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Restriction of liquid water spreading in overlaid plywood top veneer

Authors:
86 RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1
FORESTRY AND WOOD PROCESSING DOI: 10.22616/rrd.24.2018.013
RESTRICTION OF LIQUID WATER SPREADING IN OVERLAID
PLYWOOD TOP VENEER
Dace Cirule1, Edgars Kuka1,2, Anrijs Verovkins1, Ingeborga Andersone1
1Lavian State Institute of Wood Chemistry, Latvia
2Riga Technical University, Latvia
xylon@edi.lv
Abstract
Plywood overlaid with resin impregnated paper lms is used in various decorative applications for which high
stability of aesthetic qualities is of great importance. The top veneer of the plywood for these materials is perfectly
protected from a direct contact with water as far as the covering lm is not damaged. However, in case of lm
damage water can cause clearly visible defects in a relatively short period of time. To reduce these types of defects,
unsophisticated and efcient way was developed. It involves lling the vessel system of the top veneer with industrial
phenol-formaldehyde resin by using a hot-press. Inuence of some phenol-formaldehyde resin properties on its
penetration in birch veneer as well as the effect of wood moisture content were tested. The proposed top veneer
pre-treatment method with phenol-formaldehyde resin reduced the average swelling zone distance from damage site
by half for oven pre-dried plywood with 3% moisture content before treatment. Moreover, it was found that using
plywood with moisture content of 9% the swelling zone distance was reduced by two thirds compared to untreated
overlaid plywood.
Key words: plywood; phenol-formaldehyde resin; water spreading.
Introduction
Plywood manufacturing from wood veneer layers
by gluing them together with resins is one of the largest
wood-based composite industries (Chang et al., 2018).
In comparison with solid wood products, the design
of plywood with veneer sheets glued together with
adjacent layers having their wood grain perpendicular
to one another results in reduced tendency of swelling
and shrinking, providing improved dimensional
stability in changing humidity environment.
Moreover, such a design makes the plywood panel
less anisotropic regarding its strength properties.
Different varieties of plywood have been designed
and are manufactured for multitude of applications
such as furniture, engineering constructions, vehicles,
sporting goods and equipment, packaging, ships and
yachts and others (Hrázský & Král, 2007). Among
others, plywood overlaid with a resin impregnated
paper lm has gained its niche for various decorative
applications. Mostly melamine and phenolic resins
are used for production of resin lms, which are
subsequently hot-pressed onto one or both of plywood
sheet surfaces. Beside changed appearance, covering
of plywood with resin-impregnated lm signicantly
improves water and abrasion/wear resistance as well
as facilitates cleaning and maintenance of the surface.
Moreover, the resin lm coating perfectly protects
the top veneer of plywood from a direct contact with
water as long as the covering lm is intact. However,
in case of the lm damage, the inherent wood swelling
characteristics can provoke noticeable surface failure
even after a relatively short contact with water.
Intensive research has been done and is still under
experimentation to overcome the high moisture
sensitiveness of wood thus improving its utility. There
are numerous modication processes proposed to
reduce wood swelling, which can be divided into two
groups: an active modication involving changes in
wood chemical structure and passive modication,
which does not alter the chemistry of wood (Hill,
2006; Epmeier, Westin, & Rapp, 2004; Sandberg,
Kutnar, & Mantanis, 2017). The active modication
of wood comprises wood treatment methods, which
are quite sophisticated and therefore costly, such
as thermal modication, acetylation, furfurilation.
Besides, wood modication through alteration of its
chemical structure often involves reduction of strength
properties (Epmeier, Westin, & Rapp, 2004). Passive
modication mainly is performed by impregnation
with an aim to bulk the wood cell wall thus preventing
dimensional changes of wood caused by varying
humidity conditions or direct contact with liquid
water. It is found that water-soluble low molecular
weight phenol-formaldehyde (PF) resins can penetrate
into a wood cell wall and improve wood dimensional
stability (Seborg, Tarkow, & Stamm, 1962; Furuno,
Imamura, & Kajita, 2004; Kielmann, Butter, & Mai,
2018). However, even a very high resin loadings
cannot totally prevent dimensional changes of wood
(Hill, 2006). Another type of passive impregnation
involves cell lumens lling. It cannot prevent swelling
but can substantially hinder liquid water spreading
through the wood porous structure. In such a way, the
region subjected to action of water is reduced.
