• Journal of the Korean Wood Science and Technology
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• 제47권5호
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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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• 제48권3호
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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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• 제44권6호
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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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• 제49권3호
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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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• 제49권1호
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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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• 제47권2호
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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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• 제45권4호
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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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• 제45권3호
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• pp.360-367
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• 2017

본 연구에서는 단열성능 및 물리적 성질이 우수한 친환경 목섬유 단열재의 제조 기술을 확립하기 위하여 멜라민 요소 폼알데하이드(MUF), 페놀 폼알데하이드(PF), emulsified MDI (eMDI) 및 라텍스계 수지 등 서로 다른 접착제로 제조한 저밀도섬유판의 특성을 비교하였다. 그 결과 MUF, PF, eMDI 수지 접착제로부터는 경질(硬質) 저밀도섬유판이, 라텍스계 수지 접착제로부터는 연질(軟質)의 저밀도섬유판이 각각 제조되었다. 모든 저밀도섬유판은 일반 중밀도섬유판에 비해 현저히 낮은 열전도율을 나타냈으며, 압출 발포 폴리스티렌과 유사한 단열 성능을 나타내는 것으로 조사되었다. 한편 본 연구에서 제조한 저밀도섬유판 중 eMDI를 사용한 것은 흡수 두께/길이 팽창률 및 휨 강도 등 물리적 성질이 가장 우수하였다.

• Journal of the Korean Wood Science and Technology
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• 제45권1호
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• pp.126-138
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• 2017

본 연구는 주원료로 도계부산물인 닭털의 NaOH 가수분해물, 경화제로 닭털의 $H_2SO_4$ 가수분해물 및 폼알데히드계가교제를 반응시켜 접착제를 제조하고, 이에 대한 물성 및 내수성 실험을 통하여 닭털의 목질계 판상재용 접착제의 원료화 가능성을 확인하기 위하여 수행하였다. 닭털은 주로 케라틴계 단백질로 구성되어 있었으며, 중금속의 함유량이 매우 낮거나 검출되지 않았다. 알칼리에 대한 닭털의 가수분해율은 수용액 내의 NaOH 농도가 증가함에 따라 계속 증가하였으나, 과도한 가수분해에 따른 단백질 고유의 접착능 손실을 최소화하고 적정한 가수분해 조건을 찾기 위하여 닭털의 가수분해제 내의 NaOH 농도를 5%, 7.5%, 10%로 결정하였다. 접착제의 조성을 보면, 고형분 함량을 기준으로 70%의 닭털 NaOH 가수분해물 또는/그리고 경화제로 닭털의 $H_2SO_4$ 가수분해물 및 30%의 가교제로 조제하였는데, 이 접착제의 고형분 함량은 가수분해 조건 및 가교제의 종류에 따라 28.3 - 44.8% 범위에 존재하였다. 이 접착제의 점도는 상온에서 전반적으로 높았으나, $50^{\circ}C$에서 측정한 결과 분사형 접착제로서 적용이 가능한 것으로 조사되었다. 접착제의 내수성을 비교하기 위하여 측정된 열수 불용해율은 5% 농도의 NaOH 수용액에서 반응시킨 닭털의 알칼리 가수분해물(CF-AK-5%)에 경화제로 5% 농도의 $H_2SO_4$ 수용액에서 반응시킨 닭털의 산가수분해물(CF-AC-5%)을 고형분함량 기준 10% 이상 첨가하여 제조하고 경화시킨 접착제에서 높았다. 또한 접착제 제조시 고형분 함량을 기준으로 30%가 첨가된 가교제별 열수 불용해율은 phenol-formaldehyde (PF), melamine-urea-formaldehyde (MUF), formalin 순으로 조사되었다. 닭털 접착제의 열수 불용해율을 섬유판 제조에 사용되고 있는 석유화학계 합성수지와 비교한 결과, CF-AC-5%에 가교제로 PF를 그리고 경화제로 CF-AC-5%를 첨가하여 조제한 접착제는 기존 멜라민-요소수지의 열수불용해율과 통계학적으로 차이가 없었으며, 가교제로 PF를 사용한 모든 접착제와 고형분 함량을 기준으로 55%의 CF-AK-5%, 15%의 CF-AC-5% 그리고 가교제로 30%의 MUF와 함께 제조한 접착제는 기존 요소수지를 대체할 수 있는 것으로 조사되었다. 이 결과를 토대로 적정한 조건에서 가수분해한 닭털은 목질계 판상재용 접착제의 원료로 사용이 가능할 것으로 판단된다.

• Journal of the Korean Wood Science and Technology
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• 제43권5호
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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.