Project No. UNB50
Evaluation of Shear Strength and Percent Wood Failure
Criteria for Qualifying New Structural Adhesives
Value to Wood No. UNB50
Research Report 2007
by
Huining Xiao
Professor
Department of Chemical Engineering
Wenchang Wang
Graduate Research Assistant
Faculty of Forestry and Environmental Management
and
Ying H. Chui
Director and Professor
Wood Science and Technology Centre
Faculty of Forestry and Environmental Management
University of New Brunswick
July 2007
This report was produced as part of the Value to Wood Program,
funded by Natural Resources Canada
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Notice
This report was prepared with financial assistance from the Canadian Forest Service,
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© 2007 University of New Brunswick All rights reserved.
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Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Abstract
The traditional adhesives used in manufacturing of engineered wood products (EWP)
were phenolic-based. Recently wide spread use of non-phenolic adhesives, such as
polyurethane (PUR), has been observed in the engineered wood products industry. In
order to facilitate the adoption of the new generation of non-phenolic structural adhesives,
a performance-based standard, CSA O112.9 has been published by the Canadian
Standards Association for evaluation of structural adhesives for use under ‘wet’
conditions. A parallel standard is being developed for ‘dry use’ applications. The new
CSA standard adopts the traditional evaluation protocols and criteria for evaluating
structural adhesives, including delamination resistance, block shear strength and percent
wood failure. Concerns have been expressed by manufacturers of adhesives and
engineered wood products producers that these traditional protocols and criteria may not
be appropriate for the new structural adhesives, such as polyurethane. One plausible
reason for the concern was that bonding mechanism and penetration characteristics are
different between the traditional phenolic-based and the new generation of structural
adhesives. This project attempted to study the wood-adhesive bond force and penetration
characteristics of phenolic and polyurethane adhesives, and correlate them with shear
strength and percent wood failure of glue joint specimens. This project was linked to a
parallel Forintek Value to Wood project, FCC53. The specific objectives of this project
are:
1. To develop techniques of directly quantifying wood-adhesive bond force and
adhesive penetration in wood using advanced analytical techniques.
2. To correlate the wood-adhesive interaction parameters listed in 1 with percent wood
failure and block shear strength of specimens.
3. If appropriate, to recommend alternate means of evaluating new structural adhesives
for strength requirements.
The initial intent was to use Atomic Force Microscopy (AFM) to study wood-adhesive
bond force, but this was found to be unsuccessful. Therefore the focus of this project was
on understanding the influence of adhesive penetration on shear strength and percent
wood failure of adhesive joints, and how these properties are influenced by wood
properties such as density and moisture content.
This project was conducted in 3 phases. The first two phases studied the adhesive
penetrations characteristics and its possible influence on shear strength and percent wood
failure, using glued joints made with difference adhesives and wood species. Penetration
characteristics were measured using Confocal Fluorescent Scanning Laser Microscopy
3
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
(CSLM) and X-ray techniques. The adhesives used in Phase 1 were melamine
formaldehyde (MF) and polyvinyl acetate (PVAc), and the substrates were lodgepole pine,
Douglas fir and black spruce. The first part of this phase was devoted to the development
of the test procedures for Confocal Fluorescent Scanning Laser Microscopy (CSLM) and
Micro-CT. MF and PVAc were used as reference adhesives in the development of the
new CSA standard CSA O112.10, a performance based structural adhesive standard
intended for applications under limited exposure conditions. The adhesives used in Phase
2 were phenol resorcinol formaldehyde (PRF) and polyurethane (PUR), and the wood
species were black spruce and hard maple. In both phases the adhesive penetration
characteristics as well as glue line thickness were measured using CSLM. Because CSLM
is tedious to apply, X-ray techniques were also used to measure these characteristics as an
evaluation of its suitability for use as a more rapid method of measurement. Block shear
tests were carried out on the glued joint specimens that were evaluated for adhesive
characteristics. Phase 3 was a separate study aimed at correlating the long-term behaviour
of glued joints with that observed under various accelerated aging process such as
freeze-thaw and pressure-vacuum soak. The specific objective of this phase is to evaluate
the performance of glued joints that have been subjected to different exterior exposure
durations and to correlate these performance characteristics to those observed under
accelerated aging process. Two adhesives were used, PRF and PUR. Black spruce was
used as the substrate. The duration of exposure ranged from short-term to 5 years.
Therefore not all test results from Phase 3 are available at the time of writing this report,
and some of the specimens will be evaluated after the end of this project.
The key findings of this project are summarized below:
1.
General
• Fluorescent microscopy has proved to be a powerful tool in revealing penetration
of the polymeric adhesive into wood substrates quantitatively. With the
appropriate selection of a fluorescent dye, the distinction between wood and
adhesive is clear under CSLM, which enables us to obtain more detailed
information such as the location of resins and penetration depth.
• When a glue joint is prepared with the two substrates oriented horizontally,
penetration of adhesive in the lower substrate is generally higher than the upper
substrate. The difference in glue penetration is dependent on adhesive and wood
substrate characteristics. In general, those adhesives with a low viscosity exhibit a
large glue penetration values.
• Comparing the results between joints made with phenolic and polyurethane
adhesives, it appears that the smaller adhesive penetration of polyurethane
adhesive could well explain the lower shear strength and percent wood failure of
the latter. However while there is a big difference in percent wood failure, there is
4
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
a only a small difference between shear strengths of joints made with these two
adhesives. Accordingly, it may be necessary to review the criteria for percent
wood failure for adhesives that provide sufficient strength and durability but have
a lower percent wood failure because of its shallow glue penetration.
2.
Melamine formaldehyde (MF) and polyvinyl acetate (PVAc) adhesives with
Douglas fir, lodgepole pine and black spruce as substrates
• Dry shear strength of lodgepole pine specimens is higher than Douglas fir
specimens for both adhesives.
• There is a large difference in substrate glue penetration for spruce and
Douglas fir, but not for lodgepole pine.
• Glue penetration of MF is about 3 to 4 times that of PVAc for all three species.
• For MF adhesive, specimens fabricated using maximum assembly time
generally achieve a higher shear strength than those fabricated using minimum
assembly time.
3. Polyurethane adhesive with black spruce substrate
• There is no evidence to suggest a relationship between shear strength and
percent wood failure.
• Glue penetration in low density wood is higher than that in high density wood.
• Shear strength increases slightly as glue penetration depth decreases. However
this trend may be misleading as the change in shear strength may be caused by
the increase in wood density than the reduction in glue penetration.
• Glue joints fabricated with a higher initial wood MC (15%) during fabrication
generally achieve higher shear strength and percent wood failure than those
with a lower initial MC (6%).
• Temperature seems to have little effects on penetration depth. Overall,
increasing temperature to increase glue penetration may not be an effective
approach for PUR adhesives.
4. Polyurethane adhesive with hard maple substrate
• The influence of wood density on shear strength is opposite to that for black
spruce, i.e. when density increases shear strength decreases. This appears to be
related to glue penetration, since glue penetration decreases with wood density.
For high density wood, extremely poor glue bond is obtained.
• Glue penetration in hard maple is substantially less than that in black spruce.
• As in the case of black spruce specimens, shear strength is higher for the
specimens with 15% initial MC than 6% initial MC.
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Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
5.
Glue penetration values measured by X-ray technique
• X-ray technique over-estimates both the glue penetration and glue line
thickness. Further work is necessary to evaluate the reason for this
discrepancy before the technique can be used to measure these properties with
confidence.
6. Phenol formaldehyde adhesive with black spruce and hard maple substrates
• In general the results on wood property influence are similar to those of black
spruce. Shear strength increases and glue penetration decreases as wood
density increases. Increasing initial moisture content of wood also has a
positive effect on shear strength and glue penetration.
• On average, wet shear strength, percent wood failure and adhesive penetration
of PRF joints are greater than the corresponding values for PUR joints.
• Joints with low density hard maple wood and at a low moisture content have
low shear strength due to over-penetration of glue
7.
Growth ring orientation
• There is evidence to suggest that shear strength of specimens with flat sawn
board is lower than specimens with non-flat sawn boards. Possibly the
presence of rays facilitate the penetration of the adhesive, as was observed
under CSLM.
• Among the samples with non-flat sawn boards, PUR penetrates deeper into
those with 90 degree growth ring orientation but there is no distinct difference
between those with 45 and 45-90 degree growth ring orientation.
8.
Correlation between durability of glue joints subjected to outdoor exposure and
accelerated aging
• For wet strength there was virtually no reduction in strength after 2 cycles of
boil-dry-freeze. After 8 cycles, the strength retention was 77% and 88%
respectively for PUR and PRF.
• For dry strength, PUR performed (78%) better than PRF (69%) after 2 cycles.
Strength reduction was almost identical for both adhesives after 8 cycles, with
only 37% of the short-term dry strength retained.
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Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Acknowledgements
The University of New Brunswick wishes to acknowledge the financial support of
Natural Resources Canada to this research project. Thanks are also due to industry
liaisons, Mr. Pierre Audet, Boise Cascade AllJoist and Mr Ken Koo of Jager Building
Systems for their support and technical advice.
Staff
-
Dr. Huining Xiao, Project Leader
Wencheng Wang, Graduate Research Assistant
Dr. Y. H. Chui, Professor
Dr. Felisa Chan, Research Scientist
Michael Albright, Manager
Andrew Sutherland, Support Staff Supervisor
Dean McCarthy, Chief Technologist
Dave Doherty, Technician
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Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
1. Background
Wood is a construction material with excellent properties for a variety of applications.
Today, improved fabrication techniques and design methods enable some contemporary
wood structures, such as small bridges, roofs, arches and domes, to be as permanent and
economically competitive as those constructed with other materials (Triantafillou 1997).
Engineered wood products (EWPs) have been a mainstay in the wood industry for many
years. Engineered wood is a better use of trees than solid wood, because it uses less wood
to make more wood products, or same quantity of wood with a better performance.
Common examples of engineered wood products are plywood, glued-laminated timber,
oriented strand board (OSB), wood I-joist and structural composite lumber (SCL). The
increased used of EWPs has in part been due to advances made in adhesive technologies.
The primary role of an adhesive in a product is to transfer the stress experienced by one
substrate to another. This means that an adhesive bond requires sufficient strength and
durability to hold the substrates together under a defined set of conditions. Generally,
strength and accelerated tests are used to evaluate the suitability of an adhesive for
structural applications. Prior to the 1990’s phenolic-based adhesives were the main
category of adhesives used for fabricating structural wood products. In Canada the
approval of phenolic-based adhesives follows the protocol given in a number of CSA
standards that were developed specifically for this category of adhesives. The
advancement made in adhesive technologies by the adhesive industry has led to the
development of non-phenolic adhesives that are considered to have adequate durability
and strength performance for structural applications. Wide spread use of non-phenolic
adhesives, such as polyurethane (PUR), has been observed in the engineered wood
product industry. The industry has cited improvement in process efficiency, colourless
glue line, ease of application and apparent higher finger joint strength (Frangi et al. 2004)
as the main reasons for switching to non-phenolic adhesives.
In order to facilitate the adoption of the new generation of non-phenolic structural
adhesives, a performance-based standard, CSA O112.9 (CSA 2004) has been published
by the Canadian Standards Association for evaluation of structural adhesives for use
under ‘wet’ conditions. A parallel standard is being developed for ‘dry use’ applications.
The new CSA standard adopts the traditional evaluation protocols and criteria for
evaluating structural adhesives, including delamination resistance, block shear strength
and percent wood failure. Concerns have been expressed by manufacturers of adhesives
and engineered wood products that these traditional protocols and criteria may not be
appropriate for the new structural adhesives, such as polyurethane. This project will
8
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
evaluate the appropriateness of the use of block shear strength and percent wood failure
and their acceptance levels for this type of adhesive, and, if appropriate, make
recommendations on how these protocols and criteria may be modified to more
realistically reflect the performance requirements of selected end products for strength
and durability of bond line. This project was linked to a parallel Forintek Value to Wood
project, FCC53.
The objectives of this project are:
4. To develop techniques of directly quantifying wood-adhesive bond force and
adhesive penetration in wood using advanced analytical techniques.
5. To correlate the wood-adhesive interaction parameters listed in 1 with percent wood
failure and block shear strength of specimens.
6. If appropriate, to recommend alternate means of evaluating new structural adhesives
for strength requirements.
The focus of this project is on understanding the influence of adhesive penetration on
shear strength and percent wood failure of adhesive joints. Although research on the
interactions between wood and adhesives has been ongoing for at least 75 years (Frihart
2004), there are still some critical aspects and how it may lead to durable bonds, which
are not well understood. Penetration of adhesives into the porous network of wood cells is
believed to have a strong influence on bond strength (Brady and Kamke 1988; Collett
1972; Jakal 1984; Marra 1992). After the adhesive has been applied onto the wood
substrate, it flows not only into the cells on the wood surface, but also into small cavities
and cracks and thus they can penetrate deeper into the wood and even into the cell lumen
and walls (Van den Bulcke, et al. 2003). With the use of an adhesive, damaged wood cells
can be repaired, and stresses can be more effectively distributed within a larger interphase
region.
The penetration of adhesives into wood is dependent on a wide range of variables. The
chemistry of the wood and adhesives has been studied extensively to learn about the
influences of the adhesion mechanisms on adhesive penetration (Mahlberg 1999; Frazier
2002, 2004; Frihart 2004). A large amount of work has also been done to investigate the
durability of the adhesive bond (Bendtsen et al 1978; Vick and Okkonen 1998; Frihart
2005). Both bonding and bond breaking steps were studied at cellular and nano-scale, in
addition to the larger spatial scale normally examined. Systematic work of the bonding
process, the forces upon glue line, and the locus of failure using different types of
adhesives and wood species were carried out in these studies. Moreover, spectroscopic
and microscopic analyses were used as important tools to understand bond formation and
failure. In the evaluation of adhesive bond quality, shear strength and percent wood
9
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
failure have traditionally been used to evaluate adhesives for structural applications.
In order to meet CSA O112.9 requirement for evaluating structural adhesives designed
for exterior applications, this research will focus on developing various approaches or
advanced techniques for characterizing the interaction between adhesive resin and wood
and penetration. For phenolic-based resins, there is a good correlation between shear
strength and percent wood failure. However, for polyurethane (PUR), this is not
necessarily the case. This project will help to shed lights on this issue.
2. Literature Review
The performance or behavior of a wood adhesive joint is dependent on a wide range of
variables, including surface smoothness of wood substrate, presence of wood extractives
and knots, wood characteristics (growth ring orientation and density), and penetration of
adhesives. These variables are also related to the environment, such as the level and rate
of change in both temperature and relative humidity. The bonding mechanism of
adhesives is due to complex chemistry of the cellulosic substrate, i.e. hydrogen bonding
with some adhesives and weak van der Waals forces with others. This section reviews
previous research on this subject.
2.1
Characterization of strength of adhesive joints
Since an adhesive transfers stress from one substrate to another through shear, shear test
is commonly used to evaluate the bond performance of adhesive joints. Block shear test
such as ASTM D905 (ASTM 1997) is commonly used to evaluate mechanical properties
of adhesive joints. From the shear test, in addition to shear strength, percent wood failure
on the fracture surface is also measured. The premise of measuring percent wood failure
is that structural adhesives are generally assumed to be stronger than the substrate.
Therefore for properly fabricated joints, the failure plane should be in the wood and not
in the glue line. Hence a measurement of the percent wood failure after shearing test
should provide a qualitative indicator of the bond performance. Vick and Okkonen (1998)
carried out tests to quantify the strength and durability of adhesive bond made with
one-part polyurethane adhesive. In their dry test, there were four commercial one-part
polyurethane adhesives and two wood species, Douglas fir and yellow birch, which
represent moderately high density softwood and hardwood species, with yellow birch
density being higher than Douglas fir. Typically, wood failure of a high density wood is
lower than that of a low density wood. The results from their tests, which were presented
as percent wood failure and shear strength, showed that wood failure was significantly
10
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
lower for higher density wood (yellow birch) than low density wood (Douglas fir). This is
because high density wood has higher shear strength which means there is a higher
probability that the glue line will be weaker than the wood, even though the adhesive
bond is prepared properly.
