„Hardwood Science and Technology”
The 4th Conference on Hardwood Research and Utilisation in Europe 2010
Bonding of beech wood with an adhesive mixture made of liquefied
wood and phenolic resin
Ales Ugovsek1, Mirko Kariz1, Dr. Milan Sernek1
1
Department of Wood Science and Technology, Biotechnical Faculty, University of Ljubljana, Rozna
dolina, C. VIII/34, 1001 Ljubljana, Slovenia
Keywords: adhesive, beech, liquefied wood, phenol-formaldehyde, shear strength
ABSTRACT
Solid wood is often bonded with synthetic adhesives, which contain formaldehyde or other
chemicals that are harmful to human health and the environment. The development of
environmentally friendly adhesives, which could be comparable to existing synthetic adhesives,
has been the goal of many recent studies. Another option is to substitute part of the synthetic
resin with an environmentally friendly material. The liquefaction of wood, which is a novel
procedure used to convert solid wood into the liquefied state, is a promising procedure for such a
purpose. The objective of this research was, therefore, to develop an adhesive in which part of
the synthetic resin is replaced by liquefied wood.
A synthetic phenol-formaldehyde (PF) adhesive, mixed with different proportions of liquefied
wood, was used for the bonding of two beech wood lamellas. Liquefied wood made from poplar
was added in different proportions, ranging from 0 % to 100 %, with increments of 25 %. The
lamellas were bonded in a hot press at 180 °C for different press times. The test specimens were
tested according to the standard EN 12765 and EN 205.
It was found that the bond shear strength of the dry specimens increased if 25 % of the phenolformaldehyde adhesive was replaced by liquefied wood, but decreased if a higher proportion of
liquefied wood was used. When testing the specimens after immersion or boiling in water, the
bond strength decreased rapidly in the case of the samples which had been bonded with adhesive
mixtures containing more than 25 % of liquefied wood.
INTRODUCTION
Solid wood and wood based composites are nowadays usually bonded with synthetic adhesives.
These adhesives are mostly made of oil based derivatives. They are fairly cheap to manufacture,
and can provide high quality bonds, but most importantly, they contain formaldehyde, which is
harmful to human health and potentially carcinogenic. The problem of free formaldehyde,
increased ecological awareness, ever stricter environmental requirements, the introduction of
special regulations about volatile organic compounds, and the rise in price of oil derivatives, are
the main reasons for study and development of new naturally-based adhesives. Throughout
history, many adhesives have been based on natural materials, including tannin (BISANDA ET AL.,
2003; VÁZQUEZ ET AL., 2002; LI ET AL., 2004a), lignin (VELÁSQUEZ ET AL., 2003; GOSSELINK ET
AL., 2004), soy (HUANG AND SUN, 2000), and many others. Liquefied wood is another naturallybased product, which can be introduced into the adhesive mixture (KOBAYASHI ET AL., 2000; LI
ET AL., 2004b).
„Hardwood Science and Technology”
The 4th Conference on Hardwood Research and Utilisation in Europe 2010
Solid wood can be liquefied using different solvent systems. The first system uses phenols
(ALMA AND BASTÜRK, 2006), whereas the second employs cyclic carbonates (XIE AND CHEN,
2005), the third uses ionic liquids (HONGLU AND TIEJUN, 2006), the fourth introduces dibasic
esters without hydroxyl groups (WEI ET AL., 2004), and the fifth system is based on polyhydric
alcohols. Solid wood liquefied with polyhydric alcohols contains enough -OH groups, which can
potentially react with the PF resin, and take part in the adhesive mixture as a copolymer (BUDIJA
ET AL., 2009). The objective of this study was to determine the bond performance of specimens
from beech wood bonded with an adhesive mixture made of liquefied wood and phenolic resin.
