餈＆ 605 餈 J. Korean Wood Sci. Technol. 42(5): 605~614, 2014 http://dx.doi.org/DOI : 10.5658/WOOD.2014.42.5.605 Microstructure of Cured Urea-Formaldehyde Resins Modified by Rubber Latex Emulsion after Hydrolytic Degradation 1 Arif Nuryawan 2 ⋅Byung-Dae Park 2,† ABSTRACT This study investigated microstructural changes of cured urea-formaldehyde (UF) resins mixed with aqueous rubber latex emulsion after intentional acid etching. Transmission electron microscopy (TEM) was used in order to better un- derstand a hydrolytic degradation process of cured UF resins responsible for the formaldehyde emission from wood-based composite panels. A liquid UF resin with a formaldehyde to urea (F/U) molar ratio 1.0 was mixed with a rubber latex emulsion at three different mixing mass ratios (UF resin to latex = 30:70, 50:50, and 70:30). The rate of curing of the liquid modified UF resins decreased with an increase of the rubber latex proportion as determined by differential scanning calorimetry (DSC) measurement. Ultrathin sections of modified and cured UF resin films were exposed to hydrochloric acid etching in order to mimic a certain hydrolytic degradation. TEM observation showed spherical particles and various cavities in the cured UF resins after the acid etching, indicating that the acid etching had hydrolytically degraded some part of the cured UF resin by acid hydrolysis, also showing spherical particles of cured UF resin dispersed in the latex matrix. These results suggested that spherical structures of cured UF resin might play an important role in hindering the hydrolysis degradation of cured UF resin. Keywords : acid hydrolysis, cured urea-formaldehyde resin, microstructure, rubber latex, spherical particles, trans- mission electron microscopy 1 Date Received May 26, 2014, Date Accepted July 25, 2014 2 Department of Wood and Paper Sciences, Kyungpook National University, Daegu 702-701, Rep. of Korea † Corresponding author : Byung-Dae Park (e-mail: byungdae@knu.ac.kr) 1. INTRODUCTION For many decades, urea-formaldehyde (UF) resins have been the predominant thermosetting adhesive used worldwide for appearance grade wood based- composite panels. Owing to many advantages such as fast curing, good strength in the panels, colorless, and low cost, UF resins are extensively used for manufacturing particleboard or fiberboard in industrial practice. However, there are two main drawbacks of UF resins: firstly, formaldehyde emission from UF resin-bonded wood composite panels, and secondly, hydrolysis when they were exposed to water or moisture. Hydrolysis of cured UF resins was mainly responsible for the formaldehyde emission in long term and low water resistance (Myers and Koutsky 1990; Chuang and Maciel 1994; Tohmura et al. 2000). The hydrolysis of UF resins is due to rever- sible reactions to their synthesis reactions; exposing UF resin-bonded wood composite panels to water