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Structural beams from thick wood panels bonded industrially with formaldehyde-free tannin adhesives

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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 13C 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 13C NMR. The panels obtained satisfied the relevant Japanese standard specification for both internal bond strength and formaldehyde emission.
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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) -- -- -- -- -- -- 0.30 -- 12.0 0.3
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 pHs 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-
noidsC3and C4peaks 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) -- -- -- -- -- -- 0.30 -- 12.0 0.3
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 C3and C4hydroxygroups 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,C4of 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 C5site 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 C1peak 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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36 MAY 2006
... In the field of bark extraction, numerous current research projects focus on screening bark for useful components and their efficient extraction for different industries. Prominent examples are birch bark extracts for biomedical applications [10], the development of rigid tannin foams [11,12] or the usage of bark or its components as adhesives [13,14]. Regarding the use of the bulk material, research activities concentrate mostly on cork of the mediterranean cork oak (Quercus suber, L.) [6]. ...
Article
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The proportion of bark in tree trunks is in the range of ~ 10–20%. This large amount of material is currently mainly considered as a by- or even waste-product by the timber processing industry. Recently, efforts towards the use of bark have been made, e.g. as a raw material to harvest different chemical compounds or as an additive for wood particle boards. Our motivation for this work was to keep the bark in an almost natural state and explore alternative processes and applications for use. The traditional method of de-barking tree trunks by peeling was used to harvest large bark pieces. Two pieces of peeled bark were placed crosswise, with the rhytidom side (outer bark) facing each other. After different conditioning steps, bark pieces were hot pressed to panels without adding adhesives. These experiments on bark samples of different Central European tree species suggest that production of panels with species dependent properties is possible and feasible. This is a step towards producing sustainable panels by using a natural waste material, while retaining its beneficial structure and its natural chemical composition.
... Formulation 2: Water solution containing + 40% Fillaeopsis discophora tannin + 5.5% of glyoxal predissolved in water + sodium hydroxide solution (NaOH) (Pichelin et al. 2006). ...
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The resins developed with Fillaeopsis discophora tannin extract using either Vitellaria paradoxa trunk exudate as bio hardener or glyoxal as hardener were thermally analyzed using thermomechanical (TMA) and thermogravimetric (TGA) analysis. A microscopic interpretation of glass transition, physical aging, and phase segregation was analyzed and the relative activation energy was determined. The activation energies of gelling of the resins above are 57,658 and 52,967 J. mol⁻¹, respectively. Their glass transition temperatures are 172 °C and 149 °C respectively. The tannin extract used in the development of these two resins is of a condensed polyflavonoid type linked to some furan residues. The resin developed with the Vitellaria paradoxa exudate as a bio-hardener has a good thermal behavior and it degrades slower than that with glyoxal as hardener.
... Nonetheless, further research was conducted. In 2006, Pichelin et al. described the use of hexamine for the hardening of mimosa tannin resins [88]. In 2010, Moubarik et al. synthesized cornstarchtannin adhesives without the use of formaldehyde as a hardener, which was replaced by hexamine. ...
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Due to its carcinogenic properties, the presence of formaldehyde in resins and other industrial products has been a subject of great concern in recent years. The presented review focuses on modern alternatives for the production of wood-based panels; i.e., substitutes for formaldehyde in the production of amino and phenolic resins, as well as novel hardeners for formaldehyde-free wood adhesives. Solutions in which formaldehyde in completely replaced are presented in this review. Recent advances indicate that it is possible to develop new formaldehyde-free systems of resins with compatible hardeners. The formaldehyde substitutes that have primarily been tested are glyoxal, glutaraldehyde, furfural, 5-hydroxymethylfurfural, and dimethoxyethanal. The use of such substitutes eliminates the problem of free formaldehyde emission originating from the resin used in the production of wood-based panels. However, these alternatives are mostly characterized by worse reactivity, and, as a result, the use of formaldehyde-free resins may affect the mechanical and strength properties of wood-based panels. Nonetheless, there are still many substantial challenges for the complete replacement of formaldehyde and further research is needed, especially in the field of transferring the technology to industrial practice.
