Better measurement of barrier performance; PP and PLA barrier bottles coming?

By Robert Colvin
Published: May 6th, 2010

The need for an effective gas barrier in blowmolded packaging is increasing in importance for food and beverage processors with their customers’ desire to extend shelf life and cut down on product loss, says Klaus Hartwig, director of the product technology center at Nestlé Waters (Vittel, France).

Hartwig, formerly with stretch blowmolding equipment manufacturer KHS Corpoplast (Hamburg, Germany), emphasized the need to reduce the development time of improvements in introductory remarks during a session on barrier packaging at this year’s biennial plastics technology colloquium conducted by Germany’s Institute of Plastics Processing (IKV; Aachen). He says product development today takes between two and three years before new technology is marketable. This needs to be much shorter, he says.

IKV studies reveal that plasma-coated PP and PLA could eventually compete with stretch blowmolded PET bottles.

Yet, as he points out, food packagers are only willing to accept developments that offer them more cost-effective solutions than they now use, processes that are compatible with today’s recycling laws and collection means as well as offering improvements in overall shelf-life expectancy.

IKV researchers working together with industry partners presented two new processes for beverage packaging in this area. Thorsten Leopold, an IKV engineer, showed developments he and his team have made in an integrative modeling system to determine the oxygen barrier of stretch blowmolded polyethylene terephthlate (PET) bottles, depending on the biaxial stretch of the material during molding.

“Continuing downgauging and lightweighting of bottles [to save money] has led to problems of loss of barrier,” Leopold says. He points to commercial 0.5-liter carbonated soft drink (CSD) bottles that have dropped in overall weight from 30g to just 12 g/bottle in a 10-year period starting from 1997.

Leopold presented a means of calculating the permeation of gas through the walls of PET bottles by a simulation method that analyzes the influence of the area stretch ratio and the temperature on the permeation coefficient. The calculation distribution of the wall thickness and the local-area stretch ratio of the shaped bottle are included in the calculation of the temperature-dependent oxygen transmission rate (OTR). “This allows [bottlers] an evaluation method of barrier properties as a way to support development of new packaging materials,” he says.

A new take on plasma coatings

In a second presentation dealing with beer and CSD bottles by IKV engineer Karim Bahroun, a system to improve passive barrier was explained using a plasma coating for stretch blowmolded containers, rather than multiple layered preforms. Plasma-enhanced barrier coating of PET beverage bottles is an established process—for example, with Sidel’s (Le Havre, France) Actis (amorphous carbon treatment on internal surface), which uses a license of an IKV patent.

Induced by microwave energy, a plasma coating as thin as 0.1 µm on the inside of CSD and beer PET bottles is sufficient to provide good barrier protection. This method also has the advantage, says Bahroun, that it doesn’t adversely affect the recycling chain.

Bahroun wanted to see if the chemical vapor deposition (CVD) of a thin plasma barrier layer can also be applied to two materials seeing greater use in the beverage packaging market, polypropylene (PP) and polylactic acid (PLA). “A trend can be seen [within the packaging sector] in a desire to move toward alternative polymers for ecological reasons, in the case of PLA, and for economic reasons with PP,” he says. He points to prognoses that growth rates in such applications could reach up to 25% in coming years for polymers produced from renewable resources. PP’s advantage compared to PET is, he notes, its lower density, and therefore lower weight, even with higher wall thickness. Its heat stability, enabling hotter filling temperatures, is another PP plus point compared to PET, he adds.

Where PET shines is its natural gas barrier, even in an untreated form, compared to the other materials. PLA typically allows five times higher oxygen penetration and 10 times higher CO2 permeation of container walls than PET. In both categories, nonbarrier PP exhibits 30 times higher penetration. However, nonpolar PP does offer a significant moisture vapor advantage when compared with PET.

The study showed that both PP and PLA have the potential to compete with PET on gas barrier performance if they are pretreated to maintain a proper plasma adhesion to the wall surface prior to the CVD. For plasma treatment of PP, bottle interiors require a precoating of an acetylene precursor or hexamethyldisiloxan in combination with oxygen and acetylene; for PLA bottles, these also helped provide the necessary grip prior to CVD.

Once plasma is applied to 0.5-liter bottles, a 12 times better oxygen barrier, compared to untreated PP bottles, results. Barrier improvement of PLA bottles showed an 8.8 jump in CO2 protection and 3.8 times better oxygen barrier. This, says Bahroun, allows both PP and PLA bottles to offer potential in future blowmolded packaging solutions.

Robert Colvin

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