Influence of nanoclay on urea–glyoxalated lignin–formaldehyde resins for wood adhesive

In this research, the influence of nanoclay on urea–glyoxalated lignin–formaldehyde (GLUF) resin properties has been investigated. To prepare the GLUF resin, glyoxalated soda baggase lignin (15 wt%) was added as an alternative for the second urea during the UF resin synthesis. The prepared GLUF resin was mixed with the 0.5%, 1%, and 1.5% nanoclay by mechanically stirring for 5 min at room temperature. The physicochemical properties of the prepared resins were measured according to standard methods. Then the resins were used in particleboard production and the physical and mechanical properties of the manufactured panels were determined. Finally, from the results obtained, the best prepared resin was selected and its properties were analyzed by differential scanning calorimetry (DSC), Fourier transform infrared spectrometry (FTIR), and X-ray diffractometry (XRD). Generally the results indicated that the addition of sodium-montmorillonite (NaMMT) up to 1.5% appears to improve the performance of GLUF resins in particleboards. The results also showed that nanoclays improved mechanical strength (modulus of elasticity (MOE), Modulus of Rupture (MOR), and internal bond (IB) strength) of the panels bonded with GLUF resins. The panels containing GLUF resin and nanoclay yielded lower formaldehyde emission as well as water absorption content than those made from the neat GLUF resins. XRD characterization indicated that NaMMT only intercalated when mixed with GLUF resin. Based on DSC results, the addition of NaMMT could accelerate the curing of GLUF resins. The enthalpy of the cure reaction (ΔH) of GLUF resin containing NaMMT was increased compared with neat GLUF resin. Also the results of FTIR analysis indicated that addition of NaMMT change the GLUF resins structures.     

The Effect of Nanoclay on Melamine-Urea-Formaldehyde Wood Adhesives

Abstract

In this work it has been shown that the addition of small percentages of Na⁺-montmorillonite (Na-MMT) and organic modified-montmorillonite (O-MMT) nanoclay to thermosetting melamine-urea-formaldehyde (MUF) resins appeared to improve, considerably, their performance as adhesives for wood particleboard. X-ray diffraction (XRD) studies indicated that Na-MMT when mixed in small proportions, loses the periodic atomic structure during the curing of MUF resins. This can be interpreted as becoming exfoliated under such conditions. Thermomechanical analysis (TMA) on laboratory MUF-bonded particleboard indicated that small percentages of Na-MMT and O-MMT improved the internal bond strength imparted by the MUF adhesive to the panel, when tested dry and also, more importantly, when tested after 2 h of boiling. The increase in water resistance of the MUF-bonded panel was particularly noticeable. Furthermore, the addition of Na-MMT was shown to increase considerably the resistance of the MUF resin to abrasion. This is important from a wood surface finish point of view.     

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     

Influence of acrylamide copolymerization of urea–formaldehyde resin adhesives to their chemical structure and performance

To lower the formaldehyde emission of wood‐based composite panels bonded with urea–formaldehyde (UF) resin adhesive, this study investigated the influence of acrylamide copolymerization of UF resin adhesives to their chemical structure and performance such as formaldehyde emission, adhesion strength, and mechanical properties of plywood. The acrylamide‐copolymerized UF resin adhesives dramatically reduced the formaldehyde emission of plywood. The ¹³C‐NMR spectra indicated that the acrylamide has been copolymerized by reacting with either methylene glycol remained or methylol group of UF resin, which subsequently contributed in lowering the formaldehyde emission. In addition, an optimum level for the acrylamide for the copolymerization of UF resin adhesives was determined as 1%, when the formaldehyde emission and adhesion strength of plywood were taken into consideration. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Study of glycidyl ether as a new kind of modifier for urea‐formaldehyde wood adhesives

