Nanoclay synergy in flame retarded glass reinforced polypropylene/polyamide 6 blend

Abstract

Under the new European regulation on the restriction of hazardous substances in electrical and electronic equipment (RoHS 2011), owing to both the toxicity and environmental concerns with halogenated flame retardants (HFRs), some of these substances are banned and others restricted as additives. Hence, research on non-halogenated flame retardants has received great interest. Ammonium polyphosphate (APP) is the best in this category. APP is used as an intumescent flame retardant (IFR) along with synergistic agent and to improve flame retardancy of polymers without compromising the mechanical and thermal stability properties; it is also cost effective, because low loading is required. This study was conducted with three purposes The first aim was to examine the effects of the type of (1) nanoclay [(SP) or Nanofil5 (N5)] and (2) compatibilisers (SEBS-g-MA or PP-g-MA) used and (3) the loading amounts of the GFs (0, 10, 15, and 20 wt.%) and SP (0, 2, and 5 wt.%) on the flame retardancy, thermal stability, and mechanical properties of the flame-retarded PP/PA6 blend nanocomposites. The second aim was to examine the effect of intumescent flame retardant (IFR) loading (15wt.%, 17%, 18% and 20%APP765) on the flame retardancy, thermal stability, and mechanical properties. The third aim was to study the synergistic effect of SP on IFR with varying GF loading (0, 10, 15, 20%GF) to achieve the best flammability resistance. In this study, APP 765 was used as IFR and nanocomposites of polypropylene (PP)/polyamide-6(PA6) blends reinforced with glass fibres (GFs) and sepiolite (SP) were compounded using a co-rotating twin-screw extruder. The specimens for mechanical testing and analyses were injection moulded and compression moulded. The morphologies of the blends were characterized by X-ray diffraction (XRD) analyses and indicated that SP had an intercalated morphology in the PP matrix. The thermal stabilities and crystallinities of the nanocomposites were measured by thermal gravimetric analysis (TGA), and the flammability of the nanocomposites was investigated by UL-94 vertical burning and limiting oxygen index (LOI) tests and mass loss cone calorimetry. We observed that the use of 2 wt.% nanoclay and 18 wt.% APP 765 had a synergistic effect, resulting in a decrease in many of the flammability parameters of the nanocomposites. The peak heat release rate, mass loss rate, and smoke production rate were reduced by 10%, 20% and 30%, respectively, compared to those of pristine PP. A synergistic effect was also observed when 2wt.% SP and 18wt.% APP 765 were used in combination with the GFs of loading 10wt.%; the lowest PHRR and hence the best flame retardant properties were obtained under this condition. The LOI was 33%, which was significantly higher than that of pristine PP (19%). Moreover, the classification was V0, in contrast to the case for pristine PP, which has no rating. A classification of V1 was achieved when the SP content was increased to 3 wt.%. Finally, the classification was the highest when the GF content was reduced to 10 wt.% and the nanoclay content to 2 wt.%, as this resulted in a synergistic effect of glass fibre on flame retardant additives. The thermal stability also improved when the amounts of GF and SP used were increased. It was found that when the GF and SP contents were 20 and 2 wt.%, respectively, the maximum decomposition temperature was 20oC higher than that of pristine PP. Further, the decomposition rate also decreased. The char residue increased when both GF and SP increased; the maximum residue was 37 wt.%, which helped prevent heat transfer from the polymer core as well as spreading of fire, in addition to reducing the amount of combustible gases generated. Raman spectroscopy was employed to investigate the structure of the char residue after cone calorimetry and to assess the flammability of the nanocomposites. It was observed that the ID/IG ratio decreased when the nanoclay content was decreased from 5 wt. % to 2 wt.% or the IFR loading was increased (total content of IFR and SP was 20 wt.%), meaning that the structure of graphite became finer; this prevented the formation of combustible gases and the spreading of fire. It was also found that all the mechanical properties were improved (except the ductility, which decreased) when the loading amount of the IFR used was increased and that of the nanoclay was decreased. The tensile modulus was the highest, 4.63 GPa, when the nanoclay loading amount was 2 wt.%. For greater loading amounts, the modulus decreased to 3.99 GPa. The addition of GFs along with the nanoclay had a synergistic effect when the SP loading amount was 2 wt.%. All the mechanical properties increased (except the ductility, which decreased) with an increase in the GF and SP loading amounts, owing to the agglomeration of SP in the polymer blend. XRD analysis showed that the peak of the composite with SP lower than 5 wt.%, shifted from 8˚ to 7˚, with the intensity of the peak decreasing and the peak becoming broader, when the SP loading amount was 2 wt.% and the GF content was 10 wt.%. The best formulation for glass fibre loading with 2%SP is 10% GF to attain the highest retardancy and improved char strength (swelling occurs in this case). The compatibiliser SEBS-g-MA is better than PP-g-MA in term of flame retardancy and thermal stability. In terms of mechanical properties, PP-g-MA is preferred.