Phenol–formaldehyde‐type resins made from phenol‐liquefied wood for the bonding of particleboard

Liquefaction of southern pine wood in phenol in 30–40 : 70–60 weight ratios resulted in homogeneous liquefied materials, which were directly used to synthesize phenol–formaldehyde (PF)‐type resins. The synthesized resins showed good physical and handling properties: low viscosity, stability for storage and transportation, and resin applicable by a common sprayer. Particleboard panels bonded with the synthesized resins showed promising physical properties and significantly lower formaldehyde emission values than those bonded with the urea–formaldehyde resin control. One deficiency observed for the synthesized resins was lower internal bond values, which might be overcome the use of a hot‐stacking procedure. Overall, the process of wood liquefaction with limited amounts of phenol as a solvent was shown to have the potential of providing practical, low‐cost PF‐type resins with very low formaldehyde emission potentials. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Curing of low level melamine‐modified urea‐formaldehyde particleboard binder resins studied with dynamic mechanical analysis (DMA)

Effects of resin formulation, catalyst, and curing temperature were studied for particleboard binder‐type urea‐formaldehyde (UF) and 6 ～ 12% melamine‐modified urea‐melamine‐formaldehyde (UMF) resins using the dynamic mechanical analysis method at 125 ～ 160°C. In general, the UF and UMF resins gelled and, after a relatively long low modulus period, rapidly vitrified. The gel times shortened as the catalyst level and resin mix time increased. The cure slope of the vitrification stage decreased as the catalyst mix time increased, perhaps because of the deleterious effects of polymer advancements incurred before curing. For UMF resins, the higher extent of polymerization effected for UF base resin in resin synthesis increased the cure slope of vitrification. The cure times taken to reach the vitrification were longer for UMF resins than UF resins and increased with increased melamine levels. The thermal stability and rigidity of cured UMF resins were higher than those of UF resins and also higher for resins with higher melamine levels, to indicate the possibility of bonding particleboard with improved bond strength and lower formaldehyde emission. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 377–389, 2005     

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     

13C‐NMR study on cure‐accelerated phenol–formaldehyde resins with carbonates

Both liquid‐ and solid‐state ¹³C‐NMR spectroscopies were employed to investigate the cure‐acceleration effects of three carbonates [propylene carbonate (PC), sodium carbonate (NC), and potassium carbonate (KC)] on liquid and cured phenol–formaldehyde (PF) resins. The liquid‐phase ¹³C‐NMR spectra showed that the cure‐acceleration mechanism in the PC‐added PF resin seemed to be involved in increasing reactivity of the phenol rings, while the addition of both NC and KC into PF resin apparently resulted in the presence of ortho–ortho methylene linkages. Proton spin‐lattice rotating frame relaxation time (T_1ρH) measured by solid‐state ¹³C‐CP/MAS‐NMR spectroscopy was smaller for the cure‐accelerated PF resins than for that of the control PF resin. The result indicated that cure‐accelerated PF resins are less rigid than the control PF resin. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 841–851, 2000     

13C‐NMR study on cure‐accelerated phenol‐formaldehyde resins with carbonates

Both liquid‐ and solid‐state carbon‐13–nuclear magnetic resonance (¹³C‐NMR) spectroscopies were used to investigate the cure acceleration effects of three carbonates (propylene carbonate, sodium carbonate, and potassium carbonate) on liquid and cured phenol‐formaldehyde (PF) resins. The liquid‐phase ¹³C‐NMR spectra showed that the cure acceleration mechanism in the propylene carbonate‐added PF resin seemed to be involved in increasing reactivity of the phenol rings, whereas the addition of both sodium carbonate and potassium carbonate into PF resin apparently resulted in the presence of ortho–ortho methylene linkages. Proton spin‐lattice rotating frame relaxation time (T_1ρH) measured by solid‐state ¹³C cross polarization/magic‐angle spinning NMR spectroscopy was smaller for the cure‐accelerated PF resins than that of the control PF resin. The result indicated that the cure‐accelerated PF resins are less rigid than the control PF resin. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1284–1293, 2000     

