Abstract
Novel polymeric flame retardants based on two acrylamides and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) or 5,5-dimethyl-[1,3,2]dioxaphosphinane-2-oxide (DDPO) are described for several applications in HexFlow® RTM6, a high-performance epoxy resin. Neat resin samples and carbon fiber-reinforced composites were tested for their glass transition temperatures (dynamic mechanical analysis), thermal stability (thermogravimetric analyses), flammability (UL94) and flame-retardant performance (Cone Calorimetry). Additionally, the fiber degradation occurring during combustion of carbon fiber-reinforced epoxy resins was observed by scanning electron microscopy to show the fiber protecting effect of these flame retardants. Whereas DOPO-containing polyacrylamides acting mainly in the gas phase showed the best flame retardant efficiency, DDPO-containing polyacrylamides acting mainly in the condensed phase showed the best fiber protection. A mixed polyacrylamide was synthesized to combine these effects. This thermoplastic is soluble in the resin and, therefore, suitable for injection molding processes. Interlaminar shear strength measurements showed no negative effect of the flame retardant. The versatility of these flame retardants is shown by investigations dealing with boehmite as synergist in neat resin samples.
Highlights
Epoxy-based composites can be found in numerous highly sophisticated aerospace applications.In particular, carbon fiber reinforcement leads to lightweight materials with optimized mechanical properties
Oligomeric flame retardants were synthesized by Phospha-Michael addition of phosphorus compounds like the carbon rich 9,10-dihydro-9-oxa-10phosphaphenanthrene-10-oxide (DOPO) or the oxygen rich 5,5-dimethyl-1,3,2-dioxaphosphorinan-2one (DDPO) and multifunctional acrylamides with the following radical polymerization of residual double bonds (Figure 1)
Whereas DOPO, which is more likely to act in the gas-phase, shall provide mainly flame retardancy for carbon fiber-reinforced epoxy resins, DDPO may initiate the formation a protective layer on the carbon fibers to retard the formation of respirable carbon fiber fragments
Summary
Epoxy-based composites can be found in numerous highly sophisticated aerospace applications. Zinc borate is not suitable for resin transfer molding, an important process for the manufacturing of fiber-reinforced epoxy based composites. Oligomeric flame retardants were synthesized by Phospha-Michael addition of phosphorus compounds like the carbon rich 9,10-dihydro-9-oxa-10phosphaphenanthrene-10-oxide (DOPO) or the oxygen rich 5,5-dimethyl-1,3,2-dioxaphosphorinan-2one (DDPO) and multifunctional acrylamides with the following radical polymerization of residual double bonds (Figure 1). Whereas DOPO, which is more likely to act in the gas-phase, shall provide mainly flame retardancy for carbon fiber-reinforced epoxy resins, DDPO may initiate the formation a protective layer on the carbon fibers to retard the formation of respirable carbon fiber fragments. A lower surface area of boehmite results in more flame retardant species in combination with phosphinates [3] These investigations show the synergistic effect of the novel phosphorus containing polyacrylamides and two different types of boehmite in a high-performance epoxy resin.
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