... Today, research in this area focuses mainly on proteins, starches, and other polysaccharides, lignin, tannins, and chitosan as raw materials [2]. Use of natural adhesives including tannins [6,7], lignin [8], vegetable oils [9], protein, and soy flour [10,11], which have long been used in the manufacture of wood products and compressed wood panels, is one of the options that has been studied by many researchers [12][13][14]. ...
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This study aims to make environment-friendly plywood panels acceptable in terms of both mechanical and physical properties using chitosan as a natural binder. Five-layer poplar plywood specimens were prepared with two different commercial adhesives, namely urea-formaldehyde (UF) and methylene diphenyl isocyanate (MDI), and compared with chitosan (CH). For evaluation, mechanical properties such as modulus of rupture (MOR), modulus of elasticity (MOE), glue line shear strength, surface soundness, and physical properties such as thickness swelling (TS), water absorption (WA) of the samples were studied. Variable parameters were: adhesive type (UF, MDI, and CH), adhesive content (2, 4, and 6 wt%), and adhesive spread rate (150 and 180 g/m²). Other parameters such as pressing time (10 min), press pressure (30 kg/cm²) and press temperature (140 ºC) were held constant. Data analysis revealed that the mechanical properties differed significantly among the board types. Based on the findings of this study, the MOR properties of the panels slightly increased when the resin content increased from 150 to 180 g/m². The results of shear strength and surface soundness showed that boards made with chitosan as a binder, had the same results as those made with UF resin. In general, WA and TS decreased (improved) with the increase of resin content. The test results showed that the lowest TS observed in chitosan boards was 4% chitosan with a 180 g/m² spread rate, which is better than UF plywood and, close to the results of MDI specimens. According to the measured parameters, 4% chitosan with a spread rate of 180 g/m² can be considered the optimal binder composition for plywood manufacturing. The overall results show that chitosan has potential as a replacement resin material for plywood manufacturing.
... The tannins used in the production of adhesives for wood-based composites reduce the emission of formaldehyde, but in a pure state they do not provide good strengths when used in combination with classical UF or PF adhesives [22]. Particleboard made with tannin and formaldehyde adhesives had very low formaldehyde emissions and induced good mechanical strength for indoor use panels [23][24][25][26]. Another study [27] investigated the role of nano-clay (cloisite Na + ) in reducing formaldehyde emissions from particleboards. ...
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Adhesives represent an important part in the wood-based composite production, and taking into account their impact on the environment and human health, it is a challenge to find suitable natural adhesives. Starting from the current concerns of finding bio-adhesives, this paper aims to use magnesium lignosulfonate in three adhesive recipes for particleboard manufacturing. First, the adhesive recipes were established, using oxygenated water to oxidize magnesium lignosulfonate (Recipe 1) and adding 3% polymeric diphenylmethane diisocyanate (pMDI) crosslinker (Recipe 2) and a mixture of 2% polymeric diphenylmethane diisocyanate with 15% glucose (Recipe 3). The particleboard manufacturing technology included operations for sorting particles and adhesive recipes, pressing the mats, and testing the mechanical strengths and formaldehyde emissions. The standardized testing methodology for formaldehyde emissions used in the research was the method of gas analysis. Tests to determine the resistance to static bending and internal cohesion for all types of boards and recipes were also conducted. The average values of static bending strengths of 0.1 N/mm2, 0.38 N/mm2, and 0.41 N/mm2 were obtained for the particleboard manufacturing with the three adhesive recipes and were compared with the minimal value of 0.35 N/mm2 required by the European standard in the field. Measuring the formaldehyde emissions, it was found that the three manufacturing recipes fell into emission classes E1 and E0. Recipes 2 and 3 were associated with good mechanical performances of particleboards, situated in the required limits of the European standards. As a main conclusion of the paper, it can be stated that the particleboards made with magnesium-lignosulphonate-based adhesive, with or without crosslinkers, can provide low formaldehyde emissions and also good mechanical strengths when crosslinkers such as pMDI and glucose are added. In this way magnesium lignosulfonate is really proving to be a good bio-adhesive.
