Bioresource Technology 87 (2003) 325–330
Impregnation of softwood cell walls with
melamine-formaldehyde resin
W. Gindl
a,*
, F. Zargar-Yaghubi a, R. Wimmer
b
a
b
Wood Chemistry and Composites Competence Center Linz & Institute of Wood Science and Technology, University of Agricultural Sciences,
Gregor Mendel Strasse 33, A-1180 Vienna, Austria
Wood Chemistry and Composites Competence Center Linz & Institute of Botany, University of Agricultural Sciences, Gregor Mendel Strasse 33,
A-1180 Vienna, Austria
Received 8 July 2002; accepted 11 September 2002
Abstract
Melamine-formaldehyde (MF) resin impregnation has shown considerable potential to improve a number of wood properties,
such as surface hardness and weathering resistance. In this study, selected factors influencing the uptake of MF resin into the cell
wall of softwood were studied. Using UV-microspectroscopy, it could be shown that water soluble MF diffused well into the
secondary cell wall and the middle lamella. Concentrations as high as 24% (v/v) were achieved after an impregnation of 20 h. High
cell wall moisture content, high water content of the resin used for impregnation, and low extractive content are factors which are
favourable for MF resin uptake into the cell wall. For dry cell walls, solvent exchange drying improved resin uptake to a similar
extent, as was the case when cell walls were soaked in water.
Ó 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Cell wall; Impregnation; Melamine-formaldehyde; UV-microscopy; Wood
1. Introduction
Solid wood in its many forms and adaptations has
been the most versatile material for buildings, constructions, or furniture, because of superior material
properties, e.g. pleasing optical appearance, favourable
mass/strength ratio, low thermal conductance, biodegradability, and last, but not least, due to its neutral
carbon dioxide balance. There are, however, solid wood
properties that are often perceived as negative by the
end-user, such as dimensional instability with changing
moisture content, low natural durability of many species, expressed photoyellowing, or unsatisfying mechanical properties. A promising way to improve wood
properties is through controlled chemical modification.
A number of chemical substances have been tested
(Matsuda, 1996), and some have shown improvement in
the dimensional stability and/or decay resistance of
wood (Rowell, 1996; Militz et al., 1997). Regarding
*
Corresponding author. Tel.: +43-1-47654-4255; fax: +43-1-476544295.
E-mail address: [email protected] (W. Gindl).
mechanical wood properties, chemical modification has
not been proven to be satisfactory as treatments have
shown insignificant and slightly negative effects (Larsson
and Simonson, 1994; Rowell, 1996; Ramsden et al.,
1997).
Polymers of melamine (1,3,5-triamino-2,4,6-triazine)
and formaldehyde form an important class of amino
resins, which have been commercially used for over 60
years. Melamine-formaldehyde (MF) itself is one of the
hardest and stiffest isotropic polymeric materials (Hagstrand, 1999) used in decorative laminates, moulding
compounds, adhesives, coatings and other products.
Due to its advantageous properties, i.e. high hardness
and stiffness, and low flammability MF resins have
potential to improve properties of solid wood. Impregnation of solid wood with water-soluble MF resin has
led to a significant improvement of surface hardness and
MOE (Inoue et al., 1993; Miroy et al., 1995; Deka and
Saikia, 2000; Deka et al., 2002). Further, resistance to
weathering has increased (Rapp and Peek, 1999), and
colour changes due to UV-irradiation diminished with
increasing concentration of MF-resin in wood (Inoue
et al., 1993).
0960-8524/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 9 6 0 - 8 5 2 4 ( 0 2 ) 0 0 2 3 3 - X
326
W. Gindl et al. / Bioresource Technology 87 (2003) 325–330
It has been reported that MF resin may penetrate the
amorphous region of cellulose (Hua et al., 1987a, b).
Rapp et al. (1999) impregnated spruce wood samples
with a water soluble MF resin over seven days and using
electron energy loss spectroscopy penetration of MF
into secondary cell wall layers and into the middle
lamella was shown. By means of UV-microscopy, Gindl
et al. (2002) made quantitative estimates of melamine in
cell walls of specimens that were glued with melamineurea-formaldehyde resin.
The present work as part of a larger study reports
data obtained from experiments of MF resin penetrating
different types of softwood cell walls, cell wall regions,
and with different types of drying prior to impregnation.
The goal was to evaluate impregnation effects of melamine-formaldehyde resin treated softwood at the cellular level.
2. Methods
Three softwood species, two with distinct heartwood
formation, European larch (Larix decidua Mill.) and
Scots pine (Pinus sylvestris L.), and one without coloured heartwood, Norway spruce (Picea abies (L.)
