Cork

2 usd/kg
Circularity potential
Very high
Strength
Ultra low
Production energy
Ultra low
Stiffness
Ultra low
Embodied CO2
Ultra low
Density
Ultra low

Cork has some impressive qualities, unmatched by manmade materials. Once dried, it is lightweight, impermeable to liquids and gases, highly compressible and resilient. It provides excellent thermal and acoustic insulation, is a fire retardant, and highly abrasion-resistant.

Its unique mix of ingredients and honeycomb cellular structure give it perhaps its most magical quality – it has a Poisson’s ratio close to zero. This means that when squashed it resists expanding sideways, and is why it is so satisfying to cork a bottle. Most solid materials have a positive Poisson’s ratio, because they thin out as they are stretched and expand sideways as they are squashed. A rubber stopper, for example, would jam in the neck as it’s pressed.

The job of cork on the cork oak (Quercus Suber L.) is to provide protection – it is waterproof, antimicrobial and antifungal – all good qualities that can be utilised in products made of this material. Cork exists on all trees as a thin protective layer just below the bark. Only on the cork oak does it grow thick enough to be harvested and converted into products and packaging. It is native to the western Mediterranean region (Portugal produces around 50% of global production). The bark is harvested every nine years, without felling or harming the tree.

Stopper production produces waste in the form of granules. They are used to make agglomerate, which is converted into sheet, moulded and extruded products. Several other novel applications have emerged, which attempt to take advantage of all the benefits of cork, such as providing an impact absorbing base layer in artificial grass pitches (instead of rubber), mixing granules with concrete to produce a structural material with improved noise dampening, and combining granules with plastic to produce a biocomposite suitable for sports products, footwear and furniture.


Sustainability concerns
Non-renewable ingredients
Raw material generates polluting by-products
Potentially toxic in use
Microplastics


While the range of properties that can be obtained with cork cannot compete with synthetics, such as polyethylene (PE) and polyurethane resin (PUR) foam, cork competes well with synthetics in specific applications. The relatively poor mechanical properties limit engineering applications. However, cork has a unique combination of resilience, energy absorption, thermal insulation, compressive strength and strength-to-weight that are utilised in a range of technical applications.

Agglomerate cork combines all of the benefits of natural cork with sheet, moulding and extrusion – cork granules are mixed with resin to create a bio-based composite suitable for many more applications than plain cork. For example, impact protection (such as crash helmets and cycling helmets), sports equipment (dart boards, handles, balls and rackets), vibration dampening, sound dampening, gaskets (oil and fuel), seals, floatation devices, polishing products, wear-resistant flooring and automotive interior linings.

While the ratio of cork to resin can be adjusted, it is the type of resin that determines the final properties of the composite. Cork is combined with neoprene (polychloroprene) to produce a material resistant to oil and heat, with good tensile properties and tear resistance. It is mixed with PUR to produce a material with good elasticity and abrasion resistance combined with acid and weather resistance. Nitrile rubber (NBR) is used for applications in the automotive industry around fuel pumps and lines. And it is combined with styrene butadiene rubber (SBR) to make a material with very good compressibility used in matting, flooring, cushioning, noise reduction, furniture and upholstery. All of these synthetic rubbers are thermosetting – they form permanent cross-links in a one way reaction. In other words, once formed into products, they cannot be recycled.

The pleasant feel, energy absorbing and anti-sliding properties of cork make it a good material for coverings, handles and shoe soles. Birkenstock produce a high quality sole made up of layers of woven jute with a cork and latex composite footbed (providing insulation and shock absorbing properties), finished with a suede lining.


Design properties
Cost usd/kg
0.5
Embodied energy MJ/kg
25-30
Carbon footprint kgCO2e/kg
1-2
Density kg/m3
200-750
Tensile strength MPa
0.4-6
Compressive strength MPa
70-100
Hardness Mohs
0.5
Poissons ratio
0
Thermal conductivity W/mK
0.43
Temperature min-max °C
-180 to 120
Thermal
good insulator
Electrical
insulator