"A New Experimental Procedure for Incorporation of Model
Contaminants in Polymer Hosts"
C.D. Papaspyrides1*, Y. Voultzatis1, S. Pavlidou1, C. Tsenoglou1, P. Dole2, A.
Feigenbaum2, P. Paseiro3, S. Pastorelli3, C. de la Cruz Garcia3, T. Hankemeier4, S.
Aucejo4
1. Laboratory of Polymer Technology, School of Chemical Engineering, National Technical
University of Athens, Zographou, 15780 Athens, Greece
2. INRA SquAlE, Moulin de la Housse, BP 1039, 51687 Reims, France
3. Analytical Chemistry, Nutrition and Bromatology Department, Faculty of Pharmacy,
University of Santiago de Compostela (USC), 15782 Santiago de Compostella, Spain
4. Packaging Research Group, Analytical Sciences Division, TNO-Nutricion and Food
Research Institute, PO BOX 360, 3700 AJ Zeist, The Netherlands
* Corresponding Author: Tel. +30 210 772 3179, Fax +30 210 772 3180
E-mail: kp@softlab.ece.ntua.gr
1
ABSTRACT
A new experimental procedure for incorporation of model contaminants in polymers was
developed as part of a general scheme for testing the efficiency of functional barriers in food
packaging. The aim was to progressively pollute polymers in a controlled fashion up to a high
level in order to monitor migration phenomena difficult to study in real conditions. To this
end, a contamination recipe was initially formulated by a set of selected surrogates. The
experimental procedure developed, led to satisfactory results as far as homogeneity and final
concentration in the polymers concerned. High temperature data were also used in order to
evaluate the efficiency of typical thermoforming processes in reducing possible volatile and
non-volatile substances from recycled polymeric materials.
KEYWORDS: Surrogates, contamination procedure, mixing, functional barrier, diffusion
2
INTRODUCTION
Packaging has become an indispensable element in the food manufacturing process.
Presently, plastics are preferentially used for packaging foodstuffs due to their outstanding
usage properties. It is worth mentioning that more than 30 different plastics are currently
being used for this purpose. Plastic packages are capable of retarding and even preventing
detrimental changes in the packed material caused by oxygen, light and microorganisms.
Plastics are also able to greatly reduce the loss of valuable components on behalf of the
packed material, such as water or flavor (1).
Due to modern environmental concerns and associated legislation, the question of
recyclability of used plastic packaging into new food packaging applications is of increasing
interest. Consequently, recycled plastics have already been used in food-contact applications
around the world, for several years. However, these cases were considered to have more of a
pilot character than real market value and, in most cases, the mass fraction of recycled
plastics in these applications was relatively low, due to blending with virgin plastics or
sandwiching with layers of virgin plastic of relatively high thickness (1).
Plastics contain low molecular components, such as monomers and oligomers, and additives,
such as plasticizers, lubricants, stabilizers and antioxidants, which are absolutely necessary
for the processing and the stability of the final materials. The drawback in this case is the
migration of the additives from the package into the packed material (2-6).
It must be said that when a polymer is in contact with a liquid, such as foods or chemicals,
generally two mass transfers take place simultaneously: liquid enters the polymer, while
3
polymer additives diffuse from the plastic into the liquid. The following drawback is obvious,
a pollution of the stored liquid and a decrease in mechanical properties of the plastic. Thus,
the packaging can itself represent a source of contamination, through the migration of
substances from the packaging material into food. Hence, regulatory authorities around the
world have recognized that it is necessary to control such contamination, and many have
enacted extensive legislation (7). On the other hand, a large amount of research regarding the
migration of volatiles, additives, monomers and oligomers from plastic packaging materials
into food has been conducted (1, 4-16).
Approaches envisaged for producing food-packaging materials from post-consumer collected
plastic packaging include washing, which is, however, incapable of extracting all the
chemicals that have migrated into the plastic, and depolymerization, which is too costly to be
considered applicable.
