Bondability of Ipê (Tabebuia spp.) 

Wood Using Ambient-Curing 

Exterior Wood Adhesives

Daniel J. Yelle

United States Department of Agriculture

Forest

Service

Forest Products

Laboratory

Research Paper

FPL–RP–689

January

2017

From the Ipê tree to 

the wood–adhesive 

bondline to the 

compression-shear 

block test...
Abstract

Ipê is an extremely difficult species to bond because of its 

high density, interlocking grain, and high volumetric 

swelling–shrinkage under prolonged wet conditions. Despite 

its difficulties, the wood is known to be extremely durable 

in exterior conditions because of its resistance to microbial 

and insect degradation. Therefore, investigating its 

bondability with exterior wood adhesives will broaden its 

use in applications in which fasteners are not feasible. In

this study, ipê wood was bonded with five ambient-curing 

exterior-grade wood adhesives (epoxy, emulsion polymer 

isocyanate, two polyurethanes, and phenol-resorcinol 

formaldehyde). Tests for compression-shear using ASTM D 

905 and percentage wood failure using ASTM D 5266

concluded that, of the five tested adhesives, phenolresorcinol formaldehyde provided the most durable bonds 

and highest wood failure after cyclic vacuum pressure 

soaking. Fluorescence microscopy images showed that none 

of the five adhesives penetrated far into the ipê void volume. 

Transmitted-light microscopy (40× and 63×) showed that 

phenol-resorcinol formaldehyde traveled between surface 

fiber cells more effectively than did other adhesives. 

Phenol-resorcinol formaldehyde adhesive was probably able 

to encapsulate loose interfacial fibers and permeate 

microvoids between fibers, therefore acting as a stress 

reducer during moisture-induced swelling and shrinking.

Keywords: bondability, Tabebuia spp., ipê, exterior-grade 

adhesives

January 2017

Yelle, Daniel J. 2016. Bondability of ipê (Tabebuia spp.) wood using 

ambient-curing exterior wood adhesives. Research Paper FPL–RP–689. 

Madison, WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory. 6 p.

A limited number of free copies of this publication are available to the 

public from the Forest Products Laboratory, One Gifford Pinchot Drive, 

Madison, WI 53726-2398. This publication is also available online at 

www.fpl.fs.fed.us. Laboratory publications are sent to hundreds of libraries 

in the United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the 

University of Wisconsin.

The use of trade or firm names in this publication is for reader information 

and does not imply endorsement by the United States Department of 

Agriculture (USDA) of any product or service.

Contents

Introduction..........................................................................1

Experimental ........................................................................2

Results and Discussion.........................................................3

Conclusions..........................................................................4

Literature Cited ....................................................................5

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Introduction

To decrease the use of chemically treated wood, more naturally decay-resistant tropical species are being used in exterior applications for decorative trim, posts, railings and 

decking. Ipê (Tabebuia spp.) is a currently popular tropical 

hardwood of the family Bignoniaceae with high resistance 

to fungal and termite attack (Miller and others 2003, Paes 

and others 2002, Greenwood and Tainter 1993), making this 

wood an attractive choice for exterior applications.

However, bonding of ipê is a challenge because of the combination of its high density (and thus high intrinsic strength), 

swirling and interlocking grain pattern (Forest Products Research Laboratory 1951), and high concentration of certain 

extractives (Girard and others 1988). The high demand for 

ipê in exterior wood applications has prompted studies to 

broaden marketable species within the genus Tabebuia. A 

recent study found 11 Tabebuia species could meet the technological requirements and characteristics to be considered 

ipê wood (Détienne and Vernay 2011). Thus, with the increasing demand for moisture-durable wood species and the 

scarce literature about bonding high-density tropical woods, 

a study on a suitable exterior adhesive for laminating dense, 

naturally durable tropical wood such as ipê was desirable.

