Functionalization of PLA films with ALD Al2O3 coatings for safe food packaging

Abstract

This study investigates how aluminum oxide (Al2O3) coatings deposited by atomic layer deposition (ALD) can improve the surface hydrophobicity and barrier performance of polylactic acid (PLA) films for food packaging applications. An attempt was made to optimize the effect of Al2O3 ALD thin films deposited with different thicknesses on the contact angle and barrier properties. In the thickness measurements and characterizations made by FTIR, XPS, and SEM analysis confirmed the successful deposition of the Al2O3 ALD film. Surface wettability was assessed via water contact angle (CA) measurements, revealing that Al2O3 coatings significantly increased CA from 74.9 degrees (uncoated-PLA) to 98.5 degrees (20 nm-coated-PLA), indicating enhanced hydrophobicity. Bending tests showed that mechanical deformation can reduce hydrophobic performance, highlighting the coating's sensitivity to physical stress. Notably, the 20 mu m thick PLA film coated with 20 nm Al2O3 was the most significantly affected by bending, as its contact angle decreased from 98.5 degrees to 86.8 degrees. The Al2O3 coating resulted in a notable decrease in the films' water vapor transmission rate, reducing it from 150.3 g/m2/day to 4.7 g/m2/day. This considerable improvement in barrier properties is expected to contribute to a prolonged shelf life for packaged food products. Overall migration analyses confirmed that Al2O3-coated PLA films limited the total residue migration below the regulatory detection limit of 10 mg/dm2. However, specific migration analyses indicated that aluminum (Al) release (3.74-11.75 mg/kg) increased with coating thickness and was more pronounced in acidic simulants (3% acetic acid), suggesting chemical interactions between the coating and simulant. Variations in migration behavior were observed depending on PLA substrate type and coating morphology. These findings demonstrate that Al2O3 ALD coatings improve the PLA film functional properties but also highlight the need to carefully control coating quality and regulatory compliance in food-contact applications.