poster_example1 - Characterization of Wood-PMDI Bond-line...

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Thus, both the species show transition from heterogeneous to homogeneous systems with increase in the cure temperature. The graph also shows that there were morphological differences between the two species analyzed at lower cure temperatures Characterization of Wood-PMDI Bond-line Chemistry by Solid-State-NMR Sudipto Das Advisor: Dr. C.E. Frazier Excellent adhesive properties of PMDI-based wood composites are often attributed to the formation of covalent urethane linkages between wood and the resin. The resin may also react with the adsorbed water in wood to form urea and biuret-type structures. Previous studies conducted by our group have established the use of solid state NMR in characterizing wood-PMDI interphase. It consists of performing variable contact time- cross polarization-magic angle spinning (VC-CP/MAS) NMR experiments on wood bonded with doubly labeled PMDI resins. The use of doubly labeled ( 15 N- 13 C) PMDI serves two purposes: 15 N label narrows the 13 C carbonyl signals, thereby increasing the resolution of the urea and urethane peaks (figure 2), The double label gives complimentary N and C spectra (figures 1 & 2). Figure 1. Comparison of 15 N CP/MAS NMR spectra of sample prepared from doubly labeled PMDI and yellow poplar wood flakes (top) and model urethane prepared from yellow poplar wood powder and doubly labeled resin. Figure 2. C omparison of 13 C CP/MAS NMR spectra of composite sample prepared from doubly labeled resin (top), 13 C labeled resin (mid) and wood. 110 120 130 140 150 160 170 180 ppm 3 6 Precure MC = 7% Precure MC = 22% Figure 3. Effect of precure moisture on relative intensity of carbonyl signal in 13 C CP/MAS spectra. Previous have shown that the 154 ppm peak in 13 C spectra is due to urea, while the 156 ppm peak is due to urethane. In 15 N spectra, the urea & urethane peaks are 104 & 97-102 ppm respectively.
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