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Structural beams from thick wood
panels bonded industrially with
formaldehyde-free tannin adhesives
F. Pichelin
M. Nakatani
A. Pizzi✳
S. Wieland
A. Despres
S. Rigolet
Abstract
Mimosa tannin hardened with hexamine at pH 10 has shown both at the laboratory and industrial level to be a formaldehyde-
free system, within the limits of sensitivity of the method of Japanese standard JIS A 5908. This useful effect is based on the
double mechanism of slow hexamine decomposition to reactive imino-amino methylene bases and their immediately subsequent
very rapid reaction with the tannin. Decomposition to formaldehyde can never be reached under the conditions used. This yielded
a long ambient temperature pot-life coupled with the fast hardening of the adhesive and fast pressing times of the thick panels by
introducing a two-step steam-injection sequence during panel pressing. No formaldehyde emission was found in the panels
bonded with such an adhesive system once tested according to the relevant Japanese standard. No free formaldehyde was
detected by solid state
13
C NMR, nor residual hexamine, in the hardened tannin-hexamine adhesive, although these spectra have
to be taken with caution due to the usual peak enlargement and relative lack of sensitivity in solid state NMR spectra. The
reactions involved were explained by
13
C NMR. The panels obtained satisfied the relevant Japanese standard specification for
both internal bond strength and formaldehyde emission.
Japanese standard regulations have become more restric-
tive (F**** level, JIS A 5908) regarding formaldehyde emis-
sions from wood adhesives, therefore there is increased inter-
est in finding alternative adhesives that will satisfy the stricter
requirements. The use of isocyanate adhesives is not a viable
solution to the stricter regulations because residual and immo-
bilized isocyanate groups in the hardened adhesive network
have still been found, often in considerable proportion, in
hardened boards (Wieland et al. 2006, Despres et al. 2006).
Hexamethylenetetramine (hexamine) has already been
used industrially for the production of interior-grade tannin-
bonded panels of low or no formaldehyde emission (Pizzi
1977, 1999; Pizzi et al. 1994, 1997; Pichelin 1999; Pichelin et
al. 1999). In the case of the faster reacting tannins such as pine
tannins, exterior-grade panels can also be produced (Pizzi et
al. 1994, Pizzi et al. 1997, Pichelin 1999, Pichelin et al. 1999,
Pizzi 1999). In this latter application, however, hexamine’s
complexation with pine tannin does produce, sometimes too
often, aggregates that do not flow as well as one would wish
for in adhesive application (Pichelin et al. 1999, Pizzi 1999).
This affects the tannin adhesive performance as well as lim-
iting the potential use of hexamine for exterior- and semiex-
The authors are, respectively, Head R&D Panel Products, HSB,
Hochschule fur Architektur, Bau und Holz, Univ. of Applied Sci-
ence, Biel, Switzerland (Frederic.Pichelin@bfh.ch); Project Leader,
Wood Project, Urban Infrastructure and Environmental Products
Co., Sekisui Chemical Co. Ltd., Kyoto, Japan; Professor (pizzi@
enstib.uhp-nancy.fr) and PhD Students, ENSTIB-LERMAB, Univ.
of Nancy 1, Epinal, France; and Researcher, LMPC, ENSCMu, Min-
eral Materials Lab., Mulhouse, France (S.Rigolet@univ-mulhouse.
fr). This paper was received for publication in May 2005. Article No.
10057.
✳Forest Products Society Member.
©Forest Products Society 2006.
Forest Prod. J. 56(5):31-36.
FOREST PRODUCTS JOURNAL VOL. 56, NO.5 31
terior-grade tannin adhesives. In the case of other tannins such
as mimosa tannin, this aggregation problem is rather rare, but
the panels obtained are of interior grade.
Textbooks report that hexamine decomposes readily to
formaldehyde and ammonia in an acid environment and
slightly less readily to formaldehyde and to trimethylamine in
an alkaline environment (Walker 1964, Meyer 1979). Work
on fast-reacting natural and synthetic resins (all deficient in
formaldehyde), mainly tannins (Pizzi and Tekely 1995), res-
orcinol-formaldehyde resins (Pizzi and Tekely 1996), and
melamine-formaldehyde resins (Pizzi and Tekely 1996, Pizzi
et al. 1996) has shown a different behavior of hexamine. Thus,
in the presence of fast-reacting species, hexamine is not at all
a formaldehyde-yielding compound. It neither decomposes to
formaldehyde and ammonia in an acid environment nor to
formaldehyde and trimethylamine in an alkaline environment
(Pizzi and Tekely 1995, 1996; Pizzi et al. 1996; Kamoun et al.
