1. Introduction
Polystyrene homo-polymer, also known commercially as crystal polystyrene or general purpose polystyrene (GPPS), is an amorphous, clear (naturally transparent), hard and rather brittle synthetic polymer made from styrene monomer. High Impact Polystyrene (HIPS) is produced by polymerization of styrene in the presence of a polybutadiene rubber impact modifier (between 5% and 12%). Compared to GPPS, HIPS is an opaque or translucent rather than transparent polymer, with superior impact strength, improved resistance to stress cracking and to crazing caused by organic liquids, oils and fats. HIPS is used on its own or blended with GPPS in specific blend ratios depending on the final functional application requirements [
1]. Solid polystyrene can be technologically processed through injection molding, vacuum forming, sheet extrusion or thermoforming, while expanded polystyrene is either extruded or molded in a special process. A wide range of products made of polystyrene are used in several applications such as packaging, building & constructions, medical devices, electronics and consumer goods & appliances (including refrigerators). The sub-segment food packaging in particular is the most dominant application of polystyrene [
2]. GPPS is mainly used for the production of transparent containers for a variety of foods and of disposable cups for beverages. HIPS and GPPS-HIPS blends are used in the form of pots e.g., for dairy products, such as yoghurts, as vending cups for beverages such as coffee, tea, chocolate and soup, as containers for ice creams as well as disposable cutlery and utensils Foamed polystyrene trays or containers are used in contact with fast foods, meat, fish, poultry, soft fruit and eggs.
In the Europe Union (EU), the Framework Regulation (EC) 1935/2004 sets out the general principles of safety and inertness for all Food Contact Materials (FCMs). In addition to the general legislation, materials and articles made of plastics intended for food contact applications must comply with the requirements of the Regulation (EU) 10/2011, which establishes a Union List of substances that are permitted for use in the manufacture of plastic FCMs, specifies restrictions and lays down detailed rules for the migration testing conditions to be applied to determine compliance. According to the Regulation (EU) 10/2011, migration experiments shall be performed taking into account the most severe contact conditions of the material with food under foreseeable conditions of use (time/temperature conditions). The representative conditions shall be taken from Annex III (selection of proper food simulant) and Annex V (time/temperature conditions) of the Regulation. The listed food simulants are representative for a food category: acetic acid 3% (w/v) is assigned for acidic foods, Tenax® for dry food, 10% and 20% (v/v) ethanol–water mixtures for aqueous and aqueous-alcoholic food, 50% (v/v) ethanol for dairy products and vegetable oil (e.g., olive oil) for fatty food. Substitute simulants such as 95% (v/v) ethanol and isooctane can be used to replace olive oil as fatty food simulants in case testing with olive oil is not technically feasible or to screen for migration.
Depending on polarity of polymer, simulant and contact conditions, non-polar polymers are swollen by non-polar solvents and polar polymers by polar solvents. This principle was used [
3] for the rapid extraction tests as alternative migration testing and implemented for overall migration testing in the standard EN 1186-15. Polyolefins are swollen by isooctane and polar polymers like polyesters or polyamides by 95% ethanol. These polymers are not or only little swollen by simulants of the reverse polarity [
4,
5,
6]. Styrenic polymers and co-polymers are medium polar. They have strong interactions with both, isooctane and 95 % ethanol. This was recently shown for ABS (acrylonitril-butadiene-styrene-copolymer) [
7].
In the draft document “Technical guidelines for compliance testing” [
8], milder testing conditions (as normally foreseen with olive oil) for the alternative simulants 95% ethanol and isooctane are listed both for polar and non-polar polymers (including polystyrene) to account for the swelling of polymers in contact with solvent, which may occur and thus accelerate migration. Although not finally published (yet), this guideline represents the current state of scientific knowledge for migration testing. Verification of compliance with a specific migration limit for a material or article can be demonstrated using food in a migration test rather than a food simulant (Regulation EU No. 10/2011 Art.18.6). The results of specific migration testing obtained in food shall prevail over the results obtained in food simulant. However, the use of foods for specific migration testing of materials and articles not yet in contact with food, may pose practical constraints, i.e., food composition change during testing, and analytical difficulties. Nevertheless, it is recommended to compare migration into simulants with that into real foods in order to check the appropriateness of simulation especially when doubts occur. Such studies exist e.g., for PET [
9], melamine formaldehyde resins [
10], can coatings [
11] or from various materials into dry foods [
12].