In this work, the multiepoxy functional glycidyl ether (GE) modified urea‐formaldehyde (UF) resins were synthesized via a traditional alkaline‐acid process under low formaldehyde/urea (F/U) molar ratio. The synthesized resins were characterized by ¹³C magnetic resonance spectroscopy (¹³C‐NMR), indicating that GE can effectively react with UF resins via the ring‐opening reaction of epoxy groups. Moreover, the residual epoxy groups of GE could also participate in the curing reaction of UF resins, which was verified by Fourier transform infrared spectroscopy. The storage stability of GE‐modified UF resins and the thermal degradation behavior of the synthesized resins were evaluated by using optical microrheology and thermogravimetric analysis, respectively. Meanwhile, the synthesized resins were further employed to prepare the plywood with the veneers glued. For the modification on bonding strength and formaldehyde emission of the plywood, the influences of addition method, type, and amount of GE were systematically investigated. The performance of UF adhesives were remarkably improved by the modification of GE around 20–30% (weight percentage of total urea) in the acidic condensation stage during the resin synthesis. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Decreasing the formaldehyde emission in urea‐formaldehyde using modified starch by strongly acid process

The incorporation of the modified starch (MS) in urea‐formaldehyde resins at different stage of the synthesis was studied in this article. The synthesized resins were characterized by Fourier transform infrared spectroscopy, indicating that the ester bond can be introduced into the UF structure after the addition of MS. The curing reactions were examined with differential scanning calorimetry and it reveals that curing temperature of UF resin are slightly shifted to higher temperatures. To study the bonding strength and formaldehyde emission of the bonded plywood, the addition method and amount of MS are systematically investigated. The performance of the UF resins is remarkably improved by the addition of MS around 15% (weight percentage of the total resin) in the second stage. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40202.     

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     

The reaction in water of UF resins with isocyanates at short curing times: A 13C NMR investigation

CP MAS ¹³C NMR spectra of hardened resins have shown that urethane bridges derived from the reaction of the isocyanate group with the hydroxymethyl group of urea do form even at fast curing times comparable to what was used in the wood panels industry, in lower proportions than what was shown earlier. Polyureas and biurets obtained from the reaction of isocyanate with water are the predominant crosslinking reactions of pMDI alone and in UF/pMDI resin systems under fast curing conditions. Residual, unreacted isocyanate groups in the hardened network are consistently observed. Their proportion markedly decreases when the original proportion of urea–formaldehyde (UF) resin is high and that of pMDI is low. Under these fast curing conditions, the UF resin appears to self‐condense through an unusually high proportion of methylene ether links rather than methylene bridges alone. A marked proportion of residual, unreacted hydroxymethyl groups is also noticeable, initially, in the UF self‐condensation network. Direct NMR tests on thin hardboard bonded under fast pressing conditions with different proportions of UF/pMDI confirmed that crosslinking due to polyureas and biurets formation are predominant in the crosslinking of pMDI when alone and in UF/pMDI resin systems. They confirmed that residual, unreacted isocyanate groups are present in the finished panel. Their proportion is higher when the proportion of pMDI in the system is high. The presence or absence of urethanes could not be confirmed directly on the panels as the relevant peaks are masked by the wood carbohydrates signals of wood cellulose and hemicelluloses. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1624–1632, 2006     

Improving the properties of ionic liquid-treated lignin-urea-formaldehyde resins by a small addition of isocyanate for wood adhesive

The aim of this research was to investigate the effect of polymeric 4, 4 diphenyl methane diisocyanate (pMDI) on the physical and mechanical properties of plywood panels bonded with an ionic liquid (IL)-treated lignin-urea-formaldehyde resin. Soda lignin modified by 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) IL was added to a urea formaldehyde (UF) resin during resin synthesis to prepare a lignin-urea-formaldehyde (LUF) resin. pMDI at various contents (2, 4, and 6% on resin solids) was then added to prepare a LUF resin. The thermal and physicochemical properties of the resins prepared as well as the water absorption, shear strength, and formaldehyde emission of the plywood panels bonded with them were measured according to standard methods. DSC analysis indicated that the addition of pMDI decreases the gel onset and curing temperatures of the LUF resin. According to the results obtained, the addition of pMDI significantly increased the viscosity and solid content and accelerated the gelation time of LUF resins. Based on the findings of this research, the addition of pMDI dramatically improves the performance of LUF resins as a new adhesive for wood-based panels. The LUF resins with isocyanate added yielded panels presenting lower formaldehyde emission and lower water absorption content when compared to those bonded with the control LUF resins. Greater dry and wet shear strength can be obtained by a small addition of pMDI to LUF resins.     