Karst.), were selected for this study. For each of the
three species, six sample blocks, 20 20 40 mm in
size, were cut from boards, which were seasoned to 12%
moisture content. To facilitate sectioning, blocks were
immersed into boiling water for 1 h. Another set of
sample blocks was obtained from a freshly felled 20-year
old spruce tree. From all blocks 200 lm thick sections
were cut from the radial faces using a Reichert sledge
microtome, which was equipped with a type ‘‘C’’ steel
knife.
Prior to resin impregnation the samples were subjected to different drying procedures. Solvent-exchange
drying has commonly been used to prevent aspiration
and to enable unaspirated bordered pit membranes to be
examined in the microscope (Comstock and Cote, 1968;
Bauch et al., 1972; Parham and Baird, 1973). Others
have found that solvent exchange drying prevents collapse of cell wall pores during drying, facilitating an
improved penetration of substances into the cell wall
(Bower and Wellons, 1974). Therefore permeability decreases as the surface tension of the liquid from which
the wood was dried increases (Siau, 1984). The effect of
solvent exchange drying on cell wall penetration by MF
resin was tested on microtomed sections prepared from
Scots pine. Sections were subjected to a solvent exchange procedure by successively immersing them into
water, 100% ethanol, acetone and pentane, for 1 h each.
After this treatment, the pentane soaked sections were
vacuum dried at 80 °C and 8 mbar. Reference samples
were also solvent exchanged with 100% ethanol–acetone–pentane, followed by another hourly sequence of
acetone–100% ethanol–water, before they were vacuum
dried. This procedure assured extractive-free samples for
those dried from pentane as well as for the reference
samples dried from water.
A commercially available melamine-formaldehyde
resin (Hilamin M562â , Dynea, Krems) with a dynamic
viscosity of 30 mPa s and a content of solids between
55% and 60% was chosen for this study. Impregnations
were performed at room temperature by fully immersing
the microtome sections in the resin, the latter being
water diluted to a concentration of 25%. After 1, 3 or 20
h of immersion microsections were removed from the
resin, blotted dry with tissue paper and cured in an oven
at 103 °C with no catalyst added. Reference samples
were treated the same way, except for the resin impregnation. All investigations were performed on double
samples.
All samples were further dehydrated in pure ethanol
and acetone, before they were embedded in SpurrÕs resin
(Spurr, 1969). Transverse sections with a thickness of 1
lm were cut on a Leica Ultracut ultramicrotome
equipped with a Diatome Histoâ diamond knife. The
sections were sequentially picked up from the trough
and placed onto quartz glass slides by means of a platinum wire loop. With a drop of water added the sections
were covered with quartz slips for the immediate observation and quantitative measurement in the Zeiss
MPM 800 microspectrophotometer microscope. This
microscope was equipped with a UV-source, a monochromator, and a PbS detector enabling the determination of spectra at wavelengths ranging from 235 to
720 nm. At a magnification of 1000, the selected circular measuring spot was 1 lm in diameter (Fig. 1). This
small spot size allowed quantitative measurements
within latewood cell walls as well as larger cell corner
middle lamella (CCML) regions. Thin-walled earlywood
cells remained unconsidered because of biasing edge
effects that are likely to occur at cell wall thicknesses
under 2 lm.
Fig. 1. Micrograph of a cross-section from spruce wood treated with
MF resin. The arrow indicates a measuring spot (d ¼ 1 lm) in the
secondary cell wall, the arrowhead points to a CCML.
W. Gindl et al. / Bioresource Technology 87 (2003) 325–330
327
(v/v) results. Between 15 and 20 spectra were evaluated
for each sample.
3. Results
Fig. 2. Typical spectra of untreated and MF impregnated secondary
cell walls, compared to pure MF resin cured in a cell cavity.
Quantitative evaluation was achieved through UVspectra normalised against the absorbance of untreated
cell walls at the wavelength of 290 nm (A290 ). This
wavelength was chosen for normalisation because it is
close to the maximum absorbance of lignin (280 nm),
and MF does not contribute, which is not the case at 280
nm, where a slight absorbance of MF is observed (Fig. 2)
As seen in Fig. 2 pure cured MF resin had its maximum
absorbance at 245 nm (A245 ). To determine the actual
MF content in wooden cell walls, A245 of the untreated
reference was first subtracted from A245 of the MF
treated specimen. In a second step this difference was
divided by the A245 of pure cured melamine resin, which
was determined in resin filled cell cavities. Finally, the
weight-based (w/w) concentration of MF resin was divided by its density (1.5 g/cm3 ) to obtain volume-based
Fig. 2 displays typical spectra of untreated and MF
treated cell walls and pure MF resin. Unmodified cell
walls had their maximum absorbance at 235 nm. Absorbance steadily decreased from this wavelength onward reaching a local minimum at 260 nm, followed by
an upswing towards a local maximum at 280 nm, and a
levelling off to zero absorbance at around 350 nm. Pure
MF resin spectra exhibited an absorbance peak at 245
nm, with a steep decline onwards, reaching zero absorbance at 290 nm. Compared to untreated cell walls,
absorbances of MF treated cell walls were significantly
higher at wavelength ranges between 235 and 260 nm.
