Elucidating How Wood Adhesives Bond to Wood Cell Walls using High-Resolution Solution-State NMR Spectroscopy Daniel J. Yelle U.S. Forest Service, Forest Products Laboratory Madison, WI, 53726, USA [email protected] Introduction Lignin, 25-30% of wood, is a phenolic polymer made Some extensively used wood adhesives, such as up of branched phenylpropanoid units that are capable of pMDI (polymeric methylene diphenyl diisocyanate) and reacting with other phenols during adhesive bonding. PF (phenol formaldehyde) have shown excellent adhesion Model compound studies have shown that under alkaline properties with wood. However, distinguishing whether conditions and ≥100 °C, β-aryl ether linkages release for- the strength is due to physical bonds (i.e., van der Waals, maldehyde to give vinyl ether linkages (Figure 2a) [10]. London, or hydrogen bond forces) or covalent bonds be- Phenylcourmaran linkages release formaldehyde in a simi- tween the adherend and the adhesive is not fully under- lar fashion to give stilbene linkages (Figure 2b) [10]. 13C stood. Previous studies, where pMDI model compounds NMR spectroscopy showed that most of this released for- were reacted with wood, showed that carbamate (urethane) maldehyde ends up as the methylene bridge between the 5- formation with wood polymers is only possible when: 1. positions on guaiacyl units (Figure 2c) [11]. the number of moles of isocyanate is significantly higher a. HO H O than the moles of water molecules in the wood, 2. high γ γ HO β β α O 4 α β O 4 α O 4 temperatures are used, and 3. the isocyanate-based com- OCH3 OCH - OH 3 OCH3 O + CH pound is of low molecular weight [1]. A complementary OCH 2 3 OCH3 OCH3 O O O study confirmed that covalent bond formation between a I II vinyl ether pMDI adhesive and wood does not occur [2]. PF adhesive, III like pMDI, has shown strong evidence for wood cell wall b. infiltration [3-8]. Thus, there is a definite need to discern HO 5 OCH H O 5 5 3 OCH3 OCH3 γ β γ β β O O O how wood cell wall polymers interact with PF adhesive on α α α + O a molecular and Ångstrom scale. This is especially im- CH2 OCH3 OCH3 OCH3 portant to understand when attempting to utilize lignin O O O IV V stilbene byproducts as PF resin substitutes. VI c. The mechanism of alkaline catalyzed PF curing is a two stage reaction. The initial stage involves enolate for- OH + O CH2 mation with resonance stabilized ionization of the 2-, 4-, OCH3 HOH2C OCH3 H3CO C OCH3 H and 6-positions of the phenol followed by methylol for- OH OH OH 2 OH VII VIII methylene bridge mation via nucleophilic attack of the excess formaldehyde. IX During the second stage, the temperature is increased and Figure 2. Lignin hydrolysis of β-aryl ether (a) and phenylcoumaran (b) linkages and methylene bridge formation (c) via condensation of formal- the resols condense to form methylene bridges via a qui- dehyde with free-phenolic units. none methide intermediate (Figure 1). Methylene ether bridges (if formed) tend to revert to methylene bridges [9]. In this study, wood cell wall polymer reactivity is in- OH O O O O H H vestigated when PF adhesive is bonded to earlywood and + OH latewood of loblolly pine. Solution-state NMR spectrosco- H py of dissolved PF-wood sections are analyzed for changes O O O H2 1 13 H H C in chemistry using a 2D H- C-correlation experiment. If OH + H C O 2 CH2OH the wood polymers are modified during the bonding pro- cess, the chemical shifts of the native wood polymers will O CH2 move accordingly, thus revealing which wood polymer OH OH CH2OH structures are reacting with PF, and to what degree. Quinone a methide b OH OH OH OH H2 H2 Experimental C C CH2OH Microtomed wood preparation Figure 1. Alkaline catalyzed reaction of phenol with formaldehyde to 3 form reactive methylols, which condense and polymerize to form PF Loblolly pine cubes (20 mm) were selected that had resin. tangential sides exactly perpendicular to the radial sides. The cubes were placed in boiling distilled water for 10 min ate to give the vinyl ether and stilbene structures. This and then kept submerged for 24 h before sectioning. Mi- chemistry is only possible on linkages that have a free- crotomed wood was prepared using a sled microtome to phenolic group. Hydrolysis of a 4—O branch is likely to give tangential earlywood and latewood sections (0.030 occur to some degree under these conditions, thus convert- mm radial × 20 mm tangential × 20 mm longitudinal) and