Low-temperature, ultra-giant blistering of atomic layer deposited barrier coatings on polyethylene films caused by additive segregation

Abstract

Low-temperature plasma enhanced atomic layer deposition (PE-ALD) is an advanced method for coating flexible polyolefin polymers such as low-density polyethylene (LDPE) to improve their barrier properties against moisture and gases. The ALD process is highly sensitive to surface preparation, and LDPE is challenging to coat due to its highly non-polar nature. In this work, we reveal how the segregation of polymer additives to the surface (commonly known as blooming) can compromise the effectiveness of the barrier coatings. After coating a 10 μm thick LDPE film with a 50 nm thick Al₂O₃ layer using low-temperature PE-ALD, we observed the formation of ultra-giant blisters (larger than previously reported) within minutes of removing the sample from the processing chamber. Our conclusion is that the segregated antioxidant component, tris(2,4-di-tert-butylphenyl)phosphite, traps some of the trimethylaluminium (TMA) used as an ALD precursor. After deposition, the trapped TMA reacts with the moisture in the environment and the gaseous reaction product leads to the formation of ultra-giant blisters with bubble diameters ranging from hundreds of microns to over a millimeter. The bursting of these bubbles creates discontinuities and an increased number of cracks in the 50 nm thick alumina coating, hugely deteriorating its barrier properties. This is a previously unknown failure mode of ALD layers deposited on polymer films as interactions with additives are rarely investigated. We also demonstrate how the appropriate pre-treatments can effectively mitigate this problem, improving the coating's reliability.