Review and evaluation of AlOx clear barrier production processes
Aluminium oxide (AlOx) clear barrier coating has become one of the favoured choices for flexible packaging. Different processes and techniques are used for the production of AlOx on flexible films. The selection of AlOx production process is mainly based on achieved barrier level, low cost, fast production speed and the availability of vacuum web metallisers for industrial production.
The advantages of using the Plasma Assisted Deposition technique for the production of AlOx with high barrier will be discussed in details. In this technique plasma is used during deposition to enhance the structure of AlOx particularly at the coating/ film interface to increase adhesion, barrier and optical transparency.
Introduction
Most of the transparent flexible barrier coated films available at present are produced by polymeric barrier layers including EVOH and PVDC. The thickness of such layers is in the range of several micrometers to several 10’s of micrometers. However, some of these coatings may contain chemicals like chlorine, which are not considered to be environmentally friendly. Furthermore, polymer resins such as PVA or EVOH have high temperature and humidity dependency that lower gas barrier property. Boiling or retort treatment during converting affects the barrier properties of such laminated film.
The alternative to thick polymeric clear coatings such as PVDC or PVC can be a vacuum clear coating process with a coating thickness several orders of magnitude thinner and in the range of only ~ 10 nanometers achieving the same or better barrier properties than polymeric coatings. Another advantage of vacuum based processes; the coating material for the barrier layers is usually oxide like AlOx, which is non-toxic to the environment.
Although environmental aspects, barrier and optical properties are all important, equally important are cost aspects, process flexibility regarding different base films and coating robustness to survive the converting processes.
AlOx offers other advantages over conventional aluminium metallised film including transparency for easier product identification, microwave transparency, and potential recyclability.
At present, there are vacuum web metallisers available in the market that can be used for conventional aluminium metallisation as well as the production of AlOx.
This paper will outline the physical vacuum deposition processes used in the production of oxygen rich and metal rich AlOx coating onto flexible films such as PET or OPP and will discuss the mechanism of each process particularly the plasma assisted deposition of AlOx.
Vacuum deposition processes used for the production of AlOx
At present, there are two main vacuum deposition processes used for the production of AlOx clear barrier coating. These are:
a. Reactive thermal evaporation of aluminium with oxygen.
b. Plasma assisted deposition.
The selection of a particular process depends on many factors including barrier level required and the financial budget available to acquire a new generation of plasma assisted metalliser or a retrofit onto a standard aluminium metalliser.
Reactive thermal evaporation of aluminium with oxygen
This process is carried out inside a standard vacuum web metalliser. However, during the evaporation of aluminium a controlled injection of oxygen is required to oxidise aluminium and deposit AlOx coating onto the moving web. Aluminium oxide and metal/aluminium oxide can be deposited by aluminium reaction with oxygen. The overall stoichiometry is dependent upon oxygen-to-metal feed ratio during deposition.
Barrier properties of Polyester film coated with an oxygen rich or metal rich AlOx using thermal reactive evaporation of aluminium with oxygen usually shows OTR and WVTR levels < 5. However, by using a pre and post plasma treatment during the process the barrier level can be reduced to <3.
Plasma Assisted Deposition of AlOx
In this process a plasma source with a gas such as argon is employed to ionise the oxygen atoms, thus enhancing the chemical reaction with aluminium.
In this process ion gun or a magnetron plasma source is used to bombard oxygen and aluminium molecules with argon ions to increase ionisation and, thus enhance the chemical reaction. This process produces a dense AlOx coating structure with high barrier and optical transparency.
Plasma assisted reactive coating of AlOx can produce OTR and WVTR level of 0.5-0.1 in PET. The barrier level of OPP can be improved by using this process but the final level depends strongly on the initial film surface condition and the AlOx/Film interface formation.
Final product
Clear barrier vacuum coated films, particularly those produced by reactive thermal evaporation of aluminium with oxygen, are usually laminated or coated with a top coat for retort or non retort application. The laminated substrate or top coat enhances the moisture and oxygen barrier of the structure. Lamination also protects the coating from mechanical damages during converting processing.
Heat sealable resin top layer is formed by the dry lamination method. A thermoplastic polymer forming a heat sealable resin layer may be used as long as the sealant adhesive level can be sufficiently achieved. Polyethylene resins such as PE, LDPE, LLDPE, PP resin, ethylene –vinyl acetate copolymer and ionomere resin can be used to protect the AlOx coating.
Conclusion
In comparison with wet polymeric atmospheric coating, AlOx vacuum coating requires only a small fraction of the thickness of the equivalent atmospheric coated layer to produce similar functionality. Typically, atmospheric coatings are measured in micrometres (10-6 m) while, vacuum coated AlOx layer is measured in nanometres (10-9m). This reflects a huge environmental and economic advantage for vacuum coating technology.
AlOx is a natural oxide and non-dangerous to the environment or individuals. Recycling and waste management is relatively cheap and simple. In combination with a compostable substrate, it can even be fully bio-degradable.
Metallising of transparent AlOx barrier coating can be accomplished by a standard or specialised vacuum web metallisers. Selecting the most convenient clear barrier AlOx process depends on the application. For a medium barrier, reactive thermal evaporation of aluminium with oxygen is used. For a high barrier requirement plasma assisted deposition process is recommended.
Professor Nadir A. G. Ahmed
He is a Professor of Advanced Materials and Nanotechnology, and founder and managing director of Idvac ltd. His main research interests are in advanced materials using vacuum thin films and nanotechnology to develop new processes for packaging, security holograms, solar window films and smart devices. He worked at Salford and Huddersfield Universities as well as the industrial sector of advanced metallisation and optical coatings.