ALD/MLD deposited hybrid of DL-lactic acid and trimethylaluminium

Atomic and molecular layer deposition techniques have been successfully combined to produce inorganic-organic hybrid films with properties derived from both the inorganic and organic components. There are few reports dealing with further converting of ALD/MLD thin films. Typically inorganic oxide layers on polymer films in most cases require an additional top layer to enable heat sealing and to protect the thin film from mechanical stresses and defects. Adhesion of low-density polyethylene, polyethylene terephthalate and polylactic acid on ALD Al2O3 have been tested previously, where good adhesion was achieved in most cases only with the low-density polyethylene. In this study we report deposition of a hybrid ALD/MLD layer, its properties, as well as its use as adhesion layer for polyactic acid (PLA) extrusion coatings. A mixture of both lactic acid optical isomers, D- and L-lactic acids, was used as MLD precursor. 25 and 45 nm thick hybrid films were analyzed. XPS analyses gave a quite similar composition for both of these two coatings. FTIR analyses also indicated presence and bonding of hybrid components. The oxygen to carbon and oxygen to aluminum ratios were in line with theoretical calculations for such a hybrid. Deposition mechanism is expected to involve both hydroxyl groups of the lactic acid. However, the results indicated also other types of bonding between lactic acid and TMA or between the lactic acid molecules. The thickness of the hybrid remained the same for at least a week after the deposition indicating certain level of stability. In next step cellophane film deposited with 25 nm Al2O3 and 13 nm TMA-lactic acid hybrid was used as a substrate for PLA extrusion at different melt temperatures. Uncoated cellophane and BOPLA were used as reference substrates. PLA adhered only to itself and to the ALD/MLD hybrid with peel strength values exceeding 100 N/m, while PLA did not adhere to plain cellophane. Thickness of the MLD/ALD hybrid on ALD Al2O3 layer had no obvious effect on the adhesion properties at sealing temperature of 170 °C.