Chemical Modification of Soybean Flour-Based Adhesives Using Acetylated Cellulose Nanocrystals Farnaz Eslah, 1 Mehdi Jonoobi, 1,2 Mehdi Faezipour, 1 Alireza Ashori 3 1 Department of Wood and Paper Science and Technology, Faculty of Natural Resources, University of Tehran, Karaj, Iran 2 Division of Materials Science, Lulea ˚ University of Technology, Sweden 3 Department of Chemical Technologies, Iranian Research Organization for Science and Technology (IROST), Tehran, Iran In this study, two types of new bioadhesives formu- lated from abundant and renewable soybean flour (SF), acetylated soybean flour based adhesive (ASF), and soybean flour-based adhesive, were modified with acetylated cellulose nanocrystal (ACNC). The apparent viscosity and morphology of the adhesive formulations were characterized. The chemical composition of the formulations was evaluated by FT-IR spectroscopy and the effect of polyethilenimine (PEI) on the formulations was investigated using the proton nuclear magnetic resonance ( 1 H NMR) spectra. Moreover, water resis- tance of produced plywood composites bonded with the bioadhesives was measured. The results of FT-IR and 1 H NMR confirmed that chemical modifications of the SF occurred. The scanning electron microscopy (SEM) images showed less holes and cracks on the cross section of the ASF/PEI/NaOH and SF/PEI/NaOH/ ACNC formulations. The results showed that the ply- wood specimens bonded with formulations of the ASF/ PEI/NaOH (with a dry weight ratio of ASF/PEI: 5/1 and 6/1), and SF/PEI/NaOH/ACNC had good resistance to water. POLYM. COMPOS., 00:000–000, 2017. V C 2017 Society of Plastics Engineers INTRODUCTION Most adhesives used in wood composites are formaldehyde-based adhesives, such as phenol formalde- hyde (PF), ureaformaldehyde (UF), melamine–urea–form- aldehyde (MUF), etc. [1–3]. However, concerns about formaldehyde emissions and dependency on fossil resour- ces have renewed interest in natural-based adhesives made from bio-materials [4–6]. These bio-materials may include protein, starch, tannin, lignin, and bark [7]. Soy- bean is a bio-material from renewable sources and can be used as replacements for petrochemical-based polymers. Soy protein provides a good adhesion to wood surfaces, and soybean flour (SF) is one form of soy protein which is both abundant and sustainable, and has a low price with easily handled [8, 9]. However, soy-based adhesives cannot be used extensively in wood-based industry because of their poor performance, i.e., low bonding strength and weak water resistance [7, 8]. Many research- ers have attempted to modify these kind of adhesives to improve their adhesion properties. Soy adhesive modified with cellulose nanocrystal (CNC) was studied by Gao et al. [10] and their results showed that using CNC in soy adhesives improved water resistance of plywood samples about 20%. Another interesting study illustrated that sodium dodecyl sulfate can also improve the water resis- tance of soybean adhesives [11]. Xu et al. [12] reported that the water resistance of soy protein adhesives was improved by appropriate succinylation or PAE cross- linking. Gao et al. [13] showed the positive effect of cross linker species on the water resistance of soy adhesive. In addition, Liu and Li [14] found maleic anhydride to have negative effects on adhesion performance of soy protein. In recent years, much attention has been paid to nano- technology to develop a new generation of composites with high performance. The CNC is an abundant and renewable nanomaterial, which exhibits a low density as compared to mineral fillers [15]. CNC showed remarkable reinforcing properties in different matrixes such as sty- rene–acrylate latex [16], starch [17], poly hydroxyl buty- rate octanoate [18], or poly (ethylene oxide) [19]. The presence of hydroxyl groups in CNC increase its hydro- philic property and thereby increase the hydrophilic prop- erty of the polymer matrix to which they are added [20]. Although the CNC have a great potential as mentioned Correspondence to: A. Ashori; e-mail: ashori@irost.ir Contract grant sponsor: Iran Nanotechnology Initiative Council and the University of Tehran. DOI 10.1002/pc.24389 Published online in Wiley Online Library (wileyonlinelibrary.com). V C 2017 Society of Plastics Engineers POLYMER COMPOSITES—2017