Chemical Structure and Curing Behavior of Phenol–Urea–Formaldehyde Cocondensed Resins of High Urea Content

Phenol–urea–formaldehyde cocondensed (PUF) resins of high urea content were prepared by adding different forms of urea to the reaction system. The structure, curing behavior, and water resistance of the PUF resins were investigated, and their relations were also discussed by liquid ¹³C nuclear magnetic resonance (NMR) and different scanning calorimetry (DSC). The liquid ¹³C-NMR analysis showed that urea added in the form of methylolureas was well incorporated into the cocondensed resins by reacting with phenolic methylols to form cocondensed methylene bridges, and that the PUF resins had no free formaldehyde with any form. Unreacted urea and low molecule monosubstituted urea of PUF resins play a dominant role in the curing behavior and water resistance of resins. The peak temperature, curing time, and curing enthalpy (ΔH) value correspondingly increased, however, the water resistance of PUF resins decreased when urea content in PUF resins increased. The PUF cocondensed resin with up to 89.4 % (W _U/W _P) urea has relatively low cost, and moreover can pass the requirements of China Industry Standard for the exterior grade of structural plywood after 4-h cyclic boiling test.     

Time–temperature–transformation cure diagrams of phenol–formaldehyde and lignin–phenol–formaldehyde novolac resins

In this study, the time–temperature– transformation (TTT) cure diagrams of the curing processes of several novolac resins were determined. Each diagram corresponded to a mixture of commercial phenol–formaldehyde novolac, lignin–phenol–formaldehyde novolac, and methylolated lignin–phenol–formaldehyde novolac resins with hexamethylenetetramine as a curing agent. Thermomechanical analysis and differential scanning calorimetry techniques were applied to study the resin gelation and the kinetics of the curing process to obtain the isoconversional curves. The temperature at which the material gelled and vitrified [the glass‐transition temperature at the gel point (_gelT_g)], the glass‐transition temperature of the uncured material (without crosslinking; T_g0), and the glass‐transition temperature with full crosslinking were also obtained. On the basis of the measured of conversion degree at gelation, the approximate glass‐transition temperature/conversion relationship, and the thermokinetic results of the curing process of the resins, TTT cure diagrams of the novolac samples were constructed. The TTT diagrams showed that the lignin–novolac and methylolated lignin–novolac resins presented lower T_g0 and _gelT_g values than the commercial resin. The TTT diagram is a suitable tool for understanding novolac resin behavior during the isothermal curing process. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Synthesis–structure–performance relationship of cocondensed phenol–urea–formaldehyde resins by MALDI‐ToF and 13C NMR

Matrix assisted laser desorption ionization time of flight (MALDI‐ToF) mass spectrometry has consistently confirmed on a number of PUF resins that phenol–urea cocondensates exist in phenol–urea–formaldehyde (PUF) resins. A noticeable proportion of methylene‐linked phenol to urea cocondensates were detected in all the PUF resins tried, alongside methylene bridges connecting phenol to phenol and urea to urea. The PUF, PF, and UF oligomers formed were identified. Variations of the PUF preparation procedure did always yield a certain proportion of the mixed phenol to urea cocondensates. Their relative proportion was determined and related the synthesis procedure used. Comparison of the MALDI‐ToF results with a ¹³C NMR investigation showed that in a real PUF resin in which phenol to urea cocondensates were identified the methylene bridge NMR signal at 44 ppm, characteristic of phenol to urea unsubstituted model compound cocondensates, does not appear at all. This confirmed that this peak cannot be taken as an indication of the existence of phenol and urea condensation under actual resin preparation conditions. The peak indicating cocondensation in PUF resins in which the phenolic nuclei and urea are substituted appears instead at 54.7–55.0 ppm. Thermomechanical analysis has again confirmed that the resin gel times greatly accelerates with increasing urea molar content. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Fast advancement and hardening acceleration of low‐condensation alkaline PF resins by esters and copolymerized urea

