Delineating Pmdi Model Reactions with Loblolly Pine Via Solution-State NMR Spectroscopy

Article in press - uncorrected proof Holzforschung, Vol. 65, pp. 145–154, 2011 • Copyright ᮊ by Walter de Gruyter • Berlin • New York. DOI 10.1515/HF.2011.029 Delineating pMDI model reactions with loblolly pine via solution-state NMR spectroscopy. Part 2. Non-catalyzed reactions with the wood cell wall Daniel J. Yelle1,*, John Ralph2 and Charles Introduction R. Frihart1 1 U.S. Forest Products Laboratory, U.S.D.A. Forest Service, Polymeric methylene diphenyl diisocyanate (pMDI) adhe- Madison, WI, USA sive, applied in oriented strand-board (OSB), laminated 2 Department of Biochemistry (Enzyme Institute) and strand-lumber and exterior particleboard production has been D.O.E. Great Lakes Bioenergy Research Center, touted as the only adhesive that can chemically react with University of Wisconsin-Madison, Madison, WI, USA cell wall polymers since its introduction in the early 1970s *Corresponding author. (Deppe and Ernst 1971; Deppe 1977; Frisch et al. 1983). U.S. Forest Products Laboratory, One Gifford Pinchot Indeed, isocyanate adhesives have been shown to create Drive, Madison, WI 53726, USA durable bonds with wood, thus withstanding swelling and q Phone: 1-608-231-9359 shrinkage stresses at the bondline interface (Frazier 2003). Fax: q1-608-231-9592 E-mail: [email protected] To date, however, there is a lack in understanding of why these adhesives bond well to wood while others do not. Moreover, it is not clear whether covalent bonds are neces- sary for durable wood adhesion. To better formulate an Abstract adhesive with high bondline durability, the molecular char- acteristics of the interaction between the adhesive and wood Solution-state NMR provides a powerful tool to observe the should be better understood (Yelle 2009). presence or absence of covalent bonds between wood and adhesives. Finely ground wood can be dissolved in an NMR- pMDI adhesion compatible solvent system containing dimethylsulfoxide-d6 and N-methylimidazole-d6, in which the wood polymers remain largely intact. High-resolution solution-state two- In the industrial OSB manufacturing process, pMDI is typi- dimensional NMR correlation experiments, based on 13C–1H cally used for the core layer adhesive, and phenol formal- one-bond heteronuclear single quantum coherence, allow dehyde is often applied for the face layers. Bonding with structural analysis of the major cell wall components. This pMDI tolerates higher moisture contents (MC)6%) and rel- technique was applied to loblolly pine that was treated with atively fast press cycles; this allows for continuous pressing polymeric methylene diphenyl diisocyanate (pMDI) related where mats can be injected with steam to accelerate heat model compounds under controlled moisture and temperature transfer and curing (Sellers 2001). Hence, a major objective conditions. Chemical shifts of carbamates formed between is to form strong and durable bonds to wood. the pMDI model compounds and loblolly pine were deter- Several studies have been devoted to exploring how pMDI mined. The results show that: (a) under dry conditions and cures and interacts with wood during adhesive application a high concentration of isocyanate, carbamates will form and hot pressing of OSB (Wendler and Frazier 1996a,b; Ros- preferentially with side-chain hydroxyl groups on b-aryl thauser et al. 1997; Frazier and Ni 1998; Harper et al. 2001; ether and phenylcoumaran-linked lignin units in a swelling He and Yan 2005; Das et al. 2007; Riedlinger et al. 2007). solvent; (b) phenyl isocyanate is more capable of derivati- In all these studies, cell wall moisture plays an important zation in the cell wall than the bulkier 4-benzylphenyl iso- role. Bound water in the cell wall is ubiquitous and rapidly cyanate; (c) at 5% and 14% moisture content, detectable reacts with isocyanates to form urea- and biuret type struc- carbamates on lignin side-chains dramatically decrease; and - (d) under typical conditions of industrial oriented strand- tures. Thus, this bound moisture has dramatic effects on cure board production in a hot press at 5% and 14% moisture (Wendler and Frazier 1996a; He and Yan 2005) and, in some content, no carbamate formation was detected. cases, the wood species will also affect pMDI reaction kinet- ics (Das et al. 2007). In the absence of moisture, or under Keywords: b-aryl ether; 4-benzylphenyl isocyanate; carba- conditions where isocyanate molar equivalents exceed that mate; cell wall; heteronuclear single quantum coherence of water, the possibility of carbamate and allophanate for- (HSQC); g-hydroxyls; Pinus taeda; phenylcoumaran; phenyl mation exists. This is because pMDI (1) wets lumina surfaces isocyanate; polymeric methylene diphenyl diisocyanate more effectively than aqueous resins (Dunky and Pizzi (pMDI); solution-state NMR. 2002); (2) flows into macrovoids if given access by a fracture 2011/0922 Article in press - uncorrected proof 146 D.J. Yelle et al. (Roll et al. 1990; Roll 1993); and (3) has the potential to potential for answering the question whether and to what infiltrate the cell wall (Marcinko et al. 1994; Frazier 2002). degree isocyanates have reacted with wood cell wall polymers. Heterogeneous carbamylation of wood cell wall polymers Objective Isocyanate reactivity with wood and isolated cell wall poly- The objective of this study was to determine the reactivity mers, through derivatization of the wood hydroxyls via of pMDI model compounds with loblolly pine earlywood at heterogeneous carbamylation, has been the subject of con- different MCs. The working steps included: (1) derivatizing siderable study. Based on thickness swelling data, it was con- wood polymer hydroxyls with carbamates using phenyl iso- cluded that isocyanate groups must form carbamate linkages cyanate and 4-benzylphenyl isocyanate; (2) detection of car- with hydroxyl groups in lignin, hemicelluloses, and even to bamate formation with solution-state NMR and elucidating some extent with cellulose microfibrils in wood (Deppe and its regiochemistry, based on the dataset derived from model Ernst 1971). Methyl isocyanate was shown to derivatize substrates as described by Yelle et al. (2011); (3) quantifying southern pine wood blocks, which produced weight percent carbamate formation by two-dimensional volume gains (WPGs) of up to 50% in the absence of catalysts at integration of specific contours in HSQC spectra; and (4) 1208C and 1.03 MPa (Rowell and Ellis 1979). The degree determination of pMDI model compound reactivity with wood of substitution of carbamates on Klason lignin was found to under conditions that mimic industrial OSB manufacturing. be five times higher than that of holocellulose (Rowell 1980). Others found that brown rot lignin reacted approxi- mately 10 times faster than holocellulose when n-butyl iso- Materials and methods cyanate was reacted with Scots pine, catalyzed with pyridine, at 1008C (West and Banks 1986). IR absorption bands char- Chemicals acteristic of carbamate linkages in cellulosics were observed after various isocyanates were reacted under catalyzed (Wea- All reagents were from Aldrich Chemical Company (Milwaukee, ver and Owen 1995) and non-catalyzed conditions (Steiner WI, USA). Phenyl isocyanate and 4-benzylphenyl isocyanate (pMDI model compounds) and distilled N,N-dimethylacetamide (DMAc) et al. 1980; Rensch and Riedl 1993; Gao et al. 2005). ˚ were dried over 3 A molecular sieves. N-Methylimidazole-d6 was synthesized as described previously (Yelle et al. 2008). Characterizing pMDI reactivity with wood To characterize specific linkages in the cured pMDI-wood Sample preparation and composition analysis bondline, solid-state NMR spectroscopy is a logical choice. Loblolly pine (Pinus taeda) microtomed wood was prepared with a 15 13 A double-isotopically labeled (i.e., N and C) isocyanate sled microtome (serial 3423) (Spencer Lens Company, Buffalo, NY, permitted the application of one-dimensional solid-state USA). Tangential earlywood sections (50 mm thick=20 mm=20 mm, NMR to detect carbamate crosslinks in the cured pMDI and 300 mm thick=20 mm=20 mm) were cut. The sections were bondline (Zhou and Frazier 2001). However, carbamate and refluxed in acetone to remove any acetone extractables, dried and polyurea signals are not well resolved by solid-state NMR; equilibrated at 0%, 5% and 14% MC (i.e., in chambers with P2O5, therefore, covalent bonds could not be identified unambig- saturated CaBr2 and saturated NaCl, respectively) prior to reactions uously in the aforementioned study. Two-dimensional solid- with pMDI model compounds. The polysaccharide composition of state 15N–1H correlation NMR experiments have also been the loblolly pine sapwood was analyzed by high pH anion-exchange chromatography (Davis 1998). Klason lignin was determined conducted with wood and lignin powder treated with pMDI. according to the standard acid-hydrolysis method (ASTM 1996). Different urea-type structures, formed during the reaction of Results of these analyses are shown in Table 1. moisture in the wood, were characterized when the MCs of wood were 7% and 14%. Carbamate formation was sug- Reactions with pMDI model compounds gested to occur with lignin powder, but this was not con- firmed with model studies. Furthermore, the unfortunately For the reactions in DMAc, loblolly pine sections (50 mm thick) poor peak dispersion and resolution did not allow unambig- were removed from their respective humidity-controlled chambers uous distinction of the urea 15N–1H correlation from that of and 8 sections per sample treatment were weighed (approx. 100

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