Chemistry Reference
In-Depth Information
12.2 Renewable Resources Employed in Coatings
12.2.1 Vegetable Oils
Vegetable oils are an attractive resource for developing polymeric derivatives
as they are widely grown across the world for edible and non-edible pur-
poses. Technically, vegetable oils are triglyceride esters of fatty acids. Varying
proportions of three unsaturated fatty acids (oleic, linoleic and linolenic)
and two saturated fatty acids (palmitic and stearic) constitute the major
vegetable oils. Oils such as castor oil, vernonia oil and lesquerella oil possess
high levels of unique fatty acids such as ricinoleic acid, vernolic acid
and lesquerolic acid, respectively. Vegetable oils are amenable to a wide
variety of chemical modifications, e.g., epoxidation, ozonolysis, sulfonation,
maleinization, and hydroformylation. These processes enable the inherent
auto-oxidative capability of unsaturated fatty acids to be supplemented with
the reactivity of the functional groups introduced via chemical modification.
Vegetable oils that inherently possess hydroxyl groups, i.e., castor oil and
lesquerella oil, offer an even simpler modification route, e.g., reaction with
diisocyanates to form polyurethanes.
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Alkyds (oil-modified polyesters) were one of the first modifications made
to vegetable oils to render them useful for coatings. The first step involves
conversion of the triglyceride to a mixture of hydroxyl-functional mono-
glycerides and diglycerides via glycerolysis. In the second step, these alco-
hols are combined with various diacids and/or anhydrides to yield the
desired product. The diacid has a significant influence on the alkyd resin
properties, e.g., phthalic anhydride and maleic acid yield hard, brittle resins
while adipic acid and sebacic acid result in softer resins. The oil content of
an alkyd (oil length) is determined by dividing the amount of oil in the alkyd
by the total weight of the alkyd solids, and expressing it as a percentage.
Alkyds with oil lengths 460 are termed ''long oil alkyds''; those with oil
lengths between 40 and 60 are ''medium oil alkyds'', and those with oil
lengths
o
40 are designated ''short oil alkyds''. While linseed oil, soybean
oil, dehydrated castor oil, sunflower oil, saower oil, tall oil and coconut oil
are widely used in commercial alkyds, lesser known oils such as rubber seed
oil, melon seed oil, tobacco seed oil, jatropha oil, karawila (Momordica
charantia) seed oil, nahar seed (Mesua ferrea) oil, Africa locust bean (Parkia
biglobosa) seed oil and yellow oleander (Thevetia peruviana) seed oil have also
been used to synthesize alkyd coatings.
13-21
Another route for synthesizing
vegetable oil-based polyesters is by reacting epoxidized oils with dicarboxylic
acid anhydrides using catalysts such as tertiary amines,
imidazoles or
aluminum acetylacetonate.
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Epoxidation of fatty acids and vegetable oils involves the addition of an
oxygen atom to the C
ΒΌ
C bond thereby forming a three-membered epoxide
(oxirane) group. Arkema's Vikoflex
s
line consists of epoxidized soybean fatty
acid esters (7000 series) and epoxidized linseed fatty acid esters (9000 series)
while epoxidized soybean oil and epoxidized linseed oil are sold under the