... The main challenges for the development of 100% bio-based wood adhesives are the needs for additional modification of natural feedstocks to improve their chemical reactivity, the decreased dimensional stability and mechanical properties of the wood-based panels produced, and the need to modify the technological parameters, e.g., extension of pressing time (Savov et al. 2019). In terms of commercial utilization, only tannin-based bioadhesives have found wider industrial application (Pichelin et al. 2006;Valenzuela et al. 2012;Zhou and Du 2019). ...
Chapter
In recent years, bio-based wood adhesives have received a lot of attention as a sustainable and renewable alternative to the conventional synthetic adhesives used in the wood-based industry. Bio-based adhesives, on the other hand, such as protein, starch, lignin, and tannin, have inferior properties when compared with thermosetting synthetic resins. Reinforcement with nanomaterials with a high aspect ratio has the potential to improve the performance of bio-based wood adhesives. Therefore, this chapter discusses recent advances in the use of nanomaterials, such as nanocellulose, nanolignin, and nanoclay, in the synthesis of sustainable, bio-based wood adhesives for the production of wood-based composites with improved properties and a lower environmental footprint for advanced value-added applications. The majority of studies have found that nanomaterials have a positive reinforcing effect on adhesive performance. This chapter also discusses the challenges and future prospects of using these nanomaterials in bio-based wood adhesives.
... The fight against pollution caused using synthetic resins, as well as using those based on formaldehyde (a substance classified as carcinogenic), has led several researchers to develop environment-friendly bioadhesives to bond wood [4]. Pichelin et al. were able to glue structural beams with formaldehyde combined with free tannin adhesives [5]; in 2007, Liu et al. developed and characterized soy proteinbased adhesives for wood bonding [6]; Konai et al. developed bioadhesives based on Aningre superba tannin [7] with 5.5% paraformaldehyde. Only from 2013 occurred successful preparation of a 100% bioadhesive from tannins using furfuryl alcohol [8] and from 2018 with hydroxymethylfurfural as a hardener [9]; lastly, Ndiwe et al. produced for the first-time bio-adhesives using tannins and bio-hardeners from natural plants [10][11][12]. ...
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In this work, the thermal degradation and drying of bio-hardeners are investigated. Four bio-hardeners based on exudates of Senegalia senegal, Vachellia nilotica, Vachellia seyal, and Acacia Siebteriana were analyzed by FTIR and thermogravimetric analysis, and a desorption study was also conducted. The analysis by infrared spectro-scopy indicates the existence of oligomers of different types all giving 5-hydroxy-2-hydroxymethylfuran and 2, 5-dihydroxymethylfuran which are then the real hardening molecules. The pyrolysis of these extracts reveals three main regions of mass loss, a first region is located between 25°C and 110°C reflecting the loss of water from the adhesive and the formation of some traces of volatile organic compounds such as CO 2 and CO, a second zone characterized by the release of CO, CO 2 and CH 4 gases with peaks between 110°and 798.8°C. At the end of the analysis, about 22% of the initial mass remains undecomposed, this mass corresponds to the rigid segments of the bio-hardener which are not completely decomposed.