The following are some typical properties of ipê wood:

Radial shrinkage (Chudnoff 1984), 6.6%

Tangetial shrinkage (Chudnoff 1984), 8.0%

Volumetric shrinkage (Chudnoff 1984), 13.2%

Specific gravity (Chudnoff 1984; oven-dry weight/green 

volume), 0.85–0.97

Density (CIRAD 1992; wood moisture content of 12%), 

1100 kg/m3

Chalais–Meudon hardness (CIRAD 1992; wood moisture 

content of 12%), 22

Average axial compression-shear stress (CIRAD 1992; 

wood moisture content of 12%), 105 MPa

Relative area of pores in transverse direction (Almeida 

2013), 12.8%

Extractives (Almeida 2013), 13.02%

The specific gravity of ipê (approx. 0.9) is much greater 

than commercial North American hardwood species, such as 

white oak (0.64, bur oak) and hard maple (0.63, sugar maple) (Chudnoff 1984, FPL 2010). Generally speaking, the 

principal wood-related factor affecting the bonding of wood 

is its density (Lambuth 1980). Density influences the penetration of vessel and fiber lumina as well as the permeability 

of the cell wall, which is quite hindered with species such as

ipê that have low void volume. The high density of ipê also 

makes it susceptible to greater swelling and shrinkage of the 

wood, which affects moisture durability. However, the ability of moisture to enter the lumina, and thus the rate at

which it enters, is lower in dense species. The fiber cells in 

ipê are very thick-walled (Johnston 1954). The vessels in 

ipê are small but visible to the naked eye because they are

filled with a yellow powdery material called lapachol, a 

naphthoquinone (Girard and others 1988) with antifungal 

properties (Grohs and Kunz 1998, Velasquez and others

2004). This compound and other extractives from ipê may 

negatively affect bondability of the wood with some adhesives (Almeida 2013). However, it still is not known if the 

presence of ipê extractives might hinder, help, or have no 

effect on the performance of these adhesive bonds. In some 

cases, extraction of tropical woods with solvents of varying 

polarity does increase bond strength (Moredo and others

1990; Alamsyah and others 2007, 2008). However, in other 

cases, extractive removal does not correlate well with bond 

strength (Chen 1970, Moredo and others 1996). Surface 

preparation techniques, such as sanding or grooving, have 

been shown to improve the bondability of high-density Eucalyptus species (Plomley 1980). Clearly, there are mechanisms beyond wettability of the wood surface that are taking 

place with tropical woods, such as density, chemistry of the 

S-3/warty layer, and hygroscopicity (rate of water vapor 

sorption, which is related to dimensional stability). For example, studies have shown that species characterized by 

high extractive content have a decreased hygroscopicity, 

particularly as indicated by low fiber saturation points 

(Nearn 1955). Species with low fiber saturation points display low equilibrium moisture contents at high relative humidity. This is probably caused by a bulking action of the 

extractives, thus decreasing moisture sorption and dimensional changes (Wangaard and Granados 1967).

The goal of this study is to better understand what physical 

and anatomical factors affect bond durability by bonding a 

difficult-to-bond wood species. The objectives were to

(1) measure compression-shear strength of ipê bonded with 

five ambient-curing exterior wood adhesives under wet and 

Bondability of Ipê (Tabebuia spp.)

Wood Using Ambient-Curing Exterior

Wood Adhesives

Daniel J. Yelle, Research Forest Products Technologist

Forest Products Laboratory, Madison, Wisconsin
FPL–RP–689

2

10% equilibrium moisture content (EMC) conditions, (2) estimate wood failure through naked-eye and microscopy 

techniques, and (3) explore the penetrability of the adhesives into ipê’s anatomical features using fluorescence microscopy and transmitted-light microscopy and relate these 

to bondability.

Experimental

This study investigated the effect of five ambient-curing exterior wood adhesives on the shear strength of ipê during 

ambient (10% EMC) and wet conditions using a modified 

ASTM D 905 compression-shear test (ASTM 1994), followed by a naked-eye percentage wood failure analysis using ASTM D 5266 (ASTM 1999). Ipê wood was supplied 

by a local lumber company (Madison, Wisconsin) in the 

form of posts with the dimensions 4 by 4 by 48 in. (101.6 

mm by 101.6 mm by 1.22 m). The wood posts were conditioned at 70 °F (21.1 °C) and 50% relative humidity for 

three months prior to cutting specimens. The adhesives used 

were emulsion polymer isocyanate (EPI), which was ReacTITE EP-920 resin with Hardener 200 (Franklin Adhesives, 

Columbus, Ohio); epoxy (EPO), which was M1013 resin 

with M2017 hardener (Pro-Set, Inc., Bay City, Michigan);

two types of polyurethanes [one fast reacting (PUR1), which 

was ReacTITE 8143 (Franklin Adhesives, Columbus, Ohio)

and one slow reacting (PUR2), which was Gorilla Glue (The 

Gorilla Group, Santa Barbara, California)]; and phenol-resorcinol formaldehyde (PRF), which was Prefere 4001 resin 

with Prefere 5830 hardener (Dynea, Springfield, Oregon).

