Cork: a Versatile Material Employed in Many Different Sectors

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Cork is harvested from cork oak (Quercus suberus). Photo by Ryan Opaz.

The majority of people know cork only for its use in wine bottles, as stoppers. Cork, however, is a material with much greater potential, already employed in many sectors, with new applications being developed, and with a positive impact on the environment.

Cork: source and harvesting

Cork is a material obtained from a tree, the Cork Oak (Quercus suber), or more exactly from the trunk of the trees. The harvesting of the cork is a very important step in its production: the material has to be separated from the trunk, but without harming the tree. The harvest can only be performed when the cork layer reaches a minimum thickness; for this reason, the harvest only takes place every 9 years.

Cork composition may vary depending on the tree, its age, where it grows, and other factors. In any case, the main component is suberin (average of about 40%); others are lignins (22%) and carbohydrates (18%).

Characteristics of Cork

Many of cork’s properties are due to the presence of suberin in such a high concentration. Suberin is a waxy compound which gives the cork both water resistance and elasticity. For these characteristics, the material is very suitable as a bottle stopper.

Bottles of wine with cork stoppers constitute 70% of the total wine market.

Different Applications of Cork

About 60% of the harvested cork is used to manufacture bottle-stoppers; they are, therefore, the major product for this industry. At the same time, however, there is also a big section of the cork market in which this material is employed for different technological applications. For these uses, cork-based composite materials are produced.

Cork-Composite materials

Composite materials are made of two or more materials with very different physical and/or chemical properties. The various components are mixed together to obtain a material with improved or unique characteristics, if compared with each single component.

To make cork composites, cork granules are used; these granules are often by-products derived from the stopper production. They can have different dimensions: the smallest granules have, on average, 0.5 mm diameter; other granules can be as big as 2-3 centimeters.

Depending on the applications, these granules are mixed with various other materials; the proportion of each component may be different, according to the desired properties of the finished product.

Cork-based composites still maintain the properties of pure cork. At the same time, however, the presence of another component can make the material more suitable for a specific use. For instance, stronger or more resistant materials can be obtained. Some examples are described below.

Heat and sound insulation

Cork does not conduct either heat or sound very well; this means that is a very good material for both thermal and acoustic insulation. Composites of cork and polymers – generally polyurethane – are fabricated for this purpose. In these composites, cork is the main component (between 90 and 95 %); the two materials are mixed together with glue. The material is treated at about 100oC, to agglomerate the various components; the length of the treatment can be different. This kind of material is widely used for insulation in buildings.

Cork-based composite are used as thermal shields in spacecrafts. Photo Credit: NASA/Bill Ingalls

Low density and elasticity

Besides being a good insulator, cork is also an extremely light material. Its density as a dry solid is 0.24 gr/cm3. For this reason, it is used in the automotive industry, to make some parts of vehicles.

The insulating materials described above, for instance, are employed also in vehicles, such as buses, trains, etc. Furthermore, the sealing parts and gaskets of car engines can be made of cork composites; these parts show very good performance, due to cork elasticity.

Cork in Space

The co-existence of all these properties of cork makes it suitable for more unusual applications. A cork-based composite, for instance, is used as thermal shield in space crafts, both by ESA and NASA.

When prepared for these uses, others properties of the materials have be tested. In particular it is important to check that they can still show adequate performances, even in more extreme environment, and for reasonable length of time.

Cork harvest does not harm the tree, as the material is only separated from the trunk. Photo by Robert Pullar.

Cork is Good for the Environment

In addition to all these properties, another characteristic that makes cork a very good material is its positive impact on the environment.

A recent report published by WWF highlighted the importance of this industry to preserve the cork tree forests. These forests, located mainly in the South of Europe and North Africa, have very rich and diverse forms of life, both plant and animal. Without these trees, all these species will disappear very quickly. Most importantly, the tree is not harmed during the harvesting of the cork, and most trees have working lives of hundreds of years.

Furthermore, in the cork industry the majority of sub-products are valued and used; therefore the impact on the environment for the treatment of the wastes is much smaller than for other industries.

Sources

S.P. Silva, M.A. Sabino, E.M. Fernandes, V.M. Correlo, L. F. Boesel, R.L. Reis: Cork: properties, capabilities and applications. International Materials Reviews. 50(6), 345 (2005). Accessed October 3, 2011.

The Spokesman Review. Cork industry reverses slide. Accessed September 30, 2011.

WWF. Cork screwed? Environmental and economic impacts of the cork stoppers market. Accessed September 2011.

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