Content uploaded by Xiang-Ming Wang
Author content
All content in this area was uploaded by Xiang-Ming Wang on Jun 21, 2016
Content may be subject to copyright.
Wood Science and Technology 29 (1995) 253-266 Springer-Verlag 1995
The effects of temperature and humidity on
phenol-formaldehyde resin bonding
X.-M. Wang; B. Riedl; A. W. Christiansen; R. L. Geimer
Summary
The effects of temperature and relative humidity on phenol-formaldehyde
resin bonding were evaluated. Two flakes in a lap-shear configuration were bonded
under an environment of controlled temperature (110°C, 120°C, 130°C, 140°C) and
relative humidity (41%, 75%, 90%) for a series of time periods (0.25 to 16 min). The
lap-shear specimens were then shear-tested on a mechanical testing machine and the
results were used to establish a family of bond strength development curves at each
temperature and level of relative humidity. At 110°C, the higher relative humidity
appeared to retard resin bonding. The effects of relative humidity diminished as
temperature increased to 140°C. Bond strength development was chemical rate-
controlled. The rate of bond strength development at each relative humidity follows
a first order reaction mechanism. The activation energy of resin-wood bonding,
determined by bonding kinetics, was higher than that of resin alone, determined by
differential scanning calorimetry. This comparison indicates that to form a strong
resin-wood bond, a higher energy level might be required.
Introduction
Phenol-formaldehyde (PF) resin is a widely used thermosetting adhesive for
exterior-grade wood composites. During the wood-composite manufacturing process,
the resin undergoes polymerization reaction with itself and chemical reaction with
Received 9 May, 1994
Xiang-Ming Wang (Graduate Student)
Bernard Riedl (Associate Professor)
Departement des Sciences du Bois
Centre de Recherche en Sciences et Ingénierie des Macromolécules
Faculté de Foresterie et de Géomatique
Université Laval, Québec, Canada G1K 7P4
Alfred W. Christiansen (Chemical Engineer)
Robert L. Geimer (Research Technologist)
USDA Forest Service, Forest Products Laboratory
One Gifford Pinchot Drive, Madison, Wisconsin, 53705-2398 USA
Correspondence to:
Prof. Bernard Riedl
This material is based on work supported by the Ministry of
International Affairs, Quebec Government, the Natural Sciences and
Engineering Research Council of Canada, and Laval University (Quebec
City).
The
work was also supported by the U.S. Department of
Agriculture under research joint venture agreement FP-92-1835
wood under various environmental conditions, which comprise temperature, relative
humidity (RH), moisture content (MC), and water vapor pressure. These variables may
significantly affect the resin curing and bonding behavior and consequently affect the
final performance of wood composites. In addition, the resin bonding is also influenced
by some wood-related factors, such as density and porosity, shrinking and swelling,
surface texture and chemistry, and wettability. Therefore PF resin bonding to wood is
a complex process. Economic considerations require both resin suppliers and
composite manufacturers to understand the basic interaction of the resin and wood
under certain manufacturing conditions in order to provide the most appropriate resin
or to optimize process variables to produce the best quality boards.
The cure process of PF resin can be described as the conversion of small molecules to
large molecules through the processes of chain extension, chain branching and
crosslinking, which finally result in a three-dimensional network of infinite molecular
weight (Provder 1989). A number of indirect analytical techniques have been used to
characterize the cure of PF resins by responding to either the chemistry or physics of the
curing process. Such techniques include: (1) nuclear magnetic resonance (NMR)
(Woodbrey et al. 1965; Maciel et al. 1984); (2) Fourier transform infrared (FTIR)
spectroscopy (Myers et al. 1991); (3) ultraviolet spectroscopy (Chow 1969; Chow and
Hancock 1969; Chow and Mukai 1972); (4) differential thermal analysis (DTA) or
differential scanning calorimetry (DSC) (White and Rust 1965; Burns and Orrell
1967; Kurachenkov and Igonin 1971; Chow 1972; Chow et al. 1975; Kay and Westwood
1975; Christiansen and Gollob 1985); (5) torsional braid analysis (TBA) (Steiner
and Warren 1981; Kelley et al. 1986); and (6) dynamic mechanical analysis (DMA)
(Young et al. 1981; Young 1986a; Young 1986b; Follensbee 1990; Geimer et al. 1990; Kim
et al. 1991; Follensbee et al. 1993; Christiansen et al. 1993). The resin samples measured
by these indirect methods are usually in the forms of pure liquid or solids, mixtures
with various portions of wood powders, and resin-impregnated glass cloth or wood
wafers.
