«CURE KINETICS OF WOOD PHENOL-FORMALDEHYDE SYSTEMS By JINWU WANG A dissertation submitted in partial fulfillment of the requirements for the degree of ...»
H- and quantitative 13C-NMR analyses of the neat resins In order to investigate the effect of acetylation on the molecular weights measurements, the neat resins were also analyzed by 1H- and 13C–NMR in deuterium oxide (D2O, 99.9%) with 0.5% DSS (Sodium 2, 2-dimethyl-2-silapentane-5-sulfonate from Cambridge Isotope) as an internal reference. Due to the insolubility of Chromium (III) acetylacetonate in D2O, Gadolinium (III) chloride hexahydrate of 99.99% (from Aldrich) was used as the relaxation reagent with a concentration around 1 mM for 13C-NMR. A preliminary test with 20 mM of Gadolinium showed that at this concentration, the spin-spin relaxation time T2 greatly decreased, indicating that insufficient points were picked to give the correct spectrum. The 1H-NMR and C-NMR spectra of neat resins in D2O solution were recorded on a Varian Inova 500 spectrometer at 125.6 Hz. 1H- and 13C-NMR parameters were similar with those for the acetylated resins.
RESULTS AND DISCUSSION
The core PF resin also has a broader molecular weight distribution compared to the face PF resin. Average molecular weights (Mn and Mw) and polydispersity index (Mw/Mn ) calculated from the GPC results are presented in Table 2.2. The measured molecular weights confirm that the core resin has a broader molecular weight distribution and higher molecular weights than the face resin.
Figure 2.1 GPC chromatogram for core and face PF resin obtained from the differential viscometer detector.
Table 2.2 Molecular weights of acetylated resins by GPC
Mn: number average molecular weight, Mw:
weight average molecular weight.
Chemical structures and molecular weights of the acetylated PF resins by 1Hand 13C-NMR Species in the resins The typical species in PF resins are hydroxymethylated phenols (HMPs) that may be mono-di or trisubstituted at the ortho and para positions, and their dimmers and oligomers (scheme 1) (Bouajila et al. 2002).
Scheme 1. Typical PF species and carbon numbering for identification.
After acetylation, the hydroxyls are substituted by the acetoxy group (Scheme 2).
Figure 2.2 1H-NMR spectra of the acetylated face PF and core PF resins in CDCl3; Ar:
aromatic ring, Ac: acetoxy group.
Chemical shift assignments of the acetylated PF resins by 1H-NMR H-NMR spectra of acetylated face and core PF resins are shown in Figure 2.2.
The spectra of both resins are similar. Certain clusters of protons in functional groups are well resolved for the acetylated resins and can be assigned to specific chemical shift regions (Woodbrey 1965; Steiner 1975; McGraw et al. 1989). With this resolution in chemical shifts and the ability to quantify functional groups in 1H-NMR, molecular weights can be calculated for two acetylated resins using Woodbrey’s formulae (Woodbrey et al. 1965). In that objective, integration of specific chemical shift regions was performed as presented in Table 2.3.
Table 2.3 1H-NMR chemical shift of acetylated resins in CDCl3 Nature of the proton Chemical Shift region (ppm)
Scheme 3 Linear structure assumption for the acetylated PF resins, Ac: acetoxy groups.
In general, molecular size of resol resin was rather small. It was assumed that the methylene bridges did not bond the aromatic rings into cyclic structures but predominated in linear structures (Scheme 3). Hence, the average number of aromatic rings per molecule chain, n, i.e. degree of polymerization, may be calculated from Eq.
(1) (Woodbrey et al. 1965; Yazaki et al. 1994).
where Rmb in Eq. (1) and Rme in Scheme 3 represent the contents of methylene bridges and methylols per phenolic ring, respectively. For the 1H-NMR spectra of the acetylated resins in CD4Cl, Rmb and Rme can be calculated from integrals of 1H-NMR
spectra of the acetylated resins in Table 2.3 as follows:
From the degree of polymerization, the number average molecular weight for the acetylated resins, the number average molecular weight of acetylated resins, Mn(Ac), can then be calculated with Eq.(3).