EXPERIMENTAL METHODS
Sawdust (fractions 0,237 mm or smaller) of the black poplar (Populus nigra L.) was used for the
production of liquefied wood (LW). Prior to the liquefaction process, the sawdust was dried in a
laboratory oven (103 °C, 24 h). Glycerol (GLY) (p.a. grade, Kemika, Croatia) was used as the
reaction reagent, and sulphuric acid (SA) (p.a. grade, Sigma Aldrich, Germany) was used as a
catalyst. The mass ratio between the black poplar sawdust and the GLY was 1:3 (150 g of
sawdust and 450 g of GLY), and 3 % of SA, based on the GLY content, was added (13,5 g). The
liquefaction reaction was performed for 90 minutes in a glass laboratory reactor, which was
immersed in an oil bath that had been preheated to 180 °C. After reaction, the reactor was
immersed in cold water in order to quench the reaction. The liquefied product was then diluted
with a mixture of 1,4-dioxane and water (4/1, v/v) and filtered through filter disks (Sartorius
filter disks 388 grade/84/mm2) to remove the insoluble parts of the wood. After filtration, the
mixture of 1,4-dioxane and water was evaporated under reduced pressure, obtained by using a
water pump, in order to obtain the liquefied wood containing GLY.
The adhesive mixtures were prepared according to Table 1. A commercially available synthetic
PF adhesive (Fenolit d.d., Slovenia) was used to prepare the different PF-LW adhesive mixtures.
Table 1: Preparation of the adhesive mixtures
Adhesive mixture
Weight portion of PF adhesive [%]
Weight portion of LW [%]
1
100
0
2
75
25
3
50
50
4
25
75
5
0
100
Prior to bonding, all of the beech wood lamellas were planed in order to ensure a smooth and flat
surface. The two lamellas were then bonded together with the different adhesive mixtures (Table
1). The adhesive mixtures were applied with a roller, using an application rate of 200 g/m 2. The
press temperature was 180 °C, and the pressure was 1 MPa. In the first part of experiment the
press time was 600 seconds for all the adhesive mixtures, whereas in the second part the press
time was adapted to the used adhesive mixtures (Table 2). The press times used in the second
part of the experiment were determined on the basis of preliminary rheological measurements of
the adhesive mixtures.
„Hardwood Science and Technology”
The 4th Conference on Hardwood Research and Utilisation in Europe 2010
Table 2: Press times with different adhesive mixtures
Adhesive mixture
Press time-first part [s]
Press time-second part [s]
1
2
3
4
5
600
600
600
600
600
600
660
720
1200
2400
The PF-LW adhesive mixtures were tested according to the standard SIST EN 12765:2002. The
bonded samples were cut into specimens according to standard SIST EN 205. The specimens
were then conditioned for 1 week in a standard climate. The dimensions of the shear areas were
measured for all the specimens. Specimens from each press time group were divided into four
subgroups for the different pre-treatments, prior to testing according to the standard SIST EN
12765:2002. The first subgroup of specimens (pre-treatment 1) was tested in the dry state after
conditioning in a standard climate; the second subgroup of specimens (pre-treatment 2) was
soaked in cold water (20±5 °C) for 24 hours; the third subgroup of specimens (pre-treatment 3)
was soaked in hot water (67±3 °C) for 3 hours, and then cooled in cold water for 2 hours; and the
fourth subgroup of specimens (pre-treatment 4) was boiled for 3 hours, and cooled in cold water
for 2 hours. The shear tests were carried out on a ZWICK/Z100 universal testing machine
according to standard SIST EN 205, immediately after the pre-treatments had been performed.
RESULTS AND DISCUSSION
The shear strength values of the tested specimens bonded with different adhesive mixtures,
tested after different pre-treatments and a press time 600 seconds, are shown in Table 3. The
wood failure of these specimens is shown in Table 4. The specimens bonded with adhesive
mixture 2 (25 % of LW, 75 % of PF) exhibited higher shear strength than the specimens bonded
with the commercial PF adhesive after pre-treatment 1. All the other adhesive mixtures exhibited
a lower shear strength than that of the commercial adhesive. After pre-treatments 2, 3 and 4, only
the test specimens bonded with adhesive mixture 2 exhibited comparable values to the
commercial PF adhesive. All the other values were much lower, and did not attain the
requirements of the standard. According to the standard SIST EN 12765:2002, only the
commercial synthetic PF adhesive and the adhesive mixture with 25 % of added LW to the PF
adhesive met the requirements for C1, C2 and C3 durability classes, whereas for the C4
durability class only the PF adhesive was suitable.