Optimum conditions for resin curing determined by these methods may not be
suitable for optimum resin bonding, because of the interaction between the resin and
wood. To address this concern, a direct method to evaluate resin-wood bonding is
required. Humphrey and Ren (1989) and Geimer et al. (1990) separately developed
techniques to follow the strength development of a resin-bonded joint under controlled
isothermal and isohydro conditions. These studies showed that an optimum
environmental condition exist for resin bonding. Humphrey and Ren (1989) studied the
effects of temperature and equilibrium moisture content (EMC) of wood on bond
strength development of powdered PF resin between two wood disks. They observed
that at 100°C or 115°C, 10% EMC appeared to be optimal for the resin bonding,
whereas a lower EMC
(4%)
significantly retarded resin bonding and a higher EMC
(16%) caused relatively low bond strength by reducing the chemical reactivity of the
resin. Geimer and Christiansen (1994) investigated liquid PF resin bond development
between aspen flakes at
0%, 41%,
or
91%
RH at 115°C bonding temperature. They found
that bond strength increased with increased bonding relative humidity from
0%
to
41%,
but the lowest bond strength was obtained at 91% RH.
In this study, lap-shear joints were bonded in an environment of controlled
temperature and relative humidity for a preselected range of pressing times. The PF
bonded specimens were tested in tensile shear at room temperature and the results were
then used to construct bond strength development curves. In addition, the rate of bond
strength development at each relative humidity was used for the evaluation of bonding
kinetics.
Experimental
Resin and substrate preparation
The resin synthesis procedure, which has been reported in detail in a separate paper
(Wang et al. 1994), is briefly described here.
The PF resin used in this study is a 2-part laboratory-synthesized phenol-
formaldehyde resin. Part 1 is a low molecular weight, methylolated phenol oligomer.
Part 2 is a more condensed phenol-formaldehyde resin. The two parts of the resin were
prepared separately and then mixed together in a 1:1 ratio by volume. The mixed PF
resin is henceforth referred to as PF resin. The initial resin properties are shown in
Table 1. Gelation time was measured by heating a 2-ml resin sample in a 19.5-mm I.D.
test tube at 125°C (± 5°C) and is quoted in minutes between the start of the test and the
point at which bubbles ceased moving upwards.
Aspen
(Populus
spp.) flakes, with dimensions 15 mm wide by 0.89 mm thick by 76
(or 70) mm long, were used as adherends in the adhesion study. The wood grain
direction was paralleled to the length. To reduce the influence of the surface texture of
wood (variation of springwood-summerwood) on the bond strength (Marian et al.
1958), all flakes were prepared from ambiently conditioned quarter-sawn blocks. First,
the 15 × 55 × 76 mm (or 15 × 55 × 70 mm) blocks were put into a pressure vessel, which
was then filled with hot water (100°C or less), and a vacuum of 760 mm-Hg was applied
for 30 min to draw air out of the wood. Next, the wood was soaked by applying
a pressure of 3100-3600 mm-Hg in the vessel for one and a half hours. After this
vacuum-pressure soak (VPS) cycle, the wetted wood blocks were cut into flakes by using
a small, manual microtome. During cutting (knife edge parallel to the grain), it was
found that there were some visible lathe checks on the flake surfaces, even when a newly
sharpened knife was used. To eliminate these lathe checks, a very thin layer of wood was
sliced away from the top of wood block after each flake cut. This procedure guaranteed
that each flake had at least one very smooth surface, to which the resin was to be glued.
After being cut, the wet flakes were restrained between glass plates to keep them flat and
straight. These flakes held by the glass plates were then dried in an oven at 105°C for
2–3 hours, until their moisture content reached nearly zero. All flakes were conditioned
to 50% RH (22°C) for at least 24 hours before being used.
Methods
Two flakes to be bonded were both coated with liquid PF resin on their smooth surfaces.
