Wet-bonding

NR adhesives can be divided in two types wet bonding and dry bonding. Wet bonding adhesives are applied on substrates in a fluid state, the bond being formed by drying. The dry bonding NR adhesives are pressure-sensitive adhesives because the bond is created under pressure. 

For comparison, a conventional resol resin adhesive without lignin was prepared (24), and its gluability was examined. This resin was found to require a hot-pressing rate of at least 1.5 min per 1 mm plywood thickness before a satisfactory wet-bond adhesion strength was achieved at the low hot-pressing temperature of 120°C. This indicates that replacing a part of the phenol with lignin does not imply a mere extender addition, but that a positive role is achieved which enhances the reactivity of the adhesive. 

Figure 1. Comparison of wet-bond adhesion strengths of lignin-resol resin adhesives phenolated with and without acid catalyst. Legend phenola-tion with acid catalyst at 80°C for 60 min Q phenolation without catalyst at 200°C for 60 min. Note Numerical values in parentheses are percentages of wood failure hot-press temperature 120°C.

Figure 2. Relationship between hot-press time and wet-bond adhesion strength for lignin-resol resin adhesives with and without alkylresorcinol. Legend Q without alkylresorcinol 0 with 10 parts alkylresorcinol. Note Numerical values in parentheses are percentages of wood failure phenola-tion 200°C, 60 min, without catalyst hot-press 120°C, 6 min.

To determine the wet bond strength coated panels were immersed in distilled water for 1500 h, removed and discs 25.4 mm in diameter stamped from them. The surfaces were wiped with a dry tissue and bonded between two cylindrical test pieces using a polyamide cured epoxide adhesive and immediately placed in a sealed container at 100% RH for the adhesive to cure. After 16 h the specimens were broken on an Instron Universal Test Machine with minimum delay. Recovered values were measured after the panels had dried out at room temperature and humidity for 7 days. Clearly, it is unlikely that the values reported represent the minimum bond strengths, as some drying out is almost inevitable, but the values are directly comparable. 

Surface oxidation processes have also been used as pretreatments for improving the bonding strength of adhesives. Brink et al. [9] reported that the wet bonding strength of plywoods or particleboards manufactured using phenol formaldehyde increased after pretreatment of wood with nitric acid. Mari et al. [10] also reported that nitric acid oxidation reduced the amount of isocyanate resin adhesive required to manufacture particleboard and improved the mechanical properties and biological resistance of boards. 

One-sided bonding (wet bonding) In this procedure, the adhesive is applied to one adherend only. This is recommendable for bonding of solvent-permeable, respectively, absorbent materials (leather, textiles, wood products). In this case, complete solvent evaporation is not required. 

Variety of bonding methods The numerous forms and types of rubber-based adhesives and sealants also provide for numerous mechanisms for developing bonds to surfaces. Contact bond, pressure sensitive, wet bond, heat reactivation, and solvent reactivation are all feasible modes of product assembly with these products. 

As already mentioned, the etch-and-rinse technique should be employed in conjunction with the wet-bonding approach. This has been demonstrated by various in vitro studies [14,30,31], and is especially important when adhesives formulated with acetone are used [31,32]. However, this is difficult to achieve practically, as the enamel needs to be dried for bonding, and it is not straightforward to dry one without drying the other [15]. There is also uncertainty as to how wet the dentine needs to be in order... 

Y. Wang, P. Spencer, Hybridization efficiency of the adhesive/dentin interface with wet bonding, J. Dent. Res. 82 (2003) 141-145. 

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