The ow of liquid primarily moves in the path of
least resistance. In hardwoods, the least resistance
path is the capillary structure of vessel network,
which is characterized by relatively wide lumens and
end-to-end connection with perforation plates of high
permeability (Kamke & Lee, 2007). The penetration
87RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1
rate of liquid into wood varies depending on both the
wood structure and properties and the characteristics
of the penetrant solution (Meijer, Thurich, & Militz,
2001; Meijer, 2004; Kučerová, 2012). It implies
that lling of vessel lumens could hinder fast water
spreading into the top veneer under damaged covering
lm thus reducing the surface area subjected to
swelling and lessening the visual defects.
The present study was aimed at nding an
unsophisticated and effective way for the enhancement
of covered plywood resistance to rapid local swelling
in case of top veneer contact with water because of the
covering lm damage.
Materials and Methods
All of the materials used for experiments were
supplied by JSC ‘Latvijas Finieris’. Plywood overlaid
with PF resin impregnated lm was used to determine
the swelling zone expansion rate from the damaged
site. The damaged site was created by drilling in the
surface of the overlaid plywood an approximately 5
mm deep hole with diameter of 2.8 mm. Afterwards
the damaged site was exposed to liquid water and the
swelling zone distance was measured after certain
periods of time (10, 30, 60, 120, 240 and 1440 min)
with an accuracy of 1 mm.
Partly polymerised PF resin was obtained by
heating the industrial PF resin in water bath for 3 h at
50 ± 1 °C. Partly dehydrated PF resin was obtained
from the industrial PF resin by using a rotary vacuum
evaporator with process parameters: vacuum – 30 mbar,
temperature – 20 °C, rotation speed – 100 rpm. To obtain
PF resins with a different water content, dehydration
was performed for 1 h, 2 h and 4 h. Viscosity of the
PF resins was measured by using rotational viscometer
HAAKE Viscotester 6 plus. The water content of
the PF resins was determined by drying the PF resin
(~ 2 g) in an oven at 140 °C for 1 h. The mass of PF
resin was weighed before and after drying. From
these measurements the mass loss (water content) was
calculated assuming that only water evaporates.
Rotary-cut birch (Betula spp.) veneers with
thickness 1.4 mm were used for impregnation with the
industrial PF resin as well as with the resins modied
by polymerization and dehydration. The set of six
veneer specimens with dimensions 100 × 100 mm
per each resin type was used for impregnation. The
impregnation was carried out under vacuum (22 mbar)
for 10 min. The excess of resin was wiped off from
the veneer surface with a paper towel immediately
after removing the specimens from the impregnation
container. To polymerize the impregnated resin, the
specimens were kept in an oven (140 ± 2 °C) for 12
h and weight percent gain (WPG) was calculated as a
percentage increase of the dry weight of the specimens
after the impregnation.
To evaluate how the water content of the industrial
PF resin is changed after spreading it over the plywood
surface depending on the wood moisture content,
an equal amount of PF resin was paintbrush applied
on three surfaces (100 cm2 surface area): aluminium
plate (as control), conditioned plywood (RH 65 ± 5%,
temperature 20 ± 2 °C) with 9% moisture content and
oven pre-dried (102 ± 3 °C) plywood with 3% moisture
content. After ve and ten minutes the PF resin was
removed with a scraper and the water content was
determined as previously described. The control was
used to exclude from the calculations the water that
evaporates during exposure for 5 and 10 min.