Thesis
Les mousses phénoliques sont des matériaux attrayants en raison de leurs propriétés intéressantes telles qu’une faible densité, faible conductivité thermique, excellente résistance au feu (faible inflammabilité, faible émission de gaz toxiques), bonne résistance au produits chimiques, satisfaisante résistance mécanique et haute affinité de chélation envers de nombreux ions métalliques. Ces caractéristiques les rendent de plus en plus préférées dans une large gamme d’applications, notamment l’isolation thermique des bâtiments et le traitement des eaux usées. Cependant, elles sont généralement obtenues par des réactions de polymérisation des produits chimiques de type phénol et formaldéhyde, deux ressources non-renouvelables car issues du pétrole et classés comme cancérigènes pour l’homme. Ainsi, il serait d’un grand intérêt de substituer leurs principaux précurseurs par d’autres plus respectueux de l’environnement, durable, et économiquement rentable.Ces travaux de thèse proposent des formulations de mousses phénoliques biosourcées pour des applications environnementales et énergétiques. Ces mousses sont constituées d’environ 80% en masse sèche de tanins et de liqueur alcaline industrielle riche en lignine. Ces derniers ont été utilisés comme des alternatives naturelles au phénol et sont largement présents dans les sous-produits des industries locales du bois et de pâte à papier. Tout d’abord, les mousses obtenues ont été utilisées pour traiter des eaux contaminées par des métaux lourds (Cu, Cd, Zn et Pb). L’effet des paramètres expérimentaux (pH, température, concentration initiale en ion métallique et temps de contact) sur le phénomène d’adsorption a été étudié. Les capacités d’adsorption de ces mousses vis-à-vis des ions métalliques Cu2+, Cd2+, Zn2+ et Pb2+ sont estimées de 46,5 ; 41 ; 29,1 et 100,9 mg/g, respectivement. Aussi, une régression non-linéaire a été appliquée pour sélectionner les meilleures isothermes d’adsorption et isothermes cinétiques. Au regard des résultats obtenus, ces mousses biosourcées pourraient être utilisées comme adsorbant pour le traitement des eaux contaminées par des métaux lourds. Ensuite, le travail a été consacré au développement et à la caractérisation des mousses dans le but de les utiliser comme isolant thermique des bâtiments. Après résolution du problème de fissuration des mousses antérieurs par l’ajout d’un plastifiant aux formulations, une étude paramétrique a été menée pour évaluer les effets des catalyseurs, des températures de cuisson, des ratios tanins-liqueur alcaline, des agents de réticulations sur les propriétés finales des mousses. À l’aide de différentes techniques de caractérisations, il a été montré que la structurelle cellulaires, la structurelle chimique intrinsèque, le comportement thermique, la performance isolante et la résistance mécanique des mousses dépendaient fortement des paramètres expérimentaux mentionnés ci-dessous. Les mousses obtenues présentaient une bonne résistance à la compression (0,11-1,65 MPa) et une faible conductivité thermique (37-50,55 mW/m.K).L’une des originalités de ce travail consiste à produire des mousses riches en ressource largement disponible et peu valorisés jusqu’à présent (des mousses contenant jusqu’à 52% en masse sèche de liqueur alcaline industrielle). Ces nouveaux matériaux semblent être très prometteurs pour une production à l’échelle industrielle et peuvent être employées pour des applications d’isolation thermique des bâtiments.
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A considerable volume of wastewater is generally dumped into bodies of water, which has severe negative consequences on aquatic habitats. While various traditional methods are available in wastewater treatment, the development of new technologies is critical for wastewater treatment and recycling. Synthesis of biomass-based adsorbents due to their environmentally friendly properties can help sustain our lifestyles. Polymers have lately been extensively used in numerous sectors due to their unique properties. Biopolymers are a natural alternative to synthetic polymers that can be created by covalently linking monomeric units. Among these biopolymers, tannin-based biopolymers as bio sorbents are one of the potential candidates in the field of wastewater treatment. Among these studies, tannin-based biopolymers as biosorbents are one the potential candidates in the field of wastewater treatment and are considered one inexpensive adsorbent with a relatively easy combination with effective performance in water and wastewater treatment. Due to the high content of adjacent phenolic hydroxides, tannins show a high tendency to chelate against various metal ions in water. Also have a good adsorption capacity to remove dyes by establishing electrostatic interactions, ion exchange, and covalent bonds. Due to the importance of these compounds as promising materials in various applications, in this article, the classification of tannins based on structural properties, preparation methods and some of their important properties and applications in the field of adsorption are reviewed.
