Page 95 - Chemical and process design handbook

Speight_Part II_A 11/7/01 3:16 PM Page 2.36

ALKYD RESINS

The term alkyd resins represents a broad class of compounds commonly
used in coatings and is a particular type of polyester formed by the reac-
tion of polyhydric alcohols and polybasic acids.
Alkyd resins are available in several forms of which the major forms are:
(1) fibrous, in which the resins are compounded with long glass fibers (about
1 ⁄2 inch; 12 mm) and have medium strength; (2) rope, which is a medium-
impact material and conveniently handled and processed; and (3) granular,
in which the resins are compounded with other fibers, such as glass,
1
asbestos, and cellulose (length about ⁄16 inch; 2 mm). A commonly used
member of the alkyd resin family is made from phthalic anhydride and glyc-
erol. These resins are hard and possess very good stability. Where maleic
acid is used as a starting ingredient, the resin has a higher melting point. Use
of azelaic acid produces a softer and less brittle resin. Very tough and stable
alkyds result from the use of adipic and other long-chain dibasic acids.
Pentaerythritol may be substituted for glycerol as a starting ingredient.
The most common method of preparation of alkyd resins is the fatty acid
method in which a glyceride oil is catalytically treated with glycerol at 225
o
to 250 C. The glyceride oil is simultaneously esterified and deesterified to
a monoglyceride.
The esterification of a polybasic acid with a polyhydric alcohol yields a
thermosetting hydroxycarboxylic resin, commonly referred to as an alkyd
resin.
Alkyd resins are also polyesters containing unsaturation that can be
cross-linked in the presence of an initiator known traditionally as a drier.
A common example is the alkyd formed from phthalic anhydride and a
glyceride of linolenic acid obtained from various plants. Cross-linking of
the multiple bonds in the long unsaturated chain produces the thermoset
polymer.
The processing equipment (reaction kettle and blending tank) used for
unsaturated polyesters can also be used for manufacturing alkyd resins.
Alkyd resins are extensively used in paints and coatings. Some advan-

2.36