2.2
Characteristics of structural adhesives
There are several adhesives available for structural applications based on the different
end-use requirements. Each adhesive has different characteristics that should be evaluated
and selected to obtain the best property of the product. Beginning in the 1930s and
accelerated by the development in the industry, synthetic adhesives began to increase the
market share. Currently there are two common classes of structural adhesives that are
used by the EWP industries. These are phenolic-based, such as phenol-formaldehyde (PF),
resorcinol-formaldehyde (RF) and phenol-resorcinol-formaldehyde (PRF), and
polyurethane (PUR).
The phenolic-based adhesives have outstanding durability, which is derived from their
good adhesion to wood, the high strength of the polymer, and the excellent chemical
stability of the adhesive. Phenol formaldehyde (PF) polymers are the oldest class of
synthetic polymer, having been developed at the beginning of the 20th century (Detlefsen
2002). PF cures under elevated temperatures, and has been commonly used in making
wood composites such as plywood, oriented strand board (OSB) and laminated veneer
lumber (LVL). RF cures at relatively low temperature and has been widely used in
making laminated products, where curing by heat is difficult due to wood’s low thermal
conductivity and the thicknesses of laminated products. Since resorcinol is costly, this led
to the development of a related adhesive in which some of the resorcinol is replaced by
the cheaper phenol molecule, producing the phenol-resorcinol-formaldehyde (PRF)
adhesive. Resorcinol is highly reactive and so PRF adhesives are two-part cold setting
resins. PRF adhesives are widely used in wood lamination and finger jointing (Kreibich
et al. 1998).
Polyurethane (PUR) adhesives are more flexible mechanically than PRF adhesives. The
properties of PUR are determined by the choice of polyol, and the diisocyanate exerts
some influence. Isocyanates are a type of polyurethane adhesive in that they can react
with a variety of functional groups including hydroxyl groups found on cellulose, which
makes them excellent for bonding wood. When isocyanates react with diols, the result is
formation of a urethane bond. Isocyanates are usually low molecular weight compounds,
averaging 365 MW, in 100% solids converting to liquid form (Sellers 1994). PUR
11
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
adhesives are widely used in many bonding markets due to their good strength, flexibility,
impact resistance, and ability to bond different kinds of substrate. They can be used in
many applications. Some PUR glues are very sensitive to water and even sensitive to the
moisture in the air. These kinds of polyurethane are based on urethane pre-polymers made
by reacting an excess of methylene diphenyl diisocyanate (MDI) with a polyol such that a
small amount of isocyanate monomer remains. Free isocyanate groups in the adhesive
then react with moisture on substrate surface to complete the cure (Vick and Okkonen
1998). The reaction of isocyanate with water proceeds through intermediate steps to form
urea linkages with evolution of carbon dioxide, which causes the adhesive to foam and
expand and, if not properly controlled, PUR glue will react with water and become rigid.
This reaction will become acute especially in high moisture environment, which may
decrease the strength of the bond. Since the properties of PUR are dependent on the
choice of polyol, PUR adhesives with different functional groups and molecular weight
may vary significantly in terms of their behaviour.
The other less common structural adhesives are polyvinyl acetate (PVAc) and
melamine-formaldehyde (MF). PVAc is a typical thermoplastic adhesive that softens
under heat. It is cured by solvent loss and therefore has low durability and its bond
strength deteriorates at high humidity conditions. MF is a thermosetting adhesive which
is cured by reacting melamine with formaldehyde to form a durable and strong bond.
Unlike the phenolic-based adhesives, MF has a light colour appearance and can be
formulated to cure at room temperature. It has been used in making structural wood
products such as glued-laminated timber.
2.3 Potential methods of measuring adhesive penetration and
adhesion force
Some researchers indicated that the wood adhesive system needs to be considered at three
different spatial scales: millimeter and larger, micrometer, and nanometer (Frazier 2002,
Frihart 2004). Adhesive penetration is generally considered at the micro-level. Several
techniques have been used to visualize the depth of the adhesive penetration.
The use of X-ray to explore the properties of wood has become increasingly popular in
the last few years. The major and most distinguished feature of X-ray is its
non-destructive nature. X-ray is defined as electromagnetic radiation with a wavelength
between approximately 10-3nm and 10nm. In X-ray densitometry, the density of the wood
can be obtained by subjecting a sample to X-ray radiation. Since the densities of the
12
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
adhesives and wood are different, it is possible to apply this method to measure the
thickness of the glue line and adhesive penetration. During the scan, a density versus
sample length curve is obtained from which the penetration depth can be measured.
Morphological analysis is one of the most important factors in evaluating the quality of
adhesive penetration. A new nondestructive technique, confocal fluorescent scanning
laser microscopy (CSLM) or scanning laser microscopy has been put into use to
characterize the topography and morphology of the surface. CSLM can provide precise
quantitative measurement of the penetration of a resin into a wood substrate (Ling et al.
1998). With the aid of a fluorescent stain (or dye), CSLM can easily measure the
penetration of glue, enabling us to follow the movement of stain compound in wood
structure. The most distinct advantage of CSLM is in its ability to clearly resolve
penetration of the adhesives into the wood cells, rays, pores etc. According to Ducker et
al. (1992), the principle of confocal microscopy was first described by Young and Roberts
(1951). More recently, van den Bulcke et al. (2003) studied the migration of water-borne
coating into the wood substrate using confocal scanning laser microscopy (CSLM). With
the aid of a fluorescent stain or dye, CSLM can be used to distinguish the adhesive from
the wood (Frihart 2005) and because of its high resolution images and optical sectioning,
CSLM has proven to be a reliable tool to observe where and how deep the adhesive
penetrates into the wood structure.
The strength of a glue bond is dependent largely on the adhesion formed between
substrate material and adhesive. Adhesion between two surfaces has been suggested to be
a result of an interaction of two factors: intimate molecular contact and maximum
attractive force between the components (Attard et al 1999). The concept of intimate
molecular contact consists of the following theories: adsorption, diffusion, interlocking,
and weak boundary. The maximum attractive force theory involves the theories of
chemical bonding, acid-base, electrostatic, and weak boundary layer. While the
adsorption theory is based on the rules of spreading and wetting, the diffusion and
interlocking theories explain adhesion through physical contact (entanglements) and
inter-penetration (anchoring). In general, these theories deal with the fact that two
surfaces have to come into sufficient contact with each other so that the premises for
attractive forces are met. The chemical bonding theory explains that the covalent,
hydrogen, van der Waals, ionic, and metallic bonds are responsible for adhesion
(Mahlberg 1999).
Atomic Force Microscopy (AFM) is a promising tool for the investigation of interaction
and adhesion between two surfaces. Noel et al. (2004) introduced the principle of the
adhesion force measurement and conducted in situ estimation of the chemical and
13
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
mechanical contributions in local adhesion force measurement with AFM. Force
measurements can be conducted in the contact mode or tapping mode. The system is used
in detecting the deflection of a spring (or cantilever) bearing a tip at its end. The
deflection of the cantilever is detected by an optical device. Thus, one can obtain a
deflection distance (DD) curve. Moreover, provided that the spring constant of the
cantilever (ktip) and the vertical deflection (Δz) are known, we can calculate the force (F)
by using Hooke’s law, where,
F = k tip Δz
A schematic representation of a DD curve obtained is shown in Figure 2.1, whereby we
can distinguish different zones. In zone A, the cantilever is far from the surface and stays
in a state of equilibrium (no interaction with the surface). The cantilever deflection is zero.
During the approach toward (or withdrawal from) the surface, the tip interacts with the
sample and a jump-in (or jump-off) contact occurs (zones B (for loading) and F (for
unloading)). These instabilities take place because the cantilever becomes mechanically
unstable. Usually, for rigid surfaces, because of these mechanical instabilities, the
jump-in contact is not significant enough to determine attractive van der Waals forces.
However, the adhesion force (Fadhesion) between the tip and the sample is directly related
to the jump-off (Δzjump-off) through Hooke’s relation, assuming an elastic regime. This
force is calculated as follows,
Fadhesion = k tip Δz jump −off
Figure 2.1 - Schematic representation of a DD curve (Noel et al. 2004).
14
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Several methods have been used to study adhesion interactions between the surfaces. The
simplest method is to use a normal silicon or silicon nitride AFM tip. This approach is
limited and does not reflect the interaction between adhesives and wood surfaces.
Recently, chemically modified AFM probes were developed and shown to be highly
sensitive to the ionization state of surface functional groups. For instance, AFM carboxyl
terminated probe tips have been used to probe the electrostatic properties and isoelectric
point of microbial cell surfaces at high spatial resolution. D’Acunto and Ciardell (2005)
investigated adhesion forces between a phospholipid (Lecithin of Soya) and a series of
biodegradable polyurethanes (PUs) by means of an atomic force microscope.
The force sensing capacity of the AFM means that it also can be used for measuring
surface forces (Claesson et al 1996). However, measured force-distance profiles between
a scanning tip and a surface will be hard to interpret due to the difficulty of determining
the geometry relevant for the interaction zone. If the fine scanning tip is replaced by a
probe of well defined geometry, for example a spherical colloidal particle, then this
problem proves to be less severe. This technique, generally known as the "colloid probe
technique" was first devised by Ducker et al (1991, 1992) and it has been deemed a
powerful and popular approach (Figure 2.2). Using the colloid probe technique, Jones et
al. (2002) measured the pull-off forces between flat glass or silicon surfaces and silicon
AFM tips. The particles were attached to cantilevers with epoxy resin either by using a
micromanipulator. Wenzler et al (1997) studied hydrogen bond interactions between
hydroxyl- and thiol-terminated groups on the tip and surface by SiO2-coated AFM tips.
Now the tips manufactured by "colloid probe technique" have been commercialized.
Figure 2.2 - A schematic diagram of colloid probe applied in AFM (Claesson et al 1996).
15
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
3. Outline of Research Methodology
The long-term goal of this project is to evaluate the appropriateness of the use of block
shear strength and percent wood failure and their acceptance levels for both wet and dry
use applications, and eventually make recommendations on how these protocols and
criteria could be modified to more realistically reflect the performance requirements of
selected end products for strength and durability of bond line.
Specifically, this research is also intended to provide an understanding of the link
between the shear strength, percent wood failure, glue line thickness and glue penetration
for the two common types of structural adhesive: phenolic and polyurethane adhesives.
Another key objective is to identify the influence of wood characteristics (species, density)
and process parameters (moisture content, assembly time) on glue line thickness and
penetration, and in turn on shear strength and percent wood failure.
This study was conducted in three phases.
Phase 1
Phase 1 focused on two adhesives, melamine formaldehyde (MF) and polyvinyl acetate
(PVAc), applied on lodgepole pine and Douglas fir. It was performed to provide some
initial information for the development of CSA O112.10, a ‘dry use’ equivalent of CSA
O112.9. In this part of the project, glued joints were prepared for both glue penetration
measurement and block shear tests. All specimens were prepared by Forintek Canada
Corp. The initial part of this phase was devoted to the development of the test procedures
for Confocal Fluorescent Scanning Laser Microscopy (CSLM). In this phase Micro-CT
was used instead of X-ray densitometry. Essentially Micro-CT operates using X-ray
radiation to measure density of a material. Micro-CT however is a more powerful
technique than X-ray densitometry because it can provide a higher resolution and has the
capability to recreate a three-dimensional image of a scanned object. A Micro-CT
machine is available the Department of Mechanical Engineering at UNB.
Glued joints were prepared with wood from two species, Douglas fir and lodgepole pine.
Only one specimen was prepared for CSLM measurement. In order to get the average
shear strength, 30 replicates were tested for each combination of adhesive, species and
assembly time.
Phase 2
Phase 2 focused on PRF and PUR applied on black spruce and hard maple. Procedures
for measuring the penetration of adhesive resin into wood by using CSLM and X-ray
16
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
were developed. The main objective was to identify the influence of wood characteristics
(species, density, and growth ring orientation) and process parameters (moisture content,
assembly time) on glue line thickness and penetration, and in turn on shear strength and
percent wood failure.
The following test variables were studied in this test program:
Wood characteristics – species (black spruce (Picea mariana) and hard maple
(Acer saccharum Marsh.)), growth ring orientation and density
Initial moisture content – 6% and 15%
Temperature – 20℃ (room temperature) and 35℃
Adhesives: One phenol resorcinol formaldehyde and two polyurethanes
Phase 3
Phase 3 was a separate study aimed at correlating the long-term behaviour of glued joints
with that observed under various accelerated aging process such as freeze-thaw and
pressure-vacuum soak. The specific objective of this phase is to evaluate the performance
of glued joints that have been subjected to different exterior exposure durations and to
correlate these performance characteristics to those observed under accelerated aging
process. Two adhesives were used, PRF and PUR. Black spruce lumber was used as the
substrate. The duration of exposure ranged from short-term to 5 years. Therefore not all
test results are available at the time of writing this report, and some of the specimens will
be evaluated after the end of this project.
3. Materials and Method
3.1 Materials
The adhesives used in this study were Polyvinyl acetate (PVAc), Melamine formaldehyde
(MF), Polyurethane (PUR) and Phenol-resorcinol-formaldehyde (PRF). PVAc and MF
were used in Phase 1 whereas PUR (2) and PRF were used in phase 2. The species used
were Douglas fir and lodgepole pine in phase 1 and black spruce and hard maple in phase
2.
For CSLM procedure, a dye was used so that the location and dimensions of the glueline
and penetration in wood of the adhesive can be observed under the microscope. In this
project Safranin T (C20H19ClN4) with a molecular weight of 350.85 was found to be
17
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
suitable. This dye was used for all 4 adhesives, but the solution with which it was mixed
prior to adding to the adhesive solution was different, as described below.
3.2 Sample preparation
3.2.1 Preparation of the dye
Preliminary work was conducted to identify a suitable dye and concentration level for use
in the CSLM imaging. Literature review showed that Safranin is a suitable dye for use
with wood (Rijckaert et al. 2001). For PUR, the solvent for Safranin T is N,
N-Dimethylformamide (DMF) while for PRF, PVAc and MF, the solvent is distilled
water. The work involved mixing the glue with the Safranin T dye aqueous solution at
different weight concentrations. The concentrations were 0.5%, 0.25%, and 0.1%
respectively. When preparing the test specimens, the dye solution and glue liquid were
mixed at the weight ratio of 1:100.
Shear block tests were carried out to identify whether the dye has any negative effect on
shear strength of the glue bond, as will be discussed later. The results showed that the
presence of a dye did not affect shear strength of glued wood joint.
3.2.2 Preparation of wood block for shear strength test
The shear block specimens tested in phase 1 were prepared by Forintek Canada Corp. For
phase 2 the shear block specimen preparation is describe below.
Preparation of glued block shear specimens followed CSA Standard 0112.9-04 (CSA
2004). First, the lumber block defects such as machining defects (chipped grain, coarse
knife marks etc.) and drying defects (collapse, splits etc.) and knots larger than 3mm in
diameter were removed. Then the lumber was segregated into groups containing the same
range of wood characteristics. Only straight-grained wood was used to prepare the test
specimens. Wood with flat growth ring orientation was used in this study. The billets,
which were 19mm thick x 65mm wide x 350mm along the grain, were cut from the
lumber. After cutting, the billets were moved to a conditioning chamber set at the desired
conditions. Then, the density of each billet was measured and labeled. The blocks of
approximately the same density were assembled into two piles to be bonded with each
other for shear block specimens.
18
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
The wood substrates were planed just prior to gluing in order to obtain a fresh surface
which is needed for better glue penetration. Dye and adhesive were mixed at the weight
ratio of 1:100. A sufficient amount of the mixed adhesive was brush-coated to each
contacting surface of the laminations. The two glue-covered surfaces were then joined,
thus forming a wood block. The two-piece assembly was stabilized by inserting two nails
on two opposite sides of the assembly. The glued assemblies were clamped under the
pressure of 0.86 MPa (125psi) at room temperature. After assembly, all the specimens
were conditioned at 20°C and 65% relative humidity before cutting.
