Tannin adhesive has been used as a thermosetting wood adhesive
for wood panels to reduce formaldehyde emission (Kim and
Kim, 2004). Tannin adhesive gives excellent adhesive performance,
good moisture resistance, and tends to give a lower formaldehyde
emission than UF resin (Kim et al., 2007b). Before surface coating,
formaldehyde emissions from the products glued with tannin
adhesive was already less than E1 grade (under 1.5 mg/L) of formaldehyde
emission level in the Korean Standard even though paraformaldehyde
was contained in tannin adhesive, as shown in Table
2. The tannin/PVAc hybred adhesives samples were showed similar
tendency with the natural tannin adhesive. PVAc was not caused
increasing formaldehyde emission.
When fancy veneer on plywood was coasted by UV curable urethane
acrylate coating, the formaldehyde emission from this flooring
was reduced about two thirds. In the tannin/PVAc hybrid
adhesives system, the role of PVAc was not make formaldehy deemission, by replacement of the tannin adhesive with formaldehyde-
based hardener, and to increase the initial bonding strength
through PVAc’s high viscosity and room temperature curable property.
The formaldehyde emission level of the coated sample of MF/
PVAc hybrid appeared to satisfy the requirement of E0 grade when
UV coating was applied.
Fig. 2 presents the concentrations of the four indicated VOCs
from the engineered flooring bonded with each adhesive system,
as determined by the VOC analyzer. For each substance, the concentrations
were measured at 96 h. after the start of the test. In
the first phase, the concentration in the 3 L polyester bag increased
due to the constant emission of organic compounds and was limited
by the given air exchange rate. Xylene was the highest detected
compound in all samples, followed by ethylbenzene and
toluene in turn. Styrene, however, was not detected at all in any
of the systems. But TVOC emission of the PVAc added tannin adhesive
system was slightly increased compare to the natural tannin
adhesive. This tendency of formaldehyde and TVOC emission from
the natural tannin adhesive and tannin/PVAc hybrid adhesives was
confirmed by FLEC test as shown in Table 2. The difference
between each system on formaldehyde emission was not shown
in FLEC data while TVOC of tannin/PVAc hybrid adhesives were
slightly increased compare to the natural tannin adhesive.