Quantitative determination of antioxidants in latex and latex
compounds
Dr Jean-Luc Madelaine
Eliokem, 14 Avenue des Tropiques, Z A de Courtaboeuf 2, Villejust, 91955 Courtaboeuf Cedex, France
Jean-luc Madelaine received his M.S in Organic
Chemistry from the University of Caen and Ph.D. in
Chemical Engineering from the University of Compiègne
in France.
He joined Goodyear Chemicals Europe (now Eliokem) in
1986 as an engineer in charge of the process optimization
of antioxidants and antiozonants. Since then, he spent
most of his career in the field of antioxidants, from product
and process development to application development and
customer technical service.
Jean-luc is currently technical manager for the
antioxidants at Eliokem European technical center, which
employs about 30 scientists and technicians, located at
Eliokem Headquarters in Villejust, 20 km south of Paris in
France.
INTRODUCTION
Phenolic antioxidants are commonly added to synthetic
latex and latex compounds as well as to natural rubber
compounds in order to protect the polymer and the
finished articles against oxidation during storage,
transport and service life. Typical addition levels are 0.3
to 0.5 phdl (parts per hundred dry latex) in synthetic latex
and 0.5 to 1.5 phdl in natural rubber latex compounds
used for thin gloves or elastic thread manufacturing.
Quantitative determination of the antioxidant into the
latex, the latex compound or the finished product may be
necessary for various purposes such as product and
application
development,
troubleshooting
studies
(typically claims investigations, quality issues in
production), customer technical support…
Most often, such analyses are performed in order to
check that the polymer contains the expected
concentration of the additive or to assess how much of
the additive is remaining in the polymer after storage or
ageing.
This paper will review briefly the most commonly used
technique for antioxidant quantification based on
extraction followed by a chromatographic analysis and its
limitations in the case of multi-component antioxidants or
antioxidants blends.
Another approach based on the use of differential
scanning calorimetry (DSC) will then be discussed and
illustrated with some results in synthetic latex.
Antioxidants description
3 different antioxidants have been used in the various
examples reported in this paper: the monophenol BHT
and 2 antioxidants from Eliokem sold under the
®
Wingstay trade name for the protection of synthetic latex
(typically
carboxylated
acrylonitrile-butadiene
and
styrene-butadiene latex) and natural rubber articles:
-
BHT (butylated hydroxy toluene)
®
Wingstay L: the butylated reaction product of pcresol and dicyclopentadiene
Wingstay® T: butylated, octylated phenol
-
The chemical formula and average molecular weight of
these antioxidants are reported figure 1.
OH
OH
OH
CH3
CH3
CH3
1,2…n
®
BHT
Mw = 220
Wingstay L
Mw = 650
OH
R2
R1
R3
R1, R2, R3 = H, C4H9 or C8H17
®
Wingstay T
Mw =340
Figure 1: Antioxidants formula
THE “ANALYST APPROACH”
Numerous methods have been used for identifying and
quantifying additives from polymers and rubbers (1).
These methods can be divided into direct determination
methods and methods which require preliminary
extraction of the additive from the polymer matrix.
Although spectrometric techniques (UV, IR, Raman)
which belong to the first type have seen recent advances
with the use of Chemometrics, the preferred methods for
the analyst remain the extraction techniques followed by a
chromatographic analysis which allows separation of the
extract into individual additives in order to facilitate their
identification and quantification.
Various extraction procedures can be used for separation
of the additives from a polymer: solvent extraction
(Soxhlet solvent extraction, solvent reflux extraction,
superfluid extraction, microwave solvent extraction…),
fractional precipitation, fractional extraction…
The most commonly used extraction procedure for
separating antioxidants from rubbers or rubber
compounds is the solvent extraction using typically
alcohols (methanol, ethanol), ethers and ketones
(acetone, MEK).
The choice of the chromatographic technique depends on
the chemical structure of the antioxidants to be quantified.
Gas chromatography which has been used extensively for
over 25 years for measuring trace amounts of volatile
components and especially residual monomers in
synthetic polymers is the preferred method for low
molecular weight antioxidants. With the use of capillary
columns, it offers a very good separation resolution, low
detection limits as well as possible identification of the
products when coupled with Mass Spectrometry.
Liquid chromatography, especially reverse phase HPLC,
is the most widely used separative technique for additives
allowing analysing higher molecular weight additives than
Gas chromatography. Gel Permeation Chromatography
which is very close to HPLC technique can also been
used for rubber extracts.
It has to be noted that beside these separative
chromatographic methods, a popular method for
estimating phenolic antioxidants in polymer extracts is
direct analysis of the extract by UV-visible spectroscopy
which consists in oxidising or coupling the antioxidant to
form coloured products and measuring the resulting
absorbance in the visible region. However, this method is
not specific for individual antioxidants but for phenolic in
general.