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The use of engineered wood products (EWPs) is rapidly increasing in the building industry worldwide due to their reliable structural performance, lower weight, renewability, carbon storage, quick on-site installation, and lower levels of construction waste. The adhesives used for manufacturing the EWPs are mainly synthesised from petroleum and natural gas derived chemicals. However, increasing concerns regarding formaldehyde emissions, environmental sustainability and long term security of supply of petrochemicals are the biggest motivation for researchers to develop bio-based adhesives. The main objective of this article is to review recent advances in bio-based adhesives and their application in wood based composite products. Bio-adhesives derived from lignin, protein, tannin and starch and their reported performance are discussed. Bio-based adhesives at various stages of commercial development as reported by the industry and/or in patents are also discussed. Although bio-based adhesives provide a sustainable solution and significantly reduce formaldehyde and volatile emissions, they still pose several different limitations that hinder their industrial and commercial use. The major limitations include: 1) the availability of tannins 2) lack of adhesion for starches 3) poor water resistance for lignin and protein and 4) low strength properties mainly limiting their use to non-structural applications. However, the literature review demonstrates that various modifications, additives and cross-linkers can significantly improve various properties of bio adhesives. The paper also presents a brief summary of the advantages and disadvantages of synthetic adhesives.
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The same reactive intermediate compounds that react to form resins in decomposition of hexamine were also found in recomposition of hexamine from formaldehyde and an ammonium salt of a strong acid. Anion-stabilized intermediates block such recomposition of hexamine. Oligomerization resulted in short cyclic oligomers, i.e. in hexamine itself. Thus, hexamine formation is a case of polycondensation.
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Melamine-formaldehyde resins prepared with a molar defect of formaldehyde (i.e. molar ratio M : F = 1:0.5) were shown to harden in presence of hexamethylenetetramine, at acid pH. without a great proportion of decomposition of hexamethylenetetramine to formaldehyde. Approximately 2/3 of the cross-linking bridges induced by hexamethylenetetramine hardening appear to be stable aminodi- and aminotrimethylene bridges derived by reaction of Hultzsch methylene bases directly with reactive sites on melamine and on the MF resin. This confirms that hexamethylenetetramine is capable of reacting also with aminoplastic resins mostly before its degradation to formaldehyde. The existence of such a type of cross-linking was shown to lead to hardened aminoplastic resins capable of much lower formaldehyde emission from the finished board.
Article
Phenols of high reactivity with formaldehyde and phenolic resins derived from them, such as resorcinol and resorcinol-formaldehyde (RF) resins, have been shown by 13C NMR to react with hexamethylene-tetramine (hexamine) with the formation of both benzylamine and methylene bridges to form hardened resins. In the final hardened network the proportion of benzylamine bridges appears to be considerably higher than that of methylene bridges directly connecting two phenolic nuclei. The benzylamine bridges appear to have high stability and after cooling the resin at ambient temperature after hardening at 100°C remain stable, without rearrangement to methylene bridges. Heating the hardened networks at 110°C for periods of up to 30 minutes cause some rearrangement of benzylamine to methylene bridges for networks formed by direct hardening of resorcinol by hexamine. This does not appear to occur, or occurs to a minimal extent for preformed resorcinol-formaldehyde (RF) resins hardened with hexamine, with significant consequences as regards the application and the formaldehyde emission of hexamine-hardened wood adhesives systems.
