Dias nummer 1

1
Analysis of Polyadipate Ester Content in
Poly(Vinyl Chloride) Plastics by Means of
FT-Raman Spectroscopy Rolf W. Berg 1 and Amalia
Dopazo Otero 2

1Department of
Chemistry, The Technical University of Denmark,
Kemitorvet, Building 207, DK-2800 Lyngby, Denmark
2Escuela Técnica Superior de Ingeniería,
Universidad de Santiago de Compostela, Santiago
de Compostela 15782, Spain


Email rwb_at_kemi.dtu.dk
Email amalia.dopazo_at_rai.usc.es
INTRODUCTION. Synthetic plastics and polymers are
among the greatest inventions of the 20th century
and have become some of the most important and
widely-used materials. Many such products need to
be soft and flexible so that they can take on
different shapes and forms depending on their
application. This is particularly true of Poly
Vinyl Chloride (PVC) which is often 'plasticized'
or softened by use of such plasticizers as
phthalate, adipate, trimellitate and citrate
esters. 95 of the plasticizer production is for
PVC end-use. Importantly, the plasticizers are
not just additives, like fillers or pigments, but
are major components that determine the physical
properties of the polymer products. There are
more than 300 different known types of
plasticizers, of which between 50 and 100 are in
commercial use. The most commonly used
plasticizers are phthalate diesters, but there is
serious concern that several commonly used
phthalates may harm life reproduction systems
1. Alcohols of chain lengths similar to those
used for phthalate plasticizers are esterified
with adipic acid, rather than phthalic anhydride,
to produce the family of adipate plasticizers.
For example, esterification of 2-ethylhexanol
with adipic acid yields Di-2-EthylHexyl Adipate
(DEHA), also known as Di-Octyl Adipate (DOA).
Adipates typically used are based on alcohols in
the C8 to C10 range, because incompatibility
problems can be encountered at higher carbon
numbers, especially at high addition levels 2.
Adipic Acid Esters (AEs), when used in PVC,
contribute the property of improved low
temperature performance, in comparison to
phthalates, and significantly lower the viscosity
of the plastics, due to the lower inherent
viscosity of the plasticizer itself, a useful
property in many applications.
(A) (B)
Fig. 1. (A) Adipic acid structure. (B) Structure
of Di-2-ethylhexyl adipate, also known as DEHA or
Adimoll.
PURPOSE. The aim of this project was to see if
Fourier transform (FT-) Raman spectroscopy can be
used to determine the presence of adipate esters
as plasticizers in commercial flexible PVC
products and can detect what kind of ester and
how much is present. We have previously shown
that the presence of phthalate esters in PVC can
be analyzed by Fourier transform Raman
spectroscopy excited with a 1064 nm laser 3.
Here we report a similar study on detection of
AEs in soft PVC. We applied Fourier transform
Raman spectroscopy to a range of adipate ester
plasticizers in pure form as well as in samples
of poly vinyl chloride plastics. FT-Raman
spectra of 10 AEs are given below. It is
demonstrated that the presence of AEs in PVC
products is readily detectable by FT-Raman
spectroscopy. By use of proper reference samples
quantitative determination of the adipate ester
content is also possible. It is however found
that AEs as a group cannot be identified by its
characteristic Raman bands because other
aliphatic dicarboxylic esters have similar bands.
Table. Studied Neat Adipate Esters Name,
CAS-number, C-atoms and source.
EXPERIMENTAL. Spectra were obtained with a Bruker
IFS 66 Fourier-Transform spectrometer with an
FRA-106 Raman attachment. The exciting source was
a 1064 nm near-infrared Nd-YAG laser with a
nominal power of 100 mW. The scattered light was
filtered and collected on a liquid N2 cooled
germanium-diode detector giving a resolution of
approx. 2 cm-1 between individual pixels. Raman
spectra were collected over the range from 3500
cm-1 (Stokes) to -1000 cm-1 (anti-Stokes) at
approximately 23 ºC with no particular specimen
preparation. All mentioned adipate esters (Table)
were liquids at 23 oC. The esters were obtained
commercially or made in our laboratory by means
of a mixture of adipic acid and the corresponding
alcohol (1-hexyl, isoamyl, isopropyl). They were
measured in small glass test tubes. The plastics
were just placed in the beam.

