The science around modified atmosphere packaging may be well established, but advances continue in the lidding films which make it possible. Paul Gander reports

Most lidding films are designed to be as close as possible to invisible and to minimise the visual barrier between the consumer and, for example, the succulent red steak inside the pack. For this reason, it can be easy to overlook the complexity of modified-atmosphere packaging (MAP) films used for meat, poultry and fish products.

Different atmospheres suit different products. In the case of red meat, for example, an oxygen-heavy mix of gases could be used, while for sausages and other types of processed meats it might be a mix of carbon dioxide and nitrogen to combat oxidation.

Whatever the modification to the in-pack atmosphere, says Linpac Packaging's product manager for shrink barrier films Fabio Musati, a resilient gas barrier is required to contain it.

Linpac's Pontivy plant in France produces all of the group's barrier films, including fully-co-extruded multilayer polyolefin film, with an ethylene vinyl alcohol (EVOH) core barrier layer, for MAP shrink applications. The one-shot blown film can include up to seven layers, Musati explains, with orientation and thermosetting of the different polyolefins used contributing to the film's essential shrink characteristics.

"Shrink is important for improving pack appearance, avoiding wrinkles and reducing film overlap on the rim of the tray," says Musati.

A further benefit of the fully co-extruded route, Linpac argues, is that it creates films of 30-micron thickness or less, while laminated barrier films are likely to start at 40 microns and could be as much as 70 or 80 microns.

However, lamination can be used to protect print, states commercial director at BPI Films Mark Vernon, and it may be preferred where shorter run lengths are needed.

Other general characteristics required of most of today's packaging films include good machinability, good slip and mechanical properties. As Linpac's Musati puts it: "Machines are becoming longer and faster, and films need the strength to match them."

Vernon at BPI adds that the drift towards ever-thinner films means that the inherent puncture and tear resistance of the grades used has become more important. "At the same time, better gauge control in production, and an ability to keep to tighter tolerances has helped overall quality," he says.

When it comes to polypropylene (PP), developments include the new Borpact BC918CF co-polymer from resin supplier Borealis. Marketing manager for the film and fibre business unit Anton Wolfsberger says that properties such as high clarity make the film suitable for carrier layers in peelable lidding, as well as for applications in, for example, tray thermoforming.

But of course, films must be able to run across old machines too. Sealed Air Cryovac European marketing manager for case-ready products Luc Sauban says: "The tooling and the age of the fleet will vary from customer to customer, as will parameters such as the narrowness of the sealing window."

Sealing the deal
The sealing stage is clearly vital in guaranteeing a hermetic pack, as BPI's Vernon says: "If the seal fails, the pack fails". BPI supplies polyethylene (PE) for, among other applications, multilayer lidding films. In general terms, says Vernon, converters will select film grades on the basis of reel-to-reel and batch-to-batch consistency. Beyond that, selecting a polymer with a low gel count can be important in reducing interference with the contact layer's sealing properties.

At Borealis, Wolfsberger points out that the key characteristics for the sealing layer are a low initial sealing temperature, a broad sealing range and high seal strength. Its Borseal grades are designed to reflect these priorities, he says. It also has a peelable lidding grade Borpeel, which Wolfsberger identifies as a prime growth area.

At BPI, Vernon says: "A peelable seal, as opposed to a lock seal, will typically be achieved using a three-layer co-ex film, with a performance layer incorporating the peelable additive."

But Linpac's Musati is less certain about trends towards peelability. While it is an option with some Linpac grades, and customers will often be keen to trial it, they may end up seeing the result as a compromise between seal reliability and easy opening, he says. Some even change their mind and move back to a standard seal. "I don't see it as a major growth market," he says.

So what of the barrier layer which, along with the seal, ensures shelf life is maintained? "EVOH meets most needs in terms of oxygen and water vapour barrier," says Musati. There are alternatives, but they tend to be not yet commercially available or, in the case of existing materials such as polyvinylidene chloride (PVDC), less economically viable, less effective or less high-clarity.

Borealis's Wolfsberger agrees that EVOH is still the barrier most likely to be specified for multilayer structures. But he points out that aluminium or silicon oxide coating can also provide a high-performance clear barrier for MAP.

Sealed Air Cryovac, however, has doubts that there would be a real appetite among retailers for longer shelf life even if it were available. Instead, the company has focused on other benefits such as reducing the size of packs and display options. Historically, says Sauban, shrink barrier film could not stay in contact with red meat because of the discolouration, which occurred where the film touched the product.

"With our Mirabella film," he says, "the layer on the inside of the pack has no barrier, and allows oxygen to penetrate through to the meat, even where there is film-to-product contact. We've used this on foam barrier trays and are about to extend it to standard rigid trays."

Sealed Air Cryovac's Mirabella film has a number of potential supply-chain benefits. Headspace in the tray can be reduced, meaning the product takes up less space in transit and on shelf. At the same time, the support given by the film to the product means it can be displayed vertically or semi-vertically.

Sealed Air Cryovac first launched the film in France and it has since been extended to Germany and Austria. Retailer trials are currently underway in the UK.

Stretching the boundaries of MAP still further, the company has created a variant on its Darfresh vacuum system called Darfresh Bloom. This uses two separate film layers on the tray: a permeable layer vacuum-shrunk around the product, and a barrier layer over the top of the tray. The headspace between is filled with a modified atmosphere.

The system, which has so far been commercialised in Sweden and France, can be applied to red meat to provide both colour from the oxygen-rich atmosphere and support from the vacuum film. But it would be equally applicable to fish and seafood, Sauban argues, helping with odour as well as shelflife issues.

Developments in MAP have come a long way in recent years thanks to innovations in films, barriers and sealing techniques. It is now possible to use films that touch the meat and won't cause discolouration and so pack size can be reduced significantly. While not always viewed as the most glamorous area in packaging, MAP is definitely one of the most inventive.

CRITICAL BIOMASS
Given the sophisticated performance and cost-effectiveness of traditional oil-derived polymers for MAP shrink films, can biopolymers really compete in this market?

Stanelco group company Biome Bioplastics believes so. Having focused its R&D efforts on heat resistance, it is now busy tailoring versions of its biopolymers, principally starch-based, with improved moisture barrier properties.

Thermoplastic starches tend to provide a good gas barrier. The challenge hinges on the tendency of natural starches to absorb water, says Biome. This is one reason why flexible biopolymers have been associated with fresh produce, where the natural transpiration of the material is a benefit.

But while this means that the resin needs to be supplemented to curb these moisture-absorbing tendencies, any adaptation needs to maintain their compostable (or at least biodegradable) character.

As Biome managing director Paul Law says: "You could, cynically, simply fill out the structure with hydrophobic material and still say it was ‘starch-based', but it would no longer be biodegradable."

In fact, a combination of additives and a thin layer of ethylene vinyl alcohol (EVOH) need not break the 5% barrier stipulated in the PAS 2050 compostability standard, he claims.

Of EVOH, Law says: "Although it will keep moisture out, it absorbs moisture, allowing the film to eventually break down." Whether that process is sufficiently fast to allow designation as ‘compostable' remains to be seen, he adds.

R&D work has already yielded a list of potential candidates for barrier combinations, he says. The most promising will undergo independent testing, and a range of solutions should be available by the autumn.

Meat, poultry and other MAP markets have not been associated particularly with bioplastics in the past. But Law points out that demand for biodegradable absorbent pads for this type of pack has increased, making sealing films the next logical step.