Polymer Degradation and Stability 70 (2000) 31±37
www.elsevier.nl/locate/polydegstab
Quantifying rubber degradation using NMR
A.E. Somers a, T.J. Bastow b, M.I. Burgar b, M. Forsyth a,c, A.J. Hill b,*
a
Department of Materials Engineering, Monash University, Clayton, VIC 3800, Australia
CSIRO Manufacturing Science and Technology, Private Bag 33, S. Clayton, MDC 3169, Australia
c
Department of Chemistry, Monash University, Clayton, VIC 3800, Australia
b
Received 24 February 2000; accepted 23 March 2000
Abstract
Ageing can lead to the degradation of the tensile properties of natural rubber. The ageing process causes changes in the polymer
segmental motion as well as the chemical structure, both of which can be monitored using nuclear magnetic resonance (NMR)
spectroscopy. This work demonstrates that NMR can quantify rubber degradation due to ageing, and also that relatively simple
NMR equipment can be used. This simpler equipment can be made portable and so could give a simple and fast indication of the
condition of rubber in service. The 1H NMR transverse relaxation time, T2 , and the 13C NMR spectrum using cross polarization
and magic angle spinning (CP MAS) for samples taken at various levels of a degraded natural rubber liner were compared. These
experiments showed that, as the level of degradation increased, the 1H NMR transverse relaxation time decreased. The 13C spectra
showed considerable peak broadening, indicative of decreased mobility with increased level of degradation as well as the presence of
degradation products. Further investigations using lower powered NMR equipment to measure the 1H NMR transverse relaxation
times of two di€erent series of natural rubbers were also performed. This work has shown that this simpler method is also sensitive
to structural and mechanical property changes in the rubber. This method of monitoring rubber degradation could lead to the nondestructive use of NMR to determine the condition of a part in service. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Rubber degradation; NMR spectroscopy
1. Introduction
As a result mainly of its ¯exibility, wear and chemical
resistance, rubber is utilized in many applications, especially in the mining and transport industries. Rubber is
used for such things as tyres (wear resistance, ¯exibility),
conveyor belts (¯exibility, chemical and wear resistance)
and processing tank liners (chemical and wear resistance).
Unfortunately, rubber performance with time degrades
due to ageing, and the extent of this degradation is very
dicult to predict [1]. This is because the rate at which a
rubber degrades depends on many factors such as the
operating temperature, chemical environment, loading
conditions and type of rubber [1]. Ageing results in a
loss of ¯exibility, abrasion resistance and elasticity. In
some abrasive or erosive applications this time scale of
degrading properties is not a concern as the rubber will
be worn away before any serious loss of properties
* Corresponding author. Tel.: +61-3-9545-2665; fax: +61-3-95441128.
E-mail address: hill@cmst.csiro.au (A.J. Hill).
occurs. For instance in most passenger car tyres and
conveyor belts transporting highly abrasive material the
part is worn out before any signi®cant ageing occurs.
However, there are applications in which the rubber is
in service long enough for the rate of degradation to
determine its useful service life, such as conveyor belts
transporting non-abrasive material and the sidewalls of
re-treaded truck and aircraft tyres. In some cases this
unpredictability of degradation rate can lead to catastrophic failure in service such as a conveyor belt
breakage or tyre `blow out', which cause downtime and
can be very dangerous [2]. At present inspection techniques
for rubber condition rely upon observing the subsequent
e€ects of ageing, which in many cases can be too late to
prevent failure. For example, for steel cord reinforced
conveyor belts electromagnetic techniques are used to
observe the integrity of the steel cords within the rubber
[3]. This technique can identify corrosion and delamination of the steel cord exposed to the atmosphere as a
result of the rubber cracking and de-bonding due to age.
Other inspection methods rely upon visually identifying
cracks or tears in the rubber, which occur only after the
0141-3910/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0141-3910(00)00076-8
32
A.E. Somers et al. / Polymer Degradation and Stability 70 (2000) 31±37
rubber has degraded signi®cantly. These techniques do
not assess the rubber condition; they can only identify
the results of rubber failure, which often occurs in a
very abrupt manner after signi®cant degradation has
occurred.
