Synergistic Action of Montmorillonite with an Intumescent Formulation: The Impact of the Nature and the Strength of Acidic Sites on the Flame-Retardant Properties of Polypropylene Composites

Abstract

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.

The influence of montmorillonite on the flame‐retarding properties of intumescent bio‐based PLA composites

“…From the literature, 22 it is known that, for AP/PER systems, the optimum proportion for intumescent systems is 3AP:1PER. In a previous work 21 which evaluated the synergism between AP/PER and montmorillonite in a polypropylene matrix, a 3% of raw montmorillonite synergist was maintained within a proportion of 2.5 PER: 1 montmorillonite. For the current PLA/AP/lignin system, in order to keep the same proportion, a concentration of 1.2% of montmorillonite should be used for 3% of lignin Therefore, considering all these aspects and keeping the loading around 20% as currently used in the literature for fire-retardant PLA composites, 15 and the lignin concentration constant (3%), a total of six composites were prepared: two of them without montmorillonite (containing 17% and 15.8%AP) and four combining 17% and 15.8 of %AP with 3% and 1.2% of montmorillonite.…”

“…The chemical composition, the crystallinity, the textural properties, and the particle size analysis of the ANa sample have been presented in a previous work 21 …”

“…For the current PLA/AP/lignin system, in order to keep the same proportion, a concentration of 1.2% of montmorillonite should be used for 3% of lignin Therefore, considering all these aspects and keeping the loading around 20% as currently used in the literature for fire-retardant PLA composites, 15 and the lignin concentration constant (3%), a total of six composites were prepared: two of them without montmorillonite (containing 17% and 15.8%AP) and four combining 17% and 15.8 of %AP with 3% and 1.2% of montmorillonite. The effect of the montmorillonite concentration was considered in the works of Chigwada and Wilkie, 23 Martins et al 21 and Guo et al 8 The polymeric materials were processed in a BC21 Clextral 900 mm (Firminy, France) twin screw extruder. The extruder has twelve heating zones that were kept at the following temperatures: T zone 1 = 50 C, T zones 2,3 = 195 C, T zone 4 = 190 C, T zones 5-12 = 180 C. The materials were fed into zone 1, with a flow rate of 4 kg h À1 and a thread rotation of 200 rpm.…”

“…In order to enhance the flame-retardant properties, some works in the literature report the use of synergists in the intumescent PLA compositions, such as organomodified montmorillonite (Cloisite 30B), 8,19,20 multiwalled carbon nanotubes, 19 expandable graphite, 10 and modified layered double hydroxides, 17 which exhibited better performances when compared with the correspondent systems without the synergism agent. According to a previous work, 21 the addition of a raw montmorillonite, that is, a non-organomodified montmorilloniteto an intumescent polypropylene composite promotes a synergistic effect with the intumescent formulation (composed by AP and PER), improving the flammability properties. For this reason, it is important to evaluate the effect of the addition of a raw montmorillonite on the flame-retarding properties of intumescent PLA composites.…”

Thermoplastic Intumescent Coatings Modified with Pentaerythritol-Occluded Carbon Nanotubes

“…The efficiency of thermal insulation of intumescent bulk and coating materials can be significantly improved by relatively small doses of various modifiers, especially nanofillers, e.g., boron compounds [ 7 , 8 ], silica and silicates [ 9 , 10 , 11 , 12 ], metal oxides (ZnO, MnO 2 , Ni 2 O 3 , Bi 2 O 3 , TiO 2 , ZrO 2 ) [ 13 , 14 , 15 , 16 ] or bio-CaCO 3 [ 17 ]. Cellulose nanocrystals and candle soot nanoparticles seem to be interesting carbon-rich components for ICs as well [ 18 , 19 ].…”