Lab 10: TGA
1. Purpose
The purpose of this lab is to determine the composition of a piece of tire tread using
thermogravimetric analysis (TGA).
2. Theory
Thermogravimetric analysis uses heat to induce chemical and physical changes in
materials and performs a corresponding measurement of mass change as a function of
temperature or time. Thermogravimetry is performed on a wide range of materials including
polymers, composites, laminates, adhesives, food, coatings, pharmaceuticals, organic materials,
rubber, petroleum, chemicals, explosives and biological samples.
While some TGA analyses are intended to measure transitions associated with weight
gain (e.g. oxidative stability and absorption properties), most TGA experiments focus on weight
loss measurements.
Weight loss can occur through various processes—decomposition,
evaporation of water or volatiles, desorption, chemical reduction, etc.
In some advanced
instruments residual gases released from materials can be analyzed using TGA-tandem
instruments, such as TGA-FTIR or TGA-Mass Spectrometry, to determine the identity of the
released gas and give insight into the weight loss mechanism. Also, while not the focus of this
lab, kinetic models can be extrapolated from TGA weight loss data, which in turn can be used to
predict product lifetime at given temperatures and decomposition rates under various conditions.
The most common form of temperature-induced weight loss in materials investigated by
TGA is decomposition—the breaking apart of chemical bonds to release small, gaseous
molecules (carbon dioxide, carbon monoxide, water vapor, NOx’s, etc.). Thermal stability is
widely dependant on the type of material being investigated, and major decomposition
mechanisms for different materials can occur over wide temperature ranges.
This is
advantageous in composition analysis of multicomponent materials as each component can be
individually decomposed in a stepwise fashion to quantitatively determine the amount of each
component in the material (Figure 1).
Figure 1. TGA plots of various multicomponent materials.
Instrument Overview
The
TGA
thermogravimetric
is
a
analyzer,
research
whose
grade
leading
performance arises from a responsive low-mass
furnace;
sensitive
thermobalance,
and
efficient
horizontal purge gas system with flow rate control
(Figure 2).
Its convenience, expandability and
powerful, results-oriented software make it perfect for
the multi-user laboratory where a wide variety of
TGA applications are conducted.
Figure 2. A Q50 TGA from TA Instruments.
The sample rests on a small platinum or ceramic pan, which hangs from a small hook
connected to the balance. The weight of the sample pan and sample is measured using a
photodiode balance. The balance operates on a null-balance principle. At the zero mark, or “null”
position, equal amounts of light shines on the 2 photodiodes. If the balance moves out of the null
position an unequal amount of light shines on the 2 photodiodes, at which point a current is
applied to the meter movement to return the balance to the null position. The amount of current
applied is proportional to the weight loss or gain. A furnace is raised around the pan to apply
heat to the sample. This furnace is linked to computer software that controls the temperature rate
and cycle. A detailed schematic of the TGA is shown in Figure 3 below.
Figure 3. Q50 TGA instrument schematic.
While the furnace is running, there are two
different atmospheric environments in which the
sample can be exposed—nitrogen or air. Depending
on the type of decomposition and the information
desired, a specific type of purge gas is desired. In
Figure 4, a schematic representing the flow of purge
gases is shown. The balance (flowing at 40 ml/min in
Figure 4) is always kept under nitrogen gas to protect
the instrument coating from oxidative degradation, but
the sample purge gas (60 ml/min) can be varied. Also,
the
two
sample
purge
gases
can
be
used
Figure 4. Q50 TGA furnace.
interchangeably during an experiment, which is helpful, for example, in determining filler
content of composites whose matrix and reinforcement have different thermal stabilities in
different environments.
Experimental Procedure
1. Open nitrogen gas valve and the TGA release valve. Never adjust regulator valve.
2. Check that the sample pan is clean, and carefully use tweezers to place the empty pan on
the sample platform, making sure the pan is correctly oriented on the platform.
3. Press the TARE button located on the console at the bottom of the instrument. The
instrument will perform an automated tare, which should take a couple of minutes.
4. Take a small piece of tire tread and measure/record its mass using the weighing balance.
The sample should be in the range of 10-20 mg.
5. Once the sample has been prepared, carefully remove external residue (e.g. oils from
your fingers) by wiping the entire surface of the sample with a Kimwipe®.
6. After the instrument finishes its automated tare sequence, place the sample in the center
of the pan. Preferably, this should be done without removing the sample pan from the
sample platform.
7. Enter the appropriate inputs into the Q50 TGA program:
a. Summary tab
Mode: TGA 1000oC
Test: Ramp
Sample Name: Tire Tread
Pan Type: Alumina
Comments: None
Data File Name: \\GI3113-01\TA\Data\TGA\Polymer Lab\GroupX_Tire
b. Procedure tab
Test: Ramp
Heating rate: 20oC/min
Final Temperature: 900 oC
Switch to gas 2 at: check this box and enter 650 oC
c. Notes tab
Operator: Polymer Lab
Extended Text: None
Balance: #1 – Nitrogen  Flow Rate: 40 mL/min
Sample: #1 – Nitrogen  Flow Rate: 60 mL/min
8. Click Apply to save changes.
9. Press the green button on the top-left corner of the software to start the run. The TGA
instrument will automatically perform the experiment. Once the experiment is finished,
the instrument will automatically unload the sample onto the sample platform and begin
an air-cooling sequence.
10. If there is still residue on the sample pan, carefully hold the pan with long tweezers, and
clean the pan by applying a propane blow torch flame for 2-3 seconds (until residue is
gone). Make sure this is done in the fume hood and take care to keep your hand far from
the flame.
11. Raise the furnace to check the internal furnace temperature (Control – Furnace – Up).
Once the Furnace has fully sealed, check the temperature reading on software. If the
temperature has returned to room temp, proceed to the next step. If not, repeat the aircooling sequence.
12. Repeat steps 4-10 using natural rubber. On the summary tab, be sure to change the
sample name to “NR” and the data file name to “\\GI3113-01\TA\Data\TGA\Polymer
Lab\GroupX_NR”.
13. Repeat steps 4-10 using SBS rubber. On the summary tab, be sure to change the sample
name to “NR” and the data file name to “\\GI3113-01\TA\Data\TGA\Polymer
Lab\GroupX_SBS-R”.
14. Turn off the Nitrogen gas valve and TGA release valve before leaving the lab.
Assignments
1. What are the weight percents of rubber and carbon black filler in the tire tread?
2. What is the decomposition temperature of the tire tread rubber under nitrogen gas?
3. What is the weight percent of inorganic filler in the tire tread? What do you think this
could be?
4. Occasionally, TGA experiments cannot indicate “true” sample decomposition. Explain
why this is the case.
5. Why is the nitrogen gas first used as an atmospheric environment, and why is it switched
to air during the experiment?
6. SBS rubber and natural rubber are both common constituents of rubber formulas in tires
today. Based on the TGA data you collected, which type of rubber would you choose to
make a high performance tire that would be subjected to large amounts of frictional
heating?
7. Explain how sample mass, sample shape, and position of the sample on the sample pan
can affect data in a TGA experiment.
8. The sample pan used in this experiment is made from alumina. Why is alumina
preferred, and why would an aluminum sample pan (sometimes used in TGA tests) be
inadequate for this experiment?