Low‐condensation phenol‐formaldehyde (PF) resins coreacted under alkaline conditions with up to 42% molar urea on phenol during resin preparation yielded PUF resins capable of faster hardening times than equivalent pure PF resins prepared under identical conditions and presented better performance than the latter. The water resistance of the PUF resins prepared seemed comparable to pure PF resins when used as adhesives for wood particleboard. Part of the urea was found by ¹³C‐NMR to be copolymerized to yield the alkaline PUF resin; whereas, especially at the higher levels of urea addition, unreacted urea was still present in the resin. Increase of the initial formaldehyde to phenol molar ratio decreased considerably the proportion of unreacted urea and increased the proportion of PUF resin. A coreaction scheme of phenolic and aminoplastic methylol groups with reactive phenol and urea sites based on previous model compounds work has been proposed, copolymerized urea functioning as a prebranching molecule in the forming, hardened resin network. The PUF resins prepared were capable of further noticeable curing acceleration by addition of ester accelerators; namely, glycerol triacetate (triacetin), to reach gel times as fast as those characteristic of catalyzed aminoplastic resins, but at wet strength values characteristic of exterior PF resins. Synergy between the relative amounts of copolymerized urea and ester accelerator was very noticeable at the lower levels of the two parameters, but this effect decreased in intensity toward the higher percentages of urea and triacetin. ¹³C‐NMR assignements of the relevant peaks of the PUF resins are reported and compared with what has been reported in the literature for mixed, coreacted model compounds and pure PF and urea‐formaldehyde (UF) resins. The relative performance of the different PUF resins prepared was checked under different conditions by thermomechanical analysis (TMA) and by preparation of wood particleboard, and the capability of the accelerated PUF resins to achieve press times as fast as those of aminoplastic (UF and others) resins was confirmed. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 359–378, 1999     

Effect of Synthesis Conditions on the Structure and Curing Characteristics of High-Urea Content PUF Resin

Phenol-urea-formaldehyde (PUF) resins of high urea content were prepared at different hydroxyl/phenol (OH/P) mole ratios and formaldehyde/(phenol + urea) [F/(P + U)] mole ratios. The effect of synthesis parameters including OH/P and F/(P + U) mole ratios on the structure, composition, and curing characteristics of PUF resins were investigated by using both liquid ¹³C nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). The NMR analysis indicated that an increase in the OH/P mole ratio and/or F/(P + U) mole ratio decreased the amount of unreacted urea and monosubstituted urea, and promoted the formation of polysubstituted urea. The DSC results showed that the higher OH/P mole ratio and/or F/(P + U) mole ratio of PUF resins resulted in a lower curing temperature. The F/(P + U) mole ratio of PUF resins seemed to have a more significant accelerating effect on the curing reaction than the OH/P mole ratio.     

Characterization of phenol–urea–formaldehyde resin by inline FTIR spectroscopy

Phenol–urea–formaldehyde (PUF) resins were synthesized by a two‐step polymerization process. The first step was the synthesis of 2,4,6‐trimethylolphenol (TMeP) from phenol and formaldehyde, under alkaline conditions. In the second step PUF resins were synthesized by the reaction of TMeP with urea, under acidic and alkaline conditions. The influence of temperature on the synthesis of TMeP was investigated. The molar ratio between TMeP and urea was varied to study the composition effect on the second step of the PUF synthesis and final product properties. Synthesis of TMeP and PUF resins were monitored by inline FTIR‐ATR system. Analytical methods, such as differential scanning calorimetry, nuclear magnetic resonance, thermogravimetric analysis, and infrared spectroscopy were used for characterization of TMeP and PUF resins. Obtained PUF resins were cured and tested on flexural strength. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Effect of synthesis parameters on thermal behavior of phenol–formaldehyde resol resin

