Abstract
Two phosphorus-containing cage-like silsesquioxane derivatives were synthesized as reactive or additive flame retardants for epoxy resin. The silsesquioxanes were obtained via an epoxide ring-opening reaction using a 10-hydroxy-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPA). In one derivative containing in its structure 4 glycidoxypropyl and 4 phosphate groups, denoted as 4P4GS, only half of the epoxy rings was reacted with phosphate to obtain a reactive additive, while in the second derivative containing 8 phosphate groups, denoted as 8PS, all epoxy groups were converted, thus an additive modifier was obtained. The silsesquioxanes containing phosphorus atoms and the reactive phosphorus-free silsesquioxane derivative (octakis[(3-glycidoxypropyl)dimethylsiloxy]octasilsesquioxane (8GS)) were used to prepare hybrid materials based on epoxy resin. To compare the impact of the structure of silsesquioxane derivatives on the properties of hybrid materials, a number of samples containing 1, 5, and 10% of the modifiers making a series of epoxy materials containing additive or reactive modifiers, were obtained. The modified epoxies were studied using scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), nanoindentation, water contact angle, and cone calorimetry tests to assess the effects of the modifier structure on the physicochemical properties of the investigated materials.
Highlights
Epoxy resins are one of the most important and broadly explored chemo setting polymers containing reactive epoxy functional groups that are receptive to attack by both electrophiles and nucleophiles
The 8GS derivative containing eight glycidoxypropyl groups was obtained via hydrosilylation of the allyl-glycidyl ether with octakisdimethylsiloxyoctasilsesquioxane catalyzed using Karstedt’s complex, and after isolation, it was used as a substrate for the synthesis of two further derivatives
This study aimed to verify the effect of the structure of phosphate cage-like silsesquioxane derivatives and their ability to chemically bond to the polymer matrix on the surface and their mechanical and thermal properties, as well as the flammability of epoxy resins modified with their use
Summary
Epoxy resins are one of the most important and broadly explored chemo setting polymers containing reactive epoxy (oxirane) functional groups that are receptive to attack by both electrophiles and nucleophiles This attractive feature has permitted the development of a wide group of compounds applied as hardeners of different reactivity (i.a., aliphatic and aromatic amines, carboxylic anhydrides) to enable obtaining systems crosslinking in a wide temperature range and showing various physicochemical properties [1,2,3,4,5]. Several flame-retardant additives have been applied in commercially available epoxy resins, including phosphorus compounds (e.g., triphenylphosphine, tris-β-chloroethyl phosphate), hydrates (e.g., alumina trihydrate), halogenated compounds (e.g., octabromobiphenyl, decabromobiphenyl, dechlorane) in different amounts ranging from 5 to 30%, often together with synergists like antimony oxide, zinc borate, or molybdic oxide [12,13,14]
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