Polymeric diphenylmethane diisocyanate, pMDI, is a wood adhesive that provides excellent composite board properties. Much is unknown about the specific mechanism of pMDI/wood adhesion under conditions that are typical of wood gluing operations. The present research describes the use of ¹⁵N cross-polarization, magic-angle spinning (CP/MAS) NMR as a technique for probing the cure chemistry and bondline morphology of pMDI-bonded wood composites.
A 99% ¹⁵N-enriched pMDI resin with desirable adhesive properties was synthesized. A series of model cellulose/¹⁵N-pMDI composites, cured as a function of cellulose precure moisture content, were tested prior to solid wood composites in order to test the feasibility of this technique. Solid wood/¹⁵N-pMDI composites were then cured as a function of wood precure moisture content, cure temperature, and cure time.
The ¹⁵N CP/MAS NMR spectra clearly show the dominance of the isocyanate/water reaction on the cure chemistry of all composites tested, both cellulose and solid wood. Four prominent resonances are observed in each spectrum: residual isocyanate, polyurea, and the amide and imide nitrogens of biuret type structures. Different trends in the relative intensities of these resonances are observed as a function of the press variables. Significant amounts of urethane formation are not detected; however, low amounts could be obscured by signal overlap.
Relaxation studies using variable contact time experiments were complicated by excessively long cross-polarization rates for nonprotonated nitrogens. However, experiments using variable spin lock times prior to fixed contact periods indicate that the cured resin in these composites is probably a homogeneous continuum.
The utility of ¹⁵N CP/MAS NMR for elucidating fine structural and morphological information from complex isocyanate-cured wood composites is clearly demonstrated. / Master of Science
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/43617 |
| Date | 10 July 2009 |
| Creators | Wendler, Steven L. |
| Contributors | Wood Science and Forest Products |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis, Text |
| Format | xii, 129 leaves, BTD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 31467679, LD5655.V855_1994.W463.pdf |
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