of chemiluminescence and isoconversional kinetics for the prediction of organic
Scheme 1. Stress Pattern Influencing the Properties of Adhesive Bonds
Adhesives are increasingly
being used to bond materials, even where mechanical bonds were previously used.
This is due to a need for lightweight structures and the introduction of novel
materials. However, some users are skeptical about adhesive bonds for
constructions with long life cycles, such as building fronts or load-bearing
There is an almost unlimited manifold of mechanisms that lead to the failure of
adhesive bonds (Scheme 1). Bond rupture can occur in adhesion or cohesion
zones. Mechanical or chemical stress (or both) can cause bond failure. The last
case seems to occur most often in a non-linear time dependency for the process
of bond rupture.
Prediction of Bond Failure
Accelerated tests are used to
predict the performance of adhesive bonds. During these tests, environmental
conditions such as temperature, moisture, UV light or vibrations are enhanced
in order to shorten the time frame for failure. For many of these tests, industrial
standards are already in place, including tests for accelerated (forced) aging.
The intrinsic problem with data collected under such conditions stems from the
complexity of adhesive formulations. Adhesive systems have many degrees of
freedom with respect to reaction pathways. The specific conditions under which
the systems are aging will determine which kinetic pathways will take the lead.
Consequently, the prognostic reliance of such tests depends on whether the same
reaction mechanisms apply under forced conditions with respect to reality.
The lack of sufficient predictability of adhesive bonds is a source of great
concern to many engineers. Unless the adhesive industry puts itself in a
position to provide sound evidence and theoretical models pertaining to the
life expectancy of adhesive bonds, engineers will continue to resort to
alternative means, such as screws, rivets and bolts.
Influence of the Polymer Oxidation on Bond
There can be many reasons for the mechanical breakdown of an adhesive bond; the
long-term behavior of the polymers is perhaps the most difficult to predict.
Oxidation processes leading to the cleavage of the polymer chains occur
randomly and stealthily. They may not be detected for years, undergoing a
rapid, autocatalytic acceleration once the reaction has reached a critical
stage. Once the reduction of the molecular weight has reached a critical
threshold, both adhesion and cohesion properties are critically affected, which,
in turn, leads to a loss of required bond properties. Any method allowing
meaningful prediction of the polymer aging in the solid state will therefore be
most conducive to the evaluation of adhesive bonds.
Since most organic materials readily react with oxygen (even at ambient
temperature), oxidative degradation is a severe problem in materials science.
Monitoring what predicts the stability of polymeric materials against oxidation
is, therefore, crucial. Commonly applied analytical methods such as measuring
oxidation induction time (OIT) or oxidation onset temperature (OOT) using DSC
or other conventional thermoanalytical methods are not suitable for long-term
prediction of oxidative behavior because of the use of elevated temperatures
during these experiments: the high temperatures may invoke reaction pathways
that are different from those encountered under the conditions of use.
Determination of Oxidation Stability at Higher Temperatures
The oxidation of materials in
the solid state generally starts on the surface, and the oxidation progress is
mainly diffusion controlled. The general overview of commonly applied
examination methods and the mechanism of oxidation is given by
Accelerated aging is normally initialized by
extreme environmental conditions. During the induction period, stabilizers are
consumed, while the organic matter remains stable, maintaining its original
properties. At the end of the induction period, when the concentration of
stabilizers reaches a sub-critical level, oxidative decay starts and the
substance’s properties change. Frequently, oxidation reactions are
self-accelerating (auto-oxidation) and the reaction progress may rapidly
increase after the induction period.
The most common methods used to test the kinetics of thermo-oxidation of
organic substances are thermal analysis methods like differential thermal
analysis (DTA), DSC or TG. Substances are tested using isothermal or
non-isothermal temperature profiles under an oxidative atmosphere, and the OIT
or OOT are determined when the heat flow (DSC, DTA) becomes exothermal or the
sample mass loss starts (TG). In some special applications, these procedures
are standardized (such as for automotive oils, cable insulations or
OIT- and OOT-determination procedures are widely applied, especially for
industrial and commercial applications. Their advantages include an easy sample
preparation, short measurement periods and established methods of data
evaluation. A significant disadvantage of these short-time experiments becomes
apparent during the application of the high experimental temperatures, which
are generally above 180°C. Such temperatures are used to ensure that the
oxidation starts within two hours, and to provide a distinct signal that is
larger than the baseline noise: the sensitivity of conventional thermal
analysis instruments can be too low to record the beginning of the oxidation
reaction at moderate temperature profiles. The evaluation of the bad
correlation of OIT and OOT data with the observed long-term stabilities under
normal environmental conditions is reported in references 4-6. Depending on the
properties of the substance, one or several phase transitions can occur between
the temperature ranges at which the oxidative characteristic has been measured,
and those for which the oxidative properties has been predicted (lifetime
determination). It seems obvious that degradation kinetics may change for
different low- and high-temperature phases, and the extrapolation of
high-temperature experimental results to ambient temperature can be of little
In this situation, the alternative methods, based on the experiments carried
out at low temperatures, should be applied for the characterization of the
long-term stability of organic substances.
