Chemistry Reference
In-Depth Information
performance. Thakur et al.
96
took advantage of graphene oxide (GO) and
used it to prepare a castor-oil-modified hyperbranched PU-GO nano-
composite. Similarly, with only 2 wt% of GO, a tremendous enhancement of
mechanical properties was observed. The tensile strength increased from 7
to 16 MPa and the elongation-at-break increased from 695 to 810%. More-
over these nanocomposites exhibited excellent shape-memory behavior
which can be explained by an increase in stored energy because of the
homogeneous distribution of GO in the polymer matrix. The shape recovery
was also found to increase with increasing amounts of GO in the matrix.
In addition to PU, castor oil can be used to synthesize a number of dif-
ferent polymers. For example, Sathiskumar et al.
97
synthesized a new family
of castor-oil-based bio-degradable polyesters by a catalyst-free melt con-
densation reaction between castor oil and diacids with D-mannitol. The
resulting polymers were bio-degradable soft materials with a hydrophilic
surface. A star-shaped polyester polyol was synthesized by Risti´ et al.
98
via
polymerization of L-lactide with castor oil as the initiator. Saravari et al.
99
synthesized a urethane alkyd by interesterification of castor oil with jatropha
oil, followed by reaction with TDI. The castor-jatropha-oil-based urethane
alkyd had a lower molecular weight and viscosity, a slightly lower hardness
and a much longer drying time than conventional and commercial urethane
alkyds, with excellent resistance to water and acid.
5.3.2 Use of Castor Oil after Chemical Modification for
Polymer Synthesis
With both double bonds and hydroxyl groups in the fatty acid chains of
castor oil, chemical modifications are available. Hirayama et al.
100
elimin-
ated the hydroxyl groups from the fatty acid chains to obtain dehydrated
castor oil (DCO), producing a semi-drying oil, which can be used extensively
in paints and varnishes. The dehydration process is performed at about
250 1C in the presence of an acid catalyst such as H
2
SO
4
and activated earth
under an inert atmosphere or vacuum. The hydroxyl group and an adjacent
hydrogen atom from the C11 or C13 position of the ricinoleic acid portion of
the molecule are removed as water. This process forms not only conjugated
9,11-diene moieties but also non-conjugated 9,12-diene moieties with a ratio
of about 41 : 59. The average number of double bonds per triglyceride is 4.8.
The DCO is then used for the preparation of a thermosetting resin with
1,1
0
-(methylene-di-4,1-phenylene)bis-maleimide in 1,3-dimethyl-2-imidazolidi-
none. It is worth noting that almost all the conjugated diene moieties of
DCO reacted due to the Diels-Alder (DA) reaction between the maleimide
groups and the conjugated dienes.
Instead of elimination of the hydroxyl groups, new functional groups such
as dangling double bonds can be incorporated to the fatty acid chains via
reaction of hydroxyl groups with acrylic acid, resulting in acrylated castor oil
(ACO). Kim et al.
101
used ACO for the synthesis of novel cross-linked thin
