Thermal Analysis
MaxRes
Introduction
When selecting parameters for a TGA or TMA experiment, the user often has to choose
between two conflicting goals (shortest possible experimental time or optimum resolution).
The heating rate is one of the most important factors. High heating rates result in short
experimental times but may lead to poor separation of the thermal events being measured. In contrast, slow heating rates allow a better separation of close-lying effects, but
the experimental times involved are of course longer.
MaxRes resolves this dilemma by providing dynamic, event-controlled switching of the
heating rate during the measurement. It automatically adjusts the heating rate as a function of the change in weight (TGA) or dimensions (TMA) of a sample. This allows measurements to be performed more quickly without sacrificing resolution.
Features and Benefits
• Better temperature resolution of close-lying effects – makes evaluation and interpretation easier and more dependable
• Shorter experiments – saves measurement time
• Quasi-isothermal conditions during reactions – the reaction kinetics is unaffected
by temperature changes
Tutorial example
Decomposition of CuSO4·5H2O by TGA using MaxRes
The diagram illustrates different possibilities of enhancing resolution in a TGA measurement using copper sulfate
pentahydrate as an example. The three successive weight
loss steps correspond to the stoichiometric loss of 2, 2 and
1 molecules of water of crystallization.
At a constant heating rate of 25 K/min, the first and second
steps are poorly resolved and precise quantification is not
possible. At 5 K/min, the steps are well separated but the
experiment takes five times longer.
For the same quality of separation as achieved at a constant (slow) heating rate, MaxRes reduces the experiment
time by half.
Theory and application examples
MaxRes can help in the following cases:
• Overlapping reactions or transitions
• Poorly resolved events where the signal between two DTG peaks does not return to the baseline
• Shoulder-like signals
• Complex reaction kinetics
• Reaction rates and activation energies are sufficiently different
• Loss of water of crystallization (xH2O) from hydrated compounds
• Separation of volatiles / plasticizers in elastomers / plastics
• Detection of different layers (TMA)
Theory
MaxRes algorithm
MaxRes uses the derivative of the measuring curve (DTG in
thermogravimetry) to control the change of the heating rate.
Several control parameters such as threshold values and
rate factors can be entered for optimal adjustment of the
heating rate:
• Minimum and maximum heating rate
• High threshold to lower the heating rate
• Low threshold to increase the heating rate
• Timeout (time delay)
• Factor to change the heating rate
Optimized sets of default parameters for standard, slow and
fast measurements are available in the program.
Application examples
Quantification of volatiles in elastomers by TGA
Oils added to elastomers as plasticizers often vaporize
at temperatures at which pyrolysis of the polymer has already begun. For accurate compositional analysis by TGA,
improved separation of the two effects can be obtained by
performing the measurement at reduced pressure. Alternatively the oils can be extracted. Both methods are however
time-consuming and require additional instrumentation and
effort.
MaxRes has the advantage that it allows excellent separation of oil evaporation from polymer pyrolysis without the
use of further hardware. Quantification of the volatiles is
simple and precise – in the example shown, 5.5% and
7.4%.
Application examples
Decomposition of building materials - Portland cement by TGA
The hydration of Portland cement leads to the formation of
different hydrates in a complex process. This can be studied
by TGA analysis. The interpretation of the weight loss curve
in the early stages of the dehydration is however not easy
using constant heating rates because the decomposition
steps of CSH, ettringite and calcium sulfate dihydrate are
close together.
In an open crucible only two weight loss steps are observed, which makes interpretation of the process difficult.
An optimum combination of a self-generated atmosphere
(using an aluminum crucible with a 50-µm hole in the lid)
and MaxRes significantly improves the separation of the
partially overlapping dehydration processes. This then allows the ettringite to be quantitatively measured.
Rubber analysis by TGA
When rubber analysis is performed by TGA at constant heating rate, the curves obtained are often difficult to interpret
and analyze quantitatively because the decomposition steps
overlap. Due to kinetic effects, slow heating rates shift the
individual decomposition reactions to lower temperatures.
Exact quantification of components can only be achieved if
the individual weight loss steps are well separated.
The MaxRes curve obtained using automatic adjustment of
the heating rate enables three decomposition steps to be detected indicating the complex reaction kinetics involved. The
resulting DTG curves displayed in the insert clearly show
that only the DTG peaks obtained using MaxRes return to
the baseline. This allows accurate compositional analysis
of the rubber sample.
Detection of two layers of polyurethane on wood by TMA
Penetration of a TMA probe into layers of coatings due
to softening of these layers is a temperature- and time/
viscosity-dependent effect. This means that the determination of the softening behavior of two layers can be enhanced
using MaxRes heating rate control. At a constant heating
rate, penetration of the TMA probe into the two layers results
in two overlapping dimensional changes. To improve the
resolution of these two effects, MaxRes reduces the heating rate during the first indentation so that the temperature
for the softening of the second layer is not reached before
the first penetration process is completed. MaxRes allows
a better separation of the two successive processes to be
achieved than is possible with experiments using constant
heating rates.
Application examples
Separation of plasticizer evaporation from PVC/NBR blend by TGA
MaxRes not only separates steps due to differences in decomposition kinetics. Physical evaporation processes can
also be separated from overlapping decomposition reactions. This is often done using a higher heating rate to shift
the reaction toward higher temperatures.
Compositional analysis of plasticizers in PVC/NBR is a special case, however, and requires a different approach: the
pressure of the furnace atmosphere is reduced to lower the
evaporation temperature to a value at which PVC no longer
degrades. The diffusion of the plasticizer out of the sample
is a time-controlled process – lowering the heating rate allows complete evaporation before PVC degradation begins.
At constant heating rates, e.g. 5 or 10 K/min, an accurate
determination of the plasticizer content is not possible.
Study of the thermal stability of Ethylene Vinyl Acetate by EGA
Complex polymer decomposition processes can be studied
by evolved gas analysis (EGA) using a TGA-FTIR combination. However, when constant heating rates are used, the
interpretation of FTIR spectra from pyrolysis measurements
may be difficult due to presence of mixtures of reaction gases: the absorption bands of the various functional groups
often overlap.
MaxRes allows an accurate determination of the vinyl acetate content of an ethylene vinyl acetate (EVA) copolymer
to be made because the weight loss steps are now clearly
separated. EVA decomposes in two distinct steps. The C-O,
C=O and O-H absorption bands in the FTIR spectrum indicate that acetic acid (20%) is first released as a direct result
of a side-group elimination reaction. In the second weight
loss step (80%), the C-H stretching absorptions bands in
the range 2850-3000 cm-1 indicate the decomposition of
the remaining unsaturated chain.
The results show that simultaneous TGA-FTIR and MaxRes
allow a better understanding of this two-step decomposition
process to be achieved.
Mettler-Toledo GmbH, Analytical
Postfach, CH-8603 Schwerzenbach
Phone 01 806 77 11, Fax 01 806 7350
Internet: http://www.mt.com/ta
Subject to technical changes
10/2003 © Mettler-Toledo GmbH
Printed in Switzerland
ME-51724362