The amount of resin, 17-21 mg, applied on a 15 × 15 mm area at one end of each flake,
was controlled to give a thin, uniform and continuous resin film. After an open
assembly time of 20 min, the two specimens were lapped over the length of their coated
ends.
The over-lapped flake assembly was prepared for bonding in a steam treatment
chamber (Geimer et al. 1990), which is an especially designed environmental treatment
chamber that can provide a wide variety of constant relative humidity environments at
various temperatures. The chamber allows curing or bonding to be carried out at
a relative humidity of up to
100%
at any temperature below 180°C, or up to
62%
RH at
the maximum temperature of 200°C. The procedure for bonding a lap-shear specimen
we followed is outlined here. First, a desired bonding condition, i.e., a controlled
temperature and relative humidity, was established in the treatment chamber. Before
opening the chamber port for introducing the specimen, the vapor pump which was
used to circulate the humidified air or superheated steam through the chamber was
turned off. After quickly inserting the specimen into the chamber, a bonding pressure of
2.05 MPa was applied to a lap-bonded specimen (on its bonding area of 15 × 15 mm) by
a pair of 6.5-cm diameter stainless steel piston heads. Following that, the chamber was
immediately closed and the vapor pump was turned on. The bonding time is defined as
the period between the moment of turning on the vapor pump and the moment prior to
reopening the chamber port for removing the specimen. It took about 10 seconds to
place the specimen in the chamber, from the port opening to closing, or to retrieve the
specimen. A thermocouple attached to one piston head indicated the exact temperature
in the vicinity of the bonding area. The bonding conditions are given in Table 2.
After bonding, all specimens were reconditioned at
50%
RH (22°C)
for
at least 24 hr
prior to testing. The tensile lap-shear bond strength tests were performed on an Instron
mechanical testing machine at a strain rate set at 10 mm/min. The specimen was
vertically clamped between two serrated grips. The length between the grips was
73 mm.
This configuration has been shown to be most suitable for a flake bonding study with
a 15-mm lap length of specimen (Geimer and Christiansen 1994). For each test,
maximum tensile load and extension were recorded, from which curves of bond
strength development were obtained as a function of bonding temperature and relative
humidity. Under some bonding conditions, the experiment was repeated, while the data
were presented on the plots as individual points instead of averages. Although curves of
bond strength development made were based on computer fitting of data, some
man-made changes have been made to show the trends as we thought to be more
representative. The rate of bond strength development at each relative humidity was
then used to calculate an activation energy for flake bonding.
Heats of curing of the pure liquid PF resin were measured in a Mettler DSC 20 with
Mettler TA4000 Thermal Analysis System. About 10 mg of liquid resin was hermetically
sealed in a large capsule and scanned from 30°C to 250°C at a heating rate of 5°C/min.
The values of activation energy were calculated on the basis of solid resin weight and
a simple kinetic model. Comparisons of activation energies determined by DSC and by
the rate of bond strength development were used to explain the resin curing and
bonding behavior in the presence of wood.
Results and discussion
Bond strength development at 110°C
The relationship between the bond strength of the partially cured or bonded specimens
and press time (which we will henceforth referred to as a bonding curve), as a function
of relative humidity in the steam treatment chamber at 110°C, is shown in Fig. 1. The
result indicates that the extent of bond strength development was highly dependent on
the environmental relative humidity. Bond strength of PF resin built up faster and
achieved finally higher bonding at 41% RH than it did at 75% RH or 90% RH. Bond
strengths developed in the early stages of cure were higher at
90%
RH than at
75%
RH.
However with cures extended beyond 10 min. those bond strengths developed at
75%
RH were stronger. Retardation of resin bonding at the higher relative humidities can be
attributed to excess moisture present in the bondline, which likely dilutes the reactive
components of the resin and causes excessive resin penetration into wood. Although
bond strength development at relative humidities below
41%
RH was not measured in
this study, it could be estimated by comparison with a similar study by Geimer and
Christiansen (1994), which showed that an optimal resin bonding condition was 41%
RH when cured at 115°C. In our case, the bonding temperature is 110°C, which is close
to the 115°C they used. It is possible to state that a certain level of relative humidity
(around
41%)
is necessary for optimal resin bonding at a temperature around 110°C. So
if an optimum relative humidity or moisture content exists for PF resin bonding, it
would favor the interaction between resin and wood, improving resin hydrodynamic
flow while allowing maximum resin chemical reactivity.
