Civil Engineering Reference
In-Depth Information
warmed up wood tar, etc. A few paints can be dissolved in light oils, such as fish
oil, while some paints dissolve in water. Many paints, especially newer types and
binders of natural resins and wax, must have an organic solvent, usually tur-
pentine. There are two types of turpentine:
•
Vegetable turpentine
, distilled from the sap of coniferous trees or pressed out
from orange peel. Sulphate turpentine is produced from sulphate cellulose.
•
Mineral turpentine
, distilled from crude oil. It is marketed, amongst other
things, as white spirit. The ingredients for the most common oil-based sol-
vents are xylene, butanol, metylisobutylene, butyle acetate, methyl glycol
ether, toluene, methanol and petroleum.
Before crude oil-based solvents came on the market in the beginning of the twen-
tieth century, only vegetable turpentine was available. The turpentine obtained
from orange peel is widely used for dissolving natural resins, usually in combi-
nation with a mineral turpentine. The proportion of orange peel turpentine is
usually 2-10 per cent. It can also be used pure.
While mineral turpentine has crude oil as its source, vegetable turpentine is
based on renewable plant resources. In terms of primary energy consumption
and pollution during production, vegetable turpentine is a more positive envi-
ronmental choice, even if water is obviously a preferable solvent.
On the building site, vaporizing of mineral turpentines represents a major
problem and is associated with nerve damage and other serious health problems.
Many painters refuse to use paints with these solvents. The mineral turpentines
with less acute emissions are the isoaliphates, which are obtained by boiling
crude oil at a specific temperature. The vapour from vegetable turpentines is nor-
mally more irritating to the mucous membranes than that of mineral turpentine.
One constituent, pinene, can cause allergies. There is, however, no proof that
long exposure to vegetable turpentine can have the same chronic damaging
effect on the nervous system as mineral solvents.
In freshly-painted buildings the solvents release gas for shorter or longer peri-
ods depending upon the drying conditions of the building. Solvents vaporize
completely, so there are no waste problems.
Pigments
Pigments have to satisfy certain conditions such as opacity, strength of colour
and spreading rate, and they must not fade with exposure to light. Pigment
should neither smelt nor dissolve in the binders or solvents used in the paint.
Not all pigments can be used in all paints, for example pigments in a lime
paint have to be compatible with lime. White pigment is the most popular and
represents about 90 per cent of all pigments used. Pigments can be inorganic
