Chemistry Reference
In-Depth Information
structure and high synthetic potential, could be attractive and feasible re-
sources in the synthesis of bio-based chemicals.
21,22
VOs are major agri-
cultural commodities with a total production about 159million tons in 2012.
While their production has continuously increased in recent years,
23
only a
small portion of VOs are used as oleochemicals for surfactants, lubricants,
coatings, paints and bio-diesels. The industrial exploitation of VOs is mostly
based on chemical modification of the carboxyl and C
ΒΌ
C groups present in
triglycerides, i.e., the glycerol esters of fatty acids.
There are five dominating types of free fatty acids that range in length
from 14 to 18 carbon atoms, with 0 to 3 double bonds within the chain. The
common unsaturated fatty acids are oleic, linoleic and linolenic acids,
containing one (C18:1), two (C18:2) or three (C18:3) double bonds, respect-
ively. The most common saturated fatty acids are palmitic acid (C16:0) and
stearic acid (C18:0). One must bear in mind that saturated fatty acids show
no reactivity except through a telechelic carbonyl group. Highly unsaturated
fatty acids are desirable for thermoset polymer applications since double
bonds provide opportunities for development through highly cross-linked
structures, hence better thermal and mechanical strength.
Epoxidized vegetable oils (EVO) have been a frequently studied polymer
precursor in recent years.
22
Vernonia oil is a naturally occurring EVO that is
obtained from the seeds of a plant native to Africa, Vernonia galamensis. The
seeds contain up to 40 wt% oil by weight, with typical fatty acid distributions
averaging 6% oleic acid, 12% linoleic acid, and 80% vernolic acid
(Scheme 9.2).
24
Possessing low viscosity, vernonia oil has been used as a
reactive diluent in epoxy coating formulations
25
or in cationically cured
blends with commercial epoxy.
26
Epoxidized soybean oil (ESO) and epoxidized linseed oil (ELO) are cur-
rently the only bio-renewable epoxies that reach industrial-scale production.
World annual production of EVOs is greater than 200 000 tons.
27
EVO can be
prepared by the epoxidation of the double bond of fatty acids using peracids,
and such processes have been utilized since the 1940s.
28,29
Performic acid
and peracetic acid are commonly employed by the industry and are formed
in situ from hydrogen peroxide and the corresponding acid in the presence
of a strong acid catalyst such as sulfuric acid (Scheme 9.3).
30
However, strong
acids also catalyze the ring-opening reaction of the desired product, oxirane.
In order to improve epoxidation selectivity and reduce side-reactions, acidic
ion-exchange resins,
31
heterogeneous transition metal catalysts,
32
and en-
zymes
33,34
have been used as peracid catalysts for the epoxidation of VOs.
The latter have proved to be very effective for the epoxidation of VOs with
extremely high yields and fewer side-reactions. The epoxy content of VOs
Scheme 9.2 The structure of venolic acid in vernonia oil.