Figure 1. Simplified scheme
of auto-oxidation of organics including a Russel mechanism (from Lacey et
Luminescence is a term used
for various phenomena, originating from electronically excited states.
Luminescence is a “cold light,” not an incandescent light. The emission of
photons results from the relaxation of excited electrons (triplet-state) into
their ground state. This may be quite a quick process: the delay between
excitation and light emission is at least 10-10
Chemiluminescence (CL) includes all luminescence phenomena resulting from
The fact that organic substances
undergoing oxidation emit light was first recognized in the second half of the
In the past few years, chemiluminescence
has gained wide acceptance as a sensitive method by which to study the
oxidative degradation of organic solid substance.9-11
Principles of Chemiluminescence in
Light emission during the oxidative degradation process of organics is part of
the reaction course. The first step is the formation of unstable alkyl
radicals, which immediately scavenge the oxygen from the atmosphere to form
peroxy radicals. These react further and transform into different species in an
accelerating degradation cycle (auto-oxidation, see left part of Figure 1).
This is normally attributed to a transition of excited
R=O*) into their ground
state. The spectral range of the light emitted varies according to the type of
substances involved. In most cases, chemiluminescence is observed in the short
wave region of the visible spectrum from 380 to 450 nm. However, there are
well-known exceptions: the relaxation of 1
can be detected in the infrared region at ca. 1200 nm.
The energy required for this reaction (290-340 kJ mol-1
may be supplied by three different chemical mechanisms:
- The combination of two peroxy radicals with concomitant fragmentation
in a Russel mechanism13 is strongly exothermal (460 kJ
mol-1).14 The CL-emitter is an
excited “triplet” carbonyl function (see right part of Figure 1).
- The direct homolysis of hydroperoxides followed by a cage reaction
leads to an excited carbonyl-function and is combined with the evolution of 315
- The metathesis of alcoxy or peroxy radicals provides 374 kJ/mol and
323 kJ/mol, respectively.16 It has been shown that CL
signal intensity reveals the existence of two kinetic stages during oxidative
degradation of organic materials: the first one is correlated with the
concentration of peroxide groups,17 while the second
stage corresponds to the oxidation propagation by the hydrogen abstraction
responsible for the carbonyl formation.18
Figure 2. CL signals recorded
for unstabilized and stabilized cis-1,4-polyisoprene during non-isothermal
conditioning in the oxygen atmosphere. The inset presents the delayed oxidation
of the stabilized sample at higher temperature (OOT = 22°C).
The CL emission rate during oxidation of organic substances at ambient
temperatures is too low to be detected. However, only moderate temperatures are
required to provide detectable signals. The requirements for the oven used in
this process are similar to those used in conventional thermoanalytical
measurements such as DSC or TGA: exact control of the required temperature
profile is a necessity, even in long-term experiments in a gas exchange
facility. In addition, the sample compartment must be absolutely light-tight.
CL-emission detection may be achieved using a photomultiplier tube (PMT) with a
photon counting mode or a slow-scan charged coupled device (CCD) camera. PMTs
are highly sensitive devices that allow short gating times, but their dynamic
ranges are low and their use must be carried out with caution in order to avoid
the saturation of the photocathode. The advantages of solid-state detectors
(CCDs) are their simplicity in use, their high dynamic range and the feature of
imaging the sample to exhibit the inhomogeneous character of oxidation
reactions. A third way of detecting CL emission is to use micro channel plates
(MCP) or intensified CCDs. These sensors offer the best sensitivity in
combination with the imaging facility, but are characterized by an exorbitant
price, high operation complexity and a low dynamic range.