Differential scanning calorimetry (DSC) was used to investigate the influence of resin synthesis parameters on the thermal behavior of low molecular weight phenol–formaldehyde (PF) resol resins prepared with different formaldehyde/phenol (F/P) molar ratios, different sodium hydroxide/phenol (NaOH/P) molar ratios, and different catalysts. As the F/P molar ratio increased, the molecular weight and activation energy increased while the gel time, peak temperature, resin pH, and nonvolatile solids content decreased. By contrast, the molecular weight, gel time, resin pH, resin solids content, and peak temperature increased with an increasing NaOH/P molar ratio. However, the activation energy decreased with an increasing NaOH/P molar ratio. The polydispersity increased with both F/P and NaOH/P ratios. Calcium hydroxide gave a faster curing resin compared to sodium and potassium hydroxides. All DSC thermograms of this study showed just a single exothermic peak for the resins that were used. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1415–1424, 2002     

13C‐NMR study on the structure of phenol‐urea‐formaldehyde resins prepared by methylolureas and phenol

Phenol‐urea‐formaldehyde (PUF) resins were synthesized by reacting mixture of methylolureas (MMU), phenol, and formaldehyde. The structure of PUF cocondensed resins at different stages of reaction were analyzed by liquid ¹³C nuclear magnetic resonance (NMR) spectroscopy. The liquid ¹³C‐NMR analysis indicated that methylolureas had the dominant content in MMU with the reaction between urea and formaldehyde under the alkaline condition. The PUF cocondensed resins had no free formaldehyde. methylolureas were well incorporated into the cocondensed resins by reacting with phenolic units to form cocondensed methylene bridges. The second formaldehyde influenced the further reaction and the structure of the PUF resins. The resins with the prepared method of PUFB possessed relatively high degree of polymerization and low proportion of unreacted methylol groups. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effects of formaldehyde to urea mole ratio on thermal curing behavior of urea–formaldehyde resin and properties of particleboard

As a part of abating the formaldehyde emission (FE) of urea–formaldehyde (UF) resin, this study was conducted to investigate the effects of formaldehyde to urea (F/U) mole ratio on thermal curing behavior of UF resins and properties of PB bonded with them. UF resins synthesized at different F/U mole ratios (i.e., 1.6, 1.4, 1.2, and 1.0) were used for the manufacture of PB. Thermal curing behavior of these UF resins was characterized using differential scanning calorimetry (DSC). As the F/U mole ratio decreases, the gel time, onset and peak temperatures, and heat of reaction (ΔH) increased, while the activation energy (E_a) and rate constant (k) were decreased. The amount of free formaldehyde of UF resin and FE of PB prepared decreased in parallel with decreasing the F/U mole ratio. The internal bond strength, thickness swelling, and water absorption of PB was slightly deteriorated with decreasing the F/U mole ratio of UF resins used. These results indicated that as the F/U mole ratio decreased, the FE of PB was greatly reduced at the expense of the reactivity of UF resin and slight deterioration of performance of PB prepared. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1787–1792, 2006     

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     

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     

Deeper insights into the thermal properties of epoxy thermoset resins using torsion measurements

The glass transition temperature (T_g) of epoxy thermosets is a critical material property that depends on the component chemistry, the final cross-link density, and processing conditions. This study incorporates dynamic mechanical analysis (DMA) testing with a torsion clamp geometry on a TA Instruments DHR-2 and differential scanning calorimetry (DSC) to characterize five different two-component epoxy-amine systems. Investigation of the T_g dependence on DMA frequency and heating shows that lowering the frequency from 1 to 0.01 Hz results in a T_g very similar to that measured using DSC, while a heating rate of 0.3°C/min using DMA gives a T_g comparable to the DSC measured value at 30°C/min. The DMA technique reveals secondary relaxation transitions and peak broadening in the tan(δ) plots of poorly mixed epoxy blends, quantified using full width at half maximum (FWHM) of tan(δ) peaks, and are indicative of a non-homogeneous cross-linked network and off-ratio blending, respectively. The increase in the FWHM due to poor mixing ranges from 8% to 96%. These parameters are easily measurable and quantifiable in DMA, but are not observed in DSC. The additional DMA insights are valuable for process development and failure analysis, and can improve the understanding of epoxies.     

