Environmental Engineering Reference
In-Depth Information
[168-177]. The foams are produced using a mixture of tannin extract, formalde-
hyde, FA (Section 5.3), and a blowing agent, utilizing a three-step process [170].
In the first step, the mixture is stirred vigorously [170]. A strong acid, acting as the
catalyst, is then added, initiating the second step [170]. During the second step,
the exothermic condensation reaction between the FA, tannin, and formaldehyde
causes the blowing agent to boil; the evaporation of the blowing agent causes the
reaction mixture to foam and cure instantaneously [170]. During the last step, the
cross-linked foam is stabilized and hardened [170]. This approach successfully
produced foams from mimosa, quebracho, and pine tannins, with slightly differ-
ent cure kinetics for each tannin [170]. The investigation showed that an important
balance between the evaporation rate of the blowing agent and the cure kinetics
had to be met and that a mixture of tannins, including pine tannin, contributed to
this balance [170].
Mechanical compression tests of the tannin-based foams showed compressive
strengths ranging from 0.03 to 3.97 MPa, depending on relative density and direc-
tion of failure. These values compare favorably to the compressive strength of
fossil-based phenol-based foams, which range from 0.06 to 5.4 MPa for the same
relative densities [171]. In addition, it was determined that the foams had very low
thermal conductivities (0.024-0.030W m
-1
K
-1
) and were fire resistant and self-
extinguishing [171]. The combination of these properties, together with the low
material costs, suggests possible applications as insulation materials [171].
Another study examined the pyrolysis of tannin-based foams at 900°C to produce
carbon foams [172]. These carbon foams possessed the good mechanical strength
and low thermal conductivity observed in the organic foams [172]. The study showed
electrical conductivities for the carbon foams of 1.47 and 1.13 S cm
-1
along the
z
- and
xy
-directions, respectively, which represent good conductivities for the low bulk den-
sity [172]. Studies by the Pizzi group suggested applications for tannin-based foams
other than insulation, including catalyst supports and wastewater filters [178, 179].
Most recently, the Pizzi group showed that tannin-FA chemistries can be imple-
mented for wood adhesives avoiding the use of formaldehyde, for foams without
using blowing agents or formaldehyde, and for thermosetting plastics [176, 180,
181]. The first two methods are important because they reduce fossil-based formal-
dehyde and represent a greener approach to the production of solvent-free foams
[176]. The thermosetting plastics produced by condensation reactions between tan-
nin and FA could be molded and exhibited good post-curing mechanical and
thermal properties [181]. Figure 5.6 shows the cross-linked networked formed by
these materials; the incorporated aromatic and cyclic species indicate good mate-
rial strength. The break strength of tannin-furan plastics was above 190 MPa and
the Young's modulus was 2.16 GPa, rendering these materials competitive with a
number of fossil-based materials [181]. Although the synthesis of tannin-furfuryl
thermosets is tedious, once the procedure is optimized the properties of the result-
ing moldable plastics make them a competitive option for many applications.