A third route appears as an interesting compromise both from the point of view of
performance and cost. This consists of reusing old polymer packages in new packaging
products by sandwiching them through co-extrusion in bi- or tri- layer polymer
configurations, where the old polymer is located between two virgin polymer layers (1, 8, 12,
17-33). As it takes some time for the contaminant initially located in the old polymer to
diffuse through the virgin layer, the virgin polymer layer acts as a shield to food pollution,
and thus constitutes a “functional barrier”. During the co-extrusion process, the polymer is
heated up to high temperatures before being cooled down by air. In the course of this process,
contaminant diffusion accelerates and, therefore, the time of protection of the functional
barrier is significantly reduced. Thus, the main problem is to determine the period of time
over which the food is protected, either by experiments or calculations (19-31).
4
Considerable progress has been made from the scientific point of view in understanding and
modelling diffusion processes of adventitious hazardous compounds, from a recycled plastic
in direct contact with food or from a core layer, across a functional barrier. However, putting
this knowledge to use by devising industrial solutions is still in waiting.
Currently available are guidelines developed by the Food and Drugs Administration (FDA)
and the U.S. Food and Plastics Industry, which, however, were established on the basis of too
conservative an approach, and require enormous efforts concerning the implementation of the
underlying tests (7). These test schemes, also known as “challenge” tests, are simulating a
recycling process by artificially introducing model contaminants, so-called surrogates, in the
polymer; subsequently, the cleansing efficiency or surrogate removal potential of the process
is checked (7).
European project FAIR-CT98-4318, dedicated one of its research tasks to the study of
functional barriers. More specifically, one of the overall objectives of the project was to
generate an advanced scientific understanding, along with modelling, of the physicochemical
behaviour of chemical contaminants in recycled plastic layers buried by functional barrier
polymers. This will eventually constitute the basis for evaluating the safety and defining
design criteria for appropriate functional barrier protection against recycled plastics used for
food packaging (34).
Previous experimental procedures aiming to generate “real” polluted material, proposed
between 1990 and 1997, consisted in simulating pollution with cocktails of surrogates.
Simple contact of the material to be contaminated with the surrogates was suggested for
5
about two weeks at around 40°C (18, 35-39). The “real” polluted material thus obtained was
characterized by:

Low pollution levels (100 ppm).

Surface pollution, primarily; this means that aqueous washing is in general very effective,
even for low contact times (200 seconds) and high molecular weight compounds (32, 36,
37). This comprises a basic disadvantage of those earlier “model pollution” experiments
since in real life pollutants are homogeneously dispersed throughout the polymer phase.

Pollution level dependent on the nature of the surrogate (polarity, molecular weight).
In this project the aim was to pollute materials at a precontrolled and high level, of the order
of 1000 ppm, in order to monitor migration phenomena difficult to quantify in real life
situations. It should be emphasized here that the extrapolation to real cases has so far been
made only by numerical simulations based on data corresponding to low levels of
concentration (35-41). Furthermore, deep penetration and therefore, homogeneous surrogate
distribution in the material was considered here.
EXPERIMENTAL
Materials
Polymers:
The contamination procedure was tested on different polymers in order to study a wide range
of polymer matrix qualities. The following polymer hosts were supplied by Cryovac in the
form of pellets: High Density Polyethylene (HDPE), Low Density Polyethylene (LDPE),
6
Linear Low Density Polyethylene (LLDPE), Polypropylene (PP), Ethylene Propylene
Copolymer (EP), Polyamide 6 (PA-6), Ethyl Vinyl Alcohol (EVOH) and Polyethylene
Terepthalate (PET).
Contaminants:
The model set of solid and liquid contaminants, the so-called “surrogates”, represents all
general categories of chemical compounds as described in the FDA regulations: volatile and
non-polar, volatile and polar, non-volatile and polar and non-volatile and non-polar.
Additionally, a wide range of functional groups was used, in order to reflect the different
chemical and physical nature of real life contaminants. The three solid and five liquid
contaminants used are presented in table 1, along with their characteristic properties and
supply source.
