Syntheses and properties of low‐level melamine‐modified urea–melamine–formaldehyde resins

Syntheses of urea–melamine–formaldehyde (UMF) resins were studied using 2–12% melamine levels and UF base resins that were preadvanced to various different extents. The melamine reaction was carried out at pH 6.3 with F/(U + M) mole ratio of 2.1 until a target viscosity of V was reached (Gardener–Holdt) and then the second urea added at pH 8.0 to give a final F/(U + M) mole ratio of 1.15. Analyses with ¹³C‐NMR and viscosity measurements showed that MF components react fast and the UF components very slowly in the melamine reaction. Therefore, as the extent of preadvancement of UF base resin was decreased, the reaction time to reach the target viscosity became longer and the MF resin components showed high degrees of polymerization. The overpolymerization of MF components resulted in increasingly more opaque resins, with viscosity remaining stable for more than a month. As the preadvancement of UF base resin was increased, the extent of advancement of MF components decreased, to give clearer resins, with viscosity slowly increasing at room temperature. Overall, preadvancing the UF base resin components to an appropriate extent was found to be a key to synthesizing various low‐level melamine‐modified UMF resins. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2559–2569, 2004     

Low‐volatility acetals to upgrade the performance of melamine–urea–formaldehyde wood adhesive resins

1,1,2,2‐Tetramethoxyethane (TME), a high boiling point acetal derived from glyoxol, lhas been shown to upgrade the performance of melamine‐urea‐formaldehyde (MUF) and some UF resins used for wood adhesives. This affords the possibility of decreasing the percentage of resin used in the preparation of wood panels without volatilizing the TME acetal used.     

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     

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     

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     

Colloidal aggregation of MUF polycondensation resins: Formulation influence and storage stability

Colloidal particle formation followed by their clustering has been shown to be the normal way of ageing of aminoplastic resins, in particular melamine–urea–formaldehyde (MUF) resins. Ageing (or further advancement of the resin by other means such as longer condensation times) causes whitening of the resin. This is a macroscopic indication both of the formation of colloidal particles and of their clustering. Some clustering appears rather early in this process, even when the great majority of the resin does visually appear to be in colloidal state, being transparent. However, it eventually progresses to resins which are mostly in colloidal, clustered state, followed much later by a supercluster formation starting to involve the whole resin. There appears to be clear correspondence between molecular mass increases as obtained by gel permeation chromatography (GPC), low‐angle laser light scattering (LALLS) analysis, and observation by polarizing optical microscopy. LALLS, however, appears to indicate the dimensions of the colloidal particles themselves when the level of colloidal aggregation is rather low, but it indicates the dimensions of the clusters once these are mostly aggregated. The smaller visible colloidal particles, already aggregates, were found by polarizing optical microscopy to be of a mostly elongated, rodlike shape, the length of which was shown to grow much further than their width with resin advancement and ageing. As their dimensions indicate, these are already clusters; this implies that the mainly linear increase of the polycondensate chains influences also the simpler colloidal clusters' growth direction, possibly explaining the resins' lack of tridimensional hardening while still in storage. It also explains why molecules such as free urea and acetals, by disrupting these colloidal aggregation mechanisms, allow both a much longer shelf life of the resin and its better performance in hardening. These findings explained the considerable difference in the behavior and performance of different MUF resin formulations. The ageing of the MUF resins of different preparation procedures appeared then to proceed from (1) clear resin (molecular colloidal aggregation) to (2) superclusters of a whitened, heavily thixotropic resin, which is the beginning of physical gelation to (3) liquid/cluster separation, which is the terminal stage of physical gelation. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2690–2699, 2004     

Melamine salts as hardeners for urea formaldehyde resins

Various salts derived from melamine and organic acids were prepared and used as melamine substitutes for melamine urea formaldehyde (MUF) resins. The synthesis of these melamine salts and a detailed characterization of their stoichiometry are described. All salts form 1 : 1 or 1 : 2 stoichiometries in a homogeneous reaction. They crystallize during cooling of the hot and diluted reaction mixture. Both ¹³C–NMR and ¹⁵N–NMR data are reported and point toward the formation of real ionic structures. Most salts have higher water solubility than that of pure melamine and are tested for their ability to substitute melamine in MUF resins. The mechanical and chemical properties of plywood panels made up of traditional MUF resins and mixtures of UF resins with melamine salts are investigated. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1654–1661, 2001     

