Environmental Engineering Reference
In-Depth Information
5.6 The Potential of Tannins
We provide a brief introduction to the potential of tannins. Despite a promising
aromatic structure with viable reactive sites, the use of tannins in polymeric
materials has been limited. On a broader scale, tannins have some interesting
properties that fit well with the goal of creating chemicals from biomass. Outside
the scope of this chapter, tannins are used in the manufacture of leather, as wine,
beer, and juice additives, as ore flotation agents, in cement as a superplasticizer,
and more recently in pharmaceutical applications [163]. As a polymer tannins mostly
find use in adhesives, in particular as a binder for wood where the condensation
reaction between formaldehyde and tannin is exploited to form a thermosetting
resin. The use of tannins in wood adhesives is well established and is not discussed
here [163-167]. Recent work by Tondi
et al
. has produced interesting results with
regards to the condensation reaction between tannin and FA to produce a near
completely biorenewable thermoset foam for applications in insulation, which is
the focus of this discussion [168]. This section will introduce the oligomeric
structure of condensed tannins and use the tannin-based foams as an example for
the potential of this widely available chemical.
A generalized structure of a condensed tannin molecule is shown in Figure 5.5.
The two types of tannins are condensed and hydrolyzable; the latter constitutes
less than 10% of the tannins produced and is therefore not discussed here [164].
Condensed tannins are natural products of the oligomerization of aromatic flavonoids,
where the degree of oligomerization depends on the source of the flavonoids
[163]. In mimosa and quebracho tannins, the number of flavonoid repeat units
ranges between 2 and 11 with an average degree of polymerization between 4 and
5; for pine tannins the average degree of polymerization is higher (between
6 and 7) [163]. Tannins are produced by a large number of woody plants and can
be obtained through simple water extraction; common sources are the wood of
quebracho, chestnut, and mangrove and the bark of oak, black wattle, black
mimosa, and several species of pines and firs. Tannin production reached a historic
high after World War II and, after several decades of decline, it is currently
approximately 200 000 tons per year [163]. Today, countries with high tannin
productions include Brazil, South Africa, India, Zimbabwe, Tanzania (mimosa
tannin), Argentina (quebracho tannin), and Indonesia (mangrove tannin) [163].
As illustrated in Figure 5.5, flavonoids are composed of a tricyclic monomer
unit with two phenolic rings, A and B. Using the reactivity of these species with
formaldehyde and other aldehydes to form cross-linked polymers is well established
[163-167]. However, it should be pointed out that crude tannins are approximately
70-80% of these active polyflavonoid species, with the remaining ingredients act-
ing as inert impurities [163]. Purification of tannins has proven to be difficult;
tannins are therefore used with these impurities, which act as plasticizers. Resin
additives are often needed to increase the cross-linking density of the tannin matrix
[163]. In addition, the bulkiness and steric hindrance of the tannin oligomers can
cause incomplete consumption of reactive sites when reacted with formaldehyde,