The substances which can migrate from polystyrene (PS) to foods and beverages are residual monomers (i.e., styrene and butadiene), oligomers (principally styrene dimers and trimers) and any applied additives (e.g., antioxidants, antistatics and extenders). As regard styrene, it is an authorized monomer, listed without a specific restriction. Recently, the European Food Safety Authority (EFSA) was requested by the European Commission to evaluate the safety of styrene for use in plastic food contact materials following the classification by the International Agency for Research on Cancer (IARC) as ‘probably carcinogenic to humans’ [
13]. In a first scientific opinion [
14] it was concluded that the IARC conclusions mainly obtained from high dose occupational exposure and animal studies by inhalation cannot be directly applied to the assessment of consumer risk by oral exposure. Genotoxicity associated with oral exposure cannot be excluded but a systematic review of genotoxicity and mechanistic data, comparative toxicokinetics and analysis of species differences is further required for assessing the safety of styrene and styrene oligomers presence in FCM.
The migration of styrene from PS packaging into food has been extensively studied in the last decades in various foods and simulants. Styrene migration strongly depends on its residual level content in the polymer, the fat content of the food as well as the storage temperature and time conditions [
15,
16,
17,
18,
19,
20,
21,
22,
23]. An evaluation of the migration of styrene monomer and oligomers to foods and in their respective PS packagings in comparison with previous data was carried out in 2014 [
24]. The concentrations in various foods were found comparable with literature data reported since the 1980s. According to this review, the level of styrene in foods in contact with PS packaging range from 2.6 to 163 µg/kg (ppb). Styrene dimers and trimers are present in higher concentrations in PS food contact materials than the monomer however; their migration to food is limited, ranging from 2 to 4.8 µg/kg, because of their high partitioning coefficients
K values and their low diffusion coefficients
D in PS products.
Controversial data are reported concerning the migration levels of styrene monomer and oligomers into various food simulants/solvents and on the comparison of migration into real food with the corresponding authorized food simulant (especially the alternative simulants). Styrene migration in 3.5% fat milk was assumed to be comparable to that of the 50% ethanol [
25], in another study to that of 15% ethanol [
20]. Migration testing employing aqueous food simulants lead to under-estimation of the migration processes occurring in real food products [
26,
27,
28,
29,
30]. On the other hand, it has been recognized that certain food simulating liquids like ethanol/water mixtures, n-heptane or isooctane strongly interact with styrene based materials leading to an overestimation of the migration which results into too high diffusion coefficients due to swelling and stress cracks [
27,
31,
32].
The aim of this study was to systematically investigate the migration behavior from GPPS and HIPS polymers and simultaneously the swelling effect using both, food simulants stipulated in EU regulations and real foods. Migration kinetic data from extruded sheets were obtained for a set of spiked model substances as well as for the monomer styrene and styrene oligomers at different time/temperature conditions in 50% ethanol, isooctane and 95% ethanol. Migration of the volatile compounds were quantified in UHT milk (3.5% fat), UHT cream (30% fat) and olive oil. The extent of polymer swelling was characterized gravimetrically and visual changes of the test specimens were recorded after migration contact at temperatures of 20 °C, 40 °C and 60 °C with following food simulants: 3% acetic acid, 10% ethanol, 50% ethanol, 95% ethanol and isooctane. Additionally, several food types (lard, butter, fish oil, orange juice, coffee beans, coffee, noodles, oat flakes, wheat loops and water) as well as the alternative fatty food simulant Miglyol® 812 were gravimetrically investigated for their interactions with GPPS and HIPS under different storage conditions. They were selected in order to cover the majority of foreseeable food contact applications for these materials.
