Civil Engineering Reference
In-Depth Information
Plant fibre and grass boarding
The raw material for boards made from plant fibre is usually straw, but also
residue from corn winnowing and even certain types of leaf can be used.
Many of the different types of straw contain the same type of natural glue
which binds timber fibreboards. In some products, however, it is usual to add
glue.
The most common raw materials for boards or sheeting are wheat, hemp,
rye, oats, barley, reeds, rape, flax and maize. It is mainly their straw that is
used. Decomposed plant fibres in the form of peat can also be compressed
into boards. Hardboards are mainly used internally as a base cladding, but
also in some cases as external cladding. More porous boards can also be pro-
duced for use as thermal insulation. (See 'Peatboards', p. 295, and
'Strawboards', p. 291.)
Boards are not particularly resistant to vermin, and when used externally
they often have to be impregnated with fungicides. If they are rendered, the
problem is considerably reduced. The alkaline properties of the render pre-
vent the growth of mould. In Sweden there are examples of this external
cladding lasting 40 years. The raw materials used in these boards is environ-
mentally very attractive, as it is based mostly on waste from agriculture.
There are exceptions, in which glues and impregnation liquids have been
used.
In manufacture and use these products are environmentally sound. Within the
building they are good moisture-regulators. Small amounts of non-reacted iso-
cyanates can be emitted from products that contain polyurethane glue. Pure
products can be composted or energy recycled. Impregnated products or those
glued with polyurethane glue can be energy-recycled in incinerators with special
filters for the fumes. These products cannot be composted, but should be
dumped on a special tip.
Production of strawboards
Strawboards are best produced locally in small businesses. It does not matter what state
the straw is in as long as it is not beginning to rot. The moisture content before the
process starts should be 6-10 per cent. The procedure is as follows:
1.
The straw is cleaned in a ventilation unit.
2.
The fibres are straightened and put in the same direction. If extra adhesive is
required, it is added at this stage, usually in the form of a polyurethane glue in a pro-
portion of about 3-6 per cent by weight. It may be possible to find less damaging
glues. Wheat, hemp and barley do not need any added glue, even if it would give
greater solidity. Flax boards seldom contain glue. Flax straws have to be boiled under
pressure for a few hours before they can be used.
