Top Editors ... Breaking through the barrier

Title: Breaking through the barrier
Date: 29/09/2008
Autor: Lynda Searby

The food & drink industry is seeking plastics with excellent barrier properties, recyclability and often transparency, and packaging companies are offering breakthroughs to meet this demand

It’s hard to believe that the use of PET for food and beverage packaging only really took off in the early 1990s. Today, the ubiquitous plastic is the material of choice for many mass-market brands, and has wrested market share from both metal and glass.
None of this, however, would have been possible without the advent of barrier technologies such as multi-layer constructions, barrier blends, plasma coatings and oxygen scavenger technology.
And innovation in these areas continues to drive the adoption of PET packaging in new applications.
Understandably, producers of oxygen sensitive beverages like beer and wine have been cautious about switching to PET, owing to shelf-life and product quality concerns.
Italian firm PET Engineering says it has been working on this challenge for several years, and has conceived packaging concepts that use multi-layer technology and barrier blends.
One of its latest achievements has been the development of an ‘asymmetric’ beer bottle in collaboration with Husky Injection Molding Systems. The preform was designed to allow the bottle to be made from mostly mono-layer barrier resins, and the container’s concave bottom and asymmetric shape make it highly resistant to internal pressure, according to PET Engineering.

Where possible, most food and beverage producers would prefer to opt for mono-layer barrier materials - they are recyclable and inherently cheaper than multi-layer materials as no additional equipment is needed to inject the preforms or coat the containers.
However, there are some applications where mono-layer alternatives simply don’t exist. Until recently, one such area was dairy.
Milk is very light sensitive – exposure to light triggers the oxidation of certain fats and causes off-flavours. Barrier solutions which address this issue have been developed – usually three-layer PE bottles in which the layer construction consists of a ‘white and black layer sandwich’. The white layers provide an attractive outward appearance and the black layer blocks out light.
Spanish company Novapet has come up with a more economical mono-layer alternative, in the form of a PET resin based on a selection of different pigments. Branded Dairy 0+, this resin can be injected and blow moulded into PET bottles which preserve UHT milk for more than six months.
According to Novapet, the secret of Dairy PET lies in the fact the resin incorporates the necessary pigments from the start of the polymerisation process.
“Generally when you mix colour with a resin, you mix it just before extrusion.
However, attempts to make white bottles in this way have run into insurmountable problems,” explains Marco Leonardi, food packaging manager with Novapet. “To achieve adequate light protection with master batches, 10 to 12% white pigment needs to be added. However, the plasticising systems of the injection lines aren’t capable of working with these levels of inorganic additives – you can only add about 5% before you end up with bottles with inconsistent pigment distribution and poor mechanical behaviour.
Because we mix our special blend of opacifying agents during the polymerisation process, we can achieve a perfect dispersion of them within the PET matrix. This enables the production of bottles with a virtual 100% light barrier and all the elasticity and mechanical strength of PET.”
End users so far include Leche Pascual - the largest milk producer in Spain.
Further evidence that food and beverage producers are leaning towards mono-layer barrier materials comes from Danish company Færch Plast, which says that at Interpack, AMPET, its modified mono-layer PET material, was the hottest topic on its stand. Færch Plast is touting AMPET as a cost effective alternative to cans and glass for long-life ambient products. Its main benefits are that it can withstand sterilisation in retorts or autoclaves and that it doesn’t taint food over a long shelf life thanks to minimal migration from the polyester material. Larsen Danish Seafood uses trays made from AMPET to package portion-packed marinated mackerel fillets.
Where hot filling is involved, manufacturers would rather pack into PP than CPET, as it is less expensive. However, historically, PP applications have been limited to water bottles and other applications where oxygen barrier properties are not critical.
LyondellBasell Industries and Container Corp of Canada (CCC) have worked together to come up with a solution to this problem, which combines an experimental grade of LyondellBasell’s Pro-fax X11540-81-3 PP resin with a patented oxygen and CO2 barrier coating. According to LyondellBasell, they have succeeded in creating a 500ml OPP bottle with 140 times the oxygen barrier properties of an uncoated version.
“This proprietary liquid coating bonds to the exterior of the OPP bottle and quickly dries to a hard, clear coating. This coating then acts very much like EVOH (Ethylene Vinyl Alcohol) and other identified oxygen barriers,” explained Dave McKeeman, new business development manager with LyondellBasell Industries. “The challenge is