Acetals‐induced strength increase of melamine–urea–formaldehyde (MUF) polycondensation adhesives. II. Solubility and colloidal state disruption

The strength improvement induced by addition of acetals such as methylal and ethylal in melamine–urea–formaldehyde (MUF) resins could be mostly ascribed to the increased effectiveness and participation of the melamine to resin cross‐linking. This phenomenon has been shown here, by matrix‐assisted laser desorption/ionization time of flight (MALDI‐TOF) mass spectroscopy, resin aging time stability, and mainly by laser light scattering, to be due to the following: (i) the increased solubility in water afforded by the acetals cosolvents of both the unreacted melamine and of the normally very much lower solubility, higher molecular weight, lower methylolated oligomers fraction, this leading to preferentially homogeneous and hence more effective reaction rather than heterogeneous reactions; and (ii) the effect that such acetals have on the size distribution of the resin colloidal particles, with the presence of acetals such as methylals markedly decreasing the average colloidal particles diameter of the resin. This latter effect appears to be due to the disruption of the molecular clustering of the MUF resin colloidal particles, but rearrangements in the size of the colloidal particles due to the decrease in surface tension of the system, which has also been noted, cannot be excluded. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1855–1862, 2002     

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.     

Blocked melamine–urea–formaldehyde resins and their usage in agglomerated cork panels

Caprolactam and o‐p‐toluenesulfonamide are tested as chain‐growth blockers for melamine–urea–formaldehyde (MUF) resins, in an attempt to reduce the crosslinking density of the cured resin and hence improve its flexibility. Agglomerated cork panels, for which flexibility is a technical demand, were produced with the modified resins and tested. The blockers were added at three different steps in the synthesis process: methylolation, condensation, and at the end of the synthesis. Besides evaluation of standard properties, resins were characterized using gel permeation chromatography and Fourier transform infrared. Blocked resins showed better storage stability and improved water tolerance, especially when caprolactam was employed. When used as binders in agglomerated cork panels, the blocked resins allowed for significantly better flexibility, evaluated in terms of mandrel bending test. The tensile resistance of the panels remained well within the desired limits for this type of material. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46663.     

Low addition of melamine salts for improved melamine‐urea‐formaldehyde adhesive water resistance

The addition of melamine acetate salts to an adhesive glue mix can allow the use of melamine–urea–formaldehyde (MUF) resins of lower melamine contents (rather than just urea–formaldehyde resins) and lower total amounts of melamine. Performances can be obtained that are characteristic of the top‐of‐the‐line, generally higher melamine content MUF adhesive resins for the preparation of wood particleboard panels. Improvements in the panel internal‐bond strength of greater than 30% can be obtained by the addition of melamine acetate salts to top‐of‐the‐line MUF resins. The approach to the concept of increased melamine solubility with a melamine salt is compatible with the approach of increasing melamine solubility with solvents such as acetals (e.g., methylal). However, the synergy advantage of using the two approaches jointly is not very marked. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 287–292, 2003     

Variation of MUF and PMUF resins mass fractions during preparation

The variation of molecular mass distribution with the progress of the reaction was studied for the following: (i) sequential‐type melamine–urea–formaldehyde (MUF) resin formulations in which the sequence of addition of chemicals follows well‐defined species reactivity principles; (ii) a nonsequential MUF formulation in which simultaneous melamine and urea competition for formaldehyde yields a MF resin cocondensed with small amounts of urea. This resin became soaked with reacted and unreacted monomeric urea species. (iii) A PMUF resin, namely a MUF resin with a small proportion of phenol (7.8% by weight on melamine and urea) cocondensed with the main MUF fraction. All the formulations used were industrial resins formulations in current use. Development and variation of molecular mass fractions, from which performance and other useful resin parameters depend, have been found to depend on the type of resin formulation used for these type of aminoplastic resins. The two very different MUF resin formulations yielded different variations in molecular mass fractions during the progress of the reaction and during the so‐called ambient temperature “maturing” of the resin. The PMUF resin also showed both similar and different fractions present during manufacturing and during short term ageing at ambient temperature. While similarities in recurrent fractions and in trends are common to all the three different formulations, differences between them are also clearly observed. A major proportion of the reaction of some of the aminoplastic resins examined also occurs on ageing (i.e.“maturing” of the resin at ambient temperature), this appearing to be an essential phase of the resin preparation process. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4842–4855, 2006     

