HomePage Crystallography. Thesis
by Dr. V. Brodski
SUMMARY
Melamine phosphate compounds are promising environmental-friendly flame retardants. Being admixed in a polymer matrix, they are cheap and efficient materials that reduce the polymers flammability. Melamine phosphates and their derivatives are believed to replace the hazardous halogenated compounds that currently are dominating the flame-retardant market but are increasingly banned by more stringent legislation for the side effects they show when suppressing the burning process such as toxicity and corrosiveness of smoke evolved. In spite of great needs and expectations, large-scale use of melamine phosphates in flame-retardant applications is still hampered for various reasons, amongst others their limited thermal stability and their compatibility with some industrial polymers. Many parameters influencing the flame-retardant activity of melamine phosphates, in particular their thermal stability, are determined by the structural properties of the materials and therefore fundamental knowledge of melamine phosphate’s structural characteristics is required to make the search for improved flame-retardant systems systematic rather than trial-and-error based.
In
this thesis the structures of four melamine-phosphate compounds are
reported that are important for flame-retardant applications: melamine orthophosphate (MP), melamine
pyrophosphate (MPy), melamine polyphosphate (MPoly) and
tetrakis(melaminium)
bis(dihydrogenphosphate) monohydrogenphosphate trihydrate ( M4P3·(H2O)3).
The first three compounds have a melamine-to-phosphor (M:P) molar ratio
of 1.0
and have been known as flame retardants for more than half a century.
Starting
from MP, the latter two can be obtained successively by thermal
treatment as a
result of dehydration reactions. Recently, a large interest has arisen
in
melamine phosphates that have a M:P molar ratio > 1.0
because of their higher thermal stability.
The fourth compound M4P3·(H2O)3
is a prime example of this.
All structures
discussed in this thesis have
been determined using X-ray powder-diffraction data as no
single-crystals could
be prepared. For this purpose special direct-space search methodology
has been developed.
In chapter 1 a
brief introduction is given to X-rays, crystallography and the concept
of
crystal structure determination from powder diffraction data.
In chapter 2 a
new direct-space search approach is discussed, developed for structure
determination of organic compounds and used to solve the three most
complex
melamine phosphates (MPy, MPoly and M4P3·(H2O)3)
investigated in this work. The underlying approach, an energy-guiding Monte-Carlo algorithm, is based on a
simultaneous global minimization of the R-factor and a potential energy
function and employs a local minimization of the R-factor at each step
of the
Chapter 3
discusses the program suite Organa
in which a more versatile version of the above-mentioned energy-guiding
Monte-Carlo algorithm has been implemented. The initially proposed
approach has
been supplemented, amongst others with options for soft-distance
restraints and
a global optimization of preferred orientation. Also a grid-search
algorithm
was added to Organa as a complimentary tool for structure solution.
In chapter 4 the
crystal structure of MP is presented as determined from high-resolution
synchrotron powder-diffraction data. Although X-ray diffraction and
X-ray
powder diffraction in particular are known to have a limited
sensitivity with
respect to determination of the positions of the hydrogen atoms, a
hydrogen-bonding model could be proposed. This model has been
corroborated
fully by solid-state NMR experimental data and in particular the single
protonation of the melamine moieties was confirmed.
Chapter 5
presents
the crystal structure of MPy as solved using Guinier data and refined
using
synchrotron radiation data. The
proposed hydrogen-bonding model was
corroborated by solid-state NMR and periodic DFT calculations. It
appeared that
both techniques are sensitive enough to predict melamine protonation
but can
not give decisive results for the proton distribution between the
phosphate
moieties. A comparison of the crystal structures of MP and MPy allowed
to
propose a mechanism for the (de)hydration process that takes place in
the
reaction MP ↔
MPy. The analysis of the melamine packing in various melamine complexes
and
salts, including MP and MPy, revealed typical melamine-packing motifs.
It is
argued that these motifs are also likely to be present in other
melamine
phosphates and this turned out to be particularly helpful for the
structure
determination of the compounds discussed in Chapters 6 and 7.
In chapter
6 the last compound from the 1.0 series, MPoly, is analyzed. The
crystal
structure of MPoly was determined from Guinier data.
Similar to the cases of MP and MPy, a single protonation of the
melamine
moieties was corroborated by
solid-state NMR data.
A comparison of the packing in MPoly and its precursors allowed
to
accomplish the picture of the structural transformations that occur
during the
endothermic dehydration processes in the reaction path MP → MPy → MPoly.
In chapter
7 the crystal structure determination of M4P3·(H2O)3,
and its packing are discussed. The successful structure
determination of this compound with 10 independent moieties in the
asymmetric
part of the unit cell (described with 51 degrees of freedom)
illustrates the
abilities of the modern direct-space search techniques. The use of
solid-state
NMR data at the stage of structure determination allowed a 50%
reduction of the
trial structures to be considered. The structure analysis of this
compound and
its comparison with other melamine phosphates reveals packing and
bonding
characteristics that are expected to be of importance for the future
analysis
of its higher-order condensates.
The work carried out in the framework of this
thesis has given a first insight into the structural characteristics of
melamine-phosphate based flame-retardants. The developed new
methodology for
crystal determination from powder diffraction data has proven to be
able to
solve structures up to a high complexity (~ 50 degrees of freedom).
Because of
the hydrogen- bonding networks in melamine phosphates, a combined
approach of
X-ray powder diffraction and solid-state NMR analysis was essential.
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