Obtained data show that the use of isooctane as well of high concentrations of ethanol-water, especially at high temperatures, causes swelling and physical changes of polystyrene and results in significantly higher migration values compared to the investigated foods, which tested under typical contact condition do not show polymer interactions.
It is expected that the results of this study will assist in acquiring information about the selection of suitable migration testing conditions for styrenic polymers, especially if a migration limit for styrene and styrene oligomers will be set out in the future, considering the swelling effect caused by some food simulants prescribed in EU regulations.
3. Discussion
The investigated GPPS and HIPS sheets showed normal levels of residual monomer and oligomers when compared with previous reported values for PS food contact materials and articles [
24,
32,
33,
34]. Additionally, the sheets were artificially spiked during extrusion with model compounds that are widely used for migration and functional barrier studies [
35,
36,
37] as well as for so-called challenge tests for determining the cleaning efficiency of recycling processes [
38,
39,
40]. These artificially added substances represent the following general categories: volatile and non-polar (toluene and chlorobenzene), non-volatile and non-polar (phenyl cyclohexane), non-volatile and polar (benzophenone and methyl stearate). The artificially spiked substances were included in the study in order to increase the amount and variety of substances for the migration kinetics as well in order to add polar substances.
Isooctane is known to induce the swelling of non-polar and medium polar polymers like polystyrene according to the principle “like dissolves like” and consequently, to modify the transport properties of the material [
41]. The results of this study show that contact with isooctane caused a clear weight increase (i.e., uptake of isooctane) of the GPPS and HIPS polymers, which is most probably due to this swelling effect. At 60 °C the polymers are completely swollen with a weight increase after 10 days of 45% for GPPS and 30% for HIPS, respectively. The swelling affects the migration kinetics. At 60 °C testing temperature, the initial slope (only visually appreciated) of the migration curves (cf.
Figure 1) in isooctane was very high and substantially similar for all the substances, regardless of their molecular size and polarity. From the diffusion theory [
42], the level of migration from polymer material depends on the initial concentration (
Cp,0) and nature of the substance with its molecular size and solubility, the type of material and the food or food simulant in contact. In this study, the migration kinetics obtained using isooctane reflect more an extraction from the material rather than a diffusion-controlled migration process.
When testing GPPS and HIPS using isooctane at 40 °C or 20 °C (cf.
Figure 2 and
Figure 3), differences for these two polymers were observed.
GPPS was not significantly affected at 40 °C and 20 °C by isooctane with a weight increase below 1%. Migration levels from GPPS were significantly lower compared to 60 °C. At 40 °C migration of the larger components especially of the oligomers from GPPS does not follow diffusion theory. The percentage of migration of the dimers (MW 208 g/mol) is not higher than that of the trimers (MW 316 g/mol), but in a similar range. This suggests a swelling in the first micrometers of the test sheets.
In contrast to GPPS at these lower temperatures, the HIPS polymer was still strongly swollen with a weight increase of 12% and 3% respectively after 10 days at 40 °C and 10 days at 20 °C. Correspondingly, migration values in isooctane at 40 °C are similar or only slightly lower compared to the one obtained at 60 °C. Only when testing HIPS at 20 °C some differences in the migration behavior according to the nature of the substances (as expected from diffusion theory) were noted. These results are in agreement with previous studies [
43], which indicate that the rubber content (styrene-butadiene-copolymer) in HIPS can be an influencing factor for migration of chemicals such as styrene. At 60 °C in isooctane HIPS was less swollen than GPPS, which can be an effect of its increased heat resistance. However, at lower temperatures HIPS was more swollen than GPPS, possibly because of the interactions of isooctane with the rubber. The strong swelling of GPPS at 60 °C related to the lower temperatures and related to HIPS is explained by solvent crazing and stress cracking [
44].
