On the surface

This article originally published on Plastics in Packaging Magazine, June 2013

On the surface

As plastics material science continues to advance retorted packaging technology, Matthew Tabassi* reviews the future role of laminated films

Today’s consumers are seeking flexible packaging formats that protect the flavor at the same time as the quality of the contents.

As a result, retortable and microwaveable flexible packs are top of the list of solutions and these are often produced by bonding two or more flexible materials. Usually each laminate film will have its own specific properties. To achieve the best result, there are some aspects that need to be considered before deciding what type of lamination will suit a particular application.

In conventional laminating processes, aluminum is considered a major component, usually bonded to a coextruded layer of polyamide (PA), PET or polystyrene film. Since aluminum is a perfect barrier but does not seal and has low puncture resistance, it will often be used in combination with another layer that makes up for its weaknesses.

For example, a structure with good sealing might include a PET film laminated to an aluminum foil and polypropylene. PP will provide the heat sealing capability along with strong puncture resistance.

But today’s fast-moving food packaging industry is not looking for a conventional lamination structure. When industries are looking for better barrier properties and, at the same time, cost saving of the final product, innovation of new structures is necessary.

There are no universal packaging solutions on the market, but new laminated structures offer desirable properties for specific types of food packaging.

Retorting is a process that uses heat and pressure to sterilize and cook food in a strong, sealed package. Retort pouches are made of laminated materials such as PET/aluminum foil/mHDPE (sealing medium), or PP/ink/metallized PP/PE (sealing medium). The latter is suitable for snack food or soft drink pouches.

However, by increasing the number of lamination layers, product shelf-life can be increased. A perfect retorted pouch needs to be a barrier to oxygen and moisture, and to protect the food from light, while the outer layer should be tough and printable.

In recent years, metal cans have been augmented by retort pouches because they are lighter and use less storage space. Retort pouches are also easy to open because of their tear notch and can be reheated by placing in boiling water for just a few minutes.

Since only retort pouches not containing aluminum foil could be placed in the microwave for reheating, new retort structure developments were essential.

There are basically two forms of microwaveable package. The first involves transparent materials such as paper, glass and a multitude of plastics materials - polyethylene, polypropylene, polyester, nylon, polystyrene, and polyvinyl chloride.

These transparent materials allow microwaves to pass through the contents of the package and heat the product without interfering with the packaging materials. These packaging materials need to be compatible with elevated temperature and pressure demands.

The second type is called an active microwave package, which involves the use of materials that directly affect the product in the container. Here, susceptors are included into the materials, which absorb microwaves that in turn penetrate the packaging. This process raises the susceptor patch temperature to levels where it may then heat the food by conduction or by infra-red radiation.

The future of laminate applications is almost limitless, as the combination of different materials will meet almost every specific requirement. It does, of course, require state-of-the-art technology and know-how on the implementation, development and production of such packaging formats.

Shelf stability in food packaging and the emergence of newer packaging methods such as flexible pouches and paperboard cartons are key factors currently driving the retort packaging market.

The development and utilisation of retort packaging in North America and Europe has slowed due to competition from well-established frozen and canned product industries. In Japan, however, where these industries were small, the development of retort pouches has been stronger.

One of the materials that researchers have used as a retort substitute for aluminium foil is polyvinylidene chloride (PVdC), which enables the use of thinner films with improved barrier properties.

PVdC offers exceptional barrier resistance to oxygen and carbon dioxide and, unlike PA and EVOH, is not compromised by moisture.

For example, instead of a traditional lamination structure of ink/paper/LDPE/aluminum foil/primer/LDPE, we could use ink/paper/PVdC coating/primer/LDPE.

Further innovation has seen the use of laminating coating technology and done this process at on step as: PP/Tie/PVdC/Tie/PP//PET (or PS or BOPP).

The specific demands of a retort pouch remain consistent - high impact and shelf presence, resistance to high processing temperatures, and strong distribution challenges - but consumer goods companies continue to push the envelope by demanding increasingly convenient ways to build the packaging material.

And resin and machinery companies have risen to this challenge, developing clear packaging with the ability to be microwaved, along with added consumer benefits.