The shear block specimens were prepared from the glued assemblies after conditioning.
In the making of the notch on one end of the specimen, the saw cut extended through the
thickness of one ply to the bond line, and on the other end the saw cut also extended
through the thickness of the other ply to the bond line. Care was taken to ensure that both
saw cuts did not extend beyond the bond line. The shear block glue area was 40mm x
50mm in accordance with CSA O112.9-04 standard. Five replicates were cut from each
assembly.
3.2.3 Sample preparation for Micro-CT, X-ray and CSLM
The specimens used for micro-CT, X-ray and CSLM were cut from near the edge of the
glued billets from which the shear block specimens were cut. For micro-CT test, the
specimens were cut into a size of 30mm×10mm×10mm (L×W×H). For X-ray scanning,
the maximum dimensions for all the specimens were 20mm in height and 5mm in
thickness, since a smaller size specimen generally provides the best results.
For CSLM analysis, the specimens were cut into a size of 10mm×10mm×10mm
(L×W×H). Bond line cross sections were prepared with a sliding steel-bladed microtome.
The remaining dust was blown off with compressed air to make the surface as free of dust
and any other debris as possible.
3.3 Development of test procedures
3.3.1 AFM
As described above in 2.3, AFM is used in measuring the adhesion force. In the
preliminary work, the adhesion behaviour between a wood sample and PUR adhesive was
evaluated by means of AFM. We found the adhesion force decreased throughout the
curing process (Figures 3.1 and 3.2). These results led us to conclude that it may be
possible to use AFM as an approach to measure the adhesion force between wood and
19
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
adhesive.
Figure 3.1 - Force curve between PUR and wood surface (at the beginning of curing).
Figure 3.2 - Force curve between PUR and wood surface (one hour later).
In the subsequent preparation of specimens for the AFM test, 0.6g PVAc, PRF, MF and
PUR adhesives were mixed with 5ml acetone individually to produce diluted adhesive
solutions, and the test was carried out according to the method described above in 2.3.
The wood species used in these tests were black spruce, Douglas fir and lodgepole pine.
From the adhesion force versus time response, the adhesion force rose at the beginning
then started to drop. Since the wood surface was rough, so when doing the scan the force
between the wood surface and the tip varied a lot. The ideal situation is to obtain the force
of the adhesive within the same scanned surface area during adhesive curing. However,
since we used contact tip for the measurement, the tip would move from one area to
another on the wood surface during the scan, it was impossible to obtain the exact force
value. Moreover, the PUR adhesives we used in these tests were fast-curing, it was very
sensitive to water and even moisture in the air. It usually cured too fast to catch the
adhesion force. So in the rest of the project, AFM was not pursued.
20
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
3.3.2 Micro-CT
Micro-CT was used in this project in an attempt to measure the glue line thickness and
glue penetration into wood. The specimens were analyzed using a Micro-CT Skyscan
Model 1072. An appropriate methodology was developed (e.g., 7 images from -3 to +3
degree and cross the glue line to -1/2 to +1/2 thickness of glue line). The specimens were
placed inside the scanning chamber individually. A direct transmission image was
produced when the X-ray was turned on. The X-ray transmission images of the
reconstructed glue line could be viewed. The image taken when the X-ray was
perpendicular to the glue line contained the most narrow glue line. It was chosen to
present the real thickness of the glue line. In order to facilitate the determination of the
penetration thickness, a scale bar (steel wire with known precise diameter) was attached
to the samples. In this way, the thickness of the glue line and penetration depth could be
obtained. By selecting the proposed area for analysis, the density profile was obtained in
terms of gray level of the images. As a result, based on the change of the gray level value
of the images from the wood substrate to glue itself, the thickness of the glue line and
penetration were obtained.
Figure 3.3 shows the CT image of a MF glue line and the scale bar. ImageJ software was
used to analyze the images. The output from the analysis software was such that density
would increase when gray value decreased, and vice versa. A cross section of the glue
line from point A to point F was selected (shown in Figure 3.3). The corresponding data
was analysed to provide the thickness profile, as shown in Figure 3.4. When filled with an
adhesive, the density of wood cells should be higher than that without adhesive. Since
higher density meant lower gray value, wood itself had the highest gray value whereas
the adhesive had the lowest. From the data, it was obvious that AB and EF represent the
wood substrate whereas CD represents the glue line. Correspondingly, BC and DE
represent the glue penetration on each side of the glue line. The thickness of the scale bar
was 0.1651mm. By using ImageJ software, the scale bar was found to be 34 pixels thick.
So each pixel was 4.86μm wide. The number of pixels of each line and corresponding
thickness are given in Table 3.1.
Table 3.1 - Penetration and glue line thickness
Region No of pixels Thickness (μm)
BC
12
58
CD
6
29
DE
16
78
For each specimen, up to 10 cross sections were selected so that the average value could
be obtained. As an illustration, for the MF specimen shown in Figure 3.3 the thickness of
21
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
the glue line was 18μm and the penetration of the glue into the wood was 58μm and
72μm on each side of the glue line, respectively, which generated an average of 65+7μm
in penetration thickness. The density of glue line was the highest, followed by the
glue-penetrated wood and then the wood itself had the lowest density.
Figure 3.3 Micro-CT image for MF glue line and penetration.
160
Gray Value
150
A
140
B
E
130
F
120
110
100
C
90
D
80
70
0
10
20
30
40
50
60
70
80
90
Distance (pixels)
Figure 3.4 Data for the cross section from A to F for glue line in Figure 3.3.
3.3.3 X-ray densitometry
During the course of the project, an X-ray densitometer was installed at UNB and work
was conducted to evaluate the possibility of using of X-ray densitometer in place of
Micro-CT, which has been found to be rather expensive and tedious to use. Preliminary
22
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
work indicated that X-ray can provide sufficient resolution for characterizing the glue line
thickness and glue penetration into wood. Therefore a decision was made to use the X-ray
machine instead of micro-CT for the rest of this project.
Figure 3.5 shows scanned data from X-ray densitometry. The sample was coded MF
MAX ASSEMBLY BLACK SPRUCE, which was prepared by Forintek. The thickness
of the glue line and the penetration could be obtained based on the differences in glue and
wood density, similar to the approach for Micro-CT. In Figure 3.5, the x-axis is the
measurement location along a straight line going across the glue line. Since the wood had
the lowest density while the glue had the highest, it is clear that AB and EF represent the
wood. Regions BC and DE are areas of wood where glue penetrated while CD is the glue
line proper. From the calibrated machine measurements, the thickness of the glue line
(CD) is 0.06mm, while the penetration of MF in this case is 120μm (BC) and 80μm (DE).
5.00
5.20
5.40
5.60
5.80
6.00
6.20
6.40
6.60
6.80
7.00
0.00
200.00
A
B
E
F
400.00
600.00
800.00
1000.00
1200.00
C
D
Figure 3.5 X-ray measurement of sample MF MAX ASSEMBLY BLACK SPRUCE.
3.3.4 CSLM
CSLM is used to identify the location of glue in wood, glue line thickness and the
thickness of glue-penetrated wood. Literature review has identified Safranin and
Toluidine Blue as possible candidate dyes. Preliminary tests have suggested that Safranin
was a suitable dye to be used with polyvinyl acetates (PVAc), phenol resorcinol
formaldehyde (PRF), melamine formaldehyde (MF) and Polyurethane (PUR). Before it
was used, there was some concern about the influence of the dye on shear strength of glue
joint. To address this concern, glue block shear specimens were prepared and tested.
Three adhesives were used, PVAc, PRF and MF. Three groups of ten specimens with dye
and three groups without dye were prepared and tested. The wood used was black spruce.
Table 3.2 shows the mean shear strength and coefficient of variation for the test groups.
23
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Results in Table 3.2 confirm that the presence of the Safranin dye had no negative
influence on the shear strength of PVAc, PRF and MF bonded joints. It was also noted
that shear strengths of PRF and MF joints were substantially higher than that of PVAc
joint.
Table 3.2 – Summary statistics of block shear strength for PVAc, PRF and MF adhesives.
Adhesive
With dye (MPa)
Without dye (MPa)
PVAc
6.53 (4%)
5.52 (4%)
PRF
9.75 (1%)
10.61 (1%)
MF
9.27 (5%)
9.00 (5%)
Note : Value in parentheses is coefficient of variation.
After it has been confirmed that the dye has no effect on shear strengths of glued joints,
dyed samples were prepared and examined under CSLM. The CSLM measurements were
performed in the Microscopy and Microanalysis Facility at UNB using a Leica DM IRE2
confocal laser scanning microscope. The surface of the wood sample containing the glue
line was microtomed to enhance image quality. The images were obtained at a
wavelength of 488nm or 514nm with different magnifications ranging from 10× up to
150×. By altering the gain and offset options, more precise images can be obtained as
they are associated with the two power and resolution levels. All pictures were taken at
the slowest scan speed of 200 Hz, as a slower scan allows for a greater collection of
information. The resolution of the pictures was set to 1024kb ×1024kb. Figure 3.6 shows
a microtomed wood block sample with PRF as glue. The image was taken at 51% laser
power and a 488 nm wavelength. The dark green area represents the wood fluorescing
whereas the bright green line in the middle represents the fluorescing contributed from
the glue line. Clearly, the penetration depth of the glue was only about 2-3 diameter of
cells. Since the diameter of one wood cell is about 12-20μm, we could conclude that the
penetration of PRF was about 24-60 μm.
Fluorescent microscopy has proved to be a powerful instrument in investigating the
interaction between wood and wood adhesive at the micro-level. In this study, it is used
as a method of identifying the location of the glue in the wood cellular structure and
measuring the dimensions of the glue line including its thickness and the depth of
penetration into wood.
24
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Figure 3.6 - PRF glue line fluorescing under CSLM (microtomed).
3.4 Details of three phases of study
3.4.1 Phase 1
Phase 1 was initiated after a project meeting with industrial representatives on August 12,
2005. At that time, Forintek Canada Corp was conducting research work as part of
another Value to Wood FCC26 for the development of CSA O112.10, the dry use
counterpart of CSA O112.9. The industry felt that the resource of this project should be
utilized to provide some information on the influence of glue penetration on strength of
glued joints. The Forintek project used two adhesives, polyvinyl acetate (PVAc) and
melamine-formaldehyde (MF), and two wood species, Douglas fir and lodgepole pine.
These were retained for this project.
As was recommended by the Industrial Working Group, this project evaluated the glue
line characteristics using CSLM and Micro-CT for the glued specimens prepared and
evaluated by Forintek West as part of the FCC26 project which was developing technical
data in support of the drafting of CSA O112.10. As a result, instruction to prepare glued
wood specimens with a dye was given to Forintek West, so that the glue line
characteristics of their specimens can be evaluated using CSLM at UNB. The test
parameters are summarized in Table 3.3. The maximum and minimum assembly times
were as recommended by the adhesive manufacturers. Note that the FCC26 program of
work only covered Douglas fir and Lodgepole pine. Forintek West performed the shear
25
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
strength test on these specimens and measured their percent wood failure. The shear
strength and percent wood failure of the black spruce specimens were determined by
UNB.
Table 3.3 – Schedule of test specimens evaluated using CSLM and Micro-CT.
Resin
Wood
Assembly time
Number of billet
Species
assemblies
MF
Black Spruce*
Max
2
MF
Black Spruce*
Min
2
MF
Douglas Fir
Max
1
MF
Douglas Fir
Min
1
MF
Lodgepole Pine
Max
1
MF
Lodgepole Pine
Min
1
PVA
Black Spruce*
Max
1
PVA
Black Spruce*
Min
1
PVA
Douglas Fir
Max
1
PVA
Douglas Fir
Min
1
PVA
Lodgepole Pine
Max
1
PVA
Lodgepole Pine
Min
1
*Specimens prepared and tested by UNB.
3.4.2 Phase 2
At the Industry Working Group meeting in 2005, the members felt that it would be useful
to understand the influence of wood characteristics such as wood density, wood moisture
content and ring orientation on penetration of PRF and PUR in wood. This is the prime
objective of Phase 2. Two species were included, black spruce and hard maple, which are
species covered by CSA O112.9 for evaluating structural adhesives. One PRF and two
PUR adhesives were included.
In order to understand the influence of wood density on adhesive penetration, the density
range was divided into three zones. They were (1) 0.49 - 0.52; (2) 0.44 - 0.48; (3) 0.38 0.43 for black spruce. For hard maple, the ranges were (1) 0.52 - 0.57; (2) 0.57 - 0.63; (3)
0.63 - 0.68. These specimens were prepared with flat-sawn boards and the growth rings
of the two substrates were always parallel to each other as shown in lay-ups #1 to #4 in
Figure 3.7. In Figure 3.7, the red line represents the growth ring. In addition to wood
density the influence of growth ring orientation was also studied. Lay-ups #5 to #7 in
Figure 3.7 illustrate how this effect was studied in this project. For #4, the wood blocks
were cut in between the latewood so that when gluing these two surfaces together, the
26
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
glue line was in the latewood. (note this was found to be difficult to achieve in practice,
so this lay-up was not studied.) In #5, the growth rings on the one wood block were
perpendicular to the growth rings on the other wood block. In #6, one substrate was cut to
obtain a 45 degree growth ring angle, which was then bonded to the other wood block
whose growth rings are parallel to the bonding surface. For #7, each wood substrate had
growth rings with small curvature. The growth ring orientation effect was studied using
black spruce specimens only.
Since PUR adhesive is a relatively new group of adhesives used in structural wood
applications, published information on their behaviour is rather limited. Therefore two
PUR adhesives from two manufacturers were included in this project. In addition, since
the penetrating ability of PUR adhesives is thought to be rather low compared with the
traditional phenolic-based adhesives, for each PUR adhesive two adhesive temperatures
during specimen fabrication were used, at room temperature and at 35EC in an attempt to
provide adhesives with different viscosity and therefore penetration characteristics. For
PRF, only one adhesive was used. In this case, instead of temperature two assembly times
were included, maximum and minimum, according to the adhesive manufacturer’s
recommendations. The minimum assembly time was 12 minutes which consisted of 6
minutes of open time and 6 minutes of close time. For maximum assembly time the total
was 60 minutes consisting of 30 minutes of open time and 30 minutes of close time.
For the influence of moisture content, two initial wood moisture contents were covered,
6% and 15%. However in order to avoid the influence of wood MC on strength properties,
all specimens were conditioned to 12% MC before testing.
Table 3.4 presents the test program to evaluate the influence of wood density, initial wood
moisture content and preparation condition (i.e. assembly time or temperature) for black
spruce. Table 3.5 is the corresponding program for hard maple. Table 3.6 shows the test
program to evaluate the influence of growth ring orientation using black spruce as the
substrate.
All the shear tests were performed under wet condition after vacuum-pressure-soak, by
Forintek Canada Corp.
27
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Figure 3.7 - Samples with different growth ring orientations.
28
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Table 3.4 – Schedule of tests for evaluating influence of wood characteristics on bond
performance for black spruce.
Initial Temp of
Number of specimens
Total no.
Wood
wood
glue or
per density range
Adhesive
of
Species
MC Assembly
0.38-0.43 0.44-0.48 0.49-0.52 specimen
time
(%)
Min
5
5
5
15
6
Max
5
5
5
15
PRF
Min
5
5
5
15
15
Max
5
5
5
15
35EC
5
5
5
15
6
Black
PUR1
RT
5
5
5
15
spruce
35EC
5
5
5
15
15
RT
5
5
5
15
35EC
5
5
5
15
6
PUR2
RT
5
5
5
15
35EC
5
5
5
15
15
RT
5
5
5
15
Table 3.5 - Schedule of tests for evaluating influence of wood characteristics on bond
performance for hard maple.
Initial Temp of
Number of specimens
Total no.
