barrier films for food packaging: what's on the inside ... for reducing packaging weight. food...
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42 FOOD & BEVERAGE ASIA APRIL / MAY 2018
Today, 40% of all packaging is made of flexible materials, with an upward trend. This is why coating and laminating machines for the production of functional packaging films are increasingly sought after to meet this growing demand for packaging films. Barrier layers are applied to packaging films, to protect the packed product from environmental influences, such as moisture and oxygen as well as viruses, bacteria and infectious substances.
Furthermore, the barrier layer also works as a sealing layer to combine different materials together, like a top cover for a food box. Besides functioning as a protective and sealing layer, packaging materials are also expected to be “smart” – that is to be resource efficient. Take for example, materials that can extend shelf life or have a reduced CO2 footprint.
Trends in Packaging Films Currently, there are trends visible in the area of thinner substrates for packaging applications and also the resource efficient application of functional barrier layers. Thus, customised coating machines today, produced by manufacturers such as KROENERT, not only need to handle more and more thin substrates but also have to be designed for the application of various functional layers. These layers can be, for instance, protective layers as well as printing primers for
or spraying. The precise mechanical adjustment of the aggregates combined with integrated modern drive and control technology allow for exact machine speeds, the highest coating accuracy as well as the assurance of high production quality. The coating technologies will be described in detail as follows.
Barrier Films For Food Packaging:
What's on the inside matters too Since consumer trends are changing, innovative coating line concepts are important in helping food
packaging producers overcome challenges that come with manufacturing highly functional packaging films.
By Ms. Andrea Glawe, Regional Sales Director, KROENERT GmbH & Co KG
The patented MPG 600 CI (Multi-Pressure Gravure 600 Chamber Injection) technology by KROENERT uses gravure rol lers to enable the highest q u a l i t y c o a t i n g processes for various a p p l i c a t i o n s a n d media.
transparent barriers with aluminum oxide (also known as AlOx layers) or functional barrier layers such as PVdC.
The coating technology used defines the thickness of the applied layer as well as the accuracy of the layer. The various employed coating technologies are namely dipping, gravure roller coating
PROCESSING & PACKAGING
43APRIL / MAY 2018 FOOD & BEVERAGE ASIA
PROCESSING & PACKAGING
In state-of-the-art coating lines, gravure rollers with pressurised doctor chambers are in operation. The coating process can be varied between direct coating with backing roller as well as kiss-coating.
Pressurised chamber systems can be used to generate a very high flexibility in coating weight, using only one gravure roller. By adjusting the pressure in the feeding chamber through the means of the media pump, it is possible to determine the level of filling resp. over filling of the engraving. A controlled variation of the coating film thickness is achieved this way.
The pressurised chamber system is divided in three segments (seen in media flow direction): the distribution, the metering and the filling section. The metering section is separated from the filling section. In the latter, the emptied engraving of the roller is coming in contact with the coating media. With this separation, air intake to the metering system is avoided. Each air bubble in the coated substrate has the potential to reduce the function of the barrier layer. Up to four PVdC coating stations can operate inline. Each coating station is followed by a drying and curing process.
Lacquer Layers for Transparent Barrier Applications Transparent barr ier layers with high barrier properties are gaining importance. The aluminum oxide (AlOx) clear barrier coating has become one of the most popular choices for flexible packaging. However, the transparent
Layer distribution for transparent barrier layers
Coating methods for coating on AlOx
Coatings for Functional Food Packaging Concerning barrier layers, PVdC (Polyvinylidene chloride) is currently dominating the market. PVdC is used to coat papers as well as polymer substrates like PET-, PVC- or PP-foils. In most cases, 30 g/m² PVdC coating weight is sufficient as a barrier layer for food packaging. However, as the dry coating can only be applied at a maximum rate of 15 g/m² per layer, this means that lines with multiple coating heads are used or multiple passes through a single head machine are needed to achieve the requested final dry layer thickness.
The reason for this is the need for sensitive drying and cross-linking of the PVdC lattice. To avoid skin formation and to achieve a defined residual water content, pin holes and a controlled drying process are required. The industry, however, is keen to develop PVdC formulations which will allow for an increased coating weight per layer.
For sufficient adhesion between the PVdC coating and the substrate, it is mandatory to apply a primer on most of the polymer substrates. Usually, primers such as acrylic emulsions or aqueous barrier polymer solutions are used. The thickness of the primer coating is approximately one µm or less. The primer is mainly applied with gravure systems and standard anilox rollers. The engraving and the surface of the roller are designed for the application of thin coatings.
Depending on the solid content and the required dry coating weight, the PVdC layer is usually applied with a wet coating weight between 40 and 60 g/m².
ceramic barrier layers used also need to be protected as well as coated with a primer layer for printing applications.
The process steps in coating lines for transparent AlOx barriers are usually done by applying protective lacquer or an additional primer coating as the basis for digital printing processes and lamination applications. The lacquer functions as a protective layer for the scratch sensitive AlOx layer.
Both chemical solut ions, the protective barrier and the print primer, for AlOx barrier coating are applied with gravure roller technologies. The protective lacquer can be coated with smooth roller kiss-coating and Meyer Bar systems as well.
Catering to Complex Demands Changing consumer demands that have been shaped by economic and social aspects have led to the following trends today: applications need to be resource efficient and use light materials for reducing packaging weight. Food packaging not only needs to protect the food, but also prevent spoilage by extending its shelf life. It is no wonder that innovative coating line concepts are now in demand to fulfil the increasing demands for high functional packaging films.
Polymer films, thinner than 10µm need to be handled, but at the same time, the transparency of the barrier layers needs to be kept or even increased, allowing a perfect visibility of the goods inside the package. Functional protective layers will increase in versatility depending on the use of
the product. For example, pa c kages t ha t c an be used in a microwave w i l l requ i re spec ia l barrier layers.
A flexible, versatile and innovative design of the coating line that can adapt to these trends will be able to set new standards for the future. FBA