Next to the basic flame retarding potential of pure melamine, the 'derivative'
end adds additional features. Typical examples are cyanuric acid causing
a non-burning drip in polyamides, the contribution of boric acid and phosphoric
acid to char formation and the influence of the 'derivative' end on thermal
stability and water solubility (see figure 1 and 2).
Melamine cyanurate melapur® MC, melamine cyanurate, is a salt of melamine
and cyanuric acid. Melamine Cyanurate has a higher thermal stability than pure
melamine, remaining stable upto about 320 °C. For this reason Melamine
Cyanurate is often applied in polymers with higher processing temperatures,
like polyamides. Above 320°C, it undergoes endothermic decomposition
to melamine and cyanuric acid, acting as a heat sink in the process. The
vaporized melamine acts as an inert gas source diluting the oxygen and the
fuel gases present at the point of combustion (see Mechanism of action).
Figure 3 : Chemical Structrure of Melamine Cyanurate
Melamine (Mono/Pyro/Poly) Phosphates
Melamine phosphates are substances combining the synergistic effect of
melamine (thus N-containing) with P-containing components in one salt.
With increasing thermal stability the melamine phosphates can be ranked
as follows: Melamine Phosphate < Melamine Pyrophosphate < Melamine
Polyphosphate.
melapur® MP, melamine-mono-phosphate, is a salt of
melamine and phosphoric acid. Above ~200°C melamine phosphate will
react to melamine pyro-phosphate with release of reaction water, which
will result in a heat sink Above ~260°C melamine-pyrophosphate will
react under release of reaction water to melamine-polyphosphates which
again results in a heat sink effect.
Above 350°C, melamine-polyphosphate undergoes endothermic decomposition
thus acting as a heat sink and cooling the combustion source. The released
phosphoric acid acts to coat and therefore shield the condensed combustible
polymer. The phosphoric acid along with the polymer also works to form
a char around the fuel source (polymer) thus reducing the amount of oxygen
present at the combustion source. The melamine released also is a blowing
source to blow up the char resulting in an intumescent behavior.
Especially for glass fibre filled polyamide with high processing temperatures,
Melapur has developed melapur® 200, a patented
melamine polyphospate with high thermal stability. In this type of glass
filled applications Melamine phosphate and pyro-phosphate will release
water and initiate degradation of the polymer due to the high processing
temperatures.