Wood
wood
glue or
per wood density range
Adhesive
of
Species
MC Assembly
0.52-0.57 0.58-0.63 0.64-0.68 Specimens
(%)
time
Min
5
5
5
15
6
Max
5
5
5
15
PRF
Min
5
5
5
15
15
Max
5
5
5
15
35EC
5
5
5
15
6
PUR1
RT
5
5
5
15
Hard
Maple
35EC
5
5
5
15
15
RT
5
5
5
15
35EC
5
5
5
15
6
RT
5
5
5
15
PUR2
35EC
5
5
5
15
15
RT
5
5
5
15
29
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Table 3.6 - Schedule of tests for evaluating influence of growth ring orientation on
bond performance.
Initial
Growth
Temp of
Number of
wood
Species
Adhesive
ring orientation
glue
specimens
MC
(EC)
(Medium density)
(degree)
(%)
RT
5
PUR1
6
90
35
5
Black
RT
5
PUR1
6
45-90
spruce
35
5
RT
5
PUR1
6
45
35
5
3.4.3
Phase 3
The long-term performance of an adhesive joint is often predicted using an accelerated
aging protocol. In CSA O112.9 for example, two accelerated aging protocols are given.
The first protocol involves subjecting dry glued block shear test specimens to a
pressure-vacuum-soak procedure and then testing the shear block specimens while they
are wet. In the second protocol, known as boil-dry-freeze test, the dry glued block shear
specimens are subjected to eight cycles of boiling for 4 h, oven-drying at 60 ± 3 °C for 19
± 1 h, and then placing in a freezer tunnel maintained at less than –30 °C for at least 4 h.
At the end of the eight cycles, the block shear specimens were again tested wet. These
protocols, or slightly varied version of these, have been in use for decades to evaluating
long-term performance of adhesive joints of primarily phenolic-base adhesives as these
were the traditional ‘wet’ use structural adhesives.
While these protocols are considered to provide adequate long-term performance for
phenolic-based adhesives, it is of interest to evaluate their suitability for the new
generation of adhesives such as PUR. This is the goal of phase 3 of this project.
Specifically the objectives of phase 3 are:
To evaluate the long-term performance of PRF (as control) and PUR joints.
To relate the long-term performance of PRF and PUR joints with those
subjected to accelerated aging protocols in CSA O112.9.
To achieve these objectives the following procedure was followed. A sample of short 2x3
black spruce lumber pieces was obtained from an I-joist manufacturer in New Brunswick.
The lumber was of flange stock quality. For each adhesive, 120 pieces of 8 ft long finger
joined lumber, and 60 pieces of 2 ft long 2-ply edge-glued joints were fabricated at a
30
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
commercial finger-jointing facility. The 2-ply edge-glued joints were cut into two
end-matched pieces. The 120 pieces of finger joint specimens were separated into 6
groups, one as short-term control group and the other 5 groups were exposed to different
duration of outdoor condition in Fredericton, New Brunswick, Figure 3.8. The outdoor
exposure durations were 3 months, 6 months, 12 months, 24 months and 60 months as
shown in Table 3.7. These 6 groups are coded E1 to E6 respectively. Ten 2-ply joints
were also subjected to outdoor exposure at the same site as the finger joint specimens. At
the end of each target exposure time, one group of 20 finger joints and one shear block
specimen was cut from each of the ten 2-ply joint for tension and block shear test
respectively. Tension tests were conducted according to ASTM D198 (2004) whereas
block shear test was conducted according to CSA O112.9 (CSA 2004). The other finger
joint specimens and the remaining section of the 2-ply joints were left at the site for
continuation of the exposure condition until the next target exposure time was reached. At
that time, the same sampling and testing was performed.
Each of the other 10 end-match 2-ply joints was cut into seven shear block specimens,
which provided 7 groups (A1 to A7) of 10 specimens for varying levels of accelerated
aging according to the protocols in CSA O112.9 for pressure-vacuum-soak and
boil-fry-freeze conditions, as shown in Table 3.7. After the accelerated aging process,
these specimens were tested for shear strength. These will be compared with the outdoor
exposed shear specimens, and with the finger joint strength. Analysis of these test results
will provide an indication of whether the accelerated aging protocols are appropriate for
predicting long-term bond performance of PRF and PUR joints.
Table 3.7 – Long-term and accelerated aging test program.
Exterior exposure
Group
No.
finger
of
Accelerated aging in laboratory
No. of 2-ply
Duration
Group
shear
No.
of
2-ply
Conditioning and condition at
2/
shear joints
test
1/
joints
joints
E1
20
10
0 month
A1
10
CSA O112.9 - wet
E2
20
10
3 months
A2
10
CSA O112.9 boil-freeze, 2
cycles – wet
E3
20
10
6 months
A3
10
CSA O112.9 boil-freeze, 2
cycles – dry
E4
20
10
12
A4
10
months
E5
20
10
24
cycles – wet
A5
10
months
E6
20
10
60
CSA O112.9 boil-freeze, 8
CSA O112.9 boil-freeze, 8
cycles – dry
A6
10
CSA
O112.9
31
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
months
pressure-vacuum-soak - wet
A7
10
CSA
O112.9
pressure-vacuum-soak - dry
Figure 3.8 - Outdoor exposure site for finger joint and 2-ply glued joint specimens.
4. Results and Discussion
4.1 Phase 1
This phase evaluated the adhesive penetration and glueline thickness of glued joint
prepared with MF and PVAc. These two adhesives were used as ‘reference’ adhesives in
the Forintek project on the development of evaluation protocol in evaluating structural
adhesives for use under limited exposure conditions (CSA O112.10). The two species
evaluated were lodgepole pine and Douglas fir and all specimens were prepared by
Forintek Canada Corp, who also performed the block shear tests as part of its project to
develop CSA O112.10. Black spruce glued joint specimens were also prepared by UNB
for evaluation of glueline thickness and penetration. Shear test was not performed on
black spruce specimens.
Only one specimen was used for glue penetration measurement. For block shear test,
32
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
there were 30 specimens tested for each combination of adhesive, species and assembly
time (maximum and minimum).
Table 4.1 summarizes the results from the glue penetration measurement and shear tests.
Shown in Table 4.1 are the glue penetration values measured on both side of the glue line,
shear strength and number of shear specimens that had less than 100% wood failure. For
glue penetration measurements, both the range and mean penetration are given on each
side of the glue line. It was found necessary to report glue penetration on both sides of the
glue line because there was a large difference in glue penetration on either side of the
glue line. It was later confirmed that the substrate on the lower side of the joint during
clamping and glue curing generally had a deeper glue penetration. This is illustrated in
Figure 4.1. Possibly, gravity carried the glue deeper into the wood, facilitated by
clamping pressure, on the lower side substrate compared with the substrate above the
glue plane.
Figure 4.1 - Glue penetration measurement using CSLM for MF/DF/Max assembly time
specimen.
33
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Table 4.1 – Glue penetration and shear test results.
Adhesive/
Penetration
Assembly
on the
Time/
lower side
Species
(μm)
MF / Max
Average
penetration
on the
lower side
(μm)
Penetration
on the
upper side
(μm)
Average
penetration
on the
50-190
95
50-250
140
37.5-150
45
50-625
270
10-75
25
50-275
105
15-100
60
12.5-100
55
12.5-50
25
12.5-100
30
12.5-50
20
10-112.5
50
12.5-87.5
25
12.5-100
35
12.5-75
40
PVAc / BS
10 - 25
22
10 - 25
16
MF /BS
26 - 130
80
20 - 95
57
LP
MF / Max
/ DF
MF / Min /
DF
PVAc /
Max / DF
PVAc/
Min/ DF
PVAc /
Max/ LP
PVAc /
Min/ LP
(mm)
Percent of
Shear
strength*
(MPa)
(μm)
150
MF / Min /
thickness
upper side
75-300
/ LP
Glueline
specimens
below
100%
wood
failure
59
11.95
23.3%
(1.46)
61
11.73
30.0%
(1.51)
41
11.57
33.3%
(1.35)
39
11.30
26.7%
(1.51)
40
11.55
16.7%
(1.41)
21
11.54
43.3%
(1.35)
13
12.00
20.0%
(1.27)
57
11.61
13.3%
(1.31)
20
--
--
10
--
--
* Mean with standard deviation given in parentheses.
34
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Figure 4.2 – MF black spruce CSLM
micrograph.
Figure 4.3 – PVAc black spruce CSLM
micrograph.
The results for black spruce are discussed here first. Figure 4.2 shows the penetration of
MF adhesive in black spruce. The bright green area represents the adhesive penetration
and the bulk glue line. Black spruce also fluoresces itself. However, the wood is clearly
distinguishable from the MF adhesive which provides a brighter green color under CSLM.
The green solid line between the two pieces of wood is the glue line. Flow of adhesive
into rays can also be observed. Figure 4.2 illustrates a good penetration of MF adhesive
into the lumen of tracheids and rays. The glue penetration into the wood was measured to
be about 4 cell rows deep. The MF adhesive also penetrates into some tracheids next to
the rays. It seems that the adhesive entered these tracheids via rays. The penetration depth
of MF adhesive ranged from 26μm to 130μm and from 20μm to 95μm respectively on
the lower and upper side of the glue line for this specific specimen. In contrast, PVAc
exhibited a lower glue penetration into the wood, as illustrated in Figure 4.3. The
penetration was observed to be 1 to 2 cells deep which corresponds to 10μm to 25μm.
However, PVAc gave a glue line which was twice as thick as the MF glue line, Table 4.1.
Significant differences were observed between these two adhesives in black spruce. The
penetration of MF glue into the wood was much deeper than that of PVAc. Less
penetration of PVAc can be explained in terms of adhesion mechanism. The PVAc used in
this project was in a solution containing an acidic solvent with a catalyst. PVAc chains
become less mobile and eventually transform into a solid adhesive layer (or film) when
the solvent evaporates, thus adsorbing into the wood substrate (Frazier 2004). This kind
of mechanism causes PVAc to form a thick glue line and achieve lower penetration depth
than that of MF. MF used in this work was a pre-catalyzed, dry powder. The melamine
resin requires only the addition of water to complete the adhesive mix. It is a high
35
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
temperature setting adhesive. When heated, MF reacts into a highly cross-linking network
structure and becomes rigid. The higher temperature experienced by the adhesive prior to
curing likely facilitates the penetration of the adhesive into wood, as it lowers the
viscosity of the resin.
Figures 4.4 to 4.7 show respectively images of MF/Douglas fir, PVAc/Douglas fir,
MF/lodgepole pine and PVAc/lodgepole pine captured by CSLM. The numerical results
are shown in Table 4.1.
Figure 4.4 – MF / Douglas fir CSLM
micrograph.
Figure 4.5 - PVAc / Douglas fir CSLM
micrograph.
Figure 4.6 - MF / lodgepole pine CSLM
micrograph.
Figure 4.7 - PVAc / lodgepole pine CSLM
micrograph.
It can be noted from Table 4.1 that the shear strengths of all groups are similar, but there
36
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
is a large variation in glue penetration and percent wood failure characteristics. As in the
case of black spruce, glue penetration of MF is about 3 to 4 times larger than that for
PVAc. Douglas fir specimens generally had a large difference between upper and lower
side substrates. For lodgepole pine, the difference between glue penetrations in upper and
lower substrates is relatively small. The largest glue penetration was achieved for both
adhesives in Douglas fir at the maximum assembly time.
The effect of glue penetration on shear strength and percent wood failure of glue joint can
be better understood by studying Figures 4.8 to 4.11. Figures 4.8 and 4.9 provide the
shear strength and gluing characteristics results. As stated above, glue penetration for MF
is a few times greater than that for PVAc, therefore glue penetration appears to have little
impact on dry shear strength. For both adhesives, lodgepole pine specimens achieved a
higher shear strength than Douglas fir. For MF adhesive, the specimens fabricated using
maximum assembly time generally achieved a higher shear strength than those fabricated
using minimum assembly time. Figures 4.8 and 4.9 also suggest that there is no
correlation between glue line thickness and shear strength, although the range of glue line
thickness obtained in this project may be too narrow to draw any general conclusions.
For percent wood failure results shown in Figures 4.10 and 4.11, there is no specific trend
that can be detected with the gluing parameters. The conclusions that can be drawn from
this phase are:
1. Dry shear strength of lodgepole pine specimens is higher than Douglas fir specimens
for both adhesive.
2. There is a large difference in substrate glue penetration for spruce and Douglas fir,
but not for lodgepole pine.
3. Glue penetration of MF is about 3 to 4 times that of PVAc for all three species.
4. For MF adhesive, specimens fabricated using maximum assembly time generally
achieve a higher shear strength than those fabricated using minimum assembly time.
37
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
MF Adhesive - Shear strength
11.90
300
11.80
250
Shear strength (Mpa)
Thickness (micro m)
11.70
11.60
200
11.50
150
11.40
11.30
100
11.20
50
11.10
0
11.00
DF/Min
Lower side penetration
DF/Max
LP/Min
Species/Assembly time
Upper side penetration
Glueline thickness
LP/Max
Shear strength (MPa)
Figure 4.8 - MF gluing characteristics and shear strength results.
PVA Adhesive - Shear strength
60
12.10
12.00
50
11.80
30
11.70
11.60
20
Shear strength (Mpa)
Thickness (micro m)
11.90
40
11.50
10
11.40
0
11.30
DF/Min
Lower side penetration
DF/Max
LP/Min
Species/Assembly time
Upper side penetration
Glueline thickness
LP/Max
Shear strength (MPa)
Figure 4.9 - PVAc gluing characteristics and shear strength results.
38
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
MF Adhesive - Wood failure
300
30%
Thickness (micro m)
250
25%
200
20%
150
15%
100
10%
50
5%
0
Percent below 100% wood failure
35%
0%
DF/Min
DF/Max
LP/Min
Species/Assembly time
LP/Max
Lower side penetration
Upper side penetration
Glueline thickness
Percent of specimen below 100% wood failure
Figure 4.10 - MF gluing characteristics and percent wood failure results.
PVA Adhesive - Wood failure
50%
60
Thickness (micro m)
40%
35%
40
30%
30
25%
20%
20
15%
10%
10
Percent below 100% wood failure
45%
50
5%
0
0%
DF/Min
Lower side penetration
Glueline thickness
DF/Max
LP/Min
Species/Assembly time
LP/Max
Upper side penetration
Percent of specimen below 100% wood failure
Figure 4.11 - PVAc gluing characteristics and percent wood failure results.
39
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
4.2 Phase 2
4.2.1
CSLM and shear test results for PUR1(T) and PUR2(N)
4.2.1.1 Specimens with flat growth ring orientation
Tables 4.2 - 4.5 summarize the results from the glue penetration measurement and shear
tests. Shown in Tables 4.2 – 4.5 are the penetration values measured on both sides of the
glue line by CSLM, glue line thickness, shear strength and percent wood failure. The
effect of wood characteristics and processing parameters on glue penetration on shear
strength can be better understood by studying Figures 4.12 - 4.23. Note that the first
number in the group code name (e.g. 1T6HT) refers to the density range (1, 2 or 3), the
second letter refers to the PUR adhesive manufacturer (T or N), the third number is the
moisture content of wood, and the last two letters refer to the temperature of the adhesive
at fabrication (RT or HT). In the following discussion the results from black spruce are
discussed first, followed by those for hard maple.
PUR1(T) and PUR2(N) – spruce
Tables 4.2 and 4.3 summarize the statistics for CSLM and shear test results obtained from
black spruce specimens fabricated with PUR1(T) and PUR2(N) adhesive respectively. In
general, PUR2(N) specimens have a slightly higher average wet shear strength.
40
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Table 4.2 Results of the glue measurement and shear test - PUR1(T) / spruce.