Table 3: Shear strength [N/mm2] of the tested specimens after different pre-treatments and a press time of 600
seconds
Adhesive mixture
Pretreatment 1
Pretreatment 2
Pretreatment 3
Pretreatment 4
1
11,92
7,89
7,64
7,54
2
13,5
7,58
7,18
6,96
3
9,61
4
0
0
4
7,38
1,72
0
0
5
0,56
0
0
0
„Hardwood Science and Technology”
The 4th Conference on Hardwood Research and Utilisation in Europe 2010
Table 4: Wood failure [%] of the tested specimens after different pre-treatments and a press time of 600 seconds
Adhesive mixture
Pretreatment 1
Pretreatment 2
Pretreatment 3
Pretreatment 4
1
80
92
82
73
2
91
77
62
71
3
83
5
0
0
4
9
0
0
0
5
0
0
0
0
The shear strength of the specimens bonded with different adhesive mixtures, tested after different
pre-treatments and bonded with different press times, are shown in Table 5. The wood failure
values of these specimens are shown in Table 6. As can be seen from Table 5, the values decreased
drastically after the different pre-treatments. Different press times affected the shear strength values
of adhesive mixtures 3 and 4. Due to the longer press times, adhesive mixtures 2, 3 and 4 attained
the requirements for C1 durability class. All the other values decreased rapidly and did not meet the
requirements for any of the durability classes according to the standard.
Table 5: Shear strength [N/mm2] of the tested specimens after different pre-treatments and different press times
Adhesive mixture
Press time [s]
Pretreatment 1
Pretreatment 2
Pretreatment 3
Pretreatment 4
2
660
11,32
6,7
6,45
6,13
3
720
10,1
3,74
1,93
2,74
4
1200
10,12
5,41
0
0
5
2400
4,6
0
0
0
Table 6: Wood failure [%] of the tested specimens after different pre-treatments and different press times
Adhesive mixture
Press time [s]
Pretreatment 1
Pretreatment 2
Pretreatment 3
Pretreatment 4
2
660
95
96
90
92
3
720
48
13
2
3
4
1200
93
11
0
0
5
2400
35
0
0
0
CONCLUSIONS
The bond performance of specimens from beech wood, bonded with an adhesive mixture made
of liquefied wood and phenolic resin, was investigated. The bond shear strength of the dry
specimens increased if 25 % of the phenol-formaldehyde adhesive was replaced by liquefied
wood, but decreased if a higher proportion of liquefied wood was used. When testing the
specimens after immersion or boiling in water, the bond strength decreased rapidly in the case of
the samples which had been bonded using adhesive mixtures containing more than 25 % of
liquefied wood. Longer press times contributed to the shear strength values for adhesive
mixtures containing up to 75 % of liquefied wood. Up to 75 % of phenol-formaldehyde resin can
be replaced to attain the requirements of the standard for C1 class, whereas these adhesive
mixtures are not suitable for other requirements. Longer press times contribute only in the case
of adhesive mixtures which are used for interior premises, where the moisture content does not
exceed 15 %. It can be concluded that, in the case of solid wood to be used for non-structural
„Hardwood Science and Technology”
The 4th Conference on Hardwood Research and Utilisation in Europe 2010
applications in dry conditions, up to 25 % of the synthetic phenol-formaldehyde resin can be
replaced by liquefied wood if satisfactory bonding is to be achieved.
ACKNOWLEDGEMENTS
The authors acknowledge the financial support of the Slovenian Research Agency, through
Project J4-2177. Special thanks go to undergraduate student Aleš Ciber, for technical support of
the investigations.
„Hardwood Science and Technology”
The 4th Conference on Hardwood Research and Utilisation in Europe 2010
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