviour of flame retarded polymers because all these formulations are V0 rated. For the LOI me asurements, PG4A has the highest LOI (35.5%), followed by PG3B (31.5%), PG2 (28%) and PG 1 (19.5%). The fact that Alpi alone achieves a higher LOI than Alpi with Nano-MMT is so mehow unexpected. This result highlights that although LOI measurement provides a numerical result convenient to rank materials, it does not correspond to an intrinsic m aterial property and LOI value depends on experimental characteristics such as effective heat transfer coefficient to the sample for instance [8]. Clearly, the efficiency of the FR con tai ed in the polymer in real scale fire conditions cannot be revealed by UL 94 vertical and LOI tests only. In comparison, the cone calorimeter test provides much more realistic burni ng behaviours that are observed in real fire conditions and also a means to deduce the fundament l flammability properties of these materials. Table 2. Results of limiting oxygen index and UL 94 tests.