• Journal of the Korean Wood Science and Technology
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• v.47 no.5
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• pp.579-586
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• 2019

This work examined effects of the synthesis method and melamine content of melamine-urea-formaldehyde (MUF) resins on the bond-line features (i.e. resin penetration and bond-line thickness) in plywood. Two synthesis methods (MUF-A and MUF-B) and three melamine contents (5, 10, and 20%) were employed to prepare MUF resins. The MUF-A resins at three melamine contents were prepared by a simultaneous reaction of melamine, urea, and formaldehyde, while the MFU-B resins were prepared by reacting melamine at the same levels with formaldehyde followed by urea. The results showed that higher melamine content increased the viscosity of MUF-A and MUF-B resins. The resin penetration of MUF-A resins decreased by 48% while those of MUF-B resins increased by 16% at 20% melamine content. As a result, the MUF-A resins had greater bond-line thickness than those of MUF-B resins as the melamine content increased. The MUF-B resins resulted in thinner bond-line and greater resin penetration compared to those of MUF-A resins. The results suggested that MUF-B resins prepared with 20% melamine content had a proper combination of resin penetration and bond-line thickness that could produce plywood panel with a better adhesion performance.

• Journal of the Korean Wood Science and Technology
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• v.48 no.3
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• pp.291-302
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• 2020

As the properties of the melamine-urea-formaldehyde (MUF) resins were changing during their storage time, this study investigated the impacts of the synthesis method and melamine content of the MUF resins on the pH, apparent viscosity, molecular weights, and crystallinity to estimate these properties over storage times of up to 30 days. Melaminesat three addition levels (5, 10, and 20 wt% based on the resin solids) were simultaneously reacted with urea and formaldehyde (MUF-A resins), while those at the same addition levels were first reacted with formaldehyde and then with urea(MUF-B resins). The pH values of the MUF-A and MUF-B resins decreased linearly as the storage time increased; the apparent viscosity increased linearly for the low melamine contents (5% and 10%) but increased exponentially for 20%. As anticipated, the molecular weights (Mw and Mn) increased linearly with the storage time, with a steeper increase in the Mw of the MUF-B resins compared with that of the MUF-A resins. The crystallinity of the two resin types decreased with storage time at higher melamine content. The relationships between these properties and the storage time made it possible to estimate the property changes in these resins synthesized by the different synthesis methods and melamine contents; this could help predict the properties of such resins in the industry during their storage.

• Journal of the Korean Wood Science and Technology
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• v.44 no.6
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• pp.913-922
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• 2016

This study attempted to measure molecular weight (MW) of melamine-urea-formaldehyde (MUF) resins prepared by two different synthesis methods: the one-step MUF resins were synthesized in one batch procedure, while the two-step MUF resins were prepared by a physical mixing of urea-formaldehyde (UF) resin with melamine-formaldehyde (MF) resin that had been synthesized in a separate procedure. The properties of medium density fiberboard (MDF) panels bonded with two types of MUF resins were also investigated. MWs of these MUF resins were measured using gel permeation chromatography (GPC). In addition, this study measured the MWs of one-step MUF resin during its synthesis procedure. The performance of two types of MUF resins was evaluated by determining properties of MDF panels prepared in laboratory. As the synthesis procedure progressed, both number average MW ($M_n$) and weight average MW ($M_w$) of one-step MUF resin gradually increased, while the polydispersity index (PDI) decreased. And low Mw species of the resin predominantly decreased as the synthesis step progressed. The one-step MUF resin showed greater $M_n$ and $M_w$ than those of the two-step ones even though the PDI values of both resins were very similar each other. As expected, the one-step MUF resin resulted in better properties of MDF panels than those of two-step resins. In particular, the one-step MUF resin provided better internal bond (IB) strength and thickness swelling (TS) with MDF panels than those of two-step ones, indicating better water resistance of the one-step resin. These results suggest that the preparation method of MUF resins have a great impact on the MW and final panel properties.

• Journal of the Korean Wood Science and Technology
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• v.49 no.3
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• pp.274-282
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• 2021

Transparent and colorless melamine-urea-formaldehyde (MUF) resins make it difficult to identify the area of wood failure percentage (WFP) in the fracture surface of bonded wood specimens. Therefore, in this study, we develop a method of measuring WFP after the adhesion strength measurement of MUF resins under shear stress. The fractured wood surface of b lock shear strength (BSS) specimens bonded with cold-setting MUF resins at three melamine contents (20%, 30%, and 40%) was marked black, and then, WFP was accurately measured via image analysis. WFP values measured using this method consistently increased with BSS as the melamine content increased, showing the reliability of this new method. The results suggested that this new method is useful and reliable for measuring the WFP of the fracture surface of wood specimens bonded with colorless adhesives such as urea-formaldehyde, MUF, and melamine-formaldehyde resins.

