Specific migration of caprolactam and infrared characteristics of a polyamide/polyethylene composite film for food packaging under conditions of long‐term storage before use

Caprolactam is the monomer typically used for the production of polyamide 6 films. Originating from incomplete polymerization reactions, caprolactam can remain part of the final food packaging and has the potential to migrate from the plastic packaging into the packed good. For film and food producers as well as for end users, it is of great interest that the conformity of a film with migration limits is ensured over its lifetime and that environmental conditions do not affect the migration behavior inadequately. In this work, we have investigated the long‐term storage characteristics of a polyamide/polyethylene (PA/PE) film over a storage period of 40.5 months with respect to the specific migration rate of caprolactam. Within statistical indicators, this rate is stable over the complete storage period. Different exposure conditions were applied to the same PA/PE film to accelerate the aging process. Alterations under forced aging conditions (elevated temperature and ultraviolet radiation) were studied by Fourier transform infrared (FTIR) spectroscopy to detect possible chemical changes in the polymer bulk and, by the use of the attenuated total reflection method, on the sample surface. The film was subjected to an extraction study to verify the results obtained using FTIR. Additionally, mechanical tensile tests were carried out. Indication for alterations regarding caprolactam under the different storage conditions were found. The results clearly show that photo‐oxidative conditions led to changes in the chemical and mechanical properties of the film. New chemical species occurred caused by degradation of the polymers and their contribution to chemical migration is discussed.

in the composition of the food or a deterioration in its organoleptic properties." In order to regulate the migration of relevant chemical substances from plastic into food, specific migration limits were set according to EU Regulation 10/2011. 5 Aside from the overall migration, the regulation governs the specific migration of monomers used for the production of plastic packaging like caprolactam, the monomer used for the production of polyamide 6 (PA 6) films. Besides other nylon resins like polyamide 66 or polyamide 11, PA 6 films manufactured by the polymerization of ε-caprolactam are listed, for example, by the U.S. Food and Drug Administration to be safely used to produce articles intended for use in processing, handling, and packaging food. 6 If the polymerization reaction is incomplete, caprolactam remains as part of the final plastic product 7,8 and has the potential to migrate from the plastic product into the materials or food in contact with the plastic. Caprolactam is not unobjectionable in terms of consumer protection: according to EU-GHS classification, an acute toxicity (category 4) is established if inhaled or swallowed. 9 Therefore, a total specific migration limit (SML (T)) was set in the EU Regulation 10/2011 5 for the monomer caprolactam and caprolactam sodium salt to 15 mg/kg. Plastics manufactured for food contact like packaging films will be tested for legal compliance with the specific migration limit (SML) or SML (T) in general shortly after the manufacturing process. However, two aspects need to be further taken into account: during its lifetime, the film undergoes a natural aging process and the film is in constant contact with environmental conditions, which may accelerate the degradation process. 10 Therefore, it is of strong interest to investigate the chemical and physical properties of a packaging during its entire lifetime. The overall migration behavior under longterm storage was already investigated for a polyamide/polyethylene (PA/PE) film, and it was concluded that the limited migration potential is preserved over the lifetime of the film and even improved with regard to consumer protection because the overall migration rate was decreasing under prolonged time of storage. 11 However, the most important monomer of PA 6, caprolactam, is not covered by the overall migration testing procedure. 12 Only a few publications are dealing with the stability of caprolactam under storage conditions. Pramoda et al. 13 reported a thermal decomposition of PA above 300 C with the evolution of caprolactam followed by other volatiles. Contradictory results on the residual level of caprolactam after γ-irradiation were described by Araújo et al. 14 : the examined multilayer PA 6 films showed an increase, reduction, or no modification of the residual caprolactam level after irradiation when compared with nonirradiated films of the same type. The authors concluded that an increased residual caprolactam level occurs due to degradation of the polymer, whereas a reduction could be caused by crosslinking of caprolactam monomers with other compounds. In a previous work, Funk et al. 11 reported that the caprolactam migration under simultaneous ultraviolet (UV) and heat treatment appeared to be smaller than for standard testing conditions of 10 days at 40 C but without significant differences. Stoffers et al. 15

| MATERIALS AND METHODS
Three rolls of PA/PE composite films (80-μm PA; 120-μm PE) encoded RM-WS F 1, RM-WS F 11, and RM CP B F 18 were obtained from a commercial packaging manufacturer. All three films were manufactured according to the same manufacturing process. Details of the production process of the film, its field of application, and homogeneity considerations were published elsewhere. 11 All experiments were performed on these commercial composite films in order to provide relevant comparability of the results as opposed to investigations on pure PE and/or PA films, which would, however, be quite instructive for future fundamental assignments.

