Analysis of the Foaming Behavior of a Fire Retardant by TMA and TGA - METTLER TOLEDO

Analysis of the Foaming Behavior of a Fire Retardant by TMA and TGA

What Are Intumescent Fire-Retardant Coatings?

Intumescent fire-retardant paints or coatings are designed to protect the surfaces of materials from the effects of fire and heat. On exposure, intumescent systems first swell to a thick, robust, carbonaceous foam and then form a char. The closed foam structure inhibits the transport of oxygen to unburned regions beneath the char and provides a physical barrier to heat and mass transfer. Intumescent fire-retardant coating systems usually consist of a polymeric binder, polyphosphoric acid compounds (as an acid donor), carbon-rich polyhydric compounds (as a carbon donor) and a blowing agent (for gas formation). They react under the action of heat and at a certain temperature begin to foam [1] [2].


TMA and TGA Measurements in N2 and Ar

The basic mechanism can be described as follows: Decomposition of the acid donor supplies acid, which catalyzes the dehydration of the carbon donor. The mixture melts and the blower decomposes, producing gases and causing the liquid to foam. The foam simultaneously solidifies through crosslinking into a three-dimensional carbon structure and forms a char under the action of heat. The formulation is carefully designed to ensure that the different events occur in the proper order and so ultimately produce an effective char. The amount of foam produced during the intumescence reaction depends on the number of carbon atoms while the number of hydroxy-groups determines the reaction rate of the dehydration process and thereby controls the rate of foam formation [3].

Analysis of an Intumescent Coating by TMA and TGA

The coating under investigation was based on a vinylacetate polymer binder together with the fire retardant components ammonium polyphosphate, melamine, pentaerythritol and other inorganic fillers.

The purpose of the study was to measure the volume increase of an intumescent coating using thermomechanical analysis (TMA) and to determine the change in mass of the coating by thermogravimetric analysis, (TGA). A further objective was to investigate the influence of oxygen on the intumescence reaction. 

 

Experimental Details 

To determine the expansion behavior of the coating, about 2 mg of the powdered sample was put in a 100-µL aluminum crucible and covered with an aluminum disk of 5.3 mm diameter. The sample was inserted in a TMA and a force of 0.01 N applied. It was then measured in the range 50 to 450 °C at a heating rate of 10 K/min in nitrogen and in oxygen. The change in mass of a sample of about 2 mg in a 30-µL alumina crucible was measured in a TGA/DSC1 in the range 50 to 800 °C at a heating rate of 10 K/min argon and oxygen. The gas flow was 80 mL/min.

 

Results

The TMA measurements in an inert atmosphere (Figure 1, upper curve) and in an oxidizing atmosphere (Figure 2, upper curve) show that the sample shrinks up until about 270 °C. In this temperature range, the vaporization of residual solvents, decomposition reactions and melting process occur. Presumably the particles soften and coalesce leading to volume contraction of the sample. 

Conclusions

TGA and TMA measurements of an intumescent fire-retardant coating provide information about the volume change of the sample and the mass changes involved in the decomposition reaction.

The volume increase under inert conditions is about twice as large as that under oxidative conditions. In this process, only part of the gases formed are used in the foam formation process − otherwise a simultaneous loss of mass would not occur.

The TMA and TGA curves show that oxygen is an important reaction component that influences the behavior of fire-retardant coatings in a fire. Oxygen as a highly reactive biradical leads to the continuous oxidative degradation of the carbonaceous intumescent coating. This causes the insulating char to gradually lose its effect. The atmosphere is therefore a significant factor in studies on the reaction mechanisms and reaction kinetics of intumescent fire-retardant coatings. In particular, oxygen is an important component in the intumescence reaction in addition to the binder, acid donor, blower and carbon donor.

Analysis of the Foaming Behavior of a Fire Retardant by TMA and TGA | Thermal Analysis Application No. UC 295 | Application published in METTLER TOLEDO Thermal Analysis UserCom 29