No abstract

This study investigated, by differential scanning calorimetry, the thermal curing behavior of urea-formaldehyde (UF) resins modified with two formaldehyde scavengers and its impact on the adhesion performance of particleboard bonded with modified UF resin adhesives. As the concentration of the two scavengers [i.e., urea-formaldehyde prepolymer (UFP) and urea solution (US)] increased, the gel time, peak temperature, and onset temperature of the modified UF resins did not change significantly for UFPmodified UF resins, but these parameters increased for the US-modified UF resins. These results indicated that the reactivity of the UF resin modified with UFP was maintained, but it deteriorated for the US-modified UF resins. The formaldehyde emission of particleboard bonded with the modified UF resin decreased with an increase in the scavenger concentration. UFP was more effective than US in scavenging the formaldehyde emission and in achieving adhesion to the UF resin. The results showed an optimum UFP level of 20% in the UF resin for maintaining a balance between the formaldehyde emission and adhesion of the particleboard, and they indicated that both the thermal curing behavior of scavenger-modified UF resins and the properties of particleboard bonded with them must be taken into account in the evaluation of a formaldehyde scavenger system.

ABSTRACT:The purpose of this study was to investigate the effects of reaction pH conditions on thermal behavior of urea-formaldehyde (UF) resins, for the possible reduction of formaldehyde emission of particleboard bonded with them. Thermal curing properties of UF resins, synthesized at three different reaction pH conditions, such as alkaline (pH 7.5), weak acid (pH 4.5), and strong acid (pH 1.0), were characterized with multiheating rate method of differential scanning calorimetry. As heating rate increased, the onset and peak temperatures increased for all three UF resins. By contrast, the heat of reaction (⌬H) was not much changed with increasing heating rates. The activation energy (E a ) increased as the reaction pH decreased from alkaline to strong acid condition. The formaldehyde emission of particleboard was the lowest for the UF resins prepared under strong acid, whereas it showed the poorest bond strength. These results indicated that thermal curing behavior was related to chemical species, affecting the formaldehyde emission, while the poor bond strength was believed to be related to the molecular mobility of the resin used.

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.

Wood-fiber insulation boards can be utilized as a core construction material. They provide a comfortable and safe residential space and reduce energy consumption because of the ecofriendly nature and high heat insulation. In this study, wood-fiber insulation boards were prepared with different types of adhesive (melamine-urea-formaldehyde (MUF), phenol-formaldehyde (PF), emulsifiable 4,4’ methylene diisocyanate (eMDI), and latex resins), and the physical and heat insulation properties, toxic chemical emissions, and combustion characteristics were analyzed. The different adhesive types had no major effects on the insulation. With regard to the toxic emissions, all wood-fiber insulation boards showed the best rating possible except for the PF resin. In the cone-calorimeter test, the wood-fiber insulation board prepared with MUF showed a lower total heat release, mean heat release rate, smoke release, and CO and CO2 yields than the other samples because of the early formation of the carbonized layer. Based on the comprehensive evaluation, the MUF adhesive is the best choice for wood-fiber insulation boards.