Differential scanning calorimetry of urea–formaldehyde adhesive resins,synthesized under different pH conditions

The purpose of this study was to investigate the effects of reaction pH conditions on thermal behavior of urea–formaldehyde (UF) resins, for the possible reduction of formaldehyde emission of particleboard bonded with them. Thermal curing properties of UF resins, synthesized at three different reaction pH conditions, such as alkaline (pH 7.5), weak acid (pH 4.5), and strong acid (pH 1.0), were characterized with multiheating rate method of differential scanning calorimetry. As heating rate increased, the onset and peak temperatures increased for all three UF resins. By contrast, the heat of reaction (ΔH) was not much changed with increasing heating rates. The activation energy (E_a) increased as the reaction pH decreased from alkaline to strong acid condition. The formaldehyde emission of particleboard was the lowest for the UF resins prepared under strong acid, whereas it showed the poorest bond strength. These results indicated that thermal curing behavior was related to chemical species, affecting the formaldehyde emission, while the poor bond strength was believed to be related to the molecular mobility of the resin used. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 422–427, 2006     

13C NMR study on the structure of ZnO-catalyzed phenol–urea–formaldehyde resin during its synthesis process

The structure of ZnO-catalyzed phenol–urea–formaldehyde (PUF) resin at different synthesis stages was analyzed by liquid ¹³C nuclear magnetic resonance spectroscopy. The results showed that the general structure of ZnO-catalyzed PUF resin was almost the same as the control PUF resin. Addition reaction between phenol and formaldehyde mainly occurred at the first stage. Total methylol groups amount between phenols of the control resin was a little lower than that of the ZnO-catalyzed PUF resin. Co-condensation and self-condensation reaction occurred at the second stage. The preparation method of ZnO-catalyzed PUF resin favored the co-condensation reaction between phenol methylol groups and urea units, while self-condensation reaction dominated the control resin at the second stage. Formaldehyde completely reacted for both the control and ZnO-catalyzed PUF final resin. The total amount of methylol and methylene groups between urea units and phenols, respectively, was almost the same for the two final resins. The total quantity of methylol groups between phenols represented a continuing downward trend from the first stage to the final stage, and the amount of methylol group (p-Ph–CH₂OH) of ZnO-catalyzed PUF resin was 30% more than that of the control resin. Total co-condensed methylene groups amount of ZnO-catalyzed PUF resin was greater than that of the control resin, which indicated that ZnO could make the urea units well incorporated into the co-condensed PUF resin.     

Co‐polymerization analysis of thermosetting resins using 1H‐15N‐13C triple resonance NMR spectroscopy

¹H‐¹⁵N‐¹³C correlation NMR spectroscopy techniques developed to identify and characterize co‐polymer fragments in melamine‐urea‐formaldehyde (MUF) and phenol‐urea‐formaldehyde (PUF) model systems have been applied to industrially prepared MUF, PUF, and phenol‐melamine‐formaldehyde (PMF) resins. The NMR data confirm that co‐polymers form in a commercially prepared MUF resin manufactured by Momentive Specialty Chemicals Pty. Ltd. Spectra from PUF model reactions were compared with those from a PUF resin and it was determined that PUF co‐polymers did not form in the resin prepared using typical temperature and pH. Finally, NMR spectroscopy was used to identify and characterize PMF co‐polymer bonds in a phenol‐melamine‐urea‐formaldehyde (PMUF) resin prepared using a procedure from Momentive Specialty Chemicals Pty. Ltd. With these NMR techniques in hand, it is now possible to relate co‐polymer structures to properties of commercial thermosets. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Phenol–urea–formaldehyde resin co-polymer synthesis and its influence on Elaeis palm trunk plywood mechanical performance evaluated by 13C NMR and MALDI-TOF mass spectrometry