• Journal of the Korean Wood Science and Technology
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• v.49 no.1
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• pp.1-13
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• 2021

This study was conducted to investigate the effects of the synthesis method, melamine content, and GPC parameters (such as flow rate, column-detector temperature, and sample injection temperature) on the molecular weight of melamine-urea-formaldehyde (MUF) resins. Two different synthesis methods were employed. In the first method, MUF-A resins were synthesized by simultaneously reacting urea, formaldehyde, and melamine (5%, 10%, and 20%) using the reaction of alkaline-acid-alkaline steps under controlled temperature and viscosity. In the second method, MUF-B resins were synthesized by first reacting melamine at the same levels with formaldehyde and then by adding urea. The highest weight average molecular weight (Mw) of MUF-A resins was found at 10% melamine content when the flow rate was 0.3 and 0.8 ml/min; Mw decreased slightly at 20% melamine content. The results showed that Mw increased with an increase in the melamine content when the flow rate was 0.5 and 1.0 ml/min. In addition, Mw was the highest when the flow rate, column-detector temperature, and injection temperature were 0.3 ml/min, 50℃, and 25℃, respectively. On the contrary, MUF-B resins had greater Mw and number average molecular weight (Mn) than MUF-A resins. Overall, Mw and Mn increased as the melamine content increased. The optimal GPC parameter for MUF resins was determined as follows: a flow rate of 0.5 ml/min, a column-detector temperature of 50℃, and a sample injection temperature of 50℃.

• Journal of the Korean Wood Science and Technology
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• v.47 no.2
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• pp.200-209
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• 2019

To improve the water resistance of melamine-urea-formaldehyde (MUF) resins, different levels of blocked polymeric 4,4 diphenyl methane diisocyanate (B-pMDI) were blended with MUF resins to prepare B-pMDI/MUF hybrid adhesives, and their adhesion performances were evaluated for the surface lamination of fancy veneer on plywood. FT-IR spectra showed that the de-blocked -NCO groups reacted with the -OH of hydroxymethyl groups of the MUF resins to form urethane bonds at 2% B-pMDI/MUF, which was detected before and after their hydrolysis. The mass loss after the hydrolysis consistently decreased as the B-pMDI level increased, indicating an improvement in the water resistance. As the B-pMDI level increased, the activation energy of hybrid adhesives decreased, which improved the reactivity of the hybrid adhesives. Additionally, the water resistance improvement of the hybrid adhesives increased the tensile shear strength of the surface laminated plywood in semi-water proof and water-proof by 23 % and 8 %, respectively, at 2% B-pMDI level. This was likely due to the urethane linkages in the hybrid adhesives. However, the formaldehyde emission from plywood panels bonded with the hybrid adhesives increased in the dry state, indicating incomplete curing of the hybrid adhesives.

• Journal of the Korean Wood Science and Technology
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• v.45 no.4
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• pp.409-418
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• 2017

This work attempted to manufacture glued-laminated timber (Glulam) bonded with melamine-urea-formaldehyde (MUF) resin adhesives at various melamine contents from 20% to 50% under high frequency (HF) heating for a very short time. Two preparation methods were employed to prepare MUF resin adhesives with different melamine contents: one-batch method of synthesizing MUF resins in a single batch, and two-batch method of mixing urea-formaldehyde (UF) resin with melamine-formaldehyde (MF) resin that had been synthesized separately. As the melamine content increased, the gelation time and peak temperature of MUF resins decreased. The adhesion performance of plywood showed that the one-batch MUF resin adhesive with 50% melamine content only satisfied the standard requirement of water resistance. Thus, the one-batch MUF resin adhesive with 50% melamine content was applied for bonding wood lamina from four softwood species such as Japanese larch, Korean red pine, Korean pine and Japanese cedar to manufacture Glulam under HF heating. All Glulam samples bonded with the one-batch MUF resin adhesives with 50% melamine content except those from Korean Red Pine satisfied the requirement in water soaking or boiling water delamination test as an exterior grade Glulam. The presence of rosin in Korean Red Pine was believed to be responsible for its poor adhesion. These results showed that the one-batch MUF resin adhesives with 50% melamine content provided acceptable water resistance with exterior grade Glulam manufactured under HF heating.