Urea–formaldehyde–propionaldehyde physical gelation resins for improved swelling in water

Urea–formaldehyde (UF) resins' water tolerance and swelling thickness of interior‐grade wood panels bonded with UF resins were improved markedly by introducing small amounts of UFPropanal (UFP) polycondensates into the UF resin. ¹³C NMR of urea–propanal (UP) resins showed that urea and propanal do react up to the formation of dimers. The water repellancy imparted by insertion in the resin of the alkyl chain of propanal limits the proportion of propanal that can be used. Gel permeation chromatography showed that this appears to be so because UP resins and UFP resins exist as an equilibrium between two separate intermingling phases, namely one in solution and the second in a state of physical gelation. This latter is different from the state of physical gelation observed on ageing or advancement of formaldehyde‐only based polycondensation resins. This physical gelation is brought on by the insertion in the resin of the water repellant chain of the propanal reacted with urea and constitutes a new state of physical gelation of polycondensates other than what was already reported in the literature. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5131–5136, 2006     

Effects of diatomite inorganic fillers on the properties of a melamine–urea–formaldehyde resin

In this study, a low‐cost diatomite was used to partly substitute wheat flour as one type of melamine–urea–formaldehyde (MUF) resin filler. Five‐ply plywood was fabricated, and its performance was measured. The crystallinity, fracture surface, and functional groups were tested to determine the effects of diatomite on the performance of the MUF resin. The results show that diatomite was well distributed in the MUF resin system and formed an embedding structure; this improved the wet shear strength of the resulting plywood by 33% to 1.36 MPa. Diatomite captured the free formaldehyde in the resin and the microporous structure formed in the resin accelerate formaldehyde release of the plywood. Consequently, the formaldehyde emission of the plywood was reduced. The diatomite partly replaced wheat flour as an MUF resin filler and could be applied in the plywood industry. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44095.     

Testing by fourier transform infrared species variation during melamine–urea–formaldehyde resin preparation

The preparation of an industrially used sequential formulation of a melamine–urea–formaldehyde resin was followed with Fourier transform infrared (FTIR). The analysis allowed us to identify the increases and decreases of the main groups in the resin and to compare this system of resin analysis with results previously obtained by ¹³C‐NMR analysis. The FTIR analysis, although considerably more limited than ¹³C‐NMR analysis, allowed us nonetheless to identify and follow the appearance, increase, decrease, and disappearance of several of the main chemical groups during the preparation of the initial urea–formaldehyde (UF) phase of the reaction and the subsequent reaction of melamine with the UF resin that was formed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Melamine‐bridged alkyl resorcinol modified urea–formaldehyde resin for bonding hardwood plywood

A powdery product was obtained by the reaction of methylolated melamine with alkyl resorcinols to form melamine‐bridged alkyl resorcinols (MARs). The effects of the addition of this powder on the bonding strength and formaldehyde emission of urea–formaldehyde (UF) resins were investigated. Three types of UF resins with a formaldehyde/urea molar ratio of 1.3 synthesized by condensation at pH 1.0 (UF‐1.0), pH 4.5 (UF‐4.5), and pH 5.0 (UF‐5.0) were fabricated. The addition of MAR to UF‐4.5 and UF‐5.0 for bonding hardwood plywood enhanced the bonding strength and reduced formaldehyde emission. For UF‐1.0, the addition of MAR adversely affected the bonding strength. However, the UF‐1.0 resin yielded the lowest formaldehyde emission of all of the UF resins in the study. The effects of the MAR addition were related to the molecular structures of the UF resins. UF‐1.0 contained a large amount of free urea, a considerable number of urons, and a highly methylene‐linked, ring‐structured higher molecular weight fraction and had a smaller number of methylol groups. Therefore, the addition of MAR was considered to cause a shortage of the methylol groups, which in turn, led to incomplete resin curing. In contrast to UF‐1.0, UF‐5.0 contained a smaller amount of free urea and a linearly structured higher molecular weight fraction and had a larger number of methylol groups. In this case, MAR was considered to effectively react with the methylol groups to develop a three‐dimensional crosslinked polymer network to enhance the bonding strength and suppress the generation of free formaldehyde to reduce formaldehyde emission. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Synthesis and properties of high-sulfonated melamine–formaldehyde resin

