Contents lists available at ScienceDirect International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh Modification of urea-formaldehyde resin adhesives with blocked isocyanates using sodium bisulfite Muhammad Adly Rahandi Lubis a , Byung-Dae Park a, ⁎ , Sang-Min Lee b a Department of Wood and Paper Sciences, Kyungpook National University, Daegu, Republic of Korea b Department of Forest Products, National Institute of Forest Science, Seoul, Republic of Korea ARTICLE INFO Keywords: Urea-formaldehyde resin Blocked isocyanate Sodium bisulfite Hybrid resin Plywood ABSTRACT Polymeric 4-4 diphenyl methane diisocyanate (pMDI) was blocked with an aqueous sodium bisulfite solution to obtain water-dispersible blocked pMDI (B-pMDI) resin with different HSO 3 /–NCO mole ratios for the modification of urea-formaldehyde (UF) resin. Fourier transform infrared (FTIR) spectra of the B-pMDI resin clearly showed that all isocyanate groups of the pMDI resin were successfully blocked by sodium bisulfite. As the HSO 3 /–NCO mole ratio increased, the de-blocking temperature of the B-pMDI resin also increased. Two addition levels (1% and 3%) of the B-pMDI resin with different HSO 3 /–NCO mole ratios were mixed with UF resins and used as an adhesive for plywood. The gel time of the UF/B-pMDI resins decreased to a minimum at a mole ratio of 0.9 and then increased with the HSO 3 /–NCO mole ratio, and was consistent with the peak temperature (Tp). However, as the HSO 3 /–NCO mole ratio increased, the viscosity of the modified UF resins by 1% B-pMDI resin addition slightly increased, whereas those of modified resins with 3% B-pMDI resin addition rapidly increased. The adhesion strengths of plywood bonded with the hybrid resins were greater for 1% B- pMDI resin addition than for 3% B-pMDI resin addition. Formaldehyde emission of plywood bonded with the UF/B-pMDI resins significantly decreased up to 34% by the addition of B-pMDI resin at a mole ratio of 1.8. These results suggest that the modification of UF resins by mixing with water-dispersible B-pMDI resin can be a method for improving the water resistance and formaldehyde emission of UF resins for wood-based composites. 1. Introduction Urea-formaldehyde (UF) resins are the most commonly used resins, with widespread application in the manufacture of wood-based com- posite panels, such as plywood, particleboard, and medium-density fiberboard. UF resins have advantages over other adhesives, such as easy handling, low cost, and good performance in panel products [1]. They also have a high reactivity, a clear glue line, and the absence of organic solvents and are non-flammable [2]. On the other hand, low moisture resistance and formaldehyde emission from wood-based composite panels are the two most important disadvantages of UF resins [3]. Compared with UF resins, polymeric 4-4 diphenyl methane diiso- cyanate (pMDI) based resins have positive characteristics such as good bonding performance, high water resistance, aging resistance, and no issues relating to formaldehyde emission. Furthermore, the hydroxyl groups in wood may react with the isocyanate group of pMDI resins to form a polyurethane bond and provide direct covalent linkages between the adhesive and wood [4]. Several works have analyzed wood-pMDI cure using different methods, such as differential scanning calorimetry (DSC) [5], infrared (IR) spectroscopy [6], and nuclear magnetic resonance (NMR) spectroscopy [7]. They found that the formation of biuret, polyuret, and polyurea are very common in wood-pMDI cure systems. However, the formation of urethane bonds was only detected when huge excesses of pMDI were used. Unfortunately, these systems are very sensitive to moisture at room temperature before curing [8]. And, pMDI is not dispersible in water, because it is oil-borne. It is necessary to make pMDI resins compatible with water-based UF resins. Therefore, the blocking of isocyanate groups in pMDI resins is a solution to make pMDI resins dispersible in UF resins [9]. A blocked isocyanate is formed via a reaction between an isocyanate group (–NCO) and a compound containing an active hydrogen atom to block the –NCO. This product has the advantage of a long shelf life because the active isocyanate groups are masked and protected [10]. Furthermore, it has a small amount of the isocyanate groups and requires a high temperature to de-block and to make the – NCO group free. The blocked isocyanate to be applied as a wood adhesive should be de-blocked at less than 100 °C because the core temperature in the wood-based composite production during hot- pressing is not much higher than ~100 °C (usually in the range 105– http://dx.doi.org/10.1016/j.ijadhadh.2016.12.001 Accepted 26 November 2016 ⁎ Corresponding author. E-mail address: byungdae@knu.ac.kr (B.-D. Park). International Journal of Adhesion & Adhesives 73 (2017) 118–124 Available online 02 December 2016 0143-7496/ © 2016 Elsevier Ltd. All rights reserved. MARK