To assess the effect of the top veneer pre-
treatment with PF resin on reducing of the rapid
swelling zone of the overlaid plywood, PF resin was
applied with a paintbrush onto the plywood surface
(amount: 120 – 140 g m-2) prior to the covering lm
hot-pressing process. Manufacturing of overlaid
plywood specimens was performed in a laboratory
by using hydraulic laboratory press. To manufacture
the specimens, plywood with pre-treated top veneer
was covered with a PF resin impregnated lm and
pressed under 1.9 MPa pressure for 5 min at 140 °C
temperature.
To assess resin distribution in the top layer of
the pre-treated plywood, 1 cm wide bars were sawn
from specimen central parts and, after softening
by boiling in water for 3 h, samples were sliced for
a light microscopy examination. The examination
was performed with a transmitted light microscope
“Leica DMLB” connected to the video camera „Leica
DFC490”.
Results and Discussion
The results of the experiment in which the rate of
the swelling zone expansion was evaluated show that
liquid water transport in the top veneer is relatively
rapid and the covered distance can be quite large. The
average distance from the site of damage to the end of
the swelling zone after 24 hours (1440 min) was 32 ±
7 cm. Moreover, more than 60% of the total swelling
zone distance was reached during the rst 30 minutes.
The development of the swelling zone from damage
site during the rst four hours after articial swelling
initiation is presented in Figure 1.
The results show that the damage site can cause a
fast development of swelling zone on the surface of the
overlaid plywood when in contact with liquid water.
Besides, the results show that there is a limit how
far the swelling zone can develop from the damage
site, and the distance depends on the site’s specic
wood anatomic structure. The main cause of the fast
development of the swelling defects is attributed to the
rapid liquid water transport through the birch wood
vessel system. Therefore, the restriction of water ow
Dace Cirule, Edgars Kuka,
Anrijs Verovkins, Ingeborga Andersone
RESTRICTION OF LIQUID WATER SPREADING
IN OVERLAID PLYWOOD TOP VENEER
88 RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1
by lling the vessels with PF resin could be one of the
possibilities to reduce or eliminate the formation of
such aesthetic defects.
To evaluate the possibility of lling birch wood
vessels with PF resin and to estimate the effect of
resin properties on this process, vacuum impregnation
of birch veneers for 10 min was performed. The resin
penetration was evaluated by measuring the weight
percent gain (WPG) of the impregnated birch veneer
and the results are presented in Table 1.
The results show that the resin penetration into
birch veneers is signicantly affected by the resin
properties and the largest inuence is attributed
to the resin viscosity. By increasing the phenol-
formaldehyde resin temperature by 10 °C, which
resulted in a decrease of resin viscosity by 43%, the
WPG increased twice reaching the average value
of 26.4%. Also, for partly polymerised PF resin the
WPG signicantly increased by rising the resin
temperature by 20 °C which caused decrease in resin
viscosity by 47%. Moreover, partial polymerisation
of the resin reduced its ability to penetrate into the
birch veneer. However, it was still possible to achieve
high WPG (34.9%) by increasing the temperature of
the partly polymerised resin. For partly dehydrated
PF resin, the WPG values were the lowest from
all the tested resin types. As the dehydration was
performed in vacuum without increased temperature,
no substantial polymerisation of resin is supposed
to occur. Therefore, inability of partly dehydrated
resin to penetrate into veneer is mainly attributed to
the very high resin viscosity due to reduced water
content. These results suggest that viscosity is highly
important regarding the resin penetration into wood
and therefore should be well analysed. Meijer (2004)
in his review paper concluded that penetration capacity
of coating is mainly attributed to the viscosity, which
is in agreement with the results of the present study.
In addition, he concluded that wetting and surface
tension of the coating seem to play only a minor role.
As previously shown, a partial dehydration of PF
resin can cause a signicant increase in resin viscosity
that results in low WPG of the impregnated birch
veneer. Therefore, it is important to determine how
water content of phenol-formaldehyde resin inuences
the resin viscosity. The results of the experiment are
presented in Figure 2. The PF resin dehydrated for 4
h is not included in the gure because its viscosity at
20 °C was too high for measuring with the viscometer
used in the study.