Methods for compression-shear testing were similar to that 

of Okkonen and River (1988). Briefly, four assemblies (1.25 

by 9 by 0.25 in. (31.8 by 228.6 by 6.35 mm)) were prepared 

with freshly planed (within 24-h period) tangential faces for 

each adhesive, totaling 20 assemblies. Each assembly provided five block-shear specimens. Two specimens from 

each assembly were tested in compression-shear under 10% 

EMC. Two other specimens from each assembly were tested 

in compression-shear directly after a 30-min vacuum/30-

min pressure soak (VPS) cycle (full water saturation

(FWS)). The total number of block-shear specimens was 80 

(5 adhesives × 4 assemblies × 4 specimens). The bonding 

parameters used for each adhesive are summarized in Table 

1, which closely followed the adhesive manufacturer’s specifications. All adhesives were applied by hand using a 2-in.-

(50.8-mm-) wide butyl rubber roller. The assemblies were 

pressed at room temperature in a manual screw press to give 

evenly distributed squeeze-out along the bondline. Once 

fully cured, the assemblies were cut to give block-shear 

specimens with a 1- by 1-in. (25.4- by 25.4-mm) shear area,

and this area was accurately measured using a Mitutoyo 

(Kanagawa, Japan) digital micrometer to the nearest

0.0004 in. (0.01 mm) both before and after the VPS cycle. 

The specimens were randomly assigned to ambient or FWS 

for compression-shear testing. With a compression-loading 

shear tool, the specimens were tested at a rate of 0.10 in/min 

until failure (Fig. 1). The maximum load at failure was recorded, and shear stress was calculated based on the measured shear area. The two adherends from each failed specimen were separated, and the estimate of percentage wood 

failure on the shear area was recorded to the nearest 5%. 

This was done by sectioning each bond area into quadrants. 

The amount of wood still remaining adhered to the bondline 

was expressed as a percentage of the total bond area. The 

FWS specimens were dried at 122 °F (50 °C) prior to estimating wood failure. To assist in visualizing the wood versus the adhesive on the bond area, an ultraviolet (UV) lamp 

was used to fluoresce the adhesive.

To investigate how ipê anatomy might relate to bond 

strength, fluorescence microscopy was used to analyze the 

cross-sectional bondline penetrability. Therefore, the fifth 

block-shear specimen was used for fluorescence and transmitted-light microscopy. After soaking the ipê bondline microscopy specimens for several days in water:ethanol

(9:1 v/v), sections were microtomed from the cross-sectional bondline to 50-μm thicknesses using a sled microtome (Spencer Lens Company, Buffalo, New York) and the 

Figure 1. Side view of the zero-offset compression blockshear test used in this study (courtesy of Okkonen and 

River (1988)).

Table 1. Some important bonding parameters for 

adhesives used with ipê

Adhesivesa

Spread 

rate 

(g/m2)

Mix ratio by 

weight 

(resin/hardener)

Closed 

assembly 

time 

(min.)

Cold press 

pressure 

(kPa)

EPI 320 100:15 12 1,600

EPO 340 100:24 12 124

PUR1 150 N/A 2 620

PUR2 150 N/A 8 550

PRF 290 100:40 12 650

a

EPI, emulsion polymer isocyanate; EPO, epoxy; PUR1, polyurethane 

1; PUR2, polyurethane 2; PRF, phenol-resorcinol formaldehyde.
Bondability of Ipê (Tabebuia spp.) Wood Using Ambient-Curing Exterior Wood Adhesives

3

sections were viewed with a Leitz Orthoplan photomicroscope (Leica Microscopes, Wetzlar, Germany) equipped 

with a xenon lamp (emitting 200–800 nm) and Leitz H2 filter cube (exciting 390–490 nm). Three magnification scales 

(6.3×, 40×, and 63×) were selected for each adhesive analyzed by microscopy. Photos were taken using a Nikon (Tokyo, Japan) Digital Sight DS-L1.