On the Cure Acceleration of Oil-Phenol-Formaldehyde Resins with Different Catalysts

Oil-phenol-formaldehyde (Oil-PF) resins containing 50 wt% replacement of petroleum phenol with bio-oil were prepared and different catalysts [sodium carbonate (Na₂CO₃), urea, and magnesium oxide (MgO)] were added in the synthesis process of resins to accelerate the cure. The cure-acceleration effects of catalysts on cure characteristics of oil-PF resins were investigated by using differential scanning calorimetry (DSC), gel time, and a plywood panels test. The results indicated that catalysts presented different accelerating effects on the cure of the oil-PF resin. Both Na₂CO₃ and MgO can accelerate the oil-PF resin cure at a low temperature; however, urea seemed to have no significant effect on the cure of the resin. The application of Na₂CO₃- and MgO-accelerated oil-PF resins reduced hot pressing time for the manufacture of three-layer plywood panels. Compared with MgO, Na₂CO₃ had more significant accelerating effect on the cure of the oil-PF resin.     

Examination of selected synthesis and room‐temperature storage parameters for wood adhesive‐type urea–formaldehyde resins by 13C‐NMR spectroscopy. V

The effects of posttreatments of particleboard adhesive‐type urea–formaldehyde resins were studied. The resins were synthesized with formaldehyde/first urea (F/U₁) mol ratios of 1.40, 1.60, 1.80, 2.10, and 2.40 and then the second urea was added to give a final formaldehyde/urea ratio of 1.15 in alkaline pH. The resins were posttreated at 60°C for up to 13.5 h and the 2.5‐h heat‐treated resin samples were stored at room temperature for up to 27 days. Resins sampled during the posttreatments were examined by ¹³C‐NMR and evaluated by bonding particleboards. In the posttreatments, hydroxymethyl groups on the polymeric resin components dissociated to formaldehyde and reacted with the second urea, and methylene and methylene–ether groups were formed from reactions involving the second urea. Methylene–diurea and urea groups bonded to UF polymers were identified. As a result, the viscosity of the resins initially decreased but later increased along with the cloudiness of the resins. Bond‐strength and formaldehyde‐emission values of particleboard varied with posttreatment variables as well as with the F/U₁ mol ratios used in the resin syntheses. The results would be useful in optimizing resin synthesis and handling parameters. Various reaction mechanisms were considered. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1896–1917, 2003     

Examination of selected synthesis and room‐temperature storage parameters for wood adhesive‐type urea–formaldehyde resins by 13C‐NMR spectroscopy. IV

Typical particleboard wood‐adhesive urea–formaldehyde (UF) resins, synthesized with formaldehyde/first urea (F/U₁) mol ratios of 1.80, 2.10, and 2.40 and the second urea added to an overall F/U ratio of 1.15, in weak alkaline pH, were allowed to stand at room temperature over a period of 50 days. ¹³C‐NMR of time samples taken over the storage period showed gradual migration of hydroxymethyl groups from the polymeric first‐urea components to the monomeric second‐urea components and also an advancing degree of polymerization of resins by forming methylene and methylene ether groups involving the second urea. These phenomena that varied with the F/U₁ mol ratios used in the resin syntheses due to the varying polymer branching structures resulted in the first step of resin synthesis. Varying viscosity decreases and increases of the resins also occurred. Due to these chemical and physical changes, the particleboards that bonded with the sampled resins showed varying bond strength and formaldehyde‐emission values, indicating process optimizations possible to improve bonding and formaldehyde‐emission performances. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1155–1169, 2001     

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.     

