Comeback for cork
Materials scientists are returning to one of the oldest composites. Antonio Coelho, Quality and Product Development Director of Portugese company Amorim Cork Composites, describes projects involving applications with cork.
Cork products have been around for centuries, from primitive floating devices to the first cork stoppers used in wine amphora. Since the Second World War cork has found other applications.
Although the material is produced from the bark of a living tree, it is essentially a group of dead cells. Each cell is a multisided polyhedron, only 30-40µm in diameter, and is filled with a gas almost identical to the Earth’s atmosphere. One cubic centimetre of cork contains around 40 million cells.
Cork consists of more gas (90%) than solid material, making its density very low. It floats and does not rot. It is also fire resistant and displays charring behaviour for ablative materials. It is compressible but gives impressive recovery, and does not conduct heat or sound well. American space agency NASA called it ‘nature’s foam’.
It is believed that the first technical application was the creation of composites from cork and rubber for automotive gaskets. Cork does not absorb much engine oil and was mixed with the high swelling rubber grades available at the time to achieve a fully functional gasket with flange conformability and load distribution.
Another usage was discovered in the 1960s with the development of a thermal protection system for aerospace applications. This material, still used in American and European space programmes, is a combination of a phenolic binder and specially graded cork granules, taking advantage of the materials’ burning properties and low density to achieve an improved performance/cost ratio for specific heat flux conditions. The development of new cork composite materials and its introduction in technical applications has continued.
Tree of life
Cork is the outer, regenerative bark of the cork oak tree, Quercus suber, which is native to western Mediterranean countries. It is a slow growing tree that can live for 200 years, which allows it, on average, to be stripped 16 times. The first stripping takes place after 25 years with the others carried out every nine years.
Of an average of 340,000t of cork bark produced annually, Portugal accounts for 65%, Spain 26% and the remainder is from Morocco, France, Italy, Tunisia and Algeria.
An old tree with a large girth and branches can yield in excess of 455kg of cork in a single harvest. Cork oaks aged between 35-45 years typically provide 91kg per year, and trees aged 50-60 years can yield 150kg of cork.
The cork planks are seasoned for about six months in the open air. After seasoning they are boiled to remove tannic acid and resins, softening the cork and making the outermost rough surface easier to remove. The planks are sorted according to quality with the best used for bottle stoppers. The rest of the cork is either cut into sections or granulated to make composites.
Taking centre stage
Amorim Cork Composites in Portugal is working to develop new ‘green’ core materials for fibre-reinforced polymer panels, aerospace thermal protection systems and thermally efficient door panels.
These materials will also benefit from the properties of cork and will be evaluated for applications such as automotive, railway, aeronautic, aerospace and construction. These projects have partial funding from the Portuguese Innovation Agency (Agência de Inovação, QREN programme) and from the FP7 (European Community Framework Programme). The consortiums include universities, technical centres and industrial partners.
Aerocork (QREN) – In collaboration with manufacturer of ultra light planes Dyn’Aero Iberica in Ponte de Sor, Portugal, this project aims to replace up to 20% of an aircraft’s weight using Corecork materials, replacing synthetic structural foams. Website: www.aerocork.com.
Plascork (QREN) – Working with European plastics company Simoldes Plásticos, this research is providing impact protection in automotives using cork-based materials. Noise and thermal insulation will be improved as will the recyclability ratio of a vehicle.
Aerofast (7WF) – This is a large consortium led by European space organisation EADS Astrium. Amorim is responsible for developing a new generation of ablative materials for thermal protection system applications. The mission is aerocapture (using atmospheric drag to slow space vehicles) around Mars and the orbiter structure will be subjected to high heat fluxes which demand an improved ablator and thermal insulator material. Website: www.aerofast.eu.
I-Bus (QREN) – This group, led by transport company Caetano Bus of Portugal, seeks to introduce green components into a bus interior design. Website: www.ibus.pt.
EcoTrain – In partnership with Alstom Transport, headquartered in France, Eco Train hopes to produce cork-based solutions for high speed trains, improving the thermal and acoustic insulation while reducing the weight of the carriage.
Further information: Amorim Cork Composites