| Specific migration testing for caprolactam under long-term storage
For reasons of statistical evidence, the samples were examined by experienced and, to a large extend, accredited laboratories. The examination of the samples for the specific migration of caprolactam was measured in the framework of several proficiency testing (PT) schemes designed according to Eurachem Guide PT type 3.b 16  According to the current legislation, testing for 10 days at 40 C is still valid and required to test for compliance with the SML or SML (T) in long-term contact under refrigerated or frozen storage conditions. Additionally, EU Regulation 10/2011 5 allows to use migration conditions of 10 days at 40 C for long-term storage conditions at room temperature "if there is scientific evidence that migration of the respective substance in the polymer has reached equilibration under this test condition." As an additional blank experiment, migration was also studied at 60 C with the film RM-WS F 11 for the food simulant distilled water. It was reported earlier that the migration of caprolactam under simultaneous UV and heat treatment (sample heated to 60 C by UV illumination) appeared to be smaller compared with standard testing conditions of 10 days at 40 C for the food simulant distilled water, but without significant differences between both best estimates (coverage factor k = 1 for probability P = 68.3%). 11 To differentiate between a possible influence of either UV or increased temperature, it was investigated whether moderate heat (60 C) could cause changes in migration within a subsequent PT round. Furthermore, since the EU Regulation 10/2011 5 became effective, it is now valid legislation that specific migration testing conditions for 10 days at 60 C cover long-term storage at room temperature.  Different exposure conditions of either UV with simultaneous heat exposure or solely heat exposure were conducted prior to measurements to accelerate the aging of the composite film: the samples exposed solely to heat were stored at air under 48-50 C, chosen to be above 40 C which was the standard migration temperature in this work but below 60 C chosen to gain comparative migration data. A typical day-night rhythm was established with 10 h of heat exposure per day and nonexposure at ambient temperature for 14 h per day for samples exposed for 1, 2, 4, 7, and 10 days. The samples exposed for 84 days were exposed for 8 h per day on average and under nonexposure for 16 h, respectively. Blank samples (kept in the dark at room temperature) were measured at the beginning (encoded blank sample 1) of the exposure phase and at the end after 84 days (encoded blank sample 2).

| IR characterization
The exposure conditions of UV and simultaneous heat exposure were chosen identical to the conditions applied earlier for the global migration experiments 11 to examine spectroscopically whether concurrent UV and heat exposure has an impact on caprolactam migration. An ULTRA-VITALUX lamp manufactured by OSRAM was used because of its sun-like radiation spectrum. The sample temperature was adjusted to 60 C during exposure. Further details were published elsewhere. 11 The exposure conditions were established under the same day-night rhythm as for the samples stored under 48-50 C.
Additionally, one sample exposed to UV for 39 days and stored afterwards in the dark at ambient temperature for 12 days and one exposed to UV for 84 days and stored afterwards for 119 days was measured to differentiate between reversible and irreversible degradation phenomena.

| Mechanical tests
The mechanical tests were performed using a Zwick Roell Z 0.5 at an elongation speed of 200 mm/min and a 100-N load cell leading to a maximum force of 106 N. According to ISO 527-1, 20 RM-WS F 11 specimen 1.5 cm wide and about 15 cm long were cut and fixed between two pressure plates at a distance of 10 cm.
F I G U R E 1 Best estimates for the true value of the specific migration of caprolactam (with the corresponding standard deviation) from polyamide/polyethylene films into different food simulants after storage of the packaging film over different periods of time up to 40.5 months prior to testing T A B L E 1 Summary of the statistical data for the specific migration measurements for caprolactam obtained from the long-term monitoring after storage of the PA/PE films RM-WS F 1, RM-WS F 11, and RM CP B F 18 in the dark at room temperature

| Extraction study to determine NIAS
A screening of the PA/PE film for NIAS was performed at an external accredited laboratory. The sample of RM CP B F 18 exposed to UV for 39 days and a blank sample of the same film were examined. The

| Long-term migration studies
Because caprolactam can be considered the most important migrant from PA 6 films, its specific migration behavior is of particular interest.    11 Despite the lack of hard evidence, we suggest a slow postcrystallization of PA as possible origin: when exposed to humid air, PA is known to absorb water, which lowers the glass transition temperature (Tg). 22 Because a decreasing Tg increases molecular motion, ongoing crystallization appears reasonable even at room temperature under the present conditions. A decreased migration of caprolactam as indicated in our study could then result from such increased crystallinity.  Figure S1).

