Reifenhäuser Kiefel Extrusion and DuPont have developed a nine-layer barrier film for thermoforming applications that is said to offer a weight reduction of up to 20% and improved packaging performance over seven-layer PA/PE structures.
The film was developed using Reifenhäuser’s 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. The two firms say that 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 cheese or meat with bones.
Due to the high stiffness of the Surlyn sealant layer, film thickness can be downgauged by 20% with no loss in stiffness. Whilst common seven-layer film structures for thermoforming applications have typical thicknesses of around 240 µm, the thickness of the new nine-layer film is 200 µm.
The groups claim that despite this reduction, “the new film feels just as robust as current structures”. The product also reportedly displays higher gloss and transparency than competitive materials.
The firms are promising that the material costs for the new PA/Surlyn structure are around 10% lower compared to seven-layer PA/PE structures.
The seal performance of Surlyn acts against liquid or powder contamination, meaning that the number of rejects along the distribution chain is also reduced.
DuPont and Reifenhäuser are claiming that for processors looking to invest in the technology, the initial outlay for a nine-layer blown film line will pay back in the short term, because the costs for a system including nine extruders and blending units as well as a bigger die head are only 10-15% higher than those for a seven-layer line. Taking into account resin savings of 10%, the payback time for these additional costs will be less than six months, claim the firms. This can be further accelerated if higher sales prices for the superior films can be achieved.
At the DuPont and Reifenhäuser labs the optical, processing and application characteristics of the new nine-layer structure, produced with Reifenhäuser’s Evolution barrier blown film line, and with Surlyn on the internal surface, were compared to a standard seven-layer structure with a PE sealing layer, both combined with a PA layer.
In these tests, the new nine-layer structure provided a gloss gain of 10% and a 13% higher transparency over the conventional structure, and also exhibited improved thermoforming performance. Due to the stress-strain behaviour of the Surlyn sealant’s specific chemical structure with ionic crosslinking, the strength of the stretched areas of the film increases. Thanks to this '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 seven-layer film was 20% thicker before thermoforming, both structures had identical film thicknesses in the corners after thermoforming.
The structure also responds well under point loads caused, for example, when the 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. However, DuPont says that its Surlyn sealant polymers withstand such mechanisms, resulting in a 10% higher perforation resistance for the nine-layer PA/Surlyn structure compared to the 20% thicker seven-layer PA/PE based film.
Reifenhäuser Kiefel’s Evolution Ultra range of extruders were developed for easy processing of complex film structures with all kind of resins. The machinery supplier says that the range provides “outstanding flexibility in terms of choice and amount of high performance polymers including high-barrier grades, adhesives and sealants, so allowing use of the most suitable materials at minimum costs”.
The range allows for combinations of up to 11 channels, enabling the production of asymmetric films with minimum curling.