Dace Cirule, Edgars Kuka,
Anrijs Verovkins, Ingeborga Andersone RESTRICTION OF LIQUID WATER SPREADING
IN OVERLAID PLYWOOD TOP VENEER
DOI: 10.22616/rrd.24.2018.013
Figure 1. Development of swelling zone from the damage site on overlaid plywood. Error bars represent
standard deviations.
The results show that the damage site can cause a fast development of swelling zone on the surface of the overlaid
plywood when in contact with liquid water. Besides, the results show that there is a limit how far the swelling zone
can develop from the damage site, and the distance depends on the site’s specific wood anatomic structure. The
main cause of the fast development of the swelling defects is attributed to the rapid liquid water transport through
the birch wood vessel system. Therefore, the restriction of water flow by filling the vessels with PF resin could be
one of the possibilities to reduce or eliminate the formation of such aesthetic defects.
To evaluate the possibility of filling birch wood vessels with PF resin and to estimate the effect of resin properties
on this process, vacuum impregnation of birch veneers for 10 min was performed. The resin penetration was
evaluated by measuring the weight percent gain (WPG) of the impregnated birch veneer and the results are
presented in Table 1.
Table 1
Effect of phenol-formaldehyde resin properties on resin penetration in birch veneer by vacuum
impregnation
Industrial resin type
Resin
temperature
(T), C
Resin
viscosity,
mPa s
Veneer average weight
percent gain after
impregnation, %
Industrial phenol-formaldehyde resin
20
212
13.4 (3.4)
Industrial phenol-formaldehyde resin
30
120
26.4 (4.7)
Partly polymerised phenol-formaldehyde resin
30
180
6.5 (3.8)
Partly polymerised phenol-formaldehyde resin
50
96
34.9 (9.9)
Partly dehydrated (4 h) phenol-formaldehyde resin
65
2600
4.7 (1.6)
Standard deviation in parentheses
The results show that the resin penetration into birch veneers is significantly affected by the resin properties and
the largest influence is attributed to the resin viscosity. By increasing the phenol-formaldehyde resin temperature
by 10 C, which resulted in a decrease of resin viscosity by 43%, the WPG increased twice reaching the average
value of 26.4%. Also, for partly polymerised PF resin the WPG significantly increased by rising the resin
temperature by 20 C which caused decrease in resin viscosity by 47%. Moreover, partial polymerisation of the
resin reduced its ability to penetrate into the birch veneer. However, it was still possible to achieve high WPG
(34.9%) by increasing the temperature of the partly polymerised resin. For partly dehydrated PF resin, the WPG
values were the lowest from all the tested resin types. As the dehydration was performed in vacuum without
increased temperature, no substantial polymerisation of resin is supposed to occur. Therefore, inability of partly
dehydrated resin to penetrate into veneer is mainly attributed to the very high resin viscosity due to reduced water
content. These results suggest that viscosity is highly important regarding the resin penetration into wood and
therefore should be well analysed. Meijer (2004) in his review paper concluded that penetration capacity of coating
is mainly attributed to the viscosity, which is in agreement with the results of the present study. In addition, he
concluded that wetting and surface tension of the coating seem to play only a minor role.
As previously shown, a partial dehydration of PF resin can cause a significant increase in resin viscosity that results
in low WPG of the impregnated birch veneer. Therefore, it is important to determine how water content of phenol-
0
20
40
60
80
100
030 60 90 120 150 180 210 240
Proportion of total swelling
zone distance
Time, min
Figure 1. Development of swelling zone from the damage site on overlaid plywood.
Error bars represent standard deviations.
