Plant-oil-based Polymeric Materials and their Applications - Green Materials from Plant Oils - page 103

PU foam can be prepared from soybean-oil-based polyols. Yang et al. 53
prepared a series of PU rigid foams by mixing polyols with toluene diiso-
cyanate using an isocyanate index of 1.1. These rigid foams possessed high
thermal stability. When the mass fraction of soybean-oil-based polyol was
increased up to 60%, the foam had the highest compressive strength of
292.34 kPa and a density of 425.29 kg m 3 , and the cells of the foam were
small and uniform. Gu et al. 54 reported an in situ reaction of a methylene
diphenyl diisocyanate (MDI) polyuria pre-polymer and a soybean-oil-based
polyol for the synthesis of PU foam. The hydroxyl value of the soybean-oil-
based polyol exerted important effects on the cell morphologies and foam
properties. With high hydroxyl values, it exhibited superior tensile strength,
high tensile elongation, high degradability and high T g compared with low
hydroxyl values.
Recently, Miao et al. 55 developed an interesting shape-memory PU from a
series of different structural soybean-oil-based polyols with 1,6-diisocyanato-
hexane. It was found that they preserved the triglyceride structure and were
fixed in a temporary shape at 20 1C and could completely regain their
permanent shapes at 37 1C. Since several double bonds are present in vege-
table oils, the functionality of vegetable oil polyols is usually higher than 2.
Therefore, PU from vegetable oils consists of mostly cross-linked networks
that can endow the polymers with shape-memory effects. Physical interactions
such as vitrification and crystallization, might be able to fix a temporary
shape, but the cross-linking structure helps to restore the original shape.
Glycerol cross-linking and branch-linking were the two major structural pat-
terns in the PU networks, and it was found that glycerol cross-linking was
critical to the shape-recovery effect.
5.2.2.4 Other Modifications
Functional groups other than hydroxyl, acrylate and epoxide can also be
introduced to the fatty acid chains of soybean oil by reacting with different
reagents.
For example, an acrylated soybean oil can be obtained by a novel ecient
one-step method by reacting soybean oil with acrylic acid under the catalysis
of BF 3 OEt 2 (ref. 56). The number of acrylate groups could reach 3.09 per
triglyceride molecule and the conversion of the double bonds was up to
75.7%.
Biswas et al. 57 developed an environmentally friendly water-based pathway
to form the azide derivatives of soybean oil and fatty esters by first forming
epoxides and then the azidization of these epoxides. The azidization reaction
was carried out at high yields in water with a small amount of an ionic liquid
as the catalyst. In addition, new monomers were prepared by introducing
azide groups 57,58 into vegetable oils including castor, canola, corn, soybean
and linseed. Polymerization of these azidated oils with alkynated soybean
oil 58 under thermal click chemistry conditions (without using a solvent or a
catalyst) yielded fully cross-linked elastomers of almost the same density. 59