2003). The very reactive amino-immine intermediates ini-
tially formed in its decomposition do react with the phenolic
or aminoplastic species present without ever passing through
the formation of formaldehyde (Pizzi and Tekely 1995, 1996;
Pizzi et al. 1996; Kamoun et al. 2003). CP-MAS
13
C NMR
solid phase spectra of the hardened resins have confirmed this
occurrence and shown that the hardened networks present a
high proportion of di- and tribenzylamine bridges (-CH
2
-NH-
CH
2
- and -CH
2
-N(-CH
2
-)-CH
2
-), rather than methylene
bridges, connecting phenolic or melamine nuclei. These ben-
zylamine bridges (or aminomethylene bridges in the mela-
mine-formaldehyde and melamine-urea-formaldehyde cases)
are temperature stable for long periods, even at relatively high
temperatures, and dominate resin cross-linking.
This paper deals with the application of a technique called
press steam injection, which overcomes the aggregation prob-
lems of tannin adhesives mixed with hexamine and greatly
upgrades their performance as wood panel adhesives. Tech-
niques and results are reported for very thick panel products,
up to 120 mm thickness, and the structural beams cut from the
panels. The chemical modifications that led to the perfor-
mance improvement of panels bonded with tannin-hexamine
adhesives were examined by solid phase
13
C NMR and are
explained. The panels obtained and the structural beams ob-
tained from them are for interior use, hence they do not need to
withstand a boiling test. Nonetheless, the panels obtained,
while not being able to pass the relevant boiling test specifi-
cation were able to withstand 4 hours in boiling water.
Experimental
Triplicate laboratory panels of dimension 500 by 500 by 25
mm and 500 by 500 by 40 mm were prepared by using very
coarse chips (20 to 50 by 3 to 5 by 1 to 5 mm) of a number of
different mixed species obtained by recycling structural wood
taken out of service, and resinated with a solution of mimosa
tannin extract (mimosa-O and mimosa-T ex Tanzania, sup-
plied by Silva, S.Michele Mondovi, Italy) that had a Stiasny
value (Stiasny 1905, Hillis and Urbach 1959, Suomi-
Lindberg 1985) of 91.2 to 92.2, respectively, to which hex-
amine hardener was added. The tannin was applied to the
wood chips as a 45 percent solution in water. The total resin
load used was 7 percent tannin extract by weight on dry wood
chips. The tannin extract hardener content used was 5 percent
hexamine by weight on tannin extract solids content. The hex-
amine was predissolved in water to yield a 45 percent concen-
tration solution in water before being added to the tannin so-
lution to form the glue-mix.
The press times used were 380, 330, and 300 seconds and
the target density was 880 kg/m
3
. The press temperature was
180°C and it involved two steam injections, one brief one at
the beginning at a much lower steam pressure and one later at
9 bar steam pressure. The second, main steam injection time
was for 60 seconds, with the exception of some of the indus-
trial pilot plant trials where a steam injection time of 120 sec-
onds was used. The results obtained are shown in Tables 1
and 2. The gel times at 100°C and viscosity at 25°Cofthe
tannin + hexamine as a function of pH are shown in Figure 1.
Viscosity of the mimosa tannin/hexamine solutions did not
vary much in the pH range 4 to 10: values of 196, 264, and 318
centipoises at pH 4, 7, and 10, respectively. Industrial pilot
plant trials were prepared up to a thickness of 120 mm. Form-
aldehyde emission was measured on the panels according to
Japanese Standard JIS A 5908 (JIS 1994), which requires less
than 0.3 mg/L formaldehyde emission for F**** class panels.
The solid state
13
C NMR spectra of the hardened tannin-
hexamine resin systems used were obtained on a Bruker MSL
300 FT-NMR spectrometer at a frequency of 75.47 MHz and
at a sample spin of 4.0 kHz. The impulse duration at 90 de-
grees was 4.2 µs, contact time was 1 ms, number of transients
was about 1,000, and the decoupling field was 59.5 kHz.
Chemical shifts were determined relative to tetramethyl silane
(TMS) used as control. The different shifts possible for the
different structures were taken from the literature (Pizzi and
Tekely 1995, 1996; Pizzi et al. 1996; Pizzi 1999).
