Bulletin of the Transilvania University of Bra_
ov
Series I: Engineering Sciences " Vol. 5 (54) No. 2 - 2012
CHARACTERIZATION OF POLYOLEFINS
WASTES BY FTIR SPECTROSCOPY
A. MOLDOVAN1 S. PAb
ACHIA1
R. BUICAN1 M.H. b
IEREAN2
Abstract: In this paper the Fourier transform infrared spectroscopy (FTIR)
method has been used to assess the degradation state (oxidation degree) of
different polyolefinic plastic wastes from construction industry, separated in
density fractions. Polyolefin oxidation has been identified and quantified by
the presence of a strong absorption band assigned to carbonyl groups and its
possible influence on the properties and behaviour of the polymer wastes has
been critically assessed.
Key words: degradation, FTIR, polyolefin, recycling, waste.
1. Introduction recycled plastics is represented by energy
recovery and about 40% by mechanical
Due to their low biodegradability, plastic recycling [17].
wastes are a matter of great concern, due to However, plastic recycling generally
their persistence in the environment. Land encounters some major drawbacks: first of
filling or incineration, as traditional all, it is desired that the materials obtained
methods dealing with the increasing waste from plastic waste to possess mechanical
production are no longer economically strengths comparative to those of virgin
viable and possess a great environmental polymers, which is extremely difficult due
risk. Nowadays, a lot of attention has been the wide range of structural properties of
focused on recycling of post-consumer plastics, which impart different thermal
plastics. Four major categories for recycling behaviour, making blending of polymers
and reutilisation for plastic solid waste have extremely difficult. Secondly, degradation
been identified, based on their thermo- of the plastic material during its lifecycle
mechanical and compositional characteristics: caused by high temperature of initial
re-extrusion (primary), mechanical processing or by environmental conditions
(secondary), chemical (tertiary) and energy (temperature, light radiation from different
recovery (quaternary), each method being sources, humidity etc.) renders the
suitable for different areas of industry [2]. identification and sorting of the polymer
Statistical data regarding waste very difficult. Also, one must take into
accumulation and recycling in Europe (2008) account that processing parameters of the
have shown that the recovery rate of post- polymer wastes may differ substantially
consumer plastics has reached the value of from those of the corresponding virgin
51.3%. More than half of the total amount of polymers.
1
Product Design, Mechatronics and Environment Dept., Transilvania University of Bra_
ov.
2
Dept. of Materials Engineering and Weld, Transilvania University of Bra_
ov.
66 Bulletin of the Transilvania University of Bra_
ov. Series I " Vol. 5 (54) No. 2 - 2012
To overcome these drawbacks, the key industry, obtained by sink-flotation
issue is to accurately determine the method, by using the attenuated total
characteristics of the polymeric waste used reflectance Fourier transform infrared
in new materials designing and production, spectroscopy (ATR-FTIR) method, in
namely its composition and degradation conjunction with specialized software and
state (e.g. oxidation degree, polymerization spectral databases. Polymer oxidation has
degree and so forth). been identified and quantified by the
According to the reference literature, for presence of a strong absorption band at ~
determining the waste composition along 1720 cm-1, assigned to carbonyl groups
with the degree of degradation, different vibration. For a quantitative approach of
characterization methods such as: the degree of oxidation, the carbonyl index
spectroscopy (NIR, FTIR, Raman), has been calculated, and its possible
mechanical testing [3], [13], and thermal influence on the properties and behaviour
analysis (TGA and DSC) [9], [14] have of the polymer waste has been critically
been used, due to their sensitivity to assessed.
material s properties alterations.
Molecular structure and morphology 2. Materials and Characterization
modifications (that occurs during the Techniques
product life time or during the recycling
operations) [7] could be determined by Plastic wastes originating from buildings
using the above-mentioned methods. The and construction Industry (B&CW) have
most suitable and sensible analysis for been used in this study. The plastic wastes
assessing polymer degradation is FTIR samples have been kindly supplied in the
spectroscopy [14]. frame of the European W2Plastics FP7
Studies regarding the degradation of project and used without further processing.
