Author's personal copy Crystallinity and domain size of cured urea–formaldehyde resin adhesives with different formaldehyde/urea mole ratios Byung-Dae Park a,⇑ , Valerio Causin b a Department of Wood Science and Technology, Kyungpook National University, Daegu 702-701, Republic of Korea b Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131 Padova, Italy article info Article history: Received 12 July 2012 Received in revised form 22 October 2012 Accepted 31 October 2012 Available online 23 November 2012 Keywords: Urea–formaldehyde resin Crystallinity Domain radius X-ray scatterings Hydrolytic degradation abstract As a part of understanding hydrolytic degradation of cured UF resins responsible for form- aldehyde emission, this study investigated the crystallinity and domain size of cured urea– formaldehyde (UF) resins with different formaldehyde/urea (F/U) mole ratios using wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS) techniques. Both techniques were employed to calculate the crystallinity and domain size of cured UF resins as a function of F/U mole ratio. As the F/U mole ratio decreases, the crystallinity of cured UF resins increases from 26% to 48%. The SAXS provided useful information on the spatial arrangement of the crystalline domains and of their average distance. As the F/U mole ratio decreased from 1.6 to 1.0, the average distance between domains in cured UF resins decreased from 39 nm to 34 nm while the crystallite size increased from 1.3 nm to 5 nm. These results suggested that the crystallites were more closely packed with each other in cured UF resins when the F/U mole ratio decreased and the ordered domains were more randomly dispersed in high F/U mole ratio than low F/U mole ratio of UF resins. The greater amount of these ordered crystallites in cured UF resins of low F/U mole ratio was found to be correlated to the improved hydrolytic stability of the cured resins. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Urea–formaldehyde (UF) resin adhesive is a polymeric condensation product of formaldehyde with urea, and is considered as one of the most important wood adhesives together with melamine–urea–formaldehyde (MUF) res- ins, melamine–formaldehyde (MF) resins, and phenol– formaldehyde (PF) resins. The UF resin adhesive, an amino resin, is most widely used for the manufacture of wood- based composite panel such as plywood, particleboard or medium density fiberboard. Therefore, the wood panel industry is a major consumer of UF resin adhesive. In spite of some advantages such as lower cost, fast cur- ing, good performance in the panel, water solubility and colorless, UF resin adhesives also possess a critical disad- vantage: formaldehyde emission from the panels. The formaldehyde emission results primarily from UF resins in wood-based composite panels [1]. In addition, lower resistance of UF resins to water also limits the use of wood-based panels bonded with UF resin adhesives to interior applications. Furthermore, the formaldehyde emis- sion from the panels used for interior applications is known as one of the main factors causing sick building syndrome in an indoor environment. Therefore, the form- aldehyde emission issue has been one of the most impor- tant aspects of UF resin research [2–9]. For example, Que et al. reported that UF resins with low F/U mole ratios (0.97–1.27) resulted in a decreased formaldehyde emission at the expense of deteriorated mechanical properties of particleboard [10]. They suggested that a higher dosage of the resin or resin modification could compensate the deterioration. One of the UF resin modifications to reduce formaldehyde emission is to use amine compounds. The addition of different types of amine solutions such as urea, polyamine, methylamine, ethylamine or cyclopentylamine 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2012.10.029 ⇑ Corresponding author. Tel.: +82 53 950 5797; fax: +82 53 950 6751. E-mail address: byungdae@knu.ac.kr (B.-D. Park). European Polymer Journal 49 (2013) 532–537 Contents lists available at SciVerse ScienceDirect European Polymer Journal journal homepage: www.elsevier.com/locate/europolj