In an article published on July 13, 2018, in the peer-reviewed journal Food Packaging and Shelf Life, Maria Hoppe and colleagues from the Fraunhofer Institute for Process Engineering and Packaging IVV, Freising, Germany, address oligomers in polyethylene naphthalate (PEN) and polybutylene terephthalate (PBT). Both materials are used in food contact applications, such as “microwaveable dishware, kitchen utensils and coffee capsules.”
The analyzed samples included a bottle made of PEN and plates made of PBT, obtained from “local suppliers.” The scientists used dichloromethane or acetonitrile to extract the samples for one day at room temperature or 40 °C, respectively, and analyzed the extracts for the oligomers sized below 1000 Da. In the PBT extracts, seven cyclic and three linear oligomers were identified, while in the PEN extracts, five cyclic and two linear oligomers were found. The total oligomer content was 0.34% in PBT and 0.81% in PEN material.
Migration experiments were performed for the samples of PBT plates which were immersed in 20% ethanol and stored at 40 °C or 60 °C for 30 days. Only the low molecular mass oligomers, i.e. the linear and cyclic PBT dimer and the cyclic PBT trimer, could be detected in the migrates, while the oligomers with higher molecular mass “were less abundant in the polymer and are not expected to migrate to a great extent since the diffusivity decreases with increasing molecular mass.” For the two quantifiable PBT oligomers, however, “even at a storage temperature of 40 °C after three days the migration . . . into the food simulant will exceed 50 µg/kg.”
The 50 µg/kg value for oligomer migration from PBT has not been officially defined as a migration limit, but it “can be used as an indirect migration limit since this level is considered to not raise any concern from a toxicological viewpoint,” the authors explain. They suggest that the high migration observed could be due to “swelling of the PBT polymer at elevated temperatures” and argue that “the use of ethanolic food simulants at elevated temperatures to simulate long-time storage scenarios for polyesters like PBT is debatable” (FPF reported). Based on modeling estimations, the migration values at 23 °C would be “very low” even after 900 days of storage, namely at 28 µg/kg for a 0.5 L bottle made of PBT and 2.5 µg/kg for a similar bottle made of PEN. To confirm the modeling predictions and potentially “adjust the testing conditions for polyester type polymers,” the authors call for “investigating experimentally the migration of oligomers during longer storage times at room temperature and comparing these values to accelerated migration tests.”
PBT oligomers have also been studied by Fabrian Brenz and colleagues from the Department of Chemistry and Food Chemistry, Technical University of Dresden, Dresden, Germany, who summarized their findings in an article published on December 28, 2017, in the peer-reviewed journal Food Additives & Contaminants: Part A. They analyzed “PBT pellets and the slotted spoon produced from the pellets by injection molding . . . provided by a manufacturer of plastic kitchen utensils.” Isolation of PBT oligomers was performed either by extraction in dichloromethane, acetonitrile, dimethylsulfoxide, or 20% ethanol (one hour at 60 °C) or by reprecipitation where the polymer is first dissolved, then treated in an ultrasonic bath, then reprecipitated and washed with 2-propanol. The supernatant remaining after reprecipitation and washing steps is then evaporated, and “residual oligomers” are dissolved in dimethylsulfoxide before analysis by mass spectrometry.
The total content of isolated oligomers was estimated to be around 0.8% after dichloromethane extraction or around 1% after extraction by reprecipitation. In the acetonitrile extract of PBT, 26 different oligomers sized below 1000 Da were identified in total, including cyclic oligomers, linear oligomers, and dehydration products. In addition, a cyclic pentamer sized 1100 Da was identified as well. In 20% ethanol extracts, several differences were noted compared to the acetonitrile extracts, the “major difference . . . [being] that cyclic oligomers were not detectable except for the dimer.” Therefore, the authors concluded that “although the amount of cyclic oligomers is predominant in PBT it does not seem to reflect their migration potential into aqueous foods since the dominant oligomers in 20% ethanol are the linear ones.” Similar to Hoppe and co-authors, Brenz and colleagues also questioned whether ethanol “is appropriate to be used as a food simulant for migration testing of polyesters.”
They also studied the migration of PBT oligomers in water, namely by boiling the PBT spoon in distilled water for two hours, cooling, repeating the same procedure for two more times, and using the third migrate for analysis. The sum of migrating linear oligomers amounted to 0.29 mg per item, while the cyclic PBT dimer was detected at 0.05 mg per item. According to the estimations for “a scenario in which an adult would be preparing a meal with the PBT spoon in the range of once per week to once per day (worst case),” an intake would be at “0.04 up to 0.29 mg/person per day for linear oligomers,” and “0.007 up to 0.05 mg/person per day for cyclic oligomers.” These values do not exceed the daily intake limits assigned based on the threshold of toxicological concern (TTC) considerations, namely the value of 1.8 mg/person per day for linear oligomers and 0.09 mg/person per day for cyclic oligomers. Therefore, “the migration of PBT oligomers from the tested PBT spoon could be considered as safe with regard to aqueous foodstuffs,” the authors concluded.
Hoppe, M., et al. (2018). “Oligomers in polyethylene naphthalate and polybutylene terephthalate – Identification and exploring migration.” Food Packaging and Shelf Life 17:171-178.
Brenz, F., et al. (2018). “Linear and cyclic oligomers in polybutylene terephthalate for food contact materials.” Food Additives & Contaminants: Part A 35:583-598.