ing some etherified units to free phenolics. equilibrated to a moisture content of 10% using a chamber Previous literature of a vinyl ether model compound with a saturated salt solution (NaBr). showed NMR correlations to be 6.2/112.5 and 7.3/143.0 ppm (Hα/Cα and Hβ/Cβ, cis), and 5.6/109.5 and 7.3/145.2 PF resin formulation and application ppm (Hα/Cα and Hβ/Cβ, trans), respectively [12, 13]. Fig- Phenol (10.0 g) and formaldehyde (17.9 g of 37% ure 3 shows the NMR spectra of the aromatic region of solution) were charged into a cooled round bottom flask (5 both the earlywood and latewood assemblies that were °C) with stirring. After heating to 25 °C, NaOH (1.1 g, bonded with a 120 min PF cook-time. From the ear- 50%) was added dropwise. The mixture was then heated to lywood-PF samples, the vinyl ether correlations are not 70 °C and held for 60 min. After the first cook stage, the visible. Whereas, with the NMR spectra of the latewood- second NaOH (0.6 g, 50%) was added quickly to the reac- PF samples, the vinyl ether correlations are visible in every tion flask and heated to 85 °C over 15 min. After 45 min cook-time assembly, increasing at the higher cook-times. the viscosity reached 3.7-4.0 stokes. The adhesive was Stilbene model compounds have also been studied cooled in an ice bath and refrigerated until use. previously, which typically show correlations of 7.1- The reaction was monitored with thin-layer chroma- 7.4/128.5 ppm for both Hα/Cα and Hβ/Cβ, but do tend to tography using model compounds and PF standards to fol- overlap with some other lignin aromatics [12]. Thus, these low the stages of polymer growth of the resin at about 30 correlations were difficult to confirm with these HSQC min increments. At each increment, a 2 mL aliquot was experiments alone, but further exploration is underway. removed from the reaction flask and refrigerated. The in- Earlywood, 120 min cook-time assembly crements chosen were 60, 90, 120, and 150 min, with the 120 min time-point representing a typical PF adhesive for 110 G2 wood composite production (49% solids). G5/6 Each PF aliquot was weighed out onto the wood sec- G6 tions so that the weight of the resin solids was approxi- 120 H3CO 5 5 OCH3 mately twice the weight of the sections. The sections were OH OH then quickly joined together on a glass slide and secured 130 between two glass slides. The assemblies were placed into 6 2 6 2 R R + a non-spouted beaker with a watch glass on top, sat at r.t. C C C H3CO C OCH3 H2 H2 H2 H OH OH OH 2 OH 6 2 140 for 30 min, heated to 140 °C and held for 1 h. R = OH or phenol unit 5 G OMe NMR preparation and experiments OR Ball-milling of the PF bonded earlywood and late- 7.5 7.0 6.5 6.0 [ppm] wood assemblies and controls were prepared as previously Latewood, 120 min cook-time assembly described [1]. β α O 4 G2 110 One sample (20 mg) from each milled assembly was OCH3 OCH3 G5/6 directly placed into a 5 mm NMR tube, 500 µL of DMSO- OH G6 d6 was added and was sonicated at 30 °C for 1 h. O 4 120 OCH α β 3 NMR spectra were acquired at the University of Wis- H3CO 5 5 OCH3 OH OH OCH3 consin-Madison on a Bruker-Biospin (Rheinstetten, Ger- OH many) AVANCE 500 MHz spectrometer fitted with a cry- 130 6 2 6 2 ogenically cooled TCI probe as previously described [1]. R R + C C C H3CO C OCH3 H H H H 2 OH 2 OH 2 OH 2 OH 6 2 140 R = OH or phenol unit 5 G Results and Discussion OMe OR For simplicity, the 120 min cook-time assemblies are 7.5 7.0 6.5 6.0 [ppm] the only assemblies shown with NMR spectra in this pa- Figure 3. The aromatic region of NMR 1H-13C correlation spectra show- per. The 60, 90, and 150 min cook-time assemblies were ing: the earlywood (top) and latewood (bottom) 120 minute cook-time also analyzed via NMR, but focused here on the 120 min assemblies. The native guaiacyl correlations are blue, the 5-5 (biphenyl) correlations are purple, and the methylene bridge correlations are brown. since this resin gave the target viscosity. The vinyl ether correlations (red) are only seen in the latewood assem- blies. Alkaline hydrolysis of major lignin linkages Under alkaline conditions the β-aryl ether and the Aromatic condensed linkages phenylcoumaran linkages are cleaved at the Cβ—Cγ bond, The formation of the methylene bridge (aryl—CH2— releasing formaldehyde via a quinone methide intermedi- aryl), as shown in Figure 1, is the most predominant link- age formed between PF resins and guaiacyl units in lignin.