Low Formaldehyde Emission Research Articles

Characterization on the Copolymerization Resin between Bayberry (Myrica rubra) Tannin and Pre-Polymers of Conventional Urea–Formaldehyde Resin

By focusing on the disadvantages of weak water resistance and high formaldehyde emission of urea–formaldehyde resin (UF), this research provides a new method to overcome these shortages of UF resin by using tannin for partial substitution of urea. Furthermore, plasma pretreatment of wood was introduced to strengthen the bonding performance of plywood. The investigation of the chemical structure of UF resin and tannin–urea–formaldehyde resin (TUF) were performed with Fourier transform infrared spectroscopy (FT-IR) and solid-state 13C nuclear magnetic resonance (13C NMR). The results of investigations confirmed the joining of tannin into the resin structure, which may enhance structural rigidity of TUF adhesives and improve hydrolysis stability. Then, thermal performance of UF resin and TUF resins were tested by differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis. The DSC results indicated that the curing temperature did not change significantly. However, the TG analysis showed that the thermal stability of TUF resin was considerably improved. In bonding performance test, tannin–urea–formaldehyde resin (TUF) revealed an excellent water resistance, comparable to UF resin and can fulfill the standard requirement for plywood (Type II according to the Norm GB/T 17657-2013). It is interesting that the shear strength of wood specimens, bonded with TUF6 resin, after low-pressure cold plasma equipment (CLP plasma) and jet type atmospheric low-temperature plasma (JTLP plasma) treatment, reached 0.80 MPa and 0.85 MPa, respectively, after being soaked in boiling water for 3 h. In addition, most of the bonded plywood samples with TUF resin exhibited a lower formaldehyde emission, especially those prepared at 70 °C and 1.5 h, in which the formaldehyde emission amount could be reduced by approximately 39%.

Polyoxymethylene/Reduced Graphene Oxide-g-Melamine Nano-composites With Low Formaldehyde Emission: Intercalation Structure and Synergistic Thermal Oxidative Stabilization Effect

Formaldehyde emission produced by polymer degradation would cause serious environment pollution. Melamine (MA) as highly efficient formaldehyde scavenger has been widely applied in polyoxymethylene (POM) industry. However, the volatilization, migration and physical loss of MA during processing and application have become a serious problem for POM production. In this work, MA was first grafted onto graphene oxide (GO) surface through amidation reaction, and reduced graphene oxide-g-melamine (rGO-MA) hybrid was prepared by subsequent reduction. The POM/rGO-MA nano-composite was then manufactured via conventional melt processing. By incorporation of MA, hydrogen bonding was enhanced for composite, and POM molecules were intercalated into reduced graphene oxide (rGO) layers effectively with high intercalation ratio, while rGO was exfoliated and uniformly distributed in POM matrix. Compared with neat POM, the formaldehyde emission amount (FEA) of POM/rGO-MA decreased notably by 50.8%, and the thermal degradation temperature and degradation activation energy (Ea) were improved significantly. Meanwhile, the retention of mechanical property of POM/rGO-MA remained stable, while melt index increased much slowly during thermo-oxygen aging process. By grafting MA onto rGO surface, both the thermal migration resistance of MA and radical scavenging ability of GO were enhanced, while rGO-MA exhibited excellent synergistic removal effect of formaldehyde generated by POM degradation. Besides, the composite showed extraordinary oxygen barrier property, and thermal oxidation degradation of POM was delayed effectively.

Maleated lignin coreaction with phenol-formaldehyde resins for improved wood adhesives performance

This research work investigated the physical and mechanical properties of particleboard panels bonded with maleated-lignin-phenol-formaldehyde (mLPF) resin. For this purpose, lignin was modified by maleic anhydride and then various contents of unmodified/modified lignin (30, 40 and 50 wt%) were added as a substitute for phenol in the phenol-formaldehyde (PF) resin synthesis. The properties of the synthesized resins were compared with those of an unmodified LPF and a control PF resin. The structure of the modified lignin was studied by FTIR, and the changes in curing behavior of the resins prepared were analyzed by DSC. The physicochemical properties of the synthesized resins (such as gel time, viscosity, solid content and density) and physical (water absorption) and mechanical properties (flexural properties, internal bond strength) of the prepared particleboard panels bonded with LPF resins were measured. The FTIR analysis indicated that, by treatment of lignin, the COOH, C–O and CC bonds increase while the content of O–H bond markedly decreases. The DSC analysis showed that curing of the mLPF resin occurred at a lower temperature than for the control PF resin and the unmodified lignin-phenol-formaldehyde (LPF) resins. The viscosity increased and the gel time was shortened by adding maleated lignin to the PF resin. The panels bonded with the modified lignin resins presented a lower formaldehyde emission, and a higher dimensional stability and mechanical strength than those bonded with the control PF resin and the unmodified lignin adhesive. Moreover, the mechanical strength and dimensional stability of the particleboard panels were significantly improved by increasing the resin content of maleated lignin from 0 to 50 wt%.