This study evaluated a new method of producing phenol–urea–formaldehyde (PUF) adhesives formulated differently under actual “in-situ” resin synthesis conditions. This was carried out by co-polymerizing urea formaldehyde (UF) resin with phenol–formaldehyde resin in the core layer of low molecular weight (LMW) phenol–formaldehyde (PF) resin treated Elaeis palm trunk veneers during the gluing process of Elaeis palm plywood. Matrix assisted laser desorption Ionization time of flight (MALDI-TOF) mass spectrometry (MS) illustrated and confirmed a series number of the phenol–urea co-condensates repeating unit in the prepared PUF resins which corroborated well with its mechanical properties (modulus of elasticity and modulus of rupture), bonding quality (dry test and weather boil proof or WBP test) and physical properties. A series of PF, UF and PUF resins oligomers forming repeating units up to 1833 Da were identified. Besides that, the solid state ¹³Carbon nuclear magnetic resonance (NMR) interpretation identified that the signal at 44–45 ppm and 54–55 ppm corresponding to methylene bridges were co-condensated in between phenol and urea in the PUF resin system. The ¹³C NMR investigation showed that the synthesis process of PUF resin contained no free formaldehyde elements. Furthermore, the proportion of urea and methylolureas in the mixture to synthesis PUF resin were sufficient and incorporated well into the formulation by reacting with LMWPF units to form co-condensed methylene bridges. This study showed a new and useful method to synthesize PUF resin during the gluing process of manufactured Elaeis palm plywood which can also enhance the performance of Elaeis palm plywood panels for structural instead of utility grade applications.     

Synthesis of poly(urea–formaldehyde) encapsulated dibutyltin dilaurate through the self-catalysis of core materials

Polymeric microcapsules (MCs) filled with catalyst can be controlled to release the catalyst to initiate the polymerization reaction. In the present work, poly(urea–formaldehyde) (PUF) MCs filled with DBTDL (PUF/DBTDL MCs) were prepared using urea (U) and formaldehyde (F) as the wall shell materials by in situ polymerization. The U–F resins could easily polymerize in the presence of the core material DBTDL to produce PUF polymers, then they deposited on the surface of the DBTDL droplets, forming PUF/DBTDL MCs. The decomposition temperature (T _d) at 5 % weight loss of PUF/DBTDL MCs was about 245 °C. The application of PUF/DBTDL MCs to cyanate ester resins preliminarily showed the reaction control capability of the MCs due to the slow release of DBTDL through the wall shell.     

C NMR study on structure, composition and curing behavior of phenol-urea-formaldehyde resole resins

Phenol-urea-formaldehyde (PUF) resole resins were synthesized and analyzed by both liquid and solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. The liquid ¹³C NMR analysis indicated that the co-condensation reactions between the phenolic ring and the urea unit occurred during the synthesis of the resins. The addition of the urea component effectively reduced the free formaldehyde content in the resin systems. Methylene ether bridges in the resins were found to be mainly associated with the urea units. pH had significant influences on the structure and composition of the resins. Solid-state ¹³C NMR measurements of the cured resins suggested that the pH probably affected the curing mechanism. A longer time and a higher temperature can generally accelerate the curing process and increase the rigidity of the cured network.     

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     

Frontal curing of epoxy resins: Comparison of mechanical and thermal properties to batch-cured materials

The epoxy resin diglycidyl ether of bisphenol F (DGEBF) was cured by the aliphatic amine curing agent Epicure 3371 in a stoichiometric ratio both frontally and in a batch-cure schedule. Glass transition temperatures (T_g) were determined using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). DMA also was used for studying the storage modulus (E′) and tan delta (tan δ) of the cured samples. Tensile properties of epoxy samples were tested according to ASTM D638M-93. The properties of the frontally cured epoxy resin were found to be very close to that of batch-cured epoxy resin. Velocity of cure-front propagation was measured for both neat and filled epoxy. Rubber particles (ground tires) were used as a filler. The maximum percentage of filler in the epoxy resin allowing propagation was 30%. Because of convection, only descending fronts would propagate. Advantages and disadvantages of frontal curing of epoxy resins are discussed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1209–1216, 1997