A raw montmorillonite (Mt) was submitted to different acidic activation times in order to investigate the influence of the strength and the nature (Brønsted and Lewis) of acidic sites on the synergistic action with an intumescent formulation (IF) composed of ammonium polyphosphate (APP) and pentaerythritol (PER) when incorporated into a polypropylene (PP) matrix. The acidity of the Mt samples was quantified by ammonia temperature-programmed desorption (TPD-NH3) and Fourier transform infrared spectroscopy (FTIR) with pyridine adsorption. The mineral clays were also characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), nitrogen adsorption analysis and particle size distribution. Thermogravimetric analysis (TGA), limit oxygen index (LOI) and UL-94 were performed to evaluate the flame-retardant properties and the thermal stability. The TGA results show that the final residue increased 2 to 3 fold in comparison to the values predicted theoretically. The flammability properties achieved a maximum for the system containing an excess of moderate-strength Brønsted sites relative to the Lewis ones, reaching 38% in the LOI test. This result suggests that the presence of these Brønsted acidic sites is important, as they take part in the esterification reaction between APP and PER which gives rise to the char formation. The FTIR-Pyr adsorption and flammability results indicate that both the nature and strength of the acidic sites influence the flame-retardant properties.
Summary
This paper presents the results of an experimental investigation on the fire retardancy properties of glass fibre–reinforced polyester (GFRP) composites with bisphenol‐A vinylester and isophthalic polyester as matrices and low electrical conductivity E‐glass fibres as reinforcement. The fire protection systems tested were alumina trihydrate (ATH), decabromodiphenyl ether (DBDE), and antimony trioxide (Sb2O3). A mass loss cone calorimeter was used to obtain the properties of heat release rate (HRR), peak HRR, total heat released, total mass loss, time to ignition, and time of combustion. Moreover, limiting oxygen index (LOI), UL‐94, and glow‐wire tests were also performed. The fire tests were carried out in order to investigate if the combination of ATH and DBDE could have “additive,” “antagonistic,” or “synergistic” effects on the flame retardant properties of the GFRP studied in this work. In addition, the influence of the ATH content variation on flame retardant properties was also evaluated. The results indicate that the sole addition of ATH at 47.7 phr could lead to the complete inhibition of the composites ignition, while the materials containing DBDE exhibit ignition and flame propagation in the cone calorimeter test.
The influence of ammonium polyphosphate (APP) particle size on the performance of an intumescent formulation and on the synergistic action of a series of montmorillonite samples with different d‐spacings for the production of flame retardant composites was investigated. The polymer matrix employed was poly(ethylene‐co‐butyl acrylate), EBA 30, and the intumescent formulation consisted of APP and pentaerythritol (PER). After being processed, the composites were submitted to scanning electron microscopy (SEM), thermogravimetric analysis, heating microscopy, and limiting oxygen index tests. The results indicate that the greater interaction between the APP and PER molecules, caused by the increase of the contact area promoted by the reduction of the APP particle size, could favor the esterification reaction between APP423 and PER, allowing the formation of a greater amount of char precursors in shorter period of time. In addition, the montmorillonite d‐spacings had a more pronounced influence on the clays synergistic action with the intumescent formulation containing the APP with smaller particle size. Microscopy has shown to be an important tool to investigate APP particle size effect on the fire retardancy. AFM results enabled the detection of nanometric particles in the sample containing the smallest particle size of APP. SEM micrographs showed that those nanometric particles were better dispersed in the matrix, interacting more effectively with the other components, a factor probably responsible for the superior fire retardancy results. Heating microscopy revealed that the material with smaller APP particle size did show some remaining structure at the temperature of 850°C.
The influence of processing intumescent bio-based poly(lactic acid) (PLA) composites by injection and fused filament fabrication (FFF) was evaluated. A raw (ANa) and two acidic-activated (AH2 and AH5) montmorillonites were added to the intumescent formulation, composed by lignin and ammonium polyphosphate, in order to evaluate the influence of the strength and the nature (Brønsted or Lewis) of their acidic sites on the fire behavior of the composites. The thermal stability and the volatile thermal degradation products of the composites were assessed. The injected and 3D-printed composites were submitted to cone calorimeter (CC), limit oxygen index (LOI), and UL-94 flammability tests. A similar tendency was observed for the injected and 3D-printed samples. The high density of strong Lewis sites in AH2 showed to be detrimental to the fire-retarding properties. For the CC test, the addition of the intumescent composite reduced the peak of heat released (pHRR) in approximately 49% when compared to neat PLA, while the composites containing ANa and AH5 presented a reduction of at least 54%. However, the addition of AH2 caused a pHRR reduction of around 47%, close to the one of the composite without clay (49%). In the LOI tests, the composites containing ANa and AH5 achieved the best results: 39% and 35%, respectively, for the injected samples, and 35 and 38% for the 3D-printed samples. For the composite containing AH2 the LOI values were 34% and 32% for injected and 3D-printed samples, respectively. Overall, the best performance in the flammability tests was achieved by the composites containing clays with only weak and moderate strength acidic sites (ANa and AH5).
Biobased flame-retardant polylactic acid composites were prepared using ammonium polyphosphate (AP), lignin, and a raw montmorillonite (ANa) as the intumescent formulation. The concentration of AP and of ANa was varied in order to study its influence on the flammability properties of the composites.The samples were submitted to cone calorimeter test, thermogravimetric analysis coupled to Fourier-transform infrared spectroscopy (TGA-FTIR), limiting oxygen index (LOI), and UL-94 vertical burn. The cone calorimeter residues were analysed through scanning electronic microscopy, X-ray diffraction, and FTIR. The results show that the combined addition of the intumescent formulation and the ANa leads to an improvement in the fire behavior of the composites, compared with that of the neat polymer. The best fire-retardant performance was achieved by using the highest AP concentration (17%) and the lowest ANa concentration (1.2%), reaching a LOI value of 39%.
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