WOOD WATER RELATIONSHIPS

Brought to you byM.Rajagopalan

V M.Sc Life sciencesBharathidasan University

INTRO…

• Wood is our most important raw material. It is important notonly because it is used for literally hundreds of products, butalso because it is a renewable natural resource. Throughcareful planning and use, forests will provide a perpetualsupply of wood.

…….DUCTION

• One property of wood is that it is a hygroscopic, material of biological origin.

• Hygroscopic means it has the ability to attract moisture from the air.

STRUCTURE OF WOOD CELLS

• Through photosynthesis, tree produces glucose.

• Long chains of glucose form cellulose.

• Cellulose molecules combine to form elementary fibers.

• They are grouped into bundles called microfibrils.

• Microfibrils play important role in wood moisture relationships.

HYGROSCOPICITY

• Wood is hygroscopic, and can absorb water as a liquid, if in contact with it, or in the form of vapour from the surrounding atmosphere.

• Wood, either as a part of the living tree or as a material, always contains moisture.

• Moisture affects all wood properties, moisture contained in cell walls is important, moisture in the cavities merely adds weight.

• Hygroscopicity is of primary importance because moisture in wood affects all wood properties.

• It has a direct relation to weight of logs.

• Resistance to decay and insects is greatly affected

• Gluing and finishing and the mechanical, thermal, and acoustical properties of wood are all affected by its moisture content.

• Dimensional changes in wood is caused by shrinkage and swelling.

• Result in change of shape, checking (formation of cracks), warping, honeycombing, and collapse.

Equilibrium Moisture Content• Equilibrium moisture content is defined as

moisture content, where the wood neither gains nor loses moisture.

• When relative humidity increases, EMC also increases.

• Relative humidity is the ratio of amount of moisture in the air to the total amount of moisture, air can hold at that temperature.

• Moisture content of some woods can be high. Very light woods, such as balsa, can hold up to about 800 percent, pine 250 percent, beech 120 percent, and so on.

How to measure MC ??

• Oven dry method :

• Weigh the sample to get combined weight of wood and water.

• Dry the sample in a oven at 103ºC above the boiling point of water.

• Dry the wood for 24 hours without charring.

• Weigh the sample again and record the weight

• The wood sample is now called oven dry or bone dry.

• Use the following equation to determine the MC of wood.

• If the water and wood weight is same MC is 100%.

• If water weighs more than wood then the MC is more than 100 and vice versa.

FIBRE SATURATION POINT

• The theoretical point at which cell walls are completely saturated and cell cavities empty is known as the fibre saturation point.

• Water in the wood takes 2 different forms.

i. Free water

ii. Bound water

• Bound water is held within cell walls by bonding forces between water and cellulose molecules.

• Free water exists as liquid and vapor in cell cavities (lumens).

• As wet wood dries free water leave the lumens.

• After all free water is gone only the bound water remains.

• This is called the fibre saturation point (fsp).

• No water is present in the lumen and cell wall is completely saturated.

• As the wood is dried further bound water leaves the cell wall and cells start to loose moisture below the fsp.

• As water leaves microfibrils come closer and shrinkage occurs.

• When moisture is added to the wood process is reversed.

• FSP for most wood ranges from 25-30 %.

• Some woods have higher extractive contents and have lower fsp.

• Red wood - 22%, Birch – 35%.

• Wood shrinks only when the MC of wood falls below the fsp

CAPILLARY TENSION

• Capillary forces determine the movements (or absence of movement) of free water.

• Water moves through the wood as liquid or vapour through : cell cavities of fibers and vessels, ray cells, pit chambers and pit membrane openings.

• During drying it moves by capillary forces that exerts a pull on the free water deeper in the wood.

• Bound water moves as vapour through empty cell cavities and pit openings as well as directly through cell walls.

• Capillary action is through adhesion and cohesion.

• Adhesion is the attraction between water to other substances.

• Cohesion is the attraction of molecules in the water to each other.

• As wood dries, evaporation of water from the surface sets up capillary forces.

• They exert a pull on the free water in the zones of wood beneath the surfaces.

• When there is no longer any free water in the wood capillary forces are no longer of importance.

• Water in wood normally moves from zones of higher to zones of lower moisture content

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