Failed surfaces of the wet-condition bonded assemblies 

were further evaluated using environmental scanning electron microscopy (ESEM) to characterize the failure mechanisms. Both sides of failed surfaces of bonded specimens for 

all five adhesives were imaged at 25× and 50× magnification. As a control, the conventionally planed tangential surface of ipê was also imaged to visualize the actual bonding 

surface.

Results and Discussion

The bond strength of block-shear specimens of ipê under 

ambient (10% EMC) and wet (FWS) conditions were tested. 

Figure 2 shows the results of the compression-shear test. In 

the ambient (10% EMC) condition, most adhesives performed similar to each other. The EPI and PRF gave the 

highest average shear stresses of 21.9 and 20.6 MPa, respectively. This PRF shear strength was slightly higher than that 

of a similar species, Tabebuia guayacan (specific gravity 

1.06), bonded with RF and PRF adhesives, for which Troop 

and Wangaard (1950) observed an average dry shear stress 

of 16.3 MPa. The EPO, PUR1, and PUR2 gave very similar 

average shear stresses of 17.1, 17.2, and 18.4 MPa, respectively. A recent thesis by Almeida (2013) discovered that 

edge-gluing EPI and PUR adhesives to several tropical 

hardwoods gave good dry shear strength. However, edgegluing ipê (Tabebuia spp.) wood provided good dry shear 

strength (12.90 MPa) only when bonded with PUR adhesive

at a spread rate of 200 g/m2

. This is a stark contrast to what 

was observed in this study, but it may be that the EPI formulation used by Almeida required different bonding parameters (e.g., spread rate, pressure, and closed time) or that 

something was different about the ipê wood itself.

After the VPS cycle, the compression-shear stress dropped 

considerably with all adhesives (averaging approximately

5 MPa), except for the PRF adhesive, which gave an average shear stress of 14.2 MPa. The study by Almeida (2013) 

did not use PRF adhesive, but they did observe that ipê 

edge-glued with PUR adhesive (200 g/m2

) gave an average 

humid shear strength value of 5.23 MPa, which was similar 

to this study (5.32 MPa for PUR1 and 5.94 MPa for PUR2).

Conversely, Almeida (2013) observed that ipê edge-glued 

with EPI adhesive (200 g/m2

) gave an average humid shear 

strength of 1.30 MPa, whereas this study determined a

higher value of 5.22 MPa with EPI-bonded ipê. However, as 

mentioned in the previous paragraph, these strength values 

may be a reflection of several varying factors.

Wood failure analysis is used to assess the amount (percentage) of wood that has adhered to the bondline surface. Normally, high percentage wood failure is desired because this 

means that the adhesive was just as strong as the wood. Furthermore, irrespective of the direct or indirect correlation 

between absolute adhesion strength and percentage wood 

failure, the relationship between relative adhesion strength 

and percentage wood failure has been shown to be strong 

(Chow and Chunsi 1979). Under ambient conditions, almost 

all adhesives in this study showed exceedingly low percentage wood failure values after the shear test, except for EPI 

and PRF with averages of 72% and 84%, respectively

(Fig. 3). After VPS exposure, the wood failure results were

again quite dramatic, showing that most adhesives gave average values of only 0% to 5%, whereas PRF gave an average of 43%. Dimensional changes measured in the bondline 

area after the VPS cycle for all adhesives were an average of 

only 2% ± 0.2%. These collective results suggest that most 

adhesives used to bond ipê could not withstand the swelling 

stresses induced at the bondline during the VPS cycle, even 

with very small final dimensional changes. However, PRF 

appeared to stabilize the bondline region, thus maintaining a 

sufficient amount of bondline integrity to allow failure in 

the wood.

Using fluorescence microscopy, cross sections of bondlines 

were analyzed for all five adhesives. Two magnification 

scales (6.3× and 40×) were selected from all adhesives analyzed, and the 6.3× magnification images are shown in Figure 4. These images show that all adhesives had difficulty 

penetrating the ipê anatomical structure. Most were able to 

fill the vessel lumina and voids between fibers fairly well. 

