Environmental Engineering Reference
In-Depth Information
To increase the renewable content of the polyurethanes produced with vegeta-
ble-oil-based polyols, diisocyanates have been synthesized from renewable
sources. Here, the fatty-acid-derived 1,7-heptamethylene diisocyanate and the
isosorbide-based diisocyanates are of interest [36, 37]. Both diisocyanates were
used to react with vegetable-based polyols to produce polyurethanes with 100%
renewable carbon content. The 1,7-heptamethylene diisocyanate was compared to
the fossil-based 1,6-hexamethlylene diisocyanate and displayed similar properties
[36]. In addition, it was shown that 1,7-heptamethylene could be used to synthesize
thermoplastic polyurethanes from completely renewable sources [38].
In an effort to remove volatile organic compounds from the polyurethane
systems, water-borne polyurethane dispersions have grown in popularity. These
systems are used in coatings and adhesives [39-42]. Combining emulsion
polymerization of acrylic monomers with the polyurethane dispersions to form
urethane-acrylic latexes significantly increased the thermal and mechanical properties
of the resulting polyurethane films [43]. Other work showed methoxylated
soybean oil polyols with hydroxyl numbers ranging from 2.4 to 4.0 could be used
to vary the cross-linking density of the polyurethanes, producing materials ranging
from elastomeric polymers, ductile plastics, to rigid plastics, thus illustrating the
versatility of the material for a wide range of possible applications [44].
5.2.5
Polyesters
Alkyd resins are polyesters formed by esterification of polyhydroxy alcohols,
such as glycerol, with polybasic acids and fatty acids. Alkyd resins are one of the
oldest materials produced by vegetable oils, with reports of various systems for
well over 100 years [4]. The resins can be designed to exhibit good film properties
with high viscosity and good drying and hardness behavior [4]. The nature of the
components (fatty acids and glycerol) and the ester linkages provide a higher level
of biodegradability to these resins, making them suitable for possible medical
uses [4, 45].
There are three main reaction schemes to produce polyesters: polycondensation
of hydroxy acids; polycondensation of a diacid and a diol; or ring-opening polym-
erization of lactones [4]. Monoglycerides, obtained by the alcoholysis of vegetable
oils, can be directly reacted with anhydrides, such as glutaric, phthalic, maleic,
and succinic anhydride, to produce polyester resins [46, 47]. Polyester resins from
vegetable oil enjoy growing popularity because they are economical and easy to
implement [4]. The resins are classified based on their 'oil length', referring to the
percentage of oil contained in the resin. Short-oil resins contain less than 40% oil,
medium-oil lengths range from 40 to 60% oil, and long-oil alkyds contain more
than 60% oil [4]. The oil content alters the final properties of, for example, coatings;
short-oil polyester resins are used for baked finishes on automobiles and appliances,
while long-oil resins are used in brushing enamels [4].