At higher MF concentrations distinct peaks appeared
around 240 nm.
Table 1 lists MF contents (v/v) determined in latewood secondary walls (S2) from sapwood and heartwood of three species. Measurements were done at
different impregnation durations, wood types, cell wall
locations and impregnation regimes. All results for the
treated samples differed significantly from the reference
absorbance at 245 nm (t-test, p < 0:05).
For the wet larch samples, the S2 layers in the sapwood accumulated after 3 h of impregnation three times
as much MF as S2 layers in the heartwood. The same
concentration as in heartwood was found in the compound CCML of sapwood. The clear differences between sapwood and heartwood were also evident in pine
(factor 2), which was beside larch the second investigated species with obligatory heartwood formation.
Table 1
MF resin content of different wood types and cell wall locations at various conditions and impregnation regimes; larch ¼ Larix decidua Mill.,
pine ¼ Pinus sylvestris L., spruce ¼ Picea abies [L.] Karst
Species
Type of wood/cell wall location
Condition prior to
impregnation
Impregnation duration
(h)
MF resin content (v/v)
Larch
Larch
Larch
Pine
Pine
Pine
Sapwood S2
Sapwood CCML
Heartwood S2
Sapwood S2
Heartwood S2
Sapwood S2
3
3
3
1
1
1
0.15 0082
0.05 0.020
0.05 0.028
0.09 0.050
0.05 0.026
0.02 0.035
Pine
Sapwood S2
1
0.05 0.038
Spruce
Spruce
Spruce
Spruce
Spruce
Spruce
Spruce
Sapwood
Sapwood
Sapwood
Sapwood
Sapwood
Sapwood
Sapwood
Wet
Wet
Wet
Wet
Wet
Solvent exchanged/dried from
water
Solvent exchanged/dried from
pentane
Never dried
Dried/remoistened
Dry
Never dried
Dry
Dry
Dry
1
1
1
20
20
3/55–66% resin
3/25% resin
0.07 0.038
0.07 0.042
0.04 0.024
0.24 0.132
0.24 0.132
0.04 0.016
0.10 0.041
S2
S2
S2
S2
S2
S2
S2
328
W. Gindl et al. / Bioresource Technology 87 (2003) 325–330
Fig. 3. MF resin concentration in secondary cell walls of sapwood at
different durations of impregnation. The exponential trend line was
fitted to the data of the cell walls impregnated in wet condition.
MF uptake of the solvent-exchanged pine samples
dried from pentane differed clearly from the ones dried
from water. Two percent MF in normally dried samples
after 1 h of impregnation was low compared to 5%
observed after solvent exchange with drying from pentane.
A variety of test conditions are shown with spruce
sapwood. First, the MF concentration of the green
samples (freshly felled and never dried) was compared to
samples which were oven dried for 24 h with a subsequent re-moistening to full water saturation. No difference in MF resin concentration was observed between
these two types of specimens. MF impregnation of ovendried samples resulted in a significantly smaller uptake,
compared to the green and the dried/re-moistened
samples. However, after 20 h duration of impregnating
MF resin differences between these specimens were no
longer present (Fig. 3).
The final aspect for spruce sapwood was impregnating MF resin at two different concentrations. Impregnation with a 25% resin solution led to an uptake twice
as high than doing the same treatment with a 55–60%
solution.
4. Discussion
The obtained results confirm that significant portions
of MF resin have the potential to penetrate secondary
cell wall layers and middle lamella of softwoods (Rapp
et al., 1999; Gindl et al., 2002). Considering the heterogeneous size distribution of pores in cell walls (Tarkow
et al., 1966; Maloney and Paulapuro, 1999), only a
fraction of these pores reach the minimum size to become accessible to MF resin. Stamm (1964) gives a
calculated maximum volumetric swelling in organic liq-
uids of the cell wall of about 38%. This number is in the
range of the observed 24% (v/v) MF resin in spruce
sapwood after 20 h of penetration (Table 1).
In the compound CCML 5% MF resin was detected
after 3 h impregnation, which compares to 15% resin in
the S2. Several authors observed high concentrations of
extraneous substances coming from wood preservatives
in the middle lamella: i.e. vinyl polymer, silver particles,
copper, or zinc (Timmons et al., 1971; Petric et al., 2000).
