Influence of the synthesis conditions on the curing behavior of phenol–urea–formaldehyde resol resins

The curing behavior of synthesized phenol–urea–formaldehyde (PUF) resol resins with various formaldehyde/urea/phenol ratios was studied with differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The results indicated that the synthesis parameters, including the urea content, formaldehyde/phenol ratio, and pH value, had a combined effect on the curing behavior. The pH value played an important role in affecting the shape of the DSC curing curves, the activation energy, and the reaction rate constant. Depending on the pH value, one or two peaks could appear in the DSC curve. The activation energy was lower when pH was below 11. The reaction rate constant increased with an increase in the pH value at both low and high temperatures. The urea content and formaldehyde/phenol ratio had no significant influence on the activation energy and rate constant. DMA showed that both the gel point and tan δ peak temperature (T_tanδ) had the lowest values in the mid‐pH range for the PUF resins. A different trend was observed for the phenol–formaldehyde resin without the urea component. Instead, the gel point and T_tanδ decreased monotonically with an increase in the pH value. For the PUF resins, a high urea content or a low formaldehyde/phenol ratio resulted in a high gel point. The effect of the urea content on T_tanδ was bigger than that on the gel point because of the reversible reaction associated with the urea component. Too much formaldehyde could lead to more reversible reactions and a higher T_tanδ value. The effects of the synthesis conditions on the rigidity of the cured network were complex for the PUF resins. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95: 1368–1375, 2005     

Curing process of powdered phenol–formaldehyde resol resins and the role of water in the curing systems

The curing behavior of two kinds of commercial powdered resol phenolic resins was studied by differential scanning calorimetry. Liquid‐state ¹³C‐NMR spectroscopy was used to aid in understanding the curing behavior by detecting the structure of powdered resins. The reaction mechanism was interpreted with the dependency of activation energy on the degree of conversion. The results indicate that there are differences in the curing mechanism between core and face phenolic resins. The curing process of core resin was faster than that of face resin at the same reaction temperature. The water added in the curing system played an important role of plasticizer or diluent according to different curing stages and water content. In the initial curing stage, water mainly diluted the system and retarded the curing reactions. However, at the higher degrees of conversion, water played the role of plasticizer to decrease the effect of diffusion on the curing reactions to make the curing reactions more complete. The excess water added in the curing system played the role of diluent at almost all stages during the curing process. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1371–1378, 2003     

Cure kinetics of aqueous phenol–formaldehyde resins used for oriented strandboard manufacturing: Effect of zinc borate

The effect of zinc borate (ZB) on the cure kinetics of commercial phenol–formaldehyde oriented strandboard adhesives was studied using differential scanning calorimetry. ZB caused a separation of the addition and condensation reactions for both face and core resin (CR) systems with lowered cure temperature for the addition reaction. For the face resin, ZB did not change its nth‐order curing mechanism, but retarded the whole cure reactions, and increased the reaction order and the activation energy. Compared with neat CR, the addition reaction of the CR/ZB mixture, which occurred at temperatures lower than 60°C, also followed an nth‐order reaction mechanism. The condensation reaction of the mixture was changed from an autocatalytic reaction to an nth‐order one with the reaction order of about 1. The proposed models fitted the experimental data well. Relationships among cure reaction conversion (i.e., cure degree), cure temperature, and cure time were predicted for various resin/ZB systems. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3886–3894, 2006     

Comparison of model‐fitting kinetics for predicting the cure behavior of commercial phenol–formaldehyde resins

Phenol–formaldehyde (PF) resins have been the subject of many model‐fitting cure kinetic studies, yet the best model for predicting PF dynamic and isothermal cure has not been established. The objective of this research is to compare and contrast several commonly used kinetic models for predicting degree of cure and cure rate of PF resins. Toward this objective, the nth‐order Borchardt–Daniels (nth‐BD), ASTM E698 (E698), autocatalytic Borchardt–Daniels (Auto‐BD), and modified autocatalytic methods (M‐Auto) are evaluated on two commercial PF resins containing different molecular weight distributions and thus cure behaviors. The nth‐BD, E698, and M‐Auto methods all produce comparable values of activation energies, while Auto‐BD method yields aberrant values. For dynamic cure prediction, all models fail to predict reaction rate, while degree of cure is reasonably well predicted with all three methods. As a whole, the nth‐BD method best predicts degree of cure for both resins as assessed by mean squared error of prediction. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Cure kinetics of aqueous phenol–formaldehyde resins used for oriented strandboard manufacturing: Analytical technique