低甲醛释放量人造板用改性脲醛树脂制备及应用研究

为了降低脲醛树脂的游离甲醛含量及其胶接制品的甲醛释放量,本研究在脲醛树脂合成过程中加入改性剂代替部分甲醛,通过尿素-甲醛-改性剂发生共缩聚反应,合成了改性脲醛树脂。研究了改性剂取代甲醛的摩尔比对改性脲醛树脂固化速度、游离甲醛含量的影响,以及在不同的热压条件下,对胶接胶合板的胶合强度和甲醛释放量的影响。研究结果表明,改性剂的加入不仅能有效降低改性脲醛树脂的游离甲醛含量及其胶合板的甲醛释放量,还能提高胶合板的胶合强度和耐水性。     

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     

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     

木质纤维素微纤丝改性UF树脂的性能研究

将木质纤维素微纤丝（MFC）加入UF树脂,考察其热性能与力学性能的变化。DSC研究结果显示随着MFC含量的增加,UF树脂固化温度逐渐下降;热重分析显示添加MFC可改善UF树脂的热稳定性;DMA实验结果表明添加MFC的UF胶合板储能模量和玻璃化转变温度有所上升;胶合强度测试表明添加MFC的UF胶合板的胶合强度提高了29%。与未改性的UF树脂相比,木质纤维素微纤丝（MFC）的加入降低了固化温度,提高了热稳定性,改善了力学性能。     

Acetal‐induced strength increases and lower resin content of MUF and other polycondensation adhesives

Acetals such as methylal and ethylal are shown to be particularly effective additives in improving the strength of wood boards bonded with melamine‐ urea‐formaldehyde (MUF) resins, although they show some appreciable but lesser effect on other resins too, particularly phenol‐ formaldehyde resins. They equally allow a considerable decrease in resin loading, and thus, in melamine content, on the bonded wood panel and at parity of performance. Their development as additives is then primarily, but not only, targeted at the MUF adhesive resins. One‐third decreases in MUF adhesives loading at parity of performance or equally internal bond (IB) strength increases approximately up to 50% by addition of methylal are shown to be possible by laboratory particleboard as well as by thermomechanical analysis (TMA). The reasons for methylal and ethylal behavior were studied by a variety of techniques, including liquid‐ and solid‐ phase ¹³C‐NMR and could be mostly ascribed to the increased effectiveness and participation of the melamine to resin crosslinking due to its now preferentially homogeneous rather than heterogeneous reactions, consequences of the increased solubility in water afforded by the acetal cosolvents. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2561–2571, 2002     

Renewable polyol‐based polycarbamates and polycarbamate–formaldehyde thermosetting resins

Several polycarbamates and polycarbamate–formaldehyde (CF) resins were synthesized, and their properties were investigated aiming at developing of useful thermosetting polymer materials from simple polyols including those derived from renewable resources. Polycarbamates synthesized from polyols using two‐step laboratory routes showed good storage stabilities making them suitable as large volume industrial chemicals. Furthermore, syntheses and ¹³C‐NMR studies of CF resins showed the formation of oligomeric resins having hydroxymethyl and methylene groups with thermosetting curing properties that are similar to those of current urea–formaldehyde (UF) resins. Dynamic mechanical analysis studies showed somewhat slower curing rates for CF resins compared to UF resins. Bonding of particleboard and internal bond and free formaldehyde content measurements indicated high‐bond strength values and very low‐formaldehyde emission potentials for CF resins. The higher functionalities of CF resins appear to be the basis of good performances. Further investigations on scalable synthesis methods for polycarbamates and on the expansion of CF resins' bonding capabilities would need to be investigated in the future. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

低nF/nM三聚氰胺-甲醛低聚物对脲醛树脂性能的影响

采用自制的可溶性三聚氰胺-甲醛低聚物作为脲醛树脂的添加剂,通过DSC及压制胶合板等方法评价该低聚物对脲醛树脂的固化特性、游离甲醛释放量和胶合强度的影响.结果表明,对于nF/nU为1.1的脲醛树脂UF1.随着三聚氰胺-甲醛低聚物加入量的增加,树脂固化活化能和胶合板的甲醛释放量都呈现先降低后增加的趋势,而胶合强度变化趋势不...     