Synthesis and Properties of Melamine Modified Urea Formaldehyde Resin for Impregnation Under New Process

In this paper, the basic properties of UF and UMF resins for impregnation under conventional and new processes were compared, and their chemical structures were characterized by ¹³C NMR. The effects of synthetic process and melamine addition on the bonding strength and formaldehyde emission of UF and UMF resins were studied. The resin synthesized by the new process has higher linear and methylene ether bond content, relatively lower methylene content, longer curing time and storage life and lower free formaldehyde content than the conventional resin; When melamine was added at the initial stage of UF resin synthesis reaction, it did not significantly change the total amount of methylene ether and hydroxymethyl in the resin system, but the content of class I hydroxymethyl increased and the content of class II hydroxymethyl decreased. The reason may be that the added melamine and urea react not only with the remaining formaldehyde in the system, but also with the formaldehyde removed from hydroxymethyl due to dehydroxylation reaction, Hydroxymethylmelamine and monobasic substituted urea are produced. At this time, about 50% of melamine still exists in UMF resin in the form of free or low hydroxymethylate, and does not participate in the formation of the main structure of the resin. Melamine was added in the early stage of the reaction. The modified resin bonded plywood has high bonding strength and low formaldehyde emission.

Influence of wood leachate industrial waste as a novel catalyst for the synthesis of UF resins and MDF bonded with them

MDF (Medium Density Fiberboard) production processes produce a large amount of wood leachate as a hazardous waste. Wood leachate is a lignin-based waste material from the MDF production industry. It constitutes a potential environmental problem if discarded by penetrating into groundwater aquifers. Currently, there is no viable way for industrial MDF leachate waste to be transformed into environmentally friendly high value materials. The aim of this work was to present for the first time a novel approach which was successfully developed for the recovery of wood leachate industrial waste as a catalyst for the synthesis of UF adhesives for MDF panels. The leachates were examined by FTIR, 13 C NMR, and DSC as well as by evaluating the properties of fiberboards bonded with liquid UF resins. In addition, the other aim of this work was to capture the free formaldehyde gas emitted by MDF panels. It was of interest determine whether an MDF bonded panel prepared with a UF resin catalyzed by the wood leachate would exhibit lower formaldehyde emissions and the lowest thickness swelling, while still presenting a relatively good performance. The direct reuse of the fiberboard industrial leachate in the experiments demonstrated that the preparation method of the UF bonded fiberboard was environmentally friendly and sustainable. Therefore, this work arrived at some interesting conclusions that may be used as a base for future work on the recycling and reuse of MDF waste.

A Comparison among Lignin Modification Methods on the Properties of Lignin-Phenol-Formaldehyde Resin as Wood Adhesive.

The research aim of this work is to determine the influence of lignin modification methods on lignin–phenol–formaldehyde (LPF) adhesive properties. Thus, glyoxal (G), phenol (P), ionic liquid (IL), and maleic anhydride (MA) were used to modify lignin. The modified lignins were used for phenol substitution (50 wt%) in phenol–formaldehyde adhesives. The prepared resins were then used for the preparation of wood particleboard. These LPF resins were characterized physicochemically, namely by using standard methods to determine gel time, solids content, density, and viscosity, thus the physicochemical properties of the LPF resins synthesized. The panels dimensional stability, formaldehyde emission, bending modulus, bending strength, and internal bond (IB) strength were also measured. MA-modified lignin showed by differential scanning calorimetry (DSC) the lowest temperature of curing than the resins with non-modified lignin and modified with IL, phenolared lignin, and glyoxal. LPF resins with lignin treated with maleic anhydride presented a shorter gel time, higher viscosity, and solids content than the resins with other lignin modifications. Equally, the particleboard panels prepared with LPF resins with maleic anhydride or with ionic liquid had the lowest formaldehyde emission and the highest mechanical strength among all the synthesized resins. The dimensional stability of all panels bonded with modified lignin LPF resins presented no difference of any significance.