The cure kinetics of commercial phenol–formaldehyde (PF), used as oriented strandboard face and core resins, were studied using isothermal and dynamic differential scanning calorimetry (DSC). The cure of the face resin completely followed an nth‐order reaction mechanism. The reaction order was nearly 1 with activation energy of 79.29 kJ mol^?1. The core resin showed a more complicated cure mechanism, including both nth‐order and autocatalytic reactions. The nth‐order part, with reaction order of 2.38, began at lower temperatures, but the reaction rate of the autocatalytic part increased much faster with increase in curing temperature. The total reaction order for the autocatalytic part was about 5. Cure kinetic models, for both face and core resins, were developed. It is shown that the models fitted experimental data well, and that the isothermal DSC was much more reliable than the dynamic DSC in studying the cure kinetics. Furthermore, the relationships among cure reaction conversion (curing degree), cure temperature, and cure time were predicted for both resin systems. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1642–1650, 2006     

Modification of phenol–formaldehyde resol resins by lignin,starch, and urea

Lignin‐based chemicals, starch, and urea were used as modifiers for phenol–formaldehyde resol resins. The effects of the addition stage of the modifiers used in the synthesis of the resins and the type of modification reagent on the structures of the resins and their molar masses and reactivities were investigated. The modifications with corn starch and lignin promoted condensation; this was verified by increased molar masses and high ratios of methylene bridges to the sum of free ortho and para aromatic groups with respect to the corresponding reference resin without a modification reagent. The later the modifier was added to the resin condensation mixture, the more methylene bridges were formed with respect to the amounts of free ortho and para aromatic groups. In addition, when urea or wheat starch was added in the later condensation stage, the final condensation also reached high stages. The modifications with lignosulfonate and starch, as well as the early addition of urea, enhanced p–p′ bridge structures. The lowest condensation stage and, therefore, the highest reactivity were found when wheat starch was added with the starting reagents. The curing heat of the wheat‐starch‐modified resins decreased according to the deferred addition point of starch. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 582–588, 2003     

Cure kinetics of aqueous phenol‐formaldehyde resins used for oriented strandboard manufacturing: Effect of wood flour

The effect of wood flour on the cure kinetics of commercial phenol‐formaldehyde resins used as oriented strandboard face and core adhesives was studied using differential scanning calorimetry. The wood flour did not change the cure mechanism of the face resin, but lowered its cure temperature and activation energy and increased its cure reaction order. For the core resin (CR), the wood flour lowered the onset cure temperature, and caused separation of the addition and condensation reactions involved in curing of CR. Compared with neat CR, the addition reaction of CR/wood mixture also followed an nth‐order reaction mechanism but with a lower reaction order, while the condensation was changed from an autocatalytic reaction to an nth‐order one. The addition reaction happened at temperatures lower than 90°C, and the condensation reaction was dominant at temperatures higher than 110°C. The proposed models fitted the experimental data well. Relationships among cure reaction conversion (cure degree), cure temperature, and cure time were predicted. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:3774–3781, 2006     

Curing reaction of molybdenum–phenolic resins

The curing behavior and curing reaction kinetics of novel molybdenum–phenolic resins were studied with differential scanning calorimetry and thermogravimetry methods, the thermal degradation properties of the cured products were studied with thermogravimetry, and the mechanism of the curing reaction was investigated with Fourier transform infrared. When the mixing ratio of the molybdenum–phenolic resin (with 12% molybdenum) to the curing agent was 100/10 (w/w), the curing temperature and activation energy were at a minimum, the thermal degradation stability of the cured product was optimal, and the temperature corresponding to the maximum extent of curing was 200°C. The curing mechanism was similar to that of conventional phenolic systems. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 1410–1415, 2003     

Curing characteristics of urea–formaldehyde resin in the presence of various amounts of wood extracts and catalysts

The characteristics of urea–formaldehyde (UF) resin curing in the presence of wood extracts and a catalyst [ammonium chloride (NH₄Cl)] were investigated by differential scanning calorimetry (DSC). The effects of extracts from 16 wood species on resin curing behaviors were evaluated. A model developed in this study, T_p = 53.296 exp(?9.72C) + 93.104, could be used to predict the resin curing rate in terms of the DSC peak temperature (T_p) as influenced by the NH₄Cl content (C). The results indicated that the curing rate of UF resin increased as the catalyst content increased and reached a maximum when the catalyst content ranged from 0.5 to 1.0% (solid basis over liquid UF resin weight). Further increases in the catalyst content had no effect on the resin curing rate. The curing rates of UF resin in the presence of wood extracts increased with decreased pH values or increased base buffer capacities. It was also discovered that the activation energy could not fully explain the resin curing behavior when some species of wood extracts were present, and therefore, the pre‐exponential factor had to be taken into account. The concept of the equivalent catalyst content (ECC) of wood extracts to the NH₄Cl content was introduced in this study; ECCs ranged from 0.0022 to 0.0331% among the 16 wood species. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis and characterization of curable methacrylate‐based monomers