Evaluation of melamine‐modified urea‐formaldehyde resins as particleboard binders

Particleboards bonded with 6 and 12% melamine‐modified urea‐formaldehyde (UMF) resins were manufactured using two different press temperatures and press times and the mechanical properties, water resistance, and formaldehyde emission (FE) values of boards were measured in comparison to a typical urea‐formaldehyde (UF) resin as control. The formaldehyde/(urea + melamine) (F/(U + M)) mole ratio of UMF resins and F/U mole ratio of UF resins were 1.05, 1.15, and 1.25 that encompass the current industrial values near 1.15. UMF resins exhibited better physical properties, higher water resistance, and lower FE values of boards than UF resin control for all F/(U + M) mole ratios tested. Therefore, addition of melamine at these levels can provide lower FE and maintain the physical properties of boards. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

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     

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     

Hydrolytic stability of cured urea‐formaldehyde resins modified by additives

Urea‐formaldehyde (UF) resins are prone to hydrolysis that results in low‐moisture resistance and subsequent formaldehyde emission from UF resin‐bonded wood panels. This study was conducted to investigate hydrolytic stability of modified UF resins as a way of lowering the formaldehyde emission of cured UF resin. Neat UF resins with three different formaldehyde/urea (F/U) mole ratios (1.4, 1.2, and 1.0) were modified, after resin synthesis, by adding four additives such as sodium hydrosulfite, sodium bisulfite, acrylamide, and polymeric 4,4′‐diphenylmethane diisocyanate (pMDI). All additives were added to UF resins with three different F/U mole ratios before curing the resin. The hydrolytic stability of UF resins was determined by measuring the mass loss and liberated formaldehyde concentration of cured and modified UF resins after acid hydrolysis. Modified UF resins of lower F/U mole ratios of 1.0 and 1.2 showed better hydrolytic stability than the one of higher F/U mole ratio of 1.4, except the modified UF resins with pMDI. The hydrolytic stability of modified UF resins by sulfur compounds (sodium bisulfate and sodium hydrosulfite) decreased with an increase in their level. However, both acrylamide and pMDI were much more effective than two sulfur compounds in terms of hydrolytic stability of modified UF resins. These results indicated that modified UF resin of the F/U mole ratio of 1.2 by adding acrylamide was the most effective in improving the hydrolytic stability of UF resin. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Polyamine-modified urea-formaldehyde resins. I. Synthesis,structure, and properties

The objective of this study was to improve the durability and stability of urea–formaldehyde-bonded wood products by decreasing the internal stress developed during resin cure and by improving the ability of the cured resin to withstand cyclic stresses. This paper presents initial results from modifying a urea–formaldehyde resin by incorporating di- and trifunctional amines. The amines were incorporated by adding them as amines during resin synthesis, by adding urea-capped amines during resin synthesis, and by using amine hydrochlorides as acidic curing agents. Addition of amines during resin synthesis produced uncurable resins. However, modification with urea-capped amines or curing with amine hydrochlorides provided cure rates comparable to that of unmodified resin cured with ammonium chloride. These modifications also reduced the tendency of the resin to crack and fracture and substantially improved the resistance of bonded joints to cyclic stress imposed by cyclic wet–dry exposures. Resins cured with amine hydrochlorides had lower formaldehyde liberation than those cured with ammonium chloride. Thus, incorporation of flexible di- and triamines offers promise for improving the durability and stability of urea–formaldehyde-bonded wood products.     

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     

Examination of selected synthesis parameters for wood adhesive‐type urea–formaldehyde resins by 13C NMR spectroscopy. III

The varying polymer structures of wood adhesive‐type urea–formaldehyde resins resulting from different formaldehyde/first urea (F/U₁) mole ratios used in the first step of resin manufacture were investigated using ¹³C. As the F/U₁ mole ratio decreased progressively from 2.40 to 2.10 and to 1.80, the viscosity increase due to polymerization during resin synthesis became faster and resulted in decreasing side‐chain branches and increasing free urea amide groups in the resin structure. The resultant UF resins, with the second urea added to an overall F/(U₁ + U₂) of 1.15, showed viscosity decreases when heated with stirring or allowed to stand at room temperature that were also characteristic with the F/U₁ mole ratios used in resin synthesis. The formaldehyde emission levels of particleboards bonded with the freshly made UF resins showed relatively small but similarly characteristic variations. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2800–2814, 2001     

Phenol–formaldehyde–pyrolytic oil resins for wood preservation: A rheological study