Table 1
Effect of phenol-formaldehyde resin properties on resin penetration
in birch veneer by vacuum impregnation
Industrial resin type Resin
temperature
(T), °C
Resin
viscosity,
mPa s
Veneer average weight
percent gain after
impregnation, %
Industrial phenol-formaldehyde resin 20 212 13.4 (3.4)
Industrial phenol-formaldehyde resin 30 120 26.4 (4.7)
Partly polymerised phenol-formaldehyde resin 30 180 6.5 (3.8)
Partly polymerised phenol-formaldehyde resin 50 96 34.9 (9.9)
Partly dehydrated (4 h) phenol-formaldehyde resin 65 2600 4.7 (1.6)
Standard deviation in parentheses
89RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1
The results clearly show that water presence in PF
resin is highly important to ensure appropriate resin
viscosity for efcient resin penetration. The resin
viscosity increased more than 20 times when the water
content of the resin was reduced by only 6% (from
48% to 42%). A similar trend has been observed for
urea-formaldehyde and polyvinyl acetate adhesives
(Hass et al., 2012). The results suggest that any loss
of water content of PF resin could be crucial in the
process of lling wood vessels.
By using resin pre-treatment, it is possible that
plywood with low moisture content due to its inherent
hydrophilic nature could remove some of the water
from PF resin before the resin itself can penetrate
into the vessels, which subsequently would result in
higher resin viscosity and hindered penetration into
wood. Therefore, the wood moisture content effect on
removal of water from the PF resin was tested. The
results are presented in Figure 3.
The results show that a signicant decrease in
water content of PF resin is noticeable due to wood
ability to absorb water. After 5 min water content
of the resin decreased by 6% and 35%, but after 10
min by 10% and 67% compared to the control for
conditioned (moisture content 9%) and oven pre-
dried (moisture content 3%) plywood, respectively.
In case of oven pre-dried plywood, such changes in
water content of resin can dramatically affect further
resin penetration in wood vessels because of the
enormous increase in resin viscosity. These results
are in accordance with the studies in which similar
tendencies have been established regarding glue and
water-borne paint penetration into wood depending
on the wood moisture content (Hrázský & Král, 2007;
Meijer, Thurich, & Militz, 2001).
Several overlaid plywood specimens were
produced and the swelling zone from articially-
created damage site was measured to test how addition
of PF resin between the plywood surface and PF
impregnated lm can affect the average distance of the
swelling zone. The results are presented in Figure 4.
An addition of PF resin signicantly reduces the
average swellings zone distance from the damaged site
on the overlaid plywood surface. Moreover, the results
Dace Cirule, Edgars Kuka,
Anrijs Verovkins, Ingeborga Andersone
RESTRICTION OF LIQUID WATER SPREADING
IN OVERLAID PLYWOOD TOP VENEER
DOI: 10.22616/rrd.24.2018.013
formaldehyde resin influences the resin viscosity. The results of the experiment are presented in Figure 2. The PF
resin dehydrated for 4 h is not included in the figure because its viscosity at 20 °C was too high for measuring with
the viscometer used in the study.
Figure 2. Effect of phenol-formaldehyde resin water content on resin viscosity at 20 C.
The results clearly show that water presence in PF resin is highly important to ensure appropriate resin viscosity
for efficient resin penetration. The resin viscosity increased more than 20 times when the water content of the resin
was reduced by only 6% (from 48% to 42%). A similar trend has been observed for urea-formaldehyde and
polyvinyl acetate adhesives (Hass et al., 2012). The results suggest that any loss of water content of PF resin could
be crucial in the process of filling wood vessels.
By using resin pre-treatment, it is possible that plywood with low moisture content due to its inherent hydrophilic
nature could remove some of the water from PF resin before the resin itself can penetrate into the vessels, which
subsequently would result in higher resin viscosity and hindered penetration into wood. Therefore, the wood
moisture content effect on removal of water from the PF resin was tested. The results are presented in Figure 3.
Figure 3. Water content of phenol-formaldehyde resin after application on plywood surface. Error bars represent
standard deviations.
The results show that a significant decrease in water content of PF resin is noticeable due to wood ability to absorb
water. After 5 min water content of the resin decreased by 6% and 35%, but after 10 min by 10% and 67%
compared to the control for conditioned (moisture content 9%) and oven pre-dried (moisture content 3%) plywood,
respectively. In case of oven pre-dried plywood, such changes in water content of resin can dramatically affect
further resin penetration in wood vessels because of the enormous increase in resin viscosity. These results are in
accordance with the studies in which similar tendencies have been established regarding glue and water-borne
paint penetration into wood depending on the wood moisture content (Hrázský & Král, 2007; Meijer, Thurich, &
Militz, 2001).
