## 4 Functional Properties of Chitosan and Derivatives
### Adhesive Properties  
CS is an interesting candidate for adhesive applications, especially in the wood field. CS has various DD and a large spectrum of Mw. It has been reported that its adhesive properties increase when DD and Mw increase [195; 196]. The mechanisms of adhesion are multiple [1; 197]. However, the surface tension and the viscosity of the liquid adhesive are important because they influence the interlocking mechanisms and modify the interactions with the adherent. First, the viscosity of the CS solution increases with concentration. For example, the viscosity is 90.2 Pa.s for a CS solution of 4% (w/v) and increases to 7132 Pa.s for a solution of 9% (w/v) [198]. Surface tension needs to be low to easily spread out upon all types of adherent materials. Surface tension is around of 38 mN.m\({}^{-1}\) for a 2% (w/v) CS concentration in 1 at 2% (v/v) acetic acid [199]. [PERSON] et al. [200; 201; 202] estimated that the surface tension of viscoelastic thermal compressed wood ranged between 28.6 and 35.5 mN.m\({}^{-1}\). Chain link analogy for an adhesive bond in wood was proposed by [PERSON] [203]. He considered a succession of links between adhesive and wood, especially in the interface between the boundary layer and the wood structure. This interface constitutes the adhesion mechanisms: mechanical interlocking, covalent bounding, and secondary chemical bounds due to the electrostatic forces through the adhesive penetration in wood cells (Figure 10). The penetration of CS solutions into wood or porous biosourced materials is discussed by [PERSON] et al. [202] and [PERSON] et al. [203]. No penetration is observed respectively into wood [202] and sunflower [203]. However, for water-based adhesive, water is adsorbed by the wood cell wall and the high molecular weight polymer molecules are trapped by the pit membrane [204]. For [PERSON] et al., secondary forces appear to be the dominated mechanism for bonding wood [205]. CS carries polar and H-bonding functional groups. At acidic pH, positively charged CS in wet condition interacts more strongly with  
Figure 10: Schematic representation of the interfacial zone between adhesive and wood. 1: adhesive boundary layer, 2: interface between boundary layer and wood substrate which constitutes the adhesion mechanism (mechanical interlocking, covalent bonding, or secondary chemical bonds), and 3: adhesive penetration zone. the negatively charged surface _via_ electrostatic forces, H-bonds, and van der Waals forces between glucosamine and the hydrated surface of adherend [7]. The bonding strength of CS was evaluated on three plywood veneer sheets with various amounts of CS before and after water immersion treatment [206]. Water treatment consisted of immersion for 3 h at 30 \({}^{\circ}\)C. Specimens were cooled in water and tested in the wet condition. The dry bond strength increased with increasing CS to 16 g.m\({}^{-2}\) and then decreased slightly. Before water immersion, the optimum bond strength was 2.13 MPa for 16 g.m\({}^{-2}\) CS and after immersion, the maximum value of the bond strength was 1.7 MPa in the condition of 32 g.m\({}^{-2}\). [PERSON] et al. [207] showed that the dry bond strength of CS is in the range 1.1 MPa-1.6 MPa for Mw varying between 35 and 350 kDa. With glucose addition (70 wt%), the bond strength increased to 1.75 MPa for low molecular weights CS. In contrast, the bond strength tended to decrease at greater amounts of added glucose for high molecular weight CS. The Maillard reaction in the above formulation formed brownish melanoidins, which occurred between the COOH of glucose and NH\({}_{2}\) of CS, demonstrating improved adhesive properties of glucose cross-linked low molecular weight CS. [PERSON] et al. [202] evaluated the potential of CS as wood adhesive using a double lap shear test. Three formulations were tested: CS 4% (w/v); CS 6% (w/v); and a formulation of CS 6% (w/v), glycerol 1% (v/v), and trisodium citrate dihydrate 5 mmol.L\({}^{-1}\). Dry bond strengths were respectively 4.2, 6.1, and 6.0 MPa. [PERSON] et al.