The first item in the square bracket of Eq. (3) (i.e. 131) represents the molecular weight of the aromatic ring with an acetoxy group. The second item in parentheses represents the molecular weight of hydrogen that remained on the aromatic ring. The third item represents the molecular weight of acetoxymethyl, and the last item represents molecular weight of methylene bridge –CH2-. Functional groups and molecular weights thus calculated from the 1H-NMR spectra are summarized in Table
2.4. All integration values are expressed per phenolic unit (p.p.u). The core PF has a higher content of methylene bridges, a higher degree of polymerization and molecular weights, but a lower content of methylol, indicating that the core PF resin was more advanced.
Table 2.4 Summary of characteristic structures and molecular weights of two acetylated PF resins based on 1H-NMR spectra in CDCl3.
Mn: number average molecular weight; p.p.u.: per phenolic unit.
Chemical shift assignments of the acetylated PF resins by 13C-NMR C-NMR chemical Shifts of the acetylated PF resols in chloroform-d were assigned in Figure 2.3 and summarized in Table 2.5 according to literature (McGraw et al. 1989; Yazaki et al. 1994). C-NMR probed the spectrum of o-o, o-p, and p-p methylene bridges in the region of 20-40 ppm separately for the acetylated resin. The core PF resin clearly presented the signals of o-o methylene bridges at 30-32 ppm while the signal for the face PF resin was weak at this region.
Figure 2.3 13C-NMR spectra of the core and face PF resins in CDCl3 with carbon assignments shown in scheme 2.
Optimization of 13C-NMR parameters for quantitative analysis Differential NOE among the 13C nuclei and the long spin-lattice relaxation times (T1) of PF carbons prevent quantitative analysis of PF structures using C-NMR spectra. Inverse gated-decoupling technique was used to eliminate NOE variations. However, due to loss of part of the NOE, the number of scans is too high to obtain a reasonable signal-to-noise ratio. Note that there was a build-up of the NOE-effect during the acquisition period when decoupling was active. In order to Table 2.5 13C-NMR chemical shifts of the acetylated PF resol resin in chloroform-d.
* Number in parenthesis corresponds to Scheme 2 and Figure 2.3.
suppress this NOE-effect, the relaxation delay time must be at least 5 times larger than T1 for 13C (Luukko et al. 1998). For resols, the longest T1 relaxation times are those of the quaternary aromatic carbons (in the order of 15 s) (Rego et al. 2004) and result in preparation delays of at least 75 s, which need a prohibitive 53 h measuring time for 2560 repetitive scans. The use of shiftless paramagnetic relaxation reagents of Chromium (III) acetylacetonate and Gadolinium (III) chloride hexahydrate decreased the longest carbon spin-lattice relaxation T1 to approximately 0.35-0.40 s, which reduced measuring time to 3.4 h when the number of scans was set at 2560.
Furthermore, several preliminary tests were run on the acetylated PF resins to find optimum combinations of delay time, acquisition time and the number of scans. Two different regions were used to determine whether the resin spectrum was quantitative.
First, the ratio of the integration value of the phenolic carbon to the integration value of other aromatic carbons should theoretically be 1:5. The other method of determining the quantitative of the NMR analysis was to compare the integration values of the two sharp signals of acetyl methyl, -CH3 at 20 ppm (1 in Figure 2.3 ) and quaternary carbon of carbonyl C=O at 169~172 ppm (11+12 in Figure 2.3) and the ratio should be 1:1. With the delay time at 8 s, the acquisition time at 0.6 s, and the number of scans at 2560, these ratios were around 1:5 and 1:1 (Table 2.6), indicating reliable quantitative measurement.
Table 2.6 The ratios of phenolic to other aromatic carbons (1:5) and acetyl methyl to acetyl carbonyl carbons (1:1).
Measured Core PF 1:4.99 1.02:1
The numbers are corresponding to those in Figure 2.3 Table 2.7 Summary of characteristic structures and molecular weights of two acetylated PF resins based on 13C-NMR spectra in CDCl3.
Mn: number average molecular weight; p.p.u.: per phenolic unit.
