PHISICAL-CHEMICAL Properties

MECHANICAL Properties

Phisycal-chemical properties

Wood is made up of millions of different cells

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DENSITY

According to this feature we can distinguish between two groups: soft and hardwood.

To the first group belong mainly conifer trees (Pine, fur and cedar) and their density is between 50 and 100 kg/m3 (balsa wood) and 550-600 kg/m3.

The second group is mainly composed by broadleaf trees (oaks, birches, maple, cherry and walnut trees) and their density ranges between 700-800 kg/m3, but can sometimes exceed 1000 kg/m3.

Hygroscopic Behaviour

The wood internal surface is very complex and,  according to its density, pores comprises between its 50-60%.  They absorb water vapour from the surrounding air and water or other liquids (like solutions of protective substances or adhesives) through capillarity. For this reason, wood was classified as a hygroscopic porous-capillary material. Therefore, all its physical, mechanical and technological features are strongly influenced by the moisture content.

Thermal and electrical features

Thanks to its high percentage of pores, wood is an excellent heat insulation material. When evaluating the heat insulation capacity (W/(mK)) of a building material, the thermal conductivity (λ) is the main factor.

The thermal conductivity is the heat quantity crossing 1 m3 of an edge when between the two lateral parallel surfaces there is a constant temperature of 1°K.

If we consider that wood is made of ligneous material, water and air, its thermal conductivity changes according to its density, moisture, structure and temperature. Therefore, taking a piece of wood with a moisture content of 20%, its thermal conductivity perpendicularly to its grain is about 15 times less than in reinforced concrete and 10 times less than in normal concrete.

Biodegradation

Wood is subjected to various biotic and abiotic attacks because of its chemical constitution. On the one hand its mechanical features can deteriorate for environmental reasons. On the other, biotic attacks are carried out by living beings (insects, parasites, woodworms or fungi) that eats wood or use it to build their habitat.

Abiotic attacks

 Chemical attacks and solar radiations

In virtue of its high resistance against chemical attacks, wood turned out to be a good holder of aggressive substances. For example, it is used as a separator in batteries of electrical accumulators.

On the other hand, is very sensitive to alkali, even if diluted, because they destroy lignin, hemicellulose and its components and to iron salts because they can favour the hydrolytic degradation of wood under moisture.

Wood is also subjected to photolysis ageing due to the joint action of solar arrays and moisture, temperature, wind and rain.

Another key aspect is the surface deterioration caused by the UV arrays that demolish the lignin and the cellulose.

Biotic attacks

Fungi, bacteria and insects

The heterotrophs are the main enemies of wood. Since they cannot synthetize sugars they exploit nutritional substances produced by green plants.

For example, fungi and bacteria to obtain assimilable sugars of lower molecular weight they demolish the cellulose. Same happens for the insects when they colonize some ligneous parts of the plants.

In order to be able to face these attacks, the wood has a certain resistance degree called natural durability. The lignin protects against fungi also thanks to the low nitrogen quantity, which is a fundamental substance for their metabolisms.

The natural resistance is an extremely variable quality even inside the same plant, for example the laburnum can be attacked easier than the duramen because there are no extractive substances but the parenchyma contains nutritional substances for the fungi.

Moulds do not affect the mechanical features of wood, they just inhabit its surface endangering its visual aspect.

Other species like the basidiomycetes can demolish the components of the cellular wall for their metabolism using peculiar enzymes and causing losses up to the 80% of its mass.

Their diffusion occurs in more phases. The first is the vegetative, where the growing mycelium and filamentous tissues (hyphae) carry out the enzymatic destruction of all components of the wooden tissue. Then the reproductive phase occurs through the dissemination of spores.

With a moisture content of 20% is enough for the spores to germinate while getting in contact with wood giving rise to the mycelium. At this point the hyphae get nutritional substances for the fungus while spreading through the wood and destroying its components. After that, from the mycelium will bear the fruiting body (sporocarp) again, which will release other spores.

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