Is an innovative oxygen-scavenging pack about to replace the widely-used PP/EVOH container?
By Steven Pacitti, Editor, Plastics in Packaging[Editor’s note: This article was first published in the excellent Plastics in Packaging magazine, February 2014. We are grateful to both the author and the publisher, Sayers Publishing Group.]
With upwards of 18 patents to his name, Bob Tsai has pedigree, not least because he was one of the key scientists at American Can in the 1980s who pioneered the PP/EVOH container for shelf-stable food packaging. So successful was this development that it has been widely used for the past 30 years.
Now managing director of a consultancy firm, Amerasia Technologies, Tsai has worked with Mullinix Packages of Fort Wayne, IN for several years developing a PET/scavenger container which he believes will replace the former solution.
Used for packaging foods and beverages, the PP/EVOH container is well established because of its strong oxygen-barrier properties. It does, however, still allow some oxygen permeation that restricts the shelf life of the product.
“This new solution can maintain zero oxygen permeation for four years. Furthermore, at the end of four years, the headspace oxygen is still slower than the initial oxygen,” Tsai told Plastics in Packaging. In other words, he says, the PET/scavenger container is better than both metal cans and glass bottles at preventing oxidation of foods and beverages, while the crystallized PET can be heated in both microwave and conventional ovens and the amorphous PET is glass-clear.
The oxygen scavenger used in Mullinix’s OxyRx PET/scavenger container is a modified PET, which effectively makes it a single-material container that can be recycled in the same manner as other types of PET container. By contrast, PP/EVOH containers are composed of multiple materials.
It is often said that the most important performance requirement of plastics-based packaging for food applications is low oxygen permeation. This would appear to be the case more frequently in a modern world that is witnessing a rise in demand for healthy foods, such as polyunsaturated fats and whole grain flours which are more susceptible to oxidation than ‘unhealthy’ fats and highly-refined flours.
As such, the bar for barrier container performance is continually rising. But plastics have a long history in this area, having started in earnest during the 1970s. The availability of barrier EVOH polymers and the development of co-extrusion are the two technology pillars of the plastics-based barrier packages available in the market up to now.
Polyvinylidene chloride (PVdC) was the only polymer with oxygen-barrier properties good enough for food packaging during the 1950s and 60s, but the material is thermally unstable at extrusion temperatures while its corrosive nature requires a nickel-plated auger screw and barrel.
“It also has serious environmental problems during disposal [if incineration is used] because it contains chlorine,” explains Tsai, which he says limited the market.
EVOH polymers were rolled out commercially in the 1970s and, unlike PVdC polymers, they are thermally stable at the extrusion temperature. They are, however, moisture-sensitive, making their effectiveness as a barrier variable in packaging applications.
The development of subsequent technology focused on method of keeping EVOH dry so that the structure would perform similar to or better than that of a PVdC-containing structure.
“Due to the thermal instability problem of PVdC, such structures were primarily made by coating and laminating processes. However, a multilayer structure is best made by the co-extrusion process which is cheaper and quicker,” says Tsai.
Thermally-stable EVOH polymers were therefore easy to co-extrude with polypropylene, with the PP providing the mechanical strength at an affordable cost. And this simple equation was the backdrop to three decades of success.
To achieve a zero oxygen permeation rate with EVOH you would need an infinite thickness of the material, says Tsai. “Therefore, a plastics container comprising passive barrier such as EVOH is prohibitively expensive for products with very low oxygen tolerance. A new concept involving an active barrier such as oxygen scavenger was developed initially for these products with very low oxygen tolerance.”
Although oxygen scavenging sachets are well-established throughout Asia, liability concerns have caused them to struggle in penetrating the US and European markets. The key component of the sachet is iron, which requires a different formulation within the plastics to enable it to react with, and scavenge, oxygen.
In parallel to this 1990s development of PP and PE compounded with iron (PP/EVOH/oxygen scavenger) was the start of work that looked to address the modification of PET to make it capable of scavenging oxygen.
Several PET-based oxygen scavengers were commercialized in the 1990s and 2000s but, as Tsai explains, these expensive but higher-performing containers were primarily used for products with very low oxygen tolerances such as beer.
However, Tsai is convinced that a new era is emerging due to recent technology advancements in materials, processing and container design, which has driven overall costs down. And cost is the key issue.
The cost advantage of an active barrier container (compared with a passive barrier) depends on the oxygen absorption capability of that container.