The synthesis of new methacrylate‐based, curable macromonomers, 4,4′‐bis[2‐hydroxy‐3‐aminopropylmethacrylate] diphenyl ether (BHAPE) and 4,4′‐bis[2‐hydroxy‐3‐aminopropylmethacrylate] diphenyl methane (BHAPM), is reported. BHAPE and BHAPM were prepared by the reaction of glycidyl methacrylate (GMA) with 4,4′‐diaminodiphenyl ether and 4,4′‐diaminodiphenyl methane, respectively. The progress of the reaction was monitored by thin‐layer chromatography (TLC), and the structure of the monomers was characterized by Fourier transform infrared (FTIR) and ¹H‐NMR spectroscopy. Thermal curing of the monomers was conducted in a differential scanning calorimeter (DSC) with peroxide as the initiator. Thermal curing of the monomers showed the highest rate at 100°C with the activation energy value in the range 80–90 kJ distilled/mol. The water absorption properties of the cured samples in water, acidic, and basic solutions were studied. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Thermal cure behavior of unsaturated polyester/phenol blends

Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to detect and simulate the cure behavior of unsaturated polyester (UP), phenol, and UP/phenol blends and to calculate and predict the cure rate, cure temperature, conversion, and changes in the glass‐transition temperature along with various cure orders in order to obtain the optimum parameters for processing. With dynamic scanning and isothermal DSC procedures and Borchardt–Daniels dynamic software, cure data for the UP resin were obtained, 90% of the conversion rate at 100°C being achieved after 15 min. However, for the phenol and UP/phenol blends, gradually increasing the temperature was found to be best for curing according to the DSC and DMA test results. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1041–1058, 2004     

Activation energy and curing behavior of resol‐ and novolac‐type phenolic resins by differential scanning calorimetry and thermogravimetric analysis

The thermal behavior, thermal degradation kinetics, and pyrolysis of resol and novolac phenolic resins with different curing conditions, as a function of the formaldehyde/phenol (F/P) molar ratio (1.3, 1.9, and 2.5 for the resol resins and 0.5, 0.7, and 0.9 for the novolac resins) were investigated. The activation energy of the thermal reaction was studied with differential scanning calorimetry at five different heating rates (2, 5, 10, 20, and 40°C/min) between 50 and 300°C. The activation energy of the thermal decomposition was investigated with thermogravimetric analysis at five different heating rates (2, 5, 10, 20, and 40°C/min) from 30 to 800°C. The low molar ratio resins exhibited a higher activation energy than the high molar ratio resins in the curing process. This meant that less heat was needed to cure the high molar ratio resins. Therefore, the higher the molar ratio was, the lower the activation energy was of the reaction. As the thermal decomposition of the resol resins proceeded, the activation energy sharply decreased at first and then remained almost constant. The activation energy of the thermal decomposition for novolac resins with F/P = 0.5 or F/P = 0.7 was almost identical in all regions, whereas that for novolac resins with F/P = 0.9 gradually decreased as the reaction proceeded. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2589–2596, 2003     

Effect of wood on the curing behavior of commercial phenolic resin systems

Differential scanning calorimetry (DSC) was used to study the effect of wood on the curing behavior of two types of commercial oriented‐strand‐board phenolic resins. DSC analysis showed that the curing behavior of the core resin differed significantly from that of the face resin in terms of the peak shape, peak temperature, and activation energy. The addition of wood to the resins moved the two separated peaks in the DSC curves of the core resin adjacent to each other. It also accelerated the addition reactions in the curing processes of both the core and face resins. The two peaks in the DSC curves were the result of the high pH values of the resins. These two peaks became either jointed together or overlapped when the pH value of the resin was reduced. Wood also reduced the activation energies for both the core and face resins by decreasing the pH values of the curing systems. Moreover, the effects of wood on the curing behavior of the resins among the five species studied were similar. The lowest activation energy for a phenolic resin probably appeared at pH 10–11 under alkaline conditions. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 185–192, 2005     

Evaluation of isomeric composition of resol‐type phenol formaldehyde matrix resins for silica–phenolic composites and its effect on cure characteristics of the resin

