l PACKAGING
In addition to the barrier materials, it is also possible to incorporate active oxygen-scavenging components such as a cobalt salt, which catalyses a re- action that actively fixes oxygen within the PET material. Multi-layer The first successful PET bottles suit- able for beer were manufactured with multi-layer construction techniques, involving up to nine layers. Co-injection molding machines are used to produce the preforms, in which the barrier materials are formed as distinct layers within the PET structure. Multi-layer bottles have very good barrier proper- ties, often making use of EVOH as the O 2 protection layer. The multi-layer bottle ecosystem is well developed and is still the most widely used technology for O 2 -sensitive PET bottled products such as sauces, juices and beer. However, a weakness of the multi-layer design is that due to the way in which the co-injection molders make the preform, the neck and the base of the bottle cannot con- tain the barrier layers. Monolayers In this construction technique, the bar- rier material is incorporated directly into the PET during its manufacture. A pre-prepared blend may be used, from manufacturers such as Invista and M&G Chemicals, or the barrier components may be supplied sepa- rately as a concentrated ‘masterbatch’, which the container manufacturer then incorporates into their PET blend in the appropriate concentrations. To obtain a PET container with the desired properties suitable for beer, a manufacturer would select and add masterbatches for; an oxygen barrier;
a CO 2 barrier; an oxygen scavenger and UV protection pigment. Much of the difference between commercially-available PET mon- olayer containers is due to the variations in the type and quantity of masterbatch used, with the specific preparations being largely available to all. With a typical oxygen bar- rier masterbatch costing 10 times that of PET, it is not difficult to see why balancing the barrier proper- ties of the container to the shelf-life requirements of the customer, all at an attractive price, is important. Invista’s PolyShield® is a com- monly used resin with enhanced barrier properties, utilised in the construction of one-way monolayer PET kegs and bottles. Petanier’s petainerKeg TM , Dispack-Projects’ Do- lium® Keg and Lightweight Contain- ers UniKeg® all use this blend, in combination with a small percentage (usually 3-5%) of MDX6 and a UV pro- tection pigment. The exact formula- tions used, however, are not publicly available. Petainer works in co-operation with the equipment manufacturer KHS who provides specialised pack- aging lines for the kegs, and have the largest market share worldwide, at approximately 60%. Independent test- ing at VLB has confirmed Petanier’s shelf-life claims of a minimum nine months, with taste tests being unable to differentiate between beer stored in steel kegs and PET. O 2 ingress over six months is given as less than 200 ppb, with a 12% loss of CO 2 over the same period. The Dolium® Keg has at least a six-month shelf-life after filling, equivalent to the recommended time for a stainless steel keg. It also has
(a)
PET
(b)
PET/Barrier
(c)
Sealing off
The co-injection process forming a multi-layer PET preform
have to be greatly enhanced. This is achieved by the use of additional materials, which by themselves would be too costly, or might not have the desired structural or aes- thetic qualities. The type of barrier material used goes hand-in-hand with the construc- tion technique, of which there are three main types; a monolayer, in which the barriers are incorporated into the PET material itself; a layered construction, whereby the barrier material is sandwiched between struc- tural PET components; and thin film coatings, that are applied to the inner or outer surfaces. A fourth method of tackling the issue is through the use of an inner bag, which performs the barrier func- tion. In these systems, such as those from KeyKeg, Ecofass and Ecodraft, an aluminum foil multi-layer bag holds the product, which is never in direct contact with the PET shell, or the propellant gas. As such, this type of container is out of scope of this article, and will be considered separately at a future date. An effective barrier material is one that has a very low permeability to the gas that it is designed to work against, drastically reducing the rate of diffusion of that gas through the container wall. This is often achieved by increasing the complexity of the path that must be taken by a gas molecule as it diffuses through the barrier, the so-called tortuous path. These are often based on nylon, MXD6 being a common commercial prod- uct, or ethylene vinyl alcohol (EVOH). This tortuous path can be further enhanced by the addition of imperme- able nanoclay particles.
(a) Dolium, (b) Petainer and (c) UniKeg one-way PET kegs a b
c
3 z Brewer and Distiller International
www.ibd.org.uk
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