Rubbers consist of long, ¯exible polymer chains interconnected via physical or chemical cross-links. The
density of cross-links for most applications must be
sucient to give the rubber mechanical integrity so that
it can bear loads and recover deformation. However,
the cross-link density should not be so high that the
polymer chains are immobilized, which would lead to a
hard, brittle rubber. The optimum cross-linking between
polymer chains gives a rubber the properties of good
¯exibility and elasticity leading to high wear resistance
and resilience to ¯exing Ð two of the most important
features of rubbers. Rubber ageing is a process in which
a rubber loses some of its ¯exibility and elasticity with
time due to chemical reactions occurring between the
rubber and its environment. These reactions may
change the polymer chain length, cross-link density and/
or chemical structure.
The two main ageing processes in rubbers are oxidation
and ozonolysis. Oxidation ageing generally occurs quite
slowly at ambient temperatures, but is of greater consequence as the temperature is increased [4]. Oxidation
of rubbers involves the reaction of free radicals present
in the rubber with molecular oxygen. The free radicals
can be formed by the decomposition of hydroperoxides,
which are present in the rubber in minute amounts after
processing. These reactions then lead to chain scission
and/or increased cross-linking, depending on the type of
rubber. For instance, oxidation of natural rubber is
initially dominated by chain scission, which causes the
rubber to soften. This softening is followed by an
increase in cross-linking, which then leads to hardening.
In the case of styrene±butadiene rubbers increased
cross-linking dominates from the beginning leading to
hardening of the rubber [4]. Both processes lead to a
weaker, brittle polymer.
Anti-oxidants incorporated in the rubber work either
by reacting with any polymer peroxides present to form
stable end groups, or by reacting with free radicals
before they can react with the polymer main chain.
At ambient temperatures the main ageing mechanism
involves the presence of ozone in the atmosphere. Even
the low ozone levels present in the atmosphere without
pollution (as low as 0.1 ppm) are sucient to cause
degradation in rubbers with main chain unsaturation.
This is due to the high reactivity of ozone with carbon±
carbon double bonds (i.e. unsaturated carbon bonds)
present in rubbers such as natural rubber and styrene
butadiene rubber [5]. The reaction with ozone leads to
chain scission and the formation of polymeric peroxides,
which can also increase the rate of oxidative ageing [6].
Anti-ozonants added to the rubber network hinder
ozonolysis by reacting with ozone before it can react
with the carbon±carbon double bonds.
Despite the additions of anti-ozonants and anti-oxidants
to rubbers the ageing processes still occur, if at a somewhat reduced rate. As mentioned, this rate of ageing
depends on many factors, such as operating temperature, chemical environment and loading conditions,
making it dicult to predict the condition of the rubber.
It is therefore important to ®nd characterisation tools
that can be readily used to assess the extent of ageing in
rubbers.
Nuclear magnetic resonance (NMR) spectroscopy is a
characterisation technique that can identify di€erent
molecular environments and, in conjunction with NMR
relaxation techniques, can be used to characterise molecular motions occurring in the NMR time scale (kHz to
MHz frequencies). In the case of rubber ageing, processes
are occurring in which both the molecular environment
and the molecular motion of the polymer chains change
due to chain scission, increased crosslinking and the
presence of degradation products. Speci®c molecular
changes can be accurately characterized with high-®eld
laboratory NMR equipment (magnetic ®elds >7 Tesla)
[7±9] whilst bulk average motional changes can be
determined using much simpler, less costly NMR
equipment which operate at lower magnetic ®elds (<1
Tesla). This type of lower-®eld equipment can be used in
a portable form and has been utilized to investigate such
things as sulfur contents in rubbers [10,11] and water
content of mineral formations via bore holes [12].
To observe the speci®c chemical changes the resonance frequencies (chemical shifts) of the 13C nuclei,
dispersed into a multi-line spectrum of the chemically
distinguishable molecules, are used for tracking degradation reactions and products. Information about the
molecular motion can be obtained from the transverse
(spin±spin) relaxation of 1H, that is, observing the time
after the protons are pulsed for the magnetic moments
to dephase. The transverse relaxation experiments are
performed by observing the protons, as opposed to the
13
C nucleus, as it would take far too long to obtain a
useful result using 13C. This is because the natural
abundance of 13C is less than 1% and the gyromagnetic
ratio 1/4 that of 1H. This low abundance means there is
little dipole±dipole interaction between 13C nuclei and
so signals received from this nucleus are very speci®c,
leading to a spectrum with narrow peak widths, which
are extremely useful for identifying chemical structure.
