The effect of chemical treatment of wood veneer surfaces on their bondability

Abstract

In this study, the chemical treatment of veneer surfaces was applied for minimizing surface inactivation and enhancing the bondability of wood veneers. The effect of different type of activating agents (hydrogen peroxide, aluminium persulfate, acetic acid, and sodium carbonate), their concentration (1%, 2% and 3%) and their amount (10, 20 and 30 g/m2) on the physical and mechanical properties of veneer surface and veneer-based products (especially plywood) made using treated veneers, was examined through laboratory tests. The results if this study revealed that chemical treatment of veneer surfaces increased their bondability. It was found that the samples of plywood panels made using treated veneers had higher shear strength than those of the control samples made using non-treated veneers.

Full text

ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 57(2): 71−79, 2015
Zvolen, Technická univerzita vo Zvolene
DOI: 10.17423/afx.2015.57.2.07
71
THE EFFECT OF CHEMICAL TREATMENT OF WOOD VENEER
SURFACES ON THEIR BONDABILITY
Pavlo Bekhta - Ján Sedliačik - Diana Tymyk
ABSTRACT
In this study, the chemical treatment of veneer surfaces was applied for minimizing
surface inactivation and enhancing the bondability of wood veneers. The effect of different
type of activating agents (hydrogen peroxide, aluminium persulfate, acetic acid, and
sodium carbonate), their concentration (1%, 2% and 3%) and their amount (10, 20 and
30 g/m2) on the physical and mechanical properties of veneer surface and veneer-based
products (especially plywood) made using treated veneers, was examined through
laboratory tests. The results if this study revealed that chemical treatment of veneer
surfaces increased their bondability. It was found that the samples of plywood panels made
using treated veneers had higher shear strength than those of the control samples made
using non-treated veneers.
Keywords: activating agents, birch veneer, formaldehyde free, bondability, surface
activation treatment.
INTRODUCTION
The manufacture of veneer-based products (plywood, LVL) involves many different
processes; one of the most essential is the adhesive bonding of veneer sheets. Moreover,
manufacturing processes including peeling and drying can essentially change physical and
chemical surface properties of veneer. The interaction between liquid adhesive and the
veneer surface depends, first of all, on the properties of the applied adhesive and conditions
of the veneer surface. The strength of the adhesive bond is intimately associated with the
surface and sub-surface properties of veneer. In order to achieve optimum adhesion
between substrate and adhesive a clean solid surface is required. However, veneer surface
is subjected to self-contamination that according to BACK (1991) is a result of a natural
surface inactivation process where low-molecular wood extractives migrate to the surface.
In the production of veneer-based products a significant area of new wood surface is
created in a short period of time. But, in many cases, the time between creating a new
surface and adhesive spreading is too long. Within this period of time, freshly cut wood
surfaces undergo a transformation termed surface inactivation. Wood surface inactivation
is a surface phenomenon resulting in the loss of bonding ability (CHRISTIANSEN 1991).
Therefore, quality preparation of the surface is extremely important. The existing
technique of manufacturing plywood does not provide the preparation of veneer surface
before glue application, although such operation could have an essential influence on the