The effect of environmental relative humidity at 105°C on the cure of the same
PF resin as used in this study was measured by DSC in our previous study (Wang
Bonding time (min)
Fig. 1.
Bond strength development of phenol-formaldehyde resin on aspen flakes at 110°C
and
41%, 75%
or
90%
RH. All lap-shear specimens were conditioned at
50%
RH (22°C) for at least
24 hr prior to test
et al. 1994). We found that the resin cured faster at 90% RH than it did at 75% or 41%
RH. An interesting point is the effect of relative humidity on the resin cure showing
a different trend as to its effect on the resin bonding. This indicates that optimum
conditions for resin cure are not necessarily the same as those which promote the best
resin-wood bonding.
Bond strength development at 120°C
The bond strength development at 120°C is shown in Fig. 2. An increase in bonding
temperature of only 10°C caused significant changes
in
the bond strength development
overall as compared with the results shown in Fig. 1. The minimum times to achieve
bonds at 120°C were 1 min at
41%, 75%
or
90%
RH; these were much less than the times
required at 110°C: 4 min at
41%
and
90%
RH, and 6 min at
75%
RH. The bond strength
at 120°C increased linearly with increasing press time up to approximately 4 min at
each relative humidity. Increasing exposure time beyond 4 min did not improve bonds
made at 90% RH. However increasing time did improve bond strengths at the lower
relative humidities, but at a reduced rate. The absolute differences in bond strength
buildup caused by bonding relative humidity at 120°C were much less significant than
at 110°C. In Fig. 2, it is also observed that the bonding strength curves at
41%
and
75%
RH
came closer to each other with an increase in press time, especially after 6 min. The
maximum bond strength obtained at 90% RH at 120°C was also higher than it was at
110°C. This effect is probably caused by less resin cure at 110°C. The level of cure
attained by phenolic resins is limited not only by time but by the temperature during
cure, as shown by Nachtrab (1970) for phenolic novolacs and Schindlbauer and others
(1976) for phenolic resoles. The results indicate that
41%
was still the favored relative
humidity for resin bonding at 120°C but less determinant than it was at 110°C (Fig. 1).
Bonding time (min)
Fig. 2.
Bond strength development of phenol-formaldehyde
resin on aspen flakes at 120°C and
41%, 75%
or
90%
RH. All lap-shear specimens were conditioned at
50%
RH (22°C) for at least
24 hr prior to test
The importance of relative humidity thus decreased with increasing temperature. This
relationship appears to be dependent on the rate of resin cure, as will be shown in the
following discussion.
Bond strength development at 130°C and 140°C
Increasing bonding temperatures from 110°C to 140°C caused the bond strength to
develop at a much higher rate, while the absolute inhibiting effect of relative humidity
on bonding greatly decreased, as seen in Figs. 3 and 4. Formation of bonds took only
0.5 min at 130°C and 0.25 min at 140°C. Bonding curves showed linearly rising
strengths versus bonding time at each bonding condition, and the strengths reached
their plateaus at about 2 min at 130°C and 1 min at 140°C. Evaluations of bond strength
beyond the early linear regions at each bonding temperature were limited due to wood
substrate failure or partial wood failure (at the interface and in the phases of the wood
and resin) when the specimens were tested. In other words, there was often cohesive
failure in the wood, which suggests the bonds were at least as strong as the bulk wood
phase. So the actual bond strengths are possibly higher than the values shown in the
latter regions.
Bonding conditions vs. failure patterns for all measurements are summarized in
Table 2. Maximum strength of the tensile lap-shear specimens was between 60 and 85 kg
for all specimens when partial or full wood failure occurred. Before reaching the bond
strength plateaus or conditions where flake (wood) failure occurred, the bond strength
development as a function of bonding time is characterized by actual failure in the
bond. The inherent tensile strength of a single aspen flake with a length of 76 mm and
the same cross-sectional area as other flakes used in this study was approximately 120
kg. The most likely cause for the lower failure loads on the bonded joints, compared to
Bonding time (min)
Fig. 3.
