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Introduction
Urea-formaldehyde (UF) resin adhesive is a polymeric condensation product obtained by the chemical reaction of formaldehyde with urea, and it is considered as one of the most important wood adhesives. The UF resin adhesive is, an amino resin, and is most widely used for the manufacture of wood-based composite panels such as plywood, particleboard or medium density fibreboard. Therefore, the wood panel industry is a major consumer of UF resin adhesive. In addition, the presence of crystal structure in UF resin has been reported by several authors ([13] Stuligross and Kousky, 1985; [10] Pratt et al. , 1986; [9] Park and Jeong, 2011).
Nanocomposites produced by organic polymers and inorganic clay minerals containing silicate layers such as montmorillonite (MMT) have attracted great interest due to adventitious improvements in mechanical and, thermal properties, and barrier like structure with high clarity without a significant increase in density, which is not possible with conventional fillers ([14] Usuki et al. , 1993; [6] Lee et al. , 2004). Layered silicates that are used commonly in preparation of polymer layered silicate nanocomposites belong to general family of 2:1 phyllosilicates. MMT, hectorite, and saponite are the most commonly used layered silicates. In order to render layered silicates miscible with the polymer matrices, hydrophilic silicate surface should be converted to an organophilic surface, and this process makes the intercalation of many engineering polymers possible ([11] Ray and Okamoto, 2003). Polymer/layered silicate (P/LS) nanocomposites generally improve properties of polymeric materials, even at very low volume fraction loading (1-5 per cent) of layered silicates; opposed to the high volume fraction loading (»50 per cent) in the traditional advanced composites ([4] Dubois and Alexandre, 2000). Nanometer size particles in a (polymer/ceramic/metal) matrix disperse in order to form nanocomposites. The polymers containing layered silicate clay minerals as a reinforcing agent can be classified as speared (intercalated) or dispersed (exfoliated) depending on dispersion of clay in the matrix. Hybrid organic-inorganic substances composed of nanometer-sized particles are dispersed in a polymer matrix that is known as polymer/clay nanocomposites. Clays exhibit plastic behaviour upon heating, and they remain as hard materials at elevated temperatures. Polymers, resins and plastics with clay additives are used mainly as structural components for bridges, as replacements for heavier steel parts in ship...