Holzforschung 2018; aop Muhammad Adly Rahandi Lubis and Byung-Dae Park* Analysis of the hydrolysates from cured and uncured urea-formaldehyde (UF) resins with two F/U mole ratios https://doi.org/10.1515/hf-2018-0010 Received January 12, 2018; accepted April 10, 2018; previously published online xx Abstract: Uncured and cured urea-formaldehyde (UF cured ) resins prepared with formaldehyde/urea (F/U) mole ratios of 1.0 and 1.2 and at the catalyst levels of 1, 2 and 3% NH 4 Cl were hydrolyzed for 5, 15 and 30 min and the degradation products were evaluated using gel permeation chroma- tography, Fourier-transform infrared (FTIR) and liquid carbon-13 nuclear magnetic resonance ( 13 C-NMR) spectro- scopies. The molar masses of the degradation products, their functional groups and structures were determined. An extended hydrolysis time and higher catalyst levels resulted in compounds with higher molar masses. Simi- lar functional groups and distribution of chemical species were found by both FTIR and 13 C-NMR spectroscopies in the hydrolysates of UF cured , indicating water hydrolysis of hydroxymethyl groups and then methylene linkages. Methylene linkages and mono- and tri-hydroxymethyl ureas were mainly responsible for the liberation of formal- dehyde from UF cured during hydrolysis. The indicated com- pounds are believed to contribute to the long-term release of formaldehyde from the resins. This is the first system- atic report on the composition of UF cured hydrolysates. Keywords: 13 C-NMR, formaldehyde emission, FTIR, GPC, hydrolysate, molecular species, UF resin Introduction Studies on the hydrolysis of cured urea-formaldehyde resins (UF cured ) are important in understanding the performance, formaldehyde release and recycling of these resins (Park and Jeong 2011a,b; Nuryawan and Park 2017; Lubis et al. 2018). UF resin hydrolysis is the reverse reaction of its syn- thesis (Myers 1982, 1984, 1986). The penetration of water molecules into the UF resin glued joints in wood-based panels initiates the hydrolysis reaction chain (Lee et al. 1994; Kim et al. 2006). Water attacks the hydroxymethyl (CH 2 OH) end group, and then splits either C-N-C or C-N-O bonds from the methylene or ether linkages in UF cured . These reac- tions depend on many factors, such as the formaldehyde/ urea (F/U) ratio, chemical structure, degree of crystallinity, degree of cross-linking and hydrolysis conditions (Dutkie- wicz 1983; Stuligross and Koutsky 1985; Levendis et al. 1992; Park and Jeong 2011a,b; Park and Causin 2013; Nuryawan and Park 2017). A recent study focused on the hydrolytic sta- bility and degradation of UF cured by investigating their physi- cal and morphological features after hydrolysis (Nuryawan and Park 2017). However, researches on the hydrolysis deg- radation products of UF cured are limited. To the best of our knowledge, only a few studies have addressed this topic (Dutkiewicz 1983; Myers 1986; Chuang and Maciel 1994). The present work is aimed at a better understanding of the hydrolytic degradation of UF cured by measuring mass loss, liberated formaldehyde and pH of the hydrolysates. The analytic program includes gel permeation chroma- tography (GPC), Fourier-transform infrared (FTIR) and liquid carbon-13 nuclear magnetic resonance ( 13 C-NMR) spectroscopies of the hydrolysates. The expectation is to better understand the way how the formaldehyde is liber- ated from the UF cured in wood-based composites and how a typical hydrolysis pathway should be designed. Materials and methods Technical-grade urea granules (99%), formaldehyde (37%) and aqueous solutions of both formic acid (20%) and NaOH (20%) were purchased from Daejung Chemical, Korea. An aqueous solution of NH 4 Cl (20%) served as a catalyst. UF resins with inal F/U mole ratios of 1.0 and 1.2 were prepared by an alkaline–acid two-step reac- tion according to the literature (Park and Jeong 2011a,b; Lubis et al. 2017; Nuryawan and Park 2017). Approximately 500 g of formalde- hyde solution was irst mixed with 187 g urea for addition reaction (methylolation). The pH was controlled between 7.8 and 8.0 during the reaction. After that, the pH was adjusted to 4.6 for condensation. *Corresponding author: Byung-Dae Park, Department of Wood and Paper Sciences, Kyungpook National University, Daegu 41566, Republic of Korea; and Institute of Agricultural Science and Technology, Kyungpook National University, Daegu 41566, Republic of Korea, e-mail: byungdae@knu.ac.kr Muhammad Adly Rahandi Lubis: Department of Wood and Paper Sciences, Kyungpook National University, Daegu 41566, Republic of Korea Brought to you by | Kyungpook National University Authenticated Download Date | 5/11/18 10:53 AM