[Insert table 1 about here]
Contamination Recipe
The major criterion in assessing a contamination recipe was its ability to provide an initial
excess of solid and liquid surrogates, in order to accommodate the possibility of a worse case
contamination scenario during usage, disposal or storage. Preliminary experiments were
carried out to determine the initial concentrations required to obtain detectable amounts of
surrogates in the final product (i.e. in the polymer after processing). With this in mind, it was
decided to incorporate high amounts of volatile surrogates in the polymer hosts and to
exclude Dimethylsulfoxide (DMSO) from the non-polar polymers, since this polar
7
contaminant does not permeate in these polymers at all (42). The contamination recipe finally
established is shown in table 2.
[Insert table 2 about here]
Experimental Procedure
A “contamination broth” was prepared, by blending all eight surrogates together in quantities
indicated by the contamination recipe mentioned above, with a view to a 2 kg final product.
The broth was mixed with 400 g of virgin polymer in a reactor vessel, which was heated in an
oil bath. The temperature in the bath was set between 60 °C and 80 °C, depending on the
nature of the polymer. A vertical condenser was adjusted to the vessel to prevent volatile
surrogate losses. The “master-batch” produced was continuously stirred for 1 day and then a
1-week “contaminant penetration period” followed, during which, the master-batch was
periodically stirred (~ 1 h / day). After this week, the master-batch was extruded under
conditions shown in Table 3, and the extrudates were pelletized and mixed with the rest of
the virgin polymer host (1600 g). Afterwards, a second extrusion took place to improve the
penetration of the surrogates into the core of the material. The final product was then
chemically analysed.
The aforementioned experimental procedure was applied to all polyolefins, i.e. LDPE,
HDPE, LLDPE, PP, and EP without any problems. The extrusion conditions are given in
table 3.
[Insert table 3 about here]
8
On the contrary, contaminated pellets of PET and EVOH presented severe problems during
the extrusion process due to their solubility in the cocktail of surrogates employed. The
pellets of these polymers were swollen in the presence of this liquid mixture under the high
temperature process conditions, causing congestion in the feeding zone of the extruder.
This problem led to a modification of the contamination procedure. In the case of PET, where
DMSO was causing most of the problem, this particular surrogate was excluded from the
broth, in order to allow processing of the master-batch. It was incorporated in the polymer in
the second step of the mixing process, by adding DMSO containing polymer to the
masterbatch instead of virgin.
The problems were more severe with EVOH, since this polymer is soluble to all liquid
surrogates. In this case, a master-batch with lower surrogate concentrations was prepared by
mixing the same quantities of surrogates with a larger quantity of pellets (800 g), in order to
make master-batch extrusion possible. As far as DMSO is concerned, the same procedure as
in the case of PET was followed.
The extrusion conditions for these polymer hosts are also given in table 3.
Chemical Analysis
The masterbatch of each polymer was analysed at the INRA Laboratory. In the case of PP,
both the masterbatch and the final product were analysed at three different laboratories. The
analytical procedures used for chemical analysis by each laboratory are described below.
9
Each laboratory analysed five samples in order to evaluate the homogeneity of the product.
INRA Laboratory
Extruded polymer samples were ground to powder before extraction. Then, 1 g of each one of
them was soaked with 10 ml of a swelling solvent at 40 °C for 5 days. This solvent is
dichloromethane for all polymers, except for EVOH where methanol was used. Afterwards,
the extraction liquid was analysed by means of Gas Chromatography with Flame Ionization
Detector (GC-FID) with polar and non-polar column.
TNO Laboratory
Polymer samples were cut in thickness less than 1 mm and 250 mg of each were extracted
with 10 ml of dichloromethane, containing dichloroethane and dichlorobenzene as internal
standards, by shaking at room temperature for 24 h. Afterwards, the extraction liquid was
analysed by means of Gas Chromatography – Mass Spectrometry (GC-MS) and High
Performance Liquid Chromatography (HPLC).
USC Laboratory
Samples were cut in slices with thickness less than 500 μm and 250 mg of each were
extracted with 10 ml of dichloromethane at 40oC for 24 hours in a hermetically sealed glass
tube. The extract is subsequently analysed by means of GC-FID and HPLC.
