Atomic layer deposition in food packaging and barrier coatings

Atomic layer deposition is coating method for preparation of thin films down to nanometer scale. It is based on sequentially used self-terminating solid-gas reactions. Control of coating properties and good conformality can be obtained, since substrate is first subjected to one reactant, followed by purge and then same with another reactant. Thus, uncontrolled reactions between reactants in gas phase can be prevented. In certain cases second reactant can be replaced by activation treatment of surface, when only one reactant is used. Typical coating materials are ceramics. Metal oxides, for example, can be prepared using H2O, O2, O3, alcohols or oxygen plasma as oxygen source and nitrides using typically ammonia NH3 or N2/NH3 plasma as nitrogen source. Requirements for reactants include volatility (RT or elevated), they should not decompose at ALD processing temperatures, and their reactions should self-terminate. ALD temperatures vary from around 50 °C to 500 °C. Low temperature ALD enables coating of temperature sensitive materials such as polymers. ALD coating layers are typically well organized and pin hole free. They can be used as barrier materials when applied on substrates including synthetic poly-mer films and paper boards. Properties of the initial layer depend on the surface properties, chemistry and roughness. Sometimes interface between polymer and ceramic coating can be challenging. For example, lack of functional groups of the functional groups of polymer surface may cause poor bonding. Typically ALD barrier layers are thin, in a range few tens of nm. Thicker layers are in risk of cracking and thus lowered barrier properties. In this presentation use of ALD in food packaging and barrier coatings are re-viewed. Production issues and properties of such ALD based materials are discussed, and compared with other barrier materials.