Effect of novel scavengers based on phenolic compounds on formaldehyde emission and physical-mechanical properties of particleboard

ABSTRACT This study focused on the influence of some novel scavengers on the physical, mechanical, and formaldehyde emission of particleboard produced with urea-formaldehyde resin. Three different scavengers, alizarin red sulfonate, alizarin yellow-GG, and chromotropic acid, were incorporated into the UF resin at 1, 3, 5, and 7% (by weight) loading levels based on the oven-dried weight of the resin. Other manufacturing parameters were kept constant in the production of particleboards. The results indicated that the formaldehyde emission of the particleboards significantly reduced with an increasing amount of scavengers. The lowest formaldehyde emission was found in the specimens containing alizarin red sulfonate (0.38 mg/l), followed by chromotropic acid (0.43 mg/l), and alizarin yellow-GG (0.49 mg/l), respectively, at 7 wt% loading level of the scavengers. Although the physical and mechanical properties of the particleboards decreased with the increasing content of the scavengers, they met the requirements of particleboards for interior fitments (including furniture) for use in dry conditions (P2 grade) of EN 312 standard. According to the results of technological properties and formaldehyde emission of the particleboards, it can be said that alizarin red sulfonate is the best scavenger among the investigated ones.

Preparation and Properties of the Urea-Formaldehyde Res-In/Reactive Halloysite Nanocomposites Adhesive with Low-Formaldehyde Emission and Good Water Resistance.

Low-cost urea formaldehyde resin (UF)/reactive halloysite nanotubes (HNTs) nanocomposite adhesive was prepared successfully via in situ polymerization. The HNTs were modified to improve its compatibility with polymer. The XRD and FTIR results showed that physical and chemical interaction between the HNTs and polymer resin influenced the structure of UF owing to the functional groups on the HNTs. It is found from SEM images that the modified HNTs could be dispersed uniformly in the resin and the nanocomposite particles were spherical. The performance experiment confirmed that thermal stability of nanocomposite increased largely, formaldehyde emission of UF wood adhesive reduced 62%, and water resistance of UF wood adhesive improved by 84%. Meanwhile, the content of HNTs on the nanocomposites could be up to 60 wt %. The mechanism of the nanocomposites based on the reactive HNTs was proposed. The approach of the preparation could supply an idea to prepare other polymer/clay nanocomposites.

Modification of nanoclay with different methods and its application in urea-formaldehyde bonded plywood panels

ABSTRACT The reinforcement of pristine nanoclay (PNC) and modified nanoclay (MNC) in urea-formaldehyde (UF) resins at the alkaline stage of synthesis was investigated in this work. The modification of PNC using copper (II) chloride (CuCl2) as transition metal ions was carried by three different modification methods that were coded as Cu-MNC (25, 36), Cu-MNC (60, 6), and Cu-MNC (80, 3), respectively. The modification of PNC was characterized by X-ray diffraction, showing the increased d-spacing after the modification. The reinforcement of 5 wt% of PNC and Cu-MNC at the alkaline reaction of UF resins that were coded as UF/PNC, UF/MNC (25, 36), UF/MNC (60, 6), and UF/MNC (80, 3). The X-ray diffraction study showed that the addition of PNC and Cu-MNC into the UF resins decreased the crystallinity of resins from 48.1% to 45% for UF/PNC and 25% for all UF/MNC. The curing behavior exhibited that the activation energy of UF resins decreased with addition of PNC and Cu-MNC. The loading of PNC and Cu-MNC into UF resins improved the adhesion and reduced the formaldehyde emission of UF resins in plywood. Statistical analysis suggested that UF/MNC (80, 3) produced plywood with greater adhesion strength and lower formaldehyde emission over other modified resins.

