Physical & mechanical cork material properties

The cork oak is the world's only cork oak species producing such thick bark of uniform structure, which spontaneously grows as covering over dying tissue. The bark of this tree - in everyday language, "cork" – is a constantly necrotic tissue formed during the life cycle of these trees by the continually expanding cross-section of their trunks and branches. It is visible under a microscope that this tissue is made up of closed dead micro-cells with the appearance of fourteen-sided polyhedra; the intercellular spaces are completely filled with a gaseous mixture almost identical to the air environment surrounding the tree. In 1cm3 of this tissue there are more than 40 million such fourteen-sided cells. Their chemical composition includes: suberin (45%), lignin (27%), cellulose and polysaccharides (12%), tannic acid (6%), wax (5%) and other substances (5%).

Lightness. The air enclosed in the cork micro-cells makes up 90% of its volume and about 50% of the weight, which makes its specific gravity in the range of 190 to 250 kg/m3. It is therefore a material five times lighter than water, and since it does not absorb water, it is virtually unsinkable.

Impermeability to liquids and gases. Cork owes this feature to the increased presence of suberin. The content by weight is from 39 to 45% of the mass of the cork. This substance has a waterproofing structure, also causing strengthening and heat insulation.

Inertness and chemical resistance. Cork is chemically inert. Its structure is not only impervious to liquids and gases, but also in contact with them it does not chemically react and is not destroyed. Cork also retains neutrality of taste and odour, and does not absorb odours.

Resistance to biological corrosion. Corrosion is due to moisture and favourable conditions of decay. The increased resistance to biological corrosion of cork is due, among others, to it containing tannins and the absence of protein material susceptible to degradation. It also retains resistance to fungus and mould. Its surfaces and structures do not constitute a medium for mould and mildew, and do not create conditions for them to settle.

Thermal insulation properties. The thermal conductivity of this material is 0.037-0.040 W/(mK). In addition to this advantage it is worth noting the very high specific heat capacity. This value translates into high thermal inertia of cork. In contrast to other materials it retains its insulation properties over a very large temperature range. In this respect, it is far superior to, for example, polystyrene foam, which evaporates under the influence of high temperature. Due to weak thermal conduction cork is always pleasant to the touch; it neither transmits heat nor absorbs body heat. This material also has the property of stabilizing the air temperature and humidity.

Acoustic and anti-vibration properties. Cork can absorb from 30 to 70% of tones in the frequency range from 400 to 4000Hz. The structure of cork and its flexibility allows it to simultaneously suppress airborne sounds and impact sounds and eliminate sound bridge zones. Cork thanks to its specific structure it absorbs sound waves and vibrations (it does not transmit vibrations, but it absorbs them).

Flame retardance. Cork retains resistance to fire in Euro class E. This detail, however, is dependent on many factors. Typically, cork boards are flame resistant due to their high thermal inertia.

Flexibility and compressibility. The cell membranes of cork are very flexible, meaning that it remains compressible and elastic, and, after removing the pressure, it returns to its previous shape. When cork is exposed to high forces, the gas in the cells is compressed, and its structure will reduce in volume. After the cessation of pressure the cork returns to its previous shape.

Antistatic properties. The surface of cork does not accumulate electrical charges; in other words, cork is not electrified, so the phenomenon of attracting and absorbing dust does not take place. Cork surfaces are therefore easy to keep clean. For allergy sufferers and asthmatics use of cork in areas used by them acts to limit their contact with allergens.

Durability. Cork is ranked among the most durable organic materials. It is practically not subject to aging and despite the passage of years, even without undergoing impregnation procedures, it does not lose its properties, and some environments even preserve it (e.g. seawater).

Ease of processing. Cork can be treated with simple cutting and sawing tools. It adheres well to uneven surfaces because it matches their shape. It has good adhesive properties, making it easy to stick to various surfaces.

Neutrality for health. This material is not harmful to health. It is not toxic, either in contact with the skin or in case of accidental ingestion to the gastrointestinal tract. It does not cause any allergies. It does not irritate the eye conjunctiva, nasal mucous membranes or respiratory tract.

Respect for the environment. Cork is a material originating from self-regenerating trees, and so its use does not damage the environment.

Mechanical strength. Cork products have high mechanical strength and ability to retain mechanical properties in the temperature range -80C to 140C.
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