0
2000
4000
6000
8000
10000
12000
41 42 43 44 45 46 47 48 49
Viscosity, mPs
Water content, %
0
10
20
30
40
50
Control Conditioned
plywood Oven pre-dried
plywood Control Conditioned
plywood Oven pre-dried
plywood
After 5 min After 10 min
Resin water content, %
Figure 2. Effect of phenol-formaldehyde resin water content on resin viscosity at 20 °C.
DOI: 10.22616/rrd.24.2018.013
formaldehyde resin influences the resin viscosity. The results of the experiment are presented in Figure 2. The PF
resin dehydrated for 4 h is not included in the figure because its viscosity at 20 °C was too high for measuring with
the viscometer used in the study.
Figure 2. Effect of phenol-formaldehyde resin water content on resin viscosity at 20 C.
The results clearly show that water presence in PF resin is highly important to ensure appropriate resin viscosity
for efficient resin penetration. The resin viscosity increased more than 20 times when the water content of the resin
was reduced by only 6% (from 48% to 42%). A similar trend has been observed for urea-formaldehyde and
polyvinyl acetate adhesives (Hass et al., 2012). The results suggest that any loss of water content of PF resin could
be crucial in the process of filling wood vessels.
By using resin pre-treatment, it is possible that plywood with low moisture content due to its inherent hydrophilic
nature could remove some of the water from PF resin before the resin itself can penetrate into the vessels, which
subsequently would result in higher resin viscosity and hindered penetration into wood. Therefore, the wood
moisture content effect on removal of water from the PF resin was tested. The results are presented in Figure 3.
Figure 3. Water content of phenol-formaldehyde resin after application on plywood surface. Error bars represent
standard deviations.
The results show that a significant decrease in water content of PF resin is noticeable due to wood ability to absorb
water. After 5 min water content of the resin decreased by 6% and 35%, but after 10 min by 10% and 67%
compared to the control for conditioned (moisture content 9%) and oven pre-dried (moisture content 3%) plywood,
respectively. In case of oven pre-dried plywood, such changes in water content of resin can dramatically affect
further resin penetration in wood vessels because of the enormous increase in resin viscosity. These results are in
accordance with the studies in which similar tendencies have been established regarding glue and water-borne
paint penetration into wood depending on the wood moisture content (Hrázský & Král, 2007; Meijer, Thurich, &
Militz, 2001).
0
10
20
30
40
50
Control Conditioned
plywood Oven pre-dried
plywood Control Conditioned
plywood Oven pre-dried
plywood
After 5 min After 10 min
Resin water content, %
Figure 3. Water content of phenol-formaldehyde resin after application on plywood surface.
Error bars represent standard deviations.
90 RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1
for conditioned plywood with 9% moisture content
were better than for the oven pre-dried plywood with
3% moisture content, which is in accordance with
previously discussed results about the wood ability
to absorb water from PF resin depending on the
wood moisture content. The examination of the top
veneer cross-sections with microscope showed that
the reduction in the average swelling zone distance
is mainly due to PF resin penetration into wood
vessels, which restricts rapid liquid water transport
in top veneer of the overlaid plywood. These results
suggest that for more efcient lling of birch wood
vessels with PF resin and subsequent restriction of
water spreading, the plywood moisture is of a great
importance and the plywood of the highest admissible
moisture content regarding the technological process
should be used in order to ensure top veneer with
reduced water transportation ability.
However, in Figure 5 presented light microscopy
images of specimens prepared from one plywood
sample pre-treated with PF resin show that PF resin
penetration in birch wood vessels can be rather
uneven. The area with all vessel lumens empty (a),
area with only part of lumens lled (b), and the
area with completely lled vessel lumens (c) were
detected even within one specimen in some cases.