The instrumentations provided by AKTS-Chemiluminescence are fully automated and
consist of thermoelectrically cooled PMT with a photon counting mode and an
oven chamber in combination with an optical path including a shutter system (to
protect the highly sensitive detection unit against extensive light during
sample handling, as well as to provide background measurements). The
single-cell instrumentation is designed especially for sensitive measurements
at moderate temperature conditions (isothermal and non-isothermal mode), which
allows experiments to be carried out under controlled relative humidity in
temperatures up to 95°C. The multi-cell instrumentation enables the
characterization/comparison of four independent samples at the same temperature
profile without cross contamination.
of Chemiluminescence in the Investigation of
In this study, we report on a new approach to kinetic analysis of the oxidation
of organic solids at moderate temperatures using CL measurements carried out on
newly developed instrumentation. The kinetic characteristics of the oxidation
process calculated from the chemiluminescence signals are subsequently applied
to the prediction of the reaction progress under different temperature
The presented results depict the comparison of the oxidation reactions of
natural rubbers with and without stabilizer in an oxygen atmosphere. This
system is representative of many hot-melt formulations, especially HMPSA. The
results shown in Figure 2 illustrate the influence of the stabilizer (5%
565) on the oxidation behavior of the rubber
(cis-1,4-polyisoprene) during non-isothermal heating in the 30-120°C range with
a rate of 0.0132 Kmin-1
in the oxygen atmosphere.
Figure 3. Normalized
CL-emission signals of unstabilized natural rubber (cis-1,4-polyisoprene) recorded
during isothermal oxidation at 120°, 110°, 100°, 90° and 80°C. The inset
presents the Arrhenius relationship for the extrapolated onset and peak
temperatures (the activation energy values for the extrapolated onset and the
peak of oxidation are 27.8 and 16.4 kJ mole-1).
Having a set of experimental
data under different isothermal conditions, an Arrhenius relationship can be
evaluated (see Figure 3).
4. Comparison between CL emission (black) and DSC heat flow (grey) signals at
different isothermal conditions (120°, 110°, 100°, 90°C) recorded during oxidation of unstabilized
At higher temperatures
(>100°C), the CL data corresponds well with the DSC data. When testing
oxidative stabilities below 100°C, the limitations of conventional
thermo-analytical methods such as DSC become obvious: useful evaluation of such
data with regard to oxidation onset becomes tricky and is not reliable (see
Advantages of Chemiluminescence in Monitoring Oxidation
Compared to DSC and other conventional thermo-analytical methods, CL offers
many advantages. Due to its much higher sensitivity, experiments can be performed
at much lower temperatures (i.e., closer to application-related conditions).
This fact is essential to the characterization of substances with
low-temperature melting points, glass transitions, etc. The outstanding
baseline stability of CL is of great benefit when performing long-term
; moreover, the CL-signal is exclusively
related to the oxidation processes and is therefore not superposed by signals
resulting from other reactions, including phase transitions. The instrumentation
setup may be designed individually for special fields of application and goals
of research. The experiments can be performed with sample masses as low as
approx. 0.1 mg. A basic instrumentation would not be any more expensive than a
commercial DSC apparatus.
Figure 5a. Friedman
differential isoconversional analysis of the long-term oxidation process of
unstabilized (left) and stabilized (right) natural rubber.
Application of an Advanced Kinetic Analysis of CL Signals for Lifetime Prediction
of Kinetic Parameters – Isoconversional Analysis
5b. Dependence of activation energy and the pre-exponential factor of natural
rubber oxidation on the reaction progress calculated by Friedman’s differential
isoconversional method: unstabilized (left) and stabilized (right) natural
The noticeable weakness of the ‘single curve’ methods (determination of kinetic
parameters from single runs recorded with one heating rate or isothermal
condition only) has led to the introduction of ‘multi curve’ methods over the
past few years, as discussed in the International ICTAC kinetics
Degradation reactions are often too
complex to be described in terms of a single pair of Arrhenius parameters and
the commonly applied set of reaction models. As a general rule, these reactions
demonstrate multi-step characteristics. They can involve several processes with
different activation energies and mechanisms. In such situations, the reaction
rate can be explained only by complex equations where the activation energy
term is no longer constant but is dependent on the reaction progress (E ≠ const but E = E(α)).
Isoconversional methods were introduced by Friedman25
and Ozawa-Flynn-Wall.26, 27
A detailed analysis of
various isoconversional methods (i.e., the isoconversional differential and
integral methods) for the determination of activation energy has been presented
The convergence of the activation energy
values obtained by means of a differential method (Friedman) with those
resulting from the use of integral methods (Ozawa-Flynn-Wall) comes from the
fundamentals of the differential and integral calculus.
differential isoconversional method of Friedman is based on the Arrhenius
f(α): the model function
A: the preexponential factor
E: the activation energy
T: the temperature
t: the time
Friedman has applied the
logarithm of the conversion rate da/dt as a function of the reciprocal
temperature at any conversion a:
As f(α) is a constant in the last term at any fixed a, the
logarithm of the conversion rate da/dt over 1/T shows a straight-line
dependence with the slope of m = -E/R.
