Kinetics of alkaline-catalyzed cardanol–formaldehyde reaction. I

The kinetics of reaction between cardanol and formaldehyde when catalyzed by an alkali, sodium hydroxide, has been studied and effects of process parameters, e.g., cardanol formaldehyde molal ratio, catalyst concentration, and temperature have been investigated. The addition reaction has been shown to be a second-order reaction when the concentration of active positions of reactants is used instead of their molar concentrations. Specific reaction rate constants have been determined for various cardanol–formaldehyde molal ratios, catalyst concentrations, and reaction temperatures. The energy of activation, E, is found to be 67.541 MJ/kg-mol (16.132 kcal/mol). The correlation between overall reaction rate constant and the process parameters has also been developed.     

Kinetics of alkaline-catalyzed cardanol–formaldehyde reaction. III. Determination of composition of the resin

In the present study, a quick method for the determination of composition of the resin formed by alkaline-catalyzed cardanol–formaldehyde reaction has been suggested. The complete analysis of the product could be carried out by determining experimentally the concentration of only one reactant, viz., formaldehyde present in the reaction mixture at any time.     

Kinetics of the acid-catalyzed phenol–formaldehyde reaction

A kinetic study of the reaction of phenol with formaldehyde has been carried out at temperatures of 65° ± 0.05°C, 70° ± 0.05°C, 75° ± 0.05°C, and 80° ± 0.05°C using hydrochloric acid as catalyst. The pH maintained was 1.14, 1.32, 2.20, and 3.00. The reaction follows a second-order rate law. The rate is found to increase with decrease in pH. The overall rate constants are resolved into step rate constants. The values of the Arrhenius parameters and the entropy of activation for the overall reaction as well as for the step reactions have been calculated. A mechanism conforming to the energies and entropies of activation of the reaction has also been suggested.     

Kinetics and mechanism of the alkali-catalyzed p-cresol–formaldehyde reaction

The kinetics of the reaction of p-cresol with formaldehyde in relation to the functionality of p-cresol using NaOH as catalyst has been studied at temperatures of (65 ± 0.05)°C, (70 ± 0.05)°C, (75 ± 0.05)°C, and (80 ± 0.05)°C. The pH maintained was 7.0, 8.0, 9.0, 9.4, and 10.0. The reaction follows a second-order rate law. The rate was found to increase with increase in pH. The stepwise rate constants (k₁ and k₂) for the formation of monomethylol-p-cresol and dimethylol-p-cresol, respectively, were calculated from the overall rate constant k. The values of Arrhenius parameters and the entropy of activation for the overall as well as the stepwise reactions were calculated. The experimental and calculated values of k at pH 10.0 and temperatures 65, 70, 75, and 80°C were found to agree well within experimental errors. A mechanism conforming to the energies and entropies of activation of the reaction is suggested.     

Kinetics of the addition stage in the melamine–formaldehyde reaction

The addition reaction between melamine and formaldehyde has been kinetically separated from the subsequent condensation stage by suitable choice of concentration and temperature conditions. The reaction, which is reversible, has been monitored by estimation of the free formaldehyde content of the system. It has been investigated over the range of mean degree of methylolation 1 < R′ < 3.7 of the melamine nuclei, the temperature range 25–55°C., and the pH range 5.7–10.2. The rate data thus obtained have been treated according to the random reversible addition scheme for which reasonable, first approximation, agreement was obtained. Average kinetic and thermodynamic constants have been calculated and are discussed in terms of the present model. The factors which are likely to cause deviations from randomness are described. The addition of formaldehyde to melamine proceeds by superposition of an OH^?-catalyzed step with a minor solvent-catalyzed or uncatalyzed one.     

Cardanol–glycols and cardanol–glycol-based polyurethane films

The syntheses of cardanol–glycols (CGs) and CG-based polyurethane (CGPU) films have been investigated. The characterization of CGs and CGPU films were determined by IR, ¹H-NMR spectra as well as a swelling test and DSC studies. The increase of molecular weight of glycols leads to a decrease of cardanol content in CGPUs and hence decreases crosslinking density of the films, which strongly affects the swelling property and glass transition temperature. The autooxidation-polymerization of CGPUs through the double bonds of the cardanol side chain, catalyzed by cobalt salt, resulting the crosslinking films was also discussed. © 1997 John Wiley & Sons, Inc. J Appl Polm Sci 65:507–510, 1997     

Urea–formaldehyde reaction system. An experimental investigation

Batch production of urea–formaldehyde resins at temperatures up to 100°C takes several hours for completion. Reduction of the batch time may be possible with the higher reaction rates obtained at higher temperatures and increased pressures. In order to investigate this possibility, an experimental technique to obtain the necessary kinetic data, without loss of formaldehyde by evaporation, was developed. The results are compared with earlier low-temperature data extrapolated to the present range of interest. The results were interpreted on the basis of the successive reaction of two or three molecules of formaldehyde with a molecule of urea.     

