Civil Engineering Reference
In-Depth Information
steel. Linseed oil is also used on stone façades to close the pores and protect it
from aggressive air pollution. Render and concrete should not be painted in the
first year, as moisture pressing out from the inside can push the paint off. Oil
paint can be produced in matt, half-lustre and full lustre. The half-lustre and lus-
tre types are very strong and easy to clean.
Linseed oil products are generally waterproof but allow the passage of
vapour. The porosity increases with time, and is optimal after a couple of years.
In some cases, the paint may not be porous enough initially for painting mason-
ry.
Cold pressed oil is better than warm pressed oil. Cold pressing, however,
only frees about 30 per cent of the oil in the seeds. In warm pressing, the seeds
are finely ground, and pressed while warm. Both methods are usually com-
bined.
Raw linseed oil is probably the most firm, especially when cold pressed, but
it dries very slowly because of the large amount of protein substances it con-
tains. It is therefore mostly used out of doors. Boiling linseed oil to 150°C
removes the majority of the protein substances, making the product dry more
quickly. The paint can be used both indoors and outdoors. Stand oil is linseed
oil which is boiled without air to 280°C and thereby polymerized. It is consid-
ered to be more firm and elastic. It also dries more quickly than the other two
types. But even so, drying time is a problem with linseed oil products. In facto-
ry-produced oils, drying agents (siccatives) are added to a proportion of about
0.5 per cent. This also applies to products for outside use, even if the drying time
there is not too critical. For indoor use it is normal to add drying agents to all
qualities of linseed oil, but drying oils can achieve the same end. One such oil
was originally a mixture of linseed oil, Chinese tree oil and natural resins, but
today this is partly replaced with synthetic resins. Another possibility for reduc-
ing the drying time is to use linseed oil in a water-soluble emulsion with casein
paint (see p. 424).
Linseed oil paints often have fungicides added, but this is not necessary for
interior painting. Organic solvents are added to increase penetration and spread-
ing rate. This is usually unnecessary for easy-flowing oils such as cold pressed
linseed oil. The amount of solvent varies from about 10-30 per cent, and is much
lower than the equivalent in alkyde paints.
The raw materials for drying oils are renewable, and environmental problems
relating to their production are minimal. Products containing a high percentage
of drying agents or solvents such as mineral or vegetable turpentine are an
exception to this. Products containing solvents present a risk for painters in the
working environment.
In buildings, linseed oil products are not a problem, except for solvent emis-
sions during the period directly after painting. During the curing period the lin-
seed oil will emit oxidation products, mainly aldehydes, which irritate the