Wallstr€
om and Lindberg (2000) found five times more
silver particles in the CCML than in the S2 of spruce
after impregnation with K-glycerate/AgNO3 . They suggested that this finding might be caused by the fact that
the preferred path of transport goes from the lumen over
the pit membrane through the middle lamella, and not
from the lumen directly through the secondary wall.
Petric et al. (2000) found a similar distribution for copper- and zinc-containing preservatives, and argued with
the greater affinity of these substances to lignin. Since
Timmons et al. (1971) measured higher vinyl polymer
concentrations in the middle lamella than in secondary
cell walls, which opposes the finding in this study, it is
suggested that differences in the affinity to wood polymers may explain the discrepancy. Vinyl monomer is less
hydrophilic than MF-resin and may accumulate more in
lignin-rich regions like the middle lamella and CCML
(Fergus et al., 1969). On the other hand the hydrophilic
MF resin has higher affinity in less lignified cell wall
layers, as it is the case with in S2 layers.
A consistently lower MF concentration was found in
heartwood compared to sapwood (Table 1), with the
difference being more pronounced in larch than in pine.
This is in agreement with Bailey and Preston (1969) who
reported for Douglas fir a markedly lower concentration
of silver grains in heartwood cell walls compared to
sapwood. Wettability studies on sap and heartwood
observed similar differences. The contact angle of water
was lower in pine sapwood than in heartwood, which
indicated the better wettability of sapwood (Boehme
and Hora, 1996). After removal of the extractives from
the heartwood, wettability was increased strongly (Jordan and Wellons, 1977). Comparing pine with larch for
the wettability through urea-formaldehyde glue, better
wettability was always observed in sapwood with the
difference between sapwood and heartwood being more
pronounced in larch (Hameed and Roffael, 1999). It is
well known that heartwood is generally richer in extractives than sapwood (Fengel and Wegener, 1984),
and a major portion of the extractives is located in the
cell walls (Kleist and Bauch, 2001). Therefore, increasing extractive content seems to hinder directly the uptake MF resin in the cell wall of heartwood.
According to Stamm (1964) pores in wood can be
classified into permanent and transient ones. Permanent
pores, i.e. cell lumina, pit chambers, and intercellular
voids, exist regardless of the hydration status of the cell
W. Gindl et al. / Bioresource Technology 87 (2003) 325–330
wall, whereas transient pores are only present when cell
walls are hydrated and close during drying. Replacing
water in cell wall capillaries through increasingly less
polar liquids with low surface tension, the collapse of
these pores may be partially prevented, with a certain
level of porosity preserved dry (Stamm, 1964; Bower and
Wellons, 1974). Results obtained in this study show that
the MF uptake was increased due to solvent exchange
drying (factor of 2.5), however, the increase was not
dramatic (Table 1). Since similar impregnation results
were achieved with fully water saturated cell walls, the
feasibility of solvent exchange drying as a possible pretreatment to increase resin uptake can be questioned.
Our measurements show that more MF resin had
penetrated fully water saturated spruce cell walls after 1
h, than dried cell walls after 20 h. The initial difference
progressively disappeared with increased duration of
impregnation (Table 1, Fig. 3). The initial difference in
resin uptake might be explained by the fact that MF
resin per se cannot penetrate into dry cell walls. A
swelling of the cell wall is required to open the pores to
become accessible by MF. Reports are showing that
drying of the cell wall from its native, fully water saturated state leads to an unrecoverable loss of porosity
(Tynj€
al€
a and K€
arenlampi, 2001). In our study this
phenomenon had no or only a minor effect on the impregnation of cell walls using MF resin (Table 1).
The final aspect investigated in this study was the
effect of resin concentration on the penetration of MF
resin into dry cell walls. Presence of water enhanced
impregnation and more than twice the resin content was
found in the cell walls after treatment with 25% resin,
compared to impregnation with 55–60% resin (Table 1).
With this, it has to be taken into account that increased
viscosity at higher resin concentrations may have contributed to the slower resin uptake.
To summarize, it was demonstrated that MF resin
penetrates into secondary cell walls and middle lamella of
softwood. High cell wall moisture content, high water
content of the resin used for impregnation, and low extractive content are factors that promote MF resin uptake
into cell walls. A maximum resin concentration of 24%
(v/v) was obtained after an impregnation for 20 h.
Solvent exchange drying proved favourable for resin
diffusion when compared to normally dried wood, but
just by soaking the cell wall with water, prior to impregnation, an effect of similar magnitude can be achieved.
Acknowledgements
The authors wish to thank Dr. M. Dunky, Dynea, for
providing the melamine resin used in the present study.
Financial support by Agrolinz Melamin is gratefully
acknowledged.
329
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