Material of the month – cork

Materials World magazine
5 May 2015

This month, Anna Ploszajski explores the material commonly associated with popping open a bottle of wine.

In the modern age of high-tech materials, the natural world can seem somewhat unruly in comparison. Nevertheless, cork remains unrivalled by its synthetic counterparts, and we are still discovering new capabilities for this remarkable material. 

Although almost all trees grow a thin layer of cork bark, the cork oak (Quercus Suber) has evolved a particularly thick covering for protection against the harsh environments common to its native Mediterranean, where the trees are accustomed to frequent droughts, fires and temperature fluctuations. The forests of Portugal produce 50% of the annual global production of cork, a third comes from Spain and the rest from Morocco, Algeria, Tunisia, Italy and France.

The trees’ natural armour of cork has a closed cellular structure in which there are an astounding 40 million cells per cm3. Each of these cells has layered walls containing suberin, lignin, polysaccharides, tannins and ceroids, entrapping gas inside the cells that makes up 50% of their total volume. This structure gives cork its combination of properties – low density, high elasticity, buoyancy, impermeability and excellent insulation of electricity, heat, sound and vibration. Cork cells were the first to be discovered and named by Robert Hooke in the 1665 book, Micrographia.

To cork or not to cork?

These properties, as well as convenient geographical proximity to traditional European wine-making regions, have lent cork its most celebrated application – wine bottle stoppers. Cork is easily compressed into the bottle opening thanks to its porous cellular structure, but springs back to form a tight seal, which is almost impermeable. Producers of high-quality vintage wines demand cork stoppers, since they permit just the right amount of oxygen to pass into the bottle, allowing the wine to enjoy optimised ageing. 

The first known example of cork bungs is an amphora found in the ancient city of Ephesus, in modern-day Turkey, which dates back to the 1st Century. Remarkably, upon uncorking, this artefact still contained wine. Similar artefacts were found at the remains of Pompeii, the Roman city famously destroyed by the eruption of Mount Vesuvius.

Of course, cork stoppers have remained the industry standard for fine wines to this day. Nevertheless, cork’s organic synthesis means that there inevitably exist some natural flaws, channels and cracks. A study in 2005 showed that 45% of cork stoppers allowed significant gas leakage. Furthermore, if the cork bark is inadequately treated after harvesting, a chemical called trichloroanisole can leach into the wine, resulting in ‘wine taint’. This unpleasant spoiling of the wine is harmless to consume, but can significantly detract from the drinker’s enjoyment. These risks, as well as localised production, have seen many wine producers in the Americas, Australia and New Zealand eschew cork and seek synthetic alternatives, such as plastic stoppers and aluminium screw-caps. 

The natural root 
The cultivation and harvesting of cork forests have remained unchanged for hundreds of years. Once the sapling takes root, a cork oak is allowed to live for 25–30 years undisturbed. Once the trunk’s diameter measures at least 70cm at chest height, highly skilled extractors strip away the cork from the trunk by hand, using special axes. The bark is allowed to regrow for nine years and then the process is repeated – always during the summer months, when there are the best conditions for separating the bark without lasting damage to the vulnerable phellogen underneath. 

The removed planks of bark are collected, stacked and left to dry slowly in the sun for at least six months. Afterwards, they are softened and cleaned by boiling in purified water – a process that makes the cells swell by expanding the gas inside, increasing the volume of the cork by about 20%. A two to three week stabilisation stage follows, when the planks are allowed to rest and reach the correct consistency. Finally, the cork is sorted by quality, shaped and cut. The best bottle stoppers are punched from a single piece of cork, and others are made from a baked agglomerate of smaller scrap cork pieces. 

This elimination of waste is just one of the factors that makes the cork industry so sustainable. In addition, the trees are not harmed when the material is harvested, and the products are easily cleaned and recycled. The cork oak forests also give back to their natural habitat, preventing desertification and providing habitats for wildlife. This service to nature as well as the local economy has meant that Portuguese cork forests have been under protective legislation since 1209, one of the earliest examples of environmental protection laws. 

In comparison to alternative materials, the process of making plastic and aluminium stoppers releases about 10 and 26 times more carbon dioxide than cork stoppers respectively. And you can’t hang an aluminium screw-cap from your hat to keep the flies off…

Applications of cork
Aside from wine, cork crops up in a large number of surprising applications. For example, cork is able to clear up our waters. CorkSorb is a cork-based absorbent used to control oil spills. The oil is captured by capillary action in the porous structure and is retained inside the cells. Cork’s inherent hydrophobicity allows it to selectively absorb oil and not water.

The classical orchestra owes a lot to cork, since it is used to form the airtight hole stoppers in woodwind instruments and the handle of the conductor’s baton. So, too, does the world of sport, with shuttlecocks and the cores of cricket balls commonly made from cork, due to its elasticity and low density. 

Cork is also making tracks in the vehicle materials market. Manufacturers have recognised how pleasing cork is to the touch and that its surface temperature varies little with the environment, even in extreme hot and cold, lending it well to applications such as steering wheels and gear knobs as well as interior panels and decoration. Furthermore, next-generation low-emission vehicles powered by fuel cells or batteries tend to have flat-bottomed bodies, so are well suited to cork flooring, which reduces the weight of the car and, subsequently, its carbon footprint. Recently, a unique car seat has been produced from cork, which is half the volume of and three times lighter than its traditional counterparts, while offering the same degree of comfort. The seats can reduce the weight of an average car by 45kg, offering a significant positive impact on the vehicle’s overall energy use. 

Combining cork
Thanks to its cellular structure, fire resistance and hydrophobicity, traditional cork boards make for excellent acoustic and thermal insulation materials in buildings as floor, wall and ceiling tiles. What’s more, incorporating cork into composites has opened up a whole new world of superior construction materials. Cork granules integrated into cement produce a concrete composite material with lower thermal conductivity, lower density and good energy absorption properties compared with conventional mixtures. 

Cork-plastic composites beat glass fibre-reinforced plastic in mechanical tests, economic benefit and environmental impact. Using a cork core in sandwich composites with carbon fibre provides much improved acoustic insulation and increased durability compared with other synthetic cores, while maintaining mechanical performance. Finally, combining cork with other materials such as rubber, high-density polymers and adhesives achieves similarly enhanced physical properties. 

From turbine blades to protective helmets and Portuguese postage stamps to spacecraft, ingenuity and resourcefulness has landed cork in a miscellany of places. This humble tree bark brings a seal of quality in all of its guises, and its sustainable roots and undefeated combination of physical properties are sure to see it play a major role in the future of materials engineering.