RESULTS AND DISCUSSION
Reference Spectra were obtained for a number of
AEs as shown in Fig. 2. The bands at
approximately 1734 cm-1 must be assigned to the
CO stretching vibration of the ester group
-C-(CO)-O-C-, whereas the peaks at about
2950-2900 and 1450 cm-1 are due to CH2 bond
stretching and angle bending.
Fig. 5. Raman spectrum of two different PVC foil
products and its components. Arrows show the 1734
cm-1 band of the ester groups.
Fig. 3. Raman spectra of PVC samples containing
DEHA (Adimoll) in known amounts.
By using the ratio of band areas of the two
components in the soft plastics, we foresee an
easy way to identify and quantify the presence of
adipates in a range of different PVC products. To
test this possibility we have measured the
spectra for a range of different PVC consumer
products. The Raman spectra of two PVC film
products of known composition are shown in Fig.
5. The spectra clearly display peaks that can be
recognized as due to PVC and adipate esters,
respectively. Ultramoll-III is a trade name for a
highly viscous polymer plasticiser containing
adipic polyester, CAS no. 24937-93-7, from
LanXess.
The obtained FT-Raman spectra (in Fig. 3) show
that the signal for Adimoll clearly is correlated
to the amount present in the sample. To establish
this connection, we analyzed the FT-Raman
spectrum of each sample. By use of an appropriate
integration procedure, the area under the 1734
cm-1 peak over the estimated background was
determined. The same was done for the PVC peaks
at 600-700 cm-1. A graph of the area ratio
adipate-to-PVC determined in this way versus the
absolute adipate mass content (w) in the sample
is shown in Fig. 4. From this standardization
curve, it appears that it was possible from a
single Raman measurement to estimate quite
quantitatively the amount of adipate ester in our
soft PVC samples. This is probably true in
general for soft PVC. It needs, however, to be
known what kind of adipate ester has been used
and perhaps what else is present.
Fig. 2. Raman spectrum of the Adipate Esters in
the Table.
Conclusions. FT-Raman spectroscopy seems to be a
suitable and easy technique for fast analysis of
PVC samples with respect to identifying the
base-polymer (PVC) as well as the nature of the
plasticizer. This work is an extension of the
work on phthalates reported previously 3. We
estimate this FT-Raman method to be viable for
screening of large amounts of consumer products,
e.g. childrens toys, for commonly used
plasticizers, including now adipate diesters
(though they have much less characteristic
spectra). Quantitative determination of the
content of AEs should be accomplishable from a
single Raman measurement after establishing
appropriate reference data.
Quantitative Data. After this we studied home
made plastics samples containing known amounts of
PVC and a common commercial plasticizer. The
samples were prepared under different conditions
of time, temperature and agitation, during a heat
treatment in small glass bottles in an oven.
Then, when the samples were cold, FT-Raman
spectra were measured directly (Fig.3). As
plasticizer we used Adimoll, di-2-ethylhexyl
adipate, or DEHA, CAS No 103-23-1 from LanXess
(Bayer AG, Germany). Samples with good
homogeneity were achieved in the range of 40-50
wt Adimoll, and suitable conditions to obtain
good samples were about 120 ºC during 10 min.
Acknowledgement. Lykke Ryelund of Chemistry
Department in University of Copenhagen, is
thanked for measurement help. References. 1
R. Stringer et al., ESPR-Environ. Sci. Pollut.
Res., 2000, 7(1), 27-36. 2 Http//www.plasticise
rs.org/. 3 T. Nørbygaard R. W. Berg, Appl.
Spectrosc. 2004, 58(4) , 410-413.
Fig. 4. ? Graph of adipate-to-PVC area ratio
versus the adipate content (w) in soft
PVC-Adimoll samples. A dashed trend line is
included.
Poster presented at ICAVS-3, August 14-19, 2005,
Delavan, Wisconsin, USA