| IR spectroscopy
Region (b) shows a decrease in the transmission for the sample exposed to UV, however, without any characteristic spectral signature. In this spectral range, neither for PE nor for PA typical bands occurred in the ATR mode (see below). The observed decrease in transmission of the bulk films must, therefore, be caused by morphological alterations, which change the scattering characteristics. In view of the brittleness, which becomes obvious for samples exposed to UV Changes of the IR characteristics on the PA side of the films were investigated in the ATR mode for the carbonyl region. The same decreasing transmission at 1715 cm −1 , which was found for the bulk of the polymer, was found on the PA side (Figure 4), in particular in the spectra acquired after 39 and 84 days of simultaneous UV and heat exposure, confirming the formation of ketones. 23 It is worth to notice that at room temperature, the newly formed components seem to be stable because they are still detected even after storage for 119 days subsequent to the UV and heat exposure. In the case of the thermo-oxidized samples, no such change in the transmission characteristic was observed for the PA surface of RM-WS F 11 (Figure 4), indicating that under the present conditions, no thermo-oxidation of the PA part of the film occurred.
In view of the apparent changes due to accelerated aging conditions, it was examined whether other effects on the PA-part of the composite film can also be found as a consequence of forced aging.
For reasons of clarity, only the results of the sample that was exposed

| Mass spectrometry on extracts from the differently exposed films to analyze NIAS
An extraction study was performed to verify the results of IR spectroscopy and confirmed that a significant change in the film occurred under simultaneous UV and simultaneous heat treatment and newly formed substances were extracted from the UV-exposed PA/PE-films heat. 11 The results showed that the global migration after 10 days of exposure (UV and simultaneous heat treatment) for the PA/PE film increased significantly compared with standardized overall migration conditions but stayed below the limit of 10 mg/dm 2 . No data are currently available on global migration following prolonged exposure.
In the LC-QTOF-MS screening, five unknown substances were detected in the nonexposed comparison sample, as opposed to 13 unknown substances in the exposed sample. These substances also have the potential to migrate to food upon contact. A potential risk cannot be assessed nor excluded because these substances were not chemically identified.
All substances detected here, whether identified or unknown, must also be evaluated in the context of Article 3 of the framework

| Mechanical tests
To test for mechanical consequences of the chemical changes spectroscopically observed during photo-oxidative and thermo-oxidative exposure, all samples have been mechanically tested before and following exposure.  Figure 8A). The stress-strain diagram then followed a zig-zag curve with a uniform steep increase and decrease of tension by 2-3 N/mm 2 . Beyond 2 days of illumination, the damage to the composite film has progressed so far that the film lost its original tensile properties. These results showed that, on the one hand, chemical changes occurred on the interface of both polymers decreasing adhesion and, on the other hand, the PE part was changed considerably stronger in its mechanical characteristics leading to rupture and delamination following UV exposure. It is conceivable that substances that formed in the degradation processes affected the interface. Delamination of the two parts of the composite film and stretching of the PA part under stress can neither be observed for the unexposed samples nor for the samples exposed solely to heat.
A further mechanical consequence of UV exposure was noticed under prolonged exposure. Embrittlement of the composite RM-WS F 11 films was noticed after 84 days of UV exposure even without any stress extended to the samples in the framework of mechanical tests.
While cutting a piece of the samples with a knife to carry out FTIR spectroscopy, samples started to spontaneously break in longitudinal direction, parallel to the extrusion direction ( Figure 8B).
In summary, the mechanical tests revealed that alterations of the RM-WS F 11 composite film upon UV and simultaneous heat take place already after 2 days of exposure. This supports a conclusion from the spectroscopic findings where changes in the scattering characteristics in the spectral range between 1741 and 2830 cm −1 were traced back to morphological alterations.

| CONCLUSIONS
The main consequences of the present results obtained following accelerated aging conditions can be discussed regarding the following: (a) the knowledge gained on migration characteristics and UVinduced changes in the concentration of caprolactam: • Over more than 40 months of storage, the migration of caprolactam in the investigated PA/PE film is stable. A specific migration test for caprolactam for a PA/PE film at the beginning of the film's life cycle will be valid for using it throughout its entire lifetime if stored properly.
• Under the inspection conditions of 10 days at 40 C, the available caprolactam is migrating almost completely to the chosen food simulants.
• • Newly formed chemical species with characteristic carbonyl groups were detected in the bulk of the polymer as well as on both surfaces due to degradation by means of FTIR spectroscopy.
• The extraction study shows that photo-oxidative alterations led to the formation of butanal, MEK, and fatty acids. Additionally, the amount of extracted unknown substances increased as a result of exposure.
• The formation of degradation products predominantly occurs irreversibly.
Once the degradation process is initiated, the film remains damaged and degradation products become a permanent part of the film.
The presented results suggest that packaging exposed to photooxidative environmental conditions might negatively affect a packaged food with regard to its composition and organoleptic properties.
Migration of NIAS into packed foodstuff appears possible and even likely since such substances were explicitly detected on the polymer surfaces. The present results indicate that such substances need to be taken into account for the risk and conformity assessment to ensure food safety for a specific packaging material.