Vessel elements are not thought to contribute significantly 

to the overall strength of hardwood xylem. It is the fibers 

Figure 2. Strength determined using ASTM D 905 (ASTM 

1994) testing for ipê with five exterior adhesives and 

tested under ambient (10% equilibrium moisture content 

(EMC)) and vacuum pressure soak (wet, full water saturation (FWS)) conditions. (Error bars indicate standard deviation. EPI, emulsion polymer isocyanate; EPO, epoxy; 

PUR1, polyurethane 1; PUR2, polyurethane 2; PRF, phenol-resorcinol formaldehyde.)
FPL–RP–689

4

with their thick secondary cell wall layer and more substantial length that are thought to contribute most to the 

bondline integrity.

In the wood failure microscopy analysis after cyclic VPS of 

the tangentially bonded ipê using ESEM (Fig. 5), the images 

showed that the EPI, EPO, PUR1, and PUR2 bonded assemblies displayed failure within the adhesive or at the adhesive–wood interfaces. This type of failure is typical of catastrophic or instable failure. Conversely, when the PRFbonded specimens failed after cyclic VPS, the failed tangential surfaces typically displayed wood fibers a few cells deep

with tearing and gouging of the wood. This type of failure is 

commonly portrayed as a stick-slip mode of failure. The 

PRF adhesive was the only adhesive that provided significant wood failure under wet conditions according to the 

ASTM D 5366 wood failure analysis. Transmitted-light microscopy showed PRF adhesive entering and worming into 

fiber cell lumina and microvoid spaces between cells

(Fig. 6). No other adhesives showed this behavior. By effectively penetrating and encapsulating these voids, the PRF 

adhesive created a strong bondline that acted to stabilize

weak boundary layers and thus suppressed stresses caused 

by swelling and shrinking. The hypothesis put forward here 

is that PRF, being known as an adhesive with capabilities of 

permeating the cell wall, may diffuse into the first few fiber 

cells and act as a stress reducer for fractured surfaces.

Conclusions

Ipê, being an extremely difficult species to bond because of

high density, interlocking grain, and possibly extractives 

content, was examined using five exterior ambient-curing 

adhesives with ASTM D 905 and ASTM D 5266 tests. At 

ambient conditions, most bonded assemblies displayed similar shear strength values. It was found that PRF adhesive

provided the most durable bonds after full water saturation. 

Fluorescence microscopy images provided evidence that all 

five adhesives had difficulty penetrating the ipê void volume. However, close evaluation of the transmitted-light microscopic images at 40× and 63× magnification showed that 

PRF appeared to penetrate the void spaces surrounding the 

interfacial fibers better than the other adhesives. This study 

suggests that the PRF adhesion mechanism involved an encapsulation of weak fiber cells by the adhesive. This encapsulation strengthened the boundary layer between the weak

interfacial fibers and the bulk wood beneath the interface, 

improving bondline moisture durability compared with 

bondlines of the other four adhesives. Extractives were not 

specifically studied here, but because this species has such a 

high extractives content, future studies are needed to better 

understand their role in bondability.

Figure 3. Percentage wood failure for ipê bonded with five

exterior adhesives as determined using ASTM D 905 (ASTM 

1994) and ASTM D 5266 (ASTM 1999) tested under ambient 

(10% equilibrium moisture content (EMC)) and vacuum 

pressure soak (wet, full water saturation (FWS)) conditions. 

(Error bars indicate standard deviation. EPI, emulsion polymer isocyanate; EPO, epoxy; PUR1, polyurethane 1; PUR2, 

polyurethane 2; PRF, phenol-resorcinol formaldehyde.)

Figure 4. Fluorescence microscopy of the cross section 

of ipê bonded with five exterior adhesives. (EPI, emulsion 

polymer isocyanate; EPO, epoxy; PUR1, polyurethane, 

fast curing; PUR2, polyurethane, slow curing; PRF, phenol-resorcinol formaldehyde. Images are shown at 6.3×. 

The PRF image is transmitted light only because of the 

nonfluorescence of this adhesive. Scale bars are

100 μm.)
Bondability of Ipê (Tabebuia spp.) Wood Using Ambient-Curing Exterior Wood Adhesives

5

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FPL–RP–689

6

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