Improving performance of polyvinyl acetate (PVA) as a binder for wood by combination with melamine based adhesives Alireza Kaboorani, Bernard Riedl n De´partement des sciences du bois et de la forˆ et, Faculte´ de foresterie, de ge´ographie et de ge´omatique, Universite´ Laval, 2425, rue de la Terrasse, Que´bec, Que., Canada G1V 0A6 article info Article history: Accepted 28 May 2011 Available online 6 July 2011 Keywords: Adhesives for wood Mechanical properties of adhesives Durability Wood abstract In this study blending PVA with MUF and MF was evaluated as an approach to enhance the performance of PVA towards water and elevated temperatures. MF and MUF were added to PVA at different proportions: 15%, 30%, 50%, 70% and 100%. Blends of PVA with MF and MUF were used as adhesives to bond wood joints. The shear strength of wood joints was measured at dry and wet states, and elevated temperatures. Thermogravimetric analysis was used to study thermal stability of PVA and its blends with MF and MUF. The structural changes caused by the inclusions were characterized by Fourier transforms infrared spectroscopy (FT-IR). The results showed that shear strength of wood joints were improved by the addition of MF and MUF to PVA in all conditions. Adding small amounts of MUF or MF (as low as 15%) enhanced the performance of wood joints towards water and elevated temperatures. The extent of improvement was sometimes so high that the strength of glue line surpassed strength of wood in wet conditions leading to wood failure rather than glue failure. MF had more effectiveness in improving shear strength of wood joints than MUF in all conditions. Thermal stability of PVA was increased by MF but the effect of MUF on thermal stability of PVA was dependent on MUF proportions and temperatures. FT-IR analyses showed that there are some chemical bonds between PVA and MF. Considering costs, effectiveness and formaldehyde emission, adding 15% MF to PVA seems the optimal proportion of MF in the PVA blends. Crown Copyright & 2011 Published by Elsevier Ltd. All rights reserved. 1. Introduction As using adhesives allows wood industry to use small diameter trees to produce high quality products; the wood industry increas- ingly uses adhesives in its products. Wood adhesives account for a substantial part of production costs. In order to decrease the production costs, wood industry tries to use lower cost adhesives, sometimes compromising the quality of the products. Polyvinyl acetate (PVA) is one of the relatively low cost adhesives that wood industry has used for decades. Despite being non-toxic and having the least negative impact on the environment, PVA suffers a main drawback. PVA has very poor performance in fire and in wet conditions. Mechanical performance of PVA adhesive deteriorates with increasing temperature and it loses its bonding resistance capacity at over 70 1C [1–2]. Although wood products have been finding a large market in the construction field, poor performance of products bonded with PVA can limit the market share of wood products in construction. The poor performance of PVA at elevated temperatures stems from the fact that PVA is a thermoplastic polymer, which reaches Tg at relatively low temperatures. In fact, the curing process of PVA only includes water evaporation and subsequent formation of hydrogen bondings, which are very much vulnerable to water and elevated temperatures. Of course there are some cross-linked grades of PVA with improved performance at elevated temperatures and humid conditions. Research con- ducted on PVA has focused mostly on improving water resistance of PVA by several approaches [3–10] and little attempt was made on the area of heat resistance. A melamine formaldehyde adhesive (also commonly called melamine glue or an MF) is a condensation product of methylated melamine and formaldehyde. Melamine-formaldehyde adhesives have acceptable water resistance. High costs of MF have led to mixing MF with urea formaldehyde (UF) adhesive, however it was introduced many years after UF and have not attained great importance as an adhesive. The main applications of melamine based adhesives (MF and melamine–urea–formaldehyde (MUF)) are exterior and semi-exterior plywood and particleboard, and finger joints. Both MF and MUF give excellent adhesive perfor- mance, good moisture resistance and tend to produce lower formaldehyde emissions than UF adhesives. Both MF and MUF are thermosetting adhesives and during curing process they go through chemical reactions including cross- linking. Adding MF or MUF to PVA may trigger some cross-linking reactions in PVA. Such cross-linking reactions can induce some Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ijadhadh International Journal of Adhesion & Adhesives 0143-7496/$ - see front matter Crown Copyright & 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2011.06.007 n Corresponding author. Tel.: þ1 418 656 2437; fax: þ1 418 656 2091. E-mail address: Bernard.Riedl@sbf.ulaval.ca (B. Riedl). International Journal of Adhesion & Adhesives 31 (2011) 605–611