30. Furan-Based Adhesives

The interest in this type of process was, of course, the decrease of formaldehyde
content and, therefore, its lower release during the life cycle of the resin.
This approach was then extended by Pizzi’s group to other phenolic type adhesives
such as phenol–resorcinol–formaldehyde networks [19]. In this work, it was shown that
the addition of 1 gave cold setting resins with performances and costs comparable to those
made using formaldehyde alone. Thus, the phenol–resorcinol–furfural–formaldehyde cold
sets obtained appeared to have a lower bulk shrinkage compared to those prepared without
1. Moreover, it was established that the presence of furfural did not slow down the
curing rate of the resins.
Stamm [20] studied the dimensional stabilization of different woods with 2. Thus,
Douglas fir, Engelman spruce, loblolly pine, and yellow poplar woods were treated with 2
in the presence of zinc chloride, citric acid, or formic acid in order to induce their acidcatalyzed
polymerization. It was established that the maximum antishrinking efficiency
(around 72%) could be reached with a resin level of a minimum of 40% with respect to
oven dried (OD) wood. The optimal amount of each acidic catalyst was also determined.
The curing time was studied for each system and it was shown that the use of 1% zinc
chloride and 6 h of curing time at 120 C gave very satisfactory fracture moduli, toughness,
abrasion resistance, and antishrinking behavior. The only limitation associated with the
possible uses of these systems is the dark color of the final materials.
Dhamaney [21] showed that the addition of furfural into cashew nut shell liquid
adhesives based on phenol–formaldehyde resins, using CuCl2 or CaCO3 as a ‘‘hardener,’’
gave good adhesive bonding for ordinary plywood. Johns et al. [22] prepared white fir
flakeboards using an aqueous solution containing a mixture of ammonium lignosulfonate,
2, and maleic acid as a binder. Before bonding, the wood surface was activated by a nitric
acid treatment. It was shown that the panels thus obtained possessed a higher elasticity
modulus and lower thickness swell and water absorption compared with those prepared
using classical phenol–formaldehyde binders. Nevertheless, the internal bonding and the
rupture modulus were higher for panels obtained using conventional resins. It was also
established that best surface activation was achieved using a 1.5% aqueous solution of
nitric acid (25–40%) with respect to OD wood, since it gave the optimal mechanical
properties for both high and low density panels.
Gupta et al. [23] prepared plywoods from Cedrus deodora and phenol–formaldehyde
resins. They showed that the addition of 5% of 1 to this adhesive did not result in any
appreciable improvement, but the concomitant addition of 10% of coconut shell powder
gave very high failing loads and very low glue failures. Subsequently, in another context,
Pizzi et al. [24] tested different aliphatic aldehydes and 1, in tannin-based adhesives, and
showed that furfural could replace formaldehyde in the manufacture of adhesive resins for
beam lamination. Roczniak [25] studied the thermal properties of phenol–formaldehyde–1
resins, as catalyzed by dichlorohydrin of glycerol, boric acid, hexamethylenetetramine
(HMTA), or p-toluene sulfonic acid. Two main conclusions were drawn from this work:
(i) p-toluene sulfonic acid gave a faster resinification rate and (ii) HMTA led to the highest
thermal resistant resins. Krach and Gos [26] investigated the gluing of large dimension
sawn wood structures using urea–melamine–furfural as a binder. They stated that the
initial wood moisture (8 to 12%) and the time of adhesive spreading (10 to 90 min) did
not influence significantly the strength properties of the glued junction.
Philippou et al. [27] studied the bonding of wood by graft polymerization. They
produced white fir, Douglas fir, and bishop pine particleboards using 2 as well as mixtures
of ammonium lignosulfonate with 2 or with formaldehyde as cross-linking agents. Before
bonding, the wood surface was activated with different amounts of hydrogen peroxide
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