An easy case: Quantification of BHT in NBR rubber
The extraction–chromatography technique is illustrated
below by the quantification of BHT - a volatile low
molecular weight antioxidant which can be easily
quantified by Gas Chromatography- into NBR rubber.
The method consists simply in extracting BHT from the
rubber with methanol during 24 hours at room
temperature and then analysing the extract by Gas
Chromatography on a capillary column with flame
ionisation detection.
Figure 2 shows the GC
Chromatogram of the extract where BHT is easily
identified.
measured
BHT, %
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
0.5
1
1.5
2
added BHT, %
Figure 3: Measured vs. added BHT in NBR rubber
Extraction-chromatography technique for
quantification of antioxidants in latex
The above example is an ideal case with a singlecomponent antioxidant, easily extracted from the rubber,
and added at a fairly high dosage.
Most synthetic latex are today stabilised with higher
molecular weight antioxidants which are often not a single
component material but multi-component products, and
generally at quite low dosages, below 0.5 phdl.
Typical examples of chromatographic fingerprints of such
materials are reported figure 4 (CG analysis) and 5 (GPC
analysis).
Obviously,
the
quantification
by
chromatographic analysis of such materials becomes
more difficult because firstly the level of each individual
component is lower than the antioxidant dosage and
secondly the optimization of the analytical conditions,
including the calibration, for such products requires a
good knowledge of their chemical composition.
FID1 A , (WS07044\003F0301.D)
c ounts
140000
120000
FID1 A, (CGP04091\CGP00007.D)
counts
100000
BHT
60000
80000
50000
60000
40000
40000
30000
20000
20000
0
2.5
5
7.5
10
®
Figure 4: Gas chromatogram of Wingstay
octylated phenol)
10000
0
2.5
5
7.5
10
12.5
min
Figure 2: Gas chromatogram of extract from NBR rubber
3 samples of NBR stabilised respectively with 0.55, 0.90
and 1.8% BHT were analysed using this procedure. The
measured BHT content were found at 0.6, 0.9 and 1.7%
showing a good fit with the expected values as shown in
Figure 3.
12.5
min
T (butylated
In addition, it is quite common to use blends of
antioxidants in synthetic latex -such as blends of phenolic
antioxidants, synergistic blends of phenolic and thioesters
antioxidants- which also make the quantification more
difficult.
material but more difficult for the antioxidant user such as
latex producers or manufacturers of finished latex goods.
A DC1 B, A DC1 CHA NNEL B (MH01\MH010003.D)
dRI
30
29
QUANTIFICATION OF ANTIOXIDANTS BY DSC
28
27
Oxidative Induction
Induction Time
26
25
24
23
22
20
22.5
25
27.5
®
30
min
Figure 5: GPC chromatogram of Wingstay L (butylated
reaction product of p-cresol and DCPD)
Another limitation with the extraction-chromatography
technique resides in the extraction process itself which
sometimes gives yields lower than 100% due for example
to losses in the case of volatile antioxidants or incomplete
extraction with high molecular weight materials. This may
result in determined quantities lower than expected in the
latex or in the finished product. In this respect, it is
advisable to include in the analysis a reference sample
with a known antioxidant content, but such samples are
not always available.
Figure 6 below shows the result of the quantification of
®
Wingstay L, the workhorse antioxidant for natural rubber
gloves, in powdered and chlorinated gloves as a function
of the theoretical content added in the gloves, determined
by extraction with HITM solution (isopropanol / toluene /
water mixture) at room temperature followed by Gas
Chromatographic analysis of the extract. (2)
1
Temperature
and
Oxidative
Differential Scanning Calorimetry (DSC) belongs to the
thermal analysis techniques, in which a physical property
of a substance is measured versus a reference as a
function of temperature.
In DSC, energy changes (heat flows) as a function of
temperature are measured. It is used extensively in
polymer science for precise determination of glass
transition temperatures (Tg) and crystallisation and
melting temperatures of polymeric materials, due to the
endothermic
or
exothermic
nature
of
these
transformations.
When the polymer is subjected to an oxidising
atmosphere - such as air or oxygen - during the heating
process in the DSC oven, the oxidative stability of the
material can be assessed by determination of the
temperature at which the exothermic degradation begins.
Figure 7 shows a typical DSC curve for a carboxylated
SBR latex film heated from 50 to 250°C at 2°C/min under
an Oxygen flow of 50 ml/min.
The start of the oxidative process is determined as the
onset of the DSC curve and is known as the OIT,
Oxidative Induction Temperature, expressed in Celsius
degrees.
