Chemistry Reference
In-Depth Information
O
O
O
OH
+
O
HO
OH
O
O
O
O
O
O
O
O
The products from above polyesterifications are brittle materials. They are, therefore, modified
with oils, either drying or nondrying. Such oil-modified resins bear the names of
alkyds
. While
glycerol is widely used, other polyhydroxy compounds (polyols) are also utilized. These may be
trimethylolpropane, pentaerythritol, sorbitol, or others. Phthalic anhydride is usually used in alkyd
preparations. Other dicarboxylic compounds, however, may also be included for modification of
properties. Common modifiers might be isophthalic, adipic, or sebacic acids, or maleic anhydride. In
addition, many other acid modifiers are described in the patent literature.
The oils in alkyd resins are usually of vegetable origin. They are classified by the type and amount
of residual unsaturation into drying, semidrying, and nondrying oils. The drying oils contain most of
residual unsaturation, while the nondrying ones contain mostly saturated fatty acids.
Alkyds are also classified by the quantity of modifying oil that they contain into
short
,
medium
,or
long
oil alkyds. Short oil alkyds contain 30-50% oil and are usually baked to obtain a hard dry
surface. Medium oil (50-65%) and long oil (65-75%) alkyds will air dry upon addition of metal
dryers.
There are two main methods for preparation of alkyd resins. In the first one, called
fatty acid
process
, a free fatty acid is coesterified directly with the dibasic acid and the polyol at 200-240
C.
The reaction may be carried out without a solvent by first heating in an inert atmosphere. At the
end, an inert gas may be blown into the resin from the bottom of the reaction kettle to remove
water and unreacted materials. As a modification of this, a small quantity of a solvent may be used
to remove water of esterification continuously by azeotropic distillation with the aid of moisture
traps.
In the second method, known as the
, the drying oil is heated with the glycerol
in the first stage of the reaction, at about 240
C. This is usually done in the presence of a
transesterification basic catalyst to form monoglycerides
alcoholysis process
OOCR
OH
OH
+
HO
OH
RCOO
OOCR
3
HO
OOCR
2
After the first stage is complete, phthalic anhydride, with or without another dibasic acid, is added
and a copolyesterification is carried out in the same manner as in the first stage of the reaction.
The conditions under which the reaction is carried out and the rate at which the temperature is raised
during the condensation affect the molecular weight distribution of the final product. In addition, the
