# The Effect of Silica-Filler on Polyurethane Adhesives Based on Renewable Resource for Wood Bonding
## 1 Introduction  
The research on bio-based or bio-derived adhesives and polyurethanes has been carried out for years [1; 2; 3; 4; 5; 6; 7; 8]. Since [PERSON] and [PERSON] [9] stated twelve principles of green chemistry, the investigations in the field of cleaner production have intensified. One of the most important classes of adhesives is polyurethane (PURs) that are widely used in wood and furniture industries. Therefore, it is important to increase the content of bio-based components in PUR adhesives--preferably by substitution of petroleum-based polyols with renewable feedstock-derived ones. The convenient feedstock are glycerol and sucrose. The former is a by-product from vegetable oils transformations to biodiesel [10; 11] and the latter is produced in large amounts from sugarcane or sugar beet. Due to their renewability, bio-origin, low cost, and availability, both glycerol and sucrose have great potential as raw material for the development of new classes of polyurethanes [12; 13; 14]. Incorporation of disaccharides into polyurethane structure results in enhanced hydrolysis in soil which promotes biodegradation [15]. It is also anticipated that the demand on industrial sugar will increase in world markets [16]. Therefore, new ways of industrial use of sucrose should be investigated. The current trends in the adhesives for furniture sector are formaldehyde-free adhesives [17; 18], sucrose-based adhesives [19; 20], and renewable resource-based polyurethanes [21; 22]. Depletion of petroleum supplies, the fluctuating prices and environmental problems which associated with oil industry, make renewable, plant-derived resources more and more attractive for this class of adhesives [23]. Biopolyols are excellent substitutes for petroleum-derived polyols as long as the resultant PURs of high renewable content exhibit properties comparable to traditional ones. Unlike for structural applications, adhesives dedicated to furniture manufacturing--especially for veneering of medium density fiberboards or particleboards--are not required to exhibit very high strength as thesurface soundness of these materials typically varies between 0.8 MPa and 1.6 MPa depending on the density of a panel [24; 25; 26]. No lower limit for the bondline strength in the substrate-adhesive-veneer system is defined. Instead, in practice, a visual analysis of the edge after machining is used for the evaluation of bond quality. Thus, it was hypothesized that 2-component PUR adhesives of a simplified formulation limited to one biopolyol and one diisocyanate would exhibit properties sufficient for veneering in furniture manufacturing, as well as, decrease the cost of an adhesive. A great advantage of polyurethanes is the ease to tailor their properties to a specific area of application such as foams, coatings, or 1- and 2-component adhesives. A formulated adhesive contains a number of components: Isocyanates, a mixture of polyols, catalysts, chain extenders, tackifiers, fillers, and other additives [27; 28]. The use of multi-component formulations is to provide an adhesive with high strength, resistance to chemical and physical factors, as well as a long-term durability in service, which is fully justified when adhesive is dedicated to structural applications in construction or engineered wood products. In this study two PUR adhesives containing polyglycerol- or sucrose-based polyols were investigated for their application in wood bonding. Industrially feasible adhesives have to meet the requirements such as practical reactivity, viscosity, and bonding strength. These properties have to be tuned for each newly developed binder. The proposed adhesives are expected to be used in veneering in furniture industry. However, the initial experiments indicated that their physical and chemical properties had to be adjusted. The reactivity was adequate, but the low viscosity was a concern due to adhesive overpenetration through a veneer.