ADHESIVE BONDING OF ESTERN HEMLOCK GLULAM PANELS WITH E-GLASS/VINYL ESTER REINFORCEMENT.

DOUGLAS J. GARDNER [*]

ABSTRACT

The development of novel hybrid fiber-reinforced polymer (FRP) wood composites requires the study of resin matrices not commonly associated with wood adhesive bonding. Vinyl ester resin systems are used extensively in the production of FRP composite

materials due to excellent chemical resistance and tensile strength combined with low viscosity and fast curing. An efficient composite panel for decking applications is proposed by reinforcing eastern hemlock glulam panels with E-glass/vinyl ester resin face sheets. In this study, we examined the adhesive shear strength of vinyl ester resin bonded wood, and compared this to phenol resorcinol formaldehyde (PRF) bonded wood using a modified ASTM D 905 Compression Shear Test and a cyclic delamination test. A hydroxymethylated resorcinol (HMR) primer was also examined as a means to improve vinyl ester bonding with wood. Shear specimens were tested dry and tested wet after vacuum-pressure saturation. Cyclic delamination specimens were evaluated during five cycles of accelerated-aging exposure. Results indicated that vinyl ester resin with HMR primer can form strong and exterior-durable adhesive bonds with wood.

The need for replacement of aged decks in highway bridges and waterfront piers has motivated the development of new durable construction systems. Advances in research and engineering design during the last 25 years have led to the development of glued-laminated (glulam) timber panels for bridge deck construction [11,16]. Glulam decks are constructed of glulam panels that are typically 5-1/8 to 8-3/4 inches thick and 3 to 5 feet wide [11]. In a bridge deck, glulam panels are placed transverse to the supporting beams and traffic loads act parallel to the wide face of the laminations. Compared to reinforced concrete slabs, glulam panels have limited maximum deck spans, which constrains their application in bridge deck replacement projects.

One promising bridge deck system is being investigated by reinforcing glulam panels with fiber-reinforced polymer (FRP) composites using the concept of sandwich construction. The idea of strengthening wood glulam beams with FR? composite tension reinforcement is not new, and has been investigated by several researchers [4,5,13]. However, relatively little work has been done on FRP-reinforced glulam panels for decking applications.

The main advantages of FRP composites to strengthen glulam panels for decking are: 1) high tensile strength; 2) environmental protection of glulam panels; 3) ease and flexibility of installation; and 4) light weight. The FRP reinforcing faces provide tensile strength in both the positive and negative moment regions of the deck and protect the wood core from environmental effects. In addition, the top FRP composite face sheet can be bonded to a polymer concrete overlay that serves as a wearing surface [9]. The wood glulam core resists shear forces, prevents buckling of the FRP face sheets under compression, and contributes to the bending stiffness and strength of the deck. The efficiency achieved in FRP glulam panel construction is because of the optimized combination of FRP face sheets with a glulam core, which is similar to the concept used in I-beams. However, the development of a durable bond between the FRP face sheets and the glulam panel is a major challenge to the advancement of this new technology.

There are two general procedures to reinforce wood with FRP composites: bonding of consolidated laminates and wet lay-up of fabrics. Adhesive bonding of consolidated FRP composites to wood has been studied by several researchers [8,14]. In the wet lay-up process, the resin has a double function of adhesive for wood and matrix for the fabric reinforcement [6,12]. The wet lay-up method has been used with phenol resorcinol formaldehyde (PRF) adhesive to bond E-glass woven fabric to wood beams [6]. However, the PRE adhesive is not formulated to be a resin system for wetting glass fibers and binding them together into a composite. Recently, research conducted at the USDA Forest Products Laboratory [14,15] showed that epoxies develop durable bonds to wood when a coupling agent, such as hydroxymethylated resorcinol (HMR), is utilized. Although epoxies are high-performance resins for composites, their application in developing panels is restricted by the high material cost, fabrication complexity, and post-curing req uirements. Vinyl ester resin systems are used extensively in the production of FRP composite materials due to excellent chemical resistance and tensile strength combined with low viscosity and fast curing [10].

This paper presents the findings on a durable bonding system for reinforcing glulam deck panels with E-glass/vinyl ester composites for exterior decking applications. Wood used in the glulam panels was eastern hemlock, which is a low-grade, but abundant New England species. A review of the uses and properties of eastern hemlock indicates that this wood species poses special challenges for structural utilization due to its tendency to develop ring shake [7]. Reinforcement of eastern hemlock glulam panels presents an opportunity to develop a structural application for this wood species. E-glass stitched fabrics impregnated with a vinyl ester resin were bonded to the glulam panels through the wet lay-up method and compaction was achieved through vacuum bagging.