Looking at the migration and weight increase using 95% ethanol, a similar behavior was observed for GPPS and HIPS. 95% ethanol is more polar compared to isooctane, but still causes weight increase (and therefore swelling) of both polymers at all tested temperatures. At all tested temperatures relative migration values are high but depending on the nature of the substances. The time dependent migration of the most volatiles (toluene, chlorobenzene, styrene) was bigger compared to the higher molecular weight test substances (phenyl cyclohexane, benzophenone and methyl stearate with 160, 182 and 298 g/mol respectively).
According to the Regulation (EU) 10/2011, 50% ethanol is assigned to the food category dairy products. The EU 10/2011 test scheme foresees for specific migration, accelerated test conditions at elevated temperature. For example, testing for 10 days at 60 °C simulate contact times longer than 30 days at room temperature and below, while testing for 10 days at 40 °C shall cover all storage times at refrigerated and frozen conditions including hot-fill conditions and testing for 10 days at 20 °C all storage times at frozen condition. Using 50% ethanol, only little weight increase at 60 °C, 40 °C and 20 °C was found for both the polymers investigated in the present study.
Contrary, none of the tested foods did cause a significant weight increase of GPPS or HIPS material or did induce visual changes under real conditions of usage.
Alternative testing conditions (milder compared to the conventional) for the simulants 95% ethanol and isooctane depending on the polarity of the polymer are proposed in the draft document “Technical guidelines for compliance testing” [
8] in consideration of the swelling effect or as substitute test when oil testing is not feasible. In these guidelines, the polymers are divided into two groups, those that contain carbon and hydrogen only and those that contain additionally other atoms. Both polystyrenes, GPPS and HIPS belong to the first group. According [
8], migration testing for the long-term storage condition 10 days/60 °C shall be carried out with 95% ethanol under the same time-temperature conditions as oil, while with isooctane at 10 days/20 °C. For screening purposes, testing conditions are specified for the polystyrene polymer: 10 days at 60 °C, 40 °C and 20 °C in oil are reduced to 1 day for both 95 % ethanol and isooctane, except for 95 % ethanol at 20 °C, which shall be tested with 10 days contact.
In
Table 6 migration of styrene in food simulants and real foods at different time/temperature testing conditions, swelling (as % weight increase) and visual changes of the sample material are reported. For the food simulants, not always exactly 10 days were measured; data were in this case extrapolated. For the alternative fatty food simulants (95% ethanol and isooctane) official and alternative testing conditions are summarized. The alternative testing conditions are in accordance with the draft guideline [
8] (page 59
Table 7: “Test conditions for screening food simulants compared to vegetable oil for specific migration”).
The alternative fatty food simulants isooctane and 95% ethanol cause significant swelling of the HIPS polymer (both considering official and alternative testing condition), which is further increased by higher contact temperatures. The effect is that obtained migration values tend to overestimate the actual migration into real foods. In real foods even after 10 days contact at 40 °C the weight increase remained below 1%.
In case of HIPS, isooctane overestimate real migration into foods at all temperatures.
Concerning GPPS, the migration levels as well as the swelling effect of isooctane strongly depend on the temperature. At 60 °C styrene migration from GPPS reaches approx. 4000 µg/kg after 10 days and the corresponding weight increase is 45%. GPPS exposed to temperatures of 50 °C is affected by stress-cracking [
23], which lead to an overestimation of the migration under the actual condition of use. The weight increase of GPPS after contact with isooctane at 20 °C and 40 °C was below 1%; migration of styrene after 1 day and 10 days at 40 °C underestimates levels found into milk, cream and olive oil tested at 10 days 40 °C.
Testing both HIPS and GPPS with 95% ethanol at the condition 10 days at 40 °C largely overestimates the real migration into milk, cream and olive oil. While testing with 95% ethanol at the condition 1 day at 40 °C styrene migration fits to the real foods.