• Journal of the Korean Wood Science and Technology
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• v.45 no.3
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• pp.360-367
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• 2017

This study was carried out to compare the characteristics of low density fiberboards (LDFs) manufactured with different adhesive types such as melamine urea formaldehyde (MUF), phenol formalehyde (PF), emulsified MDI (eMDI) and latexes resins. As results, hard LDFs were successfully manufactured by MUF, PF and eMDI resins. Thermal conductivities of all LDFs were significantly lower than commercial medium density fiberboard. Especially, all LDFs showed comparable thermal insulation performance with extruded polystyrene foam (XPS). LDF manufactured with eMDI resins showed the highest physical properties such as thickness/length swelling by water absorption and bending strength.

• Journal of the Korean Wood Science and Technology
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• v.45 no.1
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• pp.126-138
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• 2017

This study was conducted to investigate the potential of chicken feather (CF), which is a by-product in poultry industry, as a raw material of wood adhesives. For the purpose, adhesive resins were formulated with NaOH- and $H_2SO_4$-hydrolyzed CF as well as crosslinkers, and then the properties and water resistance of the adhesive resins against hot water were measured. CF was made of mainly keratin-type protein, and no or very low content of heavy metals was detected. Hydrolysis rate of CF increased as NaOH concentration in hydrolysis solutions increased. However, in order to minimize the loss of adhesive property of protein itself by the severe hydrolysis of CF and to seek its proper hydrolysis conditions, NaOH concentrations in hydrolysis solution determined to adjust to 5%, 7.5% and 10%. In the NaOH-hydrolyzed CF, $H_2SO_4$-hydrolyzed CF as a hardener and crosslinker were added to formulate CF-based adhesive resins. Solid content of the resins ranged from 28.3% to 44.8% depending on hydrolysis conditions and type of crosslinker. Viscosity of the resins at $25^{\circ}C$ was very high. However, when the temperature of the resins was increased to $50^{\circ}C$, the viscosity decreased greatly and thus the resins could be applied as a sprayable resin. Retention rate measured to evaluate the water resistance of adhesive resins was the highest in the cured resin formulated with 5% NaOH-hydrolyzed CF and 5% $H_2SO_4$-hydrolyzed CF of 10% based on the solid weight as a hardener. Retention rate depending on crosslinkers added into adhesive resins was the highest phenol-formaldehyde (PF) followed by melamine-urea-formaldehyde (MUF) and formalin. The retention rate of CF-based adhesives formulated with 5% NaOH-hydrolyzed CF, PF and $H_2SO_4$-hydrolyzed CF of 10% and over did not differ statistically from that of commercial MUF resins. All of CF-based adhesives formulated with PF as a crosslinker and one with 5% NaOH-hydrolyzed CF of 55%, 5% $H_2SO_4$-hydrolyzed CF of 15%, and MUF of 30% on the basis of solid weight could be substituted for commercial urea-formaldehyde resins, From the results, CF can be used as a raw material of wood adhesives if hydrolyzed in proper conditions.

• Journal of the Korean Wood Science and Technology
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• v.43 no.5
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• pp.682-690
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• 2015

The bonding performance of two types of wood adhesives, namely phenol-resorcinol-formaldehyde (PRF) resin and melamine-urea-formaldehyde (MUF) resin for glued laminated timber manufactured by high frequency (HF) heating was evaluated. The HF heating system consists of HF oscillator with dielectric heating system for curing adhesives, and hydraulic press system for clamping glued laminated timber. The designed frequency and output power of the HF system was as 5 MHz and 60 kW, respectively. To verify dielectric heating mechanism under HF oscillation, the heat loss factors of laminae and adhesives were measured. The results show that it is possible to selectively heat adhesives for their curing due to the remarkably higher loss factor of the adhesives than those of wood laminae. The temperature of adhesive in the bonding line reached up to the set temperature within a few seconds by high frequency oscillating, which advanced the curing of adhesive afterwards. The bonding performance, such as shear strength of bonding line, water soaking delamination, and boiling water soaking delamination of PRF resin met the requirement of Korean Standard (KS), however the MUF resin did not meet the KS requirement of boiling water soaking delamination. These results indicate that the HF heating system is successful to manufacture glued laminated timbers with PRF resins to meet the bonding requirements.