## 4 Discussion
Considering the maximum storage modulus (E\({}^{\prime}\)) of the wood-adhesive glue line bond, for PVAc-D3 adhesive blends, D3-LIGNO 2.5% could have achieved the highest stiffness degree due to its composition. As a derivative of lignin, lignocellulosiconate is a polyphenonic compound with important volumes of available hydroxyl groups, which can form hydrogen bonds with matrices rich in OH groups [50], such as PVAc, working very well as a cross-linking agent. This could have caused D3-LIGNO 2.5% to have the highest stiffness value (E\({}^{\prime}\)) among the PVAc-D3 samples at the solid glue line. It is well known that lignin and its derivatives are capable of acting as cross-linking agents, replacing phenol in phenol-formaldehyde resins with good results, since their high volume of aliphatic and phenolic hydroxyl groups favors their reactivity and reticular structure when interacting with some kind of adhesive [51]. In the same way, the adhesive-wood bond with D3-MWCNT 0.5% in the solid state indicated that the carbon nanotubes can stiffen the glue line with a lower concentration than PVAc D3-LIGNO. In this case, the solid state of the glue line was achieved because of solvent (water) loss or evaporation by temperature rise (DMA test), where PVAc-D3 adhesive fusion causes an intimate approach with carbon nanotubes, generating a composite, which in the solid final state remained embedded in the thermoplastic matrix [52]. Both reinforcing agents (LIGNO and MWCNT) presented a higher stiffness degree at the adhesive bond line than the neat PVAc-D3 adhesive, which shows that both reinforcements significantly improve its performance. However, they did not exceed the E\({}^{\prime}\) value at the glue line solid state achieved by the PVAc-D4 adhesive, which is concordant with the evaluation carried out by FTIR analysis. According to the results of the mechanical properties, LIGNO and MWCNT fillers improve PVAc-D3 performance at dry conditions. However, D3-LIGNO blends at elevated temperature conditions had a better performance than D3-MWCNT blends. This behavior could be related to the lignocellulosic thermal stability that, as a lignin-derived compound, has exhibited extraordinary thermal stabilizer properties evaluated in different thermoplastic polymers with interesting results [53; 54; 55]. Lignin is a cross-linked bio-polymer with some amorphous zones, which is formed by phenylpropane units bound to ether and carbon-carbon links. Its chemical structure based on aromatic compounds and carbon-carbon groups makes it quite thermally stable, generating considerable char remains in the combustion process [56]. It is relevant to mention that the lignocellulosic structure is formed by 0.4 to 0.7 sulfonate groups and two times methoxyl groups per unit of phenyl propane than neat lignin (mostly hydrophobic), making it soluble in water [24; 57; 58]. This could have an effect on adequate affinity and cohesion between lignocellulosic particles and the PVAc-D3 adhesive, as well as good particles dispersion into the adhesive matrix, causing a near interaction with the hydroxyl groups (-OH) of the wood and adhesive, which generate a strong glue bond associated with its polar condition [58]. In addition, its negative charge and alkaline pH (>9) allow a good dissociation of phenolic hydroxyl groups, increasing its structural strength. The same as lignin, lignocellulosic is formed by a cross-linked molecular structure [24], which could obtain a reinforced linear-amorphous PVAc polymer due to its various functional groups that may interact with the adhesive matrix available molecules, stiffening blends structure. Because of these reasons, specimens glued with D3-LIGNO blends at dry conditions showed close values of shear strength and wood failure to the D4 adhesive. Even though shear strength reached values very close to the D4 adhesive, D3-LIGNO blends probably did not obtain a structure cross-linked enough and as chemically stable as the D4 adhesive, yielding because of the load applied after temperature and humidity treatments [1].