MATERIALS AND PROCESSES RESIN SYSTEM

The composition of the HMR coupling agent [14] adopted is shown in Table 1. The HMR primer requires a 4- to 6-hour reaction time at 70[degrees]F prior to application. The drying time of the HMR primer prior to vinyl ester application was 16 to 18 hours at room temperature.

The vinyl ester resin selected was Derakane 411-700 (from Dow Chemical Co.). Derakane 411 resins are epoxy-based vinyl esters designed to provide enhanced toughness and high corrosion resistance. The viscosity of 700 cps at 77[degrees]F was selected for ease of fabrication. Elongation to failure at room temperature is approximately 7 to 8 percent. The resin was formulated using a 1.25 percent methyl ethyl ketone peroxide (MEKP) catalyst. The spread rate of the vinyl ester resin was 30 lb./1,000 [ft.sup.2]. of single glueline (MSGL).

A PRF adhesive was selected for the control specimens. The PRF adhesive system was mixed using 70 percent resin (Resorsabond 4242 from Georgia-Pacific Resins, Inc.), 12 percent paraformaldehyde hardener (Resorsabond 4554), and 18 percent water according to a previous study [6]. The adhesive was also spread at a rate of 30 lb./MSGL.

ADHESIVE SHEAR BLOCK TESTING SPECIMENS

Green eastern hemlock (Tsuga canadensis) lumber was dried in an oven for 24 hours at 13 1[degrees]F to equilibrate at a moisture content of 13 percent. Laminations were cut (12 to 17 in. long) from the lumber according to ASTM D 905 specifications [3]. The bonding surfaces were planed. Eight laminations were bonded using PRF adhesive. A set of eight laminations was bonded using vinyl ester resin, and finally, eight laminations were primed with HMR followed by bonding with vinyl ester resin. Following lamination, the specimens were clamped and allowed to cure for 24 hours. Approximately 18 shear block test specimens, 1.75 inches by 2 inches, were cut from each laminated assembly (Table 2). One set of shear specimens was tested at ambient conditions, and the second set of shear specimens was vacuum-pressure saturated in water for 12 hours prior to testing.

CYCLIC DELAMINATION TESTING SPECIMENS

Eastern hemlock lumber was dried at room temperature and had moisture contents of 12 to 17 percent. Wood laminations from nominal 2 by 4's of a grade 2 or better were planed and bonded using PRF adhesive. Glulam panels six laminations wide were assembled.

Two layers of E-glass stitched fabric made of unidirectional continuous fibers with a backing chopped strand mat (CSM) were used for reinforcement (UM-1810 from Brunswick Technologies Inc.), as shown in Figure 1. Each layer of UM-1810 was made of two plies: 1) rovings with 18 oz./[yd.sup.2]. (610 gr/[m.sup.2]) of E-glass continuous fibers in the 0-degree direction; and 2) CSM backing with 1 oz./[ft..sup.2] (305 gr/[m.sup.2]) of E-glass binderless chopped strand mat. The weight ratio between the unidirectional rovings and the CSM backing was 2:1. The rovings, which provide tensile strength to the laminate, were aligned in the direction of the wood laminations. The CSM backing conforms to the surface of the glulam panel and promotes efficient resin flow. The CSM backing attains fast and complete wet-out, resulting in higher resin content than the rovings ply. In the first reinforcing layer, the CSM backing was placed in between the rovings and the wood substrate to enhance adhesive bonding.

The top surface of the glulam panels was planed prior to the application of the FRP face sheets. The reinforcement was applied to the wood glulam by vacuum bagging using both vinyl ester resin and HMR-primed vinyl ester resin. The vacuum bagging process [1] allows for predictable and consistent pressure application, which provides control on the FRP face sheet thickness, reduces the void content, and assists in bonding to the glulam core. The even distribution of vacuum pressure results in a better control of the fiber volume fraction, which is a design parameter that relates to the FRP laminate strength [10].

The materials and equipment required to apply the vacuum bagging process on glulam panels are: 1) bagging film (polyethylene or nylon), which provides a vacuum seal; 2) bleeder/breather cloth, which allows for airflow and acts as a sponge to absorb excess resin; 3) release peel ply, which prevents any material from adhering to the composite part; 4) sealant tape; 5) vacuum connector, which is attached through the bag; and 6) vacuum source such as a vacuum pump or a vacuum generator. Vacuum bagging was carried out under 8 inches of Hg and the vacuum was held for approximately 30 minutes until the resin gelled (Fig. 2). Following vacuum bagging, the FRP-wood lamination was allowed to cure for several days.

The geometry of the cyclic delamination specimens (Fig. 1) was adopted based on modifying the ASTM D 1101 standard [2] to reflect the actual FRP reinforcement of glulam panels. Six test specimens 3 inches in depth were cut from each panel, and the specimens were subjected to moisture cycling following ASTM D 1101 Method B [2].