Tannin-furanic foams modified by soybean protein isolate (SPI) and industrial lignin substituting formaldehyde addition

Soybean protein isolate (SPI) was tested in this study as a crosslinker and formaldehyde substitute for preparing high biomass content sustainable rigid tannin-furanic-SPI (TFS) and lignin-tannin-furanic-SPI (LTFS) versatile foams. Additionally, flame retardancy was improved by lignin used as a natural fire-retardant. Fourier-transform infrared spectroscopy (FT-IR), coupled with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF-MS), revealed a covalent cross-linking reaction between tannin and SPI. TFS and LTFS foams showed by scanning electron microscopy (SEM) a closed cell structure without any pores. The incorporation of SPI resulted in enhanced mechanical properties and reduced pulverization ratios, improved thermal stability and increased thermal conductivity (approximately 0.042–0.044 W/m K compared with control foam. Furthermore, the TFS foams exhibited outstanding flame retardancy and suppressed smoke generation while undergoing combustion. These results were supported by a higher limiting oxygen index (LOI) value, a lower heat-release rate, and a higher char residue, obtained by LOI and cone calorimetry. The addition of lignin further enhanced the thermal properties and flame retardancy of TFS foams although it decreased their mechanical performance. The TFS and LTFS foams were environmentally friendly, as shown by the low formaldehyde emission measurements. This novel sustainable TFS foam appears to have a good potential for industrial application.

The Selection of Environmentally Friendly Wood for Raw Materials in the Creative Industries

The global economic crisis that hit the world turned out to cause a decline in exports. Therefore, in need of creative industries in raw materials production process and marketing. One of the creative efforts is the selection of wood raw materials for the industry is by using environmentally friendly wood species because it can reduce global warming. Still, the selling value of products in the export market is very high. The study design used a complete randomised design with an advanced Tukey test based on one factor, which is the types of wood that are Sengon, Waru Gunung, and Johar wood. The parameters observed are specific gravity, wood colour, adhesive strength, and formaldehyde emissions. The results showed that are Sengon, Waru Gunung, and Johar wood have different specific gravity, wood colour, and formaldehyde emissions. The average density of Sengon wood is 0.236, Waru Gunung wood is 0.570, and Johar wood is 0.654. The adhesive strength of Sengon wood 41.948 kg/cm2, Waru Gunung wood is 45.555 kg/cm2, and Johar wood 54.040 kg/cm2. Besides, the formaldehyde emissions for Sengon wood is 0.575 ppm (0.575 x10-3 ug/ml), Waru Gunung wood 0.462 ppm, and Johar wood 0.473 ppm. The higher the wood specific gravity, it will result in the wood colour and lower formaldehyde emissions. Wood with medium to high density (Waru Gunung and Johar) acknowledged as the environmentally friendly woods, and it can be used as raw material for creative industries.

In-situ modification of low molar ratio urea–formaldehyde resins with cellulose nanofibrils for plywood

This study describes the in-situ modification of low molar ratio urea–formaldehyde (UF) resins with cellulose nanofibrils (CNFs) to improve the poor performance of resins synthesized with different methods (Synth 1 and Synth 2) when adding second urea. UF resins were in-situ modified with CNFs dissolved in a mixture of dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) during the alkaline reaction step. Results showed that CNF addition enhanced the resin properties, such as shorter gelation time, faster curing rates at low activation energy, higher tensile shear strength (TSS), and lower formaldehyde emission (FE), compared to those of neat resins. The dry and wet TSS values of plywood bonded with UF resins modified with 3% CNFs synthesized using Synth 2 increased by 30% and 42%, respectively, and the FE value was decreased by 22% and 42%, respectively. These results reveal that in-situ modified UF resins with CNFs showed better performance than neat UF resins and that Synth 2 was more favorable than Synth 1.

Low-formaldehyde emission composite particleboard manufactured from waste chestnut bur

Chestnut bur is an agro-waste material generated in the chestnut production. It is a tannin-rich lignocellulosic material which might be a promising raw material for low-formaldehyde composite particleboard production when using urea–formaldehyde (UF) as bonding adhesive. In this study, the characteristics of chestnut bur were analyzed to assess its application value for composite panel. Five-type particleboards were manufactured from the mixture of chestnut bur/poplar particles with the weight ratios of 0/100, 25/75, 50/50, 75/25 and 100/0, UF resin was used as bonding adhesive. The effects of the addition amount of chestnut bur on the physical, mechanical properties and formaldehyde emission of particleboard were studied. The results showed as follows: (1) chestnut bur showed low cellulose, hemicelluloses and lignin contents, but high extractives and ash contents compared with poplar wood. Chestnut bur and poplar wood had the similar fiber morphology. (2) Composite board with relatively good performances could be manufactured with mix particles of chestnut bur and poplar wood. With the increasing of chestnut bur content, the mechanical properties of the composite board decreased, whereas the dimensional stability increased. (3) The total phenol content in the chestnut bur was as high as 13.79%. The phenolic substance in the chestnut burs could react with free formaldehyde. Hence, the free formaldehyde emission of particleboard was effectively reduced. In summary, waste chestnut bur is a suitable material which can not only be utilized as the base material, but also a natural free formaldehyde scavenger for composite particleboard production.