Penetration
Specimen
on the
lower side
(μm)
1T6HT
1T6RT
2T6HT
2T6RT
3T6HT
3T6RT
1T15HT
1T15RT
2T15HT
2T15RT
3T15HT
3T15RT
1/
40.39 204.46
25.26 183.17
9.00 150.26
30.00 132.90
22.90 78.59
21.20 177.55
25.50 175.80
41.33 140.16
33.04 183.50
42.69 161.24
19.50 127.50
12.09 89.22
Average
penetration
on the
lower side
(μm)
60.59
62.88
44.43
46.14
39.72
45.92
77.53
67.29
75.52
73.34
58.87
41.68
Penetration
on the
upper side
(μm)
12.8219.50
15.26 66.32
15.00 85.27
13.02 72.03
Average
penetration
Glue line
on the
thickness
upper side
(μm)
(μm)
15.16
19.14
21.12
27.73
3.00 -
8.79
12.36
19.49 72.09
12.00 64.43
23.68 171.00
16.50 64.08
21.00 135.00
6.00 22.50
12.32 63.00
29.93
20.39
62.24
22.17
44.24
16.59
36.98
33.32 –
41.07
19.19 –
26.81
8.78 –
11.07
30.21 –
53.09
46.17 –
73.46
19.69 –
37.94
29.70 –
40.87
25.90 –
30.63
12.48 –
23.20
21.05 –
29.52
23.81 –
54.77
10.05 –
14.04
Average
glue line
thickness
(μm)
38.03
22.44
10.13
41.09
64.57
24.28
31.20
27.80
15.22
23.98
43.33
11.41
Shear
strength
(MPa)1/
Percent
wood
failure
(%)2/
4.4
10
(0.2)
(5)
4.6
7
(0.2)
(5)
4.7
27
(0.3)
(5)
4.0
50
(0.3)
(5)
4.6
14
(0.5)
(5)
5.9
15
(0.5)
(5)
4.9
69
(0.1)
(5)
4.9
5
(0.2)
(5)
4.7
43
(0.3)
(5)
5.1
12
(0.4)
(5)
4.9
72
(0.4)
(5)
4.9
4
(0.3)
(5)
2/
Mean with standard deviation given in parentheses; Number of samples that had less than 100% wood
failure given in parentheses.
41
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Table 4.3 Results of the glue penetration measurement and shear test - PUR2(N) / spruce.
Penetration
Specimen
on the
lower side
(μm)
1N6HT
1N6RT
2N6HT
2N6RT
3N6HT
3N6RT
1N15HT
1N15RT
2N15HT
2N15RT
3N15HT
3N15RT
1/
18.80 –
170.77
18.13 –
152.10
12.00 –
73.25
18.06 –
155.54
13.83 –
81.30
8.08 –
81.01
16.22 –
85.52
14.42 –
81.06
12.09 –
190.64
10.50 –
50.76
12.82 –
45.70
12.37 –
75.68
Average
penetration
on the
lower side
(μm)
63.86
35.43
49.29
35.38
51.03
31.13
29.21
35.68
33.68
22.36
25.23
21.43
Penetration
on the
upper side
(μm)
15.00 –
150.40
12.09 –
86.54
Average
penetration
Glue line
on the
thickness
upper side
(μm)
(μm)
44.93
27.42
6.00 –
9.37
20.12
12.00 –
71.64
6.19 –
28.46
9.00 –
78.70
7.50 24.97
15.00 –
63.30
12.82 –
65.74
7.65 –
56.00
11.72 –
35.65
7.50 –
31.50
13.74
14.45
14.96
12.02
21.61
29.47
11.89
19.69
11.55
24.01 –
36.00
16.55 –
30.03
15.68 –
23.25
20.67 –
44.39
15.93 –
32.74
23.25 –
26.69
54.12 –
95.62
35.57 –
51.23
28.31 –
39.00
47.41 –
56.99
38.62 –
53.22
39.46 –
90.95
Average
glue line
thickness
(μm)
30.49
24.19
18.51
31.43
25.19
25.21
72.42
45.96
31.20
51.04
46.02
48.43
Shear
strength
(MPa)1/
Percent
wood
failure
(%)2/
5.3
28
(0.2)
(5)
4.6
27
(0.3)
(5)
5.5
23
(0.3)
(5)
4.7
28
(0.4)
(5)
5.1
20
(0.2)
(5)
5.0
10
(0.2)
(5)
4.5
96
(0.3)
(2)
4.9
19
(0.2)
(5)
5.3
91
(0.3)
(5)
5.4
22
(0.3)
(5)
5.9
18
(0.3)
(5)
5.1
11
(0.5)
(5)
2/
Mean with standard deviation given in parentheses; Number of samples that had less than 100% wood
failure given in parentheses.
42
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
different density
80.00
70.00
6.0
80.00
7.0
5.0
70.00
6.0
40.00
3.0
30.00
2.0
thick ness (micro n)
4.0
50.00
Shear strength (MPa)
20.00
60.00
5.0
50.00
4.0
40.00
3.0
30.00
2.0
20.00
1.0
10.00
0.00
0.0
1T6HT
lower side penetration
2T6HT
upper side penetration
0.00
3T6HT
glue line thickness
1.0
10.00
0.0
1T6RT
lower side penetration
Shear strength
upper side penetration
3T6RT
glue line thickness
90.00
90.00
6.0
6.0
80.00
80.00
5.0
3.0
40.00
30.00
2.0
20.00
1.0
thickness (micron)
4.0
50.00
Shear strength (MPa)
60.00
5.0
70.00
70.00
60.00
4.0
50.00
3.0
40.00
30.00
2.0
20.00
1.0
10.00
0.00
10.00
0.00
0.0
1T15HT
lower side penetration
2T15HT
upper side penetration
Shear strength
different density
different density
thickness (micron)
2T6RT
3T15HT
glue line thickness
Shear strength (MPa)
Shear strength (MPa)
thickness (micron)
60.00
Shear streng th (M Pa)
different density
0.0
1T15RT
lower side penetration
2T15RT
upper side penetration
3T15RT
glue line thickness
Shear strength
Figure 4.12 Effect of wood density on gluing characteristics and shear properties for
PUR1(T) / spruce.
43
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
6.0
50.00
5.0
40.00
4.0
30.00
3.0
20.00
2.0
10.00
1.0
0.00
0.0
lower side penetration
2N6HT
upper side penetration
50.00
6.0
40.00
5.0
4.0
30.00
3.0
20.00
2.0
10.00
3N6HT
glue line thickness
1.0
0.0
0.00
1N6RT
Shear strength
lower side penetration
different density
upper side penetration
3N6RT
glue line thickness
Shear strength
different density
60.00
6.0
6.0
50.00
5.0
40.00
4.0
30.00
3.0
20.00
2.0
10.00
1.0
5.0
60.00
50.00
4.0
40.00
3.0
30.00
2.0
20.00
1.0
10.00
0.00
0.0
1N15HT
lower side penetration
2N15HT
upper side penetration
3N15HT
glue line thickness
thickness (micron)
7.0
80.00
Shear strength (MPa)
90.00
70.00
thickness (micron)
2N6RT
0.00
0.0
1N15RT
Shear strength
Shear strength (MPa)
1N6HT
thickness (micron)
7.0
60.00
Shear strength (MPa)
thickness (micron)
70.00
Shear strength (MPa)
different density
different density
lower side penetration
2N15RT
upper side penetration
3N15RT
glue line thickness
Shear strength
Figure 4.13 Effect of density on glue penetration and glue line thickness - PUR2(N) /
spruce.
From Figures 4.12 and 4.13, it can be seen that the specimens with the lowest density
achieve the deepest glue penetration. This is true for both PUR adhesives. There is a
significant difference in penetration depths between the two substrates of each joint. The
substrate on the lower side during fabrication had a deeper penetration than that on the
upper side of the joint. There is no clear relationship between glue line thickness and
wood density, nor with shear strength. The shaded bar shows the mean shear strength of
each group. There appears to be a trend, albeit a weak one, that penetration depth has
some effects on the shear strength. Shear strength increases slightly as glue penetration
depth decreases. When examining the images captured by CSLM, it was noticed that
some specimens, notably those from the lowest density range 1, had a starved/hollow
glue line. This indicates over-penetration of glue. This could explain why those having
the deepest penetration got the lowest shear strength. However this trend may be
misleading as the change in shear strength is likely due to the increase in wood density
rather than the lower glue penetration.
The influence of initial wood moisture content is illustrated in Figures 4.14 and 4.15.
44
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Results show that the joints fabricated with a higher initial wood MC (15%) during
fabrication generally achieve higher shear strength than those with a lower initial MC.
The exceptions are the two groups for PUR2(N) at HT. This result is perhaps not
surprising given that curing of PUR adhesives requires the presence of moisture. Despite
this, wood MC appears to have opposing effect on glue penetration and glue line
thickness for the two PUR adhesives. For PUR1(T) the higher MC leads to higher
penetration and generally thinner glue line. However the opposite trends are observed for
PUR2(N). Therefore the higher shear strength was likely not a result of the difference in
glue penetration. Reviewing the percent wood failure results in Table 4.2 and 4.3 shows
that the percent wood failure for both adhesives is generally higher for the higher MC
groups, especially at HT.
The influence of glue temperature during fabrication is presented in Figures 4.16 and 4.17.
Temperature seems to have little effects on the penetration depth or shear strength. No
clear trend is observed even for groups of one adhesive. For instance for PUR1(T) the
glue penetration decreases as temperature increases for the 6% MC group. However the
opposite is observed at 15% MC. This lack of trend applies to all parameters. The reason
for this is probably because while increasing temperature has the potential of increasing
glue penetration because it lowers the viscosity of the adhesive, it also increases the
reactivity of the glue, which may hinder the penetration of the glue. Different PUR glue
formulations probably are influenced to a different degree by these two conflicting factors.
Overall increasing temperature to increase glue penetration may not be an effective
approach for PUR adhesives.
Percent wood failure is usually taken as an indication of the strength of the glue bond,
with a higher percent wood failure indicating that the glue bonds are stronger than the
wood itself. However, it is not always correlated with shear strength. From Tables 4.2 and
4.3, specimens with 15% initial MC generally have a higher percent wood failure than
those at 6% MC.
45
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
different MC
90.00
6.0
80.00
5.0
60.00
4.0
50.00
3.0
40.00
30.00
2.0
Shear strength (MPa)
thickness (micron)
70.00
20.00
1.0
10.00
0.00
0.0
1T6HT
1T15HT
lower side penetration
2T6HT
2T15HT
upper side penetration
3T6HT
3T15HT
glue line thickness
Shear strength
different MC
7.0
90.00
80.00
thickness (micron)
5.0
60.00
50.00
4.0
40.00
3.0
30.00
2.0
Shear strength (MPa)
6.0
70.00
20.00
1.0
10.00
0.0
0.00
1T6RT
1T15RT
lower side penetration
2T6RT
2T15RT
upper side penetration
3T6RT
glue line thickness
3T15RT
Shear strength
Figure 4.14 Effect of initial wood moisture on gluing characteristics and shear properties
of PUR1(T) / spruce.
46
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
90.00
7.0
80.00
6.0
thickness (micron)
70.00
5.0
60.00
50.00
4.0
40.00
3.0
30.00
20.00
2.0
10.00
1.0
0.00
Shear strength (MPa)
different MC
0.0
1N6HT
1N15HT
lower side penetration
2N6HT
2N15HT
upper side penetration
3N6HT
glue line thickness
3N15HT
Shear strength
60.00
6.0
50.00
5.0
40.00
4.0
30.00
3.0
20.00
2.0
10.00
1.0
0.00
0.0
1N6RT
1N15RT
lower side penetration
2N6RT
2N15RT
upper side penetration
3N6RT
glue line thickness
Shear strength (MPa)
thickness (micron)
different MC
3N15RT
Shear strength
Figure 4.15 Effect of moisture on glue penetration and glue line thickness - PUR2(N) /
spruce.
47
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
80.00
7.0
70.00
6.0
60.00
5.0
50.00
4.0
40.00
3.0
30.00
2.0
20.00
10.00
1.0
0.00
0.0
1T6HT
1T6RT
2T6HT
2T6RT
3T6HT
S h e a r st re n g t h (M P a )
t h i c k n e ss (m i c ro n )
different T
3T6RT
lower side penetration upper side penetration glue line thickness Shear strength
different T
6.0
90.00
80.00
thickness (micron)
4.0
60.00
50.00
3.0
40.00
2.0
30.00
20.00
Shear strength (MPa)
5.0
70.00
1.0
10.00
0.0
0.00
1T15HT
1T15RT
lower side penetration
2T15HT
2T15RT
upper side penetration
3T15HT
3T15RT
glue line thickness
Shear strength
Figure 4.16 Effect of glue temperature on gluing characteristics and shear properties PUR1(T) / spruce.
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
70.00
7.0
60.00
6.0
50.00
5.0
40.00
4.0
30.00
3.0
20.00
2.0
10.00
1.0
0.00
0.0
1N6HT
1N6RT
lower side penetration
2N6HT
2N6RT
upper side penetration
3N6HT
Shear strength (MPa)
thickness (micron)
different T
3N6RT
glue line thickness
Shear strength
90.00
7.0
80.00
6.0
thickness (micron)
70.00
5.0
60.00
4.0
50.00
40.00
3.0
30.00
2.0
20.00
Shear strength (MPa)
different T
1.0
10.00
0.00
0.0
1N15HT
1N15RT
lower side penetration
2N15HT
2N15RT
upper side penetration
3N15HT
3N15RT
glue line thickness
Shear strength
Figure 4.17 Effect of glue temperature on gluing characteristics and shear properties of
PUR2(N) / spruce.
PUR1(T) and PUR2(N) – hard maple
The test results for hard maple bonded with PUR1(T) and PUR2(N) are summarized in
Tables 4.4 and 4.5. The effects of wood density, wood moisture at fabrication and
adhesive temperature at fabrication are better illustrated in Figures 4.18 to 4.23. As in the
case of black spruce, the average shear strength of PUR2(N) joints is slightly higher than
49
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
the PUR1(T) joints, although there are some extremely lower shear strength values. More
discussion on influence of fabrication factors is presented below.
Table 4.4 Results of the glue measurement and shear test - PUR1(T) / maple.
Penetration
Specimen
on the
lower side
(μm)
1T6HT
1T6RT
2T6HT
2T6RT
3T6HT
3T6RT
1T15HT
1T15RT
2T15HT
2T15RT
3T15HT
3T15RT
9.13 –
50.93
9.00 45.13
20.40 –
59.03
9.00 –
43.52
9.00 –
59.55
12.00 –
57.00
12.63 –
46.22
13.5 –
63.00
11.05 –
51.72
14.21 –
55.28
15.02 –
77.02
12.00 –
51.31
Average
penetration
on the
lower side
(μm)
30.52
31.24
30.22
29.90
27.27
23.01
34.31
39.88
30.54
27.51
25.02
24.25
1/
Penetration
on the
upper side
(μm)
10.50 –
19.50
13.58 –
27.00
10.61 –
30.04
10.50 –
48.00
12.00 –
39.00
11.42 –
20.40
9.99 –
37.53
13.5 –
57.00
14.21 –
42.63
11.05 –
39.47
13.42 –
49.13
9.00 –
27.66
Average
penetration
Glue line
on the
thickness
upper side
(μm)
(μm)
13.23
22.83
23.81
23.55
23.26
11.93
20.34
18.17
16.11
15.95
16.36
17.56
8.87 –
10.76
44.29 –
46.09
8.20 –
12.75
24.90 –
30.90
10.06 –
16.64
54.08 –
94. 59
5.10 –
20.31
59.02 –
66.49
5.34 –
12.78
83.61 –
96.97
185.68 –
189.79
8.72 –
19.77
Average
glue line
thickness
(μm)
7.48
33.80
9.37
20.25
20.01
58.53
9.23
63.58
7.32
71.89
141.09
12.31
Shear
strength
(MPa)1/
Percent
wood
failure
(%)2/
3.5
0
(0.3)
(5)
3.9
0
(0.6)
(5)
3.2
0
(1.2)
(5)
1.8
0
(0.3)
(5)
0.6
0
(0.1)
(5)
0.3
0
(0.5)
(5)
4.4
0
(0.6)
(5)
4.8
0
(2.1)
(5)
4.9
0
(1.0)
(5)
4.7
0
(1.6)
(5)
3.2
0
(1.6)
(5)
2.7
0
(0.9)
(5)
2/
Mean with standard deviation given in parentheses; Number of samples that had less than 100% wood
failure given in parentheses.