The isomeric composition of several samples of resol‐type phenol formaldehyde resins used for silica–phenolic composites was evaluated by ¹H‐ and ¹³C‐NMR spectroscopy and reverse‐phase HPLC techniques. The variations in the isomeric compositions were attributed to inadvertent variations in the process parameters. A mathematical relation was determined for calculation of free phenol from ¹H‐NMR measurements. The samples were cured at 160°C for 8 h in an inert atmosphere of N₂. The extent of cure in the hardened samples was measured by FTIR analysis. The effect of isomeric composition on the extent of cure was studied. Free phenol and p‐hydroxymethyl phenol, exhibiting a linear correlation, were found to have a pronounced effect on the extent of cure. The cure kinetics were derived by dynamic DSC measurements. Activation energy (E) for curing exhibited a near linear correlation independently with free phenol content and the extent of cure. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2517–2524, 2003     

Curing of phenol‐formaldehyde resin mixed with wood preservatives

The effects of preservatives used for glue‐line treatment on the curing of phenol‐formaldehyde resin (PF) were studied by dynamic mechanical analysis and differential scanning calorimetry. Storage modulus, G′, loss modulus, G″, and loss tangent, tan δ, of PF with and without preservatives were recorded as a function of time under isothermal heating. The time required for G′, G″, and tan δ to reach steady values increased with addition of preservative. The G′, G″, and tan δ curves of PF containing benzyl alcohol (used in the preservative as a diluent) were almost identical to those of PF containing preservative. However, the addition of antitermite and anti‐fungal compounds alone had no effect on the curing process. There were no differences in total reaction enthalpy or dependence of activation energy on degree of conversion between pure PF and mixtures. Our results indicate that benzyl alcohol in preservatives plasticizes the curing system for PF. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

A kinetic study on the autocatalytic cure reaction of a cyanate ester resin

The cure of a novolac‐type cyanate ester monomer, which reacts to form a polycyanurate network, was investigated by using differential scanning calorimeter. The conversions and the rates of cure were determined from the exothermic curves at several isothermal temperatures (513–553 K). The experimental data, showing an autocatalytic behavior, conforms to the kinetic model proposed by Kamal, which includes two reaction orders, m and n, and two rate constants, k₁ and k₂. These kinetic parameters for each curing temperature were obtained by using Kenny's graphic‐analytical technique. The overall reaction order was about 1.99 (m = 0.99, n = 1.0) and the activation energies for the rate constants, k₁ and k₂, were 80.9 and 82.3 kJ/mol, respectively. The results show that the autocatalytic model predicted the curing kinetics very well at high curing temperatures. However, at low curing temperatures, deviation from experimental data was observed after gelation occurred. The kinetic model was, therefore, modified to predict the cure kinetics over the whole range of conversion. After modification, the overall reaction order slightly decreased to be 1.94 (m = 0.95, n = 0.99), and the activation energies for the rate constants, k₁ and k₂, were found to be 86.4 and 80.2 kJ/mol. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3067–3079, 2004     

Dynamic mechanical analysis study of the curing of phenol‐formaldehyde novolac resins

The curing reaction of typical commercial phenol‐formaldehyde novolac resins with hexamethylentetraamine (HMTA) was followed by dynamic mechanical analysis. The evolution of the rheological parameters, such as storage modulus G′, loss modulus G″, and tanδ (G″/G′), as a function of time, for samples of the phenolic resins on cloth, was recorded. The curing reaction, leading to the formation of a crosslinked structure, is described by a third‐order phenomenological equation. This equation takes into account a self‐acceleration effect, as a consequence not only of the chemical reaction of crosslinking after the gel point but of phase segregation as well. This rheokinetic model of the curing of phenolic novolac resins permits the determination of the numerical values of the kinetic equation constants. The influence of the composition, structure, and physical treatment on the curing kinetics of the novolac resins is evaluated. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1902–1913, 2001     

Use of isoconversional methods to analyze the cure kinetics of isocyanate‐ended quasi‐prepolymers with water

The curing of an isocyanate (NCO)‐ended quasi‐prepolymer by reaction with water in stoichiometric ratio was monitored by using differential scanning calorimetry both under isothermal and non‐isothermal conditions. A quasi‐prepolymer containing 16 wt % free NCO prepared by reacting trifunctional polypropylene glycol (PPG) with polymeric MDI was used in this study. The variation of the effective activation energy with the extent of the curing reaction was calculated by means of model‐free differential and integral isoconversional methods. Both isoconversional methods provided similar results showingthat the activation energy depends on the extent of thereaction of the quasi‐prepolymer with water. The dependence of the effective activation energy proved to be different under isothermal and non‐isothermal curing conditions. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1049–1057, 2007