Discussion
The results of the hardening time of mimosa tannin extract,
with hexamine hardener, as a function of pH are shown in
Figure 1, which clearly illustrates the well-known fact that
the hardening time of tannin adhesives with hexamine hard-
ener lengthens with increasing pH, particularly at alkaline pH
(Pizzi 1979, Pichelin 1999, Pichelin et al. 1999). This behav-
ior depends on the rate of decomposition of hexamine, which
depends strongly on the pH. The behavior shown in Figure 1
is expected, as hexamine is a monoprotic base and to react it
has to first start decomposing. The more basic the pH, the
more difficult it is for the hexamine to start decomposing, thus
its decomposition is slower, and as a consequence there is a
Figure 1. — Hardening time as a function of pH of mimosa
tannin solutions hardened with 5 percent hexamethylenetet-
ramine, solids on solids
32 MAY 2006
slowing of the availability of the reactive species to cross-link
the tannin. The reactivity of the tannin is, however, at its high-
est at very alkaline pH levels, the same pH levels at which
hexamine is slower to decompose down to reactive species.
Thus, while the rate-determining step for the hardening time is
the slower one of the two, namely hexamine decomposition,
once the reactive species start to form, the reaction of the tan-
nin with them is extremely rapid.
It has already been shown that in the presence of fast-
reacting species, hexamine neither decomposes to formalde-
hyde nor yields formaldehyde, but it does yield very reactive
amino-imino methylene bases of the type CH
2
= N-CH
2
+
(Pichelin et al. 1999; Pizzi 1999; Kamoun and Pizzi 2000a,
2000b; Kamoun et al. 2003). This reaction mechanism is
based on the capacity of the reactive species present to be able
to react with the amino-imino methylene bases CH
2
= N-CH
2
+
before further decomposition can occur. This is considerably
more effective at pH levels where formation of the bases is
slower and the reactivity of the capturing species, the tannin,
is much higher. It is this slow generation that ensures a too-
complete reaction of any intermediate formed with the tannin
before any decomposition or evaporation of the intermediate
can occur. This is exactly the case for higher alkaline pH lev-
els such as pH 10 or higher. pH 10 is a good compromise,
however, because at higher pHs the higher level of alkali con-
tent would increase further both water absorption and thick-
ness swelling of the board bonded with such an adhesive sys-
tem. At much lower pHs, faster decomposition of the hex-
amine and lower reactivity of the tannin could lead to traces of
decomposition to formaldehyde accompanied by its volatil-
ization at higher temperature, in the press, hence leading to
loss of cross-linking and lower strength. That this is the case is
confirmed by the panel results in Table 1.
The results in Table 1 confirm that the internal bond (IB)
strength of the tannin-hexamine adhesives are good and in-
crease with increasing pH. The relevant Japanese standard JIS
A 5908 is satisfied at pH 10. But the IB strength becomes
progressively lower as the pH decreases, as expected. Table 1
shows that panel performance worsens, both IB strength and
cold water swelling, when increasing the percentage of hex-
amine hardener on tannin extract. The main problem appears
to be that the increase in the proportion of the relatively sen-
sitive aminic function of the hydroxybenzylamine bridges
formed renders the panel more sensitive to water, as can be
noted in Table 1 from the increase in the 24-hour cold water
swelling value. Panel performance also appears to improve as
resin load increases from 5 to 9 percent, but the IB values are
so much higher than what is needed, that it is not really worth-
while to increase the resin load to values as high as 9 percent.
Table 1 confirms that, at the laboratory level, faster press
times for a thicker panel are also possible, hence 330 seconds
for a 40-mm thickness is equivalent to 8.3 seconds per milli-
meter of thickness. Of particular note in Table 1 are the form-
aldehyde emission tests performed on the panels according to
the methods outlined in the Japanese standard JIS A 5908.
These results are lower than the level of sensitivity of the
method, which is why they are shown as 0.0 in the table. More
accurate determination has shown that the emission is much
lower even than the formaldehyde generated by the heating of
wood. This is quite likely because of ammonia-formaldehyde
chemical equilibria due to the presence of hexamine. The in-
dustrial pilot plant results in Table 2 indicate that the IB
strength and water swelling results obtained for industrial
panels of 40 mm and 120 mm thickness are better than those
obtained in the laboratory, and this at rather fast pressing
times, namely down to 300 seconds for the 120-mm thickness,
equivalent to 2.5 seconds per millimeter of thickness. The
formaldehyde emission tests performed on the panels accord-
ing to the methods outlined in Japanese standard JIS A 5908
again showed zero formaldehyde emission.
