REXOR

      
12 April 2017

The transparent barriers

The plastic films for food packaging are often coated with one or more oxygen and / or water vapor barrier layers, this is dependant of the food packaged.

Apart from the well-known films such as aluminum vacuum metallised or aluminum foil complexes, the packages can be made of films coated with transparent barrier layers.

The main advantage of films with transparent barrier layer is that they allow the consumer to see the packaged foods and thus reassure the consumer on what he buys.

One of the most widely used transparent barrier layers is PVdC, because PVdC has several advantages: it provides a good barrier to water vapor (WVTR) and oxygen (OTR), it can also be printed with gravure or flexo printing techniques, and can seal on to itself; but PVdC says: Chlorine.

Why is the presence of "chlorine" also decried ?

Plastic packaging is often recovered by incineration, and in this stage, packaging containing PVdC can "release" dioxins. The incineration centers are equipped with filters that prevent the release of these toxic substances into the atmosphere, so do not be afraid of PVdC coated films.

Which transparent barrier layers without chlorine ?

PVOH - EVOH

The coextruded or coated EVOH or PVOH resins provide excellent oxygen barriers (OTRs).
Consideration should be given to the construction of food packaging and to protect the EVOH or PVOH layer from moistures as these EVOH or PVOH layers will be places as far as possible from the moist environment.
An combination of an EVOH layer with a plyolefin film will provide both a good moisture barrier (provided by the PE or PP film) and an excellent oxygen barrier provided by the PVOH resin or EVOH.

Alumina oxide or silica oxide

A layer of Al deposited under vacuum giving an opaque apperance and causes the disadvantages mentioned at the beginning of the article, can be replaced by aluminum oxide (AlOx). This layer of AlOx makes it possible to obtain a transparent layer with good performances for OTR and WVTR.
This layer of alumina oxide is a layer of a few nanometers and is very sensitive to scratching.
Given this major disadvantage, the layer will have to be protected, more generally "encapsulated" by laminating.

The vacuum metallising machines can metalise, both Al and AlOx.

Another oxide such as SiO x (silica oxide) also makes it possible to obtain a transparent barrier layer with a good level of barrier to gas and water vapor, but also remains a very brittle layer that must be protected.

To conclude

There are therefore different solutions of transparent barrier layers with or without chlorine, which make it possible to produce barrier food packaging (cf.article "barrier packaging"). It will nevertheless be necessary to choose the best technical and economic solution according to the levels of barriers required.

Pascal Rousset, R & D Manager

Food
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