10
RESULTS AND DISCUSSION
The results of the analysis of the contaminated products are viewed in terms of the achieved
homogeneity and retention of the surrogates. “Retention” of a surrogate is defined as the ratio
of surrogate concentration in the masterbatch or the final product, divided by the surrogate
concentration of the contamination recipe. This ratio is a measure of the overall efficiency of
the impregnating process, as far as material utilization is concerned, and depends on
physicochemical and operational parameters. In that respect, it is worth mentioning here the
main factors affecting additive entrapment and homogeneity into a polymer (43, 44):

Ability of the additive to survive the extrusion run; this is controlled by its volatility
under the specific processing conditions. A measure of this is the difference between the
boiling point of the additive and the processing temperature.

Thermodynamic compatibility between the various species.

Extent and intensity of the mixing process vis a vis its ability to bring the components in
intimate contact: a) to generate interfacial area for liquid penetration and, b) to initially
de-aggregate and consequently distribute the solids in a uniform fashion.

Diffusivity of the substrates through the interface and into the polymer mass.
First of all it must be emphasized that the experimental procedure developed in all cases led
to an acceptable homogeneity of the sample. The average concentration data in the
masterbatch are displayed in table 4. These results are used to calculate the retention as
previously described. table 5 displays the percentage retention of the surrogates in all
masterbatch grades made.
11
[Insert table 4 about here]
[Insert table 5 about here]
As mentioned already in the Introductory Part, a major parameter influencing retention is the
volatility of the surrogate, directly related with the boiling point in the case of the liquid
contaminants. Therefore, it seemed worthwhile to correlate the aforementioned data with this
particular parameter. According to figure 1 such a correlation proves valid, indicating that the
retention of the liquid surrogates is an exponential function of the boiling point of the
surrogate. This means that the most essential factor for the retention of volatile surrogates is
the temperature difference between their boiling point and the temperature of the process
applied. Therefore, potential use of processes involving high temperatures (e.g. melt
recycling) is, as expected, a very efficient way to eliminate volatile substances.
[Insert figure 1 about here]
It is also evident that there is a distinct difference between the behaviour of the polyolefins
and the behaviour of the polar polymers. It is well known that polyolefins generally do not
have efficient barrier properties, compared with polar polymers like PET, PA-6 or EVOH.
This is confirmed by the higher levels of retention in the case of polyolefins, in contrast with
the data for the polar polymers (fig.1).
Turning to the solid surrogates, in figure 2, retention data are compared for the masterbatch
of each polymer. Again, the very high levels of retention for the case of polyolefins show
their poor barrier properties. However, surrogate retention is also high for the polar polymers,
12
since the high temperatures used throughout the contamination procedure are not sufficient to
remove these non-volatile substances.
[Insert figure 2 about here]
In particular for the case of PP, analysis of the final product was also carried out, in addition
to the analysis of the masterbatch. This was part of a “ring test” between the three analytical
laboratories involved (INRA, TNO, USC), in order to compare results and therefore evaluate
reproducibility and the efficiency of the experimental procedure in terms of achieving
homogeneity. The average concentration data are demonstrated in Table 6, indicating first of
all a satisfactory dispersion of the surrogates throughout the masterbatch and the final grade.
In other words, the experimental procedure applied resulted in dispersing all surrogates in the
final product quite successfully and at a high level of homogeneity.
[Insert table 6 about here]
The aforementioned results were again converted to percentage retention and in figure 3, for
the liquid surrogates, retention versus the boiling point of the surrogate is plotted for both the
masterbatch and the final material. It is evident that in both cases the retention of the liquid
surrogates is again an exponential function of their boiling point. However, the levels of
retention in the final product are quite lower, a fact obviously attributed to the second
extrusion step that is necessary to receive the final product from the masterbatch. By
extrapolation, it seems possible that repeated passages through the extruder could further
reduce or practically eliminate volatile substances in melt recycling.