50
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Table 4.5 Results of the glue measurement and shear test – PUR2(N) / maple.
Penetration
Specimen
on the
lower side
(μm)
1N6HT
1N6RT
2N6HT
2N6RT
3N6HT
3N6RT
1N15HT
1N15RT
2N15HT
2N15RT
3N15HT
3N15RT
27.66 –
60.00
16.50 –
62.70
19.12 –
58.92
17.08 –
49.86
15.79 –
42.66
12.63 –
50.53
9.00 –
34.50
9.00 –
54.79
14.21 –
50.53
9.47 –
43.36
15.79 –
31.73
6.51 –
46. 28
Average
penetration
on the
lower side
(μm)
37.03
33.45
35.37
32.72
Penetration
on the
upper side
(μm)
6.00 –
52.70
6.00 –
32.25
6.02 –
38.72
7.89 –
48.10
Average
penetration
Glue line
on the
thickness
upper side
(μm)
(μm)
22.73
15.16
22.66
17.61
31.24
7.24 - 30
10.00
27.38
15.79 - 30
22.84
20.42
0.00
0.00
29.31
34.37
22.71
20.97
19.92
1/
8.25 –
56.23
9.47 –
50.53
9.47 –
12.66
11.05 –
17.37
6.17 –
28.11
17.49
19.48
10.86
14.21
17.70
52.18 –
122.69
19.60 –
31.10
39.03 –
44.54
58.66 –
82.65
98.45 –
108.10
65.52 –
101.47
50.24 –
53.47
58.66 –
82.65
99.47 –
107.12
37.14 –
63.23
47.86 –
89.38
12.49 –
25.86
Average
glue line
thickness
(μm)
67.26
17.97
33.63
56.95
81.53
65.92
38.68
23.75
82.43
38.24
52.03
13.35
Shear
strength
(MPa)1/
Percent
wood
failure
(%)2/
3.6
0
(1.3)
(5)
4.8
0
(1.7)
(5)
1.1
0
(0.7)
(5)
3.1
0
(1.4)
(5)
0.8
0
(0.6)
(5)
3.2
0
(1.3)
(5)
4.9
0
(1.1)
(5)
3.7
0
(0.6)
(5)
5.6
0
(0.5)
(5)
6.5
0
(1.1)
(5)
4.0
0
(0.9)
(5)
1.8
0
(0.4)
(5)
2/
Mean with standard deviation given in parentheses; Number of samples that had less than 100% wood
failure given in parentheses.
Reviewing Figures 4.19 and 4.20 shows that the glue penetration in hard maple is
substantially less than that in black spruce. Examining the CSLM pictures showed that
the adhesive penetrated into wood cells to a shallow depth from the surface and filled any
cracks in the cellular structure. Further penetration is facilitated by the ray cells, since
rays in hard maple are numerous. Although the lower substrate still has deeper glue
penetration, the difference between lower and upper substrates is not as big as in black
51
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
spruce. The influence of wood density on shear strength is opposite to that for black
spruce, i.e. when density increases shear strength decreases. This appears to be related to
glue penetration, since glue penetration decreases with wood density. For high density
group i.e. 3, some extremely low shear strength values are observed. This indicates that
these PUR adhesives are not suited for hardwood with a high density. Shear strengths of
the density groups 1 and 2 are generally comparable with those for black spruce. As
before, there is no link between wood density and glue line thickness.
different density
different density
4.0
100.00
4.5
4.0
3.5
2.5
20.00
2.0
1.5
10.00
1.0
80.00
thickness (micron)
3.0
Shear strength (MPa)
3.5
3.0
60.00
2.5
2.0
40.00
1.5
1.0
20.00
0.5
0.5
0.00
0.00
0.0
1T6HT
lower side penetration
2T6HT
upper side penetration
glue line thickness
0.0
1T6RT
3T6HT
lower side penetration
Shear strength
upper side penetration
3T6RT
glue line thickness
Shear strength
different density
different density
6.0
120.00
6.0
200.00
5.0
100.00
5.0
80.00
4.0
60.00
3.0
40.00
2.0
1.0
20.00
1.0
0.0
0.00
4.0
150.00
3.0
100.00
2.0
50.00
0.00
1T15HT
lower side penetration
2T15HT
upper side penetration
3T15HT
glue line thickness
thickness (micron)
250.00
Shear strength (MPa)
thickness (micron)
2T6RT
0.0
1T15RT
Shear strength
Shear strength (MPa)
thickness (micron)
30.00
Shear strength (MPa)
40.00
lower side penetration
2T15RT
upper side penetration
3T15RT
glue line thickness
Shear strength
Figure 4.18 Effect of wood density on glue characteristics and shear properties - PUR1(T)
/ hard maple.
52
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
different density
140.00
4.0
100.00
3.0
80.00
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
80.00
2.0
60.00
40.00
1.0
thickness (micron)
100.00
Shear strength (Mpa)
60.00
40.00
20.00
20.00
0.00
0.0
1N6HT
lower side penetration
2N6HT
upper side penetration
0.00
3N6HT
glue line thickness
1N6RT
shear strength
lower side penetration
upper side penetration
3N6RT
glue line thickness
Shear strength
different density
6.0
60.00
7.0
100.00
5.0
50.00
6.0
80.00
4.0
60.00
3.0
40.00
2.0
20.00
1.0
0.00
0.0
1N15HT
lower side penetration
2N15HT
upper side penetration
3N15HT
glue line thickness
thickness (micron)
120.00
Shear strength (Mpa)
thickness (micron)
different density
2N6RT
5.0
40.00
4.0
30.00
3.0
20.00
2.0
10.00
1.0
0.00
0.0
1N15RT
Shear strength
Shear strength (Mpa)
thickness (micron)
120.00
Shear strength (Mpa)
different density
lower side penetration
2N15RT
upper side penetration
3N15RT
glue line thickness
Shear strength
Figure 4.19 Effect of wood density on gluing characteristics and shear properties PUR2(N) / hard maple.
The influence of initial wood moisture content on glue penetration for hard maple
parallels that for black spruce as shown in Figures 4.20 and 4.21, that is for PUR1(T),
penetration increases with MC whereas for PUR2(N) the trend is the reverse. Overall
shear strength is higher for the specimens with 15% initial MC. Specimen 3T15HT had a
very thick glue line. From the images captured by CSLM we could see that there were
many bubbles in the glue line. This is a characteristic of PUR adhesives which foam
when they react with moisture producing air bubbles in the glue line. Since the wood
density was high for the specimen, it was difficult for the glue to penetrate in the wood
and a wide glue line resulted. Temperature of glue at fabrication seems to have no effects
on the shear strength, nor glue penetration (Figures 4.22 and 4.23).
For all the specimens in these two groups, the percent wood failure readings were 0%,
which means the specimens all failed in the glue line. This suggests the glue line was
weaker than the wood for these joints.
53
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
250.00
6.0
200.00
5.0
4.0
150.00
3.0
100.00
2.0
50.00
Shear strength (MPa)
thickness (micron)
different MC
1.0
0.00
0.0
1T6HT
1T15HT
lower side penetration
2T6HT
2T15HT
upper side penetration
3T6HT
3T15HT
glue line thickness
Shear strength
120.00
6.0
100.00
5.0
80.00
4.0
60.00
3.0
40.00
2.0
20.00
1.0
0.00
Shear strength (MPa)
thickness (micron)
different MC
0.0
1T6RT
1T15RT
lower side penetration
2T6RT
2T15RT
upper side penetration
3T6RT
3T15RT
glue line thickness
Shear strength
Figure 4.20 Effect of initial wood moisture content on gluing characteristics and shear
properties - PUR1(T) / hard maple.
54
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
120.00
6.0
100.00
5.0
80.00
4.0
60.00
3.0
40.00
2.0
20.00
1.0
0.00
Shear strength (Mpa)
thickness (micron)
different MC
0.0
1N6HT
1N15HT
lower side penetration
2N6HT
2N15HT
upper side penetration
3N6HT
glue line thickness
3N15HT
Shear strength (Mpa)
different MC
100.00
7.0
thickness (micron)
5.0
60.00
4.0
40.00
3.0
2.0
20.00
Shear strength (Mpa)
6.0
80.00
1.0
0.00
0.0
1N6RT
1N15RT
lower side penetration
2N6RT
2N15RT
upper side penetration
3N6RT
3N15RT
glue line thickness
Shear strength
Figure 4.21 Effect of initial wood moisture on gluing characteristics and shear properties
- PUR2(N) / hard maple.
55
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
90.00
4.5
80.00
4.0
70.00
3.5
60.00
3.0
50.00
2.5
40.00
2.0
30.00
1.5
20.00
1.0
10.00
0.5
0.00
Shear strength (MPa)
thickness (micron)
different T
0.0
1T6HT
1T6RT
lower side penetration
2T6HT
2T6RT
upper side penetration
3T6HT
3T6RT
glue line thickness
Shear strength
250.00
6.0
200.00
5.0
4.0
150.00
3.0
100.00
2.0
50.00
1.0
0.00
0.0
Shear strength (MPa)
thickness (micron)
different T
1T15HT 1T15RT 2T15HT 2T15RT 3T15HT 3T15RT
lower side penetration
upper side penetration
glue line thickness
Shear strength
Figure 4.22 Effect of glue temperature at fabrication on gluing characteristics and shear
properties - PUR1(T) / hard maple.
56
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
120.00
6.0
100.00
5.0
80.00
4.0
60.00
3.0
40.00
2.0
20.00
1.0
0.00
Shear strength (Mpa)
thickness (micron)
different T
0.0
1N6HT
1N6RT
lower side penetration
2N6HT
2N6RT
upper side penetration
3N6HT
glue line thickness
3N6RT
Shear strength
different T
120.00
8.0
thickness (micron)
6.0
80.00
5.0
60.00
4.0
3.0
40.00
2.0
20.00
Shear strength (Mpa)
7.0
100.00
1.0
0.00
0.0
1N15HT 1N15RT
lower side penetration
2N15HT
2N15RT
upper side penetration
3N15HT
glue line thickness
3N15RT
Shear strength
Figure 4.23 Effect of glue temperature at fabrication on gluing characteristics and shear
properties - PUR2(N) / hard maple.
57
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
4.2.1.2 Samples with 45-90 degree growth ring orientations (density range 2 and 6%
MC)
Table 4.6 summarizes the results from the glue measurement and shear tests. Shown in
Table 4.6 are the glue penetration values measured on both sides of the glue line by
CSLM, glue line thickness by CSLM, and shear strength and percent wood failure from
shear tests. The effects of wood characteristics and processing parameters on glue
penetration on shear strength can be better understood by studying Figure 4.24.
Table 4.6 Results from glue measurement and shear tests - PUR1(T) / spruce.
Penetration
Specimen
on one side
(μm)
45HT
45RT
90HT
90RT
45-90HT
45-90RT
42.03 –
68.59
24.36 –
58.70
23.00 –
34.87
24.47 –
49.17
11.64 –
87.61
14.87 –
43.97
Average
Penetration
penetration
on the
on one
other side
side (μm)
(μm)
57.57
44.91
30.81
38.73
41.68
30.14
1/
38.78 –
69.58
21.57 –
50.68
20.00 –
36.80
30.67 –
77.97
13.52 –
38.24
17.48 –
26.17
Average
penetration
Glue line
on the
thickness
other side
(μm)
(μm)
52.59
42.24
30.75
49.49
24.48
22.78
15.42 –
19.65
9.85 –
13.62
10.53 –
29.91
11.20 –
12.12
12.71 –
23.97
97.71 –
12.94
Average
glue line
thickness
(μm)
17.46
11.82
16.03
11.55
15.68
11.39
Shear
strength
(MPa)1/
Percent
wood
failure
(%)2/
5.3
51
(0.2)
(5)
5.2
41
(0.6)
(5)
6.0
21
(0.6)
(5)
5.7
37
(0.5)
(5)
5.2
16
(0.4)
(5)
6.2
24
(0.4)
(5)
2/
Mean with standard deviation given in parentheses; Number of samples that had less than 100% wood
failure given in parentheses.
When analyzing the results in Figure 4.24, it should be kept in mind that only the ring
orientation of the upper side substrate was varied. All lower side substrates were
flat-sawn. Therefore to compare the differences in glue penetration, only the red bars
should be considered. The flat-sawn board results are already given in Figure 4.16. Both
the glue penetration and shear strength results seem inconsistent and no clear trend can be
detected with respect to the influence of growth ring orientation on these two properties.
However, the shear strengths of the flat sawn specimens (2T6HT and 2T6RT) appear to
be the lowest of all the groups, and their glue penetration values are among the lowest.
Possibly the presence of rays facilitates the penetration of the adhesive, as was observed
under CSLM. Again, there is no evidence to show that a relationship exists between shear
58
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
70.00
7.0
60.00
6.0
50.00
5.0
40.00
4.0
30.00
3.0
20.00
2.0
10.00
1.0
0.00
Shear strength (MPa)
thickness (micron)
strength and percent wood failure. Glue line thickness is slightly higher if glued with
heated adhesive than adhesive at room temperature.
0.0
45HT
45RT
90HT
90RT
45-90HT
45-90RT
one side penetration (with flat growth ring)
the other side penetration
glue line thickness
Shear strength
Figure 4.24 Effect of growth ring orientation on gluing characteristics and shear strength PUR1(T) / spruce.
4.2.2 Comparison of glue penetration values measured by CSLM and
X-ray
Shown in Tables 4.7 - 4.10 are the glue penetration and glue line thickness measured on
both sides of the glue line by X-ray scanner. Comparison of the measurements by CSLM
and X-ray is summarized in Figures 4.25 - 4.29.
The results from X-ray showed similar trend to those obtained from CSLM. Overall, the
X-ray measurements were much higher than CSLM. This could be caused by the size of
the beam. If the X-ray beam was too big, the X-ray would diffract and in turn generate a
wider glue line and penetration depth. Another reason could be the resolution of the X-ray.
In this study, the resolution of the X-ray scanner is 0.02mm, which is far lower than that
of CSLM. Moreover, the angle between the specimen and the X-ray beam could affect
the X-ray results. If the X-ray was not perfectly perpendicular to the specimen surface, it
was not easy to get the real value of the glue line thickness and penetration depth because
when the X-ray reached the objective, the angle of incidence changed and in turn the glue
59
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
line and penetration thickness generated from refraction changed. X-ray scanner can
provide a quick way to estimate the thickness of the glue line and adhesive penetration.
However, the density of the test samples varied between earlywood and latewood and
even from one growth ring range to another. When penetrated by an adhesive, the density
of the wood will increase. If the glue line is in between the wood with different densities,
it is difficult to identify where exactly the penetration starts. The mechanism of X-ray
relies on the difference in density, which only enables X-ray to provide an approximate
data instead of precise ones due to the variation in wood density. Difference may also be
caused by the location of the measurement since CSLM can only focus on a very small
area or a particular spot in order to enhance the resolution while X-ray has a larger scale
of scanning.
From the comparisons, it is clear that the X-ray technique over-estimates both the glue
penetration and glue line thickness. Further work is necessary to evaluate the reason for
this discrepancy. Another observation is that in the X-ray measurements, the difference in
glue penetration between the lower and upper substrates is much smaller than that for
PUR joints. Again the reason for this is unknown.
Table 4.7 – Glue penetration and thickness measurements by X-ray - PUR1(T) / spruce.