Modification of acetophenone–formaldehyde and cyclohexanone–formaldehyde resins

Cyclohexanone–formaldehyde and acetophenone–formaldehyde resin were in situ modified with phenol, Bisphenols, and substituted acetophenones. Furthermore, acetophenone–formaldehyde, cyclohexanon–formaldehyde, and in situ-modified resins were modified with anhydrides such as acetic anhydride, maleic anhydride, dodecenylsuccinic anhydride, 3,4,3′,4′-biphenyltetracarboxylic dianhydride, and 4,4′-oxydiphtalic anhydride. Modification of these resins with hydroxyl amine, semicarbazide, and phenyl hydrazine were also studied. Melting points, solubilities in organic solvents, FTIR, and NMR spectrum of the modified resins were determined. © 1996 John Wiley & Sons, Inc.     

Pyrolysis of melamine–formaldehyde and urea–formaldehyde resins

Pyrolysis of melamine–formaldehyde and urea–formaldehyde resins in helium and air was investigated by means of TG and gravimetry with isothermal heating, as well as elemental and HCN analyses. Weight loss curves suggest three kinds of reactions involved in the pyrolysis, namely, initiation reactions, reactions splitting off volatile fragments, and reactions forming stabilized structures. In TG, in both helium and air atmospheres, the active weight loss of the melamine resin was completed in two steps, and that of the urea resin was completed in one step, which, however, consisted of a few small successive steps. The isothermal heating weight losses progressed through a few stages of first- and zeroth-order reactions. Arrhenius parameters were obtained for the weight losses in TG and with isothermal heating. The residue from the melamine resin is rich in carbon and nitrogen, and poor in oxygen and hydrogen, while that from the urea resin is rich in carbon, and poor in nitrogen, oxygen, and hydrogen. The effects of temperature on HCN yield changed, depending on the amount of air fed. The melamine resin evolved much more HCN than the urea resin because of the more stable C—N linkages in the resin.     

Kinetics of the water–gas shift reaction over nanostructured copper–ceria catalysts

Water–gas shift reaction was studied over two nanostructured Cu_xCe_1−xO_2−y catalysts: a Cu_0.1Ce_0.9O_2−y catalyst prepared by a sol–gel method and a Cu_0.2Ce_0.8O_2−y catalyst prepared by co-precipitation method. A commercial low temperature water–gas shift CuO–ZnO–Al₂O₃ catalyst was used as reference. The kinetics was studied in a plug flow micro reactor at an atmospheric pressure in the temperature interval between 298 and 673 K at two different space velocities: 5.000 and 30.000 h⁻¹, respectively. Experimentally estimated activation energy, E_af, of the forward water–gas shift reaction at CO/H₂O = 1/3 was 51 kJ/mol over the Cu_0.1Ce_0.9O_2−y, 34 kJ/mol over the Cu_0.2Ce_0.8O_2−y and 47 kJ/mol over the CuO–ZnO–Al₂O₃ catalyst. A simple rate expression approximating the water–gas shift process as a single reversible surface reaction was used to fit the experimental data in order to evaluate the rate constants of the forward and backward reactions and of the activation energy for the backward reaction.     

Epoxy—phenol—cardanol—formaldehyde systems: Thermogravimetry analysis and their carbon fiber composites

Systems consisting of epoxy resin and phenol-cardanol-formaldehyde (PCF) were prepared. The thermal characterization of the systems, investigated by thermogravimetry analysis, indicated that the increase of PCF content improves the thermal stability. Chemical resistance as well as mechanical properties of their carbon-fiber composites were also determined. The matrix-fiber interfacial study by scanning electron microscopy showed that the higher the PCF amounts in the system products, the better the interfacial bonding. © 1996 John Wiley & Sons, Inc.     

Kinetics of reaction between meta-substituted long chain alkyl phenols and formaldehyde

The reaction between formaldehyde and long chain alkyl phenols as cardanol and 3-pentadecylphenol is found to be of second order, when it is studied in dioxanater (1:1) system in presence of catalyst hydroxide sodium as at 28, 50 and 70°C. Individual methylols were separated and identified by thin layer chromatography. The long alkyl chain in the meta-position of the phenolic nuclei sterically hinders The reactivity of 2-position at 28°C, but at higher temperatures i. e. 50 and 70°C this position gets activated resulting in the formation of various methylols.     

Pyrogallol–formaldehyde thermosetting adhesives

A new thermosetting wood adhesive system from pyrogallol has been developed. Pyrogallol can be easily obtained from tara pods (Caesalpinia spinosa) a native leguminosae of low cost widely distributed in Peru. In this work, polymerization of formaldehyde with pyrogallol was carried out at different pH values and optimal conditions were determined to establish the adhesive formulation. The reactivity of this resin was characterized by differential scanning calorimetry (DSC) and the results were compared with those obtained with resins made with tara tannin, gallic acid, and phenol. The results show that tara tannin and gallic acid are less reactive due to the presence of deactivating groups (i.e., carboxylates) in the phenolic moieties while their polymerization is limited to that of a bidimensional network upon curing. In contrast, pyrogallol–formaldehyde kinetic parameters (Ea and ΔH) were determined and they are comparable with those of phenol-formaldehyde adhesives. In addition, mechanical property values (MOR, MOE, and IB) of particleboards prepared with pyrogallol–formaldehyde compare favorably to those of Canadian standard requirements (CSA). Main assets of the new thermosetting adhesive is lower pressing times and temperatures than those currently used in the industry. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:399–408, 1997     

Kinetics of the reaction between urea and formaldehyde in the presence of sulfuric acid

The kinetics of the reaction between urea and formaldehyde were studied in the presence of various amounts of sulfuric acid (5–45% by weight) at different temperatures (5°, 15°, and 25°C). The reaction was shown to follow first-order kinetics. The activation energy for the reaction varies from 12.51 kcal/mole to 14.59 kcal/mole in the range of sulfuric acid concentration studied.     