polymers during their lifecycle have proven The samples have been received washed,
that generally, it involves two principal cut and separated based on their density by
steps: abiotic and biotic degradation. The sink flotation in seven fractions (0.89-0.90
abiotic degradation involves the polymeric g/cm3; 0.90-0.91 g/cm3; 0.91-0.92 g/cm3;
material degradation due to exposure to the 0.92-0.93 g/cm3; 0.93-0.94 g/cm3; 0.94-
outdoor conditions and is followed by 0.95 g/cm3; 0.95-0.96 g/cm3) as it can be
microbial assimilation (biotic degradation) seen from Figure 1. The aforementioned
[6], [9], [15]. density range (0.89-0.96 g/cm3) has been
The first indication of the sample chosen as being specific to polyolefins
oxidation, assessed by visual observation (PO). The studied fractions were assumed
(yellowing, cracking at the surface, gloss to consist only in PO, mainly polyethylene
and transparency reduction), may be due (PE) and polypropylene (PP).
to the formation of degradation products The FTIR spectra of the samples have
such as carbonyl, hydroxyl, and vinyl been obtained using a FTIR spectrometer
groups. They are considered the main (Perkin Elmer BXII) working in
initiators of polymers oxidation, by Attenuated Total Reflectance (ATR) mode,
absorbing UV light and becoming free performing a total of 4 scans with a 2 cm-1
radical sources [10], [11]. resolution, in the 4000-600 cm-1 spectral
The paper is focused on the statistical region. From each density fraction, at least
composition and degradation degree 25 representative samples have been
assessment of seven light density fractions selected based on their overall aspect
of plastic wastes from the construction (colour, gloss, transparency) - Figure 1.
Moldovan, A., et al.: Characterization of Polyolefins Wastes by FTIR Spectroscopy 67
to -CH2- asymmetric and symmetric
stretching vibration, while PP presents four
superimposed absorption bands corre-
sponding to asymmetric and symmetric
stretching vibration of methylene and
methyl groups. Different features have
Fig. 1. B&CW density fraction 0.95-0.96 been also observed in the 1470-1350 cm-1
g/cm3 (a), picked samples for ATR-FTIR domain, where absorption bands corre-
analysis from the same density fraction (b) sponding to scissoring vibration of the
methylene group (1468 cm-1 in both PE and
The samples selected for FTIR analysis PP spectra) and to symmetric deformation
represent one tenth of the received of the methylene group (1377 cm-1 only in
fraction. For each selected piece of waste, PP spectra). Supplementary, for PE, a
FTIR analysis was conducted in three doublet with the maxima at 730 and 720
different points. All the coefficients cm-1 corresponding to bending and rocking
calculated based on bands heights or areas vibrations of crystalline and amorphous
are provided as mean values. methylene group is encountered [4], [6], [8].
All these spectral differences are helpful
3. Results and Discussion in discriminating between PE and PP in the
IR spectra. Some representative FTIR
The ATR-FTIR spectroscopy analysis spectra for each density fraction with the
was used in this study as a qualitative PE-PP repartition through the seven
(aiding polymers identification based on analyzed density fraction are presented in
their specific absorption bands) and as well Figure 2. An overall view of the separated
as a semi-quantitative method, assessing waste fractions, through the information
the polymers degradation degree by revealed from the FTIR spectra analysis,
quantification of specific absorption bands reveals that in the 0.91-0.93 g/cm3 and
assigned to carbonyl groups, and by 0.95-0.96 g/cm3 density fraction, a
carbonyl index calculation. heterogeneous PE-PP mixture is
In order to discriminate between PE and encountered, while in the 0.89-0.91 g/cm3
PP the 3000-2750 cm-1 spectral region has and 0.93-0.95 g/cm3 and homogeneous
been firstly taken into consideration, where composition has been found (consisting
PE presents two absorption bands assigned either in PP or PE, respectively).
Fig. 2. FTIR spectra of each density fraction (representative spectra) and the PP/PE
repartition
68 Bulletin of the Transilvania University of Bra_
ov. Series I " Vol. 5 (54) No. 2 - 2012
An accurate identification and fraction. The correlation coefficients for
quantification of the PP and PE occurrence the compared samples range between 0.79
for each density fraction has been performed and 0.99. This range has been considered
by using specialized software (Essential acceptable due to the presence of other
FTIR - FDM Library) and spectral databases. small-molecular compounds in the analysed
Each FTIR spectrum of the polymer samples, such as additives (dyes, pigments,
waste from the mentioned density fractions plasticizers, stabilizers, fillers) as well as
has been compared with reference spectra due to possible oxidation degree associated
from the FTIR FDM Library spectral to the polymer aging. Their presence is
database (organic polymers, polymers and observed in PO spectra as new absorption
coatings). The software displays a number bands besides PO characteristic ones.
of compounds, of whose FTIR spectra The percentual (numerical) composition of
resemble more or less the spectra of the each density fraction has been determined by
analysed sample, associated with correlation dividing the number of samples that were
coefficients. A higher correlation coefficient identified as one of the two components (PE
has the significance of a better match or PP) with the total number of the samples
between the spectra of the sample and the taken into analysis (25). The composition of
compared reference. each density fraction is depicted in Figure 4.