Hydrogen-Bond-Induced Crystallization in Low-Molar-Ratio Urea–Formaldehyde Resins during Synthesis

Low-molar-ratio urea–formaldehyde (UF) resins provide low formaldehyde emission at the expense of poor adhesion due to the formation of crystalline domains. We report for the first time the crystal...

Study of the synthesis parameters of a urea-formaldehyde resin synthesized according to alkaline-acid process

This paper presents a study of two crucial synthesis parameters of the alkaline-acid process of UF resin synthesis: condensation formaldehyde/urea (F/U) molar ratio and U feed rate during the methylolation step. The differences in the polymeric structures and the performance of the ensuing resins on particleboards (PBs) properties were analyzed and compared with a standard resin. The molecular weight distribution of the resins was monitored by gel permeation chromatography/size exclusion chromatography (GPC/SEC) and the unreacted oligomers by high-performance liquid chromatography (HPLC). The PBs produced were analyzed following European standards for mechanical tests and formaldehyde emission. The data obtained were also submitted to a statistical analysis.The results obtained showed that the use of low molar ratios yield higher internal bond strength (IB) values and lower formaldehyde emissions (F). In turn, the effect of urea feed rate during the methylolation step on IB values depends on the F/UII molar ratio used: (i) a reduction for the higher final molar ratio used, and no effect for the lowest final molar ratio used. Moreover, the statistical analysis carried out showed that F/UII molar ratio has significance on almost all of the resins’ characteristics.The resin used as standard yielded the best results when the final F/U molar ratio was 1.10 and the resulting PBs presented values of internal bond (IB) of 0.51 N mm−2 and formaldehyde content (F) of 5.1 mg/100 g o.d.b., complying with the market requirements.Additionally, the effect of resin ageing was also studied and the PBs prepared using the best resin upon fifteen days of its production presented similar IB values and even lower F content in relation to those obtained using fresh resin.

Microencapsulation of polymeric isocyanate for the modification of urea-formaldehyde resins

This study was conducted to prepare the microcapsules of polymeric 4-4 diphenyl methane diisocyanate (MpMDI) by interfacial polymerization using two different surfactants (Tween 40 and Gum Arabic) for the modification of urea-formaldehyde (UF) resins. MpMDI obtained by Tween 40 (TW-MpMDI) were spherical, smooth, and small diameter, while those prepared by Gum Arabic (GA-MpMDI) were irregular, rough surface, and larger diameter. Attenuated total reflection–Fourier transform infrared (ATR-FTIR) spectra of two types of MpMDIs confirmed the successful microencapsulation of pMDI by detecting the isocyanate (NCO) groups after rupturing the shell of MpMDIs. As the MpMDI content in UF resins increased, the non-volatile solids content, pH, gelation time, viscosity, and exothermic peak temperature of UF/MpMDI adhesives slightly increased. Plywood bonded with UF/MpMDI adhesives had 15% higher tensile shear strength (TSS) and 30% higher modulus of rupture (MOR) than those of the plywood bonded with neat UF resins owing to the formation of urethane (HN-(CO)–O-R) bonds. In particular, plywood bonded with UF/TW-MpMDI had higher TSS and MOR values than those bonded with UF/GA-MpMDI adhesive. Regardless of the surfactant types, the formaldehyde emission decreased about 54% with 5% MpMDI addition into UF resins. These results and statistical analysis revealed that 1% TW-MpMDI addition into UF resins provided a proper balance between good adhesion and low formaldehyde emission for plywood.