DATA ANALYSIS

Statistical analysis of the adhesive shear strength data was performed using a two-way analysis of variance procedure using SYSTAT software. Pairwise multiple comparisons of the difference in the variable means was done using the Student-Newman-Keuls (SNK) method performed with a confidence level of [alpha] = 0.05. The variables examined included resin type, sample conditioning, and the (adhesive x condition) interaction. The following statistical model was used:

[y.sub.ijk] = [micro] + [Adhesive.sub.i] + [Condition.sub.j] + [(Adhesive x Condition).sub.ij] + [[epsilon].sub.ijk] [1]

where: y = actual physical measurement; [micro] = mean value of adhesive shear strength (psi); [Adhesive.sub.i] = PRE, vinyl ester, or vinyl ester-HMR; Condition. = dry or wet testing (j = dry or wet); [(Adhesive x Condition).sub.ij] = effect of interaction between the ith adhesive and the jth condition; [[epsilon].sub.ijk] = experimental error associated with [y.sub.ijk]; k = kth observation of the ijth treatment.

Delamination of the FRP-glulam panels was measured using magnification to the nearest 0.05 inches, and the results were recorded as a percentage of the total bondline section.

RESULTS AND DISCUSSION

ADHESIVE SHEAR BLOCK RESULTS

The adhesive shear block results for the PRF, vinyl ester, and vinyl ester-HMR bonded eastern hemlock are shown in Table 2. The statistical analysis of the adhesive shear strength is summarized in Table 3. The results from Table 2 and Table 3 indicate that HMR priming of the eastern hemlock significantly improves the adhesive bond strength and quality of the vinyl ester resin adhesive bond. Without HMR priming, vinyl ester resin does not appear to perform well as a wood adhesive. However, the adhesive bond quality of the HMR-primed vinyl ester is comparable to PRF adhesive in terms of bond strength and percent wood failure for both dry and wet conditions (Tables 2 and 3). These results suggest that vinyl ester resin with HMR priming shows promise as an exterior quality wood adhesive for decking and other construction applications.

CYCLIC DELAMINATION RESULTS

The results of the cyclic delamination test for the E-glass/vinyl ester composite bonded to eastern hemlock glulam specimens are shown in Table 4. Typical specimens after the cyclic delamination test are shown in Figure 3.

Priming of the eastern hemlock glulam with HMR prior to application of the E-glass/vinyl ester reinforcement resulted in a hybrid composite material that can withstand delamination under the exposure conditions of the accelerated-aging test ASTM D 1101 (2). After 5 cycles of hygrothermal exposure, the delamination of the HMR-primed hybrid FRP-wood composite was less than 1 percent. Without the application of the HMR primer, the E-glass/vinyl ester composite reinforcement delaminated completely from the glulam after two cycles of the accelerated-aging exposure (Fig. 3 (top)).

HMR PRIMER AND VINYL ESTER RESIN SYSTEM

HMR is referred to as a "coupling agent" (14), where it is explained that HMR covalently bonds with epoxy resin by forming ether linkages through condensation reactions between hydroxyls of the epoxy and hydroxymethyl groups of HMR. Based on this, we can hypothesize that the epoxy functionality of the vinyl ester resin may react with HMR similarly in our case. Other factors could also affect the efficacy of this coupling agent. HMR is both polar and also aromatic in functionality, and may present a slightly more compatible surface for wetting than neat wood for the less-polar adhesives including vinyl esters, which are also aromatic. HMR may possibly mechanically or physically reinforce the wood surface cells, which are damaged by machining processes. Determining how HMR actually contributes to improved bonding with vinyl ester as well as other wood adhesives needs to be further explored.

Although an exterior-quality adhesive bond was achieved using HMR primer and vinyl ester resin, further work (including creep experiments) needs to be conducted to qualify this system as an exterior structural adhesive for FRP reinforcement of wood.

CONCLUSIONS AND RECOMMENDATIONS

Based on the research presented in this paper, we arrived at the following conclusions:

1. Priming eastern hemlock with HMR provides an exterior-quality adhesive bond for vinyl ester resin that is comparable to PRF adhesive bonds.

2. A hybrid FRP-wood composite panel manufactured from HMR-primed eastern hemlock glulam and E-glass/vinyl ester reinforcement withstands the exterior exposure conditions of the ASTM D 1101 cyclic delamination test (2).

3. FRP-glulam deck panels for exterior use can be efficiently reinforced using the wet lay-up and the vacuum-bagging fabrication method.

The results of this work provide the impetus to examine the bonding of other wood species with the HMR primer and vinyl ester adhesive system. If the glulam core is treated for exterior applications, the effect of preservatives on the HMR and vinyl ester adhesive system needs to be investigated. Also, hybrid FRP-wood composites manufactured with the HMR and vinyl ester adhesive system should be evaluated using ASTM D 2559, which includes a cycle of exposure to a high temperature steam environment and resistance to deformation under static loading.

