30. Furan-Based Adhesives

Recently, Schneider et al. [47] fabricated particleboards using poly2–urea–formaldehyde
adhesives (P2-U-F). They observed that the curing time needed for P2-U-F was
double that necessary for classical urea–formaldehyde resins. They also established that
P2-U-F produced boards with lower strength properties, but with higher water resistance,
if classical processing conditions were used. However, at higher resin contents, P2-U-F
gave boards with better mechanical properties. The following optimal conditions were
derived to produce particleboards: a blending time of 10 min, a press platen temperature
of 150 C, 15% of P2-U-F resin with respect to OD softwood, 1.4 min of pressing time per
millimeter thickness, and a board density of 0.67 kg/dm 3 .
Dao and Zavarin [48,49] prepared boards using wood powder and 2 or poly2 as
binders. The wood species was white fir (Abies concolor) which was used as powder
screened to 80 mesh. Compound 2, poly2, and wood were subjected to chemical activation
with hydrogen peroxide/ferrous ions or nitric acid. It was established that an increase in
the degree of polymerization of poly2 yielded boards with increased strength properties
and that poly2 gave materials with higher strength and water resistance properties than
those obtained using 2. They also showed that the addition of the activator to poly2, rather
than to wood, was more efficient. Finally, they also isolated the acetone-soluble fraction of
poly2 (about 73%) and used it as a binder for the same wood samples. They found that the
tensile properties of the corresponding boards exceeded, by over 50%, those of composites
prepared with conventional phenol–resorcinol–formaldehyde resins.
Abd El Mohsen et al. [50] modified classical urea–formaldehyde resins by adding
different amounts of 2 and used them as binders for beech-based plywoods. These modified
resins gave materials with higher shear strength properties (100% increase) in
comparison to unmodified adhesives. They also established the following optimal
formulations: addition of 30, 45, and 60% of 2 to classical urea–formaldehyde resins
and 3, 4.5, and 6% of p-toluene sulfonic acid as a hardener, respectively. Coppock [51]
prepared durable wood adhesives from furfural-based diols, diamines, and diisocyanates.
She then made plywoods or particleboards using modified urea–formaldehyde resins, with
3 and 4 as binders and found that the materials thus obtained showed acceptable mechanical
properties. These properties were not improved by the addition of further modifiers,
such as 5,5 0 -ethylidene furfuryl amine (18). Measurements using DSC showed that 3 did
not react under alkaline conditions, but readily resinified at pH values below 3.0. These
materials were found to have lower formaldehyde emission compared with those made
with unmodified resins. The mechanical performances of flakeboards made with 14
exceeded the industrial standard requirements and were equivalent to those prepared
using MDI. Finally, materials based on 14 in the presence of 3 or 18 as modifiers were
obtained and found to have better performances in comparison to those prepared without
these additives.
Suzuki et al. [52] prepared wood-meal/plastic composites with an average thickness of
4 mm using urea–2 and phenol–1 resins as binders. The molar ratio between urea and 2 was
varied from 9:1 to 1:9. The amount of formaldehyde emission decreased with increasing
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