Monolayer PET works well as a bottle material for many beverage applications, but PET alone isn’t enough for all beverages. Sensitive beverages such as fruit juice or iced tea need extra protection against oxygen ingress, while some carbonated drinks need a barrier to prevent CO2 from escaping if they are to maintain their fizz over a long shelf life.
Where a barrier is needed, brand owners essentially have three options. They can either opt for a mono-layer bottle made from resin which incorporates a scavenger, a mono-layer bottle to which a thin external or internal surface coating is applied, or a multilayer bottle that sandwiches a plastic barrier between layers of PET.
Each solution has its pros and cons:
So really, none of the currently available solutions are perfect, as Dr Shahab Jahromi from DSM subsidiary Knowfort Technologies BV, points out: “The bottom line is that at the moment, there isn’t a solution that strikes the perfect balance between cost and performance. So if you can come up with something that delivers the right performance at the right price, there’s definitely a market for it.”
Knowfort thinks it may have just have hit on that ‘something’ with its Freshure coating technology – a transparent, high-gas barrier coating that’s created through the vapour deposition of organic compounds such as melamine.
“We’re not saying that Freshure meets those demands now, but we’re confident that it can be developed to tick all the boxes in terms of barrier, transparency and economics in the near future,” says Jahromi.
At the moment, the technology is being used commercially in flexible packaging applications, but Knowfort is keen to branch out into the beverage container market in the next few years.
“There are a number of reasons why we think it could work well for this market,” he says. “Take cost – thin-layer coatings such as oxides are applied using complicated technologies that use high vacuum and high temperatures, because they’re hard, inorganic materials. With Freshure, we’re depositing soft, organic materials, so this can be done without expensive cooling or vacuum systems with relatively low energy consumption. Under moderate vacuum conditions and above its low sublimation temperature of 200?C, melamine can cover large surface areas in a fraction of a second, creating a thin layer of transparent crystalline coating with very high gas barrier properties.”
He adds that in comparison with inorganic coatings, which are brittle and therefore fail when exposed to high pressure or temperatures, Freshure is less brittle, which means the oxygen barrier can be maintained at high temperatures and humidities.
Knowfort may have found a future solution to the cost and performance conundrum, but there are a number of internal coatings that are already in commercial use. For example, KHS Corpoplast says millions of bottles coated using its silicon oxide Plasmax technology have demonstrated stable performance in varying conditions and for different products.
Even so, the German firm recognises that there’s still room for advancing the technology and is working on making Plasmax cheaper and more effective at high temperatures.
“In general, the focus for innovation in barrier coatings is on minimising costs while increasing the quality and shelf life of the filled products,” says Arne Andersen, KHS Corpoplast’s head of sales, Plasmax. “On the one hand, we’re concentrating on the development of Plasmax coating systems with increased hourly output, which will reduce the barrier costs. On the other hand, we’re working to provide a sufficient barrier under challenging environmental conditions such as higher temperatures, knowing that the permeation speed doubles with every 10?C increase.”
In recent years, the lightweighting trend that has swept the beverage industry has added another dimension to the barrier coating challenge.
“Barrier solutions for lightweight bottles are becoming more and more important since weight reduction results in thinner container walls, so further increasing permeation of non-barrier bottles,” says Andersen. “For carbonated soft drinks, reducing weight is a challenge to the dimensional stability of the bottle and barrier performance once the expansion under internal pressure becomes excessive.”
He says lightweighting isn’t a problem for Plasmax, as long as the bottle remains within typical mechanical specifications. However, he does concede that extremely thin and weak bottles are always going to be difficult to coat, however good the barrier technology.
Competitor Sidel says its goal for any product, with regard to soft drinks and barrier technology, is to develop a lighter bottle while increasing (or at least maintaining) the product’s shelf life. It claims its Actis treatment helps to reduce the weight of carbonated soft drink bottles while ensuring a longer shelf life.
“Lightweighting PET causes a loss of CO2 and therefore reduces the shelf life of carbonated soft drinks,” says Franck Hancard, product manager for packaging and Actis with Sidel. “With an Actis coating, it’s possible to have a lightweight bottle and a longer shelf life.”