With 50% ethanol, after 10 days contact at 60 °C or 40 °C both the GPPS and HIPS polymers are slightly swollen (weight increase < 1.1%). Contrary, after 60 days contact with milk, cream and oil at 20 °C or after 10 days 40 °C, the weight increase was below 0.4%. The migration values in this food simulant at 10 days/60 °C are higher (especially in case of GPPS) compared to the real food tested at 10 days/40 °C. Testing both polymers with 50% ethanol at the condition 10 days/20 °C underestimates the real migration into foods. In case of HIPS, migration levels at 10 days/40 °C fit the styrene migration at the same condition in cream and olive oil but slightly overestimate that in milk. For GPPS, although a measurable weight increase was observed at the condition 10 days/40°C, migration of styrene remained below the detection limit.
In conclusion, contact conditions for migration testing of 10 days at 60 °C using isooctane, 95 % ethanol and 50 % ethanol are too severe for both GPPS and HIPS. Other combination of time/temperature conditions should be applied. These findings are in good agreement with previously reported results for other polymers, such as for acrylonitrile-butadiene-styrene copolymer (ABS) [
7], styrene-acrylonitrile copolymer (SAN) [
45] and polyethylene terephthalate (PET) [
46].
Migration of styrene from HIPS tested using migration cells at 10 days/40 °C into foods with 30% fat content was reported by Linssen and Reitsma [
47] in the range of 240–320 µg/kg; while the levels found in corn oil were higher reaching approx. 400 µg/kg.
It is worth to be noted that the testing conditions applied in this study for the food milk cream and olive oil do not represent a common real application of FCM made of polystyrene and have to be considered a worst-case condition. The majority of the products packed in PS, which are mostly a blend of GPPS and HIPS, are refrigerated (5 °C) with a shelf life of approx. 40 days, (e.g., yoghurt pot). Typical values found in recent food surveys [
22,
24] report styrene levels into foods taken from the manufacturer or from the retail ranging from 2.6 to 163 µg/kg (ppb) depending on the fat content and the time/temperature storage conditions.
The migration values determined within the present research work for styrene in the real foods are not representative for the levels found in real sample from the market. Additionally, the test stripes were totally immersed in the foods and the simulants. Thus, edge effects occur which lead to an overestimation of the migration under the actual condition of use where foods are packed in containers. However, the experiments for this study were carried out in the same way for food simulants and foods therefore allowing comparisons, which was the intention.
As swelling of GPPS and HIPS was little or not detectable in contact with real foods under realistic application conditions, this must have consequences for the migration testing protocol. According to the Regulation (EU) 10/2011, the use of 95% (v/v) ethanol and isooctane as simulants for compliance testing of FCM is accepted to screen for specific migration or to replace olive oil without any specification for which polymers this approach is useful. Migration testing with 95 % ethanol and isooctane is a widely used practice to simplify and reduce the time-consuming and expensive analytical procedures. On the other hand, alternative test simulants in order to screen for migration are suitable to show conformity only, but not non-conformity of a plastic food contact material. Furthermore, according to Regulation (EU) 10/2011, Chapter 2 paragraph 2.1.3 (i): in case of physical changes which do not occur under real use, “the migration tests shall be carried out under the worst foreseeable conditions of use in which physical or other changes do not take place”. Such physical changes in GPPS and HIPS were clearly found in this study.
The comparison with foods was done only for the volatile components. The results discussed for styrene were similar to those for the added test substances toluene and chlorobenzene. All three substances show similar relative migrations in the respective foods and simulants. For larger molecules like the oligomers, the swelling might have a stronger effect on the migration. Further research work is envisaged in order to better evaluate the overestimation of the laboratory simulation compared to real food contact applications and to identify the most suitable test conditions for a realistic simulation of the migration.
4. Materials and Methods
4.1. Sample Materials
The GPPS and HIPS grades used in the present study are standard industrial materials used in the European market for food contact applications. The used GPPS grade has a molecular weight of around 240.000 g/mol, melt flow index of 12 cm3/10 min at 200 °C/5 kg load, glass transition temperature of 100 °C and density of 1040 kg/m3. The used HIPS grade has a molecular weight of around 190.000 g/mol, melt flow index of 4 cm3/10 min at 200 °C/5 kg load, glass transition temperature of 100 °C and density of 1030 kg/m3. The pellets (gently provided by the Styrenics Steering Committee of Plastics Europe) were processed using a Collin Lab & Pilot Solutions co-extruder (Maitenbeth, Germany) to produce sheets with a thickness of approx. 350 µm. For both polymers, the temperature in the extruder was approx. 190 °C; the screw speed was approx. 100 rpm.