^exo
m easured
conc. (phr)
0.8
0.6
0.4
OIT=207.0°C
Onset 208.98 °C
0.2
Pow der gloves
Chlorinated gloves
60
0
0
0.4
0.6
0.8
Added level (phr)
1
1.2
®
Figure 6: Dosage of Wingstay L in NR gloves.
The amount determined in the gloves is about 70% of the
theoretical levels for the powdered gloves and 55% for
the chlorinated gloves. It is not possible to assess if these
lower than expected levels are due to incomplete
extraction from the gloves or to losses during the
production of the gloves, the later assumption being
unlikely due to the high persistence of this antioxidant in
the polymer.
As a conclusion about the analyst approach, we can state
that for the determination of low levels of complex
antioxidants in latex and latex products, the development
of specific chromatographic conditions for each
antioxidant is required, which is achievable by the
antioxidant producers knowing well the composition of the
80
10
ELIOKEM: SL
100
20
120
30
140
40
160
50
Figure 7: DSC curve of X-SBR film
180
60
200
70
220
80
240 °C
90
min
e
METTLER TOLEDO STAR System
Comparison of OIT values is widely used for investigating
the efficiency of antioxidants in polymers and more
specifically in polyolefins (3).
A variation to the measurement of the Oxidative Induction
Temperature run under a temperature gradient consists in
running an isothermal test in which the material is initially
heated in an inert gas (nitrogen) to a given temperature,
after which the atmosphere is changed to air or pure
oxygen and the time to the exotherm is measured (3).
The pattern of the curve is very similar to the figure 7
except that the x-axis is the time instead of the
temperature. This is known as the Oxidative Induction
time (expressed in minutes), which is also abbreviated as
OIT. This test is standardized for polyolefins as an ASTM
standard (4).
Variation of the OIT as a function of antioxidant
concentration
Correlation between the Oxidative Induction Time and the
concentration of a phenolic antioxidant into low-density
polyethylene has been reported earlier, the OIT being not
determined by DSC but by thermo-gravimetric analysis as
the time for the beginning of a weight change of the
sample due to oxygen absorption (5). However, little work
has been reported on the use of OIT for determination of
antioxidant levels in rubber or latex products.
The variation of the Oxidative Induction Temperature,
determined by DSC, as a function of the antioxidant
dosage in a given polymer is illustrated in the figure 8
below showing the OIT of ABS powder as a function of
®
the dosage of Wingstay L added in the ABS latex.
OIT (°C)
215
Dosage of a single antioxidant
Figure 9 shows the calibration curve for the dosage of a
®
multi-component phenolic antioxidant - Wingstay L- in a
carboxylated SBR latex. This antioxidant is quite difficult
to quantify by the extraction-chromatography technique
due to its high molecular weight and complex GPC
fingerprint (Figure 5).
After establishing the calibration curve, the OIT of 2 latex
®
samples stabilised with 0.11 and 0.26 phdl Wingstay L
were then determined and the concentration calculated
using the calibration curve (black squares on the curve) .
The calculated concentrations were respectively 0.115
and 0.22 phdl, close to the theoretical values.
The better accuracy found for the lower concentration is
not surprising considering the shape of the OIT vs.
concentration curve (see discussion about the precision
of the method)
220
OIT (°C)
218
216
210
205
214
212
200
210
208
195
190
206
204
185
180
202
200
175
0
0.05
0.1
0.15
0.2
Wingstay® L dosage, %
0.25
0
0.3
0.05
0.1
0.15
0.2
Concentration (phdl)
0.25
0.3
®
Figure 8: OIT curve for ABS powder
Figure 9: OIT of X-SBR latex vs. Wingstay L dosage
As can be seen on this curve, there is a direct correlation
between the OIT value and the antioxidant dosage. We
can therefore consider determining the antioxidant
dosage from a given sample by measuring its OIT after
establishing a calibration curve from samples containing
known amounts of the antioxidant.
Figure 10 gives the same calibration curve for a NBR
latex, showing the same shape as the X-SBR latex but
with a different OIT range.
200
OIT (°C)
195
Determination of antioxidants concentrations
synthetic latex from OIT measurements
in
The method developed for determination of the
antioxidant concentration in a synthetic latex is outlined
below:
- Preparation of standard samples: several samples with
antioxidant levels in the range of the concentration of
the sample to be analysed are prepared by adding the
antioxidant emulsion or dispersion to a latex sample
without antioxidant.
- Preparation of latex films. Thin latex films are prepared
from the standard samples and from the sample to be
analysed by coating the latex onto a glass plate, using
a 500µ calibrated coater. After drying at room
temperature and 30 minutes at 105°C to eliminate any
residual moisture, a 5 mg sample is cut from the film.
