Thermogravimetric Analysis of Highway, Offroad and Racing Tire Composition
1. Purpose
The purpose of this lab is to use thermogravimetric analysis (TGA) to compare the
composition of three types of tire tread used in various conditions: cement (car tire), soft soil
(ATV/tractor) and asphalt (racing tire).
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
Figure 4. Q50 TGA furnace.
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 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
Pre-lab
1. Open gas valve on the nitrogen (N2) and air cylinders along the north wall of the lab.
Never adjust regulator valve. The pressure should read 35 psi for N2 and 40 psi for air.
2. Your instructor or lab assistant will log on to the computer and have the instrument
initiated when you arrive to the lab. (Follow lab SOP if this is not the case)
3. Using the Pyris software, select which TGA instrument to bring online (#1 or #2) by
clicking on the appropriate button in the drop-down bar at the top of the screen.
4. In the “Methods Editor” window enter the sample name, operator and any identifying
comments. Save your data to the class folder: c\program files\pyris\data\MatE 453 Fall
2014, by clicking on the file name box then clicking on the “Browse” button.
5. In the “initial state” tab, “set the initial value” temperature to 50°C.
6. Use the “method editor” to “add a step” to your procedure.
a. Set the purge gas to N2 using the instructions in the next section.
b. Heat the sample from 50°C to 650°C at a rate of 25°C/min.
c. Switch the purge gas to air.
d. Heat the sample from 650°C to 850°C at a rate of 25°C/min.
e. Cool to 50°C at 50°C/min.
7. Cut a sample of each tire type: road, off road and racing, weighing approximately 10mg.
Remember to use weigh paper when measuring the mass of your samples on the micron
sensitive scale. Label each sample with the tire type and mass.
Lab procedures
1. Make sure that the software and instrument are connected, designated by
button on the right hand side panel of the software window.
2. On the gas control station, push the following sequence to assure purge gas in flowing:
RUN-MANUAL-N2 (or air)-START. (See images in lab TGA SOP)
3. Check that the sample pan is clean, and carefully use tweezers to place the empty pan
into the stirrup. Manually raise the sample platform, making sure the pan is correctly
oriented with the balance wire. Slowly lower the platform so that he empty pan is
suspended by the balance hook, and rotate it to the side of the instrument. (DO NOT
TOUCH THE WIRE WITH YOUR HANDS!)
4. To tare the empty pan, raise the furnace by pushing the
button with the blue arrow pointing up, on the left hand
panel. Alignment of the furnace tube may require
manual assistance.
5. Once the pan mass has equilibrated, press the “zero
weight” button on the right hand panel. (Empty scales)
The instrument will take a few moments to tare, and the
weight, on the top panel, will read a mass very close to
zero (Ex: 1.234x10-4).
6. Lower the furnace using the “lower furnace” button on
the right side panel, with the blue arrow down. Manually
raise the platform and gently remove the empty pan from
the hook.
7. Take the small piece of natural rubber sample and place it
into the center of the pan. Raise the platform gently and place on the hook. Raise the
furnace using the blue arrow up button. Lower the safety shield in front of the furnace.
8. Measure the sample mass using the “sample weight” button on the right panel, scales
with yellow load. The sample should be approximately 10 mg. This automatically enters
the sample mass weight into the “sample info” tab.
9. Equilibrate the furnace at the starting temperature by clicking on the button, “go to temp”
located on the right side panel with the thermometer and opposite pointing blue arrows.
Once the initial temperature has equilibrated, start the run using the “start/stop” button at
the top of the column on the right. Clicking on the “instrument viewer” allows you to see
the measurements as they progress.
10. Once the experiment is finished, the instrument will automatically lower the furnace.
When the furnace temperature is near room temperature, use the platform to gently
remover the pan from the hook. Clean the pan and repeat the procedure using the SBS
and tire rubber samples.
**If there is still residue on the sample pan, carefully hold the pan with tweezers, and
clean the pan by applying acetone. Acid can be used to remove sticky residue.**
11. When all runs have been completed, turn off the gas flow by pushing the following
buttons on the gas control station: STOP-BACK-BACK.
12. Collect a text file of your data for analysis using MS Excel.
13. Close the Pyris software by selecting “close all” from the “start pyris” drop down bar at
the top of the screen.
14. Turn off the Nitrogen gas valve and TGA release valve before leaving the lab.
Assignments
1. What is the weight percent of rubber and carbon black filler, respectively, in each of the
tire tread samples?
2. Why is the nitrogen gas first used as an atmospheric environment, and why is it switched
to air during the experiment? What is the decomposition temperature of each tire tread
rubber under nitrogen gas?
3. What is the weight percent of inorganic filler in each tire tread? Carry out a brief
literature search to identify potential fillers of each tire.
4. SBS rubber and natural rubber are both common constituents of rubber formulas in tires,
based on the TGA data provided and that which you collected, make a conjecture on the
type of rubber used for your samples? Comment on the comparison of rubber volume
content of the three samples.
5. Explain how sample mass, sample shape, and position of the sample on the sample pan
can affect data in a TGA experiment.