Dehydration in environmental-friendly flame retardants. Structural analysis of melamine phosphates using X-ray powder diffraction, solid state NMR and ab initio calculations

V. Brodski¹, D. J. A. De Ridder¹, R. Peschar¹, H. Schenk¹, A. Brinkmann², E. R. H. van Eck², A. P. M. Kentgens², B. Coussens³ and Ad Braam³.

1Universiteit van Amsterdam, Institute of Molecular Chemistry (IMC), Laboratory for Crystallography, Nieuwe Achtergracht 166, NL-1018WV Amsterdam, The Netherlands

2University of Nijmegen, Physical Chemistry / Solid State NMR, NSRIM Center, Toernooiveld 1, NL-6525 ED Nijmegen, The Netherlands

³DSM Research, Postbus 18, 6160 MD, Geleen, The Netherlands

Interest in melamine phosphates is high because they are attractive environmental-friendly alternatives to halogen-containing flame-retardants [1]. By combining information from three different techniques, X-ray powder diffraction, solid-state NMR and ab initio calculations, crystal structures of three melamine phosphates have been established: melamine orthophosphate [2], melamine pyrophosphate [3], and polymerized melamine phosphate [4]. The latter two arise as the result of dehydration processes that take place at elevated temperatures.

Crystal structure models were obtained on the basis of X-ray powder diffraction experiment using a newly developed Monte-Carlo approach [5]. The precise proton-bonding networks, crucial in the formation of the structures and the dehydration mechanism, are supported by ab initio energy minimizations and corroborated experimentally by solid-state NMR data.

The packing in the investigated compounds consists of infinite ribbons of melaminium cations crosslinked by chains of phosphate anions. Within the cation ribbons, adjacent melaminium moieties are linked by means of side-by-side pairs of N-H…N hydrogen bonds.

An analysis of the structural differences between the three melamine phosphates provides a first understanding of the underlying (de)hydration processes that are expected to play an important role in the flame-retardant activity of these materials.

Acknowledgement

The authors acknowledge the ESRF (Grenoble, France) for the opportunity to perform the synchrotron diffraction experiments and Dr. H. Emerich for his help at beamline BM01B (Swiss-Norwegian CRG). They also thank E.J. Sonneveld and Dr. M.M. Pop for their help in data collection and indexing, Dr. R.B. Helmholdt, Dr. V. M. Litvinov and Drs. K. Goubitz for useful discussions. This work was supported by DSM (Geleen, The Netherlands), Ciba Speciality Chemicals (Basel, Switzerland) and the Netherlands Foundation for Scientific Research (NWO).

[1] Sh. Jahromi, W. Gabriëlse, A. Braam, Polymer 44 (2003), 25-37.

[2] D. J. A. De Ridder, K. Goubitz, V. Brodski, R. Peschar, H. Schenk, J. Mat. Chem., submitted.

[3] V. Brodski, R. Peschar, H. Schenk, A. Brinkmann, E. R. H. van Eck, A. P. M. Kentgens, B. Coussens, A. Braam, Angew. Chem., submitted.

[4] V. Brodski, R. Peschar, H. Schenk, A. Brinkmann, E. R. H. van Eck, A. P. M. Kentgens, B. Coussens, A. Braam, in manuscript.

[5] V. Brodski, R. Peschar, H. Schenk, J. Appl. Cryst. 36 (2003), 239-243.