The authors are, respectively, Assistant Professor of Civil Engineering and Associate Professor of Wood Science, Advanced Engineered Wood Composites Center, Univ. of Maine, Orono, ME 04469-5793; and Visiting Research Assistant, Milwaukee School of Engineering, Milwaukee, WI 53202. The first author is grateful to the Maine Dept. of Transportation and the Federal Highway Administration for funding of this work through the Innovative Bridge Research and Construction Program, Project No. ME-98-07. The third author participated in the Research Experience in Advanced Engineered Wood Composites at the Univ. of Maine funded by the National Sci. Foundation through Grant No. EEC-9732218. The authors thank Brunswick Technologies, Inc. for supplying the glass reinforcements, North End Composites for assistance with the vacuum bagging process, and Dow Chemical for assistance with the selection of vinyl ester resins. This paper was received for publication in December 1999. Reprint No. 9073.

LITERATURE CITED

(1.) AIRTECH Advanced Materials Group. 1997. Basic Guide to Vacuum Bagging. Carson, Calif.

(2.) American Society for Testing and Materials. 1997. Standard test methods for integrity of glue joints in structural laminated wood products for exterior use. ASTM D 1101-97a. ASTM, West Conshohocken, Pa.

(3.) ______. 1998. Standard test method for strength properties of adhesive bonds in shear by compression loading. ASTM D 905-98. ASTM, West Conshohocken, Pa.

(4.) Dagher, H.J. and R.F. Lindyberg. 1999. FRP-reinforced wood in bridge applications. In: Proc. of the 1st RILEM Symposium on Timber Engineering, L. Bostrom, ed. Stockholm, Sweden. pp. 591-598.

(5.) ______, T.E. Kimball, B. Abdel-Magid, and S.M. Shaler. 1996. Effect of FRP reinforcement on low grade eastern hemlock glulams. In: Proc. National Conf. on Wood Transportation Structures. USDA Forest Serv., Forest Prod. Lab., Madison, Wis.

(6.) Foster, F. 1998. Flexural repair and strengthening of timber beams using fiber reinforced polymers. M.S. thesis, Dept. of Civil and Environmental Engineering, Univ. of Maine, Orono, Maine.

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(8.) ______, J.F. Davalos, and U.M. Munipalle. 1994. Adhesive bonding of pultruded fiber-reinforced plastic to wood. Forest Prod. J. 44(5):62-66.

(9.) Lopez-Anido, R., H.V.S. GangaRao, R. Pauer, and V. Vedam. 1998. Evaluation of polymer concrete overlay for FRP composite bridge deck. In: Proc. of the Inter. Composites Expo '98. SPI-Composites Inst., Nashville, Tenn. Pap. 13-F. Technomic Pub. Co., Lancaster, Pa. pp. 1-6.

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(15.) ______. 1997. More durable epoxy bonds to wood with hydroxymethylated resorcinol coupling agent. Adhesives Age 40(8):24-29.

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                    Composition of the HMR Primer [14]
HMR ingredients            Parts by weight
Water (deionized)               90.43
Resorcinol (crystalline)         3.34
Formalin (37% in water)          3.79
Sodium hydroxide (3 molar)       2.44
Total                          100.00
                  Adhesive shear strength and percentage
                  of wood failure of PRF and vinyl ester
                          bonded eastern hemlock.
                No. of specimens     Adhesive shear strength
Adhesive              Dry        Wet           Dry             Wet
                                              (psi)
PRF                    18        18         967 (15) [a]     614 (28)
Vinyl ester            18        18         605 (43)         289 (40)
Vinyl ester-HMR        17        17         861 (25)         645 (20)
                Wood failure
Adhesive            Dry      Wet
                    (%)
PRF                 100      100
Vinyl ester          63        2
Vinyl ester-HMR      94       95
(a.)Values in parentheses are coefficients of variation (%)
                      Statistical analysis of PRF and
                    vinyl ester bonded eastern hemlock.
                Two-way ANOVA of adhesive
                     shear strength [a]
Adhesive                   Dry            Wet
PRF                         A              C
Vinyl ester                 B              D
Vinyl ester-HMR             A              C
(a.)Adhesives and conditions with the
same capital  letter are not significantly
different (P [greater than or equal to] 0.005).
                    Delamination of E-glass/vinyl ester
                    composite bonded to eastern hemlock
                             glulam specimen.
                          Dlamination [a]
Resin            No. of         Cycle
and primer      specimens           1        2   3   4   5
                                  (%)
Vinyl ester         6            45.2     52.1  --  --  --
Vinyl ester-HMR     6             0.3      0.5 0.6 0.7 0.6
(a.)FRP-wood delamination was measured on both sides of the specimen.