The existence of the mechanism outlined above for tannin-
hexamine systems has the inherent advantage of a very long
pot-life of the glue mix at ambient temperature. It has the dis-
advantage, however, of being rather slow for normal board
pressing conditions where fast pressing rates are essential to
panel factory profitability.
It is in this context that steam injection during pressing
solves the problem of slow pressing time and slow hardening
Table 1. —Results of thick laboratory panels bonded with tanzanian mimosa tannin + hexamine. Steam injection total duration
was 60 seconds.
Tannin type Hexamine pH
Resin
load
Board
thickness
Press
time
Board
density
Dry IB
strength
IB4h
boil
24 h cold
water swelling
Formaldehyde
emission
a
(%) (%) (mm) (sec) (kg/m
3
) (MPa) (MPa) (%) (mg/L)
Mimosa-O 5 7.0 7 40 330 0.75 0.37 -- 17.1 0.0
5 8.0 7 40 330 0.75 0.57 -- 14.4 0.0
5 9.0 7 40 330 0.75 0.58 -- 14.1 0.0
5 10.0 7 40 330 0.74 0.73 -- 12.8 0.0
Mimosa-T 5 10.0 7 25 380 0.85 0.95 -- 12.0 0.0
10 10.0 7 25 380 0.84 0.71 -- 13.8 0.0
15 10.0 7 25 380 0.81 0.78 -- 15.0 0.0
5 10.0 5 25 380 0.85 0.83 0.06 15.3 0.0
5 10.0 7 25 380 0.83 0.82 0.07 12.0 0.0
5 10.0 9 25 380 0.85 0.98 0.09 10.0 0.0
5 10.0 7 40 330 0.75 0.78 -- 12.0 0.0
Standard
requirements
(JIS A5908) -- -- -- -- -- -- ⱖ0.30 -- ⱕ12.0 ⱕ0.3
a
Measured on panels according to Japanese standard JIS A 5908.
FOREST PRODUCTS JOURNAL VOL. 56, NO.5 33
reaction. Steam-injection techniques are used to considerably
accelerate the curing of wood panel adhesives, hence shorten-
ing markedly panel pressing time. The reaction of decompo-
sition of the hexamine, as well as the reaction of the amino-
imino methylene bases intermediates with tannin, are mark-
edly accelerated by the application of steam injection. The
resultant tannin-hexamine adhesive has two advantages: 1)
long pot-life at ambient temperature; and 2) a fast press time at
high temperature when steam injection is applied. It has the
added advantage that the intermediate can never reach the for-
mation of formaldehyde during hexamine decomposition.
Formaldehyde emission will then be non-existent, within the
limits of sensitivity of the method used in Japanese standard
JIS A 5908. That this is indeed the case is confirmed by the no
formaldehyde emission results of the panels in Tables 1 and 2.
Furthermore, it has the added advantage that while at pHs
much lower than 10, where hexamine decomposition is much
faster, small parts of the intermediates can still decompose to
formaldehyde, at pH 10 and higher this is not the case. This is
confirmed by the evident absence at pH 10 of any tannin-to-
tannin formaldehyde-derived methylene bridges in the
13
C
NMR spectra of hardened tannin-hexamine resins discussed
below. The mechanism is shown in Figure 2.
Steam injection has also the considerable advantage that if
any tannin-hexamine complexes are formed (Pichelin et al.
1999, Pizzi 1999), they have the appearance of non-flowing
aggregates, these are dissolved and dissolved well by using
steam injection. This is not a problem with the mimosa tannin
extract used, where these aggregates form only very rarely,
due to its lower number average molecular mass (Fechtal and
Riedl 1993, Thompson and Pizzi 1995, Pasch et al. 2001), but
it may be a problem with the even better performing pine tan-
nin extract. Steam injection then solves even this problem.
The further advantage of tannin resins of this type is that with
steam-injection hardening the tannin-hexamine system is not
washed out as instead occurs with waterborne phenol-
formaldehyde resins. This is a considerable added advantage.
Viscosity of the mimosa tannin/hexamine solutions did not
vary much in the pH range 4 to 10, varying between 210 cen-
tipoises at pH 4 and 310 centipoises at pH 10. These values
made spray application easy in all cases.
Examples of the full-scale structural beams produced are
shown in Figure 3. These beams are used vertically, as pillars,
in the interior of traditional-type Japanese wood houses and
have a structural function. The panels are instead used as in-
terior cladding.
