Improving flame retardancy of in-situ silica-epoxy nanocomposites cured with aliphatic hardener: Combined effect of DOPO-based flame-retardant and melamine

Aurelio Bifulco,Dambarudhar Parida,Khalifah A Salmeia,Sandro Lehner,Rolf Stämpfli,Hilber Markus,Giulio Malucelli,Francesco Branda,Sabyasachi Gaan

doi:10.1016/j.jcomc.2020.100022

Abstract

Silica-epoxy nanocomposites were prepared via an “in-situ” sol-gel synthesis process and a phosphorus (P) flame-retardant i.e. 6H-dibenz[c,e][1,2]oxaphosphorin,6-[(1-oxido-2,6,7-trioxa-1-phosphabicyclo[2.2.2]oct-4-yl)methoxy]-, 6-oxide (DP) and melamine (Mel) were further added to the matrix to improve its fire performance. The main components of epoxy resin were bisphenol A diglycidyl ether (DGEBA) and isophorone diamine (IPDA) hardener. The addition of DP as well as silica alone into the epoxy system stopped the melt dripping phenomena in the vertical fire test (UL 94), however, the addition of melamine was crucial for achieving the highest fire classification (UL 94-V0 rating). The presence of DP and Mel in the silica-epoxy nanocomposite promoted a large reduction (ranging from 53% up to 80%) in the heat release rate (HRR) and a delay (up to 31%) in the ignition time in the cone calorimetry experiments. Improved fire performance of the epoxy system was attributed to i) a condensed phase activity of silica, DP and melamine to form a protective thermal barrier during combustion and ii) a minor gas phase flame inhibition activity of DOPO component of DP. The mechanical characterization of the epoxy nanocomposites through tensile tests showed that the addition of DP increases the stiffness of the epoxy resin, resulting in a strong increase of Young modulus (up to 32%) and in a slight decrease of fracture strength, elongation at break and toughness. An increased glass transition temperature (up to 8%) of the epoxy system possibly due to hydrogen bonds and polar interactions of DP with the matrix was also observed.

Highlights

Epoxy resins are crosslinkable versatile polymers finding application in areas such as electronics, composites, coatings and adhesives
In this work we have evaluated the flame-retardant performance of DP in DGEBA based epoxy resin cured with a cycloaliphatic curing agent and further developed strategies to improve its fire performance by incorporation of silica precursors and a nitrogen synergist
The incorporation of DP into the in-situ silica-epoxy system resulted in the appearance of new characteristic bands

Summary

Introduction

Epoxy resins are crosslinkable versatile polymers finding application in areas such as electronics, composites, coatings and adhesives. A wide range of epoxy resins with varying properties can be synthesized by reacting different epoxides with a variety of curing agents. Curing agents such as aromatic, aliphatic and cycloaliphatic amines are preferred for hardening epoxy resins due to their high reactivity and possibility for tuning curing kinetics, which can be adapted for various applications [1]. An alternative is the use of aliphatic amines as curing agents They are very reactive and enable quick curing of the epoxy resins at room temperatures albeit the final product has lower thermal stability. Development and use of silicone and phosphorus-based flame-retardants is considered as a viable option [8,9,10,11,12]

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Conclusion