TGA of Aluminum Trihydrate and Magnesium Hydroxide
Purpose
A mixture of aluminum trihydrate and magnesium hydroxide is analyzed by TGA to demonstrate the behavior of flame retardants.
Sample
1:1 mixture of Al(OH)3 and Mg(OH)2
Conditions
Measuring cell: TGA/SDTA851e
Pan: Alumina 70 ul
Sample preparation: Approx. 20 mg powder
TGA measurement: From 35 °C to 600 °C under nitrogen atmosphere (200 ml/min), then from 600 °C to 850 °C under oxygen (200 ml/min). Heating rate: 30 K/min
Atmosphere: Nitrogen then oxygen, automatically switched
The diagram displays the TGA (black) and the DTG (red) curves as a function of temperature.
Interpretation
The sample first loses its moisture in a broad step between 35 °C and 200 °C. Water is then eliminated from the aluminum trihydroxide in the step with the point of inflection at about 300 °C. The step height is 15.05%. The Al(OH) thus eliminates about 30% water. The reaction scheme for the decomposition of Al(OH)3 with elimination of water is: 2 Al(OH)3 WR Al203+ 3 H20.
On the basis of this equation, a mass loss of 34.6% or 17.3% referred to the total sample mass is expected. The measured step height is therefore some 2.3% smaller than that expected. This could be due to the presence of aluminum oxihydroxide, AlO(OH), in the sample that remains from the manufacturing process. The shoulder in the DTG curve at 220 °C supports this explanation.
The step due to the elimination of water from the magnesium hydroxide has a point of inflection at about 415 °C. The step height is 17.7% (or 35.4% referred to Mg(OH)2). The reaction scheme for the decomposition of Mg(OH)2 with the elimination of water is:
Mg (OH)2 R MGO + H2O
A mass loss of 31% (or 15.5% referred to the total sample mass) is therefore expected. The measured mass loss is however greater. A possible reason for this could be the reaction of the magnesium hydroxide with atmospheric carbon dioxide. Magnesium carbonate eliminates CO2 in the temperature range measured. The shoulder in the DTG curve around 500 °C supports this interpretation.
Conclusions
The composition of flame retardants can be determined thermogravimetrically. The results also allow information to be obtained on the possible flame-retardant mechanisms involved.
TGA of Aluminum Trihydrate and Magnesium Hydroxide | Thermal Analysis Application No. HB 484 | Application published in METTLER TOLEDO TA Application Handbook Elastomers Volume 2