The next generation clear barrier process for sustainable flexible packaging materials
Background & Introduction
Transparent inorganic barrier layers such as aluminium oxide (AlOx) and silicon oxide (SiOx) have now been around for several decades and there is an increasing number of products that can be found on the supermarket shelf that contain such vacuum deposited coatings as their principal functional layer. With applications ranging from flow wrappers for nutraceutical and cereal bars (e.g. Kind bar), lidding material (as used for the Dole fruit pots) to microwaveable rice pouches such as Uncle Ben’s, there has been and still is an increasing interest in these barrier layers and the AlOx/SiOx market is continuing to grow at an estimated rate of 7 to 8 % per year. The clear barrier flexible packaging market is, nevertheless, still dominated by polymer based transparent barrier films, which are polyvinylidene chloride (PVDC) coated films and ethylene vinyl alcohol copolymer (EVOH) co-extruded films; however, ceramic inorganic barrier layers do offer critical advantages over these films such as their superior barrier performance, humidity insensitivity, chlorine-free nature or nm-range thickness. When compared to opaque metallized polymer films, AlOx and SiOx coated films are unique in regards to offering product visibility, microwavability and suitability for metal detectors.
In recent years, new drivers have emerged moving transparent inorganic barrier layers back into the spotlight. One of them being a delayering trend (shown in Figure 1), whereby the target is to replace triplex with duplex structures with the aim to simplify structures and reduce cost. Due to the transparent nature of AlOx and SiOx barrier layers, these can be reverse printed, hence enabling layer reduction with the barrier and print layer still being embedded in the laminate. In the case of metallized films, a duplex structure is only possible if either the sealant web is metallized or the laminate structure is surface printed (both options setting additional challenges).
The other driver being sustainability, with one main focus on the development of mono-material flexible packaging solutions for recyclability. Thereby, transparent barrier layers have shown promising recyclability when testing the physical properties of recycled material (as indicated in the REFLEX project report [1]) and, furthermore, they do not pose a potential issue during the waste sorting stage.
Bobst Manchester began development work on their original AlOx (GEN I) process in 2007 and the technology was finally fully productionized after two years, with the first machine for industrial AlOx production being released in 2010. This original work predominantly focussed on polyethylene terephthalate (PET) as a substrate, which due to its polar nature and superior mechanical and heat resistance properties was the material of choice at that time. Nevertheless, with the above mentioned sustainability drive, the focus is now increasingly shifting towards polyolefin based films such as biaxially oriented polypropylene (BOPP) and in order to meet the growing demand for sustainable mono-material packaging in terms of transparency and barrier performance, Bobst Manchester has been developing their new AlOx GEN II deposition process.
Throwback – BOBST AlOx GEN I process
The BOBST AlOx GEN I process uses reactive thermal evaporation on a standard-boat type roll-to-roll metallizer with an oxygen injection geometry that has been optimized for PET film substrates. Barrier levels, in terms of oxygen transmission rate (OTR) and water vapour transmission rate (WVTR), that can typically be achieved with this process are summarised in Table 1. Whilst the barrier levels obtained for PET readily fulfil the requirements for food packaging applications, standard packaging grade BOPP films (typically three-layer co-extruded films with co- or terpolymer skin layers and a homopolymer core) have proven to be a more difficult base material onto which to apply AlOx. Barrier levels comparable to AlOx coated PET can only be achieved with the use of ‘special’ BOPP films such as the UHB (ultra-high barrier) BOPP film referenced in Table 1. This will be discussed further in the following sections.
State of the art – AlOx BOPP barrier solutions
When looking at options for high barrier AlOx coated BOPP films with a barrier performance that is comparable to transparent or metallized barrier coated PET film, there are currently two solutions available that address these requirements and achieve the targeted OTRs and WVTRs of less than 1 cm³/(m² d) and 1 g/(m² d). Both of these options have been explored by Bobst Manchester working in close industry collaboration across the value chain of flexible packaging materials.
The first solution is the use of an inline coated (ILC) BOPP film. On this topic, Bobst Manchester has been collaborating with Brückner Maschinenbau GmbH & Co. KG (Siegsdorf, Germany), a manufacturer of film lines for biaxial orientation and Mitsui Chemical Europe GmbH (Düsseldorf, Germany; part of Mitsui Chemicals Inc., Tokyo, Japan) a manufacturer of polymer coatings and resins. The film production process and structure is schematically displayed in Figure 2. The BOPP base film is a 3-layer coextruded film with a Mitsui ADMERTM skin layer (adhesion promoting layer) onto which a Mitsui TAKELACTM coating is applied inline between machine and transverse direction orientation (MDO & TDO) on the film line.
