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The demands for modern flame retardants are higher than ever: a flame retardant must function effectively in a certain polymer matrix and avoid critical alterations to the material’s properties. Ideally, a flame retardant additive should be easily miscible and show no sign of leaching or blooming from the matrix. Additionally, the flame retardant should be non-toxic, non-accumulating and biocompatible. Hyperbranched polymers are a promising group of multifunctional flame retardants which fulfill these demands: their complex shape enables high miscibility and avoids leaching or blooming, while their high molecular weight potentially increases biocompatibility and lowers accumulation and toxicity. Moreover, they exhibit a low impact on polymer properties and a good flame retardant performance.
This work examines the efficacy and mode of action of phosphorus-based hyperbranched polymeric flame retardants in bisphenol A-based epoxy matrices. To investigate the effect of the complex shape, the hyperbranched polymers are compared to their corresponding monomeric variants. Furthermore, the materials are synthesized to contain systematically varying oxygen-to-nitrogen ratios, allowing for new insight into what role the chemical surrounding of phosphorous plays in flame retardant efficacy.
Using a multi-methodical approach, including thermogravimetric analysis coupled with Fourier transform infrared spectroscopy (FTIR), hot stage FTIR, micro combustion calorimetry, differential scanning calorimetry, oxygen index (LOI), UL-94 tests and cone calorimetry experiments, the decomposition mechanisms and the flame retardant modes of action of these flame retardants in epoxy resins are investigated, shedding new light on the chemistry of flame retardancy.
Most synthetic polymers have a high fire load, and as a result, they require flame retardants (FRs) to ensure their safe use. Phosphorus plays an important role in flame retardancy and has the potential to replace halogenated variants, which are assumed to be harmful to the environment and health. Among phosphorus-based FRs, there exists a trend towards polymeric, high molar mass molecules with complex molecular architectures. In this project, we synthesized a novel series of so-called phosphorus-based hyperbranched polymeric FRs and investigated their use as multifunctional additives to high-performance polymers, i.e. epoxy resins. By cleverly designing the chemical structure to contain varying amounts of P-O and P-N bonds, new insight into the chemical mechanism of flame retardancy was gained, and by comparing the hyperbranched polymers to their monomeric counterparts, a greater understanding of the role of complex architecture was won. This talk aims at presenting some of these results and proposes chemical mechanisms that illustrate what role these novel hyperbranched flame retardants play in molecular firefighting.
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).
Benefits of hyperbranched structure:
Molecular weight, miscibility, number of FR groups, end-group functionalization, glass transition temperature, decreased PBT.
Examples of hyperbranched FRs:
Charring agent, silicone, triazine, etc.
Systematic study of effect of chemical surrounding and impact of Complex shape of phosphorus-based hyperbranched polymers on flame retardant efficacy in epoxy resins.