Improvement in Char Strength with an Open Cage Silsesquioxane Flame Retardant

Abstract

Different characterization techniques were used to study the hydrolysis and condensation reaction kinetics of 3-methacryloxypropyltrimethoxysilane (MAPTMS) to obtain open cage silsesquioxane oligomers. The formation of hydrogen bonds, which condition the chemical structures of the resulting products, was identified. Improved thermal and fire resistant behavior of unsaturated polyester (UP) composites prepared with aluminium trihydroxide (ATH) and the synthesized oligomer were registered. Opened silsesquioxane structures also showed an improvement in the mechanical properties of the char formed after firing.

Thermal Degradation Mechanism of a Thermostable Polyester Stabilized with an Open-Cage Oligomeric Silsesquioxane

“…The organic–inorganic hybrid oligomer was synthesized by the sol–gel method from 3-methacryloxypropyltrimethoxysilane (MAPTMS) with hydrolyzable methoxy groups, which allowed the formation of siloxane bonds. The oligomer consisted mainly of mixtures of four to eight silsesquioxane molecules containing unreacted hydroxyl groups (hydrolyzed silanes, but not yet condensated) [ 12 ]. The participation of these groups in hydrogen bonds with the methacrylate groups prevented the condensation reactions from concluding.…”

“…As shown in a previous work [ 12 ], the silsesquioxane oligomer synthetized was a mixture of compounds obtained by condensation of between four to eight silane molecules, with a high number of intramolecular cycles and the complete hydrolysis of the alkoxide groups. The presence of hydrogen bonds between the carbonyl groups of the methacrylate and the hydroxyls of the hydrolyzed alkoxides prevented the condensation reactions from concluding, so open-cage structures of the polyhedral oligomer silsesquioxanes were obtained.…”

“…The presence of hydrogen bonds between the carbonyl groups of the methacrylate and the hydroxyls of the hydrolyzed alkoxides prevented the condensation reactions from concluding, so open-cage structures of the polyhedral oligomer silsesquioxanes were obtained. These open-cage structures were selected because their ashes showed better mechanical resistance [ 12 ]. Next, 5 wt % of this open-cage oligomer silsesquioxane was incorporated into the mixture of unsaturated polyester resin and styrene prior to polymerization to obtain the polyester/oligomer silsesquioxane composite studied below.…”

“…In a typical synthesis, 20 g (0.08 mole) of MAPTMS were poured into a 200 mL container, and 6.5 g (0.36 mole) of acidified water (HCl 0.01 M) was slowly added while the silane was stirred. Then, the synthesis was performed with a H 2 O:Si mole ratio = 4.5:1 with constantly stirring for 264 h. The characterization of the obtained product is described in detail in a previous paper [ 12 ].…”

“…In previous work [ 12 , 13 ], our research group improved both the thermal and fire behavior of a polyester by adding silsesquioxane oligomers together with ATH. The LOI (limiting oxygen index) value was increased from 21% for the original polyester to 51% for the samples to which ATH and silsesquioxane were simultaneously added.…”