Actis involves depositing a fine layer of hydrogenated amorphous carbon on the inside of a PET bottle. Sidel says that, to date, about 2.7 billion bottles have been coated in this way, and users include Plastipak, a major plastic packaging company that’s using Actis for customers such as PepsiCo.
Actis and Plasmax are both internal coatings, but there are also external coatings available. Enviroclear from Container Corporation of Canada can be applied externally to any polymer and PET either by dip-spray cascade or flow-coating, and is said to provide an oxygen barrier that almost approaches that of glass.
“When applied to a 500ml PET or polypropylene (PP) bottle, Enviroclear provides an oxygen barrier of 0.001cc/pkg/day (42 times that of uncoated PET) and 0.0025cc/pkg/day (25 that of uncoated PET) respectively,” says Container Corporation of Canada’s CEO, Norman Gottlieb. “When applied to a PET 350ml carbonated soft drink bottle, the barrier improvement factor is 6.4 times for carbon dioxide retention compared with a standard PET bottle.
“Enviroclear’s excellent performance as a coating for PP bottles, coupled with Enviroclear ISBM (Injection Stretch Blow Molding) bottle blowing technology, which provides consistently clear-as-glass PP bottles, means hot-fill juice is a market that could be quite appropriate for this technology.”
Historically, oxygen scavenging materials have been used in the middle layer of multilayer bottles, which require special multilayer injection machines to be produced. Today, barrier resin blends with built-in oxygen scavenging capability enable the production of mono-layer bottles with excellent oxygen barrier on standard injection machines.
PolyShield resin, a modified PET resin from Invista, is one such mono-layer option. The resins, which are designed to be blended with nylon-MXD6 from Mitsubishi Gas Chemical at the preform injection machine, provide containers with active oxygen scavenging and passive gas barrier.
“The primary advantage of PolyShield resins is that a scavenging enabler and a compatibiliser are already built into the PET base resin,” says Frank Embs,
global commercialisation manager at Invista. “As a result, tinted containers made with blends of PolyShield resin and nylon-MXD6 are virtually haze-free and provide excellent oxygen and good carbonation barrier.”
The barrier technology has been used for packaging beer worldwide and extending the shelf life of oxygen sensitive fruit juices.
This summer, Invista is planning to launch a new barrier blend based on PolyShield resin called OxyClear barrier resin, which is claimed to prevent oxygen ingress for up to 12 months in bottles with the same clarity as a standard PET bottles and has reduced yellowing during the recycling process.
While most oxygen scavengers rely on the oxidation of iron or other chemical absorbents to remove oxygen from a container, a UK company has developed a completely new method of scavenging. The technology, which is the brainchild of Emco Packaging Systems, relies on the conversion of the hydrogen present in the head space of the container to water using elemental palladium as a catalyst.
“The finely divided elemental palladium powder is anchored into a non-woven substrate
using a patented process,” explains MD, John Hirst. “A small piece of non-woven containing the palladium is placed onto a plastic base and covered with a gas-permeable membrane. The membrane is attached to the plastic base using radio frequency welding.
“The resulting wad, which is about 2cm in diameter, is suitable for inserting into the cap of a conventional screw-topped bottle, though it could be made to any size. When exposed to hydrogen and any oxygen in the container head space, a reaction is triggered that converts the hydrogen and oxygen into water. The water formed is held trapped between the gas-permeable membrane and the plastic substrate.”
Hirst says the technology has already been tested in wine applications and could also potentially be used for soft drinks and juices.
When you consider that PET packaging only really took off in the early 1990s, it’s not surprising that the barrier solutions that have evolved in the last decade or so are still, to an extent, a work in progress. However, as barrier technology companies move closer to the holy grail of a barrier solution that delivers on cost, performance and recyclability, the greater the attraction of PET will become to manufacturers of more sensitive soft drinks and juices.
**Lynda Searby is a special technical feature writer with a broad knowledge of the food and beverage industry. She has written for many respected trade journals including Food & Beverage International and Beverage Innovation.**
© FoodBev Media Ltd 2019