A mixture of five model substances, with a variety of chemical and physical properties, were spiked homogeneously during polystyrene sheets production as already described in [
7]. They were toluene (CAS No. 108-88-3), chlorobenzene (CAS No. 108-90-7), phenyl cyclohexane (CAS No. 771-98-2), benzophenone (CAS No. 119-61-9) and methyl stearate (CAS No. 112-61-8). All chemicals used in the experiments were of analytical grade: toluene, styrene and benzophenone were purchased from Sigma-Aldrich (St. Louis,MO, USA); chlorobenzene from Merck (Darmstadt, Germany); phenyl cyclohexane from Alfa Aesar (Ward Hill, USA) and methyl stearate from Fluka (Seelze, Germany).
4.2. Determination of the Initial Concentration (Cp,0) of Spiked Substances, Styrene Monomer and Styrene Oligomers
The initial concentrations of the spiked substances as well as of the styrene monomer and styrene oligomers were determined in the GPPS and HIPS spiked sheets by extraction with acetone (purchased from Chemsolute, Th.Geyer GmbH, Renningen, Germany). 1.0 g of the material was extracted with 10 mL acetone and stored at 60 °C for 3 days. Triplicate experiments were performed. A subsequent extraction (under the same condition) and recovery experiments were performed to check for exhaustiveness.For recovery experiments, reference sheets (not spiked during extrusion) were used. They were fortified at three levels with the model substances and available commercial standards for styrene dimers (PSS-266) and trimers (PSS-370) purchased from PSS Polymer Standards Service (Mainz, Germany), directly in the vial, extracted and analyzed as the samples.
The extracts were analyzed by gas chromatography with flame ionization detection (GC-FID). The Agilent 6890 GC/FID system (Agilent Technologies, Santa Clara, CA, USA) was used with the following conditions: column: DB 1 (length 20 m, inner diameter 0.18 mm, film thickness 0.18 µm); GC temperature program: 50 °C (2 min), followed by heating at 10 °C min−1 to 340 °C (15 min); pre-pressure: 50 kPa hydrogen; split: 10 mL min−1. The injection volume was 5 µL with a split ratio of 1:20. An internal standard solution of di-(tert-butyl)-hydroxyanisol (BHA purchased from Fluka, Seelze, Germany) and Tinuvin 234 (Ciba, Basel, Switzerland) was used for all GC analyses to check the stability of the retention times and the performance of the GC analysis.
Quantification of spiked substances and monomer styrene was achieved through calibration with respective standard substances, while styrene oligomers were semi-quantified using the response factor of BHA.
4.3. Migration Kinetics in Food Simulants
The migration kinetics of the artificially spiked substances as well as of the styrene monomer and oligomers were determined into the following food simulants: 50% ethanol, 95% ethanol and isooctane, at temperatures of 20 °C, 40 °C and 60 °C, monitoring several time points (from 0.5 h to 273 days, sampling times were adapted depending on the food simulant and contact condition). All solvents used in the experiments were of analytical grade and were purchased from Chemsolute, Th.Geyer GmbH, Renningen, Germany.
Migration contact with the food simulants was performed by total immersion in capped migration vials in triplicate. The used surface/volume ratio was 1 dm2 (total surface of the test specimens) per 100 mL simulant. In parallel to the samples, simulant blanks were prepared. At each kinetic time point a 0.5 mL aliquot was taken for analysis.
Quantification of migration into simulants was obtained using gas chromatography coupled with flame ionization detection (GC/FID) by external calibration with standard dilution series in the respective simulants. The same chromatographic conditions as for the quantification of initial concentrations in the polymer were used. To check stability and possible loss by evaporation of the volatile substances, control tests were performed as well. The sensitivity of the analysis for each substance in the respective food simulant was determined on a 95% confidence interval according to the calibration curve method (see
Table 7).
