Thesis
Kinetics and mechanisms of early stages of resorcinol-formaldehyde polmerization
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2012
- Thesis identifier
- T13176
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- This work focuses on the main aspects of mechanism and kinetics of resorcinol-formaldehyde sol-gel polymerization. Nuclear Magnetic Resonance, Dynamic Light Scattering, pH and gel time measurements, as well as IR and Raman spectroscopies were used to examine this process. Experiments and methods were chosen so that both chemical (NMR, IR, Raman) and physical changes (DLS, gel time) would be investigated. First of all equilibria in formaldehyde-water-methanol were examined with a fully quantitative 13C NMR and thus speciation of formaldehyde solutions as a function of its concentration in water and in methanol was successfully determined. The results confirm lack of detectable concentrations of formaldehyde in the aldehyde form and prove presence methylene glycol, its oligomers (up to trimer) and their methoxylated forms. The higher the concentration of the starting formaldehyde stock solution, the longer are the chains of oligomers. Dilution in water leads to depolymerisation to mostly methylene glycol and free methanol, while diluting in methanol yields mostly monomethoxylated form of methylene glycol and free methanol. Based on these results, confirmed by IR and Raman, values of equilibrium constants between all species found in the solution were determined. 1H NMR spectroscopy was used to both verify these results and to establish the changes of on these values in range of temperatures from 283K to 328K. Additionally performed experiments with DLS showed no phase separation caused by potential de-mixing of any of these three liquids. Formaldehyde-water-sodium carbonate solutions which were mimicking the composition of reacting mixtures without resorcinol, were also examined. The results of 13C NMR proved that the linewidth of the spectra at given peaks broadened upon addition of sodium carbonate and this effect was stronger for solutions with greater concentration of the catalyst. DLS experiments proved presence of objects with significant diameter despite filtration of the samples. These are thought to be droplets of a second phase, having a different refractive index and thus being visible in the DLS and their presence may explain changes in relaxation time T2, causing linewidth broadening in NMR experiments. Monitoring of the changes in the composition of reacting mixtures and detection of new species forming in the solutions was done primarily by 13C NMR, 1H NMR and HSQC. Unexpectedly, presence of hydroxymethyl derivatives, both mono- and di-substituted, just a few minutes after addition of formaldehyde was confirmed. Moreover, site C(5) was also found to be substituted. The results of experiments performed at 293K and 328K showed that the concentrations of both total formaldehyde and resorcinol decreased rapidly within first few minutes of mixing the two solutions, followed by a more gradual decrease over time. When plotted in scaled time, the rates of consumption of both reactants are very similar regardless of the catalyst concentrations at a given temperature, confirming that sodium carbonate serves as a catalyst for the resorcinol-formaldehyde substitution reaction. Experiments performed in 293K showed very little changes in concentrations of protons at resorcinol C(5) site, while those at 328K showed a decrease proportional to the one for C(2,4/6). This suggests that di-substituted resorcinol appears to be subject to condensation or further substitution in higher temperature, which leads to formation of either highly reactive or insoluble intermediate which then leaves solution and forms another (micro)phase, since liquid NMR signal from C(5) sites is lost. The DLS experiments proved that regardless of the R/C and R/W ratios, the growth of primary particles which subsequently form a solid network in the gel follows the same pattern. Moreover, the average hydrodynamic radius of these particles is similar across all samples. It was found that the higher the temperature, the concentration of reactants or the catalyst, the sooner the particles reach their final radius. Analysis of autocorrelation functions development in time allowed to confirm the gelation times and to determine the intermediate stage between the sol and gel forms. These novel results disprove previous suggestions that the catalyst directly controls the size of particles forming the gel structure. Experiments on the reacting systems were completed by measurements of pH changes in the course of the reaction. As it turned out, regardless of the catalyst and reactants concentration, all reacting samples exhibited a pH drop of the same magnitude within the first 30-40 minutes. A number of theories explaining this phenomenon are formed in this work, however, none of them seems to be fully explain it.
- Resource Type
- DOI
- Date Created
- 2012
- Former identifier
- 947491
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