getting a coating to stick to PP. Sandwiching an EVOH oxygen barrier between an inner and outer layer in a PP preform is also difficult.”
He says the introduction of blow moulded OPP bottles and jars with clarity equal to PET and with hot fill and barrier properties now makes PP a cost effective alternative to glass, CPET and cans in hot fill applications.
“Our initial focus is on applications like sports drinks, fruit juices and pasta sauces, which all require extended shelf life and oxygen barrier properties beyond what PP alone can supply.”
There is a lot of talk at present about ‘sustainable’ packaging materials such as PLA. However, although PLA sounds an attractive proposition to manufacturers wanting to boost their green credentials, a limiting factor is its relatively poor barrier to water vapour, oxygen and CO2.
Some progress has been made in this area. Alcan, for example, has developed a silicon oxide coating technology which is the subject of a comparative study by Agrotechnology & Food Sciences Group (AFSG), part of Wageningen University and Research Centre, to see if the same performance can be achieved for a silicon oxide coated PLA film as for CPET double-layer films.
Dr Ulphard Thoden van Velzen, senior packaging scientist at AFSG, says: “The technology looks quite promising as it offers a very good gas and water vapour barrier, but like most PLA films, it’s not always easy to work with on packaging machines and breaks easily from the stress of the rollers.”
At Interpack, Mondi Consumer Flexibles launched a new bioplastics film for form-filland- seal applications. Made from vegetable oil based biopolymers, the Sustainex film is said to have excellent print properties, workability and mechanical properties such as strength and tear-resistance. Other features include double-sided heat sealability, allowing overlapping as well as finshaped longitudinal folds, and the fact it can be run on machinery with permanently heated sealing tools. Trials are currently in progress at several frozen food producers.
Mondi has also developed extrusion-coated and laminated packaging materials that use biopolymers, as part of the Sustainex range. During the extrusion coating process, the biopolymer melt is applied as a functional coating to a carrier substrate such as paper. In extrusion laminating, the biopolymer melt is used as a glue to
combine substrates. Both technologies require the polymer to have high thermal stability and melt resistance, which biopolymers do not inherently possess.
However, Mondi engineers have found a technique that allows biopolymers to be used in these processes.
Applications could include fast food and single use paper cups and bowls.
In terms of barrier performance, Mondi admits that like all biopolymers, the properties of the Sustainex materials are very different from those of conventional polymers.
“Compared to, say PE, the oxygen transmission rates of biopolymers can be better, whereas the water vapour transmission rate is usually much higher,” explained Dr Peter Prader, project coordinator within the Mondi Innovation programme. “The latter is challenging in most packaging applications, but for some, such as fresh fruit and vegetables, it is beneficial.
“The latest developments we are working on are barrier coated biopolymers like metallised cellulose films, which are more similar to conventional polymers in terms of oxygen and water vapour transmission rates.”
Another trend which has implications for the barrier properties of a pack is the growing popularity of transparent packaging.
“We are increasingly being asked to provide solutions for the protection of transparent PET and PO packaging contents against light degradation,” says Denis Keller, global project manager, market platform packaging with Ciba.
For example, Unilever recently decided to switch from glass to squeezable plastic for its Marmite brand.
ALPHA, a specialist in plastic packaging technology, developed a squeezable PET bottle with a glass-like appearance. PET, however, only provides limited light protection and the vitamins in Marmite are sensitive to light. To protect the contents of the bottle, ALPHA added Ciba SHELFPLUS light filter solution, a resin additive that blocks light up to 500nm.
“We also reduce the amount of oxygen in the packaging with our oxygen scavenger technology, Ciba SHELFPLUS O2,” says Keller. “This solution works in combination with passive oxygen barrier materials and helps absorb the oxygen in the remaining headspace and/or in the packed product.”
In many ways, the main ‘barrier’ to the future development of barrier packaging technology is not how far science can go, but how far food manufacturers and retailers are prepared to go – or in practical terms – how much they are prepared to spend – to get the perfect barrier.
As Dr van Velzen puts it: “The trouble with packaging technology in Europe is that it can take one or two decades before a technology is accepted by the market. There is a world of difference between what is possible with packaging technology and what is actually happening.”