Engineering Chemistry S Datta

476 ENGINEERING CHEMISTRY
(iii) ‘medium oil’ containing the proportion of drying oil intermediate between the two.
The classification is based on the use of drying oil in gallons per 100 lb of resin. The
table below provides the type, field of application and properties of such varnishes:
Type Drying oil Properties Field of
per 100 lb resins
application
(i) Short 6-15 gallon Faster drying, harder Floor and furniture
and more brittle film
(ii) Medium 20-30 gallon — General purposes
(iii) Long 35-60 gallon More weather resistant Sparvarnishes
pliable films
Properties of varnishes vary greatly depending on the three following factors:
• Cooking procedure
• Type of drying oil used
• Type of resin used
Manufacture
The manufacturing procedure depends on the type of oil and resin used. The sequences
are as follows:
• In case of natural resin, when the resin is insoluble in oil, the resin is depolymerised
first by heating it upto 350°C till foaming ceases. The preheated drying oil is then
mixed and heated to obtain the desired viscosity. The mixture is cooled to 200°C and
thinner and drier are added.
• In the case of synthetic resins preheating is not required and the procedure is likewise
as above. In either case filtration is necessary.
• A solution of shellac in methanol or ethanol is produced by stirring followed by heating
to produce spirit varnishes.
The table given below furnishes the constituents of a varnish (i.e., four main ingredients
of a varnish):
Film forming
materials, i.e.,
ingredient 1
Solvents and thinners,
i.e., ingredient 2
Driers, i.e.,
ingredient 3
Anti-skinning agents
i.e., ingredient 4
(i) Oil: Linseed oil,
tung oil, cotton
seed oil, coconut
oil, saya oil etc.
(ii) Natural resins:
Shellac, Rosin,
Kauri, Copal etc.
(iii) Synthetic resins:
Alkyd, ester gum,
coumarine-indene,
chlorinated rubber,
acrylates, silicones
and vinyl resins.
Turpentine, kerosene,
aliphatic and aromatic
naphthas, xylol, alcohols.
Co, Mn, Pb, Zn
resinates, octotes etc.
Guaiacol, tertiary
amyl phenol.