By Markus Piwko and Matthias Rödl
Recyclable mono-materials are essential for enhancing sustainability in the packaging industry. Vacuum-coated inorganic transparent barrier layers with minimal thickness, such as silicon oxide (SiOx) and aluminum oxide (AlOx) are crucial for the deployment of mono-material packaging. While the process and equipment for AlOx coating are well established in the industry, the expertise of high-volume SiOx coating via electron beam evaporation remains limited to a few companies worldwide. Additionally, a fair comparison of the performance metrics for AlOx and SiOx is lacking, often leading to misleading conclusions. This article sheds light on SiOx coating and provides a comparative evaluation of AlOx and SiOx.h
Barrier films are essential for ensuring quality and extending the shelf life of food products. They provide protection against oxygen, moisture, light, and aroma loss, and enable the use of modified atmosphere packaging (MAP). Typical materials include polyethylene (PE), polypropylene (PP), as well as multilayer systems incorporating EVOH (ethylene vinyl alcohol) or metallized layers. These complex structures offer high barrier properties but significantly hinder recyclability.
With the Packaging and Packaging Waste Regulation (PPWR), the European Union (EU) is setting new standards for the circular economy in the packaging industry, requiring packaging materials to be recyclable, establishing specific recycled content targets, and placing responsibility on the manufacturers of packaging material.
One way to improve the recyclability of packaging films is the use of mono-materials instead of the predominantly used multilayer systems. Mono-materials currently make up a small but rapidly growing share—approximately 10%–15% of the global film market. Although they can also consist of multiple layers, these layers are made from a single type of plastic. In a small proportion, other materials such as coatings are permitted.
To achieve the necessary barrier properties for recyclable mono-material packaging, thin inorganic layers of AlOx or SiOx, with thicknesses between 10 nm and 50 nm are applied. These layers demonstrate excellent barrier performance even under humid conditions, while maintaining transparency and good recyclability. However, due to their minimal thickness, AlOx and SiOx layers are susceptible to mechanical damage and may require additional process steps such as substrate pre-coatings and top coatings, which can complicate manufacturing and increase costs.
Although numerous applications of SiOx and AlOx barrier layers have already been established, further development is needed to realize mono-material packaging that meets the new recycling and barrier requirements for flexible packaging. While AlOx generally suffices, especially for lower barrier demands, SiOx barrier layers are capable of meeting the more demanding recycling and barrier requirements necessary for packaging subjected to retort processing, hot filling, or liquid product packaging (see sidebar below, AlOx vs. SiOx).
Since only a few companies have expertise in AlOx and SiOx barrier coatings in their portfolio, and the properties of the final product are influenced by various factors (substrate, pre- and post-coatings, test conditions), there is market uncertainty regarding the extent to which a product should rely on AlOx or SiOx barrier coatings. This is particularly complicated by the fact that the typical barrier values under standard conditions for AlOx and SiOx are comparable and are influenced more by the substrate than by the barrier coating material itself (OTR 0.1 cm³/m²/day, WVTR 0.3–0.5 g/m²/day).
Using examples of the influence from tensile strain and storage under humid conditions, the following illustrates the difference between SiOx and AlOx barrier coatings on a material level under specific conditions.
Behavior of the coating under strain
Coatings of both materials were examined after stretching the underlying film substrates. Figure 1a shows the crack pattern of AlOx and SiOx on BOPP (pre-coated) at different strain levels. Especially at 10% strain, the smaller crack pattern size of AlOx indicates a higher degree of crack formation compared to SiOx. This is further supported by the increase in barrier values after strain shown in Figure 1b.
Behavior of the coating under storage at elevated humidity
The influence of humidity on barrier values was investigated by storing samples with AlOx and SiOx barrier coatings on PET and BOPP substrates for 72 hours at 23°C under different humidity levels. Subsequently, the oxygen barrier was measured under these conditions. It was observed that SiOx remains stable, whereas the OTR value for AlOx decreases with increasing humidity. This improvement in barrier performance is attributed to the formation of AlOH, which is associated with swelling and, consequently, densification of the layer. To what extent this reaction may lead to undesirable effects during longer-term exposure or for specific food products is currently under investigation.
Summary
It was found that SiOx barrier coatings are significantly more resistant to tensile stresses—such as those occurring during roll-to-roll processing—and more inert under humid conditions. Based on this higher resistance to tensile stress, better resistance to compressive stress can be inferred, although this is still under investigation. This is also confirmed by the common use of SiOx barrier coatings in mono-material packaging for retort, hot-filling, and liquid packaging applications.
In addition to the technical requirements for the barrier layer itself, the high cost pressure in the packaging sector makes it necessary to implement a highly efficient and quality-assuring production process. While the equipment and process technology for AlOx coating by heated boat is well established in the industry, SiOx coating by electron beam evaporation is still relatively unknown. Thus, the following section will shed light on how to implement measures to provide a high-quality cost-efficient production of SiOx barrier layers by electron beam evaporation. Those measures will be highlighted based on some examples.