Preparation of cured urea–formaldehyde resins of low formaldehyde emission

Several polymers containing amino or amido groups and biuret were tested as additives to ureaformaldehyde (UF) resin in order to neutralize its inherent acidity and combine free formaldehyde released upon hydrolysis of cured UF polycondensate. Each modifier was incorporated to liquid methylolureas at weight ratios of 1:100, 2:100, and 3:100 prior to curing with the aid of acetic acid. Over 10 days of maintaining aqueous suspensions of the ground-up resultant solid resins at ambient temperature, a neutralizing effect was exhibited most visibly by polyacrylamide, polymethacrylamide, and biuret, the test with chitosan and casein giving results slightly different from those obtained for the control nonmodified cured UF polymer. Polyacrylamide, biuret, and casein proved to be excellent inhibitors of formaldehyde release from the hardened resins which were suspended in water at ambient temperature. On the other hand, chitosan did not reduce the evolution of HCHO but, instead, augmented it when its content was 1 g/100 g of the original liquid resin before cure.     

Fibers from urea–formaldehyde resins

The preparation of fibers from aqueous urea–formaldehyde resins has been investigated; a dry spinning process has been developed based on the extrusion of catalyzed resin into a drying chamber at 180–220°C, producing a multifilament yarn at spinning speeds of up to 600 m/min. A range of UF filaments was produced with diameters between 10–70 μm; the tenacities of spun filaments were 6–10 cN/tex, initial moduli were 220–340 cN/tex, and elongation at break was 4–10%. The best tensile properties resulted from conditions that produced the smallest diameter fibers. Postspin heat treatment improved the tenacity to 14 cN/tex and the elongation to 20%. Spinnability improved with increased viscosity of the spinning solution and increased cell temperature, while tenacity and elongation increased with increasing cell temperature and spinning stretch. A correlation was found between TGA weight loss (between 105 and 200°C) and fiber tenacity. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 64–74, 2000     

Rheometry of aging of colloidal melamine–urea–formaldehyde polycondensates

Aged and whitened melamine–urea–formaldehyde (MUF) resins in a colloidal state were tested with parallel‐plate rheometry to determine the extent of their viscoelastic behavior. Only in advanced colloidal states, and so only when aggregated colloidal clusters occurred, did the resins present clear indications of viscoelastic responses, as illustrated by the crossover of elastic modulus and viscous modulus curves at lower strain percentages. These colloidal clusters were labile microstructures, which, broken by applied shear, justified the known thixotropic behavior of these resins sufficiently advanced by aging or other means. MUF resins already in the colloidal state, but for which colloidal clustering had not yet occurred, behaved exclusively as viscous liquids. Two different cases of physical gelation were observed, reversible physical gelation and irreversible physical gelation, underlying which a true gel situation possibly occurred. Physical gelation due to colloidal superstructures occurred in both, but the difference in the resin average molecular masses revealed if the physical gelation was reversible or irreversible and, therefore, if the liquid/cluster separation was defined as the terminal phase of physical gelation. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 655–659, 2005     

NMR studies of urea–formaldehyde resins

A procedure used for analyzing urea–formaldehyde resins by NMR spectroscopy was developed. Using this procedure, the condensation of dimethylolurea under acidic and alkaline conditions was sudied. It was confirmed that polymerization under acidic conditions proceeds via the formation of methylene linkages and under alkaline conditions, via the formation of dimethylene ether groups. The more highly condensed water-soluble urea resins were found to contain hemiformal groups, which could be quantitatively determined. Two butylated urea–formaldehyde resins with varying degrees of butylation were also studied and were found to differ in the number of methylene and dimethylene ether groups and degree of butoxylation. Both were found to contain hemiformal groups. The resin with lower degree of butylation was also found to contain dimethyl ether linkages. None of the resins studied showed any detectable amounts of fully substituted amide groups.     

Curing of resole-type phenol–formaldehyde resin

The process of resole-type phenol–formaldehyde resins was studied by differential thermal analysis and infrared spectroscopy. It was shown that in neutral media the first reactions that occur are those between free phenol present in the resin and monosubstituted methylol phenol with free reactive positions on the benzene ring. The formation of methylene linkages is followed immediately by the condensation of methylol groups to give dibenzyl ether linkages. These are subsequently destroyed at about 210°C. It is believed that the entire curing process is governed by a free-radical mechanism. It is also shown that oxidation of the resin occurs slowly at room temperature and humidity.