The inhomogeneous resin penetration in wood
vessels could be partly attributed to the differences in
wood anatomy (Meijer, 2004). This can account for
only reduction and not complete elimination of fast
water spreading into the top veneer of plywood with
damaged covering lm by using the proposed method.
Conclusions
1. The damage of overlaid plywood covering lm in
presence of liquid water causes a rapid formation
and spreading of well noticeable swelling zone
and during the rst 30 minutes more than 60% of
the total swelling zone is reached.
2. Resin viscosity signicantly affect its penetration
ability into veneer. Reduction of the water content
of phenol-formaldehyde resin causes an increase
in viscosity which hinders resin penetration into
wood.
3. The best penetration of the resin into veneer was
achieved for specimens with a higher moisture
content, which is in accordance with the ndings
that wood readily absorbs water from resin and the
absorption capacity is inversely related to wood
moisture content.
4. In the present study, a method for the enhancement
of covered plywood resistance to rapid local
swelling was developed comprising pre-treatment
of top veneer with phenol-formaldehyde resin
to ll wood vessels followed by hot-pressing of
covering lm.
DOI: 10.22616/rrd.24.2018.013
Several overlaid plywood specimens were produced and the swelling zone from artificially-created damage site
was measured to test how addition of PF resin between the plywood surface and PF impregnated film can affect
the average distance of the swelling zone. The results are presented in Figure 4.
Figure 4. Effect of phenol-formaldehyde resin pre-treatment and plywood moisture content on reduction of the
average swelling zone distance from the damage site. Error bars represent standard deviations.
An addition of PF resin significantly reduces the average swellings zone distance from the damaged site on the
overlaid plywood surface. Moreover, the results for conditioned plywood with 9% moisture content were better
than for the oven pre-dried plywood with 3% moisture content, which is in accordance with previously discussed
results about the wood ability to absorb water from PF resin depending on the wood moisture content. The
examination of the top veneer cross-sections with microscope showed that the reduction in the average swelling
zone distance is mainly due to PF resin penetration into wood vessels, which restricts rapid liquid water transport
in top veneer of the overlaid plywood. These results suggest that for more efficient filling of birch wood vessels
with PF resin and subsequent restriction of water spreading, the plywood moisture is of a great importance and the
plywood of the highest admissible moisture content regarding the technological process should be used in order
to ensure top veneer with reduced water transportation ability.
However, in Figure 5. presented light microscopy images of specimens prepared from one plywood sample pre-
treated with PF resin show that PF resin penetration in birch wood vessels can be rather uneven. The area with all
vessel lumens empty (a), area with only part of lumens filled (b), and the area with completely filled vessel lumens
(c) were detected even within one specimen in some cases. The inhomogeneous resin penetration in wood vessels
could be partly attributed to the differences in wood anatomy (Meijer, 2004). This can account for only reduction
and not complete elimination of fast water spreading into the top veneer of plywood with damaged covering film
by using the proposed method.
Figure 5. Top veneer cross sections of overlaid birch plywood pre-treated with phenol-formaldehyde resin: a)
empty vessel lumens; b) part of vessel lumens filled; c) completely filled vessel lumens.
Conclusions
1. The damage of overlaid plywood covering film in presence of liquid water causes a rapid formation and
spreading of well noticeable swelling zone and during the first 30 minutes more than 60% of the total swelling
zone is reached.
2. Resin viscosity significantly affect its penetration ability into veneer. Reduction of the water content of phenol-
formaldehyde resin causes an increase in viscosity which hinders resin penetration into wood.
3. The best penetration of the resin into veneer was achieved for specimens with a higher moisture content, which
is in accordance with the findings that wood readily absorbs water from resin and the absorption capacity is
inversely related to wood moisture content.
0
20
40
60
80
100
Oven pre-dried plywood
(3 % moisture content)
Conditioned plywood
(9 % moisture content)
Reduction in the avarage
swelling zone distance, %
Figure 4. Effect of phenol-formaldehyde resin pre-treatment and plywood moisture content on reduction of
the average swelling zone distance from the damage site. Error bars represent standard deviations.
