Barrier Metallised

Treofan has four main business groups, packaging, labelling, tobacco packaging and technical films. The company offers a wide range of PP films including standard and cast films, transparent, white, opaque, cavitated, metallised and high-barrier metallised. 

OPP producers have expanded the core, creating a foam stmcture with lower density, greater opacity, and a stiffer, more paper-Hke feel. Vacuum metallisation increases opacity and water-vapor barrier properties. 

Films of polyolefins, polyamides and poly(vinylidene dichloride) are made using this technique. As most of the films are used for flexible packaging, further down-stream surface treatments are usually applied to improve performance. For example, aqueous polymer emulsions, e.g., poly(vinylidene dichloride), or delaminated clay particles improve the barrier properties as will metallising with aluminium vapour. Corona discharge, causing slight surface oxidation, improves printability. 

In 2004, Treofan GmbH developed a metallised version of its PLA biodegradable film that reduces permeability aromas, oxygen and water. The metallised Biophan PLA film is said to be suitable for packaging fatty foods such as butter and cheese, as well as for confectionery, where the mirror-like finish adds a decorative feature to the barrier properties. The metallised film meets both EU and US, Food Drug Administration food contact requirements. 

A common factor of metal foodstuff packaging is that it provides long term ambient stable storage with excellent abuse resistance and protection from environmental contamination ensuring food safety and quality retention with extended shelf life. Flexible packaging which is covered elsewhere in this book may also use thin layers of metal either as discrete foil layers or as metallised plastic or paper layers for improved barrier properties, but the main structural components are non-metallic and so are not covered here. 

A laminate barrier film commonly used for the bag component of bag-inbox structures is a three-ply laminate consisting of EVA ( 50 pm)/metallised PET polyester (-325 pm)/EVA (-50 pm). Softening/melting temperatures of film components need to be considered for hot fill e.g. > 90 °C. 

The use of other plastics tends to be related to specialised needs and whether their advantages justify the additional cost, e.g. Aclar (trade name) may cost twenty times more than PET but is the nearest approach to an inert plastic and is approximately ten times less permeable than Saran (PVdC—polyvinylidene chloride) which is widely used as a film coating. However foil, even when thin (0.006 mm and above) remains the best barrier material, with newer techniques of film metallisation, especially where two contact layers are used, coming a close second best. 

Unless PVdC coated or metallised, OPP is a poor gas barrier and needs pretreatment prior to printing. 

It is heat seal resistant unless produced as a coextrusion or coated. A good barrier to most gases and volatiles, but only fair to moisture. It is easily metallised. Found as PETP and PETG (Kodak), Pet G contains an additional glycol molecule. 

Vacuumised metal coatings are gradually improving. Tests indicate that these substantially increase the protection offered by plastic materials but do not equate with a ply of foil. Two plastic plies, each with a vacuumised foil, when laminated in direct contact with one another, can give excellent barrier properties. The barrier properties achieved by metallisation may reduce somewhat once the material becomes creased. Protection from some of these creasing effects can be improved by the incorporation of a more flexible ply (e.g. LDPE), i.e. PET metallised/LDPE. PET is very resistant to tear, hence needs a tear initiation feature. It also confers child-resistance. 

Improving Barrier Properties by coating, coextrusion, lamination, metallisation and overwrapping... 

However, most commercially available metallised samples do not provide enough barrier properties for long term applications, mainly due to water transmission through the large density of pin holes and micro-defects present in the sputtered aluminium layers [20]. 

The metallised barrier here considered cannot be used for long term applications, where 10-15 years or more are targeted, since the amount of desiccant necessary to compensate for the water transmission wonld be too large. 

A comparison of some different types of improved metallised and aluminium foil-containing barriers, also including a preliminary investigation of the effect of temperature on the gas transmission rates, has been recently reported by Bonekamp [28]. 

Long lifespan is not a problem, since most of these shipping containers have a projected life of the order of weeks or months. Therefore, it is even possible to use the VIP more than once, the specifications on the permeation properties of the barrier film are less tight and the getter not necessary. A specific dryer, to adsorb water, may be used in most cases. The small size VIP and the limited projected life suggest the possibility of using metallised barriers for the skin of the panel in these applications, at least on one side, so as to overcome the edge effect, which, in this case is very critical. 

Oxygen and moisture are not the only substances that must be kept on the appropriate side of a package, since flavour and aroma barriers become more necessary. So packages are being developed to make sure that the good flavours are kept in and the bad flavours out. These materials can be polyester or oriented polypropylene metallised with a thin coat of aluminium. 

Adhesion to the film can be enhanced by surface treatment. The conventional method is corona discharge but more recently atmospheric pressure oxygen plasma treatments have been developed. The film can also be vacuum metallised to improve barrier performance, coated with copper for surface conductivity or with more advanced coatings such as sapphire. Mineral fillers can be used to provide higher modulus and to the control coefficient of thermal expansion in relation to particular coatings or specific applications. 

Manufecturers Comments Heat-seal adhesive films. Adhesion to metallised Mylar polar, non-porous surtoces. Barrier properties grease resistance. Fair washability, poor dy-dean abflity. 

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