Influencing factors of thermogravimetric test results

Sample size and sample cuvette

For thermogravimetric determination, the sample volume should be small, generally 2~5mg. On the one hand, it is because the sensitivity of the instrument balance is very high (up to 0.1μg). On the other hand, if the sample volume is large, the mass transfer resistance will be greater, the internal temperature gradient of the sample will be large, and even the thermal effect of the sample will cause the sample temperature to deviate from linearity. The temperature is programmed to change the TG curve. The finer the particle size is, the better. Flatten the sample as much as possible. If the particle size is large, the decomposition reaction will move to high temperature.

The material of the sample cuvette requires high temperature resistance, and is inert to the sample, intermediate product, final product and atmosphere, that is, it cannot have reactivity and catalytic activity. The commonly used sample dishes are platinum, ceramic, quartz, glass, aluminum and so on. Special attention should be paid to different samples to use different materials of the sample cuvette, otherwise it will damage the sample cuvette. For example: Sodium carbonate will react with SiO2 in quartz and ceramics at high temperature to form sodium silicate, so it is like sodium carbonate. For alkaline samples, do not use aluminum, quartz, glass, or ceramic sample dishes when testing. The platinum sample cuvette is active for hydrogenated or dehydrogenated organic matter, and it is not suitable for polymer samples containing phosphorus, sulfur and halogen.

Heating rate

The faster the heating rate, the more serious the temperature hysteresis. For example, polystyrene decomposes in N2. When the degree of decomposition is taken as 10% weight loss, the difference is 37 at 1℃/min and 394℃ at 5℃/min. ℃. The rapid heating speed reduces the resolution of the curve and loses the information of some intermediate products. For example, slow heating of water-containing compounds can detect some intermediates with stepwise loss of water.

Atmosphere influence

The change of the surrounding atmosphere of the thermobalance has a significant effect on the TG curve. The decomposition temperature of the TG curve of CaCO3 in vacuum, air and CO2 is nearly 600℃. The reason is that CO2 is a decomposition product of CaCO3. The presence of CO2 in the atmosphere will inhibit CaCO3. The decomposition temperature increases.

In the air, polypropylene will have significant weight gain at 150~180℃, which is the result of polypropylene oxidation, and there is no weight gain in N2. The gas flow velocity is generally 40ml/min, and a large flow velocity is beneficial to heat transfer and overflow gas diffusion.

Volatile condensation

Decomposition products volatilize from the sample and often re-condensate at low temperatures. If they condense on the wire-hanging sample cuvette, the measured weight loss will be lower. When the temperature rises further, the condensate will volatilize again, which will cause false weight loss. , To deform the TG curve. The solution is to increase the gas flow rate to make the volatiles leave the sample cuvette immediately.

buoyancy

The change in buoyancy is due to the thermal expansion of the gas surrounding the sample due to the increase in temperature, which reduces the relative density and reduces the buoyancy, which increases the apparent weight of the sample. For example, the buoyancy at 300°C can be reduced to half of the buoyancy at room temperature, and it can be reduced to about 1/4 at 900°C. The practical correction method is to do a blank test (no-load thermogravimetric test) to eliminate the apparent weight gain.