Abstract
Several studies show a synergistic effect between intumescent formulations and aluminosilicates, such as zeolites and clays, but little is known about the effect of acidity of these additives on the synergistic action. In this work, H-ZSM-5 zeolite was submitted to desilication treatments for 30 min and for 2 h, and silicalite-1 was synthesized. The objective was to obtain samples of equivalent crystalline structure, but with different amounts of acid sites, in order to evaluate the effect of acid concentration of H-ZSM-5 zeolites on the synergistic action with an intumescent formulation composed by ammonium polyphosphate and pentaerythritol in polypropylene. H-ZSM-5 zeolites and silicalite were characterized by X-ray diffraction, nitrogen adsorption analysis and temperature-programmed desorption of ammonia. The desilication produced H-ZSM-5 zeolites with similar volumes of mesopores in both treatments, but the zeolite resulting from 2 h of desilication presented a higher concentration of acid sites than the zeolite from 30 min. The flame-retardant properties were evaluated by UL-94 classification, limiting oxygen index, glow-wire, thermogravimetric analysis and heating microscopy. The results showed that increasing the concentration and accessibility of the acid sites of H-ZSM-5 zeolites the flame-retardant properties of the studied composites improved. It is suggested that the increase of acid site concentration positively influences the catalysis of the reaction between ammonium polyphosphate and pentaerythritol, favoring the production of the precursors of the intumescent layer.
Highlights
Polypropylene (PP) is indispensable for society, finding wide use every day
The H-ZSM-5 zeolite used as a starting material, the two H-ZSM-5 zeolites resulting from the desilication treatment, and the synthesized silicalite-1 were characterized with
Value obtained for the composite containing silicalite, which did not have acid sites in its structure, indicated the absence of synergistic action with ammonium polyphosphate (APP)-PER. These results showed that the presence of acid sites in H-ZSM-5 zeolites is fundamental for the occurrence of synergistic action with the intumescent formulation
Summary
Polypropylene (PP) is indispensable for society, finding wide use every day. It has high flammability due to its aliphatic structure, with high content of carbon and hydrogen which, in combustion, quickly diffuses flames, drips, and releases a lot of heat [1,2,3,4,5,6]. The development of flame-retardant polymeric materials has been growing and gaining prominence. Among the flame-retardant additives, intumescent formulations are becoming increasingly important. They are halogen free, have low toxicity, and have been widely applied in PP [7]. The intumescent formulation is usually composed of three active components: an acid source such as ammonium polyphosphate (APP), which generates acidic species during its thermal degradation at temperatures below 280 ◦ C, a carbonaceous agent, which is generally a polyhydroxylated compound such as pentaerythritol (PER), and a blowing agent, which throughout its thermal degradation releases
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