However, this low abundance and low gyromagnetic
ratio results in a relatively weak signal. Since 1H is
almost 100% abundant and has a higher gyromagnetic
ratio, a much stronger signal is obtained and so much
fewer scans are required to achieve a useful result.
However, since the protons from di€erent chemical
positions interact so well the speci®city of the signal is
reduced, resulting in broad peaks in the 1H NMR spectra.
A.E. Somers et al. / Polymer Degradation and Stability 70 (2000) 31±37
Fortunately, for relaxation experiments this general
response is sucient to obtain an accurate result for
molecular mobility. Lower-®eld NMR equipment,
robust and portable enough to be used in the ®eld, is
also better suited to 1H relaxation measurements. The
low ®eld used by such equipment means the signal to
noise ratio is lower than for higher-®eld equipment and
so it would not be possible to distinguish the signal from
the noise if the 13C nuclei were targeted.
The transverse relaxation curve obtained from a sample can give information relating to the elasticity and
¯exibility of a material. For `simple' systems, such as
single component solids or liquids, the transverse
relaxation curve generally follows the relationship:
M t† ˆ exp ÿt=T2†
1†
From Eq. (1) the transverse relaxation time, T2 , is
extracted. The more restricted a molecules motion is, i.e.
a solid as opposed to a liquid, the faster it will return to
its initial condition (relax) after being excited and so the
shorter its T2 . Unfortunately, in the case of rubber the
relaxation curve is not so simple and this is due to the
complex nature of rubber, as reported in the literature
[7±9]. Rubber is a multi-component material, showing
solid and liquid like behaviour. This leads to a complicated NMR relaxation curve, due to a distribution of
correlation times for molecular motion in the rubber.
The main forms of molecular motion within a rubber,
which a€ect the transverse relaxation, are assumed to be
the local segmental motions within the polymer chain,
the overall motion of polymer chains between cross
links and the motion of dangling chain ends [7]. The
transverse relaxation function for rubbers is generally
described as being due to two main contributing factors
[7±9]. The ®rst is that of a gaussian behaviour corresponding to slight anisotropic behaviour of the local
segmental motions within the chain, while the second is
due to a pure exponential portion corresponding to the
motions of dangling chain ends and chain lengths
between cross-links. The motion of any small free
molecules can form a long exponential tail, which is
usually subtracted or ignored. The function can be
approximated by [9]:
ÿt
t2
ÿt
ÿ qM2
2†
‡ B exp
M t† ˆ A exp
2
T2
T2
where A represents the weight fraction of the network
chains and B the weight fraction of the dangling chain
ends, T2 is the transverse relaxation time, q is a measure
of the anisotropic nature of the local chain motions and
M2 is the second moment of their interaction. For this
equation to be applicable the transverse relaxation
curve obtained, M t†, must only be due to the dipolar
interactions of a certain group of proton couples, such
33
as methylene. When using the high powered NMR
equipment it is possible to observe the relaxation of a
single group, and thus use Eq. (2). In our experiments
using equipment with a lower ®eld, however, the dipolar
interactions of all protons must be observed in order to
receive a strong enough signal. This means the transverse relaxation curve will be a composite of all these
groups relaxation, since the various groups of protons
involved in the decay of magnetization each have different responses [13]. Thus, Eq. (2) cannot be used to ®t
the transverse relaxation curve obtained from the lower
®eld equipment. For this study we assume that the rubber
relaxation curves follow a simple relationship, such as
Eq. (1), and obtain an overall relaxation time, T2 . For a
given rubber we assume that any changes in this T2 are
due to degradation. This assumption can be made since,
from Eq. (2), we see that any changes in the relaxation
behaviour are due to changes in the density of crosslinks and/or dangling chain ends. These changes will
only occur in a rubber through degradation or changes
in the formulation. Whilst this approximation means
the degradation process is not as fully characterized as it
could be, it gives a simple method for quantifying the
level of degradation. The goal is then to correlate the
level of degradation to performance.
The ultimate aim of this study is to identify a potential
method for regularly assessing the condition of a rubber
so its performance can be predicted and replacement
can be scheduled before catastrophic failure occurs. In
the present study we report on experiments using NMR
to observe changes occurring within rubber due to
degradation. We also characterize the behaviour of
rubber samples of varying formulations. The NMR
results are then compared to the mechanical performance of the rubber.
