Page 93 - Materials Chemistry, Second Edition

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Sustainable Industrial Design and Waste Management
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of wood. Small workshops depend on wholesalers of wood for their need for
raw material, which is generally not pre-treated (derosinated), and may con-
tain above standard knots, compared to large furniture industries which may
endure importing their own high quality woods. The most popular wood
types used are beech, oak, MDF, arrow, white wood, Swedish wood, rosewood,
mahogany, etc. Beech wood is characterized by being strong, heavy, resistant
to shock, and easy to machine. It is preferred for carved furniture, dining room
furniture, and luxury cabinets. Mahogany is easy to slice but difficult to
machine; it is usually used for interior woodwork. Swedish wood is usually
used for floor panels. White wood is one of the weakest types and is suscep-
tible to deform over time; because of its low price it is used to manufacture
cheap furniture.
A number of chemicals have been used in the wood industry such as
fillers, sealers, solvents, thinners, paints, pigments, binders, resins, additives,
stains, sealers, washout, topcoats, powder coating, acrylic lacquers, vinyl coat-
ings, catalyzed topcoats, urethane coating, etc.

Process description
Drying
Wood standard water content is (10–12%) by weight. Imported wood usually
has higher water content than the standard. Wood is a hygroscopic material,
which tends to take up moisture and retain it, hence water content increases
during shipment and storage. Wood must be dried before it is manipulated.
Drying increases a wood’s dimensional stability, compact ability and thus
its overall strength. It also reduces wood staining, decay, and susceptibility
to insect attack.
There are two methods for wood drying: air drying and kiln drying. Air
drying may take from two to four months depending on temperature and
humidity.
In this process wood timber is stacked in open air. As the timber tries
to achieve moisture balance with its surroundings it loses some of its water
content. Air drying is a cheap process but it takes time and it is hard to con-
trol the resultant water content.
On the other hand, kiln drying takes a much shorter time than air dry-
ing. By controlling the temperature of the kiln, specific water content can be
achieved to suit the intended use of furniture. A large factory in Damietta
uses a computerized kiln that can detect through X-rays the initial water
content of timber and automatically adjust the temperature to achieve the
aimed water content. It is capable of taking 9 meters of wood at a time and
uses surface condensation to get rid of internal water content. It should be
noted that in kiln drying the temperature should be controlled such that the
wood loses water content without creating high moisture gradients to avoid
cracking.