Reduction of Formaldehyde Emission from Particleboard by Phenolated Kraft Lignin

The aim of this study was the reduction of formaldehyde emission from particleboard by phenolated Kraft lignin. For this purpose, the lignin was extracted from black liquor and then modified by phenolation. During the urea formaldehyde (UF) resin synthesis different proportions of unmodified and phenolated Kraft lignins (10%, 15%, and 20%) were added at pH = 7 instead of the second urea. Physicochemical properties and structural changes of resins so prepared, as well as the internal bond (IB) strength and formaldehyde emission associated with the panels bonded with them were measured according to standard methods. The Fourier transform infrared (FTIR) analysis of lignin indicated that the content of O–H bonds increased in phenolated lignin while the aliphatic ethers C–O bonds decreased markedly in the modified lignin. Since both synthesis of UF resins and lignin phenolation are carried out under acid conditions, phenolation is an interesting way of modifying lignin for use in wood adhesive. The panels bonded with these resins showed significantly lower formaldehyde emission compared to commercial UF adhesives. The UF resin with 20% phenolated lignin exhibited less formaldehyde release without significant differences in internal bond strength and physicochemical properties compared to an unmodified UF resin. XRD analysis results indicated that addition of phenolated lignin decreased the crystallinity of the hardened UF resins.     

Modification of urea-formaldehyde resin adhesives with blocked isocyanates using sodium bisulfite

Polymeric 4-4 diphenyl methane diisocyanate (pMDI) was blocked with an aqueous sodium bisulfite solution to obtain water-dispersible blocked pMDI (B-pMDI) resin with different HSO₃/–NCO mole ratios for the modification of urea-formaldehyde (UF) resin. Fourier transform infrared (FTIR) spectra of the B-pMDI resin clearly showed that all isocyanate groups of the pMDI resin were successfully blocked by sodium bisulfite. As the HSO₃/–NCO mole ratio increased, the de-blocking temperature of the B-pMDI resin also increased. Two addition levels (1% and 3%) of the B-pMDI resin with different HSO₃/–NCO mole ratios were mixed with UF resins and used as an adhesive for plywood. The gel time of the UF/B-pMDI resins decreased to a minimum at a mole ratio of 0.9 and then increased with the HSO₃/–NCO mole ratio, and was consistent with the peak temperature (Tp). However, as the HSO₃/–NCO mole ratio increased, the viscosity of the modified UF resins by 1% B-pMDI resin addition slightly increased, whereas those of modified resins with 3% B-pMDI resin addition rapidly increased. The adhesion strengths of plywood bonded with the hybrid resins were greater for 1% B-pMDI resin addition than for 3% B-pMDI resin addition. Formaldehyde emission of plywood bonded with the UF/B-pMDI resins significantly decreased up to 34% by the addition of B-pMDI resin at a mole ratio of 1.8. These results suggest that the modification of UF resins by mixing with water-dispersible B-pMDI resin can be a method for improving the water resistance and formaldehyde emission of UF resins for wood-based composites.     

Study of hybrid isocyanate and high-mono-hydroxymethyl urea content urea-formaldehyde resins

Based on the difference in the reaction rate of different groups of urea-formaldehyde resins and isocyanate resins, this study designed two different urea-formaldehyde resins: a normal urea-formaldehyde resin (UF) and one with high mono-hydroxymethylurea content (UF*) to react with polymeric methylene diphenyl diisocyanate (pMDI) resin. The difference in mono- and di-hydroxymethyl urea content between UF and UF* resins was analyzed by nuclear magnetic resonance (NMR) spectroscopy, and results showed that the mono-hydroxymethyl urea content of the UF* resin was much higher than that of the conventional UF resin. The fourier transform infrared spectrometer (FTIR) analysis of differences between UF* and UF resin showed that the UF* process did not change the main structure of the conventional urea formaldehyde resin. Differential scanning calorimeter (DSC) analysis showed that the curing temperature of the hybrid UF*-pMDI resin was reduced 27.3°C compared to that of the UF-pMDI resin. When these hybrid resins were used to bond plywood respectively, test results showed that the UF*-pMDI resin improved the dry and wet bonding strength by 2.6% and 3.9%, respectively, compared with the UF-pMDI resin under the condition of hot pressing time (3 min) and temperature (140°C), meeting the requirement of Chinese standard of GB/T 9846–2015 for Class III board. This study provides a new path for further improving the performance and design of hybrid resins based on isocyanate and urea-formaldehyde resin.     

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