Bondability of Ipê (Tabebuia spp.) Wood Using Ambient-Curing Exterior Wood AdhesivesDaniel J. Yelle

United States Department of Agriculture

ForestService

Forest ProductsLaboratory

Research PaperFPL–RP–689

January2017

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, phenol-resorcinol 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.

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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 natu-rally decay-resistant tropical species are being used in exte-rior 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 com-bination of its high density (and thus high intrinsic strength), swirling and interlocking grain pattern (Forest Products Re-search 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 tech-nological requirements and characteristics to be considered ipê wood (Détienne and Vernay 2011). Thus, with the in-creasing 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 ma-ple) (Chudnoff 1984, FPL 2010). Generally speaking, the

principal wood-related factor affecting the bonding of wood is its density (Lambuth 1980). Density influences the pene-tration 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 abil-ity 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 adhe-sives (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 Eu-calyptus species (Plomley 1980). Clearly, there are mecha-nisms 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 ex-ample, 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 dis-play low equilibrium moisture contents at high relative hu-midity. This is probably caused by a bulking action of the extractives, thus decreasing moisture sorption and dimen-sional 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

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10% equilibrium moisture content (EMC) conditions, (2) es-timate wood failure through naked-eye and microscopy techniques, and (3) explore the penetrability of the adhe-sives into ipê’s anatomical features using fluorescence mi-croscopy and transmitted-light microscopy and relate these to bondability.

Experimental This study investigated the effect of five ambient-curing ex-terior 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), fol-lowed by a naked-eye percentage wood failure analysis us-ing 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 condi-tioned 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 Reac-TITE 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-res-orcinol 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 pro-vided 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 spec-ifications. 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 rec-orded, and shear stress was calculated based on the meas-ured shear area. The two adherends from each failed speci-men 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 esti-mating wood failure. To assist in visualizing the wood ver-sus 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 trans-mitted-light microscopy. After soaking the ipê bondline mi-croscopy specimens for several days in water:ethanol (9:1 v/v), sections were microtomed from the cross-sec-tional bondline to 50-μm thicknesses using a sled micro-tome (Spencer Lens Company, Buffalo, New York) and the

Figure 1. Side view of the zero-offset compression block-shear 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 aEPI, emulsion polymer isocyanate; EPO, epoxy; PUR1, polyurethane 1; PUR2, polyurethane 2; PRF, phenol-resorcinol formaldehyde.

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sections were viewed with a Leitz Orthoplan photomicro-scope (Leica Microscopes, Wetzlar, Germany) equipped with a xenon lamp (emitting 200–800 nm) and Leitz H2 fil-ter cube (exciting 390–490 nm). Three magnification scales (6.3×, 40×, and 63×) were selected for each adhesive ana-lyzed by microscopy. Photos were taken using a Nikon (To-kyo, Japan) Digital Sight DS-L1.

Failed surfaces of the wet-condition bonded assemblies were further evaluated using environmental scanning elec-tron microscopy (ESEM) to characterize the failure mecha-nisms. Both sides of failed surfaces of bonded specimens for all five adhesives were imaged at 25× and 50× magnifica-tion. As a control, the conventionally planed tangential sur-face 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 per-formed similar to each other. The EPI and PRF gave the highest average shear stresses of 21.9 and 20.6 MPa, respec-tively. 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, respec-tively. A recent thesis by Almeida (2013) discovered that edge-gluing EPI and PUR adhesives to several tropical hardwoods gave good dry shear strength. However, edge-gluing 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 for-mulation used by Almeida required different bonding pa-rameters (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 aver-age 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 (percent-age) of wood that has adhered to the bondline surface. Nor-mally, high percentage wood failure is desired because this means that the adhesive was just as strong as the wood. Fur-thermore, 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 percent-age 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 av-erage values of only 0% to 5%, whereas PRF gave an aver-age 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 ana-lyzed, and the 6.3× magnification images are shown in Fig-ure 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 satura-tion (FWS)) conditions. (Error bars indicate standard devi-ation. EPI, emulsion polymer isocyanate; EPO, epoxy; PUR1, polyurethane 1; PUR2, polyurethane 2; PRF, phe-nol-resorcinol formaldehyde.)

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with their thick secondary cell wall layer and more substan-tial 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 assem-blies displayed failure within the adhesive or at the adhe-sive–wood interfaces. This type of failure is typical of cata-strophic or instable failure. Conversely, when the PRF-bonded specimens failed after cyclic VPS, the failed tangen-tial 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 signifi-cant wood failure under wet conditions according to the ASTM D 5366 wood failure analysis. Transmitted-light mi-croscopy showed PRF adhesive entering and worming into fiber cell lumina and microvoid spaces between cells (Fig. 6). No other adhesives showed this behavior. By effec-tively 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 simi-lar 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 vol-ume. However, close evaluation of the transmitted-light mi-croscopic 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 en-capsulation of weak fiber cells by the adhesive. This encap-sulation 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 poly-mer 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, phe-nol-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.)

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Figure 5. Environmental scanning electron microscopy (ESEM) images taken from failed specimens in the wet condition (after VPS exposure). The failed surfaces of the emulsion polymer isocyanate (EPI), epoxy (EPO), polyu-rethane 1 (PUR1), and polyurethane 2 (PUR2) demon-strated catastrophic failure, whereas the failed surface of the phenol-resorcinol formaldehyde (PRF) bondline showed a stick-slip failure. The conventionally planed surface is shown in the bottom-right. (v, vessels; f, fiber cells; r, rays; a, adhesive. Scale bars are 200 μm.)

Figure 6. Transmitted-light microscopy images of the ipê–phenol-resorcinol formaldehyde (PRF) bondline. Top, PRF worming between fiber cells (40×); Middle, in-corporation of loose fiber cells into the PRF bondline (40×); Bottom, PRF traveling many fiber cells deep (63×). (PRF adhesive shows as red in images, with fiber cells as yellow-brown. Scale bars are 10 μm.)

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