Bond strength development of phenol-formaldehyde resin on aspen flakes at 130°C and
41%, 75% or 90% RH. All lap-shear specimens were conditioned at 50% RH (22°C) for at least
24 hr prior to test
strengths of simple flakes, is that the structure of the joint leads to peeling forces and
stress concentrations at the ends of lap-shear joints. The effect of stress concentrations
is evident in the pattern of failures among tested flakes. Wood failure in almost every
case occurred at the ends of the overlap. Other factors can also affect bond strength.
Variations in the temperature and relative humidity used during the bond pressing
period can cause dimensional changes in the flake adherend and/or bondline, affecting
the stress concentration as Krueger (1981) noted. Dimensional changes in the flakes can
either strengthen the wood by increasing specific gravity or decrease the strength of the
wood by inducing damage (Price 1976; Geimer 1985). In our study it was observed that
the thickness of lap bonded flakes decreased with increasing bonding temperature,
relative humidity, and time. As techniques become more refined, this side effect of
pressing environment variables on the adherend may be shown to be an important part
of the total bonding picture.
By comparing the results in Figs. 1 to 4, it is observed that scattering of data occurs
under almost every bonding condition. The variability of the data appeared more
extensive in Fig. 4 for
41%
RH than for
75%
or
90%
RH. We do not know the reason for
this, but there are many factors which could cause the variability of the data beside the
aforementioned stress concentrations. These factors include individual flake strength
and surface texture, combined effects of temperature and humidity on resin penetration
and steam hydrolysis of wood, variability in bonding chamber operation, indentation
during bonding of one flake by the other where the laps end, poor alignment of flakes in
the specimen, and possible problems during testing such as poor specimen alignment
or nonlinear force fields from bending of the flexible adherends. More data would be
necessary to clarify this issue.
Bonding kinetics
Wood contains active hydroxyl groups which enable a PF resin to bond to it in a high
temperature environment. Thus, the activation energy required to form a resin-wood
bond is influenced by both resin and wood factors. A larger value of activation energy in
wood-resin formation, as compared with curing of the resin alone, may indicate that
bond formation is actually retarded, rather than enhanced, by the presence of wood.
Bond strength development at bonding temperatures of 110°C, 120°C, 130°C and
140°C is summarized for the
90%
RH data in Fig. 5. Each bonding curve shows a linear
slope prior to reaching its plateau. Since the bonding rate, expressed as the slope of the
initial line, increased with increasing bonding temperature, it is possible to correlate the
bonding rate to bonding temperature. A plot of the logarithm of the bonding rate
(kg min-1) versus the reciprocal of absolute temperature is illustrated in Fig. 6. Since
the plot shows an excellent linear correlation (r2 = 0.99) at this humidity bonding
condition, we believe that the bond strength development follows classical kinetics, and
the activation energy can be calculated by using the Arrhenius equation:
where φ is the rate of bond strength development (kg min-1), A is the pre-exponential
factor, Ea is the activation energy (kJ mol-1), R is the universal gas constant (8.314
J K-1mol-1) and T is the absolute temperature (K).
The same approach was also used to calculate activation energy for bond formation at
41% and 75% RH. All kinetics results are shown in Table 3. The activation energies
calculated at 41%, 75% and 90% RH were 93, 99 and 98 kJ mol
-1
, respectively. Note that
little resin cure information at exposure times less 4 min was obtained for exposures at
Bonding time (min)
Fig. 5.
Bond strength development as a function of bonding temperature (110°C, 120°C,
130°C and 140°C) at
90%
RH. All lap-shear specimens were conditioned at
50%
RH (22°C) for
at least 24 hr prior to test
110°C. The activation energies observed in this study were very close to the value of
96 kJ mol-1 measured by Humphrey and Ren (1989) for the activation energy of bond
strength development of a powdered PF resin. Since the activation energies measured
here for the three relative humidities are close to each other, it indicates that the
bonding relative humidity had little effect on the rate of resin bond strength
development, especially between
75%
and
90%
RH. Considering Figs. 1-4 again, we note
that the early linear parts of the bonding curves, which are used to calculate the rate of
bond strength development (as shown in Fig. 6), are generally parallel to each other and
have the same slope, or rate, at each temperature and for all the three levels of relative
humidity. But the bond strengths at different RH values were more widely separated at
lower temperature; however this influence of humidity diminished with increasing
temperature. This indicates that relative humidity did influence the extent, but not the
rate of bond strength development in the early stages (before 4 min) at lower
temperatures (110°C – 120°C), and humidity did limit the maximum strength
development. These effects are relatively important in curing composite board resins
where heat transfer depends on the development of successive steam fronts
progressively moving into the center of the panel from the heated faces.