13
[Insert figure 3 about here]
This, however, is not the case for the solid surrogates, which are not affected under similar
conditions by the extrusion process. This is demonstrated in figure 4 by the high levels of
retention both in the masterbatch and in the final PP grade. Thus, it is safe to assume that
non-volatile substances remain in the recycled material even after repeated processes
involving high temperatures. If this is the case, it becomes necessary to estimate the time
needed for the substance to diffuse out of the polymer and into the food, or to define a
toxicologically acceptable level of the substance in the recycled polymer.
[Insert figure 4 about here]
CONCLUSIONS
The aim of this work was primarily to suggest an effective experimental procedure for the
incorporation of model contaminants or surrogates into polymers. These are to be
subsequently used for evaluating their efficiency as functional barriers in food packaging
applications. The two main objectives successfully accomplished in the procedure developed
were: pollution of the polymers at a high, precontrolled level and homogeneity of the final
grade. The procedure was successfully applied to most of the polymer hosts, while slightly
modified procedures were needed in cases of EVOH and PET, where mixing problems were
encountered.
Furthermore, retention data of the surrogates in all samples were used in order to evaluate the
effect of the extrusion in contaminant reduction, usually present in polymer recyclates. The
14
results were encouraging as far as volatile surrogates are concerned, as multiple steps of
extrusion seem to minimize the quantity of these substances in the polymer host. However,
this was not the case for the non-volatile and especially solid surrogates. Bearing this in
mind, in a future paper detailed migration data will be presented to establish a satisfactory
physicochemical model on which criteria can be based for the design of a safe functional
barrier structure.
15
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20
Table 1. Characteristic properties of surrogates used
Surrogate
Boiling
Melting
Molecular
Point
Point
Weight
(°C)
(°C)
(g / mol)
74.24
-30.3
133.42
110.78
-94.84
92.13
132.1
-45.2
112.56
189
19
78.14
238
7
160.26
Solid, polar
305.4
48.5
182.22
Solid
-
30
284.49
Solid
-
198
430.6
General Properties
Trichloroethane1
Liquid, polar, volatile
Liquid, non-polar,
Toluene1
volatile
Liquid, non-polar,
Chlorobenzene
2
volatile
Liquid, polar, nonDMSO2
volatile
Liquid, non-polar, nonPhenylcyclohexane1
volatile
Benzophenone1
Methyl
Heptadecanoate
1
UVITEX (2,5thiophenediylbis(5-tertbutyl-1,3benzoxazole))3
1. Merck KgaA
2. Fluka Chemika
3. Ciba Specialty Chemicals
21
Table 2. Contamination recipe
Final Concentration (ppm)
In non-polar
In polar
In semi-polar
polymer hosts
polymer hosts
polymer hosts
Trichloroethane
16500
16500
16500
Toluene
8250
8250
8250
Chlorobenzene
16500
16500
16500
DMSO
0
2750
1850
Phenylcyclohexane
2750
1850
1850
Benzophenone
2750
2750
2750
2750
1850
1850
1375
2750
2750
Surrogate
Methyl
Heptadecanoate
Uvitex
22
Table 3. Masterbatch extrusion conditions
Polymer
Higher Temperature (°C)
Screw Speed (rpm)
LDPE
170
30
HDPE
190
30
LLDPE
210
30
PP
240
30
EP
210
30
PET
260
30
PA-6
240
30
EVOH
230
50
23