DOI: 10.22616/rrd.24.2018.013
Several overlaid plywood specimens were produced and the swelling zone from artificially-created damage site
was measured to test how addition of PF resin between the plywood surface and PF impregnated film can affect
the average distance of the swelling zone. The results are presented in Figure 4.
Figure 4. Effect of phenol-formaldehyde resin pre-treatment and plywood moisture content on reduction of the
average swelling zone distance from the damage site. Error bars represent standard deviations.
An addition of PF resin significantly reduces the average swellings zone distance from the damaged site on the
overlaid plywood surface. Moreover, the results for conditioned plywood with 9% moisture content were better
than for the oven pre-dried plywood with 3% moisture content, which is in accordance with previously discussed
results about the wood ability to absorb water from PF resin depending on the wood moisture content. The
examination of the top veneer cross-sections with microscope showed that the reduction in the average swelling
zone distance is mainly due to PF resin penetration into wood vessels, which restricts rapid liquid water transport
in top veneer of the overlaid plywood. These results suggest that for more efficient filling of birch wood vessels
with PF resin and subsequent restriction of water spreading, the plywood moisture is of a great importance and the
plywood of the highest admissible moisture content regarding the technological process should be used in order
to ensure top veneer with reduced water transportation ability.
However, in Figure 5. presented light microscopy images of specimens prepared from one plywood sample pre-
treated with PF resin show that PF resin penetration in birch wood vessels can be rather uneven. The area with all
vessel lumens empty (a), area with only part of lumens filled (b), and the area with completely filled vessel lumens
(c) were detected even within one specimen in some cases. The inhomogeneous resin penetration in wood vessels
could be partly attributed to the differences in wood anatomy (Meijer, 2004). This can account for only reduction
and not complete elimination of fast water spreading into the top veneer of plywood with damaged covering film
by using the proposed method.
Figure 5. Top veneer cross sections of overlaid birch plywood pre-treated with phenol-formaldehyde resin: a)
empty vessel lumens; b) part of vessel lumens filled; c) completely filled vessel lumens.
Conclusions
1. The damage of overlaid plywood covering film in presence of liquid water causes a rapid formation and
spreading of well noticeable swelling zone and during the first 30 minutes more than 60% of the total swelling
zone is reached.
2. Resin viscosity significantly affect its penetration ability into veneer. Reduction of the water content of phenol-
formaldehyde resin causes an increase in viscosity which hinders resin penetration into wood.
3. The best penetration of the resin into veneer was achieved for specimens with a higher moisture content, which
is in accordance with the findings that wood readily absorbs water from resin and the absorption capacity is
inversely related to wood moisture content.
0
20
40
60
80
100
Oven pre-dried plywood
(3 % moisture content)
Conditioned plywood
(9 % moisture content)
Reduction in the avarage
swelling zone distance, %
Figure 5. Top veneer cross sections of overlaid birch plywood pre-treated with phenol-formaldehyde resin:
a) empty vessel lumens; b) part of vessel lumens lled; c) completely lled vessel lumens.
Dace Cirule, Edgars Kuka,
Anrijs Verovkins, Ingeborga Andersone RESTRICTION OF LIQUID WATER SPREADING
IN OVERLAID PLYWOOD TOP VENEER
91RESEARCH FOR RURAL DEVELOPMENT 2018, VOLUME 1
5. The results showed that by applying the proposed
method it is possible to restrict liquid water
spreading and subsequently reduce the total
average swelling zone distance from the damage
site by half.
Acknowledgements
The authors gratefully acknowledge the nancial
support by the European Regional Development Fund
in the framework of the “Forest Sector Competence
Centre” project “Wood based composites with
improved properties” (No 1.2.1.1/16/A/009)
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Dace Cirule, Edgars Kuka,
Anrijs Verovkins, Ingeborga Andersone
RESTRICTION OF LIQUID WATER SPREADING
IN OVERLAID PLYWOOD TOP VENEER
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