Although the effect of environmental relative humidity on bond strength
development could not be detected by the simple evaluation of activation energy, it still
reveals some important information about resin curing in the presence of wood and
about formation of a strong resin-wood bond. The activation energy of the liquid PF
resin determined by DSC was 80 kJ mol-1, as shown in Table 3. Since this Ea value is
smaller than that from the flake bonding, it indicates that the formation of a resin-wood
bond may require a larger energy than resin cure alone. This result agrees with Chow
(1969)’s finding in his kinetic study by ultraviolet spectroscopy of the polymerization of
PF resin mixed with wood powders. Chow (1969) pointed out that the polymerization of
PF resin involved two steps: (a) the substitution reaction between wood carbohydrate
and resin, (b) the condensation reaction of resin with resin. The former required about
one half the activation energy (28 kJ mol-1) required for resin alone (47 kJ mol-1).
Since the wood-resin reaction reduces the probability of reaction between resin
molecules, Chow (1969) proposed that to complete the resin cure or form a strong bond,
a higher energy level is required than for a resin-resin bond.
The chemical interaction between resin and wood also varies with wood species.
Mizumachi and Morita (1975) compared the activation energy of the curing reaction of
a dry PF resin alone (75 kJ mol-1) with that of the PF resin filled with various wood
powders, using differential thermal analysis. They found that some wood species
enhanced the resin curing reaction (activation energy ranged from 59-71 kJ mol-1),
and some inhibited resin cure (96–109 kJ mol-1), and some seemed to have little effect
on resin cure (75-88 kJ mol
-1
). The effects of woods on the curing reaction of PF resin
might be attributed to the wood extractives (Mizumachi and Morita 1975). Extractives
may become a serious problem to resin bonding when concentrated on the wood
surface even at low contents. Excessive amounts of water-soluble extractives may dilute
the resin system (Wellons 1981). Some resinous or oily extractives may partially block
resin bonding or diffusion (FPL 1987). Extractives also influence the chemical activity of
a wood surface. Since basic conditions are required to complete cure of PF resins
(resoles), extractives making the wood surface very acidic possibly inhibit the resin
cure, and the reverse case would favour the resin cure (Wellons
1981;
FPL 1987).
PF resin curing and bonding in the presence of wood is a complex process. Any
changes in wood aspect (such as wood surface chemistry and texture) or resin aspect
(such as viscosity and composition) may influence the rate of resin cure and bonding
quality. Kinetic study of PF resin will provide a useful and more direct way in
understanding the basic interaction between wood and resin. This information will also
help the manufacturers of wood composites to develop or formulate a suitable adhesive
for a particular wood species.
Conclusions
The bonding of PF resin under various environmental temperature and relative
humidity has been studied. The important points obtained from this work can be
summarized as below.
1. At 110°C, 41% environmental relative humidity provides better bonding
conditions than relative humidities of 75% or 90%. With increasing temperature, the
bond strength developed faster and the effect of relative humidity became less
significant.
2. Condition for optimum resin cure determined by a DSC measurement may not be
suitable for the resin-wood bonding due to the interaction between the resin and wood.
3. The activation energy required to form a resin-wood bond is larger than that
required for the resin cure alone.
4. The technique to follow the bond strength development between two flakes in
a lap-shear configuration provides a direct way (a) to monitor and characterize resin
bonding processes and (b) to understand the basic interaction between wood and resin
under various environmental conditions. Information similar to that acquired in this
study is helpful to resin manufacturers to optimize resin for specific wood composite
manufacturers, and on the other hand, wood composite manufacturers who are seeking
to improve their products by optimizing manufacturing conditions.