In Figure 3 an example of comparison The repartition of PE and PP through the
report provided by the aforementioned seven density fractions after software
software is presented. identification is similar with the one
presented in Figure 2, obtained after PO
characteristic absorption band visualisation.
In the last fraction (0.95-0.96 g/cm3) a
small amount of PP (16%) has been
identified, probably due to the presence of
different additives or degradation products
in the waste structure that are
lowering/increasing the POs density, and
making their separation difficult. It is
appreciated that the use of characteristic
software in conjunction with spectral
databases for components identification is
more reliable, especially with larger
amount of samples taken into
consideration, and reduces analysis time.
In order to determine the samples degree
of degradation, the region of (1850-1630)
Fig. 3. Essential FTIR-FDM Library
cm-1 where carbonyl group s absorption
example of an identification report
band appears in case of POs oxidation, was
investigated. All FTIR spectra present a
The report contains data related to the
weak absorption band in the investigated
spectra that are to be compared, data
interval, which indicates the macromolecular
related to the reference spectra, and also
chains oxidation. The POs and POs
the correlation coefficients. The highest
mixtures are known to degrade following
correlation coefficient was chosen in order
Norrish degradation mechanism (type I
to identify the components for each density
and II in competition) [5], [11], [12],
Moldovan, A., et al.: Characterization of Polyolefins Wastes by FTIR Spectroscopy 69
leading to chain scission and crosslinking. methylene (-C=C- at 1680-1620 cm-1 - weak
During POs degradation, hydroxyl (-OH at absorption band) and vinyl (-CH=CH2 at
3650-3200 cm-1 - a broad absorption band), 908 cm-1) groups are the main degradation
carbonyl (-C=O at 1850-1630 cm-1), products that form.
Fig. 4. Composition of the B&CW density fractions determined via FTIR analysis
Zooming in on the carbonyl regions in weight, chain branching, viscosity and so
the ATR-FTIR spectra allows us a better forth). According to literature [1], [16],
observation of the absorption bands various carbonyl containing species can be
changes of the PO wastes. In Figure 5 four found in this wave number region, such as:
samples spectra (zoom in carbonyl region) Å‚-lactones (1797 cm-1), lactones (1778 cm-1),
are presented, each one representing four per-esters (1774 cm-1), peracids (1754 cm-1),
different density fractions (Figure 5a and b aldehydes (1740 cm-1), ester (1739 cm-1),
corresponding to PP; Figure 5c and d ketone (1727-1716 cm-1), carboxylic acid
corresponding to PE). (1703-1698 cm-1), Ä…,²-keto-aldehydes
The differences between spectra, mainly (1648 cm-1), carboxylate (1592 cm-1). Most
in the carbonylic region were assigned to of these absorption bands have been found
different oxidation products that could in the analysed POs wastes spectra either
form, due to different degradation stages of as weak absorption bands, or as shoulders
the wastes samples. The degradation in the strong C=O absorption band. For all
differences that appear even in the same samples, an absorption band centered in
polymer type may be related to the the region 1655-1630 cm-1 corresponding
presence of different additives in the to alkenes C=C stretch is observed. For PP
polymeric matrix (especially the presence spectra, several differences have been
of thermal or UV-light stabilizers, may observed: FTIR spectra from Figure 5b
influence the reduction of the products present a sharper absorption band centered
oxidation), storage conditions during at 1650 cm-1 with smaller shoulders, while
products life-time and also at the end-of in Figure 5a a broad absorption band
their life (humidity, light, temperature, (1680-1550 cm-1), have been observed
oxygen content, microorganisms presence, where several absorption bands are
are increasing the degradation probability). overlapped. A more intense absorption band
Another important aspect that must be related to the presence of the double bond
brought into discussion is the variability of in comparison with the intensity of the
the components properties (molecular bands related to carbonyl groups, suggests
70 Bulletin of the Transilvania University of Bra_
