Antioxidant 168: A Plastic Additive in Polypropylene and Polyethylene

General Description

Antioxidant 168 is a plastic additive mainly incorporated into Polypropylene (PP) and Polyethylene (PE) as an antioxidant. It has been used in food packaging , supposedly explaining its detection in frozen vegetable.

Figure 1. Properties of Antioxidant 168

Applications and Degradation

Antioxidant 168 is a plastic additive used as an antioxidant in polyolefin with concentration ranging from 0.004 to 3% (wet weight). Oxidation from the reduced form to the oxidized form is a common phenomenon for this molecule, thus making the protection of the polymer against oxidation possible. In studies concerning Antioxidant 168 from food packaging, both forms are usually targeted. According to Yang et al. (2016) oxidation of Antioxidant 168 can occur either during the extrusion of the polymer, during storage in the dark or during exposure to UV; the latter being the fastest pathway. Here it is not possible to assess whether the oxidation process of Antioxidant 168 in the laboratory has occurred (i) during storage in the dark as laboratory reagents were stocked in safety cabinets or in the fridge straight from their delivery at the laboratory until their use or (ii) during the production and transport process of both the packaging and the reagent. It was recently suggested that when studying antioxidants, the oxidized form should be primarily targeted as the reduced form will react with oxygen and will not accumulate in the environment. This is confirmed in the present study as the oxidized Antioxidant 168 was systematically detected in quantifiable amounts whereas its reduced form was under the limit of detection in some laboratory reagents.^1-2

Figure 2. Transformation product of Antioxidant 168 obtained after oxidation and hydrolysis

Antioxidant 168 leached out of food packaging in contact with oil and its oxidized form, the tris(2,4-ditert-butylphenyl) phosphate, leached from PE placed in stirred water. Both forms were also detected in plastic pieces used in oyster farming or ingested by seabirds and detected in indoor dust. Antioxidant 168 can be also hydrolysed over time in the aquatic system and one of its degradation products, the 2,4-ditert-butylphenol  has been found in plastics collected on Korean beaches. A second degradation product (bis(2,4-ditert-butylphenyl) hydrogen phosphate) was reported to impair growth of human cells in culture while Chinese hamster ovary cell lines are also sensitive to this chemical. Oxidized Antioxidant 168 has been detected in one laboratory solvent, likely the result of leaching during chemical storage in the laboratory, it appears necessary to perform an inventory of potential Antioxidant 168 contaminations in the laboratory workplace prior to any leaching study, such as those using in vitro simulated gut conditions as already used to study the desorption of oestrogenic chemicals from plastic items. In the present paper, the reduced (tris(2,4-ditert-butylphenyl) phosphite) and the oxidized (tris(2,4-ditert-butylphenyl) phosphate) forms of Antioxidant 168 were quantified in both the laboratory chemicals used to mimic in vitro gut conditions and their containers to account for potential contamination.^3-4

Preparation

The preparation method of Antioxidant 168 comprises the following steps: (1) adding 2, 4-di-tert-butylphenol, a solvent and a resin catalyst into a reaction kettle, and heating while stirring at a constant speed; dropwise adding phosphorus trichloride into a reaction kettle at a constant speed, and after the reaction is finished, adding resin as a neutralizer to neutralize the reaction solution to be neutral; (2) filtering the reaction solution obtained in the step (1) to filter out solid resin; treating the filtrate to obtain an Antioxidant 168 product; (3) using a part of the resin filtered in the step (2) as a catalyst for the synthesis reaction of the next batch; the rest is regenerated and used as a neutralizer for reuse. The catalyst used in the invention can be simply, conveniently, quickly and thoroughly separated from the system after the reaction is finished, can be continuously used, does not generate solid waste, can effectively improve the quality of the Antioxidant 168 product, and reduces the environmental pollution.⁵

References

1. European Chemicals Agency, 2019. Mapping exercise – plastic additives initiative [WWW document]

2. Hahladakis, J.N., Velis, C.A., Weber, R., Iacovidou, E., Purnell, P., 2018. An overview of chemical additives present in plastics: migration, release, fate and environmental impact during their use, disposal and recycling. J. Hazard. Mater. 344, 179–199.

3. Cherif Lahimer, M., Ayed, N., Horriche, J., Belgaied, S., 2017. Characterization of plastic packaging additives: food contact, stability and toxicity. Arab. J. Chem. 10, S1938–S1954.

4. Dopico-García, M.S., López-Vilariñó, J.M., González-Rodríguez, M.V., 2007. Antioxidant content of and migration from commercial polyethylene, polypropylene, and polyvinyl chloride packages. J. Agric. Food Chem. 55, 3225–3231.

5. CN110590834A