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reduction of the glue spread and on the improvement of the plywood performance
(BEKHTA et al. 2009, BEKHTA et al. 2012, BEKHTA and MARUTZKY 2007).
Mechanical pre-treatments such as sanding and planning (AYDIN 2004) or
densification by cold rolling (BEKHTA et al. 2009) or densification by hot pre-pressing
(BEKHTA and MARUTZKY 2007, BEKHTA et al. 2012) can be applied to modify the surface
characteristics that improve glue bonding of wood. In order to improve bonding ability,
wettability and to reactivate wood surfaces for glue-wood bonds, some chemical pre-
treatments are widely applied to wood surfaces (BELFAS et al. 1993, GARDNER and ELDER
1988). In most instances, surface activation has been accomplished through the use of
various oxidizing agents, such as hydrogen peroxide (PHILIPPOU et al. 1982), nitric acid
(JOHNS et al. 1978), sodium hydroxide (YOUNG et al. 1985), and others. Surface activation
treatment of wood using hydrogen peroxide, nitric acid, and sodium hydroxide was
examined by GARDNER and ELDER (1988) to assess its effect on the gel time of phenol-
formaldehyde resin. They showed that surface activated treated wood decreases the gel
time of phenol-formaldehyde resin with hydrogen peroxide treatment having the greatest
effect followed by nitric acid and sodium hydroxide treatments. SUBRAMANIAN et al.
(1982) were found that nitric acid oxidation of wood resulted in the formation of
carboxylic acid covalently bonded to the wood. The results of CHAPMAN and JENKIN
(1986) showed that panel (fiberboard, particleboard, and plywood) pressing times could be
reduced by 30%, and in some cases better resin cure permitted a reduction in binder level
when hydrogen peroxide was used in combination with phenol-formaldehyde, urea-
formaldehyde, and tannin-formaldehyde resin-adhesives. MIRSKI et al. 2011 analysed the
potential to shorten pressing time or reduce pressing temperature for plywood resinated
with alcohol- and ester-modified PF resin resulting in the manufacture of plywood with
good properties even at a pressing temperature reduced by 20 °C or pressing time
shortened to 150 s.
In accordance with this chemical pre-treatment, the functional groups present on the
wood surface are modified so that they can react and bond with the functional groups more
effectively in the adhesive. Hereby, the surface of wood should be cleaned just before
bonding in order to remove all foreign substances that interfere with bonding.
CHRISTIANSEN (1994) summarised the mechanisms responsible for changes of wood
surfaces that may influence the physical and bonding properties of wood: 1) migration of
hydrophobic extractives during drying, 2) oxidation, 3) closure of micro-voids in the wood
surface which reduces adhesive penetration, 4) acidity or reactive of extractives affecting
cure of adhesives, 5) molecular reorientation of functional groups at the surface.
The objectives of this research were: (a) to develop and understand how various
activating agents affect the surface properties (such as hydrogen ion concentration (pH)
and wettability) of wood veneers; (b) to understand the influence of activated veneer
surface on the bondability of the adhesive bond between veneer and polymeric materials
and hence plywood performance; (c) to establish the relationships between surface
properties of activated veneer and plywood.
MATERIAL AND METHODS
Rotary cut veneer sheets of birch (Betula verrucosa Ehrh.) with dimensions of
500 mm by 500 mm and with thickness of 1.5 mm and conditioned to moisture content of
6% were chosen for the experiments. Hydrogen peroxide (H2O2), aluminium persulfate
(Al2(SO4)3), acetic acid (CH3COOH), and sodium carbonate (Na2CO3) were examined for
veneer surface activation treatments. Three levels of the concentration solution of 1%, 2%,

73
and 3% of activating agents were studied. Surface activation was accomplished by
spraying the activating agent with consumption of 10, 20 and 30 g/m2 on the veneer
surfaces before glue application. Surface activity of treated veneers was evaluated by
measuring the hydrogen ion concentration (pH) and contact angle. Three and thirty
replicates for pH and contact angle tests respectively were used.
In addition, five-layer experimental plywood panels were manufactured from veneers
with treated and non-treated surfaces using commercial phenol-formaldehyde glue resin
with a solid content of 42% and viscosity Ford 4/20 of 120 s at following pressing factors:
pressure of 1.8 MPa, temperature of 135 °C, and time of 6 resp. 8 and 10 min. The glue
spread was 150 g/m2 based on the wet mass. For each treatment, the shear strength of
plywood panels was measured according to EN 314-1. Twelve replicates were used.
RESEARCH RESULTS
The pH values of veneer surface treated with different activating agents are presented
in Table 1. Solutions (2% and 3%) of aluminum sulfate and acetic acid reduces the pH of
wood surface; but 1% solutions have almost no influence on the changing of the acidity of
wood surface. The use of 2% and 3% solutions of sodium carbonate provides an
opportunity to an increase of the acidity of wood surface. However, the use of 1% solution
of sodium carbonate is ineffective, as well as in the case of aluminum sulfate and acetic
acid. Hydrogen peroxide can lead to the decrease in surface acidity due to the
accumulation of carboxyl groups under the oxidation of wood components. However, the
used peroxide solution still had a low pH value due to the presence of stabilizers in it. This
led to the decrease in the pH of the surface under the treatment by hydrogen peroxide
solution.
The little impact of 1% solutions on an acidity of wood surface is caused by some
buffer capacity of wood components. That is, the wood components can interact with the
substances used for the treatment of wood, reducing their concentration in the solution and
consequently the activation ability of the solution.
In particular, the concentration of sodium carbonate may decrease due to the
reactions with free fatty, resin and uronic acids of wood:
2R–CООН + Na2CO3 → 2R–CООNa + Н2О + СO2
where R are the remnants of fatty, resin and uronic acids.
Ions of hydrogen which are generated under hydrolysis of aluminum sulphate and
dissociation of acetic acid, cause acidity of their solutions and can interact with anions of
uronic acids:
R–COO– + H+ → R–COOH.
The results also revealed that the surface activation treatment of veneer improved
both surface properties of veneer and mechanical properties of plywood panels. Surface
activated treated veneer decreases the contact angle (Table 2) and increases the shear
strength (Fig.1) of plywood panels with hydrogen peroxide treatment having the greatest
effect followed by acetic acid, and sodium carbonate, and aluminium persulfate treatment.
Contact angle of treated veneer surface had up to 36% improved values as compared for
those of non-treated samples. That is, the surface activation of veneer increases the
bondability of wood. Consumption of activating agents had greater influence on surface
contact angle than concentration solution. Increasing concentration solution from 1% to