High-sulfonated melamine–formaldehyde (HSMF) resins were prepared with 1.0–2.0 of sulfite/melamine (S/M) molar ratio. The factors affecting the preparation and the properties of the resin were studied. Chemical analysis indicated that there are 0.98–1.96 sulfonate groups per unit of the polymeric chain. The viscosity of HSMF resins is lower than that of sulfonated melamine–formaldehyde (SMF) resins, and the HSMF resins are more effective superplasticizers at small dosages of admixture. © 1995 John Wiley & Sons, Inc.     

Uron and uron–urea‐formaldehyde resins

The favored pH ranges for the formation of urons in urea‐formaldehyde (UF) resins preparation were determined, these being at pH's higher than 6 and lower than 4 at which the equilibrium urons ↔ N,N′‐dimethylol ureas are shifted in favor of the cyclic uron species. Shifting the pH slowly during the preparation from one favorable range to the other causes shift in the equilibrium and formation of a majority of methylol ureas species, whereas a rapid change in pH does not cause this to any great extent. UF resins in which uron constituted as much as 60% of the resin were prepared and the procedure to maximize the proportion of uron present at the end of the reaction is described. Uron was found to be present in these resins also as linked by methylene bridges to urea and other urons and also as methylol urons, the reactivity of the methylol group of this latter having been shown to be much lower than that of the same group in methylol ureas. Thermomechanical analysis (TMA) tests and tests on wood particleboard prepared with uron resins to which relatively small proportions of urea were added at the end of the reaction were capable of gelling and yielding bonds of considerable strength. Equally, mixing a uron‐rich resin with a low F/U molar ratio UF resin yielded resins of greater strength than a simple UF of corresponding molar ratio indicating that UF resins of lower formaldehyde emission with still acceptable strength could be prepared with these resins. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 277–289, 1999     

Production of melamine fortified urea‐formaldehyde resins with low formaldehyde emission

Melamine can be incorporated in the synthesis of urea‐formaldehyde (UF) resins to improve performance in particleboards (PB), mostly in terms of hydrolysis resistance and formaldehyde emission. In this work, melamine‐fortified UF resins were synthesized using a strong acid process. The best step for melamine addition and the effect of the reaction pH on the resin characteristics and performance were evaluated. Results showed that melamine incorporation is more effective when added on the initial acidic stage. The condensation reaction pH has a significant effect on the synthesis process. A pH below 3.0 results on a very fast reaction that is difficult to control. On the other hand, with pH values above 5.0, the condensation reaction becomes excessively slow. PBs panels produced with resins synthesized with a condensation pH between 4.5 and 4.7 showed good overall performance, both in terms of internal bond strength and formaldehyde emissions. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effects of acid hydrolysis on microstructure of cured urea‐formaldehyde resins using atomic force microscopy

This study investigated the effect of acid hydrolysis on the microstructure of cured urea‐formaldehyde (UF) resins using atomic force microscopy (AFM) to better understand its hydrolytic degradation process which has been known to be responsible for the formaldehyde emission of wood‐based composite panels. The AFM was scanned on both outer surface and facture surfaces of the thin films of cured UF resins that had been exposed to the etching of dilute hydrochloric acid to simulate their hydrolysis process. The AFM images showed two distinctive parts, which were classified as the hard and soft phases in cured UF resins. For the first time, this study reports the presence of thin filament‐like crystalline structures on the fracture surface of cured UF resin. The soft phase of cured UF resins by ammonium chloride was much more easily hydrolyzed than those cured by ammonium sulfate, indicating that hardener types had a great impact on the hydrolytic degradation behavior of cured UF resins. The surface roughness measurement results also supported this result. The results of this study suggested that the soft phase was much more susceptible to the hydrolysis of cured UF resin than the hard phase. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011