Results obtained using BOBST AlOx GEN I clear barrier process in combination with the above described ILC BOPP film are summarised in Table 2. Whilst the TAKELACTM coating does have some inherent oxygen barrier properties, it is only the combination with the vacuum deposited AlOx barrier layer that gives an average OTR of 1 cm³/(m² d) and WVTR of less than 1 g/(m² d).
The second option is the use of a co-extruded (typically 5 layer) BOPP film with a special skin layer polymer that results in a high surface energy and thus promotes the nucleation and growth of a dense AlOx layer with remarkable barrier performance. Since 2011, Bobst Manchester has been working with Brückner Maschinenbau GmbH & Co. KG on testing BOPP films with different skin layers in combination with transparent AlOx and also opaque Al metallization barrier layers. Such a BOPP film solution is now commercially available from Indopoly (PT Indopoly Swakarsa Industry Tbk, Jakarta, Indonesia), who collaborated as a film supplier with BOBST on mono-material PP-based flexible packing materials and structures for the 2019 K-show exhibition. More detail on the barrier performance that can be achieved using this film will be discussed in the next section.
Latest Innovation – BOBST AlOx GEN II
As indicated at the start of this article, the new BOBST AlOx GEN II process is a recent process development that started in 2018. AlOx GEN II is the latest advancement in AlOx clear barrier deposition, using a new oxygen injection geometry that has been specifically developed and optimized to give better barrier performance of AlOx coated films at reduced optical density, which in turn enables higher transparency AlOx barrier layers. This is particularly beneficial for polyolefin based substrates such as BOPP film. The AlOx GEN I process, which during its development mainly targeted PET film, involves an offline aging process that enhances transparency as well as moisture barrier properties. With AlOx GEN II, higher transparency and better barrier AlOx coated (BOPP) films can be achieved off the machine, although some degree of offline ageing is still involved.
For the work discussed here, three trials were conducted on a BOBST EXPERT K5 (using a 5 layer co-extruded BOPP film produced by Indopoly) whereby the on-machine AlOx transparency setting was varied. Barrier and optical transparency/density results are summarized in Figure 3. As can be seen, the on-machine transparency setting does have an impact on barrier performance. Whilst OTR is only slightly affected, the impact on WVTR is a lot more pronounced. Nevertheless, in all cases the average OTRs and WVTRs are below the target barrier performance for metallized PET film replacement (OTR < 1 cm³/(m² d) and WVTR <1 g/(m² d)). The OTR levels achieved are outstanding, with values of around 0.2 cm³/(m² d) or less for all settings. Average WVTR is 0.33 g/(m² d) for setting 1, 0.54 g/(m² d) for a medium setting 2 and 0.98 g/(m² d) for setting 3. When looking at the optical transparency and aging performance of setting 2 (shown in the right graph of Figure 3), it can be seen that the base film OD of 0.02 is achieved in only one day, indicating an OD drop of 0.01 during that time frame.
Based on the learning from the AlOx conversion project [2], which was running from 2013 to 2016, it is known that in order to further convert the produced AlOx UHB BOPP films a topcoat needs to be applied to protect the thin AlOx barrier layer during further conversion processes, such as printing. The BOBST specific MFB 5770 barrier topcoat was applied on an AlOx optimised BOBST CO 750 and results before and after topcoating are displayed in Table 3, along with the barrier performance of the UHB BOPP base film. As can be seen, this topcoat (despite being an oxygen barrier formulation only), further enhances OTR as well as WVTR close to foil-type barrier levels of 0.1 cm³/(m² d) and 0.1 g/(m² d)).
Summary and conclusions
With the current market driver of sustainability, transparent barrier mono-material based flexible packaging solutions have been one option addressed by the market in order to fulfil the need for fully recyclable (or better ‘recycling-ready’) flexible packaging materials. However, in order to make these solutions a potential candidate to replace existing, predominantly PET based, flexible packaging materials, equivalent barrier levels are needed. The newly developed BOBST AlOx GEN II process offers a solution to these requirements and in combination with the right BOPP film substrate shows great potential for a fully recyclable transparent barrier PP mono-material packaging solution.
www.bobst.com