- DSC measurement: The sample is placed into a
aluminium DSC crucible with a 3-hole perforated cover
and heated in the DSC oven from 50 to 250°C at
2°C/min under an oxygen flow of 50 ml/min.
- The OIT is calculated from the DSC curve as the onset
of the energy vs. temperature curve (shown in figure 7).
190
185
180
175
170
165
0
0.05
0.1
0.15
0.2
0.25
Concentration (phdl)
0.3
0.35
Figure 10: OIT of NBR latex vs. Wingstay® L dosage
Dosage of antioxidant blends
The same method can be used for measuring the
concentration of antioxidant blends, for example a blend
®
®
of 2 phenolic antioxidants (Wingstay T/ Wingstay L)
commonly used in synthetic latex as an emulsion, as
illustrated in Figure11 for the X-SBR latex.
214
OIT (°C)
214
212
212
210
210
208
208
206
206
204
204
202
202
200
OIT (°C)
200
0
0.05
0.1
0.15
0.2
0.25
Concentration (phdl)
0.3
0.35
®
Figure 11: OIT of X-SBR latex vs. dosage of Wingstay L/
®
Wingstay T blend.
Precision of the method
The magnitude of variation of the OIT over the range of
the antioxidant concentration (0-0.3 phdl) depends upon
the antioxidant and the latex type: from 10°C to 30°C in
the above reported examples.
Furthermore, the stabilisation - hence the OIT value- is
not increasing linearly with the antioxidant concentration
but is levelling out so that the slope of the OIT vs.
concentration curve becomes very low when the
concentration reaches the plateau of the curve.
It is therefore necessary to measure the OIT value with a
good precision. The repeatability of the method was
checked my measuring 5 times the same sample of X®
SBR latex stabilised with 0.2 phdl of the Wingstay L/
®
Wingstay T blend. The OIT values were 207.8, 208.1,
208.3, 208.5 and 208.7°C, i.e. a standard deviation of
0.32°C for an average value of 208.3°C which is
acceptable for getting a good precision in the
concentration calculated from the OIT especially for
concentrations in the first part of the curve.
However, when the concentration approaches the
plateau of the curve (above 0.2-0.25 phdl), the accuracy
becomes poor due to the low slope of the curve. This
likely explains the lower accuracy found in the second
sample for the X-SBR latex stabilised with Wingstay® L
(0.22 vs 0.26 phdl). One way to overcome this problem
would be to dilute the sample to be analysed with some
latex without antioxidant, in order to bring down the
antioxidant concentration to the region of higher slope.
Determination of antioxidants concentrations in latex
products
The DSC method can also be used for the quantification
of antioxidants in synthetic or natural latex finished
products.
In figure 12 below, the OIT measured on the NR
powdered gloves discussed in the first part of this paper
(figure 6) is reported as a function of the Wingstay L
dosage in the glove. Although the OIT was not measured
with a high precision in this study, the curve shows a
good correlation between the OIT and the antioxidant
dosage, after discarding the value at 0.875 phr which is
an outlier.
0.4
0.6
0.8
Concentration (phr)
1
1.2
®
Figure 12: OIT vs. Wingstay L dosage in NR powder
gloves.
CONCLUSION
Antioxidants concentrations in latex or latex finished
products can be determined with a good accuracy from
the measurements of the Oxidative Induction
Temperature
(OIT),
using
differential
scanning
calorimetry.
When compared to the analyst approach based on
extraction of the antioxidant followed by chromatographic
analysis, the DSC method is simpler to implement as it
does not require the development of chromatographic
analytical methods often specific to a given antioxidant.
However, the OIT method requires preparing standard
samples for establishing the calibration curve and shows
some limitations in the accuracy of the measurement
when the antioxidant concentration reaches the plateau of
the OIT vs. concentration curve.
Another benefit of the OIT method is that the same
procedure can be used for different antioxidants as well
as for blends of antioxidants.
REFERENCES
1.
2.
3.
4.
5.
R Crompton, Determination of additives in polymers
and rubbers, Rapra Technology, 2007.
J Braden – Protection of chlorinated and nonchlorinated gloves with phenolic antioxidants,
th
presented at the 8 International Latex Conference,
2002.
E Tury (ed.) - Thermal characterisation of polymeric
materials, Academic Press, 1981
ASTM D3895-95 – Standard test method for
oxidative induction time of polyolefins by Differential
Scanning Calorimetry, American National Standard,
1995.
H.E. Bair – Polym.Eng. Sci., 13,435-439, 1973.