New 9-Layer-Film for Thermoformed Packaging
High-Stiffness Surlyn Allows for Sealing at Lower Temperatures
In the context of their extensive collaboration to improve the efficiency, performance and sustainability of packaging applications, Reifenhäuser Kiefel Extrusion of Troisdorf, Germanys and DuPont de Nemours International Sarl. have developed a material and cost saving medium barrier 9-layer film for thermoforming applications. Thanks to the high stiffness of the Surlyn sealant layer, film thickness can be downgauged by 20 % with no loss in stiffness. Whilst common 7-layer film structures for thermoforming applications have typical thicknesses of around 240 µm, the thickness of the new 9-layer film is just 200 µm. In spite of this significant reduction, the new film feels just as robust as current structures.
Key to a weight reduction of up to 20% and improved packaging performance over incumbent 7-layer PA/PE structures are Reifenhäuser’s versatile 11/9-channel Evolution barrier blown film technology and a new high stiffness DuPont Surlyn ionomer sealant resin, which allows for sealing at lower temperatures and provides a broad operating window. Further advantages include superior perforation and abrasion resistance as well as an improved fat barrier, making the new film a first choice for vacuum packs containing hard and sharp products, such as mature cheese or meat with bones or smoked ham.
At the DuPont and Reifenhäuser labs, major optical, processing and application characteristics of the new 9-layer structure, produced with Reifenhäuser’s Evolution barrier blown film line, and with Surlyn on the internal surface, were compared to a standard 7-layer structure with a PE sealing layer, both combined with a PA layer.
In these tests, the new 9-layer structure provided a gloss gain of 10% and a 13% higher transparency over the incumbent structure, and also exhibited improved thermoforming performance. Due to the special stress-strain behavior of the Surlyn sealant’s specific chemical structure with ionic crosslinking, the strength of the stretched areas of the film increases. Thanks to this so called 'strain hardening', film thickness in the bottom corners of the thermoformed packs is higher than for PE based sealants, which get weaker in stretched areas. Thus, despite the fact that the conventional 7-layer film was 20% thicker before thermoforming, both structures had identical film thicknesses in the corners after thermoforming.
A further advantage of using the Surlyn sealing layer with its cross-linked molecular structure becomes obvious under point loads, e.g. caused when edges of vacuum-packed hard cheese, dry meats or bacon constantly press against the film. Under such conditions soft polymers in the structure can be displaced, weakening the film and creating micro holes over time. This is especially true for polyethylene copolymers used for high performance sealing applications. In contrast, the much harder Surlyn sealant polymers withstand such mechanisms, resulting in a 10% higher perforation resistance for the 9-layer PA/Surlyn structure compared to the 20% thicker 7-layer PA/PE based film – even in the corners of the thermoformed pack, which are often prone to leakers.
DuPont de Nemours (Deutschland) GmbH
Reifenhäuser Blown Film GmbH
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