Average
Average
Penetration
Penetration
Average
penetration
penetration Glue line
on the
on the
glue line
Specimen
on the
on the
thickness
lower side
upper side
thickness
lower side
upper side
(μm)
(μm)
(μm)
(μm)
(μm)
(μm)
1T6HT
80 - 100
82.50
40 - 100
67.50
200 - 220
207.50
1T6RT
40 - 100
70.00
40 - 80
62.50
20
20.00
2T6HT
60 - 120
82.50
60 - 120
80.00
20 - 40
25.00
2T6RT
40 – 80
67.50
40 - 80
65.00
20 - 40
30.00
3T6HT
40 – 80
50.00
20 - 80
47.50
20 - 100
62.86
3T6RT
40 – 80
62.50
40 - 100
62.50
20
20.00
1T15HT
60 - 120
84.29
40 - 100
57.14
40 - 100
70.00
1T15RT
60 - 100
82.50
40 - 100
72.50
20 - 100
80.00
2T15HT
60 - 100
80.00
40 -60
52.50
20
20.00
2T15RT
60 - 80
67.50
40 - 80
57.50
100 - 160
140.00
3T15HT
40 - 80
62.50
40 -60
50.00
120
120.00
3T15RT
40 - 120
67.50
40 - 120
50.00
120 - 200
145.00
60
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Upper side
100.00
100.00
80.00
80.00
thickness (micron)
thickness (micron)
Lower side
60.00
40.00
20.00
60.00
40.00
20.00
0.00
0.00
1T6HT 1T6RT
3T15RT
2T6HT
2T6RT
3T6HT
3T6RT
X-ray
1T15HT
CSLM
1T15RT
2T15HT
2T15RT 3T15HT
1T6HT 1T6RT 2T6HT 2T6RT 3T6HT 3T6RT 1T15HT 1T15RT 2T15HT 2T15RT 3T15HT
3T15RT
X-ray
CSLM
Figure 4.25 Comparison of CSLM and X-ray measurements - PUR1(T) / spruce.
Table 4.8 Glue penetration and thickness measurements by X-ray – PUR2(N) / spruce.
Average
Average
Penetration
Penetration
Average
penetration
penetration Glue line
on the
on the
glue line
on the
on the
Specimen
thickness
lower side
upper side
thickness
lower side
upper side
(μm)
(μm)
(μm)
(μm)
(μm)
(μm)
1N6HT
60 - 100
75.00
20 - 80
47.50
160 - 240
172.50
1N6RT
40 - 100
72.50
40 - 80
62.50
20
20.00
2N6HT
60 - 80
65.00
40 - 100
60.00
20 - 60
40.00
2N6RT
60 - 80
70.00
40 - 60
45.00
20
20.00
3N6HT
40 - 80
60.00
40 -60
50.00
20 - 80
43.33
3N6RT
20 - 80
62.50
40 - 80
55.00
20 - 40
25.00
1N15HT
40 - 100
72.50
40 - 80
62.50
20-60
40.00
1N15RT
60 - 80
63.33
20 - 40
36.67
100 - 160
125.00
2N15HT
40 - 80
62.50
40 - 60
47.50
100 - 120
115.00
2N15RT
40 - 80
62.50
20 - 100
45.00
100 - 140
122.50
3N15HT
40 - 60
56.67
40 - 100
53.33
120 - 180
134.29
3N15RT
40 - 80
60.00
40 - 80
60.00
120 - 180
135.00
61
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Upper side
lower side
80.00
100.00
70.00
60.00
thickness (micron)
thickness (micron)
80.00
60.00
40.00
20.00
0.00
50.00
40.00
30.00
20.00
10.00
1N6HT 1N6RT
3N15RT
2N6HT
2N6RT
3N6HT
3N6RT
X-ray
1N15HT
1N15RT
2N15HT
2N15RT 3N15HT
0.00
1N6HT
1N6RT
2N6HT
2N6RT
3N6HT
3N6RT
1N15HT 1N15RT
2N15HT
2N15RT 3N15HT 3N15RT
CSLM
X-ray
CSLM
Figure 4.25 Comparison of CSLM and X-ray measurements – PUR2(N) / spruce.
Table 4.9 Glue penetration and thickness measurements by X-ray – PUR1(T) / hard
maple.
Average
Average
Penetration
Penetration
Average
penetration
penetration Glue line
on the
on the
glue line
on the
on the
Specimen
thickness
lower side
upper side
thickness
lower side
upper side
(μm)
(μm)
(μm)
(μm)
(μm)
(μm)
1T6HT
20 - 60
40.00
20 - 40
30.00
80 - 160
110.00
1T6RT
20 - 60
40.00
20 - 40
30.00
140 - 160
150.00
2T6HT
20 - 60
37.14
20 - 60
40.00
20
20.00
2T6RT
20 - 40
30.00
20
20.00
20 - 140
82.50
3T6HT
20 - 60
42.50
20 - 60
37.50
20
20.00
3T6RT
20 - 60
31.43
20 - 40
30.00
80 - 100
97.50
1T15HT
20 - 60
37.14
20 - 40
25.00
20 - 80
50.00
1T15RT
20 - 80
40.00
20 - 40
30.00
20 - 40
30.00
2T15HT
20 - 40
30.00
20 - 40
30.00
20 - 100
50.00
2T15RT
20 - 40
31.43
20 - 40
28.00
120 - 200
152.50
3T15HT
20 - 40
25.71
20 - 40
25.00
20 - 80
32.50
3T15RT
20 - 40
27.50
20 - 40
26.67
80 - 220
152.50
62
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Lower side
Upper side
50.00
50.00
40.00
thickness (micron)
thickness (micron)
40.00
30.00
20.00
30.00
20.00
10.00
10.00
0.00
0.00
1T6HT
1T6RT
2T6HT
2T6RT
3T6HT
3T6RT
X-ray
1T15HT
CSLM
1T15RT
2T15HT
1T6HT 1T6RT 2T6HT 2T6RT 3T6HT 3T6RT 1T15HT 1T15RT 2T15HT 2T15RT 3T15HT 3T15RT
2T15RT 3T15HT 3T15RT
X-ray
CSLM
Figure 4.26 Comparison of CSLM and X-ray measurements – PUR1(T) / hard maple.
Table 4.10 Glue penetration and thickness measurements by X-ray – PUR2(N) / hard
maple.
Average
Average
Penetration
Penetration
Average
penetration
penetration Glue line
on the
on the
glue line
Specimen
on the
on the
thickness
lower side
upper side
thickness
lower side
upper side
(μm)
(μm)
(μm)
(μm)
(μm)
(μm)
1N6HT
40 - 60
45.00
20 - 40
25.00
80 - 120
100.00
1N6RT
40 - 60
50.00
20 - 100
47.50
20 - 60
42.86
2N6HT
40 - 60
42.50
20 - 60
37.50
40 - 60
57.50
2N6RT
40 - 60
42.50
20 - 40
25.00
140 - 200
172.50
3N6HT
20 - 60
40.00
20 - 40
32.50
60 - 140
102.50
3N6RT
20 - 60
37.50
20 - 40
32.50
80 - 120
95.00
1N15HT
40 - 60
52.50
20 - 60
50.00
40 - 100
48.00
1N15RT
40 - 80
52.50
40 - 60
42.50
20
20.00
2N15HT
40 - 100
60.00
40 - 60
42.50
20 - 60
26.67
2N15RT
20 - 80
50.00
40 - 80
47.50
20 - 40
25.71
3N15HT
20 - 60
40.00
20 - 60
35.00
20 - 60
28.00
3N15RT
20 - 60
37.50
20 - 40
27.50
100 - 180
135.00
63
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Lower side
Upper side
80.00
60.00
50.00
th ickn ess (m icro n)
th ick n ess (m icro n )
60.00
40.00
40.00
30.00
20.00
10.00
20.00
0.00
1N6HT
0.00
1N6RT
2N6HT
2N6RT
3N6HT
3N6RT
1N15HT 1N15RT 2N15HT
2N15RT
3N15HT 3N15RT
1N6HT 1N6RT 2N6HT 2N6RT 3N6HT 3N6RT 1N15HT 1N15RT 2N15HT 2N15RT 3N15HT 3N15RT
X-ray
X-ray
CSLM
CSLM
Figure 4.29 Comparison of CSLM and X-ray measurements – PUR2(N) / hard maple.
4.2.3
X-ray and shear test results for PRF specimens
4.2.3.1 PRF – spruce
Since the PRF adhesive has a dark brown color, several methods have been tried to make
PRF fluorescent under CSLM. A procedure was successfully developed at UNB to work
with a PRF resin. This procedure was transferred to Forintek Canada Corp, St Foy, who
fabricated the PRF glue joint specimens using materials sampled by UNB. A different
PRF was used by Forintek, and it was found, after the joint fabrication and shear tests,
that the PRF resin used by Forintek did not fluoresce under CSLM. In view of this
problem, only X-ray tests were carried out for specimens glued with PRF adhesive.
Table 4.11 provides summary statistics for all test results. Figures 4.30 to 4.33 provide
comparisons of results from different variables. Comparing the summary results shown in
Table 4.11 with those obtained for PUR adhesives, it can be observed that overall the wet
shear strength for PRF glued joints is higher than that for PUR glued joints by about 20%.
There are also more specimens that had percent wood failure equal to 100% than PUR
joints. In fact it was rare for PUR joints to have 100% wood failure in this study.
Consequently the average percent wood failure of PRF joints is much higher than PUR
joints, even though there was only a small difference in joint strength. Glue penetrates on
average, further in PRF joints than in PUR joints. Possibly this could explain the
differences in shear strength and percent wood failure. In view of this, it may be
necessary to review the criteria for percent wood failure for adhesives that provide
sufficient strength and durability but have a lower percent wood failure because of its
shallow glue penetration.
Another observation from the results shown in Figure 4.30 is that while the lower
substrate still had the deeper glue penetration than the upper one. In general, as for PUR
64
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
joints, shear strength increases and glue penetration decreases as wood density increases.
Increasing initial moisture content of wood also has a positive effect on shear strength
and glue penetration, as illustrated in Figure 4.31. On average, shear strength and glue
penetration did not change significantly with assembly time (see Figure 4.32).
Table 4.11 Results of glue measurement and shear tests - PRF / spruce.
Penetration
Specimen
on the
lower side
(μm)
Average
penetration
on the
lower side
(μm)
Penetration
on the
upper side
(μm)
Average
penetration
Glue line
on the
thickness
upper side
(μm)
(μm)
Average
glue line
thickness
(μm)
1MAX6%
80 - 100
90.00
60 - 80
75.00
40 - 60
46.67
1MIN6%
60 - 100
85.00
60 - 100
77.50
20
20.00
2MAX6%
60 - 100
82.50
60 - 80
65.00
20 - 40
35.00
2MIN6%
60 - 120
82.86
60 - 100
80.00
20 - 40
22.86
3MAX6%
60 - 100
80.00
60 - 100
77.14
20 - 40
37.50
3MIN6%
60 - 80
68.57
60 - 80
68.57
20 - 40
23.33
1MAX12%
60 - 160
112.50
80 - 140
100.00
20 - 80
50.00
1MIN12%
120 - 140
122.86
100 - 120
105.71
20 - 40
23.33
2MAX12%
60 - 140
92.50
60 - 100
77.50
20 - 80
45.00
2MIN12%
100 - 160
122.86
100 - 160
122.86
20
20.00
3MAX12%
60 - 100
85.71
60 - 100
71.43
20
20.00
3MIN12%
60 - 120
88.57
60 - 120
91.43
20 - 100
45.71
1/
Shear
strength
(MPa)1/
Percent
wood
failure
(%)2/
5.4
96
(0.2)
(4)
5.8
97
(0.3)
(3)
5.9
93
(0.3)
(5)
6.0
98
(0.1)
(2)
6.5
90
(0.3)
(5)
6.7
99
(0.1)
(1)
5.3
89
(0.3)
(5)
6.7
93
(0.1)
(5)
6.3
92
(0.2)
(5)
6.4
93
(0.2)
(3)
6.4
90
(0.4)
(4)
6.0
99
(0.1)
(1)
2/
Mean with standard deviation given in parentheses; Number of samples that had less than 100% wood
failure given in parentheses.
65
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
different density
different density
6.0
5.0
60.00
4.0
40.00
3.0
Shear strength (MPa)
2.0
20.00
1.0
0.00
0.0
1MAX6%
lower side penetration
2MAX6%
upper side penetration
130.00
120.00
110.00
100.00
90.00
80.00
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
3MAX6%
glue line thickness
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
1MAX12%
Shear strength
lower side penetration
different density
8.0
5.0
60.00
4.0
40.00
3.0
2.0
20.00
1.0
0.00
0.0
lower side penetration
upper side penetration
3MIN6%
glue line thickness
thickness (micron)
thickness (micron)
6.0
Shear strength (MPa)
7.0
80.00
2MIN6%
upper side penetration
3MAX12%
glue line thickness
Shear strength
different density
100.00
1MIN6%
2MAX12%
140.00
130.00
120.00
110.00
100.00
90.00
80.00
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
1MIN12%
Shear strength
Shear strength (MPa)
thickness (micron)
80.00
thickness (micron)
7.0
Shear strength (MPa)
8.0
100.00
lower side penetration
2MIN12%
upper side penetration
3MIN12%
glue line thickness
Shear strength
Figure 4.30 Effect of wood density on gluing characteristics and shear strength - PRF /
spruce.
66
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
140.00
8.0
120.00
7.0
100.00
6.0
Shear strength (MPa)
thickness (micron)
different MC
5.0
80.00
4.0
60.00
3.0
40.00
2.0
20.00
1.0
0.00
0.0
1MAX6%
1MAX12%
lower side penetration
2MAX6%
2MAX12%
upper side penetration
3MAX6%
3MAX12%
glue line thickness
Shear strength
140.00
8.0
120.00
7.0
100.00
6.0
5.0
80.00
4.0
60.00
3.0
40.00
Shear strength (MPa)
thickness (micron)
different MC
2.0
20.00
1.0
0.00
0.0
1MIN6%
1MIN12%
lower side penetration
2MINX6%
2MIN12%
upper side penetration
3MINX6%
3MIN12%
glue line thickness
Shear strength
Figure 4.31 Effect of initial moisture content on gluing and shear strength
- PRF / spruce.
67
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
100.00
90.00
80.00
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
8.0
7.0
6.0
5.0
4.0
3.0
2.0
Shear strength (MPa)
thickness (micron)
different assembly time
1.0
0.0
1MAX6%
1MIN6%
lower side penetration
2MAX6%
2MIN6%
upper side penetration
3MAX6%
glue line thickness
3MIN6%
Shear strength
thickness (micron)
different assembly time
140.00
8.0
120.00
7.0
100.00
6.0
5.0
80.00
4.0
60.00
3.0
40.00
2.0
20.00
1.0
0.00
0.0
1MAX12% 1MIN12% 2MAX12% 2MIN12% 3MAX12% 3MIN12%
lower side penetration
upper side penetration
glue line thickness
Shear strength
Figure 4.32 Effect of assembly time on glue penetration and glue line thickness
- PRF / spruce.
68
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
4.2.3.2 PRF – hard maple
Table 4.12 presents the summary statistics for hard maple joints made with PRF. The
shear strengths of the maple joints are substantially higher than those for black spruce,
except with a few low density groups at 6% MC.
The trends of shear strength and glue penetration with wood density are similar to black
spruce as illustrated in Figure 4.33 i.e. strength increases with wood density whereas glue
penetration decreases with wood density. The influence of initial wood MC on various
properties can be studied from Figure 4.34. As was discussed above, a few of the 6% MC
groups had extremely low shear strength. The values of percent wood failure of these low
strength groups are also low. This is likely due to starved glue line in these groups. The
trend of penetration vs MC is not consistent for minimum and maximum assembly times.
For the maximum assembly time groups, glue penetration increases with MC. However
the trend is reversed for the minimum assembly time group. The trend of the maximum
assembly time groups may be suspect because 2 of the 6 group had extremely low
strength and one group did not bond at all. It seems that for this PRF glue, the
recommended maximum assembly time is not suitable for dense wood. Increasing the
assembly time leads to a decrease in glue penetration as shown in Figure 4.35, but the
average shear strength decreases as the assembly time is reduced from minimum to
maximum.
