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The use of phosphorus-containing flame retardants as a viable alternative to their widely used halogen-containing counterparts has been the source of much recent research. As the search for an effective flame retardant for specific polymeric systems continues, a new class of flame retardants have shown promising results, namely hyperbranched polyphosphoesters and their derivatives. These macromolecules promise to combine the effects of complex, hyperbranched structures with the flame retarding effects of phosphorus, enabling a high miscibility and processability, as well as a lower impact on glass transition temperature and decreased diffusion from the polymer matrix. These and other functions enable them to act as multifunctional additives.
In recent times, phosphorus-based hyperbranched materials and their efficacy as flame retardants have been described in literature. Often, the inclusion of nitrogen atoms into the chemical surrounding of phosphorus has lead to reports of increased flame retarding performance through synergistic effects. However, a comprehensive study of the relationship between nitrogen and phosphorus in hyperbranched polymers is lacking. The aim of this work is to determine the efficacy of novel, phosphorus-based hyperbranched polymers compared to other, commercially available, previously studied flame retardants and to investigate the molecular flame retarding mechanism of these complex structured macromolecules.
Among one of the key aspects here is the modification of the O:N ratio of the phosphorus-containing repeating units of these hyperbranched polymers. The newly described synthesis route yields trifunctional monomers of the desired composition, which undergo an A2+B3-type polymerization via radical thiol-ene reactions, producing polyphosphoesters, -amidates, -diamidates, or -amides, respectively. By precisely tailoring the nitrogen and oxygen ratio in the chemical surrounding of phosphorus, a more comprehensive picture of the structure-property relationship of these materials may be gained. Furthermore, by adjusting the aromaticity of the hydrocarbon moieties in these trifunctional monomers, an optimization of flame retarding properties, such as increased charring, is aimed to be achieved. Additionally, the trifunctional monomers themselves act as low molecular weight fire retardants; therefore, by comparing the performance of these monomers to their high molecular weight, hyperbranched, polymeric counterparts, an understanding of the role of molecular architecture in designing a more effective flame retardant can be gained. Finally, several epoxy resin matrices, common in high-tech industrial applications, are investigated in the interest of comprehending the interaction between these novel hyperbranched flame retardant additives and their surrounding polymeric matrix.
In order to determine the flame retardancy mechanism of these materials, a multi-methodological approach is selected, thus offering a high volume of correlating data. Using Fourier-transform infrared spectroscopy (FTIR) coupled with thermogravimetric analysis (TGA), as well as pyrolysis combustion flow calorimetry (PCFC), provides evidence of mass loss processes, their respective decomposition products, and the heat released by volatiles in the gas phase during pyrolysis, while hot-stage FTIR offers information of the decomposition products occurring in the condensed phase. Limiting oxygen index (LOI) and UL-94 help to quantify and categorize the materials’ flammability, while the use of a cone calorimeter offers valuable insight into the fire behavior in forced flaming conditions, providing heat release rates, smoke and CO/CO2 production, amount of residue, and other important indices. Finally, the use of differential scanning calorimetry (DSC) provides information of material properties such as glass transition temperatures, and the implementation of blooming tests serves to examine the reduction of molecular mobility in the matrix.
By varying the architecture of the flame retardants – in the O:N ratio of the chemical surrounding of phosphorus and in the aromatic composition of the monomers’ hydrocarbon moieties – and by comparing low versus high molecular weight species in varied epoxy resin matrices, as well as through the use of a multi-methodological approach, new insight into the use and the mode of action of these components as flame retardants for modern materials can be won.
The fire behaviour of carbon fibre (CF) reinforced polymers differs in comparison to polymers. Fibres behave often inert with respect to pyrolysis, they change the melt flow and dripping behaviour, the heat absorption and transfer, the amount and properties of the fire residue and so on. Flame and fire retardancy concepts are needed not only suitable for the different fire protection goals typical for each application, but also tailored for composites. This field is illuminated by examples taken from different projects carried out in the group of the author in the recent years. The examples target on different applications through achieving reduction in reaction to fire controlling the fire risks (flammability, heat release) in the beginning and development of a fire and investigating the fire stability, when a severe flame is directly applied (key property in fully developed fires). Approaches to halogen-free flame retardancy in CF reinforced thermosets are presented as well as building up a bench and an intermediate scale testing of composites in fire applying mechanical load (up to 1 MN compression) and direct flame exposure (180 kW/m2) simultaneously. Indeed, e.g. we have investigated the fire stability of stringer reinforced shell components taken out from the fuselage of an aircraft.
The understanding of fire behaviour, fire resistance, and fire retardant modes of action in composites is a promising basis for target-oriented development. The role of flame inhibition, charring, and protective layer formation is discussed. Successful concepts are presented for fire retardancy tailored for different application as well as general guidelines for future development. Different phosphorus flame retardants are proposed to achieve halogen-free flame retardancy with respect to ignition and developing fires. Different protective approaches are sketched for addressing the fire stability of composites that is the most important fire risk for the fire resistance in structural applications.
The main flame retardant modes of action are well known, but in practise the detailed scientific understanding usually falls short, when it comes to modern multicomponent systems, the important tiny optimizations, or quantifying in terms of specific fire properties. This talk delivers thought-provoking impulses, picking up some overseen details as well as raising basic questions. A detailed scientific insight in flame retardant modes of action much more than an overview is presented.
Advanced analysis of flame retarded epoxy resin/melamine poly(metal phosphate)/synergist systems
(2017)
Synergistic multicomponent systems containing melamine poly(metal phosphate) have been proposed as flame retardants. The potential synergists: aluminium diethylphosphinate (AlPi), boehmite (AlO(OH)) and melamine polyphosphate (MPP) were compared in flame retardant epoxy resin (EP)/melamine poly(magnesium phosphate) (MMgP) and EP/melamine poly(zinc phosphate) (MZnP). A multi-methodical approach was used to investigate the pyrolysis, fire residues, flame retardant modes of action, and synergistic effects. The decomposition and the fuel released into the gas phase were investigated by thermogravimetry coupled with Fourier transform infrared spectroscopy (TG-FTIR). A detailed analysis of the condensed phase was done by FTIR and comprehensive solid-state nuclear magnetic resonance spectroscopy (NMR). The fire behaviour was investigated by the cone calorimeter and the morphology of the fire residues with micro-computed tomography (µCT) and scanning electron microscopy (SEM). The best performance is observed for EP/MMgP/MPP and EP/MZnP/MPP based on a synergistic effect between the flame retardants. Particular the formation of a highly voluminous fire residue plays a major role and is stabilized by the interaction between the flame retardants. As a result highly effective protection layer was formed which led to the interesting results.