212	painting materials
Surface-Active Agent (wetting agent, detergent, penetrant) is a compound
which reduces interfacial tension at the boundaries between gases, liquids, and
solids; it especially promotes wetting and penetration of liquids into solids and
serves as a detergent, emulsifying, and dispersing agent. The oldest type of surface-
active agent is a saponified vegetable oil, a soap, but in recent years many special
compounds have been developed. These include principally fatty alcohol sulphates
and sulphonated higher fatty acid esters, ethers, and amides. The latter com-
pounds have greater adaptability than soaps in respect to the physical conditions
under which they are used. They have high wetting power even in low concen-
tration, are stable in dry form and in solution, and are soluble both in water and
in organic solutions. They are not precipitated by hardness in water. They are
polar compounds and their activity is dependent upon the ability of their mole-
cules to become oriented and adsorbed at an interface (see a series of papers on
* Surface-Active Agents,' Industrial and Engineering Chemistry,, XXXI [1939],
pp. 31-69). Like soap they can be used to remove grime and greasy deposits from
paint, but there is always a risk from the action of water and possibly from
residues of the agents themselves.
Tetralin (tetrahydronaphthalene [Ci_0Hi_2]) is a colorless fluid which boils at
205°C. It is derived from the hydrogenation of naphthalene and is a good solvent
for resins, oils, fats, and waxes. Moreover, it is said to have a strong solvent effect
on linoxyn, the dried, oxidized film of linseed oil. In paint and varnish removers
it is a common ingredient. Discoloration from standing is one of its drawbacks.
Thinner, a solvent or diluent or a mixture of both used to reduce a film material
to suitable brushing or spraying consistency. The common thinners for oil paint
are turpentine or petroleum distillates. Strictly, a thinner is a Diluent but, be-
cause the difference between solvent and diluent exists only in relation to particu-
lar film-forming solids, this more general term is used for either.
Toluene (toluol, methyl benzene [CeHs-CHg]) is derived from coal-tar by
fractional distillation of commercial benzol. A clear, colorless liquid, it resembles
Benzene but boils at a higher temperature, no°C., and is less volatile. It is
miscible with most other organic solvents and is said to be the most widely used
hydrocarbon diluent for commercial cellulose lacquers. For resins and for cellulose
ethers, it has a considerable solvent power and a higher dilution ratio than the
petroleum hydrocarbons. Its vapor pressure permits rapid drying without blush.
It is low in cost and, unlike benzene, is not dangerously toxic. The less pure form
of toluene is called toluol.
Toluol is a commercial or industrial grade of Toluene.
Tricresyl Phosphate ([CHsCeHJsPO^ is generally used as a plasticizer in
commercial lacquers,, though it is a moderately good solvent for some resins,
including mastic. It has a high boiling point, 275° to 28o°C., and a high flash
point, 23O°C. At 25°C. it is 0.2 per cent soluble in water.
Triethanolamine ([CH2CH_2OBQ_3N). Though extremely active as a solvent for
oils and fats, this is best known to painters and restorers as an emulsifying agent*