“Advancements made in the 2000s in oxygen scavenger formulation, processing and container design have resulted in a cost-competitive scavenger container for higher oxygen-tolerant foods,” he says.
Specifically, Mullinix’s OxyRx container is positioned as cost-competitive with many commercial PP/EVOH containers. For applications such as small pet food containers, which require more than 10% EVOH, Mullinix’s OxyRx PET/scavenger container claims to offer cost savings.
Proving Its Credentials
Discussing the testing phase, Tsai comments that a device from US-based OxySense was used to test the oxygen transmission rate of the container.
Tsai explains: “By gluing the OxyDot on the clear inner surface of the container, the oxygen concentration inside the sealed container can be measured optically.”
Mullinix’s OxyRx container consists of two types of PET/scavenger container: CPET/scavenger container (opaque due to the crystallinity, but with high heat resistance making it suitable for retort sterilization at 260F and for heating in both microwave and conventional ovens) and APET/scavenger container (glass-clear but with lower heat resistance).
“The CPET/scavenger OxyRx container, with empty headspace, was tested side-by-side with the CPET control container. During sealing, the containers were flushed with nitrogen to 1% oxygen in the headspace. After 1000 days, the headspace oxygen of the CPET control container increased from 1.37% to 5.56% while that of the CPET/scavenger container decreased from 1.03% to 0.26%. After 1500 days, the headspace oxygen of the CPET control container increased to 7.0% while that of the CPET/scavenger OxyRx container was 0.46%, which is still lower than the initial headspace 1.03%.”
This, explains Tsai, is superior to metal cans and glass bottles which can only keep the headspace oxygen unchanged. This is achieved with a fairly thin sidewall of 20-mils.
In another test, the CPET/scavenger OxyRx container was filled with about 90% water and 10% empty headspace. A SiOx-coated barrier lidding film with an OxyDot glued on the inner surface was heat-sealed on the container. In addition to the CPET control container, the commercial PP/EVOH container was also tested side-by-side.
During water filling, the containers were flushed with nitrogen to 5% headspace oxygen and then retorted at 260F for 45 minutes.
“After one month, both the CPET control container and the PP/EVOH container showed a steady increase of oxygen concentration while the CPET/scavenger OxyRx container showed a steady decrease of oxygen concentration. The poor oxygen barrier property of the PP/EVOH container shortly after retort is due to the retort shock effect (high moisture content in EVOH).
“After 70 days, the curve of the PP/EVOH container and that of the CPET control container cross over. Normally, it takes this long for the PP/EVOH container to recover from the retort shock to match the barrier property of the CPET control container. The headspace oxygen of the CPET/scavenger OxyRx container continues to decrease.
“Two years in, the headspace oxygen changes of the PP/EVOH container, the CPET control container, and the CPET/scavenger OxyRx container are +3.0%, +9.0% and -4.4%, respectively.”
Oxygen scavengers are expensive, so it is important to avoid the oxygen scavenging capacity loss during the container inventory period before food filling. Some scavenger containers use moisture in food or UV to trigger the oxygen scavenging, but this requires additional equipment in the filling line.
Another approach is to shorten the inventory period to minimize the capacity loss, and here Tsai alludes to OxyRx’s controllable incubation period.
“During the incubation period, the container does not scavenge oxygen and the oxygen scavenging capacity is not wasted during the container inventory period.”
The advantages of having a controllable incubation time are that such a container can be used for both dry and moist foods, no triggering equipment is needed, and no scavenging capacity is lost during inventory.
“The container is effective in not only preventing oxygen permeation from outside but also competing with food in reacting with headspace oxygen inside the food container, to ultimately reduce the amount of oxygen reacting with the packaged food.”
While Mullinix will claim that OxyRx can reduce the headspace oxygen for food oxidation in both non-flushed and nitrogen-flushed situations, the relationship between food quality and oxidation is hugely complex, which means that OxyRx will need to be validated on a case-by-case basis.
But the promise is clear. Tsai is promising zero oxygen permeation for more than four years, along with high heat resistance, glass-like clarity, headspace oxygen reduction, and no delamination as in a three-layer PET/EVOH/PET container. He also says that it is cost-competitive with many PP/EVOH containers and of lower cost where more than 10% EVOH is used, as in small pet food containers.
And it can be recycled in the PET stream which, in the current environmental climate, is a real game-changer.[Reprinted with permission.]