Wine Bottle Closures – The synthetic or plastic stopper for wine bottles (part 4)

Part 4 of the series: Natural Cork versus Alu Screw Cap

The decades long negligence of the natural cork industry to address the problems of cork taint, gave room to the rise of synthetic cork stoppers.
According to a recent article in the Wall Street Journal an estimated 19% of the bottle stopper market has been replaced by synthetic (actually plastic) corks, while metal screw caps took another 11% of the market of some 20 billion wine bottles a year.

Plastic corks had been tried in the past, but were largely rejected as they were made of solid plastic, which caused difficulties to insert and extract and could leave gaps around the edges that were prone to (oxygen) leaks. All the new plastic corks are, in one way or another extruded or foamed and made of materials with low taste and odour and absence of flavour scalping. By the way, flavour scalping is a term used in the packaging industry to describe the loss of quality of a packaged product due to either its volatile flavours being absorbed by the packaging or the product absorbing undesirable flavours from the packaging.

Supreme Corq

Some 15 years ago a handful of start-ups launched a war to conquer a share of the supposedly promising new market of synthetic wine corks. All of them claimed to have (unique) patents, uniquely developed manufacturing processes, resulting in a unique wine stopper. All of them were suing one and the other about alleged infringements of the said patents, what some of them pushed into bankruptcy. The war on plastic corks has stabilized a bit with the settlement of most of the legal disputes.

One of the earliest innovators was NuKorc Pty. Ltd. in Pooraka, Australia, which began making extruded plastic corks in 1996. NuKorc filed for bankruptcy protection in March of this year, a victim of the economic downturn and the rise of alu screw caps, as this bottle closure now dominates Australia and New Zealand’s wine markets, where it isn’t associated with cheap table wine as it is in the USA and Europe.

Supreme Corq
Supreme Corq Inc. in Kent, WA/USA regards itself as the leading quality injection moulded synthetic closure manufacturer in the world. Supreme was the first (in 1993) to develop an injection moulded, foamed cork, made from a medical-grade compound made in-house out of natural rubber, waxes, and poly-olefins.

Neocork started up in 1996, with investments from several California winemakers, commercializing its co-extruded corks, with a foam core of extremely neutral Low Density Polyethylene, while the outer layer is a solid elastomer, a substance having to some extent the elastic properties of natural rubber.


LDPE was chosen for the inner foam core, not only for its well known elastic properties, but primarily for its purity, neutrality and cleanliness which has made it such a popular material for food and medical packaging. And the same goes for the outer skin made from a TPE (Thermoplastic Elastomer), well known for its purity and cleanliness and commonly used in many food and medical applications.

Neocork chose for co-extrusion because there were already injection moulded corks (and patents) in the market, which showed some problems. Inserting moulded corks into wine bottles obviously creates small surface lines that admit air. Neocork felt co-extrusion would maintain tight tolerances (0.0008 in. on the diameter), avoid ovality, and control bubble distribution better than moulding.
Neocork owns the process but has the corks made by Rubatex Corp. an extruder in Conover, NC/USA.

Nomacorc LLC in Zebulon, NC/USA, started in 1996 its co-extruded, foam-core polyethylene corks. Nomacorc’s innovation was to make corks with two types of extruded plastic. A firm inner core that would hold the shape of the cork and a spongy exterior that would fit better. The new corks have the feel of natural cork and are easier to remove with a cork screw, which addresses a key consumer objection.

Cross section Nomacorc

The patented co-extrusion process creates engineered closures with a foamed core and an outer skin. The core is said to be breathable, allowing for optimum oxygen transfer rates through the closure, while the outer skin provides a precise and consistent seal.
Nomacorc co-extrudes in two stages. First, raw materials are mixed, melted, and extruded to create a long, foamed cylinder, forming the closure’s core. Then a second extrusion process applies a flexible outer skin, which is thermally bonded to the inner cylinder. The shape is stabilized in cooling water before the high-speed cutting operation cuts the closures to the proper length.

The Germans, however, aren’t content with the results of their US colleagues. They object to the injection moulding and extrusion processes claiming an inconsistency in dimension and shape.

Cross section Swan-Kork

For example, Swan-Kork thinks its synthetic cork stopper is superior to what its American competitors bring forth. The Swan-Kork synthetic wine closure, created by mouldtec plastic GmbH in Kaufbeuren, Germay, is said to solve all well-known problems of synthetic corks available so far on the market.

Look at the detailed cross-sections of the different corks (the pictures I took from the website of Swan-Kork), showing the co-extruded cork of Nomacorc, the injection-moulded cork of German Syncor and finally at the right the surface and inner core of the Swan-Kork. Take also a look at the cross section of the Supreme Cork.

Contrary to the co-extrusion technique (manufacturing in a continuous string followed by cutting on plug length) or the classical injection moulding technique with up to approx. 48 plugs in one manufacturing cycle, Swan-Kork is manufacturing each cork individually in a separate form-mould, using the injection-pressing technique. This technique, the company claims, guarantees smaller dimension tolerances and a closed surface.
Its core shows a homogeneous, evenly fine cell structure, which is reached by using a specially developed material composition. The injection-pressing technique, in combination with this material composition, ensures a higher density of the material with at the same time a high elasticity.

CO2 footprint of plastic corks
Plastic cork producers claim that their product is 100% recyclable and uses four times less energy to produce than natural corks. But that’s not quite the complete picture. A diagram (picture at the right) used by Nomacorc on its website clearly states the differences in CO2 footprint for the several stoppers.

The last option: Metal screw caps
These closures consist of an aluminium cap with threads and a liner. While they can form a very tight seal, there is debate as to the extent of reduction as a result of insufficient oxygen ingress, resulting in sulphur-like aromas (bad eggs) and flavour suppression. Special bottling equipment and a threaded bottle must be used. Consumer acceptance is also an issue in many markets.

There are two more chapter to come. One about metal screw caps, followed by a chapter titled: “Understanding how different closures perform.”

Let’s have a look at the metal screw caps first.
to be continued

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