Comparative solid state CP-MAS
13
C NMR spectra of mi-
mosa tannin at pH 4, 7, and 10 hardened with hexamine (pre-
dissolved in solution or added as a solid in one case) were also
done. The comparative CP-MAS
13
C NMR spectra in Figure
4are hard to interpret as are all the spectra of hardened tannin
adhesives; the widening of solid state spectra peaks makes it
more difficult to observe even significant differences. The
spectra in Figure 4 show the hardened flavonoid tannin/
hexamine network when the hexamine is added as a water
solution to the tannin solution at pH’s 4, 7, and 10 or directly
as a solid at pH 4. The relation between the various atoms
numbers of the flavonoid structure in Figure 5 and the NMR
spectra is discussed below. The spectra show many similari-
ties but nonetheless also show some interesting differences
that give an idea why at pH 10 the board results are better than
those at pHs 4 and 7 (Tables 1 and 2).
The first difference noticeable is that at pH 10 the flavo-
noids’C3⬘and C4⬘peaks at 145 ppm of the flavonoid units is
Table 2. —Results of thick industrial pilot plant panels bonded with tanzanian mimosa tannin + hexamine.
Tannin type Hexamine pH
Resin
load
Board
thickness
Press
time
Board
density
Dry IB
strength
IB4h
boil
24 h cold
water swelling
Formaldehyde
emission
a
(%) (%) (mm) (s) (kg/m
3
) (MPa) (MPa) (%) (mg/L)
Mimosa-T
b
5 10.0 7 40 300 0.74 0.95 -- 9.1 0.0
Mimosa-T
c
5 10.0 7 120 300 0.75 0.79 -- 8.0 0.0
Standard
requirements
(JIS A5908) -- -- -- -- -- -- ⱖ0.30 -- ⱕ12.0 ⱕ0.3
a
Measured on panels according to Japanese standard JIS A 5908.
b
Steam injection total duration = 60 seconds.
c
Steam injection total duration = 90 seconds.
Figure 2. —Mechanism of hexamine decomposition to imino-
amino methylene bases in presence of fast-reacting species
such as tannins and their fast reaction with tannins to form
benzylamine bridges.
Figure 3. —An example of structural pillars prepared by cut-
ting thick panels of coarse chips. These pillars, used verti-
cally, have a structural function in traditional-type Japanese
houses now built according to modern principles.
34 MAY 2006
much lower than at pHs 4 and 7 in relation to the control peaks
of C5, C7, C9 at 153 to 156 ppm, which remain unaltered. The
decrease of the 145 ppm peak indicates the transformation to
phenate ions of the phenolic C3⬘and C4⬘hydroxygroups on
the flavonoid B-ring, implying a considerable increase in its
condensation reactivity, but particularly a changed shift due to
substitutions on the normally free sites on the B-ring. This is
accompanied by the more noticeable appearance of a shoulder
at 142 ppm, characteristic of C3⬘,C4⬘of a B-ring on which
multisubstitution has occurred. More direct confirmation ap-
pears to be supplied by the noticeable disappearance of the
peak at 107 to 112 ppm, indicating a decrease of the open C2⬘
and C5⬘site on the B-ring simply because these have been
substituted. An alternative interpretation of the disappearance
of the 107 to 112 ppm peak could be that marked interflavo-
noid bond cleavage has occurred at pH 10. This is unlikely in
the case of mimosa tannin, which is known to never cleave
(Pizzi 1983; Pizzi and Stephanou 1993, 1994; Thompson and
Pizzi 1995) at the interflavonoid bond but rather of being
prone to preferentially open the C-ring at C2 (Pizzi 1983;
Pizzi and Stephanou 1993, 1994; Thompson and Pizzi 1995).
The C1⬘peak at 131 to 132 ppm is also much lower, also
confirming that substitution has occurred on the B-ring. A cer-
tain extent of the C-ring opening is also noticeable in all the
different spectra. This can be deduced by the decrease at pH
10 of the C2 shoulder at 81 to 82 ppm, indicating a decrease of
the C2 of the close C-ring form, the slight downfield shift of
the C3 shoulder at 67 to 68 ppm, and the presence of the open
form of C2 at 31 to 33 ppm.