By the extension of the expression
A'(α) = A(α) f(α)
one can predict the reaction
rate or reaction progress having determined A'(α) and
E(α) using the following expression:
at any temperature profile
such as isothermal, non-isothermal, stepwise, modulated temperature or periodic
temperature variations, etc.
Description of the Oxidation of Natural Rubber
Figure 6. Reaction rates
(normalized CL-signals after baseline subtraction) for oxidation of
unstabilized (left) and stabilized (right) natural rubber with different
heating rates (0.0054 – 0.073 K min-1). Experimental
data are represented as symbols; solid lines represent the calculated signals.
The values of the heating rates are marked on the curves.
The CL signals collected during the oxidation of unstabilized and stabilized
natural rubber under non-isothermal conditions at different heating rates were
used for the determination of kinetic parameters later used for the prediction
of the reaction progress. The normalized reaction rates determined by
AKTS-Thermokinetics software are depicted in Figure 6.
Figure 7. Prediction of the
oxidation progress of unstabilized (left) and stabilized (right) natural rubber
at isothermal temperatures between 4° and 40°C.
the kinetic parameters are determined, they can be applied to predict the
course of oxidation under different temperature profiles. The presented results
clearly indicate the oxidative induction period after which the rate of
oxidation accelerates rapidly. The prediction of the oxidation of the natural
rubber under isothermal conditions at low temperatures (4-40°C) is shown in
Figure 8. Prediction of the
oxidation progress of unstabilized (left) and stabilized (right) natural rubber
at modulated isothermal temperature of 20°C,
amplitudes 0, 5, 10 and 20 K each 24 h. Note that the average temperature is
the same for all temperature profiles. Depending on the type of reactions and
stabilization, it has been shown that temperature fluctuations can have
significant influence on the reaction rate.
The most important goal in
the investigation of the kinetics of thermal decomposition is the determination
of the thermal stability of substances (i.e., the temperature range over which
a substance does not decompose at an appreciable rate). The correct prediction
of the reaction progress of materials that are unstable under ambient
conditions requires accurate calculation of both kinetic parameters and the
exact experimental temperature profile.
The example showing the prediction of the properties of rubber under a more
complicated temperature profile is illustrated in Figure 8, which presents the
oxidation progress of natural rubber at 20°C when the temperature changes with
the modulations of 0, 5, 10 and 20 K each 24 h. The dependences shown in Fig. 8
indicate that even small temperature fluctuations can significantly change the
stability of the substance (e.g., the amplitude of 10K at 20°C lowers the
oxidation stability of natural rubber by half of lifetime).
In general, calculations can be achieved for any fluctuation of temperature
that makes the prediction of thermal stability properties for varying climates
Exact consideration of the calculations of daily minimal and maximum
temperature variations of worldwide climates therefore provides very valuable
insight when interpreting and quantifying the reaction progress of materials
subjected to atmospheric conditions.
Figure 9. Average daily
minimal and maximum temperatures recorded for each calendar date between 1961
and 1990 (New York and Hong
Oxidative degradation of
polymers can be monitored by the chemiluminescence method. This method is an
order of magnitude more sensitive than conventional methods of thermal analysis
such as Differential Scanning Calorimetry, Differential Thermal Analysis or
Thermogravimetry. The data acquired during the chemiluminescence experiments
carried out iso- or nonisothermally can be evaluated by differential
isoconversional kinetic analysis to obtain meaningful and accurate predictions
of the lifetime of organic materials in temperature domains that are
representative of the life-cycle of the planned products. Within the same
context, the efficiency of stabilizers or influence of environmental factors
such as relative humidity, UV-radiation and pollutants can be forecast.
Figure 10. Prediction of the
oxidation progress of unstabilized (left) and stabilized (right) natural rubber
at different climatic locations (New York and Hong Kong).
equipment used for the data collection was developed by AKTS-Chemiluminescence.
This is a powerful tool, fully automated and supported by user-friendly yet
powerful software. It enables formulators of adhesives and coatings to evaluate
their products with respect to oxidative aging in order to make meaningful
predictions of the aging behavior.
For more information on age simulation, visit www.collano.com or