2. Experimental
2.1. Materials
Three sets of natural rubber samples were used for the
investigations detailed here:
. Series (i): Initial investigations to determine if
NMR could quantify degradation were carried out
on a tank lining that had been exposed to chemical
attack at elevated temperatures. The liner was
analyzed because of the large di€erence between
the condition of the surface, which was obviously
degraded, being hard and cracked, and the bulk,
which still appeared to be in good condition.
. Series (ii): A series of six industrial natural rubbers
were then examined. These samples were cured
using a semi-ecient vulcanization (semi EV) cure
system, plus additional free sulfur ranging from 0
34
A.E. Somers et al. / Polymer Degradation and Stability 70 (2000) 31±37
to 3.5 parts per hundred rubber (phr). Semi EV
uses low amounts of sulfur (0.5±1.5 phr) and high
amounts of accelerator (2.5±1.5 phr) as compared
to a conventional cure system (2±5 phr sulfur and 1.5±
0 phr accelerator). A sample with a semi EV cure
system and no extra free sulfur will predominately
contain mono and disul®dic cross-links [4]. As free
sulfur is added, the cross-link length and density
will increase. These materials were chosen to
investigate the e€ect of cross-link type and density
separate from any degradation products. The aim
of studying this series was to see if the low ®eld
NMR equipment could identify known changes
within the rubber to a similar level as the higher
®eld instruments.
. Series (iii): Finally, a single composition set of 16
industrial natural rubber samples were thermally aged
at 100 C for periods of 0±240 h. These samples were
investigated to evaluate low ®eld NMR techniques
when the complexities of degradation processes are
introduced in a controlled laboratory environment.
Tensile tests were also performed on series (ii) and (iii)
according to the standard method ASTM-D378 for
comparison to the T2 values.
3. Results and discussion
Fig. 1 shows the 13C CPMAS NMR spectra obtained
from sample series (i), the aged rubber liner. Spectra
were obtained for samples taken from the surface, subsurface (about 1 mm below the surface) and the bulk. At
the top of Fig. 1 the chemical structure of the main
repeat unit of natural rubber is shown, as well as the
2.2. Procedures and apparatus
Several NMR techniques were used to investigate the
above samples.
Series (i) was examined using 13C high resolution
NMR with cross polarization (CP) and magic angle
spinning (MAS). The 1H transverse relaxation times, T2 ,
were also obtained for the series using a Carr±Purcell
Meiboom±Gill (CPMG) pulse sequence [14]. These tests
were conducted using a Varian Unity 300 NMR spectrometer operating at 75.45 MHz for the 13C and 300 MHz
for the protons.
1
H transverse relaxation times were obtained for series
(ii) and (iii) using low-®eld equipment, and these were
compared to results obtained using high-®eld equipment.
For the low-®eld experiments a Bruker IBM PC/20B
Minispec spectrometer operating with a magnetic ®eld
of 0.47 Tesla and a frequency of 20 MHz was used. For
this equipment a Hahn spin-echo pulse sequence was
found to give the best results. For the high-®eld experiments series (ii) was tested using a Bruker MSL 400,
which has a magnetic ®eld of 9.4 Tesla and operates at a
frequency of 400 MHz for protons while series (iii) was
tested using the Varian Unity 300, having a ®eld of 7.1
Tesla and a frequency of 300 MHz. For these high-®eld,
transverse relaxation experiments, the CPMG pulse
sequence was used.
The 1H NMR transverse relaxation curves obtained
from the low- and high-®eld experiments for all series
were ®tted to an exponential equation of the type:
ÿ
M t† ˆ M 0† ‡ A exp ÿt=T2
from which
T2
was extracted.
Fig. 1. 13C CPMAS spectra for samples of natural rubber taken from
di€erent depths of a used liner.
