Thermal Analysis Experiments with Fire-Retarded Polymers - METTLER TOLEDO

Thermal Analysis Experiments with Fire-Retarded Polymers

Introduction

Polymer engineering and manufacturing materials consist of organic macromolecules to which a number of functional additives have been added according to the requirements specified for the particular material. Since polymer materials contain carbon and hydrogen, they are usually easily combustible. For safety reasons, high demands are therefore put on fire prevention, depending on the field of application (e.g. building industry, electrical engineering, transport, etc.). These demands cannot be met by the polymer base material itself. The addition of suitable fire retardants [1], however, allows adequate fire protection properties to be achieved even with easily flammable large-volume plastics such as ABS or polyolefines (PE, PP).

To generate and sustain a fire, three main requirements have to be fulfilled: a source of combustible material, a source of oxygen and a sufficiently high activation energy. Once a fire has started, complex, usually free radical, decomposition processes (pyrolysis and oxidation reactions) occur. As a rule, these are exothermic in nature. From this point of view, fire retardants can exert their effect by 

  • Producing competing endothermic chemical reactions to consume the exothermic combustion energy. This helps to cool the system
  • Interfering with the free radical and oxidative decomposition processes
  • Forming a non-flammable, often foamlike crust as a protective layer, e.g. through charring or carbonization, or the formation of inorganic, glassy materials 
  • Displacing or eliminating (through chemical reaction) surrounding oxygen, or the dilution of mixtures of flammable gases and oxygen

Nowadays, combinations of different chemicals are also used as fire retardants in order to achieve synergetic effects.

A number of standardized methods are used to investigate fire behavior [1]. These include procedures for determining certain characteristics of materials (e.g. the Limited Oxygen Index (LOI) according to ISO 4598 or Cone Calorimetry according to ISO 5660) as well as industry-specific methods (e.g. Bunsen burner test according to UL 94 for electrical engineering). These measurement and test procedures usually give a practical insight into the fire behavior of the materials or components under investigation and are therefore frequently specified in their requirement profiles.

The characterization of a material with regard to fire behavior according to the current standards is more difficult if only a small amount of material is available or if the geometry of the material available is unsuitable. This is usually this is case with more detailed investigations of materials and damage. In such situations, thermal analysis is then often very helpful when used together with conventional chemical analytical methods. 

 

Thermal Analysis of Fire-Retarded Polymers

Fire-retarded polymer materials can be investigated by both Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). The method used depends on the composition of the material and the specific information required. Both methods provide data of great value for fire protection studies. DSC is used to obtain calorimetric data. Possible applications are the

  • Determination of the onset of decomposition in oxygen-containing atmospheres. The decomposition reaction can be exothermic or endothermic depending on the formulation of the material.
  • Characterization of energy-consuming (endothermic) decomposition processes of additives, for example the elimination of water from metal hydroxides

TGA, with suitable heating programs and different gas atmospheres, can be used for the 

  • Determination of the approximate composition of polymer materials (possibly with the aid of other analytical chemical methods)
  • Analysis of volatile reaction products, for example water from metal hydroxides or HBr from brominated fire retardants
  • Observation of charring or carbonization, or the formation glassy crusts, in particular of the carbonization behavior of different formulations

Simultaneous differential thermal analysis SDTA (measurement of the temperature difference between the sample and the surroundings) gives a valuable insight into the energy of processes occurring in the TGA (similar to DSC). This is particularly useful for assessing decomposition behavior.

The results of thermoanalytical investigations do not usually allow direct conclusions to be drawn about other fire protective measurement results. They do however provide important information on the nature of materials and often allow simple quality control methods to be developed. Our own experience confirms that thermal analysis can sometimes be more economical and at the same time provide more decisive information than results from elaborate fire protection test procedures.

Conclusions

The results presented here show that thermoanalytical methods such as TGA and DSC can be used to obtain important information about the mechanisms and the effect of fire retardants in polymer materials. This is in particular useful for material development. 

To date no investigations have been reported that allow a direct correlation of thermoanalytical data with fireprevention tests or real fire behavior to be made. 

It is very difficult to establish such correlations because realistic fire conditions cannot be simulated in thermoanalytical experiments using just a few milligrams of sample!

Acknowledgments: The author wishes to thank Huber + Suhner AG, Pfäffikon, Switzerland and in particular Dr. R. Koeppel and his team (Material Development, Wire & Cable Division) for preparing some of the compounds used in this investigation.

Thermal Analysis Experiments with Fireretarded Polymers | Thermal Analysis Application No. UC 224 | Application published in METTLER TOLEDO Thermal Analysis UserCom 22