In principle, the SiOx deposition takes place by rastering silicon oxide-containing material in a crucible with a high-energy electron beam, causing it to sublimate. As soon as the evaporation process is initiated, a shutter will open and expose the polymer substrate, running over a process drum, to the SiOx vapor to deposit the barrier layer. While a pretreatment is required before the coating process to enable a sufficient adhesion of the SiOx to the polymer film, a post-treatment needs to be done after the coating to avoid the generation of defects by a charged substrate.
The limiting factor to achieve a high productivity and low production cost for SiOx, having a typical coating thickness between 25 nm and 40 nm, is the max. deposition rate (µm·m/min), limiting the production speed. To generate a high evaporation rate, the use of a high acceleration voltage of the electron beam has proven especially advantageous. For example, increasing the acceleration voltage from 45 kV to 50 kV can result in up to a 50% higher evaporation rate. By using a 50 kV acceleration voltage, it is possible to deposit SiOx layers of sufficient thickness for barrier properties, enabling a maximum process speed of 1200 m/min.
By achieving the capability to use a process speed of 1200 m/min, the whole winding system must be designed for this – especially considering the capability to process polymer films of various types (e.g., PET, BOPP, CPP, PE) with thicknesses between 6 µm and 25 µm, at widths between 1400 mm and 2800 mm and sufficient coil length over 60 km – potentially even over 100 km in the future. One solution, for this combination of various requirements is to integrate the winding system between two solid stainless steel plates into the coating system, allowing an excellent parallelism of the rolls that is maintained even under vacuum conditions. Finally, this enables the wrinkle- and slip-free winding of thin and soft polymer films, even under the thermal load during the SiOx coating process. In addition to the coil quality itself, this particularly reduces the risk of damage to the SiOx barrier layer. Ultimately, this leads not only to improved barrier properties but also to reduced scrap rates in general.
To further reduce substrate film waste, a film-save function can be helpful to start the production process even during ramp-up. One possible way to implement this is, to open the shutter on the process drum as soon as evaporation is stable, but before reaching the maximum process speed. However, this approach requires the exact determination of the SiOx coating thickness, with high measurement accuracy. Since transmission measurement reaches its limits in this context, XRF (X-ray fluorescence) is an option for measuring the silicon signal, allowing conclusions about the silicon loading (mgSi/cm²). By using a reference measurement and knowing the silicon signal from a given substrate (due to anti-blocking agents typically containing SiOx), the measured Si-signal can be correlated to a corresponding SiOx thickness at a given layer density.
Finally, to achieve high productivity, the ratio of coating time to maintenance time is of tremendous importance. In addition to a mature vacuum system and the ability to heat the process roller before venting to prevent condensation, the crucible must be designed to enable coating lengths as high as possible- for future even up to 200 km. Overall, it can be estimated that roll to roll ectron beam coating equipment for the deposition of SiOx barrier layer can reach an annual productivity of up to 300 million m² of polymer film. This corresponds to a production volume of about 4.5 kt per year for 16 µm BOPP, or 5.2 kt per year for 12 µm PET.
Besides high productivity, also the quality of the SiOx-coated film is crucial to enable a stable and sufficient performance of the barrier layer and to avoid scrap. In addition to the previously mentioned integration of the winding system between two substrates to prevent wrinkling and damage to the barrier layer caused by slippage, inline process control is particularly advantageous in significantly reducing waste. If the SiOx thickness is measured inline over the width of the web on several points by XRF the electron beam power can be controlled across the width. Thus, the requirements of layer uniformity across the web width of less than 7% and across the entire coil length of less than 10% can be achieved, enabling a stable barrier performance.
At the same time, a coil map can be generated from the collected data, showing the process performance of the electron beam and the measured SiOx layer thickness. When the SiOx thickness is uniform across the width, it is evident that in the overlap zones of the individual electron beams, the power is locally significantly higher than in areas where one electron beam does not overlap with another.
Finally, in case a defect occurs, a detection is important to leave this area out of further production processes and avoid the risk of a food packing material having insufficient barrier performance leading to the deterioration of the product that is packed. Especially in combination with a process speed of up to 1200 m/min this is challenging but can be solved by a high-speed camera system. If integrated and adjusted properly, such systems can not only record a coil map of the defects but can also classify them by type (e.g., pinholes, scratches) and by their impact on product properties.
Even the demand for recyclable mono-material flexible packaging drives the request for vacuum coated barrier layers, it´s still not fully clear whether AlOx or SiOx fits best to which application-specific requirements of the packaging market. The presented comparison between AlOx and SiOx barrier layers demonstrates that SiOx layers seem to be more stable towards tensile strength and humid conditions, leading to easier handling in the production process. However, in comparison, the production equipment and process technology for deposition of SiOx barrier layers using high-rate electron beam evaporation is much more complex and needs an tailored equipment design and adjusted process to enable a sufficient performance together with high productivity and low operational costs. █
Markus Piwko is industry manager, packaging applications, at VON ARDENNE.
Matthias Rödl is technical sales manager, web coating, at VON ARDENNE.