ov. Series I " Vol. 5 (54) No. 2 - 2012
Fig. 5. Carbonyl region detail for FTIR spectra of sample representing different
density fractions: a) 0.89-0.90 g/cm3; b) 0.90-0.91 g/cm3; c) 0.93-0.94 g/cm3
and d) 0.94-0.95 g/cm3
the Norrish I type degradation mechanism. The CI has been calculated as the ratio
Figure 5b, reveals that PP sample is in a between the absorbance area of carbonyl
much advanced degradation state as PP groups and the absorbance area of a
from Figure 5a, due to the higher content specific band for the PO (used as internal
in C=C. PE spectra also present some reference) encountered at 1380 cm-1 in the
differences: in Figure 5c the double bound case of PE, and at 2700-2750 cm-1 for PP,
evidences the main absorption band in the respectively [8]. The results are presented
region, and it presents small shoulders in Table 1 (average values of CI for each
corresponding to different carbonyl component of the density fractions). The
species, while PE described by Figure 5d CIaverage was calculated taking into
presents a broad absorption band with four consideration the components percent for
maxima centred at 1724, 1680, 1650, 1606 each density fraction determined earlier.
cm-1, with similar intensities. It seems that Relatively low CI values have been
PE characterized by the spectrum from obtained. Samples containing mainly
Figure 5d is in an advanced stage of polyethylene exhibit a carbonyl index
degradation as the one from 5c. around 1, by comparing with the ones with
Due to the fact that the different carbonyl a higher percentage of PP, where all
species absorption bands are not totally carbonyl indexes were above 0.7. Normally
separated, the total area of carbonyl groups this means that PE containing fractions are
absorption band, between 1850 and 1630 more degraded than PP based fractions.
cm-1 has been used for the carbonyl index But taking into account that PP has to be
(CI) calculation - Equation (1): more sensitive to oxidation (due to the
tertiary carbon presence), and that carbonyl
A1850-1650
groups could act as weak points inducing
CI = . (1)
new oxidative processes, the functionalized
APO
Moldovan, A., et al.: Characterization of Polyolefins Wastes by FTIR Spectroscopy 71
Table 1 Most of the POs fractions present a good
Calculated Carbonyl Index (CI) PE and PP separation. PP is the main
for the analyzed building and construction component in the density range of 0.89-
wastes (B&CW) 0.92 g/cm3, while above 0.92 g/cm3 density
the main component is PE.
Density
All samples present signs of degradation,
fraction CIPP CIPE CIaverage
CIaverage ranges from 0.45 to 1.14.
[g/cm3]
ATR-FTIR analysis alone is not enough
0.89-0.90 0.45 - 0.45
for determining the degradation stage of
0.90-0.91 0.46 - 0.46
polymers due to possible CO release. Its
0.91-0.92 0.49 0.76 0.61
0.92-0.93 0.28 1.26 1.14 correlation with other methods that could
0.93-0.94 - 1.13 1.13 evidence the chains scission, crosslinking
0.94-0.95 - 0.94 0.94
or crystallinity alteration, such as DSC,
0.95-0.96 0.45 1.16 1.05
XRD, MFI, Rheology, and Chromatography
is required. The present paper presents the
polymer can further degrade following two
early stages of a complex study related to
competitive degradation mechanisms. If
characterisation of the different density
the PP degradation will lead to chain
fractions separated from polymer wastes, the
scission, crosslinking or even CO release,
final aim of the study being their recycling
the carbonyl group species will be
by plastic composites manufacturing.
consumed, lowering the calculated CI, in
case which the results may lead to wrong
Acknowledgements
conclusions. In fact, the best conclusion that
may be drawn here is that only ATR-FTIR
The research work was supported by the
analysis is not enough for determining the
Sectoral Operational Programme Human
degradation stage of the waste, due to the
Resources Development (SOP HRD),
complexity of the POs degradation
ID59321 financed from the European Social
mechanism. Unfortunately, FTIR analysis
Fund and by Romanian Government, and the
is characterizing only a thin layer from the
samples have been supported by FP7 grant
material s surface (the penetration depth
W2Plastics.
being in the microns range).
A correlation of the FTIR analysis with
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