TGA-FTIR: From the Investigation of Pyrolysis to the Elucidation of Fire Retardancy Mechanisms - METTLER TOLEDO

TGA-FTIR: From the Investigation of Pyrolysis to the Elucidation of Fire Retardancy Mechanisms

Fire behavior is a key property when choosing materials, especially in the electronics, electrotechnics, transport and construction industries.

Due to their chemical structure, conventional and technical polymers are combustible and often even flammable and have to be modified by the addition of fire retardants. In the past, halogenated fire retardants have been widely used.

In recent years, various phosphorus-based alternatives have been proposed.

Introduction

Phosphorus-containing fire retardants produce their effect through a number of different fire retarding mechanisms. The two most important mechanisms are flame poisoning in the gas phase (i.e. in the flame) and charring or carbonization in the condensed phase (i.e. in the pyrolysis zone). 

The basic mechanism for flame poisoning is the release of phosphorus-containing pyrolysis products followed by the formation of PO radicals in the flame. Their reaction with the highly reactive H- and OH-radicals interferes with the chain mechanism of the oxidation of the hydrocarbons and so leads to a reduction of the heat released and flammability. Phosphorus-containing fire retardants can furthermore initiate or accelerate the charring or carbonization of the polymer matrix in the pyrolysis zone and thus prevent fuel or combustible material reaching the flame. In addition, the residues produced form a barrier against heat and mass transfer, which reduces fire risks. The barrier action is even the main mechanism in the case of intumescent systems, which nearly always contain phosphorus. These systems swell to a carbonaceous foam under the action of heat.

After ignition, polymers as a rule burn with a stable flame over the material surface. The flame is the reaction zone in which the material burns under the action of oxygen. In contrast, in the oxygen-free condensed phase below the flame, the material is degraded through pyrolysis (i.e. decomposition in an oxygen-free environment) [1].

Characterization of the pyrolysis reaction is essential in order to understand the fire behavior of polymers and in particular the fire retardancy mechanisms of phosphorus-containing fire retardants (release of phosphorus, formation of charred or carbonized and/or inorganic residues). Thermogravimetric analysis (TGA) provides important information on fire behavior and the yield of charred or carbonized residue (the char yield) with a relatively small investment of time and money.

Further information on the pyrolysis reaction can only be obtained through the combination of different techniques, for example TGA with pyrolysis gas analysis using FTIR or mass spectrometry, analysis of the residue using FTIR, REM-EDX or solid state NMR) [2, 3]. Simultaneous TGA-FTIR analysis is an excellent approach and in comparison to the other methods does not require an enormous investment. 

TGA measurements allow the number of degradation steps, the degradation temperatures, the mass loss during each degradation step and the amount of residue at different temperatures to be determined. When an FTIR spectrometer is coupled online to the TGA, the nature of the volatile decomposition products can be identified at any time during the measurement.

 

Experimental Details

The elucidation of a degradation mechanism is demonstrated using a tetrafunctional epoxy resin (ER), consisting of tetraglycidyl methylenedianiline and 4,4′ methylene-bis -(2,6-diethylaniline) and 4,4′ methylene-bis-(2,6-isopropylaniline) as aromatic hardener. The phosphorus-based fire retardant (FR) is a derivative of 9,10-dihydro-9-oxa- 10-phosphaphenanthrene-10-oxide (DOPO) with an aliphatic isocyanurate. These fire retardants were added to the epoxy resin to give a phosphorus concentration of 2 wt %. The structures of the materials used are shown in Figure 1.

Conclusions

TGA-FTIR is an ideal method to elucidate the degradation and fire retardancy mechanisms of halogen-free fire-retarded polymer materials. The method yields information about the increase of residues and the release of phosphorus-containing pyrolysis products, which are of direct relevance to the study of fire retardancy mechanisms. 

It therefore provides knowledge that is essential for understanding degradation pathways and interactions between polymer and fire retardants and hence for the development and optimization of fire-retarded polymers.

TGA-FTIR: From The Investigation of Pyrolysis to The Elucidation of Fire Retardancy Mechanisms | Thermal Analysis Application No. UC 324 | Application published in METTLER TOLEDO Thermal Analysis UserCom 32