# The Effect of Silica-Filler on Polyurethane Adhesives Based on Renewable Resource for Wood Bonding
## 3 Results and Discussion
### Bondline Shear Strength  
It is clear from the analysis of shear strength of the bondlines in solid wood (Figure 4) that pressing temperature does not have remarkable effect on that parameter neither for A nor for C polyol-based adhesive. Thus, bonding can be performed at either temperature without significantly different results. However, due to low strengths of the A polyol-based PUR without SiO\({}_{2}\) (1.8-2.8 MPa) (Figure 4) its applicability in veneering is discrepant, so that--keeping in mind that the practical surface soundness of particleboard ranging from 0.8 to 1.6 MPa [26; 39; 44]--evaluation in pull-off test is necessary. Comparable shear strengths (2.84 MPa with high rate of substrate failure were reported by [PERSON] and co-workers [22] for the adhesives derived from cellulose acetate, 1,6-hexamethylene diisocyanate and castor oil with renewable content as high as 70%. Similar results were achieved for hot melt reactive polyurethanes derived from CO\({}_{2}\)-based poly(propylene carbonate): Lap-shear tests performed after 7-day curing on aluminum and steel revealed maximum strengths, respectively, 5.5 MPa and 4.7 MPa [45] and 1.5-6.0 MPa for plastics [46]. PUR adhesives based on biopolyols from liquefied biomass were proved to exhibit comparable performance as the shear strengths ranged between 5.8 and 11.0 MPa [21]. On the other hand, addition of silica had more apparent effect on bonding strength. The comparison of the values for non-SiO\({}_{2}\)-filled and SiO\({}_{2}\)-filled adhesive formulations revealed significant increase in shear strength, but the effect was more evident for A polyol-based adhesive of low innate strength: 1.8-2.8 MPa and 7.2-7.5 MPa for non-filled and filled, respectively. A lower performance of the A polyol-based adhesive without SiO\({}_{2}\) (A0% series) can be attributed to dense crosslinking due to A polyol hydroxyl functionality as high as 9 which renders brittleness and low mechanical properties. Dispersed silica presence resulted in improving of the mechanical properties of the adhesive (A2% series) according to the mechanisms reported in [29; 32]. Unlike the A-series, the C polyol-based adhesive in shear strength tests was not apparently affected by silica addition. The used wood substrate--though conforming the standards--did not allow measure true strength of the adhesive. The observed 1.7- to 3-fold enhancement as well as high substrate failure rate indicate that the approach was more effective than that reported for thermoplastic PURs composites [34] or nanosilica-filled PUR adhesive where 8% gain was achieved [27] and coherent with the results obtained for acrylic-polyurethane coatings doped with 4-6% nanosilica where 40% enhancement in adhesion was yielded [47]. Worth mentioning is that the performance of the SiO\({}_{2}\)-filled PURs is comparable to that of the commercial adhesives not containing bio-derived components--i.e., PUR 6.25 MPa (100% substrate failure) [48]. The phenomenon can be explained: (1) by the development of the hydrogen bonds between the hydroxyl groups of the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Polyol** & **SiO\({}_{2}\)-Filler Addition[wt\%]** & **Gelling Time[s]** \\ \hline A & 0 & 510 \\ A & 2.0 & 600 \\ C & 0 & 600 \\ C & 6.0 & 2100 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Gelling time at 20 \({}^{\circ}\)C for polyglycerol- and sucrose-based PUR adhesives (NCO/OH ratio 1.0). nanoparticles and the carbonyl groups of the polyurethane [49] which intensifies interfacial forces and is accordant with the chemical bonding theory of adhesion and (2) by the role of the filler in fracture formation. According to the crack pinning mechanism, rigid particles impede crack propagating, so that fracture toughness of a filled adhesive is increased [30; 31]. The effect was not manifested for the polymer of high innate strength--i.e., PUR based on polyol C (Figure 4).