4.4. Migration of Volatile Substances in Real Foods
The migration kinetics of the volatile substances (toluene, chlorobenzene and styrene) in real foods was determined at 20 °C after 15, 20, 30, 40 and 60 days, additionally the time/temperature condition 10 days/40 °C was investigated. UHT milk (3.5% fat), UHT cream (30% fat) and olive oil were selected as real food. They were not in previous contact with polystyrene and were bought from a local market in Munich, Germany.
The migration tests in real food samples were performed by total immersion, in triplicate. The used surface/volume ratio was 0.2 dm2 (total surface) per 20 g (food) and therefore equivalent to 1 dm2/100 mL as used for the food simulants. A spatula tip of sodium azide was added to avoid spoilage. After migration contact, 5 g of each food sample (after homogenization) was placed into a headspace vial and analyzed by headspace gas chromatography with flame ionization detection (Headspace-GC-FID).
The Perkin Elmer AutoSystem XL (Perkin Elmer Industries, Shelton, USA) was used, with the following conditions: headspace auto-sampler: Perkin Elmer HS 40 XL, oven temperature: 80 °C or 120 °C (only for olive oil), needle temperature: 110 °C or 130 °C (only for olive oil), transfer line temperature: 120 °C or 140 °C (only for olive oil), equilibration time: 1 h, pressurizing time: 3 min, injection time: 0.05 min, withdrawal time: 1 min; GC column: DB 624 (length 60 m, inner diameter 0.32 mm, film thickness 1.8 µm); GC temperature program: 40 °C (6 min), heating rate 5 °C min−1 up to 90 °C and then 10 °C min−1 up to 260 °C (10 min), pressure: 120 kPa helium, split: 40 mL min−1.
For quantification, external calibration on each food matrix was carried out individually. Standard solutions (4 levels, diluted from a stock solution prepared in ethanol) were added to each foodstuff in twice, after homogenization 5 g of each spiked food level was analyzed to obtain a standard calibration curve. Control standards (known spiked amount of each food) were analyzed in parallel to the samples to check for recovery. The limit of detection of the method was defined as three times the signal-to-noise ratio (see
Table 7).
4.5. Weight Increase with Food Simulants and Real Foods
The extent of polymer swelling was characterized gravimetrically as weight increase of the test specimens after contact with the simulants or foods. Weight increase for the GPPS and HIPS polymers were obtained after contact with 50% ethanol, 95% ethanol and isooctane at the same time/temperature condition used for the migration experiments. Additionally, 3% acetic acid 10% ethanol and 20% ethanol were investigated for their interactions with GPPS and HIPS at temperatures of 20 °C, 40 °C and 60 °C, adapting sampling times (until max 273 days) on the food simulant and contact condition. As for the migration tests, experiments were performed by total immersion, in triplicate. The used surface/volume ratio was 0.2 dm2 (total surface) per 20 mL (simulant).
Weight increase of the investigated polymers in contact with UHT milk, UHT cream and olive oil were obtained simultaneously with the migration experiments (by weighing the test specimens before and after each migration contact preparation). Additionally, other foods (lard, butter, fish oil, orange juice, coffee beans, coffee, noodles, oat flakes, wheat loops and water) as well as the alternative fatty food simulant Miglyol
® 812 (CAS No. 73398-61-5: Decanoyl-and Octanoylglycerides) were tested to determine the extent of swelling. The selection of foods and testing conditions was based on the input provided by the Styrenics Steering Committee of Plastics Europe on those foods that are widely in contact with polystyrene in real applications (see
Table 4 and
Table 5). All investigations were performed in triplicate.
4.6. Testing on Visual Changes of the Polymers after Contact with Food Simulants and Real Foods
During the kinetic migration tests and the polymer swelling tests, changes in the appearance of the GPPS and HIPS test specimens were noted and photos were taken at many time intervals. All the photos of the test material taken at the different intervals during the kinetics tests are given in the
Supplementary material.