Safety Aspects of Polymer Films with Atomic Layer Deposited Thin Al2O3 Barrier Layers

Atomic layer deposition (ALD) is an efficient tool for improving the barrier properties of polymer films and polymer-coated papers/paperboards. However, adequate barrier properties are only one of the requirements set for the packaging materials. The purpose here is to present the safety aspects related to polymer films with a thin ALD deposited oxide barrier layers, especially those with Al2O3. Several approaches for testing ALD material were utilized. Based on our analyses ALD grown uniform Al2O3 thin barrier layers do not fall under the European Commission's definition of nanomaterial and the related limitations in the Commission Regulation. Plastic food contact materials may also contain non-authorized substances provided that their migration to food do not exceed very low migration limit (0.01 mg/kg food) and they are not mutagenic, carcinogenic nor toxic for reproduction, or are in nanoform. However, functional barrier concept should not be used to separate food and nanomaterial. Meanwhile, U.S. Food and Drug Administration (FDA) has a more inclusive approach to identifying products of interest in context of nanotechnology including size and/or function. FDA does not categorically judge products containing nanomaterials to be inherently harmful. Al2O3 and organic compounds of interest, such as ethylene glycol and glycerol, are accepted in Europe as additives for plastics in contact with food. Likewise, in USA these are accepted as either as indirect additives used in food contact substances or generally regarded as safe. According to some producers AlOx coatings have also been approved in USA for indirect contact with food. In our experiments abrasive testing of polymer films, although detrimental in several cases to the film itself, did not increase the number of airborne particles. ALD Al2O3 deposited at 100 ºC onto silicon and commercial polymer films were also exposed to liquid food simulants while evaluating the change in surface and barrier properties. Finally, we also tested the films for migration, although overall migration from thin films has been previously regarded as extremely low. The Al2O3 coated films were affected by the simulant exposure, but the impact depended on the substrate material and the oxidizing precursor. This indicates different thin layer growth on different polymer films. As a conclusion, ALD grown Al2O3 thin layer requires an additional, for example, laminated or extrusion coated top layer to protect it from both mechanical and chemical stresses, to prevent migration to the food, and also to act as a sealant layer in food packaging.