Table 4. Analysis of masterbatch samples (Data in ppm)
Surrogate
LDPE
LLDPE
HDPE
EP
PP
PA-6
PET
EVOH
Trichloroethane
10946±203
6979±195
10813±210
15075±188
7787±156
308±15
320±44
-
Toluene
8974±58
4418±65
11530±88
7733±125
5095±98
2656±65
791±26
63±10
Chlorobenzene
20128±183
11621±221
31807±265
19562±196
12735±189
1438±48
5614±98
32±12
Phenylcyclohexane
9170±102
9248±164
8845±120
10203±199
10593±178
3087±98
3006±85
1477±56
Benzophenone
10543±98
10120±159
8894±114
10648±218
10327±144
6464±35
5160±105
5465±155
10706±210
10798±203
8122±98
10173±156
10765±196
3615±15
2838±33
4462±176
3648±64
3510±58
3162±32
4192±68
4167±78
7411±78
3683±14
6156±116
Methyl
Heptadecanoate
Uvitex
24
Table 5. Percentage retention of surrogates in the masterbatch
Surrogate
LDPE
LLDPE
HDPE
EP
PP
PA-6
PET
EVOH
Trichloroethane
16,7
10,6
16,3
22,8
11,7
0,4
0,4
-
Toluene
24,5
13,4
34,8
23,4
15,2
7,9
2,4
0,3
Chlorobenzene
30,6
17,5
48,0
29,6
18,6
2,1
8,3
0,1
Phenylcyclohexane
83,6
79,8
79,3
93,0
93,8
42,4
40,5
32,1
Benzophenone
95,8
92,1
80,9
96,7
93,4
58,7
46,9
79,5
98,0
98,7
74,3
92,6
99,1
48,9
38,3
96,6
65,6
63,4
57,6
75,8
76,9
67,2
33,4
89,5
Methyl
Heptadecanoate
Uvitex
23
Table 6. Analysis results of masterbatch and final product of PP (Data in ppm)
INRA
TNO
USC
Surrogate
Masterbatch
Final
Masterbatch
Final
Masterbatch
Final
Trichloroethane
7787±156
775±25
10727±189
1434±45
6674±156
910±45
Toluene
5095±98
508±16
4979±125
640±12
6188±149
800±26
Chlorobenzene
12735±189
1263±59
13367±205
1845±89
15169±159
2024±33
10593±178
1399±66
12325±188
1634±48
1115±21
1805±46
10327±144
1485±48
15278±169
1930±66
26793±256
4300±123
10765±196
2347±57
17791±178
2019±65
13258±147
2468±97
4167±78
1306±23
1317±25
554±15
5794±101
1274±68
Phenylcyclohexan
e
Benzophenone
Methyl
Heptadecanoate
Uvitex
24
FIGURE CAPTIONS
Figure 1. All polymer hosts: Percentage retention of liquid surrogates in the masterbatch as a
function of the boiling point of the surrogates
Figure 2. All polymer hosts: Percentage retention of the solid surrogates in the masterbatch
Figure 3. Polypropylene: Percentage retention of liquid surrogates in the masterbatch and the
final product as a function of the boiling point of the surrogates
Figure 4. Polypropylene: Percentage retention of solid surrogates in masterbatch and final
product
25
Figure 1
LDPE
100
Retention in the masterbatch (%)
0.0098x
LDPE
y = 0.5576e
R2 = 0.9994
LLDPE
y = 0,1042e0,0129x
2
R = 0,9829
70
HDPE
60
PP
y = 0,764e0,0093x
R2 = 0,9509
y = 1,0611e0,0087x
R2 = 0,8455
90
80
50
40
EP
LLDPE
HDPE
PP
y = 0,101e0,0133x
R2 = 0,9784
30
PA-6
y = 0,0002e0,0243x
R2 = 0,7851
EVOH
y = 6E-09e0,0435x
R2 = 0,8642
PET
y = 0,0001e0,0255x
R2 = 0,8988
20
10
PA-6
EVOH
0
300
EP
350
Trichloroethane
400
450
Toluene Chlorobenzene
500
Phenylcyclohexane
550
PET
Boiling Point (K)
26
Figure 2
Benzophenone
Methyl Margarinate
Uvitex
100
Retention in the masterbatch (%)
90
80
70
60
50
40
30
20
10
0
LDPE
LLDPE
HDPE
PP
EP
EVOH
PA
PET
27
Figur e 3
100
90
y =0,101e0, 0133x
80
R2 =0,9784
Masterbatch
Final product
Retention (%)
70
60
50
40
30
y =0,037e0, 0142x
R2 =0,9725
20
10
0
300
350
Trichloroethane
400
Toluene
450
Chlorobenzene
500
550
Phenylcyclohexane
Boiling Point (K)
28
Figure 4
Masterbatch
100
Final Product
90
80
Retention (%)
70
60
50
40
30
20
10
0
Benzophenone
Methyl Margarinate
Uvitex
29