Civil Engineering Reference
In-Depth Information
polyurethane glue may be used. A German manufacturer has recently introduced a chip-
board glued with timber-based lignin glue. Waterglass glue can also be used, but is not
available commercially at the moment.
The manufacturing process of chipboard is as follows:
1.
The timber is shredded.
2.
The shredded timber is ground to shavings.
3.
The shavings are dried to a moisture content of about 2 per cent.
4.
Glue is added. The amount of glue by weight is approximately 7-12 per cent.
5.
The pulp is made into a sheet on a moving band.
6.
The sheet is then pressed at 180-200°C.
7.
The boards are dried and conditioned to the desired moisture content.
Production of plywood
Plywood is produced in different forms and from many different types of timber, including
tropical species, through sawing, cutting by knife or peeling. Sawn plywood is mainly used
in the production of furniture and is produced by sawing the log along its length in thick-
nesses of 1.5 mm or more. The other two types of cutting are used on logs that have been
boiled or steamed until they are soft and pliable. Cutting by knife is done along the length
of the log as with sawing. By peeling the veneer is peeled off the rotating log like paper
pulled from a toilet roll. A plywood board is made by gluing the veneers together. This can
be done in two ways, to make blockboard sheeting or plywood sheeting. Blockboard con-
sists of wooden core strips glued together, usually of pine, which are covered both sides
with one or two veneers. Plywood consists purely of different veneers glued together.
There is always an uneven number of veneers so that the resultant sheet has an odd num-
ber of layers. The adhesive used nowadays is usually urea or phenol glue in a proportion
of about 2 per cent by weight. Animal, casein and soya glue give good results as well.
Straw and grass sheet materials
Throughout European history many plants have been used as roof and wall
cladding, mainly the different types of straw such as wheat, rye, flax, oats, bar-
ley, marram grass, reeds, ribbon grass, greater pond sedge and eelgrass; even the
bregne species of grass. Plants can be used as they are, possibly cleaned of seeds
and leaves, and some can even be used to make sheeting. In addition to the ordi-
nary conditions a surface material has to fulfil, plant materials often give a good
level of thermal insulation and good moisture-regulating properties. It has to be
accepted that thatching is flammable. Eelgrass is less susceptible to burning