69
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Table 4.12 Results from the glue penetration measurement and shear tests - PRF / maple.
Penetration
Specimen
on the
lower side
(μm)
Average
penetration
on the
lower side
(μm)
Penetration
on the
upper side
(μm)
Average
penetration
Glue line
on the
thickness
upper side
(μm)
(μm)
Average
Shear
Percent
glue line
strength
wood
thickness
(MPa)
failure
(μm)
*
%
4.3
35
(1.6)
(4)
9.7
53
(0.9)
(4)
6.2
41
(1.5)
(4)
10.0
3
(2.3)
(4)
1MAX6%
60 - 120
85.71
60 - 120
82.86
20 – 40
36.67
1MIN6%
60 - 80
62.50
20 - 60
50.00
20
20.00
2MAX6%
60 - 120
80.00
60 - 80
75.00
20 – 100
50.00
2MIN6%
40 - 80
60.00
40 - 60
54.29
20 – 40
25.00
3MAX6%
40 - 80
56.67
x
x
X
x
3MIN6%
20 - 80
57.50
40 - 60
55.00
20
20.00
1MAX12%
60 - 120
92.50
60 - 120
82.50
20 – 40
25.71
1MIN12%
40 - 80
55.00
40 - 60
50.00
20 – 40
24.00
2MAX12%
60 - 120
90.00
40 - 100
72.50
20 – 40
28.57
2MIN12%
40 - 60
54.29
40 - 60
51.43
20
20.00
3MAX12%
80 - 100
87.50
60 - 100
72.50
20 – 40
35.00
3MIN12%
40 - 80
50.00
40 - 60
45.00
20
20.00
1/
No
bond
N/A
12.2
53
(1.2)
(4)
13.0
98
(0.1)
(2)
10.2
26
(1.2)
(4)
13.2
94
(1.6)
(1)
13.4
24
(0.5)
(4)
12.4
90
(1.9)
(3)
12.5
68
(0.3)
(4)
2/
Mean with standard deviation given in parentheses; Number of samples that had less than 100% wood
failure given in parentheses.
70
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
different density
different density
7.0
120.00
6.0
100.00
16.0
90.00
80.00
12.0
4.0
50.00
3.0
40.00
30.00
thickness (micron)
60.00
Shear strength (MPa)
5.0
70.00
80.00
10.0
60.00
8.0
6.0
40.00
2.0
4.0
20.00
20.00
1.0
2.0
10.00
0.00
0.00
0.0
1MAX6%
lower side penetration
2MAX6%
upper side penetration
3MAX6%
glue line thickness
0.0
1MAX12%
Shear strength
lower side penetration
different density
upper side penetration
3MAX12%
glue line thickness
Shear strength
different density
70.00
16.0
12.0
60.00
14.0
50.00
12.0
10.0
50.00
8.0
40.00
6.0
30.00
4.0
2.0
0.0
0.00
0.00
upper side penetration
0.0
1MIN12%
3MIN6%
glue line thickness
6.0
10.00
10.00
lower side penetration
8.0
30.00
2.0
4.0
2MIN6%
10.0
40.00
20.00
20.00
1MIN6%
thickness (micron)
14.0
70.00
Sh ear stren g th (M Pa)
80.00
60.00
thickness (micron)
2MAX12%
Shear strength
Sh ear stren g th (M Pa)
thickness (micron)
14.0
Shear strength (MPa)
100.00
lower side penetration
2MIN12%
upper side penetration
3MIN12%
glue line thickness
Shear strength
Figure 4.33 Effect of density on glue penetration and glue line thickness PRF / maple.
71
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
different MC
120.00
16.0
14.0
100.00
80.00
10.0
60.00
8.0
6.0
40.00
Shear strength (MPa)
thickness (micron)
12.0
4.0
20.00
2.0
0.00
0.0
1MAX6%
1MAX12%
lower side penetration
2MAX6%
2MAX12%
upper side penetration
3MAX6%
3MAX12%
glue line thickness
Shear strength
80.00
16.0
70.00
14.0
60.00
12.0
50.00
10.0
40.00
8.0
30.00
6.0
20.00
4.0
10.00
2.0
0.00
0.0
1MIN6%
1MIN12%
lower side penetration
2MINX6%
2MIN12%
upper side penetration
3MINX6%
Shear strength (MPa)
thickness (micron)
different MC
3MIN12%
glue line thickness
Shear strength
Figure 4.34 Effect of moisture on glue measurement and shear strength PRF / maple.
72
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
different assembly time
100.00
14.0
90.00
12.0
70.00
10.0
60.00
8.0
50.00
40.00
6.0
30.00
4.0
Shear strength (MPa)
thickness (micron
80.00
20.00
2.0
10.00
0.00
0.0
1MAX6%
1MIN6%
lower side penetration
2MAX6%
2MIN6%
upper side penetration
3MAX6%
3MIN6%
glue line thickness
Shear strength (MPa)
different assembly time
120.00
16.0
thickness (micron
12.0
80.00
10.0
60.00
8.0
6.0
40.00
Shear strength (MPa)
14.0
100.00
4.0
20.00
2.0
0.00
0.0
1MAX12%
1MIN12%
lower side penetration
2MAX12%
2MIN12%
upper side penetration
3MAX12%
3MIN12%
glue line thickness
Shear strength
Figure 4.35 Effect of assembly time on glue penetration and glue line thickness
- PRF / maple.
73
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
4.3 Phase 3
The purpose of this phase was to provide an indication on whether the accelerated aging
protocols
currently
adopted
in
structural
adhesive
standards,
namely
pressure-vacuum-soak and boil-dry-freeze allow the durability of adhesive joints to be
predicted. Two adhesives were used in this phase, PRF and PUR. The total duration of
this phase is 5 years, and is scheduled to end in August 2011, since the last group of
outdoor exposed specimens will not be tested until then.
Tables 4.13 and 4.14 present the test results obtained to-date for PRF and PUR adhesive
respectively. Figure 4.36 presents the test results for 3-month and 6-month exposed finger
joint and 2-ply joint specimens. So far, no difference in long-term performance has been
detected between the two adhesives. The results obtained to-date show that the 2-ply joint
deteriorates faster than finger joint. After 6 month exposure, the tensile strength retention
for finger joints was about 92% whereas the shear strength retention for 2-ply joints was
about 82%.
The strength retention for the accelerated aging specimens is presented graphically in
Figure 4.37 and 4.38 for wet and dry strength respectively. Note that the strength
retention in each case is relative to the short-term strength at the same moisture condition.
For the boil-dry-freeze protocol, CSA O112.9 calls for 8 cycles of exposure. This was one
of the two boil-dry-freeze regimes tested. A 2-cycle boil-dry-freeze regime was included,
to potentially provide information on the possibility of adopting this 2-cycle regime to
represent a shorter term or limited outdoor exposure for adhesive joints. The other aging
protocol was pressure-vacuum-soak. The results show that the pressure-vacuum
procedure does not lead to any reduction in strength. In effect in CSA O112.9 the
pressure-vacuum-soak procedure is used to inject water into dry adhesive joint so that its
wet strength can be determined. Again there is no significant difference between the
performances of the two adhesives.
For wet strength there was virtually no reduction in strength after 2 cycles of
boil-dry-freeze. After 8 cycles, the strength retention was 77% and 88% respectively for
PUR and PRF. For dry strength, PUR performed (78%) better than PRF (69%) after 2
cycles. Strength reduction was almost identical for both adhesives after 8 cycles, with
only 37% of the short-term dry strength retained. It will be interesting to find out if the
5-year exposed specimens will exhibit this level of strength reduction. As a preliminary
finding, it appears that the 2-cycle boil-dry-freeze strength retention is slightly lower than
the 6-month outdoor specimen strength retention, indicating that possibly the 2-cycle
regime can be used to replicate performance of adhesive joints under short-term or
74
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
limited outdoor exposure e.g. occasional wetting.
Table 4.13 – Shear strengths obtained to-date on long-term exposure and matched
accelerated aging tests on glued joints prepared using a PRF adhesive.
Exterior exposure
Group
Finger
joints
E1
Accelerated exposure
2-ply
–
joints
Duration
Group
–
2-ply joints
–
Condition
shear
tensile
shear
strength
strength
strength
(MPa)
(MPa)
(MPa)
28.3 (3.32)
10.95
0 month
A1
5.03 (0.33)
Wet
3 months
A2
5.03 (0.59)
Boil-dry-freeze,
(1.47)
E2
E3
E4
25.74
9.11
(3.72)
(0.59)
26.45
8.73
(2.60)
(1.43)
To be tested in Aug 2007
2
cycles
2
cycles
8
cycles
8
cycles
(test wet)
6 months
A3
7.61 (2.31)
Boil-dry-freeze,
(test dry)
12 months
A4
4.02 (0.70)
Boil-dry-freeze,
(test wet)
E5
To be tested in Aug 2008
24 months
A5
4.45 (1.37)
Boil-dry-freeze,
(test dry)
E6
To be tested in Aug 2011
60 months
A6
5.29 (0.51)
Pressure-vacuum-soak (test
wet)
A7
10.23 (1.77)
Pressure-vacuum-soak (test
dry)
* Values are mean with standard deviation given in parentheses.
75
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Table 4.14 – Shear strength obtained to-date on long-term exposure and matched
accelerated aging tests on glued joints prepared using a PUR adhesive.
Exterior exposure
Group
Finger
2-ply
joints
E1
E2
Accelerated exposure
–
joints
Duration
Group
2-ply
–
joints
tensile
shear
shear
strength
strength
strength
(MPa)
(MPa)
(MPa)
39.67
10.94
(5.98)
(1.54)
35.28
8.95 (0.83)
Condition
–
0 month
A1
4.77 (0.37)
Wet
3 months
A2
4.69 (0.71)
Boil-dry-freeze, 2 cycles
(8.29)
E3
E4
(test wet)
35.64
9.16
6 months
(7.86)
(1.19)
To be tested in Aug 2007
A3
8.54 (1.87)
Boil-dry-freeze, 2 cycles
(test dry)
12 months
A4
4.01 (0.92)
Boil-dry-freeze, 8 cycles
(test wet)
E5
To be tested in Aug 2008
24 months
A5
3.66 (1.69)
Boil-dry-freeze, 8 cycles
(test dry)
E6
To be tested in Aug 2011
60 months
A6
5.29 (0.58)
Pressure-vacuum-soak
(test wet)
A7
11.22
Pressure-vacuum-soak
(1.85)
(test dry)
* Values are mean with standard deviation given in parentheses.
Outdoor exposed specimens
1.2
Strength retention
1
0.8
FJ PUR
FJ PRF
Shear block PUR
Shear block PRF
0.6
0.4
0.2
0
0 month
3 months
6 months
Expoure duration
Figure 4.36 - Outdoor exposed specimen test results.
76
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
Wet shear strength
1.2
0.8
Boil-dry-freeze, 2 cycles
Boil-dry-freeze, 8 cycles
Pressure-vacuum-soak
0.6
0.4
0.2
0
PUR
PRF
Adhesive
Figure 4.37 - Shear strength retention of wet specimens.
Dry shear strength
1.2
1
Strength retention
Strength retention
1
0.8
Boil-dry-freeze, 2 cycles
Boil-dry-freeze, 8 cycles
Pressure-vacuum-soak
0.6
0.4
0.2
0
PUR
PRF
Adhesive
Figure 4.38 - Shear strength retention of dry specimens.
77
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
5. Conclusions and Key Findings
The key findings of this project are summarized below:
1.
General
• Fluorescent microscopy has proved to be a powerful tool in revealing penetration
of the polymeric adhesive into wood substrates quantitatively. With the
appropriate selection of a fluorescent dye, the distinction between wood and
adhesive is clear under CSLM, which enables us to obtain more detailed
information such as the location of resins and penetration depth.
• When a glue joint is prepared with the two substrates oriented horizontally,
penetration of adhesive in the lower substrate is generally higher than the upper
substrate. The difference in glue penetration is dependent on adhesive and wood
substrate characteristics. In general, those adhesives with a low viscosity exhibit a
large glue penetration values.
• Comparing the results between joints made with phenolic and polyurethane
adhesives, it appears that the smaller adhesive penetration of polyurethane
adhesive could well explain the lower shear strength and percent wood failure of
the latter. However while there is a big difference in percent wood failure, there is
a only a small difference between shear strengths of joints made with these two
adhesives. Accordingly, it may be necessary to review the criteria for percent
wood failure for adhesives that provide sufficient strength and durability but have
a lower percent wood failure because of its shallow glue penetration.
3.
Melamine formaldehyde (MF) and polyvinyl acetate (PVAc) adhesives with
Douglas fir, lodgepole pine and black spruce as substrates
• Dry shear strength of lodgepole pine specimens is higher than Douglas fir
specimens for both adhesives.
• There is a large difference in substrate glue penetration for spruce and
Douglas fir, but not for lodgepole pine.
• Glue penetration of MF is about 3 to 4 times that of PVAc for all three species.
• For MF adhesive, specimens fabricated using maximum assembly time
generally achieve a higher shear strength than those fabricated using minimum
assembly time.
3. Polyurethane adhesive with black spruce substrate
• There is no evidence to suggest a relationship between shear strength and
percent wood failure.
• Glue penetration in low density wood is higher than that in high density wood.
• Shear strength increases slightly as glue penetration depth decreases. However
78
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
•
•
this trend may be misleading as the change in shear strength may be caused by
the increase in wood density than the reduction in glue penetration.
Glue joints fabricated with a higher initial wood MC (15%) during fabrication
generally achieve higher shear strength and percent wood failure than those
with a lower initial MC (6%).
Temperature seems to have little effects on penetration depth. Overall,
increasing temperature to increase glue penetration may not be an effective
approach for PUR adhesives.
4. Polyurethane adhesive with hard maple substrate
• The influence of wood density on shear strength is opposite to that for black
spruce, i.e. when density increases shear strength decreases. This appears to be
related to glue penetration, since glue penetration decreases with wood density.
For high density wood, extremely poor glue bond is obtained.
• Glue penetration in hard maple is substantially less than that in black spruce.
• As in the case of black spruce specimens, shear strength is higher for the
specimens with 15% initial MC than 6% initial MC.
6.
Glue penetration values measured by X-ray technique
• X-ray technique over-estimates both the glue penetration and glue line
thickness. Further work is necessary to evaluate the reason for this
discrepancy before the technique can be used to measure these properties with
confidence.
6. Phenol formaldehyde adhesive with black spruce and hard maple substrates
• In general the results on wood property influence are similar to those of black
spruce. Shear strength increases and glue penetration decreases as wood
density increases. Increasing initial moisture content of wood also has a
positive effect on shear strength and glue penetration.
• On average, wet shear strength, percent wood failure and adhesive penetration
of PRF joints are greater than the corresponding values for PUR joints.
• Joints with low density hard maple wood and at a low moisture content have
low shear strength due to over-penetration of glue
9.
Growth ring orientation
• There is evidence to suggest that shear strength of specimens with flat sawn
board is lower than specimens with non-flat sawn boards. Possibly the
presence of rays facilitate the penetration of the adhesive, as was observed
under CSLM.
79
Evaluation of Shear Strength and Percent Wood Failure Criteria for Qualifying New Structural Adhesives
•
10.
Among the samples with non-flat sawn boards, PUR penetrates deeper into
those with 90 degree growth ring orientation but there is no distinct difference
between those with 45 and 45-90 degree growth ring orientation.
Correlation between durability of glue joints subjected to outdoor exposure and
accelerated aging
• For wet strength there was virtually no reduction in strength after 2 cycles of
boil-dry-freeze. After 8 cycles, the strength retention was 77% and 88%
respectively for PUR and PRF.
• For dry strength, PUR performed (78%) better than PRF (69%) after 2 cycles.
Strength reduction was almost identical for both adhesives after 8 cycles, with
only 37% of the short-term dry strength retained.
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