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The need to develop effective flame retardants that retain polymer properties and are safe for consumers and the environment is a continuous challenge for material scientists. While halogenated flame retardants were once commonplace, the shift to non-halogenated materials has steadily progressed due to concerns over impact on health and the environment. One prominent group of flame retardants has become a viable alternative for halogenated materials, namely phosphorus-based flame retardants. The chemical versatility of phosphorus-based flame retardants and the ability to work as reactive or additive compounds makes them ideally suited for modern materials. There exists a trend toward complex, polymeric, and multifunction flame retardants, as these materials show greater flame retardancy performance than low molecular weight counterparts and affect material properties to a much lesser extent.
One group of organophosphorus flame retardants that shows great potential for high-performance polymers like epoxy resins are hyperbranched phosphorus-based polymers. These additives exhibit great miscibility with the polymer matrix and a significantly decreased diffusion through the material, which greatly reduce leaching or blooming out of the matrix. Moreover, the material’s thermal stability remains intact at elevated temperatures due to its low impact on the glass transition temperature. Finally, following market trends and legislation such as the guidelines for the Restriction of Hazardous Substances Directive implemented by REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) in the EU, these macromolecules are non-accumulating, non-toxic and have a lower risk of leeching or blooming from the matrix, further reducing environmental impact.
The work presented herein focusses on two distinct parts studying hyperbranched polymeric flame retardants and their corresponding monomeric compounds.
The first part, involving the low molecular weight components, investigated the role of the chemical surrounding of phosphorus in terms of flame retardant efficacy. Here, a systematic variance of the surrounding of phosphorus was investigated: by changing the ratio of oxygen to nitrogen (4:0 until 1:3), four materials, namely phosphoester (4:0), phosphoramidate (3:1), phosphorodiamidate (2:2), and phosphoramide (1:3), were synthesized, characterized, and finally added to Bisphenol A based epoxy resins (10 wt.-% loading). Pyrolysis investigations showed that low molecular weight components volatize at lower temperatures than the polymer matrix. Additionally, cone calorimeter measurements and TGA-FTIR investigations show trends in respect to FR efficacy in pyrolysis and full flaming conditions.
The second part involves the hyperbranched variants of the monomeric counterparts and investigates the role of complex shape on flame retardant efficacy. By comparing the low to the high molecular weight compounds, the influence of the complex shape becomes apparent and can be quantified. Cone calorimeter measurements show an increase in flame retardancy for some materials, while for others, the mode of action is altered.
By implementing a multi-methodical approach, various flame retardancy aspects, from pyrolysis behavior in the gas and condensed phase, to ignitability / reaction-to-small-flame performance, to action in forced flammability experiments, are identified and quantified, allowing for a clearer understanding of the behavior in fire of these novel flame retardants. By comprehending the roles of chemical composition and complex shape, it opens the path for new and effective multifunctional, polymeric flame retardants with decreased PBT, higher miscibility, and low impact on Tg.
This work is funded by the Deutsche Forschungsgemeinschaft (DFG: SCHA 730/15-1; WU 750/8-1).
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.
We synthesized a library of phosphorus-based flame retardants (phosphates and phosphoramides of low and high molar mass) and investigated their behavior in two epoxy resins (one aliphatic and one aromatic).
The pyrolytic and burning behavior of the two resins (via TGA, TG-FTIR, Hot stage FTIR, Py-GC/MS, PCFC, DSC, LOI, UL-94, Cone calorimeter) are analyzed and compared to the results of flame retardant (FR)-containing composites. A decomposition pathway incorporating the identified modes of action and known chemical mechanisms is proposed. The overlap of decomposition temperature (Tdec) ranges of matrix and FR determines the efficacy of the system. Low molar mass FRs strongly impact material properties like Tg but are very reactive, and high molar mass variants are more thermally stable. Varying PeO and PeN content of the FR affects decomposition, but the chemical structure of the matrix also guides FR behavior. Thus, phosphates afford lower fire load and heat release in aliphatic epoxy resins, and phosphoramides can act as additives in an aromatic matrix or a reactive FRs in aliphatic ones. The chemical structure and the structure-property relationship of both FR and matrix are central to FR performance and must be viewed not as two separate but as one codependent system.