Nayan Chand Dhibar Please display a typical plot. Are you using air as the carrier gas? Why is this weight gain significant for you? Does this happen with every sample? Have you run a blank with the dish only? If so, what happens? Does this weight gain disappear when you heat? If so, at what temperatures? Have you tested the system for a leak? Is it a slow or rapid weight gain?

One possibly could be moisture absorption. Are the samples completely dry when placed in the pan or come from a desiccator? Also simply could be baseline correction.

This is not a regular case, however for some particular matal/metal oxide based composites like Titanium has been exhibited this types of exceptional weight gain activity due to heat which should be the result of Nitrogen or other carrier gas absorption by this particular metals during experiment. But according to your claim this is quite impossible for plastic or pure polymeric samples, however if this happening with your sample please make a blank experiment and eliminate the result with the previous experiment. Or please check the purity of both carrier gas and samples.

Hope this information would be helpful to you.

a gas may have reacted with free monomer ?

Due to absorption of moisture or atmospheric gas(s), Oxidation.

It happened to me few times during a mesurement of the humidity of the specific granulate for plastic moulding. (after measurement the weight was higher, than on the beginning). It is connected with air humidity and dynamic of the heating ramp. My opinion is, that heating is too dynamic. Hydroscopic plastics would to absorb very fast air humidity after too dynamic heating process. Is it tested sample hygroscopic?

Typically it is a local gas circulation caused by a surge in jump in heating (not by the general temperature, but by heating).

If the plastic you used was absorbing the ambient gas (purge), there is a chance.

You may try with nitrogen atmosphere to check if and reproducibility occurs.

Thermogravimetric data can effectively evaluate kinetic parameters of solid-state reactions involving weight loss or gain. (TGA) can help determine acceptable processing temperatures for hot melt extrusion of amorphous polymers, but caution is needed when extrapolating suitability of processing conditions. Weathered polyethylene films show the highest weight gain due to oxygen absorption, while no polypropylene samples show this intermediate weight increase. Because in the first exposure period the crystallinity changes little, early changes in mechanical properties and molecular masses are an evidence in favor of reactions in the amorphous phase able to modify length and structure of the plastics, even before the formation of stable bonds with atmospheric oxygen. Analysis results are affected by heating rates, sample weight, and carrier/balance gas flux, with solutions proposed for each issue.

Thermo gravimetric analysis (TGA) is a method for measuring the change in weight of a sample as a function of temperature, with advantages, disadvantages, and applications discussed in this chapter.

Thermo Gravimetric Analysis, 2020, doi:10.1007/978-981-15-1547-7_19

Best regards,

Initial Wt gain is fairly common, especially observed when the amount of most samples to be tested was small. It could occur for the larger amount of 'soft' materials under tested by TGA.

Initial Wt gain can be largely due to one or all of causes: 1) purge gas is heavier than the original absorbed with the sample, due to the effect of float force in physics. 2) chemical reaction between the purge gas or air with the sample under test.

In your case with the plastic samples, initial Wt gain can be due to both, even a fairly large quantity of the materials used.

To find the root cause of this initial Wt gain, you can perform a series of tests, the amount of materials increased by 5 to 10mg under the same procedure. Then analyse the Initial Wt gain versus the initial weight. You will be able distinguish the cause of floating force or chemical reaction. You can also try to reheat the sample to see if this initial Wt gain still there.

The above excluded the errors that were by the sample preparation, dirty sample holder, sample holder in contact with the chamber wall, miscalibration, etc.., i.e., the cause of the errors by operator/equipment