Also noticeable are three peaks at 23 to 25 ppm, at 31 to 33
ppm, and at 42 to 43 ppm. The peak at 23 to 25 ppm is that of
the unreacted C4 of the flavonoids. It is much smaller for the
pH 10 case than for the pHs 4 and 7 cases in Figure 4. The 41
to 43 ppm peak is a composition of a peak characteristic of
unreacted mimosa tannin but that can also be ascribed to
formaldehyde-derived methylene-bridges reacted on the
structure of the tannin. These are small but nonetheless no-
ticeable differences at pHs 4 and 7, indicating that some small
amounts of formaldehyde might still be produced in the de-
composition of hexamine. It is so small to be considered prac-
tically absent at pH 10. However, this is clearly not the correct
interpretation here. The 41 to 43 ppm peak is the peak of the
charged CH
2
carbon of the very reactive amino-imino meth-
ylene bases, namely the CH
2
= N-CH
2
+
produced on decom-
position of hexamine. The former is literally the reactive spe-
cies that has been proven to derive from hexamine decompo-
sition. Its intensity is particularly low in the pH 10 case,
indicating that it has indeed reacted more than in the pH 4 and
pH 7 cases. This is confirmed by the peak at 115 to 120 ppm of
increased intensity and more clearly discernible for the pH 10
case. This peak is one of the peaks that has been shown to
belong to an aromatic carbon to which is attached a benzyl-
amine bridge (Pichelin et al. 1999, Pizzi 1999). In all the spec-
tra, a low intensity broad peak at 53 to 58 ppm is present, this
being characteristic of tribenzyl amine nodes in the network
(Pizzi and Tekely 1995). It is lower in the pH 10 case, indi-
cating that at pH 10 cross-linking of the network relies less on
tribenzylamines than at lower pHs.
An interesting point to note is the absence of unreacted free
C6 and C8 sites on the very reactive A-ring, which is a clear
indication that these two sites have been totally reacted with
benzylamine bridges or they are occupied by the interflavo-
noid bond. In both cases, they contribute completely to cross-
linking. It must be clearly pointed out that this evidence is
only circumstantial and that in no way can one assume ab-
sence of free formaldehyde at such low levels from broad
peaks solid phase NMR spectra. Its absence, or its level too
low to be detected, is only based on the test of the panels ac-
cording to Japanese standard JIS A 5908.
The indication from the CP-MAS
13
C NMR spectra is that
at pH 10 the B-ring starts to react and to participate in tannin
cross-linking, and a more highly cross-linked network will
result in higher strength, supporting the results obtained in
Tables 1 and 2. Furthermore, the cross-links that exist at pH
10 are due to benzylamine bridges rather than methylene
bridges, indicating again that hexamine does not decompose
to formaldehyde under the conditions shown, and confirming
the zero-emission of formaldehyde from the wood panels pro-
duced as presented in Table 1. Thus, the main reactions in-
volved can be summarized as shown in Figure 6.
Figure 4. —Comparative solid state CP-MAS
13
C NMR spec-
tra of mimosa tannin at pH 4 (A), pH 7 (B), and pH10 (C)
hardened with hexamine predissolved in water, or added as a
solid at pH 4 (D).
Figure 5. —Schematic structure of the formula of the repeat-
ing unit of a flavonoid oligomer with the identifying atom num-
bers related to the visible NMR bands of Figure 4.
FOREST PRODUCTS JOURNAL VOL. 56, NO.5 35
Conclusions
Mimosa tannin hardened with hexamine at pH 10 has
shown both at the laboratory and industrial level to be a form-
aldehyde-free system, within the limits of sensitivity of the
method of Japanese standard JIS A 5908. This useful effect is
based on the double mechanism of slow hexamine decompo-
sition to reactive imino-amino methylene bases and their im-
mediately subsequent very rapid reaction with the tannin. De-
composition to formaldehyde can never be reached under the
conditions used. This yielded a long ambient temperature pot-
life coupled with the fast hardening of the adhesive and fast
pressing times for the thick panels by introducing a two-step
steam-injection sequence during panel pressing. No formal-
dehyde emission was found in the panels bonded with such an
adhesive system when tested according to the relevant dessi-
cator test (JIS A 5908). This appears to be also supported by
the solid state
13
C NMR spectra, where free formaldehyde
was not detected. These spectra having to be taken with cau-
tion, however, due to the usual peak enlargement and relative
lack of sensitivity in these types of spectra. In this regard, no
residual hexamine was found by solid state
13
C NMR for the
hardened tannin-hexamine adhesive. The type of reactions in-
volved were explained from the
13
C NMR. The panels ob-
tained satisfied the new, relevant Japanese standard specifica-
tion for both IB strength and formaldehyde emission.
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Figure 6. —Schematic example of tannin cross-linking
through benzylamine bridges by reaction with imino-amino
methylene bases derived by the decomposition of hexamine
in presence of tannins.
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