The rheological properties of a phenol–formaldehyde resin containing various ratios of softwood pyrolytic oil as phenol substitute were investigated using the simple Bingham rheological model for viscoplastic fluids. Flow activation energy was determined for the various resin blends and the pyrolytic oil between room temperature and 50°C and correlations relating the flow activation energy to the weight fraction of pyrolytic oil in the resin are proposed. Apparent crosslinking activation energy with and without copper chloride used as an activator was also evaluated based on two gel time measurements between 75 and 105°C. A significant decrease in activation energy was observed for the phenol–formaldehyde resin cured with copper chloride, while the effect was less important for resins containing pyrolytic oil even when gel times were much shorter for PF‐resins. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Urea–formaldehyde‐resin gel time as affected by the pH value,solid content,and catalyst

An experiment was conducted to investigate the effects of the resin solid content, catalyst content, and pH value obtained by the addition of two kinds of catalysts on the gel time of a urea–formaldehyde (UF) resin. Upon the addition of ammonium chloride, the pH value of the resin mixture decreased to 7 but not significantly further because of the limited free formaldehyde in the system. The pH values of the critical points, at which the resin‐curing rate dramatically increased and the gel time was reduced, were above 7 for both catalysts. To achieve the same gel time, the required pH value of the UF resin adjusted with ammonium chloride was higher than that of the resin modified by hydrochloric acid. This indicated that the main effects of ammonium chloride on the UF‐resin cure included both the release of hydrochloric acid and the catalysis of the reactants in the UF‐resin system. The gel time of the UF resin obviously decreased with increasing catalyst and resin solid contents and with decreasing pH. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1566–1569, 2007     

Engineering biomass into formaldehyde‐free phenolic resin for composite materials

The use of formaldehyde to prepare phenol‐formaldehyde (PF) resins is one of the primary challenges for the world‐wide PF industry with respect to both sustainability and human health. This study reports a novel one‐pot synthesis process for phenol‐5‐hydroxymethylfurfural (PHMF) resin as a formaldehyde‐free phenolic resin using phenol and glucose, and the curing of the phenolic resin with a green curing agent organosolv lignin (OL) or Kraft lignin (KL). Evidenced by ¹³C NMR, the curing mechanism involves alkylation reaction between the hydoxyalkyl groups of lignin and the ortho‐ and para‐carbon of PHMF phenolic hydroxyl group. The curing kinetics was studied using differential scanning calorimetry and the kinetic parameters were obtained. The OL/KL cured PHMF resins were tested in terms of thermal stability, and mechanical properties for their applications in fiberglass reinforced composite materials. The results obtained demonstrated that OL/KL can be promising curing agents for the PHMF resins. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1275–1283, 2015     

13C‐NMR, 13C‐13C gCOSY,and ESI‐MS characterization of ether‐bridged condensation products in N,N′‐dimethylurea‐formaldehyde systems

The reaction of urea with formaldehyde is the basis for the production of urea‐formaldehyde (UF) resins which are widely applied in the wood industry. The presence of ether‐bridged condensation products in the UF resin reaction system is an open question in the literature. It is addressed in the present work. The N,N′‐dimethylurea‐formaldehyde model system was studied since it is chemically similar to the UF resin reaction system but allows for a simple elucidation of all reaction products. It was analyzed by ¹³C‐NMR spectroscopy and ESI‐MS. In corresponding NMR and MS spectra, peaks due to methoxymethylenebis(dimethyl)urea and its hemiformal were observed. ¹³C‐¹³C gCOSY analysis was conducted using labeled ¹³C‐formaldehyde. The correlation spectra showed evidence for an ether‐bridged compound and mass spectra exhibited peaks agreeing with labeled methoxymethylenebis(dimethyl)urea and its hemiformal. Methoxymethylenebis(dimethyl)urea was characterized in N,N′‐dimethylurea‐formaldehyde systems in acidic and slightly basic media. As urea is very similar to N,N′‐dimethylurea, the results of this work strengthen the assumption that ether‐bridged condensation products are likely to form in UF resins. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013