Article
A solid state 13C NMR study of hardened networks obtained by the reaction of blocked and nonblocked isocyanates (pMDI) with urea-formaldehyde (UF) resins in water showed different results according to the temperature of the reaction. At high temperature, in water, both a nonblocked or an emulsifiable, blocked isocyanate, appear to crosslink with UF resins through the formation both of traditional methylene bridges connecting urea to urea and of urethane bridges. The latter have been confirmed by 13C NMR to form in water by reaction of the isocyanate NCO group with the hydroxymethyl groups of the UF resin. At ambient temperature, UF/pMDI resins where the pMDI is a emulsifiable blocked isocyanate, do not appear to form urethanes to any great extent but rather to crosslink through the usual UF resin urea to urea methylene bridges. Even in this case, when urethane bridges appear to be absent, evidence of crosslinking in water through reaction of the isocyanate with the NH2 and NH amide of the UF resin has not been observed. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 589–596, 2006
Article
Polyflavonoid tannins have been shown by 13C-NMR to react with hexamethylenetetramine (hexamine) at considerably higher rates than with phenol with the formation of both benzylamine and methylene crosslinks to form hardened resins. Predominantly prodelphinidintype tannins appear to present a much higher proportion of benzylamine bridges than of methylene bridges, while in procyanidin-type tannins the proportion of the two types of crosslinks appear to be comparable. The greater the nucleophilicity of the flavonoid tannin A-rings, the greater is the proportion of benzylamine bridges which appear to form. At parity of the type of tannins, the faster the reaction with hexamine, the higher the proportion of benzylamines which appear to form. © 1995 John Wiley & Sons, Inc.
Article
CP MAS 13C NMR spectra of hardened resins have shown that urethane bridges derived from the reaction of the isocyanate group with the hydroxymethyl group of urea do form even at fast curing times comparable to what was used in the wood panels industry, in lower proportions than what was shown earlier. Polyureas and biurets obtained from the reaction of isocyanate with water are the predominant crosslinking reactions of pMDI alone and in UF/pMDI resin systems under fast curing conditions. Residual, unreacted isocyanate groups in the hardened network are consistently observed. Their proportion markedly decreases when the original proportion of urea–formaldehyde (UF) resin is high and that of pMDI is low. Under these fast curing conditions, the UF resin appears to self-condense through an unusually high proportion of methylene ether links rather than methylene bridges alone. A marked proportion of residual, unreacted hydroxymethyl groups is also noticeable, initially, in the UF self-condensation network. Direct NMR tests on thin hardboard bonded under fast pressing conditions with different proportions of UF/pMDI confirmed that crosslinking due to polyureas and biurets formation are predominant in the crosslinking of pMDI when alone and in UF/pMDI resin systems. They confirmed that residual, unreacted isocyanate groups are present in the finished panel. Their proportion is higher when the proportion of pMDI in the system is high. The presence or absence of urethanes could not be confirmed directly on the panels as the relevant peaks are masked by the wood carbohydrates signals of wood cellulose and hemicelluloses. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1624–1632, 2006
Article
A simple 13C-NMR method for the quantitative determination of polyflavonoid tannin characteristics was developed. The system is effective for use on concentrated (25–50%) solutions of natural and modified tannins. It allows the determination of the average degree of polymerization (DPn) of the polyflavonoid, resorcinol vs. phloroglucinol proportion of the A-ring and catechol vs. pyrogallol proportion of the B-ring. The results obtained are consistent with existing data determined by other techniques. The method was also tried with tannin extract that was modified to form thermosetting adhesive intermediates, and with tannin modified by sulfonation, a common commercial modification for these materials. The results were again consistent with what was expected. The method affords the possibility to follow by a simple technique the variations in DPn and MM̄n (number-average molecular weight) induced by chemical modifications of polyflavonoid tannin extracts and thus to correlate them with relevant structural modifications affecting these parameters. The method is not capable of distinguishing the relative proportions of the four important flavonoid units present in commercial polymeric tannin extract. It can only distinguish the relative proportions of (i) (procyanidins + prodelphinidins) vs. (profisetinidins + prorobinetinidins) and (ii) (prorobinetinidins + prodelphinidins) vs. (profisetinidins + procyanidins). © 1995 John Wiley & Sons, Inc.