Molecular weights of the acetylated PF resins by 13C-NMR The number average molecular weights for two acetylated resins were calculated using Woodbrey’s Eq. (3) (1965) on the basis of 1H-NMR spectra as mentioned above. In this research, this method was also applied to the 13C-NMR spectra to calculate molecular weight using Eq. (3) following same principal. In this case, the Rmb and Rme were directly obtained from respective integrals of signals by setting the integral value of phenolic carbon region as unity (Rmb = 2+3+4, Rme = 5+6 in Table 2.5). Characteristic structures and calculated molecular weights were summarized in Table 2.7. All integration values were expressed per phenolic unit (p.p.u). The calculated degree of polymerization and molecular weight for the core PF resin were larger than those of face PF resins, indicating that the core PF resin was more advanced. The core resin has more methylene bridges and less methylol substitution than the face resin. This is in accordance with the results from 1H-NMR spectra.
Chemical structures and molecular weights of the neat PF resins by 13C- and 1HNMR Chemical shift assignment of the neat PF resins by 1H-NMR H-NMR spectra of the neat core PF resin is shown in Figure 2.4. The spectra of the neat face PF resins are similar. The designations of integrals and their corresponding chemical structures and shifts are presented in Table 2.8 (Woodbrey et al. 1965; Yazaki et al. 1994). The values of A1 cannot be measured experimentally for resoles and were determined by calculation from other experimental integrals (Woodbrey et al. 1965). The number molecular weight was then calculated for two
A1 = (1/5)[A2+(A3/4) +(1/2)(A4 +A5) + A6 + A7], Ar: aromatic ring.
(Notes: symbol A used here for convenient of calculation of A1) Molecular weights of the neat PF resins by 1H-NMR For the neat PF resins, the acetoxy groups (Ac in Scheme 3) should be substituted with hydrogen. Then the number average molecular weight of the neat resin, Mn(OH), could be calculated by Eq. (4) and Eq. (5) based on 1H-NMR spectra of the neat resin.
where A1, A4, A5, A6, and A7 are integrals of 1H-NMR spectra of the neat resins in Table 2.8. Characteristic structures and molecular weights were summarized in Table
2.9. Consistently, the calculated degree of polymerization and molecular weight for the core PF resin were larger than those of face PF resins. This indicates that the core PF resin was more advanced.
Table 2.9 Summary of characteristic structures and molecular weights of two neat PF resins based on 1H-NMR spectra in D2O.
Chemical shift assignment of the neat PF resins by 13C-NMR Figure 2.5 displays the typical 13C-NMR spectra of two neat resins in D2O. Typical chemical shifts were recognized according to the literature and summarized in Table 2.10 (Holopainen et al. 1997; Luukko et al. 1998, Rego et al. 2004). The most diagnostic carbon atoms found in the 13C-NMR spectra of the neat PF resins are the phenolic carbons, which are directly bonded to a hydroxyl group. Due to variations of its environment, the chemical shifts of phenolic carbon were located between 153 and 168 ppm. The wider the distribution of molecular weight, the larger the range of chemical shifts of phenolic carbons were (Holopainen et al. 1997). Comparison of the spectra of two resins found that the chemical shifts of phenolic carbons of the core PF resin had shifted to the lower field than those of the face PF. The range of chemical shifts of phenolic carbon is broader for the core PF resin implying its higher condensation alkalinity and higher molecular weight compared to face PF resin.
Free ortho and para carbons occur at 119 ppm and 121 ppm, respectively, and can be used to follow the progress of resin cure where more and more ortho and para positions become progressively substituted with formaldehyde residues. For these two resins, the intensity of free ortho and para carbons for the core PF resin is smaller than that for the face PF resin. The o-o’ bridge at 30 ppm that was nicely resolved at the C-NMR spectra of the acetylated resins (Figure 2.3) was not observed in neat resins here. Mcgraw et al. (1989) assumed that the o-p’ and o-o methylene bridges appeared together as a cluster of signals due to a strong down field shift of the o-o’ methylene.
Figure 2.5 13C NMR spectra of the core and face PF resins in D2O solvent.