A.E. Somers et al. / Polymer Degradation and Stability 70 (2000) 31±37
proposed peak assignments for the resonance frequencies of the typical carbon atoms relating to the
spectra. While only minor changes in the chemical
structure can be seen between the bulk and the sub-surface
spectra, there is a dramatic di€erence between those of
the surface and the sub-surface. Two main di€erences in
the spectra are clearly visible. Firstly, it can be seen that
the peaks for the surface spectra have become much
broader. This is mainly due to the fact that the resonance line width is related to the segmental polymer
motion. If the molecule is free to move in any direction
then the spectrum line width is narrow. If the motion of
the molecule becomes restricted then the line width
increases and this indicates that the material is becoming
more `solid-like'. In terms of rubber degradation processes
this would be consistent with the formation of extra
cross-links restricting the motion of molecules. The second
obvious di€erence in the spectra are the appearance of
extra peaks in the spectra of the surface sample, which
can be identi®ed as belonging to the degradation products
resulting from oxidation of the carbon±carbon double
bonds. In this case the oxidation products, CHnO, can
be seen in the region of 70±90 ppm. This is consistent
with studies done on natural rubber degradation using
XPS [15] and NMR [16] which shows the presence of
epoxides, peroxides, hydroperoxides and alcohols as a
result of degradation. The di€erences between the spectra
of the sub-surface and the bulk in Fig. 1 are much subtler than between the surface and the bulk. Slight
broadening of the peaks and changes in the relative
intensities of the sub-surface spectra can be seen. Since
the spectra were obtained using cross-polarization the
absolute integrated intensities of these peaks are not
related to quantities but rather the e€ectiveness of the
cross-polarization, which is clearly changing. This could
mean that the interaction between certain protons and
carbon atoms is changing, presumably due to the
degradation process.
From this experiment we can see that the degradation
of the natural rubber article can be clearly identi®ed
using NMR. By measuring the spectra at di€erent
depths in the material we have e€ectively investigated
the degradation at various stages, from the bulk, which
appears una€ected, to the surface, which is in very poor
condition. However, although the 13C NMR spectra are
qualitatively useful in identifying degradation products,
it is dicult to use such information quantitatively.
From the 13C spectra in Fig. 1 we see that the peaks
become broader as the degradation proceeds, which, as
mentioned earlier, is due to molecular motion changes.
As discussed, a quantitative measure of molecular
motion that can be obtained using NMR is the transverse relaxation time, T2 , which could be used to give an
indication of amount of degradation.
Table 1 shows the 1H NMR T2 values of the methylene group for the same samples used to obtain the
35
Table 1
Transverse relaxation time (T2 ) at various depths of a degraded natural rubber liner
Sample position
T2 (ms)
Bulk
Sub-surface
Surface
2.496
1.329
1.225
spectra in Fig. 1. In this case a dramatic di€erence can
be seen between the bulk and the sub-surface. In fact the
sub-surface value for T2 is actually much closer to that
of the surface. This is to be expected if the degradation
is progressing through the material. These result show
that the 1H T2 measurements are sensitive to the degradation of the liner. The 1H T2 measurements are less
subjective than the 13C spectra as they give a quantitative measure of the level of degradation. Another
advantage of measuring 1H T2 over 13C spectra is the
reduction from hours to minutes in the time needed to
gather reliable data.
Fig. 2 shows the results for the T2 values as a function
of free sulfur content obtained for series (ii) using highand lower-®eld NMR. It can be seen that the trends are
similar for both the high- and lower-®eld instruments.
This is an important result as it shows that both the
much simpler low-®eld equipment and the high-®eld
instrument are sensitive to changes in the rubber structure. Both results show the T2 values going through a
minimum at around 1.5±2.5 phr of free sulfur. This
indicates that the segmental polymer motion decreases
with the addition of sulfur to a point, and then begins to
increase, or level out, as more sulfur is added. In Fig. 3,
we see the tensile strength plotted against free sulfur
content for series (ii). Initially the tensile strength
increases with free sulfur amount until around 1.5 phr
of free sulfur, after which it drops rapidly and appears
to level out. This trend is almost the exact opposite as
that observed for the T2 s in Fig. 2, especially that seen
Fig. 2. Comparison of transverse relaxation times (T2 ) obtained using
low- and high-®eld NMR equipment of a semi EV cure natural rubber
with 0±3.5 phr of additional free sulfur.
36
A.E. Somers et al. / Polymer Degradation and Stability 70 (2000) 31±37
Fig. 3. Tensile strength of semi EV cured natural rubber samples with
0±3.5 phr of additional free sulfur.