Converting crystalline thermosetting urea–formaldehyde resins to amorphous polymer using modified nanoclay

Thermosetting urea–formaldehyde (UF) resins as the most common adhesives for wood-based composites emit formaldehyde, which forces producers to lower formaldehyde/urea (F/U) molar ratio for the UF resins synthesis. However, low-molar-ratio (below 1.0) UF resins have low formaldehyde emission at the expense of poor adhesion, which is responsible for the formation of crystalline domains as a result of hydrogen bonds between linear molecules. For the first time, this study reports the conversion of crystalline UF resins to amorphous polymers by blocking the hydrogen bonds, using transition metal ion-modified bentonite (TMI-BNT) nanoclay through in situ intercalation. The modified UF resins with 5% TMI-BNT showed an almost amorphous structure, faster curing and higher cross-linking density compared with those of neat resins, and resulted in 56.4% increase in the adhesion strength and 48.3% reduction in the formaldehyde emission. Thus, blocking hydrogen bonds in low F/U molar ratio UF resins with TMI-BNT converted crystalline UF resins to almost amorphous ones, resulting in a significant improvement in their adhesion with a low crystallinity.

Structures, Properties and Potential Applications of Corncob Residue Modified by Carboxymethylation.

In this study, corncob residue (CR) valorization was simply and efficiently realized via carboxymethylation, and its enhanced performance as fillers in urea-formaldehyde (UF) resin was investigated. The structures of corncob residue and carboxymethylated derivative were analyzed by nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FTIR), and Raman techniques, respectively. The thermal stability, morphology, viscosity control, and adhesive strength were then investigated to evaluate its performance as fillers in UF resin composite. Similar to commercial flour, carboxymethylated CR could effectively disperse in UF resin. It also exhibited a better initial viscosity control between 30 and 50 °C. The adhesive test analysis showed that the shear strength of resin with carboxymethylated CR addition could reach 1.04 MPa, which was comparable to flour (0.99 MPa) and significantly higher than raw CR (0.45 MPa). Moreover, a low formaldehyde emission was observed.

Effect of Veneer-Drying Temperature on Selected Properties and Formaldehyde Emission of Birch Plywood.

In this study, the effect of the veneer-drying process at elevated temperatures on selected properties and formaldehyde emission of plywood panels was determined. We assume that during the veneer drying at high temperatures, more formaldehyde is released from it, and therefore, a lower formaldehyde emission can be expected from the finished plywood. Prior to bonding, birch veneers were dried at 160 °C (control) and 185 °C in an industrial veneer steam dryer (SD) and at 180 °C, 240 °C and 280 °C in an industrial veneer gas dryer (GD). Two types of adhesives were used: urea–formaldehyde (UF) and phenol–formaldehyde (PF) resins. Bonding quality, bending strength and modulus of elasticity in bending, water absorption and thickness swelling of plywood samples were determined. The formaldehyde emission level of samples was also measured. It was concluded from the study that the effects of veneer-drying temperatures on the bonding strength and physical and mechanical properties of plywood panels were significant. Veneer-drying temperatures of 185 °C/SD, 180 °C/GD and 240 °C/GD negatively affected the bending strength and the modulus of elasticity along and across the fibres for both UF and PF plywood samples. Bonding strength mean values obtained from all test panels were above the required value (1.0 MPa) indicated in EN 314-2 standard. The lowest formaldehyde emissions for the UF and PF plywood samples were observed in the samples from veneer dried in a steam dryer at 185 °C/SD.

Influence of Initial Molar Ratios on the Performance of Low Molar Ratio Urea-Formaldehyde Resin Adhesives

In this paper, the influence of initial formaldehyde/urea (F/U) molar ratios on the performance of low molar ratio (1.0) urea-formaldehyde (UF) resin adhesives has been investigated. Two initial F/U molar ratios, i.e., the first and second initial molar ratios were used for the alkaline addition reaction. Three levels of the first initial F/U molar ratios (2.0, 3.0, and 4.0) and two levels of the second initial molar ratios (2.0 and 1.7) were employed to prepare a total of six UF resins with an identical final molar ratio (1.0). The basis properties, functional groups, molecular weight, crystallinity, and thermal curing properties of the UF resins were characterized in detail. Higher levels (3.0 and 4.0) of the first initial F/U molar ratio provided the UF resins with better properties (non-volatile solids content, viscosity, gelation time, pH, and specific gravity) than those of the resins prepared with the conventional level F/U molar ratio of 2.0. Statistical analysis suggested that combining the first and second initial molar ratio of 4.0 with 1.7 would result in UF resins with greater adhesion strength and lower formaldehyde emission than those of the resins prepared with other molar ratios. The results showed that higher levels of the first initial molar ratio resulted in a more branched structure, as indicated by GPC, FTIR, DSC, XRD, and greater adhesion strength than those of the other UF resins with an identical final molar ratio of 1.0.