74
3% results to the reduction pH of veneer surface for all activating agents except sodium
carbonate, for which the pH is increased. MATUANA et al. (1998) using four different
coupling agents to treat the wood veneer also showed that coupling treatment helped
improve the compatibility at the interface. In the study (LU and WU 2005), the morphology
of water droplets on wood veneer with different coupling agents was investigated. The
study showed that coupling agent type, molecular structure, and retention greatly
influenced the wetting behaviour of the modified wood surfaces and wood-polymer
interface.
Tab. 1 pH values for birch veneer surface treated by different activating agents.
Activating agent
Н2O2
Concentration solution,
%, (pH of solution)
1
(рН=6.46)
2
(рН=5.10)
3
(рН=3.24)
Consumption, g/m2
10
20
30
10
20
30
10
20
30
рН of veneer surface
6.85
6.75
6.64
6.52
6.41
6.32
6.24
6.01
5.93
Activating agent
СН3СОOН
Concentration solution,
%, (pH of solution)
1
(рН=2.74)
2
(рН=2.53)
3
(рН=2.39)
Consumption, g/m2
10
20
30
10
20
30
10
20
30
рН of veneer surface
6.32
6.21
6.08
5.91
5.79
5.72
5.69
5.46
5.33
Activating agent
Na2CO3
Concentration solution,
%, (pH of solution)
1
(рН=7.60)
2
(рН=7.55)
3
(рН=7.51)
Consumption, g/m2
10
20
30
10
20
30
10
20
30
рН of veneer surface
6.80
7.08
7.15
7.78
8.20
8.36
8.15
8.20
8.21
Activating agent
Al2(SO4)3
Concentration solution,
%, (pH of solution)
1
(рН=2.85)
2
(рН=2.24)
3
(рН=1.99)
Consumption, g/m2
10
20
30
10
20
30
10
20
30
рН of veneer surface
6.53
6.41
6.27
6.35
6.22
5.92
6.13
5.93
5.85
*рН of non-treated veneer surface 6.40
Tab. 2 Contact angle values for different activating agents.
Concentration
solution [%]
Consumption
[g/m2]
H2O2
CH3COOH
Na2CO3
Al2(SO4)3
Contact angle
10
48.4 (1.3)*
47.1 (2.8)
47.4 (2.7)
48.2 (2.9)
1
20
40.9 (2.7)
42.2 (3.7)
37.9 (3.3)
41.0 (2.5)
30
36.4 (2.1)
37.5 (2.8)
35.3 (2.0)
36.5 (1.7)
10
45.9 (3.5)
46.0 (3.8)
47.3 (2.7)
46.9 (1.9)
2
20
40.4 (3.2)
40.7 (2.5)
40.3 (2.5)
39.7 (2.4)
30
35.3 (2.2)
36.8 (0.6)
36.0 (1.8)
35.4 (1.6)
10
45.7 (2.9)
45.1 (3.4)
47.9 (1.7)
46.3 (2.8)
3
20
38.6 (2.3)
38.1 (2.3)
38.7 (1.9)
39.3 (2.4)
30
35.2 (2.0)
36.1 (1.4)
35.3 (3.2)
35.4 (1.6)
*Values in parenthesis are standard deviations. Contact angle for non-treated veneer surface 53.5.
Activation effect of acidic and alkaline aqueous solutions is based on the catalyzed
hydrolysis reaction of basic chemical components of wood: cellulose, hemicellulose and
lignin. The molecular weight of cellulose decreases during its hydrolysis owing to the
breaking of glycoside bonds (PAVASARS et al. 2009); as a result, there is an additional
reactive carbonyl group:

75
O
CH2OH
CH3OH
HOH
HH
HHO
O
CH2OH
OH
HOH
H
O
H
HHCH3
H+
+H2O
OH
C
CH2OH
OH
HOH
HO
H
HH
OH
O
CH2OH
OH
HOH
H
O
H
HH
+
Hydrolysis of lignin occurs in the alkaline and acidic environment (GUPALO 1993).
The molecular weight of lignin during hydrolysis is reduced because of breaking the
essential connections between the links and additional reactive (OH) groups are generated
under the scheme:
CH
CH C
H
CH
CH
C
C
C
O
C
H
C
H
C
HC
H
CC C H+
CH
CH C
H
CH
CH
C
C
C
OH
C
H
C
H
C
HC
H
CC C
OH
+
+H2O
(OH-)
The number of polar groups and reaction centers on the wood surface increases
because of such surface treatment. This contributes to an efficient intermolecular
interactions as well as the formation of strong chemical bonds between the molecules of
adhesive and molecules of the main components of wood for all investigated activating
agents. This action is further enhanced by the stitching of macromolecules of wood and
glue for radical mechanism (OMORI and DENCE 1981, PETIGARA et al. 2002). The
accumulation of carbonyl groups occurs on the wood surface treated by hydrogen
peroxide, through oxidation of carbohydrates:
O
CH2
H
H
OH
OH
H
O
H
H
OH
n
+
O
CH
H
H
OH
OH
H
O
H
H
O
n
+H2O
[O]
nn
Changes in the surface properties of wood treated by the solutions of activating
agents are caused not only by the changes of chemical structure of the main components of
wood, and largely by the changes in their supramolecular structure, due to the penetration
of water molecules between the cellulose macromolecules in the surface layers of cellulose
fibrils (MATUSIEVICH 2010):
OH+ H2OOH
HOHO
H
HO
This leads to a decrease in the degree of crystallinity of cellulose, but hydroxyl
groups which are involved in the formation of hydrogen bonds between cellulose
macromolecules are released to form hydrogen bonds with the molecules of the adhesive.

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a)
Consumption [g/m
2
]
Shear strength [MPa]
Control Hydrogen peroxi de Aceti c acid Sodium carbonate Al uminium
persulfate
010 20 30
1,2
1,4
1,6
1,8
2,0
2,2
2,4
2,6
b)
Concentration solution [%]
Shear strength [MPa]
Control Hydrogen peroxide Aceti c acid Sodium carbonate Aluminium
persulfate
0 1 2 3
1,2
1,4
1,6
1,8
2,0
2,2
2,4
2,6
c)
Time of pressing [min]
Shear strength [MPa]
Control Hydrogen peroxi de Acetic acid Sodium carbonate Aluminium
persulfate
0 6 8 10
1,2
1,4
1,6
1,8
2,0
2,2
2,4
2,6
2,8
Fig.1 Shear strength of plywood samples depending of activating agents and on consumption (а) for
concentration solution of 3% and time of pressing 6 min; concentration solution (b) for consumption of
10 g/m2 and time of pressing 6 min; time of pressing (c) for concentration solution of 3% and
consumption of 10 g/m2.

77
Thus, surface activation can markedly improve the bondability of veneer, as
evidenced by higher values (1.7‒2.4 MPa) of shear strength of plywood compared to the
non-treated surface (1.6 MPa). Though, the increasing of the consumption of activating
agents from 10 g/m2 to 30 g/m2 results even to a slight reduction by 3-7% in shear strength.
This can be explained by increased moisture content of treated veneer and thus increased
the probability of adhesive bonds failure under the effect of the excess pressure of gas-
vapor mixture during pressing. The raising concentration solution of activating agents from
1% to 3%, on the contrary, leads to increase by 6‒9% in shear strength. This may be cause
by an increase in the reaction of activating agents with the veneer surface, producing a
greater number of functional groups on the surface veneer that may react with the phenol-
formaldehyde resin (GARDNER and ELDER 1988). The highest shear strength of the panels
were found for those panels made using veneer treated with 3% solution of hydrogen
peroxide at 10 g/m2 consumption. These samples had 1.5 times higher shear strength
values than those of the control panels.
CONCLUSIONS
Chemical treatment of veneer surfaces improved both surface properties of veneer
and mechanical properties of plywood panels made using treated veneer. The wettability of
treated veneer surfaces improved due to decreased contact angle values. Contact angle
decreased after chemical treatment of veneer surfaces by all investigated activating agents.
The chemical treatment of veneer surfaces by hydrogen peroxide had the greatest positive
effect on the shear strength of plywood samples followed the treatment by acetic acid, and
sodium carbonate, and aluminium persulfate. It was not found a direct correlations between
pH, contact angle and shear strength of plywood. Based on the findings of this work such
treatment process could have potential to be used as alternative method to enhance
properties of the plywood panels.
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Acknowledgements
The authors are grateful to the Slovak Academic Information Agency (SAIA) for financial support
of this study. This research was supported by a Slovakian Agency SRDA, project No. APVV-14-
0506 “ENPROMO” and VEGA agency project No. 1/0527/14.
Author’s address
Prof. Ing. Pavlo Bekhta, DrSc.
National University of Forestry and Wood Technology of Ukraine
Department of Wood-Based Composites
Zaliznyaka 11
79057 Lviv
Ukraine
bekhta@ukr.net

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Prof. Ján Sedliačik, PhD.
Technical University in Zvolen
Department of Furniture and Wood Products
Masaryka 24
960 53 Zvolen
Slovakia
sedliacik@tuzvo.sk
Ing. Diana Tymyk, CSc.
National University of Forestry and Wood Technology of Ukraine
Department of Wood-Based Composites
Zaliznyaka 11
79057 Lviv
Ukraine
diana_tdv@ukr.net

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