Fig. 4. Comparison of transverse relaxation times (T2 ) taken using
high- and low-®eld NMR equipment for natural rubber aged in an
oven at 100 C.
using the lower ®eld equipment. From these results we
may assume that the same changes occurring within the
rubber are responsible for both T2 and the tensile
strength. As the free sulfur content is increased two
changes will occur within the rubber. Firstly, the crosslink density will increase and secondly the length of the
cross-links will increase. In the literature [17] it is stated
that this characteristic change in the tensile strength
with amount of sulfur for natural rubber is due to strain
induced crystallization. For lower amounts of sulfur,
and hence lower cross-link density, when a force is
applied to the rubber the polymer chains become
aligned in the direction of tension (i.e. strain-induced
crystallization) giving the rubber good tensile strength
properties. As more sulfur is added the cross-link density
increases and when tension is applied the chains will
again align in the direction of tension, but will not slide
past each other to the same extent since more of them
are now connected. This means more force must be
applied to either break the cross-links or polymer chains
for failure to occur. It is also known that as cross-link
length increases the tensile strength increases [4] and so
both an increase in cross-link density and cross-link
length may be contributing to the increase in the tensile
strength. However, as more sulfur is added the polymer
chains will become so highly cross-linked that they will
no longer be able to align to the direction of the applied
force when a load is applied and, thus, the tensile
strength decreases. As for the observed trend in the T2 ,
as the cross-link density increases, the polymer segmental motion becomes more restricted, thus leading to
a decrease in T2 . As the cross-link length increases, the
free mobility of the chain segments increases [4], and so
T2 becomes longer.
Presented in Fig. 4 is the change in T2 with ageing
time at 100 C for the series (iii) samples tested with
high- and lower-®eld NMR. Initially, as the ageing time
increases, the T2 decreases rapidly for both test methods,
while at longer ageing times this rate of decrease in T2
appears to plateau. The result obtained using the high
powered equipment is consistent with similar experiments reported using such equipment [9]. The initial
decrease in T2 values is obviously greater for the tests
conducted with the high-powered equipment. This
initial di€erence in T2 behaviour may be because the
lower-®eld equipment is not as sensitive to all of the
changes occurring within the rubber as the high-®eld
equipment may be. In this series of samples the changes
within the rubber are quite complex. The trend seen
using the high-®eld equipment has been explained in
work by Knorgen et al. [9]. At short ageing times (0±20
h) there is a rapid decrease in the T2 , which is said to be
due to further cross linking from any unreacted sulfur
[9], at the end of which the cross-linking agent has
reacted fully. After the initial rapid decrease there is an
increase in T2 [9] due to cross-links being broken, and
this process is known as reversion. The T2 then decreases
again as bonds in the polymer backbone are destroyed
leading to radical cross-linking and chain scission. It
appears from Fig. 4 that the lower powered equipment
is not as sensitive to the changes in the rubber over the
initial stages of ageing. This may be due to the lack of
speci®city of the low-®eld equipment. At longer ageing
times, though, the two methods follow the same trend.
These results indicate that at the long ageing times we
are concerned with, the low-®eld method is as sensitive
to the changes within the rubber due to degradation as
the higher-®eld method.
Since one of the main aims of the study was to compare the level of degradation as obtained using T2 to a
performance level, Fig. 5 shows a comparison between
tensile strength and T2 with ageing time. At the shorter
ageing times the T2 value decreases at a faster rate than
the tensile strength, while at longer ageing times the T2
seems to decrease at a slower rate than the tensile
strength. As mentioned, the changes within the rubber
due to the ageing are quite complex, and the various
stages of cross-link completion followed by reversion
and then radical cross-linking and chain scission may
e€ect the tensile and T2 values di€erently. Since the area
A.E. Somers et al. / Polymer Degradation and Stability 70 (2000) 31±37
37
References
Fig. 5. A comparison between the tensile strength and transverse
relaxation time (T2 ), obtained using low-®eld NMR equipment, for
natural rubber samples aged at 100 C.
of importance is that of longer ageing times, where the
degradation process is somewhat simpler, it appears
that a relationship between these values could be developed. Such a relationship would be very useful in the
non-destructive evaluation of rubber parts in service.
To fully understand the e€ects of both the radical
cross-linking and chain scission further investigations
are required. Work is currently being undertaken to
further develop a relationship between T2 and performance.
4. Conclusions
The problem of evaluating rubber degradation in service
over time is well known. It has been shown that high-®eld
nuclear magnetic resonance spectroscopy is a technique
that can be used to quantify this level of degradation.
Relatively simple, lower-®eld NMR equipment was also
shown to be sensitive to changes due to degradation
within rubber. It was also shown that it might be possible
to relate the degradation level evaluated with such
techniques to performance variables, such as tensile tests.
These techniques could lead to testing rubber articles
non-destructively in the ®eld using transverse relaxation
measurements and predicting when these rubber parts
should be replaced to prevent unexpected failure.
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